Introduction
1. INTRODUCTION
1.1 Background and purpose of the study
A rising concern about fires caused by electrical switchboards gave the initiative to this
study. In Sweden, many buildings are wooden structures. The combustible energy of an
apparatus enclosure made of flammable thermoplastics can be sufficient to cause severe
damage to such buildings, posing a substantial threat to human life, livestock and property.
Discussions with authorities, rescue services, insurance companies and manufacturers
revealed a general uncertainty whether insufficiently stringent flammability test methods or
a lack of adequate materials were at the root of the problem. Meanwhile, the Swedish
society, universities and enterprises have a strong commitment to environmentally benign
solutions, which called for innovative approaches to improve material properties.
The project which is summarized in this thesis set out with two objectives: The first was to
identify the main factors hindering reliable material characterization and flammability
testing. The second was to develop new materials with improved flame retardancy and
reduced environmental impact.
An obvious reason for the lacking reliability of fire tests of polymers is the materials
tendency to deform and flow under thermal impact. A material that strongly changes its
geometry is difficult to test in a meaningful way and may have an unpredictable behavior in
end product use. On the one hand, flow might lead to a withdrawal of the material from the
heat source and result in an improved ignition resistance [1, 2]. Such behavior is undesirable
for electrical appliances and cables, where the polymer is intended to insulate current
carrying parts. On the other hand, flow might lead to increases in the burning surface area
[3‐5], flame feed‐back by pools of burning materials [5], and flame‐spread to adjacent items.
The latter effects significantly influence the flame spread rate and fire growth [5, 6]. Only a
few studies have dealt with the flow of burning polymers (see section 1.5) and many
questions remain unanswered. Therefore, melt flow phenomena were selected as the
central issue for this study to make a contribution to improved flammability testing.
On a practical level, the influence of melt flow on the two most widely used flammability
tests for classification of electrical appliances, the glow wire test and the 50 W vertical flame
test (UL 94 test), was identified [7‐9] and is described in further detail as part of this work.
On a scientific level, this thesis discusses the effect of flow and surface deformation on the
flammability properties of three polymers of high industrial importance. How these
phenomena influence the results of material characterization tests, such as the cone
calorimeter test method, finds special emphasis.
The first material is polystyrene, which is abundantly used in blends to form enclosures for
electrical appliances. Polystyrene forms a low viscous melt in response to a thermal insult,
leading to a fast melt flow. An adequate description of the burning behavior of items made
of polystyrene requires knowledge of the material’s mass loss and flow behavior. Both result
directly from changes in temperature and molar mass. Detailed thermal analysis was
performed to obtain the coupling between mass loss rates and rates of molar mass
reduction. The molecular level results were applied to explain the rates of decomposition of
macroscopic samples that underwent thermal gasification and gasification/melt flow
experiments.
1
Introduction
The second material is flexible polyurethane foam, which is prone to a uniquely high flame
spread rate [2] and is a major cause of fire‐related deaths [10‐12]. A distinctive volume
change and liquid formation during combustion make the foam a suitable subject for this
study. A detailed description of the mechanism of foam collapse and the heat release rate
during that process are given. A coupling of flame heat transfer and specimen deformation
was observed that might improve the understanding of combustion rates of burning items
made of foam. The chemical composition, as well as the physical properties, of the cross‐
linked polyurethane change in a step‐wise transition during combustion, making it an
interesting counterpart to polystyrene.
As an approach to develop improved materials, the potential of inorganic nanofillers to
prevent flow and deformation was assessed. Polyurethane foam composites were prepared
which contained 4 wt.% of carbon nanofibers, or alternatively organically modified clay, to
partially substitute a conventional phosphorous‐brominated flame retardant. The carbon
nanofiber composites effectively prevented liquefaction and collapse of the foam during
combustion by forming a gelled network structure that transformed to an expanded carbon
residue. Intercalated clay composites were not effective at inhibiting collapse and
liquefaction.
The third material is a halogen‐free, mineral‐filled polyolefin cable formulation, composed of
acrylic‐ethylene copolymers, chalk and silicone. Mineral filled cables are widely used
throughout the European Union, where new legislation introduces classifications
disqualifying cables that yield flaming droplets after exposure to a burner [13]. The
flammability test performance of cables varies strongly with mounting variables, such as
bundling, spacing, clamping etc. [14]. For mineral filled polyolefin cables, it is suspected that
a significant part of the variation is caused by the mechanical instability of molten material
and the brittleness of protective mineral layers formed during combustion. A better
understanding of the transition from a polymer to a protective mineral layer was the goal of
this study. The mechanism of thermal barrier formation of different halogen‐free cable
compounds was analyzed and tested for robustness. Resistance or failure resulted from the
time‐temperature dependence of both, the heat transfer into the material and the
degradation processes that lead to thermal barrier formation.
The potential of nanosized calcium carbonate particles to accelerate the processes that lead
to the formation of a thermal barrier by the mineral filled cable material was evaluated.
1.2 Facts on fires in Sweden
Direct losses from fires amounted to 0.16 % of the gross domestic product in Sweden for the
period 2003 to 2005 [15]. Approximately the same amount, 0.17 % of gross domestic
product, was invested in fire protection of buildings [15]. Electrical fires were the origin of
11 % of all fires in the period 1998‐2007 and caused 5 % of fires that resulted in civilian
deaths in 2008 [16, 17]. The number of civilian deaths resulting from fires has slightly
decreased since 1998, whereas the economic damage caused by fires has increased [16].
2
Introduction
1.3 Illustration of relevant fire scenarios
Figure 1‐1: Left: Pool fire formed by a TV enclosure made of high impact polystyrene. With
permission from reference [18]. Right: Cable tray in a corridor yielding a rain of flaming droplets after
exposure to a burner. With permission from reference [19].
1.4 Melt flow in polymer combustion
Polymers are organic molecules consisting of long chains formed by the successive reaction
of smaller organic molecules. Solid polymer products for everyday use owe their stiffness
either to a sufficient entanglement of the chains, or to chemical or physical cross‐links that
maintain a network structure. The success of polymers in manufacturing relies on their
ability to flow at moderate temperatures (100 °C to 300 °C), which makes it easy to mold
complex shapes. For entangled materials, heat activates chain segment movement enabling
the material to flow. Physically cross‐linked materials are heated to temperatures that
exceed the stability of the physical bonds hindering chain movement. Chemically cross‐
linked materials are handled in the form of their liquid precursors and are solidified by
reaction.
The temperatures reached by a burning polymer surface (300 °C to 500 °C) generally exceed
the temperature needed to transform the material to a moldable mass of low viscosity. In
consequence, the combustion of most polymers engenders a flow of liquefied organic
material. The effect is amplified as combustion proceeds by thermal decomposition of the
macromolecular chains into smaller fragments. The smallest molecules volatize and mix with
atmospheric oxygen to form a flame. Fractions of intermediate chain length have a strongly
reduced viscosity and can accelerate the flow of burning melt. The molecular structure and
mechanism of decomposition of the polymer determines the respective size of the volatile
and liquid fractions formed. Section 3.2 presents results touching on these fundamental
concepts. The thermal degradation of polystyrene was analyzed. Random scission of the
main chain produces successively smaller chains, whereas mass loss proceeds mainly by the
repetitive removal of small fragments from the chain ends (“unzipping”) [20]. The rate of
both processes determines how much material is gasified and how much material
transforms to a low viscosity liquid. The thermal degradation of polyethylene and the
3
Intr
I rod
ducttion
opo
olym
merrs dis
d cusssed iin secctio
on 3.4
4 iss aalso
o the co
onsequ
uen
nce
e o
of ran
r dom
m scissio
on [2
21].
co
Re
epeetitiive sccisssion
n pro
p odu
ucess frag
f gmentts thaat are suffficien
ntly smaall to vo
olattize
e. Both,
po
olysstyrren
ne and
a d p
polyyethylene form
f m a deg
d grad
ded
d mel
m t w
with
h a lo
ow visscosityy and
a a heeat off
co
omb
busstio
on not
n mu
uch
h lo
owe
er than
n th
hatt off miineeral oill [3, 5,, 22
2].
An
n id
deaalizeed illu
ustrration of meelt flo
ow fro
om a bur
b rnin
ng sur
s facce typ
t ical fo
or pur
p re poly
p yolefin
ns, orr
po
olysstyrren
ne, is givven in Figurre 1‐2
2. A
An arrran
ngem
meent sim
milaar to a 5
50 W verticcal flaamee teest
(U
UL 9
94 test
t t) is taakeen as
a a faamiliar exam
mple. Th
he bur
b rnin
ng m
melt flow
f ws ffrom a th
hin
n su
urfaace layyerr
off th
he spe
s ecim
men
n aand caan sprread out
o on
n a neearby surface.. Po
ossiblyy, a burn
b ning p
poo
ol fo
orm
ms,
wh
hich hea
h ats up
p the initiaal b
burnin
ng item. In reealiity, th
his miight h
hap
ppen ffor an
n elec
e ctrical
ap
ppaarattus attop
p a taablee or
o insstalled
d in
n a wal
w l. Fig
F uree 1‐2b
1 b illusstraates tthe
e flow
w eeven
nts
en
ncountterred in this pro
p ocesss, wh
here
eass Figure 1‐2
2c sho
s owss th
he hea
h at ffluxxes invvolvved
d. The
T e flo
ow
rate of th
he degraade
ed me
elt deependss on
o thee hea
h at tra
t nsffer fro
om th
he flame
e, b
butt itt also
de
eterrminees how
h w lo
ong thee mat
m terial is eexpo
oseed to
t the
t flaamee and the
ereeby afffectts how
h w degrrad
ded
the maaterrial beco
omes. Shorrter rete
r enttion
n timees leaad to
o leess vo
olattizaatio
on and red
ducced
co
ontrribu
utio
on to the initial flaamee, but
b t migh
m ht res
r ult in a larrger poo
p ol fiire.. Tw
wo main
n faacto
ors
go
overn this proc
p cesss: (1) ‐ The
T e first faccto
or iss th
he heaat of thee flam
me, a pro
p oduct of the
e raate off
vo
olattile pro
odu
ucttion
n (m
masss losss byy gaasifficaatio
on) and the
t heeat of com
mb
busttion
no
of th
he vollatiless. A
paart of thee flaam
me h
heaat iss trransfeerre
ed b
bacck into
o th
he ma
m terrial,, geene
erattingg new
w vo
olattile
es, w
which
results in self‐su
ustain
ned bu
urn
ningg. The
T e am
mountt of heat
h t su
usttain
ningg co
om
mbustio
on conse
equ
uen
ntlyy
de
epeends on
o the
t ratte of
o m
maass loss and he
eat traansffer lim
mitaatio
onss. (2
2) ‐ Th
he sec
s ond
d faactor is the
t
vissco
osityy of th
he deg
d graded
d melt
m t, whic
w ch govverrns thee raate of maaterial reemo
ovaal.
Figgure
e 1‐2:
1 Le
eft: Photo
ograaph off melt drippiing fro
om a bur
b ning poly
p yme
er ssurfface
e. TThe specim
men was
w
13
3 mm wid
w de and
a d 2 mm
m th
hickk. C
Center: Scche
emaatic illu
ustrration of meelt flow
f w from
f m a bu
urniing po
olym
merr
specimen, sho
s win
ng ffrom
m leeft to
t rrigh
ht: (a) o
ove
ervieew,, (b) masss flo
ow, and
d (cc) heatt traansffer..
Bo
oth main
m n fact
f tors, thee rate
r e of
o maass lo
oss an
nd the
e vvisccosity,, stro
ongly depen
nd on
n the
t
temp
peraatu
ure of the mat
m terial. Geene
erally, th
he rate
r e of
o mas
m ss losss in
ncreeasses exxponen
ntiaallyy with
w
temp
peraatu
ure,, wherreaas the visscossityy off no
on‐‐degraadeed poly
p ym
merss de
ecrreasses exxpo
onentiaallyy with
w
temp
peraatu
ure.. Bo
oth
h prrop
perttiess arre linkked byy th
he mo
m lar maass disstriibution of
o the po
olym
merr. The
T
rate of maass loss iss co
onn
nectted
d to
o mola
m ar mas
m ss red
r ucttion
n by a no
on‐lineearr relatiion
nshiip. Polym
merr
osityy of
o enta
e anggled
d chaiins geenerally dec
d creaasees by
b ap
pow
wer off 3.4 with
w hd
decreaasin
ng m
mo
olarr
vissco
masss and de
ecrreassess lineaarlyy for
f veery sh
hortt cha
c inss [2
23]. This
T s redu
ucttion
n of
o vvisccossity is
uperim
mpo
osed to
t the
e eeffe
ect of te
emp
perratu
ure. In
n ord
o er to un
nde
ersttan
nd thee effe
e ect of melt
m
su
de
egraadaatio
on on meelt flo
ow in polym
merr co
omb
busstio
on, it wa
w s dee
d me
ed iinsttructivve to reccorrd the
t
4
Introduction
evolution of molar mass under conditions that match the temperatures and heating rates of
a burning polymer surface. Section 3.2 provides an analysis of molar mass changes in
gasification and melt flow/gasification experiments on polystyrene.
Flexible polyurethane foam is an example of a different molecular architecture, which leads
to a different flow behavior during combustion. In the foaming process, a macromolecular
network is formed by step growth. A short‐chain, trifunctional polyether polyol (molar mass
3000 g mol‐1) is reacted with toluene diisocyanate (TDI) and water. TDI reacts with water to
form urea segments which interlink the trifunctional polyol by urethane bonds [24]. These
reactions are reversed at 200 °C, a process that attains a very high rate in the vicinity of
300 °C [25]. The TDI molecules volatize and undergo further reactions [26], whereas the
polyether polyol, or a similar substance, remains as a low viscous fluid [27‐29]. The low
decomposition temperature of the foam, paired with its uniquely low thermal conductivity,
results in a fast flame spread and fire growth rate [2]. Meanwhile, a burning low viscous
liquid is formed that might spread out to a burning pool [22]. The morphological changes of
the foam during combustion are the subject of section 3.3. The effect of external heat flux
and flame heat flux on the rate of foam liquefaction was studied for a range of external heat
flux levels. Small scale experiments were performed in which the liquid formed by burning
foam samples was collected and could accelerate the rate of combustion of the initial
burning item.
In end product use, most polymers are blended with mineral fillers and other additives that
modify their properties. The presence of fillers can strongly alter the flow properties of the
polymer. Still, the material needs to be moldable at the processing temperature and flow is
commonly observed in the combustion of end products. Ideally, the material should be easy
to process, but should resist flow at combustion temperatures. One way to obtain such
behavior is through a high structural integrity of the filler that can maintain a solid
structure [30]. This is demonstrated for flexible polyurethane foam filled with carbon
nanofibers in section 3.3.5. Another way is to use a high filling level of inorganic material
and to activate reactions between the filler and decomposition products of the polymer to
counteract flow [31]. Such an approach is taken for the cable material studied in section 3.4.
The efficiency of measures that counteract flow depends on the severity of the thermal and
mechanical impact to the material. A methodology for characterizing the robustness of
resistance against flow is needed. Paradoxically, an improved resistance against flow usually
results in longer exposure times to the heat source. Therefore, resistance against flow can
only be part of a strategy to retard fire growth.
1.5 Literature
This section briefly refers to publications that influenced the general approach taken in this
study. More references are given in the respective sections and publications.
1.5.1 Melt flow in combustion
Melt flow phenomena are frequently reported in flammability research as side observations
and as experimental limitations. Very few studies dealt directly with melt flow phenomena.
Kashiwagi et al. [6] reported a reduction of the horizontal flame spread rate by opposed,
slow fluid motion of molten polymer against the traveling flame front. Zhang et. al. [5]
studied the effect of melt flow on horizontal flame spread and found that the formation of
pool fires can be a dominant factor of fire growth. Bulk flow of polyethylene and
5
Introduction
polypropylene was observed, whereas flow in a surface‐layer was characteristic for Nylon.
Polycarbonate and polyvinylchloride resisted flow by char formation. The work of Ohlemiller
et al. [3, 4] has been a source of inspiration for this study. Ohlemiller and Shields developed
vertically oriented test arrangements that allow to measure steady‐state flow from surfaces
exposed to an external heat flux. The flow behavior of the thermoplastics polypropylene,
polystyrene, and to lesser extent polymethylmethacrylate, was described in detail. Butler et.
al. [32, 33] pursued different approaches to model Ohlemiller´s melt flow experiments with
remarkable results.
1.5.2 Depolymerization and melt flow of polystyrene
The depolymerization of polystyrene has attracted considerable attention in the polymer
science and fire science community. A recent review outline efforts to study the degradation
pathway [34] of polystyrene. Different approaches to measure the mass loss kinetics of
polystyrene were discussed by Westerhout et. al. [35]. The significance of melt viscosity for
flammability properties of polystyrene were characterized by Kashiwagi et. al. [6, 36]. The
flow behavior of polystyrene under fire‐like conditions was characterized by Ohlemiller et al.
[3, 4]. The same material as used by Ohlemiller was characterized in this study in order to
see how changes at the macromolecular level influence the macroscopic behavior described
earlier.
1.5.3 Flexible polyurethane foam combustion
The thermal decomposition of polyurethane for flexible foams has been thoroughly studied.
A recent review [37] summarizing these efforts and the work of Ravey et al. [26] were
especially instructive. Combustion in the cone calorimeter and lateral ignition and flame
spread of different cellular insulation materials were studied by Cleary et. al. [38]. Their
study outlines the key fire properties of foams and gives recommendations for the design of
test setups. Flexible polyurethane foam combustion was tested in the cone calorimeter by
Vanspeybroeck et al. [39], who gave recommendations for suitable sample sizes and external
heat flux levels. Lefebvre et al. [40] showed that selected parameters of cone calorimeter
measurements of flexible foam combustion correlate with FMVSS no. 302 flame spread tests
and BS5852 Ignition Source Crib 5 tests. The same group investigated the effect of variations
in foam composition on cone calorimeter test performance [41]. Ohlemiller et al. [4] studied
foam combustion in vertical cone calorimeter setups, to measure the effect of melt dripping
and pool fire formation. A medium scale test in Vee‐configuration was designed that
determines the influence of heat feedback from a pool fire on flame spread [22].
1.5.4 Flammability of polyolefin‐chalk‐silicone blends
Blends of acrylate‐ethylene copolymers with chalk and silicone where originally proposed in
a patent [42]. The physical properties and mechanism of flame retardancy were first
discussed by Sultan et al. [43], following earlier studies on the thermal degradation of the
pure copolymers [21, 44, 45]. The role of chemical composition in the material´s flame
retardancy was outlined by Hermansson et. al. [46], who emphasized the intumescent
expansion of the material [31]. This study continues earlier investigations of the effect of
different copolymer structures on flammability [47], and extents investigations of the
coupling between chemical degradation and physical behavior of the material, to tests under
gravitational stress [47].
6
Introduction
1.6 Orders of magnitude
This section illustrates the order of magnitude of quantities that are repeatedly discussed
throughout this study.
Temperatures
Table 1‐1: Characteristic temperatures of familiar processes.
Item
Temperature [°C]
Ref
50 ‐ 70
105 ‐ 130
350
660
1200
1400
3000
[48]
Melting point of candle wax
Melting point of polyethylene
Surface ignition temperature of polystyrene
Melting point of aluminum
Hottest spot of a glowing copper contact
Maximal flame temperature of a candle
Temperature at the center of an electric arc
[6]
[49]
[48]
[49]
Heat release rates and heat of combustion
Table 1‐2: Heat released in the combustion of some familiar items.
Item
Heat release rate [kW]
Candle
CRT monitor
Upholstered chair
Total combustible energy [MJ]
0.077
20 ‐ 200
400 ‐ 3000
5
25 – 100
300 ‐ 700
Ref
[48]
[7]
[50]
Table 1‐3: Heat release rate and effective heat of combustion (dHc) of familiar substances measured
with the cone calorimeter at 50 kW m‐2 (40 kW m‐2 for candle wax).
Substance
Wood (southern pine)
Candle wax
High impact polystyrene*
Low density polyethylene*
Peak heat release rate [kW m‐2]
154
4200
1040
1400
dHc [kJ g‐1]
12
44
28
42
Ref
[51]
[48]
[52]
*non‐flame retarded materials
7
Introduction
Heat fluxes
The heat flux at approximately 2 cm and 4 cm distance above a candle flame is given below.
Figure 1‐3: Heat flux above a candle flame as a function of radial distance from the flame. With
permission from reference [48].
Heat fluxes measured at 45 cm distance from burning CRT computers were within 2 kW m‐2
to 24 kW m‐2 depending on the polymer used to make the enclosure [7]. A ø 18 cm cooking
plate with a power of 1.8 kW can generate a maximum heat flux of 71 kW m‐2.
Flow of viscous liquids
Table 1‐4: Typical viscosity values of familiar substances at room temperature.
Substance
Viscosity [Pa s]
Water
Octane
Olive oil
Honey
Corn syrup
Polystyrene (T = 210 °C, Mw≈105 g mol‐1)
10‐3
5 × 10‐2
10‐1
10
102
104
Molar masses
Table 1‐5: Typical molar masses of familiar substances.
Substance
Molar mass [g mol‐1]
Benzene
Candle wax
Polyethylene for plastic bags
Polystyrene for water cups
102
4 × 102
105
105
8
Experimental
2. Experimental
2.1 Materials
2.1.1 Polystyrene
High purity medical‐grade polystyrene (DOW 666D, Dow Corning, USA) was used as received
in all experiments. The material had a number‐average molar mass of 133 kg mol‐1 ± 7
kg mol‐1 and a polydispersity index of 1.59 ± 0.04. For thermal analysis, a fine powder of the
material was used to minimize thermal gradients and diffusion paths. The material was
ground after cooling in liquid nitrogen using a ZM 200 mill (Retsch, Germany) operating at
14000 rpm. Specimens for gasification, rheological and melt flow experiments were
compression molded in two steps. First, 2 mm and 6 mm thick sheets were pressed for 5 min
at 140 °C and 200 bar. Second, multiple sheets were compressed at 160 °C and 200 bar for
8 min to form 17.5 mm thick samples (Figure 2‐1). All samples were cooled with a rate
of 15 °C min‐1.
2.1.2 Flexible polyurethane slabstock foam
A polyurethane slabstock foam formulation was used that matches the composition of
formulations with widespread use in industry. All samples were prepared by the author with
the composition given in Table 2‐1. Details of the preparation can be found in Paper II. The
rise profile, final rise height and sample temperature were recorded to ensure a uniform
foam quality. All samples were cut from identical positions in the foam bun. The foam had an
open cell structure. When prepared as large cushions, using the foam formulation in Table
2‐1 results in foam with air flow values of 0.127 m3 min‐1, an internal force deflection (IFD) of
30, and a foam density of 22.4 kg m‐3. Flame‐retardant foams were prepared by adding
10.6 pph of a brominated‐phosphorous flame retardant (Firemaster 550, Great Lakes, USA).
The rise profile and physical properties of the foam were only minimally affected by the
flame retardant.
Table 2‐1: Polyurethane foam formulation
Component
Glycerol
polyol
Amount (pph)
propoxylate‐block‐ethoxylate 100.00
Product/Supplier
Voranol
Corning
235‐056,
Dow
Deionized water
3.80
Amine catalyst
0.20
Dabco 33 LV, Air Products
tin catalyst, stannous octoate
0.23
Aldrich
Blowing agent, methylene chloride
2.60
Mallickrodt
2,4 ‐ and 2,6 ‐ toluene diisocyanate
51.52
Mondur TD80, Bayer
Foam nanocomposites were prepared by dispersing the nanofiller in the polyether polyol
using a colloidal mill prior to the foaming process. Oxidized carbon nanofibers (Pyrograph III,
Grade PR 24‐PS OX, Applied Sciences Inc, USA) and organomodified montmorillonite clay
(Cloisite 30B, Southern Clay Products, USA) were added at 3.9 wt.% of the total formulation
9
Experimental
and substituted 62 % of the brominated flame retardant in the formulation. Composites of
µm‐sized talc with an equal filling level and equal substitution of the flame retardant were
used as a reference for flammability tests. The amount of catalysts had to be adapted and
cell openers used to obtain an equal density of the composite foams. Details of the
composites preparation are given in Paper III.
2.1.3 Polyolefin-chalk-silicone blends
Ethylene‐acrylate copolymers blended with chalk and silicone were used as received from
Borealis AB (Sweden). The intended use of such materials is for cable jackets or flexible
elements for electrical equipment such as plugs. Three formulations were tested. The
polymeric components were poly(ethylene‐co‐butyl acrylate), poly(ethylene‐co‐butyl
acrylate) blended with polypropylene in a 4:3 ratio, and poly(ethylene‐co‐methacrylic
acrylate). The chalk filling level amounted to 30 – 40 wt.% and the silicone amounted to 5 –
7 wt.% of the blends. All the polymers had a melt flow rate MFR2 of 0.4 at 190 °C. All data
were provided by the manufacturer.
2.1.4 Poly(ethylene-co-methacrylic acid) - calcium carbonate composites
Poly(ethylene‐co‐methacrylic acid) (EMAA) composites filled with 5 wt.% of precipitated
calcium carbonate were prepared by extrusion on a co‐rotating twin‐screw extruder (Clextral
BC 21, Clextral SAS, France). A set of reference samples were prepared on a miniature melt‐
blender (Haake Minilab, Thermo Scientific, Germany). The polymer was a commercial grade
EMAA with 9 wt.% methacrylic acid (Nucrel 0903HC, Dupont, USA). The precipitated calcium
carbonate (Specialty Minerals, United Kingdom) was in the form of fine calcitic crystals and
had an average particles size of 70 nm. Pure particles (Calofort U) and stearic acid coated
particles (Calofort S) were used.
Figure 2‐1: Left: Polystyrene samples prepared for melt dripping and gasification test. Middle: Open
cell structure of the polyurethane foam Right: Compression molded specimen of the EMAA‐chalk‐
silicone blend material.
2.1.5 Commercial materials for electrical appliances
Ten commercial materials were tested in order to evaluate flammability test methods. The
materials were provided by leading European manufacturers of low voltage switch‐gear and
control‐gear. The materials were: high‐density polyethylene (HDPE), high‐impact polystyrene
(HIPS), polyethylene copolymer (LDPE‐co), polyamide 66 (PA), polycarbonate (PC), 2 blends
of polycarbonate and acrylo‐butadiene‐styrene (PC‐ABS1/2), polypropylene (PP),
polyvinylchloride (PVC), and glass‐fiber reinforced, unsaturated polyester (UP‐GF). Eight of
10
Exp
E erimeentaal
werre h
hard p
plasstics w
with
h a mo
odu
uluss off 1 GP
Pa or
o abo
a ove and aare useed to maake
the ten maaterials w
en
nclo
osuress off electtrical aapp
pliaancees. Tw
wo maaterrials wer
w em
morre flex
f xible and
a fin
nd use
u e ass fleexib
ble
paartss. All
A ma
m terrialss co
onttain
ned
d flam
me rretaard
dants and
d fu
urtherr fillerrs aand
d ad
ddiitives. De
etails on
the m
matteriial sele
s ecttion
n caan be
b ffou
und
d in refferencce [53
3].
2..2 Fla
am
mm
mabillity
y ttes
stin
ng
g
2.2.1
1 Con
C ne cal
c orime
eter
Th
he ccon
ne calorimeeterr iss to
oday the
t mostt w
wideely useed insstru
umeentt to
o dete
erm
mine
eq
quantittative
paaram
meeterrs of
o poly
p yme
er ccom
mbu
usttion
n. A co
onee‐sh
hap
ped, raadiaantt heeate
er fforccess co
omb
busstio
on o
of the
t
mateeriaal by exp
e osu
ure to
o a uniforrm exterrnal heeatt flu
ux (Fig
( gure 2‐2)
2 ). TThe insstru
umentt mea
m asurres
etio
on in thee exh
e ausst stre
s eam
m tto dettermin
ne thee hea
h t rreleease
e p
producced
d b
by the
t
oxxygeen deeple
co
omb
busstio
on of the ma
m terrial [5
54‐5
56], usin
u ng thee pro
p porrtio
onaalityy b
betw
wee
en th
he am
mount off
oxxygeen consu
umed an
nd tthe
e heeat re
eleaased
d thatt ho
old
ds for
f a wid
w de var
v ietyy of
o o
orgaanic mat
m terials
[57, 58]]. The
T heeat release ratte is
i am
measurre of
o how
h w muc
m ch a mat
m terial can
n cconttrib
butte tto the
t
wth of a fire
f e. TThe total heeat relleassed
d per inittial masss off saamp
ple, caalle
ed eeffe
ecttive
e heeat off
grow
co
omb
busstio
on, meeasu
ure
es the po
oten
ntiaal conttrib
bution
n to
o the totaal size of a fire
f .
Co
omm
mo
onlyy em
mp
ployyed
d heeat flu
ux leve
l els are
ew
with
hin (10
0 to
o1
100) kW
W m‐22. The
T e co
one
e sh
hap
pe o
of the
t
he
eateer ens
e sure
es a h
hom
moggen
nous fllux to the
e surffacee off th
he sspeecim
men
n, w
which mustt bee flaat aand
d is
ussually 10
00 mm
mm x 10
00 mm
m w
wid
de and
d a feew millim
m me
eterrs to
t few
w ccentim
meterss th
hickk. A
As an
exxam
mplee, an
a extternal heeat flu
ux of
o 50
5 kW
W m‐2 willl heat
h t th
he surrfacce of mo
ost po
olym
merrs aabo
ove
40
00 °°C within app
pro
oxim
mattelyy 60
6 s.
s In geeneral,, the
t
polyymeer sho
ortly theere
eaftter iggnites,
triiggeereed by
b a sparrk ign
i iterr po
osittioned
d abovve thee sp
peccim
men surface.. Th
he heaat rele
r easse rratee is
meassured with thee heatt flu
ux impossed
d un
ntil flaamiingg ce
edes. Det
D tails o
on the
t insstru
um
ment caan be
found
d in
n sttandardss [55, 5
56] an
nd pub
p blicatio
onss [5
54, 59]].
Figgure
e 2‐2:
2 Le
eft: Sam
mple ccom
mpaartm
men
nt o
of the con
ne calo
orim
metter.. Rightt: SSam
mple
e ho
olde
ers forr teestss off
co
omm
merrcial mate
m eriaals and
d filled
d polyolefinss (ttop), foam
f m sam
s mplees (mi
( ddle) and
d p
polyystyyren
ne iin the
t
gasification
n ap
pparratu
us (botttom
m).
Given
n the
t well‐‐ventilateed co
onditio
ons an
nd a for
f ced
d com
c mbu
ustion off th
he maaterial, the
t daata
ob
btaiineed app
a pliess m
mosst co
orrrecttly to the
t e sp
peccificc co
omb
busstio
on sce
s narrio of a dev
d elo
opin
ng fire
f e [6
60].
Th
he p
preedicctio
on of
o m
materrial peerfo
orm
man
nce un
nder othe
o er fire
f e teestss re
equires te
estingg ovver a ran
nge
off exxperim
men
ntal co
onditio
ons an
nd det
d aileed datta ana
a alyssis [14
[ , 61
1].
11
Exp
E erimeentaal
he speecim
men geo
g ome
etriies ussed fo
or tthiss sttud
dy are
a e sh
how
wn in Figgurre 2‐2
2 . Pollyolefiin‐cchaalk‐
Th
silico
one blend
ds weere tessted aas flat
f t sp
pecimeenss, 100
1 0 mm × 100
1 mm
mw
wid
de, 4 and
d 6 mm thick.
Co
omm
meercial ma
m terrialss w
were testted ass flaat ssheeetss, 100
1 0 mm × 100
1 0 mm wid
w de and
d 3 mm thick.
Po
olysstyrren
ne sam
s mples forr gaasifficaatio
on/m
me
elt fflow
w mea
m asu
urem
meentss were
w e 80
8 mm
mm × 80
0 mm
m wid
de,
an
nd 1
17.5 mm
mm th
hickk. Foa
F m sam
mples haad a su
urfacee areaa off 85
5 mm
m ×8
85 mm
m aand
d were
e 39
9 mm
m
thickk. Conee calo
orim
metter tessts of foaamss ne
eed
d sp
peccial co
onsiiderattion
ns d
duee to
o th
he low
w de
enssityy off
the m
matterial. A fasst col
c lap
pse of the ffoam leads to a deccreeasee in
n eeffeectivve heat flux aas the
t
disstancee betw
weeen the
e heeatter and sam
s mple
e in
ncreeasses.. Fo
or 3
39 mm
mm th
hickk saamplees, this effect led
l
to
o an
n 11
1 % dec
d cay off th
he inittiall he
eatt flux. Th
he foaam saamp
ple flaankks we
w re no
ot w
wraapp
ped in
alu
um
minium
m fo
oil to
t p
prevven
nt any
a y ob
bstrrucction o
of fo
oam
m coll
c apsse and
a d to
o avvoid an
a aartiificiial distan
nce
be
etw
ween the flamee fro
ontt an
nd tthee saamp
ple surface position.
To
o ch
harractterize th
he burrnin
ng ratte of spe
ecimeens th
hat deeforrm an
nd fflow
w und
u derr grravvitattion
nal
strress, the
t e stand
darrd test
t t se
etu
up of
o the
t e co
onee caalorrim
mete
er w
was mod
m difieed in a n
num
mbeer of
o ways.
Th
he ccon
ne‐sshaapeed h
heaaterr caan b
be bro
ougght in upright possition to pe
erfo
orm
m ve
ertical tests. Sp
peccial
samp
ple ho
olde
ers weere
e deesiggneed to ho
old thee saam
mplee in
n a paarallel planee to tthe co
one
e heeatter,
wh
hilsst allo
a owing an
n unh
u hind
derred flo
ow off m
matterial fro
om th
he burnin
ng su
urfaace (FFigu
ure 2‐‐3).
Flo
owingg mate
m eriaal w
wass caapttureed in a cattch paan. A numb
ber off arrbittrarry cho
c oice
es h
havve to be
madee th
hatt stron
nglyy afffecct the
t e meassurrem
men
nt in
n vert
v ticaal co
onffigu
urattion. The
T e distaancce bet
b weeen
samp
ple ho
olde
er and
d catc
c ch pan det
d erm
min
nes whettheer, and
d how
h w muc
m ch, flaamees fro
om bu
urniing
mateeriaal in
n th
he cat
c ch pan can
c feeed bacck heaat to
t the
t e initiaal saample
e. TThe samp
ple thickn
nesss and
a
mateeriaal bac
b kin
ng tthe
e saamplee afffecct the
e h
heatingg rate
r e, d
durratiion off flow
w, aand
d an eve
e entu
ual
ovverh
heaatin
ng of a thiin film
m o
of maateriall on the
t e back
b k p
plan
ne. In
n all cas
c es, th
he settup
p was
w
co
onfiigured
d eiitheer tto am
mpliify or minim
mize tthese efffectts. It was
w s fo
oun
nd thaat tthe
e sp
park iggnitterr
waas gen
nerrallyy unsu
u uitaable for
f verticcal tesstin
ng. The com
c mbu
ustiiblee vo
olatilees flow
f w in a th
hin layyerr
up
pwaards ove
o r th
he sspe
ecim
men
n su
urface
e. TThiss makees it
i diffi
d cullt to
o posi
p itio
on the
t sparkk ignite
er iin the
t
vo
olattile strream wit
w thout influeencingg the sam
s mplee su
urfaace
e. N
Neeedlee flam
mes we
ere used forr piilotted
ign
nitiion. How
H wevver, stron
nglyy flam
me‐iinhibittingg fllam
me‐rretard
dan
nts quencch the
e need
dle flaamee and
a
ne
eed
ded to
o be
e litt ovverr an
nd oveer aga
a ain.. A co
omb
binaatio
on of a nee
n edle
e flam
me wit
w h a sp
park iggnitterr
sh
hould be
b evaaluateed.
Figgure
e 2‐3:
2
Saamp
ple ho
oldeers useed for veerticcal con
ne calorim
meter testin
ng. Lefft: Tesstin
ng w
with
h saample
suppo
orteed in a reetaiiner frram
me. Mid
ddle
e: TTestingg in
n un
nsu
uppo
orteed con
nfigguraatio
on. Rigght: Seetup
p ussed to
recovver meelt from
m po
olysstyrrene saamples in
n gaasificattion
n/m
melt flow expe
e erim
men
nts..
12
2
Exp
E erimeentaal
oam
m saample
es wer
w re test
t ted
d in a set
s up thaat w
wass designe
ed to am
mpliify tthee flaame feed
dbaackk fro
om
Fo
an
n evven
ntual poo
p ol o
of bur
b rnin
ng m
materrial forrmiingg un
ndeerne
eatth the
t e saamp
ple (Fiigure 2‐4
4, le
eft)). The
T
samp
ple waas con
ntaaine
ed in a coaarse
ely mesh
m hed
d chic
c cken w
wiree fram
f me th
hat alllow
wed
d a flo
ow off
mateeriaal to
o th
he cat
c ch pan.
Figgure
e 2‐4: Lefft: Vert
V ticaal teest set
s up for foaam com
mbu
ustion tessts in feed
f dbaack con
nfiguraation.
Rigght: Gaasification
n ap
pparratu
us.
2.2.2
2 Gas
G sific
cation
n ap
ppa
ara
atuss
It is geenerallly acc
a cepted
d thatt th
he deegraadaatio
on of a maateriall undeer a largge flaame
e o
occu
urs
esssen
ntiaallyy undeer ine
ert atmo
ospherre, ass an
ny oxxygeen is consu
umed in th
he flamee [6
62]. The
T
gaasificattion app
a paraatus iss a custtom
m‐build
d in
nstrum
ment at thee Nati
N ional Institu
ute off Sttandarrds
an
nd Teechn
nollogyy w
which
h simu
ulatess this sccen
nariio byy im
mposing a co
onsstan
nt heeat flu
ux off
co
omp
parrable ma
m gniitud
de to flamee he
eatt flu
uxees on
o to
t a poly
p ymeer spe
s ecim
men in
n in
nerrt atm
mosp
pheere
(Fiigurre 2‐4
2 , riightt) [[63
3] . Avvoid
dingg sup
s erp
possitio
on witth flam
mee heeatt flu
ux ren
ndeers thee hea
h t fluxx
moree un
nifo
orm
m, faci
f ilitaatin
ng num
n mericaal ssimulaatio
ons of the
e expeerim
ment. Th
he inteerio
or w
walls o
of the
t
ch
ham
mbeer are
a waateer‐ccooled
d an
nd painteed blaackk to
o main
m ntain an
a inssign
nificcan
nt b
bacckgrrou
und
d heeat
radiaatio
on.
he m
mass loss raatee pe
er ssurfacce are
a ao
of the maateerial is reecorrdeed in
i tthe meassurem
men
nt. V
Visu
ual
Th
ob
bservaatio
on of
o the
t maateerial beehaavio
ourr is possib
ble as vission
n iss no
ot imp
pairred byy a flam
f me or soot.
Th
he tesst allo
owss tto chaaracteerizze solid ph
hasee pro
p ocessses in
i the
e m
matteriial that afffect the
t
gaasificattion rate
r e and to de
etermine
e th
he cha
c ar yiel
y ld of
o the
t e polyyme
er. Speecimeens ussed fo
or this
stu
udyy mea
m surred
d 84
4.0 ± 0
0.1 mm
m in
n diam
metter and wer
w re 17.5
1 5 ± 0.1
1 mm
m thick.
2.2.3
3 In
n-situ tem
mp
pera
atu
ure measurrem
mentss
herm
mo
ocouplles weere inttro
oduced
d in
nto sp
peciimeenss fo
or con
c e calo
c orim
metter an
nd ggassificcatiion tests
Th
to
om
meassurre the
t e teemp
perratu
uree grradien
nt in tthe sp
pecime
en. Fo
or pol
p yollefin‐cchalk‐ssiliccon
ne bleend
samp
pless, thin
n, bare
b e wire
w e, tyypee K the
erm
moccou
uplees (CH
( HAL‐01
15‐B
BW
W, Ome
O egaa En
nginee
erin
ng Incc., UK)
U
we
eree san
s dw
wich
hed
d betw
b ween sh
hee
ets of tthee mat
m terial an
nd th
hen
n com
c mprresssion
n mo
oldeed.
Po
olysstyrren
ne sam
s mplees forr gaasificattion eexperim
meentss were
w e fitteed with
w h0
0.5 mm
md
diam
metter, tyypee K,
sh
heattheed the
t erm
mocoup
ples (K
KM
MQX
XL‐0
032
2G‐‐6, Om
megga Eng
E gineeerringg Inc., UK
K) in
n id
den
nticcal maanner.
Fo
oam
m sam
mples w
werre mounteed on
n up
pright poin
ntin
ng, 0.5 m
mm diam
metter sheaath
hed
13
Experimental
thermocouples (KMQXL‐032G‐6). The measurements were compared to thin, bare wire, type
K thermocouples (CHAL‐015‐BW) that were laterally introduced using a syringe needle.
2.2.4 Glow wire test
The glow wire test is a classification flammability test that simulates the exposure of plastic
insulation materials to an overheated electrical contact. A 4 mm thick nickel/chromium
(80/20) wire, bend to 1 cm radius, was heated to a given temperature in the range 650 °C to
960 °C. It was pressed on the surface of a thin, flat polymer specimen in vertical position
using a small force of 1 N ± 0.2 N (Figure 2‐5). The glow wire was applied for 30 s. The
current supply was held constant without compensation for the temperature change at the
glow wire tip. The tip was allowed to penetrate the sample with a maximum indentation
depth of 7 ± 0.5 mm from the specimen front surface. The time to ignition and time to
extinguishment were recorded. A paper indicator was placed 200 mm underneath the glow
wire to detect any yield of flaming droplets. Specimens used in this study were
60 mm × 60 mm wide and had a thickness of 1.5 mm or 3 mm. Two indices describe material
performance [64, 65]:
The glow wire ignition temperature (GWIT) is the temperature which is 25 °C (30 °C between
900 °C and 960 °C) higher than the maximum temperature of the tip of the glow‐wire which
does not cause ignition of a test specimen of a given thickness during three subsequent
tests. Ignition is defined by the standard as a flame that persists for longer than 5 s.
The glow wire flammability temperature (GWFI) is the highest test temperature, during
three subsequent tests for a test specimen of a given thickness, at which one of the
following conditions are fulfilled: a) flames or glowing of the test specimen extinguish within
30 s after removal of the glow‐wire and there is no ignition of the wrapping tissue placed
underneath the test specimen; b) there is no ignition of the test specimen.
2.2.5 50 W vertical flame test
The 50 W vertical flame test described by IEC 60695‐11‐10 [66] (equal to the UL 94 test) is a
classification flammability test that simulates the exposure of plastic insulation materials to a
candle sized flame. Specimen measure 125x13xt (mm)3, where t ≤ 13 mm. The specimen are
vertically hung and exposed to a blue flame with a nominal power of 50 W at the lower end
(Figure 2‐5).
Figure 2‐5: Left: Glow wire tip and sample holder of the glow wire test apparatus. Right: 50 W test
flame applied to a specimen that fails the vertical flame test and yields flaming material.
14
Experimental
The flame is applied for 10 s, and the afterflame time (t1) is recorded. The flame is applied
for a further 10 s if the specimen extinguishes and the afterflame time (t2) and afterglow
time (t3) are recorded. The specimen is placed 30 cm above a cotton pad that is used to
determine whether the material yields flaming droplets. Specimens with a thickness of
1.5 mm and 3 mm were used for this study. The performance levels used to classify
materials are given in Table 2‐2.
Table 2‐2: Vertical burning categories according to IEC 60695‐11‐10 [66].
Criteria
Category
V‐0
V‐1
V‐2
Individual test specimen afterflame time (t1 and t2)
< 10 s
< 30 s
< 30 s
Total set afterflame time tf for any conditioning
< 50 s
< 250 s
< 250 s
Individual test specimen afterflame plus afterglow
time after the second application (t2 + t3)
< 30 s
< 60 s
< 60 s
Did the afterflame and/or afterglow progress up
to the holding clamp?
No
No
No
Was the cotton indicator pad ignited by flaming
particles or drops?
No
No
Yes
2.3 Material characterization
2.3.1 Thermogravimetrical analysis
Thermogravimetrical analysis records the mass loss of a specimen undergoing a controlled
heating cycle in a controlled atmosphere. The sample is either heated at a constant rate or
kept under isothermal conditions. Experiments with low heating rates between 1 and 20 °C
min‐1 were performed on standard thermal analysis equipment (TGA/SDTA 851e, Mettler
Toledo and Q500IR, TGA Q500, TA Instruments, USA). Experiments using high heating rates
were measured on an infrared heated thermal analyzer (Q5000IR, Ta Instruments, USA). The
sample amount was kept very small in order to avoid thermal and mass transport gradients.
For this study, 1 mg to 3 mg samples were used to acquire non‐isothermal kinetic data;
3.5 mg to 10 mg samples were used for isothermal experiments and 10 mg samples for non‐
isothermal data not intended for calculation of kinetic rates.
2.3.2 Pyrolysis combustion flow calorimeter
This instrument uses a combination of initial pyrolysis of a polymer sample in inert
atmosphere and complete oxidation of the volatile products in a combustor, to measure the
maximum amount of heat that can be released by the sample [67]. The measurement aims
to simulate the conditions met on a burning polymer surface. It is generally considered that
the polymer decomposes under inert conditions beneath the flame and that the combustible
gases mix with oxygen in the flame at some distance from the surface [62]. The high
temperature of the combustor (850 °C) and high oxygen concentration used in the
instrument give results that correspond to the limiting case of complete combustion [68,
69]. The maximum heat release potential is acquired, as polymer combustion is rarely
complete in real fires. The main parameter obtained by the pyrolysis combustion flow
15
Experimental
calorimeter is the heat release capacity, which is the ratio between the maximum heat
release rate per unit mass and the heating rate and has the unit J g‐1 °C‐1. The method also
allows determining the maximum effective heat of combustion by dividing the total heat
release by the initial mass of the sample.
2.3.3 Size exclusion chromatography
Size exclusion chromatography (SEC) is a standard method used to evaluate the molar mass
distribution of polymers. It was used in this study to determine the decrease in degree of
polymerization of polystyrene undergoing thermal degradation under combustion‐like
conditions. The measuring principle of size exclusion chromatography relies on the faster
elution time of polymer chains with larger hydrodynamic radius as a solution of the polymer
passes a porous material. A comparison of the concentration leaving the column at given
times to a set of standards of well‐defined molar mass allows to determine the molar mass
distribution. Degraded polystyrene samples were recovered from thermogravimetric,
gasification and vertical cone calorimeter experiments and were dissolved in chloroform for
the SEC analysis. The solutions were filtered using 0.45 µm Teflon filters. A “717 Plus”
autosampler and model 510 solvent pump (Waters, UK) equipped with a PL‐ELS 1000 light
scattering evaporative detector and three PLgel 10 μm mixed B columns (300 x 7.5 mm)
(Polymer Laboratories, UK) were used. The flow rate was 1 ml min‐1 and the injection volume
50 µl. Calibration was performed by bracketing 4 measurements between two sets of 8
narrow molar mass polystyrene standards measured before and after the samples.
30 samples were send to Smithers Rapra Technology Ltd. (UK) to confirm the acquired data
using a different chromatographic setup. A Viscotek TDA model 301 analyzer with two mixed
bed‐B, 30 cm, 10 µm columns was used to measure solutions in chloroform at a nominal
flow rate of 1 ml min‐1. Agreement of normalized data (Mn/Mn,0 and Mw/Mw,0) from both
chromatographs was very good. There was a considerable difference in the absolute Mn
values from both instruments, as evaporative light scattering detectors (Waters instrument)
and differential refractive index detectors (Viscotek instrument) have a different sensitivity
to low molar mass fractions of the polymer. As this is a generic problem for molar mass
distribution acquired by SEC, all data is reported as normalized to reference measurements
of the pristine polymer on the respective instrument.
2.3.4 Rheological testing
Rheological tests were conducted in nitrogen in order to measure the decrease in
polystyrene melt viscosity with increasing temperature and decreasing molar mass. Constant
shear experiments were used to measure shear viscosity as a function of temperature.
Oscillative experiments were used to determine how the change in temperature and molar
mass influences the flow behavior of the material. All samples were measured in plate‐plate
configuration using electrically heated steel plates, 25 mm in diameter, on a stress‐
controlled rheometer (ARES G‐2, TA Instruments, USA). A very low constant normal force
was exerted on the samples to ensure uniform contact to the plates.
Measurements were conducted in oxidative atmosphere (air) to test the char stability of
polyolefin‐chalk‐silicone blends. A strain‐controlled rheometer equipped with a cross‐flow
furnace was used in plate‐plate configuration (ARES, Ta Instruments). A low, constant
normal force was exerted on the samples to ensure uniform contact to the plates. Both
8 mm and 25 mm aluminum plates were used for the measurements. Polyolefin‐chalk‐
silicone blends were also measured in nitrogen on a stress‐controlled instrument (MCR300,
16
Experimental
Anton Paar, Austria). All rheological experiments were terminated when bubbling of the
material prohibited the reproducible acquisition of data.
2.3.5 Infrared analysis
Infrared spectra were recorded to determine the state of degradation of sample residues
quenched from flaming combustion in the cone calorimeter. The sample residue was cut into
thin slices beginning at the top surface using a microtome. The individual thickness of the
thin slices was measured to determine the distance from the specimen surface in
combination with the use of high resolution pictures of the sample cross‐section. The slices
of material were measured in an attenuated total reflection setup (Perkin‐Elmer 2000, USA).
2.3.6 Infrared imaging
A FLIR Thermacam A40 (FLIR Microsystems, Sweden) infrared camera was used to acquire
infrared videos of glow wire and vertical flame tests. The camera was operated with a
measuring range of either (30 to 500) °C or (300 to 800) °C. As the emissivity was not known
for the various materials used in this study and changes during decomposition of the
polymers, all temperatures are reported as black‐body temperatures and give a qualitative
picture of the temperature distribution.
17
Results and discussion
3. Results and Discussion
3.1 The practical importance of melt flow in flammability testing
There is abundant empirical evidence that many thermoplastic materials cannot be tested in
a reliable manner due to deformation and melt flow of the specimens. A reproducible
exposure to the heat source and a comparable thermal impact for different materials are
hard to achieve. As will be shown in this section, a better characterization of melt flow
phenomena is necessary to improve the predictive qualities of bench scale flammability
tests. This paragraph is based on findings reported in reference [53].
A number of studies have shown that correlations of test results between different bench
scale flammability test methods [9, 61, 69], as well as between bench scale flammability test
methods and real scale fire tests [14, 70] is hard to establish. Reasonable correlations can be
found either when the scope of the study is narrowed to a single material‐flame retardant
system [71], or if very large numbers of materials with widely different performance are
tested [69]. An illustration of the problem is given in Figure 3‐1. The results of a classification
test, in which a vertically oriented specimen is exposed to a 50 W test flame, are compared
to an intrinsic material property. The heat release capacity is a measure of the specific
energy that can be released by complete combustion of the material at a certain rate of
heating. There is an overall trend that materials with a high heat release capacity fail the
vertical flame test (HB rating), and materials with heat release capacities below 200 J g‐1 K‐1
pass the vertical flame test (V‐0 rating). It is unfortunate that the bulk of commercial flame
retarded polymers for electrical engineering applications falls within the span of 200 J g‐1 K‐1
to 700 J g‐1 K‐1, where a clear lack of correlation is found. Eight out of the ten materials
tested in this study fall within this range. The latter materials are a representative selection
from the European market, delivered by 3 manufacturers of low‐voltage switchgear in with a
high sales volume (see section 2.1.5).
Figure 3‐1: Dependence of the performance level in the 50 W vertical flame test (UL 94) on the heat
release capacity, an intrinsic measure of the maximum amount of heat that can be released by the
material at a given heating rate. Hollow circles represent data reported by Lyon et al. [69], dots
materials measured in this study.
18
Results and discussion
The example given here is typical of the general observation that the test performance in
bench scale flammability tests is not exclusively linked to intrinsic material properties, but
also strongly affected by the test conditions.
End product and material flammability testing for low voltage switchgear and controlgear
according to IEC standards [72] is predominantly based on the glow wire test. Very limited or
in many cases no correlation of the glow wire test with other bench scale flammability tests
[9, 53], and end product test [70] was shown. The origin of the deficient correlation needs
clarification. To appreciate the scope of the problem, it should be considered that a glow
wire test of 650 °C is the only requirement set on the enclosure of an electrical switchboard
for household use, as long as there is no direct contact with life parts [72]. The enclosure
typically is the largest plastic component by mass and volume. An enclosure investigated in
our tests contained approximately 2.5 kg of plastic and the materials effective heat of
combustion was 28 kJ g‐1. Its potential contribution to a fire amounts to 70 MJ. Adequate
and verifiable flame retardancy is needed.
End product and material flammability testing for “household and similar electrical
appliances” (i.e. TV sets, hair dryers…) according to IEC standards is based on the glow wire
test in combination with the 50 W vertical flame test, or needle flame test [73]. Concerns
about the predictive qualities of the 50 W vertical flame test for such applications were
recently voiced [8]. It is the conviction of the author that the lack of correlation and
reliability is by large due to the difficulties arising from specimen deformation and melt flow.
The effect of melt flow and deformation on the glow wire test and 50 W vertical flame test
are discussed in the following. It is also outlined why test methods that exclude flow
phenomena need to be complemented.
3.1.1 Glow wire flammability test method
The glow wire test [64, 65] intends to mimic the ignition of a material by a glowing wire or a
glowing element. The bend of an electrically heated, 4 mm thick, nickel/chromium wire is
pressed to the vertical surface of a flat, free‐standing piece of material for 30 s (see section
2.2.4). It is observed whether the specimen ignites (flame persisting for more than 5 s)
and/or whether the sample extinguishes within 30 s after retrieval of the glowing wire (see
section 2.2.4 for a definition of the performance levels). It is further observed whether
droplets of flaming material ignite a paper indicator underneath the specimen.
Glow wire tests were performed on 10 commercial materials (see 2.1.5). A general
observation of the glow wire tests of commercial materials was that for eight out of ten
materials, the material either ignited within 5 s of contact, or did not ignite at all. It very
rarely occurred for the given sample set that a material ignited after prolonged exposure to
the glow wire. The application of the glow wire tip led to a variety of phenomena, often
immediately after the application of the glow wire tip:
- The glow wire indented all thermoplastic materials to varying extent at varying rates,
depending on the glow wire temperature, the melting behavior of the material, and the
specimen thickness.
- For some materials (HIPS, PC, PC‐ABS, PA), a gap formed between the glow wire tip and
the molten sample. Contact with the glow wire tip was generally only maintained over
19
Results and discussion
short periods of time. Generally, gaps formed more readily for thinner specimens and for
higher temperatures. Gaps formed in many cases instantaneously after ignition.
- Some materials had a “sticky” contact with the glow wire (HDPE, LDPE‐co, PP). Formation
of an inorganic deposit on the flow wire tip was observed (HDPE, LDPE‐co, PP, PA).
- The glow wire could not indent a material filled with a network of glass fibers (UP). As a
consequence, the contact area between the glow wire and the sample surface was very
small.
Due to the abovementioned effects, a wide variation in the contact area between glow wire
and sample (Figure 3‐2) and varying durations of contact were observed. Even for tests with
the same material, both contact area and duration strongly depended on the test
temperature and specimen thickness. As a consequence, test conditions in the glow wire
test were highly non‐uniform and strong correlations with intrinsic materials properties and
other test methods could not be found [70].
A paper indicator is used in the glow wire test to disqualify materials that form burning
droplets. In 240 tests performed on 10 different materials, the paper indicator alone did not
lead to disqualification of a material. Even the HIPS material that showed a very strong melt‐
flow (Figure 3‐2) did not ignite the paper indicator. The reason is that the glow wire tip acts
very locally as a heat source. The burning melt cooled and extinguished during contact with
other parts of the sample surface, which remained at room temperature. This effect
depends on the sample geometry. It can be concluded that the glow wire test is insensitive
to detect materials that might yield a low viscous burning melt. For the set of samples
tested, melt flow generally led to an improved performance due to the increased distance
between the glow wire tip and the material.
Figure 3‐2: Residues of samples which had different contact areas and durations of contact with the
glow wire. Specimens were 3 mm thick and the glow wire temperature was 960 °C. All three
materials reached a glow wire flammability index of 960 °C.
20
Results and discussion
3.1.2 50 W vertical flame test (UL 94)
The 50 W vertical flame test [66] simulates the exposure of a material to a candle sized
flame. It is the predominant test method used by material suppliers and a requirement for
materials for electrical equipment sold in the United States (as defined by the Underwriters
Laboratory standard UL 94). A Bunsen‐type burner is applied to the lower side of a thin
stripe of material, 125×13×t (mm)3 in size, where t is generally between 0.5 mm and 3.2 mm
for electrical insulation materials. Self‐extinguishment within certain time limits and ignition
of a cotton indicator underneath the specimen are observed (see section 2.2.5 for details).
The vertical orientation and complete immersion of the lower edge in the flame (Figure 3‐3)
deliberately creates a very punctual heating of the sample to very high temperatures on five
faces of the specimen. The edge application has a more severe thermal impact than an
application of a flame to a sample face, which is more probable in reality. On the contrary,
the duration of flame application, ten seconds twice, is arbitrarily short. These test
parameters are empirically chosen industry standards.
As the sample is vertically oriented with a free‐standing edge, melt flow from the sample is
facilitated and its influence on material performance shall be discussed in brief. The infrared
pictures shown in Figure 3‐3 show the widely different flow behavior that can be found for
different materials. Polytetrafluoroethylene (Teflon) was chosen to demonstrate the limiting
case of a material that extinguishes directly after flame application and does not flow. It
serves to illustrate where most heat is absorbed. The other limiting case is a polyolefin
material that clearly fails the vertical flame test by burning for more than 30 s and igniting
the cotton indicator by flaming droplets. Whereas a substantial amount of flaming material
is transferred by melt flow, flame spread on the specimen is faster than removal of the hot
material and the specimen continues to burn. Another type of behavior was shown by
materials that have both, a very low viscous melt, and a flame quenching or endothermic
flame retardant. A flame retarded polyamide 66 grade is shown in Figure 3‐3 as a
representative sample. The flow of the low viscous melt caused an effective removal of the
hottest material. At the same time, the flame retardant prohibits a flame spread over the
remaining specimen that is faster than the removal of overheated material.
Figure 3‐3: Infrared thermographs of burning specimens in the 50 W vertical flame test following the
first flame application. Materials tested from left to right: polytetrafluoroethylene (Teflon),
polyethylene, polyamide 66, polycarbonate. The test setup is described in section 2.2.5.
21
Results and discussion
Very fine droplets flowed to the cotton indicator. Depending on the amount and
effectiveness of the flame retardant, the droplets will extinguish before reaching the cotton
indicator (V‐0 classification) or ignite the cotton indicator (V‐2 classification). The last
example in Figure 3‐3 shows a polycarbonate sample that extinguished by virtue of a flow of
the burning part of the specimen to the cotton indicator (V‐2 classification).
The different flow phenomena observed in the 50 W vertical flame test reveal an ambivalent
impact of melt flow on the classification reached. On the one hand, many materials do not
reach a V‐0 classification because material can detach easily from a free‐standing, small
specimen, and even the smallest burning droplet suffices to ignite the cotton indicator. The
V‐0 classification is therefore considered as a challenging criterion for thermoplastic
materials with small amounts of additives. On the other hand, melt flow is a significant
mechanism of heat transfer that aids to prevent flame spread over the specimen surface by
a quick removal of the overheated material. The latter behavior causes an undesirable
geometry dependence of the test result that even affects V‐0 materials. The respective
contribution of flame retardants and melt flow to prevent flame spread should be much
better understood in order to achieve a robust prediction of material behavior in various
geometries. An easy removal of overheated melt should not be assumed in the confined
geometry of highly integrated electronic equipment.
Heat transfer by the flow of burning material is allowed for the V‐2 classification. As the
burning part of the specimen can be simply removed by flow, the span of intrinsic
flammability properties such as the heat of combustion or heat release capacity of the
materials reaching a V‐2 classification is too wide to be useful for regulatory purposes.
3.1.3 Calorimetric test methods
The cone calorimeter is the foremost instrument used in the research community to
determine the heat release rate, which is a measure of a material´s potential to contribute
to fire growth. There is a strong interest in using the reproducible quantitative output from
the cone calorimeter to predict the results of less reliable fire tests. Cone calorimeter
specimens are according to standard held in horizontal position, supported by a tray that
prevents the flow of molten material [55]. The specimen is heated by heat radiation and in
most cases by a flame formed on top of the specimen surface (see section 2.2.1 for details).
The external heat flux is maintained throughout the experiment, resulting in a forced
combustion [60]. The cone calorimeter was recently complemented by microscale
combustion calorimetry [67, 69]. The pyrolysis combustion flow calorimeter measures the
heat release rate of polymers that are decomposed at high heating rate in a pyrolyzer under
inert atmosphere [67]. The use of milligram samples goes a step further in eliminating the
influence of melt flow and thermal conduction to yield the heat release rate for the limiting
case of a complete combustion.
Whereas the elimination of the mass flow phenomena in the calorimetric methods serves to
obtain more reproducible data, it leaves a gap to be bridged to apply the data to more
realistic fire scenarios. As was shown in the previous sections, flame spread rate and
extinguishment are strongly influenced by the flow of overheated and burning material. This
is also true for many real scale specimens [4, 38, 70]. Whereas the chemical heat release
potential of the material can be fairly accurately determined, there are missing terms that
might tip the energy balance to extinguishment; such are flame inhibition, melt flow,
22
Results and discussion
charring and intumenscent processes [69]. The relevance of such phenomena was most
notable in our tests of mineral filled polyolefin materials, which are of high significance for
the European market. Three mineral filled polyolefins had a relatively good performance in
the cone calorimeter experiment, paired with a very weak performance in the classification
tests. Figure 3‐4 shows the heat release of three flame retarded polyolefin materials, as
compared to flame retarded grades of high impact polystyrene, polycarbonate and
polyamide. The cone calorimeter performance of the polyolefin materials was better or
equal than that of the other materials in terms of the time to ignition, peak of heat release
rate and total heat release. For such materials, a V‐2 vertical flame test rating or better
would be expected following correlations proposed in a recent study [61]. The 50 W vertical
flame test result and glow fire flammability index were however much worse for the
polyolefin materials, contradicting the cone calorimeter result. The mismatch in the test
results is due to a superimposition of at least three main effects: (1) ‐ The cone calorimeter,
as the microscale combustion calorimeter, forces combustion and thereby neglects flame
inhibition and incomplete combustion [68, 69]. This certainly applies to the HIPS, PA and PC
samples that contained flame inhibiting additives. It applies to less extent to PP and HDPE
samples containing metal hydroxides, as these are not very efficient as flame inhibitors. (2) ‐
The mechanical integrity of the mineral layer is not put to test in the cone calorimeter
experiment. In the vertical flame test and in most application areas, the mineral filled
material is subjected to gravitational or other stresses that can lead to stronger cracking.
Further, a slow flow of the material was observed, as opposed to the quick melt flow
observed for HIPS and PA. (3) ‐ Mineral filled polymers behave uniquely in the cone
calorimeter experiment as a mineral layer forms on the specimen surface shortly after
ignition [74, 75]. This led to a decay or stabilization of the heat release rate (Figure 3‐4),
which was most probably due to a reduction in the effective heat flux that reached the
sample body after reflection at the mineral layer.
Figure 3‐4: Left: Heat release rate of flame retarded polymers tested at an incident heat flux of
35 kW m‐2. Right: Comparison of heat release capacity obtained by microscale combustion
calorimetry and peak heat release rate (pHRR) from cone calorimeter experiments with an external
heat flux of 35 kW m‐2.
A comparison with microscale combustion calorimetry data shows that no reduction in heat
release capacity was observed for the milligram samples, which are less affected by re‐
radiation and mechanical stresses [69] (Figure 3‐4). The heat release capacity of the mineral
filled polyolefins was a higher than that of HIPS, PA and PC, reflecting their ability to sustain
flaming combustion.
23
Results and discussion
The flame inhibiting properties of commercial flame retardants are in many cases known.
There is however no test method that determines the influence of heat transfer by melt flow
and the stability of protective layers. Modified testing protocols with horizontal and vertical
specimen orientation, which aim at an improved description of these processes, were of
central interest to this study.
24
Results and discussion
3.2 Continuous depolymerization and melt flow of polystyrene
Flow of thermoplastics in flame spread occurs over a wide range of temperatures and flow
rates. Fluid motion can occur as a slow, sagging motion (bulk flow), and as a quick flow of
very low viscous material that easily spreads to a pool [3‐6]. The decrease of viscosity with
temperature is generally sufficient to induce flow in thermoplastics, but the flow rate can be
significantly accelerated by further reduction of viscosity through degradation. This section
investigates the role of viscosity decrease by molar mass reduction in the flow of polystyrene
under fire‐like conditions. The experimental approach was to characterize molar mass
reduction by analytical techniques and to apply the relationships obtained to macroscopic
measurements. The molar mass decrease of horizontally oriented specimen exposed to high
levels of thermal radiation was examined, followed by an investigation of molar mass
decrease in simultaneous gasification/melt flow experiments. The material used was the
same as that studied by Ohlemiller [22], which allows a comparison to Ohlemiller´s extensive
investigation of flow rates and rates of mass loss.
3.2.1 Phenomenological description
The melting behavior of polystyrene has a strong effect on the flammability behavior of end
products and the results obtained by different flammability test methods. Pure polystyrene
is rarely used for electrical appliances, but high impact modified blends (HIPS) also show
distinct melting and deformation phenomena. The series of pictures in Figure 3‐5
summarizes observations made in bench‐scale and full‐scale fire tests on a material used for
switch‐board enclosures [53]. The flow of degraded melt in combination with the use of a
small amount of a halogenated flame retardant improved the ignition resistance of the
material against a small flame. A consistent evaluation of the material in vertical flame tests
and glow wire tests was hampered by strong deformation and flow of a melt with low
viscosity.
Figure 3‐5: Left: HIPS specimen residue after a 50 W vertical flame test. Center, top: Internal stress
relaxation of an injection molded HIPS specimen in the beginning of a cone calorimeter experiment.
Center, bottom: Flaming combustion in the cone calorimeter. The external heat flux was 35 kW m‐2.
Right: Hole formed by a flame applied to the interior of a switch‐board enclosure made of HIPS.
25
Results aand
d discu
usssion
n
h at releeasee raate (79
90 kW
W m‐2) and totaal hea
h t re
eleaasee (9
91 MJ
M m‐22) of
o the HIP
PS maaterrial
A higgh hea
we
eree mea
m surred in co
onee caalorrim
mete
er ttests witth a mod
m derratee exte
ernaal hea
h at ffluxx (3
35 kW
k m‐2),
reveaalin
ng tha
t at the maaterial w
was higghlyy fllam
mmaable whe
w en flow wa
w sh
hind
dered.. In
n prracticee, the
t
ge
eom
mettry of thee prrod
ducct will
w detterrmine wh
hetherr th
he ma
m terrial can b
be rem
r movved
d byy flo
ow fro
om
the h
heat so
ourrce and whe
w ether firee grrow
wth can be
b avo
a oide
ed. The outc
o com
me is ran
r ndo
om.
A sim
mplifieed con
c nfigguraatio
on waas use
u ed tto sysstem
matticaallyy teest flo
ow beh
havviorr. TThe
e flo
ow off pu
ure
po
olysstyrren
ne from
f m a surfface
e expo
ose
ed tto a u
unifform
m ext
e ern
nal heeat flux was
w s an
nalyyzeed in
i a fiirst
levvel of appro
oxim
mation to this comp
plexx prrob
blem
m (Figgure
e 3‐‐6).. Pu
ure po
olysstyrren
ne was
w s studiied to
en
nab
ble a soun
nd analyysis an
nd tto avo
a oid varriattion
ns induce
ed by thee broaad ran
r nge of comp
positio
ons
found
d in
i comm
merrciaal b
blendss. In a firsst steep, ho
orizzon
ntal gaasifficaatio
on experrim
ments weere
pe
erfo
orm
med
d to
o ch
haractteriize thee mas
m ss lo
oss an
nd the
t erm
mal deg
d gradattion
nw
with
hou
ut the efffectt off flo
ow
(se
ee Figguree 2‐‐4 for
f a desscriiptiion off th
he app
a parratu
us).. Alll teests wer
w re p
perrforrmeed using firre‐like
he
eat flu
uxess in
n th
he abssen
nce off a flam
f me, to
o avvoid an
a inh
hom
moggen
neous heat fluxx distrribu
utio
on on
the ssam
mple sur
s facce [76
6‐78
8]. Sp
peciimeen ressidu
uess were
w e rreco
oveered
d ffrom
m the
t e expeerim
ment byy
qu
uen
nchingg, co
old‐cu
ut with
w h a waaterr je
et, aand
d th
he mo
m lar maass graadientts d
deteerm
min
ned.
Figgure
e 3‐6: Lefft: Ver
V rticaal teest settup in thee co
one caloriimeeterr. Centter: Su
urface of a po
olysstyrrene saample
exxpossed to a hea
h at fllux of 50 kW
W m‐2 by
b a ve
erticcally tilted heat
h ter in the
e co
one caloriime
eterr. As
A show
wn,
me
elt waas reco
r oveered
d frrom
m th
he streeam
m u
usin
ng ssyriingee need
n dless. Righ
R ht: Cro
oss‐sectio
on of a spe
s cim
men
residu
ue wh
w ich waas reeco
overred by quencchin
ng from
f m th
he melt fllow
w/gaasificattion
n exxperim
mentt. TThe scaale bar
b r
co
orrespo
ond
ds to
o 2 cm
m.
3..2.2
2 Th
T erma
al sta
abilitty,, dep
polym
merizzattion an
and me
eltt flow
w
On
n a burn
ningg surf
s face
e, the
e m
molar maass reeductio
on of po
olysstyren
ne sim
multtan
neo
ouslly pro
p ovid
des
vo
olattile co
omb
busstib
bless th
hat feed the fflam
me an
nd cha
c ainss of reedu
uce
ed llength
h th
hatt flo
ow reeadily.
Th
herm
maal an
nalysis and sizze excl
e lusion
n ch
hromaatoggraphyy wer
w e perf
p form
meed to
t dete
d erm
min
ne the
t rattes
off bo
oth pro
oce
essees.
Th
he mo
olar mas
m ss red
ducctio
on of polyystyyren
ne prroceeed
ds maainly byy main
m n cchaain ho
omolyysis
(re
eacctio
on 1,
1 Figu
F ure 3‐‐7),, an
nd ran
ndo
om sccission
n byy repe
r eatted hyydrrogeen ab
bstrracttion
n and
a mid‐
m
ch
hain
n β‐‐sciission (reeacctio
ons 3 aand
d 4,, Figgurre 3
3‐7) [2
20]. Chain end
e d ho
omolyysis co
ontrribu
ute
es to this
prroceesss throu
ugh
h th
he form
f mation of
o raadicals (rreactio
on 2). Masss loss is
i p
pred
dom
min
nan
ntlyy du
ue tto the
t
evvaporaatio
on of
o mo
monomeer unit
u ts (“un
nzip
pping””, reac
r ctio
on 6) , an
nd of dim
merr an
nd ttrim
mer unitss th
hat
26
6
Results and discussion
follows an intramolecular hydrogen shift (“back‐biting”, reaction 5), shown at the example of
the trimer [34]. The material studied here showed the typical pattern of degradation for
polystyrene produced by radical polymerization [34, 79]. Molar mass reduction occurred at a
high rate in the initial phase of degradation at low conversions (). Conversion is defined as:
α (t) = 1 −
m( t )
m0
(Equation 1)
With increasing conversion, an asymptotical behavior was observed. The initial strong
decrease in molar mass is partly due to the presence of so called “weak links”, head‐to‐head
configuration bonds that probably resulted from termination reactions via radical coupling
[79]. At intermediate conversion, molar mass reduction proceeds by random scission of
head‐to‐tail bonds.
Figure 3‐7: Depolymerization reactions leading to molar mass reduction and mass loss of
polystyrene. Adapted from [20].
Isothermal gravimetrical experiments at nine temperatures between 285 °C and 415 °C were
used to characterize the temperature dependence of the mass loss rate. The mass loss of
polystyrene cannot be adequately described by a simple set of parameters over a wide range
of conversions [35]. Therefore, an empirical approach was chosen that gave the best
description for low to intermediate conversions, where molar mass decreases strongly. A
combination of a 1st order process describing the weak link decomposition and a 0th order
process describing the main chain decomposition gave a good representation of the mass
loss to conversions of 0.7 (Paper I). The mathematical expression is shown in equation (2).
The limited amount of weak links caused an early decay of the first order process, which was
essentially complete at a conversion of 0.03 (Paper I). Molar mass reduction at this stage was
however considerable and could be estimated to 30 % of Mw,0. The time‐dependent
decrease of molar mass could be directly calculated from the activation energies of the 1st
and 0th order processes. A single fit parameter was needed for the calculation. The quality of
27
Results and discussion
this empirical description was good, as shown on the right hand side of Figure 3‐8. The
mathematical expression is shown in equation 3.
α (t ) = a m ⋅ (1 − exp(−k1 (T) ⋅ t )) + k 2 (T) ⋅ t
M w (t ) = A ⋅ exp(−k1 (T) ⋅ t )) +
B
1 + C ⋅ k 2 (T ) ⋅ t
(Equation 2)
(Equation 3)
Where k1(T) and k2(T) are kinetic rate constants determined from isothermal
thermogravimetric experiments. Constant am corresponds to the mass loss initiated by weak
link scission and constant A is a measure of molar mass decrease by weak link scission.
Constant B is a measure for the remaining molar mass after scission of the weak links and
constant C is a fitting parameter.
Figure 3‐8: Decrease of mass average molar mass with increasing conversion of polystyrene as
determined by size exclusion chromatography of specimens degraded in isothermal
thermogravimetric experiments. Right: Comparison of measurements of molar mass after isothermal
degradation (symbols), with values calculated from the activation energies describing the mass loss
processes using equation (3) (solid lines).
In summary, for the analysis of the melt flow scenario, the most important characteristics
obtained by thermal analysis are the strong reduction in molar mass at low conversion and
the unambiguous coupling of mass loss and molar mass reduction. Whereas it can be
concluded that a certain mass loss definitely implies a corresponding reduction in molar
mass, the inverse is not necessarily true. Molar masses determined in residues form fire
experiments are only indicative of mass loss, if mass loss is not diffusion limited, so that the
volatiles are removed from the specimen.
The relationship between viscosity and molar mass was investigated by rheological
measurements of samples that had been thermally degraded in a tube furnace. Viscosity
decreased with a power of three with decreasing molar mass, as determined by continuous
flow experiments with low shear rates in plate‐plate configuration at 200 °C (Paper I). This is
in fair agreement with the well known value of 3.4 that describes the relationship between
28
Results and discussion
zero shear rate viscosity and molar mass for polystyrene with a narrow molar mass
distribution.
3.2.3 Gradients of molar mass in partially gasified samples
The in‐depth molar mass decrease of polystyrene exposed to fire‐like heat fluxes was
studied using a unique setup. The gasification apparatus [63] at NIST was designed to
irradiate bulk samples under inert atmosphere with a uniform, constant heat flux. A
description of the instrument is given in section 2.2.2. The aim of these tests was to
determine the depth of layers with reduced molar mass for different heat flux levels in a
well‐controlled setup. The well‐defined experimental conditions and avoidance of complex
gas‐phase oxidation reactions make the data obtained suitable for numerical simulations.
During the experiment, the mass loss of the sample was recorded and bubble formation in
the sample observed through a window. The samples had a diameter of 84 mm and a
thickness of 17.5 mm.
Polystyrene samples were irradiated with heat flux levels of 35 kW m‐2, 50 kW m‐2 and
65 kW m‐2 and the mass loss rate recorded. In a second set of experiments, thermal
radiation was interrupted by closing the water cooled shutter. The mass loss at interruption
was 40 % for samples exposed to heat fluxes of 50 kW m‐2 and 65 kW m‐2. Measurements
with a heat flux of 35 kW m‐2 were interrupted earlier, at 15 % mass loss, to avoid convective
currents in the material. The samples were recovered and cut with a water jet. Extracts from
the sample cross‐sections were analyzed by size exclusion chromatography to determine
gradients of molar mass reduction. The result of the molar mass determinations is displayed
in Figure 3‐9. With increasing external heat flux, thermal degradation occurred in thinner
surface layers. This resulted from a steeper thermal wave and from a higher rate of mass
loss for the more intensely irradiated samples.
Figure 3‐9: Left: Molar mass gradient in polystyrene specimens after thermal degradation by
exposure to the indicated heat fluxes under inert atmosphere. Mass loss and experimental time at
interruption were (40 %, 370 s) for 65 kW m‐2, (40 %, 540 s) for 50 kW m‐2, and (15 %, 590 s) for
35 kW m‐2. Right: Cross‐section of a specimen residue showing the holes formed with a hand brace
to extract samples for size exclusion chromatography. The scale bar indicates 1 cm.
29
Results and discussion
It should be stressed that the gradients obtained are snap‐shots of a continuous process that
depend on the sample size. None the less, a valuable insight in the state of degradation of a
polystyrene surface is obtained. For the given configuration, mass loss stemmed from a very
thin surface layer for external heat fluxes of 50 kW m‐2 and higher. Assuming an
instantaneous evaporation of the monomer, dimer and trimer from the surface layer,
equation (2) can be used to estimate the thickness of the layer in which more than 50 %
mass loss occurred to approximately 0.5 mm for the sample irradiated with 50 kW m‐2. The
lower rate of mass loss from the surface of samples irradiated at 35 kW m‐2 led to a slower
surface displacement. In comparison to the higher heat fluxes, the thermal wave propagated
faster into the material relative to the surface displacement by mass loss. More time was
given for in‐depth degradation of the material. It emerges that the ratio between the rate of
heat transfer into the material and the rate of surface displacement by mass loss is a
meaningful parameter describing the extent of in‐depth degradation.
At this point of this investigation, the gradients of molar mass illustrate the respective time
dependence of gasification at the surface and in‐depth degradation over a range of external
heat fluxes. The data may also be used to validate computational models. The strong initial
decrease in molar mass at low conversion makes molar mass a sensitive indicator of the
production of volatiles. First calculations using equation (3) have shown that the shape of
the gradient is strongly affected by the actual heating profile. This effect is illustrated in
Figure 3‐10. The time needed for the sample surface to reach 7 mm from its initial position
(as is the case for the sample shown in Figure 3‐9) was considered. For a point at 9 mm
depth from the initial surface position, three alternative heating profiles were calculated,
such that the surface temperature is reached at the time the surface has receded by 9 mm. A
rather small variation in the heating profile led to significant changes in the computed
gradient behind the surface that reached 7 mm depth (Figure 3‐10). The calculated data
demonstrate that the temporal variation of the heating rate strongly affects the extent of in‐
depth degradation.
Figure 3‐10: Left: Three alternative heating profiles to reach the surface temperature at the time the
sample surface has receded by 9 mm. Right: Calculated molar mass gradients at the time the surface
has receded by 7 mm. Corresponding gradients and heating profiles have a corresponding line style.
30
Results aand
d discu
usssion
n
3..2.4
4 Mo
Mola
ar ma
mass
sc
cha
ang
ge
es in ga
asiific
cattio
on/me
eltt flow
we
exp
perim
ments
Th
he d
deccrease
e in
nm
molaar maass at the
e surffacee of
o vert
v ticaally orrien
nted
d ssam
mple
es (Figgurre 3‐6
3 , leeft)
waas stu
udieed using tw
wo h
heaat ffluxx le
evells. Deggraaded me
m lt w
wass saamp
pled dur
d ringg th
he p
pro
ocess for
f r
molaar mas
m ss dete
d erm
min
nation,, an
nd the
e sttate
e of degrrad
dation of thee surffacee laayer mea
m asured
d. The
T
omb
bined maass loss rate due
d e to
o gaasifficaatio
on and
a d due to meelt flo
ow waas reco
r ord
ded. The maass
co
losss rratees we
w re witthin
n 10
0 % off th
he dat
d ta repo
r orted byy Oh
hlemilllerr [4]. A
As the
t same
em
mate
eriaal and
a
the ssam
me instru
umeent weere
e ussed
d, the surface tem
mp
peraaturess mea
m surred byy O
Ohle
emiiller caan be
ap
pplied forr th
he ana
a alyssis o
of the
t exxperim
men
nts preesenteed her
h re.
Figgurre 3‐1
3 1 sho
ows a scchemaatic skketcch of
o tthee su
urfaace
e laayerr exxpo
oseed to
t hea
h at rad
r iation
n. Itt is
he
elpfful to im
magginee the su
urfaace layerr as a succcesssio
on o
of layyerss w
with
h dec
d reaasin
ng staate off
de
egraadaatio
on. In realitty, the
e trran
nsition
n be
etw
weeen the
t e laayers is smo
s ootth. Ass was sho
ow
wn in the
t
gaasificattion expe
e erim
me
entss, a thin outtmo
ostt layyerr yieelds th
he maain shaaree off vo
olattiles. M
Molar maass
is siggnifficantlyy reedu
uce
ed in a fu
urth
her deeep
per layyer due to tthe steeep
p deecrreasse iin mo
m lar maasss at
low
w ccon
nversio
onss (FFigu
ure 3‐8). Th
he tem
t mpe
eratturre of
o the
t maateeriaal iss su
ufficcientlyy high
h h att fu
urth
herr
de
epth in the maateerial to
o causse flow
w. In‐‐situ tem
mpeeratturee mea
m asurem
mentss in
n th
he ggassificcatiion
ap
ppaarattus show
wed th
hat the
e ordeer of
o m
maggniitud
de of the
e su
urfaacee he
eattingg raate waas 6 °C
C s‐‐1 and
a
‐
10
0 °C
C s‐1
f exxterrnaal h
for
heat ffluxxes off 35
5 kW
W m‐22 and 50
0 kW
W m‐22 reespecttiveely. Th
he baalan
nce
be
etw
ween hea
h at abso
a orp
ptio
on and
a d reemovaal of
o h
hot mateeriaal by mel
m t fllow
w deete
erm
mine
es tthe
e su
urfaace
‐2
temp
peraatu
ure.. For an
n exxte
ernaal heaat flux o
of 35 kW
W m , th
he surfaace te
emp
peratu
ure caan be
esstim
mated to 410 °C and for an
n exte
ernaal hea
h at fllux of 50 kW
W m‐2 to
t 425
4 5 °C
C [4
4].
Figgure
e 3‐11
1: Skketch o
of the
t po
olym
mer surrfacce exp
e oseed to
t a higgh heaat flux,, includ
ding a ten
ntative
e veeloccityy
‐‐2
profile for
f the
e saamp
pless expo
osed
d to a heeat flu
ux of
o 50
5 kW
kW m . The
T e th
hickknesssess of th
he layers weere
arbitrrarily chos
c sen
n for th
his illusstraation.
Fo
or aan exte
e ern
nal heaat flux
f x off 35
5 kW
W m‐2, th
he m
melt reco
r ove
ered
d had a mas
m ss ave
a eragge mo
molarr maasss off
0.5
5 Mw,00. The
T e polyydisspeersitty ind
dex was 5, comp
pareed to 2..7 for
f th
he pris
p stin
ne maaterriall. The
T
su
urfaace layyerr of the recovverred saamp
ple was essen
ntiaally no
on‐‐deggraadeed. The non
n n‐degrrad
ded
samp
ple surface an
nd the
t e in
nho
omo
oge
eneoussly deegraadeed meelt suggge
est thaat the
t e raate off flo
ow off
de
egraadeed and
d non‐de
egraade
ed maaterrial waas ccom
mpaarable
e, such
h th
hat a m
mixxture of deggraded
d and
a
no
on‐d
deggraded
d mat
m teriial w
was reeco
ove
ered
d. Visc
V cossity reducctio
on by tem
mperaature and
dd
degrad
datiion
we
eree off sim
milaar ord
o der of maagnitude.
Fo
or aan exte
e ern
nal heaat flux
f x off 50
0 kW
W m‐2, th
he m
melt reco
r ove
ered
d had a mas
m ss ave
a eragge mo
molarr maasss off
0.3
3 Mw,00. The
T e poly
p ydisspeersitty ind
dex was 5, ind
dicaatin
ng an in
nhomo
ogeeneouss d
degrad
datiion
n. The
T
molaar mas
m ss dist
d trib
butiion
n was shiifte
ed ffrom tthee position o
of the
t mateeriaal´s iniitiaal distrribu
utio
on,
31
Results and discussion
which suggests that all melt was degraded. The sample surface was strongly degraded
(0.2 Mw,0 in the outmost layer, approximately 200 µm thick), suggesting that flow occurred in
the strongly degraded layer. Viscosity reduction by degradation was the rate‐controlling
process, in addition to the effect of temperature.
At first sight, the lower in‐depth degradation of the sample exposed to 35 kW m‐2 seems to
contradict the observations of the gasification experiments. However, it must be considered
that melt flow had the dominant effect on the rate of surface displacement in the flow
experiments. The rate of surface displacement by flow was 1.5 mm min‐1 for a heat flux of
35 kW m‐2, as compared to 2 mm min‐1 for the samples exposed to a heat flux of 50 kW m‐2.
These values were two to three times higher than those obtained in the gasification
apparatus at a comparable mass loss. The faster surface displacement gave less time for in‐
depth heating and the process was more strongly influenced by the surface heating rate.
Calculations with equation (3) showed that molar mass reacted sensitively to the surface
heating rate and surface temperature for times comparable to the residence time at the
surface. The power law dependence of viscosity on molar mass accentuates this effect.
Hence, the lower surface heating rate, lower surface temperature and slower flow observed
for the lower external heat flux caused a more uniform heating of less degraded material.
This resulted in a simultaneous flow of degraded and non‐degraded material. The increased
heating rate and higher surface temperature of the sample exposed to 50 kW m‐2 produced
a thinner degraded layer, which had however a significantly more reduced viscosity.
In conclusion, for pure gasification, the magnitude of the surface temperature and the heat
transfer into the material were the most important factors. Higher surface temperatures led
to a higher rate of gasification relative to the rate of propagation of the thermal wave into
the material, leading to less in‐depth degradation. For gasification/melt flow experiments,
the surface temperature and surface heating rate were the dominating factors. In‐depth
degradation did not strongly influence the flow of thick samples at an external heat flux of
35 kW m‐2, as the rate of surface renewal by flow was fast in comparison to the times
needed to strongly degrade the material at the given surface temperature. For an external
heat flux of 50 kW m‐2, the flow process was controlled by degradation at the surface. The
high surface heating rate and surface temperature led to a fast molar mass reduction during
the residence time at the surface.
The ability to describe molar mass decrease by the Arrhenius parameters describing mass
loss offers a route to implement the reduction of viscosity by degradation in numerical
models. The high sensitivity of molar mass to the heating rate requires that heat transfer
into the material is modelled accurately.
32
Results and discussion
3.3 Liquefaction by step-wise depolymerization of cellular
polyurethane
Although the number of incidents has been steadily declining over the last decades,
upholstered furniture and mattresses remain the main threat to human life in home fires in
Sweden [12] and the United States [10, 11]. Flexible polyurethane foam is widely used as the
cushioning material. A combination of low thermal conductivity, low decomposition
temperature, and high effective heat of combustion of the cellular material results in high
flame spread and combustion rates [2]. Considerable efforts have been undertaken to study
the combustion properties of flexible polyurethane foams [2, 40, 41, 80]. However, the
complex physical behavior of foams during combustion remains a challenge. Recent efforts
have concentrated on describing the transformation of the foam to a liquid pool of burning
material that can additionally heat the initial burning item [4]. This feed‐back effect was
observed to accelerate the combustion rate of furniture and mattresses. The low viscosity of
the burning pool also facilitates flame spread to adjacent items. Work performed in this
study focused on obtaining a better description of foam collapse and liquefaction using the
cone calorimeter. Horizontal tests were used to study the foam collapse, as they were
shown to correlate with measures of flame spread over foam samples [40]. A vertical test
setup was designed and evaluated that allows studying the heat feedback effect for small
samples. The potential of inorganic nanoparticles to prevent the liquefaction of the foam
was assessed. Carbon nanofibers proved to be very effective at preventing collapse and
liquefaction of the foam, whereas intercalated clay composites did not have a pronounced
effect.
3.3.1 Phenomenological description
The combustion of polyurethane foam is accompanied by a very strong reduction of the
foam volume. This effect is demonstrated in Figure 3‐12, where it can be seen that a foam
sample of 9.5 cm × 8.5 cm × 4 cm transformed completely into a liquid pool within 30 s. The
vertical test setup was designed such that the burning liquid was collected in a pool that fed
back to the initial burning item. In the end of the experiment, only the pool fire remained.
Figure 3‐12: Combustion of a foam sample in a vertical cone calorimeter test (the test setup is
described in section 2.2.1). The foam liquefied and formed a pool fire that fed back heat to the
remaining sample and thereby enhanced the rate of combustion. The time elapsed between each
frame was 12 s.
33
Results and discussion
On the length scale of foam morphology, the liquefaction of the foam started as an
accumulation of liquefied material in droplets on the foam surface that started to flow after
a sufficient amount of material had formed (Figure 3‐13). The relationship between the
thermal degradation of the foam, the liquefaction process, and the heat release rate of the
foam were studied in detail, as they significantly influence the burning rate of real objects
made of foam [4, 38, 40].
Figure 3‐13: Liquefaction of flexible polyurethane foam at a surface exposed to a candle sized flame.
The foam struts coalesced to small droplets that combined to a liquid film yielding flaming droplets.
3.3.2 Thermal stability, depolymerization and melt flow
The principal components of flexible polyurethane foam are a branched polyether polyol
(68 wt.%) linked by urethane bonds to segments of a few phenyl groups, which are
interlinked by urea bonds 1. The thermal stability of the bonds decreases in the order
polyether » urea > urethane [26]. The urethane groups are the weakest link to the branched
polyether maintaining the cross‐linked structure. The decomposition of urethane and at
slightly higher temperature of urea leads to a step‐wise decomposition of the cross‐linked
network to the polyol precursor, volatile phenyl amine compounds and potentially residual
oligomeric phenyl‐urea compounds [26]. The volatile compounds account for the initial mass
loss in the thermogravimetric (TG) trace of the foam at 200 °C (Figure 3‐14). The molar mass
is reduced in a sharp transition from a network (infinite molecular mass) to an oligomer
consisting of few units of the polyether polyol ( M w ~ 104 g mol‐1) and finally only the
polyether polyol ( M w = 3 103 g mol‐1). The decomposition of the polyether completes the
degradation of the foam and accounts for the second mass loss step of the TG trace (Figure
3‐14).
The mechanisms of decomposition of polyurethane reported in the literature were based on
analytical experiments that use slow heating rates. Besides a confirmation that these
processes apply to the material studied here, it was studied whether the same mechanisms
apply at heating rates that approach those found in foam combustion. In inert atmosphere,
the typical two‐step decomposition profile was maintained, independent of the heating rate
used (Figure 3‐14). At high heating rate, even for degradation in oxidative atmosphere (air),
a two‐step behavior was observed (Figure 3‐14). At low heating rates of 2 °C min‐1 to
10 °C min‐1, degradation in air usually occurs in multiple, not well‐resolved processes (see
Paper II). This clear transition suggests that oxygen diffusion into the material was limited at
high heating rates. This is relevant for foam combustion, as the foam cells are filled with air.
1
An excess of isocyanate in the foam formulations further leads to the formation of biuret and allophanate
groups, but those are of minor importance for the foam combustion.
34
Results and discussion
The data suggests that the air in the foam cells causes a shift of the foam decomposition to
lower temperatures, but that a description of the kinetics with two rate constants of
consecutive decomposition processes is still adequate.
The heat release associated with both decomposition steps of the foam was measured by
microscale combustion calorimetry. Two consecutive steps of volatile combustion were
observed in the pyrolysis combustion flow calorimeter (Figure 3‐14). As the instrument uses
a high oxygen concentration, and a high temperature of combustion, the values obtained
give an upper limit for measurements of flaming combustion. A comparison of the
experimental values and values derived by group contribution theory agree well in that 71 %
of the heat of combustion of the foam stemmed from the polyether polyol and the
remainder was due to the compounds formed by toluene diisocyanate (Paper II).
Figure 3‐14: Left: Thermogravimetric data acquired with a high heating rate achieved by infrared
heating. Mass loss (black) and differential signal (DTG, grey) of polyurethane foam heated at a rate of
176 °C min‐1 in nitrogen (solid lines) and air (dotted lines). Right: Heat release rate measured in the
pyrolysis combustion flow calorimeter with a heating rate of 176 °C min‐1. The heat release capacity
(HRC) and effective heat of combustion (dHc) are given in the graph.
The rheological properties of the foam during combustion are characterized by the transition
from a cross‐linked network with infinite viscosity to a low molar mass liquid. The volume of
the sample changes by a factor of 40 during this transition and it is doubted that a reliable
measurement of this process with a rheometer is possible. Ohlemiller [4] observed that the
temperature range of the structural collapse occurred within 295 °C to 312 °C at a heating
rate of 5 °C min‐1 for a commercial, non‐flame‐retarded foam. The collapse occurred at
higher temperatures than the maximum rate of mass loss in the first degradation step at
equal heating rate (Tmax,1 = 281 °C). He further found that the viscosity of the liquid
recovered from foam combustion was 0.1 Pa s at relatively low temperature of 150 °C. Given
the high heating rates measured in foam combustion and a temperature of the foam surface
within 280 °C and 340 °C (see section 3.3.4), the transition of viscosity can be considered as a
step function from an infinite value to water like viscosity (≈ 10‐3 Pa s). Visual observations
indicated that surface tension effects have a strong effect on the flow rate of the liquid from
the foam surface [22]. Liquid droplets were seen to accumulate at the surface until a critical
mass of the droplet or thickness of the liquid film was reached to detach the droplet from
the surface. The ensuing flow of the droplet as it spread out on a catch pan was very fast.
35
Results and discussion
3.3.3 Reaction to thermal impact, heat release and feedback
As described above, the foam reacts to a thermal impact by a fast volume reduction and
transformation to a liquid layer. A series of horizontal cone calorimeter experiment was
performed to determine the speed of volume decrease, rate of transformation to a liquid
and heat release rate of the foam. In the cone calorimeter, the sample is exposed to a
uniform, planar heat flux, that decays at the edges of the sample with increasing distance
between cone heater and sample [81] (for distances larger or equal to 2.5 cm). Albeit the
planar heat flux applied, a distinctly rounded shape of the specimen was observed in all
experiments (Figure 3‐15). The displacement of the foam surface at the edges of the
specimen was much faster than at the center (Figure 3‐16). The non‐uniform volume
shrinkage shows that the foam decomposition was influenced by convective heat transfer
from the flame, which is significantly higher at the edges than in the center of the specimen
(Figure 3‐16) [76]. As the sample geometry changed, this effect was further amplified as the
flame licked the curved surface. The rate of surface displacement at the edges and in the
Figure 3‐15: Left: Foam combustion in the cone calorimeter with an external heat flux of 20 kW m‐2.
Right: Change of sample geometry during the experiment as deduced by image analysis. The time
step between two contour lines is 5 s.
Figure 3‐16: Left: Velocity of the surface displacement at the center (squares), at the edges
(triangles) and averaged over the whole surface (circles) of flame retarded foam specimens.
Right: Experimental variation of the mass loss rate (g m‐2 s‐1) for horizontal burning of Poly(methyl
methacrylate) with 0 kW m‐2 imposed flux (top view of 10 cm x 10 cm sample), with permission from
reference [76].
36
Results and discussion
center of the sample was compared to rates observed for non‐flaming collapse. The
comparison suggests that the difference between edge and center corresponds to a
difference in external heat flux of ca. 40 kW m‐2 (Paper II). This pronounced effect suggests
that a study of the relationship between foam shape and combustion rates in fire tests
should be rewarding.
The collapse of the foam in the cone calorimeter was followed by a pool fire in which the
liquid formed at the specimen surface is consumed. The heat release rate followed a two‐
step profile, where the first peak can be assigned to the collapse stage and the second step
to the pool fire stage, in accordance with earlier observations [37] (Figure 3‐17, left). The
similarity to the two‐steps observed in thermal degradation is striking (Figure 3‐14). This has
led to a view of the combustion process as a consecutive consumption of the compounds
formed by toluene diisocyanate (TDI) and the polyether polyol [41]. A more precise
description of this process is possible by an analysis of the effective heat of combustion and
the mass loss in both stages of foam combustion. The effective heat of combustion during
foam collapse increased with increasing external heat flux from values close to the
theoretical value for the TDI compounds to higher values (Figure 3‐17). This indicates that at
a lower external heat flux, and slower rate of collapse (Figure 3‐16), mainly the TDI
compounds were gasified. At a higher external heat flux and rate of collapse, both the TDI
compounds and the polyether were gasified to some extent. This suggests that the pyrolysis
temperature of the thin liquid layer increased with an increasing effective heat flux to the
sample.
Figure 3‐17: Left: Heat release rate in the horizontal cone calorimeter tests of pure polyurethane
foam at multiple levels of external heat flux. Right: Effective heat of combustion in the initial stage of
collapse of pure polyurethane foam.
The effect of thermal feedback could be studied in a vertical test setup (Figure 3‐18). The
main purpose of the vertical test is to study the effectiveness of flame retardant additives
that prevent a liquefaction of the foam (section 3.3.5). Details on the test design are given in
section 2.2.1. The catch pan underneath the sample was preheated by 1 min of exposure to
the cone heat flux, whilst the sample was shielded. The foam was pilot‐ignited with a
laminar flame at the beginning of the experiment. As shown in Figure 3‐18, the heat release
rate depended strongly on the physical behavior of the foam. The maximum heat release
rate was attained when the pool fire formed underneath the sample heated the remaining
sample, indicated as stage 2. A comparison with the data obtained in horizontal
37
Results and discussion
configuration (Figure 3‐17) shows that the time scale and shift with external heat flux were
very similar for experiments conducted with and without flow.
Figure 3‐18: Left: Flame spread (1), feed‐back (2) and pool fire (3) stages of foam combustion in the
vertical cone calorimeter test. The external heat flux was 30 kW m‐2. Right: Heat release rate of pure
polyurethane foam in vertical cone calorimeter tests for three external heat fluxes. The time intervals
corresponding to the three stages shown on the left hand side are indicated above the graph.
3.3.4 Gradients of temperature and mass transport
The transformation of the foam to a liquid layer was studied in detail, as it significantly
influences flame spread [40] and determines the amount of fuel that can feed a pool fire.
Foam samples were quenched from flaming combustion in the cone calorimeter in order to
study the morphology of the liquid layer. As shown in Figure 3‐19, a very thin layer of liquid
formed on essentially non‐degraded foam. No liquid trickled through the porous foam,
which collapsed only near the surface. Measurements with thermocouples placed 1 and
2 cm from the initial foam surface, showed that thermal conduction into the foam was
limited to a few millimeter wide zone (Figure 3‐19). Collapse and decomposition of the foam
occur as a translation of a liquid layer that absorbs fresh material directly underneath the
surface by coalescence of the foam struts (Figure 3‐13). The temperature of the liquid layer
results from a balance between absorbed heat radiation and the heat needed to warm up,
decompose and partially gasify the foam. A kink in the thermocouple measurements
suggests that the temperature of the liquid layer was between 280 °C and 340 °C. The
thermocouple measurements were not sufficiently precise to pinpoint the temperature
more exactly.
Figure 3‐19: Left/Middle: Cross‐section and top‐view of the surface of a foam specimen quenched
from flaming combustion in the cone calorimeter. Right: Temperature measured by thermocouples
38
Results and discussion
inserted 1 cm and 2 cm below the surface exposed to the heat flux in horizontal cone calorimeter
experiments. The external heat flux was 20 kW m‐2.
The amount of liquid produced during the collapse was determined using image analysis and
mass loss data from the cone calorimeter. First the volume decrease of the foam was
determined by image analysis of foam cross‐sections. The difference between the amount of
foam transformed to a dense liquid and the mass loss rate is the amount of liquid formed
during the foam collapse2. This can be expressed as a liquid production rate (LPR), as:
LPR =
dm liq ( t )
dt
=
dm tot ( t )
dV ( t )
− δ foam ⋅
dt
dt
Where mliq(t) is the amount of liquid formed, mtot(t) the total specimen mass, δfoam is the
initial foam density and V(t) the foam volume. The amount of liquid formed for a squared
decimeter of foam was within 0.2 g s‐1 dm‐2 and 0.3 g s‐1 dm‐2 (Figure 3‐20). It increased by
56 % for an increase in external heat flux from 11 kW m‐2 to 33 kW m‐2. The ratio of liquid
formation gasification fell within 2.2 and 2.5. That is equal to or slightly higher than the ratio
of compounds formed by TDI to polyether polyol in the foam, which was 2.2 (Paper II).
Figure 3‐20: Comparison of the rate of mass loss and the rate of liquid production during the collapse
stage in horizontal cone calorimeter experiments.
The liquid production rate gives the amount of material that can contribute to a pool fire. It
cannot be deduced from these experiment how fast the material will flow. The geometry
change in the vertical experiment is also too complex to fulfill that purpose. As mentioned in
section 3.3.1, visual observations have shown that the surface tension of the foam droplets
forming on the foam surface influences their flow [4]. An interesting subject for further work
would be to determine the threshold at which a sufficient amount of liquid accumulates
such that flow occurs.
3.3.5 Elimination of melt flow by inorganic nanoparticles
The liquefaction of the foam might result in a pool fire that boosts the burning rate, which
should be prevented by suitable additives. The conventional brominated‐phosphorous flame
retardant which was substituted by nanoparticles did not fulfil that purpose (Paper III). The
potential of inorganic nanoparticles to stop liquefaction and partially replace the
2
The volume of the liquid layer was neglected, as it was very small as compared to the foam volume.
39
Results and discussion
conventional flame retardant was assessed. Inorganic nanoparticles can be used at low filling
levels and are known to lower the heat release rate of polymers [30, 75, 82].
Composite foams containing oxidized carbon nanofibers and organically modified clay
nanoparticles were prepared using a foam optimization system. The concentration of the
inorganic filler was adjusted to 3.9 wt.% and substituted half the amount of the
phosphorous‐brominated flame retardant. The nanoparticles were dispersed in the
polyether polyol prior to the foaming process by refluxing in a colloidal mill. Small‐angle X‐
ray scattering measurements showed that the clay had an intercalated structure in both the
polyol and the foam composite (Paper III). The amount of catalysts had to be adjusted in
order to obtain foam samples of identical density, as the inorganic fillers strongly modified
the viscosity of the polyether polyol (Figure 3‐21). A titanate coupling agent was used to
reduce the viscosity of the particle‐polyol mixture. Recent, non‐published results, have
shown that the coupling agent interferes with the catalyst. Carbon nanofiber foams can be
obtained with identical quality with lower amounts of catalysts than given in Paper III.
Control samples were prepared as composites of µm‐sized talc particles, in order to account
for the replacement of the flame retardant by inorganic material. Whereas the density of the
composite and pure foam samples could be closely matched, the former had a larger
amount of closed cells. Cell‐opener additives could reduce, but not eliminate this difference.
The carbon nanofibers were observed to accumulate in the center of the foam strut during
the foaming process, whereas the cell membranes were free of particles (Figure 3‐21). The
same was observed for stacks of intercalated clay (not shown). Thus, a locally increased
concentration of nanoparticles in the skeleton structure of the foam was achieved.
Figure 3‐21: Left: Morphology of the carbon nanofiber foam composite. The scale bar indicates
1 mm. Right: Shear rate dependence of viscosity for polyether polyol mixtures with increasing filling
level of carbon nanofibers.
Thermogravimetric analysis showed that the carbon nanofibers did not have a pronounced
effect on the degradation mechanism of the foam. The mass loss curve and heat release rate
curve of the carbon nanofiber composites were shifted to lower temperatures by
approximately 10 °C (Figure 3‐22). This effect might be due to two reasons: First, a catalytic
effect of the carbon nanofibers might lead to an earlier decomposition of the foam. Such an
effect would be limited to the first mass loss step, as no catalytic effect was observed in
measurements of polyol‐carbon nanofiber mixtures. Second, the carbon nanofiber
composite foam retained an expanded structure during decomposition. Hence, the surface
40
Results aand
d discu
usssion
n
orattion
n of
o vo
olatile
es rrem
main
ned
d signiificanttly higgher, w
whiich mightt allso acccou
unt
avvailaable for evaapo
for th
he fas
f terr rate o
of ma
m ss loss
l s.
Figgure
e 3‐22
2: Le
eft: Therm
moggravvim
metrric trac
t ce of
o taalc, carrbo
on nano
n ofib
ber (CN
NF) and clay com
mpo
ositte ffoam
ms,
‐1
1
recorrded
d with
w a hea
h tingg raate of 5 °C
C min
m
in a n
nitrrogeen atm
a mospheere. Rightt: Heat
H t reelease rate
e of pu
ure
foaam and carb
c bon naanoffibeer com
c mpo
osite
e fo
oam
mm
meassureed in the
t pyrolyysiss co
omb
busttion
n flo
ow callorimeeterr
with a heeating ratte o
of 176 °C min
n‐1.
Th
he carrbon nan
n nofiibe
ers pro
oveed to
t be ve
ery efffecctivee at
a eelim
minating liq
quid
d flow
w off th
he foaam
co
omp
possitee. The
T samp
ple rettain
ned
d a dryy surffacee whe
w en exp
e posed to a fflam
me, in
n co
onttrasst tto the
t
acccum
mu
ulattion
n of d
drop
plets oth
herwisse obsserved
d (Figure
e 3‐23
3). The
e ssam
mple
e shru
unkk unifo
orm
mlyy
du
urin
ng com
mbusttion
n expe
e erim
mentss in
n co
onvven
ntio
onal cone
e ccalo
orim
metter tests an
nd a n
nettwo
ork off
caarbo
on naano
ofiberss th
hatt re
esemb
bled
d tthe initiaal foa
f m strructturre rem
r mained
d ((Figguree 3‐23
3 3). In
co
ontrrastt, the
t interrcallateed clay cou
c ld nott effectivvelyy prev
p ven
nt the
t fo
orm
mation off drrop
plets and
a
liq
quid
d flow
w of
o the
t e fo
oam
m (Fig
( guree 3‐23
3 3). Th
he elim
min
nattion
n of
o liqu
uid flo
ow resulted
d frrom
m the
t
mech
han
nicaal stre
s enggth of the eenttanggled car
c bon
n nan
n nofiibers and
d a faavo
orab
ble su
urfaace teensiion
be
etw
ween the
e fibe
f ers and th
he liq
quid
d form
f meed du
urin
ng decom
mposition thaat countteractted
disspeersiion intto dro
d ople
ets. Th
he surrfacce tten
nsio
on effe
e ect waas iinfeerreed fro
om visuall ob
bseervaatio
ons off
the ssam
mplee su
urfaacee du
urin
ng bur
b rnin
ng. Th
his hyp
h pothessis cou
uld bee su
upp
portted byy meassurrem
men
nt of
o a
higgh capaccityy of the carbo
on nanoffibeer ressidu
ue to ab
bsorb and reta
r ain th
he p
polyetthe
er p
polyyol
(Papeer III).. Th
he ressidu
ue of claay com
c mpo
ositte ffoaamss laacke
ed thee capaabilityy to
o ab
bso
orb the p
polyyol
an
nd a hig
h herr su
urface
e eenergyy betw
weeen thee clay
c y reesid
duee and po
olyo
ol was d
determ
min
ned. The
T
su
urfaace en
nerggy pro
obaablyy play
p ys a cruc
c ciall ro
ole in th
he form
f maatio
on of
o liqu
uid drrop
pletts that
t t was
w
ob
bserveed for
f thee clayy co
omp
possitees. The
T e “d
dry” surffacee off th
he car
c rbon n
nanofiberr fo
oam
ms sug
s ggests
that thee liq
quid form
f med w
wetts the naanofiber nettwo
ork an
nd ggassifiees wit
w hou
ut an
a acccum
mullatiion to
liq
quid
d drop
pletts.
Figgure
e 3‐23
3 3: Leftt/M
Midd
dle: Sam
S mplee su
urfaace off caarbon naanoffibeer com
c mpo
osite foam
f m (lefft) and
d clay
c y
naanoccom
mpo
ositte foam
m (mid
ddle
e) aafteer 6 s eexp
posu
ure to a nee
n edlee sized
d flaamee. Righ
R ht: Carrbo
on nan
n ofib
berr
co
omp
posiite foa
f m resi
r idue
e affterr a h
horrizontaal co
onee caloriimeeterr tesst.
41
Results and discussion
Tests with the vertical cone calorimeter setup showed that pool fire formation was
prevented by the addition of carbon nanofibers. A lower heat release rate was obtained,
although the sample had a larger surface area, as it shrunk but did not collapse (Figure 3‐24).
The reduction in heat release rate was largely due to the prevention of feedback from a
burning pool. In addition, thermocouple measurements in horizontal cone calorimeter tests
showed that the temperature in the foam sample rose at a significantly lower rate, as the
pyrolysis zone was covered and not directly exposed to the flame. The residual network of
carbon nanofibers had a heat shielding effect (Figure 3‐24). No remarkable heat release rate
reduction was observed for the intercalated clay composite, as the structural collapse of the
foam could not be prevented. The heat release profile resembled that of the control
formulation (Figure 3‐24).
In conclusion, addition of 3.9 wt.% of carbon nanofibers appears as a viable approach to
significantly improve the flame retardancy of flexible polyurethane foams. Remaining
challenges are an optimization of foam processing and mechanical properties. The recently
raised concern about a potentially carcinogenic action of fibrous nanoparticles with high
aspect ratio should be taken serious [83]. An evaluation of flame retardant efficiency for
different fiber lengths seems adequate.
Figure 3‐24: Left: Heat release of the nanocomposite foams compared to a talc composite in the
vertical cone calorimeter setup. The external heat flux was 11 kW m‐2. The time intervals for the
flame spread (1), feedback (2), and pool fire (3) phases of combustion are indicated above the graph.
Right: Reading of thermocouples embedded at 2 cm depth in samples tested in horizontal cone
calorimeter experiments with an external heat flux of 20 kW m‐2.
42
Results and discussion
3.4 Melt flow and expansion of a filled polyolefin stabilized
by ionic bonds
Polyolefins are along with polyvinylchloride the most widely used materials on the European
market for low voltage cables. A strong interest in environmentally benign flame retardant
solutions made polyolefins with high levels of mineral fillers the predominant choice. As was
shown in 3.1.3, the mechanical integrity of the residue strongly affects the flame resistance
of mineral filled polymers. The flow of burning material is a highly undesirable effect that
might lead to flame spread from a burning cable, e.g. installed under the ceiling, to other
parts of a room or building. This effect is accounted for by new European legislation that
prescribes a classification scheme which disqualifies materials that yield flaming droplets or
particles [84]. The new legislation gives a strong incentive for an improved understanding of
melt flow from burning cables. Materials with better flow resistance are needed. This
chapter describes the fire behavior of non‐halogenated acrylate‐ethylene copolymer
materials that contain calcium carbonate and polydimethylsiloxane (silicone). The
performance of different polymer resins was tested with regards to the stability of the
inorganic residue and melt flow phenomena.
3.4.1 Phenomenological description
Inorganic fillers can reduce the flammability of polymers by fuel dilution and the formation
of layers of inorganic residue that act as a thermal shield and diffusion barrier [74]. The
formation of an expanded inorganic layer can improve the insulating heat shield effect and is
referred to as intumescence. Effective intumescence leads to self‐extinguishment of the
material, although the term is also used if only a retardation of combustion is achieved.
The polymers studied here contain a high filling level of calcium carbonate and an inorganic
polymer, polydimethylsiloxane, to achieve a heat shield effect [46]. A moderate expansion of
the residue layer was observed caused by a blowing reaction that involved the side‐group of
the polymer and the calcium carbonate filler (Figure 3‐25, section 3.4.2). As shown in Figure
3‐25, an inorganic residue layer formed at the surface exposed to a thermal impact.
Figure 3‐25: Left: Expansion of the EMAA‐based formulation in the cone calorimeter and flame
protrusion from a crack in the surface layer. Center: Immobilization of bubbles and pore formation at
the surface exposed to the heat flux. Right: Cross‐section of a specimen quenched from combustion
in the cone calorimeter. (The brittle top layer was damaged and partially removed during cutting).
The material expanded by both, formation of small bubbles that immobilized at the surface
and larger bubbles formed in the bulk of the material. The expanded surface structure
43
Results and discussion
transformed into an inorganic residue. The structure of the polymer resin determined
whether the mechanical integrity of the char layer was maintained during combustion,
which had a strong effect on the burning behavior.
3.4.2 Thermal stability, depolymerization and melt flow
Acrylic copolymers of ethylene form the main polymeric component of the investigated
material. The performance of poly(ethylene‐co‐butyl acrylate) (EBA), poly(ethylene‐co‐butyl
acrylate) blended with polypropylene (EBA‐PP) and poly(ethylene‐co‐methacrylic acid)
(EMAA) formulations was assessed. The chemical structure of the side groups of EBA and
EMAA is shown in Figure 3‐26. Polypropylene (PP) was contained in one formulation in order
to increase its mechanical stiffness. For all formulations, 30‐40 wt.% of chalk and 5‐7 wt.% of
silicone were added to the polymer.
EMAA
EBA
Figure 3‐26: Chemical structure of the side groups.
The thermal degradation of the acrylic copolymers is initiated at lower temperatures by the
decomposition of the side groups, followed by a depolymerization of the main chain at
higher temperatures [45, 85]. For pure EMAA, side group decomposition can lead to
anhydride formation, which yields water, and decarboxylation [85]. The presence of calcium
carbonate alters the reaction of the acrylic acid side group by formation of ionic bonds
(Reaction 1) that stabilizes the side group and backbone (see 3.4.5) [86‐88]. The reaction
yields water and carbon dioxide that can act as blowing agents to expand the polymer.
Reaction 1: One pathway of the reaction of the acrylate side group with calcium carbonate to form
an ionic bond, shown at the example of EMAA.
For pure EBA, side group degradation proceeds as anhydride formation and ester pyrolysis
for temperatures above 300 °C [44, 45]. In the presence of calcium carbonate, cleavage of
the butyl group (ester pyrolysis) must occur in order to form an ionic bond between the
carboxylic acid and the calcium ion. Thermogravimetric analysis showed that mass loss by
side chain reaction was small for the pure copolymers (Figure 3‐27, left), probably hindered
by an ample spacing of the side groups [85]. The reactions were accelerated in the presence
of the calcium carbonate filler, as marked by an earlier and larger mass loss (Figure 3‐27,
right). The temperature interval between the onset of side‐group decomposition and main
chain degradation was much larger for the EMAA‐based formulation than for the EBA‐based
formulation, as it was unhindered by the ester group. The decomposition of the main chain
44
Results and discussion
reached a high rate at 450 °C for both polymers, which was not significantly altered by the
fillers.
Figure 3‐27: Left: Thermogravimetric trace of pure EBA (solid line) and EMAA (dashed line) in a
nitrogen atmosphere. Right: Thermogravimetric trace of the flame retarded formulations of EBA‐PP
(solid line), EBA (dashed line) and EMAA (dash‐dotted line) in a nitrogen atmosphere. The heating
rate was 10 °C min‐1 for all measurements.
The oxidative stability of the EMAA‐based formulation was significantly higher than that of
the EBA‐based formulation and much higher than that of the formulation containing
polypropylene (Figure 3‐28). This stability stems from the formation of a thin layer of
degraded material around the specimen acting as a diffusion barrier for oxygen. The barrier
layer has been assigned to the formation of a glassy layer formed by the oxidation of silicone
oligomers that diffuse to the surface [31]. Formation of a diffusion barrier was also observed
in measurements of EMAA with a very fine grade of calcium carbonate (see section 3.4.5) in
the absence of silicone, indicating that the chalk filler can bake to a tight layer. A
combination of both effects most likely takes place and has a more pronounced effect for
the EMAA‐based formulation (Figure 3‐28). This indicates either a higher mobility or better
dispersion of the silicone oligomer, or better dispersion of the filler in the EMAA‐based
formulation [47].
The melt viscosity of the degraded flame retarded formulations is influenced by three
factors: (1) cross‐linking by calcium ion bonds; (2) chain length reduction by
depolymerization; and (3) interaction with the filler. Rheological tests performed in nitrogen
did not show a strong effect of the calcium ion bond formation for either the EMAA‐ or EBA‐
based formulations. The melt viscosity was stable at a temperature of 300 °C over a time
period of 15 min (PAPER IV). Hence, the calcium ion formation can be attributed a stabilizing
effect of the main chain, rather than a significant cross‐linking effect. As the rate of
depolymerization of the main chain increased significantly in the vicinity of 400 °C,
meaningful rheological measurements were impossible. The very low viscosity observed in
the combustion of pure EMAA and EBA suggests however that main chain decomposition
yields a low viscous fluid of low molar mass. Rheological measurements in air showed a
pronounced increase of the viscosity of the EMAA‐based formulation at 350 °C. A similar, but
weaker effect was observed for the EBA‐based formulation (PAPER IV). These measurements
had a qualitative nature as the specimens were non‐uniformly degraded between the
rheometer plates, with a strongly degraded outer layer and a less degraded inner layer. They
indicate however that the force needed to deform the polymer reacted sensitively to the
formation of a surface layer of strongly degraded material, in which the depletion of the
45
Results aand
d discu
usssion
n
olym
mer leead
ds to
o a higheer con
c ncentrration of thee in
norrganic filller.. Th
hese obs
o ervvatiions sugggesst a
po
qu
ualitattivee piictu
ure off a rhe
eolo
ogical beehaavio
or that
t t iss de
efin
ned
d byy a temp
peratu
ure thresshold for
f r
the o
onsset of main
n ch
hain deg
d grad
dation
n and the exte
e entt off expo
osu
ure to oxxyggen (Figu
ure 3‐2
28,
rigght).
Figgure
e 3‐28
8: Le
eft:: Th
herm
moggravvim
metrric trac
t ce of
o the flam
me rettard
ded forrmu
ulattion
ns of
o EBA‐‐PP (so
olid), EBA
E A
(daash
hed)) an
nd EMA
E AA (daash‐‐dot) in
n an
n oxyggen atm
mossph
heree, re
eco
ordeed with
w h a heaatin
ng rrate
e off 10 °C min‐1.
Rigghtt: Seemi‐qu
uan
ntitaativve p
plott off th
he visc
v cosity of thee EM
MAA‐b
baseed formu
ulatiion. Att su
ufficien
ntlyy slow
w
he
eating ratee and at ssurfaces exp
e poseed to
t aair, thee mate
m eriaal co
ould
d bakee to
o a rrigid
d laayer (p
path
h A)). A
At high
he
eating ratees in
i abse
a encce o
of aiir, tthe maaterrial waas sttab
bilizeed butt visscosityy deecreease
ed due
e to
o main chain
de
ecom
mposittion
n (p
path
h B)).
3..4.3
3 Re
Reac
ctio
on to
o th
he
erm
mall im
mp
pac
ct, he
eat rele
r ease
e and
d fe
eed
db
bac
ck
Th
he fflam
me retarrded
d fo
orm
mullatiions wer
w re test
t ted in the con
c ne calo
c orim
metter in orderr to
o sttud
dy the
t
vaariation of
o the
t e heatt reeleasee raate w
with ch
hem
mical com
mposition. All fo
orm
mulaatio
ons sh
how
wed
d a
qu
uickk stab
bilizzatiion of the heeatt re
eleaase
e raatee fo
ollo
owing ignition
n, tthe tyypiccal beehaavio
or for
f r
mateeriaals tha
t at form
f m an
a ino
orgaanicc or
o ccharred surf
s face laayeer [60
[ ] (FFigu
uree 3‐‐29). The
T e to
otall heeat
releaase an
nd effe
e ective heeat off co
omb
busstio
on w
were sim
milaar ffor all formulaatio
ons.. Vaalues clo
ose to
thosee exp
e ectted fo
or the
t e co
om
mbustio
on of th
he pollym
meric ccom
mpone
entt sh
how
w tthat tthere is no
n th
he intrrinssic chaar yie
y ld (Figgurre 3‐29
3 9). The hea
h at rele
easee raate de
ecreeassed
siggnifficaant inccreasee in
wiith in
ncreeasingg ccontten
nt of th
he acrrylic cop
polyym
mer ass com
c mpaared
d to
t the mixxtu
ure with
w
po
olyp
pro
opylene. It w
wass lowe
er fo
or the
t e EM
MA
AA‐ thaan for
f the EEBA
A‐baased form
mulatiion. Th
he low
werred
he
eat releaase ratte at ideentiical efffecctivve heaat of
o com
mbusttion
n sttro
ongly sug
s ggests th
hat physical
prroceessses such as he
eat shield
din
ng and
a d diffu
usio
on barrrie
er eeffeectss caussed
d th
he deccreease
e. V
Visu
ual
ob
bservaatio
on of thee sam
s mple d
durringg com
mbu
ustion su
upp
portts this
t s view
v w, as the PP
P co
ontaainiing
formulaatio
on craacked in
nten
nsivvelyy at
a ign
nitio
on, whe
w ereaas thee cha
c ar llayeer of th
he EB
BA‐b
bassed
formulaatio
on was
w s raather staablee with
w h feew craackss and the
e EEMA
AA‐‐based
d fo
orm
mulatio
on had a veeryy
staable ino
i orgaanicc laaye
er. An
n in
nsp
pecttion
n of
o thee resid
due
e o
of the
t e EMA
E AA co
ontaainingg sp
peccim
men
reveaaled
d a co
oarsselyy laayered
d sttrucctu
ure witth a th
hin do
ome
eo
of in
norrgan
nic maate
erial fo
orm
med
d on
n to
op.
Th
he EEBA
A‐b
base
ed forrmu
ulattion
n fo
orm
med
d a sim
milar ressidu
ue, wh
hich was
w s ho
oweve
er m
mo
ore briittlee and
a
sh
how
wed
d manyy mor
m re crac
c cks. Th
he ressidu
ue of
o tthee EB
BA‐‐PP‐‐baased
d fo
orm
mulation waas esse
e enttially flat
f
an
nd h
had
d a coaarse sttructu
ure.
46
6
Results and discussion
Figure 3‐29: Left: Heat release rate of the flame retarded formulations of EBA‐PP, EBA and EMAA in
the cone calorimeter, tested with an external heat flux of 35 kW m‐2. Right: Effective heat of
combustion and time to ignition of the flame retarded formulations from the same experiment.
3.4.4 Gradients of temperature and mass transport
The temperature to which the material is heated during combustion will determine the rate
of the degradation processes and the mass loss rate for the production of combustible
volatiles. Thermocouples were inserted in cone calorimeter specimens in order to determine
how the processes described in section 3.4.2 affect the heat release rate. In addition,
specimens were quenched from flaming combustion and the cross‐sections analyzed to
determine the extent of the chemical degradation reactions throughout the specimen. The
analysis was concentrated on the EMAA and EBA‐PP formulations as these showed the
clearest differences.
The time to ignition was very important, as it determined the time given to expand the
material and form an insulating inorganic layer before the flame heat flux additionally heats
the sample. The lower oxidative stability of polypropylene and less effective barrier
formation of EBA‐PP led to much shorter time to ignition of this formulation (Figure 3‐29).
The material did not expand significantly, which was also due to a low acrylate content in a
more rigid material (Figure 3‐30, left). The lower onset of the side‐group reaction and higher
time to ignition of the EMAA material led to a more pronounced expansion prior to ignition
(Figure 3‐30, left). The mechanical strength of the inorganic surface layer is important: A large
number of cracks in the surface layer resulted in a broad flame plume on the EBA‐PP sample
surface and a strong heat flux by the flame. The temperature in the top layer of the EBA‐PP
specimen was thus very high (Figure 3‐30, top center), well above the temperatures at which
the beneficial expansion reaction could occur. Instead, the rate of depolymerization was
very high under such conditions and the high effective heat of combustion ensured that the
flame heat flux remained high. The result was a fast combustion with a steep temperature
gradient between a strongly overheated top layer and a lower layer that only passed a very
short period of time in the temperature interval in which expansion occurs. Accordingly, the
extent of the side group reaction to form beneficial ionic bonds was low and the resulting
gradient in the degradation of the side groups was steep (Figure 3‐30, right).
47
Results and discussion
Figure 3‐30: Melt expansion (left), in‐situ temperature gradient (center) and depletion of the
acrylic side groups (right) of EBA‐PP‐based (top) and EMAA‐based (bottom) flame retarded
specimens, tested in the cone calorimeter and quenched one minute after ignition. Infrared
spectra were measured on thin slices of the samples. The absorption of the carboxyl vibration of the
acrylic side group is shown as an indicator for side group depletion.
The EMAA‐based formulation had a degraded inorganic surface layer that was mechanically
stable. It showed only few, small cracks. Cracking might have been prevented by a porosity
of the top layer that prevented a strong pressure build up. Many fine holes were formed by
bubbles that opened at the surface and congealed in the process (Figure 3‐25). Flames were
positioned above the cracks and did not span the whole specimen surface. Consequently,
the temperature inside the sample remained comparatively low and the stabilizing ionomer
reaction could proceed in deeper layers of the sample. The more levelled temperature
gradient was accompanied by a wider extent of the side‐group reaction throughout the
specimen (Figure 3‐30, bottom center).
In summary, the ability to form an inorganic layer, its expansion and stability determined a
threshold behavior of the material. The material was either shielded sufficiently to remain in
a temperature range where the beneficial, retardant solid phase mechanisms could proceed,
or it bifurcated into a regime where a high temperature activated fast main chain
depolymerization and the high heat of combustion caused a run‐off of this reaction. In the
latter regime, the temperature was too high for low temperature solid phase mechanisms to
influence the combustion. Ignition resistance influenced strongly which pathway was
followed.
The flow of burning material was dominated by the bifurcation between sufficient insulation
or self‐accelerating activation of the main chain decomposition. Vertical cone calorimeter
experiments of EMAA specimens yielded results similar to horizontal measurements, if the
sample was supported in a retainer frame (Figure 3‐31).
48
Results and discussion
Figure 3‐31: Vertical cone calorimeter tests. Left: EMAA‐based specimens had a sufficient stability to
largely resist flow when exposed to an external heat flux of 25 kW m‐2 in a supported configuration.
Right (2 frames): In unsupported configuration, char rupture led to a fast rate of combustion and
feedback for an external heat flux of 35 kW m‐2.
The insulating effect of the inorganic layer prevented a significant melt flow at a
considerable external heat flux of 25 kW m‐2. Without support, and at a lower thickness of
the specimen, the insulating effect was not sufficient to prevent a self‐accelerating
combustion at 35 kW m‐2 (Figure 3‐31, right). (Tests in unsupported configuration with 25 kW
m‐2 resulted in a flow of the material before ignition could occur.) Self‐accelerating
combustion was also observed in 50 W vertical flame tests of the material. In both cases, the
isolating expansion effect was not sufficient for a thin specimen, especially when heated
from multiple directions. Self‐sustained burning and large droplets of burning melt were
observed.
Concluding from the vertical experiments, the flow behavior of the material is determined
by the heat transfer in a given setup and thereby strongly geometry dependent. The high
effective heat of combustion renders the material intrinsically instable. If it is supported and
in contact with a body that can conduct heat away (as might be the case for cables), high
temperatures may be prevented and congelation of the inorganic expanded layer proceeds
sufficiently to insulate the material (Figure 3‐28, path A). As a free‐standing material, without
connection to a heat sink, main chain decomposition is activated as higher temperatures are
reached. A run‐off reaction follows that produces a burning melt (Figure 3‐28, path B), which
retains however a considerable viscosity due to the large amount of filler in the material.
3.4.5 Potential for improvement using finer grades of the filler
As was shown above, the ability of the flame retarded formulations to withstand thermal
insults relies on the formation of stabilizing bonds between the side groups of the polymer
and calcium from the calcium carbonate filler. The higher surface area of finer grades of
calcium carbonate offers the potential for an acceleration of the stabilizing process. This
gave the incentive to study the thermal and thermo‐oxidative degradation of
nanocomposites of EMAA with a very fine grade of calcium carbonate (cubic shape, average
49
Results and discussion
width of 70 nm). Composites with a filling level of 5 wt‐% of uncoated and stearic acid
coated calcium carbonate were prepared by melt extrusion. The mechanical properties of
the composites were similar to the pure material and superior to a control sample filled with
µm‐sized particles (Paper V).
The low amount of filler added to the EMAA proofed to be very effective to cause a reaction
of the acrylic acid side groups to form calcium ion bonds. A small mass loss at low
temperatures indicated the onset of the reaction forming the calcium salt (Figure 3‐32). The
side group was stabilized to higher temperatures, resulting in a shift to a single
decomposition step, as seen in the differential thermogravimetric trace (DTG). The transition
from carboxylic acid side‐groups to calcium ion bound carboxylates was confirmed by
infrared spectroscopy (Figure 3‐32, right)
Figure 3‐32: Left: TG and DTG trace of EMAA (solid line) and an EMAA calcium carbonate
nanocomposite (dotted line) acquired in nitrogen atmosphere with a heating rate of 10 °C min-1.
Right: Ex‐situ infrared spectra showing the decrease of the carboxylic acid absorption and increase in
calcium ion absorption after isothermal degradation in nitrogen at 350 °C for the indicated times.
TG measurements in oxygen revealed that EMAA nanocomposites were much more stable
than pure EMAA. This effect is assigned to the formation of a very tight surface layer of low
oxygen permeability (Figure 3‐33), rather than an improved intrinsic stability. The surface
layer broke when the thermal degradation of the material accelerated, leading to a very fast
combustion of the material.
Figure 3‐33: Thermogravimetric trace of pure EMAA (solid line) and an EMAA calcium carbonate
nanocomposite (dotted line), acquired with a heating rate of 10 °C min‐1, using oxygen as the purge
gas.
50
Results and discussion
Simple combustion experiments showed that the addition of low amounts of the very fine
calcium carbonate could not prevent a high burning rate and formation of a low viscous
burning melt. High filling levels are still required to dilute the EMAA and to form thicker
barrier layers. The results show however clearly that the addition of low amounts of
nanoparticles can significantly accelerate both processes that are beneficial to the formation
of an expanded inorganic structure: the calcium salt formation needed to stabilize and
expand the material and the oxygen barrier formation that delays ignition. The addition of
small amounts of the nanoparticles might therefore improve the properties of the flame
retarded cable materials, whilst the mechanical properties can be maintained.
51
Conclusions
4. Conclusions
The molecular architecture and degradation mechanism of the polymers investigated had a
significant influence on their viscosity and flow behavior during combustion. The rate of heat
transfer into the materials determined the difference between the surface heating rate and
the bulk heating rate, which had a strong influence on the flow behavior of all three
polymers in this investigation:
• A transition from temperature‐driven flow to degradation‐ and temperature‐driven flow
was observed for vertically oriented, radiation heated samples of polystyrene with an
initial molar mass of 2.5 105 g mol‐1. For an incident heat flux of 35 kW m‐2, temperature‐
induced melting determined the flow rate of thick samples. At the given surface
temperature, the degradation reactions were relatively slow compared to the rate of
surface renewal by thermally driven flow, resulting in a similar flow rate of degraded and
non‐degraded melt. For an incident heat flux of 50 kW m‐2, degradation strongly reduced
the melt viscosity in a thin surface layer. Melt flow in the surface layer was in this case the
rate‐controlling process and an intensely degraded melt was recovered. The surface
temperature and the difference between surface heating rate and in‐depth heating rate
were the most important parameters affecting the flow behavior.
• Gasification experiments of polystyrene under inert atmosphere were used to determine
the gradient of molar mass at surfaces exposed to different levels of heat flux. The
thickness of the surface layer with significantly reduced molar mass markedly decreased
with increasing heat flux. The ratio between the rate of propagation of the thermal wave
into the material and the mass loss rate at the sample surface determined the extent of
in‐depth degradation.
• The uniquely low thermal conductivity of cellular polyurethane made flow a surface
phenomenon for all levels of thermal insult. The liquid formed during combustion
accumulated in a thin surface layer atop essentially non‐degraded material. The fast
shrinkage of the cellular material was very sensitive to variations in flame heat transfer.
The foam sample was first shaped by the locally varying magnitude of the convective heat
transfer from the flame. The shape of the surface then affected the convective heat
transfer resulting in an inter‐locking process. The influence of this effect on flame spread
is expected to be significant.
• Flexible polyurethane foam composites with oxidized carbon nanofibers were prepared.
An entangled network of carbon nanofibers percolated the center of the hexagonal foam
structure. The collapse and liquid formation of the foam during combustion were
eliminated. Similar composites with intercalated clay did not significantly alter the
flammability properties of the foam.
• The viscosity of poly(ethylene‐co‐methacrylic acrylate) was stabilized to the temperature
of rapid main chain scission by interaction with calcium carbonate. At slow heating rates,
52
Conclusions
• the blowing reaction of the polymer side chains with the filler could proceed for longer
times in its limited temperature range (250 °C – 400 °C), which led to a further reduction
of the heating rate and a retardation of combustion. Fast in‐depth heating led to the
activation of main chain decomposition (400 °C – 500 °C) and release of the high effective
heat of combustion of polyethylene, which caused a run‐off reaction. The mechanical
integrity of the surface layer determined the in‐depth heating rate and whether the
threshold to main‐chain decomposition was passed.
A wide variation of the contact area and of the duration of contact with the heat source
were observed in glow wire flammability tests of 10 materials from the European market for
low voltage switchgear and controlgear. For this set of samples, melt dripping uniformly led
to a better test performance, as the distance to the glow wire increased.
The edge application in the 50 W vertical flame tests (UL94) leads to a severe, highly
localized thermal impact to the sample. It also facilitates the removal of the hottest material
by flow, which is an ill‐defined process that affects the test result.
Suggestions for future work
To improve bench scale classification tests for electrical appliances, a combination of heat
release capacity measurements with tests of flame resistance for different specimen
geometries should be evaluated. The heat release capacity is a property that is fairly
independent of heat transfer limitations. As a complement, geometry variation of samples
exposed to a flame is the most cost‐effective way to amplify the effect of heat transfer, flow
and mechanical stresses. The test should comprise a setup where flow is hindered or
overheated melt collected such that it can feed back heat to the initial item. Arbitrarily
chosen boundary conditions, such as an application of the flame for precisely 10 s, should be
varied over a reasonable scale of levels. Determining the limit at which material fails is
valuable information that is today obscured by the arbitrarily chosen boundary conditions. A
failure within certain well‐defined limits should be accepted for a given application, rather
than disinformation promoted.
53
Acknowledgements
5. Acknowledgements
I sincerely want to thank my supervisor, Prof. Ulf Gedde, for his outstanding support, his enthusiasm
and his positive attitude towards my work that gave me confidence. I would like to thank Dr. Jeffrey
Gilman for being a great role model, his guidance and giving me the opportunity to work at NIST. Dr.
Mauro Zammarano and Dr. Per Blomqvist are thanked for many fine hours of working in the lab,
many interesting discussions and all I could learn from them. My thanks go to Prof. Patrick van Hees
for his excellent support and mentoring. I want to thank Dr. Takashi Kashiwagi for telling me the
spider joke and Dr. Bruska Azdhar for making things work. I highly appreciated the skilful
prearrangement and administration of the framework of this project by Sven Jansson and enjoyed
every lunch and “tea” break. I am thankful for eye‐opening advice from Dr. Gregory Linteris, I
enjoyed our discussions. Dr. Bernhard Schartel and Dr. Thomas Ohlemiller are both thanked for their
sobering advice that often, but not often enough, saved me from wasting a lot of time. The critical
spirit and jokes from Prof. Michael Hedenqvist were very welcome. I am indebted to
Prof. Wulff Possart and Dr. Carsten Wehlack for all they tought me.
Financial support from ELFORSK, BRANDFORSK, El‐Säkerhetsverket, FORMAS, the industrial
consortium supporting this project and NIST are gratefully acknowledged. NIST, SP, BOREALIS AB,
ABB, BAM, Draka Kabel, Habia Cable are gratefully acknowledged for material supply and access to
facilities.
I highly appreciate the efforts of Stefan Kjellnäs to introduce me to the world of standardization and
the resources he supplied. Lars‐Erik Ahlstrand, Bernt‐Åke Sultan, Jonas Jungqvist and Perry Nylander
are sincerely thanked for their rich support with advice and resources. Jan Berggren did an excellent
job to ground my thinking to real world scenarios. Mohsin Ali Raza is thanked for his enthusiasm and
great working spirit. I could certainly learn much from Fritjof Nilsson that went into this work (in the
last minute).The support and advice from Bertil Ahlinder, Michael Schmidt, Per Anders Högström,
Maria Conde and Kjell Oberger was extremely helpful. Thomas Korssell and Irina Ekblad are thanked
for many enriching discussions. Dick Harris, Randy Shields, Ken Steckler, Szabolcs Matko, Dr. Sameer
Rahatekar, Dr. Mark Nyden, Dr.Kathy Butler, Dr. Rick Davis, Barbara Huff, Michelle Donnelly all
greatly contributed to this work with their helpful attitude. I appreciated being around. Dr. William
Grosshandler and Dr. Anthony Hamins are thanked for making resources available. Dr. Henrik
Hillborg is thanked for writing the thesis that brought me to KTH and his continuous support. The
administrative personnel at KTH Fiber and Polymer Technology is thanked for the tremendous job
they are doing and their great attitude. Thank you Sofia for your struggle with the size exclusion
chromatograph. Håkan Svens and Kristina Karlsson are thanked for their outstanding help with the
extruder. Lars, Sven‐Ove and Britt, thank you very much for showing me how to master the cone and
putting up with a hectic guy like me. Ulf Guldvefall is thanked for his open‐mindedness and
borrowing out the infra‐red equipment. The advice from Martin Steen on electrical appliances was
very helpful.
Maria, Richard, Bruska, George, Thomas, Sung Woo, Fritjof, Viviana, Fran, Jonas, Alessandro, Camilla,
David, Johanna, Bereket, Henrik, Robert, Kattis, Linda, Anneli, Pelle, Linda, Barbara, Wenbin, Patricia,
Stacy, Sohail, Nima, Eugenia… you made my time at KTH worthwhile. Thank you for all lunch breaks,
nightshifts, boat trips, julbords…
My special thanks go to the Andersson family for making me feel welcome in Sweden.
Maik, Syad, Björn, Alex, JB, JC thanks for your friendship, wherever I am/you are.
나에게 언제나 기쁨과 행복을 주며 따뜻한 사랑으로 후원해준 세은에게 고마움과 사랑의 마음을
전합니다.
Above all, I want to thank my family for their love and being who they are to me.
54
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