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Keeping cool in the mass
Although intumescent coatings are widely used to
protect wood and metal against fire, plastics are
normally protected by adding materials to the bulk
polymer, which may impair its strength. Waterborne
intumescent coatings were shown to achieve a
high standard of fire retardancy on two types of
thermoplastic in two distinct types of fire test.
Waterborne
intumescent
coatings
can
prevent
thermoplastics from burning
Maude Jimenez*
Sophie Duquesne
Serge Bourbigot
The main way to fire retard plastics is currently by
treatment in bulk, i.e. the use of flame retardants and
stabilisers directly blended into the polymer. This effective
approach tends, however, to have some limitations.
Usually, substantial amount of additives are needed to act
efficiently, which creates three major problems:
» First, the migration of the additives from the bulk to the
surface of the polymer may form a non-uniform compound
after some time;
» Secondly, because of the weak interaction between the
additives and the polymer matrix, fragile points are formed
in the matrix, decreasing the mechanical strength of the
specimens;
» And finally, businesses seek to reduce the thickness of
their products, leading to processing issues because of the
high level of fire retardant additives.
There is also nowadays a tendency to avoid the very
efficient halogenated compounds because of their potential
secondary effects, such as the corrosive fumes and highly
toxic compounds which are emitted during burning. An
emerging problem is also the recyclability of fire-retarded
polymers, which can be particularly problematic in electrical
devices.
With phosphorus based compounds saving
polypropylene.
A good example for the limits of actual bulk treatments is
polypropylene (PP). PP is widely used in many fields, such
as wire and cables, automobiles, electronic, and electric
industry. It is highly combustible and needs to be flame
retarded. Until last years, halogen-containing compounds,
alone or in conjunction with antimony trioxide, were the main
flame retardants for PP. Phosphorus and nitrogen based
compounds, as well as metal hydroxides constitute are now
a rapidly growing group of flame retardants which are in the
focus of public interest concerning environmental friendly
chemicals. Metal hydroxides, mainly magnesium hydroxide
and aluminum hydroxide, are commonly used in the flame
retardancy of polymers due to their low toxicity and cost. But
sometimes more than 60 wt% loading of metal hydroxides is
required in PP to obtain efficient flame retardant properties,
and such high loading levels lead to a great decrease in the
mechanical properties of the filled polymer materials.
How to avoid dripping of Polycarbonate
Same kind of observations can be done for the
polycarbonate (PC): PC is known for its transparency and
it exhibits excellent mechanical strength, good electrical
properties and is widely used in a variety of fields such as
for example electric and electronic machinery, automobiles,
architecture. The aim to meet safety criteria in these various
applications is to develop a PC based component which is
an efficient flame retardant system, which prevents dripping
during fire and which allows maintaining the mechanical and
optical properties of PC. Polycarbonate resin is usually fire
retarded by incorporation of a retardant during processing.
The most common flame retardants used are brominebased e.g. decabromodiphenyl ether. Another way is to
add a relatively large amount of phosphorous based flame
retardants (10-30 %wt.). However, these phosphorous
based additives can lower the impact strength of the PC
or yellow it in high-temperature or high-humidity conditions.
Sodium and potassium perfluoroalkanesulfonic acids were
found effective in amounts well under 0.05-0.5 %, but these
components are still halogenated compounds. Some recent
works have developed fire retardant PC combined with
polymethylphenylsilsesquioxane spheres, but mechanical
resistance problems still exist.
How intumescence improves fire protection
In recent years, intumescent flame retardant (IFR) additives
have aroused great interest in relation to the flameretardancy of polymers. The IFR system is usually
composed of three components: an acid source, a charforming agent and a blowing agent which liberates gases at
high temperatures. This combination creates a foamed char
when exposed to fire temperatures, insulating the material
below and thus protecting it against heat damage.
A typical and widely studied IFR system utilises mixtures
of ammonium polyphosphate, pentaerythritol and melamine
(APP/PER/MEL). The main problems associated with these
IFR systems can be their moisture sensitivity and poor
compatibility with the polymer matrix.
Instead of incorporating these materials within the polymeric matrix by bulk treatment, the idea was developed of
applying them as an intumescent coating on the surface of
the substrate. This is an attractive alternative method as it
allows the fire retardant properties to be concentrated at
the polymer surface (where the flammability risk occurs)
and thus allows the bulk properties of the material to be
preserved.
Very few studies have investigated the use of intumescent
coatings on plastics. Intumescent coatings are used
worldwide for steel [1] and wood [2], some studies
have been carried out on textiles with a back coating
of phosphorus-containing formulations [3, 4] but only
one previous study in our laboratory has shown interest
in applying an intumescent coating on polypropylene
composites [5].
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Resolving coating adhesion problems
One problem, however, associated with the use of coatings
can be the poor adhesion of the coating onto the polymeric
substrate, particularly when waterborne coatings are used.
Various ways, such as corona treatments, cold plasma and
more recently atmospheric plasma, exist to improve the
compatibility between a (polymeric) substrate and a coating.
Flame treatment is also a well-established, low cost and
rapid method to improve adhesion. Passing the flame over
a surface leads to an increase in its surface energy and
wettability, thus assisting in providing good contact with
the fluids used for decoration of the surface. This concept,
involving the combination of an intumescent coating and
a treatment allowing adhesion to a plastic to be improved,
offers a new way to fire retard plastics.
Below, some results are presented on the use of a white
waterborne intumescent coating based on PVA resin and
of a waterborne transparent intumescent varnish based on
an acrylic resin. Both fire barrier properties and adhesion
of the coatings on flame-treated polypropylene (PP) and
polycarbonate (PC) samples were studied.
Test formulations and procedures summarised
Pure polypropylene (PP) and polycarbonate (PC) were
used as substrates. Polymer plates (100 x 10 x 3 mm, 130
x 10 x 1.6 mm and 100 x100 x 3 mm) were prepared using
a Darragon moulding press.
The waterbased intumescent varnish is an acrylic resin
based formulation, transparent and halogen free, containing
pentaerythritol (PER), silica and phosphoric acid. The white
intumescent coating is based on a polyvinyl acetate resin
("Emultex 523" from Synthomer) containing ammonium
polyphosphate (APP), pentaerythritol (PER) and melamine.
Both formulations were applied by dip coating to reach a dry
thickness of about 200 µm.
Flame treatments were carried out using an IPROS flame
apparatus (Figure 1). It is possible to modify the number
of treatments, the speed of treatment and the distance
between the substrate and the flame.
The adhesion of the film on the substrate was evaluated
according to the ASTM D3359-B standard using an
"Elcometer 107" cross-hatch cutter. The cutter chosen was
a 3 mm cutter, with 6 teeth, corresponding to thicknesses
ranging from 121 to 250 µm. Using this procedure, the best
adhesion to the substrate is classified as 5B and the lowest
is classified as 0B. Water contact angle measurements
of the flame-treated PP and PC were determined using a
"GBX 100" contact angle goniometer. The thickness of the
coating and varnish were analysed using an "Alphastep IQ"
mechanical profilometer.
Two separate fire resistance tests utilised
Two types of fire resistance tests were carried out. The
Limiting Oxygen Index (LOI) test was performed according
to the ASTM D 2863/77 standard, using an instrument
from Stanton Redcroft. This test allows the determination of
the minimum oxygen concentration, in an oxygen-nitrogen
mixture, which will ensure the combustion of a sample
positioned vertically (standard size: 100 x 10 x 3 mm³).
The vertical burning test was conducted using a vertical
burning tester (sample size 130 x 10 x 1.6 mm³ bars)
according to the ASTM D3801-1996 UL94 test. The best
ranking is V-0 when the burning time is short and when there
is no dripping of flaming particles, whereas the worst ranking
corresponds to "not classified’’ when the sample burns for
more than 30 seconds or up to the holding clamp at 130 mm
from the point of ignition.
Flame treatment greatly enhances coating adhesion
The flame treatment was applied to the substrates before
coating, in order to clean the surface from organic residues,
to oxidise the surface and thus facilitate the adhesion of the
coatings. Different flame treatments were carried out on the
PP and PC samples before coating.
Flame treatment allows the surface to be oxidised: the
contact angle with water decreases, showing that the
wettability of the samples has increased. Some reactive
functionalities such as hydroxyl groups are created on the
sample surface, which allows better compatibility with the
coating and thus better adhesion.
The best adhesion was obtained for two successive flame
passes at 200 mm/s in the case of polypropylene and
three successive flame passes at 200 mm/s in the case
of polycarbonate. As the polypropylene (thickness 1.6 mm)
began to melt after three successive flame treatments, only
two passes were utilised.
The samples were then coated and the results obtained
using the crosshatch test are presented in Table 1 for both
the coating and the varnish. The flame treatment allows the
adhesion of the coating to be increased from 0B crosshatch
classification to 5B classification for PP and 0B to 4B for PC,
and the adhesion of the clear varnish from 0B to 5B for PP
and from 1B to 4B for PC.
Optical microscopy carried out on an Olympus GX51
has been carried out on the different samples. Table 1
presents the different pictures obtained for the different
classifications.
When the sample is 0B, all the coating is detached during
the tape test. When only a major part is detached, the
sample is 1B classified. It is e.g. the case for the non
flamed PC coated with the intumescent coating. When the
coating flakes along the edges of the cuts partly or wholly
in large ribbons, as it is the case for the PC flamed one
time at 200 mm/s and coated with the intumescent coating
(Table2), the sample is 2B classified. When the coating
flakes along the edges and /or at the intersections of the
cuts, the samples are 3B classified (example of PC flamed
two times at 200 mm/s and coated with the intumescent
coating or the intumescent varnish). When there is only
small detachment of flakes of the coating at the intersection
of the cuts, the sample is 4B classified.This ranking is
the best obtained for both PP and PC coated with the
intumescent coating. Finally, when the edges of the cuts are
completely smooth and none of the squares of the lattice
is detached, the sample is 5B classified. This ranking is
the best one obtained for both PP and PC coated with the
intumescent varnish.
Both fire tests show very good results
The fire retardant performance of the coated samples was
next evaluated. The LOI and UL94 tests were carried out on
both PP and PC after first subjecting them to the appropriate
flame treatment as described above, and then coating
them by dip coating. The thickness of the coating was
about 200 µm for both formulations after drying at ambient
temperature.Table 3 presents the LOI and UL94 test results
obtained for the uncoated and coated PP and PC. The LOI
increased by 13 vol% for PP and 15 vol% for PC in the case
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of the PVA-based coating, and similarly by 14 vol% and 28
vol% in the case of the varnish. All the values obtained after
treatment are above 32 %, which is usually considered to
be commercially acceptable.
As regards the UL 94 test, the pure PP was not rated
because the whole sample burned. When the coating or
the varnish were applied, it reached the V0 rating (short
burning time, no dripping). This protection is attributed to
the formation of an intumescent coating upon heating (see
Figure 2a and 2b for the white coating, Figure 3a and 3b for
the varnish).
The pure PC is classified V2, but in both cases the
coated PC reached a V0 rating, due to the formation
of an intumescent coating when the flame was applied.
In both cases the improvement of flame retardancy was
outstanding, using only a relatively thin coating.
These results show that the preliminary studies carried
out on PP and PC with two different environmentallyfriendly coatings are extremely promising. This novel
approach to imparting fire resistance to polymers might
work satisfactorily regardless of the thickness of the
polymeric substrate. In the tests reported here, The LOI was
raised to meet the usual commercially acceptable level and
the coated samples reached the highest V0 rating in the UL
94 burning test.
Université Nord de France
T +33 3 20 33 71 96
Maude.Jimenez@univ-lille1.fr
REFERENCES
[1] Duquesne S. et al, Intumescent paints: fire protective
coatings for metallic substrates, Surface and Coatings
Technology, 2004, Vol. 180/181, pp 302-307. [2] Gu J. W.
et al, Study on preparation and fire-retardant mechanism
analysis of intumescent flame-retardant coatings, Surface
and Coatings Technology, 2007, Vol. 201 No. 18, pp
7835-7841. [3] Horrocks A. R. et al, The potential for
volatile phosphorus-containing flame retardants in textile
back-coatings, Jnl. of Fire Sciences, 2007, Vol. 25, No.
6, pp 523-540. [4] Magniez C. et al, Behavior of an
intumescent system for flame retardant materials coated
on polypropylene textiles, Jnl. of Industrial textiles, 2003,
Vol. 32, No. 4, pp 255-266. [5] Duquesne S. et al, Fire
retardancy of polypropylene composites using intumescent
coatings, ACS Symposium series, 2009, Vol. 1013, Ch. 12,
pp 192-204.
Results at a glance
Although intumescent coatings are widely and successfully
used to protect wood and metal surfaces from fire, there
is little evidence of their use on plastics. Instead, suitable
materials are added to the bulk polymer, which may reduce
its strength and cause problems with additive migration
over time. A clear and pigmented waterborne intumescent
coating were therefore applied to two test substrates (PP
and PC). Flame treatment of the plastic surface greatly
improved the adhesion of the coatings. Two different
fire retardancy tests were used. In the Limiting Oxygen
Index (LOI) test, the rating was increased by 13 vol.% or
more to comply with the normal commercial threshold of
32 %. In the vertical burning test to ASTM D3801-1996
UL94, both coatings achieved the highest rating of V0.
It therefore appears that it may be practicable to protect
plastics effectively with a relatively thin intumescent coating,
regardless of substrate thickness.
* Corresponding author:
Dr. Maude Jimenez
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Figure 1: Flame treatment apparatus
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Figure 2: (a) PC and (b) PP protected by PVA-based coating after UL94 test
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