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Synthesis Technology,
Molecular Structure, and
Rheological Behavior of
Polyethylene
Daniele Romanini
a
a
Montepolimeri S.p.A. Centro Ricerche Giulio Natta,
44100, Ferrara, Italy
Version of record first published: 06 Dec 2006
To cite this article: Daniele Romanini (1982): Synthesis Technology, Molecular
Structure, and Rheological Behavior of Polyethylene, Polymer-Plastics Technology and
Engineering, 19:2, 201-226
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P0LYM.-PLAST. TECHNOL. ENG., 19(2),201-226 (1982)
SYNTHESIS TECHNOLOGY, MOLECULAR STRUCTURE,
AND RHEOLOGICAL BEHAVIOR OF POLYETHYLENE
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DANIELE ROMANINI
Montepolimeri S.p.A.
Centro Ricerche Giulio Natta
441 00 Ferrara, Italy
I. INTRODUCTION
.........................
201
.............
202
111. TYPICAL MOLECULAR PROPERTIES
205
IV. MELT RHEOLOGICAL BEHAVIOR
..............
...............
2 13
11. SYNTHESIS INDUSTRIAL PROCESSES
V. PRACTICAL IMPORTANCE OF MOLECULAR AND
RHEOLOGICAL PROPERTIES . . . . . . . . . . .
References
.......
.............................
222
226
I. INTRODUCTION
Many thousands of tons of plastics are produced yearly all over the world.
Among these materials, polyethylene (PE) plays a very important role as the
most used product.
During 1980, the PE world market was calculated around 16,000,000
metric tons out of a total of 56,000,000 metric tons which involves all the
commercially available polymers. Out of total polymer production, PE constitutes 30%(20% low density and 10%high density) compared to 21% PVC,
9% polystyrene, and 9% polypropylene.
Polyethylene consumption in the most industrialized countries has reached
very large values; in Western Europe, the United States, and Japan the aver201
Copyright 0 1983 by Marcel Dekker, Inc.
ROMANIN I
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202
age per capita consumption exceeds 10 kg of low density and 5 kg of high
density. These values are 10 and 20 times, respectively, larger than the average
consumption recorded in the less industrialized countries. Of course, Western
Europe, the United States, and Japan, besides being the largest consumers, are
also the greatest producers of polyethylene.
Polyethylene is not only the most widespread polymer, but also the most
studied by macromolecular scientists. As a matter of fact, it can be taken as a
model to investigate and comprehend the relationships between structuralmolecular properties and the physical-rheological, mechanical, and application
properties from the other. This choice is supported by the fact that the industrial technologies employed today make it possible to produce this material
with intrinsic properties controllable within a wide range of capabilities. The
most significant physical and molecular characteristics to differentiate the various types of polyethylene are density and branching, respectively. With regard to the nominal density value, the American Society for Testing and Materials (ASTM) classifies polyethylene into three main classes [ l ] :
Low density polyethylene (LDPE)
0.9 10-0.925 g/cm3
Medium density polyethylene (MDPE)
0.926-0.940 g/cm3
High density polyethylene (HDPE)
0.941-0.965 g/cm3
Polyethylene can be branched at all density values. However, the branching type (short, long, etc), shape (comb, tree, etc.), functionality (tri or tetra),
and distribution in the polymer are strictly related to the technology and synthesis conditions employed. Branching, in particular the long type, plays a
fundamental role in polyethylene rheological behavior and thus on processing
and manufacturing performances. Linear polyethylene, which does not contain any branching, is theoretically that with the highest density.
This article deals, in general terms, with the main synthesis conditions e m
ployed for polyethylene production as well as with the typical molecular structures arising therefrom. Particular emphasis is given to both the type and content of branching. Then the main correlations between molecular properties
and melt rheological behavior and finally the practical implications on manufacturing process performance are investigated.
II. SYNTHESIS INDUSTRIAL PROCESSES
Synthesis technologies employed for the production of polyethylene substantially differ for HDPE and LDPE. Commercially available h4DPE and HDPE
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POLYETHYLENE
203
are generally produced by low pressure processes with transition metal catalysts, while LDPE is produced by a high pressure process with free radical
catalysts.
Polyethylene’s history goes back to 1933 when Fawcett and Gibson at
Imperial Chemical Industries (I.C.I.) began the study of high pressure ethylene
polymerization. The first pilot plant was built in England in 1937, while LDPE
industrial production started in 1942. Subsequently, during the 1950s, low
pressure ethylene polymerization processes of great scientific and practical interest were developed. Three catalysts, known as Ziegler-Natta, Phillips, and
Standard Oil, were mainly used. However, only the first two were industrially
used because their high activity allowed the production of uncorrosive polymers with a low ash content.
The low pressure synthesis processes most widely used today for the industrial production of HDPE and MDPE are as follows [2] :
Solution process: the polymer is formed in solution at sufficiently high
temperatures (> 100°C)
Suspension process: the polymer forms a suspension with the diluent at
temperatures of 60-80°C
Gas-phase process: the polymer consists of solid particles suspended in
gaseous ethylene
The gas-phase process can be economically attractive in comparison to the
other more conventional processes because ethylene, besides being the main
monomer, is also the fluidizing gas and the means of removing the heat of reaction. However, the final properties of the polymers so produced are not as
controllable as in the liquid-phase processes. These latter technologies are
generally preferred in the synthesis of “tailor-made” polyethylene and are particularly suitable for application fields where well-defined molecular structures
are required.
HDPE includes both ethylene homopolymer (essentially linear) and copolymer containing low amounts of short olefins (<1%)such as propylene and
butene- 1. Modification with these comonomers results in structure regulation.
In particular, short branchings are formed which reduce the polyethylene
density and crystallinity, and improve some utilization characteristics.
With regard to low and medium density polyethylene, developments in
production technologies involve three fields of interest:
204
ROMANINI
(a) Low density ethylene homopolymers (LDPE) produced by high pressure processes in which polymerization occurs by a radical mechanism
(b) Ethylene copolymers (EVA, ionomers, etc.) obtained at high pressure
with radical initiators and various polar comonomers
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(c) Low and medium density ethylene copolymers, known as “linear low
density polyethylene” (LLDPE), produced by low pressure technologies
with Ziegler-Natta or Phillips catalysts in the presence of sufficient
amounts of olefmic comonomers
The technologies generally employed in LDPE synthesis are essentially of
two types: tubular and autoclave, also known as “vessel.”
The tubular reactor consists of a spiral-wrappedmetallic pipe, the total
length of which is generally higher than 800 m while its internal diameter does
not exceed 70-80 mm. Ethylene polymerizes in a pattern where three zones
may be distinguished:
Preheating and reaction starting zone
Reaction zone
Cooling zone
In the polymerization zone, pressures of 2500-3000 bar and temperatures
of 30O-31O0Care reached. The conversion achievable with this technology
ranges around 27-28%. However, it must be pointed out that thermal and conversion profiles are strictly connected to the operative procedures used.
A vessel reactor is a constantly stirred autoclave which operates under controlled temperature and pressure conditions. The reactor may be subdivided
into reaction multiple chambers. En this case, it is called a “multizone vessel”
and LDPE with molecular and behavior characteristics approaching those of
the polyethylene produced by tubular process can be obtained. Vessel technology allows for a wider variability of the synthesis conditions and gives
products suitable to more application fields than does tubular technology.
Ethylene copolymers with polar monomers have such final properties as
transparency, adhesion, elasticity, and blend compatibility which make them
suitable for sophisticated end-uses in place of more expensive polymers. EVA
copolymers are commercially the most interesting products. They are obtained by ethylene polymerization in the presence of vinyl acetate (VA). EVA
copolymers may contain either low VA percentages (<15%)and have characteristics similar to LDPE homopolymer, or high VA contents (20-35%)and
205
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POLYETHYLENE
behave as a thermoplastic rubber or a plasticized PVC. Other ethylene copolymers studied during recent years are the “ionomers,” in which the polarity
of salified acid groups (i.e., carboxyl groups) is utilized, for instance, to support adhesion to various materials, especially to metals.
Linear low density polyethylene is particularly worth mentioning. It represents the commercial novelty of recent years. It is obtained by low pressure
processes (the same used for HDPE) for ethylene polymerization with a sufficient amount of a-olefmic comonomers while still using Ziegler-Natta or
Phillips catalysis. By the proper choice of synthesis conditions and diluent/
comonomer couples, L-LDPE with a wide range of densities (from 0.910 to
0.950 g/cm3) can be easily produced. The considerable industrial interest in
LLDPE is due to the economics allowed by the low pressure production processes as well as to its suitability in fields where it can replace conventional
HDPE and LDPE.
111. TYPICAL MOLECULAR PROPERTIES
The previously described synthesis processes transform ethylene into polyethylene through polymerization reactions which depend on the catalyst system employed.
The structure of the polymer formed is characterized by roughly linear
macromolecular chains in the case of HDPE, and branched chains in LDPE
and LLDPE products. The number of terminal methyl groups per 100 carbon
atoms as determined by IR spectroscopy is generally lower than 0.2 in HDPE
and rises to 2-3 units in LDPE and L-LDPE.
Branching is troublesome for crystallite packing and therefore promotes
lower densities. Figure 1 shows that the polyethylene density decreases with
an increase of both the CH3 number per 100 C and the bonded butene-1 percentage in the chain. However, each methyl group determined by IR may
belong to both a short chain branch (SCB) and to a long chain branch (LCB).
In order to detect the presence of long chain branching in the polymer, solution properties are applied.
An investigation on this matter was carried out on narrow fractions of
LDPE products industrially obtained by means of vessel and tubular processes
[3] . Interesting relationships between LCB degree and synthesis production
technologies were determined.
In Fig. 2 typical [77]
curves of LDPE produced both by conventional
tubular technology and by the same technology with a modification of the
temperature profile and a system of ethylene and initiator feeding to the re-
-nw
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206
0.91
0.92
0.93
DENSITY
0.94
0.95
( g/cm31
FIG. 1. Influence of the number of methyl groups and the bonded butene-1
percentage on polyethylene density.
actor are plotted on a log-log scale. At the same molecular weight, the intrinsic
viscosity markedly decreases with increasing LCB content.
Likewise, Fig. 3 shows the same curves obtained on LDPE fractions produced both by conventional vessel technology and modified vessel technology.
In the latter case the reaction zones were increased and the reactor thermal
profiles were modified.
Both high pressure processes produce polyethylene in which the LCB content increases with an increase of the molecular weight. The LCB content
can be reduced by proper modifications of technologies and synthesis conditions.
On the basis of investigations of ethylene polymerization kinetics, threefunctional branching appears to be the most probable. LCB frequency ranges
POLYETHYLENE
207
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10’ 1
I
1
1
I I I l I l
I
1
I
lo5
MOLECULAR
I I I I I
106
WEIGHT i w
-aw
FIG. 2. [ 171
curves obtained on monodispersed fractions of linear HDPE
and LDPE produced by conventional and modified tubular technology.
around 1-3 units per 10,000carbon atoms in the main chain and may cause
an increase of the average molecular weight by a factor of some units under
the same hydrodynamic volume with a linear polymer.
Figure 4 emphasizes the very slight effect of SCB content on the [17]
relationship as determined both in the case of HDPE modified with short olefins
and L-LDPE copolymer containing 8 wt% of bonded butene-1 . The hydrodynamic volume of macromolecules containing SCB remains practically unchanged when compared to that of the linear chain. In addition, the molecular weight variation for a SCB degree of about two terminal CH3 per lOOC
is evaluated to be no more than 56%. The figure shows also the curve of a
-xw
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208
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10'-L
1
I
I
I
l l l l l
I
I
1
I I I l I l
lo5
MOLECULAR
lo6
WEIGHT
i w
FIG. 3. [sl-m, curves of LDPE produced by conventional and modified
vessel technology, in comaprison to linear polyethylene.
high molecular weight HDPE. Note the presence of LCB in the high MW fractions where the deviation from the linear PE straight line is greatest.
Several authors have investigated the type and distribution of PE branchings. The model suggested by Zimm and Kilb [4] properly expresses LCB
content through the g index:
go" =
[7?1
b : hll
(1)
where [q]b and [q]l are the intrinsic viscosities, respectively, of monodispersed fractions of branched polymer and linear polymer with the same
molecular weight. The g index is the unit for linear polymers and decreases
to very small values with an increase of LCB content.
POLYETHYLENE
-
z!
U
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>
+
-
cn
209
---
-
-
0
0
cn
> loo -
-cn
0
z
-
a
+
z
-
-c
I
I
l l l l l l
I
I
I
I
1 I I I I
Figure 5 shows the distribution of the g index in the polymer as expressed
by Relation (1) as a function of the MW of low and high density PE obtained by
different synthesis technologies. The high LCB content, typical of high pressure processes, seems to be due to intermolecular transfer reactions according
to mechanisms which mainly depend on free radical concentration, reactor
residence time, and temperature. High MW fractions of LDPE produced by
tubular technology contain less LCB than the corresponding fractions of
vessel products, as clearly shown by Fig. 5.
Recent studies on the structure of high pressure produced PE made it possible to hypothesize models concerning branching topology and the shape of
macromolecules obtained by tubular and vessel processes [ 5 ] . Figure 6 presents a schematic in which it can be seen that macromolecuIes of LDPE ob-
linear
HDPE
L-LDPE
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\modified
HDPE
1o6
lo5
MOLECULAR
WEIGHT
fiw
FIG. 5. LCB index as a function of MW determined on PE fractions produced by low and high pressure processes.
TUBULAR
VESSEL
T ECHNOLOGY
TECHNOLOGY
FIG. 6. Schematic drawings of branching in macromolecules of LDPE
produced by tubular and vessel technology.
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POLYETHYLENE
21 1
tained by tubular technology show a branching of the “herringbone” type.
Such a needle-shaped structure increases the hydrodynamic volume and the
gyration radius of macromolecules compared to the statistical or “tree”-type
branching which is more probable in PE produced by vessel technology.
Knowledge of the molecular weight distribution (MWD) in a polymer is
very important for the qualitative control of the products as well as for their
optimal utilization. Regulation of MWD and achievement of the relevant process know-how are still being investigated by various researchers. Unfortunately, knowledge on this matter is rather poor and we are very far from a deep
comprehension of the synthesis parameters governing PE MWD.
The methods generally used for the control of polymer MWD are as follows:
Catalyst modification
Modification of process conditions
Blending among products with different molecular weights
Chemical, thermal, and rheological treatments.
Of course, each manufacturer uses the best solution according to the available catalysis and technology.
The MWD of PE obtained by lowpressure
- processes may be varied within
- =,/Enratios of
a wide range of polydispersity (3 <M,/M,, < 20 and over).
8-10 may be reached due to catalyst activity, while forMw/Mn values > 10,
modifications to the process must be made, the most valid of which seems to
be the multistage polymerization. MWD widening is achieved through blending and requires an additional processing phase which heavily affects production costs. Moreover, the materials so produced may negatively transmit their
heterogeneous nature to the final properties of the molded items.
MWD narrowing through chemical and thermomechanical treatments is
generally carried out in the molten state and consists in cutting the polymer
with higher molecular weight fractions. This method, also known as “controlled rheology degradation,” allows the achievement of very homogeneous
and highly fluid products and also causes a general decrease of the polymer
physicomechanical properties. Thus its use is not recommended only to control MWD. In PE produced by high pressure technology, MWD is regulated
by modification of the process conditions and the reactor thermal profile.
However, in this way the LCB degree is also affected. Therefore, more
care is taken to avoid polymer conversion than to control MWD. In practice,
LDPE produced by conventional vessel processes have wider MWD than those
produced by tubular process. However, a MWD narrowing may be achieved
by increasing the number of the reaction zones in vessel technology.
ROMANIN I
212
TABLE 1
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Molecular Properties of High Pressure Produced LDPE
Sample
LDPE-1
LDPE-2
LDPE-3
Process
Tubular
Vessel
Plurizone
vessel
Density
(g/cm3)
0.9175
0.9170
0.920
~~
M.F.R.
(g/ 10
min)
2.1
1.3
1.8
1111a
(dL/g)
0.94
1.12
0.92
mw
Aw I&
260.000
620.000
200.000
5.4
9.5
5.5
~
aIntrinsic viscosity measured in tetraline at 135OC.
As an example, Table 1 reports molecular characteristics of three LDPE
samples produced by tubular, vessel, and multizone-vessel technology. BW
(weight-average molecular weight) was determined by light scattering techniques, while TWITn
(dimensional polidispersity index) was determined by
gel permeation chromatography (GPC). It can be easily realized from the
values reported in Table 1 that the PE samples tested have nearly similar melt
flow rates (MFR) but very different molecular properties owing to their different LCB content. As a matter of fact, just this last factor considerably affects the MWD curve obtained by GPC, as clearly shown by Fig. 7. The curve
of the LDPE-2 sample, the most branched product, shows a “shoulder” due
to elution of high LCB content fractions which thickened in that range of
molecular sizes. The LDPE-3 sample, produced by a multizone vessel reactor,
has a considerably lower LCB content than does LDPE-2. In addition, its
MWD curve does not show the “shoulder” at high MW, and the results are
narrower and practically superposable with those of the LDPE-1 sample obtained by the tubular process. On the grounds of these results, it is clear that
the MWD of high pressure produced PE is strictly connected to the polymer
LCB content.
< 5 ) due to reLLDPE polymers generally show a narrow MWD
action comonomers which act as chain regulators. In fact, macromolecules
forming the polymer all have comparable length. Thus, the polydispersity result is limited and the MWD curve is nearly statistical. By proper modifications
of catalysis and the process (e.g., double-phase polymerization), LLDPE
molecular polydispersity may be increased even if with a smaller range than
those of HDPE. Due to its intrinsic properties, L-LDPE does not need a very
wide MWD for processability.
@,/an
POLYETHYLENE
20
213
'
---- L D P E - 1
LDPE-2
15
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dp
- 10
U
s
5
0
10'
1o2
lo3
MOLECULAR
1o4
SIZE
lo5
(A)
FIG. 7. Apparent curve of MWD determined by GPC of the LDPE samples
reported in Table 1.
IV. MELT RHEOLOGICAL BEHAVIOR
It is very important to be familiar with the rheological properties of molten
polymers for their correct utilization in the most suitable application fields.
PE, in particular, shows very diversified behaviors as a function of its molecular structure. To clearly determine its rheological properties, laboratory research is generally directed in two directions:
Evaluation of the behavior with regard to flow conditions (extrusion
geometry, deformation type, applied stress, etc.) and temperature of the
material
Determination of the relationships existing between the main rheological
parameters (viscosity, extrudate swelling, melt strength, etc.) and those of
molecular structure (MW,MWD, LCB, etc.)
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214
lo-’
SHEAR
10’
R A T E (s-l)
FIG.8. Typical flow curves measured at 18OoCof PE produced by different synthesis technologies.
Measurement of PE flow curves within a wide range of shear rates and melt
temperatures allows for the collection of information necessary for the selection of the most suitable conditions to transform the polymer into qualitatively and economically valid manufactured items. These curves also express
the material shear sensitivity and their behavior is mainly connected to MWD,
while viscosity at very low shear rates is highly dependent on the average MW
and the LCB content of the polymer.
Figure 8 shows the curves of some PE samples obtained by low and high
pressure synthesis technologies, the molecular properties of which are reported in Table 2.
It is interesting to point out that although having practically the same li?,
absolute molecular weight, the products show substantially different viscosity
values throughout the range of the tested shear rates. By comparing HDPE
and LLDPE curves, we note the effect of the molecular polydispersity which
combines narrower MWD with flatter flow curves, i.e., less pseudoplastic.
Both LDPE products show lower viscosity values than HDPE and LLDPE
with the same MW. This is ascribed to their LCB content which, as currently
known, reduces the local mobility of every macromolecule, thus minimizing
215
POLYETHYLENE
TABLE 2
Molecular Properties of PE Samples Reported in Fig. 8
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Sample
HDPE
L-LDPE
Process
Solution
low pressure
Slurry low
pressure
LDPE-TM Modified
tubular
LDPE-VM Modified
vessel
M.F. R.
(g/lO
min)
zw
-%uIA,
0.3
[771
(dL/g)
1.95
145.000
8.0
0.914
0.8
1.70
135.000
4.0
0.923
2.1
0.80
140.000
4.7
0.920
1.75
0.90
150.000
4.5
Density
(g/cm3)
0.945
the capability to form entanglements with the adjoining macromolecules and
promoting material flow [6].
To better emphasize the long chain branching effect on molten PE viscosity,
rheological determinations on monodispersed fractions of LDPE produced by
tubular and vessel synthesis technologies were carried out in comparison with
linear HDPE [71 .
Figure 9 shows the go melt Newtonian viscosity value as a function o f a ,
of every single fraction. It is clearly noted that, under the samea,, go undergoes a decrease up to three orders of magnitude due to LCB content. This experimental evidence seems to state that at low shear rates, i.e., at nearly static
conditions, the interaction among macromolecules is governing the flow rather
than their molecular mass.
Several authors agree in ascribing the irregular behavior of LDPE viscous
flow activation energy to LCB. In particular, its increase, in comparison to
linear HDPE, seems to be due t o the lower local mobility of the branched
macromolecules. In fact, in contrast to HDPE polymers which have an ET
(activation energy) close to 6 kcal/mol and are rather independent of T (shear
stress), LDPE samples show higher ET values which in some cases decrease with
an increase of the applied shear stress [6]. Table 3 reports typical values of
the viscous flow activation energies calculated at constant shear stresses from
the flow curves at different temperatures for PE produced by different synthesis conditions. ET energies of the materials obtained by the high pressure processes are clearly higher than those of similar low pressure products.
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21 6
lo4
lo5
MOLECULAR WEIGHT
lo6
Mw
FIG.9. Melt Newtonian viscosity qo vs weight-average molecular weight
z,+,measured on HDPE and LDPE monodispersed fractions.
The activation energy has considerable practical importance because it expresses the viscosity/temperature dependence of a material subjected to flow.
Therefore, processing machines which transform polymers into fmished products must work under more isothermal conditions as higher ET values are
reached. This is in order to avoid troubles connected with flow dishomogeneity, melt fracture, flow lines, inconstancy in the thickness of manufactured
item, etc.
POLYETHYLENE
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TABLE 3
Typical Values of the Er Viscous Flow Activation Energy of
PE Produced by Different Synthesis Technologies
Polymer
Synthesis process
HDPE
L-LDPE
L-LDPE
LDPE
Solution low pressure
Slurry low pressure
Gas-phase low pressure
Tubular high pressure
LDPE
LDPE
Vessel high pressure
Plurizone vessel high pressure
ET (kcal/mol)
5-6
6-7
7-8
10-12
9-10
10-12
HDPE
L-LDPE
LDPE-TM
LDPE-VM
FIG. 10. Melt fracture of polyethylene samples extruded at 180°C by a
capillary rheometer (D= 1.26 mm, LID = 40) at a shear rate of 1200 s-’.
Another very important feature in plastics processability is flow instability,
or “melt fracture,” which shows with surface roughness of the material when
extruded at a certain critical output rate. Figure 10 illustrates the instabilities
of the same PE samples reported in Table 2 when extruded at 180°C by a
capillary rheometer at a shear rate of 1200 s-’. It is interesting t o note that
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21 8
lo’
lo2
lo3
SHEAR
R A T E (s-l)
FIG. 11. Shear stress versus shear rate curves at 180°C of polyethylene
samples as reported in Table 2.
when melt fracture takes place, HDPE and LLDPE flow curves undergo a
clear path decrease, while this does not occur for LDPE products, as illustrated
by Fig. 11 which reports shear stress versus shear rate curves for the abovementioned polymers. The most probable explanation to such behaviors is
that, under unstable conditions, a slipping at the capillary wall exists in the
case of HDPE and LLDPE, while for LDPE, even if present, this slipping
slightly contributes to the fluid transport.
Melt fracture phenomena occur through mechanisms which are not satisfactorily known, although their occurrence seems to be strictly connected to
both the extrusion geometry and the polymer elastic component.
When dealing with elasticity, mention should also be made of extrudate
swelling, which is another feature to be carefully considered when transforming
polyethylene into finished products. As a matter of fact, high swelling causes
shrinkages, distortions, and high thicknesses, but it also results in nonbrittleness, flexibility, and good fracture energy in the manufactured items.
Even if it is now a certainty that polyethylene extrudate swelling increases
with an increase of the shear rate and shear stress, its dependence on the structural-molecular parameters is not yet clear. This is because it is very difficult
to separate the single effects of MW, MWD, and LCB from measurements on
whole polymers, i.e., unfractionated. Extrudate swelling determinations on
POLYETHYLENE
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n
.m2.0
n
219
-
I-
tn
lm4
y1
1.2
n
1.o
lo5
1
1
1
I
I
1
Ill
1
1
I 1 1 1 1 1 ,
lo7
lo6
mw product de-
FIG. 12. Die swell ratio De/D as a function of (Xw/-Tn)
termined on low and high densjty polyethylene samples.
monodispersed fractions have little reliability because the fractionation procedure impairs the elastic properties of the material.
Figure 12 reports the die swell ratio D e / D at equilibrium, expressed as the
ratio between the extrudate diameter at constant shear stress and the die
diameter as a function of (gw/&)Ewdetermined on several high and low
density PE [ 6 ] .
Extrudate swelling was noticed to increase with an increase of %
,, and
X w / X n while decreasing when (ZW/&)aw
is equal when passing from linear
HDPE to LDPE and L-LDPE products. This means that the recoverable elastic
deformation of branched polyethylene in the molten state is lower than in the
corresponding linear HDPE products. For LDPE, the reduction of the statistical molecular sizes due to the LCB effect induces a lower orientation in the
stress field and thus lower elastic recovery; the low extrudate swelling of
LLDPE products is instead due to their substabtially narrow MWD.
Another rheological feature involving many PE end-uses is melt elongational behavior. In particular, it concerns the capability of undergoing tensile
220
ROMANlNl
t
A
10
HDPE
L-LDPE
I
t
c3
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z
0
tubular
LDPE
+
vessel
LDPE
Lu
0:
t
cn5
0
lo’
lo2
lo3
MAXIMUM S T R E T C H R A T I O
FIG. 13. Dependence of melt strength versus maximum stretch ratio, determined at 18OoC on low and high density PE samples.
stresses and of elongating without breaking up into thinner pieces. These two
characteristics are in practice defined as “melt strength” and “stretch ratio.”
By plotting the melt strength as a function of the corresponding maximum
stretch ratio determined at 180°C on low and high density PE monofilaments
[ 6 ] ,we obtain the values of Fig. 13 where all products approximately cover
a curve which decreases with an increase of the monofilament maximum
stretch or break ratio. Should this last parameter be reported as a function of
the MFR fluidity index, an extremely selective map of curves for the various
polyethylene samples is obtained, as shown by Fig. 14.
In fact, under the same MFR conditions, the stretch ratio at break considerably decreases with an increase of LCB content. This enables selection of the
products according to their synthesis‘technology, with the exception of LDPE
by tubular technology is undistinguishable from that obtained by the modified vessel process.
Concerning the dependence of tensile properties on PE molecular parameters,
the result is that [6] :
POLYETHYLENE
0
--
a
-
-c
a
I
0
lo3
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v)
-
-
lo2 -
--
I
3
I
-
-
X
a
I
=
c
c
w
a
c
221
10’
=-
1
I l l 1
I
I l l 1
I
I l l l
1
1 1 1 1
FIG. 14. Maximum stretch ratio at 18OoCas a function of MFR of PE
produced by different synthesis technologies.
The melt strength increases and the stretch ratio at break decreases with an
increase of molecular weight
An increase of LCB content, the molecular weights being equal, causes a
decrease of melt strength and an increase of the maximum stretch ratio
These behaviors are in agreement with the already mentioned relationships
between the melt viscosity and the molecular structure of PE products. It is
not surprising that the LCB effect increases stretchability when the absolute
molecular weights are equal, but decreases it when the MFR are equal. This
is due to the fact that the MFR fluidity index is a function of molecular
weight, MWD, and LCB content. In addition, the molecular weight influence
on PE stretchability result is greater and opposite to that of LCB.
222
ROMAN IN I
V. PRACTICAL IMPORTANCE OF MOLECULAR AND
RHEOLOGI CA L PROPERTl ES
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The purpose of processing technologies is to transform a polymer-generally
powder or pellets-into shaped manufactured items. This transformation of
the material is related to its chemical-physical properties which affect the useability of the end products.
Transformation processes may be subdivided as follows:
(a) Simple extrusion: monofdaments, sheets, pipes, and other special profiles may be obtained by the use of extruders with different geometries
(b) Extrusion followed by a second processing phase: blow molding, coating, spinning, etc.
(c) Injection and compression molding: these are the most employed
technologies
(d) Various technologies: calendering, vacuum forming, foaming, rotational molding, etc.
Polyethylene is an extremely versatile polymer which is employed in nearly
all the above-mentioned application fields. Each practical use depends on the
specific properties arising from an optimal balance among the following parameters: density, molecular weight, molecular weight distribution, and branching
content. A proper balance determines both good processability performance
and high quality of the manufactured items, In practice, this balance assures
the commercial success of the product.
Table 4 summarizes some qualitative relationships among structural-molecular parameters and of main rheological and practical characteristics.
Due to the many features that the PE molecular structure may have, the dependence on the absolute molecular weight must not be confused with that on
the MFR fluidity index. The presence of LCB seriously modifies some performance. SCB content has no significant effect on rheological properties, but
affects crystals morphology by inducing variations of the characteristics related to it (E.S.C.R., optical properties, etc.). In the case of HDPE, the most
interesting application field is blown film with high mechanical resistance at
thicknesses lower than 10pm. These results can be obtained by using products with molecular weights high enough to give good mechanical properties,
wide MWD for good processability, and linear macromolecules for maximum
orientation and thus thin films.
POLYETHYLENE
223
TABLE 4
Relationships among Structural-Molecular Parameters and
PE Physical, Rheological, Applicative Characteristics
Characteristics
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Melt viscosity
Processability
Extrudate swelling
Melt fracture (shear rate)
Melt strength
Max stretch ratio
E.S.C.R.
Impact resistance
Stiffness
LDPE optical properties
If the
density
increases
-
If MW
increases
If MWD
widens
If LCB
increases
Increases
Decreases
Decreases
at high shears
Is lower Is lower
Is better
Is better
Increases Increases
Decreases
Decreases Decreases
Increases
Increases Increases
Decreases
Decreases Decreases
Increases
Decreases Increases Increases
Decreases
Decreases Increases Decreases
Increases No change No change
Are better Are lower Are lower
Decreases
Are lower
Other typical HDPE application fields involve extrusion of pipes and largesized containers. Requirements are high melt strength at low shear rates to
give consistency to the pipe or parison, and this involves high molecular weight
together with the good processing achievable with wide MWD polymers.
In the injection molding field it is important to use HDPE with narrow
MWD in order to minimize distortion and shrinkage of manufactured items,
and with low molecular weight in order to reach the highest injection rates
and shorten molding cycles. It is always necessary to achieve the best balance
among processing characteristics, mechanical resistance, and dimensional
stability.
In the case of LDPE, LCB considerably affects the physical, rheological,
and mechanical properties of the polymer. Therefore, for LDPE, LCB content plays the same predominant role which MWD plays for HDPE.
The characteristics determining the advantages of LDPE for some particular
uses are excellent processability at relatively low temperatures, excellent optical characteristics, impact and tearing resistance, and flexibility. These are
the properties which lead to the utilization of LDPE in industrial packaging
224
ROMANINI
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50
0
lo-’
loo
M E L T F L O W R A T E (WlOmin.)
FIG. 15. Draw down of polyethylene film obtained by industrial processing technologies.
films. LCB decreases some mechanical characteristics and decreases thickness.
To show the determinant role played by LCB on the thicknesses reachable in
blown film technology, Fig. 15 illustrates the thicknesses of products resulting
from different synthesis processes as a function of the MFR fluidity index.
LDPE products from tubular technology can be stretched thinner than those
with the same MFR from vessel technology. This is ascribed to different LCB
contents which, in the case of the “tree” branching of LDPE from vessel technology, creates macromolecules which are less oriented than those with “herringbone” branching, typical of LDPE from tubular technology.
In the field of blown film, L-LDPE is stretched at thicknesses approaching
those of HDPE while keeping the optical properties of conventional LDPE.
Table 5 reports a comparison among the physical, mechanical, and application characteristics of LLDPE, LDPE, and HDPE.
From the results reported in Table 5 , the following main qualities of L-LDPE
in comparison to LDPE may be deduced:
Heat resistance considerably higher
Higher crystallinity and consequently an increase of stiffness
PO LY ETHY LEN€
225
TABLE 5
Comparison among Typical Characteristics of LDPE, L-LDPE, and
HDPE Used in the Blown Film Field
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Characteristics
LDPE
MFR, g/ 10 min
Density, g/cm3
1.6-2.2
0.920
[ q ] THN, 135OC, dL/g
0.9
1.5-2.0
NO. CH3/ 1OOC (IR)
Melting temperature, OC
Melting energy, cal/g
Crystallinity (RX), %
Flexural stiffness, N/mm2
E.S.C.R. (Igepal lo%), h
Melt strength at 19OoC, g
Maximum stretch ratio at 190°C
Processability
Dart test,a N
Impact strength: longitudinal,a J/cm
transverse,a J/cm
L-LDPE
0.8-1.2
0.918
1.7
1.6-1.8
108-1 12
18-22
60-65
200-250
125-128
20-24
2-4
2.0-2.4
120-1 60
Good
>500
1.2-1.6
800-1 000
Fair
1.0-1.5
25
40
0.5
6
25
65-70
250-350
HDPE
0.03-0.06
0.950
2.8-3.0
<o. 2
133-1 36
40-45
80-85
900-1000
>500
4.5-6.5
20-40
Difficult
0.7
3
35
aDetermined according to ASTM test methods on films of 30 pm thickness
(1: 1.6 blow ratio) for LDPE and L-LDPE, and 20 pm thickness (1:4 blow
ratio) for HDPE.
Considerably higher E.S.C.R.
Excellent stretchability which permits thinner films to be obtained
Higher and better balanced impact resistance properties
The marketing expectations for LLDPE are therefore supported both by
savings in materials in comparison to LDPE and final properties intermediate
between the two conventional PE products ( D P E and LDPE). EVA copolymers also will take an important section of the market, particularly in the
field of industrial packaging and agricultural film,thanks to the good mechanical resistance, flexibility, and light transmittance as shown by VA.
226
ROMANlNl
Future market trends are oriented toward products whose final characteristics may be controlled within rather wide limits without substantial modifications of production and processing technologies. LLDPE seems to fit these
requirements as a polymer suitable for several fields. In addition, the production technologies and processing machines currently used for traditional polyethylene may be used.
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Acknowledgment
The author wishes to thank Prof Paolo Galli, Director, Centro Ricerche Giulio
Natta of Montepolimeri in Ferrara, for his kind suggestions and encouragement
to publish this paper.
REFERENCES
[ 1I Standard specification ANSI/ASTM D 1248-74.
[2] E. B. Mano, Proceedings of Symposium on Macromolecules (F. C . Foster,
(ed.), Elsevier, Amsterdam, 1975, p. 209.
[3] G. Gianotti, A. Cicuta, and D. Romanini, Polymer, 21, 1087 (1980).
141 B. H. Zimm and R. W . Kilb,J. Polym. Sci., 37, 19 (1959).
[51 R. Kuhn, H. Kromer, and G. Rossmanith, Angew. Makromol. Chem.,
40/41, 361 (1974).
[6] D. Romanini, A. Savadori, and G. Gianotti, Polymer, 21, 1092 (1980).
[7] D. Romanini and G. Gianotti, Proceedings o f 3 r d Meeting A.I.M. (Italian
Association of Macromolecular Science and Technology), Milan,October
17-19, 1977, p. 59.