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ENVIRONMENTALLY DEGRADABLE PLASTICS

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ETHYLENE COPOLYMERS

Introduction

Polyethylene and polystyrene are two high volume commodity polymers,
widely studied in materials research, exemplifying the classes of semicrys-
talline polymers (qv) and amorphous polymers (qv) respectively (see E

THYLENE

P

OLYMERS

, HDPE; S

TYRENE

P

OLYMERS

). There has been considerable interest to

produce copolymers from ethylene (E) and styrene (S). The incorporation of aro-
matic groups in polyethylene has been associated, for example, with enhanced ra-
diation resistance (1) or electrical-breakdown resistance by enabling space charge
dissipation (2). A variety of different copolymer molecular architectures can po-
tentially be produced. In addition to the different comonomer distributions (ran-
dom, alternating, or blocky nature), there are possibilities for chain branching and
stereoregularity in the copolymer chain microstructure.

Various references describe efforts to copolymerize ethylene and styrene.

The production of copolymers has been attempted by using conventional poly-
merization routes for either polyethylene or polystyrene with introduction of the
alternate comonomer into the process. Many reported copolymerizations produce
heterogeneous polymers consisting of mixtures of copolymers and homopolymers
(3), for example U.S. patent number 3,117,945 describes the production of a “gross”
copolymer, composed of various fractions that could be separated on the basis of
their solubility parameter. Copolymers containing low levels (

<4 mol%) of styrene

incorporation have been prepared by both conventional Ziegler-Natta Catalysts
(qv) (4–6) and free-radical processes (7). U.S. patent number 4,076,698 allegedly
has examples of copolymers of ethylene with styrene and

α-methylstyrene pro-

duced by coordination catalysts at low temperature (8), but does not give any
significant characterization data for the polymers. These early synthetic routes

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 6

ETHYLENE COPOLYMERS

163

that were investigated do not provide commercially viable routes for the produc-
tion of copolymers.

Developments published in the early 1990s demonstrated the ability of spe-

cific classes of metallocene catalysts (see M

ETALLOCENES

) to efficiently copoly-

merize ethylene and vinyl aromatic monomers, such as styrene. In particular,
INSITE Technology (The Dow Chemical Co.), including developments of single-
site, constrained-geometry, addition polymerization catalysts (9–11) has provided
an efficient route to produce essentially random copolymers based on ethylene
and styrene (see S

INGLE

-S

ITE

C

ATALYSTS

). The primary focus of subsequent sec-

tions is the preparation, characterization, and utility of poly(ethylene-co-styrene);
“ethylene-co-styrene polymer” is also used as an alternate terminology throughout
this chapter for coherence. It is pertinent to mention that INDEX Interpolymers
(The Dow Chemical Co.) is the descriptive term for the family of products, including
poly(ethylene-co-styrene), produced by INSITE technology, and many references
have used the terminology “ethylene/styrene interpolymers,” abbreviated to ESI,
as a descriptor for the copolymers.

Metallocene-Catalyzed Polymerizations

Many studies have focused on catalysts that could potentially copolymerize ethy-
lene and vinyl aromatic monomers, together with the associated polymerization
chemistry and chemical analyses of the produced polymers. It is evident from
a number of references (eg 12–14) that the catalyst structure and polymeriza-
tion conditions, such as temperature and monomer feed ratios, have major influ-
ences on the reaction product in terms of production efficiency, product composi-
tion (copolymer, homopolymer contents), and copolymer microstructure, including
stereoregularity.

Popular catalysts investigated for the preparation of copolymers have been

single-site, constrained-geometry, addition polymerization catalysts based on ti-
tanocene complexes, exemplified by dimethylsilyl(tetramethylcyclopentadienyl)-
(t-butylamido) titanium dichloride [(CH

3

)

2

Si(CH

3

)

4

Cp)(N-t-Butyl)TiCl

2

]. Other

catalysts of significant interest have been based on fluorenyl and indenyl lig-
and structures, such as those described by Mitsui Toatsu (15). A short overview
of technology (16) has included identification of the most widely investigated
catalysts cited in basic patents. Studies of catalyst systems that produce syn-
diotactic polystyrene provided the starting point for research at the University
of Salerno (17). Some of their subsequent work on the production and charac-
terization of copolymers, including aspects related to stereoregularity and crys-
tallinity of copolymers, has been referred to in a review article by Pellecchia and
Oliva (18).

Many copolymers that have been produced to date have contained less than

50 mol% styrene, in large part because the catalysts typically utilized for copoly-
merization have not allowed the incorporation of adjacent styrene units into the
growing polymer chains. This structural feature is discussed further under Prop-
erties. Limited literature (19,20) has described the production of copolymers in
which the catalyst selection and polymerization conditions employed have re-
sulted in the incorporation of adjacent styrene units in the polymer chain. Hou and

164

ETHYLENE COPOLYMERS

Vol. 6

co-workers (21) have recently reported on catalyst systems that can copolymer-
ize the monomers into block styrene–ethylene copolymers, although the reaction
products were mixtures of block copolymer with homopolymers.

A research program conducted in the group of Mulhaupt (Univer-

sity of Freiburg, Germany) included copolymerization of ethylene with
styrene using methaluminoxane-activated half-sandwich complexes (22) and
methaluminoxane-activated bis(phenolate) complexes (23). The influence of
polymerization conditions on the copolymerization of styrene with ethylene
using (CH

3

)

2

Si((CH

3

)

4

Cp)(N-t-butyl)TiCl

2

/methylaluminoxane (MAO) catalyst/

cocatalyst system was studied (14). The fractionation and comprehensive charac-
terization of a poly(ethylene-co-styrene) having 13.8 mol% styrene provided (24)
strong evidence of a single-site mechanism for the copolymerization.

Alternating ethylene–styrene (ES) copolymers, or copolymers that contain

alternating (ES)

n

sequences, have been synthesized. Suzuki and co-workers (25)

described the preparation of alternating atactic (ES)

n

copolymers via hydrogena-

tion of poly(phenylbutadienes), produced by anionic polymerization. Miyatake
and co-workers (26) have disclosed the production of copolymers having a high
degree of alternating isotactic polymer chain sequences. They further reported
(27) that the alternating isotactic copolymer had a crystalline melting point of
116

◦

C. Later references (eg 12,13,28) provide more details regarding catalysts,

reaction mechanisms, and characterization of produced copolymers having stere-
oregular, alternating ES chain sequences. For example, Venditto and co-workers
(28) described the composition, stereoregularity, and crystallinity of copolymers
produced by ansa–zirconocene and half-titanocene based catalysts, with copoly-
mers reported as having melting points in the range of 100–145

◦

C, attributed to

crystallization of alternating stereoregular (ES)

n

sequences. Izzo and co-workers

(29) have produced low molecular weight model compounds of alternating ES
copolymers with a range of catalysts, and characterization was considered to con-
firm the isotactic structure of copolymers.

Arai and co-workers (30) have shown that a copolymer having alternating

polymer chain sequences that are isotactic and crystallizable can be produced
when copolymerization is carried out at temperatures of 50

◦

C or below, using,

for example, rac-[isopropylidenebis(1-indenyl)]zirconium dichloride. Although the
presence of crystallizable (ES)

n

polymer chain sequences could contribute to prop-

erties such as mechanical strength, it appears (31) that annealing or exposure to
solvent is required to develop a significant level of crystallinity. This observation
is analogous to the behavior of isotactic polystyrene; the latter polymer also has
a slow rate of crystallization that has hindered its commercialization, because
of the restricted ability to develop crystallinity under common melt-fabrication
conditions (32).

Terpolymerizations.

A number of references, mainly patents, teach the

metallocene-catalyzed polymerization of ethylene and styrene with a range of ad-
ditional monomers, providing potential manufacturing routes to materials with
modified properties and functionality relative to ethylene-co-styrene polymers.
Dienes such as ethylidene norbornene, 1,3-butadiene, and 1,5-hexadiene (33) in-
troduce vinyl unsaturation into the copolymers, enabling more facile cross-linking
of the copolymers. Other monomers of interest include propylene, higher

α-olefins,

and norbornene (34,35).

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ETHYLENE COPOLYMERS

165

Propylene (P) is a termonomer that has received the most attention.

Structure–property relationships of terpolymers based on ethylene, styrene, and
propylene, have been published (36). The terpolymers are most differentiated from
ethylene-co-styrene polymers when there is measurable crystallinity in the solid-
state microstructure, because the introduction of methyl groups on the polymer
chain modifies both the crystalline and amorphous phases of the solid-state mi-
crostructure. In another aspect of terpolymerization, ethylene has been investi-
gated as a catalyst reactivator in propylene–styrene copolymerization by Caporaso
and co-workers (37), with the research directed at introducing styrene units into
isotactic polypropylene.

Chung and co-workers (38) have reported the synthesis of copolymers of

ethylene and paramethylstyrene and has extended these studies to produce and
characterize elastomeric terpolymers that further include propylene and 1-octene
as the additional monomers (39,40).

Copolymer Manufacture: Solution Polymerization

The two monomers of major interest, styrene and ethylene, are well known and
details can be found on all aspects of their technology elsewhere. Poly(ethylene-
co-styrene) is primarily produced via solution polymerization techniques using
metallocene catalyst/co-catalyst systems, analogous to the production of copoly-
mers of ethylene with

α-olefin monomers. Solvents that can be employed include

ethyl-benzene, toluene, cyclohexane, and mixed alkanes (such as ISOPAR E, avail-
able from Exxon). The thermodynamic properties of poly(ethylene-co-styrene), in-
cluding solvent interactions and solubility parameter assessments, are important
factors in relation to polymer manufacture and processing, and have been reported
by Hamedi and co-workers (41).

Key factors in the copolymer manufacture are

(1) Purity and cleanup of the solvent and monomers, especially styrene. Met-

allocene catalysts are noted for their susceptibility to polar molecules, and
effective removal of species such as stabilizers, water, and oxygen is required
to achieve high efficiencies.

(2) Polymer recovery via separation from unreacted monomers, especially

styrene, and the process solvent. Typically carried out by devolatilization,
as in the manufacture of polystyrene, the conditions (such as tempera-
ture and residence time) employed determine the amount, if any, of atactic
polystyrene in the final product resulting from free-radical polymerization
of styrene.

(3) Recovery and handling of the final product. A range of copolymers are ei-

ther amorphous or have low levels of crystallinity and further have glass-
transition temperatures (T

g

’s) close to or below 25

◦

C, as discussed under

Properties. These are soft tacky materials under typical ambient condi-
tions. If produced in pellet form, these copolymers can require some form
of antiblocking technology, such as the development of novel pellet coatings
(42), to allow effective material handling.

166

ETHYLENE COPOLYMERS

Vol. 6

Although classic analytical techniques can be employed, specific procedures

such as Fourier transform infrared spectroscopy for analysis of polystyrene and
copolymerized styrene (43) have contributed to the characterization and quality
assurance of copolymer products.

Properties of Poly(ethylene-co-styrene)

Poly(ethylene-co-styrene) constitutes a broad family of materials. Although
copolymers with stereoregular chain sequences could be produced, the catalyst
and process conditions most often employed for production to date give copoly-
mers that are atactic, and contain typically up to about 50 mol% (

∼ 80 wt%)

styrene content. These copolymers have been described (10) as “pseudorandom,”
since successive head-to-tail styrene chain insertions have been shown to be ab-
sent, even at high levels of styrene incorporation. This structural feature, repre-
sented schematically in Figure 1, is important; each aromatic group is typically
separated from its near neighbor by at least three methylene units, in contrast
to polystyrene. Much of the research and technology developments are reported
for copolymers with this chain microstructure. These copolymers have a contin-
uum of solid-state microstructures ranging from highly crystalline to essentially
amorphous, dependent primarily upon comonomer composition ratio.

A review (44) has provided more details on the structure, properties, and

applications of the copolymers. Researchers at Case Western Reserve University,
Cleveland, Ohio, have contributed significantly to the understanding of viscoelas-
tic behavior and mechanical properties of these copolymers (45,46).

Thermal Properties, Crystallinity, and Density.

Figure 2 correlates

crystallization (T

c

), melting (T

m

), and glass-transition (T

g

) temperatures mea-

sured by differential scanning calorimetry (DSC) with copolymer styrene content,
and Figure 3 correlates the degree of crystallinity and density with copolymer
styrene content.

Crystalline melting temperatures and crystallinity decrease with increas-

ing copolymer styrene content. Copolymers with styrene contents of less than
40 wt% (

∼15 mol%) generally exhibit a well-defined melting process. Copolymers

with styrene contents above 45 wt% (

∼18 mol%) are essentially amorphous. A

critical sequence length of (CH

2

)

n

units is required to enable crystallization of

chain segments. Thus, introduction of pendent phenyl groups by copolymeriza-
tion suppresses the crystallinity resulting from (CH

2

)

n

polymer chain sequences

because the phenyl groups are excluded from crystalline domains. It is pertinent
to comment here that subtle differences may be found in the copolymer monomer
sequence distributions as a consequence of catalyst selection and polymerization

x n

Fig. 1.

Schematic representation of so-called pseudorandom copolymer structure.

Vol. 6

ETHYLENE COPOLYMERS

167

T

m

T

C

T

g

80

Copolymer styrene, wt%

60

70

50

40

30

20

60

80

100

40

20

0

−20

T

,°

C

Fig. 2.

Thermal transitions of poly(ethylene-co-styrene) (DSC data; 10

◦

C/min heating,

cooling rates).

70

60

50

40

30

20

10

0

Cr

ystallinity

, %

Crystallinity

Density

0

10

20

30

40

50

60

70

80

Copolymer styrene, wt%

1.04

1.02

1.00

0.98

0.96

0.94

0.92

Density

, 10

−

3

kg/m

3

Fig. 3.

Density and crystallinity of poly(ethylene-co-styrene).

conditions employed. As a result of this, copolymers with nominally the same
comonomer content can exhibit some differences in structure-property relation-
ships.

The copolymers have glass-transition temperatures in the range

−20 to

+30

◦

C. Above about 45 wt% (18 mol%) styrene content, the copolymer T

g

in-

creases with increasing styrene content. Satisfactory correlation between copoly-
mer composition and T

g

has been found by considering them either as random

copolymers of ethylene and styrene or as random copolymers of ethylene and ES
dyads. Thus, T

g

data were shown by Chen and co-workers (46) to satisfactorily fit

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ETHYLENE COPOLYMERS

Vol. 6

the Fox equation in the form

1

T

g

=

(1

− 1.27w

s

)

T

g

,e

+

1

.27w

s

T

g

,es

where T

g

,e

is the glass-transition temperature of polyethylene, T

g

,es

is the glass-

transition temperature of the alternating ethylene–styrene copolymer, and w

s

is

the weight fraction of styrene incorporated into the copolymer. Extrapolation gave
a glass-transition temperature of

−66

◦

C for amorphous polyethylene and 44

◦

C for

an alternating ES copolymer.

Thermogravimetric analyses (TGA) in both air and nitrogen show weight

loss occurs at temperatures above 220

◦

C for the copolymers, reflecting random

chain scission as a predominant degradation mechanism.

Viscoelastic Properties: Dynamic Mechanical Spectroscopy (DMS).

The viscoelastic properties and time–temperature–rate dependency of properties
are key elements of the materials science of ethylene-co-styrene polymers. Dy-
namic mechanical loss spectra are presented in Figures 4a and 4b for selected
semicrystalline copolymers, and low crystallinity and amorphous copolymers, re-
spectively.

For the essentially amorphous copolymers, T

g

increases with increasing

styrene content. The mechanical loss processes associated with the amorphous-
phase T

g

have peak widths comparable to the T

g

process for atactic polystyrene,

and this provides strong evidence that the glass-transition process is character-
istic of a homogeneous amorphous material. Of further note in Figure 4 is the
so-called

γ -transition. This low temperature mechanical loss process can be asso-

ciated with local chain motions, analogous to polyethylenes (47).

For the semicrystalline copolymers, the loss peak in the temperature range

−50 to +50

◦

C shows increasing breadth of the relaxation process as the copoly-

mer styrene content decreases. The relaxation processes associated with this loss
peak are complex in nature. The long-range chain segmental motions associated
with the T

g

process become hindered because of the restrictions imposed by the

crystallites. The copolymers with less than about 25 wt% (8 mol%) comonomer
styrene exhibit complex thermal transition behavior as a result of the increas-
ing influence of the crystalline domains, restricting molecular mobility in the
amorphous region, similar to the behavior of high density polyethylene (HDPE)
(48).

Further detailed aspects of the viscoelastic behavior of amorphous copoly-

mers that have been reported includes linear stress-relaxation behavior (49) and
creep properties in the glass-transition region (50). Chen and co-workers (51) have
also reported the large strain–stress relaxation and strain recovery of copolymers
at temperatures above T

g

, and found that the observed behavior could be ratio-

nalized in terms of various network models.

Mechanical Properties.

The tensile stress–strain behavior of ethylene-

co-styrene polymers, including the effects of crystallinity and molecular weight,
has been extensively reported and analyzed. Figure 5 presents tensile stress–
strain data for a series of copolymers differing primarily in styrene content. The
copolymers generally exhibit large strain at rupture, and have been found to show
uniform deformation behavior (46).

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ETHYLENE COPOLYMERS

169

29% S

12% S

6% S

14

12

10

8

6

4

2

0

−150

−100

−50

0

50

100

T,

°C

(a)

G

′′,10

−

7

Pa

100

50

0

−50

−100

−150

0.01

0.1

1

10

T,

°C

(b)

tan

δ

γ-Transition

ES42

ES61

ES70

ES77

Fig. 4.

Dynamic mechanical loss spectra of (a) Three semicrystalline ethylene-co-styrene

polymers having different wt% comonomer compositions. (b) Amorphous or low crys-
tallinity ethylene-co-styrene polymers. (The label ES## refers to a copolymer having ##
wt% copolymerized styrene comonomer content.)

Chang and co-workers (52,53) studied the large-scale deformation behavior

and recovery behavior of semicrystalline ethylene-co-styrene polymers as a func-
tion of temperature, comonomer content, and crystallinity and compared them
to the behavior of metallocene-produced ethylene/1-octene copolymers. Chen and
co-workers (54) have provided an in-depth comparison of the morphological struc-
ture and properties of copolymers and confirmed that aspects of deformation that
depended on crystallinity, such as yielding and cold drawing, were determined
primarily by comonomer content for both sets of copolymers.

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ETHYLENE COPOLYMERS

Vol. 6

30

20

10

0

0

200

400

600

800

1000

% Strain

Stress

, MP

a

ES73

ES20

ES30

ES43

ES60

Fig. 5.

Engineering stress–strain data (23

◦

C, 5.7 min

− 1

) for ethylene-co-styrene

polymers.

The copolymers with 40–60 wt% (about 15–30 mol%) styrene comonomer

are characterized by low modulus and they show some elastomeric behavior.
D’Aniello and co-workers (55) reported the physical properties for a poly(ethylene-
co-styrene) having 20 mol% copolymer styrene, showing that the copolymer
showed good elastic properties. Information on the deformation and recovery be-
havior of selected copolymers was reported by Mudrich and co-workers (56), with
a copolymer having 45 wt% styrene of particular note in terms of strain recovery
after deformation.

The effects of styrene become dominant in the high styrene region where the

modulus and yield stress are seen to increase. Copolymers with T

g

above ambient

temperature are amorphous glasses, and these copolymers are reported (57) as
exhibiting interesting shape/reshape functionality.

Good low temperature toughness of copolymers has been found in impact

testing and low temperature tensile testing. Chen and co-workers (49) provided
estimates of the copolymer entanglement molecular weights (M

e

). The low M

e

values found suggest high entanglement density in these copolymers, and was
considered to contribute to the ability of the copolymers to shear yield at temper-
atures below T

g

rather than undergo brittle fracture.

Melt Rheology and Processability.

The melt rheological properties of

a series of copolymers having up to 20.5 mol% styrene content were reported
by Lobbrecht and co-workers (58). DMS data were generated for temperatures
ranging from 120 to 200

◦

C. Master curves were generated, and data analysis was

considered to show that there was a difference in rheological behavior for those
copolymers having greater than 16.5 mol% styrene.

The rheological properties and associated melt-processing characteristics

of ethylene-co-styrene polymers, including solid-state DMS, melt strength, and
pressure–volume–temperature (PVT) data, have been reported by Karjala and co-
workers (59,60). Rheological master curves were generated via time–temperature

Vol. 6

ETHYLENE COPOLYMERS

171

superposition, and viscosity–shear rate dependence were analyzed with a Cross
model as a function of both styrene content and molecular weight.

The melt processability of copolymers is considered as favorable toward most

fabrication techniques. Copolymers have been reported to show good thermal sta-
bility at melt-processing temperatures, viscosities that are a strong function of
styrene content, in addition to molecular weight, shear thinning behavior at high
shear rates, and good melt strength. These characteristics have allowed the fabri-
cation of articles by a wide range of standard melt-processing techniques, includ-
ing injection molding, film fabrication, blow-molding, and melt extrusion. The
potential utility in calendering operations has been described by Karjala and co-
workers (61). Selected copolymers were found to demonstrate the requisite rheo-
logical properties and thermal stability to be successfully calendered, and this was
supported by commercial-scale validation. Where required, the standard thermal-
stabilizer packages used in polyolefin formulations to improve processibility can
also be employed in ethylene-co-styrene polymers.

Copolymer Utility: Materials Engineering Aspects

For many applications, the use of poly(ethylene-co-styrene) in preference to other
ethylene homopolymers and copolymers will be determined by specific attributes
that the aromatic functionality imparts. In one aspect, the aromatic ring can be
chemically modified by sulfonation (62), and by a variety of electrophilic and nu-
cleophilic reagents (63) to produce polymers with specific functionality.

In addition to showing good compatibility with a wide range of materials,

other key characteristics, including the glass-transition temperature (T

g

), vis-

coelastic properties, and excellent processability, further suggest that copolymers
will find applications as components in formulated systems. The copolymers have
shown inherent compatibility with low molecular weight materials, including bi-
tumens (64), plasticizers (65), and tackifiers (66), in addition to a wide range of
other polymers and fillers. The technology of blend systems, which includes copoly-
mers as components and filler composites, is considered of particular relevance,
and is discussed in more detail below.

Poly(ethylene-co-styrene) as a Polymer Blend Component.

Diehl

and co-workers (67) have reported that the copolymers are compatible with many
other polymer families. The controllable combination of olefinic and aromatic
functionality contributes to the compatibility of blends containing ethylene-co-
styrene polymers. Significant toughening of atactic polystyrene (68) and syndio-
tactic polystyrene (69) can be achieved with selected copolymers. The olefinic func-
tionality helps provide compatibility with olefin homopolymers and copolymers,
including both polyethylenes and polypropylenes (70). Polymer families that have
been blended with ethylene-co-styrene polymers include styrene block copolymers
(71), poly(vinyl chloride) (PVC) based polymers (72), and polyphenylene ethers
(73). Specific copolymers have also been demonstrated as effective blend compat-
ibilizers, for example in polystyrene/polyethylene blends (74,75).

Blends of Poly(ethylene-co-styrene): Miscibility.

Blends of two ran-

dom copolymers are often immiscible when the difference in comonomer con-
tent between the copolymers is above a certain critical value. A study of

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ETHYLENE COPOLYMERS

Vol. 6

0

10

20

30

10

4

10

5

10

6

MW

Single-phase

∆

S

, v

ol%

Two-phase

Fig. 6.

Miscibility map for copolymer blends: the critical styrene content difference (

S

in vol%) between two copolymers versus molecular weight (MW) of blend components for
the onset of phase separation.

poly(ethylene-co-styrene) blends has combined predictions of miscibility with ex-
perimental observations (76). Critical composition differences for miscibility be-
tween ethylene-co-styrene polymers as a function of molecular weight are pre-
sented in Figure 6. Blends are predicted to be immiscible when the composition
difference exceeds about 10 vol% (

∼10 wt%) styrene between copolymers having

molecular weights around 10

5

(note: vol%, the model parameter used, approxi-

mates to wt% in this case). This miscibility criteria was shown to be valid for
copolymer blends from analysis of the thermal transitions of the blends and blend
components.

The above rationale regarding miscibility extends to copolymer blends with

polystyrene and polyethylene. Copolymers having less than about 90 wt% styrene
comonomer content will be immiscible with polystyrene, unless the molecular
weights of the respective polymers are very low. Similarly, copolymers with
more than about 10 wt% styrene comonomer content will be immiscible with
polyethylene.

Chen and co-workers (77) studied binary blends of copolymers over the full

range of copolymer styrene contents for both amorphous and semicrystalline blend
components. The transition from miscible to immiscible blend behavior, and the
determination of upper critical solution temperature (UCST) for blends were
uniquely evaluated by atomic force microscopy (AFM) techniques via the small,
but significant, modulus differences between the respective copolymers used as
blend components (78).

Blends of two or more copolymers differing in comonomer composition and/or

molecular weight characteristics of the individual blend components find po-
tential use in a range of applications. Thus, applications such as flooring sys-
tems, artificial leather and foams can utilize materials that have combinations of
two or more interpolymers differing in styrene content to engineer, for example,

Vol. 6

ETHYLENE COPOLYMERS

173

CaCO

3

ATH

BaSO

4

0

2

4

6

8

10

Stress

, MP

a

0

100

200

300

400

500

% Strain

Fig. 7.

Engineering stress–strain data (23

◦

C, 5.7 min

− 1

) for an ethylene-co-styrene poly-

mer (30 wt% styrene) containing 40 vol% filler.

the modulus–temperature relationship, glass-transition behavior, and damping
characteristics.

Filler Composites.

It is well known that the addition of fillers to polymers

can enhance the stiffness, dimensional stability, upper service temperature, ten-
sile strength, damping characteristics, and ignition resistance, as well as lower the
cost of fabricated parts. The performance requirements for many applications that
utilize poly(ethylene-co-styrene) can be met by modifying the balance of properties
through the addition of fillers. The copolymers show good filler acceptance with
a broad range of inorganic fillers, including calcium carbonate, barium sulfate,
alumina trihydrate (ATH) and magnesium hydroxide (Mg(OH)

2

) (79,80). Figure 7

presents tensile stress–strain data for a poly(ethylene-co-styrene) filled with 40
vol% of three different fillers. The resulting composite materials generally exhibit
very good mechanical properties, even for relatively high loadings of fillers.

The filled copolymers have shown rheological characteristics which suggest

that the compositions can be easily fabricated into final parts. Filled compositions
are expected to find broad utility in applications, including, for example, wire
and cable (81), flooring systems (82), injection-molded articles, film and sheet
structures, and profile extrusions.

Applications

Potential markets/applications for poly(ethylene-co-styrene) can be discerned
from various references and patent literature. In addition to the applications
identified in the preceding sections, important applications discussed or iden-
tified include adhesives (66), paintable injection-molded toys (83), footwear,

174

ETHYLENE COPOLYMERS

Vol. 6

Fig. 8.

Examples of foam applications utilizing ethylene-co-styrene polymers.

automobile parts, bitumen modification, packaging, and blow-molded articles.
Although intermaterial substitutions in existing applications based on thermo-
plastic elastomers such as styrene–butadiene block copolymers, flexible PVC, and
polyethylene homopolymers and copolymers are potential application areas for
copolymers, the novel combination of material attributes suggests that new ap-
plication areas will emerge.

Developments using copolymer-based materials in foam applications are of

particular note. Novel foam structures offer attractive properties and characteris-
tics, including softness, aesthetics, and drape, for a wide range of thermoplas-
tic and cross-linked foam applications. Other product technologies of interest
are as injection-molded structural foams, as foamed layers in multilayer struc-
tures and as foamed blends of interpolymers with styrenic and olefinic polymers
(Fig. 8) (84–86). Interpolymers also have potential for coextruded film and sheet
applications.

Summary.

Copolymers based on ethylene and styrene are a novel class of

polymers exhibiting unique combinations of material attributes that are not found
in polyethylenes, polystyrenes, or their blends. These copolymers offer unique
opportunities for innovative developments in basic polymer chemistry, catalyst
and process development, materials science and engineering, and application
technology.

The effective production of these novel copolymers has been enabled by IN-

SITE Technology. A product development plant to produce Interpolymers (Sarnia,
Canada) had a successful start-up in September 1999. This plant, having a name-
plate production capacity of around 22,500 metric tons per annum, has enabled
further product and process developments, and application validation.

Vol. 6

ETHYLENE COPOLYMERS

175

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M

ARTIN

J. G

UEST

Y. W

ILSON

C

HEUNG

Polyolefins R&D, The Dow Chemical Company

178

ETHYLENE–PROPYLENE ELASTOMERS

Vol. 6

ETHYLENE POLYMERS, CHLOROSULFONATED.

See Volume 2.

ETHYLENE POLYMERS, HDPE.

See Volume 2.

ETHYLENE POLYMERS, LDPE.

See Volume 2.

ETHYLENE POLYMERS, LLDPE.

See Volume 2.

ETHYLENE–ACRYLIC ELASTOMERS.

See Volume 2.

ETHYLENE–NORBORNENE COPOLYMERS.

See Volume 2.