4
Types and Basics of Polyethylene
Rajen M. Patel
The Dow Chemical Company, Freeport, Texas, USA
Contents
4.1 Introduction ............................................................................................................................... 106
4.2 Low Density Polyethylene (LDPE) ......................................................................................... 109
4.3 Ethylene Vinyl Acetate (EVA) Copolymer............................................................................. 112
4.4 Acrylate Copolymers ................................................................................................................ 115
4.5 Acid Copolymers....................................................................................................................... 116
4.6 Ionomers..................................................................................................................................... 117
4.7 High Density Polyethylene (HDPE) ....................................................................................... 118
4.8 Ultra-High Molecular Weight HDPE (UHMW-HDPE) ..................................................... 120
4.9 Linear Low Density Polyethylene (LLDPE) ........................................................................... 121
4.10 Very Low Density Polyethylene (VLDPE) ............................................................................. 123
4.11 Single-Site Catalyzed Polyethylenes........................................................................................ 124
4.12 Olefin Block Copolymers (OBC) ............................................................................................ 133
4.13 Concluding Remarks ................................................................................................................ 136
Acknowledgments .............................................................................................................................. 136
References ............................................................................................................................................ 137
Abstract
Polyethylene (PE) has a long and rich history of product, process and fabrication innovations
to meet growing market needs over the last 75 years. Many different types of ethylene homopolymer and copolymer resins have been developed with a broad range of performance to meet
the requirements of a variety of applications, and as a result PE is the highest volume plastic available today. This chapter will cover many different types of ethylene homopolymer and
copolymer resins widely used today, their molecular characterization, and structure-properties
relationships. The chapter will also cover fundamentals and a variety of applications of various
types of ethylene homopolymer and copolymer resins.
Keywords: Polyethylene, LDPE, LLDPE, VLDPE, HDPE, HMW HDPE, UHMW HDPE, EVA,
acid copolymers, acrylate copolymers, ionomers, ECO copolymers, plastomers, elastomers,
metallocene LLDPE, EPDM, olefin block copolymers
Corresponding author: rmpatel@dow.com
Mark A. Spalding and Ananda M. Chatterjee (eds.) Handbook of Industrial Polyethylene and Technology, (105–138)
© 2018 Scrivener Publishing LLC
105
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Handbook of Industrial Polyethylene and Technology
4.1 Introduction
Polymers are typically classified as thermoplastic or thermoset polymers, among several classification systems. Thermoplastic polymers can be melted and reprocessed
multiple times at high temperatures whereas thermoset polymers cannot be melted
and reprocessed due to the presence of cross-links between molecules. This classification of polymers is based on the processing aspects of polymers. However, polymers
can also be classified as either amorphous or semicrystalline based on structural order.
Amorphous polymers have zero degree of crystallinity and molecules are packed in a
random manner (no order). Semicrystalline polymers typically have 1 wt% to about 80
wt% crystallinity, with the rest being an amorphous phase (assuming two-phase model).
The crystalline phase consists of crystallites which are typically in a chain folded lamellar form. Note that polymers cannot be 100 wt% crystalline due to their high molecular
weight and therefore a high degree of entanglements. Amorphous and semicrystalline
polymers can be further classified based on whether their glass transition temperature
is above or below room temperature (~25 °C). The glass transition temperature (Tg) is a
key property of the amorphous phase of polymers. It is the temperature at which amorphous phase transitions from glassy (rigid) to a rubbery behavior. Below Tg, molecules
in the amorphous phase are not mobile, hence the polymer is rigid (glassy). Above Tg,
molecules in the amorphous phase become mobile, leading to a rubbery behavior.
Semicrystalline polymers having glass transition temperatures (Tg) below room temperature include PE (ethylene-based polymers with >50 mol% units derived from ethylene) and polypropylene (PP) (propylene-based polymers with >50 mol% units derived
from propylene) resins; both referred to as part of the polyolefin family. Since the
amorphous phase of polyolefins is in a rubbery state, they exhibit much lower modulus
compared to amorphous (e.g., polystyrene) or semicrystalline polymers (e.g., polyethylene terephthalate) having the amorphous phase in a glassy state due to Tg above room
temperature. Modulus, tensile strength, elasticity, and polarity of PE can be dramatically changed by changing the degree of crystallinity of the resins via copolymerization
with a wide range of comonomers. As a result, PE is a very versatile polymer exhibiting
a very broad range of properties, ranging from stiff/brittle to ductile/tough to elastomeric, depending upon the degree of crystallinity. This tremendous flexibility in tuning
mechanical properties has led to a wide range of applications for PE. Therefore, PE is
commercially very important and is the highest volume thermoplastic produced, having a rich history of major innovations in products, processes, and breadth of applications [1, 2]. Global demand for PE in 2014 was about 85 million metric tons (187 billion
pounds). Global demand is expected to increase to about 105 million tons (230 billion
pounds) by 2019.
Polyethylene is composed of only carbon and hydrogen (with some notable exceptions) which can be combined in a number of ways to make different types of PEs.
There are various molecular architectures that have been commercialized over the last
70 years to make different types of ethylene homopolymers and copolymers. These can
be generally grouped into eleven major types:
Low density PE (LDPE);
Ethylene vinyl acetate copolymer (EVA);
Types and Basics of Polyethylene
107
Acrylate copolymers such as ethylene methyl acrylate (EMA), ethylene
ethyl acrylate (EEA) and ethylene n-butyl acrylate (EnBA);
Acid copolymers such as ethylene acrylic acid (EAA) or ethylene methacrylic acid (EMAA) copolymers;
Ionomers;
High density PE (HDPE);
Ultra high molecular weight high density PE (UHMWPE);
Linear low density PE (LLDPE);
Very low or ultra low density PE (VLDPE or ULDPE);
Homogeneous PE produced via single-site catalysts (including metallocene) (polyolefin plastomers, polyolefin elastomers, mLLDPE,
mVLDPE) [3];
Olefin block copolymer (e.g., INFUSE OBC) [4].1
There are still other minor types of ethylene copolymers including:
Chlorinated PE (CPE) made via chlorination of HDPE powder;
Ethylene/carbon monoxide (ECO) copolymers, which are photodegradable and made using a high pressure free-radical polymerization process,
and are used to make six-pack loop carriers for beverage;
Ethylene/vinyl trimethoxy silane (VTMOS) copolymers, used in the wire
and cable industry for moisture curing;
Ethylene/maleic anhydride (MAH) copolymers made using the high
pressure process and used mainly as a compatiblizer;
Specialty ter-polymers made in a high pressure process such as ethylene/acrylic acid/acrylate; ethylene/butyl acrylate/carbon monoxide; ethylene/vinyl acetate/carbon monoxide; ethylene/butyl acrylate/glycidyl
methacrylate, etc.;
Cyclic olefin copolymers (COC), made from ethylene and norbornene
comonomer using a single-site catalyst;
MAH-grafted-PE resins, used in tie-layers, tying ethylene vinyl alcohol
(EVOH) or polyamide to a PE layer in multilayer co-extruded barrier
films. Such resins typically contain about 0.07% to 0.15% of MAH in
ready-to-use formulations to provide good adhesion to polyamide and
EVOH.
These eleven major types of ethylene homopolymers and copolymers can
be grouped into three categories: high pressure process resins, heterogeneous PEs catalyzed by Ziegler-Natta (Z-N)/Chromium Oxide catalysts,
and homogenous PE catalyzed by single-site (including metallocene)
catalysts. This is shown in Figure 4.1.
Most of the ethylene polymer types listed above consist of a polymer backbone composed of carbon and hydrogen with different types of branches from the backbone
[5]. The branches range from alkyl in LDPE, LLDPE, and VLDPE to acetoxy and
1
INFUSE is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow.
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Handbook of Industrial Polyethylene and Technology
High pressure
Z-N and chromium
oxide catalyzed
LDPE
EVA
Acrylate copolymers
acid copolymers
ionomers
HDPE
UHMWPE
LLDPE
ULDPE/VLDPE
Plastomer/elastomers &
mLLDPE/mVLDPE
Olefin block copolymer
(OBC)
Single site
catalyzed
Figure 4.1 Grouping of eleven types of ethylene homopolymers and copolymers into three buckets.
alkyl branches in EVA to carboxyl branches in EAA or EMAA. The degree and type
of branches control the degree of crystallinity of PE by introducing defects in regular
chain architecture, thereby affecting the solid-state properties of PEs. Molecules in the
crystalline phase typically are packed tightly together in a regular 3-dimensional order.
Hence, the crystalline phase typically has a higher density than the amorphous phase.
As a result, the density of PE is directly related to the degree of crystallinity and, in fact,
nonpolar PEs (LDPE, LLDPE and HDPE resins) are classified and specified by resin
density. The density of PE can be used to estimate the wt% and vol% crystallinity in PE
as follows:
Wt% Crystalinity =
c
a
c
a
Vol% Crystalinity =
c
100
(4.1)
100
(4.2)
a
a
where is the nonpolar PE density, a = 0.855 g/cm3 for nonpolar PE, and c = 1.00 g/cm3;
is the amorphous phase density and c is the crystal density.
a
The ability to control types and degree of branching via the incorporation of different comonomers allows one to make PE from nonpolar to polar, from stiff and rigid
plastics to soft and flexible elastomers, from having a high melting point to a low melting point, etc. As a result, a wide range of properties are obtained from the abovementioned ethylene homopolymer and copolymer types, making ethylene polymer one
of the most versatile plastics.
Even though there are eleven major types of ethylene homopolymers and copolymers, generally PE resins are classified into three broad categories; LDPE, LLDPE
and HDPE. Global demand for these three broad categories of PE in 2014 is shown
in Figure 4.2. HDPE represents the largest fraction of the PE produced, followed by
Types and Basics of Polyethylene
2014 Global demand
109
2019 Global demand
LDPE
LDPE
HDPE
HDPE
LLDPE
LLDPE
Growth forecast
24%
Total global demand = 84.8
Million metric tons
Total global demand = 105.4
Million metric tons
Figure 4.2 Global demand for the three broad categories of PE. (Source: IHS, Polyolefins conference, 2015).
Figure 4.3 Molecular structures, properties and general applications of three broad categories of PE resins.
LLDPE and LDPE. Global demand for LLDPE resin is expected to increase as a percentage, mainly at the expense of LDPE.
Molecular structures, properties, and general applications of these three broad categories of PE resins are shown in Figure 4.3. The demand for PE can also be classified
by the type of conversion process used to make the final end-use part. This is shown in
Figure 4.4.
The eleven types of ethylene homopolymers and copolymers will be described in
more detail in the following sections.
4.2 Low Density Polyethylene (LDPE)
Low density polyethylene was accidentally discovered at Imperial Chemical Industries
(ICI) by Fawcett and Gibson as a waxy solid powdery substance found while
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Handbook of Industrial Polyethylene and Technology
Rotomolding
2%
Blow molding
12%
Wire & cable
2%
Fiber
1%
Raffia
1%
Other
7%
Film & sheet
52%
Extrusion coating
3%
Pipe & profile
7%
Injection molding
13%
Figure 4.4 PE demand by conversion processes (2013). (Source: The Dow Chemical Company)
investigating high-pressure and high-temperature reactions of ethylene and benzaldehyde. Researchers at ICI realized after months of experimentation that oxygen had to
be present to initiate the conversion of ethylene to PE under high pressure. The presence of oxygen leads to the formation of free-radical initiators (peroxides) that subsequently decompose to start the free-radical addition polymerization of LDPE. ICI filed
a series of patents to claim the inventions, claiming continuous production of LDPE
under pressures greater than 500 atmospheres and temperatures greater than 100 °C,
preferably in the presence of 0.01 wt% to 5 wt% oxygen [6]. Polymerization of ethylene
to PE is a highly exothermic reaction, releasing approximately 3370 J/g of ethylene.
Removal of the exothermic heat of polymerization determines the percent conversion
of ethylene to PE per pass.
Low density polyethylene is made using either an autoclave or a tubular process,
using a free radical polymerization process initiated by organic peroxides. The autoclave reactor is a continuous stirred-tank reactor (CSTR) with an agitator designed
to promote good mixing. Autoclave reactors are essentially adiabatic and rely on back
mixing of the hot reactants with the cold incoming ethylene feed stream. The heat of
polymerization is removed with effluent from the reactor to keep the reaction stable.
Autoclave reactors typically operate at 1200 to 2400 bar. Tubular reactors are non-adiabatic reactors that allow additional heat removal via a coolant in the reactor jacket
(containing high pressure hot water) in addition to heat removal via cold ethylene feed.
High pressure hot water (120 °C to 180 °C) in the reactor jacket prevents fouling of the
reactor wall by preventing the crystallization of LDPE. Tubular reactors typically operate at 2000 to 3200 bar. Conversion (ethylene to PE) per pass is typically about 14 to
20 wt% for the autoclave process and about 21 to 38 wt% for the tubular process.
Types and Basics of Polyethylene
111
The free radical polymerization process leads to both long-chain branching (via
chain transfer to another molecule) and short-chain branching (via back-biting mechanism) in LDPE resins. LDPE has ratio of about ten short-chain branches to every longchain branch. LDPE density (crystallinity) is controlled by the reactor temperature and
pressure. A small reduction in density can also be achieved by adding a small amount
of comonomer like propylene or 1-butene (both of which also act as a chain transfer
agent reducing molecular weight). Higher reactor pressure (via increasing propagation rates) and lower reactor temperature (via decreasing back-biting rates) increases
LDPE density, and conversely, higher reactor temperature and lower reactor pressure
decreases LDPE density. Due to the limitations in the reactor pressure and temperature
range, LDPE resins can be commercially produced only in the density range of about
0.915 to 0.935 g/cm3 (about 45 to 59 wt% crystallinity).
A key molecular feature of LDPE resins is the presence of a high level of “tree-like”
long-chain branching (LCB), typically about 2 to 3 LCB/1000 carbon atoms. It has been
proposed that the length of LCB is greater than the critical entanglement molecular
weight [7]. For PE, this equates to 270 carbon atoms or greater [5]. As a result of a
high level of LCB, LDPE resins exhibit excellent extrusion processability (due to a high
degree of shear thinning) and melt strength properties but inferior impact, tear, abrasion, and environmental stress-cracking resistance (ESCR) properties. Though crystalline domains control low deformation (strain) properties of PE, such as modulus and
yield stress, it has been clearly established by many researchers that large deformation
(strain) properties, such as stretchability, impact, tear, failure processes, etc., are also
controlled by the amorphous region, particularly by tie chains , the amorphous chains
that bridge adjacent lamellae [8, 9]. Inferior mechanical properties of LDPE are caused
by the compact/smaller size of LDPE molecular coil (lower radius of gyration) in the
molten state due to the presence of a high degree of LCB, resulting in reduced tie-chain
concentration [10]. LDPE resins are widely used as a blend component with LLDPE
resin to improve bubble stability, gauge uniformity, optics, and rates in the blown film
process [11]. LDPE resins having fractional melt indices are also widely used as a blend
component in collation shrink film (shrink bundling) applications because of the need
for cross-direction (CD) film shrinkage (~10–25%). Due to their very high melt strength
as a result of the high level of long-chain branching in the resin, LDPE resins are especially suitable for extrusion coating and foam applications. The high melt strength of
LDPE resins, especially autoclave LDPE resins, allows lower neck-in (change in the
width of the molten web) and high line speed during the extrusion coating operation.
Autoclave-produced LDPE has broader molecular weight distribution than tubular
LDPE [12, 13]. This is shown in Figure 4.5. Molecular weights were measured using a
low angle laser light scattering detector to accurately measure the absolute molecular
weight of the LDPE resins containing high levels of long-chain branching. Autoclave
LDPE also has a higher level of long-chain branching and, hence, a higher melt strength
than tubular LDPE at similar melt indices. Melt strength of an autoclave resin is compared with a tubular grade in Figure 4.6. Melt strength was measured using a Goettfert
Rheotens apparatus. Autoclave LDPE is generally the preferred resin for extrusion coating applications due to its lower neck-in performance.
Due to its high melt strength and, in particular, strain hardening in elongational flow
(increase in elongational viscosity with deformation/strain) during foaming process,
Handbook of Industrial Polyethylene and Technology
dWf / {dLog molecular weight (by LALLS)}
112
1
0.8
Autoclave
Tubular
Tubular (1.9 MI)
0.6
0.4
Autoclave (2.3 MI)
0.2
0
2
3
4
5
6
Log molecular weight (by LALLS)
7
8
Figure 4.5 Molecular weight distributions of autoclave and tubular LDPE resins at similar melt indices.
20
190 °C
18
Autoclave (2.3 MI)
Melt strength (cN)
16
17.0 cN
Autoclave
Tubular
14
12
10
Tubular (1.9 MI)
8
7.6 cN
6
4
2
0
0
50
100
150
200
Velocity (mm/s)
250
300
350
Figure 4.6 Melt strengths of autoclave and tubular LDPE resins at similar melt indices.
LDPE resins allow the production of foams with nearly 100% closed cells,. Blown films
made from LDPE resins with a relatively narrow molecular weight distribution and low
melt strength, typically made via the tubular process, exhibit very good optics and are
used in high clarity applications such as in soft bread packaging. Other applications of
LDPE include trash bags, food storage bags, food wrap film, and injection molded articles. LDPE, as well as silane copolymers or silane-grafted LDPE, is widely used in wire
and cable insulation applications due to being a “clean” resin having no metal/catalyst
residues, yielding excellent insulation properties (high electrical breakdown potential).
4.3
Ethylene Vinyl Acetate (EVA) Copolymer
Ethylene vinyl acetate is a random copolymer of ethylene and vinyl acetate monomers
made using a high pressure free-radical polymerization process. Vinyl acetate (VA)
wt% crystallinity
Types and Basics of Polyethylene
50
45
40
35
30
25
20
15
10
5
0
113
y = –1.034x + 47.7
R2 = 0.9936
0
5
10
15
20
25
30
wt % VA
Figure 4.7 Wt% crystallinity of EVA resins as a function of wt% VA content.
(Courtesy of Dr. Ken Anderson, Celanese)
content has two fundamental effects that influence the properties of EVA copolymers.
The first effect is to disrupt the crystallinity formed by long ethylene sequences of the
copolymer. The second key effect of VA content is the polar nature resulting from acetoxy side chains. EVA resins have a lower degree of crystallinity and a lower melting
point compared to high pressure LDPE resin; both properties are controlled by the
VA content. Weight percent vinyl acetate in the EVA copolymer typically ranges from
about 4 to 40%. EVA polymers with VA content higher than 40 weight percent create
pellet handling issues; e.g., pellet blocking and stickiness. At about 45% VA content
by weight, depending on molecular weight and molecular weight distribution, EVA
becomes completely amorphous (non-crystalline). A plot of the crystallinity of EVA
resins as a function of VA content is shown in Figure 4.7.
EVA copolymers have been commercially available. EVA is the largest volume polar
ethylene copolymer used today. Major suppliers of EVA include DuPont (ELVAX ),
and Nexxstar ), Celanese (Ateva ), LyondellBasell
ExxonMobil (Escorene
(Ultrathene ), Arkema (EVATANE ), etc.2 As the vinyl acetate level is increased, the
degree of crystallinity (and hence, stiffness) decreases but density increases. This is due
to bulky acetoxy side groups in EVA which increases the amorphous phase density
and therefore overall density even though crystallinity is lower. As the vinyl acetate
level increases, the EVA copolymer becomes more polar. Melt indices of EVA resins
typically range from 0.35 to 2500 dg/min (190 °C, 2.16 kg). EVAs have a high degree
of long-chain branching which make them easy to extrude and melt process. However,
they are thermally less stable compared to other ethylene copolymers due to presence
of weaker carbon-oxygen bonds which can lead to the generation of acetic acid (vinegar) and the formation of cross-linked gels during high temperature melt processing.
Therefore, EVA resins are generally melt processed below 230 °C. Also, the high degree
of long-chain branching in EVA resins leads to inferior puncture, dart, and tear properties compared to linear PE resins (e.g., ATTANE from The Dow Chemical Company)
at similar crystallinity.
2
ELVAX is a trademark of E.I. du Pont de Nemours and Company or its affiliates; Escorene and Nexxstar
are trademarks of ExxonMobil Corporation; Ateva is a trademark of Celanese Corporation; Ultrathene is
a trademark of LyondellBasell; EVATANE is a trademark of Arkema.
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Handbook of Industrial Polyethylene and Technology
EVAs are widely used in film applications that require a lower crystallinity and/or a
lower melting point; e.g., sealants, barrier shrink bags, high clarity films, applications
requiring low temperature toughness such as ice bags, stretch hood films, and thermal
lamination films. EVA resins having wt% VA levels from 7 to 18% VA are widely used
in the core layer of stretch hood film formulations, depending upon on-pallet stretch
level requirements. Both EVA and LDPE resins exhibit very low hot-tack strength as a
sealant. This is due to a very high degree of long-chain branching which slows down
diffusion across the molten sealant interface significantly. EVA resins are also used as
a tie-layer resin for tying polyvinylidene chloride (PVDC) to a PE layer in multilayer
co-extruded barrier films. EVA resins having 18 to 20% VA content and 15 to 20 melt
index are extrusion coated (8 to 15 micron thick) onto a biaxially oriented polypropylene (BOPP) film and such films are widely used as thermal lamination films for
laminating onto glossy printed paper for use as magazine covers. EVA resins are also
widely used in wire and cable applications such as heat shrinkable insulation, semiconductive insulation jackets, and flame retardant formulations [14]. EVA resins are
used to make cross-linked foams used as mid-soles in footwear applications as well
as injection molded articles. Other uses for EVAs include medical applications such
as IV bags and cryo-containers. Ease of sterilization of EVAs using gamma radiation
makes them very suitable in such medical applications. High melt index EVA resins
(melt index typically greater than 50) having 18 to 33 wt% VA levels are widely used for
hot melt adhesive (HMA) applications. EVA-based hot melt adhesives are formulated
with waxes and tackifiers and are used in applications such as case and carton closing,
labeling, lamination, book binding, furniture and wood working. EVA-based hot melt
adhesives exhibit poor thermal and oxidative stability leading to shorter pot life due to a
rapid rise in viscosity, yellowing, and gel/char formation (which can clog the applicator
nozzle). EVA-based hot melt adhesives also exhibit inferior low temperature adhesion
due to higher Tg of EVA resins (Tg of about −30 °C).
Figure 4.8 compares the glass transition temperature of EVA versus single-site catalyzed ethylene/1-octene polyolefin plastomer (POP) and polyolefin elastomers (POE)
ENGAGE /AFFINITY resins from The Dow Chemical Company.3 Weight % crystallinity at 23 °C was obtained by dropping perpendicular at 23 °C in the differential
scanning calorimeter (DSC) melting endotherm during the heat of fusion integration
(area under the curve). EVA resins exhibit a higher Tg compared to POP/POE resins at
a given crystallinity. The Tg of POP/POE resins increase with the increase in resin density/crystallinity. Note that above 0.91 g/cm3 density, Tg cannot be observed as a step
change in heat capacity in a DSC melting endotherm.
EVA copolymers at 28 to 33 wt% VA content and high melt indices (40 to 50 dg/min)
are widely used to make an encapsulant film (0.3 to 0.8 mm thick) for photovoltaic
(solar) cells. Such soft thick cross-linked encapsulant films exhibit excellent light transmission and help to protect fragile silicon wafer solar cells. However, under exposure to
atmospheric water and/or ultraviolet radiation, EVA will decompose to produce acetic
acid, lowering the pH and increasing the surface corrosion rates of embedded devices
leading to deterioration of module performance. Also, the glass transition of EVA,
3
ENGAGE and AFFINITY are trademarks of The Dow Chemical Company (“Dow”) or an affiliated
company of Dow.
Types and Basics of Polyethylene
115
0
0
10
20
30
40
50
–10
Tg (ºC) by DSC
–20
POP & POE
EVA
28% VA
–30
25% VA 18% VA 15% VA
0.909 g/cc
0.897 g/cc
–40
y = 0.7316x – 62.746
–50
–60
0.876 g/cc
0.863 g/cc
0.879 g/cc
0.857 g/cc
–70
wt% Crystallinity at 23 ºC
Figure 4.8 Glass transition temperature (Tg) of EVA and ethylene/octene POP/POE (AFFINITY/
ENGAGE from The Dow Chemical Company) resins as a function of crystallinity at 23 °C, measured
using differential scanning calorimetry (DSC). %VA levels are shown for the EVA resins and measured
densities are shown for the 1-octene-based POP/POE resins.
measured using dynamic mechanical analysis, begins at about −15 °C. Temperatures
lower than this can be reached for extended periods of time in some climates. Because of
increased modulus below the glass transition temperature, a module may be more vulnerable to damage if a mechanical load is applied by snow or wind at low temperatures.
The Dow Chemical Company has developed ethylene/alpha-olefin copolymer-based
encapsulant film under the trade name of ENLIGHT . The ENLIGHT encapsulant
film provides improved moisture and UV resistance, high volume resistivity and low
leakage current compared to traditionally used EVA films, enabling long-lasting protection to significantly extend solar module service life.
EVA copolymers also provide an alternative to polyvinyl chloride (PVC) in medical
applications because of migratory plasticizer and incineration issues of the PVC resins.
EVA copolymers are also used as a carrier resin to make compounds and masterbatches
as well as in automotive applications (e.g., sound dampening materials), often with calcium carbonate fillers
4.4
Acrylate Copolymers
Acrylate copolymers are made using a high pressure free-radical polymerization process typically using autoclave reactors. There are three types of acrylate copolymers
depending upon the type of acrylate comonomer used; ethylene methyl acrylate
(EMA), ethylene ethyl acrylate (EEA), and ethylene butyl acrylate (EBA). The glass
transition temperature of acrylate copolymers is in the order of EMA > EEA > EBA at
a given crystallinity. Acrylate comonomers add polar functionality to ethylene polymers, decreasing the crystallinity and melting point. Acrylate copolymers made using a
tubular reactor results in a more heterogeneous molecular structure and a higher crystalline melting temperature compared to those made with autoclave reactors. Acrylate
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Handbook of Industrial Polyethylene and Technology
copolymers containing up to 35 wt% comonomer level are commercially available. Due
to their low glass transition temperature, EBA resins exhibit superior low temperature
toughness and impact resistance. Acrylate copolymers have a high degree of long-chain
branching (and exhibit high melt strength), similar to all PE resins made using a high
pressure free-radical polymerization process. A key feature of acrylate copolymers is
that they have very good thermal stability (similar to LDPE) and, hence, can be melt
processed at very high temperatures, up to 325 °C. This is in contrast to EVA resins
which are thermally not as stable and are melt processed below 230 °C. Acrylate copolymers are also non-corrosive to equipment and are not hygroscopic. Acrylate copolymers, especially EMA, are used in extrusion coating and lamination applications as
extrusion coated tie layers for adhesion to printed oriented polypropylene (OPP), oriented polyethylene terephthalate (OPET), and PVDC coated substrates. Acrylate copolymers are also used as sealant layer resins, in flexible hoses and tubing, as well as carrier
resins for making masterbatchs and compounds that allow for an increased loading of
fillers, pigments, and additives. Acrylate copolymers, especially EEA and EBA, are also
used in wire and cable applications such as semiconductive layers and insulation layers. They have also found applications in hot melt adhesive formulations and asphalt
modification. Suppliers of acrylate copolymers include Arkema, DuPont, ExxonMobil
Chemical, The Dow Chemical Company, Westlake Chemical, and Mitsubishi.
4.5 Acid Copolymers
Copolymers of ethylene and acrylic acid (EAA) or methacrylic acid (EMAA) are commonly referred to as “acid copolymers.” Acid copolymers are made using a high pressure free-radical polymerization process using autoclave reactors. They have a high level
of long-chain branching and therefore exhibit high melt strength. EAA and EMAA
are nearly equivalent in performance at the same mole percent acid comonomer level.
For equivalent polar functionality (mole % comonomer), EMAA copolymers have a
slightly higher comonomer (acid) weight % than EAA copolymers. Major suppliers of
EAA include The Dow Chemical Company (PRIMACOR ), DuPont (Nucrel ), and
ExxonMobil chemical (Escor ).4 Major suppliers of EMAA include DuPont (Nucrel).
The acid content generally ranges from 3% to 20 wt% and melt index ranges from 1 to
1300 dg/min (190 °C, 2.16 kg). Increasing the acid level increases adhesion to polar
substrates and decreases adhesion to nonpolar substrates. Increasing acid level also
improves resistance to grease and oil. Acid functionality enables excellent adhesion
to aluminum foil, metalized substrates, and paper, and hence acid copolymers are
widely used as tie layers in many flexible packaging applications. These applications
include laminated tubes, aluminum foil packaging, and metalized aluminum packaging. Laminated tubes are used in oral care (e.g., toothpaste tube), food, and pharmaceutical applications. Packaging applications utilizing acid copolymers include condiment
pouches and sachets, aseptic packaging, and processed meat and cheese packaging.
4
PRIMACOR is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow;
Nucrel is a trademark of E.I. du Pont de Nemours and Company; Escor is a trademark of ExxonMobil
Corporation.
Types and Basics of Polyethylene
Polar Ethylene Co-polymers
Ethylene vinyl acetate (EVA)
H2C
– 0.4 to 2500 MI
– 3-20% AA, 4-12% MAA
– Tie-layers (adhesion to metal)
– Partially neutralized to make
Ionomers
CH3
CH
CH2 CH2
O
H
H
H
H
H H
C
C
C
C
C C
H
H
H
C O
x
y
z
H C O
O H
O– Na+
CH2 CH2
CH2
– EEA (Ethylene Ethyl Acrylate)
– EnBA (Ethylene n-Butyl Acrylate)
H
CH2 C
O
H3C
Acrylate copolymers
– EMA (Ethylene Methyl Acrylate)
n
C
O
– High clarity film, sealants & hot
melts adhesives
Ethylene acrylic acid (EAA),
ethylene methacrylic acid
(EMAA) & Ionomers
EVA
Vinyl acetate (VA)
– 5-40% wt VA
117
C
m
O
x = ethylene
y = acrylic acid
z = sodium neutralized acrylic acid
x, y, z are randomly distributed
H
C
C
RO
O
– Wire & cable, tie-layers
Figure 4.9 Chemistry and key attributes of polar ethylene copolymers made using high pressure process.
Non-packaging applications of acid copolymers include protective metal coating for
corrosion protection, powder coating (e.g., football face masks, playground equipment,
and outdoor furniture), and wire and cable. Ethylene methacrylic acid is also used as an
intermediate for producing ionomers.
Ethylene acrylic acid copolymers having high acid levels (~20 wt%) are also converted into water-based dispersions. Primary applications of the dispersions include
priming (coating weight of 0.5 to 1.2 g/m2) and lamination, paper and textile sizing,
paper and paperboard coating, metal coating, and heat seal coating.
Chemistry and key attributes of PE polar copolymers are summarized in Figure 4.9.
Note that all polar copolymers of ethylene are made using the high pressure free-radical polymerization process. Such polar copolymers cannot be made using Ziegler-Natta
or current single-site catalysts (coordination catalysis) because the polar comonomers
are catalyst poisons. Catalysts that allow the copolymerization of ethylene with polar
comonomers using coordination catalysis are still of academic and commercial interest.
4.6 Ionomers
Ionomers are copolymers of ethylene and methacrylic or acrylic acid that are ionically
cross-linked with zinc, sodium, or lithium cations [15, 16]. Ionomers are prepared from
acid copolymer via the partial neutralization of the acid groups using a base such as
sodium hydroxide (to form sodium ionomers) or zinc oxide (to form zinc ionomers).
Since ionomers are partially neutralized acid copolymers, they have some acid functionality enabling adhesion to foil, polyamide, and paper. Ionic functionality/crosslinking imparts unique properties such as improved optics, toughness, tensile strength,
abrasion, and oil/grease resistance. Ionomers also exhibit higher stiffness/hardness and
lower peak crystallization temperature (freeze point) at a given crystallinity, higher melt
118
Handbook of Industrial Polyethylene and Technology
strength, and hot-tack strength. DuPont is the major supplier of ionomer resins under
the trade name of Surlyn.5 Surlyn ionomers are derived from ethylene methacrylic acid
copolymers, having about 4 to 12 wt% acid level. The ionic nature of ionomers makes
them very sensitive to high humidity environments. They must be packaged in foillined bags or boxes to prevent moisture pick-up.
Ionomers are widely used as extrusion coatings or co-extruded sealants in food
packaging with an emphasis on processed meat, medical device packaging, and skin
packaging. Ionomers used as an inner sealant layer exhibit very good adhesion to processed meat, helping to minimize migration of liquids in the packaging for a more
appetizing appearance. Due to its low adhesion to HDPE, ionomers are used in delamination peel seal applications such as cereal liners. They are also used in durable applications such as golf ball skins, bowling pin covers, athletic footwear, ski boots, perfume
bottle stoppers, and the impact modification of polyamide. Ionomers have excellent
melt strength at thermoforming temperatures, desired for deep-draw thermoforming
applications. Ionomers are also used as interlayers for laminated glass in safety and
security applications (e.g., hurricane-proof glass). Such laminated glass is capable of
stopping projectiles from penetrating glass windows during a hurricane.
4.7 High Density Polyethylene (HDPE)
Polyethylene resins having a density greater than 0.940 g/cm3 are defined as HDPE.
HDPE is the highest volume type of PE used today. The need of pressures in excess
of 2000 bars (30,000 psi) for the manufacturing of LDPE resins requires thick-walled
autoclave and tubular reactors, and large compressors leading to high capital cost, high
maintenance and energy cost. As a result, a large research effort was undertaken to
enable polymerization of ethylene at lower pressures to improve the process economics. HDPE was first synthesized by Prof. Karl Ziegler of the Max Planck institute in
Germany using titanium and zirconium halides with aluminum alkyls at much milder
process conditions (much lower pressure and temperature) compared to LDPE. Prof.
Ziegler shared the Nobel Prize for his discovery with Prof. Giulio Natta, who discovered
that polypropylene can also be produced using the same catalyst. These catalysts, in
general, are referred to as Ziegler-Natta (Z-N) catalysts. Around the same time, Hogan
and Banks at Phillips Petroleum synthesized HDPE using silica/alumina supported
chromium oxide catalyst at relatively low pressures. Phillips Petroleum commercialized
chrome catalyzed HDPE resins using a loop slurry process in late 1950s [17]. Hoechst
commercialized Z-N catalyzed HDPE resins using a continuous stirred tank reactor
(CSTR) slurry process in the late 1950s. Subsequently Union Carbide developed a gas
phase process to make HDPE resins also in the late 1960s.
Completely linear HDPE resins (no comonomer) are quite brittle and prone to environmental stress cracking. To overcome this, small amounts of alpha-olefin comonomer (1-butene or 1-hexene or 1-octene) are incorporated into the PE backbone. This
decreases the density/crystallinity and increases environmental stress crack resistance
(ESCR) due to an increase in tie-chain concentration. Use of a higher alpha-olefin
5
Surlyn is a trademark of E.I. du Pont de Nemours and Company or its affiliates.
Types and Basics of Polyethylene
Weight percent
119
High MW copolymer
fraction for tie chains
for improved ESCR,
impact, burst, slow
crack growth
Low MW homopolymer
fraction for stiffness
and processability
Comonomer
content (wt%)
Mol. Wt.
Figure 4.10 Molecular weight distribution of Z-N catalyzed bimodal HDPE resin [1].
comonomer (1-hexene or 1-octene) leads to a significant improvement in ESCR compared to using 1-butene at the same final density. Subsequent advances led to the development of bi-modal HDPE resins, having improved mechanical properties using Z-N
catalysts in a dual reactor slurry or gas phase process. Such bi-modal HDPE resins have
a very high molecular copolymer fraction (using 1-hexene or 1-butene as a comonomer) and a very low molecular weight homopolymer fraction (little or no comonomer).
The very high molecular weight copolymer fraction provides improved environmental
stress crack (ESCR), impact, and slow crack growth resistance due to a high tie-chain
concentration. The very low molecular weight homopolymer fraction provides stiffness
and processability, as shown in Figure 4.10. Bimodal HDPE resins exhibit superior balance of stiffness/impact toughness/ESCR balance compared to unimodal chromium
oxide catalyzed HDPE resins. Attempts have been undertaken to produce such bimodal
resins in a single reactor, using bimodal catalyst systems (e.g., a dual-site catalyst or
bimetallic catalyst) or catalyst blends [18]. However, product design freedom is limited
compared to the use of multiple reactor systems.
Due to a high degree of crystallinity, HDPE resins exhibit very high modulus (stiffness) and excellent chemical resistance. The first very big application of HDPE was
the “hula hoop” made by a California-based toy manufacturer, the Wham-O company.
Subsequently, blow molded bottles were developed from chromium oxide catalyzed high
molecular weight high density PE (HMW-HDPE). Chromium oxide catalyzed HMWHDPE unimodal resins typically exhibit very high weight average molecular weight (MI
< 0.1 dg/min), a broad molecular weight distribution (MWD), and often a very low level
of long-chain branching. Chromium oxide catalyzed HMW-HDPE unimodal grades
for blow molding exhibit the desired parison swell, top load, and adequate environmental stress crack resistance (ESCR) and toughness. Blow molded bottles made from unimodal and bimodal HDPE resins are used in household and industrial chemical (HIC)
bottle applications such as detergent, bleach, fabric softener, and agricultural chemicals
such as pesticides and herbicides. Such bottles are also used in dairy, water, and juice
packaging applications as well as pharmaceutical, medical, and cosmetics applications.
Both unimodal and bimodal HMW HDPE resins are also widely used to make pipes for
gas and water distribution, blow molded fuel tanks, motor oil containers, in automotive
applications, and also large canisters, tanks, and drums, requiring specific approvals for
the storage and transportation of dangerous filling goods (UN certification). Bimodal
120
Handbook of Industrial Polyethylene and Technology
HDPE resins are used to make T-shirt bags used for grocery shopping applications,
and oriented tapes for woven raffia applications. A unique application of HDPE is high
moisture barrier liners for dry food packaging such as cereals, crackers, and cookies.
Such very high moisture barrier grades are broad MWD bimodal HDPE resins with a
density typically greater than 0.960 g/cm3. Other HDPE applications include injection
molded drink cups, crates, pails, small tanks, and containers. HDPE resins are used
to make monocomponent and bi-component fibers (HDPE sheath and polypropylene
or polyethylene terephthalate core) to make spunbond fabric as well as bicomponent
staple fibers for hygiene applications [19]. The Dow Chemical Company has commercialized ASPUN fiber grade resins since 1986 for spunbond fabric and staple fiber
applications.6
4.8 Ultra-High Molecular Weight HDPE (UHMW-HDPE)
Ultra-high molecular weight HDPE (UHMW-HDPE) resins have a molecular weight
greater than about three million Daltons. UHMW-HDPE has many outstanding properties such as high abrasion resistance, impact strength, chemical resistance, environmental stress-crack resistance, and a low coefficient of surface friction. However, such
polymer cannot be processed using conventional melt processing techniques in plastics
fabrication due to its extremely high melt viscosity. UHMW-HDPE is produced in a
commercial gas or slurry phase process as a powder. Suppliers of UHMW-HDPE resin
include Celanese corporation (trade name is GUR ), Mitsui Chemicals (trade name
is Mipelon), etc.7 The powder is directly compression molded, sintered and machined
into the final shape of products for some biomedical applications such as hip, knee, and
spine implants, or applications requiring low friction surfaces such as alpine skis, bearings, and gliding parts. UHMW-HDPE powder is also used as an additive to enhance
the lubricity and abrasion for compounding of rubbers and engineering plastics.
UHMW-HDPE is also converted into very high strength fibers (e.g., Dyneema from
DSM, Spectra from Honeywell, and Tekmilon from Mitsui Chemicals) using a gel
spinning process [20–23].8 A gel consisting of 0.5–10 wt% UHMW-HDPE in decalin or
xylene is processed by an extruder through a spinneret. The extrudate is drawn through
the air and then cooled in a water bath. The resulting fiber has a very high degree of
molecular orientation, and therefore exceptional tensile strength. These fibers are used
in body armor, fishing line, bow strings, and high performance sailing. UHMW-HDPE
is also used to make microporous battery separator membranes using solution processing techniques.
6
ASPUN is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow.
GUR is a trademark of Celanese Corporation; Mipelon is a trademark of Mitsui Chemicals.
8
Dyneema is a trademark of DSM; Spectra is a trademark of Honeywell International; Tekmilon is a
trademark of Mitsui Chemicals.
7
Types and Basics of Polyethylene
121
4.9 Linear Low Density Polyethylene (LLDPE)
Since Z-N and chromium oxide catalyzed HDPE resins (density > 0.940 g/cm3) exhibit
much higher stiffness and inferior dart impact, tear, and optics compared to LDPE
resins (typical density of ~0.920 g/cm3) they are not suitable for many flexible packaging applications. Hence, there were strong commercial incentives to manufacture Z-N/
Chromium oxide catalyzed PE resins using low pressure processes (low capital cost) at
lower densities (0.915 to 0.940 g/cm3), by incorporating higher levels of alpha-olefin
comonomer. These resins were termed as “linear” low density PE (LLDPE), reflecting
the lack of long-chain branching in the resins. Note that sometimes the density range
of 0.915 to 0.940 g/cm3 is sub-classified into LLDPE (0.915 to 0.930g/cm3) and MDPE
(0.930 to 0.940 g/cm3) resins. However, for the purpose of this chapter, the entire density range of 0.915 to 0.940 g/cm3 is termed as LLDPE resins.
DuPont in Canada was first to commercialize Z-N catalyzed LLDPE resins made in
a solution process in the early 1960s. However, the commercial potential of the Z-N
LLDPE resins was not realized for the next 15 years due to the poor extrusion processability and poor bubble stability of LLDPE resins compared to LDPE resins as a
result of the lack of long-chain branching. Union Carbide adapted their HDPE gas
phase process to make LLDPE resins in the late 1970s. The UNIPOL LLDPE gas phase
process was made available for worldwide licensing in 1977 to make PE resins above
0.915 g/cm3, which accelerated the commercialization of LLDPE resins [24].9 Currently,
UNIPOL gas phase process is licensed by Univation technologies. British Petroleum
announced its own gas-phase process for LLDPE in the early 1980s. This gas phase process is currently licensed by INEOS technologies under the trade name of Innovene
G.10 Initially, gas-phase LLDPE resins were made with 1-butene as the comonomer.
In 1978, The Dow Chemical Company commercialized LLDPE resins based on a
1-octene comonomer using their proprietary solution process, under the trade name
of DOWLEX .11 Octene-based Z-N LLDPE resins exhibited superior film mechanical
properties such as dart impact and tear compared to the 1-butene-based gas-phase Z-N
LLDPE resins. This led gas phase resin suppliers to develop their own LLDPE resins
using 1-hexene as a comonomer. For the chromium oxide catalyst-based slurry process,
the lowest density achievable is about 0.920 g/cm3.
Z-N LLDPE resins exhibit a relatively broad molecular weight distribution (MWD
from about 3.5 to 4.5) and a broad short-chain branching distribution (comonomer
distribution). This is due to the multi-site nature of the Z-N catalyst with differences
in each site’s ability to incorporate alpha-olefin comonomer. Catalyst sites that readily
incorporate alpha-olefin comonomer produce lower molecular weight chains as the
incorporation of alpha-olefin slows down the rate of polymerization. Catalyst sites that
do not readily incorporate alpha-olefin produce higher molecular weight chains. Thus,
Z-N catalyzed LLDPE resins are molecular blends of high molecular weight lightly
short-chain branched molecules, lower molecular weight highly short-chain branched
molecules and everything in between. Hence, LLDPE (and VLDPE/ULDPE) resins are
9
UNIPOL is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow.
Innovene is a trademark of INEOS Europe Limited.
11
DOWLEX is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow.
10
122
Handbook of Industrial Polyethylene and Technology
Melt strength of Z-N LLDPE (DOWLEX) and LDPE resins
Goettfert Rheotens Data @ 190 C
14
DOWLEX 2045
LDPE A
12
LDPE A
LDPE B
Force (cN)
10
8
LDPE B
6
Dowlex 2045
4
2
0
0
50
100
150
200
250
Velocity (mm/s)
DOWLEXTM 2045, MI = 1.0, 0.920 g/cc
300
350
LDPE A, MI = 1.0, 0.923 g/cc
LDPE B, MI = 2.0, 0.925 g/cc
Figure 4.11 Rheotens melt strength of Z-N LLDPE and LDPE resins.
classified as heterogeneous PEs. Note that HDPE and LLDPE resins produced with
chromium oxide catalysts exhibit much broader molecular weight distribution (MWD
~10 to 20) compared to Z-N catalyzed HDPE and LLDPE resins. Chromium oxide
catalyzed LLDPE/HDPE resins exhibit very good melt strength during film blowing
due to their broad MWD [25]. These resins are used in thick gauge film applications
such as geomembranes as well as a blend component in film formulations to improve
melt strength/bubble stability.
Due to lack of long-chain branching, Z-N LLDPE resins exhibit poor extruder processability (high torque, amps and back pressure) and melt strength/bubble stability
compared to LDPE resins. Melt strength of Z-N LLDPE and LDPE resins are compared
in Figure 4.11. LDPE exhibits about three times the melt strength compared to a Z-N
LLDPE resin at the same melt index (MI=1 dg/min). A 2 dg/min melt index LDPE resin
exhibits about twice the melt strength of a 1 dg/min melt index Z-N LLDPE resin.
The processing deficiencies of LLDPE resins have been systematically addressed
using lower (L/D) screws, barrier-flighted screws, improved die designs and cooling air-ring designs, and the use of wider die gaps for extrusion and film processes.
Blown film properties of LLDPE resins strongly depend upon fabrication conditions
such as blow up ratio (BUR), die gap, melt temperature, and frost line height [26].
Z-N LLDPE resins exhibit substantially improved toughness (ESCR, tensile strength,
dart impact, puncture and tear) properties compared to high pressure LDPE resins.
This has resulted in the growing use of LLDPE resins in flexible packaging applications since their introduction in the late 1970s, mainly at the expense of LDPE resins.
LLDPE/LDPE blends are also widely used in blown film processes to achieve a balance
of toughness and processability and bubble stability, to allow down-gauging and higher
rates. Many of the LLDPE/LDPE blends show a synergy in melt strength over the entire
composition range [27]. LLDPE and LLDPE/LDPE blends are used in applications such
Types and Basics of Polyethylene
123
as trash bags, heavy duty shipping sacks, food storage bags, food and frozen-food packaging, bag-in-box films, lamination films, stretch wrap, stretch hood, greenhouse films,
mulch films, silage films, breathable diaper back sheets, and garment bags. One of the
key and high volume applications of LLDPE resins is stretch films for wrapping pallets to impart pallet stability. LLDPE blown film does not exhibit cross-direction (CD)
shrinkage and cannot be used as a collation shrink film. LDPE is blended in LLDPE to
achieve the desired CD shrinkage (10–25%) and to obtain the desired “bullseye” in the
package for some of the collation shrink bundling applications. Z-N LLDPE resins are
prone to shark skin melt fracture for resins with fractional melt indices due high shear
stress levels at the die lip wall. Melt fracture and die lip build up can be mitigated with
the addition of fluoropolymer processing aids. Z-N LLDPE resins are widely used to
make double bubble oriented shrink film for retail high clarity shrink wrapping and
bundling applications. Such films exhibit balanced shrinkage in the MD and CD directions. Double bubble oriented shrink film process can be described generically by the
following steps: Extrusion-quenching-reheating-biaxial stretching-cooling. The film is
reheated after quenching to a temperature below the melting point of the LLDPE resin
and subsequently biaxially stretched followed by cooling to lock in the orientation.
LLDPE resins are also used in rotomolding applications for making toys, water tanks,
and kayaks. High melt index LLDPE resins are used to make injection molded articles
such as lids and tubs. The higher toughness and ESCR coupled with a faster molding cycle due to the higher crystallization temperatures have made LLDPE resins very
attractive for injection molding applications, displacing LDPE resins. LLDPE resins are
also used to make pipes for raised temperature applications (PERT) such as underfloor
heating applications.
Suppliers of LLDPE resins include The Dow Chemical Company under the trade
name of DOWLEX and TUFLIN , Chevron Phillips Chemical under the trade
name of MARLEX and MARFLEX , ExxonMobil Chemical, Nova Chemicals under
the trade name of SCLAIR , Westlake Chemicals under the trade name of HIFOR and
HIFOR XTREME, LyondellBasell under the trade name of Petrothene , and Borealis
under the trade name of Borstar .12
4.10 Very Low Density Polyethylene (VLDPE)
Very low density polyethylene (also known as ultra low density PE, ULDPE) resins
are Z-N catalyzed resins having densities in the range of 0.885 to 0.915 g/cm3. A lower
density/crystallinity is achieved by incorporating even higher levels of alpha-olefin
comonomer into the copolymer. A key limitation of Z-N catalysts is the inability to incorporate very high levels of alpha-olefin comonomer (1-butene, 1-hexene, or 1-octene)
to produce resins with a density less than 0.885 g/cm3. VLDPE resins exhibit improved
puncture, dart impact, tear, ESCR, low temperature toughness, heat seal, and hot-tack
12
TUFLIN is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow;
MARLEX and MARFLEX are trademarks of Chevron Phillips Chemical Company; SCLAIR is a trademark of Nova Chemicals Corporation; HIFOR is a trademark of Westlake Chemical; Petrothene is a trademark of LyondellBasell; Borstar is a trademark of Borealis AG.
124
Handbook of Industrial Polyethylene and Technology
properties compared to EVA and LLDPE resins. Applications of VLDPE resins include
flexible tubing, bag-in-box, sealants, low-temperature packaging (e.g., ice bags), barrier
shrink bags for packaging of primal and subprimal meat cuts, medical packaging, flexible hoses, and as a cling layer in stretch films. Suppliers of Z-N catalyzed VLDPE resins include The Dow Chemical Company (ATTANE and FLEXOMER ),13 Westlake
Chemical (MXSTEN ),14 and Nova Chemicals (SCLAIR ).
4.11 Single-Site Catalyzed Polyethylenes
The most recent commercially significant advancement in PE technology is the development and commercialization of ethylene homopolymers and copolymers produced
using single-site catalyst (SSC) technology (e.g., metallocene). The discovery of methylaluminoxane (MAO) as a cocatalyst to activate dicyclopendadienyl (bisCp)-based
metallocene catalysts for highly improved efficiency by Sinn and Kaminsky in 1980
marked the most significant breakthrough in the field of metallocene catalysis [28].
Three major families of high efficiency single-site catalysts (SSC) have been commercially used for the preparation of PE copolymers. These are bis-cyclopentadienyl
single-site metallocene catalyst (also known as a Kaminsky catalyst), a half sandwich, constrained geometry mono-cyclopentadienyl single-site catalyst (known as a
Constrained Geometry Catalyst, CGC, under the trademark of INSITE technology
by The Dow Chemical Company), and post-metallocene catalysts. “Post-metallocene”
refers to a class of single-site catalysts, which are not metallocenes. The use of these
catalyst technologies has allowed a very rapid development of olefin copolymers with
a wide range of structures and related properties [29–31]. Since the early 1990s, SSC
technology has initiated a major revolution in the polyolefin industry.
A key feature of single-site catalyzed PE is the narrow composition (intermolecular) and molecular weight distribution [32]. Such resins are classified as homogeneous
PE. Single-site catalysts have enabled the commercial production of homogeneous PE
resins over a broad density range of 0.857 to 0.965 g/cm3, thus overcoming the key
limitations of Z-N catalysts. These homogeneous PE resins exhibit a broad range of
morphology (from lamellar morphology at high crystallinity to granular morphology
at low crystallinity) and solid-state properties (from necking and cold drawing at high
crystallinity to uniform drawing and high elastic recovery at low crystallinity) [33]. The
morphology of 0.920 g/cm3 and 0.87 g/cm3 ethylene/octene copolymers made by constrained geometry catalyst (CGC) technology is shown in Figure 4.12.
The narrow composition (comonomer) distribution of single-site catalyzed versus
Z-N catalyzed LLDPE is readily observed in crystallization elution fractionation (CEF)
profiles as shown in Figure 4.13. The narrow molecular weight distribution of singlesite catalyzed versus Z-N catalyzed LLDPE is shown in Figure 4.14.
Homogeneous PE random copolymer resins produced by a single-site catalyst
(e.g., metallocene or constrained geometry catalyst) with a density range of 0.885 to
13
ATTANE, FLEXOMER, and INSITE are trademarks of The Dow Chemical Company (“Dow”) or an
affiliated company of Dow.
14
MXSTEN is a trademark of Westlake Chemical.
Types and Basics of Polyethylene
(a)
125
(b)
Figure 4.12 TEM micrographs of 0.920 g/cm3 (A) and 0.87 g/cm3 (B) ethylene/octene homogeneous PE
made using constrained geometry catalyst (CGC) technology [3].
9
8
AFFINITY PL1880G
DOWLEX 2045G
Mass (dWf/dT)
7
6
5
4
3
2
1
0
20
30
40
50
60
70
80
90
100
110
120
–1
Temperature (°C)
Figure 4.13 Crystallization elution fractionation (CEF) of SSC (AFFINITY PL1880G – 1 dg/min MI,
0.902 g/cm3) vs. Z-N catalyzed LLDPE (DOWLEX 2045 – 1 dg/min MI, 0.920 g/cm3) resins.
0.910 g/cm3 are known as polyolefin plastomers (POP), and copolymers with density
below 0.885 g/cm3 are known as polyolefin elastomers (POE). Several families of SSC
technology-based PE random copolymers have been commercialized since the 1990s.
These include polyolefin elastomers (e.g., ENGAGE from The Dow Chemical Company,
TAFMER from Mitsui Chemicals),15 polyolefin plastomers (e.g., AFFINITY from
The Dow Chemical Company; EXACT from ExxonMobil Chemicals, Queo from
Borealis, KERNEL from Japan Polychem Corporation),16 EPDM (NORDEL IP
from The Dow Chemical Company),17 enhanced PEs such as ELITE and ELITE
AT from The Dow Chemical Company, SURPASS single-site catalyzed resins from
15
TAFMER and Evolue are trademarks of Mitsui Chemicals.
EXACT, Exceed and Enable are trademarks of ExxonMobil Corporation; Queo is a trademark of
Borealis AG; KERNEL is a trademark of Japan Polychem Corporation.
17
NORDEL is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow.
16
126
Handbook of Industrial Polyethylene and Technology
1.2
Mn
1
Mw
AFFINITYPL 1880G 38,594 86,598
DOWLEX2045G
MWD
2.24
25,981 111,612 4.30
dWf / {dLogM}
0.8
AFFINITY PL 1880G
DOWLEX 2045G
0.6
0.4
0.2
0
2.00
3.00
4.00
5.00
Log (molecular weight)
6.00
7.00
Figure 4.14 Gel Permeation Chromatography (GPC) of SSC POP (AFFINITY PL1880G – 1 dg/min MI,
0.902 g/cm3) vs. Z-N catalyzed LLDPE (DOWLEX 2045G – 1 dg/min MI, 0.920 g/cm3) resins.
Nova Chemicals,18 NEXLENE single-site catalyzed resins from SK Chemicals;19 gas
phase metallocene LLDPE/VLDPE (EXCEED and ENABLE from ExxonMobil
Chemicals, EVOLUE from Mitsui Chemical, HARMOREX from Japan Polychem
Corporation); slurry metallocene LLDPE/VLDPE (mPACT from Chevron Phillips
Chemical Company, Lumicene from Total Petrochemicals)20.
Some of the POP/POE (e.g., AFFINITY, ENGAGE, Queo) and mLLDPE resins
have a small amount of long-chain branching and are referred to as substantially linear
homogeneous PEs. The polymer with a vinyl chain-end (formed due to chain termination via a beta-hydride elimination mechanism) gets incorporated into a growing
polymer chain (in addition to ethylene and a-olefin), leading to the formation of longchain branching. Substantially linear homogeneous PE resins exhibit improved shear
thinning (e.g., as measured using I10/I2 or I21/I2 ratio) and extruder processability due to
the presence of a small amount of long-chain branching [34].
The narrow composition distribution (short-chain branching distribution) of singlesite catalyzed homogeneous PE leads to a lower melting point compared to heterogeneous Z-N catalyzed LLDPE/VLDPE resins at a similar density. This is illustrated
in Figure 4.15. Homogeneous octene-based POP resin (AFFINITY PL1880) exhibit a
sharp/single melting point (with a shoulder at lower temperature) versus a broad melting distribution of Z-N catalyzed octene-based VLDPE resin (ATTANE 4203). Note that
the heat of fusion (area under the melting curve) of both resins is similar due to similar density. Single-site catalyzed POP resins exhibit narrow crystallite sizes (thickness)
18
SURPASS is a trademark of Nova Chemicals.
NEXLENE is a trademark of SK Chemicals.
20
mPACT is a trademark of Chevron Phillips Chemical Company; Lumicene is a trademark of Total
Petrochemicals.
19
Types and Basics of Polyethylene
1
AFFINITY PL 1880
0.902 g/cc
0.8
Heat flow (watts/gram)
127
ATTANE 4203
0.905 g/cc
Tm = 98 °C
Tm = 123 °C
0.6
0.4
0.2
0
30
40
50
60
70 80 90 100 110 120 130 140
Temperature (°C)
Figure 4.15 DSC comparison of ethylene/1-octene POP (AFFINITY PL1880) and Z-N catalyzed
VLDPE (ATTANE 4203) resin at similar density.
1.6
POP, 0.909 g/cc
Z-N LLDPE
0.922 g/cc
Heat flow (Watts/gm)
1.4
POP, 0.902 g/cc
1.2
1
POP, 0.896 g/cc
0.8
0.6
Z-N VLDPE
0.912 g/cc
POE, 0.885 g/cc
POE, 0.870 g/cc
0.4
0.2
–20 –10 0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
Temperature (°C)
Figure 4.16 DSC melting curves of SSC (POP and POE) and Z-N catalyzed LLDPE /VLDPE ethylene/
octene copolymers at various densities.
distribution (due to narrow composition distribution) and hence a lower melting point.
Z-N catalyzed LLDPE/VLDPE resins exhibit a broad crystallite size (thickness) distribution due to the presence of a chain fraction having a higher density (low comonomer
incorporated) resulting in thicker crystallites exhibiting a high melting point. Note that
the melting point of a lamella is related to the thickness of the lamella.
DSC melting curves of various octene-based POP and POE resins are compared
with Z-N catalyzed octene-based LLDPE/VLDPE resins in Figure 4.16 (based on
128
Handbook of Industrial Polyethylene and Technology
140
SSC
SSC PE, LLDPE/VLDPE, LDPE & EVA RESINS
SSC
Regression
Melting peak temp. (°C)
120
LLDPE/VLDPE
100
Heterogeneous
LDPE
9% VA
12% VA
18% VA
EVA
80
Homogeneous
60
40
0.86
0.88
0.9
0.92
Density (g/cc)
28% VA
0.94
0.96
Figure 4.17 Melting point of 1-octene-based SSC, Z-N LLDPE/VLDPE, LDPE, and EVA resins as a
function of density [3].
ASTM D-3417 – cooling and heating rate of 10 °C/min). The melting peak temperature
of POP/POE resins decreases with density whereas the melting peak temperature of
Z-N catalyzed LLDPE/VLDPE resins is insensitive to density (due to broad composition distribution).
A plot of melting points of octene-based SSC resins (POP/POE), octene-based Z-N
LLDPE/VLDPE (except resin at 0.885 g/cm3, which is a butene-based Z-N FLEXOMER
VLDPE made in a gas phase process), LDPE, and EVA resins as a function of density is
shown in Figure 4.17.
All samples were cooled and heated at 10 °C/min using a Perkin-Elmer DSC-7. The
LDPE resins have slightly lower melting peak temperature compared to octene-based
SSC resins (made using CGC technology) at a given density. The density of EVA resins increases as vinyl acetate content increases, even though the degree of crystallinity
and melting peak temperature decreases. This is due to the bulky nature of the acetoxy
group increasing the amorphous phase density, as mentioned earlier. Therefore, the
density of EVA resins cannot be compared with the density of ethylene/alpha-olefin
copolymer resins. A plot of melting points of LDPE, LLDPE/VLDPE and SSC ethylene/octene copolymers, and EVA resins as a function of resin crystallinity is shown
in Figure 4.18. Melting points of EVA resins are very close to that of SSC PE resins at
the same crystallinity. The optical properties of LLDPE resins (both Z-N and singlesite catalyzed) depends upon molecular weight and composition distribution and can
be significantly affected by LDPE addition, especially for single-site catalyzed PE. For
a wide variety of PE blown films, haze shows a complex parabolic relationship with
the logarithm of the recoverable shear strain parameter (melt elasticity) [35]. The large
majority of the contribution to the total haze in blown and cast films is observed to
come from the surface roughness of the films, with the bulk (internal) contribution
being relatively minor [11, 36, 37].
Types and Basics of Polyethylene
140
SSC
SSC PE, LLDPE/VLDPE, LDPE & EVA RESINS
0.922 g/cc 0.928 g/cc 0.937 g/cc
0.907 g/cc
Regression
120
Melting peak temp. (°C)
129
LLDPE/VLDPE
0.914 g/cc
0.885 g/cc
LDPE
4% VA
100
9% VA
12% VA
EVA
18% VA
80
SSC
28% VA
60
40
0
10
20
30
40
50
60
70
80
Wt% crystallinity
Wt% crystallinity = heat of fusion/292 (J/gm) * 100
Figure 4.18 Melting point of octene-based SSC (POP/POE), Z-N LLDPE/VLDPE, LDPE, and EVA
resins as a function of DSC crystallinity [3].
Compression molded plaques
Young's modulus (Psi)
105
POP
10
4
POE
Single-Site
LLDPE/VLDPE
Single-Site
103
Regression
102
0.84
0.86
0.88
0.90
Density (g/cc)
0.92
0.94
Log(Modulus) = -130.0787+270.267*den-134.832*den*den, r
0.96
2=0.99
Figure 4.19 Plot of Young’s modulus of nonpolar PE resins as a function of density (single-site and
Ziegler-Natta catalyzed resins). Filled circles are constrained geometry catalyst-based single-site resins.
Filled triangles are bis-cyclopentadienyl metallocene catalyst-based single-site resins.
Figure 4.19 shows the plot of Young’s modulus of compression molded plaques of
single-site and Ziegler-Natta catalyzed resins as a function of density. It can be seen
that Young’s modulus of nonpolar PE resins is primarily a function of resin density
[26]. Mechanical properties such as impact and tear strength of single-site (and Z-N)
catalyzed resins having 1-octene as a comonomer are far superior compared to the
resins having 1-butene as a comonomer [38, 39]. The mechanical properties of resins
having 1-hexene as a comonomer are closer (but still lower) to resins having 1-octene
as a comonomer [39].
130
Handbook of Industrial Polyethylene and Technology
Heat sealing is a key functionality in packaging applications [40, 41]. The lower melting point of homogeneous PE (POP) at the same density is advantageous in key application functionalities such as heat sealing and heat shrinkage. Before the 1990s, EVAs and
ionomers were the primary choice for heat sealant layers due to their low melting point
and low heat-seal-initiation temperatures. However, EVA resins exhibit very low hot-tack
strength due to the presence of high levels of long-chain branching inhibiting molecular
diffusion across the molten interface. They can also have taste and odor issues due to
poor thermal stability during extrusion. Ionomers exhibit higher hot-tack strength compared to EVA resins due to the presence of ionic domains and were predominantly used
where higher hot-tack strength was needed such as in vertical form fill and seal (VFFS)
machines. However, ionomers are more expensive resins than EVA. Introduction of
single-site catalyzed POPs in the 1990s exhibiting lower heat-seal and hot-tack initiation temperatures (due to lower melting points), and lower levels of extractibles to meet
FDA direct food contact regulations, has allowed the packaging industry to extensively
use POPs in the sealant layers of various packaging applications. The lower heat-seal
and hot-tack initiation temperature allows faster packaging line speeds and therefore
improved productivity. Single-site catalyzed POP resins also exhibit significantly higher
hot-tack strength compared to EVA. This is illustrated in Figure 4.20 for AFFINITY
POPs of various melt index and densities. Single-site catalyzed POP resins also exhibit
low heat seal temperatures comparable to EVA resins, as shown in Figure 4.21. As a
result, these POPs are widely used for sealant applications (e.g., in VFFS machines).
These resins are also used in fresh product packaging applications (e.g., fresh cut salad)
due to their high oxygen transmission rate (OTR). High melt index (MI 8 to 20 dg/min)
POPs are blended with LDPE for use in extrusion coated sealants for packaging applications such as sachets. POPs are used for the impact modification of polypropylene for
clear, tough polypropylene-based containers (impact needed at typically > 0 °C). The
rapid growth of multilayer packaging applications in the last 20 years has led to significant growth in the usage of single-site catalyzed POP resins as sealants.
Single-site-catalyzed (e.g., metallocene) LLDPE/VLDPE resins exhibit improved
dart impact and puncture, and improved optics (especially when blended with LDPE),
compared to standard Z-N LLDPE/VLDPE resins. These resins are widely used in flexible packaging applications such as stretch wrap film, heavy duty shipping sacks, food
and frozen-food packaging, lamination films, bag-in-box films, stretch hood, greenhouse film, mulch film, and silage film, as well as hygiene applications such as breathable diaper back sheets.
Another key feature of single-site catalysts is their ability to incorporate a very high
level of alpha-olefin comonomers to make polyolefin elastomers (POE). Such low density POE resins exhibit very low modulus and low Shore A hardness for enhanced flexibility and soft touch, very low glass-transition temperature, and high elastic recovery.
Before the 1990s, ethylene-propylene rubber (EPR) and ethylene-propylene-diene
monomer (EPDM) resins based upon Ziegler vanadium catalysts were predominantly
used in applications requiring high UV and oxidation resistance as well as compatibility
with polypropylene. Thus EPDM materials were used as impact modifiers for polypropylene resins. Since the 1990s, metallocene POE resins with densities from 0.857 to
0.87 g/cm3 have found wide commercial acceptance as impact modifiers for polypropylene to make thermoplastic olefins (TPO) primarily used in automotive applications.
Types and Basics of Polyethylene
16
AFFINITY POP
1 MI, 0.908 g/cc
14
Hot-tack strength (N/inch)
131
AFFINITY POP
1 MI, 0.902 g/cc
12
AFFINITY POP
1.6 MI, 0.896 g/cc
10
EVA
0.4 MI, 12% VA EVA
8
EVA
0.8 MI, 18% VA EVA
6
4
2
0
60
70
80
90
100
110
Temperature (°C)
120
130
140
Figure 4.20 Hot-tack curves of ethylene/octene AFFINITY POP vs. EVA resins. Polyamide/EAA/Sealant
(1/1/1.5 mil) blown co-extruded film structure was used [3].
14
AFFINITY POP
0.908 g/cc
Seal strength (lb/inch)
12
AFFINITY POP
0.902 g/cc
10
AFFINITY POP
0.896 g/cc
8
EVA
0.4 MI, 12% VA EVA
6
EVA
0.8 MI, 18% VA EVA
4
2
0
60
70
80
90
100
Temperature (°C)
110
120
130
Figure 4.21 Heat seal curves of ethylene/octene AFFINITY POP vs. EVA resins. Polyamide/EAA/
Sealant (1/1/1.5 mil) blown co-extruded film structure was used [3].
Both 1-octene – and 1-butene-based POE resins are used in these applications. POEs
made using 1-octene as a comonomer exhibit the lowest glass transition temperature and
highest melting point (e.g., DSC Tg ~ −55 °C for 0.87 g/cm3 ethylene/octene POE) and
best compatibility with polypropylene resins compared to either ethylene/butene copolymers (EB) or EPR and EPDM resins, leading to improved low-temperature toughness
and stiffness balance in TPO resins. Availability of POE resins in pellet form vs. bales
for EPDM and EPR grades is also advantageous for compounding with polypropylene.
132
Handbook of Industrial Polyethylene and Technology
Table 4.1 Features, benefits, and applications of ethylene-based plastomers (POPs) and elastomers (POEs).
Features
Benefits
Applications
Narrow Composition
Distribution (CD) and
Lower Tm at a given density
Excellent optics
Low heat seal initiation
temperature (HSIT)
Excellent hot-tack
strength
Sealants
Packaging
Shrink film
Controlled Long-Chain
Branching (LCB)
(Substantially Linear
homogeneous PE)
Improved processability
Extrusion
Ability to make very low
density ~0.86 g/cc
Soft and Flexible due to
lower modulus
High elastic recovery
Impact modification of
PP (TPO)
Elastic films, Fibers
Soft/Flexible goods
With the rapid growth of TPO in the last 15 years, especially in automotive applications such as bumper fascia and instrument panels, the use of POE resins, especially
those with 1-octene as a comonomer, for the impact modification of polypropylene has
increased significantly. POE resins have also found commercial acceptance in soft and
flexible goods, footwear (e.g., cross-linked foam for midsoles), adhesives, cling layers in
stretch films, and elastic films and fibers. Features, benefits and applications of ethylenebased plastomers (POPs) and elastomers (POEs) are summarized in Table 4.1.
The Dow Chemical Company has developed high melt index single-site catalyzed
resins (melt index from 500 to 1200 dg/min, density less than 0.875 g/cm3), under the
trade name of AFFINITY GA, for hot melt adhesive applications as an alternative to
EVA. These resins exhibit much improved thermal and oxidative stability compared
to EVA. Such metallocene-based HMAs give clean and char-free operation leading to
reduced maintenance cost such as time and money to change filters and nozzles, excellent viscosity stability, improved color, and virtually no smoke/odor; all of these lead
to reduced total system cost. AFFINITY GA-based HMA also exhibits aggressive bond
strength over a wide range of service temperatures versus EVA resins due to the lower
Tg of 1-octene-based AFFINITY GA resins.
Single-site catalysts are also used along with modern manufacturing techniques to
yield very high purity, high consistency EPDM elastomers. In addition to ethylene and
propylene incorporation, EPDM elastomers also incorporate dienes as termonomers,
having a pendant unsaturated olefin group to enable sulfur curing. The most commonly used diene is ethylidene norbornene (ENB). Due to the allylic hydrogens associated with ENB, it reacts readily with sulfur and sulfur donors. Less commonly used
are dicyclopentadiene (DCPD) and vinyl norbornene (VNB). VNB exhibits better cure
response to peroxide than ENB, but sulfur cure response is poorer. Figure 4.22 illustrates commercially used dienes for EPDM. Incorporation of the diene termonomer
Types and Basics of Polyethylene
Ethylidene
norbornene
Dicyclopentadiene
133
Vinyl
norbornene
Figure 4.22 Commercially used polymerizable dienes as termonomer for EPDM elastomer.
enables EPDM to exhibit a high degree of cure reactivity for traditional thermoset elastomer applications. The alpha-olefin incorporating capability of metallocene catalysts is
employed to incorporate from 30 to 60 wt% propylene to create semicrystalline to fully
amorphous elastomers.
4.12 Olefin Block Copolymers (OBC)
In the case of the single-site catalyzed random copolymer of ethylene and alpha-olefin
copolymers, incorporating more comonomer along the polymer backbone reduces density, decreases the melting point, and decreases the crystallization peak temperature. As
a result, the heat resistance decreases and cycle times in injection molding increases for
POE. These deficiencies have limited the use of POEs in applications where heat resistance, high temperature compression set, and faster cycle times are desired.
The most recent advancement in PE technology is by The Dow Chemical Company,
overcoming most of the deficiencies of POEs via the introduction of olefin block copolymers (OBCs), commercialized under the trade name of INFUSE . OBCs are block
copolymers of ethylene and alpha-olefin comonomers arranged into alternating “soft”
and “hard” blocks [4]. Soft blocks contain a high level of alpha-olefin comonomer (e.g.,
1-octene) and have a low density/crystallinity and a low melting point. Hard blocks
contain almost no or a very low level of alpha-olefin comonomer and have a high density and crystallinity, and high melting and crystallization temperatures. The soft blocks
deliver flexibility/softness and the hard blocks deliver improved heat resistance and
compression set at 70 °C and faster cycle time via a higher crystallization temperature. Therefore, OBCs combine flexibility and softness properties of POEs but with an
improved heat resistance, elastic recovery, compression set at 70 °C, abrasion resistance,
and faster cycle/setup times.
In order to shuttle or transfer growing chains between two distinct catalysts having
different comonomer (alpha-olefin) selectivity, OBCs are made using a chain-shuttling
agent (CSA). This is shown in Figure 4.23. OBCs are produced in a continuous solution
polymerization process.
The DSC melting curves and peak melting temperatures of OBCs are compared with
those of POE random copolymers in Figure 4.24. It can be seen that at the same density, OBCs exhibit a much higher melting point compared to POP and POE random
copolymers, resulting in an improved heat resistance. The melting point of OBC resins
is almost independent of density due to the presence of high density blocks which melt
at around 120 °C.
Handbook of Industrial Polyethylene and Technology
134
Figure 4.23 Catalytic block technology used to make OBCs [1].
130
OBC (1MI, 0.877 g/cc)
120
OBC (5 MI, 0.865 g/cc)
0.7
Random (3 MI, 0.875 g/cc)
0.6
Random (0.5MI, 0.863 g/cc)
110
Melting temperature (C)
Heat Flow, W/g
0.8
Density versus peak melting temperature
DSC melting curves
0.9
0.5
0.4
0.3
0.2
90
80
70
60
0.1
0
–60
100
50
–10
50
Temperature, C
90
140
40
0.865
OBCs
Random copolymers
0.875
0.885
Density (g/cc)
0.895
Figure 4.24 DSC melting curves and peak melting temperature of OBCs and POE/POP as a function of
density [1].
The DSC crystallization curves and crystallization peak temperatures of OBCs are
compared with POE random copolymers in Figure 4.25. Again, it can be seen that, at
the same density, OBCs exhibit a much higher crystallization peak temperature compared to POP and POE random copolymers. This results in much faster cycle time for
OBCs than for POEs in injection molding applications and much faster setup time in
profile extrusion applications. The OBCs also exhibit improved elastic recovery (lower
permanent set) compared to POEs, as shown in Figure 4.26. OBCs at 0.865 g/cm3 density exhibit elastic recovery similar to styrenic block copolymers such as Kraton G1657
(from Kraton Polymers). Applications of OBC resins include elastic film and laminate,
Types and Basics of Polyethylene
Density versus peak crystallization temperature
110
DSC cooling curves (10C/min)
0
100
Crystallization temperature (C)
–0.2
Heat flow, W/g
–0.4
–0.6
–0.8
–1
–1.2
OBC (1MI, 0.877 g/cc)
OBC (5MI, 0.865 g/cc)
–1.4
135
90
80
70
60
50
Random copolymers
40
Random (3MI, 0.875 g/cc)
OBCs
Random (0.5 MI, 0.863 g/cc)
–1.6
–50 –30 –10 10 30 50 70
Temperature, C
90 110
30
0.865
0.875
0.885
Density (g/cc)
0.895
Figure 4.25 DSC crystallization curves and peak crystallization temperature of OBCs and POE/POP as a
function of density [1].
300
Random copolymer: 0.870 g/cc, 5 MI
Permenant set (%)
250
SEBS
OBC: 0.865 g/cc, 0.8 MI
200
150
100
50
0
0
100
200
300
400
Applied strain (%)
500
600
Figure 4.26 Permanent set of OBC, ethylene/octene random copolymer (POE) and SEBS resins as a
function of applied elongation.
profile extrusions, soft and flexible goods, soft touch over-moldings, and adhesives
applications. OBCs have expanded the competitive space for polyolefin elastomers
against a range of flexible materials and compete with high-value styrenic block elastomers (SBS, SIS, and SEBS).
In 2013, Dow introduced a new addition to the OBC family. These new products are
marketed as propylene-based OBCs and have the INTUNE Olefin Block Copolymers
trade name.21 INTUNE Olefin Block Copolymers contain segments of both isotactic
PP and PE, bridging the gap between the two largest volume thermoplastic polymers
produced today. Because PP and PE are immiscible, blends of these key thermoplastics
21
INTUNE is a trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow.
136
Handbook of Industrial Polyethylene and Technology
Table 4.2 Summary of density, melting point, degree of crystallinity, and year of commercial
introduction of major types of PE resins.
Polyethylene technology
Melting point
(°C)
% Crystallinity
Year
0.915–0.931
106–120
45–56
1939
EVA
0.93–0.95*
40–105
5–40
1955
HDPE
0.94–0.967
125–134
62–80
1955
LLDPE
0.915–0.94
120–125
45–62
1975
VLDPE
0.885–0.915
118–122
23–45
1983
SSC PE
0.857–0.967
40–134
2–80
1991
Type
Density (g/cm3)
LDPE
*Density of EVA is higher due to bulky vinyl acetate comonomer. Hence, EVA density cannot be compared
to nonpolar polyethylenes.
typically have poor physical properties, and engineered multilayered structures with PP
and PE show interlayer adhesion failures due to incompatibility. The use of INTUNE
OBCs expands the range of useful blends and resolves interlayer adhesion issues in
multilayered structures, facilitating unique combinations of stable properties in fabricated articles and engineered multilayered systems.
4.13 Concluding Remarks
Ethylene homopolymers and copolymers have a long and rich history of product,
process, and fabrication innovations that has met the growing market needs over the
last 75 years. Many different types of ethylene-based resins have been developed with
broad performance ranges, to meet the requirements of a variety of applications, and
as a result, PE is the highest volume plastic sold today. Ethylene homopolymers and
copolymers constitute a broad and diverse family of large-volume commodity and differentiated resins, exhibiting a very wide range of properties, covering rigid plastics to
elastomers and nonpolar to polar copolymers. Table 4.2 provides a brief summary of
some of the key product categories and lists density, melting point, degree of crystallinity, and year of commercial introduction. This very wide range of properties is accomplished by molecular design, primarily enabled by catalysts, and has led to high growth
rates in applications and the usage of these polymers, especially in flexible and rigid
packaging. Our industrial world would indeed be much poorer without them.
Acknowledgments
The author would like to thank his colleagues Herbert Bongartz, Shaun Parkinson,
Hrishikesh Munj, Teresa Karjala and Sanjib Biswas for their critical review and suggestions.
Types and Basics of Polyethylene
137
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