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light weight, and superior aesthetics (as compared with

foil trays).

Copolymers

Commercially available copolymers offer improved melt

strength for the vertically positioned extrusion blow-mold-

ing process. They are used to some extent for bottles, and

to a greater extent as extruded sheet for blister-pack

applications. Since the FDA regulation permits certain

copolymers for food-contact use, new applications include

packages for noncarbonated drinks, cooking oil, and vita-

min preparations.

In India, work on a blend of PET with poly(trimethy-

lene naphthalate) is being used for beer bottles. This blend

exhibits 18 times the carbon dioxide barrier of PET and 9

times the barrier to oxygen (4).

Hot-ﬁll applications is a growing market. PET suffers

from a low glass-transition temperature (701C), which

causes severe bottle distortion if the contents of the bottle

approaches this temperature. A recent approach to the

hot-ﬁll problem is by DuPont. An amorphous isosorbide

homopolyer is blended with PET. This blend raises the

glass-transition temperature to 941C (5).

Polymer options, manufacturing processes, and appli-

cations are summarized in Table 2.

The manufacturing process of PBT is very similar to

that of PET. Instead of EG, 1,4-butandiol

OH])—CH—(HO[

is used to react with either TPA or DMT. PBT is rarely

used in packaging applications.

RECYCLING

About 1 billion pounds of postconsumer PET containers

were recycled in 2004, a recycling rate of 21.4%. Cleaning

of the recovered plastic comprises washing, rinsing,

and drying. PVC is a common impurity in PET. Melting

PET containing PVC will produce black spots due to

charring. It is difﬁcult to separate the two polymers after

the bottles have been ground into small particles. Froth

ﬂotation has been shown to be an effective means of

separation. Originally, PET was used exclusively for

packaging soft drinks. As a result, the waste stream was

fairly homogeneous consisting of unpigmented 1- and 2-L

bottles. However, PET is now used to package a wide

variety of beverages including water, fruit juices, tea,

and beer plus other products such as edible oils, butter,

and ketchup. These containers come in a variety of

sizes and shapes, and various label adhesives and barrier

resins are combined with the PET. Careful sorting of these

containers is needed and can reduce the quality and yield

of product. In the United States, PET from recycled bottles

is used primarily in ﬁber applications. Recycled PET for

food containers typically requires FDA approval. Two

tertiary recyling technologies have been used: methanoly-

sis and glycolysis. Suitability for food contact is a critical

property in selling recycled PET. The FDA has issued

letters of nonobjection to operators using methanolysis

or glycolysis processes. Two-liter soda bottles containing

recycled PET manufactured by glycolysis or methanolysis

are widely available. The FDA has also issued a letter

of nonobjection for a trilayer PET material having a middle

layer sandwiched between two virgin layers of PET (6).

BIBLIOGRAPHY

‘‘Polyesters, Thermoplastic’’ in A. J. Brody and K. S. Marsh, eds.,

The Wiley Encyclopedia of Packaging Technology, 2nd edition,

Wiley, New York, 1997, pp. 742–745.

Cited Publications

1. A. J. East and co-workers, ‘‘Polyesters, Thermoplastic’’ in Kirk–

Othmer Encyclopedia of Chemical Technology, Vol. 20, 5th

edition, Wiley, Hoboken, NJ, 2006.

2. Plastics World (September 22, 2003).

3. P. Aspy and co-workers, ‘‘Controlled crystallization—PET Co-

polyester,’’ Paper presented at the Seventh Annual Interna-

tional Conference on Oriented Plastic Containers, Atlanta,

March 1983, Ryder Associates, Whippany, NJ, 1983.

4. ‘‘New PET Alloy Makes Monolayer Beer Bottle,’’ Asia Paciﬁc

News, September 25, 2005.

5. J. Bradenberg and R. A. Hayes, U.S. Patent 6,818,730 (to E. I.

du Pont de Nemours & Co., Inc.), November 16, 2004.

6. J. K. Borchardt, ‘‘Recycling, Plastics’’ in Kirk–Othmer Encyclo-

pedia of Chemical Technology, Vol. 21, 5th edition, Wiley,

Hoboken, NJ, 2006.

Table 2. PET Polymer Resin Options for Different Manufacturing Processes and Applications

Manufacturing Process or Application (PET

Resin Type)

Standard

Homopolymer

Standard Bottle

Resin

Slow-Crystallizing

Copolymer

Fast-Crystallizing

Homopolymer

Injection blow-molded bottles (nonoriented) X

Injection-stretch blow-molded bottles (biaxially

oriented)

Extrusion blow-molded bottles X

Sheet extrusion

XXX

Film casting

(nonoriented) XXX

Biaxially oriented ﬁlm X

Heat-set ﬁlm X

Crystallizable PET trays X

PET coating for paperboard X

Resin option depends in desired application.

978 POLYESTERS, THERMOPLASTIC

POLYETHYLENE, HIGH-DENSITY

STEPHEN J. CARTER

Solvay Polymers, Inc.

Deer Park, Texas

Updated by Staff

INTRODUCTION

High-density polyethylene (HDPE) is a semicrystalline

polymer, conveniently deﬁned by ASTM as having a

density of 0.941 or greater, with a typical upper limit of

0.965. For commercial purposes, HDPE is described as a

homopolymer when the density is Z0.960, but as a

copolymer when the density is below this ﬁgure. The

polymer is available in a wide range of molecular weights

as determined by either MI or HLMI (melt index or high-

load melt index).

HDPE is one of the largest-volume plastics used in

packaging for the simple reason that it can be successfully

employed in numerous transformation processes. Blow

molding and injection molding account for 45–54% of total

consumption of HDPE (1). Table 1 lists some of the major

processes used and the products that can be obtained from

them.

MANUFACTURE

HDPE is commonly sold in pellet form and is manufac-

tured in a two-stage process. Polymerization takes place in

a reactor on a continuous basis using either of two types of

catalyst: Ziegler or Phillips. The choice of hardware today

is limited to the slurry and gas-phase processes, the

solution process having largely been bypassed on both

cost and environmental counts, except for specialty

grades. The slurry (Phillips) process was developed by

the Phillips Petroleum Company and was licensed world-

wide in the 1960s (2). Figure 1 shows a simpliﬁed view of

the process. Puriﬁed ethylene, isobutane (the slurry car-

rier), activated catalyst, and any comonomer are circu-

lated within loop-style reactors under relatively narrow

ranges of pressure and temperature. At periodic intervals,

portions of slurry are withdrawn and separated into

unreacted materials, carrier, and HDPE. The raw polymer

powder is dried and then stored in tanks until com-

pounded in an extruder, together with the requisite

additives (stage 2 of the process).

The newer gas-phase process was originally developed

by Union Carbide in the 1970s and was named Unipol; BP

Chemicals licensed a competing process about 10 years

later. The gas-phase hardware dispenses with liquid

carrier and, as the name suggests, permits direct polymer-

ization of ethylene using living polymer as a ﬂuid-bed

medium and using ethylene as the ﬂuidizing gas. From a

distance, a GP reactor looks very much like a water tower

because of the elongated bulb sitting on top of the cylind-

rical tower housing the ﬂuid bed. The bulb is the portion of

the reactor where ﬁnes deentrainment takes place. Newer

world-scale HDPE plants are typically rated at 400–

500 MM lb/yr and are composed of one or more gas phase

units closed-coupled to twin-screw compounding trains.

Whatever the hardware, the core of the polyoleﬁn

manufacturing operation is the catalyst. Ziegler formula-

tions are traditionally based on combinations of aluminum

alkyls with titanium compounds, often with proprietary

modiﬁers to improve productivity, comonomer incorpora-

tion, or other parameters. The catalyst may be chemically

or physically attached to a variety of bases (e.g., silica or

magnesium chloride) to improve productivity and ﬂow-

ability through controlled particlesize distribution (many

catalysts are self-supporting). Ziegler catalysts tend to be

unrestricted in molecular-weight (MW) terms, although

they produce relatively narrow molecular-weight distribu-

tions (MWD). Phillips catalysts, on the other hand, tend to

be restricted to fractional melt-index (MI) grades, produ-

cing broader MWD HDPE with better environmental

stress-crack resistance (ESCR) properties (3).

In the realm of catalysts, the most recent development

has been the commercialization of the metallocene type.

Metallocene resins have been used to produce extremely

high-clarity resins (4). New non-metallocene bimetallic

catalysts are being developed to produce blow-molding

resin a a single reactor (5).

MOLECULAR STRUCTURE

There are four principal features of HDPE that affect its

packaging and processing properties: density, degree and

type of branching, molecular weight, and molecular-weight

distribution. As comonomer is introduced into the reactor

to control density (typically 1-butene or 1-hexene), short-

chain branching (SCB) is incorporated into the backbone of

the polymer (catalysts can also create some SCB in the

Table 1. Examples of Packaging Applications

Conversion Process Examples of Packages

Blow molding 0.1 to 5.0-gal juice, water, or milk bottles

Bottles containing pet food to bleach

industrial 55-gal drums

Bottles for cosmetics and

pharmaceuticals

Injection blow

molding

Bottles for cosmetics, personal care,

pharmaceuticals

Injection molding Caps and closures

Pails and buckets

Thin-walled dairy cups/tubs

Crates

Foamed packaging

Blown or cast ﬁlm Multiwall bag liners

Grocery, merchandise or produce bags

Breakfast-cereal wrappers

Snack-food wrappers

Thermoforming Prepackaged-food containers

Dairy cups and containers

Tubs for personal care, hygiene items

Pallets

Sheet extrusion Foamed sheet for packaging

Rotational molding Water and chemical tanks and shipping

containers

POLYETHYLENE, HIGH-DENSITY 979

absence of comonomer). The nature and frequency of the

SCB induce major disruptions in the HDPE crystallites,

with the result that the density and degree of crystallinity

are decreased. As little as one ethyl branch per 1000

carbon atoms can produce a density change of 0.01. As

implied, the density of HDPE enjoys a rough linear

relationship with regard to crystallinity. Commercial poly-

mers exhibit degrees of crystallinity of 70–80% at 0.960,

depending on the MW (MI) and type of SCB. A drop of 10%

in crystallinity as one progresses from 0.960 to 0.940 is

typical. There is increasing evidence that the ‘‘blockiness’

of the branches is as important as the overall frequency (6).

New information has been obtained using temperature-

rising-elution fractionation (TREF), a relatively new tech-

nique for studying SCB that is independent of molecular

weight and that fractionates polymers according to crystal-

linity, composition, and tacticity differences.

As a consequence of SCB, the morphology of HDPE in

the solid state consists of three very different regions. The

amorphous region contains polyethylene chains that are

randomly oriented and acrystalline in nature, while the

crystalline regions contain HDPE chains that are tightly

packed in folds to yield differing types of geometries.

Joining the two regions together is the interface. The

crystalline regions imbue PE with its rigidity, tensile

strength, and lack of permeability toward small molecules,

while the extent of the amorphous areas affect impact

resistance and ESCR.

Table 2 lists some of the major properties associated

with changes in density, MW, and MWD. An increase in

density causes a rise in tensile strength at yield, stiffness,

and chemical resistance, while a decrease improves both

ESCR and impact resistance. This results in compromises

for many applications. For example, a 1-gal milk jug is

manufactured from a homopolymer to extract the highest

levels of top-load and lightweighting capability, while a

bottle containing industrial chemicals (bleach, detergent,

etc.) is blown from a lower-density resin to take advantage

of the higher ESCR. Density is commonly measured by the

gradient column method or, more recently, by an ASTM-

approved acoustics technique.

The term molecular weight colloquially refers to the

length of a polyethylene chain. Since all HDPE polymers

possess a variety of chain lengths, it becomes necessary to

mathematically deﬁne some MW terms in order to obtain

meaningful information. Thus M

, the number-average

molecular weight, is SN

/SN

,whereN = number of

polyethylene molecules and M =combined mass of the

polyethylene molecules (e.g ., 100 molecules at 10,000 + 100

molecules at 20,000 = 15,000). The weight-average molecu-

lar weight, M

,isSN

/SN

(e.g ., 100 molecules at

10,000 + 100 molecules at 20,000 = 16,667). For packaging

applications, M

ranges from about 7000 to 19,000 and M

ranges between 80,000 and 250,000. When plastics people

talk about MW, in reality they are referring to M

Molecular weights are commonly estimated by rheo-

logical techniques (capillary/dynamic oscillatory rheo-

metry), solution (intrinsic) viscosity, or the popular

size-exclusion method, gel-permeation chromatography

(GPC). However, each of these methods does give rise to

considerable variation, especially for high-molecular-

weight polymers, and any numbers quoted should be

used only as a guide.

The most frequently used way of determining a mea-

sure of MW is the melt index (MI). For HDPE, this is

condition E of ASTM D1238, which uses a well-delineated

Figure 1. Simpliﬁed ﬂow diagram for the Phillips (slurry) manufacturing process.

980 POLYETHYLENE, HIGH-DENSITY

capillary rheometer, a temperature of 1901C, and a 2.16-

kg load to push the extrudate out of the barrel.

The MI is simply the weight of polymer collected during

a 10-min period under these conditions. For higher-MW

polymers, the load is changed to 21.6 kg and the result is

referred to as high-load melt index (HLMI). The ratio of

HLMI/MI represents a crude measure of the molecular-

weight distribution and is often the simplest technique for

quality-control purposes. Polydispersity, formally M

,is

a m ore rigorous gauge of MWD and is usually obtained by

GPC. Because this method gives a direct visual

representation of the MWD, it can also supply other in-

formation that is absent from other techniques. Figure 2

shows typical MWD curves for a variety of polymer types.

Note that such features as modality, symmetry, and MWD

breadth are easily determined.

From Table 2 it can be seen that both MW and MWD

affect virtually all common properties, especially melt

parameters, processing, and impact strength.

One other type of branching is long-chain (LCB).

These longer side chains to the PE backbone are char-

acteristically 6–12 carbons long. They impact both melt

elasticity (important for foamability, large parison stabi-

lity, etc.) and processability (how easily a material can be

transformed). In other end uses, LCB sites can be used to

initiate crosslinking with a peroxide or other suitable

radical generator. LCB can be imaged through elonga-

tional or short-die, capillary-die rheometry techniques.

PROPERTIES

Chemical

HDPE resists most chemicals, including dilute acids and

bases, oxidizing agents, hydrocarbons, and some alde-

hydes/ketones. The major exceptions are chlorohydrocar-

bons and aromatics, which tend to swell and soften the

polymer. Even some of these chemicals can be packaged in

HDPE if the interior surface is ﬂuorinated or sulfonated,

or if a coextrusion with a barrier layer is incorporated

(e.g., nylon).

The ability of an agent to degrade HDPE over a period of

time is dependent on the ESCR of the polymer. The failure

mechanism is the extension of a crack at a focal point, or

local area of concentrated stress. On a microscopic level,

one might consider this to be a plasticization of the strands

of PE that link the crystals of HDPE together in the

interfacial zone—in other words, a dissolution of the

‘‘glue’’ that binds the crystalline regions. Both density and

MW have a profound effect on this parameter, so the trick is

to make a grade of HDPE that has the highest density and

lowest melt viscosity compatible with the application.

ESCR is usually measured by the bent-strip test (ASTM

D1693), which involves taking a strip of HDPE, making a

notch in it, and bending it in a jig, so that the notch

provides a narrow area of constant strain. The strip is

immersed in either 100% detergent (condition A), or 10%

solution in water (condition B). The latter is considered a

much more aggressive condition. For bottles, it is also

possible to employ a more sophisticated, real-world test by

applying a low constant pressure to a bottle that contains

a small amount of water or other liquid [constant-pressure

bottle test (CPBT)]. Failure occurs when either the bent

strip snaps or, in the case of the CPBT test, the bottle

bursts and leaks liquid. In either case, ESCR is reported in

average number of hours to failure based on a statistical

approach, or ﬁrst to fail.

The permeability of HDPE toward small molecules is

most dependent on the density, because it is the crystalline

regions that present the highest impedance. Longer, more

entangled, higher-MW chains also cause small molecules

Figure 2. GPC curves for various types of HDPE polymers:

curve 1—low-MW, symmetrical, narrow-MWD injection molding

resin; curve 2—medium-MW unsymmetrical monomodal blow-

molding resin; curve 3—high-MW bimodal ﬁlm resin. The dashed

line in the lower-left corner represents the oligomer region.

Table 2. Inﬂuence of Density, Molecular Weight, and

Molecular-Weight Distribution on Selected HDPE

Properties

Property Density m MW m

MWD

Broadens

Tensile strength at yield mmm(sl)

Tensile elongation at break k (sl)

Flexural modulus mksl

Stiffness mm(sl) k (sl)

Impact strength kmk

Melt strength mm

Melt viscosity mk

Die-swell ratio (DSR)

Heat-deﬂection temperature mmk(sl)

Permeability kk

ESCR kmm

Chemical resistance m

Gloss m

Haze mm

Shrinkage from melt

m (sl) m (sl) k (sl)

Flowability and

processability

sl=slightly.

Attack by strong oxidizing agents excepted (e.g., hydrogen peroxide,

concentrated nitric acid, concentrated bleach).

Highly dependent on polymer type.

Subject to cooling rate, part thickness, etc.

POLYETHYLENE, HIGH-DENSITY 981

such as oxygen or water to take a more tortuous path. The

transmission rates of gases such as moisture are depen-

dent on the thickness of PE that they have to traverse, as

well as the temperature and concentration of the gas. As

the thickness of the HDPE is increased, the ability of the

layer to prevent small molecules from passing through it

decreases exponentially. As barrier polymers go, HDPE is

only a fair performer. To get the smallest transmission

rates through a ﬁlm or bottle wall, it may be necessary to

make a multilayer construct using coextrusion.

Preparation of the surface of HDPE for decoration

is most commonly achieved by treating it with either a

gas ﬂame or corona discharge. In the case of either

method, time should be taken to optimize the process.

The polyethylene on the surface should become sufﬁ-

ciently oxidized so that the surface tension rises from 29

to at least 36 dyn/cm. Simple tests based on water dissipa-

tion or dye retention can often be used as a quick check.

One should be aware that many additives such as anti-

stats, lubricants, and slip agents can migrate quickly

through the plastic and cause print delamination over a

period of days. The same result is seen when the oligomer

content is high.

Physical

Impact resistance of a bottle or container is one of the

most important parameters in packaging. This property is

improved by decreases in density and MWD, and an

increase in MW. Often the needs for impact resistance

are in direct conﬂict with load-deformation requirements.

The degree of impact resistance can be assessed by a

variety of tests performed on compression-molded pla-

ques, such as Izod or Charpy, in both notched and un-

notched conditions. Dropping heavy weights on plaques in

ambient or cold temperatures using the Gardner appara-

tus is also practised widely (dart impact for ﬁlms). Many

manufacturing companies also drop loaded containers in

various attitudes to verify performance and ensure that

the design or molding process itself has not introduced a

weakness.

Load deformation comes in two ﬂavors: (i) instant, or

top load, in which the stress is exerted on a bottle or other

container during a momentary situation (e.g., during

ﬁlling or capping); and (ii) constant loading, in which

bottles are stacked on top of one another during a long

storage period. If the material being contained is nonche-

mically aggressive, a homopolymer of medium MW should

prove sufﬁcient for the application. A more demanding

ESCR service will require an HDPE of higher MW, or

lower MI to compensate for the reduced stiffness of the

lower-density material. The conventional gauge of stiff-

ness is ﬂexural modulus and typical numbers for HDPE

range from 250,000 for a homopolymer to half that value

for a 0.940-density material.

APPLICATIONS

Blow molding and injection molding are the dominant

processes for preparing end products of HDPE. The

important packaging use is for extrusion molding

of bottles for milk, water, and liquid detergent (1, 5).

HDPE coextrusions with LDPE, ethylene, and vinyl acet-

ate copolymer or ionomer are widely used in liners for

food cartons. Injection molding produces a wide variety

of products including cups, pails, crates, trays, food

containers, and caps and closures. Blown ﬁlms are

used for sacks and bags, trash can liners, and food packa-

ging (5).

RECYCLING

In 2006, 17,645 10

lb of HDPE was produced in

the United States (7). About 24.8% of produced HDPE

is recycled in the United States, mainly milk and water

jugs and liquid detergent bottles. Cleaning of the recovered

plastic comrprises washing, rinsing, and drying. Removal of

the labels is the worst problem in the washing and drying

stages. Detergents are usually used to improve the efﬁ-

ciency of label removal. Metal-foil labels can introduce

metal into the polymer. When metal-foil labels are heat-

sealed onto the plastic, the only way to remove them is

by using an extrusion-melt ﬁlter. This leads to plugging of

the ﬁlter screens, causing more frequent changes and

the increasing production costs. During the rinse cycle,

polyethylene particles sink to the bottom of the bath and

can be separated from the polyethylene. Recycled HDPE is

used for nonfood bottles, pipes, lawn and garden products,

ﬁlm, lumber, pallets, crates, buckets, and automotive

parts (8).

BIBLIOGRAPHY

Stephen J. Carter, ‘‘Polyethylene, High-Density’’ in The Wiley

Encyclopedia of Packaging, 2nd edition, John Wiley & Sons,

New York, 1997, pp. 745–748.

Cited Publications

1. A. Borruso, ‘High-Density Polyethylene Resins’’ in Chemical

Economics Handbook, SRI Consulting, Menlo Park, CA,

2008.

2. J. P. Hogan and R. L. Banks, U.S. Patent 2,825,721 (to Phillips

Petroleum Co.), March 4, 1958.

3. M. B. Welch and H. L. Hsieh, ‘‘Oleﬁn Polymerization Catalyst

Technology’’ in C. Vasile and R. B. Seymour, eds., Handbook of

Polyoleﬁns, Marcel Dekker, New York, 1993, pp. 21–38.

4. E. Benham and M. McDaniel, ‘‘Polyethylene, High Density’’ in

Kirk–Othmer Encyclopedia of Chemical Technology, Vol. 20,

5th edition, John Wiley & Sons, Hoboken, NJ, 2006.

5. R. I. Mink and co-workers, U.S. Patent 7,129,302 (to Univation

Technologies, LLC), October 3, 2006.

6. L. T. Wardhaugh and M. C. Williams, ‘‘Blockiness of Oleﬁn

Copolymers and Possible Microphase Separation in the Melt,’’

Polym. Eng. Sci. 35, 18–27 (1995).

7. Year End Statistics, American Chemistry Council, December

28, 2007.

8. K. Borchardt, ‘‘Recycling, Plastics’’ in Kirk–Othmer Encyclope-

dia of Chemical Technology, Vol. 21, 5th edition, John Wiley &

Sons, Hoboken, NJ, 2006.

982 POLYETHYLENE, HIGH-DENSITY

POLYETHYLENE, LINEAR AND VERY

LOW-DENSITY

V. F IRDAUS

P. P. T ONG

Mobil Chemical Company

Edison, New Jersey

Updated by Staff

INTRODUCTION

Polyethylene is a thermoplastic material well suited for

packaging applications. It is a large-volume commodity

resin. In 2006, U.S. and Canadian production of linear

low-density polyethylene (LLDPE) was 13,037 10

lb.

Sales and captive use totaled 13,050 10

lb (1). Market

share corresponded to 28.7% of total consumption (2).

LLDPE has established itself as the third major member

of the global polyethylene market, which includes

low-density polyethylene and high-density polyethylene.

Polyethylene resins are available in a variety of mole-

cular weights and densities tailored to speciﬁc end-use

markets. Polyethylene can be generally classiﬁed into

the product types listed in Table 1 on the basis of its

density.

HISTORY

In the early 1950s, 20 years after ICI pioneered LDPE

in the United Kingdom using a high-pressure process,

Phillips Petroleum Company commercialized catalysts

containing chromium oxide supported on silica. These

catalysts were used to produce HDPE, which became the

ﬁrst commercial products of catalytic ethylene polymeriza-

tion. Almost simultaneously, K. Ziegler and co-workers (2)

discovered a new group of transition-metal catalysts that

polymerize ethylene and other a-oleﬁns under mild condi-

tions. The ﬁrst commercial introduction of LLDPE was

made in a solution process by DuPont Canada in

1960 (3) using these catalysts. In 1976, W. Kaminsky and

H. Sinn discovered a new family of catalysts for ethylene

polymerization using metallocene complexes (4). These

catalysts afford the synthesis of ethylene copolymers

with a high degree of compositional uniformity. Currently,

there are several commercial processes that produce

LLDPE and VLDPE as described in the next section (5).

PROCESS

LLDPE and VLDPE resins are commercially produced

by several processes, including gas-phase, solution, and

slurry polymerization. The ﬁrst gas-phase process using

the ﬂuid-bed process was developed by Union Carbide in

1979 and has been licensed worldwide. Another gas-phase

process used globally is one developed by British Petro-

leum Company. Ethylene copolymers with butene, hexene,

and 4-methyl-1-pentene are produced in the ﬂuidized-bed

reactors, a highly versatile, economical process that can

accommodate various types of catalysts. Although Ziegler–

Natta catalysts are most widely used, metallocene-based

catalysts are also used in this gas-phase process (6).

The ﬁrst solution processes were introduced in the late

1950s. Currently, there are two types of solution processes

commonly used for the production of ethylene copolymers.

The ﬁrst process used heavy solvents (C

–C

hydrocar-

bons) to solubilize the ethylene and polyethylene at high

temperatures and pressures. The process utilizes Ziegler–

Natta or metallocene catalysts. The second type of solution

polymerization process uses mixtures of supercritical

ethylene and molten polyethylene as the medium for

ethylene copolymerization reactions. In this case, retro-

ﬁtted high-pressure reactors that were used previously for

producing LDPE are currently being used to produce

LLDPE and VLDPE.

The slurry polymerization process is the oldest cataly-

tic polyethylene production technology. The main disad-

vantage of this technology, however, lies in the fact that

the LLDPE resins produced by this process exhibit high

swelling even when light solvents are used. The resultant

stickiness in the polymer particles signiﬁcantly limits

the polymer density range and production rates that

can be achieved via this process. Recent developments

in this process have allowed the production of broad-

MWD LLDPE using light solvents such as isobutane and

isopentane.

POLYMER STRUCTURE AND PROPERTIES

Properties of LLDPE and VLDPE are usually speciﬁed in

terms of density (ASTM method D792) and melt index

(ASTM method D1238). Density is a measure of crystal-

linity, and melt index is related to the polymer molecular

weight (see also Polymer properties).

Density is the most important parameter governing

resin properties. Polyethylene is essentially a composite

material consisting of a rigid crystalline phase and an

elastic amorphous phase. As crystallinity decreases with

decreasing density, the product becomes softer and more

pliable, clarity increases, and toughness also increases.

Density is controlled by the concentration of short-chain

branching (SCB), which is introduced in polyethylene resins

via copolymerization with a-oleﬁns . Comonomers widely

used are 1-butene, 1-hexene , 4-methyl 1-pentene, and

1-octene. The side branches serve to disrupt the polyethy-

lene crystals, channel polymer chains into the amorphous

phase, and thus reduce overall density. The comonomer

type determines the length of the side branch. The short

Table 1. Commercial Classiﬁcation of Polyethylene

Resins

Product Density (g/mL)

High-density polyethylene (HDPE) 0.94–0.97

Medium-density polyethylene (MDPE) 0.926–0.939

Linear low-density polyethylene (VLDPE) 0.915–0.926

Very low-density polyethylene (VLDPE) 0.89–0.915

Low-density polyethylene (LDPE,

produced via free-radical polymerization)

0.915–0.940

POLYETHYLENE, LINEAR AND VERY LOW-DENSITY 983

side branches of a butene copolymer can be partially

incorporated into the polymer crystal, whereas those from

the higher a-oleﬁns (HAOs) cannot. As a result, the HAO

copolymer chain is likely to leave a given crystal and enter

another one to form a ‘‘tie chain’’ that helps bind the crystals

together. For this reason, HAO copolymers are generally

stronger and tougher than butene copolymers.

Another important structural characteristic is the poly-

mer chain length or molecular weight (MW) and molecu-

lar-weight distribution (MWD). It is a common practice to

characterize MW by means of the melt index (MI), which

bears an inverse relation with MW; thus, lower MI values

correspond to higher MW. The melt-ﬂow ratio (MFR) is a

ratio of the polyethylene viscosity at two different shear

rates and is an indicator of the MWD. In general, long

chains are entangled with one another and entanglements

contribute to the resin’s strength and toughness. However,

the desire for higher strength needs to be balanced against

processabilty, since MW and MWD have a profound inﬂu-

ence on the ﬂow behavior or rheological characteristics of

the molten polymer. MW controls the overall viscosity

level and MWD controls how viscosity depends on shear

rate—that is, its shear-thinning response. Processability,

related to ﬂow at high shear rates, is therefore a function

of both MW and MWD. Most LLDPE and VLDPEs have

relatively narrow MWD and show low degree of shear-

thinning (see Figure 1).

When compared to LDPE (whose shear-thinning is

further enhanced by the presence of long-chain branching)

of a similar melt index, LLDPE and VLDPE resins tend to

build up high stresses during processing at relatively low

shear rates. Other consequences of their narrow MWD are

low-melt elasticity and ‘‘soft’’ stretching ﬂow behavior.

Low-melt elasticity implies rapid relaxation of polymer

chains in the melt when subject to stress. For this reason,

the tendency to induce molecular orientation during pro-

cessing is low, and the product physical properties are not

as sensitive to processing conditions. The ‘‘soft’’ stretching

behavior implies a low extensional viscosity that enables

easier drawdown of the molten resin bubble to thin

gauges. By the same token, LLDPE and VLDPE resins

have low melt strength and can be sensitive to bubble

instability during blown-ﬁlm extrusion.

A third inﬂuential factor in the LLDPE/VLDPE pro-

perties and processing is the short-chain branching

distribution (SCBD) of the polymers. In general, LLDPE

resins are structurally heterogeneous. Not only is there a

distribution of MW, but there is also a distribution of SCB.

The SCB is not uniform across all polymer chains; some

chains have many branches, while others may be essen-

tially linear. This characteristic of heterogeneous SCBD

generally contributes to thicker crystallites, higher melt-

ing point, higher stiffness, and lower optical clarity in

LLDPE when compared to LDPE at the same density. The

SCBD also affects the fraction of the resin that can be

extracted by solvents such as hexane. This fraction,

commonly referred to as the ‘‘FDA extractables,’’ is highly

branched and low in MW. Both SCBD and MWD are

determined by the catalyst system and, to a lesser extent,

the reactor conditions. Polyethylene produced using me-

tallocene catalysts have a very narrow MWD as well as a

much more homogeneous SCBD. This leads to very low

extractable levels in the ﬁnal product.

Since VLDPE resins have lower density compared

to LLDPE resins, the short-chain branching levels are

higher. Conventional VLDPE resins (catalyzed by Ziegler–

Natta catalysts) have high extractables, high strength,

and clarity. Metallocene-catalyzed VLDPE resins, in spite

of the low density, still offer the additional advantage of

having very low extractable levels. This is a major advan-

tage in medical applications where high purity is essen-

tial. Also due to the homogenous SCBD, the melting point

of metallocene-catalyzed resins is signiﬁcantly lower com-

pared to conventional LLDPE and VLDPE. This leads to

improved sealability in the ﬁnal product.

In 2004, B15% of LLDPE in the United States was

produced using metallocene catalysts. This usage is ex-

pected to rise to B25% by 2010. The latest addition to the

catalyst family is a group of organometallics of various

transition metals (6).

APPLICATIONS

LLDPE competes with LDPE for many applications, and

has made signiﬁcant inroads into traditional LDPE mar-

kets. This is mainly due to the signiﬁcantly better down-

gauging potential, which allows thinner ﬁlms with

enhanced physical properties.

Film

Film is the largest application for LLDPE resins, greater

that 55% in the United States in 2005 (6). This ﬁlm has a

high tensile strength and puncture resistance. The tough-

ness and low-temperature properties of LLDPE ﬁlm ex-

ceed those of conventional LDPE. The trash-bag market

exceeds 700,700 t/yr: it is the largest market for LLDPE

(see Table 2). Ethylene-1-butene copolymers, which are

the cheapest, have a lower ﬁlm strength compared to

those of the high-strength ethylene-1-hexene and ethy-

lene-1-octene copolymers. Bags for merchandise packa-

ging manufactured from thin LLDPE ﬁlm have excellent

tensile strength, puncture resistance, and seal strength at

thin gauges. This application includes bags on a roll, such

as garment bags, laundry and dry cleaning bags, and ice

Figure 1. LDPE/LLDPE viscosity comparison.

984 POLYETHYLENE, LINEAR AND VERY LOW-DENSITY

bags. Several LLDPE and VLDPE ﬁlms are competing for

a special ﬁlm market; elastic stretch ﬁlm for packaging.

The resins based on ethylene-1-hexene and ethylene-

1-octene copolymers are particuarly suited for these

purposes. A signiﬁcant volume of LLDPE is used for

manufacturing large-size packaging for food and textiles.

These bags are easily reused, take up less space, and are

moisture-resistant.

Several VLDPE grades, especially those produced via

metallocene catalysts, are used to prepare very clear ﬁlm.

High-clarity ﬁlm is necessary in many packaging applica-

tions. Barrier properties and clarity are attractive proper-

ties for packaging poultry, frozen foods, and vegetables.

Medical applications include ﬁlm for blood bags and

surgical disposable bags. Compositionally uniform LLPDE

and VLDPE are especially attractive because of their

sealability.

Injection Molded Products

Injection molded products is the second largest market for

LLDPE. It accounted for B7% of its consumption in 2005.

Over one-half of the LLDPE consumed is used to produce

housewares These housewares are stiffer and more resis-

tant to impact and distortion at elevated temperatures

than those of LDPPE. Lids for HDPE jars and containrs

are made of LLDPE. They have excellent gloss and low

warpage.

FABRICATION PROCESSES

As indicated in the previous section, LLDPE and VLDPE

are used in a large variety of product types. Since ﬁlm is

the largest market segment, ﬁlm extrusion is brieﬂy

summarized in the following paragraphs (7).

Film Extrusion

Polyethylene is converted from pellets into ﬁlm using one

of two techniques: blown- or cast-ﬁlm extrusion. Depend-

ing on the end-use property requirements, the resin is

either blown into a tubular ﬁlm or cast into a thin ﬂat

sheet. Compared to cast ﬁlms, blown ﬁlms typically have

higher haze, more balanced orientation, and good impact

strength, and can generally achieve gauge uniformity

within 10%. On the other hand, cast ﬁlm tends to have

better clarity and is highly oriented in the machine

direction. Excellent gauge uniformity (within 3%) can be

achieved by casting the ﬁlm, which allows ﬁlm production

at very thin gauges (sometimes less than 0.5 mil). The ﬁlm

is quenched much sooner on cast ﬁlm lines, and signiﬁ-

cantly higher extrusion rates are achieved. Some basics of

ﬁlm extrusion are explained below.

Blown Film. The process of converting pellets into

blown tubular ﬁlm consists of four basic steps: feed/

recycle, extrusion, ﬁlm blowing, and winding.

1. The feed section consists of a hopper in which virgin

polymer is loaded. Usually, there is some mechanical

means of blending in some recycled material. This could

be either postconsumer recycle (PCR), which is mandated

in some states and in some applications, or merely scrap

ﬁlm and edge trim that is fed through a grinder and

combined into the feed stream with the virgin resin. The

feed stream also may contain additives such as pigments

and processing aids.

2. The function of the extruder is mainly to compress,

melt, and convey the polymer to the discharge end. There

are several different types of extruder screws. The original

ﬁlm equipment consisted of screws that were optimized for

LDPE resins. When LLDPE was ﬁrst introduced, it be-

came evident that the LDPE screws would require some

modiﬁcation to process LLDPE, since the power efﬁciency

was very low because of the rheological behavior of these

mostly linear resins. The processing requirements of

LLDPE were met by designing screws such as the barrier

screw or decreasing-pitch screw that could handle a more

viscous melt. By far the most commonly used screws for

extruding LLDPE or LLDPE/LDPE blends are the barrier

screws.

3. After the extruder, the polymer melt is forced into an

annular blown-ﬁlm die for extrusion into a tube. The

thickness of the annular opening, the die gap, is generally

between 25 and 100 mils. When processed through a

narrow die gap, LLDPE resins generally require a proces-

sing aid to avoid melt fracture, a ﬁlm deformity that

occurs when a certain critical shear stress is exceeded.

Although melt fracture does not cause ﬁlm property loss

(8) except at severe levels, it poses an appearance problem

that can be a critical factor in several applications. After

the polymer exits the die, it is drawn down in the machine

and transverse directions (MD and TD) to achieve the

desired ﬁlm thickness. Film cooling is also an important

part of the process. The frost-line height (FLH), where the

polymer melt crystallizes, plays an important role in

determining ﬁnal ﬁlm properties (9).

4. The ﬁnal step consists of the buddle being ﬂattened

by collapsing frames, drawn through nip rolls and over

idler rolls to a winder to produce rolls of blown ﬁlm.

Cast Film. The cast-ﬁlm process is similar in most

aspects to the blown-ﬁlm process outlined above, except

in the ﬁlm formation step. Here, the polymer melt exits

the extruder and enters a ‘‘T’’ or coat-hanger die. The

orientation of the die can vary depending on processing

conditions. Typically, a vertically downward die is pre-

ferred. The molten polymer web is cast onto a polished roll

that has an internal cooling mechanism to maintain a

constant temperature. An air knife is generally used to

Table 2. Typical LLDPE/VLDPE Film Applications

Agricultural Rollstock Ice Bags

Bakery bags Industrial liners and sheet

Diaper backsheet Medical packaging

Garbage bags, consumer Newspaper bags

Garbage bags, institutional Produce bags-on-roll

Grocery sacks Shopping (retail) bags

Household food bags Shrink wrap

Heavy-duty sacks Stretch wrap

POLYETHYLENE, LINEAR AND VERY LOW-DENSITY 985

press the molten polymer against the roll. Usually, two

chill rolls are used to provide appropriate tension to the

ﬁlm. After this point, the ﬁlm goes though nip rolls and

into the winder to produce rolls of cast ﬁlm.

Coextrusion

The reasons for using coextrusion are usually to achieve

speciﬁc physical properties such as sealability, barrier,

and strength. Also, there are economic beneﬁts to using

coextrusion including reduced costs (by using an inexpen-

sive polymer in the noncritical layer) and reduced waste

(by introducing a recycle layer). Coextruded ﬁlm can be

produced on both blown- and cast-ﬁlm equipment, and

involves multiple extruders feeding a single die. As many

as nine-layer lines are now being used commercially,

although two and three-layer lines remain the most

common.

PHYSICAL PROPERTIES

Although LLDPE resins are generally considered as hav-

ing superior physical properties compared to LDPE, the

actual impact or tear strength is strongly dependent on

the comonomer used, as well as on the melt index and

density. Some key ﬁlm properties that are critical for

several ﬁlm applications are explained below:

Dart Impact (ASTM D1709). This test measures the

ability of a ﬁlm to withstand the force of a falling

dart, and is an indicator of ﬁlm toughness. Dart

impact is strongly affected by the molecular weight,

density, and comonomer type.

Elmendorf Tear (ASTM D1922). The resistance to tear

in either the machine direction (MD) or transverse

direction (TD) is measured by this test. Typically,

LLDPE ﬁlms tend to have lower MD tear strength

compared to TD. Since that becomes the limiting

direction, only the MD tear values are often measured

and reported, whereas the TD tear is not considered

critical in most LLDPE ﬁlm applications. Ideally, a

balanced orientation, or MDT:TDT ratios close to 1.0,

is desirable to maximize ﬁlm performance.

Other Properties. Tensile and yield strength (ASTM

D638) of the LLDPE resins are also considered

critical end-use properties. Typically, the tensile

strength of butene LLDPE is somewhat lower than

the HAO-LLDPEs. Yield strength is primarily a

function of resin-base density. Hexane and xylene

extractables are also measured to ensure that the

ﬁlm meets FDA limits for food-packaging applica-

tions. Film blocking, the tendency of a bag to resist

opening, is a function of resin-base density and

MWD. Other properties such as puncture resistance,

puncture propagation tear (PPT), sealability, haze,

and gloss are also considered important, depending

on the application.

Comonomer type plays a strong role in inﬂuencing the

ﬁnal ﬁlm properties. In general, butene copolymers tend

to have lower ﬁlm strength, and they are used in low-end

applications. Standard hexene copolymers show signiﬁ-

cantly improved strength properties over butene, but are

not considered as strong as octene copolymers. However, a

new class of ‘‘superstrength’’ hexene copolymers actually

exceed the strength properties of standard octene copoly-

mers. Film strength properties of some commercially

available resins are shown in Table 3.

VLDPE resins generally offer much better optical

properties and impact strength, due to (a) the increased

level of comonomer or (b) low base density. As in the case

of LLDPE, ﬁlm properties vary with respect to melt index,

density, and comonomer type. In general, VLDPE resins

exhibit outstanding puncture resistance, ﬂexibility, low-

temperature stability, and good sealability.

The use of LLDPE in injection molding offers such

advantages as higher stiffness, improved ESCR, and heat-

distortion resistance, which can be coupled with higher

melt indexes to provide faster cycle times and down-

gauged articles.

SAFETY AND HEALTH

Polyethylene is one of the most inert polymers and con-

stitutes no hazard in normal handling (resin suppliers will

provide Material Safety Data Sheets on request). It is

generally recognized as a safe packaging material by the

FDA. Resin suppliers will state which of their products

comply with regulations governing polyethylenes used in

food-contact applications.

LLDPE can present a certain health hazard when

burned because smoke, fumes, and toxic decompositon

products can be formed in the process.

BIBLIOGRAPHY

V. Firdalis and P. P. Tong, ‘‘Polyethylene, Linear and Very Low-

Density (LLDPE and VLDPE)’’ in The Wiley Encyclopedia of

Table 3. Typical Properties of Commercially Available LLDPE Resins

Comonomer Butene Standard Hexene ‘‘Super’’ Hexene Octane

Melt index (dg/min) 1.2 0.9 0.9 1.0

Density (g/mL) 0.918 0.917 0.917 0.920

MFR 24 28 25 30

Dart impact (g) 85 180 480 250

Elmendorf tear MD (g) 80 300 450 350

3.5-in. Glouceste r, 2:1 BUR, 4301F, 100-mil die gap, 250 lb/h, 1.0-mil ﬁlm.

986 POLYETHYLENE, LINEAR AND VERY LOW-DENSITY

Packaging, 2nd edition, John Wiley & Sons, New York, 1997,

pp. 748–752.

Cited Publications

1. Year End Resin Statistics, American Chemistry Council, De-

cember 2007.

2. A. Borruso, ‘‘Linear Low-density Polyethylene (LLDPE) Re-

sins,’’ in Chemical Economics Handbook, SRI Consulting,

Menlo Park, CA, July 2007.

3. A. W. Anderson, U.S. Patent 4,076,698 (1978).

4. W. Kaminsky and H. Hahnsen, U.S. Patent 4,545,762 (1985).

5. B. A. Krensel, Y. V. Kissin, V. I. Kleiner, and L. L. Stotskaya,

Polymers and Copolymers of Higher Alpha-Oleﬁns, Hanser

Publishing, Dublin, NH.

6. Y. Kissin, ‘‘Polyethylene, Linear and Low Density, (LLDPE)’’ in

Kirk–Othmer Encyclopedia of Chemical Technology, Vol. 20,

5th edition, John Wiley & Sons, Hoboken, NJ, 2006.

7. Film Extrusion Manual, TAPPI Press, Norcross, GA, 1992.

8. V. Firdaus and P. P. Tong, SPE 1993 ANTEC Proceedings,

p. 2550.

9. P. P. Shirodkar, V. Firdaus, and H. Fruitwala, SPE 1994

ANTEC Proceedings,p.211.

POLYETHYLENE, LOW-DENSITY

NORMA J. MARASHIN

Union Carbide Corporation,

Bound Brook New Jersey

Updated by Staff

INTRODUCTION

Polyethylene is a thermoplastic polymer formed from the

polymerization of ethylene. It is available in a variety of

molecular weights and densities, which have been tailored

to speciﬁc end-use markets. ASTM has divided polyethy-

lene into four general categories according to density:

Type

III

Nominal Density (g/mL)

0.910–0.925

0.926–0.940

0.941–0.959

Z0.960

In general, the polyethylene industry does not

always follow these designations but has broken polye-

thylene into two broader categories: high-density poly-

ethylene (HDPE), density Z0.940; and low-density

polyethylene (LDPE), density 0.915–0.939. There is also

now a type of polyethylene, very low-density polyethylene

(VLDPE), with densities of o0.915. Dow Chemical offers

VLDPEs under the trade name ATTANE and polyethylene

with densities of 0.895–0.913 under the trade name

FLEXOMER.

Low-density polyethylene was ﬁrst produced in Eng-

land in 1933 by Imperial Chemical Industries labora-

tories, when ethylene gas was compressed to high

pressures and heated to high temperatures (1). ICI’s

development of a commercial process for the manufacture

of LDPE was closely followed by the wartime use of the

product in critical areas such as high-frequency cables for

ground- and airborne radar equipment.

In the early 1950s, another type of polyethylene,

HDPE, was commercially introduced by several compa-

nies who had developed new low-pressure processes for its

production (2). HDPE is a linear polymer without any of

the long-chain branching characteristics of LDPE.

Because of its different structure, HDPE possessed

properties different from and complementary to those of

LDPE and was quickly utilized in many new packaging

applications. These two polyethylenes—differentiated

with respect to density, properties, and manufacturing

processes—coexisted and grew until the early 1960s, when

a third type of polyethylene, LLDPE, was introduced.

Because the molecular structure of this new type of

LDPE was more similar to that of HDPE, the term linear

low-density polyethylene (LLDPE) was coined. Therefore,

the polyethylene industry today is composed of three types

of polyethylene: HDPE, LDPE (sometimes also referred to

as high-pressure LDPE or HP-LDPE), and LLDPE. There

is some confusion in the terminology used for LDPE and

LLDPE. In some articles and publications, LDPE is used

as a generic term for polyethylene below 0.935, thus

covering both HP-LDPE and LLDPE. In other cases,

LDPE is used to cover only HP-LDPE and LLDPE is a

separate category of polymer. In this article, the second

type of categorization is used. LLDPE is treated as a

separate type of polyethylene and referred to only as

LLDPE, while LDPE covers only low-density polyethylene

produced by the high-pressure process. When written out

(low-density polyethylene), both LDPE and HP-LDPE are

included. LLDPE has made inroads in many of the

markets currently served by LDPE and, in addition, is

competing with HDPE in some new applications.

However, by the late 1980s, the penetration of LDPE

markets by LLDPE appeared to be leveling off and LLDPE

producers looked for new technology to regain momentum.

The early 1990s saw an exciting explosion of new low-

pressure, low-density PE products and technology de-

signed to address some of the deﬁciencies of LLDPE

compared to LDPE. For example, Phillips Petroleum

announced LDLPE (low- density linear PE) and claimed

that it was easier to process than conventional LLDPE (3).

Union Carbide Corp. announced Unipol II, a technology

that would be capable of making low-pressure products

that were true ‘‘drop-ins’’ for conventional, commodity

LDPE products (4). Himont built a new U.S. Gulf Coast

facility to utilize its Spherilene process, which is claimed

to produce polyethylene resins in spherical form directly

from the reactor (5). The development of single-site cata-

lyst technology that could make polyethylene homo-, co-,

or terpolymers from 0.865 to 0.96 density was reported (6,

7). By early 1995, most major polyethylene producers

began using metallocene or single-site programs and

many had formed joint ventures to commercialize the

POLYETHYLENE, LOW-DENSITY 987