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products. At present, post-metallocene catalysts are being

developed. Salicylaldimine complexes of group IV metals

may show higher activity and the possibility of further

reﬁnement of molecular structure.

CHARACTERIZATION OF POLYETHYLENE

The properties of products made from polyethylene are

dependent on some basic characteristics of polyethylene

itself. Several of the terms commonly used to describe

polyethylene are discussed in the following paragraphs.

Melt Index

The melt index (MI) of a polymer is used as an empirical

measure of its molecular weight. To measure melt index

according to ASTM D1238, a polymer sample is melted

and forced through a small oriﬁce of ﬁxed size under a

ﬁxed pressure. The weight of polymer that is extruded in

10 min under 44 psi (303 kPa) of pressure is called the melt

index. When the pressure is increased to 440 psi

(3030 kPa), the weight of polymer extruded in 10 min is

the ﬂow index (FI). Since a polymer with very high

molecular weight will be very viscous and resistant to

ﬂow, it will not pass through the small oriﬁce quickly and

the weight obtained in 10 min (or melt index) will be low.

Melt index is therefore inversely proportional to molecular

weight. Typical low-density polyethylene melt indices

range from 0.2 to over 150 dg/min. In general, products

in the lower-melt-index range are used for ﬁlm extrusion

and the higher-melt-index products for molding and ex-

trusion coating. Within any fabrication process, the use of

a lower-melt-index resin will result in a stronger product,

although usually with some sacriﬁce in extrudability.

Melt-Flow Ratio

The melt-ﬂow ratio (MFR) is a rough estimate of the

molecular-weight distribution (MWD) of a resin. Since

all polymer chains in a given resin are not exactly the

same length, an MWD measurement will describe how

dissimilar the chains are from each other. Melt-ﬂow ratio

is the ratio of the ﬂow index to the melt index. The higher

the ratio, the broader the molecular-weight distribution

and the more dissimilar the chains are from each other. A

polymer with every chain exactly the same length would

have a very narrow molecular-weight distribution and a

very low melt-ﬂow ratio. Melt-ﬂow ratios of commercial

low-density polyethylenes vary from about 20 (very nar-

row) to about 100 (very broad). Polymers with narrow

molecular-weight distributions give stronger products, but

are more difﬁcult to extrude than those with broad

molecular-weight distributions.

Density

The density of a polymer is a measure of its crystallinity.

Density measured according to ASTM D1505 consists of

taking a small sample of polymer that has been molded in

a carefully prescribed manner and dropping it into col-

umns with solutions of different viscosities. The position of

the unknown polymer is the column is then compared to

standard samples of known density. The density of a ﬁlm

or molded article is only partly controlled by the density of

the resin used to make the product. The rate at which the

product is cooled also plays an important role. The faster a

ﬁlm or molded article is cooled, the less time there is for

the polymer chains to crystallize and the lower the density

of the ﬁnal product. For example, the density of a sample

cut from a blow-molded polyethylene bottle was 0.945 g/

mL. The same polyethylene resin, when compression-

molded into a plaque and cooled according to ASTM

D1505, measured 0.954 g/mL. Product properties such as

stiffness, rigidity, environmental stress-crack resistance

(ESCR), and moisture vapor transmission rate (MVTR)

are affected by density. The lower the density of a product,

the more limp and ﬂexible it is. A 0.918-g/mL polyethylene

product will have better ESCR and higher MVTR than a

product with a density of 0.930 g/mL.

LDPE PROCESS

LDPE is made by the high-pressure polymerization of

ethylene (2, 8). In either a tubular or autoclave reactor,

ethylene is pressurized to more than 20,000 psi (138 MPa)

and heated to W150 1C. Small amounts of an initiator,

typically oxygen or peroxide, are added to start the poly-

merization process. Comonomers such as vinyl acetate or

ethyl acrylate can be added to make EVA and EEA

copolymers, respectively. Critical molecular characteris-

tics such as molecular weight, molecular-weight distribu-

tion, and density are controlled by reaction temperature,

ethylene pressure, and the concentration of chain-transfer

agents. Constraints on the viscosity of the polymer solu-

tion in the reactor limit the rate at which products with

high molecular weight and/or high density can be made in

the high-pressure process. It has been found that the

tubular high-pressure process gives products that differ

subtly in the type and degree of branching from products

made in the autoclave process. Density depression occurs

in all types of polyethylene because of the presence of

short branches along the backbone of the polyethylene

chain. These short branches, typically one to ﬁve carbon

atoms long, prevent the long polyethylene chains from

folding together and forming crystals. However, early in

the development of LDPE, it was found that the presence

of these short branches did not totally explain some of the

rheological properties of LDPE. It was then hypothesized

and proved that LDPE also contains low levels of long-

chain

branching (9). These

long chains, which can be over

1000 carbon atoms in length, have a very small effect on

the density of LDPE but have major impact on the

processing and properties of LDPE (Figure 1). The tubular

reactor makes LDPE with a large number of long-chain

branches. The branches are of relatively short length,

however. The autoclave reactor, on the other hand, gives

products with low levels of long-chain branches of extre-

mely long length. Because of the difference in long-chain

branch type and frequency, some specialization in product

applications has occurred with these products. The auto-

clave reactors produces products that are especially useful

988 POLYETHYLENE, LOW-DENSITY

in high-speed extrusion coating and in ﬁlm applications

requiring toughness. The tubular reactor gives products

with optimal clarity and processing characteristics.

LLDPE PROCESS

Low-pressure processes for the polymerization of ethylene

were developed in the 1950s (2, 8). These processes used

organometallic catalysts to polymerize ethylene to form

HDPE at moderate pressures (approximately 300 psi) and

temperatures (approximately 100–200 1C). Three basic

types of low pressure systems developed: solution, where

the polymer was completely dissolved in a solvent at high

temperatures; slurry, where the solid polymer particles

were physically suspended in a solvent at lower tempera-

tures; and gas phase, where the solid polymer was in

contact with only the polymerization gases. Again, be-

cause of process constraints in the reactor, molecular

weights and densities were thought to be limited, espe-

cially in the solution and slurry systems. The density

limitations of the low-pressure and high-pressure pro-

cesses were thought to form incompatible regions, with

the low-pressure process making polythylenes above 0.935

density and the high-pressure process making polyethy-

lene below 0.935 density.

The ﬁrst commercial production of LLDPE was made in

a solution process by DuPont Canada in 1960 (10). A large

market for this new polymer did not develop until 1977,

when Union Carbide started licensing their gas-phase

process for the manufacture of linear low-density poly-

ethylene. Since that time, several other polyethylene

manufacturers have announced the conversion of low-

pressure HDPE processes or even high-pressure LDPE

processes to make LLDPE and some of these processes are

available for licensing.

The structure of these new linear low-density polymers

is very different from the LDPE made from the high-

pressure process. In LLDPE, there is no long-chain

branching. Density is controlled by the addition of como-

nomers such as butene, hexene, or octene to the ethylene.

These comonomers give rise to short-chain branches of

different lengths, two carbon atoms for butene, four for

hexene, and six for octene. The length of the short-chain

branches determines some of the strength characteristics

of LLDPE. The absence of long-chain branches in LLDPE

plays a signiﬁcant role in the difference in extrusion

characteristics between LLDPE and LDPE, as discussed

in the sections on speciﬁc application areas.

LOW-DENSITY POLYETHYLENE PROPERTIES

Low-density polyethylene is one of the most widely used

packaging materials in the market. Its utility in a variety

of different applications is due not to some single out-

standing property or characteristic, but usually to a

combination of properties. The low price of polyethylene

compared to wood, metal, and other polymers has accel-

erated its penetration into many applications. In addition

to the low cost of polyethylene, the excellent toughness,

ﬂexibility, moisture barrier, chemical resistance, electrical

insulation, and light weight of polyethylene ﬁlms, bottles,

pipes, cables, and other articles make them superior to

articles made from conventional materials of construction.

Since the introduction of LLDPE into the market, the

superior properties of LLDPE have led to its use in new

applications for polyethylene as well as the replacement of

LDPE and/or HDPE in some areas. Compared to LDPE,

LLDPE at the same melt index and density offers better

toughness, rigidity, stress-crack resistance, elongation,

melting point, and moisture barriers. (See article on

polymer properties for more details.)

MARKETS/APPLICATIONS

Low-density polyethylene was the ﬁrst thermoplastic

polyoleﬁn used commercially. The free-radical initiated

polymerization process leads to a polymer with large

amounts of long chain branching. This imparts an unu-

sual rheological behavior in both shear and extension.

LDPE is used in a variety of applications such as ﬁlm,

coating, molding, and wire and cable insulation.

The global market for polyethylene is undergoing ex-

tensive changes. Increases are concentrated in China and

the Middle East. Consumption growth of LDPE is rather

strong, despite the fact that a decline was expected with

the advent of LLDPE to the market (11).

In 2006, 7.853 10

lb of LDPE were produced in the

United States (12). Film applications are by far the largest

market for LDPE, accounting for 55% of world use in 2006.

Film demand is split between packaging and nonpacking

uses. Extrusion coating is the second largest market for

LDPE accounting for 9.9% of total in 2006.

Figure 1. Molecular structures for LLDPE and HP-LDPE: (a)

HP-tubular process; (b) HP-autoclave processes; (c) LLDPE.

POLYETHYLENE, LOW-DENSITY 989

Film and Sheet

About 50% of all the low-density polyethylene used in the

world today goes into the ﬁlm and sheet area. These

applications include garbage bags, grocery sacks, garment

bags, heavy-duty sacks, shrink ﬁlm, stretch ﬁlm, carrier

ﬁlm, pond liners, construction and agricultural ﬁlm, and

food packaging. Food applications include packaging for

baked goods, dairy products, frozen food, meat and poul-

try, cookies, and candy. Polyethylene used in the ﬁlm area

range in melt index from 0.2 to 6 and in density from 0.915

to 0.935. The advantages of LDPE/LLDPE include low

cost, ﬂexibility, toughness, and chemical- and moisture-

barrier properties (Tables 1–3). This has been the market

most rapidly penetrated by LLDPE in the United States.

This rapid penetration is due, in part, to the outstanding

physical properties of LLDPE ﬁlm (Table 4). Dart drop and

Elmendorf tear measure the toughness of ﬁlm in a high-

speed test and give an indication of the failure behavior of

ﬁlm under catastrophic conditions. Puncture and tensile

tests are done at relatively slow speeds and give an

indication of intrinsic properties and failure modes that

can occur with long-term use.

The differences in molecular structure between LLDPE

and LDPE (linear vs branched molecules and molecular-

weight distributions) affect the rheology of the two mate-

rials. LLDPE is more viscous at extrusion shear rates and

requires more power to extrude. In addition, it is neces-

sary to use a wide dip gap to avoid melt fracture when

extruding 100% LLDPE. Therefore, in the early days of

LLDPE introduction, minor modiﬁcations to the screw

and die gap had to be made in order to extrude LLDPE

on extruders designed for HP-LDPE and obtain optimum

ﬁlm properties Most extruders today have been designed

to handle the different rheology of LLDPE, and such

modiﬁcations are not necessary. In addition, there are

additives now available that allow the extrusion of con-

ventional LLDPE through narrow die gaps without melt

fracture. The new generation of LLDPE ﬁlm products,

designed to be ‘‘drop-in’’ for LDPE products, will not

require any modiﬁcations to extrusion equipment.

LDPE remais the resin of choice for shrink wrap and

clarity ﬁlms (13).

Injection Molding

This market includes lids, buckets, wash basins, house-

wares, toys, freezer containers, and general housewares.

The advantages of low-density polyethylene include

good low-temperature properties, low cost, light weight,

and ﬂexibility (Table 5). Low-density polyethylene in the

melt-index range of 2–150 and density range of 0.920–

0.930 is commonly used in injection-molding applications.

LLDPE has made signiﬁcant penetration into certain

markets such as lids and housewares, where property

advantages, such as higher stiffness and improved envir-

onmental stress-crack resistance and heat-distortion

resistance, can be coupled with higher melt indices to

result in faster cycle times and downgauged articles

(Table 6) (14).

Extrusion Coating

In this application, polyethylene is used as a coating on

another material such as paper, aluminum foil, and card-

board. These coated products are used for liquid packa-

ging, such as milk and juice, and for the new aseptic

packages of nonrefrigerated juices. The polyethylene

serves as an adhesive, moisture barrier, seal layer, prin-

table surface, and/or barrier to tear. Because of its mole-

cular structure, that is, the type of long chain branching,

polyethylene made in tbe high-pressure autoclave reactor

is the most successful in this market. Resins with melt

indices of 4–10 and densities of 0.920–0.930 are commonly

used. Because of its linear structure, LLDPE has not

penetrated the extrusion coating market, except as blends

with LDPE.

Blow Molding

High-density polyethylene is the preferred material for

this market because of its rigidity and barrier properties.

Table 1. HP-LDPE Homopolymer Film Resins

Type: HP-LDPE HP-LDPE HP-LDPE HP-LDPE HP-LDPE HP-LDPE

Comonomer: None None None None None None

Melt index: 0.2–0.8 1.5–2.0 1.5–2.0 1.2–2.0 1.0–2.0 5.0–10.0

Density (g/cm

): 0.919–0.923 0.922–0.925 0.930–0.935 0.918–0.924 0.923–0.927 0.917–0.930

Molecular-weight

distribution:

Broad Broad Broad Broad Broad Broad

Process: Blown Blown Blown and/or

cast

Blown Cast Cast

Applications: Shipping sacks;

heavy-duty

applications

Bread and

bakery;

general-

purpose

packaging

Overwrap General-

purpose

packaging

Bread and

bakery;

general-

purpose

packaging

Extrusion

coating

Critical properties: Toughness Clarity Clarity; stiffness Extrudability;

toughness

Extrudability;

good tear

Drawdown

adhesion;

pinhole

resistance

990 POLYETHYLENE, LOW-DENSITY

Low-density polyethylene may be used in those segments

where ﬂexibility and excellent stress-crack resistance are

required, such as squeeze bottles, toys, and drum liners.

Fractional-melt-index polyethylene (density 0.920–0.935)

is typically used in this market. LLDPE offers better

stiffness and improved stress-crack resistance compared

to LDPE.

SAFETY AND HEALTH

Polyethylene is generally recognized as a safe packaging

material by the Food and Drug Administration. Resin

suppliers will state which of their resins comply with

regulations governing polyethylenes used in food-contact

applications. These regulations are covered in the U.S.

Food, Drug, and Cosmetic Act as amended under Food

Additive Regulation 21 CFR 177.1520. Polyethylene is a

very stable polymer. However, proper material handling

procedures are required to control dust and smoke and to

provide adequate ventilation during extrusion. Copoly-

mers containing EEA, EMA, and EVA should be used

selectively on food contact applications. Resin suppliers

will provide Material Safety Data Sheets and Materials

Handling Guides for polyethylene resins on request.

Table 2. HP-LDPE Copolymer Film Resins

Type: HP-LDPE HP-LDPE HP-LDPE HP-LDPE

Comonomer: 2–5% VA

3–5% VA

7% VA

15–18% EA

Melt index: 1.5–2.0 0.2–0.4 0.2–4.0 2.0–6.0

Density (g/cm

): 0.925–0.930 0.923–0.927 0.927–0.945 0.927–0.940

Molecular-weight

distribution:

Broad Broad Broad Broad

Process: Blown Blown Blown; extrusion coating Blown; extrusion coating

Applications: Frozen food Ice bags Sealing layer; liquid

packaging

Disposable gloves; ID cards

Critical properties: Clarity; low-temperature

properties

Low-temperature

properties

Adhesion; low-temperature

properties

Low stiffness; adhesion to polar

substrates

Vinyl acetate.

Ethyl acrylate.

Table 3. LLDPE Film Resins

Type: LLDPE LLDPE LLDPE LLDPE LLDPE

Comonomer: Butene Hexene; octene Hexene; octene Hexene; octene Hexene; octene

Melt index: 0.8–2.5 2.0–4.0 2.0–5.0 0.7–1.5 0.8–1.5

Density (g/cm

): 0.917–0.922 0.912–0.919 0.928–0.935 0.924–0.928 0.917–0.923

Molecular-weight

distribution:

Narrow Narrow Narrow Narrow Narrow

Process: Blown Cast Cast Blown Blown

Applications: General-purpose

packaging

Stretch wrap Bread and bakery;

overwrap

Grocery sack Blending; ice bags

Critical properties: Extrudability;

toughness

Puncture and tear

resistance

Stiffness; moisture

barrier

Stiffness; tear

resistance

Excellent

toughness; Low-

temperature

properties

Table 4. Blown-Film Properties

of LLDPE and HP-LDPE

Property: ASTM test HP-LDPE HP-LDPE LLDPE LLDPE LLDPE

Melt index: 2.5 0.2 1.0 1.0 1.0

Density (g/cm

): 0.921 0.923 0.918 0.918 0.920

Comonomer: None None Butene Hexene Octene

Dart drop, gf/mil (N/mm): D1709 0.75 (29) 185 (71.4) 100 (38.6) 200 (77.2) 250 (96.5)

Puncture energy, in. lbf/mil (kJ/m): 6 (26.7) 5 (22.2) 16 (71.2) 17 75.6)

Elmendorf tear, gf/mil (N/mm): D1922 MD 160 (61.8) 90 (34.7) 140 (54) 340 (131.2) 370 (142.8)

XD 110 (42.5) 100 (38.6) 340 (131.2) 585 (225.8) 800 (308.8)

Tensile strength, psi (MPa): D882 MD 2900 (20) 2800 (19.3) 5000 (34.5) 5200 (35.9) 6500 (44.8)

XD 2700 (18.6) 3000 (20.7) 3800 (26.2) 4700 (32.4) 5100 (35.2)

Tensile impact strength, ft lbf/in. (MPa): MD 440 (36.4) 500 (41.4) 1200 (99.3) 1930 (159.7)

XD 650 (53.8) 1050 (86.9) 900 (74.5) 1760 (145.6)

All properties measured on 1.5-mil (38-mm) ﬁlm produced at 2:1 blowup ratio.

POLYETHYLENE, LOW-DENSITY 991

ENVIRONMENTAL IMPACT

In recent years there has been quite a bit of adverse

publicity around the use of plastics and the impact of

litter on the environment. In the 1980s, the decision of fast

food restauraants to discontinue the use of the expanded

polystyrene clamshell burger container is one example of

how plastics growth and use can be changed by public

opinion. Many U.S. states have considered or passed

regulations specifying the amount of recycled plastic

that must be incorporated into new plastic articles. This

has had a large impact on the polyethylene industry since

polyethylene, including LDPE, is one of the more recycl-

able plastics being used today. The recycling effort has

been hampered by the higher cost of collecting, separating,

and cleaning the polyethylene containers compared to the

cost of the virgin polymer.

Photo- and biodgradable LDPE copolymers have been

offered as a solution to the litter problem. LDPE copoly-

mers containing carbon monoxide have been produced.

These photodegradable copolymers have been used to

make beverage carrier rings, but have not been used in

other commodity applications. The search for a biodegrad-

able LDPE continues. Blends of starch-like ﬁllers improve

biodegradation, but at a signiﬁcant sacriﬁce of physical

properties. A new biodegradable/compostable technology

has been developed, such that biodegradability can be

triggered by heat, light, or stress. Products made with this

technology are being marketed in Europe and South

America (13).

BIBLIOGRAPHY

Norma J. Maraschin, ‘‘Polyethylene, Low-Density’’ in the Wiley

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

New York, 1997, pp. 752–758.

Cited Publications

1. British Patent 471,590 (September 6, 1937) and U.S. Patent

2,153,553 (April 11, 1933), E. W. Fawcett and coworkers (to

ICI, Ltd.).

2. A. Renfrew and P. Morgan, eds., Polyethylene, Interscience

Publishers, New York, 1960, Chapter 2.

3. Chem. Week, 12 (May 5, 1993).

4. Plast. News, 22 (March 8, 1993).

5. Chem. Week, 8 (June 12, 1991).

6. Chem. Market. Reporter, 5 (December 23, 1991).

7. Chem. Eng. News, 20 (September 18, 1989).

8. R. A. V. Raff and J. B. Allison, Polyethylene, Interscience

Publishers, New York, 1956, Chapter 3.

9. J. J. Fox and A. E. Martin, Proc. Roy. Soc. London Ser.A175,

211, 216 (1940).

10. A. W. Anderson, U.S. Patent 4,076,698 (to E. I. DuPont de

Nemours & Co., Inc.), February 28, 1978.

11. A. Borruso, ‘‘Low-Density Polyethylene Resins,’’ Chemical Eco-

nomics Handbook, SRI Consulting. Menlo Park, CA, Oct. 2007.

12. Year End Statistics, American Chemistry Council, December

2007.

13. N. J. Maraschin, ‘‘Polyethylene, Low-Density’’ in Kirk-Othmer

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

John Wiley & Sons, Hoboken, NJ, 2006.

Table 5. HP-LDPE and LLDPE Molding Resins

Type: HP-LDPE HP-LDPE HP-LDPE LLDPE LLDPE LLDPE

Melt index: 6–10 15–25 35–50 0.8–1.2 12–30 50–150

Density (g/cm

): 0.924–0.926 0.914–0.918 0.923–0.925 0.918–0.922 0.920–0.926 0.926–0.935

Molecular-weight

distribution:

Broad Narrow Narrow Broad Narrow Narrow

Process

: IM IM, BM, PE IM BM, IM, PE IM IM

Applications: Bottle closures Drug bottles;

aseptic

packaging

Lids Drum liners;

irrigation

tubing; spouts

Industrial

containers;

Speciality

housewares

Lids;

housewares

Critical properties: ESCR;

stiffness ESCR;

low

modulus

Cycle time ESCR;

processability

ESCR;

low-

temperature

impact

Cycle time

Key; IM, injection molding; BM, blow molding; PE, proﬁle extrusion.

ESCR, environmental stress-crack resistance.

Table 6. Comparison of LLDPE and HP-LDPE Molded Properties

Type: LLDPE LLDPE LLDPE HP-LDPE HP-LDPE

Melt index: 20 50 100 23 49

Density (g/cm

): 0.924 0.926 0.931 0.924 0.924

Dishpan impact, ft lbf (J) at – 20 1C: 30 (40.7) 24 (32.5) 3 (4.1) 9 (12.2) 3 (4.1)

Failure mode: Ductile Ductile Ductile Shatter Shatter

ESCR

: 150 3 o2 o1

Environmental stress-crack resistance.

At 50 1C, 100% Igepal, no slit, ASTM D1693.

100

992 POLYETHYLENE, LOW-DENSITY

POLYMER PROPERTIES

RUBEN J. HERNANDEZ

Michigan State University, East

Lansing, Michigan

The properties of a polymer result from its chemical

nature, morphology, formulation, processing, and even

use conditions. Intrinsic polymer properties depend pri-

marily on the chemical nature of the polymer; but since

most polymers are polymorphic materials, their intrinsic

properties may also depend on the polymer’s morphology.

Morphological changes at room temperature, however, are

very slow and highly time-dependent. Formulation (com-

pounding with additives) and processing have a direct

impact in the ﬁnal properties of a polymeric material.

Furthermore, storage and use conditions (e.g., humidity

and aging) may affect a plastic’s performance.

Intrinsic properties of a polymer can be classiﬁed as

either molecular or bulk. Intrinsic molecular properties

depend mainly on the chemical structure of the polymer’s

constitutional units and, to a lesser degree, on its macro-

molecular character. The chemical nature of a constitu-

tional unit, which results from the type and number of

atoms, existing side groups, charge distribution, and type

of secondary molecular forces, controls important proper-

ties. These properties are cohesive energy, molecular

packing density, molecular relaxation (including glass-

transition temperature), barrier, mechanical strength,

frictional forces, surface tension, and adhesion character-

istics. In PE, for example, the –CH

– group yields a set of

properties values quite different from the constitutional

unit –OC(CH

)

NH– of nylon 6. The latter unit is larger

and more polar than the ﬁrst, and it tends to develop

strong intermolecular hydrogen bonding.

Intrinsic bulk properties such as stiffness, melting

temperature, heat-sealing temperature range, melt-ﬂow

index, and viscosity are largely inﬂuenced by the molecu-

lar mass and distribution, as well as the architecture of

the polymer chain. Molecular mass and molecular distri-

bution include the average molecular mass, dispersion

index, and single- or multimodal distribution. The ‘‘archi-

tecture’’ of a polymer refers to the monomers layout in the

polymer chain. In this respect, polymers can be linear,

branched, or crosslinked, have different tacticity (e.g.,

atactic, isotactic, and syndiotactic polypropylene), and

show various copolymer arrangements such as random,

block, alternating, or graft.

Phase morphology affects, in varying degrees, the

physical, mechanical, and optical properties of a polymer.

Polymers can be isotropic (amorphous); that is, they are

glassy (hard and brittle) below the glass-transition tem-

perature and are rubber-like (soft and elastic) above it. As

temperature increases, amorphous polymers become true

liquids without any thermodynamic discontinuity. Poly-

mers that are nonisotropic can crystallize in several

arrangements; even two different crystal structures can

coexist, depending on the values of temperature and

pressure. Sometimes a crystalline polymer remains a

liquid below the melting temperature, thus producing a

supercooled material. In semicrystalline polymers, crys-

talline and noncrystalline regions may coexist, forming

microcrystalline or paracrystalline regions. Polymers may

also have a one- or a two-dimensional molecular order in

the liquid state that forms thermotropic or lyotropic liquid

crystalline mesophases. A simple example of how the

morphology affects the properties is illustrated by the

density of PE; the density of PE may range from 0.90 to

0.97 g/mL as the percent of crystallinity increases. Besides

density, there are other properties proportional to the

degree of crystallinity: rigidity, heat resistance, barrier,

abrasion resistance, gloss, shrinkage, and parting-line

difference. However, stress-crack resistance, clarity, fold-

ing endurance, impact strength, and parison sag decrease

as crystallinity increases.

Process operation, on the other hand, can alter the

morphology of a material. Film orientation, for instance,

directly affects the polymer morphology by producing a

slightly more compact molecular packing and a more

transparent ﬁlm. Similarly, a rapid cooling process will

increase the amorphousness of a supercooled phase, while

slow cooling will increase crystallinity. Thus, process

conditions may affect properties such as the heat of fusion

of a material and may also affect operations such as

thermoforming operation and heat sealing. Since trans-

parency (or opacity) is directly controlled by the crystal-

linity, morphological changes can inﬂuence the optical

properties of a polymer. When additives are incorporated

into a resin, however, they can substantially alter the

resin’s original properties.

DETERMINATION OF POLYMER PROPERTIES

Polymer properties can be measured experimentally

(usually according to standardized methods) or be esti-

mated from semiempirical correlations. Many intrinsic

properties related to the molar constitutional unit can be

estimated from group contributions or increment meth-

ods. Some polymer properties that can be estimated by

group contribution include density, thermal expansion

coefﬁcient, thermal conductivity, speciﬁc heat, speciﬁc

entropy of fusion, melting temperature, glass-transition

temperature, cohesive-energy density, solubility para-

meter, surface tension, viscosity coefﬁcient, dielectric con-

stant, magnetic susceptibility, speciﬁc shear modulus,

speciﬁc bulk modulus, and sound velocity (1).

The most common properties of polymers related to

packaging applications are described next. For easy re-

ference they are grouped under these headings: (1) density

and thermophysical properties, (2) mechanical properties,

(3) solubility and degradation properties, (4) barrier prop-

erties, (5) surface and adhesion properties, (6) electrical

properties, and (7) optical-appearance properties.

DENSITY AND THERMOPHYSICAL PROPERTIES

Density. The density of a plastic is directly proportional

to its crystallinity: d = d

+ C(d

d

)/100, where d is the

density, C is percent volumetric crystallinity, and d

and

POLYMER PROPERTIES 993

are the amorphous and crystalline density, respectively.

For example, d

= 0.855 g/mL while d

= 1.0 g/mL for PE

(2). Polymer density values vary from 0.87 g/mL for poly-

propylene up to 1.86 g/mL for PVDC (3). The standard

ASTM D1505 (4) describes the density gradient method to

evaluate the density of ﬁlms and resins.

Glass-Transition Temperature. The glass-transition tem-

perature T

, which is actually a relaxation temperature, is

an important property in amorphous polymers. As a

relaxation temperature, T

is associated with the onset

of the rotation and mobility of chain segments involving

several monomers. Rather than a single-point tempera-

ture, T

in semicrystalline polymers is a range of tem-

perature values. The broadness of this range depends on

the sample morphology. When the polymer is at a tem-

perature below T

, it becomes glassy and stiff, while above

it shows a plastic or rubbery behavior (5). The T

values

for polymers range from about 251C(1251C is also

commonly reported) in polyethylene to 3651C for thermo-

plastic polyimide (1, 6). Both the T

and the melting

temperature set the application temperature range of a

plastic container. For instance, a polypropylene container

may become brittle at freezing temperature if its T

near 01C; ‘‘crystal’’ polystyrene is brittle at room tempera-

ture because it has a T

of B801C and does not contain

plasticizer. On the other hand, a container made of a

low-melting-temperature (T

) plastic will soften if heat-

sterilized. Both differential thermal analysis (DTA) and

differential scanning calorimetry (DSC) are used to deter-

mine T

and T

(these methods are described in ASTM

D3418). Van Krevelen (1) showed that polymers have

ratios ranging from 0.5 to 0.76. Most polymers,

however, have the T

ratio centered at 0.67.

Heat Capacity. Heat capacity (or speciﬁc heat), c, is the

amount of energy needed to change a unit of mass of a

material one degree of temperature. The heat capacity of

plastics, which are obtained at constant pressure, are

temperature-dependent, especially near the glass-transi-

tion temperature. In a semicrystalline polymer, the heat

capacity of the amorphous phase is greater than the heat

capacity of the crystalline phase implying that the c values

depend on the percent of polymer’s crystallinity. The heat

capacity values of polymers at 251C vary from 0.9 to 1.6 J/

(g K) for amorphous polymers and from 0.96 to 2.3 J/

(g K) for crystalline polymers. Reliable data of the heat

capacity of amorphous and crystalline phases are avail-

able for only a limited number of polymers (1, 2). Values of

c for polymers can be found in the review by Wunderlich

(7). The usual techniques to measure c are differential

thermal analysis (DTA) and differential scanning calori-

metry (DSC).

Heat of Fusion. The heat of fusion, DHm, is the energy

involved during the formation and melting of crystalline

regions. For semicrystalline polymers, the heat of fusion is

proportional to the percent of crystallinity. Amorphous

polymers or amorphous polymer regions do not have heat

of crystallization, since amorphous structures have a

smooth transition from the liquid amorphous state to the

liquid state. Experimental values of crystalline heat of

fusion for common packaging plastics vary from 8.2 kJ/mol

for polyethylene to 43 kJ/mol for nylon 6,6 (1). ASTM

D3417 describes a method for measuring the heat of fusion

and crystallization of a polymer by differential scanning

calorimetry (DSC).

Melting Temperature. The melting temperature T

is a

true transition temperature. This means that at T

both

the liquid and solid phases have the same free-energy

value. Most semicrystalline polymers, instead of having a

sharp melting point, show a melting temperature range,

and for amorphous polymers there is no T

. Similar to T

can be estimated from contribution groups and em-

pirical relationships between T

and T

(as indicated

above). Values of T

range from as low as 275 K (for

polyisobutylene), to up to 728 K (for polyethylene ter-

ephthalamide) (1, 6). ASTM D2117 and D3418 describe

methods for measuring T

Thermal Conductivity. Thermal conductivity k is the

parameter in Fourier’s law relating the ﬂow of heat to

the temperature gradient. In practical terms, k is a

measure of the ability of a material to conduct heat. The

thermal conductivity of a polymer is the amount of heat

conducted through a unit of thickness per unit area, unit

time, and degree of temperature. Thermal conductivity

values control the heat-transfer process in applications

such as plastic processing, heat sealing, cooling and heat-

ing a package, and sterilization processes. The k values for

plastics are much lower than those for metals; for in-

stance, thermal conductivity for plastics ranges from

3 10

4

for PP to 12 10

4

cal/(s cm 1C) for HDPE,

while for aluminum k = 0.3 cal/(s cm 1C) and for steel

k = 0.08 cal/(s cm 1C) (8). Plastic foams show k values

much lower than those for the nonfoamed plastics because

of the presence of air trapped within the cellular struc-

ture. Enhanced by low thermal conductivity of air, foams

make excellent insulating materials and good cushioning

packaging materials. Plastic ﬁllers may increase the

thermal conductivity of plastics. Methods for measuring

k are given in ASTM D4351, C518, and C177.

Thermal Expansion. The coefﬁcient of linear (or volume)

thermal expansion is the change of length (volume) per

unit of length (volume) per degree of temperature change

at constant pressure: b = (1/L)(dL/dT) and a = (1/V)

(dV/dT). Units of a and b are K

1

or 1F

1

(reciprocal

degrees Kelvin and Fahrenheit). Compared to other ma-

terials, polymers have high values of thermal expansion

coefﬁcients. While metals and glass have values in the

range 0.9–2.2 K

1

polymers range from 5.0 to 12.4 K

1

(8).

Thermal expansion coefﬁcients can be measured by ther-

momechanical analysis (TMA). ASTM D696 gives a

method using a quartz dilatometer, and ASTM E831

describes the determination of the linear thermal expan-

sion of solid materials. Volume contraction of a container

from the molding operation temperature down to room

temperature is called shrinkage, and its measurement is

described in ASTM D955, D702, and D1299-55.

994 POLYMER PROPERTIES

MECHANICAL PROPERTIES

The response of polymers subjected to mechanical forces is

determined by the polymer’s isoentropic elastic and viscoe-

lastic behavior. Ideally, elastic behavior should be reversi-

ble for small deformations. Two types of small deformation

are possible: tensile and shear. Hooke’s law describes the

response of plastics to tensile forces by relating tensile

stress to elongation, while the law of shear deformation

relates shear stress to both shear modulus and angle of

shear. Viscoelastic behavior is observed in large deforma-

tions of solid polymers and polymer melts (9). In a creep

experiment, the elongation of solid sample increases slowly

as the weight hangs from the sample. When the force is

released, the sample partially recovers its shape, decreas-

ing the elongation. This behavior is due to the viscoelasti-

city behavior of the polymers: It partially recovers because

it is elastic, and it creeps because it is viscous. This behavior

is due to the continuous-chain molecular rearrangement

that takes place at all temperatures. The time lapses

associated with molecular rearrangement are much larger

below than above the glass-transition temperature. The

viscoelastic behavior affects almost all mechanical proper-

ties of a polymer and should be carefully considered in

packaging design. Polymer strength increases with increas-

ing molecular mass and with increasing intermolecular

forces, but it decreases with the presence of plasticizers.

For example, at the same molecular mass, polyamides,

polyesters, or polyacrylonitriles are stronger than polyole-

ﬁns, and plasticized PVC is weaker than rigid PVC.

Bursting Strength. This is the hydrostatic pressure,

usually in pascal (or psi), required to produce rupture of

the material when the pressure is applied at a controlled

increased rate through a circular rubber diaphragm of

30.48 mm (1.2 in.) in diameter. ‘‘Points bursting strength’’

is the pressure expressed in psi. The measurement of the

bursting strength of plastic ﬁlms is described in ASTM

method D774.

Dimensional Stability. Dimensional stability refers to

the capability of a structure to maintain its dimensions

under the changing conditions of temperature and humid-

ity. Machine and transverse directions may produce dif-

ferent changes in dimensional stability, which is

important in any ﬂexible material converting process.

During printing, for example, even small changes in

dimensions may lead to serious problems in holding a

print pattern. ASTM D1204 describes a standard method

for linear dimensional changes of ﬂexible thermoplastic

ﬁlms and sheets at elevated temperature.

Folding Endurance. This is a measure of the resistance

of the material to ﬂexure or creasing. Folding endurance is

greatly inﬂuenced by the polymer’s glass-transition tem-

perature and the presence of plasticizers. ASTM D2176

describes the procedure to determine the number of folds

necessary to break the sample ﬁlm.

Impact Strength. Impact strength is the material’s re-

sistance to breakage under a high-velocity impact. Widely

used impact tests are Izod (ASTM D256A) and Charpy

(ASTM D256) for rigid materials, and for dart-drop impact

(ASTM D4272) and pendulum-impact resistance (ASTM

D3420) for ﬂexible structures. A free-falling dart method

for polyethylene ﬁlms is described in ASTM D1709. Unlike

low-speed uniaxial tensile tests, the pendulum-impact test

measures the resistance of ﬁlm to impact puncture simu-

lating high-speed enduse applications. Dart drop mea-

sures the energy lost by a moderate-velocity blunt

impact passing through the ﬁlm. Both pendulum and

impact tests measure the toughness of a ﬂexible structure.

Melt-Flow Index. Most polymer melts follow a pseudo-

plastic behavior, which implies that the viscosity de-

creases with shear rate. In common extrusion processes,

the shear-rate value varies from 100 to 100,000 s

1

(per

second). It follows that a complete rheological description

of an extrusion process should include a wide range of

shear-rate values at suitable temperature range. The

melt-ﬂow index (MFI) is a widely used test to measure

polymer ﬂow properties, but it provides ﬂow values at only

one shear-rate value. MFI, also called extrusion plast-

ometer test, is described in ASTM D1238. The melt-ﬂow

index measures the mass of polymer extruded during

10 min. Values of MFI vary between 0.3 and 20 g/10 min,

corresponding to shear-rate values ranging from 1 to

50 s

1

. Variables that affect the MFI of a resin include

average molecular mass, distribution of molecular mass,

branching, and temperature (10).

Pinhole Flex Test. Pinhole ﬂex resistance is the property

of a ﬁlm to resist the formation of pinholes during re-

peated folding. A related test is the folding endurance.

Film having a low value of pinhole ﬂex resistance will

easily generate pinholes at the folding line, under re-

peated ﬂexing. The test is described by the standard

ASTM F456.

Poisson’s Ratio. Poisson’s ratio is the ratio of lateral

strain to longitudinal strain measured simultaneously in

a creep experiment. Its value varies from 0.5 (for totally

deformed but nonelastic liquids), to 0 (for pure elastic

incompressible solid materials). Typical values for rubber

materials range from 0.49 to 0.499; and for plastic, from

0.20 to 0.40. This shows that when a polymer sample is

elongated, its volume will increase. Because polymers are

viscoelastic materials, the Poisson-ratio value is morphol-

ogy- and time-dependent (1).

Tensile Properties. The mechanical behavior of a poly-

mer can be evaluated by its stress–strain tensile charac-

teristics. The stress is measured in force/area, which can

be given in pascals or psi, and the strain is the dimension-

less fractional length increase. Modulus of elasticity, E,is

the elastic ratio between the stress applied and the strain

produced, giving the material’s resistance to elastic defor-

mation. The tensile modulus also gives a measure of the

material stiffness; the larger the modulus, the more brittle

the material. For example, E of LDPE is 250 mPa, while

for ‘‘crystal’’ PS it is 2500 mPa. Comparatively, values of

tensile modulus in polymers (1.9 10

mPa for nylon) are

POLYMER PROPERTIES 995

much lower than for glass (60 10

mPa) or mild steel

(220 10

mPa) (6). Elastic elongation is the maximum

strain under elastic behavior. Ultimate strength or tensile

strength is the maximum tensile stress the material can

sustain. Ultimate elongation is the strain at which the

sample ruptures. Toughness is how much energy a ﬁlm

can absorb before rupturing, and it is measured by the

area under the stress–strain curve. Brittleness is the

lack of toughness (11). Amorphous and semicrystalline

polymers become brittle when cooled below their glass-

transition temperature. Tests for tensile properties are

described in ASTM D882, for ﬂexural strength in ASTM

D790, and for ﬂexural modulus in ASTM D790 M.

Tear Strength. Tear-strength measurement considers

the energy absorbed by the ﬁlm sample in propagating a

tear. Standard methods available are (a) ASTM standard

D1004, which describes the measurement for initial tear

resistance, and (b) ASTM D1922 and ASTM D1938, which

refer to the energy absorbed by a test specimen in propa-

gating the tear that has already been initiated. The value

of tear strength in one ﬁlm depends on the orientation

stretching ratio, whether the measurement is performed

along the crossdirection or along the machine direction

(CD or MD).

Viscosity. Viscosity, a fundamental rheological property

of ﬂuids, is a measure of the molecular resistance to shear

ﬂuid deformation, generated by the action of external

forces. When the external action is given by the shear

stress and the shear ﬂow deformation given by the rate of

strain or shear rate, the absolute viscosity is equal to the

ratio of shear stress/shear rate. The viscosity of a polymer

will increase with the length of the polymer chain, with

smaller dispersion index (narrow distribution), and with

increasing intermolecular forces. An increase of tempera-

ture, however, will decrease the viscosity. The rheological

behavior of thermoplastics is, in most cases, Newtonian or

pseudoplastic (10). The viscosity of Newtonian ﬂuids is

constant at any shear rate value. This behavior is char-

acteristic of ﬂuids with low molecular mass. In pseudo-

plastic materials, the viscosity decreases with shear rate;

that is, the viscosity depends on the value of shear rate.

Pseudoplastic behavior is characteristic of polymer melts.

The viscosity of polymer melts ranges between 100 and

N/(s m

2

). Viscosity is also known as either kinematic

viscosity or intrinsic viscosity. Kinematic viscosity is the

absolute viscosity divided by the density. Intrinsic viscos-

ity (IV) can be obtained as follows: (1) Solutions of differ-

ent concentrations c of the polymer in a solvent (having

viscosity Z

) are prepared; (2) the viscosity Z of the solu-

tions are measured; and (3) from the plot of viscosity

number, [(Z Z

)/Z

c], versus concentration, the IV is

obtained as the value of the viscosity at concentration

zero (11). Therefore, intrinsic viscosity, is a measure of the

polymer’s capacity to enhance the viscosity of the solvent.

The units of IV are mL/g. For PE, the relation between

molecular weight (MW) and IV is accepted to be as follows:

MW = 5.3 10

(IV)

1.37

(8). An IV of 5, for example, corre-

sponds to an MW of 500,000. Intrinsic viscosity values of

polymers can be estimated by group contribution in terms

of molar intrinsic viscosity (1). ASTM has published

several methods for measuring viscosity.

SOLUBILITY AND CHEMICAL DEGRADATION

Plastics are chemically resistant to many gases, liquids,

and solid products within a wide range of pH. Never-

theless, polymers are not inert materials. Given the right

conditions (temperature, time, and concentration), poly-

mers can be depolymerized, transformed by chemical

reactions, and penetrated by solvents and vapors. They

can be affected by environmental agents such as visible

and UV radiation, oxygen, microbes, solvents, and organic

compounds (12). The extent of the transformation depends

on the thermodynamic and kinetics parameters, which, in

turn, depend heavily on temperature. In hydrophilic

polymers (e.g., nylons, EVOH, and PET) water acts as a

plasticizer, consequently lowering the glass-transition

temperature and changing oxygen permeability and other

mechanical properties (13). Water can dissolve certain

polymers such as PVOH. Hydrophobic polymers such as

polyoleﬁns, on the other hand, are not affected by water.

Chemical Degradation. Chemical degradation of poly-

mers results in the fragmentation of large chains into

smaller ones and, eventually, into atomic elements. One

important type of polymer chemical degradation is oxida-

tion (12). Often oxidation is promoted or induced by

electromagnetic radiation (photodegradation) and by ther-

mal energy at high temperature (thermal oxidation).

Polymers such as PE, PP, and PVC are particularly prone

to oxidation during melt processing. Tensile strength,

elasticity, and impact strength are drastically affected by

thermodegradation processes. Polymers may also be de-

graded by acids and alkalis. Test for resistance to acids,

bases, and solvents is described in ASTM D543. Plastic

resistance to grease and oil is covered in ASTM D722. The

ASTM D756 test covers the effect of atmospheric humidity

and temperature on plastics, and ASTM D570 covers the

sorption of water of immersed plastics.

Environmental Stress Cracking. A plastic material may

be resistant to a chemical compound in no-stress condi-

tions. Nevertheless, the plastic may crack when subjected

to a mechanical stress during exposure to that compound.

Almost all plastics can show environmental stress crack-

ing (ESC), when exposed to gases, liquids, or solids while

under stress. The mechanism by which a plastic shows

ESC is often complex. A practical evaluation of a plastic’s

resistance to liquids can be done by immersing a polymer

sample in the liquid and then checking for the appearance

of crazing, cracks, or even total failure of the sample. Also,

a plastic container can be tested by ﬁlling it with the

liquid, causing ESC. Stress conditions applied during

testing may be of two types: constant stress or constant

strain. The selection of the test method should correspond

with the intended use of the container. Common ESC tests

for PE bottles include bottle stress crack (ASTM D2561),

top-load stress crack, and internal pressure (ASTM

996 POLYMER PROPERTIES

D2561). For some applications, the temperatures recom-

mended by ASTM for these tests may be too mild (14).

Flammability. The ﬂammability behavior of a material

is a broad term related to the easiness to ignite, burn,

produce smoke, and endure burning. A ﬂammable mate-

rial ignites easily and has a rapid ﬂaming combustion

process. The initial step is decomposition (or pyrolysis),

which is an endothermic process (the material absorbs

heat). After ignition, products from pyrolysis are com-

busted and consequently, heat Q is generated. Some

ﬂame-retardant agents tend to minimize Q. The ﬂamm-

ability characteristics of a polymer may be predicted from

its chemical structure (1), and a method to measure ﬂame

resistance is the oxygen index (15). ASTM test D3713

describes the response of solid plastics to ignition by a

small ﬂame, and the ASTM D4100 method covers the

gravimetric determination of smoke-particulate matter

produced from combustion or pyrolysis of plastic materi-

als. Testing plastics for practical ﬁre situations is very

complex and depends on many variables other than just

chemical structure (16, 17).

Photodegradation. Sunlight is the source of energy for

polymer photochemical degradation. Near-ultraviolet and

visible radiation (290–710 nm) carry enough energy to

break single covalent bonds such as C–C, C–O, or C–N.

The absorption of these radiations is sometimes attributed

to impurities in the polymer. Photodegradation processes

generate chain ruptures and free-radical formation, re-

sulting in color changes and increasing fragility (because

of chain crosslinking and chain scissions). UV light stabi-

lization of PP and other polyoleﬁns can be achieved by the

addition of hindered amine light stabilizers (HALSs) (18).

These additives reduce the rate of photoinitiation and

chain-reaction propagation by their ability to trap free

radicals. ASTM D4674 describes the accelerated testing

for color stability of plastics exposed to indoor ﬂuorescent

lighting and window-ﬁltered daylight.

Solubility Parameter. Polymer solubility properties in

organic liquids are controlled by the polymer’s cohesive

energy. The cohesive energy of a solid (or liquid) is the

total energy necessary to remove one molecule from

the bulk of the solid (or liquid). It is also an indication of

the molecular internal pressure. The solubility of a poly-

mer is given by its solubility parameter d, which is the

square root of the cohesive-energy density (CED),

d = (CED)

1/2

. In addition to cohesive energy density, poly-

mer morphology and molecular mass also strongly affect

the tendency of a polymer to dissolve. Crystalline struc-

tures, on the other hand, are insoluble below their melting

temperature. Polymers have solubility parameter’s ran-

ging from 12 to 29 (J/cm

)

1/2

. Substances that have similar

delta values [o0.5 (J/cm

)

1/2

apart] will tend to mutually

dissolve. This substantiates the rule of thumb that ‘‘like

dissolves like.’’ Solubility parameters can be determined

by solvent swelling (19) or intrinsic viscosity methods (20).

Values of CED and solubility parameter of a polymer

can be estimated by group contribution methods, based

on the contribution of intermolecular forces: polar forces,

dispersion forces, and hydrogen bonding (1). Extensive

tables of d are presented by Barton (21).

Solubility in Organic Solvents. Many organic compounds

interact with polymers to produce slight swelling or

staining, or total dissolution, depending on the polymer

molecular mass, crystallinity, and solubility parameter.

ASTM D2299 gives a method to assess the susceptibility of

plastic staining by incidental contact with organic pro-

ducts. Common solvents for plastics are boiling xylene and

trichlorobenzene; solvents for polyoleﬁns, are styrene

polymers, vinylchloride polymers, and polyacrylates; the

solvent for polyamides and polyvinyl alcohol derivatives is

formic acid; tetrahydrofuran is the solvent for all uncros-

slinked polymers; nitrobenzene and m-cresol are the

solvents for PET; water is the solvent for polyacrylamide

and polyvinyl alcohol; and dimethylformamide is the

solvent for polyacrylonitrile. In addition, polyvinyl acetate

is dissolved by benzene, chloroform, methanol, acetone,

and butyl acetate (22). ASTM D543 covers the testing of

all plastic materials for resistance to chemical reagents.

Thermodegradation. Thermal degradation of polymers

is the breaking of chain bonds by heat, in the absence of

oxygen. Heat-stabilizing additives help prevent degrada-

tion, but impurities may have the contrary effect. Ther-

mally stable polymers resist thermal degradation and will

have high values of bond dissociation energy (1). As the

temperature increases, chemical bonds with the lowest

values of dissociation energy will be the ﬁrst to break.

Thermodegradation affects PE, PP, and PVC (18). The

poor thermal stability of the PVC resin is caused mainly

by dehydrochlorination. For each HCl eliminated from the

polymer chain, a double bond is created, which, in turn,

promotes oxygen chemical degradation. Heat stabilizers

include inorganic and organic compounds. In the case of

ﬂexible PVC ﬁlms, barium–zinc and calcium–zinc stabili-

zers are used, while organotin compounds are commonly

used in PVC bottle containers.

BARRIER PROPERTIES

The barrier property of a material indicates its resistance

to diffusion and sorption of foreign molecules. A high-

barrier polymer has low values of both diffusion (D) and

solubility (S) coefﬁcients. Since the permeability coefﬁ-

cient P is a derived function of D and S, a high-barrier

polymer has low permeability. The diffusion coefﬁcient

indicates how fast a penetrant will move within the

polymer, while the solubility coefﬁcient gives the amount

of the penetrant taken (or sorbed) by the polymer from a

contacting phase. Both diffusion and solubility can be

applied to the reverse process of sorption—that is, the

migration of compounds from the polymer to a surround-

ing medium. Several factors inﬂuence the effective value

of the diffusion and solubility coefﬁcients in polymers:

(1) chemical compositions of the polymer and permeant,

(2) polymer morphology (since diffusion and sorption occur

mainly through the amorphous phase and not through

crystals), (3) temperature (as temperature increases,

POLYMER PROPERTIES 997