Yam, Kit L. (ed.). The Wiley encyclopedia of packaging technology

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the reason to blow-mold PP today. In detergent exposure

tests that crack HDPE in a day or two, PP does not fail even

after many weeks. Random and impact copolymers are most

often used. The development of high-melt-strength resins

will assist the growth of PP blow molding. Today, injection

stretch blow-molded PP bottles are biaxially oriented and

rival the clarity of PET. If the chains of PP are stretched,

they line up to give remarkable strength, clarity, and

toughness (even at low temperature). This process uses a

preform that is stretched and blown while warm. Random

copolymers are used since low temperature toughness is

provided by biaxial orientation. Typical resin MFR is 1–3 g/

10 min for these applications.

Polypropylene ususally has a milky appearance (pati-

cularly when empty). However, the contact clarity is really

quite remarakalbe, making a ﬁlled bottle look attractive.

Full-shrink-sleeve decoaration changes the whole visual

impact if total clarity is an issue.

Film

The use of PP in ﬁlm is very large. Oriented ﬁlms typically

have high toughness and excellent clarity (see Film,

oriented polypropylene). They can be produced by a high-

expansion bubble process or a tenter process. Product

variations are possible based on the amount of transverse-

and machine-direction orientation. Nonoriented cast ﬁlms

are usually made by a chill-roll process, but there are also

water-quench and water-quenched bubble processes in use

(see Extrusion; Film, nonoriented polypropylene). Or-

iented ﬁlms can have a stiff feel or ‘‘hand.’’ They sparkle

and tend to ‘‘crinkle’’ audibly. These ﬁlms are employed as

cigarette wrap, candy wrap, and snack-food pouches, often

in replacement of cellophane (see Cellophane). Oriented

ﬁlms have been tailored for superior barrier properties (by

coating), heat-seal strength (by resin modiﬁcation or coat-

ing), heat-shrink properties (see Films, shrink), printabil-

ity, and electrical properties. A relatively new addition to

the family of oriented ﬁlms is opaque ﬁlm. The opacity is

produced by a ﬁller and by controlled voiding. A type of

decorative ribbon is foamed uniaxially oriented PP with a

colorant (see Colorants). Foods stored in restaurants or

microwaved are wrapped in ﬁlms containing chloride.

Vinyl chloride produces unsafe emissions upon burning.

Polypropylene, as a nonchlorine-type resin, is being stu-

died as an alternative wrap. Various additives have been

proposed to overcome insufﬁcient gas barrier and adhesion

properties (5).

The use of woven slit tape or slit ﬁlm for heavy-duty

agricultural bags is not a major use in the United States,

but these bags compete with jute and other natural

ﬁber in carrying much of the world’s grain. A related

product is strapping, which is made by extruding either

sheet or tapes. If sheet is extruded, it is subsequently slit

(see Slitting/rewinding). The ﬁlaments are then stretched

while warm to give tensile strength values of Z50,000 psi

(W345 MPa), ten times that of unoriented PP.

Nonoriented ﬁlm is available in thicker gauges and

has a softer ‘‘hand’’ at the same thickness. Some applica-

tions are release sheets, sanitary products, disposable-

diaper layers, bandages, and apparel packaging. The use

of PP in composite ﬁlm and sheet materials is small, but

growing rapidly. PP is used in combination with other PP

structures, paper, metal foils, fabric (woven and nonwoven),

and other plastics. Such composites can be made by coex-

trusion (see Coextrusion), lamination (see Laminating), or

extrusion coating (see Extrusion coating). The motivation

for making such structures is often related to barrier

properties, temperature resistance, chemical resistance,

and cost. The principal market for these products is in

food packaging.

Most ﬁlm processes use PP grades with a MFR in the

range of 2–10 g/10 min. Extrusion coating uses materials

with ﬂow ranging from 10 to over 60 g/10 min. Selection of

the additive package is an important consideration. Print-

ing, winding, static-charge buildup, blocking (sticking

together), odor, color, heat-seal strength, and other proper-

ties are inﬂuenced by the additives.

Injection Molding

Injection molding produces many familiar packaging

items, including threaded, dispensing, and pump closures

(see Closures); aerosol valves and overcaps (see Pressur-

ized packaging); wide-mouthed jars, totes, crates (see

Crates, Plastic), yogurt cups, snuff boxes, cosmetic con-

tainers, drug syringes, barrel bungs, dairy tubs, and many

others. Most general-purpose PP molding grades are well-

suited for packaging. In food and drug packaging, one

must ensure that the particular grade is suitable for the

product to be packaged. Conventional injection-molding

grades span a MFR range of 2 to W70 g/10 min.Generally,

the lower-ﬂow materials are tougher and process less

rapidly. New polymer production processes can produce

very high-ﬂow PP grades without narrowing the MWD.

Although less tough than narrow-MWD grades of similar

MFR, these grades are more easily molded. Copolymers

with MFR up to 70 g/10 min are being used in thin-walled

injection-molded (TWIM) containers, notable for quality

appearance and ease of printing or decorating. Thin walls

allow high productivity, low part weight, and low con-

tainer cost. By using narrow-MWD or ‘‘controlled-rheol-

ogy’’ resins, high MFR grades with good toughness can be

made. These grades are appropriate choices for thin-

walled moldings and for large multiple-cavity molds.

They offer better dimensional control, but are not as stiff

as broader-MWD material.

Normal homopolymers are brittle at refrigerator

temperatures. The use of copolymers is recommended

when shipping or use expose the part to low temperature

impact.

HEALTH AND SAFETY ISSUES

Except

for ﬁre-retardant grades

containing antimony, PP

is generally a nontoxic material. Many grades are avail-

able that comply with FDA requirements for

food packaging. PP is used in drug packaging and medical

devices. It is ﬁber-spun for use in undergarments,

1008 POLYPROPYLENE

upholstery, sanitary products, and bandages. It is not

soluble at normal temperatures in food and beverages.

PP is combustible and burns completely when adequate

air is available to the ﬂame. As sold by resin manufac-

turers, PP is usually in a coarse granular form and

presents no unusual ﬁre hazard. If it is ﬁnely divided

(ﬁner than 200 mesh or 74 mm), however, polypropylene

can present a combustible dust hazard as do most organic

materials. Like other organic materials (e.g., wood, wool,

ﬂour), the products of incomplete combustion include

carbon monoxide and can include a number of unpleasant,

partially oxidized pyrolysis products (aldehydes, ketones,

etc.).

Under most processing conditions, little hazard exists

with the use of PP. One should avoid contact with the

molten polymer. If the plastic is exposed to air at tem-

peratures above 5001F (2601C), proper ventilation should

be used. The autoignition temperature is 675–7001F (357–

3711C) for most grades. In summary, there are no unusual

risks associated with the use of PP.

MARKETS AND APPLICATIONS

Polypropylene resins, along with polyester resins, are the

fastest growing of commodity thermoplastics in the world.

In 2006, production was 41.5 million tons with an esti-

mated value of $66 billion (6).

Isotactic polypopylene is well suited for a variety of end

uses. Polypropylene is recyclable, and this is an important

consideration for packaging products. It can be incinerated

without toxic emissions. Typical applications for polypro-

pylene products include ﬁbers and ﬁlaments, oriented and

cast ﬁlm, injection-molded products, blow-molded bottles

and parts, and thermoformed containers. Random copoly-

mers can be reproduced with excellent optical properties

and sealing initiation temperatures in compliance with the

U.S. Food and Drug Administration regulations for food

contact (1). Polypropylene can be combined with polyethy-

lene as blends for liquid packaging ﬁlms (7).

BIBLIOGRAPHY

R. C. Miller, ‘‘Polypropylene’’ in A. J. Brody and K. S. Marsh, eds.,

The Wiley Encyclopedia of Packaging, 2nd edition, Wiley, New

York, 1997, pp. 765–768.

Cited Publications

1. R. B. Lieberman, ‘‘Polypropylene’’ in the Kirk–Othmer Ency-

clopedia of Chemical Technology, Vol. 20, 5th edition, Wiley,

Hoboken, NJ, 2006.

2. J. R. Fried, ‘‘Polymer Technology—Part 1: The Polymers of

Commercial Plastics,’’ Plast. Eng. 38, 49–55 (1982).

3. J. R. Fried, ‘‘Polymer Technology—Part 3: Molecular Weight

and Its Relation to Properties,’’ Plast. Eng. 38, 27–33 (1982)

(Parts 4, 5, and 6 followed).

4. Annual Book of ASTM Standards, Vol. 8.0, ASTM Interna-

tional, West Conshohocken, PA, issued annually.

5. M. Iriya and T. Nakao, U.S. Patent 7,323,240 (to Asahi Kasei

Life and Leasing Corp.), January 29, 2008.

6. A. Borruso, ‘‘Propylene Resins,’’ Chemical Economics Hand-

book, SRI Consulting, Menlo Park, CA, 2007.

7. A. K. Breck, N. Farkas, and Y. Cai, U.S. Patent Application

2008/8090062, April 17, 2008.

General Reference

N. Pasquini, Propylene Handbook, Hanser Publishers, New York,

2005.

POLYSTYRENE

PHILLIP A. WAGNER

JOHN SUGDEN

The Dow Chemical Company

Midland, Michigan

Updated by Staff

INTRODUCTION

Polystyrene is the parent of the family of styrene-based

plastics. By copolymerization of styrene with other mono-

mers, a wide range of properties is obtainable. Polystyrene

resin is one of the most versatile, easily fabricated, and

cost-effective plastics. The largest end-use is in packaging,

which accounts for 18% of expanadable polystyrene world

use and 30% of general-purpose polystyrene world use. In

2004, world consumption of expandable polystyrene was

3.7 X10

t and general-purpose polystyrene was 11.5 X10

t. An annual average increase of about 3.6% is expected

through 2009 (1).

General-purpose polystyrene (Table 1) is often called

crystal polystyrene, which refers to the clarity of the resin.

The commercial success of this resin is due to its trans-

parency, ease of fabrication, thermal stability, low speciﬁc

gravity, high modulus, excellent electrical properties, and

low cost. It is commercially produced by two processes.

Suspension polymerization is used primarily to produce

foam-in-place beads and ion-exchange resin. The bulk of

the resin is produced by solution polymerization in a

continuous process consisting of one or more vessels

followed by volatile removal at high temperatures and

high vacuum. There are three common commercial grades

of general-purpose polystyrene: easy ﬂow, medium ﬂow,

and high heat. The choice of resin depends mostly on the

fabrication method. Easy ﬂow and medium ﬂow are used

primarily for injection molding and the high-heat extru-

sion applications.

High-heat polystyrene, such as Styron 685D (trade-

mark of The Dow Chemical Company), has the highest

molecular weight and contains the fewest additives. High-

heat resins normally do not contain mineral oil or other

ﬂow additives and generally contain a mold release

POLYSTYRENE 1009

additive or extrusion aid. The higher heat distortion

and toughness are offset by low melt-ﬂow rates.

The high molecular weight improves the mechanical

properties of the resin and also the ability to induce

orientation during fabrication. Orientation of the poly-

meric chains tremendously increases the toughness of

resin. For food-packaging applications, low residual levels

are desirable to minimize taste or odor transmission to the

food.

High-impact polystyrene (Table 2), sometimes called

rubber-modiﬁed polystyrene, is normally produced by

copolymerization of styrene and a synthetic rubber. This

produces a two-phase system consisting of a dispersed

rubber phase and a continuous polystyrene phase. This

system uses a unique feature of polystyrene to allow

elongation by the formation of energy-absorbing crazes.

The dispersed rubber particles initiate astronomical num-

bers of crazes without crack formation, thus contributing

to the development of very tough products. By varying the

amount of rubber, normally 2–15 wt%, and morphology

of the rubber-phase physical properties of the resin can

be varied considerably. High-impact polystyrene has

lower tensile strength and a tremendous increase in

elongation and impact strength. The opacity of high-

impact polystyrene precludes its use in applications re-

quiring transparency.

FABRICATION

Processing of polystyrene can be accomplished by most

fabricating techniques. Polystyrene resins are among the

most widely used thermoplastics for both extrusion and

injection-molding applications. Various formulations offer

different properties specially suited for speciﬁc applications .

This also results in slightly different processing conditions .

P olystyrene resins are usually processed at melt tempera-

tures of 360–5001F. In extrusion applications, extruders

vary signiﬁcantly in size. Extruder screws can range from

small (

in:-dia) to large (12-in.-dia) with varying L/D

ratios from 20:1 to 42:1. The current trend in extruder

design is toward 30:1 and 36:1 ratios using two-stage single

screws and vented barrels. Both single- and two-stage

screws are used because of polystyrene’s excellent stability

compression ratios, between 3:1 and 5:1, which are used to

achieve good mixing and uniform delivery. Screw cooling of

the feed section of the screw is recommended on large

extruders when operating at high output rates or when

high levels of regrind are being used. Because polystyrene is

nonhygroscopic, drying of the granules prior to extrusion is

usually unnecessary.

An optimum set of operating conditions can be deter-

mined only through experience with a particular extruder.

Optimum running conditions require the proper balancing

Table 1. Typical Properties for General-Purpose Polystyrene

Property ASTM Test Method Units High Heat Medium Flow Easy Flow

Melt-ﬂow rate (condition G; 2001C/5 kg) D1238 g/10 min 1.6 7.5 16.0

Vicat softening temperature D1525 1C 108 102 88

Deﬂection temperature under load annealed

264 psi

D648 1C 103 84 77

Tensile strength at break D638 psi (MPa) 8200 (56.6) 6500 (44.8) 5200 (35.9)

Tensile elongation at break D638 % 2.4 2.0 1.6

Tensile modulus D638 psi (MPa) 485,000 (3340) 460,000 (3175) 450,000 (3100)

Izod impact D256 ft lb/in. (J/m) 0.5 (24) 0.4 (21) 0.3 (16)

Rockwell hardness D785 M scale 76 75 72

Molecular weight SEC Daltons 300,000 225,000 210,000

Molecular number SEC Daltons 130,000 92,000 75,000

Table 2. Typical Properties of High-Impact Polystyrene

Properties ASTM Test Method Units Extrusion Resin Injection-Molding Resin

Melt-ﬂow rate (condition G, 200 1C/5 kg) D1238 g/10 min 3.0 7.5

Vicat softening point D1525 1C 100 94

Deﬂection temperature under load annealed

264 psi

D648 1C92 82

Tensile rupture D638 psi (MPa) 2400 (16.6) 2300 (15.9)

Tensile yield D638 psi (MPa) 2600 (17.9) 2700 (18.6)

Tensile modulus D638 psi (MPa) 240,000 (1655) 260,000 (1793)

Tensile elongation D638 % 40 40

Flexural modulus D790 psi (MPa) 280,000 (1931) 260,000 (1793)

Flexural strength D790 psi (MPa) 5600 (38.6) 4700 (32.4)

Izod impact notched 731F D256 psi (J/m) 1.5 (80.1) 1.6 (85.5)

Rockwell hardness D785 M scale 29 25

1010 POLYSTYRENE

of many variables such as screw design, extruder size,

extrusion rate, extruder conditions, polymer used, and

desired end product. Suggested starting conditions using a

typical polystyrene screw are as follows:

Hopper

Feed section

Transition section

Metering section

Adequate Flow of Cooling Water

3501F (1761C)

4001F (2041C)

4401F (2261C)

Extruder head pressures range from 1500 to 4000 psi.

The die gap is normally set at 10–20% greater than the

desired sheet thickness. Polystyrene resins are notably

stable materials. Evidence of this is seen in the ability to

use regrind and to recycle these resins. Many new packa-

ging applications are being developed through the use of

coextrusion. By incorporating layers of other resins such

as EVOH and PVDC, barrier properties can be achieved,

and the ease of polystyrene fabrication and effective cost

and performance of polystyrene can be maintained.

In injection-molding applications, machines vary in

size and capacity to permit molding very small to very

large parts. Machine cost is primarily a function of

clamping capacity, which is the force necessary to hold

the mold halves together during high-pressure injection.

This is directly related to the pressure needed to ﬁll the

mold cavity. Typical sizes are listed in Table 3. For best

operating conditions, the preferred shot size is usually

about 50–75% of the maximum shot size of the machine.

Cooling time is the major portion of an injection-mold-

ing cycle. The cooling times are inﬂuenced mainly by part

wall thickness and the heat capacity of the polymer. In the

case of crystalline polymers (e.g., polypropylene), the heat

of crystallization is a large part of the heat that must be

removed. The shorter cooling times for amorphous poly-

mers, such as polystyrene resins, allow for higher produc-

tion rates than are possible with many crystalline

polymers. Typical injection-molding conditions for poly-

styrene are listed in Table 4.

Blow-molding is a multistep fabrication process for

maufacturing hollow symmetrical objects. Styrene-based

plastics are blow-molded to make speciality bottles and

containers. However, blow-molding is primarily a process

for polyethylene and poly(vinyl chloride) (2).

APPLICATIONS

Polystyrene is widely used in many packaging applica-

tions. These include injection-molded products such as

beverage containers, dairy product containers, and packa-

ging for personal care items. Extruded solid-sheet poly-

styrene products include salad boxes, dairy product and

baked good containers, and vending cups and lids. Ex-

truded foam sheet polystyrene packaging products include

poultry and meat trays, produce trays, hinged lid contain-

ers, egg cartons, and foam cups. Blow- and foam-molded

polystyrene products include vitamin bottles and loose ﬁll

packaging. Reported newly investigated products are (a) a

paper-wrapped polystyrene foam beverage container that

provides thermal insulation as well as a means to display

high-quality printed matter (3) and (b) a food packaging

tray of expanded polystyrene with the ability to absorb

liquids inside (4).

RECYCLING

Polystyrene items separated from solid-waste streams are

subjected to one or more of the following unit operations:

densiﬁcation, granulation, washing, drying, extrusion, and

pelletizing. The pellets obtained have properties similar to

virgin resin. Dissolution of polystyrene in various solvents

removes solid contaminants. Volatilization and recovery of

the solvent produces solid polystyrene for reuse (5). Solid

Table 3. Typical Sizes of Injection-Molding Machines

Nominal Rating of Clamping

Capacity (kN)

Shot Size (g PS)

Rate of Fill

(mL/min)

1335 170–340 131–262

4450 1360–2155 410–738

8900 4536–5100 1148–1476

To convert kN to tons, divide by 8.9.

To convert g to oz, divide by 28.35.

To convert cm

/min to in.

/min, divide by 16.4.

Table 4. Typical Injection-Molding Conditions

Controls English Units SI Units

Barrel temperatures

Zone 1, feed 375–4201F 190–2151C

Zone 2 400–4501F 204–2321C

Zone 3 430–4751F 221–2461C

Zone 4 430–4751F 221–2461C

Zone 5, adapter 430–4751F 221–2461C

Nozzle 425–4601F 218–2381C

Melt temperature 430–4751F 221–2461C

Mold temperature 70–1201F 20–501C

Injection pressure 1100–2300 psi 8–16 MPa

Injection pressure Adjust to control part weight and dimensions

Injection speed Variable control, slow–fast–slow, adjust to control appearance

Backpressure 0–150 psi, higher with concentrates

POLYSTYRENE 1011

waste packing material dissolved in a dimethylbenzene-

based solvent produces a viscous ﬂuid that can be used as

an adhesive or coating material (6). Food packaging mate-

rial is not recycled at this time because it is not an

economically sustainable process (7). There is a market

for nonfood service polystyrene recycling.

BIBLIOGRAPHY

P.A. Wagner and J. Sugden, ‘‘Polystyrene’’ in A. J. Brody and K. S.

Marsh, eds., The Wiley Encyclopedia of Packaging, 2nd edition,

Wiley, New York, 1997, pp. 768–771.

Cited Publications

1. A. Borruso, ‘‘Polystyrene,’’ Chemical Economics Handbook,

SRI Consulting, Menlo Park, CA, 2005.

2. D. B. Priddy, ‘‘Styrene Plastics,’’ in Kirk–Othmer Encyclopedia

of Chemical Technology, Vol. 23, 5th edition, Wiley, Hoboken,

NJ, 2007.

3. L. Bresler, U.S. Patent Application 20060131316, June 22,

2006.

4. C. Christodoulou, U.S. Patent Application 20060060478, March

23, 2006.

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

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

Hoboken, NJ, 2006.

6. Z.-Z. Yuan, U.S. Patent Application 20070249741 (to U.S. Hu

Bang Co., Ltd.), Oct. 25, 2007.

7. ‘‘Polystyrene, General Information,’’ Plastics Foodservice

Packaging Group, American Chemistry Council, www.ameri-

canhemistry.com/s_plastics (accessed March 2008).

POLYVINYLIDENE CHLORIDE (PVDC)

DONG SUN LEE

Department of Food Science

and Biotechnology, Kyungnam

University, Masan, Gyeongnam,

Republic of Korea

KIT L. YAM

Department of Food Science,

Rutgers University,

New Brunswick,

New Jersey

Polyvinylidene chloride (PVDC) is a copolymer of vinyli-

dene chloride (usually 70–90 mol %) and other monomers

including vinyl chloride, acrylronitril, and alkyl acrylate.

Homoploymer of vinylidene chloride (VDC) is not used in

commerce. PVDC has been an important contributor to

the plastic packaging that has increased the fraction of

processed foods reaching the dinner table. PVDC provides

exceptionally low permeabilities to gases and liquids.

Furthermore, they are resistant to attack by many che-

micals and are inert to most foodstuffs.

The overwhelming fraction of PVDC is used for food

packaging. Lesser amounts are used for other packaging

applications, ﬁbers, specialty binders, and molded parts.

The volume of PVDC use in packaging is small with the

proportion less than 0.5% of the total polymers in Europe.

The world market demand of PVDC resins is estimated as

about 80,000 metric tons, in which 66% are for extrusion,

22% for solvent coating, and 12% for latex.

CHEMISTRY

Homopolymer of vinylidene chloride is not commercially

useful because of its crystallinity and thermal instability.

The melting temperature of the homopolymer is about

2021C, and the level of crystallinity is greater than 60%.

Crystallization from the melt is rapid. As an extrudable

resin, the polymer needs to be heated above the melting

temperature for a few minutes. The degradation rates

above 2001C are too rapid to allow time for extrusion. As a

latex, the homopolymer can crystallize in the particles.

Then, during drying after coating, the particles will not

fuse to give a continuous ﬁlm. As a resin for solvent

coating, the crystals are too stable to be dissolved by

convenient methods.

Copolymerization alleviates the problems with the

homopolymer. A comonomer decreases the lamellar thick-

ness of the crystals and lowers the melting temperature. A

comonomer decreases the rate of the crystallization and

allows a latex to remain completely amorphous longer.

Hence, the casting will fuse easily and then crystallize. A

comonomer gives a semicrystalline material that can

dissolve in convenient solvents.

Vinylidene chloride is colorless liquid readily dissolving

in most organic solvents, and it undergoes free-radical

polymerization with many other monomers to form solid

particles. The commercially important comonomers are

vinyl chloride (VC), acrylonitrile, and methacrylonitrile,

along with various methacrylates and alkylacrylates. For

extrudable resins, vinyl chloride or methyl acrylate is

commonly used to depress the melting temperature to

the range of 140–1751C. For latex coatings, an alkylacry-

late or alkylmethacrylate is used as comonomer. Terpoly-

mers are common, and in some cases a surface active

comonomer is included such as acrylic acid or 2-sulfoethyl

methacrylate. For solvent coatings, acrylonitrile and

methacrylonitrile are the most common comonomers.

Most extrudable resins are made in suspension by free-

radical polymerization. The monomers are dispersed as

droplets in water and stabilized by a protective suspend-

ing agent such as polyvinyl alcohol or methylcellulose. An

oil-soluble free-radical initiator is used. The result is a

free-ﬂowing collection of nearly spherical, porous particles

with diameters in the range of 200–300 mm.

Latexes are made by a free-radical, emulsion polymer-

ization. Often, monomers are continuously added to the

reaction vessel during polymerization. The latexes must be

stabilized with surface active agents to prevent coagulation.

Resins for solvent coating are also made through a free-

radical, emulsion polymerization. Often, monomers are

continuously added into the reaction. A minimum amount

1012 POLYVINYLIDENE CHLORIDE (PVDC)

of surface active agent is used because the higher amount

deteriorates the stability of the polymer which should go

through coagulation, dewatering, and drying after the

polymerization step. Common solvents used in the coating

include tetrahydrofuran (THF) or methyl ethyl ketone

with toluene.

In addition to those materials that are required for

polymerization, certain additives are commonly used with

PVDC copolymers. Plasticizers, stabilizers, and extrusion

aids are included in extrudable resins at low levels for

high barrier applications and moderate levels for less

demanding applications. A latex may contain an antimi-

crobial compound. A resin for solvent coating will contain

some of the salt used to cause coagulation.

PVDC can degrade at elevated temperatures, thereby

releasing HCI and leaving a double bond in the polymer

backbone and consequently generating an allylic dichlor-

omethylene unit susceptible to facile thermal radical

dehydrohalogenation. Hence, at low levels of degradation

a series of conjugated double bonds are formed which may

impart color to the polymer. In extreme cases, the degra-

dation can lead to a brittle char and even to carbon. The

degradation is also catalyzed by Lewis acids, particularly

iron cations formed from the interaction of process equip-

ment with evolved hydrogen chloride. Hence, extrusion of

PVDC resins should be done using the proper materials of

construction and with training by experts. Strong bases

can also cause degradation of PVDC. For example, contact

with organic amines is not recommended, because such

agents act to introduce the double bond sites initiating

rapid thermal degradation.

GENERAL PRINCIPLES

All PVDC copolymers have certain common characteris-

tics. Commercial copolymers have molecular weight

ranges of about 65,000–150,000. Molecules of lower mole-

cular weights tend to be brittle and have limited commer-

cial value. The physical and chemical properties of

copolymers depend largely on the VDC content, which

ranges from 72 to 94 wt% in most commercial products.

The axial symmetry of the VDC molecule permits tight

packing of molecular chains with concomitant formation

of crystals constituting 25–45 vol% of the total structure.

Speciﬁc gravities range from 1.65 to 1.75.

These characteristics of PVDC resins are not signiﬁ-

cantly altered by the small amounts of processing and

stabilizing aids used to facilitate extrusion and molding. A

representative extrudable resin is a VDC–VC copolymer

with about 85% VDC and a molecular weight of 120,000,

with plasticizer sufﬁcient for ease of extrusion.

Note that the barrier and strength properties of PVDC

depend on the chemical compositions of the molecules, the

degree of molecular orientation, and the direction of

orientation in the ﬁnished form. Relatively high VDC

content imparts relatively high barrier properties and

gas resistance. A comparatively high degree of orientation

imparts tensile strength. Higher crystallinity correlates

with lower permeability.

PROPERTIES

The properties of PVDC copolymers most pertinent to

food packaging include a unique combination of low

permeability to atmospheric gases, moisture, and most

ﬂavor and aroma bodies and stress-crack resistance to a

wide variety of agents. In addition, the ability to with-

stand the rigors of hot ﬁlling and retorting is important in

commercial sterilization of foods in multilayer barrier

containers.

The range of barrier properties for small molecules in

extrudable resins is shown in Table 1. Furthermore,

PVDC is excellent barrier to ﬂavor and aroma compounds.

The three major types of household ﬁlm are compared in

Table 2. A high-barrier PVDC ﬁlm would have much lower

permeabilities. The general-purpose and high-barrier

ﬁlms are compared in Table 3.

The bulk mechanical properties of PVDC-coated pa-

pers, ﬁlms, and structures such as formed containers and

blown bottles depend almost entirely on the properties of

Table 1. Transport Properties of a High-Barrier PVDC

Extrudable Resin in Film or Sheet Form

Property Value

Gas permeabilities at 231C (731F)

(nmol m

1

GPa

1

)

Oxygen 0.04–0.30

Nitrogen 0.02–0.10

Carbon dioxide 0.10–0.50

Water vapor transmission rate (WVTR) at 381C

(1001F), 90% RH (nmol m

1

)

0.01–0.10

Multiply by 0.50 to obtain cm

mil/(100 in.

day atm).

Multiply by 3.95 to obtain g mil/(100 in.

day).

Table 2. Permeability of Household Films at 251C to Selected Permeants

Permeant

Plasticized PVDC

(Saran Wrapt)

Film Type Plasticized PVC

(Reynolds Plastic Wrapt)

Polyethylene

(Handi-Wrap IIt)

Oxygen (nmol m

1

GPa

1

)

1.9 220 640

Water vapor at 381C, 90% RH (nmol m

1

)

0.055 0.30 0.19

d-Limonene (MZU)

1.3 10

1.1 10

3.3 10

Dipropyl disulﬁde (MZU)

1.1 10

3.3 10

6.8 10

Multiply by 0.50 to obtain cm

mil/(100 in.

day atm).

Multiply by 3.95 to obtain g mil/(100 in.

day).

MZU = 10

20

kg m/(m

s Pa).

POLYVINYLIDENE CHLORIDE (PVDC) 1013

the substrate material. On the other hand, the PVDC

coating controls the specialized attributes providing bar-

rier to permeation, chemical resistance, or heat or di-

electric sealability to the substrate (see Table 4).

APPLICATIONS: EXTRUDABLE RESINS

Flexible Films

Monolayer ﬁlms are used for household wrap, as unit-

measure containers for pharmaceuticals and cosmetics,

and as drum liners and food bags. Multilayer ﬁlms,

generally coextruded with polyoleﬁns, are used to package

meat, cheese, and other moisture- or gas-sensitive foods.

The structures, which contain 80–90% polyoleﬁn with an

inner layer of PVDC, are usually shrinkable ﬁlms that

provide a tight barrier seal around the food product.

Semirigid Containers

PVDC is used as barrier layer in semirigid thermoformed

containers. The sheet can be produced by coextrusion or by

laminating monolayer or coextruded PVDC ﬁlms to semi-

rigid styrenic or oleﬁnic substrates. PVDC can also be

used as barrier layer in blow-molded bottles.

APPLICATIONS: COATINGS

Generally preferred forms of PVDC coatings on various

substrates are shown in Table 5. Pretreatment with

primers or electrotreatment may be required on some

substrates. The form of the package (i.e., ﬁlm, thermo-

formed sheet, or blown bottles) inﬂuences the choice as

well, but the key differentiating elements are the porosity

and chemical composition of the substrate.

Table 3. Properties of PVDC Films

Type of Film

Property ASTM Test Method General-Purpose High-Barrier

Speciﬁc gravity D 1505 1.60–1.71 1.73

Yield (m

ﬁlm/kg resin) 23 23

Haze (%) D 1003 5 2–3

Clarity (%) D 1746 52–65 80

Tensile strength (MPa) D 882

48.3–100 82.8–86.2

89.7–93.1 138–148

Elongation (%) D 882

40–100 95–100

40–100 50–60

Tensile modulus, 2% secant (MPa)

345–759 1,103–1,138

310–724 931–1,034

Tear strength propagating (N mm

1

) D 1922 3.9–38.6 3.9–38.6

Folding endurance (cycles) D 2176 W500,000 W500,000

Change in linear dimensions at 1001C (2121F) for 30 min (%) D 1204

12–22 6–7

6–18 3–4

Service temperature (1C) 18 to 135 18 to 135

Heat-seal temperature (1C) 121–149 121–149

Oxygen permeability at 231C (731F), 50% RH

(nmol m

1

GPa

1

) D 1434 1.6–2.2 0.16

WVTR at 381C (1001F), RH 90% (nmol m

1

) 0.06 0.02

Coefﬁcient of friction, face-to-face, back-to-back, at 231C (731F) and 50% RH D 1894 0.3 to no slip No slip

Film 1 mil (25.4 mm) thick. Not to be confused with Saran Wrapt household brand ﬁlm.

Machine (longitudinal) direction.

Transverse direction.

Humidity has no effect on the permeability of PVDC ﬁlms.

Table 4. Properties of PVDC Films

-Made from Aqueous Latexes and from Solutions

Latexes

-Selected for

Properties High Barrier High Seal Strength

Solution Resins

Oxygen permeability at 231C(731F) (nmol m

1

GPa

1

) 0.06–0.20 0.20 0.04-0.20

WVTR at 381C (1001F) and 90% RH (nmol m

1

) 0.012–0.015 0.012–0.380 0.005–0.025

Minimum heat-seal temperature (1C) 132 82 104–129

Oven-dried ﬁlms.

Cast on PET ﬁlm from dispersions.

Heat and dielectric-sealing grades available.

Cast from solutions of up to 20 wt% solids in 65% THF or methyl ethyl ketone/35% toluene (wt/wt) at 23–351C (73–951F).

1014 POLYVINYLIDENE CHLORIDE (PVDC)

Paper and Paperboard

Paper and paperboards coated with PVDC latex are used

where moisture resistance, grease resistance, oxygen bar-

rier, and water-vapor barrier are required.

Cellophane

About 90–95% of all cellophane produced in North Amer-

ica is coated with PVDC solution coatings to render the

ﬁlms moisture-resistant (thereby retaining the high gas

barrier inherent to dry cellophane) and provide the needed

moisture barrier.

Plastic Films

Latex coatings on plastic ﬁlms provide barrier to gases,

moisture, ﬂavors, and odors, and, in some packaging, heat-

seal capability. Heat-seal latexes are not often good gas

barriers, and, conversely, the best PVDC barriers do not

usually provide the best seals. When both heat sealability

and barrier are required, it is best to apply two coatings,

each designed for one purpose.

Semirigid Containers

Latex coatings impart barrier properties to thermoform-

able plastic sheet used to produce high-barrier food con-

tainers. Latex coatings on polyethylene terephthalate

(PET) bottles impart barrier to oxygen, carbon dioxide,

water, ﬂavors, and odors.

PRODUCERS

The principal producer/suppliers of PVDC in forms of

extrudable resin, latex emulsion, and solvent soluble resin

include Dow Chemical, Asahi Kasei, Kureha Corp.,

Owensboro Specialty Polymers, Rohm and Haas, and

Solvin. The Dow Chemical Company, the ﬁrst developer

and producer of PVDC ﬁlm, produces Sarant resins and

ﬁlms plus SARANEXt multilayer ﬁlms coextruded with

polyoleﬁns.

REGULATORY STATUS

VDC–VC copolymers containing about 10–27 wt% VC are

considered to comply with the food-additive provisions of

the Federal Food Drug and Cosmetics Act on the basis of

‘‘prior sanction’’. A variety of speciﬁc regulations govern

PVDC copolymers other than those of VC. The potential

user is encouraged to work with the supplier to compare

the current regulations with the application.

BIBLIOGRAPHY

General References

P. T. DeLassus, W. E. Brown, and B. A. Howell, ‘‘Vinylidne

chloride copolymers’’ in A. L. Brody and K. S. Marsh, eds.,

The Wiley Encyclopedia of Packaging Technology 2nd edition,

John Wiley & Sons, New York, 1997, pp. 958–961.

R. A. Wessling, D. S. Gibbs, B. E. Obi, D.E. Beyer, P.T. DeLassus,

and B. A. Howell, ‘‘Vinylidne chloride polymers’’ in Kirk–

Othmer Encyclopedia of Chemical Technology (Online),

http://www3.interscience.wiley.com/cgi-bin/mrwhome/

104554789/HOME, John Wiley & Sons, Hoboken, NJ, 2005.

DOI: 10.1002/0471238961.2209142523051919.a01.

R. A. Wessling, Polyvinylidene Chloride, Gordon and Breach, New

York, 1977.

PRESSURE CONTAINERS

MONTFORT A. JOHNSEN

Montfort A. Johnsen &

Associates, Ltd., Danville,

Illinois

Several distinct classes of pressure containers exist. All

are used to dispense the contents into either open or closed

environments. Among the largest are Hortonspheres, with

capacities exceeding 1 million gal (3,850,000 L). For trans-

port purposes, the largest pressure containers are ‘‘jumbo’’

tankcars, with capacities of about 31,000 gal (117,000 L).

Cylinders down to about 500 gal (1900 L) are designated as

bulk tanks. Smaller ones are variously known as ‘‘pig

tanks,’’ ‘‘ton cylinders,’’ or simply as cylinders. Sometimes

the intended use serves to identify the general size

and shape as in the case of ‘‘propane cylinder’’ or ‘‘lecture

bottle.’’

Pressure containers in sizes ranging from 3 to 1000 mL

are used to dispense consumer specialties. In two of these,

the primary container is pressurized. The most popular is

the aerosol, where pressure is generated by a propellant

gas. The other uses mechanical pressure from springs or

elastomers under tension to propel products from a ﬂex-

ible inner container. A third form is the mechanical pump,

such as the ﬁnger-pump or trigger-spray dispenser, in

which pressure is generated by the user, but only within

the valve body. In 2006 an estimated 3.65 billion aerosols

were produced in the United States. The remainder of

Table 5. Suggested Coatings for Various Substrates

Coatings

Substrates Latexes Solutions

Cellophane X

Nonporous papers

Porous papers X

Polyoleﬁns XX

Polyesters XX

Polyamides (nylons) XX

Styrenics X

Vinyls (PVC)

Includes coated and dense forms, such as glassine.

Plasticizers in highly plasticized PVC may migrate to the PVDC coating,

thereby damaging the coating’s effectiveness as a barrier.

PRESSURE CONTAINERS 1015

this article is restricted to aerosols (see also Aerosol

containers).

An aerosol composition normally consists of a combina-

tion of concentrate and propellant. The propellant portion

can range from about 0.5% to 100%, and at the 100% end

the propellant functions as the product itself. The regula-

tory deﬁnition of an aerosol [by the U.S. Department of

Transportation (DOT)] is a product that consists of ‘‘che-

mical ingredients pressurized by compressed gas.’’ In a

1991 clariﬁcation the DOT further required that the

product name reﬂect the intended use of the concentrate

ingredients. This was designed to halt interstate ship-

ments of commodities, under aerosol exemptions,

which contained ﬂammable propellants—to which trace

amounts of two or more incidental chemicals had been

added, with the sole purpose of satisfying the aerosol

description in the tariff.

Virtually all aerosols are ﬁtted with valves, for the

purpose of dispensing the contents—most often in spray

forms, but also as foams, streams, gels, gases, lotions,

pastes, ointments, and so forth. Limited numbers of aero-

sols are packaged with screw-threaded closures, designed

to be connected to valve-and-hose assemblies for such

purposes as air-conditioner reﬁlls. About a dozen ﬁrms

produce aerosol valves, and they offer a bewildering

array of diverse styles, oriﬁces, materials, and associated

ﬁtments. The scope of aerosol products is surprisingly

large. Every consumer and institutional and/or industrial

product category is included, such as pesticides, household

products, pharmaceuticals, drugs, foods, cosmetics,

and medical devices. Some liquid streams can be propelled

25 ft (8 m) to reach wasp nests and kill the insects

and their eggs. Compressed-gas jets may be used to dedust

the surfaces of electronic circuit boards, tapes, and

CDs (compact disks). Sterile saline solutions are available

for ﬂushing cleaning solution residues from contact eye-

wear. Cheese and jelly products can be cleanly extruded

from special valves onto other food items. Gels can be

produced that magically spring into foams when stroked

with the ﬁngertip. Unlike pump sprayers, aerosols can

produce sprays with an optimum particle size distribution

for maximizing their knockdown and cidality to ﬂying

insects. Particles are produced that are small enough to

remain airborne for long periods, yet not so small that

they airﬂow around the bodies of ﬂying pests. As a result,

a maximum number of insects are impacted by lethal

doses.

The scope of aerosols has been enhanced by the steadily

growing numbers of bag-in-can and piston can products

now on the market. The product is partitioned from the

propellant by placing it inside a collapsable plastic, metal,

or composite laminated bag, or above a piston. The

propellant then simply acts to pressurize the system.

Nonaerated products, such as lithium grease, toothpaste,

gels, artist’s pigment dispersions, and standard chemical

solutions, can then be dispensed. Product viscosities up to

about 2,000,000 cP can be easily handled. Originally re-

stricted to the 2.125-in. (52-mm)-diameter aerosol can,

these compartmentalized aerosols are now available in

diameters ranging from about 1.378 in. (35 mm) to

2.588 in. (65.7 mm) and in several heights.

Among the myriad aerosol valve designs is the

meter-spray type. Originally designed to dispense costly

perfumes in dosages of about 0.02–0.05 mL, the primary

use has now shifted to the metered-dose inhalents

(MDIs).

AEROSOL CONTAINERS

About 82% of all aerosols produced in the United States

during 2006 were steel [actually about 80% electrotinplate

(ETP) and 5% electrochrome-coated steel (ECCS)]. Alumi-

num accounted for about 18%, while glass, plastic-coated

glass, stainless steel, and plastic totaled about 1%. Tere-

phthalate (B-OPET) should decrease because of reinter-

pretations of certain DOT shipping regulations.

The percentage of aluminum cans in U.S. aerosol units

is perhaps the lowest in the world. Most countries use

about 60% steel, 40% aluminum, and virtually no other

container materials. The low percentage of aluminum in

the United States is considered to be due to three factors:

The consumer culture demands less packaging elegance,

more large-size utility products are sold, and the prices of

aluminum cans are signiﬁcantly higher.

Steel

The usual steel aerosol can is produced from electrolyti-

cally tinplated sheet, and it consists of three segments: top

(dome), body, and bottom (base). In the United Kingdom

the top is called the cone and the base is known as the

dome, which has led to some confusion in nomenclature

from time to time. In 2007, the U.S. suppliers are the

Crown Cork & Seal Company, B’way Corp. Aerosols Inc.,

Ball Corporation Aerosols and Packaging, The Sexton Can

Company, and McKernan Packaging and Clearing House.

The Sexton Can Company produces both one-piece (35-

and 38-mm) drawn cans and a line of two-piece cans,

where a top shell is sealed to the base by double-seaming.

A major portion of these last cans is used for pure HCFC-

22 and HFC-134a air-conditioning and refrigerant gases,

under two special DOT exemptions, since their pressures

exceed the standard aerosol limits.

DC Containers Inc. is a newer ﬁrm. Their containers

are unique in that about 92% of the sidewall is drawn and

ironed to a thickness of only 0.0038 in. (0.096 mm). This

operation acts to elongate the base–body ‘‘cup,’’ formed by

ordinary extrusion techniques. The cup is then trimmed to

height and eventually double-seamed to an aerosol dome.

For the small (35- and 38-mm diameter) cans, the drawn

and

ironed bottom shells

are trimmed and then formed

into a rounded dome and curl by a multistage (‘‘female’’

only) die-shaping operation. They also produce two large

sizes of 65-mm steel cans using the Protact process (Aero-

sol Containers).

It is not unusual for the can body of a three-piece steel

aerosol can to represent about 65–75% of the total weight,

depending on the can height. If the draw–iron process

effectively thins some 92% of the body metal to half the

normal thickness, the weight of the body will be reduced

by some 46% and the weight of the entire can will

1016 PRESSURE CONTAINERS

diminish by about 32%. As a rule, about half the ﬁnal net

cost of the aerosol can is due to the cost of tinplate, so it

follows that a 32% reduction in metal will equate to about

a 16% reduction in can cost. This economic fact has many

marketers interested in testing these new cans. Claims of

environmental source reduction, lack of the slightly un-

sightly side seam and bottom double seam, and some other

stated advantages serve to enhance interest.

The thickness of tinplated steel plate is speciﬁed in

terms of the weight per ‘‘basis box’’ (see Cans, steel; Tin-

mill products). The basis box was an accounting term

developed about 1730 in Pontypool, England and was

used to note the weight of 112 tinplated steel sheets,

14 in. wide by 20 in. long (35.6 mm 50.80 mm), thus

having an area of 31,360 in.

(20.2325 m

) on each side.

The Pontypool steel plate averaged 100 lb (45.45 kg) per

basis box. After tinplating, it weighed about 101–102 lb

per basis box; when crated, it constituted a load that was

not overly heavy for Welsh longshoremen employed in

lading ships bound for canmaking shops in London and

continental Europe.

Untinned steel plate weighing 100 lb (45.45 kb) per

basis box has an average thickness of 0.0110 in.

(0.280 mm). The plate is simply called ‘‘100-lb stock.’’

The thinest base weight steel now made is 47 lb

(0.0052 in.) thick. It is experimental, but may ﬁnd use in

beer and beverage cans. For aerosol cans the minimum

plate thickness is often important in meeting the DOT

requirements. The steel-mill tolerance is usually 710% on

thin, canmaking plate, but the can companies have been

successful in insisting that the 3s (three-sigma) tolerance

be 6% to 10%. For example, the very strong double-

reduced temper 8 (DR-8) 75-lb stock widely used for

aerosol can bodies will have a steel thickness range of

0.00776–0.00908 in. (averaging 0.00825 in. or 0.210 mm).

The cause of this apparently large variability comes from

the upward distortion of the heavy-duty rollers, used to

make the plate, causing the center portion of the typical

54-in. (1.37-m)-wide strip to be thicker than at the edges.

The old English basis-box system was introduced to the

United States in 1829, when the ﬁrst tinplate was im-

ported from Wales. It continued to be used in 1858 when

the ﬁrst tinplate was manufactured here. Although the

English have (ofﬁcially) converted from basis box to metric

measurements, the United States has yet to do so.

Prior to 1937, all tinplate was produced by the hot-dip

method, with both sides being coated equally. If 1.00 lb of

tin was deposited per basis box, this meant that 0.50 lb

was laid down on each side. Technically, such plate could

be designated as 0.50/0.50-lb (227/227-g) ETP. When

electrodeposition of tin became practical on a large scale,

it also became possible to differentially coat the two sides

of a given steel sheet. This advance led to some changes in

nomenclature. The rule is to designate the tin weight on

each side as if the plate were nondifferentially coated. For

example, if 1.00 lb of tin were deposited per basis box, but

with 0.75 lb on one side and 0.25 lb on the other, the

designation would be D-150/50, meaning that the more

heavily tinplated side would have the same weight of tin

as nondifferential 1.50-lb plate and the light side would

have the equivalent of nondifferential 0.50-lb plate. The

nominal tin-coating weight on each side would be 0.375/

0.125 lb (170/42.5 g). In the metric system, this plate

would be designated as having an average of 16.8 g/m

tin on the more heavily plated side and an average of 4.2 g/

on the lighter side.

Differential plate is normally marked on the more

heavily plated side, sometimes with a ‘‘D,’’ but also with

various lines, hexagons, triangles, or other ﬁgures, im-

printed by roll printing a dilute solution of sodium carbo-

nate onto the tin coating, prior to ﬂow melting. This

prevents plate reversal problems during canmaking. As

a rule, the more heavily tinplated surface is oriented

toward the product, to reduce possible corrosion. In a

few instances, very lightly tinplated plate, such as D-05/

15, is placed with the heavily tinned surface facing the

atmosphere. An example is where the product is quite

alkaline, such as some oven cleaners or other hard-surface

cleaners, which quickly dissolve the amphoteric tin metal.

In fact, the 0.05-lb tin coating is so thin and incomplete

that the dark gray SnFe

alloy layer and gray steel itself

can be seen through it. In this case the free tin is typically

1.36 min. thick and the underlying FeSn

layer is about

0.19 min. (0.08 g/m

). By using the heavier 0.15-lb tin

coating on the outside of the can, the white, shiny surface

of the tin improves appearance. It even brightens the

lithography in some instances.

Different thicknesses of tinplated steel plate are used

in the preparation of three-piece aerosol cans. As can size

increases, heavier plate must be used to obtain the same

pressure resistance. The end sections (dome and base) are

typically 60–80% thicker than the can body. They must

resist eversion and possible separation, under high-pres-

sure conditions. Even with can bodies as thin as about

0.004 in. (0.10 mm), the so-called ‘‘hoop effect’’ prevents

any swelling or bursting, unless the body metal is softened

by strong heating—as in a ﬁre.

Plates used for three-piece cans extend from about 60

to 135 lb per basis box. Bodies range from 60 to 90 lb

(0.0066–0.0099-in., or 0.168–0.251-mm average thick-

ness), while end sections extend from 90 lb on small 112-

diameter (45-mm) cans to 135 lb on the large 300-diameter

(76-mm) containers. The use of these plate thicknesses,

along with the basic pressure-resistant architecture of

aerosol containers, allows even the lowest-strength cans

to resist pressures to at least 160 psig (lb/in.

gauge)

(1.2 MPa) without deformation, and up to at least

226 psig (1.7 MPa) without bursting.

The canmaking plate is also continuously annealed to

provide various tempers, or stiffness levels. Annealing

also serves to remove certain work-hardening effects,

obtain the desired mechanical properties, and achieve an

optimum grain structure. The annealing process is fairly

rapid. Plate is unreeled into an inert atmosphere oven and

heated to about 12501F (6771C) for about 20 s, then

allowed to cool to 9001 F (4821C) across the following 25–

30 s. The rate of this initial cooling determines temper.

The highest tempers are achieved by using the slowest

rates of cooling—to allow the formation of a larger number

of cementite (Fe

C) grains per unit volume of the steel. For

example, the very stiff temper No. 5 (or T-5) plate contains

about 15,000 grains of cementite per mm

. The various

PRESSURE CONTAINERS 1017