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type of blowing agent control the density of the foam

produced. Figure 2 shows the relationships for three of

today’s most commonly used blowing agents. The foam can

also be colored for aesthetics by the addition of color

concentrates (see also Colorants).

A large percentage of all foam sheet is thermoformed to

produce the ﬁnal product. To achieve good postexpansion of

the sheet, the sheet should be allowed to age for 3–5 days.

This aging allows cell gas pressure to reach equilibrium.

Matched metal molds are normally used in the thermo-

forming process. Oven temperature control is critical for

consistent thermoforming. Scrap produced from the extru-

sion and thermoforming process can be reprocessed by

grinding the scrap and densifying it in an extruder.

ENVIRONMENTAL CONCERNS

Ecologically, polystyrene foam is very easily recycled, and

signiﬁcant quantities are presently being reused. It also

conforms favorably to current forms of disposal, such as

landﬁll and incineration. In landﬁll, foam remains inert,

but it packs and crushes easily and there is no pollution of

underground water streams by decaying material. In

incineration the chief products of combustion are water,

carbon dioxide, and carbon monoxide, which are typical of

organic materials, and the Btu (British thermal unit)

value is high. If burned without enough oxygen, polycyclic

aromatic compounds, carbon mooxide, and carbon black

are produced. Use of polystyrene for food packaging has

been banned in some U.S. cities (San Francisco and Oak-

land are the latest to pass bans). Recycled polystyrene can

be used for park benches, ﬂower pots, and toys.

BIBLIOGRAPHY

P. A. Wagner, ‘‘Foam Extruded Polystyrene’’ in: A. J. Brody and

K. S. Marsh, eds., The Wiley Encyclopedia of Packaging

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

pp. 449–450.

Cited Publications

1. M. Xanthos and D. B. Todd, ‘‘Plastics Processing’’ in the Kirk–

Othmer Encyclopedia of Chemical Technology, Vol. 19, 5th ed.,

John Wiley & Sons, Hoboken, NJ, 2006.

2. C. I. Chung, Extrusion Principles and Practise, Carl Hanser

Verlag, Munich, 2000.

FOAM PLASTICS

K. W. SUH

M. H. TUSIM

The Dow Chemical Company,

Midland, Michigan

Foamed plastics, which are also known as cellular plastics

or plastic foams, have been important since primitive

Figure 2. Blowing-agent requirements for polystyrene foam.

HFC-152a (Formacel Z-2) data supplied by DuPont. To convert

lb/ft

to g/cm

, multiply by 0.0162.

Figure 1. A two-extruder tandem system for the production of foamed sheet: (1) continuous feeding and blending system; (2) volumetric

pump system; (3) process control; (4) primary extruder; (5) secondary extruder; (6) annular die and cooling mandrel; (7) draw rolls and

winders.

518 FOAM PLASTICS

humans began to use wood, a cellular form of the polymer

cellulose. Cellulose is the most abundant of all naturally

occurring organic compounds, which represents approxi-

mately one third of all vegetable matter in the world. The

high strength:weight ratio of wood, the good insulating

properties of cork and balsa, and the cushioning proper-

ties of cork and straw have contributed to the development

of the board range of cellular synthetic polymers in use

today.Cellular polymers have been commercially accepted

for a wide variety of applications since the 1940s. The total

usage of foamed plastics in the United States rose from

about 2.1 10

tons in 1993. It is expected to rise to about

2.9 10

metric tons in 1998 (1).

NOMENCLATURE

A cellular plastic is deﬁned as a plastic whose apparent

density is decreased substantially by the presence of

numerous cells disposed throughout its mass (2). In this

article, the terms cellular polymer, foamed plastic, ex-

panded plastic, and plastic foam are used interchangeably

to denote all two-phase gas–solid systems in which the

solid is continuous and composed of a synthetic polymer or

rubber.

THEORY OF THE EXPANSION PROCESS

Foamed plastics can be prepared by various methods. The

most widely used, which is called the dispersion process,

involves dispersion of a gaseous phase throughout a ﬂuid

polymer phase and the preservation of the resultant state.

Other methods of producing cellular plastics include

sintering dispersed small particles and dispersing small

cellular particles in the plastic. The latter processes are

relatively straightforward techniques of lesser commercial

importance.

The expansion process has been the subject of extensive

investigation because it is the foundation of foamed plas-

tics (3–6). In general, the expansion process consists of

three steps: creation of small discontinuities or cells in a

ﬂuid or plastic phase, growth of these cells to a desired

volume, and stabilization of the resultant cellular struc-

ture by physical or chemical means.

Bubble Initiation. The development of bubbles within a

liquid or polymer solution is generally called nucleation,

although the term actually refers only to those bubbles

that separate from the supersaturated liquid or polymer

solution in the presence of an initiating site such as a

surface irregularity. This process has several sources: (a)

dissolved gases that are normally present in the liquid or

polymer solution and forced into supersaturation by in-

creased temperature; (b) low-boiling liquids that are in-

corporated into the system as blowing agents and forced

into the gas phase by increased temperature and de-

creased pressure; (c) gases produced as blowing agents,

eg, by the water—isocyanate reaction used for CO

pro-

duction in polyurethane foams; and (d) chemical blowing

agents that decompose thermally to form a gas.

Bubble nucleation is affected by many conditions.

Physically, the effects of temperature, pressure, and in

some cases humidity are fairly obvious. Other important

parameters are surface smoothness of the substrate, sur-

face characteristics of ﬁller particles, presence and con-

centration of certain surfactants or nucleators, size and

amount of second-phase liquid bubbles, and the rate of gas

generation.

In many cases, bubbles of gas and other contaminants

are already present in the liquid or polymer solution, and

these serve as sites into which the gas may diffuse. The

number and size of these gas bubbles may be another

important factor in bubble development.

Bubble Growth. The initial bubble is ideally a sphere

that grows as a result of the interaction of the differential

pressure (DP) between the inside and outside of the cell

and the interfacial surface tension (g). The radius (r) of the

bubble at equilibrium is related to these factors as shown

in equation (1):

DP ¼

ð1Þ

The differential pressure is larger for a small bubble at

a ﬁxed surface tension. Accordingly, smaller bubbles tend

to equalize these pressures by growing faster, breaking

the wall separating the cells, or by diffusion of the blowing

agent from the small to the large cells as indicated by

equation (2):

1:2

¼ 2g





ð2Þ

where DP

1.2

is the difference in pressure between cells of

radius r

and r

. Therefore, the rate of growth of the cell

depends on the viscoelastic nature of the polymer phase,

the blowing-agent pressure, the external pressure on the

foam, the cell size, and the permeation rate of blowing

agent through the polymer phase.

Bubble Stabilization. Bubble stability during growth is

a function of the type and concentration of surfactant, rate

of bubble growth, viscosity of the ﬂuid medium, differen-

tial pressure variations, and presence of cell-disrupting

agents such as solid particles, liquids, or gases. As the cell

walls are squeezed into polyhedra, a wall-thinning effect

takes place, and liquid is drained from cell-wall faces into

the lines of cell intersections to form ribs or struts, which

are typically triangular in cross section. This cell-wall

membrane thinning can continue to the point where the

cell walls collapse and the cells open. This becomes an

important characteristic of most plastic foams and affects

properties such as thermal conductivity, moisture absorp-

tion, breathability, and load bearing.

Ultimate stabilization occurs as a result of either

chemical reaction continuing to the point of complete

gelation or the physical effect of cooling below the sec-

ond-order transition point to prevent polymer ﬂow. As the

ﬁnal solidiﬁcation is approached, the previously formed

bubbles may be distorted by the system ﬂow or gravity,

FOAM PLASTICS 519

which thereby produce anisotropy in the cellular struc-

ture. This effect must be considered when evaluating the

physical properties of plastic foams by obtaining samples

oriented in speciﬁc directions to the process ﬂow.

PROPERTIES

The data in Table 1 show the broad ranges of properties of

rigid foamed plastics (7); the manufacturer should be

consulted for the properties of a particular product. The

properties of some commercial ﬂexible foamed plastics are

given in Table 2 (7). These values depend on several

structural variables and should be used only as general

guidelines (see also the Polymer properties article).

The properties of a foamed plastic depend on composi-

tion and geometry, which are often referred to as struc-

tural variables. Furthermore, they are inﬂuenced by the

foam structure and the properties of the parent polymer.

The polymer phase description must include the additives

present.

Cells. A complete knowledge of the cell structure of a

particular polymer would require the size, shape, and

location of each cell. Because this is impractical, approx-

imations are employed. Cell size has been characterized

by measurements of the cell diameter and as a measure-

ment of average cell volume. Mechanical, optical, and

thermal foam properties depend on cell size.

Geometry. Cell shape is governed predominantly by

ﬁnal foam density and the external forces exerted on the

cellular structure before its stabilization in the expanded

state. In the presence of external forces, the cells may be

elongated or ﬂattened. Cell orientation can inﬂuence

many properties.

Fraction open cells. An important characteristic of the

cell structure is the extent of communication with other

cells. This is expressed as fraction open cells.

Gas composition. In closed-celled foams, the gas phase

in the cells can contain blowing agent (so-called captive

blowing agent), air, or other gases generated during

foaming. Thermal and electrical conductivity can be pro-

foundly inﬂuenced by the cell-gas composition.

Mechanical Properties. In mechanical properties, rigid

foams differ from ﬂexible foams. The tests used to char-

acterize them are therefore different, as are their applica-

tion properties. In the last two decades, a separate class of

high-density, rigid cellular polymers called structural

foams (density W0.3 g/cm

) have become commercially

signiﬁcant.

Structural variables that affect the compressive

strength and modulus of a rigid plastic foam are, in order

of decreasing importance, plastic-phase composition, den-

sity, cell structure, and plastic state. The effect of gas

composition is minor with a slight effect of gas pressure in

some cases.

Structural foams or foams with integral skins are

usually produced as fabricated articles in injection-mold-

ing or extrusion processes. They have relatively high

densities, and cell structures are composed primarily of

holes in contrast to a pentagonal dodecahedron structure

in low-density plastic foams. Because structural foams are

generally not uniform in cell structure, they exhibit con-

siderable variation in properties with article geometry.

In ﬂexible foams, which are used in comfort cushioning,

packaging, and wearing apparel, different mechanical

properties are emphasized than for rigid foams. The

compressive nature of ﬂexible foams, both static and

dynamic, is their most signiﬁcant mechanical property.

Other important properties are tensile strength and elon-

gation, tear strength, and compression set.

Thermal Properties. The thermal conductivity of cellu-

lar polymers has been thoroughly studied in heteroge-

neous materials and plastic foams (8). Heat transfer can

be separated into its component parts as follows:

k ¼ k

þ k

ð3Þ

where k = total thermal conductivity and k

, k

, and k

represent solid conduction, gases conduction, radiation,

and convection, respectively.

As a ﬁrst approximation, the heat conduction of low-

density foams through the solid and gas phases can be

expressed as the product of the thermal conductivity of

each phase and its volume fraction. In most cellular

polymers, the conduction through the solid phase is

determined primarily by the density and the polymer-

phase composition. Although conductivity through gases

is much lower than through solids, the amount of heat

transferred through the gas phase in a foam is usually the

largest component of a total heat transfer because of the

large gas-phase volume. Ordinarily, convection cannot be

measured in cells of diameter less than B4 mm (8).

Because most cellular polymers have cell diameters

of o4 mm, convection can be ignored. Radiant heat trans-

fer through cellular polymers has also been studied (8).

The increase in k at low densities is caused by increased

radiant heat transfer; at high densities, by an increasing

contribution of k

The thermal conductivity of most materials decreases

with temperature. When the foam structure and gas

composition are not inﬂuenced by temperature, the k of

the cellular material falls with decreasing temperature.

The thermal conductivity of a cellular polymer can change

on aging under ambient conditions if the gas composition

is inﬂuenced by aging. This is the case when oxygen or

nitrogen diffuses into cellular foams that initially have an

insulating ﬂuorocarbon blowing agent in the cells (8). The

thermal conductivity of foamed plastics varies with thick-

ness (9). This has been attributed to the boundary effects

of the radiant contribution to heat transfer.

The speciﬁc heat of a cellular polymer is the sum of the

speciﬁc heats of each of its components. The contribution

of the gas is small and can often be neglected.

520 FOAM PLASTICS

Table 1. Physical Properties of Commercial Rig

id Foamed Plastics

Polyurethane

Polystyrene

Isocyanurate

Property

ASTM

Test

Cellulose

Acetate Epoxy Phenolic

Extruded Plank

Expanded Plank Extruded Sheet

PVC

Polyether Bun Laminate

Urea—

formaldehyde

Density, kg/m

96–128 32–48 32–64 35

53 16 32 80

96 160 32 64

32–48 64–128 32 32

13–19

Mechanical

properties,

compressive

strength, kPa

at 10%

D1621 862

138–172 138–620 310 862

90–124 207–276 586–896 290

469 345 1,035 138–344

482–1896 210 117–206 34

Tensile strength,

kPa

D1623 1172

138–379 517

145–193 310–379 1020–

1186

2070–

3450

4,137 551 1,207 138–482 620–2000

250 248–290

6,900

Flexural strength,

kPa

D790 1104

172–448 1138

193–241 379–517

586 1,620 413–689 1380–2400

Shear strength,

kPa

C273 965

103–207 241

241

241 793 138–207 413–896

180 117

Compression

modulus, MPa

D1621 38–90 3.9

10.3

3.4–14

13.1 35 2.0–4.1 10.3–31

Flexural modulus,

Mpa

D790 38

9.0–26

10.3 36 5.5–6.2 5.5–10.3

Shear modulus,

MPa

C273

2.8–4.8 10.3

7.6–11.0

6.2 21 1.2–1.4 3.4–10.3

1.7

Thermal

properties,

thermal

conductivity W/

(m K)

C177 0.045–0.046 0.016 0.029

0.030

0.037 0.035 0.035 0.035

0.035 0.023

0.016–

0.025

0.022–

0.030

0.054 0.019 0.026

0.022 0.032

0.030

Coefﬁcient of

linear

expansion,

5

/1C

D696

1.5 0.9 6.3 6.3

5.4–7.2 5.4–7.2

5.4–7.2 7.2 7.2

Maximum service

temperature,

177

205–260 132 74

74–80 74–80 74–80 77–80

93–121 121–149 149 149

Speciﬁc heat, kJ/

(kg

K)

C351

1.1

B0.9

Electrical

properties

1.19–

1.20

o1.05

o1.05 1.02 1.02 1.02

1.27 1.28

1.05 1.1 1.4

Dielectric constant D1673 1.12

0.028–

0.031

o0.0004

o0.0004 0.0007 0.0007 0.0007

0.00011 0.00014

Dissipation factor

Moisture-

resistance

water

absorption,

vol%

C272 4.5

13–51 0.02 0.05 1–4

1–4 1–4

Moisture-vapor

transmission,

g/(m

s GPa)

E96

o120 35–120 23–35 86

56 15

35–230 50–120

230

1,610–2,000

Source:

Suh and Webb (7).

To convert kg/m

to lb/ft

, multiply by 0.0624.

To convert kPa to psi, multiply by 0.145.

To convert kJ/(kg

K) to Btu(lb

1F), divide by 4.184.

Table 2. Physical Properties of Commercial Flexi

ble Foamed Plastics

Polyethylene

Polypropylene

Polyurethane

Silicone

Property

ASTM

Test

Expanded

Natural Rubber

Expanded

SBR

Latex Foam

Rubber

Extruded

Plank

Extruded

Sheet Crosslinked Sheet Modiﬁed

Sheet

Standard

Cushioning

High-

Resilience

Type

Poly(vinyl

Chloride) Liquid Sheet

Density, kg/m

56 320 72 80 130

35 43

26–28

64–96 10

24 40 56 112 272

160

Cell structure

Closed Closed Closed Open Open

Closed Closed Closed

Closed

Open Open Closed Closed

Open Open

Compression

strength 25%,

deﬂection, kPa

D33574

206

4.8

5.7 4.6

D3575

Tensile strength,

kPa

D33574

206 551 103

138 41

344

118 103 10.3 24 36

20%

Tensile elongation,

D33574

310

60 276 276–480

1380 138–275 205 160

227 310

Rebound

resilience, %

D3574

40 62

Tear strength, (N/

10

D3574

10.5 26

4.4 2.4

Maximum service

temperature,

70 70 70

82 82

79–93

135 121

350 260

Thermal

conductivity, W/

(m K)

C177 0.036 0.043 0.030

0.050 0.053 0.040–0.049 0.036–0.040

0.039 0.039

0.03 0.040 0.078 0.086

Source:

Suh and Webb (7).

To convert kg/m

to lb/ft

, multiply by 0.0624.

To convert kPa to psi, multiply by 0.145.

To convert kJ/(kg

 K) to Btu/(lb

1

F), divide by 4.184.

522

The coefﬁcients of linear thermal expansion of poly-

mers are higher than those of most rigid materials at

ambient temperatures because of the supercooled liquid

nature of the polymeric state. This large coefﬁcient is

carried over directly to the cellular state. A variation of

this property with density and temperature has been

reported for foams in general (10).

Moisture Resistance. Plastic foams perform better than

other thermal insulation materials when exposed to

moisture, particularly when subjected to a combination

of thermal and moisture gradients.

Moisture absorption and freeze–thaw resistance of

various insulations and the effect of moisture on thermal

performance show that in protected-membrane rooﬁng

applications, the order of resisting moisture pickup is

extruded polystyrene g polyurethaneWmolded polystyr-

ene (11). Water absorption values for insulation in use for

5 years were 0.2 vol% for extruded polystyrene, 5 vol% for

polyurethane without skins, and 8–30 vol% for molded

polystyrene. These values correspond to increases in k of

5–265%. For below-grade applications, extruded polystyr-

ene was better than molded polystyrene or polyurethane

without skins in terms of moisture and thermal

resistance.

Electrical Properties. Electrical insulation takes advan-

tage of the toughness and moisture resistance of polymers

along with the decreased dielectric constant and dissipa-

tion factor of the foams. In addition, because of the low

dissipation factor and rigidity, plastic foams are used in

the construction of radar domes.

Environmental Aging. The response of cellular materials

to light and oxygen is governed almost entirely by the

composition and state of the polymer phase. Expansion

into the cellular state increases the surface area; reactions

of the foam with vapors and liquids are correspondingly

faster than those of solid polymer. All cellular polymers

deteriorate under the combined effect of light or heat and

oxygen; this may be alleviated by additives.

Other Properties. Cellular polymers by themselves are

poor materials for reducing sound transmission. They are,

however, effective in absorbing sound waves of certain

frequencies. Materials with open cells on the surface are

particularly effective in this respect. The presence of open

cells in a foam allow gases and vapors to permeate the cell

structure by diffusion and convection, thus yielding high

permeation rates. In closed-cell foams, the permeation of

gases or vapors is governed by the composition of the

polymer phase, gas composition, density, and cellular

structure. The penetration of visible light through foamed

polystyrene has been shown to follow approximately the

Beer–Lambert law of light absorption (10).

The resistance to rot, mildew, and fungus is related to

moisture absorption. Therefore, open-cell foams support

such growth better than do closed-cell foams.

MANUFACTURING PROCESSES

Cellular plastics and polymers have been prepared by

processes that involve many methods of cell initiation,

growth, and stabilization. The most convenient method of

classifying these methods seems to be based on cell growth

and stabilization. According to equation (1), the growth of

the cell depends on the pressure difference between the

inside of the cell and the surrounding medium. Such

pressure differences may be generated by lowering the

external pressure, i.e., decompression; or by increasing

the internal pressure in the cells, i.e., pressure generation.

Other methods of generating the cellular structure are by

dispersing gas or solid in the ﬂuid state and stabilizing

this cellular state, or by sintering polymer particles in a

structure that contains a gas phase (see Table 3).

Foamable compositions in which the pressure within

the cells is increased relative to that of the surroundings

are called expandable formulations. Both chemical and

physical processes are used to stabilize plastic foams for

expandable formulations. There is no single name for the

group of cellular plastics produced by the decompression

processes. The operations used are extrusion, injection

Table 3. Production of Cellular Polymers

Polymer Extrusion

Expandable

Formulation Spray

Forth

Foam

Compression

Mold

Injection

Mold Sintering Leaching

Cellulose acetate X X

Epoxy resin X X X

Phenolic resin X

Polyethylene X X X X X X

Polystyrene X X X X X

Silicones X

Urea–formaldehyde

resin

Urethane polymers X X X X

Latex foam rubber X

Natural rubber X X X

Synthetic elastomers X X X

Poly(vinyl chloride) X X X X X X

Ebonite X

Polytetraﬂuoroethylene X

FOAM PLASTICS 523

molding, and compression molding. Physical or chemical

methods may be used to stabilize the products.

EXPANDABLE FORMULATIONS

Physical Stabilization. Cellular polystyrene, poly(vinyl

chloride) (PVC), polyethylene, and copolymers of styrene

and acrylonitrile can be manufactured by this process.

Chemical Stabilization. This method has been used

successfully for more materials than the physical stabili-

zation process. It is generally more suitable for condensa-

tion polymers than the vinyl polymers because of the fast

yet controllable curing reactions. Chemical stabilization

can be used for foam formation from polyurethane, poly-

isocyanurate, phenolic, epoxy, and silicone resins.

DECOMPRESSION EXPANSION PROCESSES

Physical Stabilization. Cellular polystyrene, cellulose

acetate, polyoleﬁns, and poly(vinyl chloride) can be man-

ufactured by this process.

Chemical Stabilization. Cellular rubber and ebonite are

produced by chemical stabilization processes.

Dispersion. In several techniques for producing cellular

polymers, the gas cells are produced by the dispersion of a

gas or solid in the polymer phase followed, when neces-

sary, by stabilization and treatment of the dispersion. In

frothing, a quantity of gas is mechanically dispersed in the

ﬂuid polymer phase and stabilized. Latex foam rubber was

the ﬁrst cellular polymer produced by frothing. In another

method, solid particles are dispersed in a ﬂuid polymer

phase, the dispersion stabilized, and the solid phase

dissolved or leached, thus leaving the cellular polymer.

Cellular polymers called syntactic foams are made by

dispersing an already cellular solid phase in a ﬂuid

polymer and stabilizing the dispersion.

USES

Concern over energy conservation and safety has stimu-

lated growth in applications for insulation and cushioning

in transport. A healthy economy is also expected to

increase the demand for cushioning in furniture, bedding,

and ﬂooring as well as for packaging. Structural foams are

widely used as substitutes for wood, metal, or unfoamed

plastics. Table 4 shows the demand for plastic foams in the

United States.

Cushioning. The properties of signiﬁcance in the cush-

ioning applications of cellular polymers are compression–

deﬂection behavior, resilience, compression set, tensile

strength and elongation, and mechanical and environ-

mental aging. The broad range of compressive behavior

of ﬂexible foam is one of the advantages of cellular

polymers because the needs of almost any cushioning

application can be met by changing either the chemical

nature or the physical structure of the foam. Flexible

urethanes, vinyls, latex foam rubber, and oleﬁns are

used to make foamed plastic cushioning for automobile

padding, seats, furniture, ﬂooring, mattresses, and

pillows.

Thermal Insulation. Thermal insulation is the largest

application for the rigid materials because of their ther-

mal conductivity, ease of application, cost, moisture ab-

sorption, and transmission permeance. Plastic foams that

contain a captive blowing agent have much lower thermal

conductivities than do other insulating materials. The

low-thermal-conductivity polyurethane foams are used

in refrigerators and freezers. Polystyrene foam is popular

where cost and moisture resistance are important, and

polyurethane foams are used in spray applications.

Extruded polystyrene foam is found in residential

construction as sheathing, perimeter, and foundation in-

sulation. Both polystyrene and polyurethane foams are

highly desirable roof insulators.

Packaging. The entire range of cellular polymers from

rigid to ﬂexible is used for packaging because of their low

cost, ease of application or fabrication, moisture suscept-

ibility, thermal conductivity, consumer appeal, and me-

chanical properties, especially compressive properties.

Extruded Polystyrene Foam Sheet. A polystyrene

foam sheet is made from a combination of poly-

styrene resin, blowing agents, nucleating agents, and

pigments in an extrusion process (12, 13). A polystyrene

foam sheet is used in many shapes and can usually be

characterized by its stiffness and low density, generally

2–12 lb/ft

(32–190 kg/m

). Sheet products are typically

0.015–0.150 in. thick.

Table 4. U.S. Cellular Polymer Market (million lb)

Foam 1990 1992 1993 1998

Annual Growth, 1993–1998

Polyurethane 2681 2752 2848 3509 4.3

Polystyrene 1417 1366 1416 1624 2.8

PVC 320 340 350 395 2.5

Polyoleﬁns 100 105 110 135 4.2

Other 62 67 70 80 2.7

Total 4580 4630 4794 5743 3.7

Source: Courtes y of Business Communications Co., Inc.

Projected.

524 FOAM PLASTICS

The major uses for foam sheets are for disposable

applications such as carryout cups and clam shells, plates,

bowls, egg cartons, and meat and poultry trays. New

market growth has come in the applications of a polystyr-

ene foam sheet for use in art boards, insulated boxes, and

bottle wraps. For applications that require extra stiffness

or modulus, ﬁlms or paper may be laminated to the foam.

The foam sheet can be fabricated by die cutting and

scoring, in addition to thermoforming to shape and bend

the foam.

Typically, the foam-sheet process uses an annular die in

the production of thin sheets. Single-screw, twin-screw,

long-barrel single-screw, and tandem-extruder systems

can be used with the latter being most common.

Expanded Polystyrene Beadboard. Expanded polystyr-

ene (EPS) beadboard is produced with expandable poly-

styrene beads. These beads are impregnated with 5–8%

pentane and sometimes with ﬂame retardants during

suspension polymerization. The beads are preexpanded

by fabricators with steam or vacuum and aged. Then they

are fed to steam-heated block molds where expansion and

fusion of beads continue. The molded blocks are sliced

after curing. Block densities range from 13 to 48 kg/m

with 24 kg/m

the most common for cushion packaging

and 16 kg/m

for insulation.

Expandable polystyrene bead-molding products ac-

count for the main portion of the drink-cup market and

are used in packaging materials, insulation board, and ice

chests.

Extruded Polystyrene Loose-Fill Packaging. Loose-ﬁll

packaging particles are used in void-ﬁlling packaging

applications. The foamed particles provide cushioning for

lightweight, often fragile products. The foamed particles

are usually resilient and are low in density [bulk density

properties under o0.30 lb/ft

(4.8 kg/m

)] and can be

reused many times. Polystyrene loose-ﬁll particles can be

recycled. The growth of this market has been affected by

the development of starch-based extrusion products.

Expandable polystyrene beads for loose-ﬁll packaging

are produced by the extrusion process. Virgin and recycled

polystyrene with nucleating agents and additives are fed

into the extruder, and blowing agents are usually fed

downstream into the extrusion process. The blowing

agents used have progressed from chloroﬂuorocarbons

(CFCs) to hydrochloroﬂuorocarbons (HCFCs) to hydrocar-

bons or blends of hydrocarbons with carbon dioxide. Some

extrusion processes can use expandable beads (pentane/

isopentane blowing agent) for feedstock.

Loose-ﬁll particles are produced by a foam-at-the-die

process, or by an expandable-bead process. In the foam-at-

the-die process, the molten gel is allowed to foam to a

density of approximately 1–2 lb/ft

. The foam is then cut at

the die face or downstream after some cooling has oc-

curred. The subsequent density reduction is accomplished

by steam treatment, followed by air aging. The foam may

be steamed and aged 2–3 times to decrease the density to

below 0.25 lb/ft.

The expandable-bead process produces an unfoamed

particle that has the blowing agent incorporated into the

bead. The expandable beads are produced at a central

extrusion location and then shipped to local expanders.

Local expansion facilities are needed because of the light-

weight nature of the ﬁnal product. The expandable beads

are exposed to steam as done with the foam-at-the-die

process but will require an extra expansion step.

Polyoleﬁn Foam Sheet and Plank. Polyoleﬁn foams are

closed-cell foams that are tough, ﬂexible, and resilient and

are used in applications such as cushioning, ﬂotation,

furniture overwrap, water sports, surface protection, and

many other uses. The energy-absorbing characteristics of

polyoleﬁn foams make them excellent foams for use in

cushioning applications. Polyethylene and polypro-

pylene foams are low-density products, generally less

than 10 lb/ft

(160 kg/m

), which are made by the extru-

sion process. These foams are made in many forms—

plank, sheet, rounds, and multiple strands—and can be

made with crosslinked or non–crosslinked formulation.

Polyethylene and polypropylene foams are usually

produced from a combination of virgin and recycled resins,

blowing agents, nucleating agents, and other additives

such as colorants, antistatic additives, and ﬂame retar-

dants. Both polyethylene and polypropylene can be made

into sheet-type products, typically 0.06–0.625 in. thick

(2–15 mm) by up to 72 in. (1800 mm) wide. Because of the

low melt strengths typical of polypropylene, only poly-

ethylene is used in the manufacture of large cross sections

of foam of 2–4-in. (50–100-mm) thickness and 24–48-in.

(600–1200-mm) width. Planks of foam are often fabricated

into speciﬁc shapes to meet the packaging requirements of

electronic components such as computers and their moni-

tors, the handling requirements for water sports uses such

as boogie boards, or the part separation requirements of

multiple-use industrial applications.

Noncrosslinked polyoleﬁn sheet foams can be made on

single-, dual-, and twin-screw extrusion lines that are

designed to mix in large quantities of blowing agents

and then extruding the supersaturated gel through an

annular die at the proper temperature, usually very near

the crystalline melting point. Figure 1 shows the theore-

tical relationship between the blowing-agent content and

the expected foam density. To produce a foam with a

density of B2 lb/ft

(32 kg/m

), the extrusion system

must be able to mix in 6–16 lb of blowing agent per

100 lb of resin, depending on the blowing agent selected.

The gel pressure at the die is also very important. If the

CFC-12 HCFC-22 HFC-134a

pentane

HFC-152a butane CO

Figure 1. Effect of blowing-agent concentration of theoretical foam density.

FOAM PLASTICS 525

pressure is too low, then the blowing agent will begin to

come out of solution and will allow the molten gel to begin

foaming prior to exiting the die lips. If this occurs, then

severe disruption of the skin surface will occur and a poor-

quality foam will result. Blowing agents are injected into

the molten resin gel downstream from the hopper. The

exact location will depend on the type of equipment being

used. To extrude sheet products that are very wide

and very thin, an annular die and forming mandrel is

used. The proper-size forming mandrel allows the ex-

truded tube to be stretched both radially and downstream

at the same time, and the mandrel provides a means to

cool the inner surface of the foam tube. The cooled tube is

then slit at one or two locations and subsequently wound

into rolls. Plank extrusion is usually through a more

rectangular-shaped die onto a conveyor belt or some other

means to move the foam away from the die. Plank extru-

sion is usually through a more rectangle-shaped die onto a

conveyor belt or some other means to move the foam away

from the die. Plank extrusion usually requires higher

rates than sheet because of the larger die opening and

the need for maintaining sufﬁcient pressure to prevent

prefoaming.

Polyoleﬁn foam manufacture generally uses physical

agents, which are materials that are a gas at the foaming

temperature (usually at the resin melting point). Chemi-

cal blowing agents are rarely used in noncrosslinked

applications. Blowing agents for polyoleﬁn foam extrusion

are now typically hydrocarbons (14) or a blend of hydro-

carbon and carbon dioxide. Prior to the Montreal Protocol

agreements, CFC-114 was the main foaming agent used in

polyoleﬁn foams. CFC-114 was an excellent foaming agent

because of its solubility and permeability characteristics.

Another strong attribute was its nonﬂammable character-

istic. With the Montreal Agreement, use of CFCs was

prohibited after 1991 in noninsulating applications. The

CFC-114 replacement list for foaming agents was not long.

Hydrocarbons (propane, butane, and isobutane), HCFCs

(22,142b), HFCs (152a, 134a), and carbon dioxide were

considered. Most of the replacement options included an

increase in ﬂammability and an increase in cost to the

manufacturing operations. The use of carbon dioxide is

attractive in terms of cost, ﬂammability, and environmen-

tal safety, but the processing aspect makes total substitu-

tion difﬁcult. Because of the high vapor pressure of this

physical gas, maintaining the proper resin-blowing-agent

solution characteristics at the die is very difﬁcult. The

Clean Air Act of 1990 (CAA) has since further reduced

the blowing-agent alternatives. The CAA has prohibited

the use of HCFCs, leaving only hydrocarbon and their

blends, HFCs (15), and carbon dioxide, or blends thereof.

Crosslinked foams offer higher temperature stability,

more ﬂexibility, ﬁner cell size, and better thermoforming

properties than do noncrosslinked foams. Crosslinking of

the oleﬁn resin is accomplished by chemical crosslinking

(such as dicumyl peroxide or silanes) or radiation cross-

linking via X-ray or electron-beam exposure (7). Typically,

the polyethylene resin, additives, crosslinking agents, and

chemical blowing agents (such as azodicarbonamide) are

mixed together at temperatures below the activation

temperature of the blowing agents, and then extruded

into a ﬂat sheet that can be rolled up, or into some other

proﬁle, prior to expansion into a foam product.

Crosslinking occurs before the foam-expansion step.

Expansion is done by exposure of the crosslinked sheet

to hot (B2001C) air. Generally, chemical crosslinking is

used in the production of thick products, whereas radia-

tion crosslinking is used for thinner foams.

HEALTH AND SAFETY FACTORS

Flammability. Plastic foams are organic and therefore

combustible. All plastic foams should be handled, trans-

ported, and used according to manufacturers’ recommen-

dations and local and national regulations.

Virtually all plastic foams are blown with inert gases,

chemical blowing agents that release inert gases, hydro-

carbons containing three to ﬁve carbon atoms, chlorinated

hydrocarbons, and hydrochloroﬂuorocarbons such as

HCFC-22, HCFC-141b, and HCFC-142b.

Atmospheric emissions. Certain organic compounds

generate smog photochemically. Interaction with the total

environment must be considered in developing environ-

mentally acceptable blowing agents. The products of

combustion of plastic foams are usually carbon monoxide

and carbon dioxide with smaller amounts of many other

substances.

The presence of additives or unreacted monomer in

certain plastic foams can limit their use where food or

human contact is anticipated. The manufacturers’ recom-

mendations and existing regulations should be followed.

BIBLIOGRAPHY

1. H. Kibbel, P-120R, Polymeric Foams, A Huge Sub-Industry,

Business Communications Company, Inc., Norwalk, CT, 1994.

2. ASTM D883-80C, Deﬁnitions of Terms Relating to Plastics,

American Society for Testing and Materials, Philadelphia, PA,

1982.

3. K. C. Frisch and J. H. Saunders, Plastic Foams, Vol. 1, Parts 1

and 2, Marcel Dekker, New York, 1972 and 1973.

4. C. J. Benning, Plastic Foams, Vol. 1 and 2, Wiley-Interscience,

New York, 1969.

5. N. C. Hilyard and co-worker, Mechanics of Cellular Plastics,

Macmillan, New York, 1982.

6. D. Klempner and K. C. Frisch, Handbook of Polymeric Foams

and Foam Technology, Hanser Publishers, New York, 1991.

7. K. W. Suh and D. D. Webb, Encyclopedia of Polymer Science

and Engineering, Vol. 3, 2nd edition, Wiley, New York, 1985,

pp. 1–59.

8. R. E. Skochdopole, Chem. Eng. Prog. 57(10), 55 (1961).

9. B. Y. Lao and R. E. Skochdopole, paper presented at 4th SPI

International Cellular Plastics Conference, Montreal, Ca-

nada, SPI, New York, Nov. 1976.

10. J. D. Grifﬁn and R. E. Skochdopole, ‘‘Plastic Foams,’’ in

E. Baer, ed., Engineering Design for Plastics, Reinhold, New

York, 1964.

11. F. J. Dechow and K. A. Epstein, ASTM STP 660, Thermal

Transmission Measurements of Insulation, ASTM, Philadel-

phia, PA, 1978, p. 234.

526 FOAM PLASTICS

12. U.S. Pat. 5,250,577 (Oct. 5, 1993), G. C. Welsh (to The Dow

Chemical Company).

13. U.S. Pat. 5,364,696 (Nov. 15, 1994), P. A. Wagner (to The Dow

Chemical Company).

14. U.S. Pat. 4,640,933 (Feb. 3, 1987), C. P. Park (to The Dow

Chemical Company).

15. U.S. Pat. 5,411,684 (May 2, 1995), M. H. Tusim and C. P. Park

(to The Dow Chemical Company).

FOIL, ALUMINUM

Foil Division, Aluminum

Association

Updated by Staff

INTRODUCTION

Aluminum foil is a thin-rolled sheet of pure or alloyed

aluminum, varying in thickness from about 0.00017

(4.3 mm) in. to a maximum of 0.0059 (150 m) in. (1). By

industry deﬁnition, rolled aluminum becomes foil when it

reaches a thickness less than 0.006 in. (152.4 mm) (see

Table 1). Aluminum foil is widely sold in the consumer

market in rolls of 50 cm in width and in varying lengths.

Aluminum, from which the foil is made, is a bluish

silver-white trivalent metallic element that is very malle-

able and ductile. Noted for its light weight, good electrical

and thermal conductivity, high reﬂectivity, and resistance

to oxidation, aluminum is the third most abundant ele-

ment in the earth’s crust (1).

Aluminum always occurs in combination with other

elements in mineral forms such as bauxite, cryolite, cor-

undum, alunite, diaspore, turquoise, spinel, kaolin, feld-

spar, and mica. Of these, bauxite is the most economical

mineral for production of aluminum. It can contain up to

60% alumina, which is hydrated aluminum oxide. It takes

about 4 kg of bauxite to produce 1 kg of aluminum (2).

Alumina is converted into aluminum at a reduction

plant or smelter. In the Hall–He

roult process, the alumina

is dissolved in a molten salt called cryolite. The action takes

place in steel boxes lined with carbon called pots. A carbon

electrode or anode is lowered into the solution, and electric

current of 50,000–150,000 A ﬂows from the anode through

the mixture to the carbon-cathode lining of the steel pot.

The electric current reduces, or separates, the alumina

molecules into aluminum and oxygen. The oxygen com-

bines with the anode’s carbon to form carbon dioxide. The

aluminum, heavier than cryolite, settles to the bottom of

the pot from which it is siphoned into crucibles. The molten

aluminum is eventually processed into products.

FOIL MANUFACTURE

One of aluminum’s most common uses is as foil. Alumi-

num foil is generally produced by passing heated

aluminum-sheet ingot between rolls in a mill under

pressure. Ingot is ﬂattened to reroll sheet gauges on sheet

and plate mills and ﬁnally to foil gauges in specialized foil-

rolling mills.

A second method for producing aluminum foil, rapidly

gaining popularity, involves continuous casting and cold

rolling. This method can eliminate the conventional en-

ergy-intensive and costly steps of casting ingot, cooling,

transporting to rolling plants, and then reheating and hot

rolling to various gauges. Some lubrication is needed

during the rolling stages, otherwise the foil surface be-

comes marked in a herringbone pattern. These lubricants

are sprayed on the foil before passing through the rolls.

Only oils approved for food contact must be used for foil

intended for food packaging (3).

First produced commercially in the United States in

1913, aluminum foil became a highly marketable com-

modity because of its protective qualities, economic pro-

duction capability, and attractive appearance. The ﬁrst

aluminum foil laminated on paperboard for folding car-

tons was produced in 1921. Household foil was marketed

in the late 1920s, and the ﬁrst heat-sealable foil was

developed in 1938.

World War II established aluminum as a major packa-

ging material. During the war, aluminum foil was used to

protect products against moisture, vermin, and heat da-

mage. It was also used in electrical capacitors, for insula-

tion, and as a radar shield.

Table 1. Physical Properties of Aluminum Foil

Property Value

Density 0.0976 lb/in.

(2.70 g/cm

)

Speciﬁc gravity 2.7 (approx.)

Melting range 1190–12151F (643–6571C)

Electrical conductivity 591 IACS. vol., 200% IACS

(approx.), weight

Thermal conductivity 53 W/(m K) at 251C

Thermal coefﬁcient of linear

expansion

13.1 10

6

per 1F, 68–2121F

(23.6 10

6

per 1C, 29–1001C)

Reﬂectivity for white light,

tungsten ﬁlament lamp

85–88%

Reﬂectivity for radiant heat,

from source at 1001F

(37.81C)

95% (approx.)

Emissivity, at 1001F (37.81C) 5% (approx.)

Atomic number 13

Atomic weight 26.98

Valence 4

Speciﬁc heat at 201C 0.21–0.23

Boiling point 32001F (17601C)

Temperature coefﬁcient of

resistance (representative

values per 1C)

0.0040–0.0036

at 201C

at 1001C 0.0031–0.0028

Low-temperature properties—

aluminum increases in

strength and ductility as

temperature is lowered,

even down to 3201F

(195.61C)

FOIL, ALUMINUM 527