30079_08.pdf

CHAPTER 8

PLASTICS AND ELASTOMERS

Edward N. Peters

General Electric Company

Selkirk, New York

8.1 INTRODUCTION

115

8.4 FLUORINATED

THERMOPLASTICS

124

8.4.1 Poly(tetrafluoroethylene)

124

8.2 COMMODITY

THERMOPLASTICS

8.4.2 Poly(chlorotrifluoroethylene)

124

116

8.4.3 Fluorinated Ethylene-

Propylene

8.2.1

Polyethylene

116

125

8.2.2

Polypropylene

116

8.4.4 Polyvinylidine Fluoride

125

8.2.3

Polystyrene

117

8.4.5 Polyethylene

chlorotrifluoroethylene)

8.2.4

Impact Polystyrene

117

128

8.2.5 SAN (Styrene/Acrylonitrile

Copolymer) 117

8.2.6 ABS 118

8.2.7 Poly vinyl Chloride 118

8.2.8 Poly(vinylidine chloride) 119

8.2.9 Poly(methyl Methacrylate) 119

8.2.10 Polyethylene Terephthalate) 119

8.4.6 Poly(vinyl

fluoride)

128

8.5 THERMOSETS

128

8.5.1 Phenolic Resins

128

8.5.2 Epoxy Resins

128

8.5.3 Unsaturated Polyesters

128

8.5.4 Alkyd Resins

129

8.5.5 Diallyl Phthalate

129

8.3 ENGINEERING

THERMOPLASTICS 120

8.3.1 Polyesters (Thermoplastic) 120

8.3.2 Polyamides (Nylon) 120

8.3.3 Polyacetals 121

8.3.4 Polyphenylene Sulfide 121

8.3.5 Polycarbonates 122

8.3.6 Polysulfone 122

8.3.7 Modified Polyphenylene Ether 123

8.3.8 Polyimides

8.5.6 Amino Resins

129

8.6 GENERAL-PURPOSE

ELASTOMERS

129

8.7 SPECIALTYELASTOMERS

129

123

8.1 INTRODUCTION

The use of plastics has increased almost 20-fold in the last 30 years. Plastics have come on the scene

as the result of a continual search for man-made substances that can perform better or can be produced

at a lower cost than natural materials such as wood, glass, and metal, which require mining, refining,

processing, milling, and machining. Plastics can also increase productivity by producing finished

parts and consolidating parts. Thus, an item made from several metal parts that require separate

fabrication and assembly can often be consolidated into one or two plastic parts. Such increases in

productivity have led to fantastic growth.

Plastics can be classified in several ways. The two major classifications are thermosetting materials

and thermoplastic materials. As the name implies, thermosetting plastics or thermosets are set, cured,

or hardened into a permanent shape. The curing that usually occurs rapidly under heat or UV light

leads to an irreversible cross-linking of the polymer. Thermoplastics differ from thermosetting ma-

terials in that they do not set or cure under heat. When heated, thermoplastics merely soften to a

mobile, flowable state where they can be shaped into useful objects. Upon cooling, the thermoplastics

harden and hold their shape. Thermoplastics can be repeatedly softened by heat and shaped.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Thermoplastics can be classified as amorphous or semicrystalline plastics. Most polymers are

either completely amorphous or have an amorphous component even if they are crystalline. Amor-

phous polymers are hard, rigid glasses below a fairly sharply defined temperature, which is known

as the glass transition temperature. Above the glass transition temperature, the amorphous polymer

becomes soft and flexible and can be shaped. Mechanical properties show profound changes near the

glass transition temperature. Many polymers are not completely amorphous but are semicrystalline.

Semicrystalline polymers have melting points that are above their glass transition temperatures. The

degree of crystallinity and the morphology of the crystalline phase have an important effect on

mechanical properties. Crystalline plastics will become less rigid near their glass transition temper-

ature but will not flow until the temperature is above the crystalline melting point. At ambient

temperatures, crystalline/semicrystalline plastics have greater rigidity, hardness, density, lubricity,

creep resistance, and solvent resistance than amorphous plastics.

From a cost and performance standpoint, polymers can be classified as either commodity or

engineering plastics.

Another important class of polymeric resins are elastomers. Elastomers have glass transition tem-

peratures below room temperature. Thus, elastomeric materials are rubber-like polymers at room

temperatures, but below their glass transition temperature they will become rigid and lose their

rubbery characteristics.

8.2 COMMODITY THERMOPLASTICS

The commodity thermoplastics include polyolefins and side-chain-substituted vinyl polymers.

8.2.1 Polyethylene

Polyethylenes (PEs) have the largest volume use of any plastic. They are prepared by the catalytic

polymerization of ethylene. Depending on the mode of polymerization, one can obtain a high-density

(HDPE) or a low-density (LDPE) polyethylene polymer. LDPE is prepared under more vigorous

conditions, resulting in short-chain branching. Linear low-density polyethylene (LLDPE) is prepared

by introducing short-branching via copolymerization with a small amount of long-chain olefin.

Polyethylenes are crystalline thermoplastics that exhibit toughness, near-zero moisture absorption,

excellent chemical resistance, excellent electrical insulating properties, low coefficient of friction, and

ease of processing. Their heat deflection temperatures are reasonable but not high. The branching in

LLDPE and LDPE decreases the crystallinity. HDPE exhibits greater stiffness, rigidity, improved heat

resistance, and increased resistance to permeability than LDPE and LLDPE. Some typical properties

of PEs are listed in Table 8.1.

Uses. HDPE's major use is in blow-molded bottles, drums, carboys automotive gas tanks; injec-

tion-molded material-handling pallets, trash and garbage containers, and household and automotive

parts; and extruded pipe.

LDPE/LLDPEs find major applications in film form for food packaging, as a vapor barrier film,

plastic bags; for extruded wire and cable insulation; and for bottles, closures and toys.

8.2.2 Polypropylene

Polypropylene (PP) is prepared by the catalyzed polymerization of propylene. PP is a highly crys-

talline thermoplastic that exhibits low density, rigidity, excellent chemical resistance, negligible water

absorption, and excellent electrical properties. Its properties appear in Table 8.2.

Table 8.1 Typical Property Values for Polyethylenes

Property

Density (Mg /m 3 )

Tensile modulus (GPa)

Tensile strength (MPa)

Elongation at break (%)

Flexural modulus (GPa)

Vicat soft point ( 0 C)

Brittle temperature ( 0 C)

Hardness (Shore)

Dielectric constant (10 6 Hz)

Dielectric strength (M V /m)

Dissipation factor (10 6 Hz)

Linear mold shrinkage (in. /in.)

HOPE

0.96-0.97

0.76-1.0

25-32

500-700

0.8-1.0

120-129

-100 to -70

D60-D69

LLDPE/LDPE

0.90-0.93

4-20

275-600

0.2-0.4

80-98

-85 to -35

D45-D55

2.3

9-21

0.0002

0.015-0.035

0.007-0.009

Table 8.2 Typical Property Values for Polypropylenes

Density (Mg /m 3 )

Tensile modulus (GPa)

Tensile strength (MPa)

Elongation at break (%)

Heat deflection at 0.45 MPa ( 0 C)

Heat deflection at 1.81 MPa ( 0 C)

Vicat soft point ( 0 C)

Linear thermal expansion (mm/mnrK)

Hardness (Shore)

Volume resistivity (H-cm)

Linear mold shrinkage (in. /in.)

0.09-0.93

1.8

10-60

100-105

60-65

130-148

3.8 X IQ- 5

D76

1.0 X 10 1 7

0.01-0.02

Uses. End uses for PP are in blow-molding bottles and automotive parts; injection-molding clo-

sures, appliances, housewares, automotive parts, and toys. PP can be extruded into fibers and filaments

for use in carpets, rugs, and cordage.

8.2.3 Polystyrene

Catalytic polymerization of styrene yields polystyrene (PS), a clear, amorphous polymer with a mod-

erately high heat deflection temperature. PS has excellent electrical insulating properties, but, it is

brittle under impact and exhibits very poor resistance to surfactants and solvents. Its properties appear

in Table 8.3.

Uses. Ease of processing, rigidity, clarity, and low cost combine to support applications in toys,

displays, and housewares. PS foams can readily be prepared and are characterized by excellent low

thermal conductivity, high strength-to-weight ratio, low water absorption, and excellent energy ab-

sorption. These attributes have made PS foam of special interest as insulation boards for construction,

protective packaging materials, insulated drinking cups, and flotation devices.

8.2.4 Impact Polystyrene

Copolymerization of styrene with a rubber, polybutadiene, can reduce brittleness of PS, but only at

the expense of rigidity and heat deflection temperature. Impact polystyrene (IPS) or high-impact

polystyrene (HIPS) can be prepared, depending on the levels of rubber. These materials are translucent

to opaque and generally exhibit poor weathering characteristics. Typical properties appear in Table

8.3.

8.2.5 SAN (Styrene/Acrylonitrile Copolymer)

Copolymerization of styrene with a moderate amount of acrylonitrile provides a clear, amorphous

polymer (SAN) with increased heat deflection temperature and chemical resistance compared to

polystyrene. However, impact resistance is still poor. Typical properties appear in Table 8.3

Uses. SAN is utilized in typical PS-type applications where a slight increase in heat deflection

temperature and/or chemical resistance is needed, such as housewares and appliances.

Table 8.3 Typical Properties of Styrene Thermoplastics

Property

SAN

IPS/HIPS

ABS

Density (Mg/m 3 )

1.050

1.080

1.02-1.04 1.05-1.07

Tensile modulus (GPa)

2.76-3.1

3.4-3.9

2.0-2.4

2.5-2.7

Tensile strength (MPa)

41-52

65-76

26-40

36-40

Elongation at break (%)

1.5-2.5

—

15-25

Heat deflection temperature at 1.81 MPa ( 0 C)

82-93

100-105

80-87

80-95

Vicat soft point ( 0 C)

98-107

110

88-101

90-100

Notched Izod (kJ/m)

0.02

0.1-0.3

0.1-0.5

Linear thermal expansion (10~ 5 mm/mm-K)

5-7

6.4-6.7

7.0-7.5

7.5-9.5

Hardness (Rockwell)

M60-M75

M80-M83

M45, L55 R69-R115

Linear mold shrinkage (in./in.)

0.007

0.003-0.004

0.007

0.0055

8.2.6 ABS

ABS is a terpolymer prepared from the combination of acrylonitrile, butadiene (as polybutadiene),

and styrene monomers. Compared to PS, ABS exhibits good impact strength, improved chemical

resistance, and similar heat deflection temperature. ABS is also opaque. Properties are a function of

the ratio of the three monomers. Typical properites are shown in Table 8.3.

Uses. The previously mentioned properties of ABS make it suitable for tough consumer products;

automotive parts; business machine housings; telephones; appliances; luggage; and pipe, fittings, and

consuits.

8.2.7 Polyvinyl Chloride

The catalytic polymerization of vinyl chloride yields poly vinyl chloride. It is commonly referred to

as PVC or vinyl and is second only to polyethylene in volume use. Normally, PVC has a low degree

of crystallinity and good transparency. The high chlorine content of the polymer produces advantages

in flame resistance, fair heat deflection temperature, good electrical properties, and good chemical

resistance. However, the chlorine also makes PVC difficult to process. The chlorine atoms have a

tendency to split out under the influence of heat during processing and heat and light during end use

in finished products, producing discoloration and embrittlement. Therefore, special stabilizer systems

are often used with PVC to retard degradation.

There are two major sub-classifications of PVC: rigid and flexible (plasticized). In addition, there

are also foamed PVC and PVC copolymers. Typical properties of PVC resins appear in Table 8.4.

Rigid PVC

PVC alone is a fairly good rigid polymer, but it is difficult to process and has low impact strength.

Both of these properties are improved by the addition of elastomers or impact modified graft copol-

ymers, such as ABS and impact acrylic polymers. These improve the melt flow during processing

and improve the impact strength without seriously lowering the rigidity or the heat deflection

temperature.

Uses. With this improved balance of properties, rigid PVCs are used in such applications as door

and window frames; pipe, fittings, and conduit; building panels and siding; rainwater gutters and

down spouts; credit cards; and flooring.

Plasticized PVC

Flexible PVC is a plasticized material. The PVC is softened by the addition of compatible, nonvo-

latile, liquid plasticizers. The plasticizers, which are usually used in > 20 parts per hundred resins,

lower the crystallinity in PVC and act as internal lubricants to give a clear, flexible plastic. Plasticized

PVC is also available in liquid formulations known as plastisols or organosols.

Uses. Plasticized PVC is used for wire and cable insulation, outdoor apparel, rainwear, flooring,

interior wall covering, upholstery, automotive seat covers, garden hose, toys, clear tubing, shoes,

tablecloths, and shower curtains. Plastisols are used in coating fabric, paper, and metal; and rotation-

ally cast into balls, dolls, and so on.

Table 8.4 Typical Property Values for Polyvinyl Chloride Materials

General

Purpose

1.40

3.45

8.7

113

0.53

Rigid

Foam

0.75

Property

Density (Mg /m 3 )

Tensile modulus (GPa)

Tensile strength (MPa)

Elongation at break (%)

Notched Izod (kJ/m)

Heat deflection temperature

at 1.81 MPa ( 0 C)

Brittle temperature ( 0 C)

Hardness

Rigid

1.34-1.39

2.41-2.45

37.2-42.4

Plasticized

1.29-1.34

Copolymer

1.37

3.15

52-55

> 13.8

> 40

> 0.06

14-26

250-400

0.74-1.12

73-77

0.02

-60 to -30

A71-A96

(Shore)

D85

(Shore)

7.00

R107-R122

(Rockwell)

5.94

D55

(Shore)

5.58

Linear thermal expansion

(1(T 5 mm/mm-K)

Linear mold shrinkage (in. /in.)

0.003

Foamed PVC

Rigid PVC can be foamed to a low-density cellular material that is used for decorative moldings and

trim.

Uses. Foamed plastisols add greatly to the softness and energy absorption already inherent in

plasticized PVC, giving richness and warmth to leather-like upholstery, clothing, shoe fabrics, hand-

bags, luggage, and auto door panels; and energy absorption for quiet and comfort in flooring, carpet

backing, auto headliners, and so on.

PVC Copolymers

Copolymerization of vinyl chloride with 10-15% vinyl acetate gives a vinyl polymer with improved

flexibility and less crystallinity than PVC, making such copolymers easier to process without detract-

ing seriously from the rigidity and heat deflection temperature. These copolymers find primary ap-

plications in flooring and solution coatings.

8.2.8 Polyfvinylidene chloride)

Poly(vinylidene chloride) is prepared by the catalytic polymerization of 1,1-dichloroethylene. This

crystalline polymer exhibits high strength, abrasion resistance, high melting point, better than ordinary

heat resistance (10O 0 C maximum service temperature), and outstanding impermeability to oil, grease,

water vapor, oxygen, and carbon dioxide. It is used for packaging films, coatings, and monofilaments.

When the polymer is extruded into film, quenched, and oriented, the crystallinity is fine enough

to produce high clarity and flexibility. These properties contribute to widespread use in packaging

film, especially for food products that require impermeable barrier protection.

Poly(vinylidene chloride) and/or copolymers with vinyl chloride, alkyl acrylate, or acrylonitrile

are used in coating paper, paperboard, or other films to provide more economical, impermeable

materials.

A small amount of poly(vinylidene chloride) is extruded into monofilament and tape that is used

in outdoor furniture upholstery.

8.2.9 Poly(methyl Methacrylate)

The catalytic polymerization of methylmethacrylate yields poly(methyl methacrylate) (PMMA), a

strong, rigid, clear, amorphous polymer. PMMA has excellent resistance to weathering, low water

absorption, and good electrical resistivity. PMMA properties appear in Table 8.5.

Uses. PMMA is used for glazing, lighting difrusers, skylights, outdoor signs, and automobile

taillights.

8.2.10 Polyethylene Terephthalate)

Poly(ethylene terephthalate) (PET) is prepared from the condensation polymerization of dimethyl

terephthalate and ethylene glycol. PET is a crystalline polymer that exhibits high modulus, high

strength, high melting point, good electrical properties, and moisture and solvent resistance. PET

crystallizes slowly, hence blow-molded and extruded objects are clear. Injection-molding grades are

nucleated to facilitate crystallization and shorten the molding cycle. Nucleated PET resins are opaque.

Uses. Primary applications of PET include blow-molded beverage bottles; fibers for wash and

wear, wrinkle-resistant fabrics; and films that are used in food packaging, electrical applications

(capacitors, etc.), magnetic recording tape, and graphic arts.

Table 8.5 Typical Properties of PolyQnethyl Methacrylate)

Property

PMMA

Density (Mg/m 3 )

1.18-1.19

Tensile modulus (GPa)

3.10

Tensile strength (MPa)

Elongation at break (%)

Notched Izod (kJ/m)

0.4

Heat deflection temperature at 1.81 MPa ( 0 C)

Continuous service temperature ( 0 C)

Hardness (Rockwell)

M90-M100

Linear thermal expansion (10~ 5 mm/mm-K)

6.3

Linear mold shrinkage (in./in.)

0.002-0.008

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