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Source: Handbook of Plastics, Elastomers, and Composites

Chapter

Thermoplastics

Anne-Marie M. Baker

Joey Mead

Plastics Engineering Department

University of Massachusetts Lowell

Lowell, Massachusetts

1.1 Introduction

Plastics are an important part of everyday life; products made from plastics range from so-

phisticated articles, such as prosthetic hip and knee joints, to disposable food utensils. One

of the reasons for the great popularity of plastics in a wide variety of industrial applica-

tions is the tremendous range of properties exhibited by plastics and their ease of process-

ing. Plastic properties can be tailored to meet speciﬁc needs by varying the atomic

composition of the repeat structure; and by varying molecular weight and molecular

weight distribution. The ﬂexibility can also be varied through the presence of side chain

branching and according to the lengths and polarities of the side chains. The degree of

crystallinity can be controlled through the amount of orientation imparted to the plastic

during processing, through copolymerization, by blending with other plastics, and via the

incorporation of an enormous range of additives (ﬁllers, ﬁbers, plasticizers, stabilizers).

Given all of the avenues available to pursue in tailoring any given polymer, it is not sur-

prising that the variety of choices available to us today exists.

Polymeric materials have been used since early times, even though their exact nature

was unknown. In the 1400s, Christopher Columbus found natives of Haiti playing with

balls made from material obtained from a tree. This was natural rubber, which became an

important product after Charles Goodyear discovered that the addition of sulfur dramati-

cally improved the properties; however, the use of polymeric materials was still limited to

natural-based materials. The ﬁrst true synthetic polymers were prepared in the early 1900s

using phenol and formaldehyde to form resins—Baekeland’s Bakelite. Even with the de-

velopment of synthetic polymers, scientists were still unaware of the true nature of the ma-

terials they had prepared. For many years, scientists believed they were colloids—a

substance that is an aggregate of molecules. It was not until the 1920s that Herman

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Thermoplastics

Chapter One

Staudinger showed that polymers were giant molecules or macromolecules . In 1928,

Carothers developed linear polyesters and then polyamides, now known as nylon. In the

1950s, Ziegler and Natta’s work on anionic coordination catalysts led to the development

of polypropylene; high-density, linear polyethylene; and other stereospeciﬁc polymers.

Materials are often classiﬁed as metals, ceramics, or polymers. Polymers differ from

the other materials in a variety of ways but generally exhibit lower densities, thermal con-

ductivities, and moduli. Table 1.1 compares the properties of polymers to some representa-

tive ceramic and metallic materials. The lower densities of polymeric materials offer an

advantage in applications where lighter weight is desired. The addition of thermally and/or

electrically conducting ﬁllers allows the polymer compounder the opportunity to develop

materials from insulating to conducting. As a result, polymers may ﬁnd application in

electromagnetic interference (EMI) shielding and antistatic protection.

TABLE 1.1 Properties of Selected Materials 451

Material

Speciﬁc

gravity

Thermal

conductivity,

(Joule-cm/°C cm 2 s)

Electrical

resistivity,

µΩ-cm

Modulus

MPa

Aluminum

2.7

2.2

2.9

70,000

Brass

8.5

1.2

6.2

110,000

Copper

8.9

4.0

1.7

110,000

Steel (1040)

7.85

0.48

17.1

205,000

A1 2 O 3

3.8

0.29

>10 14

350,000

Concrete

2.4

0.01

–

14,000

Bororsilicate glass

2.4

0.01

>10 17

70,000

MgO

3.6

–

10 5 (2000°F)

205,000

Polyethylene (H.D.)

0.96

0.0052

10 14 –10 18

350–1,250

Polystyrene

1.05

0.0008

10 18

2,800

Polymethyl methacrylate

1.2

0.002

10 16

3,500

Nylon

1.15

0.0025

10 14

2,800

Polymeric materials are used in a vast array of products. In the automotive area, they

are used for interior parts and in under-the-hood applications. Packaging applications are a

large area for thermoplastics, from carbonated beverage bottles to plastic wrap. Applica-

tion requirements vary widely, but, luckily, plastic materials can be synthesized to meet

these varied service conditions. It remains the job of the part designer to select from the ar-

ray of thermoplastic materials available to meet the required demands.

1.2 Polymer Structure and Synthesis

A polymer is prepared by stringing together a series of low-molecular-weight species

(such as ethylene) into an extremely long chain (polyethylene), much as one would string

together a series of bead to make a necklace (see Fig. 1.1). The chemical characteristics of

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Thermoplastics

the starting low-molecular-weight species will determine the properties of the ﬁnal poly-

mer. When two different low-molecular-weight species are polymerized the resulting

polymer is termed a copolymer such as ethylene vinylacetate. This is depicted in Fig. 1.2.

Plastics can also be separated into thermoplastics and thermosets. A thermoplastic mate-

rial is a high-molecular-weight polymer that is not cross-linked. It can exist in either a lin-

ear or a branched structure. Upon heating, thermoplastics soften and melt, which allows

them to be shaped using plastics processing equipment. A thermoset has all of the chains

tied together with covalent bonds in a three dimensional network (cross-linked). Thermo-

set materials will not ﬂow once cross-linked, but a thermoplastic material can be repro-

cessed simply by heating it to the appropriate temperature. The different types of

structures are shown in Fig. 1.3. The properties of different polymers can vary widely; for

example, the modulus can vary from 1 MPa to 50 GPa. Properties can be varied for each

individual plastic material as well, simply by varying the microstructure of the material.

There are two primary polymerization approaches: step-reaction polymerization and

chain-reaction polymerization. 1 In step-reaction (also referred to as condensation poly-

merization ), reaction occurs between two polyfunctional monomers, often liberating a

small molecule such as water. As the reaction proceeds, higher-molecular-weight species

Figure 1.1 Polymerization.

Figure 1.2 Copolymer structure.

Figure 1.3 Linear, branched, and cross-linked polymer structures.

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Thermoplastics

Chapter One

are produced as longer and longer groups react together. For example, two monomers can

react to form a dimer, then react with another monomer to form a trimer. The reaction can

be described as n -mer + m -mer → ( n + m )mer, where n and m refer to the number of

monomer units for each reactant. Molecular weight of the polymer builds up gradually

with time, and high conversions are usually required to produce high-molecular-weight

polymers. Polymers synthesized by this method typically have atoms other than carbon in

the backbone. Examples include polyesters and polyamides.

Chain-reaction polymerizations (also referred to as addition polymerizations ) require

an initiator for polymerization to occur. Initiation can occur by a free radical or an anionic

or cationic species, which opens the double bond of a vinyl monomer and the reaction pro-

ceeds as shown above in Fig. 1.1. Chain-reaction polymers typically contain only carbon

in their backbone and include such polymers as polystyrene and polyvinyl chloride.

Unlike low-molecular-weight species, polymeric materials do not possess one unique

molecular weight but rather a distribution of weights as depicted in Fig. 1.4. Molecular

weights for polymers are usually descri bed by two different average molecular weights,

t he number average molecular weight, , and the weight average molecular weight,

. These averages are calculated using the equations below:

M n

∞

∑

n i M i

n i

i 1

∞

∑

n i M i

M w

-------------

n i M i

i 1

where n i is the number of moles of species i, and M i is the molecular weight of species i .

The processing and properties of polymeric materials are dependent on the molecular

weights of the polymer.

Figure 1.4 Molecular weight distribution.

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M w

-----------

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