Page 71 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.11

may give off hydrogen halides at elevated temperatures. However, PVF has a greater ten-
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dency to crystallize and better heat resistance than PVC.
2.2.5 Nylons
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Nylons were one of the early polymers developed by Carothers. Today, nylons are an
important thermoplastic, with consumption in the United States of about 1.2 billion lb in
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1997. Nylons, also known as polyamides, are synthesized by condensation polymeriza-
tion methods, often an aliphatic diamine and a diacid. Nylon is a crystalline polymer with
high modulus, strength, and impact properties, and low coefficient of friction and resis-
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tance to abrasion. Although the materials possess a wide range of properties, they all
contain the amide (-CONH-) linkage in their backbone. Their general structure is shown in
Fig. 2.8.

FIGURE 2.8 General structure of nylons.

There are five main methods to polymerize nylon. They are

• Reaction of a diamine with a dicarboxylic acid
• Condensation of the appropriate amino acid
• Ring opening of a lactam
• Reaction of a diamine with a dicarboxylic acid
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• Reaction of a diisocyanate with a dicarboxylic acid
The type of nylon (nylon 6, nylon 10, etc.) is indicative of the number of carbon atoms
in the repeat unit. Many different types of nylons can be prepared, depending on the start-
ing monomers used. The type of nylon is determined by the number of carbon atoms in the
monomers used in the polymerization. The number of carbon atoms between the amide
linkages also controls the properties of the polymer. When only one monomer is used (lac-
tam or amino acid), the nylon is identified with only one number (nylon 6, nylon 12).
When two monomers are used in the preparation, the nylon will be identified using two
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numbers (nylon 6,6, nylon 6,12). This is shown in Fig. 2.9. The first number refers to the
number of carbon atoms in the diamine used (a) and the second number refers to the num-
ber of carbon atoms in the diacid monomer (b + 2), due to the two carbons in the carbonyl
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group.
The amide groups are polar groups and significantly affect the polymer properties. The
presence of these groups allows for hydrogen bonding between chains, improving the in-
terchain attraction. This gives nylon polymers good mechanical properties. The polar na-
ture of nylons also improves the bondability of the materials, while the flexible aliphatic
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carbon groups give nylons low melt viscosity for easy processing. This structure also
gives polymers that are tough above their glass transition temperature. 94
Nylons are relatively insensitive to nonpolar solvents; however, because of the pres-
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ence of the polar groups, nylons can be affected by polar solvents, particularly water.
The presence of moisture must be considered in any nylon application. Moisture can cause

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