Page 201 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 7 Polymers: Structure, General Properties, and Applications

TABLE 7.2
Glass-transition and Melting Temperatures
nf Some Polymers

Material Tg (°C) Tm (°C)
Nylon 6,6 57 265
Polycarbonate 150 265
Polyester 73 265
Polyethylene
High density -90 137
Low density -110 115
Polymethylmethacrylate 105 -
Polypropylene - 14 1 76
Polystyrene 100 239
Polytetrafluoroethylene -90 327
-
Polyvinyl chloride 87 212
Rubber f 73

7.2.4 Polymer Blends
The brittle behavior of amorphous polymers below their glass-transition tempera-
ture can be reduced by blending them, usually with small quantities of an elastomer
(Section 7.9). The tiny particles that make up the elastomer are dispersed through-
out the amorphous polymer, enhancing its toughness and impact strength by
improving its resistance to crack propagation. These polymer blends are known as
rubber-modified polymers.
Advances in blending involve several components, creating polyblends that
utilize the favorable properties of different polymers. Miscible blends (mixing with-
out separation of two phases) are created by a process similar to the alloying of
metals that enables polymer blends to become more ductile. Polymer blends account
for about 20% of all polymer production.

7.3 Thermoplastics

It was noted earlier that within each molecule, the bonds between adjacent long-
chain molecules (secondary bonds) are much weaker than the covalent bonds
between mers (primary bonds). It is the strength of the secondary bonds that deter-
mines the overall strength of the polymer; linear and branched polymers have weak
secondary bonds.
As the temperature is raised above the glass-transition temperature, Tg, or
melting point, Tm, certain polymers become easier to form or mold into desired
shapes. The increased temperature weakens the secondary bonds (through thermal
vibration of the long molecules), and the adjacent chains can then move more easily
when subjected to external shaping forces. When the polymer is cooled, it returns to
its original hardness and strength; in other words, the process is reversible. Polymers
that exhibit this behavior are known as thermoplastics, common examples of which
are acrylics, cellulosics, nylons, polyethylenes, and polyvinyl chloride.
The behavior of thermoplastics depends on other variables as well as their
structure and composition. Among the most important are temperature and rate
of deformation. Below the glass-transition temperature, most polymers are glassy
(brittle) and behave like an elastic solid. (That is, the relationship between stress and