Page 91 - Mechanical Behavior of Materials

90 Chapter 3 A Survey of Engineering Materials

Figure 3.16 Molecular structures of a phenolic thermosetting plastic and of a synthetic similar
to natural rubber, cis-polyisoprene. In the phenolics, carbon–carbon bonds form cross-links,
whereas in polyisoprene cross-links are formed by sulfur atoms.

(phenolic), the structure of which is shown in Fig. 3.16. The common plastic Bakelite is a phenolic.
Some other common thermosetting polymers are the epoxy adhesives and polyester resins used for
fiberglass.
The cross-linking (thermosetting) chemical reaction occurs during the final stage of processing,
which is typically compression molding at elevated temperature. Following this reaction, the
resulting solid will neither soften nor melt upon heating, but will usually decompose or burn instead.
The network structure results in a rigid and strong, but brittle, solid.
Recall that thermal expansion and the resulting increased free volume produces the glass
transition temperature (T g ) effect in thermoplastics, above which deformation may occur by relative
sliding between chain molecules. This situation contrasts with that for a thermosetting plastic where
relative motion between the molecules is prevented by strong and temperature-resistant covalent
bonds. As a result, there is no distinct T g effect in highly cross-linked thermosetting plastics.

3.5.4 Elastomers
Elastomers are typified by natural rubber, but also include a variety of synthetic polymers with
similar mechanical behavior. Some elastomers, such as the polyurethane elastomers, behave in
a thermoplastic manner, but others are thermosetting materials. For example, polyisoprene is a
synthetic rubber with the same basic structure as natural rubber, but lacking various impurities found
in natural rubber. This structure is shown in Fig. 3.16. Adding sulfur and subjecting the rubber to
◦
pressure and a temperature around 160 C causes sulfur cross-links to form, as shown. A greater
degree of cross-linking results in a harder rubber. This particular thermosetting process is called
vulcanization.
Although cross-linking results in the rigid network structure of thermosetting plastics, typical
elastomers behave in a very different manner because the cross-links occur much less frequently
along the chains, specifically at intervals on the order of hundreds of carbon atoms. Also, the