Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51

2nd Revised Pages

280 • Chapter 8 / Deformation and Strengthening Mechanisms

In most vulcanizing reactions, sulfur compounds are added to the heated elastomer;

chains of sulfur atoms bond with adjacent polymer backbone chains and crosslink

them, which is accomplished according to the following reaction:

(8.12)

C

H

H

C

CH 3

C

H

C

H

H

C

H

H

C

 (m  n) S (S)m (S)n

CH 3

C

H

C

H

H

C

H

H

C

CH 3

C

H

C

H

H

C

H

H

C

CH 3

C

H

C

H

H

in which the two crosslinks shown consist ofmandnsulfur atoms. Crosslink main

chain sites are carbon atoms that were doubly bonded before vulcanization but, after

vulcanization, have become singly bonded.

Unvulcanized rubber, which contains very few crosslinks, is soft and tacky and

has poor resistance to abrasion. Modulus of elasticity, tensile strength, and resistance

to degradation by oxidation are all enhanced by vulcanization. The magnitude of the

VMSE

Polymers/

Rubber

modulus of elasticity is directly proportional to the density of the crosslinks. Stress–

strain curves for vulcanized and unvulcanized natural rubber are presented in Figure

8.31. To produce a rubber that is capable of large extensions without rupture of

the primary chain bonds, there must be relatively few crosslinks, and these must be

widely separated. Useful rubbers result when about 1 to 5 parts (by weight) of sulfur

are added to 100 parts of rubber. This corresponds to about one crosslink for every

10 to 20 repeat units. Increasing the sulfur content further hardens the rubber and

also reduces its extensibility. Also, since they are crosslinked, elastomeric materials

are thermosetting in nature.

Stress (10

3 psi)

10

8

6

4

2

0 0

0

50

60

40

30

20

10

12345 6

Strain

Stress (MPa) Vulcanized

Unvulcanized

Figure 8.31 Stress–

strain curves to 600%

elongation for

unvulcanized and

vulcanized natural

rubber.