Callister - An introduction - 8th edition

15.9 Deformation of Elastomers • 589
temperature at which rubberlike behavior persists for many of the common elastomers
is between 50 and 90C (60 and 130F). Below its glass transition temperature,
an elastomer becomes brittle such that its stress–strain behavior resembles
curve A in Figure 15.1.
vulcanization
Vulcanization
The crosslinking process in elastomers is called vulcanization, which is achieved by
a nonreversible chemical reaction, ordinarily carried out at an elevated temperature.
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:
H
CH 3
H
H
H
CH 3
H
H
C
C
C
C
C
C
C
C
H
H
H
H
(m n) S
H
H
(S) m (S) n
H
H
(15.4)
C
C
C
C
C
C
C
C
H
CH 3
H
H
H
CH 3
H
H
VMSE
Polymers: Rubber
in which the two crosslinks shown consist of m and n sulfur 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 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 15.16. 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
Stress (MPa)
60
50
40
30
Vulcanized
10
8
6
4
Stress (10 3 psi)
Figure 15.16
Stress–strain curves to
600% elongation for
unvulcanized and
vulcanized natural
rubber.
20
10
Unvulcanized
2
0
0
0
1 2 3 4 5 6
Strain