Chemistry Reference
In-Depth Information
can be polymerized at about 50
Cinn-pentane with either butyl lithium or tita-
nium tetrachloride/triisobutyl aluminum catalysts. The product, which is the cis
form of 1,4-polyisoprene,
H
CH
2
C
CCH
2
x
CH
3
1-18
has a structure close to that of natural rubber. The residual double bond in each
repeating unit is essentially inert toward further polymerization under these reac-
tion conditions, but it activates the repeating unit for subsequent reaction with sul-
fur during vulcanization after the rubber has been compounded with other
ingredients and shaped into articles like tires. The chemistry of the vulcanization
reaction is complex and depends partly on ingredients in the mix other than just
sulfur and rubber. Initially discrete rubber molecules become chemically linked
by structures such as
CH
3
H
CH
2
CC
CH
2
C
C
C
CH
2
H
CH
3
H
H
CH
3
S
x
CH
2
C
C
C
CH
2
H
1-19
Here x is 1 or 2 in efficient vulcanization systems but may be as high as 8
under other conditions where cyclic and other structures are also formed in the
reaction. The rubber article is essentially fixed in shape once it is vulcanized, and
it is necessary that this cross-linking reaction not occur before the rubber has
been molded into form.
1.3.3.2
Epoxies
Epoxy polymers are used mainly as adhesives, surface coatings, and in combina-
tion with glass fibers or cloth, as light weight, rigid structural materials. Many dif-
ferent epoxy prepolymers are available. (A prepolymer is a low-molecular-weight
polymer that is reacted to increase its molecular size. Such reactions are usually
carried out during or after a process that shapes the material into desired form.
The polyisoprene mentioned in
Section 1.3.3.1
is not called a prepolymer, since
the average molecule contains about 1000 repeating units before vulcanization.)
The most widely used epoxy prepolymer is made by condensing epichlorohydrin
