Page 265 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.57

Mechanisms for Protection against the Effects of Ozone Attack. The mechanism
of antiozonant protection is still not fully understood. However, there are several theories,
which detail the mechanism of protection by chemical antiozonants: inert barrier, compet-
itive reaction, reduced critical stress, and polymer back-bone chain repair.
The inert barrier theory says that a material that is nonreactive migrates to the surface
and forms a physical barrier that prevents the ozone from reaching the reactive double
bonds in the polymer. Waxes are thought to behave in this manner.
According to the competitive-reaction or “scavenger” model, the antiozonant migrates
to the surface of the rubber and then selectively reacts with ozone so that the rubber is not
harmed until the antiozonant is consumed. A protective-film theory suggests, that after the
antiozonant has done the above and behaved as a “scavenger,” the reaction products be-
come an inert film. Any chemical antiozonant might function in both of these ways. There
is much evidence to support the “scavenger” mechanism as the dominant one. There is
also good support for the formation of a protective film. Surface films on rubber have been
seen visually and microscopically. With partial removal of the film and reexposure to
ozone, only the cleaned surface is degraded.
According to the reduced critical stress theory, certain materials migrate to or near the
rubber surface and modify the internal stress of the polymer such that cracks do not ap-
pear. Although this phenomenon is poorly understood, it is easy to observe. The use of in-
creasingly higher levels of antiozonant raises the critical stress level required for cracks to
form.
The chain repair theories suggest that severed polymer chains (terminated by carboxy
or aldehyde groups) can be relinked by reaction with the antiozonant or that the antiozo-
nant reacts with the ozonide or zwitterion (carbonyl oxide) to give a low-molecular-
weight, inert, self-healing film. Either way, the antiozonant would be chemically linked to
the rubber. However, the chain repair or self-healing film theories do not appear to be as
strongly supported as the other theories.
Ideal Antiozonants. An ideal antiozonant should be competitively reactive with
ozone in the presence of carbon-carbon double bonds in the rubber-molecule backbone.
However, it should not too reactive with ozone (or even oxygen) lest it not persist to give
long-term protection. It should not react with sulfur accelerators or other ingredients in the
cure package. It should be nonvolatile and persist at the surface of the rubber. In addition,
the ideal antiozonant should not discolor the rubber. Unfortunately, an ideally active non-
staining chemical antiozonant has not yet been found.

4.5.3 Types of Vulcanizable Elastomers and their Applications
4.5.3.1 Natural Rubber (NR). Natural rubber, as stated above, was the first elastomer to
be used in commercial applications. Although the polymer (cis-1,4-polyisoprene) occurs
in over 200 plants, the rubber tree, hevea brasiliensis, is the source of essentially all that is
used. The chemical structure of the polymer is given here:

There are two possible structures for poly-1,4,polyisoprene. Natural rubber structure is
of the cis form. The trans forms (the structure of guta perch or balata gum) have higher
melting point and higher glass transition temperatures (see below).

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.