Page 258 - Handbook of Plastics Technologies

ELASTOMERS

4.50 CHAPTER 4

4.5.2 Preservation of Vulcanizates
As with other polymers, elastomers are subject to degradation due to elements of their envi-
ronment. These elements include oxygen, atmospheric ozone, and ultraviolet light as from
sunlight. Molecular chain scission or cross-linking can be the result, giving rise to substan-
tial losses in performance properties (ultimate elongation, strength, flexibility, fatigue life,
and so on). Thus, it is generally necessary to include additives in rubber recipes or formula-
tions to protect elastomeric or rubber compositions from damage by the environment.
4.5.2.1 Oxidation of Polymers. The need for stabilization of organic polymers is essen-
tial, because they are exposed to oxygen throughout phases of their lifetime: the polymer
production phase, the fabrication phase, and the application stage. Thus, antioxidants are
added to polymers just prior to isolation, before exposure to oxygen. Such a stabilizer is
expected to maintain polymer properties and to suppress gel formation or changes in vis-
cosity. This protection is then expected to continue during storage before fabrication.
Fabrication involves shear and thermal energy, e.g., by Banbury mixing or mill mixing,
extrusion, and calendering. Additional antioxidants are frequently added. The end-use
product is expected to survive the environmental stresses throughout its service life. The
amount and type of stabilizer chosen to protect a product will depend on the type of poly-
mer and its use.
The oxidative degradation of the polymer proceeds by a free-radical chain reaction
mechanism. Initiation usually occurs by exposure to heat, light, or mechanical stress. The
process is sometimes catalyzed by certain transition metal-ion impurities. The oxidation of
hydrocarbon or related polymers by oxygen is an autocatalytic process with primary prod-
ucts being hydroperoxides.
Autoxidation occurs in three mechanistic phases: initiation, propagation, and termina-
tion steps:
Initiation R-R → 2R•
ROOH → RO• + HO•
2ROOH → RO• + RO • +H O
2
2
Propagation RO • + RH → ROOH + R•
2
R• + O → RO •
2
2
Termination 2R• → R-R
R• + RO • → ROOR
2
2 RO • → Nonradical products + O 2
2
This oxidation model, however, ignores factors such as relativity differences between
polymers and differences in oxygen permeability. Also, it does not account for the fact that
the nature and character of the propagating radicals is not the same for all polymers. Poly-
mers show a wide variation in susceptibility to thermal autoxidation. The ease of autoxida-
tion depends primarily on the relative C-H bond dissociation energies for the component
parts of the polymer structure. Once the free-radical process is initiated, autoxidation pro-
ceeds. The abstraction of the most labile hydrogen atom in the polymer by alkylperoxy
radical predominates in the propagation step. This follows this order:
Strongest C-H bonds Weakest C-H bonds
Primary Secondary Tertiary Allylic
RCH 2 -H R 2 CH 2 -H R 3 C-H RCH=CH-CH 2 -H

Also, radicals also differ in relative lifetime.

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