ISSN:
0022-3832
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Cracking on the surface of vulcanizates exposed to ozone under strain has been studied quantitatively. Butyl rubber vulcanizates were used to allow ready variation of polymer unsaturation. A relaxed sample under tension when exposed to ozone undergoes a work change manifested by increased length and decreased stress as cracking occurs. The change in work is considered related to generation of new surface in the form of cracks. Cracking is found to be primarily a surface phenomenon and in this wise permeation of the ozone into the sample assumes an important role in this phenomenon since it defines the “depth of the surface.” Polymer structure as it affects permeation appears as important as chemical inertness in controlling ozone cracking. The amount of cracking judged by change of work measured in these experiments is related linearly to the time squared. The rate of cracking is related linearly to ozone concentration squared over the range 0.03-0.20 volume per cent in air. The rate and type of cracking is dependent on extension. The analysis of this relation is complicated by relaxation effects. The state of relaxation of a sample under tension has a profound effect on the rate of ozone cracking. This effect is discussed in terms of distribution of strain of the network chains. Although cracking requires a stored energy to produce new surface, the chemical action of ozone proceeds in the absence of strain. Whether the effects of polymer degradation under conditions of zero strain become large enough to be measurable depends on the rate of permeation of ozonized air into the specimen.Vulcanization has an important bearing on ozone cracking in Butyl rubber. This is probably due to a significant decreases in the percentage of unreacted double bonds and also to a local densification of the network in the region of the double bond involved in a crosslink. Cracking is particularly sensitive to the onset of reversion. This sensitivity may arise because state of reversion may be a gradient, greatest on the surface and least at the center of the sample.
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1956.120199115
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