Page 246 - Handbook of Plastics Technologies

ELASTOMERS

4.38 CHAPTER 4

SCHEME 8

SCHEME 9

dangling from the rubber molecular chains. Higher ratios of sulfur concentration to accel-
erator concentration give both more polysulfide cross-links and more sulfur combined
with the rubber chains, forming sulfur-containing six-membered heterocyclic rings along
the rubber molecular chains. These features are indicated by Fig. 4.24. Such changes in
the vulcanizate network structure are responsible for changes in vulcanizate properties due
to changes in the curing-system.
Effects of changes in vulcanizate properties due to changes in accelerator and sulfur
concentrations were studied with the following recipe (parts by weight): natural rubber,
100; N330 carbon black, 50; N-isopropyl-N´-phenyl-p-phenylenediamine (IPPD anti-
degradant) 2; zinc oxide, 5; stearic acid, 3; plasticizer, 3; sulfur, variable; N-cylohexylben-
zothiazolesulfenamide (CBS), variable.
The effects of changes in the accelerator concentration on 300 percent modulus (jargon
for stress at 300 percent tensile strain), thermal-oxidative aging, and fatigue life (DeMattia
flex crack) are given in Fig. 4.25. The effects on 300 percent modulus are indicated by the
diagonal contours of negative slope. They are parallel and show that the stress at 300 per-
cent strain increases with an increase in either sulfur or accelerator concentration. The
contours for percent retention of ultimate elongation after hot air aging (at 100°C for two
days) indicate that oxidative aging, in the presence of IPPD, depends only on the concen-
tration of sulfur. Higher concentrations of sulfur give poor aging characteristics in correla-
tion with the higher number of points of chain sulfuration.

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