RELAXATION OF ORIENTATION DEVELOPED DURING THE MIXING PROCESS AND ITSEFFECT ON THE TENSILE-STRENGTH OF NATURAL-RUBBER VULCANIZATES

A study was made of the effect of the localized alignment of polymer c hains, occasioned by the high shearing forces operating in the mixing process, on the ultimate tensile strength of natural rubber compounds. On heating to vulcanization temperatures (150-degrees-C) a partial re laxation of induced orientation occurs rapidly (10 min), i.e. during t imes commensurate with scorch times in accelerated sulfur vulcanizatio n. Further relaxation of orientation is slow. Thermal relaxation prior to crosslinking did not affect the stress-strain curves for unfilled natural rubber, indicating that the absence of an induction period in peroxide cures does not detrimentally affect tensile properties. Stres s relaxation can also be brought about by swelling of the polymer in a solvent or by compounding in the presence of a solvent. Compounding n atural rubber in the presence of small amounts of solvent (5 phr) prod uced a structure in which larger extensions were required before the o nset of strain induced crystallization and the upturn in the stress-st rain curve was observed. These higher elongations did not affect the e nergy to break.