Tear strength of pre-stressed NR black-filled vulcanisates

Parallel-sided test-pieces prepared from conventional natural rubber (nr) sulfur vulcanisates filled with 50 p.p.h.r. haf black were pre-stressed to a pre-determined stress level for one minute,after which the loading stress was removed. they were swollen in toluene for 72 hours after which time they were deswollen to remove residual strain,and to allow for rapid elastic recovery. the test-pieces were dried down to a constant weight before carrying out the tear test. tear deviations from the intended tear path took place if the stress level during pre-stressing was lower than 18 - 20 mpa (ca. 350% strain). above 20 mpa stress level,the tear did not deviate but propagated along the intended tear path. however,if the samples were cycled a few times even below 18 mpa,no tear deviations from the intended path were observed. tear measurements on all samples pre-stressed to 23 mpa were conducted at four different temperatures and at various different test speeds. within the range of test speeds and test temperatures covered in this investigation,tearing energy increased with increasing test speeds. the tearing energy data were amenable to wlf transformations indicating that the tearing process was predominantly associated with viscous processes. tearing energy of the pre-stressed samples was closely associated with the amount of permanent set produced during pre-stressing (single loading or cyclic loading). the higher the amount of set,the lower the tearing energy.