Tear Strength of Elastomers under Threshold Conditions

Abstract

Measurements have been made of the tear strength of several elastomeric materials under threshold conditions, e.g., in the swollen state and at high temperatures when dissipative contributions to the work of fracture are minimized. The materials studied were Neoprene WRT, BR, SBR, EPDM, and a castable polysulfide material, LP-32. They were crosslinked to different degrees, using a free-radical source, or sulfur, or a metal oxide as the crosslinking reagent. Values obtained for the threshold tear strength were similar in all cases, lying in the range 50–200 J/m2. They appeared to be approximately proportional to E−1/2, where E is the tensile (Young's) modulus of the elastomer, as predicted by the theoretical treatment of Lake and Thomas. Values for networks with polysulfidic crosslinks were significantly higher than with monosulfide or C—C crosslinks, by about a factor of 2. Values for carbon black-filled materials were higher also, by about the same factor, in comparison with the equivalent unfilled materials. These effects are attributed to rupture of labile crosslinks, or bonds to carbon black, prior to main-chain rupture.

Effect of crosslink type on the fracture of natural rubber vulcanizates

“…Lake and Thomas [1] have reported that under threshold tearing conditions (where energy dissipative processes are minimized) an elastomer with mainly polysulfidic crosslinks has a higher threshold tear strength than one with predominately monosulfidic crosslinks. This result has been confirmed by other workers [2] and leads to the conclusion that enhanced energy dissipation cannot account for the increase in strength due to polysulfidic crosslinking. The basic network strength (as quantified by the threshold tearing energy) is directly affected by the type of crosslink.…”

“…Payne [27] has shown that increased hysteresis correlates with increased fatigue life. However other researchers [1,2] have shown that in the absence of energy dissipation, a difference in tear strength between polysulfidic, monosulfidic and carbon-carbon crosslinked elastomers is still apparent. Therefore, although hysteresis differences (if they exist between polysulfidic and monosulfidic networks) may account for a portion of the fatigue life differences under the conditions tested here, the intrinsic nonhysteretic strength of the network as quantified by the threshold tearing energy is still a major factor.…”

“…Many researchers have investigated the effect of crosslink type on the threshold tear energy [1,2], tear energy master surfaces [3], and tensile strength [4][5][6][7]. The effect ofcrosslink type on the strength properties of elastomers is still under debate.…”

On the origin of brittle fracture of entangled polymer solutions and melts

“…The threshold fracture energy C 0 can be measured directly in tearing tests by extrapolating the energy required to create a unit of facture surface to zero crack speed, thereby eliminating other dissipative processes [37]. Equation 3is in qualitative agreement with data of elastomers [37] and hydrogels [38].…”

Reinforcement of Elastomers by Particulate Fillers

“…Under these conditions, the tearing energy of the reinforced rubber obeys the same WLF transform as the gum rubber [84][85][86] (or, at higher temperatures, the same Arrhenius dependence [86]), but is somewhat higher (by a factor of 1.6-2.0) throughout the range of temperature and tearing rates studied. The relatively small increase in tearing energy caused by the carbon black when tear deviation is prevented appears to be the result of an increase in the threshold tearing energy, T {) (see Chapter 10) [87][88][89]. The additional hysteresis caused by the carbon black does not contribute to the strength of the vulcanizate.…”