Tear Phenomena of Polymer Films

Abstract

The force required to tear polymer films was measured at various temperatures, and the rates of the tear of commercial films of cellulose triacetate, poly (ethylene terephthalate), and polycarbonate. One of the arms of the test-piece with a definite length of cut was clamped to a movable crosshead, and the other to a stress transducer. The force for tearing during the tear process was dependent not only upon the type of the polymer but also upon temperature, the rate of the tear, the thickness of the film and its fine structure, and the condition of annealing before the measurement. As the force required to deform the test-piece was found to be negligible in comparison with the tearing force F, the energy for tearing E_t is identical with 2F/t where t is the thickness of the specimen. The energy for tearing E_t was measured at the rate of tear 0.125cm/sec and 24°C, and the relationship, E_t=αtⁿ, was obtained over the range of thickness from 0.01 to 0.2mm where α was defined as the coefficient of tearing being independent of the shape factor of the specimen and the value of n was nearly 0.6 for all the films tested. The coefficient of tearing obtained on assuming n=0.6 was compared with the energy for tensile rupture for several kinds of polymer films, and it was found that the tear strength was not necessarily predictable from the energy for tensile rupture.
The energy for tearing of cellulose triacetate and poly (ethylene terephthalate) films plotted against the temperature shows maximum value in the neighborhood of the temperature corresponding to the glass transition temperature of these crystalline polymers. For polycarbonate films, however, the energy for tearing steeply decreases in the vicinity of the glass transition temperature without remarkable peak, and the curve shifts toward lower temperature as the rate of the tear decreases. The result of the measurement shows that there are several regions of temperature according to the different mechanisms of tear. The cellulose triacetate film shows stick-slip type of tearing below -50°C, steady from -50 to 70°C, irregular from 70°C to 155°C with the maximum value of energy for tearing, and irregular above 155°C with the energy increasing with temperature. The poly (ethylene terephthalate) film shows steady type of tearing below 10°C, irregular from 10 up to 120°C with the maximum value of energy for tearing, and steady above 120°C.
The Griffith theory on brittle fracture gives the relationship between the tensile strength and the size of crack in the specimen. The surface energy was obtained from the tensile strength of the specimens containing artificially induced cut of known length in tensile measurement. For the cellulose triacetate film the value of the surface energy at rate of extension 10%/min was found to be 1.6×10⁶ergs/cm². Though both the value of energy for tearing and the surface energy are in the order of 10⁶ergs/cm², the former is dependent on the thickness of the specimen and the latter not.