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Short‐time stress relaxation and toughness of rubber‐modified polystyrene
Author(s) -
Schmitt J. A.,
Keskkula Henno
Publication year - 1960
Publication title -
journal of applied polymer science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.575
H-Index - 166
eISSN - 1097-4628
pISSN - 0021-8995
DOI - 10.1002/app.1960.070030802
Subject(s) - materials science , composite material , crazing , polystyrene , natural rubber , toughness , stress relaxation , stress (linguistics) , breakage , strain rate , polymer , creep , linguistics , philosophy
A short‐time stress‐relaxation apparatus similar to that of Watson, Kennedy, and Armstrong was built. A large compressed spring is used to attain high strain rates which m.ere kept constant at 3GO in./min. by controlling the spring motion hydraulically. Measurements wcre made at room temperature on injection‐molded bars of polystyrene, styrene. ticrylonitrile copolymer, and a rubber‐styrene polymer. No correlation could be seen between the short‐time stress‐relaxation rates and impact resistance or “toughness.” The apparatus, however, has proved to be very useful, because it furnishes a means of straining a specimen rapidly and recording the stress and its decay with time. Stress‐strain curves for a high straining rate have been constructed from the data. A high‐speed flexural test can be made by modifying the specimen holders. This flexural test has shown a correlation between the dart‐drop impact strength of an object and the deflection a t breakage of specimens cut perpendicular to the flow of plastic in the mold. In addition, it was found that stress relaxation data can be used to measure critical elongation (or critical stress) as a function of time, without observing the formation of craze cracks. A mechanism of reinforcement of polystyrene by rubber particles is proposed. A phase‐contrast photomicrograph is presented as evidence that the energy of nondestructive impact is used up in forming many short cracks radiating out around the rubber particles. A general whitening in the specimen results which is caused by reflection and scattering of light from the many planes.