Gutta-percha endodontic filling points were found to contain approximately 20% gutta-percha (matrix), 66% zinc oxide (filler), 11% heavy metal sulfates (radiopacifier), and 3% waxes and/or resins (plasticizer). The mechanical properties were indicative of a partially crystalline viscoelastic polymeric material. They were found to obey Hooke's law and displayed a prominent upper and lower yield point when stressed beyond the proportional limit. The essential differences in mechanical properties of individual brands were found to be a function of the gutta-percha and zinc oxide concentration.
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Friedman et al. (1975) studied this question.