Experimental Identification of the Deformation Mechanisms During Sintering of Cold Compacted Polytetrafluoroethylene Powders

The influence of the cooling rate after melting on the crystalline weight fraction of pure poly(tetrafluoroethylene) (PTFE) was derived from differential scanning calorimetry tests. Specimens made of pure or filled PTFE were manufactured by isostatic or constrained uniaxial cold pressing, up to different values of the porosity. Dilatometry tests were carried out on these specimens while imposing various heating-cooling treatments. The deformations recorded during these tests strongly depend on the mode and level of compaction. The original procedures presented, herein, made it possible to identify that these macroscopic deformations, which are strongly anisotropic for uniaxially pressed specimens, result from the combination of different independent mechanisms. The mechanisms that were distinguished are reversible thermal expansion, void closure, crystalline to amorphous and vice versa transformations, and recovery, which were not observed for isostatically pressed specimens. It was also shown that, after being subjected to a standard sintering treatment, anisotropically pressed specimens still exhibit anisotropic crystallization strains. POLYM. ENG. SCI., 51:2220-2235, 2011. (C) 2011 Society of Plastics Engineers