Process for processing a semicrystalline thermoplastic
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United States of America Patent
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May 21, 2002
Issued Date -
N/A
app pub date -
May 23, 2000
filing date -
Sep 30, 1996
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Expired
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Abstract
A process for processing a thermoplastic, which includes, in succession, the following steps: (1) a molten semicrystalline thermoplastic extrudate is cooled to a temperature T.sub.1 which is greater than the crystallization temperature (T.sub.c) of the thermoplastic and is less than T.sub.f +5.degree. C., T.sub.f denoting the melting point of the thermoplastic; (2) next, the extrudate is drawn in the molten state; (3) next, the extrudate is relaxed, at a temperature T.sub.3 which is between T.sub.c and T.sub.f, for a duration at least equal to the average relaxation time (.tau..sub.3) of the thermoplastic at the relaxation temperature, and after the drawing; (4) next, the extrudate is quenched to a temperature T.sub.4 which is less than T.sub.c. In particular, this process makes it possible to manufacture a film essentially consisting of at least one propylene homopolymer, with a thickness of from 100 to 250 .mu.m and having a hot shrinkage of at most 20%, a haze of less than or equal to 3% and a total transparency with respect to air of greater than 70%.
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