Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6-naphthalate) films and bottles

Amorphous preforms of poly(ethylene 2,6-naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat-setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat-setting temperature and moved from 60degreesC without heat treatment up to a temperature of 255degreesC by a heat-setting temperature of 255degreesC; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80degreesC. For heat-setting temperatures higher than 220degreesC, the structural variations changed rapidly as a function of the heat-setting time, and the corresponding shrinkage of the heat-set samples sank below 1% in a timescale of 30-60 s for a film thickness of 500 mum. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat-set films was zero for temperatures up to the heat-setting temperature, between 220 and 265degreesC, if the heat-setting time was sufficient. According to the results obtained, a heat-setting time of 30 s, for a film thickness of 500 mum, was sufficient at a heat-setting temperature of 255degreesC to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151degreesC and another rigid amorphous phase, more tempera ture-stable than the first one, that shrank in the temperature range of 200-310degreesC. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat-set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. (C) 2002 Wiley Periodicals, Inc.