Double crystallization of biaxially oriented polypropylene

Abstract

The melting conditions of biaxially oriented polypropylene (BOPP) have a very strong effect on the courses of both non-isothermal and isothermal crystallization from the melt. It has been shown that the crystallization proceeds by two mechanisms. Depending on the melting conditions, both crystallization mechanisms can proceed individually or simultaneously (double crystallization). The high rate mechanism, occurring in non-isothermal crystallization at low undercooling and observed in isothermal crystallization, can be attributed to the ordered structure of the melt preserved after melting, presumably a mesomorphic state. The second component corresponds to crystallization from the isotropic melt.Anomalous crystallization of BOPP samples can be eliminated by energetic heat treatment, by the destruction of ordered structures. As a result of this treatment, the characteristics of crystallization and its quantitative parameters will become similar to those of unoriented polypropylene.The characteristics of the crystallization of isotactic polypropylene (PP) have been studied by several research groups [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. It has been established on the basis of the dilatometric [1 -4, 6-8], optical polarization [5,10] and calorimetric [9, 10] investigations of isothermal crystallization that the crystallization isotherms can be described by the Avrami equation. Moreover, it has been observed that the rate of crystallization and its quantitative characteristics depend strongly on the thermal history of the samples [3,4,7,15].Our present work reports on the effect of the thermal history (melting conditions) of biaxially oriented polypropylene samples (BOPP) on the courses of isothermal and non-isothermal crystallization from the melt. A new type of crystallization memory effect, the phenomenon of "double crystallization", has been detected in these investigations. ExperimentalSamples cut out from biaxially oriented polypropylene flasks manufactured by two-step technology (Hercules Inc.) have been used for the investigations. The basic substances is PP of Pro-Fax 6723 type. According to X-ray analysis made with a Mfiller Micro 111 instrument, the a-modification of PP is present in the samples, and the orientation along the two axes is homogeneous. On the basis of our thermorelaxation investigations, the elongation during orientation is 300 -400 %.

Crystallization, melting and spherulitic structure of Β-nucleated random propylene copolymers

“…It may be explained by the formation of a mixed polymorphic structure on one hand and, similarly to the observations with low-density polyethylene [13], by a fractional crystallization according to the chain regularity on the other hand.…”

The melting of high-pressure polyethylene subjected to stepwise heat treatment

“…3 shows the melting curves of polyethylene foils treated by the combined method, various numbers of temperature steps being applied. (More complicated cases of stepwise crystallization and heat treatment are discussed in other papers of our research group [7,8].…”

“…We have found that the peaks of the melting curves are due to melting and recrystallization processes [7], and that the melting of high-pressure polyethylene faithfully reflects the thermal history of the sample. This phenomenon was given the name "thermal memory effect".…”

“…Thus, the stepwise heat treatment of polyethylene crystallized from melts [2], or polypropylene [3], or rolled and stretched polyethylene foils [4,5] may produce samples the melting curves of which have several peaks. However, different opinions exist as regards the origin of the melting peaks of heat-treated high-pressure polyethylene [2,4,5].As reported in our previous papers [6,7], an arbitrary number of melting peaks can be caused to appear in the melting curve of high-pressure polyethylene by applying the method of stepwise isothermal crystallization. We have found that the peaks of the melting curves are due to melting and recrystallization processes [7], and that the melting of high-pressure polyethylene faithfully reflects the thermal history of the sample.…”

“…Polymer samples prepared under certain conditions have frequently been observed to give melting curves containing several peaks [2][3][4][5][6][7]. Thus, the stepwise heat treatment of polyethylene crystallized from melts [2], or polypropylene [3], or rolled and stretched polyethylene foils [4,5] may produce samples the melting curves of which have several peaks.…”