Characterization Of Crystallization And Orientation Structures Formed In Manufacturing Of BOPET

In order to overcome the drawing crack problem of common polyethylene terephthalate (PET) during the film processing, various amount of the multi functional comonomer, glycerine, were added into the polycondensation system of terephthalic acid and ethylene glycol to prepare the branched copolyesters. After extruding the copolyesters into amorphous slabs, the stretching tests were performed for those slabs under different drawing temperatures and different drawing ratios. The crystalline and orientational structures of copolyesters formed in stretch processing were investigated.Moreover, an off-line birefringence test system based on a spectrometer and an on-line birefringence test system based on an interferometer were constructed for the convenient, instant orientational structure characterization of PET film.The traditional DSC analysis results of glycerine modified copolyesters showed the weak branched molecular structures would enhance the segment movement abilities, which was identified by the decreasing of glass transition temperature and cooling crystalline temperature as well as the increasing crystallinity of copolyesters. On the other hand, it was found from the modulated DSC analysis of stretched films that a high draw ratio would result in a high crystallinity and a high orientation, however, increasing drawing temperature would lead to an opposite effect. Especially, those stress-induced crystallization effects were found more obvious in branched copolyesters than in pure PET. Actually, the improvement the crystalline properties would enhance the intermolecular action so as to overcome the drawing crack problems.Based on the interference mechanism of parallel polarized beams, the transmittance intensity distribution of white polarized light resource through the copolyester film could be obtained with a spectrometer. After the data processing with a Visual Basic code, the birefringences of copolyester films upon whole visual light wavelength band were produced. Compared with the common POM measurement, spectrometer measurement could give the results with a relative error less than 5%, showing a high precision. In order to test the change of orientation of copolyester films just during the deformation, an on-line birefringence measuring equipment was established, from which the retardation gathered from the stretching deformation of copolyester films could be recorded. After the data processing with a Matlab program, the relationship between the birefringence and the stretching stress of the films was obtained, from which the stress-optical coefficient was calculated and it was well coincided with the reported result.