Effect of Tensile Drawing on the Molecular Orientation in Poly(ethylene terephthalate) Film

• Main Authors: LI, CHUN-SHENG, 李峻昇
• Other Authors: Mao, Ching-Feng
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/27152447827414717056

碩士 === 南臺科技大學 === 化學工程與材枓工程系 === 105 === The characteristic properties of poly(ethylene terephthalate) (PET) films, such as mechanical properties, crystallinity, oxygen permeability and transparency, are determined by the orientation of PET molecules, which can be manipulated by the processing conditions. In this study the effects of processing conditions, including draw ratios, draw rates, types of stretching (uniaxial or sequential biaxial stretching) on properties of PET films were investigated using a homemade stretching device modified from a commercial tensile testing machine. The validity of the device was evaluated and confirmed by reproducibility of test, and then different draw ratios of PET films were prepared by changing the operating time. The molecular orientation in PET films can be quantified and measured in terms of birefringence (n), which can be determined by the difference in refraction index between the fast axis and slow axis of a sample when using a polarized light source. In this study the Senarmont device consisting of a He-Ne laser (632.8 nm), a set of polarizer-quarter wave plate-analyzer, and a detector was used to measure n. The device was first calibrated and confirmed by a Scotch tape measurement. The excellent agreement between the literature data and the measured value validates the reliability and accuracy of the device. The stretching results showed that the birefringence and crystallinity of PET films obtained from the uniaxial stretching increased with increasing draw ratio and draw rate due to the induced crystallization. Similarly, the Tg values increased with increasing draw ratio, and the Th,c values shifted to lower temperature due to the presence of induced crystallization. In contrast, the crystallinity of PET films obtained from the sequential biaxial stretching remained unchanged, which was explained by the fact that the crystallinity reached a maximum value after the MD (machine direction) stretching, thus no further increase in crystallinity could be obtained from the TD (transverse direction) stretching. The birefringence, on the other hand, decreased with increasing draw ratio of TD and became almost null when MD and TD were identical.