Study On Thermal Shrinkage Mechanism Of Poly(Ethylene Terephalate) Industrial Yarn

The industrial yarn of PET is a system of polymer of high crystallinity and high orientation. Though such kind of structure affords high strength and high modulus, it belongs to an unstable thermodynamic system because of the reduction of entropy caused by high orientation, which will spontaneously transform to a state of low free energy and a macroscopic shrinkage will be observed. Shrinkage of industrial yarn of PET was set as starting point for study and its characteristics of shrinkage behaviors was summarized. A relationship between macroscopic shrinkage properties and microscopic structure parameters was established, by which a texture structure model was suggested and mechanism of shrinkage was explained. After comparing properties and microscopic structures of different types of PET yarns, an ideal structure of PET yarn of high modulus and low shrinkage (HMLS) was proposed.1. A PET industrial yarn of high strength and obvious shrinkage (SH8) was selected as sample. Its shrinkage percent and shrinkage force were measured (isothermal and programmed heating) by using self-designed device and a phenomenal analysis was provided. The results showed: a) The shrinkage process of PET industrial yarn of high strength and high orientation was a relaxation process of internal stress of oriented units, so its thermal shrinkage behavior depended on not only temperature, but also the length of observation time. Temperature and time had somehow equivalent effects on shrinkage behavior; b) The PET industrial yarn of high crystallinity and high orientation showed the shrinkage percent and shrinkage stress stepped up as increased temperatures or extended times at the beginning and then a maximum value may be observed and a dropping process was followed at suitable conditions; c) there were two kinds of shrinkage mechanism in the course of the shrinkage: the relaxation of local oriented units with the nature of entropy elasticity and the relaxation of long range orientation units (molecular chains) in the amorphous region at higher temperatures or longer times, which was accompanied by change of shrinkage velocity or reduction of stress.2. The structure changes in the course of shrinkage were characterized and a relationship between macroscopic shrinkage behavior and microscopic structure changes was discussed. The simple two-phases model was not good enough to describe the shrinkage behavior of polymers of high crystallinity and high orientation, because their thermal process included the contributions from the structure changes in the crystalline region, amorphous region and the region with intermediate orientation order. A modified Prevorsek model was suggested. The crystalline regions showed an enhancement of the degree of crystallinity and size of crystallite and partially recrystallization. The non-crystalline regions showed the following changes: the decreasement of f_am in two stages corresponded to that in thermal shrinkage behavior; Along c axis, partially disoriented segments were packed together forming small amount of psudo-crystalline boundary layers, which were unstable and could be destroyed by extension at room temperature. Main external factors that had effects on the thermal shrinkage included temperature, tension and observation time, temperature and time showed equivalent effects.3. Based on the comparisons of the structures and properties of 10 types of PET industrial yarn, which were produced by 5 factories and classified as different models, the following conclusions were obtained: High-strength model was differenced from HMLS model in thermal shrinkage, their L_s,177 were 7.5% and <3% respectively; There were no differences in orientation factors of crystalline region between them; HMLS showed lower values of orientation factors of non-crystalline regions; the crystallinity of HMLS model and high-strength model were approximately 53-60% and 44-50% respectively; their estimated areas of average crystallite were 20-25nm2 and 14-18nm2; the difference in the shape of melting peak between them was that high-strength model showed a small endothermic peak before main melting peak; HMLS had higher T_σ; the difference in the dynamic behavior them was high-strength model showed a peak of tg& at high temperature.4. The effects of time and temperature on physical ageing were performed. HMLS showed high anti-physical ageing properties.