Thermotropic liquid crystal polyarylate (TLCP) fiber is a high-performance industrial fiber with high strength and high modulus, high temperature resistance, chemical corrosion resistance, extremely small linear expansion coefficient, aging resistance and other excellent properties. At present, the development of heat treatment technology for TLCP virgin fibers is very weak. TLCP polyarylate fiber is generally prepared by melt spinning method. Since the viscosity of polyarylate liquid crystal melt increases sharply with the increase of molecular weight, there is a certain limit on its molecular weight in spinning applications, which often makes the fiber strength fail to meet the requirements. This is encountered in melt-type liquid crystal spinning. main problem. In order to further improve fiber performance, heat treatment (ie, post-solid-state polymerization) is usually used to increase the molecular weight of polyarylate fiber and improve fiber crystallization, thereby further improving the strength of the fiber. The solid-state polymerization reaction during the heat treatment process involves the transesterification reaction of the polyarylate molecular chain and the esterification reaction at the end of the molecular chain. Both reactions will produce small molecule by-products such as water and acetic acid, which will not cause harm to the environment. The TLCP fiber used in this test was obtained from commercial Vectran liquid crystal polyester chips through the self-assembled twin-screw melt spinning machine equipment of the High Performance Fiber Research Laboratory of the Sichuan Institute of Textile Science. The spinneret aperture is 0.30mm and the number of holes is 0.30mm. is 50, and the average linear density of the fiber is 367.99dtex. By drawing on the research on the solid-state polycondensation kinetics of PET and polyamide, using thermotropic liquid crystalline polyarylate as-spun fiber as raw material, the post-solid-state polymerization macro-kinetics of TLCP fiber was studied, and the thermal properties of TLCP fiber before and after heat treatment were studied. and surface morphology were studied. After analysis, it was found that TLCP fiber is widely used in defense fields such as aerospace, armor protection, ship ropes and cables, and in the fields of military and civilian dual-use reinforcement materials such as high-temperature filter materials.
Homemade vacuum oven for heat treatment of TLCP fiber
During the heat treatment process, small molecule by-products produced by solid-state polymerization after TLCP fiber are continuously released, and the quality of TLCP fiber continues to decrease with the change of heat treatment time. The amount of small molecule by-products produced by TLCP fiber at different heat treatment times is studied by changing with time. Post-solid-state polymerization macrodynamics. During the experiment, the thermal history of the fibers at different heat treatment times at the same temperature was ensured to be the same. The weight loss rate was defined as the amount of small molecule by-products produced per unit mass of TLCP fibers at different heat treatment times at the same temperature.
The relationship between weight loss rate and strength of TLCP fiber at 220℃ heat treatment temperature
It can be seen from the figure that the fiber strength and weight loss rate increase with the extension of heat treatment time. In the early stage of heat treatment, the fiber strength increases faster than the fiber weight loss rate; in the later stage, the fiber strength increases faster than the fiber weight loss rate. The increase in fiber strength during the heat treatment of TLCP fiber is due to the increase in the molecular weight of polyarylate and the increase in crystallinity. Therefore, in the early stage of the heat treatment process at 220°C (before 9 hours), the increase in fiber strength may be mainly due to the increase in fiber crystallinity and molecular weight. The influence of growth and the increase in strength in the later period are mainly caused by the increase in the molecular weight of polyarylate. In addition, it can be seen from the figure that the strength of the fiber reaches 9.73cN/dtex after 90 hours of heat treatment at 220°C, indicating that in order to obtain high-strength TLCP fibers at a heat treatment temperature of 220°C, the heat treatment time must be ≧90h , it takes a long time. Under other heat treatment temperature conditions, bonding occurs between fibers, making it difficult to obtain fiber bundle samples, and bonding has a significant impact on the mechanical properties of the fibers, so further testing is needed.
Analysis of the influence of heat treatment temperature on post-solid-state polymerization
As can be seen from the above figure, under the same heat treatment time, the weight loss rate of TLCP fiber increases with the increase of heat treatment temperature, and the higher the heat treatment temperature, the faster the weight loss rate increases. In addition, it can be seen from the figure that the heat treatment temperature around 250°C is the turning point in the change of weight loss rate. On the one hand, increasing the heat treatment temperature increases the diffusion rate and activity of the end groups of the polyarylate molecular chain, speeding up the transesterification reaction and condensation reaction; on the other hand, the diffusion rate of by-product small molecules is accelerated, thus promoting the reaction balance to the direction of the forward reaction. conduct. The influence of post-solid-state polymerization heat treatment temperature is much more obvious than time. Therefore, in the post-treatment production process of TLCP fiber, the heat treatment temperature should be increased as much as possible on the premise of avoiding adhesion. In addition, from the perspective of improving fiber processing efficiency, the appropriate heat treatment temperature should be higher than 250°C.
Analysis of apparent reaction rate constant values after solid-state polymerization of TLCP fibers at different temperatures
It can be seen from the table that�Under different heat treatment temperatures, the apparent reaction rate constants in the early and late stages of post-solid-state polymerization increase with increasing temperature. At the same heat treatment temperature, the apparent reaction rate constants in the early stages of heat treatment are about one order of magnitude higher than those in the later stages.
Analysis of the causes of changes in reaction rate constants before and after polymerization
During the heat treatment process, the polyarylate chain growth reaction mainly occurs in the amorphous region of the fiber. In the early stage of heat treatment, the crystallinity of the fiber is low, the chain end groups are easy to move and the by-product small molecules diffuse easily. At the same time, the initial end group concentration is high, so the early stage of heat treatment is The reaction rate is large. With the prolongation of heat treatment time, the crystallization of TLCP fiber continues to improve, resulting in the movement of chain end groups slowing down, the concentration decreasing, and the chance of collision of reactive groups decreasing, so the reaction rate slows down; in addition, the increase in crystallinity also leads to smaller by-products. The diffusion rate of molecules from the inside of the TLCP fiber to the fiber surface slows down, which increases the concentration of small by-product molecules in the amorphous region of the TLCP fiber and inhibits the forward reaction.
DSC analysis of TLCP fiber
As the heat treatment temperature increases, the area of the DSC melting peak continues to decrease. This may be due to the destruction of the ordered long molecular chains inside the fiber due to excessive temperature. The higher the temperature, the higher the melting point of TLCP fiber. This shows that increasing the heat treatment temperature can not only increase the molecular weight of TLCP fiber quickly, but also increase the melting point and crystallinity of TLCP fiber, and increase the strength and service temperature of TLCP fiber. In addition, after heat treatment of TLCP as-spun fiber at 250°C for 7 hours, the melting point of the fiber increased from 277.9°C to 300.1°C, while after heat treatment at 280°C for 1 hour, the melting point of the as-spun fiber increased from 277.9°C to 307.4°C. This shows that the heat treatment temperature has a more significant impact on the melting point than the heat treatment time. This also shows that in the TLCP fiber heat treatment process, the heat treatment temperature of the TLCP fiber should be increased as much as possible.
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