Investigation Of Low-temperture Dissolution Process Of PPTA Polymer

Poly-p-phenylene terephthalamide（PPTA）fiber has received much attention as a high-performance polymer material due to its excellent properties.This fiber has high strength and modulus,outstanding heat resistance,good chemical stability,as well as being lightweight.PPTA fiber has been widely used in many fields.The PPTA condensation process generates a polymer-solvent system that is prone to phase separation,making it unsuitable for spinning.Consequently,traditional para-aramid spinning uses a two-step method.Firstly,the PPTA polymer is synthesized via low-temperature condensation,followed by the removal of the polymer solvent.Subsequently,the polymer undergoes washing and drying to ensure complete removal of trace water content before being dissolved again.Due to the strong hydrogen bond interactions between PPTA molecular chains,it can only be dissolved in solvents such as sulfuric acid for liquid crystal spinning.However,the dissolution process of sulfuric acid can easily lead to polymer degradation,severely affecting the fiber’s properties and performance.Reducing degradation during the dissolution process of PPTA is crucial for producing high-performance PPTA fibers.This article systematically discusses the effects of pre-dissolution and solvent dissolution processes on the degradation of PPTA resin.Firstly,through observing the swelling process of PPTA resin,it has been found that finely elongated PPTA resin particles have better swelling effect.By determining the temperature of the PPTA/H₂SO₄ system with changes in PPTA resin mass fraction,it was found that the temperature of the PPTA/H₂SO₄ system is linearly positively correlated with the resin mass fraction.When the dissolution time reaches 7-8 min,the system temperature reaches its peak and a high temperature will lead to degradation.Changing the pre-dissolution conditions can determine the transparency of the PPTA resin.The best results were obtained when the pre-dissolution temperature was at 40℃and the PPTA resin exhibited good swelling and transparency.Altering the low-temperature pre-dissolution temperature and time of the PPTA resin in liquid sulfuric acid,the spinning solution had the highest inherent viscosity and lower k_w when the pre-dissolution was conducted for 10 min at 40℃.Meanwhile,in the method where solid sulfuric acid was mixed with PPTA resin prior to dissolution,the spinning solution had the highest inherent viscosity and lower k_w when pre-dissolving at 40℃for 15 min.Secondly,through observing the dissolution process of PPTA resin using a hot-stage polarizing microscope,a liquid crystal phenomenon was observed in the PPTA/H2SO4 solution,which became more apparent under shear force.By studying the dissolution process of PPTA resin in liquid and solid sulfuric acid,the influence of sulfuric acid concentration and form on the inherent viscosity and blockage coefficient of the PPTA/H2SO4 solution was explored.When 100%solid sulfuric acid was used as the solvent to dissolve PPTA resin,the resulting spinning solution showed the least degradation with good uniformity.The impact of resin particle size on the blockage coefficient of the spinning solution was more pronounced,where smaller resin particle size led to lower blockage coefficient.Through investigating the influence of dissolution process on the inherent viscosity and blockage coefficient of the spinning solution,when pure solid sulfuric acid with a concentration of 100%was used as the solvent at a dissolution temperature of83℃,with a PPTA mass fraction of 20%,stirring speed of 50 r/min and a dissolution time of 1 h,the resulting solution had minimal degradation.