Study On The Preparation Of Para-Phenylene Terephthalamide Pulps And The Formation Mechanism

Poly(para-Phenylene Terephthalamide) (PPTA) pulps have excellent properties as reinforced materials besides having kept many merits of PPTA fibers and have been used widely in the fields of friction, sealed and composite materials. Spinning-cutting process is a mature and industrialized method to produce PPTA pulps. In this paper, PPTA pulps were prepared directly from the precipitation of PPTA solution that obtained from solution polycondensation at low temperature. The formation mechanism of PPTA pulps is investigated by means of mathematic analysis.The process conditions for the preparation of PPTA pulps by direct polycondesaton were investigated firstly in the solvent system of NMP-CaCl2 at low temperature. The sequence for the effect of different factors on the relative molecular mass and aspect ratio is: stirring speed > initial monomer concentration > the first addition ratio of TPC and the effect of stirring speed on the aspect ratio of PPTA pulps is very remarkable. Washing time can be shortened and the amount of ashes within the polymer can be reduced when the resultant PPTA pulps were washed in ultrasonic bath.With the increase in the length of molecules, the polycondensation speed was controlled by diffusion of the end group. There are two conditions should be satisfied for the copolycondensation of two relative groups. The position of thecenter of mass of the diffusing molecules is given by the condition . The near parallel alignment required for reaction between two PPTA molecules is given byθ≤θc. Flow can induce the molecular orientation increasing the number of pairs of molecules with relative orientation suitable reaction and improve the polycondensation rate and the degree of the polymerization of the resultant polymer. On the other hand, high shear rate is in favor of orientation of PPTA molecules alaong the flow direction and the orientation will remain in the gel after its formationthat will benefit the fiber formation.The effect of two new nontoxic additive ( PVP with low relative molecular mass and ammonia) on the properties of the resultant PPTA pulps was studied too. By the addition of the PVP, the polycondensation can be accelerated, the inherent viscosity of the polymer can be increased and the polymers can be fibrillated more easily, meanwhile PVP did not affect the chemical structure of the resultant PPTA pulps. The relative molecular mass of the resultant PPTA pulps can be improved by the addition of ammonia as the acid accepted agent and it has no influence on other properties.PPTA pulps are prepared by means of twin screw as main reactor. Thin PPTA gel can reduce the temperature grads during maturing process and is in favor of improving the relative molecular mass and aspect ratio of PPTA pulps. Low temperature is needed for pre-polycondensation. The temperature for the main reaction in the twin screw should be about 70 and the resident time of the reactivematerials in twin screw should be longer than 3 minutes. Increasing the rotation speed of the screw has almost no effect on the relative molecular mass and aspect ratio of PPTA pulps but has negative effect on the performance of the pulps due to too short residue of the material in the twin screw..A new approach to forming PPTA pulps is introduced in this paper -precipitation. That is, a PPTA concentrated sulfuric acid solution was injected into a rapidly stirred coagulant and the pulps formed. A proposed mechanism of the process is that when the viscous PPTA solution is introduced into a rapidly stirred coagulant, it is dispersed into small drops, elongated by action of the extensional flow field, then coagulated and solidified at last with the time being. At high stirring speed, the solidified PPTA fibers are fibrillated into fibrils and become PPTA pulps with high specific surface area and certain mean length. Mathematic analysis of the droplet deformation and coagulation was carried out individually, according to the forming mechanism of PPTA pulps. The final length of PPTA pulps depends on t...