The influence of sintering process on the sintering degree of PTFE

PTFE is an extremely widely used engineering plastic. It has excellent properties such as excellent resistance to media, low coefficient of friction, and a wide operating temperature range. It plays an important role in sealing and lubricating fields. Due to the high melting temperature and melt viscosity of PTFE, it is not easy to process, so a common processing method is to process by cold pressing and sintering similar to metal powder metallurgy. In the actual production process, sintering is an important part of the PTFE molding process. The sintering temperature, holding time, heating/cooling rate, and other process conditions directly affect the sintering degree of the blank and then affect the final performance of the product.

During the sintering process of PTFE, the cooling rate affects the crystallinity and performance of filled PTFE. It is found that quenching can reduce the crystallinity and hardness of PTFE composites and increase the tensile strength. The difference in the surface micromorphology of PTFE raw materials and cold-pressed blanks before and after sintering and at different sintering times. The influence of different sintering processes and cooling conditions on the mechanical properties of PTFE composites. However, for the common cases in which the sintering degree of the blank is too low or too high, the effective monitoring of the relevant indicators of the sintering degree of the PTFE blank is essential for the quality control of the final product in mass production.

The differential scanning calorimetry (DSC) method is used to study the melting and crystallization curves of blanks under different sintering process conditions. The two parameters of crystallinity and molecular weight reduction are used as the basis for measuring the sintering degree of PTFE, and the sintering temperature and holding time are studied. And the effect of heating rate on the degree of sintering of the blank.

The DSC analysis of the PTFE blank shows that different sintering processes have a greater influence on the sintering degree of the blank. If the sintering temperature is too low, the melting curve of the surface area of ​​the billet shows a double melting peak of low and high temperature and the crystallinity is high. At this time, the degree of sintering of the billet is insufficient. If the sintering temperature of the blank is too high or the holding time is too long, the crystallization enthalpy of the surface area of ​​the blank will increase, and the number average molecular weight will decrease significantly. At this time, the blank will be over-sintered. If the heating rate is too high, a high temperature melting peak appears on the melting curve and the crystallinity is high. At this time, the sintering degree inside the billet is insufficient. At the same time, when the heating rate is too high, the sintering degree of each area of ​​the blank is significantly different. From the center of the blank to the surface area along the radial direction, the crystallinity gradually decreases and the sintering degree gradually increases.

The degree of sintering has a greater impact on the properties of PTFE materials. Too low or too high a degree of sintering will cause abnormal changes in the crystallinity and number average molecular weight of the blank, resulting in a decrease in the tensile properties of the blank. DSC analysis shows that the ideal PTFE sintering process parameters are: sintering temperature 370℃, holding time 20～140 min, heating rate 50～100℃/h. At this time, the crystallinity of the material and the reduction rate of the number average molecular weight are maintained at a low level, the sintering degree of the blank is relatively ideal, and there will be no “underburning” or “overburning” phenomenon.