Low Pre-oxidized Polycarbosilane Fiber Thermal Cross-linking Technology

Continuous Nicalon-type SiC fibers with high tensile strength and high modulus can be produced from polycarbosilane,or PCS,by the processes of spinning,air oxidation curing and heat treatment. In order to decrease the content of oxygen in SiC fibers,a new method of curing was studied systematically,that is:low-oxidaiton+thermal crosslinking,or LOTC. The work includes following parts:1,search of lowest oxidation degree of PCS fiber and the corresponding oxidatioin conditions,2,investigation of thermal crosslinking conditions with a manufacture-scale batch-type stove,3,heat treatment of PCS fibers to prepare SiC fibers,4,characterizations of the materials and the products with IR,TG-DTA,SEM,EA,XRD and so on,5,comparison of LOTC-PCS fibers and the SiC fiber therefrom (KDT-type) with the high-oxidaiton PCS and the corresponding SiC fiber (KD-type),and 6,studies of the mechanism of thermal crosslinking.In this work,one found that the oxidation weight gain 9% is the lowest requirement of thermal crosslinking,which is just the threshold of gelation or the surface oxidation crosslinking. A time-temperature equivalence relationship for air-curing was established. To reach a weight gain,one can adopt different heating system in air. However,generally,lower temperature with longer time leads a higher reaction degree of Si-H than high temperature with shorter time for a certain weight gain. The oxygen content of the KDT SiC fiber is 13.8%,that is much lower than that of KD fiber,20.3%. Therefore,the crystallinity of 0-SiC in KDT SiC fiber is more evident than that of KD fiber. With the structure advantages,KDT SiC fiber shows a higher tensile strength and longer high-temperature duration than KD fiber. In the process of thermal crosslinking,the Si-H in PCS is mostly consumed and some Si-CH2 -Si is formed. With the results,the mechanism of thermal crosslinking was discussed.