Study On Modification Of High Ortho Phenolic Fibers

Phenolic resin and its fibers are one of the representative materials in the fields of flame retardancy,heat resistance and thermal insulation.However,phenolic fibers produce a large amount of small molecules of water during the curing process,which leads to the loosening of the fiber structure and reduces the mechanical properties.Commonly used solution curing methods have problems such as severe pollution,difficulty in recovering waste liquid,and low efficiency.Therefore,in this study,the green preparation of phenolic fibers was achieved by using melt spinning and addition curing with the epoxy resin F-704 as an entry point.Thermal curing and microwave curing methods are used to improve production efficiency while greatly reducing environmental stress.（1）High ortho phenolic resin was synthesized through the catalysis of phenol and formaldehyde in acids and divalent metal ions.It was blended with epoxy resin F-704,and a small amount of curing agent was added.After melt spinning,thermal curing,or microwave curing treatment,high-performance thermoplastic high ortho phenolic epoxy fibers were obtained.Structural characterization in FTIR,Micro FTIR and ¹³C NMR,demonstrating that ring-opening etherification of epoxy groups and phenolic hydroxyl groups,among others,occured during the curing reaction.The activation energy E_a=86.12 k J·mol^-1and order of reaction n=0.92 was calculated through curing kinetics analysis.Thermogravimetric and mechanical test analyses show that the cured phenolic epoxy fibers had excellent properties.The initial weight loss temperature and the residual carbon rate at 800°C for thermal-cured and microwave-cured phenolic epoxy fibers were 188°C,56.09%and 260°C,60.56%,respectively.The breaking strength and elongation at break for both were 190.0 MPa,5.2%and 151.7 MPa,3.7%,respectively.In SEM analysis,the toughness fracture characteristics of the thermal curing was more obvious than those of the microwave curing,which also reflected the difference in the mechanical properties of the two curing methods.（2）Introducing hydroxymethyl groups into the structure of high ortho phenolic resin,thermosetting high ortho phenolic resin was synthesized through a two-step method.It was blended with epoxy resin F-704 and subjected to melt spinning,thermal curing,or microwave curing treatment to obtain high-performance thermosetting high ortho phenolic epoxy fibers.Their structures were characterized by FTIR,Micro FTIR and ¹³C NMR,etc.It was determined that the molecular structure contained hydroxymethyl groups.During the curing reaction,phenolic hydroxyl,hydroxymethyl,etc.all participated in the ring opening etherification of epoxy groups,forming a cross-linked network structure.The activation energy E_a=89.75 k J·mol^-1and order of reaction n=0.92 was calculated through curing kinetics analysis.The thermogravimetric test results showed that the initial weight loss temperature and residual carbon content of the thermal curing and microwave curing phenolic epoxy fibers at 800℃were 206℃,56.86%,and 315℃,59.66%,respectively.The breaking strength and elongation at break of thermosetting phenolic epoxy fibers were 205.7 MPa and 14.5%,respectively,while those cured by microwave were 176.3 MPa and 7.1%.SEM analysis of phenolic epoxy fibers shows that the cross-section of thermal-cured phenolic epoxy fibers with better mechanical properties exhibited more obvious ductile fracture characteristics.This study prepared phenolic epoxy fibers with excellent heat resistance and mechanical properties through physical blending.After structural characterization and performance testing analysis,epoxy group made the phenolic fiber mainly undergo addition reaction during curing,which reduced the influence of small molecule formation on the properties of phenolic fiber.it was found that the mechanical properties of thermal-cured phenolic epoxy fibers were stronger than those of microwave-cured fibers due to their moderate crosslinking degree.Microwave curing phenolic epoxy fibers have better heat resistance due to their more uniform crosslinking structure.