Toughening And High-temperature Matching Properties Of Phenolic Composites

Phenolic products are not only widely used in molding compounds,friction materials,insulating materials,casting,construction and mi PCFRoelectronics,but also in high-end aerospace fields,military equipment,whose market is very broad.Due to the huge methine groups in phenolic resin,methyl bridge is easily broken and embrittlement failure occurs when resin is subjected to external force,limiting its application.Furthermore,the methine group is easily oxidized and broken at high temperature.With science and technology develops,the requirements for various performance of phenolic resin are growing,and improving toughness and thermal matching characteristic of the matrix is a key task of phenolic resin modification.In this paper,a series of nitrile rubber/phenolic resin（NBR/BPF）composites were prepared,and the effect of rubber on curing reaction of resin,mechanical properties,thermal stability and microstructure of phenolic composites was studied.The effect of fiber lay-up direction on the properties of composites was investigated.Porcelain feasibility of nitrile rubber toughening composites was discussed.Firstly,nitrile rubber was introduced into fiber-reinforced phenolic resin composites for composites with high coefficient of thermal expansion（CTE）.The properties of modified composites were tested and analyzed.The results showed that rubber reduced the tensile strength of composites and increased the elongation at break:the tensile strength decreased from 82 MPa to 38MPa,and the elongation increased from 6.6mm to 11.7mm.The 10phr rubber-modified composite material reached186MPa optimal flexural properties.20 phr rubber-modified composites reached the maximum impact strength of 45.73KJ/m².The addition of rubber reduced the heat resistance of composites.The initial decomposition temperature decreased from427.6℃ to 343.5℃.Rubber increased composites’CTE,which increased from8.9×10^-6/K to 1.5×10^-5/K.The SEM results showed that the pure composites belonged to brittle fracture,the NBR/BPF composites had insufficient wettability at the interface,there were defects in the interface,the sample was complicated in failure,and the fracture path is tortuous,showing a certain toughness.Considering gigantic CTE difference of fiber and resin,thermal effects of fiber lay-up direction on NBR/BPF composites were discussed by presetting the fiber angles in±9°,±18°,±27°,±36°and 45°.The results showed that the fiber content in the composites is basically unchanged.The flexural properties of the laminate design composites decreased from 145Mpa to 126Mpa.The impact strength continued to decrease,reaching a minimum value of 30.2 KJ/m² at±36°fiber lay-up direction.CTE of the composites increases with the angle increase,and the average CTE of the±36°lay-up direction reached the peak of 1.89×10^-5/K.The volume effect calculation method could obtain more accurate thermal expansion parameters when a correction factor was introduced.There was a gap between the thermal expansion parameters calculated by the laminate theory and the measured values.The CTE calculated by the two methods were different from the measured values at±36°fiber lay-up direction.We speculated that the agglomeration and twisting deformation of the fibers leaded to the change of stress and strain.In order to expand the application range of flexible composites,modification of NBR/BPF composites is carried by ceramic-forming fillers.The results showed that the ceramic-forming filler did not change the infrared absorption spectrum of the matrix.The thermal conductivity of the composites gradually increased with the increase of the rubber content in the system,and it was 0.42W/（m·K）at 50 phr.The addition of ceramic-forming filler improved the thermal stability of the composites.The thermal weight loss rate of the porcelainizable composites in high temperature region was significantly lower than that of the rubber phenolic resin composites,and the former only had a relatively gentle thermal weight loss reaction around 500℃.After ceramic modification,the 600℃ residual weight rate of the composites could reach 86.0%,which was much higher than 63.15%of the composites without filler.ceramic-forming filler slowed down the increasing trend of composites’CTE,and the maximum CTE of filler modified samples was 1.4×10^-5/K.The flexural strength of the filler modified samples at room temperature decreased from 159 MPa to 90.8 MPa,and fillers could retain high mechanical strength after high temperature treatment.SEM showed that the composites formed a better surface melt liquefaction layer at 1200℃,and the interface between fibers and other components became blurred.XRD analysis showed that anorthite began to form in the ceramic composites at 800℃,mullite began to form at1000℃,and the anorthite component basically disappeared at 1200℃.