Study On The Preparation And Properties Of Syntactic Foam Used As Thermal Insulation Of Ablative Materials

With the rapid development of astronautical technology,it has given a higher request to heat protection and heat insulation of materials.How to combine heat protection and heat insulation to achieve integration of structure and functionality has become a research emphasis of thermal protection materials.This paper has taken thermal insulation material as the research objects,HGM and short glass fiber are filled in boron-phenolic resin to prepare syntactic foams.The effect of filler(HGM and short glass fiber)content and heat treatment temperature on mechanical properties,heat stability and thermal insulation of syntactic foams are systematically studied,which will provide a theoretical basis for the development and design of polymer-based anti thermal insulation materials.In this paper,we treat the surface of HGM with different coupling agent such as APTES,NDZ-311 and GA,the result shows that surface modification is helpful for improving the compatibility and interfacial properties between HGM and phenolic matrix.Besides,it could effectively relieve particle aggregation.Especially for HGM-APTES-GA,the stable chemical connection to phenolic matrix could give syntactic foam a better performance.Filling HGM has an apparent effect in improving the thermal stability of system,the residual rate at 900? under nitrogen atmosphere of syntactic foam containing 50 vol.% HGM is 71.45%,which has increased for 22.32% compared with neat phenolic cured resin.Under the condition of room temperature,the compressive strength and flexural strength decreases with increasing HGM content,that of syntactic foams containing 50 vol.% HGM has decreased for 55.08% and 67.00% respectively compared with neat phenolic cure resin.With the increase of heat treatment temperature,the pyrolysis of resin matrix greatly diminishes the mechanical properties of material,the existence of HGM forms effective reinforcement on mechanical properties.After heat treatment under 400?,600? and 800?,the compressive strength and flexural strength of syntactic foams containing 50 vol.% HGM have improved by 246.67% and 173.02%,199.07% and 81.60%,107.87% and 71.43%,respectively.The increase on flexural strength is less than that on compressive strength because matrix fracture is the main failure mode under bending laod,especially after high temperature heat treatment,amorphous carbon generated in the structure makes material brittle,leading to typical characteristics of brittle fracture of specimens.The introduction of short fiber is confined by the viscosity of resin mixture due to the existence of HGM,filling a small amount of fiber has a slight improvement on thermal stability of system.The residual rate at 900? under nitrogen atmosphere of syntactic foam reinforced with 1.5wt.% fiber has increased for 3.54% compared with syntactic foam containing 50 vol.% HGM.Under the condition of room temperature,filling 1.25 wt.% fiber makes up for the deficiency of mechanical properties to a great extent,has improved by 137.61% and 80.27% on compressive strength and flexural strength.After heat treatment under 400?,600? and 800?,the compressive strength and flexural strength of syntactic foams containing 1.25 wt.% fiber have improved by 44.07% and 89.51%,157.59% and 51.35%,77.73% and 156.32%,respectively due to the micro-buckling and bridging of fiber.By studying the heat transfer mechanism of syntactic foam,it is found that gas convection heat transfer in the pore is neglected for syntactic foam with micron grade stomatal aperture.The proportion of radiation heat transfer in total thermal conductivity is less than 1% at room temperature,so the heat transfer in syntactic foam is composed of matrix solid-phase heat conduction and gas-phase heat conduction.Filling HGM could effectively reduces the thermal conductivity of materials,although the proportion of radiation heat transfer increases with temperature increases,solid-phase heat conduction is still the main heat transfer path when temperature is below 1000?.Because of randomly arrangement and low volume fraction,the effect of filling short fiber on thermal conductivity is not obvious.Bulk syntactic foam and sandwich structure have been prepared,by comparing the back temperature of them,it could be seen that the outermost fiber sheet of the sandwich structure works as heat shield,which is helpful for reducing back temperature and improving the overall heat insulation performance.Under the heating condition of alcohol blast burner,sandwich structure has a significant hindrance function on heat transfer,making back temperature at 60 s decrease for 39.51%,from 156.4? to 94.6?compared with bulk syntactic foam.