Research On Hydrophobicity And Thermostability Of Organic-inorganic Hybrid Modified Phenolic

Phenolic resin matrix composite have been wildly used in ablative thermal protection systems due to its excellent ablative properties. Traditional full-density carbon/phenolic composite is not suitable for long-time reentry environment with high heat load because of high density and thermal conductivity, while the novel carbon/phenolic composite which employ phenolic aerogels as matrix, can effectively reduce their bulk density and thermal conductivity. But the lower char yield and easily moisture absorption blocks the application of phenolic aerogels. Aiming to solve the above problems, an organic-inorganic hybrid modified processing is proposed to introduce silica inorganic phase into phenolic aerogel, the hybrid silicon modified phenolic aerogel and its composites has been designed and successfully prepare. Microstructure, thermal stability, wettability, compressive properties and thermal conductivity have been studied detaily.In this dissertation, phenolic aerogels are prepared by sol-gel method with resorcinol, furfural, hexamethylenetetramine, three-aminopropyl triethoxysilane and methyl trimethoxysilane. The results show that, the silica network structure can form chemically link with phenolic macromolecular chains and form a three-dimensional interpenetrating network structure. Phenolic particles attach to the surface of the preferentially generated silica structure to grow and form slits and wedge pores. Pores size distribute from 1nm to over 100 nm and focus on 30 nm. Compared to hexamethylenetetramine and phenol/formaldehyde ratio, the introduction of silane-modified phenolic greatly affect the performance of the prepared phenolic aerogels. The particle size of phenolic aerogels range from nearly 200 nm to 50 nm as the mole ratio of silane material increase from 0 to 0.6, the amount of micro- and mesopore in aerogels increase, and the proportion of mesopores also increase. The char yield of the silicon modified phenolic aerogels increase from 46.84% to 61.52%(1000°C, nitrogen atmosphere).The static contact angle of silicon modified phenolic aerogels with water is 80°; the static contact angle can reach 135° after grafting trimethylethoxysilane. Furthermore its char yield increase by 3.86% and the initial thermal decomposition temperature increase by 18.5oC.Lightweight carbon/phenolic composites are prepared using silicon phenolic aerogels impregnated low-density and low thermal conductivity short carbon fiber skeleton. Silicon modified phenolic aerogels can uniformly distribute in short carbon fiber skeleton and wrap the fiber surfce with good interfacial bonding. The static contact angle of composite after grafting reach 132°, which meets the hydrophobic requirements. The young’s modulus and yield strength through the thickness direction of phenolic/carbon composite material increase from 18.8MPa and 0.63 MPa to 22.1MPa and 1.08 MPa, respectively. Lightweight carbon/phenolic composites are proved to have relatively good thermal insulation properties based on the examined results of hot disk method, which test the thermal conductivity along different directions with different component.