| Carbon fiber, with low density and high specific strength, has the outstanding heat resistant performance and chemical properties. Phenol formaldehyde resin, with perfect wettability as agglomerant, has high strength and temperature resistance after carbonization. In the engineering practice, pyrolytic carbon structure of phenolic resin bonded carbon fiber (C/C composite for short) is often used in the thermal insulation structure of new type of space craft. The liquid impregnation technology was applied to make carbon fiber as the main framework and water-soluble phenolic resin as the binder with the process of vacuum filtration and integrally forming by molding, then the carbon fiber/formaldehyde resin(CF/PF) composite was prepared by curing, and the lightweight C/C composite thermal insulation materials of different density were prepared after carbonization. The poriferous microstructure and mechanical behaviors of thermal insulation materials were analyzed, and the kinetics of curing and carbonization as well as the mass transfer mechanism during procuring of the resin and fracture mechanism, thermal physical properties of insulating materials were investigated.Kinetics during curing and pyrolysis behavior of resin were reseached by DTA and TG. While raising the heating rate, curing exothermic peak moved gradually owards higher temperature. The range of curing temperature increased obviously, but the reaction time became short. The curing process for the CF/PF composite was 101℃/2h+108℃/2h+137℃/2h with the heating rate of 1℃/min and the carbonization process for C/C composite was 210℃/0.5h+380℃/0.5h+580℃ /0.5h+900℃/2h with the heating rate of 2℃/min under N2 protection.Because of the exorbitant initial curing temperature, the structure of phenolic resin was loose with many defects after direct curing. Thus procuring process was applied to deal with the prepreg. The mechanism of resin migration during procuring could be analyzed as follows. Due to the capillary force of porous structure during free procuring, moisture volatilization would make the phenolic resin migrate upward. As a result, uneven distribution of resin happened in the CF/PF composite. On the contrary, the moisture gathered in confined space after evaporating when the procuring happened under sealing condition, almost remaining saturated vapor pressure, greatly reduced the evaporation rate of the water and solvent. In consequence, the phenolic resin gelled first in the original location, solving the problem of uneven distribution of resin in the C/C composite insulation material.C/C composite of different bulk densities were prepared by controlling the pressure when prepreg was molded, with the modal pore diameter about 60μm and porosity over 70%. With the increase of material density, layer spacing and porosity decreased while the compressive strength and flexural strength improved. The compression strength was highest at 2.08MPa and the flexural strength was highest at 4.79MPa when the loads were parallel to carbon fiber layer of composite with the density of 0.260g/cm3. C/C composite have obvious layered structures. Its microstructure and mechanical properties went as follows. Evenly dispersed carbon fiber constituted the main framework of the composites with fiber nodes bonded by resin to fasten the frame. The compressive strength was low as yield occured between the weak layers when load was parallel to layers, resulting in crack propagation. The layer spacing decreased when load was vertical to layers, the compact layers beared pressure instead of porous structure to make stress increased rapidly. The flexural strength was higher when the load was parallel to the layers than vertical to the layers. This is because the bearing capacity along the fiber layer 5 higher than vertical to layer.Lightweight C/C composite possess excellent thermophysical properties. The coefficients of thermal expansion (CTE) during 100℃～400℃ approximatly linearly increased not more than 0.72 × 10-6℃-1 at 400℃, showing the perfect dimensional stability when heated of this material. In addition, the coefficients of thermal conductivity of the composites during 30℃～240℃ were between 0.23 and 0.26W/(m-K). This low coefficients of thermal conductivity are determined by disorderly dispersed carbon fiber, loose interlayer structure and high porosity.Forming mechanism of C/C composite is closely connected with phenolic resin curing. The migration of phenolic resin can be controlled by sealed curing. The resin in situ gels, so as to make the C/C composite structure.The porous structure of C/C composite make it very good thermal insulation. |
PDF Full Text Request