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In the thermal protection design of aerospace products, for components requiring substantial thickness, possessing complex geometries, or being susceptible to stress cracking, composite materials are frequently employed as a substitute for traditional thermal protection coatings. Furthermore, composite thermal protection structures offer versatile fabrication options: they can be rapidly formed directly onto the specific thermal protection site, or they can be bonded into place using prefabricated composite thermal protection sleeves, thereby offering the distinct advantages of an efficient and convenient preparation process. Currently, phenolic glass fiber composites are widely utilized in thermal protection structures due to their numerous merits, including structural stability, high-temperature resistance, high char yield, low ablation rate, high mechanical strength, and excellent impact toughness.

Phenolic glass fiber composites hold significant potential for widespread application within the aerospace sector. Their thermal protection mechanism involves a synergistic interplay of multiple protective processes, including the inhibition of heat conduction, endothermic pyrolysis, thermal insulation provided by the char layer, and structural support. This paper will primarily analyze the thermal protection mechanism from the perspectives of heat conduction inhibition, endothermic pyrolysis, and thermal insulation provided by the char layer.

1. Inhibition of Heat Conduction

Within phenolic glass fiber composites, the phenolic resin matrix itself possesses an inherently low thermal conductivity, which serves to significantly retard the conduction of external heat. When the composite material is subjected to the high-temperature environments encountered during flight, heat initially propagates inward from the outer surface. However, the macromolecular chain structure inherent to the phenolic resin matrix obstructs the pathways of heat conduction, thereby causing a marked reduction in the rate of heat transfer. Concurrently, the presence of glass fibers within the composite further alters the internal heat conduction pathways. Moreover, the interface between the glass fibers and the phenolic resin exhibits high thermal resistance; consequently, heat encounters additional impedance as it traverses this interface, further inhibiting the heat conduction process and effectively lowering the rate of temperature rise within the composite material.

2. Endothermic Pyrolysis

When exposed to high-temperature external environments, the phenolic resin undergoes continuous pyrolysis reactions. This pyrolysis process is an endothermic reaction: upon absorbing thermal energy, the chemical bonds within the phenolic resin molecules rupture, causing the material to decompose into small-molecule gases and carbonaceous residues. This process effectively consumes the thermal energy transferred from the external environment to the composite’s surface, thereby simultaneously lowering the surface temperature of the composite material. 3. Thermal Insulation via the Char Layer

As the pyrolysis reaction proceeds, a char layer gradually forms on the surface of the phenolic resin composite. This char layer possesses a porous structure, with its pores filled with gases exhibiting extremely low thermal conductivity; this feature further enhances the composite’s thermal insulation capabilities. Concurrently, the char layer presents a hard surface texture capable of withstanding specific aerodynamic pressures and mechanical loads, thereby shielding the underlying uncharred material from the scouring and erosion effects of high-temperature gas flows. The char layer generated on the composite surface not only serves an insulating function but also, to a certain extent, mitigates mass loss in the composite, thereby extending the service life of the thermal protection structure.