Research On Ceramifying Mechanism On Mg-Al-Si/Boron Phenolic Resin Composite

Ceramifying polymer composites have been regarded as a new type of thermal protection material,the key feature of which is a controlled transition from a degraded polymer composite to a ceramic residue under controlled conditions,forming a high-dense multiphase-ceramic-composite-structure to meet the requirement of high-temperature-resistant ablation.A novel carbon-based polymers matrix ceramifiable composite?Mg-Al-Si/BPF?,based on nano MgO-Al₂O₃-SiO₂?MAS?modified boron phenolic resin?BPF?,was prepared with fusing agent of B₂O₃ and Bi₂O₃ by compression molding which applied in the surroundings of high temperature and abundant oxygen.An improved thermal stability of MAS/BPF obtained.Containing 40Wt%MAS,MAS/BPF heated at different temperature for 2h reserved a preferable dimensional stability limited to 17.3%in radial and 8.33%in vertical dimension,and a lower mass loss rate under 44.9%.The flexural strength of residues first decreased and then increased as the heating temperature elevated,and the flexural strength reached 10.2MPa when heating at 1200?.At 1200?for different heating time,the residues of MAS/BPF exhibited an excellent anti high temperature oxidation resistance with slightly variation in mass loss rate and dimensional change rate.The results demonstrated that the eutectic reaction occurred between the fillers and pyrolysis product or pyrolysis atmosphere when treated at 1200?,attributing to the catalyzed-effect of activated carbon.The liquid,firstly forming in the surface,played a great role as liquid bridge connection,which enhanced the densification of the surface structure.As heating time prolonged,large quantities of molten liquid obtained and gradually penetrated into the interior to make up for internal voids,resulting in forming framework structure with mutual–linked glass phase and ceramic phase.As a result,the residues of MAS/BPF could maintain high mechanical strength and structural integrity.In order to facilitating ceramifying reaction to shift at lower temperature and improving the integrated performance of MAS/BPF at low temperature stage,the fusing agent?B₂O₃ and Bi₂O₃?was introduced.Under low temperature,the fused fusing agent could tightly bond the MAS and residual carbon,and facilitated the ceramifying reaction,promoting the formation of the mutual-linked glass phase and ceramic phase structure.The flexural strength of modified composites significantly strengthened at 800?¹000?.The flexural strength of B₂O₃ modified composites after fired at 800?and 1000?was 5.45MPa and 7.36MPa,and that of Bi₂O₃ was4.79MPa and 9.01MPa.The composites have an excellent high temperature performance with the mass loss rate limited to 52.5%and dimensional change rate controlled to 14.7%.According to orthogonal experiment,it was thought that the optimum parameter of the content of inorganic fillers was MAS/B₂O₃/Bi₂O₃ being35/2/4.