Abstract
In this paper, a set of silicon hybrid phenolic resins (SPF) with high Si-content were prepared by mixing phenolic resins with self-synthesized silicon resins. In order to obtain the nanoscale phase structure, condensation degree and the amount of Si-OH groups in silicon resins were controlled by the amount of inhibitor ethanol in the hydrolytic condensation polymerization of siloxane. Increasing the amount of ethanol resulted in more silanol groups and a lower degree of condensation for silicon resins, which then led to more formation of Si-O-Ph bonds in hybrid resin and improved compatibility between silicon resin and phenolic resin. When 400% ethanol by weight of siloxane was used in the sample SPF-4, nanoscale phase separation resulted. The residual weight of the cured SPF-4 at 1000 °C (R1000) significantly increased compared to pure phenolic resins. The result of the oxyacetylene flame ablation and the Cone Calorimeter test confirmed the improved ablative property and flammability after the modification. The performance improvement of the cured SPF-4 was attributed to the nanoscale phase structure and high silicon content, which promoted the formation of dense silica protective layers during pyrolysis.
Highlights
Phenolic resins, a kind of classical thermosetting resins, are widely used as the matrix for ablative composites or a precursor for C/C composites in the aerospace field owing to their excellent properties such as flame retardancy, low cost, high char yield, and high strength of pyrolysis products [1,2,3].phenolic hydroxyls (Ph-OHs) are converted into hydroxyl radicals in the pyrolysis process due to the low bond dissociation energy of O-H and C-O bonds, as well as methylene groups can decompose into methyl groups
The structure of Q0, Q1, Q2, Q3, Q4, T0, T1, T2, T3 for silicon resins were displayed in Scheme 1 [33,34]
It revealed that the structure of the oligomer was formed, while the existence of T0, T1, T2, Q0, Q1, Q2, Q3 indicated that the condensation reaction between MTMS and TEOS was incomplete
Summary
A kind of classical thermosetting resins, are widely used as the matrix for ablative composites or a precursor for C/C composites in the aerospace field owing to their excellent properties such as flame retardancy, low cost, high char yield, and high strength of pyrolysis products [1,2,3].phenolic hydroxyls (Ph-OHs) are converted into hydroxyl radicals in the pyrolysis process due to the low bond dissociation energy of O-H and C-O bonds, as well as methylene groups can decompose into methyl groups. Auto-oxidation occurs during thermal degradation, which seriously affects the anti-oxidation and thermal stability of cured phenolic resins and limits their application under the long-term high-temperature conditions [4,5,6,7]. Introducing other elements such as silicon, boron, and phosphorous is an effective approach to improve the oxidation resistance and ablative properties of phenolic resins [8,9,10] due to the oxides with high melt point formed by these elements can increase the residual weight of phenolic resins at high temperature. Since a Si-O bond has higher bond energy (443.7 KJ/mol) and better molecular chain flexibility than a C-C bond
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