Linear and branched alkyl chain modification of PF resin: Synthesis, pyrolysis and ablative performance at high heat flux

Abstract

Alkyl modified phenolic resins were synthesized by acid catalysed etherification of Phenol formaldehyde resin (PF) using linear (n-propyl-NPA and n-butyl-NBA) and branched (iso-propyl-IPA and t-butyl-TBA) chain alcohols and characterized. Spectroscopic characterization (FTIR and ¹H NMR) and hydroxyl value revealed the extent of etherification to be ca. 10–13 %. Enthalpy of curing was not significantly varied with etherification from the control PF resin. Mechanical property evaluation indicated an increase in elongation by 60–80 %. Glass transition temperature was about 135–144 °C vis-a- vis 148 °C for the unmodified PF resin. Thermal stability of the synthesized PF resins was not significantly altered and a marginal improvement in char residue (PF- 49 % and alkyl modified PF- 54–57 %) was observed at 900 °C. Low density carbon composites (0.70 g/cc) were processed using these PF resins as matrix resins and porous graphitic felt as reinforcement and evaluated their thermal and ablative characteristics. Alkyl modification improved the wettability of the graphitic felt and a reduction in backwall temperature was observed when the cured composites were subjected to Plasma arc jet at 220 W/cm² at the end of 500 s.