Thermal characteristics of addition-cure phenolic resins C.P. Reghunadhan Nair*, R.L. Bindu, K.N. Ninan Propellant and Special Chemicals Group, Vikram Sarabhai Space Centre, Trivandrum-695 022, India Received 3 January 2001; accepted 6 February 2001 Abstract The thermal and pyrolysis characteristics of four different types of addition-cure phenolic resins were compared as a function of their structure. Whereas the propargyl ether resins and phenyl azo functional phenolics underwent easy curing, the phenyl ethynyl- and maleimide-functional ones required higher thermal activation to achieve cure. All addition-cure phenolics exhibited improved thermal stability and char-yielding property in comparison to conventional phenolic resole resin. The maleimide-functional resins exhibited lowest thermal stability and those crosslinked via ethynyl phenyl azo groups were the most thermally stable systems. Propargylated novolac and phenyl ethynyl functional phenolics showed intermediate thermal stability. The maximum char yield was also given by ethynyl phenyl azo system. Non-isothermal kinetic analysis of the degradation reaction implied that all the polymers undergo degradation in at least two steps, except in the case of ethynyl phenyl azo resin, which showed an apparent single step degradation. The very low pre-exponential factor common to all polymers implied the significance of volatilisation process in the kinetics of degradation. Isothermal pyrolysis studies led to the conclusion that in the case of nitrogen-containing polymer, the pyrolysis occurs via loss of nitrogenous products, which is conducive for enhancing the carbon-content of the resultant char. FTIR spectra of the pyrolysed samples confirmed the presence of C–O groups in the char. XRD analysis of the partially carbonised polymers did not give any indication of crystallites except in the case of ethynyl phenyl azo system. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Maleimide-phenolics; Propargyl ether resins; Addition-cure phenolics; Phenyl ethynyl phenolic resin; Acetylene functional phenolics; Thermal decomposition kinetics; Pyrolysis 1. Introduction Phenol-formaldehyde resins are known for their high temperature resistance and high char yielding properties [1,2]. Phenolic resins with improved thermal and pyrolysis characteristics are desirable for application in compo- sites for thermo-structural applications [3] and could be better matrices in carbon/carbon composites [4]. Although conventional phenolics are already con- sidered as temperature resistant polymers and are cur- rently in use for the above-mentioned applications, some of the properties of phenolics need further improvement for meeting the ever-increasing performance require- ments. The adverse properties, which require research attention, include the moderate thermo-oxidative stabi- lity, inherent brittleness, condensation-cure nature lead- ing to evolution of volatiles etc. According to Conley [5], the thermo-oxidative process is the major path for degradation of phenolic resins. The degradation pro- cess, accelerated by the presence of phenolic groups adjacent to the methylene bridges, is hence, dependent on the dihydroxy phenyl methane units in the resin. Phenolic resins curing via the addition polymerisation of thermally stable groups could obviate some of these problems. In this perspective, we synthesised a series of addition curable phenolic resins. These are non-con- ventional phenolic resins curing principally by the poly- merisation of groups such as phenyl maleimide [6,7] terminal acetylene [8], phenyl acetylene [9], propargyl ether [10] etc. Previous papers describe their syntheses, characterisation and mechanical properties of compo- sites. In some cases, their thermal stability has also been evaluated. To assess their utility as thermally stable materials for related applications, it is necessary to examine their relative thermal stability and degradation. This paper focuses on the relative thermal behaviour, kinetics of thermal degradation and the pyrolysis under selected temperature conditions, of four different addi- tion-cure phenolic resins. 0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00076-3 Polymer Degradation and Stability 73 (2001) 251–257 www.elsevier.nl/locate/polydegstab * Corresponding author. Fax: +91-471-461-795. E-mail address: cprnair@eth.net (C. P. Reghunadhan Nair).