R. L. Bindu

Thermal characteristics of addition-cure phenolic resins

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.

Bis propargyl ether resins : synthesis and structure-thermal property correlations

Abstract Bis propargyl ethers of bisphenol-A, (BPA), bisphenol ketone (BPK) and bisphenol sulfone (BPS) were synthesised and characterised. These monomers were thermally polymerised to the corresponding poly (bischromenes). The cure behaviour, as monitored by differential scanning calorimetry, depended on the structure of the monomer. The non isothermal kinetic analysis of the cure reaction using four integral methods revealed that the presence of electron-withdrawing group did not favour the cyclisation reaction leading to formation of chromene, which precedes the polymerisation and this is in conformation to the proposed mechanism of polymerisation. Thus, the cure temperature and activation energy for the reaction increased in the order BPA

Phenolic resins bearing maleimide groups : Synthesis and characterization

Novel phenolic novolac resins, bearing maleimide groups and capable of undergoing curing principally through the addition polymerization of these groups, were synthesized by the polymerization of a mixture of phenol and N-(4-hydroxy phenyl)maleimide (HPM) with formaldehyde in the presence of an acid catalyst. The polymerization conditions were optimized to get gel-free resins. The resins were char- acterized by chemical, spectral, and thermal analyses. Differential scanning calorime- try and dynamic mechanical analysis revealed an unexpected two-stage curing for these systems. Although the cure at around 275°C was attributable to the addition polymer- ization reaction of the maleimide groups, the exotherm at around 150 to 170°C was ascribed to the condensation reaction of the methylol groups formed in minor quantities on the phenyl ring of HPM. Polymerization studies of non-hydroxy-functional N-phenyl maleimides revealed that the phenyl groups of these molecules were activated toward an electrophilic substitution reaction by the protonated methylol intermediates formed by the acid-catalyzed reaction of phenol and formaldehyde. On a comparative scale, HPM was less reactive than phenol toward formaldehyde. The presence of the phenolic group on N-phenyl maleimide was not needed for its copolymerization with phenol and formaldehyde.

Addition‐cure‐type phenolic resin based on alder‐ene reaction: Synthesis and laminate composite properties

A maleimide-functional phenolic resin was reactively blended with an allyl-functional novolac in varying proportions. The two polymers were coreacted by an addition mechanism through Alder-ene and Wagner–Jauregg reactions to form a crosslinked network system. The cure characterization was done by differential scanning calorimetry and dynamic mechanical analysis. The system underwent a multistep curing process over a temperature range of 110–270°C. Although the cure profiles were independent of the composition, the presence of maleimide led to a reduced isothermal gel time of the blend. Increasing the allylphenol content decreased the crosslinking in the cured matrix, leading to enhanced toughness and improved resin-dominant mechanical properties of the resultant silica laminate composites. Changing the reinforcement from silica to glass resulted in further amelioration of the resin-reinforcement interaction, but the resin-dominant properties of the composite remained unaltered. Increasing the maleimide content resulted in enhanced thermal stability. Integrating both the reactive groups in a single polymer and its curing led to enhanced thermal stability and Tg, but to decreased mechanical properties of the laminate composites. This can be attributed to a brittle matrix resulting from enhanced crosslinking facilitated by interaction of the reactive groups located on the polymer of an identical backbone structure. The cured polymers showed a Tg in the range of 170–190°C.

Phenyl ethynyl functional addition cure phenolic resins: Synthesis, characterisation and thermal properties

Phenyl ethynyl functional addition curable phenolic resins were synthesised by reacting a mixture of phenol and 3-(phenylethynyl) phenol (PEP) with formaldehyde in presence of an acid catalyst. Relatively narrow molar-mass distributed polymers were obtained in good yield. The presence of PEP led to reduced molar-mass and narrow distribution of the copolymers. The resin underwent thermal curing at around 250–275°C, much lower than the cure temperature of PEP. The thermal stability and anaerobic char residue of the cured resins increased with increase in ethynyl-content and these properties were more than those of resol resin. These addition cure phenolics provided an overall increase in char of about 70% vis-à-vis resol resin when compared on the basis of uncured resins.

Thermal characteristics of propargyl ether phenolic resins

Addition curable propargyl ether phenolic resin of varying degrees of propargylation showed noticeably improved thermal stability in the lower temperature regime vis-à-vis cured resole. The crosslinking generated by the cycloalipahtic and linear polynene groups caused rapid decomposition at higher temperatures leading to a reduced char-content. The kinetics of non-isothermal decomposition showed the degradation occurring in two defined kinetic steps. The kinetic parameters computed from the mass-loss data showed an inconsistent variation in activation energy with composition. However, the computed rate constants confirmed that the first stage decomposition is facilitated by an enhanced propargyl content. The second step, associated with the carbonization process was systematically facilitated by enhanced crosslinking. Moderate propargyl functionalization was conducive for better thermal stability and anaerobic char residue. Analyses of char obtained by pyrolysis at 900°C showed it to be incompletely carbonized and amorphous in nature.