C. Gouri

Thermal decomposition characteristics of Alder-ene adduct of diallyl bisphenol A novolac with bismaleimide: effect of stoichiometry, novolac molar mass and bismaleimide structure

The addition-cured blends of diallyl bisphenol A formaldehyde resin (ABPF) with various bismaleimides (BMIs) were evaluated for thermal stability and degradation behavior by thermogravimetric analysis (TGA). TGA of the blend of ABPF and 2,2-bis 4-[(4-maleimido phenoxy) phenyl] propane (BMIP) with varying maleimide to allylphenol stoichiometry indicated that the thermal stability of the system was only marginally improved by the increase in BMI stoichiometry in the blend. The effect of BMI structure on thermal stability was studied using four different BMIs, viz. bis (4-maleimido phenyl) methane (BMIM), bis (4-maleimido phenyl) ether (BMIE), bis (4-maleimido phenyl) sulfone (BMIS) and BMIP. TGA showed a two stage decomposition pattern for BMIS system and a single stage for all the other three. The thermograms of BMIM and BMIE were identical and superior to that of BMIS; the latter showing a relatively poor performance at lower temperatures. Compared to the BMI-adduct of monomeric diallyl bisphenol A (DABA), the polymeric analog viz. ABPF system exhibited better thermal stability. Non-isothermal kinetic analyses of the different systems showed the decomposition occurring in at least two kinetic steps. The computed activation energy exhibited a direct correlation to the relative thermal stability of the systems.

Adhesive and thermal characteristics of maleimide-functional novolac resins

A novel, addition-curable maleimide-functional novolac phenolic resin was evaluated for adhesive properties such as lap shear strength and T-peel strength using aluminium adherends, when thermally self-cured and cocured with epoxy resins. The adhesive properties of the self-cured resin, although inferior at ambient temperature, improved at high temperature and were found to depend on the cure conditions. When cocured with epoxy resin, the adhesive properties improved significantly and showed a strong dependence on the nature of the epoxy resin used, on the stoichiometry of the reactants, on the concentration of imide groups in the phenolic resin, and on the extent of polymerization of the maleimide groups. Optimum adhesive properties were obtained for novolac resins with a moderate concentration of maleimide groups, taken on a 1 : 1 hydroxyl–epoxy stoichiometry with a novolac epoxy resin. In comparison to the conventional novolac, the imide–novolac contributed to improved adhesion and better adhesive property retention at higher temperature when cured with the novolac–epoxy resin.

Adhesive Characteristics of Alder-Ene Adduct of Diallyl Bisphenol a Novolac and Bisphenol a Bismaleimide

Diallyl bisphenol A–formaldehyde copolymer (ABPF) was addition cured with bisphenol A bismaleimide (BMIP) making use of the Alder-ene reaction at high temperatures. The lap shear strength (LSS) of the system was found to depend on the conditions of cure and the stoichiometry of the reactants. Moderate cross linking achieved at a 1:1 maleimide:allylphenol stoichiometry and a stepwise cure, up to a maximum of 250 °C for 2 h, was found to be the most effective in achieving the optimum LSS properties. The system exhibited greater than 100% retention of the LSS at temperatures up to 250 °C. Matrix modification using polysulfone (PS) and polycarbonate (PC) resulted in a remarkable improvement in the adhesive characteristics, although the high-temperature retention was marginally adversely affected. The performance advantage both at room temperature (RT) and at high temperature was greater in the case of PS modification, showing an optimum improvement at 20% loading as against PC modification, exhibiting maximum properties at 10% loading. Scanning electron microscopy (SEM) analysis confirmed that the fine dispersion of PS, rather than large size nodules found in PC, was conducive for the better performance of the former. Dynamic mechanical analysis (DMA) corroborated the observations made in SEM. The existence of co-continuous phases of thermoplastic, matrix and thermoplasticdissolved matrix was evidenced in the PS modification and a clear phase separation was evident in the case of the PC modified system, manifesting independent glass transitions by the individual phases.

Effect of elastomer modification on the adhesive characteristics of maleimide-functional phenolic resins

The effect of addition of elastomeric modifiers on the adhesive properties like lap shear strength and T-peel strength of an addition curable, maleimide functional novolac phenolic resin (PMF), self-cured and cocured with a novolac epoxy resin, was studied using aluminium adherends. The modifiers used were (1) two grades of carboxyl terminated butadiene acrylonitrile copolymer (CTBN) of different molecular weights, (2) a low molecular weight, epoxidized hydroxyl-terminated polybutadiene, and (3) a high molecular weight acrylate terpolymer containing pendant epoxy functionality. The adhesive properties, when examined as a function of the varying concentrations of the additives, ranging from 10 to 30 parts per hundred parts (phr) of the resin, were found to depend on the nature of the matrix being modified as well as on the nature and concentration of the elastomer. The adhesive properties at ambient temperature of the self-cured, highly brittle PMF resin were dramatically improved by the inclusion of all the elastomers, the increase being substantial in the case of high molecular weight CTBN. For the more rigid, less ductile, epoxy-cured PMF system, the adhesive properties were marginally improved by the high molecular weight CTBN, whereas the other elastomers were practically ineffective. For both self-cured and epoxy-cured PMF systems, the inclusion of these elastomers generally decreased the high-temperature adhesive properties, implying impairment of thermal characteristics, evidenced also from their dynamic mechanical spectra. The presence of phase-separated elastomer particles in the modified systems has been evidenced from scanning electron micrographs.

Rheokinetic cure characterization of epoxy–anhydride polymer system with shape memory characteristics

An epoxy resin capable of exhibiting shape memory property was derived by curing diglycidyl ether of bisphenol A (DGEBA) with a blend of carboxy telechelic poly(tetramethyleneoxide) (PTAC) and pyromellitic dianhydride (PMDA). The cure kinetics of DGEBA/PTAC/PMDA blend of varying compositions was investigated using isothermal rheological analysis. The overall reaction conformed to a second-order autocatalytic model. The kinetic parameters including reaction order, kinetic constants and activation energy were determined. The results showed that increase of PTAC decreased the overall activation energy and frequency factor of the cure reaction. This effect resulted in a diminution of the overall rate of curing. The catalysis by PTAC has its origin from the activation of epoxy groups by the protons of the COOH groups. The autocatalysis was caused by the COOH groups generated by the reaction of alcohol groups with anhydride. The activation energy for the autocatalysis was more than that for the primary reaction as the COOH groups responsible for autocatalysis were generated on a sterically hindered polymer backbone. The kinetics helped generate a master equation conforming to second-order autocatalytic model that could predict the cure profile of a specified resin system at a given temperature, leading to cure optimization.

Thermosetting film adhesives based on maleimide-modified-phenol-functional acrylic copolymers

Pendant phenol-functional thermoplastic butylacrylate-acrylonitrile-maleimidophenol (BNM) terpolymers synthesized by free radical copolymerization of butyl acrylate (BuA), acrylonitrile (AN), and varying proportions of N-(4-hydroxy phenyl) maleimide (HPM) were cross-linked through different phenol group reactions. This included curing of the phenol group with a diepoxide and hexamethylene tetramine (HMTA), and thermal curing of the propargyl ether and cyanate ester groups derived from it. These thermosets exhibited good film-forming properties and their adhesive properties were evaluated using aluminium adherends. Cross-linking the polymers through phenol-epoxy reaction was found to be very effective in enhancing the mechanical and adhesive properties and for imparting a better thermoadhesive profile in comparison to the thermoplastic adhesive. Among the epoxy cross-linked (EpBNM) polymers, the optimum adhesive properties at ambient temperature were obtained for a polymer containing 9.5 wt% HPM. A single component thermosetting polymer (GlyBNM) containing the optimum epoxy and phenol content was synthesized by copolymerization of glycidyl methacrylate (GMA) with BuA, AN, and HPM. When compared to EpBNM of identical composition, the adhesive properties of GlyBNM were inferior. Cross-linking of BNM through HMTA produced a brittle polymer with reduced adhesive properties. Thermal curing of the BNM polymer, after transformation of the phenol groups to propargyl ether and cyanate ester, did not enhance the adhesive properties, implying the superiority of the epoxy cross-linked system over the other systems.

Thermoplastic film adhesives based on phenol-functional acrylic copolymers: synthesis, mechanical and adhesion properties

Acrylic polymers possessing varying proportions of pendant phenol groups were synthesized by the free radical copolymerization of N-(4-hydroxyphenyl) maleimide (HPM) with butyl acrylate (BuA) and acrylonitrile (AN) and characterized. These thermoplastics form excellent films and their mechanical and adhesion properties were evaluated as a function of the phenol content. Enhancing the HPM content increased both the tensile strength and the modulus but decreased the elongation. A nominal increase in the phenol content was found to be conducive for improving the adhesion properties of the films. At higher concentrations, the adhesion properties showed a decreasing trend due to the embrittlement caused by the rigid maleimide groups. The adhesion property at 50°C increased linearly with the HPM content due to an increased T g, whereas a reverse trend was observed for the adhesion property measured at-196°C, due to the dominance of the embrittlement effect. The reduced flow characteristics of the high HPM-loaded systems led to a diminished honeycomb flat-wise tensile strength. Enhancing the HPM concentration in the chain promoted the adhesion properties for the vulcanization bonding of nitrile rubber to aluminium. Addition of silica filler marginally improved the lap shear strength (LSS) for the metal-metal system, but was detrimental for rubber-metal bonding; a reverse trend was observed for the carbon-filled system. The diminished performance for metal-metal bonding by carbon could be attributed to the weakening of the interphase, whereas the enhanced rubber-metal bonding could be due to possible reinforcement of the rubber phase by carbon. The fillers generally improved the high temperature adhesion. However, they impaired the flow properties of the resin and, thereby, adversely affected the flat-wise tensile strength in both cases.

High-temperature adhesives based on Alder-ene reaction of diallyl bisphenol A novolac and bismaleimide: Effect of BMI structure and Novolac molar mass

Diallyl bisphenol A formaldehyde novolac (ABPF) resin was cured with four structurally different bismaleimides (BMIs) at high temperatures through an Alder-ene reaction which resulted in thermally stable network polymers. The adhesive characteristics of the different BMI-ABPF systems were evaluated in terms of the lap shear strength (LSS) on aluminium substrates at varying temperatures up to 250°C. The LSS properties were not significantly affected by the structure of the BMI. Although the LSS of BMI-ABPF systems per se were not particularly high due to the brittle nature of the cross-linked structures, all the systems exhibited remarkably good retention of LSS at high temperatures. Replacing ABPF with its monomeric analogue i.e. o,o′- diallyl bisphenol A (DABA) resulted in better adhesion, but in a poorer thermo-adhesive profile. Comparison of DMA and thermo-adhesive profiles implied that in the majority of the cases molecular relaxations at higher temperature are conducive to matrix toughening which results in enhanced adhesion properties.

Dual-cure propargyl novolac-epoxy resins: Synthesis and properties

A partially propargylated oligomeric phenolic novolac (PPN) was synthesised by the Williamsons reaction of a novolac with propargyl bromide and was characterised. Reactive blending of the PPN resin with epoxy resin resulted in a dual cure thermoset. The curing occurred through the phenol-epoxy reaction at about 135°C together with a Claisen rearrangement and the addition polymerisation of propargyl ether groups at around 235°C. The phenol-epoxy reaction could be catalysed by triphenyl phosphine, without affecting the curing of the propargyl ether groups. The cure characterisation was done by DSC and DMA. The effect of the phenol-epoxy ratio on the adhesive properties of the PPN-epoxy blend between aluminium adherends was evaluated. The system exhibited moderately good lap shear strength which was optimised for a phenol:epoxy equivalent ratio of 2:1. Good retention of the properties was observed at 100°C. On a comparative scale, the diglycidyl ether of bisphenol-A was better than the novolac epoxy, in improving the adhesion. Addition of conventional matrix tougheners was not conducive to improved adhesion. The system formed a good composite with glass fabric. The mechanical properties of the glass laminates were independent of the phenol-epoxy stoichiometry. The cured system possessed good thermal stability and a Tg>300°C. The epoxy adversely affected the Tg and the thermal stability.