K. Ambika Devi

Studies on allophanate-urethane networks based on hydroxyl terminated polybutadiene: effect of isocyanate type on the network characteristics

Abstract Polyurethane networks containing allophanate groups, based on hydroxyl terminated polybutadiene and various diisocyanates, such as toluene diisocyanate, 4,4′ di(isocyanatocyclohexyl) methane and isophorone diisocyanate were synthesized at various NCO/OH equivalence ratios (r-values) ranging from 1.0 to 1.5. Mechanical properties such as tensile strength, stress at 100% elongation, equilibrium relaxation modulus increase and % elongation at break decreases with r-value. Crosslink density (νe) for the networks was determined using swell methods. Crosslink density values calculated from equilibrium relaxation modulus were found to be close to those obtained from swell data. Crosslink density (νt) was theoretically calculated assuming complete reaction between the functional groups. The ratio, νe/νt decreases with r-value. Linear fits with good correlation coefficients were obtained between network parameters and mechanical properties. Dynamic mechanical studies show that the r-value does not influence the glass transition temperature. Thermal decomposition studies using thermogravimetric technique indicate that the decomposition is in multiple stages.

Rheo-kinetic evaluation on the formation of urethane networks based on hydroxyl-terminated polybutadiene

Rheo-kinetic studies on bulk polymerization reaction between hydroxyl-terminated polybutadiene (HTPB) and di-isocyanates such as toluene-di-isocyanate (TDI), hexamethylene-di-isocyanate (HMDI), and isophorone-di-isocyanate (IPDI) were undertaken by following the buildup of viscosity of the reaction mixture during the cure reaction. Rheo-kinetic plots were obtained by plotting ln (viscosity) vs. time. The cure reaction was found to proceed in two stages with TDI and IPDI, and in a single stage with HMDI. The rate constants for the two stages k1 and k2 were determined from the rheo-kinetic plots. The rate constants in both the stages were found to increase with catalyst concentration and decrease with NCO/OH equivalent ratio (r-value). The ratio between the rate constants, k1/k2 also increased with catalyst concentration and r-value. The extent of cure reaction at the point of stage separation (xi) increased with catalyst concentration and r-value. Increase in temperature caused merger of stages. Arrhenus parameters for the uncatalyzed HTPB-isocyanate reactions were evaluated.

Dual cure phenol - epoxy resins: Characterisation and properties

Reactive blends of 2, 2′- diallyl bisphenol A (DABA) and a novolac epoxy resin (EPN) were investigated for their cure behaviour, and their rheological, physical, mechanical and thermal properties. Cure characterisation was by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The system underwent dual curing through a sequential phenol-epoxy reaction and allyl polymerisation. The former reaction was catalysed by triphenyl phosphine (TPP). Whereas the phenol-epoxy reaction was completed, the allyl polymerization was limited to 40% by the regulation of cure conditions. Increase of epoxy concentration in DABA-EPN blends led to an improvement in the tensile strength and flexural strength of the neat castings. The flexural strength and interlaminar shear strength of the glass laminate showed an improvement with an increase in EPN concentration. Although the crosslink density of the neat casting was enhanced by epoxy-concentration, this did not result in any significant variations in the glass transition temperature (Tg) of the cured matrix. The Tg was in the range 78-82°C. Complete polymerization of all allyl groups resulted in an increase in the Tg and thermal stability while the mechanical properties of the neat system remained practically unaltered. The dual curing of the matrix system resulted in considerable improvement in the flexural properties of their glass composites.

Studies on bismaleimide co-cured Novolac epoxy–diallyl bisphenol-A system

The novolac epoxy (EPN)–2-2′-diallyl bisphenol-A (DABA) system was modified by reactive blending with bisphenol-A bismaleimide (BMI) to improve its high temperature properties. Molar concentration of BMI in the stochiometric (1:1) blend of EPN and DABA was varied and the cure conditions were optimized for these systems by DSC, DMA and rheological studies. The glass composites processed using these ternary blends were characterized for their thermal, mechanical and dynamic mechanical properties. The incorporation of BMI into the epoxy–phenol system yielded a matrix system with improved glass transition temperature and comparable mechanical properties. The increase in BMI molar concentration beyond 1.0 resulted in deterioration in the mechanical properties and fracture toughness. But the high temperature performance of the composites, determined from their inter-laminar shear strength retention, showed systematic improvement with concentration of BMI in the system. The toughening of the system using polyether sulphone improved its mechanical properties with a marginal penalty on its glass transition temperature. The dependence of properties of composites on the type, orientation and stacking sequence of the laminates was investigated. The influence of the nature of the bismaleimide on the properties of the ternary reactive blend and its glass laminates was also examined. On changing the BMI, it was seen that the rigidity of the molecular backbone of the bismaleimide contributed to the high temperature properties of the ternary blend, while BMI with better flexibility has improved the mechanical properties of the resultant composite systems.

Triphenyl Phosphine Catalyzed Curing of Diallyl Bisphenol A‐Novolac Epoxy Resin System—A Kinetic Study

The kinetics of triphenyl phosphine (TPP) catalyzed phenol‐epoxy reaction in the blend of 2,2′‐diallyl bisphenol A (DABA) and an epoxy novolac (EPN) was investigated using differential scanning calorimetry (DSC). The kinetic parameters viz., activation energy (E) and the pre‐exponential factor (A) were calculated by the multiple heating rate methods of Ozawa and Kissinger. The effect of catalyst concentration on the kinetic parameters of the cure reaction was studied and E and A were found to depend on catalyst concentration. The apparent activation energy values normalised to a fixed A value decreased systematically and the corresponding rates of reaction showed an increasing trend with increase in catalyst concentration. The iso‐conversional kinetic analysis done for different catalyst concentrations and for different conversion levels implied that the overall kinetics does not vary much with conversion level. The activation parameters were used to predict the cure profile of the resin under given conditions of temperature and catalyst concentration.

Bismaleimide Modified Epoxy-Diallylbisphenol System ― Effect of Bismaleimide Nature on Properties

The properties of the ternary matrix system (EPB), obtained by the reactive blending of epoxy resin - diallyl bisphenol and a bismaleimide, depend on the structure and properties of its components. The influence of the structural variations of bismaleimide on the thermal, physical and mechanical properties of the ternary blend was examined. EPB compositions were prepared using novolac epoxy, diallyl phenol A and three different bismaleimide systems viz. 2, 2-bis 4-(4 maleimidophenoxy) phenyl propane-BMIP, bis (4-maleimidophenyl) methane-BMIM and bis (4-maleimido phenyl) ether-BMIE. The EPB systems with different types of bismaleimides were processed and characterized for their thermal, physical and rheological properties. EPB-glass composites processed using these matrix systems were characterized for their thermo-physical and mechanical properties. High temperature properties of these systems were assessed by their adhesive strength retention at different temperatures. The EPB system formed by the reactive blending of epoxy- phenol system with BMIP was found to yield the blend with improved mechanical performance at ambient conditions, while that with BMIM proved to be the best for high temperature application.

Influence of Epoxy Nature on the Properties of an Epoxy– Diallylbisphenol A–Bismaleimide Matrix System

The ternary matrix system comprising a reactive blend of epoxy, diallylbisphenol A, and bisphenol A bismaleimide (EPB) was found to be comparable with the epoxy–phenol (EP) system in terms of its mechanical properties, and superior to it in terms of its high-temperature performance. The influence of structural variations of the epoxy resin on the thermal, physical, and mechanical properties of the ternary blend was examined. EPB compositions were prepared using a combination of bisphenol A bismaleimide and diallylbisphenol A with three different epoxy resin systems, namely novolac epoxy (E1), bisphenol A diglycidyl ether (E2), and tris(4-glycidyloxyphenyl)methane (E3). Cure characterisation of these reactive blends (EPB–E1, EPB–E2 and EPB–E3) was carried out using IR spectroscopy, differential scanning calorimetry, and rheological analysis. The trend in their thermal stability, based on the temperature of the onset of decomposition, is EPB–E3 > EPB–E1 > EPB–E2. The mechanical properties – compressive, flexural, and interlaminar shear strength – of the glass composites of these matrices were found to depend on the epoxy structure. The higher crosslink density of the trifunctional epoxy improved the high-temperature properties of the system. The ternary system served as a very good adhesive, where its strength as well as its retention at high temperature increased in proportion to the functionality of the resin. In general, the maximum strength values were observed for the system containing the novolac epoxy.