Curing and thermal behavior of epoxy resins of hexafluoro - bisphenol –A and bisphenol-A been reported in the literature by several methods[20-24]. But the expression used by Dharwadkar and Kharkhanawala for the calculation of activation energy is independent of sample size and heating rate. Ea100θ ln ln = +C (1 − a )−1 RTi2 (T f − Ti )
(1)
where α, fraction reacted; Ti, the temperature of inception of reaction; Tf, temperature at the point of inflection on the thermogram; θ = (T-Ts) (where T is the temperature under consideration and Ts is the maximum temperature); C, −1 constant; R, gas constant. A plot of ln ln (1 − a ) versus θ gave a straight line was equal to m=
100 Ea
RTI 2 (T f −Ti )
(2)
From this equation, Ea was calculated.
Activation energies of decomposition (Ea) were calculated by using Equations 1 and 2 and the values of E have been summarized in Table 2. On comparing the activation energies of decomposition reaction of EFN and EBN novolac epoxy resins cured with DDM/ DDS/ IMAM curing agents respectively, the following trend was observed for activation energies [(EFN / EBN) + IMAM ] > [(EFN / EBN) + DDM ] > [(EFN / EBN) + DDS]. These results also substantiate the fact that the presence of fluorinated groups in the curing agent IMAM have also contributed towards the improvement of thermal stability of both the cured epoxy resins. From Table 2 it was also observed that the cured samples having high Rw value showed higher activation energy which led to the increase in their thermal stability.
4. Conclusions EFN and EBN novolac epoxy resins were synthesized, which were then thermally cured with aromatic amines (DDM, DDS) and synthesized aromatic imide-amine (IMAM) curing agents. The investigation of curing behavior of cured epoxy resins revealed that IMAM was found to be the least reactive curing agent towards both EFN and EBN as compared to their counterparts DDM and DDS. This was attributed to the electronic effects, high molecular weight and physical properties of IMAM curing agent. The experimental results for the thermal degradation demonstrated that the major weight loss for all the samples occurred in the temperature range of 300-600°C. Both the EFN and EBN samples cured with IMAM exhibited higher thermal stability probably due to the solid state aromatization reactions which may lead to the formation of cross-links between the carbonyl and trapped or unreacted amino groups. The presence of CF3 groups in IMAM may also have played its role in the enhancement of thermal stability of such cured resins. The EFN cured resins were found to be more heat resistant as compared to their corresponding EBN resins. This was owing to the ease of oxirane ring opening due to the presence of CF3 groups in EFN that might have been a good ignition for Polímeros, 26(1), 11-20, 2016
the formation of better crosslinked network structure than EBN cured resins.
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