Search Results
You are looking at 1 - 10 of 22 items for
- Author or Editor: K. Varma x
- Refine by Access: All Content x
Abstract
Transparent glasses of SrBi2B2O7 (SBBO) were fabricated via the conventional melt-quenching technique. The amorphous and the glassy nature of the as-quenched samples were, respectively, confirmed by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC). The glass transition (T g) and the crystallization parameters [crystallization activation energy (E cr) and Avrami exponent (n)] were evaluated under non-isothermal conditions using DSC. There was a close agreement between the activation energies for the crystallization process determined by Augis and Bennet and Kissinger methods. The variation of local activation energy [E c(x)] that was determined by Ozawa method, decreased with the fraction of crystallization (x). The Avrami exponent (n(x)) increased with the increase in fraction of crystallization (x) suggesting that there was a change over in the crystallization process from the surface to the bulk.
Lacunary interpolation by splines. II
(0, 4) and (0, 1, 3) cases
4,4′-bismaleimidophenyl methane (BM) and 3,3′-bismaleimidophenyl sulfone (BS) were blended in solution using weight ratios 3∶1 (MS31), 2∶1 (MS21), 1∶1 (MS11), 1∶2 (MS12) and 1∶3 (MS13). Chain extended bismaleimide resins were also prepared by treating BS/BM with 4,4′-diaminodiphenyl ether in molar ratios of 1∶0.3 (BM-E and BS-E resins). These resins were also blended with bismaleimides and the curing characteristics were evaluated by differential scanning calorimetry. Increase in BM content in BM∶BS blends or increase in chain extended bismaleimide content in BM-E∶BS or BS-E ∶ BM blends resulted in a reduction of melting and curing temperatures. Indication about the extent of cross-linking was obtained from solubility measurements (in DMF) of isothermally cured resins (180 °C, lh and 220 °C, lh in an air oven). Thermogravimetric analysis of samples isothermally cured at 180 °C and 220 °C (lh each) was carried out in nitrogen atmosphere. Improvement in thermal stability of chain extended bismaleimides was observed on blending.
The curing behaviour of bismethacryloxy derivative of diglycidyl ether of bisphenol A (vinyl ester resin) containing styrene as the reactive diluent (40% w/w) was studied using gel point determination method and DSC. Seven samples of styrene/α-methylstyrene in the ratio 40∶0, 35∶5, 30∶10, 25∶15, 20∶20, 15∶25 and 0∶40 were studied. Delayed curing was observed in samples containing increasing proportions of α-methylstyrene. The energy of activation decreased from 869 kJ mol−1 to 333 kJ mol−1 as the concentration of α-methylstyrene increased in the formulations. However, no difference in thermal stability was observed by replacement of styrene by α-methylstyrene. It was concluded that in vinyl ester resin samples 10–15% α-methylstyrene and 30-25% styrene can be used as reactive diluent.
