Abstract
The influence of the end groups of two liquid rubbers on curing kinetics, morphology, and hardness behavior of diglycidyl ether of bisphenol-A based epoxy resin (DGEBA) has been studied. The rubbers are silyl-dihydroxy terminated (PDMS-co-DPS-OH) and silyl-diglycidyl ether terminated (PDMS-DGE). Crosslinking reactions, investigated by shear rheometry, ranged 90–110 °C, using a constant concentration (5 phr) of liquid rubbers and 1,2-Diamino cyclohexane (1,2-DCH) as hardener agent. The gel time, t gel, of the neat epoxy significantly decreased when adding the elastomers, more so for the silyl-dihydroxy terminated elastomer; at 110 °C the reaction was nearly complete before rheological test started. The results suggest that the elastomers induced a catalytic effect on the curing reaction. Scanning electron microscopy revealed phase separation of the elastomer during the curing reaction with rubber domains about 5 μm size. However, the DGEBA/dihydroxy terminated elastomer composite cured at 110 °C exhibited a homogenous morphology, that is, the rapid reaction time would not allow for phase separation. Water contact angle tests evidenced either more hydrophilic (silyl-diglycidyl ether terminated rubber) or more hydrophobic (silyl-dihydroxy terminated rubber) behavior than the neat epoxy. The latter effect is attributed to the presence of aromatic rings in the backbone structure of PDMS-co-DPS-OH. Microindentation measurements show that the elastomers significantly reduced the hardness of the epoxy resin, the DGEBA/ether terminated composite exhibiting the lowest hardness values. Moreover, hardness increased as reaction temperature did, correlating with a reduction of microdomains size thus enabling the tuning of mechanical properties with reaction temperature.
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Acknowledgments
C. Valerio-Cardenas was supported by a postdoctoral fellowship from DGAPA–UNAM. This research was financially supported by PAPIIT (IG101313) and CONACyT, program CB-2011 (Grant 168095). A. E. G. is grateful to DGAPA-UNAM (PAPIIT Project IN-106008) for partial support. One of the authors (AF) wishes to thank the MICINN (Ministerio de Ciencia e Innovación), Spain, for financial support of the research (Grant FIS2010-18069).
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Romo-Uribe, A., Arcos-Casarrubias, J.A., Flores, A. et al. Influence of rubber on the curing kinetics of DGEBA epoxy and the effect on the morphology and hardness of the composites. Polym. Bull. 71, 1241–1262 (2014). https://doi.org/10.1007/s00289-014-1121-6
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DOI: https://doi.org/10.1007/s00289-014-1121-6