Study of Epoxy Based GFRP Nanocomposites Modified With Polymer Fiber/Elastomer

Abstract

Commercially used epoxy based glass fiber reinforced polymer composites (epoxy based GFRPs) show good specific strength and stiffness and are employed in many structural applications. However, safe operation of structures for the required lifetime demands that in addition to good static mechanical properties, these composites also possess high impact strength. Among the various approaches studied for an improvement in impact strength of epoxy based composites, significant efforts have been made by addition of nanoclay or polymer/elastomer fillers. Addition of nanoclay is reported to improve almost all properties of epoxy, but the impact strength has not shown significant improvements. Further, addition of elastomers to GFRPs is reported to show significant toughening of epoxy but other properties such as tensile strength etc. show significant deterioration. So, literature reports that addition of nanoclay leads to significant improvements in static properties and addition of polymer/elastomer filler leads to improved toughness of epoxy. Therefore, a combination of both, nanoclay and polymer/elastomer filler in epoxy based GFRPs, may provide a new route for high impact strength along with good tensile properties. Thus, in the present work, epoxy based GFRP nanocomposites reinforced with clay as the nanofiller and thermoplastic fibers/elastomer particles as the micro-filler were processed using vacuum assisted hand lay-up technique. Polymer fibers namely polypropylene (PP) fibers and polyethylene terephthalate (PET) fibers, and an elastomer namely ethylene propylene diene monomer (EPDM) particles were used separately as the second filler in GFRPs containing nanoclay as the first filler. Nanoclay was added in a fixed amount of 1 phr in various nanocomposites. Low concentration of clay was chosen in order to avoid excessive increase in viscosity of the resulting system on addition of polymer/elastomer micro-fillers along with clay. The amount of PP fibers, PET fibers, and EPDM rubber particles were varied in the range of 0–3 phr, 0–3 phr, and 0–10 phr respectively (‘phr’ is per hundred resin, by weight). As postulated and specified in the present work, the main challenge was to improve the compatibility of polymer/elastomer fillers with other constituents of the nanocomposites. For this, surface treatment (compatibilization) of the micro-fillers was done using two methods (i) coating of silane coupling agents on micro-filler surface, and (ii) ultraviolet assisted maleic anhydride grafting on micro-filler surface. For maleic anhydride grafting of micro-fillers, the optimum treatment time for exposure of MAH-acetone solution to UV radiations was determined to be 30 h, 08 h, and 20 h respectively for PP fibers, PET fibers, and EPDM particles. The processing methodology including the sequence of steps and range of process parameters (v) used in the present work were effective in fabrication of epoxy based GFRPs possessing significantly improved mechanical performance. XRD results and TEM analysis showed that the processing methodology was effective in dispersing the nanoclay and obtaining the desired exfoliated clay morphology in nanocomposites. Addition of nanoclay to the reference GFRP slightly improved the impact strength along with significant improvements in tensile strength and modulus. Addition of untreated thermoplastic polymeric fibers (PP fibers/PET fibers) to the nanocomposite system resulted in deterioration of impact strength as well as tensile properties. This decrease was due to the relatively inert nature of thermoplastic fibers resulting in their poor compatibility with other constituents of the nanocomposite system. However, addition of untreated elastomeric particles (EPDM rubber) to the nanocomposite system did improve the impact strength till an optimum concentration of 5 phr elastomer loading. This improvement was mainly due to cavitation of rubber particles in the nanocomposite system. Compatibilization procedures used for surface modification of micro-fillers were successful in enhancing the interfacial adhesion of micro-fillers with other constituents of the composite system and resulted in significant improvements in impact strength of resulting GFRPs. For a given composition, impact strength and tensile properties of nanocomposites reinforced with compatibilized polymer fibers/elastomer fillers was significantly higher than their counterparts reinforced with untreated micro-fillers. The optimum loading of polymer fibers (PET fibers/PP fibers) in the nanocomposite system for maximum improvement in impact strength was 2 phr, with silane treatment. Addition of silane treated PP fibers and PET fibers resulted in maximum improvement of 44% and 19%, respectively in impact strength of resulting nanocomposites with a minor drop in tensile properties. For fiber concentration beyond this optimum value (i.e. at 3 phr), a decrease in impact strength was observed due to poor dispersion of PET fibers/PP fibers in nanocomposites because of concomitant high viscosity of resin at such high concentration of reinforcement. Further, the optimum loading of EPDM filler in nanocomposite system for maximum improvement in impact strength was 5 phr, with silane treatment. Addition of silane treated EPDM rubber particles resulted in maximum improvement of 68% in impact strength of resulting nanocomposites. This increase in impact strength was due to the combined effect of improved compatibility between EPDM filler and glass fibers, along with cavitation phenomenon of rubber particles. Similar to the case of silane treated micro-fillers, addition of MAH grafted micro-fillers also improved the impact strength and tensile properties of nanocomposites compared to when nanocomposites were reinforced with untreated micro-fillers. Further, for a given composition of nanocomposites, improvement in impact (vi) strength with MAH grafted micro-fillers was less than that obtained by silane treated microfillers. However, MAH grafting resulted in better recovery of tensile properties of nanocomposites. Thus, for the first time, GFRP nanocomposites with clay as the nano-filler and compatibilized polymer fibers/elastomer particles as the micro-sized filler were fabricated successfully providing improved impact strength without much drop in tensile properties. Processing of these nanocomposites from the highly viscous formulations was a challenging task and great care was taken to prevent any agglomeration.