Polymeric Filler Reinforced Glass Fiber Epoxy Nanocomposites for Improved Impact Strength

Abstract

Commercially used fiber reinforced polymer composites mostly comprise of glass fibers as the reinforcement and epoxy as the matrix (epoxy based GFRPs). These GFRPs have high specific strength and stiffness and are used in various structural applications. However, safe operation of structures for the required lifetime demands not only good static mechanical properties but also high impact strength. For this reason, the current research in this field of work focuses on improvement in impact strength of epoxy based GFRPs. Epoxy toughening has remained an interesting and challenging topic. Significant efforts have been paid on epoxy-based systems toughened by reinforcement of micrometre-sized liquid rubbers, core-shell rubber particles, thermoplastic particles etc. Addition of rubbery toughening agents provides impressive toughening effect but tends to cause severe deterioration in strength and processing difficulties due to high viscosity. Further, introduction of thermoplastic particles typically gives a moderate toughening effect only. Nanoclay reinforcement to glass fiber reinforced epoxy based composite system shows substantial improvements in static mechanical properties (tensile properties, flexural properties, microhardness etc.) but only marginal improvements in impact strength of the resulting GFRP nanocomposite. So, the impact behaviour of GFRP nanocomposites is still an area of concern. In the present research, it was envisaged that incorporation of thermoplastic fillers as an additional second filler along with nanoclay as the first filler can considerably increase the impact strength and tensile properties of epoxy based GFRPs. Thus, the research work was designed to process epoxy based GFRPs through addition of nanoclay (nano-filler) and thermoplastic fibers (micro-filler) using vacuum assisted hand lay-up technique. Nanoclay was added in a fixed loading of 2 phr in GFRPs. Nanoclay concentration was kept low in order to avoid abrupt increase in viscosity of resulting GFRPs on addition of thermoplastic fibers and nanoclay. Three different thermoplastic fibers viz. (i) ultra high molecular weight polyethylene fiber (loading: 0.125–0.500 phr), para-aramid fibers (0.50–2.00 phr), and Inviya (spandex) fibers (0.50–2.00 phr) were used separately as the micro-filler in GFRPs. A major challenge was to improve the compatibility of thermoplastic fibers with other constituents of epoxy based GFRPs. Surface treatment (compatibilization) of the nano-filler was done with silane agent (silanization) and that of thermoplastic micro-fillers was done using various methods viz. potassium permanganate treatment for UHMWPE fibers, phosphoric acid treatment for para-aramid and Inviya fibers, silanization with 3-aminopropyltriethoxy silane agent, UV-assisted maleic v anhydride grafting (MAH) grafting, and/or a combination of these treatments. For MAH grafting of thermoplastic fibers, the optimum treatment time (for exposure of MAH-acetone solution containing thermoplastic fibers to UV radiations) was determined to be 4.5 h, 02 h, and 4.5 h for UHMWPE fibers, para-aramid fibers, and Inviya fibers respectively. Processing of multi-scale filler reinforced epoxy GFRPs was done using a series of processing steps including homogenization, probe ultrasonication etc. Characterization techniques including XRD and TEM analysis confirmed the dispersion of nanoclay platelets at a nano-level in developed GFRPs. Three different types of montmorillonite nanoclays viz. (i) CA, (ii) IE, and (iii) PG were separately reinforced in the reference epoxy based GFRP to investigate the effect on Izod impact strength and tensile properties of resulting GFRPs. PG nanoclay showed the best combination of mechanical properties, and thus, was chosen as the nano-filler for the present research. PG nanoclay was subjected to silanization for further improvement in mechanical properties of GFRPs containing the nanoclay. For silanization, silane agent concentration was varied in the range of 100–400% as a proportion of nanoclay loading (1X, 2X, 3X, and 4X). Silanization was confirmed through FTIR analysis. GFRPs reinforced with 2 phr of 3X silanized nanoclay of ‘nowashing’ case (3X2PGAW*C) showed maximum improvement of 27% and 16% in Izod impact strength and tensile strength respectively over the reference sample. Addition of pristine thermoplastic polymeric fibers to GFRP nanocomposite system resulted in deterioration of mechanical properties. Pristine thermoplastic fibers did not interact effectively with other constituents of GFRP system due to lack of polar functional groups owing to chemically inert and mechanically smooth surface. To resolve this issue, thermoplastic fibers were subjected to surface treatment. Surface modification of thermoplastic fibers improved their interfacial adhesion with other constituents of GFRP based nanocomposite system and resulted in significant improvements in impact strength of resulting nanocomposites. Epoxy based GFRPs (containing 2 phr of silanized nanoclay) reinforced with 0.250 phr of treated UHMWPE fibers (potassium permanganate treatment followed by silanization) showed 30% and 17% improvement in impact strength and tensile strength respectively over the reference composite. Epoxy based GFRPs (containing 2 phr of silanized nanoclay) reinforced with 1.00 phr of MAH treated para-aramid fibers showed 34% and 6% improvement in impact strength and tensile strength respectively over the reference composite. Epoxy based GFRPs (containing 2 phr of vi silanized nanoclay) reinforced with 1.50 phr of treated Inviya fibers (phosphoric acid treatment followed by silanization) showed 150% and 4% improvement in impact strength and tensile strength respectively over the reference composite. These newly developed multi-scale filler reinforced epoxy GFRPs displaying significantly improved impact strength along with good tensile properties can be utilized in high impact applications where safety is a critical prerequisite like dashboards, bumpers, and other structural components of automobiles. In addition to this, the newly developed GFRPs can be used in aviation parts (radome, stabilizers etc.), marine industry (decks, hulls etc.), and sports industry (vault poles, archery bows etc.).