Experimental Investigation of Cenosphere Reinforced HDPE Syntactic Foam Composite

Abstract

Polymer matrix composites can reduce the structural weight and result in improved fuel efficiency and performance in transportation applications. Thermoplastic matrix composites have been used for semi-structural and engineering applications. In addition to the ease of fabrication using a wide range of forming processes, thermoplastic polymers are recyclable, which are the strong driving forces for their current and future applications. Rapid production of high quality components is the key to cost reduction in industrial applications. The present work is the first attempt of manufacturing syntactic foams, hollow particle filled lightweight composites, using an industrial scale Polymer Injection Molding (PIM) process. High Density Polyethylene (HDPE) is used as the matrix material and fly ash cenospheres as the filler. Development of syntactic foams with cenospheres serves dual purpose of beneficial utilization of industrial waste fly ash and reduction in the component cost. Pressure and temperature used in PIM are optimized to minimize cenosphere fracture and obtain complete mixing of cenospheres with HDPE. The optimized parameters are used for manufacturing syntactic foams with 20, 40 and 60 wt.% cenosphere without any surface treatment initially. With increasing cenosphere content, density and tensile strength reduce and modulus increases. A theoretical model based on a differential scheme is used to estimate the properties of cenospheres by conducting parametric studies because of inherent difficulties in direct measurement of cenosphere properties. Further, the influence of cenosphere surface treatment, functionalization of HDPE and blending method on tensile properties are investigated. Cenospheres are treated with silane and HDPE is functionalized with 10% dibutyl maleate. Tensile test specimens are cast with 20, 40 and 60 wt.% of cenospheres using injection molding. Modulus and strength are found to increase with increasing cenosphere content for composites with treated constituents. Highest modulus and strength were observed for 40 and 60 wt.% untreated mechanically mixed and treated brabender mixed cenospheres/HDPE blends, respectively. These values are 37 and 17% higher than those for virgin andfunctionalized HDPE. Theoretical models are used to assess the effect of particle properties and interfacial bonding on modulus and strength of syntactic foams. Brabender mixing method provided highest ultimate tensile and fracture strengths, which is attributed to the effectiveness of brabender in breaking particle clusters and generating the higher particle-matrix surface area compared to that by mechanical mixing method. Theoretical trends show clear benefits of improved particle-matrix interfacial bonding in the strength results. Effect of surface treatment and blending method on flexural properties is dealt next. Flexural test specimens are cast with 20, 40 and 60 wt.% of cenospheres using PIM. The flexural modulus and strength are found to increase with increasing cenosphere content. Particle breakage increases with the cenosphere content and the measured properties show increased dependence on processing method. Untreated constituents blended by mechanical mixing provide the highest benefit in flexural modulus. Modulus of syntactic foams is predicted by two theoretical models. Bardella-Genna model provides close estimates for syntactic foams having 20 and 40 wt.% cenospheres, while predictions are higher for higher cenosphere content, likely due to particle breakage during processing. The uncertainty in the properties of cenospheres due to defects contribute to the variation in the predicted values. Untreated constituents blended by mechanical mixing route as observed in tensile and flexural characterization registered higher tensile modulus and better flexural performance. Thereby, characterization of cenosphere/HDPE syntactic foams synthesized by mechanical mixing route for untreated constituents is dealt in the subsequent investigations. Quasi-static and high strain rate compressive response is investigated later. Thermoplastic matrix syntactic foams have not been studied extensively for high strain rate deformation response despite interest in them for lightweight underwater vehicle structures and consumer products. Quasi-static compression tests are conducted at 10-4, 10-3 and 10-2 s-1 strain rates. Further, a split-Hopkinson pressure bar (SHPB) is utilized for characterizing syntactic foams for high strain rate compression.The compressive strength of syntactic foams is higher than that of HDPE resin at the same strain rate. Yield strength shows an increasing trend with strain rate. The average yield strength values at high strain rates are almost twice the values obtained at 10-4 s-1 for HDPE resin and syntactic foams. Further, HDPE matrix syntactic foams are characterized for their viscoelastic properties by dynamic mechanical analysis. Tests are conducted over 35-130°C temperatures and 1-100 Hz frequency range and combined using the time-temperature superposition principle to generate a set of isothermal master curves. Storage and loss modulus increase with increasing weight fraction of cenospheres, but with little difference between 40 and 60 wt.%, at all temperatures. The sensitivity of storage modulus to weight fraction of cenospheres increases with increasing frequency. Storage and loss modulus decrease with increasing temperature in the range of 35- 130°C, while tan δ increases. The Williams-Landel-Ferry (WLF) constants are a linearly increasing function of cenosphere weight fraction. Structure-property correlations of all the investigated properties are presented with the help of exhaustive SEM images to understand underlying mechanisms. Finally, the potential for using the optimized parameters of injection molding process is demonstrated by casting several industrial components as a deliverable of this work.