Experimental Investigation of Glass Microballoon/Hdpe Syntactic Foam Composite

Abstract

Polymer matrix composites inherit good specific values and reduced structural weight making them more promising in automobiles and aerospace applications. Thermoplastic polymers are moldable into different shapes, recyclable and reusable leading to wide usage in semi-structural and engineering applications. These materials are widely used in consumer products and industrial components. Reducing weight of thermoplastic components has been always a high priority in transportation, aerospace, consumer products and underwater vehicle structures. Their current and future potential are driven by processing flexibilities using variety of industrial scale manufacturing techniques and material innovations therein like in foams. Foams are lightweight cellular materials that are widely used in applications such as packaging, thermal insulation, sound absorption, underwater vehicle structures and as the core in sandwich structures used in aircraft. Rapid production of such high quality foam components for industrial applications reduces matrix material requirement and the associated cost. The present study is focused on developing an industrial scale compression molding based processing method for glass microballoon/high density polyethylene (GMB/HDPE) syntactic foams and studying their mechanical properties to develop structure-property correlations. Although glass hollow particle filled lightweight syntactic foams with thermoset matrices have been studied in detail, studies on thermoplastic syntactic foams are scarce. Despite continued interest in developing lightweight thermoplastic syntactic foams, they have not been studied extensively with focus on volume fraction and wall thickness variations. Matrix material used in the present investigation is high density polyethylene (HDPE) and the filler is glass microballoon (GMB), both in as received conditions. Syntactic foam (SF) developed by glass microballoons have benefits like low density, good dimensional stability, high stiffness, material saving and reduced component cost without compromising the specific properties. Blending of GMB in HDPE is carried out using a Brabender mixer with processing parameters optimized for minimal filler breakage. The optimized parameters are used for manufacturing HDPE syntactic foam lumps (brabender output) with 20, 40 and 60 volume % glass microballoon. SF lumpsare processed through compression molding route to form SF sheets that are used for mechanical characterization. In total NINE types of syntactic foams are prepared with three different GMB true particle densities (200, 270 and 350 kg/m3) varying by 20, 40 and 60 volume % in HDPE resin. Different density particle resemble varying wall thickness of GMBs. Lower and higher density values represent thin and thick walled GMBs. Neat HDPE samples are also prepared with similar processing condition as that of foams for comparative analysis. Minimum of five replicates are tested and average values are used for analysis. Uniform distribution of GMBs is observed through micrography affirming the good quality of GMB/HDPE syntactic foam sample processed through the adopted compression molding route. Experimental and theoretical densities of developed syntactic foams are computed. Measured density of all the syntactic foams is lower than neat HDPE resin. Weight saving potential of 10-36% is observed by using GMBs in HDPE matrix. For all particles types, GMB failure is observed to be the highest for syntactic foams containing 60 vol. % GMBs. Increasing glass microballoon content increases particle to particle interaction during processing resulting in particle breakage. Additionally, increasing wall thickness makes GMBs stronger resulting in reduced particle fracture. Particle failure forms glass debris that gets embedded in HDPE matrix. Although fractured particles do not provide reduction in density as planned, they still help in replacing more expensive HDPE resin. Tensile test is conducted at a constant strain rate of 5 mm/min strain rate on trimmed GMB/HDPE foam samples as per ASTM D638-14. Tensile modulus is observed to be highest for the thick walled microballoon having highest filler content as compared to neat resin. Increasing filler content and the wall thickness increases modulus, effect of volume fraction being more prominent. Ultimate tensile strength is seen to be decreased by 32-66% with increasing filler content as compared to neat HDPE. The fracture strength of all the GMB/HDPE foams is 1.3-2.9 times lower than that of the neat HDPE. Neverthless, specific modulus is highest for syntactic foam with thick walled microballoon at 60 vol. % filler loading as compared to neat resin and other foams. Specific strength of GMB/HDPE foams is less compared to neat resin. Highervalues of specific tensile modulus affirm the use of these syntactic foams for weight sensitive applications demanding higher modulus in molded components. Further, tensile test is carried out for lower strain rates (1.6×10-5, 1.6×10-4 and1.6×10-3 s-1). Highest tensile modulus is observed in foams with thin walled microballoons at highest filler loading as compared to neat HDPE at 1.6×10-3 s-1 strain rate. The effect of wall thickness on the modulus of syntactic foams with the same GMB volume fraction is greater at lower strain rates compared to higher ones. Tensile modulus is found to be relatively insensitive to GMB wall thickness variations. Ultimate tensile strength decreases with increasing filler content. Compared to neat HDPE, syntactic foams fracture at lower strain. The fracture strength of all the developed syntactic foams is 1.5-3 times lower than that of the neat HDPE. No clear trend is observed for specific tensile strength. GMB/HDPE foams samples are subjected next to flexural test as per ASTM D790-10. Foams exhibited higher flexural modulus as compared to neat HDPE. Flexural modulus increases while strength decreases with increasing filler content. Additionally, increase in wall thickness increases the flexural modulus. Specific flexural modulus and strength of SF with 350 kg/m3 particle density having 60 vol. % GMB and 200 kg/m3 GMB having 60 vol. % are observed to be 147 and 8% higher compared to neat HDPE samples. Flexural properties are sensitive to volume fraction variations as compared to wall thickness variation. Two theoretical approaches, Porfiri-Gupta and Bardella- Genna model are used to estimate tensile and flexural modulus of syntactic foams. Bardella-Genna model predicts values closer with experimental results for all GMB/HDPE foams tested under tensile (except lower strain tests) and flexural conditions. Outcome of existing literature on tensile and flexural studies is compared with the experimental results of the present work is presented in the form of property maps which helps in material selection for the material scientist/design engineer based on the suitable application. Quasi-static compressive behavior of GMB reinforced HDPE syntactic foams are investigated next. Compression molded GMB/HDPE sheets are subjected to 0.001,0.01 and 0.1 s-1 strain rates. Compressive modulus of foams is higher compared to neat HDPE. Increasing strain rates and decreasing filler content increases yield strength for all the foams investigated compared to neat HDPE. Yield strain and energy absorption of GMB/HDPE foams increases with an increasing strain rate and wall thickness. Specific compressive modulus and strength of GMB/HDPE foams are superior and are comparable to neat HDPE. GMB/HDPE foam achieved high stiffness to weight ratio making them suitable for wide variety of applications. Porfiri-Gupta model based on differential scheme predicts a good estimate of compressive modulus for all the type of GMB/HDPE foams. Property maps are exhibited to present comparative studies of quasi-static compression with existing literature. Further, GMB/HDPE foams are characterized for viscoelastic properties by dynamic mechanical analysis. Test is ramped from 35-150°C at a rate of 5°C/min with the deformation occurring at a constant frequency of 1 Hz. With increase in temperature, storage and loss modulus decreases while tanδ increases with increase in filler loading and wall thickness. Storage modulus and loss modulus increases with increasing wall thickness and volume fraction of GMBs. Damping factor (tanδ) shows an increasing trend with increase in GMB content and wall thickness. Damping factor is less sensitive to glass microballoon content as compared to storage and loss modulus. Structure-property correlations of all the investigated properties are presented with the help of exhaustive SEM images to understand underlying mechanisms. Finally the behavior of material is analyzed using the crystallinity measurement. Crystallinity is observed to be highest for the HDPE as compared to GMB/HDPE foams. Inclusion of GMB decreases the crystallinity signifying stiffness rise of the polymer backbone resulting in ductile to brittle behavioral change. Developed GMB/HDPE syntactic foams achieved better physical and mechanical properties as compared to other thermoplastic foams studied in recent past as exhibited by property maps. Consumption of expensive matrix is reduced by dispersing GMBs leading to lower cost of these syntactic foams. GMB/HDPE foams developed in the present work have a weight saving potential of 36% with betterspecific mechanical properties making them candidate material in weight sensitive and buoyant applications.