Optimising the Properties of Polymer Composites: Low Weight / High Strength Composites for Commercial Applications M. Hayward * , J. Johnston * , T. Dougherty ** and K. de Silva *** * Victoria University of Wellington, School of Chemical and Physical Sciences, PO Box 600, Wellington, New Zealand, matilda.hayward@vuw.ac.nz ** Nuenz Limited, Lower Hutt, New Zealand *** Product Accelerator, University of Auckland, New Zealand ABSTRACT This paper presents our research on the use of dry, expanded, hollow polymer microspheres (PMS) as an additive in a commercial vinyl ester resin to reduce the density of the composite material. PMS were functionalised with hydroxyethyl methacrylate (HEMA) monomers as a method to maximise the interfacial adhesion between the dispersed PMS and the polymer matrix. Stronger interfacial adhesion is known to improve the mechanical performance of the composite material by reducing the number of interfacial voids, which can be weak points in the composite structure. The PMS and HEMA-functionalised PMS were compared via FT-IR to confirm that the functionalisation process was successful. During polymerisation, the surface- bound HEMA co-polymerised with the vinyl ester to form a strong interfacial bond between the surface of the spheres and the resin matrix. SEM imaging of the fracture surface of the composite confirmed improved interfacial adhesion. Mechanical testing showed an increase in the tensile strength and hardness of the HEMA-PMS composite compared to the unfunctionalised PMS composite. Keywords: polymer composites, interfacial adhesion, polymer microspheres, vinyl ester 1 INTRODUCTION Vinyl ester (VE) resins are thermosetting polymers, produced by the esterification of epoxy resins with a monocarboxylic acid group. They are cured via addition of a catalytic amount of an organic peroxide, such as methyl ethyl ketone peroxide (MEKP). Vinyl ester polymers have an excellent resistance to harsh and corrosive environments. This results in their wide-spread use as a resin for multiple applications in various industries such as marine composites, swimming pools and solvent storage tanks. Alongside corrosion resistance, which results in a significant reduction in maintenance time and costs compared to other polymer materials, vinyl ester resins also have favourable mechanical properties, a low viscosity and are low cost.[1,2] Fibre- reinforced polymer (FRP) composites, which use vinyl ester resins as the matrix material, are low-density with superior mechanical properties, such as strength and stiffness. Further improvements to the strength-to-weight ratio of the vinyl ester matrix are of constant interest to maximise the performance of the FRP composite in high-end commercial applications. There are many examples in the literature of polymer composite materials filled with hollow glass [3,4] or ceramic microspheres. Tagliavia et al. studied the flexural properties of hollow glass particle – vinyl ester composites, and found an increase in the volume fraction of microspheres resulted in an increase in the specific elastic modulus of the composite, but a decrease in composite specific strength.[5] The authors have suggested this is due to interface de- bonding followed by matrix cracking which is prominent in composite structures. PMS have an extremely low density and also exhibit high elasticity and low moisture absorption. PMS can be incorporated into a polymer composite to reduce the overall density, improve workability and increase resilience of the material. Ahmad et al. demonstrated that the incorporation of polymeric hollow microspheres into a polymer composite can increase the resilience and elasticity of the composite.[6] Foon et al. studied the effect of epoxy-PMS in vinyl ester resins and found that an increase in PMS resulted in an increase in tensile, flexural and impact strength. This trend occurred up until 5 wt.% filler loading at which point the strengths decreased with an increase in filler content due to the onset of agglomeration of PMS.[7] Sphere dispersion, adhesion and wettability can all be increased via surface functionalisation techniques. Addition of a compatible surface group with the resin can onset chemical binding between the additive surface and the resin allowing for further improvements to the composite performance, and also reduces agglomeration which means filler loading can be increased without decreasing the properties. In this paper, HEMA was used to functionalise the surface of the polymer spheres, which co-polymerises with the vinyl ester resin forming a strong adhesive bond across the composite interface. 2 EXPERIMENTAL The PMS used for this study were EXPANCEL 461 DET 40 d25 provided by Akzonobel (New Zealand) with sphere diameters of 35-55 μm and true densities of 25 kg m -3 . 2- Hydroxyethyl methacrylate (97%) was from Sigma Aldrich. The vinyl ester resin was a commercial Derakane™ 8084 Advanced Materials: TechConnect Briefs 2018 109