A new generation of high stiffness rotational moulding materials

Bhabha, Hashim (2015) A new generation of high stiffness rotational moulding materials. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Polyethylene (PE), particularly linear medium density PE (LMDPE), is the most widely used thermoplastic in the rotational moulding (RM or rotomoulding) industry, possessing a balance between melt flow characteristics and mechanical properties best suited to the RM process relative to alternative thermoplastics. Reliance of the RM industry on LMDPE limits the application envelope for manufacturers due to the inherently low modulus of the material; manufacturers overcome this low modulus by increasing the wall thicknesses of their products which is costly and energy intensive. The addition of filler particles to PE as a method of modulus enhancement was considered a feasible alternative to increasing the wall thickness. The resulting composite material could down gauge part thickness and potentially expand the application envelope of RM. Phase 1 of this study observed the behaviour of RM grade PE’s with the introduction of filler particles in order to double the modulus (namely garnet, sand, cenospheres or fly-ash and the latter two combined). The PE/filler composites were mixed by dry blending or melt compounding, moulded and mechanically tested in tensile, flexural and Charpy impact mode. The aim of doubling the tensile modulus of rotomoulding grade PE was achieved by the melt compounded, rotomoulded PE/fly-ash composites. The introduction of maleic anhydride grafted linear low density polyethylene (MA-g-LLDPE) coupling agent also increased the modulus and tensile yield stress of LMDPE with the addition of fly-ash. However, the beneficial melt flow rate and impact toughness of PE decreased significantly with the addition of fly-ash. The latter was especially true for rotomoulded samples. As the RM industry typically uses finite element analysis (FEA) to numerically approximate the stress or deflection of load-bearing parts, phase 2 of this study focused upon developing numerical material properties for FEA of the new PE/fly-ash composites. Physical measurements from compression tests on rotomoulded PE/fly-ash safety steps were close to FEA approximations (confirming the practical value of the numerical materials data), except in the case of the unfilled and highest filled PE samples. The significant differences observed between physical measurements and FEA were probably due to complex factors such as the non-linear behaviour of PE and the variation in wall thickness of rotomoulded parts, highlighting the importance of properly understanding the finite element method (FEM) for RM.