Rotational Moulding innovation as a way to sustainability in polymer processing

Nowadays, the research on sustainable polymer materials and processing is focused on reduction of energy consumption both in the manufacturing step as well as in the transport and use of final products. In ARRED'ARTE project, rotational moulding (RM), also known as rotomoulding, was investigated as a relatively simple technology that offers the advantage of supplying one-piece hollow plastic items ranging from small to large size, also in complex shapes impossible to fabricate by other processes (i.e. injection moulding or blow-moulding). Polyethylene (PE) resin currently represents about 85% of worldwide RM production. In order to achieve lightweight PE-based rotomoulded items and thus saving energy during the transport, composite materials based on PE and hollow glass microspheres (HGM) were developed. HGM consist of a shell of stiff and chemically stable soda-lime-borosilicate glass and an empty core filled with an inert gas. The main properties of HGM are very low density (0.46 g/cm3), low thermal and electrical conductivity, and high crush strength, this making them suitable as low-density filler and reinforcement of polymer matrices in several applications. In details, the adhesion between the glass particles and PE matrix was improved surface modifying the HGM with dodecyl(triethoxy)silane or adding a suitable coupling agent, namely a PE grafted with maleic anhydride, during melt-compounding. The PE-based materials, prepared at different HGM loads, were thermally, rheologically, mechanically and morphologically studied. Several prototypes were successfully manufactured at laboratory scale, using a benchtop uni-axial rotomachine, and then in an industrial site, using bi-axially rotating moulds. The PE/HGM composite items, compared to the counteparts based on neat PE, are characterized not only by reduced weight and improved mechanical properties but also by very attractive aesthetic effects of light diffusion, especially when lit internally. Prototypes based on polycaprolactone, a biodegradable polymer, were as well fabricated by rotomoulding using epoxy resin moulds and low processing temperatures (with consequent energy savings). RM is known as high energy consuming process where the heating of the moulds is done mainly using conventional gas fired oven. In order to improve the process sustainability and lower the energy costs a new approach using microwave (MW) assisted heating was studied in the ROPEVEMI Project. Various MW-active materials were tested in order to modify the moulds and make them rapidly heatable in a microwave oven. We started at laboratory scale, testing different MW-active inorganic powders (Fe2SiO4 fine and coarse, SiC, TiO2, Fe2O3 and BaTiO3) and various binding matrices (cement mortar, epoxy resins, silicon resins and liquid glass) obtaining several formulations. We tested them in a laboratory MW oven, properly modified to reproduce uniaxial RM, in order to investigate the feasibility of the MW-assisted process and choose the better formulation to be scaled up to a pilot industrial plant. At laboratory scale we demonstrated the possibility of producing rotomoulded items up to 60 g PE in about 6 min of MW heating at 600 W. The best formulation (coarse Fe2SiO4/liquid glass/cement mortar) was scaled up and used to modify industrial moulds then used in bi-axial RM within a purposely projected and built industrial MW oven. Optimization of the MW-assisted RM process and of the modified moulds was done leading eventually to the production of PE items identical to those made with conventional heating, demonstrating the industrial feasibility of the RM-MW process.