Lightweight Composites with Hollow Glass Microspheres for Rotational Molding Technology

The present work is aimed at obtaining lightweight materials suitable for the manufacturing of plastic products by rotational molding. Rotational molding or rotomolding (RM) is a low-shear technology used to produce one-piece hollow items for various applications, like liquid storage tanks, containers, toys, indoor and outdoor furniture [1]. About 85% of RM worldwide production is based on different grades of polyethylene (PE) resin. Hollow glass microspheres (HGMs) are mainly used as an inert, low density filler in polymeric matrices. In particular, HGMs made of chemically stable soda-lime-borosilicate glass with high crush-strength allow to attain, also in high-shear processing conditions, lightweight composites with a reduced resin content [2]. In this work, we reinforced medium density polyethylene (MDPE) and poly(?-caprolactone) (PCL) with HGMs to study innovative composites. In the first case, materials were prepared with MDPE, which is traditionally used in RM manufacturing. HGMs were used as such or surface modified by treatment with dodecyl(triethoxy)silane (DDTES), a commercial coupling agent bearing an alkyl chain, in order to enhance compatibility between the inorganic particles and the polyolefin matrix. The effectiveness of the silanization process was established using FTIR spectroscopy and TGA. A polyethylene grafted with maleic anhydride (PE-g-MA) was also tested as coupling agent to enhance material properties. MDPE composites at various HGMs content were prepared by melt blending at 200°C, 10 min, and 60 rpm in a Brabender internal batch mixer. In the second case, composites were prepared with PCL, an aliphatic polyester fully biodegradable and biocompatible. Although PCL has a good flexibility, characterized by high fracture strain, it shows low thermal resistance and heat deflection temperature due to its low melting point (~60°C). Moreover, the Young modulus of PCL is very low, typically 200-400 MPa. These drawbacks have limited its commercial application to some extent. The HGMs were used as such or surface modified by treatment with (3-aminopropyl)triethoxysilane (APTES) [3] in order to enhance the compatibility between the inorganic particles and the PCL matrix. PCL-based composites at different compositions were analogously prepared using the Brabender mixer in the following conditions: 100°C, 10 min and 60 rpm, under nitrogen atmosphere. The composites either based on MDPE or PCL were characterized in terms of morphological, rheological and mechanical properties. On the basis of scanning electron microscopy (SEM) inspections, a good filler dispersion in the composites was found as a result of HGM-surface modifications. The addition of HGM particles has relevant implications on the rheological and mechanical properties enhancing the stiffness of the composites. In particular, the MDPE-based composite reinforced with 20 wt% of silanized HGM exhibits an increase of Young modulus up to 90% compared to neat MDPE accompanied by a reduction of density of about 10%. The PCL-based composite reinforced with 20 wt% of silanized HGM exhibits better mechanical properties (Young modulus enhancement of 120%, tensile strength enhancement 20%) and a lightening of about 12% compared to the neat polymer matrix. Moreover, several rotomolded parts in composites based on MDPE and PCL were prepared successfully through uni-axial benchtop rotomachine showing their potential use in RM technology. As an example, in Fig. 1 PCL-based parts at increasing percentage (0, 5, 10 wt% from left to right) of HGMs prepared with the uni-axial benchtop rotomachine are shown. A few formulations based on MDPE were then selected and used to successfully produce some prototypes in the actual industrial environment, that is under bi-axial rotation in the experimental conditions normally used for polyethylene. In Fig. 2 MDPE-based prototypes prepared by RM are shown. With respect to the neat MDPE (Fig. 2, left), the presence of 10 wt% HGMs (Fig. 2, right) confers to the composite item a good aesthetical quality, characterized by a uniform translucent finishing put in evidence by interior lighting.