Stiffness Enhancement of Rotationally Moulded Products

Abstract

The demand for high-performance solutions for rotational moulding (RM) of plastics is steadily increasing. However, the materials currently available are significantly limiting the feasibility and design of products that can be manufactured. There are substantial challenges for overcoming these limitations and finding new materials or composites that can enhance the performance of products and subsequently expand the field of application for rotational moulding. This research project focused on closing gaps in the existing knowledge of reinforced plastics in the RM industry, and on the development of a new composite that provides enhanced stiffness and impact strength, without forfeiting other properties. The characteristics of RM, typically using low-cost raw materials and equipment, often impede performance increases as they usually are accompanied by significantly increased material costs or machine adaptations. Therefore, investigations were focused on composites which are easy to manufacture and can be used on the majority of existing RM machines in the industry. Additional processing methods that introduce shear and pressure to the composite during initial blending were studied for their ability to enhance filler-matrix bonding compared to dry-blended composites. The biggest obstacle in this additional processing is achieving the correct post-preparation of the material. This is required to produce particles with proper melt and flow behaviour that is essential to produce an excellent end product. Natural nano-sized clay, halloysite nanotubes (HNT), was incorporated into three different polyethylenes (PE) to develop a composite based on an abundant and renewable resource. The addition of HNT led to an increase in Young’s modulus, flexural modulus and flexural strength, but a loss in impact strength of PE. Another approach was the utilisation of micron-sized and nano-sized synthetic reinforcements, such as ultra-thin glass fibres (UTGF) and carbon nanotubes (CNT), which have previously not been widely considered in the field of RM. The influence of aspect ratios of various reinforcement materials and the benefits of their higher values were investigated. The use of a compounded UTGF/HDPE masterbatch resulted in significantly increased impact strength and Young’s modulus, which were only accompanied by a small decrease in flexural modulus. Low weight fractions of CNT were able to enhance the overall performance of HDPE (improved tensile, flexural and impact properties), making it a desirable candidate for further, industrial-scale manufacturing. A low-cost alternative which requires less processing, is glass fibre powder with an aspect ratio of 12:1; offering 53% increased Young’s modulus compared to the pure matrix material. However, this glass fibre powder has reduced flexural and impact properties. Theoretical models were evaluated in terms of their accuracy and ability to predict Young’s modulus and tensile strength of the composites investigated in this research. Limitations of these observed models include assumptions of perfect fibre-matrix bonding, lack of defects, and the high influence of aspect ratio on the predicted values. The latter is the most substantial with very high aspect ratios as seen in UTGF and CNT, where models led to vastly overpredicted and impossible values. Considering the advances in performance of certain composites used in this research - especially with synthetic reinforcements - their cost-effectiveness and possible implementation in most existing industrial RM machines may be beneficial. These composites have the potential to expand the applications for RM products and give manufacturers a range of new material options without having to upgrade or adapt current RM machinery and technology. Additionally, such composites may allow for the manufacture of larger products with RM, without major design changes or the introduction of stiffening elements.