Experimental evaluation of the mechanical properties of recycled high-density polyethylene (rHDPE) blended with talc filler, for engineering applications

Abstract

The extensive use of thermoplastic products, particularly high-density polyethylene (HDPE), has led to significant plastic waste, posing environmental threats. To manage thermoplastic waste, recycling is the preferred method; however, this has not been wholly effective due to technological and economic challenges and limitations. Large-scale applications of recycled HDPE (rHDPE) can incentivize recycling and create new revenue streams. HDPE is a well-established thermoplastic for engineering applications, and components made of HDPE have desirable properties such as high strength-to-weight ratio, ease of processing, availability, low cost, and excellent chemical and corrosion resistance. With concerns about the fate of plastics at end-of-life, there is a growing interest in strategies to utilize rHDPE in place of virgin HDPE (vHDPE). This study focused on investigating the mechanical and thermal properties of rHDPE-talc blends across various talc filler contents and temperatures, and across four recycling generations, as understanding these properties is crucial for application. Following ASTM standards, tests for tensile strength, elastic modulus, storage modulus, nominal yield stiffness, nominal yield strain, impact strength, and melt flow index were performed. Dynamic mechanical analysis and differential scanning calorimetry were also carried out. Results show that talc content and temperature affect tensile strength, elastic modulus, nominal stiffness, yield strain, impact strength, and storage modulus. Melting temperature decreased while crystallinity increased with talc filler content increase. Compared to neat HDPE and in most cases vHDPE-talc blends, rHDPE-talc blends perform better. Response Surface Methodology was applied using the Central Composite Design statistical experimental design approach to further study the stiffness and strength of rHDPE as functions of temperature and talc filler content. It revealed significant correlations for practical applications. Increasing the number of thermal reprocessing cycles decreased tensile strength, elastic modulus, impact strength, and storage modulus, while nominal yield strain and melt flow index increased. Crystallinity and melting temperature minimally decreased with increased thermal reprocessing cycles. Despite these changes, most of the properties of both the neat rHDPE and its talc blends remain comparable to the virgin counterparts, even after the fourth recycling generation. This implies that the recycled materials can be suitable for use in existing applications of vHDPE.