Development of Recycled Polypropylene Composite Materials for Applications in 3D Printing

The adverse effects that waste plastics are having on the environment is becoming increasingly apparent. However, the plastics recycling industry in New Zealand is entirely market driven, necessitating the development of new markets to account for increasing quantities of waste. Innovations in additive manufacturing (AM) have presented opportunities to recycle thermoplastics for use as AM feedstock material. Using waste thermoplastic materials to fabricate composites in this way, adds value to the polymer by enhancing mechanical and aesthetic properties. The main objective of this research was to produce strong, stiff and dimensionally consistent composite AM feedstock material using recycled polypropylene. Alkali treated natural hemp and harakeke fibres were chosen as composite constituents based on the advantages gained in terms of mechanical performance as well as low environmental impact compared to synthetic fibres. In addition, recycled gypsum powder (which is a currently underutilised waste product), was selected as a composite constituent primarily to investigate the influence it has on polymer shrinkage. Initial screening tests using 20 wt% harakeke fibre were conducted to determine effective AM feedstock filament fabrication parameters, which were then used to fabricate all subsequent composite filaments. The parameters that provided the most significant improvements in tensile properties were: 105oC constituent drying temperature for approximately 24 hours (as opposed to 80oC), Maleated polypropylene (MAPP) coupling agent in powdered form (as opposed to granulated form) and initial compounding carried out on a 16mm twin screw extruder (as opposed to a larger more intensive, 20mm extruder). A range of composite filaments with differing fibre and gypsum weight contents were then produced using pre and post-consumer polypropylene (PP). The most successful filaments in terms of tensile properties consisted of 30 wt% harakeke in a post-consumer PP matrix which had a tensile strength and Young’s modulus of 41MPa and 3.8 GPa respectively. Comparing these results to those of plain PP filament, showed improvements in tensile strength and Young’s modulus of 77% and 275% respectively. To investigate the effects of 3d printing on tensile properties, feedstock filaments were 3d printed into tensile test samples. Compared to the filaments, 3d printed samples showed a reduction in tensile strength and Young’s modulus as large as 40% and 60% respectively. An investigation was conducted to determine the cause of this reduction which was considered possibly due to moisture absorption and/or reduction of printing pressure relative to extrusion pressure. Pre-drying materials prior to printing resulted in strength and stiffness improvements, relative to undried filament of up to 26% and 44% respectively supporting moisture absorption to be an issue. However, densities were also found to be reduced, predominantly in the fibre reinforced materials supporting reduced pressure to also be a contributing factor. Specific strength and stiffness values of filament and printed materials were found to be closer than strength and stiffness, further supporting the relationship between a decrease in density and mechanical properties. Unlike fibre reinforced samples, the difference in specific strength and stiffness for plain PP filament and printed materials was equal to the difference in strength and stiffness. This could also suggest loss of polymer orientation to be a factor. Finally, a novel method of measuring shrinkage in 3d printed components was developed and used to compare relative shrinkage of different composites. Natural fibre composites showed less shrinkage than gypsum composites with 10 wt% natural fibre showing a similar shrinkage value (1.17 %). as 50 wt% gypsum. The composite that showed the least shrinkage consisted of 30 wt% harakeke with a shrinkage value of 0.34% corresponding to a net reduction of 84% relative to plain PP