Thermo-Mechanical Recyclability of Additively Manufactured Polypropylene and Polylactic Acid Parts and Polypropylene Support Structures

Abstract

Polymers have a reputation for several advantageous characteristics like chemical resistance, weight reduction, and simple form-giving processes. The rise of additive manufacturing technologies such as Fused Filament Fabrication (FFF) has introduced an even more versatile production process that supported new product design and material concepts. This led to new investigations and innovations driven by the individualization of customized products. The other side of the coin contains an increasing resource and energy consumption satisfying the growing demand for polymer products. This turns into a magnitude of waste accumulation and increased resource consumption. Therefore, appropriate product and material design, taking into account end-of-life scenarios, is essential to limit or even close the loop of economically driven product systems. In this paper, a comparison of virgin and recycled biodegradable (polylactic acid (PLA)) and petroleum-based (polypropylene (PP) & support) filaments for extrusion-based Additive Manufacturing is presented. For the first time, the thermo-mechanical recycling setup contained a service-life simulation, shredding, and extrusion. Specimens and complex geometries with support materials were manufactured with both, virgin and recycled materials. An empirical assessment was executed through mechanical (ISO 527), rheological (ISO 1133), morphological, and dimensional testing. Furthermore, the surface properties of the PLA and PP printed parts were analyzed. In summary, PP parts and parts from its support structure showed, in consideration of all parameters, suitable recyclability with a marginal parameter variance in comparison to the virgin material. The PLA components showed an acceptable decline in the mechanical values but through thermo-mechanical degradation processes, rheological and dimensional properties of the filament dropped decently. This results in significantly identifiable artifacts of the product optics, based on an increase in surface roughness.

Evaluation of the Viability of 3D Printing in Recycling Polymers

“…Similarly, the elastic modulus of polypropylene decreases by 20% and the elastic modulus decreases by 12.9% with every recycling cycle [ 87 ]. The results corroborate those from Sun et al [ 75 ] and Nagengast et al [ 84 ] that the strength of polymers declines following exposure to UV light, leading to the shortening of the long-chain hydrocarbon bonds in the polymeric structure. Likewise, Gaikwad et al [ 77 ] acknowledged that recycled ABS has less than 90% of the strength of virgin ABS.…”

“…Like [ 68 ], the authors of [ 71 , 72 ] found that internal defects are common in recycled polymers exposed to either extreme temperatures or UV light. Both [ 71 ] and Nagengast et al [ 84 ] agree that the internal defects in recycled polymers render them functionally unreliable, and, therefore, they are disposed of as waste. These defects imply a limitation on using 3D printing as a plastic recycling method.…”

“…Polymers undergo degradation by photolysis when exposed to ultraviolet light [ 75 ]. The synthesis of [ 75 ] and Nagengast et al [ 84 ] indicates that the long-chain hydrocarbons break upon exposure to light or excess heat, rendering a polymer material brittle. Using an experimental research design involving exposure of polymers to UV light, [ 75 ] found that 3D printed filaments deteriorate on exposure to ultraviolet light, rendering them mechanically weaker.…”

“…However, there might be challenges in the frequent recycling of agricultural plastics. For instance, Sun et al [ 75 ] and Nagengast et al [ 84 ] found that recycled plastics rapidly deteriorate on exposure to UV light, indicating that they are less durable. Regardless, all wastes can be recycled within farms with locally distributed recycling.…”

“…However, factors other than the strength of polymers should also be studied to provide a comprehensive understanding of the different plastic waste types that can yield reliable 3D printing feedstock. On the other hand, a future development to addressing photolysis cited by Sun et al [ 75 ] and Nagengast et al [ 84 ] as a setback to the durability of recycled polymers concerns the addition of anti-photolysis agents, which inhibit degradation when exposed to light. According to El-Hiti et al [ 94 ], a PVC film blended with Schiff, Thiadiazole Moiety, and Nickel Chloride is resistant to photolysis.…”