Influence of Soybean Hull Fiber Concentration on the Water Absorption and Mechanical Properties of 3D-Printed Thermoplastic Copolyester/Soybean Hull Fiber Composites

In this work, fused filament fabrication 3D-printed parts of soybean hull fiber thermoplastic copolyester (TPC) composites with soybean hull fiber concentrations of 0-35 wt.% were tested to understand the influence of mechanical properties and moisture sensitivity on the soybean hull fiber concentration. The composites were analyzed for their microstructures and mechanical properties in as-printed condition and after immersion in deionized water for 168 h. The printed parts with ≥ 25 wt.% soybean hull fiber were found to have more porosity (9-12%) leading to high rate of water absorption with a maximum weight gain of ~ 8% and up to 4% volumetric swelling. However, in the as-printed condition, these composites exhibited significantly higher elastic modulus of 80 ± 3 MPa than pure TPC (36 ± 3 MPa) and their strength improved by 40%. The toughness of the composites decreased below that of pure TPC when the fiber concentration was 35 wt.% due to significant drop in the elongation. The composites with ≤ 15 wt.% soybean hull fiber showed marginal drop in the mechanical properties due to water absorption. Additionally, the microstructural analysis showed good fiber–matrix interfacial characteristics in as-printed condition, which were damaged due to moisture absorption in addition to defragmentation of fiber bundles. Interestingly, the toughness of TPC–soybean hull fiber composites was immune to water absorption and the deleterious effect of moisture on the mechanical appears to be partly reversible after drying the composites.