Impacts of multiple recycling on bamboo flour–thermoplastic polyurethane composite filaments used in 3D printing applications

The effects of multiple recycling cycles on bamboo flour−thermoplastic polyurethane filaments and their properties in fused filament fabrication were investigated. Successive recycling induced progressive thermomechanical degradation of the thermoplastic polyurethane matrix, decreasing the average number and average molecular weight, while increasing polydispersity as shown by chain scission. Scanning electron microscopy images revealed increasingly rough and irregular filament surfaces with recycling, whereas gel permeation chromatography and differential scanning calorimetry confirmed reduced chain length and crystallinity, followed by partial recrystallization after three recycling cycles due to increased chain mobility. Mechanical testing indicated significant property deterioration, with the tensile strength, modulus and elongation at break decreasing by more than 50% after three recycling cycles. The decline in tensile properties was attributed to reduced thermoplastic polyurethane molecular weight, thermal degradation of bamboo flour, and persistent porosity, despite some compensatory effects from improved the adhesion between bamboo flour and matrix. With respect to printing by fused filament fabrication with bamboo flour−thermoplastic polyurethane parts from recycled filaments, additional property losses occurred because of weakened layer bonding, although the change in tensile modulus was not influenced signifcantly. Additionally, correlations between weight-average molecular weight and tensile properties highlighted molecular weight as a reliable processing–structure–property indicator. Despite property reductions, the recycled parts maintained mechanical properties comparable to those of elastomers and leather, underscoring their potential for applications in additive manufacturing and footwear applications. • Recycling cycles progressively degrade mechanical properties of BTC filaments. • After three recycling cycles, tensile strength and elongation of BTC filaments drop by over 50%. • Monitoring molecular weight helps predict BTC filament performance during recycling. • Layer bonding weakens in 3D-printed parts from recycled filaments, reducing toughness. • Despite degradation, recycled parts’ properties remain comparable to elastomers and leather.