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Thermomechanical pulp fines as additives for 3D printing filaments Chen, Siwei


To enhance the sustainability of fused deposition modeling (FDM) 3D printing filaments, wood materials are emerging as eco-friendly fillers in thermoplastics. This study introduced thermomechanical pulp (TMP) fines, a by-product from industrial pulping process, as fillers in polylactic acid (PLA) for 3D printing filaments. To address the limited compatibility between hydrophilic fines and hydrophobic PLA matrices, three strategies were adapted to prevent fine agglomeration and enhance filler-matrix adhesion. Mechanical treatment using ball milling, chemical modification by (2-Dodecen-1-yl)succinic anhydride (DDSA) esterification, and additives of coupling agents (maleic-anhydride grafted polypropylene [MAPP] and maleic-anhydride grafted PLA [MAPLA]) were employed and systematically studied. Tensile, rheology, thermal analysis and printability tests were conducted to evaluate the effect of incorporating fines and the efficacy of above three methods. Further characterization by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) were used to analyze fracture surfaces and understand fine distribution. The PLA/TMP fines composites exhibited higher tensile modulus compared to pure PLA (increased from 792 MPa to 1084 MPa with a 20% w/w loading content), while it lowered the tensile strength, elongation at break and toughness. Compared to untreated TMP fines at the same loading level (20% w/w), the composites with ball-milled (Bm) fines presented a 33% increase in tensile strength, a 115% increase in elongation at break, and a 192% increase in toughness due to smaller fine particles and improved dispersion. DDSA-treated fines showed increased hydrophobicity but limited mechanical improvement. By adding MAPP or MAPLA into the PLA/TMP fines composites, both groups exhibited toughening effects, especially with MAPP. Combining MAPP with ball milling further enhanced the composites (10 % w/w untreated TMP fines in PLA) with an increase of elongation at break by 139% and toughness by 170%, demonstrating the modifications’ synergistic benefits. The PLA/TMP fines composites exhibited pronounced shear thinning behavior and reduced complex viscosity, storage modulus, and loss modulus at higher frequencies compared to PLA. While those incorporating ball-milled fines closely mirrored the rheological characteristics of pure PLA. Particularly, PLA mixed with 10% Bm fines demonstrated excellent printability, offering a sustainable 3D printing material.

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