A comprehensive review on polylactic acid/lignin composites - Structure, synthesis, performance, compatibilization, and applications

Today, the world is facing a great problem of plastic pollution due to its non-degradable nature. Alternatively, polylactic acid (PLA), a bio-based and biodegradable polymer, is emerging as a promising substitute for conventional, non-biodegradable plastics. However, its high cost, limited properties, and single functionality hinder its wide application. Lignin, a natural and sustainable biomass derived from plant cell walls, has become a promising filler for PLA. The integration of lignin into PLA composites holds the potential to realize the trifecta of low cost, high performance, and multifunctional properties while maintaining the principles of biodegradation and sustainability. However, the poor compatibility between PLA and lignin severely affects their overall performance, which creates a major challenge for the development of PLA/lignin composites. In recent years, a significant of advancements have been achieved in addressing this challenge. In this review, we provide a systematic insight into PLA/lignin composites, focusing on numerous compatibilization strategies including physical addition and chemical modification, and the progress on the structural characteristics, synthesis methods, performance improvements brought by lignin, and multiple applications. Finally, the existing problems and developmental direction of PLA/lignin composites are discussed. We believe that this review can be useful for future research prospects and industrial applications.

Lignin: A multi-faceted role/function in 3D printing inks

3D printing technology demonstrates significant potential for the rapid fabrication of tailored geometric structures. Nevertheless, the prevalent use of fossil-derived compositions in printable inks within the realm of 3D printing results in considerable environmental pollution and ecological consequences. Lignin, the second most abundant biomass source on earth, possesses attributes such as cost-effectiveness, renewability, biodegradability, and non-toxicity. Enriched with active functional groups including hydroxyl, carbonyl, carboxyl, and methyl, coupled with its rigid aromatic ring structure and inherent anti-oxidative and thermoplastic properties, lignin emerges as a promising candidate for formulating printable inks. This comprehensive review presents the utilization of lignin, either in conjunction with functional materials or through the modification of lignin derivatives, as the primary constituent (≥50 wt%) for formulating printable inks across photo-curing-based (SLA/DLP) and extrusion-based (DIW/FDM) printing technologies. Furthermore, lignin as an additive with multi-faceted roles/functions in 3D printing inks is explored. The effects of lignin on the properties of printing inks and printed objects are evaluated. Finally, this review outlines future perspectives, emphasizing key obstacles and potential opportunities for facilitating the high-value utilization of lignin in the realm of 3D printing.

Effects of grafting and long-chain branching structures on rheological behavior, crystallization properties, foaming performance, and mechanical properties of polyamide 6

Polyamide 6 (PA6) was modified with ethylene maleic anhydride syndiotactic copolymer resin (ZeMac), and triglycidyl isocyanurate (TGIC) as modifiers to prepare a grafting structure and a long-chain branching structure, respectively. The effects of two modifiers on the rheological behavior, crystallization properties, foaming performance, and mechanical properties of PA6 were systematically studied by rotating rheometry, differential scanning calorimetry and scanning electron microscopy. The results showed that there were differences in crystallization properties between the two modification methods, but they significantly improved the rheological, foaming performance, and mechanical properties of PA6. In particular, PA6 with long-chain branching structure through TGIC modification showed better performance in various physicochemical characterizations. The introduction of ZeMac reduced the average diameter of bubbles in pure PA6 from 146.32 to 88.12 µm, and the density of bubbles increased from 1.69 × 105 to 5.35 × 105 cells·cm−3. The introduction of TGIC reduced the average diameter of bubbles in pure PA6 from 146.32 to 64.36 µm, and the density of bubbles increased to 1.31 × 106 cells·cm−3. Moreover, the mechanical properties of both nonfoamed and foamed samples were improved after modification.