Several efforts have been dedicated to the development of lignin-based polyurethanes (PU) in recent years. The low and heterogeneous reactivity of lignin hydroxyl groups towards diisocyanates, arising from their highly complex chemical structure, limits the application of this biopolymer in PU synthesis. Besides the well-known differences in the reactivity of aliphatic and aromatic hydroxyl groups, experimental work in which the reactivity of both types of hydroxyl, especially the aromatic ones present in syringyl (S-unit), guaiacyl (G-unit), and p-hydroxyphenyl (H-unit) building units are considered and compared, is still lacking in the literature. In this work, the hydroxyl reactivity of two kraft lignin grades towards 4,4′-diphenylmethane diisocyanate (MDI) was investigated. ³¹P NMR allowed the monitoring of the reactivity of each hydroxyl group in the lignin structure. FTIR spectra revealed the evolution of peaks related to hydroxyl consumption and urethane formation. These results might support new PU developments, including the use of unmodified lignin and the synthesis of MDI-functionalized biopolymers or prepolymers.

Owing to its chemical structure, wide availability and renewable nature, lignin is a promising candidate for the partial replacement of fossil-based raw material in the synthesis of epoxy resins. Its poor compatibility has been reported to be one of the main drawbacks in this domain. On the other hand, a well-established modification method for lignin epoxidation has been used for many years for the improvement of lignin compatibility. However, the extent of the effect of lignin epoxidation on the improvement of bio-based epoxy mechanical properties, applied as adhesives, is still an open question in the literature. In this context, a pristine and industrial grade kraft lignin (AKL) was reacted with epichlorohydrin to yield epoxidized lignin (E-AKL) in this work. Afterwards, AKL or E-AKL were separately blended with petroleum-based epoxy resin at 15 and 30 wt% and cured with a commercial amine. The adhesive curing kinetic was evaluated using a novel technique for thermal transition characterization, Temperature Modulated Optical Refractometry (TMOR); the results showed that the incorporation of AKL reduces the crosslinking rate, and that this effect is overcome by lignin modification. Mechanical tests revealed an improvement of impact and practical adhesion strength for samples containing 15 wt% of E-AKL. These results elucidate the effect of lignin epoxidation on the application of lignin-based epoxy adhesives, and might support the further development and application of these bio-based materials.

Lignin is a natural polymer composed primarily of phenylpropanoid structures with an abundance of reactive groups: aliphatic and aromatic hydroxyls, phenols, and carbonyls. Considering the large quantity of hydroxyl groups, lignin has significant potential as a replacement for petroleum-based polyols in polyurethane (PU) synthesis and as a value-added, renewable raw material for this purpose. Several methods of lignin-based polyurethane synthesis are reviewed in this paper for reactive and thermoplastic systems: direct lignin incorporation, chemical lignin modification and depolymerization. Despite the unmodified lignin low reactivity towards diisocyanates, its direct incorporation as polyol generates highly brittle PUs, but with proper performance when applied as adhesive for wood. PU brittleness can be reduced employing polyols obtained from lignin/chain extender blends, in which glass transition temperature (Tg), mechanical properties and PU homogeneity are strongly affected by lignin content. The potential applications of lignin can be enhanced by lignin chemical modifications, including oxyalkylation and depolymerization, improving polyurethanes properties. Another PU category, lignin- based thermoplastic polyurethane (LTPU) synthesis, emerges as a sustainable alternative and is also presented in this work.

The main objective of this work is the study of the influence of microwaves devulcanization of the elastomeric phase on dynamically revulcanized blends based on Ground Tire Rubber (GTR)/High Density Polyethylene (HDPE). The devulcanization of the GTR was performed in a system comprised of a conventional microwave oven adapted with a motorized stirring at a constant microwaves power and at various exposure times. The influence of the devulcanization process on the final properties of the blends was evaluated in terms of mechanical, viscoelastic, thermal and rheological properties. The morphology was also studied.

The main objective of this work is to study the influence of clay addition on dynamically revulcanized blends of Ground Tire Rubber (GTR)/High Density Polyethylene (HDPE). GTR was previously devulcanized in a system comprised of a conventional microwave oven adapted with a motorized stirring, with a fixed microwave power and at various exposure times. The influence of clay addition on the final properties of the blends was evaluated in terms of mechanical, viscoelastic, thermal and rheological properties, with morphology being also analyzed. The results depict that the clay can modify the rheological behavior of the GTR phase, in addition to the thermal and mechanical properties of some blends.

Additive manufacturing, particularly Stereolithography (SLA), has gained widespread attention thanks to its ability to produce intricate parts with high precision and customization capacity. Nevertheless, the inherent low mechanical properties of SLA-printed parts limit their use in high-value applications. One approach to enhance these properties involves the incorporation of nanomaterials, with graphene oxide (GO) being a widely studied option. However, the characterization of SLA-printed GO nanocomposites under various stress loadings remains underexplored in the literature, despite being essential for evaluating their mechanical performance in applications. This study aimed to address this gap by synthesizing GO and incorporating it into a commercial SLA resin at different concentrations (0.2, 0.5, and 1 wt.%). Printed specimens were subjected to pure tension, combined stresses, and pure shear stress modes for comprehensive mechanical characterization. Additionally, failure criteria were provided using the Drucker-–Prager model.