Phytochemicals from Bark Extracts and Their Applicability in the Synthesis of Thermosetting Polymers: An Overview

This review focuses on recent research on the phytochemicals found in bark from different trees and their potential to be used as substrates for the synthesis of thermosetting resins. Recent studies about the influence of each bark harvesting step on the extracted phytochemicals, from debarking to extraction, are investigated. A comparison of bark extracts in terms of the correlation between extraction conditions and efficiency (based on the total phenolic content (TPC) and extraction yield) is presented for six groups of trees (Norway spruce, pine species, other conifers, oak species, other deciduous trees of the north temperate zone, tropical and subtropical trees) and evaluated. The evaluation revealed that there is an interesting relationship between the extraction time and the type of solvent for some types of tree bark. It was found that a relatively short extraction time and a solvent temperature close to the boiling point are favourable. The latest research on the application of bark extracts in different types of thermosetting resins is described. This review discusses the attractiveness of bark extracts in terms of functional groups and the possibilities arising from extractable phytochemicals. In addition, different approaches (selective versus holistic) and methods of application are presented and compared.

Engineered Interleaved Random Glass Fiber Composites Using Additive Manufacturing: Effect of Mat Properties, Resin Chemistry, and Resin-Rich Layer Thickness

Standard lay-up fabrication of fiber-reinforced composites (FRCs) suffer from poor out-of-plane properties and delamination resistance. While advanced manufacturing techniques (e.g., interleaving, braiding, and z-pinning) increase delamination resistance in FRCs, they typically result in significant fabrication complexity and limitations, increased manufacturing costs, and/or overall stiffness reduction. In this work, we demonstrate the use of facile digital light processing (DLP) technique to additively manufacture (AM) random glass FRCs with engineered interleaves. This work demonstrates how vat photo-polymerization techniques can be used to build composites layer-by-layer with controlled interleaf material, thickness, and placement. Note that this engineering control is almost impossible to achieve with traditional manufacturing techniques. A range of specimens were printed to measure the effect of interleaf thickness and material on tensile/flexural properties as well as fracture toughness. One important observation was the ≈60% increase in interlaminar fracture toughness achieved by using a tough resin material in the interleaf. The comparison between AM and traditionally manufactured specimens via vacuum-assisted resin transfer molding (VARTM) highlighted the limitation of AM techniques in achieving high mat consolidation. In other words, the volume fraction of AM parts is limited by the wet fiber mat process, and engineering solutions are discussed. Overall, this technique offers engineering control of FRC design and fabrication that is not available with traditional methods.

Machine-Learning-Assisted Design of Highly Tough Thermosetting Polymers

Despite advances in machine learning for accurately predicting material properties, forecasting the performance of thermosetting polymers remains a challenge due to the sparsity of historical experimental data and their complicated crosslinked structures. We proposed a machine-learning-assisted materials genome approach (MGA) for rapidly designing novel epoxy thermosets with excellent mechanical properties (high tensile moduli, high tensile strength, and high toughness) through high-throughput screening in a vast chemical space. Machine-learning models were established by combining attention- and gate-augmented graph convolutional networks, multilayer perceptrons, classical gel theory, and transfer learning from small molecules to polymers. Proof-of-concept experiments were carried out, and the structures designed by the MGA were verified. Gene substructures affecting the modulus, strength, and toughness were also extracted, revealing the mechanisms of polymers with high mechanical properties. The developed strategy can be employed to design other thermosetting polymers efficiently.

Comparative Analysis of Erosive Wear Behaviour of Epoxy, Polyester and Vinyl Esters Based Thermosetting Polymer Composites for Human Prosthetic Applications Using Taguchi Design

In polymer composites, synthetic fibers are primarily used as a chief reinforcing material, with a wide range of applications, and are therefore essential to study. In the present work, we carried out the erosive wear of natural and synthetic fiber-based polymer composites. Glass fiber with jute and Grewia optiva fiber was reinforced in three different polymer resins: epoxy, vinyl ester and polyester. The hand lay-up method was used for the fabrication of composites. L16 orthogonal array of Taguchi method used to identify the most significant parameters (impact velocity, fiber content, and impingement angle) in the analysis of erosive wear. ANOVA analysis revealed that the most influential parameter was in the erosive wear analysis was impact velocity followed by fiber content and impingement angle. It was also observed that polyester-based composites exhibited the highest erosive wear followed by vinyl ester-based composites, and epoxy-based composites showed the lowest erosive wear. From the present study, it may be attributed that the low hardness of the polyester resulting in low resistance against the impact of erodent particles. The SEM analysis furthermore illustrates the mechanism took place during the wear examination of all three types of composites at highest fiber loading. A thorough assessment uncovers brittle fractures in certain regions, implying that a marginal amount of impact forces was also acting on the fabricated samples. The developed fiber-reinforced polymer sandwich composite materials possess excellent biocompatibility, desirable promising properties for prosthetic, orthopaedic, and bone-fracture implant uses.

Additive-Free and Support-Free 3D Printing of Thermosetting Polymers with Isotropic Mechanical Properties

The democratization of thermoplastic 3D printing is rooted in the ease of processing enabled by economical melting and shaping. Thermosetting polymers, on the other hand, have not enjoyed this advantage given that thermosetting resins cannot hold their shape without cross-linking or excessive fillers, and once cross-linked, they cannot be extruded for printing. Due to this formidable challenge, thus far, 3D printing of thermosetting polymers has been limited to the photopolymerization of specialized photosensitive resins or extrusion of resins loaded with large fractions (as high as 20 wt %) of rheology modifiers. Here, we report a rheology-modifier- and photoinitiator-free process for the 3D printing of a pure commercial epoxy polymer, without any resin modification and using a conventional 3D printer. A low-cost non-Newtonian support material that switches between solid-fluid states under a nozzle shear stress enables the printing of complex 3D structures and the subsequent and ″one-step″ curing. Our results show that the one-step curing eliminates the often-compromised interlayer adhesion common in layer-by-layer 3D printing processes and results in unprecedented isotropic mechanical properties (strength, elastic modulus, tensile toughness, and strain to failure). This in-bath print and cure (IBPC) 3D printing process for thermosetting polymers is low-cost, scalable, high-speed (nozzle speeds exceeding 720 cm/min), and high-resolution (down to 220 μm filament size). We demonstrate potential applications for hobbyists, structural and aerospace components, and fiber-reinforced composites, among others.

Enzyme-catalysed polymer cross-linking: Biocatalytic tools for chemical biology, materials science and beyond

Intermolecular cross-linking is one of the most important techniques that can be used to fundamentally alter the material properties of a polymer. The introduction of covalent bonds between individual polymer chains creates 3D macromolecular assemblies with enhanced mechanical properties and greater chemical or thermal tolerances. In contrast to many chemical cross-linking reactions, which are the basis of thermoset plastics, enzyme catalysed processes offer a complimentary paradigm for the assembly of cross-linked polymer networks through their predictability and high levels of control. Additionally, enzyme catalysed reactions offer an inherently 'greener' and more biocompatible approach to covalent bond formation, which could include the use of aqueous solvents, ambient temperatures, and heavy metal-free reagents. Here, we review recent progress in the development of biocatalytic methods for polymer cross-linking, with a specific focus on the most promising candidate enzyme classes and their underlying catalytic mechanisms. We also provide exemplars of the use of enzyme catalysed cross-linking reactions in industrially relevant applications, noting the limitations of these approaches and outlining strategies to mitigate reported deficiencies.

Interpretation and Physical Meaning of Kinetic Parameters Obtained from Isoconversional Kinetic Analysis of Polymers

Several successful examples—where physically sounded kinetic information was obtained from thermoanalytical data in different application fields, such as polymerization of thermosetting resins, biobased polymers and nanocomposites, crystallization and glass transition of semi-crystalline polymers and their nanocomposites—are here presented and discussed. It is explained how the kinetic parameters obtained from advanced isoconversional methods can be interpreted in terms of reaction mechanisms or changes in the rate-limiting step of the overall process, in the case of complex chemical reactions or complex physical transitions, and how these parameters can be used to extract model-fitting parameters.

Antibacterial Nanocomposites Based on Thermosetting Polymers Derived from Vegetable Oils and Metal Oxide Nanoparticles

Thermosetting polymers derived from vegetable oils (VOs) exhibit a wide range of outstanding properties that make them suitable for coatings, paints, adhesives, food packaging, and other industrial appliances. In addition, some of them show remarkable antimicrobial activity. Nonetheless, the antibacterial properties of these materials can be significantly improved via incorporation of very small amounts of metal oxide nanoparticles (MO-NPs) such as TiO2, ZnO, CuO, or Fe3O4. The antimicrobial efficiency of these NPs correlates with their structural properties like size, shape, and mainly on their concentration and degree of functionalization. Owing to their nanoscale dimensions, high specific surface area and tailorable surface chemistry, MO-NPs can discriminate bacterial cells from mammalian ones, offering long-term antibacterial action. MO-NPs provoke bacterial toxicity through generation of reactive oxygen species (ROS) that can target physical structures, metabolic paths, as well as DNA synthesis, thereby leading to cell decease. Furthermore, other modes of action-including lipid peroxidation, cell membrane lysis, redox reactions at the NP-cell interface, bacterial phagocytosis, etc.-have been reported. In this work, a brief description of current literature on the antimicrobial effect of VO-based thermosetting polymers incorporating MO-NPs is provided. Specifically, the preparation of the nanocomposites, their morphology, and antibacterial properties are comparatively discussed. A critical analysis of the current state-of-art on these nanomaterials improves our understanding to overcome antibiotic resistance and offers alternatives to struggle bacterial infections in public places.

Synthesis and Properties of Tung Oil-Based Unsaturated Co-Ester Resins Bearing Steric Hindrance

New tung oil (TO)-based, unsaturated, co-ester (Co-UE) macromonomers bearing steric hindrance were synthesized by modifying a TO-based maleate (TOPERMA) monomer with an anhydride structure with hydroxyethyl methacrylate (HEMA) and methallyl alcohol (MAA), respectively. The obtained Co-UE monomers (TOPERMA-HEMA and TOPERMA-MAA) were then characterized by ¹ H NMR and gel permeation chromatography (GPC). For comparison, hydroxyethyl acrylate (HEA)-modified TOPERMA (TOPERMA-HEA) was also synthesized and characterized. Subsequently, the obtained Co-UEs were thermally cured with styrene, and the ultimate properties of the resulting materials were studied. It was found that by introducing the structure of steric hindrance into the TO-based Co-UE monomer, the tensile strength and Young's modulus of the resulting materials were improved. Furthermore, by reducing the length of the flexible chain in the Co-UE monomer, the tensile strength, Young's modulus, and glass transition temperature (T_g) of the resultant materials were also improved. The TOPERMA-MAA resin gave the best performance in these TO-based Co-UE resins, which showed a tensile strength of 32.2 MPa, Young's modulus of 2.38 GPa, and T_g of 130.3 °C. The developed ecofriendly materials show promise in structural plastic applications.