Cure‐on‐demand wood adhesive based on the frontal polymerization of acrylates

Polymer Hydrogels and Frontal Polymerization: A Winning Coupling

Polymer hydrogels are 3D networks consisting of hydrophilic crosslinked macromolecular chains, allowing them to swell and retain water. Since their invention in the 1960s, they have become an outstanding pillar in the design, development, and application of engineered polymer systems suitable for biomedical and pharmaceutical applications (such as drug or cell delivery, the regeneration of hard and soft tissues, wound healing, and bleeding prevention, among others). Despite several well-established synthetic routes for developing polymer hydrogels based on batch polymerization techniques, about fifteen years ago, researchers started to look for alternative methods involving simpler reaction paths, shorter reaction times, and lower energy consumption. In this context, frontal polymerization (FP) has undoubtedly become an alternative and efficient reaction model that allows for the conversion of monomers into polymers via a localized and propagating reaction-by means of exploiting the formation and propagation of a "hot" polymerization front-able to self-sustain and propagate throughout the monomeric mixture. Therefore, the present work aims to summarize the main research outcomes achieved during the last few years concerning the design, preparation, and application of FP-derived polymeric hydrogels, demonstrating the feasibility of this technique for the obtainment of functional 3D networks and providing the reader with some perspectives for the forthcoming years.

Kinetic and Chemical Effects of Clays and Other Fillers in the Preparation of Epoxy-Vinyl Ether Composites Using Radical-Induced Cationic Frontal Polymerization

Addition of fillers to formulations can generate composites with improved mechanical properties and lower the overall cost through a reduction of chemicals needed. In this study, fillers were added to resin systems consisting of epoxies and vinyl ethers that frontally polymerized through a radical-induced cationic frontal polymerization (RICFP) mechanism. Different clays, along with inert fumed silica, were added to increase the viscosity and reduce the convection, results of which did not follow many trends present in free-radical frontal polymerization. The clays were found to reduce the front velocity of RICFP systems overall compared to systems with only fumed silica. It is hypothesized that chemical effects and water content produce this reduction when clays are added to the cationic system. Mechanical and thermal properties of composites were studied, along with filler dispersion in the cured material. Drying the clays in an oven increased the front velocity. Comparing thermally insulating wood flour to thermally conducting carbon fibers, we observed that the carbon fibers resulted in an increase in front velocity, while the wood flour reduced the front velocity. Finally, it was shown that acid-treated montmorillonite K10 polymerizes RICFP systems containing vinyl ether even in the absence of an initiator, resulting in a short pot life.

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

The synthesis and processing of most thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization is an attractive, scalable alternative due to its exploitation of polymerization heat that is generally wasted and unutilized. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully cured thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat- or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Numerous applications of frontally generated materials exist, ranging from porous substrate reinforcement to fabrication of patterned composites. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications.

Anisotropic frontal polymerization in a model resin-copper composite

This work investigates experimentally and numerically frontal polymerization in a thermally anisotropic system with parallel copper strips embedded in 1,6-hexanediol diacrylate resin. Both experiments and multiphysics finite element analyses reveal that the front propagation in the thermally anisotropic system is orientation-dependent, leading to variations in the front shape and the front velocity due to the different front-metal strip interaction mechanisms along and across the metal strips. The parameters entering the cure kinetics model used in this work are chosen to capture the key characteristics of the polymerization front, i.e., the front temperature and velocity. Numerical parametric analyses demonstrate that the front velocity in the directions parallel and perpendicular to the metal strips increases as the system size decreases and approaches the analytical prediction for homogenized systems. A two-dimensional homogenized model for anisotropic frontal polymerization in the metal-resin system is proposed.

High-performance water-borne fluorescent acrylic-based adhesive: synthesis and application

Water-borne adhesives have immense importance in cellulose-based materials, where their durability, handling, and strength remain to be a major concern. The present work demonstrates the development of three water-borne adhesives, namely, poly(1-vinyl-2-pyrrolidone-co-acrylic acid), poly(acrylonitrile-co-acrylic acid), and poly(1-vinyl-2-pyrrolidone-co-acrylonitrile-co-acrylic acid) applicable for cellulose-based materials. These acrylic-acid based adhesives were characterized by Fourier-transform infra-red spectroscopy, thermogravimetric analysis, X-ray diffraction, gel permeation chromatography, and universal testing machine. The synthesized polymer adhesives can be stored in the powder form for a longer period, thus utilizing less space. In order to use as adhesives, suitable formulations can be prepared in water. The adhesives show thermal stability up to 300 °C. Our studies show that poly(1-vinyl-2-pyrrolidone-co-acrylonitrile-co-acrylic acid) showed higher lap shear strength (ASTM D-906) than commercially available adhesives. In addition, these adhesives, being fluorescent in nature, can be detected under UV light and thus are applicable for the detection of fractured joints of any specimen. This property also helps in anti-counterfeiting applications, thus adding further to their utility.

Synthesis and application of a water-borne fluorescent acrylic adhesive, which can be stored as a powder for long-term use.

Rapid Synthesis of Elastomers and Thermosets with Tunable Thermomechanical Properties

Rapid, solvent-free synthesis of poly(1,4-butadiene) in ambient conditions is demonstrated by frontal ring-opening metathesis polymerization (FROMP) of 1,5-cyclooctadiene (COD). Furthermore, cross-linked copolymers with a wide range of tunable properties are readily prepared by FROMP of mixtures of COD and dicyclopentadiene (DCPD). Specifically, glass transition temperature and tensile modulus are varied from -90 to 114 °C and 3.1 MPa to 1.9 GPa, respectively, by controlling the comonomer ratio. Copolymers with subambient glass transition temperature exhibit robust elastomeric behavior, with the ability to repeatedly recover from large elastic deformations. As a demonstration of the capability of this manufacturing strategy, gradient materials are fabricated in less than a minute with spatially controlled properties for multistage shape memory actuation. This simple yet powerful manufacturing strategy enables rapid synthesis of copolymers ranging from elastomers to thermosets with precise control over thermomechanical properties.

Frontal Polymerization of a Thin Film on a Wood Substrate

Frontal polymerization has been explored as a technique to form two-dimensional thin films (<0.5 mm) on wood. We used trimethylolpropane triacrylate with a thermal free-radical initiator. The viscosity of the resin was adjusted by incorporating fumed silica within the formulation. As filler materials, either calcium carbonate or graphene nanoplatelets was used to evaluate the effect of filler type and content on front propagation. We observed that resin viscosity and film thickness critically affected the qualitative and quantitative propagation of the thermal front resulting in the formation of the coating. A workable coating was formed at a viscosity of 0.6 Pa·s, which was obtained when 3 phr (parts per hundred resin) of fumed silica was used in the resin formulation. Wet film thickness for this resin system was also found to have a limiting value, and full propagation of the front to result in a conformal coating required at least 15 mil (1 mil = 25 μm) of wet film thickness. Filler materials affected film propagation as a function of particle size and thermal properties. While 15 phr calcium carbonate could be incorporated with the resin, only 5 phr graphene nanoplatelets could be loaded within the matrix to ensure complete propagation of the front. Interestingly, for graphene fronts, velocity and temperature reduced systematically as a function of filler content. Filler type and content affected porosity and roughness of the coating, which was quantified by computerized tomography to understand the relationship between porosity and adhesion of the coated film with the wood substrate.