Bio-based epoxy vitrimer with inherent excellent flame retardance and recyclability via molecular design

The development of biobased fire-safe thermosets with recyclability heralds the switch for a transition towards a circular economy. In this framework, we introduced a novel high-performance bio-epoxy vitrimer (named GVD), which was fabricated by forming a crosslinking network between bio-epoxy glycerol triglycidyl ether (Gte), varying amounts of reactive flame-retardant agent 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) (0-7 wt%) and a vanillin-based hardener (VA) with imine bonds. For instance, the epoxy vitrimer GVD5, featuring a DOPO content of 5 wt%, achieved a V-0 rating in the vertical burning test (UL-94) and obtained a limiting oxygen index (LOI) value of 31 %, surpassing the performance of pristine epoxy. Furthermore, the peak heat release rate and total heat release of GVD5 were reduced by 38.2 % and 26.3 %, respectively, compared to pristine epoxy. The GVD vitrimers further demonstrated exceptional reprocessability and recyclability, attributed to the presence of dynamic imine bonds within the topological crosslinking network. Remarkably, the epoxy vitrimers maintained the mechanical properties of the parent epoxy. Therefore, this work provides a facile strategy for fabricating high-performance and multi-functional bio-epoxy thermosets.

Electrical and Joule Heating Capabilities of Multifunctional Coatings based on Recycled Carbon Fiber from Prepreg Scrap

The continuous growth in the use of preimpregnated semielaborated products to manufacture continuous carbon fiber-reinforced polymer parts has led the industry to face an important challenge in the management of the prepreg scrap, as the amount of waste produced can reach almost 75% due to the inefficiency of the cutting phase. In this context, this industry is pushed to move toward a circular economy approach by conferring a new use to their waste. The main problem arises from the fact that shortening carbon fiber leads to nonefficient mechanical reinforcement and that other thermal or chemical recycling approaches are environmentally hazardous. In this work, mechanical recycling of carbon fiber prepregs from expired virgin prepregs or scrap from an automated manufacturing operation is proposed as an economical and environmentally efficient method to obtain multifunctional coatings with Joule effect heating capabilities, which are demanded by different industries as a high-value product. As a coating, mechanical performance is not so relevant; nevertheless, obtaining high electrical conductivity by the incorporation of proper size and distributed short recycled carbon fiber can lead to a self-heating coating that could be used for anti- and deicing purposes or any other thermally activated function with very low power consumption. In this way, electrical conductivities up to 2 S/m were obtained, which allowed for achieving temperatures of 200 °C by the Joule effect in all samples in less than 17 s by the application of voltages below 48 V for bulk materials and 100 V for the coating.

Recent Progress on Multifunctional Thermally Conductive Epoxy Composite

In last years, the requirements for materials and devices have increased exponentially. Greater competitiveness; cost and weight reduction for structural materials; greater power density for electronic devices; higher design versatility; materials customizing and tailoring; lower energy consumption during the manufacturing, transport, and use; among others, are some of the most common market demands. A higher operational efficiency together with long service life claimed. Particularly, high thermally conductive in epoxy resins is an important requirement for numerous applications, including energy and electrical and electronic industry. Over time, these materials have evolved from traditional single-function to multifunctional materials to satisfy the increasing demands of applications. Considering the complex application contexts, this review aims to provide insight into the present state of the art and future challenges of thermally conductive epoxy composites with various functionalities. Firstly, the basic theory of thermally conductive epoxy composites is summarized. Secondly, the review provides a comprehensive description of five types of multifunctional thermally conductive epoxy composites, including their fabrication methods and specific behavior. Furthermore, the key technical problems are proposed, and the major challenges to developing multifunctional thermally conductive epoxy composites are presented. Ultimately, the purpose of this review is to provide guidance and inspiration for the development of multifunctional thermally conductive epoxy composites to meet the increasing demands of the next generation of materials.

An Analysis of the Effect of Activation Temperature and Crack Geometry on the Healing Efficiency of Polycaprolactone (PCL)/Epoxy Blends

Self-healing materials have attracted great interest in recent years. Particularly, the use of thermoset/thermoplastics blends has emerged as a good option with relatively low activation temperatures and potential infinite healing cycles. Nevertheless, a methodical study of healing conditions and evaluation is still required for further industrial development. The effect of activation temperature and crack morphology in polycaprolactone (PCL)/epoxy blends are explored. For this purpose, PCL content was varied (5, 10, and 15 wt %) with contents lower than critical composition. Therefore, the morphology of all studied blends is the epoxy matrix with a separated PCL phase. In this sense, an increase in PCL content leads to a reduction in the Tg, due to the partial PCL miscibility, and the presence of larger PCL domains. It was observed that a higher temperature (150 °C) and PCL content led to a more efficient self-healing process because of both the lower viscosity of the melted PCL at higher temperatures and the presence of larger PCL reservoirs when increasing the PCL content. Crack morphology influence was studied by inducing cracks with different tools: a custom crack machine with a cutting blade, a scalpel, and a pin. The results show that the recovery was better when the cracks were smaller and shallower, that is, with the pin. In addition, the healing efficiency by means of both parameters, crack volume and depth change, showed more similar results in slimmer cracks, due to a lower crack width-to-depth ratio.

New Manufacturing Process of Composites Reinforced with ZnO Nanoparticles Recycled from Alkaline Batteries

A new manufacturing method of thermosetting resins reinforced with dense particles is developed in the present work. A rotary mold is used, avoiding the natural sedimentation of particles through applying centrifuge forces. A deep study of the sedimentation phenomenon is carried out in order to evaluate the main experimental parameters which influence the manufacturing of composite. The used reinforcement is zinc oxide (ZnO) obtained by a new recycling method from spent alkaline batteries. In order to compare the benefits, commercial ZnO nanoparticles are also analyzed. Recycled ZnO particles enhance the interaction of the epoxy matrix due to their inner moisture, allowing the manufacture of composites with relatively high ceramic content. Moreover, an increment in the glass transition temperature of the epoxy matrix and in the mechanical properties, such as its stiffness and hardness, is achieved.

Influence of Morphology on the Healing Mechanism of PCL/Epoxy Blends

Polycaprolactone (PCL) is being researched as a self-healing agent blended with epoxy resins by several reasons: low melting point, differential expansive bleeding (DBE) of PCL, and reaction induced phase separation (RIPS) of PCL/epoxy blends. In this work, PCL/epoxy blends were prepared with different PCL ratios and two different epoxy networks, cured with aliphatic and aromatic amine hardeners. The curing kinetic affects to the blend morphology, varying its critical composition. The self-healing behavior is strongly affected by the blend morphology, reaching the maximum efficiency for co-continuous phases. Blends with dispersed PCL phase into epoxy matrix can also show high self-healing efficiency because of the low PCL domains that act as reservoir of self-healing agent. In this last case, it was confirmed that the most efficient self-healable blends are one whose area occupied by PCL phase is the largest. These blends remain the good thermal and mechanical behavior of epoxy matrix, in contrast to the worsened properties of blends with bicontinuous morphology. In this work, the self-healing mechanism of blends is studied in depth by scanning electron microscopy. Furthermore, the influence of the geometry of the initial surface damage is also evaluated, affecting to the measurement of self-healing efficiency.

Synthesis and characterisation of epoxy resins reinforced with carbon nanotubes and nanofibers

Epoxy nanocomposites were fabricated using two kinds of nanofiller, amino-functionalized multi-walled carbon nanotubes (CNTs) and non-treated long carbon nanofibers (CNFs). The non-cured mixtures were analysed through viscosity measurements. The effect of the nanoreinforcement on the curing process was determined by differential scanning calorimetry. Finally, the characterisation of cured nanocomposites was carried out studying their thermo-mechanical and electrical behaviour. At room temperature, the addition of CNTs causes a viscosity increase of epoxy monomer much more marked than the introduction of CNFs due to their higher specific area. It was probed that in that case exists chemical reaction between amino-functionalized CNTs and the oxirane rings of epoxy monomer. The presence of nanoreinforcement induces a decrease of curing reaction rate and modifies the epoxy conversion reached. The glass transition temperature of the nanocomposites decreases with the contents of CNTs and CNFs added, which could be related to plasticization phenomena of the nanoreinforcements. The storage modulus of epoxy resin significantly increases with the addition of CNTs and CNFs. This augment is higher with amino-functionalized CNTs due, between other reasons, to the stronger interaction with the epoxy matrix. The electrical conductivity is greatly increased with the addition of CNTs and CNFs. In fact, the percolation threshold is lower than 0.25 wt% due to the high aspect ratio of the used nanoreinforcements.