Thermoset Matrix Research Articles

Preparation of fluorinated epoxy‐phthalonitrile resins with excellent thermal stability and low dielectric constant

AbstractFluoropolymers find applications in heat‐resistant cables, chemical‐resistant linings, electronic components, cladding materials, and weather‐resistant films. Therefore, it is imperative to improve their temperature resistance level and dielectric properties. In this study, a series of new fluorinated epoxy‐phthalonitrile resins with different mass ratios were prepared by adding phthalonitrile to the epoxy resin matrix, followed by a two‐step reaction of the amine with the epoxy resin at low temperature, and then by the reaction of the nitrile with the epoxy resin and the nitrile group at high temperature. The thermal stability and thermal oxidation stability of the cured products were improved; the initial decomposition temperature for 5% weight loss in air was 375.3°C, indicating good heat resistance performance. In addition, the glass transition temperature and storage modulus of the fluorinated epoxy‐phthalonitrile resins cured products increased with an increase in phthalonitrile content. The storage modulus remained above 1500 MPa until 150°C. The glass transition temperature of fluorinated epoxy‐phthalonitrile resins (at a mass ratio of 5:5) was 180°C, much higher than that of the epoxy resin (which was 140°C). Moreover, the dielectric constant of fluorinated phthalonitrile‐epoxy resin (5:5 mass ratio) was 2.01, which was 39.63% lower than that of fluorinated epoxy resin. The thermoset matrix has potential applications in the fabrication of a variety of low dielectric constant composites for electronic device related industries.

3D Printing of continuous fiber composites using two-stage UV curable resin.

3D printing allows for moldless fabrication of continuous fiber composites with high design freedom and low manufacturing cost per part, which makes it particularly well-suited for rapid prototyping and composite product development. Compared to thermal-curable resins, UV-curable resins enable the 3D printing of composites with high fiber content and faster manufacturing speeds. However, the printed composites exhibit low mechanical strength and weak interfacial bonding for high-performance engineering applications. In addition, they are typically not reprocessable or repairable; if they could be, it would dramatically benefit the rapid prototyping of composite products with improved durability, reliability, cost savings, and streamlined workflow. In this study, we demonstrate that the recently emerged two-stage UV-curable resin is an ideal material candidate to tackle these grand challenges in 3D printing of thermoset composites with continuous carbon fiber. The resin consists primarily of acrylate monomers and crosslinkers with exchangeable covalent bonds. During the printing process, composite filaments containing up to 30.9% carbon fiber can be rapidly deposited and solidified through UV irradiation. After printing, the printed composites are subjected to post-heating. Their mechanical stiffness, strength, and inter-filament bonding are significantly enhanced due to the bond exchange reactions within the thermoset matrix. Furthermore, the utilization of the two-stage curable resin enables the repair, reshaping, and recycling of 3D printed thermosetting composites. This study represents the first detailed study to explore the benefits of using two-stage UV curable resins for composite printing. The fundamental understanding could potentially be extended to other types of two-stage curable resins with different molecular mechanisms.

Assessing novel thermoset materials from Brazil nut oil: Curing, thermal and mechanical properties

Assessing novel thermoset materials from Brazil nut oil: Curing, thermal and mechanical properties

Recyclable thermosets based on modified epoxy-amine network polymers.

The development of high performance, recyclable thermoset materials for applications in plastics, composites, coatings and adhesives requires a synthetic approach where recyclability is designed into the molecular structure of the material. This paper describes a single stage process for the creation of materials from simple, low-cost molecular building blocks, where the polymerisation of liquid epoxy resins and aliphatic amines in the presence of an n-butyl diboronic ester, delivers epoxy-amine-dioxazaborocane materials with tunable physical properties including glass transition temperature (Tg). Mechanical (thermal) recycling and reprocessing of the epoxy-amine-dioxazaborocane thermoset is demonstrated, with retention of Young's modulus and ultimate tensile strength. Most notably, an efficient and low-cost process for the chemical recycling, disassembly and dissolution of the thermoset is demonstrated via two complementary processes using either pinacol (diol) or mono-functional phenylboronic ester.

Expanding lignin thermal property space by fractionation and covalent modification.

To fully exploit kraft lignin's potential in material applications, we need to achieve tight control over those key physicochemical lignin parameters that ultimately determine, and serve as proxy for, the properties of lignin-derived materials. Here, we show that fractionation combined with systematic (incremental) modification provides a powerful strategy to expand and controllably tailor lignin property space. In particular, the glass transition temperature (Tg) of a typical kraft lignin could be tuned over a remarkable and unprecedented 213 °C. Remarkably, for all fractions the Tg proved to be highly linearly correlated with the degree of derivatisation by allylation, offering such tight control over the Tg of the lignin and ultimately the ability to 'dial-in' this key property. Importantly, such control over this proxy parameter indeed translated well to lignin-based thiol-ene thermosetting films, whose Tgs thus covered a range from 2-124 °C. This proof of concept suggests this approach to be a powerful and generalisable one, allowing a biorefinery or downstream operation to consciously and reliably tailor lignins to predictable specifications which fit their desired application.

Comparing the Principles of Reversible Covalent Chemistry and Supramolecular Chemistry Points to New Directions in the Development of Polymers

Polymers are some of the most widely used materials for human use and have greatly facilitated people's lives. However, with the use of polymer materials, traditional thermoplastic and thermoset materials are unable to meet the more diverse needs, and traditional processing methods are not able to significantly improve the performance of polymer materials. Some researchers have found that by applying the principles of reversible covalent and supramolecular chemistry in dynamic chemistry in the development of polymers, the properties and functions of polymers can be changed from the bottom up. Therefore, this paper analyses the similarities and differences between the principles of reversible covalent chemistry and supramolecular chemistry by collecting applications of reversible covalent chemistry and supramolecular chemistry in the field of polymer synthesis and comparing the two to provide assistance for future developments in the field of polymers.

Use of different kinds of waste in the construction of new polymer composites: review

Waste valorization is one of the current research areas that has attracted a great deal of attention over the past few years as a potential alternative to the disposal of a wide range of residues in landfill sites. In particular, the development of environmentally friendly and innovative strategies to process several kinds of wastes is an area of increasing importance in our current society. Landfill, incineration, and composting are common, mature technologies for waste disposal. However, they are not satisfactory due to the generation of toxic methane gas and bad odors, high energy consumption, and slow reaction kinetics. The scope of this manuscript is to give a clear overview on the recent development of waste valorization strategies in polymeric matrices for the sustainable production of new polymer matrix composites. In other words, this work should be regarded as a review, providing guidelines for the sustainable development of various industries for the benefit of the society. In fact, the incorporation of waste and/or by-products into polymer composites may represent a new resource and a successful approach to materials suitable for specific applications that promote a circular economy approach. According to this study, there is a considerable amount of research on the application of solid wastes, in particular agricultural wastes, as fillers for thermoplastic and thermoset matrices to obtain composites suitable for application in components subjected to light to moderate loads. As a main drawback, surface pre-treatment is necessary in many cases to increase the adhesion between matrix and fillers and the final mechanical, thermal, and chemical properties of polymer composites.

Novel reactive benzoxazine resins using renewables: Leveraging inherent chemistries to produce novel thermoset materials

Novel thermoset tannin-based oxazolidine materials have been prepared in a one pot reaction combining condensed tannins with N,N-alkyldiamines and various aldehydes. Reaction stoichiometry was important in O,N-oxazole hetereocycle formation with N-methylene coupling and ammonium ion formation also observed. For this tannin-diamine system, a tannin :1,6-hexanediamine:formaldehyde mole ratio of 1:1:2 was favoured for oxazole product formation, cure and material properties. Thermal analysis (DSC, DMTA) established that these materials exhibit benzoxazole crosslinking reactivity, with higher alkyldiamine contents found to contribute to greater cure onset temperatures that ranged between 130 and 180 °C. Consolidated, crosslinked tannin-diamine materials exhibit consistent mechanical properties at elevated temperatures (>200 °C). Tannin-diamine materials could be combined with wood fibre , either as a powder or aqueous dispersion, to give composite materials (30–40 % fibre fractions, o.d.) with similar high temperature mechanical properties. Substitutions of glyoxal or furfural for formaldehyde produced comparable tannin-diamine products exhibiting similar material performance when hot-pressed. These tannin-based benzoxazoles have potential for utilisation as a matrix or binder with natural fibre reinforcements and composite materials. • Tannin-based oxazolidine materials have been synthesised via a one pot reaction. • Condensed tannins react with N,N-alkyldiamines and aldehydes to form O,N-oxazoles. • O,N-oxazoles are formed via N-methylene coupling and ammonium ion formation. • Tannin-based benzoxazoles exhibit similar crosslinking as synthetic oxazole systems. • Tannin-diamines can be consolidated either pure or with fibre to form composites.

Silane and silazane surface modification of recycled glass fibers for polypropylene composites

AbstractGlass fiber (GF) composites are one of the significant challenges in recycling thermoset materials. After pyrolysis, the glass fibers lack sufficient strength and show poor matrix compatibility. Here we have investigated a series of multifunctional silane and silazane agents for surface modification of recycled glass fibers that provide a combination of hydrophobic properties and residual reactive groups on the surface. This allowed testing of interfacial effects from the surface modification as well as a potential synergistic compatibilization using maleated PP (MAPP). The treated GFs were used to prepare new polypropylene (PP) composites by multiple extrusion steps, resulting in a series of composites where the dispersion efficiency was attributed mainly to the surface chemistry and compatibilization effects. The amino‐silane modifications of the recycled fibers resulted in further improvements in the mechanical properties of the PP composites in comparison with the hydrophobized GFs. Moreover, synergistic effects from the addition of MAPP were observed with scanning electron microscopy. The results clearly demonstrate that the surface modifications were effective and good alternatives to currently used methods.

Competing Effects of Cohesive Energy and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

To develop structure–property relationships for cross-linked thermosetting polymers, it is crucial to better understand key factors that control their segmental dynamics and macroscopic properties. Here, we employ a coarse-grained (CG) polymer model to systematically explore the combined effect of varying the cohesive energy (ε) and cross-link density (c) on the segmental relaxation time and mechanical properties for a model cross-linked glass-forming thermoset material. We find that increasing c increases both the glass transition temperature Tg and fragility of glass formation, while the fragility decreases with an increase in ε. These competing effects of ε and c on fragility are practically important since fragility determines the overall temperature width of the glass formation over which the non-Arrhenius temperature dependence is observed. Our simulation results show that the basic mechanical properties (i.e., bulk and shear moduli) of cross-linked thermosets are mainly influenced by ε. More interestingly, the macroscopic mechanical properties are found to be strongly correlated with the Debye–Waller parameter ⟨u2⟩, a measure of material “stiffness” at a molecular level. In particular, the distribution of local molecular stiffness, 1/⟨u2⟩, exhibits a nearly universal Gaussian distribution at a fixed reduced temperature T/Tg. Our work reveals the key and competitive roles of cohesive energy and cross-link density in controlling the segmental dynamics, large scale, and local mechanical properties of cross-linked thermosets, providing an understanding that should be useful in the molecular design of these materials.

Characterization of Polyurethane Shape Memory Polymer and Determination of Shape Fixity and Shape Recovery in Subsequent Thermomechanical Cycles.

Multifunctional polyurethane shape memory polymers (PU-SMPs) have been of increasing interest in various applications. Here we report structure characterization, detailed methodology, and obtained results on the identification of functional properties of a thermoset PU-SMP (MP4510) with glass transition temperature of 45 °C. The stable, chemically crosslinked network of this thermoset PU-SMP results in excellent shape memory behavior. Moreover, the proximity of the activation temperature range of this smart polymer to room and body temperature enables the PU-SMP to be used in more critical industrial applications, namely fast-response actuators. The thermomechanical behavior of a shape memory polymer determines the engineering applications of the material. Therefore, investigation of the shape memory behavior of this class of commercial PU-SMP is of particular importance. The conducted structural characterization confirms its shape memory properties. The shape fixity and shape recovery properties were determined by a modified experimental approach, considering the polymer's sensitivity to external conditions, i.e., the temperature and humidity variations. Three thermomechanical cycles were considered and the methodology used is described in detail. The obtained shape fixity ratio of the PU-SMP was approximately 98% and did not change significantly in the subsequent cycles of the thermomechanical loading due to the stability of chemical crosslinks in the thermoset materials structure. The shape recovery was found to be approximately 90% in the first cycle and reached a value higher than 99% in the third cycle. The results confirm the effect of the thermomechanical training on the improvement of the PU-SMP shape recovery after the first thermomechanical cycle as well as the effect of thermoset material stability on the repeatability of the shape memory parameters quantities.

Bench-scale fuel fire test for materials of rechargeable energy storage system housings

The fire behaviour of electric vehicles (EVs) differs from that of vehicles with combustion engines. Especially the rechargeable energy storage system (REESS) requires special fire protection measures. The fire behaviour of materials for REESS housings plays an important role in the fire resistance of such systems. Full-scale fire resistance tests like gasoline pool fire tests on complete REESS according to the UNECE-R100-8E standard are mandatory for EVs. However, these tests are not applicable for materials used for REESS housings in the material and process development state, due to the high material demand and production costs of the REESS. Standard tests like the limiting oxygen index test, UL94-V test or cone calorimeter test are insufficient to analyse the fire behaviour of thermoplastic and thermoset materials in a gasoline pool fire. This paper describes a bench-scale fuel fire test including several test criteria to evaluate materials for REESS housings on a laboratory scale. This bench-scale fire test is demonstrated on two case studies: fibre-reinforced thermoset plates and thermoplastic sandwich structures.

High content chitosan-based materials with high performance properties

Chitosan is a valuable biopolymer with a great potential to be used in the design of sustainable materials. Its use typically requires converting the solid powder into a quite dilute solution by disrupting the hydrogen bonding between primary amine and hydroxyl groups. In this work we show that chitosan can be reacted with a tris-aromatic tris-epoxy monomer, generating thermoset materials. The design of the new structures adopted a strategy where the chitosan was mixed in its solid form, to avoid the use of solvents and additional processing steps. A combined polymerization mechanism was proposed, including growth chain polymerization and polyaddition. The obtained materials containing different epoxy/chitosan weight percentage ratios show outstanding properties: high glass transition ~230 °C, high Young's modulus ~2116 and 1716 MPa, tensile strength of ~35 MPa and T5% ~ 300 °C.

Additive Manufacturing of Degradable Materials via Ring-Opening Metathesis Polymerization (ROMP).

Thermoset materials comprise a significant proportion of high-performance plastics due to their shape permanence and excellent thermal and mechanical properties. However, these properties come at the expense of degradability. Here, we show for the first time that the industrial thermoset polydicyclopentadiene (PDCPD) can be additively manufactured (AM) with degradable 2,3-dihydrofuran (DHF) linkages using a photochemical approach. Treatment of the manufactured objects with acid results in rapid degradation to soluble byproducts. This work highlights the potential of ring-opening metathesis polymerization (ROMP) chemistry to create degradable materials amenable to advanced manufacturing processes.

Facile Catalyst‐Free Approach toward Fully Biobased Reprocessable Lignin Thermosets

AbstractReprocessable and biobased thermosets are prepared from two renewable feedstocks, lignin and polyamine (Priamine 1071). Lignin is oxidized to produce polycarbonyl and further reacts with polyamine to form a crosslinked network of imines. The thermoset materials are formed under heat and pressure in the absence of any catalysts. The mechanical strength and thermal properties of thermosets are tunable by changing feedstock ratios. The dynamic imine crosslinks can be associatively reformed.

Recycling of Thermoset Materials and Thermoset-Based Composites: Challenge and Opportunity.

Thermoset materials and their composites are characterized by a long life cycle with their main applications in aircrafts, wind turbines and constructions as insulating materials. Considering the importance of recovery and valorization of these materials at their end-of-life, avoiding landfilling, the interest concerning their recycling grows continuously. The thermoset materials and their composites, to be successfully recovered and valorized, must degrade their three-dimensional structures and recover the mono-oligomers and/or fillers. The thermoset materials could successfully degrade through thermal treatment at different temperatures (for example, above 1000 °C for incineration, ca. 500 °C for oxidation/combustion of organic constituents, etc.), chemical degradation by catalyst, irradiation with or without the presence of water, alcohol, etc., and mechanical recycling, obtaining fine particles that are useful as filler and/or reinforcement additives. Among these recycling methods, this mini-review focuses on the formulation and recovery method of innovative thermoset with in-build recyclability, i.e., materials having chemical links that could be degraded on-demand or containing dynamic covalent bonds to have re-processable and/or recyclable thermoset. This issue could be considered the future perspective in developing novel thermoset materials. The aim of this review is to get an overview of the state of the art in thermoset recycling and of the most commonly used thermoset composites, recovering valuable reinforcing fibers. Additionally, in this work, we also report not only known recycling routes for thermoset and thermoset-based composites, but also new and novel formulating strategies for producing thermosets with built-in recyclability, i.e., containing chemical-triggered on-demand links. This mini-review is also a valuable guide for educational purposes for students and specialized technicians in polymer production and recycling.

Oxidative Degradation of Thermosets Based on Thioketal Cleavable Linkages in Aqueous Environment

Thermosets are rigid, infusible, and unmolded materials containing three-dimensional (3D) cross-linked structures. They are considered a fundamental pillar in the international economy, which are produced by 65 million tons annually. The responsive cross-linking moieties provide the thermosets characterized with outstanding physicochemical properties such as stiffness, degradability, and chemical and thermal resistance. We prepared degradable thermoset materials using thioketal (TK) cross-linkers, which underwent main-chain or side change degradation in the presence of hydrogen peroxide in water. TK cross-linkers at different concentrations (5, 10, and 20% wt %) were polymerized with 2-hydroxyethyl acrylate (HEA) or with 2-hydroxyethyl methacrylate (HEMA) and 1-vinyl-2-pyrrolidone (PD) to produce cross-linked poly(HEA) and poly(HEMA-PD) by free radical polymerization, respectively. The resultant polymer materials completely degraded in hydrogen peroxide/water (3–30%, vol). Using isophorone diisocyanate, we also produced degradable polyurethane based on TK-bearing diol. We prepared a 3D degradable thermoset using the Direct-Ink-Writing (DIW) 3D printing technology, which was charged by diethylene glycol diacrylate (15%, wt %) and a prepolymer (isophorone diisocyanate terminated by acrylate moieties) containing diol-thioketal linkage (15%, wt %). Finally, we found that TK-poly(HEA) underwent microbial degradation by Lactobacillus jensenii at 37 °C, which indicates a benign eco-friendly effect.

Closed‐loop recycling of thermoset composites from electric motor assembly process into flexible polyurethane foams

AbstractPolyurethane foams (PU) are used in many automotive applications due to their durability and sound absorption properties. This paper examines the effects of reusing predominantly silica‐filled thermoset composites as a reinforcing agent when ground to a particle size less than 106 μm (0%, 2.5%, 5%, 7.5% by wt%) on mechanical, physical, thermal, and sound absorption properties of PU foam. Young's modulus improved by 27%, and tensile strength increased by 18% with 7.5% reinforcement. Compression stress improved at 25%, 50%, and 65% strain by 37%, 41%, and 44%, respectively, with the 7.5% filler. Compression modulus increased by 66% with 7.5% filler. The addition of the filler did not impact tensile extension or tear resistance. TGA showed increased thermal stability, and the sound absorption coefficient increased over a spectrum of frequencies. This research demonstrates a promising recycling opportunity for thermoset materials and how Ford Motor Company can create a closed‐loop recycling process from an assembly aid material into another vehicle application.

Composites based on biogas digestate

The extraction of fibres from digestate can enhance the profitability of biogas stations. Furthermore, fibre from digestate can be an environmentally friendly alternative to commonly used fibres. In this study a nonwoven of the solid fibrous components of biogas digestate was used. The nonwoven was infiltrated and consolidated with a biobased thermoset matrix by a hot-pressing process. Multiple processing parameters were varied (matrix amount, pressing conditions) to evaluate mechano-physical properties. A pressure of 4.5MPa resulted in a high-quality composite with a matrix ratio of 47% by weight. A young’s modulus of approximately 5500MPa and a strength of approximately 70MPa is reached. Different applications are conceivable, e.g. indoor furniture or decorative surfaces.

Thermoset nanocomposites reinforced by vinyl‐functionalized halloysite

AbstractIn the current study, naturally occurring halloysite nanotube (HNT) is first modified by vinyltriethoxysilane to obtain vinyl‐functionalized HNT (HNT‐Vinyl). Subsequently, the HNT‐Vinyl is added as nanofiller by varying weight from 0 to 8% in the presence of multifunctional methacrylates (bisphenol A‐dimethacrylate and triethylene glycol dimethacrylate). Free radical photocrosslinking polymerization of these materials provides the in‐situ formation of HNT/thermoset nanocomposites. Comparison of characteristic bands of the nanocomposite components by Fourier transform infrared spectroscopy confirms the presence and covalent bonding of HNT nanofiller in the thermoset matrix. Thermal properties of obtained HNT/thermoset nanocomposites have been compared with neat thermoset as a reference sample using thermogravimetric analysis. Moreover, the existence and dispersion of nanofillers in the nanocomposites have been proved by scanning electron microscopy and transmission electron microscopy. The HNT nanofiller has been detected as agglomerated/non‐agglomerated morphology in the thermoset matrix. On the other hand, the addition of HNT‐Vinyl to the thermoset matrix has improved the mechanical properties of nanocomposites. Especially, a dramatic increase on tensile strength values has been determined with 4% HNT‐Vinyl loading. Tensile strength and elastic modulus values of the corresponding nanocomposite are 4.9 and 1.5 times higher than those of the neat thermoset.