Development of Bio-Based and Recyclable Epoxy Adhesives by Modification with Thermoplastic Polymers

This paper deals with the design of novel epoxy adhesives by incorporating thermoplastic polymers such as polyetherimide (PEI) and poly(ε-caprolactone) (PCL) into a bio-based and recyclable epoxy resin, known as Polar Bear. The adhesives were characterized by their mechanical (quasi-static and dynamic) and rheological properties, thermal stability, and adhesion properties in single-lap joints tested at three different temperatures (i.e., -55 °C, 23 °C, 80 °C). The experimental results indicated that low PEI content substantially improved the mechanical performance and toughness of the adhesive, while preserving good processability. Nonetheless, exceeding 3% weight percentage adversely affected the adhesives' mechanical resistance and workability. Conversely, while PCL addition enhanced the adhesives' viscosity, it also decreased mechanical performance. However, its eco-friendliness offers potential for sustainable adhesive applications. It is worth noting that regardless of temperature, the modified adhesives consistently outperformed the commercial epoxy adhesive (DP-460), used as reference, in single-lap shear joint tests. Additionally, both PEI- and PCL-modified epoxy adhesives have demonstrated recyclability through a simple acid-based process, enabling joint disassembly and recycling of the adhesive into a thermoplastic polymer. Overall, the modified adhesives represent a promising eco-friendly, high-performance alternative for structural applications, aligning with sustainable and circular practices.

Quantum Chemical Model Calculations of Adhesion and Dissociation between Epoxy Resin and Si-Containing Molecules

There is no doubt that when solid surfaces are modified, the functional groups and atoms directly bonded to solid atoms play a major role in adsorption interactions with molecules or resins. In this study, the adhesion and dissociation between epoxy resin and molecules containing Si atoms were analyzed. The analysis, conducted in contact with the solid surface of silicon, utilized quantum chemical calculations based on a molecular model. We compared some Si-containing molecular models to test quantum chemical calculations that contribute to the study of adhesion and dissociation between epoxy resins and solid surfaces somehow other than simple potential energy curve calculations. The AFIR (artificial force induced reaction) method, implemented in the GRRM (global reaction route mapping) program, was employed to separate an epoxy resin model molecule and three types of silicon compounds (Si(CH3)2(OH)2, Si(CH3)4, and (CH3)2SiF2) in three directions, determining their minimum dissociation energy when changing the applied energy by 2.5 kJ/mol. In systems with weak hydrogen bonds, such as Si(CH3)4 or (CH3)2SiF2, the energy required for dissociation was not large; however, in systems with strong hydrogen bonds, such as Si(CH3)2(OH)2, dissociation was more difficult in the vertical direction. Although anisotropy due to hydroxyl groups was calculated in the horizontal direction, dissociation remained relatively easy.

The Effect of Carbon Nanofibers on the Mechanical Performance of Epoxy-Based Composites: A Review

This review is a fundamental tool for researchers and engineers involved in the design and optimization of fiber-reinforced composite materials. The aim is to provide a comprehensive analysis of the mechanical performance of composites with epoxy matrices reinforced with carbon nanofibers (CNFs). The review includes studies investigating the static mechanical response through three-point bending (3PB) tests, tensile tests, and viscoelastic behavior tests. In addition, the properties of the composites' resistance to interlaminar shear strength (ILSS), mode I and mode II interlaminar fracture toughness (ILFT), and low-velocity impact (LVI) are analyzed. The incorporation of small amounts of CNFs, mostly between 0.25 and 1% by weight was shown to have a notable impact on the static and viscoelastic properties of the composites, leading to greater resistance to time-dependent deformation and better resistance to creep. ILSS and ILFT modes I and II of fiber-reinforced composites are critical parameters in assessing structural integrity through interfacial bonding and were positively affected by the introduction of CNFs. The response of composites to LVI demonstrates the potential of CNFs to increase impact strength by reducing the energy absorbed and the size of the damage introduced. Epoxy matrices reinforced with CNFs showed an average increase in stiffness of 15% and 20% for bending and tensile, respectively. The laminates, on the other hand, showed an increase in bending stiffness of 20% and 15% for tensile and modulus, respectively. In the case of ILSS and ILFT modes I and II, the addition of CNFs promoted average increases in the order of 50%, 100%, and 50%, respectively.

Polymer chain extenders induce significant toxicity through DAF-16 and SKN-1 pathways in Caenorhabditis elegans: A comparative analysis

Polymer chain extenders, commonly used in plastic production, have garnered increasing attention due to their potential environmental impacts. However, a comprehensive understanding of their ecological risks remains largely unknown. In this study, we employed the model organism Caenorhabditis elegans to investigate toxicological profiles of ten commonly-used chain extenders. Exposure to environmentally relevant concentrations of these chain extenders (ranging from 0.1 µg L-1 to 10 mg L-1) caused significant variations in toxicity. Lethality assays demonstrated the LC50 values ranged from 92.42 µg L-1 to 1553.65 mg L-1, indicating marked differences in acute toxicity. Sublethal exposures could inhibit nematodes' growth, shorten lifespan, and induce locomotor deficits, neuronal damage, and reproductive toxicity. Molecular analyses further elucidated the involvement of the DAF-16 and SKN-1 signaling pathways, as evidenced by upregulated expression of genes including ctl-1,2,3, sod-3, gcs-1, and gst-4. It implicates these pathways in mediating oxidative stress and toxicities induced by chain extenders. Particularly, hexamethylene diisocyanate and diallyl maleate exhibited markedly high toxicity among the chain extenders, as revealed through a comparative analysis of multiple endpoints. These findings demonstrate the potential ecotoxicological risks of polymer chain extenders, and suggest the need for more rigorous environmental safety assessments.

Effects of Curing Agents on the Adhesion of Epoxy Resin to Copper: A Density Functional Theory Study

Epoxy resins are widely used adhesives in industrial fields. To use epoxy resin as an adhesive, it is necessary to mix the epoxy resin with a hardener. Hardeners have various functional groups and skeletons, and the properties of epoxy resins vary depending on the hardener. Although the adhesion of epoxy resins has been extensively studied using density functional theory (DFT) calculations, few studies have evaluated the effect of hardener molecules. Therefore, in this study, DFT calculations of adhesion energies and bonding structures on Cu (111) and Cu2O (111) surfaces are performed for model molecules of adducts of epoxy resin with hardeners having various functional groups and skeletons to evaluate the influence of the hardeners on the adhesion of epoxy resin to the metal surface. The adhesion energy to the Cu (111) surface is governed by the energy due to dispersion forces. Hardeners of the thiol type, which contain relatively heavy sulfur atoms, and hardeners with aromatic rings, displaying high planarity, enable the entire molecule to approach the metal surface, resulting in a relatively high adhesion strength. The calculations for the Cu2O (111) surface show the adhesion strength is more strongly influenced by interactions such as hydrogen bonds between the surface and adhesive molecules than by dispersion forces. Therefore, in adhesion to Cu2O (111), the benzylamine-epoxy adduct with hydrogen bonding and OH-π interactions with the surface, in addition to having a relatively flexible framework, shows a high adhesion strength.

Core-Shell Rubber Nanoparticle-Modified CFRP/Steel Ambient-Cured Adhesive Joints: Curing Kinetics and Mechanical Behavior

Externally bonded wet-layup carbon fiber-reinforced polymer (CFRP) strengthening systems are extensively used in concrete structures but have not found widespread use in deficient steel structures. To address the challenges of the adhesive bonding of wet-layup CFRP to steel substrates, this study investigated the effect of core-shell rubber (CSR) nanoparticles on the curing kinetics, glass transition temperature (Tg) and mechanical properties of ambient-cured epoxy/CSR blends. The effects of silane coupling agent and CSR on the adhesive bond properties of CFRP/steel joints were also investigated. The results indicate that CSR nanoparticles have a mild catalytic effect on the curing kinetics of epoxy under ambient conditions. The effect of CSR on the Tg of epoxy was negligible. Epoxy adhesives modified with 5 to 20%wt. of CSR nanoparticles were characterized with improved ductility over brittle neat epoxy; however, the addition of CSR nanoparticles reduced tensile strength and modulus of the adhesives. An up to 250% increase in the single-lap shear strength of CFRP/steel joints was accomplished in CSR-modified joints over neat epoxy adhesive joints.

Evidence through Thermal Analysis of Retro Diels-Alder Reaction in Model Networks Based on Anthracene Modified Polyester Resins

The present work is focused on polyester resins obtained from the diglycidyl ether of bisphenol A and anthracene modified 5-maleimidoisophthalic acid. Because the maleimide-anthracene Diels-Alder (DA) adduct is stable at high temperatures, it is considered a good option for high performance polymers. However, the information related to the retroDA reaction for this type of adduct is sometimes incoherent. A detailed thermal study (conventional TGA, HiRes TGA, MTGA, DSC, MDSC) was performed in order to establish whether the rDA reaction can be revealed for this type of anthracene modified polyester resins. The TGA method confirmed the cleavage of the anthracene-maleimide DA adduct, while the DSC demonstrated the presence of anthracene in the system. At high temperatures, unprotected maleimide homopolymerizes and/or reacts with allyl groups according to the -ene reaction. Therefore, the thermal DA reaction is not displayed anymore upon the subsequent cooling, and the glass transition region is registered at a higher temperature range during the second heating. The use of sample-controlled thermal analysis (HiRes TGA) and MTGA improved the TGA result; however, it was not possible to separate the very complex degradation processes that are interconnected.

Examining the Quasi-Static Uniaxial Compressive Behaviour of Commercial High-Performance Epoxy Matrices

Four commercial high-performance aerospace aromatic epoxy matrices, CYCOM®890, CYCOM®977-2, PR520, and PRISM EP2400, were cured to a standardised 2 h, 180 °C cure cycle and evaluated in quasi-static uniaxial compression, as well as by dynamic scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The thermoplastic toughened CYCOM®977-2 formulation displayed an overall increase in true axial stress values across the entire stress-strain curve relative to the baseline CYCOM®890 sample. The particle-toughened PR520 sample exhibited an overall decrease in true axial stress values past the yield point of the material. The PRISM EP2400 resin, with combined toughening agents, led to true axial stress values across the entire plastic region of the stress-strain curve, which were in line with the stress values observed with the CYCOM®890 material. Interestingly, for all formulations, the dilation angles (associated with the volume change during plastic deformation), recorded at 0.3 plastic strain, were close to 0°, with the variations reflecting the polymer structure. Compression data collected for this series of commercial epoxy resins are in broad agreement with a selection of model epoxy resins based on di- and tetra-functional monomers, cured with polyamines or dicarboxylic anhydrides. However, the fully formulated resins demonstrate a significantly higher compressive modulus than the model resins, albeit at the expense of yield stress.

Effect of Carbon Nanofibers on the Strain Rate and Interlaminar Shear Strength of Carbon/Epoxy Composites

The static bending properties, different strain rates and interlaminar shear strength (ILSS) of carbon-fiber-reinforced polymers (CFRP) with two epoxy resins nano-enhanced with carbon nanofibers (CNFs) are studied. The effect on ILSS behavior from aggressive environments, such as hydrochloric acid (HCl), sodium hydroxide (NaOH), water and temperature, are also analyzed. The laminates with Sicomin resin and 0.75 wt.% CNFs and with Ebalta resin with 0.5 wt.% CNFs show significant improvements in terms of bending stress and bending stiffness, up to 10%. The values of ILLS increase for higher values of strain rate, and in both resins, the nano-enhanced laminates with CNFs show better results to strain-rate sensitivity. A linear relationship between the logarithm of the strain rate was determined to predict the bending stress, bending stiffness, bending strain and ILSS for all laminates. The aggressive solutions significantly affect the ILSS, and their effects are strongly dependent on the concentration. Nevertheless, the alkaline solution promotes higher decreases in ILSS and the addition of CNFs is not beneficial. Regardless of the immersion in water or exposure to high temperatures a decrease in ILSS is observed, but, in this case, CNF content reduces the degradation of the laminates.

Reinforced Epoxy Binder Modified with Borpolymer

Polymer binders based on epoxy resins have unique properties that contribute to their use in many composite industries. The potential of using epoxy binders is due to their high elasticity and strength characteristics, thermal and chemical resistance, and resistance to climatic aging. This is the reason for the existing practical interest in modifying the composition of epoxy binders and understanding the strengthening mechanisms in order to form reinforced composite materials with a required set of properties based on them. This article presents the results of a study of the process of dissolving the modifying additive of polymethylene-p-triphenyl ether of boric acid in the components of an epoxyanhydride binder applicable to the production of fibrous composite materials. The temperature and time conditions for the dissolution of polymethylene-p-triphenyl ether of boric acid in anhydride-type isomethyltetrahydrophthalic anhydride hardeners are presented. It has been established that the complete dissolution of the borpolymer-modifying additive in iso-MTHPA occurs at a temperature of 55 ± 2 °C for 20 h. The effect of the modifying additive of polymethylene-p-triphenyl ether of boric acid on the strength properties and structure of the epoxyanhydride binder has been studied. Increases in transverse bending strength up to 190 MPa, elastic modulus up to 3200 MPa, tensile strength up to 0.8 MPa, and impact strength (Charpy) up to 5.1 kJ/m² are observed when the content of the borpolymer-modifying additive in the composition of the epoxy binder is 0.50 mass. %.

Excellent Energy Storage Performance in Epoxy Resin Dielectric Polymer Films by a Facile Hot-Pressing Method

Epoxy resin (EP), as a kind of dielectric polymer, exhibits the advantages of low-curing shrinkage, high-insulating properties, and good thermal/chemical stability, which is widely used in electronic and electrical industry. However, the complicated preparation process of EP has limited their practical applications for energy storage. In this manuscript, bisphenol F epoxy resin (EPF) was successfully fabricated into polymer films with a thickness of 10~15 μm by a facile hot-pressing method. It was found that the curing degree of EPF was significantly affected by changing the ratio of EP monomer/curing agent, which led to the improvement in breakdown strength and energy storage performance. In particular, a high discharged energy density (U_d) of 6.5 J·cm^-3 and efficiency (η) of 86% under an electric field of 600 MV·m^-1 were obtained for the EPF film with an EP monomer/curing agent ratio of 1:1.5 by hot pressing at 130 °C, which indicates that the hot-pressing method could be facilely employed to produce high-quality EP films with excellent energy storage performance for pulse power capacitors.

New Epoxy and Hardener System Based on an Imidazolium Ionic Liquid as an Anticorrosive Coating for Steel in the Marine Environment

The large sizes of cations and anions of organic salts are the driving force for the application of ionic liquids (organic salts) in harsh salty conditions. Moreover, the formation of crosslinked ionic liquid networks as anti-rust and anticorrosion protective films on the substrate surfaces repels seawater salt and water vapor from their surface to prevent corrosion. In this respect, an imidazolium epoxy resin and polyamine hardener as ionic liquids were prepared by the condensation of either pentaethylenehexamine or ethanolamine with glyoxal and p-hydroxybenzaldehyde or formalin in acetic acid as a catalyst. The hydroxyl and phenol groups of the imidazolium ionic liquid were reacted with epichlorohydrine in the presence of NaOH as a catalyst to prepare polyfunctional epoxy resins. The chemical structure, nitrogen content, amine value, epoxy equivalent weight, thermal characteristics, and stability of the imidazolium epoxy resin and polyamine hardener were evaluated. Moreover, their curing and thermomechanical properties were investigated to confirm the formation of homogeneous, elastic, and thermally stable cured epoxy networks. The corrosion inhibition and salt spray resistance of the uncured and cured imidazolium epoxy resin and polyamine as coatings for steel in seawater were evaluated.

Contribution of the Surface Treatment of Nanofibrillated Cellulose on the Properties of Bio-Based Epoxy Nanocomposites Intended for Flexible Electronics

The growing interest in materials derived from biomass has generated a multitude of solutions for the development of new sustainable materials with low environmental impact. We report here, for the first time, a strategy to obtain bio-based nanocomposites from epoxidized linseed oil (ELO), itaconic acid (IA), and surface-treated nanofibrillated cellulose (NC). The effect of nanofibrillated cellulose functionalized with silane (NC/S) and then grafted with methacrylic acid (NC/SM) on the properties of the resulted bio-based epoxy systems was thoroughly investigated. The differential scanning calorimetry (DSC) results showed that the addition of NCs did not influence the curing process and had a slight impact on the maximum peak temperature. Moreover, the NCs improved the onset degradation temperature of the epoxy-based nanocomposites by more than 30 °C, regardless of their treatment. The most important effect on the mechanical properties of bio-based epoxy nanocomposites, i.e., an increase in the storage modulus by more than 60% at room temperature was observed in the case of NC/SM addition. Therefore, NC's treatment with silane and methacrylic acid improved the epoxy-nanofiber interface and led to a very good dispersion of the NC/SM in the epoxy network, as observed by the SEM investigation. The dielectric results proved the suitability of the obtained bio-based epoxy/NCs materials as substitutes for petroleum-based thermosets in the fabrication of flexible electronic devices.

Bio-Based Vitrimers from 2,5-Furandicarboxylic Acid as Repairable, Reusable, and Recyclable Epoxy Systems

In this work, a series of bio-based epoxy vitrimers were developed by reacting diglycidyl ether of bisphenol A (DGEBA) and bio-based 2,5-furandicarboxylic acid (FDCA) at different molar ratios. Triazabicyclodecene was used as a transesterification catalyst to promote thermally induced exchange reactions. Differential scanning calorimetry, gel content measurements, and Fourier transform infrared spectroscopy were used to study the FDCA-DGEBA crosslinking reaction. The transesterification exchange reaction kinetics of such crosslinked systems was characterized via stress relaxation tests, evidencing an Arrhenius-type dependence of the relaxation time on temperature, and an activation energy of the dynamic rearrangement depending on the molar composition. In addition, self-healing, thermoformability, and mechanical recycling were demonstrated for the composition showing the faster topology rearrangement, namely, the FDCA/DGEBA molar ratio equal to 0.6. This work provides the first example of bio-based epoxy vitrimers incorporating FDCA, making these systems of primary importance in the field of reversible, high-performance epoxy materials for future circular economy scenarios.

Biological Effects and Toxicity of Compounds Based on Cured Epoxy Resins

The aim of this work was to investigate selected biological and toxicity properties of cured epoxy resin-based compounds based on a bisphenol A epoxy resin, cold-cured by a polyamide and containing two types of metal powders (aluminum and copper). This study involved cytotoxicity analysis, pH measurements, absorbance measurements and sterilization. The cytotoxicity analysis was conducted to determine the harmful degree of the cured epoxy resin. Aimed at identifying toxic agents in cured compounds, the cytotoxicity analysis involved absorbance measurements in an entire wavelength range. Cytotoxicity and absorbance results demonstrated that the extracts of all the tested resin samples had no cytotoxic effects on the cells of living organisms. The absorbance values obtained over the entire wavelength range did not point to the formation of aggregations, which proved that no toxic agents harmful to living organisms were extracted from the resin samples. Based on the results obtained, it can be concluded that all tested compounds, based on epoxy resins, which are also used as adhesives in various applications, are essentially safe materials when using such formulations in a cured state.

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

New Application Field of Polyethylene Oxide: PEO Nanofibers as Epoxy Toughener for Effective CFRP Delamination Resistance Improvement

Delamination is the most severe weakness affecting all composite materials with a laminar structure. Nanofibrous mat interleaving is a smart way to increase the interlaminar fracture toughness: the use of thermoplastic polymers, such as poly(ε-caprolactone) and polyamides (Nylons), as nonwovens is common and well established. Here, electrospun polyethylene oxide (PEO) nanofibers are proposed as reinforcing layers for hindering delamination in epoxy-based carbon fiber-reinforced polymer (CFRP) laminates. While PEO nanofibers are well known and successfully applied in medicine and healthcare, to date, their use as composite tougheners is undiscovered, resulting in the first investigation in this application field. The PEO-modified CFRP laminate shows a significant improvement in the interlaminar fracture toughness under Mode I loading: +60% and +221% in G I,C and G I,R, respectively. The high matrix toughening is confirmed by the crack path analysis, showing multiple crack planes, and by the delamination surfaces, revealing that extensive phase separation phenomena occur. Under Mode II loading, the G II enhancement is almost 20%. Despite a widespread phase separation occurring upon composite curing, washings in water do not affect the surface delamination morphology, suggesting a sufficient humidity resistance of the PEO-modified laminate. Moreover, it almost maintains both the original stiffness and glass transition temperature (T g), as assessed via three-point bending and dynamic mechanical analysis tests. The achieved results pave the way for using PEO nanofibrous membranes as a new effective solution for hindering delamination in epoxy-based composite laminates.

Adhesive and Impact-Peel Strength Improvement of Epoxy Resins Modified with Mono and Diamine Functionalized Elastomers

Epoxy resins are widely applied in the automotive and electronic industries. However, pure epoxy resins are brittle and thus possess poor mechanical properties. Herein, we report a facile method for improving the impact-peel and adhesive strengths of epoxy resins via the incorporation of two different types of polyether amines (monoamine-based and diamine-based prepolymers). A comparative study was performed to investigate the potential advantages of incorporating a prepolymer into an epoxy resin matrix. It was discovered that the incorporation of a diamine prepolymer significantly improved the impact-peel strength of the epoxy resin system at low (-40°C) and room (23°C) temperatures. For 15 wt% adhesive loading, the diamine prepolymer-based epoxy system demonstrated a 130% (low temperature) and 32% (room temperature) higher impact-peel strength than the monoamine prepolymer-based epoxy system. Moreover, the 15 wt% diamine prepolymer-based epoxy system exhibited a significantly improved shear strength (~36 MPa) and T-peel strength (260 N/25 mm) owing to the effectively reduced crack propagation and cohesive interactions between the epoxy molecules. Our results suggest that the modification of epoxy resins with an appropriate amount of mono and diamine-functionalized elastomers provides a novel route for the development of highly efficient adhesive materials for various applications.

Glycidate as a High-Strength Epoxy Adhesive Curable with Amine under Ambient Conditions

This paper reports that glycidates bearing epoxy moieties with adjacent ester can be cured with diethylenetriamine (DETA) under mild conditions and exhibit high adhesiveness. Curing of bifunctional glycidates with DETA gave cross-linked products. The curing started at a lower temperature (7 °C) than the analogous glycidyl ether (27 °C), while the rate of the curing was slower due to the lower activation energy (Ea = 57 kJ/g) and exothermicity (ΔH = 58 J/g) as confirmed by DSC analysis. The curing system of neopentyl glycol diglycidate and DETA effectively adhered aluminum plates by curing at 25 °C, and the strength was more than five times higher than the curing with analogous glycidyl ether. The higher adhesive strength under curing of ambient conditions and facile preparation of monomers are the significant advantages of this curing.

Preparation of Gellan Gum-Inorganic Composite Film and Its Metal Ion Accumulation Property

Gellan gum is one of the water-soluble anionic polysaccharides produced by the bacteria Sphingomonas elodea. In this study, we prepared gellan gum-inorganic composite films by mixing the gellan gum and a silane coupling reagent—3-glycidoxypropyltrimethoxysilane (GPTMS). These gellan gum-GPTMS composite films were stable in an aqueous solution and showed a thermal stability. In addition, these composite films indicated a mechanical strength by the formation of the three-dimensional network of siloxane. We demonstrated the accumulation of metal ions from a metal ion-containing aqueous solution by the composite film. As a result, although the composite film indicated the accumulation of heavy and rare-earth metal ions, the light metal ions, such as Mg(II) and Al(III) ions, did not interact with the composite material. Therefore, the accumulative mechanism of metal ions using a composite film was evaluated by IR measurements. As a consequence, although the accumulation of heavy and rare-earth metal ions occurred at both the −COO− group and the −OH group in the gellan gum, the accumulation of light metal ions occurred only at the −OH group.

Extrusion Dwell Time and Its Effect on the Mechanical and Thermal Properties of Pitch/LLDPE Blend Fibres

Mesophase pitch-based carbon fibres have excellent resistance to plastic deformation (up to 840 GPa); however, they have very low strain to failure (0.3) and are considered brittle. Hence, the development of pitch fibre precursors able to be plastically deformed without fracture is important. We have previously, successfully developed pitch-based precursor fibres with high ductility (low brittleness) by blending pitch and linear low-density polyethylene. Here, we extend our research to study how the extrusion dwell time (0, 6, 8, and 10 min) affects the physical properties (microstructure) of blend fibres. Scanning electron microscopy of the microstructure showed that by increasing the extrusion dwell from 0 to 10 min the pitch and polyethylene components were more uniformly dispersed. The tensile strength, modulus of elasticity, and strain at failure for the extruded fibres for different dwell times were measured. Increased dwell time resulted in an increase in strain to failure but reduced the ultimate tensile strength. Thermogravimetric analysis was used to investigate if increased dwell time improved the thermal stability of the samples. This study presents a useful guide to help with the selection of mixes of linear low-density polyethylene/pitch blend, with an appropriate extrusion dwell time to help develop a new generation of potential precursors for pitch-based carbon fibres.

Synthesis, Characterization and Sorption Ability of Epoxy Resin-Based Sorbents with Amine Groups

Water pollution by toxic substances, such as azo dyes, is a serious environmental problem that needs to be addressed. This study presents the synthesis and characterization of new polymeric sorbents, based on the epoxy resin Epidian^® 5 (Ep5), as a potential adsorbent for the removal of the toxic azo dye C.I. Acid Violet 1 (AV1). Triethylenetetramine (TETA) was applied as a cross-linking agent in the amounts of 1 g (6.67 wt %), 1.5 g (10 wt %), and 2 g (13.33 wt %). The use of a compound with amino groups allows for the simultaneous functionalization of the obtained material. The reaction was carried out in an environment of ethylene glycol, with the addition of a porophore solvent (toluene) and bis(2-ethylhexyl)sulfosuccinate sodium salt (S). The attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) revealed the existence of a strong band in the 828–826 cm⁻¹ range corresponding to the second-order amine group, which indicates their incorporation into the epoxy structure. The glass transition and decomposition temperatures of the resins decreased with the increasing amounts of amine in the material. The thermogravimetry (TGA) analysis demonstrated that all products are thermally stable up to 340 °C. The surface morphology and microstructural properties of the obtained sorbents were determined using scanning electron microscopy (SEM) images and showed an irregular star shape, with dimensions ranging from 400 to 1000 µm. The adsorption capacities of Ep5-TETA1, Ep5-TETA1.5, Ep5-TETA2 and Ep5-TETA1.5 + S for AV1 evaluated during batch experiments were found to be 2.92, 3.76, 7.90 and 3.30 mg/g, respectively.

Mechanical Properties of the Carbon Nanotube Modified Epoxy-Carbon Fiber Unidirectional Prepreg Laminates

The effect of plasma treatment of the multi-walled carbon nanotube (MWCNT) surface on the fracture toughness of an aerospace grade epoxy resin and its unidirectional (UD) carbon fiber prepreg laminates has attracted scientific interest. A prepreg route eliminates the possible risk of carbon nanotube filtration by unidirectional carbon fibers. X-ray photoelectron spectroscopy results suggested that oxygen atom concentration at the nanotube surface was increased from 0.9% to 3.7% after plasma modification of the carbon nanotubes. A low number (up to 0.5 wt.%) of MWCNTs was added to epoxy resin and their carbon fiber prepreg laminates. Transmission electron micrographs revealed that the plasma treatment resulted in a better dispersion and distribution of MWCNTs in the epoxy resin. Plasma-treated MWCNTs resulted in a more pronounced resistance to the crack propagation of epoxy resin. During the production of the reference and nanotube-modified prepregs, a comparable prepreg quality was achieved. Neat nanotubes agglomerated strongly in the resin-rich regions of laminates lowering the interlaminar fracture toughness under mode I and mode II loading. However, plasma-treated nanotubes were found mostly as single particles in the resin-rich regions of laminates promoting higher energy dissipation during crack propagation via a CNT pull-out mechanism.

New Imidazolium Ionic Liquids from Recycled Polyethylene Terephthalate Waste for Curing Epoxy Resins as Organic Coatings of Steel

Imidazolium ionic liquid (IIL) was prepared from aminolysis of polyethylene terephthalate (PET) waste with pentaethylenehexamine (PEHA) to apply as hardener of epoxy resin. Its purified chemical structures, thermal stability, and thermal characteristics were identified as well as amino phthalamide aminolyzed products. The thermal, thermomechanical, and mechanical properties of the cured epoxy resins with different weight percentages of IIL were investigated to optimize the best weight ratio to obtain homogeneous networks. The adhesion, durability, and corrosion resistance of the cured epoxy resins on the steel surfaces were tested to confirm that the best weight ratio of epoxy: IL was 2:1. This ratio achieved higher adhesion strength and salt spray resistance to seawater extended to 1500 h.

Halloysite Nanotubes as an Additive to Ensure Enhanced Characteristics of Cold-Curing Epoxy Resins under Fire Conditions

At present, the most commonly used electrical insulating materials, including cold-curing epoxy resins, are well designed for normal operating conditions. However, new generations of materials should also be capable of withstanding extreme emergency conditions, e.g., in case of fire. For this reason, this study presents the possibilities of an improved cold-curing epoxy resin using halloysite nanotubes (HNTs) to increase its operational safety. The positive effect of HNT addition is indicated mainly in terms of the suppression of thermo-oxidation processes, which has been demonstrated by the decreases in the maximum heat flow peaks as well as the specific enthalpy values during the thermal decomposition of the epoxy resin. The observed dielectric parameters of the HNT-added materials differ only slightly from those without a filler, whereas their mechanical properties strongly depend on the amount of dispersed HNTs.

Epoxy toughening through high pressure and shear rate preprocessing

We have successfully conceived and demonstrated a simple, scalable process for improving the fracture energy of epoxy resins. The process is based on the combined application of high pressures (in the order of GPa) and shear rates (in the order of 10⁶ s⁻¹) in the pre-cured polymer, obtaining mechanical forces sufficiently high to increase the reactivity of the monomers due to the scission of the epoxy groups. To achieve these high values of pressure and shear rate, we take advantage of the elastohydrodynamic phenomena occurring at lubricated higher kinematic pairs as, for example, the rolling element – track pair in ball bearings. Experimental results show that, under certain combinations of pressure and shear rate, a substantial improvement in fracture toughness is obtained. SEM observations, Raman spectroscopies, nanoindentation and GPC and NMR measurements show that the process is able to increase the polymer chain length before curing, reducing the number of potential anchor points during the subsequent curing and hence reducing the crosslinking density. The chain lengths obtained are big enough to guarantee adequate stiffness and strength due to increased chain tangling, hence overcoming the drawbacks associated with other toughness promotion methods, such as stiffness and strength reduction.

Epoxy resin composites with commercially available graphene: toward high toughness and rigidity

Some commercially available graphene (CG) samples, actually graphite nanoplates, are difficult to be applied in polymers due to their large thickness, even though their fabrication is more cost-effective. In this work, a CG sample with a thickness of about 25 nm is pre-treated via a facile acid soaking technique, and then the acid-treated CG is introduced into epoxy resin (EP) to improve the toughness while maintaining the high rigidity of the EP. The thickness of the acid-treated CG is dramatically decreased to about 1 nm, well-dispersing CG in the EP matrix. The tensile strength and the fracture energy of EP composite with 0.2 wt% acid-treated CG are increased to 85.2 MPa and 3.6 kJ m-2 from 74.3 MPa and 2.3 kJ m-2 of pure EP, respectively. In addition, the tensile-fractured surfaces of EP composites reveal that the toughening mechanism is mainly attributed to the crack deflection and crack pinning caused by the firmly-embedded CG nanosheets in the EP matrix. Besides, the glass transition temperature of EP composites is increased to 170.5 °C from 163.9 °C of pure EP. The present study provides a valuable approach for making use of the cost-effective graphite nanoplates to achieve a similar performance as graphene in EP.

Preparation and Properties of Polyether Aliphatic Polymerized Amide as a Vegetable Oil-Based Epoxy Curing Agent

An epoxy curing agent polyether aliphatic polymerized amide (PEAPA) was synthesized using epoxy fatty acid methyl ester and diethylenetriamine. The Fourier transform infrared spectra and ¹H NMR analysis indicated successful synthesis of PEAPA. Gel permeation chromatography showed a high degree of polymerization. The obtained PEAPA was used to cure E51 epoxy resin and partially replace rigid 1,3-cyclohexanediamine curing agent. A series of epoxy resins with varying rigidities were prepared. The mechanical and thermal properties of the materials were analyzed. Mechanical property tests showed that the tensile strength and hardness of the materials decreased gradually with increased PEAPA content. However, elongation at breaks of the prepared materials increased with increased PEAPA content. Micromorphological investigation indicated excellent compatibility between PEAPA and the curing system. Furthermore, a dynamic mechanical thermal analysis demonstrated that the glass transition temperature of the epoxy resin decreased with increased PEAPA content. Thermal stability, while still excellent, decreased slightly with the addition of PEAPA. At the primary weight loss stage, initial decomposition temperatures for all resins were above 330 °C.

Highly Toughened and Transparent Biobased Epoxy Composites Reinforced with Cellulose Nanofibrils

Biobased nanofillers, such as cellulose nanofibrils (CNFs), have been widely used as reinforcing fillers for various polymers due to their high mechanical properties and potential for sustainable production. In this study, CNF-based composites with a commercial biobased epoxy resin were prepared and characterized to determine the morphology, mechanical, thermal, and barrier properties. The addition of 18–23 wt % of CNFs to epoxy significantly increased the modulus, strength and strain of the resulting composites. The addition of fibrils led to an overall increase in strain energy density or modulus of toughness by almost 184 times for the composites compared to the neat epoxy. The addition of CNFs did not affect the high thermal stability of epoxy. The presence of nanofibrils had a strong reinforcing effect in both glassy and glass transition region of the composites. A significant decrease in intensity in tan δ peak for the epoxy matrix occurred with the addition of CNFs, indicating a high interaction between fibrils and epoxy during the phase transition. The presence of highly crystalline and high aspect ratio CNFs (23 wt %) decreased the water vapour permeability of the neat epoxy resin by more than 50%.

A New Way of Toughening of Thermoset by Dual-Cured Thermoplastic/Thermosetting Blend

The work aims at establishing the optimum conditions for dual thermal and electron beam curing of thermosetting systems modified by styrene/butadiene (SB)-based triblock copolymers in order to develop transparent and toughened materials. The work also investigates the effects of curing procedures on the ultimate phase morphology and mechanical properties of these thermoset–SB copolymer blends. It was found that at least 46 mol% of the epoxidation degree of the SB copolymer was needed to enable the miscibility of the modified block copolymer into the epoxy resin. Hence, an electron beam curing dose of ~50 kGy was needed to ensure the formation of micro- and nanostructured transparent blends. The micro- and nanophase-separated thermosets obtained were analyzed by optical as well as scanning and transmission electron microscopy. The mechanical properties of the blends were enhanced as shown by their impact strengths, indentation, hardness, and fracture toughness analyses, whereby the toughness values were found to mainly depend on the dose. Thus, we have developed a new route for designing dual-cured toughened micro- and nanostructured transparent epoxy thermosets with enhanced fracture toughness.

Intertidal exposure favors the soft-studded armor of adaptive mussel coatings

The mussel cuticle, a thin layer that shields byssal threads from environmental exposure, is a model among high-performance coatings for being both hard and hyper-extensible. However, despite avid interest in translating its features into an engineered material, the mechanisms underlying this performance are manifold and incompletely understood. To deepen our understanding of this biomaterial, we explore here the ultrastructural, scratch-resistant, and mechanical features at the submicrometer scale and relate our observations to individual cuticular components. These investigations show that cuticle nanomechanics are governed by granular microinclusions/nanoinclusions, which, contrary to previous interpretations, are three-fold softer than the surrounding matrix. This adaptation, which is found across several related mussel species, is linked to the level of hydration and presumed to maintain bulk performance during tidal exposures. Given the interest in implementing transfer of biological principles to modern materials, these findings may have noteworthy implications for the design of durable synthetic coatings.

There is interest in the development of mussel inspired materials; however, this requires an understanding of the materials. Here, the authors report on an investigation into the properties of mussel cuticle from different species that challenges conventional wisdom about particle filled composites.

Triblock Copolymer Toughening of a Carbon Fibre-Reinforced Epoxy Composite for Bonded Repair

Epoxy resins are the most widely used systems for structural composite applications; however, they lack fracture toughness, impact strength and peel strength due to high cross-linking densities. Use of conventional toughening agents to combat this can lead to reductions in mechanical, thermal and processability properties desirable for bonded composite applications. In this work, an asymmetric triblock copolymer of poly(styrene)⁻b⁻poly(butadiene)⁻b⁻poly(methylmethacrylate) was used to modify an epoxy resin system, with the materials processed using both vacuum bag and positive pressure curing techniques. Interlaminar fracture toughness testing showed improvements in initiation fracture toughness of up to 88%, accompanied by a 6 °C increase in glass transition temperature and manageable reductions in gel-time. Shear testing resulted in a 121% increase in ultimate shear strain with only an 8% reduction in shear strength. Performance improvements were attributed to nano-structuring within the toughened resin system, giving rise to matrix cavitation and dissipation of crack front strain energy upon loading.