High-strength Epoxy Research Articles

Fire safety and high mechanical strength epoxy resin enabled by a bis-benzimidazole-primed phenyl phosphonic acid

Preparation of fire safety and high mechanical strength epoxy resin (EP) has become a hot topic for its valuable applications in construction and other fields. Here, bis-2-aminobenzimidazole-modified phenyl phosphonates (PP-BMI) with symmetric structures were synthesized by a simple chemical modification strategy, and its fine structure and composition have been thoroughly characterized. Due to the multifaceted functions of PP-BMI in both gaseous and condensed phases, EP composite with 5 wt% PP-BMI reached a UL-94 V-0 rating and high LOI of 34.3 %. The excellent fire safety of EP/5PP-BMI was further verified with a significant reduction of 57 %, 52 % and 46 % in peak heat release rate (PHRR), total heat release (THR) and peak CO production (P-COP), respectively, compared to the original EP. The flame retardancy of PP-BMI on EP can be attributed to the dilution of non-combustion gases, free radical trapping and catalytic charring functions. More importantly, PP-BMI can further enhance the mechanical properties of EP, which is closely related to the interaction between PP-BMI and EP matrix. Furthermore, EP/PP-BMI composites have good resistance to acids and alkalis, ensuring long-lasting flame retardancy and mechanical properties. This work provides a new strategy for realizing epoxy composites with high fire safety and excellent mechanical properties in construction and multifaceted areas.

Sustainably Sourced Tannic Acid Enables Fast-Curing High-Strength Epoxy Adhesives with Increased Toughness

Sustainably Sourced Tannic Acid Enables Fast-Curing High-Strength Epoxy Adhesives with Increased Toughness

The super-fast synthesis of high-strength epoxy adhesives by UV-initiated frontal polymerization at room temperature

The synthesis and processing of most polymer adhesives rely on energy-inefficient and environmentally burdensome manufacturing approaches. In this paper, a novel synthesis strategy, UV-initiated frontal polymerization (FP) for the fast synthesis of high-strength, self-propagating epoxy adhesives at room temperature, was presented, in which the self-propagating rate reached 71.4 mm min−1. CuCl2 was used as a catalyst to decrease the onset temperature of polymerization and ensure sufficient polymerization on the surface of the material to be bonded. Mild steel, glass, ceramic, wood, PMAA, and concrete were successfully bonded within several minutes at room temperature. In the same material system, the adhesive strength with the FP curing approach was greater than that of thermal curing, and the former had a better pore structure, as determined by MIP analysis. This paper provides a promising idea for the in-situ fast synthesis of high-strength epoxy adhesives at room temperature.

Flame retardant, high mechanical strength, transparent and water-resistant epoxy composites modified with chitosan derivatives

Adding bio-based flame retardants to improve the flame retardancy of polymer materials without sacrificing other properties is a great challenge. Herein, a novel flame-retardant CS-DOPA was prepared from chitosan and 10-hydroxy-9,10-dihydro-9-oza-10-phosphaphenanthrene-10-oxide by acid-base neutralization reaction and fully characterized. The 4 wt% CS-DOPA modified EP showed good flame retardancy in both gaseous and condensed phase. The peak heat release rate, total smoke production, CO production, and smoke production rate of EP composites containing 4 wt% CS-DOPA were reduced by 55 %, 34 %, 45 %, and 46 %, respectively, to pass the UL-94 V-1 rating with a limiting oxygen index of 34.1 %. The CS-DOPA contributes to the formation of the condensed phase of the thermo-oxidation-resistant high-quality char layer with non-flammable other and phosphorus-containing free radicals released in the gas phase. In addition, EP/4CS-DOPA has good water resistance, mechanical properties, and transparency, with tensile and flexural strength improved by 12.7 % and 13.9 %, respectively, and still has high strength even after water treatment. The present work provides a green and facile strategy to use chitosan as a main raw material to manufacture EP materials with high performance.

High-Strength Epoxy Nanocomposites Reinforced with Photochemically Treated CNTs.

A carbon nanotube (CNT)/epoxy nanocomposite was prepared using a photochemical surface modification process of CNTs. The vacuum ultraviolet (VUV)-excimer lamp treatment created reactive sites on the CNT surface. Increasing the irradiation time increased the oxygen functional groups and changed the oxygen bonding state such as C=O, C-O, and -COOH. By the VUV-excimer irradiation on CNTs, the epoxy infiltrated well between the CNT bundles and formed a strong chemical bond between CNT and epoxy. The tensile strength and elastic modulus of the nanocomposites with VUV-excimer irradiated sample during 30 min (R30) were found to increase by 30 and 68% compared to using pristine CNT, respectively. R30 was not pulled out and remained embedded in the matrix until the fracture occurred. The VUV-excimer irradiation is an effective surface modification and functionalization method for improving the mechanical properties of CNT nanocomposite materials.

Shear design of RC members strengthened with steel reinforcing bars embedded through section

The shear strengthening technique investigated in this paper consists of inserting steel reinforcing bars into holes drilled into a shear-deficient existing concrete member and bonding these bars to the concrete with high-strength epoxy adhesive. It is important to realize that current shear design methods are applicable to RC members with conventional stirrups and hence overestimate the shear capacity of members with epoxy-bonded bars. In order to overcome this difficulty, a shear design method for RC members strengthened with epoxy-bonded shear reinforcement is presented. The proposed method was developed considering the development of epoxy-bonded bars as well as the effect of crack width and aggregate interlock on shear capacity. The predicted shear capacities were compared with beam test results and provide accurate predictions.

Study on the application of a granite composite material in five-axis CNC-VMC machine Tool

A five-axis CNC-VMC machine tool generally has a high-speed spindle spinning at a maximum rotation of 50,000 rpm, under which the machine tool bed will violently vibrate that leads to poor machining precision and surface quality of workpiece. The gray cast iron, as the typical material of a machine tool, exhibits poor dynamic suppression of vibration that its processing yield and quality can no longer satisfy the product demand of current market. Consequently, in this study, we proposed granite composite material (GCM) in place of gray cast iron for the structure of machine tool, which would be mixed with main ingredients of high-strength epoxy resin and cemented carbon fibers and fillers to maximize the structural properties of the machine tool. The sample of granite composite material would first be tested for its mechanical properties. The results showed that the GCM cast with only 1/3 density of gray cast iron could withstand a maximum compressive stress of 125.8 MPa. With this granite composite material used in a digitally controlled machine tool bed, the maximum stress and the maximum strain exerted on the machine tool were reduced by 68.93% and 72.6%, respectively, when compared with the cast iron, while the 1st to the 6th-order natural frequencies of the machine tool of mineral composite was significantly increased by 20% to 30%, giving it more resistance to vibration than the gray cast iron. The stability and precision of the machine tool were shown to have been drastically improved by using the granite composite material.

A fully bio-based, anti-flammable and non-toxic epoxy thermosetting network for flame-retardant coating applications

A fully bio-based epoxy thermosetting network was prepared by curing a renewable luteolin-derived epoxy resin (DGELU) with a furan-derived hardener, 5, 5′-methylenedifurfurylamine (DFA). For comparison, a control sample was prepared by curing the petroleum-based commercial epoxy diglycidyl ether of bisphenol A (DGEBA) with a petroleum-based hardener, 4, 4′-diaminodiphenylmethane (DDM). The glass transition temperature (Tg) was found to be 216 °C for the cured DGELU/DFA, which was 67 °C higher than that of the cured DGEBA/DDM. The storage modulus at 30 °C and the tensile strength of the cured DGELU/DFA were 82.5 % and 23.2 % higher than those of the cured DGEBA/DDM. The benzopyrone unit of the luteolin structure was conducive to exceptional charring, and the char yield for thermal decomposition in the nitrogen of the cured DGELU/DFA was 3.2-fold higher than that for the cured DGEBA/DDM. As expected, the cured DGELU/DFA displays a relatively high limiting oxygen index (LOI) of 38.0 % and a UL-94 V-0 classification, whereas the petroleum-based counterpart exhibited a much lower LOI of 24.0 % and no classification in the UL-94 test. An in-vitro cytotoxicity study indicated that the cured DGELU/DFA showed no cytotoxic effect against normal fibroblast cells after 24 and 72 h. Because of these fascinating features, the DGELU/DFA thermosetting network was deposited onto wood surfaces for evaluation as a flame-retardant coating. Compared with the wood treated using the cured DGEBA/DDM coating, wood coated with the cured DGELU/DFA showed a super fire-safe property that could lead to a 55.8 %, 12.4 %, and 11.5 % decrease in peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP), respectively. An anti-flammable, high strength and non-toxic bio-derived epoxy thermosetting network for highly safe epoxy coating applications has been developed.

Processing of post-industrial unidirectional prepreg tapes using SMC equipment

The aerospace industry has an issue of disposing of waste, unused carbon fiber unidirectional prepreg tapes. This paper explores producing chips using sheet-molding compounding equipment (SMC) for conversion into products via compression molding, which are stronger than traditional glass-reinforced SMC parts due to the carbon fiber and high strength epoxy resin. Without a means to convert the continuous tape prepreg into chips, it will not be possible to convert the prepreg into a useful product and provide a way to reduce flow to the landfill and produce less expensive, lighter, and stronger parts for the transportation industry. The cure state of the epoxy resin in the prepreg tapes, which is affected by the amount of time the material has spent stored at elevated or room temperatures, significantly changes the ability of the material to be chopped into short fiber chips. In addition, equipment-related factors affect the material’s ability to be chopped by the SMC equipment. This paper accounts for each of these factors and uses statistical methods to design experiments to investigate the impact each of these material-related and equipment-related factors on the success of the cutting portion of the process. The level of cure, as defined by endothermic peak, glass-transition temperature (Tg), or heat of reaction/degree of cure, has the greatest impact on the cutting process. For material with a moderate level of cure, the sharpness of the blades in the cutting roller also has a major impact on the success of the process. The cutting pressure, roller speed, and tape tension also have an impact on the success of the process, but impact of these factors lessens for feedstock prepreg with a sufficiently high level of cure. An optimal setup was determined, and general rules of thumb for selecting material and equipment settings were established.

The Behavior of RC Beams Retrofitted with Carbon Fiber Reinforced Polymers (CFRP)

The need for the introduction of economical and quicker retrofitting techniques is increasing due to the ever-aging infrastructure and damages produced by major catastrophic events around the world. The application of Carbon Fiber Reinforced Polymers (CFRP) for strengthening and retrofitting of reinforced concrete structures is gaining popularity due to its higher strength, lightweight, durability, corrosion resistance, and aesthetic value. This study presents the results of two strengthened and two retrofitted beams in comparison to control specimens. Two specimens were strengthened and two were retrofitted by attaching CFRP (Sika Carbo-Dur S812 or Sika-Wrap 230C) to the tension side of the beams using high strength epoxy. The results show that one CFRP strip/wrap simply attached at the tension side can help the damaged beam regain/pass the original strength. All specimens fail due to debonding of CFRP from the concrete surface emphasizing the need for efficient anchorage systems. Among the four patterns adopted, CFRP strips along with u-shaped anchorages at the ends provided the highest strength enhancement of 17.36%.

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.

A lightweight and high-strength epoxy composites based on graphene oxide modified kapok fibers

Lightweight and high-strength (LWHS) composites are widely used in aerospace, automotive and marine fields. However, it is difficult to maintain the strength of the material while decreasing its density, let alone increasing. In this work, inspired by the mechanism of fiber-reinforced polymers, a LWHS epoxy composite was constructed through hollow kapok fibers. Computational fluid dynamics simulations show that the hollow structure of kapok fiber can be effectively retained. The results show that the graphene oxide effectively improves the interfacial adhesion between kapok fiber and epoxy matrix. Decrease in specific gravity and the enhancement of mechanical properties can simultaneously realize in the LWHS composites via the interweaving of kapok fibers. At an optimal 1.5 wt% fiber content, the specific gravity of LWHS composites decreased by 20.3% while tensile strength, bending strength and compressive strength increased by 21.5%, 9.56% and 51.92%, respectively. Tensile modulus, bending modulus and compression modulus increased by 48.35%, 92.18% and 49.84%, respectively.

Fully aminated rigid-rod aramid reinforced high strength epoxy resin and its composite with carbon fibers

Poly(p-aminophenylene aminoterephthalamide) ((NH2)2-PPTA), a new organo-soluble fully aminated rigid-rod aramid has been synthesized by the polycondensation of nitroterephthalic acid and 2-nitro-1,4-phenylenediamine and then hydrogenation-reduction of the nitro side groups. (NH2)2-PPTA was then used as the modifier of epoxy resin (EP) and carbon fiber (CF)/EP composites and their mechanical properties were systematically investigated. Because all the benzene rings in (NH2)2-PPTA contain amino side groups, (NH2)2-PPTA has excellent solubility in organic solvents and EP. In addition, the amino side groups are able to react with the matrix so as to effectively integrate the rigid-rod (NH2)2-PPTA into the cross-linked molecular network of EP and facilitate the stress transfer between EP and (NH2)2-PPTA. The tensile and flexural properties and notch impact strength of EP are greatly increased with the addition of only 0.5 wt% (NH2)2-PPTA. Unusually high tensile strength and flexural strength have been achieved, which reach 140.6 ± 4.1 MPa and 237.9 ± 12.3 MPa, respectively. Moreover, addition of a small amount of (NH2)2-PPTA also significantly improves the flexural strength, toughness and the interlaminar shear strength of CF/EP composites. Compared with previously reported nanofillers and other rigid-rod polymers, (NH2)2-PPTA shows better reinforcing and toughening effect for EP and CF/EP composites owing to its great solubility, reactivity and inherent great mechanical properties.

Self-healing and shape-memory epoxy thermosets based on dynamic diselenide bonds

The fabrication and development of excellent thermoset polymers with self-healing and shape memory properties is highly desirable. Herein, a novel high-strength epoxy resin (EP-SeSe) with diselenide bonds was rationally designed for rapid repair materials in building constructions and electrical packaging. The resultant resins exhibit excellent shape memory performance and self-healing performance due to the dynamic diselenide bonds (SeSe), while maintaining a high mechanical strength (tensile strength = 105 MPa and E = 2.0GPa), and exceptional thermal stability. It is especially noteworthy that the diselenide bonds of the bis(4-aminophenyl) diselenide (BAPDSe) allow EP-SeSe to exchange molecular chains in a dynamic manner, facilitating transport of chain segments while the ortho-phenyl groups prevent the side effects of weak bonds on mechanical properties. Furthermore, when the temperature exceeds Tg, EP-SeSe displays fast stress relaxation, self-healing, remodeling, and weldability due to the [2 + 1] radical-mediated mechanism of the diselenide bonds. We envision that this study provides a facile method for constructing robust multifunctional epoxy resins, which have great potential applications for self-healing and shape-memory epoxy resins.

Investigation on repair of tension cracks in reinforced concrete panels

This research concentrates on repair of tension cracks in RC panels subjected to direct tensile loading and hydrostatic water pressure by incorporating high-performance materials including High-strength epoxy, Glass Fiber Reinforced Polymer (GFRP) laminate, Engineered Cementitious Composite (ECC) slag, and ECC fly ash. The findings are directly applicable to construction and repair of structures in which crack control and liquid tightness are critical such as liquid containing structures. The test setup simulates repairing internal surface of a cracked wall segment of a containment exposed to internal hydrostatic pressure. The study showed the applicability of ECC fly ash in repairing RC specimens with the ability to restore the structural strength along with improving the cracking and leakage behavior. GFRP exhibited high effectiveness in waterproofing and repair of damaged RC structures. Applicability of epoxy injection in local repair of cracked RC specimens was confirmed, while failure of epoxy resulted in substantial growth in crack width and leakage rate of the panel.

Lateral Cyclic Load Test of Masonry Infilled RC Frames Strengthened with Ultra-High Performance Concrete Diagonal Strips

Infilled frame structures have been a common form of construction. However, their behavior is one of the least understood and investigated among all the major construction systems. For a long time, these structures were built with a general lack of conclusive research and design information. An experimental program was undertaken which aims at investigating the behavior of masonry infilled RC frames strengthened with Ultra-High Performance Concrete (UHPC). In this study, UHPC cross diagonal strips, having a width of 355 mm, were bonded onto the masonry substrate using high strength epoxy mortar. Single-side strengthening layout was adopted due to the fact that it is frequently implemented in the case of external facade walls of existing masonry buildings to preserve the building exterior aesthetics and also on internal walls which have some sorts of artworks on one side. Regarding the CFRP strengthened infilled frame, 60% and 160% increase was achieved in the lateral load carrying capacity and energy dissipation, respectively, when compared to the reference frame. Furthermore, the drift capacity at failure was improved by 1.58 and 1.40 times for UHPC and CFRP strengthened specimens, respectively, when compared with the reference specimen.

Comparative characterization of the surface state of Ti-6Al-4V substrates in different pre-bonding conditions

This paper deals with the interfacial adhesion properties of Ti-6Al-4V alloy substrates, prepared by using different treatment protocols selected on the basis of their different effectiveness, and bonded using a structural, high-strength epoxy adhesive. The alternative pre-bonding treatments were sodium hydroxide anodization and low-pressure-plasma treatment, the effects of which were compared to that of a base preparation via solvent degreasing of the substrates’ surface. The treated surfaces were joined according to standard protocols, and then tested for shear strength. The mechanical results were then correlated to surface characteristics of the substrates such as oxidation state and wettability. Parallel scanning Kelvin-probe measurements allowed us to focus our attention on the possible role of the electrical properties of the substrates. We observed that each treatment entails different behavior of the electrical potential of the surface, which correlates with the mechanical strength of the joints. The results suggest that an evaluation of the surface potential of titanium-alloy substrates might be a promising, indicative supplementary parameter for the evaluation of their pre-bonding surface conditions, allowing correlations with presence/absence of an oxide layer at the resin-substrate interface.

A modified method to measure delamination strength of stabilizer free REBCO coated conductor under transverse tension

A setup of transverse tension measurements of high temperature superconductor (HTS) RE-Ba-Cu-O coated conductors are modified for simplicity. Modification includes using high strength epoxy films instead of solder as adhesive agents and changing the shape of contact surface between test samples and anvils. This method was designed to measure delamination strength of stabilizer free REBCO coated conductors with a width more than 10 mm. It is also applicable to REBCO coated conductors with copper or stainless-steel stabilizers. Samples made by the IBAD/PLD process were tested by this test. Effect of operation temperature on the delamination strength of coated conductors was investigated. In addition, fracture surfaces of delaminated samples were studied.

Exploitation of rubbery electrospun nanofibrous mat for fracture toughness improvement of structural epoxy adhesive bonded joints

The improvement of the fracture toughness of adhesive joints is a key factor in many structural applications. The ability of nylon electrospun nanofibrous mat to act as an adhesive carrier and reinforcing web in adhesive bonding has been demonstrated by the Authors in previous works. It has been shown that the impregnation method developed and refined during the previous studies allow generating high-quality pre-preg nanomats out of a 2k unfilled epoxy resin. By applying this methodology, in the present work, rubbery nanofibrous mats have been adopted for the first time to reinforce and increase the fracture toughness of adhesive joints. Rubbery nanofibers were produced by electrospinning of nitrile butadiene rubber (NBR) and poly(ε-caprolactone) (PCL). The addition of the semi-crystalline polymer (PCL) is exploited to maintain the nanofibrous morphology, which the rubber alone (NBR) would not be able to ensure due to its low glass transition temperature (Tg). The nanofibers thus obtained have been integrated into a two-component high strength epoxy resin for structural applications. S235 steel adherends for Double Cantilever Beam (DCB) tests have been manufactured and sandblasted to improve adhesion. An optimization of the sandblasting parameters (distance, pressure, angle and time) has been carried out evaluating the shear strength and the fracture surfaces on S235 steel Single Lap Joints (SLJ). Finally, DCB tests have been performed to compare the mode I fracture toughness with and without the rubbery electrospun nanomats.

Integration of nylon electrospun nanofibers into structural epoxy adhesive joints

The fracture toughness is a key parameter in the development of bonded joints for several structural applications. Adhesives are commonly toughened with fillers or modifying the resin chemical composition. Many studies also suggest that resin toughening could be achieved through electrospun polymer nanomat. In previous works, the Authors proved that nylon nanomats can be used as an adhesive carrier and reinforcing web for the adhesive layer. This allowed developing a laboratory route to produce high-quality prepregs of electrospun nylon carrier using medium viscosity, two-component, unfilled epoxy adhesive. By applying the same methodology, in the present work, electrospun nylon prepregs were produced using a high strength and high toughness 2k structural epoxy adhesive to toughen the joint. The wet nano-reinforced strips were placed between S235 steel sandblasted adherents and oven-cured to obtain Double Cantilever Beam (DCB) joints. DCB tests have been performed to compare the mode-I fracture toughness with and without the nanofibrous mat. Unlike previous works with medium-low toughness epoxies, this time the fracture toughness is reduced after the integration of an electrospun nano-reinforcement. From the Scanning Electron Microscope (SEM) images it seems that the nanomat hinders the ductile failure mechanism which instead develops in the neat resin.