Fast-curing Epoxy Research Articles

Enhance the performance of steel-wood composite screw connections with fast-curing adhesive

In composite floors composed of the H-steel and the wood panel, screw connection is one of the design keys. In this paper, the method of using fast-curing epoxy resin to anchor the screw tip was proposed, and six sets of pull-out tests and five sets of push-out tests of epoxy-wood-combined anchored screws were conducted to analyze the influence of the effective meshing length and the epoxy meshing ratio on the connection performance, respectively. Firstly, compared with ordinary screws, the pull-out stiffness and strength of epoxy-anchored screws were increased by 79.8 % and 153.9 %, respectively. Secondly, compared with the ordinary screw connection, the initial shear stiffness and capacity of the steel-wood composite connection were increased by 60.0 % and 22.8 % when the epoxy meshing ratio was 0.25, respectively. Thirdly, the modified effective meshing length of epoxy-anchored screws was proposed. Finally, based on the calculation formula for screws with two plastic hinges in Eurocode 5, the two parameters of pull-out resistance and yield moment for the screw were modified, thus improving the calculation accuracy of shear capacity for combine-anchored screw connections by about 21.0 %.

The distinctions of the interfacial nanostructures and properties between carbon fiber reinforced fast and conventional curing epoxy composites subjected to hydrothermal treatments

The vital role of the interphase in loading transfer in carbon fiber reinforced resin composites has received considerable critical attention. In this study, to quantitatively distinguish between the effects of hydrothermal aging on the interfacial nanostructures and properties of carbon fiber reinforced fast and conventional curing epoxy composites, peak force quantitative nano-mechanics (PF-QNM) technique by atomic force microscopy (AFM) was utilized to determine the interfacial modulus, adhesion and thickness. In addition, the shear behavior was studied by short-beam strength test, and the water uptake was investigated by water adsorption test. Based on the modulus contrast of the interphase in PF-QNM modulus image, the average interfacial thickness of the fast curing epoxy composites after 18 h of hydrothermal aging was 67 nm, increased 236 % when compared with the unaged composites. It was much higher than the rise of 70 % of the conventional curing epoxy composites after 18 h of hydrothermal aging while the interfacial thickness was 69 nm. Consistently, the average interfacial thickness of the fast and conventional curing epoxy composites determined by the adhesion were 68 nm (widened 247 %) and 68 nm (widened 67 %). In a word, the interfacial thickness of the fast curing epoxy composites was about 4 times than that of the conventional curing epoxy composites after 18 h of hydrothermal aging, because of the fast curing caused microcracks and flaws. Then, after 192 h of hydrothermal aging, the decreasing of short-beam strength of the fast curing epoxy composites were almost 1.6 times than that of the conventional curing epoxy composites, due to the faster growing of microcracks and delamination on the side surface of the fast curing epoxy composites short-beam samples during testing. Lastly, the maximum water absorption and diffusion coefficient of the fast curing epoxy composites were 1.2 % and 5.5 × 10−7 cm2/s respectively, which were both 1.3 times that of the conventional curing epoxy composites. Significantly, all the results proved that, attributing to the fast curing caused microcracks and flaws in the fast curing epoxy composites, through which water preferred to enter a composite along the fibers and interphase, the interphase in the fast curing epoxy composites were more sensitive to the hydrothermal aging than the conventional curing epoxy composites. Therefore, the water uptake, interfacial thickness and short-beam strength of the fast curing epoxy composites changed faster.

Optimization of prepreg compression molding process for production of structures with recycled carbon fibers

Carbon fibers are widely used in a growing number of industrial sectors. Their high strength to weight ratio makes carbon fibers suitable for high-value-added applications. However, significant environmental and economic problems are related to composite waste disposal and consumption of non-renewable resources for their production. European legislation has imposed more controls on composite materials waste management. Consequently, development of processes for the recovery of the added value of wastes, instead of its simple disposal, is crucial to promoting the application of EU requirements and the realization of a circular economy model. Currently, thermal processes represent the most promising technology in this direction. Thermal processes include pyrolysis, fluidized bed pyrolysis, and microwave-assisted pyrolysis, but only pyrolysis is implemented on an industrial scale in Europe because of its economic viability and technical feasibility. Although the recovery of carbon fibers has reduced cost, compared to virgin fibers, length, orientation, and sizing of the fibers are lost. In this context the challenge is to realize composites reinforced with reclaimed fibers that can compete economically and in terms of mechanical performance with composite with virgin fibers. In this work, prepreg compression molding technique was used to produce a window frame with non-woven fabrics of recycled carbon fibers impregnated with a fast-curing resin. The fibers were coated with a sizing developed by CETMA and compatible with fast-curing epoxy resin.

Development of a High-Fidelity Framework to Describe the Process-Dependent Viscoelasticity of a Fast-Curing Epoxy Matrix Resin including Testing, Modelling, Calibration and Validation.

Fast-curing epoxy resins enable substantial reduction of cycle times during production of thermoset polymer matrix composites. Due to the snap-cure behaviour, both characterisation and processing of these resins are associated with high complexity which motivates the development of a high-fidelity framework for the prediction of the process-dependent behaviour ranging from experiment to model validation. In order to determine influence of time, temperature, and degree of cure, a multitude of rheometer and dynamic mechanical analysis experiments are conducted and evaluated. Building on the experimental results, a material model based on a generalised Maxwell model is developed. It is calibrated on the results obtained in the tests and shown to describe the material’s behaviour with high accuracy under all investigated conditions. The model’s predictive capabilities are further tested by applying it to a dynamic mechanical analysis, exposing the model to previously unknown loading and temperature conditions. It is demonstrated that the model is capable of predicting such changing boundary conditions with high accuracy.

Kinetics of fast-curing epoxy resin cationic thermopolymerization: Propagated by ACE and AM mechanism

Kinetics of fast-curing epoxy resin cationic thermopolymerization: Propagated by ACE and AM mechanism

The Mechanical Characterization of Welded Hybrid Joints Based on a Fast-Curing Epoxy Composite with an Integrated Phenoxy Coupling Layer.

The joining of composites mostly relies on traditional joining technologies, such as film or paste adhesives, or mechanical fasteners. This study focuses on the appealing approach of using standard thermoplastic welding processes to join thermosets. To achieve this, a thermoplastic coupling layer is created by curing with a thermoset composite part. This leads to a functional surface that can be utilized with thermoplastic welding methods. The thermoplastic coupling layer is integrated as a thin film, compatible with the thermoset resin in the sense that it can partially diffuse in a controlled way into the thermoset resin during the curing cycle. Recent studies showed the high affinity for the interphase formation of poly hydroxy ether (phenoxy) film as coupling layer, in combination with a fast-curing epoxy system that cures within 1 min at 140 °C. In this study, an investigation based on resistance and ultrasonic welding techniques with different testing conditions of single-lap shear samples (at room temperature, 60 °C, and 80 °C) was performed. The results showed strong mechanical strengths of 28.9 MPa (±0.7%) for resistance welding and 24.5 MPa (±0.1%) for ultrasonic welding, with only a minor reduction in mechanical properties up to the glass transition temperature of phenoxy (90 °C). The combination of a fast-curing composite material with an ultra-fast ultrasonic joining technology clearly demonstrates the high potential of this joining technique for industrial applications, such as automotive, sporting goods, or wind energy. The innovation allowing structural joining performance presents key advantages versus traditional methods: the thermoplastic film positioning in the mold can be automated and localized, joint formation requires only a fraction of a second, and the joining operation does not require surface preparation/cleaning or structure deterioration (drilling).

Characterization and modeling cure- and pressure-dependent thermo-mechanical and shrinkage behavior of fast curing epoxy resins

A fast crosslinking reaction is crucial for the economical production of automotive components in liquid composite molding (LCM) processes. Although a large number of works have focused on modeling of shrinkage induced surface waviness and part distortions and the underlying phenomena an accurate determination of material properties under consideration of processing parameters still represents a major challenge for fast-curing epoxy resins. Fast-curing epoxy polymers allow manufacturing of thermoset composite parts in minutes while entailing a highly exothermic heat generation. Cure is carried out under elevated temperatures and pressures and is accompanied by a change in density due to chemical shrinkage and followed by thermal contraction. In the present study, comprehensive results on the characterization of thermo-mechanical properties, pressure dependent reaction kinetics and shrinkage are presented for a fast curing epoxy resin. The obtained results give important advice for improved process modeling and facilitate prioritisation for future characterisation of fast-curing epoxy resins.

Fast curing unidirectional carbon epoxy prepregs based on a semi-latent hardener: The influence of ambient aging on the prepregs Tg0, processing behavior and thus derived interlaminar performance of the composite

The aging behavior of a fast-curing epoxy resin (EP) was examined over three months in unidirectional carbon fiber prepregs for the first time. The semi-latent, 120 °C curing one-pot formulation was developed based on industry-grade diglycidyl ether of bisphenol A (DGEBA) resins without the necessity of B-staging, to fully monitor the property change by aging without any post-processing influence. To extend the shelf-life compared to standard wet EP systems, the latent dicyandiamide (DICY) and a semi-latent urea-based catalyst were used, leading to full conversion within 15 min. The conversion was monitored by increasing Tg0 and rheology. High quality unidirectional carbon prepregs were manufactured and then investigated over three months regarding tackiness and resin flow. In parallel, laminates were autoclave-manufactured to evaluate the aging influence on the interlaminar properties. Thus, a correlation between decreasing gel-time, interlaminar adhesion and resulting multiple crack growth was observed via GIC and interlaminar shear strength (ILSS) testing. The investigations show a clear influence of the consolidation quality of the prepregs based on ILSS values and GIC values. A decrease of 19% in ILSS and combined with an erroneous increase of 63% of the GIC due to multiple delamination was observed in the worst case.

In situ curing and bonding of epoxy prepregs in epoxy thermoset injection molding

In this study, epoxy molding compounds are combined with fast-curing epoxy prepregs in thermoset injection molding using a new integrative process. The combination is carried out under the varied parameters of mold temperatures and curing times, which are dominant factors in thermoset processing. The focus of the investigations is the bond strength in the interface resulting from these parameters, as the interface is known as the weak point of hybrid components. To identify causes for possible increases and decreases of the bond strength, additional rheological and thermoanalytical analyses are done under near-process conditions. The influence of prepreg pre-crosslinking, a function of the mold temperature, is also described by means of additional tests in which specific pre-crosslinking of the prepreg is adjusted by the temperature storage and then functionalized in integrative process combination. The aim of the study is to identify and understand initial process limits for the integrative process combination for a potential process window.

Effect of carbazole coating on TiO2 nanoparticles as a photosensitizer and MWCNTs on the performance of epoxy composites

Over a short period of time, electronic assemblies have been dramatically developed with smaller size and lighter weight and 3D printing plays a crucial role in their future production. However, it is difficult to develop epoxy resins and other thermoset resins for this process because of the polymer's liquid state during processing. This work uses a UV curing technique for making flexible epoxy/multi-walled carbon nanotubes (MWCNTs) composites that are suitable for future 3D printing applications. The synthesized carbazole-coated TiO 2 nanoparticles, used as a photosensitizer, were characterized by their photoabsorbance, morphology, and surface property before use. The results indicated that the TiO 2 nanoparticles coated with 75 wt% carbazole were appropriate for UV-cured flexible epoxy/MWCNT composites because of its fast cure kinetics after irradiation by a UV light source. The epoxy/MWCNT composites displayed low tensile properties because of the agglomeration of MWCNTs inside the epoxy matrix. In addition, the epoxy/MWCNT composites showed slightly lower thermal expansion with high electrical properties with a large amount of MWCNTs in the epoxy composites. • The synthesis and characterization of carbazole coating on TiO 2 nanoparticle as a new photosensitizer for fast curing epoxy composite with MWCNTs • The morphological, mechanical, thermal, and electrical properties of flexible epoxy composites after complete cure by UV technique.

Investigation of the interphase mechanisms and welding behaviour of fast-curing epoxy based composites with co-cured thermoplastic boundary layers

This study focuses on investigating thermoplastic polymers and their interphase behaviour with the fast-curing epoxy system Araldite LY3585/Aradur 3475. The three compatible thermoplastic polymers poly(methyl methacrylate) (PMMA), amorphous polyamide (PA12TR90), and poly(hydroxy ether) (phenoxy) were chosen to determine their affinity and interphase formation with the epoxy system. The final concept involves the use of a thermoplastic as a boundary layer on top of the epoxy to establish a connection between two joined parts by resistance welding. The thermoplastic boundary layer was generated by co-curing during the curing process. The experiments resulted in a high compatibility between phenoxy and PMMA in the selected epoxy system which leads to excellent mechanical properties after welding. Phenoxy showed the strongest joints, with an average lap shear strength of 25 MPa, demonstrating the high potential of this joining technique for industrial applications in the automotive and wind energy fields.

Investigation of the Shrinkage Behaviour of a Fast Curing Epoxy Resin as a Function of Moisture and Gas Content and the Mixing ratiohttps://www.kunststoffe.de/a/article/article-245087

Investigation of the Shrinkage Behaviour of a Fast Curing Epoxy Resin as a Function of Moisture and Gas Content and the Mixing ratiohttps://www.kunststoffe.de/a/article/article-245087

Thermomechanical Characterization of a Fast Curing Epoxy Resin for Simulation of Surface Waviness

The interaction of process parameters and material properties represents a major challenge for reproducible manufacture of high quality composite parts with polymer matrix. Major phenomena that have to be taken into account are thermal expansion, chemical shrinkage as well as process-dependent curing and mechanical properties that lead to residual stresses and geometrical deviations on different length scales. The analysis of these effects is even more challenging when considering fast curing resins. This contribution presents the development of a simulation approach dedicated to the prediction of surface waviness originating from liquid injection molding (LCM). A fast curing epoxy resin used in automotive industry was characterized in terms of curing kinetics and thermomechanical properties. A generic cure cycle was derived. A simulation approach is presented which aims at the numerical analysis of process-related parameters and their influence on surface waviness. In a first step, the influence of the resin thermomechanical behavior and the tool boundary condition on surface deformation is evaluated during cooling. The presented work provides a basis for a comprehensive understanding of the underlying effects and prospectively allows a linkage to internal residual stresses.

Determining the interphase thickness and properties in carbon fiber reinforced fast and conventional curing epoxy matrix composites using peak force atomic force microscopy

Quantitative analyses of the thickness and properties of the interphase in carbon fiber reinforced resin matrix composites, which play an important role in loading transfer, are essential and intuitive to accurately design the properties of the composites. In this study, to know the distinctions of the interphase between the carbon fiber reinforced fast and conventional curing epoxy matrix composites quantitatively and completely, in addition to modulus map, adhesion map by Peak Force Quantitative Nano-Mechanics technology was newly introduced and applied to determine the interphase thickness and properties. The interphase in the fast curing epoxy matrix composites in modulus image was found to be soft with an average thickness of 20.03 ± 2.04 nm, while it was 40.48 ± 4.17 nm in the conventional curing epoxy matrix composites, and the modulus of the interphase formed a depression between the fiber and the matrix. Consistent with the interphase thickness determined by the modulus, the average interphase thickness in the fast and conventional curing epoxy matrix composites were 19.45 ± 0.68 nm and 41.01 ± 3.98 nm, respectively, based on the adhesion contrast in the interphase, and the adhesion of the interphase reached a peak between the fiber and the matrix. Compared with the conventional curing epoxy matrix composites, the modulus and the adhesion of the interphase changed more sharply in the fast curing epoxy matrix composites. Obviously, numerous distinctions of the interfacial properties existed between the fast and conventional curing epoxy matrix composites.

Curing kinetics and mechanical properties of fast curing epoxy resins with isophorone diamine and N‐(3‐aminopropyl)‐imidazole

ABSTRACTFast curing epoxy resins were prepared by the reactions of diglycidyl ether of bisphenol A with isophorone diamine (IPD) and N‐(3‐aminopropyl)‐imidazole (API), and their curing kinetics and mechanical properties influenced by IPD content were also investigated. The analysis of curing kinetics was based on the nonisothermal differential scanning calorimetry (DSC) data with the typical Kissinger, Ozawa, and Flynn–Wall–Ozawa models, respectively. The glass‐transition temperature was also measured by the same technique. Additionally, the mechanical properties including flexural, impact, and tensile performances were tested, and the curing time was estimated by isothermal DSC. The degree of cure (α) dependency of activation energy (Ea) revealed the complexity of curing reaction. Detailed analysis of the curing kinetics at the molecular level indicated that the dependence of Ea on the α was a combined effect of addition reaction, autocatalytic reaction, viscosity, and steric hindrance. From the nonisothermal curves, the curing reaction mechanism could be proposed according to the increasingly obvious low temperature peaks generated by the addition reaction of epoxy group with the primary amines in API and IPD molecules. Using the preferred resin formulation, the resin system could be cured within 10 min at 120 °C with a relatively good mechanical performance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47950.

The role of ionic-liquid extracted lignin micro/nanoparticles for functionalisation of an epoxy-based composite matrix

The present study aims to design high performance functionalized lignin-epoxy composites. In this work, the triethylammonium hydrogen sulphate ionic liquid (IL) was introduced onto the surface of lignin micro/nanoparticles while it was extracted from biomass, to prepare a highly functional and reinforcing IL-Lignin filler in an epoxy matrix. A bio-composite of IL-Lignin epoxy was prepared from IL-Lignin and a fast curing epoxy pre-polymer hardened with an anhydride-based curing agent using a simple one-step method. The thermal degradation mechanisms and the influence of the IL-Lignin on the thermal stability and cure kinetics of IL-Lignin epoxy networks have been investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Fourier Transformation Infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analysis confirm that the ammonium-based IL coupled with lignin is an effective promoter for crosslinking of the epoxy pre-polymer resulting in higher and faster degrees of conversion. Nuclear magnetic resonance spectroscopy reveals the characteristics of IL extracted lignin. Using atomic force microscopy (AFM) and dynamic light scattering (DLS), topographical features of the IL-Lignin surfaces and particle size of lignin at nanoscale were investigated and supporting evidence inferred underpinning the improved mechanical properties. The addition of 2 wt% IL-Lignin yielded an 80% increase in flexural strength (99.10 MPa), a 57% increase in flexural modulus (2.84 GPa), a 52% increase in tensile strength (40.90 MPa) and a 23% increase in toughness (86.08 kJ/m3), compared to the neat epoxy matrix. There was, however, a softening effect on the glass transition temperature due to the addition of IL-Lignin. This particular system of composite matrix is prone to controlled breakdown at the end of its lifecycle even after post cure due to biodegradable lignin linkages.

Investigation of fast curing epoxy resins regarding process induced distortions of fibre reinforced composites

Unavoidable deformations occur during part production due to the non-isotropic nature of carbon fibre reinforced plastics (CFRP). These deformations often lead to dissatisfaction of tolerances or result in cost and time intensive rework of the tooling. In a cost driven production environment, similar to the automotive industry, it is essential to predict the deformations early on in the part development process in order to compensate toolings accordingly. In future applications, Fast Curing Epoxy Resins (FCER), with curing times of less than 20 min, will play a key role in high rate CFRP-production at low cost.The present paper reports on a comprehensive experimental study on different FCER systems. It includes the thermo-chemical characterization of neat resin samples as well as the quantification of spring-in deformations of L-profiles. Essential part and processing parameters, as the lay-up, the curing temperature, the cooling rate and the fibre volume fraction are varied and their effect on process induced deformations is quantified. Results for FCER system are compared to slower curing systems to assess differences.

Investigation of strain history in fast and conventional curing epoxy matrix composites by FBGs

A comprehensive understanding of strain history in resin matrix composite, which is caused by variability of thermo-mechanical properties of the resin during composite processing, is essential to allow better design and control of properties of the resin matrix composite. In this paper, to know strain history of fast curing epoxy matrix composite and differences of strain history between fast and conventional curing epoxy matrix composites well, temperature and strain history at different locations in ten-ply unidirectional carbon-fiber fabrics reinforced the fast and conventional curing epoxy matrix composite laminates manufactured by wet lay-up method were measured by fiber Bragg grating (FBG) sensors. Results shown that the peak temperature due to curing exothermal reaction was 133.7 °C in both the 1st ply and the 5th ply in the fast curing composite when cure temperature profile settled at 80 °C, which was 27.4 °C higher than that in the conventional curing composite. Cure residual strain in the 1st ply and the 5th ply in the fast curing composite were −5183.3 με and −4074.7 με, respectively; while they were −2975.9 με and −2660.8 με in the conventional curing composite. The related properties of rheology and cure kinetics of the epoxy resin were thus given in advance.

Core-shell rubber nanoparticle reinforcement and processing of high toughness fast-curing epoxy composites

To simultaneously address the lower toughness and the build-up of internal heat for fast-curing epoxy matrices, the influence of nominal 100 nm and 300 nm core-shell rubber (CSR) particles on the properties and rheo-kinetics were studied. The fracture energy was enhanced by a factor of 14.5, up to 2572 ± 84 J m−2 with 14.5 wt% of the nominal 300 nm diameter CSR particles, with evidence of cavitation and plastic void growth of the rubber core combined with shear band yielding of the epoxy matrix. These toughening mechanisms were modelled with an approximately linear increase up to 10 wt% for both particle types. At higher concentrations, deviation between the measured and modelled data was observed due to insufficient epoxy to dissipate additional energy. The CSR particles were not filtered out or damaged during the manufacturing of composites and reduced the total heat of reaction with a linear correlation, demonstrating a multi-functionality of simultaneous toughening and reduction of the exothermic peak.

Formability of complex composite structures with ribs made of long carbon-fiber-reinforced prepregs

A long fiber prepreg sheet (LFPS) composed of long carbon fibers with lengths of 50mm and a fast-curing epoxy resin was used to fabricate complex-shaped structures with ribs using thermal compression molding. The formability of the composite structures was evaluated for various compression pressures (10, 30, and 60MPa) and times (5 and 10min) at a fixed curing temperature (155°C). Using a single layer of LFPS, the highest geometric accuracy of the composite structure with ribs was 99.64%, and the highest average fiber volume fraction of the rib part was 29.82% under 30MPa for 10min. Furthermore, the highest flexural strength and modulus of the ribs were 253MPa and 18GPa, respectively, under the condition of 30MPa and 10min. These values increased to 366MPa and 26GPa, respectively, when two layers (a fiber volume fraction of 38.15%) were applied. Among the various thermoforming conditions, an applied pressure of 30MPa and a pressing time of 10min at 155°C were the most appropriate to achieve relatively excellent net-shape molding with improved mechanical properties.