Rheological properties of neat epoxy exposed to in‐service aerospace contaminants

We present an experimental study on the rheological properties of a commonly used epoxy resin system (EPIKOTE‐862 resin and EPIKURE‐W curing agent), exposed to a variety of fluids typical of aerospace operations (jet fuel, hydraulic fluids, deicing, detergents, etc.), for a period of up to 6 months, at room temperature for most conditions, and with no concurrent mechanical loading or prior degradation. The specimens were subjected to stress and frequency sweeps with a shear rheometer, while a limited set received also a temperature sweep in a range consistent with aircraft operations. Results indicate that the treated resin samples are linear viscoelastic under these testing conditions. The resin has reasonable chemical resistance to most contaminants of this study, with the exception of two commonly used detergents: an aircraft surface cleaning compound, Penair C5572, and a nonionic detergent, Methyl Ethyl Ketone (MEK). The durability change of the first compound appears triggered by high temperatures only, while the second compound causes a very drastic stiffness loss under several conditions. This drop of performance occurs within a 3‐months period, with no apparent color change or fracture that could prompt visual inspection and repair. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3961–3971, 2013     

Development of high Tg epoxy resin and mechanical properties of its fiber‐reinforced composites

A new epoxy resin with high glass transition temperature (T_g) (～ 180°C) and a viscosity low enough for infiltration into dry reinforcements at 40°C was developed for the vacuum‐assisted resin transfer molding process. To study the curing behavior and viscosity, several blends were formulated using multifunctional resin, aromatic hardener, and reactive diluents. Effects of these components on the viscosity and T_g were investigated by thermomechanical analysis, dynamic scanning calorimetry, and rheometer. Experimental results showed that a liquid aromatic hardener and multifunctional epoxy resin should be used to decrease the viscosity to <1 Pa·s at 40°C. Moreover, the addition of a proper reactive diluent decreased the viscosity and simultaneously minimized the deterioration of T_g. Mechanical properties of the composite produced with the optimized blend were evaluated at both room‐temperature and high‐temperature conditions. According to the results, the composite showed comparable mechanical properties with that of the current commercial resin. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Shape memory CTBN/epoxy resin/cyanate ester ternary resin and their carbon fiber reinforced composites

Carboxylated-terminated liquid acrylonitrile rubber (CTBN) and epoxy resin (JEF-0211) were coreacted with cyanate ester (CE) to form CTBN/EP/CE ternary resin systems. Further, the ternary resin system was applied as prepreg for carbon fiber composites with vacuum bag degassing molding process. CTBN/EP/CE ternary shape memory polymer (SMP) exhibited relatively high tensile strength, Young's modulus, impact strength, and excellent shape memory properties. Compared with CTBN/EP/CE ternary SMP, CTBN/EP/CE carbon fiber composites showed much higher mechanical properties, such as their tensile strength and Young's modulus were high to 570 MPa and 36.7 GPa, respectively. Furthermore, CTBN/EP/CE carbon fiber composites exhibited good shape memory properties, their shape fixity ratio and shape recovery ratio were more than 95% after 30 times repeating shape memory tests. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48756.     

Tailoring the viscoelasticity of epoxy thermosets

This research work is aimed to understand the effect of epoxy resin chemistry on the viscoelasticity of the cured epoxy thermosets. In this article, two model systems are selected based on epoxy‐amine reactants. In the model Systems I, the functionality of the epoxy is varied. In model System II, the pendant (side) chain length in the amine is varied. It is found that by varying the initial properties (e.g., functionality, pendant chain length, mixing ratio (r), aromaticity, etc.) the network properties (crosslink density and flexibility) of the cured (or hardened) epoxy changed. The changes (or shift) in the viscoelastic properties (or viscoelasticity) of the cured epoxy is mainly due the changes in the network properties. Further, to study the time and temperature effects the viscoelastic master curves for the two model systems are generated using Time‐Temperature‐Superposition (TTS) principle. The shift in the viscoelastic master curves is modeled with a simple Kohlrausch‐Williams‐Watts (KWW) fit function. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Time-dependent properties of epoxy resin with imidazolium ionic liquid

This study investigates creep and viscoelastic behavior of the diglycidyl ether of bisphenol A (DGEBA) epoxy resin and triethylenetetramine (TETA) system containing an imidazolium ionic liquid (IL), the 1-n-butyl-3-methylimidazolium chloride ( C ₄ MImCl ). Different time-dependent analysis methods are studied using data from tensile creep, tensile creep/recovery, and three-point and four-point flexural creep tests of epoxy with 1.0 or 4.0 phr of IL. From the results, the composition containing 1.0 phr of C ₄ MImCl , cured at 60°C, presented greater viscoelasticity and crosslink density compared to compositions cured at 30 and 40°C, which was attributed to higher free volume and higher molecular mobility induced by the presence of the IL. In tensile creep tests using the stepped isostress method (SSM), no important degrading effects were found after the addition of 1.0 phr of IL over long time periods. This composition also showed the best overall performance in flexural SSM creep tests.     

Modification of polystyrene properties by CO2: Experimental study and correlation

The sorption of CO₂ in polymers entails their swelling and plasticization whose study is crucial for the design of processes and further applications. The operating conditions during foaming, purification, or impregnation of polymers in CO₂ are mainly determined by the mentioned binary system. In this work, the modification of polystyrene's physical properties (glass transition temperature and viscosity) has been experimentally studied. Since plasticization phenomena are very valuable for the processing of polymers, the amount of CO₂ absorbed into the polymer is related with the changes in the described properties. Furthermore, interfacial tension is also correlated with the sorption of CO₂ from literature data. The proposed correlation fits pretty well the properties shifts in the studied working conditions. Finally, the influence of pressure and temperature on the diffusivity of the CO₂ in the polystyrene is calculated through the measurement of viscosity along time. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41696.     

Stress relaxation behavior of starch powder‐epoxy resin composites

The purpose of the present study is to investigate the quasi‐static and the viscoelastic behavior of epoxy resin reinforced with starch powder. An increase in the elastic modulus on the order of 42% was achieved; a behavior that was predicted by the modulus prediction model (MPM). Next, the composite was subjected to flexural relaxation experiments, in order to determine the relaxation modulus, at different filler‐weight fractions and flexural deflections imposed. The viscoelastic models of the standard linear solid, the power law model and the residual property model (RPM) were applied in order to simulate/predict the stress relaxation curves. Predicted values derived from the application of the above models were compared to each‐other as well as to respective experimental findings. From the above comparison it was proved the superiority of the RPM model in predicting both the linear and the nonlinear viscoelastic response of the materials investigated. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41697.     

Structure and properties of a low viscosity acrylate based flexible epoxy resin

A series of low viscosity acrylate‐based epoxy resin (AE)/glycol diglycidyl ether (GDE) systems were prepared. The effect of GDE and low molecular weight polyamide (LPA) content on the rheological behavior, phase structure, damping, and mechanical properties were studied by differential scanning calorimeter (DSC), viscometer, scanning electron microscopy (SEM), dynamic mechanical thermal analysis (DMTA), and electro mechanical machine. The viscosity of the uncured AE systems decreased significantly after the incorporation of GDE. The damping properties were found to decrease slightly with the increasing GDE and LPA content. The tensile strength of the cured AE/GDE samples enhanced significantly after the incorporation of GDE with at least 150% improvement for all the samples while it decreased slightly with increasing LPA content. The AE/GDE cured systems were intended for future use as structural damping materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42959.     

Tack properties and adhesion mechanism of two different crosslinked polyacrylic pressure-sensitive adhesives

Tack properties of cross-linked random poly(n-butyl acrylate-acrylic acid) (A) and poly(2-ethylhexyl acrylate-acrylic acid) (B) copolymers as pressure-sensitive adhesives (PSAs) were compared by a probe tack test to know the optimal application in the industrial field. Tack increased remarkably with temperature, reached a peak, then decreased. The peak of tack appeared at higher temperature for B. Tack increased with increasing contact time and decreasing crosslinking agent level. The fracture energy at higher temperature was higher for B than A. From the observation of debonding behavior, the fibrillation occurred at the edge of probe. The wettability and deformability of PSA were larger for B than A. From a dynamic mechanical analysis, the shear storage modulus (G') in the rubbery plateau region was lower for B than for A. The good wettability and deformability were improved as a result of its lower G'. The relaxation behaviors of PSAs and vulcanized isoprene rubber were measured by ¹H pulsed nuclear magnetic resonance. This technique is found to be useful for estimating the degree of intermolecular interactions. The crosslinking degree hardly influenced. The intermolecular interaction was weaker for B. This was the reason of the lower G' for B.     

Curing behavior and thermal properties of multifunctional epoxy resin with methylhexahydrophthalic anhydride

The curing behavior and thermal properties of bisphenol A type novolac epoxy resin (bisANER) with methylhexahydrophthalic anhydride (MHHPA) at an anhydride/epoxy group ratio of 0.85 was studied with Fourier‐transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetry. The results showed that the FTIR absorption intensity of anhydride and epoxide decreased during the curing reaction, and the absorption peak of ester appeared. The dynamic curing energies were determined as 48.5 and 54.1 kJ/mol with Kissinger and Flynn–Wall–Ozawa methods, respectively. DSC measurements showed that as higher is the curing temperature, higher is the glass transition. The thermal degradation of the cured bisANER/MHHPA network was identified as two steps: the breaking or detaching of ? OH, ? CH₂? , ? CH₃, OC? O and C? O? C, etc., taking place between 300 and 450°C; and the carbonizing or oxidating of aromatic rings occurring above 450°C. The kinetics of the degradation reaction was studied with Coats–Redfern method showing a first‐order process. In addition, vinyl cyclohexene dioxide (VCD) was employed as a reactive diluent for bisANER (VCD/bisANER = 1 : 2 w/w) and cured with MHHPA, and the obtained network had a higher T_g and a slight lower degradation temperature than the undiluted system. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2041–2048, 2007     

Synthesis of isocyanurate-based imidazole carboxylate as thermal latent curing accelerator for thermosetting epoxy resins

A novel thermal latent curing accelerator, 1-(2-cyanoethyl)-2-methylimidazole/tris (2-carboxyethyl) isocyanurate adduct (2MICN-T), was successfully synthesized through an acid–base neutralization of tris(2-carboxyethyl)isocyanurate (TCEIC) and 1-(2-cyanoethyl)-2-methylimidazole (2MICN). It was further added into diglycidylether of bisphenol A based epoxy resin/methylhexahydrophthalic anhydride mixture to form one-component curing systems. With the addition of 2 wt% of 2MICN-T, the one-component system could be steadily stored for more than 1 month at room temperature, while the shelf life of 2MICN curing system was only 2 days. Nonisothermal differential scanning calorimeter also demonstrated the excellent thermal latency of 2MICN-T in low-temperature region and rapid initiation of the curing reaction when raising temperature. Compared to the cured resins with original 2MICN as accelerator, the resulted thermosets exhibited enhanced glassy storage modulus, glass transition temperature, and thermal stability when 2 wt% of 2MICN-T was applied. It was attributed to the chemical incorporation of the isocyanurate moieties with multi carboxyl groups and nitrogen-contained heterocyclic ring, effectively increasing the crosslinking density, chain rigidity, and heat resistance of the cured resin. Thus, it is suggested that 2MICN-T can play both roles as latent curing accelerator and modifier for one-component epoxy compounds, and is particularly recommended for application in electronic packaging fields.     

Acrylic‐based epoxy resin damping systems: Synthesis,characterization, and properties

Acrylate‐based epoxy resin (AE)/low molecular weight polyamine (LPA) composites were developed. The chemical structure, curing behavior, fracture morphology, damping properties, and mechanical properties were evaluated by Fourier transform infrared (FTIR), ¹H‐nuclear magnetic resonance (¹H‐NMR), gel permeation chromatography (GPC), Differential scanning calorimeter (DSC), scanning electron microscope (SEM), Dynamic mechanical thermal analysis (DMTA), and electro mechanical machine. Transmission electron microscope (TEM) and SEM pictures exhibited nanoscale micro‐phase separation between epoxy and acrylic segmers. DMTA results indicated that the loss factor of cured AE/LPA system could reach 1.84 and temperature range of tan δ > 0.5 was about 84 °C. Tensile strength and elongation at break of the cured AE samples can reach 6.5 MPa and 185%, respectively. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43654.     

Processing and mechanical properties of bio‐derived vinyl ester resin‐based composites

A “green” vinyl ester resin (GVER) is investigated for use in structural applications. The GVER was formulated using a monodisperse vinyl ester created via a novel synthetic route capable of using bio‐waste material from paper and biodiesel industries. The GVER was used either as a neat resin or as blended with a commercial vinyl ester resin. The processing viscosity and gel times are investigated. The GVER reaches a similar viscosity as the commercial resin with only half the styrene monomer content, thereby reducing the volatile organic compounds associated with manufacturing. Composites of the GVER matrix reinforced by carbon fabric were tested for their tensile and flexural properties. The mechanical performance of the GVER compares favorably with commercial resin and provide a route for composites manufacturing from sustainably sourced vinyl ester matrix. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44642.     

Insights into the synergistic mechanism of reactive aliphatic soft chains and nano-silica on toughening epoxy resins with improved mechanical properties and low viscosity

Toughening epoxy resin (EP) without sacrificing strength, modulus, and processing performance is always a harsh task. Here, a series of epoxy systems containing soft butyl glycidyl ether (BGE) and rigid nano-silica (nano-SiO₂) were prepared. Micro-phase separation structures derived from the self-assembly effect of BGE can be observed in atomic force microscopy images by controlling the total amount of BGE and nano-SiO₂ at 2 wt% for the EP_C:Si-m:n (m + n = 4) systems. Due to the synergistic effect of self-assembly effect of BGE and the rigid effect of well dispersed nano-SiO₂, EP_C:Si-2:2 system exhibited improvement of tensile strength of 59.3% (92.63 MPa), tensile modulus of 24.8% (3.52 GPa), elongation at break of 78.6% (4.84%), and glass transition temperature of 2.4% (138.4°C) compared with Pure EP system. Besides, due to the low loading of nano-SiO₂ (≤2 wt%) and the dilution effect of BGE, the viscosity of all the toughening systems is lower than 600 mPa·s, which can provide this toughening system with superior processing performance for large production of composites by automotive manufacturing methods such as vacuum assistant resin infusion technology.     

Multiscale thermal modeling of cured cycloaliphatic epoxy/carbon fiber composites

Cycloaliphatic epoxies (CEs) are commonly used for structural applications requiring improved resistance to elevated temperatures, UV radiation, and moisture relative to other epoxy materials. Accurate and efficient computational models can greatly facilitate the development of CE‐based composite materials for applications such as Aluminum Conductor Composite Core high‐voltage power lines. In this study, a new multiscale modeling method is developed for CE resins and composite materials to efficiently predict thermal properties (glass‐transition temperature, thermal expansion coefficient, and thermal conductivity). The predictions are compared to experimental data, and the results indicate that the multiscale modeling method can accurately predict thermal properties for CE‐based materials. For 85% crosslink densities, the predicted glass‐transition temperature, thermal expansion coefficient, and thermal conductivity are 279 °C, 109 ppm °C^?1, 0.24 W m^?1 K^?1, respectively. Thus, this multiscale modeling method can be used for the future development of improved CE composite materials for thermal applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46371.     

Glass‐transition temperature based on dynamic mechanical thermal analysis techniques as an indicator of the adhesive performance of vinyl ester resin

Vinyl ester resins are being used extensively as matrices in fiber‐reinforced polymer composite materials, but their use as a structural adhesive has been limited. Initial studies investigating the durability of a vinyl ester as a wood adhesive showed unsatisfactory performance in comparison with other adhesives. In this work, the glass‐transition temperatures (T_g's) of a vinyl ester and a E‐glass/vinyl ester composite material, fabricated by the Composites Pressure Resin Infusion System, were determined with dynamic mechanical thermal analysis. The results indicated that the resin cured under ambient conditions had a much lower T_g (～60°C) than the postcured material (～107°C). This suggested undercuring, that is, incomplete crosslinking, of the resin when it was cured at room temperature. E‐glass/vinyl ester samples, however, showed virtually no difference in T_g between room‐temperature‐cured and postcured samples. The exact reasons for this are not currently known but are thought to be both mechanical and chemical in nature. On the basis of the findings presented in this article, it can be concluded that if this vinyl ester resin is to be used as a structural adhesive, postcuring or formulation to ensure a high degree of crosslinking under ambient conditions is necessary. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2221–2229, 2005     

Impact of water absorption on the creep performance of epoxy/microcrystalline cellulose composites

Recently, considerable effort has been made to study cellulose/epoxy composites. However, there is a gap when it comes to understanding the post-conditioning anomalous effect of moisture uptake on their mechanical and dynamic-mechanical properties, and on their creep behavior. In this work, up to 10.0 wt% microcrystalline cellulose (MCC) was incorporated into epoxy resin by simple mixing and sonication. Epoxy/MCC composites were fabricated by casting in rubber silicone molds, and rectangular and dog-bone test specimens were produced. The moisture uptake, dynamic mechanical, chemical, tensile, and creep behavior were evaluated. The incorporation of MCC increased the water diffusion coefficient. The changes in storage modulus and glass transition temperature, combined with Fourier-transform infrared spectroscopy analysis, evidenced that water sorption in epoxies causes both plasticization and additional resin crosslinking, although the latter is prevented by the addition of MCC. The creep strain of the composites increased by 60% after conditioning, indicating that plasticization induced by water sorption plays an important role in the long-term properties of the composites.     

Effects of polycaprolactone molecular weights on thermal and mechanical properties of polybenzoxazine

Polymer blends of polybenzoxazine (PBA‐a) and polycaprolactone (PCL) of different molecular weights (M_n = 10,000, 45,000, and 80,000 Da) were prepared at various PBA‐a/PCL mass ratios and their properties were characterized. The results from dynamic mechanical analyzer (DMA) revealed two glass transition temperatures implying phase separation of the two polymers in the studied range of the PCL contents. Moreover, a synergistic behavior in glass transition temperature (T_g) was evidently observed in these blends with a maximum T_g value of 281°C compared with the T_g value of 169°C of the PBA‐a and about ?50°C of the PCL used. The blends with higher M_n of PCL tended to provide greater T_g value than those with lower M_n of PCL. The modulus and hardness values of PBA‐a were decreased while the elongation at break and area under the stress?strain curve were increased with an increase of the content and M_n of PCL, suggesting an enhancement of toughness of the PBA‐a. Scanning electron micrographs (SEM) of the sample fracture surface are also used to confirm the improvement in toughness of the blends. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41915.     

Mechanical and thermal properties of biphenyldiol formaldehyde resin/gallic acid epoxy composites enhanced by graphene oxide

In this study, the gallic acid‐based epoxy resin (GA‐ER) and alkali‐catalysed biphenyl‐4,4′‐diol formaldehyde resin (BPFR) are synthesized. Glass fibre‐reinforced GA‐ER/BPFR composites are prepared. Graphene oxide (GO) is used to improve the mechanical and thermal properties of GA‐ER/BPFR composites. Dynamic mechanical properties and thermal, mechanical, and electrical properties of the composites with different GO content are characterized. The results demonstrate that GO can enhance the mechanical and thermal properties of the composites. The glass transition temperature, T_g, of the BPFR/GA‐ER/GO composites is 20.7°C higher than the pure resin system, and the 5% weight loss temperature, T_d5, is enhanced approximately 56.6°C. When the BPFR: GA‐ER mass ratio is at 4 : 6 and GO content is 1.0–1.2 wt %, the tensile and impact strengths of composites are 60.97 MPa and 32.08 kJ/m² higher than the pure resin composites, respectively. BPFR/GA‐ER composites have better mechanical properties, and can replace common BPA epoxy resins in the fabrication of composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42637.