Uncured Epoxy Research Articles

3D Printing of Thermally Responsive Shape Memory Liquid Crystalline Epoxy Networks.

A two-component liquid crystalline epoxy network (LCEN) with shape memory behavior was developed and evaluated as a candidate material for 3D printing. The cure kinetics of the uncured material and the shape memory properties of the cured LCEN were investigated by using parallel plate rheology and dynamic mechanical analysis, respectively. A commercially available fumed silica additive was introduced to the neat, uncured material to improve the rheological properties for 3D printing. The addition of fumed silica was found to increase the yield stress, shear-thinning behavior, and toughness of the uncured epoxy ink. Polarized light microscopy, differential scanning calorimetry, and wide-angle X-ray scattering measurements between the neat and additive-modified LCEN suggested a reduction in liquid crystalline alignment in the modified LCEN, owing to interactions between crystalline domains and fumed silica, which in turn influenced the mechanical behavior. Overall, the additive was found to be successful in preserving the shape memory properties of LCEN while improving its printability.

The effect of absorbed moisture and resin pressure on porosity in autoclave cured epoxy resin

AbstractOne of the main issues in epoxy‐based composite manufacturing is the formation of porosity derived from moisture absorption during storage and layup due to the high hydrophilicity of epoxy matrices. During the curing process, the presence of moisture and other volatile compounds can initiate the nucleation and growth of voids. In this study, the effect of both the initial water content absorbed in the uncured resin and the pressure on the porosity development in an epoxy resin was investigated. In particular, Kardos' and Ledru's models, aimed at predicting void formation in polymers, were applied to study the effect of different hydrostatic pressures in an epoxy resin during curing up to the gel point, after conditioning it at two different relative humidity levels, 50% and 95%. Subsequently, the porosity of the cured resin samples was quantified through density measurements. Comparative analysis of the microscopy images of cured samples and the predictions of both models revealed an overestimation of the final void sizes by both models, with the Kardos' model exhibiting a higher deviation. Additionally, a finite element model was employed to investigate the conditions leading to void formation, aiming to understand the factors influencing the porosity development and properly set the process parameters during composite manufacturing.Highlights Evaluation of the conditions leading to void growth in epoxy resin during curing Moisture sorption in uncured epoxy resin Effect of curing pressure on pore development Finite element analysis for void growth during resin curing

Electrothermal free-form additive manufacturing of thermosets

Despite the extensive use of thermosetting resins, additive manufacturing of such materials is limited. Traditional processing requires either (i) long curing schedules to avoid the collapse of an uncured printed part or (ii) changes to the resin chemistry and curing mechanism. This work demonstrates a material-extrusion based setup capable of printing and curing any industry-standard thermoset resin through the use of electric fields and carbon nanomaterial susceptors. A material extrusion printer is used to deposit the liquid ink, composed of uncured epoxy with a carbon nanotube filler. After each layer is deposited, the nanotube-loaded ink is exposed to radio frequency (RF) fields generated by a coaxial applicator suspended over the part. The resulting current generates heat rapidly and volumetrically, thus curing the resin, preventing part collapse, and enabling further printing. This methodology enables rapid additive manufacturing of arbitrarily large structures with low viscosity resins that are otherwise impossible to print.

An approach for improving SHM systems with GUW detection and embedded sensors by a planar gradient acoustic impedance matching

AbstractFor technical structures, such as airplanes, the service life must be increased to satisfy the requirements for high efficiency in terms of costs and sustainability. Monitoring the system's current health and providing solutions to maintain it reliably is one way to accomplish this objective. A structural health monitoring system, in which actuators induce a guided ultrasonic wave or wavefield monitored by a sensor or a network of sensors, is a standard for thin‐walled structures, which are made of, for example, fibre‐metal‐laminates. The sensor measurements provide information that can be used to determine the respective health state of the system. However, embedding the actuator and sensors in the system typically results in a distortion of the wavefield, for example, mode‐conversion and reflections. The distortion can lead to over‐ or underestimation of the quantity, location and severity of the damage. Due to these false detections, both an unnecessary high effort for retrofitting the structure and a reduction of the structure's technical reliability may be needed. The objective of the current contribution is to reduce the wave‐field distortion/reflections caused by the sensors to prevent or minimise these false detections. To reduce the distortion and reflection of the wavefield, a functionally graded material with an acoustic impedance matching based on a mechanical model is used. In order to control and adjust the acoustic impedance, which is the leading parameter concerning distortion and reflections of waves, tungsten particles were added to the uncured epoxy resin. The design of an interphase between the sensor or its glass housing and the surrounding structure is achieved by this novel approach. The interphase properties are varying in the radial direction to the sensor and are depending on the tungsten particle content in the epoxy resin. In this work, several models regarding the radial distribution of the tungsten particle content and the excitation frequency are investigated. Additionally, the dependence on geometrical influence is also addressed. The approach results, for specific configurations, in (i) reduced reflections from the sensor (less distortion) and (ii) amplified measuring signals of the sensor. Numerous numerical examples demonstrate the benefit of the presented approach.

Impact of the Force Field on the Calculation of Density and Surface Tension of Epoxy-Resins.

The molecular simulation of interfacial systems is a matter of debate because of the choice of many input parameters that can affect significantly the performance of the force field of reproducing the surface tension and the coexisting densities. After developing a robust methodology for the calculation of the surface tension on a Lennard-Jones fluid, we apply it with different force fields to calculate the density and surface tension of pure constituents of epoxy resins. By using the model that best reproduces the experimental density and surface tension, we investigate the impact of composition in mass fraction on uncured epoxy resins and the effects of degree of cross-linking on cured resins.

Microscopic testing of carbon fiber laminates with shape memory epoxy interlayer

For the first time, microscopic testing has been performed on shape memory polymer composites (SMPCs) which were manufactured by commercial materials already used in aerospace. Results from micro-tests have been compared with those from conventional memory-recovery cycling on macro-scale. Two shape memory polymer composite (SMPC) laminates were fabricated with different shape memory (SM) interlayer: one in the form of an uncured epoxy powder and the other in the form of a thin epoxy foam. The latter, in particular has been studied to evaluate lightweight and stiff sandwich structures with SM properties. The assessment of the manufacturing process by a hot press moulding technique was assessed through micro scale analysis using SEM and MicroCT analysis. DMA analyses were carried out to understand the interaction mechanisms between raw constituents. A Vickers micro-indentation examination before and after heating was able to assess the shape recovery behaviour at the micro-scale level. A nano-instrumental indentation was used to characterise the shape memory response under different loads at elevated temperatures. Whilst an instrumented thermo-mechanical test allowed to investigate the shape memory behaviour at macro-scale level. Results allow identifying the recovery mechanisms at the micro-scale which are responsible for the shape memory performances at the macro-scale. The higher recovery ability of the SM foam is confirmed in comparison with bulk interlayers.

Molecular-Level Correlation between Spectral Evidence and Interfacial Bonding Formation for Epoxy Adhesives on Solid Substrates.

Interfacial bonding strength of an epoxy-based adhesive depends on the interfacial interaction between the adhesive and the substrate. Normally, the curing process at the interface accompanied by the interfacial bonding formation is different from that in the bulk, and it is still a big challenge to probe the interfacial bonding formation at a molecular level. In this study, to trace the interfacial structural evolution of a representative formula of epoxy (digylcidyl ether of biphenyl A, DGEBA) and amine hardener [1,2-bis(2-aminoethoxy)ethane, EDDA] with the sapphire and silica substrates upon curing and post-curing steps, sum frequency generation (SFG) vibrational spectroscopy is employed to detect the molecular-level interfacial structural information. For the sapphire substrate, upon curing, backbone methylene (CH2) stretching signals decrease, indicating the formation of a rigid chain network structure and thus losing the local methylene order, while vibrational signals of the sapphire surface hydroxyl (OH) groups (including hydrogen-bonded and unbonded) increase significantly, indicating the formation of a strong hydrogen-bonding and polar interaction between the epoxy adhesive and the sapphire surface. Upon post-curing, increased backbone CH2 signals and decreased sapphire OH signals suggest interfacial chemical bonding formation due to the reaction between the epoxy rings and the sapphire surface OH groups. Orientation analysis confirms the enhanced ordering of the sapphire surface OH groups upon curing and post-curing, in comparison to the uncured epoxy formula. As for the fused silica, weak vibrational signals of the methylene (CH2) and methyl (CH3) groups are observed before curing, while both of them increase slightly for the cured and post-cured epoxy formulae, suggesting relatively less hydrophilic nature of the silica surface compared to that of the sapphire surface, also evidenced by the very weak OH signals upon curing and post-curing. Further measurement on the adhesion strength matches up with the above spectroscopic experimental results, substantiating the correlation between the macroscopic bonding strength of the epoxy adhesive and the microscopic molecular-level structure.

Determination of Mechanical Properties of Epoxy Composite Materials Reinforced with Silicate Nanofillers Using Digital Image Correlation (DIC).

In this study, silicate nanofillers; dicalcium silicate, magnesium silicate, tricalcium silicate, and wollastonite; were synthesized using four different methods and incorporated into the epoxy resin to improve its mechanical properties. Characterization of the newly synthesized nanofillers was performed using Fourier-transformation infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The purpose of this study was to analyze newly developed composite materials reinforced with silicate nanoparticles utilizing tensile testing and a full-field non-contact 3D Digital Image Correlation (DIC) method. Analysis of deformation and displacement fields gives precise material behavior during testing. Testing results allowed a more reliable assessment of the structural integrity of epoxy composite materials reinforced using different silicate nanofillers. It was concluded that the addition of 3% of dicalcium silicate, magnesium silicate, tricalcium silicate, and wollastonite lead to the increasement of tensile strength up to 31.5%, 29.0%, 27.5%, and 23.5% in comparison with neat epoxy, respectively. In order to offer more trustworthy information about the viscoelastic behavior of neat epoxy and composites, a dynamic mechanical analysis (DMA) was also performed and rheological measurements of uncured epoxy matrix and epoxy suspensions were obtained.

A Study on Temperature Distribution within HVDC Bushing Influenced by Accelerator Content during the Curing Process

Power transmission technology plays an important role in energy sustainability. Bushing is an indispensable type of equipment in power transmission. In production, the accelerator changes the temperature distribution during the curing process, influencing the formation of defects and thus the safety output of renewable energy. In this study, uncured epoxy resin samples with different accelerator contents were prepared and measured by differential scanning calorimetry (DSC). The obtained heat flow curves were analyzed for curing kinetics. Then, the curing process of large length–diameter ratio bushings was simulated by using the finite element method combined with a curing kinetics model, transient Fourier heat transfer model, and stress–strain model. The study reveals that the curing system can be established by the Sestak–Berggren autocatalytic model with different accelerator contents. The overall curing degree and the maximum radial temperature difference of the capacitor core tend to increase and then decrease with the accelerator content. This is mainly attributable to the rapid exotherm excluding the participation of some molecular chains in the reaction, resulting in permanent under-curing. As the accelerator content increases, the strain peak decreases and then increases. This paper provides guidance for the comprehensive evaluation and manufacturing of the low-defect capacitor cores of large-size high voltage direct current (HVDC) bushings.

Hierarchical carbon fibre composites incorporating high loadings of carbon nanotubes

Uncured solid bisphenol-A epoxy resins containing up to 20 wt% carbon nanotubes (CNTs) were prepared using melt blending in a high shear mixer. The extrudate was ground to produce fine nanocomposite (NC) powders. This simple method produced well-dispersed NC, with CNT agglomerate sizes below 1 μm. Consolidated NCs displayed improved tensile moduli and strengths up to 3.3 GPa (+32%) and 78 MPa (+19%), respectively at 15 wt% CNT, compared to the pure cured epoxy matrix. The relatively high Tg of 39 °C for the uncured NC powders simplified the manufacture of composite prepregs using wet powder impregnation. The prepregs were laminated into hierarchical carbon fibre reinforced composites with improved through-thickness properties. Interlaminar shear strength improved for intermediate CNT loadings in the matrix up to 65 MPa (10 wt% CNT, +19%) but decreased at higher concentrations. Compression moduli remained constant irrespectively of CNT loading but compression strength increased with a CNT loading of 2.5 wt% to 772 MPa (+31%). The mechanical properties of the hierarchical composites reflect good consolidation (void content <3%) and excellent fibre alignment (<±0.8°). In addition to the improved mechanical properties, incorporation of CNTs improved the through-thickness electrical conductivity up to 115 S/m.

Effects of Thermal Activation on CNT Nanocomposite Electrical Conductivity and Rheology.

Carbon-based nanocomposites featuring enhanced electrical properties have seen increased adoption in applications involving electromagnetic interference shielding and electrostatic dissipation. As the commercialization of these materials grows, a thorough understanding of how thermal activation affects the rheology and electrical performance of CNT–epoxy blends can inform quality decisions throughout the production process. The aim of this work was the identification of the effects that thermal activation has on the electrical and rheological properties of uncured epoxy mixtures and how those may be tied to the resulting cured composites. Herein, three distinct CNT-loaded composite mixtures were characterized for changes in electrical resistivity and viscosity resulting from varying activation times. Electrical conductivity decreased as activation time increased. Uncured mixture viscosity exhibited a strong dependence on CNT loading and applied strain, with activation time being found to significantly reduce the viscosity of the uncured mixture and surface profile of cured composite films. In all cases, cured composites featured improved electrical conductivity over the uncured mixtures. Factors contributing to the observed behavior are discussed. Raman analysis, optical microscopy of CNT networks, and data from silica bead mixing and dispersion studies are presented to contextualize the results.

Numerical Simulation of the Rheological Behavior of Nanoparticulate Suspensions

Nanoparticles significantly alter the rheological properties of a polymer or monomeric resin with major effect on the further processing of the materials. In this matter, especially the influence of particle material and disperse properties on the viscosity is not yet understood fully, but can only be modelled to some extent empirically after extensive experimental effort. In this paper, a numerical study on an uncured monomeric epoxy resin, which is filled with boehmite nanoparticles, is presented to elucidate the working principles, which govern the rheological behavior of nanoparticulate suspensions and to simulate the suspension viscosity based on assessable material and system properties. To account for the effect of particle surface forces and hydrodynamic interactions on the rheological behavior, a resolved CFD is coupled with DEM. It can be shown that the particle interactions caused by surface forces induce velocity differences between the particles and their surrounding fluid, which result in increased drag forces and cause the additional energy dissipation during shearing. The paper points out the limits of the used simulation method and presents a correction technique with respect to the Péclet number, which broadens the range of applicability. Valuable information is gained for a future mechanistic modelling of nanoparticulate suspension viscosity by elucidating the interdependency between surface forces, shear rate and resulting drag forces on the particles.

Morphology, rheological, and electrical properties of flexible epoxy/carbon composites cured by UV technique

Abstract

Influence of Aggregate Packing on the Performance of Uncured and Cured Epoxy Asphalt Mixtures

AbstractThe skeleton structure has a strong influence on the uncured and cured performances of the epoxy asphalt mixture. Based on the Bailey method, the different skeletal structures of the mixtur...

Inverse infusion processed hierarchical structure towards superhydrophobic coatings with ultrahigh mechanical robustness

Poor mechanical durability still hinders artificial superhydrophobic coatings from practical application, although these coatings were created years ago and still attract wide research interest. Previous epoxy/nanoparticle works seem to encounter a dilemma: a delicate nanostructure and low surface energy without a proper and strong microstructure to provide protection and strength naturally leads to weak mechanical adhesion and poor wear robustness. Herein, we develop an inverse infusion process (IIP) to construct hierarchical structures via newly synthesized hydrophobic and tough epoxy resin as well as rationally selected alumina nanoparticles (NPs). The IIP process forms an uncured epoxy layer first on the substrate and a preliminary nanocomposite coating next using an air spraying method, after which the epoxy layer inversely infuses into the micro/nanostructure and constructs a tougher microstructure on the top as well as a continuous nanocomposite layer throughout the coating after curing. This results in superhydrophobic coatings on a substrate with excellent adhesion and mechanical robustness, and our coatings can tolerate 600 cycles of tape peeling tests (with 3 M high-tack tape), 20 m sandpaper abrasion (with a pressure of ~5 kPa and 80 grit sandpaper), 115 cycles of Taber abrasion (with 250 g load), and 600 g of sand impact from a height of 110 cm, which are the highest values achieved by superhydrophobic coatings to the best of our knowledge. Furthermore, our coatings sustained highly corrosive media, such as sodium hydroxide and hydrochloric acid solutions. A combination of the established IIP process and the rationally selected ingredients dramatically enhanced the mechanical durability of the superhydrophobic coatings, and these attractive materials may attract attention for self-cleaning, anti-icing, and anti-fouling applications in industry.

Preparation and properties of flame-retardant epoxy resins containing reactive phosphorus flame retardant

To prepare flame-retardant epoxy resin, phosphorus compound containing di-hydroxyl group (10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phospha phenanthrene-10-oxide, DOPO-HQ) was reacted with uncured epoxy resin (diglycidyl ether of bisphenol A, YD-128) and then cured using a curing agent (dicyandiamide, DICY). This study focused on the effect of phosphorus compound/phosphorus content on physical properties and flame retardancy of cured epoxy resin. The thermal decomposition temperature of the cured epoxy resins (samples: P0, P1.5, P2.0, and P2.5, the number represents the wt% of phosphorus) increased with increasing the content of phosphorus compound/phosphorus (0/0, 19.8/1.5, 27.8/2.0, and 36.8/2.5 wt%) based on epoxy resin. The impact strength of the cured epoxy resin increased significantly with increasing phosphorus compound content. As the phosphorus compound/phosphorus content increased from 0/0 to 36.8/2.5 wt%, the glass transition temperature (the peak temperature of loss modulus curve) increased from 135.2°C to 142.0°C. In addition, as the content of phosphorous compound increased, the storage modulus remained almost constant up to higher temperature. The limiting oxygen index value of cured epoxy resin increased from 21.1% to 30.0% with increasing phosphorus compound/phosphorus content from 0/0 to 36.8/2.5 wt%. The UL 94 V test result showed that no rating for phosphorus compounds less than 19.8 wt% and V-1 for 27.8 wt%. However, when the phosphorus compound was 36.8 wt%, the V-0 level indicating complete flame retardancy was obtained. In conclusion, the incorporation of phosphorus compounds into the epoxy chain resulted in improved properties such as impact strength and heat resistance, as well as a significant increase in flame retardancy.

Short carbon fibre-reinforced epoxy foams with isotropic cellular structure and anisotropic mechanical response produced from liquid foam templates

In this work, we show that mechanically anisotropic short carbon fibre (sCF)-reinforced epoxy foams with an isotropic cellular structure can be fabricated from liquid foam templates. Short carbon fibres were mechanically frothed in an uncured liquid epoxy resin to produce an air-in-resin liquid foam template, followed by subsequent polymerisation. Fracture toughness test showed that the incorporation of short carbon fibres into the epoxy foams led to a significant increase in their critical stress intensity factors. It was also observed that neat epoxy foams failed catastrophically whilst sCF-reinforced epoxy foams failed in a progressive manner. Compression test further showed that the in-plane compressive moduli of the mechanically frothed sCF-reinforced epoxy foams were significantly higher than their out-of-plane compressive moduli, signifying an anisotropic mechanical response. This anisotropic mechanical response stemmed from the radial flow generated by the high intensity mechanical frothing process, facilitating the preferential orientation of the added short carbon fibres in-plane whilst the entrained air bubbles during the mechanical frothing process were in equilibrium with the surrounding uncured liquid epoxy resin, resulting in an epoxy foam with an isotropic (spherical) cellular structure.

Manufacturing and properties of biomimetic graphite nanoplatelets foils

Among the various architectures developed for composite materials, nanolaminated architecture is a promising candidate for the design of new materials for structural and functional applications. Nanolaminates mimic the natural occurring architecture of nacre and it is formed by highly oriented nanolaminae connected by a very low amount of matrix. Due to their architecture, inherently impermeable to fluid, such kind of materials is difficult to manufacture unless using layer‐by‐layer processes. Here, we describe the fabrication of films with nanolaminated architecture containing very high content (&gt;80% w/w) of graphite nanoplatelets (GNPs) within an uncured epoxy matrix (prepregs), suitable for the conventional prepregs manufacturing technologies: film stacking and consolidation under temperature under pressure. Nanolaminated prepregs have been manufactured through the deposition of a paste of GNPs in epoxy enriched acetone solvent. Mainly, scanning electron microscopy has investigated the nanolaminated architecture of the prepregs. Stress–strain behavior of nanolaminated prepregs has been investigated as a function of their composition and consolidation pressure. Finally, a laminate made of six nanolaminated prepregs (about 0.3 mm thick) has been manufactured by hot pressing revealing interesting mechanical properties: a tensile modulus of 30 GPa and a damping coefficient, tan δ, of 0.03. POLYM. ENG. SCI., 59:2443–2448, 2019. © 2019 Society of Plastics Engineers

Molecular resiliency and chemical bond richness of interfacial epoxy-polyurea matrix linked to characteristics of glass transition temperature

A unique chemically hybridized interfacial epoxy-polyurea matrix (IEPM) has been identified through non-negative matrix factorization (NMF) and characteristics of the glass transition temperature (Tg) controlled by epoxy curing time (tc), epoxy-resin viscosity (ν), and polyurea type (PU). IEPM, if annexed to brittle carbon-fiber reinforced epoxy via curing epoxy, forming a new C-IEPM system, is shown to substantially enhance loss modulus property. IEPM is produced as a chemical reaction between a thermoset epoxy-based structure, with ongoing crosslinking morphology, and isocyanate and amine moieties of a lightly-crosslinked polyurea structure. A cogent link is found to exist between “richness” of chemically bonded structures in well-designed IEPM and tc. Furthermore, thermomechanical analysis (TMA) of six IEPM samples educed unique Tg characteristics as a function of tc, ν, and PU after applying pre-polymerized polyurea (reactive isocyanate and amine groups) to uncured epoxy. The results reveal that by maintaining relatively small ΔTg,IEPM – PU (difference between Tg,IEPM and Tg,polyurea) and small Tg,polyurea (via lower ν and aliphatic polyurea) along with smaller tc (effectuating a sharp reduction in slope of the TMA curve), IEPM can catalyze quickly and evoke IEPM re-generativity, or ‘molecular resiliency,’ that may maximize loss modulus. The results are confirmed using dynamic mechanical analysis (DMA).

Modification of cellulose nanofibre surfaces by He/NH3 plasma at atmospheric pressure

Cellulose nanofibre coatings were treated by a dielectric barrier discharge plasma in a He/NH3 gas mixture at atmospheric pressure. Ultrasound was optionally irradiated during the treatment. The treatment enhanced the wetting of deionized water, glycerol, and uncured epoxy. Irradiation of ultrasound did not significantly change optical emission from the plasma, but increased the oxygen contents and enhanced etching and roughening at the nanofibre coating surfaces. Furthermore, the irradiation of ultrasound enhanced the wetting of deionized water and glycerol drastically, while that of uncured epoxy to some extent.