Cold-curing Epoxy Resin Research Articles

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

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

Investigation of in-plane wave motion in a Fibre Metal Laminate using PVDF foil sensors

Structural Health Monitoring (SHM) via Guided Ultrasonic Waves (GUW) has been the subject of research for years and has already been described in detail for purely isotropic and purely anisotropic materials. Fibre Metal Laminates (FML) combine isotropic metal layers with anisotropic fibre composite layers to a laminate. They are mainly used in the aerospace industry to reduce mass while optimizing stiffness and strength properties. On the other hand, the combination of different materials leads to high differences in the acoustic impedance at the interfaces, which in turn leads to the question to what extent these materials can be monitored via GUW. This research investigates the applicability of polyvinylidene fluoride (PVDF) foil sensors for the detection of GUW in the interior of an Aluminium Glass Fibre laminate. Preliminary analytic investigations based on new adaptions of the partial-wave method gave insight into general GUW characteristics along fibre-metal interfaces and within an FML plate waveguide. Analytic and transient finite element simulation results were used to predict displacement fields. Composition and dimension of fabricated FML plates as well as excitation frequency were chosen based on these predictions, particularly maximizing dominance of the in-plane displacement components. For manufacturing the FML specimen with embedded PVDF sensors, a cold-curing epoxy resin was used to prevent the temperature-sensitive sensors from being destroyed or depolarized during production. After the design and application of a PZT actuator on the laminate, GUW were excited with particular attention to the frequencies in which a particularly dominating in-plane displacement field was assumed. Laser-Doppler Vibrometry (LDV) was used for measurements of the GUW's out-of-plane component.

Effect of synergistic environmental conditions on thermal properties of a cold curing epoxy resin

Abstract This study presents an investigation addressing the effect of environmental conditions on the thermal properties of a cold curing epoxy resin used in repair and strengthening of masonry structures. The exposure conditions consist of laboratory indoor conditions (IC), long-term water immersion (WI), hygrothermal exposure (HG) and outdoor real exposure conditions (OC). The changes in the glass transition temperature, Tg, relaxation enthalpy and enthalpy peak temperature are monitored with Differential Scanning Calorimetry (DSC). Different measurement techniques for obtaining the Tg from DSC curves are also presented and discussed. The results showed a reduction of Tg in WI tests due to plasticization, which reversed at long-term periods. In HG conditions, post-curing of epoxy resin led to an increment of Tg. It was observed that the changes of Tg in OC conditions in the studied environment have a good correlation with the moisture absorption level and the obtained results from WI tests.

Amines-boron trifluoride complexes as catalyst for cold curing epoxy resin ED-20 using anhydride curing agents

The reaction of the anhydride curing epoxy resin ED-20 in the presence of boron trifluoride-amines was investigated in the temperature range 303–393 K. An effect of the basicity of the amine in the complex on the speed of the process was assessed. Based on correlations of a structure of the catalyst-temperature epoxy compositions are developed with a preset temperature-time mode of curing and set of required physical and mechanical properties.

Improved mechanical properties of CFRP laminates at elevated temperatures and freeze–thaw cycling

Externally bonded Fiber Reinforced Polymer (FRP) laminates are recognized as an effective system to repair or strengthen existing structures in the field of buildings. Despite the evident advantages of FRPs over traditional materials, the greatest impediment to worldwide utilization is represented by the limited knowledge of composites behavior at elevated temperature and/or freeze–thaw cycling exposure. In order to investigate this aspect, a series of mechanical tests have been carried out on innovative epoxy based-matrices and on carbon FRP coupons exposed to different temperature and humidity conditions in an environmental chamber. Since commercially available resins exhibit a reduced capacity to transfer loads over fibers around glass transition temperature (Tg) which, for cold-cured systems (i.e. systems cured around the ambient temperature), is somewhat comparable to the operating temperature, the innovative matrices have been formulated with the aim to attain Tg values significantly higher than those exhibited by presently used epoxy based-systems. The strategy adopted for the formulation of such resins is based on the key idea that for cold-cured epoxy resin, it is essential to increase the exothermal crosslinking reaction heat allowing the curing reaction in the bulk of the material to be carried out at temperatures higher than ambient temperature.The results of Differential Scanning Calorimetry (DSC), Dynamic-Mechanical Analysis (DMA) and Thermo-Gravimetric Analysis (TGA) as well as results of tension tests on FRP laminates subjected to severe operating conditions are presented and widely discussed in this paper. The experimental results pointed out that the innovative epoxy-based formulations provide significant improvements on the mechanical properties of FRP laminates exposed to elevated temperatures or to freeze–thaw cycles.

Phase-stepping photoelasticity for the measurement of interfacial shear stress in single fibre composites

Abstract Phase-stepping photoelasticity has been used to study the fragmentation of an E-glass fibre in epoxy resin and evaluate quantitatively its interfacial adhesion. To investigate the interfacial response of an E-glass fibre with differing degrees of adhesion, it was coated by plasma-polymerisation with a crosslinked conformal film of 90% acrylic acid and 10% 1,7-octadiene. The stress distribution within the resin at the fibre–matrix interface and the adjacent matrix has been described in detail using contour maps of fringe order. From these, the interfacial shear stress profiles at fibre-ends have been calculated. With a cold-cured epoxy resin a good interface was formed between uncoupled unsized E-glass, while a poor interface was achieved with the plasma polymer coated fibre. Further, it was clearly demonstrated that chemical bond formation between the functional plasma polymer coating and the matrix was incomplete leading to the formation of an interphase epoxy resin of low yield strength. We now have a technique capable of probing the micromechanical response of interphases as a function of chemical bond formation.

Phase-stepping photoelasticity for quantifying the interfacial response in fibre composites at fibre-breaks

A phase-stepping photoelastic technique has been used to study the fragmentation of E-glass fibre and evaluate quantitatively the effect of an interphase region on interfacial shear strength at a fibre-break and hence the efficiency of the interfacial bond. To investigate the interfacial response of an E-glass fibre in the presence of a soft interphase, it was coated with an epoxy resin of lower Young's modulus and yield strength than that of the matrix resin. This enables the effect of a sizing or modified matrix region to be quantified. The micro-mechanical response in the matrix at the interface/interphase has been described in detail using contour maps of fringe order. From these, the profile of the interfacial shear stress at fibre-breaks has been calculated, which demonstrated that in this cold-cured epoxy resin a good interface formed with uncoupled and unsized E-glass. In the presence of a modified matrix or interphase region the stress transfer process was modified and the stress concentration associated with a transverse crack reduced.

Analysis of the single-fibre pull-out test by means of Raman spectroscopy: Part II. Micromechanics of deformation for an aramid/epoxy system

The single-fibre pull-out test has been analysed for Kevlar-49 fibres in a cold-cured epoxy resin by using both a conventional pull-out experiment and Raman spectroscopy. The interfacial shear strength (ISS) has been estimated from the pull-out force for fibres with a range of embedded lengths. Raman spectroscopy has been used to analyse the distribution of fibre strain in the pull-out test by mapping the variation of strain along an aramid fibre undergoing pull-out from the epoxy resin matrix. At low strains the behaviour follows elastic shear-lag analysis but, as the fibre strain is increased, debonding takes place at the fibre/matrix interface. It is found that this debond propagates along the interface until the entire fibre is debonded. The fibre is then pulled out of the resin matrix by a frictional pull-out process. It is shown that the conventional pull-out experiment produces only an apparent value of ISS and that through a partial-debonding model it is possible to use the interfacial parameters obtained from the Raman analysis to predict the data from the conventional test.

Surface properties and adhesion in bonding aluminium alloys by adhesives

Abstract To achieve adhesively bonded metal joints which are durable it is necessary to gain a better understanding of the processes (both on a macro- and a micro-scale) which take place at every stage of their preparation. In an attempt to elucidate a few of these aspects, the surfaces of two aluminium alloys — 2024 T3 and 7075 T6 — were examined using scanning electron microscopy after various etching and anodizing pretreatments. The effects of adsorption of a phenolic resin, used typically in a primer, on 2024 T3 aluminium were examined using spectrophotometric measurements. Finally, the effects of the ingress of water into joints formed by bonding the alloys with a cold-curing epoxy resin were studied.

Assessment of certain epoxy resins for replicating metal surfaces

Replicating features of four cold-curing epoxy resin systems were assessed for metal surfaces with finishes of 0.5–6.2 μm CLA. A dilute solution of methyl silicone rubber in trichloroethane gave satisfactory replica release. Surface detail was well reproduced by all the resins: the most effective was a Ciba-Geigy system MY753/HY951, with a maximum linear shrinkage of 0.165%. Linear shrinkage of one system was reduced to 0.063% by adding inert slate powder, but inferior replication resulted

Dental plaque morphology as revealed by direct observation and by replicating techniques

The ability of four elastomer impression materials to reproduce details of bacterial plaque structures was studied by comparing areas of the replica models with corresponding areas of the replicated plaque. Plaque was grown on hydroxyapatite splint segments for 48 hours and replica models were made from three different resins. One of the resins, a methacrylate material, was not suited for this purpose due to its content of spherical particles. The polysulfide and polyether impression materials reacted with the heat curing epoxy resin, and less satisfactory results were obtained with cold curing epoxy resin. Best results were obtained by a combined use of low viscosity silicone impression materials and cold or heat curing epoxy resins as model materials. However, a considerable loss of detail occurred in the replica models compared with observations of plaque directly on splint surfaces. Outlines of individual bacteria could sometimes be seen in the models, but generally several cells seemed to fuse, and appeared like small globular structures where cells and pellicle were difficult to distinguish. The bacterial colonization started near the gingival border of the surface, initiated by attachment of individual bacteria to the pellicle surface.

Photoelastic stress analysis in fibre reinforced plastics

A room temperature technique for experimental stress analysis in three–dimensional fibre reinforced plastics is proposed. The technique is verified experimentally in two dimensions for the particular case of a cold–curing epoxy resin subject to load before the resin has fully hardened. This technique allows strains which are produced by loading to be permanently locked into the epoxy model by the curing process and enables slices to be cut for analysis without disturbance of the fringe pattern. A simple three–dimensional model analysis is presented to demonstrate possible advantages of the technique.