Epoxy Adhesive Formulation Research Articles

Predicting the Properties of High-Performance Epoxy Resin by Machine Learning Using Molecular Dynamics Simulations.

Epoxy resin is an of the most widely used adhesives for various applications owing to its outstanding properties. The performance of epoxy systems varies significantly depending on the composition of the base resin and curing agent. However, there are limitations in exploring numerous formulations of epoxy resins to optimize adhesive properties because of the expense and time-consuming nature of the trial-and-error process. Herein, molecular dynamics (MD) simulations and machine learning (ML) methods were used to overcome these challenges and predict the adhesive properties of epoxy resin. Datasets for diverse epoxy adhesive formulations were constructed by considering the degree of crosslinking, density, free volume, cohesive energy density, modulus, and glass transition temperature. A linear correlation analysis demonstrated that the content of the curing agents, especially dicyandiamide (DICY), had the greatest correlation with the cohesive energy density. Moreover, the content of tetraglycidyl methylene dianiline (TGMDA) had the highest correlation with the modulus, and the content of diglycidyl ether of bisphenol A (DGEBA) had the highest correlation with the glass transition temperature. An optimized artificial neural network (ANN) model was constructed using test sets divided from MD datasets through error and linear regression analyses. The root mean square error (RMSE) and correlation coefficient (R2) showed the potential of each model in predicting epoxy properties, with high linear correlations (0.835–0.986). This technique can be extended for optimizing the composition of other epoxy resin systems.

Thermally Controlled Acceleration of Epoxy Resin Curing through Polymer-Bound Imidazole Derivatives with High Latency

Imidazole-containing polymeric accelerators were synthesized through free-radical polymerization and applied in the curing of diglycidyl ether of bisphenol A resin with the hardener dicyandiamide. Reactivity and storage stability were followed via differential scanning calorimetry, and an increased pot life was observed after incorporation into polymers. Depending on the employed monomers and composition, the compatibility with epoxy resin was adjusted and the storage stability thereby improved, while a high reactivity is retained under curing conditions. Microscopy imaging showed the dissolution of the polymeric accelerators in epoxy resin above a critical temperature enabling a high reactivity. The best performing accelerator was then applied in an epoxy adhesive film formulation. The strength of the bonding of stainless-steel test bodies was examined in a lap shear test showing superior long-term stability while maintaining efficient acceleration of this type of polymeric accelerator for adhesive applications.

Prediction of Lap Shear Strength and Impact Peel Strength of Epoxy Adhesive by Machine Learning Approach.

In this study, an artificial neural network (ANN), which is a machine learning (ML) method, is used to predict the adhesion strength of structural epoxy adhesives. The data sets were obtained by testing the lap shear strength at room temperature and the impact peel strength at −40 °C for specimens of various epoxy adhesive formulations. The linear correlation analysis showed that the content of the catalyst, flexibilizer, and the curing agent in the epoxy formulation exhibited the highest correlation with the lap shear strength. Using the analyzed data sets, we constructed an ANN model and optimized it with the selection set and training set divided from the data sets. The obtained root mean square error (RMSE) and R2 values confirmed that each model was a suitable predictive model. The change of the lap shear strength and impact peel strength was predicted according to the change in the content of components shown to have a high linear correlation with the lap shear strength and the impact peel strength. Consequently, the contents of the formulation components that resulted in the optimum adhesive strength of epoxy were obtained by our prediction model.

Properties of high-temperature epoxy/DDS resin systems for bonding application

The choice of basic epoxy resin (ER) is especially important for the design of epoxy adhesive formulations. In the present study, performance of several high-temperature ER systems, prepared using 4,4-diaminodiphenylsulfone as the curing agent by the same curing process, was investigated. The curing behavior was studied by dynamic rheometry and differential scanning calorimetry. The thermal properties of the cured resins were investigated by dynamic thermomechanical analysis, thermomechanical analysis, and thermogravimetric analysis. The peeling properties, mechanical behaviors, and moisture absorptivities of the cured resins were also studied. The results showed traditional diglycidyl ether of bisphenol A epoxy to be insufficient in heat resistance. Naphthalene-based ER and bifunctional cardo ER showed higher glass transition temperature values. However, the processability and adhesive properties may not comply with the application requirements. Biphenyl novolac ER has excellent performance in all aspects, which is suitable for use as a high-temperature adhesive.

Epoxy adhesives toughened with waste tire powder, nanoclay, and phenolic resin for metal-polymer lap-joint applications

Brittleness and low thermal stability of epoxy confines its usage as adhesive for metal-polymer interfaces. In this work, epoxy formulations containing phenolic resin (X1 (wt.%), for thermal stability), recycled tire powder (X2 (wt.%), economically reasonable for toughening), and clay nanoplatelets (X3 (wt.%), for modulus enhancement) were optimized based on L9 Taguchi experimental design approach. The main effects of X1, X2, and X3 on mechanical properties of epoxy adhesives including tensile strength, Young’s modulus, and toughness, were studied by taking into account signal-to-noise ratio (S/N) as well as the analysis of variance (ANOVA). It was found that at X1 level of 50 wt.%, X2 level of 10 wt.%, and X3 level of 2 wt.% an increase of about 39% was observed in the single-lap shear strength compared to blank epoxy adhesive formulation. Scanning electron microscopy revealed crack mechanism in the form of deviation of crack growth pathway when encountered with large platy nanoclays. Thermal stability of epoxy was also improved by 16% based on degradation peak temperature monitoring.

Aligned multi-walled carbon nanotubes (MWCNT) and vapor grown carbon fibers (VGCF) reinforced epoxy adhesive for thermal conductivity applications

Present study deals with the development of aligned multi-walled carbon nanotubes (MWCNT) and vapour grown carbon fibers (VGCF) reinforced epoxy adhesive system used as an alternative to Pb/Sn soldering element. The alignment of MWCNTs was carried out in a specific designed instrument exerting of an external electric field. Solution casting technique was used for the fabrication of epoxy adhesive system incorporated with aligned MWCNT and VGCF. Scanning electron microscopy and Atomic force microscopy was carried out to determine the degree of alignment of MWCNTs. Owing to the formation of continuous path for the flow of electrons, 3 wt% of aligned MWCNT with 3 wt% of VGCF reinforced epoxy adhesive achieved about five orders of magnitude higher in the thermal conductivity compared to pure epoxy. Further, the experimental results showed that the shear strength of epoxy/3% MWCNT/3% VGCF adhesive system was 112% higher than the strength of pure polymer, emphasizing the advantages of aligned MWCNT on stiffness and strength. Dynamic Mechanical Analysis and Thermo-gravimetric analysis was carried out to study the thermo-mechanical and thermal degradation behaviour of epoxy adhesive formulations.

Designing the Epoxy Adhesive Formulations for Manufacturing Engineered Woods

The design of an epoxy adhesive was investigated by a 2K design of experiment (DOE). All the assigned parameters showed no significant effect for both curing and mechanical properties, except for bisphenol A (BPA), which showed a significantly negative effect on the heat distortion temperature (HDT) of the cured samples. Adding dicyandiamine (DICY) into the hardener retarded cure time and also caused an incomplete curing at room temperature. Curing at 110 °C and 150 °C post curing were the optimal conditions and 20 g of DICY with 50 g of triethylenetetramine (TETA) was optimized. Adduct obtained from aliphatic epoxy (RD108, 14.63 g) and TETA (7.71 g) were selected and employed as hardener ingredients. The incomplete crosslinking reaction was the main reason for the inferior properties at high RD108 loadings. The toughening by blending with polycarbonate (PC) was explored, and 5 phr of PC was selected. Limitation of resin/fiber infusion due to high viscosity was observed. Dilution of the solvents using ethyl acetate (EA) and methyl ethyl ketone (MEK) to reduce viscosity was explored. The mechanical properties of the wood samples manufactured from the EA-diluted epoxy were superior to the MEK dilution. The lower boiling point and good solubility of EA were explained.

Mechanisms of Interfacial Degradation of Epoxy Adhesive/Galvanized Steel Assemblies: Relevance to Durability

This study reports on the durability of galvanized steel joints bonded with toughened epoxy adhesive formulations under the influence of temperature and moisture aging. The influence of hydrothermal aging on the substrate and adhesive properties was first determined and then correlated to the durability of bonded joints. The mechanical properties of the adhesively bonded joints were assessed by a shear stress-strain analysis on single lap joints. Experimental results have shown a loss of adherence during aging. Scanning electron microscopy, fourier transform infra-red spectroscopy, and differential scanning calorimetry were conducted to analyze precisely the locus of failure and the failure faces after separation of the interface. Consequently, physical and chemical modifications of the adhesive and surface corrosion of the substrate were observed. A three-step mechanism has been proposed to explain the loss of adhesion during aging. The first step was attributed to adhesive and interface hydrolysis of the lap-shear sample's Highly stressed zones, the second step to hydrolysis of the centre of the bonded joint and initiation of corrosion. The last one is due to an important growth of corrosion products which favours rupture in a weak boundary layer at the metal/polymer interphase. The aging model proposed bears out that the higher the corrosion resistance of the metal surface the lower the loss of adhesion under a wet environment.

Effects of Hardeners and Catalysts on the Reliability of Copper to Copper Adhesive Joint

As the performance of microelectronic devices is improved, the use of copper as a heat dissipation member is increasing due to its good thermal conductivity. The high thermal conductivity of copper, however, leads to difficulties in the joining process. Satisfactory bonding with copper is known to be difficult, especially if high shear and peel strengths are desired. The primary reason is that a copper oxide layer develops rapidly and is weakly attached to the base metal under typical conditions. Thus, when a clean copper substrate is bonded, the initial strength of the joint is high, but upon environmental exposure, an oxide layer may develop, which will reduce the durability of the joint. In this study, an epoxy adhesive formulation was investigated to improve the strength and reliability of a copper to copper joint. Epoxy hardeners such as anhydride, dihydrazide, and dicyandiamide and catalysts such as triphenylphosphine and imidazole were added to an epoxy resin mixture of DGEBA and DGEBF. Differential scanning calorimetry (DSC) analyses revealed that the curing temperatures were dependent on the type of hardener rather than on the catalyst, and higher heat of curing resulted in a higher Tg. The reliability of the copper joint against a high temperature and high humidity environment was found to be the lowest in the case of dihydrazide addition. This is attributed to its high water permeability, which led to the formation of a weak boundary layer of copper oxide. It was also found that dicyandiamide provided the highest initial joint strength and reliability while anhydride yielded intermediate performance between dicyandiamide and dihydrazide.

Epoxy adhesive formulations using latent imidazole metal cation complexes

Epoxy adhesive formulations using latent imidazole metal cation complexes Complexes of 2-methylimidazole with cations from several metal sulfates were prepared and investigated as curing agents for epoxy resins. The reactivity of one-part formulations of these complexes with a bisphenol A type epoxy resin was determined by the differential scanning calorimetry and the pot life observed by viscosity measurements. Tensile lap shear tests at room temperature and at 120°C were used to evaluate the adhesive strength of the formulations directly after preparation as well as after one and three months of storage at room temperature. The DSC measurements showed much lower reactivity (7 - 32%) and higher reaction temperatures of the complex formulations in comparison to the mixtures with pure 2-methylimidazole. The viscosity of most formulations remained almost unchanged over the observed period of three months. The adhesive strength of the freshly prepared complex formulations is comparable to a formulation with pure 2-methylimidazole and decreases over time, depending on the type of metal cation and the cation-to-imidazole molar ratio. The obtained results indicate that complexes of 2-methylimidazole with cations are suitable as latent curing agents for epoxy resins.

Shelf Life and Controlled Cure of Epoxies by Loaded Zeolite

Epoxy formulations based on the multi-functional amine hardener, dicyandiamide (Dicy), regularly contain a free accelerator for reducing the curing temperature and the time needed to complete the network formation. Unfortunately, all accelerators reduce the shelf life of these adhesives at 25°C. In order to solve this problem, accelerator-loaded zeolites fillers were developed, optimised with respect to host–guest interactions and characterised by Fraunhofer IFAM (Bremen, Germany) with regard to the release and curing behaviour in epoxy adhesive formulations. They are added to an epoxy adhesive (diglycidylether of bisphenol A (EP) and dicyandiamide (Dicy), mass ratio 100:6.7), stored at 25°C in regular air or cured (heated with β = 10 K/min to 170°C subsequent isothermal curing for 45 min). That shelf life and curing behaviour are investigated by FT-IR spectroscopy and modulated DSC. Compared to the EP containing free accelerator, the zeolite-filled EP possesses a threefold increase in shelf life at 25°C due to the immobilization of the accelerator in the pores of the zeolites. While the free accelerator acts steadily during heating, it is shown that the loaded zeolite releases the accelerator at about 76°C. Surprisingly, the released accelerator is not only involved in the chemical formation of the epoxy network but it accelerates the dissolution of Dicy considerably. As the result, network formation at 170°C finishes after not more than 19 min and the starting temperature for curing could be reduced to 140°C.

Parametric study of factors affecting the pull-out strength of steel rods bonded into precast concrete panels

This paper deals with the effect of bond thickness, embedment length and type of epoxy adhesive formulation on the adhesion of steel anchors to concrete. The test results indicate that the shear strength of the epoxy adhesive formulation prepared from a diglycidylether of bisphenol A (DGEBA) and an ethyleneamine curing agent is independent of bond thickness from approximately 1–4 mm. However, the same is not found to be true for the DGEBA epoxy resin cured using an alkylenediamine curing agent where considerably more complex behaviour is observed. The adhesive shear strength of this system is shown to pass through a maximum value at 2 mm bond thickness. At thickness beyond 2 mm, the strength undergoes a reduction until a value is reached which remains essentially constant with increased thickness. The variation in concrete strength is regarded as a main factor responsible for this complexity. The addition of filler to epoxy adhesive formulations results in a reduction in the adhesive shear strength, the magnitude of the decline being largely dependent upon the quantity of filler used in the mixture. The quadratic equations reviewed in this study provide a basis for assessing trends in the mechanical behaviour of filled epoxy systems. The shear strength of an epoxy adhesive anchor can be determined by an elastic behavioural model. The curves for epoxy adhesives indicate that the elastic solution does fit the experimental data very well. Increasing embedment of an anchor beyond a certain point does not increase the ultimate tension capacity of the anchor.

The application of static secondary ion mass spectrometry to the study of epoxy adhesive constituents

The application of fast atom bombardment secondary ion mass spectrometry (FAB-SIMS) with quadrupole mass analysis to the study of some of the common constituents present in epoxy adhesive formulations is described and the potential of the technique evaluated. The compounds investigated were Epikote 828 (mainly diglycidyl ether of bisphenol-A) and the cross-linking agent dicyandiamide. The characteristic secondary ion spectra recorded for both materials exhibited a number of identifiable and uniquely diagnostic signals, which should be of use in the study of surface and interface adhesive chemistry.

Modification of Free Volume in Epoxy Adhesive Formulations

Abstract Additives are described which modify the free volume available for segmental motion in epoxy adhesives. Such a mechanism can produce an increase in the tensile modulus of conventional epoxy-amine systems of>60% (e.g. to>4.1 GPa) and in tensile strength of>50% (e.g. to 125 MPa), while also producing a ductile mode of failure (stress-strain curve has negative slope before failure). At low strains, a reduction in free volume hinders polymer segmental motion and so increases the modulus. However, these materials also exhibit a very low Poisson's ratio and strains of ca. 5% cause a sufficient increase in free volume that ductile failure can occur. Improvements in low temperature cure properties (e.g. 118 MPa tensile strength at 60°C cure) together with reductions in the coefficient of thermal expansion and water uptake are also reported. These improvements in bulk adhesive properties are shown to translate into improved adhesive joint performance.

The use of 50 per cent phosphoric acid as an etching agent in orthodontics: A rational approach

The chemical treatment of enamel surfaces is an approach used in obtaining increased adhesion of dental materials to tooth surfaces. One of the conditioning solutions extensively used prior to the direct bonding of orthodontic attachments is a 50 per cent phosphoric acid solution. The object of this investigation was to study the effect of various concentrations of phosphoric acid on the bond strength of an epoxy adhesive formulation developed for the direct bonding of orthodontic attachments. Furthermore, an attempt was made to quantitate the etching action of the phosphoric acid solutions by determining the depth of etch on polished enamel surfaces and recording the surface profiles of etched enamel surfaces by means of a surface-roughness measuring instrument. The use of 50 per cent phosphoric acid solution as a conditioning agent prior to the use of the epoxy adhesive formulation is advocated. The rationale supporting this recommendation includes (1) the bond strength of the epoxy adhesive system to enamel surfaces etched with 50 per cent H 3PO 4 is not significantly improved by the use of phosphoric acid at any other concentration; (2) the depth of etch obtained with 50 per cent H 3PO 4 is less than that observed with H 3PO 4 solution at concentrations which produce comparable bond strengths; (3) the surface roughness of enamel surfaces etched with 50 per cent H 3PO 4 is greater than that obtained with more dilute acid solutions. The optimal phosphoric acid concentration should be determined for each adhesive system.

Epoxy adhesives for orthodontic attachments: Progress report

The following article is a progress report on a basic epoxy adhesive formulation employed by the author experimentally and clinically to bond plastic orthodontic attachments to tooth surfaces. l%e adhesive elements (epoxy resins and uaring agents) are formulated for orthoamtic purposes and are not industrial adhesives. Caution should be exercised by the practitioner in duplicating the described formulation not to we Gdustrial adhesives prior to biologic testing, since contact dermatitis may be a problem. Examination of the surfaces of hundreds of extracted and clinically bonded teeth has revealed no deleterious effects, macroscopically and microscopically, after bonding with the reported epoxy adhesive formulation. Fnrthe-rmore, hygiene is improved olinically when the plastic attachments cover the entire labial wrface, since decalcification resdting from poor tooth brushing of the gingival area3 cannot occur. It is the p-arpose of this article, also, to introduce a novel concept in attachment placement which is more esthetic and hygienic. Additional modified formnlations and promising adhesive systems are being investigated.