Structural Epoxy Adhesives Research Articles

POSS‐reinforced covalent network in epoxy adhesive and its improvement in the high‐temperature adhesion, toughness, and transparency

AbstractDeveloping epoxy structural adhesives with excellent high‐temperature adhesion remains a great challenge. The incorporation of fillers can enhance the performance of epoxy adhesives, albeit at the expense of reduced high‐temperature adhesion properties. Polyhedral oligomeric silsesquioxanes (POSS) is considered a promising filler for enhancing high‐temperature adhesion properties due to its excellent compatibility and nano cage‐structured. Herein, the rigid cage‐structured was covalently introduced epoxy adhesives to improve the high‐temperature adhesion properties. The introduction of 1 wt% POSS significantly increased the high‐temperature (120°C) adhesion strength by 197% and toughness by 283%. The POSS‐reinforced adhesive also featured excellent wide temperature adaptability, indicating that it maintained high adhesion strength at various temperatures. Furthermore, POSS drastically enhanced the transparency of epoxy adhesive by three times. It is highly anticipated that this work will open a new avenue of designing epoxy adhesives with the excellent high‐temperature adhesion properties and transparency.

Standard specimen geometries do not always lead to consistent fatigue results for epoxy adhesives

This paper questions the recommendation regarding the use of standard specimen geometries, (Type I, Type II, and Type IV), for estimating the tensile quasi-static and fatigue properties of structural epoxy adhesives. The work presents results from an experimental program investigating the performance of structural epoxy adhesives indicating a significant effect of the specimen geometry, especially when referring to fatigue loading. Simple finite element models are also developed to facilitate the comparison of the stress distribution along the three specimen geometries. The fatigue experimental results allowed the derivation of probabilistic S-N curves, showing higher fatigue sensitivity of Type I specimens compared to Type II and IV. Furthermore, probability distribution function (PDF) curves of the equivalent static strength estimated by using Sendeckyj’s wear-out model attributed lower mean strength and higher variance for Type I specimens validating the fatigue data.

Accelerated‐curing epoxy structural film adhesive for bonding lightweight honeycomb sandwich structures

AbstractAccelerating the curing of epoxy/aromatic amine adhesives and improving their toughness are challenges in heat‐resistant epoxy structural adhesives. Herein, we report an epoxy/aromatic amine adhesive accelerated curing system with an oxo‐centered trinuclear (chromium III) complex, which is toughened using a thermoplastic block copolymer (TPBC). The reaction characteristics, heat resistance, microstructure, and bonding properties of the accelerated epoxy adhesives were analyzed. The reaction peak temperature of the epoxy with 3% catalyst was 113.1°C, which was 113.6°C lower than that of epoxy without catalyst, and the modified epoxy resin demonstrated a potential for rapid curing at medium temperature. The glass transition temperature of the TPBC‐toughened epoxy adhesive was 125°C after curing, indicating excellent thermal stability after medium temperature curing. The introduction of the TPBC increased the single‐lap shear strength of the epoxy adhesive without reducing its heat resistance. The shear strength at room temperature and 120°C of the modified epoxy adhesive with 50 phr of TPBC was 25.2 and 10.9 MPa, respectively. Moreover, the epoxy film adhesive exhibited outstanding bonding properties when used in the bonding of lightweight honeycomb sandwich structures.

Empirical investigations on the effects of ionizing radiation on epoxy structural adhesives and resins: An overview

Depending on the type of application, ageing effects can highly influence the properties of adhesives. Radiation environment as it can be found in space missions, nuclear reactors – both fusion and fission and particle detectors represents hazardous conditions for adhesives due to the polymer chains of which it is composed. Ionizing radiation interacts through the mechanism of chain scission and crosslinking with these chains resulting in a change of physical properties. In the present study, a comprehensive literature review is conducted regarding the influence of ionizing radiation on the mechanical properties of epoxy adhesives. Radiation damage tests are expensive and complex in terms of operational safety. Therefore, it is desirable to screen the results of previously conducted experiments. 126 data sets collected from about 46 articles are analysed, homogenised, and visualised to provide an overview of the current state of this field of research. The data is categorised according to the performed test and the type of applied load. Mechanical properties are plotted over irradiation dose. Since conducting irradiation tests are often costly and includes hazards, the collection of data sets presented in this study brings huge benefit to the adhesive community. In the future, high demand for such data is becoming apparent due to the recent boom in the space industry and the major efforts made in the field of nuclear fusion development. The provided data can be used by designers, engineers, and scientists for sizing adhesives joints under radiation environment by taking this particular ageing effect into account.

Super-Flexibility and High-Temperature Adhesion of Epoxy Structural Adhesives Endowed by Homogeneous Rigid–Flexible Crosslinking Networks

Epoxy adhesives that combine flexibility, high-temperature adhesion property, and medium–low-temperature curing ability are an urgent need in the microelectronic device era. Unfortunately, it is still a challenge so far. In this context, two kinds of curing agents are combined to construct homogeneous rigid–flexible crosslinking networks via the stepwise curing method for preparing an epoxy adhesive with prominent deformation ability and high-temperature adhesive strength. Detailed analysis showed that the crosslinking network constructed by the stepwise curing method was higher and more homogenous than that of the one-step curing method. Benefitting from the homogeneous rigid–flexible crosslinking network, the elongation at break and high-temperature adhesion strength (200 °C) of the epoxy adhesive prepared by the stepwise method were up to about 100% and 1.60 MPa, respectively, which were about 1677.7 and 112.5% higher than those of the epoxy adhesive cured by the one-step curing method. The epoxy adhesive with medium–low-temperature curing ability, excellent flexibility, and high-temperature adhesion was first reported. It is expected that this work could provide some inspiration to design the special epoxy adhesives.

Effect of Diluents on Mechanical Characteristics of Epoxy Compounds.

The aim of this work is to assess the influence of different commercial diluents on some mechanical properties of two bisphenolic epoxy compounds, cold-cured by a polyamide curing agent, to be employed as epoxy structural adhesives for building and industrial applications. The diluents under analysis were epoxy, bituminous, nitro, acrylic and extraction. The choice of these products was made on the basis of their wide commercial availability as diluents for epoxies used as adhesives and in different industrial and construction applications. The diluents were all added in small proportions, i.e., from 1 to 10 g per 100 g of epoxy resin. The cold-cured epoxy compounds were subjected to compressive (according to ISO 604) and static tensile (according to ISO 527-1) tests. The same mechanical tests were performed on both unmodified epoxy resins, for comparison purposes. On the basis of the obtained results, it was concluded that the influence of the presence of a diluent, and of its amount, on the mechanical properties of epoxy compounds depends on the type of resin and of diluent, as well as on the mechanical characteristics analyzed.

Investigation of Forming Behaviour of Metal-Polymer Sandwich Composite through Limit Dome Height Test Simulations

Metal-polymer-metal (MPM) sandwich composites are in the class of proficient engineering materials which give outstanding strength-to-weight ratios because of their comparatively low density. These materials are vital constituents within the automobile, aerospace, marine, and civil construction industries as substitutes for sheet metals that considerably reduce weight while not compromising functionality. Moreover, these materials have supplementary qualities like sound dampening and thermal insulation capabilities. For these materials to be utilized within the aforesaid industries, they need to bear numerous forming processes that are essential in product manufacturing. This paper investigated formability analysis of metal-polymer sandwich composites made of “AW 6082-PVC-AW 6082 (APA)” and “galvanized steel-PVC-galvanized steel (GPG)” sandwich sheets, considering epoxy structural adhesives as the binding agent, via FEA simulation. All the FEA simulations were performed using Altair HyperWorks software. For evaluating the formability, the actual limit dome height (LDH)—biaxial strain path—tests were simulated using FEM software. The results analyzed are forming limit diagram (FLD), punch force distribution, and a dome height at diverse conditions of punch velocity and friction. A comparison is made to represent the best combinations for formability of the sandwich composites. Maximum formability and dome height are attained at low friction conditions and forming speed. It has also been observed that LDH simulations are very sensitive to friction, and it has a substantial impact on the test outputs. Maximum thinning (or fracture) generally moves away from the apex of the dome towards the die corner radius as the friction increases from zero upwards.

Quasi-static and intermediate test speed validation of SHPB specimens for the determination of mode I, mode II fracture toughness of structural epoxy adhesives

The study of the behaviour of adhesive joints under impact loadings is a very active field of research, driven by significant industrial interest, such as the automotive industry, in order to improve mechanical behaviour, reduce weight and simplify manufacturing. Furthermore, due to high standards in terms of safety required in the automotive industry, namely the occupant’s safety during collisions, the behaviour of adhesives under impact loads is becoming a prominent object of study. This work proposes a novel and simplified method to experimentally obtain a custom set of TSLs (traction-separation laws) and estimate the fracture energy both for mode I and II that can be tested from quasi-static to impact conditions resorting to a SHPB (Split Hopkinson Pressure Bar) equipment with especially designed specimens. The method is validated by comparing those results to fracture energy values obtained using DCB (Double Cantilever Beam), ENF (End Notch Flexure) and prior validated mixed mode apparatus Finally, the SLJ (Single Lap Joint) behaviour is predicted by resorting to the custom TSLs and comparing model performance to experimental results. With the proposed method the obtained fracture energy was found to be equivalent to that obtained with more traditional fracture mechanical tests used in the field of adhesive bonding. The custom TSLs were able to predict the behaviour of SLJ fairly accurately.

Environmental durability of epoxy-bonded CFRP-to-steel joints in mode I fracture

Exposure to environmental stresses can weaken the connection of epoxy-bonded CFRP-strengthened steel elements. As a result, designers often express concern about possible premature delamination arising from combined peel and shear effects at the CFRP-steel interface. This study was undertaken to further knowledge in relation to the peel failure of epoxy-bonded CFRP-to-steel joints subjected to hygrothermal conditioning. The effect of the bond-line thickness on the bond strength was also examined in this study. A total of 54 specimens were tested including 30 adhesive CFRP-to-steel specimens and 24 adhesive coupons. Two commercially available structural epoxy adhesives were examined. Hygrothermal conditioning may cause the adhesion failure near to the edges of the adhesive joints. The load-carrying capacity and strain energy release rate of adhesive joints depend on the adhesive and hygrothermal environment, and can be degraded up to 15%. The statistical analysis using two-way ANOVA method was applied and showed that the wet conditioning has a significant effect on the maximum failure load of CFRP-to-steel bonded specimens. Reducing the bond-line thickness to less than 1 mm can result in a major reduction in load-carrying capacity and strain energy release rate of CFRP-to-steel bonded specimens. Humid and wet conditioning cause up to 44% and 63% drop in elastic modulus, 31% and 55% reduction in tensile strength, and 32% and 44% increase in average maximum strain of adhesive materials, respectively.

Shear performance of timber-GFRP double-lap adhesive joints for civil engineering applications

Adhesive bonding technology make it possible to join different materials and to assemble lightweight joints with high mechanical performance. These advantages over conventional technologies have led to a growing interest in these joints in timber construction. This study reports the results of an experimental campaign on double-lap adhesive joints between timber and GFRP adherends assembled with two different commercial structural epoxy adhesives. The aim of the present study is to evaluate the applicability of the bonding technique in the field of civil engineering and in the manufacture of window frames with adhesive joints (patent application No. 1020000023128, inventor Prof. Munafò). The results obtained after curing under laboratory conditions are compared with those obtained after exposure to high temperatures (40 °C) and relative humidity (100% RH). The results show that the effect of artificial ageing varies depending on the adhesive and generally leads to a reduction in the ultimate strengths and stiffness for both adhesives. The best mechanical performance in terms of ultimate load and stiffness is obtained by the epoxy adhesive EPX1 in each configuration tested, with values for ultimate load and displacements sufficient to ensure the mechanical performance in the range of applications studied. The epoxy adhesive EPX2 showed a significant decay of mechanical performance after artificial ageing and exhibited lower ultimate displacements and loads.

Low Viscosity Epoxy Structural Adhesive Cured at Room Temperature for Crack Repair of Subway Tunnel

Epoxy structural adhesive has good mechanical properties, especially high adhesion, low shrinkage rate, and high stability, widely used in steel plate reinforcement, crack repair, bridge splicing, and concrete bonding, etc. However, due to the high cross-linking density and high internal stress after curing, its shortcomings, such as brittleness, the poor performance of fatigue resistance and heat resistance, limit its application in special track engineering. In this work, two kinds of epoxy structural adhesives (EPA-1, EPA-2) with low viscosity and room temperature curing were synthesized by selecting different bisphenol A epoxy resin and active diluent as component A and modified amine curing agent as component B. The results showed that EPA-1 and EPA-2 presented significant toughness compared with the classic construction adhesive (AralditeXH160). The yield strengths of EPA-1 and EPA-2 were 4.88 MPa and 13.13 MPa, and the shear strengths were 11.93 MPa and 13.08 MPa, respectively, showing good adhesive properties. In addition, the viscosities of EPA-1 and EPA-2 were 127 mPa·s and 308 mPa·s, respectively, and their decomposition temperatures were all above ∼280 °C, which indicated that the self-made epoxy structural adhesives could be used for the repair of cracks in the vibration subway tunnel.

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.

Durability of Epoxy Adhesives and Carbon Fibre Reinforced Polymer Laminates Used in Strengthening Systems: Accelerated Ageing versus Natural Ageing.

This work addresses the durability of structural epoxy adhesives and carbon fibre reinforced polymer (CFRP) laminates typically used in strengthening of existing reinforced concrete structures exposed to natural ageing. The experimental program included four natural (real) outdoor environments inducing ageing mainly caused by carbonation, freeze-thaw attack, elevated temperatures, and airborne chlorides from seawater. Moreover, a control (reference) environment (20 °C of temperature and 55% of relative humidity) and an environment involving water immersion of the materials under controlled temperature (20 °C of temperature) were also included in this investigation. The characterization involved the assessment of the physical, chemical and mechanical properties along a study period of up to two years. Furthermore, comparisons between the natural ageing tests developed in the scope of the present work and accelerated ageing tests existing in the literature were performed. Regarding to the epoxy adhesives, an increase in the glass transition temperature with the time was observed, while the tensile properties decreased, regardless of the outdoor environment. The CFRP laminates were marginally affected by the studied environments. Despite the remarkable dispersion of the results observed in the accelerated ageing tests for the period investigated, this testing protocol yielded higher mechanical degradation than under natural ageing.

Adhesive hardening and plasticity in bonded joints

While strain-hardening mechanisms for many metals are well understood, polymers in general, and adhesives in particular, have received less attention. Thin adhesive joints are difficult to characterize, for instance, where the constraint of relatively rigid adherends can mask plastic response. The aim of this work was to quantify adhesive plastic strain hardening and evaluate the effectiveness of a nonlinear hardening model. The plastic response of two structural epoxy adhesives was shown to be dominated by kinematic hardening. A numeric model was developed to describe the shear stress-strain response of scarf and lap shear joints. Experimental adhesive joint responses were compared with linear and nonlinear hardening models. A nonlinear kinematic hardening model described the shear stress-strain response within 3% for a toughened adhesive. Agreement with a second adhesive, using a nonlinear combined hardening model, was within 2%.

Comparative Study of the Impact Wedge-Peel Performance of Epoxy Structural Adhesives Modified with Functionalized Silica Nanoparticles

Epoxy structural adhesives have strong adhesion, minimal shrinkage and high thermal and chemical resistance. However, despite these excellent properties, their high-energy impact resistance should be improved to satisfy the increasing demands of the automotive industry. For this reason, we used four types of silica nanoparticles with different surface groups, such as polydimethylsiloxane (PDMS), hydroxyl, epoxy and amine groups, as toughening agents and examined their effect on the glass transition temperature (Tg), crosslinking density and phase separation of epoxy structural adhesives. High-energy impact resistance, mode I fracture toughness and lap shear strength were also measured to explain the effect of surface functional groups. Silica nanoparticles with reactive functional groups increased the mode I fracture toughness of epoxy structural adhesives without sacrificing the crosslinking density. Although the mode I fracture toughness of epoxy structural adhesives could not clearly show the effect of surface functional groups, the dynamic resistance to cleavage obtained by impact wedge-peel tests showed quite different values. At a 0.3 vol% content, epoxy-functionalized silica nanoparticles induced the highest value (40.2 N/mm) compared to PDMS (34.1 N/m), hydroxyl (34.9 N/mm), and amine (36.1 N/m). All of these values were significantly higher than those of pristine epoxy structural adhesive (27.7 N/mm).

Behaviour of the adhesive bond between low-grade wood and GFRP reinforcements using epoxy resin

In this study, shear tests and pull-off tests were carried out to investigate the bond performance of glass fibre-reinforced polymer (GFRP) strips bonded to low-grade poplar wood (Populus × euroamericana I-214). Accordingly, 180 shear specimens and 72 pull-off specimens were produced using three different structural epoxy adhesives. These specimens were divided into two joint types (wood-wood and wood-GFRP configurations), which were tested under dry conditions and after exposure to hydrothermal cycles (i.e., accelerated ageing). The results showed that the epoxy adhesives exhibited good behaviour in the wood-GFRP samples exposed to different hydrothermal conditions. The samples exhibited shear strength values between 4.93 MPa and 8.49 MPa and pull-off strength values between 1.72 MPa and 2.69 MPa. Additionally, there were different main failure modes between the dry specimens and the aged specimens.

Mechanical Behavior of Toughened Epoxy Structural Adhesives for Impact Applications

The focus of our study is to identify physical properties of different impact-resistant/toughened structural adhesives and identify/develop an elastic-viscoelastic-plastic model as a function of loading rate by using Ludwik-type equations to be able to predict adhesive behavior at higher loading rates and to make cars more crashworthy. For this purpose, we first characterized eight different commercial toughened epoxy structural adhesives to provide detailed information about their constituents using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDS). Most (but not all) of the model adhesives contained organic tougheners in the form of carboxyl terminated butadiene acrylonitrile (CTBN) copolymer, as well as polyurethane adducts. The main crystalline inorganic phases were found as calcite (CaCO3), wollastonite (CaSiO3) or calcium silicate (CaSiO3), talc (Mg3Si4O10 (OH)2), zeolite which is an alumina silicate based mineral and has many different elements in its composition (M2/nO·Al2O3·xSiO2·yH2O, M can be Mg, Na, Ca, K, Li). The total amount of inorganic fillers was found to be different in each adhesive. Material behavior of the model adhesives were determined via tensile tests and Single Lap Joint (SLJ) tests in shear. Split Hopkinson pressure bar (SHPB) was also used to measure the strain and stress values at higher strain rates in the order of 102 s−1, which is generally encountered in impact related loading situations. Toughness values in the range ~0.5 to ~1.35 MJ/m3 were observed with the model adhesives tested in tensile mode within the ~3 × 10−3 to 0.18 m/m/s strain rate range. The softening behavior of the elastic moduli at higher strain rates observed during tensile testing was also observed with SHPB testing. It is remarkable that, overall, the modulus magnitudes seem to be similar between the tensile test and SHPB specimens within this softening range of the initial bilinear elastic behavior observed. When the results from bulk (tensile) and bonded (shear) specimens were compared, it was clearly seen that the toughness responses of the adhesives to (tensile/shear) strain rates in the bulk and bonded forms, respectively, were different, with the bonded shear toughness values in the ~25 to ~120 MJ/m3 range within ~1.25 to ~25 mm/mm/s shear strain range. The model adhesive which included just inorganic fillers had the lowest tensile toughness at the lowest tensile strain rate, but the highest slope in its tensile toughness regression line, exhibited the second highest bonded shear toughness. When tested at the extension rates of 25 mm/min and 100 mm/min in bonded lap shear, the same adhesive exhibited limited interfacial failure areas, however the dominant failure mode was cohesive failure. When the extension rate increased further, transition to interfacial (adhesive) failure was observed revealing that interfacial failures do not necessarily diminish adhesive bond toughness. Our observations point to the fact that cohesive deformation/failure processes indicating interfacial separations, inter-particle interactions as well as polymer matrix deformation in high deformation loading scenario as in bonded shear loadings may provide the highest toughness. Apparently, a large inorganic filler weight fraction is not necessary to obtain high shear toughness in bonded form since the highest bonded shear toughness was obtained with the adhesive which had the least amount of inorganic fillers among the model adhesives with 14.72 wt %.

Epoxy‐based composite adhesives with improved lap shear strengths at high temperatures for steel‐steel bonded joints

AbstractHigh‐performance room temperature‐cure epoxy structural adhesives utilizing simplified formulation are developed. The developed structural adhesive consists of diglycidyl ether of bisphenol A (DGEBA) and novolac epoxy blend as a base resin, micrometer‐sized silica particles as a reinforcing filler, and triethylenetetramine as a curing agent. The developed ambient temperature‐cure epoxy structural adhesive with optimized formulation exhibits outstanding properties including high glass transition temperature of 95°C, high thermal stability with degradation temperature at 5% weight loss of 364°C, exceptionally high rubbery plateau modulus of 320 MPa, good flame‐retardant characteristics with limiting oxygen index of 40, and high single lap shear strength for single lap steel‐steel bonded joint of 548 MPa at the temperature of 80°C. The silica‐filled DGEBA/novolac epoxy composite adhesive is a potential candidate for applying as a structural adhesive for construction with long‐term durability.

Synthesis of phosphorus-containing polyol and its effects on impact resistance and flame retardancy of structural epoxy adhesives

Structural epoxy adhesives are commonly used in many industries but are limited by their flammability and low impact resistance. In order to improve the flame retardancy of epoxy adhesives, phosphorus-containing species are mainly used as reactive flame retardants, but these generally cause a decrease in mechanical strength. In this study, an oligomeric phosphorus-containing polyol (P-polyol) was synthesized by step polymerization between phenylphosphonic dichloride and ethylene glycol. The curing behavior, viscoelastic properties, adhesion performance, impact resistance, and flame retardancy of an epoxy resin containing P-polyol were characterized. Especially, impact resistance was evaluated by the impact wedge-peel test, an industry-standard testing method. The flame retardancy of the structural epoxy adhesive improved steadily with the addition of p-polyol. Impact resistance also improved at moderate P-polyol contents (8–10 parts per 100 parts of resin (8–10 phr)) but drastically decreased at a high content (15 phr). Based on this study, by incorporating P-polyol, it is possible to simultaneously improve the impact resistance and flame retardancy of structural epoxy adhesives. We obtained both a high impact resistance (dynamic resistance to cleavage: 16.9 N/mm) and flame retardancy (heat release capacity: 255 J/(g∙°C)) at an optimum P-polyol content (8 phr).

On the influence of mechanical loadings on the porosities of structural epoxy adhesives joints by means of in-situ X-ray microtomography

Structural bonding is a beneficial technique extensively used in numerous industrial fields. This technique is however prone to structural defects such as pores, which are created during the mixing of the adhesive and during the shaping of the joint. Depending on their characteristics, these pores are likely to influence the mechanical behaviour of adhesively bonded joints, as they induce local decreases in the cross-section of the bonds and they may also create threatening stress concentrations. It is also fair to assume that the characteristics of the pores within an adhesive joint are subject to changes when the assemblies are submitted to external loads. In order to investigate these changes, adhesively bonded samples were made using two different bicomponent epoxy structural adhesives. These samples were placed inside an X-ray tomograph, containing a tensile machine. In-situ X-ray tomography measurements were made simultaneously with the application of a tensile load on the samples. It was therefore possible to characterise the porosity states of each sample under mechanical loading, and to compute various quantities (porosity volumetric ratio, the pores number, equivalent diameters distributions, etc.). It was found that the pores in the joints are impacted by the increasing mechanical stress, resulting in pore nucleation, pore growth and coalescence. Moreover, the present study shows that this microstructural behaviour cannot be generalised, as different adhesives may display different properties.