Diluted epoxy adhesives II. Relationship of shrinkage during cure to lap shear strengths

The shrinkage of a series of epoxy adhesives diluted with either phenyl glycidyl ether (PGE) or 1,4-butanediol diglycidyl ether (BDE) and cured with either diethylenetriamine (DTA) or diethylaminopropylamine (DPA) was measured and compared with the steel-to-steel lap shear strength obtained with these adhesives. It was found that the adhesives with the highest level of shrinkage also gave the highest strengths. Samples cured initially at room temperature with DPA underwent a significant expansion when postcured at elevated temperatures. The amount of shrinkage was found not to be dependent on sample size.     

Thermal conductivity and lap shear strength of GNP/epoxy nanocomposites adhesives

The addition of graphene nanoplatelets (GNPs) into the epoxy adhesives has been studied in order to increase their thermal conductivity. Thermally conductive adhesives are often used as thermal interface materials (TIMs). The incorporation of 8 and 10 wt% GNPs reinforcement caused a thermal conductivity enhancement of ~206 and ~306%, respectively. The wettability seems to decrease with low GNPs content (2–3 wt%) in comparison with the neat epoxy adhesive but the contact angle remains constant for higher GNPs contents. Lap shear strength remains constant for neat adhesives and resins doped with GNPs. The lack of enhancement of adhesive properties of doped resins is due to a weak interface reinforcement-matrix. In fact, the joint failure is in the adhesive except for high GNPs content (10 wt%) where a cohesive failure mode is observed.     

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

High-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.     

Synergistic effect of graphene and carbon nanotube on lap shear strength and electrical conductivity of epoxy adhesives

In this study, synergy between graphene platelets (GnPs) and carbon nanotubes (CNTs) in improving lap shear strength and electrical conductivity of epoxy composite adhesives is demonstrated. Adding two-dimensional GnPs with one-dimensional CNTs into epoxy matrix helped to form global three-dimensional network of both GnPs and CNTs, which provide large contact surface area between the fillers and the matrix. This has been evidenced by comparing the mechanical properties and electrical conductivity of epoxy/GnP, epoxy/CNT, and epoxy/GnP-CNT composites. Scanning electron microscopic images of lap shear fracture surfaces of the composite adhesives showed that GnP-CNT hybrid nanofillers demonstrated better interaction to the epoxy matrix than individual GnP and CNT. The lap shear strength of epoxy/GnP-CNT composite adhesive was 89% higher than that of the neat epoxy adhesive, compared with only 44 and 30% increase in the case of epoxy/GnP and epoxy/CNT composite adhesives, respectively. Electrical percolation threshold of epoxy/GnP-CNT composite adhesive is recorded at 0.41 vol %, which is lower than epoxy/GnP composite adhesive (0.58 vol %) and epoxy/CNT composite adhesive (0.53 vol %), respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48056.     

Viscoelastic properties and lap shear strength of EVA/aromatic hydrocarbon resins as hot-melt adhesives

A series of ethylene vinyl acetate (EVA) copolymers were blended with aromatic hydrocarbon resins in the molten state in various ratios and possible relations between viscoelastic and adhesion properties as hot-melt adhesives (HMAs) were investigated. When the softening point of aromatic hydrocarbon resin was high and the concentration of aromatic hydrocarbon resin was low, the tan δ peak height decreased and broadened. This result corresponds to the miscibility of the blend. The single lap shear strength increased with increasing softening point of the aromatic hydrocarbon resin and it reached a maximum value with increasing temperature. A large scatter was observed in lap shear strength values, which were higher at higher test rates and lower temperatures, and under these conditions interfacial failure occurred.     

Material characterization of structural adhesives in the lap shear mode

A general method for characterizing structural adhesives in the bonded lap shear mode is proposed. Two approaches — semi-empirical and theoretical — are assessed. The semi-empirical approach includes Ludwik's and Zhurkov's equations to describe, respectively, the failure stresses in the constant strain-rate and constant stress loading modes with the inclusion of temperature effects. The theoretical approach is used to describe adhesive shear stress/strain behaviour by utilizing viscoelastic or non-linear elastic constitutive equations. On the basis of this approach an empirical extension of Crochet's delayed failure equation is proposed to account for the effects of high temperature.     

Rheology and adhesive properties of filled PIB-based pressure-sensitive adhesives. I. Rheology and shear resistance

Two nanosized fillers (fumed silica Rosil-175 up to 20 wt% and halloysite nanotubes up to 40 wt%) were used to suppress cold flow of polyisobutylene (PIB)-based adhesives. Rheological measurements revealed the presence of the yield stress in the PIB-halloysite system which is indicative of the three-dimensional network formation in the bulk of the matrix. However, further rheology tests identified low strength of the network. In case of Rosil, no yield stress was detected, only gradual increase in the viscosity. Shear bank testing showed 4–5 times increase in the PIB-halloysite system time to failure when 40 wt% of halloysite was incorporated into the PIB matrix. Increase in the time to failure for PIB-Rosil systems was over two orders of magnitude – at the Rosil content up to 20 wt%. Such superior properties of the Rosil adhesion joints are considered to be the result of Rosil high specific area and low strength of the halloysite nanotubes network.     

Liquid-rubber-modified epoxy adhesives cured with dicyandiamide. II.–Morphology and adhesion strengths

The lap shear and T-peel strengths of n-butylacrylate/acrylic acid (nBA/AA)-copolymer-modified DGEBA adhesive were evaluated. Adherends used in this study included carbon-fiber-reinforced plastics (CFRP), glass-fiber-reinforced plastics (GFRP), and aluminum. It was found that the adhesive strengths could be improved by incorporating nBA/AA copolymer in the DGEBA matrix. An optimum functionality of the copolymer existed for maximum lap shear strength. Morphological studies revealed that the adhesive strength was also influenced by the compastibility between the copolymer and the matrix, the property of the adherend as well as the addition of the bisphenol A.     

Liquid-rubber-modified epoxy adhesives cured with dicyandiamide. I. Preparation and characterization

n-Butylacrylate/acrylic acid (nBA/AA) copolymers were synthesized and used to modify epoxy resin (DGEBA) cured with dicyandiamide (DICY). The precuring reaction between nBA/AA copolymer and DGEBA, the curing cycle of DGEBA, and the effects of DICY and aluminum powder upon the adhesive strengths of modified DGEBA were studied. It was found that the optimum DICY/DGEBA ratio was 6 g/100 g, and lap shear strength and T-peel strength increased with increasing amount of aluminum powder. The curing cycle for modified DGEBA was determined to be 1 h at 177°C.     

Dynamic mechanical properties and adhesive strengths of epoxy resins modified with liquid rubber. I. Modification with ATBN

The dynamic mechanical properties and the adhesive strengths of Epikote 828 and Epikote 828-ATBN blend systems were investigated. The ATBN blend systems were proved to be completely incompatible with the dynamic mechanical measurement and also fitted well with Takayanagi's model which was designed for completely incompatible two-phase systems. The epoxy resin had a nonreacted part when cured at room temperature. The blending of ATBN reduced the nonreacted part of the epoxy resin, and made contributions to the adhesive strengths. In the case of tensile test of crosslap specimens using aluminium as adherends, the adhesive strengths of ATBN blend systems were almost 1.5-fold of those of epoxy resin without blending of ATBN. As for wood adherends, the maximum of the adhesive strengths was found at 60°C for epoxy resin without blending of ATBN, and at 0°C for ATBN blend systems. The facts meant that there were mutual interactions between the adhesive strengths and the viscoelastic behavior of the adhesive polymers in the two-phase systems as observed in completely miscible polymer blends. There was not pronounced distinction between epoxy resins without blending of ATBN and ATBN blend system, as to the shear adhesive strengths.     

Network structure and properties of imidazole-cured epoxy novolac adhesives

The structural characteristics of four epoxy adhesives, obtained by crosslinking an epoxy novolac with various levels of a substituted imidazole curing agent, were investigated and correlated with thermal and mechanical properties. Variations in network structure were characterized by measuring crosslink densities and by qualitatively assessing glassy state free volume from densities and coefficients of thermal expansion. Differential scanning calorimetry was used to obtain glass transition temperatures, and dynamic mechanical thermal analysis was used to follow primary (alpha) and secondary (beta) transitions. Bulk behavior was characterized by tensile modulus, strength, and toughness, together with compressive modulus and yield strength. The effect of sub-T_g aging on compressive yield strength was investigated as well. As the level of imidazole increased, crosslink density, and hence network packing efficiency and free volume, decreased. For fully cured networks, both the glass and the alpha transition temperatures increased with crosslink density. Calculated activation enthalpies and entropies indicated significant degrees of network cooperativity in the alpha transitions, particularly for the more highly crosslinked systems. Beta transition temperatures, however, were found to be independent of crosslink density. Bulk properties generally showed a dependence both on crosslink density and free volume. Yield stress, for example, was highest for the network with lowest crosslink density and free volume. Volume relaxation associated with physical aging also caused yield stress to increase.     

新型双组分丙烯酸环氧酯胶粘剂的制备与性能研究

利用丙烯酸环氧酯及环氧树脂E-51为主体,以酸酐为固化剂研制一系列双组分胶粘剂,并对其凝胶化时间、吸水性、拉伸剪切强度、接触角、表面能等各项性能进行了研究.该系列胶粘剂吸水性较低(1.18％～1.78％)、疏水性强、剪切强度最高达30.9 MPa,综合性能良好.     

Polyester hot-melt adhesives. I. Factors affecting adhesion to epoxy resin coatings

The peel strength and tensile shear strength of polyester hot-melt adhesives on metals coated with epoxy resins are affected by four characteristics of the polyester: (1) inherent viscosity, (2) glass transition temperature (T_g), (3) degree of crystallinity, and (4) melting point. The inherent viscosity affects the strength, toughness, and crystallinity of the adhesive. The T_g and degree of crystallinity affect the low-temperature adhesive properties; the peel strength is relatively low when the T_g is appreciably above the use temperature. The T_g, degree of crystallinity, and melting point affect the high-temperature adhesive properties. A hot-melt adhesive with high peel and tensile shear strengths from 0° to 120°C is the polyester of 1,4-butanediol and trans-1,4-cyclohexanedicarboxylic acid.     

Effects of symmetrical bonding defects on tensile shear strength of lap joints having ductile adhesives

In this paper we examine the effect on joint strength of depleting the bond line of a relatively flexible adhesive (polyethylene) while maintaining a constant adhesive film thickness. It is shown that the tensile shear strength of a lap specimen is not governed by edge effects but rather by the bonded area. By using limit analysis of the plasticity theory, we demonstrate why the tensile shear strength of the joint is insensitive to stress concentrations at the bonding defects.     

Effect of bonding defects on shear strength in tension of lap joints having brittle adhesives

In this paper we examine how the joint strength of lap joints containing a brittle adhesive may be affected by partial removal of adhesive from the bonded area. It is found that the shear strength in tension of a lap joint specimen is governed essentially by the leading edges of the joint and not by the bonded area.     

Aging effects on the electrical properties of silver-filled epoxy adhesives

Electrically conductive adhesives, such as silver-filled epoxies, are used in many applications where standard soldering or direct metal-to-metal connections are not feasible. Anomalous behavior at these connections has been observed as the development of increasing resistance over time in the absence of mechanical failure between the adhesive and substrate. The results of aging of electrically conductive adhesive joints to aluminum- and gold-metallized substrates show that little change occurs at low humidity under an ambient flow of nitrogen. With increasing humidity and temperature, there is a measurable increase in resistance over time, particularly in the case of aluminum-metallized substrates. Secondary ion mass spectrometry (SIMS) was used to investigate interfacial phenomena that may be responsible for increasing resistance observed in the aluminum-metallized samples exposed to accelerated aging. Preliminary results from this technique indicate the presence of an oxide layer between the adhesive and the metallization layer that is consistent with increasing resistance measured for samples exposed to accelerated aging. The results also support the conclusion that deterioration at the adhesive-aluminum interface, and not degradation in the bulk adhesive, is the primary cause of increasing resistance.     

Relationship between mechanical properties of and crack progogation in epoxy resin adhesives

Crack propagation in a series of amine-cured epoxy resin adhesives in both the bulk material and adhesive joints has been studied as a function of the formulation of the resins and the conditions of testing using a linear elastic fracture mechanics approach. In both cases propagation was found to take place in either a stick—slip (unstable) or a continuous (stable) manner; the particular type of propagation depending upon the amount and type of hardener used and the temperature and rate of testing. A constant crack opening displacement (δ) has been shown to be a unique failure criterion for continuous propagation with δ having approximately the same value in both bulk material and joints. However δ was found to increase in the stick—slip mode of propagation. These crack propagation characteristics have been related to the compressive yield behaviour of each material as determined by uniaxial compression tests performed on the epoxy resins. Possible mechanisms of crack propagation have also been discussed.     

Elastomer modification of epoxy based film adhesives: adhesive and mechanical properties

Three approaches were employed to improve the flow and sandwich bonding properties of a nylon-carrier supported film adhesive based on carboxyl terminated butadiene acrylonitrile (CTBN)-modified novolac epoxy resin. These included the addition of a commercial acrylate flow modifier, replacement of novolac epoxy partly with solid diglycidyl ether of bisphenol A (DGEBA) resins, and replacement of CTBN partly with an epoxy functional acrylate terpolymer (EPOBAN). Adhesive properties such as lap shear strength (LSS), T-peel strength (TPS) and flatwise tensile strength (FTS) on honeycomb core bonded sandwich specimens were evaluated using aluminium adherends. The addition of the flow modifier in low concentrations enhanced the flexibility of the system and resulted in a marginal increase in LSS, TPS and FTS. Replacing novolac epoxy partly with solid DGEBA resulted in a less brittle system with enhanced LSS and TPS, but with reduced FTS due to the decreased flow characteristics. A substantial increase in FTS was observed when CTBN was partly replaced with EPOBAN. The introduction of EPOBAN resulted in good flow and fillet properties and the optimum FTS was obtained for the composition based on 25/75 CTBN/EPOBAN ratio. Mechanical properties of selected systems were also studied in addition to adhesive properties.