Mechanical and thermal behavior of modified epoxy–novolak film adhesives

Modified epoxy-based film adhesives were developed for bonding structural joints. Film adhesives with different compositions were prepared by hot pressing the molten resins. Peel and shear tests were carried out to evaluate the adhesion properties. Dynamic mechanical thermal analyses were conducted to follow the changes in the adhesive structure and also the trend of impact strength. Incorporation of thermoplastic poly(vinyl butyral) (PVB) into an epoxy- novolak combination resulted in higher cohesive strength, better film-forming ability, enhanced adhesive shear and peel strengths, but decreased thermostability. However, due to the lower chemical functionality of PVB, a lower crosslink density was achieved. Incorporation of a small amount of ethylene glycol dimethacrylate (EGDM) as a flexibilizer led to improved mechanical properties, easy handling and facile application. Finally, good shear strength retention up to 200 °C for 1 h was observed in the case of EGDM-modified adhesives.     

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.     

The effect of curing temperature on thermal,physical and mechanical characteristics of two types of adhesives for aerospace structures

The effects of cure temperatures on the thermal, physical and mechanical characteristics of two types of thermosetting structural epoxy film adhesives were determined in detail. The aim of this paper is to assess the effect of cure temperatures (82–121 °C) on the degree of cure of the two adhesives and the relevant void formations that need to be addressed in bonded part production and repair. Two thermal parameters were used to characterize the advancement of the reaction, such as degree of cure and glass transition temperature. The joint properties with respect to the cure temperatures were characterized by void content and bond-line thickness measurements and lap shear strength tests. Experimental results presented that all lap shear strengths were well within minimum shear strength (29 MPa) required by the specification of the film-type adhesive. However, the lap shear strength testing after aging at 82 °C and 95%R.H for 1000 h showed that the improved durability when the adhesive is cured at 121 °C did not occur for the 82 °C cure. Low curing conversion (75–77% degree of cure) combined with high voids (over 2 areal%) has a catastrophic effect on the bonding qualities at the metal-adhesive interface and due to lack of cohesion in the adhesive. The changes in the interface caused by the low temperature curing may contribute to an increased susceptibility of the bonded joint to moisture and consequent bond-line degradation.     

Adhesion strength of sodium dodecyl sulfate-modified soy protein to fiberboard

Thermal and rheological properties of sodium dodecyl sulfate (SDS)-modified soy protein isolate (SPI) adhesives were studied using differential scanning calorimetry (DSC) and rheometry. The ordered structure of native SPI was denatured as SDS concentration increased, and thermal stability of native SPI decreased at high SDS concentration. The enthalpy of SPI denaturation decreased significantly with increasing SDS concentration. Apparent viscosity of the SPI adhesives increased as SDS concentration increased. The SPI adhesives modified by high concentrations of SDS exhibited characteristics of a Newtonian-type flow. The SDS-modified SPI adhesives were applied to fiberboard, and effects of SDS concentration, press conditions, and assembly time on bond strength were investigated. Shear strength of the SPI adhesives increased with SDS concentration, reaching its maximum value at 3 wt% of SDS, and then decreased significantly. The shear strength increased as press time and/or press temperature increased. High press temperature (100 °C) and long press time (5 min) are needed to achieve relatively good adhesion properties. The shear strength also increased as assembly time increased. The shear strength of the SDS-modified SPI adhesives decreased after soaking in water for 24 h.     

New two-part epoxy paste structural adhesives for a thick bondline

The properties of three different adhesive pastes J-101, J-114, and EA9309.3N/A are evaluated. These adhesives are two-part thermosetting pastes which can be cured at 80-90°C and are designed for inlay or dowel bonding application, and thus are especially suitable for a bondline thickness from 0.8 to 1.6 mm. The compositions are based on CTBN-modified DGEBA and multicomponent polyamine curing agent systems. Durability and fatigue life tests are also discussed. The microstructure and fracture behavior of these adhesives is studied by scanning electron microscopy (SEM). Different toughening mechanisms are proposed to account for the toughness of these adhesives. The experimental results show that both J-101 and J-114 adhesives develop good mechanical properties when cured at 80-90°C. Furthermore, their durability performance compares favorably with, or even exceeds, that of commercially-available EA9309.3N/A adhesive.     

A Method to Evaluate Water Resistance of Acrylic Adhesives

A number of two-part acrylic adhesives were examined for the effect of immersion time in water at 23°C, 80°C and 100°C on the tensile lap shear strength. The mechanism of acrylic bonded joint failure in water was discussed based on the observed results. The relative water resistance between these adhesives was compared from shear strength retention values. The dependence of the water resistance of these adhesives at 23°C and at elevated temperatures was discussed. It has been found that for acrylic adhesives, the water resistance at room temperature can be evaluated on the basis of the shear strength retention values after aging in water at 80°C.     

Preparation and characterization of novel film adhesives based on cyanate ester resin for bonding advanced radome

In this paper, the novel film adhesives based on phenolphthalein poly(ether sulfone) (PES-C) and epoxy (EP) modified cyanate ester resin (CE) were prepared for bonding an advanced radome. The film adhesives are convenient for applying to manufacture, possessing good adhesion strength, thermal durability and excellent dielectric property. The curing behaviors were confirmed by differential scanning calorimetry (DSC), showing that the main reaction pathways are not varied with adding PES-C but the reaction rates are evidently accelerated, and the film adhesives can be well cured at lower temperature of 177 °C. The adhesion strength was evaluated in lap shear strength and peel strength, indicating that the better adhesion strength is obtained with increasing in PES-C. The maximum value of lap shear strength is 33 MPa at room temperature. The thermal durability was determined by thermal aging tests of lap shear specimens, showing that the decrease in strength gets faster with adding PES-C, and the usability of film adhesives over 2000 h at 200 °C. The dielectric properties were measured by dielectric resonator methods, finding that the introduction of PES-C brings a positive effect on dielectric properties. The lowest value of determined dielectric loss is 0.0075 at 10 GHz.     

Soy-based,tannin-modified plywood adhesives

In this research, two different types of commercial tannins, namely a hydrolysable tannin (chestnut) and a condensed flavonoid tannin (mimosa), were used to prepare two types of soy-based (soy flour (SF) and soy protein isolate) adhesives for making plywood. Thermogravimetric properties (TGA) and its derivative as function of temperature (DTG) of different soy-based adhesive were measured in the range 40°C–300°C. Thermomechanical analysis (TMA) from 25°C to 250°C was done for the different resin formulations. Duplicate three-ply laboratory plywood panels were prepared by adding 300 g/m² of the adhesives’ total resin solid content composed of SF or isolated soy protein (ISP), urea, chestnut, and mimosa tannin extracts with hexamine as hardener. Based on the results obtained, tannins can improve SF adhesion properties. The TMA showed that chestnut tannin extract appeared to react well with SF, while mimosa tannin extract appeared to react well with ISP. Matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry also showed that among other reactions, the soy protein amino acids reacted with the tannins. Furthermore, delamination and shear strength test results showed the good water resistance of plywood bonded with soy-based tannin modified adhesive.     

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

Accelerating 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.     

Photoelastic thermal stress measurements in scarf adhesive joints under uniform temperature changes

This study deals with the investigation of thermal stresses and delamination growth in scarf joints under a uniform temperature change by photoelastic measurements and a two-dimensional finite element analysis. The adherends were fabricated from aluminum plates, and an adhesive layer was modeled and fabricated from an epoxide resin plate. The adherends and the epoxide resin plate were bonded using a heat-setting and one-component-type adhesive. The adhesive was cured at 85 °C and cooled down to room temperature. The thermal stress was then generated in the scarf joint under a temperature change and measured by photoelasticity. After the scarf joints were cooled in a stepwise manner, the delamination growth, which initiates from the edge of the interface, was measured. It was found that the delamination initiates from the edge of the interface with the acute angle side and it never initiates from the edge with the obtuse angle side. When the scarf angle is 90°, i.e. in adhesive butt joints, the resistance against the delamination is minimal. The thermal stresses in the scarf joints with a thin adhesive layer were also analyzed. It was found that the thermal strength increases as the adhesive thickness decreases. The stress singularity near the edge of the interface was calculated from the stress distributions in the joints with different scarf angles. As a result, it was found that the stress singularity in the scarf joints under thermal loads is quite different from that under static tensile loads.     

Preparation of epoxy hardeners from waste rigid polyurethane foam and their application

Aliphatic amines were used as decomposer to decompose waste rigid polyurethane and polyisocyanurate foams, and the obtained decomposed products were directly used as curing agent of epoxy resin. Effects of the decomposing condition including amine type, foam–decomposer ratio, and reaction temperature on the decomposition reaction and properties of the decomposed products were investigated. Using amines with low molecular weight could enhance decomposition reaction rate and total amine number and lessen viscosity of the obtained decomposed products. Viscosity of the decomposed products decreased with increase of reaction temperature, but increased with increment of foam–decomposer ratio. Shear strength of adhesives consisting of decomposed products and epoxy resin was measured, and their thermal properties were analyzed. The adhesives could be cured completely over 60°C and their shear strength enhances with adding coupling agent in the adhesive system. The adhesives have good thermal stability and show satisfactory shear strength with more than 15, 15, 7, and 3 MPa at 25, 60, 100, and 150°C, respectively. The results demonstrate that the obtained adhesive systems can be used as structural adhesive. © 1995 John Wiley & Sons, Inc.     

Wood–phenol adhesives prepared from carboxymethylated wood. I

Preparation of carboxymethylated wood (CM wood)–phenol resin adhesives has been attempted by two methods, “kneading method” and “solvolysis method,” and their adhesion strength has been studied. The two preparation methods differ in the dissolution step. In the case of the kneading method, CM wood was dissolved in aqueous phenol by kneading at 100–120°C under shear, whereas, in the case of the solvolysis method, the dissolution was facilitated by phenolysis at 80°C in the presence of appropriate amounts of hydrochloric acid. The wood-based adhesive prepared by the solvolysis method revealed excellent and enhanced applicability compared with that of the adhesive prepared by the kneading method, although the latter can be used as an adhesive for wood. Adhesion strength of these adhesives was enhanced when poorly substituted CM wood and appropriate amounts of formaldehyde were used in the resin preparation. A crosslinking agent for carboxymethyl cellulose, that is, polymeric MDI, was also added just before application. The water-proof adhesion strength was higher than the JIS specification for phenol resin adhesives for this modification.     

Improvement of the Molar Tube Bond Durability with Preheated Orthodontic Adhesive

This study aimed to evaluate the effect of preheated orthodontic adhesives and thermal cycling on the bond strength of molar tubes. One hundred sixty molar tubes were bonded to acid-etched bovine incisors using a conventional orthodontic adhesive (Transbond XT), two microhybrid (Wave and Permaflo), and a nanofilled (Filtek Z350) flowable composite resins, at room temperature or preheated at 60°C. Transbond XT primer and Single Bond 2 adhesive system were used in association with Transbond XT and the flowable composites, respectively. The specimens were stored in water (37°C) for 24 h, and half of the sample was subjected to thermal cycling for 6000 cycles. Ashear bond strength (SBS) test was performed, followed by the appraisal of the adhesive Remnant Index (ARI). Three-way analysis of variance (ANOVA) and the Tukey test were performed at a significance level of 95% (P < .05). Samples bonded with preheated adhesives showed higher SBS regardless of the aging method. Only samples bonded with preheated conventional orthodontic adhesive maintained their bond strength after thermal cycling for 6000 cycles. Preheating orthodontic adhesives improved the bond strength of molar tubes, but only the preheated conventional orthodontic adhesive was capable of maintaining bond strength after thermal cycling.     

Sequential Fractionation of Cottonseed Meal to Improve Its Wood Adhesive Properties

For better understanding of the adhesive properties of different fractions of cottonseed protein, cottonseed meals from both glanded and glandless cotton varieties were separated into several fractions. Each meal was sequentially extracted with water and 1 M NaCl solution, or with phosphate buffer and NaCl solution. Adhesives were prepared from the recovered fractions and hot-pressed onto maple veneer strips and tested for their properties. The adhesive strength of the water- and buffer-washed solid fractions (i.e., the un-extractable residues of the meals) from the glanded seed ranged from 1.32 to 1.62 MPa and were unchanged or increased compared with the adhesive strength of the original meal that varied from 0.98 and 1.49 MPa. Soaking the wood specimens bonded at 80 °C revealed that the water resistance of these water- and buffer-washed adhesives was significantly improved in that they exhibited no delamination during soaking compared with the meal adhesive that showed some delamination (20–30 % of the samples). Furthermore, the water resistance of these fractions with wet shear strength around 1.5 MPa was comparable to that of cottonseed protein isolate (>90 % protein) when the joints were bonded at 100 °C. The preparations from glandless cottonseed meals showed similar adhesive performances. Additional extraction of the meals with NaCl solution reduced adhesive performance. The results suggest that water- or buffer-washed cottonseed meal fractions can be used as wood adhesives and would be less costly to prepare than cottonseed protein isolates.     

Effect of natural and synthetic tackifiers on viscosities,adhesion properties and thermal stability of SNR and DPNR solution adhesives

Styrene-grafted natural rubber (SNR) and deproteinized natural rubber (DPNR) latexes were formulated with coumarone-indene (CI), gum rosin and petro resin (PR) tackifiers into solution adhesives with toluene as a solvent. The solution viscosities were evaluated by a Brookfield viscometer DV-II Plus with spindle No. 3. Pressure sensitive adhesives (PSAs) films were made and the adhesion properties were evaluated with loop tack, peel strength and shear strength tests. Thermal stability of the film was evaluated via Perkin-Elmer Pyris 6^TM thermogravimetric analysis at temperatures ranging from 30 to 600?°C at a heating rate of 10?°C per minute in nitrogen environment. Results indicate that as the tackifiers content increased, the solution viscosities increased with SNR/PR and DPNR/PR formulations showing the highest viscosities. Adhesion test also indicates that loop tack and peel strength of the adhesive solution increased but their shear strength decreased; increase of CI tackifier loadings conferred the highest peel strength for both SNR- and DPNR-based PSAs. Thermal analyses show that the addition of 40 phr CI tackifiers improved thermal stability of SNR adhesives based on their higher T_max and integral procedural decomposition temperature properties.     

Evaluation of wood bonding performance of water-washed cottonseed meal-based adhesives with high solid contents and low press temperatures

Water-washed cottonseed meal (WCSM) has been shown as a promising bio-based wood adhesive. In this work, we tested the bonding strength of WCSM slurries with high solid contents and low press temperatures per industrial input for non-structural applications as European Standard Class D1 wood adhesives. Increasing the WCSM content from 11 to 20% and 30% did not substantially change the adhesive strength but increased the viscosity of WCSM slurries dramatically. The shear strength at break of the maple wood pairs bonded at 40 and 60 °C was lower than that of maple pairs bonded at 100 °C. However, the shear strength of the pairs bonded at lower temperatures (40 and 60 °C) could be improved by extending the press time from 20 to 120 min. Addition of citric acid (CA) improved the viscosity of the WCSM adhesive at 20% solid content, but lowered the adhesive strength. The addition of denaturing reagent sodium dodecyl sulfate (SDS) showed reverse impacts on the adhesive strength and viscosity, compared to CA addition. Based on these observations, eight adhesive slurries were formulated with 20 and 30% of WCSM, 3% of CA or SDS, or 9.6% or 19.1% of denaturing reagent guanidine hydrochloride (GdmCl), and their bonding strengths were tested. These formulations could be used as the basis for developing low temperature WCSM-based wood adhesives to meet the criteria of both operational flowability and shear strength of the domestic furniture and small utensils niche markets for forest products.     

Investigation of formaldehyde-free wood adhesives from kraft lignin and a polyaminoamide–epichlorohydrin resin

A formaldehyde-free wood adhesive system consisting of kraft lignin and a polyaminoamide-epichlorohydrin (PAE) resin (a paper wet strength agent) has been investigated in detail. The lignin-PAE adhesives were prepared by mixing an alkaline kraft lignin solution and a PAE solution. Mixing times longer than 20 min had little impact on the shear strength of the wood composites bonded with the lignin-PAE adhesives. The shear strength of the wood composites bonded with the lignin-PAE adhesives increased and then flattened out when the press time and the press temperature increased. The shear strength and water resistance of the wood composites bonded with the lignin-PAE adhesives depended strongly on the lignin/PAE weight ratio. Of the weight ratios studied, the 3:1 lignin/PAE weight ratio resulted in the highest shear strength and the highest water resistance of the resulting wood composites. The wood composites bonded with the lignin-PAE adhesives did not delaminate and retained very high strengths even after they underwent a boiling-water test. The lignin-PAE adhesives could be stored at room temperature for two days without losing their adhesion ability. PAE was the crosslinking agent in this lignin-PAE adhesive. Possible reactions between lignin and PAE are discussed in detail.     

Environmental aging of the Ti-6Al-4V/FM-5 polyimide. adhesive bonded system: implications of physical and chemical aging on durability

Ti-6A1-4V/FM-5 polyimide adhesively bonded double cantilever beam (DCB) specimens were aged for 12 months at elevated temperatures (177°C and 204°C) in one of three different environments: ambient atmospheric air pressure and reduced air pressures of 2 psi (13.8 kPa) and 0.2 psi (1.38 kPa), to assess bond durability. The FM-5 polyimide adhesive (Tg~ 250°C) is based on a polyimide developed by NASA Langley Research Center and is produced by Cytec Industries, Inc. Bonds aged for different times were tested to measure the critical strain energy release rate as a function of the temperature and environment. The greatest loss in bond strength occurred after aging in air at 204°C. Following thermal rejuvenation of the aged bonds at 300°C for 2 h, part of the strength loss could be recovered. This strength recovery was attributed to the reversal of physical aging in the adhesive resin. Further evidence for physical aging, which is a thermo-reversible phenomenon, was obtained from tests conducted on neat resin specimens using DMA (dynamic mechanical analysis) and DSC (differential scanning calorimetry). The unrecovered portion of the loss in bond strength following longer-term aging was attributed to chemical aging/degradation of the bonded 'system'. The 'system' in this study includes the adherends, the adhesive, the surface pretreatment (chromic acid anodization, CAA), and their respective interphase/interface regions. Evidence for chemical aging was also seen from weight loss, and Soxhlet extraction data on neat resin specimens.     

Adhesive Properties of Cyanoethyl Starch

Cassava starch was cyanoethylated to different degrees of substitution (DS) and then used in adhesive preparations. The adhesives were used in bonding wood substrates and the adhesive strength of the bonded substrates was evaluated using a tensometer. The effects of the DS, solids content, pH and moisture on adhesive strength were investigated. The adhesive strength increased with increasing DS; with respect to the percentage solids, it increased to a maximum and then decreased again; with pH, it showed a minimum at a neutral pH; while it decreased with length of exposure to saturated moisture conditions. The experimental data were subjected to regression analysis to find the best fitting models. The results showed that the adhesive strength varied polynomially with DS (R ² = 0.97), pH (R ² = 1.0), percentage solids content (R ² = 0.94), and exponentially with the length of exposure to saturated moisture conditions (R ² = 0.96). Cyanoethylation significantly enhanced the adhesive property of starch (p < 0.05) and the cyanoethyl starch adhesives were found effective for bonding wood. However, cyanoethyl starch adhesive is not suitable as structural adhesive in a high moisture environment. The performance of cyanoethyl starch adhesive was compared with two commercial adhesive pastes.     

Selected aspects of epoxy adhesive compositions curing process

The paper focuses on selected parameters of curing process – temperature and time. The tests aimed at evaluating the impact of short-term thermal recuring on 1050A and 2017A aluminium alloy sheet adhesive joints strength. Joints were formed with two different adhesives, the main component of which was in both cases epoxy resin Epidian 53 and two different cure agents – poliamineamide C (PAC) and triethylenetetraamine (PF) curing agents. Curing conditions – first curing time, recuring time and recuring temperature – were modified for each of the four tests conducted. For the sake of comparative analysis, adhesive joints were subjected to a single-stage cure cycle at ambient temperature. A two-stage cure cycle of both Epidian 53 compositions at 80?°C for 1 and 2?h produces a material of different mechanical properties than the same material which submits a single-stage cure cycle at ambient temperature, as well as at 60?°C for 30?min. Simultaneously, Epidian 53/PF/100:50 composition proves to produce higher joint strength after recuring than Epidian 53/PAC/100:80; the strength of a joint formed with the former composition increases up to 50% when compared with joints subjected to a single-stage cure cycle. Moreover, tests show that recuring of the adhesive joint formed with both compositions at 60?°C for 30?min does not have a considerable influence on either 1050A or 2017A aluminium adhesive joint strength.