Adhesion of elastomers: Dwell time effects

The strength of adhesion of elastomers to rigid substrates generally increases with time of contact. This effect has been studied for samples of butyl and chlorobutyl rubber adhering to some rigid substrates. The peel strength increased continuously over long periods of contact until in some cases failure became cohesive within the elastomer layer. At higher temperatures the strength increased more rapidly, consistent with the WLF relation governing molecular motions. It is postulated that slow molecular rearrangements occur at the interface and increase the bond strength. A criterion for the observed transition from interfacial to cohesive failure is suggested.     

Adhesion of Natural Rubber Compounds to Plasma-Polymerized Acetylene Films

Plasma-polymerized acetylene films were shown to be novel, highly effective primers for rubber-to-steel bonding. However, the performance of the primers depended strongly on processing variables such as the substrate pretreatment and the carrier gas. Miniature lap joints were prepared by using natural rubber as an “adhesive” to bond together pairs of pretreated steel adherends primed with plasma-polymerized acetylene films which were deposited using various carrier gases. The initial strength of joints prepared from substrates which were mechanically polished and then coated with plasma-polymerized acetylene films deposited using an argon or nitrogen carrier gas was 2000 N for a bonded area of 64 mm² and failure was 100% cohesive in the rubber. Similar results were obtained for joints prepared from mechanically-polished brass substrates. However, the initial strength of joints prepared from polished substrates which were coated with plasma-polymerized films deposited using oxygen as a carrier gas was lower by a factor of two and there was only 30% rubber coverage on the substrate failure surfaces. demonstrating the importance of the carrier gas.

The initial strength of joints prepared from substrates which were pretreated by alkaline cleaning, acid etching, or mechanical polishing and then coated with plasma polymers using argon as the carrier gas was also approximately 2000 N/64 mm² and failure was again 100% cohesive in the rubber. However, the strength of joints prepared from substrates which were pretreated by ultrasonic cleaning in acetone and then coated with plasma polymers using argon as the carrier gas was lower by a factor of almost two, demonstrating the significance of substrate pretreatment.

During exposure to steam at 121°C, the durability of miniature lap joints prepared from polished steel substrates primed with plasma-polymerized acetylene films using argon as a carrier gas was excellent. After exposure for 3 days, the breaking strength of the joints decreased slightly, from 1740 to 1410 N/64 mm², but the locus of failure remained cohesive in the rubber, implying that effect of steam was mostly to reduce the cohesive strength of the rubber. Similar results were obtained from joints prepared from polished brass substrates. However, the durability of joints prepared from polished brass substrates and from polished steel substrates primed with plasma-polymerized acetylene was poor during exposure to aqueous salt solutions for three days. Although all of the joints decreased significantly in breaking strength, the strength of the joints prepared from brass substrates was about 400 N/64 mm² higher than that of joints prepared from steel primed with plasma-polymers. Most of the joints prepared from steel primed with plasma-polymerized acetylene films failed near the interface between the primer and the steel substrate although some specimens had 20-40% rubber coverage on the failure surfaces.     

Ice Adhesion to Hydrophilic and Hydrophobic Surfaces

The adhesional shear strength has been determined for ice formed against polished steel, monolayers adsorbed on steel and thin plastic coatings painted on metal surfaces. The adsorbed monolayers reduced the shear strength to about one-third of that for ice on clean steel. The monolayers also had the effect of changing the character of the breaks from clearly cohesional to apparently adhesional failure. The shear strength from the plastic coatings ranged from values equal to that of ice against clean steel to values 70 to 80% lower. The reduction in shear strength did not correlate with the water contact angle on the coatings but was usually found to be due either to air entrapment at the ice/coating interface or to cohesive failure of the coating itself.

The ice separated from the various substrates was examined microscopically by forming plastic replicas of the ice surface. These studies helped determine the mechanism of failure and, since one of the replicating solutions was also an ice etchant, much was learned about the crystal structure and the crystal defects of ice near surfaces. The ice lattice was found to be highly defective near the substrates and this is discussed in connection with the “liquid-like,” behavior of interfacial ice.     

Ice Adhesion to Hydrophilic and Hydrophobic Surfaces

The adhesional shear strength has been determined for ice formed against polished steel, monolayers adsorbed on steel and thin plastic coatings painted on metal surfaces. The adsorbed monolayers reduced the shear strength to about one-third of that for ice on clean steel. The monolayers also had the effect of changing the character of the breaks from clearly cohesional to apparently adhesional failure. The shear strength from the plastic coatings ranged from values equal to that of ice against clean steel to values 70 to 80% lower. The reduction in shear strength did not correlate with the water contact angle on the coatings but was usually found to be due either to air entrapment at the ice/coating interface or to cohesive failure of the coating itself.

The ice separated from the various substrates was examined microscopically by forming plastic replicas of the ice surface. These studies helped determine the mechanism of failure and, since one of the replicating solutions was also an ice etchant, much was learned about the crystal structure and the crystal defects of ice near surfaces. The ice lattice was found to be highly defective near the substrates and this is discussed in connection with the “liquid-like,” behavior of interfacial ice.     

Effect of adhesive characteristics on static strength of adhesive-bonded aluminum alloys

The use of adhesive is posed to increase dramatically for application to the next generation of vehicle structures as is the use of aluminum. In this study, the effect of adhesive characteristics on the strength of adhesive-bonded lap shear aluminum was investigated. It was found that the joint strength depended on not only the adhesive properties but the bond adhesion between the adhesive and adherend. For the given selected aluminum substrates, to ensure the cohesive failure mode and consistent joint strength it is necessary to select an adhesive which had a weaker than or comparable strength to the bond adhesion. To improve the failure mode from adhesive to cohesive, atmospheric pressure plasma surface treatment of X610-T4PD and X626-T4P aluminum was performed and results showed that it improved not only the joint strength but degree of cohesive failure mode.     

Reversible Bonding of Aromatic Thermosetting Copolyesters for In‐Space Assembly

Reversible bonding is an attractive option for assembly and disassembly of reconfigurable space structures due to the simplicity of the fastening concept. Interchain transesterifications reaction [ITR—a type of dynamic covalent exchange reactions afforded by aromatic thermosetting copolyesters (ATSP)] between two ATSP coatings can successfully be used as a reversible bonding concept, provided that the mode of debonding is completely cohesive (rather than adhesive or delaminatory from metal substrate). An optimization study is carried out on the ITR bonding for which ATSP coating is applied on 7075 aluminum substrates and bond/debond experiments are carried out using a custom‐built tool kit. The toolkit enables precise control over bonding pressure, temperature, and contact time. Bonding conditions are optimized to produce complete cohesive failure with maximized bonding strength. Optimized bonding parameters are successfully implemented to realize 50 cycles of bond/debond process without compromising adhesive strength. Experiments show a debonding strength of 28.7 MPa for the 51st cycle at room temperature—significantly in excess of prior highest reversible bonding strength results found in the literature. These results, in addition to the high thermal stability and glass transition temperature of the base polymer, indicate viability of this reversible bonding concept for in‐space assembly.     

Film Structure and Adhesion

Paint films although attached to a substrate on one side only may be subjected to stresses, comparable to those in structural adhesives. These stresses result from shrinkage during film formation and subsequent ageing, mechanical strains, relative thermal movements of film and substrate and from osmotic pressure due to soluble material under or within the film. The adhesive strength required to prevent detachment varies from very little for weak, highly porous coatings to 10,000 lb/in² for tough coatings of high elastic modulus. Generally, adhesive strength both to the substrate and between coats in a paint system must exceed cohesive strength, under the conditions when failure is likely to develop. Dispersion and other forces, such as hydrogen bridging, between coatings and clean metal substrates should suffice to ensure adhesion but most practical surfaces carry contaminants, which interfere with wetting and intimacy of contact. Solvents and other low molecular weight components may also provide a weak interfacial layer, at least for a period after application. Modification of polymer structure to improve contaminant displacement and to increase polymer/substrate interaction forces, for example by the introduction of polar substituent or end groups will be discussed and potentialities of adhesion-promoting surface treatments reviewed.     

Film Structure and Adhesion

Paint films although attached to a substrate on one side only may be subjected to stresses, comparable to those in structural adhesives. These stresses result from shrinkage during film formation and subsequent ageing, mechanical strains, relative thermal movements of film and substrate and from osmotic pressure due to soluble material under or within the film. The adhesive strength required to prevent detachment varies from very little for weak, highly porous coatings to 10,000 lb/in² for tough coatings of high elastic modulus. Generally, adhesive strength both to the substrate and between coats in a paint system must exceed cohesive strength, under the conditions when failure is likely to develop. Dispersion and other forces, such as hydrogen bridging, between coatings and clean metal substrates should suffice to ensure adhesion but most practical surfaces carry contaminants, which interfere with wetting and intimacy of contact. Solvents and other low molecular weight components may also provide a weak interfacial layer, at least for a period after application. Modification of polymer structure to improve contaminant displacement and to increase polymer/substrate interaction forces, for example by the introduction of polar substituent or end groups will be discussed and potentialities of adhesion-promoting surface treatments reviewed.     

Effect of surface modifications of leather on its joint strength with polyvinyl chloride

The treatment of bovine leathers with wetting and lyotropic agents followed by heating produced a strengthening of the leathers which increased their joint strength properties to polyvinyl chloride (PVC). A cohesive failure of leather was always obtained. The highest cohesive strength (or point peel strength) was obtained when the treatment was carried out at 140°C with the surfactant NFOE (8.5) (nonylphenol polyoxyethylene with 8.5 mol of oxyethylene). The lyotropic agents (CaCl₂, urea) gave very high values (a five-fold increase), whereas the water-dimethyl ketone blends and pure water resulted in a smaller improvement in cohesive strength (a three-fold increase). The improved cohesive strength of leather was mainly due to the destruction of the ordered structure of collagen fibres and to the creation of a complex entanglement network among the collagen fibres. The treatments applied to a bovine leather produced a shrinkage of 65%; the degree of shrinkage was not a function of the kind of treatment, but of the structure of the leather. The application of surface treatments to leather prior to its bonding to other substrates may mean that the roughening process of the leather, a tedious and difficult operation which is necessary in order to obtain adequate adhesive joints, can be avoided.     

Study on effects of pre-treatment and surface roughness on tensile-shear strength of 2060 Al–Li alloy adhesive joints

The forces between adhesive and adherend mainly influenced by the pre-treatment technology of the substrates have important effects on the bonding strength. In this paper, the influence of different pre-treatment processes and surface roughness on the tensile-shear strength of 2060 Al–Li alloy adhesive joints as well as related mechanism was investigated. In this perspective, substrates were processed by mechanical abrasion at different levels and by phosphoric acid anodizing, which resulted in different surface topographies that were characterized by means of roughness measurements. Single-lap joints were prepared using a two-component epoxy adhesive. The tensile-shear strength of joints was measured via destructive testing and the failure modes were analyzed to evaluate the quality of bonding. Results showed that with the increase of surface roughness of Al–Li alloy, the tensile-shear strength of the adhesive joints increased and the failure modes changed from interfacial failure to cohesive failure. The groove structures formed during mechanical abrading were regarded as being responsible for this strengthening behavior. Moreover, a rough porous membrane was produced on adherents’ surface by phosphoric acid anodizing, causing a consolidation of adhesion at the adhesive-substrate interface.     

Consideration of Energy Dissipation for the Strength of Adhesive Joints

For some adhesive joints where the main difference is the degree of contact at the interface, failure occurs not at the interface, but some distance away in the polymer itself. This cohesive mode of failure in the polymer was always found to be the case in our studies of cupric oxide to branched polyethylene interfaces, even where the joint was so weak that the peeled surface seemed clean of the polymer to the naked eye. It was observed that the strength of the joint was associated with the coarseness of the texture of the peeled surface of the polymer. With a differential scanning calorimetry technique we have shown that the coarseness of the surface texture and therefore the strength of the joint, is a direct function of the amount of polymer involved in plastic deformation. The strength criteria for the adhesive joint of this kind is thus the energy of deformation and not the maximum tensile stress that the material can withstand.     

On the evaluation of adhesion of coatings by automatic scratch testing

Automatic scratch testing is an expedient technique for comparatively evaluating the cohesive failure load and adhesion failure load of thin coatings on various substrates. In combination with SEM examination of the scratch track, this technique has been used herein to detect and evaluate various effects on coating strength and adhesion. For soft Triballoy T-800 and Stellite SF-6 cobalt-base coatings on 4340 low alloy steel, adhesion was found to be strong and failure was found to be cohesive in the coating. In the presence of a plated chromium interlayer, pre-existing cracks lowered substantially the cohesive failure load, which was also lowered by an increase in the coating deposition pressure. The spacing of transverse cracks within the coating was found in all cases to decrease with increasing applied normal load. In soft aluminum coatings on depleted uranium (DU)-0.75% Ti alloy specimens, alloying aluminum with magnesium or zinc enhanced the coating strength and adhesion. In (Al-Mg) coatings on this substrate, a smoother surface led to a lower friction coefficient and a higher adhesion failure load. In hard, thin TiN coatings on 17-4 PH steel, a lower bias voltage applied to the substrate yielded higher cohesive and adhesion failure loads. In hydrogenated amorphous SiC thin coatings on 4340 steel, loss of hydrogen by annealing converted the residual compressive stresses into tensile stresses and lowered both the cohesive and the adhesion failure loads. Finally, automatic scratch testing proved helpful in determining delamination loads in multilayer TiN/Ti/TiN coatings on DU-0.75% Ti alloy.     

Consideration of Energy Dissipation for the Strength of Adhesive Joints

For some adhesive joints where the main difference is the degree of contact at the interface, failure occurs not at the interface, but some distance away in the polymer itself. This cohesive mode of failure in the polymer was always found to be the case in our studies of cupric oxide to branched polyethylene interfaces, even where the joint was so weak that the peeled surface seemed clean of the polymer to the naked eye. It was observed that the strength of the joint was associated with the coarseness of the texture of the peeled surface of the polymer. With a differential scanning calorimetry technique we have shown that the coarseness of the surface texture and therefore the strength of the joint, is a direct function of the amount of polymer involved in plastic deformation. The strength criteria for the adhesive joint of this kind is thus the energy of deformation and not the maximum tensile stress that the material can withstand.     

Strength and bonding characteristics of adhesive joints with surface-treated titanium-alloy substrates

This paper presents a study on the effect of surface treatments on the mechanical behavior of adhesively bonded titanium alloy joints. Several different treatments were selected for the preparation of Ti-6Al-4V alloy faying surfaces, and bonded joints were fabricated using surface-treated titanium alloy substrates and a film adhesive. Tensile tests were performed on single-lap specimens to evaluate the joint strength and to assess the failure mode, i.e. cohesive failure, adhesive (interfacial) failure or a mix of both. Contact angle measurements were also carried out, and the surface free energies of titanium alloys and the thermodynamic works of adhesion for the adhesive/titanium alloy interfaces were obtained. A three-dimensional finite element analysis was used to predict the strength of the specimens exhibiting cohesive failure. In addition, an expression of the relationship between the joint strength corresponding to interfacial failure and the thermodynamic work of adhesion was introduced based on the cohesive zone model (CZM) concept. It is shown that two surface treatments, Itro treatment and Laseridge, lead to cohesive failure and a significant increase in the joint strength, and the numerically predicted strength values are fairly close to the experimental values. These surface treatments are possible replacements for the traditional surface treatment processes. For degreasing, emery paper abrasion, atmospheric plasma treatment, sulfuric acid anodizing, nano adhesion technology and high-power lasershot, the specimens fail at the adhesive/substrate interface and the joint strength increases linearly with the thermodynamic work of adhesion as expected from our CZM-based expression.     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90-100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90–100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

Effect of work of adhesion on contact of an elastica with a flat surface

The JKR technique has enjoyed widespread usage in recent years for measuring the work of adhesion between an elastomeric cap and substrates of interest. Although some success has been seen in coating the cap with other materials, the requirement that one of the contacting members be elastomeric remains an important feature of the test. The use of a soft structure instead of a soft material for the flexible contact element is proposed here. A flexible strip is bent and then pushed onto a rigid horizontal surface. First a JKR type of analysis is carried out, in which the surface energy is assumed to be proportional to the contact area, and the effects of adhesion are represented by a bending couple at the two separation points of the strip with the surface. For a given work of adhesion, the magnitude of the unknown couple is varied until the total energy is minimized. Then a DMT type of model is considered, with forces acting in the cohesive zone outside of each separation point. The forces are either constant or are linear functions of the gap. For both analyses, the effect of adhesion on the contact length, displacements, and forces is investigated. The results can be applied to determine the work of adhesion, based on measurements taken from such a contact problem.     

Dismantlable adhesion properties of reactive acrylic copolymers resulting from cross-linking and gas evolution

ABSTRACT

The acrylic copolymers involving 2-hydroxyethyl acrylate (HEA) and tert-butyl acrylate (tBA) units as reactive units behave as pressure-sensitive adhesive type dismantlable adhesive materials. In order to clarify the individual role of HEA and tBA units on dismantlability, the 180° peel behavior after the dismantling treatment, i.e., heating in the presence of given amount of acid catalysts, was systematically investigated using the acrylic copolymers involving different amounts of the reactive units. It was revealed that transesterification of HEA units resulted in an increase in the cohesive force and modulus due to an increase in the molecular weight and cross-linking. Deprotection of tBA units, i.e., transformation of tBA to acrylic acid (AA) unit with isobutene evolution, promoted cross-linking by the esterification of AA units and tended to reduce a cohesive force by forming voids in the adhesive layer due to the evolution of isobutene gas. Interfacial failure in the peel tests corresponded with a high degree of cross-linking and increased modulus of the adhesive. Conversely, cohesive failure was associated with reduced cohesive strength of the adhesive layer and a low peel strength.     

Adhesion and interfacial healing between supramolecular hybrid networks and glass substrates

Adhesion towards glass and interfacial healing of partially supramolecular hybrid polymer networks featuring a range of H-bonds content were investigated through two dedicated adhesion test methods. In a first series of tests, adhesion strength was measured by separating two substrates containing a cured inner resin layer, and shown to decrease with increasing H-bonds content in the polymer network (from 0 to 50%) as the mechanical strength of the polymer also decreased while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with glass substrates. In a second step, the test was used to evaluate interface restoration through healing of the polymer matrices and results showed an increased from none to a tensile strength recovery up to 70% after 1 h healing time for the 50% H-bond polymer. Then, self-adhesion of freshly cut polymer surfaces to glass substrates was investigated, showing increasing tack with increasing H-bonds content. The influence of glass surface treatments on adhesion and interfacial recovery properties was also explored: while aminosilanes did not influence the interfacial behavior of partially supramolecular self-healing polymers towards glass, trimethoxy (octadecyl)silane (ODS) modification strongly hindered their adhesion abilities, further highlighting the fundamental role of hydrogen bonds interaction with the substrates.     

Modelling Environmental Degradation in EA9321-Bonded Joints using a Progressive Damage Failure Model

A progressive cohesive failure model has been proposed to predict the residual strength of adhesively bonded joints using a moisture-dependent critical equivalent plastic strain for the adhesive. Joints bonded with a ductile adhesive (EA9321) were studied for a range of environmental degradations. A single, moisture-dependent failure parameter, the critical strain, was calibrated using an aged, mixed-mode flexure (MMF) test. The mesh dependence of this parameter was also investigated. The parameter was then used without further modification to model failure in aluminum and composite single-lap joints (SLJ) bonded with the same adhesive. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. The elastic-plastic response of the adhesive and the substrates, both obtained from the bulk tensile tests, were incorporated. Both two-dimensional and three-dimensional modelling was undertaken and the results compared. The predicted joint residual strengths agreed well with the corresponding experimental data, and the damage propagation pattern in the adhesive was also predicted correctly. This cohesive failure model provides a simple but reliable method to model environmental degradation in ductile adhesive bonded joints, where failure is predominantly within the adhesive layer.