Wetting Kinetics and the Strength of Adhesive Joints

The work of adhesion, depending solely on the contact angle and surface tension, is unreliable as a guide to the strength of a joint. The kinetics of joint formation, exemplified by the rate of wetting, is also important and can be measured by rate of change of contact angle, a process for which an empirical equation has previously been suggested. A proposed mechanism introduces a viscosity term into the differential form of this equation.

A series of metal lacquers was prepared from epoxy, U/F and M/F resins in various ratios and also epoxy/P/F and epoxy/acrylic mixtures. Stainless steel substrates, coated with these lacquers were then cemented together using polyethylene as an adhesive. It is shown that joint strength correlates better with a high wetting constant (γ/ηL) than with a low contact angle except where this is very low.     

Wetting Kinetics and the Strength of Adhesive Joints

The work of adhesion, depending solely on the contact angle and surface tension, is unreliable as a guide to the strength of a joint. The kinetics of joint formation, exemplified by the rate of wetting, is also important and can be measured by rate of change of contact angle, a process for which an empirical equation has previously been suggested. A proposed mechanism introduces a viscosity term into the differential form of this equation.

A series of metal lacquers was prepared from epoxy, U/F and M/F resins in various ratios and also epoxy/P/F and epoxy/acrylic mixtures. Stainless steel substrates, coated with these lacquers were then cemented together using polyethylene as an adhesive. It is shown that joint strength correlates better with a high wetting constant (γ/ηL) than with a low contact angle except where this is very low.     

Effects of wood-surface roughness,adhesive viscosity and processing pressure on adhesion strength of protein adhesive

The objective of this research was to study the effects of wood-surface roughness, adhesive viscosity and processing pressure on adhesion strength between soybean protein adhesive and wood, and to seek the relative importance of the individual factors in determining adhesion strength. Processing pressure was found to be the most important factor in determining adhesion strength. An optimum pressure, which was about 4.55 MPa in this research, is needed for development of a strong bond. A higher pressure resulted in reduced adhesion strength, possibly due to damage to the wood surface; a lower pressure also resulted in decreased adhesion strength because of the lack of bond formation. Adhesive viscosity had greater effect on adhesion strength than surface roughness. Contact angle, which was found to be mainly determined by adhesive viscosity and surface roughness, was a major factor controlling adhesive penetration. A smaller contact angle, resulting from lower viscosity and rougher surface, produced deeper penetration, while a larger contact angle, resulting from higher viscosity and smoother surface, produced shallower penetration. An optimum penetration is needed to enhance adhesion strength by developing a three-dimensional interactive zone at the interface. Too deep or too much penetration would result in 'dry-out' at the interface; less penetration would limit the formation of the three-dimensional zone at the interface. Both cases resulted in reduced adhesion strength. Contact angles ranging from 35 to 47° provided the optimum penetration needed for good adhesion. The results of this research could be used as reference to determine optimum process parameters in plywood manufacturing when an aqueous based adhesive is used.     

Effect of Surface Roughness on the Adhesive Strength of the Heat-Resistant Adhesive RTV88

Heat-resistant adhesive RTV88 is a hyper-elastic material and so far there have been little research on using RTV88 in adhesive joints. In this study, the effect of surface roughness on the adhesive strength of RTV88 was examined. Aluminum adherends were first sandblasted in order to generate rough surfaces, and then tensile–shear tests on Al/RTV88 single lap joints were performed. The shear strength was shown to be influenced by surface roughness. Peel failure was dominant when the surface roughness was at a low level. However, cohesive failure was the major type of failure when the surface roughness was at a high level. Effective area, peel failure area, and cohesive failure area were introduced to explain the effects of surface roughness on the adhesive strength. An empirical relation for the failure force was proposed, based on these parameters. Tensile tests of the RTV88 bonding was performed in order to obtain the necessary data. Finally, the empirical relation for the failure force was verified by tensile–shear test results.     

Effect of surface pre-treatment on surface characteristics and adhesive bond strength of aluminium alloy

In this work aluminium alloy surfaces have been subjected to three different methods of surface pre-treatments such as solvent degreasing, FPL (Forest Products Laboratory) etching and priming using an epoxy based primer. The treated surfaces were evaluated for surface energy, contact angle, surface topography, surface roughness and adhesive strength characteristics. The influence of surface pre-treatments on the variation of polar, dispersive and total surface energy of the surfaces is addressed. A wettability test was performed on the surfaces using an epoxy adhesive in order to assess the influence of the pre-treatment techniques on substrate/adhesive interaction. Theoretical work of adhesion values for the various pre-treated surfaces were calculated using the contact angle data and further tested experimentally by adhesive bond strength evaluation by tensile testing of a single lap aluminium-epoxy-aluminium assembly. The method of surface pre-treatment showed a profound effect on the surface topography and roughness by AFM. This study reveals that a combination of high surface energy and high surface roughness of the substrate along with good wettability of the adhesive contributed to the highest joint strength for the aluminium alloy through the FPL etching pre-treatment.     

Peel Strength of Aluminium-Aluminium Joints Bonded by Adhesive Based on Carboxylated Nitrile Rubber-Chlorobutyl Rubber Blend

The peel strength of aluminium-aluminium joints bonded by an adhesive based on carboxylated nitrile rubber and chlorobutyl rubber was found to depend on surface topography and use of a silane primer. Anodization causes a marginal increase in bond strength while the silane primer improves the adhesive joint strength remarkably.

The peel strength was also found to be dependent on test conditions (test rate and temperature). The threshold peel strength value obtained by measurements at low peel rate and high test temperature was found to depend on the type of failure during peeling (cohesive or interfacial) which, in turn, is controlled by the presence of silica filler in the adhesive. Two different threshold values of peel strength were obtained: 60 N/m for interfacial failure (in silica-filled adhesive), 140 N/m for cohesive failure (in unfilled adhesive).     

聚烯烃的粘接及其粘合机理的探讨

探讨了热熔胶中EVA以及填料对聚丙烯粘接强度的影响;研究了控制粘接强度的润湿因子,并推断了在两相界面之间是分子扩散在起主要作用。     

Geometric Factors Impacting Adhesive Lap Joint Strength and Design

Too often adhesive thickness, adherend thickness and other geometric factors are not explicitly considered in adhesive joint design. This study includes experimental and computational research exploring the means of enhancing the engineering design process for adhesive lap joints to include such effects. It clearly demon-strated that both the cleavage stresses and the shear stresses, near the bond termini, play important roles in lap 'shear' joint failure. Finite Element and Fracture Mechanics analyses were used to examine the energy release rate applied to growth of cracks in adhesive lap joints. Lap joints with similar geometries to those analyzed were designed, fabricated and tested. In a separate set of experiments the bond termini were constrained in the direction normal to the uniaxial loading. If the strength of lap shear joints is dominated by the adhesive shear strength, then constraining the lateral motion of the bond termini should have little or no effect on the overall shear strength of the adhesive joint. This work clearly demonstrates that this is not the case. If cleavage stresses are important in lap joints then constraining the bond termini, in a direction normal to the bond area, should have a commensurate effect on the overall strength of the lap joint. None of the ASTM standardized 'lap shear tests' provide any insight into this premise. This paper also presents analyses and experimental results for lap joints to which several methods of lateral constraint were applied near the bond termini. The analytical and numerical methods described and used for explaining and predicting such effects might be a useful adhesive joint design tool.     

双组分黏合剂对聚脲层间黏结强度影响因素的研究

探讨了基材表面粗糙度、层间黏合剂养护龄期、环境温度和湿度对聚脲层间黏结强度的影响规律.实验结果表明：打磨处理后聚脲层间的黏结强度普遍提高;层间黏合剂养护龄期在2～4 h之间,黏结强度较高,在4.2～4.8 N/mmm范围内;随着环境温度的升高,黏结强度增大,高温下达到4.8 N/mm;环境湿度小时可获得较高的黏结强度.     

An Experimental Study of Fatigue Strength for Adhesively Bonded Tubular Single Lap Joints

In this paper, manufacturing technology of the tubular single lap adhesive joint was studied to obtain reliable and optimal joint quality. In addition, a surface preparation method and a bonding process for the joint were devised. The effect of the adhesive thickness and the adherend roughness on the fatigue strength of the joint was experimentally investigated. From experiments, it has been found that the fatigue strength of the joint increased as the adhesive thickness decreased and the optimal arithmetic surface roughness of the adherends was about 2 μm.     

Air entrainment in dip coating under reduced air pressures

This study examines experimentally and for the first time the effect of reduced air pressure on dynamic wetting. The purpose is to assess the role of air viscosity on dynamic wetting failure which hitherto has been speculated on but not measured. In this paper we used dip coating as the model experimental flow and report data on air entrainment velocity we measured with a series of silicone oils in a range of viscosities in a vacuum chamber where the pressure can be reduced from atmospheric down to a few mbar when the mean molecular free path of air is large and air ceases to have a viscosity. To complement earlier work, we carried out the experiments with a range of substrates of varying roughness. The substrates were chosen so that for each one, their two sides differ in roughness. This enables simultaneous comparative observation of their wetting performance and reduces the experimental error in assessing the role of roughness. The data presented here capture the effects of viscosity, roughness and air pressure but the important result of this study is that the air entrainment velocity can be increased considerably (exponentially) when the pressure is reduced with the suggestion that it approaches infinity as pressure approaches zero. In other words, the role of the surrounding air viscosity is important in dynamic wetting. The data from this study have significant implication to the fundamental understanding of dynamic wetting. Indeed they form the missing data link to fully understand this phenomenon. The data presented in this work also confirm the complex role of roughness, in that it can increase or decrease the air entrainment speed depending on the value on the viscosity of the coating solution. The results presented in this paper are very useful in practice as they imply that if one chooses carefully roughness one can coat viscous formulation at unexpectedly very high speeds with a moderate low pressure (50 mbar typically).     

Adhesive Bonding of Clean and Oil-Contaminated Electrogalvanized Steel Substrates

The performance of two-part, amidoamine-cured epoxy adhesives on clean and oil-contaminated electrogalvanized steel (EGS) was studied using screening and lap shear tests. On exposure to boiling water, the cured epoxy adhesives with amidoamines having higher amine value delaminated from the clean and oil-contaminated EGS surfaces before those cured with amidoamines having low amine value. The results of X-ray photoelectron spectroscopy (XPS) showed that the adhesives cured with amidoamines having high amine value were unable to displace the oil from the EGS substrate. However, the durability and the strength of the adhesive bonds on the oiled EGS could be improved by adding proper amounts of silane or wetting agent to the adhesive. The preferential adsorption of amino curing agents occurred on the clean EGS surface, confirmed by XPS and reflection absorption infrared spectroscopy, and this decreased the durability of the bonds in boiling water. In addition, from XPS analyses of various specimens, different amounts of cured resins were detected in the adhesive/EGS interfacial regions which affecting the durability of the adhesive bonds. In addition, the amidoamine curing agents may form complexes on the EGS surface.     

An Ultrasonic Evaluation and Quality Control Tool for Adhesive Bonds

The problem of predicting adhesive bond performance for both surface preparation and undercure defects has been studied using an ultrasonic, experimental test bed system. This experimental test bed incorporates the ultrasonic and computer equipment necessary to acquire and process data from various types of adhesively bonded test specimens. The computer hardware and software has been developed to allow the design of reliable pattern recognition algorithms for the evaluation of surface preparation and bond cure. The specific problem studied is the inspection of the adhesive bond in an aluminum to aluminum step-lap joint whose strength could be affected by improper surface preparation or undercure. A set of 154 bond specimens was used to design an algorithm that is 91% reliable for separating the specimens into a good class, those bonds with no defects, or a weak class, bonds with poor surface preparation or undercured adhesive layer. A Fisher Linear Discriminant function was selected by the test bed as the best pattern recognition routine for this classification problem.     

Air entrapment in sheet molding compounds (SMC)

Entrapped air can commonly lead to large delaminations in thick walled sheet molding compound (SMC) products. In this work different sources of air entrapment in SMC are investigated. The critical process is shown to be the impregnation of the fibers. If no surface active additives are used, large volumes of air may be entrapped in this process, unless the viscosity of the compound is very low. In this situation of poor wetting, the viscosity of the compound during fiber impregnation will critically determine the interlaminar, tensile strength of the product. However, if surface active additives are used, the air escapes entrapment even at relatively high viscosities. The lowering of the viscosity, which is a side effect of the additives, has practically no importance under these conditions of good wetting. Large numbers of small air bubbles are also entrapped during the mixing of the components, but it is shown that these bubbles have very little effect on the mechanical properties of the finished part.     

Effect of autoclave cure time and bonded surface roughness on the static and fatigue performance of polyurethane film Adhesive Single lap joints

This study investigates the effect of autoclave cure time and bonded surface roughness on the static and fatigue performance of film-adhesive single lap joints. Joint static performance is assessed in terms of its load transfer capacity in a quasi-static tensile-shear test to failure. Effect on fatigue life under a mean and cyclic tensile-shear amplitude is also investigated. Two levels autoclave cure (soak) time and two levels of bond surface roughness are investigated. All other autoclaving process variables are kept constant; namely, the ramp rate of temperature rise/cooling, pressurization/depressurization, as well as the cure temperature and cure pressure levels. Test joints are made of aluminium-aluminium or aluminium-magnesium adherends, joined with a polyurethane film adhesive. The results suggest that: an increase of the surface roughness is beneficial to static strength and detrimental to fatigue strength; an increase of the autoclave soak (cure) time is beneficial both to static and fatigue strength. Test data, failure mode analysis, discussion, observations and conclusions are provided.     

Some Aspects of Bond Formation and Failure in Adhesive Wood Joints

An investigation has been made of the effect of varying glue-spread on the bond strength of holly (Ilex aquifolium) using three adhesives and three different wood sections. The glue-spreads are lower than those normally used, and it has been found with edge-grain joints that 100% cohesive wood failure can occur with a glue-spread as low as 2.7mg/cm². Scanning electron microscopy shows that interlocking between adhesive and adherend does not occur. Factors leading to delamination and joint failure are discussed.

Lignin, without further addition, has been shown to be a useful wood adhesive. It has also been shown that it is possible to make end-grain joints without the use of an adhesive; the lignin present in the wood specimens is considered to be responsible for such joints.     

A study on the lap shear strength of a co-cured single lap joint

In this paper, the lap shear strength of a co-cured single lap joint subjected to a tensile load was investigated by experimental analysis. Co-cured joint specimens with several different bonding parameters such as bond length, surface roughness, and stacking sequence of the composite laminate were fabricated and tested. The dependence of the lap shear strength of the co-cured joint on the bonding parameters was investigated from the experimental results. The failure mechanism of the co-cured single lap joint was partially cohesive failure. The lap shear strength of the co-cured single lap joint was significantly affected by the bond length and the stacking sequence of the composite laminate. However, the effect of surface roughness on the lap shear strength of the co-cured single lap joint was not so significant.     

Some Aspects of Bond Formation and Failure in Adhesive Wood Joints

An investigation has been made of the effect of varying glue-spread on the bond strength of holly (Ilex aquifolium) using three adhesives and three different wood sections. The glue-spreads are lower than those normally used, and it has been found with edge-grain joints that 100% cohesive wood failure can occur with a glue-spread as low as 2.7mg/cm². Scanning electron microscopy shows that interlocking between adhesive and adherend does not occur. Factors leading to delamination and joint failure are discussed.

Lignin, without further addition, has been shown to be a useful wood adhesive. It has also been shown that it is possible to make end-grain joints without the use of an adhesive; the lignin present in the wood specimens is considered to be responsible for such joints.     

Studies on Effective Bond Strength of Kendu Fruit Adhesive

Certain investigations on the effective bond strength of gum adhesive extracted from kendu (Diospyros cordifolia Roxb) fruit have been studied with respect to different interfaces such as general-purpose wood-wood, teakwood-teakwood, plywood-plywood, glass-glass, PMMA-wood, PVC-PVC. LDPE-LDPE, HDPE-HDPE, and PMMA-PMMA. The effects of different parameters such as curing temperature, curing time, thickness of specimens, and presolvent treatments on the bond strength of kendu fruit adhesive (KFA) are investigated. The bond strengths of KFA under different hazardous environments such as boiling water, saline water, and kerosene oil, acidic as well as alkaline solutions for the teakwood-teakwood interface have also been examined. A comparative account of studies of the effective bond strength of KFA with three different commercial synthetic adhesives such as Varnicol, Quickfx, and Araldite is reported. The KFA shows a superior bond strength for general-purpose wood-wood adhesion. Its bond strength is also comparable with synthetic adhesives for certain plastic-plastic as well as teakwood-teakwood adhesions. The retention of effective bond strength of KFA in various hazardous atmosphere remarkably encourages an industrial-grade adhesive formulation through proper R&D investigations. Adequate modifications and informations are required for an improved adhesive formulation from this natural adhesive to meet the industrial standard.