Influence of Acrylic Adhesive Viscosity and Surface Roughness on the Properties of Adhesive Joint

In adhesion, the wetting process depends on three fundamental factors: the surface topography of the adherend, the viscosity of the adhesive, and the surface energy of both. The aim of this paper is to study the influence of viscosity and surface roughness on the wetting and their effect on the bond strength. For this purpose, an acrylic adhesive with different viscosities was synthesized and some properties, such as viscosity and surface tension, were studied before adhesive curing took place. Furthermore, the contact angle and the lap-shear strength were analyzed using aluminum adherends with two different roughnesses. Scanning electron microscopy was used to determine the effect of the viscosity and the roughness on the joint interface. The results showed that the adhesive exhibits an optimal value of viscosity. Below this value, at low viscosities, the low neoprene content produces poor bond strength due to the reduced toughness of the adhesive. Additionally, it also produces a high shrinkage during curing, which leads to the apparition of residual stresses that weakens the interfacial strength. However, once the optimum value, an increase in the viscosity produces a negative effect on the joint strength as a result of an important decrease in the wettability.     

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.     

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.     

Atmospheric plasma modification of polyimide sheet for joining to titanium with high temperature adhesive

This investigation highlights the rationale of adhesive bonding of atmospheric pressure plasma treated high temperature resistant polymeric sheet such as polyimide sheet (Meldin 7001), with titanium sheets. The surface of polyimide (PI) sheet was treated with atmospheric pressure plasma for different exposure times. The surface energy was found to increase with increase in exposure time. However, longer exposure time of plasma, results in deterioration of the surface layer of PI resulting in degradation and embrittlement.Contact angle measurements with sessile drop technique were carried out for estimation of surface energy. SEM (EDS) and AFM analyses of treated and untreated specimens were carried out to examine the surface characteristics and understanding morphological changes following surface treatment. Untreated samples and atmospheric pressure plasma treated samples of polyimide Meldin 7001 sheet were bonded together as well as with titanium substrates to form overlap joints. Single lap shear tensile testing of these adhesively bonded joints was performed to measure bond strength and to investigate the effect of surface treatment on adhesive bond strength. An optimized plasma treatment time results in maximum adhesive bond strength and consequently, this technology is highly acceptable for aviation and space applications.     

Improvement of adhesion and long‐term adhesive reliability of liquid crystalline polyester film by plasma treatment

Surface modification of thermotropic liquid crystalline aromatic polyester (LCP) films was examined by low‐pressure plasma treatment to improve initial adhesion and long‐term adhesive reliability between the LCP film and an epoxy bonding sheet for printed circuit boards. Plasma irradiation was carried out in various plasma gases with different plasma modes as reactive‐ion‐etching (RIE), direct‐plasma (DP) with pressures ranging from 4.0 to 26.6 Pa. Although the initial adhesion of the DP‐treated film increased, the long‐term adhesive reliability estimated by pressure cooker test (PCT) decreased with decreasing the plasma gas pressure in every gas. The higher concentration of the generated polar groups such as phenolic hydroxyl group and carboxyl group enhanced the initial adhesion by the increase in the chemical interaction; however, it damaged the long‐term adhesive reliability due to the acceleration of the penetration of water molecules into the interface. The large surface roughness was effective to enhance the initial adhesion and the long‐term adhesive reliability. The RIE‐treatment generated the polar groups and the larger surface roughness than the DP‐treatment. The RIE‐treatment in the O₂ atmosphere at the gas pressure of 13.3 Pa was the best plasma condition for both the initial adhesion and the long‐term adhesive reliability. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

水基型金属—木复合胶粘剂研制与应用

研究了铝板、钢板、镀锌板等金属与木芯板、木层压板、高压板、刨花板等木材复合粘接用的水基型胶粘剂的制备工艺、粘接工艺,探讨了产品玻璃化温度、固体含量、粘度、酸度、乳化剂及缓蚀剂用量等对粘接强度的影响。     

Investigation of the relationship between statistical characteristics of substrate surface roughness and the strength of adhesive joints

The relationship between statistical characteristics of butadiene styrene rubber (BSR) surface roughness and shear strength of adhesive joints has been investigated. The assumption of stationary normal distribution of coordinates of surface points was made to determine the statistical characteristics of surface roughness. The profile length above the selected level l ₁ (u) was introduced as a new surface roughness parameter to characterize adhesive penetration depth. The validity of simulated l ₁ (u) value was verified experimentally. A good correlation between experimental and calculated results was found. A relationship between adhesive penetration depth and the bonding pressure during adhesive joint preparation was also obtained. The dependences among lap shear joint strength, bonding pressure and roughness characteristic l ₁ (u) were determined.     

Plasma Treatment of Wood–Plastic Composites to Enhance Their Adhesion Properties

In this study, the adhesion properties of adhesives and paints on wood–plastic composites (WPCs) after plasma treatment at atmospheric pressure and ambient air were investigated. Surface energy determination by means of contact angle measurements according to the Owens–Wendt approach and atomic force microscopy to detect changes in surface topography were carried out. An increase in the polar component of surface energy and an increase in surface roughness after plasma treatment were detected, indicating enhanced bond strength. To confirm these results, bond strength tests were conducted. By tensile bond strength tests, increased adhesion of waterborne, solventborne and oil-based paints on plasma treated surfaces was found. Furthermore, by shear bond strength tests, an increase in bond strength of plasma treated WPCs bonded with poly(vinyl acetate) and polyurethane adhesives was ascertained.     

Influence of steam-based pre-treatment using acidic chemistries on the adhesion performance of powder coated aluminium alloy AA6060

In this study, the adhesion of a commercially applied powder coating on a steam treated AA6060 surface with pure steam and steam with citric and phosphoric acid chemistries has been investigated. Contact angle, roughness, and nanoscale pull off forces were determined as a function of the steam treatment prior to application of the powder coating. A focussed ion beam technique was used to examine the cross section of the powder coating, interface adhesion, and fracture morphology after the boiling test and interface indentation method. Transmission electron microscopy was used to study the fracture after indentation. Regardless of the steam treatment method, the wettability of the AA6060 surface was increased after the steam treatment. Addition of citric and phosphoric acid resulted in a low degree of hydrophobicity for the oxide layer compared to the use of pure steam. Steam generated oxide with citric and phosphoric acid resulted in poor penetration of the adhesive over the intermetallic particles, and the interface showed mixed (cohesive/adhesive) fracture during interface indentation, while the pure steam treated surface showed dominant cohesive fracture.     

Atmospheric pressure plasma surface modification of titanium for high temperature adhesive bonding

In this investigation surface treatment of titanium is carried out by plasma ion implantation under atmospheric pressure plasma in order to increase the adhesive bond strength. Prior to the plasma treatment, titanium surfaces were mechanically treated by sand blasting. It is observed that the contact angle of de-ionized water decreases with the grit blast treatment time. Optical microscopy and scanning electron microscopic (SEM) analysis of untreated and atmospheric plasma treated titanium are carried out to examine the surface characteristics. A substantial improvement in the surface energy of titanium is observed after the atmospheric pressure plasma treatment. The surface energy increases with increasing exposure time of atmospheric pressure plasma. The optimized time of plasma treatment suggested in this investigation results in maximum adhesive bond strength of the titanium. Unmodified and surface modified titanium sheets by atmospheric pressure plasma were adhesively bonded by high temperature resistant polyimide adhesive. The glass transition temperature of this adhesive is 310 °C and these adhesively bonded joints were cured at high temperature. A substantial improvement in adhesive bond strength was observed after atmospheric pressure plasma treatment.     

硅烷偶联剂处理木塑复合材胶接接头的耐水性

利用硅烷偶联剂KH560对木粉/聚乙烯复合材料进行表面处理以改善其胶接性能。利用接触角、吸水量、表面形貌以及胶接强度测试等分析方法,研究了硅烷偶联剂处理聚乙烯木塑复合材料胶接接头在水环境中的胶接耐久性能。试验结果表明,机械打磨并偶联剂处理后,聚乙烯木塑复合材表面接触角增加,表面粗糙度增大,胶接强度和耐水性明显提高。偶联剂分子链上环氧基团的"架桥"作用以及甲氧基的憎水作用,是粘接强度和耐水性能提高的主要原因。浸水环境下,聚乙烯木塑复合材料表面粗糙度略有降低;随着浸水时间的延长,表面接触角下降,胶接接头的吸水量增加,胶接强度下降。水环境下聚乙烯木塑复合材料中木质纤维成分的吸水膨胀,是造成胶接强度下降的主要原因。     

双组分高强度环氧胶粘剂的研制

根据车间内钢梁上吊车轨道安装底板与钢梁粘接的具体要求,研制了一种中温固化双组分环氧胶粘剂。探讨了E-51、E-39D和纳米碳酸钙用量对甲组分粘度的影响,测试了不同促进剂的胶粘剂凝胶时间并研究了粘接表面处理、中温固化时间对胶粘剂剪切强度的影响。结果表明,通过选用不同粘度的环氧树脂并添加纳米碳酸钙,控制甲组分粘度在8～20Pa.s,选用促进剂M3份,表面制备并采用偶联剂处理后,100℃下固化2h后,该胶铝-铝、钢-钢剪切强度可达45MPa和51MPa,实现了胶粘体系中温高强度快速固化。室温放置20h后钢-钢剪切强度为5.8MPa,可以安装加热设备以便后固化。     

Soy flour dispersibility and performance as wood adhesive

Soy flour adhesives using polyamidoamine-epichlorohydrin (PAE) resin as the curing agent are being used commercially to make bonded wood products. The original studies on the soy-PAE adhesives used purified soy protein isolate, but the much lower cost soy flour is now used commercially. We examined the performance of commercially available soy flours that have their proteins either mainly in their native (90 protein dispersibility index (PDI)) or denatured (70 and 20 PDI) states. We expected that the more native state soy proteins with their better dispersibility would provide better adhesion to wood surfaces and enhanced reaction with PAE resin. Small-scale wood bonding tests showed that neither of these effects was observed without and with a low level of PAE. In these tests, the solids content of the soy formulations had a large influence on adhesive viscosity but little influence on bond strength. Additionally, little difference was observed in any of the adhesive or viscosity properties between the soy flours having either a 0.152 or 0.075?mm (100 or 200 mesh) particle size.     

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.     

An eco-friendly and high shear strength adhesive from catechol-modified polyethyleneimine

Conventional adhesives often emit volatile organic compounds (VOCs), which have a negative impact on human health. In this paper, an environmentally-friendly supramolecular adhesive PD which has high adhesive and low VOCs emission is prepared by the reaction between polyethyleneimine (PEI) and 3,4-dihydroxybenzaldehyde (DBA). PD containing abundant catechol groups exhibit excellent adhesion to wood substrates and is able to reach a maximum shear strength of 5.20 ± 0.39 MPa. One factor is attributed to multiple reactions between PEI, and DBA and oxidation of DBA. These reactions construct a complex three-dimensional cross-linked structure which is very helpful to improve the bonding performance of the adhesive. Another reason refers to the fact that a large number of catechol groups in PD can form a lot of hydrogen bonds with the amino group in PEI and the hydroxyl group in the wood substrate. These hydrogen bonds play an important role in enhancing shear strength. PD adhesives have a stronger bond strength than commercial chloroprene rubber (CR, Pattex-PXL) and polyvinyl acetate adhesives (PVAc, JUJU-8708) and have lower emissions of VOCs. As an environmentally-friendly adhesive for wood-based substrates, this adhesive may have potential applications in the wood processing industry.     

Development of a chitosan‐based adhesive. Application to wood bonding

The potential of chitosan (CS) as wood adhesive was investigated in this study. The measurement of the bond strength of different CS‐based formulations was carried out using double‐lap shear tests. The best formulation tested was an adhesive composed of 6% of CS, 1% of glycerol, and 5 mmol/L of trisodium citrate dehydrate. Best bond strength from this formulation was found to be equal to 6.0 MPa in dried conditions and 1.6 MPa in wet conditions (specimens immersed for 3 h at 30°C in water). The failure in the double‐lap joint tests mainly occurred in wood. Finally, the penetration of the rhodamine‐labeled CS at 4% in the pinewood matrix was also studied using microtome and microscopy techniques to show interactions between CS‐based adhesive and wood. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2013     

The effect of surface properties on the adhesion of modified polychloroprene used as adhesive

The adhesion properties of polychloroprene can be improved by addition of such materials as piperylene–styrene co-polymer (PSC), VeoVa-10 polymer, VeoVa-11/methyl methacrylate/2ethylhexyl acrylate co-polymer (VeoVa-11/MMA/2EHA) and poly(vinyl acetate) waste (wPVAc). Here, the relationship between adhesion properties and surface tension of polychloroprene was investigated. Contact angle measurements have been used to study the effects of nature and content of polymeric additives on the adhesion and surface properties of polychloroprene. Low-surface-tension VeoVa-10 polymer has the tendency to migrate to the surface of polychloroprene; thus, adhesion is determined mainly by this additive property. Enrichment of polychloroprene film bottom layer by the additive was observed using high-surface-tension PSC and wPVAc. In this case, the adhesion properties of polychloroprene depend on the interactions at the interface. Adhesion properties of polychloroprene were found to depend not only on compatibility between adhesive components, but also on compatibility between the adherend and the adhesive.     

Surface energy, surface topography and adhesion

In this paper are discussed some of the fundamental principles which are relevant to an understanding of the influence that interfacial roughness may have on adhesion. The surface energies of the adhesive, substrate and of the interface between them determine the extent of wetting or spreading at equilibrium. Numerical values for surface energies may be obtained either from contact angle measurements or from analysing force–displacement curves obtained from the surface forces apparatus. The extent to which the relationships, appropriate for plane surfaces, may be modified to take into account interfacial roughness are discussed. For modest extents of roughness, the application of a simple roughness factor may be satisfactory, but this is unrealistic for many of the practical surfaces of relevance to adhesive technology which are very rough, and is ultimately meaningless, if the surface is fractal in nature. Some examples are discussed of published work involving polymer–metal and polymer–polymer adhesion, where the roughness of the interface exerts a significant influence on the adhesion obtained. Roughness over a range of scales from microns to nanometres may strengthen an interface, increasing fracture energy by allowing bulk energy dissipating processes to be activated when the bond is stressed.     

Quantitative Analysis of Gross Adhesive Penetration in Wood Using Fluorescence Microscopy

An optimum amount of adhesive penetration is desirable for economy of production and development of bond strength in wood composites. A general method that allows quantitative measurement of gross adhesive penetration in wood is described. Staining techniques have been developed that can provide sharp contrast between a cured adhesive and the wood substrate using fluorescence microscopy. An image analysis system utilizes this contrast to quantify gross adhesive penetration in wood. An example of this technique is provided, whereby the effect of molecular weight distribution of phenol formaldehyde prepolymers on gross adhesive penetration into yellow poplar (Liriodendron tulipifera) flakes is observed and quantified. Adhesive penetration into wood flakes was shown to be correlated with the molecular weight distribution of the prepolymer, decreasing with higher weight average molecular weight. Gross adhesive penetration into hardwoods is likely to be dominated by flow into vessel elements, as demonstrated by the wood species studied here.