The effect of primers on adhesive properties and strength of adhesive joints made with polyurethane adhesives

The paper presents selected aspects of the effect of primers on adhesive properties and strength of aluminium sheet adhesive joints, made using polyurethane adhesives. The strength of adhesive joints was determined based on two cure time variants: 15 and 64 h. It was found that the longer cure time at a humidity of 33% is more desired, as it leads to a substantial increase in strength of the tested adhesive joints. In addition, two variants of surface preparation were applied: degreasing and degreasing followed by the application of a primer (a pro-adhesive agent). It was observed that the primer application prior to the application of an adhesive leads to a significant increase in strength compared to the variant where the adhesive application is preceded only by degreasing. Moreover, the aluminium sheet surface that was subjected to cataphoretic painting and priming exhibits better adhesive properties. It has a higher value of both surface free energy and its dispersion and polar components compared to the surface that was only subjected to degreasing.     

AFM adhesion force measurements on conversion-coated EN AW-6082-T6 aluminium

Atomic force microscopy (AFM) has been used to measure the adhesion force between functionalised AFM tips and smooth surfaces of an EN AW-6082-T6 aluminium alloy, both before and after application of different conversion coatings. In addition, the surface of a sapphire sample was studied as a model aluminium surface. The results obtained for the sapphire surface were highly reproducible, and were used as a mean to establish proper routines for the more complex industrial surfaces. The adhesion force between a chromate conversion-coated (CCC) EN AW-6082-T6 aluminium alloy and a COOH functionalised tip was significantly increased compared to the uncoated surface, probably as a result of strong hydrogen bonding. However, the adhesion force decreased with time during the first 24 h after treatment due to aging of the CCC. Chromate-free Ti–Zr-based treatment also increased the adhesion, but the adhesion force varied significantly due to non-uniform deposition and composition of the conversion coating. The measured AFM adhesion forces correlated qualitatively with macroscopic adhesion test results obtained previously for these specific conversion coatings. The AFM technique may thus provide useful information on the adhesion behaviour of heterogeneous conversion-coated aluminium surfaces.     

Surface Free Energy and 7075 Aluminium Bonded Joint Strength Following Degreasing Only and Without Any Prior Treatment

Abstract

Adhesion is a surface phenomenon occurring in many processes, e.g., bonding, painting or varnishing. Knowing the adhesion properties is critical for evaluating the usability or behaviour of materials during these processes. Good adhesion properties favour the processes of bonding, resulting in high strength of adhesive joints. Adhesive bonded joints are used in many industries, and the subject of this study was 7075 aluminium alloy sheet bonded joints as typically used in the aviation or construction industry. Surface free energy (SFE) can be used to determine the adhesion properties of the materials. The SFE of the tested sheets was determined with the Owens–Wendt method, which consists in determining the dispersion and polar components of SFE. The purpose of this work was to correlate the bonded joint strength of selected aluminium alloy sheets to the surface free energy of the sheets that had been subjected to degreasing only and no other prior treatment was used. Single-lap bonded joints of 7075 aluminium alloy sheets were tested. Higher joint strength was measured for the thinner sheets, while the lowest strength was measured for the thickest sheets. This suggests that the thickness of the joined parts is an important factor in the strength of bonded joints. The comparison of adhesion properties to the strength of adhesive joints of tested materials shows that there is no direct relation between good adhesion properties (i.e., high SFE) and joint strength. As for degreasing, the highest joint strength was observed for aluminium alloy sheets with the lowest SFE; the sheets which were not degreased gave the highest SFE and highest joint strength.     

The effect of water on the adhesion of organic coatings on aluminium

Water normally decreases the strength of adhesive joints. In the case of epoxy coatings on aluminium, however, after an initial decrease the adhesive strength increased with the time of exposure to water. It is suggested that this increase is caused by the hydration of aluminium oxide adjacent to the adhesive joint, thus enabling additional hydrogen bonding between the organic coating and its support.

Results obtained by measuring adhesion with peel and tape tests on aluminium foil and an alloy with different surface pretreatments and different curing conditions have been compared. It is shown that the tape test is useful for the semi-quantitative determination of the stability towards water of an adhesive joint.     

Improving the adhesion strength of polymers: effect of surface treatments

Abstract

To improve their adhesion strength, polymeric surfaces are usually modified through different treatments. This study investigates the effect of mechanical, chemical, and energetic treatments on the bonding strength of ethylene propylene diene methylene (EPDM), polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS) materials. Three adhesives based on different chemical compositions, namely silicone, polyurethane, and modified-silane (MS) polymer, were considered. Results show that the effect of the applied treatments on the adhesion strength of EPDM surfaces is insignificant. Only a slight improvement is obtained in the case of polyurethane-based adhesive, while the failure modes remained adhesive. As for PVC, most treatments were effective in the case of the silicone-based adhesive, especially grit blasting, primer, and UV/ozone treatments. Only UV/ozone treatment improved the adhesion strength and altered the failure mechanisms of this material when polyurethane and MS-based adhesives are used. The adhesion of ABS increased and the failure modes changed from adhesive to cohesive for most treatments. Particularly, a significant improvement is obtained when primer coating and UV/ozone radiation are applied. This comparative study paves the way for the design of polymeric joints with highly enhanced adhesion performance.     

Development of improved polypropylene adhesive bonding by abrasion and atmospheric plasma surface modifications

The present work deals with the problematic adhesive bonding of substrates with low surface energy. Different approaches have been explored with the aim of creating adequate adhesive joints based on polyolefinic substrate and polyurethane adhesive. The selected material under study was polypropylene (PP) as adherend, and a commercial Sikaflex^®-252 polyurethane one component based structural adhesive (PU) as joint fluid. Among the diverse pre-treatments typically used to prepare surfaces prior to bonding, mechanical abrasion with emery paper of 80 grain size, the use of a chemical primer and atmospheric pressure air plasma torch (APPT) were the selected methods to facilitate the application of the PU by means of surface energy enhancement as well as to create a correct mechanical interlocking of the adherent–adhesive interface. Changes in the wettability of the polymer were evaluated by contact angle measurements following the UNE EN 828:2010. Surface energy was calculated both in terms of Owens approximation and acid–base considerations, leading to the possibility of determining a relationship between changes in surface energy and adhesion. Changes in the chemical composition of the surface were studied by X-ray photoelectron spectroscopy (XPS), electron diffraction X-Ray (EDX) probe and attenuated total multiple reflection mode infrared spectroscopy (ATR-FTIR). Morphological modifications were investigated with scanning electron microscopy (SEM). Variations in the strength of single-lap PP–PP joints with the treatments were evaluated by lap shear tests following the UNE-EN 1465:2008 standard. Experimental evidence supports the superiority of the APPT treatment to increase wettability and adhesion of polyolefinic surfaces, especially when combined with the use of a primer.     

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.     

The Effect of Different Surface Pretreatment Methods on Nano-adhesive Application in High Strength Steel and Aluminum Bonding

Epoxy adhesives (single and two components) modified with SiO₂ nano-particles were used in this investigation to glue aluminum alloy and also two types of high strength steel (dip-galvanized steel DP 600 and micro-alloyed steel ZStE340). To improve the adhesion between metal surfaces and adhesives, the metal surfaces were pretreated with: a self-indicating pretreatment (SIP^*); corundum blasting; corundum blasting + a SIP coating; and a Pyrosil^® treatment + SurALink^® primer (PG 15 for epoxy adhesive). A single-lap shear tension test, done in accordance to DIN EN 1465, was used to determine the adhesive strength. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) analysis were used to analyze fractures that took place in the samples. The results showed that the adhesion strength of glued samples, containing the nano-particles modified adhesive, had significantly higher strength than unmodified ones. Pretreatment of the metal surfaces affected the adhesion, using nano-adhesives, only slightly. The adhesive strength values for single component epoxy resins were higher than those for two component epoxy resins. It was found that steel samples fractured adhesively at the steel surfaces. Aluminum treated samples indicated after pretreatment an increase in adhesive strength and the fracture occurred adhesively at the aluminum surfaces. Aluminum glued with two-component adhesives and pretreated with corundum blasting plus a SIP coating showed a mixed fracture mode; adhesively at the aluminum surface and cohesively in the adhesive layer.     

Adhesion Primer Properties: Microstructural Changes Induced by Ageing

The use of primers in adhesively bonded joints becomes more and more important in industrial process for improving the environmental strength of bonded structures. In this study we consider bonded joints comprising an aluminium alloy substrate clad with pure aluminium, an alumina layer obtained by surface treatment, a corrosion-inhibiting primer and a structural adhesive film. Observations and analyses (TEM or SEM imaging, electron diffraction, x-ray micro-analysis …) show that important microstructural changes of the metal/oxide/primer interphase are induced by ageing. The main modifications observed are: (a) appearance of crystalline areas in the primer layer near the oxide/primer interface, (b) complete disorganization of the columnar oxide morphology, (c) dissociation of the corrosion inhibitor. These phenomena are directly correlated to the strength losses of the bonded joints that are evaluated by mechanical experiments.     

CRH3高速动车组空调通风口胶接结构设计

通过理论分析和计算确定了动车组空调通风口部件与铝合金车体胶接用胶粘剂的强度指标。介绍了胶粘剂的选择及胶接结构的设计原则,考查了搭接长度、搭接宽度、胶层厚度和被粘接材料厚度等对胶接件粘接强度的影响。结果表明:车体与空调通风口部件的胶接接头选择受剪切应力作用的搭接接头较适宜,并且搭接接头的承载能力随搭接长度或宽度增加呈先快速上升后趋于稳定态势;当搭接长度为10 mm、胶层厚度为6 mm、铝合金板厚度为5 mm且常温湿固化型单组分PU(聚氨酯)胶粘剂的剪切强度超过0.23 MPa时,搭接接头的承载能力相对最大。     

Effects of different surface cleaning procedures on the superficial morphology and the adhesive strength of epoxy coating on aluminium alloy 1050

This study reports the effects of different cleaning procedures on the surface characteristics of the aluminium alloy 1050 substrates and on the adhesive strength of the epoxy coating to this alloy's surface. The cleaning procedures used in this study were (1) degreasing by acetone, (2) alkaline etching by 5 w/w% NaOH solution and (3) alkaline etching by 5 w/w% NaOH solution followed by acid cleaning by 50 v/v% HNO₃ solution. The surface morphology, chemical composition and topography of the cleaned substrates were investigated by field-emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS) and atomic force microscope (AFM), respectively. The effectiveness of the cleaning procedures was also studied by polarization test and open circuit potential (OCP) measurements. The surface free energy and work of adhesion were obtained on the cleaned samples using contact angle measuring device. Pull-off test was conducted to evaluate the adhesion strength of the epoxy coating on the aluminium substrates. Results revealed that the surface cleaning of aluminium alloy by alkaline etching followed by acid cleaning method was the most efficient procedure for removing the oxide layer from the surface of aluminium compared to other cleaning procedures. The surface roughness, surface free energy, electrochemical activity and adhesion strength of the epoxy coating to the aluminium surface were significantly increased using this surface cleaning procedure.     

Silane Coupling Agents as Adhesion Promoters for Aerospace Structural Film Adhesives

The performance of eight organofunctional silane coupling agents as adhesion promoters for the bonding of aluminium with two 121°C and two 177°C curing structural film adhesives was investigated and compared to the chromic acid (FPL) etch pre-treatment process and two non-chemical pretreatments. Aspects considered were shear strength of joints at ambient and elevated temperatures and durability, as judged by the wedge test.

The epoxy silane, γ-glycidoxypropyltrimethoxy silane, was found to be a very efficient adhesion promoter with all film adhesives evaluated. The cationic styryl silane, a neutral diamine monohydrochloride, showed promise with two adhesive systems. Four other neutral silanes were less effective.

Performance of amine functional silanes was mixed. Although the shear strength of joints with the primary amine silane at its natural pH of ∼10.3 was relatively good, durability was poor. However, good durability was obtained if the primer was first adjusted to pH 8 with hydrochloric acid, but not if acetic or phosphoric acids were used. Diamine silane was not an effective adhesion promoter at either its natural pH or when acidified with hydrochloric acid.     

铝合金与硅橡胶的粘接

进行铝合金表面处理工艺、胶粘剂粘接与硫化工艺对粘接强度影响的研究,结果表明采用化学氧化处理方法,可以使铝合金与硅橡胶的粘接强度稳定在３．０ＭＰａ以上。     

环氧-聚砜结构胶粘剂耐环境性能的研究

报导了SY-14环氧-聚砜胶粘剂体系的耐环境性能研究结果。在北京、南昌、广州和海口四个不同气候条件下的10年大气曝晒试验表明,各项胶接强度均末发生明显变化。含有抑制腐蚀底胶的胶接试板进行的10年室温盐水浸泡试验也取得了优良的试验结果。     

Degradation of Fatigue Properties of Weld-Bonded Aluminum Exposed to Moisture and Elevated Temperature

Experiments have been completed in order to characterize the fatigue behavior of weld-bonded aluminum 5754-O/bis-phenol-A epoxy adhesive joints subjected to 100% relative humidity at 38°C. It was found that the presence of water vapor at elevated temperature decreases the fatigue strength of weld-bonded joints by as much as 33% at 5 × 10⁶ cycles. Optical microscopy, scanning electron microscopy, dynamic mechanical analysis, and tensile testing of fatigued specimens and exposed bulk adhesive revealed that fatigue strength degradation is mainly due to the plasticization and micro-cracking of adhesive by the water vapor.     

Degradation of Fatigue Properties of Weld-Bonded Aluminum Exposed to Moisture and Elevated Temperature

Experiments have been completed in order to characterize the fatigue behavior of weld-bonded aluminum 5754-O/bis-phenol-A epoxy adhesive joints subjected to 100% relative humidity at 38°C. It was found that the presence of water vapor at elevated temperature decreases the fatigue strength of weld-bonded joints by as much as 33% at 5 × 10⁶ cycles. Optical microscopy, scanning electron microscopy, dynamic mechanical analysis, and tensile testing of fatigued specimens and exposed bulk adhesive revealed that fatigue strength degradation is mainly due to the plasticization and micro-cracking of adhesive by the water vapor.     

Hydroxymethylated Resorcinol (HMR) and Novolak-Based HMR (n-HMR) Primers to Enhance Bond Durability of Eucalyptus globulus Glulams

The effectiveness of the hydroxymethylated resorcinol (HMR) and the novolak-based HMR (n-HMR) primers to enhance bond durability of Eucalyptus globulus glulams was evaluated according to EN 391 (Delamination) and EN 392 (Shear strength) for outdoor load-bearing timber structures (Service Class 3). All glulams prepared under increasingly more stringent conditions with both primers, surpassed the requirements for delamination and shear strength. Specifically, when used with a one-component polyurethane adhesive: lamellas did not need to be planed before bonding; the primer amount could be reduced to 50%; and the adhesive could be applied to only one of the bonding surfaces. Additionally, lamellas primed with the n-HMR were effective for 2 weeks prior to bonding. The novolak-based primer overcomes the main limitations in using the original HMR primer, offering more flexibility for the same efficiency, and consequently, it is a more attractive option for manufacturing Eucalyptus globulus glulams.     

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.     

A comparative study on the use of photoreagents for the enhancement of adhesion on heavy-duty leather (Salz leather)

This paper aims to expand on work presented by us in previous publications regarding the enhancement of adhesion on leather substrates with photoreagent type primers. Two new reagents were synthesized: 3,5-dihydroxyphenyl-4-azidobenzoate (7) and 4,4′-ethylene-1,1′-diphenyl azide (8) and used as an intimate mixture with commercial polyurethane adhesive (Solibond PU39), to enhance the adhesion of Salz leather surfaces. The molecular design of the primer molecule was intended to provide 2- and 3-point fixation between the substrate and adhesive.

The strength of adhesive joints do not seem to be highly dependent on the molecular structure of the primers investigated in respect to the type of OH groups present (aliphatic or aromatic), both confer high adhesive strengths under dry and wet testing conditions. Cyanuric chloride (CNCl)₃ a commercially used primer when tested under the same conditions for comparison purposes, gave good results but with a high standard deviation.

Overall, the azide primers generate good, reliable joint strengths under extreme testing conditions and we believe they present negligible technological problems to industrial scale implementation.     

Improvement of the resistance to water of an adhesive joint between polymers and aluminium by using thin adhesion layers

The influence of a thin adhesive layer (AL) of a polymer on the wet adhesion of an epoxy coating on an aluminium substrate has been studied by the peel and tape test method. It is shown that thin layers considerably improve the stability of adhesive joints in the presence of water. The mechanism of this improvement and the mechanical properties required for such thin layers are discussed. Some experimental evidence for improved wet adhesion of polymer binders to an aluminium support is presented and the mechanism explained. These results on interfacial processes contribute to a better understanding of the mechanism of adhesion.