Interactions of BTESPT silane and maleic anhydride grafted polypropylene with epoxy and application to improve adhesive durability between epoxy and aluminium sheet

For improvement of adhesive strength and durability of adhesion between epoxy and aluminium sheet bis-(triethoxysilylpropyl)tetrasulfide (BTESPT) silane and maleic anhydride grafted polypropylene (PP-g-MAH) are chosen for surface pretreatment of the aluminium sheet respectively. Fourier transform infrared (FTIR) spectroscopy was used for characterization of the structure and the interactions in the systems. It is shown that BTESPT silane and the anhydride on PP-g-MAH take part in the curing reactions of the epoxy/polyamide system. The adhesive shear strength of the samples, prepared under different curing temperatures, and after immersion in boiling water and 3.5% NaCl water solution respectively, was tested. The features of the shear fractured surfaces were examined by scanning electron microscope (SEM). For the aluminium sheet pretreated by BTESPT silane, the maximum adhesive shear strength is 22.2 MPa, which is higher than that of 17.5 MPa for aluminium sheet without pretreatment by the silane. After immersion in boiling water for 80 h and in NaCl water solution at 50 °C for 180 h the adhesive shear strengths are 13.39 MPa and 18.4 MPa respectively, which are higher than these (below 6 MPa) for aluminium sheet without pretreatment by the silane. As for the aluminium sheet pretreated by PP-g-MAH, the maximum adhesive shear strength is 13.17 MPa. After immersion in boiling water for 80 h and in NaCl water solution at 50 °C for 180 h the adhesive shear strengths decline to 10.67 MPa and 8.1 MPa respectively.     

Model epoxy powder coatings and their adhesion to steel

Model epoxy powder coatings differing in crosslink density were applied to steel. Their adhesion loss upon immersion in water at 35 and 55°C, as well as recovery of the adhesion upon drying were determined. Permeability of free films to gaseous water and oxygen were measured. Impedance measurements were used to study the diffusion characteristics of water into the coatings on steel. The permeability data of the formulations were similar. However, the adhesion behavior was very different. The adhesion in the dry state proved highly dependent on the rheology during the cure process. The adhesion recovery after immersion was apparently determined by the effectiveness of water as a plasticiser in the polymer network.     

Laboratory evaluation of epoxy coatings with an adhesion promoter by impedance

The corrosion behaviour of glassflake epoxy coatings on steel substrate with and without an adhesion promoter was examined by electrochemical impedance spectroscopy. The impedance data show that an improvement in the protective behaviour of the coating is connected with a large increase in the resistance in the pores of the coating. These effects are probably due to an improvement in the initial, wet and recovered adhesion between glassflakes and epoxy binder. The results of pull-off adhesion tests are presented.     

Effect on the toughness and adhesion properties of epoxy resin modified with silyl-crosslinked urethane microsphere

In order to give a toughness and improve adhesion properties of the cured epoxy system, modified epoxy resins, which have pre-reacted urethane microspheres formed using dynamic vulcanization method in liquid diglycidylether of bisphenol A, were prepared. It was found that the size of the particles decreased to sub-micro order with increase in solubility of urethane oligomers in epoxy resin, and coefficient of variance in the particle size distribution resulted in less than 15%. Fracture energy G_1c of the cured system was highly improved. Lap shear strength and peel strength were also improved. These mechanical and adhesion properties do not depend on any curing condition of epoxy resin because of the existing stable particles in the epoxy resin before curing.     

Changes in epoxy-coated aluminium due to exposure to water

An investigation was performed of the changes that occur in a typical epoxy-coated aluminium system due to exposure to water. The adhesion of the epoxy coating upon exposure to water was evaluated for different exposure temperatures and periods. The adhesion test results showed an initial loss of adhesion of the coating but after this the adhesion improved again and even significantly exceeded adhesion prior to exposure. The amount of adhesion improvement and the speed with which adhesion improvement occurred was found to be larger for higher exposure temperatures. The changes that occur in the epoxy-coated aluminium system due to exposure to water were investigated using a number of different analytical techniques. Based on this, a model was proposed for the processes that take place during exposure and which can explain the adhesion test results. First, the adhesion of the epoxy coating is lost upon exposure due to the accumulation of a significant amount of water at the interface. The water at the interface causes formation and growth of a thin oxyhydroxide layer underneath the epoxy coating. After some time, this oxyhydroxide layer re-establishes contact with the epoxy coating and forms a new, water-stable bond, hence explaining the improvement of the adhesion after its initial loss.

The temperature-dependence observed in the adhesion test results is explained by the fact that at a higher exposure temperature, more water accumulates at the interface, the oxyhydroxide layer grows faster and also attains a larger limiting thickness.     

Correlation between water diffusion and adhesion loss: Study of an epoxy primer on steel

Epoxy adhesion to steel is known to be weakened by the presence of water.     

Effects of carbon nanotube content on adhesion strength and wear and corrosion resistance of epoxy composite coatings on AA2024-T3

The effects of multiwalled carbon nanotube (MWCNT) content on the adhesion strength and wear and corrosion resistance of the epoxy composite coatings prepared on aluminum alloy (AA) 2024-T3 substrates were evaluated using atomic force microscopy (AFM), blister test, ball-on-disk micro-tribological test and electrochemical impedance spectroscopy (EIS). The adhesion strength of the epoxy composite coatings improved with increasing MWCNT content. Increased MWCNT content also decreased the friction coefficient and increased the wear resistance of the epoxy composite coatings due to improved solid lubricating and rolling effects of the MWCNTs and the improved load bearing capacity of the composite coatings. Finally, EIS indicated that increased MWCNT content increased the coating pore resistance due to a decreased porosity density, which resulted in an increase in the total impedance of the coated samples.     

The influence of ageing of epoxy coatings on adhesion of polyurethane topcoats and protective properties of coating systems

The dependence of adhesion and protective properties of coating systems on surface properties of epoxy intermediate coatings, aged and non-aged before an application of polyurethane topcoats, were examined. The intermediate coatings were aged 500 h in UV chamber. The surface free energy and polar groups were estimated after ageing. After applying polyurethane topcoats on aged and non-aged epoxy coatings, resistance to salt spray and thermal shocks were tested as well as internal stresses were measured before and after corrosion tests.The results showed that adhesion in coating systems with polyurethane topcoats applied on aged epoxy coatings depends strongly on the degradation degree of epoxy intermediate coatings and the value of generated internal stresses. Coatings with good adhesion retention in corrosion environments have good protective properties even when temporary blistering has occurred.     

Effect of cross linking degree and adhesion force on the anti-corrosion performance of epoxy coatings under simulated deep sea environment

Deep sea exploitation represents the future direction for its abundant resources, while organic coatings can play a very important role in ocean engineering for corrosion protection. In this paper, under simulated deep sea environment in the laboratory, the effect of the crosslink density and the adhesion force of the coatings on the anti-corrosion performance of two different types of epoxy coatings was studied by using electrochemical impedance spectroscopy (EIS) and adhesion test. The results showed that the deterioration of coatings’ property of corrosion protection under simulated deep sea environment was much faster than that of under normal pressured seawater. However, the diffusion velocity of water molecule through epoxy coatings decreased with increasing of the crosslink density and the adhesion force of the coatings. Therefore, the capability of corrosion protection could be improved by increasing the density of crosslink of the coatings within some extent and by increasing the adhesion force of the coatings.     

Adhesive strength of powder coated aluminium substrates

Adhesive strengths of differently pretreated powder coated aluminium substrates were evaluated using the pull-off and the tape tests in the dry and wet conditions. Various accelerated test chambers were employed to evaluate the corrosion behaviour of the samples. Scanning electron microscopy (SEM), with energy dispersive X-ray analysis (EDX) were also employed to the better understanding of the nature and morphology of the differently pretreated aluminium substrates prior to and after the accelerated tests.

Accelerated tests and wet adhesion measurements showed that the degreased samples demonstrated a better performance than Polyacrylic acid (PAA) treated samples. The chromate/phosphate conversion coating (CPCC) treated samples gave the best performance and the Polyacrylic acid/ hexafluorozirconic acid treated samples (PZr) gave marginally second best performance in anti-corrosive behaviour.

Based on the results obtained from the different experiments, it is argued that these studies may provide sufficient evidence to show that hydrolysis may destroy interface bonds between PAA and aluminium substrate. Moreover, SEM/EDX analysis, suggest that, the thin PAA polymeric layer was replaced by a crystalline, thick hydrated oxide layer, when samples were exposed to humid conditions.     

The influence of aluminium surface pretreatment on the corrosion stability and adhesion of powder polyester coating

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.     

Silane and epoxy coatings: A bilayer system to protect AA2024 alloy

This work has proved that a good combination of a simple and fast metal pre-treatment, followed by the deposition of a thin layer of an organic–inorganic silane coating and further layer of epoxy coatings, are able to protect the aluminium alloy AA2024-T3 against corrosion in high concentrations of NaCl solution. The alloy AA2024 is one of the most employed aluminium alloy in structural applications due to its good mechanical properties. However, AA2024 alloy series commonly presents galvanic corrosion due to the rich content of copper element. The influence of different surface pre-treatments, the presence of a silane layer as pre-coating treatment and the influence of phosphonic acids combined with the silane layer on the corrosion protection and adhesion to the aluminium alloy have been examined using accelerated corrosion tests. High roughness and the presence of a pre-coating film between the metal surface and the organic coating were essential for a good protection and resistance to blistering appearance in the surface of AA2024-T3.     

Adhesion characteristics and corrosion stability of epoxy coatings electrodeposited on phosphated hot-dip galvanized steel

The influence of hot-dip galvanized steel (HDG) surface pretreatment with phosphate coatings on the corrosion stability and adhesion characteristics of epoxy coatings electrodeposited on HDG steel was investigated. Phosphate coatings were deposited on hot-dip galvanized steel from baths with different concentrations of NaF (0.1, 0.5 and 1.0 g dm⁻³) and at different temperatures (50, 65 and 80 °C). The influence of fluoride ion concentration in the phosphating bath, as well as the deposition temperature of the bath, on the adhesion characteristics and corrosion stability of epoxy coatings on phosphated HDG steel was investigated. The dry and wet adhesions were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test, while corrosion stability was investigated by electrochemical impedance spectroscopy (EIS).     

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.     

Degradation of epoxy coatings in humid environments: the critical relative humidity for adhesion loss

An abrupt loss of adhesion at interfaces between epoxy and inorganic substrates has been shown to occur whenever the epoxy is equilibrated in air whose relative humidity exceeds a critical value. We report that the critical relative humidity marking the onset of adhesion loss is associated with (1) a sudden increase of water solubility in the epoxy, (2) a corresponding increase in the volume of the epoxy, and (3) a strong decrease in the mobility of absorbed water. Possible mechanisms such as capillary condensation, osmotic cell formation, and a decrease in glass transition temperature induced by water are ruled out on the basis of available data. Instead, it is proposed that the observed behavior is linked to water 'condensation' into traps or clusters in the polymer above the critical humidity. The solubility behavior of a liquid diol that models the diol-terminated epoxy chain tails suggests that the excess sorption above the critical humidity involves trapping of the water by hydroxyl groups which become available as inter-chain hydrogen-bonded structures are broken above the critical relative humidity. Pronounced irreversibility of solubility and swelling is due to a reaction of the absorbed water with residual oxirane groups, leading to the formation of diols. This explains the upward shift of the solubility and swelling curves toward asymptotic behavior upon relative humidity cycling. We also show that osmotic cells that form around artificially introduced water-soluble impurities can cause a loss of adhesion at a predictable critical relative humidity.     

Thermal and radio-oxidation of epoxy coatings

Degradation induced by thermal (50–110 °C) and radio-oxidation of low T_g epoxy-amine networks has been studied. It has been found that oxidation leads mainly to amide groups formation at the vicinity of tertiary amines whatever ageing conditions (thermal or radio-oxidation at 200 Gy h⁻¹). In addition, some species as acids, peracids or formates have been revealed indicating a chain scission process. Physical modifications as T_g decrease and soluble fraction increase due to chain scission process, have been correlated with chemical modifications.     

An original epoxy-stamp on glass-disc specimen exhibiting stable debonding for identifying adhesive properties between glass and epoxy

An original axisymmetric specimen composed of a concave epoxy-stamp bonded on a glass-disc, which exhibits stable decohesion under traction, is proposed for identifying the adhesive properties, namely the adhesion surface energy, the adhesion peak stress and the decohesion rupture gap between glass and epoxy. Debonding stability is obtained by combining two stabilizing effects of the epoxy-stamp geometry: axisymmetry and concavity. This stamp design is believed to be an important discovery since stable crack propagation is the exception rather than the rule in fracture mechanics. The increasing graph of the traction force vs. the specimen elongation enables to measure the adhesion surface energy upon unloading, calculate the adhesion peak stress and deduce the decohesion rupture gap. Preliminary experiments confirm the stability of the debonding process and show excellent reproducibility.     

Improvement of mechanical properties and electrical conductivity of polythiourethane-modified epoxy coatings filled with aluminium powder

Electrically conductive epoxy-polythiourethane aluminium-filled composites have been developed and characterized. The influence of aluminium loading levels on electrical and mechanical properties was explored. The distribution of the conductive filler and morphology of epoxy matrix were studied by SEM and energy dispersive X-ray analysis (EDX). Microscopy observation provided useful information for understanding the conductivity behaviour and mechanical properties of the conductive epoxy coatings. The strength of adhesion of the filled resins to metal substrates was also determined. Impact strength measurements and adhesion tests demonstrated that curing conditions significantly affect the properties of epoxy-polythiourethane composites. The experimental results showed that polythiourethane hardener greatly enhances most of these properties in comparison to standard epoxy formulations with amine hardeners.     

Evaluation of parameters affecting adhesive strength of high build epoxy coatings on textured and plasma treated metallic substrates

Epoxy coating systems have been used throughout the years for protecting critical industrial assets. To ensure superior coating adhesion and longevity, abrasive blasting has been traditionally employed to texture metallic surfaces. Though a large variety of abrasives are available in the industry, coal slag (aluminium silicate) and garnet (mineral abrasive) have remained popular choices until now owing to their easy availability. However, depleting reserves of coal slag owing to stricter environmental regulations and shortage of good quality garnet have fueled the need to identify substitutes. This work is focused on evaluating the effect of surface texturing on adhesive strength of epoxy coatings using two novel abrasive types – olivine (obtained from dunite rock) and calcium silicate (iron furnace slag) which are available in abundance and present an excellent alternative. The surface textures of various blast cleaned mild steel (A36) and stainless steel (316L) substrates were evaluated using a combination of techniques namely, optical profilometry (spatial parameters), replica tape method as per ASTM D 4417 (Method C) (height parameter) and scanning electron microscopy (Secondary electron imaging/energy dispersive X-ray spectroscopy analyses) for embedded particle composition analysis. Correlations were drawn to the wettability of metallic substrates textured using the selected abrasives. Adhesive strength tests on high build epoxy coatings, performed in accordance with ASTM D4541 revealed exceptional performance using various grades of both olivine and calcium silicate abrasives. The effect of air plasma treatment on adhesive strength of epoxy coatings applied on mild steel substrates were also studied.     

The influence of zinc surface pretreatment on the adhesion of epoxy coating electrodeposited on hot-dip galvanized steel

The adhesion and electrochemical properties of epoxy coatings electrodeposited on hot-dip galvanized steel with and without passive films were investigated during exposure to 3% NaCl. The passive films were formed in hot air, in boiling water and by chromating. Adhesion was measured both by a standardized pull-off method and by swelling in N-methyl pyrrolidone. Pretreatment of hot-dip galvanized steel with passive film formed in hot air increases both dry and wet adhesion strength of the epoxy coating compared to pretreatment with passive film formed in boiling water and chromate coating. The overall increase of wet adhesion for this sample was maintained throughout the whole investigated time period. It was shown that the change in adhesion of epoxy coating on a chromate coating is smallest of all investigated samples, although the initial value of adhesion on this surface had the lowest value. The corrosion stability of coated Zn samples pretreated by different methods, was investigated by electrochemical impedance spectroscopy and in the initial time of exposure to NaCl the highest values of pore resistance were also obtained for the epoxy coating on Zn pretreated in hot air, whereas the epoxy coating on a HDG steel with a chromate coating showed the smallest change in electrochemical properties (pore resistance, coating capacitance, charge-transfer resistance) during prolonged exposure time.