Improving the adhesion between epoxy coatings and aluminium substrates

Two different methods were followed to improve the adhesion and durability of the adhesion of a commonly used epoxy coating on an aluminium substrate. The first method was by application of a thin polymeric layer, having a thickness of around 10 nm, on the aluminium substrate prior to application of the epoxy coating. The functional groups in the polymers were chosen so as to be capable of chemisorption to the oxide surface and should also to be capable of being involved in the curing reaction of the epoxy resin. These polymers were poly(acrylic acid) (PAA), poly(ethylene-alt-maleic anhydride) (PEMah) and poly(vinyl phosphonic acid) (PvPA). An investigation of the interphasial region between the epoxy coating and the aluminium substrate in the final cured system showed that the polymeric layers were indeed involved in the curing reaction with the epoxy.

For the poly(ethylene-alt-maleic anhydride)-based system, this resulted in the formation of a cured, mixed poly(ethylene-alt-maleic anhydride)/epoxy interphasial region between coating and substrate while for the two other polymers, a weakly cured interphasial region was formed. The second method of adhesion and durability improvement was by hydration of the aluminium substrates, performed by immersion in boiling water. This procedure results in the formation of a porous pseudoboehmite oxyhydroxide layer. The epoxy coating was found to be capable of fully penetrating into the layer. The adhesion of the epoxy coatings was tested initially and after exposure to 40 °C water and 40 °C 5% acetic acid. The poly(ethylene-alt-maleic anhydride)-based system resulted in a very good initial adhesion and durability in presence of water for the epoxy coating, while the systems based on the other two polymers did not. The pseudoboehmite-based system also resulted in very good initial adhesion and durability in the presence of water. None of the improved systems were, however, found to be able to withstand 40 °C 5% acetic acid and showed severe corrosion underneath the epoxy coating.     

In situ ATR-FTIR studies of the aluminium/polymer interface upon exposure to water and electrolyte

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with the Kretschmann configuration was applied for in situ studies of the transport of water and ionic species through a polymer film to an aluminium/polymer interface. The time dependent intensity changes of the infrared bands of water were used to follow the transport of water to the aluminium/polymer interfacial region and a NaSCN solution was employed as model electrolyte to follow the transport and accumulation of thiocyanate ions. Apart from water sorption and ion transport, the main processes identified were corrosion/oxidation of the aluminium surface and swelling of the polymer film. The method proved to be useful for detailed in situ studies of changes at a polymer coated metal surface, such as oxidation and surface film formation on the metal. It should also be possible to study the effects of defects and pores in the polymer film on the transport properties of water and ions to the metal/polymer interface, as well as adsorption and other chemical reactions and physical interactions in the metal/polymer interfacial region.     

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.     

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.     

Changes in the dielectric relaxations of water in epoxy resin as a function of the extent of water ingress in carbon fibre composites

Dielectric relaxation measurements are reported over a frequency range from 10⁻¹ to 10⁹ Hz as a function of exposure time for an epoxy resin-carbon fibre composite, ageing at 60 °C in water. Investigation of the nature of the dipole relaxation of the water molecules, indicates the nature of their interaction with the polymer matrix. Analysis of the dielectric relaxation spectra allow identification of processes that can be attributed to ‘free’ and ‘bonded’ water, water in micro-cracking, located in carbon fibre disbonds and plasticizing the polymer matrix. Identification of the various types of location in which water exists was aided by use of the N_g factor from the Kirkwood-Frölich equation, which describes the constraints on free dipole ration nature imposed by the environment in which it is located. These data indicate the power of the dielectric technique for quantitative analysis of water ingress into epoxy composites.     

Fluorescence labels to monitor water absorption in epoxy resins

Water uptake phenomena was studied in a group of commercial epoxy based thermosets using gravimetric and fluorimetric analysis. The different epoxy formulations were labeled with two dansyl derivatives differing in the spacer length between the chain and the fluorophore moiety. The fluorimetric method consisted of monitoring the changes in the first moment of the dansyl emission band as a function of water immersion time. Using the fluorescence, it was possible to obtain the parameters that govern the water diffusion process and there was a good concordance with gravimetric results. Furthermore, the fluorescence response of the dansyl moieties was used to study the effect of the molecular structure of the polymers in the water absorption process.     

Encapsulated aluminium pigments

Silica encapsulation of aluminium flakes by a new type of sol/gel process provides silver pigments with long-term stability in water-based coatings. This new, heavy metal-free stabilisation technology guarantees gassing stability and optical properties comparable to chromate-passivated pigment types. No degradation under high shear stress can be observed, an important property for circulation lines and pumps of the automotive industry. Finally, the silica-coated pigments show an excellent weather fastness (Florida testing passed) and best adhesion if the pigmented coatings are exposed to humidity tests.     

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.     

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.     

Effect of tie-layer thickness on the adhesion of ethylene-octene copolymers to polypropylene

The adhesion of some ethylene-octene copolymers to polypropylene (PP) and high density polyethylene (HDPE) was studied in order to evaluate their suitability as compatibilizers for PP/HDPE blends. A one-dimensional model of the compatibilized blend was fabricated by layer-multiplying coextrusion. The microlayered tapes consisted of many alternating layers of PP and HDPE with a thin tie-layer inserted at each interface. The thickness of the tie-layer varied from 0.1 to 15 μm, which included thicknesses comparable to those of the interfacial layer in a compatibilized blend. The delamination toughness was measured in the T-peel test. Generally, delamination toughness decreased as the tie-layer became thinner with a stronger dependence for tie layers thinner than 2 μm. Inspection of the crack-tip damage zone revealed a change from a continuous yielded zone in thicker tie layers to a highly fibrillated zone in thinner tie layers. By treating the damage zone as an Irwin plastic zone, it was demonstrated that a critical stress controlled the delamination toughness. The temperature dependence of the delamination toughness was also measured. A blocky copolymer (OBC) consistently exhibited better adhesion to PP than statistical copolymers (EO). A one-to-one correlation between the delamination toughness and the reported performance of the copolymers as compatibilizers for PP/HDPE blends confirmed the key role of interfacial adhesion in blend compatibilization.     

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.     

Critical current degradation and delamination crack observation of epoxy-coated REBCO superconducting tapes after thermal cycles in liquid nitrogen

The delamination and critical current (I_c) degradation caused by thermal stress after epoxy impregnation are threats for the application of REBa₂Cu₃O_7-x(REBCO, RE = Rare earth) superconducting magnets. In this work, two types of REBCO tapes were coated by Stycast 2850FT with controlled coating geometries. Critical currents of coated samples after thermal cycles in liquid nitrogen were measured. I_c degradation was found in coated samples with a free no-coating edge, when the surface coating layer was thicker than 1000 μm. It was also found that additional edge coating can help to suppress the I_c degradation. Samples with degraded I_c after thermal cycles showed an obvious delamination phenomenon. The morphology and location of delamination cracks were carefully observed by using focused Ion beam, scanning electron microscope, and transmission electron microscope. Delamination cracks propagated within the REBCO layer and stopped at reaching the Silver/REBCO interface. Simulations by finite element method suggest that delamination cracks are generated by the stress accumulation within the REBCO layer, which could be reduced by a full epoxy coating on both tape surface and edges.     

Coagulation of humic acid by aluminium sulphate in saline water conditions

Chemical coagulation has been applied in practice for the pre-treatment of saline waters in the process of water treatment by membrane desalination. The relative magnitude of the ionic strength and individual ion concentrations (cations and anions) in saline waters leads to differences in the nature of coagulation by hydrolysing metal salts compared to the treatment of freshwaters with respect to colloid destabilisation, coagulation mechanisms, and colloidal removal. In this study, the coagulation of saline solutions containing humic acid by aluminium sulphate has been examined systematically. Laboratory experiments have been undertaken over a wide range of pH (4 to 10) and alum dose (3-1000 μM Al) conditions. Coagulation has been assessed principally in terms of colloidal particle removal (change in UV-absorbance) and colloid surface charge (zeta potential). The experimental results have shown that the coagulation ofhumic substances in seawater has some distinctly different characteristics compared to the well-established coagulation behaviour in surface freshwaters. These differences can be attributed to the solution chemistry and its modification of the aluminium hydrolysis reactions and the nature of the precipitates formed. Based on the results, coagulation performance domains (Al dose vs. pH) are defined, and the mechanisms of coagulation are discussed.     

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.     

Delamination Mechanics of a Clamped Rectangular Membrane in the Presence of Long-Range Intersurface Forces: Transition from JKR to DMT Limits

A 1-dimensional rectangular freestanding membrane clamped at opposite ends adheres to the planar surface of a rectangular punch. A tensile load applied to the punch causes the membrane to deform and gradually delaminate from the substrate. At equilibrium, the applied load is balanced by the disjoining pressure at the membrane-punch interface with range, y, and magnitude, p. Applying the Dugdale-Barenblatt-Maugis cohesive zone approximation, the disjoining pressure is taken to be uniform and confined to a finite cohesive length at the contact edge. For a fixed adhesion energy, γ = p y, we investigate the following: (i) the Derjaguin-Muller-Toporov (DMT) limit where y → ∞ and p → 0, (ii) the Johnson-Kendall-Roberts (JKR) limit where y → 0 and p → ∞, and (iii) the general case for intermediate but finite y and p. Delamination continues until the contact area shrinks to a line prior to “pinch-off”. The results are compared with the 2-dimensional axisymmetric membrane counterpart.     

三氧化二铝对对氯苯酚吸附的研究

研究了水体中三氧化二铝对对氯苯酚稀溶液的吸附能力,测定了不同温度下的吸附平衡等温线,不同酸碱度以及离子强度下的吸附平衡等温线。从温度、离子强度和pH三方面比较了三氧化二铝对对氯苯酚降解效果的影响。研究表明：对氯苯酚的最适宜温度为25-35℃,最适pH为8—8．5。     

Simulation of pH-controlled dissolution of aluminium based on a modified Scanning Electrochemical Microscope experiment to mimic localized trenching on aluminium alloys

Some constituent intermetallic (IMPs) particles at the surface of aluminium alloys are considered as preferential sites for the initiation of structural corrosion resulting in localized trenching around the particles and the surrounding Al matrix. In this work, a modified scanning electrochemical microscope (SECM) experiment was used to induce such phenomena via a local alcalinisation on 200 nm thick aluminium coatings promoting their local dissolution in an aerated 0.1 M NaCl electrolyte. The local alcalinisation was induced by the oxygen reduction reaction on the tip of a SECM which mimics the surface of an isolated IMP. From a phenomenological point of view, reproducible cylindrical damage develops in front of the platinum ultramicroelectrode (UME). Using a 2D finite element modelling to simulate the SECM experiments, the role of the local alcalinisation was validated and the calculated Al dissolution rate was found in agreement with the experimental evaluation.     

Improved resistance of alumina to mild wear by aluminium titanate additions

The wear behaviour of an alumina (Al₂O₃)–aluminium titanate (Al₂TiO₅) composite containing 10 vol.% of second phase is studied and compared to that of single phase alumina. A careful control of the microstructure has been done in order to compare materials with similar alumina grain sizes. Wear tests have been performed on a pin on disk tribometer with an alumina ball as pin, at room temperature, under a normal force of 10 N and at sliding speeds from 0.06 to 0.15 m/s. Extensive analyses of the microstructural modifications due to wear have been done by a combination of field emission scanning microscopy and confocal microscopy.Mild wear conditions were attained for both materials. The main wear mechanism identified in both materials involves the formation of a hydroxide film and its cracking and delamination. The composite specimens presented increased wear resistance compared to the single phase ones.     

Critical Tractions for Initiating Adhesion Failure at Interfaces in Encapsulated Components

Determining the initiation of adhesive failure at a surface buried deep within the bulk of an epoxy is qualitatively different from measuring the propagation of an existing surface crack. Most current tests are shown to be unsuitable for assessing the critical traction at initiation. A new test geometry is presented that initiates failure away from an air interface, produces a slowly varying stress distribution near the initiation site and minimal contributions from thermal residual stresses, and enables tests with mixed modes of loading. This new geometry is used to examine temperature-dependent adhesive failure in tensile, shear, and mixed modes of loading for both smooth and rough surfaces. Some of the experimental results are unexpected. As examples, the critical traction at initiation of adhesive failure is apparently insensitive to surface roughness, and the critical normal traction is independent of temperature while the critical tangential traction tracks the shear yield stress.     

Critical Tractions for Initiating Adhesion Failure at Interfaces in Encapsulated Components

Determining the initiation of adhesive failure at a surface buried deep within the bulk of an epoxy is qualitatively different from measuring the propagation of an existing surface crack. Most current tests are shown to be unsuitable for assessing the critical traction at initiation. A new test geometry is presented that initiates failure away from an air interface, produces a slowly varying stress distribution near the initiation site and minimal contributions from thermal residual stresses, and enables tests with mixed modes of loading. This new geometry is used to examine temperature-dependent adhesive failure in tensile, shear, and mixed modes of loading for both smooth and rough surfaces. Some of the experimental results are unexpected. As examples, the critical traction at initiation of adhesive failure is apparently insensitive to surface roughness, and the critical normal traction is independent of temperature while the critical tangential traction tracks the shear yield stress.