Electrochemical impedance spectroscopic investigation of adhesion formation mechanism at an epoxy/aluminum interface

A theoretical method for characterizing the structure of a coating/metal interface by electrochemical impedance spectroscopy using water molecules as the probe was established. The properties of coating/metal interfaces for a series of epoxy resins with different water affinities were studied using this method. It was found that as the water affinity of the coating decreased, it became much more difficult for water molecules to reach the coating/metal interface. This suggests that, during the adhesion formation, a more hydrophobic layer is formed along the epoxy/metal interface.     

Adhesiveness of an epoxy oligomer filled with aluminum oxide powders

Concentration dependences of the strength of filled epoxy resin-steel wire (diameter 150 μm) adhesive joints are studied. Nanostructural powders of aluminum oxide with cylindrical particles (diameter 4–6 μm and length 30–40 μm) are used as fillers. The structure of cured compositions is studied by X-ray diffraction techniques and it is established that nanostructural Al₂O₃ fibers are porous. The effect of the scale factor is studied. It is shown that the breaking force increases nonlinearly and the strength of the adhesive joint decreases with an increase in the bonding area. It is established that the dependences of adhesive strength on concentration are described by curves with maxima. This makes it possible to recommend highly dispersed fillers as a method for the control of interfacial strength.     

Studies on metal/benzocyclobutene (BCB) interface and adhesion

Interfacial characteristics such as chemical reaction, metal diffusion, and morphology were investigated for Cu/BCB, Cr/BCB and Ti/BCB structures. Using Auger and XPS depth profiling, the formation of titanium carbide and chromium oxide was confirmed at the metal/BCB interface. Annealing at 250°C for extended periods resulted in the diffusion of Cu, Cr and Ti into the BCB and subsequent formation of Cu-Si, CrSi₂ and Ti-Si compound precipitates. The reaction is a thermal diffusion controlled process which is dependent on time and temperature. Ar sputtering treatment of BCB film before metallization was found to roughen the surface, resulting in metal spikes which penetrate into the roughened BCB film. However, the peel strength of metals on BCB was only about 177 g cm^_1presumably due to the brittleness of the BCB film. The etch rates of the BCB film in a reactive ion etcher (RIE) and a plasma etcher were measured using Ar, O₂, O₂ + CF₄, and O₂ + SF₆ gas mixtures. Faster etch rates were obtained when CF₄ and SF₆ were added to oxygen, since the presence of atomic fluorine enhances the etch rate of organics, while also etching Si and SiO₂ formed by exposure of Si-containing BCB film to oxygen gas. Surface compositional changes on the BCB film were observed by XPS after plasma modification. Pure O₂ and O₂ + CF₄ plasmas oxidized the carbo-siloxane linkage (C_—Si_—O) of the BCB, resulting in the formation of SiO₂ on the surface. The O₂ + SF₆ plasma, however, did not produce the surface SiO₂, because of its faster Si and SiO₂ etch rates.     

Theoretical analysis of the decay of shear strength of adhesion in metal/epoxy/metal joints in an aqueous environment

A theory for the decay of shear adhesion strength has been applied to lead-alloy/ epoxy/lead-alloy joints in water at 65°C, 75°C and 85°C. Theoretical values for the retention of wet shear strength over time coincided with experimental data. From experiments on wet shear adhesion strength and water absorption, it has been clarified that the following two cases exist: (1) in adhesive system A, the diffusion coefficient of water is greater at the interface than in the adhesive; (2) in adhesive system B, the diffusion coefficient of water is lower at the interface than in the adhesive, and water at the interface does not immediately contribute to adhesion failure — ie, there is a time lag between contact with water and bond breakage.     

Thermal and insulating properties of epoxy/aluminum nitride composites used for thermal interface material

We synthesized an epoxy matrix composite adhesive containing aluminum nitride (AlN) powder, which was used for thermal interface materials (TIM) in high power devices. The experimental results revealed that adding AlN fillers into epoxy resin was an effective way to boost thermal conductivity and maintain electrical insulation. We also discovered a proper coupling agent that reduced the viscosity of the epoxy‐AlN composite by AlN surface treatment and increased the solid loading to 60 vol %. For the TIM sample made with the composite adhesive, we obtained a thermal conductivity of 2.70 W/(m K), which was approximately 13 times larger than that of pure epoxy. The dielectric strength of the TIM was 10 to 11 kV/mm, which was large enough for applications in high power devices. Additionally, the thermal and insulating properties of the TIM did not degrade after thermal shock testing, indicating its reliability for use in power devices. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Electrospinning of aqueous lignin/poly(ethylene oxide) complexes

Microfibers of kraft lignin blended with poly(ethylene oxide) (PEO) were produced by electrospinning of the solution of lignin and high molecular weight poly(ethylene oxide) (PEO) in alkaline water. Interactions between lignin and PEO in alkaline aqueous solutions create association complexes, which increases the viscosity of the solution. The effect of polymer concentration, PEO molecular weight, and storage time of solution before spinning on the morphology of the fibers was studied. It showed that after one day the viscosity dropped and fiber diameter decreased. Results from the solutions in alkaline water and N,N‐dimethylformamide (DMF) with different polymer concentrations were compared. The 7 wt % of (Lignin/PEO: 95/5 wt/wt) in alkaline aqueous solution was successfully spun and the ratio of PEO in lignin/PEO mixture could be further reduced. In comparison, higher concentrations were needed to prepare a spinning solution in DMF and fiber diameters were in a much smaller range. The final target of spinning lignin is to produce carbonized fibers. Fibers spun from aqueous solutions had lower PEO content, which is a big advantage for the carbonization process as it reduces the challenges regarding melting of the fibers or void creation during carbonization. Furthermore, the larger diameter of these fibers inhibits disintegration of the carbonized fibers, which happens due to the mass loss during the process. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41260.     

Lamb waves characterization of adhesion levels in aluminum/epoxy bi-layers with different cohesive and adhesive properties

This paper presents a study of the detectability of the level of adhesion at the Aluminum/Epoxy interface using the propagation of ultrasonic guided waves. The topic is relevant to the long-range ultrasonic inspection of structural adhesive bonding. The samples considered are epoxy-coated aluminum: samples were fabricated with partially or totally crosslinked epoxy groups and with different surface treatments of aluminum substrates. Experimentally measured dispersion curves and attenuation coefficients of a selected mode are used to quantitatively track changes at the epoxy-aluminum interface. The results show that modes could be very sensitive to the nature of the interface, that is to say to the level of adhesion, if properly selected. Numerical predictions via the Jones rheological model and their comparison with experimental results highlight three adhesion levels: weak, medium and high one.     

Effect of the microstructure of copper oxide on the adhesion behavior of epoxy/copper leadframe joints

The thermal oxidation of copper leadframe was carried out at 175°C and the adhesion behavior of the epoxy/copper leadframe joint was analyzed by investigating the microstructure changes of copper oxide with the thermal oxidation time of copper. The peel strength increased sharply at an early stage of oxidation (~20 min) followed by a slight increase. After further oxidation (120 min), the peel strength showed a slight decrease. The contact angles of water and diiodomethane decreased sharply at an early stage of oxidation with negligible change afterwards. As the oxidation time increased, X-ray photoelectron spectroscopy (XPS) results revealed that the chemical composition of copper oxide had changed (Cu/Cu₂O → Cu₂O → CuO); this change improved the wettability of the copper surface, which affected the peel strength. Increase of the surface roughness of copper oxide, investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), causes the epoxy resin and copper oxide to undergo mechanical interlocking, which increases the peel strength. Failure analysis by SEM and XPS indicated that failure was largely in the copper oxide, and the amount of copper oxide on the peeled epoxy increased as the oxidation time increased, due to the weak mechanical strength of the oxide layer. However, a small portion of the epoxy resin was also fractured during the failure process, regardless of the oxidation time. Consequently, fracture proceeded mainly in the copper oxide close to the epoxy resin/copper oxide interface.     

The effects of zinc aluminum phosphate (ZPA) and zinc aluminum polyphosphate (ZAPP) mixtures on corrosion inhibition performance of epoxy/polyamide coating

The epoxy/polyamide coating was loaded with different pigment mixtures of the zinc phosphate (ZP), zinc aluminum phosphate (ZPA) and zinc aluminum polyphosphate (ZAPP) pigments. The electrochemical impedance spectroscopy (EIS) and salt spray test were used to investigate corrosion inhibition performance of the coatings. The adhesion strengths of the coatings were measured by a pull-off test. Results revealed lower coating pull-off strength loss when the ZPA and ZAPP pigments were used. A significant decrease in number of blisters together with low pull-off strength loss and best corrosion inhibition properties were observed when the mixture of 80:20 of ZAPP:ZPA was used.     

The effect of silane layer drying temperature on epoxy coating adhesion on silane-pretreated aluminum substrate

In this study, 3-glycidoxypropiltrimethoxysilane was used as an adhesion promoter to enhance the adhesion strength of epoxy coating on an aluminum (Al) substrate. Silane layer drying temperature was investigated as a factor that has an influence on the adhesion of polymeric coating on metal substrate and also on its performance in wet and corrosive environments. FTIR tests were carried out to study Al/silane interactions. Drying the silane layer at high temperatures formed a condensed siloxane layer that improved the bonding strength as well as the performance of the protective coating in corrosive environments. The highest dry and wet pull-off strengths were obtained at drying temperatures of 100 and 125°C, respectively.     

Using heterogeneous silane patterns to maintain adhesion and decrease water penetration into epoxy/aluminum interfaces

Silanes are used for surface modification to improve dispersion, bind biomaterials, improve adhesion, etc. Their use in improving adhesion in composite materials is widespread, and for these systems there is a growing need for both increased adhesion performance and resistance to water penetration across polymer/oxide interfaces. The present work reports adhesion results obtained using patterned, binary combinations of adhesion-promoting and non-adhesion-promoting silanes patterned onto an oxide surface. The effects of pattern shape, texture (feature size) and the fractional coverage of the adhesion promoter are explored for the bonding of epoxy matrices to aluminum oxide surfaces using combinations of γ-glycidoxypropyltrimethoxysilane (GPS), an adhesion promoter, and either octadecyltrichlorosilane (ODTS) as a hydrophobic non-adhesion promoter, or vinyltrimethoxysilane (VMS) as a water scavenger. In addition to dry tests, samples are submerged in boiling water for 48 h, and in 50°C water for 4 h to determine if water penetrated into the interface, thereby reducing the adhesion. Climbing drum peel tests reveal that heterogeneity of silane primer substrates can influence the adhesion and increase the durability to water penetration. Adhesion enhancement is attributed to the blocking of lateral diffusion of water by barriers due to the presence of the hydrophobic silane. Results show that due to exposure to water at 50°C for 4 h, the adhesion is reduced by 46% in the homogeneous samples but only by 20% in the heterogeneous samples. In contrast, the use of VMS was ineffective in preventing the loss of adhesion due to water penetration.     

The performance of epoxy coatings containing polyaniline (PANI) nanowires in neutral salt,alkaline, and acidic aqueous media

PANI/epoxy coatings have great promise applications in the industry as the metal corrosion protection coating, and their performance directly determines the life span of the coating and equipment durability. In this study, the performance of epoxy coatings with and without PANI nanowires immersed in 12 wt% NaCl, 5 wt% HCl, and 5 wt% NaOH solutions at different temperature were investigated for the first time. The performance and the degradation reactions of the coating cooperated with PANI nanowires were characterized by the variety of techniques and methods, including ultraviolet–visible spectrophotometry (UV–vis), field emission scanning electron microscopy (SEM), Attenuated Total Reflectance-Fourier transform Infrared spectroscopy (ATR-FTIR), and thermogravimetric analysis (TGA). The experiment results indicated that the failure mechanism of the different coatings varied with the different temperatures and solutions. Electrochemical impedance spectra (EIS) results showed that an appropriate content of PANI nanowires improve the protection performance of epoxy coatings in 12 wt% NaCl, 5 wt% HCl, and 5 wt% NaOH solutions, which is attributed to the passivation ability and shielding effect of PANI nanowires.     

Phase separation during crosslinking of epoxy resin/poly(ethylene oxide) blends

Summary Poly(ethylene oxide) (PEO) was found to be miscible with uncured epoxy resin of DGEBA type (ER) as shown by the existence of a single glass transition temperature (Tg) in each blend. However, PEO with ¯Mn = 20,000 was judged to be immiscible with the highly aminecross linked ER. It was observed that the phase separation in the ER/PEO blends occurred as the crosslinking progressed. This is considered to be due to the dramatical change in the chemical and physical nature of ER during the crosslinking.     

Extraction of Cu(II) ions from aqueous media using PEG/Sulphate salt aqueous two-phase system

A simple ATPS of polyethylene glycol/sulphate salt was proposed for the extraction of Cu(II) ions from aqueous media. Maximum extraction efficiency of Cu ions was obtained in the aqueous PEG2000/ sodium sulphate two-phase system. In the following, design of experimental methodology using Taguchi orthogonal array was applied to evaluate the effects of four independent variables on the partitioning behavior of Cu ions from aqueous media. The results indicated that the temperature and pH of the system were the major contributory factors for Cu ions partitioning. The maximum extraction efficiency of Cu ions was determined to be 77.89% under the optimal conditions.     

Influence of nanoparticles and nanofibers of aluminum oxide on the properties of epoxy composites

The strength properties of epoxy composites filled with aluminum oxide nanoparticles and reinforced by aluminum oxide nanofibers are compared. It is demonstrated that these fillers have a specific effect on the properties of the epoxy matrix. Original Russian Text ? B.N. Dudkin, G.G. Zainullin, P.V. Krivoshapkin, E.F. Krivoshapkina, M.A. Ryazanov, 2008, published in Fizika i Khimiya Stekla.     

Investigation of oxide film formation on 316L stainless steel in high-temperature aqueous environments

Oxide films were grown on the surface of 316L stainless steel subjected to high temperatures and a high-pressure aqueous environment (250 °C and 7 MPa). The morphology, chemical compositions and corrosion properties of oxide films were investigated by scanning electron microscopy (SEM), auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The results indicated that oxide films formed at 250 °C were more corrosion resistant and thicker than were oxide films formed in air at room temperature (25 °C). These distinctions are correlated with the structure and chemical compositions of oxide films. It was found that both films contained a double-layer structure comprised of mixed iron–nickel oxides and chromium oxides. Iron was present as FeO, Fe₂O₃ and FeOOH; Cr was present as Cr₂O₃, Cr(OH)₃ and CrO₃; and Ni existed as Ni(OH)₂ within the oxide films formed at 250 °C.     

Turbulent friction reduction by aqueous poly(ethylene oxide) polymer solutions as a function of salt concentration

Turbulent friction reduction by high molecular weight poly(ethylene oxide) polymers has been examined in a series of salt solutions ranging from pure water to nearly theta solvent conditions. The effects of polymer homology and solvent character have been successfully analyzed under these conditions and relationships are proposed for the observed effects. The reduction in turbulent friction (drag reduction) has been catalogued through evaluation of the polymer intrinsic concentration—an index of drag reduction effectiveness. Plots of the reciprocal of the polymer intrinsic concentration versus salt molarity are approximately linear and are similar to the plots of intrinsic viscosity versus molarity reported by other workers. An attempt is made to graphically and numerically combine these results. The suggestion is advanced that those solvent properties which bring about decided conformational changes in these polymer molecules (as indexed by intrinsic viscosity effects) also affect, in an apparently analogous fashion, the turbulent friction reduction efficiencies of these molecules. The decreases in turbulent friction reduction resulting from the increasingly collapsed state of the polymer coil suggest the possibility of correlating friction reduction with changes in the polymer expansion factor α. On the basis of the limited data available, the suggestion is also made that drag reduction studies might best be made under theta solvent conditions where different polymer families might be more meaningfully compared in the absence of solvent effects.     

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.     

提高芳纶纤维与橡胶界面粘合性能的方法

综述提高芳纶纤维与橡胶界面粘合性能的方法。化学改性主要包括表面基团活化（基于苯环和酰胺基团的取代反应）、表面浸溃和涂层，物理改性主要包括等离子体表面改性（冷等离子体处理和等离子体表面接枝）、超声波改性和γ射线辐射。物理改性方法对环境污染小，可连续化操作且不影响本体性能，发展优势大。