Post-manufacturing thermal treatments are commonly employed in the production of hip replacements to reduce shrinkage voids which can occur in cast components. Several studies have investigated the consequences of these treatments upon the alloy microstructure and tribological properties but none have determined if there are any biological ramifications. In this study the adsorption of proteins from foetal bovine serum (FBS) on three Co-Cr-Mo ASTM-F75 alloy samples with different metallurgical histories, has been studied as a function of protein concentration. Adsorption isotherms have been plotted using the surface concentration of nitrogen as a diagnostic of protein uptake as measured by X-ray photoelectron spectroscopy. The data was a good fit to the Langmuir adsorption isotherm up to the concentration at which critical protein saturation occurred. Differences in protein adsorption on each alloy have been observed. This suggests that development of the tissue/implant interface, although similar, may differ between as-cast (AC) and heat treated samples.
The development of state-of-art time-of-flight secondary ion mass spectrometry (ToF-SIMS) results in extremely large datasets. In order to perform multivariate analysis of such datasets without loss of mass and spatial resolution, appropriate data handling methods must be developed. The work in this paper presents an approach that can be taken to perform non-negative matrix factorisation (NMF) of large ToF-SIMS datasets. A large area stage raster scan of a chemically contaminated fingerprint is used as an example and the results show that the fingerprint signal was successfully separated from the substrate signal. Pre-processing challenges and artefacts that arises from the results are also discussed and an alternative approach, using the MapReduce programming model, is suggested for even larger datasets.
Time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) have been employed to study the interfacial interaction between polymeric methylene diphenyl diisocyanate (PMDI) and aluminium produced by the deposition of a thin PMDI layer on the aluminium, in order to improve adhesion and/or abhesion performance. When the PMDI concentration increases, the intensity ratio fragments indicative of the reaction product with water (m/z = 106 u: C7H8N+) to that of isocyanate group (m/z = 132 u: C8H6NO+) decreases. A very thin MDI layer on oxidised aluminium samples exhibits lower 106/132 ratio than degreased samples as a result of less hydroxide/hydroxyl spices on the surface. This suggests that water reactions occur both at the surface of PMDI and at the PMDI/aluminium interface. The variation of the PMDI chemistry has also been studied by exposing PMDI treated samples to the air for various periods of time (a few hours to 14 days), in order to assess the reaction of the PMDI surface and PMDI/aluminium interface. At the interface, the yield of reaction with water is limited because of the finite amount of hydroxyl groups on the aluminium surface, and the water reaction is completed in a short period of time. However, the PMDI surface continues to react with water from the atmospheric. This methodology was also used to establish the presence of specific interactions at the PMDI/aluminium interface, and a fragment indicative of covalent bond formation between PMDI and aluminium (AlCHNO3-) is observed at the interface.
This work combines large area ToF-SIMS imaging with multivariate analysis methods in order to characterise a cold-atmospheric plasma treatment of the surface of an automotive grade polypropylene. Data pre-processing steps are also presented alongside their main challenges. The results enabled the investigation of the spatial distribution of the treatment for different standoff distances as well as the effects of plasma plume mixing with atmosphere.
The very thin native oxide film on stainless steel, of the order of 2 nm, is known to be readily modified by immersion in aqueous media. In this paper, XPS and ToF-SIMS are employed to investigate the nature of the air-formed film and modification after water emmersion. The film is described in terms of oxide, hydroxide and water content. The preferential dissolution of iron is shown to occur on immersion. It is shown that a water absorbed layer and a hydroxide layer are present above the oxide-like passive film. The concentrations of water and hydroxide appear to be higher in the case of exposure to water. A secure method for the peak fitting of Fe2p and Cr2p XPS spectra of such films on their metallic substrates is described. The importance of XPS survey spectra is underlined and the feasibility of C60+ SIMS depth profiling of a thin oxide layer is shown.
Organic primer coatings loaded with environmentally harmful Cr(VI) corrosion inhibitive pigments still play an important role in corrosion protection of aluminium alloys for the aerospace industry. A potential green alternative coating system has recently been developed, loaded with lithium salt corrosion inhibitors. Under exposure to neutral salt spray, lithium salts leach from the organic coating into coating defects to induce the formation of a corrosion protective layer. In this work the composition and growth of this protective layer is investigated by time-of- ight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS imaging is successfully applied to monitor the lateral spread of leaching lithium salts in artificial one-millimeter-wide scribes. The chemical composition of the protective layer is revealed by comparing the mass spectra of salt spray exposed scribe areas to the mass spectra of pseudoboehmite and aluminium-lithium layered double hydroxide reference samples. The insights obtained in this work have led to a thorough understanding of the formation mechanism of the protective layer and provide local chemical and structural information which can be linked to corrosion protection behavior.
A thermodynamic study of the adsorption of an epoxy acrylate resin used for UV-cured coatings on two different anticorrosion pretreatments on aluminium alloys relevant to aerospace industry has been undertaken. Aluminium alloy Al2219 specimens, treated with an inorganic chromate based conversion coating (Alodine 1200S) and an organic titanium based conversion coating (Nabutan STI/310), were immersed in solutions of different concentrations of the resin and adsorption isotherms were determined by assessing the uptake of the adsorbate, as a function of solution concentration, by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results show different behaviour for the two substrates, which can be attributed to the organic component of the titanium based coating. In the case of the inorganic conversion coating a clear plateau is achieved at relatively low concentrations and at a lower level of adsorption than for the hybrid coating. The data for both the coatings conform well to the Langmuir model, the organic coating, as well as showing a higher level of adsorption of the resin, also presents oscillatory behaviour at low concentration, which is shown to be complementary to the behaviour of the reactive diluent included with the epoxy acrylate to aid processing. A discussion of this competitive adsorption of the epoxy resin and the diluent on the different substrates is presented, based on considerations of the chemistry of the systems under investigation.
The extent to which an organosilane surface treatment regime can promote durability enhancement of an adhesively bonded aluminium alloy system has been determined. Results have revealed the range of application and film-conditioning parameters which contribute to joint durability in a simple Boeing wedge joint. Organosilane solution parameters relating to solvent type, solution concentration, pH and hydrolysis time have all been shown to influence resultant durability. Interestingly, parameters such as film drying temperature and in-process time delay (time interval between application of the organosilane to the alloy surface and subsequent bonding) have little influence on joint performance. The factors responsible for the durability variations observed have been considered using various surface analytical techniques. Superficially, failure surfaces indicative of interfacial failure between substrate and adhesive have been observed. More detailed characterisation using both XPS and SIMS has indicated failure processes associated with a 'diffusion zone' comprising aluminium oxide and the organosilane. Crown Copyright © 2005 Published by Elsevier Ltd. All rights reserved.
Protecting an aircraft from the extremes of environments during service begins at the interface between topcoat and environment. The topcoat considered here is an aliphatic polyurethane (PU) based matte coating. This paper examines the degradation of the PU topcoat through the use of a novel HyperTest which combines ultra-violet (UV) and ozone as the degradation method. To benchmark the technique against accepted accelerated testing methods, QUV was used and samples were tested between two and 56 days. For The HyperTest, samples were degraded at increments between one to 120 minutes. X-ray photoelectron spectroscopy (XPS) determined that 56 days of UV exposure was equivalent, in terms of the extent of the chemical degradation of the topcoat, to one to two minutes of UV/ozone (UV/O3) exposure. There was a significant increase in carbonyl component with increasing oxygen concentration for samples treated with The HyperTest, whereas no clear degradation trend was observed for the samples exposed to UV alone. After 60 minutes of UV/O3 exposure a steady-state mechanism is established as the oxidative decomposition of the PU coating. The proposed degradation mechanism of the PU topcoat, through UV/O3 exposure, is the reaction of atomic oxygen with the polymer matrix/binder through hydrogen abstraction producing a hydroxyl group. This further decomposes to produce a carbonyl component observed in the XPS analysis. The products of degradation are simple volatile molecules such as CO2 and H2O for both testing methods used here. However, the efficient nature of The HyperTest, requiring only minutes to degrade samples as shown here, proves it to be a viable complementary technique to established methods of laboratory accelerated testing.
Coil Coating is an industrial process widely used to apply a paint layer to metal coil stock, in order to improve the resistance to corrosion as well as the aestetics of the surface. Three thermally cured coatings, formulated on a low Tg ispophtalic based polyester, were investigated by XPS and ToF-SIMS. A model formulation was employed for all three coatings investigated, however, the crosslinking agents used were varied for each formulation. Hexamethoxymethyl melamine (HMMM), tris isocyanurate (TIC) and a combination of HMMM and TIC were included as the cross linking agents in the three coating formulations. The use of TIC alone required a Sn based catalyst to promote the curing reaction. The aim of this work was to investigate the difference in the surface composition of the three coatings and the different behaviour of the different crosslinking agents used. This was in preparation for further studies which will involve interfacial analysis in order to elucidate the mechanism behind intercoat adhesion. The XPS analysis of the air/coating surfaces revealed a nitrogen concentration consistent with the concentration expected from the formulation for the coating containing HMMM. In the other two formulations a lower concentration then that calculated from the formulation was observed. The surface concentration of the two crosslinking agents is not influenced by the presence of the others, indeed the formulation containing both crosslinking agents is, in term of nitrogen concentration, merely a simple combination of the other two coatings. Peaks diagnostic of the cross linking agents were observed in ToF-SIMS spectra acquired from the air/coating surfaces. The coating with HMMM revealed peaks not present on the coating with TIC and vice versa, however the coating with a combination of HMMM and TIC showed all the peaks, with different relative intensities. In the positive spectrum of the sample with TIC the characteristic pattern of the tin catalyst can be observed. By XPS and ToF-SIMS analysis, we have been able to determinate that the HMMM and the TIC have a different distribution on the coating/air surface. Their distribution is not affected by the presence of the other crosslinking agent.
Thin films of aminopropyltriethoxysilane (APS) have been deposited on grit-blasted aluminium and dried at four different temperatures: room temperature (RT), 50, 93 and 120°C respectively. These specimens were prepared in order to assess the occurrence of the three important reactions known to take place when using silanes as films and/or primers: hydrolysis in the absence of water, condensation with the substrate, i.e. covalent bond formation and crosslinking or self-condensation. Analyses performed using X-ray photoelectron spectroscopy (XPS) indicate that the films reduce in thickness with temperature and that the type of silicon bonding changes mostly above 50°C. Time of flight secondary ion mass spectrometry (ToF-SIMS) reveals that covalent bonding of APS on aluminium occurs at all temperatures used in this study as well as showing that the films are close to being fully hydrolysed. It is also possible to assess the presence of crosslinking within the films.
Chemical throwing power, being the distance over which an inhibitor is able to protect a defect effectively, is an important parameter for active protective coatings. This study investigates the chemical throwing power of lithium-based leachable corrosion inhibitors exhibiting different leaching kinetics, from coatings at different inhibitor loading concentrations. The results demonstrate that Li-salt loaded coatings provide corrosion protection of defect areas up to a width of 6 mm. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to detect the lateral spread of Li in the defect areas and provide the chemical speciation of corrosion protective layers in the defect areas.
The present work has been conducted in order to investigate the quality of impregnation of unsaturated polyester resin across the bleached alfa pulpboard, and evaluate the potential for the obtained compound to serve a purpose as a useful composite material. The ultra-low-angle Microtomy (ULAM) method was used to study the quality of impregnation of alfa fibres by unsaturated polyester resin. The cross-section through the bleached alfa pulpboard/unsaturated polyester material, which emphasised the impregnation zone, was analysed using X-ray photoelectron spectroscopy (XPS). The profiles obtained by the XPS line scan analysis of the tapered surface show a well-defined impregnation region. The depth of this region has been characterised using two chemical constituent peaks, which showed an approximate depth of impregnation of about 20 to 40 μm, on the basis of C-OH/C-O-C, components which are exactly complementary to the β-shifted carbon signal, employed as an analogue for the unsaturated polyester. A comparative study was also carried out between the mechanical and tribological properties of the composite material and the pure unsaturated polyester resin, in order to complement the results obtained by surface analysis. The comparison shows better stiffness, tensile strength at break and wears resistance in the reinforced unsaturated polyester resin. The results of this research demonstrate a good quality of impregnation of unsaturated polyester resin across the short alfa fibres which present the matrix and reinforcement of the studied composite material.
The effects of flame treatment on the surface characteristics of four injection moulded, automotive grade, polypropylene samples, pigmented with carbon black, have been studied. The changes in wettability have been monitored by water contact angle and Dyne inks, whilst XPS has been used to establish the changes in oxygen surface concentration as a function of flame treatment. As expected carbon pigmented and carbon plus talc filled samples showed a significant increase in oxygen concentration and surface wettability with increasing flame treatment. For the glass filled sample this effect was not so pronounced. Inspection of the XPS valence band shows initial attack in the flame treatment process to be at the pendant methyl group of the poly(propylene) molecular architecture. XPS in conjunction with cluster ion bombardment shows the depth of surface treatment to range from ca. 7 nm at one pass of flame treatment to some 15 nm following seven passes. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) shows the segregation of characteristic additives during the injection moulding process which are subsequently greatly reduced during the flame treatment. As treatment level increases oxygen increases from mono-atomic to diatomic attachment. This work extends the understanding of the flame treatment of moulded polyolefines and establishes that the beneficial properties conferred are the result of the conjoint effect of the oxygenation of the bulk polymer along with the removal of surface segregated processing aids.
Photocatalytic experiments on the pharmaceutical pollutant carbamazepine (CBZ) were conducted using sol-gel nitrogen-doped TiO-coated glass slides under a solar simulator. CBZ was stable to photodegradation under direct solar irradiation. No CBZ sorption to the catalyst surface was observed, as further confirmed by surface characterization using X-ray photoelectron spectroscopic analysis of N-doped TiO surfaces. When exposing the catalyst surface to natural organic matter (NOM), an excess amount of carbon was detected relative to controls, which is consistent with NOM remaining on the catalyst surface. The catalyst surface charge was negative at pH values from 4 to 10 and decreased with increasing pH, correlated with enhanced CBZ removal with increasing medium pH in the range of 5-9. A dissolved organic carbon concentration of 5mg/L resulted in ∼20% reduction in CBZ removal, probably due to competitive inhibition of the photocatalytic degradation of CBZ. At alkalinity values corresponding to CaCO addition at 100mg/L, an over 40% decrease in CBZ removal was observed. A 35% reduction in CBZ occurred in the presence of surface water compared to complete suppression of the photocatalytic process in wastewater effluent. © 2012 Elsevier B.V.
This work presents a data analysis extension to a well-established methodology for the assessment of organic coatings using imaging time-of-flight secondary ion mass spectrometry (ToF-SIMS). Such an approach produced results that can be analysed using a multivariate analysis (MVA) procedure that performs the simultaneous processing of spatially and chemically related datasets. The coatings consist of two commercial resins that yield extremely similar spectra and there are no peaks of sufficient intensity that are uniquely diagnostic of either material to provide an unambiguous identification of each. In order to resolve the problem, in addition to microtome-based sample preparation steps of tapers for the analysis through sample thickness, standard samples in cured and uncured conditions are introduced and measured in the same fashion as the specimens under investigation. The resulting ToF-SIMS imaging datasets have been processed using non-negative matrix factorisation (NMF), which enabled identification of phase separation in the cured coatings.
Polymeric methylene diphenyl diisocyanate (PMDI) is the major component of polyurethane formulations and as a result the adhesion, or indeed abhesion, of polyurethanes, in a variety of forms (foams, coatings and adhesives), to metal substrates will be a function of the interactions between PMDI and metal surfaces. In this paper the adsorption of PMDI on oxidised metal (aluminium and iron) substrates has been investigated. The thermodynamics of adsorption has been examined by the construction of adsorption isotherms derived from ToF-SIMS data. At low solution concentration, the adsorption isotherns of PMDI are not of the Langmuir type, but are shown to conform to Langmuir adsorption at higher solution concentrations (> 1 g L-1). The interaction between the PMDI and iron is probably an acid-base interaction, and thus the adsorption of small PMDI molecules is displaced by larger PMDI molecules on the iron surface above a critical solution concentration. By contrast, such displacement is small on the aluminium surface as a result of dominance of covalent bond formation between PMDI and the metal substrate.
The present work has used an accelerated test method, based upon cyclic-fatigue testing using a fracture-mechanics approach, to study the durability of organosilane-pretreated joints which were adhesively-bonded using a hot-cured epoxy-film adhesive. The silane primer investigated was γ-glycidoxypropyltrimethoxysilane (GPS). The cyclic-fatigue tests were mainly conducted in (a) a 'dry' environment of 25±2 °C with a relative humidity of 55±5% and (b) a 'wet' environment where the joints were fully immersed in distilled water at 28±2 °C. However, tests were also undertaken by varying the relative humidity of the environment. The loci of failure of the joints were identified using various surface specific techniques. The use of the GPS-pretreatment has been shown to be effective in increasing the joint durability, compared with a simple grit-blast and degrease (GBD) treatment and, indeed, gives comparable results to using a chromic-acid etch (CAE) pretreatment. Crown Copyright © 2005 Published by Elsevier Ltd. All rights reserved.
Time of flight secondary ion mass spectrometry (ToF-SIMS) has been employed for the study of the adsorption of epoxy resin molecules, diglycidyl ether of bisphenol A (DGEBA), on aluminium substrates treated with an organosilane, γ-glycidoxypropyltrimethoxysilane (GPS). Both the kinetics of adsorption and the thermodynamics have been examined, by the construction of adsorption isotherms. The kinetics of adsorption was investigated to establish an exposure equilibrium time for DGEBA adsorption onto GPS-treated aluminium. Specimens were treated with three different concentrations of DGEBA solution and six times of treatment. It was found that DGEBA molecules were adsorbed on the aluminium substrate and reached a true adsorption equilibrium after 10 min. Adsorption isotherms were then produced to define the type of adsorption. Two possible types of adsorption isotherms, Langmuir and Temkin, were examined, and DGEBA adsorption was found to be of the Langmuir type, providing a better fit than Temkin adsorption. Using a modified form of the Langmuir equation, the fractional monolayer coverage of DGEBA molecules was determined. © 2005 Elsevier Ltd. All rights reserved.
Peak-fitting has been performed on a series of peaks obtained by ToF-SIMS analysis in order to assess whether information may be obtained from this procedure on the samples’ characteristics. A variety of samples were examined including a range of treatments for aluminium leading to different surface roughnesses, polymer films with a range of polydispersities, molecular weight and thicknesses as well as aluminium samples with adsorbed adhesion promoters on the surface. Variation of peak-fitting was assessed by varying the peak intensity, full width at half maximum and peak asymmetry. Although further studies are needed it is possible to say that the peak width increases with roughness whereas peak asymmetry seems to be related to oxide thickness. Polymer characteristics do not seem to influence the width whereas the peak asymmetry increases either versus molecular weight or polydispersity. A possible assumption is that the peak asymmetry relates to the ion formation processes. Additional work with varying polymer films thickness indicates that both FWHM and peak asymmetry may be related to sample charging and this could be used for assessment of film thicknesses. Finally, peak-fitting was used to obtain a more reliable peak area when peaks are too close in mass to use current methods.
As a further extension of the role of surface analysis in the study of early polymeric conservation artefacts, experiments were conducted to study the morphology and chemistry of fractured thermoplastic beads originating from 1970s jewellery. Two different methods were used to prepare the surfaces, either by breaking the object in compression or using liquid nitrogen to render it brittle. Scanning electron microscope (SEM) was used to obtain the morphology, whereas x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS) provided the chemistry of the system. The major component of the objects is established as being polystyrene and the presence or absence of certain additives may be observed, particularly at the surface of the bead or at the surface of samples cracked in a vice. The beads exhibit discolouration and cracking. Hexagonal structures of a few micrometers in diameter are also visible if the morphology is examined at high magnification. Such structures are consistent with stress, crazing, photodegradation and exposure of polystyrene to solvents. Furthermore, it was possible to correlate the presence or absence of additives with the morphologies encountered on the surfaces. Indeed the most damaged samples seemed to be characterised by the absence of additives while exhibiting a composition close to pure polystyrene. Copyright © 2008 John Wiley & Sons, Ltd.
The transfer of organic materials from a polyolefin-based film onto glass was studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Polyolefin films were brought in contact with glass and then removed either by peeling or twisting. The polyolefin films include minor additives, the transfer of which was followed by analysis of the glass side of the interface. In this study, we consider the effects of two different treatments in relation with the migration of the organic additives to the glass. ToF-SIMS images are obtained over a large area using macrorastering. These images, and also spectra recorded in discrete areas, show the presence of organics from polyolefin films and, hence, transfer of minor organic components to the glass. Comparison of imaging data between the two different treatments confirms that not only do additives migrate to the surface but also segregate with clear patterns on the glass surface as a result of one particular treatment. Principle component analysis (PCA) was also carried out on all data and the results obtained show good agreement with the imaging results (ion-selected images). ToF-SIMS and PCA results show that samples produced with treatment 1 exhibit a more organic-like surface on the glass side than samples obtained with treatment 2 which exhibit a more glass-like surface. Copyright © 2010 John Wiley & Sons, Ltd.
The interactions of polymeric methylene diphenyl di‐isocyanate (pMDI) and a model Fe–Cr alloy have been studied by X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Films of two different thicknesses have been investigated: one with an extremely thin pMDI layer in which the interfacial chemistry can be probed directly and a thicker one in which sputter profiling using cluster ions is necessary to expose the interface chemistry for direct analysis. Multivariate analysis (MVA), using principal component analysis (PCA) and nonnegative matrix factorisation (NMF), has been used to identify specific ions associated with the interfacial region of the ToF‐SIMS sputter depth profile and chemical species from the XPS sputter depth profile. As an unsupervised method, this avoids an unconscious bias on the part of the analyst. Specific ions associated with pMDI interactions with both Fe and Cr allow the proposal of two complementary reaction mechanisms, supported by the XPS data. A range of cluster ions is used in this investigation, but the bulk of the work used argon clusters for the XPS depth profiles and Buckminster Fullerene projectiles for the ToF‐SIMS analyses. To ensure that such data were directly comparable, the ToF‐SIMS sputter profiles were repeated in a different system of the same type using argon cluster ions.
A study of the effect of methanol solution composition (in terms of methanol and water proportions) on the hydrolysis of γ-glycidoxy propyl trimethoxy silane (GPS) was conducted using proton nuclear magnetic resonance (H1 NMR). In conjunction with this study, adsorption experiments were performed with the same solutions and the films deposited on aluminium surfaces studied by both X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). Hydrolysis experiments were performed until completion of the hydrolysis reaction and it is shown that the addition of methanol to an aqueous silane solution prevents hydrolysis and slows down the hydrolysis kinetics dramatically when added at a proportion of 90% (v/v). The use of a pure methanolic solution, however, stops the reaction totally. XPS data also indicated that adsorption from a solution containing methanol is less favourable than in pure water. An unexpected "inversion" of the adsorption isotherm plateaux obtained for solutions containing 10% and 90% methanol, respectively, as well as with the variation of the fragment ions characteristic of the silane epoxy and silicon atom suggest a variation in the adsorption conformation. A correlation was obtained between the state of hydrolysis deduced from both NMR data and ToF-SIMS intensities of specific fragments. This indicates that the concentrations in solution and the amount adsorbed are either correlated or anti-correlated according to the molecule under consideration. These correlations are more convincing when fragments characteristic of no hydrolysis or with one silanol are considered. © 2005 Elsevier Ltd. All rights reserved.
Linear saturated dicarboxylic acids are a class of organic chemical compounds with two carboxyl functional groups (-COOH) at the extremities of their aliphatic chains. This class of organic acids can be represented by the general molecular formula HOOC-(CH2)n-COOH. The most common values for n with their respective acid names are present in Table I. The general chemical behavior and reactivity of these compounds are similar to monocarboxylic acids, and they are all widely used in the production of copolymers, such as polyamides and polyesters (Refs. 1–3). The easy conversion of carboxyl groups to esters has industrial importance since many esters are used as taste and odor enhancers. Carboxylic acids are also used as catalysts, replacing ecologically unfavorable organic halides (Ref. 4). Over the last three decades, interest in such acids has increased, specifically regarding their application to improve the corrosion resistance of metallic substrates such as zinc, copper, iron, and aluminum (Refs. 5–12). Research has also shown that carboxylic acids can be used as additives for the electro synthesis of polymeric protective coatings. Such coatings promote passivation of different metallic substrates, allowing the oxidation of the carboxylic acid monomers without concomitant reactions (Refs. 10–13). More recently, carboxylic acids have been used to generate hydrophobic surfaces on various metallic substrates (Fe, Al, Cu, Mg, Zn, Ti, etc.) forming self-assembled layers by adsorption, via carboxyl groups, to the positively charged metal surfaces (Refs. 14–18).
Model samples of the interface of an adhesive joint containing small levels of aminopropyl triethoxysilane (APS) have been prepared in order to examine the interface formed with an aluminium substrate. X-ray photoelectron (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS) has been used to analyse and image the interface region in between the aluminium and an epoxy adhesive in order to ascertain the reactions by the organosilane, present as a minor component within the system. It was found that APS was present at the interface between the adhesive and the substrate and that it had reacted with the substrate forming a covalent bond and was also crosslinked within the adhesive. Evidence of near to full hydrolysis of APS is also present within the spectra.
The interfacial interactions between methylene diphenyl di-isocyanate (MDI) and polymeric MDI (PMDI) with 316L stainless steel have been studied in order to understand the adhesion properties of polyurethane based adhesives (containing free isocyanate groups) on metal surfaces. A thin (< 5 nm) layer of MDI and PMDI was deposited on the surface of clean 316L stainless steel and then analysed by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). By using density functional theory (DFT) methods for the interpretation of XPS data, the presence of specific interactions between the adsorbate and the substrate was established. The reaction between isocyanate and metal hydroxyl groups with the formation of urethane-like bonding with the metal was observed. The formation of hydrogen bonds between the isocyanate nitrogen and the hydroxyl groups and the formation of nitrogen-metal double bond, as consequence of the cycloaddition reaction between isocyanate and metal oxide, are also proposed.