Depth profiling of coil coating using step-scan photoacoustic FTIR

Photoacoustic (PA) Fourier Transform Infrared Spectroscopy (FTIR) has been demonstrated to be very useful in the analysis of molecular distribution and/or degradation in polymeric materials in a non-destructive manner. Step-scan (SS) PA FTIR has been found to be especially suitable to depth profile multi-layered polymer coating/laminate systems. In this current study, the capability of SSPA-FTIR in assessing industrial coil coatings was evaluated. Two multi-layered model coil coating samples were prepared, one had a pigmented Polyvinylidene Fluoride (PVdF) top coat and the other had a clear Polyethylene Terephthalate (PET) laminate film on the surface; both were depth profiled by SSPA-FTIR. The signal magnitude and phase angle were used to obtain a modulation frequency and a phase angle resolved depth profile, respectively. The advantages and disadvantages of the technique were also investigated. Optical microscopy was used to determine the true thicknesses of the PET and PVdF layers from the sample cross-sections. The values were compared with those predicted by SSPA-FTIR. It was found that a precise depth profile was only obtained with the PET sample whereas in the high pigmented coating system, the predicted values were smaller than the true PVdF thickness, possibly due to the high thermal diffusivity of the inorganic pigment.     

A comparison of different approaches for depth profiling of films and coatings by confocal Raman microscopy

We compare and analyze different approaches to perform depth profiling of polymer films and coatings by confocal Raman microscopy (CRM). Data were generated using three methodologies: conventional metallurgical objectives, oil-immersion optics and numerical post processing of the as-measured intensity profiles, via an optimized deconvolution technique adapted to CRM. A series of bi- and multi-layered polymeric films were used as test systems. Strengths and weaknesses of each methodology are evaluated in terms of delivered depth resolution, signal throughput and flexibility. It is shown that the application of regularized deconvolution on data obtained from dry objectives yielded intensity profiles with a quality comparable, in some cases superior, to those obtained with immersion objectives, with the advantage of being totally non-invasive.     

Depth profiling of UV cured coatings containing photostabilizers by confocal Raman microscopy

The chemical imaging possibility of confocal Raman microscopy was used to characterize UV cured coatings layers. Depth profiles of acrylate curing conversion were recorded in order to elucidate the interaction of photoinitator, photostabilizer, and irradiation source. Formulations containing acyl phosphine oxides resulted in a better through cure.     

Confocal Raman microscopy study of the melamine distribution in polyester–melamine coil coating

Hexamethoxymethylmelamine, a common crosslinker used in combination with polyester resins to prepare thermosetting coatings, can self-condense during curing and in theory build up regions with high crosslink density (melamine enrichment). This distribution may affect such properties as the “local” glass transition temperature and structural heterogeneity and thereby modify formability, stain resistance, and weatherability. The self-condensation regions have not previously been observed in detail so their distribution in the film is unclear. Confocal Raman microscopy (CRM) has been used to characterize the melamine distribution in three polyester–melamine paint systems: one clear and two pigmented coil coatings. Both the surface mapping and depth profiling features of CRM were used to perform a 3D analysis. The depth profiling of the clear coating was performed in a nondestructive way, but due to the opaque nature of the pigments, cross sections of the pigmented coatings were prepared in order to facilitate the analysis of the melamine distribution through the film thickness. Melamine-enriched zones were found on the surfaces of all three samples. They appeared to be approximately spherical with diameters of around 5 μm. It was also observed from the cross sections that the melamine-enriched zones appeared as “particles” distributed randomly through the coating. The Raman spectra collected at these regions show significantly higher triazine ring band intensity. All of these findings confirm the hypotheses constructed over the past 20–30 years. This paper was awarded Second Place in the 2008 FSCT Roon Awards competition held as part of the FutureCoat! conference sponsored by the Federation of Societies for Coatings Technology, in Chicago, IL, on October 14–16, 2008.     

A depth-resolved look at the network development in alkyd coatings by confocal Raman microspectroscopy

The formation of molecular networks related to the consumption of unsaturated carbon-carbon double bonds (CC) during oxidative drying of alkyd coating films incorporating unsaturated fatty acids was studied. The concentration of CC bonds was measured as a function of drying time and distance from the exposed film surface (depth) using confocal Raman microspectroscopy (CRM). The change in spatial distribution of the CC double bond concentration across the film cross section provides information on the kinetics of the oxidative cross-linking process in the alkyd films. It was found that the CC bond consumption is not homogeneous across the depth of the drying film. The results obtained allowed us to quantitatively monitor the progress of the drying process and the movement of the ‘drying front’ within the coating films. The drying profiles suggest that oxygen penetration into the coating film is a rate-limiting factor in the drying process. Depth profiles during the film forming process develop due to local variations in the oxygen solubility, diffusion coefficient of oxygen, and available amount of double bonds for cross-linking. The influence of several industrially relevant factors, like oil length of the alkyd resin, thickener, solvent, and drier on the film formation process is discussed. Depth resolution of the analytical approach and spatial accuracy of confocal Raman microspectroscopy are also treated.     

Study of the porous structure of white chocolate by confocal Raman microscopy

Confocal Raman microscopy has been shown to be a useful technique for investigation of white chocolate surfaces. The appearance of protrusions and pores, and the distribution of fat, sucrose, and milk powder at and below the surface of white chocolate pralines were investigated using confocal Raman microscopy. Raman horizontal and depth scans showed that the protrusions and pores continue at least 10 µm into the chocolate shell and that some protrusions and channels mainly consist of fat, while some consisted of a fat layer, leaving a hollow space underneath. Further, the pores and their continuing channels consisted of nothing but air. These findings indicate that the protrusions might be connected to channels where we suggest a pressure driven convective flow of liquid fat from within the chocolate matrix that, depending on temperature, moves up to the surface or goes back into the matrix, leaving an empty pore with a shell of fat at the surface, which in some cases collapse and leaves a hollow pore and channel. Therefore, these findings support that the protrusions could be connected to oil migration in chocolate and, thus, further to fat bloom development. Practical applications: Confocal Raman microscopy can be used to investigate the local distribution of different components in white chocolate. This technique offers the possibility to acquire the local distribution of different components within the sample, with a resolution down to the optical diffraction limit. Further, the analysis can be performed at ambient conditions, without requiring any special sample preparation or marker molecules. The results obtained by using this technique suggest that specific surface imperfections on chocolate could be part of a network of pore structures at and beneath the chocolate surface, which could be related to oil migration and thus, to fat bloom formation.     

Polyamide membrane precipitation studied by confocal backscattering microscopy

A wide variety of commercial polymeric membranes are manufactured by a non-solvent immersion precipitation process, yet the detailed mechanism and kinetics of membrane formation are poorly understood. We have used a confocal microscope, with fluorescence filters removed, to observe backscatter from precipitating nylon 6 films. Nylon 6 in formic acid/water solutions was spread on a glass substrate, which was then quickly immersed into water or water/formic acid non-solvent baths. Two slope-casting chambers were designed in order to observe the precipitation process from both the substrate side and the bath side (in separate experiments). From the substrate side, a scattering front was observed to form within a few seconds after immersion, and then to progress toward the substrate as the precipitated film formed. When observed from the bath side, the scattering front first moved toward the bath for approximately 5-10 s, then reversed direction and moved toward the substrate. This later motion toward the substrate can be explained by the contraction of the film as water is expelled. The initial motion toward the bath implies that the initial point of precipitation is not at the film surface, but rather at some depth (ca. 60+ μm) within the film. This new application of confocal microscopy has enabled measurements of the dynamics of nylon membrane precipitation with sufficient time and depth resolution to permit quantitative testing of models of the precipitation process.     

Analysis of composite nanofibrous layers by confocal Raman microscopy

A method for the determination of nanofibrous mats chemical composition based on Raman spectroscopy and singular value decomposition is presented. Two different composite samples – a simultaneously spun and a layer-by-layer sample – consisting of polycaprolactone and poly(ethylene) oxide were prepared by electrospinning. Using Raman mapping, we were able to determine the chemical composition in various local areas within each sample, and to distinguish and image substantial changes in the distribution of the polymers caused by various preparation parameters. The ratio between both polymers was expressed as the relative fraction of the particular chemical compound. The results were validated by the Fourier transform infrared spectroscopy. The presence of two different substances in the nanofibrous mats was also proved by scanning electron microscopy. Confocal micro-Raman spectroscopy with its chemical sensitivity and spatial resolution may be used to identify inhomogeneities in the nanofibrous materials and could be of great help in the optimization of the electrospinning process.     

Multiphase imaging of freezing particle suspensions by confocal microscopy

Ice-templating is a well-established processing route for porous ceramics. Because of the structure/properties relationships, it is essential to better understand and control the solidification microstructures. Ice-templating is based on the segregation and concentration of particles by growing ice crystals. What we understand so far of the process is based on either observations by optical or X-ray imaging techniques, or on the characterization of ice-templated materials. However, in situ observations at particle-scale are still missing. Here we show that confocal microscopy can provide multiphase imaging of ice growth and the segregation and organization of particles. We illustrate the benefits of our approach with the observation of particles and pore ice in the frozen structure, the dynamic evolution of the freeze front morphology, and the impact of PVA addition on the solidification microstructures. These results prove in particular the importance of controlling both the temperature gradient and the growth rate during ice-templating.     

Stainless steel as new substrate for coil coating

Increasing needs of very high resistance to cosmetic corrosion, of more extended service life and reduced maintenance costs for infrastructures, civil and industrial buildings open new fields of application for coil coated stainless steel. This paper describes the adhesion and corrosion properties of new coil coated stainless steel materials produced in industrial coil coating lines. The use of an electrochemical test (electrochemical impedance spectroscopy) can give detailed information on the reactivity of the system and allow the performance of different substrates (AISI 409, 430, 316 and 304) coated with different polymers (polyvinylidene and polyester) to be compared. The results obtained show the interesting properties of this new class of coil coated products. The materials were tested for a long time (about 200 days) in an aggressive environment (3.5% sodium chloride solution) also in the presence of macrodefects. In particular, VIVINOX 430, 304 and 316 revealed no reactivity, corrosion or disbonding, thus supporting the expectancy of very long trouble free exposure also in very aggressive natural environments. (VIVINOX is the brand name of the AST (Acciali Speciali Terni) line of coil coated stainless steel.)     

Fast-responsive semi-interpenetrating hydrogel networks imaged with confocal fluorescence microscopy

The composition of semi-interpenetrating polymer networks (semi-IPNs) based on responsive N-isopropylacrylamide (NIPAAm) hydrogels has been shown to affect the kinetics of the volume phase transition. Several N-alkyl-substituted acrylamides were used as the linear polymers in a crosslinked NIPAAm network, and the kinetics was observed as a function of crosslinking density and linear polymer concentration. The time required for collapse of the network could be reduced by as much as 90%, with little change to the corresponding swelling ratio and volume phase transition temperature. However, the underlying changes in network morphology are not known, and here we present kinetics data in combination with imaging of the resulting hydrogel networks. The crosslinked networks and the linear polymers were fluorescently labeled, and the resulting morphology was imaged with confocal fluorescence microscopy and two-photon laser scanning microscopy. The most hydrophilic of the linear polymers was acrylamide, which was shown to phase separate during polymerization. The hydrophilic domains become more interconnected at higher concentrations of the crosslinker and the linear polymers. This correlates well with the kinetics of the volume phase transition for the corresponding networks. The semi-IPNs containing more hydrophobic linear polymers had very similar morphology, but some domains were present, ranging from 500 nm to 2 μm and increasing in size with increased linear polymer concentration. The time scale of collapse was an order of magnitude faster than expected, based on size of the hydrophobic N-alkyl group, when the linear polymer had the same lower critical solution temperature as the hydrogel network. This is an indication that the simultaneous collapse of the linear polymer and the crosslinked network contributes to the fast response of these semi-IPNs.     

Non-destructive 3-dimensional mapping of microcapsules in polymeric coatings by confocal Raman spectroscopy

Nowadays, the properties of polymeric coatings are enhanced by various additives mixed into the resin. Recently, embedding of polymeric microcapsules into the coating matrix has been investigated to provide special on-demand features to the coating. The detection and characterization of such microcapsules in a polymeric coating are of major importance but difficult, because both are built up by similar molecules with similar densities. Current analysis methods require complex sample preparation to allow reliable measurements.In contrast, confocal Raman spectroscopy allows fast and non-destructive differentiation between characteristic molecular bonds at a spatial resolution below one micrometer. Hence, the objective of this research was to apply this technique on microcapsules embedded in a coating and provide answers to the following questions: Can one detect microcapsules embedded in a coating and clearly identify them? Can one differentiate between full and empty microcapsules and the coating matrix? Can one determine the exact location of the capsules and their distribution in the coating?Therefore, several two-dimensional confocal Raman spectroscopy mappings recorded at different depths allowed a three-dimensional reconstruction of the polymeric coating with the polymeric microcapsules in it. Thereby, the distribution of the capsules within the coating could be determined with micrometer resolution. As a result Raman tomography provides a more detailed insight into the distribution of microcapsules through the possibility of three-dimensional reconstruction.     

Structures and morphologies of cast and plastically strained polyamide 6 films as evidenced by confocal Raman microspectroscopy and atomic force microscopy

Both confocal Raman microspectroscopy and atomic force microscopy (AFM) have been undertaken to study the crystalline and the morphological aspects of cast PA 6 films at a sub-microscopic scale. The percentages of the different crystalline structures present within PA 6 cast films, i.e. the monoclinic α, the pseudo-hexagonal β, and the monoclinic γ, have been measured by confocal Raman microspectroscopy. In cast films, the prevailing structure is the β one. AFM has been used to characterize the morphology of the PA 6 films. Simultaneously, the deformed state has been considered as well. Our main interest has been to follow the evolution of the percentage of each crystalline structure as a function of the plastic deformation mechanisms which are responsible of the yielding of PA 6 films: shear banding for temperatures T lower than 160 °C and formation of fibrils for      

An investigation on the melamine self-condensation in polyester/melamine organic coating

Melamine is a common crosslinker widely used in the coil coating industry to crosslink hydroxyl functional polyesters. Theoretically, melamine can also self-condense with the consequence regions with higher crosslink density build up in the coating. Although the melamine self-condensation has been studied, most of works were based on melamine segregation on the coating surface; the existence of melamine self-condensation has only been modelled rather than observed. Utilising confocal Raman microscopy and nanoindentation, the regions with higher melamine concentration were clearly observed and characterised for the first time.     

Wetchemical surface modification of plasticized PVC. Characterization by FTIR-ATR and Raman microscopy

The chemical modification of plasticized PVC films with amino thiophenol in mixtures of DMF/H₂0 has been studied. The degree of modification and the distribution of modified groups within the films were determined using Raman spectroscopy and FTIR-ATR. The amounts of plasticizer (bis-2-ethylhexyl phtalate, DOP) bleached out during the reaction and the amount of dimethylformamide (DMF) penetrated into the film were analysed in control reactions without the reactant using ¹H NMR spectroscopy and Raman spectroscopy.Reaction kinetics and surface selectivity of the modification reaction were compared with the corresponding reactions of PVC films without plasticizer.     

Characterization of radiation-grafted polymer films using CP/MAS NMR spectroscopy and confocal Raman microscopy

Preirradiated poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) base film were grafted with different amounts of an α-methylstyrene (AMS) and methacrylonitrile (MAN) copolymer. The molar ratio of AMS and MAN in the grafted polymer was determined using ¹³C- CP/MAS NMR spectroscopy and compared with the molar ratio determined with FTIR spectroscopy. The distribution of the components across the thickness of the grafted films was determined using confocal Raman microscopy. The validation of the confocal Raman microscopy was performed with FEP films grafted with MAN only, where pronounced grafting fronts were observed. The local degree of grafting for AMS/MAN co-grafted FEP films was calculated for each sample based on the intensity profiles, taking the mass of the grafted polymer and its molar ratio into account. The grafting of the AMS/MAN co-grafted films was found to be homogeneous over the thickness, even in case of small amounts of the copolymer (15 mass%). The homogeneity of the grafting across the film thickness is a prerequisite to obtain sufficient proton conductivity after sulfonation of the radiation-grafted films. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Morphology of wax modified coil coatings without and with pigmentation

Wax modified coil coatings without and with white pigmentation were analyzed by optical light microscopy, Raman spectroscopy and Atomic Force Microscopy. By optical light and Raman microscopy dispersed spherical wax domains in the micrometer range were ascertained on the surface and within the bulk of the coatings. By the combined Atomic Force microscopical and Raman spectroscopical approach it was possible to detect resin and pigment residues in the lubricant features with diameters above 2 μm. Vice versa indications for wax inclusions with diameters below 2 μm in the matrix and rather thin wax layers expanding from the spherical lubricant domains onto the surface were obtained. As to the wax distribution evaluated by optical light and Raman microscopy significant differences between clearcoat and white topcoat were detected. The non-pigmented coating exhibited smaller wax features and an inhomogeneous distribution with less wax on the surface.     

The depth profiling of TiO2 pigmented coil coatings using step scan phase modulation photoacoustic FTIR

The depth distribution of a TiO₂ pigment within the polyurethane (PU) coil coatings is investigated using step scan phase modulation photoacoustic (SS-PM-PA) FTIR. Coil coatings with different pigment contents were prepared and the modulation frequency (MF) of the SS-PM-PA FTIR varied to record the depth distribution of the pigment within the coating. The TiO₂ pigment was shown to contribute significantly to the SS-PM-PA FTIR signal. A TiO₂ aggregated region within the topcoat is found close to the topcoat-primer interface and further away from the topcoat surface. A deeper TiO₂ aggregated region can be identified when pigment content is relatively low. The SS-PM-PA FTIR signal shows a considerable contribution from the primer originated signal, provided the TiO₂ pigment content is sufficiently high and the modulation frequency applied is relatively low. SEM cross-section imaging results show a strong correlation of the TiO₂ depth distribution with SS-PM-PA FTIR results, which confirms the applicability of the SS-PM-PA FTIR technique to the depth profiling study of TiO₂ pigmented coil coatings.     

Protein adsorption dynamics in cation-exchange chromatography quantitatively studied by confocal laser scanning microscopy

A self-contained research system based on the technique of confocal laser scanning microscopy (CLSM) was put up to quantitatively analyze the dynamics of protein adsorption to porous cation exchanger by mathematical modeling. Bovine serum albumin adsorption to the cation exchanger SP Sepharose FF was performed by batch adsorption and micro-flow cell in which protein concentration in single absorbent was visualized by CLSM. The effects of ionic strength and the protein concentration in liquid phase (50 mmol/L acetate buffer, pH 5.0) on the adsorption dynamics were examined. The intraparticle concentration profile data experimentally obtained from CLSM were quantitatively analyzed by three diffusive mass transfer models (i.e., pore diffusion, surface diffusion and Maxwell-Stefan models (MSM)) in virtue of the attenuation equation for the CLSM visualization developed earlier. The nuance between the model simulations and experimental results of the developing protein distribution in a single adsorbent particle could thus be found out. Without salt addition to the buffer, the adsorption isotherm was strongly favorable, and the pore diffusion model (PorDM) and MSM gave similarly good simulations of the experiments, whereas the surface diffusion model was unreasonable in the model presumption. Moreover, it was observed that the experimentally obtained adsorption front was relative flatter as compared with the calculated results from the PorDM, which implied the possible existence of surface diffusion. With increasing salt concentration, the simulations became to deviate from the experiments. Especially, when the salt concentration approached 50 mmol/L, all the three mass transfer models could hardly give good simulation of the experiment. This was considered due to the difference in adsorption behavior between the fluorescence labeled and unlabeled proteins therein.     

Characterization of coating systems by scanning electrochemical microscopy: Surface topology and blistering

Operation of the scanning electrochemical microscope used in feedback mode over a coated metal allows changes in the state of the coating surface to be monitored during immersion in aqueous electrolytes. This paper reports changes in the coating induced by specific anions in the electrolyte in situ during immersion. Significant surface roughening is observed for immersion times shorter than 1 day when the electrolyte contains chloride ions. This effect is also observed when the oxygen dissolved in the electrolytic phase is employed as redox mediator for SECM imaging. The coated system exposed to chloride-free electrolytes containing sulphate or nitrate maintains a featureless topography within the same time scale. The observed features are due to the nucleation and growth of blisters at the metal/coating interface induced by chloride ions in the environment. The implication is that ionic migration occurs simultaneously with the absorption of water by the coating already from the beginning of exposure to the aqueous environment. The unique role of chloride ions compared with sulphate or nitrate ions towards coating performance has been established at a very early stage following immersion of the sample.