Detection of different chemical binders in coatings using hyperspectral imaging

Epoxy (EPIKOTE Resin 828EL, Hexion) with epoxy equivalent weight (EEW) of 186–190 g/equivalent, epoxy Novolac (D.E.N.\(^{\mathrm{TM}}\) 425, Olin Epoxy) with epoxy equivalent weight (EEW) of 169–175 g/equivalent, Polysiloxane resin (PS) (SILRES® REN 50, Wacker Silicones), Hydroxy functional acrylic (SYNOCURE-854-BA80, Arkema), Hardtop XP component B (Jotun AS), and N-(2-aminoethyl)-3-aminopropylmethyl -dimethoxysilane (Dynasylan 1401, Evonik) were used in this study to prepare the clearcoats.

The hardener used to cure stoichiometrically the epoxy and novolac epoxy was the amine curing agent commercially available under the name of Jotamastic 90 Standard Comp B containing 10–16% xylene as the solvent.

Mixing ratio used for the stoichiometric curing of epoxy with the amine curing agent was 100 g of epoxy : 52.54 g of the curing agent. Mixing ratio used for the stoichiometric curing of epoxy Novolac with the amine curing agent was 100 g of epoxy Novolac : 57.30 g of the curing agent.

One epoxy-amine clearcoat was prepared by over-curing the epoxy with curing agent in 30% excess of the stoichiometric amount.

PS clearcoat was made by mixing the PS with Dynasylan 1401.

Polyurethane (PU) clearcoat was made by reacting the hydroxy functional acrylic resin with the poly(isocyanate) curing agent (Hardtop XP component B).

All clearcoats were prepared by applying the respective films on \(7 \times 15\,\hbox {cm}^{2}\) sandblasted steel (Ca 2.5 cleanliness) with an applicator. Wet film thickness (WFT) for all films was 300 microns.

In formulations with extenders, 15 weight percent of an extender (commercial grade of feldspar, calcium carbonate, alumina, talc) was dispersed in the epoxy using a speed mixer. This epoxy part containing the extender was cured stoichiometrically with the amine curing agent (i.e., Jotamastic 90 Standard Comp B).

After application, all samples were cured at room temperature for 7 days before scanning with the HSI camera.

Table 1 shows a list of all samples used in this study. Each test panel in the table has an identification, a short description, and an aspect that we want to detect. There are 18 different test samples in all. Samples with uncured resin and curing agents were also analyzed with the HSI camera to obtain the reference spectra for the unreacted chemicals. The spectra of cured systems were then compared with the spectra of the unreacted chemicals to confirm the occurrence of chemical reaction between the reactive species. These panels consist of different cured binders (BND1-4), epoxy binders including different extenders (EXM1-4), and epoxy binder with different curing degrees (EXC1-2). In addition, 8 reference panels are also included in which correspond to the unreacted chemicals used in this work (BND5-12).

The panels have been scanned by HySpex SWIR-384 hyperspectral camera at NEO lab (Norsk Elektro Optikk AS, Norway) as shown in Fig. 2. This is a “push-broom” line scanning short-wave infrared camera which spans wavelengths from 970 nm to 2500 nm in NIR range of electromagnetic spectrum. It collects photons in 288 spectral bands with a spectral sampling of 5.3 nm. The images were acquired using a broadband halogen lamp to illuminate the test panels from above, while the camera captured the signal scattered by the surface. The camera used a close-up lens to cover a field of view (FOV) of 290 mm from a 100 cm working distance and with a spatial resolution of 750 µm. The samples were scanned using a translation stage, moving perpendicularly to the linear FOV of the camera with a steady speed proportional to the exposure time. A Zenith Lite reflectance target (SphereOptics GmbH, Germany) was also imaged together with the test panels. The target is a calibrated, PTFE-based diffuse reflectance panel with 50% reflectivity (nominal). The spectrum recorded from the target was used to correct for variations in the spatial distribution of the illumination source on the test panels and to transform the spectral raw data from the camera to reflectance.

The reflectance data are used for the analyses in this paper, which is a property of observed material based on occurring absorption features. Reflectance is the ratio of the intensity of light leaving a target to the intensity of light striking the target.

Many different analysis techniques can be used to unravel the mixed spectral signal and produce abundance maps of observed features. It is possible to model reflectance spectra of a solid surface as a series of deep dips due to absorption features, superimposed on a background continuum.²⁹ These absorption features are characterized by their wavelength position, band width, depth of absorption feature, and shape symmetry. The wavelengths at which absorption features are detected are important inputs for analyses, since they designate the chemical composition types present at the observed scene.^30,31 In the following, the methods employed for analyses in this study are described.

Figure 6a shows the mean spectra of the uncured epoxy novolac (red line, BND8 panel in Table 1), uncured amine curing agent (green line, BND9 panel), and the cured epoxy novolac-amine system (blue line, BND2 panel). As discussed in the previous section, the peak at 2207 nm is attributable to the epoxy ring in the uncured epoxy novolac, whereas the peaks at 1542 nm and 2032 nm in the spectrum of unreacted curing agent can be attributed to the amine groups in it. The other peaks observed in the spectrum of epoxy novolac are similar to those characterized by Yi-Hui et al.³⁷ using a laboratory FT-NIR spectrometer. After the reaction, these groups are expected to disappear. Therefore, in the mean spectrum of the cured epoxy novolac-amine, there are no peaks attributable to these functional groups. In addition, the spectrum of cured epoxy novolac-amine shows a stronger peak around 1433 nm which can be attributed to the increased number of hydroxyl groups formed due to the chemical reaction between the epoxy rings and the amine groups of the curing agent.^13,14

k-means clustering was performed on the cured epoxy novolac panel (BND2 panel), as depicted in Figs. 6c and 6d, to determine the uniformity of pixels throughout panel. The defined parameters for analysis were 5 clusters and 30 iterations; however, only 3 clusters were achieved after 30 iterations. An RGB image of the panel is shown in Fig. 6b, and the spatial distribution of pixels belonging to each cluster is demonstrated in Fig. 6d by different colors. The spectra of each cluster center are shown in Fig. 6c. The spectrum of each cluster center has the same color and number as those in the spatial overview of pixels. The percentages of pixels in each cluster are mentioned in the legend.

The spectra of the cluster centers show that the properties of pixels over the whole panel are homogeneous and suggest that the mean spectrum represents the whole panel well. The spectra of cluster centers are similar to each other which is an evident for identical chemical structures over all of the panel. These results confirm that NIR-HSI is a powerful tool for understanding the spatial differences in the applied coatings and relating them to the observed differences in the mechanical properties of the applied coatings.

Figure 7a shows the mean spectra of a commercial methyl phenyl group containing silicone resin solution in xylene and n-butanol (red line, BND12 panel in Table 1), a commercial organosilane (with a chemical name of n-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane) (green line, BND11 panel) and the crosslinked silicone resin containing the organosilane (blue line, BND3 panel).

The peaks in the spectrum of liquid silicone resin at 1144, 1182, 1400, 1678, 1744, and 1776 nm are attributed to the vibrational modes of –\(\hbox {CH}_3\) groups present in a silicone resin. Phenyl group’s presence in the PS can be confirmed by the peak at 2174 nm whereas the SiH group can be confirmed by the peak at 2332 nm.³⁸ As previously mentioned, the peaks at 1547 nm and 2022 nm present in the spectrum of organosilane are attributable to the amine groups in it.^39,40 After physical drying, the peaks attributable to the amine groups are still present in the spectrum of cured PS (i.e., the blue line in Fig. 7) indicating that the amine groups of the organosilane did not react with other moieties of the PS. This was expected because the silicone resin used in this work did not have functional groups which could have reacted with the amine functional groups of the used organosilane at room temperature. Based upon these results, it can be concluded that the NIR-HSI camera can detect as supplied and physically dried silicone resins, organofunctional silanes with an acceptable accuracy.

Figures 7c and 7f show the spatial heterogeneity in the PS sample based upon two absorptions, i.e., the 1547 nm absorption related to the amine group of organosilane and 1678 nm absorption related to the –\(\hbox {CH}_3\) groups of PS. It is evident that by using different absorption peaks, one can explore different defects in a coating. For example, by using 1678 nm absorption, the spatial distribution of craters in the PS sample (indicated by the dark spots in Fig. 7f) can be detected more clearly than by using the 1547 nm absorption.

k-means clustering analysis of PS sample (BND3 panel) is depicted in Figs. 7d and 7e. The spectra of the cluster centers in Fig. 7d are very similar to each other, but Fig. 7e shows that there is spatial heterogeneity in the chemical composition of the applied film which may be attributed to the reasons discussed above, such as nonideal mixing of the components, slow diffusion of the mixture components, and faster evaporation of solvents (liquid silicone resin used was dissolved in xylene/n-butanol mixture).

Polyurethane (PU) (BND4 panel in Table 1) was formed by reacting a commercial hydroxy functional acrylic resin (BND7 panel) with a poly(isocyanate) curing agent (BND10 panel). Figure 8a shows the mean spectra of the uncured hydroxy functional acrylic (red line), poly(isocyanate) (green line) and the cured PU (blue line). In the spectrum of commercial hydroxy functional acrylic resin, the peaks present at 1140, 1417 and 2043 nm, 1913 nm can be attributed to the –CH groups, hydroxyl (OH) groups and carbonyl (–C=O) groups, respectively.⁴¹ In the spectrum of poly(isocyanate) curing agent, the peaks at 1482 nm and 2049 nm can be attributed to the overtone of -NH and stretch overtone of the hydrogen bonded carbonyl group present in the urea, respectively, and the peak at 1913 nm can be attributed to the carbonyl group of the isocyanurate present in the curing agent.^16,42,43 Hexamethylene diisocyanate (HDI)-based poly(isocyanate) curing agents are composed of several oligomers depending upon the polymerization mechanism and isocyanurate group is present in most of these oligomers. Detailed discussion about the structure of various oligomers present in HDI-based poly(isocyanate) curing agents can be found elsewhere.⁴⁴ Some researchers have attributed the broad peak around 1920 nm to the overlapping bands resulting from the carbonyl groups and the adsorbed water.⁴⁵ The shoulder at 2146 nm has also been attributed by different researchers to the isocyanate group.^16,46 It is important to mention that urea in the curing agent was probably produced by the well-known side reactions.⁴⁷ After reaction, the peaks confirming the formation of urethane group (i.e., the repeating unit in a PU polymer) can be seen at 2027 and 2179 nm.^16,43

Figures 8c and 8f show the spatial variation of the isocyanate group in the cured PU sample obtained by using the absorptions at 1482 and 1913 nm. Since both the absorptions are related to the isocyanate group, it appears that the distribution of the curing agent is uniform over the whole sample. However, the presence of residual isocyanate absorption peaks indicate that the reaction has not completed. The two bright spots on the left side of the sample indicate a defect during sample preparation.

k-means clustering analysis of PU sample (BND4 panel) is shown in Fig. 8d and 8e. The spectra of the cluster centers in 8d are very similar to each other except the small cluster 1. This cluster is also detected in Figs. 8c and 8f demonstrating the presence of two bright spots on the left side of the sample corresponding to a defect during sample preparation. The spectrum of the cluster 1 shows the peaks at 1482 nm and 1913 nm attributed to the residual isocyanate groups and it can be seen that the unreacted curing agent is accumulated at two places mainly. The reason for this accumulation of the unreacted curing agent is unknown but most probably it is related to a mixing issue during sample preparation or film application. With the naked eye, it would have not been possible to identify such a spot concentrated with the residual curing agent (compare RGB image shown in Fig. 8b with 8e and 8f). In general, it is evident that by employing k-means clustering spatial heterogeneity of the model, PU coating becomes visible.

A variety of extenders are used to enhance the properties (e.g., mechanical and/or anticorrosive) and reduce the production cost of the commercial organic coatings. Therefore, to study more realistic coatings, some model epoxy-amine coatings were prepared with different extenders. The extenders used in this work included \(\hbox {CaCO}_{{3}}\), talc, feldspar, and alumina. Note that, only one extender was dispersed at a time in the epoxy followed by mixing with the curing agent just before the application with an applicator. In all formulations, extender content was kept constant at 15 wt%. All applied films were cured for 7 days at room temperature before scanning with the HSI camera.

Figure 9 shows the mean spectra of cured model coatings with and without extenders. It can be seen that the mean spectra of the model epoxy-amine coatings with or without different extenders are very similar and the observed peaks are attributable to the organic part of the coatings i.e., the (partially) cured epoxy-amine network. The peaks around 1390 nm and 1908 nm are attributed to the hydrogen bonded –OH groups and the interactions between the adsorbed water and –OH groups, respectively. According to Mijovic et al.⁴⁸ peak around 1908 nm can be used to estimate the moisture content in different materials. Krauklis et al.5 have shown that for monitoring the content of adsorbed water in cured epoxy-amine samples the peak around 1908 nm is more reliable. The sharp peak at 1389 nm in the spectrum of epoxy-amine coating filled with talc can probably be attributed to the combined effect of structural -OH groups present in talc and –OH groups produced during epoxy-amine cure reaction. Figure 9 also shows that the formulation with talc probably had the highest amount of adsorbed water (comparing the peak height at 1908 nm) which was in agreement with supplier data about the moisture content of different extenders used here (i.e., the talc had the highest moisture content of around 0.5% among different extenders used in this work). The reference spectra of the extenders were not obtained in this work, and the aim here is not to study the factors influencing the peak heights of hydroxyl groups in a NIR spectrum of filled coatings. It is important to mention that the concentration of different extenders used in the commercial formulations can be significantly different from that used in this work (i.e., 15 wt%). Furthermore, commercial coatings mostly use combinations of different extenders. Therefore, the results shown in this work may not represent typical more complex coatings. Future work could investigate how the concentration of different extenders affects the spectra and whether NIR-HSI cameras can be used to detect the differences between various extenders (i.e., chemical composition, particle sizes, and moisture content) used in the organic coatings.

To show the potential of using NIR-HSI combined with k-means clustering in distinguishing chemically different binders, all four panels coated with the cured binders (i.e., epoxy, phenolic, PS, and PU) were considered within one frame as shown in Fig. 10a for the analysis and the k-means clustering was applied on them as an assembly (Fig. 10b). The spectra of the cluster centers are shown in Fig. 10c. Four cluster centers were used because there are four different binders in the fused image. Each of the clusters found by k-means was related to the binder which constituted the majority of its pixels.

The confusion matrix of this analysis which shows the accuracy of distinction between different binders is shown in Fig. 10d. Each row of the confusion matrix represents the classification percentages of pixels for each panel, whereas each column of the confusion matrix shows the percentages of pixels classified as specific binder in each panel. It is evident that the phenolic and PS binders were distinguished with more than 90% accuracy, while that of epoxy and PU was around 70% which may be attributed to the very similar spectra of cured epoxy and PU used in this work (blue spectra shown in Figs. 4a and 8a, respectively). These results confirm the capability of NIR-HSI in distinguishing chemically different (cured) binders with an acceptable level of accuracy. However, it is important to mention that more detailed work is required in this regard as the samples used in this work were model coatings which do not represent either the actual commercial coatings with complex formulations or the environments in which inspection would occur. This example of k-means clustering demonstrates the potential of hyperspectral data for distinguishing between coatings. Since k-means, even though it is an extremely simple form of clustering, achieves decent classification accuracy of the 4 clearcoats, more sophisticated clustering or classification algorithms will likely achieve even higher accuracy.