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Application of Spectroscopy andMicroscopy Techniques in SurfaceCoatings Evaluation: A ReviewSaeed Farrokhpay aa Julius Kruttschnitt Mineral Research Centre (JKMRC), TheUniversity of Queensland, Indooroopilly, Queensland, Australia

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Applied Spectroscopy Reviews, 47:233–243, 2012Copyright © Taylor & Francis Group, LLCISSN: 0570-4928 print / 1520-569X onlineDOI: 10.1080/05704928.2011.639424

Application of Spectroscopy and MicroscopyTechniques in Surface Coatings Evaluation:

A Review

SAEED FARROKHPAY

Julius Kruttschnitt Mineral Research Centre (JKMRC), The University ofQueensland, Indooroopilly, Queensland, Australia

Abstract: This article presents a review of the published articles related to the novelapplication of spectroscopy and microscopy methods in paint and coatings quality eval-uation. Traditional and simple techniques have been used in paint and coating industryfor many years and proven to be effective. However, the paint and coating industryfaces new formulations with nontraditional applications. Therefore, the industry needsto adjust itself with the current sophisticated production and testing methods. Thereare a number of modern microscopy and spectroscopy techniques that can be utilizedin the paint and coating industry for a better understanding of the product qualityand/or application performance. This, in particular, is highly applicable in modernand nontraditional applications such as nanotechnology and smart coatings. Thoughimportance of spectroscopy and microscopy methods is being increasingly recognizedin the industry, there is no current comprehensive review available to highlight the needfor novel application of these techniques in surface coatings evaluations.

Keywords: Surface coatings, paint, spectroscopy

Introduction

The paint manufacturing process involves many steps of quality evaluation. The raw materialand the production process undergo several tests, and the quality of the final product(properties such as viscosity, fineness, and density) is often checked. The product is thenapplied to a surface and parameters such as drying time, color and gloss, hardness, adhesion,and its resistance against different conditions is also evaluated.

The paint and coatings industry is growing day by day around the globe. The currenttrends and challenges in paints and coatings technology have been recently reviewed (1).Traditional and simple techniques have been used in paint and coatings evaluation formany years. Although these techniques have been effective, today the surface coatingsindustry faces new and nontraditional applications. For example, using pigments as smallas 20–30 nm (nanotechnology), coatings that react to external stimuli in an intelligent way(smart coatings), and nontoxic protective pigments (environmentally friendly coatings).Therefore, the industry needs to adjust itself with the current sophisticated production and

Address correspondence to Saeed Farrokhpay, Julius Kruttschnitt Mineral Research Centre(JKMRC), The University of Queensland, 40 Isles Road, Indooroopilly, QLD 4068, Australia. E-mail:s.farrokhpay@uq.edu.au

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testing methods. In this article, such techniques will be reviewed, and their applicationsin the surface coatings industry will be highlighted. The importance of the application ofmicroscopy and spectroscopy methods in paint and coatings evaluation is being increasinglyrecognized. A comprehensive review of the application of spectroscopy in the coatingsindustry was published in 1975 (2). Although the science and technology has been rapidlygrowing since then, there is no current review available. It should be noted that the orderin which the various techniques are discussed in this article is a matter of convenience anddoes not necessarily relate to their importance.

Microscopy Techniques

Microscopy is a technical term for using a microscope to view objects that cannot beseen with the naked eye. There are three well-known branches of microscopy: optical,electron, and scanning microscopy. In general, paint films are opaque; therefore, microscopymethods can only be used for surface characterization. However, there are some microscopytechniques that can be used for paint film depth analysis, as will be explained.

Atomic force microscope (AFM) is a technique for measuring surface topography,and it is an important tool in colloid and interface analysis (3, 4). The vertical deflectionof the cantilever is measured by a detection apparatus indicating the local sample heightand produces a surface topographic image. The phase shift between the driving force forthe cantilever vibration and the optically detected motion of the cantilever is recorded inphase imaging (5). Two basic imaging techniques are tapping (noncontact) and contactmode (Figure 1 (6)). In the former, the surface–tip interactions are attractive, and in thelatter, topographic images are derived from repulsive forces. Both methods have beenused to image dry paint films (7, 8). Although Biggs and his coworkers (8) have found aclearer representation of the pigment–binder composite structure in exterior paints usingthe tapping mode, it is generally agreed that the same detail of surface topography can beobtained by both methods. The photooxidation of polystyrene and the changes in surfacemorphology of coating systems have also been investigated using AFM (9, 10).

Figure 1. Two basic AFM imaging techniques: tapping mode (left) and contact mode (right).

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AFM in combination with laser confocal scanning microscopy (LSCM) can cover awide length-scale range, from micrometers to millimeters. This combination is a powerfultechnique for quantifying topographic changes of polymeric coatings resulting from surfaceroughening, pitting, and cracking (11–14). AFM and LSCM have also been used to analyzesurface topography changes during outdoor exposure (15, 16). AFM and LSCM have beenused to measure morphological changes of the surface of pigmented coatings during UVexposure and it has been shown that both pigment dispersion and thickness of the clear layerplayed a role in the resulting topography. The increase in surface roughness measured byAFM resulted in a significant decrease in gloss (7, 17). LSCM has shown that gloss changesin pigmented coatings formulated with dispersants is mainly dominated by an increase insurface roughness (15).

Scanning electron microscopy (SEM) can be used to examine paint surface defects,such as pits and cracks, and loss of adhesion. For example, SEM with energy-dispersiveX-ray spectroscopy (EDX) has been used to identify the reasons for paint adhesion failuresfrom a steel structure caused by welding splatter (18, 19). Chalking, which is a commonproblem with exterior paints, has also been investigated using SEM (20–22). Chalkingoccurs as a result of oxidation of the surface layer of polymers containing white pigmentssuch as anatase TiO2. SEM has been also used to obtain high-resolution images to showthe extent of latex deformation resulting from particle–substrate adhesion (23). Pigmentdispersion characterization in coatings has been performed using SEM (15). SEM imagesof a typical paint film are presented in Figure 2 (24).

Transmission electron microscopy (TEM) is a microscopy technique in which a beamof electrons is transmitted through an ultra-thin specimen and interact with the specimen.An image is formed from the interaction of the electrons transmitted through the specimenand can be detected by a sensor. TEM is a popular analysis method in a range of scientificfields, including surface science, providing information on the particle shape, size, and

Figure 2. SEM image of a paint film (dimensions 12.8 × 8.9 µm).

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Figure 3. TEM images showing pigment particle size and morphology (left) and dried paint filmlayer thickness (right).

surface topography. Farrokhpay (6, 25), Morris et al. (26), and Farrokhpay et al. (27) haveused TEM to identify TiO2 pigment particle size and morphology and have successfullyshown a very thin inorganic coating (alumina type) layer present on the pigment surface(Figure 3). TEM is an ideal method to study pigment dispersion in dry paint films, becauseit allows viewing inside the film (7). The thickness of paint film has also been measuredusing TEM (Figure 3) (7). TEM and SEM both have been used to determine the contactdiameters of various sizes of latex (28, 29) and dispersion of nano alumina and silicaparticles in automotive polyurethane coatings (30).

In conventional SEM, a high vacuum is required; therefore, it is not capable of analyz-ing a wet sample. Furthermore, in order to obtain clear images of nonconducting samples,the sample must be coated in carbon, gold, or platinum. Environmental or universal scan-ning electron microscopy can be now used to avoid these disadvantages. This allows theobservation of wet samples without the need for dehydration and coating. These instrumentshave been used to study emulsion as well as drying paint films (31).

It should be noted that the microscopy techniques are, in fact, complementary toeach other. For example, AFM provides a picture of the surface only, and TEM providesthin depth (cross-section) details. Therefore, one can hardly expect complete results usinga single technique. It is also worth mentioning that though the microscopy techniquesdescribed thus far are useful because they provide a direct picture of the actual film, theycan only target a very small area of the paint film (32).

Image Analysis for Quantitative Data

Although microscopy techniques provide comparative and qualitative results, a quantitativeunderstanding of, for example, the degree of pigment dispersion, cannot be made throughvisual observations of electron micrographs. This is partially due to the tendency of humaneye to see nonexisting patterns and the inability to intuitively judge whether a distributionof objects is truly random, ordered, or artificially disordered (24). A technique for quan-tifying the degree of TiO2 dispersion in paint films has been developed based on electronmicroscopy and image analysis (24). Results obtained using this technique also provide anupper limit for improvement of pigment dispersion. This technique includes imaging thepaint film with an electron microscope (such as SEM), determining the coordinates of theparticle centers, and analyzing the resulting coordinates using algorithms that divide theimage into a large number of subareas (Figure 4). The particle attributes in each subarea are

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Figure 4. Dispersion analysis via comparison of image subareas using SEM images of paint film.The degree of variability between subareas increases from left to right (Color figure available online).

then compared to those in other subareas. A high degree of variability between subareasindicates poor dispersion and a low degree of variability shows that the particles are welldispersed. Farrokhpay et al. (7, 17) have analyzed TEM and AFM images using analySIS (acommercially available software from Olympus Company, Hamburg), to show the degreeof pigment dispersion and the presence of pigment aggregates in paint films. Image analysiscan also be used as a straightforward and reliable method for characterization of porosity(33). It should be noted that the main concern in applying image analysis technique is thatthey depend heavily on the quality of images. Therefore, sufficient contrast between thematrix and particles is often required to obtain meaningful data (34).

Spectroscopy Techniques

There is a key difference between spectroscopy and microscopy techniques. In spectroscopy,the data are usually in the form of a spectrum, which contains a series of points plotted alongtwo axes. On the other hand, in microscopy analysis, the information is assimilated by acomputer into a comprehensive image of the sample, as discussed in the previous section.

X-ray is being used widely for material structural analysis due to its strong transmissioncapability (35). Small-angle X-ray scattering (SAXS) is a nondestructive method that canbe applied to soft materials, such as liquids and suspensions (36–38). SAXS complementsmicroscopic techniques by providing statistically averaged information on the samplemorphology. Although SAXS is a powerful tool for structural analysis, it has rarely beenapplied in paint and coatings. The maximum measurable range of SAXS is usually in therange of hundreds of nanometers and does not cover the larger size region that is desirablein the surface coatings industry. Therefore, ultra-small-angle X-ray scattering (USAXS)has been specially designed for such micrometer-size applications. USAXS can measureup to several micrometers using monochromatized and parallel X-ray (32, 38–41) and canbe used to analyze paint and coating films. Previous ultra-small-angle neutron scattering(USANS) experiments have been performed on wet paint systems under varying shearrates (42). The result of this study (42) has shown that the degree of flocculation is shearrate dependent. USANS measurements cannot be used in real-time experiments due toits low signal, whereas USAXS has high scattering signals and is suitable for real-timemeasurements.

Raman spectroscopy (43) is a spectroscopic technique based on inelastic scatteringof monochromatic light. Raman spectroscopy has been widely used to study the pigmentspresent in both prehistoric and historic items (44). It has been used to characterize pigmentsused on prehistoric rock art (45, 46), Greek and Roman murals (47–50), medieval frescos(51), and painted pottery (52). Raman spectroscopy has been used to investigate blue andgreen pigments used on the wall paintings at the Maya site of Ek’Balam (53). Micro-Raman

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spectroscopy is an ideal nondestructive technique for studying the fine paint layers of thesehistoric samples. The individual particles within each paint layer can also be identified dueto the high spatial resolution of this technique (44). Laser-induced breakdown spectroscopy(LIBS) has also been used in combination with Raman microscopy to identify the pigmentsapplied in different types of painted art. This combination has led to a detailed characteriza-tion of the pigments used in old paintings (54). Raman spectroscopy is a suitable techniquefor in situ identification of synthetic organic pigments in complex binding media (55).

Electrochemical impedance spectroscopy (EIS) is a nondestructive method used toevaluate the performance of organic coating/metal systems (56, 57). EIS has been widelyused to investigate the corrosion performances of protective coatings, including nano coat-ings, applied onto mild steel substrates (58–63). EIS has also used to understand the degra-dation mechanism by studying defective areas in coating, as well as the electrochemicalperformance of zinc-rich paints in artificial seawater (64, 65).

It should be noted that EIS data are usually difficult to interpret, due to the fact thatthe results actually represent an average response for the entire surface, though coatingdegradation (such as blistering) generally occurs locally. Therefore, the reproducibilityof the impedance data is usually low and statistical data analysis is often necessary (66,67). Moreover, EIS does not provide any information about the failure site location or thedegradation mechanism. To overcome such limitations, new techniques that perform localmeasurements have been developed such as scanning vibrating electrode technique (SVET)(68–70), scanning Kelvin-probe (SKP) (71–73), localized electrochemical impedance spec-troscopy (LEIS) (74, 75), and scanning acoustic microscopy (SAM) (76–78). SAM hasbeen used to characterize delamination processes at the water-borne epoxy coating–steelinterface (76) as well as coating adhesion (79–82).

Another microelectrochemical technique that can be used to study corrosion of coatedmetals is scanning electrochemical microscopy (SECM) (83), which combines scanningprobe techniques with electrochemistry. This technique is applicable for both insulatingand conducting surfaces (84, 85) and can be used to quantitatively detect the reactants andproducts involved in corrosion reactions (84, 86, 87). SECM has been used to observeddamage to paint and coatings resulting from immersion in aqueous brine solutions(88). Indications of coating failure cannot be observed by conventional electrochemicaltechniques or visual observation. It has been reported that chloride increases coatingdegradation at a very early stage (85).

X-ray photo spectroscopy (XPS) and Fourier transform infrared spectroscopy in atten-uated total reflectance mode (FTIR-ATR) have been used to show that latex films containingdifferent surfactants behave differently during film maturation (89, 90). According to thesestudies, some surfactants do not migrate to the interfaces probably due to greater com-patibility with the copolymer system. Probe microscopy methods, in particular scanningelectrical potential microscopy (SEPM), have been also successfully used to produce thefirst electrical maps of polymer films and particles (91–93). It has been demonstrated thattransparent films from low-Tg latex contain electrically positive boundaries between par-ticles (94). ATR-FTIR is also commonly used to measure chemical changes of coatingsduring ultraviolet (UV) degradation (95, 96).

Surface defects in dry paint films are a major problem in the coatings industry.Although these defects are very small, they are detectable by the naked eye. Surface defectsare often caused by substances in the first few monolayers; therefore, a surface-sensitivetechnique is required to characterize them. Laser microprobe mass analysis (LAMMA) andtime-of-flight secondary ion mass spectroscopy (ToF-SIMS) are often used for this purpose(97). These two methods similarly characterize and identify paint film defects; however,whereas LAMMA mainly provides information on inorganic materials, ToF-SIMS is usedto characterize organic materials. In particular, ToF-SIMS is used to distinguish between

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different silicone oils based on the same monomers (97). Recently, a combination ofRaman and XRF spectroscopy has been used in forensic examinations of different kindsof trace evidence (98). Combinations of electrochemical techniques (EIS) and surfaceanalysis techniques (AFM, SEM, EDX) have been used to evaluate protective propertiesof paint films (99).

Another study has shown that LIBS is a suitable technique for detecting lead in paintat the hazard levels defined by federal agencies. Although, this technique offers a way toobtain unique information, its current costs limit its practical application (100).

Conclusions

There are a number of novel applications of spectroscopy and microscopy techniques inpaint and coating industry to obtain a better understanding of the product quality and/orapplication performance. These methods are highly applicable in modern and nontradi-tional applications such as nanotechnology and smart coatings. These techniques oftenrequire special sampling methods, highly trained operators, and high capital cost invest-ment. However, the surface coatings industry needs to apply these new techniques due tothe complexity of the current sophisticated production and testing methods.

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