Anal. Chem. 2001, 73, 2701-2704
Coatings Dennis Anderson
Crompton Corporation, P.O. Box 646, 5777 Frantz Road, Dublin, Ohio 43017 This review covers analytical techniques applicable to the examination of coatings and coatings’ raw materials, substrates upon which coatings are placed, etc., since the last review in 1999 (1). The emphasis in this review has changed from previous years, in that only the most pertinent and inovative references are cited. Readers are encouraged to survey the entire review, since the analysis of specific paints, coatings, or related materials may be found throughout. Measurement of trace elements in coatings continues, with the determination of copper content in antifouling paints (2), the examination of lead via chelatometry (3), and the certification of alkyd resin reference materials for arsenic, barium, cadmium, chromium, lead, antimony, and selenium content (4). Other interesting studies of note included the determination of formaldehyde in food packaging coatings (5), the evaluation of UV absorber effectiveness in automotive topcoats (6), the use of excimer fluorescence for determining the curing of release coatings (7), and a reexamination of the concept of “critical pigment concentration” (8). Gas chromatography remains a powerful tool for the characterization of volatile species in coatings. Nonvolatile species in coatings can also be examined following degradation or derivitization to enhance volatility. The measurement of residual toluene diisocyanate in polyurethane coatings received special attention during this period (9). Pyrolysis techniques were the subject of an extensive review (10) as well as specific studies of latex system microstructure (11) , coating colors for paper (12), and identification of binders used in Texas rock paintings (13). Coupling a mass spectrometer to the gas chromatograph improves analysis capability considerably. Studies of note during this period include the analysis of paints in general (14) and varnishes on coated wires (15) in particular. The nonvolatile nature of many coatings components makes high-performance liquid chromatography useful in many cases. High-performance gel permeation (size exclusion) chromatography was used for the characterization of a styrene-modified alkyd paint system (16). An excellent review was also published relative to liquid chromatographic techniques to determine booster biocides used in antifouling paints (17). The use of capillary electrophoresis was also the analytical method of choice in three useful articles dealing with the determination of various species present in electrodeposition coatings (18-20). Infrared spectroscopy continues to provide extensive information concerning coating composition. During this period, two general studies were reported (21, 22), as were specific studies examining yellow (23) and red (24) organic pigments in automotive top coats. Attenuated total reflectance measurements were used to advantage in the study of polyester/melamine resin degradation (25) and in-depth profiling of butyl acrylate/poly10.1021/ac0103571 CCC: $20.00 Published on Web 04/11/2001
© 2001 American Chemical Society
urethane latexes (26). The use of infrared spectroscopy for the prediction of coating lifetime was studied in depth during this period (27), as was the identification of red pigments (28) and the spatial distribution of species on matte finish coating surfaces (29) using Fourier transform Raman spectroscopy. Nuclear magnetic resonance spectroscopic investigations of interest include the examination of acrylic polyurethane hybrid dispersions (30) and isocyanate-free waterborne acrylic coatings (31). Nuclear magnetic resonance spectroscopy was also used to study the degradation of clear automotive coatings in work highlighting coating composition, mechanism of solidification, and cured coating structure (32, 33). In addition, a unique study employing 15N-enriched hindered amine light stabilizers permitted the following of nitroxide formation in acrylic/melamine and acrylic/urethane coatings during accelerated weathering (34). The use of thermoanalytical measurements continues as a mechanism to evaluate the physical properties of coatings. Recent studies of interest include the examination of binder content in latex paint films (35) and the determination of conversion at the gel point of powder paints (36). Dynamic mechanical analysis was also used to advantage for the characterization of polyurethane coatings (37) and alkyd ceramers (38). Methodology for the preparation of specimens for microscopic examination led to a reexamination of microtomed sections (39) and cross sections for film thickness determination (40). Light microscopy was one of a number of techniques used for the analysis of coating failures (41). Scanning probe microscopy was useful in the study of mar resistance (42), while atomic force microscopy was employed in the evaluation of pigment dispersion and film formation with latex systems (43). The unique capabilities of scanning acoustic microscopy were helpful in two studies examining coating degradation (44, 45). Volatile organic compound (VOC) emissions from latex paints continue to be studied (46, 47) as well as the determination of VOCs in polymer matrixes by microdistillation (48). Other unique studies reported during this period include curing emissions from conversion varnishes (49) and volatiles from coated wood-based products and furniture (50). Electron spin resonance studies continue for the prediction of coating durability in general (51), the production of photoinduced radicals in polyurethanes (52), and the longevity of hindered amine light stabilizers in automotive coatings (53). Surface leveling in thermosetting powder coatings was examined in detail (54), as was the aspect ratio of extender pigments (55), the surface tension of leveling additives in powder coatings (56), and the development and evaluation of improved scratch adhesion procedures (57-59). Measurement of rheological properties for waterborne automotive coatings received special attention (60) as did the examination Analytical Chemistry, Vol. 73, No. 12, June 15, 2001 2701
of dynamic mechanical properties for a novel polyurethane coating used in military applications (61). Two quantitative studies of wet adhesion for low-VOC coatings on wood were published (62, 63) during this period. Physical measurements continue to provide useful information: including an analysis of the notched film adhesion test (64), a physical analysis of gravelometry (65), the use of scribe techniques to assess painted galvanized steel underfilm corrosion (66), and methodology to evaluate scratch durability for clear automotive coatings (67). Methodology for the estimation of coating service lifetime included an examination of conventional and reliability-based procedures (68), as well as the effects of weathering on the physical properties of clearcoats (69) and global exposure models for automotive coating photooxidation (70). Failure models for predicting weatherability in automotive coatings (71) were presented as was the commercial introduction of the “Video Image Enhanced Evaluation of Weathering” system (72). In addition, studies concerning chemical depth profiling of automotive coating systems (73), the effects of UV radiation, thermal shock, and weathering in epoxy coatings (74), and the use of time-of-flight secondary Ion mass spectrometry in the investigation of coatings defects (75) were published. The use of electrical measurements included numerous studies incorporating electrochemical impedance spectroscopy (EIS) (7678). EIS was found to be particularly useful in monitoring the barrier properties of several coatings systems (79), the corrosion protection ability of epoxy and polyester clearcoats (80), and the location of microblisters on painted surfaces (81). EIS-based corrosion data were compared with other mechanisms to study corrosion protection: including salt spray testing (82), water diffusion measurements (83), and natural plus artificial weathering (84). Cathodic delamination and filiform corrosion were compared using EIS (85), as were a number of regimens for marine corrosion control (86). Electrochemical noise analysis of polyurethane-coated steel subjected to erosion-corrosion indicated that nonporous coatings remained fully protective over extended time periods (87). Changes in coating capacitance (88) and dielectric constant (89) were also found to be predictive of weathering properties. Finally, the deterioration of coatings under cathodic protection was primarily related to the level of delamination and appearance of blisters in the exposed coatings (90). ACKNOWLEDGMENT
Many thanks to Chemical Abstracts Service, which provided the literature search, and to Crompton Corporation for permission to publish this work. Dennis G. Anderson is Manager of Analytical Sciences for the Industrial Specialties Business Unit of Witco (a Crompton Business). Prior to assuming this responsibility 10 years ago, Mr. Anderson was employed for 24 years with DeSoto, Inc., where he was involved with the analysis and characterization of polymers and coatings using chemical and instrumental techniques. He received B.S. and M.S. degrees in chemistry from Roosevelt University where he was also a faculty member. He has authored 35 publications dealing with the analysis of polymers and coatings and was coauthor of An Infrared Spectroscopy Atlas for the Coatings Industry. He is also the recipient of three Roon Foundation Awards for distinguished service to the coatings industry.
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