Chapter 17
Depth Profiling of Automotive Coating Systems on the Micrometer Scale Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 21, 2016 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch017
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Karlis Adamsons , Lance Litty , Kathryn Lloyd , Katherine Stika , Dennis Swartzfager , Dennis Walls , and Barbara Wood 2
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Marshall R & D Laboratory, DuPont Company, Philadelphia, PA 19146 Central Research & Development, Corporate Center for Analytical Sciences, Experimental Station, Wilmington, D E 19898 2
Techniques for obtaining chemical composition and component distribution depth profiles for automotive coating systems have been identified, developed and applied. Primary reasons for this work were to determine general system composition, as well as component or additive distribution, as a function of locus (i.e., surfaces, interface/interphase regions, depth profiles) and exposure history (i.e., outdoor or accelerated exposures). Depth profiling studies have been applied to both freshly prepared systems, as well as samples subjected to outdoor and/or accelerated weathering. In-plane (or slab) microtomy is demonstrated as a powerful tool for preparing samples for depth resolved analysis using techniques which otherwise would not have direct depth specificity. (1) IR-microscopy using transmission mode analysis provides general chemical composition. (2) Solvent extraction followed by HPLC chromatography gives information on the extractable materials, such as UVA's, surfactants, and degradation products. Cross-section microtomy is used as another technique for preparing samples for depth profiling. These cross-sections are used in a variety of techniques, including Raman-microscopy, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), optical-microscopy, and transmission electron microscopy (TEM) analyses. (1) Raman microscopy provides chemical information with high spatial resolution. Raman imaging approaches can be used to map chemical heterogeneity at surfaces or cross sections. (2) ToF-SIMS analysis provides ion-imaging capability with isotopic sensitivity to establish chemical distribution and spacially resolved durability profiles. (3) Optical microscopy gives information on component distribution and coating layer thickness. (4) TEM analysis of very thin (< 1μ)multi-coat cross-sections, together with appropriate functional group or sub-
© 1999 A m e r i c a n C h e m i c a l Society
Bauer and Martin; Service Life Prediction of Organic Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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258 structure staining techniques, provides information on component distribution and coating layer thickness. INTRODUCTION Tools for effective surface/near-surface and interface/interphase characterization, as well as depth profiling, are critical for effective development of current and future automotive coating systems, including those targeted for original equipment manufacturer (OEM) and refinish markets *' . The detailed study of chemical composition, physical properties, mechanical performance, and appearance characteristics of freshly prepared and weathered (either outdoor or accelerated) coating systems has been our primary focus . A better understanding of the overall chemistry, including network heterogeneity, component stratification or segregation, additive migration, and/or formation of degradation products, is believed to be essential for the creation and service lifetime prediction of high performance
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 21, 2016 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch017
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products Current automotive coating systems are complex in design, being both multicomponent and multi-layered . For example, a typical automotive coating on a metal substrate usually has four layers - Electrocoat (E-coat), Primer (PR), Basecoat (BC), and Clearcoat (CC) - each designed with specific function. Each of these layers is a complex mixture of different components, designed to impart a specific (i.e., pigment, latex, microgel, mica and/or metallic flake) distribution, and binder chemistry. As a system, they are carefully designed to provide customers with a desired set of properties and acceptable durability under a range of weather exposure 3 8 9
conditions ' ' . Note, however, that the ultimate customer - the vehicle owner typically has only one major concern with regard to the coating on their vehicle: the appearance. The responsibility for providing the customer with what they want belongs to the supplier(s) of the different coatings and to the manufacturer applying them according to specifications. The challenge of living up to our responsibility is quite substantial; we must understand the individual components, as well as the integration of components in the architecture of the overall coating system. The enormity of the challenge has started to bring together the combined measurement technology resources of raw material suppliers, coating system developers, manufacturers, universities, and government/private laboratories ' . Work that is reported herein derives, in large part, from efforts that evolved from many of these relationships. 5
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There are many potential contributing factors in the failure of a coating system ' ' The common root and basic faults associated with the failure of coating systems have been previously detailed . The study of the effects of weathering exposure on the properties of coating systems is a very challenging problem, due to the number of contributing variables ' ' . The ultimate goal of a weathering study is to identify the changes that occur in a coating system that eventually lead to systemic failure 1 3
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and the environmental factors that contribute
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Bauer and Martin; Service Life Prediction of Organic Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
259 can be studied in a variety of ways, including natural weathering in a given climate or with the use of accelerated testing equipment which usually allows for precise control of a subset of desired exposure factors (i.e., temperature, relative humidity, and irradiation conditions). From these tests, we hope to develop protocols for predicting service lifetime . However, the task of reliably predicting service lifetimes is an enormous challenge that is not yet well understood. This approach assumes that the variability of material properties, material processing, design, and application consideration is minimal, relative to the impact of environmental factors Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 21, 2016 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch017
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Understanding the chemical and morphological changes that accompany weathering could play an important role in understanding the relationship between weathering exposure and predicting, and ultimately improving, service lifetime. We currently are examining this approach to understand the complex interrelationships inherent to these complex problems. Some of these studies highlight the utility of multitechnique approaches in monitoring changes in binder chemistry as a function of outdoor or accelerated exposure conditions. Other studies demonstrate the utility to obtain information at a given locus in a coating system (i.e., surface/near-surface, bulk, interface/interphase, or as a depth profile) . Sampling for depth profiling includes in-plane ' and cross-sectional ' microtomy techniques. In-plane microtomy is typically done with a large-scale, or slab type, microtome. The sampling in the reported work was done co-planar to the surface of the coatings. Cross-sectional microtomy is readily done by large- or small-scale microtoming techniques. T E M cross-sectional sampling is unique in that cryo-microtomy is used to generate very thin sections. 7
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Table 1. Analysis. Analytical Depth Technique Profiling Applied (Chem/Phys)
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Methods & Locus of Automotive Coating System Cross-Sectional
Surface
Analysis
Morphology
Analysis
(Chem/Phys)
(Physical)
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IR-Micro Chem Raman-Micro ' Chem ToF-SIMS ' Chem Opt-Micro Phys TEM ' ' Phys Extr/HPLC Chem
Interface
Chem
(Chem/Phys)
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Chem
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Phys
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Bauer and Martin; Service Life Prediction of Organic Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 21, 2016 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch017
260 cutting, coating systems were potted in an acrylic or epoxy based polymer to provide mechanical support during handling. For practical considerations, in order to avoid, or minimize, contamination from the potting polymer a composition unique to the coating system layer(s) under study is chosen (i.e., epoxy for acrylicbased CC/BC bilayers). In addition, microtoming is done along the direction of the interface, thus minimizing cross-contamination. . Note that potting was not used in the more surface specific ToF-SIMS experiments due to the risk of crosscontamination. Such contamination can result in misleading data where a significant amount of the spectral information is due the potting compound. Instead, specialized cryo-ultra-microtomy techniques were employed to generate the ultrathin (< 1 p) sections required. IR-Microscopy Analysis: Transmission mode analyses were done on a Nicolet Magna-IR 760 FT-IR Spectrophotometer equipped with a Nicolet Nic-Plan microscope. Attenuated total reflectance (ATR) mode analyses were done on a Nicolet 20-SXC FT-IR Spectrophotometer equipped with a Spectra Tech IR-Plan microscope and an ATRobjective using a ZnSe internal reflection element. Solvent Extraction & Chromatography Analysis: Methylene chloride (CH2CI2) was used as solvent to extract benzotriazole type ultra-violet screeners (UVAs) from in-plane (slab) microtomed sections. At typical loading levels for these additives (-1-3 % by weight), it was observed that a l"x 1.5"x IOJLI sized section extracted by 5 mL solvent resulted in UVA concentrations sufficient for easy detection and identification by HPLC methods. Solvent extraction was done for a period of at least 24 hours using a lab benchtop rocker/mixer device. Raman Microscopy (Microprobe) Analysis: A Renishaw Raman imaging microscope was used in depth profiling of specific coating system components (i.e., organic pigments used in BC's). This instrument consists of a conventional optical microscope integrated to a Raman spectrometer with a laser for excitation. The coating systems reported herein were studied using an argon-ion-laser-pumped Ti-sapphire laser (Coherent Inc.) for excitation. The wavelength was tuned to 780 nm, corresponding to the center frequency of the notch rejection filters available in our lab. This wavelength is presently the best compromise available to maintain a high degree of fluorescence rejection while still maintaining an adequate spectral range and sensitivity. Most of this work on automotive coatings would not be possible without the availability of near-infrared excitation sources, due to characteristic sample fluorescence. Power levels of -1 mW were typical in these experiments. High spatial resolution analyses were performed using a high numerical aperture 100X objective, which provides a diffraction limited laser spot of approximately 1 JH in diameter. These spot sizes are much smaller than can be provided by infrared microscopy, which, at best,
Bauer and Martin; Service Life Prediction of Organic Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 21, 2016 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch017
261 provides a diffraction limited spot size on the order of ~10p (with a more practical minimum spot size of ~30p with conventional laboratory equipment). . A motorized X-Y translating stage (Prior) was used for area point mapping measurements. Raman maps were generated by taking spectra point-wise across an area of a paint film cross-section at 1 urn intervals with the use of the mapping stage which is under software control. Sample cross-sections were prepared by potting the coating systems in an epoxy matrix and microtoming, polishing, or by simply cracking the coating after immersion in liquid nitrogen. Background intensity due to fluorescence is subtracted out using spline baseline fitting routines (GRAMS/386 ™, Galactic Industries). Peak area integration was performed on the successive Raman spectra to generate pigment concentration Raman images. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) Analysis: The Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analyses were performed on a TRIFT II Spectrometer using a 15 keV pulsed (-5 nsec) and bunched (-1 nsec) Ga ion gun. The beam size is about 1 jam and the rastered area is 200x200 urn. Typically the mass resolution after Oxygen-16 (negative ion mode) is -2000. During the 30 min. acquisition (dose ~5xl0 ions/cm ) a mass spectrum and 16 mass selected ion images (256x256 pixels) can be obtained. 12
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The experiments described herein illustrate the sensitivity and high spatial resolution of the technique which can be used to determine the in depth distributions of elements and molecular species in automotive finishes. Optical Microscopy Analysis: The images were obtained with a Leica DM RXA Microscope equipped with an MTI 3CCO color video camera and Kodak Digital Science 8650 PS color printer. A 200X magnification of the sample was used. Transmission Electron Microscopy (TEM) Analysis: The images reported in these studies were obtained using a JEOL 2000FX Transmission Electron Microscope (TEM) operated at various accelerating voltages (in the 80-200 KV range) and recorded on large format sheet film. Automotive coating systems were found to be sufficiently stable in the TEM electron beam permitting imaging of ultra-thin cryo-microtomed (
