Advanced techniques for nanocharacterization of polymeric coating surfaces

Surface properties of a polymeric coating system have a strong influence on its performance and service life. However, the surface of a polymer coating may have different chemical, physical, and mechanical properties from the bulk. In order to monitor the coating property changes with environmental exposures from the early stages of degradation, nondestructive techniques with the ability to characterize surface properties with micro- to nanoscale spatial resolution are required. In this article, atomic force microscopy has been applied to study surface microstructure and morphological changes during degradation in polymer coatings. Additionally, the use of AFM with a controlled tip-sample environment to study nanochemical heterogeneity and the application of nanoindentation to characterize mechanical properties of coatings surfaces are demonstrated. The results obtained from these nanometer characterization techniques will provide a better understanding of the degradation mechanisms and a fundamental basis for predicting the service life of polymer coatings. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology on November 12–14, 2003, in Philadelphia, PA.     

Electrochemical and mechanical studies on influence of curing agents on performance of epoxy tank linings

Effect of different curing agents on the mechanical, chemical resistance and anticorrosive properties of tank linings based on phenolic + novolac epoxies has been studied. Proportion of two epoxies was chosen by monitoring the processing viscosity of the coating. Parts of hardener required for 100 g of the above resin (phr) was scrutinized using differential scanning calorimetry and flexibility measurements. As a result, five coatings were finalized and their mechanical properties were investigated by dynamic mechanical analyzer. Anticorrosive properties of these coatings were studied using electrochemical impedance measurements and neutral salt spray exposure test. Chemical resistance of these coatings was investigated by monitoring the weight change of the coating during immersion. Coatings cured with modified cycloaliphatic amine (amine value = 350), in spite of lower cross-link density, exhibited superior anticorrosive properties and reasonably good chemical resistance. Electrochemical impedance results correlated fairly well with actual solvent resistance property of the coating.     

Monte carlo approach to estimating the photodegradation of polymer coatings

The degradation of a polymer coating and predicting the coating lifetime based on physical properties and distribution within the coating of the polymer binder, pigments, and fillers are economically very important. As technologies advance and allow for control of coatings at the nanoscale level, methods such as Monte Carlo can be used not only to predict the behavior of a nanodesigned coating with time but also to design coatings, such as optimizing pigment particle distributions or optimum hard and soft phase distributions of the binders in multiphase systems for maintaining the desired property with time. Erosion of the coating surface was simulated using Monte Carlo techniques where terrestrial solar flux is the initiator for polymer segment cleavage and removal. The impact on the sensitivity of the polymer adjacent to the detached polymer segment can be increased or decreased in the model based on the chemistry and surface energy of the remaining polymer matrix. Multiple phases with varying sensitivity to degradation can be modeled. The Monte Carlo generates a statistically similar surface topography and chemistry of the coating. The results of the Monte Carlo model are compared to measurable properties such as gloss, fracture toughness, and wetting contact angle, using various published correlations of the property to the surface topology. The simulated properties change through the life-time of the coating in ways that are consistent with observed behavior. Apparently, complicated changes in many properties can be described by the repeated application of simple, random processes. Presented at the 79th Annual Meeting of the Federation of Societies for Coatings Technology, October 28–November 1, 2002, in New Orleans, LA.     

Estimating service lifetimes in weathering: an optimistic view

Directly correlating lifetime to coating composition by using standardized, artificial exposures, or even natural exposure, is often very difficult. However, significant progress can be made by breaking down the problem into smaller questions, which can be separately addressed. If one understands the physical parameters that affect end-use properties, then one can also group, and thus correlate, properties according to whether they depend on processes at the surface or in the bulk of a coating, or whether they depend on defects. A scheme is presented that shows how one can use knowledge from analytical physical or chemical materials science in a statistical model related to the “chemical paradigm.” Simple physical models that use this information, about the initial state of the coating and its rate of degradation, can be used to compare the performance of coatings and estimate, simply, service lifetime depending on the property of interest, in the environment of interest. One can see that different properties are sensitive in different ways to the degradation process and decay with a different rate. Thus, although properties may be determined by the same degradation process, and location within a coating, they do not correlate directly. These approaches show how to organize our knowledge of degradation processes, and environments, and be able to make some testable predictions on how coating properties deteriorate. Presented at the 2006 FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA, on November 1–3, 2006     

Scratch resistance behavior of model coating systems

Coatings are often subjected to physical deformations caused by car wash brushes, tree limbs, keys, fingernails, and the like, which may result in mechanical abrasion. The resistance of the coating to scratches imposed by such mechanical abrasions has been studied through the utilization of laboratory tests such as nanoindentors, crockmeters, scanning probe microscopes, and taber abraders. Little emphasis, however, has been placed on the influence of coating attributes on measured scratch resistance. In this study we attempt to relate the effect of coating glass transition temperature, crosslink density, and crosslink type in a series of formulated waterborne polyurethane dispersion clearcoats on resultant scratch resistance. Methods utilized to impart the scratches, e.g., scanning probe microscopy, weighted fingernail, and crockmeter, as well as related coating physical properties measured, e.g., viscoelastic behavior, hardness, and tensile strength, are discussed. The scratch resistance of model coating systems analyzed was found to be dependent upon the base resin T_g, which affected the surface hardness of the coating and the toughness of the crosslinked network, as measured by the method of essential work. Coatings that exhibited both hard surfaces and tough-elastic network integrity afforded the optimized scratch resistance behavior. Presented at the 26th International Waterborne, High-Solids, and Powder Coatings Symposium, February 10–12, 1999, New Orleans, LA. 401 Southfield Rd., P.O. Box 6231, Dearborn, MI 48121-6231.     

Novel, water-based fluorinated polymers with excellent antigraffiti properties

In this article we describe novel, water-based, crosslinkable fluorinated polymers that form coatings with excellent antigraffiti properties. The synthesis of the binders and the surface and bulk properties of their coatings are discussed. The surface properties of these coatings are characterized in terms of their surface-free energy, as calculated from static contact angle measurements. The distribution of the fluorine atoms throughout the coating is measured by X-ray photoelectron spectroscopy (XPS). The bulk properties are studied by determining the crosslink density through dynamic mechanical thermal analysis (DMTA), and the effect of the crosslinking conditions on the crosslink density and the antigraffiti properties is discussed. The results indicate that a combined action of surface and bulk properties gives these coatings their excellent antigraffiti properties. The applicability of these polymers as protective coatings for metal and concrete surfaces are demonstrated. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL     

Use of a scanning thermal microscope to examine corrosion protective coatings in exposure

As the capabilities for control of coatings properties at the nanoscale advance, characterization of these properties at these same length scales is needed. One set of organic coatings properties that is very important to understand and control at these length scales are the thermomechanical properties of the coatings. A new hybrid instrument, a scanning thermal microscope was used to examine the morphology and the thermal property simultaneously for two component polyurethane coatings under exposure to corrosive environments. Through the morphology measurement, the surface appearance on the micro-level was monitored and the influence of different corrosive factors such as wet/dry cycling, thermal cycling and condensation on the appearance was studied. At the same time, the micro-scale thermal measurements and concurrent FTIR testing provided the information on the relationship between the change of the structure and the properties of the coating. The information gained from these measurements on the changes in local coatings properties will be presented and correlated to other studies in our lab on nanoscale properties as well as global coating property changes due to exposure.     

Fracture toughness of inorganic-organic hybrid coatings

The concepts of fracture toughness and the energy release rate at fracture for thin polymeric films are introduced. Fracture toughness and energy release rate data for ceramer films based on a linseed oil alkyd, a sunflower oil alkyd, and a commercial alkyd with titanium diisopropoxide bis(acetylacetonate), titanium(IV) isopropoxide, and zirconium(IV) propoxide are presented and compared to previously reported tensile data. Differences between the fracture data and the tensile data demonstrate the usefulness of fracture toughness testing. The energy release rate at fracture may be the one property to maximize in order to optimize all of the other coating properties. It may therefore be a great aid in the optimization of coating formulations. Data from dynamic mechanical thermal analysis indicate that there may be a correlation between fracture properties and secondary relaxation processes in the films. Department of Polymers & Coatings, Fargo, ND 58105.     

Using electrochemical measurements to estimate coating and polymer film durability

Electrochemical measurements are increasingly being used to evaluate the durability of coatings and polymer films in significantly shorter times than those needed to complete long-term exposure tests. Electrochemical measurements have their limitations and nuances. Variability of data, collecting data before samples reach steady state, and misinterpretation of the data can lead to erroneous estimations of coating and polymer film durability. However, replicate samples, monitoring open circuit potential, and capacitance magnitudes can be used to develop more accurate estimations of polymer film, coating, or coated metal durability from electrochemical data. Presented at the 80th Annual Meeting of the Federation of Societies for Coatings Technology, in New Orleans, LA, October 30–November 1, 2002. 510 Charmany Dr., Ste. 55, Madison, WI 53719, rustdr@pairodocspro.com, www.pairodocspro.com.     

Low VOC carbamate functional coatings compositions for automotive topcoats

Polymers and oligomers having carbamate functional groups have been used in a variety of curable coating compositions. Carbamate functional polymers offer many advantages for automotive topcoats, such as outstanding resistance to environmental etching, scratching and marring, humidity, and UV exposure. Hydrophobic carbamate oligomers suitable for crosslinking with standard amino resins were synthesized and formulated into stable one-pack automotive clearcoats with low volatile organic compound (VOC) and excellent physical properties. Because of their unusually steep thermal viscosity curves, these oligomers are particularly adaptable to hot spray techniques that enable coatings in the 85–90% weight solids range to be applied with conventional electrostatic mini-bells. Presented at the 27th Annual International Waterborne, High-Solids, and Powder Coatings Symposium, in New Orleans, LA, March 2000. Automotive Coatings Technical Center, Southfield, MI 48034.     

Mica/polypyrrole (doped) composite containing coatings for the corrosion protection of cold rolled steel

Micas/polypyrroles (PPys) doped with molybdate, p-toluene sulfonate, dodecyl benzene sulfonate, and 2-naphthalene sulfonate composite pigments were synthesized by chemical oxidative polymerization and characterized in coatings for corrosion protection on cold rolled steel substrate by various electrochemical techniques. Synthesized composite pigments were characterized for morphology by scanning electron microscopy, which indicated physical formation of PPy on the surface of mica. Chemical composition of the composite pigments was analyzed by X-ray photoelectron spectroscopy which chemically confirmed doped PPy formation on the mica surface. Coatings were formulated at 20% pigment volume concentration (composite pigments or as-received mica pigment) and were applied on cold rolled steel substrate. Coatings were exposed to salt spray test conditions (ASTM B117) for 30 days and were periodically assessed for corrosion with electrochemical impedance spectroscopy (EIS), open circuit potential (OCP), and potentiodynamic polarization. EIS and circuit modeling results demonstrated higher coating resistance (R _c) for mica/PPy (doped) composite coatings as compared to as-received mica pigment containing coating after 30 days of salt spray exposure. Lower current density and more positive corrosion potential values were observed for mica/PPy (doped) composite coatings as compared to mica pigment-based coating in potentiodynamic polarization measurements, indicating improved corrosion protection for cold rolled steel substrate. OCP measurements revealed more positive values for mica/PPy (doped) composite coatings as compared to mica pigment-based coating suggesting superior corrosion protection for mica/PPy (doped) composites.     

Concurrent physical aging and degradation of crosslinked coating systems in accelerated weathering

Coating degradation is a combination of both chemical and physical processes; however, physical processes have not received much attention. Physical aging has a non-negligible effect on coatings’ mechanical properties and permeability etc. through the densification that continues as a polymer approaches its thermodynamic equilibrium below the glass transition temperature, T _g. Observations in recent work showed that physical aging affects coatings’ mechanical property response during accelerated weathering and is, itself, affected by the associated chemical degradation. Two crosslinked coating systems were studied in order to compare different chemical compositions, their T _g, and their thermal response in accelerated weathering. During thermal cycling, physical aging measured by enthalpy recovery exhibited different trends in the two coatings. A “rejuvenation” mechanism was observed in the coating with a T _g between the top and bottom limits of the exposure cycle; continued aging was observed for the coating with a high T _g. Stress relaxation tests detected aging and “memory” behavior over periods comparable with accelerated weathering cycles. Both thermal and mechanical responses changed in complicated and different ways as the coatings degraded. Different degrees of coating thickness reduction were observed in both isothermal relaxation and degradation. When various coatings are evaluated, simply judging their performance under the same weathering environment is not reliable since polymer relaxation behavior depends on the relationship between the exposure temperatures and the T _g of each polymer. This paper was awarded First Place in the 2007 Gordon Awards technical paper competition, held as part of the FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in Toronto, ON, Canada, on October 3–5, 2007.     

Mechanical property changes and degradation during accelerated weathering of polyester-urethane coatings

Chemical changes, measured using spectrocopy, and crosslink density, measured by mechanical thermal analysis, were determined during accelerated weathering on a model polyester-urethane coating of known composition. The tensile modulus, measured above the glass transition temperature, and thus the crosslink density, decreased with exposure, as expected from the chemical changes. However, the tensile modulus, measured at room temperature, increased with exposure. Physical aging of the polymer network was found to occur concurrently with photodegradation and accounts for much of the increase in room temperature modulus. Increased hydrogen bonding in the increasingly oxidized polyester-urethane may also contribute to the increase in modulus at room temperature. Both physical and chemical changes must be determined if changes, and rates of change, in performance due to weathering are to be understood. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL     

Effects of additives on the performance of two-component waterborne polyurethane coatings

The effects of various additives and solvents on a coating’s performance in a two-component waterborne polyurethane coating system are studied using a statistical experimental design method. Using a blocked arrangement of fractional factorial design matrices, six different independent variables, including the use of different additives, catalysts, as well as solvents at different levels, were examined for their effects on various coating performance parameters, such as gloss, hardness, viscosity, and chemical resistance. Statistical analysis revealed several significant effects of these additives on the coating performances. Most importantly, several two-factor interactions between additives were also found to significantly influence the performance properties of the coating. These interactions could be very difficult to detect using traditional one-at-a-time experimental approaches. The information obtained from this study could be used for designing coating systems with superior performance properties. Presented at the 26th International Waterborne, High-Solids and Powder Coatings Symposium, February 10–12, 1999, New Orleans, LA. Coatings and Colorants Division, 100 Bayer Rd., Pittsburgh, PA 15205-9741.     

A model of the infrared cure of powder coatings based on surface absorptivities in-situ measurements

The efficiency of radiative powder coatings curing depends both on the coating radiative properties and the spectral emissivities of the infrared emitters. This investigation is structured to develop a mathematical model using the radiative properties of the coating to describe precisely the coating temperature and deduce the degree of polymerization conversion during the cure. The reflectance measurements results obtained with a FTIR equipped with an integrating sphere are analyzed and implemented in a thermal model. These results show that reflectance values vary with the chemical composition, the pigment nature, and are influenced by the powder coatings cure. The thermal model, solved with the finite volume method, permits to compute the varying temperature within the thickness of the powder-coated metal sample and provides a good prediction of the temperature within the “coating + substrate” system. The experimental cure of powder coatings supporting this study was performed in a small test oven equipped with electrical infrared emitters. Presented at 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 1–3, 2006, in New Orleans, LA.     

Through thickness changes to stiffness and thermal conductivity in thermal barrier coatings subjected to gradient exposure

Sintering-induced increases in the elastic modulus and thermal conductivity of thermally sprayed Thermal Barrier Coatings (TBCs) under constraint by a bond-coated substrate due to thermal gradient exposure have been measured. The results suggest the degree of sintering due to this constrained thermal exposure to be significantly lower when compared to free standing exposed specimens. Overall, a ∼30% increase in elastic modulus and ∼12% increase in thermal conductivity have been measured in a ∼ 450 µm thick TBC after 12 h of gradient exposure. Here, the top surface of the TBC was kept at 1250 °C, and the gradient was determined based on a constant backside cooling air flow rate. The elastic modulus, measured across the thickness of coating, reveals that in the region near the top surface, the elastic modulus increased by ∼ 50%. Contrastingly, no significant increase in elastic modulus was observed near the metal/coating interface. The dynamic evolution of thermal conductivity and sintering under gradient exposure and constraint was calculated using an analytical model which was then used to rationalize the experimental observations.     

A macroscopically nondestructive method for characterizing surface mechanical properties of polymeric coatings under accelerated weathering

The surface of coatings and plastics is the first target in any degradation process initiated by ultraviolet (UV) radiation or mechanical stress (via scratch and abrasion). Surface damage can lead to changes in optical, morphological, and mechanical properties and can result in pathways for ingress of moisture and corrosive agents. Current test methods for monitoring performance of protective coatings focus on chemical properties and optical properties, such as color and gloss measurements, or invasive tests such as abrasion and cross-cut adhesion. In this study, a macroscopically nondestructive performance protocol using nanoindentation metrology via a well-controlled scratch test was applied to evaluate the scratch resistance and monitor the surface mechanical property changes in a protective coating under accelerated weathering. Polyurethane (PU) coatings with different polyol compositions were chosen for this study. Coating specimens were exposed to high-intensity UV radiation at 55°C and 75% RH conditions. Exposed specimens were removed at specified UV exposure times for surface modulus/hardness and scratch resistance characterization via nanoindentation and scratch test. The effect of polyol type and UV radiation dose on the scratch damage (scratch morphology) was investigated and correlated with the surface hardness and modulus of the materials.     

Mg-rich coatings: A new paradigm for Cr-free corrosion protection of Al aerospace alloys

Environmentally acceptable alternative coatings to chromate pigments and pretreatments for the corrosion control of Al alloy 2024 T-3, commonly used in aircraft, were designed, formulated, and tested as primer coatings to provide protection using particulate Mg-rich pigmentation. The system was designed by analogy to pigmented Zn-rich primer coatings used for the protection of steel. In the current study, four coating polymer systems were examined as possible candidates as polymer matrices for Mg-rich cathodic protect coatings. Mg-rich primers were formulated with ∼50-micron average particle size magnesium powder, near to the critical pigment volume concentration (CPVC) for this system. Top-coated scribed coatings systems have been subjected to Prohesion^™ exposure in dilute Harrison’s solution for up to 5000 hr. These coatings are the first nonchromated coatings to satisfy 3000 hr of such exposure and remain shiny and undamaged in the scribe area, only showing damage at about 4800 hr. The corrosion byproducts generated in the scribe areas during Prohesion exposure were examined by energy dispersive X-ray analysis (EDXA), and the local pH of the coating determined by the nature of the salt formed as a function of exposure conditions and time, did not cause Al corrosion. Presented at the 81 st Annual Meeting of the Federation of Societies for Coatings Technology, November 12–14, 2003, in Philadelphia, PA.     

Performance of polyester and modified polyester coil coatings exposed in different environments with high UV radiation

Coil coating is a continuous strip coating technology, which enjoyed rapid growth. It provides a cost-effective method of applying a coating to a metal substrate with a low solvent emission, and nowadays paints of different nature are applied this way on a range of metal substrates, for a large variety of products and uses.

Coil coatings can be more or less degraded by high UV radiation. However, differences in distribution of light intensity as a function of wavelength between exposure environments, can lead to significant differences in the degradation results, depending on coating nature. In this work, the behaviour of polyester and silicone polyester coatings exposed in three different high UV environments (natural atmosphere and two accelerated UV tests), was studied. Coatings of two different colours (blue and brown) were considered for each paint system. Colour change, gloss and chalking measurements and, visual inspections of coating defects were carried out. In order to explain their corresponding performance, chemical changes on coil coated exposed surfaces were studied by to Fourier transform infrared spectroscopy equipped with a photoacustic detector (PAS/FTIR).     

Study of PANI-MeSA conducting polymer dispersed in UV-curing polyester acrylate on galvanized steel as corrosion protection coating

Polyaniline (PANI) was synthesized by chemical oxidative polymerization using methane sulfonic acid (MeSA) as dopant and ammonium peroxodisulfate as oxidizer. Coatings of PANI-MeSA dispersed in polyester acrylate resin were applied on galvanized steel and UV-cured. The UV-curing resin base was studied to provide high performance and environmental friendly coating system. Morphology and mechanical properties of the coatings were investigated by scanning electron microscopy (SEM) and microindentation hardness tests. Long-term open-circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements were performed in 3 wt.% NaCl solution. SEM images show dispersion of the PANI particles in the coating, and electrochemical studies show long term active anodic ennoblement introduced to the protective system in the presence of PANI, shifting OCP value to nobler region. The changes in impedance value of the system during long-term exposure to the electrolyte give useful information about the PANI mechanism of action in corrosion protection and indicate the redox action (changing of states) of PANI under the defects healing process.