As the Waterborne Symposium prepares for its 40th anniversary celebration, we track the origins of this prestigious technical event, and recognize those who have been instrumental in its development.
This paper describes the structure and design rationale for a new ultra accelerated weathering system. The system allows a 63 year (approximate) equivalent South Florida UV radiant exposure within a single year of ultra accelerated exposure. The system provides high fidelity to natural solar UV spectral power distributions while attenuating visible and IR wavelengths to maintain acceptable specimen exposure temperatures. The paper includes data showing correlation between ultra accelerated exposure and real world exposure. This paper is the first in a series describing the system and exposure results and reviews accomplishments of a cooperative development effort between Atlas Material Testing Technology, the National Renewable Energy Laboratory and the Institute for Laser Optical Technology under the Department of Energy’s Global Initiative for Proliferation Prevention program.
Atomic force microscopy (AFM) has been used to study the morphology and microstructure of an amine-cured epoxy before and after outdoor exposure. Measurements were made from samples prepared in an essentially CO2-free, H2O-free glove box and from samples prepared in ambient conditions. For those prepared in a CO2-free glove box, AFM imaging was conducted on (1) an unexposed air/coating surface, (2) an unexposed coating bulk, (3) an unexposed coating/substrate interface, and (4) a field exposed air/coating surface. For samples prepared in ambient conditions, only the unexposed air/coating surface was investigated. The same regions of the exposed samples were scanned periodically by the AFM to monitor changes in the surface morphology of the coating as UV exposure progressed. Small angle neutron scattering and Fourier transform infrared spectroscopy (FTIR) studies were performed to verify the microstructure and to follow chemical changes during outdoor exposure, respectively. The results have shown that amine blushing, which occurs only under ambient conditions, had a significant effect on the surface morphology and microstructure of the epoxy. The surface morphology of the samples prepared under CO2-free, dry conditions was generally smooth and homogeneous. However, the interface and the bulk samples clearly revealed a two-phase structure consisting of bright nodular domains and dark interstitial regions, indicating an inhomogeneous microstructure. Such heterogeneous structure of the bulk was in good agreement with results obtained by small angle neutron scattering of unexposed samples and by AFM phase imaging of the degraded sample surface. The relationship between submicrometer physical changes and molecular chemical degradation is discussed.
Two ductile coating ma-terials were subjected to a combined indentation and scratch test procedure de-signed to screen a predetermined pattern of many small sample surfaces in a limited time. The screening of 50 surface spots ordered in a matrix pattern on the surface was carried out in 4.5 hr. The test provides reproducible data in terms of indentation modulus, elastic recovery, scratch penetration depth, and scratch residual depth, and also offers the possibility of detecting critical mechanical transitions such as rupture. The presented procedure produces sufficient data in a limited time scale to fulfill the requirements for a fast method to screen coating compositions.
Changes in the intrinsic structure of paint surfaces resulting from extended UV exposure can significantly alter the appearance of paint due to a breakdown in the resin that binds the pigments and flattening agents. In this study, the coating structure of a solvent-based poly-urethane was analyzed to establish correlations between the intrinsic spatial scaling properties of the coating and UV exposure time. Atomic force microscopy and laser scanning confocal microscopy were employed to map surface structures over a range of scales from 80 nm to 80 εm. The roughness of the polyurethane surface was characterized in terms of scaling exponents using detrended fluctuation analysis to identify long-range, power law relations, and to correct for inhomogeneities in the surface structure. The time-dependence of the roughening process was also determined and correlated with changes in gloss.
The new analytical expression introduced in this study shows that a plot of the accumulated volume fraction of pigment particles vs. the square root of particle size should be a straight line for the theoretically preferred particle size distribution. This linear theoretical model was also found to be consistent with the experimentally developed linear model proposed by Kaeuffer. However, Kaeuffer proposed that his straight line went through the origin, but the analytical model developed in this paper has shown that this intercept is not zero. Thus, Kaeuffer’s experimental model is only partially correct.
Preparation and characterization of weather resistant silicone/acrylic resin coatings were conducted. In order to prepare these coatings, a silicone/acrylic resin (KLD) was first prepared by an addition polymerization reaction of monomers, including n-butyl acrylate, methyl methacrylate, n-butyl methacrylate, and 3-methacryloxypropyltrimethoxysilane (MPTS). In the preparation of the silicone/acrylic resin, Tg of the acrylic copolymer was fixed at 40°C and the contents of MPTS were varied to be 10, 20, and 30 wt%. The weather resistant silicone/acrylic resin coatings were then prepared by blending the synthesized silicone/acrylic resin and TiO2. The viscosity of the synthesized resin decreased with the content of MPTS, whereas the thermal stability at high temperature increased. The prepared coatings exhibited excellent adhesion to various substrates, and various physical properties of the coatings were satisfactory. The weatherability of the coatings was tested three ways: outdoor exposure test, Weather-Ometer (WOM), and QUV accelerated weatherability tester (QUV). The gloss retention, yellowness index difference, color difference, and lightness index difference were improved at high MPTS concentration. The coatings containing 30 wt% MPTS have especially good weather properties.
The curing of polyester-based clearcoats has been studied by using thermal analysis methods. Polyester resins are used as raw materials for coil coatings. The aim of this work was to study the viscoelastic properties of coil coatings. Differential scanning calorimetry (DSC) was found to be useful in curing studies of lacquers, for example, to predict conversion as a function of cure time or cure temperature. Dynamic mechanical analysis (DMA) was used to determine the gel times of some clearcoats. The DSC measurements were done with different apparatus and software programs. This study concentrates on the use of these modern techniques to predict the curing and correlation of the results.
Accelerated weathering of wood surfaces coated with hexadecyltrimethoxysilane (HDTMOS) in the presence of methyltrimeth-oxysilane (MTMOS) by the sol-gel process was investigated. The sol-gel process allowed the deposition of a covalently bound thin layer of polysiloxane networks on the wood surface that was resistant to water sorption and water leaching. The rate of weight loss resulting from surface erosion and the extent of surface color loss caused by photo-induced discoloration were decreased for coated specimens compared to uncoated specimens.
Phase-change, or hot melt, inks aresolid at ambient temperature and liquid at the moment of printing. Such inks contain no volatile solvent to be trapped in ink film or to produce Volatile Organic Compounds (VOC). The phase-change inks found application in non-impact printing technology by either thermal transfer or “solid ink” digital printing. The environmental and health problems caused by inks containing volatile organic compounds, such as toluene, call for new solutions in the rotogravure printing industry. Because of their low VOC content, phase-change inks might be a reasonable solution for gravure printing.
Surfactants, varying in their chemical composition and hydrophobic behavior, are used in the formulation of a waterborne coating. These differences influence their aggregation in micellar structures, their interaction with associative thickeners, and in particular, the synergies present in their competitive adsorptions on the disperse phases in a waterborne coating. Adsorption of HEUR thickeners on latexes and the ability of surfactants to displace them from those surfaces is an important variable in the dispersion's viscosity. With large particle latexes, viscosity increases arise primarily from the network built through the interaction of HEURs with surfactants in the aqueous phase. Fluorescence is used to verify the mechanism by which surfactants enhance associative thickener viscosities. That is best achieved with nonionic surfactants, because of their synergies with large hydrophobe HEURs at low concentration. With decreasing latex particle size the adsorbed species is an important contributor to the dispersion's viscosity through its contribution to the latex's effective volume fraction increase and when the size of the adsorbed HEUR is matched to the separation distances of the latex at 0.25 volume fraction. Achieving controlled shear-thinning behavior in small particle size latex paints with the economic constraints on the amount of HEUR required to obtain 90 KU viscosities are discussed.
Model coatings with known composition were assessed for liquid water permeability both as single coat systems and combined as primers and topcoats to form regularly used coating systems—in total 38 different systems. The water permeability of the coatings, expressed as water absorption value, was measured by a method similar to the method that now is proposed as a European standard for the assessment of liquid water permeability. During artificial weathering, the samples were measured for a water absorption value, thus showing the change in water permeability as a function of weathering.
During the accelerated exposure of a model polyester-urethane coating, measurements were taken at intervals to determine how the ultraviolet absorption, erosion, and chemical change proceeded during degradation. Infrared spectra showed that the carbonyl concentration increased with exposure as the material oxidized, but the ultraviolet results showed carbonyl peaks that diminished with exposure. Changes in optical path length, carbonyl concentration (both infrared and ultraviolet), and C–H concentration appeared linear with the exposure period. Knowing the chemical composition of the polyester and its reaction with isocyanurate made it possible to construct a representative model of the polymer network. Computational chemistry provided confirmation of the literature on which elements of the network were susceptible to degradation. As the polymer eroded, the aromatic content including some carbonyl was lost, but the remaining polymer was oxidized. Quantitative estimates were made for the mass lost and the change in concentration of the carbonyl and CH concentrations that agreed well with results from both the infrared and ultraviolet spectroscopy. Knowing the polymer composition allowed thickness change, a macroscopic quantity, to be connected to chemical changes via the polymer network model.
An epoxy primer commonly used in the automotive industry was applied cataphoretically (using different times and potentials) over metallic substrates, and was investigated by means of different techniques such as accelerated cathodic disbonding, EIS, and AC/DC/AC tests. The variables considered in this study were the metallic substrate (cold rolled steel and phosphatized cold rolled steel), the thickness (20 and 25 µm), and finally the deposition potential in the cataphoretic cell (140 V and 190 V). The primer performed better when applied over phosphatized steel at the maximum thickness and when the lowest cataphoretic potential was used. Primer quality results obtained by the different techniques used in this study were quite similar and led to almost the same conclusions, although the AC/DC/AC and cathodic disbonding techniques provided results in less time than the EIS technique.
Electrochemical impedance technique was used to study the corrosion resistance of polymer-coated steels in hydrochloric acid solutions. The mechanism of the degradation process of the coating can be described using a Randles-type equivalent circuit model with the inclusion of Warburg diffusion impedance. The impedance parameters: coating resistance R po , coating capacitance C c , charge transfer resistance R ct and double layer capacitance C dl of the metal/coating interface, and the Warburg coefficient σ were calculated using a nonlinear least-square minimization scheme. The time dependency of impedance parameters was compared to the method of breakpoint frequency. It was found that the delamination of the metal/coating interface is more associated with the charge transfer resistance than the coating resistance. Open circuit potential measurement is indicative of the corrosion status of the coating.
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.
Poly(methyl methacrylate) (PMMA) discs in various molecular weights, M w , were prepared by free-radical polymerization of methyl methacrylate (MMA). Pyrene (P) was introduced during polymerization as a fluorescence probe to monitor the gelation and dissolution processes in chloroform vapor and solvent, respectively. In-situ steady state fluorescence (SSF) experiments were performed to monitor vapor uptake and chain desorption processes. Direct illumination of PMMA discs were performed to excite the P molecules embedded inside the PMMA glass. Variation in P intensity, I, was monitored during the swelling of the PMMA material exposed to chloroform vapor. It was observed that PMMA film swells like a crosslinked polymeric gel at early times by obeying the Li-Tanaka equation. Swelling time constants, τ c , of PMMA discs were measured and found to have a strong correlation with the molecular weight, of PMMA. In a separate experiment, when the PMMA discs were in chloroform, desorption of PMMA chains from glass discs was monitored by observing the change of pyrene fluorescence intensity. A diffusion model with a moving boundary was employed to quantify the fluorescence data observed from dissolving PMMA discs made at various molecular weights. It was observed that desorption coefficient, D, decreased by increasing M w by obeying the D≈M w −1 law.
A wide variety of radiometric instruments are now available for measuring the radiant characteristics of industrial and laboratory UV lamps. Relating these characteristics to the performance of a UV-cured product depends on how well the selected parameters match the critical factors of the cure process. Further, the distinction between process design and process monitoring is significant, especially in the UV exposure characteristics to be measured. The differences in instruments and the reasons for these differences are followed by suggestions of how to avoid some common errors and how to report data more completely. Optical terminology, specifications, methods, and how radiometric measures are used in system design and in production monitoring are presented.
Waterborne coatings that rapidly set and become tack-free can be prepared from polymers containing both pendant anionic (acidic, carboxylate, or strong-acid groups such as sulfonate) and cationic functionality (quaternary ammonium groups). This phenomenon is related to anion-cation interactions that function as ionic crosslinks and dramatically enhance the physical properties and water resistance of the coatings. We define this process as “controlled ionic-coacervation.” The best coating properties can only be obtained by using a “critical solvent combination.” The critical solvent combination requires water plus at least two organic solvents: (1) a lower boiling (70 to 134°C) water-soluble organic solvent having at least one hydroxyl group and (2) a higher boiling (135 to about 250°C) organic solvent. Loss of only a small amount of solvent causes a coating to rapidly become tack-free. Ionization of acid functionalities on the polymers by an increase in pH (e.g., through the loss of CO2) can initiate controlled ionic interactions. The influence of polymer and solvent compositions on coating properties is discussed.
