Journal of Coatings Technology and Research (v.5, #2)

Modern organic coatings are complex materials which must be observed and interpreted from the viewpoint of what has become known as Materials Science. Paints, as most of us first learned to name organic coatings, were originally designed and conceived as art forms. Visual artists developed their own applied colorants; part of an artist’s success depended on these skills (consider Leonardo da Vinci). Today, one’s skills as a materials scientist are perhaps the most crucial attributes of the true coatings scientist. Considering pigmented organic coating as random heterogeneous composite materials was the key to my research group’s contributions in improving the understanding of pigment volume concentrations (PVC) and critical PVC effects in coatings. Seeking to develop and improve electrochemical materials characterization techniques for measuring and predicting the corrosion protective properties of coatings has driven our research at North Dakota State University (NDSU) in this area. Analyzing the performance of protective coatings for exterior bronze statues and viewing these coatings from the materials requirements of the art conservator has directed our activities in conservation science. The use of the methods of modern electrochemical materials science has been the key to our development of Mg-rich primers for entirely Cr-free corrosion protection of aircraft alloys. Our work on developing methods to perform material measurements directly on coatings during their field use has directed our recent studies of embedded sensors in coatings. Our work in these areas will be reviewed and current results presented. Forecasts of future developments in these and other areas of coatings science will be presented, extrapolating from present developments in materials science. Incorporating materials concerns, especially the methods and concepts of nanomaterials science, into coatings science will be of paramount importance to future research and development in coatings.
Keywords: Physical chemistry; Materials science; Corrosion protection; CPVC; Electrochemistry; Chromate replacement; Room temperature ionic liquids

The diffusion of polymer chains across the interface between distinct latex particles is the final step in latex film maturation. This step drives the transformation of a honeycomb of compacted latex particles bound by weak surface forces into a mechanically robust film. Knowledge of the onset of this diffusion process is limited. We have examined film formation in butyl acrylate-methyl methacrylate copolymer latex containing 1 wt% methacrylic acid. These films dry via a propagating drying front. We were able, via fluorescence resonance energy transfer measurements, to determine the extent of polymer interdiffusion at 23°C as a function of distance from the edge of the drying front for a series of partly wet latex films. Our apparatus allows us to arrest the latex drying process and to extract interdiffusion information from sub-millimeter regions of the drying film. We have tracked the latex drying process and subsequent polymer diffusion as a function of humidity. We find that adjacent to the drying front, increasing humidity initially delays the onset of interdiffusion, but once this initial barrier is overcome increasing humidity increases the rate of diffusion. This transition occurs within 1–2 mm of the drying front.
Keywords: Fluorescence resonance energy transfer; Hydroplasticization; Film formation; Poly(methyl methacrylate); Poly(butyl acrylate)

Composite latex particles made from sequential processing steps may have a wide range of morphologies. The particular structure achieved is the result of a number of complex interactions between the chemical and physical aspects of the emulsion polymerization process. The intention is often to produce a core-shell particle structure, but in reality this is often a nontrivial task. In addition, it is challenging to develop confident conclusions about the detailed morphology of such particles, even when the two-component polymers are well phase-separated. This paper reports on an inter-laboratory study of acrylic-styrene copolymer latices in which the two-component polymers were not well phase-separated—a common result when attempting to make core-shell latex particles. Six different organizations each performed a number of analytical measurements to characterize two different latex systems. Their collective results were presented at a workshop in which the group strived to reach consensus on the detailed structures of the particles. Their conclusions were that multiple, complementary analytical results are required to reach a sound decision. While electron microscope results were always judged to be necessary, considered by themselves, it is often found that one can be led to false conclusions, especially for systems that are not well phase-separated. Given the results of this study it is clearly inappropriate to assume that the polymer formed in the second stage of a semibatch latex polymerization process forms a shell, giving rise to a core-shell morphology.
Keywords: Core-shell; Latex; Morphology; Characterization; Synthesis

Considerations for characterizing moisture effects in coatings weathering studies by Henry K. Hardcastle; William L. Meeks (181-192).
This article investigates the effects of weathering factors on moisture absorption of coatings and presents results from several projects including real-time and accelerated weathering studies of coatings moisture absorption. Moisture variable characteristics, observed in outdoor exposure environments, are reviewed. Data are presented from the experiments performed in end-use and accelerated weathering environments to characterize moisture characteristics on coatings. The information from natural outdoor environment characterizations and design of experiments (DOEs) indicates the new approaches for characterizing coatings weathering with regard to moisture effects. Considerations and new approaches for performing coating weathering studies are discussed.
Keywords: Automotive coatings; Weathering; Moisture; Gloss; Degradation; Florida; Arizona; Accelerated weathering

Enhancing exterior performance of clear coatings through photostabilization of wooden surfaces. Part 1: Treatment and characterization by Bernard S. W. Dawson; Adya P. Singh; Hank W. Kroese; Michael A. Schwitzer; Suzanne Gallagher; Stephen J. Riddiough; Shuhong Wu (193-206).
The aim of this work was to produce a less photodegradation-prone substrate for clear coating by partially delignifying the surface cells of Pinus radiata boards to a depth of 2–3 mm while maintaining the integrity of the wood surface tissues in the delignified zone. To achieve this, several surface oxidative delignification treatments were trialled in the method development process and peracetic acid was chosen as the method for refinement and deployment. The treatment method was optimized to yield a significant degree of delignification compatible with the aim of producing a photostabilized yet intact wooden surface. A preweathering technique was also used as a second delignification method for producing photostabilized boards. Microscopic and chemical techniques were used to characterize the effects of chemical and preweathering treatments, which produced delignified surface envelopes 2–3 mm and 100 μm deep, respectively. Acetyl bromide lignin analyses, infrared spectroscopic analyses, and density changes of 30-μm-thick sections of peracetic-acid-treated samples, as a function of depth from the wood surface, suggested that a partial delignification had occurred that diminished with depth. Light and transmission electron microscopy provided evidence of delignification at the cellular level. In the surface layers of peracetic-acid-treated boards, all cell wall regions were delignified, with the middle lamella being the most severely affected. Lignin appeared to be completely removed from the cell corner middle lamella regions, but tracheids were still joined in other parts of the middle lamella. The S1, S2, and S3 walls were also delignified. In subsurface layers, cell walls were only partially delignified and the tissues held their integrity. In contrast, in the preweathered boards, cell walls in the outermost layers were completely separated at the middle lamella from photodegradation. Preferential lignin removal over that of hemicelluloses was achieved via oxidative treatment of solid wood. The outcome of this was the successful fulfillment of our aim to produce partially delignified wooden surfaces that retained sufficient strength and aesthetic appearance and were suitable for application in wooden structures, provided that the necessary protection measures were carried out. Due to the narrow surface zone produced, machining of such surfaces would not be recommended as the most delignified zone would be removed first. In Part 2 of this series, the photostability of treated boards, which had been clear coated and weathered, will be considered.
Keywords: Peracetic acid treatment; Delignification; Photostabilization; Preweathering treatment; Pinus radiata

Enhancing exterior performance of clear coatings through photostabilization of wood. Part 2: coating and weathering performance by Bernard S. W. Dawson; Adya P. Singh; Hank W. Kroese; Michael A. Schwitzer; Suzanne Gallagher; Stephen J. Riddiough; Shuhong Wu (207-219).
Clear-coated boards have not been recommended for use in exterior conditions since irradiation with visible and UV radiation darkens them and photodegrades the lignin in the wooden surface beneath the coating, leading to delamination and subsequent catastrophic coating failure due to the continued action of sun, rain, and biological factors. Many approaches to rectify this problem have been explored. Chemical modification of the surface with hexavalent chromium, reaction with various anhydrides, grafting of UV absorbers, and esterification are among the methods attempted. A second approach has been via the clear coating itself where UV absorbers, antioxidants, and ultrafine titanium and iron oxides have been added. However, these have had limited or no success in stopping photodegradation processes. Since the main cause of photodegradation is photooxidation of lignin in the wooden surface as a consequence of free radical reactions initiated by UV irradiation, the approach taken in the present study, in an attempt to enhance the weathering performance of clear-coated boards outdoors, was to delignify the surfaces of wooden boards and then apply clear coatings to try and retard possible photodegradation. Two different pretreatments were used. Firstly, chemical surface delignification with a peracetic acid treatment created a partial delignification to a depth of 2–3 mm while still retaining the structural integrity of the surface. Secondly, a preweathering treatment, which resulted in a 100-μm-deep delignification zone, was compared. The coatings applied to the exposure surface of the pretreated boards were either polyurethane or an acrylic varnish. The clear-coated boards were exposed to exterior and accelerated weathering regimes for 3 years or 3000 h, respectively. Pretreated coated boards did not darken and yellow on exposure but untreated coated boards did. However, despite apparently arresting photodegradative processes on board surfaces, there were no significant gains in the performance ratings of coated pretreated boards over those of coated untreated control boards. Explanations for this involve the effectiveness of design factors incorporated into boards for exposure trials. These factors were the fungicidal dipping of boards before coating, precoating the exposure surface with a reactive primer, and applying a full polyurethane system to the back side and edges of boards. Both pretreatments resulted in clear-coated board surfaces that performed very similarly on exposure outperforming systems reported previously. It was surprising to observe that the preweathering treatment, which resulted in a 100-μm-deep delignification zone, performed as effectively as the chemically pretreated boards with 2- to 3-mm treatment zone. However, preweathered surfaces had lost all lignin in the middle lamella and there was cell separation, whereas in peracetic acid-treated boards, there was more or less complete lignin removal from the cell corner middle lamella only and partial lignin removal from other cell wall regions. Furthermore, it is anticipated that refinements in treatment methods and coating formulations will bring desired benefits and future work should focus in this area.
Keywords: Delignification; Acrylic and polyurethane coatings; Pinus radiata ; Photodegradation; Photostabilization; Weathering performance

Experimental and theoretical behavior of exterior wood coatings subjected to artificial weathering by Jan Van den Bulcke; Joris Van Acker; Marc Stevens (221-231).
Several solvent- and water-borne exterior wood coatings were artificially weathered to study their performance behavior. Penetration and layer thickness were measured on unweathered samples and compared to theoretical estimates by means of measured basic parameters of coating and substrate. Color, surface roughness, gloss, and adhesion were monitored during aging, and the latter two were also calculated and compared to their experimental values. Theoretical values of gloss, and especially those of adhesion, were less successful on an absolute scale, but were in accordance with practical values on a relative scale. The influence of solid content, drying speed, and viscosity on penetration depth manifested itself clearly both in theory and in practice. In general, solvent-borne coatings performed well, but some water-borne coatings also showed good performance. By measuring the characteristics of a coating as described in this article, it is possible to rank the coatings and follow their weatherability. Finally, calculation of theoretical values proved a promising method for initial screening purpose.
Keywords: Exterior wood coating; Artificial aging; Gloss; Penetration; Adhesion

Assessment of UV-permeability in nano-ZnO filled coatings via high throughput experimentation by Michael S. Lowry; David R. Hubble; Amy L. Wressell; Menas S. Vratsanos; Frank R. Pepe; Charles R. Hegedus (233-239).
The degree of UV protection afforded by nano-ZnO in a polyurethane/acrylic clear topcoat was investigated. The influence of nano-ZnO concentration and dry film thickness on the optical properties (e.g., UV permeability and visible transmittance) of the coating was probed using a library of 28 samples that were prepared by high throughput techniques. A model for predicting the UV permeability of nano-ZnO filled coatings was developed and the nano-ZnO loading condition required to block >99% UV radiation was determined to be 2.0 g/m2. This model can be used to assist in the development of coatings and other polymeric systems embedded with nano-ZnO to protect the coating and underlying materials (i.e., substrate) from UV degradation.
Keywords: Nano-ZnO; ZnO; Polyurethane/acrylic coatings; UV permeability; UV transmission; Beer–Lambert’s Law; Beer’s law; High throughput experimentation

Structure, properties and corrosion resistivity of polymeric nanocomposite coatings based on layered silicates by Davood Zaarei; Ali Asghar Sarabi; Farhad Sharif; Seid Mahmood Kassiriha (241-249).
This paper reviews the recent research and development of polymeric nanocomposite coatings based on layered silicates. In the past few decades, extensive research activities have been conducted on clay minerals due to their unique layered structure, rich intercalation chemistry and availability at low cost, environmental stability, and good processability. One of the most important categories of layered silicates is nanoclays. The nanoclay is considered as reinforcement for polymers in the manufacture of low-cost, lightweight and high performance nanocomposite coatings. In this paper, we try to introduce the structure, properties, and surface modification of clay minerals. Different properties of polymer clay nanocomposite coatings consisting of different polymers are also reviewed. These coatings may consist of conductive and nonconductive polymers. The corrosion resistance of each type is discussed separately. Some novel properties can be observed from the interaction of two dissimilar chemical components at the molecular level that posses enhancements in corrosion inhibition on metallic substrates. Finally, the prospective problems of industrial usage of these materials are mentioned.
Keywords: Clay minerals; Montmorrilonite; Polyaniline; Conductive polymers

A UV crosslinkable waterborne siloxane–polyurethane dispersion, PDMS-PEDA-PU, was synthesized by incorporating acrylate groups into the side chain of the polyurethane using pentaerythritol diacrylate (PEDA). Polysiloxane groups were introduced into the soft segment of the polyurethane using polydimethylsiloxane (PDMS). They formed a crosslinking structure after UV radiation. The structure of PDMS-PEDA-PU was confirmed by the use of Fourier transform infrared spectroscopy (FTIR). X-ray diffraction (XRD) analysis indicated that the formed PDMS-PEDA-PU was amorphous. The curing rule of PDMS-PEDA-PU was investigated on the basis of the change in C=C conversion during the UV curing process. The influence of the PDMS/PEDA ratio was studied with respect to the water resistance, the thermal property, and the mechanical property of the films.
Keywords: Crosslinkable; Waterborne; Siloxane–polyurethane

Preparation of light color antistatic and anticorrosive waterborne epoxy coating for oil tanks by Zhong-Hua Chen; Ying Tang; Fei Yu; Jian-Hua Chen; Hai-Hong Chen (259-269).
Utilizing waterborne epoxy resin as a main film-forming material, a kind of light color antistatic and anticorrosive waterborne epoxy coating with volume resistivity of 106 $$Upomega , hbox{m}$$  was prepared by adding light color conductive pigments and anticorrosive pigments. The coating has excellent corrosion protective and outstanding decorating properties, which is applicable for the inner coating of oil tank. The effects of the type and the amount of conductive pigments, the film thickness, the curing temperature, and the curing time on the volume resistivity of the film were discussed. The properties of the several kinds of waterborne epoxy resin were compared and the determining of anticorrosive pigments was discussed.
Keywords: Light color; Antistatic; Anticorrosive; Conductive pigment; Waterborne epoxy resin; Oil tank