Light-stabilization of polymeric materials by grafted UV-cured coatings

The weathering resistance of organic materials has been substantially increased by protecting their surface with a photocured coating containing both a UV absorber (UVA) and a radical scavenger (HALS). A kinetic study by real-time IR spectroscopy has shown that HALS have no effect on the cure rate, whereas UV absorbers slow down the cure process, due to their radiation inner filter effect. 3D analysis of the depth of cure profile revealed an incomplete cure at the coating/substrate interface, leading to adhesion failure. To prevent this detrimental effect, the UV-cured coating was photochemically grafted onto the substrate. The polymer material was first coated with a thin layer of a benzophenone solution in a diacrylate monomer. Polymer radicals, generated by hydrogen abstraction from the substrate by the excited benzophenone molecules, effectively initiate the polymerization of the acrylate functions, thus ensuring a chemical bonding between the coating and the substrate. The grafting reaction was characterized by ATR spectroscopy analysis and by surface energy measurements. Excellent adhesion was achieved by applying to the treated substrate a photocurable acrylate coating, containing the light stabilizers, because of the copolymerization reaction taking place with the unreacted acrylate double bonds of the base coat, upon UV exposure. The efficiency of this on-line stabilization process has been demonstrated on poly(vinyl chloride) that was made eight times more resistant to accelerated weathering. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2571–2580, 1998     

Highly cross-linked UV-cured siloxane copolymer networks as icephobic coatings

Preventing ice growth on infrastructure, vehicles, and appliances remains a significant engineering challenge. Damage caused by ice growth on these installations can be expensive to repair, and their failure can be dangerous. Materials such as cross-linked polymer networks make effective anti-ice coatings and can prevent ice growth: reducing the cost of infrastructure repairs and limiting downtime. A link between cross-link density and ice adhesion has been demonstrated, such that lower cross-link density materials tend toward lower ice adhesion. Here we describe a method of lowering cross-link density by incorporating the covalently bound comonomers methyl methacrylate, lauryl methacrylate, and styrene into UV-cured PDMS-based polymer networks. Cross-link density, hardness, surface roughness, and ice adhesion on these materials are tested, showing the influence of comonomer proportions on their properties. Durability is found to increase with the addition of 5, 10, and 25 wt% comonomer, with little to no effect on ice adhesion until 25 wt%, where increases in ice adhesion are observed. Coatings show promisingly low ice adhesion of ~50 kPa, maintaining this low adhesion for up to 50 deicing cycles.     

Powder coatings for corrosion protection

Powder coatings found a wider use in corrosion protection of steel structure. In Europe very often double-layer systems are used, based on an adhesion promoting epoxy (EP) primer and a weathering stable top coat, mostly polyester (SP) sometimes EP/SP-hybrid powders. An interesting development is the use of zinc filled EP powders as primer to offer a cathodic protection to the steel surface. Powder systems with and without zinc were compared to proved coating systems based on liquid paint materials, where powder coating systems showed results comparable to these systems. Besides many advantages of powder coatings for corrosion protection there are still some problems. The workshops carring out the powder coating have to be in control of the surface pretreatment like chromating, but espescially phosphating and the work with the chromate-free pretreatment methods for galvanized steel. As always in the field of corrosion protection it is the surface pretreatment and preparation which determines the quality of the whole coating system decisively. This problem can be solved by appropriate working. In some years the problem with the general maintenance of powder coatings after weathering and ageing will be actual. This problem should be solved because of the homogeneous coatings on larger areas. Of importance will be the adhesion on the old coating and the appearance of the maintenance coating. The touch up of smaller parts as transport damages will be much more difficult in order to the appearance.     

Photodegradation of UV-cured coatings II. Polyurethane–acrylate networks

Highly crosslinked aliphatic and aromatic polyurethane-acrylate (PUA) coatings have been obtained by photopolymerization of multifunctional monomers. The discoloration and chemical modifications occurring upon accelerated QUV aging were monitored by UV and IR spectroscopy. The polymers were found to undergo photooxidation and loss of carbamate and phenyl groups, with lower quantum yields (10^?3 mol photon^?1) than in related linear polymers. Hydroxy-phenyl benzotriazole UV-absorbers have a limited effect on the degradation rate, at the concentration used (0.5%). Hindered amines (HALS) are substantially more effective, especially in aliphatic PUA. A 20-fold increase in the stabilization efficiency was found with the UVA + HALS combination. Aromatic PUA are more difficult to stabilize, because of the strong absorption and photolysis of the phenyl group which yields colored products. Radical-induced oxidation is predominant in aliphatic PUA and develops with long kinetic chains, while in aromatic PUA it competes with direct photolysis.     

Characterization of UV-cured polyester acrylate films containing acrylate functional polydimethylsiloxane

Acrylate functional polydimethylsiloxane (AF-PDMS) was tested as a reactive additive in UV-curable coating formulations. Pencil hardness, solvent resistance, and gloss of the UV-cured films were measured to study the influence of AF-PDMS content on coating properties. Depth-profile analysis by FTIR-ATR and Raman spectroscopy was also performed to investigate the effect of AF-PDMS on the behavior of film formation during UV curing. The kinetics of photopolymerization were monitored by photo differential scanning calorimetry (photo-DSC). Our results show that AF-PDMS containing coating formulations are very sensitive to oxygen inhibition, so that an inert environment such as nitrogen purging is required to avoid coating defects.     

Influence of photoinitiator and curing conditions on polymerization kinetics and gloss of UV-cured coatings

Polymerization kinetics and gloss of different formulations of coatings at different UV curing conditions were studied. The results showed that the photoinitiator type, its concentration, sample coating thickness, as well as the UV light intensity were the most significant factors affecting the polymerization course and the gloss of UV-cured films. The increasing concentration of the photoinitiator and the UV light intensity significantly decreased the gloss of the cured surface. The influence of the sample coating thickness on the kinetics and final gloss was also considerable.     

In-line determination of the thickness of UV-cured coatings on polymer films by NIR spectroscopy

We report on the development of a measuring method based on near-infrared (NIR) spectroscopy, which is able to determine the thickness of UV-cured coatings and which can be used for in-line monitoring in technical coating processes. In particular, acrylate coatings, which were applied to transparent polymer films with a typical thickness of 5–35 μm, were investigated. NIR spectra were recorded in transflection mode. Quantitative analysis of the spectral data was carried out with partial least square (PLS) regression. In-line measurements were performed on a pilot-scale roll coating machine at web speeds up to 50 m/min. It was shown that quantitative data with excellent precision (i.e. with a standard deviation lower than ±1 μm) and high time resolution (2.5 spectra/s) can be obtained.     

Zinc coatings for oxidation protection of ferrous substrates

A common phenomenon in the process industries is the oxidation of the exterior surface of steel pipes used in superheated steam or hot oils networks. For their protection different coatings could be used. In the present work the performance of zinc coatings deposited with hot-dip galvanizing, pack cementation and thermal spraying was considered, in order to protect industrial equipment up to 400°C. For that purpose coated carbon steel coupons were exposed at 400°C and their behavior was examined with light microscopy, scanning electron microscopy and X-ray diffraction. Thermogravimetric analysis was also used in order to observe in situ the oxidation phenomena. From this investigation it was deduced that in every coating a scale is formed that is mainly composed of ZnO, while Fe oxides were also detected in galvanized and pack coatings. The growth of this scale took place at the metal/scale interface. Moreover, as far as it regards the kinetics of the oxidation, it was concluded that the increase of the mass of the specimens is a function of the square root of the exposure time, which means that the scale formed is rather protective for the underlying zinc. From the above observation it seems that the behavior of zinc coatings would be excellent at 400°C. However, the presence of the Fe/Zn phases inside the galvanized and pack coatings led to the formation of cracks, which could expose the substrate and thus destabilize the coating. This phenomenon does not take place in the thermal sprayed coatings, where the Fe/Zn phases are absent.     

Zinc coatings for oxidation protection of ferrous substrates

The oxidation resistance of ferrous materials at elevated temperatures is limited. For that purpose the performance of zinc coatings deposited with hot-dip galvanizing, pack cementation and thermal spraying was considered. In the present work the oxidation resistance of these coatings at 400°C was estimated with light microscopy, thermogravimetric analysis and X-ray diffraction. From this examination it was deduced that in every coating a scale that was mainly composed of ZnO was formed, while Fe oxides were also detected in galvanized and pack coatings. However, the presence of the Fe/Zn phases inside the galvanized and pack coatings led to the formation of cracks, which could expose the substrate and thus destabilize the coating. This phenomenon was not observed in the thermal sprayed coatings, where the Fe/Zn phases were absent. In any case these cracks are not likely to jeopardize the resistance of the coating because zinc is anodic to steel. Hence, from the above examination it was deduced that the behavior of zinc coatings would be sufficient at 400°C.     

Sol-gel coatings for chemical protection of stainless steel

Sol-gel thin coatings of ZrO₂, SiO₂, 70SiO₂-30TiO₂ and 88SiO₂-12Al₂O₃ compositions (mole %) have been prepared from sonocatalyzed sols and deposited by dip-coating technique on 316L stainless steel foils. The influence of the coatings on the chemical corrosion of the substrate has been measured through potentiodynamic polarization curves in aqueous 15% H₂SO₄ solution between 25 and 50°C. The values of the corrosion potential, polarization resistance and corrosion rate have been determined. Analysis of the data combined with scanning electron microscopy studies indicate that the films act as a geometric blocking against exposure to the corrosive media and increase the lifetime of the substrate up to a factor 8.5.     

Wet-chemical deposition of functional coatings for bone implantology

Since the early 1980s, research on the modification of bone implant surfaces by applying coatings has mainly focused on the application of inorganic calcium phosphate (CaP) coatings using physical deposition techniques. Organic components of the extracellular bone matrix, on the other hand, play an essential role in the process of bone healing, but cannot be applied using these physical techniques. Therefore, a recent trend in biomaterials research involves development of novel wet-chemical deposition techniques for both inorganic and organic coatings. This study reviews the major wet-chemical coating techniques that are used for the deposition of inorganic CaP coatings and organic biomolecules coatings.     

Sol-gel alumina coatings on stainless steel for wear protection

The aluminium oxide films on austenitic steel are prepared from sols of re-dispersed boehmite nano powders in water. After dip-coating of the sol, a heat treatment including drying, calcination and annealing in vacuum at temperatures up to 1100°C is performed to obtain crack-free coatings of a thickness up to 6 μm. XRD measurements detect α- and γ-alumina, a TiO_x-phase at the metal/coating interface and a gradient of phase formation in the coating. The strong adhesion on the substrates is due to the layered assembly and gradient composition of the coating caused by an inter-diffusion of metal cations and oxygen in the metal/oxide interface during heat treatment. Residual stress measurements by X-rays result in compressive stresses of 2–4 GPa in the alumina coatings. The pin-on-disc test shows a remarkable improvement of wear resistance obtained by sol-gel coatings. The α-alumina content and the compressive stress of the coatings correlate with wear resistance of the coatings.     

Nanosilica-reinforced UV-cured polyurethane dispersion

UV-curable polyurethane dispersions (UV–PUDs) have been reinforced with hydrophobic and hydrophilic modified silicas, respectively, and the effects have been studied with dispersion and dispersion cast films. It has been found that particle size increased and water swell decreased, tensile modulus, strength, and thermal stability increased with the addition and increasing amount of silica. These effects were more pronounced with the hydrophilic modified one than the hydrophobic modified one.     

Sol-gel ZrO2 coatings for chemical protection of stainless steel

ZrO₂ coatings deposited on 316 L stainless steel sheets were synthesized by sol-gel method using Zr(OC₃H₇)₄ as precursor and isopropanol, glacial acetic acid, and water as solvents for application with ultrasounds. Different solutions for dip-coating were prepared with compositions varying between 0.025 and 0.9 mol/dm³ of ZrO₂. X-ray diffraction shows that the films densified at 800°C are crystalline with a tetragonal structure. The thickness of the coatings varied from 0.35–0.75 m. The influence of the ZrO₂ coatings on the corrosion behavior of stainless steel substrates in aqueous NaCl was studied through potentiodynamic polarization curves at 1 mV/s. The values of the electrochemical parameters allow for an explanation of the role of the films in the increased resistance of steel against corrosion in moderately aggressive environments.     

Electrochemical characterization of organic coatings for protection of historic steel artefacts

Electrochemical techniques are mainly known in the field of cultural heritage conservation as a tool for the elimination of corrosion layers or the removal of chlorides. However, these techniques are also a valuable tool for assessing the anti-corrosive efficiency of protective coatings. The aim of this study was to evaluate the performance of different coatings for their use in metallic heritage conservation using polarisation resistance (R _p) and electrochemical impedance spectroscopy (EIS). Carbon steel samples were prepared to simulate the surface composition and morphology of historic steel artefacts, and coated by a conservator–restorer following the common practices in conservation treatments. Three commercial organic coatings have been studied: a microcrystalline wax (Renaissance™) and a methyl acrylate/ethyl methacrylate copolymer resin (Paraloid™ B-72) dissolved in acetone—both them commonly used in conservation and restoration treatments—and an ethylene copolymer wax emulsion in water (Poligen™ ES-91009) that has not been used so far for this purpose. Four commercial corrosion inhibitor additives were added to the Paraloid™ B-72 resin and Poligen™ ES-91009 wax. The additives were commercial preparations with the following known active components: a blend of triazoles (M435), an ammonium salt of tricarboxylic acid (M370), a calcium sulphonate (M109), and a bis-oxazoline (Alkaterge-T™). R _p and EIS results showed that the best protection of the steel specimens was afforded by Poligen™ ES-91009 when applied in thick layers. None of the additives have shown a clear improvement of the protection properties of the coatings, and one of them impaired the barrier effect of the coating.     

Time-temperature effects in polymer coatings for corrosion protection as analyzed by EIS

More information is contained in Electrochemical Impedance Spectroscopy (EIS) results at steady-state vs. Temperature data sets than just the low frequency limit/polymer coating film resistance data most often cited. We have analyzed various EIS-Temperature data sets for several coating films in immersion and present the results considering the coating film dielectric constant vs. frequency and temperature. Water up-take can be analyzed by these methods and can be used to estimate the role of this process in the failure of corrosion protective coatings. For coating films that have been subjected to cyclic exposure, analysis of the dielectric constant vs. frequency data resulting from EIS data on these films indicate that cyclic exposure contributes significantly to ‘physical aging’ of the coating polymer.     

Application of functional coatings by sol-gel method

Nippon Sheet Glass Co. has developed many sol-gel derived products during the last decade. In this paper, the history of the production development, the details of the products and the production process for automotive windows using sol-gel thin-film coatings are introduced. These sol-gel thin-film coatings include UV absorbing, hydrophobic, low reflective and solar control coatings.     

Fabrication of coatings based on epoxy resins for environmental protection of microelectronics

Modified coating for protection of microelectronics from external effects, including radiation, was created on the basis of epoxy resins. The coating was produced with a modifier, thioalkaphene B, containing polysulfide derivatives of ortho-tert-butyl phenol. The mechanism of its action was explained. The technology for coating deposition was tested on an industrial automated line; the thus protected semiconductor devices successfully passed tests in extreme working conditions.     

Fire protection of polypropylene and polycarbonate by intumescent coatings

An innovative way to improve fire‐retardant properties of different polymers by applying intumescent coatings on their surface has been studied. Two polymers have been investigated: polypropylene and polycarbonate. The surfaces were first subjected to a flaming treatment in order to clean them and to increase their wettability and thus improve the adhesion of the coatings. Two different formulations were then applied: a transparent intumescent varnish, based on an acrylic resin, and an intumescent coating based on polyvinylacetate resin. Different parameters have been obtained using several fire tests. The cone calorimeter, the limiting oxygen index and UL94 tests have been carried out to evaluate the fire‐retardant properties obtained for both the intumescent coating and the intumescent varnish. Results clearly evidence an outstanding improvement of the fire‐retardant properties using intumescent coatings without any incorporation of flame retardants in the bulk. Copyright © 2010 John Wiley & Sons, Ltd.     

Robust self-healing waterborne polyurethane coatings via dynamic covalent Diels-Alder bonds for corrosion protection

For waterborne polyurethanes (PUs), balancing robust mechanical performances and excellent self-healing ability is a great challenge. Here, we show that this goal can be achieved by a rational tuning of the PU chemistry. In particular, we synthesized an anionic self-healing waterborne PU using acetone process, in which 2,2-bis(hydroxymethyl)propionic acid (DMPA) serves as inner emulsifier, thermally dynamic Diels-Alder bonds act as healing motifs and hexamethylene diisocyanate trimer is the crosslinker. The mechanical performance can be tuned by increasing DMPA concentration due to the gradually increased hard segment contents and ionic interactions. The tensile stress and elongation at break of films containing 5.6 wt% of DMPA are 24.9 MPa and 911.9%, respectively. Moreover, dynamic reversible Diels-Alder bonds located in main chains and cross-linking points ensure excellent self-repairing capability. Upon mechanical damage, the tensile stress can be restored to 95% of its initial value. Electrochemical impedance spectroscopy also points out an outstanding barrier ability and excellent corrosion protection performance of the coatings, which can be recovered even after serious damages.