Effects of highly absorbing pigments on near infrared cured polyester/melamine coil coatings

In order to expand available colour range for an industrial coil coating line a range of 25 μm polyester melamine coatings were applied to galvanised steel substrate and rapidly cured using near infrared (NIR) radiative curing. The purpose was to improve understanding of this relatively new curing technology and identify any problems associated with differing absorption of a range of coloured coatings. It has been suggested that in order to increase efficiency of NIR cure, NIR absorbers should be added to the coating formulation. UV/Vis/NIR spectroscopy was used to deduce the parts that coating and substrate absorption play in topcoat cure and lab scale trials were run on coatings throughout the colour range with their heating profiles and surface finish being recorded and assessed. The results showed that in this particular application having a coating that absorbs too strongly in the NIR region can actually result in solvent boil defects due to cross linking and film formation occurring prior to solvent removal.     

Inhibition of corrosion-driven organic coating disbondment on galvanised steel by smart release group II and Zn(II)-exchanged bentonite pigments

Bentonite pigments exchanged with either zinc or group II cations are characterised as inhibitors of corrosion-driven cathodic disbondment of model polyvinylbutyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanised steel. An in situ scanning Kelvin probe (SKP) technique is used to quantify rates of coating delamination as a function of pigment volume fraction (?_pt) and draw up a ranking order of inhibitor efficiency. Group II cation-exchanged bentonites show a moderate degree of inhibition, where rates of coating disbondment are reduced by up to 60-70% compared to the unpigmented case. In contrast, bentonite pigments containing exchangeable Zn²⁺ ions are markedly more effective, and no delamination is observed over periods of up to 24 h when ?_pt ≥ 0.1. The efficiency of in-coating Zn²⁺ is attributed to the ability to block underfilm cathodic oxygen reduction by reinforcing a pre-existing zinc (hydr)oxide layer.     

Novel approaches in NIR curing technology

Near infrared (NIR) curing technology is used more and more in coating processes where high belt speeds are needed to reduce the curing time from minutes down to seconds [1], [2].In times of increased concern for energy consumption it is vital to optimize the uptake of energy by the coating system caused by differences in pigmentation. As standard clear coats typically absorb NIR radiation only to a small extent, the heat uptake proceeds mainly indirectly via thermal conduction from the substrate e.g. steel and by convection. NIR absorption in pigmented systems is strongly dependent on the choice of inorganic and/or organic pigments. In particular, pale shades absorb only a small amount of NIR radiation.Efficient, colorless and transparent NIR absorbers are highly desirable for further penetration of NIR curing technology into industrial reality.The presented data in this paper demonstrates for the first time the use of an efficient NIR absorber (NIR-A 1) in different pigmented as well as transparent coating formulations. Its performance against 3 commercially available near infrared absorbers was evaluated concerning absorption in the near infrared region (800–1500 nm) as well as heat uptake during near infrared curing. NIR-A 1 reaches absorption values as high as 30% calculated versus carbon black (FW^® 200) as 100% reference, with a broad absorption profile. It outperforms the 3 commercial near infrared absorbers also concerning heat uptake after near infrared curing. An evaluation of the weathering stability shows nearly no impact on a clear coat after 1500 h WOM CAM 7. The addition of 0.05 w%/w of NIR-A 1 to a 45.0 w%/w TiO₂ pigmented formulation leads to more than a doubled belt speed to cure. Only a low impact on the color of the coating formulation can be seen.     

A study of the corrosion resistance of a waterborne acrylic coating modified with nano-sized titanium dioxide

The incorporation of nano-sized inorganic pigment particles into organic coatings may offer the potential for improving many of their properties, including corrosion resistance, at relatively low loadings. In the present research, titanium dioxide with a crystallite size of 5-10 nm was added to a waterborne organic primer formulation at loadings from 0.1 to 5% (w/w) and applied to hot-dip galvanized steel (HDG) panels. The corrosion resistance of the modified coatings was measured by neutral salt spray corrosion testing and electrochemical impedance spectroscopy (EIS), with an unpigmented film tested for comparison. 3% (w/w) TiO₂ appeared to produce an optimum improvement in the corrosion resistance.     

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.     

Comparative electrochemical studies of zinc chromate and zinc phosphate as corrosion inhibitors for zinc

The anticorrosive performance of two inhibitive pigments, zinc chromate and zinc phosphate, was compared using electrochemical impedance spectroscopy (EIS) and the scanning vibrating electrode technique (SVET) in pigment extracts in 0.1 M NaCl. It was observed that zinc was protected from corrosion in both extracts. In tests using hot dip galvanised steel painted with an epoxy primer incorporating the pigments, the SVET detected the anodic and cathodic distribution along the scribes, although no significant differences were observed among the various primers. On the contrary, EIS was able to distinguish processes occurring on the metal surface exposed by the scribe in different samples. For primers with anticorrosive pigment, a time constant at high frequencies was attributed to a layer of protective nature, probably formed by metal ions from the substrate and inhibitive ions leached from the anticorrosive pigments.     

Anticorrosive painting for a wide spectrum of marine atmospheres: Environmental-friendly versus traditional paint systems

This work presents some of the main results obtained in different marine atmospheres by Working Groups 1 and 2 (dedicated to anticorrosive protection of steel by paint coatings) of the Ibero-American PATINA network, developed in the context of the CYTED Programme. As marine atmospheres it includes natural atmospheres with a salinity level above S1 classification of ISO standard 9223 (>3 mg Cl⁻ m⁻² day⁻¹), and sulphur dioxide contamination only up to classification P1 of the same standard (maximum of 35 mg SO₂ m⁻² day⁻¹). Consideration is also made of accelerated tests traditionally used to assess the anticorrosive behaviour of the substrate/paint coatings contemplated in the study, namely salt spray, artificial weathering and different cycles involving ultraviolet radiation, humidity, temperature and different contamination conditions. The substrates were steel, hot-dip galvanised steel and electrogalvanised steel (Zincorr^® sheet). The paint systems applied on these substrates, with or without pretreatments, were solventborne, waterborne, high solid and powder paint systems. As a result it has been possible to conclude which of the studied anticorrosive coating types were the most suitable for each of the different types of marine atmospheres considered in the study.     

Studying the corrosion protection properties of an epoxy coating containing different mixtures of strontium aluminum polyphosphate (SAPP) and zinc aluminum phosphate (ZPA) pigments

Epoxy/polyamide coatings were loaded with different mixtures of strontium aluminum polyphosphate (SAPP) and zinc aluminum phosphate (ZPA) pigments. Moreover, a coating containing zinc phosphate (ZP) was prepared as a reference sample. The coatings were applied on St-37 steel substrates and then were exposed to 3.5 wt% NaCl solution up to 35 days. The corrosion inhibition properties of the pigments extracts were studied on bare steel samples by a potentiodynamic polarization technique after 24 h immersion. The morphological properties and corrosion resistance of the coatings were investigated by scanning electron microscope (SEM), optical microscope, electrochemical impedance spectroscopy (EIS) and salt spray tests.     

A study on the anticorrosion performance of the epoxy-polyamide nanocomposites containing ZnO nanoparticles

Epoxy nanocomposites were prepared using different loadings (2, 3.5, 5 and 6.5 wt%) of ZnO nanoparticles. Nanocomposites were applied on steel substrates. Samples were immersed in 3.5 wt% NaCl solution for 1344 h. Corrosion resistance of the coatings was studied by an electrochemical impedance spectroscopy (EIS). The effects of addition of nanoparticles on the mechanical properties of the epoxy coating were studied by a dynamic mechanical thermal analysis (DMTA). Curing behavior of the coatings containing nanoparticles was studied by a differential scanning calorimeter (DSC). Atomic force microscope (AFM) was utilized to investigate the surface topography and surface morphology of the coatings. Coating resistance against hydrolytic degradation was studied by FTIR (Fourier Transform Infrared).Results showed that addition of low loadings of nanoparticles can increase T_g of the composite. Decrease in T_g and cross-linking density of the coating were observed at high loadings of nanoparticles. It was found that nanoparticles can influence the curing behavior of the epoxy coating. Nanoparticles improved the corrosion resistance of the epoxy coating. Increase in coating resistance against hydrolytic degradation was obtained using nanoparticles.     

Electrochemical and anticorrosion performances of zinc-rich and polyaniline powder coatings

In this work, hydrochloride polyaniline (PANI-Cl) powder was incorporated as a conductive pigment into powder zinc-rich primer (ZRP) formulations in order to enhance the electronic conduction paths between zinc particles inside the coating and the steel substrate (i.e. percolation). Coatings were applied onto steel substrates and immersed in a 3% NaCl solution at ambient temperature.The protective properties and electrochemical behaviour of coatings were investigated by monitoring the free corrosion potential versus time and by using EIS. It was found that corrosion potential remains cathodic and constant for a long time up to 100 days of immersion. From EIS results, it was shown that the coatings exhibit larger impedance values than those observed with liquid or other zinc-rich powder formulations containing carbon black. From Raman spectroscopy results, it may be proposed that zinc particles in contact with PANI-Cl pigments were passivated. Other zinc particles remain still active which ensures the cathodic protection of the substrate. Moreover, coatings exhibit good barrier properties.     

Adhesion enhancement of powder coatings on galvanised steel by addition of organo-modified silica particles

The addition of organo-modified silica particles (OSP) to organic monolayer coatings has been investigated as an alternative to the use of primers or surface pretreatments in galvanised steel substrates. A commercial additive consisting of trifunctional organosilane (alkyl-triethoxysilane) grafted on silica particles was directly incorporated at different concentrations (1, 2.5, 3.5 and 4.5 wt%) as an integral additive in a polyester powder coating. The OSP were characterised physicochemically by means of FTIR and TGA, and the coating formulated by DSC. The anticorrosive properties of the systems were evaluated by means of electrochemical impedance spectroscopy (EIS), showing improvements with all the formulations containing the OSP, especially in the coating with 2.5% OSP. In order to explain this behaviour, morphological (using SEM) and adhesion studies were done. The formation of agglomerates in the powder coatings was detected when the concentration was over 2.5%. There was an improvement in the adhesion of the coating to the substrate for all the samples containing the OSP but especially for that containing 2.5%. The impact resistance was increased too, especially in the formulations with 2.5% and 3.5%.     

Corrosion protection of galvanized steel by polyimide coatings: EIS and SEM investigations

Coatings of two types of polyimides (PI), as poly(4,4′-oxydiphtalic anhydride-co-2,5-bis(4,4′-methylenedianiline)-1,4-benzoquinone) (AQ) and poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PM), were synthesized on galvanized steel panels and studied and compared in terms of chemical structure, microstructure and corrosion performance in 0.5 M NaCl solution. Infra-red spectroscopy, scanning electron microscopy and in situ electrochemical impedance spectroscopy were employed in the investigations. The results showed that, although both studied PI coatings provided the galvanized steel substrate with corrosion protection during the test period, there were evident differences in electrochemical behaviour of the coatings, which could be primarily explained by the different nanostructures. FE-SEM examinations revealed AQ PI coating to be heterogeneous and discontinuous but PM PI coating homogeneous and continuous in nanostructure. Electrochemical behaviour of AQ PI coated galvanized steel obeyed that of a defect-containing coating and indicated gradual decrease in protectivity. In contrast, PM PI coating behaved like a defect-free coating and it provided the galvanized steel substrate with effective corrosion protection all through the 960 h test. The explanations for these observations and the mechanisms of coating damage are discussed in this paper.     

Application of EIS and salt spray tests for investigation of the anticorrosion properties of polyurethane-based nanocomposites containing Cr2O3 nanoparticles modified with 3-amino propyl trimethoxy silane

The Cr₂O₃ nanoparticles were modified with 3-amino propyl trimethoxy silane in order to obtain proper dispersion and increment compatibility with the polyurethane coating matrix. The nanocomposites prepared were applied on the St-37 steel substrates. The existence of 3-amino propyl trimethoxy silane on the surface of the nanoparticles was investigated by Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric analysis (TGA). Dispersion of the surface modified particles in the polyurethane coating matrix was studied by a field emission-scanning electron microscope (FE-SEM). The electrochemical impedance spectroscopy (EIS) and salt spray tests were employed in order to evaluate the corrosion resistance of the polyurethane coatings. Polarization test was done in order to investigate the corrosion inhibition properties of the Cr₂O₃ nanoparticle on the steel surface in 3.5 wt.% NaCl solution. The adhesion strengths of the coatings were evaluated by pull-off adhesion tester before and after 120 days immersion in 3.5 wt.% NaCl solution. FT-IR and TGA analyses revealed that surface modification of the nanoparticles with 0.43 silane/5 g pigment resulted in the greatest amount of silane grafting on the surface of particles. Results obtained from FE-SEM analysis showed that the surface modified nanoparticles dispersed in the coating matrix properly. Results obtained from EIS and salt spray analyses revealed that the surface modified particles enhanced the corrosion protection performance of the polyurethane coating considerably. The improvement was more pronounced for the coating reinforced with 0.43 g silane/5 g pigment. Moreover, the adhesion loss decreased in the presence of surface modified nanoparticles with 0.43 silane/5 g pigment.     

Process control of printing processes with in-line NIR spectroscopy and elimination of the influence of the substrate on the prediction of the coating weight

Near-infrared (NIR) reflection spectroscopy assisted by chemometric methods was used for in-line monitoring of the coating weights of thin printed layers of clear varnishes applied to papers or polymer foils. The coating weights of the acrylate-based UV-curable layers were in the range between 1 and 7 g m⁻². The precision of the multivariate calibration models was found to be strongly affected even by minor variations of the substrate which may lead to considerable mispredictions of the coating weight. The interfering effect of the substrate absorption was compensated by special calibration procedures. Two different approaches were used. Multistage calibration models combining an identification step with quantitative analysis allowed a clear assignment of any spectrum to specific calibration models. Alternatively, the variation of the substrate was included into the calibration model by use of a broader range of calibration samples. The efficiency of both approaches was demonstrated by the prediction of the coating weights of independent test samples. Moreover, in-line measurements were carried out at a sheet fed offset printing press. It was shown that the coating weights of printed layers can be determined with a prediction error of about 0.15 g m⁻² for layers on paper or 0.3 g m⁻² on PET foils.     

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.     

In-line monitoring of the conversion in photopolymerized acrylate coatings on polymer foils using NIR spectroscopy

Near-infrared (NIR) reflection spectroscopy was used to monitor the conversion of double bonds in acrylate coatings after irradiation with UV light or electron beams. Quantitative analysis of the spectroscopic data was performed either with a chemometric method on the basis of the PLS algorithm or according to the Beer-Lambert law. FTIR spectroscopy was used for calibration. In-line monitoring of the conversion in pilot-scale was carried out on clear and pigmented coatings, which were applied to polymer foils or paper by roll coating. Useful data were obtained from layers with a thickness from 4 g/m² upwards and at line speeds of at least up to 120 m/min. It was shown that any change of the irradiation dose or other parameters such as inertization leads to an immediate response in the conversion record. Similar investigations were also performed on layers of UV-curable adhesives on the basis of acrylic hot-melts.     

Monitoring of a military vehicle coating under Prohesion exposure by embedded sensors

Two-layered organic coatings protect metallic structures against corrosion by reducing the transport of water, ions, and oxygen from the environment to the substrate and storing species that provide corrosion inhibition. The topcoat and the primer provide the former and latter, respectively, with the primer also providing adhesion between the topcoat and the substrate. A standard testing method for coatings is the ASTM G85 Prohesion^® test. This test involves alternating between a wet exposure of 1-h salt spray at 25 °C and a dry exposure of 1-h air at 35 °C. Apart from visual inspection, conventional monitoring of coating properties is achieved by ex situ electrochemical tests under immersion in a suitable electrolyte. In situ monitoring during a Prohesion^® test was performed for a military vehicle coating system using embedded sensors located between the topcoat and primer. In situ results associated with the dry and wet exposures are presented from electrochemical impedance spectroscopy and electrochemical noise measurements. In situ single-frequency impedance measurements were used to monitor the water uptake/loss associated with the exposures of the test conditions. Ex situ monitoring was also performed for comparison with the in situ embedded sensor monitoring.     

Study of the anticorrosive behaviour of epoxy binders containing non-toxic inorganic corrosion inhibitor pigments

The anticorrosive performance of epoxy coatings pigmented with non-toxic corrosion inhibitors pigments was investigated in this work. The coatings used contained the following pigments: zinc phosphate (ZP), zinc phosphomolybdate (ZPM) and zinc calcium phosphomolybdate (ZCPM). For comparative studies epoxy coatings with the following compositions were made up: one only with filler (CRG); one without pigments, varnish (VR) and other with zinc chromate (ZC) pigment. The corrosion inhibitor performance of the coatings was evaluated by immersion tests in 0.01 mol L⁻¹ NaCl aqueous solutions and accelerated tests in a salt spray chamber. The corrosion inhibitor performance of the samples was monitored using open-circuit potential (E_oc) measurements and electrochemical impedance spectroscopy (EIS) technique. Complementary tests were carried out using water vapour permeability of free-standing films and thermogravimetric (TG) analysis. The permeability test showed that the addition of the studied pigments did not modify the barrier properties of the free-films in comparison that pigmented with chromate. Thermal analysis indicated that the addition of the pigments improved the thermal stability of the coatings and it suggested a resin/pigment interaction. The total immersion tests and salt spray tests demonstrated that the barrier properties of the coatings pigmented with the inhibitors were not degrading as much as that pigmented with ZC. Therefore, all the three pigments could replace ZC as an anticorrosive pigment in similar conditions to those described here. The best corrosion inhibitor performance in the total immersion test was presented by the ZPM and ZCPM coatings while in the salt spray test the corrosion inhibitor performance of all the three pigmented coatings was similar, suggesting that only in the less aggressive test is possible to detect any difference between the coatings with the non-toxic pigments.     

Preparation and characterization of hydroxyapatite-forsterite-bioactive glass nanocomposite coatings for biomedical applications

In order to improve biological and mechanical properties of hydroxyapatite, the concept of hydroxyapatite-included nanocomposite coatings was introduced. By judiciously choosing constituent ceramics for composites preparation, the biological and mechanical performance of coatings can be tailored in order to meet various clinical requirements. The aim of this work was fabrication, development and characterization of novel hydroxyapatite-forsterite-bioactive glass nanocomposite coatings. The sol-gel technique was used to prepare hydroxyapatite-forsterite-bioactive glass nanocomposite in order to apply coating on 316L stainless steel (SS) by dip coating technique. The X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were used to investigate the phase structure, microstructure and morphology of the coating. In order to evaluate the forsterite incorporation influence upon bioactivity, the changes on the surfaces of the prepared composite coatings after the predicted days of contact with simulated body fluid (SBF) were investigated by SEM. Results showed that the suitable calcined temperature for nanocomposite coatings with different amounts of forsterite was 600 °C. At this temperature, the homogenous and crack-free coating could attach to the 316L SS substrate. The crystallite sizes of the prepared coatings were lower than 100 nm. The EDX analysis of hydroxyapatite-forsterite-bioglass, coated 316L SS surface, indicated consisting elements of prepared coatings and the substrate. During immersion in the SBF at pre-determined time intervals, apatite layer was formed and stimulation for apatite formation was increased with increase in forsterite amounts. It seems that hydroxyapatite-forsterite-bioactive glass nanocomposite coatings might be good candidates for biomedical applications.     

Resistance of metallic substrates protected by an organic coating containing aluminum powder

The corrosion behavior of an epoxy primer containing aluminum powder (10 vol.%) applied on carbon steel and on galvanized steel was examined by electrochemical impedance spectroscopy (EIS). The data show that this coating is more protective when applied onto carbon steel substrates, and that on galvanized steel thicker coatings allow to achieve similar protection levels as those obtained for carbon steel. These effects are probably due to aluminum pigments providing a cathodic protection of the substrate, and to the resulting products precipitating inside the pores of the polymeric coating. Three stages can be distinguished during exposure of the coated specimens. Upon immersion of the coated samples in the test solution, a pre-saturated stage is observed. After a certain period of immersion, which strongly depends on the thickness of the applied coating, a saturation stage is reached in which an effective protection of the metallic substrate against corrosion is achieved. Finally, at sufficiently long exposure times, swelling through the coating eventually leads to the detachment of the coating.