Surface degradation process affected by heterogeneity in nano-titanium dioxide filled acrylic urethane coatings under accelerated UV exposure

The objective of this study was to investigate the effect of nanoparticle dispersion on surface morphological changes and degradation process in polymeric coatings during exposure to ultraviolet (UV) radiation. Three types of nano-titanium dioxide (nano-TiO₂) were selected and dispersed into acrylic urethane (AU) coating to generate degrees of nanoparticle dispersion states. Two accelerated exposure conditions: wet (30 °C and 75% relative humidity (RH)) and dry (30 °C and 0% RH), were selected. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was used to monitor surface chemical degradation. Laser scanning confocal microscopy (LSCM) was used to characterize nanoparticle dispersion and surface/subsurface morphological changes in the AU coatings during UV exposure. For a given nanoparticle, similar surface morphological changes of the coatings indicated the similar degradation processes under the wet and dry conditions, but the degradation was faster under the wet condition. Surface morphological changes were closely related to the nanoparticle dispersion in three coatings, and the heterogeneity in nanoparticle dispersion significantly affects the degradation process and dominates the degradation patterns.     

Use of laser scanning confocal microscopy for characterizing changes in film thickness and local surface morphology of UV-exposed polymer coatings

Laser scanning confocal microscopy (LSCM) has been used to characterize the changes in film thickness and local surface morphology of polymer coatings during the UV degradation process. With the noninvasive feature of LSCM, one can obtain thickness information directly and nondestructively at various exposure times without destroying the specimens or deriving the thickness values from IR measurement by assuming uniform film ablation. Two acrylic polymer coatings were chosen for the study, and the physical and chemical changes of the two systems at various exposure times were measured and analyzed. Those measurable physical changes caused by UV exposure include film ablation, formation of pits and other surface defects, and increases in surface roughness. It was found in both coatings that changes in measured film thickness by LSCM were not correlated linearly to the predicted thickness loss using the changes in the CH band obtained by the Fourier Transform Infrared (FTIR) spectroscopy measurements in the later degradation stages. This result suggested it was not a uniform film ablation process during the UV degradation. At later stages, where surface deformation became severe, surface roughness and profile information using LSCM were also proven to be useful for analyzing the surface degradation process Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 13–14, 2004 in Philadelphia, PA.     

Evaluation of the ASTM D7869-13 test method to predict the gloss and color retention of premium architectural finishes-I

A recently developed xenon arc-based accelerated weathering cycle, ASTM D7869-13, has been validated for automotive and aerospace coatings, but its ability to predict the gloss and color retention of premium architectural finishes has not yet been evaluated. We review new weathering data comparing the performance of poly(vinylidene fluoride) (PVDF) architectural finishes in south Florida exposure as well as several accelerated exposure methods including ASTM D7869-13. ASTM D7869 accurately reproduced Florida rank order gloss and color retention trends for coatings made with PVDF-acrylic blends and inorganic pigments, as well as the gloss and color changes seen in Florida for 70% PVDF masstone coatings made with a number of single organic pigments. However, the D7869 cycle has difficulty predicting the rank order of rutile TiO₂ grades for the gloss retention of PVDF coatings in Florida, as well as the magnitude and direction of color fade from organic pigment degradation in organic pigment/inorganic pigment blends. One open question that remains is whether the ASTM D7869 cycle might have some utility for industry standard or specification purposes, if the test is limited to specific reference colors or more ideally to specific reference pigments.     

Relating gloss loss to topographical features of a PVDF coating

Semi-gloss commerical poly(vinylidene fluoride) (PVDF) coatings typically have 60° gloss values between 20 and 50. Gloss is affected by PVDF crystallite structures and by the pigmentation. In this article, we have demonstrated that for some pigmented PVDF coatings, after 10 years of Florida exposure, the principal proximal cause of gloss changes is the formation of micron-scale pits, rather than the emergence of pigment particles at the coating surface. We have used laser scanning confocal microscopy (LSCM) and light scattering to characterize the surface topography and near-surface structure of weathered and unweathered PVDF coatings. Florida-weathered PVDF coatings show only a modest increase in the root mean square (RMS) roughness of the surface, even when oticeable gloss loss has occurred. Changes in gloss can be correlated with surface roughness and other topographical, features, including the formation of pits and the emergence of pigments. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 25–27, 2004, in Chicago, IL.     

Photopolymerisable formulations for enhanced properties of pigmented films

Abstract

Ultraviolet (UV) curing is an important and widely used surface technology for coatings. The curable formulations containing: monomer: epoxydiacrylate, photoinitiating system: 4-methoxybenzoyldiphenylphosphine oxide/2-hydroxy-2-methyl-1-phenyl-propan-1-one, additive reactive: tertiary amine, and inorganic thermoresistant pigments: white (TiO₂) and blue (Al₂O₃–Cr₂O₃), were cured like films by UV exposure. A series of experiments was carried out in order to optimise the photoinitiator ratios in photoinitiating system, and pigment content in the photocurable formulation. Films with enhanced mechanical properties (pendulum hardness, scratch resistance), high resistance at solvents and good appearance were developed. In order to provide adequate hiding power of the pigmented formulation, the pigment content has been chosen in the range 5–15%. Up to 15% pigment content may affect the efficiency of photoinitiator. The polymerisation efficiency (conversion and polymerisation rates) of photoinitiating systems in pigmented formulations was performed by differential scanning photocalorimetry (photo DSC) in comparison with the original systems without pigments.     

Relationship between chemical degradation and thickness loss of an amine-cured epoxy coating exposed to different UV environments

The relationship between chemical degradation and thickness loss of an unpigmented, non UV-stabilized, crosslinked amine-cured epoxy coating exposed to three UV conditions was investigated. Spin-coated samples having a thickness of approximately 7 μm on an Si substrate were prepared from a stochiometric mixture of a bisphenol A epoxy resin and a tetra-functional amine curing agent. Samples were exposed outdoors and to two accelerated laboratory UV environments. Chemical degradation and thickness loss were measured by transmission Fourier transform infrared spectroscopy (FTIRS) and laser scanning confocal microscopy (LSCM), respectively. In addition, surface roughness and morphological changes were measured by atomic forcemicrosocopy (AFM) and LSCM. Substantial chemical degradation, thickness loss, and morpholocal changes occurred in the exposed films, and the rate of chemical degradation was greater than that due to the thickness loss. This additional chemical loss was attributed to an inhomogeneous degradation process in which nanoscale localized depressions initiate at certain sites on the surface, which then enlarge and deepen with exposure time. The results of this study provide a better understanding of the degradation mechanism and should lead to the development of scientific-based models for predicting the service life of crosslinked amine-cured epoxy coatings. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL     

The effect of TiO2, pigmentation on the hydrolysis of amino resin crosslinked epoxy can coatings

Pigmented (TiO₂), amino resin crosslinked coatings, designed for applications in can coatings’ internal lacquers, were formulated, characterized, applied, and cured. Three grades of a pigmentary form of TiO₂ were characterized in terms of their particle size, their particle morphology, their zeta potential, and their moisture retention behavior. Epoxy coatings that were crosslinked using one of several, different amino crosslinkers were prepared. The effect of the presence of the TiO₂ pigments on the hydrolysis of the cured coatings was monitored via the controlled retorting of the coatings. The different grades of TiO₂ pigment were selected, to establish whether or not they could be used interchangeably with respect to hydrolysis and to melamine release. Also, the effects of the aging of the fluid coatings on the amount of melamine released from the coatings (after curing and retorting) were monitored. Storage under laboratory conditions for 2, 20, and 40 weeks was used for this purpose. The TiO₂ pigment contributed significantly to the hydrolysis behavior of the epoxy coatings in that their presence substantially reduced the amount of melamine released and the extent of crosslinker hydrolysis. Typical results show that excluding the TiO₂ pigment particles from the formulation results in there being 50% more hydrolysis of the crosslinker to melamine. With respect to the melamine release and crosslinker hydrolysis, the different grades of the pigment gave similar results.     

Preparation of electrochromic Ni1−xO and TiO2 coatings from pigment dispersions and their application in electrochromic foil based devices

Thin (100–400 nm) electrochromic TiO₂ and Ni_1−xO coatings providing transmissive light modulation were made from an anatase pigment dispersion obtained by co-grinding nanocrystalline titanium particles (6–10 nm in size) with trisilanol heptaisobutylsilsesquioxane as dispersant, while Ni_1−xO based pigment dispersions were made by milling pre-prepared Ni_1−xO pigment with nickel oxyhydroxide (NiO_xH_y) dispersant. Dispersions were obtained by milling the pigments with zirconia beads of various sizes (0.1, 0.2 and 0.4 mm) and the particle size was determined with the dynamic light scattering technique (DLS). Pigment dispersions were deposited by spin-coating on glass and plastic (PET) film and thermally treated at 150 °C to obtain thin TiO₂ and Ni_1−xO pigment coatings. SEM and AFM were used for determination of the surface morphology, revealing their homogenous structure and low surface roughness (up to 20 nm). The optical transmittance and haze of the coatings deposited on glass and PET film were determined from the UV–vis spectra. Their electrochromic effect was analyzed by electrochemical charging/discharging the coatings in a LiClO₄/PC electrolyte. The results demonstrated a convenient, simple and robust technique for making “electrochromic paint” coatings. Pre-prepared TiO₂ and pigments were used for construction of foil-based electrochromic devices with transmissive modulation of light.     

Enhanced photocatalytic activity of highly transparent superhydrophilic doped TiO2 thin films for improving the self-cleaning property of solar panel covers

Undoped and metal doped nanocrystalline TiO₂ transparent thin films were synthesized on glass substrates via sol-gel/dip-coating method. TiO₂ thin film coatings can be applied to the surfaces of solar panels to impart self-cleaning properties to them. The structural and optical properties of few nanometer-thick films were characterized by XRD, SEM, CA, AFM, XPS, and UV–Vis spectrophotometry techniques. The stoichiometric TiO₂ films crystallized in anatase phase, with a particle size of ～100 nm, which were uniformly distributed on the surface. The prepared films with a roughness of ～1–5 nm, increased the hydrophilicity of the glass surface. Reducing the amount of Ti precursor (X) favored the improvement of film quality. To improve the photocatalytic activity of the TiO₂ thin film, it was doped with Ni, Cd, Mo, Bi and Sr metal ions. The effect of metal doping on the photocatalytic activity of the films was investigated using the degradation process of methylene blue (MB) dye as the model contaminant. Among the prepared coatings, the Sr–TiO₂ film showed the highest efficiency for MB degradation. It increased the dye degradation efficiency of the films under both UV and Vis lights. The kinetic investigations also showed that the degradation of MB by TiO₂ and M ? TiO₂ films obeyed the pseudo-first order kinetics.     

The effect of TiO2 nanopigment on the optical properties of polyester fabric in UV–VIS–NIR regions

The particle size parameter is a key factor that affects radiative properties of nanopigments, and consequently, pigments of the same type but different sizes represent different spectral performance. Therefore, current study dealt with a systematic experimental investigation on the effect of TiO₂ pigmented coatings on spectral reflectance and color performance of white and colored polyester fabrics in UV, VIS, and NIR region of electromagnetic spectrum, with a special emphasis on VIS region. In order to accomplish this target, polyester fabrics were coated with TiO₂ nanopigment with various concentrations and different diameters, and their reflectance spectra were measured using spectrophotometric method. Two‐way analysis of variance (ANOVA) was utilized to investigate the significance of the effect of TiO₂ nanopigment on the color performance of coated fabrics. According to experimental observations, an organized color shift appears in color coordinate of fabrics coated with TiO₂ nanopigment of various sizes. Moreover, although TiO₂ nanopigment with 35 nm diameter has the most significant impact on short wavelength region (UV region), the effect of pigment with 250 nm diameter is more noticeable on NIR region as long wavelength region.     

Characterization of pigment-leached antifouling coatings using BET surface area measurements and mercury porosimetry

In this work BET surface area measurements and mercury porosimetry are used to characterize leached layers formed when seawater-soluble pigments (Cu₂O and ZnO) dissolve during accelerated leaching of simple antifouling coatings. Measurements on single-pigment coatings show that an increasing fraction of Cu₂O or ZnO pigment particles becomes unavailable for dissolution when the concentration of the pigment decreases in the coating and the interparticle distance in the binder matrix becomes larger. Experimental data for a coating initially containing a mixture of Cu₂O and TiO₂ pigments suggest that a substantial fraction of the smaller and inert TiO₂ particles may be lost from the coating upon dissolution of the larger Cu₂O particles. This inert particle translocation effect is important to take into account when interpreting polishing and leaching data and when developing mathematical models of antifouling coating behaviour because the active binder surface area and porosity of the leached layer are substantially increased. A similar effect was not observed for a coating with a mixture of ZnO and TiO₂ pigments. The two experimental methods are expected to be useful for practical analysis of leaching of seawater-soluble components from commercial antifouling coatings.     

Photocatalytic enamel/TiO2 coatings developed by electrophoretic deposition for methyl orange decomposition

The aim of this study was to obtain photocatalytic coatings, capable to decompose organic pollutants, through Electrophoretic Deposition (EPD) of enamels containing respectively 0%, 5%, 10%, 15% (in wt%) of TiO₂ onto carbon steel substrates. High quality and homogeneous coatings were obtained by applying 12.5?V during 10?s, as the best EPD conditions. The layers were subsequently heat treated at 740?°C for 10?min, in order to obtain dense glazes.Rietveld refinement of XRD patterns and Raman results show that, after the heat treatment at 740?°C, TiO₂ mostly exists as anatase, responsible of the photocatalytic effect. Semi-quantitative chemical analysis indicate segregation of TiO₂ on the coatings surface, reaching saturation in the sample with 10?wt% TiO₂. FEG-SEM observations reveal rod-like and spherical Ti-rich phases along the cross section of the coatings; some Ti was also dissolved into the enamel. 3D topographical mapping shows that, by adding TiO₂, surface roughness increases significantly.Photocatalytic tests were carried out using a 2?×?10^?5 M aqueous solution of Methyl Orange (MO) as an organic pollutant. By comparing the decomposition rate of MO achieved with the pure enamel (0% of TiO₂) and with the sample with 10% of TiO₂, it was shown that the addition of 10% of TiO₂ results in 90% photocatalytic efficiency.Moreover, the permeation of organic compounds and their UV degradation were studied by measuring the water contact angle onto the enamel surface directly after dipping into oleic acid and after various UV irradiation times. The longer the UV irradiation time, the lower the contact angle, down to a minimum of 14.54° after 8?h of UV irradiation. This means, the compound was initially adsorbed on the enamel/TiO₂ coating surface (10?wt% TiO₂) but was efficiently decomposed upon UV irradiation.     

Studies of the aging effect on the level of isocyanate residues in polyester-based can coating systems

A methyl ethyl ketone oxime-blocked isocyanate was used as a crosslinking agent in the curing of polyester–polyurethane (PEPU) can coatings. The completeness of cure was assessed from the amount of residual (un-reacted) isophorone diisocyanate (IPDI) trimer that was presented in the coating after curing. This amount of residual trimer depended strongly on the temperature at which the curing was undertaken and time given for curing. The degree of cure was also assessed, correlating with the reduced IPDI trimer migration potential. The amount of residual IPDI trimer that was extractable from a typical PEPU coating rose markedly (15- to 20-fold) if the wet coating was aged up to 6 months before use. The presence of a TiO₂ pigment increased the aging effect. An aluminum flake pigment had an even greater effect. The TiO₂ pigment increased aging by different degrees according to the type of surface modification that had been applied to the pigment. The grade having the greatest alumina content, as a result of surface modification, gave the greatest aging. The undesirable effects of aging were counteracted by the addition of more catalyst to the aged coating just before the coating was used to coat panels. Deactivation of the acidic polymerization catalyst system (based on organotin compounds) is thought to be the cause of this aging. These findings have allowed improvements to be made to the specifications of the pigments and the catalysts used, with respect to the consistent industrial production of low migration coatings.     

Role of phase content on the photocatalytic performance of TiO2 coatings deposited by suspension plasma spray

In this work, suspension plasma spraying (SPS) with different hydrogen (H₂) flow rates was employed to produce TiO₂ coatings with various phase contents, oxygen contents, and roughnesses. To eliminate the role of the morphology and oxygen content on the photocatalytic activity, all coatings were polished to reach the same roughness followed by heat-treatment at 550 °C in air for 48 h. Then coatings were analyzed by X-ray diffractometer (XRD), confocal laser microscope, scanning electron microscope (SEM), UV–visible spectrometer, Raman microscope, and thermogravimetric analyzer. The XRD data indicated that the percentage of anatase decreased as function of H₂ flow rates, and almost 46% of anatase transformed to rutile during SPS process at the highest H₂ flow rate. Moreover, the photocatalytic performance was evaluated by monitoring the degradation of methylene blue under visible light irradiation, and the results indicated that anatase phase positively enhances the photocatalytic activity of TiO₂ coatings.     

Use of poly(2-vinylpyridine)-b-poly(?-caprolactone) copolymers as pigment stabilizers in powder coatings

Four poly(2-vinylpyridine)-b-poly(ɛ-caprolactone) copolymers, differing in buoy block length and anchor/buoy block length ratio, have been used as TiO₂ pigment dispersants in a polyester powder coating resin. When the TiO₂ surface was fully covered with the block copolymers, the colloidal stability of the TiO₂ dispersions at typical curing temperatures was significantly improved because of the formation of a steric layer around the pigment particles. As a result, powder coatings with excellent flow, high gloss, and low haze values were obtained. Because of the high affinity of the dispersants for the TiO₂ surface, pretreatment of the pigments with the block copolymers was not necessary. At full pigment surface coverage, the chain length of the stabilizing polymer had little effect on the performance of the dispersants used. Dept. of Polymer Chemistry and Coatings Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.     

Characterization of absorptivities to solar radiation for colored pigments in coatings

Colored pigments absorptivities to solar radiation have been characterized quantitatively by measuring the absorption coefficients of the colored pigments mixed in white titanium dioxide standard coating. The standard coating contains white TiO₂ with 17% pigment volume concentration (PVC). Dry white coating with more than 134 μm thickness is opaque and used as a dispersion medium for colored pigments. The reflectances of the coatings of white TiO₂ and three kinds of colored pigments (perylene black, toluidine red, and benzidine yellow) to solar radiation are measured by means of a UV-3100 recording spectrophotometer operating in the 250∼2500 nm spectral range. It is found that the plots of lg 1/R (R, reflectance of coating to solar radiation) againstV _coloredV_{Ti0 2} (volume ratio of the colored pigment to TiO₂ in coating) are straight lines of increasing slopes in the range ofV _coloredV_{Ti0 2} less than five percent. From the slopes of the straight lines, the absorption coefficients of colored pigments can be measured quantitatively. Beijing, 100029, China. Email: guohy@public.fhnet.cn.net.     

Organosilane coatings applied on bronze: Influence of UV radiation and thermal cycles on the protectiveness

3-Mercapto-propyl-trimethoxy-silane (PropS-SH) coatings, with or without the addition of three types of oxide particles (CeO₂ (nano-sized), TiO₂ (nano-sized) and La₂O₃ (micro-sized)), were applied on quaternary bronze (Cu-Sn-Zn-Pb) and tested through natural and artificial exposures. In particular, uncoated and coated samples were exposed both in the coastal town of Rimini (Italy) under sheltered/unsheltered conditions and in a climatic chamber where the samples underwent temperature/UV cycles, at constant humidity. The sample surfaces were periodically characterized by SEM, EDS, micro-Raman, FTIR and XRD techniques. Moreover, during natural exposures, runoff rainwater was collected monthly, in order to evaluate the release of the alloying metals from bronze by atomic absorption spectrometry (AAS). The results showed that micro-scale cracking and spalling phenomena took place during natural exposure, partly limiting the protective efficiency of the organosilane coating. In fact, UV radiation has a strong influence on coating performance, inducing structural modifications of the polymer. Oxide particle- and especially TiO₂-charged coatings proved to be more resistant to UV degradation.     

Relationship between the passivation of TiO2 particles and LLDPE photodegradation: a comparison between bulk and surface impacts

The effect of the concentration of alumina (Al₂O₃) and silica (SiO₂) used to passivate titanium dioxide (TiO₂) particles on the photodegradation of plastic films containing these particles was investigated. The films were made of linear low-density polyethylene (LLDPE) containing four different types of passivated TiO₂ particles. The UV degradation of the films was evaluated for the surface and the bulk by measuring the physical and chemical changes as a function of time. The surface chemical and physical degradation effects were measured by ATR-FTIR and AFM, respectively. A statistical Gaussian adjustment was proposed to correlate the AFM depth profiles of the eroded surfaces of the films after the photodegradation process. The bulk physical effect was evidenced by the loss of mechanical properties in the films. The results showed that the higher the concentrations of Al₂O₃ are, the better the inhibition of the photodegradation of the LLDPE films. In this study, it was confirmed that the observed UV degradation effect correlated at both the surface and bulk levels. The results showed not only the reduction of the photodegradative effect as the passivation of the TiO₂ particles increased but also the possibility of using these particles as UV stabilizers of LLDPE films. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47026.     

The depth profiling of TiO2 pigmented coil coatings using step scan phase modulation photoacoustic FTIR

The depth distribution of a TiO₂ pigment within the polyurethane (PU) coil coatings is investigated using step scan phase modulation photoacoustic (SS-PM-PA) FTIR. Coil coatings with different pigment contents were prepared and the modulation frequency (MF) of the SS-PM-PA FTIR varied to record the depth distribution of the pigment within the coating. The TiO₂ pigment was shown to contribute significantly to the SS-PM-PA FTIR signal. A TiO₂ aggregated region within the topcoat is found close to the topcoat-primer interface and further away from the topcoat surface. A deeper TiO₂ aggregated region can be identified when pigment content is relatively low. The SS-PM-PA FTIR signal shows a considerable contribution from the primer originated signal, provided the TiO₂ pigment content is sufficiently high and the modulation frequency applied is relatively low. SEM cross-section imaging results show a strong correlation of the TiO₂ depth distribution with SS-PM-PA FTIR results, which confirms the applicability of the SS-PM-PA FTIR technique to the depth profiling study of TiO₂ pigmented coil coatings.     

Effect of anisotropy on photo-mechanical oxidation of polyethylene

Blown polyethylene (PE) film was exposed to UV irradiation while under tensile stress. Tests were conducted on unpigmented PE and on a series of five PEs containing TiO₂ pigments with different photo-activities. The yield strengths measured in tensile tests on unexposed films were typically 10% greater in the longitudinal (machine) direction than in the transverse direction. UV exposures were conducted with stress applied both parallel and transverse to the machine direction. The chemical degradation was followed using the FTIR carbonyl index. For all six materials, tensile stress accelerated carbonyl group development. Some samples cracked during UV exposure in tension and did so in a shorter time if the stress was applied in the machine direction than if it was transverse to it. The carbonyl index at the onset of cracking was lower for longitudinal samples than for transverse samples.Unpigmented PE was slightly more sensitive to transverse strain and the anatase-pigmented PE slightly more sensitive to longitudinal strain. The carbonyl production rate for two of the rutile pigmented grades showed very little dependence on the straining direction whereas for a third the increase in rate was significantly greater when the deformation was applied in the machine direction than for transverse stretching. With the fourth rutile-pigmented grade, transverse stretching produced the greater effect. These differences cannot be correlated with the photoactivity of the different pigments. The highest ratios of longitudinal/transverse enhancement were found with the least photoactive rutile pigment and with anatase—the most photoactive of all those tested.