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.     

Effect of TiO2 pigment type on the UV degradation of filled coatings

The objective of this study was to investigate the effect of the photoreactivity of titanium dioxide (TiO₂) pigments on the photodegradation of polymeric coatings used in exterior applications. Two polymer matrices, an amine-cured epoxy (EP) and an acrylic urethane (AU), containing three types of TiO₂ pigments, classified by different levels of photoreactivity, were studied. Specimens were exposed on an ultraviolet (UV) weathering chamber, the Simulated Photodegradation by High Energy Radiant Exposure device at the National Institute of Standards and Technology. Two exposure conditions were used: ambient, dry condition [25°C and 0% relative humidity (RH)] and high temperature, wet condition (55°C and 75% RH), which is similar to more severe outdoor exposures. The physical and chemical degradations of the filled coatings were monitored at periodic intervals using a combination of laser scanning confocal microscopy (LSCM) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Progression of degradation on the coating surfaces was characterized by LSCM in terms of changes in surface roughness and morphology, pigment agglomerate size, and the occurrence of pits or holes in the coatings. The observed physical changes were correlated to the chemical changes measured by ATR-FTIR as a function of UV exposure time. Both EP and AU systems showed less degradation in terms of surface roughness and morphological changes under the dry conditions compared to the wet exposure conditions. It was observed that both the pigment type (and hence photoreactivity) and particle dispersion strongly affected the degradation of both EP and AU systems.     

Microstructure and morphology of amine-cured epoxy coatings before and after outdoor exposures—An AFM study

Atomic force microscopy (AFM) has been used to study the morphology and microstructure of an amine-cured epoxy before and after outdoor exposure. Measurements were made from samples prepared in an essentially CO₂-free, H₂O-free glove box and from samples prepared in ambient conditions. For those prepared in a CO₂-free glove box, AFM imaging was conducted on (1) an unexposed air/coating surface, (2) an unexposed coating bulk, (3) an unexposed coating/substrate interface, and (4) a field exposed air/coating surface. For samples prepared in ambient conditions, only the unexposed air/coating surface was investigated. The same regions of the exposed samples were scanned periodically by the AFM to monitor changes in the surface morphology of the coating as UV exposure progressed. Small angle neutron scattering and Fourier transform infrared spectroscopy (FTIR) studies were performed to verify the microstructure and to follow chemical changes during outdoor exposure, respectively. The results have shown that amine blushing, which occurs only under ambient conditions, had a significant effect on the surface morphology and microstructure of the epoxy. The surface morphology of the samples prepared under CO₂-free, dry conditions was generally smooth and homogeneous. However, the interface and the bulk samples clearly revealed a two-phase structure consisting of bright nodular domains and dark interstitial regions, indicating an inhomogeneous microstructure. Such heterogeneous structure of the bulk was in good agreement with results obtained by small angle neutron scattering of unexposed samples and by AFM phase imaging of the degraded sample surface. The relationship between submicrometer physical changes and molecular chemical degradation is discussed. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL.     

Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation

Photodegradation of polymer coatings generally involves photooxidation, resulting in the formation of oxidized products, chain scission, and crosslinking. On severe exposure to ultraviolet (UV) light in the presence of air, chemical degradation transforms into substantial changes in the physical and mechanical properties, leading to failures of the coatings. Systematic research by NIST on service life prediction of polymeric coatings indicates that the degradation of polymer coatings starts from the sub-micrometer degradation-susceptible regions at the surface and then grows in width and depth. Additionally, due to the oxygen diffusion effect and the attenuation of the UV light passing through the polymer, the degradation can be spatially heterogeneous. In this study, the changes with depth of the mechanical and chemical properties of a UV-exposed epoxy/polyurethane system were measured by nanoindentation and Fourier transform infrared spectroscopy (FTIR) microscopy using cross-sectioned specimens. Multilayers of epoxy/polyurethane samples were prepared by a draw-down technique. After curing, samples were exposed to the outdoors in Gaithersburg, MD, for four months. Cross-sectioned slices of the exposed and unexposed samples, approximately 500 nm thick as-prepared by microtoming, were used for micro-FTIR imaging. Samples for nanoindentation were prepared by embedding the epoxy/polyurethane multilayers (both exposed and unexposed) in a molding compound, followed by microtoming and polishing the embedded films in the thickness direction. Micro-FTIR images clearly show that, for the outdoor exposed samples, substantial amounts of oxidation products are distributed in the 60 μm deep region from the surface to the epoxy bulk, decreasing in the center of epoxy region and increasing again toward the epoxy/urethane interface. Nanoindentation results also show that the modulus significantly increases in the first 60 μm region after UV degradation, and then decreases gradually with depth until a value slightly higher than the modulus of the undegraded epoxy is reached. The modulus rises again in the region near the epoxy/urethane interface. These similarities in the depth profiles of the properties indicate the linkage between the chemical degradation and the mechanical degradation. The study clearly shows that the spatial distribution of chemical species and mechanical properties is heterogeneous in the thickness direction for polymer coatings after UV degradation. It also demonstrates that cross-sectional analysis using nanoindentation and micro-FTIR imaging techniques is a useful method to characterize the mechanical and chemical depth profiles of polymer coating degradation.

New aspects of aging in epoxy networks. I. Thermal aging

The thermal aging of an amine‐cured epoxy in the glassy state is studied for two network states by using DSC and attenuated total reflection‐infrared (IR‐ATR). The “low‐crosslinked” network possesses a relatively high molecular mobility and a considerable amount of residual reactive groups. In the low crosslinked matrix, the presence of high crosslinked regions is revealed. In contrast, the “highly crosslinked” epoxy system has a reduced molecular mobility and only small reactive groups. The high crosslinked matrix contains low crosslinked regions. Thermal loading for both networks is performed below their glass transition. During thermal aging, an ongoing curing reaction takes place in the low‐crosslinked epoxy. Thermooxidative degradation and the disintegration of short‐range ordering are observed as well. The highly crosslinked epoxy system undergoes a phase separation of relatively mobile segments in the low mobile matrix, which is a reversible process on heating. Thermooxidative degradation is also detected for this kind of network. In summary, for the “low” and the “highly” crosslinked epoxy, significant chemical and structural changes take place during thermal aging even though the networks are vitrified. It is convincing that these changes in the cured epoxy should exert an influence on the mechanical properties of a bonded structure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 361–368, 2004     

Kinetics of photodegradation and nanoparticle surface accumulation of a nanosilica/epoxy coating exposed to UV light

Temperature effect on the kinetics of photodegradation and surface accumulation of nanoparticles in an epoxy nanocoating exposed to ultraviolet light (UV) was investigated. A model epoxy coating containing 5% untreated nanosilica was selected. Exposed film specimens were removed at specified UV dose intervals for measurements of chemical degradation of the epoxy component, and nanosilica accumulation on specimen surface release as a function of UV dose for four temperatures. The chemical degradation was measured using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV–visible spectroscopy. Atomic force microscopy was employed to determine the kinetics of nanosilica accumulation on the nanocoating surface during UV exposure. The temperature dependence behaviors of kinetic parameters obtained by various measurement techniques will be used to better understand the degradation mechanism and surface accumulation of nanoparticles in exterior nanocoatings.     

Synthesis,characterization and end-use evaluation of epoxy resins prepared from diallyl and dipropyl bisphenol A

We have synthesized 2,2'(6')-diallyl-4,4'-isopropylidenediphenol (diallyl bisphenol A), 2,2'(6')-dipropyl-4,4'-isopropylidenediphenol (dipropyl bisphenol A), and epoxy resins (glycidyl ether oligomers) of different average molecular weights. The acrylic and methacrylic esters were prepared by reacting the epoxy resin with acrylic and methacrylic acids, respectively. The amine-cured epoxy resins showed promise as general-purpose adhesives, while the epoxy acrylates were found to be useful as reactive oligomers in ultraviolet (UV) radiation curable coatings, and the epoxy methacrylates as anaerobic adhesives.     

New aspects of aging in epoxy networks. II. Hydrothermal aging

An amine cured epoxy is prepared in two different network states for hydrothermal aging. The “low‐crosslinked” network has a considerable amount of residual reactive groups and a relatively high‐molecular mobility. The low‐crosslinked matrix contains high‐crosslinked regions. In contrast, the “highly crosslinked” epoxy system has little reactive groups and a lower molecular mobility. Here, low‐crosslinked regions are found in a high‐crosslinked matrix. Hydrothermal loading for both networks is performed in demineralized water at temperatures below their glass transition. The water plasticizes both kinds of networks which remain in the glassy state, however. As a consequence, in the low‐crosslinked epoxy, the increased molecular mobility promotes an ongoing curing reaction leading to the consumption of epoxy groups until an almost complete network has formed. As a new aging process, phase separation occurs in the highly crosslinked epoxy. The new phase is more mobile than the matrix because it has its own glass transition at a lower temperature. In addition, thermooxidative degradation is observed for both network states. Certainly, these chemical and structural changes in the epoxy networks should influence the performance of an adhesive joint, a coating, or a fiber‐reinforced composite. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 369–377, 2004     

Accelerated aging versus realistic aging in aerospace composite materials. II. Chemistry of thermal aging in a structural composite

Samples of an aerospace structural epoxy composite (8552/IM7) were subject to long‐term (≈ 1 year) thermal aging at temperatures of 70°, 120°, 170°, and 200°C (in air). The changes to the chemical and physicochemical structure of the composite were analyzed by a range of different techniques, including gravimetric analysis, Fourier transform infrared (FTIR), and dynamic mechanical analysis (DMA) to compare the effects of different severities of degradation treatment. The results highlighted the large differences in chemical effects between the surface and the interior of the composite with very minor changes in the latter even at quite high aging temperatures and long aging times. The oxidative changes at the surface, however, varied from highly selective molecular changes for particular chemical groups at the lower aging temperatures (70° and 120°C), to quite general and extensive oxidative degradation at the higher aging temperatures (170° and 200°C). The results indicated that the mechanical changes in an aged composite of this type will vary greatly with the material thickness and surface protection as well as the aging temperature the composite is exposed to. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3221–3232, 2006     

Surface degradation and nanoparticle release of a commercial nanosilica/polyurethane coating under UV exposure

Many coating properties such as mechanical, electrical, and ultraviolet (UV) resistance are greatly enhanced by the addition of nanoparticles, which can potentially increase the use of nanocoatings for many outdoor applications. However, because polymers used in all coatings are susceptible to degradation by weathering, nanoparticles in a coating may be brought to the surface and released into the environment during the life cycle of a nanocoating. Therefore, the goal of this study is to investigate the process and mechanism of surface degradation and potential particle release from a commercial nanosilica/polyurethane coating under accelerated UV exposure. Recent research at the National Institute of Standards and Technology (NIST) has shown that the matrix in an epoxy nanocomposite undergoes photodegradation during exposure to UV radiation, resulting in surface accumulation of nanoparticles and subsequent release from the composite. In this study, specimens of a commercial polyurethane (PU) coating, to which a 5 mass% surface-treated silica nanoparticle solution was added, were exposed to well-controlled, accelerated UV environments. The nanocoating surface morphological changes and surface accumulation of nanoparticles as a function of UV exposure were measured, along with chemical change and mass loss using a variety of techniques. Particles from the surface of the coating were collected using a simulated rain process developed at NIST, and the collected runoff specimens were measured using inductively coupled plasma optical emission spectroscopy to determine the amount of silicon released from the nanocoatings. The results demonstrated that the added silica nanoparticle solution decreased the photodegradation rate (i.e., stabilization) of the commercial PU nanocoating. Although the degradation was slower than the previous nanosilica epoxy model system, the degradation of the PU matrix resulted in accumulation of silica nanoparticles on the nanocoating surface and release to the environment by simulated rain. These experimental data are valuable for developing models to predict the long-term release of nanosilica from commercial PU nanocoatings used outdoors and, therefore, are essential for assessing the health and environmental risks during the service life of exterior PU nanocoatings.     

Novel water-dispersible glycidyl carbamate (GC) resins and waterborne amine-cured coatings

Water-dispersible glycidyl carbamate (GC) functional resins were synthesized and crosslinked using a water-dispersible amine to form coatings. GC functional resins are synthesized by the reaction of an isocyanate functional compound with glycidol to yield a carbamate (urethane) linkage (–NHCO–) and reactive epoxy group. The combination of both functionalities in a single resin structure imparts excellent mechanical and chemical properties to the coatings. Previous studies on the development of GC coatings have focused on solvent-borne coating systems. In this study, GC resins were modified by incorporating nonionic hydrophilic groups to produce water-dispersible resins. To determine the influence of the content of hydrophilic groups on dispersion stability, aqueous dispersions were made from a series of hydrophilically modified GC resins and characterized for particle size and dispersion stability. The composition of a typical, dispersed GC resin particle was predicted using Monte Carlo simulations. Stable GC dispersions were used to prepare amine-cured coatings. The coatings were characterized for solvent resistance, water resistance, hardness, flexibility, adhesion, and surface morphology. It was observed that GC resins were able to be dispersed in water without using any surfactant and by minimal mixing force (hand mixing) and produced coating films with good properties when crosslinked with a compatible waterborne amine crosslinker.     

The thermooxidative stability of cured epoxy resins. I

Prior studies have shown that for many epoxy resin systems significant oxidative degradation occurs in air at temperatures as low as 100°C and that thin oxidized surface layers serve as crack initiators in flexural samples at low strains and can reduce flexural strengths to less than 25% of the original values. This study was undertaken in an attempt to identify predominant degradation pathways and cured resin systems that are thermooxidatively stable in 125°C air. Based on flexural property retention and IR spectral data, the thermooxidative resistance ranking of D.E.R.*332 epoxy resin polymerized with the following is p-toluenesulfonamide > 4,4′-diaminodiphenyl sulfone or sulfanilamide > methylene dianiline ? triethylenetetraamine > 2,5-dimethyl-2,5-diaminohexane. Oxidation of aliphatic amine-cured D.E.R. 332 epoxy resin is initiated by electrophilic attack of oxygen on the lone-pair electrons of the nitrogen to form an amine oxide. Polymer chain cleavage then occurs via Cope reactions. A newly proposed oxidative degradation pathway is described wherein the hydroxylamine products of Cope reactions are further oxidized to nitrones, which then decompose to amides via oxaziridine intermediates. Commercial antioxidants added to an aliphatic amine-cured epoxy resin were ineffective, supporting the conclusion that the predominant degradation mechanism is not free radical in nature. © 1993 John Wiley & Sons, Inc.     

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.     

Multiscale physical characterization of an outdoor-exposed polymeric coating system

Surface topography and gloss are two related properties affecting the appearance of a polymeric coating system. Upon exposure to ultraviolet (UV) radiation, the surface topography of a coating becomes more pronounced and, correspondingly, its gloss generally decreases. However, the surface factors affecting gloss and appearance are difficult to ascertain. In this article, atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM) measurements have been performed on an amine-cured epoxy coating system exposed to outdoor environments in Gaithersburg, Maryland. The formation of the protuberances is observed at the early degradation stages, followed by the appearance of circular pits as exposure continues. At long exposure times, the circular features enlarge and deepen, resulting in a rough surface topography and crack formation. Fourier Transform Infrared Spectroscopy (FTIR) study indicates that the oxidation and chain scission reactions are likely the origins of the surface morphological changes. The relationship between changes in surface roughness and gloss has been analyzed. The root mean square (RMS) roughness of the coating is related to nanoscale and microscale morphological changes in the surface of the coating as well as to the gloss retention. A near-linear dependence of RMS roughness with the measurement length scale (L) is found on a double logarithmic scale, i.e., RMS ∼ L ^f. The scaling factor, f, decreases with exposure time. The relationship between surface topography, on nano- to microscales, and the macroscale optical properties such as gloss retention is discussed. Moreover, a recent development in using an angle-resolved light scattering technique for the measurement of the specular and off-specular reflectance of the UV-exposed specimens is also demonstrated, and the optical scattering data are compared to the gloss and the roughness results.

Growing more ductile epoxies: an essential work of fracture study

Ambient, amine-cured epoxy compositions exist within the dual constraints of VOC regulation and the vitrification effect, which limits the ultimate T _g of these materials. These combined constraints result in epoxy products that are densely crosslinked and which contain appreciable quantities of nonfugitive diluents or plasticizers. Such materials tend to be more brittle than traditional solvent-based epoxy coatings based on solid epoxy resin and polyamide hardeners. There is thus a need for a practical method to measure their fracture toughness. This work introduces the method of essential work as a useful way to determine the fracture toughness of thermoset systems. This method is then used to relate fracture toughness to the crosslink network structures of amine-cured epoxy compositions. The test compositions are varied systematically with a view to structure/properties interpretation, and employ an in situ chain extension approach to “grow” more ductile networks. Solvent uptake of selected compositions is also determined, and the relative trade-off between ductility vs. solvent uptake is examined. This article was awarded the Outstanding Paper Award in New Coatings Technology at the 33rd Annual International Waterborne, High-Solids, and Powder Coatings Symposium in New Orleans, LA, February 2006, and was presented at the Thermoset Resins Formulators Association Conference in Montreal, Quebec, Canada, September 2006.     

The influence of lixiviates on the thermal degradation of diglycidyl ether of bisphenol A n=0/1,2‐diaminecyclohexane studied by dynamic mechanical analysis and thermogravimetry‐fourier transform infrared spectroscopy

The influence of the lixiviates originated in a municipal landfill on the thermal degradation of a polymeric system composed of a diglycidyl ether of bisphenol A (n = 0) and 1,2‐diaminecyclohexane was studied by dynamic mechanical analysis. Storage modulus (E′), loss modulus (E″), and glass transition temperature were measured to make a comparative study between the samples before and after being exposed to the chemical compounds in the lixiviate agents. The different data obtained were analyzed to check the resistance of these materials to chemical attack and the possibility of their use as coating materials in plants where those reagents were present. Thermal stability of the system diglycidyl ether of bisphenol A/1,2‐diaminecyclohexane exposed to the attack of lixiviates has also been studied by thermogravimetric analysis. A quantitative study of the gases originated during thermal degradation of the epoxy/diamine system made by infrared spectroscopy. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 443–453, 1999     

The influence of ageing of epoxy coatings on adhesion of polyurethane topcoats and protective properties of coating systems

The dependence of adhesion and protective properties of coating systems on surface properties of epoxy intermediate coatings, aged and non-aged before an application of polyurethane topcoats, were examined. The intermediate coatings were aged 500 h in UV chamber. The surface free energy and polar groups were estimated after ageing. After applying polyurethane topcoats on aged and non-aged epoxy coatings, resistance to salt spray and thermal shocks were tested as well as internal stresses were measured before and after corrosion tests.The results showed that adhesion in coating systems with polyurethane topcoats applied on aged epoxy coatings depends strongly on the degradation degree of epoxy intermediate coatings and the value of generated internal stresses. Coatings with good adhesion retention in corrosion environments have good protective properties even when temporary blistering has occurred.     

新型光稳定剂应用于水性环氧涂料的研究

水性环氧涂料已发展成为解决一般常用传统高VOC溶剂型涂料的高污染问题,它具有有效减少或不使用溶剂的优点,而环保型涂料所遭遇的挑战之一是需要光稳定剂来保护涂料预防光降解产生。针对水性环氧涂料开发了一种新型复配型光稳定剂EVERSORB EP5,应用实验设计(DOE)研究结果表明:3种不同类型光稳定剂添加至2种不同类型溶液-水和水性环氧涂料中进行分散过滤试验,EVERSORB EP5分散效果最好,人工加速老化试验结果显示:增加EVERSORB EP5在水性涂料中的有效浓度,可以增强相对的保护效果,增加涂料的膜厚无法得到某种程度的保护效果。     

化工厂重防腐涂料及涂装

研制了用芳香胺固化酚醛环氧树脂配合惰性填料、鳞片的耐酸耐碱涂料系列,分析了在产品设计中应关注的主要因素,阐述了两个配套涂层在化工设备上的应用优势。研究与应用证明,酚醛环氧芳香胺玻璃鳞片胶泥配套体系能在100~150℃强酸、强碱浸泡下的钢结构储罐、物料塔中应用较长时间。     

Aging behavior of a hot‐cured epoxy system

The thermal and hydro‐thermal aging of a hot‐cured epoxy system (diglycidylether of bisphenol A (DGEBA) + dicyandiamide (DDA)) in the glassy state is revisited using DSC and IR attenuated total reflection spectroscopy. Because of the diffusion of DDA from the solid particles into the liquid DGEBA matrix, curing produces a highly crosslinked amorphous matrix that contains low crosslinked amorphous regions. After full curing, the network possesses a relatively low molecular mobility and no residual reactive groups. Thermal and hydro‐thermal loading is performed at 60°C, well below the principal glass transition temperature (T_g1 = 171°C). Both aging regimes cause significant chemical and structural changes to the glassy epoxy. It undergoes a phase separation of relatively mobile segments inside the low mobile matrix, providing a second glass transition that shifts from T_g2 = 86–114°C within 108 days of aging. This phase separation is reversible on heating into the viscoelastic state. Hydro‐thermal aging leads to a reversible and a nonreversible plasticizing effect as well. On thermal aging, no chemical changes are observed but hydro‐thermal aging causes significant chemical modifications in the epoxy system. These modifications are identified as a partial degradation of crosslinks produced by the cyano groups of the DDA and correspond to the nonreversible plasticitation. These changes in the cured epoxy should exert an influence on the mechanical properties of an adhesive bond. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006