An improved accelerated weathering protocol to anticipate Florida exposure behavior of coatings

A new accelerated weathering protocol has been developed which closely replicates the performance of automotive and aerospace coating systems exposed in South Florida. IR spectroscopy was used to verify that the chemical composition changes that occurred during accelerated weathering in devices with a glass filter that produced a high fidelity reproduction of sunlight’s UV spectrum matched those that occurred during natural weathering. Gravimetric water absorption measurements were used to tune the volume of water absorption during accelerated weathering to match that which occurred during natural weathering in South Florida. The frequency of water exposure was then scaled to the appropriate UV dose. A variety of coating systems were used to verify the correlation between the physical failures observed in the accelerated weathering protocol and natural weathering in South Florida. The new accelerated weathering protocol correctly reproduced gloss loss, delamination, cracking, blistering, and good performance in a variety of diverse coating systems. For automotive basecoat/clearcoat paint systems, the new weathering protocol shows significant acceleration over both Florida and previous accelerated weathering tests. For monocoat aerospace systems, the new weathering protocol showed less acceleration than for automotive coatings, but was still an improvement over previous accelerated tests and was faster than Florida exposure.     

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.     

Analysis of test methods for UV durability predictions of polymer coatings

The purpose of this paper is to review procedures which are used for the evaluation of the durability of polymer coatings. In particular, methods of environmental acceleration and techniques of assessment of coating degradation have been examined, with an emphasis upon those which may produce reliable fast answer durability predictions. The advantages and disadvantages of the various exposure regimes currently used (such as Florida exposure, EMMAQUA or artificial light sources) have been discussed in terms of correlation with actual durability data and of degree of acceleration. A general rule of thumb is that the correlation of accelerated methods with natural exposure is inversely proportional to the degree of acceleration used. The common physical methods used to assess the extent of coating durability have been presented, with the general drawbacks to these techniques being highlighted. Finally, the benefits and drawbacks of a number of chemical techniques (in particular electron spin resonance (ESR), Fourier transform infrared spectroscopy (FTIR), hydroperoxide determination and chemiluminescence), which in principal could provide durability information in a fraction of the time of the physical techniques mentioned earlier, have been discussed. Of these, both ESR and FTIR spectroscopy show particular potential because of the short exposure times necessary to obtain significant results under UVA exposure.     

A new acceleration factor for the testing of corrosion protective coatings: flow-induced coating degradation

For corrosion protective coatings that are designed to give lifetimes of protection that may extend to 50 years, valid accelerated test methods are necessary to develop improved systems and validate performance. Fluid flow over metals has long been believed to influence the corrosion process. Studies have been focused on the effects of flow rate on the corrosion of bare metals. The influence of fluid flow on the degradation of metal-protective coatings has received less attention. This paper describes a preliminary study on the influence of laminar flow on organic coatings. A Hele-Shaw cell and its associated fluid control apparatuses are incorporated into the electrochemical cell setup. The barrier properties of the coating as a function of immersion time and flow rate have been monitored by electrochemical impedance spectroscopy. We observe that the barrier properties of the coating measured electrochemically decrease exponentially with the increasing flow rate. We propose that the flowing electrolyte solution could be used in acceleration tests for the lifetime prediction of organic coatings as the acceleration of failure we have observed does not appear to change the mechanism of failure. Further analysis is proposed to validate immersion flow rate as a universal accelerating parameter for coating failure.     

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     

涂料老化机理及寿命评估方法研究进展

从引起涂料老化最主要的几个方面,理论上分析了涂料的老化机理,并列举了涂料寿命评估方面存在的困难,总结了目前采用的最主要的实验室涂料寿命评估试验及其取得的一些成果,还指出,要推算涂料的使用寿命,首先要了解涂料的基本种类、使用要求及使用环境,据此设计相应的实验室加速老化试验与户外暴露环境试验的对比,经过较长时间的试验比对,才能得出相对准确的涂料使用寿命。     

The interplay of physical aging and degradation during weathering for two crosslinked coatings

Polymer molecular relaxation, or ‘physical aging’, is a very important influence on permeability and mechanical properties of any polymer below its glass transition. ‘Physical aging’ occurs as even an unstressed polymer gradually relaxes towards its equilibrium conformation. This and the shorter term response to stress happen over periods much longer than the typical cycle of an accelerated weathering test, thus important properties of a polymeric coating may be affected by the difference in frequency between natural and artificial exposures, in addition to other factors. Further, ‘physical aging’ is affected by chemical changes to the polymer network caused by the degradation during a weathering exposure. In this investigation, purely physical aging was compared with the effect of concurrent chemical degradation by measuring ‘enthalpy recovery’ and mechanical stress relaxation at a variety of temperatures and at various stages during accelerated weathering exposure. The effect of physical aging was quite apparent in both an epoxy-polyamide coating and a polyester-urethane coating. Changes in physical aging behaviour during degradation were different for the two coatings, which points to further reasons for discrepancy between accelerated weathering and natural exposure.     

Characterization of interphase adhesion in multi-layer coating systems

Multi-layer coating systems are the state of the art for painting of metal and plastic surfaces e.g. in the automotive industry. Utilizing such systems consisting of several specialized coating materials, a high adhesion and a good corrosion protection can be reached, along with good barrier properties, chemical and scratch resistance as well as appealing surface appearance. The interphase regions between single layers of such systems represent areas of a potential weakness, where a delamination and a premature coating degradation can start. To test in a short time the protective properties of the coating, a special hydrothermal cyclic loading test procedure was developed, which causes accelerated build-up of internal stress within the coating system. Using an electrochemical impedance spectroscopy with various optimized electrode geometries, it was possible to detect the early stages of coating degradation and to compare different coating systems. Internal stress measurements contributed additionally to a better understanding of the coating degradation.     

Acid-resistant organic coatings for the chemical industry: a review

Industries that work with acidic chemicals in their processes need to make choices on how to properly contain the substances and avoid rapid corrosion of equipment. Certain organic coatings and linings can be used in such environments, either to protect vulnerable construction materials, or, in combination with fiber reinforcement, to replace them. However, degradation mechanisms of organic coatings in acid service are not thoroughly understood and relevant quantitative investigations are scarce. This review describes the uses and limitations of acid-resistant coatings in the chemical industry, with a comparison to alternative resistant materials based on metals or ceramics. In addition, coating degradation phenomena, caused by acid exposure, are mapped to the extent possible, and analysis methods for detecting coating degradation type and severity are listed and discussed. It is concluded that more knowledge on chemical and physical degradation mechanisms is required, and that improvements in resistance to elevated temperatures and abrasion would decrease the risk of use and increase the potential application areas of organic coatings exposed to acidic environments in the chemical industry.     

汽车内饰材料自然加速曝晒试验箱的研制

在汽车整机自然暴露试验过程中,汽车内饰件因长期处于太阳光辐射和高温环境而造成变形、破损等,为能更真实反映饰件老化性能状态,我们研发了用于汽车内饰件试验的新型自然加速试验箱,解决了室内加速老化试验模拟差的问题。     

On the use of the atomic force microscope to monitor physical degradation of polymeric coating surfaces

The atomic force microscope (AFM) was used to monitor changes in surface features of an acrylic melamine coating that was exposed to a variety of conditions. Exposure to ultraviolet (UV) radiation and high relative humidity caused general roughening of the surface and the formation of pits. Further, the damage of the coating surface was much more substantial for exposure to high relative humidity compared to exposure to dry environments. This difference in degradation rates correlated with measurements of chemical degradation determined using infrared spectra that were acquired along with the AFM images. Building Materials Division, 100 Bureau Dr., Stop 8621, Gaithersburg, MD 20899-8621.     

A macroscopically nondestructive method for characterizing surface mechanical properties of polymeric coatings under accelerated weathering

The surface of coatings and plastics is the first target in any degradation process initiated by ultraviolet (UV) radiation or mechanical stress (via scratch and abrasion). Surface damage can lead to changes in optical, morphological, and mechanical properties and can result in pathways for ingress of moisture and corrosive agents. Current test methods for monitoring performance of protective coatings focus on chemical properties and optical properties, such as color and gloss measurements, or invasive tests such as abrasion and cross-cut adhesion. In this study, a macroscopically nondestructive performance protocol using nanoindentation metrology via a well-controlled scratch test was applied to evaluate the scratch resistance and monitor the surface mechanical property changes in a protective coating under accelerated weathering. Polyurethane (PU) coatings with different polyol compositions were chosen for this study. Coating specimens were exposed to high-intensity UV radiation at 55°C and 75% RH conditions. Exposed specimens were removed at specified UV exposure times for surface modulus/hardness and scratch resistance characterization via nanoindentation and scratch test. The effect of polyol type and UV radiation dose on the scratch damage (scratch morphology) was investigated and correlated with the surface hardness and modulus of the materials.     

Accelerated protocol for measurement of carbonation through a crack surface

This paper introduces a method for accelerating experiments to quantify gaseous carbonation of cementitious materials through a sheltered crack surface. To date the majority of measurements of carbonation have focused upon the determination of the carbonation reaction through an open material face with no restriction to gaseous exposure. Experiments to determine the extent of carbonation through a crack surface can verify the extent to which restrictions of gaseous exposure can alter rates of carbonation into the crack surface as well as the depth into the crack to which the reaction occurs.The paper demonstrates that with experimental data the accelerated protocol can produce differences in outcomes in time intervals that are short relative to those in which the reaction occurs naturally. The experiment conducted to demonstrate the viability of the accelerated protocol involved measuring differences in the penetration of carbonation into the crack surface that resulted from differences in crack width. A byproduct of this experiment was a measurement of the depth into the crack (from the material face) to which carbonation occurs. It is not the intent of the paper to develop a theory of rates of carbonation, but rather to demonstrate that statistical differences are obtainable with the accelerated protocol.     

Estimating service lifetimes in weathering: an optimistic view

Directly correlating lifetime to coating composition by using standardized, artificial exposures, or even natural exposure, is often very difficult. However, significant progress can be made by breaking down the problem into smaller questions, which can be separately addressed. If one understands the physical parameters that affect end-use properties, then one can also group, and thus correlate, properties according to whether they depend on processes at the surface or in the bulk of a coating, or whether they depend on defects. A scheme is presented that shows how one can use knowledge from analytical physical or chemical materials science in a statistical model related to the “chemical paradigm.” Simple physical models that use this information, about the initial state of the coating and its rate of degradation, can be used to compare the performance of coatings and estimate, simply, service lifetime depending on the property of interest, in the environment of interest. One can see that different properties are sensitive in different ways to the degradation process and decay with a different rate. Thus, although properties may be determined by the same degradation process, and location within a coating, they do not correlate directly. These approaches show how to organize our knowledge of degradation processes, and environments, and be able to make some testable predictions on how coating properties deteriorate. Presented at the 2006 FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA, on November 1–3, 2006     

Simulations of nanoscale and macroscopic property changes on coatings with weathering

Coatings are designed for and applied on a surface for both aesthetics and protection of the substrate. Many properties are measured to indicate performance, and eventual failure, of a coating under these two broad categories. Monte Carlo simulations have shown success in predicting trends in macroscopic properties during exposure. The Central Limit Theorem (CLT) is applicable because damage made to a coating can come from the accumulation of a vast number of very small damage events. Application of the CLT to property equations has generated additional equations for the prediction of properties of a coating with exposure, including measurable properties such as gloss, color, fracture toughness, and contact angle. These equations, when fitted to measured data, provide in sight into the mechanisms of degradation processes, since the fitting parameters are physically based. They also offer a means to scale accelerated testing measurements to early field measurements of the property of interest for predicting lifetime in varied environments. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL.     

聚脲弹性体/纤维增强树脂基复合材料抗冲击性能研究

玻璃纤维增强树脂基复合材料在使用过程中极易受到外力冲击,造成复合材料结构破坏,严重威胁其安全使用寿命。研究了聚脲弹性体涂层对玻璃纤维增强乙烯基树脂复合材料抗冲击性能的影响。通过简支梁摆锤冲击试验和光学显微镜对前涂覆(FCGF)、后涂覆(BCGF)及未涂覆(NCGF)试样进行对比测试。研究结果表明,聚脲弹性体涂层的弹性形变和断裂破坏能够大幅增加冲击能耗,提高整体的冲击强度。当试样聚脲涂层厚度相同时,前涂覆(FCGF)试样聚脲弹性体冲击后并未完全断裂,主要依靠弹性形变吸收冲击能量,起缓冲减震作用;后涂覆(BCGF)试样聚脲弹性体发生断裂破坏能够消耗更多的冲击能量,其整体结构破坏最小,冲击强度更高。     

Oligomers in the evolution of automotive clearcoats: mechanical performance testing as a function of exposure

Automotive coatings must provide excellent resistance to chemical and mechanical damage in order to maintain a vehicle's long-term appearance and the owner's long-term satisfaction. The Automotive Industry and coating suppliers are partners in design and delivery of future coatings capable of meeting customer demanded performance. As a result of this partnership, new coating materials are being explored based on oligomer chemistry that show promise in providing improvements in both physical and chemical properties/performance and the long-term maintenance of those properties. Oligomeric systems are also useful in design of low VOC coatings. These supersolids coatings will be capable of meeting current and future air quality standards. In this paper measurement techniques for monitoring chemical and mechanical property changes, including cure rate, crosslinking, tensile properties, rheology and scratch and mar performance, were explored. Laboratory mar tests, wet and dry rub tests, which have been validated by commercial experience, are currently used as the basis for comparison of a coating's mechanical performance. QUV accelerated weathering was combined with micro-scratch experiments, atomic force microscopy, optical microscopy, image analysis and IR surface characterization techniques to provide correlation's between chemical composition and mechanical performance, and an indication of service life.     

Role of promoters in improving adhesions of organic coatings to a substrate

An organic coating used as a protective layer is expected to withstand the widely varying environmental conditions which cause changes in its structure and eventually lead to its failure in service. Among the prime properties, adhesion — the interfacial force, which depends on penetration of the coating phase into the micropores or crevices or surface irregularities of the substrate, forms a mechanical interlocking. It may fail due to the degradation of the binder, or the stress and strain developed during application, curing and prolonged exposure. The science of adhesion has been studied extensively and the interfacial molecular forces involved in this phenomenon are discussed. The formation of a chemical bond linkage between an organic coating and a non-organic surface, the role of coupling agents as adhesion promoters, the phenomena involved and the findings of earlier workers are summarized in this review.     

Studying the effects of the chemical structure of an automotive clearcoat on its biological degradation caused by tree gums

The effects of artificial and natural tree gums on the mechanical, chemical, and aesthetic performances of two automotive acrylic/melamine clearcoats were studied. To this end, two clearcoats with different acrylic/melamine ratios were investigated. Biological experiments were performed under post-aging conditions using an accelerated weathering test. Analytical techniques including optical microscopy, scanning electron microscopy (SEM), gloss measurement, FTIR, and DMTA analyses were utilized to reveal the responses of the coating system upon exposure to the aforementioned biological materials. Contact angle measurements were also conducted to estimate the surface energy of the coatings. Greater crosslinking density, together with a higher T _g and damping behavior of the clearcoat, indicative of a greater degree of cure, were obtained as the ratio of melamine crosslinker increased. It was shown that both Arabic and natural tree gums could strongly attach to the clearcoats’ surface, imposing a significant stress during the drying process, thereby leading to a physical failure. In addition, the acidic nature of these biological materials leads to a chemical alteration in the clearcoats’ structure. The greater crosslinking density and lower hydrophilicity of the clearcoats containing higher melamine crosslinker were responsible for the weaker interaction of gums with the surface. This showed a greater capability for stress damping. Small surface cracks with fracture morphology on the coatings exposed to biological materials at higher exposure times (in the xenon test) were also observed. This is discussed based on the adhesion of the coatings to gums at longer exposure times, because of significant stress.