Monte carlo approach to estimating the photodegradation of polymer coatings

The degradation of a polymer coating and predicting the coating lifetime based on physical properties and distribution within the coating of the polymer binder, pigments, and fillers are economically very important. As technologies advance and allow for control of coatings at the nanoscale level, methods such as Monte Carlo can be used not only to predict the behavior of a nanodesigned coating with time but also to design coatings, such as optimizing pigment particle distributions or optimum hard and soft phase distributions of the binders in multiphase systems for maintaining the desired property with time. Erosion of the coating surface was simulated using Monte Carlo techniques where terrestrial solar flux is the initiator for polymer segment cleavage and removal. The impact on the sensitivity of the polymer adjacent to the detached polymer segment can be increased or decreased in the model based on the chemistry and surface energy of the remaining polymer matrix. Multiple phases with varying sensitivity to degradation can be modeled. The Monte Carlo generates a statistically similar surface topography and chemistry of the coating. The results of the Monte Carlo model are compared to measurable properties such as gloss, fracture toughness, and wetting contact angle, using various published correlations of the property to the surface topology. The simulated properties change through the life-time of the coating in ways that are consistent with observed behavior. Apparently, complicated changes in many properties can be described by the repeated application of simple, random processes. Presented at the 79th Annual Meeting of the Federation of Societies for Coatings Technology, October 28–November 1, 2002, in New Orleans, LA.     

Novel techniques to investigate the impact of cellulose esters on the rheological properties and appearance in automotive basecoat systems

The viscous response of a coating formulation changes over several orders of magnitude after application onto a substrate. Furthermore, the performance and application attributes for a refinish basecoat formulation are slightly different from those of an OEM basecoat. The absence of a high temperature baking step during the refinish process necessitates that the paint exhibit very good antisag behavior as well as quick ‘dry to touch’ without compromising the flow and leveling properties of the coating as it dries. As most paint formulations are multicomponent in nature, it is always a challenge to precisely measure the impact of specific formulation components on the collective performance attributes of the applied coating. In this work, the focus has been to develop novel techniques that can be used to demonstrate a more quantitative measure of some of the performance attributes that rheological additives like cellulose esters can provide to automotive coatings. A series of pigmented basecoat formulations were prepared which contained cellulose acetate butyrates (CABs) of varying molecular weights at different levels based on the total solids of the coating formulation. The nonvolatile content of the formulations was also varied. The viscoelastic behavior of a typical automotive basecoat formulation during the drying process was then investigated using a novel rheological technique. Complex viscosity data (including storage and loss moduli as well as tan delta) were determined as a function of drying time and then compared to the macroscopic properties typically associated with a coating film as it dries. Thermogravimetric analysis (TGA) was also used to correlate the rheology of the metallic basecoat formulations with drying behavior of the coating. The final appearance of the coating was investigated by several microscopic techniques such as Laser Scanning Confocal Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy, and Surface Profilometery, and attempts were made to correlate bulk measurements like ‘flop index’ with the microstructure of the coating. 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.     

Statistical approaches for predicting weathering degradation and service life

Predicting the service lifetime of any material is very important. One of the major difficulties is relating individual, nanoscale, degradation events to the eventual deterioration in macroscopic properties. Monte Carlo simulations and the Central Limit theorem provide approaches to the kinetics of how general coating topology, or bulk morphology, changes during long term weathering. Results on structural changes, from these approaches, may then be translated, via well-known models, into the degradation of macroscopic, “use” properties. Thus service lifetime may be related to composition and degradation mechanisms. Results on pigmented, acrylic coatings demonstrate the applicability and utility of this statistical approach. Changes in gloss and contact angle were related, via these models and provide insights into the processes of degradation in pigmented coatings. Values of parameters for the models that relate surface roughening with exposure are consistent with results from atomic force microscopy. The models provide justification for using simple expressions for trends in non-destructive monitoring, e.g. gloss, or contact angle or for comparing different systems and estimating long-term changes in properties.     

Rapid-set, waterborne coatings from polyzwitterionic polymers formulated with a critical solvent combination

Waterborne coatings that rapidly set and become tack-free can be prepared from polymers containing both pendant anionic (acidic, carboxylate, or strong-acid groups such as sulfonate) and cationic functionality (quaternary ammonium groups). This phenomenon is related to anion-cation interactions that function as ionic crosslinks and dramatically enhance the physical properties and water resistance of the coatings. We define this process as “controlled ionic-coacervation.” The best coating properties can only be obtained by using a “critical solvent combination.” The critical solvent combination requires water plus at least two organic solvents: (1) a lower boiling (70 to 134°C) water-soluble organic solvent having at least one hydroxyl group and (2) a higher boiling (135 to about 250°C) organic solvent. Loss of only a small amount of solvent causes a coating to rapidly become tack-free. Ionization of acid functionalities on the polymers by an increase in pH (e.g., through the loss of CO₂) can initiate controlled ionic interactions. The influence of polymer and solvent compositions on coating properties is discussed. 1712 Building, Midland, MI, 48642. Email: rosegd@dow.com.     

Exterior wood coatings. Part-2: Modeling correlation between coating properties and their weathering performance

Understanding how a coating’s properties are related to its performance characteristics and how measuring its properties can be used for predicting the performance of coating in service was the main focus of this study. A number of exterior penetrating wood stains were characterized in terms of glass transition temperature (T _g), solid content, viscosity, and surface tension. The contact angles of liquid coatings were measured on wood treated with chromated copper arsenate, alkaline copper quat, and copper azole as well as untreated wood. Also, the film thicknesses of the cured coatings on wood surfaces were measured by back-scattered electron imaging of osmium-treated samples. Partial least square-regression (PLS-R) modeling was used to correlate measured coating properties with their weathering performance on the preservative-treated and untreated wood substrates, based on water repellency, color retention, and visual ratings after 3 years of natural weathering. A PLS model was developed with 77% fitting accuracy and 69% prediction ability. This model indicated that preservative treatments significantly affected coating performance, and among coating properties, film thickness, viscosity, and glass transition temperature had the greatest effects on the coatings’ performance in exterior.     

Bridging the PE lifetime under fatigue and creep conditions with its crystallization behavior

The service lifetime for several linear polyethylene copolymers was studied by fatigue-type accelerated tests. The material morphology and crystallization behavior were correlated with the lifetime and the failure modes. The correlation is based on the rate constant of material degradation (RCMD) recently introduced by the authors within a mathematical model for crack layer growth kinetics. RCMD is found to depend on the loading conditions (e.g., creep or fatigue) and on material morphology reflected in crystallization kinetics. The ratio of RCMDs for fatigue and creep is a scaling factor that allows one to correlate fatigue and creep lifetimes. The dependence of the RCMD's ratio on the morphological features associated with the primary and secondary crystallization kinetics is also reported in this paper. © 1993 John Wiley & Sons, Inc.     

Wet film application techniques and their effects on the stability of RuO2–TiO2 coated titanium anodes

The effect of wet film application techniques on the physical and electrochemical properties and operational stability of RuO₂–TiO₂ coated titanium anodes was evaluated. Four compositions of RuO₂–TiO₂ coatings were applied to Ti substrates by three different wet coating methods—brush, dip and spin. Changing the coating technique resulted in different morphologies. Electrochemically active surface area of the coatings was related to the morphology. A shift in Ru(III)/Ru(IV) oxidation potential occurred upon changing the application technique. For lower ruthenium content coatings, this shift was related to coating lifetime. Anode stability in accelerated lifetesting showed that dip coated samples lasted up to three times longer than brush coated samples for lower ruthenium content.     

Molecular relaxation phenomena during accelerated weathering of a polyurethane coating

Results from a polyester-urethane (PU) coating system under accelerated weathering showed that crosslink density, obtained from high-temperature modulus data, diminished due to chain scission. However, T _g and room-temperature tensile modulus both increased with weathering. Molecular relaxation phenomena in polymers have long been investigated to explore changes occurring in a polymer at temperatures below its glass transition and were explored for an explanation to supplement chemical degradation for these observations. Relaxation was quantified using “enthalpy recovery” which first increased with exposure, then diminished. The concurrent physical and chemical aging effects were characterized by tracking nonexponentiality in the spectrum of relaxation times, and the size of “co-operatively relaxing regions” deduced from relaxation around the glass transition. Mechanical relaxation in this coating extended longer than cycle periods typical of accelerated weathering, suggesting that frequency effects might be important when comparing accelerated to natural weathering. This paper was awarded Third Place in the John A. Gordon Best Paper competition, presented as part of 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 1–3, 2006, in New Orleans, LA.     

Concurrent physical aging and degradation of crosslinked coating systems in accelerated weathering

Coating degradation is a combination of both chemical and physical processes; however, physical processes have not received much attention. Physical aging has a non-negligible effect on coatings’ mechanical properties and permeability etc. through the densification that continues as a polymer approaches its thermodynamic equilibrium below the glass transition temperature, T _g. Observations in recent work showed that physical aging affects coatings’ mechanical property response during accelerated weathering and is, itself, affected by the associated chemical degradation. Two crosslinked coating systems were studied in order to compare different chemical compositions, their T _g, and their thermal response in accelerated weathering. During thermal cycling, physical aging measured by enthalpy recovery exhibited different trends in the two coatings. A “rejuvenation” mechanism was observed in the coating with a T _g between the top and bottom limits of the exposure cycle; continued aging was observed for the coating with a high T _g. Stress relaxation tests detected aging and “memory” behavior over periods comparable with accelerated weathering cycles. Both thermal and mechanical responses changed in complicated and different ways as the coatings degraded. Different degrees of coating thickness reduction were observed in both isothermal relaxation and degradation. When various coatings are evaluated, simply judging their performance under the same weathering environment is not reliable since polymer relaxation behavior depends on the relationship between the exposure temperatures and the T _g of each polymer. This paper was awarded First Place in the 2007 Gordon Awards technical paper competition, held as part of the FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in Toronto, ON, Canada, on October 3–5, 2007.     

Advanced techniques for nanocharacterization of polymeric coating surfaces

Surface properties of a polymeric coating system have a strong influence on its performance and service life. However, the surface of a polymer coating may have different chemical, physical, and mechanical properties from the bulk. In order to monitor the coating property changes with environmental exposures from the early stages of degradation, nondestructive techniques with the ability to characterize surface properties with micro- to nanoscale spatial resolution are required. In this article, atomic force microscopy has been applied to study surface microstructure and morphological changes during degradation in polymer coatings. Additionally, the use of AFM with a controlled tip-sample environment to study nanochemical heterogeneity and the application of nanoindentation to characterize mechanical properties of coatings surfaces are demonstrated. The results obtained from these nanometer characterization techniques will provide a better understanding of the degradation mechanisms and a fundamental basis for predicting the service life of polymer coatings. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology on November 12–14, 2003, in Philadelphia, PA.     

聚碳酸酯老化行为研究进展

双酚A型聚碳酸酯性能优异,应用广泛。但在户外使用时,由于光、一氧和湿度等环境因素的作用,导致聚碳酸酯力学强度和外观发生变化。为提高PC的抗老化性能,必须通过研究。充分认识聚碳酸酯的老化反应和老化进程。综述了近年来聚碳酸酯老化行为的研究,并提出存在的问题和今后研究的方向。     

A modern analytical toolbox: defect (W&D) studies of automotive coating systems, including depth profiling studies

Chemical and microscopy measurement technologies have proven to be very powerful in our failure analysis efforts to detail composition and morphology changes of multilayered automotive coating systems obtained from the field. Weathering and durability (W&D) analyses used include those following chemical composition and property changes, at surface/near-surface and interface/interphase loci, as well as depth profiling through one or more system layers. Microscopy tools include a variety of techniques to study surfaces and normal cross sections. Both macro- and micro-morphology details can be effectively imaged to help investigators correlate coating system appearance changes with the mechanisms, and associated kinetics, of degradation. This article will offer an overview of commonly used analyses for developing new coating systems and for tracking appearance, chemical, physical, and mechanical changes due to common mechanisms causing coating systems to fail during service in the field.     

The dilaton approach to evaluating the limiting mechanical properties of aramid fibres in high-speed pulsed loads

The dilaton mechanism can be used to estimate the real lifetime of aramids in conditions of pulsed loads. The dependence of the power time parameters makes it possible to characterize the performance properties of aramids of the Rusar type. __________ Translated from Khimicheskie Volokna, No. 6, pp. 9–12, November–December, 2006.     

Effects of UV degradation on surface hydrophobicity, crack, and thickness of MWCNT-based nanocomposite coatings

Surface degradation is a common problem in polymeric coatings when they are exposed to sunlight, moisture, and oxygen. In order to reduce their surface degradation, thus keeping the coatings’ original properties, multi-wall carbon nanotubes (MWCNTs) were added, and the coatings were exposed to UV light and salt fog for various lengths of time. At 0 days of UV exposure, contact angle values of 0%, 0.25%, 0.5%, 1%, and 2% MWCNT-based nanocomposite coatings of 75 μm (∼3 mil) thickness were between 85° and 89°. However, after 16 days of UV exposure, contact angle values of the same samples were reduced to 11°, 13°, 34°, 50°, and 54°, respectively. Longer UV exposures resulted in several microcracks on the surface of the coated samples in the absence of nanoscale inclusions, while very minimal cracks or degradation appeared on the MWCNT-loaded samples. Test results also showed that UV exposure along with salt fogging reduced the coating thickness up to 24% at 0% CNTs; in contrast, this reduction was only 7% with a 2% MWCNT coating. These results clearly indicate that MWCNTs added to polymeric coatings reduce UV degradation, lessen surface cracks, protect the film thickness, and hence increase the lifetime of the polymeric coatings.     

纳米TiO_2/纳米ZnO杂化改性内墙涂料的研究

采用物理掺和法,在不同内墙涂料中加入纳米TiO2和纳米ZnO,制备纳米复合涂料,进行抗菌试验和甲醛降解实验。结果表明,随着纳米粒子杂化含量的增加,甲醛降解能力和抗菌性能增强,当1#内墙涂料∶纳米TiO2∶纳米ZnO含量比=100∶1.6∶1.6;2#内墙涂料∶纳米TiO2∶纳米ZnO含量比=100∶1.8∶1.8时,效果最好。     

Hydrolytic cleavage of epoxy ring during the epoxidation of alkenes over boron modified titanium silicalites of MFI topology

Boron incorporated bifunctional microporous titanium silicalites of MFI topology, which contained both oxidizing and acidic sites, can effectively catalyse the selective epoxidation of α-olefins, followed by ring opening of the epoxide through hydrolysis reactions. Data are presented that correlate the oxidative and acidic functions of these catalysts. The rate of hydrolysis is seen to depend on the phases generated by incorporation of the boron atoms. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rheological prediction of the physical stability of concentrated dispersions containing particulates

A dynamic rheological technique is presented that can be used for the prediction of physical stability of concentrated dispersions containing particulates, as well as product development and quality control. Typical physical evaluation frequently requires weeks or months before an assessment can be made. By using dynamic rheology, the prediction of physical stability can often be accomplished in less than three (3) hours. The dynamic or oscillatory rheology technique is performed in two parts. First, a strain sweep test (frequency constant, strain variable) defines the linear viscoelastic range (LVR). Second, frequency scans (from 0.01 to 5.0 Hz) are performed at various temperatures, from 5 to 49°C (40–120°F). It is critical that the frequency scans are performed in the LVR. Dynamic rheology measurements yield data on the elastic (G′) and viscous (G″) moduli. Plotting the elastic to viscous moduli ratio as a function of frequency and temperature generates a 3-D surface which is a “fingerprint” of the dispersion’s colloidal stability. The G′/G″ ratios correlate well with observed physical stability properties. A volume index or a simple mean/variance calculation can be used to assign a value to the graphical 3-D representations. Presented April, 1990, at the AOCS National Meeting, Baltimore, MD.     

Deposition temperature effect on sputtered hydroxyapatite coatings prepared on AZ31B alloy substrate

The corrosion of Mg alloys is a provocative topic and it is still a challenge to find a solution for the improvement of their degradation rate into solution found in human body (Simulated Body Fluid, SBF). The aim of the present paper is to coat AZ31B alloy by hydroxyapatite (HAp) as a possible solution in order to change its degradation behaviour for medical implants. Since the Mg alloy is sensible to temperature while the HAp properties depend on the deposition temperature, in this study, the effect of deposition temperature on the properties of the AZ31Balloy was evaluated. The HAp coatings were prepared using the RF magnetron sputtering technique, ranging the temperature from the room one to 400 °C. It was found that the grain size of the investigated Mg alloy increased more than 100% when the deposition temperature was increased. By increasing the temperature, the hardness level was reduced of about 15%. All HAp coatings revealed corrosion behaviour much better than the uncoated AZ31B alloy; in particular, the coating deposited at 200 °C exhibited the best corrosion behavior. Moreover, the best protection to the corrosive attack of SBF was found for the HAp coating deposited at 200 °C (97.3%), which was also characterized by the lowest porosity. To conclude, HAp coatings can be used to improve the properties of AZ31B alloys, but just up to 200 °C; beyond this temperature, the mechanical and the anticorrosion properties are lost.     

Reaction pathways in the electrochemical degradation of thiocarbamate herbicides in NaCl solution

We have identified the intermediates and end products which are formed during the electrolytic degradation of thiocarbamate pesticides in aqueous NaCl solutions and investigated how the intermediate and end product volumes and ratios depend on reaction conditions. Further, we have defined both the reaction pathways leading to intermediate and end product formation and the methods affecting this process. The degradation of the molecular part containing the N atom is accounted for, as are the listed reactions and reaction pathways that affect degradation. If pH is changed, the ratio of degradation products NO ₃ ⁻ and N₂ can be influenced in favour of N₂ gas. This pH change can then be produced without additional material by changing the construction of an electroflotation cell. To implement degradation, “flotation electrolysis” has been found to be an effective method as it facilitates a regulated pH-shift, foam handling and gas bubble rising velocity. The efficiency of the method can be further enhanced by UV photolysis.