Stuart Croll

Blistering and degradation of polyurethane coatings under different accelerated weathering tests

An epoxy primer with a high gloss polyurethane topcoat coating system was exposed either only in a QUV chamber or exposed in a QUV chamber and a Prohesion chamber, alternatively, in this study. AFM studies found that micro blisters formed on the coating surface after both exposures. Blisters formed under the QUV exposure developed in their size with increasing exposure time while those formed under the QUV/Prohesion alternating exposure retained their size throughout of the exposure time. The development of blisters caused the increase of surface roughness and the initial gloss loss of the coating. SEM showed that the QUV exposure is more damaging than the QUV/Prohesion alternating exposure.

Degradation of low gloss polyurethane aircraft coatings under UV and prohesion alternating exposures

In this work, scanning electron microscopy (SEM)/energy dispersive X-ray analysis (EDX), atomic force microscopy (AFM), and electrochemical impedance spectroscopy (EIS) were used to study the accelerated weathering coating degradation that occurred in two low gloss polyurethane aircraft coating systems. One was epoxy primer with a traditional matte polyurethane topcoat and the other was the epoxy primer with a fluorinated polyurethane topcoat. The coated panels were exposed in a QUV chamber and a Prohesion chamber alternately. It was found that the primer with the matte topcoat system could not stop water penetration effectively, and the coating system was destroyed after 25 weeks. In contrast, the primer with the fluorinated polyurethane topcoat system formed a good barrier of water on the substrate, and the coating system was still functional after 2 years of exposure. Adhesion tests showed that the loss of coating adhesion caused the eventual failure of the low gloss coatings.

DLVO theory applied to TiO2 pigments and other materials in latex paints

Understanding how a paint formulation translates into comparative numbers of particles, how the spacing between particles compares to their size and what controls their stabilization mechanisms improves efficient formulation design. The application of Derjaguin, Landau, Verwey and Overbeek (DLVO) theory of the electrostatic stabilization of colloids is reviewed by calculating the inter-particle potentials for typical titanium dioxide pigment particles in a conventional aqueous paint formulation. The calculations show that composition and structural details of the particles need to be input, in order for DLVO theory to model the stability of the pigment particles. It is necessary to include the extent of the surface treatment on the pigment particles, the ionic strength of the continuous phase and the thickness of any adsorbed layer of dispersant polymer as well as knowing the zeta-potential of the particles under the prevailing conditions. Ionic strength determines the range of DLVO forces, so the conductivity of various salt solutions was determined in order that ionic strength could be measured from simple determinations of paint conductivity. Other calculations compare the likely stability of an extender and a typical latex to that of the pigment particles in both DLVO calculations and in calculations of settling rates. The spacing between particles in a random dispersion is estimated from their particle sizes, their concentrations and the characteristic packing fraction assigned to the dispersion.

A Quantitative study of polymeric dispersant adsorption onto oxide-coated titania pigments

Polyacrylic acid salts and similar copolymers are used extensively in the coatings industry to disperse oxide-coated titania pigments. The effect of polymer adsorption onto the pigment can vary widely depending on polymer composition, pigment surface treatment, and solution properties such as pH and ionic strength. This investigation examines the effect of molecular weight on the adsorbed layer thickness and stabilizing action of polyacrylic acid dispersants. In order to understand the stabilization mechanism, a DLVO model was used in which the surface treatment layer of the pigment and the adsorbed layer thickness of the dispersant were taken into account. It was found that only this level of detail could account for the degree of stability found in pigment suspensions. Layer thickness and adsorption isotherms indicate that the dispersant molecules do not adsorb completely flat to the pigment surface but with enough loops and tails to provide some electrosteric stabilization.

Thermal Cycling of Epoxy Coatings Using Room Temperature Ionic Liquids

Nonaqueous room-temperature ionic liquids (RTILs) offer a method of using electrochemical impedance spectroscopy (EIS) to analyze coatings without the confounding effects, such as plasticization, associated with an aqueous working fluid. Thermal cycling has been used as an accelerated testing method for ranking the corrosion protection of coatings. The influence of thermal cycling on the degradation of a coating was investigated using a hydrophilic RTIL as the electrolyte for the drying cycle of EIS experiments. The change in diffusion coefficient, saturated water volume fraction, and activation energy for the absorption and desorption of water by the coating was obtained.

Simulation of Wet-Dry Cycling of Organic Coatings Using Ionic Liquids

The capacitance evolution of an organic coating undergoing cyclic wetting and drying conditions was monitored by single frequency electrochemical impedance spectroscopy. Monitoring of the drying condition was possible with the use of a hydrophilic room temperature ionic liquid and the methodology employed is presented. Experimental results associated with cyclic dilute NaCl wetting and ionic liquid drying are presented for an epoxy coating on an AA 2024-T3 substrate. The calculated capacitance evolutions associated with wetting and drying were generally consistent with Fick’s second law. The calculation of the water ingress and egress diffusion coefficients using a short-time approximate solution and a series solution to Fick’s second law are presented. The latter solution is shown to address the capacitance evolution better than the former with the ingress coefficient larger than the egress coefficient for a given exposed coating surface. There was agreement between the calculated diffusion coefficient ingress values for coating areas exposed to cyclic NaCl wetting—ionic liquid drying and cyclic NaCl wetting—natural drying conditions. Comparison of the impedance spectra for test areas indicated that the use of the ionic liquid as a drying medium influenced the electrochemical properties of the coating only after a number of cycles.

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.

Morphological changes in polyurethane coatings on exposure to water

When a polyurethane self-priming coating on a sol-gel treated aluminum panel was immersed in dilute Harrisons solution, subsequent change of the polyurethane coating surface was inspected with atomic force microscopy (AFM) and scanning electron microscopy (SEM). After immersion, filiform threads formed in the coating surface. The mechanism suggested for the formation of filiform structures is that when the solution penetrates into the coating, osmotic cells form under the surface layer of the coating. The formation of these osmotic cells may be caused by the existence of soluble impurities in the coating and these soluble impurities concentrate in the solution and promote water adsorption into the coating. As a result, blisters form on the surface, especially at flexible, loosely cross-linked locations. The combination of blisters appears as filiform structures on the coating surface.

Spectroscopic adsorption and effective dosage in accelerated weathering of a polyester-urethane coating

Topcoat integrity is a crucial property for a protective coating system for protecting metal substrates in conjunction with anti-corrosion primers. Infrared spectroscopy was used to examine the chemical changes seen during accelerated weathering in a model topcoat urethane polymer and to measure the coating ablation. During weathering the ultraviolet absorbance of the urethane coating showed a typical tail (yellowing) into the visible region that increased with exposure period. Effective ultraviolet dosage can be calculated by integrating the spectrum of the incident radiation with the quantum yield for the degradation process and the ultraviolet absorption of the material under investigation. Depending on the form of the quantum yield, there is a clear acceleration of the absorption of damaging radiation because the absorbance increases with exposure. This non-linear relationship offers possibilities on how to estimate a service lifetime; one could choose a value of the exposure period characteristic of the start of the acceleration in dosage, or one might choose the asymptote at which the dosage rate becomes very great. The ultraviolet tailing into the visible region is analyzed as an example of an “Urbach” tail which is usually attributed to structural disorder that introduces energy levels between the principle electronic states.

Quantitative ultraviolet spectroscopy in weathering of a model polyester urethane coating

Abstract Spectroscopy was used to quantify the effects of ultraviolet light on a model polyester–urethane coating as it degraded in an accelerated exposure chamber. An explorative calculation of the effective dosage absorbed by the coatings was made and, depending on the quantum yield chosen to represent the damaging processes, was found to follow an accelerating trend with exposure period that may permit estimation of a service lifetime by spectroscopy. The accelerating trend was due to an increase in the absorption “yellowing” tail as degradation proceeded. Quantification as an “Urbach” tail, and the underlying electronic transition by an analysis due to Tauc et al., showed that the Urbach energy was consistent with values found elsewhere. It is possible that, for this urethane at least, any explanation of “yellowing” may not require the development of specifically yellow chromophores.