Water transport in multilayer coatings

Coatings form the interface between structures and the environment in many application domains. They play a crucial role in providing protection, e.g. against corrosion, they form a barrier against an aggressive environment and they create the aesthetic appearance. To fulfill such functionalities, coatings are often composed of multiple layers, wherein each layer is designed to add a specific feature to the coating. The top coat serves as a barrier, whereas a base coat provides color and primers secure a good adhesion and corrosion protection. Nowadays, it is common that most objects are protected and decorated by coatings.Water penetration often forms a key factor in deterioration of a coating and its substrate. For example, water penetration may contribute to blister formation, leeching of water soluble components, hygroscopic stresses or freeze-thaw damages. Furthermore, water may act as a solvent or an electrolyte carrier, and may promote biodegradation. Consequently, a profound understanding of water penetration into (multilayer) coatings lays the foundation for improved product durability. Such a fundamental approach includes water penetration kinetics, water induced structural changes in coatings and clarification of the state of water in coatings. Most of the published studies are devoted to single layer coatings and only little is known about the water penetration in multilayer systems. Investigation of water transport in multilayer systems requires experimental techniques that measure transient water distributions and evaluate water-polymer interactions. The only technique combining these features is high resolution NMR (nuclear magnetic resonance) imaging. This thesis presents a study into water transport in multilayer coatings. This investigation was performed in two ways: experimentally with high resolution NMR imaging, combined with relaxometry and theoretically with an introduction and verification of a model for water transport. The thesis starts with an overview of knowledge about water in coatings in Chapter 2. This chapter focuses on water in polymers, since in organic coatings water mainly permeates in polymeric binder. The relation between the state of water in the polymer matrix and equilibrium sorption is addressed and the mechanisms of water transport kinetics are discussed. Further, the thesis proceeds with a description of NMR principles in Chapter 3. It discusses the NMR basics, principles of imaging and how information about molecular motions of the measured species is present in the NMR signal. The key results of the study are presented in the Chapters 4-8. First, water uptake was studied in two-layer systems consisting of hydrophilic base coats and hydrophobic top coats. The base coat consisted of acrylic, polyurethane and pigment particles and the formulation included a polymeric pigment dispersant. Chapter 4 presents the results of visualization of water transport in the two-layered systems on the basis of high resolution NMR imaging. The water transport rate in the studied systems appeared to be barrier limited. Mainly the base coat absorbed water, where it quickly redistributed. The swelling of the base coat showed to be linearly related with the absorbed water quantity. Further, Chapter 5 presents the results of a relaxation analysis of the NMR signal, which was introduced to identify water and polymeric phases in the base coat. Polymeric dispersant plasticization was observed. This polymeric dispersant appeared to be a major contributor to the water uptake capacity of the base coat. It was found with NMR diffusometry that high water mobility occurred under saturated conditions, whereas in partially saturated systems water mobility decreased and bonding to the polymer played a key role. In Chapter 6 a theoretical model was formulated for the transport in multilayer coatings, composed of a hydrophobic top coat and a hydrophilic base coat. This model basically assumed instantaneous equilibration of water in the base coat and Fickian water transport in the top coat. According to the model, the sorption isotherm of the base coat is the driving force of the process and the top coat permeability to water determines the timescale of transport. The asymmetry of the observed uptake/drying rates appeared to be due to the non-linear sorption isotherm of the base coat. Furthermore, top coats applied on base coats showed a higher permeability than free films and water uptake capacities of base coats decreased, when thicker top coats were applied. The ability of the model to predict water transport using the base coat sorption isotherm and the top coat permeability was validated experimentally. Practically, it is sufficient to measure the base coat sorption isotherm and the top coat permeability to understand water transport. This enables design of a simple and efficient procedure to test water resistance of this type of coatings. Such procedure will include only basic and cost-efficient equipment, like balances for gravimetric measurements of sorption isotherms and permeability. In the next step, the model was used for a theoretical investigation of the response of multilayer coatings to relative humidity fluctuations, which is presented in Chapter 7. The theoretical investigation showed that in most of the considered systems the response time is in the order of days for high humidities and has values of a few hours at low humidities. The response time is determined by the product of the timescale of penetration through the top coat and the differential solubility of the base coat. This means that most two-layer polymeric base coat/top coat systems with hydrophilic base coats are relatively inert to most humidity fluctuations, as the response in terms of water content will be damped. As accelerated and artificial weathering tests include only humidity fluctuations with typical timescales less than a day, this raises concerns if these tests can be compared to natural moisture exposure. Finally, the thesis addresses migration of plasticizer into a top coat. It was found that stress relaxation results in non-Fickian kinetics of the uptake process. The plasticizer, which is tributyl phosphate, causes a steep increase in the permeability of the film and may cause degradation of mechanical properties of the coating. This implies that, practically, that tributyl phosphate (the main component of common aerospace hydraulic fluids) makes multilayer protective coatings more permeable and causes significant stresses in the system.