Protective Coatings

Precursors of cross-linked polymer systems are blends of many compounds, some of them forming series (distributions) of compounds of increasing molecular weights, type and number of functional groups, or some other property. The distributions may arise from impurities in the raw materials, or a result of side reactions. In most cases, the distributions are generated intentionally; functional copolymers, hyperbranched polymers, off-stoichiometric highly branched polymers, chain extended systems, or precursors prepared in several stages can serve as examples. The distributions affect processing and materials properties. We show ways to generate these distributions from bond formation kinetics and reaction mechanisms. Statistical branching theories based on assemblage of branched molecules and gel structure from building units in different reaction states are used to model the evolution of the cross-linked system.

Crosslinked network formation occurs when multifunctional precursors react to form a three-dimensional polymer network. In attempting to link network topology to physical properties, materials are typically characterized by the functionality of the crosslink nodes, the typical chain length between crosslinks and the concentration of the crosslink junctions. Statistical models that are in common usage calculate average values of these quantities and do not determine the possible variation in these properties. Molecular dynamics studies show not only how networks form and whether the average values conform to the predictions of these statistical theories, but also how they vary locally. An important result from the simulations is that there are significant regions where there are few bonds connecting neighboring chains. In addition, very long loops and dangling chains occur. None of this should be surprising in a system where the reactions and spatial arrangements are influenced by random chance and there are such a huge number of precursor molecules and possibilities that a number of nonideal configurations are possible. The simulations permit visualizations of these imperfections and show how large they are. Many of the end use properties of polymer networks depend on the continuity of the network, so this heterogeneity must have an impact on the physical properties. Here, the simulations use coarse-grained techniques that are not specific to any reactive chemistry, but such research may indicate how networks can be assembled that are less prone to large heterogeneities and are thus more robust.

A rigidity percolation model was developed to simulate the growth of elastically active bonds and evolution of Young’s modulus during free-radical crosslinking polymerization. The polymerization process was followed by a kinetic gelation modeling. The bonds between monomer sites that form a network were represented as beams with rigid joints. Young’s modulus appears at the percolation threshold, and then monotonically rises with conversion. As the initiation rate is raised, Young’s modulus appears at a higher conversion, but it grows faster beyond the percolation threshold. Consequently, Young’s moduli attained at high conversions are not affected by the initiation rate. As primary cyclization is enhanced, Young’s modulus appears at a higher conversion due to delayed percolation threshold. The difference of Young’s modulus at different levels of primary cyclization is most significant at the percolation threshold regions. After that the difference becomes smaller even though it is still appreciable. The formation of elastically active bonds follows a similar trend as the Young’s modulus to which they give rise. Furthermore, the dependence of Young’s modulus on the number of elastically active bonds indicates that changing the initiation rate does not affect their bonding structure, and that enhancing primary cyclization causes them to form less rigid bonding structure because of its heterogeneity. Finally, irregular distribution of bonds in the network causes stress to be higher in some and lower in others by orders of magnitude.

This chapter provides background and practical use of viscometers and rheometers for automotive paint formulators. Focus is balanced between use of common test methods and how they relate to paint processes. Commercial rheometers measure mechanical properties of liquid samples in a simple shear geometry. By varying the duration and amount of applied stress or strain, we can study non-Newtonian behaviors that dictate end-use performance. Methods discussed can quantify pseudoplastic, thixotropic, and yield stress behaviors. After some discussion of steady shear viscometry, we introduce dynamic oscillatory methods and provide examples useful to paint formulators. Finally we point to potential for further development with study of applied film rheology.

This chapter reviews the technique of magnetic microrheology for the characterization of the viscosity of coatings. In magnetic microrheology, a liquid containing a dilute solution of micron-sized magnetic probe particles is placed in a magnetic field gradient and the motion of the probe particles is tracked with a microscope. The probe particle velocity found from image analysis is then used, along with the particle and system parameters, to find the local viscosity of the liquid. The application to coatings requires an apparatus designed for tracking particles in thin liquid coating layers. In the chapter, an overview of magnetic microrheology is provided, including the design of an apparatus for monitoring coating viscosity. Examples are provided for the use of the method to track viscosity as a function of time during drying and curing, and position through the thickness of the coatings. The chapter explores the application of the method to understand the effects of process variables on the viscosity development of a coating used in the manufacture of tissue paper.

This chapter provides a brief overview of cryogenic scanning electron microscopy (cryoSEM) and how it can be used to characterize wet coating microstructure and its development. As one of the few tools capable of studying wet coating microstructure, cryoSEM reveals transient states in the microstructure that develop as the coating dries. In this chapter, the basic concepts of cryoSEM sample preparation are reviewed including sample vitrification, cryo-fracture, and sublimation. Additionally, several possible artifacts of cryoSEM sample preparation are highlighted and the challenges of image interpretation are discussed. Lastly, several case studies are presented where cryoSEM was used to characterize the microstructure of wet coatings. The examples include: drying-induced gradients in particle concentration, understanding the origin of dry film microstructure from the microstructure of the initial dispersions, and understanding the fundamentals of drying-induced stress development in hard particle coatings.

In latex polymer-based coatings, volatile organic compounds (VOC) are used as coalescent to soften the latex polymer to ensure film formation when the coating is applied and then the coatings become harder, tack-free, and durable after the VOC evaporates during the drying. There has been increasing pressure to reduce VOC and to maintain the minimum film formation temperature (MFFT) and hardness-related properties such as block, print, and dirt pick-up resistance at the same time. Traditional methods to lower VOC and maintain film hardness include lowering the dominant phase polymer glass transition temperature (T _g) and blending in small amounts of hard polymer to reinforce a soft film or manipulating latex morphology. In this chapter, a new strategy, namely, Film Formation through Designed Diffusion Technology is presented to advance the balance of VOC, MFFT, and hardness-related properties with mechanistic understanding and application results. The Designed Diffusion™ (DD) technology uses a two polymer system, in which small amounts of a specially designed, soft Polymer DD is added to a high T _g dominant phase Polymer A to accelerate early property development. The polymers (Polymer A and Polymer DD) have been chosen so that the coalescent partitions selectively into Polymer A in the wet state. Film formation triggers a change in the solubility of the coalescent in the two polymers. Transport of the coalescent into Polymer DD facilitates the removal of the coalescent from the dominant phase (Polymer A) and thus the coating develops properties at a faster rate.

In situ FTIR has been widely used to study polymerization kinetics of different reaction systems. However, lack of humidity control during these studies has generally limited this technique to ambient or nitrogen atmosphere conditions. In this work, a relatively small humidity control chamber was newly designed and introduced into the original FTIR sample chamber in transmission mode to control the humidity during in situ kinetic studies of coatings over a broad range from 5 to 80% RH. With the current setup, it only takes 2–3 min to get to the desired humidity level. Factors that affect the humidity control and FTIR data collection are discussed. Two reaction systems were selected to study the effect of moisture on the kinetics. One is an aliphatic polyisocyanate reacting with moisture in the air, and the other is an acrylic polyol reacting with a polyisocyanate. Relative humidity was studied at three levels: 15, 45, and 75%. In both cases, the reaction rate increases under higher humidity. Formation of different forms of urea in the first system and formation of both urethane and urea in the second system were analyzed. In the second reaction system, more urea is formed under higher humidity or under a higher NCO/OH molar ratio. The humidity control within the transmission FTIR chamber has turned out to be a useful tool to monitor the effect of humidity on the curing kinetics of organic coatings.

The chapter first describes fundamentals and applications of UV-curing technology and brings out the most important problem in this area: shrinkage. In the following text, the reason for shrinkage formation, its influence in UV-curing, and the methods to measure the shrinkage are described. The methods reviewed include dilatometry, pycnometer, buoyancy, bonded-disk, interferometer, laser displacement, laser scanning, video-imaging device and some combined methods. After that, several ways to control or overcome the problem of shrinkage are summarized, including changing the condition of UV-curing, adding an inert component, adjusting the structure of monomers, such as decreasing the density of functional groups, introducing rigid structure, applying ring-opening polymerization, using thiol-containing system and hybrid system (e.g., free-radical/cationic hybrid system, thiol-ene/acrylate hybrid system and organic/inorganic hybrid system), and adopting solid-state photopolymerization. The developing prospects of the research for shrinkage in UV-curable coatings are given in the concluding section.

As a latex coating dries, volume shrinkage that occurs during film formation is constrained by the substrate, resulting in the development of a bi-axial tensile stress in the film. In situ measurements of stress development can be made using the cantilever beam bending technique. Modified walled cantilevers can be used to limit lateral drying and promote drying uniformity resulting in more accurate stress measurements. In this work, film formation and stress development were characterized for a series of acrylic latexes with varying glass transition temperature (T _g ) of the latex polymer. Both the minimum film formation temperature (MFFT) and the maximum stress measured in the film were shown to increase with the polymer T _g . Furthermore, the use of coalescing aids (both conventional and low-VOC) to enhance film formation was explored for a latex with an MFFT of 18 °C, and their role in stress development was characterized. All coalescing aids investigated were effective in lowering the MFFT and reducing the maximum stress developed in the coating. The maximum stress was greatly reduced as the concentration of the coalescing aid increased up to three weight percent, while only minor further reductions in the maximum stress were observed at higher concentrations.

The chapter describes the internal stress development in reactive coatings during the process of film formation over a long period of time, e.g., well over 1000 h. Measurements of internal stress were made in situ using a cantilever method. Deflection of the cantilever was measured by two means: the traditional optical microscope method and a novel capacitive sensor method. The capacitive sensor approach was developed to determine the stress of coatings with higher film thickness. It offers unique advantages of high sensitivity and ease of testing multiple coatings simultaneously. The effects of acrylic polyol binder structure on the stress development of polyurethane coatings were examined in detail. The acrylic polyol binders are copolymers with defined structures synthesized by group transfer polymerization. The effects of copolymer structure, such as rigid vs. less rigid segments in the main chain, copolymer molecular weight, and crosslink density of the formed polyurethane network on the internal stress are discussed. Applying rigid segments in the polymeric chain leads to higher stress mainly in the rubbery region and around the vitrification point of the coating. Increasing the crosslinking density increases the plateau value of internal stress. Molecular weight of the copolymer does not affect the internal stress. Addition of a polyester polyol with low glass transition temperature can effectively lower the stress of a polyurethane coating. Addition of solvent to a polyurea coating with 100% solids was also investigated. Finally, the effect of baking, i.e., curing at an elevated temperature, was studied on a polyurea/polyurethane coating. These studies and the capacitive sensor approach developed will help understand long-term performance of reactive coatings.

The factors and theoretical approaches to equilibrium swelling of cross-linked polymer systems were analyzed in relation to the application of the equilibrium swelling method for characterization of structure of cross-linked coating films. Special attention was paid to the possibility of characterizing the cross-link density and its changes during film formation. For swelling of highly cross-linked networks (coating films) in good solvents, the finite extensibility of network chains was respected. The effect of adhesion of the film to the substrate was also considered. If equilibrium swelling method (swell test) is used for characterizing changes in cross-link density of drying (reacting) films, the results are often not meaningful for several reasons such as (a) dependence of interaction parameter on conversion; (b) if the sample is in its glassy state and contains unreacted functional groups, increase of conversion occurs during the test when swelling induces transition from the glassy to the rubbery state.

Slab microtomy combined with Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR) analysis is a useful tool to determine the chemical depth profiles of multilayer coating systems. This approach allows relatively small steps (down to ~5 μm per section) to be used to document chemical composition throughout a coating system or at/near a given locus of interest such as a clearcoat/basecoat interface. In this work, two automotive refinish coating systems were studied: a fast-curing, solvent-borne clearcoat on a standard, non-crosslinked waterborne basecoat, and on a waterborne basecoat containing an isocyanate crosslinker. The objective is to understand the effect of crosslinker addition to the basecoat on the interlayer diffusion of coating materials and the chemical composition of the clearcoat/basecoat system, particularly at the clearcoat/basecoat interface and inside the basecoat. Analysis of depth profiling results shows that in the unmodified basecoat there is a limited amount of isocyanate diffusing from the clearcoat to the basecoat to convert to urea and urethane groups, compared to the system with a crosslinker-containing basecoat. When a crosslinker solution (i.e., crosslinker and solvent) is added to the basecoat, it leads to a more gradual change of the chemical compositions going from the clearcoat to the basecoat. The addition of the crosslinker solution into the basecoat not only helps in crosslinking the basecoat, it also promotes diffusion of the clearcoat materials into the basecoat, making the interface and basecoat stronger than the system using the standard basecoat. The difference of these two coating systems helps to further understand the effect of adding isocyanate crosslinker on coating system properties.

The physical interactions between components in waterborne coatings impact in-can properties as well as the migration of additives during the drying of coated films. The distribution of additives determines their influence on film properties and can significantly diminish the appearance and performance of surface coatings. The tendency of various chemical species to move to surfaces during the drying process is believed to depend on the conditions of film formation, as well as the chemical compositions, molecular weights, and rheological properties of both additives and latex components. In this chapter, we review a study on the influence of a mid-range molecular weight alkali-soluble resin or ASR on a model acrylic latex containing a practical polymer composition with a low glass transition temperature (T _g). The film formation behavior of the acrylic latex in the absence and presence of the ASR was characterized using standard tests and microscopy techniques. The combination of atomic force microscopy (AFM) with confocal Raman microscopy (CRM) was used to characterize spatial compositional inhomogeneity of the multicomponent polymer systems. CRM was also used to obtain depth profiles of the ASR and polymerization surfactant in the films of neat acrylic latexes and pigmented paints.

The primary functions of automotive clearcoats are to protect the underlying layers and maintain a glossy showroom appearance for as long as possible. One of the most common sources of damage is from polymeric carwash brushes and small embedded dirt and grit particles that create small scratches in the clearcoat surface. Complex multiasperity contact, unknown geometries, and stress states make it difficult to quantify this damage. Since the mid-1990s, researchers have developed sophisticated single indenter nano- and microscratch techniques to quantify the surface mechanical properties and to understand the connection to scratch performance. Indenter size and shape were selected so that the scratches produced would be similar in size and appearance to the damage produced under real-world conditions. In this chapter, four automotive clearcoats with different crosslink densities, including a 2K urethane, a 1K melamine, and two experimental clearcoats, are evaluated and compared. New statistical methods for creating and analyzing scratch damage based on NanoScratch testing are discussed. Recovery of scratches in both short timescale (minutes) and long timescale (days) is analyzed by the NanoScratch testing and Atomic Force Microscope (AFM) analysis. The connection between the scratch morphology and the visual appearance is explored using dark field imaging as an objective surrogate for appearance. Two components to scratch resistance, damage resistance and scratch visibility, are analyzed. The damage resistance can be characterized by residual depth and fracture threshold, and the scratch visibility can be characterized by the contrast and size of a scratch image. Crosslink density appears to affect the residual depth and scratch visibility. A coating with good damage resistance does not automatically lead to low scratch visibility. The methods presented offer new ways to further understand scratch performance of coatings.

Several of the most popular scratch and mar test methods and their scratch failure mechanisms are reviewed. These include Crockmeter-2 μm, Crockmeter-9 μm, Amtec Kistler Car Wash, and Nano Scratch. Plastic deformation is identified as the primary failure mechanism for the Crockmeter tests, but fracture type scratches are more associated with Amtec Kistler Car Wash test. Nano Scratch quantitatively measures both plastic deformation and fracture load. From the coating design perspective, crosslink density and chain elasticity are the most important factors for scratch and mar resistance. Higher crosslink density is correlated to lower plastic deformation, while higher chain elasticity results in higher fracture threshold. Aging of polyisocyanate type coatings helps scratch and mar resistance due to postcure reaction from the isocyanate residual groups. Weathering hurts scratch and mar resistance due to network degradation. For technology comparison, two-component (2K) isocyanate was superior in comparing to one-component (1K) acrylic melamine due to its better weathering resistance. Impact of nanoparticles on scratch and mar resistance appeared to be related to the cluster formation of the particles. The tightly clustered nanoparticles appeared to improve scratch and mar resistance through arresting the propagating fronts of cracks initiated during the scratching process.

This chapter presents a discussion of organic coating defects that occur on application or during drying and baking. The greatest emphasis is on cratering and its mechanisms, but a number of other defects, including dewetting, telegraphing, soak-in, pinholes, sagging, orange peel, flooding and floating, Bénard cells, wrinkling, popping, gassing, and air entrapment are described and discussed along with their mechanisms. Suggestions for reduction or prevention of many of the defects are included and relevant references are listed. Much of the discussion is based on long experience with automotive coatings, but the information is relevant to other industrial coatings, and many of the same defects also occur with architectural and maintenance coatings.

Degradation of polymer coatings in service occurs due to the cumulative effect of a vast number of aggressive photons, molecules, or other assaults that occur at random locations and random intervals such that the Central Limit Theorem can be applied. This permits use of the Normal distribution to describe the extent and variation in the damage that occurs. In addition, initial network formation is also governed by stochastic processes as small reactive molecules react to form the polymer. Molecular dynamics simulations show that extensive variation is possible in the connectedness and local topology of a crosslinked polymer, so there are statistically occurring flaws and inconsistencies, even before exposure in service. The original defects, together with the damage caused by degradation, determine the performance of coatings. For deterioration in service, accumulation of damage is modelled as a stochastic process with the properties of the Normal distribution, using the mean to represent the most likely state of the material. Inserting this in very well-known theories of material properties such as gloss and toughness shows how the kinetics of failure of coatings may arise. Models include basic physical property values of materials and show how they affect the outcome. This approach also shows how nonsimultaneous degradation leads to an apparent induction period in the change of properties that may not originate with autocatalytic degradation chemistry. Equations generated in this approach demonstrate that different properties are sensitive in different ways to the degradation process and thus decay with a different rate, even though the basic mechanism of deterioration is identical.

The long-term durability of a coating is strongly dependent upon the coating’s resistance to crack growth, which is governed by the stress in the coating and the coating’s fracture energy. Stresses in a coating generally increase as weathering time progresses due to continued cure of the coating, physical aging, and cyclic stresses from thermal and moisture expansion mismatch between the coating and substrate. A coating’s fracture energy generally decreases as weathering progresses due to degradation of the binder network induced by both photooxidation and hydrolysis. The regular cracking patterns observed in most coatings is a result of stress relief around cracks that have already propagated.

In recent years, the technology for automotive coating application has evolved from manual art to robotic science. However, despite the radical change in application technology, the principles of spray coating remain unchanged. This chapter reviews the major advances in coating application and develops several important concepts that are foundational for coating process engineers. Beginning with practical paint application, attention is given to the importance of spray fan pattern and overlap in obtaining a smooth uniform film. Then through the derivation of a coating equation, improved comprehension of the factors related to deposited film thickness can be gained. Practical application is followed by a more technical overview of the characteristics of atomization by pneumatic spray guns and high-speed rotary bells. Equations for each provide insight to the associated atomizer and paint variables. At the end, a brief introduction of electrostatic deposition is also given.