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108 Corrosion Control Through Organic Coatings have a coefficient of thermal expansion that is twice that of aluminium or zinc and four times that of steel [29]. Another factor that must be considered at elevated temperatures is the glass transition temperature (T g) of the polymer used in the binder. This is the temperature above which the polymer exists in a rubbery state and below which it is in the glassy state. Using coatings near the T g range is problematic, because the binder’s most important properties change in the transition from glassy to rubbery. For example, above the T g, polymer chain segments undergo Brownian motion. Segments with appropriate functional groups for bonding are increasingly brought into contact with the metal surface. An increase in the number of bond sites can dramatically improve adhesion; wet adhesion in particular can be much better above the T g than below it. Increased Brownian motion is also associated with negative effects, such as increased diffusion. Above the T g, the Brownian motion gives rise to the continuous appearance and disappearance of small pores, 1 to 5 nm or smaller, within the binder matrix. The size of these small pores compares to the ‘‘jump distance” of diffusing molecules — the distance that has to be covered by a molecule moving from one potential-energy minimum to a neighboring one in the activated diffusion process. The permeation rate through these small pores is linked to temperature to the same degree that the chain mobility is. That is, the chain mobility of elastomeric polymers shows a high degree of temperature dependence and thus favors activated diffusion at higher temperatures. As the crosslink density of the binder increases, segmental mobility decreases, even at elevated temperatures. Diffusion still occurs through large pore systems whose geometry is largely independent of temperature. The temperature dependence of diffusion in highly crosslinked binders is a result of the temperature dependence of the viscous flow of the permeating species. Miszczyk and Darowicki have found that the increased water uptake at elevated temperatures can be to some extent irreversible; the absorbed water was not fully desorbed during subsequent temperature decreases. They speculate that the excess water may be permanently located in microcracks, microvoids, and local delamination sites [29]. 6.4 CHEMICAL DEGRADATION All breakdowns in polymers could, of course, be regarded as chemical degradation of some sort. What is meant here by the term ‘‘chemical degradation’’ is breakdown in the paint film that is induced by exposure to chemical contaminants in the atmosphere. Atmospheric contaminants play a more minor role in polymer breakdown than do UV exposure, moisture, and (to a lesser degree) temperature. However, they can contribute to coating degradation, especially when they make the coating more vulnerable to degradation by UV light, water, or heat. Mayne and coworkers have shown that the organic coating and the ions (e.g., sodium, potassium, calcium) in a solution interact, causing a gradual reduction in resistance of the coating (the “slow change,” see Chapter 1, Section 1.2, “Protection Mechanisms of Organic Coatings”) [30-34]. One interesting aspect is that, as long as it doesn’t go too far, the reduction in resistance is reversible. The process is of course accelerated by heat; raising the temperature increases the amounts of ions © 2006 by Taylor & Francis Group, LLC
Weathering and Aging of Paint 109 exchanged, reducing the resistance and thus aging the coating. Coatings vary in their ability to resist this, but all exhibit the trend to some extent. Chemical species, such as road salts and atmospheric contaminants in the wind or rain, are routinely deposited on paint surfaces. There, they combine with condensation to form aggressive, usually saline or acidic solutions. Most polymers used in modern coatings have good resistance to acids and salts; however, modern coatings also contain a large number of additives (see Chapter 2), which can prove vulnerable to chemical attack. For example, many coatings contain light stabilizers based on hindered amines to aid UV resistance. It is well known that the performance of these stabilizers is diminished by acids and pesticides [35]. When this occurs, chemical exposure makes the coating vulnerable to UV breakdown. The number of studies in this area is limited, but a few have shown exposure of coatings to atmospheric contaminants to be detrimental. Sampers [35] reports that, in a study of polyolefin samples exposed both in Florida and on the Mediterranean coast of France, a dramatic difference was seen in polymer lifetime. Samples exposed on the Mediterranean had only half the life of those in Florida. The two stations had broadly similar weathering parameters; the differences should have led to longer lifetimes in France. Sampers concluded that constituents in the rain or wind had chemically interacted with the hindered amine light stabilizers in the polymers exposed in France, causing these samples to be especially vulnerable to UV degradation. In a study of gloss retention of coatings exposed for 2 1/2 years at weathering sites in Kuwait, Carew and colleagues [36] reported a probable link between industrial pollution and coating damage, although in this case the damage seems to have been caused by dust from a cement factory. The sites in this study are described in Table 6.2. Because all the sites are located in the Shuaiba area of Kuwait’s industrial belt, they should be very similar in temperature and humidity. The difference between sites is the distance from the Arabian Gulf and the amount and type of atmospheric pollution. Carew and colleagues found that coatings consistently showed the worst performance at site C, although this site is farther from the sea than sites A and B TABLE 6.2 Description of Sites in Kuwait Study Site Distance from Sea Pollution Notes A 0.2 km Heavy Downwind from refinery and salt and chlorine plant B 0.55 km Heavy Next to refinery and desalination and electricity production plant C 1.5 km Heavy Upwind from refinery, next to cement clinker factory D 3 km Mild Rural area Data from: Carew, J.A. et al., Weathering performance of industrial atmospheric coatings systems in the Arabian Gulf, Proc. Corros. ’94, NACE, Houston, 1994, Paper 445. © 2006 by Taylor & Francis Group, LLC
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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Introduction 9 9. Thomas. N.L., Pro
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