© 2006 by Taylor & Francis Group, LLC

More documents

Recommendations

Info

100 Corrosion Control Through Organic Coatings of course, interactions between these stresses are to be expected; for example, as the polymeric backbone of a coating is slowly being broken down by UV light, the coating’s barrier properties can be expected to worsen. Ranby and Rabek [1] have shown that under UV stress, polyurethanes react with oxygen to form hydroperoxides and that this reaction is accelerated by water. Another example is the temperaturecondensation interaction. Elevated temperatures by themselves can damage a polymer; however, they can also create condensation problems, for example, if high daytime temperatures are followed by cool nights. These day/night (diurnal) variations in temperature determine how much condensation occurs, as the morning air warms up faster than the steel. Various polymers, and, therefore, coating types, react differently to changes in one or more of these weathering stresses. In order to predict the service life of a coating in a particular application, therefore, one must know not only the environment — average time of wetness, amounts of airborne contaminants, UV exposure, and so on — but also how these weathering stresses affect the particular polymer [2]. 6.1 UV BREAKDOWN Sunlight is the worst enemy of paint. It is usually associated with aesthetic changes, such as yellowing, color change or loss, chalking, gloss reduction, and lowered distinctness of image. More important than the aesthetic changes, however, is the chemical breakdown and worsened mechanical properties caused by sunlight. The range of potential damage is enormous [3-7] and includes: • Embrittlement • Increased hardness • Increased internal stress • Generation of polar groups at the surface, leading to increased surface wettability and hydrophilicity • Changed solubility and crosslink density In terms of coating performance, this translates into alligatoring, checking, crazing, and cracking; decreased permeation barrier properties; loss of film thickness; and delamination from the substrate or underlying coating layer. All the damage described above is created by the UV component of sunlight. UV light is a form of energy. When this extra energy is absorbed by a chemical compound, it makes bonds and break bonds. Visible light does not contain the energy required to break the carbon–carbon and carbon–hydrogen bonds most commonly found on the surface of a cured coating. However, just outside of the visible range light in the wavelength range of 285 to 390 nm contains considerably more energy, commonly enough to break bonds and damage a coating. The 285 to 390 nm range causes almost all weathering-induced paint failure down at ground level [4]. At the short end of the UV range, we find the most destructive radiation. The damage caused by short-wave radiation is limited, though, to the topmost surface layers of the coating. Longer wave UV radiation penetrates the film more deeply, but causes less damage [8-10]. This leads to an inhomogeneity in the coating, where the top surface can be more highly © 2006 by Taylor & Francis Group, LLC
Weathering and Aging of Paint 101 crosslinked than the bulk of the coating layer [4]. As the top surface of the film eventually breaks up, chalking and other degradation phenomena become apparent. (The light located below 285 nm, with even higher energy, can easily break carboncarbon and carbon-hydrogen bonds and has enough energy left over for considerably more mischief as well. However, Earth’s atmosphere absorbs most of this particular wavelength band of radiation and, therefore, it is a concern only for aircraft coatings, which receive less protection from the ozone layer.) The interactions of coatings with UV radiation may be broadly classed as follows: • Light is reflected from the film. • Light is transmitted through the film. • Light is absorbed by a pigment or by the polymer. In general, reflectance and transmittance pose no threats to the lifespan of the coating. Absorption is the problem. When energy from the sun is absorbed, it leads to chemical destruction (see Section 6.1.3). 6.1.1 REFLECTANCE Light is reflected from the film by the use of leafy or plate-like metal pigments located at the top of the coating. These are surface-treated so that the binder solution has difficulty wetting them. When the film is applied, the plate-like pigments float to the top of the wet film and remain there throughout the curing process. The dried film has a very thin layer of binder on top of a layer of pigment that is impermeable to light. The binder on top of the pigment layer may be broken down by UV radiation and disappear; but as long as the leafy pigments can be held in place, the bulk of the binder behind the leafy pigments are shielded from sunlight. 6.1.2 TRANSMITTANCE Transmitted light, which passes through the film without being absorbed, does not affect the structure of the film. Of course, if a coating layer underneath is sensitive to UV radiation, problems can occur. Epoxy coatings, which are the most important class of anticorrosion primer, are highly sensitive to UV radiation. These primers are generally covered by a topcoat whose main function is to not transmit the UV radiation. 6.1.3 ABSORPTION Light can be absorbed by a pigment, the binder, or an additive. Light absorbed by the pigment is dissipated as heat, which is a less destructive form of energy than UV light [4]. The real damage comes from the UV radiation absorbed by the nonpigment components of the coating — that is, the polymeric binder or additive. UV energy absorbed by the binder or additive can wreak havoc in wild ways. The extra energy can go into additional crosslinking of the polymer, or it can start breaking the existing bonds. © 2006 by Taylor & Francis Group, LLC
Page 1 and 2:
© 2006 by Taylor & Francis Group,
Page 3 and 4:
Corrosion of Ceramic and Composite
Page 5 and 6:
Published in 2006 by CRC Press Tayl
Page 7 and 8:
Preface This book has been written
Page 9 and 10:
Author Amy Forsgren received her ch
Page 11 and 12:
Contents Chapter 1 Introduction ...
Page 13 and 14:
2.4.2 Reactive Reagents ...........
Page 15 and 16:
6.2.4.1 Alkaline Blistering........
Page 17 and 18:
1 Introduction This book is not abo
Page 19 and 20:
Introduction 3 • A dissolution pr
Page 21 and 22:
Introduction 5 1.2.2 ELECTROLYTIC R
Page 23 and 24:
Introduction 7 to the metal is a ne
Page 25 and 26:
Introduction 9 9. Thomas. N.L., Pro
Page 27 and 28:
12 Corrosion Control Through Organi
Page 29 and 30:
Page 31 and 32:
Page 33 and 34:
Page 35 and 36:
Page 37 and 38:
Page 39 and 40:
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
Page 57 and 58:
Page 59 and 60:
Page 61 and 62:
Page 63 and 64: 48 Corrosion Control Through Organi
Page 81 and 82: 4 Blast Cleaning and Other Heavy Su
Page 83 and 84: Blast Cleaning and Other Heavy Surf
Page 99 and 100: 5 Abrasive Blasting and Heavy-Metal
Page 101 and 102: Abrasive Blasting and Heavy-Metal C
Page 113: 6 Weathering and Aging of Paint Thi
Page 117 and 118: Weathering and Aging of Paint 103 I
Page 119 and 120: Weathering and Aging of Paint 105 c
Page 121 and 122: Weathering and Aging of Paint 107 S
Page 123 and 124: Weathering and Aging of Paint 109 e
Page 125 and 126: Weathering and Aging of Paint 111 I
Page 127 and 128: 7 Corrosion Testing — Background
Page 129 and 130: Corrosion Testing — Background an
Page 143 and 144: 130 Corrosion Control Through Organ
Page 165 and 166:
152 Corrosion Control Through Organ
Page 167:
154 Corrosion Control Through Organ
show all

© 2006 by Taylor & Francis Group, LLC

Create successful ePaper yourself

Delete template?

Save as template?