© 2006 by Taylor & Francis Group, LLC

More documents

Recommendations

Info

102 Corrosion Control Through Organic Coatings Because the polymer chains in the cured film are well anchored and already crosslinked, further crosslinking results in additional tightening of the polymer chains [7]. This increases the internal stress of the cured film, which in turn leads to hardening, decreased flexibility, and embrittlement. If the internal stresses overcome the cohesive strength of the film, then the unfortunate end is cracking; if failure takes the form of lost adhesion at the coating/metal interface, then delamination is seen. Both, of course, can happen simultaneously. Instead of causing additional crosslinking, the UV energy could break bonds in the polymer or another component of the coating. Free radicals are thus initiated. These free radicals react with either: • Oxygen to produce peroxides, which are unstable and can react with polymer chains • Other polymer chains or coating components to propagate more free radicals Reaction of the polymer chain with peroxides or free radicals leads to chain breaking and fragmentation. “Scissoring,” a term used to describe this reaction, is an apt description. The effect is exactly as if a pair of scissors was let loose inside the coating, cutting up the polymer backbone. The destruction is enormous. When scissoring cuts off small molecules, they can be volatilized and make their way out of the coating. The void volume necessarily increases as small parts of the binder disappear (and, of course, ultimately the film thickness decreases). The internal stress on the remaining anchored polymer chains increases, leading to worsened mechanical properties. After enough scissoring, the crosslink density has been significantly altered for the worse, loss of film thickness occurs, and a decrease in permeation barrier properties is seen. The destruction stops only when two free radicals combine with each other, a process known as termination [4, 11]. Table 6.1 summarizes the effects on the coating when absorbed UV energy goes into additional crosslinking, scissoring or generating polar groups at the coating surface. TABLE 6.1 Effects of Absorbed UV Energy Absorbed UV energy goes into… …which causes …and ultimately Additional crosslinking Increased internal stress, leading to hardening, decreased flexibility, and eventually embrittlement Scissoring Increased internal stress Increased void volume Worsened crosslink density Generation of polar groups at the surface © 2006 by Taylor & Francis Group, LLC Increased surface wettability and hydrophilicity Cracking, delamination, or both Loss of film thickness Decrease of permeation barrier properties Decrease of permeation barrier properties
Weathering and Aging of Paint 103 Ideally, selection of binders that absorb little or no UV radiation should minimize the potential damage from this source. In reality, however, even paints based on these binders can prove vulnerable because other components — both those intentionally added and those that were not — often compromise the coating as a whole. Components that can be said to have been added intentionally are, of course, pigments and various types of additives: antiskinning, antibacterial, emulsifying, colloid-stabilizing, flash-rust preventing, flow-controlling, thickening, viscositycontrolling, additives, ad infinitum. Examples of unintentional components are catalysts or monomer residues left over from the polymer processing; these may include groups that are highly reactive in the presence of UV radiation, such as ketones and peroxides. Interestingly, impurities can sometimes show a beneficial effect. When studying waterborne acrylics, Allen and colleagues [12] have found that low levels of certain comonomers reduced the rate of hydroperoxidation. The researchers speculate that the styrene comonomer reduced the unzipping reaction that the UV otherwise would cause. 6.2 MOISTURE Moisture (water or water vapor) can come from several sources, including water vapor in the surrounding air, rain, and condensation as temperatures drop at night. Paint films constantly absorb and desorb water to maintain equilibrium with the amount of moisture in the environment. Water is practically always present in the coating. In a study of epoxy, chlorinated rubber, alkyd and linseed oil paints, Lindqvist [13] found that even in stagnant air at 25˚C and 20% relative humidity (RH), the smallest equilibrium amount of water measured was 0.04 wt %. Water or water vapor is taken up by the coating as a whole through pores and microcracks; the binder itself also absorbs moisture. Water uptake is not at all homogeneous; it enters the film in several different ways and can accumulate in various places [13, 14]. Within the polymer phase, water molecules can be randomly distributed or aggregate into clusters, can create a watery interstice between binder and pigment particle, can exist in pores and voids in the paint film, and can accumulate at the metal-coating interface. Once corrosion has begun, water can exist in blisters or in corrosion products at the coating-metal interface. Water molecules can exist within the polymer phase because polymers generally contain polar groups that chemisorb water molecules. The chemisorbed molecules can be viewed as bound to the polymer because the energy for chemisorption (10 to 100 kcal/mole) is similar to that required for chemical bonding. The locked, chemisorbed molecule can be the center for a water cluster to form within the polymer phase [13]. When water clusters form in voids or defects in the film, they can behave as fillers, stiffening the film and causing a higher modulus than when the film is dry. Funke and colleagues [14] concluded that moisture in the film can have seemingly contradictory effects on the coating’s mechanical properties because several different — and sometimes opposite — phenomena are simultaneously occurring. Two of the most important parameters of water permeation are solubility and diffusion. Solubility is the maximum amount of water that can be present in the © 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:
Page 65 and 66: 50 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 and 114: 6 Weathering and Aging of Paint Thi
Page 115: Weathering and Aging of Paint 101 c
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 167:
154 Corrosion Control Through Organ
show all

© 2006 by Taylor & Francis Group, LLC

Create successful ePaper yourself

Delete template?

Save as template?