Document dQOkv6Ox3q5nBzJYrgwe9rbdb

Paint failures -- An examination with use of a simple dynamic model (First In a three-part series) iit t i By Clive H. Hare and Stephen J. Wright Clive H. Hare and Stephen J. Wright away by gradual erosion as oxygen, are associated with Clive H. Hare, Inc., moisture, and sunlight combine to slowly 2 Mear Rd., Holbrook, Mass. 02343. break down the film and wear it away. In this manner, they may last 10,20, even 50 IN MANY WAYS, paint films are like years, but eventually they will powder people; they are organic, made up of away to nothing. high polymeric materials, and are subject If a paint Him lasts 20 or even 10 years, to a surprisingly similar process of it must be considered successful. The term degeneration. They do not last forever, "failure" is more appropriately reserved but like us are heir to a million and one for paint jobs that undergo catastrophic possible terminations. They have a finite 'premature deterioration within the first service life, and the most that can be done one or two years of life. It is these failures is to see that they last for as long as possi that are considered here. ble. There are many causes of early paint Like all soldiers, the best of them never film failure and a concise treatment ofthe die catastrophically, they simply fade subject is all but futile. Many common August 2, 1982 Vc^. Lr'ij 3 \ \ failures are, however, related, and caft be heterogeneous, is subject to dimensional understood by the use of a simple model, changes as alternating bands of sum- and by examining the way it responds to merwood and springwood respond to stresses in paint application and from the climatic conditions. environment. By use of the model a First to be considered is how dements variety of apparently unrelated paint of composite paint systems interact. failures will be discussed and they will be Failures can be related to mismatched shown to be individual manifestations of strengths of elements of a paint system similar fundamental problems. and their adhesion to each other. The paint Him model consists of: a sub Under ideal conditons where no exter strate, a primer, and a top coat, held nal stresses are applied to a system and together by a series of connected forces. whore age-related phenomena do not take (Figure 1). In more complex systems place, tiie composite will maintain its where other intermediate coats are pres equilibrium indefinitely, regardless of the ent, the composite may have a number of quality of the adhesive and cohesive A\ alternating interfaces and paint film con forces that hold the system together. tinual held together by a series of In the real world, however, neither sub adhesive and cohesive forces in strate nor paint film are static entities, equilibrium. (Figure 2.) and when toe system is put into service it The substrate, of course, varies. Each must exist in toe stress-producing en substrate has its own peculiarities, which vironment. The elements of the sub impact upon the design and effectiveness strate/paint system -composite are of the applied coating system. Cemen dynamic entities, which, inevitably, res titious substrates, for example, are often pond to hot/cold and humid/dry cycling alkaline; metal surfaces tend to corrode; as well as actual mechanical movements plastic surfaces may be subject to attack such as caused by impact, vibration, or < from the solvent carriers of the coating bending. Certain substrates, such as system; and wood, being highly wood, do not respond uniformly to such . 44 American Paint < Coatings Journal -1 - -.o Y-:v;J . '; < stresses, and its heterogeneity is what makes wood so difficult a substrate to paint. In fact, no substrate nor paint film will respond in quite the same degree as another to such environmental stresses and this differential is often instrumental in paint failure. The substrate continuum of the mold is held together by lateral cohesive forces. The primer is anchored to the substrate by vertical adhesive forces and is itself held together by cohesive strength. Similarly, the continua of all films above the primer are held together by their own lateral cohesive forces and are affixed to the coatings beneath them by vertical forces, which may or may not be different from the force that anchors the primer to the substrate. As long as each composite layer adheres to those layers that sandwich it and as long as each layer retains sufficient cohesive strength to maintain its own in tegrity, responses in any one component to stress wifi cause a response in other components, producing a complex series of strainings throughout the system in each individual component and across each interface. As long as such straining forces do not exceed the cohesive strengths of continua or the adhesive strengths across any interface, equilibrium wifi be maintained and the paint system composite win successfully work. Paint system failures occur when the cohesion of any element of the system or the adhesion of any element to another is insufficiently strong to withstand the de mands that the total composite exacts upon the individual film, as the system responds to the service environmental de mands. Let us examine a situation, for exam ple, where the applied film has acceptable adhesion between ail coats, but the finish coat is more brittle a"d less strong than the underlying coats. Unless the brit tleness of tile topcoat is overly marked, application will produce little problem and the total composite will behave well until it responds to a significant stress. For example, if there is a prefinished board that is coated indoors and it is put outside to respond to a typical summer/winter cycle, the board will expand in hot conditions and contract in cold or dry conditions. These changes will not take place once, but will be repeated. Their effect on the coating depends on the viscoelastic properties and the coef ficient ofexpansion of the elements of the total composite. Continual climatic cy cling produces -a considerable flexing of the system. Under such conditions, the cohesion of the topcoat is progressively weakened until failure occurs as the builtup strains in the topcoat are dissipated by cracking. (Figure 3). If the topcoat film is very weak, such as August 2,1982 45 ; i * i might be found in a poor underbound will be the corresponding demand that film (rare in exterior systems), the strains the system exerts on its adhesive forces, will dissipate as a fine mud-cracking, which anchor it to lower film or substrate. which may not be visible. In most exterior Unfortunately, the adhesion of a system systems, having better general cohesion, is finite. Measurements can be taken to the cracking will be more macro in artificially improve adhesion, but once nature, and thus result in a severe check dried, paints will never increase their ing or cracking failure visible in the top adhesion. If the cohesion becomes too coat. strong, under service induced stresses, the Oil paints are oxidizing systems that adhesion may not be able to cope with the begin life as flexible systems, but, as they demands of the coating system. age, progressively undergo chemical The stonger cohesive forces will in this changes such as autoxidation, excessive way literally pull the underlying film from crosslinking, and chain schism. In prac the substrate and the system will peel. tical terms the film gradually becomes Such adhesive failures need not neces more and more brittle. sarily take place across the interface Thus, films, which once could with between the strong film and the film stand a great deal of flexing, inevitably, directly *below it. If the adhesion across 4 $ become just too brittle to withstand such this interface is good, and better than the flaring, and the resultant strains may be adhesion across a second lower interface, dissipated in deep cracks. Films lower in the resulting failure will occur Anther the composite are older, and therefore, down the composite at the lower inter generally more brittle than the upper face, and the lower film will be stripped films. If adhesion is good across all from its substrate or film to which it coating/coating inerfaces, cracks in One adheres. ((Figure 5), coat will often result in cracks in lower On the other hand, if the continuum of films, and vice versa. the lower film is weaker than the adhesion The strength of a cohesive paint system across both interfaces, the strain -in the will, of course, increase with the film system may be dissipated by the cohesive thickness of the system. (Figure 4). As the failure in the lower film. In this case, frag cohesive strength increases, the greater ments of the lower film will be seen on the 46 American Paint A Coatings Journal V. Figure 5 Finish Hist Intermediate Second Intermediate Primer Substrata Finish First Intermediate Second intermediate Primer Substrate underside of the composite film tion of latex house paints, there was a delaminating and on the surface bared by fashion among home owners to abandon the delamination. (Figure 6). It is impor the old solvent-thinned oil paints for the tant to realize that under stress, a coating convenience of the water-reducible latex system will always fail at its weakest finishes. point, whether this be at a poorly Unfortunately, the latex finishes are prepared interface, or in the continuum of thermoplastic materials (unlike oil paints, a poorly cohesive film. which are thermosetting systems), and The two foregoing failure models are well illustrated by practical paint film failures. respond greatly to differentials in temperature and humidity. The latex film is as flexible as the old oil film is brittle. As the adhesion of the latex film to the ' >I Ou t illustration of adhesive failure at old oil, paint is, under normal circum lower, less secure interfaces caused by stances, very good, the marked expansion ! strain produced by a strong or flexible and contraction of the latex Him puts the 1 ; topcoat is the cause of what has become system under very significant stress. i * f one of the most common types ofexterior Generally, the oil film is cohesively very - t paint failures in the Northeast. In this strong, and so this stress is all too often part of the country, there are many older felt most keenly at the interface of the -> homes that have for years been painted first coat of oil paint and the wooden sub with oil paints. These oxidizing systems strate. over a period suffer from the embrittling Caught between these two stresses, the processes described above. old oil paint is literally stripped from the As these aging processes occur, the wood by the latex film (bringing with it all \ coating film responds progressively less intermediate oil films that had been ap easily to climatic differentials. In the late plied in the interval between the last 1 1950s and early 1960s, with the introduc painting and the original painting) and August 2, 1982 47 -t. V. * r - -;W -re: -VV ,'CC -] i j] W; the house is bared of paint to the original wooden substrate. The phenomena becomes worse as the cohesion of the latex film increases. Thus, the thicker the latex paint or the greater the number of latex coats, or the higher its gloss (the lower its pigment volume concentration), the greater is the stress that is exerted on the, total system. Similarly, the older and more brittle the oil paint, or, for any other reason, the lower the security of its adhesion to the wood, the more that paint system will be subject to this type of failure. The cardinal rule of house painting, particularly on old houses, is to apply oil paint over oil paint and latex paint over latex paint. Never apply a latex paint (especially a high-gloss latex paint) to the siding of a house that bears an old oil paint system next to the wood. This is particularly true ifthe old oil paint system shows signs, before painting, ofweakened adhesion at the original wood interface. Equally commonplace is the illustra tion of strong topcoats producing cohesive failure in poorly bound paint films that lie lower (nearer the substrate) in the total paint composite. In this case, the failure is typically found on ceilings in old houses, apartments, schools and other institutional buildings. For years, the practice was for ceiling paints to be for mulated from limed oils, calcicoater vehi cles or simple calamine and distempers. All these materials were cheap, opaque, but quite underbound. In many cases, there was little real vehi cle present, and the film could be readily removed by washing with soapy water. Few folks had cause to wash ceilings, however,. and for years these paints produced good service, as ceilings were continuously recoated with similar products as needs arose. With the advent of the better-bound ceiling paints, such as fiat alkyds and latex fiats, the situation became more complicated. These paints, 48 while in their own right of better quality than the calcicoater materials, were inap propriate for use on this type cf ceiling. Being more cohesive and much stronger than the poorly bound ceiling paints to which they were applied, these paints induced considerable stress into the system. As film thicknesses built up, the strength of the upper layers of the composite greatly increased in com parison to the strength of the lower layers of the composite to the point that as the system was put under stress, the com posite failed within the continuum of the underbound calcicoater films deeply embedded within the total composite. Stress was often derived from the dif ferences in humidity and temperature during the summer and winter. More often than not in such situations, however, failure is more likely to occur at, or just after the application of an ad ditional coat of paint, than in the long run as a result of climatic differentials. As new coatings are applied, the water or sol vents that carry the new paint tend to penetrate into the older films and produce considerable expansion. This expansion is followed by significant contraction as the water or solvents evaporate from the system and the system dries out. These two marked changes produce even , greater stresses than normal en vironmental stresses, sufficient to cause significant delamination. tins particular failure had caused many headaches in the renovation of older structures, where the interior paint systems have been subject to rather more extreme conditions of cold and humidity that occur when buildings are left open during partial renovation and closed just before painting. As the heat is restored, or after new paint ap plication, significant failure of the ex isting composite occurs, with large amounts of eating areas peeling and re quiring complete refinishing. /- Ci- American Paint A Coatings Journal I\f