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Dirt pick-up.

A freshly applied coating usually has an excellent appearance that beautifies the object or structure that has been painted. Unfortunately, that appearance does not always last long. One problem faced by architectural coatings in particular, but that can also occur with industrial and automotive coatings, is the accumulation of dirt on the painted surface. The dirt may come from rain, air pollution fall-out, by the spattering of mud and dirty water from the ground or other surfaces, or the settling on, and adherence of, road and construction dust and dirt. The dirt often adheres very well and may be difficult or impossible to remove. The appearance is hurt and the dirt layer provides a medium on which mildew or mold may grow. This problem or defect is referred to as dirt pick-up or environmental soiling.

A number of things contribute to dirt pick-up, including the amounts and kinds of dirt to which the coating is exposed, the mechanical properties of the coating, and the climatic conditions experienced by the coating. The more dirt the coating contacts, the more likely that some will stick. Soft coatings are notorious dirt catchers, particularly where hard dirt particles are involved. Particles that fall on a soft surface can settle down into the coating, especially in hot weather, and wind-driven or vehicle-driven particles can strike the surface and become embedded. The morphology of the coating surface also affects soiling. Dirt adheres more readily to rough, porous surfaces than to smooth ones. Plasticizers added to many coatings to make them soft and/or flexible can migrate to the surface, form a sticky exudate layer, and catch and hold additional dirt. Moisture absorbed from the air during times of high humidity and precipitation can plasticize and soften coatings, which leads to an increase in dirt pick-up.

A few descriptions of devices or chambers for accelerated testing of dirt pick-up may be found in literature. However, most testing appears to be based on outdoor exposure of white or other light-colored panels followed by inspection and rating or by measurement of reflectance or color difference. ASTM D 3274, "Evaluating Degree of Surface Disfigurement of Paint Films by Microbial (Fungal or Algal) Growth or Soil and Dirt Accumulation" provides photographic reference standards and a numerical rating system. ASTM D 3719, "Quantifying Dirt Collection on Coated Exterior Panels" is based on reflectance. A digital camera, by itself or coupled to a low power microscope, along with a computer and image analysis software, could be used to count dirt particles, measure darkening or degree of dirt coverage, and make other evaluations.

How do we prevent or minimize dirt pick-up? Hard coatings (those with high glass transition temperatures) are less likely to pick up and retain dirt and grow mildew, but they are more apt to have other problems such as cracking and high internal stress. Smooth, low porosity coatings pick up less dirt than rough, porous ones. High PVC coatings are better than low PVC products (probably due to having less resin at the surface) as long as the critical PVC is not exceeded. As coatings degrade, they become rougher and more porous, which favors the adherence of dirt. Therefore, highly durable coatings tend to retain their initial resistance to soiling.

One school of thought says that surfaces should be made as hydrophobic as possible so that rain and other water beads up and rolls off. This worked for styrene-acrylic latex paints where silicone modification and separately applied silicone water repellents both gave low dirt pick up. It did not hold for solventborne alkyds or waxed automotive topcoats where more dirt was collected when water beaded on the surface. When water wet the surface and sheeted off, only a thin film of water remained and little dirt was left behind. There also is evidence that hydrophilic surfaces are easier to clean. In recent years, work has been done on self-cleaning surfaces that use a low rate of chalking (release of pigment on the surface) to keep the surface clean. Dirt that settles on the surface is washed off by rain or by spraying with water.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email publications@coatingstech.org.
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Title Annotation:Coatings Clinic
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:May 1, 2006
Words:741
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