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Abrasion, scratch, and mar.

Organic coatings are resistant to most insults and stresses, but they can be damaged, including by wear that abrades the surface, scraping or scratching that breaks the surface, or compressive stress that causes indentation. Abrasion is the process of wear in which there is displacement of material from a surface during rubbing against hard particles or protuberances. We want most coatings to be sandable, but we do not want them to be rubbed thin or completely off when in use. A scratch is a mark or injury (usually tearing or cutting) produced by the movement of a sharp or jagged object across a surface. Scratches are particularly noticeable on glossy surfaces such as automobile finishes and floor coatings. The definitions of abrasion and scratching are generally accepted, but there are differences of opinion as to the meaning of the term mar. Some people think of it as anything in or on the surface of a coating that hurts appearance, whereas others (including me) favor a definition that is restricted to permanent deformation in the form of scuffing, multiple fine scratches, or indentation. Sometimes mar is thought to be synonymous with abrasion. This has been reinforced by several ASTM methods that are called mar tests, but use abrasion as the means of testing the surface of the coating.

I have considerable experience with automotive topcoats where surface damage can arise from a number of sources, including the trim process in the auto factory, keys and fingernails, dragging objects (even protective covers) across painted surfaces, airborne sand and grit, and encounters with branches and shrubs, as well as polishing and car washing. A particularly destructive practice is the wiping of dust from a vehicle with a cloth (dry wiping) rather than by washing. The dirt and dust particles on the coating are highly abrasive and rubbing a cloth across them has much the same effect as rubbing with sandpaper. Although there have been recent improvements in topcoats, consumers continue to demand better abrasion and scratch resistance.

There are strategies for developing coatings that are resistant to damage, although not every option works with every technology and improvements in mar and scratch may come at a cost of hurting other properties. Coatings that must resist abrasion need to be tough, that is, have both high tensile strength and high elongation. They also may be formulated to have slippery surfaces by the addition of waxes or other materials that reduce friction. This is unacceptable for floor coatings (except perhaps dance floors) and auto finishes where the surfaces must be recoatable in case a need for repairs arises. Raising crosslink density via higher bake temperatures, longer bakes, use of catalysts, or increasing resin or crosslinker functionality improves mar and scratch resistance for most coatings, but may also lower impact resistance, make cracking more likely, and reduce recoat adhesion. Mar and scratch resistance appears to be independent of Tukon and other indentation hardness results, which is a little surprising. Some work has been done to relate mar and scratch resistance to fracture toughness (the ability to resist crack propagation). I have seen fractures under the microscope, but also a lot of what looked like slicing or shearing as well as grooves with no break in the surface.

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There are several mar and scratch tests, some of which, as noted previously, are abrasion tests. One common type involves the abrading of the coating surface with a wool felt cloth moving across an abrasive powder. The cloth is attached to a "toe" or cylinder connected to a reciprocating device (crockmeter). Gloss measurements are made in marred and unmarred areas. Mar resistance is defined as percent gloss retention. This is the basis for ASTM D6037, "Dry Abrasion Mar Resistance of High Gloss Coatings." Abrasive papers have been used in place of the cloth/abrasive powder combination and D6279, "Rub Abrasion Mar Resistance of High Gloss Coatings" employs a wet rub technique as well as a dry one. It is difficult to compare laboratory test results and field damage as field scratches vary greatly in depth, width, length, shape, and frequency. However, I found that a dry abrasion test similar to ASTM D6037 gave surface damage much like many of the more severe field scratches and differentiated well between different coating technologies.

One test for mar resistance uses a scraping loop (D5178, based on the Balanced Beam Scrape Adhesion and Mar Tester). Other test instruments include single stylus scratching devices capable of variable loading. Microscratchers such as that in D7027, "Scratch Resistance of Polymeric Coatings and Plastics using an Instrumented Scratch Machine," move in a rapid, realistic manner, but tend to cause a lot of damage; nanoscratchers produce more realistic damage, but the stylus moves slowly. D7187, "Measuring Mechanistic Aspects of Scratch/Mar Behavior of Paint Coatings by Nanoscratching" is excellent for testing mar and scratch resistance and also provides data on the relationship between force and scratch depth. This "scratch modulus" can be used to compare coatings and chemistries. Nanoscratchers are expensive and are not practical for routine testing, but they are powerful tools for understanding the mechanical properties needed for mar and scratch resistance.

By Clifford K. Scholl, Schoff Associates
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Title Annotation:COATINGS CLINIC
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Aug 1, 2012
Words:864
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