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Slip and friction on coated surfaces.


A slippery surface can be an advantage for some coatings and a dangerous disadvantage for others. Can coatings need to have slippery surfaces to reduce wear on can-making and filling equipment and to allow the cans to move easily on filling lines. Slick coating surfaces are less apt to be abraded, scratched, or to hold on to dirt compared to standard coatings. Blocking (sticking together of coated items that have been stacked or rolled) can be prevented by using slip-aids. Coatings are applied to nuts and bolts to prevent corrosion from contact with dissimilar metals, but the coating surface must be slippery enough to allow nuts to be tightened, but not so slippery that they come loose with vibration. Dance floors are expected to be slippery enough to allow the sliding of dancers' shoes (usually by wax rather than coatings), but not so slippery that the dancers fall down. However, coatings for most floors and steps must be the opposite of slippery (i.e., there must be resistance when a shoe sole contacts a coated surface). The surface may need texture or roughness, sometimes to the point of adding sand to the formulation. This resistance is called friction, which is the amount or degree of resistance to motion that one surface or object encounters when resting on another (static friction) or when it is moving against another (kinetic or sliding friction). We define these properties in terms of the coefficient of friction (COF, often denoted as g), a dimensionless number, which describes the ratio of the force of friction between two bodies and the force pressing them together. The COF depends on the materials involved: steel on ice has a very low value (0.03), whereas rubber on concrete has a high value (1.0). The Wikipedia article on "Friction" is a good general reference on the subject.

Perhaps the description of a simple horizontal pull technique will help the reader to better understand the concept of COF. This method involves fixing a coated metal panel onto a stable base, then placing a test sled (a metal block of known mass with either a polished face or a rubber facing) on the panel. The sled then is pulled by a motor or pulley and a set of hanging weights and increasing force is applied until the sled begins to slide across the panel. The force needed to cause the onset of motion is equal to the frictional force. Its value is divided by the mass of the block, which is the force pushing the block and panel together, to give the static coefficient of friction, SCOF.

SCOF = [micro]s = frictional force/mass of sled If the investigator gave the sled a nudge and measured the force that enabled the motion to just keep going, then the equation would provide a coefficient of kinetic or sliding friction, KCOF or [micro]k. Static friction coefficients for a given pair of surfaces usually are higher than those for kinetic friction. Most dry, clean surface combinations, including coatings, have SCOF values between 0.2 and 0.6. Not surprisingly, wet surfaces have lower SCOF values than dry ones, going down to 0.1 in many cases.

Recommended SCOF values for coatings that I have seen are 0.5-0.6 for floor coatings (rubber/coating), 0.1-0.25 for coatings for nuts and bolts (coating/ coating), 0.25 for bowler approach sections in bowling alleys (rubber/coating), and approximately 0.1 for can coatings (coating/coating or metal/coating). However, empirical tests related to end use are much more likely to be done than measurements of SCOF. Examples include testing whether use of a fluoropolymer or addition of a slip-aid has increased resistance to abrasion, blocking, or dirt pick-up or allowed coated nuts to be turned on coated bolts. Slip-aids include silicones, waxes, polyethylene beads, and fine particle silicas (see November/December Buyers' Guide issues of JCT CoatingsTech under Anti-Blocking Agents, Surface Conditioners/Lubricants, and Waxes for suppliers). Nonskid properties are much more difficult to evaluate empirically, so SCOF and other index-type measurements have been done by floor coating, polish, and wax suppliers for many years (see the Wikipedia article "Floor slip testing").

A number of ASTM methods for SCOF have been developed, but most were withdrawn for various reasons. I have used one now withdrawn method, D4518, "Measuring static friction of coating surfaces" many times. The horizontal pull method used to describe SCOF above is in D4518, but I used another technique in that standard, one based on placing the coated panel on an inclined plane. The operator raises the angle until the sled just begins to slide. The tangent value for the angle is the SCOF. D4518 still is available from ASTM. There are ASTM SCOF methods still in force, including D2047 for polish coated flooring (which has been used for coatings) and D1894 for plastic films, which might be useful for coatings.
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Title Annotation:Coatings Clinic
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Jul 1, 2016
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