WEST CONSHOHOCKEN, PA - ASTM International has introduced two new standards. ASTM D7490 uses contact angel measurements to determine the wettability of coatings. ASTM D7233 allows for more effective quality testing of the synthetic, hollow filaments that are often used in the manufacture of lower-cost paintbrushes.
The wettability of a particular solid coating, substrate or pigment on a surface is a key to its effectiveness, as wetting is important to adhesion as well as to the prevention of surface defects such as craters, dewetting and crawling. The new standard, D7490, Test Method for Measurement of the Surface Tension of Solid Coatings, Substrates and Pigments Using Contact Angle Measurements, was developed by Subcommittee D01.23 on Physical Properties of Applied Paint Films, part of ASTM International Committee D01 on Paint and Related Coatings, Materials and Applications.
Clifford Schoff, a longtime member of Committee D01, said that D7490 will be used to determine the wettability of a given primer or substrate. It can also be used to explain why there are wetting problems or defects over a certain primer or substrate.
“The standard can also be used to characterize pigments that have been pressed into disks and to determine how easy the pigment is to wet,” said Schoff. “This is useful in deciding whether a given dispersing resin solution or paint vehicle will wet the pigment, or explaining why it does not.”
According to Schoff, the technique used for contact angle measurement is the same as that described in another D01.23 standard, D7334, Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement.
“We built on Practice D7334 and developed D7490 for measuring the surface tensions of solid coatings and related materials,” said Schoff. “The key to the test method is that liquids that wet a surface flow out and give a low contact angle, whereas those that do not wet bead up and give a high contact angle.”
The new standard, ASTM D7233, Test Method for Testing Fracture of Level Paintbrush Filaments, was developed by Subcommittee D01.61 on Paint Application Tools.
John Feathers, Technology Associate for DuPont Filaments, and Chair of Subcommittee D01.61, said that previously, the low-cost paintbrush market was served by brushes that used natural hog bristle, while professional brushes were made with solid filaments.
“While the hollow filaments did provide lighter weight and cheaper manufacturing cost, they’ve often had quality issues such as fracturing when bent during painting,” said Feathers. “Failure to recover to a straight position after flexing during painting often results in filaments sticking out from the brush, resulting in poor control of the paint application.”
Feathers said that ASTM D7233 and the equipment detailed within it will allow suppliers of hollow filaments for paintbrushes to better assess the tendency of the filaments to fracture.
“This method can be used between filament suppliers and brush manufacturers to establish an acceptable quality specification for the filaments,” notes Feathers. “In addition, the method is useful when developing new hollow filament cross-sectional shapes to reduce or eliminate fracture when bending.”
