ASTM International has established a new standard for determining paint “open time” and revised its standard for testing water resistance of coatings under 100% relative humidity.
ASTM D7488 - Test Method for Open Time of Latex Paints provides a means of determining the “open time” during which paint can be worked into a previously painted area. Open time is a key performance property for coatings, particularly for brush applications.
“Historically, solvents used in coating formulations assisted in delivering good open time of a formulated coating,” says Carol Hawkins, a scientist with Dow Chemical Co. and chairman of ASTM Subcommittee D01.42, which developed the standard.
“As we move away from using solvents in coatings due to changes in regulations for Volatile Organic Compounds, it is important to develop a test method that formulators can use to determine open time of a coating.”
The standard will prove useful to formulators who represent manufacturers of architectural paints and paint additives, Hawkins said.
Coatings Humidity Testing
ASTM has also revised D2247-11 - Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity. “Water can cause the degradation of coatings, so knowledge of how a coating resists water is helpful for assessing how it will perform in actual service,” the revised standard notes.
“Failure in tests at 100% relative humidity may be caused by a number of factors, including a deficiency in the coating itself, contamination of the substrate, or inadequate surface preparation. This practice is therefore useful for evaluating coatings alone or complete coating systems.”
Tests at 100 % relative humidity are used for specification acceptance, quality control, and research and development for coatings and substrate treatments, the standard says.
| Knowing how a coating resists water is helpful for assessing how it will perform in actual service, ASTM notes.|
Basic Principles and Procedures
According to ASTM: “This practice covers the basic principles and operating procedures for testing water resistance of coatings by exposing coated specimens in an atmosphere maintained at 100% relative humidity so that condensation forms on the test specimens.
“This practice uses the technique of creating a slight temperature differential within the exposure area to form condensation on the coated specimens. As the warmer saturated air passes the cooler specimens, water is deposited onto the specimens in the form of condensation.
“This practice places the entire specimen in the exposure area allowing condensation to form on all surfaces. This makes this practice suitable for flat panels as well as large or 3D objects.”
Results obtained from this test “should not be represented as being equivalent to a period of exposure to water in the natural environment, until the degree of quantitative correlation has been established for the coating or coating system,” the standard warns.
ASTM International is one of the largest international standards development and delivery systems in the world.