By Cynthia A. Gosselin, Ph.D., The ChemQuest Group
Assuming that paint chemistry, film thickness, and application techniques are matched to the intended service environment, the degree of cure is the single most important characteristic required for a painted system to perform well. During the coating development phase, proper curing parameters are determined through a variety of sophisticated techniques monitoring the glass transition temperature. Thermomechanical Analysis (ASTM E1545) and/or Differential Scanning Calorimetry/Differential Thermal Analysis (E1356) are the usual methods for dissecting the degree of cure on a molecular level to inform the needed curing temperature and time information. Unfortunately, each of these techniques takes a fair amount of time that is inconsequential in a laboratory environment, but impractical for the fastpaced need of a production setting.
Testing has been developed to harness the laboratory curing information by correlating the glass transition temperature with a solvent (resistance) rub test. The standard for this testing is ASTM D5402 Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs.
This is a quick way for a production quality lab to simulate the degree of cure by measuring the number of double solvent rubs it takes to damage the surface. This manual method is quick, surprisingly simple, and exhibits a 95% confidence limit for reproducibility between round robin operators. Coating thickness is also an important variable. Thicker coatings provide higher rub numbers because it takes longer to expose the substrate. Very thin films with pencil hardness of 6H fall a little outside of the 95% confidence limit, at 90%. The film thickness range required number of rubs and type of solvent are listed on the Technical Data Sheet.
Historically, many quick property tests were developed within the coil coating industry where 40,000+ pound coils take only minutes to run through the line, with mere seconds of curing time. This production cycle requires the absolute minimal test completion time. In 1980, the test was codified as National Coil Coating Association No. 11-18. Revised in June 1996, it became NCCA Technical Bulletin 4.2.11 Test Method for Evaluation of Solvent Resistance by Solvent Rub. In 1993, ASTM D5402 was written for use in the laboratory, the field, or the factory.
Coatings that chemically change during the curing process become more resistant to solvents as they cure. They reach specific levels of solvent resistance prior to topcoating or being placed into service. Epoxies, vinyl esters, polyesters, alkyds, and urethanes are typical chemistries that fit this paradigm. In this standard, no solvent is specified. Rather, the producer and user agree on the solvent and the number of double rubs based on formulation chemistry.
ASTM Test Method D4752 Standard Practice for Measuring MEK Resistance of Ethyl Silicate (Inorganics) Zinc-Rich Primers by Solvent Rub is the preferred solvent rub method for ethyl silicate zinc-rich primers as the cure is a reaction with moisture, providing a binder. In this case, MEK is the specified solvent correlating very well with the chemical changes occurring during cure as identified by diffuse reflectance infrared spectroscopy.
The test is remarkably simple. First, the dry film thickness is verified to be within the required range. Cheesecloth is saturated with the solvent of choice. An index finger is placed in the center of the cloth, with the thumb and remaining fingers holding the cloth in place. The index finger is held at an approximately 43° angle as the cloth is rubbed onto the paint, first away from the operator, then back over the same line. The complete forward and back motion takes about one second. The testing continues, adding solvent as needed without lifting the finger off the substrate until either the required double rubs are achieved or the substrate surface is exposed. If the substrate is exposed before the required number of rubs, this is reported as a failure.
In addition to reporting the number of double rubs, sometimes a rating system is used to further describe any damage to the surface, such as burnishing, marring, or depressions in the film, particularly in zinc-rich systems.
In 2013, a mechanical rubbing machine was developed and codified through ASTM D7835/D7835M Standard Test Method for Determining the Solvent Resistance of an Organic Coating using a Mechanical Rubbing Machine. It provides consistent stroke length, rate, pressure, and contact area not subject to variables such as human fatigue and tabulates the number of rubs achieved.
Whether conducted manually or by machine, this simple test is a rapid, effective way to ascertain that the required level of cure is achieved for optimal service life.
Cynthia A. Gosselin, Ph.D., is director at The ChemQuest Group, www.chemquest.com. Email: cgosselin@chemquest.com.