Assessing Solvent Resistance of Coatings using Solvent Rub Technique
ASTM D 4752-03 This test method describes a solvent rub technique for assessing the MEK resistance of ethyl silicate inorganic zinc (IOZ) rich primers. The MEK resistance of some IOZ primers has been shown to correlate well with the cure of the primer as determined by diffuse reflectance infrared spectroscopy. 1 Using this analytical testing, an evaluation scale of 0 to 5 was developed to visually assess the cure of an IOZ primer by solvent rubbing with MEK solvent. As we know, the ethyl silicate cures by the reaction r eaction of the vehicle with moisture to provide the complete binder matrix. Curing is a function of the amount of available relative humidity and temperature. A certain level of cure is necessary for the IOZ primer to support a topcoat. The cure rating required for the application of specific t opcoats must be agreed upon before the test method is used. The testing procedure for this standard is as follows: Select an area on the primer surface at least 6-in. high. Clean the surface with tap water or a dry cloth cl oth to remove any loose material. Mark a 6-in. high by 1-in. wide rectangular test area and measure the dry film thickness (DFT) of the primer. Saturate the 100% cotton cheesecloth (or other mutually agreed upon cloth) to a dripping wet condition with MEK and place the index finger into the center of the double f olded rag while holding the excess cloth with the thumb and remaining fingers of the same hand. The index finger should be held at a 45 degree angle to the test surface. Rub the rectangular test area with moderate pressure using an away and back motion. One away and back cycle is considered a double rub, and should be completed at the rate of 1/s. Continue rubbing the surface with the MEK saturated rag, or if necessary wet the r ag without lifting it from the surface via a squeeze bottle, until 50 double rubs have been completed. If the metal substrate is exposed prior to completion of 50 double rubs, record the number of rubs when the substrate was exposed. Select an adjacent area as a control and repeat the process with a dry cheesecloth to establish the effect of burnishing without the influence of MEK . Inspect the test areas and rags and visually rate the results in accordance with the MEK Resistance Rating Scale included in the AS TM standard. This procedure can be used in the laboratory, field, or in the fabricating shop. Although this standard is a visual test that yields quantitative results, many inspectors only perform the 50 double rub requirements and neglect to completely provide the reporting data. As indicated above, the inspector is to record the dry film thickness of the primer prior to performing MEK double rub testing. A dditionally, the inspector is to report the elapsed time between application of the IOZ primer and performing the testing. Rub results have been found to be cure time dependent, meaning, higher rub scores are associated with longer cure periods, and lower rubs scores are associated with shorter cure periods.1 Complete reporting assists the specifier or other Protective Coatings Specialist in evaluating the overall testing results. Another common mistake made by many inspectors is to apply this test method on organic coatings. As clearly stated in the scope of ASTM D 4752, this standard only pertains to testing of IOZ primers. The appropriate test method that should be used to determine the solvent resistance of organic coatings is ASTM D 5402. ASTM D 5402-93 (1999) This practice describes a solvent rub technique for assessing the solvent resistance of an organic coating that chemically changes during the curing process, such as epoxies, vinyl esters, polyesters, alkyds, and urethanes. Before being placed into service, these coatings should reach specific levels of solvent resistance before being placed into service. This solvent rub technique is one method to determine if such a condition has been achieved. However, the standard states, the level of solvent resistance by itself does not indicate full cure and some coatings become solvent resistant before they become 2 sufficiently cured for service." The time required to reach a specific level of solvent resistance is influenced by temperature, film thickness, air movement, thinning (type and amount of solvent used to reduce the coating), and relative humidity (especially water reactive coatings). This standard differs from ASTM D 4752 in that it does not specify the test solvent, the number of required double rubs, or the expected test result. The onus falls to the coatings manufacturer to specify the appropriate test solvent, the number of double rubs for the specific organic coating, the temperature at which to conduct the testing, and the specific test results needed in order to constitute solvent resistance of the coating film. Lets take a look at how this test is conducted.
The test procedure calls for the selection of areas on the primer surface at least 6-in. high on which to run the test. Clean the surface with tap water to remove any loose material and allow t o dry. Mark a 6-in. high by 1-in wide rectangular test area and measure the dry film thickness of the primer; it is advisable to take several DFT readings in the test area. Saturate the 100% cotton cheesecloth (or other mutually agreed upon cloth) to a dripping wet condition with the appropriate solvent (specified by the coatings manufacturer) and place the index finger into the center of the double folded rag while holding t he excess cloth with the thumb and remaining fingers of the same hand. The index finger should be held at a 45 degree angle to the test surface. Rub the rectangular test area with moderate pressure using an away and back motion. One away and back cycle is considered a double rub, and should be completed at the rate of 1/s. Continue rubbing the surface with t he solvent saturated rag until 25 double rubs have been completed. If additional solvent double rubs are required, reposition the index finger on an unused portion of the rag and resaturate with the appropriate solvent, allowing no more than 10s t o elapse prior to continuing with the testing procedure of an additional 25 double rubs. Repeat this process until t he required number of double rubs has been performed. The standard then instructs the tester to inspect the middle 5 in. of the test area for fingernail hardness and visual appearance changes comparative to the adjacent control area. The t ester can use ASTM D 3363 "Pencil Hardness" and ASTM D 523 "Gloss" if quantitative data is desired. Measure the film thickness after completing the rubs to determine any film loss. Finally, visually examine the cloth f or indications of coating removal. This procedure can be used in the laboratory, field, or in the fabricating shop. As required by the former standard, the reporting data is absolutely essential for the specifier or Protective Coatings Specialist to adequately assess the condition of solvent resistance of the organic coating. The inspector should report the following: • • • • • • •
Solvent used Number of solvent rubs performed Film thickness before and after rubbing Results of visual examinations of cloth Ambient and surface temperature Humidity and weather conditions Elapsed time between coating application and testing
A reporting form is included in the appendix of this standard and may be helpful in reporting the test results. SUMMARY Two common industry standards are available to inspectors to assess the solvent resistance of coatings. ASTM D 4752, which is utilized to check the solvent resistance of IOZ primers, i s the standard with which most inspectors seem to be the most familiar. Rather, this test method is only specific to IOZs and should not be used on organic coatings. Instead, the testing parameters outlined in ASTM D 5402 should be used on organic coatings. Inspectors should also be cognizant of the significant differences between these two standards, as well as properly performing and reporting data. References: 1. ASTM D4752-03, "Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub", (West Conshohocken, PA: Annual Book of ASTM : 2003). 2. ASTM D5402-93, "Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs", (West Conshohocken, PA: Annual Book of ASTM: 1999).
