Saul Vance counted to himself as he rubbed the coating panel, trying to keep his index finger at a 45-degree angle and not pushing too hard. After already running a previous test, he had to concentrate to keep track of the number of double rubs. The odor of the MEK (Methyl Ethyl Ketone) vapors coming off the saturated double thickness of cheesecloth he was pushing along the length of the metal panel didn’t help his concentration. He timed the back and forth movement of his arm with the metallic banging of the coil coating line accumulators moving up and down outside the small QC lab. The accumulators moved up as much as 80 feet to take up the slack in the coil line so that another coil coating panel could be cut from the line. Saul knew from the sound that the QC chemist was taking another sample from the line to test. As he stared at the coating break in the panel beneath his finger, he noticed sweat dripping from his forehead onto the panel, not from the heat, but from sheer terror. Now his coating formulation had failed twice to meet his own 200 double rub specification.
Saul’s coil line customers that purchased his coatings wanted to see how well the formulation resisted MEK solvent. Coil coating lines are highly mechanized and automated to give high-quality coated metal that can then be fabricated into parts for appliance, automotive and building products. This pre-painted metal process without overspray saves money because coil lines use close to 100% of the coating. The continuous process generates large quantities of coated metal in a short time. Over the years, the MEK double rub test had become a quick and easy way for coil coaters to determine the cure and crosslink density of the final baked coating. The solventborne formulations are applied and baked under controlled environments on the coil line. The cured coil coatings are tested on the line to meet tight specifications that ensure proper curing of the thermoset polyester formulations.