The UV and EB Curing Process
Prior to discussing the testing of the cure of UV/EB curable coatings let’s review what UV/EB curing is. Curing or polymerization is the process of reacting prepolymer monomers and oligomers together in a chemical reaction that forms polymer chains, or cross-linked three-dimensional networks.
UV curing is a process that converts 100% solids chemistry from a liquid to a solid in a fraction of a second as a result of exposure to UV light. UV light is sourced from high-intensity mercury vapor lamps or UV light-emitting diodes (LED)s. In curing, photo-initiator components of the chemistry absorb UV light energy to form free radicals, which cause polymerization and cure.
The EB or electron beam curing process utilizes a stream of accelerated electrons produced by a high voltage electron generator to cure 100% solids chemistry. The power of the high-energy electrons is capable of initiating the curing process without the need for photo-initiators. Significantly EB energy has the ability to break bonds of polymers, producing free radicals which drive free-radical chemical reactions. Cure is instantaneous, taking place in milliseconds.
The curing of a UV curable coating is dependent on a number of variables, including coating thickness, opacity, formulated cure speed, and the substrate involved. Similarly, the adequate curing of an EB curable coating is dependent on application and EB process variables being optimized. Coating opacity is not a factor with EB curing, but coating density is.
Cure may be said to be inversely related to the extractable reactive monomer and oligomer level remaining after cure has taken place. Proper cure, or functional cure can be best defined as the cure obtained when the target performance parameters of the coating have been optimized for the end-user. Cork UV/EB coatings have migration levels below those published in the Swiss Ordinance guidelines and comply with the EuPIA Good Manufacturing Practices.
UV coatings are challenging to test for cure since they typically cure from top to bottom. Air-born oxygen inhibits the cure of the topmost film layer, while internal film cure occurs as UV light penetrates. Adhesion to a substrate takes place when UV light reaches the interface. Good adhesion requires good surface wetting and low film shrinkage. Cure is approximately 80% complete when UV light exposure stops. Post-cure takes place over the next 24 hours or longer. Post cure typically improves adhesion.
Determining adequate functional cure is not easy, however, certain simple tests have been developed, which have been proven useful in place of expensive extractable testing.
TESTING THE CURE of Ultraviolet And Electron Beam Coatings
MEK RUB TESTING
MEK Rub Testing checks the relative solvent resistance of a cured sample and is used as a relative indicator of cure. Film thickness and cure affect test results. The test uses a Q-Tip saturated with MEK which is rubbed across a coated sample in a back and forth 2-4 inches (double rub) firm and constant motion. The test is stopped when the coating is worn through with the number of double rubs recorded. ASTM 5042.15
PERMANGANATE STAINING TESTING
Permanganate Staining Testing measures residual unsaturation or the unreacted double bonds in a molecule indicating the amount of cure. The intensity of stain color is proportional to the amount of unsaturation. The test is conducted using a 1% solution of potassium permanganate (KMNO4) in distilled or deionized water. The solution is applied to a coated sample covering a ½ in. diameter area which is allowed to remain on the surface for 5 min. before rinsing with water. A brown stain will be seen with the darker the color indicating more unreacted material. The resulting stain color is compared to that of a well-cured retained standard.
Scratch Testing can assist in determining if a coating is under-cured. Adhesion to a substrate develops when enough UV light reaches the coating substrate interface, with good adhesion being seen immediately after a coated piece exits the curing unit. A quick adhesion test may be done by rubbing the edge of a nickel coin across the coated sample, or by fingernail scratching. A cured coating should not peel or flake off. Adhesion failures can be caused by under-cure, over-cure, film shrinkage, film brittleness, poor substrate, or ink wetting.
Block Testing is designed to simulate a stack of coated/printed sheets traveling in a truck, the test is conducted using individual sheets. These are tested face-to-face and face-to-back at 140° F., 70% R.H. for 24 hours under a 2 PSI load. Any sticking indicates a cure issue.
PENCIL HARDNESS TESTING
The Pencil Hardness Testing test method is used to test certain hard-coats for their hardness and resistance to scratches and wear. Pencils ranging from 6B to 8H are used to determine resistance to scratching. The 5021 Pencil Hardness Tester is sold by Gardco. ASTM D3363.
SCOTCH TAPE TESTING
Tape adhesion tests are unable to reveal much about cure because slip additives that migrate to the surface of a coating can prevent a tape from sticking effectively. However, the test can have value as an indicator of expected coating performance. Scotch brand #610 tape is used. ASTM D 3359 describes the procedure of attaching a 3” strip of tape to a coated sample. The tape is removed within 90 seconds by pulling the free end back rapidly, without jerking. It is examined looking for any coating and/or underlying ink that is removed.
FLAIR PEN TESTING
The Flair Pen Test is a quick test that can indicate that a coating is under-cured. It is based on the pen’s ink immediately beading up and reticulating on the surface of a coating. This is an indicator of wax and silicone bloom, which is an indicator of an adequate cure. A solid ink line (no reticulation) results when cure is inadequate or when no silicone, and/or little wax blooms.
Abrasion tests use devices such as the Taber Abraser ASTM D4060-19, the GA-CAT ASTM D5181-09, and the Sutherland ASTM D5264-98. The tests are an indicator of surface cure and slip. Generally, under-cured coatings offer poorer abrasion resistance. These tests are best used as a gauge of a coating’s required performance criteria.
The listed tests, while valuable in assessing the degree of cure, they do not actually test absolute cure. They only test relative cure response, and the coating properties developed from curing.
Contact Cork Technical experts if you require a UV/EB coating with enhanced performance properties for a demanding application. Cork’s business is the development and formulation of Aqueous, energy-curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives.