U.V. Inks - Adhesion vs. Cure

In this article we will clarify one of the more difficult aspects of using U.V. inks - distinguishing between U.V. ink that does not adhere due to inadequate cure versus ink that does not adhere due to ink/substrate incompatibility. The easier of these scenarios to troubleshoot is ink/substrate incompatibility. No one ink system is universal to all substrates or applications. Varying performance objectives prevent the formulation of a "Universal Ink."
The first cut at ascertaining ink/substrate compatibility is to check the ink manufacturer's recommendations or to consult with a service technician to determine if you are using the proper ink for your substrate.
With the wide range and grades of substrates in use today, it may be necessary to forward your substrate sample to your ink supplier's technical service team for their recommendation. When time constraints do not allow for testing by the ink manufacturer, you will need to make this call yourself.
Typically, if adhesion failure is the result of incompatibility between the ink and substrate, the cured ink film will have a tendency to flake or pop off the substrate with minimal mechanical abrasion or tape test. When this occurs, there is normally little or no ink residue left on the surface of the stock. The entire ink film releases from the substrate demonstrating no "bite" into the material. Often these ink flakes will have a dry appearance and will crumble when rubbed between your fingers rather than roll up into a gelatinous lump. To eliminate the affect of postcure, allow the printed ink to remain untouched on the substrate for up to two hours and re-test. If the ink still releases cleanly and in relatively large flakes or sheets, it is likely that you will need to evaluate other inks for this application. However, before you throw in the towel on the ink/substrate combination you are evaluating, you may want to check with the manufacturer one more time to determine if there is a recommended adhesion modifier that may be added to a given ink line to improve adhesion characteristics on the most difficult substrates.
Care should be taken in that most additives have potential trade-offs of pot life, a postcure requirement, or loss of ink film flexibility. Once satisfied that an adhesion failure is not due to lack of ink/substrate compatibility, direct your attention to the most frequently encountered technical issue with U.V. inks - adequacy of cure. Thoroughness of cure and adhesion are closely related.
The portion of ink film that actually provides adhesion to the substrate, called the contact layer, is the last layer to receive the crosslinking dosage of ultraviolet energy. If U.V. energy does not reach this contact layer due to excessive ink deposit or inadequate energy exposure, the contact layer remains un-cured or under cured, resulting in adhesion failure. In this case, when you put pressure on the ink film with your thumb and twist, the undercured contact layer will appear soft and gelatinous. Often the cured surface of the ink may slide or move over the liquid layer beneath. Frequently, when an ink exhibits adhesion to a material but is grossly undercured, there will be an ink residue on the substrate surface indicating the contact layer is attempting to bite into the substrate. Seldom will the ink layer completely release in the form of ink flakes or large pieces from the substrate as it did with ink substrate incompatibility. In less severe cases of undercure, the thumb twist test would indicate that the ink film is at least partially cured at the contact layer by not readily twisting. At this time a cross-hatch and tape test (described in Sericol Solutions Number Three late last year) should be used to confirm the level of adhesion.
If a significant portion of the ink film releases with the tape, but the ink film appears to be splitting in thickness with a portion of the contact layer remaining on the substrate, it indicates that the ink is exhibiting adhesion characteristics but is undercured. The portion of the contact layer remaining on the substrate will appear soft, indicating a lack of ink film through cure. To resolve this marginal cure we need to either decrease ink film deposit or increase U.V. energy exposure.
The last three issues of Sericol Solutions reviewed the variables that affect ink film deposit and suggested methods controlling the ink deposit including: mesh, stencil, squeegee, ink viscosity, press setup and substrate influences. Most U.V. inks require an exposure of 350 to 400 millijoules/cm squared to facilitate cure. If, after passing a U.V. energy monitor or "light bug" through the curing reactor at these energy levels we still have not achieved cure, we need to review these deposit variables. If millijoule levels are below this range, first check lamp use hours, cleanliness of the lamp/reflector and maintenance history of the curing unit. Provided these issues are all satisfactory, we must either slow down the conveyor speed or increase lamp wattage output to bring millijoule readings into that 350 to 400 range. Care must be exercised while increasing U.V. energy because this may increase the substrate temperature which can cause the substrate to distort. However, by experimenting with belt speed and lamp energy output, you should be able to strike a balance between heat related problems and ink film through cure.
For more help distinguishing between adhesion failure due to ink/substrate incompatibility versus undercure, contact Sericol's Technical Service Department or the Screen Printing Technical Foundation (S.P.T.F.).