UV Inks and Coatings: Considerations for Plastic Substrate
Editor's Note: UV/EB Corner, a feature created in conjunction with RadTech and written by its members, highlights whats new and exciting in the field of graphic arts.
By Akeru Fukazawa
Printing on plastic is a growing and challenging market for UV-curable inks and coatings. They have been successfully used for many years, but new opportunities arise daily that remind printers and formulators how difficult achieving adhesion can be.
This article gives a brief overview of four properties that are important to consider when printing on plastic.Surface Tension Surface tension is the most fundamental parameter to consider when printing on plastic. Most plastic films start out with a very low surface tension and are typically treated to increase the surface tension to 40 dynes/cm or higher. Substrate suppliers usually perform this treatment, but the effects will start to dissipate quickly.
In-line treatment is the best way for printers to ensure that the surface tension is in the acceptable range at the time of printing. Corona discharge treatment (oxidative) is the most common treatment method. It can be used on numerous types of substrates without damaging heat-sensitive plastics. Gas flame treatment is used on more tolerant plastics. Chemical treatments and primers are also used, often in conjunction with corona treatment. To monitor the effectiveness of these treatments, a dyne level test kit is an indispensable tool for plastic printers.
The surface tension of the ink must be lower than the substrate and, likewise, the coating lower than the ink, to allow for proper wetting and adhesion between the layers. UV formulators are careful to choose certain types of raw materials to help achieve acceptable levels. By comparison, solventborne formulations have naturally low surface tensions and can easily wet most substrates. Substrate treatment is, therefore, especially critical when printing UV materials.
Substrate Swelling/ Penetration
Unlike most paper and board substrates, typical plastic films do not have voids in the surface that allow an ink or coating to penetrate into it. However, certain plastics can be attacked or swelled by specific UV raw materials. These can be matched up with the type of substrate and allow the ink/coating to penetrate the film. When cured, the plastic and the ink/coating film form a strong bond with good adhesion properties. In addition, this type of penetration can usually be enhanced by increased temperature.
Glass Transition Temperature (Tg)
Compared to conventional inks and coatings, the UV variants are generally comprised of lower molecular weight materials that react to make denser, highly cross-linked networks. They also often have higher Tg and make harder films with good abrasion and chemical resistance.
If the Tg is above the temperature used in the hot-stamping or lamination process, then it will not allow the foil or plastic to adhere. Using raw materials with a lower Tg will help achieve good hot stamp and lamination results and/or allow the printer to reduce the temperatures in his process.
Degree of Cure
As with any UV formulation, the right combination of photoinitiators (catalyst) must be used to optimize the films final properties. UV inks and coatings are formulated to work with a certain type, and sufficient amount, of UV energy. If that varies, so too will the films final properties.
A UV ink or coating may feel cured at the surface, but not be cured all the way through the film. The degree of cure near the bottom of the film is critical to achieving good adhesion. Any penetration achieved by the ink/coating will not be utilized without sufficient through and bottom cure. The surface tension can also change if the amount of UV energy is varied.
The plastic market is an exciting area for packaging. The use of UV-cured inks and coatings is a good fit for this market. Instant cure means faster production. No heat requirement means less environmental and workplace impact.
Adhesion is one of the few issues that continually appear. This is often due to the steady stream of new substrates and new applications that are being developed by our customers. Formulators will continue to meet and overcome these challenges by applying some of the basic principles presented here.
About the Author Akeru Fukazawa is currently visiting the U.S. as a technical adviser for UV/EB curable inks and coatings at INX International Ink Company. Mr. Fukazawa has been employed with Sakata INX Corporation since 1982 as a chemist with the R&D and technical service group for offset inks, coatings and specialty inks. For more information on RadTech, please contact Gary M. Cohen, RadTech International North America, at (240) 497-1242 or visit www.radtech.org.