Why some UV inks work on coated metals and others don't
Editor's note: This article was updated from an original version that appeared in the January 1996 issue of Screen Printing magazine.
If your business involves screen printing on metal, then based on your application and your know-how, you may be using solvent-based inks and/or UV inks. But do you know which type of ink is best suited for your product and the type of metal you're printing? Do you know why your UV ink is successful sometimes but fails at other times? Do you know why a conventional solvent-based ink seems to perform where a UV ink seems to fail? This article will attempt to answer these questions and explore the use of UV screen-printing inks on metal versus solvent-based inks. To explain these answers, it will address the following areas: the substrate (coatings) the inks performance parameters required the market (past, present, and future)
Before beginning, however, it may help to define the metal-coating market, which includes the following major product segments: decorative signage (Figure 1), nameplates, appliance trim, and printed circuits. While circuit printing does fall under the "metal" umbrella, the special requirements for this application make it a unique product specialty. Thus, the recommendations in this article pertain to the other product segments.
When you talk about printing on metal, what are you really talking about? Are you printing directly on "raw" metal? In most cases, no. Typically, metal screen-printing applications do not involve raw metal. The ink is really being printed onto a coated metal (Figure 2). If you're printing on primed or treated metal, this is also a coated metal.
Now it's time to consider the types of coatings used on metals, which fall into two categories:
A thermoplastic coating is a coating that does not chemically react during "drying." This drying could be by evaporation or fusion (heat). The most important thing to remember about a thermoplastic coating is that after drying, the coating is not normally resistant to high heat or aggressive solvents. You can remelt a thermoplastic coating with heat or redissolve it with a strong solvent.
A thermoset coating is a coating that cures by a chemical reaction called polymerization and/or crosslinking. When a thermoset coating is properly cured, it is resistant to heat, will not remelt, and will generally have excellent solvent and scuff resistance. This polymerization or crosslinking is accomplished by baking the coatings at high temperatures for long periods of time.
Resin systems used to coat metals can be vinyls (either plastisol or solution systems), polyesters, or acrylics. Vinyl coatings are thermoplastic, drying by solvent evaporation when solution systems are used or by heat fusion when plastisols are used. Polyester and acrylic are the most common resins used for thermoset coatings. The softer thermoplastic coatings have a surface that can be solvated by an ink, while the harder crosslinked finish of a thermoset coating is difficult for an ink to adhere to (Figure 3).
Although a thermoplastic coating is much softer and easier for ink to adhere to than a thermoset coating, the thermoplastic coating has limited chemical and abrasion resistance, which makes it unsuitable for most screen-printing applications. Thus, most screen printers will be dealing with thermoset-coated metal substrates. When ordering coated-metal substrates from your supplier, make sure you ask what type of coating is on the metal (acrylic, polyester, etc.) and whether any slip agents or other surfactants have been added to the coating that may make it more difficult to print.
Printing UV Inks on Metal
The most successful inks currently used for coated-metal decorating are solvent based. Increasingly, UV inks are being used, but they are just a small part of the market. Why is this so? To answer this question, you must look at how and why both types of ink work.
Solvent-based ink Conventional solvent-based inks can be broken into two categories, based on how they cure:
1. air drying (dry by evaporation)
2. crosslinking (chemically harden when cured or "baked")
The air-dried inks are normally vinyls, acrylics, or lacquers. The cured inks are normally epoxies, enamels, and polyesters or acrylics modified with a crosslinking resin, such as melamine. (Note: Epoxy resins are not recommended for extended outdoor durability.)
Air-dried solvent-based inks, which do not chemically react, will have poor or no resistance to solvent, gasoline, or aggressive cleaning solutions. When compared to a chemically reactive or "baked" ink film, the surface of the air-dried ink film will be softer and susceptible to scratching and scuffing. Softer ink films could also lead to blocking if not dried properly, and because metal is heavy, blocking in the stack could be a problem. Most air-dried inks will have good flexibility and fair to good outdoor durability.
The epoxies, acrylics, or polyesters, which are chemically reactive, need to be cured at high temperatures, often for long periods of time (Table 1). After curing, these inks generally exhibit very hard ink films, which are more resistant to solvents, gasoline, and cleaning solutions, as well as scratching and scuffing. These baked inks tend to be less flexible than air-dried inks, but they may be suitable for embossing or forming. When properly cured and cooled before stacking, they will not block.
|Table 1: Typical Baking Cycles of Solvent-Based Inks|
| * Not recommended for outdoor exposure |
Epoxy, enamel, acrylic, and polyester inks need toe cured at high temperatures, for long periods of time, or both. After curing, the ink films generally will be very hard.
UV inks Two types of chemistry are involved in curing UV inks:
The cationic curing system is a chemistry based on epoxide resins. These ink systems are slower curing than traditional free-radical or acrylate systems. The free-radical systems are based on acrylated polymers such as polyesters or urethanes. These ink systems are generally fast curing and may have good outdoor durability, depending on the formulation. Both the cationic and free-radical UV ink systems are chemically reactive and can produce hard ink films with chemical, scuff, and flexibility properties similar to baked solvent-based inks.
Any screen-printing ink will have to exhibit certain properties to satisfy the requirements for coated-metal printing. The ink may have to provide a hard film to resist scuffing, scratching, blocking, and solvents, but be flexible enough to meet bending, diecutting, and embossing requirements.
If the end product is for interior use, epoxies would be suitable in addition to other resin systems. If the product doesn't need solvent resistance, an air-dried ink may suffice. If both solvent resistance and outdoor durability are important criteria, most air-dried inks and any epoxy-based inks are not suitable; conventional baked inks or free-radical UV inks could be. In any case, the most important parameter is that the ink film adheres after proper drying or curing. Some do--some don't. Why?
To understand why, you must go back to the coating on the metal. If this coating is thermoplastic, many inks will adhere to it. But if the coating is thermoset (and in most screen-printing applications it is), printing with UV, air-dried, or baked inks is more difficult.
The difficulty has to do with the hardness of the thermoset coating on the metal. This hardness depends on the resin system of the coating, the method of coating application, and the curing (usually baking) conditions of the coating. Usually, the higher the temperature and the longer the cure, the harder the surface becomes. This surface hardness is the root of the problems that screen printers encounter in trying to get free-radical UV inks to adhere to coated metals as compared to solvent-based inks.
One key difference here is the shrinkage factor of the ink films. Depending on the formulation, a free-radical UV ink deposit may shrink up to 50% during curing due to polymerization and crosslinking of resin components. With conventional baked inks, shrinkage is based on solvent content and tends to be more limited than with UV inks.
After printing a coated metal with a conventional baked ink, the printed product must be baked at high temperatures for long periods of time. This will soften the surface of the metal coating to some degree and allow the ink to "wet" the surface better to achieve good adhesion. As the metal and the coating cool, the ink film also cools, and any shrinking occurs in slow stages, allowing for good adhesion. Thus, the ink film does not "pop" off the metal coating (Figure 4).
UV printing has one system, cationic, that exhibits very little shrinkage, but is slow curing and based on cycloaliphatic epoxides. The inks are susceptible to blocking if not cured properly. Since little shrinkage is involved, the cationic ink system tends to adhere better to the thermoset-coated metals than the free-radical systems. Cationic inks continue to cure over long periods of time, however, and care must be taken to prevent intercoat adhesion problems.
A free-radical UV ink, on the other hand, reacts rapidly during the curing cycle, cools quickly, and shrinks on a surface that is hard and nonshrinking, such as thermoset-coated metal. This could cause a loss of adhesion (Figure 5). The reason is that free-radical UV inks are cured in a UV-curing unit in seconds. The metal and coating are not affected--they don't heat up so they neither expand nor shrink, and the substrate surface remains hard. The UV ink deposit, however, shrinks very rapidly. This puts stress on the adhesion of the ink to the substrate, to the point that the ink film may pop off the coated metal.
Because most commercially available coated metals are processed by a variety of coil coaters, whose primary markets have no requirements for printing (e.g., the commercial building industry), these metals are often poorly suited for screen printing. The coatings vary considerably in base chemistry and crosslink density, relating to surface hardness variables that often go beyond a UV ink's ability to wet the surface (that is, the surfaces are too hard for the ink to adhere to). While one batch of metal may process well, the next could be too hard, exhibiting poor adhesion.
To increase your chances of success with these commercially available metals, you should conduct a cross-hatch test on each batch before the production run. Additionally, surface-tension testing may be helpful in detecting any high levels of surfactants in the coating that could cause adhesion problems.
However, the best chance you will have for successfully processing UV inks on coated metal surfaces is to control the coating and its resultant cure density, or hardness, yourself. This is done by working with your metal supplier and your ink supplier to get a coating that is more receptive to UV inks. (Testing each batch of metal is still advisable.)
At this point, you can logically say, "Wait. There are free-radical UV inks being used today on coated metal. As a matter of fact, they are winning Golden Image Awards." You, of course, would be right.
Some printers have had success with free-radical UV inks on metals whose coatings have been qualified for the window of performance required. In most of these instances, the metal supplier has worked with the printer and ink manufacturer to develop a softer coating that meets printing requirements. Most of these applications have been more in point-of-purchase decorative signage (not exterior qualified), short-term display, and indoor signage.
As a rule, a baked conventional ink (except an epoxy) will exhibit three- to five-year durability, and a clear coating will further enhance exterior performance. UV inks (not epoxy based) will exhibit two- to three-year durability, although a UV clear coat will further enhance exterior performance of these inks, as well.
Yes, both cationic and free-radical UV inks can be used for screen printing on coated metal. Free-radical UV inks are successfully being used as halftone inks for decorative signage and are being used with some success for nameplates and appliance trim. For long-term outdoor signage, however, solvent-based inks still rule the market.
Technology will someday resolve UV-printing obstacles. Raw materials will eventually allow manufacturers to produce a UV ink with properties comparable to those of a solvent-based baking ink. The question is, will market opportunity merit the investment in R&D? While you'll find no definitive answer to this question, the fact remains that the coated-metal printing market is shrinking. More and more, plastics are being specified for signs, nameplates, appliance trim, and automotive trim. This shift from metal to plastics is occurring for several reasons, including these:
1. Plastics are lighter than metal. Plastic sheets can be stacked higher with less weight, which means that blocking problems are less probable. And they are more economical then metals. 2. The baking of inks for coated metals consumes high energy levels and requires long curing-time cycles, both of which slow production.
With these concepts in mind, it will be interesting to see if UV inks for coated-metal printing ever grow to the size or ability of conventional baked inks.
I have attempted to present an open-minded view of the status and future of UV inks in the coated-metal market. Seeing the positives and the negatives, I have tried to explain this information in an easily understandable form and hope I have been successful.
UV inks are the inks of the future, but they are not magic inks. They will perform outstandingly in some areas; and acceptably in others. Learn about them, understand them, use them.
About the author
Frank Blanco, Jr. is senior vice president of Nazdar, Shawnee, KS. He has been involved in the screen-printing industry for 30 years. Blanco holds a bachelor of science degree in chemistry and an MBA from Fairleigh-Dickinson University. He is a frequent speaker at industry events and a contributing author to several industry journals.
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