Various Ink Systems for Metals

First, I want to touch briefly on the metals that are used in the screen-printing industry. Pound-wise, the major one would be steel as supplied by both domestic and foreign manufacturers. If you buy steel from the mill, you can specify cold-rolled dry steel. Dry means oil-free. It is relatively clean. You can purchase the steel in sheet or coil form if you have the equipment to do your own sheeting.

If ultimate corrosion- resistance is required, you can specify a surface-treated steel - phosphatized zinc grip, paint grip or some other chemical treatment. You may prefer to purchase precoated panels, sheets or coils. In this case, there is a paint coating applied at the mill. You can also specify what type of coating you might want, whether it be alkyd, polyester or the newer acrylics.

If you order precoated sheets, in most cases you are going to be ordering it roller-leveled–and this may pose some problems, because in the leveling or flattening of the sheets the surface may become contaminated with some sort of oil. It could merely be from fingerprints, but oil is a major problem.

To ensure proper adhesion, that surface must be completely free from oil and grease. If you were to purchase bare steel from a warehouse (and I would include aluminum here), it would probably come with a coating of oil applied to prevent rusting or corrosion while in the warehouse. This oil must be removed prior to printing. Other forms of steel used to a significant degree are stainless and galvanized and, to a lesser degree, template. Regardless of which one you utilize, cleanliness is vital. Aluminum is another major metal. It can be one of many different alloys supplied to your specification or to the specification of your customer. As in the case of steel, you may purchase it precoated and ready for decorating.

Aluminum can also be supplied in the form of cast or machine parts, or extruded, plain or even anodized. The aluminum most used for exterior signs generally are two types of alloys, either 6061T6 or 5052H38. These alloy grades are commonly used and specified by federal and state highway departments. To achieve proper adhesion, the aluminum, like steel, must be surgically clean. You can purchase aluminum that has been pretreated or has a chemical conversion coating known as Bondrite, Aladyne, Uradite or by other names. These are primarily used to prevent corrosion, and they assist in the adhesion of any paint or ink you might apply.

Anodizing is another form of surface treatment for aluminum. It also provides corrosion-resistant surfaces on which you can obtain better ink adhesion. Anodizing is more expensive than other surface treatments, and it is used only when its properties are desirable to you or your customer. The anodic film on the surface is very brittle and reduces the ductility of the aluminum. Forming and cutting can become problems with anodized aluminum. You can create crazing or cracking of the film at points of high stress. Other metals used to some extent are copper and brass, with copper’s major use being in the printed circuit industry. But, like other metals, for proper adhesion both copper and brass must be free from any surface oil or other contaminants that might interfere with the performance of the end-product.

Precoated Metal

Precoated metal products may eliminate the need for you to handle the material in your shop. The surfaces have been treated and they have a coating applied to them. When they first came out, these products had the conventional alkyd-type coating. This presented no problem (as far as adhesion was concerned) with any of the conventional inks on the market. Then they went to polyester and made it a little more difficult for the screen printer. When they went to acrylic, it further magnified the problems. The acrylic surface is, as with the other coatings, a thermosetting system. Being thermoset, it has a very tight, hard surface on which it is very difficult to achieve excellent adhesion. If you prefer to buy the precoated material, it is necessary to pretest almost every load or shipment of that material.

You can have excellent results for months and then get in a new load of material whose surface could be of such a nature that the inks you used in the past no longer adhere. This is especially true in cases where you are going to be using very low temperature baking or air-drying. We have found in many cases that to achieve desired properties, you must reach a temperature of 127° C to 138° C (260° F to 280° F) before you can attain adhesion on some of the acrylic precoated surfaces.

Ink Products

The ink products used can be classified as either air-dried, thermoset or catalytic. The decoration or printing of metal products will involve one or all of the following (depending on the product end-use or customer specifications): If you are in the metal sign industry, you should always use a primer.

The primer can either be applied by roller coating, curtain coating, spraying or, in some cases, printing. It is usually formulated with vehicles that are going to exhibit excellent adhesion to the metal surface as well as excellent flexibility, enabling the final sign or product to withstand the shock of shearing, punching or subsequent forming. It will also contain either additives or pigments that resist lateral corrosion, as exemplified by the salt corrosion test. The next product used would be background coating. This product provides the background color to the sign. It also serves the purpose of protecting the primer. The primer resins that give you tenacious adhesion are not known for their exterior durability. They tend to chalk or degrade when exposed to ultraviolet light, so they must be protected.

The main purpose of the ink is either decorative or informative. It must be compatible with the previously applied coating and, to assure proper intercoat adhesion, it must be formulated specifically to meet the requirements you or your customer may have regarding permanence or fade resistance. In most metal decorating applications, the final printing operation is clear coating. The clear can be applied by roller coating, spray coating, curtain coating or printing. The main function of the clear is either to protect against the elements or for aesthetic purposes. Most of the clears used for exterior purposes contain ultraviolet inhibitors.

Those are chemicals that will minimize or decrease the passage of UV light through the clear, which can degrade or fade the colors applied previously. Thus, the ultraviolet inhibitors can prolong the life of a specific product by up to 50%. If the application demands other properties, the clear may contain additives for abrasion resistance, or be formulated to resist specific chemicals or other production operations.

Ink Considerations

The basic considerations for the product you choose to use on a metal surface will be one or more of those listed below:

The application: You may choose a product that prints better than another, but will that product meet other requirements or specifications that you have in your own shop? Drying: Will your equipment have the necessary drying capabilities required by the product you choose to use? Are your ovens going to be hot enough? Are they going to be long enough? Are you going to have enough racks if the product is slower drying?

Adhesion: In pretest application in your shop, is the adhesion suitable for the intended end-use or specifications? Whether the test be crosshatch tape, fingernail scratch, flex/bend/impact resistance or whatever, is the adhesion test going to be realistic for the end-use of the finished product? Formability: If the product is to undergo subsequent operations, will it perform as intended without cracking, chipping, or flaking? Will it take the stretch or the bend required? Abrasion resistance: Some specifications may require a tabor abrasion test. Will the system meet these requirements or other handling problems that you may encounter in your shop?

Chemical resistance: This brings us to the appliance field. Will the product meet appliance specifications and needs for resistance to chemicals? One of the appliance specifications requires an immersion in a 5% detergent/water solution for a period. The chemical resistance of the ink you choose is going to depend a lot on the specifications, so be sure that you know what those requirements are. Heat resistance: Space heaters, ovens, dishwashers, etc., generally require a good deal of heat resistance without a change of color or deterioration of the film. Can your product meet those requirements?

Durability: Is the system, both vehicle and pigment, suited for your intended end-use? If it is set up for exterior exposure, how long will that system last? Any film of ink or coating immediately starts to degrade upon exposure to ultraviolet light, some faster than others. Any pigment will start to fade upon exposure to ultraviolet light, although at different rates - some fast, some extremely slow. The rate of fading or deterioration of the color will depend a great deal on the pigment, the degree of protection offered by the vehicle and the overprint or clear coat that you use.

Surface Treatments

The basic types of products available for use on metal surfaces include: Nitrocellulose lacquers, which dry by solvent evaporation. They may be modified with an oxidizing vehicle to enhance heat resistance and formability. Generally, the exterior exposure for nitrocellulose systems should be limited to six months to a year.

They will dry in 30 to 45 minutes at room temperature and in less than a minute at 82 degree C (180° F). Temperatures well in excess of 107° C (225° F) generally are not recommended for nitrocellulose, especially if you are going to put it through a multiple bake. Forming should be kept to a minimum unless a good, compatible primer is used. These lacquers generally have fair-to-good abrasion resistance; however, they are thermoplastic. Chemical and heat resistance would be considered very poor.

Durability is only fair, but you can extend the life of a nitrocellulose system by choosing a specific clear, such as an enamel, to give you needed protection. For decorating anodized aluminum, there are specifically designed dye systems available; the aluminum, in this case, must be decorated after anodizing but prior to sealing. The lacquer system is made with dyes which penetrate the pores of the anodized surface. After sealing and removal of the residual on the surface, the result is dye encapsulated in the anodic film.

There is no ink on the surface, nothing to scratch off, scrape off or mar. The only way to destroy the color would be to destroy the anodic film. Permanency of the dye systems is generally acceptable in most applications; they will withstand up to one-year exterior exposure. They have been used on automotive trim (rather than an organic surface coating), grill trim and hubcaps, and are being used quite a bit on interior trim. The next surface ink is enamels. We could break down classification very simply: Fast-drying, slow drying or thermoset. Fast-drying inks are manufactured with alkyds, which dry by oxidization.

But to make the ink dry fast, these alkyds are highly modified with other resins which detract from the overall properties of the alkyds themselves. They are designed primarily to dry fast at room temperature or low heat. Again, they should not be subjected to extremely high baking cycles. They are similar in physical properties to the lacquers; in that they have limited exterior durability. Because they do contain a percentage of alkyds, they will last longer than nitrocellulose - one year is the most you should expect. They have limited formability because of the modifying resins. They tend to be a little brittle.

Abrasion resistance and chemical resistance are less than you might desire for a good quality, long-life outdoor sign. Slower-drying alkyd systems not highly modified show considerable improvements in flexibility, formability, adhesion, abrasion and chemical resistance. These are the products primarily used on long-life outdoor signs today. This type of product will air-dry set-to-touch in four hours at room temperature or can be baked 30 minutes at 121° C (250° F) in the oven. Heat resistance is better than mentioned before, but with this type of system (which is still an alkyd system), the enamel will tend to yellow if you reach temperatures of 149° C (300° F) or more for any extended period. Thermosetting enamels are made with oil-free alkyds, but they are combined with a resin that will cross-link at temperatures in excess of 149° C (300° F).

The resins, either melamine or urea-formaldehyde resins, create the hard chemical-, abrasion-, and heat resistant surfaces you expect in a thermosetting enamel. The formability of the thermosetting system can be fair-to-good depending on the ratio of alkyds to cross-linking resins. Generally, the faster the cure the less the formability and flexibility. The exterior durability is good for systems that have been formulated for exterior exposure. If a thermosetting enamel was initially designed for appliance finishes or possibly the printed circuit area, it is not necessarily going to be suitable for exterior use. There is a class of thermosetting enamels that has a slightly different formulation from the one just discussed. Instead of using the most common ingredient in an alkyd resin (phthalic anhydride), a silicone polymer is substituted, which really increases the durability of the system.

It gives good formability, abrasion, chemical and heat resistance – everything you might desire. It is not widely used within the industry because of some problems that naturally occur with the product due to its reactivity. It has very poor shelf stability - less than a year. Similar coatings, silicone polyesters as they might be classed, have been used for house siding, etc., in one case a siding manufacturer guarantees his product for 20 years. It is an extremely durable, but extremely expensive system.

The acrylics are available in either air-dry or thermoset systems. The air-dry have good exterior durability, good color and retention. They have relatively good formability and are quick drying. They have one drawback in that they are thermoplastic like the lacquers, and to some extent like the fast-drying type of enamel. They cannot be stacked while they are hot; they must be stacked cool.

If you do not have high-temperature ovens and cannot bake, but you need to use a fast-drying ink that will stand up well outdoors, acrylics would be a good choice. Thermosetting acrylics have all the desirable properties: Adhesion, formability, abrasion resistance, heat resistance and durability. The one critical factor is their conversion temperature of 149° C to 176° C (300° F to 350° F). These temperatures or times can be decreased by using acid catalysts. The thermosetting acrylics must reach a conversion temperature to be completely cross-linked. They cannot be halfway or part way cross-linked or the whole sign will fall upon exterior exposure. It will not have chemical resistance or resistance to UV degradation.

The polymer is simply not formed unless it is baked sufficiently. Epoxies are available in both single-part thermosetting and two-part catalytic systems. Neither is suitable for exterior exposure beyond three to six months. Not that the film is going to fall apart or anything disastrous is going to happen - the film just chalks. The main usage in the metal sign industry is not an epoxy as described above, but a derivative with the backbone of epoxy and that would be as a primer.

The epoxy-ester is an alkyd formed with the epoxy backbone and is used primarily because of the tenacious adhesion and resistance to corrosion that epoxies have. In the appliance industry, epoxies are used because they have the gloss, resistance to chemicals, formability, flexibility, adhesion and heat resistance required for most applications. They exhibit excellent formability, much better than thermosetting enamels, and the single-part epoxy shows much better shelf stability than thermosetting enamels. Baking times and temperatures for thermosetting epoxies are not very different from those required for thermosetting enamels.

UV

One of the primary reasons to go into ultraviolet curing of inks would be because of the almost instant drying, or curing, through cross-linking. These inks exhibit excellent chemical resistance as well as abrasion and heat resistance. However, the start of the art today leaves a lot to be desired in adhesion, formability - and because of these two drawbacks - durability.

The same UV ink may perform excellently on a piece of vinyl film or plastic, and may wear out your weatherometer, but on metal it may fail within 500 hours or less. These inks are now in general use on metal where the properties of formability and durability are not critical. I know several nameplate manufacturers who are designing nameplates using the UV system on metal. They may be doing something they are not telling us about, but they are completely satisfied.

There is a lot of work being done to improve the characteristics of UV ink on metal, but it is going to be a little while before all these problems are solved. In the meantime, there are conventional solvent-based primers or base coats that can be applied to metal that will accept and be compatible with UV inks so the UV inks can be applied over these primers or base coats.