Screen Printing Inks and Plastic Substrates

Much of the screen printing industry’s growth is due to the introduction and increased use of new plastic substrates coupled to the development of compatible ink systems. Since the printer’s end-product consists basically of ink and substrates, ink and plastic manufacturers have collaborated closely in order to increase their knowledge of each other’s products.

From the screen printer’s standpoint, a close collaboration with his ink and substrate suppliers is of utmost importance. The purpose of this paper therefore is to discuss some of the principal groups of plastic substrates, their recognition and compatibility with specialized ink systems.

Principle Groups of Plastics

The two terms with which we are basically concerned are thermoplastic and thermosetting plastic substrates. Thermoplastics are those that can be reshaped or reformed by the application of heat. They include acrylics, cellulose derivatives, polystyrene and vinyl.

Thermosetting plastics, once they have been shaped and cured, can not be reconverted. These include phenolics, polyesters, melamines and epoxys. It should be noted, however, that polyesters may also be thermoplastic such as polyethylene terephthalate (MylarÒ). Thermosetting polyesters are crosslinked types as used in glass reinforced plastics.

It is important that the printer know this: before accepting a job he should be fully aware of the end use of the product to be imprinted and requirements such as heat/abrasion/solvent and chemical resistance, as well as weathering and light fastness requirements of color must be understood to ensure the proper selection of ink for each job.

In many cases plastic substrates contain plasticizers which tend to migrate into the ink printed on the surface. This applies particularly to thermoplastic products although even here there are exceptions: MylarÒ, ABS, polystyrene and acrylics do not contain migratory plasticizers.

When these plasticizers are present, however, it is necessary to allow the ink to reach a stage of equilibrium with the plastic stock. In most cases, this will not be reached until at least 24 hours after printing and this is the reason why this minimum time lapse is essential after test printing and before starting a production run.

The required tests will vary, but apart from the obvious one for adhesion, the printer will have to test for distortion of the plastic surface, the suitability for embossing, for thermal diecutting as well as product resistance: Will the ink resist the product with which the plastic surface may subsequently come into contact? When printing plastic containers, for example, the printer must ascertain the nature of the product with which the container will subsequently be filled, as this will have considerable bearing upon ink selection.

Principle Groups of Inks

Inks for plastics can be characterized by different drying systems: they may dry by evaporation, oxidation, or polymerization. Lacquers, for example, dry exclusively by solvent evaporation and form a physical- chemical bond which includes solvent action on the surface which partially dissolves the plastic surface and may cause etching. (This physical-chemical bond is sometimes incorrectly called a chemical bond but this indicates a chemical reaction which does not occur in this case.)

Oxidizing and polymerizing inks, on the other hand, will bond as a result of a high degree of specific adhesion. This is called a mechanical bond. Among oxidizing and polymerizing inks are alkyd enamels, modified alkyds, epoxy esters and epoxy inks. They find their widest application in the printing of thermosetting plastics, that is those that cannot be reconverted after they have been shaped and cured.

This group of inks may, however, also be used on some high solvent resistant thermoplastic substrates such as polyethylene and polyesters such as MylarÒ. They may be used to decorate other thermoplastics, provided the item will not be formed after printing. The drying and curing of these inks are highly crosslinked and will, therefore, not possess good forming characteristics.

When considering the end use of the product to be imprinted, it is important to check whether this requires a high degree of solvent and abrasion resistance. If so, epoxy inks are called for. On the other hand, items such as polyethylene bottles often require a high degree of solvent or product resistance and only moderate abrasion resistance because they are frequently discarded after use.

Alkyd enamels will, in many cases, prove satisfactory on such products and should be given preference over epoxy inks because the latter do not exhibit good outdoor durability and are either two-phase systems consisting of ink and catalyst or single-phase systems which require curing under high temperatures. Lacquer type screen inks are primarily used in printing of thermoplastic materials.

The inks are based on many different types of resins: acrylics, cellulose acetates, cellulose acetate butyrate, ethylcellulose, nitrocellulose, polyvinyl chloride and some which use a combination of several of these. Ink manufacturers strive to formulate inks which will allow the printer to considerably reduce his ink inventory. Inks of the lacquer type, for example, can be used on a wide variety of plastics and their versatility exhibits excellent adhesion.

They are particularly useful in the area of vacuum forming of rigid plastics and even if initial adhesion after air drying is only marginal, this can often be improved during the vacuum forming process which will result in partial fusion of the ink to the substrate. What this means is that adhesion tests should be carried out only after the vacuum forming process.

Lacquer type inks, however, must be checked very carefully for weathering characteristics because on some plastic surfaces they will tend to lose adhesion after exposure to ultraviolet light or heat. Ink manufacturers use a carefully balanced solvent system, designed to minimize printing problems.

For this reason, it is most important that the screen printer uses only those solvents supplied or recommended by his ink supplier. An excessive amount of the wrong retarder, for example, may trap-in what appears to be a dried ink film and finally will evaporate after the sheets have been stacked.

This will cause blocking or sheets sticking together. At this point mention must be made of a formulation which allow the screen printer to stack printed vinyl pressure sensitive films in high piles immediately after jet drying without danger of blocking. These systems have the added advantage of rapid drying at very low temperatures. It means that pressure sensitive decals can be successfully dried on conveyor jet dryers without curling or on very short wickets without heat application.

Of ecological advantage is the fact that these inks lend themselves to formulations which conform to antipollution regulations. I have referred to the use of wrong retarders and no doubt many readers will have experienced warpage of substrates which causes register problems when printing subsequent colors.

This warpage or “puckering” is usually caused by excessive amounts of solvents used in thinning inks. Not all lacquer type inks are dependent on solvent action for adhesion. In certain instances, the presence of active solvents in the lacquer can cause problems and this is particularly true when printing on plasticized thermoplastics such as styrenes or acrylics.

These products, particularly after they have been preformed or injection molded, are subject to severe “crazing”: an effect caused by the cracking of the dried ink film. These cracks are actually surface cracks in the plastic material which transfer themselves to the dried ink film.

They result from a release of the internal stresses and strains of the plastic material when too active solvent systems are used in the inks. Crazing can occur on non-plasticized polystyrene and acrylics and often happens when printing on a molded sheet whereas printing on the same substrate before molding with the same ink may prevent this effect.

To eliminate crazing on most of these stocks, however, specific inks have been formulated but the printer must realize that crazing effects may be noticed too late and sometimes long after the job has been completed: this is another reason why substrates must be pretested or supplied for testing to ink manufacturers with sufficient lead time before going into production.

Adhesion of inks to plastic surfaces may also be adversely affected by the presence of mold release agents. Many of these lubricants used in the manufacture of plastics can be removed from the plastic surface by using inert solvents which will not attack the plastic itself. Silicon lubricants are a special problem and these may require the use of alkaline cleaners.

Many plastics are treated with surface active agents to reduce their static charge and to minimize dirt and dust collection. Ink manufacturers use a wide variety of pigments in their formulations, and light fastness and durability indications for each color should easily be available to the printer.

The terms “light fastness” and “durability” are frequently misunderstood: a single color may, in itself, possess excellent permanency but, if this color is mixed with white or shaded with other colors, the permanency may be drastically reduced. By the same token, if a color is reduced with compatible clear to obtain various degrees of transparency or translucency, a marked reduction in permanency may again take place.

Recognition of Plastics

When in doubt about the plastic substrate to be imprinted, it is advisable to perform some simple tests to ascertain the nature of the substrate and its generic name. There are various ways of testing these substrates and the following hints may be useful:

Vinyls

Vinyls are thermoplastic and are supplied in both rigid as well as flexible form. One may recognize vinyl by applying what I call the “hot copper wire test”. When heated copper wire is placed onto the surface of the vinyl film and immediately returned to the flame, it will produce a yellow flame which will cease burning as soon as the wire is removed. If a flame is allowed to contact the vinyl sheet, a green coloration is always visible at the bottom of the flame if the material is polyvinyl chloride. Burning vinyl will produce black soot and have a sharp odor reminiscent of chlorine.

Acrylics

Like vinyls, acrylics are thermoplastic but have otherwise entirely different characteristics. To the screen printer, they are important for outdoor displays and they can, of course, be vacuum formed although great care must be taken in the choice of ink when forming these products. Inks for acrylics contain special resins which are based on the same systems as the acrylic substrates.

With acrylics, solvent resistance must be checked because many are attacked by cleaning fluids, acetones and oxidizing agents. It is advisable to use specific acrylic ink formulations although synthetic enamels as well as some lacquer type inks are often suitable. Acrylic substrates are most easily identified visually but the flame test may be used. It will produce a yellow flame and, unlike vinyls, will continue burning after the flame is removed.

Acrylics also have a very unpleasant odor which is unique to acrylics. In this case, one may employ a further recognition test: While burning, acrylics will produce a distinct crackling noise. Another way of identifying acrylics is to either file the edge or use a hacksaw to cut a piece: The characteristic odor of monomer can then be noticed.

Cellulose Derivatives and Cellulose Ester Plastics

These are the oldest known thermoplastic materials. One of the first of these had the trade name “celluloid”. Their use is decreasing because some are highly flammable and have poor light resistance, although they have high impact strength. We are concerned with two principal products which fall into this group:

Cellulose Acetate — This may be imprinted with many gloss vinyl inks as well as lacquers and some so-called multipurpose plastic inks. Cellulose acetate is available in two principal types: cellulose diacetate — which usually presents no severe problem for screen inks; and cellulose triacetate — which causes considerable adhesion problems and may require special formulations. It will burn with a yellow flame center surrounded by blue and will continue burning after the flame is removed.

The initial odor upon burning is reminiscent of vinegar. Cellulose Acetate Butyrate — This can be used for outdoor point of purchase displays and has better water and weathering resistance than cellulose acetate. The most obvious difference is that butyrate has a somewhat objectionable odor at close quarters.

It may be imprinted with certain acrylic type formulations as well as some lacquer inks. Upon burning it will show the same flame characteristics as cellulose acetate but will produce drops of melted plastic. The odor is similar to that of rancid butter.

Polyolefins

The best known of these are polyethylene and polypropylene. Polyethylene is most widely used in the container industry because it is strong, lightweight and highly resistant to breakage. It may be molded, extruded or formed and is easily colored. Polyethylene must be pretreated either by flame treatment or corona discharge prior to printing.

Epoxy inks and synthetic enamels are generally recommended for printing on this product. Polypropylene is one of the lightest commercially available thermoplastics. It has greater rigidity than polyethylene as well as excellent surface hardness and good scratch and abrasion resistance.

It has wide uses in the manufacture of bottle caps, medicine cups and housewares. It also usually requires pretreatment and epoxy type inks are most suitable. Polyolefins can be recognized by a flame with a blue core surrounded by yellow. In other words, the flame produced will be opposite to that of the cellulose derivatives. Polyolefins will melt, forming into tiny drops and their odor upon burning is similar to that of paraffin. There is a class of substrates consisting of two or more groups of plastics, and one of these is extruded, fluted, polyethylene/ polypropylene copolymers.

These have a wide variety of point of purchase as well as packaging applications and are known under different trade names. The products are usually supplied in either corona-treated or top coated form. It must be remembered, however, that corona treatment will not last indefinitely and special ink formulations are available which provide good adhesion even to indifferently treated stock. These special formulations also have the added advantage of being highly water resistant. Even if some inks show good initial adhesion to this material, continued rain water may weaken this considerably and that is why such special formulations are recommended.

Polystyrenes

These belong to the thermoplastic group. Low impact polystyrene is distinguished by its hardness, rigidity, heat and dimensional stability as well as ease of fabrication. In spite of its hardness, however, polystyrene is often easily breakable. Low impact polystyrene is also very sensitive to solvent attack.

We are, however, also concerned with other copolymers such as butadiene-styrene of the high impact type and acrylonitrile butadiene-styrene known as ABS as described later in this paper. These types are much tougher and have less sensitivity to solvents than low impact polystyrene.

Orientated polystyrene is produced by biaxial orientation during extrusion and overcomes the brittleness inherent in polystyrene and increases the strength while retaining the desirable characteristics of regular polystyrene such as clarity and dimensional stability. There is also polystyrene foam which is, however, extremely sensitive to solvents and usually requires special ink formulations. For most polystyrene, acrylic type inks are safest. Polystyrene can be recognized by its bright yellow flame which will continue burning after the flame is removed.

ABS

ABS stands for acrylonitrile-butadiene-styrene. As the name implies, its principal components are acrylics and styrenes and for printing purposes, therefore, ABS has similar qualities and accepts many types of acrylic ink formulations. It may be vacuum formed, blow molded, extruded or calendered and has exceptional chemical resistance, toughness as well as a high gloss. Unfortunately, the flame test for ABS is not conclusive due, no doubt, to its styrene and acrylic composition.

Phenolics

Phenolics are thermosetting and have wide uses in the electronic and appliance industries. They have great heat resistance and excellent insulation qualities and may be molded into rigid shapes with good surface finish. While they have fairly good solvent resistance and low water absorption, they tend to become yellow when exposed to actinic light. Epoxy inks are recommended when printing these products.

When holding a flame to phenolics they will not burn but turn into a carbon-like substance. They will give off a unique odor peculiar to phenolics.

Amine plastics

This thermosetting group includes melamines and ureas. They are available in a wide range of translucent and opaque colors and are quite hard, scratch resistant and unaffected by alcohols, oils and grease. They are also used as closures in the packing industry as well as in electrical components. Epoxy inks should be used. When applying a flame, phenolics will at first burn quite brightly but the flame will soon extinguish itself to form a soot-like substance. In other words, melamines and ureas are difficult to distinguish from other phenolics.

Polycarbonates

These are thermosetting plastics which have recently become of interest to the printer. They may be molded, cast or extruded and have very good moisture as well as high impact and temperature resistance. They are virtually impossible to break and are used for vandal proof sign work.

They are also used in the manufacture of containers which have to be sterilized. It must be pointed out, however, that polycarbonates are more sensitive to solvent attack than acrylics. Some vinyl inks as well as acrylic formulations are suitable. There is no foolproof flame test to recognize polycarbonates but to distinguish them from acrylics, the break test may be applied: acrylic will shatter with a blow from the hammer, while polycarbonate is more likely to break the hammer!

Polyesters

These find their widest application in the pressure sensitive field. The most commonly known product is DuPont’s Mylar“ film. They are thermoplastic and special ink formulations must be used unless the material is pretreated with a base coat, after which they can often be overprinted with synthetic enamels or lacquers.

Suggested Precautions

While there are, of course, many other plastics, the ones described above are those of prime interest to the screen printer. Since they show varying degrees of ink compatibility, the screen printer’s choice of ink has therefore become very confusing. When printing on these surfaces, it is absolutely imperative to schedule production timetables to allow for sufficient testing procedures.

Every reputable ink manufacturer will make this recommendation in order to avoid unnecessary problems after the job is rejected. Ink manufacturers will test printers’ substrates under their own laboratory conditions and will basically advise on adhesion. Given time, however, the ink manufacturer can make recommendations concerning solvent resistance, weatherability and other specifications.

Even then, however, it must be remembered that an ink laboratory operates under conditions different from those at most screen printing shops. Allowances must therefore be made and the only foolproof tests should be carried out right at shop level. The printer should be warned not to rely merely upon verbal or even written recommendations from the substrate ink supplier.

As an example, it should be remembered that if a screen printer prints on stock supplied by his customer, on the understanding that it is a polyester, his natural inclination will be to ask the ink supplier what ink to use. The ink supplier will then make certain recommendations but without having seen and physically tested the substrate, he should not make specific warranties: in the first instance, there is no guarantee that the screen printer received the correct information from his customer.

Secondly, if it is polyester, it may be topcoated and not even require a special polyester ink formulation. The procedure therefore should be to send the substrate to the ink supplier to request him to submit printed samples for examination.