Introduction:

The methods used to apply paints to plastics are not much different than those employed for painting metals. While certain specific metal painting methods such as chemical deposition and electro-deposition coating are not possible with plastics, other techniques such as brushing, dip coating, flow coating, curtain coating, plus all the variations of spraying and rotary atomization are used to coat both metals and plastics. With the advent of UV radiation and low temperature curing, even powder coating on certain plastics is now being done.

The paintability of all metals is rather similar, despite the minor differences that exist among them. This is decidedly not true for plastics. The values for the surface energies, electrical conductivities, heat conductivities, and heat resistances of metals are all quite close together. IN general, the values for these attributes of plastics are much different than for metals. In addition the values among the various plastics show extreme variation from one to another. Even within a given type of plastic, the values are dissimilar due to differences among samples in molecular weights of the resin and the formulation of the material. The organic base plastic composition might consist of just a single resin or be a blend of two, three or more different resins. Formulated into a resin composition may be an assortment of fillers, extenders, and plasticizers. Various additives are often added to produce desirable physical and chemical properties, and pigments may be introduced to alter gloss or to give them color. The nature and amounts of these additives are likely to significantly change the paintability of that particular plastic formulation. Parts made of the same resin formulation are not necessarily equal in paintability. The molding, extruding, or other forming process used to produce the part can play an important role in the paintability of the part as well.

The chemical nature of the resin to a large measure determines the surface energy of the plastic. In general, a surface with a higher surface energy is more readily "wetted" by a paint and hence is more "paintable" and coating adhesion will be better. Metals typically have substantially higher surface energies than plastics. The low polarity of the molecules in plastics such as polyethylene and polypropylene is the cause of the low surface energy (and poor paintability) of these plastics. Increasing the surface energy is one of the major purposes of pretreatment for such plastics.

Cleaning Plastic Substrates:

As with any other surface to be painted, plastics need to be reasonably free of any soils or foreign materials. Common soils found on plastics items which are to be coated include fingerprints, dust, lint, and mold release residues. Detergent cleaning can usually satisfactorily remove the salt and oils deposited on plastic by touching them with bare hands. Fingerprint soils can be totally avoided if workers handling the parts from the forming process to the paint application step wear lint-free gloves. The gloves must be changed periodically to prevent them from becoming contaminated and acting as a dirt transfer medium.

Most plastics are poor electrical conductors. As a result, they have a tendency to build up static charges that attract and tenaciously hold particles of lint and dust. Wiping with a tack cloth may not remove all of these contaminants. An excellent method of removing statically-attracted lint and dirt is to use a destaticizing air blow-off. It should generate both positive and negative charges, utilizing a weak radioactive emission source in the blow-off air source. The air is filtered and blown across the part, and the positive and negative ions neutralize all static charges. The air stream gently blows away dust and lint particles into a vacuum to prevent re-deposition of the contaminant particles. Destaticizing needs to be performed immediately before painting so that the parts are clean going into the coating process. Delays between destaticizing and painting will allow charges to reform and as a consequence parts will attract particulates to their surface.

By installing the plastic painting operations inside a "clean room", dirt such as dust and lint is more easily controlled. Clean rooms must be constructed of clean, fiber-free, and lint-free materials. The air supply should be filtered and kept at around 50% relative humidity. Only authorized persons wearing lint-free coveralls, hair nets, and shoe covers should be permitted in this room at any time.

Some paintable mold releases are available, but other mold releases adversely affect paint adhesion. Various techniques may be required to remove these agents used to facilitate the separation of plastic parts from the molds. Wax type mold releases can sometimes be removed by solvent cleaning, but this type is not recommended for parts to be painted. Solvent use is almost automatically discouraged due to VOC emission restrictions and the potential fire and health dangers of many solvents. Water-soluble mold releases are much preferred. Removal of these from the plastic surface is readily accomplished with ordinary aqueous detergent solutions.

Mold release agents may also be blended into plastic formulations, termed "internal" mold release agents. Internal mold releases must be avoided whenever possible. Paintability may or may not be visibly impaired immediately. In some cases internal releases have migrated to the part surface and caused paint adhesion failure months after a part was painted. Plastic raw materials can become contaminated by accident with mold release either by the supplier or by the molder. This most often is the result of the plant failing to keep separate the formulations of a given type plastic, one which has internal release agents in it being used for parts that are not painted, and another formulation being used for painted parts.

Certain plasticizers, which may be added to various molding resins to increase its impact strength, can decrease paint adhesion just as do mold releases. Plasticizers can slowly migrate to the surface and soften the interface between the plastic and the paint film, resulting in adhesion loss. Although all of the initial paint adhesion tests might have been completely satisfactory, subsequent lifting or separation of the paint film from the plastic surface may occur. This may result in a field failure complaint later, long after the part has been painted and put in service.

Achieving Robust Paint Adhesion:

Most plastic surfaces are not only low in surface energy but also inherently low in surface profile. Smooth surfaces will tend to give poor paint adhesion unless the surface is first roughened by chemical or mechanical means and then painted. Conversion coatings on metals contribute to paint adhesion in part by the micro-rough surface of the inorganic layer that is produced on the metal. The most common way of overcoming surface smoothness of plastics is to micro-etch the surface with a chemical agent to generate micro-roughness that will provide adhesion anchoring sites for the paint. If possible the etching is done by the solvents present in the paint being applied. The solvent is rather critical because different solvents etch plastics at varying rates. Both over-etching and under-etching are to be avoided. Insufficient etching will not provide proper adhesion; excessive etching can damage the plastic. It may warp the part, expose particles of additive fillers and extenders, and perhaps even creating areas where materials in the plastic may bleed into the coating. Some plastics, polycarbonate and polystyrene for example, will crack or their surface will become overly crazed from attack by solvents to which they are especially sensitive. If plastics have areas that are highly stressed from the molding process, solvents can form visible cracks in these areas due to stress-relief so some care is required when solvent etching is used.

When some parts are molded there are areas where the rapid plastic injection flow produces significant frictional heating of the plastic part. In those areas a highly crosslinked (glazed) skin is formed that is resistant to solvent etching. Paint adhesion will be poor in these areas unless steps are taken to remove the overly-hard plastic skin. These areas can be de-glazed enough to allow satisfactory paint adhesion by tumbling with a moderately abrasive media, or by blasting the surface with a mildly aggressive grit material. Brief hot solvent or solvent vapor immersion treatment is also effective for some parts. But creating micro-roughness to increase paint adhesion is not very effective if the plastic itself is not at least somewhat polar in nature.

When de-glazing or solvent etching is not effective or otherwise not desirable, it may be necessary to use a chemical reaction to create polar oxidized groups on the surface. This is especially true for extremely non-polar plastic surfaces, Two examples of low polar plastics treated oxidatively are polypropylene and polyethylene. These resins and similar low-polar plastics may be briefly exposed to an open flame from a gas burner. This initiates an oxidative chemical reaction that forms enough polarity on the surface to provide excellent paint adhesion. Passing plastic parts through an electrical corona discharge that generates ozone has also been used to cause surface oxidation. The corona produces excited oxygen atoms that form ozone, which in turn oxidatively attacks the plastic to produce polar groups such as hydroxyl, carbonyl and carboxylic acid.

Low polarity plastics can also be oxidatively surface treated using light sensitive chemicals called photosensitizers, and then exposure to ultraviolet light. The UV light "cracks" the molecules of the photosensitive compounds to form free radicals. Free radicals are extremely reactive species that in this process combine with oxygen in the air. Oxygen free radicals in turn react with the plastic to produce polar groups on the surface of the types listed above.

Cold gas-plasma technology can be employed to pretreat plastics and oxidize the surface to dramatically improve surface properties for paint adhesion. When a gas is forced to absorb enough energy, it becomes ionized, or a "plasma". Excitation is provided by a radio-frequency generator. Arc welding and fluorescent lighting are both examples of a phenomenon in which a "glow" is caused by excited ions falling back to their stable energy state. Within the safety limits of the system this process can use any gas, or mixture of gases, such as oxygen, nitrogen, helium, argon, air, and ammonia can be used.

The plasma reactor is typically a vacuum vessel fitted with a door for loading parts in and out. A glow discharge can be observed when the reactor is running. Gas-plasma treatment micro-etches and activates the surface. A brief treatment will make a polar surface that has a high surface energy, enabling it to be wet completely and uniformly by paints. Plasma processes usually do not change the surface appearance, so inert materials can be treated without causing discoloration. Plasma conditioning allows plastics to be painted with good adhesion. This excellent adhesion can be achieved with the same paints used on metals, an important feature to some manufacturers since both the plastic and metal components of an assembly can be painted simultaneously using the same coating.

Less effective is the use of chemical oxidizing agents in the paint itself to oxidize the plastic surface enough for improved paint adhesion. This will oxidize the surface of some plastics to achieve a degree of polarity sufficient enough to provide good paint adhesion. Reflectance infrared spectroscopy has verified that these treatments produce the same oxygenated (hydroxyl, carbonyl and carboxylic acid) groups on the plastic surface as the other oxidizing processes.

This article was written by TSG associate Dr. Norman R.Roobol, Industrial Painting Consultant