PODER JUDICIAL

PAINTING

The most common reason for painting or coating thermoplastic parts is to enhance aesthetics and provide uniform color and texture to assemblies made of different materials or by different processes. Paints and coatings can hide some molding defects, such as gate blush or foam swirl. They also offer colors or surface effects that resins cannot, such as certain metallic or stippled effects. In addition, some paints perform a function, such as electrically conductive paints for EMI/RFI shielding.

Paints and coatings can also protect the plastic substrate from chemicals, abrasion, or environmental attack. For instance, paint prevents many colored plastics from fading and becoming brittle when exposed to the elements and/or UV radiation from sunlight or artificial lighting. Coatings can also prevent attack from cleaning solvents, lubricants, and other substances encountered in-use or during manufacture. Commercial scratch-resistant coatings commonly provide abrasion resistance for lenses made of Makrolon polycarbonate. Contact your Bayer sales representative for the latest information on scratch coatings and treatments for Bayer thermoplastic resins.

While some plastic parts require painting, plating, and/or decorating for aesthetic or functional concerns, most do not for two reasons: first, the injection-molding process accommodates a diversity of high-quality surface finishes and textures; second, thermoplastic resins can be produced in a rainbow of colors. Some specific instances where painting or plating may be needed include: protecting final assemblies from harsh chemicals or UV degradation, shielding electronic devices from EMI radiation, or adding graphics or labeling in contrasting colors. Painting, plating, and decorating, as well as their design considerations, are discussed in this chapter.

Types of Paints

Paints are generally made up of four components: a polymeric resin or resin components that form the coating; pigments or dyes for color; a solvent or carrier for thinning, delivery, and uniform coverage; and additives to enhance or modify application, adhesion, and appearance. A variety of paints have been developed based on different chemistries and polymer types. The common types of paints used on plastics include polyurethane, acrylic, alkyd, epoxy, and vinyl.

• Polyurethane paints provide a flexible, durable finish, cure without heat, and are compatible with most plastics, including many chemically sensitive, amorphous plastics, such as polycarbonate and polycarbonate blends.

• Epoxies typically produce hard, tough, glossy finishes.

• Vinyls tend to produce soft, rubbery finishes.

• Acrylic paints give brittle, scratch- resistant finishes that resist most common oils.

Paint Curing

There are a variety of methods to cure paints:

• Air-curing paints solidify as the solvent evaporates, leaving the resin to polymerize on the part surface. • Heat-curing systems bake parts for

rapid and complete curing. The curing temperature for these paints may limit your choice of plastics on which these paints can be used. Parts must withstand the required curing temper- ature. Polycarbonate parts can usually withstand paint bake temperatures of about 120°C (250°F).

• Two-component paint systems use a chemical reaction to drive the curing process. These systems generally give off very few volatiles, but have a short pot life after mixing: often only minutes.

• Other paints rely upon exposure to oxygen or UV radiation to completely cure.

Paint-Selection Considerations

Semicrystalline plastics, such as nylons, tend to be chemically resistant to most solvent systems and often require special pretreatments or primers. Acetal, polypropylene, and polyethylene, which Several factors determine the type of

paint systems you choose, including the specific plastic substrate, the type of finish required, available painting facili- ties, and local regulatory restrictions. To some degree, the paint system should chemically react with the plastic surface for good adhesion, but it should not damage the plastic substrate.

Paint systems also differ in the types of solvent system used. Solvent systems generally fall into two types: organic- solvent systems or water-based systems. Organic solvents penetrate the plastic substrate to form strong chemical bonds for superior adhesion. Excessively aggressive solvents may damage the substrate. For example, many solvent systems severely damage parts made of polycarbonate resins. Damage and chemical attack tend to be worse in areas of high molding or assembly stresses. Always test your solvent system on an actual, finished part to determine its suitability.

Water-based systems are generally less aggressive to plastic parts but tend to form slightly weaker bonds. An increasingly important advantage, water-based systems avoid most of the environmental, health, and safety issues associated with organic-solvent systems.

have waxy surfaces, are chemically resistant to most solvent systems as well. Amorphous plastics, such as polycarbonate or ABS, because they are less chemically resistant, achieve good adhesion with many more paint systems.

Look for a system that is not too chemi- cally aggressive: especially for polycar- bonate and polycarbonate blends. To achieve the optimum match of substrate and paint system, consult both your resin and paint suppliers before making your final selection. The cost of the paint is usually insignificant compared to the labor and overhead costs, and the cost of complying with environmental protection regulations. Be sure to consider the cost of the entire process when making your selection. Government regulatory agencies, especially OSHA and EPA, limit the emission of volatile organic compounds (VOCs) into the air. Many organic- solvent-based paint systems and application systems cannot meet current emission limits without elaborate and expensive environmental-protection equipment. Generally, waterborne coat- ings and high-solid polyurethane systems comply with most government regula- tions. Check the current and near-future regulations in your area, because these regulations vary.