Tight adherence of a coating depends on the nature of the plastic and matching of the paint system with the plastic.
Design guide for the paint selection:
1. Heat distortion point and heat resistance: This determines whether bake type paint can be used and if yes, the maximum baking temperature the plastic can tolerate.
2. Solvent resistance: The susceptibility of the plastic to solvent attack dictates the choice of paint system. Some softening of the substrate is desirable to improve adhesion, but a solvent that aggressively attacks the surface and results in cracking or crazing must be avoided.
3. Residual stress: Application of coating to the localized areas of stress may swell the plastic and cause crazing. Annealing of the part before coating will minimize or eliminate the problem. It can be completely avoided by careful design of the moulded part to prevent locked-in stress.
4. Mould release residues: Excessive mould release can cause lack of adhesion. The plastic surface must be rinsed or cleaned of the release agents to assure satisfactory adhesion.
5. Plasticizers and other additives: Most plastics are compounded with plasticizers and chemical additives which usually migrate to the surface; thus softening the coating by destroying the adhesion. Hence the coating should be checked for short and long term softening problem for the specified formulation on which it is to be used.
6. Other factors: Stiffness or rigidity, dimensional stability and coefficient of expansion of the plastic affect the long term adhesion of the coating. The physical properties of the paint film should be compatible with those of the plastic substrate.
LESS CONVENTIONAL TYPES OF COATING: These types of coatings are applied to different plastics for specific purpose.
1. Intumescent coatings: Fire resistant coating is applied to a part in a thickness of 50-60 mil. It foams up to a thickness of ½ to 1”, during a fire, to protect the part from the flame and high temperature.
2. Anti-static coating: It is applied to minimize the tendency of the part to acquire or hold a static charge which may attract dust or dirt. The coating absorbs moisture from the air for its function. It is not effective in very dry atmosphere.
3. Abrasion resistance coating: Transparent coatings are used on acrylic and PC to increase the abrasion resistance of the surface.
4. Electrically conductive coatings: Coatings usually containing metal particles (Usually Ag or Cu or graphite) renders the coating electrically conductive. Some with fairly high resistance are used only as static charge dissipaters, others with much lower electrical resistance are used to conduct current so they can be used as heating films, or as RF shields in electronic applications. Transparent, electrically conductive coatings are also available, but require special application techniques.
5. Organosol coatings: they are used where a thick coating, 50-6- mil is required (in spray or dip application) and where the coating is required to exhibit a slight amount of resilience.