From Chapter 12 of Skoog et al.
Adhesives and Adhesion Published March 18, at 9: With the advent of UV radiation curing and low temperature curing of select powdercoatings, powder application on certain somewhat heat-resistant plastics is now being performed, although this technology is still in its infancy.
The paintability of all the metals commonly utilized in manufacturing is rather similar. This is possible in part because values for the surface energies, electrical conductivities, heat conductivities, and heat resistances of metals are all quite close together.
This is decidedly not true for plastics. In general, the values for these attributes of plastics are much lower than for metals, and also, the values among various plastics show extreme variation.
Even within a given family of plastics, their paintabilities can be surprisingly dissimilar due to differences in molecular weights of the plastics and the incorporation of different additives in the manufacture of the various resins.
The chemical nature and amounts of these additives are likely to change the paintability of that particular formulation significantly. Even parts manufactured from the same resin formulation are not necessarily equal in paintability.
The chemical nature of the resin largely determines the surface energy of the plastic. In general, a surface with a higher surface energy is more readily wetted by paint and hence is more paintable, meaning that coating adhesion will be better.
It has been noted that paint adhesion to metals is often superior to adhesion on plastics because 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, and hence paint adhesion, is one of the major purposes for pretreating 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 plastic items to be coated include fingerprints, dust, lint, and mold release residues.
Usually, detergent cleaning can satisfactorily remove salts 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.
Most plastics are either insulators or ineffective 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 blowoff. The destaticizer should generate both positive and negative charges; most units use a weak radioactive emission source in the blow-off air source.
Intake air is filtered and ion-laden air is blown across the part so that positive and negative ions neutralize all static charges. Destaticizing needs to be performed immediately before painting so that the parts are as clean as possible just going into the coating process.
Delays between destaticizing and painting will allow charges to reform and as a consequence, parts will re-accumulate particulate on their surface. By installing the plastic painting operations inside a clean room.
Mold Releases and Plasticizers Some paintable mold releases are available, but many 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 of release is not recommended for parts to be painted.
Solvent use is almost automatically discouraged due to VOC emission restrictions and potential fire and health dangers with 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; these are termed internal mold release agents.
Internal mold releases must be avoided whenever possible. Paintability may or may not be impaired immediately, and QA adhesion tests may show excellent initial adhesion. In some cases, however, internal mold releases have slowly migrated to the part surface and caused paint adhesion failure months after a part was painted.
Some plasticizers will slowly migrate to the surface and soften the interface between the plastic and the paint, resulting in adhesion loss.
In this case, too, initial paint adhesion tests may be completely satisfactory, but subsequent lifting or separation of the paint film from the plastic surface occurs later.Shop eBay for great deals on SpaRoom Diffuser Aromatherapy Supplies.
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