Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 93 of 331

combination of excellent aesthetic surface appearance (wet-look), acid-rain etching resistance, and weatherability, are the best choice. In recent years, the use of two-component solvent-based polyurethane coatings has grown rapid- ly in plastic exterior component applications, replacing traditional two-com- ponent acrylic/melamine coatings. In the meantime, environmental concerns and VOC regulations have provided an excellent opportunity for waterborne polyurethanes to become the dominant choice for plastic interior component applications in the years to come. The painting process of plastic products is quite similar to the process used for metal substrates. For example, today's automotive plastic component paint lines use the same sequence of primer, base or color coat, and clearcoat as is used for metal body frame; however, there are two unique plastic characteristics that require pre- treatment of plastic components. First, plastics in general have low surface tension; therefore, the adhesion of coatings to a plastic surface may be more difficult than on metal substrates. This is especially true for the olefinic plastics such as TPO or polypropylene. For these plastic substrates, standard three-stage or five-stage cleaning prior to painting operation may not be enough. In order to improve surface wetability and adhesion of coatings to these plastic substrates, additional surface pretreatment such as plasma discharge, flaming, and chemical or solvent pretreatment, may be needed. All these pretreatment methods not only remove the problematic weak boundary layer from the plastic substrates,they also increase the surface roughness of plastic substrates to facilitate the diffusion of coatings. Furthermore, a thin layer (<0.4mil) of adhesion promoter is often used to improve the adhe- sion between coating film and olefinic plastic substrates. On the other hand, plastic components made from thermoplastic polyurethane (TPU) and reaction injection molded (RIM) polyurethane have good adhesion with polyurethane coatings without these pretreatments. Unlike metal, plastic is generally nonconductive. Today, most of the paint lines designed to have high transfer efficiency use an electrostatic spray gun. In order to make this process work, plastic substrates have to either be prepared with conductive pigment or filler, or be coated first with a layer of conductive precoat before they can be painted using an electrostatic spray gun. Plastic substrates are more flexible than metal substrates; therefore, coatings designed for plastic substrates also have to be flexible, with comparable elonga- tion to the plastic substrates. Otherwise, any dynamic deformation of the sub- strates, such as the impacting or denting of plastic parts, will cause premature adhesion failure of the coatings. The typical two- component solvent-based polyurethane coatings formulated for plastics most likely are based on flexible polyester resins. For the same reasons, the polyisocyanates most often used are the biuret or trimer adducts derived from HDI. These polyurethane coatings provide excellent flexibility and film properties, low baking temperature re- quirements, good surface gloss and DOI, and excellent acid-etching resistance and weatherability; however, these coatings also have lower surface hardness and lower mar resistance, which may not withstand repeated automatic commer- cial car washes. Today, more plastics are used in exterior applications, and cus- tomers demand longer good appearance and maintenance-free products. How to improve the scratch and mar resistance of polyurethane coatings for plastic substrates while maintaining their other performance features will be the chal- lenge for coating formulators and their raw material suppliers. 92

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