HIRATSUKA, JAPAN - One of the last technical roadblocks to greater plastics utilization in the Japanese automotive industry is painting. ``A bad color match will detract from an otherwise engaging design,'' said veteran designer Larry Shinoda, a consultant for Nippon Paint Co. Ltd. of Osaka, Japan.
But even if a good initial match is achieved, problems inherent in painting plastics often cause trouble later. Chief among those is discoloration and fading. Paints for plastics are less durable than those for metals.
In particular, bumpers are a source of concern. An estimated 70 percent of bumpers on Japanese motor vehicles today are made of plastics, mainly polypropylene and reaction injection molded polyurethane. Including only passenger cars, the percentage exceeds 95 percent. And in almost all cases, manufacturers try to match colors between bumpers and the body.
Part of the problem is that, under normal conditions, bumpers are painted off-line and brought to the main assembly line after the body is painted. This means a color specialist must be on-site at all times to ensure paints aremixed properly.
A more serious issue involves the characteristics of paints for plastics and paints for metals. Yoshio Sasaki, managing director of the technical division at Kansai Paint Co. Ltd. of Osaka, says paints for plastics should be flexible to avoid peeling and cracking when the material deforms.
``Naturally, this limits our ability to protect against scratches, as paints for metals are inherently denser,'' Sasaki said.
Because of this feature, standard bumper paint begins to lose luster after only 18 months, half the life of body paint.
Plastics have the added disadvantage of not retaining paint as well as metal, forcing carmakers to add steps and cost to the paint process. The transfer efficiency for nonconductive primers is 20-30 percent, which means as much as 80 percent of the paint is lost. But by employ-ing a conductive primer, the efficiency rate increases to 60 percent.
However, the conductive primer adds 30 percent to cost. A move toward water-based painting using a water-based primer will add another 40 percent, according to analysts. At present, the material cost for painting a car in Japan is about $120. That could increase to more than $200.
Although water-based primers are not yet in mass production, that may change soon as Honda of America already has begun switching to an operation not based on solvents. The subsidiary's Japanese parent, and Toyota, Nissan and Mitsubishi have yet to do so. But all are conducting feasibility studies.
One solution to the problem is to use electroconductive plastic panels, which would permit electro-dipping of the primer coat. Researchers say the materials would be similar to the middle plastic layer of sandwich steel sheet, which now finds use for damping purposes in upscale Japanese cars like the Lexus LS400 and Infiniti Q45. In the case of Toyota, which developed its sheet jointly with Nippon Steel Corp. of Tokyo, nickel powder is added to the resin middle layer.
Mitsubishi Chemical's Takeshita, while declining to confirm such a product is under development, said recycling electrostatic materials would pose no special problem. However, he cautions that this issue will not be resolved until after the year 2000.
In a related technological area, Honda Motor Co. Ltd. also aggressively is promoting film coating technology for its cars. The Tokyo firm, beginning with the 1991 Civic, has applied the special anti-corrosion treatment to all models for the Japanese and North American markets.
Known as Eco-Wrap, the process involves applying a clear, scratch-resistant film on the upper part of the body, including the roof, hood and upper front fenders. The process was developed jointly by Honda and Kansai Paint. Besides Honda, Nissan has shown the greatest interest in the process. Toyota, Mitsubishi and Mazda have not.
