When a molded article of a polyolefin resin having relatively low surface polarity, typically polypropylene, is processed by, for example, coating, the surface of the molded article is generally cleaned and modified (grained) with a solvent, e.g., trichloroethane, prior to primer coating or plasma treatment. Such pretreatments including modification with a solvent polarize the surface of the molded article to bring about firm adhesion between the surface and a topcoating paint applied thereon. However, with the recent increasing need of limiting use of an organic solvent, especially a halogenated hydrocarbon such as trichloroethane, as a cleaning agent, it has been demanded to develop a new approach for surface modification of a polyolefin resin molded article in place of solvent cleaning.
The technique disclosed in JP-A-3-103448 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") is among such approaches. According to this technique, a polypropylene resin molded article is treated with an ozone stream to oxidize the surface and thereby to render the surface hydrophilic.
The problem associated with this method is that uniform modification of all the surface of a resin molded article cannot be achieved without difficulty. That is, in order to modify the surface uniformly, an ozone stream must be applied to all the surface uniformly while equalizing the time of application on every part of the surface. Therefore, where a resin molded article to be treated has a complicated shape, either the molded article or an ozone stream should be moved properly. Otherwise, all the surface of the molded article cannot be modified uniformly, ultimately resulting in a failure of uniform coating.
In order to eliminate the above-mentioned disadvantage, the inventors of the present invention previously proposed a method for modifying the surface of a polyolefin resin molded article which comprises contacting a polyolefin resin molded article with an aqueous ozone solution to oxidize the surface of said molded article, as disclosed in JP-A-5-163619. By this method, the surface of a polyolefin resin molded article can easily and uniformly be modified without using an organic solvent, thereby making it possible to form a coating layer on the molded article with high adhesive strength.
The above technique has achieved certain excellent results of surface modification on account of the immersion system adopted in which a polyolefin resin molded article is immersed in an aqueous ozone solution at a prescribed temperature. However, it involves a fear of the following disadvantages.
As shown in FIG. 12, molded article 51 is usually cleaned by power wash (washing with running water) using water jet nozzles 52 before it is immersed in an aqueous ozone solution. Since the washing is carried out with resin molded article 51 on belt conveyor 53, molded article 51 should be removed from conveyor 53, transferred into container 54 for immersion, and kept immersed there. Since the power washing is not so effective to clean the side of resin molded article 51 facing to conveyer 53 (i.e., the reverse side of molded article 51), it is very likely that contaminant remaining on the reverse side of resin molded article 51 may be dissolved in the aqueous ozone solution in container 54 while resin molded article 51 is immersed in the aqueous ozone solution. As a result, the aqueous ozone solution tends to be contaminated, sometimes failing to obtain satisfactory results of modification.
Further, in order to maintain the aqueous ozone solution in container 54 at a constant temperature, it is necessary to adjust the temperature of the whole aqueous ozone solution in container 54 by heating with heater 55, which may incur an increased energy cost.
Furthermore, because the aqueous ozone solution is retained in container 54 for a long period of time, there is a tendency that ozone in the aqueous solution decomposes by itself, resulting in a failure of keeping a necessary ozone concentration.