Numerous articles employed in a lot of fields present a coupling between a plastic material and the surface of a metal part. The process of the present invention is suitable for preparing the said articles. The reasons for making the said coupling are various. For example, the coating of metal parts is sometimes necessary to prevent the metal surface from coming in contact with foods or being exposed to chemical substances during use. Examples of the articles obtainable from the process of the present invention are, therefore, closures. Bottle closures of a plastic material are joined to a metallic material, aluminium for example, in order to prevent the oxygen diffusion in bottles.
In industry it is also necessary to connect different parts, such as metal valves, metal fittings or metal pipes with pipes or fittings of a plastic material.
Methods for joining metal surfaces to plastic materials are already known. In particular a method for coupling a metal part with plastic materials by a injection moulding process is known. International patent application WO 96/14533 describes such a process. According to the described invention, the method involves the steps of applying an organic or inorganic primer to the metallic pipe or pipe fitting, then applying heat to the metallic pipe end or fitting and, subsequently, hot dipping the metallic pipe or pipe fitting into a fluidised bed of plastic material. The subsequent and last step is that of overmoulding the metallic pipe or pipe fitting with a plastic material by an injection moulding process.
The problem with the method disclosed in said patent application is that the adhesion strength between the metallic pipe and the plastic material is not high.
It has now been found a process that improves the adhesion strength between the metallic surface and the plastic material.
In addition, thanks to the process of the present invention a higher adhesion strength is obtained without damaging the coating of plastic material. In fact the chemical degradation of the polymer does not occur or is reduced to the minimum.
Moreover, the present process does not even cause the loss of the shape of the overmoulded plastic material due to the action of heat.