This invention relates to production of breathing composite sheets comprising a porous substrate such as nonwoven fabric and a thermoplastic film.
Recently, various types of breathing composite sheets comprising a porous substrate in combination with a perforated film component that functions to regulate water permeability, moisture permeability, water resistance and other properties have been used in a wide variety of applications. Typical examples of the products which may be mentioned include paper diaper; hygenic pad; electrochemical cell separator; packages for desiccant, disoxidant and the like; dust proof clothing; filter; dew-proof wall sheet; and water transmitting sheet used in concrete mold. The methods for producing these breathing sheet materials may fall generally into the following two categories:
1. Methods in which a film is prepared from a thermoplastic resin blended with an inorganic or organic filler and the film is subjected to a treatment for perforating the film, for example, by orientation of the film or by removal or extraction of the filler from the film, and thereafter the perforated thermoplastic film is bonded to a porous substrate.
2. Methods in which a thermoplastic resin film and a porous substrate are bonded together and then the thermoplastic film is perforated, for example, by corona discharge treatment or by needle-punching.
All these methods comprise at least two separate steps, i.e. a step of perforating a thermoplastic film and a step of bonding the film to a porous substrate. Naturally, these separate steps are undesirable and disadvantageous from the standpoints of the quality control and the process control and need two kinds of apparatuses, i.e. a perforator apparatus and a bonding equipment, which cost a good deal of plant and equipment investment. These result in a drawback that it is difficult to manufacture the products at a low cost.