The present invention relates to a process for producing photographic paper base. More particularly, it relates to a process for producing paper for photographic paper base having a high degree of whiteness.
As photographic paper bases, those produced by applying a colloidal solution of gelatin-barium sulfate to paper and thereafter drying, and those produced by extruding or laminating polyolefin and, particularly, polyethylene on paper according to an extrusion coating method have been generally used. It has been believed hitherto that, because high degree of whiteness is generally required for the paper used as a photographic paper base, highly bleached cellulose pulp having high degree of whiteness must necessarily be used as a raw material.
Many processes for bleaching such cellulose pulp have been known hitherto. Particularly, as a bleaching means for obtaining pulp having high degree of whiteness, a method that has been widely used includes a multiple-stage bleaching method effectively combining oxidation bleaching, reduction bleaching and alkali extraction to bleach chemical pulp (CP), such as kraft pulp (KP) or sulfite pulp (SP).
In all of these processes, the pulp is bleached in a step prior to the paper making step(s). For example, an aqueous dispersion of pulp fibers having a suitable concentration can be held in a bleaching tower or a bleaching bath for a desired time, and bleached by adding a suitable amount of a bleaching agent solution while controlling suitably the temperature and the pH. Furthermore, there are also processes in which the bleaching is carried out in a pulper or a beater in the paper stock preparation step. Though these processes are effective as processes for uniformly bleaching the pulp, they have the problem that a large amount of a bleaching agent solution is required, because the concentration of the bleaching agent to pulp is necessarily very low in the pulp slurry.
Furthermore, in the case of photographic paper, it is sufficient that only the surface part of the base has high degree of whiteness, therefore the above-described processes are uneconomical, because unnecessary bleaching is carried out.