METHODS FOR PRODUCING ULTRAFINE FIBER AND ULTRAFINE FIBER-CONTAINING SHEET, SHEET OBTAINED THEREBY, AND RESIN COMPOSITE COMPRISING LAMINATED RESINS

An object of the present invention is to provide a method for producing an ultrafine fiber-containing sheet with suppressed yellowing. Another object of the present invention is to provide an efficient method for eliminating an introduced substituent, and a composite sheet comprising an organic layer and/or an inorganic layer laminated on the obtained sheet. The present invention provides a method for producing an ultrafine fiber-containing sheet, comprising (a) introducing a substituent having electrostatic and/or steric functionality to a fiber raw material to obtain substituent-introduced fiber; (b) machine-processing the substituent-introduced fiber obtained in the step (a) to obtain substituent-introduced ultrafine fiber; (c) preparing a sheet from the substituent-introduced ultrafine fiber obtained in the step (b); and (d) eliminating at least a portion of introduced substituents from the sheet obtained in the step (c). In a preferred aspect, the substituent having electrostatic and/or steric functionality is a phosphoric acid-derived group, and the method may further comprise, after the step (a) and before the step (c), the step of (e) changing the degree of neutralization of the substituent-introduced fiber or the substituent-introduced ultrafine fiber.