Process for producing leather

Claim 1

A multiple-stage tanning process for producing an improved, tanned, heavy leather, comprising the steps, carried out in a series of aqueous tanning baths, of: (1) providing a piece of wet leather stock selected from the group consisting of pickled leather stock, bated leather stock, and limed leather stock; (2) adjusting the pH of the leather stock provided in step (1) to obtain a pH of the leather stock in the range of 4.5-5.5; (3) treating the leather stock from step (2), while establishing and maintaining the pH thereof at 4.5-5.5, with 1-50% by weight, based on the initial wet weight of the leather stock, of a first tanning composition comprising an aqueous dispersion or solution of a polymer polymerized from a monomer mixture comprising at least one member selected from a group consisting of acrylic acid, methacrylic acid, mixtures of acrylic acid and methacrylic acid, and mixtures of a major proportion of at least one member selected from the group consisting of acrylic acid and methacrylic acid with a minor proportion of at least one member selected from the group consisting of alkyl esters of acrylic acid, alkyl esters of methacrylic acid, and partially sulfated unsaturated drying oils, until the leather stock is penetrated therewith; (4) adjusting the pH of the leather stock from step (3) to 1.5-3.3 to exhaust the first tanning composition and to obtain an optimal pH for the subsequent second tanning treatment; (5) treating the leather stock of step (4), while establishing and maintaining the pH thereof at 1-3.3, with 5.5-20% by weight, based on the initial wet weight of the leather stock, of a second tanning composition comprising a zirconium tanning compound having 0-45% basicity calculated on the Schorlemmer scale, the amount of the zirconium tanning compound being sufficient to provide an amount of zirconium calculated as the oxide of about 1.8-6.6% by weight, based on the initial wet weight of the leather stock, until the leather stock is tanned to the desired extent; (6) neutralizing the tanned leather stock from step (5) to a pH of 3.5-5 by the addition to the tanning bath of an aqueous solution of a weak base; and (7) washing the tanned leather stock from step (6) with water, optionally further treating the washed tanned leather stock with conventional adjuvants, and drying the tanned leather stock in conventional operations.