Flame-retardant finishing of fiber materials

A process for treating fiber materials comprising:

• a) applying to a fiber material an aqueous mixture comprising a tetrakishydroxymethyl-phosphonium compound and/or a product formed by reaction of a tetrakishydroxymethyl-phosphonium compound with an organic nitrogen compound as well as water,
• b) drying the fiber material obtained after step a),
• c) treating the material obtained after step b) with gaseous ammonia and/or ammonia dissolved in a solvent, wherein when ammonia dissolved in a solvent is used the fiber material is optionally dried after step c),
• d) optionally treating the material obtained after step c) with an oxidizing agent,
• e) washing the fiber material obtained after step d) with an aqueous solution having a pH of at least 7.5 at 20° C.,
• wherein furthermore the fiber material is treated with an aqueous solution A whose pH at 20° C. is in the range from 0.5 to 6.5 or whose pH after 5 minutes of heating said solution A to a temperature in the range from 80° C. to 170° C. is in the range mentioned,
• wherein the treating of the fiber material with said solution A is carried out either between said steps c) and d) or between said steps d) and e) or after step e), wherein the fiber material is optionally washed with water between step c) and step d) and/or between step d) and step e),
• and wherein when the treatment of the fiber material with said solution A is carried out after step e) a step e) is carried out again after the treatment with said solution A.