Abstract: An elastic fiber of a mixture containing a copolyether ester or a copolyester ester and a chemically crosslinked rubber, which fiber has a permanent elongation after 100% stretching of at most 9% and an elongation at break of at least 450% and a process for producing an elastic fiber comprising the melt spinning of a mixture of a copolyether ester or a copolyester ester and a rubber to form a fiber, in which the rubber is completely or almost completely crosslinked at the moment the fiber is formed, and are disclosed.

Abstract: The invention relates to an acid-dyeable polyamide fiber which can be obtained very reproducibly and unsusceptibly to faults during the production process. The fiber is characterized in that the amino end group content is less than 10 meq/kg. A very reproducible affinity for dyestuffs is obtained by copolymerization of a compound containing an acidic end group and having a basic nature. Compounds derived from melamine are particularly suitable.

Abstract: The invention relates to a polyamide fiber whose polyamide is heavily branched. The branched polyamide preferably has 3-20 branches which are joined to a core molecule which is smaller than the polyamide arms. The arms originate preferably from functional groups which are identical and are preferably carboxyl groups. The polyamide is chosen from the group of polyamides which are produced by polycondensation of .alpha.,.omega.-aminocarboxylic acids or by ring-opening polymerization of lactams. Preferably, the polyamide is nylon-6. The properties of the fiber according to the invention are at least at the same level as those of fibers obtained by spinning linear polyamide. The spinning process for the fiber according to the invention is less susceptible to faults. Inter alia, the viscosity of the polyamide no longer changes during the spinning process.