Binding fiber for bonding of flat material containing natural fibers

A binding fiber for bonding of flat materials containing natural fibers through activation of the binding fiber by heat in a water saturated condition, wherein said binding fiber is a bicomponent fiber of the core/sheath type, wherein the sheath is made from a copolyamide as a first thermoplastic component, said copolyamide composition comprising:

• caprolactam 40-70 mol-%;
• laurolactam, and/or 12-aminododecanoic acid, 20-50 mol-%;
• aminoundecanoic acid, 0-10 mol-%;
• and
• further consisting of one dicarboxylic acid 10-30 mol-% selected from the group of compounds consisting of:
• aliphatic α,ω-dicarboxylic acids with 4 to 14 C atoms;
• aromatic dicarboxylic acids, where the percentage of aromatic dicarboxylic acids is smaller than 15 mol-% relative to the total quantity of all dicarboxylic acids;
• wherein said copolyamide composition further comprises, with respect to the one dicarboxylic acid, an equimolar quantity of at least one diamine selected from the group consisting of hexamethylene diamine, piperazine, 2-methyl-1,5-diaminopentane and other aliphatic long-chain diamines, wherein the molar percentage of hexamethylene diamine, relative to the diamine mixture, is between 50 and 100 mol-%;
• wherein the sum of all lactames, all amino acids and all dicarboxylic acids is 100 mol-%;
• wherein the copolyamide has a relative viscosity of at least 1.50 when measured at 0.5% in m-cresol at 25° C., an MVR of maximum 20 cm3/10 min. when measured at 190° C. and 2.16 kg load, a hydrophobicity having a ratio of alkyl groups to amino groups in the monomers of smaller than 7.2, and has a melting point temperature of less than 105° C. in the water saturated condition and a higher melting point temperature in the dry condition;
• and
• wherein the core is made from a second thermoplastic component having a melting temperature above 130° C. in the water saturated condition and a melting temperature of at least 150° C. in the dry condition;
• wherein, when the binding fiber for bonding of flat materials containing natural fibers is subjected to a temperature greater than the melting temperature of the copolyamide sheath of the first thermoplastic component and less than the melting temperature of the core of the second thermoplastic component, the copolyamide sheath melts for enabling bonding with the natural fibers of the flat material, and the thermoplastic core does not melt for enabling improved strength of the bonded flat material.