Process for dyeing and foaming thermoplastic polyurethane

A process for the preparation of coloured and expanded thermoplastic polyurethane material which comprises the following steps:

• a) providing thermoplastic polyurethane material and at least one gaseous fluid wherein the melting temperature of the thermoplastic polyurethane material is above the supercritical temperature of the at least one gaseous fluid;
• b) placing the thermoplastic polyurethane material in an autoclave together with a colorant and/or a fluorescent whitening agent, wherein the colorant is selected from at least one of a disperse dye, an acid dye and a pigment;
• c) increasing the pressure in the autoclave by introducing the at least one gaseous fluid at a temperature below the melting point of the thermoplastic polyurethane material and at least above the supercritical temperature of the at least one gaseous fluid at the applied pressure;
• d) allowing the non-expanded thermoplastic polyurethane material to saturate;
• and
• e) decreasing the pressure in the autoclave down to ambient pressure at a temperature between the melting temperature and the glass transition temperature of the thermoplastic polyurethane material at such a rate that the thermoplastic polyurethane material expands to obtain the coloured expanded thermoplastic polyurethane material;
• and
• f) removing the coloured expanded thermoplastic polyurethane material from the autoclave;
• wherein the at least one gaseous fluid is selected from CO2 and the saturation step is performed at a pressure in the range of 73-500 bar and at a temperature below the melting point of the thermoplastic polyurethane material and at least above the supercritical temperature of supercritical CO2 (SC CO2) at the applied pressure.