A method of introducing an additive into a polymeric material comprises using a liquid formulation comprising an additive, for example a colourant, and a vehicle comprising an aliphatic or aromatic tri- or di-carboxylic acid covalently linked by ester bonds to two or more chains. The method involves contacting the liquid formulation with the polymeric material in a melt processing apparatus. The cavity transfer mixer may be used in the process. A fibre is suitably subsequently produced.
