A method for the manufacture of plasticised polyvinyl alcohol polymer, the method comprising the steps of: introducing a polyvinyl alcohol polymer, manufactured by hydrolysis of polyvinyl acetate wherein the extent of hydrolysis is 98% or greater, or a blend of the polymer, into a mixing reactor; wherein the mixing reactor comprises a blending chamber having a primary inlet, a primary outlet and at least two inter-engaging components extending between the primary inlet and primary outlet, the components being arranged to apply a shearing force to the polymer while the polymer is conveyed by the components from the inlet through a reaction zone to the outlet; one or more secondary inlets located downstream from the primary inlet for introducing reactants comprising a processing agent and a plasticiser to the chamber to form a reaction mixture; wherein the plasticiser is selected from the group consisting of: polyethylene glycols, propylene glycol, ethylene glycol, mannitol, erythritol, pentaerythritol, sorbitol, dipentaerythritol, trimethylolpropane and mixtures thereof; wherein the blending chamber comprises a plurality of heated regions arranged so that the mixture is subjected to a controlled temperature profile, having a maximum of 220°C; wherein the location of the reaction zone is controlled by changing one or more of: the temperature profile, rotational speed of one or more screws and the rate of feed of the reaction mixture into the mixing reactor; a secondary outlet located between the reaction zone and primary outlet arranged to allow removal of processing agent from the chamber; reacting a reaction mixture comprising the processing agent, plasticiser and polymer in the reaction zone to form plasticised polymer; wherein the processing agent is water and the amount of water in the reaction mixture from 3wt% to 18wt%; and allowing the plasticised polymer to pass from the primary outlet. introducing a polyvinyl alcohol polymer, manufactured by hydrolysis of polyvinyl acetate wherein the extent of hydrolysis is 98% or greater, or a blend of the polymer, into a mixing reactor; wherein the mixing reactor comprises a blending chamber having a primary inlet, a primary outlet and at least two inter-engaging components extending between the primary inlet and primary outlet, the components being arranged to apply a shearing force to the polymer while the polymer is conveyed by the components from the inlet through a reaction zone to the outlet; one or more secondary inlets located downstream from the primary inlet for introducing reactants comprising a processing agent and a plasticiser to the chamber to form a reaction mixture; wherein the plasticiser is selected from the group consisting of: polyethylene glycols, propylene glycol, ethylene glycol, mannitol, erythritol, pentaerythritol, sorbitol, dipentaerythritol, trimethylolpropane and mixtures thereof; wherein the blending chamber comprises a plurality of heated regions arranged so that the mixture is subjected to a controlled temperature profile, having a maximum of 220°C; wherein the location of the reaction zone is controlled by changing one or more of: the temperature profile, rotational speed of one or more screws and the rate of feed of the reaction mixture into the mixing reactor; a secondary outlet located between the reaction zone and primary outlet arranged to allow removal of processing agent from the chamber; reacting a reaction mixture comprising the processing agent, plasticiser and polymer in the reaction zone to form plasticised polymer; wherein the processing agent is water and the amount of water in the reaction mixture from 3wt% to 18wt%; and allowing the plasticised polymer to pass from the primary outlet.