A process for obtaining a plant extract containing condensed tannins, with astringent characteristics, modified by copolymerization, to be used in tannage and retanning of skins, comprising:
- (a) adding water to a reactor at between 2.0 and 5.8%, by weight, to the total desired weight of a load;
- (b) adding to the reactor an amount between 1.2 and 2.5%, by weight, of a copolymerization agent to form a reaction mixture, selected from the group consisting of acrylic acid, methacrylic acid, mixtures of acrylic acid and methacrylic acid and esters thereof;
- all the while stirring the reaction mixture;
- (c) adding thioglycolic acid at between 0.1 and 1.7%, by weight, while cooling the reaction mixture, allowing the same to react whereupon there occurs an exothermal reaction and a temperature reaches 95° C., and after the reaction a product obtained from the reaction being left to cool;
- (d) adjusting the pH to values in the range between 3.5 and 7.0, by adding an alkali to the reaction mixture;
- (e) either mixing a powdered extract of a plant with water at an amount about 0.5 to 2.0 kg per liter of water;
- or
- mixing a solution of tannin in liquid form in a concentration of 25% to 60%;
- (f) raising the temperature of the mixture to a range between 67 and 98° C., to carry out the reaction, for a period of 2 to 7 hours
- (g) adding sodium bisulphite at 0.5 to 4.0%, by weight, of the amount of tannin;
- (h) raising the temperature of the reaction mixture, keeping the reaction system heated at a temperature between 45 and 127° C., the pressure being between 0.2 and 7 kgf/cm2;
- and
- (i) allowing a reaction time of about 4 hours.