kept at 100 to 110 u00b0 c until a clear copolymer is obtained and no active hydrogen is detectable any longer the polysiloxane with lateral polyether groups has a viscosity of 870 mm 2 s 25 u00b0 c in comparative experiment 2 a polypropylene oxide with a mean molar mass number average m n of 400 g mol which is not in accordance with the invention is used polymer c1 is heated to 130 u00b0 c and water traces are removed at 1 hpa thereafter 7 g of hexamethylene diisocyanate are metered in and the mixture is homogenized for 20 minutes the isocyanate reaction is started with 1 drop of dibutyltin laurate dbtl after two hours the nco content has fallen below the detection limit ir 20 ppm and so 120 g of a surfactant commercially available under the emulan u00ae he 50 from basf se ludwigshafen germany are metered in after cooling to 25 u00b0 c the 80 copolymer solution has a viscosity of 2100 mm 2 s and a urethane content of 0 139 meq g polymer c4 is a polydimethylsiloxane having a chain length of approx 200 as described in claim 1 in de 1444442 the amounts of the polymers or mixtures thereof added to the black liquor are reported in the table the results of the testing of the efficacy in deaeration are compiled in the table as can be inferred from the table deaeration in comparative experiments 1 and 2 is much poorer than in examples 1 to 4 in comparative experiment 1 an unbranched linear polyether polysiloxane copolymer analogous to gb 2 350 117 a is used in comparative experiment 2 a.