/000 ml and a flow rate of 00.0 g/s. Example 00 0000 g of the water-absorbent polymer particles prepared in Example 0b (base polymer) having a content of residual monomers of 0000 ppm were put into a laboratory ploughshare mixer (model MR0, manufactured by Gebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). A surface-postcrosslinker solution was prepared by mixing 0 g of 0-oxazolidinone as described in Table 0 and 00 g of deionized water, into a beaker. At a mixer speed of 000 rpm, the aqueous solution was sprayed onto the polymer particles within one minute by means of a spray nozzle. The mixing was continued for additional 0 minutes. The product was removed and transferred into another ploughshare mixer (model MR0, manufactured by Gebrüder Lödige Maschinenbau GmbH, Paderborn, Germany) which was heated to 000° C. before. After mixing for further 00 minutes at 000° C. with sample taking every 00 minutes, the product was removed from the mixer and sifted from 000 to 000 μm. The samples were analyzed. The results are summarized in Table 00. The resulting polymer particles that were surface-postcrosslinked with 0,0-oxazolidin-0-one had a bulk density of 00.0 g/000 ml and a flow rate of 00.0 g/s. Example 00 0000 g of the water-absorbent polymer particles prepared in Example 0b (base polymer) having a content of residual monomers of 0000 ppm were put into a laboratory ploughshare mixer (model MR0, manufactured by Gebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). A surface-postcrosslinker solution was prepared by mixing 0 g of 