Preparation of super absorbent polymers

A process for preparing coated water-absorbing compositions, especially for use in personal hygiene products, comprises coating a super-absorbent polymeric material with an aqueous coating medium which besides water comprises at least one (or more) polyhydric alcohol(s) having hydroxyl substituents on non-adjacent carbon atoms, the said medium being substantially free of organic solvents.

EXAMPLES AND DRAWINGS In order that the invention can be fully understood it will now be further described, though only for purposes of illustration, with reference to the following Examples and FIGS. 1 to 7 of the accompanying drawings. The base polymer employed was a conventional partially neutralised sodium salt of a cross-linked polyacrylic acid. CRC (centrifugal retention capacity) was determined by edana method 441.1-99 and AAP (absorption against pressure) was determined by edana method 442.1-99 modified so as to give a working pressure of 49 g/cm 2 . These edana methods are standards of the European Disposables and Non-wovens Association. 
 Example 1 Samples (15 to 20 g) of powdered base polymer were mixed with a coating solution of 2-methyl-1,3-propanediol dissolved in water and heated for 60 minutes at 190° C. In each case the weight of coating solution and weight of polymer were such as to give a coating ratio (CR) of 4%. However the concentration of diol with respect to polymer was varied in each case. The results are shown in Table 1 below and graphically in FIG. 1 where the y axis gives the weight (g) of solution absorbed per gram of polymer for the two tests. 1 TABLE 1 Diol wt % CRC (g/g) AAP (g/g) 0.4 25.9 24.2 0.8 25.4 24.8 1.2 25.6 24.6 1.6 25.3 24.9 
 Example 1a Example 1 was repeated using a coating solution of 1,4-butanediol dissolved in water. The results are shown in Table 1a below, and graphically in FIG. 1 a . 2 TABLE 1a Diol wt. % CRC (g/g) AAP (g/g) 0.4 26.7 24 0.6 25.6 24.5 0.8 25.7 24.1 1.0 25.7 24.7 1.2 25 23.7 1.4 25.9 24.2 1.6 25.2 25.3 
 Comparative Example 1b Example 1a was repeated by employing as solvent a solution consisting of 50/50 (by weight) water/methanol. The results are shown in Table 1b below and graphically in FIG. 1 b. It will be seen from a comparison of Example 1a and this Comparative Example that satisfactory values for CRC and AAP can be obtained in the absence of methanol using water only as solvent without the need for a solvent such as methanol. 3 TABLE 1b Diol wt. % CRC (g/g) AAP (g/g) 0.2 27.7 23.2 0.4 25.8 24.2 0.6 25.7 25.2 0.8 26.6 24.8 
 Example 2 Example 1 was repeated using a concentration of 2-methyl-1,3-propanediol of 1.0 wt %, (based on weight of polymer powder), a cure time of 60 minutes and a coating ratio of 4%, but varying the temperature employed. The results are shown in Table 2 below and graphically in FIG. 2 . 4 TABLE 2 Temp ° C. CRC (g/g) AAP (g/g) 170 30.0 10.1 180 29.1 19.2 190 26.2 24.9 200 23.0 23.7 210 21.7 22.6 
 Example 2a Example 2 was repeated using 1,4-butanediol. The results are shown in Table 2a below and graphically in FIG. 2 a . 5 TABLE 2a Temp ° C. CRC (g/g) AAP (g/g) 170 30.3 9.9 180 28.9 18.9 190 25.9 25.6 200 22.7 23.8 210 20.7 22.5 
 Example 3 Example 2a was repeated using a cure time of 16 hours (overnight) but varying the cure temperature. The results are shown in Table 3 below and graphically in FIG. 3 . 6 TABLE 3 Temp ° C. CRC (g/g) AAP (g/g) 140 15.8 150 28 24.6 160 25.6 24.9 170 22.3 24.7 
 Example 4 Example 1a was repeated using a concentration of diol of 1.0 wt. %, a cure temperature of 190° C. and a coating ratio of 4%. The cure time was varied for each sample. The results are shown in Table 4 below and graphically in FIG. 4 . 7 TABLE 4 Time CRC (g/g) AAP (g/g) 20 30 11.1 40 28.2 22.9 60 26.9 25.7 80 25.1 24.3 100 24.1 23.8 
 Example 5 Example 4 was repeated but using a cure temperature of 220° C., and using 2-methyl-1,3-propanediol. The results are shown in Table 5 below and graphically in FIG. 5 . 8 TABLE 5 Time (min) CRC (g/g) AAP (g/g) 5 28.4 9.4 10 29.2 9.8 15 29.6 12.2 20 29.0 22.2 25 26.2 23.4 
 Example 5a Example 5 was repeated using 1,4-butanediol. The results are shown in Table 5a below and graphically in FIG. 5 a . 9 TABLE 5a Time (min) CRC (g/g) AAP (g/g) 5 28 8.9 10 29.5 9.5 15 29.7 11.2 20 28.5 21.6 25 25.4 23.7 
 Example 6 Example 4 was repeated but using a cure temperature of 250° C. The results are shown in Table 6 below and graphically in FIG. 6 . 10 TABLE 6 Time (min) CRC (g/g) AAP (g/g) 10 27.8 19.5 15 22.1 22.7 20 19.4 20.7 
 Example 7 A pilot scale experiment was carried out using 10 kg of powdered base polymer The experiment was carried out at a temperature of 220° C. with a coating ratio of 4% and a diol concentration of 1%. The results are shown in Table 7 below and graphically in FIG. 7 . 11 TABLE 7 Time (min) CRC (g/g) AAP (g/g) 4 27.3 22.7 6 26.9 24.4 8 24.7 24.1 10 24.7 24.2 12 24.5 24.4