Polyoxymethylene with improved resistance to acids and its use

1. A polyoxymethylene composition comprising

A from 84 to 99.79% by weight of at least one polyoxymethylene homo- or copolymer,

B from 0.1 to 5% by weight of at least one polyalkylene glycol,

C from 0.1 to 10% by weight of zinc oxide, and

D from 0.01 to 1% by weight of one or more nitrogen-containing costabilizer and said nitrogen-containing costabilizer comprises melamine.

2. The polyoxymethylene as claimed in claim 1 , wherein the amount of polyalkylene glycol is from 0.5 to 5% by weight.

3. The polyoxymethylene as claimed in claim 1 , wherein the amount of zinc oxide is from 1 to 3% by weight.

4. The polyoxymethylene as claimed in claim 2 , wherein the amount of zinc oxide is from 0.5 to 3% by weight.

5. The polyoxymethylene as claimed in claim 1 , wherein the amount of nitrogen-containing costabilizer is from 0.03 to 0.3% by weight.

6. The polyoxymethylene as claimed in claim 2 , wherein the amount of nitrogen-containing costabilizer is from 0.03 to 0.3% by weight.

7. The polyoxymethylene as claimed in claim 1 , wherein test specimens in the form of ISO ¼ tensile specimens of thickness 1 mm produced from pellets obtained by melting and palletizing a mixture made from components A to D, which on each of five days in succession were fully immersed for 20 seconds in an aqueous solution made from 10% by weight of phosphoric acid and 1% by weight of an ionic surfactant, were then removed and, without wiping off any adhering acid/sufactant solution, aged freely suspended for 24 hours in an environment with controlled temperature and humidity, at 23° C. and relative humidity of about 30%, and then aged for further 9 days suspended in the environment with controlled temperature and humidity, show a relative weight difference of less than 15% compared with test specimens produced in the same way but not exposed to the acid/surfactant solution.

8. The polyoxymethylene as claimed in claim 7 , wherein the relative change in weight of the test specimens made from components A to D is one third or less of the relative change in weight determined on test specimens made from 100% by weight of component A.

9. The polyoxymethylene as claimed in claim 1 , wherein said polyalkylene glycol is polyethylene glycol.

10. The polyoxymethylene as claimed in claim 1 , wherein said polyalkylene glycol is polypropylene glycol.

11. The polyoxymethylene as claimed in claim 1 , wherein said at least one polyoxymethylene homo- or copolymer is a homopolymer of formaldehyde.

12. A molding made from the polyoxymethylene as claimed in claim 1 wherein the molding when in contact with aggressive acids or with aggressive acid-containing cleaning agents is resistive to said aggressive acids or said aggressive acid-containing cleaning agents.

13. The molding as claimed in claim 12 , wherein the molding is used in the sanitary sector or the white goods sector.

14. A method for improving the acid resistance of the polyoxymethylene homo- or copolymer which comprises making the polyoxymethylene as claimed in claim 1 , by mixing components A–D together to form a mixture and then melting said mixture.

15. A molding made from a polyoxymethylene composition comprising

A from 84 to 99.79% by weight of at least one polyoxymethylene homo- or copolymer,

B from 0.1 to 5% by weight of at least one polyalkylene glycol,

C from 0.1 to 10% by weight of zinc oxide, and

D from 0.01 to 1% by weight of one or more nitrogen-containing costabilizer wherein the molding when in contact with aggressive acids or with aggressive acid-containing cleaning agents is resistive to said aggressive acids or said aggressive acid-containing cleaning agents and said nitrogen-containing costabilizer comprises melamine.

16. The polyoxymethylene as claimed in claim 1 , wherein nitrogen-containing costabilizer comprises a combination of melamine and at least one amino compound, amide compound, hydrazine compound, urea compound or a hindered amine.

17. The molding as claimed in claim 15 , wherein nitrogen-containing costabilizer comprises a combination of melamine and at least one amino compound, amide compound, hydrazine compound, urea compound or a hindered amine.