Method for increasing the recyclability of a polyamide used in sintering

1. A method for increasing the recyclability of a polyamide in a sintering process, comprising incorporating 4000 to less than 10,000 ppm of at least one acid in the polyamide, said acid being an acid of formula HxPyOz in which x, y and z are integers of 1 to 7, boric acid, or a salt, ester, anhydride of these acids or a mixture thereof wherein the acid is added during treatment of the polyamide with water by contacting the polyamide in solid state with water or steam containing the acid at a temperature near its crystallization temperature Tc for a sufficient time to effect the increase, then the water or steam is separated from the polyamide and the polyamide is dried.

2. The method according to claim 1 , wherein said at least one acid is hypophosphorous acid H 3 PO 2 , phosphorous acid H 3 PO 3 , phosphoric acid H 3 PO 4 , pyrophosphoric acid H 4 P 2 O 7 , a metal phosphate, a metal phosphite, a metal hypophosphite, or a phosphoric or phosphorous ester, anhydride, or mixture thereof.

3. The method according to claim 1 , wherein said at least one acid comprises a mixture of hypophosphorous acid H 3 PO 2 and of phosphoric acid H 3 PO 4 , in a weight ratio in the range from 10/90 to 90/10.

4. The method according to claim 1 , wherein the acid is incorporated by at least one of the following methods: impregnation of the polyamide in an aqueous dispersion of the acid, addition of acid during synthesis of the polyamide, at the beginning or at the end of synthesis, addition by mixing by compounding, or during any step of a process for manufacture of powder from said polyamide, by dissolution-precipitation of polyamide in a solvent containing the acid.

5. The method according to claim 1 , comprising:

Mixing the polyamide with a solution of water or of steam comprising 0.1 to 5% of acid, the powder/solution weight ratio being in the range from 5 to 75%;

Subjecting the mixture to a temperature plateau in the range from Tc−20° C. to Tc+10° C.

Separating the water or steam from the polyamide to recover the dry polyamide.

6. The method according to claim 1 , wherein the polyamide is PA11, PA12, PA10.10, an aliphatic polyamide resulting from the condensation of an aliphatic diamine having from 6 to 12 carbon atoms and an aliphatic diacid having from 9 to 12 carbon atoms or a copolyamide 11/12 having either more than 90% of units 11 or more than 90% of units 12.

7. The method according to claim 1 , wherein the polyamide is in the form of granules or powder.

8. The method according to claim 1 , wherein the polyamide is in the form of granules or powder having a volume-median diameter D50 of 10-150 μm.

9. The polyamide powder obtainable according to the method of claim 1 , containing from 4000 to less than 10000 ppm of acid.

10. A sintering process, comprising subjecting a polyamide powder according to claim 9 to sintering conditions.

11. A manufactured article obtained by fusion of a powder according to claim 9 by electromagnetic radiation.