Process for preparing a stable aqueous dispersion of polyamide-imide which is free of carcinogenic, mutagenic or reprotoxic substances, and application to coatings

1. A process for preparing a stable aqueous polyamide-imide dispersion, the process comprising:

selecting a polyimide-imide powder; and

producing a dispersion comprising the polyamide-imide powder, a polar aprotic solvent having a boiling point of more than 180° C. at 760 mm Hg and water, wherein the polar aprotic solvent is at least one selected from the group consisting of dimethylsulphoxide, sulpholane, and γ-butyrolactone, and

wherein the aqueous polyamide-imide dispersion comprises a thixotropic agent which is thermally stable at a temperature of 250° C. or more and is at least one selected from the group consisting of an organically modified sheet silicate of laponite, saponite, bentonite, and smectite type having a pH of from 9 to 11 in a 2% by weight suspension.

2. The process of claim 1 , further comprising:

dissolving the polyamide-imide powder in the polar aprotic solvent having a boiling point of more than 180° C. at 760 mm Hg, thereby obtaining a polyamide-imide solution;

precipitating the polyamide-imide solution in an aqueous mixture at ambient temperature; and

dispersing the precipitated solution and stabilizing by milling polyamide-imide particles.

3. A process for preparing a stable aqueous polyamide-imide dispersion according to claim 1 , characterized in that the dispersion is produced with at least:

a step in which the polyamide-imide powder is dispersed by mechanical agitation in a homogeneous mixture containing water and a polar aprotic solvent with a boiling point of more than 180° C. at 760 mm Hg;

a step in which the polyamide-imide dispersion is stabilized by means of a treatment with ultrasound or by milling.

4. The process of claim 1 , wherein an amount of polyamide-imide is 35% by weight or less.

5. The process of claim 2 , wherein the dissolving of the polyamide-imide powder is carried out at a temperature of more than 50° C.

6. The process of claim 2 , wherein the precipitation is carried out at ambient temperature in an aqueous mixture containing distilled water and at least one non-ionic surfactant, wherein the aqueous mixture is free of polyoxyethylenated alkylphenyl ethers.

7. A process for preparing a stable aqueous polyamide-imide dispersion according to claim 3 , characterized in that the step for dispersion of the polyamide-imide powder by mechanical agitation is carried out at a temperature in the range 60° C. to 80° C. in a homogeneous mixture containing distilled water, a polar aprotic solvent and at least one non-ionic surfactant.

8. A process for preparing a stable aqueous polyamide-imide dispersion according to claim 7 , characterized in that the non-ionic surfactant selected from the group constituted by polyoxyethylenated acetylenic diols and high molecular weight block copolymers containing groups with a high affinity for pigments preferably has a hydrophilic-lipophilic balance of 13 or less, preferably 8 or less.

9. A process for preparing a stable aqueous polyamide-imide dispersion according to claim 3 , characterized in that the stabilization by ultrasound treatment is carried out at a minimum frequency of 20 kHz and a minimum power of 200 W.

10. The process of claim 2 , wherein the milling of the polyamide-imide particles is carried out by vertical or horizontal bead milling, wherein a yield of the milling is from 40% to 95%.

11. The process of claim 1 , wherein a size of the polyamide-imide polymer particles in dispersion is less than 70 μηι.

12. The process of claim 1 , wherein the aqueous polyamide-imide dispersion comprises from 0.05% to 0.4% by weight of a bactericidal and/or fungicidal agent, for protection during storage and for protection of a film, selected from the group consisting of iodopropynyl-butyl-carbamate, benzisothiazolinone, chloromethyl-isothiazolinone and methylisothiazolinone.

13. The process of claim 1 , wherein the aqueous polyamide-imide dispersion comprises a spreading agent, wherein the spreading agent is a polyether modified dimethylpolysiloxane.

14. The process of claim 1 , wherein the aqueous polyamide-imide dispersion comprises an anti-foaming agent which is an emulsion of a hydrophobic component containing a silicone oil, paraffinic mineral oil, or a combination thereof.

15. The process of claim 1 , wherein the polyamide-imide powder is an aromatic polyamide-imide powder.

16. The process claim 15 , wherein the aromatic polyamide-imide powder is obtained by reacting a trimellitic acid anhydride chloride and an aromatic diamine selected from compounds other than 4,4′-methylenedianiline.

17. The process claim 15 , wherein the polyamide-imide powder is selected from the group consisting of aromatic polyamide-imide polymers having a molecular mass of from 10,000 to 30,000 g/mole and a glass transition temperature of from 220° C. to 285° C.

18. The process claim 15 , wherein a maximum particle size in the polyamide-imide powder is less than 150 μιηand 95% of the particles have a size of less than 75 μιη.

19. The process claim 15 , the wherein a mean volumetric granulometry of the polyamide-imide particles is from 30 to 40 μιη.

20. The process claim 1 , wherein the polar aprotic solvent is dimethylsulphoxide.

21. A process for producing a dry polyamide-imide film on a substrate, the process comprising:

preparing a stable aqueous polyamide-imide dispersion according to claim 1 ;

applying the stable aqueous polyamide-imide dispersion to the substrate by spraying;

curing the stable aqueous polyamide-imide dispersion applied to the substrate at a temperatures of from 230° C. to 325° C.

22. The process of claim 21 , wherein after the curing, the substrate with the applied stable aqueous polyamide-imide dispersion is cooled at ambient temperature.