OA17208A - Process for preparing a stable aqueous dispersion of polyamide-imide which is free of carcinogenic, mutagenic or reprotoxic substances, and application to coatings. - Google Patents

Abstract

The invention concerns a process for preparing an aqueous polyamide-imide dispersion, characterized in that it comprises at least one step of selecting a polyamide-imide powder and a step of producing a dispersion comprising the polyamide-imide powder, a polar aprotic solvent with a boiling point of more than 180°C at 760 mm Hg, and water. The invention also concerns a process for producing a dry polyamide-imide film on a substrate.

Description

[001 ] The présent invention relates to a process for the production of a dry film of polyamidc-imidc from a stable, ready-to-use aqueous dispersion which is free of carcinogenic, mutagcnic or reprotoxic substances, said process comprising either a step for dispersion by means of a spécifie milling process preceded by a step for dissolution and précipitation, or a step for de flocculation by ultrasound treatment preceded by a dispersion step.

[002] In order to ovcrcome difïïculties in using them, novel polyimides hâve been manufactured from the beginnîng ofthe 1970s. Such thermoplastic polyimides cany chemical groups which can render them fusible and soluble in solvents without significantly altering their properties. The principal thcrmoplastic polyimides are polycthcr-imidcs and polyamide-imidcs.

[003] Polyamide-imides, which arc high performance amorphous thermoplastic polymers, are widely used as imprégnation restas and protective anti-corrosion vamishes for applications rcquiring good thermal behaviour or as binders for décorative, non-stick and self-lubricating paints.

[004] Its principal qualifies consist of very good film-forming properties, good surface hardness, good abrasion résistance, excellent adhesion to any metallic support, good résistance to solvents, fuels, acids, exccptional electrical rigidity and rcmarkable fiexibility of the film above 220°C.

[005] One of the main applications conccms glide coatings. In fact, the polyamide-imide polymer has superior mcchanical properties as well as excellent chemical résistance. A

Rockwcll M hardness in the range 110 to 125, a résistance at break and compressive strength in the range 110 to 220 MPa mean that it can copc with the pressure and shear type loads occurring in a mctal-mctal contact.

[006] Since a polyamidc-imide polymer is practically insoluble, application in the form of a coating nécessitâtes a vital step of dissolution in an appropriate dipolar aprotic solvent. For applications as a protective layer of a fuser élément, Xcrox Corporation proposes, for example, dissolving an Amoco® AI-10 polyamide-imidc in a mixture of acetone and dipolar aprotic solvent with a high boiling point, namely N-methyl-2-pyrrolidone (NMP) (EP 1 291 734).

[007] In addition, in order to faci litate the application of polyamide-imides and to limit the use oforganic solvents with a high boiling point in non-negligîble quantifies, or solvents considered to be highly volatile oxygen compounds (tetrahydrofuran, THF), the ski lied pereon will strive to find versions of functionalized polymère which are soluble in water or aqueous dispersions for a large variety of applications. The use of polyamide acid salts is thus at the origin of inventions in that field.

[008] One of the very first patents mentionmg taking polyamide acid salts into aqueous dispersion is patent US 4 087 394 filed by EI Du Pont de Nemours and Co. That patent pertains to the préparation of coatings with lubricant, unmoulding, chemical résistance, high température résistance and electrically insulating properties from an aqueous dispersion containing a fluoropolymer as the major ingrédient and a polyimîde precureor in solution which is capable of forming a film below the melting point of the fluoropolymer (PTFE). The solution of polyamide acid sait, polyimîde precureor, is constitutcd by a carboxylic polyamide acid with a water-soluble tertîary amine (tricthylamine and/or diethylethanolamine). However, a coalescing agent, exclusively N-mcthylpyrrolidonc, is systematically added to fàcilîtate film formation.

[009] Patent US 4 259 221 from General Electric, based on earlier work, proposes applying a single polyamide-imide coating which is soluble in water or, preferably, in a *

water/organic solvent mixture in order to reducc costs arising from organic solvents, such as their environmental impact Dispersing the polymer is facilitated by forming a sait with a tertiary amine (dimethylethanolamine). The powdered PAI polymer (Amoco® AI-10) is a polyamide-imide pre-polymer obtained by synthesis in two reactions: a reaction between a trimcllitic anhydride and an aromatic polyamine, resulting in a polyamide-imidc prepolymer, followed by reaction between the pre-polymer and a polyisocyanate. The organic solvent selected front a list of dîpolar aprotic solvents, in particular comprising: cresol, phénol, xylene, NMP, DMF, DMAc, DMSO, is exclusively NMP.

[0010] In patent US 2011 0081493, T Matsushita (Hitachi Chemical Ltd) typically reused the same process for obtaining a polyamide-imide resin in solution to applying a layer which is not adhesive as regards ink in a printing System. However, the whole range of soluble polyamide-imidcs marketed under the trade name HPC by Hitachi Chemical Ltd, in particular the HPC-1000 version given as an example in said patent, exclusively contains organic solvents such as γ-butyrolactone, NMP, NEP or, as is preferred, NMP/water mixtures. In another patcnL US 5 037 862, Hitachi Chemical Ltd also form a pastc from at least 40% of an aromatic polyamide-imide resin in a solvent for scrigraphy applications. The solvents could be selected from the following substances: lactones, aliphatic ketoncs, ethers, phénols, DMSO and sulpholane. However, the process consiste ofdispersing fine particles of aromatic polyamide-imide resin (40 pm) in a solution of polyamide-imide resin, preferably dissolved in an ether such as THF, triethylene glycol dimethyl ether, without providing details as to whether the pastc obtained can be dispersed in water.

[0011] AkzoNobel proposes using the same type of polyamide-imide in aqueous dispersion in an application conceming a paint for aluminium cooking utensils. The corresponding EP 0 724 915 concems the application of an aqueous dispersion of PTFE containing a polyamide-imide binder. The proportion ofNMP in the final aqueous polyamide-imide dispersion is 11% by weight.

[0012] The same approach is ctaimed by DuPont de Nemours in patent US 5 789 083.

[0013] Patent US 6 479 581 from Solvay Advanced Polymers illuminâtes the preferential molecular structure of a soluble polyamide-imide prc-polymcr for gcncrating a non-tacky, adhesîvc film on a widc variety of ferrous and non-ferrous substrates including steel, aluminium and coppcr. A solution of polyamide-amie acid obtained by synthesis in an atiphatic tertiary amine (triethylaminc) would considerably favour dispersion in water. The invention stipulâtes that the quantity ofresidual organic solvent in the prc-polymcr is in the range 0.1% to 5% by weight In realîty, the moist Torfon® AI-30 powdcr corresponding to the soluble prc-polymcr obtained by synthesis contains at least 2% by weight of residual NMP solvent.

(0014] In response to the above technique, which consiste of functionalizing the polymer, in patent US 5 468 798, Whitford Corp proposed dissolving a polyamide-imide resin in a suitable polar aprotic solvent such as NMP then mitling the resin precipitated in water by means of a bail mill. Starting from this stable aqueous dispersion of PAI resin particles in a watcr/NMP mixture, Whitford proposed producing a dry adhesion-promoting layer of between 20 and 30 pm, rcndcred sufficiently rough and adhesîvc by means of filamcntary nickel. The scratch résistance and abrasion résistance of a PAI polymer means that the traditional “tefloning” in cooking utensils, electrical applianccs or printing machines can readily be replaced.

(0015] However, even though a number of tcchnological advances claim the possibility of preparing an aqueous dispersion or a polyamide-imide resin which is soluble in water, none of them is completely free of CMR category substances and there is cunently no ready-touse commcrcially available dispersion ofthis polymer on the market. In other words, this type of alternative simply results in a drastic réduction in the volatile organic substances released. For this reason, ail of the aqueous solutions cunently on the market for application in the form of a coating (Altana, Fujifilm Hunt Chemicals, Okitsumo, Solvay) in fact contain CMR category substances, somctimes in rclatively large quantifies.

[0016] In view of the régulations rcgarding the working environment (Directive

2004/37/EC from the European Parliament and Council dated 29^ώ April 2004 regardîng the protection of workers against risks linked to exposure to carcinogenic or mutagcnic agents in the workplacc) and the classification of evaporated substances (Commission Directive 2009/2/EC dated 15^ώ January 2009, amending, for the purposes of its adaptation to technical progress, for the 31* time, Council Directive 67/548/EEC on the approximation of dispositions rclating to the classification, packaging and labelling of dangerous substances; (EC) Rule no 1272/2008 European Parliament and Council dated 16^Λ Deccmber 2008 rclating to the classification, labelling and packaging of substances and mixtures), the risk to the user is particularly high.

[0017] Po lar so Ivents (toluène, xylcnc), in particular polar aprotic solvents containing nitrogen, such as NMP, Ν,Ν-dtmcthylacctamidc (DMAc), Ν,Ν-dimethylformamidc (DMF), hexamethylphosphoramide (HMPA), etc, employed when forming a polyamide-imidc film are substances which are classified as carcinogenic, mutagcnic or reprotoxie (CMR) which arc subject to prohibition or restricted use in many applications.

[0018] The limiting concentration thresholds for the most sensitive carcinogenic and mutagcnic substances, classcd as 1A (can cause cancer), IB (may induce a genetic anomaly), are 0.1% or less in mixtures and 0.3% or less for reprotoxie substances. Other CMR substances of class 2, deemed to be of conccm for fertility and human development, hâve to be in concentrations of 1% or less.

[0019] The classification o f the polar aprotic solvents used and their known toxicity is given in the table below.

Substance name	CMR classification
N,N-dimethylacetamide	Reprotoxic cat. IB
N,N-dimethylformamide	Reprotoxic cat. IB
Hexamcthylphosphoramidc	Carcinogenic and mutagenic cat. 1B
N-methylpyrrolidone	Reprotoxic cat. IB
Toluene	Reprotoxic cat. 2

[0020] The new classification for CMR substances in classes 1 A, IB and 2 will be applicable to mixtures from Junc 2015, and so it would be appropriate to provide a novel alternative in ordcr to prcvent the risk of chemical exposure to CMR agents in application of the Frcnch Labour Code (article R4411-73 and articles R231-56 to R231-56-12).

[0021 ] Starting from this point, the présent invention proposes preparing an aqueous dispersion of an aromatic polyamîde-imide polymer containing no CMR substances in order to generate a dry coating of polyamide-imide that is easy to apply, adhèrent to a wide variety of substrates, particularly to ferrous or non-ferra us metals, and that can be protectivc, lubricating and abrasion-résistant [0022] In particuiar, the présent invention concems a process for preparing a stable aqueous polyamide-imide dispersion, characterized in that it comprises at least the following steps:

• select ing a polyamide-imide powder;

• producing a dispersion comprising the polyamide-imide powder, a polar aprotic solvent with a boiling point of more than 180°C at 760 mm Hg, and water.

[0023] In a first variation, the dispersion is produced with at least:

• a step in which the polyamide-imide powder is dissolved in the polar aprotic solvent with a boiling point of more than 180°C at 760 mm Hg in order to obtain a polyamide-imide solution;

• a step in which the polymer solution is precipitated in an aqueous mixture at ambient température;

• a step in which the precipitated solution is dispersed and stabilized by milling the polyamide-imide particles.

[0024] In a second variation, 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 uhrasound or by milling.

[0025] Optional characteristics, which may be complementaiy or substitutional, are defined below.

[0026] The proportion of polyamide-imide may be 35% by weight or less.

[0027] The step for dissolving the polyamide-imide powder in the first variation may be carried out at a température of more than 50°C, preferably more than 80°C.

[0028] The précipitation step in the first variation may be carried out at ambient température in an aqueous mixture containing distillcd water and at least one non-ionic surfactant, the aqueous mixture being free of polyoxyethylenated alkylphenyl ethers.

[0029] The step for dispersion of the polyamide-imide powder by mechanical agitation in the second variation may be carried out at a température 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.

[0030] The non-ionic surfactant may be selected from the group constituted by polyoxyethylenated acctylenic diols and high molecular weight block copolymers containing groups with a high affinity for pigments, and may hâve a hydrophilic-lipophilic balance of 13 or less, preferably 8 or less.

[003 ! ] The stabilization by ultrasound treatment in the second variation may be carried out at a minimum frequency of 20 kHz and a minimum power of200 W.

[0032] The step for milling the polyamide-imidc particles in the first variation and in the second variation may be carried out by vertical or horizontal bcad milling, the milling yield being in the range 40% to 95%, preferably more than 90%.

[0033] The size of the polyamide-imide polymer particles in dispersion may bc less than 70 μτη, preferably less than 20 μπι and primarily less than 10 μητ [0034] The aqueous polyamide-imide dispersion may comprise a thixotropic agent which is thcimally stable at températures of250°C or more selected from the group constitutcd by organically modified sheet silicates of the laponite, saponite, bentonite or smectite type with a pH in 2% by weight suspension in the range 9 to 11.

[0035] The aqueous polyamide-imide dispersion may comprise in the range 0.05% to 0.4% by weight of a bactericidal and/or fbngicidal agent for protection during storage and for protection of the film, selected from the group constitutcd by iodopropynyl-butyl-carbamate, benzisothiazolinone, chloromethyl-isothiazolinone and methylisothiazolinone.

[0036] The aqueous polyamide-imide dispersion may comprise a spreading agent of the polycther modified dimethylpolysiloxane type to reduce the surface tension.

[0037] The aqueous polyamide-imide disp ersion may comprise an anti-fb aming agent which may bc an émulsion of hydrophobie components containing silicone and/or parafïïnic minera! oils.

[0038] The polyamide-imide (PAI) powder may be selected from aromatic polyamideimide powdcrs.

[0039] The selected aromatic polyamide-imide may bc obtained from a reaction between a trimellitic acid anhydride chloride and an aromatic diamine selected from compounds other than 4,4*-mcthylencdianilinc.

[0040] The selected polyamide-imide may preferably bc selected from the group formed by aromatic polyamide-imidc polymers with a molecular mass in the range 10000 to 30000 g/mole and a glass transition température in the range 220°C to 285°C.

[0041J The maximum particle size in the polyamidc-imide powder may be less than 150 gm, 95% of the particles having a size of less than 75 gm.

[0042] The mean volumétrie granulometry of the polyamide-imide particles may be in the range 30 to 40 gm.

[0043] The polar aprotic solvent may be selected from the group constituted by dimethylsulphoxide, sulpholane and γ-butyrolactonc.

[0044] The polar aprotic solvent may be dimethylsulphoxide.

[0045] The présent invention also pertains to a process for producing a dry polyamideimide film on a substrate, characterized in that it comprises:

• preparing a stable aqueous polyamide-imide dispersion in accordance with the first or second variation of the process described above;

• applying said aqueous dispersion to said substrate by spraying;

• curing said aqueous dispersion applied to the substrate at températures in the range 230°C to 325°C.

[0046] Optionally, after curing, the substrate coated thereby may be cooled at ambient température.

[0047] Some characteristics and advantages of the invention are set out in more detail in the description below, made with référence to the accompanying drawings.

[0048] Figure 1 represents a polycarboxylic diimide monomer and an aromatic diamine comonomer, [0049] Figure 2 represents an imide unit on the left and an amie acid on the right;

[0050] Figure 3 represents a 1,3-polyamide-inride configuration on the left and a 1,4polyamide-imide configuration on the right;

[0051 ] Figure 4 represents the granulometry of an aqueous dispersion of polyamide-imide obtained by a process in accordance with the invention;

t [0052] Figure 5 represents the freezîng point for a watcr-dimcthylsulphoxide mixture as a function of the fraction of dimethylsulphoxide.

[0053] The process consists of preparing a ready-to-usc aqueous dispersio n of a polyamideimidc polymer.

[0054] Amorphous polyamide-imide polymers (P Aïs) hâve mechanical bending, impact résistance and résistance at break properties which are superior to other hctcrocyclic polymers, namely polyimides (PI), polyethcrimides (PEI), phenylene polysulphide (PPS) or polyether etherketone (PEEK). It can also bc used to improve extrusion or moulding operations (as well as overmoulding), due in part to their having glass transition températures which arc lower than the corresponding polyimides (Sladc H Gardner “An investigation ofthe structure-propcrty relatîonships for high performance thermoplastic matrix, carbon fibre composites with a tailored polyimide interphase”, 1998).

[0055] The polyamide-imide polymer is preferably selected from aromatic polyamideimides. Aromatic polyamide-imides are of particular interest in producing high tcmpcraturc-rcsistant films and fibres.

[0056] The aromatic polyamide-imide polymer may be synthesized by a polycondensation reaction between a polycarboxylîc diîmide mono mer and a diamine or diisocyanate comonomer in a polar solvent. Figure 1 represents an example with a dibasic carboxylic acid imide on the left and a diamine on the right [0057] Synthesis of the polymer may also involve polycondensation of a diamine and a trimellitic acid anhydride chloride in a bipolar aprotic solvent The reaction between the diamine and an anhydride produces an intermediate product, polyamic acid, which has to be cyclized. Cyclization or imidization of this precursor polymer by a chemical or thermal pathway gives rise to the final polyamide-imide. As can be seen in Figure 2, the precursor obtained may hâve altcmating imide and amie acid units, and preferably an imide unit n

proportion of more than 90%. Figure 2 shows an ùnide unit on the left and an amie acid unit on the right in more detail.

[0058] As can be seen in Figure 3, depending on the diamine selected, the polymer may hâve a 1,3-polyamide-imide configuration, a 1,4-polyamide-imidc configuration or a mixture. Figure 3 shows a 1,3-polyamide-imide configuration on the left and a 1,4polyamide-imidc configuration on the right in more detail.

[0059] For the purposes of the invention, the selected aromatic polyamidc-imide polymer is preferably obtained by a pathway which consiste of reacting a trimellitic acid anhydride chloride and an aromatic diamine selected from compounds other than 4,4’methylenedianilinc, which is classifîcd as a CMR substance.

[0060] The selected polyamide-imidc polymer is preferably selected from the group formed by aromatic polyamide-imide polymers with a molecular mass in the range 10000 to 30000 g/mole, a glass transition température in the range 220°C to 285°C, a molar fraction of polyamide-amie acid of 0.1 or less or an acid index of20 mgKOH/g or less.

[0061 ] The thermal résistance of the polyamidc-imide polymer increases with its molecular mass and its degree of imidization, and so an aromatic polyamide-imide polymer with a high molecular mass will advantageously be selected in order to increase the performance of the film (Robertson G, Polymers 2004, vol 45, pp 1111-1117).

[0062] In an advantageous embodiment, the Applicant proposes preparing an aqueous dispersion from a polyamide-imide powder with a maximum particle size of less than 150 pm wherein 95% of the particles hâve a size of less than 75 pm. Preferably, the mean volumétrie granulomctry is in the range 30 to 40 pm.

[0063] In a first variation and in a first step, the polyamide-imide polymer is dissolved in a polar aprotic solvent with a boiling point of more than 180°C at 760 mm Hg for which the Hansen total solubility parameters, expressed as MPa^l/2, arc close to those of the polyamide imide polymer. The polymer solubili ty parameters, extracted from the European Polymer Journal, volume 22, Issue 10, 1986, pages 821-825, are approximately 3 MPa^1/2.

[0064] In order to ensurc good stability on dispersion, the différence between the solubility parameters, ôsolvent and ôpolymer, should preferably be less than a value of 4 where there arc strong spécifie solvcnt-polymer interactions, as shown by PAI dispersions in a water/NMP mixture, while the respective solubilities of water and NM P arcrcspcctivcly 47.9 MPa^1/2 and 22.9 MPa^1/2.

[0065] The polar aprotic solvents are advantageously selected from the group constituted by the following compounds: dimcthylsulphoxide, sulpholanc and γ-butyrolactone, preferably dimcthylsulphoxide (DMSO) which has a zero-danger classification. In fact, DMSO is known to be one ofthe least toxic compounds. Its uses and applications form a compound that is in common use in the pharmacy field (Parcell S, “Sulfur in human nutrition and applications in medicinc”, Altem Med Rev 2002, vol 7, pp 22-44). DMSO has approximately 40 pharmacobgical properties which may be bénéficiai in the prévention, relief or régression of many diseases (Morton W, “DMSO, Naturc’s Healer”, New York;

Avery 1993).

[0066] Other solvents such as acetone, ethyl acetate, cyclohcxanonc, 13-dioxoIane, mcthylcthyl ketone, tetrahydrofuran or triethylamine, which arc capable of complying with the solubility requirements, arc clearly prohibited in view of their being dassified as highly 20 flammable substances.

[0067] The polyamidc-imide polymer is preferably dissolved in DMSO at a température of more than 50°C with mechanical agitation, preferably at more than 80°C, in order to facilitate dissolution and to avoid rccrystallization phenomena at ambient storage température.

[0068] The proportion of polymer dissolved in the solvent is 35% by weight or less, advantagcously 30% or less in order to providc a kincmatic viscosity which is acceptable in view of the second step.

[0069] In a second step, the solution of polymer, maintained at a minimum température of

50°C, is slowly prceipitated in an aqueous mixture at ambient température. Addition is preferably carried out drop by drop with vigorous agitation and high shear (minimum 7000 rpm) using an IKA® or Ultra Turrax® dispcrscr-homogcnizcr to obtain a maximum grain size of 1 mm. The aqueous mixture contains distilled water and at least one non-ionic surfactant selected from the group constituted by polyoxycthylenated acetylenic diols. The non-ionic surfactant preferably has a hydrophilic-lipophilic balance of 13 or less, preferably 8 or less, in order to rcducc the surface tension at equilibrium and the dynamic surface tension and to ensure that the polymer-solvent System is wetted in the water. The aqueous mixture is free of polyoxycthylenated alkylphenyl ethers.

[0070] The aqueous mixture may also comprise a thixotropic agent which is thcimally stable at températures of250°C or higher selected from the group constituted by organically modified sheet silicates of the laponite, saponite, bentonite or smectite type with a pH in suspension at 2% by weight in the range 9 to 11.

[0071 ] The aqueous mixture may also comprise in the range 0.05% to 0.4% by weight o f a bactericidal/fungicidal agent or a synergy to providc protection during storage and protection of the film selected from the group constituted by the following biocidally active substances: iodopropynyl-butyl-carbamate, benzisothiazolinone, chloromethylisothiazolinone and methyl isothiazolinone.

[0072] The présent invention comprises a third step with the aim of reducing the size of the suspended partides by fine milling using a colloïdal bcad or basket mill. The colloïdal bead or basket mill is preferably a modular vertical System with a recirculation System sold, for example, by Brant Industrie under the trade namc D1SPERMAT®. The diameter ofthe »

ceramic or metallic bcads is 1 mm or less, preferably 0.4 mm or less. Dispersion by mitting is carried out at a minimum rate of600 rpm and at constant power in order to ensure mechanical de-agglomeration of the assodated particles (agglomérâtes and aggregates) into fincr particles and stabilization of the fincr particles produced during dispersion to prevent them from re-agglomerating (or “flocculating). After a purge in compressed air and washing with distitled water, the milting yield is in the range 40% to 95%, preferably more than 90%.

[0073] Altematively, fine mitting may be carried out by recirculating the dispersion in a horizontal colloïdal bead mill with a cooled, pressurized mitting chamber (for exampie an HMP type SUSSMEYER® mill).

[0074] The particle size may be measured in the liquid phase using a CILAS 1064 laser granutometer. The size of the potyamidc-imidc polymer particles in dispersion is preferably less than 20 pm and primarily less than 10 pm. Figure 4 illustrâtes the granulomctry of an aqueous potyamîde-imide dispersion obtained by said process.

[0075] Functional pigment fîllers may be added, preferably during mitling or as a subséquent addition.

[0076] In a second variation, the Applicant proposes preparing an aqueous polyamide-imide dispersion in a first step by dispersion with mechanical agitation (300 to 600 rpm) using a propeltcr mixer in a watcr/DMSO mixture with proportions of water and DMSO varying between 60/40 and 20/80 depending on the quantity ofpolymer to be dispersed. The efïiciency of dispersion is given by the shear rate at constant pcripheral speed as a fonction ofvolume and diameterof the propclter. The température of the mixture is advantageously more than 60°C in order to fâcilitate dispersion and less than 80°C in order to limit évaporation. The aqueous mixture may contain at least one non-ionic surfactant selected from the group constituted by polyoxyethylenated acetylenic diols and high molecular weight block copolymers with groups having a high affinity for the pigments. In a second step, disintegration of the particles and homogenization of the dispersion are ensurcd by milling or an ultrasound treatment (ultrasonication) either in an ultrasound bath with mechanical agitation, or using an ultrasound probe (HIELSCHER® UIP, SON1TUBE®). The frcqucncy and the minimum powcr are respectively 20 kHz and 200 W. In the case in which functional pigment fi 11ers are added during mechanical dispersion, the stability of the dispersion is preferably ensurcd by bead milling.

[0077] The functional pigment fïllers may also be added during milling or post addition.

[0078] The proportion ofpolyamide-imide in the final dispersion îs preferably in the range

5% to 20%. If the concentration is greater than 20% by weight, the fluidity is reduced, which renders application di fïïcult; if the concentration is less than 5% by weight, it becomes more complicated to form a film of suffi oient thickness.

[0079] The proportion of co-solvent with respect to the water in the dispersion is preferably in the range 40% to 80% in order to guarantee good stability during cold storage. Figure 5 illustrâtes the freezing point for a watcr-dimethylsulphoxide mixture as a function of the dimethylsulphoxide fraction.

[0080] The aqueous dispersion may also comprise a modified polyether dimethylpolysiloxane type spreading agent in order to rcduce the surface tension and to improve film formation.

[0081 ] Finally, the aqueous dispersion may comprise an anti-foaming agent such as an émulsion of hydrophobie components containing silicone and parafïinic minerai oils in order to fàcilitate film formation, particularly for high concentrations by volume of pigments.

[0082] In conclusion, the aqueous dispersion prepared thereby contains a minimum of 7% by weight of the polyamide-imide polymer formulation.

[0083] The aqueous dispersion contains no concentrations of a carcinogenic, mutagenic or rcprotoxic substance appcaring on the CIRC (1ARC), ACGIH, NTP and/or OSHA lists in a concentration of 0.1% or higher.

[0084] The aqueous dispersion has a neutre! pH in the range 7 to 73, a dynamic viscosity in the range 0.1 to 1 Pa.s at 20°C dcpcnding on the proportion of polymer and a stability on storage of at least two weeks at a température of-5°C or 40°C.

[0085] The polyamide-imide film may be obtained by pneumatic spraying or cold electrostatic spraying. The aqueous dispersion is sufficicntly thixotropic to allow it to be applied to a vertical surface. The solvents are evaporated off during a drying phase at 80°C. The dry film is formed after a curing phase at températures in the range 230°C to 325°C depending on the polymer, preferably at a température which is at least 10°C higher than the glass transition température ofsaid polymer. Cooling may be carried out at ambient température.

[0086] The DMSO co-solvenL which is moderately volatile and thermally stable at 150°C, is evaporated off completely during the drying step, principally between 40°C and 80°C by elution with water. No décomposition product apart from CO2, anaiysed by ATG-IRTF, is detected during the curing step between 230°C and 325°C. The volatile organic compounds given off during the drying phase, principally DMSO and dimcthyl sulphide (DM S), is readily recyclcd by re-condensation.

[0087] Functional additives or pigment fîllers may be added to the aqueous dispersion, preferably during milling in order, inter alia, to guarantee the lubrication and wcar résistance properties.

Claims (24)

1. !. A process for preparing a stable aqueous polyamide-imide dispersion, characterized in that it comprises at least the following steps:

   • sel cct ing a polyami dc-imide powdcr;

   • producing a dispersion comprising the polyamide-imide powdcr, a polar aprotic solvent with a boiling point of more than 180°C at 760 mm Hg and water.
2. 2. 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 powdcr is dissolved in the polar aprotic solvent with a boiling point of more than 180°C at 760 mm Hg in order to obtain a polyamide-imide solution;

   • a step în which the polymer solution is precipitated in an aqueous mixture at ambient température;

   • a step in which the precipitated solution is dispersed and stabilized by milling the polyamide-imide particles.
3. 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 powdcr is dispersed by mechanical agitation in a homogcncous 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. 4. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding claims, characterized in that the proportion of polyamide-imide is 35% by weight or less.
5. 5. A process for preparing a stable aqueous polyamide-imide dispersion according to claim

   2, characterized in that the step for dissolving the polyamidc-imidc powder is carried out at a température of more than 50°C, preferably more than 80°C.
6. 6. A process for preparing a stable aqueous polyamide-imide dispersion according to daim

   2 or daim 5, characterized in that the précipitation step is carricd out at ambient température in an aqueous mixture containîng distilled water and at least one non-ionic surfactant, the aqueous mixture being free of polyoxyethylenated alkylphenyl ethers.
7. 7. A process for preparing a stable aqueous polyamide-imide dispersion according to daim

   3, characterized in that the step for dispersion of the polyamide-imide powder by mechanical agitation is carried out at a température in the range 60°C to 80°C in a ho mo g en cous mixture containîng distilled water, a polar aprotic solvent and at least one non-ionic surfactant.
8. 8. A process for preparing a stable aqueous polyamide-imide dispersion according to daim

   7, characterized in that the non-ionic surfactant selected from the group constitutcd by containîng groups with a high affinity for pigments preferably has a hydrophilic-lipophilic balance of 13 or less, preferably 8 or less.
9. 9. A process for preparing a stable aqueous polyamide-imide dispersion according to daim

   3 or claim 7 or daim 8, characterized in that the stabilization by ultrasound treatment is carricd out at a minimum frequency of 20 kHz and a minimum power of200 W.
10. 10. A process for preparing a stable aqueous polyamide-imide dispersion according to daim

    2 or daim 3, characterized in that the step for milling the polyamide-imide particles in claim 2 or claim 3 may be carried out by vertical or horizontal bead milling, the milling yield being in the range 40% to 95%, preferably more than 90%.
11. 11. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding daims, characterized in that the size ofthe polyamide-imide polymer *

    particles in dispersion is less than 70 gm, preferably less than 20 pm and primarily less than 10 gm.
12. 12. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding claims, characterized in that the aqueous polyamide-imide dispersion comprises a thixotropic agent which is thermally stable at températures of250°C or more selected from the group constituted by organically modified shcct silicates of the laponitc, saponitc, bentonitc or smcctitc type with a pH in 2% by weight suspension in the range 9 to 11.
13. 13. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding claims, characterized in that the aqueous polyamide-imide dispersion comprises in the range 0.05% to 0.4% by weight of a bactericidal and/or fùngicidal agent for protection during storage and for protection of the film, selected from the group constituted by iodopropynyl-butyl-carbamate, benzisothiazolinonc, chloromethyl-isothiazolinonc and methylisothiazolinone.
14. 14. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding claims, characterized in that the aqueous polyamide-imide dispersion comprises a spreading agent of the polycthcr modified dimethylpolysiloxane type to reduce the surface tension.
15. 15. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding claims, characterized in that the aqueous polyamide-imide dispersion comprises an anti-foaming agent which may be an émulsion of hydrophobie components containing silicone and/or paraffinic minerai oils.
16. 16. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding claims, characterized in that the polyamide-imide (PAI) powder is selected from aromatic polyamide-imide powders.
17. 17. A process for preparing a stable aqueous polyamide-imide dispersion according to claim

    16, characterized in that the selected aromatic polyamide-imide is obtained from a reaction between a trimcllitic acid anhydride chloride and an aromatic diamine selected from compounds other than 4,4’-methyIenediani!ine.
18. 18. A process for preparing a stable aqueous polyamide-imide dispersion according to claim

    16 or claim 17, characterized in that the selected polyamide-imide is preferably selected from the group formed by aromatic polyamide-imide polymers with a molecular mass in the range 10000 to 30000 g/molc and a glass transition température in the range 220°C to 285°C.
19. 19. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of daims 16 to 18, characterized in that the maximum partîdc size in the polyamideimide powder is less than 150 pm, 95% ofthe particles having a size of less than 75 pm.
20. 20. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of daims 16 to 19, characterized in that the mcan volumétrie granulometry ofthe polyamide-imide particles is in the range 30 to 40 pm.
21. 21. A process for preparing a stable aqueous polyamide-imide dispersion according to any one of the preceding daims, characterized in that the polar aprotic solvent is selected from the group constituted by dimethylsulphoxide, sulpholanc and γ-butyrolactone.
22. 22. A process for preparing a stable aqueous polyamide-imide dispersion according to daim

    21, characterized in that the polar aprotic solvent is dimethylsulphoxide.
23. 23. A process for producing a dry polyamide-imide film on a substrate, characterized in that it comprises:

    • preparing a stable aqueous polyamide-imide dispersion according to any one of daims 1 to 22;

    • applying said aqueous dispersion to said substrate by spraying;
24. 24.

    • curing said aqueous dispersion applied to the substrate at températures in the range 230°C to 325°C.

    A process for producing a dry polyamide-imide film on a substrate according to claim 23, characterizcd in that after curing, the substrate coated thereby is cooied at ambient