Classifications

• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK  ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
  • C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
  • C09C1/62—Metallic pigments or fillers
  • C09C1/64—Aluminium
  • C09C1/644—Aluminium treated with organic compounds, e.g. polymers

Definitions

• the present invention relates to a process for producing the deposition of thin polymer layers on the surface of aluminum metal particles, this coating of the aluminum particles leading to the production of aluminum pigment compositions adapted in particular to the formulation of metallized paint, and particularly the formulation of powder-type paints for electrostatic application.
• a "powder-like" metallic paint based on aluminum particle is most often a powdery solid composition which contains a thermosetting mixture of resins and metallic aluminum particles.
• a powder-type paint composition is generally used for the production of coatings on metallic surfaces such as automobile body parts, the application being carried out electrostatically (the powder paint, electrically charged, is deposited on the surface of the object to be covered which is connected to the mass).
• This electrostatic application is most often a problem in the case of powder-type paints containing metal particles, namely that the metal particles tend to separate from the resin particles, especially in view of differences in conductivity and density between these materials, which affects the efficiency and homogeneity of the application, insofar as the metal / resin ratio changes during application.
• This evolution of the metal / resin ratio is further accentuated when the powder paint which is not deposited is recycled and reinjected into the spray system, as is most often the case in the case of electrostatic application.
• the present invention aims to provide a process for coating aluminum particles with polymers which, like the method of Batzilla and Tulke, do not require the use of an organic solvent medium, but which makes it possible to achieve this goal at a reduced cost, and in addition obtaining aluminum particles having a quality at least as good as that of the aluminum particles coated with polymers obtained by processes using solvent media such as those described in US 4,434,009. .
• the subject of the present invention is a process for coating aluminum particles with a polymer layer, which comprises the successive steps of:
• step (B) carrying out an emulsion-type polymerization in the aqueous dispersion obtained at the end of step (A), preferably by introducing into said dispersion monomers whose radical polymerization is initiated by the persulfate anions, or alternatively, by introducing monomers and a polymerization initiator of said monomers into said dispersion, whereby a polymer layer is formed on the surface of the aluminum particles.
• the present invention is based on an unexpected observation made by the inventors, namely that the presence of persulfate anions S 2 O 8 2 " in an aqueous medium placed in contact with metallic aluminum particles induces a decrease in rate of corrosion and oxidation of the surface of the metal aluminum particles by the aqueous medium, which results in particular in a strong inhibition of the formation of an oxidation layer on the surface of the particles and therefore by a maintenance gloss properties of the particles if the particles are not left for too long periods in contact with the aqueous medium.
• step (A) this effect of temporary protection of the aluminum particles by the S 2 O 8 2 " anions discovered by the inventors is exploited to effect the dispersion of the aluminum particles of step (A). ) without the presence of the aqueous medium inducing a decrease in the gloss qualities of these particles
• step (B) definitive protection of the particles from corrosion and oxidation is achieved by the polymer layer that covers the particles.
• the work carried out by the inventors made it possible to establish that the protective effect of the aluminum particles with respect to an aqueous medium conferred by persulfate anions S 2 O 8 2 " is significantly It is important that the effect obtained by using the more elaborate and expensive protective agents which are currently envisaged to obtain such a protective effect, such as the complexing agents of the organophosphate type used in the Batzilla and Tulke article cited above.
• the inventors have demonstrated that, surprisingly, the persulfate anions S 2 Os 2 " make it possible to obtain a protection such that the polymerization step (B) can be carried out at much higher temperatures. with the complexing agents of organophosphate type Batzilla and Tulke.
• the persulfate anions S 2 O 8 2 " which provide the temporary protection of the aluminum particles with respect to corrosion in aqueous medium in step (A) are agents known as radical polymerization initiators which are advantageously usable in step (B) of the process, hence the use of the persulfate anions S 2 O 8 2 " in the process of the invention as a protective agent; is most economically interesting, especially when the monomers of step (B) are compounds whose radical polymerization is initiated by the persulfate anions introduced in step (A): in this case, indeed , the temporary protection against corrosion in step (A) and the initiation of the polymerization in step (B) are provided by one and the same inexpensive compound.
• step (A) of the process of the invention consists in producing an aqueous dispersion of metal aluminum particles in the presence of surfactants, and putting the aluminum particles in contact with S 2 O persulfate anions. 8 2 "
• metal aluminum particles (or “aluminum particles”) is meant, in the sense of the present description, particles comprising aluminum in the metallic state.
• the total amount of elemental aluminum present preferably represents at least 50% by weight, advantageously at least 70% by weight, and even more preferably at least 90% by weight relative to the amount of metal elements present. in said particles.
• the amount of aluminum in the metallic state represents at least 90% of the total amount of aluminum (advantageously at least 95%, and still more preferably at least 98%, and particularly preferably at least 99%, or even at least 99.5%).
• the particles used in the invention are particles essentially based on metallic aluminum, that is to say particles consisting of at least 99.5% by weight (preferably at least 99.7% by weight). % by weight, and even more preferably at least 99.9% by weight) of aluminum in the metallic state. It is also most often particles that have never been in contact with an aqueous medium and which preferably have the highest possible gloss.
• the particles used in step (A) are advantageously anisotropic particles of average dimensions less than or equal to 500 microns, preferably less than 300 microns and preferably less than 100 microns, which behave, schematically, as small reflecting mirrors reflecting the light.
• it is flake-like particles having a mean transverse diameter less than or equal to 500 microns, preferably between 1 and 400 microns, this average diameter being more preferably less than or equal to 250 microns and more advantageously less than or equal to 100 microns, and of average thickness less than or equal to 3 microns, and preferably between 0.1 and 2 microns.
• the particle form factor used in step (A) is between 1/5 and 1/1000, and preferably between 1/10 and 1/100.
• the particles used in step (A) most often have a specific surface area of between 0.5 and 100 m 2 / g, this specific surface area being advantageously between 1 and 10 m 2 / g, for example of the order of 5 m 2 / g.
• aluminum particles that are particularly suitable for carrying out the process of the invention, mention may be made especially of the particles marketed under the name of Alpate by the companies Toyal Europe SA, Toyal America Inc. or Toyo Aluminum KK
• step (A) the presence of persulfate anions S 2 Oe 2 " provides temporary protection of the surface of the particles against corrosion by water.
• These persulfate anions are generally introduced into the medium of step (A) under the form of water-soluble salts, such as, for example, in the form of sodium persulfate, potassium persulfate, and / or ammonium persulfate.
• the persulfate anions provide particulate protection all the more important that their content is high in the medium, when used in small proportions.
• the ratio persulfate / aluminum is at least 1 micromole d persulfate anions, and advantageously at least 5 micromoles of persulfate anions per m 2 of surface developed by the aluminum particles, and that this ratio remains less than or equal to 1000 micromoles of persulfate anions per m 2 , and, most often, at 500 micromoles of persulfate anions per m 2 of surface developed by the aluminum particles.
• this ratio is advantageously between 5 and 100 micromoles (for example between 10 and 50 micromoles) of persulfate anions per m 2 of surface developed by the aluminum particles.
• step (A) the aluminum particles are brought into contact with the persulfate anions S 2 Os 2 " before any contact with water, which makes it possible in particular to avoid at most a degradation of the gloss qualities of the aluminum particles
• the step (A) comprises, first of all, a step of pre-treatment of the aluminum particles with persulfate anions S 2 ⁇ 8 2 " in a non-aqueous medium, then the particles thus treated are then only dispersed in an aqueous medium.
• the non-aqueous medium used in the pre-treatment step may advantageously be an alcohol such as an alkanol, an alkoxyalkanol or a mixture of such compounds, for example.
• Derivatives of the abovementioned alcohols may also be used.
• a pretreatment step of aluminum particles with persulfate anions S 2 O 8 2 " in a non-aqueous medium is, according to the process of the invention, preferably , carried out by contacting salts of persulfate anions S 2 Os 2 - , generally in the pulverulent solid state, with aluminum particles in the form of a paste, that is to say of a concentrated medium in aluminum particles wetted by a non-aqueous medium (dispersion in the non-aqueous medium, generally containing 60 to 80%, for example 65 to 75% by weight, of aluminum particles).
• such a paste of aluminum particles in a non-aqueous medium can be made starting from aluminum particles in the form in which they are most commonly marketed, namely in the form of dispersions in a solvent such as white spirit.
• An aluminum paste in a given non-aqueous solvent can be obtained by filtering these commercial-type dispersions and then washing the resulting filter cake with the given non-aqueous solvent. After several washes, a wet filter cake is obtained which is in the form of a paste of aluminum particles having the same characteristics as in the initial solvent, but dispersed in another medium, such as an alcohol.
• the process of the invention is carried out according to this particular mode, it is most often advantageous for the filtration steps to be carried out without drying the aluminum particles, that is to say always preserving the particles in the form a dispersion or a wet cake, preferably ensuring that at no time the aluminum content in the dough does not exceed 80% by weight, this content advantageously remaining less than or equal to 75% by weight.
• This precaution makes it possible in particular to avoid interparticle agglomeration.
• step (A) does not comprise a step of specific pre-treatment of the particles by the anions S 2 Os 2 ' prior to contacting the particles with the aqueous medium .
• step (A) generally consists in directly producing the dispersion of the aluminum particles in an aqueous medium in the presence of persulfate anions and surfactants, step (A) then being preferably carried out at a temperature below 60 ° C., and preferably below 50 ° C.
• the dispersion of step (A) is advantageously carried out by dispersing the aluminum particles within an aqueous medium initially containing the persulfate anions, avoiding contacting the aluminum particles with an aqueous medium not containing the persulfate anions, so as to avoid corrosion by the aqueous medium. Nevertheless, it is not excluded to temporarily contact the aluminum particles with an aqueous medium containing no persulfate anions, provided however that this contacting is carried out for a sufficiently short time and at a temperature sufficiently low to prevent corrosion of the particles, which would induce in particular a degradation of the brightness qualities of the particles.
• step (A) comprises intermediately temporary contacting of the aluminum particles with an aqueous medium in the absence of persulfate anions
• this contacting is preferably carried out at a temperature less than 60 ° C., more advantageously less than 50 ° C. and even more advantageously less than 40 ° C.
• the contact time between the aqueous medium and the aluminum particles in the absence of persulfate anions is advantageously the most low possible, especially when the temperature is high.
• this contact time is preferably of the order of a few minutes at the most.
• this contact time is preferably less than or equal to 5 minutes. This contact time can nevertheless be higher, especially if the temperature is lower.
• a contact time of up to 20 minutes can be envisaged.
• step (A) the presence of additional agents providing protection of the surface of the particles with respect to the water, other than the persulfate anions are certainly not excluded, but it is absolutely not required. Therefore, most often, especially for reasons of cost, step (A) is conducted in the absence of any agent providing protection of the aluminum particles with respect to water, other than anions persulfate.
• step (A) the dispersion of the aluminum particles is specifically carried out by means of surfactants.
• surfactants chemical species with an amphiphilic character, that is to say having hydrophobic zones and zones of a more hydrophilic character, these chemical species being capable of modifying the surface tension between the particles. of aluminum and the aqueous medium when they are introduced into the medium in sufficient quantity.
• these surfactants allow dispersion of the particles in the aqueous medium.
• the surfactants used are advantageously ionic surfactants.
• zones of hydrophobic nature correspond, schematically, to a grouping of hydrophobic zones of the surfactants at the periphery of the aluminum particles, the surfactants being organized generally around the aluminum particles in the form of a lipid bilayer type system. , or the hydrophilic parts of the surfactants orient themselves towards the surface of the particles and towards the aqueous medium and where the hydrophobic parts are oriented towards the inside of the system. It is in these zones of hydrophobic nature that the "emulsion" type polymerization of step (B) will then take place.
• the nature of the surfactant used generally influences the size and the stability of the hydrophobic zones obtained around the aluminum particles and, consequently, the conditions of the polymerization of step (B).
• the aluminum particles used in stage (A) are particles which initially have on their surface molecules comprising hydrocarbon chains, these molecules preferably being fatty acids, such as acid molecules.
• fatty acids such as acid molecules.
• stearic, oleic acid, isostearic acid, or lauric acid which are commonly used for the preparation of aluminum flakes, especially according to conventional methods, such as the "Hall" method.
• the surfactants added in step (A) form a lipid bilayer type system with the initially present layer of fatty acids, which has two advantages in particular.
• the fatty acids initially present on the surface of the aluminum particles constitute "primers" for the formation of the lipid bilayer around the aluminum particles already present on the particle, which generally makes the formation of the lipid bilayer type system around the aluminum particles.
• the initial presence of hydrocarbon chains on the surface of the aluminum particles also limits the amount of surfactants to be used, which is reflected in particular in terms of reduced operating costs.
• the aluminum particles used in step (A) may also be more specific particles, such as, for example, aluminum particles having, on the surface, hydrocarbon chain-type groups covalently bound to the surface of the particles.
• aluminum such as those described in FR 02 12273, for example.
• the aluminum particles may carry on the surface groups capable of participating in the polymerization of step (B). So, these groups may, for example, carry functional groups which confer on them properties of initiation of the polymerization.
• These groups may also be unsaturated hydrocarbon chains capable of acting as monomers in step (B), the presence of such monomer groups on the particles then serving as initial anchoring point for the polymers on this surface of the particles. whereby a particularly effective anchoring of the polymer chains formed in step (B) on the particles is obtained.
• the exact nature of the surfactants to be used in step (A) can vary to a rather large extent.
• the surfactants used are long-chain carbon ionic surfactants. It is preferably tetraalkylammonium halides (preferably bromides) in which at least one of the 4 alkyl chains contains from 6 to 20 carbon atoms, and preferably from 10 to 18 carbon atoms, these tetraalkylammonium halides. being more preferably selected from halides di (C 6 -C 2 o) -dimethyl-ammonium.
• the surfactant of step (A) are selected from di bromides alkyl (Cio-Ci 8) -dimethyl-ammonium.
• DODAB didodecyldimethylammonium bromide
• DDAB didecyldimethylammonium bromide
• Step (B) of the process of the present invention consists in carrying out an emulsion-type polymerization within the dispersion obtained at the end of step (A).
• this polymerization can schematically be considered analogous to an emulsion polymerization of conventional type, where the polymerization is carried out in micelles of surfactants dispersed in an aqueous medium.
• Step (B) is substantially a polymerization of this except that the micelles conventionally used are replaced by larger systems consisting of aluminum particles surrounded by surfactants.
• step (B) leads to polymer formation in the hydrophobic areas surrounding the aluminum particles, thus leading to encapsulation aluminum particles by a polymer layer.
• step (B) the usual conditions of an emulsion polymerization of conventional type can generally be used in step (B).
• step (B) the polymerization essentially leads to polymer formation in the form of a layer around the particles and that A minimum of free polymer chains is formed within the aqueous medium, ie, elsewhere than on the surface of the particles.
• the formation of such free chains involves the implementation of a larger amount of monomers to obtain effective coverage of the aluminum particles.
• an interesting solution consists in using the surfactants at a concentration such that the residual concentration of free surfactants in the medium obtained at the end of step (A) is below their critical micellar concentration, whereby no micelles are formed in the medium, in which the emulsion polymerization could take place.
• the monomers in a progressive and controlled manner during step (B). preferably by jointly introducing one or more initiators, also in a progressive and controlled manner.
• the monomers used are monomers which have an affinity for the lowest water possible, so that they migrate and polymerize preferentially in the zones hydrophobic medium.
• the monomers of step (A) are chosen so that the polymer layer formed is a transparent layer, which advantageously modifies as little as possible the metallic appearance and the brightness of the initial particles. .
• the monomers used to be monomers whose radical polymerization is initiated by the persulfate anions introduced in step (A): in fact, in this case, the present process is the advantage of not requiring the addition of additional polymerization initiators, which is reflected in particular in terms of reduced costs.
• advantageous monomers for the implementation in step (B) are, for example, acrylate and / or methacrylate monomers, preferably acrylates or metahrylates comprising from 2 to 15 carbon atoms, advantageously from 2 to 10 carbon atoms, such as acrylates and / or alkyl methacrylates, preferred (meth) acrylates being acrylate and / or butyl methacrylate, acrylate and / or ethyl methacrylate, acrylate and and / or methyl methacrylate, acrylate and / or 2-ethylhexyl methacrylate, and mixtures of these monomers.
• acrylate and / or methacrylate monomers preferably acrylates or metahrylates comprising from 2 to 15 carbon atoms, advantageously from 2 to 10 carbon atoms, such as acrylates and / or alkyl methacrylates, preferred (meth) acrylates being acrylate and / or butyl methacrylate, acryl
• step (B) can be carried out at relatively high temperature without impairing the brightness properties of the initially introduced aluminum particles, although this step is conduct in an aqueous medium.
• step (B) can be carried out at a temperature greater than 60 ° C., which is particularly surprising in view of the results obtained in the state of the technique where the implementation of such temperatures was not possible for a process conducted in an aqueous medium.
• step (B) can be conducted in a very wide range of temperatures, typically between 15 and 100 ° C., step (B) being advantageously conducted between
• step (B) allows in particular to benefit from a significant margin of maneuver when it is desired to adapt the conditions of the polymerization of step (B), in the extent that the wide range of possible temperatures allows the use of many monomers, with a possible modulation of the conditions of their polymerization.
• the process of the present invention is a particularly alternative to the polymer coating process of aluminum particles known from the state of the art. It also has the advantage of leading to particularly interesting pigment compositions.
• These pigment compositions that can be obtained according to the process of the invention constitute, according to a particular aspect, another object of the present invention.
• These pigment compositions generally comprise persulfates, at least in trace amounts, or persulfate derivatives used in the process.
• the pigment compositions that can be obtained according to the process of the invention are in the form of a powder, which can in particular be obtained by filtration and drying of the aqueous medium obtained at the end of step (B). ) of the process of the invention. They may also be in the form of a dispersion of the polymer-coated particles in an aqueous or non-aqueous medium, these dispersions preferably being concentrated dispersions in the form of a paste typically containing from about 60 to 90 % by weight (for example 65 to 85% by weight) of particles.
• the pigment compositions obtained according to the method of the present invention are suitable for many fields of application. Indeed, these pigment compositions based on aluminum particles coated with a polymeric protective layer generally have a high resistance to oxidation and corrosion by water and air, associated with a very good compatibility with polymeric materials, such as, in particular, resins. These compositions can therefore be used in particular for the formulation of paints or inks with metallized appearance, these formulations can be aqueous, or for the preparation of metallized plastics. More specifically, the pigment compositions obtained according to the process of the present invention are particularly well suited for the formulation of metallized-aspect paints, in particular for the formulation of metallized powder-type paint, of the type intended for an electrostatic application.
• Powder-like metallized paint compositions which comprise thermosetting resin particles and a pigment composition obtainable by the process of the present invention as a metallized pigment are another specific object of the invention. the invention.
• Metallic-based paint compositions based on water which contain a pigment composition that can be obtained according to the process of the invention in an aqueous medium constituting yet another subject of the invention.
• DODAB didodecyldimethylammonium bromide
• the mixture obtained was stirred for 3 minutes on a magnetic stirrer and then added to a 2L reactor containing 500 ml of demineralized and degassed water, stirred at 350 rpm and heated to 80 ° C.
• the beaker was rinsed with 0.3 L of degassed water and the washings were introduced into the reactor.
• the medium obtained was stabilized maintained at 80 ° C with stirring at 350 rpm for 10 minutes.
• Step (b1) Polymerization Within the Dispersion Performed
• step (a1) 2 ml of butyl methacrylate (MABu) were introduced over one hour, with a constant flow rate of 2 ml per hour. After the complete addition of the 2 mL of MABu, the medium was allowed to stir for 10 minutes.
• MABu butyl methacrylate
• the contents of the reactor were drained into a beaker containing 2 L of demineralized water and the medium obtained was stirred overnight (12 hours) until cooling.
• the medium was then filtered on frit and rinsed with 2L of demineralized water.
• the particles recovered on the filter were dried at 20 ° C. under vacuum for 2 hours and they were then placed in an oven at 40 ° C. for 7 days.
• the preparation of the dispersion was performed by using the pre-dispersion of aluminum particles in Dowanol ® PM (1-methoxy-2-propanol), which improves the quality of the dispersion of aluminum particles prior to their coating by the polymer layer.
• DDAB didecyldimethylammonium bromide
• the mixture thus obtained in the beaker was stirred for 3 minutes on a magnetic stirrer, then it was added to a 3 L reactor containing 500 mL of demineralized and degassed water, stirred at 250 rpm, and brought to room temperature. 7O 0 C. the beaker was rinsed with 0.5 L of degassed water and the washings were introduced into the reactor.
• the medium obtained was maintained at 70 ° C. with stirring at 250 rpm for 2 minutes.
• step (a2) 16 ml of MABu were introduced, with a constant flow rate of 8 ml per hour. After complete addition of the 16 mL of MABu, the medium was allowed to stir for 40 minutes.
• the reactor was then heated to a temperature of 90 ° C. and the medium was left at this temperature for 90 minutes.
• the contents of the reactor were drained into a beaker containing 2 L of demineralized water and the medium obtained was stirred overnight (12 hours) until cooling. The medium was then filtered on frit and rinsed with 2 L of demineralised water. The particles recovered on the filter were placed in an oven at 40 ° C. for 2 days.
• the preparation of the dispersion was performed by using the pre-dispersion of aluminum particles in Dowanol PM ® as in Example 2.
• this dispersion without intermediate drying the aluminum particles, in particular to minimize interparticle agglomeration phenomena.
• a paste of dispersed aluminum particles of white spirit ("grade 7601 NP" sold by the Toyal company: dispersion of lenticular particles having a d50 of 23 microns).
• grade 7601 NP sold by the Toyal company: dispersion of lenticular particles having a d50 of 23 microns.
• a paste containing 67% by weight of aluminum particles and 35% by weight of Dowanol® PM was obtained.
• the particle size of the aluminum particles in the dough thus obtained is the same as that of the initial standard dough.
• This paste was used for producing a dispersion of aluminum particles that can be used according to the method of the invention under the following conditions.
• the mixture thus obtained in the beaker was stirred for 3 minutes on a mechanical stirrer (motor unit driving a turbine type blade at 300 rpm), then spent 30 seconds with ultrasound.
• the mixture thus produced was introduced into a 3 L reactor containing 1 L of demineralized and degassed water, stirred at 300 rpm and heated to 70 ° C.
• the beaker was rinsed with 0.75 L of water. degassed water and the washings were introduced into the reactor.
• the medium obtained was maintained at 70 ° with stirring at 300 rpm for 10 minutes.
• Step (b3) Polymerization Within the Dispersion Performed
• step (a4) 10 ml of MABu were introduced, with a constant flow rate of 10 ml per hour. After the complete addition of the 10 mL of MABu, the medium was allowed to stir for 180 minutes. The heating was then stopped and the reactor allowed to cool with stirring. After this cooling, the contents of the reactor were drained, and the medium was sieved at 500 and 200 microns, then filtered on frit and washed with 10L of demineralised water. The particles recovered on the filter were dried in an oven at 40 ° C. for 15 days. At the end of these different steps, 233 g of aluminum particles (passing at 200 ⁇ m) coated with a polymer layer were obtained.
• Example 3 the preparation of the dispersion in a manner similar to Example 3, starting from a paste of aluminum particles dispersed in white spirit, grade 7601 NP, where the white spirit was substituted by Dowanol PM ® , by a succession of mixtures and filtrations, without intermediate drying of the particles.
• the solvent exchange in the pigment paste was carried out under the same conditions as in Example 3, with the difference that, during the last mixing, DDAB was added to the medium in the form of an aqueous gel containing 75% by weight (ie 15 g) of DDAB, and that the paste obtained after the last filtration was brought to the oven at 40 ° C. until a content of
• the DDAB is partially fixed on the aluminum particles, and the excess of this surfactant, not localized on the particles, is essentially eliminated during the filtration steps.
• the paste thus obtained was used for the production of a dispersion of aluminum particles that can be used according to the process of the invention under the following conditions, similar to those of Example 4.
• the mixture was mixed under stirring, 600 mL of degassed water at 70 ° C. and 6 g of an aqueous gel containing 75% by weight of DDAB, then 10 g of persulfate of potassium.
• 100 g of the previously prepared aluminum paste was gradually added, and still stirred.
• the mixture thus obtained in the beaker was stirred for 3 minutes on mechanical stirrer and then spent 30 seconds with ultrasound.
• the mixture thus produced was introduced into a 3 L reactor containing 1 L of demineralized and degassed water, stirred at 250 rpm and heated to 70 ° C.
• the beaker was washed with 0.4 L of water. degassed water and the washings were introduced into the reactor.
• the medium obtained was maintained at 70 ° C. with stirring at 250 rpm for 2 minutes.
• step (a4) 10 ml of MABu were introduced, with a constant flow rate of 10 ml per hour. After the complete addition of the 10 mL of MABu, the medium was allowed to stir for 20 minutes.
• the contents of the reactor were drained, and the medium was screened at 500 and 200 microns, then filtered on frit and washed with 3L of demineralized water. The particles recovered on the filter were dried in an oven at 40 ° C. for 15 days.
• Example 4 the particles obtained at the end of Example 4 were compared with the initial particles of grade 7601 NP, within a blue formulation.
• an identical amount of metallic aluminum was introduced and the colorimetric coordinate b (in the CIE Lab system), which corresponds to the axis from yellow (positive values of b) to blue (values negative b's).
• b the colorimetric coordinate
• the blue formulation obtained from the standard paste of grade 7601 NP has a value of b of -29.9 + 1, 0, whereas the blue formulation obtained from the particles synthesized at the end of Example 4 has a value of b of -29.3 + 1.0.
• the coated aluminum particles visually exhibit optical characteristics similar to those of the initial aluminum particles.

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