WO2007110185A1

WO2007110185A1 - Method for coating solid surfaces

Info

Publication number: WO2007110185A1
Application number: PCT/EP2007/002507
Authority: WO
Inventors: Giovanni Manfre
Original assignee: Nanophoton S.R.L.
Priority date: 2006-03-24
Filing date: 2007-03-21
Publication date: 2007-10-04
Also published as: ITMI20060560A1

Abstract

A method for forming functional coatings on solid surfaces by means of liquid suspensions containing micro- or nanoparticles, an apparatus for performing the method, a coating obtained with the method and a solid surface coated with the method according to the invention.

Description

METHOD FOR COATING SOLID SURFACES Technical Field

The present invention relates to a method for forming functional coatings on solid surfaces with liquid suspensions containing micro- and/or nanoparticles, to an apparatus for carrying out said method, to a coating obtained with said method, and to a solid surface coated with the method according to the invention. Background Art

Coatings with liquid suspensions containing micro- or nanoparticles (hereinafter referred to as LSMN) are useful for giving a particular functional performance to the coated surfaces. Coatings with LSMN are used for example to give hydrophobic and/or hydrophilic behavior (for example to achieve self-cleaning of coated surfaces) and resistance to one or more among abrasion, printing, fingerprinting, environmental chemical stability, UV degradation.

Conventional processes for coating solid surfaces can be divided into two large categories: coatings which do not form droplets and coatings which form droplets.

The methods of the first group are the most widespread and substantially comprise so-called "immersion" techniques (dip coating, rod coating, knife coating, blade coating, gravure coating, roll coating, slot and extrusion coating, slide coating, curtain coating). Immersion methods suffer several drawbacks, including agglomeration (clustering) of particles during drying, which makes the coating less uniform, the inability to obtain uniform coatings (especially in terms of thickness uniformity), difficulty in coating surfaces having complex geometric shapes or microporous surfaces, a situation in which it is easy to lose most of the applied suspension, with a corresponding increase in industrial costs; finally, they are scarcely versatile because they do not allow to obtain "hard" coatings. Methods of the second group usually generate droplets by using spray systems. They comprise a first step of mixing the particles with the liquid vehicles to form a colloidal liquid suspension. Mixing is then followed by the formation of droplets, which as mentioned occurs by means of spray systems with nozzles, and by the deposition of the droplets on the solid surfaces to be coated. Finally, evaporation, drying or hardening of the suspension medium occur, forming the film. During the deposition and evaporation, drying or hardening steps, the microdroplets coalesce, leading to the formation of the final coating.

A first drawback of spray techniques resides in that the physical stresses to which the suspensions are subjected during spraying from the nozzle (generation of high-pressure streams through said nozzle) inevitably cause a separation of phases which makes the suspension lose the uniformity of dispersion which is instead necessary in order to obtain coatings having uniform and constant thickness and physical properties. Uniformity of dispersion is required in particular in the case of film coating of glass-like surfaces, in which it is necessary to maintain the transparency of the glass, a characteristic which however is compromised by particle clustering or uneven dispersion.

A second drawback of spray techniques resides in the physical mechanism by means of which they produce the droplets. The diameter of the droplets normally depends on several factors, including intrinsic chemical-physical parameters of the suspensions (such as viscosity, density, surface tension), process parameters (such as the internal and external radius dimensions and length of the nozzle and the flow-rate (volume/time) of the stream through the nozzle), and the percentages by weight or volume of the micro- and nanoparticles in the suspension.

In conventional spray systems, the droplet forms along the fluid thread (jet) which exits at high pressure from the nozzle, specifically at the distance at which the amplitude of the oscillations encountered by the jet exceeds the diameter of such jet. However, in order to produce droplets of useful size, conventional spray systems require extremely high operating pressures (up to approximately 20,000 atm). For example, in order to form microdroplets of water with a diameter on the order of 10 μm, with a nozzle having a diameter of 20 μm and a length of 200 μm, the required pressure is 5,000 atm. With liquids having a higher viscosity than water, or to obtain smaller diameters, this pressure increases further. Moreover, an attempt to reduce operating pressures entails producing droplets having an excessively large diameter, such as to contain so much suspension medium that the evaporation or drying of said suspension medium from the deep layers of the film being formed produces porous coatings. It has been demonstrated that porosity worsens over time the physical and mechanical properties of the coating (lifetime).

In order to reduce the pressures, dynamic spray systems have been studied which amplify the fluid dynamic oscillations of the jet (for example by means of pumps with piezoelectric injectors) or, as an alternative, systems have been studied in which the jet is atomized by making it strike (splushing) against the walls of the system or against air streams which are present in the system.

One particularly appreciated solution is to atomize the jet in static environments, generating droplets having a diameter of less than 3 μm by using ultrasonic transducers (Ultrasonic Atomizing Transducers) which generate ultrasonic frequencies which have a very high resonance (above 2 MHz) and act on the jet as it leaves the nozzle. However, such systems require tens of watts of power, low liquid flow-rates (at the most 400 cc/hour), and are not effective for vaporizing suspensions. In particular, it has been determined experimentally that ultrasonic transducers are not suitable in spray systems for vaporizing LSMNs, because the introduction of particles in suspension either prevents the formation of droplets or causes rapid nozzle clogging. Disclosure of the Invention The aim of the present invention is therefore to provide a method for producing functional coatings of solid surfaces which overcomes the drawbacks of the background art.

An object of the invention is to provide a method for producing functional coatings of solid surfaces starting from liquid suspensions containing microparticles and/or nanoparticles (LSMN) which allows to obtain coatings which are uniform in terms of thickness, density and properties imparted to the coated surface.

Another object of the invention is to provide a method as above which converts the LSMNs into droplets while maintaining the homogeneous distribution of the LSMNs inside the droplets, said method allowing to also coat surfaces which are scarcely accessible due to geometric reasons and allowing to obtain soft and hard coatings; a method which, without resorting to high operating pressures, converts the suspensions into vapors of droplets having a diameter which is small enough to avoid clustering or formation of porosities in the final film, said method minimizing suspension wastes, allowing to work with flow-rates of liquid which are compatible with industrial practice and allowing, if applied to glassy surfaces, to maintain their transparency if required. Another object of the invention is to provide a method as above which allows to obtain coatings which are thinner than currently obtainable, and are multifunctional site-specific coatings (i.e., capable of imparting different properties depending on the area).

Another object of the invention is to provide an apparatus for carrying out the method cited above.

Another object of the invention is to provide a coating which is prepared with the method cited above.

Still another object is to provide a surface which is coated externally with a coating prepared with the method cited above. This aim and these and other objects are achieved by a method as defined in appended claim 1.

The aim and objects of the invention are also achieved by a coating as defined in appended claim 13.

The aim and objects of the invention are also achieved by a solid surface which is coated externally with a coating as defined in appended claim 15.

The aim and objects of the invention are also achieved by an apparatus as defined in appended claim 18.

It is understood that any characteristic cited with reference to only one of the aspects of the invention but can refer also to other aspects must be considered equally valid as regards said other aspects even though it is not repeated explicitly. Brief Description of the Drawings

The present invention is described with reference to Figures 1-5, which are provided here merely by way of non-limiting example and wherein:

Figure 1 is a block diagram of an embodiment of an apparatus for carrying out the method according to the invention, from which the steps of the method according to the invention can also be deduced; Figure 2 is a diagram of the annular magnets used in a preferred embodiment of the invention to polarize an LSMN suspension; and

Figures 3-5 are a sectional view (Figure 5) and perspective views (Figures 3 and 4) of a diagram of the two-cylinder capacitor used in a preferred embodiment of the invention for producing gaseous ions. Ways of carrying out the Invention

In a first aspect, the invention relates to a method for providing functional coatings on solid surfaces which comprises the steps of: i) polarizing, by means of one or more magnetic fields, a liquid suspension which comprises at least one liquid suspension medium and at least one type of micro- and/or nanoparticles suspended in said suspension medium, ii) subjecting an ionizable gas, preferably air, to the action of one or more electric fields in order to ionize one or more components which are present in said gas, iii) mixing the ionized gas of step ii) with the polarized suspension of step i) inside one or more containers, each container comprising at least one ultrasonic transducer, where steps i) and ii) can occur in any mutual order, preferably simultaneously and in parallel, step iii) must occur after the two preceding steps, and step iii) determines the vaporization of the liquid suspension.

Although one does not wish to be constrained to any particular theory or physical phenomenon, for the sake of full comprehension of the invention the Inventor believes that it is useful to consider the following.

A liquid which is already in itself polar or polarizable has a molecular state which, in normal conditions, has the polarization vectors of the individual molecules oriented chaotically and randomly. However, when the liquid passes within strong magnetic fields, as in the present invention, its polarization becomes uniform. The intensity of these magnetic fields can be selected according to the degree of polarization that one wishes to impart to the suspension, i.e., the degree of "order" that one wishes to produce within the suspension. In this regard, it can be seen that any presence of micro- and nanoparticles suspended in the liquid might interfere with the external magnetic field depending on the dielectric and paramagnetic properties of the material of which they are made. However, due to matters related to energy stability, the polarization vectors of nano- and microparticles tend in any case to orient themselves so as to add to each other and increase the intensity of the external field.

Step iii) as described above determines the vaporization of the suspension following the mechanical, physical and chemical impact between ionized gas (preferably ionized air) and the polarized liquid, in the presence of ultrasound.

In the invention, the expression "vaporization" or "atomization" or "nebulization" is understood to reference the conversion of the suspension into a mist composed of droplets having dimensions preferably ranging from 0 to 3 μm, lower limit not included.

The impact between ionized gas and polarized liquid is facilitated by the ultrasound of the one or more containers, which accordingly become resonant cavities activated by the ultrasound transducer. However, said transducer can, for example, be located within the walls of the container or on its bottom.

An essential aspect of the present invention resides in that the vaporization of the LSMNs is achieved without resorting to nozzles or to the high pressures which are typical of spray systems, so that there is no risk of causing phase separations between the liquid component of the suspension and the particulate component, which have different densities. Very low pressures, not comparable with the pressures required to form the jet in classic spray systems, are required to make the suspension flow in the ducts which run through the magnetic fields meant to polarize said suspension. In one embodiment of the invention, the method comprises an additional step iv), which follows step iii), of conveying and depositing the atomized suspension on a solid surface to be coated.

In one embodiment, the method according to the invention comprises an additional step v), which follows step iv), of causing the solidification of the LSMN suspension deposited on the solid surface to be coated. LSMN liquid suspensions can be made to solidify for example by simple cooling, since the diameter of the droplets is so small as to cause an extremely swift evaporation of the suspension medium contained therein.

As an alternative, it is possible to determine solidification by triggering, for example by means of thermal or UV radiation, chemical reactions (catalysis) which involve one or more components possibly added in an initial step to said suspensions and vaporized together with the nano- and microparticles. These reactions produce a matrix inside which the micro- and nanoparticles then remain embedded. In the embodiment in which one wishes to achieve solidification by chemical reaction, it is therefore advantageous to add to the suspension at least one optional ingredient, which if stimulated adequately (for example by cooling, thermal radiation or UV radiation), produces the solidification reaction. These optional ingredients are for example selected among liquid and non-liquid monomers and prepolymers (homo- and co-prepolymers), capable of polymerizing once they have been deposited if they are subjected to radiation (such as UV and thermal radiation). Optionally, it is also possible to facilitate polymerization also by adding chemical catalysts, also to be added during the initial step to the suspension. As an alternative, it is possible to avoid adding other ingredients to the suspension and select a suspension medium having a chemical structure which is capable of solidifying if it is stimulated adequately (for example by cooling, thermal radiation or UV radiation).

With the method according to the invention, functional coatings are thus produced which have uniform thicknesses which range preferably from 5 run to 6 μm and are characterized by complete absence of porosity.

The term "functional" means that the coating according to the invention gives particular physical or chemical properties to the coated surface, wherein said properties are a function of the quantity and quality of the deposited nano- and microparticles. In this regard, it should be noted that the method according to the invention allows to impart different properties to a same surface in a region-specific manner. In other words, by varying the quantity and quality of the composition of nano- and microparticles deposited in each region of the surface, it is possible not only to modify the thickness of the coating and therefore the "quantity" or intensity of the imparted property, but also to modify the "quality" of said property. For example, it is possible to obtain coated surfaces in which one part is water-resistant, one part has acid corrosion resistance properties, and a third part has the solar energy capture properties which are typical of a photovoltaic panel.

The expression "to ionize a gas" references, in the invention, the negative charging of one or more components of said gas. In the preferred case in which the gas is air, this ionization involves oxygen. The ionization of one or more components depends substantially on the selected intensity of the electric field generated by the capacitor or capacitors, as described in greater detail hereafter.

The expression "surface" or "solid surface to be coated" references a surface made of plastic elements (including transparent plastics), metal, ceramic, and preferably glass, of any shape, form and size. The expression "liquid suspensions containing microparticles and/or nanoparticles" references a suspension which comprises one or more liquid suspension media and at least one type of microparticles or nanoparticles.

The expression "suspension medium" references a liquid which is unable to dissolve the particles that one intends to use. Once the type of particle to be used has been selected, the person skilled in the art will know immediately and easily how to select accordingly the suitable suspension medium or mixture of suspension media. One preferred suspension medium is water or mixtures containing water. It should be noted that the suspension medium must be polarizable, but it does not need to already have polarity inherently, since the action of the magnetic fields to which the suspension is subjected is capable not only of orienting dipoles which are already present but also of generating induced dipoles in normally nonpolar media.

The terms "microparticles and nanoparticles" reference particles which have micrometer- and nanometer-size, are metallic, organic or ceramic, such as for example paint pigments. Preferred particle types are particles of ITO, ATO, TiO₂, CaCO₃, silica, silicon, and mixtures thereof. Preferred types of micro- and nanoparticles are conventional micro- and nanoparticles for producing coatings for photovoltaic panels, particularly micro- and nanoparticles used in the so-called "quantum dot" technology. The expression "polarization of an LSMN suspension" references the induced orientation of the dipoles that are present in the suspension, be they inherent to the molecules of the suspension medium (as for example in the case of water) or be they inherent to suspended nano- and microparticles (as in the case in which the suspension medium is inherently nonpolar). The expression "magnetic fields" references magnetic fields generated by means I) as described hereinafter with reference to the apparatus according to the invention.

The expression "electric fields" references electric fields generated by one or more capacitors II) as described hereafter with reference to the apparatus according to the invention.

In a highly advantageous embodiment of the invention, the range of percentages by volume of the particles that can be used in LSMNs is from 10 to 74% on the total volume of the suspension. If one wishes to obtain transparent coatings, it is preferable to resort to low percentages, ranging advantageously from 10 to 20% by volume on the total volume of the suspension.

In a further aspect, the invention relates to an apparatus for carrying out the method described above, said apparatus comprising:

I) means, preferably one or more magnets, which are adapted to generate magnetic fields for polarizing a liquid suspension containing nano- and/or microparticles (LSMNs) as defined above.

II) one or more capacitors adapted to generate electrical fields for ionizing a gas,

III) one or more containers comprising at least one ultrasonic transducer, and IV) means adapted to connect the means I), II) and III) mentioned above and to convey the polarized liquid suspension and the ionized gas into the one or more containers III), where the means I) and II) are preferably arranged in parallel and the means III) are arranged downstream of both means I) and II).

In one embodiment, the apparatus also comprises means V), which are arranged downstream of the means III) and are adapted to convey the atomized LSMN suspension onto a solid surface to be coated. The surface to be coated can assume any mutual position with respect to the direction of the fluid of droplets which exits from the container III), but is preferably in a vertical position.

In one embodiment, the apparatus also comprises means VI), which are arranged downstream of the means V) and are adapted to determine the forced solidification of the LSMN suspension deposited on the solid surface to be coated. For example, the means VI) can be means for heating the solid surface to be coated or means for irradiating the solid surface to be coated with UV radiation.

In one embodiment, the apparatus also comprises means VII), which are arranged upstream of the means II) and are connected thereto and are adapted to introduce by forcing the gas to be ionized within the ionizing means II). The means VII) are constituted for example by a pumping system. The means VII) can also be used to facilitate the exit of the atomized suspension from the one or more containers, directing it toward a solid surface to be coated. By way of example, the apparatus is now described with particular reference to Figure 1, in which the gas to be ionized is air (and in which therefore ionization determines the production of charged ozone), wherein:

- the element 1 is a container in which the LSMN is contained after the step (not shown) for mixing nano- and/or microparticles with the liquid suspension medium, and optionally with the other ingredients which might be necessary for physical or chemical curing of the final coat;

- the elements 2 and 3 are respectively magnets 2, which are adapted to generate the magnetic field which determines the polarization of the LSMN, and ducts 3 for conveying the polarized suspension to the container 7 in which the suspension is to be mixed with the ionized air. Along the ducts 3, polarization can be maintained or, if so wished, even increased by means of the presence of additional optional magnets (not shown in the figure);

- the elements 4 and 5 are a highly advantageous embodiment of a capacitor according to the invention, in which said capacitor is divided into two distinct regions (4 and 5 respectively). This embodiment is described in greater detail hereinafter;

- the element 6 represents means for controlling the percentage of air which enters the two-cylinder capacitor and the percentage of charged ozone which exits from the ozonizer. The element 6 is preferably an electronic circuit;

- the element 7 represents the container inside which the ultrasonic transducer 8 is arranged and in which the impact between the polarized liquid suspension that arrives from the element 3 and the ionized air that arrives from the element 5 occurs;

- the element 9 represents the solid surface to be coated, onto which the atomized LSMN suspension is conveyed in output from the container 7, for example through an opening in said container 7, which is advantageously circular and has a diameter ranging from approximately 2 to 3 cm.

Figure 1 does not show the pumping system, which is composed for example of at least one fan and at least one low-pressure dosage pump, which introduces air to be ionized into the elements 4 and 5. Said pumping system may further facilitate the outflow of the atomized suspension from the container 7 by directing the stream of droplets toward the surface 9. Once the deposition of the LSMN on the solid surface has occurred, drying can occur by simple evaporation and in an extremely short time in view of the limited size of the generated droplets, especially in the advantageous embodiment in which the suspension medium is water. As an alternative, solidification can be forced by chemical or physical means (for example by thermal or UV irradiation). In this case, the apparatus must comprise means adapted to force solidification and optionally the vaporized suspension can contain one or more ingredients capable of polymerizing or curing if subjected for example to thermal or UV irradiation. By way of example, an advantageous embodiment of the means I) is now described with particular reference to the magnets of Figure 2.

Figure 2 illustrates four annular magnets (designated respectively by the reference numerals 10, 11, 12 and 13) which are arranged in series. The technical specifications of said magnets, such as the intensity of the generated fields, depend on different factors, such as:

- the degree of polarization that one wishes to give to the suspension of the liquid, and

- any magnetic contribution given to the suspension by the electrical properties of the nano- and microparticles, which as mentioned can modify the energies involved.

In Figure 2, in addition to the magnets 10, 11, 12 and 13, one can also see the element 14, which represents a duct, preferably made of copper, in which the liquid suspension flows at a given flow-rate. The choice of the flow-rate is among the decisions that a person skilled in the art can take by referring to ordinary knowledge in the field, depending for example on the specific application and/or selected thickness of the coating. The duct 14 is preferably located at the center of the magnets 10, 11, 12 and 13.

In a highly preferred embodiment, the magnets 10, 11, 12 and 13 are adapted to generate magnetic fields with an intensity ranging from 200 to 300 J/cm³«10^'3. These magnets are obtained advantageously by sintering neodymium. Commercial examples of useful magnets are for example those which can be purchased under the trademark MagNEO®. In a preferred manner, in the context of the present invention it is possible to use MagNEO® magnets from grade N30 to grade N43, preferably grade N38 magnets, were N references the magnetic number and is therefore an indicator of the intensity of the field that it generates.

By way of example, a preferred embodiment of the one or more capacitors II) is now described. In this embodiment, the electrical fields are generated advantageously by a two-cylinder capacitor which is constituted by coaxial cylinders which are both internally hollow and in which the cylinders having a smaller diameter are inserted within a cylinder having a larger diameter, leaving however an air space between the coaxial cylinders.

The two cylinders comprise laminas, preferably made of copper, which represent the two plates of the capacitor. The expressions "inner surface (or lamina or wall)" of the outer cylinder and "outer surface (or lamina or wall)" of the inner cylinder are used to reference, unless otherwise specified, the areas of the cylinders which face the interspace between the two cylinders.

As mentioned, an interspace is provided between the inner surface of the outer cylinder (having a larger diameter) and the outer surface of the inner cylinder (having a smaller diameter), and the gas to be ionized flows therein. The interspace has a fluid connection to a point of entry into the capacitor and to a discharge point, so as to allow the supply of the gas and its outflow from the interspace once it has been ionized. The outer lamina of the smaller-diameter cylinder is coated with an insulating material, preferably glass, so as to avoid electrical discharges between the two walls of the capacitor without affecting the electrical field generated between the two plates of the capacitor.

In the case of a generic gas, a potential difference occurs between the plates of the cylindrical capacitor which is adapted to generate an electrical field sufficient to ionize negatively at least part of the gas molecules which pass through the interspace. The value of this potential difference can be calculated easily in each case by the person skilled in the art on the basis of ordinary knowledge in the field, once the gas to be used has been selected. At the output of the cylindrical capacitor there is therefore a gas flow which contains negative ions; said flow is subsequently directed and made to collide with the polarized suspension.

In the preferred embodiment in which the gas is air, the two-cylinder capacitor is as shown in Figures 3-5. The capacitor 24 is divided into two physically distinct regions 26 and 27, which are arranged in series coaxially and are separated by an electrically insulating perforated partition 25. Each of the two regions 26, 27 is in turn constituted by two coaxial hollow cylinders 15, 16 and 15', 16', as described above, which are kept connected by an external connecting structure 17. The pairs of coaxial cylinders 15, 16, 15' and 16' form two interspaces 18 and 18', which have a fluid connection between them. In particular, the distal portion of the interspace 18 in the proximal region 26 of the capacitor with respect to the direction of flow of the gas has a fluid connection to the proximal portion of the interspace 18' of the distal region 27 of the capacitor by means of the hole 21 in the insulating partition 25.

The two outer cylinders 16 and 16' of the two regions 26, 27 have mutually identical diameters and so do the two internal cylinders 15 and 15'. For the sake of simplicity, the two regions of the two-cylinder capacitor are referenced here respectively as "ionizing" region 26 and "ozonizing" region 27, where the ionizing region 26 is arranged upstream of the ozonizing region 27 with respect to the direction of the air stream.

The proximal portion of the interspace 18 in the ionizing region 26 contains a point 22 for inflow of the gas into the capacitor 24 and has a fluid connection to the optional pumping system which forces the gas to be ionized into said capacitor 24. The distal portion of the interspace 18¹ in the ozonizing region 27 contains a point 23 for the outflow of the ionized gas from the capacitor 24 and has a fluid connection to the means IV) which are adapted to connect the two-cylinder capacitor 24 to the one or more containers III). If the gas is air, it has been found that ionization involves the oxygen that is present therein, generating negatively charged ozone. In this embodiment, the two regions, the ionizing region 26 and the ozonizing region 27, have mutually different voltages, in that the potential difference between the lamina 19 or 19' of the outer cylinder 16 or 16' and the lamina 20 or 20' of the internal cylinder 15 or 15' is different depending on whether one is in the region 26 or in the region 27. In the first region 26 (ionizer), the potential difference between the two plates of the capacitor is (in modulus) 15 kV or higher. In the second region 27 (ozonizer), the potential difference between the two plates of the capacitor is (in modulus) 40 kV or higher. The result of this combination is that at the output of the second region 27 of the cylindrical capacitor 24 one obtains an air stream in which part of the oxygen has been converted into negatively charged ozone.

It is possible to resort to any known technical solution to obtain the potential differences indicated above. However, in a highly advantageous embodiment, in order to obtain a negative potential of at least -15 kV at the level of the lamina of the internal cylinder of the ionizer, said lamina is connected to the output of an ordinary electrical circuit which is supplied at low voltage (for example approximately 10 V), in which however said circuit comprises a transformer and voltage booster stages or voltage multiplier stages which are arranged at the output of the secondary winding of said transformer. Simply by selecting the number of boosters or multipliers, this solution allows to increase the low input voltage to the selected value (preferably at least -15 kV).

In a preferred embodiment, in order to obtain a positive potential which is at least equal to +40 kV at the level of the lamina of the inner cylinder of the ozonizer, said lamina is connected to the output of a conventional electrical circuit which is supplied at a low voltage (for example approximately 10 V). The circuit comprises an ordinary electrical pulse generator which is connected downstream to a coil. The pulse generator is also capable of boosting the low voltage in input to the circuit. The coil is arranged directly upstream of the output of said circuit. By simply selecting the structural characteristics of the coil and the boosting capacity of the pulse generator, this solution allows to obtain in output from the circuit a high positive voltage, equal for example to at least +40 kV. In a preferred embodiment, it is possible to install in parallel a plurality of two-cylinder capacitors which are fed by a single gas intake duct and have output ducts which are aligned so as to merge into a single duct which conveys the gas toward the container or containers in which impact with the polarized suspension occurs. This embodiment allows to multiply the laminas of the two cylinders of the two-cylinder capacitor, offering significant operating versatility. The differences in potential of the several capacitors in parallel might in fact be different, so as to have the possibility to control even more finely the quantity and quality of the ionization of the gas, adapting it for example according to the type of micro- and nanoparticles to be deposited.

The flow of the gas in the interspace 18 and 18' that is present between the plates of the cylindrical capacitor 24 is optionally but advantageously controlled by an electronic circuit 6. The circuit 6 is designed and provided in order to be able to vary the quantity (flow rate) of gas in input to the capacitor 24 and thus ultimately the percentage of ionized ions in output from the capacitor 24. It is in fact noted that by varying the quantity of ions it is possible to modify the impact force on the suspension and therefore the degree of atomization of said suspension. In the case of a cylindrical capacitor divided into two sections such as the one described above if the gas is air, the circuit 6 controls both the air flow-rate in input to the ionizer and the flow-rate of the ionized gas which passes between the ionizer and the ozonizer.

As regards the force of the impact between the gas and the suspension, it is noted that the momentum transferred by the ions to the surface of the suspension to be atomized is a direct function of their charge and of course of their mass. Therefore, for an equal charge, working with charged ozone instead of with oxygen, which is lighter (or even with ions which are even heavier than ozone), has allowed in practical terms to increase the force of the impact between the ions and the suspension. It is thus possible to vary at will the rate of vaporization, the quantity of vaporized suspension per unit time, and most of all the size of the droplets, simply by varying the percentage of ions that are produced or by using different gases in successive steps of a same deposition cycle or also by mixing in suitable quantities different ionizable gases, so as to generate ion streams with a broad weight range. The higher or lower energy transferred from ions having a different molecular weight to the suspension or to droplets which have already formed in fact determines easier or more difficult overcoming of the surface attraction forces which act at the air/liquid interphase and tend to contrast the vaporization of the suspension or the further breakup of the droplets.

Another parameter which affects the choice of the weight of the ions to be used is the percentage and chemical makeup of the suspended particles. One of the consequences of the presence of nanoparticles or micrometer-size particles within the suspension is in fact the reduction of the surface tension forces which are typical of the pure liquid, i.e., without suspended particles. The effect of this decrease is to make vaporization of the liquid even more difficult, since a higher surface tension would tend to facilitate the immediate generation of droplets as a consequence of the impact with the ions. By modifying the composition of the gas (by choosing for example to use heavier ions), it is thus possible to overcome this drawback easily, achieving vaporization of the suspension despite the forces that normally contrast it.

A preferred embodiment of the ultrasonic transducer (element 8 in Figure 1) is now described by way of example. As mentioned, the transducer is arranged in the container 7, where the polarized suspension in output from the magnetic fields and the ionized gas that exits from the electrical fields are made to collide.

The impact between the two is facilitated by the electrical attraction between the positive part of the dipoles of the suspension and the fixed negative charge of the gas ions. In this situation, however, it has been found inexplicably and surprisingly that the ultrasound causes an acceleration of the ions, causing rapid atomization of the suspension. During this process, the suspension is subjected to physical stresses which are so low as to maintain perfect homogeneity and uniformity of the dispersion of nano- and microparticles within the formed droplets. In other words, the combination of ultrasound, prior polarization of the suspension and prior negative ionization of a gas stream allows atomization of part of the suspension in droplets characterized by an average diameter of 3 μm or less and by a uniform concentration of nano- and microparticles within said droplets. Atomization occurs as a consequence of the impact of the ions on the surface of the suspension. In a preferred embodiment, the duct that conveys the ionized gas toward the container or containers in which the impact with the polarized suspension occurs is inserted in a region which is located toward the bottom of said container or containers. If some suspension is already present in the container and the connecting point of the duct that conveys the ionized gas lies below the level of said suspension, the continuous flow of gas prevents the suspension from entering the duct.

Preferred transducers which have proved to be useful in terms of atomization are transducers with a power ranging from 30 to 100 W, with a resonance frequency ranging from 1.4 to 2.0 MHz, with a resonance impedance ranging from 2.0 to 4.0 R, with a capacitance measured at 1 kHz of 2000 pF, with a life ranging from 6000 to 10,000 hours (values supplied by the manufacturer S. Square Enterprise Company Ltd, Pro Wave Electronics Corporation, Taiwan). The waves generated by transducers which match the technical specifications given above apply on average a pressure of approximately 1.2 g/cm² and have proved to be adapted to atomize into droplets of a diameter of less than 3 μm approximately 400 to 600 cc of suspension per hour.

In a preferred embodiment, conventional commercially available transducers having the technical specifications cited above can be partially modified structurally. In particular, it has been verified that it is useful to provide the transducers with an aluminum pedestal which keeps them raised with respect to the bottom of the container in which they are located. This solution has allowed to make the transducers work so as to prevent the ultrasound from causing the temperature of the polarized suspension to rise above 50 ⁰C, a temperature above which the electrical performance of the suspension has been found to degrade rapidly.

If one wishes to increase the quantity of atomized suspension per unit time, it is possible to arrange in parallel more than one container and then convey into a single output stream the droplets produced in the several containers. It has in fact been found that the atomization rate depends substantially on the density of the suspension and therefore on the quantity of nano- and microparticles present per unit volume. Accordingly, the splitting or division in general of the flow-rate of the suspension in output from the magnetic fields and concerning more than one container does not affect the atomization rate, but indeed increases it directly in proportion to the number of containers arranged in parallel. In a preferred embodiment, there are two containers.

It has also been found surprisingly that the droplets obtained with the method according to the invention maintain a partial charge which is positive or negative depending on the situations (for example as a function of the suspension liquid and of the contained nano- and microparticles), so that their subsequent deposition on a solid surface to be coated can be accelerated by imparting to said surface a charge whose sign is opposite to the sign of the charge of the droplets.

Other characteristics and advantages of the present invention will become better apparent from the description of the following preferred embodiments, intended exclusively by way of non-limiting example. Example 1 Tests were conducted with the Nanotop® hydrophobic suspension by the Flexotec® company, adapted to make coated surfaces easy to clean. The suspension comprises nanoparticles of copper crystals having dimensions ranging from 10 to 20 nm, associated with structures of more complex particles, which are still nanometer-sized but sponge-like. The particles are suspended in a quantity ranging from 10 to 20% by volume in a liquid composed of two phases. The first phase is a solvent which can be polarized at low vapor pressure, while the second phase is a liquid which also can be polarized and solidifies as soon as the solvent has evaporated.

Coating a glass surface with traditional techniques in order to maintain the transparency of the glass produced layers which were not uniform in terms of quantity of deposited nanoparticles and excessive thicknesses, which in some points caused opacity. Moreover, poor chemical stability was observed.

The experiment was then repeated by using instead the apparatus according to the present invention. Using air as the gas to be ionized and also resorting to electrical fields of -15 kV and electrical fields of +40 kV applied in succession along the path of the air as shown in Figure 5, the suspension was vaporized into droplets with an average diameter of less than 3 μm and at a rate of 400 cc/hour. The glass surface was coated completely in approximately 10 minutes and the evaporation time of the solvent was less than 10 seconds. The droplets, in view of their extremely small diameter, allowed to deposit the nanoparticles in areas which are usually inaccessible, such as porosities or microcracks of the glass surface. The result was that the glass plate became hydrophobic (subsequent experiments performed after coating showed the forming of droplets with contact angles of more than 130°) and resistant to so-called fingerprinting, a property which also is in high demand. Moreover, in view of the thinness of the coating, transparency was maintained. The coating exhibited remarkable chemical stability against treatment with aggressive chemical products, such as those used to remove limescale.

Finally, a great saving of suspension was also observed thanks to the possibility to direct the stream of droplets toward the surface arranged in a vertical position with respect to the direction of said stream. Example 2 Two types of tests were carried out: the first one to obtain coatings with hydrophobic and limescale-preventing properties for glass panes specifically for shower and bath enclosures (minimizing maintenance cleaning) and the second one to obtain coatings with oxygen impermeability properties for polymeric materials commonly used in food packaging. The suspension can be purchased commercially under the name of

Tecnoclean® of the company Tecnocer®. The suspension contains crystalline nanoparticles of copper and nanoparticles of CaCO₃ (kaolin), both having stated dimensions ranging from approximately 10 to 20 run. The suspension liquid is a mixture of decamethyl cyclopentasiloxane and a low vapor pressure solvent produced by Union Carbide and/or Dow Corning.

By using current technologies such as spray deposition or simple spreading (both with cloths or by spinning), coatings with uniform thicknesses were obtained which had even significant variations in terms of performance. In particular, coatings which had poor chemical stability and low adhesion to the substrate to be coated were obtained in some areas. The fragility of the coating forced to repeat the treatment with a high periodic rate, in order to restore deteriorations of said coating. Moreover, in each coating process there were losses of suspension of even 30% by volume with respect to the original total volume.

The experiment was then repeated, by using instead the apparatus according to the present invention with the same technical specifications as Example 1. In this case, the solvent evaporated substantially immediately (on the order of hundredths of a second) after the deposition of the droplets and the decamethyl cyclopentasiloxane polymerized until it provided a solid layer with a thickness of approximately 2 μm which incorporated approximately 5% by weight of nanoparticles on the total weight of the coating.

The coating was found to be completely dry after a few minutes. Using a microscope, it was verified that the nanoparticles of kaolin had entered the pores and surface roughness of the coated materials, providing a coating which, over time, did not cause delamination from the surface due for example to thermal variations (which can, for example, freeze the water that has accumulated at the surface, which by becoming ice produces a separation pressure at the interface between the coating and the surface).

Example 3

Tests were performed with a paint based on a polar solvent for transparent applications on plastic surfaces for motor vehicle components. The paints used were manufactured by DuPont and PPG. The suspension medium of the paints had polarization properties similar to those of water, and organic and inorganic pigments with dimensions ranging from 100 to 200 nm were suspended therein.

Using current technologies such as spray deposition with nozzles measuring 0.8-0.6 mm subjected to a pressure of approximately 3 bars and flow-rate of the paints of approximately 13-15 m/s, microdroplets having average diameters of approximately 15 μm were produced. Painting cost was very high, since over 30% by volume on the volume of the suspension is lost. The resulting coating, however, had poor thickness uniformity and opaque areas. An additional problem was the need to dispose of the suspension lost during the coating step. In view of the toxicity of the suspension being considered, it was in fact necessary to deal with the high costs typical of disposal compatible with currently applicable statutory provisions regarding environmentally hazardous waste. The experiment was then repeated, using instead the apparatus according to the present invention with the same technical specifications as in Example 1. The use of the invention allowed to obtain the following advantages:

- reduction of the overall treatment times thanks to the faster application and the almost imperceptible drying time of the paint. It should be noted that a further reduction in the treatment times can be achieved by applying the apparatus according to the invention also to the steps which prepare for actual painting. For example, the step for prewashing the surface, to be performed with ordinary water, is highly accelerated if it is performed according to the method described here. In particular, not only the duration of the wash is shortened, but the subsequent drying step becomes entirely unnecessary, due to the substantially instantaneous evaporation of the water deposited on the surface;

- sharp reduction of treatment costs, especially thanks to the reduction of the time requirements, of the waste of suspension and of the electric power used for painting;

- the possibility to perform the entire application as a single in-line method.

Example 4 Tests were performed with heat-polymerizing clear paints for coating surfaces of different materials of different kinds.

Repeating two series of experiments, the first with conventional spray techniques and the second with an apparatus according to the present invention having the same technical specifications as in Example 1, the following advantages of the invention described here were observed:

- better deposition of the paint, providing a thinner and more uniform coating;

- reduced use of heat energy to polymerize the coating;

- the coating had a higher scratch resistance; - the coating was easier to clean;

- reduction of waste and costs, since the atomization process requires, as a whole, a lower energy expenditure.

Example 5

To demonstrate the usefulness of the invention in obtaining IR- cutting coatings in the field of so-called "solar control" glazing, a glass pane suitable for providing photovoltaic panels received the deposition of a mixture composed of nanoparticles of ITO, ATO and of the molecular organic type having an average size of 20 run. Said mixture proved to be able, once deposited on the surface of the glass pane, to transmit sunlight in the visible range TV (400-750 ran wavelength) for over 75% and to absorb or disperse (by scattering) the infrared sunlight IR. In this manner, the total solar energy TE transmitted by a glass pane coated with said mixture did not exceed 40%, therefore far from the values of 60-70% typical of conventional panels. Accordingly, the TV/TE ratio (solar control efficiency coefficient) was found to be 1.9-2 or higher, entailing considerable energy savings in lighting and electric power consumption.

Example 6

Tests were carried out with a suspension of ITO (indium and tin oxide) nanoparticles mixed with a liquid alkoxide containing silica and other metals such as titanium and nickel, which acted as a solvent of the sol- gel type. The coated surface was a glass pane of the so-called "float" type.

The apparatus according to the invention allowed to deposit the particles uniformly and, surprisingly, avoid the so-called clustering (agglomeration) of said particles which instead occurred unavoidably with conventional depositions such as spray, rolling, deeping, spinning or blading.

Deposition of the microdroplets having a diameter of less than 3 μm and the subsequent almost immediate evaporation of the suspension medium therefore allowed to obtain a deposition of the ITO nanoparticles (not yet in the final form of solid and compact coating, since the suspension did not also contain components capable of polymerizing and forming a final coating) on the surface of the glass pane in which the most significant agglomerations comprised at most three nanoparticles, a value well below the degree of agglomeration which is typical of conventional techniques. Although only some preferred embodiments of the invention have been described in the text, the person skilled in the art will understand immediately how to apply some simple modifications to the invention, obtaining in any case other embodiments which are equally advantageous and preferred. The disclosures in Italian Patent Application No. MI2006A000560 from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

1. A method for providing functional coatings on solid surfaces, comprising the steps of: i) polarizing, by means of one or more magnetic fields, a liquid suspension which comprises at least one liquid suspension medium and at least one type of micro- and/or nanoparticles suspended in said suspension medium, ii) subjecting an ionizable gas to the action of one or more electric fields in order to ionize one or more components which are present in said gas, iii) mixing the ionized gas of step ii) with the polarized suspension of step i) inside one or more containers, each container comprising at least one ultrasonic transducer, where steps i) and ii) can occur in any mutual order, step iii) must occur after the two preceding steps, and step iii) determines the vaporization of the liquid suspension.

2. The method according to claim 1, wherein types of micro- and/or nanoparticles are selected among micro- and/or nanoparticles of ITO, ATO, TiO₂, CaCO₃, silica, silicon, micro- and nanoparticles to produce coatings for photovoltaic panels, and mixtures thereof.

3. The method according to claim 2, wherein the micro- and/or nanoparticles are micro- and nanoparticles for producing coatings for photovoltaic panels.

4. The method according to claim 1, wherein the liquid suspension medium is water.

5. The method according to claim 1, wherein the one or more magnetic fields have an intensity ranging from 200 to 300 J/cm^3»10^"3.

6. The method according to claim 1, wherein the ionizable gas is air.

7. The method according to claim 6, wherein the one or more electrical fields consist of a first electrical field generated by a potential difference of at least 15 kV in modulus and a second electrical field generated by a potential difference of at least 40 kV in modulus.

8. The method according to claim 1, wherein steps i) and ii) occur simultaneously and in parallel.

9. The method according to claim 1, wherein step iii) determines the vaporization of the liquid suspension in droplets having a diameter ranging from 0 μm, limit not included, to 3 μm.

10. The method according to claim 1, further comprising a step iv) which follows the step iii), for conveying and depositing the vaporized liquid suspension on a solid surface to be coated.

11. The method according to claim 10, further comprising a step v) which follows the step iv), for determining the solidification of the liquid suspension deposited on the solid surface to be coated.

12. The method according to claim 1, wherein the liquid suspension contains suspended micro- and/or nanoparticles in a quantity ranging from

10% to 74% by volume on the total volume of the suspension.

13. A functional coating which can be obtained with the method according to one or more of claims 1 to 12.

14. The functional coating according to claim 13, having a uniform thickness ranging from 5 to 6 nanometers and without porosities.

15. A solid surface coated with a functional coating defined according to one or more of claims 13 to 14.

16. The coated solid surface according to claim 15, selected from the group that consists of surfaces of elements made of plastics, metal, ceramic and glass.

17. The coated solid surface according to claim 16, wherein the surface belongs to a glass element.

18. An apparatus for performing a method as defined according to one or more of claims 1 to 12, said apparatus being characterized in that it comprises: I) means adapted to generate magnetic fields for polarizing a liquid suspension which contains nano- and/or microparticles,

II) one or more capacitors which are adapted to generate respective electrical fields for ionizing a gas, III) one or more containers comprising at least one ultrasound transducer and arranged downstream of the means I) and II), and

IV) means adapted to connect the means I), II) and III) mentioned above and to convey the polarized liquid suspension in output from the means I) and the ionized gas in output from the one or more capacitors II) into the one or more containers III).

19. The apparatus according to claim 18, characterized in that it further comprises means V), which are arranged downstream of the means III) and are adapted to convey the vaporized suspension in output from the means III) onto a solid surface to be coated.

20. The apparatus according to claim 19, characterized in that it further comprises means VI), which are arranged downstream of the means V) and are adapted to determine the forced solidification of the suspension deposited on the solid surface to be coated.

21. The apparatus according to claim 18, characterized in that it further comprises means VII), which are arranged upstream of the means II) and are connected thereto and are adapted to introduce by forcing the gas to be ionized in the electrical fields generated in the means II).

22. The apparatus according to claim 18, characterized in that the means I) are four annular magnets arranged in succession with respect to a path of a liquid suspension to be polarized which contains nano- and/or microparticles, a duct being provided which passes through the rings of the magnets and in which said liquid suspension flows.

23. The apparatus according to claim 22, characterized in that the four magnets have a magnetic number ranging from N30 to N43.

24. The apparatus according to claim 18, characterized in that the one or more capacitors are of the two-cylinder type, in which there is an interspace, in which the gas to be ionized flows, between the two coaxial cylinders which comprise the plates of the one or more capacitors.

25. The apparatus according to claim 24, characterized in that the two-cylinder capacitor is constituted by two regions, a proximal one and a distal one with respect to the direction of the flow of gas, wherein the two regions are physically distinct, are arranged in series with respect to the flow of the gas, and are separated by a partition which is perforated and electrically insulating; further characterized in that each of the two regions is in turn constituted by two internally hollow cylinders, in which the cylinder having the smaller diameter is inserted within the cylinder having the larger diameter, wherein between the two coaxial cylinders there is an interspace in which the gas to be ionized flows; further characterized in that the distal portion of the interspace in the proximal region of the capacitor is in fluid connection with the proximal portion of the distal region by means of the hole in the insulating partition, the proximal portion of the interspace in the proximal region containing a point for the inflow of the gas into the capacitor, the distal portion of the interspace in the distal region containing a point for the outflow of the ionized gas from the capacitor and being connected to the means IV); further characterized in that the two outer cylinders of the ionizing and ozonizing regions have mutually identical diameters and the two inner cylinders of the ionizing and ozonizing regions have mutually identical diameters.

26. The apparatus according to claim 24 or claim 25, characterized in that the surfaces of the outer cylinder and of the inner cylinder each comprise a copper lamina, and wherein the lamina located within the inner cylinder is coated with glass.

27. The apparatus according to claim 26, insofar as it depends on claim 25, characterized in that the potential difference between the laminas located within the cylinders in the proximal region is at least 15 kV in modulus, while the potential difference between the laminas located in the cylinders in the distal region is at least 40 kV in modulus.

28. The apparatus according to claim 18, characterized in that the transducer located within the one or more containers III) is a transducer with a power level ranging from 30 to 100 W, with a resonance frequency ranging from 1.4 to 2.0 MHz, with a resonance impedance ranging from 2.0 to 4.0 R, with a capacitance measured at 1 kHz of 2000 pF, and with a life ranging from 6,000 to 10,000 hours.