Field of application

The present invention regards a process for producing a composite material for decorative articles and a composite material obtained by means of the aforesaid process according to the preamble of the respective independent claims.

The process and the composite material, subject of the present invention, are inserted in the field of production of decorative objects and jewelry. The process and the composite material are intended to be advantageously employed in the manufacturing industry for producing precious semifinished products.

State of the art

In the gold-working field, and more generally in the field of production of decorative articles made of precious and semi-precious material, the possibility to use composite materials comprising a polymer material in which particles of precious metal material are dispersed, such as gold in particular, has for some time been studied. Indeed, the use of such composite materials would in principle allow attaining several particularly attractive advantages for the gold-working manufacturing industry, such as in particular the possibility to use polymer material technologies rather than metal material technologies, thus being able to obtain finished products by means of the use of simple and inexpensive industrial production processes.

In addition, the decorative articles obtained with the abovementioned composite materials, due to the polymer material, have a resistance to oxidation and corrosion that is improved with respect to some metal alloys used in the field of reference.

Nevertheless, the production techniques for decorative articles made of composite material - that have been implemented up to now - have not allowed attaining appreciable results, such to stimulate the gold-working industry to invest in this field.

The processes implemented up to now for obtaining the composite materials described briefly above have, in fact, in practice demonstrated that they do not lack drawbacks.

A composite material of the abovementioned type and a process for the obtainment thereof are for example described in the patent US 5,578,383. In particular the composite material described herein comprises a polymer material, constituted by a thermosetting resin, in which precious metal particles are dispersed that have average dimensions comprised between 1 and 100 micrometers. The process for producing the composite material described briefly above provides for adding the precious metal particles to the thermosetting resin, mixing the latter in order to allow a distribution of the metal particles at its interior that is as uniform as possible, and casting the resin with the metal particles dispersed therein in a mold in order to directly obtain the decorative article. The above-reported operative steps must all be executed so that the thermosetting resin is provided with a sufficiently low viscosity to allow the workability thereof.

The composite material described in the patent US 5,578,383 advantageously has a low density, mainly due to the low density of the thermosetting resin, which confers a lower weight to the decorative articles obtained therewith given the same volume with respect to the decorative articles made entirely of metal. Nevertheless, the process for obtaining decorative articles described in the patent US 5,578,383 is difficult to achieve, in particular at the industrial level, given the limited time interval within which the composite material is workable and within which it is necessary to execute all the operative steps of the aforesaid process, time interval that depends on the pot-life of the thermosetting resin.

A different composite material and a different process of production of such composite material are described in the patent US 2008/0319034. The composite material in accordance with the invention described herein comprises a transparent elastomeric or thermoplastic polymer material, in which particles of a metal powder with dimensions smaller than 0.5 micrometers are dispersed. The process of production of such composite material provides for introducing the polymer material and the metal powder in the chamber of an extruder, their mixing by means of the screw of the extruder at a temperature comprised between 120° and 220°C so that the polymer material is found in melted state and the extrusion and cutting of the mixture thus obtained in order to obtain chips (or pellets) of composite material.

Nevertheless, the composite material described in the patent US 2008/0319034 does not have entirely satisfactory aesthetic characteristics. The dimensions of the metal powder particles, in fact, while they allow introducing high percentages of metal powder into the polymer material, even 75% (corresponding to 18 carats), maintaining good mechanical properties of the composite material, they also involve a dark brown color of the composite material. The metal particles having dimensions smaller than 0.5 micrometers are in fact affected by surface plasmon resonance phenomena, with consequent absorption of part of the incident light radiation, in the visible region, on their own surface. Indeed, in the particles the conduction electrons vibrate and resonate in certain frequencies of the visible spectrum, absorbing the incident light waves. Depending on the specific size of the single particles, the absorbed or transmitted frequencies change, causing a change of color that is translated, in the particles having the size indicated in the patent US 2008/0319034, into the abovementioned dark coloring.

The patent US 4,282,174 describes a composite material comprising a thermoplastic or thermosetting polymer material and metal powder particles with substantially spherical shape, with dimensions comprised between 0.5 and 50 micrometers, dispersed in the material. The process of production of decorative articles with the aforesaid composite material provides for obtaining powders of the polymer material with very thin size, mixing such powders with the metal powders and molding such mixture of powders at high pressures and at temperatures comprised between 100° and 250°C in order to obtain the articles according to the desired shapes and sizes.

The composite material described in the patent US 4,282,174, nevertheless, is not provided with suitable gloss and sheen characteristics, since the spherical shape of the particles as well as the wide range of grain size distribution of the same particles do not allow an optimal reflection of the light radiation. In addition, the process of production of decorative articles described therein is rather complex to achieve, especially regarding the difficulty of obtaining the polymer powders.

A further composite material comprising a polymer material and metal powder particles dispersed therein is described in the patent WO 2012/101568. The polymer material is preferably constituted by silicone and the metal powder particles have a flattened shape, in particular with triangular or hexagonal geometry, with a thickness comprised between 3 and 300 nm and dimensions on the preferred extension plane comprised between 0.6 and 100 micrometers. In particular the patent WO 2012/101568 teaches to obtain such metal powder particles by means of the so-called polyol method and teaches to add the metal powder to the silicone in liquid phase.

The method proposed therein for obtaining the metal powder particles nevertheless involves toxicity risks and is characterized by low yields, which makes the use thereof difficult on an industrial level. In addition the process described in the patent WO 2012/101568 for obtaining the composite material is difficult to carry out on an industrial level, especially with regard to the difficulty of mixing the metal powder with the liquid silicone, preventing the sedimentation of such powder.

Presentation of the invention

In this situation, main object of the present invention is to provide a process for producing a composite material for decorative articles, which allows obtaining products whose exterior color appears to be gold.

Another object of the present invention is to provide a process for producing a composite material for decorative articles which is simple and safe to achieve at the industrial level.

Another object of the present invention is to provide a process for producing a composite material for decorative articles which is characterized by high yields in each of its operative steps.

Further object of the present invention is to provide a process for producing a composite material for decorative articles which is entirely safe for the health of the operators who make it.

Another object of the present invention is to provide a process for producing a composite material for decorative articles which is eco-sustainable.

Another object of the present invention is to provide a composite material for decorative articles and decorative articles obtained with such composite material which are provided with optimal aesthetic gloss characteristics.

Further object of the present invention is to provide a composite material for decorative articles and decorative articles obtained with such composite material which are provided with suitable mechanical characteristics.

These and still other objects are all attained by the process for obtaining a composite material for decorative articles and by the composite material obtainable in particular by means of such process that is the subject of the present invention.

Brief description of the drawings

The technical characteristics of the invention, according to the aforesaid objects, can be clearly seen in the contents of the below-reported claims and the advantages thereof will be more evident in the following detailed description, made with reference to the enclosed drawings, which represent a merely exemplifying and non-limiting embodiment of the invention, in which:

FIGS. 1 and la respectively show an image at the scanning electron microscope of a first specimen of gold metal powder obtained by means of a synthesis step provided in the process in accordance with the present invention according to a first embodiment, and an enlarged image at the scanning electron microscope of the same first specimen;

FIGS. 2 and 2a respectively show an image at the scanning electron microscope of a second specimen of gold metal powder obtained by means of a synthesis step provided in the process according to the present invention in accordance with a second embodiment, and an enlarged image at the scanning electron microscope of the same second specimen;

FIG. 3 shows a graph relative to the progression of the specific weight of a composite material according to the present invention, as a function of the percentage of gold powder comprised therein;

FIG. 4 shows a table reporting the values of several mechanical characterization parameters of specimens of a composite material containing gold particles according to the present invention, as a function of the percentage by weight of gold powder contained therein.

Detailed description

As specified above, the process and the composite material, subject of the present invention, are intended to be advantageously employed in the manufacturing industry for producing finished objects or precious semifinished products. In particular, the process and the material according to the invention are inserted in the field of production of decorative articles and jewelry which contain percentages of precious metal equal to those of any fine alloy, improving the tactile characteristics thereof, the comfort of use and several chemical-physical and mechanical properties, such as in particular the density and elasticity.

The process and the composite material according to the invention can be advantageously used for producing decorative jewelry articles as well as for producing accessories or inserts of products in the watches, eyeglasses, clothing, shoes and/or leather fields, given that the composite material is provided with mechanical characteristics of elasticity and tenacity that allow the elastic deformation thereof and, at suitable thicknesses, also for example the sewability.

In addition, the composite material according to the present patent can also have use in manufacturing products or parts of products for health and medical applications, as will be better explained hereinbelow.

In accordance with the idea underlying the present invention, the process for producing a composite material for decorative articles provides for a step for arranging a polymer material having a transmission of the light radiation in the visible spectrum (i.e. for wavelengths comprised between about 380 nm and about 750 nm), in particular measured according to the ASTM D1003 standard test, equal to at least 70% and preferably equal to at least 80%, for thicknesses of the polymer material of 2 mm. Preferably the polymer material employed in the present process is transparent, being substantially limpid when observed in transmission; nevertheless the same material can also be translucent, i.e. it has a minimum degree of light diffusion, without departing from the protective scope defined by the present patent, so long as it is still provided with the above- specified light radiation transmission characteristics.

The process further comprises one or more steps for synthesizing one or more metal powders, each of a precious metal in particular selected within the group containing gold, silver or platinum. Each step for synthesizing a powder of a precious metal is obtained starting from a solution comprising one or more compounds of the precious metal, one or more reducing agents and one or more selector agents for the crystallographic growth. The metal powder is formed for at least 60% by weight by plate-like metal particles of the corresponding precious metal, such particles having a preferred extension on a two-dimensional surface with at least one dimension comprised between 0.8 and 75 μπι and a thickness equal to at least 500 nm, and advantageously less than 2000 nm. In addition, preferably, the aforesaid metal particles (i.e. at least 60% with at least one dimension comprised between 0.8 and 75 μηι) have thickness comprised between 600 and 2000 nm and advantageously still more preferably between 800 and 1600 nm. Each of the aforesaid plate-like metal particles therefore preferably has two main extension faces, substantially parallel and opposite each other, mainly on the thickness.

The solution is an at least 0.01 molar solution and, preferably, at least 0.05 molar solution of the corresponding precious metal in order to facilitate, during the synthesis step, the growth of the plate-like metal particles until the desired thicknesses are reached, equal to at least 500 nm (and preferably greater than 600 nm or, still more advantageously, greater than 800 nm, preferably always remaining below 2000 nm and still more preferably below 1600 nm).

The process then comprises a step for adding a dose of one or more metal powders in a percentage by weight comprised in the interval between 2 and 90% to a dose of the polymer material in a percentage by weight comprised in the interval between 10 and 92%. Preferably the process according to the invention provides for the addition of a dose of only one metal powder comprising particles of a single precious metal. Nevertheless, the addition of a dose of multiple metal powders can be provided, each comprising particles of a different precious metal, mixed together in order to obtain different properties of the composite material or a different aesthetic effect, without departing from the protective scope defined by the present patent.

Also provided are a step for closely mixing the dose of polymer material and the dose of metal powder, at the end of which the metal powder is dispersed in a substantially uniform manner in the polymer material, and a step for cross- linking the dose of polymer material with the dose of metal powder distributed substantially uniformly therein. In order to obtain a dispersion of the metal powder particles inside the polymer material that is as uniform as possible, the close mixing step is obtained in an extremely precise manner, facilitating the distribution of the metal powder particles in the mass of the polymer material and preventing the same from being affected by sedimentation phenomena before or during the cross-linking step.

The thickness equal to at least 500 nm, advantageously lower than 2000 nm, and in particular preferably comprised between 600 and 2000 and still more advantageously comprised between 800 and 1600 nm, contributes, substantially in an analogous manner to the main faces of the particles, to the reflection of the light radiation, allowing the obtainment of a composite material having improved color characteristics with respect to the composite materials of known type. Metal particles having thicknesses lower than 500 nm, in fact, when irradiated edgewise, i.e. at the thickness thereof, can give rise to surface plasmon resonance phenomena as specified above, causing the absorption or transmission of specific frequencies of the incident light radiation, according to the thickness of the single irradiated particles, causing undesired color alterations. Therefore the composite material obtained with the process according to the present invention, comprising metal particles having the abovementioned thickness characteristics, has characteristics of interaction with the light radiation and, in particular, of reflection and gloss, that are greatly improved with respect to known composite materials. Hence, in order to obtain desired gloss and color characteristics of the composite material obtained by means of the present process, it is essential that the arranged metal powder comprises at least 60%, and preferably at least 80%, plate-like metal particles with thickness of at least 500 nm. Preferably such percentage of particles has maximum thickness that preferably does not exceed 2000 nm or, still more preferably, 1600 nm, while the minimum thickness greater than 500 nm is preferably also above 600 nm or, still more preferably, above 800 nm.

The step for synthesizing the process according to the present invention allows obtaining particles with the aforesaid thickness, due to the high concentration of precious metal present in the starting solution of the synthesis step, which as specified above is an at least 0.01 molar solution and, preferably, at least 0.05 molar solution of the corresponding precious metal. Such molarity with respect to the precious metal of the synthesis solution in fact allows shifting the equilibrium of the synthesis reactions of the particles towards the grown of the particles themselves, which are therefore able to reach the desired thicknesses. The high concentration of the starting synthesis solution also involves a decrease of the volume of the reaction environment, which in turn facilitates the increase of the particles as well as allows an improved control of the synthesis reaction.

As previously specified, the synthesis solution comprises one or more compounds of the precious metal, one or more reducing agents and one or more selector agents for the crystallographic growth.

More in detail, the compound of the precious metal can be any chemical compound containing such precious metal, such as an oxide or a salt. In particular, if the precious metal is for example gold, the compound can be selected from among: gold chloride I (AuCl), gold chloride III (AuCl ₃ ), chloroauric acid hydrate HI (HAuCl ₄ *4H ₂ 0 or HAuCL»*3H ₂ 0), potassium tetrachloroaurate III (KauCl ₄ *0.5 H ₂ 0), gold oxide III (Au ₂ 0 ₃ ), gold hydroxide III Au(OH) ₃ , potassium aurate (KAu0 ₂ *3H ₂ 0), gold sulfide I (Au ₂ S), gold sulfide II (AuS), gold sulfide III (Au ₂ S ₃ ) or gold carbide (Au ₂ C ₂ ).

Preferably, as better explained hereinbelow, the reducing agent is selected from among alcohols comprising at least two hydroxyl groups in vicinal position, in order to allow a pinacol rearrangement reaction to occur with formation of a carbonyl, or among organic carboxylic acids comprising at least one hydroxyl group and preferably two or three hydroxyl groups, such as tartaric acid, citric acid, oxalic acid, malic acid, ascorbic acid, succinic acid, sorbic acid or the like.

The selector agent for the crystallographic growth will preferably be selected from within a group of compounds containing aniline, polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyacrylic acid, methyl polyacrylate and ethyl polyacrylate.

The experimental tests that have been carried out for implementing the process and the composite material in accordance with the present invention have allowed identifying the conditions of synthesis reaction which allow obtaining plate-like metal particles having the above-reported optimal size characteristics. Such reaction conditions were summarized in the above-indicated composition and reaction parameters, also reported hereinbelow. Nevertheless, the exact growth mechanism of the precious metal particles during the synthesis step, which leads to the obtainment of particles having the aforesaid form, is not yet entirely known.

During the synthesis reaction of the metal particles, the reducing agent facilitates the reduction of the compound of the precious metal, with the consequent release, into solution, of atoms of the precious metal and nucleation of pure precious metal particles. The precious metal particles thus nucleated, due to the presence of the selector agent for the crystallographic growth, are grown along preferred crystallographic directions, assuming a planar form and a polyhedral geometry, in particular triangular or hexagonal. Presumably, such growth of the metal particles, in the presence of the selector agent for the crystallographic growth, is due to a preferred adsorption of the selector agent for the crystallographic growth on the faces of the metal crystal being formed that are provided with a smaller adsorption coefficient and with a consequent anisotropic growth of the crystals, i.e. of the particles, along preferred growth directions.

For example, in the case of gold's face-centered cubic crystalline lattice, the crystallographic planes having the lower adsorption coefficient are the planes (1 1 1), which have a lower surface energy than the other planes. Therefore the selector agent for the crystallographic growth will preferably be adsorbed on the gold particles being formed at crystallographic planes belonging to the family of planes { 1 1 1 }, consequently slowing or even preventing the growth of the metal particles on the planes (1 1 1), promoting a strongly anisotropic growth of the crystals, in particular along the preferred growth direction <1 10>, and leading to the formation of plate-like gold metal particles having basal planes belonging to the family of planes { 1 1 1 } .

Otherwise, in the case of silver's face-centered cubic crystalline lattice, the selector agent for the crystallographic growth will preferably be adsorbed on the silver particles being formed at crystallographic planes belonging to the family of planes { 100} , promoting an anisotropic growth of the crystals, in particular along the preferred growth direction <1 1 1>.

At a later time, the metal particles being grown coalesce together in the direction of the crystalline lattice, unimpeded by the presence of the selector agent for the crystallographic growth adsorbed thereon, growing and acquiring the planar form and polyhedral geometry as previously described.

Advantageously the synthesis solution of the plate-like metal particles comprises, in addition to the abovementioned compound and agents, one or more dispersing agents, adapted to prevent the adhesion between the metal particles being formed in the solution, i.e. adapted to maintain the metal particles separated from each other. The limited volume of the reaction environment in fact facilitates the packing of the already- formed metal particles, which subsequently would be difficult to separate. Hence the presence of one or more dispersing agents, such as a polyol, facilitates the mutual moving away of the particles, maintaining them separate in the solution.

Advantageously the selector agent for the crystallographic growth and the precious metal are present in the solution in a molar ratio comprised between 5 and 40 and, preferably, comprised between 15 and 30. Indeed it was verified that a molar ratio between the selector agent and the precious metal comprised within the aforesaid interval allows obtaining a powder comprising plate-like particles with the above-indicated size characteristics, regardless of the temperature at which the reaction is made to occur, or the temperature at which the synthesis step is conducted. For values of the molar ratio less than or greater than the values comprised in the aforesaid interval, it was verified that it is possible to obtain a powder with particles having the desired size characteristics only if the reaction was made to occur within precise temperature intervals. In particular it was also verified that for values of the molar ratio between the selector agent for the crystallographic growth and the precious metal that are lower than 10, many of the particles obtained during the synthesis step have nanometric size and a rounded form, thus being poorly suitable for reflecting the light radiation in different directions and for conferring the desired gloss characteristics to the final composite material.

Preferably, the synthesis step is conducted at a temperature sufficiently high for allowing the formation of the metal particles within an industrially convenient time interval, generally on the order of several hours, and at the same time sufficiently low to prevent the nucleation of new particles from prevailing over the growth of already-nucleated particles, with the consequent undesired formation of numerous nanometric particles. In particular the optimal synthesis temperature for the metal particles is comprised between 25 and 180°C and is suitably selected as a function of the precious metal to be reduced. For example, if it is desired to obtain gold particles, the optimal synthesis temperature is preferably comprised between 100 and 150°C, while if it is desired to obtain silver particles, the optimal temperature is preferably comprised between 60 and 80°C.

In accordance with a preferred embodiment of the process according to the present invention, the reducing agent present in the synthesis solution is a weak reducing agent, i.e. it facilitates a low reduction speed of the precious metal compound. The phenomenon that leads to the formation of the plate-like metal particles is in fact governed by the reaction kinetics, rather than by thermodynamics. Therefore, a reducing agent which promotes, in the synthesis solution comprising the compound of the precious metal and the selector agent for the crystallographic growth, a low reaction kinetics, is adapted to be used for obtaining metal particles in the process in accordance with the present invention.

More clearly, the reducing agent that is preferably employed is not a strong reducing agent and therefore does not promote, at the reaction conditions, and, in particular at the reaction temperature, a quick reduction of the compound of the precious metal and, consequently, a quick release of atoms of the precious metal into the synthesis solution. The greater the force of the reducing agent and the greater its concentration in solution, in fact, the lower the size of the precious metal particles obtained by means of the synthesis step, since the nucleation of new particles would be facilitated to the detriment of the growth of those already- formed particles. In addition, the obtained metal particles would preferably have a substantially spherical form, since there would not be the conditions for developing the desired plate-like geometry for the metal particles in accordance with the present invention.

The use of a mild reducing agent, instead, promotes the growth rather than the nucleation of the precious metal particles and, in particular, the preferred growth of such particles allows the selector agent for the crystallographic growth to be adsorbed on the surface of the nuclei of formed particles and to direct the growth of the latter.

In particular, as specified above, alcohols comprising at least two hydroxyl groups in vicinal position have proven to be particularly suitable for being employed as reducing agents in the process according to the invention; this in order to allow a pinacol rearrangement reaction to occur with formation of a carbonyl, or organic carboxylic acids comprising at least one hydroxyl group and preferably two or three hydroxyl groups. The reducing effect of such reducing agents is due to the fact that, in the synthesis solution, they are oxidized to aldehydes and ketones, thus reducing the ions of the precious metal.

For example, at temperatures greater than 60°C, if the reducing agent employed is ethylene glycol, the AuCl ₄ ^" ions are reduced according following reactions:

- first oxidation reaction of the ethylene glycol to acetaldehyde:

- second oxidation reaction of the acetaldehyde to 2-3 butanedione:

preferably to at least 5 hours, during which the solution is maintained under stirring. The greater the time interval during which the synthesis step is continued, the greater the yield of the synthesis reaction, where by yield of the synthesis reaction it is intended to indicate the percentage by weight of precious metal introduced into the synthesis solution which was transformed into metal powder having the above-indicated characteristics.

The synthesis reactions obtained in accordance with the present invention have reached very high yield percentages, greater than 90%.

As specified above, during the synthesis step the solution is maintained under stirring, e.g. by means of a magnetic stirrer, in order to prevent the precious employed is ethylene glycol, the AuCU ^" ions are reduced according to the following reactions:

- first oxidation reaction of the ethylene glycol to acetaldehyde:

Advantageously, the synthesis step is continued for a time interval equal to at least one hour, and preferably equal to at least 3 hours and still more preferably to at least 5 hours, during which the solution is maintained under stirring. The greater the time interval during which the synthesis step is continued, the greater the yield of the synthesis reaction, where by yield of the synthesis reaction it is intended to indicate the percentage by weight of precious metal introduced into the synthesis solution which was transformed into metal powder having the above-indicated characteristics.

The synthesis reactions obtained in accordance with the present invention have reached very high yield percentages, greater than 90%.

As specified above, during the synthesis step the solution is maintained under stirring, e.g. by means of a magnetic stirrer, in order to prevent the precious

INCORPORATED BY REFERENCE (RULE 20.6) metal particles that have formed and are being formed from being packed and forming agglomerates of particles which would subsequently be hard to separate.

In accordance with a preferred embodiment of the process according to the present invention, the synthesis step is preceded by a step for obtaining the synthesis solution; such step comprises the arrangement of a first pre-solution and a second pre-solution. More in detail, the first pre-solution comprises the selector agent for the crystal lographic growth (or the selector agents for the crystallographic growth) and the reducing agent (or the reducing agents), while the second pre-solution comprises the compound of the precious metal (or the compounds of the precious metal). The step for obtaining the synthesis solution then comprises the joining of the first pre-solution and the second pre-solution by adding the second pre-solution, containing the compound of the precious metal, dropwise to the first pre-solution.

• The obtainment of the synthesis solution in the above-described manner facilitates a dispersion of the precious metal compound within the synthesis solution that is as uniform as possible, thus facilitating its contact with the reducing agent and with the selector agent for the crystallographic growth.

The process according to the present invention then comprises, advantageously, at the end of the synthesis step, a step for separating the precious metal powder thus obtained from the depleted solution, e.g. by means of centrifugation or filtration and subsequent washing, drying and sterilization (e.g. by means of UV irradiation) of the powder or by means of spray drying.

The depleted solution is then advantageously neutralized and recovered for a new use. In particular, the precious metal which was not transformed into metal powder - hence still present therein - is recovered. Reported hereinbelow are two embodiments of the step for synthesizing the metal powder in a process for producing a composite material according to the present invention.

In accordance with a first embodiment of the step for synthesizing a metal powder of a precious metal, and in particular a gold metal powder, of the process according to the present invention, gold particles were produced starting from a synthesis solution comprising chloroauric acid, as gold compound, in aqueous solution, tartaric acid as reducing agent and polyvinylpyrrolidone as selector agent for the crystallographic growth. The molar ratio between polyvinylpyrrolidone (selector agent for the crystallographic growth) and gold (precious metal) introduced in solution was equal to 20 and the synthesis step was conducted at a temperature of 100°C.

In particular the chloroauric acid in aqueous solution was obtained by means of a solution of gold in aqua regia, by means of the following reactions:

Au (s) + 3 NO ^"3 (aq) + 6 H ⁺ (aq)→ Au ³⁺ (aq) + 3N0 ₂ (g) + 3H ₂ 0 (I)

Au ³⁺ (aq) + 4 CI ^" (aq)→ AuCl ^"4 (aq)

More in detail, tartaric acid and polyvinylpyrrolidone were first solubilized in bidistilled water. The chloroauric acid in aqueous solution was then added dropwise to the pre-solution of the selector agent for the crystallographic growth and the reducing agent in distilled water, while such pre-solution containing tartaric acid and polyvinylpyrrolidone was maintained under stirring. Following the introduction of the gold compound, the synthesis solution was maintained under stirring for 8 hours.

At the end of the synthesis step, the formed particles were separated from the solution, washed and sterilized. The particles thus obtained were observed at the scanning electron microscope. As is visible in the enclosed Figures 1 and la, the gold particles obtained by means of this first embodiment of the synthesis step comprised about 80% by weight of plate-like particles of desired geometry and size, while the remaining fraction by weight was mainly constituted by nanometric particles with substantially spherical form.

The specimens of composite material made with the powders thus obtained had non-optimal color, in particular dark yellow, probably due to the presence of the aforesaid undesired fraction of nanometric particles which due to the surface plasmon resonance interact with the incident light, modifying the diffused or transmitted wavelength thereof.

In accordance with a second embodiment of the step for synthesizing a metal powder of a precious metal, and in particular of a gold metal powder, of the process according to the present invention, gold particles were produced starting from a synthesis solution comprising chloroauric acid, such as gold compound, in aqueous solution, ethanediol (or ethylene glycol) as reducing agent and polyvinylpyrrolidone as selector agent for the crystallographic growth. The molar ratio between polyvinylpyrrolidone (selector agent for the crystallographic growth) and gold (precious metal) introduced into solution was equal to 15 and the synthesis step was conducted at a temperature of 150°C.

The chloroauric acid in aqueous solution was obtained by means of a solution of gold in aqua regia, as specified in the preceding embodiment. The ethanediol and polyvinylpyrrolidone were first solubilized in a further dose of ethanediol, which was then also used as solvent. The chloroauric acid in aqueous solution was then added dropwise to the pre-solution of the selector agent for the crystallographic growth and the reducing agent in distilled water, while such pre- solution containing ethanediol and polyvinylpyrrolidone was maintained under stirring; Following the introduction of the gold compound, the synthesis solution was maintained under stirring for 8 hours.

At the end of the synthesis step, the formed particles were separated from the solution, washed and sterilized.

Upon observation at the scanning electron microscope of the particles thus obtained, it was possible to verify that the fraction of nanometric particles with substantially spherical form was considerably reduced, since over 90% by weight of the particles consisted of plate-like particles with desired geometry and size, as is quite clear in the enclosed Figures 2 and 2a.

The composite material specimens made with the powders thus obtained were characterized by a greater gloss and by a brighter gold color with respect to the specimens made from the powders obtained in accordance with the above- described first embodiment, in line with the lower presence of nanometric gold particles with substantially spherical shape.

As specified above, the process according to the present invention comprises a step for arranging at least one polymer material, having the above- specified physical characteristics for the transmission of the light radiation, a step for closely mixing a dose of polymer material and a dose of metal powder and a step for cross-linking the dose of polymer material with the dose of metal powder distributed substantially uniformly therein.

Of course, without departing from the protective scope of the present invention, the composite material according to the present invention can also comprise other components such as additives (in particular dyes), fibers etc. In accordance with a preferred embodiment of the process according to the present invention, the polymer material used in the process is a high consistency silicone rubber (HCR) and the close mixing step is preferably obtained by means of a cylinder mixer. In particular the silicone rubber HCR employed preferably has a Shore hardness comprised between 20 and 70. The use of such polymer material, already having a pasty consistency before the cross-linking step, facilitates the dispersion of the metal powder at its interior, which can be made as specified above by means of a machinery diffused in the rubber-plastic industry; the polymer material also facilitates the maintenance under dispersion of the metal powder, in particular preventing the formation of sedimentation phenomena during the cross-linking step. The latter can be advantageously conducted at high temperatures, generally comprised between 100°C and 200°C, in accordance with the specific silicone rubber employed.

In particular, the cross-linking step is advantageously preceded by a step of forming a decorative article. The latter can be advantageously obtained via molding, e.g. with injection-compression molding or via pressing, or via extrusion.

If the step for forming the decorative article is obtained by means of molding, the heating of the mold can be advantageously provided, in order to facilitate at least one partial cross-linking of the polymer material already inside the mold and to allow a facilitated extraction thereof from the latter. In order to make the extraction of the article from the mold even more facilitated, a release agent is preferably distributed on the internal walls of the mold.

The silicone rubbers HCR have suitable physical characteristics for the light radiation transmittance, which render them suitable for being employed for obtaining composite materials in accordance with the present invention. In addition, they are characterized by high mechanical strength, generally comprised between 5 and 14 MPa, by an optimal elongation at break, generally comprised between 50% and 800%, as well as optimal properties of wear resistance, chemical resistance and aging resistance. In addition, such polymer materials are non-toxic and non-allergenic, hence they can come into contact with the skin even for prolonged time periods without causing any undesired effect. In addition, the aforesaid materials form a very strong interface with the precious metal powder, remedying phenomena of release of the powder particles which could occur over time following mechanical stresses imparted to the decorative articles or following wear.

Otherwise, liquid silicones could be used as polymer materials in the process according to the present invention. In the latter case, after the close mixing step and after a step for depositing the liquid silicone - with the metal powder dispersed therein - in a mold having the form of the decorative article that one wishes to obtain, the cross-linking can also be attained at ambient temperature. In particular, such liquid silicones must advantageously have a viscosity comprised between about 15,000 mPa*s and about 80,000 mPa*s, in order to be mixable and at the same time to prevent that the metal powder contained therein can sediment during their cross-linking. Liquid silicones which are adapted to being employed in the production process according to the present invention are for example silicones that can be found on the market with the commercial names Silpuran® 6000/05 AB and Lumisil® LR 7600/60 A/B by Wacker Chemie AG, or Silopren® LSR 7060 by Momentive Performance Materials Inc., or Dow Corning® OE-6636 Optical Encapsulant by Dow Corning Corporation.

The latter polymer materials have the advantage of having a high transmission of the light radiation, even greater than 90% for thicknesses of 2 mm; nevertheless they have several process limits, requiring particular care in the mixing and cross-linking steps in order to prevent the metal powder from sedimenting within the polymer material, in particular if long cross-linking times are provided for.

Generally, thermoplastic materials are adapted to be employed as polymer materials in the process according to the present invention; such thermoplastic materials are provided with the previously-specified optical characteristics of transparency and transmission of the light radiation, and can for example include polycarbonate.

The process in accordance with the present invention is particularly simple and inexpensive to achieve and allows obtaining composite materials provided with optimal color and gloss characteristics. In particular the step for synthesizing the metal powder implemented for the process, subject of the present invention, is characterized by high yields (even greater than 90%, as specified above) which make it particularly appealing for industrial use, unlike the processes known up to now.

Also forming the object of the present invention is a composite material for decorative articles in particular obtained by means of a process of the above- described type, which comprises at least one polymer material having a transmission of the light radiation in the visible spectrum, measured according to the ASTM D1003 standard test, equal to at least 70% and preferably equal to at least 80% for thicknesses of the polymer material of 2 mm, in which one or more metal powders are dispersed in a substantially uniform manner, each of a corresponding precious metal and formed for at least 60% by weight by plate-like metal particles having preferred two-dimensional extension with dimensions comprised between 0.8 and 75 μπι and having thickness equal to at least 500 nm (but advantageously comprised between 600 and 2000 and still more advantageously between 800 and 1600 nm). In particular, as previously specified, the composite material can comprise particles of only one metal powder or of multiple metal powders that are mixed together.

The composite material in accordance with the present invention has optimal color and gloss characteristics, due to the metal powder contained therein, comprising particles having the size and geometry characteristics that are specified above. More in detail, the polyhedral geometry of the particles facilitates the reflection of the light radiation in different directions, conferring gloss and sheen to the articles obtained with the composite material according to the invention that are greater than those of the normal metal alloys generally used in the gold-working field.

In addition, the predominant dimensions of the plate-like particles, comprised between 0.8 and 75 μιη, ensure an optimal reflection of all the wavelengths of the light radiation, nevertheless without being distinguishable to the naked eye, as would otherwise occur if the metal particles had greater size. Indeed, the thickness of the particles, equal to at least 500 nm and in particular comprised between 500 nm and 2000 nm (and preferably comprised between 600 and 2000 and still more advantageously between 800 and 1600 nm), contributes the reflection of the light radiation, such that all the particles irradiated by the light radiation, even when arranged edgewise, substantially reflect the wavelengths of the latter, conferring precious aesthetic characteristics of color and gloss to the article obtained with the composite material, subject of the present invention.

As indicated above, the composite material according to the present invention can exclusively comprise the polymer material and the metal powder, or it can also provide for other components such as additives (in particular dyes), fibers etc.

Preferably, the polymer material can then be part of a matrix of polymer material containing, in addition to the polymer material, one or more additives to which the precious metal powders will then be added in order to form the composite material according to the invention.

The composite material according to the present invention, having the characteristics specified above during the description of the process for the production thereof, is also provided with optimal mechanical characteristics, corrosion resistance characteristics and aging resistance characteristics.

Merely by way of example, the table of Figure 4 reports the results of several mechanical tests carried out on specimens of composite material according to the present invention that were made with a silicone rubber matrix HCR and with increasing quantities of metal powder and in particular of dispersed gold powder. In particular the following were executed on such specimens: the measurement of the Shore A hardness according to the ASTM D2240 standard, a tear resistance test according to the ASTM D624-00 standard, a mechanical tensile strength test according to the ASTM D638 standard and an elongation at break test according to the ASTM D1708 standard.

During the mechanical tests, the specimens were V-cut, in order to be evaluated in their most critical use conditions, i.e. in order to evaluate their mechanical strength following the formation of a crack.

The specific weight was also measured for each specimen according to the ISO 1 183 standard. The progression of the specific weight of the composite material as a function of the percentage of gold powder loaded in the polymer material is reported in the graph of Figure 3.

As can be seen from the results of the mechanical tests, the specimens demonstrated optimal mechanical properties even in critical conditions (i.e. in the presence of a crack, as specified above). In particular, with the increase of gold powder content, the specimens showed an increasing hardness and decreasing values of tear resistance, tensile strength and elongation at break.

The composite material in accordance with the present invention is adapted to be employed for obtaining decorative articles, as reported above. In addition, the present composite material can be advantageously employed also for the obtainment of health or medical articles or parts of health or medical articles, such as portions of toothbrushes, night bite guards or the like, in particular when at least partly loaded with silver powder that is capable of conferring microbicidal and fungicidal characteristics to the final product.

The finding thus conceived therefore attains the pre-established objects. Of course, in the practical achievement thereof, it can also assume shapes and configurations that are different from that illustrated above, without departing from the present protective scope.

In addition, all details can be substituted with technically equivalent elements and the sizes, shapes and materials used can be of any type according to requirements.