WO2014122574A2

WO2014122574A2 - Process of activation of metal silver

Info

Publication number: WO2014122574A2
Application number: PCT/IB2014/058773
Authority: WO
Inventors: Angelo Ramponi; Sandro RECCHIA
Original assignee: Conceria Stefania S.P.A.; Università Degli Studi Dell'insubria
Priority date: 2013-02-11
Filing date: 2014-02-04
Publication date: 2014-08-14
Also published as: WO2014122574A3; ITMI20130191A1

Abstract

The present invention relates to a process for surface antibacterial activation of metal silver comprising the following steps: a) oxidative aggression in an acid aqueous environment of a portion of the surface of the metal silver with formation of silver ions; b) raising the pH value and adjusting the redox potential of the material in the suspension obtained in step a); c) fixing the silver ions in the form of insoluble or slightly soluble, inorganic and organic salts on the surface of the metal silver as treated in step a) by the addition of a precipitating reagent corresponding to the insoluble or slightly soluble, inorganic or organic silver salt of interest; d) drying the metal silver coated by salified ions as obtained in step c); e) further optional step of partial chemical reduction of the salified silver ions present on the surface of the metal silver.

Description

"PROCESS OF ACTIVATION OF METAL SILVER"

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Field of the invention

The present invention relates to a method for treating metal silver surfaces having a non- nanometric size (typically yarns and particles) , suitable to provide the same with strong antibacterial capabilities.

The present invention is particularly meant for manufacturing activated silver, to be used advantageously as additive for manufacturing items which need to have strong antibacterial features. Possible examples of such items are: a) items meant to come at least partially into contact with the skin of end users, such as clothing items (for example jackets, shoes, watchstraps, sofa and seat covers) ; b) medical, pharmaceutical and cosmetic preparations (gauzes, tools, ointments and creams) for which it is desirable to avoid the use of antibacterial chemicals; c) surfaces of containers which must ensure asepsis of the contents. The list is of course by way of example and not by way of limitation.

State of the art

A large number of methods and processes have been developed over the years to blend silver into the most varied items in order to exploit advantageously its antibacterial properties. By way of example, some of the most recent patents envisage the use of silver in deodorants, in gauzes for treating open wounds, in manufacturing packaging paper with antibacterial properties, in polymer materials, in the surface treatment of fabrics and hides and in many other fields.

Scientific research highlighted that the antibacterial properties of silver are mainly related with the interaction of silver ions with bacterial cell membranes. Silver ions perform their antibacterial action both in free form (in the form of aquo ions) and in a bound or weakly-bound form (organic and inorganic complexes, slightly soluble or completely insoluble salts) .

Silver in metal form is able to perform an antibacterial action comparable to that of silver in ionic form only provided that nanometric-sized particles (having a diameter size of less than 100 nanometers) are used. Such evidence is to be associated with the high defectiveness of surface silver atoms found in metal nanoparticles , which makes said atoms similar to bound silver ions from the standpoint of their reactivity. The antibacterial features of silver metal nanoparticles are accordingly rapidly lost as the size of the same increases: metal silver shows no antibacterial activity already at a micrometric level.

In some applications, such as for example the manufacturing of fabrics meant to come into direct contact with the skin of end users, silver nanoparticles are preferable to ionic silver for basically two reasons: i) metal silver is more stable in time, since it gives rise to a very limited transfer to sweat; ii) silver is the element which in its metal form features the highest level of thermal conductivity, hence ensuring the excellent thermal comfort of the fabrics containing it.

By contrast, it should be highlighted that over the last few years alarm with regard to the dangerousness of nanoparticles for human health has increased exponentially. However, research worldwide and the relevant knowledge in the field of the toxicology of nanomaterials are still at an early stage, so that new results about the dangerousness of nanomaterials follow one another at an unrelenting pace. The dangerousness of nanomaterials is basically related to the extremely reduced size of particles, which enables the same to reach the innermost tissues of the human organism.

By way of examples, recent studies proved that metal silver nanoparticles are able to effectively attack not only bacteria, but also the cells of human liver tissue.

The growing level of toxicological knowledge hence makes it conceivable to anticipate that items based on the use of nanomaterials which are currently being marketed shall be withdrawn on account of their toxicity.

In the field of silver nanoparticles, in order to overcome such problems, processes have been envisaged entailing the blend of nanoparticles in polymer matrixes based on polystyrene or even polyvinylchloride . With regard to the use of nanoparticles in the textile field, however, two drawbacks arise from this solution: i) it is no longer possible to claim that the fabrics are manufactured by using exclusively natural substances; ii) the thermal comfort provided by the use of silver alone strongly decreases.

Alternatively, some solutions envisage the arrangement of silver nanoparticles on microparticles of inorganic oxides such as for example silica and titania. In this case, it is certainly possible to state that said materials are of a natural origin, although the problem of the scarce thermal conductivity of the same remains.

It should be highlighted, however, that, although these materials are safe for end users, their production at any rate entails problems and non negligible additional costs to monitor and protect work environments and operators.

There is accordingly a strong need to overcome the above-mentioned objective limits, in view of developing a material based on silver and featuring antibacterial properties, having a non nanometric size and being attainable in great amounts at a reduced cost and without particular risks for the health of both manufacturing operators and end users.

The notion on which the steps of the process covered by the present invention is based essentially relates to a method which alters the nature of the surface of non-nanometer metal silver particles so as to provide the same with strong antibacterial capabilities .

Summary of the invention

By accomplishing research in the present technical field, the applicant surprisingly and unexpectedly implemented a process for the surface antibacterial activation of metal silver, a process for the preparation of metal silver with enhanced antibacterial activity comprising on the surface insoluble or slightly soluble, inorganic or organic salts of silver ions, as well as the metal silver hence modified having enhanced antibacterial properties and the use of said metal silver for antibacterial purposes.

The process for the surface antibacterial activation of metal silver comprises the following steps :

a) oxidative aggression in an acid aqueous environment of a portion of the surface of the metal silver with formation of silver ions: this step is performed by using oxidising acids in aqueous phase

(such as for example nitric acid) or else by using mixtures of non oxidising acids + oxidising agents

(such as for example sulphuric acid + hydrogen peroxide) ;

b) raising the pH value and adjusting the redox potential of the material in the suspension obtained in step a) : step b) is closely linked to the reagents used in steps a) and c) , so that in some cases a dilution is sufficient, whereas in other cases the pH must be raised by using a base (such as for example sodium hydroxide) , in other cases still it is necessary to add a sacrificial reagent in order to reduce the redox potential of the suspension;

c) fixing the silver ions in the form of insoluble or slightly soluble, inorganic and organic salts on the surface of the metal silver as treated in step a) by the addition of a precipitating reagent corresponding to the insoluble or slightly soluble, inorganic or organic silver salt of interest: for step c) it is possible to use precipitating agents corresponding to the form of salification of interest, in solid form or in aqueous solution, such as for example sodium sulphide, sodium chloride, sodium carbonate, sodium thiocyanate and others;

d) drying the metal silver coated by salified ions as obtained in step c) : depending on the choices made in the preceding steps and on the type of metal silver being treated, this step can be performed either at ambient temperature or at higher temperatures, by using proper drying equipment, the duration of this step ranging from a few minutes to several hours depending on the conditions used;

e) further optional step of partial chemical reduction of the salified silver ions present on the surface of the metal silver: for this optional step, depending on the chemical nature of the salified ionic silver on the surface of the metal silver and on the use envisaged for the silver thus attained, a partial chemical reduction of the surface ionic silver can be advantageously performed by means of photo-reduction, reduction by metal hydrades or electrochemical reduction processes.

A further purpose of the present invention is: the process for the preparation of metal silver comprising on the surface insoluble or slightly soluble, inorganic or organic salts of silver ions, comprising the same steps of the activation process as described above,

the metal silver thus obtained,

metal silver comprising on the surface insoluble or slightly soluble, inorganic or organic salts of silver ions,

process for the preparation of metal silver with enhanced antibacterial activity comprising on the surface insoluble or slightly soluble, inorganic or organic salts of silver ions, wherein said surface has a minimum thickness of 50 nanometers,

use of the metal silver as obtained according to the present invention for manufacturing items and semi-finished products wherein silver is introduced to provide the item or the semi-finished product with a significant antibacterial activity. Detailed description of the invention

A purpose of the present invention is accordingly a process for the antibacterial activation of the metal silver surface comprising the following steps:

a) oxidative aggression in an acid aqueous environment of a portion of the surface of the metal silver with formation of silver ions;

b) raising the pH value and adjusting the redox potential of the material in the suspension obtained in step a) ;

c) fixing the silver ions in the form of insoluble or slightly soluble, inorganic and organic salts on the surface of the metal silver as treated in step a) by the addition of a precipitating reagent corresponding to the insoluble or slightly soluble, inorganic or organic silver salt of interest;

d) drying the metal silver coated by salified ions as obtained in step c) ;

e) further optional step of partial chemical reduction of the salified silver ions present on the surface of the metal silver.

According to the process of activation of the present invention, depending on the type of metal silver to be treated, step a) of oxidative aggression can be advantageously preceded by a step consisting of washing with organic solvents the metal silver surface to be treated. It is indeed known that, for example, powder silver obtained by grinding is normally mixed with additives consisting of organic compounds used to avoid agglomeration during grinding. The presence of said compounds may interfere with all the process steps, so that this prior washing of the surfaces to be treated may be advisable .

Step a) in itself can be performed by using the following standard treatments:

use of oxidising acids in aqueous phase. This kind of treatment envisages the use of acids which are soluble in water with sufficient oxidising power to change the surface metal silver into ionic silver. Acids such as, for example, nitric acid or perchloric acid can be advantageously used in this step. The acid concentration must not be too low, otherwise the aggression of the surface would be too time-consuming. By way of example, it was experimented that concentrations of nitric acid equalling or exceeding 3% can be advantageously used in this step. The amount of acid to be used depends on the extent of the transformation which is desired and at any rate must be determined by means of stoichiometrical calculations.

use of mixtures of non oxidising acids + oxidising agents. Should the use of oxidising acids be incompatible with the following steps of the treatment, preferably mixtures of non oxidising acids and oxidising agents and more preferably oxidising agents which are easily removable from the reaction suspension can be used. By way of example, the mixtures of sulphuric acid + hydrogen peroxide can be advantageously used since any excess of hydrogen peroxide can be decomposed by simply heating the suspension .

Step b) of the process according to the present invention is aimed at achieving the quenching of the oxidative aggression reaction produced in step a) and at creating a favourable environment for the process of fixation of the following step c) . As a consequence, this step is closely linked to the reagents used in steps a) and c) , so that the following standard treatments are possible:

dilution with distilled water. In some cases it is sufficient to perform a dilution to stop the aggression reaction. By way of example, if nitric acid is used, a dilution bringing the acid concentration below 3% can basically stop the reaction .

pH increase by using bases. Should step c) be performed at a pH above 2, it may be preferable to stop the reaction of step a) by using aqueous bases at intermediate concentrations, such as for example aqueous solutions of sodium carbonate or potassium carbonate. It is advisable to use intermediate concentrations in order to avoid an excessive increase of temperature;

use of a sacrificial reagent: depending on the chosen fixation process, it may be preferable to preserve the extremely acid pH conditions of step a) , stopping the aggression reaction by using a sacrificial reagent in order to reduce the redox potential of the suspension. By way of example, solutions of iron salts (II) and tin (II) but also solutions of organic reducers such as calcium oxalate. The amount of sacrificial agent to be used must be determined by means of stoichiometrical calculations .

For step c) of fixation of silver ions in the form of insoluble or slightly soluble, inorganic and organic salts on the surface of the metal silver, two possible kinds of treatment are envisaged:

fixation of ionic silver in the form of slightly soluble salts. This treatment can be performed by adding directly a precipitating reagent in powder or in aqueous solution. The precipitating agent must be added with caution so as to avoid that the nucleation process may be favoured compared to the process of growth of the surface activation layer. By way of example, sodium chloride, sodium carbonate, sodium thiocyanate and others can be used in this step. By operating in these conditions, a surface layer which is strongly adherent to the silver surfaces is achieved;

fixation of ionic silver in the form of insoluble salts: if it is deemed appropriate to use a completely insoluble ionic form of surface silver, such as for example silver sulphide, it is preferable to avoid direct fixation with a sulphide source and instead act through a substitution reaction of an already activated phase according to step c) type 1). This avoids the formation of insoluble salt particles not adhering to the metal silver surface. By way of example, a silver sulphide activated surface can be achieved by adding, in appropriate conditions, an aqueous solution of sodium sulphide to a suspension containing silver chloride activated silver.

For step d) , depending on the choices made in the previous steps and on type of silver being treated, drying can be performed at temperatures above ambient temperature by using proper drying equipment, such as stoves or ventilated dryers. Before performing this step it is preferable to wash the activated silver with distilled water, in order to avoid the undesired crystallization of salts on the surface of the activated silver. The duration of this step is extremely variable and depends on the size of the silver being treated and on the amount of water still present in the suspension. Typically, the duration of this step can range from a few minutes to several hours.

For the further optional step e) , if ionic silver is fixed through the formation of slightly soluble salts, it may be advantageous to perform a partial reduction of the surface ionic silver. It has been remarked that the formation of a reduced layer does not bring prejudice to the antibacterial activity and can advantageously provide higher stability against ionic silver transfer. This step can be advantageously performed by means of three different types of processes:

photo-reduction: to this end the sunlight, either natural or simulated, but also an ultraviolet light source, can be used. Said process can be used if the activation layer is light sensitive and if a partial reduction of the activated layer is desired. By way of example, an activation layer of silver chloride can be advantageously reduced by exposure to sunlight for at least 4 hours.

reduction by metal hydrades: if the activation layer is not light sensitive the advantageous results arising from reduction can be achieved through reduction by metal hydrades. By way of example, a layer of silver phosphate can be advantageously reduced by using in appropriate conditions a solution of lithium aluminium hydride.

electrochemical reduction: if the activation layer is not too thick and hence the activated silver still shows a good electric conductivity and sufficiently low electric resistance values, it may be advantageous to perform an electrochemical reduction by applying a sufficiently negative electric potential to the activated silver. The potential value depends on the nature of the activated layer and at any rate is never higher than +0.4V compared to the standard reference potential of the hydrogen electrode.

As a further form of preferred embodiment of the process of antibacterial activation of the metal silver surface according to the present invention: step a) consisting of oxidative aggression is preferably performed:

by the use of oxidising acids in aqueous phase, i.e. acids soluble in water with sufficient oxidising power to transform the surface metal silver into ionic silver, such as for example nitric acid or perchloric acid; or

by the use of mixtures comprising non- oxidising acids and oxidising agents, the latter preferably of the type which can easily removed from the reaction suspension, mixtures such as sulphuric acid and hydrogen peroxide wherein any excess of hydrogen peroxide can be decomposed by simply heating the suspension;

step b) consisting of raising the pH and adjusting the redox potential of the material in the suspension in order to stop step a) is preferably performed:

by dilution with water, preferably distilled, or by using bases, preferably aqueous bases at intermediate concentrations such as for example aqueous solutions of sodium carbonate or potassium carbonate, or

by using a sacrificial reagent in order to reduce the redox potential of the suspension wherein step a) is performed, for example by using solutions of iron salts (II) and tin (II) or even solutions of organic reducers such as calcium oxalate;

step c) consisting of fixing silver ions on the surface of metal silver particles in the form of salts is performed:

for slightly soluble, inorganic or organic salts by the gradual, direct addition of a corresponding precipitating reagent in powder or in aqueous solution, such as for example sodium chloride, sodium carbonate, sodium thiocyanate, or

for insoluble, inorganic or organic salts, by the formation of a slightly soluble salt with the addition of the corresponding precipitating reagent, for example obtaining silver chloride through the addition of sodium chloride, followed by a substitution reaction using a specific precipitating agent, for example using a solution of sodium sulphide to obtain the silver sulphide of interest; step d) consisting of drying the particles obtained in step c) is preferably performed both at ambient temperature and at higher temperatures by using proper drying equipment such as stoves or ventilated dryers;

the further optional step e) consisting of the partial chemical reduction of the salified silver ions present on the surface of the metal silver is preferably achieved by means of photo-reduction, reduction by metal hydrades or electrochemical reduction processes, as previously described.

A further purpose of the present invention is a process for the preparation of metal silver comprising on the surface insoluble or slightly soluble, inorganic or organic salts of silver ions, comprising the same steps included in the process of antibacterial activation of the metal silver surface as described above.

In particular, as forms of preferred embodiments of the processes covered by the present invention, both the process of antibacterial activation of metal silver and the process of preparation of metal silver comprising on the surface insoluble or slightly soluble, inorganic or organic salts of silver ions, it is highlighted that: in step a) consisting of the oxidative aggression in an acid aqueous environment of a portion of the surface of the metal silver with formation of silver ions, said surface has a minimum thickness of 50 nanometers;

step c) consisting of fixing the silver ions in the form of insoluble or slightly soluble, inorganic or organic salts on the surface of the silver particles is preferably performed by the addition of a precipitating reagent in the form of a soluble salt, preferably a sodium salt, corresponding to the slightly soluble silver salt which is meant to be fixed; preferably said precipitating reagent is sodium carbonate, sodium oxalate, sodium chloride, sodium sulphate, sodium iodate, sodium sulphite;

step c) consisting of fixing the silver ions in the form of insoluble or slightly soluble, inorganic or organic salts on the surface of the silver particles is preferably performed by the addition of a precipitating reagent in the form of a soluble salt, preferably a sodium salt, corresponding to the insoluble silver salt which is meant to be fixed; preferably said precipitating reagent is sodium iodide, sodium bromide, sodium thiocyanate, sodium sulphite, sodium phosphate. Step c) consisting of fixing the silver ions in the form of salts on the surface of the metal silver particles preferably entails the formation of salts chosen from the group comprising: silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate, silver thiocyanate.

A further purpose of the present invention is the metal silver comprising on the surface silver ions in the form of insoluble or slightly soluble salts, in particular what is directly obtained from the processes covered by the present invention, more preferably metal silver whose surface comprising silver salts has a minimum thickness of 50 nanometers, even more preferably the metal silver comprising on the surface organic or inorganic salts chosen from the group comprising silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate, silver thiocyanate.

The metal silver covered by the present invention features an enhanced antibacterial activity and said metal silver, covered by the processes of antibacterial activation or of preparation of metal silver comprising on the surface silver salts, may take on the most varied shapes, morphologies and sizes, such as for example: nets, fabrics, yarns, bars, laminated pieces, sponges, particles of different shapes and sizes (spheres, flakes, powders obtained through grinding or electrochemical treatment) . This confirms the further benefit of the processes covered by the present invention, their versatility. Said processes can indeed be applied regardless of the nature, shape, morphology and size of the metal silver to which the processes can be applied.

Further benefits related to the processes covered by the present invention are the combined effectiveness, straightforwardness, ease of implementation of the steps which constitute the processes, the low costs for implementing said steps, the moderate costs of the materials needed for their implementation and the moderate costs of the implementation and combination of the individual steps. All this makes said processes readily applicable on an industrial scale.

A further purpose of the present invention is the use of metal silver comprising on the surface silver ions in the form of insoluble or slightly soluble salts according to the present invention for the antibacterial treatment of materials or items or for providing them, in bulk or in the surface, with antibacterial properties. The above uses wherein the metal silver surface comprising silver salts has a minimum thickness of 50 nanometers are particularly preferred; even more preferred are the above uses wherein the metal silver comprises on the surface organic or inorganic salts chosen from the group comprising silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate, silver thiocyanate.

The uses specified above hence make it possible to manufacture materials or items with enhanced antibacterial properties on account of the presence, either in bulk or on the surface of said materials or items, of the metal silver comprising on the surface silver ions in the form of insoluble or slightly soluble, organic or inorganic salts according to the present invention.

Said materials or items containing metal silver according to the present invention may be of the most varied kinds, such as for example natural and synthetic fibres and fabrics for use in several fields (clothing, underwear, footwear, gauzes, sofa and seat covers), polymer films, metal and plastic surfaces (also medical tools), food containers, drug containers (ointments, gels, creams) and containers in general eligible to contain easily biodegradable materials, packages and deodorants.

As pointed out, the metal silver treated according to the processes covered by the present invention features enhanced antibacterial properties. Said enhanced antibacterial properties have been tested on metal silver treated with the process covered by the present invention and have been compared, under the same conditions, to the antibacterial properties of the same untreated metal silver .

MIC (Minimum Inhibitory Concentration) tests have been performed in accordance with protocol ASTM E2149 -10 on both untreated metal silver and metal silver being treated according to the process covered by the present invention.

Protocol ASTM E2149 - 10 "Standard method for determining the antimicrobial activity of immobilized antimicrobial agent under dynamic contact condition" tests the antibacterial activity of materials, using known amounts of bacterial strains of Escherichia coli and counting progressively in time the residual bacterial charge: the antibacterial capabilities are hence tested on an immediate level as well as in the course of time.

Untreated silver does not show significant reduction rates, at most less than 50% of bacterial charge reduction on an immediate level and down to less than 30% in the course of time (after 6 hours) , whereas, with the same concentration, the metal silver treated according to the present invention shows 100% of bacterial charge reduction both on an immediate level and 6 hours after the test start.

In order to better understand how the different kinds of treatment may be favourably combined, some of the possible combinations depending on the material which is to be achieved are schematically shown. If the activation of a silver net is desired wherein activation is limited and forms an activation layer in insoluble ionic form of silver sulphide, the steps can be combined sequentially according to the following procedure:

The metal silver net is submitted to oxidative aggression by using a mixture of sulphuric acid and hydrogen peroxide, wherein hydrogen peroxide in excess is removed by simply heating the reaction bath. The pH value of the suspension wherein the net remains soaked is then raised by adding an aqueous solution of sodium carbonate. Having achieved a pH value of about 7, a diluted aqueous solution of sodium chloride is added slowly and under strong stirring (to this end the net itself may act as an agitator) until the ionic silver is completely fixed in the form of silver chloride. In order to fix silver in the insoluble form of silver sulphide, an aqueous solution of sodium sulphide is then added under stirring: the substitution reaction from silver chloride to silver sulphide has a duration which depends on the thickness of the activation layer and typically requires at least 30 minutes. The activated net is washed and dried for 2-4 hours at temperatures ranging from 40 to 60°C and is ready for use.

If an activation of micrometric powder silver is wished wherein the activation is substantial and forms an activation layer in the insoluble ionic form of silver phosphate, the steps can be combined sequentially according to the following procedure:

The micrometric powder metal silver is submitted to oxidative aggression by using aqueous nitric acid at concentrations ranging from 5% to 10%. The reaction is blocked by adding a solution of sodium oxalate (sacrificial agent) in large excess. The pH value of the suspension under stirring is raised by adding slowly an aqueous solution of sodium carbonate until a pH of about 9 is achieved. While this step of pH increase is under way, the ionic silver is simultaneously fixed in the form of silver oxalate. In order to fix silver in the insoluble form of silver phosphate, an aqueous solution of sodium phosphate is then added under stirring: the substitution reaction from silver chloride to silver phosphate has a duration which depends on the thickness of the activation layer and typically requires at least 4 hours. The activated micrometric silver powder thus obtained is washed and dried for at least 8 hours at temperatures over 80 °C and is ready for use.

If an activation of micrometric powder silver is wished wherein the activation is substantial and forms an activation layer in slightly soluble, light sensitive, partially reduced ionic form, the steps can be combined sequentially according to the following procedure:

The micrometric powder metal silver is submitted to oxidative aggression by using aqueous nitric acid at concentrations ranging from 10% to 30%. The reaction is blocked by adding distilled water until the concentration of nitric acid falls below 3%. having blocked the aggression reaction at the desired level, a diluted aqueous solution of sodium chloride is added slowly and under strong stirring until the ionic silver is completely fixed in the form of silver chloride. The micrometric silver powder thus obtained is washed and dried for at least 8 hours at temperatures over 80°C. In order to perform photo activation, the powder is placed into a container arranged on an orbital agitator and is irradiated with simulated sunlight for at least 4 hours. The powder thus obtained is ready for use.

A further purpose of the present invention is metal silver, both the one directly obtained through the process covered by the present invention and the one comprising on the surface insoluble or slightly soluble, organic or inorganic silver salts, preferably metal silver whose surface comprising silver salts has a thickness of 50 nanometers, more preferably metal silver according to the present invention, particularly when it is in the form of micrometric particles, characterised by a ratio in minimum weight between the salified surface and the silver mass amounting to 1%.

A further purpose of the present invention is the use of the process and of the products directly obtainable from said process, as well as of the products characterised by a minimum weight ratio between salified surface and silver mass amounting to 1% in the industrial fields where enhanced antibacterial activity is to be achieved.

The following forms of embodiment of the invention are only by way of example and not by way of limitation.

EMBODIMENT EXAMPLE No . 1. Achievement of photo- reduced micrometric powder silver.

Starting from pure powder metal silver obtained by electrochemical reduction, to which no anticaking agent has been added, the metal silver can be transformed by using agueous nitric acid, preferably at a concentration above 3% and more preferably at a concentration ranging from 10% to 30% for no longer than 10 minutes. The treatment duration depends on the percentage of silver mass which is to be transformed, excellent antibacterial properties are achieved with transformation rates preferably higher than 3% and more preferably higher than 10%.

Aqueous sodium hydroxide is then added, preferably at a concentration above 1 molar, until pH values exceeding 2 are achieved. A diluted aqueous solution of NaCl in stoichiometric excess compared to the amount of silver submitted to aggression is added slowly and under strong stirring to the suspension thus obtained.

The micrometric powder of activated silver is washed at least 3 times with distilled water and is then reclaimed by filtration or centrifugation . The residual water is removed by a conveyed air flow dryer at temperatures preferably above ambient temperature and more preferably ranging from 40°C to 60°C.

After drying, the activated silver micropowder is partially photo-reduced, preferably by exposure to natural or simulated sunlight or to ultraviolet light and more preferably to simulated sunlight, making sure that the powder is stirred through orbital agitation .

The powder thus obtained is ready to be scattered on surfaces through specific processes which depend on the envisaged uses, ensuring a rate of bacterial reduction amounting to 100% or at any rate exceeding 98% already at a concentration of 0.1 grammes per square meter of treated surface. The powders obtained through the process described above have been tested in order to check for their antibacterial properties by comparing them, under the same conditions, with the antibacterial properties of metal silver powders not submitted to treatment according to the present invention.

MIC (Minimum Inhibitory Concentration) tests have been performed in accordance with protocol ASTM E2149 -10 on both untreated powder metal silver and powder metal silver on whose surface AgCl according to the process covered by the present invention was scattered.

Protocol ASTM E2149 - 10 "Standard method for determining the antimicrobial activity of immobilized antimicrobial agent under dynamic contact condition" tests the antibacterial activity of materials such as the exemplary powders, using known amounts of bacterial strains of Escherichia coli and counting progressively in time the residual bacterial charge: the antibacterial capabilities are hence tested on an immediate level as well as in the course of time.

Untreated silver does not show significant reduction rates, at most less than 50% of bacterial charge reduction on an immediate level and down to less than 30% in the course of time (after 6 hours) , whereas, with the same concentration, the metal silver treated according to the present invention shows 100% of reduction both on an immediate level and 6 hours after the test start.

EMBODIMENT EXAMPLE No. 2. Achievement of activated, micrometric powder silver with high stability .

Starting from pure powder metal silver obtained by electrochemical reduction, to which no anticaking agent has been added, the procedure is as shown in the embodiment example No. 1 until powder is achieved which is activated by fixing ionic silver in the form of silver chloride. An aqueous solution of sodium sulphide is then added to perform the substitution reaction: the duration of this step closely depends on the entity of the initial aggression on the powder and at any rate exceeds 2 hours.

The micrometric powder of activated silver is washed at least 3 times with distilled water and is then reclaimed by filtration or centrifugation. The residual water is removed by a conveyed air flow dryer at temperatures preferably above ambient temperature and more preferably ranging from 40°C to 60°C.

The powder thus obtained is ready to be scattered on surfaces through specific processes which depend on the envisaged uses, ensuring a rate of bacterial reduction amounting to 100% or at any rate exceeding 98% already at a concentration of 1.5 grammes per square meter of treated surface.

Claims

1. A process for surface antibacterial activation of metal silver comprising the following steps:

d) drying the metal silver coated by salified ions as obtained in step c) ;

2. A process as claimed in claim 1 wherein step a) of oxidative aggression is performed:

- either by the use of oxidising acids in aqueous phase, i.e. acids soluble in water with sufficient oxidising power to transform the surface metal silver into ionic silver;

- or by the use of mixtures comprising non- oxidising acids and oxidising agents.

3. A process as claimed in claim 1 wherein step b) of raising the pH value and adjusting the redox potential of the material in suspension in order to stop step a) is performed by:

dilution with water, preferably distilled, or

the use of bases, preferably medium concentration aqueous bases, or

the use of a sacrificial reagent to reduce the redox potential of the suspension in which step a) takes place, or solutions of inorganic reducing agents or solutions of organic reducing agents.

4. A process as claimed in claim 1 wherein step c) of fixing the silver ions on the surface of the metal silver particles in the form of salts is performed:

in the case of slightly soluble inorganic or organic salts by the gradual direct addition of a corresponding precipitating reagent in powder or in aqueous solution, or

in the case of insoluble inorganic or organic salts by the formation of a slightly soluble salt with the addition of the corresponding precipitating reagent, followed by a substitution reaction using a specific precipitating agent to obtain the insoluble salt of interest.

5. A process as claimed in claim 1 wherein step d) of drying the particles obtained in step c) is performed both at ambient temperature and at higher temperatures with suitable drying equipment.

6. A process as claimed in claim 1 wherein the optional additional step e) of partial chemical reduction of the salified silver ions present on the surface of the metal silver is performed by means of photo-reduction processes, reduction with hydrides or electrochemical reduction.

7. A process as claimed in claim 1 wherein in step a) of oxidative aggression in an acid aqueous environment of a portion of the surface of the metal silver with formation of silver ions, said surface has a minimum thickness of 50 nanometres.

8. A process as claimed in claim 1 wherein step c) of fixing the silver ions in the form of salts on the surface of the metal silver particles preferably entails the formation of salts chosen from the -group comprising: silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate.

9. A process for the preparation of metal silver comprising on the surface insoluble or slightly soluble, inorganic or organic salts of silver ions comprises the following steps:

c) fixing the silver ions in the form of insoluble or slightly soluble, inorganic and organic salts on the surface of the metal silver as treated in step a) by the addition of a precipitating reagent corresponding to the insoluble or slightly soluble, inorganic or organic salt of interest;

d) drying the metal silver coated by salified ions as obtained in step c) ;

10. A process as claimed in claim 9 wherein step a) of oxidative aggression is performed: - either by the use of oxidising acids in aqueous phase, i.e. acids soluble in water with sufficient oxidising power to transform the surface metal silver into ionic silver;

or by the use of mixtures comprising non- oxidising acids and oxidising agents.

11. A process as claimed in claim 9 wherein step b) of raising the pH value and adjusting the redox potential of the material in suspension in order to stop step a) is performed by:

dilution with water, preferably distilled, or

the use of bases, preferably medium concentration aqueous bases, or

12. Ά process as claimed in claim 9 wherein step c) of fixing the silver ions on the. surface of the particles of metal silver, in the form of salts, is performed:

in the case of insoluble inorganic or organic salts, by the formation of a slightly soluble salt with the addition of the corresponding precipitating reagent, followed by a substitution reaction using a specific precipitating agent to obtain the insoluble salt of interest.

13. A process as claimed in claim 9 wherein step d) of drying the particles obtained in step c) is performed both at ambient temperature and at higher temperatures with suitable drying equipment.

14. A process as claimed in claim 9 wherein the optional additional step e) of partial chemical reduction of the salified silver ions present on the surface of the metal silver is performed by processes of photo-reduction, reduction with hydrides or electrochemical reduction.

15. A process as claimed in claim 9 wherein in step a) of oxidative aggression in acid aqueous environment of a portion of the surface of the metal silver with formation of silver ions, said surface has a minimum thickness of 50 nanometres.

16. A process as claimed in claim 9 wherein step c) of fixing the silver ions in the form of salts on the surface of the metal silver particles preferably entails the formation of salts chosen from the group comprising: silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate.

17. A metal silver comprising on the surface silver ions in the form of insoluble or slightly soluble salts.

18. A metal silver as claimed in claim 17 wherein the surface of the metal silver comprising silver ions in the form of insoluble or slightly soluble salts has a minimum thickness of 50 nanometres.

19. A metal silver as claimed in claims 17 or 18 wherein the insoluble or slightly soluble salts of silver ions included in the surface of the metal silver are organic or inorganic salts chosen from the group comprising silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate.

20. A use of metal silver comprising on the surface silver ions in the form of insoluble or slightly soluble salts for the antibacterial treatment of materials or articles.

21. A use as claimed in claim 20 wherein the surface of the metal silver which comprises silver ions in the form of insoluble or slightly soluble salts has a minimum thickness of 50 nanometres.

22. A use of metal silver comprising on the surface silver ions in the form of insoluble or slightly soluble, organic or inorganic salts to give antibacterial properties in bulk or on the surface of materials or articles.

23. A use as claimed in claim 22 wherein the surface of the metal silver which comprises silver ions in the form of insoluble or slightly soluble salts has a minimum thickness of 50 nanometres.

24. A use as claimed in claims 20, 21, 22 or 23 wherein the insoluble or slightly soluble salts of silver ions included in the surface of the metal silver are organic or inorganic salts chosen from the group comprising silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate.

25. Materials or articles comprising, in bulk or on the surface, metal silver comprising on the surface silver ions in the form of insoluble or slightly soluble salts.

26. Materials or articles as claimed in claim 25 wherein the surface of the metal silver which comprises silver ions in the form of insoluble or slightly soluble salts has a minimum thickness of 50 nanometres .

27. Materials or articles as claimed in claims 25 or 26 wherein the insoluble or slightly soluble salts of silver ions included in the surface of the metal silver are organic and inorganic salts chosen from the group comprising silver chloride, silver iodide, silver bromide, silver sulphate, silver sulphide, silver iodate, silver phosphate, silver sulphite, silver carbonate, silver oxalate.