GB2498877A - Aqueous silver halide and polyvinyl alcohol antimicrobial composition - Google Patents

Abstract

An aqueous antimicrobial composition comprises particles of a silver halide salt and polyvinyl alcohol (PVA). Preferably the silver halide is silver chloride and has a particle size of 100-200nm. The composition may further comprise a corrosion-inhibiting surfactant, a benzotriazole stabiliser substituted by one or more electron-withdrawing substituents, a hydrophobic polymer emulsion resin binder and/or an inorganic oxide. In another embodiment is a method for preparing the composition by controlled precipitation of silver halide in the presence of polyvinyl alcohol. Also claimed is the use of the antimicrobial composition to provide antimicrobial properties to compositions such as fabrics, textiles and wound dressings; or to articles such as furniture and medical devices; or to building materials; or to materials such as plastics and packaging; or to formulations such as paints, detergents and personal care product; or to films, coatings and surfaces.

Description

Silver Halide Compositions

Field of the Invention

The present invention relates to antimicrobial silver halide compositions, and in particular to aqueous silver halide antimicrobial compositions comprising polyvinyl alcohol. The invention also relates to a method of preparation of the compositions and to their use.

Backeround of the Invention It has long been known that silver metal and certain silver compounds show antimicrobial activity, and such materials have been in use as antimicrobial agents for many years For instance, an early medicinal use of silver was the application of very dilute aqueous silver nitrate solutions to prevent eye infection in newborn babies. Various other silver salts, compounds, colloids, and complexes have also been shown to provide antimicrobial behaviour even in minute quantiLies.

In recent years, build up of resistance to conventional organic antimicrobial agcnts has [ccl to increased interest in the use of silver and other inorganic antimicrobial agents. Silver halides, especially silver chloride, represent a particularly useful class of antitnicrobial silver compound.

For instance, PCT WO 2006/036909 discloses an antimicrobial silver halide composition comprising at least 50 % water, silver halide particles, and a hydrophilic polymer wherein the composition does not substantially gel or solidify at 25°C. Use of this composition for application to a fibre is also disclosed: the preferred hydrophilic polymer is gelatin, and the composition may further comprise a hydrophobic polymer binder, thoLigh the hydrophobic polymer is generally kept separate from the hydrophilic polymer/silver halide particle composition until a short tune prior to use.

One problem that may arise with such formulations is that of poor colloidal stability of the aqueous dispersions of fine silver halide particles resulting in aggregation and settling out of the relatively dense particles from the composition on storage, and also during transport, particularly in hot climates. This can cause unpredictable changes in the concentratiOil of the antimicrobial agent in the composition and also cause inefficient or incomplete transfer of the material from its original container to the end-user.

Thus the compositions need to be stirred or agitated before or during use to redisperse the particles and to maintain homogeneity and consistency in the antibacteriaL concentratation.

However, if the aggregation or agglomeration of particles is not completely reversible, then only liartial redispersion of the particles by the end-user will be possible.

A means of improving the colloidal stability of aqueous antimicrobial silver halide compositions comprising gelatin is disclosed in PCT WO 201 0/02 1662 that provides additives comprising N-lielerocyclic acids of a specific acidity range to improve the redispersihility and the colloidal stability of the dipersions following extended storage.

The additives extend shelf-life of the aqueous silver halide and gelatin formulations, but do not prevent settling of the particles.

It is well known that many silver salts, including the silver halides, are light sensitive and will darken on exposure to light. WO 2006/036909 and 2010/02 1662 both mention that 1.5 the degi-ee of light sensitivity of the gelatin based silver halide compositions they disclose may be niinimised by several techniques known to those skilled in the art including, in particular, storage of the silver halide particles in a low p1-I environment with a pH below 4.5 preferred in this respect. Further, WO 2006/036909 and 20i0/OTI 662 mention that adding compounds of elements such as iron, iridium, ruthenium, paLladium, osmium, gallium, cobalt, rhodium, and the like, to the silver halide particles will also inhibit discoloration of the composition.

An additional issue that has recently arisen is that several silver-containing substances, including silver chloride, have become classified as corrosive for transport, and therefore many antimicrobial silver compositions also have to be classified as corrosive. This causes difficulty and greater expense in transporting the compositions, and thus there has arisen a desire for antimicrobial silver compositions that do not demand cLassification as corrosive.

One consequence of the use of a relatively low pH in silver-containing antimicrobial compositions to reduce darkening on exposure to light is that the low pH tends to increase any corrosive effect. On the other hand, raising the pH of an antimicrobial silver halide composition to improve the corrosivity will make darkening oii exposure to light worse.

Thus there is still a need to provide aqueous antimicrobial silve.r halide compositions that are colloidaily stable, do not settle out on storage, and wherein the particles do not tend to aggregate or agglomerate. Further there is also a requirement that such compositions should not be classified as corrosive for transport.

Summary of the Invention

According to this invention there is provided an aqueous antimicrobial composition comprising: * particles of a silver halide salt; and polyvinyl alcohol.

Detailed Description of the invention

By an aqueous composition is meant a composition that is based on water, though low levels of organic cosolvents up to 1% may sometimes be present.

The silver halide may include silver chloride, bromide, iodide, and mixtures thereof such as, for example, silver chlorobromide. Preferably the silver halide particles are predominantly silver chloride, that is to say the particles are greater than 50 mole percent silver chloride, but more preferably they are greater than 90 mole percent silver chloride and most preferably they comprise silver chloride without deliberate addition of other halides. [he silver halide particles may be homogeneous in composition or the composition may vary within the particle, such as for example a core-shell particle.

Thc-average particle size of the silver halide is not limited, but is preferably less than about I jim, more preferably in the range from 10 to 500 nm, with a size in the range ol 1 00 to 200 niu being especially preferred-The shape of the silver halide particles is not restricted, but the preferred silver chloride particles of the invention generally crystallise as cubes or irregular cLibes.

The proportion of silver halide in the composition is selected to provide a suitable actJvity.

The compositions of the invention are particularly suitable as concentrated fornnilatioiis, intended to be diluted as required by the final user to provide a suitable silver concentration br the particular use envisaged. For a concentrate the silver halide concentration may be up to 20% by weight of the total composition, preferably from 0l0/n to t5% by weight, more preferably from i% to 10%, and especially from 1% to 5% by weight. SLLch concentrates will gcnerally be diluted by the final user to provide a suitable silver concentration for the particular use envisaged such that the final silver concentration is generally below 1%, commonly below 0.1%.

The poly vinyl alcohol of the composition of the invention may be fully saponified or partially saponified, but is preferably at least 80% saponified, and most preferably 88% saponified. The molecular weight of the polyvinyl alcohol is not particularly restricted, bat i.s typically greater than 100,000. Suitable grades of polyvinyl alcohol are L5 commercially available. -Further, the polyvinyl alcohol may be a modified or substitited polyvinyl alcohol, for example caionic polyvinyl alcohol, acetoacetylated polyvinyl alcohol, silylated polyvinyL alcohol, and the like. Such niodified polyvinyl alcohols are commercially available.

Furthermore, mixtures of two or more types of polyvinyl alcohol may be used.

The proportion of polyvinyl alcohol in the composition may be up to 20% by weight of the total composition. For a concentrate it may be up to 20% by weight ol the total composition, preferably from l% to 10% by weight, and most preFerably from 4% to 100/) by weight, especially between 5% and 7% by weight. The type and concentration of the polyvinyl alcohol may be selected such that the concentrate formulation is not too viscous to be handled, stirred, or pumped, but is sufficiently viscous to retain colloidal stability oF the silver halide particles in the preferred size range thus avoiding settling out on storage.

Thus the viscosity of the compositions should be typically no lower than approximately 0.05 Pas when determined at relatively low shear rates and at room temperature, that is to say between approximately 15°C and 25°C, especially at approximately 25°C Preferably viscosity may be in the raiige from approximately 0.05 Pas to approximately I Pas at room temperature and when determined at a shear rate of approximately 100 sec', especially between 0.1 Pas and 1 Pas.

in contiast to the gelatin based silver halide compositions provided by WO 2006/03 6909 and 201 0/021662, the polyvinyl alcohol based compositions of the present invention do not gel when cooled. The type and concentration of the polyvinyl alcohol may be selected such that the compositions of the invention do not increase viscosity excessively when stored cold. For example, at 4°C the viscosity may be up to approximately 5 Pas, preferably between 1 and 2 Pas measured at a shear rate of approximately 100 secL Typically, if allowed to warm above room temperature the viscosity of the compositions at 40°C is rather greater than half that of the same composition at 25°C and at the same shear rate.

it is found that dispersions of silver chloride particles in the preferred siz.e range of 100 to iim are stable in compositions of this viscosity range, hut are not too viscous to be handled or pumped conveniently. By a stable composition is meant a dispersion that is colloidafly stable and resistant to settling out on storage and wherein tile particles do not tend to aggregate or agglomerate. Further the compositions are also stable to storage in the cold and at temperatures above normal room temperature. It is also found that such concentrated formulations are readily diluted for use by the end user. This reduces the polyvinyl alcohol concentration, but storage stability is not important during such use.

The pH of the composition is not particularly limited, hut is generally between approximately 4 and approximately 7, preferably betwecn approximately 5 and 6. If desired, pH may be controfled by appropriate addition of acid or alkali.

According to one preferred embodiment of the invention, there is also present in the composition of the invention a corrosion-inhibiting surfactant, Any suitable corrosion-inhibiting surfactant may be used, but surfactants of the sulphosuccinate class are especially useful. A preferred compound is sodium dioctyl sulphosuccinate that is available commercially and is commonly trivially abbreviated as DOSS. Therefore according to this embodiment of the invention there is provided an antimicrobial composition comprising silver halide, polyvinyl alcohol, and the sodium salt of dioctyl suiphosuccinate. The concentration of the surfactant may be up to approximatcly I 0°A by weight of the composition, preferably up to 6%, especially between 2% and 3%.

Fui-ther. there may he present iii the composition additional optional components known in the art such as, for example, stabilisers. There are many silver halide stabilisers that are well known in the photographic art, and described, for example, in the book Stabilisers for Photographic Silver Halide Emulsions by Gunter Fischer published in 2004 by Kiuwer Academic! Plenum Publishers, New York. Numerous silver halide stabilisers are disclosed in this book, inclLtding for example various nitrogen-containing heterocyclic 1.0 compounds such as benzotriazo]e compounds.

For the current invention, henzotriazole compounds substituted by one or more electron-withdrawing groups are especially preferred.

Therefore according to another embodiment of the invention there is provided an antimicrobial composition comprising silyer halide, polyvinyl alcohol, and a benzotriazole IS compound substituted by one or more electron-withdrawing substituents. Suitable electron-withdrawiiig substituents include fluoro, chloro, bromo, iodo, nitro, trifluoromethyl, eyano, and carboxy groups. Such substituted bcnzotriazole compounds are known in the art. The concentration of the benzotriazole may be up to approximately 1% by weight of the composition, preferably up to 0.1 u/a, especially between 0.001% and 0.05%.

Further there may also be present in the composition of the invention additional optional components to aid in the application of the inventive composition to the end use. For example a hydrophobic polymer emulsion resin binder may he present to assist in adhesion and to iniprove the durability and, robustness of the silver halide particles once applied to the substrate surface. Such polymer emulsions are well known in the art.

Suitable hydrophobic polymer emulsion binder resins include acrylic, polyurethane, polyester, polyvinyl acetate, polyvinyl acetal, styrene-butadiene, vinyl chloride, and vrnyhdine chloride polymers, including copolymers thereof, of which acrylic polymers and polyurethane polymers are preferred. The hydrophobic polymer emulsion may be chosen according to S]Decific requirements of the end use, for instance wash resistance, abrasion, feel, and the like.

The hydrophobic polymer emulsion should be a film-forming binder resin having a retatively low glass transition temperature (Tg), especially a Tg below 0°C with a Tg in the range from about -30°C to about 0°C particularly suitable. The particle size of the emulsion polymer is not particularly restricted, and may be from about 10 ran to about 10 jim with an average size in the range from about 100 nm to about 1 jim especial iy suitable; the particle size may be polydisperse in distribution. Polymer emulsions of this type may comprise up to 50% or more by weight of polymer particles suspended in water, and the emulsion polymer may be added to the silver halide particles in such a ratio that the hydrophobic polymer content may comprise up to about 40% or more of the total composition by weight, preferably from 1% to 10% by weight, especially in the range from 1% to 5% by weight.

One advantage of the silver halide compositions of the present invention is that they are compatible and stable in admixture with the emulsion polymer, and thus it is possible to provide a useful concentrated formulation comprising a single part rather than the two part formulations provided by WO 2006/036909.

Further there may also be present in the composition of the invention an additional inorganic particulate component such as, for example, an inorganic oxide. Suitable inorganic oxides include especially white or colourless particulate inorganic oxides such as, for example, oxides of aluminium, silicon, zirconium, titanium, and zinc, of which zinc oxide is prefcrrred. The inorganic oxide may be treated to aid dispersion in the formulation. Such treated oxides are krown in the art. The particle size of the inorganic oxide is not particularly restricted, but is preferably less than about 1 jim, more preferably in the range from 10 to 500 nm, with a size in the range of 100 to 500 nm being especially preferred.

The concentration of the inorganic oxide may be up to 10% by weight of the total composition, preferably from 0.1% to 5% by weight, especially from 1% to 5% by weight.

S

Furthermore, the composition of the invention may also comprise one or more crosslinkers. Suitable crosslinkers for polyvinyl alcohol arc known, and include in Particular boric acid and salts thereof. Alternatively, the composition may comprise a modified polyvinyl alcohol such as acetoacetylated polyvinyl alcohol and the crosslinker may comprise a crosslinker to react with the modified polyvinyl alcohol such as, for example, a zirconium salt. Still further, the crosslinker may comprise a ci-osslinlccr for the polymer of hydrophobic polymer emulsion. The crosslinker may be kept separately from the silver halide particle composition until a short time prior to use.

The silver halide particles of the invention may be prepared by any method known in the art, generally by allowing a suitable silver salt, for example silver nitrate, to react with a suitablc halide salt, for example sodium chloride, in aqueous solution in the presence of the polyvinyl alcohol. it is especially preferred that the silver halide particles he prepared by a controlled precipitation technique.

Therefore according to another aspect of the invention. there is provided a method for IS preparation of a silver halide antimicrobial composition by controlled precipitation of silver halide in the presence of polyvinyl alcohol. Controlled precipitation of silver halidc is well known in the photographic industry for the preparation of silver halide photographic emulsions and enables control of the size, size distribution, morphology, composition, etc. of the precipitated silver halide particles.

Preferably, the precipitation is carried out by the double jetting method; that is to say that separate aqueous solutions of the silver salt and halide salt are simultaneously pumped into a precipitation vessel, and the reaction mixture is stirred during the precipitation reaction. Good mixing is necessary for control of the reaction.

It is convenient to start with a solution of polyvinyl alcohol in the precipitation vessel to permit the reaction to be stirred at the commencement of jetting and then to jet the silver and halide solutions into this solution to form the silver halide salt. It is to be understood that in the method of the invention, the starting concentration of polyvinyl alcohol has to he greater than that in the final product because it is diluted durthg the reaction by the addition of the silver and chloride solutions. Typically, the initial concentration of polyvinyl alcohol is up to 10% or greater, though this may be limited by the viscosity of the solution. To Sonic extent, this viscosity limitation may be alleviated by performing the reaction at elevated temperature and therefore, although the precipitation reaction may be carried out at ambient Lemperaturc or below, higher temperatures are generally preferred S and temperatures in the range of 40°C to 60°C are normally suitable.

Control of the precipitation may be achieved by appropriate choice of the solution concentrations and jetting rates, or may be controlled by feedback from the silver or halide concentration in the precipitation vessel using a suitable ion selective electrode.

Alternatively control may be by means oj a suitable mathematical program. Further, the temperature may be controlled during the pi-ecipitation. Such methods are known in the photographic art for production of silver halide photographic emulsions.

For the preferred silver chloride of the invention, control of the precipitation may conveniently be achieved by control of the chloride ion concentration, it is especially preferred that the precipitation be carried out in the presence of a small excess of chloride iS ions. The chloride ecess may be achieved by an initial addition of a small amount of a suitable soluhle chloride salt, for example sodium chloride, to the precipitation vessel before the reaction or by jetting some of the chloride solution before starting to jet the silver. The chloride excess during the reaction may be maintained, for example, by *use of a chloride solution that is slightly more concentrated than the silver solution, or alternatively by pumping the chloride solution at a slightly greater rate. Further, additional chloride may be added after completion of precipitation to maintain an excess of chloride ions and minimise solubility of the silver haiide.

The minimum concentration of chloride ions during the precipitation reaction should be at least 0.5xi0' molar, preferably at Least lO molar. There is no particular tipper limit to the chloride ion concentration, but a concentration in the range of 10.2 to 1 0 molar is especially suitable. The term molar implies moles! litre. In the method of the invention, control of the chloride ion concentration during precipitation determines the shape, size.

and size distribution of the silver chloride particles.

Deionised water may be used for the precipitation reaction. The pt-1 of the reaction is not particularly limited but may, for example, be in the range between 4 and 7 and may be adjusted as appropriate with acid or alkali if desired. Further there may also be present during the precipitation reaction additional auxiliary agents as are known in the art, such S as anlifoams, stabilisers, surfactants, and the like.

The product of the precipitation reaction may be used as produced, in which case the concentration of silver and polyvinyl alcohol at the end of the reaction is that of the final producL Alternatively, the concentration at completion of the reaction may be greater Ehan that intended in the final product to allow incorporation of additives, suchas iii particular the corrosion-inhibiting surfactant, stabi I iser, hydrophobic polymer emulsion, and inorganic oxide as hereinbefore described. Further, the composition may be diluted by addition of water, or other additives may be incorporated as known in the art. Further, the concentration of polyvinyl alcohol may be adjusted by addition of further polymer, o' the composition may be desalinated to remove the sodium nitrate byproduct and increase the IS concentration of the other components.

The antimicrobial compositions of the invention are useful for any application wherc antimicrobial properties are important. Such uses include providing antimicrobial properties to fibres, yarns, fabrics, textiles, cloths, bandages, masks, woun.d dressings, gauze, fitters, tents, canvas, sails, ropes, pool covers, clothing, bedding, brushes, fishing nets, and the like; providing antimicrobial properties to articles such as1Iriiiture, equipment, machinery, containers, pipework, medical devices, patio upholstery, camping gear, geotexriles, tanks, seals, spray-heads, pump units, doorknobs, toys, telephones, mobile phones, keyboards, cameras, and the like; providing antimicrobial properties to building materials such as drywall, insulation, housewrap, roof wrap, wall paper, cement, concrete, mortar, sealing materials, grout, adhesives, and the like; providing antimicrobial properties to materials such as plastics, packaging, papers, wood composites, and the like; providing antimicrobial properties to formulations such as paints, detergents, adhesives, sealants, personal care products, and the like; and disinfection and preservation of films, coatings, and surfaces such as walls, floors, and the like. 1]

According to one particular embodiment of the invention, the silver halide composition is used to provide an antimicrobial coating, For this use a suitable coating formulation may be provided, with the silver halide and polyvinyl alcohol composition of [lie invention dispersed in a suitable carrier medium, suitably diluted if necessary, together with other addenda as known in the art. Preferably, the coating formulation is water-based. The formulation may be coated by any suitable coating method known in the art, and after being coated may be dried by evaporation of the carrier medium or may be cured by exposure to air, to radiation, including light, or by other methods generally known.

The silver halide concentration in the coating formulation may be selected as necessary to provide a suitable coating weight, but is typically up to 5% by weight of the formulation, especially between 0.1% and 1% by weight. Depending on the specific use the formrdation may be coated at a suitable wet laydown to provide a silver coverage on the substrate ofup to 1 gui2, especiaUy between 0.001 and 0.1 grn2.

The coating formulation may also comprise additives as are known in die art, for example the stabiliser, hydrophobic polymer emulsion, crosslinker, and inorganic pigment as hereinbefore described. Further the coating fornmlation may also comprise one or more surfactants to provide suitable wetting of the substrate. Additionally the coating fonnulation may be diluted by addition of water or other carrier medium, or the concentration of polyvinyl alcohol may be adjusted by addition of further polymer.

The formulation may be coated on any suitable surface, such as for example a substrate base such as glass, paper, polycarbonate, polyacrylate, polyester, and the like.

Ahernatively the formulation may be used to coat an article, such as a doorknob, toy, or a consumer eectronics device, for example a telephone, mobile phone, computer keyboard, digital camera, or the like. Any suitable coating method as is known in the art may be used.

According to another embodiment of the invention, the silver halide composition is used to provide an antimicrobial textile substrate such as a fibre, yarn, thread. lbric, or ctoth For this use, the textile substrate may be treated with the silver halide and polyvinyl alcohol composition of the invention, suitably diluted if necessary, together with other addenda as known in the art. The silver halide concentration in the treating forniLliation may be selected as necessary to provide a suitable laydown on the substrate. Depending on the specific use the silver halide concentration in the treating formLflation may be LII) to 1% by weight of tile formulation, but is typically between 0.01% and 1% by weight. The S treating formulation may also comprise additives as arc known in the art, for example the stabiliser, hydrophobic polymer emulsion, crosslinker, and inorgaliic pigment as hereinbeforc described.

Suitable methods of treating the fibre or cloth with the inventive composition are known in the art, but typically the composition may be used as an additive to an exisLing treating process such as, for example, a padding bath or dye bath. The composiLion nay provide a silver coverage on the fabric o. up to 1 gm'2, especially between approximately 0.001 and 0.1. gm'2 depending on the specific use.

The following examples will serve to illustratc the invention,

EXAMPLE 1

2001.6g of a 9.7% solution of 40-88 polyvinyl alcohol were heated to 50°C; the pH was measured at 5.3. Antifoani (1.0 ml) and an initial charge of 2.5 molar sodium chloride solution (2.Sml) were added, and the mixture stirred well. Silver chloride was then precipitated by double-jetting 2.5 molar sodium chloride solution at 15.8 mI/mm and 2.3 molar silver nitrate solution at 15.5 mi/mm for 6 minutes. The jetting rates were then adjusted to 31.6 mI/mm for the sodium chloride and 30.8 mI/mu for the silver nitrate, and cloubie-jetung continued for a further 9 minutes. Finally the reaction was then sti red for an additional half minute. The product was a whiLe mobile suspension. with. a final p14 of:' 5.35. On cooling to 5°C the dispersion became quite viscous but was still niobile and did not settle out even after five months stored at this temperature. Inspection by Scanning Electron Microscopy showed that the silver chloride was in the form of cubes, and the mean particle size was determined at l 9nm. Calculation gives the silver chloride concentration in the product as 4.5% by weight (3.4% expressed as sitver) and the polyvinyl alcohol concentration as 6.7%.

786 grams of the dispersion prepared above was heated to 50°C and stirred well. A further 852.1 g of9.?% 40-88 polyvinyl alcohol solution was added, and then, slowly, 128.1 g of a commercial sample of sodium dioctyl sulphosuccinate (75% in ethanol! water). Finally 4.4 grams of deionised water were added and the mixture stirred for one hour and stood overnight. The pH was 5.63. The formulation comprises 2% silver chloride, 7.6% polyvinyl alcohol, 5.4% dioctyl sulphosuccinate, and 0.65% ethanol by weight iii water.

1500 grams of this composition was used for a corrosion test. Three aluminium coupons were used, suspended by a ptfe tape such that one was immersed below the surface of the thrmulation, one was half-immersed, and one was suspended in the headspace of the vessel. The mixture was heated to 55°C for one week iii the dark. A control experiment was set up at the same time wit]] a representative gelatin-based silver halide antimierobal eompositioh comprising 5.3% silver chloride, 1.4% gelatin, and 0.022% benzotriazole stabiliser, and having a pH of 4.6.

At the end of the week, the coupons were removed from the formulations, cleaned, and is examined. Results observed were as follows: Sample Fully immersed Half immersed coupon 1-Ieadspace coupon ______________ coupon _________________________ [nvention No corrosion, one Discoloration at Slight brown colour small dark patch on interface, some marks on bottom pait of surface. but no corrosion. coupon.

Comparison Big pits on bottom Corroding at interface; Brown mark covering part of coupon and line of small pits part much of surface.

aroLind hole for way down coupon I______________ suspension tape. where inimersed. _______________________ lt is seen that the inventive sample passes the corrosion test whereas the comparison sample fails because of pitting. The weights of the. three coupons for the inventive sample were compared before and after the test: the fully immersed coupon had neither gained nor lost weight; the half immersed coupon had lost 0.0002 g (0.003%); and the headspace coLipon had gained 0.0057 g, thus confirming the result.

Finally, the state of the formulations after completion of the test and removal of the coupons was recorded: the inventive sample was still suspended with no deposit in the bottom of the vessel but with a crust on the surface of the solution. Ft is notable that even after this severe test the inventive formulation remains stably suspended. By contrast, and despite the presence of the stabiliser, the comparison sample had completely set±led out to give a dense white precipitate at the bottom of vessel with sonic grit that was difficult to remove.

S EXAM PLIE 2 2000.6g of a 10% solution ol' 26-88 polyvinyl alcohol was heated to 50°C; the pH was measwed at 5.07. Antifoam (1.0 ml) and an initial charge of 2.52 molar sodium chloride solution (2.Sinl) were added, and the mixture stirred well. Silver chloride was then precipitated by double-jetting 2.52 molar sodium chloride solution at 15.7 mI/ruin and 2.52 molar silvcr nitrate solution at 15.3 mI/mm for 6 minutes. The jetting rates were then adjusted to 31.3 mI/mm for the sodium chloride and 30.4 mI/mm for the silver nitrate, double-jetting was continued for a further 9 minutes, and the reaction was then stirred for an additional half minute. The product was a white mobile suspension that was clearly less viscous than the product of example 1, with a final pH of 5.05. inspection by Scanning Electron Microscopy showed that the silver chloride was in the ibrm of cubes, and LIte mean particle size was determined at 98nni. Calculation gives the silver chloride concentration in the product as 4.5% and the polyvinyl alcohol concentration as 6.86%.

EXAMIPLE 3 3 000g. of a 9.7% solution of 40-88 polyvinyl alcohol was heated to 50°C. Antifoam and an initial charge of 4.7 molar sodium chloride solution (2m1) were added, and the mixture stirred well. Silver chloride was then precipitated by double-jetting 4.7 molar sodium chloride solution at 1.2.5 mI/mm and 4.7 molar silver nitrate solution at 12.5 mI/mm for 6 minutes. The etting rates were then adjusted to 25 mI/mm for the sodium chloride and 24.8 mI/nun for the silver nitrate, and double-jetting continued for a further 9 minutes.

Finally the reaction was then stirred for an additional half minute. The product was a white mobile suspension, with a final p1-I of 5.25. On cooling to 5°C the dispersion became quite viscous but was still mobile and did not settle out even alter seven, months stored at this temperature. Inspection by Scanning Electron Microscopy showed that the silver chloride was in the form of cubes, and the mean particle size was determined at 128nm. Calculation gives the silver chloride concentration in the product as 5.3% and the polyvinyl alcohol concentration as 7.7%.

EXAMPLE 4

2000.6g of a 9.7% solution of 40-88 polyvinyl alcohol were heated to 50°C; the pH. was measured at 5.25. Antifoam (1.0 ml) and an initial charge of 4.7 molar sodium chloride SOiLtilon (lml) were added, and the mixture stirred well. Silver chloride was then precipitated by double-jetting 4.7 molar sodium chloride solution at 12.5 mI/mm and 4.7 molar silver nitrate solution at 12.4 mI/mm for 6 minutes. The jetting rates were then adjusted to 25.0 mI/pith for the sodium chloride and 24.8 mI/mm for the silver nitrate, and double-jetting continued for a further 36 minutes. Finally the reaction was then stirred for an additional minute. The product was a white mobile suspension, with a final p1-I of 5.2.

Inspection by Scanning Electron Microscopy showed that the silver chloride was in the form 0.1: cubes, and the mean particle size was determined at 148nm. Calculation gives the silver chloride concentration in the product as 13.75% by weight and the polyvinyl 1.5 alcohol concentration as 4.1%. On cooling to 5°C the dispersion became fairly viscous but was stilt mobile and did not settle out even after five months stored in the dark at this temperature.

EXAMPLE S

1502.9g of a [3.27% solution of a commercial cationic polyvinyl alcohol was diluted with 498.1 g deionised water and heated to 50°C; the pH was measured at 5.16. Antifoam (1.0 nil) and an initial charge of 2.5 molar sodium chloride solution (2.Snil) were added, and the mixture stirred well. Silver chloride was then precipitated by double-jetting 2.5 molar sodiuni chloride solution at 15.7 mI/mm and 2.5 molar silver nitrate sotution at 15.4 mi/mm for 6 minutes. The jetting rates were then adjusted to 31.3 mI/mm for the sodium chloride and 30.6 mI/mm for the silver nitrate, and double-jetting continued for a further 9 minutes. Finally the reaction was then stirred for an additional half minute. The product was a white mobile suspension, with a final pH of 5.3. Inspection by Scanning Electron Microscopy showed that the silver chloride was in the form of cubes, and the mean particte size was determined at lO7nm. Calculation gives the silver chloride concentration iii the product as 4.5% by weight and the polyvinyl alcohol concentration as 6.8%. The dispersion produced had a viscosity similar to that from example 2 and did not settle out even after five months stored in the dark at 5°C.

EXAMPLE 6

ViscositY Measurements and Stability Tests A sample of a silver chloride dispersion comprising 4.55% silver chloride and 6.84% polyvinyl alcohol and with pH 5.4 was prepared as in example I. A series of samples all having a silver chloride concentTatiOfl of 2% and comprising varying concentrations of polyvinyl alcohol was prepared by diluting portions of this dispersion with combinadons of water and a 10% 40-88 polyvinyl alcohol solution as appropriate with Stirling at 50°C.

The resultant concentrations of polyvinyl alcohol are shown in the foliowing table; sample A in the table was prepared by diluting with water and sample H with just 10% polyvinyl alcohol solution. The viscosities of these solutions were measured with a.Bohlin C-VOft rlteorneter at 25°C and a shear rate of 101 sec', and the results are given in the following

table:

irnplc Polyvinyl alcohol Viscosity A 2.98% 0.0i7 Pas B 4.64% 0.059 Pas C 5.20% 0.088 Pas LP 5.76% O.l36Pas E 6.31% 0.260 Pas F 6.87% 0.322 Pas 7.42% 0.380 Pas H 8.53% 0.770 Pas Samples of all compositions were stored in the dark at anibieni temperature, and the stability ol: the samples was observed on keeping: after 14 weeks samples A and B werc showing slight signs of settling out, showing that a concentration below 5% ol this particular grade of polyvinyl alcohol does not provide full stability to these compositions.

A further sample of a silver chloride dispersion comprising 4551⁄4 silver chloride antI 6.86% polyvtnyl alcohol was prepared as in example I. The mean particle size was I lO urn and the pH was 5.4. This is labelled as sample I in the following table; sample I was then prepared by treating part of this dispersion with the commercial sample of sodium clioctyl sulphosucciiiatc as in example 1, and stirring well at 50°C for 30 minutes. Finally a 1% solution of the stabiliser 4-nitrobenzotriazole in dilute sodium hydroxide soluLion was added with stirring to give sample K. The viscosities of these solutions were measured with a Bohlin C-VOR rheometer at 25°C at a shear ratc of 100 sec', and the results are given in the following table, together with the concentration by weight of the components in the formulation: Sample Si]ver Chloride Polyvinyl alcohol Surfactant Stabiliser Viscosity ________ 4.55% 6.86% _________ 0.43 Pas J 4.38% 6.6% -2.87% _________ 0.60 his K 4.33% 6.53% 2.83% 0.0]% 0.63 his Samples of all three compositions were stored in a refrigerator at 5°C, and further samples tO were incubated at 40°C in the dark. The stability of the samples was observed on keeping: all samples were stable after 6 weeks.

EXAMPLE 7

Coating Tests A coating formLllation was made up by diluting 20.Og of a sample of a silver chloride IS dispersion comprising 4.56% silver chloride and 6.65% polyvinyl alcohol prepared as in example t with 27.Og additional 9.7% 40-88 polyvinyl alcohol solution and 0.5g of an ethoxylated 2,4,7,9-tetramethyl-5-decyne-4,7-diol surfactant containing 10 mol o.f ethylene oxide per molecule, and the total made up to 200g with cleionised water. The formulation contains 0.45% silver chloride and 2.02% polyvinyl alcohol arid was then coated on a clear polyester filmic suhstrate at a wet coating weight of 6 gm'2 to provide a coating with a silver chloride coating weight of 0.027 gm'2. After drying the coating was almost clear with very slight opalescence showing the suitability of the formulauon. This is coating A. A second coating formulation was made up by diluting 20.Og of the silver chloride dispersion prepared in example 5 comprising 4.5% silver chloride and 6.8% cationic polyvinyl alcohol with l9.7g additional 13.27% cationic polyvinyl alcohol solution and 0.5g of the same surfactant as for coating A, and the total made up to 200g with deionised water. The formulation contains 0.45% silver chloride and 2.48% polyvinyl alcohol and was then coated on a clear polyester filmic substrate at a wet coating weight of 6 gut2 to provide a coating with a silver chloride coating weight of 0.027 gm2. After drying the coating was almost clear with very slight opalescence showing the suitability of the formulation. This is coating B. A third coating formulation was made up by diluting I0.Og ofa sample ofa silver chloride dispersion comprising 4.56% silver chloride and 6.65% polyvinyl alcohol prepared as in example I with 90.Og of an acrylic polymer emulsion of 11% soLids content and having a Tg 01: -30°C, and the total made up to 200g with deionised water. Thc formulation 0 contains 0.45% silver chloride, 0.3 3% polyvinyl alcohol, and 4.95% hydrophobic polymer to give 5.28% total polymer binder and was then coated on a clear polyester filmic substrate at a wet coating weight of 6 gui2 to provide a coating with a silver chloride coating weight of 0.027 gm'2. After drying the coating was almost clear with very slight opalescence showing the suitability of the formulation. This is coating C. I 5 The adhesion of these coatings was tested by applying an adhesive tape to part of tile coated area, and then tearing off the tape to see whether the coating adheres to the tape or the polyester film. Coatings A and B were removed by this test, but coating C passes the test and remains adhered to the polyester showing the advantage of the hydrophobic polymer binder.

EXAMPLE 8

Inorczanic Oxide The inorganic oxide used was a commercial sample of hydrophilic transparent microfiie Zinc Oxide having an average particle size of less than 200 nut A lormulation was made up by suspending 91,2 g of this zinc oxide in 400 g deionised water and 400 g 9.7% 40-88 polyvinyl alcohol solution, and dispersing for llfteen minutes at 6800 rpm hi a Silverson mixer. This was then added to 2 Kg of a silver chloride dispersion comprising 4.56% silver chloride and 6.65% polyvinyl alcohol prepared as in example I and mixed well.

Then 113.6 g of the same commercial sample of sodium dioctyl sulphosuccinate as in example I was added with good stirring over about 30 minutes, and finally 63.6 g of a 1% solLition of the stabiliser 4nitrobenzotriaiOle in dilute sodium hydroxide solution was added with stirring to give a composition comprising 2.97% zinc oxide, 2.97% silver chloride. 6.0% polyviiiyl alcohol, 2.8% dioctyl suiphosuccinate, 0.02% sLabiliser, and.

0.33% ethanol by weight iii water. SEM showed separate populations of silver chloride and zinc oxide particles.

i 500 grams of this composition was used for a corrosion test with aluminium coupons as described in example 1. At the end of the test, the coupons were removed from the ftrmuladons, cleaned, and examined. The fully immersed coupon was clean, there was some etching at the interface on the half immersed coupon but no deep corrosion pits, and the headspace coupon had extensive brown mottling but was otherwise unaffected. The weights of the three coupons were compared before and after the test: the fully immersed coupon had gained 0.00 15 g; the half immersed coupon had lost 0.0002 g (0.004%); and the headspace coupon had gained 0.0048 g. Thus the sample passed the test.

Finally, the state of the formulation after completion of the test was noted: the is composition was still suspended with no deposit in the bottom of the vessel hut with a clear layer on the surface of the formulatioll.

EXAMPLE 9

Fabric Treatment A sample of a silver chloride! polyvinyl alcohol compositioll prepared as in example 1.

was dilLited with deionised water at a ratio of S g of composition 10 i litre to provide a tbric treating formulation comprising 0.023% silver chloride and 0.034% poLyvinyl alcohol. A second treating formulation was prepared by dilution the silver chloride! cationic polyvinyl alcohol composition of example S at the same ratio. Samples of coLton cloth and of Dacron''M 54 synthetic cloth were separately padded with these tormttlations and allowed to dry at ambient tmperatLlre. Examination of the samples by Scanning Electron Microscopy showed numerovs small silver chloride cubes adhered to th.e fibres of the fabric. The silver coating weights (figures being given as silver) measLired [or the four samples are given in the following table.

Sam Ic Cloth Silver Exam Ic I Cotton 0.11 grn2 Exam Ic 1 Dacron 0.05 rn2 Exam le 5 Cotton o. ió gnY2 Example 5 Dacron 0.05 gnt2

EXAMPLE 10

FabHc Treatrneiit The silver ch[oride/ zinc oxide composition of example 9 was diluted tli deionisecl water at a ratio of 12 g of composition to 1 litre to provide a fabric wash bath comprising 0.36% silver chloride, O.36% zinc oxide, and 0.7% polyvinyl alcohol. Anoiller sample of the DacronRTM 54 syithetic ciot]i as in example 9 was completely immersed in this bath.

allowed to drain, and dried. The washed sample was exposed to sunlight-, hut even after a day there was n visible darkening of the sample.

Claims (1)

1. <claim-text>Claims An aqueous antimicrobial composition comprising: * particles of a silver halide salt; and * polyvinyl alcohol.</claim-text> <claim-text>2. A composition according to c]aim 1 wherein the silver halide is silver chloride.</claim-text> <claim-text>3. A composition according to claim I or 2 wherein the mean particle size of the silver liabde is from IOU to 200 am.</claim-text> <claim-text>4. A composition according to any of claims 1 -3 wherein the polyvinyl alcohol has a degree of hydrolysis of at least 80%.</claim-text> <claim-text>5. A composition according to any of claims 1 -3 wherein the polyvinyl alcohol comprises a modifled polyvinyl alcohol.</claim-text> <claim-text>6. A coniposition according to any of claims 1 -5 which also comprises a corrosion-inhibiting surfactant.</claim-text> <claim-text>7. A composition according to claim 6 wherein the srfactaiit*is the sodium saIl o± dioctyl sulphosuccinate.</claim-text> <claim-text>8. A composition according to any of the preceding claims which also comprises a benzotriazole stabiLiser wherein the benzotriazole is substituted by one or more e Iectron-withdi-awi ng substituents.</claim-text> <claim-text>9. A composition according to claim 8 wherein the benzotriazole compound is 4-nitro benzotriazole.</claim-text> <claim-text>1.0. A composition according to any of the preceding claims which also comprises a hydrophobic polymer emulsion resin binder.</claim-text> <claim-text>II. i-\ composition according to any of the preceding claims which also comprises an inorganic oxide.</claim-text> <claim-text>12. A composition according to claim 11 wherein the inorganic oxide is an oxide of aluminium, silicon, zirconium, titanium, or zinc.13. A method for preparing a silver halide antimicrobial composition by controlled precptation of silver halide in the presence of polyvinyl alcohol.4. Use of an antimicrobial composition comprising silver halide and polyvinyl alcohol to provide antimicrobial properties to fibres yarns. ihbrics, textiles, cloths.bandages, masks, wound dressings, gauzes, filters, teuLs. canvas, sails, ropes, pool covers, clothing, bedding, brushes, and fishing nets; articles sueh as furniture, equipment, machinery, containers, pipework. medical devices, patio upholstery.camping gear, gcotextiles, tanks, seals, spray-heads, pump units, doorknobs, toys telephones. mobile phones, keyboards. and cameras; hL[ilding materials sLich as drywall, insulation, bousewrap, roof wrap, walipaper, cement, concrete. inuriju.sealing materials, grout, and adhesives; materials such as plastics. packaging, papers, and wood composites; formulations such as paints, detergents, adhesives.sealants, and personal care products; and films, coatings, and surftices sLleh as walls and floors.</claim-text>