Process for preparation of water-based gel ink with fixed color containing silver nanoparticles

Abstract

The present invention relates to a process for preparing an aqueous gel ink with a fixed color, comprising the steps of: (i) preparing a gel-based matrix of an aqueous ink; (ii) preparing an aqueous suspension of silver nanoparticles with a fixed color by mixing a silver salt with: - water; - a mixture of at least an alkali metal or alkaline earth metal salt of citrate; - an oxidizing agent, preferably hydrogen peroxide HO; - and polyvinylpyrrolidone; and (iii) adding the aqueous suspension of silver nanoparticles obtained in step (ii) to the gel-based matrix of the aqueous ink obtained in step (i) while stirring, to obtain an aqueous gel ink with a fixed color having silver nanoparticles dispersed therein.

Description

[Technical field]

[0001] The present invention relates to a process for preparing an aqueous gel ink with fixed color, as well as to an aqueous gel ink with fixed color, free of any dyes and pigments, comprising silver nanoparticles stabilized by polyvinylpyrrolidone obtainable according to the process of the present invention. The present invention also relates to a writing instrument comprising the aqueous gel ink with fixed color according to the present invention.

[0002] One of the main objectives of the present invention is to replace all types of dyes and pigments normally present in water-based gel inks, which have the drawback of being expensive and causing high manufacturing costs.

[0003] Another object of the present invention is to replace all types of dyes and pigments normally present in aqueous gel inks, which have the drawback of being irritating to biological membranes, such as the skin and eyes, and potentially causing allergies.

[0004] The inventors have surprisingly found that the new nanoparticle-based water-based inks are also resistant to UV light, thereby improving light stability over time.

[0005] Additionally, silver nanoparticles have antibacterial properties and therefore the amount of other antibacterial agents can be reduced.

[0006] For this purpose, the inventors have developed a specific process that allows to obtain new aqueous inks with fixed color when written by replacing conventional aqueous gel inks containing dyes and pigments with new ones based on nanoparticles. The process developed within the framework of the present invention also presents the advantage of being carried out in an aqueous medium and is therefore an "environmentally friendly process". Moreover, the process of the present invention is carried out in a low temperature range and works in an ecologically viable manner, also taking into account ecological requirements.

[0007] The present invention relates to a process for preparing an aqueous gel ink having a fixed color, (i) preparing a gel-based matrix of a water-based ink; (ii) A silver salt: -water, at least an alkali metal or alkaline earth metal salt of citrate, preferably an alkali citrate, and an alkali metal hydride, preferably NaBH 4 and a mixture of - an oxidizing agent, preferably hydrogen peroxide H 2 O 2 , and polyvinylpyrrolidone, by mixing with said aqueous suspension of silver nanoparticles having a fixed color; (iii) adding, under stirring, the aqueous suspension of silver nanoparticles obtained in step (ii) to the gel-based matrix of the aqueous ink obtained in step (i) to obtain an aqueous gel ink having a fixed color with the silver nanoparticles dispersed therein.

[0008] The present invention also relates to a process for preparing an aqueous ink, which prepares the matrix of the aqueous ink in step (i). The present invention also relates to an aqueous ink obtainable through such a process. The various embodiments described below with respect to the process of preparation of an aqueous gel ink and the aqueous gel ink obtainable through this process can be considered as well with respect to the process of preparation of an aqueous ink and the aqueous ink thus obtained, in particular the nature and / or content of the components. These embodiments with respect to the aqueous ink, the process of its preparation and the matrix of the aqueous ink are also part of the present invention.

[0009] According to a preferred embodiment of the present invention, the process for preparing an aqueous gel ink having a fixed color comprises the steps of: (i) preparing a gel-based matrix of a water-based ink; (ii) A silver salt: -water, - At least sodium citrate and sodium borohydride (NaBH 4 and a mixture of -Hydrogen peroxide H 2 O 2 , and polyvinylpyrrolidone, by mixing with said aqueous suspension of silver nanoparticles having a fixed color; (iii) adding, under stirring, the aqueous suspension of silver nanoparticles obtained in step (ii) to the gel-based matrix of the aqueous ink obtained in step (i) to obtain an aqueous gel ink having a fixed color with the silver nanoparticles dispersed therein.

[0010] The process according to the invention makes it possible to obtain aqueous ink compositions exhibiting the plasmonic effect (also called plasmonic effect), thus compositions of different plasmonic colours can be obtained depending on the content of the components used.

[0011] For the purposes of the present invention, the term "ink" is intended to mean a "writing ink" intended to be used in a writing instrument, in particular a pen. Writing inks should not be confused with "printing inks" that are used in printing presses and do not have the same technical constraints and therefore the same specifications. In fact, writing inks must not contain solid particles of a size larger than the channels of the writing instrument, in order to avoid clogging of the writing instrument, which would inevitably and irreversibly stop writing. In addition, they must allow an ink flow rate suitable for the writing instrument used, in particular a writing flow rate of 100 to 500 mg / 200 m, in particular a writing flow rate of 150 to 400 mg / 200 m. They must also dry sufficiently quickly to avoid soiling the writing medium. They must also avoid problems of migration (bleed-through) over time. The ink according to the invention will therefore be suitable for the writing instrument for which it is intended, in particular a pen.

[0012] Furthermore, the "writing ink" must not be too fluid to avoid leakage during writing, but must be sufficiently fluid to facilitate the flow of the writing action.

[0013] In the particular case of the present invention, the writing ink may more specifically be a "gel ink" (and thus corresponds to a thixotropic ink), in particular at 20° C. and at rest (0.01 s -1 The viscosity measured at a shear rate of 100 s at 20 °C using the same rheometer, for example a cone-plate rheometer such as the Malvern KINEXUS with a 60 mm cone and 1° angle. -1 In certain embodiments, the viscosity of the gel ink measured under these conditions is different from and particularly higher than the viscosity measured at a shear rate of 1 s. -1 At shear rates of 1,000 to 7,000 mPa.s, specifically 2,000 to 5,000 mPa.s, more specifically 2,500 to 3,500 mPa.s, and 5,000 to 6,000 mPa.s, -1 Specifically, the viscosity is stable during storage for at least 3 months at 40° C. and 20% relative humidity, in particular the viscosity does not decrease by more than 50%. More specifically, the return to the viscosity at rest after shearing is very rapid, in particular a maximum of a few minutes, in order to avoid static leakage within a few minutes after writing.

[0014] In the sense of the present invention, the term "fixed color" is intended to mean that the color of the aqueous gel ink by visual observation is the same within 7 calendar days (1 week) as it was before application onto an absorbent support, in particular paper, cardboard or textile, and after application onto the absorbent support.

[0015] The content of some components, especially sodium borohydride (NaBH 4Depending on the content of sodium citrate and / or polyvinylpyrrolidone, the color of the composition can change due to the plasmonic effect, in particular, the color of the composition can change depending on the light absorption by the silver nanoparticles and the space between the silver nanoparticles in the ink composition.

[0016] Indeed, the plasmonic color is due to light absorption by the silver nanoparticles and / or the spacing between the silver nanoparticles in the material.

[0017] Depending on their size, shape and distance, the color of the nanoparticle dispersion, as well as its properties, can change. This is due to plasmon resonance. When silver nanoparticles are exposed to waves of a certain frequency, electrons gather in certain places, which change depending on the size and shape of the silver nanoparticles. This electron aggregation causes anisotropy of the nanoparticles, which then leads to changes in light absorption and scattering, resulting in a certain color. Plasmon resonance is also influenced by the distance between the silver nanoparticles due to their bonding. In fact, the closer the silver nanoparticles are, the more they interact with each other, increasing their bonding effect, also called the plasmonic effect. Similarly, shape affects plasmon resonance. In particular, such plasmonic effects can be characterized by UV (ultraviolet)-visible-NIR (near infrared) absorption spectroscopy.

[0018] The present invention also relates to an aqueous ink with a fixed colour obtainable by the process of the present invention, said aqueous ink comprising silver nanoparticles and polyvinylpyrrolidone.

[0019] In the present invention, the gel-based matrix of the aqueous ink prepared in step (i) may contain 50-95% by weight, specifically 60-90% by weight, more specifically 70-85% by weight of water.

[0020] The gel-based matrix of the water-based ink prepared in step (i) may also contain classical gel ink ingredients such as co-solvents, antimicrobial agents, corrosion inhibitors, antifoaming agents, rheology modifiers, etc. The gel ink ingredients used to prepare the gel-based matrix of the water-based ink of step (i) are largely described below in connection with the subject matter of the water-based gel ink with fixed color of the present invention.

[0021] The gel-based matrix of the water-based ink is prepared by methods well known to those skilled in the art, such as by simple mixing of its ingredients.

[0022] In a specific embodiment, the gel-based matrix of the water-based ink does not contain a colorant, i.e., does not contain any dyes or pigments, in which case the gel-based matrix of the water-based ink is transparent.

[0023] In another specific embodiment, the gel-based matrix of the water-based ink does not contain any reducing agents or any oxidizing agents.

[0024] In the present invention, the silver salt is specifically AgNO 3 , AgClO 4 , Ag 2 SO 4 , AgCl, AgBr, AgOH, Ag 2 O, AgBF 4 , AgIO 3 , AgPF 6 and mixtures thereof, more specifically silver nitrate, AgNO 3 In particular, the silver salt is in the form of an aqueous solution of silver nitrate.

[0025] In a specific embodiment, the total amount of silver salt added in step (ii) is in the range of 0.0005-0.006 wt. %, specifically 0.001-0.005 wt. %, based on the total weight of the aqueous suspension.

[0026] Silver nanoparticles are formed when a silver salt is contacted with a reducing agent and an oxidizing agent.

[0027] In the present invention, at least an alkali metal or alkaline earth metal salt of citrate, preferably an alkali citrate, and an alkali metal hydride, preferably NaBH 4 The mixture of is responsible for the color of step (ii).

[0028] In the present invention, the alkali metal of the citrate is selected from lithium citrate, sodium citrate, potassium citrate, rubidium citrate, cesium citrate, and francium citrate, preferably sodium citrate or potassium citrate, more preferably sodium citrate.

[0029] In a specific embodiment of the invention, the alkali metal salt of citrate is sodium citrate.

[0030] In the present invention, the alkaline earth metal salt of citrate is selected from beryllium citrate, magnesium citrate, calcium citrate, strontium citrate, barium citrate, and radium citrate, preferably magnesium citrate or calcium citrate, more preferably calcium citrate.

[0031] In a specific embodiment of the invention, the alkaline earth metal salt is calcium citrate.

[0032] In the present invention, the alkali metal hydride is sodium borohydride (NaBH 4) , sodium cyanoborohydride (NaBH 3 CN ) , sodium triacetoxyborohydride (NaHB(OAc) 3 ), sodium tri-sec-butylborohydride, potassium tri-sec-butylborohydride, potassium triethylborohydride, lithium triethylborohydride, lithium tri-sec-butylborohydride, nickel borohydride, lithium aluminate hydride, diisobutylaluminum hydride, and sodium bis(2-methoxyethoxyaluminum hydride.

[0033] In a specific embodiment of the invention, the alkali metal hydride is sodium borohydride (NaBH 4 ).

[0034] According to a preferred embodiment of the present invention, sodium citrate and sodium borohydride (NaBH 4 ) and a mixture of is involved in the colouring step (ii).

[0035] In particular, alkali metal hydrides, preferably NaBH 4 The ratio of isopropyl alcohol to butyl alcohol isopropyl alcohol. 4 In particular, this is due to the plasmonic effect exhibited by the composition. For example, the color can be obtained by the addition of an alkali metal hydride, preferably NaBH 4 The more you add, the more you can change from red to pink to purple to blue.

[0036] In a specific embodiment, the amount of citrate added in step (ii) is in the range of 0.001-0.08 wt.%, specifically 0.002-0.005 wt.%, based on the total weight of the aqueous suspension.

[0037] In a specific embodiment, the amount of alkali metal hydride added in step (ii) is in the range of 0.0005-0.005 wt.%, specifically 0.0006-0.003 wt.%, based on the total weight of the aqueous suspension.

[0038] The mechanism of formation of a colloidal solution from the reduction of silver ions consists of two steps: nucleation and growth. The nucleation step requires a high activation energy, while the growth step requires a low activation energy.

[0039] According to a preferred embodiment of the present invention, process (ii) for preparing an aqueous suspension of silver nanoparticles with a fixed color comprises an additional heating step, for example when the alkali metal hydride is added.

[0040] According to a preferred embodiment of the present invention, the process (ii) for preparing an aqueous suspension of silver nanoparticles with a fixed color comprises: - First, the silver salt is dissolved in water and at least an alkali metal or alkaline earth metal salt of citrate, preferably an alkali citrate and NaBH 4 and an oxidizing agent, preferably hydrogen peroxide, H 2 O 2 and polyvinylpyrrolidone (step: nucleation); - an optional heating step; - Compiling the previous composition with a silver salt and an alkali metal hydride, preferably NaBH 4 and (step: growing).

[0041] In a specific embodiment, a silver salt and at least an alkali metal or alkaline earth metal salt of citrate, preferably an alkali citrate and an alkali metal hydride NaBH 4 The molar ratio between the mixture and is in the range of 0.02:1 to 0.10:1, preferably 0.03:1 to 0.06:1.

[0042] According to the present invention, the oxidizing agent is C 1 -C 8Alkyl peroxy acids, such as peracetic acid, acetylcyclohexanesulfonyl peroxide, diisopropyl peroxydicarbonate, tert-amyl perneodecanoate, tert-butyl perneodecanoate, tert-butyl perpivalate, tert-amyl perpivalate, bis(2,4-dichlorobenzoyl) peroxide, diisononanoyl peroxide, didecanoyl peroxide, dioctanoyl peroxide, dilauroyl peroxide, Bis(2-methylbenzoyl)peroxide, disuccinic acid peroxide, diacetyl peroxide, dibenzoyl peroxide, tert-butyl per-2-ethylhexanoate, bis(4-chlorobenzoyl)-peroxide, tert-butyl perisobutyrate, tert-butyl permalleinate, 1,1-bis(tert-butylperoxy)3,5,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, tert -butyl peroxy isopropyl carbonate, tert-butyl perisononaoate, 2,5-dimethylhexane 2,5-dibenzoate, tert-butyl peracetate, tert-amyl perbenzoate, tert-butyl perbenzoate, 2,2-bis(tert-butylperoxy)butane, 2,2-bis(tert-butylperoxy)propane, dicumyl peroxide, 2,5-dimethylhexane-2,5-di-tert-butylperoxide, 3- tert-Butylperoxy 3-phenylphthalide, di-tert-amyl peroxide, α,α'-bis(tert-butylperoxyisopropyl)benzene, 3,5-bis(t-butylperoxy)3,5-dimethyl 1,2-dioxolane, di-tert-butyl peroxide, 2,5-dimethylhexyne-2,5-di-tert-butyl peroxide, and 3,3,6,6,9,9-hexamethyl-1,2,4,5-tetraoxacyclononane, hydrogen peroxide H 2 O 2 and mixtures thereof.

[0043] Hydrogen peroxide (H 2 O 2) can be regarded as a "green" reagent in that its decomposition ultimately leads to the formation of water and oxygen.

[0044] According to a preferred embodiment of the present invention, the oxidizing agent is hydrogen peroxide H 2 O 2 .

[0045] In a specific embodiment, the amount of the oxidizing agent added in step (ii) ranges from 0.03 to 0.1% by weight, specifically from 0.04 to 0.08% by weight, based on the total weight of the aqueous suspension.

[0046] The aqueous suspension in step (ii) also contains polyvinylpyrrolidone. In fact, it stabilizes the silver nanoparticles in the suspension and thus avoids any aggregation.

[0047] In a specific embodiment, the amount of polyvinylpyrrolidone added in step (ii) ranges from 0.05 to 0.1% by weight, specifically from 0.07 to 0.09% by weight, based on the total weight of the aqueous suspension.

[0048] The aqueous suspension of silver nanoparticles obtained in step (ii) has a fixed color. The color depends on the proportion of alkali metal hydride, preferably NaBH 4 . For example, the color can change from red to pink, purple, and blue as the amount of alkali metal hydride, preferably NaBH 4 , increases.

[0049] In a specific embodiment, the silver nanoparticles obtained in step (ii) have a spherical shape.

[0050] Specifically, the silver nanoparticles of the present invention have an average particle size in the range of 1 to 100 nm, more specifically 10 to 50 nm. This average particle size is measured by the analysis of 2D images (microscope: JEOL ARM 200) according to the standard ISO9001:2015.

[0051] In one aspect, the present invention also relates to a process for preparing an aqueous suspension of silver nanoparticles with a fixed colour according to step (ii) and to the aqueous suspension obtainable according to step (ii).

[0052] The present invention also relates to an aqueous gel ink with a fixed color obtainable by the process of the present invention, said aqueous gel ink comprising silver nanoparticles and polyvinylpyrrolidone, in particular said aqueous ink comprising silver nanoparticles stabilized by polyvinylpyrrolidone.

[0053] In particular, the composition according to the present invention exhibits a plasmonic effect.

[0054] In a specific embodiment, the amount of polyvinylpyrrolidone ranges from 0.05 to 0.1% by weight, specifically 0.05 to 0.08% by weight, based on the total weight of the aqueous gel ink with fixed color obtainable by the process of the present invention.

[0055] In the aqueous gel ink with fixed color of the present invention, the silver nanoparticles specifically have a spherical shape.

[0056] In the aqueous gel ink with fixed color of the invention, the silver nanoparticles of the invention have an average particle size in particular ranging from 1 to 100 nm, and more particularly from 10 to 50 nm, which is measured by analysis of 2D images (microscope: JEOL ARM 200) according to standard ISO 9001:2015.

[0057] According to a preferred embodiment, the distance between the silver nanoparticles in the aqueous gel ink of the present invention is lower than 100 nm, specifically varies between 10 and 50 nm, more specifically varies between 15 and 30 nm.

[0058] The fixed color of the aqueous gel ink of the present invention is the same as the fixed color of the aqueous suspension of silver nanoparticles obtained in step (ii). Thus, specifically, the silver nanoparticles are the only colorant of the aqueous gel ink of the present invention. In this case, the aqueous gel ink according to the present invention does not contain any other colorant other than the silver nanoparticles.

[0059] In the aqueous ink having a fixed color of the present invention, more specifically in the aqueous gel ink, the amount of silver nanoparticles is specifically in the range of 0.0005 to 5% by weight, more specifically 0.0007 to 3% by weight, based on the total weight of the aqueous ink.

[0060] In the aqueous gel ink having a fixed color of the present invention, the amount of silver nanoparticles is specifically in the range of 0.0005 to 0.005% by weight, more specifically 0.0007 to 0.002% by weight, based on the total weight of the aqueous gel ink.

[0061] In the aqueous gel ink having a fixed color of the present invention, the amount of water is specifically in the range of 50 to 95% by weight, more specifically 60 to 90% by weight, and even more specifically 70 to 85% by weight, based on the total weight of the aqueous gel ink.

[0062] The aqueous ink with fixed color of the present disclosure and the aqueous suspension obtainable in step (ii) contain in particular an alkali metal salt, more particularly a sodium salt. In fact, this composition is intended to be obtained by the above process using at least an alkali metal hydride, and also an alkali metal salt of citrate. When the above process uses an alkaline earth metal salt of citrate, the resulting aqueous ink composition and the aqueous suspension obtainable in step (ii) contain an alkaline earth metal salt in addition to the alkali metal salt.

[0063] In particular, the amount of alkali metal salt, such as sodium salt, is at least 0.0003% by weight, based on the total weight of the aqueous ink and / or based on the total weight of the aqueous suspension, and in particular ranges from 0.0003 to 0.01% by weight.

[0064] In particular, when present, the amount of alkaline earth metal is at least 0.00009% by weight, based on the total weight of the aqueous ink and / or based on the total weight of the aqueous suspension, and more particularly in the range of 0.00009% by weight to 0.007% by weight.

[0065] The aqueous gel inks with fixed color of the present invention may also contain classical gel ink ingredients such as co-solvents, antimicrobial agents, corrosion inhibitors, antifoam agents, rheology modifiers, etc., as described below. These gel ink ingredients are added to the gel-based matrix of the aqueous ink in step (i) of the process of the present invention.

[0066] The aqueous gel ink of the present invention may contain a co-solvent. Among the co-solvents that can be used are the following: - glycol ethers such as triethylene glycol, polyethylene glycol, diethylene glycol monoethyl ether, diethylene-glycol-monobutyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, phenoxyethanol, phenoxypropanol, -Alcohol: Isopropanol, butanol, isobutanol, pentanol, benzyl alcohol, glycerin, diglycerin, polyglycerin, etc. 1 -C 15 a linear or branched chain alcohol of esters such as ethyl acetate or propyl acetate, Carbonate esters such as propylene carbonate or ethylene carbonate, - ketones such as methyl isobutyl ketone (MIBK), acetone or cyclohexanone, and - Water-miscible polar solvents, such as mixtures thereof.

[0067] In specific embodiments, the co-solvent is selected from the group consisting of glycol ethers, more specifically, triethylene glycol, polyethylene glycol, diethylene glycol monoethyl ether, diethylene-glycol-monobutyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, phenoxyethanol, phenoxypropanol, and mixtures thereof. In even more specific embodiments, the co-solvent is selected from the group consisting of triethylene glycol, polyethylene glycol, and mixtures thereof.

[0068] Specifically, the co-solvent is present in the aqueous gel ink of the present invention in an amount ranging from 5 to 35% by weight, more specifically from 9 to 30% by weight, and even more specifically from 11 to 25% by weight, based on the total weight of the aqueous gel ink.

[0069] The aqueous gel inks of the present invention may include an antimicrobial agent, such as an isothiazolinone (ACTICIDE® from Thor), specifically selected from the group consisting of 1,2-benzisothiazolin-3-one, 2-methyl-4-isothiazolin-3-one, and mixtures thereof.

[0070] Specifically, the antimicrobial agent is present in the aqueous gel ink of the present invention in an amount ranging from 0.01 to 0.5% by weight, more specifically 0.1 to 0.2% by weight, based on the total weight of the aqueous gel ink.

[0071] The aqueous gel inks of the present invention may specifically include a corrosion inhibitor selected from the group consisting of tolytriazole, benzotriazole, and mixtures thereof.

[0072] Specifically, the corrosion inhibitor is present in the aqueous gel ink of the present invention in an amount ranging from 0.05 to 1 wt %, more specifically from 0.07 to 0.5 wt %, and even more preferably from 0.08 to 0.15 wt %, based on the total weight of the aqueous gel ink.

[0073] The aqueous gel ink of the present invention may contain an antifoaming agent, specifically a polysiloxane-based antifoaming agent, more preferably an aqueous emulsion of modified polysiloxane (such as MOUSSEX (registered trademark) manufactured by Synthron, TEGO (registered trademark) Foamex manufactured by Evonik, etc.).

[0074] Specifically, the antifoaming agent is present in the aqueous gel ink of the present invention in an amount in the range of 0.05 to 1% by weight, more specifically 0.1 to 0.5% by weight, and even more specifically 0.2 to 0.4% by weight, based on the total weight of the aqueous gel ink.

[0075] The aqueous gel ink of the present invention is particularly capable of generating a thixotropic phenomenon in which the viscosity depends on the time of the shear rate, and may contain a rheology modifier capable of generating a gelling effect, which is selected from the group consisting of polysaccharides such as xanthan gum, gum arabic, and mixtures thereof.

[0076] Specifically, the rheology modifier is present in an amount in the range of 0.08 to 2% by weight, more specifically 0.2 to 0.8% by weight, and even more specifically 0.3 to 0.6% by weight, based on the total weight of the aqueous gel ink.

[0077] The aqueous gel ink having a fixed color of the present invention also - pH adjusters such as sodium hydroxide and triethanolamine, - Lubricants, - Binders, - Crosslinking agents, - Wetting agents, - Plasticizers, - Antioxidants, and - Other additives such as UV stabilizers.

[0078] When present, these additives are added to the gel-based matrix of the aqueous ink in step (i) of the process of the present invention.

[0079] In one aspect, the present invention relates to a process for preparing an aqueous ink having a fixed color, (i) preparing a matrix of an aqueous ink; (ii) A silver salt: -water, at least an alkali metal or alkaline earth metal salt of citrate, preferably a mixture of an alkali citrate and an alkali metal hydride, preferably NaBH4, an oxidizing agent, preferably hydrogen peroxide H2O2, and polyvinylpyrrolidone, by mixing with said aqueous suspension of silver nanoparticles having a fixed color; (iii) adding, under stirring, the aqueous suspension of silver nanoparticles obtained in step (ii) to the matrix of the aqueous ink obtained in step (i) to obtain an aqueous ink having a fixed color with the silver nanoparticles dispersed therein.

[0080] In one aspect, the present invention relates to an aqueous ink with fixed color obtainable by the process of the present invention, said aqueous ink comprising silver nanoparticles and polyvinylpyrrolidone, in particular an alkali metal salt (such as the sodium salt) as specifically defined in the present disclosure, which may also comprise an alkaline earth metal salt.

[0081] The aqueous ink with fixed color of the present invention may also contain classical ink ingredients such as co-solvents, antimicrobial agents, corrosion inhibitors, antifoam agents, rheology modifiers, etc., as mentioned above, which are added to the aqueous ink matrix in step (i) of the process of the present invention.

[0082] In one aspect, the present invention relates to the use of a water-based ink, more particularly a water-based gel ink, of fixed colour as defined above, for writing on an absorbent support, which in one embodiment is a porous substrate, in particular paper, cardboard or textile.

[0083] The present invention also relates to a method of writing with an aqueous ink, more particularly an aqueous gel ink, of fixed color, comprising the step of writing with an aqueous ink having a fixed color according to the present invention on an absorbent support, including a porous substrate such as paper, cardboard, or textile.

[0084] After writing on an absorbent substrate with the fixed color, aqueous ink, more specifically aqueous gel ink, of the present invention, the distance between the silver nanoparticles in the aqueous ink, more specifically aqueous gel ink, coated on the absorbent substrate is lower than 4 μm, more specifically varies from 50 nm to 3 μm, more specifically varies from 500 nm to 1 μm.

[0085] Finally, the present invention relates to a writing instrument, comprising: an axial barrel containing an aqueous ink according to the invention, more particularly an aqueous gel ink; a pen body that delivers water-based ink stored within the axial barrel.

[0086] The writing instrument of the present invention may be selected from the group consisting of gel pens, felt tip pens, correction fluids, markers, and specifically gel pens.

[0087] The invention will be better understood with reference to the examples given in a non-limiting manner.

[0088] Example 1: Preparation of water-based gel ink with fixed color by the process of the present invention Preparing a gel-based matrix for water-based inks (step (i)) In the first step (i), a gel-based matrix for the water-based ink was prepared by mixing 15 g of triethylene glycol (co-solvent), 4 g of polyethylene glycol (co-solvent), 0.19 g of Acticide® MBS (antimicrobial agent), and 0.1 g of Additin® RC8221 (corrosion inhibitor). The mixture was mixed in a homogenizer mixer for 15 ms. -1The mixture was homogenized for 15 minutes at a speed of 1500 rpm and heated at a temperature of 35° C. Then, 0.4 g of xanthan gum (rheology modifier) ​​was added to the mixture. The mixture was mixed in a homogenizer mixer at 15 m.s. -1 The mixture was homogenized for 15 minutes at a temperature of 35° C. at a speed of 1500 rpm. 80.01 g of deionized water was slowly added to the mixture. The mixture was allowed to stand for 2 hours and 30 minutes. Then 0.3 g of Moussex® S 9092 (antifoaming agent) was added. The mixture was homogenized in a homogenizer mixer at a speed of 15 m.s. -1 The mixture was homogenized at a speed of 1000 rpm for 30 minutes at a temperature of 35° C. The obtained gel-based matrix of the water-based ink was cooled at room temperature (25° C.).

[0089] Preparing an aqueous suspension of red silver nanoparticles (step (ii)) In a second step (ii), an aqueous suspension of silver nanoparticles with a fixed color was prepared by mixing 42.527 mL of distilled water, 0.473 mL of silver nitrate solution (10 mM) (9370.1 Cark Roth), 3.68 mL of trisodium citrate (30 mM) (S1804-500G Sigma Aldrich), 3.68 mL of polyvinylpyrrolidone 2% (PVP40-100G Sigma Aldrich), and 120 μL of hydrogen peroxide 0% (412071 Carlo Erba), 100 μL of sodium borohydride NaBH 4 (100 mM) (71321-25G Fluka Analytical).

[0090] The mixture was homogenized in a homogenizer mixer at a speed of 400 rpm for 15-30 minutes.

[0091] Then, 840 μL of AgNO3 (10 mM) was added to the mixture in a homogenizer mixer at a speed of 400 rpm for 10 min.

[0092] The mixture was heated at 100 °C and then 350 μL of NaBH 4 (10 mM) was added.

[0093] NaBH 4When was added to the mixture, the resulting solution instantly changed from clear to colored, the color being attributed to the plasmonic effect.

[0094] Mixing is stopped when the expected red color is obtained.

[0095] The color of the aqueous suspension of silver nanoparticles is determined by the reducing agent NaBH 4 Note that the color depends on the ratio of . The resulting composition exhibits a plasmonic effect, which means that its color is due to the plasmon effect, i.e., due to light absorption by the nanoparticle dispersion.

[0096] for example: 200 μL of NaBH in the above mixture 4 Addition of 0.1% NaOH ensures a pink suspension of silver nanoparticles (Test 1). 250 μL of NaBH in the above mixture 4 Addition of 0.1% NaOH ensures a bright purple suspension of silver nanoparticles (Test 2). 300 μL of NaBH in the above mixture 4 Addition of 0.1% NaOH ensures a purple suspension of silver nanoparticles (Test 3). 600 μL of NaBH in the above mixture 4 Addition of 0.1% NaOH ensures a blue suspension of silver nanoparticles (Test 4). 970 μL of NaBH in the above mixture 4 Addition of 0.1% NaOH ensures a red suspension of silver nanoparticles (Test 5).

[0097] Preparing an aqueous gel ink with a fixed color (step (iii)) In a third step (iii), 1 mL of the aqueous suspension of silver nanoparticles obtained in step (ii) is added to 1 mL of the gel-based matrix of the aqueous ink obtained in step (i) to obtain an aqueous gel ink having a fixed color (red) in which the silver nanoparticles are dispersed and stabilized by 0.07% polyvinylpyrrolidone.

[0098] Test 1: After addition of the water-based ink to the gel-based matrix, the initial pink appears pink instantly. Test 2: After addition of the water-based ink to the gel-based matrix, the initial light purple appears instantly light purple. Test 3: After addition of the water-based ink to the gel-based matrix, the initial purple color appears purple immediately. Test 4: After addition of the water-based ink to the gel-based matrix, the initial blue appears blue instantly. Test 5: After addition of the water-based ink to the gel-based matrix, the initial blue instantly appears red.

[0099] Test 1: When the aqueous gel ink having the obtained fixed color was written on cellulose paper, the color appeared pink immediately and did not change over time. Test 2: When the aqueous gel ink having the obtained fixed color was written onto cellulose paper, the color appeared immediately as a light purple and did not change over time. Test 3: When the aqueous gel ink having the obtained fixed color was written on cellulose paper, the color appeared purple immediately and did not change. Test 4: When the aqueous gel ink having the obtained fixed color was written onto cellulose paper, the color appeared blue immediately and did not change over time. Test 5: When the aqueous gel ink with the obtained fixed color was written on cellulose paper, the color appeared red immediately and did not change over time. Furthermore, a visual evaluation of the color of this aqueous gel ink was realized over time (Tests 1, 2, 3, 4, 5).

[0100] As can be seen from Table 1, the color of the water-based gel ink did not change over time. [Table 1]

Claims

1. 1. A method for preparing an aqueous gel ink having a fixed color, comprising: (i) preparing a gel-based matrix of a water-based ink; (ii) a silver salt: -water, - mixtures of at least an alkali metal or alkaline earth metal salt of citrate with an alkali metal hydride, - an oxidizing agent, - and polyvinylpyrrolidone, to prepare an aqueous suspension of silver nanoparticles with a fixed color; (iii) adding, under stirring, the aqueous suspension of silver nanoparticles obtained in step (ii) to the gel-based matrix of the aqueous ink obtained in step (i) to obtain an aqueous gel ink having a fixed color with silver nanoparticles dispersed therein.

2. 2. The method of claim 1, wherein the amount of the citrate added in step (ii) is in the range of 0.001 to 0.08 wt. %, based on the total weight of the aqueous suspension.

3. The method according to claim 1 or 2, wherein the total amount of silver salt added in step (ii) is in the range of 0.0005 to 0.006% by weight, based on the total weight of the aqueous suspension.

4. 4. The process according to any one of claims 1 to 3, wherein the amount of alkali metal hydride added in step (ii) is in the range of 0.0005 to 0.005 wt.%, based on the total weight of the aqueous suspension.

5. 5. The method according to any one of claims 1 to 4, wherein the amount of oxidizing agent added in step (ii) is in the range of 0.03 to 0.1 wt.%, based on the total weight of the aqueous suspension.

6. 6. The method according to any one of claims 1 to 5, wherein the amount of polyvinylpyrrolidone added in step (ii) is in the range of 0.05 to 0.1 wt.%, based on the total weight of the aqueous suspension.

7. 7. The method according to any one of claims 1 to 6, wherein the silver nanoparticles obtained in step (ii) are silver nanoparticles having a spherical shape.

8. An aqueous writing gel ink having a fixed color, comprising silver nanoparticles and polyvinylpyrrolidone, not comprising any other colorant other than the silver nanoparticles, and further comprising an alkali metal salt, and exhibiting a plasmonic effect.

9. 9. The aqueous gel ink of claim 8, further comprising an alkali metal salt, in particular an amount of alkali metal salt of at least 0.0003 wt. %, in particular in the range of 0.0003 to 0.01 wt. %, based on the total weight of the aqueous gel ink.

10. 10. The aqueous gel ink of claim 8 or 9, wherein the silver nanoparticles have an average particle size in the range of 1 to 100 nm, in particular 10 to 50 nm.

11. 11. The aqueous gel ink of claim 8, wherein the amount of silver nanoparticles is in the range of 0.0005 to 5 wt.%, particularly 0.0005 to 3 wt.%, more particularly 0.0005 to 0.005 wt.%, based on the total weight of the aqueous gel ink.

12. 12. The aqueous gel ink according to claim 8, wherein the amount of water is in the range of 50 to 95% by weight, based on the total weight of the aqueous gel ink.

13. a co-solvent, specifically in an amount ranging from 5 to 35% by weight, relative to the total weight of the aqueous gel ink; and / or an antimicrobial agent, specifically in an amount ranging from 0.01 to 0.5% by weight, relative to the total weight of the aqueous gel ink; and / or a corrosion inhibitor, specifically in an amount ranging from 0.05 to 1% by weight, relative to the total weight of the aqueous gel ink; and / or an antifoaming agent, specifically in an amount ranging from 0.05 to 1% by weight, relative to the total weight of the aqueous gel ink; and / or - An aqueous gel ink according to any one of claims 8 to 12, further comprising a rheology modifier in an amount ranging from 0.08 to 2% by weight, in particular with respect to the total weight of the aqueous gel ink.

14. A writing implement, - an axial barrel containing an aqueous gel ink with a fixed color according to any one of claims 8 to 13; a pen body delivering the aqueous gel ink according to any one of claims 8 to 13 stored within said axial barrel, More specifically, said writing instrument is selected from the group consisting of gel pens, felt tip pens, correction fluid, markers, and specifically gel pens.

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