FIELD OF THE INVENTION

[00011 The present invention rel tes io dispersions of metal particles and. in particular, to methods of making and stabilizing silver particle dispersions.

BACKGROUND

[0002] Silver dispersions and silver colloids can be produced in a number of ways including nanoparticie synthesis from soluble silver ions. One method is known in the art as the polyoi process and involves the reduction of silver nitrate by ethylene glycol in the presence of

polyvinylpyrolidone (PV.P) at elevated temperatures. The dispersions may be used as produced or the silver particles may be precipitated, dried and then made up to a desired concentration in a liquid vehicle.

SUMMARY

f 0O3] In one aspect a sliver particle dispersion is provided, the dispersion comprising an aqueous vehicle, at least 0.1% metallic silver particles by weight, and at least 0.1% of a stabilizing compound by weight, the stabilizing compound associated with silver particles and having the formula E _x -R-A _y , where E is a Lewis basic group comprising at least one atom selected from the group consisting of O, S, N, and P; R. is a hydrocarbyl group; A is an ionizable or ionic group, and x and y are each independently a non-zero integer and wherein E can be the same or different when x>l and A can be the same or different when. y>l . The dispersion may comprise greater than 3 % silver particles by weight, and the I Q partic le size distribution of the metallic silver particles may change- by less than 20% after aging the dispersion for- 7 days at 70°C. The stabilizing compound can be an amine that may be an aniline derivative, such as p- amiiiophenyiacetic acid or a salt thereof. The ionizable group can be a carboxyl group,

(0004} In another aspect, a method of producing an aqueo us sil ver dispersion is provided, the method comprising replacing an adsorptive substance on a silver particle in a first dispersion with a stabilizing compound to produce a second dispersion, and filterin the second dispersion to remove at least a portion of the adsorptive substance and to produce a third dispersion comprising a liquid vehicle comprised of at least 90% water, wherein the silver particles have a particle size d _$ o that is less than 20% greater than the d _$ o of the particles in the first dispersion. The particle size d$o of silver particles can be maintained within +/- 10% after displacing the adsorptive substance and within -*·/- 10% after filtration. The third dispersion can be essentially free of organic compounds other than the stabilizing compound- The fi ltration step can be a diafiltration step that is carried out using a membrane having a molecular weight cut off of greater than or equal to 500 kD or greater than or equal to 100 kD. The liquid fraction of the first dispersion can include greater than 80% by volume of one or more non-aqueous solvents and the liquid fraction of the second, and third dispersion can be greater than 80% water. A non-aqueous solvent can be exchanged for water via diafiltration.

[0005] in another aspect, a method of producing an aqueous silver dispersion is provided, the method comprising diafiltering a first silver dispersion to produce a second silver dispersion, wherein the dgo particle size increases by less than 40% between the first dispersion and the second dispersion. The liquid fraction of the second dispersion can comprise greater tha 95% water, and the diafiltering may include exchanging a non-aqueous solvent for water. The silver particle size in each of the first and second dispersi ons can ha ve a d _$ o particle size of less than 100 nm or less than 50 nra measured by disc centrifuge. The first silver dispersion can be an aqueous dispersion wherein the liquid fraction of the dispersion comprises at least 50% water by weight. The first silver dispersion may also be a non-aqueous dispersion including less than 10% water. The method can include removing essentiaiiy all .of the organic compounds other than one or more stabilization compounds. For instance, the method can reduce the concentration of po 1 y vi n i ro I idone to less than 1,000 ppra by weight in the dispersion.

BRIEF DESCRIPTION OF DRAWINGS

(0006} The detailed description belo ma be better understood with reference to the accompanying figures which are provided for illustrative purposes and are not to be considered as limiting any aspect of the invention.

FIG. 1 provides a graph illustrating a distribution curve of silver particle size for one embodiment;

FIG. 2 provides a graph illustrating a distribution curve of silver particle size for the samples of FIG. 1 after heat treatment; FIG. 3 provides a graph illustrating a distribution curve of silver particle size for three different examples; and

FIG. 4 provides a focused view of a portion of the graph of FIG, 3.

DETAILED DESCRIPTION

[0007J In one aspect, an aqueous silver dispersion is provided that can exhibit, for example, improved colloidal stability. Dispersions may be concentrated to greater than 1%, greater than 3%, greater than 5%, greater than 10% or greater than 20% silver particles by weight in water and can retain their particle size distribution (e.g., djo and d ) during processing steps such as purification, concentration and heat aging. The particles can be treated with a stabilizing compound that may replace at least some of the PVP that may be associated with silver particles produced using the polyol process. The stabilizing compound may be a substance that includes a Lewis basic group and a charged or iomzable .group,. The Lewis basic group may associate with the silver particles while the ionixable or charged group may contribute to charge stabilization, improving the stability of the colloidal dispersion.

[0008] The dispersions may be treated using diafiitration (cross-flow ultrafiltration) that can, for example, concentrate the silver particles as well as remove organic compounds including, for instance, PVP and by-products thereof. This treatment may result in dispersion that is free of undesirable compounds such as organic compounds that are not associated with the silver particles. For instance, a dispersion may be considered to be essentially free of organic compounds other than the stabilization compound(s) if there is less than .1.000 pprri of organic compounds in the dispersion that are not associated with silver particles. As used herein, an aqueous dispersion includes a liquid vehicle in which at least 10% of the liquid vehicle is water. The silver particles may be dispersed in a liquid vehicle where the liquid fraction of the dispersion includes greater than 50%, greater than 75%, greater than 90% or greater than 95% water by weight. The dispersions may be essentially free of non-aqueous solvents. These stable aqueous dispersions may be used, for example, to produce inks capable of depositing metallic silver on a substrate via an inkjet printer.

[0Θ09] Colloidal silver produced using the polyol process is typically purified by

precipitation to eliminate both the solvent and excess PVP, The resulting powder may be dried and subsequently redispersed into various solvents using high shear mixing and comminution techniques. See, for example, U.S. Patent No. 7,575,621 and U.S. Patent Application

Publication Nos. 2008/0034921 and 2007/0034052, each of which is incorporated by reference herein. For instance, the powder may be redispersed in water to produce an aqueous dispersion that may be used, for instance, to produce an inkjet ink. Often, this redispersion process results in a significant increase in both the average particle size and the breadth of the particle size distribution of the final dispersion compared to the initial as-formed dispersion. In particular, when the nanoparficles are redispersed from the powder form into water, the resulting silver dispersions exhibit an increase in average particle size and in the breadth of the particle size distribution over time, sometimes including settling of the colloidal dispersion,

[0010] The silver particles and dispersions used herein may initially be obtained using any source or technique capable of providing silver particles of desired size. Silver particles may be formed by methods known to those of skill in the art using, for instance, precipitation methods such as the polyol. process. In this process, dissolved silver ions can be reduced to metallic silver particles in a polyol such as ethylene glycol in the presence of a vinyl pyrre!idone polymer, such as vinyl pyrroiidone homopoiymer (PVP). This process is described fully in U.S. Patent Application Publication No. 2007/0034052, which is hereby incorporated by reference herein, initially produced dispersions may have an average particle size of less than 200 nm, less than 150 nm, less than 100 nm, less than 75 mti, less than 50 nm. less than 30 ran or less than or equal to 20 nm. Unless otherwise stated, the silver particle sizes provided herein are determined by CPS disc centrifugation using the technique described in the Examples section below,

[0011] The stabilization procedure may start with initial silver nanoparticles that are

dispersed in a solvent such as ethylene glycol, as may be common with particles produced using the polyol process. For example , one embodiment of the polyol process combines silver nitrate. PVP and ethylene glycol, A stabilizing compound can be introduced that may serve to replace some or all of the PVP associated with the silver particles. This step may be accompanied by a pH adjustment. The amount of stabilizing compound introduced may be in the range of 0.1 to 100 % by weight of the silver particles present, and in specific embodiments may be in the range of 1 to 30 % or 2 to 15 % of the weight of the silver particles. If a purification procedure such as diafiltration is to be used, the stabilization compound may be added prior to, during, and/or after, the purification process. [0012J The stabilizing compound may be soluble in water and may include a portion or functional group that associates with the silver particles and another portion that is hydrophilic. A compound is "associated" with a particle if it does not m ove independently of the particle in the dispersion. The stabilizing compound may include a polymer or may be polymer tree. In one set of embodiments the stabilizing compound is represented by the formula E _S -R-A _y , where E is a Lewis basic group comprising at least one atom selected from the group consisting of O, S, N, and P on which at least one lone pair of electrons is available to act as a Lends basic group. R can be a hydrocarbyl group. The Lewis basic group, E, may be, for example, more nucleophi!ic than n-methylpyrrolidone (NMP), When more than one Lewis basic group Is present the groups may be the same or different, and when x>l . E can be the same or different. The term

"hydrocarbyl group," as used herein, denotes a divalent, linear, branched, cyclic, or po!ycyclic group that contains carbon and hydrogen atoms. The hydrocarbyl group may optionally contain atoms in addition to carbon and hydrogen se lected from Groups 13, 14 , 15, 16, and 17 of the Periodic Table. Examples of divalent hydrocarbyls include the following: Cr -Cso alkyl _\ -C30 alkyl substituted with one or more groups selected from C _\ -€3 ₀ aikyl, C3 - Cis cycloalkyl or aryl; C3 - C 15 cycloalkyl; C3 - CJ S cycloalkyl substituted with one or more groups selected from C _\ - C ₂ 0 alkyl, C ₃ - C _I5 cycloalkyl or aryl; C _& ~ C _\ , aryl; and C _¾ - C _t? aryl substituted with one or more groups selected from Cj -Cj alkyl,€ ₃ - C55 cycloalkyl or aryl; where aryl preferably denotes a substituted or unsubsiituted phenyl, napthyl, or anthracenyi group.

j¾013J "A" can be an ionizable or charged group, and x and y can each be independently a non-zero integer. The term "ionizable or charged group," as used herein, denotes a group which bears a positive or negative charge in water or protic solvent and is optionally dependent or independent of the pH of the solution. Examples of an ionizable group are the conjugate bases of compounds containing acidic hydrogens, the conjugate acids of compounds which can be protonated in water, and the like. Examples of a charged group are quaternary ammonium salts, quaternary phosphonlum salts, and the like. In some embodiments, "A" can be an acid group such as carboxylic acid, sulfonic acid, phosponic acid and/or phosporic acid. When more than one ionizable or charged group is present, the groups may be the same or different, and when y>l , A can be the same or different.

[0014] Examples of Lewis basic groups that may form a portion of a stabilizing compound include, for example, primary .amines, secondary amines, tertiary amines and deprotonated acid groups,. Additional examples include deprotonaied carboxylic acids, carboxylic acid amides, carboxylic acid phosphides, thioearboxyiic acids, ditliiocarboxylic acids, thiocar oxylic acid amides, thiocarboxylic add phosphides, carbonic acid, carbamic acids, ureas, thiocarbonic acid, thioureas, thiocarbaroic acids, dithiocarbamic acids, hydroxycarboxylic esters,

hydroxycarboxylic acid amides, amino acid esters, hydroxythiocarboxylic esters,

hydroxydithiocarboxylic esters, hydroxythiocarboxylic acid amides, hydroxycarboxylic

thioesters, hydroxythiocarboxylic thioesters, hydroxydithiocarboxylic thioesters,

mercaptocarboxylic esters, mercaptocarboxylic acid amides, mercaptothiocarboxylic esters, mercaptoditbiocarboxyiic esters, mercaptothiocarboxylic acid amides, mercaptocarboxylic thioesters, mercapiothiocarboxyMc thioesters, mereaptodithiocarboxyik thioesters,

hydroxyketones, hydroxyaldehydes, hydroxyimines, mercaptoketones, niercaptoa!dehydes, mercaptoimines, .hydroxytliioke tones, hydroxythioaldehydes, mercaptothioketones,

mercaptothioaidehydes, 2-hydroxybenzaldehydes, 2-mercaptoberi2aldehydes, 2- aminobenzaldehydes, 2-hydroxybenzthioaldehydes. 2-hydroxybenzoate esters, 2- hydroxyberizamid.es, 2-hydroxybenzoate thioesters, 2-hydfoxythiobenzoate esters, 2- hydroxythiobenzaniides, 2-hydroxybenztliioaIdehydes, 2-mercaptobenzthioaidehydes, 2- aminoberr thioaldehydes, 2-hydroxyarylkeiones, 2~.roercapioary!ketones, 2-aniinoaryiketones, 2- hydiOxyary!iiTiines _f 2-mercaptoarylimmes, 2-aminoaryIimines, 2-hydroxyaryIthioketones, 2- mercaptoarylthioketones, 2-aminoarylthioketones, . benzoins,2-pyrroleearboxadehydes _} 2- pyrro!ethi ocarboxadehydes, 2-pyrrolecai-boxai dimines, hy drocarb l 2-pyrrolyi ketones ,

hydrocarbyi 2-pyrrolyl .mines, hydrocarbyl 2-pyrrolyl thioketones, 2 ndoleearhoxadehydes. 2- mdolethiocarboxadehydes, 2- dolecarhoxaldirnines, hydrocarbyl 2-indolyl ketones, hydrocarbyi 2-mdolyi imines, hydrocarbyl 2-indolyl thioketones, hydroxyquinolines, tropolones,

aminotropolones, aminotropone imines, and the like. In one set of embodiments the stabilizing compound may be an aniline derivative including an amino group as the Lewis basic group and a CHaCOOH group as the charged or ionizable group. One example of an aniline derivative is p- aminophenylacetic acid (APAA). Some additional compounds that may serve as stabilizing compounds include, for example, sulianilic acid, gamm aminobutyric acid, para aminobenzoic acid and/or taurine,

[§0151 desired amount of the stabilizing compound may be added to the starting dispersion as an aqueous solution. The pH of the mixture, which may be acidic after the polyol process. may be raised from about 3 to about 10 using sodium hydroxide. The resulting dispersion, which may include silver particles, PVP, ethylene glycol and stabilizing compound may then be subject to a diafiltration procedure to remove undesirable components. Diafiltration may be miderstood herein as a cross-flow membrane based separation that is used to selectively remove ingredients from a given liquid or dispersion. More specifically, upon passage of the colloid through the membrane, the pore size of the membrane may regulate the retention and elution of ingredients from the colloid environment. Preferably, the silver dispersion may be pumped under pressure across the diafiltration membrane and the PVP, ethylene glycol, organic byproducts and excess stabilizing compound are elated in the permeate (that which passes through the membrane). The silver particles and a portion of the stabilizing compound associated with the silver particles may be retained. In addition, the diafiltration ^' process may be made continuous where water may be replenished as the make-up solution. For example, any non-aqueous solvent may be exchanged for water so that the retentate dispersion includes a liquid vehicle fraction that Is greater than 50%, greater than 80%, greater than 90%, greater than 95%, greater than 99% or greater than 99.9% water by volume. In some embodiments, less than 5%, less than 1% or less than 0,1% of the silver particles in the original dispersion are passed through the membrane with the filtrate. PVP however may pass through the membrane efficiently, resulting in .greater than 90%, greater than 99% or greater man 99>9% of the PVP being separated from the silver particles.

Diafiltration is a continuous process and additional solvent (e.g., water) may be pumped through the system to further purify the silver particle dispersion. As additional water passes through the sy stem more organic and/or inorganic impurities can be removed while all or most of the stabilized si l ver particles are retained without aggregating. In one embodiment, 10 volumes of sodium hydroxide solution (about 2 L each) at pH 10 are pumped through the system, to further purify the silver particle dispersion and to adjust the pH of the dispersion. This is followed with 10 volumes (2 L each) of DI water to complete the purification process,

[0016] Di filtration membranes may be characterized by a molecular weight cut off

(MWCO) which is initially defined as the lowest molecular weight of the solute in which 90% of the solute is retained by the membrane. It has been found that membranes exhibiting a MWCO of greater than 100 kD, greater than 200 kD or greater than or equal to 500 kD ears retain silver particles having a size of less than 100 nm, less than 50 nm, less than 40 ran, less than 30 nm or less than 25 nm, while allowing for the removal of organic components, such as PVP, from the dispersion. Surprisingly, a 500 kD membrane has been shown to retain over 99% of silver particles m this size range (e.g., 30 nm) while allowing high molecular weight PVP to be removed.

[0017) Using diaiiitration, any non-aqueous solvents may be exchanged partially or entirely for water. This procedure may also be used to concentrate the silver particles. For example, the concentration of silver particies in the dispersion may be increased by more than a factor of 3X, 5X or 10X. in one embodiment, an initial dispersion having a silver particle concentration of 3% by weight is increased to 30% by weight using diafiitration. in contrast to precipitation techniques _* the dso and d values, can remain essentially unchanged. For instance, the d > and d _% values may increase by less than 30%, less than 20%, less than 10% or less than 5%.

Working Exam les

Comparative Example .1.

[00.18] A. mixture of Ag PVP in waier w s prepared from dry silver-PVP particles prepared using the polyol process and separated by the process disclosed in U.S. Patent No. 7,575,621 , whereby the silver polyol was added to water to result in a 80. % solids mixture. The mixture, was mechanically agitated overnight and transferred to a Dispermat™ mixer where it was mixed for about. 15 hours at an. rpm of 9500 and a temperature of 10°C at a solids loading of 78 wt%, Water was added to the resulting silver mixture to give a 50% solids loading dispersion. The 50% dispersion was mixed on the Dispermat at 4000 rpm for i hour at 10°C. The dispersion was then lei down to 20% solids and mixed with a Silverson high shear mixer for 20 minutes at 7200 rpm in an ice bath. The .resulting dispersion was allowed to settle for 60 hours, the dispersion was then decanted, and then filtered through a 0.45 micron nylon capsule filter. The silver particle size was measured using disc centrifuge before and after heat treatmeni for 7 days at 70 degrees. This material is designated as sample CE.

Example 2

[0019] A sample of about 3.0 wt, % silver as Ag/PVP in ethylene glycol was produced, using the polyol process of comparative example 1 and as described in U.S. Patent Application

Publication No. 2007/0034052. The sample was diluted 1 : 1 v/v with water. The pH was adjusted to about 10.5 with sodium hydroxide and 0.1 grams of the sodium salt of p- aminophenyiacetjc acid (APPA) was added per 1.0 g of Ag. The dispersion was then mixed for 15 minutes at room temperature. The sample was concentrated to 1.5 wt % silver, and ethylene glycol was then exchanged for water by dtafiltering the sample using a membrane (GE model UFP-500-C-4MA) having a MWCO of 500 kD. Prior to using the membrane for the first time, it was conditioned by passing a 50/50 solution of water/ethylene glycol through the membrane. The sample was then diafiltered for 10 volumes with sodium hydroxide to produce an aqueous mixture ai pH -10. Using the same membrane the dispersion was subsequently diafiltered with 10 volumes of DI water to reduce the pH and further remove any residual PVP or ethylene glycol. The sample was then concentrated to approximately 30,0 wt % silver. Five different samples were made in this manner and are designated as _. samples 1-5 i the tables below.

§020| Initial particle size distribution measurements were made using a CPS disc centrifuge. A CPS model DC24000 disc centrifuge with autogradient features was operated under the following instrument parameters:

Disc Speed - 17,000 rpm

Gradient - ~ 15ml 8% to 24% sucrose solution formed by autogradient accessory

Particle Density - 10,5 g/cm.

Refractive Index - 0.1435

Particle Absorbance - 2.56?

Run Collection - collect data from 200 - l Onni

[0021] The reagents used included DI water and 8% and 24% sucrose solutions capped with dodecane. Samples were diluted to a final concentration of 0.003% in DI water (17 M hm) and 0.1 niL sample was injected into the instrument. Results were obtained and dso and <¾ο values determined for each sample.

[0022J initial size measurements were made before purification and particle size distribution was measured again after samples 1-5 were diafiltered and after sample CE was redispersed in water. Each of the samples was then heat, aged at 70°C for a period of 7 days and the particle size distribution, of each sample was determined for a third time. Results are provided in Table 1 (dso) and Table 2 (d%) below. The djo value provides the particle size at which half the particles are larger and half are smaller than the stated value. The d _? o value provides the particle size at which 10% of the particles present are larger and 90% are smaller. It is often useful to avoid the presence of even a small number of large particles. Therefore the value can be useful in evaluating the suitability of the dispersion for a number of applications, including as an Inkjet composition. In each of Tables 1 and 2. column 2 provides the initial particle size measurement; column 3 provides the same measurement after diafiltraiion (or redispersion in the case of sample CE); column 4 provides size measurement after heat treatment. Column 5 provides the percent change in size from the initial to the diaftltered (or redispersed) sample. Column 6 provides the percent change in particle size from the diafiltered (or redispersed) sample to the heat treated sample; and column 7 provides the total percent change in size from the initial dispersion through the heat treatment.

PARTICLE SIZE BY CPS -<¾»

Post Post Heat Total

Initio! Filial Heat Processin Trea ment % iple U Dispersion Aged % change % cfesng© ensftge

I 22.9 23.2 24.0 1.1 3.4 4.6

2 23.1 23..3 23.7 1.0 1.7 2.7

23.9 24.2 24.7 1.4 2.1 3.5

4 23.7 24.1 24.9 1.7 3.3 5.1

5 23.8 24.1 24,9 1.3 3.3 4.6

.; [ ^■ : 22.0 28.6 34.3 30.0 19.9 555 Table .1

PARTICLE SIZE BY CPS -d

Post Post Heat

Final Processing Tefal %

% chasige change

1 30.2 31.8 0.6 6.0

2 31.0 31.2 0.6 4.2

3 32.7 33.5 34.0 2.4 1 ,5

4 31.8 2 7 3.3.8 2.8 3.4

5 32 J 32.7 33.7 i .9 3 , 1 3,y 45.2 76,0 45.8 145.2

[0023] The size distribution, of ^' the particles is illustrated in FIGS. 1-4. FIG. 1 provides the size distribution curves of samples 1-5 prior to heat treatment. FIG. 2 provides the size distribution curves of samples 1-5 after heat treatment. FIG. 3 provides the curves for

comparative sample CE as prepared, as redispersed, and after heat treatment FIG. 4 provides the same data as FIG. 3 but with a cutoff at 45 nra. FIGS. 3 and 4 each show a significant increase in particle size after redispersion and an additional increase in particle size after heat treatment of the comparative sample.

| 024| A comparison of FIGS, ί and 2 shows that there is little if any variation in the particle size distribution of samples 1-5 before (FIG. 1 ) and after (FIG, 2) heat treatment, in comparison, FIGS. 3 and 4 show that the sample prepared by the precipitation method without the addition of a stability compound and without diafiltration exhibits a significant increase in particle size upon redispersion and upon heat treatment. The difference in particle size, and in particular at ye, illustrates that the experimental samples 1-5 provide a significantly more consistent and stable dispersion than does comparative example CE.

[00251 While several embodiments of the present invention have been described and

illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art. will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or

configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein, it is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equi valents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, .and or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is Included within the scope of the present invention.

1102 1 Λϋ definitions, as defined and used herein, should be understood to control over dictionar definitions, definitions in document incorporated by reference, and/or ordinary meanings of the defined terms.

[0027] The indefinite articles ⁱ⁴ a' ^* mid "an." as used herein in the specification and in the claims, unless clearly indicated to- the contrary, should be understood to mean "at least one." 028] The phrase "and/or, ^5* as used herein in the specification and in the claims, should be unde rstood to mean "either or both" of the- elements so conjoined, i.e. , elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the "and/or" clause, whether related or unrelated to- those elements specifically identified, unless clearly indicated to the contrary.

{0029] All references, patents and patent applications and publications that are cited or referred to in this application are incorporated in their entirety herein b reference.