JP2007145947A - Electroconductive ink composition and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electroconductive ink composition which formes a film excellent in adhesion to a base material and smoothness of the film surface and can fire the film at a comparatively low temperature, and its manufacturing method. <P>SOLUTION: The electroconductive ink composition comprises a solid in which the surfaces of fine particles of a nobel metal are enveloped with a protective colloid, and the above protective colloid comprises at least one type of a 5-20C amine compound and at least one organic compound selected from a 4-30C amide compound, an aldehyde compound, an ester compound, and a ketone compound. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a conductive ink composition that suppresses metal grain growth, has excellent adhesion to a substrate such as non-alkali glass and smoothness of the film surface, and can fire a thin film at a relatively low temperature. It relates to a manufacturing method.

  Conventionally, metal thin films such as silver thin films have been provided for various applications such as various electrodes, circuits, electric field shields, and reflective films. As a method for producing such a metal thin film generally formed on a glass substrate or a semiconductor substrate, a vacuum deposition method or a paste coating method is known.

  The vacuum deposition method is a method in which a substrate is placed in a vacuum deposition apparatus, and a metal thin film is deposited thereon. The film thickness can be precisely controlled, and a high-quality metal thin film can be provided. .

  In the paste coating method, commercially available metal fine particles are dispersed in a matrix component composed of at least a resin component and an organic solvent, and if necessary, glass frit is added to a glass substrate, and the liquid component is evaporated by heating. This is a method of forming a metal thin film. Specifically, there are screen printing, dip coating, spin coating, and the like, which are characterized by a simple and inexpensive film forming process as compared with vacuum deposition.

  Patent Document 1 discloses a gold paste composed of gold fine particles having a particle size of 1.0 μm or less and ethyl cellulose. By controlling the particle size of the gold fine particles, sintering between the gold fine particles is improved, and 500 ° C. A gold thin film having a low resistance value can be formed by firing at the following relatively low temperature. Further, Patent Document 2 discloses a silver fine particle dispersed paste stably dispersed in an organic solvent, which can be fired at a temperature of 250 ° C. or lower.

However, the vacuum deposition method has a problem that the apparatus is large and expensive, and a long vacuum is required to achieve the required degree of vacuum. On the other hand, the paste coating method has made it possible to fire at a relatively low temperature of 250 ° C. or less, but such a low temperature generally has a problem that the adhesion of the metal thin film to the substrate is generally insufficient. was there.
Japanese Patent Laid-Open No. 10-340619 JP 2002-299833 A

  That is, since noble metals are generally stable, the reactivity with the substrate that is the object to be coated is low, and the adhesion with the substrate is poor. In addition to silver particles, there is a problem that when the reactivity and adhesion to other substances are low, such as noble metals, they are easily peeled off from the substrate. For this reason, it was necessary to surface-treat a glass substrate and to improve adhesiveness with a silver thin film.

Moreover, when silver is deposited on a glass substrate, the grain growth becomes significant when the firing temperature exceeds 250 ° C., and therefore the smooth surface becomes rough and surface turbidity occurs, resulting in poor surface properties during high temperature firing. It was difficult to use the obtained metal film as a wiring as it was.

  The present invention has been made in view of the above points, and is a conductive ink composition capable of forming a thin film excellent in adhesion to the substrate and smoothness of the film surface and firing the thin film at a relatively low temperature. It aims at providing a thing and its manufacturing method.

  That is, the present invention is a conductive ink composition comprising a solid material in which noble metal fine particles are surrounded by a protective colloid, and the protective colloid is at least one amine compound selected from 5 to 20 carbon atoms, A thin film formed on a substrate is formed by using at least one organic compound selected from amide compounds, aldehyde compounds, ester compounds, and ketone compounds having 4 to 30 carbon atoms. Even when fired at a low temperature of ˜150 ° C., the fired film suppresses metal grain growth, has excellent adhesion to the substrate and smoothness of the film surface, and has a low volume resistivity.

  In the present invention, by using a protective colloid having a small molecular weight, in which both the number of carbons of the amine compound and the number of carbons of at least one organic compound selected from aldehyde compounds, ester compounds, and ketone compounds are used, compared to the conventional case. Thus, there is an effect that a fired film can be formed at a relatively low firing temperature of 80 to 150 ° C. Since the firing temperature can be set lower than that of conductive ink containing conventional nanoparticles, it can be widely applied to plastic substrates. That is, it has a great feature that it can be fired at a relatively low temperature by controlling the temperature at which the protective colloid evaporates and decomposes by the molecular weight.

  In the conductive ink composition of the present invention, the fired film obtained has excellent surface smoothness and low volume resistivity, particularly when the organic compound is an amide compound or an aldehyde compound.

  In the present invention, when the noble metal is silver or gold, the ratio of the noble metal is 60 to 95% by mass and the ratio of the protective colloid is 5 to 40% by mass. The case where the enclosed solid is dispersed in an organic solvent is also included.

  Furthermore, the present invention also includes a method for producing a conductive ink composition including a solid material in which noble metal fine particles are surrounded by a protective colloid, and in this case, at least one kind selected from 5 to 20 carbon atoms is used. Compounds and / or mixtures obtained by stirring raw materials containing amines and at least one organic compound selected from amide compounds, aldehyde compounds, ester compounds, and ketone compounds having 4 to 30 carbon atoms as protective colloids A reducing agent is added to the solution containing the protective colloid and noble metal ions and stirred, and then the excess protective colloid and the solvent are removed. Thereby, metal grain growth can be suppressed, adhesion to a substrate such as non-alkali glass, excellent film surface smoothness, and a metal film can be formed at a low temperature of 80 to 150 ° C.

  In the present invention, the protective colloid is at least one organic compound selected from carbon atoms of 5 to 20, and at least one organic compound selected from amide compounds, aldehyde compounds, ester compounds, and ketone compounds having 4 to 30 carbon atoms. Even if a thin film produced on a substrate such as alkali-free glass or plastic is fired at a low temperature of 80 to 150 ° C. by using a protective colloid composed of a compound and having a molecular weight limited, the fired film is a metal particle. It has the effects of suppressing growth, excellent adhesion to the substrate and smoothness of the film surface, and low volume resistivity.

  In the method for producing a conductive ink composition, at least one amine selected from 5 to 20 carbon atoms, an amide compound having 4 to 30 carbon atoms, an aldehyde compound, an ester compound, and a ketone compound were selected. A compound and / or mixture obtained by stirring a raw material containing at least one organic compound is used as a protective colloid, and after adding a reducing agent to the solution containing the protective colloid and the noble metal ion and stirring, an extra protective colloid and The solvent is removed, and even when the metal film is baked at a temperature of 80 to 150 ° C., the grain growth of the noble metal is suppressed, the surface smoothness is maintained, and the adhesion to the substrate is improved.

  The conductive ink composition of the present invention is an ink material capable of forming a metal thin film by being applied to a substrate and then baked by drying or heat treatment. At least fine particles of a noble metal surrounded the surface with a protective colloid. Contains solids (colloidal particles). Two or more precious metals may be used, and two or more protective colloids may be used.

  In the said conductive ink composition, the ratio of a noble metal is 60-95 mass%, the ratio of a protective colloid is 5-40 mass%, and the sum total of both is 100 mass%. In addition, the conductive ink composition is a mixture of alcohol, such as methanol, ethanol, isopropanol, butanol, pentanol, hexanol, cyclohexanol, peptanol, α-terpineol, and the like, in which the surface of noble metal fine particles is surrounded by a protective colloid. It can also be used as ink by being dispersed in an organic solvent (dispersion medium).

  The protective colloid is a compound of at least one amine compound selected from 5 to 20 carbon atoms and at least one organic compound selected from amide compounds, aldehyde compounds, ester compounds, and ketone compounds having 4 to 30 carbon atoms. Or a mixture of these. By using a protective colloid with a small molecular weight in this way, it has the advantage that a metal film can be formed at a relatively low firing temperature of 80 to 150 ° C. compared to the prior art, and the fired film suppresses metal grain growth. Excellent adhesion to the substrate and smoothness of the film surface and low volume resistivity.

  Specifically, examples of the amine compound include pentylamine, hexylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine, dodecylamine, hexadecylamine, octadecylamine and the like.

  The amide compound is N-methylpropionamide, acetamide, methylacetamide, dimethylacetamide, N-methylpyrrolidone, dimethylsulfoamide, and the aldehyde compound is butanal, pentanal, hexanal, heptanal, octanal, nonanal, decanal, undecanal, Dodecanal, tridecanal, tetradecanal, pentadecanal, hexadecanal, heptadecanal, octadecanal, etc., ester compounds are methyl propionate, methyl octanoate, methyl acetoacetate, etc., and ketone compounds are 3 -Pentanone, 2-octanone, acetylacetone and the like. In particular, when an amide compound and an aldehyde compound are used, low volume resistivity and excellent surface smoothness can be obtained even when the conductive ink composition thin film is baked at a relatively low temperature of 80 to 150 ° C. preferable.

  The noble metal fine particles are so-called ultrafine particles (nanoparticles) having a particle size of several nm to several hundred nm, and specifically, metal fine particles having a particle size in the range of, for example, 1 nm or more and 100 nm or less are preferable. Used for. Specific examples include gold, silver, platinum, palladium, rhodium, ruthenium, osmium, iridium and the like, but are not particularly limited. Only one kind of these noble metals may be used, or an alloy combined appropriately. Among the noble metals, gold, silver, and platinum are preferable, but silver is most preferable.

  In the method for producing the conductive ink composition, specifically, at least one amine selected from 5 to 20 carbon atoms, an amide compound having 4 to 30 carbon atoms selected from 4 to 30 carbon atoms, an aldehyde compound, A compound and / or mixture obtained by stirring a raw material containing an ester compound and at least one organic compound selected from ketone compounds is used as a protective colloid, and divided into a solution containing the protective colloid and a noble metal ion in multiple times. After the reducing agent is added and stirred, excess protective colloid and solvent are removed. If the combination of the protective colloids described above, even if a paste in which noble metal fine particles are dispersed in an organic solvent is baked at a temperature of 80 to 150 ° C., the grain growth of the noble metal is suppressed and the surface smoothness is maintained. Improves the adhesion.

  That is, in the method for producing a conductive ink composition, (1) at least one amine compound selected from 5 to 20 carbon atoms and (2) 4 to 30 carbon atoms as a protective agent (protective colloid). A combination of at least one organic compound selected from an amide compound, an aldehyde compound, an ester compound, and a ketone compound is dissolved in an organic solvent, and a reducing agent is dropped over 10 minutes to 2 hours to protect the agent (protection Prepare noble metal fine particle / solvent dispersion protected with colloid).

  As the above-mentioned noble metal fine particles, as mentioned above, gold, silver and platinum are preferable, but silver or gold is most preferable.

  The noble metal ion is a metal compound that is soluble in an organic solvent, or can be prepared by dissolving this metal. The metal compound is not particularly limited, and examples thereof include chloroauric acid / or a salt thereof, silver nitrate, silver chloride, silver acetate, chloroplatinic acid / or a salt thereof, palladium chloride, palladium nitrate, palladium acetate, palladium nitrate and the like.

  As the organic solvent, for example, an organic solvent having 6 to 18 carbon atoms in the main chain is preferably used. If the carbon number is less than 6, the volatility is too high and handling becomes difficult. Conversely, if the carbon number exceeds 18, the viscosity becomes too high and handling becomes difficult, and it becomes difficult to concentrate. Is also not preferred. Specifically, hydrocarbons such as hexane, heptane, octane, decane, undecane, dodecane, tridecane, and trimethylpentane, toluene, xylene, tetralin, hexadecane, tetradecane, heptamethylnonane, and the like are preferable.

  The concentration of the noble metal ion solution is not particularly limited as long as it can be dissolved in a solvent, and is preferably 100 mM or more. If it is less than 100 mM, no sufficiently high concentration of noble metal fine particles can be obtained continuously, which is not preferable.

  As the reducing agent, a reducing agent-containing solution dissolved in a solvent is used. For example, sodium borohydride, hydrazine compound, citric acid or a salt thereof, succinic acid or a salt thereof, ascorbic acid or a salt thereof, phosphinic acid or Its salt, tartaric acid / or its salt, etc. can be used.

  The concentration of the reducing agent-containing solution is not particularly limited as long as it can be dissolved in a solvent, and is preferably 0.1 times or more with respect to the noble metal ion molar concentration. If it is less than 0.1 times, a larger amount of reducing agent-containing solution is required than the amount of the noble metal ion-containing solution added, which is not preferable because it is industrially disadvantageous.

  The reducing agent-containing solution is preferably added in a plurality of times, not all at once, and the rate is preferably constant.

  As a protective agent (protective colloid) for surrounding the surface of the noble metal fine particles, (1) an amine compound having 5 to 20 carbon atoms in a range of 50 to 99 mol%, (2) an amide compound having 4 to 30 carbon atoms, A compound obtained from a raw material containing at least one organic compound selected from an aldehyde compound, an ester compound, and a ketone compound in a range of 1 to 50 mol% is preferable.

  At least one organic compound selected from the amine compounds, amide compounds, aldehyde compounds, ester compounds, and ketone compounds is as described above.

  Then, the solid (colloid particles) in which the noble metal fine particles are surrounded by the protective colloid and the organic solvent are stirred for 0.5 to 3 hours, and a liquid obtained by concentrating the obtained filtrate with an evaporator is obtained. An organic solvent is added to this liquid to form a precipitate, which is then filtered with suction and dried to obtain a solid of precious metal fine particles protected with a protective colloid.

  As the organic solvent used here, alcohols such as methanol, ethanol, isopropanol, butanol, pentanol, hexanol, cyclohexanol, peptanol and α-terpineol can be mixed and used.

  As described above, the precious metal fine particles protected with the protective colloid which is a solid matter described above have a precious metal fine particle ratio of 60 to 95% by mass and a protective colloid ratio of 5 to 40% by mass, and the total of both is 100% by mass. is there.

  And this solid substance is organic solvent (dispersion) such as toluene, hexane, cyclohexane, heptane, cycloheptane, octane, decane, undecane, dodecane, tridecane, trimethylpentane, benzene, xylene, tetralin, hexadecane, tetradecane, heptamethylnonane, etc. It can be dissolved in a medium and finished into a paste-like ink composition. In this case, the concentration of the noble metal fine particles is preferably adjusted to 10 to 95% by mass.

  The ink composition forms a thin film on a substrate by a spin coating method, a screen printing method, a dip coating method, an ink jet printing method, or the like, and the thin film is baked at 80 to 200 ° C., thereby causing noble metal grain growth. It suppresses and maintains the smoothness of the surface, and further improves the adhesion to the substrate.

  Hereinafter, the present invention will be described based on examples. These examples are merely illustrative and do not limit the present invention in any way.

Example 1
7.5 g of silver acetate, 19.6 g of octylamine as a protective agent, and 4.89 g of (2) N-methylpropionamide were added to 0.15 L of 2,2,4-trimethylpentane and stirred. The mixed solution was stirred, and 0.3 L of a propanol solution containing 0.28 mol / L sodium borohydride as a reducing agent was dropped over 30 minutes to reduce silver.
The obtained liquid was poured into 1.0 L of methanol to produce a brown precipitate, and then the precipitate was collected by suction filtration. The precipitate was redispersed in 2,2,4-trimethylpentane and filtered, and then the filtrate was dried to obtain 1.5 g of a silver nanoparticle solid.

  The obtained solid was dissolved in a nonpolar solvent so that the metal concentration was 43% by mass to prepare a silver fine particle dispersed ink. And this was spin-coated on the board | substrate, and the thin film was produced. Since the silver fine particles have good dispersibility with respect to a nonpolar solvent, good stability with time, and hardly form an aggregate, a continuous film having a smooth surface was formed. The thin film was baked on a hot plate at 80 ° C., 100 ° C., 120 ° C., 150 ° C., 180 ° C., and 200 ° C. for 30 minutes.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The volume resistivity of the fired film is 54.3 μΩ · cm at 80 ° C., 29.6 μΩ · cm at 100 ° C., 7.8 μΩ · cm at 120 ° C., 5.3 μΩ · cm at 150 ° C., and 5.3 μΩ · cm at 180 ° C. cm and 5.8 μΩ · cm at 200 ° C.

Example 2
1.9 g of silver nanoparticle solid was obtained in the same manner as in Example 1 except that 5.76 g of (3) octanal was used instead of (2) used in Example 1 as a protective agent.
The obtained black solid was dissolved in 2,2,4-trimethylpentane so that the metal concentration was 38% by mass to prepare a silver fine particle dispersed ink. This was spin-coated on a glass substrate to produce a thin film. Since the silver fine particles have good dispersibility with respect to a nonpolar solvent, good stability with time, and hardly form an aggregate, a continuous film having a smooth surface was formed. The thin film was fired on a hot plate at 100 ° C., 120 ° C., 150 ° C., 180 ° C., and 200 ° C. for 30 minutes.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The volume resistivity of the fired film is 1535 μΩ · cm at 100 ° C., 17.2 μΩ · cm at 120 ° C., 5.3 μΩ · cm at 150 ° C., 4.1 μΩ · cm at 180 ° C., and 2.9 μΩ · cm at 200 ° C. there were.

Example 3
The same procedure as in Example 1 was carried out except that (1) 17.4 g of octylamine and (4) 3.97 g of methyl propionate were used as the protective agent, to obtain 3.0 g of a silver nanoparticle solid.
The obtained black solid was dissolved in 2,2,4-trimethylpentane so that the metal concentration was 38% by mass to prepare a silver fine particle dispersed ink. This was spin-coated on a glass substrate to produce a thin film. Since these silver fine particles have good dispersibility in nonpolar solvents, good stability with time, and hardly form aggregates, a continuous film having a smooth surface was formed. The thin film was baked on a hot plate at 80 ° C., 100 ° C., 120 ° C., 150 ° C., 180 ° C., and 200 ° C. for 30 minutes.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The volume resistivity of the fired film is 580 μΩ · cm at 80 ° C., 538 μΩ · cm at 100 ° C., 18.9 μΩ · cm at 120 ° C., 6.6 μΩ · cm at 150 ° C., 9.0 μΩ · cm at 180 ° C., 200 ° C. 7.5 μΩ · cm.

Example 4
The same procedure as in Example 1 was carried out except that (1) 11.6 g of octylamine and (5) 7.11 g of methyl octoate were used as the protective agent, to obtain 1.0 g of a silver nanoparticle solid.

  The obtained black solid was dissolved in 2,2,4-trimethylpentane so that the metal concentration was 38% by mass to prepare a silver fine particle dispersed ink. This was spin-coated on a glass substrate to produce a thin film. Since the silver fine particles have good dispersibility with respect to a nonpolar solvent, good stability with time, and hardly form an aggregate, a continuous film having a smooth surface was formed. The thin film was fired on a hot plate at 80 ° C., 120 ° C., 150 ° C., and 180 ° C. for 30 minutes.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The volume resistivity of the fired film was 485 μΩ · cm at 80 ° C., 57.8 μΩ · cm at 120 ° C., 55.5 μΩ · cm at 150 ° C., and 25.1 μΩ · cm at 180 ° C.

Example 5
The same procedure as in Example 4 was carried out except that 5.3 g of methyl acetoacetate (6) was used as a protective agent to obtain 2.0 g of a silver nanoparticle solid.
The obtained black solid was dissolved in 2,2,4-trimethylpentane so that the metal concentration was 38% by mass to prepare a silver fine particle dispersed ink. This was spin-coated on a glass substrate to produce a thin film. Since these silver fine particles have good dispersibility in nonpolar solvents, good stability with time, and hardly form aggregates, a continuous film having a smooth surface was formed. The thin film was fired on a hot plate at 80 ° C., 150 ° C., 180 ° C., and 200 ° C. for 30 minutes.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The volume resistivity of the fired film was 121 μΩ · cm at 80 ° C., 68.0 μΩ · cm at 150 ° C., 44.6 μΩ · cm at 180 ° C., and 35.0 μΩ · cm at 200 ° C.

Example 6
The same operation as in Example 4 was carried out except that (7) 4.50 g of acetylacetone was used as a protective agent to obtain 2.0 g of a silver nanoparticle solid.
The obtained black solid was dissolved in 2,2,4-trimethylpentane so that the metal concentration was 38% by mass to prepare a silver fine particle dispersed ink. This was spin-coated on a glass substrate to produce a thin film. Since these silver fine particles have good dispersibility in nonpolar solvents, good stability with time, and hardly form aggregates, a continuous film having a smooth surface was formed. The thin film was fired on a hot plate at 80 ° C., 120 ° C., 150 ° C., 180 ° C., and 200 ° C. for 30 minutes.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The volume resistivity of the fired film was 2780 μΩ · cm at 80 ° C., 773 μΩ · cm at 120 ° C., 141 μΩ · cm at 150 ° C., 124 μΩ · cm at 180 ° C., and 107 μΩ · cm at 200 ° C.

Example 7
The same procedure as in Example 4 was carried out except that 5.76 g of (8) octanone was used as a protective agent, to obtain 0.6 g of a silver nanoparticle solid.
The obtained black solid was dissolved in 2,2,4-trimethylpentane so that the metal concentration was 38% by mass to prepare a silver fine particle dispersed ink. This was spin-coated on a glass substrate to produce a thin film. Since the silver fine particles have good dispersibility with respect to a nonpolar solvent, good stability with time, and hardly form an aggregate, a continuous film having a smooth surface was formed.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The thin film was fired on a hot plate at 80 ° C., 120 ° C., 150 ° C., 180 ° C., and 200 ° C. for 30 minutes. The volume resistivity of the fired film was 1400 μΩ · cm at 80 ° C., 112 μΩ · cm at 100 ° C., and 15.1 μΩ · cm at 150 ° C.

Example 8
The same procedure as in Example 4 was carried out except that 3.88 g of (9) 3-pentanone was used as the protective agent to obtain 0.4 g of a silver nanoparticle solid.
The obtained black solid was dissolved in 2,2,4-trimethylpentane so that the metal concentration was 38% by mass to prepare a silver fine particle dispersed ink. This was spin-coated on a glass substrate to produce a thin film. The silver fine particles have good dispersibility in nonpolar solvents, have good temporal stability, and do not easily form aggregates, so that a continuous film with a smooth surface is easily formed.

  As a result, the obtained fired film had a smooth surface and a metallic luster. The thin film was baked on a hot plate at 100 ° C. for 30 minutes. The volume resistivity of the fired film was 168 μΩ · cm at 100 ° C.

  As a result, in the examples, it can be seen that the fired film has a smooth surface and a metallic luster even when fired at a temperature of 80 to 150 ° C.

In the conductive ink composition of the present invention and the method for producing the same, a thin film excellent in adhesion to the substrate and surface smoothness at the same time while suppressing the firing cost by firing at a relatively low firing temperature of 80 to 150 ° C. And can be used for a wide variety of precious metal thin films such as various electrodes, circuits, electric field shields, and reflective films.

Claims (6)

  The conductive ink composition includes a solid material in which fine particles of a noble metal are surrounded by a protective colloid, and the protective colloid includes at least one amine compound selected from 5 to 20 carbon atoms and 4 to 30 carbon atoms. A conductive ink composition comprising: an amide compound, an aldehyde compound, an ester compound, and at least one organic compound selected from ketone compounds.   The conductive ink composition according to claim 1, wherein the organic compound is an amide compound or an aldehyde compound.   The conductive ink composition according to claim 1, wherein the noble metal is silver or gold.   The conductive ink composition according to any one of claims 1 to 3, wherein a ratio of the noble metal is 60 to 95% by mass and a ratio of the protective colloid is 5 to 40% by mass.   The conductive ink composition according to any one of claims 1 to 4, wherein a solid substance in which noble metal fine particles are surrounded by a protective colloid is dispersed in an organic solvent. A method for producing a conductive ink composition comprising a solid in which noble metal fine particles are surrounded by a protective colloid, and having at least one amine selected from 5 to 20 carbon atoms and 4 to 30 carbon atoms A compound and / or mixture obtained by stirring a raw material containing at least one organic compound selected from an amide compound, an aldehyde compound, an ester compound, and a ketone compound is used as a protective colloid, and the protective colloid and the noble metal ion are contained. A method for producing a conductive ink composition, comprising adding a reducing agent to a solution and stirring, and then removing excess protective colloid and solvent.