Silver flakes coated with specific additives for conductive applications and method of manufacture

Abstract

The invention is directed to flaky silver pigments having a d50 in a range of 2.0 to 100.0 µm and an aspect ratio of more than 25 which are coated at least partially with an additive component, wherein the additive component is: I) a thiol represented by the general formula (Ia): A1-S-A2 or a dithiol represented by the general formula (Ib): A1-S-S-A2, wherein A1 and A2 represent either an alkyl having 1 to 8 C-atoms with A1=A2 or A1 and A2 independently represent a moiety containing polar or polarizable functionalities selected from the group consisting of ester, carboxyl, amine, hydroxyl, benzyl, benzoic, furyl, furfuryl, pyrrolyl, pyridyl, pyrazolyl, pyrazonolyl, oxazolyl, isooxazolyl, ether and combinations thereof and wherein the polar or polarizable functionalities are spaced apart from the sulphur atom or disulphur atoms by a maximum of 3 non-aromatic C-atoms and wherein the maximum total length of the thiol or dithiol involves a chain containing a maximum of 8 non-aromatic C-atoms and the maximum length difference of A1 and A2 is within 2 atoms of each other, or II) a thiol represented by the general formula (IIa): HS-(CH(A3))x-(R1)y-A3 or represented by the general formula (IIb): HS-R4-CO-O-R5, wherein R1 is an alkylene having 1 to 6 C-atoms or a phenylene, and wherein A3 is independently selected from the group consisting of NR2 2, OH, COOH, COOR3, N-succinimide, and 1-amino 2-carboxylethyl, wherein integers x and y are either 0 or 1 and x + y = 1 or 2, and wherein R2 is independently selected from the group consisting of H, methyl, ethyl and propyl and wherein R3 is selected from the group consisting of H, methyl, ethyl or propyl, butyl, pentyl, hexyl, N-succinimide and 2-aminoacetyl acid, and wherein R4 is methylene or ethylene, and wherein R5 is selected from the group consisting of an alkyl having 1 to 6 C-atoms and N- succinimide, and wherein the maximum total length of the thiols of formulas (IIa) or (IIb) involves a chain containing a maximum of 8 non-aromatic C-atoms, or III) an amine having an ether function and no more than 8 non-aromatic C-atoms, and wherein the additive component can be one molecule or mixtures of two or more of any of the molecules represented by any of formulas (Ia), (Ib), (IIa), (IIb) and (III).

Description

[0001] Silver Flakes coated with specific additives for conductive applications and method of manufacture

[0002] The present invention relates to silver flakes with high aspect ratio, specifically high aspect silver flakes that are coated with specific additives.

[0003] Electrically conductive traces requiring relatively low temperature curing (< 150° C) often utilize metallic silver as the primary conductive element due to the low bulk electrical resistivity of silver (~1 .7 pQ cm). When contact traces are drawn, micron-size spheroidal silver particles are often utilized, which offer high resolution edge formation and good packing efficiency. At elevated curing temperatures (typically >700° C), silver can fully sinter, leading to line densification (partial melting), strengthened mechanical integrity, and full particle welding, and line resistivities near the bulk resistivity of silver (1 .7 pQ cm). However, curing at lower temperatures does not allow for densification, mechanical strengthening, or high interparticle contact area. The lack of strengthening requires utilization of a persistent, non-volatile binder, and the low temperature does not allow effective particle welding, both of which introduce additional silver particle-silver particle resistance. Additionally, the lack of densification reduces silver particle-silver particle contact area, leading to increased resistance. Thus, low temperature curing of spheroidal particles results in non-uniform current density between silver particles, resulting in a reduction of conductivity relative to a solid silver line or require a very high loading of the silver particles which causes high costs.

[0004] Flaky silver flakes offer a unique solution to the above challenges. Silver flakes with low thicknesses produced by PVD (physical vacuum deposition) methods are known from e.g. JP 2020152961 A1.

[0005] JP 2005015647 A discloses small flaky silver pigments with thicknesses of about 20 to 50 nm which are prepared by reduction of silver cations.

[0006] Silver flakes with thicknesses of lower than about 100 nm exhibit very high surface energy, which dramatically reduces the temperature necessary to weld particles, effectively decreasing line resistance. The high surface area afforded by these flakes also increases the flake-flake contact area if well-oriented, further reducing line resistance and affording a high degree of mechanical flexibility without electrical contact breaks. However, this lack of passivation may result in undesired particle welding during homogenization and dispersion storage. Moreover, exceptionally high surface passivation will increase required flake- flake welding temperature beyond acceptable application limits (typically <150° C). Thus, care must be taken to choose surface passivation that balances welding temperature and dispersion stability. Due to the high potential particle aspect ratios, conductivity is anisotropic and high conductivity can only be achieved toward the in-plane direction. This suggests that random particle orientation may lead to reduced conductivity as a substantial fraction of the silver flakes would orient away from the direction of current flow in the line. However, silver flakes which are highly oriented in the direction of the plane mitigate this challenge, and plates may be stacked in parallel to increase conductivity. Thus, excellent surface orientation is important for thin flaky silver flakes to become a viable material for electrical traces. Additionally, non-passivated I untreated silver flakes tend to display poor orientation performance in binder systems, especially near the binderair interface. Furthermore, the adhesion in a binder system is rather low.

[0007] To stabilize the thin silver flakes US 5492653 A proposes a coating composition, wherein a silver flake is coated with a lubricant comprising at least one straight-chain carboxylic acid or salt of said acid, said lubricant having from 6 to 18 carbon atoms. The silver flakes were obtained by milling processes therein.

[0008] There is a need for conductive pigments which overcome the described disadvantages. The conductive pigments should have a very low resistivity when applied in a coating system and have a good adhesion therein.

[0009] This need is addressed by providing flaky silver pigments having a dso in a range of 2.0 to 100.0 pm and an aspect ratio of more than 25 which are coated at least partially with an additive component, wherein the additive component is:

[0010] I) a thiol represented by the general formula (la): A1-S-A2 or a dithiol represented by the general formula (lb): A1-S-S-A2, wherein A1 and A2 represent either an alkyl having 1 to 8 C-atoms with AI=A2 or A1 and A2 independently represent a moiety having polar or polarizable functionalities selected from the group consisting of ester, carboxyl, amine, hydroxyl, benzyl, benzoic, furyl, furfuryl, pyrrolyl, pyridyl, pyrazolyl, pyrazonolyl, oxazolyl, isooxazolyl, ether and combinations thereof and wherein these polar or polarizable functionalities are spaced from the sulphur atom or disulphur atoms by a maximum of 3 non-aromatic C-atoms and wherein the maximum total length of the thiol or dithiol involves a chain containing a maximum of 8 non-aromatic C-atoms and the maximum length difference of A1 and A2 is within 2 atoms of each other, or II) a thiol represented by the general formula (Ila): HS-(CH(A3))x-(R1)y-A3 or represented by the general formula (lib): HS-R4-CO-O-R5, wherein R1is an alkylene having 1 to 6 C-atoms, a phenylene or a pyridylene, and wherein A3 is independently selected from the group consisting of NR22, OH, COOH, COOR3, N-succinimide, and 1 -amino 2-carboxylethyl, wherein integers x and y are either 0 or 1 and x +y = 1 or 2, and wherein R2is independently selected from the group consisting of H, methyl, ethyl and propyl and wherein R3is selected from the group consisting of H, methyl, ethyl or propyl, butyl, pentyl, hexyl, N-succinimide and 2-aminoacetyl acid, and wherein R4is methylene or ethylene, and wherein R5is selected from the group consisting of an alkyl having 1 to 6 C-atoms and N- succinimide, and wherein the maximum total length of the thiols of formulas (Ila) or (lib) involves a chain containing a maximum of 8 non-aromatic C-atoms, or

[0011] III) an amine having an ether function and no more than 8 non-aromatic C-atoms, or mixtures of two or more of any of the molecules represented by any of formulas (la), (lb), (II), and the amine III).

[0012] Preferred embodiments can be found in claims 2 to 12.

[0013] Furthermore, these pigments may be manufactured in a simple way. A method of manufacture for flaky silver pigments comprising: i) depositing a silver film, under ultra-high vacuum conditions to a predetermined thickness, on a flexible substrate which was precoated with a polymeric film and a release coat, ii) stripping the silver film from the polymeric film in a solvent in a stripping chamber to form silver film pieces, iii) separating the silver film pieces from the polymeric film and release coat material, iv) crushing the silver film pieces into silver flakes with the desired particle size properties in a solvent, v) optionally exchanging solvents and vi) targeting the solvent content to the desired content to obtain a flaky silver pigment dispersion, wherein in at least one of the steps ii), iii), iv), v) or vi) the additive component of any of formulas (la), (lb), (Ila), (lib) or III) is dissolved in the solvent.

[0014] A preferred embodiment can be found in claim 14.

[0015] Detailed description:

[0016] The terms “silver flakes” and “flaky silver pigments” are used as equivalents in this invention.

[0017] Surface treatment of the flaky silver pigment needs to be selected to provide both appropriate surface passivation of the flakes and appropriate functionality to properly orient the flakes for a maximized plane-parallel conductivity. With appropriately surface passivated / functionalized thin silver flakes, line traces may be produced with an acceptable curing temperature which have similar conductivity, reduced silver content, and increased mechanical flexibility relative to traces generated using micron-sized spheroidal particles and / or which allow, due to increased conductivity, higher currents to flow. Furthermore, the high aspect ratio afforded by these silver flakes allows for achievable continuity of the flakes in the coated films with rather low pigment to binder ratios.

[0018] In some embodiments the silver flakes are made by PVD method and in other embodiments they are produced by milling techniques of a silver powder and particularly of spherical silver powder.

[0019] In some embodiments the dso is in a range of 3.0 to 80.0 pm and more preferably in a range of 3.5 to 60 pm. The pigment size is typically indicated using quantiles (d values) from the volume averaged particle size distribution. Here, the number indicates the percentage of particles smaller than a specified size contained in a volume-averaged particle size distribution. For example, the dso value indicates the size where 50% of the particles are smaller than this value. These measurements are conducted e.g. by laser granulometry using a particle size analyzer manufactured by Horiba such as a Horiba LA 950 instrument. The measurements are conducted using Fraunhofer approximation for equivalent spheres and suitable parameters according to information from the manufacturer.

[0020] Particularly for PVD silver pigments the dso is preferably in a range of 6.0 to 80.0 pm and more preferably in a range of 8.0 to 30 pm.

[0021] In preferred embodiments for PVD-pigments the average thickness tiAg of the silver flakes is in a range of 15 to 100 nm, more preferably in a range of 20 to 80 nm and most preferably in a range of 30 to 60 nm. As it is desired to have a large contact area between the flakes the average thickness should not be more than 100 nm, since the surface area of the flakes is inversely proportional to the thickness. If the thickness is very thin the conductivity of the silver flakes decreases due to quantum confinement effects of the free electrons in this material. Additionally, the stabilization of the silver flakes against agglomeration in a dispersion is more difficult for smaller thicknesses. Therefore, the average thickness should not be lower than 15 nm and more preferably not lower than 20 nm. The thickness of the silver flakes can be determined by SEM which allows determination of the whole thickness distribution. Herein the flaky silver pigments are first washed with acetone and then dried. The silver flake powder can then be poured onto a conductive adhesive label (Spectro tabs from Plano GmbH, Germany). By this procedure, a certain amount of the flaky silver pigments become fixed into an upright position. Under the SEM these particles can be well identified and their thickness at the pigments’ edge be determined. For each sample about 50 particles may counted and the average thickness hAg can be determined.

[0022] The flaky silver pigments have an aspect ratio defined as dso / hAg of higher than 25. Such high aspect ratios allow a large contact area between the flakes in the final application film. More preferably the aspect ratio for PVD-pigments is more than 50, even more preferably more than 100, furthermore preferably more than 150 and most preferably equal to and more than 225. In preferred embodiments the aspect ratio is in a range of more than 50 to 2000, more preferred in a range of 75 to 1000, even more preferred in a range of 100 to 600 and most preferred in a range of 150 to 400. Such silver pigments are also called “higher aspect ratio silver pigments” within this invention.

[0023] For embodiments using milled silver pigments the aspect ratio is higher than 25, preferably higher than 35, more preferably higher than 40, even more preferably higher than 50 and most preferably higher than 75. Typical ranges of aspect ratios for milled silver pigments are a range of 25 to less than 100, preferably in a range of 40 to 90 and most preferably in a range of 50 to 80. Such silver pigments are also called “lower aspect ratio silver pigments” within this invention.

[0024] In principle the flaky silver pigments may be produced by any method, but it is preferred that they are made by techniques such as PVD (physical vapor deposition) method or by milling of spherical silver powder particles. A preferred method to manufacture the silver flakes is the PVD method. Although more costly, silver flakes with very low thicknesses and rather small thickness variation can be obtained and such silver flakes are supposed to orient extremely well in the final application film having large contact areas and therefore low resistances. PVD silver flakes enable films with low resistivity and thus high conductivity to be already obtained at low pigment to binder ratios.

[0025] In case of milled silver pigments films containing these pigments have a significant decrease of resistivity and thus increase of conductivity by the additive treatment described in this invention.

[0026] It was surprisingly found that coating of flaky silver pigments with high aspect ratios with specific additive components can impart the desired properties.

[0027] The additive components employed here are mainly based on thiols which are known to bind strongly via the sulphur atoms to silver surfaces. Also certain amines were found to be useful. However, all these molecules need to be restricted regarding their length. The additive will form a film on the silver surface which constitutes a barrier for electron transport from silver pigment to pigment and which needs to be passed twice if the silver pigment particles are not welded together. Without being bound to a theory it is assumed that an effective charge transfer across the silver / additive interface, which most likely occurs via a tunneling mechanism, is possible only when the additives have limited molecular dimensions and, in many cases, have additional functionalities which are polar or polarizable.

[0028] The additive components may be summarized by the following generic structures.

[0029] In a first variant (I) a thiol may be represented by the general formulas A1-S-A2 (la) or a dithiol represented by the general formula: A1-S-S-A2 (lb).

[0030] In some embodiments, A1 and A2 represent an alkyl having 1 to 8 C-atoms with AI=A2, resulting in a symmetrical short-chained thiol or dithiol. More preferred for A1 or A2 is an alkyl having 1 to 7 C-atoms. In another embodiment A1 or A2 is an alkyl having 2 to 6 C- atoms.

[0031] Alternatively, A1 and A2 independently represent a moiety containing polar or polarizable functionalities selected from the group consisting of ester, carboxyl, amine, hydroxyl, benzyl, benzoic, furyl, furfuryl, pyrrolyl, pyridyl, pyrazolyl, pyrazonolyl, oxazolyl, isooxazolyland ether. These polar or polarizable functionalities are spaced from the sulphur atom or disulphur atoms by a maximum of 3 non-aromatic C-atoms. More preferred Ai and A2 independently are selected from the group consisting of ester, carboxyl, hydroxyl, benzyl, benzoic, furyl, furfuryl, pyrrolyl, pyridyl and ether and even more preferred the group consists of ester, carboxyl, benzyl, benzoic, furfuryl and ether. It is believed that these functionalities support a charge transfer across the silver / additive interface. Non-aromatic C-atoms are more likely to block electron transfer as electrons cannot be delocalized like in aromatic moieties or transported via N- or O-atoms.

[0032] Furthermore, the maximum total length of the thiol or dithiol involves a chain containing a maximum of 8 non-aromatic C-atoms. With “maximum total length” it is meant that the maximum chain length which is possible according to the molecular formula of the thiol is determined and then the number of non-aromatic C-atoms are counted. A1 and A2 may contain aromatic C-atoms. Preferably they contain up to 12 aromatic C-atoms and more preferably up to 6 aromatic C-atoms. Also, it has been found that the moieties A1 and A2 need to match their size in a way which leads to substantially symmetrical structures. Therefore, the maximum length difference of A1 and A2 is within 2 atoms of each other. More preferably the maximum length difference of A1 and A2 is within 1 atom of each other and most preferably the length difference A1 and A2 is within 0 atoms of each other. In most preferred embodiments A1 and A2 are the same (A1 = A2). As the bonding of the thiols or dithiols of variant (I) to the silver surface is most likely realized by sulphur atoms the moieties A1 and A2 should not differ too much in their length. Surprisingly it was found that the thiols of this type need a certain symmetry with respect to their length to achieve the desired effects.

[0033] In a second variant II) the additive component is a thiol which is represented by the general formula:

[0034] HS-(CH(A3))x-(R1)y-A3(Ila) or is represented by the general formula: HS-CH2-CH2-CO-O-R4(lib).

[0035] R1is an alkylene having 1 to 6 C-atoms, a phenylene or a pyridylene and preferably an alkylene having 1 to 4 C-atoms or a phenylene. A3is a moiety containing polar or polarizable functionalities and is selected from the group consisting of NR22, OH, COOH, COOR3, N-succinimide and 1 -amino 2-carboxylethyl and more preferably is selected from the group consisting of NH2, OH, COOH and COOR3. The integers x and y are independently either 0 or 1 and x + y = 1 or 2. Therefore, at least one of -CH(A3) or R1 moiety must be present in the molecule. R2is independently H, methyl, ethyl or propyl. R3is H, methyl, ethyl or propyl, butyl, pentyl, hexyl, N-succinimide or 2-aminoacetyl acid. The alkyl moieties from propyl or higher may be linear or branched.

[0036] R4is an alkyl having 1 to 6 C-atoms or N-succinimide, preferably an alkyl having 1 to 5 C- atoms, more preferably an alkyl having 1 to 4 C-atoms and most preferably an alkyl having 2 to 3 C-atoms.

[0037] The maximum total length of the thiols of formulas (HA) and (lib) involve a chain containing a maximum of 8 non-aromatic C-atoms and preferably of 7 non-aromatic C-atoms and most preferably of 6 non-aromatic C-atoms.

[0038] Examples of the thiols of formulas (la) and (lb) are 3,3'-dithiodipropionic acid, 4,4'- dithiodibutyric acid, dimethyl 3,3’-dithiodipropionate, 2,2'-dithiodibenzoic acid, methyl furfuryl mercaptopropionate, 2,2’-(dithiodimethylene)difuran, diethyl sulfide, dipropyl sulfide, dibutyl sulfide, dihexyl sulfide, diheptyl sulfide, and mixtures thereof. More preferred are 3,3'-dithiodipropionic acid. 4,4'-dithiod ibutyric acid, dimethyl 3,3’- dithiodipropionate and 2,2’-(dithiodimethylene) difuran and most preferred are 3,3'- dithiodipropionic acid and 4,4'-dithiod ibutyric acid.

[0039] Examples of the thiols of formulas (Ila) or (lib) are butyl 3-mercaptopropionate, ethyl thioglycolate, mercapto succinic acid, 4-mercapto benzoic acid, 3-mercaptopropanyl-N- hydroxysuccinimide ester, 4-aminothiophenol, cysteamine, L-cysteine, L-cystine, 6- mercapto-1 -hexanol, 3-mercaptopropanoic acid, methyl 3-mercaptopropionate, 6- mercapto pyridine-3-carboxylic acid, and mixtures thereof. A preferred embodiment is butyl 3-mercaptopropionate.

[0040] In a third variant III) an amine having an ether function and no more than 8 non-aromatic C-atoms was found to be useful.

[0041] Amines of variant (III) have at least one ether function and no more than 8 non-aromatic C-atoms. They may have up to 12 aromatic C-atoms and more preferably up to 6 aromatic C-atoms and most preferably no aromatic C-atoms. Examples of the amines of variant III) are 3-methoxypropyl amine, 3-ethoxypropyl amine, 3-propoxypropyl amine, 2- propoxypropyl amine, 2-ethoxyethyl amine, 2-methoxyethyl amine, and mixtures thereof. Preferred embodiments are 3-methoxypropyl amine and 3-ethoxypropyl amine. In this invention the term “additive component” has the meaning that one type of molecules or mixtures of two or more of any type of the molecules represented by any of formulas (la), (lb), (Ila), (lib) and (III) are used for coating at least partially the surface of the flaky silver pigments.

[0042] A preferred embodiment is directed to a mixture of a thiol according to formula (lib) with any of the thiols of formulas (la), (lb), (Ila) or (lib) which is used for coating the flaky silver pigments and particularly preferred is a mixture of butyl 3-mercaptopropionate with any other of the thiols of formulas (la), (lb), (Ila) or (lib).

[0043] Another preferred embodiment is directed to a mixture of an amine according to variant III) with a thiol and preferably one thiol of formulas (la), (lb), (Ila) or (lib) which is used for coating the flaky silver pigments and particularly preferred is a mixture of 3-methoxypropyl amine or 3-ethoxypropyl amine with any of the thiols of formulas (la), (lb), (Ila) or (lib).

[0044] In a preferred embodiment the flaky silver pigments which are coated by the additive as described before are mixed with a solvent to form a paste or a dispersion.

[0045] In case of silver flakes obtained by milling a paste will be formed. The non-volatile content of this paste is attributable mostly to the content of silver flakes and is preferably in a range of 20 to 60 wt.% and more preferably in a range of 25 to 50 wt.%, each based on the total weight of the paste.

[0046] A dispersion is typically formed when silver flakes obtained by PVD process are utilized. Here the non-volatile content of this dispersion is preferably in a range of 15 to 30 wt.% and more preferably in a range of 18 to 25 wt.%, each based on the total weight of the dispersion.

[0047] In preferred embodiments the silver flake paste or silver flake dispersion is composed of silver flake, additive component as described above and solvent in an amount in a range of 97 wt.% to 100 wt.% and more preferred in an amount in a range of 98 wt.% to 100 wt.%, each based on the total weight of the paste or dispersion. In these preferred embodiments only small amounts or no amount of further ingredients is added. In some embodiments further ingredients like dispersing additives or rheological additives may be added. Additionally, little amounts of residual material used as release coat may be present here.

[0048] The solvents of the silver flake containing paste or dispersion are typically esters, alcohols or blends thereof. Examples of an ester-type solvent may include, but are not limited to, methyl acetate, ethyl acetate, n-propyl acetate, 1 -propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, sec-butyl acetate, tert-butylacetate, ethyl butyrate, ethyl lactate, hexyl acetate, isoamyl acetate, methyl propionate, and mixtures and blends thereof.

[0049] Examples of an alcohol-type solvent include, but are not limited to, methanol, ethanol, n- propanol, 1 -propanol, isopropanol, n-butanol, 1 -butanol, 2-butanol, isobutanol, tertbutanol, benzyl alcohol, ethylene glycol, diethylene glycol, 2-ethylhexanol, glycerol, methyl carbitol, 2-methyl-1 -butanol, 3-methyl-2-butanol, 1 ,2-propanediol, 1 ,3-propane diol, ethylene glycol mono butyl ether, ethylene glycol mono propyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, propylene glycol mono n- propyl ether, propylene glycol mono-n-butyl ether, propylene glycol mono-t-butyl ether, propylene glycol monophenyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monomethyl ether acetate, dipropylene glycol dimethyl ether, dipropylene glycol mono-n-butyl ether, dipropylene glycol mono-t-butyl ether, dipropylene glycol mono-n- propyl ether, tripropylene glycol monomethyl ether, tripropylene glycol mono-n-propyl ether, tripropylene glycol mono-n-butyl ether and mixtures and blends thereof.

[0050] Furthermore, the solvent may contain small residual amounts of acetone or other solvents due to the PVD processing.

[0051] In some embodiments the total amount of the additive component is in a range of 0.02 to 3.0 wt.%, based on the total weight of the paste or dispersion. Herein the total amount of the additive component includes additive component dissolved in the solvent and additive component adsorbed on the surface of the silver flakes. In preferred embodiments the total amount of additive component is in a range of 0.05 to 2.0 wt.%, more preferred in a range of 0.08 to 0.50 wt.% and most preferred in a range of 0.1 to 0.30 wt.%, each based on the total weight of the paste or dispersion.

[0052] Method of manufacture of coated silver flakes:

[0053] A preferred method of manufacture of the flaky silver pigments is a PVD method, wherein the method comprises the following steps: i) depositing a silver film, under ultra-high vacuum conditions to a predetermined thickness, on a flexible substrate which was precoated with a polymeric film and a release coat, ii) stripping the silver film from the polymeric film in a solvent in a stripping chamber to form silver film pieces, iii) separating the silver film pieces from the polymeric film and release coat material, iv) crushing the silver film pieces into silver flakes with the desired particle size properties in a solvent, v) optionally exchanging solvents and vi) targeting the solvent content to the desired content to obtain a silver pigment dispersion, wherein in at least one of the steps ii), iii), iv), v) or vi) the additive component of any of formulas (la), (lb), (Ila), (lib) or III) is dissolved in the solvent.

[0054] The flexible substrate is usually a web made from polymers and preferably a PET polymer. As release agents those common in the art can be used. Usually, the release agents are polymers such as acrylics, methacrylics or polystyrol. They can be also other organic materials as described e.g. in US 2004 / 0131776 A1 or in US 2010 / 0062244 A1 . In a preferred embodiment all the evaporation steps of step i) are done by a roll-to-roll process. Step i) can be conducted as an electron beam process, magneton sputtering, resistive evaporation, or inductive heating.

[0055] Preferred solvents for stripping the silver film from the flexible substrate in step ii) are acetone, ethyl acetate, propylene glycol methoxy ether, isopropyl alcohol, ethanol or mixtures thereof. Most preferred solvents are acetone, ethyl acetate or isopropyl alcohol. During the stripping step the silver film may be already crushed to a certain degree by appropriate energy impact (ultrasound or stirring) to form silver film pieces. This procedure may be called pre-particle sizing. These silver film pieces are still much larger than the final silver pigments.

[0056] The separation of step iii) is usually made by settling the silver film pieces and decanting the supernatant solution accompanied by additional washing and decantation steps with a solvent. Here, the separation of silver film pieces and release coat will not be perfect as some residual release coat can usually be found in PVD pigment products. Steps iv) to vi) are common and very well known in PVD process technology.

[0057] The additive component can be brought into contact with the silver film pieces or final pigments during the PVD process steps by dissolving the additive of any of formulas (la), (lb), (Ila), (lib) or III) in the solvent utilized in at least one of steps ii), iii), iv), v) or vi). Herein, typical concentrations of the additive component are about 0.05 to 0.5 wt.% and more preferred 0.10 to 0.30 wt.% of the respective solution (without silver particles). This kind of processing saves an extra coating step when the final pigment dispersion would be utilized. In other embodiments the additive component is dissolved in at least two or three of any of the steps ii), iii), iv), v) or vi). Also, mixtures of additive coatings are easily accessible by dissolving a first additive in a first of any of the steps ii), iii), iv) or v) and dissolving a second additive in a second step following this first step.

[0058] A further aspect of this invention is directed to the use of the flaky silver pigments coated by the additive component. The flaky silver pigments can be used as conductive particles in application fields like printed electronics, sensing, photovoltaics, thick polymer film applications, EMI shielding, antennas, RFID and smart packaging. Further examples of applications in printed electronics are flexible hybrid electronics, in-mold electronics, 3D electronics, circuits for e-textiles and circuit prototyping. Further examples of applications in sensing are capacitive sensors, pressure sensors, biosensors, strain sensors and electrodes. Further examples for thick polymer film application are heaters, OPV (organic photovoltaic), flexible substrates and thermoforming. The additive component coated flaky silver pigments are usually employed as highly conductive pigments or particles in electrical traces of any of the above mentioned applications.

[0059] Another aspect of this invention is directed to an ink or a coating formulation containing the flaky silver pigments coated by an additive component according to any of the above- mentioned embodiments for applications in the fields of printed electronics, sensing, photovoltaics, thick polymer film applications, EMI shielding, antennas, RFID and smart packaging.

[0060] Due to their extraordinary low resistance (and therefore high conductivity) these materials may be used here at low concentrations leading to low pigment to binder ratios. This advantage may outweigh their potential high costs when the additive coated silver flakes are produced via the PVD process.

[0061] EXAMPLES

[0062] Preparation of Examples and Comparative Examples:

[0063] Comparative Example 1 : Procedure of manufacturing uncoated PVD-Ag flakes:

[0064] A 30 cm wide clear polyester (PET) film disposed on a rolling band was coated with a polyurethane polymer used as releasing agent. Using ebeam PVD evaporation a film of silver (purity: 99.99 %) was applied to this substrate at an optical density (OD) of about 1 .2. The ebeam source accelerating voltage was held at a constant 10 kV throughout the run and the vacuum in the chamber was 2.7x10-5mbar.

[0065] The silver film coated foil was then introduced into a stripping chamber filled with acetone as processing solvent. Here the films were treated first with a High Shear Blader (to a dso of approximately 180 pm) and thereafter with an Ultra Turrax down to a dso in the range of 60 - 70 pm (a pre-particle sizing step). In order to separate the silver film pieces from the release coat this slurry was settled, decanted and mixed with acetone three times. Finally a solid content of about 5 wt.% was obtained. The slurry was placed into a stainless pot and was further processed by an T50 Ultra Turrax down to the final dso-value (11.8 pm). The slurry was centrifuged, decanted and the solvent exchanged by adding n-propyl acetate (NPAC). The final solid content was adjusted to 22 wt.%. The thicknesses of the flakes were investigated with a SEM and an average thickness of 44 nm was found.

[0066] Example 1a: coating with 3-mercaptopropionate

[0067] The PVD coating process of silver was performed as described in Comparative Example 1 , but the optical density of the silver film was 1.1 OD. In the stripping chamber the solvent acetone further contained 0.25 wt% of butyl 3-mercaptopropionate (BMP). Also, the washing step after the pre-particle sizing step was conducted twice with acetone containing 0.25 wt.% of butyl 3-mercaptopropionate. The particle sizing step in the stainless pot was conducted in acetone containing 1 .25 wt.% of butyl 3- mercaptopropionate. The solvent exchange process was conducted using a 0.1 wt.% solution of butyl 3-mercaptopropionate in n-propyl acetate. The average thickness of the Ag-flakes was 41 nm as determined by SEM.

[0068] Example 1 b:

[0069] The PVD coating process was performed as in Example 1a, but butyl glycol was used as exchange solvent instead of n-propyl acetate. Example 2a: coating with BMP and 3,3'-dithiodipropionic acid

[0070] The whole pigment preparation procedure was performed as in Example 1a, but in the solvent exchange step a solution of 0.1 wt.% of 3,3'-dithiodipropionic acid (DTDP) was used instead of BMP.

[0071] Example 2b: coating with BMP and 3,3'-dithiodipropionic acid

[0072] The whole pigment preparation procedure was performed as in Example 1 b, but in the solvent exchange process a solution of 0.1 wt.% of 3,3'-dithiodipropionic acid (DTDP) was used instead of BMP.

[0073] Example 3: coating with butyl 3-mercaptopropionate and 4,4'-dithiodibutic acid

[0074] The whole pigment preparation procedure was performed as in Example 2a, but 4,4'- dithiodibutic acid (DTDB) was used as further thiol in the solvent exchange step instead of 3,3'-dithiodipropionic acid.

[0075] Example 4: coating with 3-methoxy propylamine

[0076] The PVD coating process of silver and the stripping process were performed as in Comparative Example 1 . The stripping chamber contained only acetone with no additive. The washing step after the pre-particle sizing step was conducted three times with acetone only and then a solution of 1 .0 wt.% solution of 3-methoxy propylamine in acetone was used. The solvent exchange process was conducted using a 0.1 wt.% solution of 3- methoxy propylamine in n-propyl acetate. The average thickness of the Ag-flakes was 41 nm as determined by SEM.

[0077] Example 5:

[0078] The PVD coating process was performed as in Examples 1a or 1 b, but the OD of the silver film was set to 1 .25 and during the particles sizing step an acetone solvent containing 2.5 wt.% of 4,4'-dithiodibutyric acid (DTDB) as additive was used instead of butyl 3- mercaptopropionate. In the solvent exchange step the final solvent added was n-propyl acetate which contained 0.1 wt.% of 4,4'-dithiodibutyric acid dissolved therein.

[0079] Examples 6 to 9 and Comparative Examples 2 to 5:

[0080] The PVD coating process was performed as in Example 3, but different additives were used during particle sizing and solvent exchange steps. These additives and their concentrations are shown in table 1a. Example 10: coating with 3-methoxy propylamine and butyl 3-mercaptopropionate

[0081] The same procedure as in Example 4 was employed, but instead of a 1 .0 wt.% solution of 3-methoxy propylamine a solution of 0.80 wt.% of 3-methoxy propylamine and 0.20 wt.% of butyl 3-mercaptopropionate was used.

[0082] Examples 11 to 17 and Comparative Example 6:

[0083] Silver was deposited on a 30 cm wide clear polyester film coated with a CAB / acrylic-based releasing agent using e-beam PVD evaporation. The e-beam source was positioned 36 cm below the web during process and conditions were modified to achieve an optical density ranging between 1 .0 and 1 .5 OD. The e-beam source accelerating voltage was held at a constant 10 kV throughout the run. The silver was stripped from the polyester film in an acetone bath, resulting in a suspension (slurry) at a concentration of ~1 .5% solids. The slurry was divided into two equal master batches and processed, through a combination of high-speed dispersion and rotor-stator homogenization, to a particle size of ~70 pm (D50 value). The first batch of slurry was processed as is, while the second batch was processed with an addition of 0.25% butyl 3-mercaptopropionate (BMP), based on total slurry weight. After settling, the supernatants were then decanted to a solid concentration of ~5%.

[0084] Each master batch was divided into 16 equal aliquots and further processed with the additives listed in Table 1 b below to a particle size with a D50 ranging little between 14.2 - 14.6 pm. Samples denoted with an “a” after the number refer to a mixture with BMP and samples denoted with a “b” refer to the single thiols without BMP pretreatment.

[0085] Comparative Example 7:

[0086] Commercially available Ag-flakes Metalor AA-3462 with a dso of 3.6 pm and a specific surface (BET) of 2.25 m2 / g. These silver flakes are obtained by milling of silver powder and are delivered with a surface coating of stearic acid as stabilizer.

[0087] Example 19:

[0088] In order to remove stearic acid at least partially the Ag flakes of Comparative Example 7 in powder form were washed with fresh acetone solution, left standing for 30 min and then centrifuged at 2500 rpm for 10 min. The same procedure was repeated once. Then 20 g of the Ag-pigment (powder) were dispersed under stirring in 200 ml acetone having 0.25 wt.% in total of additives which were a mixture of BMP:DTDP in a 1 :3 ratio.

[0089] Comparative Example 8: Commercially available Ag-flakes Metalor AA-4077 with a dso of 7.0 pm and a specific surface (BET) of 1 .3 m2 / g. These silver flakes are obtained by milling of silver powder and are delivered with a surface coating of stearic acid as stabilizer.

[0090] Example 20: The silver pigments of Comp. Example 8 were treated with the same procedure as described in Example 19 to obtain thiol coated silver pigments with a minimum amount of stearic acid.

[0091] In Figures 1 a and 1 b an overview is given on all additives utilized with their chemical formulas and their appropriate generic formulas related to claim 1 .

[0092] Table 1a: Further experimental details of Examples 5 to 9 and Comparative Example 2 to 5

[0093] Table 1b: Further experimental details of Examples 10 to 17 and Comparative Example 7: B Testing methods and results:

[0094] The pigment dispersion samples of the previous Examples and Comparative Example 1 were formulated at different silver pigment to binder ratios by using the following formulation:

[0095] Table 2a: Components of test Formulation

[0096] The formulation was adjusted to a spraying viscosity of approximately 12 - 13 sec. in a DIN-cup 4 mm 120 °C. The pigment to binder ratio (P / B) was varied by proper variation of the amount of silver pigment and binder utilized.

[0097] Drawdowns of the samples were prepared by the following procedure:

[0098] Drawdowns were made onto a byko-chart 2812 opacity chart with a wire wound bar to a 40 |j.m wet film thickness and the drawdowns were put into an oven at 75°C for 10 minutes.

[0099] Two strips of length of about % inch were cut from the centre of a drawdown and the resistance R was measured across a six-inch section using a Keithley DMM6500 6 % Digit Multimeter (4-point measurement). From the values of the resistance the resistivity p can be evaluated by formula (IV):

[0100] P = R ttot Wd / ld (IV) Herein ttot is the calculated total thickness of the dried drawdown and w9is the width and Id the length of the stripe in cm which were configured by the tips of the multimeter.

[0101] The parameter ttot is the sum of the calculated parameters fog and ts. These are the total thicknesses of all silver flakes and the total thickness contribution of the binder in the dried drawdowns. They can be calculated theoretically assuming an additive behaviour of these parameters which can be regarded to be at least a fair approximation.

[0102] These can be calculated by the following formulas (V): and

[0103] Herein the symbols pAg and ps are denoted to the literature values of the densities of silver and the solvent used, and ps is the density of the binder which was assumed to be 1.2 g / cm3which is a typical value for dried binders. Parameter twet is the wet thickness of the drawdown in pm which was always 40 pm, mA .inkand ms, ink are the mass fractions of the silver ink and the binder ink utilized initially (the sum of two being equal to 1) and NVMA9and NVMB are the non-volatile contents of the initial silver and binder inks expressed as dimensionless ratios. NVMA9was always about 0.21 for all samples utilising PVD Ag flakes and 0.5 for Example 19 and Comparative Example 7 and NVMB was always 0.035. Results are depicted in tables 3a and 4a, whereas for the samples of table 4a only two drying temperatures (75 °C and 125 °C) were evaluated.

[0104] From these drawdowns the gloss was measured at 60° using a BYK Micro Tri-gloss meter.

[0105] Furthermore, the adhesion was measured with the remaining section of the drawdown using a Scotch 81 1 % inch removable tape for Examples 1 to 6 and Comparative Example 1 . For this test a three-note system was used:

[0106] S: success

[0107] P: partial success F: fail Then the temperature of the oven was raised to 110°C and the chart and % inch strips were heated in the oven for 10 minutes. Afterwards the measurements of gloss, resistivity and adhesion were repeated. The same procedure was repeated for 120°C and 125°C temperatures. For the drying temperature of 125 °C also the quality of the surface of the drawdown was evaluated visually using a microscope and the following noting system was applied:

[0108] Note 1 : no loose flakes on the surface (mirror status)

[0109] Note 2: small loose flakes on the surface

[0110] Note 3: poor surface quality

[0111] For Examples 5 to 9 and Comparative Examples 2 to 5 the parameters were evaluated at only two temperatures (75 °C and 125 °C).

[0112] For Examples 10 to 18 and Comparative Example the parameters were evaluated for three temperatures (75 °C, 125 °C and 150 °C) and four different pigment / binder ratios. The relation of pigment to binder ratios and mAg and ms for this experimental series was always as follows:

[0113] Pigment / Binder ratio: mAg: ms:

[0114] 3 31.3 68.7

[0115] 6 47.8 52.2

[0116] 9 57.8 42.2

[0117] 12 64.7 35.3

[0118] Results are presented in table 5a.

[0119] For Examples 19 and 20 and Comp. Example 7 and 8 the parameters were evaluated for three temperatures (75 °C, 110 °C and 125 °C) and four different pigment / binder ratios which were a little bit higher than in other examples utilizing PVD pigments. The relation of pigment to binder ratios and mAg and ms for this experimental series was always as follows:

[0120] P / B mAgm ratioB

[0121] 4.99 25.9 74.1 10 41.2 58.8

[0122] 15.04 51.8 49.2

[0123] 20.05 58.4 41.6

[0124] Results are depicted in table 5b.

[0125] Naturally the resistivities decrease with increasing pigment to binder ratios. Inventive Examples for the “large aspect ratio silver pigments” which are in fact the PVD pigments need to have the following resistivities for certain pigment to binder ratio ranges as depicted in table 2b:

[0126] Table 2b: Criteria for Inventive Examples with high aspect ratio: of These resistivity to P / B relations have to be fulfilled for at least for one of the temperatures measured.

[0127] In Fig. 3 a plot is made for various Examples and Comparative Examples for the calculated resistivities against the pigment to binder ratios for the large aspect ratio silver pigments”. Here for each sample the lowest resistivities are chosen from the various drying temperatures of the draw down preparations. The data therefore do not represent results at a constant drying temperature. This appears to be justified as the temperature behavior differs for particulate samples. Some of the thiols may undergo reactions at higher temperatures and therefore perform better at lower drying temperatures. The skilled person can in any case find the optimal drying temperature with respect to a particular additive component and particularly thiol and the coating formulation used. The borders of resistivity outlined above in table 2b are visualized in Fig. 3 (“linear”). Furthermore, if the resistivity is plotted against the pigment / binder ratio in the region of a pigment to binder ratio of 6.5 to 20 and more preferably to 25 a curve with positive slope should develop. This is interpreted to occur for silver pigments which were well dispersed in the dispersion and having no agglomeration problems. This criteria is important when the first criteria is passed. It would be an indication that the particles are still somehow agglomerated although they possess pretty low resistivities. In Fig. 2 it can be well observer that this criteria is not fulfilled for Comparative Example 1 in contrast to Examples 1a to 4.

[0128] The tape test may be passed (notes: S and PS) but does not necessarily need to be passed. The gloss results are meaningless if an application with purely conductive properties of the Ag flakes is intended. In some applications also optical performance may be important and therefore a gloss above 115 (at any of the temperatures 75 or 110 °C and above 110 for temperature of 125 °C would be appreciated. The glosses measured for temperatures of 125 °C often decreased which is probably attributed to beginning crosslinking of the binder.

[0129] Regarding the visual inspection of the drawdown at 125 °C curing temperature notes 1 and 2 would be well appreciated for possible applications where optical properties play a role and note 3 is rather not acceptable here. But altogether these notes are not mandatory to be qualified as inventive example. The results of tape test, visual inspection of drawdown and especially the gloss measurements are an indication of good pigment flake orientation in the dried drawdown.

[0130] Table 3a: Results of Resistance measurements and calculated resistivities at various temperatures for Example 1 to 4 and Comp. Example 1 : For Example 10 drawdowns were prepared with pigment to binder ratios of 20 and 40 and here the drying temperature was 150 °C. For these samples resistivities of 10.1 pWcm and 7.8 pW cm were calculated respectively.

[0131] The conductivity o of the dried films can be easily calculated from the calculated resistivities p using formula (VI): ct = 10-4 / p [MS / m] In Figure 2 the calculated conductivities are plotted against the pigment to binder ratio for some of the Examples and Comparative Example 1 . It can be seen that for the uncoated silver pigments (Comparative Example 1) the slope is rather negative which seems to be caused by agglomeration of the particles in the suspension and an unfortunate orientation of the flakes in the dried film resulting from this.

[0132] Table 3b: Results for optional parameters like gloss at various temperatures and the tape test at 125 °C: Table 4a: Results of Resistance measurements and calculated resistivities at various temperatures for Examples 5 to 9 and Comparative Examples 2 to 5

[0133] * Additive might have decomposed and formed an insulating layer

[0134] ** Could not be determined

[0135] Table 5a: Results of Resistance measurements and calculated resistivities at various temperatures for Examples 10 to 18 and Comp. Example 6: Table 5b: Measured Resistances and calculated resistivities for Examples with milled silver pigments: C Discussion:

[0136] All of the Examples 1a to 4 show remarkably low resistivity values and thus high conductivities in the drawdowns. The conductivities increase with increasing pigment to binder ratio as demonstrated in Figure 2. Especially Examples 2a and 2b show very high calculated conductivities which correspond to about 27 % of bulk silver (Example 2b for pigment / binder = 25). The high orientation and thus high contact area of the flakes are also well demonstrated by high gloss values, successful tape-test results and best visual inspection notes. Additionally crosscut sections were evaluated by SEM and high orientation and large contact areas were confirmed. Such pigments would qualify especially for high-current applications. The pigments of Example 3 had quite good resistance values while the orientation of the flakes was not as good as for the samples with thiol coatings. This may be attributed possibly to less surface passivation as the amine group is known to bind less strong to silver surfaces as thiols and also less compatibility to the binder might exist here. The uncoated silver PVD flakes of Comparative Example 1 show comparable conductivities at lower pigment binder ratios, but at higher ratios the conductivity decreases leading to a negative slope. This is attributed to poor orientation in the binder system. Correspondingly, gloss, mechanical stability and visual inspection were rather bad for the uncoated pigment. Very high pigment to binder ratios are not feasible with appropriate high conductance with the uncoated silver flakes. Also the viscosity of the initial silver ink was observed to be too high which lead to these agglomeration problems and would also decrease the shelf-life of such suspensions.

[0137] Examples 5 to 9 show resistivities of lower than 60 pQ cm for pigment to binder ratios below 3.0 for at least one of the two temperatures evaluated and also resistivities of lower than 100 for pigment to binder ratios of more than 3.0.

[0138] The resistivity values were much higher for the Comparative Examples 2 to 5. It is therefore demonstrated that not every thiol used for coating the silver PVD flakes is qualified to impart a low resistance and therefore high conductivity and conductance to films made with these coated silver flakes. In these Comparative Examples the silver flakes were at least partially coated with the inventive butyl-3-mercaptopropionate, but apparently the additional coating with any of the thiols employed here, which did not belong to thiols of claim 1 decreased the resistivity dramatically. These effects are not understood in detail, but it is assumed that the electron transfer via a tunnelling mechanism through the thiol coating layer is hindered here. Comparative Examples 2 and 3 employed molecules with several thiol groups. Such molecules may bond to different silver flakes and may therefore cause agglomeration of them leading to high resistivity. In Comparative Example 5 (methyl furyl disulfide) a molecule was used which would fit to formula (lb) with respect to the general structure, but the two moieties have different length (difference larger than two molecules). Such unsymmetrical molecules of were surprisingly found to not perform well. In Comparative Example 5 the additive was a molecule (tetramethylthiuram disulfide) which had polar and polarizable moieties and a symmetrical structure related to the -S-S- center, but it is assumed that the additional sulphur atoms of the thiuram groups cause additional bonding sites leading to agglomeration of silver flakes.

[0139] In the series of Examples 10 to 18 in table 5 most of the Examples had a calculated resistivity within the limits defined herein in form of a mixture with pretreatment of BMP as well as a single thiol. Examples 10 a,b, 11 a,b and 12, a,b show that sulfides with dialkyl moieties which are not too long-chained can give surprisingly low resistivities. Comparative Example 6 was a mercapto substance with a carboxylic group but an alkylene moiety with a long carbon chain of ten C-atoms which is too long to fall under the desired range of R1.

[0140] In cases of Example 13a with 4-mercapto benzoic acid as additive and of Example 16a with 4-amino thiophenol as additive the pretreatment with BMP did perform well, but the single additive (Comp. Example 13b and Comp. Example 16b) did not perform well.

[0141] On the other hand Example 15b with cysteamine only performed well in absence of BMP. It is assumed that cystamine reacts with BMP in an aminolysis reaction at elevated temperatures yielding an amide with an unfavorable a,co-mercapto structure which does not fall under this invention.

[0142] Regarding the samples with milled silver pigments a significant decrease of the resistivity can be noticed when the pigments were treated with the respective additive mixture and stearic acid was removed at least partially before by washing. To further illustrate the effects conductivities were calculated according to formula (VI) and plotted against the pigment to binder ratio in Fig. 4. For no readable resistances a conductivity of zero was assumed. Especially for high pigment to binder ratios a clear improvement of conductivity can be seen. Additionally one curve with a PVD-pigment (Example 2a) was added in that diagram to illustrate the overwhelming increase of conductivity of such high aspect ratio pigment at all pigment to binder ratios.

[0143] The “lower aspect ratio” silver pigments have no or very low conductivities at low pigment to binder ratios. PVD silver pigments coated with additive components according to this invention could be used already at rather low pigment to binder ratios with comparable or even better performance with respect to conductivity which could even outweigh the higher cost associated with a PVD manufacturing process compared to milled silver pigments.

Claims

Claims:1 . Flaky silver pigments having a cteo in a range of 2.0 to 100.0 pm and an aspect ratio of more than 25 which are coated at least partially with an additive component, wherein the additive component is:I) a thiol represented by the general formula (la): A1-S-A2 or a dithiol represented by the general formula (lb): A1-S-S-A2, wherein A1 and A2 represent either an alkyl having 1 to 8 C-atoms with AI=A2 or A1 and A2 independently represent a moiety containing polar or polarizable functionalities selected from the group consisting of ester, carboxyl, amine, hydroxyl, benzyl, benzoic, furyl, furfuryl, pyrrolyl, pyridyl, pyrazolyl, pyrazonolyl, oxazolyl, isooxazolyl, ether and combinations thereof and wherein these polar or polarizable functionalities are spaced apart from the sulphur atom or disulphur atoms by a maximum of 3 non-aromatic C-atoms and wherein the maximum total length of the thiol or dithiol involves a chain containing a maximum of 8 non-aromatic C-atoms and the maximum length difference of A1 and A2 is within 2 atoms of each other, orII) a thiol represented by the general formula (Ila): HS-(CH(A3))x-(R1)y-A3 or represented by the general formula (lib): HS-R4-CO-O-R5, wherein R1is an alkylene having 1 to 6 C-atoms or a phenylene, and wherein A3 is independently selected from the group consisting of NR22, OH, COOH, COOR3, N-succinimide, and 1 -amino 2-carboxylethyl, wherein integers x and y are either 0 or 1 and x + y = 1 or 2, and wherein R2is independently selected from the group consisting of H, methyl, ethyl and propyl and wherein R3is selected from the group consisting of H, methyl, ethyl or propyl, butyl, pentyl, hexyl, N-succinimide and 2-aminoacetyl acid, and wherein R4is methylene or ethylene, and wherein R5is selected from the group consisting of an alkyl having 1 to 6 C-atoms and N-succinimide, and wherein the maximum total length of the thiols of formulas (Ila) or (lib) involves a chain containing a maximum of 8 non-aromatic C-atoms, orIII) an amine having an ether function and no more than 8 non-aromatic C-atoms, and wherein the additive component can be one molecule or mixtures of two or more of any of the molecules represented by any of formulas (la), (lb), (Ila), (lib) and (III).

2. Flaky silver pigments according to claim 1 , wherein the dso is in a range of 3. Oto 80.0 pm and preferably in a range of 3.5 to 60 pm.

3. Flaky silver pigments according to claim 1 or 2, wherein the average thickness of the silver flakes is in a range of 15 to 100 nm.

4. Flaky silver pigments according to any of the preceding claims, wherein the flakes are made by PVD method or by milling techniques of a silver powder.

5. Flaky silver pigments according to any of the preceding claims, wherein for the thiols according to formulas (la) or (lb) Ai = A2 and Ai and A2 represent a moiety containing polar or polarizable functionalities.

6. Flaky silver pigments according to any of the preceding claims, wherein the additive component is a mixture of a thiol according to formula (lib) with any of other thiols of formulas (la), (lb), (Ila) or (lib).

7. Flaky silver pigments according to any of the preceding claims, wherein the additive component is a mixture of an amine according to variant III) with a thiol of any of formulas (la), (lb), (Ila) or (lib).

8. Flaky silver pigments according to any of the preceding claims, wherein the thiols of formulas (la) and (lb) are selected from the group consisting of 3,3'-dithiodipropionic acid, 4,4'-dithiodibutyric acid, dimethyl 3,3-dithiodipropionate, 2,2'-dithiodibenzoic acid, methyl furfuryl mercaptopropionate, 2,2'-(dithiodimethylene)difuran, diethyl sulfide, dipropyl sulfide, dibutyl sulfide, , dihexyl sulfide, diheptyl sulfide, and mixtures thereof and the thiols of formulas (Ila) or (lib) are selected from the group consisting of butyl 3-mercaptopropionate, ethyl thioglycolate, mercapto succinic acid, 4-mercapto benzoic acid, 3-mercaptopropanyl-N-hydroxysuccinimide ester, 4-aminothiophenol, cysteamine, L-cysteine, L-cystine, 6-mercapto-1 -hexanol, 3-mercaptopropanoic acid, methyl 3-mercaptopropionate, 6-mercapto pyridine-3-carboxylic acid, and mixtures thereof.

9. Flaky silver pigments according to any of claims 1 or 7, wherein the amines of formula iii) are selected from the group consisting of 3-methoxypropyl amine, 3-ethoxypropylamine, 3-propoxypropyl amine, 2- propoxypropyl amine, 2-ethoxyethyl amine, 2- methoxyethyl amine, and mixtures thereof.

10. Flaky silver pigments according to any of the preceding claims, wherein the flaky silver pigments are mixed with a solvent to form a paste or a dispersion.11 . Flaky silver pigments according to claim 10, wherein the amount of the additive component is in a range of 0.02 to 3.0 wt.%, based on the total weight of the paste or dispersion of the silver flake.

12. Flaky silver pigments according to claims 10 or 11 , wherein the amount of silver pigment, additive component and solvent of the paste or dispersion is in a range of 97 to 100 wt.%, each based on the total weight of the dispersion or paste.

13. Method of manufacture the flaky silver pigments according to any of claims 1 to 12, wherein the method comprising: i) depositing a silver film, under ultra-high vacuum conditions to a predetermined thickness, on a flexible substrate which was precoated with a polymeric film and a release coat, ii) stripping the silver film from the polymeric film in a solvent in a stripping chamber, to form silver film pieces, iii) a separating the silver film pieces from the polymeric film and release coat material, iv) crushing the silver film pieces into silver flakes with the desired particle size properties in a solvent, v) optionally exchanging solvents and vi) targeting the solvent content to the desired content to obtain a silver pigment dispersion, wherein in at least one of the steps ii), iii), iv), v) or vi) the additive component of any of formulas (la), (lb), (Ila), (lib) or III) is dissolved in the solvent.

14. Method of manufacture the flaky silver pigments according to claim 13, wherein the solvent is acetone, ethyl acetate, propylene glycol methoxy ether, isopropyl alcohol, ethanol or mixtures thereof.

15. Use of the flaky silver pigments coated by an additive component according to any of claims 1 to 12 as conductive particles in fields like printed electronics, sensing,photovoltaics, heaters, EMI shielding, antennas, RFID and smart packaging.

16. Ink or coating formulation containing the flaky silver pigments coated by an additive component according to any of claims 1 to 12 for applications in printed electronics, sensing, photovoltaics, heaters, EMI shielding, antennas, RFID and smart packaging.

Images