Anisotropic conductive polymer material

Abstract

A method for forming a body comprising a mixture of a matrix and conductive particles, whereby the conductive particles are formed into aligned conductive pathways in an alignment step by applying an electric field between alignment electrodes and thereafter stabilizing the mixture wherein the conductive particles have a low aspect ratio; and a polymeric composition and method for producing such composition which is curable by UV light to an anisotropic electrically conductive polymer layer, comprising i) providing a non-conductive matrix of a flowable polymer composition having inherent photocurability, ii) adding to matrix conductive particles having low aspect ratio in an amount to allow the concentration of the conductive particles to be maintained at a level lower than the percolation threshold, and iii) placing the formed composition in a receptacle where exposure to UV light is prevented, and a method for establishing an anisotropic electrically conductive, optionally thermally conductive.

Claims

1. A method for producing a polymer composition with the ability to be cured by UV light to an anisotropic electrically conductive polymer layer, comprising the steps of: providing a non-conductive matrix of a flowable polymer composition having inherent photocurability; adding to said matrix conductive particles having a predetermined aspect ratio in an amount sufficiently low to allow the concentration of the conductive particles to be maintained at a level lower than the percolation threshold of isotropic mixture; and placing the thus formed composition in a receptacle in which exposure to UV light is prevented; wherein the aspect ratio is defined as the ratio between the largest linear dimension of a particle and the largest dimension perpendicular to said largest dimension, wherein the aspect ratio for a majority of the conductive particles is in the range below 5, wherein the aspect ratio for at least 75% of the particles is in the range below 10, and wherein the aspect ratio for at least 90% of the conductive particles is in the range below 20.

2. A method in accordance with claim 1, wherein the conductive particles are chosen from carbon particles, metallic particles, metal coated particles, and metal oxide particles or any combination thereof.

3. A method in accordance with claim 2, wherein the conductive particles are carbon particles comprising particles of carbon black, or carbon nanocones, or graphitic particles, or graphene or any combination thereof.

4. A method in accordance with claim 1, wherein the polymer matrix is adhesive in nature.

5. A method in accordance with claim 1, wherein the conductive particles being present in the non-conductive matrix in a concentration in the range 0.1-10% by volume.

6. A method in accordance with claim 1, wherein the conductive particles being present in the non-conductive matrix in a concentration in the range 0.1-2% by volume.

7. A method in accordance with claim 1, wherein the conductive particles being present in the non-conductive matrix in a concentration in the range 0.1-1.5% by volume.

8. A method in accordance with claim 1, wherein the conductive particles are added as a predominantly non-aqueous dispersion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 show optical micrographs of assemblies of 0.2 vol-% CNC particles dispersed into the adhesive (A) and aligned by the electric field (B) as well as the schematic of the situation (C).

(2) FIG. 2 shows a plot of the dependence of DC conductivity of 0.2 vol-% CNC particles dispersed into the adhesive against the alignment time. The solid line is guide to the eye.

(3) FIG. 3 shows aligned film with (A-B) and without (C-D) electrical contacts between electrodes.

(4) FIG. 4 shows schematics of the UV curing technique.

(5) FIG. 5 shows optical micrographs showing the healing of a scratch.

(6) FIG. 6 a-c shows aligned and cured conductive particle polymer system in in-plane geometry.

(7) FIG. 7 shows aligned material with arbitrary alignment geometry and arbitrary electrode shape.

(8) FIG. 8 shows an optical micrograph of aligned and cured film of nanocone adhesives in in-plane geometry after pyrolysis.

(9) FIG. 9 illustrates the steps to produce aligned conducting.

(10) FIG. 10 illustrates dendritic structures maximizing the contact area between conductive item and matrix.

(11) FIG. 11 shows optical micrographs of conductive particle assemblies before and after alignment.

(12) FIG. 12 shows a plot of the dependence of DC conductivity of 0.2 vol-% CNC particles dispersed into the adhesive against the alignment time. The solid line is guide to eye.

(13) FIG. 13 shows aligned film with (A-B) and without (C-D) electrical contacts between electrodes (a) and material (b).

(14) FIG. 14 shows schematics of the UV curing technique.

(15) FIG. 15 a-c depicting aligned and cured conductive nanocone adhesives in in-plane geometry.

(16) FIG. 16 shows aligned material with arbitrary alignment geometry and arbitrary electrode shape.

(17) FIG. 17 illustrates dendritic structures maximizing the contact area between conductive item and matrix.

(18) FIG. 18 A-B show optical micrographs of assemblies of 0.2 vol-% CNC particles dispersed into the adhesive and aligned by the electric field. FIG. 18 C shows the applied geometry of joint electrodes (a) and adhesive with aligned pathways (b).

(19) FIG. 19 A-F illustrates the connection of solar cell electrodes by conductive adhesive with aligned particles.

(20) FIG. 20 plots the dependence of DC conductivity of 0.2 vol-% CNC particles dispersed into the adhesive against the alignment time. The solid line is a guide to the eye.

(21) FIG. 21 A-E shows the connection of solar cell electrodes by conductive adhesive with aligned particles in a schematic process line.

(22) FIG. 22 shows optical micrographs showing the healing of a scratch.

(23) FIG. 23 illustrates how this method can be incorporated into a pick and place device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(24) The present invention will be described below with reference to examples and figures. It is to be understood that the present invention is by no means limited to these examples and figures.

(25) I. Production

(26) The method can be used in a production line for ESD (electrostatic dissipation or discharge, also known as antistatic) devices, such as films for antistatic packaging or antistatic mats or boards. A thermally conductive film can also be made, that can e.g. be used for lighting reflectors or electronic parts, or a thermally conductive mat that e.g. can be used for form a heat sink. The method comprises the following steps: i. a matrix is formed from epoxy mixed with conducting particles, according to the present invention ii. the matrix is applied to a substrate e.g. by spraying, pouring or dipping iii. an electrical field in the range of 0.01 to 20 kV/cm is applied iv. the matrix is cured, using e.g. UV light or heat v. optionally the matrix is reduced, so as to expose the conducting pathways vi. optionally steps ii to v is repeated to create several layers, e.g. for creating conductive pathways in different directions.

(27) The method can also be used in a production line for e.g. solar cells or electronics. The method comprises the following steps: i. epoxy is mixed with conducting particles to form a matrix with conducting particles ii. the matrix is applied between surfaces that shall be electrically and mechanically connected iii. an electrical field in the range of 0.01 to 20 kV/cm is applied over the matrix iv. the matrix is cured, using e.g. UV light or heat.

Example 1

(28) This example concerns the preparation of a mixture of conductive particles and polymer matrix that in this example is an thermally cured polymer adhesive; as well as determination of conductivity as a function of particle load; and how the step-like increase in conductivity with increasing particle load can be explained by formation of conductive paths between particles when the contacts are formed with increased particle fraction.

(29) This example concerns moreover the preparation of the same mixture when the particle load is low, for example 10 times less than the observed percolation threshold, the limit where the isotropic non-aligned mixture is not conductive; as well as the alignment of this mixture using electric field so that the aligned particles form conductive paths resulting in a conductive material, whose conductivity is directional. The example, moreover, shows change of the viscosity of so obtained material, by curing, so that the alignment and directional conductivity obtained in the alignment step is maintained.

(30) The employed conductive particles were CB from Alfa Aesar, CNC from n-Tec AS (Norway) and iron oxide (FeO.Fe.sub.2O.sub.3) from Sigma-Aldrich.

(31) The employed polymer matrix was a two component low viscosity adhesive formed by combining Araldite AY 105-1 (Huntsman Advanced Materials GmbH) with low viscosity epoxy resin with Ren HY 5160 (Vantico AG).

(32) The conductive particles were mixed in the adhesive by stirring for 30 minutes. Due to the high viscosity of mixture, efficient mixing is possible only up to 20 vol-%. of particles.

(33) Estimated percolation threshold of these materials are at 2 vol-%. The mixtures are conductive above and insulators below this threshold. The conductivity is due to the conductive particles and the polymer is essentially insulator.

(34) To illustrate the benefit of alignment, the particle loads of 1/10 of the estimated percolation threshold were used.

(35) FIG. 1 illustrates, using optical micrographs, the mixing of assemblies of 0.2 vol-% CNC particles dispersed into the example adhesive before (FIG. 1A) and after an electric field alignment and curing (FIG. 1B).

(36) The scheme shows the applied alignment (out-of-plane) geometry (FIG. 1C). This alignment geometry was used to cover conductive path distances from 10 m to 2 mm. For an out-of-plane alignment 2 mm3 cm wide layer of material is injected between two metal electrodes with spacing of 2 mm.

(37) Mixture was aligned using an AC source. In this example the alignment procedure 1 kHz AC-field (0.6-4 kV/cm, rms value) was employed for >10 minutes for >1 mm electrode spacing and <10 minutes for <1 mm electrode spacing.

(38) FIG. 2 shows the conductivity as a function of alignment time illustrating orders of magnitudes conductivity enhancement.

(39) The curing was performed immediately afterwards at 100 C. for 6 minutes.

(40) The material remains aligned after curing and conductivity level obtained by alignment is maintained.

Example 2

(41) This example concerns versatile choice of alignment conditions and illustrates how the present invention can be employed not only with electrodes connected to the orientation material but also with electrodes electrically isolated from the material.

(42) The procedure was otherwise similar to that in example 1, but instead of having material directly connected to the alignment electrodes, the electrodes were electrically disconnected from the material by an insulating layer, for example by 0.127 mm Kapton foils. Alignment occurred exactly as in Example 1.

(43) This procedure allows removal of electrodes after alignment and thus freestanding aligned film even in the case where the matrix is adhesive. The alignment also occurs if the electrodes do not touch the material and so the alignment can be performed from the distance. When the material and electrodes are moved, continuous or stepwise, with respect to each others during the alignment, this allows continuous alignment processing. Three possible options for the alignment settings are illustrated in FIG. 3 that shows aligned film with (A-B) and without (C-D) electrical contacts between electrodes (a) and material (b). In the case (A) the aligned film forms permanent connection between the electrodes. In the case (B) the electrodes and material are only loosely joined together and can be moved apart after alignment. In the case (C) there are insulating layers (c) between the material and electrodes and they are easily moved apart after the alignment even in the case where the material is an adhesive. In this case the obtained material is a multilayer consisting of aligned layer (b) and two insulating layers (c) In the case (D) the alignment is carried out from the distance and the mutual location of electrodes and film can be additionally moved during the alignment.

Example 3

(44) This example concerns the applicability of the alignment method, the use of alignment for particular application of UV-curing. This emphasises the benefit of low particle fraction which makes the material more transparent for UV light for curing.

(45) The procedure was otherwise similar to that in example 1 or 2 but the thermally cured polymer matrix was replaced by UV-curable Dymax Ultra Light-Weld 3094 adhesive and the curing step was done by the UV-light with the wavelength 300-500 nm.

(46) FIG. 4 illustrates the alignment of 0.2 vol-% CNC dispersion in out-of-plane geometry. The mixture was formed following the guideline of example 1 (FIG. 4a) but spread on the alignment electrode using RK Print Paint Applicator that uses a moving bird applicator to level the adhesive layer to the predetermined thickness (the idea is schematically illustrated in FIG. 4b). This admixture was aligned following the method outlined in example 2 but the upper electrode was not in contact with the material by use of an insulating layer such as Kapton (FIG. 4c); this allows removal of electrodes after alignment and thus freestanding aligned film even in the case where the matrix is adhesive. After alignment, the upper alignment electrode is removed and aligned admixture cured by UV or blue light. (FIG. 4d). The lower electrode can be optionally removed (FIG. 4e) to form a fully free-standing film.

(47) FIG. 4 also gives the schematics of UV curing. Conductive particles are dispersed with UV-curable polymer matrix (a). This mixture is spread to form a predetermined layer on the substrate (that acts also as an alignment electrode) using an applicator (b). The material is aligned by electric field using lower electrode and another top-electrode that does not touch the material (c). The upper electrode is removed and the aligned mixture is cured using a light (UV/vis) source, which leads to a semi-freestanding aligned film (d). If required, the lower electrode can be additionally removed leading to a fully free-standing aligned film (e).

Example 4

(48) This example shows how the present invention can be employed with thermoplastic or thermotropic polymer matrix.

(49) The procedure was otherwise similar to that in Example 1 or 2 but thermoplastic or thermotropic polymer is used instead of thermoset polymer. In this example alignment was performed when the material was fluid at elevated temperature above the melting point of material. Permanent alignment was achieved when the temperature of fluid matrix with aligned particles was decreased below its glass transition or melting point, which resulted in the stabilization of material.

(50) The used matrix material was polyfluorene polymer (American Dye Source, with melting point at 180 C.)

Example 5

(51) This example illustrates how the invention can be employed with polymer matrix and co-solvent.

(52) The procedure was otherwise similar to that in examples 1, 2, 3, or 4 but the polymer matrix contains solvent. The alignment was performed with the presence of solvent and the solvent was evaporating off after alignment. This can occur with or without curing of thermoset polymer or cooling thermoplastic or thermotropic polymer.

(53) In the case of thermoset polymer matrix this solvent decreases the viscosity of matrix polymer. This means that the solvent acts as thinner. An example solvent in the thermocured polymer in example 1 is benzylalcohol that is a good solvent for epoxy resin and hardener.

(54) In the case of thermoplastic or thermotropic polymer matrix in example 1 this solvent makes the mixture fluid already below the melting point of matrix and allows thus alignment at lower temperature. A possible solvent in example 4 is toluene that is a good solvent for polyfluorene.

Example 6

(55) This example shows the robustness of the procedure and shows how electric field heals macroscopic defects in a conductive particle adhesive mixture.

(56) The materials and procedure was similar to that in examples 1, 2, 3, 4, or 5, but a macroscopic scratch defect was made by a sharp spike; and the electric field was reapplied. FIG. 5 a-e are optical micrographs showing the healing of the scratch in the case of CNC particle mixture.

Example 7

(57) This example concerns versatile choice of alignment geometries and illustrates how the invention can be employed not only in the geometry shown in Example 1 but also in (i) thin films and (ii) in in-plane geometry. This example underlines the generality of the method.

(58) The material was the same and the procedure similar as in Example 1, but instead of out-of-plane alignment geometry, in-plane alignment geometry was used.

(59) For the in-plane alignment 10 m thick layer was spread either by spin-coating or by plastic spatula over 1 cm1 cm area of metal finger grid where the thickness and width of fingers, respectively, were 50-200 nm and 2-10 m. The spacing between fingers was 10-100 m.

(60) FIG. 6 illustrates aligned and cured conductive CNC adhesives in in-plane geometry. FIG. 6a shows an optical micrograph 0.2 vol-% aligned material. Schematic (FIG. 6b) illustrates the alignment setting. In this geometry the alignment occurs typically in seconds or tens of seconds.

(61) In another version the alignment electrodes were electrically insulated for example by SiO.sub.2 layer following the idea of example 2. Alignment was achieved exactly as without insulating layer.

Example 8

(62) This example concerns versatile choice of alignment geometries and illustrates how the invention can be employed not only in the out-of-plane and in-plane geometries with flat well defined electrodes but also when the geometry and electrode shape is arbitrary. This example underlines the generality of the method. This also illustrates that the alignment does not require a surface or interface parallel to the emerging aligned pathways.

(63) The materials were otherwise the same and the procedure similar as in Example 1, 2, 3, 4, or 5 but instead of out-of-plane or in-plane alignment geometry and flat electrodes, arbitrary geometry and arbitrary electrode shape were used. FIG. 7 shows an optical micrograph of aligned material when arbitrary geometry and arbitrary electrode shapes have been used.

Example 9

(64) This example concerns another feature of the invention, the reduction of matrix after alignment and stabilization. This illustrates how the present invention can be employed in (i) thin films and (ii) in in-plane geometry so that the outcome forms solitary network of aligned pure conductive particles or aligned channels with conductive core and insulating mantle.

(65) The material was otherwise the same and the procedure similar as in example 1 but all or part of the matrix was removed from the aligned and cured film. In typical procedure the aligned and cured film was heated at 450 C. from 10 minutes to 2 hours. As a result of this procedure step, the thickness of matrix was greatly reduced between the conductive channels and instead of a uniform film with aligned conductive channels embedded into it, a film with distinctive solitary network was achieved (see FIG. 8).

(66) This procedure can be performed similarly to the materials examples shown in examples 1, 2, 3, 4, or 5. Alternative overall steps are illustrated in FIG. 9, which illustrates the steps to produce aligned conducting film. From left to right: Molecules are dispersed into fluid which can be thermoset, thermoplastic or lyotropic material. Thin film of this dispersion is spread over a substrate. Aligned particle channels forming conductive channels are formed by applying an electric field. Solid uniform film with aligned conductive channels is formed by changing the viscosity. In the case of thermoset matrix this is achieved by curing the matrix polymer. In the case of thermoplastic matrix this is achieved by decreasing the alignment temperature below a phase transition such as melting point or glass transition of the matrix. In the lyotropic case the alignment is performed with the presence of solvent and the solidification obtained by evaporating solvent off. A network of separated aligned wires may be formed by removing part or the entire matrix, for instance by a selective solvent or by pyrolysing a part of the solid matrix.

Example 10

(67) This example concerns further versatility of the invention, the use of electric field alignment when preparing electrodes with very large contact area dendrimer surface.

(68) The procedure was otherwise similar to that in examples 1, 2, 3, 4, 5, 7, 8, or 9 but the alignment was terminated before the chains reached from electrode to electrode. FIG. 10 shows so obtained electrodes with dendritic surface.

(69) II. Electrostatic Discharge

(70) The method of the present invention is for manufacturing an ESD device which has at least one anisotropic conductive layer comprising a mixture of a matrix and conductive particles. The steps are: a. applying a layer of the mixture over a first surface of the ESD device, the mixture having a first viscosity which allows the conductive particles to rearrange within the layer; b. applying an electric field between two alignment electrodes, over the layer, so that a number of the conductive particles are aligned with the field, thus creating conductive pathways; c. changing the viscosity of the layer to a second viscosity, said second viscosity being higher than the first viscosity in order to mechanically stabilise the layer and preserve the conductive pathways.

(71) Note that the first surface could be used as an alignment electrode, so there is no need to use a separate electrode. The electrodes may also be remote and thus insulated from the mixture.

(72) The method can be performed in a production line for ESD devices; the production line may comprise the steps: i. Polymer resin is mixed with CB according to the present invention to form a matrix ii. the matrix is formed to a film, or an object is dipped in the matrix or it is sprayed on or poured over a. for films the layer has a thickness from 0.1 to 5 mm, preferably less than 3 mm. b. for thin mats the layer has up to 3 cm in thickness, preferably less than 2 cm in thickness c. for thick mats the layer has up to 50 cm in thickness, preferably less than 5 cm in thickness iii. an electrical field according to the present invention is applied iv. the matrix is cured, using e.g. UV light or heat v. optionally the matrix is reduced, so as to expose the conducting pathways vi. optionally steps ii to v is repeated

(73) The method can also be performed in a production line where a conductive layer or wires are to be connected or laminated. The production line may comprise the steps: i. epoxy is mixed with CB according to the present invention to form a matrix ii. the matrix is formed to a film or paste and used as glue where conductivity between layers or components or wires are wanted iii. an electrical field according to the present invention is applied iv. the matrix is cured, using e.g. UV light or heat.

Example 11

(74) This example concerns the preparation of a mixture of conductive particles and polymer matrix which is a thermally cured polymer adhesive. It also shows the conductivity as a function of particle load and how the step-like increase in conductivity with increasing particle load can be explained by formation of conductive paths between particles when the contacts are formed with increased particle fraction.

(75) This example concerns moreover the preparation of the same mixture when the particle load is low, for example 10 times less than the observed percolation threshold, the limit where the isotropic non-aligned mixture is not conductive; as well as the alignment of this mixture using electric field so that the aligned particles form conductive paths resulting in a conductive material, whose conductivity is directional, for example below the percolation threshold of non-aligned material. The example, moreover, shows the change of the viscosity of the resulting material obtained, for instance by curing, so that the alignment and directional conductivity obtained in the alignment step is maintained. The employed conductive particles were CB from Alfa Aesar, CNC material from n-Tec AS (Norway) and iron oxide (FeO.Fe.sub.2O.sub.3) from Sigma-Aldrich.

(76) The employed polymer matrix was a two component low viscosity adhesive formed by combining Araldite AY 105-1 (Huntsman Advanced Materials GmbH) with low viscosity epoxy resin with Ren HY 5160 (Vantico AG).

(77) The conductive particles were mixed in the adhesive by stirring for 30 minutes. Due to the high viscosity of mixture, efficient mixing is possible only up to 20 vol-%.

(78) Estimated percolation thresholds of these materials are at 2 vol-%. The mixtures are conductive above and insulators below this threshold. The conductivity is due to the conductive particles and the polymer is an insulator.

(79) To illustrate the benefit of alignment, the materials were the same and similarly prepared as in above but ten times lower particle loads were used.

(80) FIG. 11 illustrates, using optical micrographs, the mixing of assemblies of 0.2 vol-% CNC particles dispersed into the example adhesive before (FIG. 11A) and after an electric field alignment and curing (FIG. 11B).

(81) The scheme shows the applied alignment (out-of-plane) geometry (FIG. 11C). This alignment geometry was used to cover conductive path distances from 10 m to centimeters, preferentially to millimeters. For an out-of-plane alignment 2 mm3 cm wide layer of material is injected between two conductive layers (a).

(82) Mixture was aligned using an AC source to obtain aligned pathways (b). In this example the alignment procedure 1 kHz AC-field (0.6-4 kV/cm (rms value)) was employed for 10 minutes for >1 mm electrode spacing and <10 minutes for <1 mm electrode spacing.

(83) The dependence of DC conductivity of 0.2 vol-% CNC particles dispersed into the adhesive against the alignment time is shown in FIG. 12. The solid line is guide to eye. The curing was performed immediately afterwards at 100 C. for 6 minutes.

(84) The material remains aligned after curing and conductivity level obtained by alignment is maintained.

Example 12

(85) This example concerns versatile choice of alignment conditions and illustrates how the present invention can be employed not only with electrodes connected to the orientation material but also with electrodes electrically isolated from the material.

(86) The procedure was otherwise similar to that in Example 11, but instead of having material directly connected to the alignment electrodes, the electrodes were electrically disconnected from the material by an insulating layer, for example by 0.127 mm Kapton foils. Alignment occurred exactly as in Example 11.

(87) This procedure allows removal of electrodes after alignment and thus freestanding aligned film even in the case where the matrix is adhesive. The alignment also occurs if the electrodes do not touch the material and so the alignment can be performed from a distance. When the material and electrodes are moved, continuous or stepwise, with respect to each others during the alignment, this allows continuous alignment processing. Three possible options for the alignment settings are illustrated in FIG. 13 that shows aligned film with (A-B) and without (C-D) electrical contacts between electrodes (a) and material (b). In the case (A) the aligned film forms permanent connection between the electrodes. In the case (B) the electrodes and material are only loosely joined together and can be moved apart after alignment. In the case (C) there are insulating layers (c) between the material and electrodes and they are easily moved apart after the alignment even in the case where the material is an adhesive. In this case the obtained material is a multilayer consisting of aligned layer (b) and two insulating layers (c) In the case (D) the alignment is carried out from the distance and the mutual location of electrodes and film can be additionally moved during the alignment.

Example 13

(88) This example concerns the applicability of the alignment method, the use of alignment for particular application of UV-curing. This emphasises the benefit of low particle fraction which makes the material better transparent for UV light for curing.

(89) The procedure was otherwise similar to that in Example 11 or 12 but the thermally cured polymer matrix was replaced by UV-curable Dymax Ultra Light-Weld 3094 adhesive and the curing step was done by the UV-light with the wavelength 300-500 nm.

(90) FIG. 14 illustrates the alignment of 0.2 vol-% CNC dispersion in out-of-plane geometry. The mixture was formed following the guideline of Example 11 (FIG. 14a) but spread on the alignment electrode using RK Print Paint Applicator that uses a moving bird applicator to level the adhesive layer to the predetermined thickness (the idea is schematically illustrated in FIG. 14b). This admixture was aligned following the method outlined in Example 12 but the upper electrode was not in contact with the material by use of an insulating layer such as Kapton (FIG. 14c); this allows removal of electrodes after alignment and thus freestanding aligned film even in the case where the matrix is adhesive. After alignment, the upper alignment electrode is removed and aligned admixture cured by UV or blue light. (FIG. 14d). The lower electrode can be optionally removed (FIG. 14e) to form a fully free-standing film.

(91) FIG. 14 also gives the schematics of UV curing. Conductive particles are dispersed with UV-curable polymer matrix (a). This mixture is spread to form a predetermined layer on the substrate (that acts also as an alignment electrode) using an applicator (b). The material is aligned by electric field using lower electrode and another top-electrode that does not touch the material (c). The upper electrode is removed and the aligned mixture is cured using a light (UV/vis) source, which leads to a semi-freestanding aligned film (d). If required, the lower electrode can be additionally removed leading to a fully free-standing aligned film (e).

Example 14

(92) This example concerns versatile choice of alignment geometries and illustrates how the invention can be employed not only in the geometry shown in Example 11 but also in (i) thin films and (ii) in in-plane geometry. This example underlines the generality of the method.

(93) The material was the same and the procedure similar as in Example 11, but instead of out-of-plane alignment geometry, in-plane alignment geometry was used.

(94) For the in-plane alignment a 10 m thick layer was spread either by spin-coating or by plastic spatula over 1 cm1 cm area of metal finger grid where the thickness and width of fingers, respectively, were 50-200 nm and 2-10 m. The spacing between fingers was 10-100 m.

(95) FIG. 15 illustrates aligned and cured conductive CNC adhesives in in-plane geometry. FIG. 15a shows an optical micrograph of 0.2 vol-% aligned material. Schematic (FIG. 15b) illustrates the alignment setting. In this geometry the alignment occurs typically in seconds or tens of seconds.

(96) In another version the alignment electrodes were electrically insulated. Alignment was achieved exactly as without insulating layer.

Example 15

(97) This example concerns versatile choice of alignment geometries and illustrates how the invention can be employed not only in the out-of-plane and in-plane geometries with flat well defined electrodes but also when the geometry and electrode shape is arbitrary. This example underlines the generality of the method. This also illustrates that the alignment does not require a surface or interface parallel to the emerging aligned pathways.

(98) The materials were otherwise the same and the procedure similar as in Examples 11, 12, 13, or 14 but instead of out-of-plane or in-plane alignment geometry and flat electrodes, arbitrary geometry and arbitrary electrode shape were used. FIG. 16 shows an optical micrograph of an aligned material when arbitrary geometry and arbitrary electrode shapes were used.

Example 16

(99) This example concerns further versatility of the invention, the use of electric field alignment when preparing electrodes with very large contact area dendrimer surface.

(100) The procedure was otherwise similar to that in Examples 11, 12, 13, 14, or 15 but the alignment was terminated before the chains reached from electrode to electrode. FIG. 17 shows thus obtained electrodes with dendritic surface.

(101) This can be used for making a film for use in batteries or capacitors.

Example 17

(102) This example concerns the materials selection for the procedure described in Example 11, 12, 13, 14, 15 or 16.

(103) Polymeric material, including polyvinyl chloride resin, suitable for the described alignment process for flooring may be homopolymers, or copolymers, consisting of vinyl chloride and other structural units, such as vinyl acetate. To protect the polymeric material from degradation during processing and during its use as flooring material, vinyl compounds may be stabilized against the effects of heat and ultraviolet radiation, using e.g. soaps of barium, calcium and zinc; organo-tin compounds; epoxidized soy bean oils and tallate esters or organic phosphites.

(104) Polymeric materials may contain plasticizers to provide flexibility and to facilitate processing. One suitable plasticizer is dioctyl phthalate (DOP). Others suitable ones may include butylbenzyl phthalate (BBP), alkylaryl phosphates, other phthalate esters of both aliphatic and aromatic alcohols, chlorinated hydrocarbons, and various other high boiling esters.

(105) The stabilized and plasticized vinyl formulation is mixed with varying amounts of inorganic filler to provide mass, colour and thickness at a reasonable cost. The fillers may be calcium carbonate, talcs, clays and feldspars. White pigment can be titanium dioxide and coloured pigments are preferably inorganic.

(106) Other additives can be used to avoid flame spread and smoke generation during a fire. These compounds include alumina trihydrate, antimony trioxide, phosphate or chlorinated hydrocarbon plasticizers, zinc oxide, and boron compounds. Cushioned flooring containing chemically expanded foam can be compounded with azobisformamide blowing agents. Various other processing aids and lubricants may also be employed.

(107) The amount of filler can be less than 1% or up the 80% of the weight, whereas as vinyl resin, other resins, plasticizer and stabilizer can be less than 1% or amount to 20%.

Example 18

(108) The procedure was otherwise similar to that in Examples 11, 12, 13, 14, 15 or 16 but the aligned material was used produce a top layer that is laminated with the flooring material.

Example 19

(109) The procedure was otherwise similar to that in Examples 11, 12, 13, 14, 15 or 16 but the aligned material was used as a part of furniture or work-station.

Example 20

(110) The procedure was otherwise similar to that in Examples 11, 12, 13, 14, 15 or 16 but the aligned material was used as a part of shoe or a gasket.

Example 21

(111) The procedure was otherwise similar to that in Examples 11, 12, 13, 14, 15 or 16, but the aligned material was use as a part of packaging material.

Example 22

(112) The procedure was otherwise similar to that in Examples 11, 12, 13, 14, 15 or 16 but the aligned material was use as a part of a battery or capacitor.

Example 23

(113) The procedure was otherwise similar to that in Examples 11, 12, 13, 14, 15 or 16 but the aligned material was made into a sheet of up to 5 cm in thickness, preferably less than 1 cm in thickness and less than 10 m wide. Said sheet can then be stored and used in the production of large parts for use in vehicles, computers and printers, for example, by cutting or thermoforming.

(114) III. Solar Cell

Example 24

(115) This example concerns the preparation of a mixture of conductive particles and polymer matrix that in this example is an thermally cured polymer adhesive; as well as determination of conductivity as a function of particle load; and how the step-like increase in conductivity with increasing particle load can be explained by formation of conductive paths between particles when the contacts are formed with increased particle fraction.

(116) This example concerns moreover the preparation of the same mixture when the particle load is low, for example 10 times less than the observed percolation threshold, the limit where the isotropic non-aligned mixture is not conductive; as well as the alignment of this mixture using electric field so that the aligned particles form conductive paths resulting in a conductive material, whose conductivity is directional, for example below the percolation threshold of non-aligned material. The example, moreover, shows change of the viscosity of so obtained material, for instance by curing, so that the alignment and directional conductivity obtained in the alignment step is maintained.

(117) The employed conductive particles were carbon black from Alfa Aesar, carbon cones (CNCs) from n-Tec AS (Norway) and iron oxide (FeO.Fe.sub.2O.sub.3) from Sigma-Aldrich.

(118) The employed polymer matrix was a two component low viscosity adhesive formed by combining Araldite AY 105-1 (Huntsman Advanced Materials GmbH) with low viscosity epoxy resin with Ren HY 5160 (Vantico AG).

(119) The conductive particles were mixed in the adhesive by stirring for 30 minutes.

(120) Estimated percolation threshold of these materials is at 2 vol-%. The mixtures are conductive above and insulators below this threshold. The conductivity is due to the conductive particles and the polymer is essentially insulator.

(121) To illustrate the benefit of alignment, the materials were the same and similarly prepared as in above but ten times lower particle loads were used.

(122) FIG. 18 illustrates, using optical micrographs, the mixing of assemblies of 0.2 vol-% CNC particles dispersed into the example adhesive before (FIG. 18A) and after an electric field alignment and curing (FIG. 18B).

(123) The scheme shows the applied alignment (out-of-plane) geometry (FIG. 18C) that corresponds to that illustrated in FIG. 19. This alignment geometry was used to cover conductive path distances 1 from 10 m to centimeters, preferentially to millimeters. For an out-of-plane alignment 2 mm15 cm wide layer of material is injected between the solar cell busbar 3 and the solar cell tab 2.

(124) Mixture was aligned using an AC source to obtain aligned pathways (b). In this example the alignment procedure 1 kHz AC-field (0.6-4 kV/cm (rms value)) was employed for 1 minutes for <1 mm electrode spacing.

(125) FIG. 20 shows the conductivity as a function of alignment time illustrating orders of magnitudes conductivity enhancement.

(126) The curing was performed immediately afterwards at 100 C. for 1 minute.

(127) The material remains aligned after curing and conductivity level obtained by alignment is maintained.

(128) FIG. 19 A shows a solar cell 4 with tabs 5 collecting the current produced by the photovoltaic effect. FIG. 19 B illustrates an isotropic dispersion 8 of conductive particles 6 in an adhesive 7 having a first viscosity. The dispersion 8 is spread onto the solar cell tabs 5 forming a layer of adhesive on each tab as shown in FIG. 19 C. The external electrodes, the busbars 10 are placed on the adhesive layer, where after alignment of the conductive particles 6 is effected by application of an electric field over the electrodes 5, 10, FIG. 19 E, indicated by the AC symbol. Stabilisation of the adhesive dispersions, e.g. by curing, to a second viscosity, higher than the first viscosity, will secure the mechanical strength of the adhesive dispersion and support the aligned conductive particles thus making the adhesive dispersion conductive. The solar cell 4 is now in contact with the busbars 10, because conductive paths have been formed in the adhesive dispersion 8.

(129) The solar cell combines the above-illustrated settings with out-of-plane geometry and short alignment distances plus conveniently low alignment voltages. In a typical example, a 1 mm8 cm wide layer of described anisotropic adhesive with 0.2 vol-% carbon load was injected between the silver and copper electrodes of a solar cell. In this case the electrodes were pressed together and the resultant spacing was less than 100 m. This is followed by electric field alignment and curing, the whole procedure taking typically in the order of ten minutes.

(130) FIG. 21 A-E shows a top view of the sequence described in FIG. 19. The solar cell 4 with tabs 5 are shown in FIG. 21A, The isotropic dispersion 8 of adhesive 7 and conductive particles 6 are spread on the solar cell tabs (FIG. 21 B). The busbars 10 are placed on the adhesive (FIG. 21C) and alignment of particles by applying a voltage 12 over the electrodes 5,10, FIG. 21D. Stabilisation of the adhesive e.g. by curing using e.g. UV light or heat, FIG. 21E.

Example 25

(131) This example shows the robustness of the procedure and shows how electric field heals macroscopic defects in a conductive particle adhesive mixture.

(132) FIG. 22 is optical micrographs showing the healing of the scratch in the case of CNCs. The materials and procedure was similar to that in Example 24, but a macroscopic scratch defect was made by a sharp spike; and the electric field was let on. The optical micrographs showing the healing of the scratch in the adhesive layer, an electric field of 1 kHz, 500 V/cm was let on and the conductive pathways is gradually reforming. After reforming basically all conductive particles are forming conductive pathways in the matrix.

Example 26

(133) This example shows, as illustrated in FIG. 23 how a pick and place device is fitted with an electrical field applicator so that the present invention, as explained in Example 24, can be used in solar panels with back-side contact cells. The pick and place lifts the solar cell (a) on to the encapsulation foil. Once placed the field is applied from the pick and place head (b). At this stage curing may occur via heating or UV curing if the connecting ribbons are transparent, or curing may occur during the lamination stage at the end of the production line.

Example 27

(134) A conductive adhesive according to the present invention is used in thin-film solar panel production where transparent electrodes are used. A thin-film flexible solar cell is built on a plastic substrate using a cadmium telluride p-type layer and a cadmium sulfide n-type layer on a plastic substrate. The semiconductor layers can be amorphous or polycrystalline. A transparent conductive oxide layer overlaid by a busbar network is deposited over the n-type layer. A back contact layer of conductive metal is deposited underneath the p-type layer. The adhesive is applied and becomes conductive as in Example 24.

Example 28

(135) For a solar cell one or more wiring members for collecting current and for transmitting current in the solar cell are made of a dispersion of a matrix and conductive particles. The concentration of said conductive particles is below a percolation threshold, so that the dispersion is not conductive. The dispersion has aligned conductive particles in areas where wiring members for collecting current meet wiring members for transmitting current.

(136) Conductive wires are in this way formed directly to connect the solar cell devices, such that the tab or busbar is not needed to make the circuit. The adhesive dispersion of the present invention is used in one or more layered structures, e.g. one layer that is directional conductive replacing the tab, and one layer directional conductive so as to replace the conductive bars. The matrix can be reduced, e.g. by using a solvent, to expose the conductive pathways, so that the next layer can contact to these. The electrical field is applied in the corresponding directions, e.g. using a mask and a remote field, or by using parts of the solar panel under construction as electrodes, so that the conductive particles in the matrix are aligned to form the needed conductive wires.

(137) IV. UV Polymer

(138) The above mentioned objects are achieved by the present invention which in accordance with a first aspect of the invention concerns a method for producing a polymer composition with the ability to be cured by UV light to an anisotropic electrically conductive layer.

(139) According to a second aspect of the invention the method concerns a polymer composition producable by means of the method according to the first aspect of the invention.

(140) Finally, according to a third aspect the invention concerns methods for establishing an anisotropic electrically conductive layer.

(141) Preferred embodiments of the invention are disclosed.

(142) While it is essential to store polymer compositions which are not immediately used in a container or receptacle in which exposure to UV light is prevented, it is preferred that it is stored in a manner in which exposure to any light is prevented and in a manner in which contact with oxygen is also suppressed.

(143) The subsequent step of making the layer, which may be in the form of a glue joint, a film, coating or free standing mat or film product, may take place weeks or months later than the production of the composition. If it is desired to obtain a product in which the conductive strings of particles (pathways) are mainly mutually parallel, an AC electric field should be applied to align the particles.

(144) It is however worth noticing that for achieving the benefits of the polymer composition according to the present invention optimally, a number of parameters should be taken into consideration and controlled as described below.

(145) The conductive particles are typically infusible conductive particles such as carbon particles, metal or metal coated particles, or metal oxide particles. The conductive particles show low molecular or particle anisotropy and thus the major part of the conductive particles have low aspect ratios; i.e. aspect ratio ranges of 1-5, 1-10 or 1-20 are typical. The terms low molecular or particle anisotropy and low aspect ratio have the same meaning herein. This is the case with spherical carbon black or disk-like or conical carbon particles or graphitic particles. The conductive particles can be a mixture of different carbon particles. Also other conductive particles can be used, like metal, such as silver or metal oxide particles or colloidal metal particles. The particles are typically added to the polymer preferably in the form of a non-aqueous dispersion, since significant amounts of water tend to have a negative influence.

(146) A non-aqueous dispersion is preferable since the aqueous dispersion can experience hydrolysis under electric field (H.sub.2O.fwdarw.H.sup.++OH.sup.) if the field is too high. The hydrolysis may be avoided by lowering the field. Also, hydrolysis is avoided if the water content is relatively small. This level would typically correspond at least to the impurity levels in typical organic solvents or polymer materials. It can even correspond to the water levels in azeotropic mixtures of alcohols and water. For example ethanol contains more than 11 mol-% water.

(147) The flowable polymer composition may have inherent adhesive properties and can be based on a broad range of polymers comprising one or several polymer components and additives commonly used therewith. In particular, it can be a thermoset polymer system which is solidified by cross-linking reactions. The polymer can also be a thermoplastic polymer system or a lyotropic polymer system. It can also be any compatible combination of such polymers.

(148) UV curing polymers generally have desirable properties in the form of rapid curing time and strong bond strength. They can cure in as short time as a second or a few seconds and many formulations can bond dissimilar materials and withstand harsh temperatures.

(149) These qualities make UV curing polymers important in the manufacturing of items in many industrial markets such as electronics, telecommunications, medical, aerospace, glass, and optical. Unlike traditional adhesives, UV light curing polymers and polymer adhesives not only bond materials together but they can also be used to seal and coat products.

(150) When exposed to the correct energy and irradiance in the required band of UV light, polymerization occurs, and so the polymer compositions harden or cure. The types of UV sources for UV curing include UV lamps, UV LEDs and Excimer Flash lamps.

(151) Laminates can be built up with successively applied UV cured layers. This obviates the need for adhesive or primer layers. Thin layers can be formed in very short time, in the range of one second. There are a wide variety of UV curable vinyl monomers, particularly acrylics, with a wide variety of properties that can be combined by means of copolymers or laminates. For example strong acrylics can be combined with the fracture resistant acrylates. Acrylics could be combined with intermediate layers of cross-linked elastomers for maximizing tear strength while retaining surface hardness. Certain fluoracrylates are hard, and antireflective. They have higher specular transmission than a commonly used fluoropolymer, because fluoroacrylates can be completely amorphous and have no scattering centers. Epoxy resins have tightly linked adhesive polymer structures and can be used in surface adhesives and coatings. Such epoxy resins forms cross-linked polymer structures that have strong adhesion and low shrinkage.

(152) There are many systems available for UV curing an adhesive, coating or film. The Dymax Heavy-Duty UV curing Widecure Conveyor Systems is an example of a system mounted on a conveyor belt. Dymax BlueWave LED Prime UVA used LED light and thus use less effect and have constant high intensity.

(153) A highly desirable characteristic when using the object of present invention is that conductive paths can be formed of predominantly low aspect ratio particles like carbon black, carbon nanocones and/or graphene and the formation can take place at low electric field strengths. This simplifies the production equipment and enables both larger surfaces and thicker films to be produced. The carbon black and carbon nanocones and graphitic particles are considerably less expensive than the carbon nanotubes and can be produced in sufficient quantities by industrial methods. Moreover, it is more difficult to form uniform dispersions with carbon nanotubes.

(154) Another highly desirable characteristic of the present invention is that a comparably low concentration of conductive particles may be used. For conductive mixtures a percolation threshold is defined as the lowest concentration of conductive particles necessary to achieve long-range conductivity in the random system. With the polymer composition according to the present invention the concentration of conductive particles required for achieving conductivity in a predefined direction is not determined by the percolation threshold and can be much lower. For practical reasons the concentration of particles is determined by the requirements on the conductive paths desired to build when using the polymer composition, there usually being no reason to have excess amounts of conductive particles not arranged into the conductive paths. The concentration of conductive particles in the polymer composition can be up to 10 times lower than the percolation threshold or even lower. Concentrations of conductive particles is typically in the range of 0.2-10% by volume or 0.2-2 or 0.2-1.5% by volume. It could even be less than 0.2% in some embodiments, e.g. 0.1% by volume.

(155) Such a low particle concentration has several advantages. The tendency of particle segregation in the dispersion is reduced and the shelf life thereby correspondingly increased, the cost of the components is reduced, the mechanical strength of the subsequently formed anisotropic conductive film is increased and the optical transparency is increased thereby enhancing the sensivity to UV light, allowing a more rapid and less power consuming curing process. The increased transparency may also be seen to improve the aestetic properties of the cured products and gives mechanical and optical properties closer to that of polymers without conductive particles.

(156) The subsequent use of the object of the present invention includes but is not limited to:

(157) electrostatic discharge (ESD) devices, conductive glue and adhesives for use in solar panels and electronics or to suppress electromagnetic interference (EMI). Also possible is to apply material on cellulose based paper that would not allow thermal curing.

(158) The aspect ratio as discussed herein is defined as the ratio between the largest linear dimension of a particle and the largest dimension perpendicular to said largest dimension. Low aspect ratio as used herein refers to an aspect ratio less than 20, more preferably less than 10 and even more preferably less than 5.

(159) The conductive particles are typically chosen among the groups comprising metal particles, metal coated particles, metal oxide particles and carbon particles as well as any combination of particles from two or more of said groups.

(160) The composition according to the present invention may be used as glue, i.e. to glue two objects together as well as establish an anisotropic electrically conductive layer on top of a single surface (substrate).

(161) Alternatively, the composition according to the present invention may be used to establish an anisotropic thermally conductive layer. An adequate use for such thermal conductive layers may be to dissipate heat from certain electronic components, e.g. within a computer.

(162) The method and composition can also be used to produce a free-standing polymer film.

(163) In a preferred embodiment the aspect ratio for a majority (more than 50%) of the conductive particles is in the range below 5, the aspect ratio for at least 75% of the conductive particles is below 10, and the aspect ratio for at least 90% of the conductive particles is below 20.