EP4372146A1 - Electrically conductive paper structure, method for producing the same and use thereof - Google Patents

Abstract

Electrically conductive paper structure with cellulose-containing fiber materials and electrically conductive fibers, wherein the paper structure includes at least a first surface region and at least a second surface region, wherein the at least one first surface region of the paper structure has a substantially constant layer thickness and the at least one second surface region of the paper structure has alternately arranged locations with a higher layer thickness and locations with a lower layer thickness, so that the at least one second surface region of the paper structure has a higher electrical resistance compared to the at least one first surface region of the paper structure.

Description

The invention relates to an electrically conductive paper structure, a method for producing the same and a use of the electrically conductive paper structure.

Electrically conductive paper structures based on e.g. cellulose-containing fibers and carbon fibers are known in the state of the art, see e.g. WO 2020/224800 A1 and the EP 2 770 104 B1 It is therefore generally known to equip flat paper substrates with conductive fibers, in particular carbon fibers, metal fibers or graphitized carbon fibers, or other materials that provide conductivity, e.g. carbon nanotubes, in such a way that the electrical current flows through the flat paper substrate. Depending on the specific resistance present, the paper substrate can be used for different purposes, e.g. as a heating element, as an element for electromagnetic shielding or as an element for signal detection.

The present invention is based on the object of providing an electrically conductive paper structure that is improved compared to the prior art. In particular, the electrically conductive paper structure should have an improved physical quality with regard to the flow of electrical current through the flat paper substrate in order to advantageously increase the variety of applications.

This object is achieved by the feature combinations defined in the independent claims. Further developments of the invention are the subject of the dependent claims.

Summary of the invention

• 1. (First aspect of the invention) Electrically conductive paper structure with cellulose-containing fiber materials and electrically conductive fibers, wherein the paper structure includes at least a first surface region and at least a second surface region, wherein the at least one first surface region of the paper structure has a substantially constant layer thickness and the at least one second surface region of the paper structure has alternately arranged locations with a higher layer thickness and locations with a lower layer thickness, so that the at least one second surface region of the paper structure has a higher electrical resistance compared to the at least one first surface region of the paper structure.
• 2. (Preferred embodiment) Electrically conductive paper structure according to clause 1, wherein the at least one first surface region of the paper structure heats up more strongly than the at least one second surface region of the paper structure when the paper structure is supplied with electrical current.
• 3. (Preferred embodiment) Electrically conductive paper structure according to clause 1 or 2, wherein the paper structure has two current conducting rails suitable for introducing electrical current into the paper structure, wherein the two current conducting rails are arranged in parallel, ie one current conducting rail at one end of the paper structure and the further current conducting rail at the opposite end of the paper structure.
• 4. (Preferred embodiment) Electrically conductive paper structure according to one of clauses 1 to 3, wherein the alternately arranged locations with a higher layer thickness of the paper and locations with a lower layer thickness of the paper of the at least one second surface region each have the shape of strips, wherein the strips are arranged alternately in a so-called first direction of the paper structure arranged perpendicular to the longitudinal axis of the respective strip.
• 5. (Preferred embodiment) Electrically conductive paper structure according to clause 4 with reference to clause 3, wherein the longitudinal axis of each of the two parallel current conducting rails runs along a so-called second direction of the paper structure arranged perpendicular to the first direction of the paper structure.
• 6. (Preferred embodiment) Electrically conductive paper structure according to one of clauses 1 to 5, wherein the paper structure includes a plurality of first surface regions and a plurality of second surface regions.
• 7. (Preferred embodiment) Electrically conductive paper structure according to one of clauses 1 to 6, wherein with respect to the alternately arranged locations with higher layer thickness and locations with lower layer thickness in at least one second surface area of the paper structure, the percentage layer thickness ratio of the lower layer thickness based on the higher layer thickness is in a range from 0% to 90%, preferably in a range from 30% to 90%, particularly preferably in a range from 40% to 80%.
• 8. (Preferred embodiment) Electrically conductive paper structure according to clause 3 or one of the clauses 4 to 7 referring back to clause 3, wherein the both current conducting rails are each based on a continuous, electrically conductive thread for contacting the electrically conductive paper structure, wherein the thread is embedded from one end to the opposite end of the paper structure and is in particular a metal wire, a metal thread, a metal strip, a thread based on a carrier substrate such as a plastic film and coated with a metal, a laminate of (plastic) films and metal foils, a metal braid, a braided braid, a knitted fabric or a lamella tape.
• 9. (Preferred embodiment) Electrically conductive paper structure according to one of clauses 1 to 8, wherein the alternately arranged locations with a higher layer thickness of the paper and locations with a lower layer thickness of the paper in at least one second surface area of the paper structure are obtainable by means of a partial removal of fiber material, in particular by means of milling or by means of a round-wire paper machine or by means of a fourdrinier paper machine or by means of a watermark dandy roll or by means of an embossing cylinder or by means of a moulette cylinder.
• 10. (Second aspect of the invention) A method for producing an electrically conductive paper structure according to any one of clauses 1 to 8, comprising:
  • providing a suspension of cellulosic pulp and water in a paper machine;
  • optionally adding at least one chemical additive;
  • adding electrically conductive fibers; and
  • the formation of a paper web.
• 11. (Preferred embodiment) The method according to clause 10, wherein the step of providing a suspension of cellulose-containing pulp and Water in a cylinder mold paper machine and the step of forming a paper web comprises introducing two continuous, electrically conductive threads into the stock suspension in the cylinder mold paper machine, wherein the threads are brought to the cylinder mold in such a way that during the formation of the paper web the threads are embedded in the fiber structure in order to produce current-conducting rails.
• 12. (Preferred embodiment) A method according to clause 10 or 11, wherein the paper structure is formed so that it is composed of two separate paper layers and the electrically conductive threads are arranged between these paper layers.
• 13. (Third aspect of the invention) Use of the electrically conductive paper structure according to any one of clauses 1 to 9 as a heating element, an electromagnetic shielding element or a signal detection element.

Detailed Description of the Preferred Embodiments

Electrically conductive paper structures can be used, for example, as surface heating. Usually, the surface conductivity of an electrically conductive paper structure is essentially constant across the surface as a result of the manufacturing process. With a constant distance between the current conducting rails feeding the electrical current, the constant surface resistance of the electrically conductive paper structure leads to a homogeneous temperature with respect to the entire area supplied with electricity. A locally limited lower or higher temperature is therefore not possible. which is why both the application range and the energy efficiency of conventional electrically conductive paper structures are limited.

The present invention is based on the idea of providing an improved electrically conductive paper structure by using the watermark technology known in the field of banknote production and other suitable methods for controlling the paper thickness. With the help of the technology for producing watermarks, thinner and thicker areas can be created in the paper. By locally varying the layer thickness of the paper, e.g. parallel to two current conducting rails feeding the electrical current, the electrical resistance of the electrically conductive paper structure increases, i.e. the electrical resistance of the paper can be increased to a certain extent by locally redistributing the paper material. In this way, the heat generation can be changed locally and used advantageously for various applications. For example, it is possible to provide surface heating in such a way that, on the one hand, there are first surface areas with a constant layer thickness of the paper and, on the other hand, there are second surface areas that have both thin paper areas and thick paper areas. When electrical current is fed in, the smooth and unstructured first surface areas heat up somewhat more than the irregular and structured second surface areas, i.e. the second surface areas remain somewhat cooler than the first surface areas when the electrically conductive paper structure is put into operation.

• Die Heizleistung des elektrisch leitenden Papiergefüges kann auf spezielle Bereiche fokussiert werden, in denen die Wärme gewünscht ist. Entsprechend kann die Heizleistung des elektrisch leitenden Papiergefüges in weiteren Bereichen reduziert werden, um auf diese Weise die Energieeffizienz zu steigern.
• Eine lokal unterschiedliche elektromagnetische Abschirmwirkung des elektrisch leitenden Papiergefüges kann bewerkstelligt werden, sodass besondere Antennenstrukturen oder dergleichen realisierbar sind.
• Optische Muster, insbesondere Flächenbereiche, die sowohl dünne Papierstellen als auch dicke Papierstellen aufweisen, sind z.B. mittels IR-Strahlung erkennbar, sodass auf diese Weise versteckte Sicherheitsmerkmale oder Produktkennungen erzeugbar sind.
• Im Bereich der den Strom einspeisenden Stromleitschienen kann das Papiermaterial ggf. mindestens zum Teil entfernt werden, sodass z.B. im Zuge eines Aufwickelns des elektrisch leitenden Papiergefüges zu einem Rollenmaterial eine zu hohe Wellenbildung oder Faltenbildung in bestimmten Bereichen vermieden werden kann.

Based on the local variation of the electrical resistance in the paper, a wide range of applications are possible:

• The heating power of the electrically conductive paper structure can be adjusted to specific Areas where heat is required can be focused on. Accordingly, the heating power of the electrically conductive paper structure can be reduced in other areas in order to increase energy efficiency.
• A locally different electromagnetic shielding effect of the electrically conductive paper structure can be achieved, so that special antenna structures or the like can be realized.
• Optical patterns, especially surface areas that have both thin and thick paper areas, can be recognized using IR radiation, for example, so that hidden security features or product identifiers can be created in this way.
• In the area of the current conducting rails that feed the current, the paper material can be removed at least in part if necessary, so that, for example, when the electrically conductive paper structure is wound up into a roll material, excessive corrugation or wrinkling in certain areas can be avoided.

The electrically conductive paper structure according to the invention comprises cellulose-containing fiber materials and electrically conductive fibers, in particular carbon fibers, wherein the paper structure comprises at least one first surface area and at least one second surface area, wherein the at least one first surface area of the paper structure has a substantially constant layer thickness and the at least one second surface area of the paper structure has alternating locations with a higher layer thickness and locations with a lower layer thickness, so that the at least one second surface area of the paper structure has a higher electrical resistance compared to the at least one first surface area of the paper structure. In particular in the case of a supply of the paper structure With electric current, the at least one first surface area of the paper structure heats up more than the at least one second surface area of the paper structure.

According to a preferred embodiment, the electrically conductive paper structure has two current conducting rails suitable for introducing electrical current into the paper structure, the two current conducting rails being arranged in parallel, i.e. one current conducting rail at one end of the paper structure and the other current conducting rail at the opposite end of the paper structure. The wording "at the end of the paper structure" is to be understood broadly, i.e. the respective current conducting rail does not necessarily have to be arranged at the end, but the corresponding current conducting rail can also be arranged essentially at the end of the paper structure. The respective current conducting rail can, for example, be based on a continuous, electrically conductive thread for contacting the electrically conductive paper structure, wherein the thread is embedded from one end to the opposite end of the paper structure and is in particular a metal wire, a metal thread, a metal strip, a thread coated with a metal based on a carrier substrate such as a plastic film, a laminate of (plastic) films and metal foils, a metal braid, a braided braid, a knitted fabric or a lamella tape.

According to a further preferred embodiment, the alternately arranged locations with a higher layer thickness of the paper and locations with a lower layer thickness of the paper of the at least one second surface area each have the shape of strips, wherein the strips are arranged alternately in a so-called first direction of the paper structure arranged perpendicular to the longitudinal axis of the respective strip. It is particularly preferred that the longitudinal axis of each of the two parallel current conducting rails runs along a so-called second direction of the paper structure that is arranged perpendicular to the first direction of the paper structure. The individual strips each have a preferred width in a range from 1 mm to 20 mm, more preferably in a range from 3 mm to 10 mm and particularly preferably in a range from 5 mm to 7 mm.

According to a further preferred embodiment, the electrically conductive paper structure includes a plurality of first surface regions and a plurality of second surface regions.

According to a further preferred embodiment, the electrically conductive paper structure is designed such that, with regard to the alternately arranged locations with a higher layer thickness and locations with a lower layer thickness in at least one second surface area of the paper structure, the percentage layer thickness ratio of the lower layer thickness relative to the higher layer thickness is in a range from 0% to 90%, more preferably in a range from 30% to 90% and particularly preferably in a range from 40% to 80%.

The alternately arranged areas with a higher layer thickness of the paper and areas with a lower layer thickness of the paper in at least one second surface area of the paper structure are obtainable in particular by means of a partial removal of fiber material, e.g. by means of milling or by means of a round-wire paper machine or by means of a fourdrinier paper machine or by means of a watermark dandy roll or by means of an embossing cylinder or by means of a moulette cylinder.

The electrically conductive paper structure according to the invention is preferably produced using a round screen technology. In this way, it is fundamentally possible to process varying fiber compositions. It is expedient to add electrically conductive short-cut fibers, in particular carbon fibers, to the cellulose-containing fiber materials. The electrically conductive short-cut fibers in particular have a fiber length in the range of 3 to 6 mm. The amount in which the electrically conductive short-cut fibers are added is expediently selected so that sufficient fiber-to-fiber contacts are provided and thus a suitable electrical current flow is ensured. The electrically conductive paper structure can contain other natural and/or synthetic fiber materials, optionally chemical additives and optionally residual moisture. Furthermore, the electrical conductivity can be achieved not only by electrically conductive short-cut fibers, in particular carbon fibers or metallic short-cut fibers, but also by adding carbon particles or carbon nanotubes.

The cylinder screen technology makes it possible to embed a continuous, conductive thread into the core of the electrically conductive paper structure, so that the electrically conductive paper structure is permeated with the continuous, conductive thread from one end to the opposite end, see e.g. the WO 2020/224800 A1 . The continuous, conductive thread embedded in the electrically conductive paper structure in this way thus represents a preferred current conducting rail or a contact thread. The term "contact thread" is to be understood here as a thread for contacting the electrically conductive paper structure, which within the electrically conductive paper structure is directly in contact at many points with the electrically conductive (short-cut) fibers, carbon particles and/or Carbon nanotubes are conductively connected. The contact thread embedded in the electrically conductive paper structure is comparable in terms of its arrangement in the paper, in particular, to a security thread embedded in banknote paper. It is expedient not to introduce just one continuous, conductive thread into the electrically conductive paper structure, but rather to advantageously introduce a plurality of continuous, conductive threads into the electrically conductive paper structure. On the one hand, the contact threads enable easy contact at the ends of the threads and, on the other hand, by introducing them over the entire surface of the electrically conductive paper structure, they establish a local power supply to all areas. The contact threads thus fulfill a dual function in a particularly advantageous manner. The electrically conductive paper structure according to the invention, provided with contact threads, is also characterized by its robustness and long-term durability.

The cellulose-containing fiber materials that can be used for the electrically conductive paper structure according to the invention can be selected, for example, from fiber materials of natural origin or from synthetic fiber materials. Cellulosic fiber materials of natural origin include, for example, wood fibers, semi-cellulose, thermomechanical pulp, cotton fibers, chemically digested cellulose such as sulfate or sulfite pulp, wood pulp, chemically modified wood pulp, recycled fiber materials and combinations of two or more of the above-mentioned elements.

Conductive materials, in particular conductive fibers, such as carbon fibers, metallic (short-cut) fibers or metallized plastic fibers, and/or conductive particles, such as carbon particles, carbon nanotubes or fullerenes, are added to the cellulose-containing fibers. For example, the proportion of electrically conductive Short-cut fibers that are added to the cellulose-containing fibers, depending on the application, vary in a range from a few percent by weight (wt.%) to 50 wt.% or more. It is advisable to add enough conductive fibers to exceed the so-called percolation threshold in order to ensure sufficient conductivity. As a result, a sufficient network of conductive fibers is required to ensure the flow of electrical current.

Carbon fibers in particular are added to the cellulose-containing fibers as conductive materials. Carbon fibers are preferably understood to be industrially produced fibers that are made from carbon-containing material and converted into a graphite-like carbon structure, for example by pyrolysis. Such fibers can be produced isotropically or anisotropically and usually have a diameter in the range of 5 µm to 8 µm. During processing, these individual fibers are combined into bundles (roving) with 1,000 to 400,000 individual fibers and can then be processed further. Carbon fibers are good electrical and thermal conductors. In addition, carbon fibers are understood to be fibers from a group that includes high-strength carbon fibers, high-stiffness carbon fibers and/or high-strength carbon fibers. According to a further preferred embodiment of the present invention, the proportion of carbon fibers in the paper structure (under controlled climatic conditions of 23 °C and 50% relative humidity) is in a range from 5% to 20% by weight, in particular in a range from 10% to 15% by weight. It should also be taken into account that with the increase in the proportion of primary and/or recycled carbon fibers, the specific resistance of the paper structure formed thereby may decrease. and the conductivity of the paper thus formed increases. The advantages of an increased proportion of carbon fibers in the paper structure include improved conductivity, a reduction in electrical resistance in the sheet or paper structure and the associated higher power consumption. The paper fiber can be based on cellulosic fibers, e.g. cotton fibers, with synthetic fibers (e.g. polyethylene terephthalate (PET) fibers, bicomponent fibers, polylactic acid (PLA) fibers) optionally mixed in. The paper fiber can be fibrillated in a targeted manner by means of targeted grinding (e.g. Dutch grinding) in order to provide a particularly high strength potential for the paper structure to be produced. The specific resistance of the paper structure according to the invention is, for example, in the range from 10 ^-2 Ωm to 10 ^-5 Ωm.

Chemical additives can optionally be added to the electrically conductive paper structure according to the invention, which are selected, for example, from a group which includes in particular retention agents, drainage aids, retention agent dual systems or microparticle systems, wet and dry strength agents, sizing agents, fillers and/or pigments, in particular selected from a group of talc, titanium dioxide, aluminum hydroxide, bentonite, barium sulfate, calcium carbonate, kaolin, defoamers, deaerators, biocides, enzymes, bleaching aids, optical brighteners, dyes, shading dyes, impurity scavengers, precipitants (fixing agents), wetting agents, pH regulators. Alternatively or in combination, the chemical additive can also be selected from a group of preferably water-soluble polymers, which include in particular amine-containing polymers, polyethyleneimine, pyrolydine, polyamides, polyacrylamide, aridine, proteins, peptides, polyether-containing polymers, in particular polyethylene oxide, polyethers, hydroxyl-containing polymers, in particular starch, carboxymethylcellulose, Polyvinyl alcohol, charged polymers, in particular cationic polymers, in particular cationic starch, corn starch, potato starch, wheat starch, rice starch, polymers containing ammonium groups, anionic polymers, in particular anionically modified polyacrylamides, sulfonated polymers, inorganic salts with high charge density, in particular aluminum salts, aluminum (III) chlorides, aluminum sulfate, sodium aluminate, inorganic, charged particles/pigments, in particular bentonite, montmorillonite, sodium silicate, wet strength agents, in particular epichlorohydrin resins, glyoxal, zirconium salts, zirconium carbonate, combinations of anionic polymers and cationically modified pigments, aids for reducing the flash point, combinations thereof and the like. Furthermore, surface adhesives can be added if required, e.g. based on acrylates, styrene acrylates, polyurethanes or mixed polymers of acrylates and polyurethanes.

According to a further particularly preferred embodiment, the paper structure according to the invention has a basis weight according to DIN EN ISO 536 which is in a range from 15 g/m ² to 1000 g/m ² , preferably in a range from 20 g/m ² to 300 g/m ² .

The paper structure according to the invention further has, for example, a power consumption which is in the range from 50 W/m ² to 5000 W/m ² . In this case, a temperature in the range from 15 °C to 180 °C can be achieved on the surface of the paper structure.

The electrically conductive paper structure according to the invention can be provided with additional reinforcing fibers to control the desired properties. In addition, surface sizing or surface impregnation is possible.

Different designs are conceivable with regard to the current conducting rails, in particular (contact) threads. It is important in all designs that contact is ensured with the conductive components in the paper structure, in particular metallic fibers and/or carbon fibers. In the simplest case, a metallic thread, e.g. made of rolled metal, a metal strip or a metal wire can be used, the metal being selected in particular from a highly conductive metal such as silver, copper, gold, aluminum, tungsten, iron or the like or an alloy of one or more of the aforementioned elements. Furthermore, the use of a metallized thread is also possible, e.g. the use of a thread based on a plastic carrier film as a carrier substrate and metallized with a highly conductive metal such as silver, copper, gold, aluminum, tungsten, iron or the like. Polyethylene terephthalate (PET) in particular can be used as the plastic carrier film. Furthermore, a metallized film or a laminate made of films and rolled metal films can be used as the (contact) thread. A particularly reliable contact using crimp contacts or ZIF connectors (ZIF = Zero Insertion Force) is possible with purely metallic threads that are designed as a metal strip, e.g. with a width in the range of 2 to 5 mm. An increased metal thickness improves the permanent contact. Metallized threads of this type can also be provided with an adhesive, at least on one side, to better fix the contact thread in the electrically conductive paper structure. This adhesive is preferably a conductive adhesive. It is also possible for the (contact) thread embedded in the electrically conductive paper structure to be partially exposed in the area of the contact point, as is the case with so-called window threads in banknotes.

Furthermore, the (contact) threads can additionally be provided with a protective layer or protective film on the upper side, which can be removed as required in the contact area, i.e. in the window area.

The term (contact) thread is not necessarily limited to the sole design as a (rather narrow) thread, which for example has a width of 2 mm or less, but designs such as (rather wide) strips or bands are also conceivable, which for example have a width of 4 mm to 20 mm, or even a width of up to 30 mm. In principle, it is also conceivable that a simple conductive metal wire or a metal braid is used as the (contact) thread. Design variants such as flat strands, braids, knitted fabrics, lamella tapes and the like are also possible. The thickness of the (contact) thread can be selected, for example, in a range from 10 to 300 µm, preferably in a range from 10 to 200 µm, more preferably in a range from 10 to 100 µm and particularly preferably in a range from 10 to 50 µm.

The electrically conductive paper structure according to the invention can additionally be printed with a conductive pattern of conductor tracks in order to reduce the distances between two (or a plurality greater than two) contact threads serving as electrodes. The printing, ie the provision of the printed conductor tracks, can be carried out, for example, by means of a screen printing process. As conductive paints that produce the conductive pattern, aqueous screen printing inks based on soot particles, silver particles or other particles that produce conductivity can be used, for example. The conductive pattern of printed conductor tracks is expediently produced in such a pattern that the distances between two of the two electrodes are approximately similar in all areas. is. According to a preferred variant, the conductive pattern can be contacted with the contact threads embedded in the substrate in such a way that the contact is made via the points (so-called thread windows) where the contact threads are partially exposed. The measure of additional printing with a conductive pattern means that the product can be operated as a conductive surface element with relatively low voltages. Due to the reduced distance between two electrodes, a current required for the respective application, e.g. a heater, can be achieved with a smaller number of conductive fibers in the substrate. Alternatively, with the same number of conductive fibers in the substrate, a desired current can be achieved with a lower voltage. By suitably adapting the dimension of the conductive fibers and the printed, conductive pattern, the percolation threshold can even be reduced. The latter describes the minimum proportion of conductive fibers that are necessary to achieve a continuously conductive fiber network between two contact threads serving as electrodes and thus a relevant current flow. This results in cost savings with regard to the expensive conductive fibers. A lower operating voltage also reduces the cost, e.g. for the control electronics, and increases work safety.

The electrically conductive paper structure can be connected to a power source in particular by means of adhesive contacts or so-called crimp contacts.

The electrically conductive paper structure can in particular be a single-layer paper structure. In a special embodiment, the electrically conductive paper structure is a multi-layer paper structure, which is in particular of lamination. If necessary, the current conducting rails can be present between the individual layers of the multi-layer paper structure.

• das Bereitstellen einer Stoffsuspension aus cellulosehaltigem Faserstoff und Wasser in einer Papiermaschine, insbesondere eine Rundsiebpapiermaschine oder eine Langsiebpapiermaschine;
• gegebenenfalls das Zugeben wenigstens eines chemischen Additivs;
• das Zugeben elektrisch leitfähiger Fasern; und
• das Bilden einer Papierbahn.

The present invention further comprises a method for producing an electrically conductive paper structure, comprising:

• providing a stock suspension of cellulosic fiber and water in a paper machine, in particular a cylinder wire paper machine or a fourdrinier wire paper machine;
• optionally adding at least one chemical additive;
• adding electrically conductive fibers; and
• the formation of a paper web.

According to a preferred embodiment, the step of providing a stock suspension of cellulose-containing fiber material and water takes place in a cylinder-forming paper machine, wherein the step of forming a paper web comprises in particular the introduction of two continuous, electrically conductive threads into the stock suspension located in the cylinder-forming paper machine, wherein the threads are guided to the cylinder-forming machine in such a way that during the formation of the paper web the threads are embedded in the fiber structure in order to produce current-conducting rails in this way.

The round screen technology is known, for example, from the production of banknote paper, see for example the EP 0 279 880 A1 and the EP 0 492 407 A1 . In the case of round screen machines, the thread is introduced into the pulp and brought to the screen in such a way that the thread is embedded in the fibre structure during sheet formation. The thread can be completely embedded in the paper structure so that the thread is not visible to the observer from either the front or the back. The thread can but also embedded in such a way that it is freely accessible on one side after embedding in the paper structure. This is possible, for example, by mechanically removing, in particular by suction, the paper layer deposited on one side of the thread. The formation of freely accessible areas on at least one side of the embedded thread can also be achieved by choosing the width of the thread sufficiently high, see, for example, the EP 0 625 431 A1 . Furthermore, the thread can be embedded in the paper structure in such a way that the thread is exposed at least at one point on the surface of the paper structure in order to form a so-called window thread. The production of window security threads is known in the field of banknote paper production, see for example the EP 0 059 056 A1 The thread is guided outside the pulp to the paper screen in such a way that the thread comes to rest on raised areas (or bumps) on the paper screen. At the points where the thread rests on the bumps, no paper can form on the side facing the screen, so that the thread is freely accessible at precisely these points in the later finished paper.

According to a preferred embodiment, the paper structure is formed in such a way that it is composed of two separate paper layers and the electrically conductive threads are arranged between these paper layers. Such a production is known from the production of banknote paper with embedded security thread, see e.g. EP 0 229 645 A1 .

The present invention further comprises a use of the electrically conductive paper structure as a heating element, as an element for electromagnetic shielding or as an element for signal detection. The use of the electrically conductive paper structure as a heating element is particularly Suitable for use in floors, walls, wallpaper, containers, fabrics, clothing, table tops, heating plates, heating mats, car interior heaters, especially door, seat or dashboard heaters.

Further embodiments and advantages of the invention are explained below with reference to the figures, in which a true-to-scale and true-to-proportion reproduction was omitted in order to increase clarity.

Figuren 1-3

ein elektrisch leitendes Papiergefüge gemäß einem Vergleichsbeispiel;

Figuren 4-6

ein erstes Ausführungsbeispiel eines erfindungsgemäßen, elektrisch leitenden Papiergefüges;

Figur 7

ein zweites Ausführungsbeispiel eines erfindungsgemäßen, elektrisch leitenden Papiergefüges; und

Figur 8

ein drittes Ausführungsbeispiel eines erfindungsgemäßen, elektrisch leitenden Papiergefüges.

Show it:

Figures 1-3

an electrically conductive paper structure according to a comparative example;

Figures 4-6

a first embodiment of an electrically conductive paper structure according to the invention;

Figure 7

a second embodiment of an electrically conductive paper structure according to the invention; and

Figure 8

a third embodiment of an electrically conductive paper structure according to the invention.

Fig.1 shows a top view of a comparative example of an electrically conductive paper structure 1 with two current conducting rails 3, in particular contact threads, embedded in the paper substrate 2 and suitable for introducing electrical current. The two current conducting rails 3 are arranged in parallel, ie one current conducting rail 3 is located at one end of the paper substrate 2 and the another current conducting rail 3 at the opposite end of the paper substrate 2. The block arrow "A" illustrates the so-called first direction of the electrically conductive paper structure 1. The longitudinal axis of each of the two parallel current conducting rails 3 runs along the so-called second direction (see block arrow "B") of the paper structure, which is arranged perpendicular to the first direction "A" of the paper structure 1. The electrically conductive paper structure 1 is based on a paper substrate 2, which has a mixture containing paper fiber and electrically conductive short-cut fibers, in particular carbon fibers. The current conducting rails 3 were produced by means of a round-wire paper machine, as is particularly known in the Figure 3 the EP 0 279 880 A1 shown, embedded in the paper substrate 2. Based on the Figure 1 It can be seen that the current conducting rails 3 pass through the electrically conductive paper structure 1 from one end to the opposite end.

The Figure 2 schematically illustrates the electrically conductive paper structure 1 of the Figure 1 in cross-sectional view, ie along the Figure 1 shown line C-C'. The double arrow "d" illustrates the layer thickness of the paper substrate 2. The Figure 2 The dashed area 4 is shown in the Figure 3 shown enlarged. From the Figure 3 It can be seen that the paper substrate 2 has a constant layer thickness d.

Basically, the electrical resistance R ₀ depends on the width and length of the heating surface as well as on the surface conductivity of the paper 2 between the current conducting rails 3.

The Fig.4 shows in plan view a first embodiment of an electrically conductive paper structure 5 according to the invention with two embedded in the paper substrate 6, suitable for the introduction of electrical current Current conducting rails 7, in particular contact threads. The two current conducting rails 7 are arranged in parallel, i.e. one current conducting rail 7 is located at one end of the paper substrate 6 and the other current conducting rail 7 at the opposite end of the paper substrate 6. The block arrow "A" illustrates the so-called first direction of the electrically conductive paper structure 5. The longitudinal axis of each of the two parallel current conducting rails 7 runs along the so-called second direction (see block arrow "B") of the paper structure, which is arranged perpendicular to the first direction "A" of the paper structure 5. The electrically conductive paper structure 5 is based on a paper substrate 6 which has a mixture containing paper fiber and electrically conductive short-cut fibers. The current conducting rails 7 were produced by means of a round-wire paper machine, as is used in particular in the Figure 3 the EP 0 279 880 A1 shown, embedded in the paper substrate 6. Based on the Figure 4 it can be seen that the current conducting rails 7 run through the electrically conductive paper structure 5 from one end to the opposite end. In the area of the two current conducting rails 7, the respective surface area of the paper substrate 6 has a constant layer thickness. In the hatched surface area outside the two current conducting rails 7, the paper substrate 6 contains alternating locations with a higher layer thickness and locations with a lower layer thickness. The alternating locations with a higher layer thickness of the paper and locations with a lower layer thickness of the paper each have the shape of strips that are alternately arranged in the direction of the block arrow "A". The longitudinal axis of the individual strips extends along the direction of the block arrow "B".

The Figure 5 schematically illustrates the electrically conductive paper structure 5 according to the invention of Figure 4 in cross-sectional view, ie along the Figure 4 shown line D-D'. The double arrow "d" illustrates the Layer thickness of the paper substrate 6. The Figure 5 The dashed area 8 is shown in the Figure 6 shown enlarged. From the Figure 6 It can be seen that by means of the technology for producing watermarks, locations 9 with a higher layer thickness and locations 10 with a lower layer thickness can be produced in the paper 6. In the example according to the invention according to Figures 4 to 6, the locations 9 with a higher layer thickness and the locations 10 with a lower layer thickness each have the shape of stripes.

By locally varying the layer thickness d parallel to the busbars 7, the electrical resistance increases to the value R _neu . For example, if any number of thinner and thicker strips with a constant width are created, a short calculation yields the formula (1-1) shown below: $$R_{\mathit{new}}=\frac{{\mathrm{<figure-callout\; id="1"\; label="R\; O"\; filenames="imgf0001.png"\; state="\{\{state\}\}">R</figure-callout>}}_O}{1-{\left(\raisebox{1ex}{$\mathit{<figure-callout\; id="2"\; label="\Delta d\; d"\; filenames="imgf0001.png"\; state="\{\{state\}\}">\Delta d</figure-callout>}$}\!\left/ \!\raisebox{-1ex}{$d$}\right.\right)}^2}$$

The local "redistribution" of the paper can thus increase the electrical resistance.

If you combine the first surface areas in the paper that have a constant layer thickness (see e.g. the Fig.3 ), with second surface areas that have alternating areas with higher layer thickness and areas with lower layer thickness (see e.g. the Fig.6 ), then an electrically conductive paper structure results in which, when supplied with electrical current, the first, smooth, unstructured surface areas heat up more than the structured, second surface areas of the paper structure. In other words, the second surface areas of the paper structure remain somewhat cooler than the first surface areas during heating.

The Fig.7 illustrates in plan view a second embodiment of an electrically conductive paper structure 11 according to the invention with two current conducting rails 13, in particular contact threads, embedded in the paper substrate 12 and suitable for introducing electrical current. First surface areas 15, which have a constant layer thickness (see e.g. the Fig.3 ), with second surface areas 14, which have alternately arranged areas with higher layer thickness and areas with lower layer thickness (see e.g. the Fig.6 ), combined. The second surface areas 14 of the paper structure remain somewhat cooler than the first surface areas 15 during heating. In the example according to the Figure 7 the first surface areas 15 and the second surface areas 14 are each linear, resulting in a one-dimensional alternating sequence of the surface areas.

The Fig.8 illustrates in plan view a third embodiment of an electrically conductive paper structure 16 according to the invention with two current conducting rails 18, in particular contact threads, embedded in the paper substrate 17 and suitable for introducing electrical current. First surface areas 19, which have a constant layer thickness (see e.g. the Fig.3 ), with second surface areas 20, which have alternately arranged areas with higher layer thickness and areas with lower layer thickness (see e.g. the Fig.6 ), combined. The second surface areas 20 of the paper structure remain somewhat cooler during heating than the first surface areas 19. In the example according to the Figure 8 the first surface areas 19 and the second surface areas 20 together form a mosaic, in particular a pattern according to a chessboard, so that a two-dimensional alternating sequence of the surface areas results.

According to the above embodiments, the heating power can be focused on the areas in which high heat generation is particularly desired. Reduced heating in the other areas increases energy efficiency.

In a further embodiment, which is not shown in the drawing, the current guide rails (see reference number 3 in the Figure 2 and reference number 7 in the Figure 5 ) are embedded in the paper structure in such a way that they are freely accessible on a main surface of the paper structure, e.g. in analogy to the Figures 4 and 5 the WO 2020/224800 A1 .

In a further embodiment, which is not shown in the drawing, the current guide rails (see reference number 3 in the Figure 2 and reference number 7 in the Figure 5 ) completely embedded in the paper structure, e.g. in analogy to the Figures 2 and 3 the WO 2020/224800 A1 If necessary, the embedded current guide rails can be partially exposed, e.g. in analogy to the Figure 6 or the Figure 7 the WO 2020/224800 A1 .

In a further embodiment, which is not shown in the drawing, the current conducting rails are created by copper strips subsequently glued to the paper structure.

In a further embodiment, which is not shown in the drawing, the current guide rails are based on conductive tracks (e.g. Zn tracks), which are generated, for example, by a plasma process.

Claims (13)

Electrically conductive paper structure with cellulose-containing fiber materials and electrically conductive fibers, wherein the paper structure includes at least a first surface region and at least a second surface region, wherein the at least one first surface region of the paper structure has a substantially constant layer thickness and the at least one second surface region of the paper structure has alternately arranged locations with a higher layer thickness and locations with a lower layer thickness, so that the at least one second surface region of the paper structure has a higher electrical resistance compared to the at least one first surface region of the paper structure. Electrically conductive paper structure according to claim 1, wherein the at least one first surface region of the paper structure heats up more strongly than the at least one second surface region of the paper structure when the paper structure is supplied with electrical current. Electrically conductive paper structure according to claim 1 or 2, wherein the paper structure has two current conducting rails suitable for introducing electrical current into the paper structure, wherein the two current conducting rails are arranged in parallel, i.e. one current conducting rail at one end of the paper structure and the further current conducting rail at the opposite end of the paper structure. Electrically conductive paper structure according to one of claims 1 to 3, wherein the alternately arranged locations with a higher layer thickness of the paper and locations with a lower layer thickness of the paper of the at least one second surface area each have the shape of strips, wherein the strips are arranged alternately in a so-called first direction of the paper structure which is perpendicular to the longitudinal axis of the respective strip. Electrically conductive paper structure according to claim 4 with reference to claim 3, wherein the longitudinal axis of each of the two parallel current conducting rails runs along a so-called second direction of the paper structure arranged perpendicular to the first direction of the paper structure. An electrically conductive paper structure according to any one of claims 1 to 5, wherein the paper structure includes a plurality of first surface regions and a plurality of second surface regions. Electrically conductive paper structure according to one of claims 1 to 6, wherein with respect to the alternately arranged locations with higher layer thickness and locations with lower layer thickness in at least one second surface area of the paper structure, the percentage layer thickness ratio of the lower layer thickness based on the higher layer thickness is in a range from 0% to 90%, preferably in a range from 30% to 90%, particularly preferably in a range from 40% to 80%. Electrically conductive paper structure according to claim 3 or one of claims 4 to 7 referring back to claim 3, wherein the two current conducting rails are each based on a continuous, electrically conductive thread for contacting the electrically conductive paper structure, wherein the thread is embedded from one end to the opposite end of the paper structure and in particular is a metal wire, a metal thread, a metal strip, a metal strip based on a carrier substrate such as a plastic film, with a metal-coated thread, a laminate of (plastic) foils and metal foils, a metal braid, a plaited braid, a knitted fabric or a lamella tape. Electrically conductive paper structure according to one of claims 1 to 8, wherein the alternately arranged locations with a higher layer thickness of the paper and locations with a lower layer thickness of the paper in at least one second surface area of the paper structure are obtainable by means of a partial removal of fiber material, in particular by means of milling or by means of a round-wire paper machine or by means of a fourdrinier paper machine or by means of a watermark dandy roll or by means of an embossing cylinder or by means of a moulette cylinder. A method for producing an electrically conductive paper structure according to any one of claims 1 to 8, comprising: - providing a suspension of cellulosic pulp and water in a paper machine; - optionally adding at least one chemical additive; - adding electrically conductive fibres; and - forming a paper web. A method according to claim 10, wherein the step of providing a stock suspension of cellulosic fiber and water takes place in a cylinder wire paper machine and the step of forming a paper web comprises introducing two continuous, electrically conductive threads into the stock suspension in the cylinder wire paper machine, wherein the threads are fed to the cylinder wire in such a way that during the formation of the paper web the threads are embedded in the fiber structure in order to produce current-conducting rails in this way. A method according to claim 10 or 11, wherein the paper structure is formed such that it is composed of two separate paper layers and the electrically conductive threads are arranged between these paper layers. Use of the electrically conductive paper structure according to one of claims 1 to 9 as a heating element, as an element for electromagnetic shielding or as an element for signal detection.

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