PROCESS FOR MANUFACTURING A FUNCTIONAL FLEXIBLE CELLULOSIC SUBSTRATE, SETUP FOR IMPLEMENTING SAID PROCESS

Abstract

A process for manufacturing a flexible cellulosic substrate comprises at least one functional circuit and/or at least one functional board. The flexible cellulosic substrates are made functional by printing with a functional ink, which provides good performance (signal speed/dielectric properties of the substrate), is economical, thermally and dimensionally stable, and is able to be produced simply and reproducibly at an industrial rate. The process starts with an aqueous fibrous suspension comprising paper pulp and/or a pulp of (micro/macro) cellulose fibrils and produces a wet fibrous mat from this suspension. One of the faces of the wet fibrous mat is printed by means of at least one functional ink capable of transmitting, emitting, and/or processing at least one signal in order to produce at least one topography comprising at least one track for circulation of the signal. Printed circuits and functional boards are obtained by the manufacturing process.

Claims

1. Process for manufacturing a flexible cellulosic substrate which comprises at least one functional circuit and/or is akin to at least one functional board, this circuit and/or this board being capable of carrying, circulating, and/or processing a signal, in particular an electrical signal in the case of printed electrical circuits or circuit boards, characterized in that it essentially consists of: a) preparing or making use of an aqueous fibrous suspension comprising paper pulp and/or pulp of cellulose fibrils; b) producing a wet fibrous mat from this suspension; c) draining this wet fibrous mat; c) optionally, pressing this wet fibrous mat; d) printing one of the faces of the wet fibrous mat using at least one functional ink capable of transmitting, emitting, and/or processing at least one signal, in order to produce at least one topography comprising at least one track for circulation of the signal, optionally at least one component capable of acting on the signal; e) optionally, coating the printed face of the fibrous mat by means of at least one wet, preferably fibrous, layer; f) optionally at least partially eliminating the water contained in the fibrous mat optionally coated in accordance with step e); g) optionally, coating the printed face of the fibrous mat by means of at least one layer composed of inorganic pigments and of binders; h) optionally, printing one of the faces of the substrate capable of being obtained as a result of at least one of steps e) to g), by means of at least one functional ink capable of transmitting, emitting, and/or processing at least one signal, in order to produce at least one topography comprising at least one track for circulation of the signal, optionally at least one component capable of acting on the signal.

2. Process according to claim 1, wherein the suspension used in step a) has a dry matter concentration of between 0.1 and 1% by weight.

3. Process according to claim 1, wherein the printing according to step d) is carried out by depositing the functional ink on one of the faces of the fibrous mat, after disappearance of the surface film of water during the draining of step (c), this phenomenon occurring at an overall dryness of the fibrous mat that is greater than 1% by weight and less than or equal to 30% by weight, preferably 15% by weight.

4. Process according to claim 1, wherein it is integrated into a discontinuous manufacturing of paper involving at least one disperser, at least one filtration/draining column equipped with at least one filtration fabric, at least one calibrated cylinder for compression, and a device for drying the sheet under load.

5. Process according to claim 1, wherein the functional ink is chosen from inks composed of at least one low-polarity solvent slightly miscible in water, preferably from the group comprisingideally consisting of: colored inks, electrically conductive inks, thermally conductive inks, semiconductive inks, insulating inks, magnetic inks, dielectric inks, and mixtures thereof.

6. Process according to claim 1, wherein it is integrated into an industrial continuous manufacturing of paper making use of a papermaking machine comprising a headbox, a fourdrinier wirepreferably with a flat table, a press section, a dryer, and a reel.

7. Process according to claim 1, wherein the manufactured object is a circuit board comprising at least one printed circuit and at least one electronic component, the latter preferably being an interdigital capacitor or a sandwich capacitor.

8. Flexible cellulosic substrate capable of being obtained by the process according to claim 1, and which comprises at least one functional circuit and/or is akin to at least one functional board, single or multilayer, wherein it has a thickness between 100 and 500 m, preferably between 200 and 400 m.

9. Circuit board manufactured by the process according to claim 6, wherein it is composed of a grid of sandwich capacitors each formed at the intersection of electrically conductive tracks P1, P2 forming the grid.

10. Circuit board according to claim 8, wherein it constitutes a keyboard of an electronic device, preferably a computer, a digital tablet, or a smartphone.

11. Setup for implementing the process according to claim 1, comprising: I. optionally at least one device for preparing and refining paper pulp and/or pulp of (micro/macro) cellulose fibrils; in a discontinuous production mode: at least one system for manufacturing sheets of paper preferably of the type specified in standard ISO 5269 (Rapid-Kthen method); or in a continuous production mode: at least one papermaking machine comprising a headbox, a fourdrinier wirepreferably with a flat table, a press section, a dryer, and a reel; II. a contactless deposition/printing system, in particular by extrusion, spraying, or inkjetting; and/or a contact deposition/printing system, in particular by screen printing, preferably rotary screen printing, flexography, pad printing, gravure printing, or offset printing; at least one functional ink chosen from inks composed of at least one low-polarity solvent slightly miscible in water, preferably from the group comprisingideally consisting of: colored inks, electrically conductive inks, thermally conductive inks, semiconductive inks, insulating inks, magnetic inks, dielectric inks, and mixtures thereof.

Description

[0198] The description of these examples is made with reference to the attached figures in which:

[0199] FIG. 1a is a photograph showing the deposition of conductive carbon-based ink, using a modified Prusa i3 3D printer, on a wet fibrous mat in accordance with step b) of the process implemented in Example 1;

[0200] FIG. 1b is a photograph of part of the printed circuit encapsulated in wet paper after pressing and drying, respectively in accordance with steps (f.1) and (f.2) of the process implemented in Example 1;

[0201] FIG. 1c is a photograph of the printed circuit encapsulated in wet paper after rolling, produced in accordance with the process used in Example 1;

[0202] FIG. 1d is a photograph under an optical microscope at magnification 400 of a transverse section along section line D-D of FIG. 1b;

[0203] FIG. 1e is a photograph under an optical microscope at magnification 400 of a top view of the paper of FIG. 1b, encapsulating a printed circuit, showing the stripping by abrasion [step (i)] of a portion of the encapsulation layer in order to uncover an electrical contact;

[0204] FIG. 1f is a photograph of a printed and encapsulated circuit in accordance with the process according to the invention implemented in Example 1, in which the printed circuit is a straight electrically conductive track 3 mm wide, 44 mm long, and 0.209 mm thick;

[0205] FIG. 2 is a diagram showing an example setup for the preferred discontinuous mode of implementing the process according to the invention;

[0206] FIG. 3 is a diagram showing an example setup for a continuous mode of implementing the process according to the invention;

[0207] FIG. 4 is a diagram of an element of the circuit board comprising: a printed flat sensor formed by an interdigital capacitor and conductive tracks supplying power to an LED, and an LED component installed in this printed circuit, in accordance with Example 3;

[0208] FIG. 5 is a curve showing the capacitance (pF) of the interdigital capacitor of FIG. 4 as a function of time, before and after contact between a human finger and the portion of the paper encapsulation layer located immediately above the capacitor;

[0209] FIG. 6 is a photograph showing the touch of a human finger above the capacitor mentioned in the description of FIG. 5, said capacitor being part of a circuit board comprising several elements identical to the one referred to in FIG. 4;

[0210] FIG. 6 is a diagram of an element of the circuit board comprising a matrix of flat capacitors formed at the intersection of 44 conductive parallel tracks printed and encapsulated in accordance with Example 4 to form a touch keyboard;

[0211] FIG. 8 is a curve showing the capacitance (pF) of a flat capacitor of the matrix of FIG. 7 as a function of time, before and after a brief breath emitted by a person on the portion of the paper encapsulation layer located just above said capacitor;

[0212] FIG. 9 is a general photograph from above of the circuit board forming a touch keypad, manufactured in accordance with Example 4 of implementing the process according to the invention.

EXAMPLE 1

Manufacture of a Flexible Cellulosic Substrate Composed of a Circuit Formed by One or More Printed Conductive Tracks, on a Wet Fibrous Mat, Said Tracks Encapsulated by a Superimposed Wet Fibrous Layer, the Whole being Dried Under Load to Consolidate the Cohesion of the Double Layer. (FIG. 1b)

[0213] Discontinuous setup presented in FIG. 2 and supplemented by the system for preparing the aqueous cellulosic suspension and by a printing system.

[0214] Raw materials: cellulosic pulp (bleached hardwood fibers refined to 50 SR) and commercial conductive ink based on carbon in non-aqueous solvent.

[0215] Methodology: In the embodiment used for this example, the following operations were carried out: [0216] a) Suspension of cellulosic fibers created in a standard device (Lhomargy type) (according to standard ISO-5263-1: 2004) [0217] b) & c) Consolidated fibrous mat created using a machine in accordance with the preparation methods described in standard ISO-5269. This is the so-called German or Rapid Kthen method which, at the end of the draining phase, makes it possible to obtain a dryness of around 10-15% and a dry matter content of approx. 60 g/m.sup.2, [0218] c) Optionally the fibrous mat is pressed using a flexible roller of 3 kg (corresponding to a linear pressure of 15 kg/m). [0219] d) The wet sheet is then printed according to the following step (d).

[0220] The wet fibrous mat of step (c) is printed with conductive tracks using a volumetric dosing system (such as a syringe pump or a Moineau positive displacement micropump). [0221] e) Superimposition of a wet sheet previously prepared according to step (b). [0222] f) The double layer is dried by following the end of the ISO-5269 procedure. [0223] i) The contacts are exposed by localized abrasion of the surface cellulosic layer. In the case of this example, a conical grinding wheel on a silicon carbide rod (i.e. 4.8 mm Dremel wheel) was used.

[0224] Method and instruments used for characterization by microscopy of the obtained product. The developed sample is analyzed by optical microscopy (Dino-Lite type of instrument); the electrical resistance of the conductive track or tracks is measured using a Fluke 116 type of multimeter.

[0225] FIG. 1:

a) Deposition of commercial conductive ink based on carbon in non-aqueous solvent, with modified Prusa i3 3D printer, b) printed circuit encapsulated in wet paper after drying, c) rolled encapsulated circuit, d) lateral section of an encapsulated track, e) image of the track exposed by abrasion of the paper layer, f) conductivity measurement of a track (track width 3 mm, average thickness 0.209 mm, length 44 mm, resistance 170 Ohm, conductivity 410 S/m.

EXAMPLE 2: CIRCUIT BOARD COMPRISING PRINTED FLAT INTERDIGITAL CAPACITORS USEFUL AS TOUCH SENSORS

2.1 Without LED Controller

[0226] Paper, and more precisely cellulose, is a dielectric, i.e. an insulator, that is not electrically inert: that has, on the atomic scale, electrostatic dipoles which interact with external electromagnetic fields. The quantity that characterizes the dielectrics is dielectric permittivity, which describes the polarization of the material.

[0227] This example illustrates the use, in accordance with the invention, of paper as a dielectric in printed flat interdigital capacitors. More specifically, the porosity and the variation in wetness of the paper have the effect of varying the capacitance of these capacitors.

[0228] Thus, in this example, circular sheets of paper of 314 cm.sup.2 (called hand sheets) are manufactured which integrate touch sensors (switches) each associated with an LED indicator, using the printing/encapsulation process that is the object of the present invention.

Step a)

[0229] 1 liter of fibrous suspension of 2 g/l bleached kraft cellulose fibers (resinous) refined to 57 SR is used.

Step b)-Step c)

[0230] A circular wet fibrous mat of 314 cm.sup.2 (20 cm in diameter) was manufactured by filtration/draining (Rapid-Kthen method), to obtain a dryness of approximately 10-15% and a dry matter content of approx. 60 g/m.sup.2.

Step c)

[0231] The fibrous mat is then pressed using a 3 kg flexible roller (corresponding to a linear pressure of 15 kg/m).

Step d)

[0232] A commercial conductive ink based on carbon in non-aqueous solvent is deposited using a system of direct dosing by screw pump with a deposition nozzle having an internal diameter of 400 m (nozzle-fibrous mat distance approximately 300 to 500 m, deposition rate approximately 200 mm/min) As shown in FIG. 4, each printed capacitor 1 comprises a positive comb 2 and a negative comb 3. Each comb is formed by a stem 4.2 & 4.3 having perpendicular teeth 5.1 & 5.2 extending from its end in an interdigital manner, meaning nested between one another. The circuit comprises parallel printed connectors 6 and 7, connected to an LED 8.

[0233] The tracks 4.2, 4.3, 5.1, 5.2 forming the interdigital capacitor 1 and the connectors 6, 7 of the LED have a width of 1 mm and a thickness of 400 m (before drying).

C=S/d(1),Using equation (1)

where C is the capacitance of the capacitor, c the relative permittivity of the separator (paper), S the electrode cross-sectional area, and d the distance between the electrodes;

[0234] the capacitor 1 is sized to reach a capacitance of approx. 1 pF, which dictates a spacing of 500 m between the teeth 5.1 & 5.2 of the capacitor 1 and a cumulative length of the interdigital electrodes of 9 cm (FIG. 4).

Step e)

[0235] After the tracks have been deposited, the LED is positioned and a layer of wet fibrous mat (prepared according to steps a, b, c and c) is superimposed in order to encapsulate the printed circuits.

Step (f.1)

[0236] The mat/circuit/encapsulation layer assembly is pressed using a 3 kg flexible roller (corresponding to a linear pressure of 15 kg/m).

Step (f.2)

[0237] The pressed mat/circuit/encapsulation layer assembly is dried under load (approximately 0.5 to 2 bars) at 95 C. for 20 min (Rapid Kthen method, Franck type dryer, TAPPI T 205 standard practice).

[0238] Several flexible cellulosic substrate sheets are obtained, each comprising several capacitor 1/LED 8 assemblies.

Characterization

[0239] The interdigital capacitor thus produced shows a variation of more than 75% of the value of its initial capacitance (i.e. from 15 to 35 pF, FIG. 4), at the approach of a human finger (relative permittivity of approximately 60). This variation in the capacitance value makes it possible to very clearly detect the approach to and touching of the sensor by a human finger.

[0240] The application of a voltage of 5 V across the terminals of the connectors 6, 7 connected to the encapsulated LED 8 makes it possible to light the diode, which proves that the encapsulation process guarantees good electrical contact and makes it possible to keep the LED 8 in place without the need to use conductive adhesives or solder paste.

[0241] FIG. 5 shows that after touching the sensor, some time (approximately two seconds) is necessary for the evaporation/dispersion of the humidity in the fibrous structure of the paper in order to return to the initial state of the capacitor.

2.2 with LED Controller

[0242] Example 2.1 is reproduced while integrating, into each interdigital capacitor 1/LED 8 assembly, an external controller (Arduino MEGA 2560) allowing the supply of power to the LED 8 when the capacitance of the capacitor varies significantly (a detection threshold is fixed at around 25 pF, so that the small variations generated by external interference are not detected). The use of the LED 8 provides a simple way to illustrate that the sensor has been activated (see FIG. 6). It is important to note that this circuit is completely integrated into the paper.

EXAMPLE 3: CIRCUIT BOARD COMPRISING PRINTED FLAT SANDWICH CAPACITORS USEFUL AS BREATH SENSORSBREATH-SENSITIVE KEYBOARD OBTAINED USING THIS BOARD

3.1 Printed Flat Sandwich Capacitors Useful as Breath Sensors

[0243] This example concerns the manufacturing of a circuit board according to the invention, the board comprising breath-sensitive sensors, where the goal is to create keyboard keys which are therefore activated by localized blowing through a straw. This application most particularly concerns quadriplegic users.

[0244] The printed capacitors in this example are sandwich sensors in a matrix. The circuit board 10 shown in FIGS. 7 and 9 comprises a printed circuit formed by four parallel conductive tracks 11, each connected by one of their ends to an Arduino MEGA 2650 controller 12 which allows detecting variations in capacitance, and by four parallel conductive tracks 13, perpendicular to tracks 11 and capable of each being connected by one of their ends to one of the poles of a generator not shown in FIG. 7 and FIG. 8.

Methodology:

Step a)

[0245] 1 liter of fibrous suspension of 2 g/I bleached kraft cellulose fibers (resinous) refined to 57 SR is used.

Step b)-Step c)

[0246] A circular wet fibrous mat of 314 cm.sup.2 (20 cm in diameter) was manufactured by filtration (Rapid-Kthen method), to obtain a dryness of approximately 10-15% and a dry matter content of approx. 60 g/m.sup.2.

Step c

[0247] The fibrous mat is then pressed using a 3 kg flexible roller (corresponding to a linear pressure of 15 kg/m).

Step d

[0248] The parallel tracks 11 are printed on the wet fibrous mat using a solvent-based (non-aqueous) carbon-based conductive ink and a system of direct dosing by screw pump with a deposition nozzle having an internal diameter of 400 m (nozzle-fibrous mat distance approximately 300 to 500 m, deposition rate approximately 200 mm/min).

[0249] The printing conditions are summarized in the following table:

TABLE-US-00002 Dimensions Number of capacitors 16 Width of electrodes 4 mm Thickness of deposited ink (not dried) 500 m Printing Parameters Ink Carbon-based solvent Paper substrate Resinous fiber Refining 57 SR Number of layers 2 Printing speed 300 mm/min

[0250] The printed tracks are covered with a superimposed fibrous encapsulation layer (prepared according to steps a, b, and c), parallel tracks 13 then being printed on the free face thereof.

Step e

[0251] A superimposed fibrous encapsulation layer, prepared according to steps a, b, c and c, is then placed and compacted on parallel tracks 13.

[0252] Tracks 11 and 13 have a width of 4 mm and a length of 100 mm.

[0253] The areas located at the intersections of the tracks 11 and 13 form sixteen capacitors 14. This matrix topography makes it possible to increase the density of the sensors while reducing the number of necessary connections to the controller 12 (i.e. for a grid of 16 capacitors, only four rows and four columns, or eight connections, to the controller are required).

[0254] Interdigital sensors require two connections per capacitor: 32 for a grid of 16 capacitors 14.

Step f

[0255] The multilayer structure was compacted by applying a linear pressure of 15 kg/m and dried under compression at 95 C. (Rapid-Kthen method).

Characterization

[0256] The Arduino MEGA 2560 controller measures the variations in capacitance in the capacitor and transfers the data to a spreadsheet enabling their display in FIG. 8. This figure shows a sudden increase in capacitance (from 3.8 to 6.2 pF) corresponding to the moment when the user blows through a straw on the key of the keyboard. As in Example 3.1, it takes approximately four seconds to return to the value of the initial capacitance. The use of sandwich capacitors is therefore very advantageous in that it allows breath detection. A sensitive keyboard has therefore been produced. As shown in Example 2.1 with flat interdigital capacitors, sandwich capacitors are also subject to a variation in their capacitance when a finger approaches, which allows them to be used for the production of sheets that are touch- and breath-sensitive.

3.2 Breath- (or Contact-) Sensitive Keyboard

[0257] The breath-sensitive keyboard 10 shown in FIG. 9 is almost identical to the circuit board 10 comprising the grid of sandwich capacitors integrated into the paper (encapsulation) of Example 3.1. Numeric keys (keypads), such as the ones located on the right side of standard computer keyboards, have been marked on the upper face of the circuit board. The sixteen keys thus correspond to the sixteen capacitors 14.

[0258] Four tracks 13 (horizontal electrodes) are successively supplied power and the output voltage of four tracks 11 (vertical electrodes) is measured to evaluate the variation in capacitance of each capacitor 14, and thus to detect the variations in humidity of the paper, generated by the user's breath (or the presence of a finger in contact with the paper). The capacitance variations in the sixteen capacitors 14 are detected by the controller 12 which is not shown in FIG. 9 but is present in FIG. 8 showing the board of Example 4. A threshold, making it possible to define a limit to the rate of variation in capacitance beyond which an event is detected, is implemented and integrated in a controller 12 which, by communicating with a computer via a USB connection, will write the symbol corresponding to the key activated by the breath, as a standard keyboard would do.