Method of production of a conductive layer for sensors and conductive layer for sensors obtained by said method

Abstract

The invention relates to a method of production of a conductive layer (3) for sensors (1), comprising the steps of: a) dissolving thermoplastic polyurethane in a solvent, b) adding graphite, c) processing the thus obtained mixture in a mixer, preferably in high shear mixer, even more preferably in a planetary high shear mixer, d) casting said homogeneous mixture onto one or more supports thereby making one or more samples and e) drying said one or more samples to remove the solvent. Each of said samples constitutes a conductive layer. Therefore, the method of production according to the invention comprises a limited number of steps compared to currently known methods. Furthermore, these steps are easy to implement and do not require complex and / or expensive processes. Thanks to this method of production, it is therefore possible to obtain a conductive layer (3) and, correspondingly, a sensor (1) in a cost-effective manner.

Description

Method of production of a conductive layer for sensors and conductive layer for sensors obtained by said method

[0001] The present invention relates to a method of production of a conductive layer for sensors.

[0002] The present invention further relates to a conductive layer for sensors that is obtained by said method.

[0003] By way of non-limiting example, the conductive layer of the present invention can be used inside a tactile sensor.

[0004] In particular, said tactile sensor can find application within the automotive field and can be installed within the cockpit of a vehicle.

[0005] As is known, the cockpit of a vehicle is the inner space in which the driver can monitor and control operation of the vehicle by means of several components, including steering wheel, gear lever, display, indicator panels (speed, fuel, etc.), controls and switches.

[0006] To enable interaction between driver and vehicle, conventional cockpits are often equipped with single-command buttons, the operation of which is based on mechanical components.

[0007] Due to the presence of such mechanical components, the buttons can be only positioned in some specific areas of the cockpit, this leading to remarkable limitations in the designing of the cockpit itself.

[0008] Technological development in recent years has also led to an increasing number of functions available to the driver and passengers and, consequently, to the introduction of an increasing number of buttons within the vehicle.

[0009] The need for a large number of buttons, combined with the use of mechanical components, resulted, from the standpoint of cockpit manufacture, in greater difficulties in the assembly of the buttons themselves and additional labor costs.

[0010] Furthermore, the combination of numerous buttons and mechanical components has caused several problems from the standpoint of driving safety.

[0011] Such combination, in fact, necessarily requires some buttons to be placed at points distant from the steering wheel of the vehicle, thereby forcing the driver, in the act of locating and reaching the most remote buttons, to take their eyes off the road, thereby increasing the likelihood of losing control of steering and the number of road accidents.

[0012] As if this were not enough, it must also be considered that, in the event of an accident, there is the risk that mechanical components, flying off their seats due to the impact, will hit the driver and passengers, thereby aggravating their condition.

[0013] In an attempt to overcome the limitations described above, tactile sensors have recently been introduced in the cockpit to control some of the vehicle functions, such as, for example, lowering the windows, controlling the interior temperature and steering wheel controls, such tactile sensors replacing, at least in part, mechanical buttons.

[0014] As is known, the operation of said tactile sensors is based on a conductive element, formed as a conductive layer suitable for detecting a stimulus, i.e. a command given by the driver or any other user.

[0015] In most cases, such stimulus takes the form of a force or pressure, applied to said conductive layer by the driver or the passengers, which force, or pressure causes a change in electrical resistance within the conductive layer.

[0016] This change in electrical resistance is detected by electronics, usually contained inside a further layer of the tactile sensor, said electronics controlling, based on such change, the function associated with the tactile sensor.

[0017] The conductive layer and the electronics-containing layer are often made of flexible materials.

[0018] This makes it possible to place said sensors at any point within the cockpit, allowing using said sensors as actual touch surfaces covering the cockpit.

[0019] As a result, said tactile sensors offer greater design freedom compared to buttons and allow a large number of functions to be concentrated close to the steering wheel, thereby reducing the risk of road accidents.

[0020] In addition, the structure of said tactile sensors, based on the conductive layer and the electronics-containing layer, makes it possible to simplify the installation of such devices within the cockpit and to reduce labor costs.

[0021] However, currently known and used tactile sensors have some limitations in terms of implementation costs.

[0022] It should in fact be considered that the conductive layer characterizing tactile sensors is often made of a composite, i.e. a polymer matrix to which one or more conductive fillers are added.

[0023] In order to manufacture said composite, numerous processing steps and expensive manufacturing techniques are required.

[0024] The process for producing said composite may include, for example, several steps of mixing said polymer matrix and / or said one or more conductive fillers, extruding filaments and / or overlaying several layers, consisting of the composite still in its fluid state, and a final step of curing by means of reagents.

[0025] The lead time and high cost of the process of manufacturing the composite inevitably has an impact on the cost of the final product, thereby limiting the application of said conductive layer and thus of said tactile sensors.

[0026] The complexity of the manufacturing process is also due to the fact that it is mandatory that the one or more conductive fillers are homogeneously dispersed in the polymer matrix.

[0027] The main object of the present invention is therefore to overcome the above-described limitations by providing a method of production of the conductive layer that is simple, fast and cost-effective.

[0028] A further object of the present invention is to provide a conductive layer that has a competitive cost and allows obtaining tactile sensors with high performance both in terms of detection of the command given by the user and in terms of easy installation within the cockpit.

[0029] These and other objects of the present invention are achieved with the method of production and the conductive layer as claimed in the appended claims.

[0030] According to the invention, the method of production of a conductive layer for sensors comprises the steps of:dissolving thermoplastic polyurethane (TPU) in a solvent, thereby obtaining a base mixture;adding graphite to said base mixture, thereby obtaining a heterogeneous mixture;processing said heterogeneous mixture in a mixer, until a homogeneous mixture is obtained;casting said homogeneous mixture on one or more supports so as to make one or more samples;drying, preferably in an oven, said one or more samples to remove said solvent, each of said one or more samples constituting a conductive layer.

[0031] According to a preferred embodiment of the invention, the solvent employed in step a) is a dipolar aprotic solvent, even more preferably dimethyl formamide (DMF).

[0032] Even more preferably, the weight ratio of TPU to DMF is between 1:1 and 1:10.

[0033] According to a further preferred embodiment of the invention, during step a), one or more polymers are introduced together with TPU, said one or more polymers being soluble in the solvent employed in this step.

[0034] The percentage by mass of TPU to said one or more polymers is preferably between 1 and 99 wt%, more preferably between 10 and 90 wt%.

[0035] According to a first preferred embodiment, said one or more polymers are added directly to the solvent, together with TPU, during phase a).

[0036] According to a second preferred embodiment, before being dissolved in the solvent during phase a), TPU and said one or more polymers are blended together (by melt mixing) in an extruder, optionally using one or more processing aids, extenders, plasticisers and / or thermal stabilizators.

[0037] Regardless of the embodiment, when the solvent employed in step a) is DMF, said one or more polymers used together with TPU are preferably polystyrene (PS), polyvinyl chloride (PVC), polylactic acid (PLA) and / or their derivations.

[0038] According to a further preferred embodiment of the invention, during step b), between 20 and 70 wt% graphite, even more preferably between 40 and 50 wt% graphite, is added to the base mixture.

[0039] Also, during step b), besides graphite, one or more fillers can be added to said base mixture.

[0040] Preferably, said one or more fillers are conductive fillers.

[0041] Even more preferably, said one or more conductive fillers are selected from carbon black (CB), graphene (GFN), carbon nanofibers (CNF), carbon nanotubes (CNT), derivatives thereof or a combination of at least two of said conductive fillers.

[0042] Preferably, the graphite (and said one or more fillers, if any) is added to the base mixture without having previously undergone any steps of mixing with solvents.

[0043] Alternatively, prior to being added to said base mixture, the graphite (and said one or more fillers, if any) can be introduced into a further solvent, which can be the same or different from the solvent employed in step a), and subjected to a step of mixing together with said further solvent.

[0044] According to a preferred embodiment of the invention, during step c) a high shear mixer is used.

[0045] Even more preferably a planetary high shear mixer (PHS) is used.

[0046] Planetary high shear mixers provide for simultaneous shear and rotational movement and offer several advantages for mixing polymer nanocomposites, namely: efficient dispersion of nanoparticles, high viscosity handling, shorter mixing time, scalability and homogeneous mixing of multi-phase systems.

[0047] Therefore, planetary high shear mixers allow for the homogeneous distribution of graphite, and other conductive fillers when present, in the base mixture.

[0048] The homogeneous distribution of graphite (and conductive fillers) in the base mixture - in particular when the base mixture includes TPU or other polymers - is essential to achieve uniform conductivity of the conductive layer and, consequently, uniform detection behavior of the sensor.

[0049] If graphite (and conductive fillers) is not well dispersed, the sensor's detection behavior may be inaccurate. According to a further preferred embodiment of the invention, in step d), said one or more supports on which the homogeneous mixture is cast are made as molds. The shape and size of said molds, which determine the shape and size of said one or more samples, vary according to the specific application for which the conductive layer obtained by said method is intended.

[0050] According to a further preferred embodiment, alternative or complementary to the previous embodiment, said method of production comprises, after step e), a further step of cutting said one or more samples by following shapes and sizes which, similarly to the previous embodiment, vary according to the specific application for which the conductive layer is intended.

[0051] It is clear that the method of production according to the invention provides for a limited number of steps compared to currently known and used methods.

[0052] Each of said steps is easy to implement and does not require complex and / or expensive processes.

[0053] In particular, the method of production of the conductive layer according to the invention requires neither overlaying several layers of mixtures, nor extruding a filament, let alone a process of curing the TPU (or any polymers added thereto).

[0054] Therefore, said method of production makes it possible to obtain a conductive layer, and thus a sensor, in a cost-effective manner.

[0055] In addition, by varying the percentage of TPU relative to the percentage of graphite – and possibly combining the TPU with said one or more polymers and the graphite with said one or more fillers – it is possible to accurately calibrate the flexibility, hardness and electrical conductivity of the conductive layer, thereby allowing adapting the features of the conductive layer, and thus of the sensor based on said conductive layer, to the specific applications required.

[0056] Indeed, according to another aspect of the present invention, there is provided a conductive layer made as a composite consisting of a base matrix and a filler material.

[0057] Said base matrix comprises TPU and, possibly, said one or more polymers.

[0058] Preferably, said conductive layer is obtained by the method described above.

[0059] Further advantages and features of the present invention will become apparent from the ensuing detailed description of preferred embodiments of the invention, given by way of non-limiting examples with reference to the annexed drawings, in which:Fig.1

[0060] shows an exploded view of a tactile sensor containing a conductive layer according to the invention;Fig.2

[0061] shows an exploded view of a tactile sensor containing a conductive layer according to a constructional variant of the invention;Fig.3

[0062] shows an exploded view of a tactile sensor containing a conductive layer according to a further constructional variant of the invention;Fig.4

[0063] shows the block diagram of the method of production of the conductive layer of

[0064] In the following description, reference will be made to a conductive layer applied to a tactile sensor, in particular to a tactile sensor used within the cockpit of a vehicle to allow interaction between a user – for example, the driver of the vehicle – and the electronics of the vehicle.

[0065] However, this embodiment should not be interpreted in a limiting sense, and the conductive layer according to the invention could be applied to other types of sensors.

[0066] A tactile sensor incorporating the invention is shown inand is indicated as a whole with reference numeral 1.

[0067] By acting onto said tactile sensor 1 by touching it, the user can control a vehicle function (or more vehicle functions) associated with said tactile sensor, such as, for example, the adjustment of air conditioning, of electric window lifters or electric side-view mirrors.

[0068] In a manner known per se, said tactile sensor 1 comprises a conductive layer 3, sensitive to the command given by the user to the tactile sensor, and a control layer 5, adjacent to the conductive layer 3 and comprising an electronics 5a suitable for detecting a change in the electrical features of the conductive layer 3.

[0069] Said conductive layer 3 is made of a flexible composite sensitive to the pressure or force applied by the user when they touch the surface of the tactile sensor 1.

[0070] In particular, said flexible composite is based on a piezoresistive mechanism, whereby the pressure or force applied by the user causes a change in electrical resistance of the conductive layer 3.

[0071] The electronics 5a detects said change in electrical resistance, thus recognizing the command given by the user, and activates the function associated with the tactile sensor 1.

[0072] Therefore, the control layer 5 with its electronics 5a acts as a probe.

[0073] Preferably, said control layer 5 and said electronics 5a constitute a printed circuit board, even more preferably a flexible printed circuit board.

[0074] The flexibility of the composite constituting the conductive layer 3 and of the printed circuit board constituting the control layer 5 and the electronics 5a allows adapting the tactile sensor 1 to the profiles and complex and irregular shapes present within the cockpit. Therefore, said tactile sensor can easily be placed, for example, on the door inner panels and on the central dashboard, as it can be used over the cockpit cover layer or as a substitute therefor.

[0075] Preferably, as can be seen in, the tactile sensor 1 further comprises a protection layer 7, acting as a cover and arranged to protect said tactile sensor 1 from shocks or moisture, and a fixing layer 9, arranged to fix the conductive layer 3 to the cockpit surface.

[0076] Of course, these two layers, too, can be made of flexible materials.

[0077] According to the embodiment shown in, the control layer 5 is interposed between the protection layer 7 and the conductive layer 3, which is situated between the control layer 5 and the fixing layer 9.

[0078] In an alternative embodiment, however, said conductive layer 3 could be interposed between the protection layer 7 and the control layer 5, and the control layer 5 could be interposed between the conductive layer 3 and the fixing layer 9.

[0079] Furthermore, the tactile sensor 1 could comprise multiple conductive layers, control layers, protection layers and fixing layers.

[0080] The tactile sensor 1 could further be coupled to a lighting system, preferably a LED lighting system, even more preferably an RGB LED lighting system, suitable for signaling the function associated with the tactile sensor, the state (active, inactive) of said function and / or the adjustment level, for example for controlling the inner temperature.

[0081] In more detail, the flexible conductive material constituting the conductive layer 3 is a composite consisting of a base matrix enriched with a filler material.

[0082] According to the invention, said base matrix comprises thermoplastic polyurethane (TPU) and said filler material comprises graphite.

[0083] The composition of TPU and graphite has a percentage by mass that is preferably between 20 and 70 wt%, even more preferably between 40 and 50 wt%.

[0084] Said base matrix can comprise one or more polymers besides TPU, for example polystyrene (PS), polyvinyl chloride (PVC), polylactic acid (PLA) and / or their derivations.

[0085] The percentage by mass of TPU to said one or more polymers is preferably between 1 and 99 wt%, more preferably between 10 and 90 wt%.

[0086] The introduction of said one or more polymers can be used, advantageously, to adjust the flexibility and rigidity of the conductive layer 3, according to the specific applications of the tactile sensor 1 within the vehicle cockpit, and to reduce the costs of the conductive layer according to the type and quantities of the polymers combined with TPU.

[0087] Furthermore, said filler material can comprise one or more fillers besides graphite.

[0088] Preferably, said one or more fillers are conductive fillers.

[0089] Even more preferably, said one or more conductive fillers can be selected from carbon black (CB), graphene (GFN), carbon nanofibers (CNF), carbon nanotubes (CNT), derivatives thereof or a combination of at least two of these fillers.

[0090] The combinations of said one or more conductive fillers with graphite (GFT) and the base matrix with TPU can generate, for example, one of the following hybrid composites:GFT / CB / TPU;GFT / CB / GFN / TPU;GFT / CB / CNF / TPU;GFT / CB / CNT / TPU;GFT / GFN / TPU;GFT / GFN / CNF / TPU;GFT / GFN / CNT / TPU;GFT / CNF / TPU;GFT / CNF / CNT / TPU;GFT / CNT / TPU;GFT / CB / CNF / CNT / GFN / TPU.

[0091] The combination of TPU and graphite advantageously allows obtaining a flexible conductive material, and thus a lightweight, stable conductive layer 3.

[0092] Advantageously, said conductive layer 3 can be obtained by means of a fast, simple and low-cost process.

[0093] Indeed, according to a further aspect of the present invention, there is provided a method of production of said conductive layer 3.

[0094] Said method of production, the block diagram of which is shown in, comprises the steps of:dissolving TPU (and said one or more polymers, if any) in a solvent, thereby obtaining a base mixture;adding graphite (and said one or more polymers, if any) to said base mixture, thereby obtaining a heterogeneous mixture;processing said heterogeneous mixture in a high shear mixer, preferably in a planetary high shear mixer until a homogeneous mixture is obtained;casting said homogeneous mixture on one or more supports so as to make one or more samples;drying, preferably in an oven, said one or more samples to remove the solvent employed in step a).

[0095] At the end of step e), each of said one or more dried samples constitutes a conductive layer.

[0096] According to a preferred embodiment of the invention, the solvent employed in step a) is a dipolar aprotic solvent, even more preferably dimethyl formamide (DMF).

[0097] Preferably, the weight ratio of TPU to DMF is between 1:1 and 1:10.

[0098] Also, when, besides TPU, said or more polymers are added, these polymers, too, are soluble in the solvent.

[0099] According to a first preferred embodiment, said one or more polymers are added directly to the solvent, together with TPU, during phase a).

[0100] According to a second preferred embodiment, before being dissolved in the solvent during phase a), TPU and said one or more polymers are blended together (by melt mixing) in an extruder, optionally using one or more processing aids, extenders, plasticisers and / or thermal stabilizators.

[0101] Regardless of the embodiment, when the employed solvent is DMF, said one or more polymers are preferably polystyrene (PS),polyvinyl chloride (PVC), polylactic acid (PLA) and / or their derivations.

[0102] According to a further preferred embodiment of the invention, 20 to 70 wt% graphite, even more preferably 40 to 50 wt% graphite, is added to the base mixture.

[0103] Furthermore, graphite (and said one or more fillers, if any) can be added to the base mixture without previously undergoing any steps of mixing with solvents.

[0104] Alternatively, prior to being added to said base mixture, the graphite (and said one or more fillers, if any) can be introduced into a further solvent, which can be the same or different from the solvent employed in step a), and subjected to a step of mixing together with said further solvent.

[0105] Preferably, when the solvent employed in step a) is DMF, then said further solvent is also DMF.

[0106] According to a further preferred embodiment of the invention, during step c), the heterogeneous mixture is processed in a high shear mixer, preferably a planetary high shear mixer (PHS) capable of mixing at high revolutions per minute the graphite (and said one or more fillers, if any) introduced in the mixture.

[0107] The use of such mixer is particularly suitable when said one or more conductive fillers are added to the graphite, because conventional mixers can hardly achieve homogeneous dispersion of different fillers, especially when fillers are provided as nanoparticles.

[0108] During step c), said heterogeneous mixture is processed by the high shear mixer over a period of time between 30 seconds and 3 minutes, preferably between 50 seconds and 130 seconds, even more preferably for 90 seconds.

[0109] Of course, said heterogeneous mixture can undergo several processing cycles inside the mixer, although a single processing cycle is sufficient to achieve a sufficient level of dispersion of the graphite (and said one or more fillers, if any) within the base mixture.

[0110] A high shear mixer can also be used to mix the graphite (and said one or more fillers, if any) in said further solvent when such a step is provided.

[0111] According to a further preferred embodiment of the invention, in step d), said one or more supports on which the homogeneous mixture is cast are made as molds. The shape and size of said molds, which determine the shape and size of said one or more samples, vary according to the specific application of the tactile sensor within the vehicle cockpit.

[0112] According to a further preferred embodiment, alternative or complementary to the previous embodiment, said method of production comprises, after step e), a further step of cutting said one or more samples, thereby allowing obtaining, again, samples of various shapes and sizes.

[0113] In particular, for applications providing for a single touch or double touch on the tactile sensor, the shape of the conductive layer can be circular like the one shown in

[0114] Alternatively, for applications providing for combined touch-sliding of the finger on the tactile sensor, the shape of the conductive layer can be, for example, a rectangular strip as shown inor a square layer as shown in

[0115] Of course, the conductive layer according to the invention could be made with any other shape including but not limited to circles, ellipses, squares, rectangles, triangles and so on.

[0116] It will be evident to the person skilled in the art that the embodiment described in detail above is in no way to be interpreted in a limiting sense and that numerous modifications and variants are possible without thereby departing from the scope of protection as defined by the appended claims.

Claims

Method of production of a conductive layer (3) for sensors (1), comprising the steps of:dissolving thermoplastic polyurethane in a solvent, thereby obtaining a base mixture;adding graphite to said base mixture, thereby obtaining a heterogeneous mixture;processing said heterogeneous mixture in a mixer until a homogeneous mixture is obtained;casting said homogeneous mixture on one or more supports so as to make one or more samples;drying said one or more samples to remove said solvent, each of said one or more samples constituting a conductive layer.Method of production according to claim 1, wherein the solvent employed in step a) is dimethyl formamide.Method of production according to claim 2, wherein the weight ratio of thermoplastic polyurethane to dimethyl formamide is between 1:1 and 1:10.Method of production according to claim 1 or 2 or 3, wherein, during step a), one or more polymers are introduced together with the thermoplastic polyurethane, said one or more polymers being soluble in the solvent employed in said step a).Method of production according to claim 4, wherein, before being dissolved in the solvent during phase a), the thermoplastic polyurethane and said one or more polymers are blended together, by melt mixing, in an extruder.Method of production according to any one of the preceding claims, wherein, during step b), 20 to 70 wt% graphite is added to said base mixture.Method of production according to claim 6, wherein, during step b), 40 to 50 wt% graphite is added to said base mixture.Method of production according to any one of the preceding claims, wherein, during step b), besides graphite, one or more conducting fillers are added selected from carbon black, graphene, carbon nanofibers, carbon nanotubes, derivatives thereof or a combination or at least two of said fillers.Method of production according to any one of the preceding claims, wherein, during step c), said heterogeneous mixture is processed in a high shear mixer.Method of production according to claim 9, wherein, during step c), said heterogeneous mixture is processed in a planetary high shear mixer.Method of production according to claim 9 or 10, wherein, during step c), said heterogeneous mixture is processed over a period of time between 30 seconds and 3 minutes, preferably between 50 seconds and 130 seconds, even more preferably for 90 seconds.Method of production according to any one of the preceding claims, wherein, in step d), said one or more supports are made as molds.Method of production according to any one of the preceding claims, wherein said method of production comprises, after step e), a further step of cutting said one or more samples.Method of production according to any one of the preceding claims, wherein, during step e), said one or more samples are dried in an oven.Conductive layer (3) for sensors (1), made of a composite consisting of a base matrix enriched with a filler material, characterized in that said base matrix comprises thermoplastic polyurethane, and in that said filler material comprises graphite.Conductive layer (3) according to claim 15, wherein the composition of TPU and graphite has a percentage by mass between 20 and 70 wt%, preferably between 40 and 50 wt%.Conductive layer (3) according to claim 15 or 16, wherein said conductive layer (3) is obtained by the method of production according to any one of claims 1 to 14.Sensor (1) comprising a conductive layer (3) according to any one of claims 15 to 17.

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