Polymer-based ink compositions for use in the production of piezoresistive films

Abstract

The present invention relates to polymer-based ink compositions comprising graphene nanoplatelets and graphite powder dispersed in an aqueous solution of a polymer matrix, the piezoresistive films obtained from the ink compositions, and methods for production thereof.

Description

[0001] POLYMER-BASED INK COMPOSITIONS FOR USE IN THE PRODUCTION OF

[0002] PIEZORESISTIVE FILMS

[0003] Technical Field of the Invention

[0004] The present invention relates to polymer-based ink compositions comprising graphene nanoplatelet and graphite powder reinforcement, production methods thereof, and usage areas.

[0005] Background of the Invention

[0006] Piezoresistive films are materials that experience a change in their electrical resistance when exposed to mechanical deformation or pressure. The change in resistance can be attributed to a change in the dimensions or atomic arrangement of the material under stress. When stress is applied to the material, the atomic structure of the material changes, which affects the communication networks of the electrons in the material, thereby resulting in a change in the resistance of the material. Due to such features, piezoresistive films are used as sensors in electronic assemblies in order to sense and measure physical interactions such as deformation, pressure and vibration.

[0007] Pressure sensors, force sensors, strain gauges, accelerometers, vibration sensors, touch sensors, wearable devices may be given as the main areas where piezoresistive films are used. When used as a strain gauge, a piezoresistive film is applied to a surface, and the change in the electrical resistance of the film is measured in order to measure the strain on the surface. The pressure sensitivity of the piezoresistive films makes them suitable for touch-sensitive interfaces. It can sense the pressure exerted by the touch of the user, enabling such features as pressure-sensitive drawing on touch screens. Piezoresistive films are also used in wearable devices to measure various physiological parameters such as respiratory rate, muscle contractions, and body movements.

[0008] Although the features expected from the piezoresistive films used in the art vary according to the intended use and applications, such features include high sensitivity, a broad range of measurement, a fast response time, linearity, a low noise level, and durability. Sensitivity is one of the most critical features expected from the piezoresistive films, which is the ability to respond with high sensitivity to the changes of mechanical stress or deformation. The ability to make precise measurements ensures the film to produce accurate results, and increases its reliability. Accordingly, fatigue resistance and accordingly multi-cycle sensitivity are indispensable features of the piezoresistive films, which indicate the ability of the film to reproduce the same conductivity value if the same loading or deformation is applied sequentially. That is, they demonstrate whether the measurement results of the film are consistent when the same load, stress or deformation is applied multiple times. Fatigue resistance is important in terms of the reliability and reproducibility of a sensor. If a piezoresistive film has a high fatigue resistance, no variation in the gauge factor is observed after the measurements repeated under the same conditions, thereby obtaining similar conductivity results. This emphasizes the stability, linearity and predictability of the sensor. In the case of a piezoresistive film with a low fatigue resistance, large deviations will be observed between the measurements.

[0009] Polymer-based piezoresistive films obtained from mixtures containing carbon and / or metal particles dispersed in the polymer matrix are known in the art. The sensitivity of these films is directly related to the quality and quantity of the reinforcement materials incorporated thereto.

[0010] For example, EP4O13829(A1) describes the production of a deformable conductor suitable for use as a piezoresistive sensor, and an ink composition prepared for the production thereof. It also discloses that carbon-based or metal-based conductive fillers are used in a polymer matrix in order to achieve conductivity in the final film to be obtained by using the ink composition. In this prior art document, it is claimed that, in addition to the conductive fillers, the addition of a polymerization initiator to the ink composition improves the electrical conductivity of the final film under stress. It proposes the use of transition metal ions such as nickel, palladium, copper, and silver as the conductive fillers. However, the intense metal content suggested in the said document limits the usage areas of the final film.

[0011] In a study published by Gongalves BF et al. in 2016 (doi: 10.1016 / j.compositesb.2016.12.047), polyvinyl alcohol-based piezoresistive sensors reinforced with carbon nanotubes were produced. In this context, the effects of different carbon nanotube proportions in the polymer matrix on the piezoresistance values of the final films produced were measured. 6% and 15% carbon nanotube proportions by weight were studied therein, wherein multi-cycle studies of the samples were presented. The graphs presented therein show that the produced films experience a loss of sensitivity even with successive applications of deformation up to 10 cycles, regardless of the proportion of the carbon nanotubes.

[0012] In another study carried out in the art, in an article published by P. Costa et al. in 2019 (https: / / pubs.acs.org / doi / 10.1021 / acsami.9bl9294), the production of piezoresistive films from SEBS matrices containing graphene oxide or graphene nanoplatelets is described. The analyzes regarding the measurement of their piezoresistance properties also show that the sensitivity of measurement decreases in multiple successive applications of stress / deformation. This study actually reveals that the use of graphene nanoplatelets or graphene oxide, alone, in the ink composition is insufficient to achieve the desired sensitivity.

[0013] In view of the above information, considering the piezoresistive films that already exist in the art and are used in sensor applications, it is apparent that there is still a need for piezoresistive films with reduced fatigue resistance and thus increased measurement sensitivity in multi-cycle applications.

[0014] Objects of the Invention

[0015] The main object of the present invention is to eliminate the above-mentioned shortcomings and disadvantages of the prior art.

[0016] Another object of the present invention is to provide polymer-based piezoresistive films with concurrent increased mechanical strength and electrical conductivity, thus having improved piezoresistance properties.

[0017] Another object of the present invention is to provide polymer-based piezoresistive films with increased fatigue resistance and therefore increased deformation sensitivity in multi-cycle applications.

[0018] Another object of the present invention is to provide polymer-based piezoresistive films in which variations in the gauge factor value are minimized, and thus can take precise measurements when used repeatedly.

[0019] Another object of the present invention is to provide polymer-based piezoresistive films with extended range of measurement and thus usage area. Another object of the present invention is to provide polymer-based piezoresistive films that show high linearity and low noise level performance in time-deformation analysis.

[0020] Another object of the present invention is to provide a polymer-based ink composition that facilitates the application of the polymer-based piezoresistive films of the invention on an application surface.

[0021] Another object of the present invention is to provide a fast and easy production method to obtain the polymer-based ink composition of the invention.

[0022] Still another object of the present invention is to provide an easy and reliable method for applying the polymer-based ink composition of the invention on the application surface.

[0023] Summary of the Invention

[0024] The present invention describes a polymer-based ink composition suitable for use in the production of piezoresistive films, comprising graphene nanoplatelets and graphite powder dispersed in an aqueous solution of a polyvinyl alcohol-containing polymer matrix. With the combined use of graphite powder and graphene nanoplatelets, the rheological properties of the ink composition are improved and therefore the application of the said composition in the form of a film on a surface is facilitated.

[0025] The present invention also describes a method for the production of the inventive ink composition. In the said method, the following steps are sequentially followed: (a) preparing a polymer-based aqueous solution by dissolving a polymer material in distilled water, (b) adding graphene nanoplatelets to the said aqueous solution, (c) performing the mixing process until a homogeneous mixture is obtained, (d) adding graphite powder to the aqueous solution containing graphene nanoplatelets, and (e) performing the mixing process until a homogeneous mixture is obtained.

[0026] The present invention also describes a polymer-based piezoresistive film obtained by applying the ink composition proposed by the invention onto an application surface using screen-printing and / or spraying techniques. Here, it is intended that graphene nanoplates initiate establishing electrical conduction lines within the film, while graphite powder forms a skeletal structure for the conduction lines. With the synergistic effect of graphite powder and graphene nanoplatelets in the film structure, it is observed that the film of the invention does not lose sensitivity in its piezoresistance property, even after being exposed to the same rates of stress / deformation (strain) at least 30 times successively. In other words, no change is observed in the gauge factor value of the piezoresistive film of the invention even after 30 cycles of application of stress / deformation, i.e., the film gives the same conductivity value after each stress / deformation application up to at least 30 cycles.

[0027] The present invention also describes a method for the production of the polymer-based piezoresistive film of the invention. In the said method, the following steps are successively followed: (i) selecting a mask material, (ii) cutting the mask material and attaching it on an application surface, (iii) applying the said piezoresistive ink composition on the mask by means of a spatula, (iv) drying the ink composition, (v) peeling and separating the mask material.

[0028] The present invention also describes the use of the polymer-based piezoresistive film of the invention as a sensor in electronic assemblies.

[0029] Brief Description of the Drawings

[0030] Fig. 1 - A piezoresistance graph of a polymer-based piezoresistive film according to an embodiment of the invention with the application of 15-cycle sequential deformation.

[0031] Fig. 2 - A piezoresistance graph of the polymer-based piezoresistive film according to the same embodiment of the invention with the application of 15-cycle sequential deformation following the application of 15-cycle sequential deformation shown in Fig. 1.

[0032] Fig. 3 - A comparative representation of the linearity analyzes of the polymer-based piezoresistive films according to different embodiments of the invention on a single graph.

[0033] Fig. 4 - A graph formed by taking the derivative of the graph presented in Fig. 3.

[0034] Fig. 5 - A comparative representation of the initial electrical resistance of the polymer- based piezoresistive films before deformation, according to different embodiments of the invention, on a single graph.

[0035] Fig. 6 - A comparative representation of a gauge factor value of the polymer-based piezoresistive films according to different embodiments of the invention, on a single graph. Detailed Description of the Invention

[0036] The present invention will now be detailed and its advantages and preferred embodiments will be explained. However, the embodiments given herein are provided for better understanding of the subject, and the invention is not limited thereto.

[0037] In the most general sense, the invention relates to polymer-based piezoresistive films and polymer-based ink compositions suitable for use in obtaining same. A polymer- based ink composition proposed within the scope of the invention comprises graphene nanoplatelets and graphite powder dispersed in an aqueous solution of a polymer matrix.

[0038] In an embodiment of the invention, said polymer matrix comprises at least one polymer selected from the group comprising polyvinyl alcohol (PVA), polydimethylsiloxane (PMDS), polyvinylidene fluoride (PVDF), polylactic acid (PLA), polyaniline (PANI), polymethyl methacrylate (PMMA), polyethylene glycol (PEG), polyethylene terephthalate (PET), and thermoplastic polyurethane (TPU). All of the said polymer matrices provide the desired viscosity, fluidity, and toughness in the ink composition of the invention, as well as the desired homogeneous film-forming capacity during the application of the ink composition on a surface.

[0039] In a preferred embodiment of the invention, the polymer matrix comprises polyvinyl alcohol.

[0040] According to a preferred embodiment of the invention, the density of the aqueous solution of the polymer matrix is in the range of lOOmg / mL to 400mg / mL.

[0041] According to the most preferred embodiment of the invention, said polymer matrix comprises polyvinyl alcohol (PVA) with a molecular weight in the range of 10000 to 150000. In cases where the molecular weight of the PVA is below 10000, a decrease in the film-forming capability of the ink composition as well as disintegration and ruptures in the formed films have been observed. At molecular weight values above 150000, a loss of conductivity was observed in the final film structure obtained using the said ink composition, due to an increase in the distance between the graphene nanoplatelets and graphite powder particles.

[0042] According to an embodiment, the density of the aqueous solution of the PVA- containing polymer matrix is in the range of lOOmg / mL to 400mg / mL. This range value guarantees the desired balance in the ink composition of the invention in terms of fluidity, toughness, and surface applicability. Since the ink composition is highly fluidic at density values lower than lOOmg / mL, there are difficulties in applying it to the surface as a film. At density values higher than 400mg / mL, a homogeneous film cannot be obtained because the ink composition tends to solidify quickly. This is a factor that directly affects the piezoresistive property of the final film.

[0043] According to an embodiment of the invention, the proportion of the graphene nanoplatelets in the ink composition is in the range of 1-50% by weight. In a more preferred embodiment of the invention, the proportion is in the range of 5-50%. According to the most preferred embodiment of the invention, the proportion of the graphene nanoplatelets in the ink composition is in the range of 20-40% by weight.

[0044] According to an embodiment of the invention, the proportion of the graphite powder in the ink composition is in the range of 1-50% by weight. In a more preferred embodiment of the invention, this proportion is in the range of 5-40%. According to the most preferred embodiment of the invention, the proportion of the graphite powder in the ink composition is in the range of 10-40% by weight.

[0045] In a preferred embodiment of the invention, the ink composition of the invention comprises graphene nanoplatelets in the range of 20-40% and graphite powder in the range 10-40% by weight.

[0046] In the studies carried out within the scope of the invention, it is observed that there is a relation between the proportion of the graphene nanoplatelets and that of the graphite powder. Accordingly, it is possible to achieve the desired synergistic effect when the ratio of graphene nanoplatelets to graphite powder is in the range of 1:2 to 1:0.25.

[0047] In a preferred embodiment, the ratio of graphene nanoplatelets to graphite powder is in the range of 1: 1.6 to 1: 1. In this range value, the synergistic effect increases unexpectedly. In other words, there is a surprising increase in the deformation sensitivities of the polymer-based ink compositions having a proportion of the graphene nanoplatelets and graphite powder in this range of ratio, in multi-cycle applications. The studies carried out in the invention show that, in certain ranges of ratio, a maximum synergistic effect is achieved when an inverse correlation is achieved between the proportions of graphene nanoplatelets and graphite powder.

[0048] Accordingly, in an embodiment, the ink composition comprises 5% graphene nanoplatelets and 40-45% graphite powder by weight.

[0049] Accordingly, in another embodiment, the ink composition comprises 10-20% graphene nanoplatelets and 35-40% graphite powder by weight.

[0050] Accordingly, in another embodiment, the ink composition comprises 30% graphene nanoplatelets and 10-20% graphite powder by weight.

[0051] Accordingly, in another embodiment, the ink composition comprises 35% graphene nanoplatelets and 5-15% graphite powder by weight.

[0052] Accordingly, in another embodiment, the ink composition comprises 40-45% graphene nanoplatelets and 5-10% graphite powder by weight.

[0053] In a preferred embodiment of the invention, the ink composition comprises 25% graphene nanoplatelets by weight. According to this embodiment, the proportion of the graphite powder in the same composition is adjusted to remain in the range of 10- 40%, more preferably 25-40% by weight. It has been observed that said synergistic effect at these proportions gives results much higher than expected (Fig. 3 to Fig. 6).

[0054] The particle size of the graphene nanoplatelets and graphite powder in the ink composition of the invention are important in terms of the desired consistency, fluidity, and film-forming capability in the ink composition. Moreover, the fact that the ink composition has the desired properties in this sense also affects the characteristic features of the formed film structure. Therefore, the particle size of the nanoplatelets and graphite powder can also be considered as an effective factor in determining the conductivity and piezoresistance measurement sensitivity of the final film.

[0055] Accordingly, the average particle size of the said graphene nanoplatelets of the invention is provided in the range of 0.01 to 100 pm. Graphene nanoplatelets are known in the art for their two-dimensional structure, and their thickness is typically only a few atomic layers, being less than 1 nanometer. Therefore, the "average particle size" as used herein corresponds to the length and width values of the said particles. The average particle size values can be measured by differential light scattering devices, which are capable of taking measurements from different angles by rotating the particles, thus providing homogeneous particle size distribution. In a preferred embodiment of the invention, the average particle size of the graphene nanoplatelets is in the range of 0.05 to 25 pm.

[0056] Accordingly, the average particle size of the graphite powder mentioned herein is provided to be in the range of 0.1 to 200 pm. As also known in the art, the graphite powder has a structure in which multiple graphene layers are positioned on top of each other. The layers are held together by weak van der Waals forces, allowing the layers to slide over each other easily. The particles in graphite powder have a plate-like shape, so their length and width values are quite high compared to their thickness, just like in graphene nanoplatelets. Therefore, the "average particle size" values given here also correspond to the length and width values of the particles. In a preferred embodiment of the invention, the average particle size of the graphite powder is in the range of 1 to 100 pm.

[0057] The invention also relates to a method for the production of the polymer-based ink compositions suitable for use in the production of piezoresistive films. The method of the invention is also suitable for the production of an ink composition according to any one of the embodiments described above.

[0058] The method of the invention mainly includes the following steps: a. Preparing a polymer-based aqueous solution by dissolving a polymer material in distilled water b. Adding graphene nanoplatelets to the said aqueous solution c. Performing the mixing process until a homogeneous mixture is obtained d. Adding graphite powder to the aqueous solution containing graphene nanoplatelets e. Performing the mixing process until a homogeneous mixture is obtained

[0059] According to an embodiment of the invention, dissolving a polymer material in distilled water as described in step (a) can be carried out at room temperature or at higher temperatures up to 100°C. In this step, a faster dissolution of the polymer material can be achieved by heating the solution and / or mixing it with a magnetic mixer. According to an embodiment of the invention, step (a) is carried out at a temperature of 30- 100°C. If the heating process is applied, this step is carried out in a closed system in order to condense the evaporated water to recover it. According to the most preferred embodiment of the invention, step (a) is carried out in a closed system at a temperature of 70-90°C. According to a preferred embodiment of the invention, the dissolving process in step (a) is carried out using a magnetic mixer until the dissolution process is completed.

[0060] According to an embodiment of the invention, said polymer material in step (a) comprises at least one polymer selected from the group comprising polyvinyl alcohol (PVA), polydimethylsiloxane (PMDS), polyvinylidene fluoride (PVDF), polylactic acid (PLA), polyaniline (PANI), polymethyl methacrylate (PMMA), polyethylene glycol (PEG), polyethylene terephthalate (PET) and thermoplastic polyurethane (TPU).

[0061] In a preferred embodiment of the invention, said polymer material in step (a) comprises polyvinyl alcohol (PVA). In a more preferred embodiment, said polymer material is polyvinyl alcohol. The molecular weight of the polyvinyl alcohol mentioned in these embodiments is in the range of 10000 to 150000. In cases where the molecular weight of the PVA is below 10000, a decrease in the film-forming capability of the ink composition and disintegration and ruptures in the formed films have been observed. At molecular weight values above 150000, a loss of conductivity was observed in the final film structure obtained using the said ink composition, due to an increase in the distance between the graphene nanoplatelets and graphite powder particles.

[0062] According to an embodiment of the invention, the density of the polymer-based aqueous solution prepared in step (a) is in the range of lOOmg / mL to 400mg / mL. In a more preferred embodiment, this value is between 150mg / mL and 300mg / mL. The fact that the density is between these values is important in terms of ease of production, especially when a film is to be produced from the inventive ink composition.

[0063] According to an embodiment of the invention, the average particle size of the said graphene nanoplatelets in step (b) is in the range of 0.01 to 100 pm, more preferably in the range of 0.05 to 25 pm. Since the thickness values of the graphene nanoplatelets are very small (below 1 nanometer) compared to their length and width values, the "average particle size" mentioned herein corresponds to the length and width values of the nanoparticles having a square or rectangular plate shape. The average particle size values can be measured by differential light scattering devices, which are capable of taking measurements from different angles by rotating the particles, thus providing homogeneous particle size distribution. The range values given here are critical in ensuring that the solution can be mixed easily and homogeneously, and therefore the particles are effectively distributed in the final film.

[0064] In an embodiment of the invention, the mixing process in step (c) is carried out by sonication or a mechanical mixer. Said mixing process is preferably carried out using sonication in the frequency range of 20-40 Hz for 3-8 hours. At this stage, sonication is more useful, especially for mixing and homogenizing fine and nanometer-sized particles. Given that the shear stress applied by sonication is lower than mechanical mixing, fine and sensitive structures such as graphene nanoplatelets are gently mixed and the particles are prevented from being damaged.

[0065] In an embodiment of the invention, the average particle size of the said graphite powder in step (d) is in the range of 0.1 to 200 pm, preferably in the range of 1 to 100 pm. The particles in graphite powder have a plate-like shape, so their length and width values are quite high compared to their thickness, just like in graphene nanoplatelets. Therefore, the "average particle size" values given here also correspond to the length and width values of the particles. The average particle size values can be measured by differential light scattering devices, which are capable of taking measurements from different angles by rotating the particles, thus providing homogeneous particle size distribution. The range values given here are critical in ensuring that the solution can be mixed easily and homogeneously, and therefore the particles are effectively distributed in the final film.

[0066] In an embodiment of the invention, said mixing process in step (c) is carried out by sonication or a mechanical mixer. In this step, mixing is preferably carried out using a mechanical mixer, as the proportion of the particles in the solution and therefore the density of the mixture increase. According to a preferred embodiment, the mixing process in step (e) is carried out using a mechanical mixer at a mixing speed of 400 rpm for 15-30 minutes. The invention also relates to polymer-based piezoresistive films. The polymer-based piezoresistive films of the invention are obtained by applying the ink composition according to any one of the embodiments described above to an application surface using screen-printing and / or spraying techniques. The method preferred may vary depending on the application surface.

[0067] In order for the piezoresistive film of the invention to be used in sensor applications effectively, it is preferred that the thickness of the film is in the range of 10-500 pm. At thickness values below 10 pm, ruptures in the film are observed. The thickness values above 500 pm negatively affect the piezoresistance property of the film, making it difficult to make measurements. In a more preferred embodiment of the invention, the thickness of the piezoresistor film is in the range of 200-400 pm. It has been observed that the gauge coefficient of the piezoresistive films proposed in the present invention does not change up to 45 cycles and they exhibit an electrical resistance in the range of 1-10 kOhm. This shows that, with the synergistic effect of the graphene nanoplatelets and graphite powder used in combination in the ink composition, the conductivity values and the fatigue resistance and therefore the measurement sensitivity of the films of the invention are unexpectedly increased.

[0068] According to an embodiment of the invention, it is preferred that the width value of the piezoresistive film of the invention is at least 100 pm. There are difficulties in taking measurements in the films having a width below this value, and the conductivity value of the film decreases. The length value of the film of the invention may change depending on the area and application in which it is used.

[0069] The invention also relates to a method for the production of the polymer-based piezoresistive film according to any one of the above-described embodiments. The film production method of the invention mainly includes the following steps: i. Selecting a mask material ii. Cutting the mask material and attaching it on an application surface iii. Applying the said piezoresistive ink composition on the mask by means of a spatula iv. Drying the ink composition v. Peeling and separating the mask material According to an embodiment of the invention, said mask material in step (i) is selected from the group comprising an acetate paper, a plate made of polylactic acid, and a foil paper.

[0070] In the preferred embodiment of the invention, steps (iii) and (iv) are repeated at least once before step (v). Preferably, steps (iii) and (iv) are repeated once in order to apply two layers of the ink composition onto the application surface. The main purpose is to provide a homogeneous film thickness that provides high elastic strength and conductivity. According to an embodiment of the invention, said drying in step (iv) is carried out at a temperature of 20-90°C.

[0071] Example 1

[0072] In an exemplary study carried out to prepare an ink composition of the invention, a PVA material was first dissolved in distilled water using a closed system. In this step, the closed system was heated to 80°C and the mixture was mixed with a magnetic mixer until a homogeneous solution was obtained, preferably for 3-8 hours. The density of the prepared polymer solution was adjusted to approximately 300mg / mL. Then, graphene nanoplatelets were added to the prepared polymer solution. The proportion of the graphene nanoplatelets was adjusted to 25% by weight in the final ink component. The average particle size of the graphene nanoplatelets was in the range of 0.05 to 25 pm. The resulting mixture was mixed by sonication at a frequency range of 20-40 Hz for about 4 hours. Graphite powder was added into the polymer solution in which graphene nanoplatelets were homogeneously distributed. The proportion of the graphite powder was adjusted to 35% by weight in the final ink component. The average particle size of the graphite powder was in the range of 1 to 100 pm. The resulting mixture was mixed by using a mechanical mixer at a mixing speed of 400 rpm for about 20 minutes. As a result of such processes, a final ink composition was obtained in the invention.

[0073] Example 2

[0074] The analyzes of the piezoresistance property of the film obtained from the ink composition obtained in Example 1 were carried out. For this purpose, first of all, said ink composition was applied in two layers on a plexiglass material using the screenprinting technique, with a length of 150 mm and a width of 50 mm. A foil paper was used as the mask material. The thickness of the film obtained upon drying was in the range of 200-400 pm. The films prepared were successively exposed to constant deformation cycles as part of the deformation tests. A total of 30 cycles were applied in 2 parts and said analysis results are presented in Fig. 1 and Fig. 2.

[0075] As seen from Fig. 1 and Fig. 2, the film gave almost the same conductivity value after each successive deformation cycle. Here; (R2 / Ri)-(R7Ri) =~0.01, Rn: electrical resistance of a cycle number n upon deformation, Ri: initial electrical resistance. In other words, with the piezoresistive film obtained from the ink composition proposed by the invention, measurements that change from cycle to cycle, i.e., variations in the Gauge factor, are reduced. These analyzes show that the piezoresistive film of the invention does not lose sensitivity up to 45 cycles.

[0076] Example 3

[0077] In the invention, the effects of different proportions of the graphite powder on the ink composition and therefore on the final film were comparatively examined. For this purpose, by following the method steps proposed for the production of the polymer- based ink compositions of the invention, comparative analyzes were carried out in which the amount of graphene nanoplatelets (GNP) was kept constant at 25% by weight in the ink composition while the proportion of the graphite powder was changed. In this context, 9 different ink compositions were generated by varying the proportion of the graphite powder (GP) between 0% and 40% by weight. From these compositions, nine different piezoresistive films were generated by following the method steps proposed for the production of the polymer-based piezoresistive films of the invention.

[0078] Deformation was applied to the films generated in Example 3 with an UTM (universal testing machine) device, and linearity levels were measured via a time-deformation graph. Said graph represents the change in the electrical resistance of the films before and after deformation, which is presented in Fig. 3. As seen from the graph, the ink compositions in which 25-40% by weight of graphite powder (GP) is added exhibit unexpectedly linear behavior.

[0079] Linearity in piezoresistive films is a property in which the electrical resistance of the film changes linearly with the applied mechanical stress or deformation. It shows that the electrical properties of the film operate stably and produce reliable results under various conditions of application and measurement. Such linear behavior is important in many sensor applications of the piezoresistive film because a linear response is helpful in analyzing the measured mechanical variable in an accurate manner. For example, a piezoresistive film used as a pressure sensor must respond linearly to applied pressure so that pressure changes can be measured precisely.

[0080] The sensitivity of the said films to mechanical deformation was measured by taking the derivative of the graph presented in Fig. 3. The graph showing the measurements is presented in Fig. 4. As seen from the said graph, the films obtained from the ink compositions in which 25-40% by weight of the graphite powder (GP) is added exhibited a very low noise and an unexpectedly high sensitivity.

[0081] In another analysis, the initial electrical resistances of the nine different films prepared in Example 3 were measured before the application of deformation. The results are available in the graph presented in Fig. 5. Again here, the films obtained from the ink compositions in which 30-40% by weight of the graphite powder (GP) was added provided the most desired results, i.e., the highest electrical conductivity and accordingly the lowest resistance. It is an expected result that graphite powder increases the electrical conductivity, and within the scope of the invention, it is aimed to increase the electrical conductivity, but to achieve this without compromising the mechanical properties, and in this sense to improve the piezoresistance properties of the films produced. However, the point here is that an increase in the amount of graphite powder added to the ink composition is surprisingly reflected exponentially in the conductivity of the film. Therefore, for example, compared to the ink composition containing 20% graphite powder by weight, the proportion of the graphite powder in the ink composition containing 30% graphite powder by weight increases by 1.5 times, while the electrical resistance exhibited by these compositions decreases approximately 10 times.

[0082] Using the measured initial electrical resistance values and the time-deformation values from the UTM device, the gauge factors of the nine discrete films were calculated using the following equation.

[0083] Gauge Factor (GF) = AR / (Ri * As)

[0084] Here; AR is a change in electrical resistance (AR = R - Ri) R is electrical resistance upon deformation

[0085] Ri is initial electrical resistance

[0086] As is a change in applied mechanical stress or deformation.

[0087] The calculated gauge factors are presented in the graph given in Fig. 6. Gauge factor is a coefficient that indicates the sensitivity of the electrical resistance of a piezoresistive material against mechanical stress or deformation. This factor represents how the effect of the mechanical deformation is reflected on the material as changes in the electrical resistance. A piezoresistive material with a high gauge factor gives a more sensitive response to small mechanical changes. This allows the measured mechanical variable to be determined more precisely and accurately. A high gauge factor also enables the material to work in a wide range of mechanical deformation and therefore extends the usage areas of the material. The graph presented in Fig. 6 shows that the films obtained from the ink compositions containing 25% graphene nanoplatelets as well as 25-40% graphite powder (GP) have gauge factor values much higher than expected.

[0088] The present invention also relates to the usage areas of the polymer-based piezoresistive film of the invention. The piezoresistive film of the invention is suitable for use as a sensor in electronic assemblies. Said sensor application may be a pressure sensor, a vibration sensor, a flow sensor, a touch sensor, a weight measurement sensor, a deformation sensor, etc.

Claims

CLAIMS1. A polymer-based ink composition for use in the production of piezoresistive films, comprising graphene nanoplatelets and graphite powder dispersed in an aqueous solution of a polymer matrix.

2. The composition according to claim 1, wherein said polymer matrix comprises at least one polymer selected from the group comprising polyvinyl alcohol, polydimethylsiloxane, polyvinylidene fluoride, polylactic acid, polyaniline, polymethyl methacrylate, polyethylene glycol, polyethylene terephthalate, and thermoplastic polyurethane.

3. The composition according to claim 2, wherein the polymer matrix comprises polyvinyl alcohol.

4. The composition according to any one of claims 1 to 3, wherein the density of the aqueous solution of the said polymer matrix is in the range of lOOmg / mL to 400mg / mL.

5. The composition according to any one of claims 1 to 4, wherein the proportion of graphene nanoplatelets in the ink composition is in the range of 1-50% by weight.

6. The composition according to claim 5, wherein the proportion of graphene nanoplatelets in the ink composition is in the range of 5-50% by weight.

7. The composition according to any one of claims 1 to 6, wherein the proportion of graphite powder in the ink composition is in the range of 1-50% by weight.

8. The composition according to claim 7, wherein the proportion of graphite powder in the ink composition is in the range of 5-40% by weight.

9. The composition according to any one of claims 1 to 8, wherein the composition comprises 20-40% of graphene nanoplatelets and 10-40% of graphite powder by weight.

10. The composition according to claim 9, wherein the ratio of graphene nanoplatelets to graphite powder is in the range of 1:2 to 1:0.25.

11. The composition according to claim 10, wherein the ratio of graphene nanoplatelets to graphite powder is in the range of 1: 1.6 to 1: 1.

12. The composition according to any one of claims 1 to 11, wherein the average particle size of graphene nanoplatelets is in the range of 0.01 to 100 pm.

13. The composition according to claim 12, wherein the average particle size of graphene nanoplatelets is in the range of 0.05 to 25 pm.

14. The composition according to any one of claims 1 to 13, wherein the average particle size of graphite powder is in the range of 0.1 to 200 pm.

15. The composition according to claim 14, wherein the average particle size of graphite powder is in the range of 1 to 100 pm.

16. A method for the production of the ink composition according to any of the preceding claims, comprising the following steps: a. Preparing a polymer-based aqueous solution by dissolving a polymer material in distilled water b. Adding graphene nanoplatelets to the said aqueous solution c. Performing the mixing process until a homogeneous mixture is obtained d. Adding graphite powder to the aqueous solution containing graphene nanoplatelets e. Performing the mixing process until a homogeneous mixture is obtained17. The method according to claim 16, wherein step (a) is carried out in a closed system at a temperature of 30-100°C.

18. The method according to claim 17, wherein step (a) is carried out in a closed system at a temperature of 70-90°C.

19. The method according to any one of claims 16 to 18, wherein said dissolving process in step (a) is carried out using a magnetic mixer.

20. The method according to any one of claims 16 to 19, wherein polyvinyl alcohol is used in step (a).

21. The method according to claim 20, wherein the molecular weight of the polyvinyl alcohol used is in the range of 10000 to 150000.

22. The method according to any one of claims 16 to 21, wherein the polymer-based aqueous solution prepared in step (a) has a density in the range of lOOmg / mL to 400mg / mL.

23. The method according to claim 22, wherein said aqueous solution has a density in the range of 150-300 mg / mL.

24. The method according to any one of claims 16 to 23, wherein the average particle size of the graphene nanoplatelets in step (b) is in the range of 0.01 to 100 pm.

25. The method according to any one of claims 16 to 24, wherein said mixing process in step (c) is carried out using sonication for 3-8 hours.

26. The method according to any one of claims 16 to 25, wherein the average particle size of the graphite powder in step (d) is in the range of 0.1 to 200 pm.

27. The method according to any one of claims 16 to 26, wherein said mixing process in step (e) is carried out using a mechanical mixer at a mixing speed of 400 rpm for 15-30 minutes.

28. A polymer-based piezoresistive film, obtained by applying a piezoresistive ink composition according to any one of claims 1 to 15 onto an application surface using screen-printing and / or spraying technique.

29. The polymer-based piezoresistive film according to claim 28, wherein the thickness of the film is in the range of 10-500 pm.

30. The film according to claim 29, wherein the thickness of the film is in the range of 200-400 pm.

31. The film according to claim 29 or 30, wherein the width of the film is greater than 100 |jm.

32. A method for the production of the film according to any one of claims 28 to 31, wherein the method is performed using the screen-printing technique in the following steps: i. Selecting a mask material ii. Cutting the mask material and attaching it on an application surface iii. Applying the piezoresistive ink composition on the mask by means of a spatula iv. Drying the ink composition v. Peeling off the mask material33. The method according to claim 32, wherein in step (i), the mask material is selected from the group comprising an acetate paper, a plate made of polylactic acid, and a foil paper.

34. The method according to claim 32 or 33, wherein steps (iii) and (iv) are repeated at least once before step (v).

35. The method according to any one of claims 32 to 34, wherein said drying process in step (iv) is carried out at a temperature of 20-90°C.

36. Use of the film according to any one of claims 28 to 31 as a sensor in electronic assemblies.