Method for producing porous composites of nanocellulose, natural rubber latex and zinc oxide, composite and device for ultraviolet light photosensing

Abstract

The present description relates to a method for producing porous composites of nanocellulose with natural rubber latex (LBN) functionalized with zinc oxide (ZnO) for UV radiation sensing. A production route is described for producing said porous composites by means of a mechanical step of agitating and sonicating an aqueous solution of nanocellulose and natural rubber latex (LBN) with zinc oxide added in powder form. The resulting composite is coated with natural rubber latex (LBN) and has anchored ZnO. A device for UV radiation photosensing is described, comprising a device body (1), formed from said porous composite (1) and a pair of electrodes (2), one electrode (2) being provided at each of the opposite ends of the device body (1).

Description

PRODUCTION PROCESS OF POROUS COMPOSITES OF NANOCELLULOSE, NATURAL RUBBER LATEX AND ZINC OXIDE, COMPOSITE AND DEVICE FOR ULTRAVIOLET LIGHT PHOTOSENSING TECHNICAL FIELD OF THE INVENTION

[0001] The present invention is in the field of composites with materials of renewable origin. The present description is also in the field of light-sensitive devices. FUNDAMENTALS OF THE INVENTION

[0002] Cellulose is the most abundant natural polymer in nature, renewable and biodegradable, formed from glucose (C6H 10 O5) with f3-(1,4) bonds in the D-glucose units. The polymer chains interact via intra- and intermolecular interactions, forming hydrogen bonds through the hydroxyl groups present in the monomeric units.

[0003] Nanoscale structures (on the order of 10' 9m) extracted from cellulose fibers arouse considerable interest due to their intrinsic characteristics (different morphological structures, hydrophilicity, possibility of functionalization and abundance), and this enables applications in various segments. Nanocellulose extracted from plant biomass can be classified into two different names, according to its structure and extraction method: nanocrystalline cellulose (NCC), the crystalline region of the polymer, forming shorter and more rigid fibers; and nanofibrillated cellulose (NFC), consisting of the crystalline and amorphous region of the polymer forming longer and more flexible fibers.

[0004] The process of obtaining nanocellulose requires cellulose fibrillation, a method called “top-down”, in which the biomass “bulk” consisting of fibers and microfibers is comminuted into fibrils with nanometric dimensions. This process can be carried out via chemical and / or mechanical methods, thus altering its final properties.

[0005] The most common chemical process for obtaining nanocellulose is acid hydrolysis, assisted with strong acids (sulfuric or hydrochloric). In this process, the amorphous regions of the cellulose fibers are hydrolyzed, isolating the CNCs. This process promotes the removal of the amorphous regions of the material, generating well-defined crystals of stable nanocellulose in colloidal suspension.

[0006] These nanostructures can be used in their original form as a substrate in composites, since they have low density, high rigidity, surface modification capacity, and a high aspect ratio, and are also capable of supporting three-dimensional structures.

[0007] Natural rubber latex (NRL) is another type of renewable material obtained from the sap of species such as the rubber tree (Hevea brasiliensis). It is a versatile material with applications in a variety of products, such as mattresses, pillows, cushions, upholstery, footwear, medical devices, orthopedic products, soundproof panels, toys, gaskets, seals, cushioning materials in packaging and insulation in machines and equipment.

[0008] In addition to these applications, LBN can be used in sensors designed to detect and measure various physical or chemical parameters. Examples of latex foam applications in these devices include measuring mechanical deformation, detecting changes in pressure, force, stretching or elongation of materials. In this sense, they are integrated into surfaces and touch screens to measure physiological parameters such as skin temperature, sweat or pulse.

[0009] In the field of electronics in general, electronic devices made of natural inputs are of great interest because they offer the potential to reduce the environmental impact of electronic waste generated, contributing to a more sustainable future for the planet. CNC has shown promise in the development of sensors due to its unique properties, including high surface area, biocompatibility and ability to be modified for specific functionalities, such as creating biosensors designed to detect specific biological molecules or pathogens, developing chemical sensors for detecting specific gases or chemical compounds, building humidity, strain or stress detection, optical, electrochemical and temperature sensors. LBN, on the other hand, can be present in strain, pressure and touch sensors, in wearable sensors and in stretchable electronics. One drawback of CNC and LBN, however, is the fact that they have an electrically insulating nature, which may restrict their application to electronic parts. Especially with regard to LBN, documents CN110643079 A, ​​CN103756026 B and CN107778535 A, for example, bring elements such as sealing rings and capacitor coating plates made from LBN due to its insulating properties.US20170027168, on the other hand, claims a method for producing products from CNC comprising, among its steps, its processing with at least one metal alloy and at least one coating agent to increase hardness, compressibility, corrosion resistance, greater electrical resistance, greater specific heat capacity, ductility and lower thermal conductivity.

[0010] But a special type of electronic device of great importance is ultraviolet (UV) radiation sensors, designed to detect and measure light at wavelengths between 200 and 400 nm. They work by detecting UV radiation from changes in the electrical behavior of oxides. These sensors are used in a variety of applications, including scientific research, industrial processes, environmental monitoring, and electronics. They are used in dental and medical equipment for sterilization and curing, and in photolithography, where they monitor UV exposure during the semiconductor manufacturing process.

[0011] Specifically with regard to UV radiation sensors, LBN and CNC face an application obstacle not only due to their electrical insulation characteristics, but also due to the fact that they, by themselves, are not capable of absorbing or reflecting UV radiation. Their functionalization with compounds that have such capacity, such as ZnO and TiO2, does not necessarily guarantee the electronic percolation necessary for the composite to act adequately as an electronic device that converts UV radiation into electrical current. Document ES277475, for example, deals with an article that comprises a substrate electrostatically coated with at least one layer of a nanocomposite comprising a mixture of CNC and nanoparticles selected from AI2O3, TiO2, SiO2 and / or ZnO to form a gas and moisture barrier and block infrared or UV radiation.CN115232488B describes a photoelectric shield for a cathode, composed of TiO2, WO3 and two binding agents, one of which may be a synthetic latex. This shield requires a conductive agent, which may be acetylene black, carbon fiber, carbon nanotube, conductive graphite or Ketjen Black, to accelerate the transfer of photoinduced electrons from TiO2 to WO3 and to the cathode metal. It is worth noting that, in the case of the combination of LBN foam with ZnO, there are documents that use this oxide as a curing activator (FR2062511) or as an aid in the composition of a high-elasticity, low-density foam with a uniform structure, ideal for pillows and mattresses (CN111286091 A).

[0012] The combination of CNC and LBN functionalized with ZnO to act as a device for UV light photosensing has not been revealed to date in the state of the art and is of fundamental importance in renewable electronics to achieve a device capable of acting in this segment. STATE OF THE TECHNIQUE

[0013] The state of the art includes some ultraviolet (UV) radiation sensors based on composites that use cellulose as a substrate. The prior art also includes the application of zinc oxide (ZnO) for radiation sensing.

[0014] Patent document KR102184508 discloses a sensor based on a CsPbBr3 composite on a cellulose fiber substrate. The technical difficulty of stabilizing CsPbBr3, which oxidizes easily under ambient conditions, is overcome by its immobilization on a cellulose substrate in a process that involves at least one surfactant as an additive.

[0015] Patent document W02007015710 discloses a spinning process for producing sheets, tapes and balls of yarn from various fibers. A method comprising ZnO nanowires and cellulose fibers for producing the aforementioned articles is described. The use of such articles as radiation sensors is also disclosed. A limitation of the described technique is the productivity of the articles per spinning process, which presents complex mechanical steps, very specific geometries and low scalability compared to production processes in bulk substrate.

[0016] Patent document CN107305913 describes a sensor produced by spray deposition of ZnO on a substrate. Resins and polymers (e.g., ethyl cellulose) are cited as suitable substrates for the production of the described sensor. Limitations of this prior art include problems with the homogeneity of the spray dispersion of ZnO, in addition to the use of non-biodegradable and non-flexible matrices.

[0017] Therefore, prior art that teaches a direct synthesis route of three-dimensional nanocellulose structures combined with LBN and functionalized with ZnO for UV radiation sensing is not relevant. BRIEF DESCRIPTION OF THE INVENTION

[0018] It is one of the objectives of the present description to reveal a process of functionalization of CNC and LBN with ZnO by simple mixing, which does not generate waste and results in a porous biodegradable cellulose composite with a stable, flexible, resilient three-dimensional structure, with high surface area and adequate electronic percolation, giving the necessary sensitivity for a UV light photosensing device based thereon. A device for UV light photosensing based on said porous CNC and LBN material functionalized with ZnO is also disclosed. BRIEF DESCRIPTION OF THE FIGURES

[0019] The present invention is illustrated in the embodiments represented in figures, as briefly described below.

[0020] Figure 1 is a schematic representation of a device for UV light photosensing based on a porous CNC / LBN composite functionalized with ZnO, according to one embodiment of the present disclosure.

[0021] Figure 2 is a schematic representation of the experimental procedure for the ZnO functionalization of CNC / LBN-based cryogels according to embodiments of the present disclosure. Figure 2A demonstrates the process of formation and functionalization of the cryogels simultaneously by formulating the CNC suspension with LBN and ZnO, followed by freezing this suspension in a freezer and drying it through the lyophilization process. Figure 2B shows an X-ray tomogram of the CNC / LBN-based cryogel with highly oriented walls.

[0022] Figures 3A, 3B, and 3C are scanning electron microscopy (SEM) images of the walls of cryogels based on pure CNC, CNC / LBN, and CNC / LBN functionalized with ZnO, respectively. Figure 3D is the replica of Figure 3C in backscattered electron (BSE) mode. The blue colors indicate the presence of highly dispersed ZnO distributed along the walls of the cryogels described by the orange color.

[0023] Figure 4A is an illustrative diagram of the setup containing gold electrodes coupled to the potentiostat used in the chronoamperometric characterization of the CNC / LBN composites functionalized with ZnO under the cyclic incidence of UV radiation. Figure 4B is the visual representation of the CNC / LBN composite with ZnO under exposure to UV radiation.

[0024] Figure 5A presents an illustrative schematic together with a 3D reconstruction performed by X-ray tomography demonstrating the photosensitivity of ZnO to UV radiation, and its arrangement in the structure of the cryogels forming an electronic path. Figures 5B, 5C and 5D show the photocurrent curve responses of the ZnO sensors under intermittent exposure to UV radiation in relation to the ZnO concentration, the LBN concentration and the cryogel height, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention relates to a porous composite of crystalline nanocellulose (CNC) and natural rubber latex (LBN) functionalized with zinc oxide (ZnO) and its production process. A device for ultraviolet (UV) light photosensing based on said porous composite is also described.

[0026] The objectives of the present description are achieved by a production process of a porous composite of CNC and LBN functionalized with ZnO, based on a mechanical route, which comprises: (a) mechanically mixing ZnO powder in water followed by resuspension of CNC and addition of LBN, wherein the aqueous suspension of CNC together with LBN has a concentration of 4% m / v, and wherein the ZnO:CNC mass ratio is between 100 and 300% m / m and the LBN:CNC mass ratio is between 20 and 70% m / m; (b) sonicate the suspension under mechanical agitation; (c) freeze-drying the suspension in a mold, forming a porous composite of CNC with LBN functionalized with ZnO, forming a three-dimensional oriented structure.

[0027] In one embodiment of the process, sonicating the aqueous suspension in step (b) comprises sonication with intermittent pulses (on / off) at 60 second intervals, at a power of 450 W, for at least 2 h.

[0028] In one embodiment of the process, lyophilizing the suspension in step (c) comprises freezing the suspension at a constant temperature of -20 °C for 24 h so that the growth of the ice promotes the functionalization of the cryogel with ZnO and then drying for at least 48 h using a common lyophilization technique.

[0029] By way of example, the mold in step (c) can be cylindrical or prismatic.

[0030] The present invention also relates to the CNC composite with LBN functionalized with ZnO obtained by the process claimed herein. Figure 2B is an X-ray tomogram of a porous composite of the present invention. It is possible to observe that there is the formation of a lamellar structure with high surface area, which may indicate an excellent optical path for UV radiation, thus enhancing the performance of the device for UV light photosensing based on this porous composite.

[0031] Figures 3A, 3B and 3C are SEM images of a porous material of pure CNC, CNC / LBN and CNC / LBN functionalized with ZnO according to the present invention, respectively. Figure 3A shows the completely smooth surface containing only CNC, while Figure 3B demonstrates a surface with reliefs, indicating the total coating with LBN. Figure 3C shows the surface coated with LBN and ZnO, in which the ZnO is anchored to the coated surface. Figure 3D represents Figure 3C in the backscattered electron (BSE) mode, in which the bluish coloration contrasts the good dispersion and distribution of ZnO along the coated wall, represented by the orange color and showing that the process of the present invention generates a composite with good uniformity, important for the adequate percolation of electrons for photosensing of UV light.Thus, the composite of the present invention is characterized by being constituted by a porous CNC framework coated with LBN and with ZnO particles anchored to the coated surface.

[0032] The present invention also relates to a device for photosensing UV light. As illustrated in Figure 1, a device body (1) formed of a porous nanocellulose / LBN composite functionalized with ZnO is provided, and a pair of electrodes (2) are arranged in the device body (1), with one electrode (2) being provided at each of two opposite ends of the body (1). The electrodes (2) can be electrically connected to a setup, and these to an electrical control and reading system. The porous nanocellulose / LBN material functionalized with ZnO is a semiconductor material sensitive to UV radiation.

[0033] In one embodiment of the device, the body (1) is formed by a porous nanocellulose / LBN material functionalized with ZnO having a shape selected from a group comprising cylindrical shape, prismatic shape and combinations of these shapes.

[0034] In a preferred embodiment of the device, the electrodes (2) consist of a layer of silver paint deposited directly on two opposite ends of the body (1) by simple and low-cost deposition techniques, for example, the silver paint can be deposited manually using a brush. Figure 2A represents an exemplary embodiment of the device of the present invention.

[0035] Figures 4A and 4B represent the experimental procedure used for the electrical characterization of CNC / LBN composites functionalized with ZnO. To evaluate the electrical response of the composites, conductive silver paint was applied to the ends of the samples, aiming at better electrical contact with the gold electrodes of the setup. The setup was then connected to a potentiostat, in which a constant electrical voltage of 10 V was applied to the samples, as shown in Figure 4A. During the chronoamperometry analysis, UV light was incident on the samples intermittently over time, aiming to analyze the sensitivity of the composite to this type of radiation, as shown in Figure 4B.

[0036] Figure 5A is a detailed illustration corresponding to the sensitivity of ZnO to UV radiation, in which the creation of the electron-hole pair occurs in the conduction band of ZnO due to the energetic correspondence between the band gap of this oxide and the energy associated with the UV vibration. Considering the ordered and aligned structure of the walls of the CNC / LBN cryogels, ZnO creates an electronic path along the structure of the composite prepared by the process of the present invention.

[0037] Figures 5B, 5C and 5D are test curves of photosensor devices based on porous CNC / LBN composites functionalized with ZnO, varying according to the ZnO concentration (Fig. 5B), LBN concentration (Fig 5C) and body height (Fig 5D). The response of the sensors as a function of time was investigated with cyclic incidence of UV.

[0038] Based on Figure 5B, it is clear that the responsiveness of the electrical signal to the incidence of UV begins to be detected from a concentration of 54% (m / m) of ZnO in relation to the total mass of the composite, however, the response presents a relatively low scale compared to other sensors of the state of the art. On the other hand, the concentration of 64% (m / m) of ZnO in relation to the total mass of the composite presented a significant variation of electric current of 0.2 pA, being the maximum value obtained in relation to the variation of ZnO mass.

[0039] In contrast, the increase in LBN concentration resulted in greater sensor responsiveness to UV radiation, while maintaining the ZnO concentration constant (Figure 5C). The increase in LBN concentration suggested a potential optimization in the dispersion and distribution of ZnO throughout the composite matrix, together with an adhesion of the oxide to its wall. These factors contribute to the more efficient formation of an electronic percolation network when the composite is exposed to UV radiation.

[0040] The variation in body height did not significantly influence the responsiveness of the device (Figure 5D). However, bodies with heights greater than 3.32 mm and less than 2.29 mm did not show any signal to UV. Thus, the balance between the lateral area exposed to UV and the distance between the electrodes proved to be ideal in this body height range.

[0041] Although exemplary embodiments of the processes and products described have been presented in this report, it is not intended that the scope of protection be limited to the literal meaning thereof. Therefore, the description should be interpreted not as limitative, but merely as examples of particular embodiments that preserve the inventive concept presented herein. A person skilled in the art may readily apply the teachings presented herein to analogous solutions arising therefrom, limited only by the scope of the claims of this application.

Claims

CLAIMS 1. Process for producing porous composites of nanocellulose, natural rubber latex and zinc oxide, characterized by comprising: a) mechanically mixing ZnO powder in water followed by resuspension of CNC and addition of LBN, in which the aqueous suspension of CNC together with LBN has a concentration of 4% m / v, and in which the ZnO:CNC mass ratio is between 100 and 300% m / m and the LBN:CNC mass ratio is between 20 and 70% m / m; b) sonicating the suspension under mechanical agitation; c) lyophilizing the suspension in a mold, forming a porous nanocellulose material with LBN functionalized with an oriented three-dimensional structure.

2. Process, according to claim 1, characterized by the fact that sonicating the suspension in step (b) comprises sonication with intermittent pulses at intervals of 60 seconds, at a power of 450 W, for at least 2 h.

3. Process, according to claim 1, characterized by the fact that lyophilizing the suspension in step (c) comprises freezing the suspension at a constant temperature of -20 °C, for 24 h, and then drying for at least 48 h, by a common lyophilization technique.

4. Porous composite of nanocellulose, natural rubber latex and zinc oxide characterized by being produced according to the process of claim 1.

5. Porous composite of nanocellulose, natural rubber latex and zinc oxide characterized by being constituted by a three-dimensional porous CNC framework coated with LBN and with ZnO particles anchored to the coated surface.

6. Device for photosensing ultraviolet light characterized by comprising a device body (1) formed by a porous material of nanocellulose, natural rubber latex and zinc oxide according to claims 4 or 5, and a pair of electrodes (2), one electrode being provided at each of the opposite ends of the device body (1).

7. Device according to claim 6, characterized in that the electrodes are made of silver paint deposited on the ends of the device body (1).

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