UV-curable graphene ink for producing flexible NFC antenna and methods of manufacturing thereof

Abstract

A UV-curable graphene-based conductive ink comprising graphene-based colloid, wherein the graphene-based colloid comprises reduced graphene oxide and silver nanoparticles; photocurable polymer resin, photoinitiator and dispersant. A method of manufacturing a near field communication, NFC antenna, wherein the method comprises the steps of (i) inkjet printing a first conductive layer on a flexible substrate using a UV-curable graphene-based conductive ink; (ii) curing the first conductive layer obtained from step (i); (iii) inkjet printing dielectric layer onto the cured first conductive layer obtained from step (ii); (iv) curing the dielectric layer obtained from step (iii); (v) inkjet printing a second conductive layer on the cured dielectric layer obtained in step (iv); (vi) curing the second conductive layer obtained from step (v) to produce a flexible NFC antenna; and (vii) curing the flexible NFC antenna obtained from step (vi) to produce the NFC antenna of the present invention.

Description

[0001] UV-CURABLE GRAPHENE INK FOR PRODUCING FLEXIBLE NFC ANTENNA AND METHODS OF MANUFACTURING THEREOF

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to UV-curable graphene-based conductive ink for producing flexible near-field communication (NFC) antenna and methods of manufacturing thereof, in particular, the UV-curable graphene-based conductive ink is subsequently used in manufacturing flexible NFC antenna that exhibits desirable characteristics.

[0004] BACKGROUND OF THE INVENTION

[0005] Near field communication (NFC) technology is a short-range wireless communication technology that allows data exchange between two devices. It works using electromagnetic induction at frequency 13.56MHz. This technology enables devices to communicate between devices such as between two smartphones, between smartphone and NFC tag or between a smartphone and NFC-enabled sale terminal, that are close to each other, typically within a few centimetres. NFC is used in applications that require a secure and convenient method of exchanging data wirelessly such as mobile payments, contactless transaction and data exchange between devices.

[0006] One of the required process steps to produce the NFC tag is fabricating NFC antenna. The antenna is responsible for transmitting and receiving the NFC signals. Typically, a coil of wire or a circuit design is printed on a substrate such as a PCB. As technology advances, a flexible NFC tag is currently a demand technology that allow it to be bent, curved or attached to uneven surfaces. This flexibility makes them suitable for a wide range of applications such as wearables, medical and health, packaging and labels, where the traditional rigid NFC tags might not be practical. Further, some of the issues in current fabrication of the NFC antenna are such as requirement of complex techniques such as screen printing, metal plating and pressing conductive foil; limitation for smaller devices due to limited printing resolution and low durability due to low bending performance. As such, there is a need to identify a method and material to manufacture NFC antenna that exhibit desirable characteristics.

[0007] SUMMARY OF THE INVENTION

[0008] The present invention relates to a UV-curable graphene-based conductive ink comprising graphene-based colloid, wherein the graphene-based colloid comprises reduced graphene oxide and silver nanoparticles, wherein the graphene-based colloid is used in an amount ranging between 82% to 92% by weight of the UV-curable graphene-based conductive ink, wherein the reduced graphene oxide is used in an amount of 20% to 22% by weight of the graphene-based colloid and wherein the silver nanoparticles are used in amount of 19% to 21 % by weight of the graphene-based colloid; photocurable polymer resin, wherein the photocurable polymer resin is used in an amount ranging between 8% to 12% by weight of the UV-curable graphenebased conductive ink; and photoinitiator, wherein the photoinitiator is used in an amount ranging between 0.06% to 0.12% by weight of the UV-curable graphenebased conductive ink; and dispersant, wherein the dispersant is used in an amount ranging between 1.2% to 1.6% by weight of the UV-curable graphene-based conductive ink.

[0009] The present invention further relates to a method of manufacturing a near field communication, NFC antenna, wherein the method comprises the steps of (i) inkjet printing a first conductive layer on a flexible substrate using a UV-curable graphenebased conductive ink; (ii) curing the first conductive layer obtained from step (i) to produce a cured first conductive layer; (iii) inkjet printing dielectric layer onto the cured first conductive layer obtained from step (ii) to produce a dielectric layer; (iv) curing the dielectric layer obtained from step (iii) to produce a cured dielectric layer; (v) inkjet printing a second conductive layer on the cured dielectric layer obtained in step (iv) to produce a second conductive layer; (vi) curing the second conductive layer obtained from step (v) to produce a flexible NFC antenna; and (vii) curing the flexible NFC antenna obtained from step (vi) to produce the NFC antenna of the present invention.

[0010] Additional aspects, features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the accompanying drawings and preferred embodiments of the invention. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0011] The present invention will be fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, wherein:

[0012] In the attached drawings:

[0013] FIGURE 1 represents the illustration of a method of manufacturing a near field communication, NFC antenna.

[0014] FIGURE 2 represents the illustration of a UV-curable graphene inkjet printed-film a flexible substrate.

[0015] FIGURE 3 represents the illustration of an NFC that is produced via inkjet printing using UV-curable graphene ink of the present invention with the resolution between 30 pm to 100 pm.

[0016] FIGURE 4 represents the illustration of an NFC that is produced via inkjet printing using UV-curable graphene ink of the present invention with the resolution between 30 pm to 100 pm.

[0017] FIGURE 5 represents the illustration recognition distance test that is conducted using the NFC that is produced via inkjet printing using UV-curable graphene ink of the present invention.

[0018] FIGURE 6 represents the graph plotted for resistance changes for up to 20,000 bending cycles for the bending test.

[0019] DETAILED DESCRIPTION OF THE INVENTION

[0020] Detailed description of preferred embodiments of the present invention is disclosed herein. It should be understood, however the embodiments are merely exemplary of the present invention, which may be embodied in various forms. Therefore, the details disclosed herein are not to be interpreted as limiting, but merely as the basis for the claims and for teaching one skilled in the art of the invention. The numerical data or ranges used in the specification are not to be construed as limiting. The present invention relates to UV-curable graphene-based conductive ink for producing flexible near-field communication (NFC) tag and methods of manufacturing thereof, in particular, the UV-curable graphene-based conductive ink is subsequently used in manufacturing flexible NFC antenna that exhibits desirable characteristics.

[0021] First aspect of the present invention discusses on a UV-curable graphene-based conductive ink, where the ink comprises graphene-based colloid, photocurable polymer resin, photoinitiator and dispersant (composition as described in Table 1 ).

[0022] The graphene-based colloid is used in an amount ranging between 82% to 92% by weight, preferably 85.2% to 90.5%, most preferably 87.85 % by weight of the UV- curable graphene-based conductive ink. The graphene-based colloid comprises reduced graphene oxide and silver nanoparticles, wherein the reduced graphene oxide is used in an amount of 20% to 22% by weight, preferably 20.4% to 21 .6%, most preferably 21 % by weight of the graphene-based colloid and wherein the silver nanoparticles are used in amount of 19% to 21 % by weight, preferably 19.7% to 20.9% preferably 20.3% by weight of the graphene-based colloid. For the purpose of the present invention, the graphene oxide is used in a dispersion form and it has been dispersed in water prior to being used.

[0023] The photocurable polymer resin is used in an amount ranging between 8% to 12% by weight, preferably 10.3% to 10.9% by weight, most preferably 10.62% by weight of the UV-curable graphene-based conductive ink. The photocurable polymer resin is selected from the group consisting of aliphatic urethane acrylate, polyethylene glycol diacrylate and mixtures thereof, preferably polyethylene glycol diacrylate.

[0024] The photoinitiator is used in an amount ranging between 0.06% to 0.12% by weight, preferably 0.08% to 0.1 %, most preferably 0.09% by weight of the UV-curable graphene-based conductive ink. The photoinitiator is preferably 2-Hydroxy-4’-(2- hydroxyethoxy)-2-methylpropriophenone. For the purpose of the present invention, the photoinitiator is used in a powder form.

[0025] The dispersant is used in an amount ranging between 1.2% to 1.6% by weight, preferably 1 .3% to 1 .5% by weight, preferably 1 .4% by weight of the UV-curable graphene-based conductive ink. The dispersant is selected from the group consisting of ethylene glycol, terpineol, propylene glycol, ethanol, emulsion based on 2,4,6- trimethylbenzoyldiphenylphosphine oxide, a-hydroxyketones and benzophenone derivatives and mixtures thereof, preferably ethylene glycol.

[0026] Table 1 shows the chemical components and compositions thereof used in the UV- curable graphene-based conductive ink of the present invention.

[0027] Table 1 : Chemical components and compositions thereof used in the UV-curable graphene-based conductive ink of the present invention

[0028] Second aspect of the present invention discusses on a method of preparing the UV- curable graphene-based conductive ink. The method adapts the preparation of graphene-based colloid; followed by premixing and sonicating photocurable polymer resin and photoinitiator mixture; and preparing the UV-curable graphene-based ink.

[0029] Preparing graphene-based colloid

[0030] The first stage is the stage of preparing the graphene-based colloid of the present invention, wherein the first stage comprising the steps of: i. sonicating graphene oxide dispersion to produce sonicated graphene oxide dispersion, wherein the sonication process is carried out at a temperature ranging between 25°C to 35°C at a frequency of 37kHz for a duration ranging between 30 minutes to 60 minutes and wherein the graphene oxide dispersion has a concentration of ranging between 0.8 mg / mL to 1 .2 mg / mL, preferably 1 mg / mL; ii. stirring the sonicated graphene oxide obtained from step (i) to produce stirred graphene oxide, wherein the stirring is carried out at a speed ranging between 300 rpm to 500 rpm for a duration ranging between 45 minutes to 60 minutes at a temperature ranging between 25°C to 35°C; iii. adding polyacrylic acid, diethanolamine and water dropwise while stirring to produce reduced graphene oxide, wherein the stirring is carried out at a speed ranging between 300 rpm to 500 rpm for a duration ranging between 45 minutes to 60 minutes at a temperature ranging between 25°C to 30°C, wherein the polyacrylic acid is used in an amount ranging between 1 % to 2% by weight, preferably 1 .4% by weight of the graphene-based colloid, wherein the diethanolamine is used in an amount ranging between 25% to 35% by weight, preferably 29.2% by weight of the graphene-based colloid, wherein the water is used in an amount ranging between 30% to 40% by weight, preferably 35.3% by weight of the graphene-based colloid and wherein the polyacrylic acid, diethanolamine and water are pre-mixed prior to being added; iv. adding silver nitrate solution dropwise while stirring to the reduced graphene oxide obtained from step (iii) to produce a first mixture, wherein the silver nitrate solution has a concentration ranging between 6M to 8M, wherein the addition of silver nitrate is done at a flow rate setting ranging between 4 mL / min to 6 mL / min, wherein the stirring is carried out at a speed of 1000 rpm for a duration ranging between 30 minutes to 45 minutes at a temperature ranging between 25°C to 30°C and wherein the silver nitrate is simultaneously reduced for a duration ranging between 20 hours to 24 hours; v. sonicating the second mixture obtained from step (iv) to further grow the silver nitrate nanoparticles, wherein the sonication process is carried out at a frequency of 37kHz for a duration ranging between 60 minutes to 70 minutes at a temperature ranging between 60°C to 65°C; vi. adding solvent into the second mixture obtained from step (v) in order to clean the excess diethanolamine to produce cleaned second mixture, wherein the solvent is such as but not limited to ethanol and wherein the ratio of mixture to solvent is 1 :3; vii. centrifuging the cleaned second mixture obtained from step (vi) to obtain centrifuged second mixture, wherein the centrifuging process is carried out at a speed of 9000 rpm for a duration of 20 minutes at a temperature of 25°C; viii. removing the supernatant and collecting the remaining composite from the centrifuged second mixture obtained from step (vii) to obtain the graphenebased colloid of the present invention, wherein the composite is further dispersed in water to produce 0.1 g / mL of graphene-based colloid dispersion of the present invention; and ix. ultrasonicating the graphene-based colloid dispersion of the present invention obtained from step (viii) to produce nano particles of the graphenebased colloid of the present invention, wherein the ultrasonication process is carried out by firstly mixing at a speed of 80 rpm for a duration of 5 minutes, followed by dispersing at a speed of 80 rpm at a sonicating frequency of 40kHz for a duration of 30 minutes, followed by mixing at a speed of 80 rpm for a duration of 10 minutes and dispersing at a speed of 80 rpm at a sonicating frequency of 40kHz for a duration of 60 minutes. mixture

[0031] The second stage is the stage of preparing photocurable polymer resin and photoinitiator mixture, wherein the stage comprising the steps of: i. adding photocurable polymer resin and photoinitiator into a glass vial to produce a mixture, wherein the photocurable polymer resin is used in an amount ranging between 8% to 12% by weight, preferably 10.62% by weight of the UV-curable graphene-based conductive ink and wherein the photoinitiator is used in an amount ranging between 0.06% to 0.12% by weight, preferably 0.09% by weight of the UV-curable graphene-based conductive ink; and ii. sonicating the mixture obtained from step (i) to produce a mixture of photocurable polymer resin and photoinitiator, wherein the sonicating process is carried out in a sonicator bath at a temperature ranging between 25°C to 35°C at a frequency of 37kHz for a duration of 5 minutes to 10 minutes.

[0032] I the UV-curable a ra phene- based ink of the present invention

[0033] The third stage is the stage of preparing the UV-curable graphene-based ink of the present invention, wherein the stage comprising the steps of: i. adding the graphene-based colloid obtained from the first stage into the mixture of photocurable polymer resin and photoinitiator obtained from the second stage to produce a mixture; ii. adding dispersant into the mixture obtained from step (i) to produce the UV- curable graphene-based conductive ink of the present invention, wherein for the purpose of the present invention, the addition of the dispersant is to modify the surface tension of the UV-curable graphene-based conductive ink to be in a range between 40 mN / m to 60mN / m; and iii. homogenizing the UV-curable graphene-based conductive ink of the present invention, wherein the homogenization process is carried out by firstly mixing in a mixer at a speed of 2000 rpm for a during of 1 minute, followed by degassing at a speed of 800 rpm for a duration of 1 minute and finally mixing and degassing at a speed of 800 rpm for a duration of 30 second.

[0034] The UV-curable graphene-based conductive ink is further filtered using 5 pm and 0.2 pm filter disc into the inkjet printer cartridge to ensure a smoother flow of ink and a better printing quality.

[0035] Third aspect of the present invention discusses on the substrate of the present invention, wherein the method of preparing the substrate comprises the steps of cleaning the substrate prior to being used in inkjet printing to ensure no residue that could affect the adherence of the ink to the substrate, wherein the substrate are such as but not limited to polyethylene terephthalate (PET), polyvinyl alcohol (PVA) and polyimide and wherein the cleaning process can be done by: i) wiping the substrate surface with lint-free wipes; or ii) wiping the substrate surface with 70% alcohol and drying the substrate with nitrogen air.

[0036] Fourth aspect of the present invention discusses on a method of manufacturing the NFC antenna of the present invention (1000) as illustrated in FIGURE 1 , wherein the method comprises the steps of: i. inkjet printing a first conductive layer on the flexible substrate (100) as discussed in the third aspect using the UV-curable graphene-based conductive ink as discussed in the second aspect of the present invention, wherein the printing process is carried out at a drop spacing ranging between 10 pm to 20 pm, preferably 15 pm, with cartridge height ranging between 0.5 mm to 1 .0 mm, preferably 0.7 mm, at a jetting voltage ranging between 20 V to 25 V, at a frequency ranging between 20kHz to 30kHz, preferably 25kHz; at a meniscus point ranging between 3.0 to 4.0 in water, preferably 3.2 in water; at a platen temperature ranging between 35°C to 45°C, preferably 40°C; and a cartridge temperature ranging between 25°C to 30°C, preferably 28°C; ii. curing the first conductive layer (200) obtained from step (i) to produce a cured first conductive layer, wherein the curing process is carried out via UV-light having power ranging between 5% to 15%, preferably 10% for a duration ranging between 100 seconds to 150 seconds, preferably 120 seconds; iii. inkjet printing dielectric layer onto the cured first conductive layer (300) obtained from step (ii) to produce a dielectric layer, wherein the printing process is carried out at a drop spacing ranging between 10 pm to 20 pm, preferably 15 pm, with cartridge height ranging between 0.5 mm to 1 .0 mm, preferably 0.7 mm, at a jetting voltage ranging between 20 V to 25 V, at a frequency ranging between 20kHz to 30kHz, preferably 25kHz; at a meniscus point ranging between 3.0 to 4.0 in water, preferably 3.2 in water; at a platen temperature ranging between 35°C to 45°C, preferably 40°C; and a cartridge temperature ranging between 25°C to 30°C, preferably iv. curing the dielectric layer (400) obtained from step (iii) to produce a cured dielectric layer, wherein the curing is carried out at a temperature less than 60°C; v. inkjet printing a second conductive layer (500) on the cured dielectric layer obtained in step (iv) to produce a second conductive layer, wherein the printing process is carried out at a drop spacing ranging between 10 pm to 20 pm, preferably 15 pm, with cartridge height ranging between 0.5 mm to 1 .0 mm, preferably 0.7 mm, at a jetting voltage ranging between 20 V to 25 V, at a frequency ranging between 20kHz to 30kHz, preferably 25kHz; at a meniscus point ranging between 3.0 to 4.0 in water, preferably 3.2 in water; at a platen temperature ranging between 35°C to 45°C, preferably 40°C; and a cartridge temperature ranging between 25°C to 30°C, preferably 28°C; vi. curing the second conductive layer (600) obtained from step (v) to produce a flexible NFC antenna, wherein the curing process is carried out via UV- light having power ranging between 5% to 15%, preferably 10% for a duration ranging between 100 seconds to 150 seconds, preferably 120 seconds; and vii. curing the flexible NFC antenna (700) obtained from step (vi) to produce the NFC antenna of the present invention, wherein the curing process is carried out at a temperature of 40°C for a duration of 30 minutes.

[0037] FIGURE 2 shows the UV-curable graphene inkjet printed film on a flexible substrate based on the above method.

[0038] FIGURES 3 and 4 shows the examples of NFCs that are produced via inkjet printing using UV-curable graphene ink of the present invention with the resolution between 30 pm to 100 pm. This is important for precision printing that requires detailing and sharpness of the printed design such that the occurrence of short-circuits between conductive lines can be avoided.

[0039] The following example is constructed to illustrate the present invention in a non-limiting sense. The NFC antenna of the present invention is prepared using the UV-curable graphenebased conductive ink’s composition as described in Table 1 adopting methods as described in the second to fourth aspects of the present invention.

[0040] TEST RESULTS Test results for the NFC antenna of the present invention

[0041] The NFC antenna of the present invention is also subjected to a distance recognition test. It should be noted that the NFC antenna of the present invention is manufactured according to the ISO / IEC 14443 standard wherein a Proximity Integrated Circuit Cards (PICC) are intended to operate within a distance of approximately 10 cm. As illustrated in FIGURE 5, the recognition distance is shown to be less than 10 cm, which indicates that the present invention is readable within the standard requirement.

[0042] The NFC antenna of the present invention is subjected to a bending test, wherein the bending test is carried out up to 20,000 cycles. The bending test is tested according to ASTM D790 test method. For the purpose of the present invention, the bending test helps to evaluate the NFC antenna to withstand repeated bending which stimulates the conditions during the handling of the NFC tag having an NFC antenna.

[0043] Table 2 shows the outcome of bending test of the NFC antenna of the present invention.

[0044] Table 2: Outcome of bending test of the NFC antenna of the present invention

[0045] Based on Table 2 above and as illustrated in FIGURE 6, it is evident that the NFC antenna of the present invention exhibits an improved resistance although the bending cycles increases.

[0046] As a whole, the NFC tag of the present invention is able to overcome the conventional shortcomings since the present invention is able to exhibit desirable outcomes for distance recognition test and bending test. The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises", "comprising", “including”, and “having” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups therefrom.

[0047] The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. The use of the expression “at least” or “at least one” suggests the use of one or more elements, as the use may be in one of the embodiments to achieve one or more of the desired objects or results.

Claims

CLAIMS1 . A UV-curable graphene-based conductive ink comprising: graphene-based colloid, wherein the graphene-based colloid comprises reduced graphene oxide and silver nanoparticles, wherein the graphenebased colloid is used in an amount ranging between 82% to 92% by weight of the UV-curable graphene-based conductive ink, wherein the reduced graphene oxide is used in an amount of 20% to 22% by weight of the graphene-based colloid and wherein the silver nanoparticles are used in amount of 19% to 21 % by weight of the graphene-based colloid; photocurable polymer resin, wherein the photocurable polymer resin is used in an amount ranging between 8% to 12% by weight of the UV-curable graphene-based conductive ink; and photoinitiator, wherein the photoinitiator is used in an amount ranging between 0.06% to 0.12% by weight of the UV-curable graphene-based conductive ink; and dispersant, wherein the dispersant is used in an amount ranging between 1.2% to 1.6% by weight of the UV-curable graphene-based conductive ink.

2. The UV-curable graphene-based conductive ink as claimed in claim 1 , wherein the photocurable polymer resin is selected from the group consisting of aliphatic urethane acrylate, polyethylene glycol diacrylate and mixtures thereof.

3. The UV-curable graphene-based conductive ink as claimed in claim 1 , wherein the photoinitiator is preferably 2-Hydroxy-4’-(2-hydroxyethoxy)-2- methylpropriophenone.

4. The UV-curable graphene-based conductive ink as claimed in claim 1 , wherein the dispersant is selected from the group consisting of ethylene glycol, terpineol, propylene glycol, ethanol, emulsion based on 2,4,6-trimethylbenzoyldiphenylphosphine oxide, a-hydroxyketones and benzophenone derivatives and mixtures thereof.

5. A method (1000) of manufacturing a near field communication, NFC antenna, wherein the method comprises the steps of: i. inkjet printing a first conductive layer on a flexible substrate (100) using a UV-curable graphene-based conductive ink; ii. curing the first conductive layer (200) obtained from step (i) to produce a cured first conductive layer; iii. inkjet printing dielectric layer onto the cured first conductive layer (300) obtained from step (ii) to produce a dielectric layer; iv. curing the dielectric layer (400) obtained from step (iii) to produce a cured dielectric layer; v. inkjet printing a second conductive layer on the cured dielectric layer (500) obtained in step (iv) to produce a second conductive layer; vi. curing the second conductive layer (600) obtained from step (v) to produce a flexible NFC antenna; and vii. curing the flexible NFC antenna (700) obtained from step (vi) to produce the NFC antenna of the present invention.

6. The method as claimed in claim 5, wherein the flexible substrate in step (i) are such as but not limited to polyethylene terephthalate, polyvinyl alcohol and polyimide.

7. The method as claimed in claim 5, wherein the printing process in steps (i) and (iii) are carried out at a drop spacing ranging between 10 pm to 20 pm, with cartridge height ranging between 0.5 mm to 1 .0 mm, at a jetting voltage ranging between 20 V to 25 V, at a frequency ranging between 20kHz to 30kHz; at a meniscus point ranging between 3.0 to 4.0 in water; at a platentemperature ranging between 35°C to 45°C; and at a cartridge temperature ranging between 25°C to 30°C.

8. The method as claimed in claim 5, wherein the curing process in step (ii) is carried out via UV-light having power ranging between 5% to 15%, preferably 10% for a duration ranging between 100 seconds to 150 seconds.

9. The method as claimed in claim 5, wherein the curing process in step (iv) is carried out at a temperature less than 60°C.

10. The method as claimed in claim 5, wherein the printing process in step (v) is carried out at a drop spacing ranging between 10 pm to 20 pm, with cartridge height ranging between 0.5 mm to 1.0 mm, at a jetting voltage ranging between 20 V to 25 V, at a frequency ranging between 20kHz to 30kHz; at a meniscus point ranging between 3.0 to 4.0 in water; at a platen temperature ranging between 35°C to 45°C; and at a cartridge temperature ranging between 25°C to 30°C.1 1 .The method as claimed in claim 5, wherein the curing process in step (vi) is carried out via UV-light having power ranging between 5% to 15%, preferably 10% for a duration ranging between 100 seconds to 150 seconds.

12. The method as claimed in claim 5, wherein the curing process in step (vii) is carried out at a temperature of 40°C for a duration of 30 minutes.

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