Method for synthesizing silica reinforced polyester adhesive

Abstract

Disclosed is a method for synthesizing a silica reinforced polyester adhesive. The method comprises: dissolving an organic acid and a polyol under a first predetermined set of conditions to obtain an adduct; adding silica to the adduct to obtain a silica-adduct mixture under the first predetermined set of conditions; and adding a Lewis acid catalyst to the silica- adduct mixture under a second predetermined set of conditions to obtain the silica reinforced polyester adhesive. Disclosed also is a method for obtaining a silica from a silica-rich lignocellulosic biomass.

Description

[0001] METHOD FOR SYNTHESIZING SILICA REINFORCED POLYESTER ADHESIVE

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to methods for synthesizing silica reinforced polyester adhesives. Moreover, the present disclosure relates to methods for obtaining silica from silica-rich lignocellulosic biomasses.

[0004] BACKGROUND

[0005] Polyester-based adhesives have long been recognized for their versatility and compatibility with a wide range of substrates, making them a preferred choice for bonding applications across industries. Despite the versatility of conventional polyester adhesives, they often fall short in meeting the stringent requirements of modem industrial applications, particularly those involving high-stress environments or exposure to extreme temperatures and chemicals. The main challenge lies in achieving optimal adhesive performance under these demanding conditions while maintaining cost-effectiveness and ease of application. Conventional polyester adhesives may exhibit limitations such as inadequate mechanical strength, poor thermal stability, and limited resistance to chemical degradation, compromising the reliability and longevity of bonded assemblies. Addressing these shortcomings is crucial for ensuring the widespread adoption of polyester-based adhesives in critical applications where performance and durability are paramount.

[0006] Various solutions have been proposed to enhance the performance of polyester adhesives, including the incorporation of reinforcing fillers and additives to improve mechanical properties and resistance to environmental factors. However, many of these approaches have inherent limitations, such as difficulties in achieving uniform dispersion of fillers within the adhesive matrix, leading to inconsistencies in adhesive performance and potential bonding failures. Additionally, some reinforcement materials may introduce undesirable side effects, such as increased viscosity or reduced adhesion to certain substrates, limiting their practical utility in diverse applications. Moreover, the use of conventional catalysts in polyester adhesive formulations may not always yield optimal results, as they may exhibit limited catalytic activity or compatibility with specific reactants, hindering the efficiency of the adhesive synthesis process. The need for polyester adhesives with enhanced performance characteristics, particularly in terms of strength, durability, and resistance to environmental factors, has driven the development of innovative adhesive formulations, such as silica reinforced polyester adhesives, which is prepared by integrating silica particles in the structural matrix of the polyester adhesive.

[0007] In this regard, silica powder, also known as silicon dioxide powder, serves as a versatile material with numerous applications across various industries. Conventionally, silica powder is produced through methods such as Silica Sand Grinding, Precipitation, Pyrolysis, Sol-Gel Process, Flame Spray Pyrolysis, and Chemical Vapor Deposition (CVD). These methods allow for customization of silica powder properties, including particle size, morphology, and surface characteristics, tailored to specific application requirements.

[0008] Despite the effectiveness of conventional methods, they are associated with several drawbacks. High energy consumption, environmental impact, challenges in particle size distribution control, equipment complexity and maintenance, safety concerns, high production costs, and difficulties in scaling up production are some of the key issues plaguing conventional silica powder production methods. Additionally, these methods generate by-products and waste, further adding to environmental concerns.

[0009] Biological methods offer promising alternatives by harnessing natural processes for silica generation. However, challenges such as low silica content in plant sources, variability in silica content, low extraction efficiency, contamination and impurities, high processing costs, seasonal availability of biomass, and difficulties in scaling up and commercialization hinder the widespread adoption of biological methods.

[0010] While conventional chemical and physical methods provide effective means for silica powder production, they entail significant drawbacks. These methods require substantial energy inputs, leading to high operational costs and environmental impact. Moreover, challenges in controlling particle properties and ensuring product purity pose additional hurdles. On the other hand, biological methods offer eco-friendly alternatives but face limitations such as low silica content in biomass sources, inconsistent extraction efficiency, and high processing costs. Therefore, in light of the foregoing discussion, there exists a need to overcome the aforementioned drawbacks.

[0011] SUMMARY

[0012] The aim of the present disclosure is to provide a method to efficiently synthesize silica- reinforced polyester adhesive having strength, durability, and resistance to environmental factors and which can be used in applications requiring stable thermochemical properties, strength and structural stability. The aim of the present disclosure is achieved by a method for synthesizing a silica reinforced polyester adhesive, as defined in the appended independent claims to which reference is made to. Advantageous features are set out in the appended dependent claims. Another aim of the present disclosure is to provide a method for generating silica from silica-rich lignocellulosic biomass, in an ecofriendly and sustainable manner.

[0013] Throughout the description and claims ofthis specification, the words "comprise", "include", "have", and "contain" and variations of these words, for example "comprising" and "comprises" , mean "including hut not limited to", and do not exclude other components, items, integers or steps not explicitly disclosed also to be present. Moreover, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is an illustration of a flowchart depicting steps of a method for synthesizing a silica reinforced polyester adhesive, in accordance with an embodiment of the present disclosure.

[0016] DETAILED DESCRIPTION OF EMBODIMENTS

[0017] The following detailed description illustrates embodiments of the present disclosure and ways in which they may be implemented. Although some modes of carrying out the present disclosure have been disclosed, those skilled in the art would recognize that other embodiments for carrying out or practising the present disclosure are also possible.

[0018] In one aspect, the present disclosure provides a method for synthesizing a silica reinforced polyester adhesive, the method comprising: dissolving an organic acid and a polyol under a first predetermined set of conditions to obtain an adduct; adding silica to the adduct to obtain a silica-adduct mixture under the first predetermined set of conditions; and adding a Lewis acid catalyst to the silica-adduct mixture under a second predetermined set of conditions to obtain the silica reinforced polyester adhesive.

[0019] The aforementioned method provides a novel adhesive formulation (namely, silica- reinforced polyester adhesive) which is bio-compostable, aligning with growing environmental concerns and regulatory requirements. Moreover, the aforementioned method allows efficient extraction of silica from various agricultural by-product with minimal waste - generation during the process. Advantageously, the aforementioned method provides an innovative and sustainable solution to achieve efficient utilization of resources in biomass (agricultural by-product) conversion technologies. Utilizing bio-silica derived from selected lignocellulosic biomass further enhances the sustainability aspect of the adhesive, contributing to eco-friendly manufacturing practices. Additionally, the flexibility to tailor adhesive properties by selecting different organic acids enables customization to meet specific application needs, providing versatility in adhesive performance. Furthermore, reinforcing the adhesive formulation (by surface modification) using silica, the method provides the silica-reinforced polyester adhesive with enhanced strength, thermochemical properties, compatibility with various other raw materials, and therefore expanding the range of potential applications across industries such as automotive, construction, and electronics. Furthermore, the disclosed method allows efficient silica cross-linking to integrate a structural matrix of polyester adhesives. The cross-linking step of the disclosed method, further improves adhesive performance, enhancing durability and resistance for demanding applications. In other words, silica cross-linking process (as described in the disclosed method) enhances the mechanical strength, thermal stability, and chemical resistance of the resulting adhesive product, making it suitable for a wide range of high-performance applications, including adhesives, sealants, and composite matrices. Thus, the aforementioned method provides an adhesive formulation that can be tailored to meet specific application requirements, offering versatility and performance in various bonding applications across industries such as automotive, construction, and electronics. Advantageously, compared to existing options, the incorporation of silica particles within the polyester resin enhances mechanical strength, offering superior resistance to stress and wear. Beneficially, the thermal stability of silica enables the adhesive to withstand higher temperatures without degradation, while its chemical resistance expands the adhesive's suitability for diverse applications. In summary, the disclosed method presents a versatile, high-performing adhesive solution by combining polyester resin with silica reinforcement, addressing key challenges and offering enhanced performance across various industrial applications.

[0020] Throughout the present disclosure, the term "agricultural by-products" refers to residual materials or substances generated as a result of various agricultural processes or activities. In other words, agricultural by-products are secondary products that are generated during the production, processing, or harvesting of agricultural crops. Specifically, agricultural byproducts typically include any organic matter or residues that remain after the primary agricultural product has been harvested or processed. Notably, agricultural biomass is a lignocellulosic biomass rich in silica contents. Notably, the agricultural by-products may be, but not limited to, crop residues, straw, husks, stems, leaves, stalks, shells, pomace, and other biomass materials. It may be appreciated that throughout the present disclosure the agricultural by-products is selected from at least one of: rice husk, rice straw, wheat straw, sugarcane bagasse, sorghum straw. In a preferable embodiment, agricultural by-products used in production of silica derivative, is rice straw. It may be appreciated that, throughout the present disclosure, the agricultural by-products may also be referred to as lignocellulosic biomass or silica-rich lignocellulosic biomass.

[0021] It may be appreciated that the lignocellulosic biomass (i.e., the agricultural by-products) comprise various biomass constituents such as hemicellulose, cellulose, lignin, silica, pectin, proteins, sugars, organic and inorganic biomass constituents found in plant cells. The term "biomass constituent" refers to various fundamental chemical and structural components that make up the bio-physical structure of the lignocellulosic biomass (i.e., the agricultural byproducts). The aforementioned biomass constituents can be used directly as raw materials to produce various consumable goods and products or can be used after processing to produce various consumable goods and products. Moreover, such biomass constituents can be utilized in industries such as construction, manufacturing, pharmaceuticals, cosmetics and consumer goods. It may be appreciated that the biomass constituents are value-added materials, chemicals, or items obtained from the agricultural by-products or waste that are utilized for applications other than fuel or energy generation. It may be appreciated that, throughout the present disclosure, the biomass constituents extracted from the lignocellulosic biomass is silica.

[0022] The term "silica" refers to an amorphous silicon dioxide (SiO2), encompassing both hydrated and anhydrous forms of amorphous silica that are substantially free of crystalline phases, and which may be obtained through thermal or chemical treatment processes. In particular, the silica referenced herein is characterized by its high surface area, chemical inertness, and lack of long-range atomic order, distinguishing it from crystalline forms such as quartz or cristobalite. Silica is used in various industries such as manufacturing industries, cosmetics industries and pharmaceutical industries. The silica extracted is used to produce silica based adhesives which possess high thermal stability, chemical resistance, and strong bonding capabilities and are used in a various industries such as, construction, medical, and electronic applications.

[0023] Throughout the present disclosure, the term " silica-reinforced polyester adhesive" refers to adhesive produced by integrating silica into a structural matrix of a polyester adhesive, to improve bonding strength, durability, stiffness, and resistance to heat, chemicals, and moisture, thereof. Throughout the present disclosure, the term "synthesize" refers to a process of preparing, producing, or creating the silica-reinforced polyester adhesive through a thermochemical reaction, a series of thermochemical reactions, a chemical reaction or a series of chemical reactions. In this regard, synthesizing involves combining different chemical components or precursors in a controlled manner to form the desired adhesive material. It may be appreciated that synthesizing specifically involves integrating / introducing silica into the structural matrix of the polyester adhesive.

[0024] Throughout the present disclosure, the term "organic acid" refers to an organic compound with acidic properties, typically comprising at least one of: a carbon group (such as carboxylic group (-COOH)), other acidic functional groups (such as sulfonic acid (-SO3H), phenol (-OH), and enol groups). The organic acids are typically weak acid and water soluble in nature. Throughout the present disclosure, the term "polyol" refers to a compound comprising a plurality of hydroxyl groups (-OH) in its structural matrix. Notably, the carbon group, other acidic functional groups and the plurality of hydroxyl groups are reactive groups of the organic acids and polyols respectively. In this regard, the reactive groups are prone to interact with each other when the organic acid and the polyols are combined, under specific physical and thermochemical conditions.

[0025] The method comprises the step of dissolving the organic acid and the polyol under a first predetermined set of conditions to obtain an adduct. The term "adduct" refers to product of a direct addition of two or more distinct reactants (namely, the organic acid and polyol), resulting in a single reaction product. In other words, the adduct refers to a homogeneous mixture of the organic acid and the polyol. It may be appreciated that the adduct refers to an esterified complex (namely, a polyester adhesive) formed due to interaction / chemical reaction between the organic acid and the polyol. The adduct serves as a base media for further processing during preparation of silica-reinforced polyester adhesive. The term "first predetermined set of conditions" refers to the specific physical and thermochemical conditions (such as temperature, pressure and so on) which are set at a predetermined value to achieve a desired chemical reaction between various ingredients / substances (namely, the organic acid and the polyol).

[0026] In an embodiment, the first predetermined set of conditions comprises: a concentration of the organic acid in a range of 25 to 75 % by volume of the adduct; a temperature in a range of 65-95°C; a time period in a range of 20-90 minutes; continuous stirring at a speed in a range of 200-800 rotation per minute (RPM); and an equimolar concentration of the polyol.

[0027] In this regard, the concentration of the organic acid is in the range from 25, 30, 35, 40, 45, 50, 55, 60, 65 or 70% up to 30, 35, 40, 45, 50, 55, 60, 65, 70 or 75% by volume of the adduct. The temperature which is suitable for promoting and enhancing reaction between the organic acid and the polyol is in the range from 65, 70,75, 80, 85 or 90°C up to 70,75, 80, 85, 90 or 95°C. Similarly, the organic acid and the polyol should be allowed to interact at the aforementioned temperature and for the time period, with continuous stirring. The time period in range from 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80 or 85 minutes up to 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or 90 minutes, and the stirring speed is in the range of 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700 or 750 RPM up to 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750 or 800 RPM, in order to successfully form the adduct. Notably, the organic acid is combined with the polyol of equimolar concentration to promote desired chemical reaction between them thereby forming the adduct having required physical and chemical properties. In this regard, under the first predetermined set of conditions, when the organic acid is mixed with the polyol (in equimolar proportion), the reactive groups of the organic acid and the polyol interact with each other to undergo (at least partial) esterification and / or polymerisation to form the adduct. Moreover, continuous stirring (at the aforementioned stirring speed range) and heating (at the aforementioned temperature range) of a mixture of the organic acid and the polyol, promote thorough mixing and enhance miscibility, and thereby facilitating the complete dissolution of the organic acid and polyol. Optionally, throughout the process of mixing / dissolving, various monitoring techniques were implemented to ensure the progression of dissolution, with periodic assessments confirming the formation of the adduct as required / desired. The technical effect of dissolving the organic acid and the polyol under the first predetermined set of conditions is efficient preparation of the adduct having required physical and chemical properties.

[0028] In an embodiment, the organic acid is selected from at least one of: citric acid, adipic acid, maleic acid, oxalic acid, acetic acid, furan dicarboxylic acid (FDCA); and the polyol is selected from at least one of: ethylene glycol, propylene glycol, glycerol, sorbitol, mannitol, erythritol. In this regard, the aforementioned organic acids are highly reactive when allowed to interact with the aforementioned polyols. The aforementioned organic acids and the polyols when dissolved together form undergo various chemical reactions (such as at least partial esterification and polymerisation) to form a polyester adhesive (namely, the adduct). In an exemplary embodiment, the organic acid is citric acid, and the polyol is glycerol. In an exemplary embodiment, citric acid is initially dissolved in a polyol (such as glycerol) under aforementioned first predefined set of conditions (for example, at equimolar concentration of glycerol, at a temperature of 80°C for approximately a time period of 40- 60 min under continuous stirring) to prepare the silica-reinforced polyester adhesive. Notably, the choice of organic acids and the polyols determines various properties (such as viscosity, solubility, adhesive properties, biodegradability, thermal resistance and other physical and chemical properties) of the adduct. In other words, the adhesive properties of the silica reinforced polyester adhesive can be tailored for specific needs based on the organic acid. It may be appreciated that the aforementioned organic acids and polyols and / or any combination thereof are selected based on the type of adduct required. The technical advantage is versality in adduct preparation by providing a diverse range of raw materials (i.e., the organic acid and the polyol). The method comprises the step of adding silica to the adduct, in order to obtain a silica- adduct mixture. Notably, the step of silica-addition is performed under the first predetermined set of conditions (i.e., at the temperature in the range of 65-95°C, for the time period in the range of 20-90 minutes while continuously stirring the adduct at the stirring speed in the range of 200-800). It may be appreciated that the silica which is added to the adduct is in form of amorphous silicon dioxide (SiO2) obtained from the lignocellulosic biomass. This step aims to enhance the adhesive's mechanical properties and durability through the incorporation of silica nanoparticles or a silica precursor. It may be appreciated that the adduct may be subjected to mechanical stirring or sonication to facilitate thorough mixing and prevent agglomeration / clumping of silica particles and obtain silica-adduct mixture.

[0029] In an embodiment, the silica is added to the adduct directly as powered silica or as silica suspension. In this regard, the silica obtained from the agricultural by-products is typically amorphous silica. The silica (obtained from the silica-rich lignocellulosic biomass) is either powdered using suitable milling techniques. In this regard, milling is a mechanical process that involves reducing the size of particles by applying mechanical force. In this case, the silica is subjected to milling equipment such as ball mills, jet mills, or attrition mills, which effectively break down the particles to achieve the desired size range. During milling, the silica powder is introduced into the mill along with grinding media, such as balls or beads, which collide with the powder and break it down into smaller particles. The duration and intensity of milling are controlled to ensure uniform particle size distribution and achieve a desired / target particle size. Once the milling process is completed, the resulting powered silica is characterized by its fine particle size and consistent particle distribution, making it suitable for various applications such as in the production of paints, coatings, adhesives, and composites. The controlled particle size ensures optimal performance and functionality in the final product. It may be appreciated that the particle size of the silica in the powered silica (obtained from the silica-rich lignocellulosic biomass) is milled in the range of 50 to 110 microns. For example, the particle size of powered silica may be from 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100 or 105 microns up to 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105 or 110 microns, using suitable milling techniques. Notably, the particles of powered silica (of the aforementioned size range of 50 to 110 microns) have greater surface area and less settling tendency, promoting better dispersion in in the adduct. Similarly, the silica, when added in suspension form (as silica suspension), then it disperses uniformly and rapidly throughout the adduct. Notably, the silica suspension is prepared by forming a homogenous mixture of silica in a suitable / compatible solvent. In this regard, the homogenous mixture namely, the silica suspension is prepared by dispersing the silica in the suitable / compatible solvent, ensuring optimal dispersion and compatibility. Subsequently, the silica suspension is gradually introduced to the adduct obtained in the prior step, with continuous mixing to achieve uniform distribution of silica within the adduct matrix. The concentration of silica in the silica suspension is in a range of 5 to 15 w / w percent. The suitable solvent is selected depending on what organic acid and polyol are used to prepare the adduct in order to avoid any unnecessary reaction from occurring. The suitable solvent may be selected from: water, water-ethanol mixture (in 9: 1 ratio), water-ethanol mixture (in 8:2 ratio), water-methanol (in 9: 1 ratio), water-methanol (in 8:2 ratio), any suitable water-alcohol mixture or any other suitable solvent. The technical benefit of adding silica in powdered form or in suspension form is that particles of silica are more uniformly distributed throughout the adduct thereby enhancing efficiency of further processing to form silica reinforced polyester adhesive.

[0030] In an embodiment, the silica is obtained from a silica-rich lignocellulosic biomass by: pre-treating the silica-rich lignocellulosic biomass to obtain lignin-silica hydrolysate; precipitating the lignin-silica hydrolysate at a first predefined pH to obtain a precipitated silica therefrom; subjecting the precipitated silica to a heat treatment at a temperature in a range of 545-580°C, in an inert environment to obtain a residue; dissolving the obtained residue in a suitable alkali solution to isolate a supernatant therefrom; and precipitating the supernatant at a second predefined pH, to obtain the silica.

[0031] In this regard, the aforementioned method comprises steps for obtaining silica from agricultural by-products (i.e., the silica-rich lignocellulosic biomass). The silica-rich lignocellulosic biomass is pretreated to obtain lignin-silica hydrolysate. Notably, the lignin- silica hydrolysate is a crucial precursor in the extraction of silica from silica-rich lignocellulosic biomass. The term " pretreatment'' refers to a thermochemical process which involves subjecting the agricultural by-products to specific physical and thermochemical conditions (such as temperature, pressure and so on) in the presence of specific chemicals (such as acids and alkalis) to alter their bio-physical structure and composition. Notably, the pretreatment aids in breaking down complex biomass constituents (such as polymers, cellulose, silica, lignin and hemicellulose contained in the silica-rich lignocellulosic biomass) into simpler compounds, thus improving the accessibility of the biomass constituents for ease of extraction thereof. It may be appreciated that by pre-treating the silica-rich lignocellulosic biomass, the disclosed method herein optimizes the efficiency and effectiveness of extraction of silica in relatively pure form. Moreover, the pretreatment step aims to enhance the accessibility of silica within the biomass matrix, facilitating its subsequent extraction. Various pre-treatment methods may be employed, including physical, chemical, or biological approaches, tailored to efficiently disrupt the lignocellulosic bonds while minimizing energy and chemical inputs. The resulting lignin-silica hydrolysate serves as a key intermediate for further processing, enabling the extraction of silica with enhanced efficiency and yield.

[0032] Notably, the lignin-silica hydrolysate produced as result of pretreatment refers to a hydrolysed product comprising both lignin and silica. The lignin-silica hydrolysate is then subjected to precipitation under specific conditions such as at a first predefined pH which results in separation of silica from the lignin-silica hydrolysate. The separated silica is obtained as precipitated silica. In this regard, the first predefined pH is in a range of 8-9. For example, the first predefined pH may be 8, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8 or 8.9 up to 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9 or 9. This range of first predefined pH allows effective precipitation of silica from the lignin-silica hydrolysate. This adjustment range is chosen to optimize the precipitation of silica while ensuring compatibility with subsequent processing steps. Moreover, this step entails controlling the pH of the lignin-silica hydrolysate solution to induce silica precipitation, followed by filtration to separate the precipitated silica from the solution. Notably, pH adjustment is typically achieved by adding acidic or alkaline agents to the hydrolysate solution, causing a shift in pH towards the desired range for silica precipitation. The pH adjustment triggers the formation of insoluble silica particles, which precipitate out of the solution. Filtration is then employed to separate the precipitated silica from the remaining liquid phase, resulting in a clarified solution and a solid precipitate containing silica. This step ensures the efficient recovery of silica from the lignin-silica hydrolysate, laying the groundwork for further processing to obtain purified silica suitable for various applications.

[0033] Optionally, suitable acidic or basic agents may be selectively added to the lignin-silica hydrolysate solution to modulate its pH within the specified range. Acidic agents such as hydrochloric acid or sulfuric acid lower the pH, while basic agents such as sodium hydroxide or ammonium hydroxide elevate the pH, as necessary to induce silica precipitation. Maintaining the pH within the aforementioned range facilitates efficient silica precipitation while minimizing the risk of unwanted side reactions or precipitation of other compounds. This optional feature allows for flexibility in pH adjustment methods, accommodating variations in raw materials and process conditions, while ensuring consistent and reliable silica recovery from the lignin-silica hydrolysate.

[0034] Optionally, the precipitated silica is subjected to centrifugation at 5000-8000 RPM. Optionally, the precipitated silica is subjected to filtering using a suitable filter. The suitable filter is selected from at least one of: cellulose filter, nylon filter, polyether sulfone filter, capable of removing unwanted residues (such as unwanted lignin residues). The aforementioned filters provides flexibility in the filtration process, based on factors such as desired particle size retention, chemical compatibility, and cost-effectiveness. Cellulose filters, composed of cellulose fibres, offer broad compatibility and are commonly used for general-purpose filtration. Nylon filters, constructed from nylon polymers, provide enhanced chemical resistance and mechanical strength, making them suitable for filtration of aggressive solutions or high-pressure applications. Polyether sulfone filters, made from thermoplastic polymers, exhibit excellent chemical resistance and thermal stability, making them ideal for filtration of harsh chemicals or elevated temperature solutions. By optionally choosing from these filter mediums, the filtration process can be tailored to meet specific requirements, ensuring efficient separation of precipitated silica from the solution while maintaining product integrity and purity.

[0035] Optionally, the precipitated silica is treated with water to remove the salts. Notably, after the precipitation of silica from the lignin-silica hydrolysate and filtration to separate the solid precipitate, the resulting precipitated silica is crude from and may contain residual salts from the precipitation process. Specifically, treating the precipitated silica precipitate with water allows for the dissolution and removal of these salts, thereby improving the purity of the silica product. In this regard, water is added to the precipitated silica precipitate, and the mixture is stirred or agitated to facilitate the dissolution of salts. Subsequently, the mixture is subjected to filtration or centrifugation to separate the dissolved salts from the silica particles. This step ensures the removal of impurities and enhances the quality of the final silica product, making it suitable for various industrial applications where high purity is required.

[0036] Afterwards, the precipitated silica is subjected to the heat treatment which causes the precipitated silica to dehydrate / dry (removal of moisture) and thus the residue is obtained. The precipitated silica is subjected to heat treatment in order to vaporize organics (such as salts) therefrom. Notably, the precipitated silica, obtained from the previous steps of the process contains residual organic compounds that may have been introduced during the pretreatment and precipitation stages. In order to obtain pure silica, these organic compounds need to be removed. This is achieved through heat treatment, where the precipitate is subjected to elevated temperatures to facilitate the vaporization of organic materials. During the heat treatment process, the organic compounds undergo thermal decomposition and vaporize, leaving behind purified silica. The temperature and duration of the heat treatment can be optimized based on the composition of the precipitate and the desired purity level of the final silica product. This step ensures the production of high-purity silica suitable for a wide range of applications, including electronics, coatings, and advanced materials.

[0037] The heat treatment is carried out at a temperature in the range of 545, 550, 555, 560, 565, 570 or 575°C up to 550, 555, 560, 565, 570, 575 or 580°C. During the heat treatment process, the precipitate is subjected to elevated temperatures to facilitate the vaporization of organic materials. By maintaining the temperature within the specified range, optimal conditions are created for the efficient removal of organic compounds while minimizing the risk of undesired reactions or degradation of the silica product. This temperature range ensures thorough vaporization of organics, resulting in the production of high-purity silica with minimal contamination.

[0038] Notably, the heat treatment is carried out in an inert environment to avoid triggering any unnecessary chemical reaction. In this regard, to maintain the inert environment, inert gases such as nitrogen or argon are commonly employed during heat treatment processes to prevent oxidation and ensure the stability of the materials being processed. Furthermore, conducting the vaporization of organics in an inert atmosphere helps to minimize the risk of unwanted chemical reactions between the organic compounds and atmospheric oxygen. By creating a controlled environment with low reactivity, the inert atmosphere ensures the integrity and purity of the silica product during the heat treatment process, ultimately enhancing the quality and suitability of the final product for various applications. The residue (obtained after the heat treatment of the precipitated silica) is subsequently dissolved in a suitable alkali solution to isolate a supernatant therefrom. The suitable alkali solution is selected from: sodium hydroxide, calcium hydroxide, lithium hydroxide, potassium hydroxide, any other suitable alkali having strong alkaline properties. The suitable alkali solution interacts / reacts with the silica and organic compounds contained in the residue, resulting in the formation of soluble silicate compound and other soluble salts. For example, when the residue (containing silica, typically as silicon dioxide SiO2) is treated with the suitable alkali solution such as sodium hydroxide solution, then the residue (containing silica) reacts with sodium hydroxide to form sodium silicate, which is soluble in water. Optionally, the dissolution process involves mixing the residue with the suitable alkali solution and agitating the mixture to ensure thorough dissolution. The concentration of suitable alkali solution may vary depending on factors such as the composition of the residue and the desired dissolution rate. Typically, a high concentration of the suitable alkali solution is used to facilitate efficient dissolution of the silica and organic residues. Once the residue is dissolved, the resulting solution is then subjected to filtration to remove any remaining solid impurities, including carbon residue and undissolved particles. Filtration can be carried out using various filter media such as filter paper, filter membranes, or other porous materials capable of retaining solid particles while allowing the passage of the dissolved silica solution. The filtration process yields a clarified solution (namely, the supernatant) containing dissolved silica along with sodium silicate and other soluble salts. This solution can then be further processed through various purification steps to obtain high-purity silica or utilized directly in applications such as in the production of silica-based materials, catalysts, or as a precursor for silica nanoparticles. The term "supernatant" typically refers to a liquid / fluid portion obtained, when the resulting solution is allowed to rest (preferably, after centrifugation) or is filtered. Then the supernatant is further processed to obtain pure silica.

[0039] Next step in obtaining silica, is precipitating the supernatant at a second predefined pH, to obtain the silica. In this regard, the pH of supernatant obtained from the previous step is adjusted by adding an acid agent or a basic / alkaline agent. After filtration to remove carbon residue and other impurities, the supernatant solution contains dissolved silica along with sodium silicate and other soluble salts. Adjusting the pH of this solution (namely, supernatant) to the second predefined pH facilitates the precipitation of silica. The second predefined pH is in a range of 4-7. For example, the second predefined pH may be 4, 4.5, 5, 5.5, 6 or 6.5 up to 4.5, 5, 5.5, 6, 6.5 or 7. Once the pH is adjusted (to the second predefined pH), silica begins to precipitate out of the solution as insoluble silica particles. Optionally, the precipitated silica is then separated from the solution, typically by filtration or centrifugation, and washed to remove any remaining soluble salts or impurities. After washing, the silica precipitate is dried to remove moisture and obtain silica powder. Drying is carried out using various methods such as air drying, vacuum drying, or freeze drying, depending on the desired properties of the final silica powder. The dried silica powder is collected and can be further processed or utilized in various applications such as in the production of ceramics, rubber, paints, or as a filler in plastics and composites. Optionally, the obtained silica, is subjected to milling to obtain the powered silica.

[0040] The technical advantage of obtaining silica from silica-rich lignocellulosic biomass using aforementioned method is that this method allows efficient, ecofriendly and sustainable extraction of silica, with excellent condition and purity.

[0041] In an embodiment, the step of pre-treating the silica-rich lignocellulosic biomass comprises subjecting the silica-rich lignocellulosic biomass to at least one of: an acid treatment, an alkaline treatment. The term "acid treatment" refers to an acid solution to treat the silica-rich lignocellulosic biomass (i.e., the agricultural by-products. It may be appreciated that a suitable acid (in a solid form or in a liquid form) of a required concentration, is dissolved in a solvent thereby forming the acid solution. The technical effect of acid treatment is fractionation of the silica-rich lignocellulosic biomass thereby aiding in efficient extraction of silica. The term fractionation refers to sever / spilt chemical bonds and bindings of individual biomass constituents contained in the silica-rich lignocellulosic biomass so that they can be easily treated and extracted. Optionally, the acid treatment is performed at under specific physical and thermochemical conditions which refers to carefully selecting suitable acid (based on type of the silica-rich lignocellulosic biomass / agricultural by-products used), temperature, pressure and other such parameters that affects efficacy of fractionation of the silica-rich lignocellulosic biomass / the agricultural by-products. Notably, the acid treatment is performed in an enclosed chamber which may be continuous stirred-tank reactor, plug flow and batch reactor and any such suitable reactors, where the specific physical and thermochemical conditions can be implemented. As a result of the acid treatment, the silica content of the lignocellulosic biomass are effectively fractionated forming lignin-silica hydrolysate. The term "alkali treatment" refers to treatment of the lignocellulosic biomass (acid treated) using an alkali solution of a suitable alkali. The alkali pretreatment further facilitate fractionation of silica (contained in the lignocellulosic biomass) from other available biomass constituents based on their solubility (in the alkali solution). Notably, similar to the acid treatment, the alkali pretreatment may also be performed in an enclosed chamber which may be continuous stirred-tank reactor, plug flow and batch reactor and any such suitable reactors, where the specific physical and thermochemical conditions can be implemented. The technical advantage (of the acid treatment and / or alkali treatment) is achievement of adequate and effective fractionation of the silica-rich lignocellulosic biomass to easily extract silica therefrom, while eliminating a loss / decomposition / degradation thereof.

[0042] It may be appreciated that the acid and / or alkaline treatment techniques are utilized to effectively disrupt the lignocellulosic structure of the rice straw and facilitate the release of silica content. Specifically, acid pre-treatment involves the use of acids such as sulfuric acid, hydrochloric acid, or phosphoric acid, that hydrolyse hemicellulose and break down lignin bonds, thereby enhancing silica accessibility. Conversely, alkaline pre-treatment utilizes bases such as sodium hydroxide or ammonium hydroxide to solubilize lignin and remove lignin-carbohydrate complexes, leading to increased silica liberation. The aforementioned acid or alkaline pre-treatment methods can be optimized based on factors such as concentration, temperature, and duration to achieve maximal silica yield while minimizing energy and chemical consumption. By employing acid or alkaline pre-treatment, the method ensures efficient conversion of rice straw into lignin-silica hydrolysate, facilitating subsequent silica extraction processes.

[0043] In an embodiment, the acid treatment uses an acid selected from at least one of: sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid, citric acid, malic acid, nitric acid; and the alkali treatment uses an alkali selected from at least one of: sodium hydroxide, ammonium hydroxide, calcium hydroxide, potassium hydroxide, mono-ethanol amine, urea. It may be appreciated that the suitable acid is selected from amongst aforementioned acids or a combination thereof as required based on the type, and volume of the agricultural byproducts to be treated. In this regard, the aforementioned suitable alkali (namely, sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide, mono-ethanol amine, and urea) are selected based on the type of the lignocellulosic biomass and content of silica therein. For example, the content of silica in rice husk is different from that of rice straw. So, to effectively fractionate the cellulose content from the rice husk, the suitable alkali may be sodium hydroxide whereas for rice straw potassium hydroxide may be required. Similarly, the concentration of the alkali may be selected as per requirement of the fractionation process. The technical advantage is efficient separation of silica from the biophysical structure of the lignocellulosic biomass.

[0044] Notably, after addition of silica to the adduct (forming a uniform and homogeneous silica- adduct mixture), the method comprises the step of adding a Lewis acid catalyst to the silica- adduct mixture to obtain the silica reinforced polyester adhesive. Notably, the silica reinforced polyester adhesive is synthesized through esterification of the organic acid and the polyol in presence of the Lewis acid catalyst. The "Lewis acid catalyst" refers to a chemical reagent which facilitate the silica reinforcement without being part of any possible chemical reaction during the process. The Lewis acid catalyst serves a crucial role in catalysing and promoting a desired cross-linking reaction between the adduct and silica, a process essential for enhancing mechanical strength, thermal stability, and chemical resistance of the resulting adhesive product (i.e., the silica-reinforced polyester adhesive). In other words, the Lewis acid catalyst is introduced in the silica-adduct mixture to catalyse the esterification of organic acid and polyol therein and promote integration of the silica particle in the structural matrix of the polyester adhesive.

[0045] In this regard, this step (step of adding the Lewis acid catalyst to the silica-adduct mixture) is carried out under a second predetermined set of conditions. The term "second predetermined set of conditions" refers to a specific physical and thermochemical conditions set to promote the reaction between the silica and the adduct (the polyester adhesive formed due to interaction of the organic acid and polyol) so that silica particles integrate into a structural matrix of the adduct / polyester adhesive.

[0046] In an embodiment, the second predetermined set of conditions comprises: a temperature in a range of 85-115 °C; a time period in a range of 2-6 hours; and a concentration of the Lewis acid catalyst in a range of 0.01-1.0% w / w.

[0047] In this regard, the temperature (in the step of adding the Lewis acid catalyst) is in the range of 85, 90, 95, 100, 105 or 110°C up to 90, 95, 100, 105, 110 or 115°C. The temperature of aforementioned range is crucial during the Lewis acid catalysed reaction (which occurs during the step of adding Lewis acid catalyst to silica-adduct mixture), particularly for promoting efficient esterification of the organic acid and polyol components within the silica-adduct mixture. The temperature range during the step of adding Lewis acid catalyst to silica-adduct mixture is of a higher value (elevated value) compared to temperature during the formation of silica-adduct mixture. The elevated temperature (during the step of adding Lewis acid catalyst to silica-adduct mixture) facilitates the activation of the Lewis acid catalyst and accelerates the esterification reaction kinetics. Maintaining the reaction temperature within this specified range ensures optimal conditions for the formation of ester bonds and the subsequent development of a polyester-condensate network structure reinforced with silica.

[0048] Similarly, the time period for which concoction of Lewis acid catalyst plus silica-adduct mixture is processed is in the range of 2, 2.5, 3, 3.5, 4, 4.5, 5 or 5.5 hours up to 2.5, 3, 3.5, 4, 4.5, 5, 5.5 or 6 hours. The time period plays a significant role in achieving a desired degree of esterification and cross-linking within the silica-adduct mixture. During this time, the Lewis acid catalyst catalyses the esterification reaction between the organic acid and polyol components, leading to the formation of condensed polyester chains. The prolonged reaction duration allows for sufficient interaction between the reactants and promotes the development of a robust adhesive material with enhanced mechanical properties and durability.

[0049] Similarly, the ideal value for the concentration of the Lewis acid catalyst should lie in the range of 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08 or 0.09% w / w up to 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 1.0% w / w, for achieving a desired degree of esterification and cross-linking within the silica-adduct mixture. Notably, the concentration of the Lewis acid catalyst is a critical parameter that influences the rate and extent of the esterification reaction. In an exemplary embodiment of the present disclosure, a concentration of 0.05% w / w a Lewis acid catalyst having ZnC12 group is employed as the catalyst. This concentration is selected to promote efficient esterification of the organic acid and polyol components, facilitating the formation of a polyester-condensate network structure reinforced with silica. The optimized concentration of the Lewis acid catalyst contributes to the synthesis of a silica reinforced polyester adhesive with enhanced mechanical strength, thermal stability, and chemical resistance. The technical effect, achieved by executing the step of adding Lewis acid catalyst to the silica-adduct mixture, is efficient integration of silica into the structural matrix of the polyester adhesive.

[0050] In this regard, the second predetermined set of conditions may comprise any other additional physical and thermochemical conditions such as pressure, as required to facilitate reaction rate, optimize reaction time and obtain desirable outcome. Under the second predetermined set of conditions, the Lewis acid catalyst is introduced into the reaction mixture (namely, the silica-adduct mixture). Addition of Lewis acid facilitates the esterification reaction between the organic acid and polyol components within the silica-adduct mixture. The condensed product of organic acid and polyol contribute to the adhesive properties of the mixture, while the Lewis acid catalyst promotes esterification reaction between the components. As the reaction progresses, condensed polyester chains form, contributing to the adhesive properties of the mixture. Simultaneously, the Lewis acid catalyst promotes cross-linking of these chains through ester bonds, leading to the formation of a robust and durable adhesive material. Silicon dioxide reinforces the condensed polyester chains through cross-linking randomly leading to the formation of a strong and durable adhesive. By incorporating the Lewis acid catalyst into the silica-adduct mixture, practitioners can reliably catalyse the esterification reaction, ensuring the successful synthesis of a silica reinforced polyester adhesive with superior properties.

[0051] In an embodiment, the Lewis acid catalyst is selected from a group comprising at least one of: metal chlorides, metal sulphates, metal alkoxide. In this regard, the Lewis acid catalyst selected for the aforementioned step (i.e., the step of adding the Lewis acid catalyst) is at least one of: Lewis acid catalyst comprising a metal chloride group such as zinc chloride (ZnC12), magnesium chloride (MgC12), aluminium chloride (AICI3), and iron chloride (FeC13), Lewis acid catalyst comprising a metal sulphate group such as aluminium sulphate (A12(SC>4)3), ferric sulphate (Fe2(SO4)3), zinc sulphate (ZnSOq). copper sulphate (C11SO4). titanium oxysulphate (TiOSC>4), and Lewis acid catalyst comprising a metal alkoxide group such as titanium alkoxides (Ti(0R)4), aluminium alkoxides (Al(0R)3), zirconium alkoxides (Zr(OR)4), tin(IV) alkoxides (Sn(0R)4), vanadium alkoxides (V0(0R)3). It may be appreciated that any other suitable Lewis acid catalyst may also be used, without limiting only to aforementioned examples. Notably, careful consideration is given to selecting an appropriate Lewis acid catalyst, ensuring compatibility and high catalytic activity within the silica-adduct mixture. In other words, the selection of the Lewis acid catalyst is based on its compatibility with the silica-adduct mixture and its ability to efficiently catalyse the esterification of the organic acid and polyol components within the silica-adduct mixture. Notably, the selection of the Lewis acid catalyst contributes to the successful synthesis of the silica reinforced polyester adhesive with desired properties. In other words, the type of Lewis acid catalyst and / concentration are selected based on which polyol and organic acid are used to prepare the polyester adhesive. The technical effect is efficient esterification and integration of silica into the structural matrix of the polyester adhesive.

[0052] It may be appreciated that the method for generating / obtaining silica from silica-rich lignocellulosic biomass targets agricultural residues such as rice straw and rice husk and the like, which are known to possess significant silica content due to the accumulation of silica bodies within their cellular structure. These biomass sources are readily available from agricultural activities and offer a sustainable and renewable feedstock for silica production. For example, rice straw, being a readily available agricultural residue abundant in silica, is often used in generation of silica therefrom. In the described pretreatment step, complex lignocellulosic structure of the rice straw is broken down and the silica content therein is released. The silica-rich lignocellulosic biomass is harvested from fields after rice cultivation or other suitable sources, then undergoes cleaning and preparation to remove extraneous matter and ensure the purity of the biomass prior to silica extraction. Harvesting from such agricultural residues provides a cost-effective and abundant source of raw material and also aligns with the principles of sustainability by utilizing waste materials from agricultural processes.

[0053] The method for synthesizing a silica reinforced polyester adhesive, as disclosed in the present disclosure, is eco-friendly and cost effective method. The disclosed method aids in efficient synthesis the silica reinforced polyester adhesive with minimal environmental impact, and carbon footprint. The disclosed method helps in reducing energy usage and waste generation during extraction of the non-biofuel based products. It will be appreciated that the silica reinforced polyester adhesives synthesized using the method described hereinabove exhibit remarkable superiority in strength, durability, thermal stability, and chemical resistance. Through meticulous control of the synthesis process, including the dissolution of organic acid and polyol to obtain the adduct, addition of silica to form the silica-adduct mixture, and incorporation of a suitable / compatible Lewis acid catalyst, the resulting adhesive material possesses exceptional mechanical properties. The cross-linking of polyester chains reinforced with silica nanoparticles or precursor materials leads to a robust adhesive with enhanced tensile and shear strength, making it suitable for a wide range of demanding applications. Additionally, the adhesive formulation demonstrates heightened thermal stability, retaining its adhesive properties across a broad temperature range, from sub-zero temperatures to elevated temperatures encountered in industrial processes or outdoor applications. Furthermore, the chemical resistance of the adhesive ensures its longevity and reliability in harsh environments, where exposure to corrosive chemicals or solvents may compromise the integrity of conventional adhesives. Overall, the superior performance of these silica reinforced polyester adhesives underscores their suitability for applications requiring durable and high-performance materials, such as automotive assembly, construction, aerospace, electronics, and beyond. Furthermore, the disclosed method eliminate use of harsh and carcinogenic chemical which are traditionally employed to obtain silica and polyester adhesives and thereby promoting green technology.

[0054] DETAILED DESCRIPTION OF THE DRAWINGS

[0055] Referring to FIG. 1, illustrated is a flowchart 100 depicting steps of a method for synthesizing a silica reinforced polyester adhesive, in accordance with an embodiment of the present disclosure. At step 102, an organic acid and a polyol are dissolved under a first predetermined set of conditions to obtain an adduct. At step 104, silica is added to the adduct to obtain a silica-adduct mixture under the first predetermined set of conditions; and at step 106, a Lewis acid catalyst is added to the silica-adduct mixture under a second predetermined set of conditions to obtain the silica reinforced polyester adhesive. As shown, at step 108, a silica-rich lignocellulosic biomass is pretreated to obtain lignin-silica hydrolysate. At step 110, the lignin-silica hydrolysate is precipitated at a first predefined pH to obtain a precipitated silica therefrom. At step 112 the precipitated silica is subjected to a heat treatment at a temperature in a range of 545-580°C, in an inert environment to obtain a residue. At step 114, the obtained residue is dissolved in a suitable alkali solution to isolate a supernatant therefrom. At step 116, the supernatant is precipitated at a second predefined pH, to obtain the silica which is utilized in step 104.

Claims

CLAIMS l / Wc Claim:

1. A method for synthesizing a silica reinforced polyester adhesive, the method comprising: dissolving an organic acid and a polyol under a first predetermined set of conditions to obtain an adduct; adding silica to the adduct to obtain a silica-adduct mixture under the first predetermined set of conditions; and adding a Lewis acid catalyst to the silica-adduct mixture under a second predetermined set of conditions to obtain the silica reinforced polyester adhesive.

2. The method as claimed in claim 1, wherein the silica is obtained from a silica-rich lignocellulosic biomass by: pre-treating the silica-rich lignocellulosic biomass to obtain lignin-silica hydrolysate; precipitating the lignin-silica hydrolysate at a first predefined pH to obtain a precipitated silica therefrom; subjecting the precipitated silica to a heat treatment at a temperature in a range of 545-580°C, in an inert environment to obtain a residue; dissolving the obtained residue in a suitable alkali solution to isolate a supernatant therefrom; and precipitating the supernatant at a second predefined pH, to obtain the silica.

3. The method as claimed in claim 2, wherein the step of pre-treating the silica-rich lignocellulosic biomass comprises subjecting the silica-rich lignocellulosic biomass to at least one of: an acid treatment, an alkaline treatment.

4. The method as claimed in claim 3, wherein the acid treatment uses an acid selected from at least one of: sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid, citric acid, malic acid, nitric acid; and the alkali treatment uses an alkali selected from at least one of: sodium hydroxide, ammonium hydroxide, calcium hydroxide, potassium hydroxide, mono-ethanol amine, urea.

5. The method as claimed in claim 1, wherein the silica is added to the adduct directly as powered silica or as silica suspension.

6. The method as claimed in claim 1, wherein the first predetermined set of conditions comprises: a concentration of the organic acid in a range of 25 to 75 % by volume of the adduct; a temperature in a range of 65-95°C; a time period in a range of 20-90 minutes; continuous stirring at a speed in a range of 200-800 rpm; and an equimolar concentration of the polyol.

7. The method as claimed in claim 1, wherein the organic acid is selected from at least one of: citric acid, adipic acid, maleic acid, oxalic acid, acetic acid, furan dicarboxylic acid (FDCA); and the polyol is selected from at least one of: ethylene glycol, propylene glycol, glycerol, sorbitol, mannitol, erythritol.

8. The method as claimed in claim 1, wherein the second predetermined set of conditions comprises: a temperature in a range of 85-115 °C; a time period in a range of 2-6 hours; and a concentration of the Lewis acid catalyst in a range of 0.01-1.0% w / w.

9. The method as claimed in claim 1, wherein the Lewis acid catalyst is selected from a group comprising at least one of: metal chlorides, metal sulphates, metal alkoxide.

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