Defect-free dense flat sheet membrane, process of fabricating the same, and uses thereof

Abstract

The present invention provides a green, solvent-efficient, and industrially scalable membrane fabrication process that overcomes the drawbacks of conventional toxic-solvent-based processes. The resulting membranes combine environmental sustainability with enhanced structural, thermal, and operational performance, offering a high-quality, reproducible platform for advanced separation and purification applications across chemical, petrochemical, and environmental industries.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of polymer membrane technology, and more particularly to a defect-free dense flat sheet membrane, and a process of fabricating the same. The present invention also relates to the use of said defect-free dense flat sheet membrane for sustainable separation technologies.BACKGROUND OF THE INVENTION

[0002] The background description includes information that may be useful in understanding the present invention. It is not an admission that any information provided herein is prior art or relevant to the present invention, or that any publication specifically or implicitly referenced is prior art.

[0003] Polymeric membranes are widely utilized in diverse applications such as filtration, gas separation, solvent recovery, and biomedical devices due to their favorable mechanical strength, selective permeability, and ease of processing. Among these, cellulose-derived polymers are extensively employed because of their biocompatibility, renewability, and excellent film-forming characteristics.

[0004] Conventionally, the fabrication of such membranes involves dissolving the polymer in organic solvents followed by casting and controlled evaporation to obtain thin films of uniform morphology. However, the solvents commonly used in these processes are often toxic, non-biodegradable, and derived from petrochemical sources, posing environmental and health hazards during production, handling, and disposal. The continued use of such solvents contradicts current trends toward sustainable and green manufacturing practices.

[0005] Efforts have been made to identify safer and environmentally benign alternatives capable of dissolving cellulose-based polymers effectively while maintaining membrane quality and performance. Nevertheless, challenges remain in achieving complete polymer dissolution, homogeneous mixing, and defect-free membrane formation when employing alternative solvent systems. Furthermore, the resulting membranes often exhibit inconsistent morphology, poor mechanical stability, or reduced permeability due to uncontrolled solvent evaporation or inadequate process optimization.

[0006] There is therefore a need for a reproducible and environmentally sustainable process for preparing polymeric membranes from cellulose-derived materials using non-toxic solvent systems. The present disclosure overcomes the limitations of current technologies by providing a process for fabrication of a defect-free dense flat sheet green membranes, which have consistent membrane properties, while minimizing ecological impact.OBJECTS OF THE INVENTION

[0007] The object of the present invention is to provide a sustainable and efficient process of fabricating polymeric membranes derived from cellulose-based materials using environmentally acceptable solvent systems.

[0008] An object of the present invention is to provide a process for producing defect-free membranes having uniform thickness, improved mechanical stability, and consistent morphology.

[0009] Another object of the present invention is to provide a process for producing defect-free membranes that minimizes environmental and health hazards by eliminating or reducing the use of toxic and non-biodegradable organic solvents.

[0010] Yet another object of the present invention is to provide polymeric membranes that exhibit superior performance, environmental compatibility, and application versatility in areas such as filtration, separation, purification, and biomedical use.SUMMARY OF THE INVENTION

[0011] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0012] Aspects of the present disclosure provides a green, solvent-efficient, and industrially scalable membrane fabrication process that overcomes the drawbacks of conventional toxic-solvent-based processes. The resulting membranes combine environmental sustainability with enhanced structural propertyoffering a high-quality, reproducible platform for advanced separation and purification applications across chemical, petrochemical, and environmental industries.

[0013] In one embodiment, the present disclosure relates to a process for preparing a defect-free dense flat sheet membrane derived from a cellulose-based polymer using a bio-based solvent system under controlled processing conditions.

[0014] According to one aspect of the present disclosure, the process comprises the steps of:(a) dissolving a cellulose-based polymer in a bio-based solvent to form a homogeneous dope solution;(b) continuously stirring the mixture for about four hours at 85°C to achieve complete dissolution;(c) allowing the resulting solution to stand at room temperature for approximately 24 hours to remove entrapped air bubbles;(d) casting the degassed solution onto a suitable substrate by coating or drop-casting to form a uniform film;(e) drying the film in a vacuum oven at 80°C for 24 hours to remove solvent and obtain a dense, void-free membrane; and(f) washing the membrane with deionized water to remove residual solvent, followed by drying for 24 hours to yield the defect-free, dense flat sheet membrane.

[0015] In an aspect, the cellulose-based polymer used in the process is cellulose acetate having an average molecular weight of approximately 30,000 g / mol.

[0016] In another aspect, the bio-based solvent employed is Dihydrolevoglucosenone, a renewable, biodegradable dipolar aprotic solvent that enables complete dissolution of cellulose acetate and uniform film formation without the use of hazardous organic solvents.

[0017] In an aspect, the polymer concentration in the dope solution is maintained within 8 to 14 wt%, providing optimal viscosity and film-forming characteristics.

[0018] According to another aspect of the present disclosure, the process yields a cellulose acetate membrane that is dense, homogeneous, and mechanically stable, exhibiting thermal stability up to 250°C as determined by thermogravimetric analysis.

[0019] In yet another aspect, the invention provides a separation device for gases or liquids incorporating the cellulose acetate membrane prepared by the disclosed process.

[0020] The process of the present invention thus enables the fabrication of eco-friendly, defect-free, and thermally stable dense membranes using a bio-based solvent system, representing a sustainable alternative to conventional solvent-based membrane manufacturing techniques.

[0021] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.BRIEF DESCRIPTION OF DRAWINGS

[0022] Characteristics and advantages of the subject matter as disclosed in the present disclosure will become clearer from the detailed description of an embodiment thereof, concerning the attached drawing, given purely by way of an example, in which:

[0023] FIG. 1 relates to schematic representation of fabrication process of the solvent-cast membrane using cellulose acetate in Cyrene.

[0024] FIG. 2 relates to the SEM images of solvent casted CA-Cy membranes (CAS) surface and cross-section.

[0025] FIG. 3 relates to thermogravimetric analysis of the fabricated membrane at different compositions.DETAILED DESCRIPTION OF THE INVENTION

[0026] The following is a detailed description of the embodiments of the disclosure. The embodiments are in such detail as to communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

[0027] All publications herein are incorporated by reference to the same extent as if each publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0028] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0029] In some embodiments, numbers have been used for quantifying weight percentages, angles, and so forth, to describe and claim certain embodiments of the invention and are to be understood as being modified in some instances by the term "about." Accordingly, in some embodiments, the numerical parameters outlined in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values outlined in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0030] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0031] As used in the description herein and throughout the claims that follow, the meaning of "a," "an," and "the" includes plural reference unless the context dictates otherwise. Also, as used in the description herein, the meaning of "in" includes "in" and "on" unless the context dictates otherwise.

[0032] Unless the context requires otherwise, throughout the specification which follow, the word "comprise" and variations thereof, such as, "comprises" and "comprising" are to be construed in an open, inclusive sense that is as "including, but not limited to."

[0033] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each value is incorporated into the specification as if it were individually recited herein.

[0034] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any examples, or exemplary language (e.g. "such as") provided concerning certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0035] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified.

[0036] The description that follows, and the embodiments described therein, are provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for explanation, and not of limitation, of those principles and the disclosure.

[0037] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0038] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0039] Various terms are used herein. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0040] While a particular form of the invention has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention.

[0041] Embodiments of the present disclosure relate to a defect-free dense flat sheet membrane, and a process of fabricating the same. The present invention also relates to the use of said defect-free dense flat sheet membrane for sustainable separation technologies.

[0042] In an embodiment, the present disclosure provides a green, solvent-efficient, and industrially scalable membrane fabrication process that overcomes the drawbacks of conventional toxic-solvent-based processes. The resulting membranes combine environmental sustainability with enhanced structural, thermal, and operational performance, offering a high-quality, reproducible platform for advanced separation and purification applications across chemical, petrochemical, and environmental industries.

[0043] In an embodiment, the present disclosure provides a process of fabricating the defect-free dense flat sheet membrane comprising the steps of preparing a dope solution, followed by casting the same to obtain said defect-free dense flat sheet membrane.

[0044] In an embodiment, the present invention provides a process for preparing a polymeric dope solution comprising the dissolution of a cellulose-based polymer in an environmentally acceptable solvent system. The polymer and solvent are weighed according to the desired polymer concentration and mixed in a closed vessel to prevent solvent loss. The mixture is subjected to continuous stirring under controlled temperature until a clear and homogeneous solution is obtained.

[0045] In some embodiments, the process is applicable to a variety of cellulose-based polymers, including but not limited to cellulose acetate, cellulose nitrate, cellulose propionate, cellulose acetate butyrate, hydroxypropyl cellulose, carboxymethyl cellulose, and ethyl cellulose. In an embodiment, any cellulose derivative that is soluble in a bio-based solvent and capable of forming a film may be used. In a preferred embodiment, the cellulose-based polymer is cellulose acetate, which provides excellent film-forming ability, mechanical stability, and compatibility with the bio-based solvent system. The use of cellulose acetate results in membranes with uniform morphology, smooth surface finish, and enhanced mechanical and thermal stability.

[0046] In some embodiments, the polymer concentration in the solvent system is maintained within a range of 5 to 20 wt%, and the mixture is stirred for a period of 2 to 8 hours at a temperature between 60°C and 100°C to ensure complete dissolution of the polymer. In a preferred embodiment, the polymer concentration is maintained within a narrower range of 8 to 14 wt%, the stirring period is approximately 4 hours, and the temperature is around 85°C, resulting in uniform dissolution and formation of a transparent dope solution.

[0047] In a preferred embodiment, the present disclosure employs a bio-based solvent system for dissolving the cellulose-based polymer to form a homogeneous dope solution suitable for membrane casting. The solvent serves to solubilize the polymer effectively while offering low toxicity, high biodegradability, and environmental compatibility. In some embodiments, the solvent may be selected from bio-derived polar aprotic solvents that are obtained from renewable carbohydrate-based feedstocks. Such solvents possess physicochemical characteristics similar to conventional dipolar aprotic solvents but with reduced environmental and health hazards. Examples of suitable solvents include lactones, cyclic ketones, esters, and carbohydrate-derived aprotic solvents capable of dissolving cellulose-based polymers.

[0048] In a preferred embodiment, the bio-based solvent used is Dihydrolevoglucosenone (Cyrene), a bio-based dipolar aprotic solvent derived from cellulose. Dihydrolevoglucosenone is characterized by high polarity, strong hydrogen-bonding capability, and an ability to dissolve a wide range of cellulose derivatives such as cellulose acetate, cellulose nitrate, and hydroxypropyl cellulose. In another embodiment, the Dihydrolevoglucosenone solvent may be used alone or in combination with one or more compatible co-solvents to tailor the viscosity, volatility, and polymer-solvent interaction characteristics. The co-solvent facilitates control of the casting behavior and film formation rate during membrane fabrication.

[0049] In some embodiments, the solvent-to-polymer ratio ranges between 80:20 and 95:5 by weight, depending on the molecular weight and substitution degree of the polymer. In a preferred embodiment, the solvent-to-polymer ratio is maintained between 86:14 and 92:8 by weight, providing an optimal viscosity range for efficient mixing, degassing, and uniform film casting.

[0050] In some embodiments, the homogeneous dope solution obtained from the dissolution step is allowed to stand undisturbed and is maintained at ambient temperature for a duration of 12 to 36 hours to remove entrapped air bubbles formed during stirring. Degassing ensures defect-free film formation and uniform membrane structure. In a preferred embodiment, the solution is allowed to stand for approximately 24 hours at room temperature, ensuring complete removal of air bubbles before the casting step.

[0051] In still another embodiment, the degassed polymer solution is cast onto a suitable casting substrate, for example a glass plate, Petri dish, or metallic surface, to form a uniform film. Casting may be carried out using a spreader (for example, by coating or drop casting), doctor blade, or equivalent apparatus to regulate film thickness and ensure even distribution of the dope solution. In some embodiments, the wet film thickness may range from 100 to 300 micrometers, depending on the intended application and polymer concentration. In a preferred embodiment, the film thickness is maintained between 100 to 150 micrometers, producing a uniform and defect-free membrane surface.

[0052] In yet another embodiment, the cast film is subjected to a controlled drying process to remove the solvent and facilitate membrane formation. Drying may be performed under ambient, vacuum, or reduced-pressure conditions depending on the nature of the solvent. In some embodiments, the drying temperature ranges from 60°C to 100°C, and the drying time ranges from 12 to 48 hours. In a preferred embodiment, the film is dried in a vacuum oven at 80°C for approximately 24 hours to avoid macrovoid formation, allowing gradual solvent removal and formation of a dense and uniform membrane.

[0053] In a further embodiment, the dried membrane is separated from the casting substrate and washed using a neutral washing medium such as deionized water to remove residual solvent or impurities. The washed membrane is then subjected to final drying to achieve dimensional stability and mechanical strength. In some embodiments, the final drying temperature ranges from 60°C to 90°C for a duration of 12 to 36 hours. In a preferred embodiment, the membrane is dried at 80°C for 24 hours, yielding a finished defect-free dense flat sheet membrane with consistent structure and performance.

[0054] In a preferred embodiment, the present disclosure provides process for preparing a defect-free dense flat sheet membrane, comprising:(a) dissolving a cellulose-based polymer in a bio-based solvent to form a dope solution;(b) continuously stirring the mixture for 4 hours at 85°C to obtain a homogeneous solution;(c) allowing the homogeneous solution to stand at room temperature for 24 hours to degas entrapped air bubbles;(d) casting the degassed solution onto a casting substrate to form a uniform film;(e) drying the film in a vacuum oven at 80°C for 24 hours to remove solvent and form a membrane; and(f) washing the obtained membrane with deionized water to remove residual solvent, followed by drying at 80°C for 24 hours to obtain the defect-free dense flat sheet membrane.

[0055] In additional embodiments, the membrane post-treatment step can be added after making the defect-free dense flat sheet membrane to improve selectivity without changing or damaging the membrane, or causing the membrane to lose performance with time. The membrane post-treatment step can involve coating the selective layer surface of the defect-free dense flat sheet membrane with a thin layer of material such as a polysiloxane, a fluoro-polymer, a thermally curable silicone rubber, or a UV radiation curable silicone rubber.

[0056] In alternative embodiments, the defect-free dense flat sheet membrane may comprise cellulose acetate as the primary polymer, Dihydrolevoglucosenone as the bio-based solvent, and one or more functional additives uniformly dispersed within the polymer matrix. For example, the additives may be selected from: Nanoparticles such as titanium dioxide, silicon dioxide, zinc oxide, aluminum oxide, or graphene oxide, in an amount between 0.5 and 5 wt% of the polymer; Pore-forming agents such as polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), or lithium salts in an amount between 1 and 8 wt%; and Plasticizers such as glycerol, triethyl citrate, or sorbitol in an amount between 0.5 and 3 wt%.

[0057] In an embodiment, the defect-free dense flat sheet membrane prepared according to the process of the present invention exhibits thermal stability, making it suitable for use in elevated-temperature separation, filtration, and processing applications. The improved thermal behavior is attributed to the intrinsic stability of the cellulose-based polymer, the strong intermolecular hydrogen bonding within the polymer matrix, and the use of a bio-based solvent that promotes uniform polymer packing during membrane formation. In some embodiments, the membrane obtained by the process described herein exhibit thermal stability in the range of 180°C to 270°C, as determined by thermogravimetric analysis (TGA) or an equivalent thermal characterization technique. In a preferred embodiment, the membrane exhibits thermal stability up to approximately 250°C.

[0058] In an embodiment, the defect-free dense flat sheet membrane prepared according to the process of the present invention is a dense, defect-free flat sheet membrane possessing uniform thickness, smooth surface morphology, and high structural integrity. The membrane exhibits excellent selectivity and permeability due to its homogeneous polymer structure and absence of surface or sub-surface voids. The membrane is suitable for a wide range of liquid, gas, and vapor separation processes that require precise molecular discrimination through a dense polymeric matrix. The membranes may also be used for organic solvent recovery, vapor permeation, and selective removal of volatile organic compounds (VOCs).

[0059] While the foregoing description discloses various embodiments of the disclosure, other and further embodiments of the invention may be devised without departing from the basic scope of the disclosure. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.EXAMPLES

[0060] The present disclosure is further explained in the following examples. However, it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.

[0061] Materials:Cellulose Acetate (CA) with an average molecular weight of 30,000 g / mol and Cyrene (dihydrolevoglucosenone, 99% purity) were procured from Sigma-Aldrich. All chemicals were used as received without further purification. Deionized water was used as the non-solvent during the washing steps. The purity of the materials was above 99%, ensuring minimal interference from impurities during membrane fabrication.

[0062] Example 1: Fabrication of cellulose acetate membranesPreparation of Dope Solution:The dope solutions were prepared by dissolving cellulose acetate in Cyrene at different polymer loadings of 8 to 14 wt%. The required amounts of CA and Cyrene were weighed precisely according to the desired concentration and mixed in a sealed vessel. The mixture was stirred continuously for 4 hours at 85°C to ensure complete dissolution and formation of a homogenous solution. After stirring, the solution was allowed to stand for 24 hrs at room temperature to remove entrapped air bubbles before casting. Membrane Fabrication by Solvent Casting:The solvent casting process was performed by pouring the prepared dope solution into a Petri dish. The solution was spread evenly to achieve a uniform thickness and allowed to dry in a vacuum oven at 80°C for 24 hours. This slow evaporation process enabled controlled solvent removal and uniform membrane formation. The resulting membranes were then carefully removed from the Petri dish, washed with deionized water to remove film from dish then dried for 24 hrs. (FIG. 1).

[0063] Example 2: Characterization of cellulose acetate membranesSEM images showed that the solvent-cast cellulose acetate membranes have a dense and smooth structure without defects, i.e, macro voids, confirming uniformity and good dense structure (FIG. 2). TGA analysis revealed initial weight loss up to 150°C due to residual solvent and moisture, followed by major decomposition between 300°C and 400°C. The membranes exhibited excellent thermal stability up to 250°C, indicating their suitability for separation applications (FIG. 3).ADVANTAGES OF THE PRESENT INVENTIONThe process of the present invention utilizes Cyrene, a fully bio-based and non-toxic solvent, which enables the fabrication of polymeric membranes in an environmentally sustainable manner. The process eliminates the need for hazardous, non-biodegradable organic solvents such as N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF), thereby reducing ecological and occupational health risks associated with conventional membrane fabrication methods .The process of the present invention further facilitates the production of defect-free, dense cellulose acetate membranes exhibiting uniform morphology.

Claims

1. A process for preparing a defect-free dense flat sheet membrane, comprising: (a) dissolving a cellulose-based polymer in a bio-based solvent to form a dope solution; (b) continuously stirring the mixture for 4 to 8 hours at 80-90°C to obtain a homogeneous solution; (c) allowing the homogeneous solution to stand at room temperature approximately 24 hours to degas entrapped air bubbles; (d) casting the degassed solution onto a casting substrate to form a uniform film; (e) drying the film in a vacuum oven at 80°C for 24 hours to remove solvent and form a membrane; and (f) washing the obtained membrane with deionized water to obtain the defect-free dense flat sheet membrane, followed by drying the same.

2. The process as claimed in claim 1, wherein the cellulose-based polymer is cellulose acetate having an average molecular weight of 30,000 g / mol.

3. The process as claimed in claim 1, wherein the bio-based solvent is Dihydrolevoglucosenone.

4. The process as claimed in claim 1, wherein the dope solution has a polymer loading in the range of 8 to 14 wt%5. The process as claimed in claim 1, wherein the casting is effected by coating or drop casting.

6. The process as claimed in claim 1, wherein the film has a wet thickness of 150 microns and a dry thickness of 100 microns.

7. The process as claimed in claim 1, wherein the drying step is performed under vacuum to ensure uniform solvent removal and prevent macrovoid formation.

8. A cellulose acetate membrane obtainable by the process as claimed in any one of claims 1-7.

9. The cellulose acetate membrane as claimed in claim 8, wherein the membrane exhibits thermal stability up to 250°C.

10. The cellulose acetate membrane as claimed in claim 8, have macrovoid free cross section.