Description:BACKGROUND

FIELD OF THE DISCLOSURE
[0001] Various embodiments of the disclosure relate generally to recyclable epoxy thermosets. More specifically, various embodiments of the disclosure relate to epoxy vitrimers and carbon fiber reinforced polymers comprising the epoxy vitrimers.

DESCRIPTION OF THE RELATED ART

[0002] Thermosets are covalently cross-linked polymer networks that, unlike thermoplastics, cannot be reprocessed by melting, or dissolving in solvents. Thermosetting polymers or thermosets are widely used in industrial applications such as coatings, adhesives, and fiber reinforced polymer composites for high-performance applications due to their superior mechanical strength, chemical resistance, and thermal stability. A key limitation of thermosets lies in their defining characteristic of intractable three-dimensional structure which, while beneficial for performance, renders them non-recyclable and non-processable, contributing to significant material waste.
[0003] Epoxy polymers or epoxy resins, a subclass of thermosets, are quite diverse in their chemical properties. Epoxy monomers can be readily synthesized from various unsaturated precursors through oxidation reactions or via nucleophilic substitution reactions, such as with epichlorohydrin due to which a wide range are available such as epoxidized soybean oil, epoxidized polyisoprene, and bisphenol A diglycidyl ether for epoxy polymer formation. Furthermore, epoxy monomers are highly reactive and can engage in crosslinking reactions with a broad range of functional groups, such as thiols, amines, carboxylic acids, and anhydrides.
[0004] Epoxy polymers are extensively utilized due to their exceptional chemical resistance, mechanical properties, electrical insulation, and strong adhesion, making them indispensable in high-performance applications such as aerospace. However, despite their versatility and widespread use, traditional epoxy polymers are irreversible thermosets that cannot be repaired, reshaped, or recycled. This inherent limitation presents significant environmental challenges, particularly due to the accumulation of epoxy polymer waste.
[0005] Vitrimerization is an emerging technology for converting thermoset polymers into reprocessable polymers. Vitrimer chemistry is based on dynamic covalent bonds, which transform permanent covalent linkages into covalent adaptable networks (CANs). These adaptable networks enable polymer reprocessing and recycling. To replace conventional epoxy resins with epoxy vitrimers in demanding applications such as aerospace, stringent quality standards must be met. Novel vitrimers must be meticulously engineered to match or surpass the mechanical and thermal properties of the conventional epoxy resins.
[0006] Epoxy vitrimers synthesized through reaction of epoxy monomers with carboxylic acids or acid anhydrides are well-established, where the dynamic covalent network (CAN) operates via ester bond exchange. The dynamic behavior of the CAN in these epoxy vitrimers is achieved by reducing the crosslinking density of the network. However, this approach results in vitrimers with high ester group content and lower crosslinking density, leading to compromised thermal and mechanical properties, as well as diminished resistance to acidic and alkaline environments.
[0007] Epoxy vitrimers employing disulfide exchange reactions are widely recognized for their self-healing properties, attributed to their low thermal activation energy. However, under the high processing temperatures typically required for polymer processing, these vitrimers may undergo irreversible structural damage, thereby adversely affecting their recyclability.
[0008] Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.
SUMMARY

[0009] According to embodiments of the present disclosure, an epoxy vitrimer is provided. The epoxy vitrimer comprises an epoxy unit derived from an epoxy monomer. The epoxy vitrimer further comprises a first dynamic crosslinker unit derived from a first dynamic crosslinker. The first dynamic crosslinker is covalently attached to the epoxy unit and able to form a first covalent adaptive network through transesterification exchange reaction. The epoxy vitrimer further comprises a second dynamic crosslinker unit derived from a second dynamic crosslinker. The second dynamic crosslinker is covalently attached to the epoxy unit and the first dynamic crosslinker unit and able to form a second covalent adaptive network through silyl ether exchange reaction. The second dynamic crosslinker is able to form a third covalent adaptive network through disulfide exchange reaction.
[0010] In another embodiment, a method of recycling the epoxy vitrimer is provided. The method comprises heating the epoxy vitrimer dispersed in a solvent to a temperature greater than 200°C for a period of time in a range of 2 to 10 hours. The solvent comprises dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone, or combinations thereof.
[0011] In yet another embodiment, a method of preparing an epoxy vitrimer is provided. The method comprises introducing a first dynamic crosslinker and a second dynamic crosslinker into an epoxy monomer in liquid form to obtain a reaction solution. The method further comprises providing a catalyst in the reaction solution to form a reaction mixture. The method further comprises curing the reaction mixture at a temperature in a range of 150°C to 200°C for a period of time in a range of 2 to 4 hours to form the epoxy vitrimer.
[0012] According to embodiments of the present disclosure, a carbon fiber reinforced polymer (CFRP) is provided. The carbon fiber reinforced polymer comprises at least one carbon fiber layer impregnated with an epoxy vitrimer. The epoxy vitrimer comprises an epoxy unit derived from an epoxy monomer. The epoxy vitrimer further comprises a first dynamic crosslinker unit derived from a first dynamic crosslinker. The first dynamic crosslinker is covalently attached to the epoxy unit and able to form a first covalent adaptive network through transesterification exchange reaction. The epoxy vitrimer further comprises a second dynamic crosslinker unit derived from a second dynamic crosslinker. The second dynamic crosslinker is covalently attached to the epoxy unit and the first dynamic crosslinker unit and able to form a second covalent adaptive network through silyl ether exchange reaction. The second dynamic crosslinker is able to form a third covalent adaptive network through disulfide exchange reaction.
[0013] In another embodiment, a method of recycling the carbon fiber reinforced polymer is provided. The method comprises dispersing the carbon fiber reinforced polymer in a solvent and heating it to a temperature in a range of 160°C to 200°C for a period of time in a range of 2 to 10 hours, wherein the solvent comprises dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone or combinations thereof.
[0014] In yet another embodiment, a method of forming a carbon fiber reinforced polymer is provided. The method comprises providing at least one carbon fiber layer. The method further comprises impregnating the at least one carbon fiber layer with an epoxy vitrimer precursor. The epoxy vitrimer precursor comprises an epoxy monomer, a catalyst, a first dynamic crosslinker, and a second dynamic crosslinker. The first dynamic crosslinker comprises an aliphatic carboxylic acid, or an aromatic carboxylic acid containing 2 to 13 carbon atoms and having at least two carboxylic acid groups, and the second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT). The method further comprises curing the epoxy vitrimer precursor impregnated on the at least one carbon fiber layer at a temperature in a range of 150°C to 200°C for a period of time in a range of 2 to 4 hours to form the carbon fiber reinforced polymer.
[0015] In yet another embodiment of the present disclosure, an article comprising the carbon fiber reinforced polymer is provided.

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a flow chart that illustrates a method of preparing an epoxy vitrimer, in accordance with an exemplary embodiment of the disclosure;
[0017] FIG. 2 is a representative reaction scheme, in accordance with an exemplary embodiment of the disclosure;
[0018] FIG. 3 is a flow chart that illustrates a method of preparing a carbon fiber reinforced polymer, in accordance with an exemplary embodiment of the disclosure;
[0019] FIG. 4 is a bar chart of tensile strengths plotted against various vitrimers;
[0020] FIG. 5 are plots of relaxation modulus versus time at different temperatures;
[0021] FIG. 6 is an Arrhenius plot to determine activation energy; and
[0022] FIG. 7 displays micro-computed tomography images of self-healing.
[0023] Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present disclosure.
DETAILED DESCRIPTION OF EMBODIMENTS

[0024] The following description illustrates some exemplary embodiments of the disclosed disclosure in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this disclosure that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present disclosure.
[0025] The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements, or process steps.
[0026] As used herein, the term 'or combinations thereof' means that the listed components may be used individually or in any combination thereof.
[0027] All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.
[0028] These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.
[0029] As used herein, the term “copolymer” refers to a polymer derived from more than one species of monomer, where the copolymer includes repeating units of each of the monomers.
[0030] As used herein, the term “blend” refers to a mixture of two or more polymers or copolymers that have been blended together to create a new material with different physical properties.
[0031] As used herein, the term “epoxy polymer” refers to a thermoset polymer that is made from monomers that contain oxirane groups or epoxide groups.
[0032] The term “carbon fiber reinforced polymers (CFRPs)”, as used herein refers to composite materials or composites made by embedding carbon fibers in an epoxy vitrimer matrix, or carbon fibers impregnated with an epoxy vitrimer. The carbon fibers provide strength and stiffness to CFRP, while the epoxy vitrimer holds the fibers together and protects them from environmental damage.
[0033] The term, "elastic modulus" of a material is defined as a gradient of a stress-strain curve of the material in an elastic deformation region and is a measure of a stiffness of the material. The unit of elastic modulus is the Pascal (Pa). Stiffer materials have higher values of elastic modulus. The elastic modulus is otherwise known as the "modulus of elasticity" or "Young's modulus".
[0034] As used herein, the term “cycle life” refers to a number of times a polymer may be recycled without adversely affecting its mechanical or thermal properties.
[0035] Plastic “recycling” refers to a process whereby useful products may be produced from waste plastics after reprocessing or melting the waste plastics. However, after recycling, the recycled polymer usually possesses inferior properties compared to its virgin counterparts. The extent of degradation may depend on degradation during use, cycle life, and the severity of conditions applied during reprocessing.
[0036] Vitrimers are a class of polymers containing reversible dynamic covalent bonds that can reorganize upon application of external stimuli such as heat, light, chemical agents, and/or pH. Polymers containing dynamic covalent bonds are known to form covalent adaptable networks (CANs). Epoxy vitrimers of the present disclosure, form associative CANs, which means that existing covalent bonds are only broken when new ones are formed on application of heat. The formation of CANs through dynamic crosslinkers renders the epoxy vitrimers recyclable, repairable, or processable.
[0037] According to embodiments of the present disclosure, a method of preparing an epoxy vitrimer is provided. The method comprises introducing a first dynamic crosslinker and a second dynamic crosslinker into an epoxy monomer in liquid form to obtain a reaction solution. The method further comprises providing a catalyst in the reaction solution to form a reaction mixture. The method further comprises curing the reaction mixture at a temperature in a range of 150°C to 200°C for a period of time in a range of 2 to 4 hours to form the epoxy vitrimer.
[0038] FIG. 1 is a flow chart 100 that illustrates a method of preparing an epoxy vitrimer through exemplary steps 102 through 106, according to embodiments of the present disclosure. At step 102, a first dynamic crosslinker and a second dynamic crosslinker are introduced into an epoxy monomer in liquid form (also termed as liquid epoxy monomer) to obtain a reaction solution.
[0039] The epoxy monomers comprise at least one epoxide group. As used herein, the term “at least one” refers to having one, or more than one. The epoxy monomers include epoxy monomers of glycidyl type where the epoxy monomers are prepared by condensation reaction of a diol, diacid, or diamine with epichlorohydrin. In some embodiments, the epoxy monomer molecule may contain some rigidity by having phenylene rings in the molecule. In other embodiments, the epoxy monomer molecule may contain some flexibility by having linear or branched alkylene or poly(alkyleneoxide) unit in the molecule. Non-limiting examples of epoxy monomers include bisphenol A diglycidyl ether (BADGE), diglycidyl ether of bisphenol F, hydrogenated bisphenol A diglycidyl ether, tetraglycidyl methylene dianiline, pentaerythritol tetraglycidyl ether, trimethylol triglycidyl ether (TMPTGE), tetrabromo bisphenol A diglycidyl ether, or hydroquinone diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, butylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol A polyethylene glycol diglycidyl ether, bisphenol A polypropylene glycol diglycidyl ether, 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), tetraglycidyl diaminodiphenyl methane (TGDDM), or combinations thereof. In one embodiment, the epoxy monomer comprises 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), bisphenol A diglycidyl ether (BADGE), tetraglycidyl diaminodiphenyl methane (TGDDM), diglycidyl ether of bisphenol F, or combinations thereof.
[0040] The first dynamic crosslinker comprises an aliphatic carboxylic acid, or an aromatic carboxylic acid containing 2 to 13 carbon atoms and having at least two carboxylic acid groups. Non-limiting examples of aromatic carboxylic acid include terephthalic acid, phthalic acid, isophthalic acid, trimellitic acid, hemimellitic acid, trimesic acid, 2′-Bipyridine-5,5′-dicarboxylic acid, 2',5'-Dimethyl-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid, [1,1':4',1''-Terphenyl]-4,4''-dicarboxylic acid, 2',5'-dimethoxy-[1,1':4',1''-Terphenyl]-4,4''-dicarboxylic acid, 4,4''-dihydroxy-[1,1':4',1''-Terphenyl]-3,3''-dicarboxylic acid, 3,3''-Diamino-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid, or combinations thereof. Non-limiting examples of aliphatic carboxylic acid include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanediocic acid, dodecanedioic acid, brassylic acid, or combinations thereof.
[0041] A concentration of the first dynamic crosslinker is in a range of 3.5 wt% to 10 wt% to the total weight of the epoxy monomer.
[0042] The second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT), tert-Butyldimethylsilyl ethers (TBDMS), 1,2-Bis(triethoxysilyl)ethane (BTSE), 1,4-Bis(triethoxysilyl)benzene, or combinations thereof. In one embodiment, the second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT).
[0043] A concentration of the second dynamic crosslinker is in a range of 10 wt% to 40 wt% to the total weight of the epoxy monomer.
[0044] Suitable epoxy monomers are in liquid form at room temperature. In certain embodiments, the epoxy monomers may be heated to reduce a viscosity of the liquid epoxy monomer before introducing the first dynamic crosslinker and the second dynamic crosslinker.
[0045] At step 102, the first dynamic crosslinker and the second dynamic crosslinker are added to the liquid epoxy monomer with constant mixing to obtain the reaction solution, as in a one-pot synthesis. Mixing may be performed using any apparatus or method known in the art, such as manual mixing in a beaker using a spatula, high-shear mixing, rotor-stator mixing, and the like.
[0046] In some embodiments, step 102 may be performed in two steps. For example, in the first step, the first dynamic crosslinker and the second dynamic crosslinker are mixed to form a first solution, and in the subsequent second step, the first solution is added to the liquid epoxy monomer to form the reaction solution. The reaction between the epoxy monomer, the first dynamic crosslinker, and the second dynamic crosslinker is initiated at step 102.
[0047] In one embodiment, the reaction solution is degassed to remove any dissolved gases.
[0048] At step 104, a catalyst is provided in the reaction solution, or degassed reaction solution to form the reaction mixture. The catalyst accelerates the reaction between the epoxy monomer, the first dynamic crosslinker, and the second dynamic crosslinker initiated at step 102. It is believed that the catalyst forms a lower activation energy reaction intermediate through ring-opening of the epoxide ring of the epoxy monomer. As the catalyst-initiated reaction modifies the activation energy, the reaction between the epoxy monomer, the first dynamic crosslinker, and the second dynamic crosslinker may be performed at a lower temperature.
[0049] Examples of catalysts include 1-(3-aminopropyl)imidazole (API), 2,4,6-Tris(dimethylaminomethyl)phenol, benzyldimethyl amine, methyl imidazole, 2-ethyl-4-methyl-1H-imidazole, benzyl triethyl ammonium chloride, boron trichloride, boron trifluoride monoethyl amine, or combinations thereof. In one embodiment, the catalyst comprises 1-(3-aminopropyl)imidazole.
[0050] In one embodiment, the catalyst is provided at a concentration in a range of 5 wt% to 20 wt% to the total weight of the epoxy monomer.
[0051] At step 106, the reaction mixture is cured to form the epoxy vitrimer. During curing, the extent of reaction or a degree of dynamic crosslinking increases between the epoxy monomer, the first dynamic crosslinker, and the second dynamic crosslinker to form a 3-dimensionally crosslinked epoxy vitrimer. The reaction mixture, in liquid or semi-liquid form in step 102 and 104, solidifies in step 106 on crosslinking to form the epoxy vitrimer.
[0052] The reaction mixture is poured into a mold, in one embodiment, at step 106 of curing. However, other processing methods such as casting, filament coiling, continuous molding or molding between film coatings, infusion, pultrusion, resin transfer molding or RTM, reaction injection molding (or RIM) or any other methods known to those skilled in the art, as described in “Epoxy Polymers: New Materials and Innovations,” edited by J. P. Pascault and R. J. Williams, Wiley-VCH, Weinheim 2010, may be employed.
[0053] As is known in the art, curing generally requires time, where the speed of curing is dependent on the particular epoxy monomers, crosslinkers, and the catalyst employed. The step 106 of curing may be accelerated by applying heat. In certain embodiments, curing agents and/or accelerants may be employed as known in the art.
[0054] In one embodiment, curing the reaction mixture is performed at a temperature in a range of 150°C to 200°C for a period of time in a range of 2 to 4 hours.
[0055] The epoxy vitrimer obtained using the method as illustrated in FIG. 1 may further comprise components such as a polymer, an additive, a filler, a UV additive, a flame-retardant additive, an antimicrobial additive, a pigment, or combinations thereof to form an epoxy formulation. The components may be added at steps 102, 104, 106, or after step 106 of epoxy vitrimer preparation to obtain the epoxy formulation.
[0056] Examples of polymers include epoxy polymers, copolymers, and blends. Examples of polymers that may be blended with epoxy polymers include, but are not limited to, polysulfone (PSF), poly(ether sulfone) (PES), poly(ether imide) (PEI), polyamides (PA), poly(ether ether ketone) (PEEK), poly(phthalazinone ether) or combinations thereof. Examples of copolymers include, but are not limited to, polymethyl methacrylate, polystyrene, or combinations thereof. Examples of epoxy polymers include epoxy virgin epoxy polymers, post-consumer recycled (PCR) epoxy polymers, post-industrial recycled (PIR) epoxy polymers, or combinations thereof.
[0057] Non-limiting examples of additives include flame-retardant additives, antioxidants, UV additives (stabilizer), antimicrobial additives, or the like.
[0058] Non-limiting examples of fillers include silica, clays, carbon black, kaolin, talc, calcium carbonate, whiskers, glass fibers, carbon fibers, polyester fibers, polyamide fibers, aramid fibers, cellulose-based and nanocellulose-based fibers, plant-based fibers such as flax, hemp, sisal, bamboo, or combinations thereof.
[0059] The term “pigment” as used herein refers to colored material, or materials intended to provide color to the vitrimer and is insoluble in the vitrimer. Examples of pigments include titanium oxide, carbon black, carbon nanotubes, metal particles, silica, metal oxides, metal sulfides, phthalocyanines, anthraquinones, quinacridones, dioxazines, azo pigments, natural pigments such as madder, indigo, cochineal, or combinations thereof.
[0060] The formation of the epoxy vitrimer, as outlined in the method illustrated in FIG. 1, can be better understood through a representative reaction, in accordance with the embodiments of the disclosure. A reaction scheme 200, as shown in FIG. 2, illustrates one possible mechanism by which the representative reaction may proceed. It is understood that alternative pathways, intermediates, or mechanisms could also account for the observed results, and the invention is not limited to the mechanism depicted below.
[0061] In the reaction scheme 200, as shown in FIG. 2, the catalyst is 1-(3-aminopropyl) imidazole, epoxy monomer is bisphenol A diglycidyl ether (BADGE), the first crosslinker is terephthalic acid and the second crosslinker is bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT). Referring to FIG. 2, the catalyst 1-(3-aminopropyl) imidazole attacks the epoxide ring of the epoxy monomer (BADGE) resulting in bond formation between carbon atom of the epoxide and ring nitrogen atom of the catalyst. The first dynamic crosslinker, terephthalic acid, through acidic hydrogen atom of the carboxylic acid group forms hydrogen bonding with the oxygen atom of the epoxide ring to form a first reaction intermediate [I].
[0062] The reaction proceeds through the formation of a second intermediate [II] where the catalyst is linked to the epoxy monomer, and the terephthalic acid is converted to a terephthalate anion (leaving group) with the loss of the acidic hydrogen atom.
[0063] In the next step, the terephthalate anion attacks other epoxy monomers, as shown in structure [III], to form a chain of covalently bound repeating units of an epoxy unit and a first dynamic crosslinker unit as shown in structures [IV] to [VI]. As used herein, the term “epoxy unit”, refers to a structural moiety derived from the epoxy monomer through ring-opening reaction of the epoxide group, and forms part of the crosslinked network in the resulting epoxy vitrimer. As used herein, the term “first dynamic crosslinker unit”, refers to a structural moiety derived from the first dynamic crosslinker through reaction of the acidic hydrogen of the carboxylic acid group to the epoxy unit, and forms part of the crosslinked network in the resulting epoxy vitrimer. A second dynamic crosslinker unit is attached to the epoxy unit and the first dynamic crosslinker unit through silyl ether linkage as shown by structure [V]. As used herein, the term “second dynamic crosslinker unit”, refers to a structural moiety derived from the second dynamic crosslinker through silyl ether linkage with the first dynamic crosslinker unit, and forms part of the crosslinked network in the resulting epoxy vitrimer.
[0064] A first covalent adaptive network is formed through a transesterification exchange reaction between the epoxy unit, and the first dynamic crosslinker unit, as shown in in structures [IV] to [VI]. A second covalent adaptive network is formed through silyl ether exchange reaction between the epoxy unit, the first dynamic crosslinker unit, and the second dynamic crosslinker unit as shown by structures [V] and [VI]. A third covalent adaptive network is formed through disulfide exchange between the second dynamic crosslinker units of the same chain, or different chains. In FIG. 2, the bonds and functional groups marked in red colour indicate dynamic bonds, these bonds will break and re-form due to their dynamic nature.
[0065] As described, the inventive epoxy vitrimer comprises an epoxy unit derived from an epoxy monomer. The epoxy vitrimer further comprises a first dynamic crosslinker unit derived from a first dynamic crosslinker. The first dynamic crosslinker is covalently attached to the epoxy unit and able to form a first covalent adaptive network through transesterification exchange reaction. The epoxy vitrimer further comprises a second dynamic crosslinker unit derived from a second dynamic crosslinker. The second dynamic crosslinker is covalently attached to the epoxy unit and the first dynamic crosslinker unit and able to form a second covalent adaptive network through silyl ether exchange reaction. The second dynamic crosslinker is able to form a third covalent adaptive network through disulfide exchange reaction.
[0066] The presence of three covalent adaptive networks (CAN) provides the epoxy vitrimer with enhanced mechanical properties when compared to an epoxy vitrimer having single CAN, or double CAN. The mechanical properties of the epoxy vitrimer may be characterized in terms of tensile strength. As used herein, the term “tensile strength” is defined as the maximum tensile load a material can withstand before it breaks. The tensile strength is determined using a tensile test and the tensile strength corresponds to the highest point of the stress-strain curve plotted from the test. The tensile strength of the inventive epoxy vitrimer is in a range of 50 MPa to 55 MPa. When the mechanical properties such as tensile strength is enhanced upon processing it implies that the processed epoxy vitrimer or formulations derived therefrom is of greater mechanical strength than that of the epoxy it is obtained from.
[0067] The presence of three CANs in the inventive vitrimer with an enhanced 3-dimensional network modifies a degradation temperature. As used herein, the term “degradation temperature” refers to a maximum temperature at which a polymer can be used without undergoing damaging chemical changes. The degradation temperature is an important parameter to consider when processing or manufacturing polymers, as it can affect the properties of the polymer and may even limit its range of applications. In one embodiment, the inventive epoxy vitrimer has a degradation temperature of 250°C, or above 250°C. A vitrimer system having triple CAN, for example, the inventive epoxy vitrimer, has a higher degradation temperature than vitrimer systems having single or double CAN. The inventive epoxy vitrimer maintains its mechanical properties and dimensional stability even at elevated temperatures, owing to its higher degradation temperature. This makes the epoxy vitrimer more resistant to thermal degradation and aging, ensuring a longer service life and improved durability across various applications.
[0068] The relaxation time is another important parameter with a view to the processability of vitrimers, influencing, for example, a time required for a material to be shaped, or recycled. Stress relaxation tests are typically performed using a dynamic mechanical analyzer to determine the relaxation time. The sample is subjected to a preload force and allowed to equilibrate at appointed temperatures for a set time and then a constant strain (1%) is applied. The relaxation modulus (also termed stress relaxation) is recorded at set times. The relaxation time (τ) is defined as the time when the relaxation modulus is 37% (1/e) of its original modulus. An activation energy Ea may be calculated using Arrhenius equation: τ(T) = τ0 exp (Ea/RT). The presence of triple CAN in the inventive epoxy vitrimer lowers the activation energy when compared to an epoxy vitrimer system comprising single or double CAN. Lower activation energy facilitates faster and more efficient reprocessing and/or self-healing at lower temperatures. As will be appreciated, the covalent adaptive networks present in the inventive vitrimer are dynamic in nature and the networks may be broken and re-formed on application of heat. As a result, the inventive vitrimer may be processed, or repaired (self-healable) post-formation.
[0069] Processing the epoxy vitrimer, in one instance, corresponds to recycling the epoxy vitrimer. In some embodiments, processing the epoxy vitrimer corresponds to recycling the epoxy vitrmer along with epoxy polymers. The epoxy polymers include virgin epoxy polymers, post-consumer recycled (PCR) epoxy polymers, post-industrial recycled (PIR) epoxy polymers, or combinations thereof. Post-consumer recycled (PCR) plastics refer to plastic waste generated by consumers, or after-use plastic products. The composition of PCR plastics can vary significantly due to the diverse mix of polymers and additives used by different manufacturers. The variation in composition makes recycling of PCR plastics more complex and challenging. In contrast, post-industrial recycled (PIR) plastics are derived from plastic waste produced during industrial and manufacturing processes. PIR plastics are generally easier to recycle as they typically originate from a single source and are of known composition.
[0070] In accordance with embodiments of the present disclosure a method of recycling the epoxy vitrimer is provided. The method comprises heating the epoxy vitrimer dispersed in a solvent to a temperature of 200°C, or less than 200°C for a period of time in a range of 2 to 10 hours. Examples of solvents include dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP) and the like. In some embodiments, epoxy polymers such as virgin epoxy polymers, post-consumer recycled (PCR) epoxy polymers, post-industrial recycled (PIR) epoxy polymers, or combinations thereof may be recycled along with the epoxy vitrimer.
[0071] The terms “self-repairing” or “self-healing”, refer to the ability of a material to eliminate or reduce defects in the material. A material that exhibits self-healing is said to be self-healable. In one embodiment, scratches on a surface of the epoxy vitrimer can be repaired on heat treatment at a temperature in a range of 150 to 180 ° C. The surface self-heals after the application of heat within a period of less than about 12 hours, preferably less than about 6 hours.
[0072] The epoxy vitrimers may be reprocessed or self-healed multiple times without degradation of their mechanical properties when compared to processing epoxy polymers made from similar epoxy monomers but without the dynamic crosslinkers. In one embodiment, the epoxy vitrimer has a cycle life of more than 3 times. Further, the epoxy vitrimers may be blended with virgin epoxy polymer, filled epoxy polymer, PCR epoxy polymer, PIR epoxy polymer, or epoxy copolymers.
[0073] In yet another embodiment, an article formed using the inventive epoxy vitrimer is provided. The articles may be formed by molding, blow molding, injection molding, filament winding, continuous molding, film-insert molding, infusion, additive manufacturing, pultrusion, RTM (resin transfer molding), RIM (reaction-injection molding), 3D printing, or any other method known to those skilled in the art. The article may further contain epoxy polymer, virgin epoxy polymer, PIR epoxy polymer, PCR epoxy polymer, filled epoxy polymer, epoxy polymer blends, epoxy copolymers, and other additives.
[0074] According to embodiments of the disclosure, a carbon fiber reinforced polymer (CFRP) is provided. The carbon fiber reinforced polymer comprises at least one carbon fiber layer impregnated with an epoxy vitrimer. The epoxy vitrimer, is as described previously, and comprises the epoxy unit derived from the epoxy monomer. The epoxy vitrimer further comprises the first dynamic crosslinker unit derived from the first dynamic crosslinker. The first dynamic crosslinker is covalently attached to the epoxy unit and able to form the first covalent adaptive network through transesterification exchange reaction. The epoxy vitrimer further comprises the second dynamic crosslinker unit derived from the second dynamic crosslinker. The second dynamic crosslinker is covalently attached to the epoxy unit and the first dynamic crosslinker unit and able to form the second covalent adaptive network through silyl ether exchange reaction. The second dynamic crosslinker is able to form the third covalent adaptive network through disulfide exchange reaction.
[0075] Carbon fibers are made up of long chains of carbon atoms that are fused together. The carbon in the carbon fiber may be in the form of graphite, amorphous carbon, carbon nano-tubes, or combinations thereof. Typically, any carbon-containing material can be ‘carbonized’ to form the carbon fiber by heating it to a temperature of around 1,000°C, or more. The properties of carbon fibers primarily depend on the nature and molecular weight of the carbon-containing material from which they are synthesized, the rate of heating, and the carbonization conditions. For instance, the carbonization of polyacrylonitrile produces carbon fibers with a small crystal size that offers good flexibility, high tensile strength, and good electrical conductivity. While carbonization of pitch leads to carbon fibers that possess large crystal sizes offering stiffness, good tensile strength, excellent electrical conductivity, and excellent thermal conductivity. Commercially, carbon fibers are made from polyacrylonitrile, cellulose-based polymers such as cotton and rayon, pitch, nonheterocyclic aromatic polymers, aromatic heterocyclic polymers, linear polymers, and coal.
[0076] The carbon fibers may have a diameter in a range of five to seven micrometres with a volumetric mass between 1.74 grams per cubic centimeter (g/cm3) and 1.95 g/cm3. The carbon fibers have an elastic modulus in a range of 230 GPa to 800 GPa.
[0077] The fibers are woven together to form a carbon fiber fabric. Typical weave styles include plain, satin, and twill weaves. The carbon fiber fabric may have a fiber areal weight in a range of 150 to 1000 gm-2. As used herein, the term fiber areal weight refers to a mass of fiber per unit area, and depends on the weave and a size of the tow, where tow is a bundle of individual fibers. In some embodiments, the carbon fibers may be layered in a unidirectional, biaxial, or random alignment to form a carbon fiber sheet, or a non-woven sheet. A surface mass of fibers in the sheet is in a range of 80 to 4,000 g/m2. The number of carbon fibers per tow is in a range of 3,000 to 320,000.
[0078] The carbon fiber fabric (also termed as fabric) or carbon fiber sheet (also termed as sheet) may be made using different types of fibers providing different characteristics, in one embodiment. In some embodiments, the fabric or sheet may be made using single type of fibers.
[0079] The at least one carbon fiber layer comprises carbon fiber in sheet form (also termed as non-woven), fabric form (woven), or combinations thereof. The at least one carbon fiber layer has a thickness in a range of 0.2 millimeters (mm) to 0.3 mm. As used herein, the term at least one corresponds to having one, or more than one layer. The at least one layer may be arranged one over the other when there is more than one carbon fiber layer. In some embodiments, one layer of carbon fiber is impregnated with the epoxy vitrimer and a second layer is placed over the impregnated first layer to form more than one layer of carbon fibers in the CFRP. A number of layers of the at least one carbon fiber layer in the CFRP is in a range of 1 to 15. In some embodiments, the number of layers of the at least one carbon fiber layer is in a range of 5 to 10.
[0080] The at least one carbon fiber layer is impregnated with the epoxy vitrimer using resin transfer molding (RTM) processes. Examples of RTM processes include ScRIMP (Seeman Composites Resin Infusion Molding Process), VARTM (Vacuum-Assisted Resin Transfer Molding), VAP (Vacuum-Assisted Processing), and RFI (Resin Film Infusion). In RTM methods, a resin or polymer of desired viscosity is injected into a mold containing the carbon fiber layer during the injection stage, followed by curing.
[0081] A ratio of the at least one carbon fiber layer to the epoxy vitrimer is in a range of 70:30 to 50:50 when the number of layers of the at least one carbon fiber layer is 10 layers.
[0082] FIG. 3 is a flowchart 300 outlining a method for preparing the CFRP according to embodiments of this disclosure. Referring to FIG. 3, the flow chart 300 illustrates the method of forming a carbon fiber reinforced polymer (CFRP) through exemplary steps 302 through 306, according to embodiments of the present disclosure. At step 302, at least one carbon fiber layer is provided. In one embodiment, the at least one carbon fiber layer is provided in a mold cavity and placed over a non-stick substrate such as a Teflon sheet. A peel layer may be provided between the non-stick substrate and the at least one carbon fiber layer for ease of removal of CFRP post-formation.
[0083] The carbon fiber constituting the carbon fiber layer, and the at least one carbon fiber layer are as described previously.
[0084] At step 304, the at least one carbon fiber layer is impregnated with an epoxy vitrimer precursor. The precursor is in liquid or semi-liquid form. The epoxy vitrimer precursor comprises an epoxy monomer, a catalyst, a first dynamic crosslinker, and a second dynamic crosslinker. The epoxy monomer, the catalyst, the first dynamic crosslinker, the second dynamic crosslinker are as described previously, with respect to FIG. 1.
[0085] The formation of the epoxy vitrimer from the epoxy vitrimer precursor proceeds through a catalyst-assisted reaction resulting in the formation of a first crosslinker anion. The anion attacks the epoxy monomer to form a chain of covalently bound repeating units of the epoxy unit and the first dynamic crosslinker unit, and is described in detail in FIGs. 1-2. The impregnation of the at least one carbon fiber layer is performed using vacuum assisted resin transfer moulding (VARTM), in one embodiment.
[0086] In some embodiments, a bleeder layer is provided over the at least one carbon fiber layer. The bleeder layer advantageously slows the flow of the epoxy vitrimer precursor over the at least one carbon fiber layer on application of vacuum.
[0087] A vacuum bag is placed over the at least one carbon fiber layer, and/or the bleeder layer to create vacuum between the at least one carbon layer and the vacuum bag, or between the vacuum bag and the bleeder layer to optimize the flow and distribution of the epoxy vitrimer precursor. The non-stick substrate, the peel layer, the at least one fiber layer, the bleeder layer, and the vacuum bag are all sealed together along all sides to create the vacuum. The application of vacuum compresses the impregnated carbon fiber layer. At step 304, the epoxy precursor is filled in the mold cavity through an inlet channel. The epoxy vitrimer precursor, in some embodiments, is applied or poured evenly over the at least one carbon fiber layer to impregnate the at least one carbon fiber layer.
[0088] A vacuum is generated in the mold cavity through vacuum channels placed between the vacuum bag and the at least one carbon fiber layer, or between the vacuum bag and the bleeding layer. During the process of filling the mold cavity, a negative pressure is generated whereby the epoxy precursor is drawn into the mold cavity via the inlet channels in order to fill the mold cavity. From the inlet channel, the precursor disperses in all directions in the mold cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus, positions of the inlet channels and vacuum channels are placed optimally in order to obtain a complete filling of the vacuum cavity. The application of vacuum, draws and distributes the epoxy vitrimer precursor over the at least one carbon fiber layer.
[0089] At step 306, the epoxy vitrimer precursor impregnated on the at least one carbon fiber layer is cured at a temperature in a range of 150°C to 200°C for a period of time in a range of 2 to 4 hours to form the carbon fiber reinforced polymer. In certain embodiments, vacuum is maintained during step 306 of curing. During curing, the extent of reaction or a degree of dynamic crosslinking increases between the epoxy monomer, the first dynamic crosslinker, and the second dynamic crosslinker to form a 3-dimensionally crosslinked epoxy vitrimer. The epoxy vitrimer precursor, in liquid or semi-liquid form in step 304 solidifies in step 306 on crosslinking to form the epoxy vitrimer bonded to the at least one carbon fiber layer to form the CFRP.
[0090] The CFRP may further comprise components such as a polymer, an additive, a filler, a UV additive, a flame-retardant additive, an antimicrobial additive, a pigment, or combinations thereof. Examples of polymers include epoxy polymers, copolymers, and blends. Examples of polymers that may be blended with epoxy polymers include, but are not limited to, polysulfone (PSF), poly(ether sulfone) (PES), poly(ether imide) (PEI), polyamides (PA), poly(ether ether ketone) (PEEK), poly(phthalazinone ether) or combinations thereof. Examples of copolymers include, but are not limited to, polymethyl methacrylate, polystyrene, or combinations thereof. Examples of epoxy polymers include epoxy virgin epoxy polymers, post-consumer recycled (PCR) epoxy polymers, post-industrial recycled (PIR) epoxy polymers, or combinations thereof.
[0091] As will be appreciated, the covalent adaptive networks present in the epoxy vitrimer are dynamic in nature and the networks may be broken and re-formed on application of heat. As a result, CFRP comprising the epoxy vitrimer may be processed, or repaired (self-healing).
[0092] In some embodiments, the carbon fiber reinforced polymer has a ratio of concentration of the at least one carbon fiber layer to the epoxy vitrimer is in a range of 70:30 to 50:50 when the number of layers of the at least one carbon fiber layer is 10 layers.
[0093] In some embodiments, the carbon fiber reinforced polymer has a thickness in a range of 2 mm to 5 mm.
[0094] In accordance with embodiments of the present disclosure, a method of recycling the CFRP is provided. The method comprises heating the CFRP dispersed in a solvent to a temperature of 200°C, or preferably less than 200°C for a period of time in a range of 2 to 10 hours. Examples of solvents include dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone, or combinations thereof. The epoxy vitrimer dissolves in the solvent to form an epoxy vitrimer solution and the at least one carbon fiber layer separates from the epoxy vitrimer solution. The at least carbon fiber layer after drying may be reused. The epoxy vitrimer solution may further be processed to retrieve and re-form the epoxy vitrimer.
[0095] Traditional CFRP based on epoxies, degrade over time, or exhibit aging due to deterioration in epoxy structure or bonding between epoxy chains on prolonged exposure to environmental factors and stress. The drawbacks of traditional CFRPs are addressed in the present disclosure by employing epoxy vitrimers that may be re-processed, or self-healed on application of heat.
[0096] In one embodiment, scratches or mar on a surface of the CFRP can be repaired on heat treatment at a temperature in a range of 150 to 180 ° C. In some embodiments, the internal structure of the CFRP can self-heal due to the presence of CANs in the epoxy vitrimer. The CFRP self-heals after the application of heat within a period of less than about 6 hours. In one embodiment, the carbon fiber reinforced polymer has a self-healing efficiency of 85%, or greater than 85%. As used herein, the term self-healing efficiency is defined as the ratio of defects on a specimen after self-healing to a ratio of defects on the specimen before self-healing. The self-healing may be analyzed using micro-computed tomography techniques which may reveal internal as well as external (surface, such as mar or scratches) defects.
[0097] The CFRP may be reprocessed or self-healed multiple times without degradation of their mechanical properties when compared to CFRP containing epoxy polymers without the dynamic crosslinkers. In one embodiment, the CFRP has a cycle life of more than 3 times. Further, the epoxy vitrimers in the CFRP may be blended with virgin epoxy polymer, filled epoxy polymer, PCR epoxy polymer, PIR epoxy polymer, or epoxy copolymers, fillers, pigments, and additives, as described earlier.
[0098] In yet another embodiment, an article formed using the CFRP is provided. In some embodiments, the articles are shaped in accordance with a contour of the mold cavity during the formation of the CFRP, as illustrated in FIG. 3, by utilizing an appropriately designed mold cavity. In certain embodiments, the article may be formed by molding, blow molding, injection molding, filament winding, continuous molding, film-insert molding, infusion, pultrusion, RTM (resin transfer molding), RIM (reaction-injection molding), 3D printing, or any other method known to those skilled in the art on application of heat to reprocess the CFRP to obtain a desired shape of the article. The article may further contain epoxy polymer, virgin epoxy polymer, PIR epoxy polymer, PCR epoxy polymer, filled epoxy polymer, epoxy polymer blends, epoxy copolymers, fillers, pigments and additives.

EXAMPLES
EXAMPLE 1
Preparation of epoxy vitrimer
[0099] About 6 grams (g) of bisphenol A diglycidyl ether (BADGE) monomer was taken in a beaker and stirred for 10 minutes. The beaker was warmed to reduce a viscosity of the BADGE monomer. About 0.3 g of terephthalic acid (TA) and 1.2 g of bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT) were added simultaneously to BADGE monomer and the resulting solution was stirred for 5 minutes. The solution was degassed till all the air bubbles in the solution disappeared. About 0.96 g of 1-(3-aminopropyl)imidazole (API) catalyst was added to the solution and stirred for 30 seconds to form a reaction mixture. The reaction mixture was poured into a tensile mold and the mold was kept in the hot oven for 3 hours at 180°C for curing to obtain the epoxy vitrimer. The formation of the epoxy vitrimer was confirmed by Fourier Transform Infrared (FTIR) spectra.
Mechanical testing
[00100] According to ASTM D638 (type V) stress-strain properties of the epoxy vitrimer samples were measured using Universal Testing Machine at room temperature. The testing parameters were load cell: 5 kN, preload force: 0.1 N, cross head speed: 1 mm/minute, gauge length: 15mm, number of Samples: 3 per batch for consistency, sample dimension: 50 mm length x 3.21 mm width x 3.17 mm thickness.
[00101] The samples were placed between clamps of the Universal Testing Machine - Tensile Testing Module such that the edges of the samples were parallel to the direction of the load. The grips were then tightened to hold the sample securely within the jig. The test sample was then pulled apart at a tensile speed of 1 mm/min until it broke. Tests were performed on vitrimer formed using BADGE monomer and terephthalic acid alone (labelled as TA), vitrimer formed using BADGE monomer and bis 3-(triethoxysilyl) propyl tetrasulfide (labelled as TSPT) and the inventive vitrimer including both TA and TSPT corresponding to Example 1 (labelled as TA + TSPT). FIG. 4 is a bar chart 400 of tensile strengths of the vitrimers where bar diagram 402 corresponds to tensile strength of vitrimer formed using BADGE monomer and terephthalic acid (labelled as TA), bar diagram 404 corresponds to tensile strength of vitrimer formed using BADGE monomer and bis 3-(triethoxysilyl) propyl tetrasulfide (labelled as TSPT), and bar diagram 406 corresponds to tensile strength of the inventive vitrimer (labelled as TA + TSPT). The inventive epoxy vitrimer (TA+TSPT) had a tensile strength of 53 MPa compared to TA (43 MPa) and TSPT (48 MPa). The results confirmed the superior mechanical performance of the inventive vitrimer when compared to vitrimers comprising single or double CAN.
[00102] FIG. 5 is a plot of stress relaxation against temperature recorded at three different temperatures of 160 °C, 180 °C, and 200°C, respectively. Stress relaxation is a characteristic feature of epoxy vitrimers with dynamic crosslinks, where the exchange of dynamic bonds within the crosslinking networks leads to a network rearrangement effect, resulting in stress relaxation. Consequently, rapid stress relaxation is essential to demonstrate the material's ability to be recycled or self-healed. The relaxation time for the triple CAN systems was found to decrease with increasing temperature at constant strain, with relaxation times of 280 seconds (s), 166 s, and 116 s at 160°C, 180°C, and 200°C, respectively. As shown in FIG. 5, the relaxation time decreased from 280s to 116s as the temperature increased from 160 to 200 °C.
[00103] FIG. 6 is a plot of activation energy (ln(τ)) against 1000/Temperature (T), which yielded a straight line and an activation energy of 56 kJ/mol. The inventive epoxy vitrimer was found to follow Arrhenius's behavior, which is indicative of a reversible CAN system.

Self-healing test
[00104] The self-healing ability of the epoxy vitrimer was determined using a scratch test. The reaction mixture obtained from Example 1 was poured on a glass slide and cured for 3 hours at 180°C to form a film. A scratch was marked on the film and the film was left for self-healing for 6 hours at 200°C. The scratch was hardly visible after 6 hours confirming the remarkable healing capability of the epoxy vitrimer. This self-healing ability of the epoxy vitrimer is attributed to the dynamic bonds present in the vitrimer.

Recyclability test
[00105] Chemical recycling of the epoxy vitrimer was performed by dissolving a sample of the epoxy vitrimer obtained from Example 1 in dimethyl sulfoxide (DMSO) at 110°C for 10 hours. This was followed by compression molding at 3 MPa pressure for 2 hours, resulting in a thin sheet, which confirmed the recyclability of the epoxy vitrimer.

EXAMPLE 2
Formation of carbon fiber reinforced polymer (CFRP) comprising epoxy vitrimer
[00106] A Teflon release film was kept on a glass substrate in a tray. A peel ply was cut into many pieces of desired size and kept on the Teflon release film. Commercially procured carbon fiber fabric of 13x13 size was placed over the peel ply pieces. A porous film was placed over the carbon fiber fabric and a bleeder layer was placed over the porous film. Sealants were attached on all 4 sides. A breather was placed on the sides and at a bottom portion of the layers to absorb excess vitrimer precursor. Finally, a spiral pipe was attached to the top and bottom of the tray covered by a vacuum bag.
[00107] An epoxy vitrimer precursor, containing 8 grams (g) of bisphenol A diglycidyl ether (BADGE) monomer, 0.4 g of terephthalic acid (TA), 1.6g of bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT) and 1.28 g of 1-(3-aminopropyl)imidazole (API) catalyst was flowed in through the spiral pipe under vacuum. The vacuum was maintained and the epoxy vitrimer precursor was cured at a temperature of 180 °C for 3 hours to obtain the CFRP. The CFRP had a thickness of 2 mm.
Mechanical testing
[00108] Interlaminar shear strength (ILSS) and Flexural strength (FS) of the CFRP of Example 2 were recorded. A CFRP sample of Example 2 was measured for ILSS using a universal tester manufactured by Instron to which a 3-point bending jig (indenter 3.2 mmR, support 1.6 mmR, distance between supports 8 mm) was installed at conditions of a 1 mm/min crosshead speed. ILSS refers to the maximum shear stress that a material can withstand along a plane of its layered structure (for example CFRP) before failure occurs. A typical epoxy laminate typically exhibits an ILSS between 10-25 MPa depending on laminate quality and cure. The CFRP of Example 2 exhibited ILSS of 38 MPa thus exhibiting superior mechanical performance.
[00109] Flexural strength refers to the maximum stress a material can withstand before it yields or breaks. The CFRP of Example 2 exhibited a flexural strength of 520 MPa.
Self-healing test
[00110] The self-healing ability of the CFRP was tested by monitoring the samples subjected to mechanical testing. The sample (before self-healing) exhibited slight bending upon interlaminar shear strength (ILSS) test and had an ILSS value of 38 MPa. The bent sample was subjected to a compression molding process at 200°C for 30 minutes under a pressure of 10 MPa. Following this treatment, the ILSS of the sample (after self-healing) was retested and found to be 32 MPa. The self-healing efficiency was estimated from the ILSS value before seld-healing and after self-healing and was found to be 86%.
[00111] The samples were analyzed using micro-computed tomography (Micro-CT) to investigate the self-healing behavior of the CFRP. FIG.7 displays the Micro-CT images 700 of the samples before and after self-healing. The sample before self-healing had numerous large voids, as seen from images 710 and 720, with void lengths reaching up to 850 micrometers. The void length decreased to 383 micrometers as shown in images 730 and 740 for the sample after self-healing. A significant reduction in number of voids was also observed on self-healing. The improvement in void morphology is attributed to the presence of triple CAN dynamic bonds, which facilitated the healing process of the CFRP.
Recyclability test
[00112] A 5x5 cm2 sample was cut from the CFRP obtained from Example 2. The sample was immersed in DMSO solvent for 10 hours at 110°C to facilitate the separation of the epoxy vitrimer from the carbon fiber fabric. The epoxy vitrimer which remained in solution may be reused for other applications. The carbon fiber fabric obtained was tested for purity for further reuse.
[00113] Raman spectra of virgin carbon fiber and carbon fiber fabric obtained after recycling were recorded. An ID/IG value indicative of the purity of carbon of carbon fiber was calculated for both the samples. Here ID corresponds to the intensity of Raman band at 1350 cm-1 and IG corresponds to the intensity of Raman band at 1580 cm-1. The virgin carbon fiber had an ID/IG value of 1.151 while the carbon fiber obtained after recycling had an ID/IG value of 1.158 respectively. There was no change in the ID/IG ratio confirming that the carbon fiber was not damaged upon recycling and hence can be reused again.
[00114] It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the disclosure is not limited to the embodiments described but can be practiced with modification and alteration within the scope of the appended claims.
, C , Claims:We Claim:
1. An epoxy vitrimer comprising:
an epoxy unit derived from an epoxy monomer;
a first dynamic crosslinker unit derived from a first dynamic crosslinker, wherein the first dynamic crosslinker unit is covalently attached to the epoxy unit and able to form a first covalent adaptive network through transesterification exchange reaction; and
a second dynamic crosslinker unit derived from a second dynamic crosslinker, wherein the second dynamic crosslinker unit is covalently attached to the epoxy unit and the first dynamic crosslinker unit and able to form a second covalent adaptive network through silyl ether exchange reaction, and wherein the second dynamic crosslinker unit is able to form a third covalent adaptive network through disulfide exchange reaction.
2. The epoxy vitrimer as claimed in claim 1, wherein the epoxy monomer comprises bisphenol A diglycidyl ether (BADGE), diglycidyl ether of bisphenol F, hydrogenated bisphenol A diglycidyl ether, tetraglycidyl methylene dianiline, pentaerythritol tetraglycidyl ether, trimethylol triglycidyl ether (TMPTGE), tetrabromo bisphenol A diglycidyl ether, or hydroquinone diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, butylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol A polyethylene glycol diglycidyl ether, bisphenol A polypropylene glycol diglycidyl ether, 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), tetraglycidyl diaminodiphenyl methane (TGDDM), or combinations thereof.
3. The epoxy vitrimer as claimed in claim 1, wherein the first dynamic crosslinker comprises an aliphatic carboxylic acid, or an aromatic carboxylic acid having at least two carboxylic acid groups and containing 2 to 13 carbon atoms.
4. The epoxy vitrimer as claimed in claim 1, wherein the second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT), tert-butyldimethylsilyl ether (TBDMS), 1,2-bis(triethoxysilyl)ethane (BTSE), 1,4-bis(triethoxysilyl)benzene, or combinations thereof.
5. The epoxy vitrimer as claimed in claim 1, wherein a concentration of the first dynamic crosslinker is in a range of 3.5 wt% to 10 wt% to the concentration of the epoxy monomer, and wherein a concentration of the second dynamic crosslinker is in a range of 10 wt% to 40 wt% to the concentration of the epoxy monomer.
6. The epoxy vitrimer as claimed in claim 1, wherein the epoxy vitrimer comprises a catalyst, wherein a concentration of the catalyst is in a range of 5 wt% to 20 wt%, and wherein the catalyst comprises 1-(3-aminopropyl)imidazole, 2,4,6-Tris(dimethylaminomethyl)phenol, benzyldimethyl amine, methyl imidazole, 2-ethyl-4-methyl-1H-imidazole, benzyl triethyl ammonium chloride, boron trichloride, boron trifluoride monoethyl amine, or combinations thereof.
7. The epoxy vitrimer as claimed in claim 1, wherein the epoxy vitrimer is processable, or self-healable, or both.
8. An epoxy formulation comprising the epoxy vitrimer as claimed in claim 1.
9. The epoxy formulation as claimed in claim 8, wherein the formulation comprises a virgin epoxy polymer, a post-consumer recycled (PCR) epoxy polymer, post-industrial recycled (PIR) epoxy polymer, or combinations thereof.
10. A method of recycling the epoxy vitrimer as claimed in claim 1 comprising:
heating the epoxy vitrimer dispersed in a solvent to a temperature greater than 200°C for a period of time in a range of 2 to 10 hours, wherein the solvent comprises dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone, or combinations thereof.
11. An article formed using the epoxy vitrimer as claimed in any of the claims 1-7.
12. A method (100) of preparing an epoxy vitrimer comprising:
providing a first dynamic crosslinker and a second dynamic crosslinker in an epoxy monomer in liquid form (102) to form a reaction solution;
providing a catalyst in the reaction solution (104) to form a reaction mixture; and
curing the reaction mixture (106) at a temperature in a range of 150°C to 200°C for a period of time in a range of 2 to 4 hours to form the epoxy vitrimer.
13. The method (100) as claimed in claim 12, wherein the epoxy monomer comprises bisphenol A diglycidyl ether (BADGE), diglycidyl ether of bisphenol F, hydrogenated bisphenol A diglycidyl ether, tetraglycidyl methylene dianiline, pentaerythritol tetraglycidyl ether, trimethylol triglycidyl ether (TMPTGE), tetrabromo bisphenol A diglycidyl ether, or hydroquinone diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, butylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol A polyethylene glycol diglycidyl ether, bisphenol A polypropylene glycol diglycidyl ether, 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), tetraglycidyl diaminodiphenyl methane (TGDDM), or combinations thereof, wherein the first dynamic crosslinker comprises an aliphatic carboxylic acid, or an aromatic carboxylic acid having at least two carboxylic acid groups and containing 2 to 13 carbon atoms, and wherein the second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT), tert-butyldimethylsilyl ethers (TBDMS), 1,2-bis(triethoxysilyl)ethane (BTSE), 1,4-bis(triethoxysilyl)benzene, or combinations thereof.
14. The method (100) as claimed in claim 12, wherein the catalyst comprises 1-(3-aminopropyl)imidazole, 2,4,6-Tris(dimethylaminomethyl)phenol, benzyldimethyl amine, methyl imidazole, 2-ethyl-4-methyl-1H-imidazole, benzyl triethyl ammonium chloride, boron trichloride, boron trifluoride monoethyl amine, or combinations thereof, and wherein a concentration of the catalyst is in a range of 5 wt% to 20 wt%.
15. The method (100) as claimed in claim 12, wherein a concentration of the first dynamic crosslinker is in a range of 3.5 wt% to 10 wt% to the concentration of the epoxy monomer, and wherein a concentration of the second dynamic crosslinker is in a range of 10 wt% to 40 wt% to the concentration of the epoxy monomer.
16. A carbon fiber reinforced polymer comprising:
at least one carbon fiber layer impregnated with an epoxy vitrimer, wherein the epoxy vitrimer comprises;
an epoxy unit derived from an epoxy monomer;
a first dynamic crosslinker unit derived from a first dynamic crosslinker, wherein the first dynamic crosslinker unit is covalently attached to the epoxy unit and able to form a first covalent adaptive network through transesterification exchange reaction; and
a second dynamic crosslinker unit derived from a second dynamic crosslinker, wherein the second dynamic crosslinker is covalently attached to the epoxy unit and the first dynamic crosslinker unit and able to form a second covalent adaptive network through silyl ether exchange reaction, and wherein the second dynamic crosslinker is able to form a third covalent adaptive network through disulfide exchange reaction.
17. The carbon fiber reinforced polymer as claimed in claim 16, wherein a number of layers of the at least one carbon fiber layer is in a range of 1 to 15.
18. The carbon fiber reinforced polymer as claimed in claim 16, wherein the carbon fiber reinforced polymer has a self-healing efficiency of 85%, or greater than 85%.
19. The carbon fiber reinforced polymer as claimed in claim 16, wherein the epoxy monomer comprises bisphenol A diglycidyl ether (BADGE), diglycidyl ether of bisphenol F, hydrogenated bisphenol A diglycidyl ether, tetraglycidyl methylene dianiline, pentaerythritol tetraglycidyl ether, trimethylol triglycidyl ether (TMPTGE), tetrabromo bisphenol A diglycidyl ether, or hydroquinone diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, butylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol A polyethylene glycol diglycidyl ether, bisphenol A polypropylene glycol diglycidyl ether, 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), tetraglycidyl diaminodiphenyl methane (TGDDM), or combinations thereof.
20. The carbon fiber reinforced polymer as claimed in claim 16, wherein the first dynamic crosslinker comprises an aliphatic carboxylic acid, or an aromatic carboxylic acid having at least two carboxylic acid groups and containing 2 to 13 carbon atoms.
21. The carbon fiber reinforced polymer as claimed in claim 16, wherein the second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT), tert-butyldimethylsilyl ether (TBDMS), 1,2-bis(triethoxysilyl)ethane (BTSE), 1,4-bis(triethoxysilyl)benzene, or combinations thereof.
22. The carbon fiber reinforced polymer as claimed in claim 16, wherein the at least one carbon fiber layer comprises 2 layers, and wherein the epoxy monomer comprises bisphenol A diglycidyl ether (BADGE), the first dynamic crosslinker comprises terephthalic acid, and the second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT).
23. The carbon fiber reinforced polymer as claimed in claim 16, wherein a ratio of the at least one carbon fiber layer to the epoxy vitrimer is in a range of 70:30 to 50:50 when the number of layers of the at least one carbon fiber layer is 10 layers.
24. The carbon fiber reinforced polymer as claimed in claim 16, wherein the carbon fiber reinforced polymer comprises a catalyst, and wherein the catalyst comprises 1-(3-aminopropyl)imidazole, 2,4,6-Tris(dimethylaminomethyl)phenol, benzyldimethyl amine, methyl imidazole, 2-ethyl-4-methyl-1H-imidazole, benzyl triethyl ammonium chloride, boron trichloride, boron trifluoride monoethyl amine, or combinations thereof.
25. A method (300) of forming a carbon fiber reinforced polymer comprising:
providing at least one carbon fiber layer (302);
impregnating the at least one carbon fiber layer with an epoxy vitrimer precursor (304), wherein the epoxy vitrimer precursor comprises an epoxy monomer, a catalyst, a first dynamic crosslinker, and a second dynamic crosslinker, wherein the first dynamic crosslinker comprises an aliphatic carboxylic acid, or an aromatic carboxylic acid containing 2 to 13 carbon atoms and having at least two carboxylic acid groups, and wherein the second dynamic crosslinker comprises bis 3-(triethoxysilyl) propyl tetrasulfide (TSPT); and
curing the epoxy vitrimer precursor (306) impregnated on the at least one carbon fiber layer at a temperature in a range of 150°C to 200°C for a period of time in a range of 2 to 4 hours to form the carbon fiber reinforced polymer.
26. A method of recycling the carbon fiber reinforced polymer as claimed in claim 16 comprising:
dispersing the carbon fiber reinforced polymer in a solvent and heating it to a temperature in a range of 160°C to 200°C for a period of time in a range of 2 to 10 hours, wherein the solvent comprises dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone, or combinations thereof.
27. An article formed using the carbon fiber reinforced polymer as claimed in any of the claims 16-24.