EP4731604A1

EP4731604A1 - A crosslinker and carbon/glass fiber-reinforced epoxy composites therefrom

Info

Publication number: EP4731604A1
Application number: EP24831283.7A
Authority: EP
Inventors: Sushanta Kumar SAHOO; Arya VIJAYAN; Venkatasubramaniam LAKSHMANAN; Ravi Rajimol PUTHENPURACKAL
Original assignee: Council of Scientific and Industrial Research CSIR
Current assignee: Council of Scientific and Industrial Research CSIR
Priority date: 2023-06-26
Filing date: 2024-06-26
Publication date: 2026-04-29
Also published as: WO2025004100A1

Abstract

The present invention relates to covalent network-based polymers derived from imine and disulphide based crosslinkers with the epoxy. The recyclability of the high-cost fibres is achieved by the synthesis of epoxy vitrimers based on dynamic imine and imine-disulphide bonds. Schiff base chemistry and condensation reaction are used to prepare the crosslinkers with imine linkages as well as imine-disulphide linkages. Various aromatic carbonyl compounds like divanillin, terepthaldehyde, vanillin, 4-hydroxy benzaldehyde, isatin etc. are used for the reaction with various aliphatic and aromatic amines like IPDA, 2-AFD, 4-AFD etc. The prepared epoxy vitrimers showed comparable thermal, mechanical and thermomechanical properties with the conventional DGEBA epoxy-amine system. The carbon/glass fiber-reinforced composite with the vitrimers showed superior mechanical properties beneficial for many engineering applications. The recycling of the vitrimers and composites can be achieved by simple immersion in a suitable solvent system with stirring or agitation at ambient temperature (30-60ºC) conditions. The fibers were easily recovered without losing their morphology, strength and structural integrity. With this invention, wastage of reinforcement carbon fibers and glass fibers can be minimized and the recovered fibers can be used for other applications with a circular economy approach.

Description

A CROSSLINKER AND CARBON/GLASS FIBER-REINFORCED EPOXY COMPOSITES THEREFROM

FIELD OF THE INVENTION

The present invention relates to a crosslinker, recyclable epoxy vitrimers and carbon/glass fiber- reinforced epoxy composites for engineering applications. Particularly, the present invention relates to a process for the development of a series of imine and imine-disulfide -based crosslinker. More particularly, the present invention relates to a process for the preparation of recyclable epoxy vitrimers and their carbon/ glass fiber-reinforced composites using the crosslinker.

The present invention further relates to the recovery of carbon or glass fiber from the carbon/ glass fiber-reinforced composites through a reversible chemical degradation of epoxy vitrimers, while retaining the structural morphology of the carbon/ glass fiber.

BACKGROUND OF THE INVENTION

Epoxy resins are the most widely used thermoset resins for various engineering applications due to their high glass transition temperature (T_g), modulus, and chemical resistance. However, these resins cannot be recycled, reprocessed, or dissolved due to their permanent crosslinking structures after curing with hardeners, making them less environmentally friendly than thermoplastics. Carbon/glass fiber-reinforced epoxy composites are currently used for high-performance applications due to high strength and stiffness, higher dimensional stability, excellent mechanical properties, solvent and corrosion resistance, etc. Due to the insoluble and infusible three- dimensional spatial network of crosslinked epoxy, it is difficult to recover high-value carbon and glass fibres in a continuous manner from the composite materials for reuse.

It has been a significant challenge to develop effective methods for producing reprocessable and recyclable epoxy with mechanical and thermal properties comparable to that of non -recyclable epoxies used for structural engineering applications. Through several covalently adaptable network (CAN) approaches, epoxy vitrimers can be prepared and might undergo an associative bond exchange mechanism or dissociative bond exchange mechanism.

Reference may be made to the patent document CN111040132A which discloses the development of a degradable amine epoxy curing agent containing a -C-N- dynamic covalent bond through nucleophilic addition reaction of paraformaldehyde (PFA) and various aliphatic diamines. The epoxy resin's mechanical properties and heat resistance has been improved by introducing a hexahydrotriazine ring structure. The dissociation of the -C-N bonds in the cured epoxy matrix has been carried out in acidic conditions. However, the fiber recovery process from the composite involves higher processing temperature up to 200 °C, longer dissolution period of 6-120 hours and a harsh pH of less than 2 (highly acidic).

Reference may be made to the patent document CN109320918B which discloses that a recyclable carbon fiber-reinforced epoxy resin composite material was fabricated using Schiff base chemistry and was recycled under acidic conditions. Epoxy functionalization of aromatic aldehydes like vanillin, p-hydroxy benzaldehyde has been carried out and subsequently cured with commercial crosslinkers like ethylene diamine, butane diamine etc. The developed composite had excellent mechanical properties and higher glass transition temperature. The fiber recovery process involves the use of mineral acids solutions like 0.1 M HC1.

Reference may be made to the patent document WO2020161538A9 which discloses a recyclable epoxy-amine system under the brand name “Recyclamine,” which involves unique amine -based curing agents containing specifically engineered cleavage points at cross-linking sites. The reworkable epoxy system involves the synthesis of bi -functional, tri-functional epoxy resins with several ether linkages, wherein few systems include silane linkages. Conventional amine -based curing agents like isophorone diamine have been used as crosslinker. The resin dissolution has been carried out at a temperature up to 130 °C in a mixture of mineral acidic medium like sulphonic acid (7 wt.%) and tetrahydrofuran for complete dissolution.

Reference may be made to the patent document US2020/0247937A1 which discloses the invention of an anhydrous route to highly processable covalent network polymers and blends. The covalent network polymers prepared from an imine - linked oligomer and an independent crosslinker comprising reactive moieties selected from the group consisting of epoxy, isocyanate, bismaleimide, sulfide, polyurethane, anhydride, polyester, and combinations. The polymers showed high glass transition temperatures and the composite made of the polymer and the fibers showed easy recyclability. The imine-containing oligomer has been used as the primary component of the matrix (60-90%) and the resins containing epoxy functionality has been used as a crosslinker. The mechanical performance of the composites, fiber recovery and recyclability has not been dealt in detail.

Reference may be made to the Journal “Journal of Materials Science, 2021, Volume: 56, Pages: 15733-15751 which discloses degradable bio-based epoxy vitrimers based on imine chemistry and their application in recyclable carbon fiber composites constituting dynamic imine bonds that were synthesized by the lignin-derived vanillin and 1,6-hexylenediamine and m-xylylene diamine, respectively for CF-based epoxy vitrimer composite. The carbon fiber vitrimer composites showed a tensile strength of 584-622 MPa, however the fiber recovery process involves use of strong mineral acids and toxic tetrahydrofuran solvents.

Reference may be made to the Journal “ACS Sustainable Chemistry & Engineering, 2021, Volume:9, Issue: 12, Pages: 4638-4647” which discloses the synthesis of recyclable epoxy vitrimers from glycerol triglycidyl ether (Gte) and an imine -containing hardener consisting of vanillin and 4- aminophenol. The carbon fiber vitrimer composite showed good dissolution in diamine solvents like ethylene diamine, but reduced mechanical property like tensile strength of 449 MPa (62 wt.% fiber) and Young’s Modulus of 12.9 GPa

Reference may be made to the journal “Composites Science and Technology, 2020, Volume: 199, Article Number: 108314” which discloses an imine-containing epoxy vitrimer with versatile recyclability and its application in fully recyclable carbon fiber reinforced composites” and a recyclable CF composite prepared from Imine-containing hardener synthesized from lignin- derived vanillin and methylcyclohexanediamine. The vitrimer composites showed resin dissolution and fiber recovery in diamine solvents. The synthesis of the crosslinker involves a complex procedure of three-hour reaction in nitrogen atmosphere, followed by removal of excess reactants under reduced pressure and recrystallization. The removal of high boiling reactants like methylcyclohexanediamine (191 °C) involves high energy consumption and the recrystallization procedure involves the use of excessive solvents. Reference may be made to the Journal “Composites Part B: Engineering, Volume 224,2021,109188” which discloses a carbon fiber composite with good mechanical properties and recyclability and re-processibility. This work involves the synthesis of a poly-imine resin using vanillin and hexachloro-cyclotriophosphazene with multi-aldehyde functionalities, which is subsequently cured with commercial diamine crosslinkers. The fiber recovery process from the vitrimer composite involves the use of a solution containing strong mineral acids (IM HC1) and highly toxic solvents like tetrahydrofuran.

Reference may be made to the Journal “Composites Part B -engineering, 2020, Volume: 199, Article Number: 108278” which discloses the synthesis of a vitrimers based on disulfide exchange from bis(4-glycidyloxyphenyl) disulfide (BGPDS) and the amine curing agent such as 4- aminophenyl disulfide (AFD) that both contained aromatic disulfide bonds, namely dual disulfide vitrimers for carbon fiber composite in wind power and automobile industry. The carbon fiber vitrimer composites showed resin dissolution and fiber recyclability in thio-based solvents, but showed a lower tensile strength of 334 MPa.

Reference may be made to the Journal “ACS Sustainable Chemistry and Engineering, 2023, Volume: 11, Issue: 15, Pages: 6021-6031” which discloses the preparation of fully biobased epoxy vitrimers containing aliphatic disulfide bonds from cystamine, 4-AFD, and vanillin-derived biosourced epoxy. Schiff base and di-sulfide bond containing epoxy resin has been synthesized from bio-based vanillin, 4-AFD, cystamine and epichlorohydrin which is subsequently crosslinked again using 4-AFD. The epoxy vitrimer prepared containing two different type of dynamic bonds shows complete resin dissolution in DTT/DMF solution at 65 °C. The study on development of carbon fiber composites and fiber recovery has not been performed.

In view of these prior arts, significant progress has been done in developing recyclable epoxy resins (resin containing dynamic imine and / or disulfide bonds) and poly-imine resins, which are further crosslinked using commercial di-amines to prepare epoxy vitrimers and their fiber reinforced composites. Though vanillin-based recyclable epoxy resin or other aldehyde based poly-imine based resins show excellent dissolution and fiber recovery in acidic medium, the resin synthesis involves multiple steps. Few research work have been carried out in development of dynamic crosslinkers based on Schiff-base chemistry using vanillin and diamines like methylcyclohexanediamine, 4-aminophenol, 1,6-hexylenediamine and m-xylylene diamine, however other aliphatic/aromatic amines as well as amine with disulfide linkages are not explored yet along with aromatic di-aldehydes like divanillin, terepthalaldehyde. Further, the synthesis process of these crosslinkers usually involves recrystallization to extract. The resin dissolution and fiber recovery process utilizes harsh solvents like mineral acids, THF, DMF, etc. which needs to be avoided at industrial volume.

The present invention is focused on the synthesis of crosslinkers containing dynamic imine bonds only or both imine and disulfide bonds for curing the DGEBA epoxy resin and development of recyclable carbon / glass fiber epoxy composites. The prepared epoxy composites showed mechanical and thermal properties comparable to that of non-recyclable epoxy thermoset. The recyclable carbon fiber composites showed >99 % resin dissolution in selected solvent systems, and -100% fiber recovery while retaining the surface morphology. The carbon fiber recovery process also involves moderate temperatures (not exceeding 60 °C), dilute organic acids like acetic acid and relatively less harsh solvents like hexylamine, 2-mercaptoethanol solutions.

OBJECTIVES OF THE INVENTION

The main objective of the present invention is to provide a series of imine and imine-disulfide- based crosslinkers through Schiff base chemistry.

Another objective of the present invention is to prepare a series of recyclable epoxy vitrimers and carbon/ glass fiber-reinforced vitrimer composites.

Yet another objective of the present invention is to provide a process for the preparation of a series of imine and imine-disulfide -based crosslinker.

Yet another objective of the present invention is to provide a process for the preparation of recyclable epoxy vitrimers and their carbon/ glass fiber-reinforced composites using the crosslinker.

Yet another objective of the present invention is to analyze the thermal, mechanical, and thermomechanical properties of the developed reversible epoxy vitrimers and achieve properties comparable with non-reversible DGEBA epoxy-amine systems. Yet another objective of the present invention is to recover the carbon and glass fiber from the carbon/ glass fiber-reinforced vitrimer composites through a chemical dissolution process at moderate temperatures and relatively safe solvents.

Yet another objective of the present invention is to study the surface structure and morphology of the recovered fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates the synthesis of crosslinker 1.

Fig. 2 illustrates the synthesis of crosslinker 2.

Fig. 3 illustrates the synthesis of crosslinker 3.

Fig. 4 illustrates the synthesis of crosslinker 4.

Fig. 5 illustrates the synthesis of crosslinker 5.

Fig. 6 illustrates the Differential Scanning Calorimetry (DSC) graphs of developed epoxy vitrimers showing the glass transition temperature.

Fig. 7 illustrates the thermogravimetric analysis of vitrimers.

Fig. 8 illustrates the thermomechanical property comparison of vitrimers.

Fig. 9 illustrates the thermomechanical property comparison of carbon fiber vitrimer composites.

Fig. 10 illustrates the thermomechanical property comparison of glass fiber vitrimer composite.

Fig. 11 illustrates the resin degradation study of carbon and glass fiber vitrimer composites.

Fig. 12 illustrates the SEM images of virgin carbon fiber and carbon fiber recovered from vitrimer composites.

Fig. 13 illustrates the XRD and XPS comparison of virgin carbon fiber and recovered carbon fiber from the vitrimer composite.

Fig. 14 illustrates the cylinder- or ring-shaped carbon fiber vitrimer composite manufactured through filament winding process.

Fig. 15 illustrates the carbon fiber vitrimer composite laminate prepared through the compression moulding technique.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a crosslinker comprising a carbonyl compound and an amine in an equivalent weight ratio ranging from 1:0.5 to 1:6; wherein said crosslinker having one or more primary amine and /or secondary amine and/or hydroxyl groups as epoxy curing moieties.

In an embodiment of the present invention, the carbonyl compound is mono or dicarbonyl compound selected from the group consisting of divanillin, vanillin, terephthalaldehyde, 4- hydroxybenzaldehyde or isatin.

In another embodiment of the present invention, the amine used is an aliphatic or aromatic diamine selected from the group consisting of Isophorone diamine (IPDA), 4- Aminophenyl disulfide (4-AFD) or 2- Aminophenyl disulfide (2-AFD).

In yet another embodiment of the present invention, the crosslinker is selected from the group consisting of: i. Divanillin-Imine crosslinker (DVI) [crosslinker 1]; ii. Terephthaldehyde-Imine crosslinker (TAI) [crosslinker 2]; iii. 4-hydroxy benzaldehyde-disulfide crosslinker (HB_DS) [crosslinker 3]; iv. vanillin-disulfide crosslinker (VA_DS) [crosslinker 4]; and v. isatin-disulfide crosslinker (IS_DS) [crosslinker 5].

In yet another embodiment, present invention provides a process for preparation of crosslinker comprising the steps of: i. adding the carbonyl compound and the amine in a ratio ranging from l:0.5.to 1 :6 in a solvent with stirring at a temperature in the range of 25 to 35°C followed by raising the temperature in the range of 60-75 °C and stirring for 2 to 3 hrs to obtain a mixture; ii. cooling the mixture as obtained in step (i) followed by distillation to obtain the crosslinker.

In yet another embodiment of the present invention, the solvent used is selected from ethanol or methanol.

In yet another embodiment, present invention provides a high-strength carbon or glass fiber vitrimer composite using the crosslinker comprising: a) at least 26 to 38 wt.% Diglycidyl ether of bisphenol A (DGEBA) epoxy resin with epoxy equivalent weight of 180-200 gm/eq; b) at least 12 to 24 wt.% crosslinker; c) at least 45 to 50 wt.% carbon fiber or glass fiber. In yet another embodiment, present invention provides a process for the preparation of carbon or glass fiber vitrimer composite comprising the steps of: i. mixing DGEBA epoxy resin and the crosslinker in a weight ratio ranging between 3:1 to 4: 1, curing at a temperature in the range of 30-35 °C for a period in the range of 20 to 24 hrs followed by post-curing at a temperature in the range of 80-180°C for a period in the range of 2 to 3hrs to obtain epoxy vitrimers; ii. fabricating 40-50 wt.% carbon fiber/glass fiber in epoxy vitrimers as obtained in step (i) through the hand layup method followed by curing at temperature in the range of 30-35 °C for a period in the range of 20 to 24 hrs followed by post curing at temperature in the range of 80-180°C for a period in the range of 2 to 3 hrs to obtain carbon or glass fiber vitrimer composite.

In yet another embodiment of the present invention, said composite shows a glass transition temperature in the range of 90-95 °C while scanned with heating rate of 10 °C/min; Flexural strength in the range of 44-121 MPa, Flexural modulus in the range of 2.2-3.6 GPa, Tensile strength in the range of 13-50 MPa, Tensile modulus in the range of 1.67-3.07 GPa and Storage modulus at 30°C in the range of 1192-2425 MPa.

In yet another embodiment of the present invention, said composite shows resin dissolution with carbon or glass fiber recovery in selected solvent systems like a) 75-80 wt. % dissolution in 20 % v/v acetic acid at 60 °C for 48 hours (for all composites). b) >90 wt. % dissolution in 2-mercaptoethanol at 60 °C for 48 hours, (for composites with crosslinker containing both imine and disulphide bonds) c) >99 wt. % dissolution achieved in two-step approach, initial dissolution in hexylamine at 60 °C for 48 hours followed by dissolution in 20 % v/v acetic acid at 60 °C for 6 hours, (for composites with only imine bonds).

In yet another embodiment of the present invention, the carbon or glass fiber vitrimer composite shows an increase in Tensile strength by 14-46 folds, Tensile modulus by 4-24 folds, Flexural strength by 4-17 folds and Flexural modulus by 6-25 folds when compared to their corresponding vitrimers. In yet another embodiment of the present invention, composite shows an increase in Tensile strength by 11-31 folds, Tensile modulus by 3-5 folds, Flexural strength by 4-6 folds and Flexural modulus by 6-14 folds when compared to their corresponding vi trimers.

In yet another embodiment of the present invention, for the preparation of composites can be used for the preparation of continuous fiber laminates or cylinder-shaped components following the wet lay-up method, filament winding, compression moulding, etc., for engineering applications.

DETAILED DESCRIPTION OF THE INVENTION

Recyclable epoxy vitrimers are made from different crosslinkers with dynamic imine and/or disulfide bonds, which are synthesized by adopting Schiff base chemistry via a condensation reaction between aldehydes and amines, which are used for making recyclable fiber -reinforced vitrimer composites.

The carbonyl compounds used to synthesize new crosslinkers are divanillin, terepthaldehyde, vanillin, 4-hydroxy benzaldehyde, isatin, etc. and, Amines used are aromatic and aliphatic amines like IPDA, 4-AFD, 2-AFD etc. Crosslinker 1 and crosslinker 2 are developed from divanillin and terephthalaldehyde respectively by the reaction with isophorone diamine (IPDA) using the Schiff base chemistry. The crosslinker 3 and crosslinker 4 are developed from 4 -hydroxybenzaldehyde and vanillin respectively through the condensation reaction with 4-aminophenyl disulfide (4-AFD) and resulted in more than one type of recyclable bond viz. imine and disulfide. Crosslinker 5 was derived from isatin and 2-aminophenyl disulfide (2-AFD) via a condensation reaction resulting in a compound with both imine and disulfide recyclable bonds.

Diglycidyl ether of bisphenol A (DGEBA) epoxy with epoxy equivalent weight (EEW) in a range of 180-200 gm/eq. is used for making epoxy vitrimers. The epoxy vitrimer developed from each crosslinker may require a different curing profile, some requiring post-curing temperatures of 180°C.

Unidirectional high-strength carbon fiber (400 gsm) and s-glass fiber (1200 gsm) were used to prepare fiber vitrimer composites through hand layup method with a minimum fiber content of 50 wt. %. The same curing profile adopted for vitrimers was followed to cure the corresponding fiber vitrimer composites. The developed vitrimers and fiber vitrimer composites showed excellent glass transition temperature (>90 °C), and mechanical and thermo-mechanical properties comparable to that of the DGEBA epoxy-IPDA system. The vitrimers and fiber vitrimer composites developed in the present invention showed the varied extent of degradation in different solvent systems. All the vitrimer composites showed 75-80 % dissolution on treating with 20% v/v acetic acid within 24-48 hours. Vitrimer composites showed a higher dissolution of 85-99% when subjected to a two-stage dissolution comprising hexylamine and 20% v/v acetic acid solutions. Vitrimer composites with disulfide linkages resulted in a dissolution of >90% when treated with 2-mercaptoethanol solution. Subsequently, the carbon or glass fibers were recovered while retaining the inherent morphology through SEM, XRD and XPS for reuse.

EXAMPLES

Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

EXAMPLE 1: SYNTHESIS OF CROSSLINKERS

Example la: Synthesis of Divanillin-Imine crosslinker (DVI) [Crosslinker 1]

Isophorone diamine (IPDA) was introduced to a two-necked round bottom flask with a reflux condenser. Under stirring and nitrogen atmosphere at room temperature, divanillin in ethanol was added dropwise using a dropping funnel. Once the addition of divanillin was completed, the temperature was raised to 60 °C and the mixture was stirred for 3 hours at 100-200 rpm. After cooling to room temperature the mixture was distilled under reduced pressure to remove ethanol and excess IPDA to obtain the crosslinker. The divanillin and IPDA were mixed in a ratio starting from 1: 1 to 1:6. Better disssolution was optimised for 1: 1.5 equivalent ratio.

The structure of DVI has been confirmed by ¹ H NMR (500 MHz, DMSO-de, 8 in ppm) 8.14 (2H) , 8.33 (2H), 7.14-7.52 (8H), 3.74 (12H) and through FTIR spectroscopy (imine peak at 1637 cm' ¹ ). Further, the molecular mass has been confirmed through HRMS (M+H)⁺, calculated: 873.4802, observed: 873.4799.

Example lb: Synthesis of Terephthaldehy de -Imine crosslinker (TAI) [Crosslinker 2]

Terephthaldehyde-imine crosslinker was prepared in the same procedure as described above in Example 1. Terephthaldehyde and IPDA were mixed in an equivalent weight ratio of 1:1 to 1:3. Among different ratios adopted for the synthesis of the cross-linker, the 1: 1.5 equivalent ratio is optimized based on dissolution property along with comparable mechanical properties.

The structure of TAI has been confirmed by ¹ H NMR (500 MHz, DMSO-de, 8 in ppm) 8.22 (4H) , 7.63 (8H) and through FTIR spectroscopy (imine peak at 1638 cm ¹). Further, the molecular mass has been confirmed through HRMS (M+H)⁺, calculated: 537.3957, observed: 573.3978.

Example 1c: Synthesis of 4-hydroxy benzaldehyde-disulfide crosslinker (HB_DS) [Crosslinker 3]

The same procedure as mentioned in Example 1 was adopted to make an HB -disulphide -based crosslinker but the reaction was carried out at 70 °C. 4-hydroxy benzaldehyde and 4-AFD were used as reactants and were mixed in an equivalent ratio of 1:0.5 to 1:1. Among different ratios adopted for the synthesis of the cross-linker, the 1 :0.5 ratio is optimized based on better dissolution property.

The structure of HB_DS has been confirmed by ’ H NMR (500 MHz, DMSO-de, 8 in ppm) 8.34 (2H), 6.83-7.70 (16H) and through FTIR spectroscopy (imine peak at 1601 cm ¹). Further, the molecular mass has been confirmed through HRMS (M+H)⁺, calculated: 457.1044, observed: 457.1055.

Example Id: Synthesis of vanillin-disulfide crosslinker (VA_DS) [Crosslinker 4]

The procedures described in Example 1 were used to synthesize a vanillin-disulfide -based crosslinker but the reaction was carried out at 70 °C. Vanillin and 4-AFD were used as reactants and mixed in an equivalent weight ratio of 1:0.5 to 1:1. Among different ratios adopted for the synthesis of the cross-linker, the 1:0.5 ratio is optimized based on dissolution property along with better mechanical properties.

The structure of VA_DS has been confirmed by ’H NMR (500 MHz, DMSO-de, 8 in ppm) 8.18 (2H), 6.45-7.48 (14H), 3.73 (6H) and through FTIR spectroscopy (imine peak at 1622 cm ¹). Further, the molecular mass has been confirmed through HRMS (M+H)⁺, calculated: 517.1256, observed: 517.1263.

Example le: Synthesis of isatin-disulfide crosslinker (IS_DS) [Crosslinker 5]

The same procedure as mentioned in Example 1 was adopted and the temperature was modified to 75 °C for the synthesis of IS-disulphide -based crosslinker. Isatin and 2-AFD were used as reactants and were mixed in a ratio of 1:0.5 to 1: 1. Among different ratios adopted for the synthesis of the cross-linker, the 1 :0.5 equivalent ratio is optimized based on better dissolution property. The structure of IS_DS has been confirmed by ¹ H NMR (500 MHz, DMSO-de, 8 in ppm) 6.34- 7.56 (16H) and through FTIR spectroscopy (imine peak at 1648 cm ¹) Further, the molecular mass has been confirmed through HRMS (M+Na)⁺, calculated: 529.0769, observed: 529.0781.

EXAMPLE 2: PREPARATION OF EPOXY VITRIMERS

The crosslinkers (Crosslinker 1-5) were used to make epoxy vitrimers by mixing it with epoxy resin in the weight ratio of 3: 1 to 4:1 (Epoxy: crosslinker) and casting the resin to a thickness of 3-4 mm. Epoxy vitrimers were cured at room temperature (25 to 35°C) for 24 hrs and post-cured at 80°C for 2 hours, 100°C for 2 hours, 120°C for 2 hours, etc. Depending on the crosslinker, the post-curing time and temperature were increased upto 180°C for 2 hours in some cases. Table 2 shows thermogravimetric analysis (TGA) data of epoxy vitrimers and table 3 shows the mechanical properties of the epoxy Vitrimers.

Table 3: Mechanical properties of the epoxy vi trimers

EP_IP: Epoxy IPDA thermoset;

DVI_EP: Divanillin imine epoxy vitrimer;

TAI_EP: Terephthalaldehyde imine epoxy vitrimer;

HB_DS_EP: 4-hydroxy benzaldehyde-disulphide epoxy vitrimer;

VA_DS_EP: Vanillin-disulphide epoxy vitrimer;

IS_DS_EP: Isatin-disulfide epoxy vitrimer.

EXAMPLE 3: PREPARATION OF CARBON/GLASS FIBER REINFORCED VITRIMER COMPOSITES

Carbon/glass fiber-reinforced vitrimer composites were fabricated using the developed epoxy vitrimers and the curing profile was followed according to the corresponding epoxy vitrimer. Unidirectional carbon fiber (BhorForce 400 gsm) or s-glass fiber (Owen Corning, 1200 gsm) was used as the fiber reinforcement to prepare fiber vitrimer composites through hand layup method with a minimum fiber content of 50 wt. %. The same curing profile adopted in Example 2 for vitrimers was followed to cure the corresponding fiber vitrimer composites.

The thickness of the carbon/glass fiber composite was maintained between 3-4 mm with a minimum fiber: resin ratio of 50:50 (w/w). Table 4: Mechanical properties of the carbon fiber reinforced vitrimer composites

EP_IP_CF: Epoxy IPDA carbon fiber composite;;

DVI_EP_CF: Divanillin imine epoxy vitrimer carbon fiber reinforced composite; TAI_EP_CF: Terephthaldehyde imine epoxy vitrimer carbon fiber reinforced composite;

HB_DS_EP_CF: 4-hydroxy benzaldehyde-disulphide epoxy vitrimer carbon fiber reinforced composite;

VA_DS_EP_CF: Vanillin-disulphide epoxy vitrimer carbon fiber reinforced composite;

IS_DS_EP_CF: Isatin-disulfide epoxy vitrimer carbon fiber reinforced composite;

Table 5: Mechanical properties of the glass fiber reinforced vitrimer composites

EP_IP_GF: Epoxy IPDA glass fiber composite

DVI_EP_GF: Divanillin imine epoxy vitrimer glass fiber reinforced composite;

TAI_EP_GF: Terephthalaldehyde imine epoxy vitrimer glass fiber reinforced composite;

VA_DS_EP_GF: Vanillin-disulphide epoxy vitrimer glass fiber reinforced composite;

HB_DS_EP_GF: 4-hydroxy benzaldehyde-disulphide epoxy vitrimer glass fiber reinforced composite;

IS_DS_EP_GF: Isatin-disulfide epoxy vitrimer glass fiber reinforced composite.

EXAMPLE 4: SOLVENT RESISTANCE / CHEMICAL STABILITY

The chemical stability of the vitrimers, prepared by curing the epoxy resin with the different crosslinkers, is evaluated by immersing the pre-weighed samples in various solvent systems like IM H2SO4, IM HC1, water, IM NaOH, ethanol, acetone, DCM, DMSO, THF, and DMF for 10 days. The percentage of weight loss due to the degradation in solvents was evaluated after drying.

Table 6: The chemical stability evaluation of the different vitrimer systems in different solvents

It was observed that even after 10 days of immersion in solvents, all the vitrimer systems were recovered without much degradation and weight loss.

EXAMPLE 5: CHEMICAL DISSOLUTION AND RECYCLABILITY

The chemical dissolution of epoxy vitrimers and fiber-reinforced composites has been checked in different solvents. The solvent systems used were effective for the recovery of the fiber by simple immersion and occasional stirring. Mild temperature conditions of 30-60 °C aided the speed of recycling. The following solvent systems and the dissolution procedures were found to be effective for fiber recovery involving moderate conditions and relatively safe solvents.

METHOD 1

The recyclability of imine and/or imine-disulfide -based epoxy vitrimers and its fiber-reinforced composites have been studied in a solvent system of 20% v/v acetic acid for 48 hours at 60 °C. All the composites showed 75-80% dissolution.

METHOD 2

The recyclability of imine -based epoxy vitrimers and fiber-reinforced composites have been checked in different solvent systems like hexylamine and followed by 20% v/v acetic acid. The initial dissolution was tested in hexylamine at 60 °C for 48 hours with frequent stirring which ensured maximum dissolution of the vitrimer. A final treatment for ensuring the complete dissolution was done with 20% v/v acetic acid at 60 °C for 6 hours. All the systems exhibited 85- 99% dissolution.

METHOD 3

The recyclability of disulfide -based epoxy vitrimers and fiber-reinforced composites have been checked in different solvent systems like 2-mercaptoethanol and 2-mercaptoethanol/ any organic solvent (with varying ratios of 70:30 to 100:0) etc. for 48 hrs at temperature 60 °C with proper stirring. All the disulphide -based vitrimers showed >90% dissolution in 48 hrs.

Table 7 showing the percentage of dissolution of each vitrimer in different solvent systems

ADVANTAGES OF THE INVENTION

• The preparation of cross-linkers is a simple one -pot synthesis with green solvents in a cost- effective method.

• Prepared epoxy vitrimer exhibited thermal, mechanical and thermomechanical properties comparable to non-degradable DGEBA cured with an amine -based crosslinker.

• Recovery of expensive carbon/glass fiber from fiber-reinforced composites using a simple chemical recycling method at moderate temperature (< 60 °C).

• The recycling methods don’t require mineral acidic conditions like the use of H2SO4/ HC1 and high-temperature conditions above 80 °C. • More than 99% recovery of carbon fiber or glass fiber through chemical dissolution of resin and the recovered fiber exhibited retention of structural morphology.

Claims

WE CLAIM

1. A crosslinker, comprising: a carbonyl compound and an amine in an equivalent weight ratio ranging from 1:0.5 to 1:6; wherein the crosslinker having one or more primary amine and /or secondary amine and/or hydroxyl groups as epoxy curing moieties.

2. The crosslinker as claimed in claim 1, wherein the carbonyl compound is mono or dicarbonyl compound selected from the group consisting of divanillin, vanillin, terephthalaldehyde, 4-hydroxybenzaldehyde or isatin.

3. The crosslinker as claimed in claim 1, wherein the amine is aliphatic or aromatic diamine selected from the group consisting of Isophorone diamine (IPDA), 4- Aminophenyl disulfide (4-AFD) or 2- Aminophenyl disulfide (2-AFD).

4. The crosslinker as claimed in claim 1, wherein the crosslinker is selected from the group consisting of: i. Divanillin-Imine crosslinker (DVI) [crosslinker 1]; ii. Terephthaldehyde-Imine crosslinker (TAI) [crosslinker 2]; iii. 4-hydroxy benzaldehyde-disulfide crosslinker (HB_DS) [crosslinker 3]; iv. vanillin-disulfide crosslinker (VA_DS) [crosslinker 4]; and v. isatin-disulfide crosslinker (IS_DS) [crosslinker 5].

5. A process for preparation of crosslinker as claimed in claim 1, comprising the steps of: i. adding the carbonyl compound and the amine in a ratio ranging from 1 :0.5 to 1 :6 in a solvent with stirring at a temperature in the range of 25 to 35 °C followed by raising the temperature in the range of 60-75 °C and stirring for 2 to 3 hrs to obtain a mixture; ii. cooling the mixture as obtained in step (i) followed by distillation to obtain the crosslinker.

6. The process as claimed in claim 5, wherein the solvent is selected from ethanol or methanol.

7. A carbon or glass fiber vitrimer composite using the crosslinker as claimed in claim 1, comprising: a) at least 26 to 38 wt.% Diglycidyl ether of bisphenol A(DGEBA) epoxy resin with epoxy equivalent weight of 180-200 gm/eq; b) at least 12 to 24 wt.% crosslinker as claimed in claim 1; c) at least 45 to 50 wt.% carbon fiber or glass fiber.

8. A process for the preparation of carbon or glass fiber vitrimer composite as claimed in claim 7, comprising the steps of: i. mixing DGEBA epoxy resin and the crosslinker as claimed in claim 1 in a weight ratio ranging between 3:1 to 4: 1, curing at a temperature in the range of 30-35 °C for a period in the range of 20 to 24 hrs followed by post-curing at a temperature in the range of 80- 180°C for a period in the range of 2 to 3hrs to obtain epoxy vi trimers; ii. fabricating 40-50 wt.% carbon fiber/glass fiber in epoxy vitrimers as obtained in step (i) through the hand layup method followed by curing at temperature in the range of 30-35 °C for a period in the range of 20 to 24 hrs followed by post curing at temperature in the range of 80-180°C for a period in the range of 2 to 3 hrs to obtain carbon or glass fiber vitrimer composite.

9. The carbon or glass fiber vitrimer composite as claimed in claim 7, wherein said composite shows a glass transition temperature in the range of 90-95 °C while scanned with heating rate of 10 °C/min; having flexural strength in the range of 44-121 MPa, flexural modulus in the range of 2.2-3.6 GPa, tensile strength in the range of 13-50 MPa, tensile modulus in the range of 1.67-3.07 GPa and storage modulus at 30°C in the range of 1192-2425 MPa.

10. The carbon or glass fiber vitrimer composite as claimed in claim 7, wherein said composite shows resin dissolution with carbon or glass fiber recovery in selected solvent systems like; a) 75-80 wt. % dissolution in 20 % v/v acetic acid at 60 °C for 48 hours (for all composites). b) >90 wt. % dissolution in 2-mercaptoethanol at 60 °C for 48 hours, (for composites with crosslinker containing both imine and disulphide bonds) c) >99 wt. % dissolution achieved in two-step approach, initial dissolution in hexylamine at 60 °C for 48 hours followed by dissolution in 20 % v/v acetic acid at 60 °C for 6 hours, (for composites with only imine bonds).