Liquid cross-linking boronic esters for epoxy-based vitrimers, production process thereof and vitrimers produced with these esters

Abstract

The present invention relates to boronic esters which find use as reversible cross-linking agents for epoxy-based vitrimers, the production process thereof, and the vitrimers produced using these boronic esters.

Description

[0001] LIQUID CROSS-LINKING BORONIC ESTERS FOR EPOXY-BASED VITRIMERS, PRODUCTION PROCESS THEREOF AND VITRIMERS PRODUCED WITH THESE ESTERS

[0002] *** *** ***

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to boronic esters which find use as reversible crosslinking agents for epoxy-based vitrimers, the production process thereof, and the vitrimers produced using these boronic esters.

[0005] STATE OF THE ART

[0006] Polymers have traditionally been divided into two large classes, thermoplastic polymers and thermosetting polymers, distinguished based on the functional and mechanical features thereof.

[0007] In parts made with thermoplastic polymers, the adjacent polymer chains are not connected to each other by chemical bonds and are held together by weak interactions (for example, van der Waals forces) and by poor mobility at low temperatures. As the temperature increases, however, the mobility of the chains increases and the forces of interaction are overcome by the movement of the chains themselves. Thermoplastic materials may be reversibly deformed above the glass transition temperature (Tg) or the crystalline melting point and be processed by means of extrusion, injection molding, and welding. This allows easy processing by means of melting and allows subsequent remelting cycles to recycle the polymer after a first use. At the same time, however, these features make the polymer susceptible to sliding of the polymer chains, and therefore to deformation, under the application of a mechanical stress; consequently, parts made with thermoplastic polymers generally have poor mechanical properties. Thermoplastic polymers also have poor solvent resistance.

[0008] Thermosetting materials, instead, consist of polymer chains interconnected by covalent bonds which form a stable three-dimensional network. Therefore, they have excellent mechanical properties and thermal and chemical resistance. By virtue of these features, thermosetting polymers are used in a wide range of applications, for example in the aerospace, automotive, high-performance sports equipment, adhesives and coatings fields. Thermosetting polymers are also used as matrices in fiber- reinforced composite materials ("FRPC", in which glass, carbon, aramid or polyester fibers are used for example), a market that has experienced enormous growth in recent decades. Thermosetting polymers employed in such composites include epoxy polymers, polyesters, vinyl esters, phenolics, cyanate esters (CE), bismaleimides (BMI), and polyimides.

[0009] Among these, epoxy polymers are the most used thermosets for high performance composites when the service temperatures are below 200°C, by virtue of superior mechanical performance, lighter weight and environmental degradation resistance with respect to other thermosetting resins. Epoxy resins are two-component chemicals based on a prepolymer obtained by polymerizing a diether, commonly bisphenol A diglycidyl ether (known in the art by the abbreviation DGEBA), and a polyamine hardener; the two components are both viscous liquids before hardening.

[0010] Despite these excellent technological features, thermosetting polymers, including epoxy resins, give rise to problems at the end of life of the products in which they are present.

[0011] Due to the irreversible chemical bonds between the different polymer chains, once cross-linking has been completed (i.e. , the formation of bonds between adjacent chains, a phenomenon commonly referred to in the field as curing) it is no longer possible to process the material, for example by molding, and it is not possible to recycle these polymers. For the same reason, it is not possible in the case of thermosetting polymeric matrix composite materials, to recover the material that forms the dispersed phase (typically, the aforementioned fibers). The only possibilities of reusing parts made with, or comprising, thermosetting polymers, is by grinding in order to form fillers to be used in other polymeric compositions, which however represents a considerable loss of value; or by incineration, with the consequent emissions of greenhouse gases or even potentially harmful species; or finally by commitment to landfills, also in this case giving rise to environmental problems.

[0012] In recent years a new class of polymeric compounds has emerged, called vitrimers, that have together the best properties of thermoplastics and thermosets. Vitrimers contain dynamic bonds between polymer chains, capable of exchanging, i.e., detaching from one point of a chain and reconnecting at a different point thereof (or of another one) thus allowing rearrangements of the three-dimensional network of bonds. At service temperatures, they behave like permanently cross-linked polymers, and thus have stability features typical of thermosets, but at high temperatures the bond rearrangement reactions within their network accelerate, making the flow of polymer chains possible, while maintaining a constant number of chemical bonds and crosslinks. By virtue of this, the vitrimers can be reprocessed, repaired and remodeled, using techniques typically used for thermoplastic materials, such as extrusion, injection or compression molding, significantly reducing costs and environmental impact. Vitrimers are therefore an excellent alternative to traditional epoxy resins also in fiber-reinforced composites since they allow the recycling not only of the matrix but also of the precious fibrous reinforcement.

[0013] Chemical systems that can give rise to the dynamic cross-linking described above have been described in the literature, including simple transesterification, or the use of cross-linking compounds such as disulfides, imines, or boronic or borate esters. Crosslinking compounds, normally referred to in the field as "cross-linkers", are generally small organic molecules which form bridged bonds between two adjacent polymer chains.

[0014] Vitrimers that employ boronic esters as dynamic cross-linkers can be excellent candidates to replace traditional thermosets since the production thereof follows the same procedure as synthetic epoxy resins.

[0015] Examples of boronic esters useful for producing vitrimers are reported in patent application US 2020 / 0399412 A1 (although in the text of the application, the polymers obtained are referred to as "comb copolymers" and the term vitrimers does not appear).

[0016] Boronic esters useful as cross-linkers in epoxy-based vitrimers are disclosed for example in the review article, "Boronic acid esters and anhydrates as dynamic crosslinks in vitrimers", M. Gosecki et al., Polymers 2022, 14, 842.

[0017] The currently known boronic esters useful as cross-linkers are solid compounds. This involves difficulties in the reactions between the starting components of the epoxy resins which, as mentioned, are generally viscous liquids: in fact, the dispersion of solids in viscous liquids is not efficient, and this can lead to inhomogeneity in the distribution of the cross-linking points, with non-uniform properties, and not completely reproducible between different production batches, of the final epoxy resin.

[0018] The object of the present invention is to provide cross-linking compounds for epoxy resins based on boronic esters that are effective at the epoxy resin production temperatures, as well as the production process thereof.

[0019] SUMMARY OF THE INVENTION

[0020] This and other objects are achieved with the present invention, which in a first aspect thereof relates to a boronic ester useful as a cross-linking agent for epoxy resins formed by reaction between a boronic acid and a compound comprising at least two diol functions, in which said boronic acid is selected from 2-aminophenyl boronic acid, 3-aminophenyl boronic acid, 4-aminophenyl boronic acid, 3-carboxybenzene boronic acid, 3-hydroxybenzene boronic acid and 3-mercaptobenzene boronic acid.

[0021] In a second aspect thereof, the invention relates to a process for the production of the above boronic esters.

[0022] Finally, in the last aspect thereof, the invention relates to an epoxy vitrimer produced using the boronic esters indicated above.

[0023] BRIEF DESCRIPTION OF THE FIGURES

[0024] The invention will be described in detail below with reference to the figures, in which:

[0025] - Figures 1 to 4 show the1H NMR spectra of four preferred boronic esters of the invention;

[0026] - Figure 5 shows the appearance of the mixtures between boronic ester and an epoxy prepolymer before polymerization;

[0027] - Figure 6 shows the appearance of vitrimers of the invention after polymerization;

[0028] - Figure 7 shows a vitrimer of the invention before and after mechanical recycling;

[0029] - Figure 8 shows a vitrimer of the invention before and after chemical recycling;

[0030] - Figure 9 shows the1H NMR spectra of a vitrimer of the invention, before and after dissolution in aqueous DMSO-d6.

[0031] DETAILED DESCRIPTION OF THE INVENTION

[0032] In the following description the following abbreviations and definitions are adopted:

[0033] - double diol: a compound comprising at least two diol functions;

[0034] - Hydrolyzed BDGE: hydrolyzed 1 ,4-butanediol diglycidyl ether, i.e. the compound having the IUPAC name 3,3’-(butane-1 ,4-diylbis(oxy))bis(propane- 1 ,2-diol);

[0035] - TGPh: phloroglucinol (1 ,3,5-trihydroxybenzene) triglycidyl ether;

[0036] - ambient temperature: a temperature between 15 and 30 °C.

[0037] In the first aspect thereof the invention relates to a boronic ester effective at the temperatures typical of epoxy resin production processes.

[0038] The identification of boronic esters with these features was not obvious before the present invention: in fact, the previously known boronic esters were not functional in the reversible cross-linking of epoxy resins either because they are solid or because they are poorly reactive at the production temperatures of these resins; the poor reactivity obviously prevents obtaining the cross-linking and therefore the good mechanical properties of the vitrimers, while the solid boronic esters are not homogeneously distributed in the reagent mixture, giving rise to products with non- uniform and uncontrolled features.

[0039] The boronic esters of the present invention have instead proved effective in obtaining epoxy-based vitrimers.

[0040] The boronic esters of the invention derive from the double condensation of a compound comprising a boronic acid group with a compound comprising at least two diol functions; the condensation is herein referred to as double because in the formation of the ester, two -OH groups of a boronic acid react with the two -OH groups of the diol. In the following description, unless it is necessary to specify the exact nature of a compound, "diol" means a double diol. The diols employed for preparing the esters of the invention are preferably vicinal diols, i.e., such that pairs of -OH groups are present on adjacent carbon atoms of the molecule.

[0041] Preferred boronic esters of the invention are:

[0042] - 3,3’-((oxybis(methylene))bis(1 ,3,2-dioxaborolan-4,2-diyl))dianiline (abbreviated meta-DGBEA), having the formula shown below:

[0043] - 3,3’-(((butan-1 ,4-diylbis(oxy))bis(methylene))bis(1 ,3,2-dioxaborolan-4,2- diy l))dian il ine (abbreviated meta-BDBEA), having the formula shown below:

[0044]

[0045] - 3,3’-((oxybis(methylene))bis(1 ,3,2-dioxaborolan-4,2-diyl))diphenol

[0046] (abbreviated meta-HDGBE), having the formula shown below: - 3,3’-(((butan-1 ,4-diylbis(oxy))bis(methylene))bis(1 ,3,2-dioxaborolan-4,2- diyl))diphenol (abbreviated meta-HBDBE), having the formula shown below:

[0047] In a second aspect thereof, the invention relates to a process for the production of the above boronic esters. The process consists of reacting the chosen diol and boronic acid in a solvent. The diol is typically a viscous liquid while the boronic acid is a solid in the form of a powder. The molar ratio of boronic acid to diol used in the reaction is between 1.5:1 and 2.5:1 , and it is preferably 2:1. The solvent must be capable of dissolving the boronic acid powders, and it is preferably a “green” solvent such as ethanol, isopropanol or ethyl acetate. The reaction is carried out under stirring at a temperature between 20 and 30 °C and it requires a time between 1 and 24 hours.

[0048] At the end of the reaction (that can be determined by known methods, for example by plate chromatography) the solvent is removed by evaporation, preferably at a pressure lower than atmospheric pressure, for example by employing a rotavapor.

[0049] The process of the invention has a series of advantageous features: the synthesis of the boronic esters is very simple, including only the steps of mixing the initial reagents and final elimination of the solvent; no by-products to be eliminated are formed in the process and the solvent can be recovered and reused; the process has a low environmental impact, not requiring the use of a catalyst (typically the catalysts comprise metals, even heavy ones) or other dangerous or toxic compounds, or no environmental impact in the case of use of the preferred solvents indicated above.

[0050] Finally, in the last aspect thereof the invention relates to a vitrimer obtained with a boronic ester as described above.

[0051] Each boronic ester molecule contains two functional groups, each of which is capable of reacting with an epoxide functional group present on the epoxy compound (typically a prepolymer).

[0052] For carrying out the process, the boronic ester of the invention and the epoxy prepolymer are mixed in a weight ratio such that the molar ratio between the epoxy groups of the prepolymer and the functional groups of the ester is between 0.5:1 and 1.5:1 , and preferably it is 1 :1. The mixture is fluid also at ambient temperature, but to improve the homogeneity thereof it is preferably brought to a temperature between ambient temperature and 60 °C. The homogenization step of the mixture has a duration lower than 10 minutes, generally about 5 minutes, times not sufficient to carry out the polymerization of the system.

[0053] The polymerization reaction can be carried out at temperatures between ambient temperature and 120°C, and it requires times between 12 and 30 hours, typically about 24 hours. When the polymerization is carried out at ambient temperature or slightly higher, it can be followed by a post-polymerization treatment, typically carried out at about 60°C for 2-5 hours; this treatment is known to those skilled in the art of polymers and serves to complete the polymerization of the non-polymerized or insufficiently polymerized resin.

[0054] Further features and advantages of the invention will emerge more from the following experimental part. MATERIALS, INSTRUMENTS, AND METHODS

[0055] The phloroglucinol triglycidyl ether (TGPh) was purchased from Specific polymers. The prepolymer EPP C-Systems 10 10 CFS was purchased from the Cecchi online site. The diglycerol was supplied by Spiga Nord SpA. The boronic acids used were purchased from FluoroChem. Pentaerythritol, deuterated dimethyl sulfoxide (DMSO-d6) and deuterated chloroform (CDCI3) were purchased from Merck.

[0056] The1H NMR analyses were performed at ambient temperature using 5 mm tubes on a Broker Avance III 400 MHz spectrometer, provided with a reverse broadband probe (BBI). The chemical shifts (in ppm) were determined with reference to tetramethylsilane (TMS).

[0057] The success of the polymerization reactions was verified by measuring the gel fraction of the samples. The tests were performed by weighing a small amount (~ 200 mg) of a material and placing it in a vial with 10 ml of ethyl acetate for 24 hours. The ethyl acetate was then removed, and the undissolved part of the material was washed with ethyl acetate, dried under vacuum at 50 °C for 24 hours and weighed again. The gel fraction was calculated from the mass ratio of the polymer before and after exposure to ethyl acetate.

[0058] The glass transition and melting temperature values of the samples were determined by differential scanning calorimetry (DSC) with a Discovery DSC 250 instrument, TA Instruments. The boronic esters analyses were carried out at heating and cooling rate of 10 °C / min from -50 °C to 150 °C, the vitrimer measurements were carried out at heating and cooling rate of 10 °C / min from 0 °C to 200 °C with nitrogen as cell purge gas (50 ml / min). The glass transition temperatures were deduced from the second heating.

[0059] The optical microscope images were taken with Leica DFC 420 optical microscope at 10x magnification.

[0060] The mechanical reprocessability tests of the vitrimers were carried out using a Carver 3853 CE press.

[0061] The chemical recyclability tests of the vitrimer were carried out by hydrolysis and dissolution of about 200 mg of vitrimer in a 9:1 v / v ethyl alcohol / water mixture for 3 hours at 60 °C and subsequent melting of the film to regenerate the original vitrimer.

[0062] EXAMPLES

[0063] Example 1 This example relates to the preparation of a boronic ester of the invention, according to the reaction: acjd

[0064] 12.13 g of diglycerol and 20.00 g of 3-aminophenyl boronic acid were weighed together with 300 ml of ethanol; these amounts correspond to a 1 :1 molar ratio between the total number of diol groups of the diglycerol and the moles of boronic acid. The mixture was placed in a glass round-bottom flask with a magnetic stirrer at ambient temperature. The flask was closed and stirred until the mixture became homogeneous (about 5 minutes). The mixture was allowed to react under these conditions for 24 hours. At the end of the reaction, the solvent was removed with rotavapor. The product was found to be a viscous liquid having a glass transition T (Tg) of 10 °C and the1H NMR spectrum reported in Fig. 1. In this figure, as in the following Figures 2-4, the positions on the molecule of the hydrogen atoms corresponding to the peaks of the spectrum are identified with graphic symbols.

[0065] Example 2

[0066] This example relates to the preparation of a boronic ester of the invention, according to the reaction: boronic acid

[0067] The preparation of Example 1 was repeated, with the difference that 17.39 g of hydrolyzed BDGE was used instead of diglycerol.

[0068] The product was found to be a viscous liquid having a Tgof 9 °C and the1H NMR spectrum reported in Fig. 2.

[0069] The preparation of this Example was also repeated using acetone, ethyl acetate, methyltetrahydrofuran and anisole as solvents, obtaining results identical to those reported above. Example 3

[0070] This example relates to the preparation of a boronic ester of the invention, according to the reaction: i-hydroxypnenyi boronic acid

[0071] The preparation of Example 1 was repeated, using 20.00 g of 3-hydroxyphenyl boronic acid instead of 3-aminophenyl boronic acid and 12.04 g of diglycerol.

[0072] The product was found to be a viscous liquid having a Tgof 0 °C and the1H NMR spectrum reported in Fig. 3.

[0073] Example 4

[0074] This example relates to the preparation of a boronic ester of the invention, according to the reaction: boronic add

[0075] The preparation of Example 2 was repeated, using 20.00 g of 3-hydroxyphenyl boronic acid instead of 3-aminophenyl boronic acid and 17.28 g of hydrolyzed BDGE.

[0076] The product was found to be a viscous liquid having a Tgof -9 °C and the1H NMR spectrum reported in Fig. 4.

[0077] Example 5

[0078] This example relates to the preparation of various epoxy polymers by employing the boronic esters produced in examples 1 -4.

[0079] Each of the four boronic esters was mixed at 60 °C for 5 minutes with the prepolymer EPP in the weight ratios indicated below. Fig. 5 shows the appearance of the mixtures between boronic ester and prepolymer EEP after this first mixing step (the number in the four photographs in the figure indicates the boronic ester preparation example).

[0080] The prepolymer EPP was reacted with the boronic esters of the invention in the following weight ratios and under the conditions shown in Table 1. In the table, "pol." stands for polymerization and the column related to the boronic ester in parentheses shows the number of the Example with which it was prepared. The reaction time was 24 hours in all cases.

[0081] Table 1

[0082] Fig. 6 shows (with the same numbering pattern as Fig. 5) the photographs obtained on the four polymer samples obtained in this example; the images first confirm the obtainment of polymers, and demonstrate, by means of transparency, the homogeneity thereof.

[0083] Example 6 (Comparative)

[0084] The preparation procedure of Example 5 was repeated with a solid boronic ester, obtained by condensation of two molecules of thioglycerol with one molecule of 1 ,4- phenyldiboronic acid.

[0085] As epoxy compound the same EPP as in Example 5 and TGPh were used.

[0086] The polymerization was initially tested at 25 °C for 24 hours, but it was not possible because at this temperature the cross-linker remained solid and did not mix with the epoxy prepolymer. Heating the mixture to 120°C until the cross-linker dissolved also did not allow cross-linking the system, as the cross-linker crystallizes again and precipitates when the mixture is cooled to ambient temperature.

[0087] Example 7

[0088] This example relates to a mechanical reprocessing test for a vitrimer of the invention.

[0089] The vitrimer obtained in Example 5 employing the boronic ester of Example 1 was reduced into pieces and the pieces were subjected to compression molding under a pressure of 4 tons for 5 minutes at 100 °C using the Carver 3853 CE press. The test result is shown in Fig. 7, which shows the pieces of initial vitrimer on the left side, and a transparent film of recycled vitrimer under a new geometric shape on the right side.

[0090] Example 8

[0091] This example relates to a chemical reprocessing test for a vitrimer of the invention.

[0092] 1 g of a sample of the same vitrimer used in Example 7 was treated with 15 ml of 90% v / v ethanol (mixture with water) at 60 °C; complete dissolution of the sample was obtained in 20 minutes. In other tests, the inventors verified that dissolution is also obtained at ambient temperature, but with longer times. The solution thus obtained was cast into a mold and after evaporation of the solvent it was possible to obtain a vitrimer solid part. The three phases of this test, from initial vitrimer fragment, to polymer dissolved in EtOH, and final solid part, are shown in Fig. 8.

[0093] To confirm that the hydrolysis of the boronic ester bonds is responsible for the dissolution of the vitrimer, a1H NMR spectrum of the vitrimer dissolved in aqueous DMSO-d6 was recorded, reproduced in Fig. 9: also in this figure, as in Figures 1 -4, the positions on the molecule of the hydrogen atoms corresponding to the peaks of the spectrum are identified with graphic symbols.

[0094] -OH and diglycerol groups were detected and the epoxy signals of the EEP disappeared due to successful cross-linking.

Claims

CLAIMS1 . Boronic ester useful as a cross-linking agent for epoxy resins formed by reaction between a boronic acid and a compound comprising at least two diol functions, wherein said boronic acid is selected from 2-aminophenyl boronic acid, 3- aminophenyl boronic acid, 4-aminophenyl boronic acid, 3-carboxybenzene boronic acid, 3-hydroxybenzene boronic acid and 3-mercaptobenzene boronic acid.

2. Boronic ester according to claim 1 , wherein said compound comprising at least two diol functions is selected from diglycerol, pentaerythritol and 3,3’-(butane-1 ,4- diylbis(oxy))bis(propane-1 ,2-diol).

3. Boronic ester according to claim 1 or 2, chosen from:- 3,3’-((oxybis(methylene))bis(1 ,3,2-dioxaborolan-4,2-diyl))dianiline, having the formula shown below:- 3,3’-(((butan-1 ,4-diylbis(oxy))bis(methylene))bis(1 ,3,2-dioxaborolan-4,2- diy l))dian il ine, having the formula shown below:- 3,3’-((oxybis(methylene))bis(1 ,3,2-dioxaborolan-4,2-diyl))diphenol, having the formula shown below:- 3,3’-(((butan-1 ,4-diylbis(oxy))bis(methylene))bis(1 ,3,2-dioxaborolan-4,2- diyl))diphenol, having the formula shown below:

4. Process for the synthesis of a boronic ester according to any one of claims 1 to 3, comprising the reaction between a boronic acid and a compound comprising at least two diol functions in a solvent, in a molar ratio between boronic acid and said compound which includes at least two diol functions between 1.5:1 and 2.5:1 , at a temperature between 20 and 30 °C and for a time between 1 and 24 hours.

5. Process according to claim 4 wherein the molar ratio between boronic acid and the compound comprising at least two diol functions is equal to 2:1 .

6. Process according to any one of claims 4 or 5 wherein said solvent is selected from ethanol, isopropanol and ethyl acetate.

7. Process according to any one of claims 4 to 6 in which at the end of the reaction the solvent is eliminated by evaporation.

8. Process according to claim 7 in which evaporation is carried out at a pressure lower than atmospheric pressure.

9. Process for the production of an epoxy vitrimer comprising as a cross-linking agent a boronic ester of any one of claims 1 to 3, wherein said boronic ester and an epoxy prepolymer are mixed in quantities such that the molar ratio between the epoxide groups of the prepolymer and the functional groups of the ester is between 0.5:1 and 1.5:1 , and the polymerization reaction is carried out at a temperature between 15 and 120 °C for a time between 12 and 30 hours.

10. Process according to claim 9, in which before the polymerization reaction the reagent mixture is treated at a temperature between 15 and 60 °C for a time of less than 10 minutes, to improve the homogeneity of said mixture.11 . Process according to any one of claims 9 or 10 in which, when the polymerization reaction is carried out at ambient temperature, said reaction can be followed by a post-polymerization treatment carried out at approximately 60 °C for a time between 2 and 5 hours.

12. Epoxy vitrimer obtained according to any one of claims 9 to 11 .

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