Process for preparing thermosetting epoxy resins

Abstract

The invention relates to a process for cleaving and at least partially recycling the starting components of a thermosetting epoxy resin which is a polymerisation product of a dicarboxylic acid diglycidyl ester of formula (I) and a diamine of formula (II): (I) wherein R1 represents a C1-20 alkylene group, H2N-R2-NH2 (II) wherein R2 represents a C1-20 alkylene group, but the two amino groups are not directly bonded to aromatic rings; the thermosetting epoxy resin having substantially the following structure: (III) wherein the wavy lines each symbolise the attachment of the nitrogen atom to a further 2-hydroxypropylene moiety; the process comprising the following steps a) to e): a) an alcoholysis of the resin to obtain the corresponding diester of the dicarboxylic acid according to formula (I) in question and the 2,3-dihydroxypropyl group-substituted diamine of formula (II) ("polyol") in question: polyol; b) an acetolysis at elevated temperature to obtain the acetamides of the diamines of formula (II) ("diamide") and of triacetin: triacetin diamide; c) the deacetylation of the diamide to obtain the diamine monomer of formula (II); d) the deacetylation of triacetin to obtain glycerol, which is subsequently converted to glycidol; and e) the transesterification of the diester obtained in step a) with the glycidol obtained in step d) to obtain the dicarboxylic acid diglycidyl ester of formula (I) in question; and furthermore a process for the preparation of such thermosetting epoxy resins and new resins prepared in this way.

Description

[0001] Process for the production of thermosetting epoxy resins

[0002] The present invention relates to a process for producing thermosetting epoxy resins by polymerizing diepoxide and diamine monomers, to novel thermosetting epoxy resins produced in this way, and to a process for splitting and recycling the same.

[0003] The subject matter of the present invention was already partially presented to the public by the inventors themselves in a publication dated April 12, 2024, which thus falls under the statutory grace period: Xianyuan Wu et al. Science 384, eadj9989 (2024); DOI: 10.1126 / science.adj9989.

[0004] STATE OF THE ART

[0005] Epoxy thermosets (EPDs) are an important class of polymer materials characterized by long durability and excellent thermal stability. They are indispensable in many industrial applications, such as packaging, composites manufacturing, transportation, building construction, and aerospace. However, due to the strong covalent bonds within the main chains of the cured EPDs, these materials are generally not recyclable, meaning they are unsustainable end products that generally have to be incinerated or landfilled. Furthermore, a variety of EPDs have long been manufactured using bisphenol A (BPA), derived from fossil resources, which has been identified as an endocrine disruptor.To improve the situation in this area, thermosets, especially EPDs, would be desirable, as they can be recycled at least to a significant extent.

[0006] Against this background, the aim of the invention was to identify and develop EPDs that are as widely and easily recyclable as possible, especially those whose starting materials are obtained from renewable raw materials. SUMMARY OF THE INVENTION

[0007] This object is achieved by the present invention in a first aspect by providing a process for splitting and at least partially recycling the starting components of a thermosetting epoxy resin (EPD) which is a polymerization product of diepoxide and diamine monomers, wherein the diepoxide monomers are dicarboxylic acid diglycidyl esters according to the following formula (I) and the diamine monomers are diamines of the following formula (II): where R1 represents a linear, branched or cyclic, saturated, unsaturated or aromatic C 1-20 alkylene group in which one or more carbon atoms are optionally replaced by a heteroatom selected from O, N and S, preferably a divalent radical of an aromatic ring selected from monocyclic heteroaromatics and benzene,

[0008] H2N-R 2 -NH2

[0009] (H) where R 2 for a linear, branched or cyclic, saturated, unsaturated or aromatic C 1-20 alkylene group, in which one or more carbon atoms are optionally replaced by a heteroatom selected from O, N and S, with the proviso that the two amino groups are not directly bonded to aromatic rings; and wherein the thermosetting epoxy resin essentially has the following structure: wherein the wavy lines each symbolize the bonding of the nitrogen atom to another 2-hydroxypropylene group; wherein the process comprises the following steps a) to e): a) an alcoholysis of the resin to obtain the corresponding diester of the respective dicarboxylic acid according to formula (I) and the respective 2,3-dihydroxypropyl group-substituted diamine of formula (II) ("polyol"): wherein the alcoholysis is preferably carried out essentially without the addition of a catalyst, followed by separation of the diester from the polyol and optionally evaporation of the alcohol and any other solvents used for the separation; b) acetolysis, i.e. cleavage of the polyol with acetic acid, at elevated temperature to obtain the acetamides of the respective diamine of formula (II) ("diamide") and triacetin:

[0010] Diamide triacetin and the separation of the solid diamide from the liquid triacetin; c) the deacetylation of the diamide to obtain the diamine monomer of formula (II) required for a later resynthesis of the epoxy resin; d) the deacetylation of triacetin to obtain glycerol, which is subsequently converted to glycidol; and e) the transesterification of the diester obtained in step a) with the glycidol obtained in step d) to obtain the respective dicarboxylic acid diglycidyl ester of formula (I) and thus the diepoxide monomer required for a later resynthesis of the epoxy resin.

[0011] The inventors have discovered, quite surprisingly, that special thermosetting epoxy resins consisting of combinations of these two monomers have not only proven to be highly recyclable, but can also be cleaved in a simple and cost-effective manner in a relatively short time by means of a sequence of standard chemical reactions, starting with an alcoholysis of the respective EPD, and that the starting monomers of the respective EPD can be at least partially recycled from the cleavage products, which for some of the starting products even achieves yields of around 90%.

[0012] And in preferred embodiments, this is even achieved entirely without the addition of an external catalyst or other additives. Without wishing to be limited to a specific theory, the inventors assume that the nucleophilic attack of the oxygen atoms of the respective alcohol on the carbon atoms of the carboxyl groups of the dicarboxylic acid moieties in the EPD is sufficiently catalyzed by the interaction between the hydroxyl hydrogen atoms of the alcohol and the nitrogen atoms of the diamine moieties, forming hydrogen bonds between them. However, in aromatic diamines, the electron density at the nitrogen atoms is apparently reduced by the +M effect to such an extent that this catalytic mechanism does not function for EPD from aromatic diamines, as the later comparative examples demonstrate.

[0013] In further preferred embodiments of the process according to the invention, the EPD to be cleaved contains

[0014] R 1 represents a divalent radical of an aromatic ring selected from furan and benzene, preferably a furandiyl radical; and / or R 2 - for a Ci-20-alkylene group which comprises at least one cycloalkylene radical from a number of eight carbon atoms and two cycloalkylene radicals from a number of fourteen carbon atoms,

[0015] - for a C2-6 alkylene group,

[0016] - represents an optionally alkylated cyclohexanediyl radical or

[0017] - an optionally alkylated 4,4'-methylene-bis(cyclohexyl) radical, preferably an optionally methylated 4,4'-methylene-bis(cyclohexyl) radical.

[0018] And in particularly preferred embodiments, the thermosetting epoxy resin was obtained from 2,5-furandicarboxylic acid diglycidyl ester (DGF) as the diepoxide monomer and 4,4'-methylene-bis(cyclohexylamine) (MBCA) or 4,4'-methylene-bis(2-methylcyclohexylamine) (MBMCA) as the diamine monomer:

[0019] MBCA MBMCA and essentially has the following structure: or where the wavy lines each symbolize the bonding of the nitrogen atom to another 2-hydroxypropylene group.

[0020] These special selection options of the residues R 1 or R 2not only leads to the obtainment of EPD according to the further aspects of the invention, which deliver very good results in terms of mechanical properties, but also significantly increases the rate of cleavage of the respective EPD by alcoholysis and the yields during recycling. As the later examples demonstrate, this also significantly simplifies recycling, since the diesters formed during the alcoholysis of these aromatic dicarboxylic acids with the respective alcohol are obtained as solid precipitates that can be easily filtered off from the alcoholic solution of the polyol. In such cases, the two EPDs obtained from DGF and MBCA or MBMCA have proven to be by far the most recyclable using the process according to the invention, since over 90% of the dicarboxylic acid could be recovered as diesters after just a few hours of alcoholysis.

[0021] In the case of dicarboxylic acid diesters that are liquid at room temperature, their separation from the polyols after alcoholysis can be carried out, for example, by distillation or by extraction, optionally after concentrating a combined alcoholic solution of the diester and the polyol, for example by partitioning the two components between an aqueous, e.g., alkaline, phase and an organic phase, for example in a sodium (hydrogen) carbonate solution of the polyol and a solution of the diester in ethyl acetate, dichloromethane, or the like. A person of ordinary skill in the art will readily be able to select a suitable separation process for the particular combination of alcoholysis products. In some preferred embodiments, the thermosetting epoxy resin to be cleaved according to the invention is furthermore part of a glass fiber-reinforced composite material, wherein in step a) after alcoholysis, the glass fibers are separated from the diester and the polyol.In the case of such glass fiber-reinforced composites, the process according to the invention even enables almost 100% recycling of the glass fibers contained. As an alternative to glass fibers, the epoxy resin can also be part of a composite reinforced with natural fibers, although this is not preferred according to the invention, since natural fibers can undergo changes during the cleavage and further processing of the cleavage products into the starting monomers to be recycled and therefore may not be 1:1 recyclable.

[0022] In preferred embodiments where R 1represents a divalent radical of an aromatic ring selected from monocyclic heteroaromatics and benzene and the diester of the corresponding dicarboxylic acid is obtained in step a) as a solid precipitate and the separation of the diester from the polyol is carried out by filtering the precipitate from the resulting alcoholic solution of the polyol, the glass fibers can be separated from the latter simply by rinsing or by dissolving the solid diester in a solvent.

[0023] In preferred embodiments, the alcoholysis in step a) is carried out with a lower alcohol, more preferably with methanol or ethanol, to accelerate the reaction, preferably at a temperature in the range of the boiling point of the alcohol or above (i.e. under pressure). Methanol has proven to be the most effective alcohol, with which at least the two EPDs obtained from DGF and MBCA or MBMCA were completely cleaved even at room temperature after a few days, and at 70°C, methanol brought about 100% cleavage after just 18 hours. In any case, a balance must be struck between the energy expenditure and the duration of the alcoholysis. For methanol, a temperature between 60°C and 70°C is preferable according to the invention, whereas when using ethanol, a temperature of at least 90°C or at least 120°C should be selected in order to also enable complete cleavage in the same time.Also to accelerate the reaction, the acetolysis in step b) is carried out in preferred embodiments of the process according to the invention at a temperature above 150°C, more preferably above 200°C, and especially above 250°C, without intending to limit the invention thereto. And the deacetylations in steps c) and d) are preferably carried out with aqueous HCl, preferably under reflux, for economic reasons, without being limited thereto. Any person of ordinary skill in the art will be able to adapt this section of the process according to the invention to the particular combination of components used.

[0024] In a second aspect, as previously indicated, the invention also relates to a process for the preparation of thermosetting epoxy resins as defined above by polymerizing a combination of the corresponding diepoxide and diamine monomers, either in solution in a suitable solvent or solvent mixture or in bulk at a temperature above the melting point of the monomers, wherein the polymerization is optionally carried out in the presence of reinforcing fibers to produce a fiber-reinforced composite material.

[0025] The EPDs thus obtained are naturally characterized by the advantages already mentioned above, in particular by their cleavability and at least partial recyclability when using the process according to the invention according to the first aspect, which vary according to the monomer combination selected in each case.

[0026] If the selection is made according to the broadest definitions of R above 1 and R2 , i.e. a combination in which R 1 and R 2each represents a linear, branched, or cyclic, saturated, unsaturated, or aromatic C1-20 alkylene group, in which one or more carbon atoms are optionally replaced by a heteroatom selected from O, N, and S, but the two amino groups of the diamine are not directly bonded to aromatic rings. This offers the possibility of synthesizing cleavable and at least partially recyclable EPDs whose properties vary within relatively broad limits. Not only do they undergo alcoholysis at varying rates—depending on the chain lengths, molecular structures, and the respective spatial arrangement of the groups—but the extent of their recyclability also varies within certain limits. Above all, their thermomechanical properties can be specifically varied within relatively broad limits.For example, the glass transition temperature when using an aromatic dicarboxylic acid for the diepoxide monomer is approximately 2.5 times higher than that with adipic acid. However, in the latter case, both the decomposition temperature (slightly) and the elasticity (significantly) are higher than in EPD with aromatic dicarboxylic acids.

[0027] The preparation process according to the invention is not specifically limited with regard to the other reaction conditions, as these must also be adapted to the respective combination. In preferred embodiments, however, the diepoxide and diamine monomers are polymerized in a molar ratio of 2:1 to ensure the most complete conversion of the functional groups during polymerization. However, to control the properties of the respective EPD, a ratio of 1:1, 1:5, or even 1:10 can also be chosen.

[0028] To accelerate the polymerization, a reaction temperature of at least 50 °C or at least 80 °C is preferably selected, wherein in particularly preferred embodiments the monomers are first heated to 80 °C for 2 hours and then to 150 °C for 8 hours in order to increase the reaction rate and the conversion achieved.

[0029] In particularly preferred embodiments, the EPDs thus obtained can be subjected to a thermal post-treatment to ensure complete conversion. For this purpose, the EPD can be thermally post-treated, for example, at least at 170 °C for at least 1 hour.

[0030] Finally, in a third aspect, the present invention also relates to a thermosetting epoxy resin obtainable according to the second aspect of the invention and cleavable and at least partially recyclable according to the first aspect. This is composed of the same monomer components as defined above, ie, dicarboxylic acid diglycidyl esters of the formula (I) and diamines of the formula (II), wherein R 1 and R 2 each represents a linear, branched or cyclic, saturated, unsaturated or aromatic Ci-2o-alkylene group in which one or more carbon atoms are optionally replaced by a heteroatom selected from O, N and S, but the two amino groups of the diamine are not directly bonded to aromatic rings and where R 1 preferably represents a divalent radical of an aromatic ring selected from monocyclic heteroaromatics and benzene.

[0031] However, even more preferably, the EPD according to the invention again contains

[0032] R 1 for a divalent radical of an aromatic ring selected from furan and benzene, preferably a furandiyl radical; and / or

[0033] R 2 for a C1-20 alkylene group which, from a number of eight carbon atoms, comprises at least one cycloalkylene radical and, from a number of fourteen carbon atoms, two cycloalkylene radicals, for a C2-6 alkylene group, for an optionally alkylated cyclohexanediyl radical or for an optionally alkylated 4,4'-methylene-bis(cyclohexyl) radical, preferably for an optionally methylated 4,4'-methylene-bis(cyclohexyl) radical.

[0034] And in particularly preferred embodiments, the thermoset epoxy resin is in turn obtained from 2,5-furandicarboxylic acid diglycidyl ester (DGF) as the diepoxide monomer and 4,4'-methylene-bis(cyclohexylamine) (MBCA) or 4,4'-methylene-bis(2-methylcyclohexylamine) (MBMCA) as the diamine monomer and essentially has a structure as previously shown in which essentially all nitrogen atoms are connected to two 2-hydroxypropylene moieties each, which indicates essentially complete conversion during the polymerization process.

[0035] As previously mentioned, these preferred embodiments offer significant advantages, especially in the separation and recycling of EPDs according to the process of the invention. BRIEF DESCRIPTION OF THE FIGURES

[0036] The present invention will now be described in more detail by way of example and with reference to the accompanying figures, in which:

[0037] Fig. 1 shows the FTIR spectra and Fig. 2 the 13 C-NMR spectra of the EPD from several examples, based on which the determination of the complete conversion during polymerization and the elucidation of the structure were carried out in an analogous manner for all examples and comparative examples.

[0038] Fig. 3 shows the DSC thermograms, Fig. 4 the TGA thermograms and Fig. 5 the DMA thermograms of the EPD from several examples, on the basis of which the determination of the glass transition and decomposition temperatures and the storage modulus was carried out in an analogous manner for all examples and comparative examples.

[0039] Fig. 6 shows photographs of a solubility test with the EPD from Example 1 in different solvents.

[0040] Figures 7A, 7B and 8 show the results of preliminary tests on the alcohol content of the EPD from Example 1.

[0041] Fig. 9 is a representation of the yields in the recycling of dicarboxylic acids from the seven EPD resins according to the invention by alcoholysis with methanol.

[0042] EXAMPLES

[0043] Materials and reagents

[0044] 4,4'-Methylenebis(cyclohexylamine) (mixture of isomers) (MBCA) (>97.0%, TCI), 4,4'-Methylenebis(2-methylcyclohexylamine) (mixture of isomers) (MBMCA) (>99.0%, TCI), 1-Amino-3-aminomethyl-3,5,5-trimethylcyclohexane (mixture of isomers) (IPDA) (>99.0%, TCI), 1,6-Diaminohexane (Hexamethylenediamine, HDA) (>99.0%, TCI), Adipic acid (AA) (>99.0%, TCI), 2,5-Furandicarboxylic acid (FDCA) (>98.0%, TCI), Thionyl chloride (>99.0%, Sigma Aldrich), Triethylamine (>99.5%, Sigma Aldrich), Glycidol (96%, Sigma Aldrich), Acetic anhydride (>99.0%, Sigma Aldrich), terephthalic acid (TPA) (>99.0). %, TCI), 4,4'-diaminodiphenylmethane (DDM) (>98.0%, TCI), N,N'-dimethylcyclohexylamine (A3) (>98.0%, TCI), conc.H2SO4 (95.0-98.0%, Sigma Aldrich), anhydrous dichloromethane (DCM) (>99.8%, Sigma Aldrich), anhydrous N,N'-dimethylformamide (DMF) (>99.8%, Sigma Aldrich), anhydrous sodium sulfate (Na2SC) (>99.0%, Sigma Aldrich), sodium bicarbonate (NaHCOa) (99.7%, Sigma Aldrich), tetrahydrofuran (THF) (>99.0%, Sigma Aldrich), and silica gel for column chromatography (Sigma Aldrich) were purchased from Sigma Aldrich or TCI. All chemicals were used as received.

[0045] General

[0046] Column chromatography was performed using Merck silica gel type 9385, 230-400 mesh, and typically with DCM and methanol or EtOAc and pentane as eluent.

[0047] Thin-layer chromatography (TLC): Merck silica gel 60, 0.25 mm. Components were visualized by UV or KMnCh staining.

[0048] Gas chromatography (GC) was used for product identification and to determine conversion and selectivity values. Product identification was performed using GC-MS (5975C MSD) equipped with an HP-5 MS column and helium as the carrier gas. The temperature program started at 50 °C for 5 min, followed by heating at 10 °C / min to 325 °C and holding for 5 min. Conversion and product selectivity were determined using GC-FID (Agilent 8890 GC) equipped with an HP-5 MS column and using nitrogen as the carrier gas.

[0049] Nuclear magnetic resonance spectroscopy (NMR): The 1 Dog 13 C-NMR spectra were recorded on a Bruker Avance III 300 MHz (300 and 75 MHz, respectively), and 2D-NMR spectra were recorded on a Bruker Avance III 700 MHz with a cryoplatform and a 5 mm triple-resonance cryoprobe (700 and 175 MHz, respectively). All 1 H-, 13C-NMR and 2D-NMR spectra were recorded at room temperature. Chemical shift values ​​are given in ppm, with the solvent resonance serving as an internal standard (CDCh: 7.26 for 1 H, 77.0 for 13 C; CD3OD: 3.31 for 1 H, 49.0 for 13 C; DMSO-de: 2.50 for 1 H, 39.5 for 13 C). The data are given as follows: chemical shifts, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, br = broad, m = multiplet), coupling constants (Hz), and integrals. Fourier transform infrared spectroscopy (FTIR): The Fourier transform infrared (FTIR) spectra were recorded with a Nicolet 210 using attenuated total reflection (ATR) in transmission mode. The characteristic IR absorptions mentioned in the text are in cm -1 specified.

[0050] Thermogravimetric analysis (TGA): Thermogravimetric analyses (TGA) were performed using a TG 209F1 instrument (Netzsch). Typically, the sample (~10 mg) was weighed into an aluminum crucible and heated from 40 °C to 550 °C at a heating rate of 20 °C / min and in a N2 flow of 60 ml / min. The temperature at which a 5% weight loss (T5%) and a 90% weight loss (T90%) occurred was recorded.

[0051] Differential scanning calorimetry (DSC): Differential scanning calorimetry (DSC) analyses were performed using a Netzsch DSC200F3 calorimeter calibrated with indium, n-octadecane, and n-octane standards. In a typical procedure, the sample (~10 mg) was weighed into an aluminum DSC crucible and capped with a lid. The sample was tightly sealed and heated from 25 °C to 200 °C at a heating rate of 20 °C / min. It was then cooled to 25 °C at a heating rate of 20 °C / min. A second heating scan was then performed at the same heating rate to 250 °C. All experiments were conducted under a N2 stream at a flow rate of 20 mL / min.

[0052] Dynamic Mechanical Analysis (DMA): DMA measurements were performed using Metravib DMA 25 with Dynatest 6.8 software. Samples were tested in uniaxial tension mode at a frequency of 1 Hz with a preset strain of 10 -5m while applying a temperature increase at a rate of 3 °C / min from 25 °C to 250 °C.

[0053] Gel content (GC): Three samples of the same material, each weighing approximately 20 mg, were tested separately and immersed in THF for 24 h. These samples were then dried in a ventilated oven at 70 °C for 24 h. The gel content (GC) was calculated using the following equation, where m2 is the mass of the dried material and rm is the initial mass. The gel contents given are the respective averages of the three samples:

[0054] GC (%) = m2 / mi x 100 of 2,5-furandicarboxylic acid acid (DGB) and adipic acid (DGA)

[0055] General procedure

[0056] Step 1: In a typical procedure, 5 g of the respective dicarboxylic acid, 1.2 eq. of SOCl2, and 5 drops of DMF were charged into a 100 mL Schlenk flask equipped with a magnetic stir bar. The Schlenk flask was tightly sealed and purged three times with Ar. The mixture was then heated to 80 °C overnight with stirring. After completion of the reaction, the Schlenk flask was cooled to room temperature. The mixture was dissolved in DCM and washed successively with water and saturated aqueous NaHCO3 solution and dried with anhydrous Na2SO4 to give the respective diacid chloride.

[0057] Step 2: The diacid chloride in 20 ml of dry DCM was added dropwise via a dropping funnel over 30 min to a stirred mixture of 1.2 eq. glycidol, 1.2 eq. triethylamine, and 50 ml of dry DCM, cooled in an ice bath. The mixture was then warmed to room temperature with stirring over 4 h. After completion of the reaction, the mixture was washed twice with water and filtered through a thin layer of silica gel with DCM to remove discoloration. The organic phase was collected and dried over anhydrous Na2SO4. The resulting product was dissolved in DCM (5 ml) for DGF and DGA and precipitated in diethyl ether, yielding a white powder in each case. In this way, the diglycidyl ester of 2,5-furandicarboxylic acid (DGF) (yield: 60.5%) and the diglycidyl ester of terephthalic acid (DGB) (yield: 54%) were obtained. The diglycidyl ester of adipic acid (DGA) was obtained by vacuum distillation (yield: 65.4%). DGF: <h2 style=";text-align:left;direction:ltr">

[0058] <h2 style=";text-align:left;direction:ltr"> 1 <h2 style=";text-align:left;direction:ltr"> H-NMR (300 MHz, CDCb) Ö: 7.26 (s, 2H), 4.67-4.63 (m, 2H), 4.21-4.15 (m, 2H), 3.36-3.30 (m, 2H), 2.94-2.92 (m, 1H), 2.76-2.75 (m, 1H).<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0059] <h2 style=";text-align:left;direction:ltr"> 13 <h2 style=";text-align:left;direction:ltr"> C-NMR (75 MHz, CDCb) Ö: 157.56, 146.48, 119.02, 66.02, 49.08, 44.79.<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0060] <h2 style=";text-align:left;direction:ltr"> Fp.: 89 °C<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0061] <h2 style=";text-align:left;direction:ltr"> DGB:<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0062] <h2 style=";text-align:left;direction:ltr"> 1 <h2 style=";text-align:left;direction:ltr"> H-NMR (300 MHz, CDCb) Ö: 8.15 (s, 2H), 4.72-4.69 (m, 1 H), 4.22-4.18 (m, 1 H), 3.39-3.36 (m, 1 H), 2.91 -2.82 (m, 2H), 2.73-2.71 (m, 2H).<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0063] <h2 style=";text-align:left;direction:ltr"> 13 <h2 style=";text-align:left;direction:ltr"> C-NMR (75 MHz, CDCb) Ö: 165.41, 133.68, 129.77, 65.96, 49.34, 44.72.<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0064] <h2 style=";text-align:left;direction:ltr"> Fp.: 106.5-107 °C<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0065] <h2 style=";text-align:left;direction:ltr"> DGA:<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0066] <h2 style=";text-align:left;direction:ltr"> 1 <h2 style=";text-align:left;direction:ltr"> H-NMR (300 MHz, CDCb) Ö: 4.42-4.38 (m, 2H), 3.92-3.87 (m, 2H), 3.20-3.18 (t, 2H), 2.85-2.82 (m, 2H), 2.65-2.63 (m, 2H), 2.38-2.37 (d, 4H), 1.68-1.64 (d, 4H).<h2 style=";text-align:left;direction:ltr"> <h2 style=";text-align:left;direction:ltr">

[0067] <h2 style=";text-align:left;direction:ltr"> 13 <h2 style=";text-align:left;direction:ltr"> C-NMR (75 MHz, CDCb) Ö: 172.59, 64.60, 49.00, 44.28, 33.25, 29.90.

[0068] Bp.: 150-155 °C (0.06 mmHg)

[0069] Examples 1 to 7 - Comparison examples 1 to 2

[0070] Production and thermomechanical analysis of thermosetting epoxy resins

[0071] General procedure

[0072] The respective combination of diamine (1 eq.) and diglycidyl ester (2 eq.) was thoroughly mixed in a SpeedMixer for 3 minutes (a solvent may be added to ensure optimal mixing, but this must be removed under vacuum after mixing). The resulting mixture was then poured into an aluminum mold and cured for 2 hours at 80 °C, then for 8 hours at 150 °C, and then post-cured for 1 hour at 170 °C.

[0073] In this way, EPDs were obtained from the following combinations: Example 1 :

[0074] 2,5-Furandicarboxylic acid diglycidyl ester / 4,4'-methylene-bis(cyclohexylamine)

[0075] DGF MBCA

[0076] Example 2:

[0077] 2,5-Furandicarboxylic acid diglycidyl ester / 4,4'-methylene-bis(2-methylcyclohexylamine)

[0078] DGF MBMCA

[0079] Example 3:

[0080] 2,5-Furandicarboxylic acid diglycidyl ester / Isophoronediamine Example 4:

[0081] 2,5-Furandicarboxylic acid diglycidyl ester / hexamethylenediamine

[0082] DGF HDA Example 5:

[0083] Diglycidyl adipic acid / 4,4'-methylenebis(cyclohexylamine)

[0084] Diglycidyl adipic acid / 4,4'-methylene-bis(2-methylcyclohexylamine)

[0085] Diglycidyl terephthalate / 4,4'-methylenebis(cyclohexylamine)

[0086] 2,5-Furandicarboxylic acid diglycidyl ester / 4,4'-diaminodiphenylmethane

[0087] DGF DDM Comparison Example 2:

[0088] Bisphenol A-dig lycidyl ether / 4,4'-diaminodiphenylmethane

[0089] DGBA MBCA

[0090] The characterization of this EPD was carried out using Fourier transform infrared spectroscopy (FTIR) and 13 C NMR spectroscopy.

[0091] Fig. 1 shows the FTIR spectra of the EPD from Example 1, Example 7 and Comparative Example 1. It can be seen that the characteristic stretching vibration bands of the oxirane ring are at 865, 906 and 1235 cm -1 have completely disappeared, reflecting the reaction of all epoxide groups with amines. The presence of the stretching vibration bands at 1720 cm -1 confirms the presence of ester functionalities in the DGF / MBCA, DGF / DDM and DGB / MBCA epoxy resin matrix.

[0092] In Fig. 2 13C-NMR spectra of the EPD from Example 1, Example 7 and Comparative Example 1 are shown, with the individual peaks marked with the numbering of the carbon atoms in the corresponding formula. A comparison with a previously recorded spectra for 2,5-furandicarboxylic acid dimethyl ester 13 The C spectrum shows that the chemical shifts at 158, 147, and 119 ppm are assigned to carbon atoms 4, 5, and 6 of DGF / MBCA, respectively. The signals between 80 and 45 ppm originate from carbon atoms 1, 2, 3, and 11, indicating successful ring opening of the epoxide group by the MBCA reaction. The signals between 35 and 15 ppm refer to the remaining carbon atoms 7-10 present in the cyclohexane rings.

[0093] In the same way, the peaks of all nine EPDs prepared could be assigned to the respective groups and atoms. Furthermore, the glass transition temperatures (Tg) were determined using differential scanning calorimetry (DSC) analysis, the decomposition temperatures, more specifically the temperatures at which a 5% mass loss occurred (Tzs%), using thermogravimetric analysis (TGA), the storage moduli (E') at 0-50 °C as a measure of elasticity, and the gel contents (GG) of all EPDs.

[0094] Fig. 3 shows the DSC thermograms and Fig. 4 the TGA thermograms of the EPD of Examples 1 and 7 as well as those of the two Comparative Examples 1 and 2. The glass transition temperature (T g) the temperature at the respective inflection point of the base curve was used, which is marked with a dashed line in Fig. 3. The decomposition temperatures were determined using the TGA thermograms shown in Fig. 4. Finally, Fig. 5 shows the DMA thermograms of the EPDs from Example 1 and Comparative Examples 1 and 2, from which the storage modulus (E') and the associated tan δ were determined. All of these values ​​were determined in an analogous manner for all of the nine EPDs produced. The results are given in Table 1 below for all examples and comparative examples.

[0095] Table 1 - Properties of the produced EPD nb: cannot be determined exactly due to blistering

[0096] The first thing that is striking about these values ​​is that the use of adipic acid, a linear and relatively short-chain dicarboxylic acid, in Examples 5 and 6 results in a significant reduction in the glass transition temperature while simultaneously increasing elasticity compared to the other EPDs. The same applies to the EPD with hexamethylenediamine in Example 4. On the other hand, the EPD from Comparative Example 7, which was made from bisphenol A diglycidyl ether (DGBA) and MBCA and thus has two ether bonds instead of two ester bonds in the basic unit, had by far the highest 5% decomposition temperature measured. While the other six EPDs have an average decomposition temperature of just under 290 °C ± 10%, that of the EPD from Comparative Example 7 is around 25% higher.

[0097] From these fluctuations in the thermomechanical properties, it is quite clear to the average person skilled in the art that by a targeted selection of appropriate starting monomers, i.e. special combinations of diglycidyl esters and diamines within the quite broad ranges for R 1 and R 2 according to the manufacturing process of the present invention, a series of EPDs with the desired characteristics can be obtained.

[0098] Splitting of the thermosetting epoxy resins and recycling of the components

[0099] Solubility tests

[0100] First, dissolution tests were carried out on the very first EPD produced, the one from Example 1, which was made from DGF / MCBA, in various organic and aqueous solvents, initially at room temperature.

[0101] Attempt 1

[0102] For this purpose, 100 mg of EPD was placed in 4 ml of solvent, and its behavior was observed at room temperature for 1 week. The following solvents were used: 1 M aqueous HCl, 1 M aqueous NaOH, methanol (MeOH), ethanol (EtOH), propanol (PrOH), tetrahydrofuran (THF), dimethylformamide (DMF), dichloromethane (DCM), acetone, and ethyl acetate (EtOAc). The gel content of each of the placed samples was determined as previously described. The results are shown in Table 2 below. Table 2 - Gel content of the EPP from Example 1 (DGF / MCBA)

[0103] As previously observed for all nine EPDs prepared according to the invention, the gel contents are essentially within 2-3%, demonstrating their resistance to cleavage. The exception is methanol, in which the EPD from Example 1 was completely dissolved.

[0104] Fig. 6 shows photographs of the respective containers, in which it is clearly evident that the precipitate had completely dissolved in methanol after 1 w. Thus, complete alcoholysis of the EPD must have occurred to the methanol-soluble 2,5-furandicarboxylic acid dimethyl ester (DMFD) and the 4,4'-methylenebis(cyclohexylamine) substituted with four 2,3-dihydroxypropyl groups, shown below.

[0105] - even at room temperature: Subsequently, further experiments were carried out with methanol and other lower alcohols and also at different elevated temperatures.

[0106] Attempt 2

[0107] First, 100 mg of the EPD from Example 1 were placed in methanol in a tightly sealed 10 ml pressure tube at temperatures of 30 °C, 40 °C, 50 °C, 60 °C and 70 °C for 18 h each, and then the molar fraction of dissolved DMFD, which corresponds 1:1 to the degree of decomposition induced by alcoholysis, was determined by means of 1 H-NMR (using p-xylene as an internal standard). As can be seen from the graph in Fig. 7A, this fraction and thus the degree of decomposition increased with increasing temperature from less than 10% at 30 °C to almost 90% at 60 °C and finally to 100% at 70 °C, with the amount of DMFD actually isolated being approximately 5-10% lower in each case.

[0108] Thus, the EPD from Example 1 proved to be 100% alcoholizable in methanol at 70 °C - and this within only 18 hours.

[0109] Attempt 3

[0110] Subsequently, the same EPD was tested again in various other alcohols, namely ethanol, propanol, and butanol, but this time at at least 60 °C. As can be seen from the graph in Fig. 7B, no alcoholysis was observed with either propanol or butanol within 18 h, while methanol again yielded a value of 90%. Although no reaction was observed with ethanol at 60 °C, approximately 65% ​​and over 95% alcoholysis to form diethyl 2,5-furandicarboxylate (DEFD) were observed at 90 °C and 120 °C, respectively.

[0111] It is assumed that the values ​​could also be increased for ethanol to butanol with longer residence times in the alcohol and / or (even) higher temperatures. Thus, other alcohols—at least ethanol—are also suitable for use in the inventive splitting and recycling process. Experiment 4

[0112] In light of these results, other EPDs were subsequently tested for their suitability for alcoholysis. First, 100 mg of the corresponding EPD were placed in 4 ml of methanol and heated to 70 °C under pressure for 20 hours. In this case, the EPD from Example 1 (DGF / MBCA), that from Example 7 (DGB / MBCA), as well as those from Comparative Example 1 (DGF / DDM) and Comparative Example 2 (DGBA / MBCA) were tested again.

[0113] Since dimethyl terephthalate (DMTA) is poorly soluble in methanol even at 70 °C, but readily soluble in DCM even at room temperature, the methanol was distilled off after 20 hours in all four batches, and the residue was treated with DCM to dissolve the diesters. This solution was filtered through a thin layer of silica gel, thus removing any solid components.

[0114] The graph in Fig. 8 clearly shows that none of the EPDs from the two comparative examples showed any conversion with methanol during the test period, whereas those from Examples 1 and 7 were indeed alcoholyzed to form the diesters DMFD and DMTA. Due to the slightly modified test protocol, the result for the DGF / MBCA product from Example 1 in this case at 70 °C was not 100%, but only around 90% DMFD, whereas the DMTA obtained from the EPD from Example 1 was isolated at around 35%. Thus, the latter EPD obtained from the DGB / MBCA combination also proved to be alcoholyzable with methanol, whereas neither the EPD formed from DGBA and MBCA with ether instead of ester bonds nor the EPD formed from the aromatic diamine with DGF showed any reaction with methanol.

[0115] Further tests with the two EPDs of the comparison examples in combination with other alcohols could therefore be safely omitted.

[0116] Attempt 5

[0117] As final alcoholysis preliminary tests, all seven EPDs according to the invention were therefore tested for their suitability for alcoholysis with methanol in parallel batches, which were designed analogously to those in Experiment 4: 100 mg of EPDs from Examples 1 to 7 were heated to 70 °C in 4 ml of methanol for 18 hours in a tightly sealed 10 ml pressure tube, after which the methanol was distilled off and the solid residue was dissolved in DCM and filtered, and then the molar proportion of dissolved diester was determined using 1 H-NMR (with p-xylene as internal standard).

[0118] The graph in Fig. 9 clearly shows that all seven EPD resins had undergone alcohol degradation with methanol, with the following yields (in ascending order and rounded): Example 4, DGF / HDA, 20% Example 3, DGF / IPDA, 35% Example 7, DGB / MBCA, 40% Example 5, DGA / MDCA, 45% Example 6, DGA / MBMCA, 65% Example 2, DGF / MBMCA, 90% Example 1, DGF / MBCA, 90%

[0119] Although the combination of DGF and MBCA from Example 1, as well as the combination of DGF and MBMCA from Example 2, which differs only by two methyl groups in the base unit, proved to be the most alcoholyzable, even the combination with the worst results, i.e., the EPD from Example 4 with two short linear alkylene groups in the base unit, could be alcoholized to approximately 20% within 18 hours. This value can also presumably be significantly increased by increasing the temperature and / or the reaction time.

[0120] Step a) Alcoholysis

[0121] To each of 5 g of the corresponding EPD from Examples 1 to 7, 40 ml of methanol were added to a 100 ml single-neck round-bottom flask, after which the flask was sealed and heated to 70 °C. After 48 h, the solution for Examples 1 to 6 was subjected to fractional distillation to obtain the dimethyl esters of 2,5-furandicarboxylic acid (Examples 1 to 4) and adipic acid (Examples 5 and 6), as well as the corresponding polyols of MBCA (Examples 1 and 5), MBMCA (Examples 2 and 6), and IPDA (Example 3).

[0122] In Example 7, the solid residue of dimethyl terephthalate (DMTA) and not the alcoholyzed residue of the DGB / MBCA-EPD was filtered off, after which the diester was dissolved in DCM. After filtration, the DCM was removed under vacuum to obtain pure DMTA. The methanol was distilled off under vacuum from the MBCA-polyol solution.

[0123] The yields of the diesters for the seven examples were in the range of 20-90% of theory, while the yields of the polyols were in the range of 45-85% of theory.

[0124] Step b) Acetolysis

[0125] 50 mg of each of the polyols obtained in step a) were treated with 3 ml of acetic acid and heated under pressure to 270 °C for 24 h, as this had resulted in essentially complete conversion in preliminary experiments. The diamide obtained as a solid precipitate was filtered off, washed, and dried under vacuum. The combined liquid phases were combined and extracted with diethyl ether. The organic phase was dried over anhydrous Na2SO4, and the solvent was then distilled off to obtain liquid triacetin.

[0126] Diamide Triacetin

[0127] The yields of the diamides were in the range of 15-22% of theory.

[0128] Step c) Deacetylation of the diamides

[0129] The deacetylation of the diamides was carried out according to Gribble et al. ("Bis-intercalating agents, synthesis and antitumor activity of N,N-(methylenedi-4,1-cyclohexanediyl-bis(9-acridinamine) isomers", Heterocycles 88, 535-546 (2014)). The resulting diamides were treated with 1 M HCl, and the mixture was heated to reflux for 18 h. After this time, the acidic solution was basified with NaHCO3 solution and the solid residue of the respective diamines was filtered off, thoroughly washed, and dried in vacuo. The yields of the diamines of formula (II) recovered as starting products for the preparation of the EPD were consistently above 80% of theory.

[0130] Step d) Deacetylation of triacetin

[0131] The deacetylation of triacetin to obtain glycerol was also carried out according to the literature (Wann et al., J. Org. Chem. 46, 2579-2581 (1981)). In a 100 ml two-necked round-bottom flask equipped with a magnetic stir bar, 3.5 g (16 mmol) of triacetin, 1.61 g (40 mmol) of sodium borohydride (NaBH4), and 20 ml of dimethyl sulfoxide (DMSO) were placed. A solution of 3.4 ml (55 mmol) of methanesulfonic acid in 20 ml of DMSO was added with vigorous stirring. Upon completion, the reaction was quenched by adding 20 ml of aqueous NaOH (10%). The product was extracted from the quenched solution with diethyl ether (5 x 10 ml), and the organic phases were combined and dried over anhydrous NaSO4. Pure glycerol was obtained by distillation in a yield of 74% of theory. This was subsequently purified according to the literature (Cespi et al., "A simplified early-stage assessment of process intensification: Glycidol as a value-added product from epichlorohydrin industry wastes," Green Chem.18, 4559-4570 (2016)) first converted to 3-chloro-1,2-propanediol and then to glycidol.

[0132] For this purpose, glycerol (150 g) and acetic acid (7.8 g) were placed in a 300 cm 3 A Hastelloy steel reactor was filled with a magnetic stirrer bar and heated to 100 °C. HCl(g) was introduced into the reactor under constant pressure (8 bar) for 4 h, after which excess HCl was purged with N2 through NaOH traps, and the system was cooled to room temperature. The reaction mixture was then neutralized with NaHCO3, after which 20 ml of 1.0 M Na2CO3 solution was added, the mixture was heated to 100 °C for 30 min, and the solids were removed by centrifugation.

[0133] From the solution, 3-chloro-1,2-propanediol (α-MCH) and 2-chloro-1,3-propanediol (β-MCH) were separated from dichlorohydrins by distillation. A mixture of α-MCH and β-MCH was dissolved in 200 ml of EtOH and added to a KOH solution (1 mol in 400 ml of EtOH). The reaction mixture was stirred at room temperature for 30 min, then filtered, and the EtOH was removed using a rotary evaporator. β-MCH was separated by vacuum distillation and then converted to glycidol using KOH under the same conditions, which was obtained in this way in an overall yield of >80% of theory.

[0134] Step e) Transesterification of the dimethyl esters to the dicarboxylic acid diglycidyl esters

[0135] The recycled glycidol and the dimethyl esters obtained in step a) were subjected to transesterification according to the literature (Marotta et al., "Green approaches in the synthesis of furan-based diepoxy monomers", RSC Adv. 8, 16330-16335 (2018)). The respective dimethyl esters were mixed with 6 mol% KOCN under reduced pressure (0.3 bar) and gently heated to 60 °C. Upon heating, the mixture yielded a clear and transparent solution. After 6 h, it was cooled and extracted with a mixture of ethyl acetate and water. The diglycidyl ester products were each isolated and purified by column chromatography. In this way, the diglycidyl esters of the respective dicarboxylic acid according to formula (I), used as starting materials for the preparation of the EPD, were obtained in yields of 63-82% of theory.

[0136] The present invention thus provides, in addition to a process for producing thermosetting epoxy resins and the resulting EPDs, primarily a process for splitting and recycling the starting components, by means of which the monomers of formula (I) and (II) can be at least partially recovered by relatively simple chemical reactions—and, with respect to the diglycidyl esters of formula (I), even in a yield of approximately 90%. This was previously unknown in the prior art.

Claims

PATENT CLAIMS 1 . A process for splitting and at least partially recycling the starting components of a thermosetting epoxy resin which is a polymerization product of diepoxide and diamine monomers, wherein the diepoxide monomers are dicarboxylic acid diglycidyl esters according to the following formula (I) and the diamine monomers are diamines of the following formula (II): where R 1 represents a linear, branched or cyclic, saturated, unsaturated or aromatic C 1-20 alkylene group in which one or more carbon atoms are optionally replaced by a heteroatom selected from O, N and S, preferably a divalent radical of an aromatic ring selected from monocyclic heteroaromatics and benzene, H2N-R 2 -NH2 (II) where R 2for a linear, branched or cyclic, saturated, unsaturated or aromatic C 1-20 alkylene group, in which one or more carbon atoms are optionally replaced by a heteroatom selected from O, N and S, with the proviso that the two amino groups are not directly bonded to aromatic rings; and wherein the thermosetting epoxy resin essentially has the following structure: wherein the wavy lines each symbolize the bonding of the nitrogen atom to another 2-hydroxypropylene group; wherein the process comprises the following steps a) to e): a) an alcoholysis of the resin to obtain the corresponding diester of the respective dicarboxylic acid according to formula (I) and the respective 2,3-dihydroxypropyl group-substituted diamine of formula (II) ("polyol"): Polyol, wherein the alcoholysis is preferably carried out essentially without the addition of a catalyst, followed by separation of the diester from the polyol and optionally evaporation of the alcohol and any other solvents used for the separation; b) acetolysis, i.e. cleavage of the polyol with acetic acid, at elevated temperature to obtain the acetamides of the respective diamine of formula (II) ("diamide") and triacetin: and the separation of the solid diamide from the liquid triacetin; c) the deacetylation of the diamide to obtain the diamine monomer of formula (II) required for a subsequent resynthesis of the epoxy resin; d) the deacetylation of triacetin to obtain glycerol, which is subsequently converted to glycidol; and e) the transesterification of the diester obtained in step a) with the glycidol obtained in step d) to obtain the respective dicarboxylic acid diglycidyl ester of formula (I) and thus the diepoxide monomer required for a subsequent resynthesis of the epoxy resin.

2. Process according to claim 1, characterized in that in the thermosetting epoxy resin R 1 represents a divalent radical of an aromatic ring selected from furan and benzene; and / or R 2 - for a Ci-20-alkylene group which comprises at least one cycloalkylene radical from a number of eight carbon atoms and two cycloalkylene radicals from a number of fourteen carbon atoms, - for a C2-6 alkylene group, - represents an optionally alkylated cyclohexanediyl radical or - represents an optionally alkylated 4,4'-methylenebis(cyclohexyl) radical.

3. Method according to claim 2, characterized in that R 1 represents a furandiyl residue; and / or R 2 represents an optionally methylated 4,4'-methylene-bis(cyclohexyl) radical.

4. Process according to claim 3, characterized in that the thermosetting epoxy resin was obtained from 2,5-furandicarboxylic acid diglycidyl ester (DGF) as diepoxide monomer and 4,4'-methylene-bis(cyclohexylamine) (MBCA) or 4,4'-methylene-bis(2-methylcyclohexylamine) (MBMCA) as diamine monomer: DGF MBCA MBMCA and essentially has the following structure: where the wavy lines each represent the bond of the nitrogen atom to another symbolize 2-hydroxypropylene grouping.

5. Process according to one of claims 1 to 4, characterized in that the thermosetting epoxy resin is part of a glass fiber reinforced composite material and in step a) after alcoholysis the glass fibers are separated from the diester and the polyol.

6. Method according to one of claims 2 to 5, characterized in that R 1 represents a divalent radical of an aromatic ring selected from monocyclic heteroaromatics and benzene, wherein the diester of the corresponding dicarboxylic acid is obtained as a solid precipitate during the alcoholysis in step a) and the separation of the diester from the polyol is carried out by filtering off the precipitate from the resulting alcoholic solution of the polyol; Any glass fibers present are separated from the solid diester by rinsing or dissolving it in a solvent.

7. Process according to one of claims 1 to 6, characterized in that the alcoholysis in step a) is carried out with methanol or ethanol at a temperature in the range of the boiling temperature of the alcohol or above.

8. Process according to claim 7, characterized in that the alcoholysis in step a) is carried out with methanol at a temperature between 60 °C and 70 °C.

9. Process according to one of claims 1 to 10, characterized in that the acetolysis in step b) is carried out at a temperature of above 150 °C, above 200 °C or above 250 °C.

10. Process according to one of claims 1 to 9, characterized in that the deacetylations in steps c) and d) are carried out with aqueous HCl, optionally under reflux.

11. A process for the preparation of thermosetting epoxy resins as defined in any one of the preceding claims by polymerizing the corresponding diepoxide and diamine monomers, either in solution in a suitable solvent or solvent mixture or in bulk at a temperature above the melting point of the monomers, the polymerization optionally being carried out in the presence of reinforcing fibers to produce a fiber-reinforced composite material.

12. Process according to claim 11, characterized in that the diepoxide and diamine monomers are polymerized in a molar ratio of 2:

1.

13. The process according to claim 11 or 12, characterized in that the polymerization is carried out at a temperature of at least 80 °C.

14. The process according to claim 13, characterized in that the monomers are first heated to 80 °C for 2 hours and then to 150 °C for 8 hours.

15. Process according to one of claims 11 to 14, characterized in that the resulting thermoset is subjected to a thermal aftertreatment.

16. The process according to claim 15, characterized in that the resulting thermoset is thermally post-treated at at least 170°C for at least 1 hour.

17. A thermoset epoxy resin obtainable by a process according to any one of claims 11 to 16 and cleavable and recyclable by a process according to any one of claims 1 to 10.

18. Thermosetting epoxy resin according to claim 17, in which the dicarboxylic acid component and the diamine component are as defined in any one of claims 2 to 4, optionally as defined in claim 4

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