Method for preparing depolymerized lignin from kraft lignin, fractions obtained and resins that contain them

Abstract

The present invention relates to the chemical industry and preferably to the industry relating to novel products derived from or based on lignin, and even more preferably to products derived from or based on modified lignin, phenolic adhesive resins comprising same and use thereof as adhesive for plywood panels, particle boards, chipboards and pine veneers, inter alia. The starting lignin is Kraft lignin.

Description

[0001] METHOD OF PREPARATION OF DEPOLYMERIZED LIGNIN FROM

[0002] KRAFT LIGNIN, FRACTIONS OBTAINED AND RESINS THAT CONTAIN THEM

[0003] FIELD OF INVENTION

[0004] The present invention relates to the chemical industry and preferably to the industry related to new products derived from or based on lignin, and even more preferably to products derived from or based on modified lignin, phenolic adhesive resins comprising it, and their use as adhesives for plywood panels, particle boards, chipboards, and pine veneers, among others. The starting lignin is Kraft lignin.

[0005] BACKGROUND

[0006] Lignin is one of the wood components available in large quantities throughout the world. Its polyphenolic chemical structure has been of interest for its valorization and industrial application. Because of this, the chemical and materials industry has focused on harnessing lignin biopolymers as a viable alternative renewable raw material for the synthesis of new bio-based materials and prepolymeric structures of industrial interest. Considering lignin's polyphenolic structure, there is an opportunity to convert lignin polymers into a wide range of oligomeric lignophenolic chemical structures and simple structures. Lignin has been described as prone to structural degradation during lignocellulose fractionation, resulting in a depletion of ether bonds and an increase in carbon-carbon bonds relative to native lignin.Lignin depolymerization is challenging due to the different bond strengths of ether (COC) and carbon-carbon (CC) bonds and the tendency of low-molecular-weight species to undergo condensation reactions. Given the heterogeneity of low-molecular-weight species and their functional diversity, lignin products are also difficult to valorize. Lignin can be broken down into various building blocks, and their reduction can yield commercially interesting chemical structures such as vanillin, sihngaldehyde, vanillic acid, phenol, bisphenol, xylene, among others.

[0007] Once lignin is obtained, due mainly to its high heterogeneity, additional steps are necessary to isolate and purify it to facilitate future processing. Currently, three main methods for lignin fractionation are used: selective precipitation, membrane filtration, and extraction with organic solvents.

[0008] Selective precipitation is the most widely used methodology for recovering lignin generated in the pulp and paper industry. It involves first solubilizing biomass lignin in an alkaline solution and then precipitating it by adding an acidic solution with different pH values. Lignin isolated by selective precipitation typically has a high molecular weight (Mw) and a broad molecular weight distribution (Mn); however, this methodology involves the use of corrosive chemicals and therefore requires costly waste treatment and disposal.

[0009] Membrane filtration is another technique for fractionating technical lignins through ultrafiltration or nanofiltration with a certain cross-flow filtration permeability, which allows lignin fractions with similar molecular weight distribution ranges to pass through. However, the high cost of large-scale implementation makes its implementation difficult. Extraction using organic solvents provides an alternative way to isolate lignin, based on lignin's solubility in different solvents. A variety of organic solvents (e.g., acetone, butanol, texane, and dichloromethane) can be used for sequential extraction processes. Low-boiling organic solvents, such as methanol and ethanol, are preferred over others because these solvents can be easily recovered and reused. Lignin fractions isolated by organic solvents exhibit a narrower molecular weight distribution range.

[0010] Specific chemical methods for lignin depolymerization have been described. Authors such as Rahimi, A., Azarpira, A., Kim, H., Ralph, J., Stahl SS, (2013). Chemoselective Metal-Free Aerobic Alcohol Oxidation in Lignin. Journal of the American Chemical Society 135: 6415-6418 have described a chemoselective method for depolymerizing lignin, which highlights the use of oxidation and depolymerization using the TEMPO reagent (2,2,6,6-tetramethylpiperidine-1-oxyl) and another method with hydrogen peroxide (H2O2). TEMPO is a solid heterocyclic compound of orange-red color and that in its structure presents a stable aminoxyl radical. This method has very long reaction times.The results shown are very preliminary, not Kraft lignin but different lignin models are used, what is concluded is that an effective catalytic method is developed for the chemoselective aerobic oxidation of secondary benzyl alcohols in the presence of unprotected primary alcohols.

[0011] On the other hand, there has also been progress in the development of lignin deconstruction methods using physical mechanisms, specifically through microwave and ultrasound assisted catalysis. In the particular case of ultrasonic mechanisms, the use of this physical method has been described in lignocellulosic biomasses, resulting in an increase in lignin extraction yields, mainly due to the breaking of chemical bonds between lignin-hemicelluloses and degradation of the lignin itself, (SASMAL, S et al. Ultrasound Assisted Lime Pretreatment of Lignocellulosic Biomass toward Bioethanol Production, (2012), Energy Fuels, 26: 3777).This paper presents the optimization of ultrasound-assisted lime pretreatment to reduce the lime pretreatment time of three lignocellulosic biomass materials: areca nut shell (Areca catechu), bon bogoh (Ziziphus rugosa), and moj (Albizia lucida). Ultrasound was explored for the pretreatment of lignocellulosic biomass in conjunction with lime, which enhances delignification. However, the ultrasonic cavitation method can also generate increased reactivity, leading to polymerization of the products depending on the operating conditions.

[0012] One potential offered by lignin biopolymers from low molar mass fractions is the use of lignin as a substitute for phenol in the generation of adhesives, noting that the presence of hydroxyl groups in its structure presents a potential use for these applications. Likewise, research has indicated that it is possible to partially replace phenol with lignin in phenol-formaldehyde resins; however, one of the greatest obstacles to their use is the low reactivity and high degree of substitution present in their polymeric structure. The highest substitution percentages are only achieved after chemical modifications of the polymer, granting the lignin the necessary reactivity for polymerization. However, these referenced products do not achieve the optimal adhesive and mechanical resistance capacity obtained with commercial phenol-formaldehyde resol adhesive resins.Traditional formulations contain between 25-30% phenol in their resins.

[0013] Regarding patent documents W02018205020A1 (FPInnovations) discloses a method for depolymerizing a raw material comprising lignin producing depolymerized lignins in a reactor that is loaded with raw material, a catalyst, a polyol and water, allowed to heat to approximately 150-300 ° C and a mixture of depolymerized lignin is produced, stopping the depolymerization reaction with cold water.The by-products of the depolymerized lignin mixture are acidified / neutralized; depolymerized lignin and solid residues are precipitated from the depolymerized lignin; the precipitated depolymerized lignin and solid residues are dissolved in acetone; and the depolymerized lignin is separated from the solid residues by filtration, producing a solid depolymerized lignin, which can be used in the production of lignin-based rigid polyurethane foam, a phenol-formaldehyde resol, and a lignin-based epoxy resin with a high percentage of bio-content (> 50% by weight).

[0014] As seen in the state of the art, a method has not yet been developed to depolymerize Kraft lignin, so as to obtain reactive fractions in the sense that these can be useful for replacing phenol in adhesive resins and that said lignin fractions in the resin provide sufficient mechanical resistance and adhesive capacity to it.

[0015] BRIEF DESCRIPTION OF THE INVENTION

[0016] The present invention proposes a method of oxidation of Kraft lignin to depolymerize and simultaneously activate it, to obtain a depolymerized and reactive lignin to be applied in the synthesis of biomaterials, which involves the oxidation mechanism with TEMPO reagent and assisted by a physical mechanism of ultrasonic cavitation.

[0017] The present invention relates to a method for preparing depolymerized lignin that shortens reaction times without decreasing the degree of depolymerization. Furthermore, the method is based on Kraft lignin. Kraft lignin is much more difficult to oxidize and depolymerize than other lignins, which is why it is currently normally burned as fuel for papermaking processes. The starting lignin used in the depolymerization processes is relevant since, due to the heterogeneity of different lignins, they do not respond equally to different treatments.

[0018] The method of the invention makes it possible to obtain depolymerized lignin fractions from Kraft lignin with sufficient reactivity to be used in adhesive resins. The lignin fractions have very favorable adhesive and mechanical strengths. Finally, the method of the invention reduces production times without compromising depolymerization.

[0019] The present invention relates to a method for preparing depolymerized / activated lignin from Kraft lignin, to the fractions obtained, to the uses given to the fractions, to the lignophenolic adhesive resins (lignin-phenol-formaldehyde) comprising said fractions of depolymerized / activated lignin, the method for obtaining the resin and the use of the resin as an adhesive in plywood boards, particle boards, agglomerated boards and pine wood veneers, among others.

[0020] The lignin fractions obtained using the inventive process were tested, and their reactivity toward formaldehyde (hydroxymethylation reaction) was measured. It was observed and demonstrated that the fractions are more active and reactive than a lignin to which the inventive method is not applied.

[0021] In order to verify the potential of the lignin fractions obtained by the present Kraft lignin depolymerization method, lignophenolic adhesive resins (Lignin-Phenol-Formaldehyde) with different compositions were prepared and used to partially replace different percentages of phenol with depolymerized lignin obtained by the method described in the first aspect of the invention. The percentages of phenol replacement by depolymerized / activated lignin fractions tested were 15%, 30% and 50%. The prepared lignophenolic resins were characterized according to their chemical characteristics by means of FTIR, their dynamic viscosity, adhesiveness, temperature tolerance (TGA) and a differential scanning calorimetry (DSC) study to determine the curing behavior. The depolymerized lignin fractions of the invention allowed the production of resins with high adhesive performance in pine wood veneers.

[0022] BRIEF DESCRIPTION OF THE FIGURES

[0023] Figure 1. FTIR of unfractionated Kraft Domtar DKL lignin and lignin fractions.

[0024] Figure 2. Yields of the soluble (organic phase) and insoluble (residue) fractions of depolymerized lignins resulting in MIBK.

[0025] Figure 3. Molecular weight distribution of lignins depolymerized with TEMPO and / or ultrasound.

[0026] Figure 4. Combined graph with recovery percentages and molecular weight distribution.

[0027] Figure 5. Reactivity of lignins obtained by depolymerization with TEMPO / Ultrasound.

[0028] Figure 6. Adhesiveness of R-Control-FF, R-DLK-Control, R-DLK-O, R-DLK-R and R-DLK-RO resins evaluated in ABES equipment at a pressing temperature of 130°C.

[0029] Figure 7. Adhesiveness of the resins R-Control-FF, R-DLK-Control, R-DLK-TEMPO / US (15%), R-DLK-TEMPO / US (30%), R-DLK-TEMPO / US (50%) evaluated in ABES equipment at a pressing temperature of 130°C.

[0030] Figure 8. FTIR analysis of lignophenolic resins made with different percentages (15%, 30% and 50%) of partial phenol replacement by Kraft lignin depolymerized with TEMPO and ultrasound. DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention relates to a method for preparing depolymerized / activated lignin by means of the oxidation reaction of Kraft lignin with TEMPO and ultrasound, said depolymerized / activated lignin, and its use for preparing a phenolic adhesive resin (resol type), useful as an adhesive for plywood type boards, particle boards, chipboards and pine wood veneers.

[0032] In particular, the present invention relates to a method for preparing depolymerized lignin from Kraft lignin, comprising the steps of: a) oxidizing a mixture of Kraft lignin with the TEMPO reagent (2,2,6,6-tetramethylpiperidine-1-oxyl) in a ratio (w / w) of Kraft lignin to TEMPO reagent in the range of 100:1 to 10:1; in an acidic and oxidizing medium in a polar solvent; b) bubbling air into the mixture from step a); c) subjecting the mixture to physical cavitation in the ultrasound range, in the interval between 95,000 J and 380,000 J until the reaction is complete; d) washing the residue obtained in c) with an organic solvent, to extract the soluble organic fraction of depolymerized lignin, and drying; and optionally e) filtering the insoluble fraction obtained in step d) to recover the residual insoluble fraction of depolymerized lignin.

[0033] The term Kraft lignin refers to lignins obtained using the Kraft process. Domtar Kraft lignin is most preferred.

[0034] Preferably, the depolymerized / activated lignin corresponds to a mixture of low molecular weight monomers / oligomers, that is, in the range of 1,170-2,286 g / mol, and high molecular weight reactive oligomers / polymers, that is, in the range of 4,001-9,508 g / mol, preferably 100:3.7.

[0035] Preferably, the acidic and oxidizing medium in a polar solvent comprises nitric acid, more preferably nitric acid in acetonitrile, even more preferably 69% v / v nitric acid in 50 mL of acetonitrile. Preferably, the acidic and oxidizing medium in a polar solvent comprises hydrochloric acid, preferably hydrochloric acid in acetonitrile, even more preferably 37% v / v hydrochloric acid in 50 mL of acetonitrile, in a percentage of between 0.5% and 10% of 37% hydrochloric acid dissolved in 100 mL of acetonitrile per gram of lignin, both dissolved in a solution with a volume ratio of acetonitrile and water in a proportion (v / v) of 12:1 to 8:1, preferably 10:1; and

[0036] Preferably in step b) after bubbling air, the reaction medium is kept at a temperature between -5 S C and 5 S C. Particularly 0°C.

[0037] Preferably, step c) is carried out for a period of between 1.5 and 2.5 hours. In particular, step c) is carried out at 190,000 J, equivalent to 40% amplitude, for 2 hours.

[0038] Preferably in step d) of washing the residue with an organic resin solvent, this is selected from: 4-methyl-2-pentanone (MIBK) or ethyl acetate, petroleum ether, dichloromethane, diethyl ether, hexane and toluene, and any combination of the above, preferably with 4-methyl-2-pentanone (MIBK), to extract the soluble organic fraction of depolymerized lignin.

[0039] Preferably in step d) it is dried with anhydrous sodium sulfate, evaporating the solvent.

[0040] Preferably, step e) filtration is carried out by vacuum filtration.

[0041] Another aspect of the invention is the soluble organic fraction of depolymerized lignin obtained in step d) by the method defined above, and the insoluble residual fraction of depolymerized lignin obtained in step e) of said method is also an aspect of the invention. Another aspect of the invention is the use of the aforementioned fractions in phenolic resins as phenol substituents.

[0042] Due to the phenolic and reactive nature of the products developed, the present invention also relates to a lignophenolic adhesive resin comprising replacing a portion of the phenol in the phenolic resin with one or both fractions obtained using the method of the invention. The fractions obtained were used for the synthesis of resole-type phenolic adhesive resins based on lignin-phenol-formaldehyde, where the depolymerized lignin is selected from the fractions of the invention.

[0043] Likewise, one aspect of the invention is a phenolic adhesive resin comprising the soluble organic fraction of lignin in a percentage between 15% and 50% by weight with respect to the weight of phenol in the resin or the insoluble residual fraction of lignin or a mixture of both, in a percentage between 15% and 50% by weight with respect to the weight of phenol in the resin. More preferably, the percentage is between 15% and 30%.

[0044] Another aspect of the invention is the method for preparing a lignophenolic adhesive resin, comprising replacing 15-50% by weight of the phenol in the phenolic resin with the soluble organic fraction of depolymerized lignin or replacing 15-50% by weight of the phenol in the phenolic resin with the insoluble residual fraction of depolymerized lignin or a combination of both. More preferably, the percentage is between 15% and 30%.

[0045] Finally, one aspect of the invention is the use of the lignophenolic adhesive resin defined above as an adhesive for plywood panels, particle boards, chipboards and pine wood veneers.

[0046] The resulting depolymerized lignin fractions were chemically characterized by their reactivity toward formaldehyde (hydroxymethylation) and FTIR; the adhesive resins made with depolymerized / activated lignin were evaluated for their viscoelastic, physicomechanical, and adhesive properties using viscosity measurements, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and adhesiveness using an Automated Bonding Evaluation System (ABES) system.

[0047] For the depolymerization of Domtar kraft lignin (BioChoice® lignin), a series of experiments were carried out using TEMPO reagent or without it as a catalyst for the oxidation-depolymerization of Kraft lignin with the aim of obtaining monomeric / oligomeric fractions and / or oligomeric / polymeric fractions with capacities relevant for their industrial application. Table 1 of Example 1 shows the results obtained. The depolymerized lignin in each experiment was characterized according to the weight yield of the soluble and insoluble fractions and by molecular weight distribution by means of gel permeation chromatography (GPC). Undepolymerized Kraft lignin was used as a control sample.

[0048] In the chemical depolymerization / activation of Kraft lignin with TEMPO only using the TEMPO reagent for oxidation-depolymerization (control). Domtar Kraft lignin was added TEMPO and nitric acid (69% v / v) in acetonitrile and hydrochloric acid (37% v / v dissolved in acetonitrile), and then acetonitrile and water were added. The reaction mixture was bubbled with air keeping the reaction medium at 0 ° C and then increasing the temperature of the medium to the reaction temperature (65 ° C), and once completed, the mixture was cooled. Then the solvent was evaporated and the residue was washed with 4-methyl-2-pentanone (MIBK) to extract the soluble fraction which was subsequently dried with anhydrous sodium sulfate and then the solvent was evaporated to obtain the yield of this soluble fraction. The insoluble residue was recovered by vacuum filtration.

[0049] In the chemo-physical depolymerization / activation of lignin with TEMPO / Ultrasound (invention), to achieve the chemo-physical oxidation-depolymerization of Kraft lignin, TEMPO was added to the Domtar Kraft lignin, and then, nitric acid (69% v / v) in acetonitrile and hydrochloric acid (37% v / v dissolved in 50 mL of acetonitrile) were added and finally acetonitrile and water were added. The reaction mixture was bubbled with air while maintaining the reaction medium at 0 ° C. Then, the mixture was subjected to physical cavitation with ultrasound and air bubbling. To avoid overheating of the reaction medium, the reaction vessel was subjected to cooling. Upon completion of the reaction, the solvent was evaporated, and the residue was washed with MIBK to extract the soluble fraction. The fraction was then dried with anhydrous sodium sulfate, followed by evaporation of the solvent. The insoluble residue was recovered by vacuum filtration.A second control experiment was carried out under the same reaction conditions, but without TEMPO in the reaction medium.

[0050] Phenolic resins were prepared from Kraft lignin recovered from oxidation processes, and the depolymerized / activated lignin of the invention was used as a partial replacement for phenol. The phenolic nature and adhesion capacity of the prepared resin allow it to be considered an excellent complement or co-reactant for adhesive systems and, due to its origin and abundance, it can be considered an important source of renewable biomass with potential applications in the synthesis of value-added products. In this sense, it is highlighted that, in the area of ​​biomaterials, lignin has been used to produce different types of resins, where phenol-formaldehyde resol resins stand out.However, as described in the prior art, thermally or chemically depolymerized lignin can only be used as a partial substitute for phenol in lignin-phenol-formaldehyde phenolic resins, and therefore, the challenge is to develop a depolymerized lignin to produce alternative biobased products to those currently on the market.

[0051] The present depolymerized / activated lignin was used by means of the chemo-physical reaction with the TEMPO reagent and assisted by ultrasound (cavitation), respectively, for the synthesis of a phenolic type resin (lignin-phenol-formaldehyde) for its application in plywood type boards, and two specific tests were carried out: hydroxymethylation and adhesiveness in ABES (Automated Bonding Evaluation System) equipment to confirm this potential use.

[0052] In the hydroxymethylation, the formaldehyde reactivity assay was performed with the present depolymerized and activated lignin, specifically, the depolymerized / activated lignin from depolymerization experiment 13 was used (Table 2), see examples, corresponding to the organic fraction and residual fraction, respectively. In addition, a mixture of organic and residual fractions and undepolymerized Kraft lignin (control) and the phenol reagent as a witness were incorporated, respectively. The hydroxymethylation assays were performed based on the general procedure described by Wooten, AL, Sellers, TJ, Tahir, PM (1988) Reaction of formaldehyde with lignin. Forest and Products Journal, 38, 45-46, in which the lignin is dissolved in NaOH at 40°C and 60°C and subsequently formaldehyde is added, taking periodic samples until reaching 4 hours of reaction.For the hydroxymethylation reaction, the temperature was set at 45°C, then the lignin was solubilized in 0.25 M NaOH, formaldehyde was added, and it was left to react for 4 hours, a sample was taken and immediately brought to a temperature of -4°C, until free formaldehyde was determined. The determination of free formaldehyde was carried out using the hydroxylamine hydrochloride (NH2OH-HCI) method with titration to the equivalence point. The procedure consisted of thawing the vial containing the sample. Distilled H2O and the contents of the vial were placed in a beaker, then the remains of the mixture were washed from the vial with water and placed in the beaker, and the mixture was homogenized with magnetic stirring, and acidified with 3% H2SO4 (v / v) to pH 2 and then the pH of the mixture was adjusted to the reference pH (4.0) with 1 M NaOH and 10% (w / v) NFLOH-HCI was added, adjusted to the reference pH with 1 M NaOH.Finally, it is allowed to react with formaldehyde and reacts with hydroxylamine hydrochloride (NF OH-HCI) forming CH2NOH, water and hydrochloric acid (HCI).

[0053] The back reaction of the hydrochloric acid formed in this reaction was carried out with 1 M NaOH (titrated by a primary standard, potassium hydrogen phosphate, KHC8H4O4) to a reference pH of 4.0. Using the spent volume of 0.1 N NaOH, the free HCHO in the sample was calculated, from which the values ​​of HCHO reacted with lignin were finally extrapolated.

[0054] For the synthesis of phenolic adhesive resin (phenolic type resin in alkaline medium (resol)) based on lignin-phenol-formaldehyde, a reaction system consisting of a 3-necked glass flask, with a reflux system and magnetic stirring, was used. Phenol was dissolved in distilled water and 2 / 3 of 33% NaOH, then the amount of depolymerized / activated lignin of the present invention was slowly added at 45 ° C. After the start of the chemical addition stage, 1 / 3 of the formaldehyde was added, and the reaction was allowed to begin with, before adding formaldehyde again, raising the temperature to 85 ° C. Finally, the last 1 / 3 of formaldehyde was added and allowed to react, ending the synthesis in a final stage without reflux.

[0055] Five resins were prepared with 15% phenol replacement by the present depolymerized / activated lignin, using the lignins from experiment 13, where for each resin the following were used: i) organic fraction from experiment 13, ii) the residual fraction from experiment 13, iii) the mixture of the organic and residual fractions from experiment 13, iv) undepolymerized Kraft Domtar lignin (control), and v) phenol and formaldehyde resin (control), under the same conditions. Viscosity and Adhesiveness were evaluated on the resins prepared as above.

[0056] The chemical characterization of the present depolymerized / activated lignin and the produced lignophenolic adhesive resins (lignin-phenol-formaldehyde) was carried out by means of Fourier transform infrared (FTIR) analysis. Viscosity measurements were performed according to the analysis conditions specified in the ASTM D2556-14 (2018) testing method, and a quantity of each sample was conditioned at 25°C. The viscosity was measured by maintaining the temperature until the samples reached a stable viscosity value.

[0057] To determine the temperature tolerance of each of the resins, a thermogravimetric study (TGA) was conducted based on the ASTM E1 131 standard. The analysis was carried out on a thermobalance and evaluated the stability of the resins at temperatures between 40°C and 700°C, at a heating rate of 10°C / min and with a nitrogen flow of 20 mL / min.

[0058] To determine the viscoelastic behavior and glass transition temperature of the adhesive resins produced, a differential scanning calorimetry (DSC) study was performed based on ASTM D3418. The DSC analysis of the resins was carried out on a Mettler Toledo differential scanning calorimetry analyzer (STAR ​​SW 8.10). DSC analyses of all resins were carried out over a temperature range of 30°C to 250°C at a rate of 10°C / min and with a nitrogen flow rate of 20 mL / min.

[0059] The adhesiveness of the lignin-phenol-formaldehyde resins was measured using the Automated Bonding Evaluation System (ABES) methodology using standardized equipment manufactured by Adhesive Evaluation Systems (AES). Samples of Pinus radiata wood were used, to which the adhesive was applied (wet base). The effect of pressing time (30, 60, 90, 120, 150, and 180 seconds) was also evaluated at a temperature of 130°C.

[0060] An elemental analysis was performed to determine the total CHNS (Carbon, Hydrogen, Nitrogen and Sulfur) content in samples of each DKL fraction (F1 and F2) obtained by fractionation methodology. The results of this analysis are presented in Table 2. It can be observed that there are no significant differences in the elemental analysis of CHNS between the DKL lignin fractions compared to the unfractionated DKL. Fraction 1 has a higher amount of N being 0.08% higher compared to the unfractionated DKL lignin. The amount of C for the 2 fractions is lower, in a range of difference between 2% and 3% compared to the unfractionated DKL lignin. The amount of H is lower in the ethyl acetate-insoluble lignin (F2) compared to the ethyl acetate-soluble fraction and the unfractionated control lignin. The S in the 2 fractions is lower compared to the unfractionated DKL lignin.

[0061] In the results shown in Table 2, a difference can be observed between the two fractions F1 and F2, where F1 is a fraction of lower weight average molecular weight (Mw), with an Mw of 2040 g / mol compared to fraction F2 which has an Mw of 8346 gr / mol. The F1 of DKL soluble in ethyl acetate is also a DKL fraction that has a lower Mw than the unfractionated control lignin. F1 is also a lignin with a lower polydispersity index (13), that is, it is a more homogeneous DKL fraction if compared to fraction 2 insoluble in ethyl acetate (2.47) and to the unfractionated DKL (6.5) which is much more polydisperse. The results of the characterization of molar mass distribution by GPC correlate with those described in Jiang, X., Savithri, D., Du, X., Pawar, S., Jameel, H., Chang, HM., Zhou, X. (2017). Fractionation and characterization of kraft lignin by sequential precipitation with various organic solvents. ACS Sustainable Chem. Eng., 5, 835-842.

[0062] To determine the presence of the main bonds and functional groups in unfractionated and ethyl acetate fractionated DKL, an FTIR analysis was carried out as shown in Figure 1. In the three lignins characterized by FT-IR (fractionated and unfractionated) shown in Figure 1, it is observed that the main bonds and functional groups characteristic of lignin are the [3-O-4 (1120 nm), C=O (1720), and OH-phenolic (1375 nm) bonds. Particularly, fractionated lignin F2 has a higher intensity of the [3-O-4 (1120 nm) bonds compared to fraction 1; It is observed through the peak at 1375 nm that the fraction soluble in ethyl acetate (F1) has a slightly higher presence of phenolic OH groups compared to the fraction insoluble in ethyl acetate (F2) and the unfractionated DKL, which would confirm the obtaining of the desired fraction of lignin, with greater reactive OH groups.The majority presence of G-type lignin vs. S-type lignin can also be determined for all lignins characterized by FTIR, consistent with the type of raw material being worked with, which is softwood.

[0063] Figure 2 shows the results of the recovery yields obtained from the soluble (organic) and insoluble (residue) fractions of Kraft lignin depolymerized with TEMPO and / or ultrasound. The soluble fractions of Kraft lignin from experiments 2-6 and 11-13 show an increase in yield when compared to the control sample, going from a recovery percentage of 8% to a range of approximately 15-25%. Meanwhile, the yield in experiment 4, which corresponds to oxidation-depolymerization with TEMPO / Ultrasound, is similar to experiment 2, where oxidation-depolymerization took place only with TEMPO reagent. It is noteworthy that the reaction times for each experiment were very different, being 2 hours for experiment 4 vs. 24 hours for the sample in experiment 2.Experiment 6 corresponds to a replicate of experiment 4 where the only difference lies mainly in the application of magnetic stirring to the reaction medium. In experiment 11, a similar effect is observed with twice the energy applied to the reaction medium. Regarding experiment 12, practically the same percentage of soluble fraction recovery was obtained as in experiment 2 without ultrasound, although it was carried out with twice the lignin and energy at an amplitude of 75% to keep the reaction time below 2 hours. Starting with experiment 12, with a higher percentage of soluble phase recovery, a scaling up of 20 grams was carried out in a total volume of 1.5 L corresponding to experiment 13. Figure 6 shows the molecular weight distribution of the different fractions recovered from the different oxidation-depolymerization experiments carried out with TEMPO and / or ultrasound.

[0064] As shown in Figure 3, all organic fractions have a lower average molecular weight than the organic fraction of the control lignin. These fractions present a fairly homogeneous weight distribution despite the different conditions used, with weights ranging from 1,170 to 2,286 g / mol. Regarding the residues, the weight distribution is much more uneven, ranging from 4,001 g / mol to 9,508 g / mol. Comparing sample 2 (TEMPO oxidation) with sample 4 (TEMPO + Ultrasound), the latter presents a higher molecular weight (Mw) both in the organic phase and, above all, in the residue. When observing samples 4 (TEMPO + Ultrasound) and 5 (Ultrasound), there is a higher molecular weight in sample 4, which contains TEMPO. Experiment 7 was carried out with twice as much lignin and despite this, similar results or even lower Mws were obtained than in sample 4. Experiments 8, 9 and 10 show similar results.It is worth noting that the organic fraction of Experiment 9 shows the lowest Mw of all, at 1,170 g / mol. Experiment 11 was carried out by applying double the energy, obtaining very similar results and reducing the time by half. Experiment 12 was carried out by doubling both the lignin concentration and the energy. Molecular weights very similar to those obtained previously were obtained. Finally, Experiment 13 represents a scaling of the reaction with 20 grams of lignin, in which weights of 1,603 g / mol were obtained in the organic fraction and 6,338 g / mol in the residual fraction (Figure 4).

[0065] As previously mentioned, all experiments performed show an organic fraction with a lower molecular weight than the control lignin. Furthermore, experiments 2-6 and 11-13 show a greater amount of recovered organic phase.

[0066] Regarding the residues, experiments 2, 3, and 13 show a lower molecular weight than the control lignin, while the other samples have a higher molecular weight. Broadly speaking, ultrasound significantly reduces depolymerization time. In contrast, applying energy increases the average molecular weights. Agitation has virtually no impact on the results obtained. It is possible to compare experiments 4 and 6, where the only difference is the use of agitation in experiment 6, and the results for both recovery and weight distribution are practically identical. Increasing lignin concentration also does not influence the results (experiment 4 vs. 12, the lignin concentration doubles), and therefore, it would be possible to depolymerize twice as much lignin under the same conditions without significantly affecting the result. Finally, scaling the experiment does not alter the results either.

[0067] Regarding the parameters that do interfere with the result, the application of energy through ultrasound significantly reduces the time, going from 24 hours to 2-4 hours. The use of air also shows a positive effect, since in experiments 9 and 10, where air was not used, the organic fraction is much lower compared to that of experiments 3 and 4. Meanwhile, the application of ultrasound pulses results in a very low recovery of the organic fraction (experiment 8 vs. 4). A similar result occurs with the application of more energy per gram of lignin, which results in a low recovery of the organic fraction, although markedly higher than that resulting from comparing experiment 1 vs. experiment 4.

[0068] A hydroxymethylation assay was also performed before processing lignophenolic resins with depolymerized / activated Kraft lignin to determine the ability of formaldehyde to bind to the activated lignin fractions. The hydroxymethylation result and the reactivity correlated with this hydroxymethylation are shown in Figure 5.

[0069] Figure 6 shows that the organic fraction (DL-Organic) and the residual fraction (DL-Residue) from the TEMPO / Ultrasonic depolymerization (experiment 13) have greater reactivity than the control sample of undepolymerized Kraft Domtar lignin (DKL). The residual sample also shows greater reactivity compared to the organic fraction. The hydroxymethylation analysis was complemented with an adhesiveness study using ABES methodology to confirm its contribution to the synthesis of resol-type adhesive resin.

[0070] In the synthesis of lignophenolic resin with fractions obtained from depolymerized / activated lignin by TEMPO / ultrasound reaction, 5 resins were first synthesized (Figure 7) with 15% replacement of phenol in the formula by depolymerized / activated Kraft lignin with TEMPO / ultrasound. The synthesized resins were: a phenol-formaldehyde control resin (without lignin, R-Control-1); a resin using undepolymerized Kraft lignin (R-DLK-Control); a resin using the organic fraction of depolymerized / activated lignin (R-DLK-O); a resin using the non-organic or residual lignin fraction of depolymerized / activated lignin (R-DLK-R); and a resin using a mixture of both fractions (organic and residual) of depolymerized / activated lignin (R-DLK-RO). The viscosity and ABES adhesion capacity of each resin were determined.

[0071] The viscosities obtained from each resin are presented in Table 3 and show the variability of the same in the different adhesive resins. The resin synthesized with the organic fraction of depolymerized / activated lignin (R-DLK-O) has the highest viscosity, with a viscosity of -730 cP. It is followed by the resin synthesized with undepolymerized Kraft lignin (R-DLK-Control), with a viscosity of -430 cP, then the resin synthesized with the mixture between the organic fraction and the residual fraction of depolymerized / activated lignin (R-DLK-RO) with a viscosity of -245 cP, and the resin synthesized with the residual fraction of depolymerized / activated lignin (R-DLK-R) with a viscosity of -170 cP, and lastly is the phenol-formaldehyde control resin with a viscosity of 72 cP.The viscosities determined for almost all resins are suitable for application as adhesives in plywood panels, where viscosities in the range of approximately 100–1000 cP are required. The exception is the phenol-formaldehyde control resin (R-Control-FF), which shows a lower value and is not within the aforementioned viscosity range. Without being bound by theory, this could be due to the fact that this resin is based on a reaction between two monomers (phenol and formaldehyde), and therefore, the synthesis probably involved partial condensation in the resin.

[0072] Figure 6 shows the adhesiveness evaluation in ABES equipment, and shows that the control resin of undepolymerized Kraft lignin (R-DLK-CONTROL) reaches its maximum performance at 120 seconds of pressing while the phenol-formaldehyde resin (R-CONTROL-FF) reaches its maximum performance at 150 seconds of pressing. On the other hand, the resin synthesized with the residual fraction of depolymerized / activated lignin (R-DLK-R) and the resin synthesized with the organic fraction of depolymerized / activated lignin (R-DLK-O) reach their maximum performance at 90 seconds of pressing, and the resin synthesized from the mixture of organic fraction and residual fraction of depolymerized / activated lignin (R-DLK-RO) reaches its maximum performance at 120 seconds of pressing.The results confirm that the adhesive resins R-DLK-R, R-DLK-0 and R-DLK-RO reach their highest performance at a lower temperature and, therefore, are useful as adhesive resins for industrial use, including R-DLK-RO since its maximum adhesive point (measured at 120 seconds) is very similar to that reached with only 60 seconds of pressing, and its application could be sufficient depending on the type of wood. The R-DLK-R resin is the resin that requires the greatest mechanical stress to be torn in the test, reaching a maximum stress of 4.5 N / mm at 100 seconds of pressing, being the resin with the highest performance under the adhesiveness parameter in the test conditions carried out and its evaluation in ABES equipment.

[0073] To determine the percentage of partial replacement of phenol with the lignophenolic resin with residual fraction of depolymerized / activated lignin by TEMPO / ultrasonic reaction, three other resins were synthesized only using the residual fraction of the product obtained from the reaction with TEMPO and ultrasound due to its greater reactivity and better adhesiveness result as indicated before. Therefore, partial replacement percentages of phenol were used, 15%, 30% and 50% by residual Kraft lignin obtained from the reaction with TEMPO / ultrasonic. These 3 resins were compared with a control phenol-formaldehyde resin and by the lignophenolic resin with untreated Kraft lignin. Viscosity was determined, chemical characterization was performed by FTIR, and its adhesion capacity was determined by ABES equipment and its temperature tolerance by TGA, and a differential calorimetry study was performed by DSC.

[0074] Table 4 shows the viscosities of the three aforementioned lignophenolic resins. The lignin concentration influences the viscosity. For example, the viscosity of the resin with 30% activated Kraft lignin (R-DLK-TEMPO / US (30%)) was 404 cP, reaching a viscosity similar to the viscosity of the control phenol-formaldehyde (FF) resin, 360 cP, making this resin useful as a gluing agent for wood-based panels. Meanwhile, a lignin content greater than 30% causes a substantial increase in viscosity, reaching 4764 cP, making it difficult to handle when attempting to apply it in adhesiveness tests, as described below.On the other hand, the viscosity of the control resin based on 15% replacement, but without treated lignin (R-DLK-Control), reaches a value somewhat desired for the intended application, being 430 cP, however, as could be seen previously in the adhesiveness tests with this same resin, its low adhesion performance demonstrates the importance of the depolymerization / activation treatment by means of the TEMPO reaction and ultrasound.

[0075] Figure 7 shows the adhesiveness result determined by means of the ABES equipment of the 3 lignophenolic resins mentioned above, where the adhesion capacity of the resin made with 30% phenol replacement with depolymerized lignin / activated by TEMPO / Ultrasound (R-DLK-TEMPO / US-30%) can be observed.Based on the shear strength parameter of this result, it can be observed that already from the first 60 seconds of pressing there is a direct competition with the phenol-formaldehyde control resin (R-Control 1 (FF)), where both reach a resistance of ~4.2 N / mm of resistance, and where this same competition behavior is maintained until 180 minutes of pressing at 130 ° C, where even in this last time the resistance reached by this resin (R-DLK-TEMPO / US (30%)) is slightly higher than that reached with the phenol-formaldehyde control resin, with a resistance of ~6.0 N / mm vs 5.25 N / mm reached with the control resin (FF).

[0076] On the other hand, if the result of this resin (R-DLK-TEMPO / US (30%)) is compared with the other two partial phenol replacement lignophenolic resins, with 15% and 50% (R-DLK-TEMPO / US (15%), R-DLK-TEMPO / US (50%)), respectively, it can be seen that in the case of these two resins, their adhesion strength is lower than that of 30% replacement, with a resistance value in both cases no greater than ~4.4 N / mm after 180 seconds of pressing, and highlighting that their adhesion strength began to decline after 120 seconds of pressing where they reached their maximum resistance of ~4.5 N / mm. In the case of these two resins (15% and 50% replacement), it can also be observed that the one with the lowest adhesion performance is the one corresponding to the 50% replacement.This result could also be associated with its high viscosity, which could hinder its penetration into the fibers of the wood slats used for the adhesiveness study, contrary to what was obtained with the resin with 30% replacement, in which its high adhesion capacity is consistent with the viscosity obtained and described above. In contrast to the untreated Kraft lignin-based lignophenolic resin (R-DLK-Control), it can be determined and established that all resins made with depolymerized / activated Kraft lignin have a much higher adhesion capacity than the untreated lignin control resin, which would confirm the relevance of the treatment carried out to obtain a depolymerized / activated lignin suitable for the formulation of phenolic resins to replace phenol.

[0077] In order to establish and confirm the difference in the results obtained, the chemical characteristics of these resins were determined by FTIR, in addition to determining the temperature tolerance properties by TGA, and the viscoelastic properties in a differential scanning calorimetry by DSC.

[0078] Figure 8 shows the chemical characterization by FTIR of the lignophenolic resins made with partial replacement of phenol by residual lignin obtained from depolymerization / activation with TEMPO and assisted by ultrasound. Similarities and differences in the chemical characterization by FTIR are established for the manufactured lignophenolic resins, compared to each other and to the control phenol-formaldehyde phenolic resin (R-Control FF). In all resins, including the controls, signals associated with OH groups (3350-3400 cm -1 ), to C=C groups (1600 cm -1 , 1450 cm -1), to C-0 groups (1230 cm' 1 ). While resins containing depolymerized / activated lignin versus the phenol-formaldehyde control without lignin, show the amount of phenolic OH groups at 1375 cm -1 , which characterizes the lignin polymer. Furthermore, resins containing depolymerized / activated lignin, especially in the control resin with untreated lignin and in the resin containing 50% replacement, show greater intensity in the signals associated with methyl and methylene groups at 2935 and 2835 cm 1, respectively. In resins with 15% and 30% phenol replacement by depolymerized / activated lignin, these signals are lower, which could be due, without adhering to any theory, to the more efficient condensation reactions to form the resol-type adhesive resin, compared to resins with 50% replacement and the control resin with untreated lignin. This is consistent with the intensity of methyl groups (characteristic groups of phenolic resins) assigned to the signal at 1015 cm . 1 , where this signal can be clearly observed, except for the resin sample with 50% replacement, where the signal is weaker and narrower. The broader and more intense signal corresponds to the signal obtained with the lignophenolic resin with 30% phenol replacement by activated lignin derived from the reaction with Tempo and ultrasound, confirming the efficiency of this product in the higher adhesion performance obtained and discussed above.

[0079] Table 5 shows the curing and thermal degradation behavior results determined by DSC and TGA of lignophenolic resins made with 15%, 30% and 50% depolymerized / activated lignin obtained from the reaction with Tempo and ultrasound and compared to the control phenolic resin phenol-formaldehyde (FF). The lignophenolic resins based on depolymerized / activated lignin with Tempo and ultrasound have a curing behavior similar to that observed with the control phenol-formaldehyde phenolic resin.For example, the start of the curing process for lignophenolic resins with 15% and 30% phenol replacement occurs at approximately 120°C vs 15°C obtained with FF resin, however when 50% phenol replacement was used, the start of the curing process occurs much earlier, at 80°C, which would directly correlate to the adhesiveness and viscosity result discussed above, since, without adhering to any theory, it is very likely that with this amount of lignin, the resin being more viscous, presents a polymerization before being used in the gluing process, making it difficult to impregnate the cellulose fibers of the wood sheets with the adhesive resin.This result can also be correlated to the glass transition temperature (Tg) and the maximum curing temperature (Tp), where for example for the resins with 15%, 30% and FF control, the Tg is ~140°C and the Tp is between 160-185°C, whereas for the resin with 50% lignin the Tg is 81°C and the Tp is ~90°C.

[0080] Therefore, the appropriate ratio for the incorporation of depolymerized / active lignin into the resin to replace phenol is up to 30%, which is also supported by the adhesiveness results, where the resin with 30% replacement of phenol by activated lignin resulted in a higher adhesion capacity. The results obtained from DSC and shown in Table 5 are consistent with what has been reported in the literature, where for example as described by Yan, L., Cui, Y., Gou, G., Wang, Q., Jian, M., Zhang, S., Hui, D., Gou, J., Zhou, Z. (2017). Liquefaction of lignin in hot compressed water to phenolic feedstock for the synthesis of phenol formaldehyde resins. Composite Part B, 1 12, 8-14, the start temperature of curing of phenolic and lignophenolic resins would be at an approximate temperature of 1 10°C and an average Tp of ~150°C.

[0081] On the other hand, if the results of tolerance to thermal degradation are analyzed, a greater difference can be observed between the lignophenolic resins and the FF resin control, clearly the FF control resin has a greater tolerance to degradation, where a 50% mass loss was obtained at 385 ° C, while the developed lignophenolic resins have a lower tolerance, determining that the 50% mass loss is achieved at a lower temperature, in a range between 175 ° C and 210 ° C. This result could be correlated, without adhering to any theory, to the greater resistance of the methylene bonds and bridges obtained with the FF control resin, and where at a higher temperature they are stronger to degradation.Finally, there is no major difference in the final ash content in all phenolic resins, where small variations could be due, without adhering to any theory, to the solids content of each resin and water content, and shown in the viscosity value which, in all resins, was more or less variable.

[0082] Thus, low-molecular-weight lignin oligomers or fractions were obtained using three different processes: a solvent fractionation method, on the one hand, and a chemophysical depolymerization process based on a TEMPO reaction assisted by ultrasound using Kraft lignin as the feedstock. The depolymerization mechanism integrates a catalyst-based process involving TEMPO oxidation and ultrasonic frequency waves. Kraft lignin fractions are obtained within 3.5 hours, which is much shorter compared to the oxidation-depolymerization mechanism using only the TEMPO reagent, which requires 24 hours to obtain the results presented above.The low-molecular-weight lignin oligomers or fractions obtained had a mass distribution range of 1000–2500 g / mol for the organic fractions, compared to 6000 g / mol for the undepolymerized Kraft control lignin. Greater variability in molecular weight was observed in the residual fractions; however, these lignin molecules with a wider distribution range were expected to have a reactivity consistent with the proposed oxidation mechanism.

[0083] The lignophenolic products derived from the proposed chemophysical depolymerization mechanism of lignin by means of TEMPO and ultrasound are products of greater chemical reactivity and solubility compared to undepolymerized lignin, which was determined by the hydroxymethylation reaction.

[0084] It was established that the residual Kraft lignin depolymerized / activated with TEMPO and ultrasound is a lignin with greater reactivity toward formaldehyde than that determined by the hydroxymethylation reaction, confirming its usefulness in the production of lignophenolic resins with partial phenol replacement. In fact, the preparation of lignophenolic resins was successfully carried out, resulting in resins with viscosity and chemical properties suitable for subsequent studies of adhesion strength, curing behavior by DSC, and tolerance to thermal degradation with TGA.

[0085] The curing behavior of almost all the lignophenolic resins produced (15% and 30% replacement) have a curing process very similar to that obtained with the phenol-formaldehyde control resin, with the exception of the lignophenolic resin with 50% activated lignin, which results in low adhesiveness and the beginning of the curing process at temperatures much lower than desired, which could be related, without adhering to any theory, to the viscosity and chemical characterization by FTIR described above. The three lignophenolic resins produced have a lower tolerance to thermal degradation compared to the phenol-formaldehyde control resin determined by TGA, an analysis that also made it possible to specify that the ash content for all cases was very similar, and that the variability depended on the specific solids content and viscosity of each resin in particular.According to the adhesiveness results, the resin with 30% residual Kraft lignin, depolymerized / activated by means of reaction with Tempo and assisted by ultrasound, turns out to be the best partial replacement of phenol, where even the adhesiveness obtained at 130°C and 180 seconds of pressing is slightly higher than that obtained with the phenol-formaldehyde control resin, confirming its capacity to be used as a replacement for phenol in phenolic resins.

[0086] Example 1: Lignin Fractionation and Depolymerization

[0087] Kraft Domtar lignin (DKL) was used, previously washed (3 times with 3 volumes of distilled water) and fractionated using ethyl acetate (fraction 1, F1). The fraction insoluble in ethyl acetate was called fraction 2 (F2). 15 g of Kraft Domtar lignin (DKL) were solubilized in 300 mL of ethyl acetate (ratio (w / v), g lignin to my solvent, 3:100) and kept under constant magnetic stirring for 2 hours at room temperature, then the mixture was filtered in a Buchner funnel with Whatman filter paper No. 2. The filtrate obtained (fraction 1, soluble in ethyl acetate) was concentrated through rotary evaporation at 45°C until obtaining an approximate quantity of 30 mL of concentrated lignin, which was then precipitated in 200 mL of diethyl ether (ratio (v / v) of concentrated lignin / precipitation agent, 3:20) which was finally filtered and dried at room temperature to calculate the yield and perform its chemical characterization.The ethyl acetate-insoluble lignin fraction (F2) was dried at room temperature to calculate the yield.

[0088] A control experiment was performed using only the TEMPO reagent in the oxidation-depolymerization mechanism, based on that reported by Rahimi, A., Azarpira, A., Kim, H., Ralph, J., Stahl SS, (2013). Chemoselective Metal-Free Aerobic Alcohol Oxidation in Lignin. Journal of the American Chemical Society 135: 6415-6418. 2 g of Kraft Domtar lignin and 73 mg of TEMPO reagent (weight / weight lignin ratio Kraft / TEMPO, 2000: 73) were added to a 3-necked round-bottom flask. Then, 0.15 mL of nitric acid (69% v / v) in 100 mL of acetonitrile and 0.1 mL of hydrochloric acid (37% v / v dissolved in 100 mL of acetonitrile) were added. Finally, 200 mL of acetonitrile and 20 mL of water were added. The reaction mixture was bubbled with air for 30 min, keeping the reaction medium at 0°C. Then, the temperature of the medium was increased to the reaction temperature of 65°C. The reaction with TEMPO was maintained for 24 hours at 65°C.After the reaction was complete, the mixture was cooled. The solvent was then evaporated, and the residue was washed with 4-methyl-2-pentanone (MIBK) to extract the soluble fraction of the resulting sample. The insoluble residue was recovered by vacuum filtration. The soluble fraction was dried with anhydrous sodium sulfate, and then the solvent was evaporated to obtain the soluble fraction.

[0089] In the chemo-physical depolymerization with TEMPO / Ultrasonic, to achieve the chemo-physical oxidation-depolymerization of Kraft lignin, the following procedure was used: In a beaker, 1 g of a sample of Kraft Domtar lignin and 36.5 mg of TEMPO reagent were added. Then, 75 pL of nitric acid (69% v / v) in 50 mL of acetonitrile and 50 pL of hydrochloric acid (37% v / v dissolved in 50 mL of acetonitrile) were added. Finally, 100 mL of acetonitrile and 10 mL of water were added. The reaction mixture was bubbled with air for 30 min, keeping the reaction medium at 0 ° C. The mixture was then subjected to air bubbling ultrasound for 2 hours at 40% amplitude, equivalent to 190,000 J. To prevent overheating of the reaction medium, the beaker where the reaction was carried out was connected to a water chiller.Upon completion of the reaction, the solvent was evaporated, and the residue was washed with MIBK to extract the soluble fraction of the resulting sample. The insoluble residue was recovered by vacuum filtration. The soluble fraction was dried with anhydrous sodium sulfate, and then the solvent was evaporated. A control experiment was carried out under the same reaction conditions, but without TEMPO in the reaction medium. Table 1. Oxidation-depolymerization experiments with TEMPO and / or ultrasound on Kraft lignin. a Application of ultrasound with 5-second pulses and a 2-second rest period. b Application of ultrasound with an amplitude of 75%. c Application of ultrasound to 20 g of Kraft lignin in a total volume of 1.5 L and with an amplitude of 50% (1000 W ultrasound). Figure 2 shows the results of the recovery yields obtained from the soluble (organic) and insoluble (residue) fractions of Kraft lignin depolymerized with TEMPO and / or ultrasound. Figure 3 shows that all the organic fractions have an average molecular weight lower than that of the organic fraction of the control lignin. These fractions present a fairly homogeneous weight distribution despite the different conditions used, obtaining monomeric / oligomeric fractions with a molecular mass distribution in the range of 1500 g / mol to 3000 g / mol, preferably in the range of 1 170-2286 g / mol.

[0090] Example 2: Preparation of lignin-phenol-formaldehyde phenolic resins based on recovered and depolymerized Kraft lignin

[0091] Lignin-phenol-formaldehyde phenolic resins were prepared from the recovered and depolymerized Kraft lignin obtained from examples 1 and 2 above, and were subjected to two specific tests: hydroxymethylation and adhesiveness in ABES (Automated Bonding Evaluation System) equipment.

[0092] Hydroxymethylation assay

[0093] The raw materials used in the formaldehyde reactivity test were the products obtained from experiment 13 (see Table 1), corresponding to the organic and residual fractions obtained, respectively. In addition, a sample of the mixture of organic and residual fractions and two control samples (unpolished Kraft lignin and the reagent phenol) were added.

[0094] Hydroxymethylation assays were performed in a 250 mL, 3-necked glass flask using the general procedure described by Wooten, AL, Sellers, TJ, Tahir, PM (1988) Reaction of formaldehyde with lignin. Forest and Products Journal, 38, 45-46, in which 50 g of lignin are dissolved with 0.25 moles of NaOH at 40° and 60°C and then formaldehyde (0.633 moles) is added. The reaction time was 4 hours with samples taken during the first 30 minutes and then every 1 hour. For the hydroxymethylation reaction, the temperature was set at 45°C, then 5 g (1 / 10 of the amount of the reference method) of lignin was solubilized in 100 mL of 0.25 M NaOH, then left to solubilize for approximately 10 minutes. Subsequently, formaldehyde (0.127 moles) is added, and it is left to react for 4 hours.Sampling is carried out by taking 1.5 mL of sample in an Eppendorf tube and immediately taking it to -4°C, until free formaldehyde is determined.

[0095] Free formaldehyde was determined using the hydroxylamine hydrochloride (NH2OH-HCl) method with equivalence point titration. The sample vial was thawed, and 20 mL of distilled H2O and the contents of the vial were placed in a 125 mL beaker. The mixture was then washed from the vial with 20 mL of water and placed in the beaker. The mixture was then homogenized with magnetic stirring. The mixture is acidified with 3% (v / v) H2SO4 to pH 2. The mixture was adjusted to reference pH (4.0) with 1 M NaOH and 2.0 mL of 10% (w / v) NF H-HCI solution adjusted to reference pH with 1 M NaOH was added. Finally, it was left to react for 10 minutes where formaldehyde reacts with hydroxylamine hydrochloride (NH2OH-HCI) forming CH2NOH, water and hydrochloric acid (HCI).

[0096] The back reaction of the hydrochloric acid formed in this reaction was carried out with 1 M NaOH (titrated by a primary standard, potassium hydrogen phosphate, KHC8H4O4) up to reference pH (4.0). With the spent volume of 0.1 N NaOH, the free HCHO in the sample was calculated, where the values ​​were finally extrapolated to moles of HCHO reacted per 100 g of lignin. The results are shown in Figures 5 and 6.

[0097] Resin Preparation

[0098] A reaction system consisting of a 3-necked glass flask with a reflux system and magnetic stirring was used. Phenol was dissolved in distilled water and 2 / 3 of 33% NaOH, then the required amount of lignin was slowly added, all at 45°C. Subsequently, at the beginning of the chemical addition stage, 1 / 3 of the required formaldehyde (32 mL) was added, the reaction was left for 30 minutes, and then 32 mL of formaldehyde was added again and the temperature was raised to 85°C. Finally, the last 1 / 3 of formaldehyde was added and left to react for approximately 90 minutes, ending the synthesis in a final stage without reflux for approximately 1 hour.Five resins were made with 15% replacement of phenol by lignin, using the lignins from experiment 13, where for each resin the following were used: i) organic fraction from experiment 13, ii) the residual fraction from experiment 13, iii) the mixture of the organic and residual fractions from experiment 13, iv) Kraft Domtar lignin without depolymerization (control), and v) control resin of phenol and formaldehyde under the same conditions. The evaluated resins were taken to study their viscosity and adhesiveness in ABES equipment.

[0099] Infrared analysis by FTIR

[0100] Approximately 150 g of phenolic resin were synthesized in an alkaline medium (resol) and a Fourier transform infrared (FTIR) analysis was performed. A Cary 630 FTIR-ATR (Agilent) system was used for this analysis. Samples of recovered and depolymerized lignin were prepared using KBr capsules. The resin samples were prepared by controlled drying at 40°C for 24 hours or until a dry and homogeneous film was obtained. The spectrum was measured in the range of 4000 - 650 cm 1 , with a total of 32 scans at a resolution of 8 cm 1 Figure 1 shows the presence of the main bonds and functional groups in unfractionated and ethyl acetate fractionated DKL.

[0101] ABES adhesiveness

[0102] The adhesiveness of lignin-phenol-formaldehyde resin samples was measured using the ABES methodology, using standardized equipment manufactured by Adhesive Evaluation Systems (AES). Pinus radiata wood specimens measuring 1.17 x 20 x 0.7 mm were used, to which a dosage of 20 mg of adhesive (wet basis) was applied on a 100 mm surface. 2 (20 x 5 mm). The effect of pressing time (30, 60, 90, 120, 150, and 180 seconds) was evaluated at a temperature of 130°C. Adhesiveness is expressed as shear strength (Stress, N / mm) in relation to pressing temperature.

[0103] Total CHNS Content

[0104] An elemental analysis was performed to determine the total CHNS (Carbon, Hydrogen, Nitrogen, and Sulfur) content in samples of each DKL fraction (F1 and F2) obtained from the fractionation. Table 2 shows the results of this analysis. Table 2. Results of the characterization of the lignin fractions by GPC and elemental analysis.

[0105] Viscosity A total of 200 mL of each sample was poured into a 250 mL beaker. The beaker was then heated to a temperature of 25°C in a glycerin bath. After approximately 10 minutes, the viscosity was read using a Brookfield viscometer (Fungilab Smart SMAL 201 103). Measurements were made at 25°C for 5 minutes (the time required for the samples to reach stable viscosity values). Each measurement was made in triplicate. The viscosities obtained for the aforementioned resins are presented in Table 3.

[0106] Table 3. Viscosities of the resins synthesized and evaluated by ABES. The viscosity of the other 3 lignophenolic resins with partial replacement of phenol by depolymerized / activated lignin, partial replacement percentages of phenol were used, 15%, 30% and 50% by residual Kraft lignin obtained from the reaction with TEMPO / Ultrasonics, are shown in Table 4.

[0107] Table 4. Viscosities of the resins synthesized and evaluated by ABES

[0108] Figure 7 shows the adhesiveness results determined using ABES equipment for lignophenolic resins. Figure 8 shows the chemical characterization using FTIR of lignophenolic resins made with partial replacement of phenol with residual lignin obtained from ultrasound-assisted TEMPO depolymerization / activation. Figure 8 shows the chemical characterization using FTIR of the lignophenolic resins described above.

[0109] Thermogrammetric Study (TGA)

[0110] TGA analysis was performed on a TA Instruments thermobalance (model Q50). Resin stability was evaluated between 40°C and 700°C, at a heating rate of 10°C / min and a nitrogen flow rate of 20 mL / min.

[0111] Differential Calorimetry (DSC)

[0112] DSC analysis of the resins was performed on a Mettler Toledo differential scanning calorimetry analyzer (STAR ​​SW 8.10). DSC analysis of all resins was carried out over a temperature range of 30°C to 250°C at a rate of 10°C / min and with a nitrogen flow rate of 20 mL / min.

[0113] The summary of the results of the curing behavior and thermal degradation determined by DSC and TGA of the lignophenolic resins made with 15%, 30% and 50% with depolymerized / activated lignin obtained from the reaction with Tempo and ultrasound compared to the control phenolic resin of phenol-formaldehyde (FF) is described in Table 5.

[0114] Table 5. Curing behavior and thermal degradation of lignophenolic resins.

Claims

CLAIMS 1. Method for preparing depolymerized lignin from Kraft lignin, characterized in that it comprises the steps of: a) oxidizing a mixture of Kraft lignin with the TEMPO reagent (2,2,6,6-tetramethylpipehdine-1-oxyl) in a ratio (w / w) of Kraft lignin to TEMPO reagent in the range of 100:1 to 10:1; in an acidic and oxidizing medium in a polar solvent; b) bubbling air into the mixture from step a); c) subjecting the mixture to physical cavitation in the ultrasound range, in the interval between 95,000 J and 380,000 J until the reaction is complete; d) washing the residue obtained in c) with an organic solvent, to extract the soluble organic fraction of depolymerized lignin, and drying; and optionally e) filtering the insoluble fraction obtained in step d) to recover the residual insoluble fraction of depolymerized lignin.

2. The method of claim 1 wherein the acidic and oxidizing medium in a polar solvent comprises nitric acid in acetonitrile.

3. The method of claim 1 to 2 wherein the acidic and oxidizing medium in a polar solvent comprises hydrochloric acid in acetonitrile.

4. The method of claim 1 wherein the organic solvent of step d) is selected from: 4-methyl-2-pentanone (MIBK), or ethyl acetate, petroleum ether, dichloromethane, diethyl ether, hexane and toluene.

5. Soluble organic fraction of depolymerized lignin obtained by the method according to claims 1 to 4 in step d).

6. Insoluble residual fraction of depolymerized lignin obtained by the method according to claims 1 to 4 in step e).

7. Use of the fraction defined in claim 5 and / or the fraction defined in claim 6 in phenolic resins as phenol substituents.

8. Phenolic adhesive resin comprising the organic lignin fraction defined in claim 5 in a percentage between 15% and 50% by weight with respect to the weight of phenol in the resin.

9. Phenolic adhesive resin comprising the insoluble residual fraction of lignin defined in claim 6 or a mixture of the organic fraction defined in claim 5 and the residual fraction defined in claim 6, in a percentage between 15% and 50% by weight with respect to the weight of phenol in the resin.

10. Method for preparing a lignophenolic adhesive resin, comprising replacing 15-50% by weight of the phenol in the phenolic resin with the depolymerized organic lignin fraction defined in claim 5. 1 1. Method for preparing a lignophenolic adhesive resin, comprising replacing 15-50% by weight of the phenol in the phenolic resin with the insoluble residual fraction of depolymerized lignin defined in claim 6.

12. Method of preparing a lignophenolic adhesive resin, comprising replacing 15-50% by weight of the phenol in the phenolic resin with a combination of the organic fraction of depolymerized lignin defined in claim 5 and the insoluble residual fraction of depolymerized lignin defined in claim 6.

13. Use of the lignophenolic adhesive resin defined in claim 8 or 9, as an adhesive for plywood panels, particle boards, chipboards and pine wood veneers.

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