Self-polymerized kraft lignin, method for producing them and their uses

Abstract

It is provided a process of preparing self-polymerized kraft lignin comprising the steps of thermally treating kraft lignin producing self-polymerized lignin; and isolating the self-polymerized lignin. Also provided is the use of self-polymerized kraft lignin as an adhesive for wood panels.

Description

SELF-POLYMERIZED KRAFT LIGNIN, METHOD FORPRODUCING THEM AND THEIR USESCROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is claiming priority from U.S. Provisional Application No. 63 / 512,354 filed July 7, 2023, the content of which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] It is provided a process of preparing self-polymerized kraft lignin and the use of same as an adhesive.BACKGROUND

[0003] Black liquor is a high pH (ca. 13-14) aqueous by-product of the kraft pulping process primarily composed of degraded lignin, organic acids, and inorganic salts. The lignin present in black liquor can be precipitated with acids and separated by filtration or centrifugation. Kraft lignins can be efficiently produced in this manner at industrial scale using the LignoForce™, LignoBoost™ and Westvaco processes.

[0004] In these processes, black liquor acidification is performed by using either carbon dioxide or a mineral acid (e.g. sulfuric acid) or a combination of the two to drop the pH of the black liquor from about 13-14 to about 10. At this point, lignin precipitates out of solution in colloidal form. The lignin suspension is then filtered to produce a lignin cake of high residual content hereby referred to as high residual content (HRC) lignin which is, subsequently, washed with acid (e.g. sulfuric acid) and water to produce a purified lignin hereby referred to as low residual content (LRC) lignin.

[0005] Kraft lignin are cheap, abundant, green, and renewable materials. The physicochemical properties of kraft lignins are however rarely suited for use in commercial applications, being outperformed by conventional petroleum-sourced chemicals and materials.

[0006] It has been reported that fractions of kraft lignin with high molar mass improves the adhesion properties by reducing curing time. These high molar mass fractions are obtained by fractionation and not by self-polymerization.

[0007] The self-polymerization of kraft lignin has been observed and is considered a problem to avoid.

[0008] It is thus highly desired to be provided with new means to produce kraft lignins with tailored properties in order to be used as an adhesive.SUMMARY

[0009] It is provided a process of preparing self-polymerized kraft lignin comprising the steps of thermally treating an acid form of kraft lignin producing self-polymerized lignin; and isolating the self-polymerized lignin.

[0010] In an embodiment, the kraft lignin is a powder or in an aqueous suspension.

[0011] In another embodiment, the kraft lignin comprises 0-20% water of water.

[0012] In a further embodiment, the kraft lignin comprises >20% water.

[0013] In another embodiment, an acid is added to the kraft lignin prior to thermally treating the kraft lignin.

[0014] In a further embodiment, the acid is HCI, H3PO4, NaHSO4, or H2SO4.

[0015] In an embodiment, the process encompassed further comprises the step of dissolving the self-polymerized lignin in an aqueous sodium hydroxide solution.

[0016] In an embodiment, the kraft lignin is at a pH between 3.5 and 8.8.

[0017] In a further embodiment, the kraft lignin is thermally treated in stainless- steel rotating autoclave, a rotary digester or in a glass flask.

[0018] In an embodiment, the kraft lignin is thermally treated at a temperature of between 120 °C and 200 °C.

[0019] In a further embodiment, the self-polymerized kraft lignin has an average molar masse of about 5,000 g / mol up to 1 ,000,000 g / mol.

[0020] In another embodiment, wherein the self-polymerized kraft lignin has an intrinsic viscosity of about 6 mL / g up to more than 20 mL / g.

[0021] In an embodiment, the self-polymerized kraft lignin solution has a complex viscosity from 0.5 Pa-s to more than 1 ,000 Pa-s.

[0022] In another embodiment, the kraft lignin produced is powders or an aqueous suspension.

[0023] Also provided is an adhesive comprising self-polymerized kraft lignin produced by the process as defined herein.

[0024] In an embodiment, the adhesive described herein further comprises a phenol-formaldehyde resin.

[0025] In an embodiment, the self-polymerized kraft lignin is admixed with LiOH, KOH, NH3, or NaOH.

[0026] In another embodiment, the adhesive comprises up to 20% w / w of the selfpolymerized kraft lignin.

[0027] It is also provided wood panels bonded with the adhesive provided herewith.

[0028] In an embodiment, the panels are plywood panels.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Reference will now be made to the accompanying drawings.

[0030] Fig. 1 illustrates a mass average molar mass (Mw) of LRC kraft lignin (4.6% water content) heated to 130-150 °C for 0-140 min with as determined by SEC-MALS.

[0031] Fig. 2 illustrates a mass average molar mass (Mw) of LRC kraft lignin (4.6% water content) heated to 130-150 °C for 0-140 min with as determined by SEC-RI.

[0032] Fig. 3 illustrates a Mark-Houwink-Sakurada plot: intrinsic viscosity ([q]) versus mass average molar mass (Mw) in log-log scale of self-polymerized LRC kraft lignins (4.6% water content heated to 130-150 °C for 0-140 min) as determined by SEC-MALS-RI-IV.

[0033] Fig. 4 illustrates a mass average molar mass (Mw) of LRC kraft lignin heated to 100 °C for 0-100 min in the presence of aqueous sulfuric acid as determined by SEC-MALS-RI.

[0034] Fig. 5 illustrates a Mark-Houwink-Sakurada plot: intrinsic viscosity ([q]) versus mass average molar mass (Mw) in log-log scale of self-polymerized LRC kraft lignins (heated to 100 °C for 0-100 min in the presence of aqueous sulfuric acid) as determined by SEC-MALS-RI-IV.

[0035] Fig. 6 illustrates a mass average molar mass (Mw) of HRC kraft lignin heated to 100 °C for 0-80 min in the presence of aqueous sulfuric acid as determined by SEC-MALS-RI.

[0036] Fig. 7 illustrates a Mark-Houwink-Sakurada plot: intrinsic viscosity ([q]) versus mass average molar mass (Mw) in log-log scale of self-polymerized HRC kraft lignins (heated to 100 °C for 0-80 min in the presence of aqueous sulfuric acid) as determined by SEC-MALS-RI-IV.

[0037] Fig. 8 illustrates a complex viscosity (q*) at 0.25 Hz of aqueous alkaline solutions of self-polymerized LRC kraft lignins as a function of mass average molar mass (Mw) as determined by SEC-MALS-RI.

[0038] Fig. 9 illustrates a complex viscosity (q*) at 0.25 Hz of aqueous alkaline solutions of self-polymerized HRC kraft lignins as a function of mass average molar mass (Mw) as determined by SEC-MALS-RI.

[0039] Fig. 10 illustrates ABES shear strength (pressed at 130 °C and 150 °C for 90 s) measured for adhesive solutions of self-polymerized kraft lignins (ADSOL).

[0040] Fig. 11 illustrates ABES shear strength (pressed at 110 °C, 130 °C and 150 °C for 90 s) measured for commercial PF resin (PF-01 ) and 30 / 70 mixtures of adhesive solutions of self-polymerized kraft lignins (ADSOL).

[0041] Fig. 12 illustrates ABES shear strength (pressed at 110 °C for 90 s) measured for commercial PF resin (PF-02) and 30 / 70 mixtures of adhesive solutions of self-polymerized kraft lignins (ADSOL).DETAILED DESCRIPTION

[0042] In accordance with the present description, there is provided a process of preparing self-polymerized kraft lignin and the use of same as an adhesive.

[0043] It is thus described the self-polymerization of kraft lignin and process to prepare high viscosity lignins that have desirable properties for applications such as adhesives.

[0044] Accordingly, as described herein, the process of preparing modified lignin products as encompassed herein comprises thermally treating kraft lignin either as powders (0-20% water content) or as aqueous suspensions (>20% water content) producing self-polymerized lignin with controllable molar mass distribution and rheological properties upon cooling.

[0045] As encompassed herein, self-polymerized lignin with controllable molar mass distribution means yields kraft lignins with average molar masses of about 5000 g / mol up to more than 1 ,000,000 g / mol including all intermediate values.

[0046] For controllable rheological properties, it is intended to encompass intrinsic viscosity of about 6 mL / g up to more than 20 mL / g including all intermediate values, including complex viscosity solutions from 0.5 Pa.s to more than 1000 Pa.s including all intermediate values for the same concentration of lignin.

[0047] In an embodiment, an acid is used to accelerate the self-polymerization process.

[0048] In an embodiment, the self-polymerized lignin is dissolved in aqueous sodium hydroxide to yield aqueous lignin solutions of adjustable viscosity.

[0049] In a further embodiment, the aqueous self-polymerized lignin is used as an adhesive either as sole component or as part of a formulation.

[0050] In a further embodiment, the aqueous self-polymerized lignin is mixed with phenol-formaldehyde resin and used as an adhesive either as sole component or as part of a formulation.

[0051] The self-polymerization of kraft lignin alters the properties of kraft lignins to the point where they have far more desirable properties for applications such as adhesives compared to unmodified kraft lignins. The two main properties that are altered by the self-polymerization process is the molar mass distribution and the viscosity. The self-polymerization can be achieved simply by thermally treating kraft lignins at 130-180 °C for definite periods of time and can be accelerated in the presence of acids (e.g. sulfuric acid). In an embodiment, it is encompassed thermallytreating kraft lignins at a temperature between 120 °C and 200 °C. The selfpolymerized kraft lignins can then be used to prepare alkaline adhesive solutions that have improved bonding capabilities compared to unmodified kraft lignins. While it has been observed in the past that kraft lignins with relatively high molar mass perform better as adhesive than those of relatively low molar mass, these kraft lignins have been produced exclusively through fractionation and average molar mass were limited to 50,000 g / mol. Fractionation requires solvents, chemicals or complex filtration apparatus and ends up using only a fraction of the original kraft lignin. In the present process, the whole mass of kraft lignin is used and average molar masses above 500,000 g / mol can easily be produced.

[0052] The process described herein is both simple and cheap to implement. The bonding properties of kraft lignins is greatly improved for use as sole adhesive or as part of formulations. The self-polymerized kraft lignins can be used as thickening agents in aqueous alkaline media.EXAMPLE 1 Determination of the molar mass distribution and of the intrinsic viscosity of kraft lignins

[0053] Samples of lignins were dissolved in LiBr / DMSO (0.5 % w / v) at 50 °C for 2 h at concentrations of 2-5 mg / mL. Once cooled to ambient temperature, the resulting solutions were filtered through 0.2 pm PTFE filters prior to analysis. Size-exclusion chromatography (SEC) was performed using an Agilent Model 1200 high performance liquid chromatography system equipped with a Wyatt Dawn® multi-angle light scattering detector (MALS) with a laser operating at 785 nm, a Wyatt Optilab® differential refractive index detector (Rl), and a Wyatt ViscoStar® on-line differential viscometer (IV). The separation was performed with two columns in series: one PolarGel M column (7.8 mm x 300 mm, 8 pm) followed by one PolarGel L columns (7.8 mm x 300 mm, 8 pm). The columns were kept at 35 °C. The SEC system was operated under the following conditions: LiBr / DMSO (0.5 % w / v) eluent, 0.5 mL / min flow rate; 50 pL injection volume; 65 min run time. A set of 10 pullulan polymers standards (Mp= 710, 343, 202, 116, 50.6, 23.0, 9.90, 6.00, 1.26 and 0.342 kg / mol) was used to construct a conventional narrow standard calibration curve for relative molar mass determination.EXAMPLE 2Preparation of self-polymerized kraft lignin (dry process)

[0054] 100 g of LRC kraft lignin with a water content of 4.6% w / w and a pH of 3.5(measured at 2% w / w aqueous suspension) was charged in a stainless-steel rotating autoclave. The headspace of the vessel was purged with nitrogen, and the vessel was hermetically sealed. The vessel was heated while rotating in an oil bath maintained at 130 °C for 20 min. After cooling to ambient temperature, the self-polymerized lignin was isolated as a dry solid. Analogous experiments were carried out at temperatures of 130-150 °C and heating periods of 40-120 min. The properties of the self-polymerized kraft lignins are listed in Table 1 and further illustrated in Figs. 1-3.Table 1 : Molar mass distribution and intrinsic viscosity of self-polymerized LRC kraft lignin (4.6% water content, pH = 3.5) at 130-150 °C for 20-120 min (dry process) as determined by SEC-MALS-RI-IV.EXAMPLE 3Preparation of self-polymerized kraft lignin (aqueous process)

[0055] 158 g of LRC kraft lignin with a water content of 63.4% w / w and a pH of 3.9(measured at 2% w / w aqueous suspension) and 900 mL deionized water were charged in a stainless-steel rotating autoclave. The headspace of the vessel was purged with nitrogen, and the vessel was hermetically sealed. The vessel was heated while rotating in an oil bath maintained at 130 °C for 20 min. After cooling to ambient temperature, the resulting suspension was filtered, and the lignin was air-dried. Analogousexperiments were done with heating periods of 40-240 min. The properties of the selfpolymerized kraft lignins are listed in Table 2.Table 2: Molar mass distribution and intrinsic viscosity of self-polymerized LRC kraft lignin (90.5% water content, pH = 3.9) at 130 °C for 20-240 min (aqueous process) as determined by SEC-MALS-RI-IV.EXAMPLE 4Preparation of self-polymerized kraft lignin (steam process)

[0056] 100 g of LRC kraft lignin with a water content of 66.0% w / w and a pH of 3.5(measured at 2% w / w aqueous suspension) was charged in a rotary digester. The lignin was steamed at 130 °C for 60 min. After cooling to ambient temperature, the lignin was air-dried. The properties of the self-polymerized kraft lignins are listed in Table 3.Table 3: Molar mass distribution and intrinsic viscosity of self-polymerized LRC kraft lignin (66.0% water content, pH = 3.5) at 130 °C for 60 min (steam process) as determined by SEC-MALS-RI-IV.EXAMPLE 5Preparation of self-polymerized kraft lignin (aqueous acid process)

[0057] 5 g of LRC softwood kraft lignin with a water content of 4.6% w / w and a pH of 3.8 (measured at 2% w / w aqueous suspension) and 90 ml_ deionized water were charged in a glass flask. The resulting suspension was magnetically stirred, and the flask was heated in an oil bath until the internal temperature reached 100 °C then 10 ml_ of 5.0 M H2SO4 was added. The mixture was kept at 100 °C for 10 min. After cooling to ambient temperature, the resulting suspension was filtered. The solid lignin residue was thoroughly washed with water and air-dried. Analogous experiments were done with heating periods of 20-100 min. The properties of the self-polymerized kraft lignins are listed in Table 4 and further illustrated in Figs. 4 and 5.Table 4: Molar mass distribution and intrinsic viscosity of self-polymerized LRC kraft lignin (90.5% water content, pH = 3.8) at 100 °C for 10-100 min (aqueous acid process) as determined by SEC-MALS-RI-IV.EXAMPLE 6 Self-polymerization of HRC kraft lignin (dry process)

[0058] 118.7 g of HRC kraft lignin with a water content of 15.7% w / w and a pH of8.8 (measured at 2% w / w aqueous suspension) was charged in a stainless-steel rotating autoclave. The headspace of the vessel was purged with nitrogen, and the vessel was hermetically sealed. The vessel was heated while rotating in an oil bath maintained at 130-160 °C for 20-240 min. No significant changes in molar mass distribution or viscosity were observed.EXAMPLE 7Self-polymerization of HRC kraft lignin (aqueous acid process)

[0059] 118.7 g of HRC kraft lignin with a water content of 15.7% w / w and a pH of8.8 (measured at 2% w / w aqueous suspension) was suspended in 1800 mL of distilledwater in a 2-L glass reactor. The dark brown mixtures were placed into an oil bath (120 °C) and heated to an internal temperature of 100 °C. Then, 65 ml_ of 3 M H2SO4 was added dropwise. The reaction mixture was refluxed for 10 min, and then quickly cooled in ice baths to about 20 °C. The reaction mixture was filtered, and the solid was thoroughly washed with water. Analogous experiments were done with heating periods of 20-80 min. The properties of the self-polymerized kraft lignins are listed in Table 5 and further illustrated in Figs. 6 and 7.Table 5: Molar mass distribution and intrinsic viscosity of self-polymerized HRC kraft lignin in the presence of sulfuric acid at 100 °C for 10-100 min (aqueous acid process) as determined by SEC-MALS-RI-IV.EXAMPLE 8Viscoelastic behaviour of self-polymerized LRC kraft lignins (dry process)

[0060] Concentrated solutions of self-polymerized LRC kraft lignins (dry process) were prepared in the following way: 3.97 g of self-polymerized LRC kraft lignin was slowly added over 60 min to a mechanically stirred solution of 1.03 g NaOH in 3.56 g deionized water at 20 °C. The resulting dark brown solution was further stirred for 60 min at 20 °C.

[0061] Small amplitude oscillatory testing was done using the following parameters: Temperature of measurement = 20.0 °C;Gap between the plates = 0.2 mm;Plate diameter = 35 mm;Oscillation frequency = 0.01 Hz to 100 Hz; andConstant stress of 10 Pa.

[0062] The properties of the alkaline solutions of self-polymerized LRC kraft lignins are listed in Table 6 and further illustrated in Fig. 8.Table 6: Molar mass distribution, complex viscosity (q*), storage modulus (G’), loss modulus (G”) and loss tangent (tan<5) at 0.25 Hz of alkaline solutions of selfpolymerized LRC kraft lignin (dry process) as determined by SEC-MALS-RI and small amplitude oscillatory testing.EXAMPLE 9Viscoelastic behaviour of self-polymerized HRC kraft lignins (aqueous acid process)

[0063] Concentrated solutions of self-polymerized HRC kraft lignins (aqueous acid process) were prepared in the following way: 3.97 g of self-polymerized HRC kraft lignin was slowly added over 60 min to a mechanically stirred solution of 1.03 g NaOH in 3.56 g deionized water at 20 °C. The resulting dark brown solution was further stirred for 60 min at 20 °C.

[0064] Small amplitude oscillatory testing was done using the following parameters: Temperature of measurement = 20.0 °C;Gap between the plates = 0.2 mm;Plate diameter = 35 mm;Oscillation frequency = 0.01 Hz to 100 Hz; andConstant stress of 10 Pa.

[0065] The properties of the alkaline solutions of self-polymerized LRC kraft lignins are listed in Table 7 and further illustrated in Fig. 9.Table 7: Molar mass distribution, complex viscosity (q*), storage modulus (G’), loss modulus (G”) and loss tangent (tan5) at 0.25 Hz of alkaline solutions of self-polymerized HRC kraft lignin (aqueous acid process) as determined by SEC-MALS-RI and small amplitude oscillatory testing.EXAMPLE 10 Self-polymerized kraft lignin adhesives

[0066] Adhesive solutions (40% solids) of self-polymerized LRC kraft lignins (dry process) were prepared in the following manner.

[0067] 20 g of self-polymerized LRC kraft lignin was dispersed in 30 g of deionized water with mechanical stirring, then 10 g of 40% NaOH solution was added slowly over 20 min at 20 °C. The resulting mixture was continuously stirred until a uniform solution formed.

[0068] The adhesive properties of the solutions of self-polymerized LRC kraft lignins were evaluated using an automated bond evaluation system (ABES). This system is pressing two veneers (hard maple wood of 20 mm * 117 mm * 0.6 mm) together with adhesive on a bonding area of 20 mm * 5 mm, cooling, and pulling them automatically. The following parameters were used:Adhesive solution per pull: 4.0 mgPlate temperature: 130 °C or 150 °CPress time: 90 sPull speed: 500 N / s

[0069] The physicochemical properties of the self-polymerized kraft lignins solutions and their bonding properties are listed in Table 8 and further illustrated in Fig. 10. The values given are the average of 5 samples evaluated.Table 8: Molar mass distribution, Brookfield viscosity (q), and shear strength of alkaline solutions of self-polymerized LRC kraft lignin as determined by SEC-MALS-RI, rheometry, and automated bond evaluation system (ABES).EXAMPLE 11Self-polymerized kraft lignin adhesives (PF replacement)

[0070] Adhesive solutions of self-polymerized LRC or HRC kraft lignins (dry process) were prepared in the following way.

[0071] 20 g of self-polymerized LRC or HRC kraft lignin was dispersed in 30 g of deionized water with mechanical stirring, then 10 g of 40% NaOH solution was added slowly over 20 min at 20 °C. The resulting mixture was continuously stirred until a uniform solution formed. Self-polymerized kraft lignin-PF glue mixes were produced by adding a calculated amount of lignin solution to a calculated amount of PF resin at designed ratio on solids basis, with mechanical stirring at room temperature until homogenous.

[0072] The adhesive properties of the mixtures of self-polymerized LRC or HRC kraft lignins with PF resin were evaluated using an automated bond evaluation system (ABES). This system is pressing two veneers (hard maple wood of 20 mm * 117 mm * 0.6 mm) together with adhesive on a bonding area of 20 mm * 5 mm, cooling, and pulling them automatically. The following parameters were used:Adhesive solution per pull: 4.0 mgPlate temperature: 110 °C, 130 °C or 150 °CPress time: 90 sPull speed: 500 N / s

[0073] The physicochemical properties of the self-polymerized kraft lignins solutions and the bonding properties of their mixtures with commercial PF resin arelisted in T able 9 and further illustrated in Fig. 11. The values given are the average of 5 samples evaluated.Table 9: Molar mass distribution, Brookfield viscosity (q), and shear strength of alkaline solutions of self-polymerized LRC kraft lignin as determined by SEC-MALS-RI, rheometry, and automated bond evaluation system (ABES).a Properties of the ADSOL solutions prior to mixing with PF-01 . b The self-polymerized kraft lignin used to prepare ADSOL-03 was not fully soluble in the SEC eluent.EXAMPLE 12Self-polymerized kraft lignin adhesives (PF replacement)

[0074] Adhesive solutions of self-polymerized LRC or HRC kraft lignins (dry process) were prepared in the following way.

[0075] 20 g of self-polymerized LRC or HRC kraft lignin was dispersed in 30 g of deionized water with mechanical stirring, then 10 g of 40% NaOH solution was added slowly over 20 min at 60 °C. The resulting mixture was continuously stirred until a uniform solution formed. Self-polymerized kraft lignin-PF glue mixes were produced by adding a calculated amount of lignin solution to a calculated amount of PF resin at designed ratio on solids basis, with mechanical stirring at room temperature until homogenous.

[0076] The adhesive properties of the mixtures of self-polymerized LRC or HRC kraft lignins with PF resin were evaluated using an automated bond evaluation system (ABES). This system is pressing two veneers (hard maple wood of 20 mm * 117 mm * 0.6 mm) together with adhesive on a bonding area of 20 mm * 5 mm, cooling, and pulling them automatically. The following parameters were used:Adhesive solution per pull: 4.0 mgPlate temperature: 110 °C Press time: 90 sPull speed: 500 N / s

[0077] The physicochemical properties of the self-polymerized kraft lignins solutions and the bonding properties of their mixtures with commercial PF resin are listed in Table 10 and further illustrated in Fig. 12. The values given are the average of 5 samples evaluated.Table 10: Molar mass distribution, Brookfield viscosity (q), and shear strength of mixtures of alkaline solutions of self-polymerized LRC or HRC kraft lignin and commercial PF resin as determined by SEC-MALS-RI, rheometry, and automated bond evaluation system (ABES).a Properties of the ADSOL solutions prior to mixing with PF-02.EXAMPLE 13Evaluation of self-polymerized kraft lignin as a replacement of PF resin in softwood plywood glue mix

[0078] Solutions of LRC kraft lignin and of self-polymerized LRC kraft lignins (dry process) were first prepared in the following way.

[0079] 310 g of LRC kraft lignin or of self-polymerized LRC kraft lignin was dispersed in 440 g of deionized water with mechanical stirring, then 150 g of 40% NaOH solution was added slowly over 20 min at 25 °C. The resulting mixture was continuously stirred until a uniform solution formed.

[0080] Three glue mixes at 0 and 20% (w / w) PF replacement levels with solutions of kraft lignin were prepared. The composition of these mixtures is given in Table 11. As shown in this table, all glue mixes had viscosities suitable for plywood gluing through a roll-spreader (glue applicator).Table 11 : Composition and properties of plywood glue mixtures in which 0-20% of the PF resin was replaced by solutions of either LRC kraft lignin or self-polymerized LRC kraft lignin.a Mw(SEC-RI) = 8,160 g / mol.bMw(SEC-RI) = 39,000 g / mol.

[0081] A total of 12 plywood panels (3-ply, 24" x 24") were produced using 1 / 8" thick Douglas-fir veneers, of which 4 panels each were bonded, respectively, with glue mixes in which 0 and 20% (w / w) of PF resin was replaced by lignin solutions. The plywood panels were manufactured according to the parameters listed in Table 12.Table 12: Conditions used for 3-ply plywood making.

[0082] The resulting panels were cut in accordance with the CSA standards. Ten test specimens were tested after subjecting them to the vacuum / pressure treatment. The vacuum / pressure test was performed after treatment in water at 21 °C and a vacuum of -25 inHg for 0.5 hour, followed by 5 atm (75 psi) pressure for 0.5 hour. The resulting samples were then tested for shear strength and wood failure rate.

[0083] The average test results of shear strength and wood failure rate are summarized in Table 13. Based on the data in Table 13, replacing PF resin with LRC lignin in the glue mixes resulted in reduced shear strength and wood failure, while replacing PF resin with LRC lignin in the glue mixes resulted in reduced shear strength but increased wood failure.Table 13: Composition and properties of plywood glue mixtures in which 0-20% of the PF resin was replaced by solutions of either LRC kraft lignin or self-polymerized LRC kraft lignin.a Standard deviation given in parentheses.

[0084] While the disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations and including such departures from the present disclosure as come within known or customary practice within the art and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A process of preparing self-polymerized kraft lignin comprising the steps of: a) thermally treating an acid form of kraft lignin producing self-polymerized lignin; and b) isolating the self-polymerized lignin.

2. The process of claim 1 , wherein the kraft lignin is a powder or in an aqueous suspension.

3. The process of claim 1 or 2, wherein the kraft lignin comprises 0-20% water of water.

4. The process of claim 1 or 2, wherein the kraft lignin comprises >20% water.

5. The process of any one of claims 1-4, wherein an acid is added to the kraft lignin prior to thermally treating the kraft lignin.

6. The process of claim 5, wherein the acid is HCI, H3PO4, NaHSC , or H2SO4.

7. The process of any one of claims 1-6, further comprising the step of dissolving the self-polymerized lignin in an aqueous sodium hydroxide solution.

8. The process of any one of claims 1-7, wherein the kraft lignin is at a pH between 3.5 and 8.8.

9. The process of any one of claims 1-8, wherein the kraft lignin is thermally treated in stainless-steel rotating autoclave, a rotary digester or in a glass flask.

10. The process of claim 9, wherein the kraft lignin is thermally treated at a temperature of between 120 °C and 200 °C.

11. The process of any one of claims 1-10, wherein the self-polymerized kraft lignin has an average molar mass of about 5,000 g / mol up to 1 ,000,000 g / mol.

12. The process of any one of claims 1-11 , wherein the self-polymerized kraft lignin has an intrinsic viscosity of about 6 mL / g up to more than 20 mL / g.

13. The process of any one of claims 1-12, wherein the self-polymerized kraft lignin solution has a complex viscosity from 0.5 Pa-s to more than 1000 Pa-s.

14. The process of any one of claims 1-13, wherein the kraft lignin produced is powders or an aqueous suspension.

15. An adhesive comprising self-polymerized kraft lignin produced by the process defined in any one of claims 1-14.

16. The adhesive of claim 15, further comprising a phenol-formaldehyde resin.

17. The adhesive of claim 15 or 16, wherein the self-polymerized kraft lignin is admixed with LiOH, KOH, NH3, or NaOH.

18. The adhesive of any one of claims 15-17, comprising up to 20% w / w of the selfpolymerized kraft lignin.

19. A wood panel bonded with the adhesive of any one of claims 15-17.

20. The panel of claim 19, wherein said panels are plywood panels.

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