Method for preparing lignin formaldehyde-free adhesive and co-producing fermentable sugar from wheat straw

Abstract

The invention discloses a method for preparing a lignin formaldehyde-free adhesive and co-producing fermentable sugar from wheat straw, and belongs to the technical field of adhesives. The preparation method comprises the following steps: pre-impregnating wheat straw at low temperature by adopting sodium hydroxide, pre-treating the impregnated wheat straw through a disc mill machine, separating fermentable sugar and lignin through enzymolysis reaction, and mixing the lignin with water to prepare the lignin formaldehyde-free adhesive. According to the invention, the lignin prepared by enzyme hydrolysis of a material obtained by low-temperature sodium hydroxide preimpregnation combined with disc milling pretreatment is used as a formaldehyde-free adhesive to bond wood for the first time; the lignin formaldehyde-free adhesive prepared by the invention can realize high-strength bonding of wood boards, the dry strength of three layers of poplar veneers after bonding can reach 0.8-1.9 MPa, the class II wet strength can reach 0.6-1.2 MPa, the national standard 0.7 MPa requirement is met, and the bonding strength of plywood prepared under partial hot pressing conditions is far higher than that of plywood bonded by industrial urea formaldehyde and phenolic resin glue.

Description

Technical Field

[0001] The present invention belongs to the technical field of adhesives, and more specifically, relates to a method for preparing a lignin-based formaldehyde-free adhesive and co-producing fermentable sugars by utilizing wheat straw. Background Art

[0002] Lignocellulosic feedstocks are primarily composed of carbohydrates (cellulose and hemicellulose) and lignin. Lignin acts as a binder, tightly binding the three components and providing the lignocellulosic feedstock with a degree of resistance to external attack. Pretreatment effectively breaks down the protective effect of lignin on the carbohydrates in the lignocellulosic feedstock, separating them from the carbohydrates and facilitating the high-value utilization of each component.

[0003] Pretreatment methods can be divided into physical methods (ball milling, crushing, steam explosion, etc.), chemical methods (dilute acid, organic solvents, ionic liquids, etc.) and biological methods (cellulase, laccase, etc.). Conventional physical and biological methods have problems such as low pretreatment efficiency and high cost, making them difficult to produce on a large scale. Some chemical pretreatment methods such as ionic liquids and organic solvents have a series of problems such as environmental pollution and high cost. Most of the above methods remain in the laboratory stage and have not been able to achieve industrial application. The sodium hydroxide pre-impregnation combined with disc grinding mechanical pretreatment process is highly consistent with the existing alkaline hydrogen peroxide hot mill chemical mechanical pulp (APMP) production process. The production line can be quickly constructed by appropriately adjusting the production process flow, with low construction cost and stable large-scale production. Unlike traditional chemical pulp production, the pulp prepared by this method does not require bleaching, so there is no need to add hydrogen peroxide, which has the advantages of less investment and higher lignin retention rate.

[0004] The production of phenolic resins using lignin as a phenol substitute typically requires modification, but this modification is costly and the resulting wood adhesives struggle to meet market demand. Furthermore, while partial lignin substitution for phenol can effectively reduce fossil fuel use, the risk of carcinogenic formaldehyde remains. In recent years, research on soy protein adhesives for plywood production has been extensive, demonstrating that plywood produced with soy protein adhesives can meet national standards. However, the shortcomings of soy protein adhesives, such as susceptibility to mildew and low wet strength, have severely limited their development.

[0005] Currently, wood adhesives are primarily based on phenolic and urea-formaldehyde adhesives. These adhesives are primarily produced through the polycondensation of formaldehyde with its corresponding monomers, resulting in a heavy reliance on fossil resources. Furthermore, the resulting boards release free formaldehyde, which is hazardous to human health. While there has been considerable research into using lignin as a partial replacement for phenol in the production of wood adhesives, lignin suffers from the disadvantages of high molecular weight and polydispersity, resulting in a limited number of reactive sites for condensation with formaldehyde and difficulty in reacting due to steric hindrance. Consequently, the resulting lignin-based phenolic resins suffer from drawbacks such as high curing temperatures and high viscosity.

[0006] Patent CN117467388A discloses a fully bio-based formaldehyde-free adhesive and its preparation method. The preparation process is as follows: S1. The biomass extract is evaporated and concentrated, followed by a dehydration reaction; S2. An appropriate amount of alkali is added to the reaction system, and the in-situ generated bio-based aldehyde substances and lignin undergo a cross-linking reaction under the catalytic action of sodium hydroxide to obtain a formaldehyde-free adhesive. However, both steps require the use of a catalyst and high-temperature heating conditions. Summary of the Invention

[0007] In response to the aforementioned problems in the prior art, the present invention aims to provide a method for preparing a lignin-based formaldehyde-free adhesive using wheat straw and co-producing fermentable sugars. This method utilizes a wide range of raw materials, is simple to prepare, and achieves a higher lignin retention rate. Another technical problem addressed by the present invention is to provide a lignin-based formaldehyde-free adhesive prepared by the above method that meets national standards and is non-toxic and environmentally friendly.

[0008] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0009] A method for preparing a lignin-formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw comprises the following steps: pre-impregnating the wheat straw with sodium hydroxide at low temperature, mechanically pre-treating the impregnated wheat straw with a disc mill, separating the fermentable sugars and lignin through an enzymatic hydrolysis reaction, and mixing the lignin with water to produce the lignin-formaldehyde-free adhesive.

[0010] Preferably, the mass ratio of the wheat straw to sodium hydroxide is 10:1, the temperature of the low-temperature pre-impregnation is 40° C., and the pre-impregnation time is 1 hour.

[0011] Preferably, the number of disc grinding pretreatments is 3 to 11 times, and the disc grinding gap is gradually shortened from 1 mm to 0.1 to 0.8 mm as the number of disc grindings increases.

[0012] More preferably, the number of disc grinding pretreatments is 5 to 9 times, and the disc grinding gap is gradually shortened from 1 mm to 0.1 to 0.6 mm as the number of disc grindings increases.

[0013] Preferably, the enzymatic hydrolysis reaction process is: enzymatically hydrolyzing the pretreated wheat straw with a solid content of 5 wt%, adding cellulase and xylanase, and performing hydrolysis in a constant temperature shaker at 50°C.

[0014] Preferably, the dosage of the cellulase is 20 FPU / g, the dosage of the xylanase is 150 U / g xylan, the substrate concentration is 5 wt%, and the enzymatic hydrolysis is carried out in a constant temperature shaker at 50°C / 150 rpm for 72 hours.

[0015] Preferably, the mass ratio of lignin to water is 1:4.

[0016] The lignin formaldehyde-free adhesive is prepared by the method of preparing the lignin formaldehyde-free adhesive by utilizing wheat straw and co-producing fermentable sugar.

[0017] Application of the lignin formaldehyde-free adhesive in the preparation of plywood.

[0018] The application is to use lignin formaldehyde-free adhesive to bond poplar veneer, with a single-sided adhesive amount of 50g / m 2 After hot pressing curing, the hot pressing pressure is 1MPa, the hot pressing curing temperature is 150-190℃, and the hot pressing time is 5-20min to obtain plywood.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1) The present invention achieves effective dissociation of the wood fiber raw material through improved low-temperature pre-impregnation and combined sodium hydroxide disc grinding pretreatment. After subsequent enzymatic hydrolysis and saccharification of the pretreated wheat straw, the carbohydrates in the straw can be largely converted into corresponding fermentable monosaccharides. The resulting lignin can be used as a bio-based green formaldehyde-free adhesive for the production of plywood.

[0021] 2) The preparation process of the present invention is compatible with existing chemical-mechanical pulp production lines and can simultaneously produce fermentable sugars and green lignin-based formaldehyde-free adhesives, paving a new path for the high-value utilization of agricultural wheat straw waste and the development of environmentally friendly formaldehyde-free wood adhesives.

[0022] 3) Compared with the traditional alkaline hydrogen peroxide chemimechanical pulping process, the present invention does not add hydrogen peroxide, has a lower pre-impregnation temperature, a higher lignin retention rate, and less degradation and removal of wheat straw components. After pretreatment, both the carbohydrates (hemicellulose, cellulose) and lignin in the raw material can be efficiently utilized. The present invention proposes for the first time the use of low-temperature sodium hydroxide pre-impregnation combined with disc milling pretreatment, followed by enzymatic hydrolysis of the material, to prepare lignin as a formaldehyde-free adhesive for bonding wood. This method can provide a new method for the production of fermentable sugars and high-performance lignin-based formaldehyde-free adhesives by enzymatic hydrolysis of materials after alkaline disc milling pretreatment.

[0023] 4) The lignin formaldehyde-free adhesive prepared by the present invention can achieve high-strength bonding of wood boards. The dry strength of three-layer poplar veneer after bonding can reach 0.8-1.9 MPa, and the Class II wet strength can reach 0.6-1.2 MPa, meeting the national standard requirement of 0.7 MPa (GB / T9846-2015). The plywood prepared under some hot pressing conditions far exceeds the bonding strength of industrial urea-formaldehyde and phenolic resin adhesives.

[0024] 5) When the lignin formaldehyde-free adhesive prepared by the present invention is used to bond plywood, the amount of glue applied is only 50g / m 2The hot pressing temperature, time and pressure range are wide, and it is applicable to most woods and hot pressing equipment. It has good industrialization prospects and is a new method for the full utilization of lignocellulosic raw material biomass. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a graph showing the changes in the content of the main components in wheat straw after pretreatment with different refining times in Examples 1-5;

[0026] Figure 2 The results of enzymatic hydrolysis and saccharification of glucan and xylan in wheat straw after pretreatment with different refining times in Examples 1-5 are shown;

[0027] Figure 3 The graph shows the bonding strength of different plywood panels. a is the bonding strength of three-layer plywood panels bonded using different adhesives, and b is the bonding strength of three-layer plywood panels made using different hot pressing conditions.

[0028] Figure 4 The energy consumption diagram of disc grinding under different disc grinding times in Examples 1-5;

[0029] Figure 5 This is a graph showing the changes in the content of the main components in wheat straw after pre-soaking with different amounts of sodium hydroxide in Comparative Example 1;

[0030] Figure 6 This is a graph showing the saccharification yields of glucan and xylan in wheat straw after pre-soaking with different amounts of sodium hydroxide in Comparative Example 1;

[0031] Figure 7 This is a graph showing the bonding strength of three layers of plywood bonded using different adhesives in Comparative Example 1;

[0032] Figure 8 This is a graph showing the main component contents in wheat straw recovered at different disc grinding times in Comparative Example 2;

[0033] Figure 9 This is a graph showing the saccharification yields of glucan and xylan in wheat straw after different disc grinding times in Comparative Example 2;

[0034] Figure 10 This is a graph showing the bonding strength of three-layer plywood bonded with adhesives prepared with different grinding times in Comparative Example 2;

[0035] Figure 11 This is the energy consumption diagram of the disc grinding under different disc grinding times in Comparative Example 2. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described below in conjunction with specific examples. In the following examples, unless otherwise specified, the technical means used are conventional means well known to those skilled in the art. In the examples, if specific conditions are not specified, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be purchased commercially.

[0037] Example 1

[0038] A method for preparing a lignin-based formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw comprises the following steps:

[0039] 1) 2 kg of wheat straw raw material and sodium hydroxide solution were mixed in a mass ratio of 1:4, and pre-soaked at 40°C for 1 hour (the mass fraction of sodium hydroxide was equivalent to 10 wt% of the mass of the absolute dry wheat straw);

[0040] 2) The pre-impregnated wheat straw obtained in step 1) was subjected to disc grinding pretreatment, and the grinding was repeated three times, with the disc grinding gap gradually shortened from 1 mm to 0.8 mm with the number of disc grindings. The disc ground sample was thoroughly washed with tap water in a pulp bag and stored at 4°C until use;

[0041] 3) Weigh 20 g of the absolutely dry material prepared in step 2) into a 1000 mL conical flask, add 20 mL of acetic acid / sodium acetate buffer to adjust the pH of the system to 4.8, add cellulase (20 FPU / g glucan) and hemicellulase (150 U / g xylan) and add water to 400 g, place the conical flask in a shaker at 50 ° C and 150 rpm for hydrolysis for 72 h, take 1 mL of the hydrolyzate after centrifugation and dilution, and determine the fermentable sugar content by high performance liquid chromatography. The lignin obtained after enzymatic hydrolysis was centrifuged and washed with water, then freeze-dried and stored in a desiccator for later use;

[0042] 4) The lignin obtained in step 3) was crushed and sieved (>80 mesh) by a grinder to obtain a homogenized product, and then mixed with deionized water at a mass ratio of 1:4 to obtain a lignin formaldehyde-free adhesive.

[0043] Example 2

[0044] When wheat straw is used to prepare a lignin formaldehyde-free adhesive and co-produce fermentable sugars, the disc grinding pretreatment in step (2) is repeated 5 times, and the disc grinding gap is gradually shortened from 1 mm to 0.6 mm with the number of disc grinding times. The remaining preparation methods and parameters are the same as those in Example 1, and a lignin formaldehyde-free adhesive and fermentable sugars are obtained.

[0045] Example 3

[0046] When wheat straw is used to prepare a lignin formaldehyde-free adhesive and co-produce fermentable sugars, the disc grinding pretreatment in step (2) is repeated 7 times, and the disc grinding gap is gradually shortened from 1 mm to 0.4 mm with the number of disc grinding times. The remaining preparation methods and parameters are the same as those in Example 1, and a lignin formaldehyde-free adhesive and fermentable sugars are obtained.

[0047] Example 4

[0048] When wheat straw is used to prepare a lignin formaldehyde-free adhesive and co-produce fermentable sugars, the disc grinding pretreatment in step (2) is repeated 9 times, and the disc grinding gap is gradually shortened from 1 mm to 0.2 mm with the number of disc grinding times. The remaining preparation methods and parameters are the same as those in Example 1, and a lignin formaldehyde-free adhesive and fermentable sugars are obtained.

[0049] Example 5

[0050] When wheat straw is used to prepare a lignin formaldehyde-free adhesive and co-produce fermentable sugar, the disc grinding pretreatment in step (2) is repeated 11 times, and the disc grinding gap is gradually shortened from 1 mm to 0.1 mm with the number of disc grinding times. The remaining preparation methods and parameters are the same as those in Example 1, and a lignin formaldehyde-free adhesive and fermentable sugar are obtained.

[0051] Example 6

[0052] The main component contents of wheat straw pretreated with sodium hydroxide and disc grinding in Examples 1-5 were determined according to the standard method of the U.S. National Energy Laboratory (NREL / TP-510-42618). Figure 1 and as shown in Table 1.

[0053] Table 1 Contents of main components in wheat straw after different refining times

[0054]

[0055]

[0056] From Table 1 and Figure 1As can be seen, with increasing disc refining cycles, the glucan content gradually increased from 58.59% (third) to 62.17% (eleventh), indicating a relative increase in the glucan content of the material. This result is due to the higher ash content in the wheat straw and the dissolution of some extractives, lignin, and hemicellulose. Compared to cellulose, hemicellulose, due to its primarily amorphous, loose structure, is easily dissolved by sodium hydroxide during the disc refining process. The xylan content initially increased from 15.94% (third) to 18.87% (seventh). This increase was due to the removal and dissolution of ash, extractives, and some lignin, resulting in an increase in the relative xylan content. With further disc refining cycles, xylan dissolution continued to increase, and its content in the material decreased from 18.87% (seventh) to 15.22% (eleventh). With increasing pretreatment times, the lignin content decreased from 15.19% (third) to 13.97% (seventh), and then increased to 15.01%, indicating that the lignin content remained around 15% with increasing disc refining cycles. These results indicate that with increasing sodium hydroxide pre-impregnation combined with disc refining pretreatment, the glucan (cellulose) content in the material increased significantly. This increase in cellulose ensures a high yield of reducing sugars in subsequent enzymatic hydrolysis. Lignin content, however, fluctuated around 15%, suggesting that a higher lignin content is beneficial for plywood bonding.

[0057] Example 7

[0058] The calculation of the glucan and xylanase hydrolysis saccharification yields of wheat straw and wheat straw raw materials pretreated with sodium hydroxide-disk grinding in Examples 1-5 is as follows: Figure 2 and as shown in Table 2. The calculation formula is as follows:

[0059] Glucan enzymatic hydrolysis yield (%) = (glucose mass in enzymatic hydrolysis solution × 0.9) / glucan mass in initial substrate × 100%;

[0060] Xylan enzymatic hydrolysis yield (%) = (mass of xylose in the hydrolyzate × 0.88) / mass of xylan in the initial substrate × 100%.

[0061] Table 2 Glucan and xylanase hydrolysis saccharification yields in wheat straw after different refining times

[0062]

[0063]

[0064] From Table 2 and Figure 2It can be seen that the enzymatic hydrolysis yields of glucan and xylan in wheat straw raw materials were only 29.12% and 12.5%. After pretreatment, the enzymatic hydrolysis yields of glucan and xylan increased significantly to 75.57% and 65.17%, and continued to increase to 100% (fifth time) with the increase in the number of disc grinding pretreatments. The increase in the enzymatic hydrolysis yield in the pretreated material can effectively improve the purity of the prepared lignin. Lignin with higher purity is considered to be the key to providing the bonding strength between wood. In summary, the enzymatic digestibility of wheat straw after sodium hydroxide pre-impregnation combined with disc grinding pretreatment is significantly enhanced, and the enzymatic hydrolysis yield is significantly improved, indicating that this pretreatment method can effectively deconstruct wheat straw, improve its enzymatic hydrolysis yield and obtain lignin that can be used as plywood adhesive.

[0065] Example 8

[0066] The specific process of using the adhesive to bond three layers of plywood is as follows: the plywood is made of poplar veneer with a thickness of 1.5 mm, the poplar veneer is cut into lengths and widths of 200 mm and 200 mm respectively, and the amount of glue applied on one side is 50 g / m 2 Adjacent veneer layers were placed with the fiber grain perpendicular to each other and then subjected to a single-stage hot press curing process. Molded plywood was cut into standard test pieces according to the national standard (GB / T 17657-2013). Dry strength and Class II wet strength were measured using a universal tensile testing machine (SUNS UTM6503) at a tensile speed of 5 mm / min using a 5 kN load cell.

[0067] 1. Using the above plywood preparation process, the lignin formaldehyde-free adhesive, industrial phenolic resin glue, and industrial urea-formaldehyde resin glue prepared in Examples 1-5 were respectively used to bond plywood. The hot pressing temperature was 180°C, the pressure was 1.0 MPa, and the hot pressing time was 10 min. The results are as follows: Figure 3 a and Table 3.

[0068] Table 3 Bonding strength of plywood bonded with different adhesives

[0069]

[0070] From Table 3 and Figure 3As shown in Figure 1, the lignin obtained after enzymatic hydrolysis of pretreated wheat straw exhibits strong bonding properties for plywood, with the bonding strength showing an initial increase followed by a decrease. The dry and wet strengths increased from 1.07 / 0.95 MPa (3rd pass) to 1.84 / 1.12 MPa (5th pass), respectively. This is due to the increased enzymatic hydrolysis yield, which leads to higher purity of the prepared lignin. During the hot pressing process, more covalent bonds are formed between the higher-purity lignin and the main components of the board, such as lignin, thereby increasing the bonding strength of the board. However, as the number of discing cycles increases from 5 to 11, the bonding strength of the obtained lignin to plywood decreases, with the wet strength falling below the national standard requirement of 0.7 MPa. This decrease in bonding strength is attributed to the increased number of discing cycles, which leads to more depolymerization and polycondensation reactions in the lignin. The active sites of the polycondensed lignin are occupied, which in turn inhibits the formation of covalent bonds between the lignin and the lignin in the wood board during the hot pressing process, resulting in a decrease in the strength of the prepared plywood.

[0071] 2. Using the above plywood preparation process, the lignin formaldehyde-free adhesive prepared in Example 2 was used to bond plywood. The hot pressing temperature was 150-190°C, the pressure was 1.0 MPa, and the hot pressing time was 5-20 min. In order to reduce the hot pressing temperature, an appropriate amount of sulfuric acid was added to the lignin formaldehyde-free adhesive dispersion system to adjust its pH to 2. The hot pressing was carried out at 150°C for 10 min. The results are shown in FIG. Figure 3 b and Table 4.

[0072] Table 4 Bonding strength of plywood bonded under different hot pressing conditions

[0073]

[0074] From Table 4 and Figure 3 As shown in Figure 2, at a hot-pressing temperature of 160°C / 10 min, insufficient bonding between lignin and the board material was observed, making it difficult to meet the requirements for plywood use. Increasing the hot-pressing time to 20 min significantly increased the dry and wet bond strengths of the plywood to 1.07 and 0.84 MPa, respectively. Increasing the hot-pressing temperature to 170°C also significantly improved the dry and wet bond strengths, demonstrating that increasing the hot-pressing time and temperature positively impacts the bond strength of plywood. Further increasing the hot-pressing temperature to 180°C achieved optimal dry and wet bond strengths. Further increasing the temperature while decreasing the hot-pressing time yielded still high dry and wet bond strengths of 1.82 and 0.97 MPa, respectively. While the lignin-based formaldehyde-free adhesive prepared under these pretreatment conditions can achieve high bond strengths, the higher hot-pressing temperature increases the risk and energy consumption.

[0075] Because lignin's polycondensation reaction intensifies under acidic conditions, this application adds sulfuric acid to the adhesive system to promote its polycondensation reaction with the lignin in the board, increasing the number and strength of bonds formed, thereby reducing the hot-pressing temperature. However, the addition of excessive sulfuric acid can cause severe corrosion to the board. Therefore, this application adds sulfuric acid to adjust the pH value of the adhesive system to 2 under hot-pressing conditions of 150°C / 10 minutes. After hot-pressing, the dry and wet strengths of the plywood reached 1.13 and 1.0 MPa, respectively, significantly exceeding the national standard of 0.7 MPa. This shows that the introduction of an appropriate amount of acid can significantly reduce the hot-pressing temperature.

[0076] Example 9

[0077] The specific energy consumption of disc grinding of wheat straw pretreated with sodium hydroxide-disc grinding in Examples 1-5 was calculated, and the results are as follows: Figure 4 As shown in Table 5, the calculation formula for disc grinding specific energy consumption is as follows:

[0078] Disc grinding specific energy consumption = (disc grinding total energy consumption - idling total energy consumption) / feed amount;

[0079] The unit of specific energy consumption is kW h / t, the unit of total energy consumption is kW h, and the unit of feed amount is t.

[0080] Table 5 Disc grinding energy consumption at different disc grinding times

[0081] Disc grinding times Energy consumption (kW h / t dry wheat straw) Example 1 3 times 148 Example 2 5 times 213 Example 3 7 times 270 Example 4 9 times 317 Example 5 11 times 357

[0082] Depend on Figure 4 As shown in Table 5, as the number of disc grinding increases from 3 to 11 times, the disc grinding energy consumption increases from 148 to 357 kWh / t of dry wheat straw, indicating that the pretreatment method of the present application can produce a green, high-strength, formaldehyde-free wood adhesive with low energy consumption.

[0083] Comparative Example 1

[0084] The amount of sodium hydroxide used was 10 wt%, 20 wt%, 30 wt%, 40 wt% and 50 wt% relative to the absolute dry wheat straw. The impregnation conditions were the same as in Example 1. After the impregnation was completed, no mechanical pretreatment was performed by grinding. The pre-treated material was directly washed to neutrality using a pulp bag and dehydrated to a moisture content of about 60 wt% and stored at 4°C for future use. The subsequent component analysis, enzymatic hydrolysis step and adhesive preparation steps were the same as in Example 1. The hot pressing process for the adhesive to bond plywood was the same as in Example 8. The results are shown in Tables 6-8 and Figure 5-7 shown.

[0085] Table 6 Content of main components in wheat straw after pre-soaking with different amounts of sodium hydroxide

[0086] Sodium hydroxide dosage (wt%) Glucan% Xylan % Lignin% 10 39.52 22.17 20.14 20 41.21 21.35 20.98 30 42.58 22.79 21.58 40 43.19 21.58 22.36 50 43.47 21.74 22.67

[0087] From Table 6 and Figure 5As can be seen, with the increase in sodium hydroxide dosage, the glucose content in the recovered material increased from 39.52% to 43.47%, while the xylan and lignin contents did not change much, each maintaining at around 20%. This result suggests that sodium hydroxide solution pre-impregnation may increase the cellulose (glucan) content by dissolving some ash and extracts, but has a poor effect on the degradation and removal rate of lignin and xylan.

[0088] Table 7 Glucan and xylanase hydrolysis saccharification yields in wheat straw after pre-soaking with different sodium hydroxide dosages

[0089] Sodium hydroxide dosage (wt%) Glucan enzymatic hydrolysis rate % Xylan enzymatic hydrolysis rate % 10 30.86 13.75 20 32.86 15.78 30 35.58 17.36 40 36.17 17.64 50 37.63 18.36

[0090] From Table 7 and Figure 6 It can be seen that the enzymatic hydrolysis yields of glucan and xylan of wheat straw raw material are 29.12% and 12.5% ​​respectively. After pre-impregnation with 10wt% sodium hydroxide (compared to absolute dry wheat straw), the enzymatic hydrolysis yields of glucan and xylan of the material are slightly increased to 30.86% and 13.75%. The improvement of the enzymatic hydrolysis yield may be due to the swelling effect of sodium hydroxide solution on wheat straw fiber. As the amount of sodium hydroxide increases from 10% to 50wt%, the enzymatic hydrolysis yields of glucan and xylan increase to 37.63% and 18.36%. This result shows that only using sodium hydroxide pre-impregnation without subsequent disc grinding mechanical pretreatment can only slightly improve the enzymatic hydrolysis saccharification yield of the material, and increasing the amount of sodium hydroxide has no obvious promoting effect on the enzymatic hydrolysis yield of the material.

[0091] The residual lignin after enzymatic hydrolysis is crushed and mixed with water and then used for bonding plywood. The specific conditions and steps are the same as those in Example 8.

[0092] Table 8 Bonding strength of plywood bonded with different adhesives

[0093]

[0094]

[0095] From Table 8 and Figure 7 It can be seen that the plywood prepared under different conditions can hardly provide bonding strength. This result is due to the low saccharification yield in the enzymatic hydrolysis process. The residue after enzymatic hydrolysis still contains a large amount of carbohydrates, which cannot effectively provide bonding strength during the hot pressing process of the plywood.

[0096] Comparative Example 2

[0097] The wheat straw was not pre-impregnated and was directly subjected to disc grinding pretreatment. The disc grinding times and disc grinding gap changes were the same as in Examples 1-5. After pretreatment, the recovered materials were subjected to component analysis, enzymatic saccharification, and adhesive preparation. The specific experimental steps were the same as in Example 1. After the disc grinding, the wheat straw was enzymatically hydrolyzed and saccharified, and the lignin residue was recovered and used to prepare formaldehyde-free wood adhesive. The hot pressing conditions and experimental steps were the same as in Example 8. The results are shown in Tables 9-12 and Figure 8-11 shown.

[0098] Table 9 Contents of main components in wheat straw recovered at different disc grinding times

[0099] Disc grinding times Glucan% Xylan % Lignin% 3 39.8 21.62 20.52 5 41.1 22.11 20.74 7 41.79 22.01 21.52 9 42.07 22.3 22.08 11 42.73 22.71 22.57

[0100] From Table 9 and Figure 8 It can be seen that the glucan content in the recycled material increased slightly to 42.73% with the increase in the number of disc milling times, and the xylan and lignin contents both increased slightly to 22.71% and 22.57%. This result was mainly due to the dissolution of some extractives and ash during the disc milling pretreatment process.

[0101] Table 10 Glucan and xylan hydrolysis saccharification yields in wheat straw after different disc grinding times

[0102]

[0103]

[0104] The enzymatic hydrolysis yields of glucan and xylan from wheat straw were 29.12% and 12.5%, respectively. Figure 9 It can be seen that the enzymatic hydrolysis yields of glucan and xylan after pretreatment with different disc grinding times increased slightly with the increase in disc grinding times, but even after 11 disc grinding times, the enzymatic hydrolysis yields of glucan and xylan were only 34.6% and 16.81%. This result shows that without pre-immersion in sodium hydroxide solution, the enzymatic hydrolysis yields of glucan and xylan are difficult to increase by mechanical disc grinding pretreatment alone.

[0105] Table 11 Bonding strength of plywood prepared with adhesives prepared with different grinding times

[0106] Disc grinding times Dry strength (MPa) Wet strength (MPa) 3 0 0 5 0 0 7 0 0 9 0.15 0 11 0.17 0

[0107] From Table 11 and Figure 10 It can be seen that the adhesives prepared with different disc grinding times can hardly provide any bonding strength. This result is mainly because it is difficult to achieve effective exposure of carbohydrates in wheat straw through disc grinding mechanical pretreatment alone, and carbohydrates are difficult to be effectively consumed in the enzymatic hydrolysis process. Therefore, the content of the active lignin adhesive component in the prepared adhesive is low and cannot provide an effective bonding network.

[0108] Table 12 Energy consumption of wheat straw without pre-preg after mechanical pretreatment with different grinding times

[0109] Disc grinding times Energy consumption (kW h / t dry wheat straw) Example 1 3 times 558 Example 2 5 times 624 Example 3 7 times 658 Example 4 9 times 695 Example 5 11 times 730

[0110] From Table 12 and Figure 11 It can be seen that after three disc grindings, the energy consumption reached 658kW h / t, which is 3.77 times the energy consumption of disc grinding after pre-impregnation. As the number of disc grindings increased to 11, the disc grinding energy consumption reached 730kW h / t, which is much higher than the disc grinding energy consumption after sodium hydroxide solution pre-impregnation and disc grinding pretreatment. This result shows that the material without sodium hydroxide pre-impregnation experiences large mechanical friction during the disc grinding process, resulting in excessive mechanical energy consumption. After the sodium hydroxide solution pre-impregnation treatment, the wheat straw is fully swollen, and the fiber cells are easily dissociated during the mechanical disc grinding process, which helps to significantly reduce the disc grinding energy consumption.

[0111] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a lignin-based formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw, characterized in that: Wheat straw is pre-impregnated with sodium hydroxide at low temperature. The impregnated wheat straw is mechanically pretreated by a disc mill, and fermentable sugars and lignin are separated by enzymatic hydrolysis. The lignin is mixed with water to produce a lignin-free formaldehyde adhesive.

2. The method for preparing lignin formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw according to claim 1, characterized in that: The mass ratio of the wheat straw to sodium hydroxide is 10:1, the temperature of the low-temperature pre-impregnation is 40° C., and the pre-impregnation time is 1 hour.

3. The method for preparing lignin formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw according to claim 1, characterized in that: The number of disc grinding pretreatments is 3 to 11 times, and the disc grinding gap is gradually shortened from 1 mm to 0.1 to 0.8 mm as the number of disc grinding times increases.

4. The method for preparing lignin formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw according to claim 3, characterized in that: The number of disc grinding pretreatments is 5 to 9 times, and the disc grinding gap is gradually shortened from 1 mm to 0.1 to 0.6 mm as the number of disc grinding times increases.

5. The method for preparing lignin formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw according to claim 1, characterized in that: The enzymatic hydrolysis reaction process is as follows: enzymatically hydrolyzing the pretreated wheat straw with a solid content of 5 wt%, adding cellulase and xylanase, and performing hydrolysis in a 50° C. constant temperature shaker.

6. The method for preparing lignin formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw according to claim 5, characterized in that: The dosage of the cellulase was 20 FPU / g, the dosage of the xylanase was 150 U / g xylan, the substrate concentration was 5 wt %, and the enzymatic hydrolysis was carried out in a constant temperature shaker at 50° C. / 150 rpm for 72 h.

7. The method for preparing lignin formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw according to claim 1, characterized in that: The mass ratio of the lignin to water is 1:

4.

8. The lignin formaldehyde-free adhesive prepared by the method for preparing a lignin formaldehyde-free adhesive and co-producing fermentable sugars using wheat straw according to any one of claims 1 to 7.

9. Use of the lignin formaldehyde-free adhesive according to claim 8 in the preparation of plywood.

10. The use according to claim 9, characterized in that Poplar veneer is bonded with lignin-free formaldehyde adhesive, with a single-sided adhesive amount of 50g / m 2 After hot pressing curing, the hot pressing pressure is 1MPa, the hot pressing curing temperature is 150-190℃, and the hot pressing time is 5-20min to obtain plywood.

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