Water-resistant modified starch adhesive and preparation method thereof

Abstract

The invention discloses a water-resistant modified starch adhesive and a preparation method thereof, and relates to the technical field of adhesives. The preparation method comprises the following steps: sequentially adding pretreated starch and a hydrochloric acid solution into deionized water, then adjusting the pH value, gelatinizing, cooling, carrying out ultrasonic oscillation, adding an initiator, adjusting the pH value, dropwise adding a grafting solution, reacting, adding an adjusting solution, continuing reacting, heating, reacting, and then adjusting the pH value again, so as to obtain the modified starch. After cooling, adding adipic dihydrazide, uniformly stirring, finally adding the modified starch composition B and glycerol, stirring for reaction, adjusting the pH value, and performing ultrasonic dispersion to obtain the water-resistant modified starch adhesive. The corn starch is pretreated, so that the uniformity and reaction activity of starch particles are effectively improved; by introducing the grafting liquid, the modified starch composition B and other components, the bonding strength, the water resistance and the storage stability are further improved. Therefore, the adhesive disclosed by the invention has a wider application prospect.

Description

Technical Field

[0001] The invention relates to the technical field of adhesives, and in particular to a water-resistant modified starch adhesive and a preparation method thereof. Background Art

[0002] Wood adhesive is a soft substance that can make different forms of wood tightly glued together through adhesion under certain conditions. Wood adhesive has a wide range of uses, including laminated beams, integrated materials, plywood, particleboard, fiberboard, composite flooring, decorative veneer, etc. The wood-based panel industry, wood products industry and house decoration industry need to consume a large amount of adhesives. Most of the wood adhesives currently sold on the market are urea-formaldehyde resins, phenolic resins and melamine-formaldehyde resins synthesized with aldehyde compounds as the main raw materials, and are collectively called "trialdehyde glue". However, if the process parameters are not properly controlled during the production of trialdehyde glue, it is inevitable that harmful aldehyde components will remain in the finished product, causing great harm to the environment and human body. In summary, it is urgent for people to develop new environmentally friendly wood adhesives.

[0003] Starch is a green material and is widely used in food, medicine, industrial production and other fields. After the starch structure is opened, the molecular cohesion and stability can be improved by starch modification, and it is regarded as a highly potential raw material for biomass adhesives. Starch adhesives have the advantages of low raw material cost, wide sources and environmental friendliness. Although starch-based adhesives have been used in some wood processing fields, their limitations in bonding strength, water resistance and storage stability seriously restrict their widespread application. In order to solve this technical problem, the patent technology document CN105754516A proposes a starch / phenolic resin composite adhesive and a preparation method thereof, which is composed of the following raw materials in parts by weight: 162 parts of starch, 243-648 parts of water, 216-378 parts of polyvinyl alcohol aqueous solution, 0.22-0.76 parts of ammonium persulfate, 1.62-4.86 parts of sodium hypochlorite, 47.1-94.1 parts of phenol, 40.6-162.3 parts of formaldehyde solution and 2-16 parts of sodium hydroxide. However, the bonding strength, water resistance and storage stability of starch adhesives prepared by these existing solutions still need to be improved. Summary of the invention

[0004] The purpose of the present invention is to provide a water-resistant modified starch adhesive and a preparation method thereof, to solve the following technical problems: Existing starch adhesives still have problems with unsatisfactory bonding strength, water resistance and storage stability.

[0005] The purpose of the present invention can be achieved through the following technical solutions: A method for preparing a water-resistant modified starch adhesive comprises the following steps: Step S1: adding pretreated starch and hydrochloric acid solution to deionized water in sequence, then adjusting the pH value to 4-4.5, gelatinizing at 85-90° C. for 1 hour, and cooling to 45-60° C. to obtain gelatinized starch solution; Step S2: After ultrasonically oscillating the gelatinized starch solution for 10 minutes, an initiator is added, and the pH value is adjusted to 4-4.5. After 10-15 minutes, a grafting solution is added dropwise, and the reaction is continued for 0.5-1 hour. After adding the adjusting solution, the reaction is continued for 1-1.5 hours, and then the pH value is adjusted to 6-7 after reacting at 82.5-85° C. for 1-1.5 hours, and the modified starch composition A is obtained after cooling to 25-35° C.; Step S3: Add adipic acid dihydrazide to the modified starch composition A and stir for 15-30 minutes, then add the modified starch composition B and glycerol and stir for 30-60 minutes, adjust the pH value to 7-8, and obtain a water-resistant modified starch adhesive after ultrasonic dispersion.

[0006] Preferably, the usage ratio of deionized water, pretreated starch and hydrochloric acid solution in step S1 is 150-200 mL: 150-170 g: 30 mL; The concentration of the hydrochloric acid solution in step S1 is 6 mol / L.

[0007] Preferably, the method for preparing the pretreated starch in step S1 is as follows: Corn starch is added to deionized water, and after ultrasonic dispersion at 45-60° C., the pH value is adjusted to 8-9 with a sodium hypochlorite solution. After secondary ultrasonication, the pretreated starch is filtered, dried at 50-65° C., ground, and sieved.

[0008] Preferably, the amount of deionized water and corn starch is 1000mL: 200-230g; The ultrasonic dispersion lasts for 25-30 minutes, the power is 280-300W, and the frequency is 35-40kHz; The concentration of the sodium hypochlorite solution is 5 mol / L; The duration of the secondary ultrasonic treatment is 35-50 min, the power is 280-300 W, and the frequency is 35-40 kHz; The sieve used during the sieving process has a mesh size of 200-300.

[0009] Preferably, the amount of the gelatinized starch solution, initiator, grafting solution, and regulating solution in step S2 is 180-230 mL: 14.3-15 mL: 55-60 mL: 30-35 mL; The duration of the ultrasonic oscillation in step S2 is 10-15 minutes, the power is 280-300W, and the frequency is 35-40kHz.

[0010] Preferably, the preparation method of the initiator in step S2 is as follows: Dissolve 5-5.2 g of ammonium cerium nitrate in 14.3-15 mL of deionized water, then add 0.72-0.75 g of nitric acid and stir for 10-15 minutes to obtain an initiator.

[0011] Preferably, the preparation method of the grafting solution in step S2 is as follows: Dissolve 40-50 g of diacetone acrylamide in 55-60 mL of deionized water to obtain a grafting solution.

[0012] Preferably, the preparation method of the regulating solution in step S2 is as follows: Dissolve 5 g of hexadecyltrimethylammonium bromide in 30-35 mL of deionized water to obtain a regulating solution.

[0013] Preferably, in step S3, the usage ratio of the modified starch composition A, adipic acid dihydrazide, the modified starch composition B, and glycerol is 279-340 mL: 23-26 g: 125-190 mL: 5-10 g; The duration of the ultrasonic dispersion in step S3 is 10-15 min, the power is 280-300 W, and the frequency is 35-40 kHz.

[0014] Preferably, the preparation method of the modified starch composition B in step S3 is as follows: Step B1: Add 5-10 g of polyvinyl alcohol to 20-30 mL of deionized water, and stir at 80-90° C. for 15-30 min to obtain a polyvinyl alcohol solution; Step B2: Add 2-5 g of melamine to 5-10 mL of deionized water and adjust the pH to 3-5, and stir at 60-80° C. until dissolved; Step B3: 50-60 g of pretreated starch, 100-150 mL of deionized water, 20-30 mL of polyvinyl alcohol solution and 5-10 mL of melamine solution are mixed and stirred evenly, then 0.5-1 g of ammonium persulfate is added and reacted at 50-70° C. for 1-3 h to obtain a modified starch composition B.

[0015] As a further embodiment of the present invention.

[0016] Beneficial effects of the present invention: The present invention provides a water-resistant modified starch adhesive and a preparation method thereof. The present invention effectively improves the bonding strength, water resistance and storage stability of the starch adhesive through the following method.

[0017] (1) The functional groups such as carboxyl introduced during the preparation of pretreated starch can react with diacetone acrylamide, melamine and other substances added later to form more stable chemical bonds and cross-linked structures. These cross-linked structures can effectively prevent the intrusion of water, provide stronger cohesion and adhesion, improve the water resistance of the adhesive, and make the adhesive more secure when bonding objects. In addition, the uniformity, reactivity, structure and performance stability of the starch particles after pretreatment are improved, making the subsequent grafting and cross-linking reactions more sufficient and uniform, and also improving the spreadability and wettability of the adhesive on the surface of the bonded material, further enhancing the water resistance of the adhesive, enhancing the bonding effect and storage stability.

[0018] (2) The acid treatment during the preparation of gelatinized starch liquid shortens the starch molecular chain and increases the fluidity of the starch molecules. The acid treatment and gelatinization process change the structure of the starch molecules and expose more reaction sites, which enables the starch molecules to mix better with other substances in the subsequent processing and react more effectively with grafting monomers such as diacetone acrylamide to form grafted copolymers. At the same time, the viscous network structure formed in the gelatinization process can form a continuous film on the surface of the bonded object, firmly bonding the objects together through the intermolecular force, so that the adhesive has good bonding effect, water resistance, flexibility, etc.

[0019] (3) During the preparation of the modified starch composition A, the gelatinized starch liquid generates free radicals under the action of the initiator, and diacetone acrylamide acts as a grafting monomer to undergo a grafting reaction with the starch, thereby increasing the spatial structure of the starch molecules and the number of active groups, thereby improving the bonding performance of the adhesive. The added adipic acid dihydrazide can also react with diacetone acrylamide under the specific reaction conditions of the present invention to form a cross-linked structure that can effectively prevent the intrusion of moisture, thereby improving the water resistance of the adhesive. Hexadecyltrimethylammonium bromide reduces the surface tension of the system and increases the fluidity of the system, allowing the adhesive to better wet and penetrate the surface of the bonded object, increasing the contact area between the adhesive and the bonded object, and further improving the bonding effect. Ultrasonic oscillation makes the gelatinized starch liquid evenly dispersed, ensuring that the initiator and the grafting liquid can evenly contact and react with the starch molecules, thereby forming a stable chemical structure.

[0020] (4) The melamine in the modified starch composition B can form a three-dimensional network structure with starch and polyvinyl alcohol that contains a large number of active groups and can significantly improve the bonding strength and bonding stability of the adhesive. When it is mixed with a specific proportion of the modified starch composition A, adipic acid dihydrazide will further react with the carbonyl groups on the diacetone acrylamide grafted chain in the composition A. At the same time, the network structure of the composition B itself will intersperse and cross-link with the composition A to form a more complex and dense multiple cross-linked network. This structure can effectively block the intrusion of moisture, greatly improve the water resistance of the adhesive, and enable it to maintain good bonding strength and stability in a humid environment. Moreover, under the action of ultrasonic dispersion, after the composition B is mixed with the specific proportion of the composition A, the composition B can make the components of the adhesive more evenly distributed on the surface of the bonded material, avoiding the situation of insufficient local bonding strength.

[0021] Therefore, the modified starch adhesive prepared by the present invention has more excellent bonding strength, water resistance, storage stability, and broad application prospects. DETAILED DESCRIPTION

[0022] The following will be described clearly and completely in conjunction with the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] Example 1: A method for preparing a water-resistant modified starch adhesive is as follows: S1: Add 200 g corn starch to 1000 mL deionized water, perform ultrasonic dispersion at 45°C for 25 min, power 280 W, frequency 35 kHz, adjust the pH to 8 with 5 mol / L sodium hypochlorite solution, perform secondary ultrasonic treatment for 35 min, power 280 W, frequency 35 kHz, filter, dry at 50°C, grind, and sieve through a 200-mesh sieve to obtain pretreated starch; S2: 150 g of pretreated starch and 30 mL of 6 mol / L hydrochloric acid solution were added to 150 mL of deionized water, and then the pH value was adjusted to 4 with 1 mol / L sodium hydroxide solution, and then gelatinized at 85 ° C for 1 hour, and cooled to 45 ° C to obtain gelatinized starch solution; S3: Dissolve 5 g of ammonium cerium nitrate in 14.3 mL of deionized water, then add 0.72 g of nitric acid and stir for 10 min to obtain an initiator; S4: dissolving 40 g of diacetone acrylamide in 55 mL of deionized water to obtain a grafting solution; S5: dissolving 5 g of hexadecyltrimethylammonium bromide in 30 mL of deionized water to obtain a regulating solution; S6: 180 mL of gelatinized starch solution was subjected to ultrasonic oscillation for 10 min at a power of 280 W and a frequency of 35 kHz, and then 14.3 mL of initiator was added, and the pH value was adjusted to 4 with a sodium hydroxide solution with a concentration of 1 mol / L. After 10 min, 55 mL of grafting solution was added dropwise, and after reacting for 0.5 h, 30 mL of adjusting solution was added, and the reaction was continued for 1 h. After reacting at 82.5° C. for 1 h, the pH value was adjusted to 6 with a sodium hydroxide solution with a concentration of 1 mol / L, and the mixture was cooled to 25° C. to obtain a modified starch composition A; S7: Add 5 g of polyvinyl alcohol to 20 mL of deionized water and stir at 80 °C for 15 min to obtain a polyvinyl alcohol solution; S8: Add 2 g of melamine to 5 mL of deionized water and adjust the pH to 3 with nitric acid, and stir at 60 °C until dissolved; S9: 50 g of pretreated starch, 100 mL of deionized water, 20 mL of polyvinyl alcohol solution and 5 mL of melamine solution were mixed and stirred uniformly, and then 0.5 g of ammonium persulfate was added and reacted at 50° C. for 1 h to obtain a modified starch composition B; S10: 23 g of adipic acid dihydrazide was added to 279 mL of modified starch composition A and stirred for 15 min, then 125 mL of modified starch composition B and 5 g of glycerol were added and stirred for 30 min, the pH value was adjusted to 7 with a sodium hydroxide solution having a concentration of 1 mol / L, and finally ultrasonic dispersion was performed for 10 min, the power was 280 W, and the frequency was 35 kHz to obtain a water-resistant modified starch adhesive.

[0024] Example 2: A method for preparing a water-resistant modified starch adhesive is as follows: S1: Add 215 g corn starch to 1000 mL deionized water, perform ultrasonic dispersion at 50°C for 28 min, power 290 W, frequency 38 kHz, adjust the pH to 8.5 with 5 mol / L sodium hypochlorite solution, perform secondary ultrasonic dispersion for 40 min, power 290 W, frequency 38 kHz, filter, dry at 55°C, grind, and sieve through a 250-mesh sieve to obtain pretreated starch; S2: 160 g of pretreated starch and 30 mL of 6 mol / L hydrochloric acid solution were added to 180 mL of deionized water, and then the pH value was adjusted to 4.3 with 1 mol / L sodium hydroxide solution, and then gelatinized at 88 ° C for 1 h, and cooled to 50 ° C to obtain gelatinized starch solution; S3: 5.1 g of ammonium cerium nitrate was dissolved in 14.7 mL of deionized water, and then 0.74 g of nitric acid was added and stirred for 13 min to obtain an initiator; S4: dissolving 45 g of diacetone acrylamide in 58 mL of deionized water to obtain a grafting solution; S5: dissolving 5 g of hexadecyltrimethylammonium bromide in 33 mL of deionized water to obtain a conditioning solution; S6: 210 mL of gelatinized starch solution was subjected to ultrasonic oscillation for 13 min at a power of 290 W and a frequency of 38 kHz, and then 14.7 mL of initiator was added, and the pH value was adjusted to 4.3 with a sodium hydroxide solution having a concentration of 1 mol / L. After 13 min, 58 mL of grafting solution was added dropwise, and after reacting for 0.8 h, 33 mL of adjusting solution was added, and the reaction was continued for 1.3 h. After reacting at 84° C. for 1.3 h, the pH value was adjusted to 6.5 with a sodium hydroxide solution having a concentration of 1 mol / L, and the mixture was cooled to 30° C. to obtain a modified starch composition A; S7: Add 8 g of polyvinyl alcohol to 25 mL of deionized water and stir at 85° C. for 20 min to obtain a polyvinyl alcohol solution; S8: Add 4 g of melamine to 8 mL of deionized water and adjust the pH to 4 with nitric acid, and stir at 70 °C until dissolved; S9: 55 g of pretreated starch, 125 mL of deionized water, 25 mL of polyvinyl alcohol solution and 8 mL of melamine solution were mixed and stirred uniformly, and then 0.8 g of ammonium persulfate was added and reacted at 60° C. for 2 h to obtain a modified starch composition B; S10: 25 g of adipic acid dihydrazide was added to 315 mL of modified starch composition A and stirred for 20 min, then 158 mL of modified starch composition B and 8 g of glycerol were added and stirred for 45 min, the pH value was adjusted to 7.5 with a sodium hydroxide solution having a concentration of 1 mol / L, and finally ultrasonic dispersion was performed for 13 min, with a power of 290 W and a frequency of 38 kHz to obtain a water-resistant modified starch adhesive.

[0025] Example 3: A method for preparing a water-resistant modified starch adhesive is as follows: S1: Add 230 g corn starch to 1000 mL deionized water, perform ultrasonic dispersion at 60° C. for 30 min, power of 300 W, and frequency of 40 kHz, adjust the pH to 9 with 5 mol / L sodium hypochlorite solution, perform secondary ultrasonic dispersion at 50 min, power of 300 W, and frequency of 40 kHz, and then filter, dry at 65° C., grind, and sieve through a 300-mesh sieve to obtain pretreated starch; S2: 170 g of pretreated starch and 30 mL of 6 mol / L hydrochloric acid solution were added to 200 mL of deionized water, and then the pH value was adjusted to 4.5 with 1 mol / L sodium hydroxide solution, and then gelatinized at 90 ° C for 1 h, and cooled to 60 ° C to obtain gelatinized starch solution; S3: 5.2 g of ammonium cerium nitrate was dissolved in 15 mL of deionized water, and then 0.75 g of nitric acid was added and stirred for 15 min to obtain an initiator; S4: dissolving 50 g of diacetone acrylamide in 60 mL of deionized water to obtain a grafting solution; S5: dissolving 5 g of hexadecyltrimethylammonium bromide in 35 mL of deionized water to obtain a conditioning solution; S6: 230 mL of gelatinized starch solution was subjected to ultrasonic oscillation for 15 min at a power of 300 W and a frequency of 40 kHz, and then 15 mL of initiator was added, and the pH value was adjusted to 4.5 with a sodium hydroxide solution with a concentration of 1 mol / L. After 10-15 min, 60 mL of grafting solution was added dropwise, and after reacting for 1 h, 35 mL of adjusting solution was added, and the reaction was continued for 1.5 h. After reacting at 85° C. for 1.5 h, the pH value was adjusted to 7 with a sodium hydroxide solution with a concentration of 1 mol / L, and the mixture was cooled to 35° C. to obtain a modified starch composition A; S7: Add 10 g of polyvinyl alcohol to 30 mL of deionized water and stir at 90° C. for 30 min to obtain a polyvinyl alcohol solution; S8: Add 5 g of melamine to 10 mL of deionized water and adjust the pH to 5 with nitric acid, and stir at 80 °C until dissolved; S9: 60 g of pretreated starch, 150 mL of deionized water, 30 mL of polyvinyl alcohol solution and 10 mL of melamine solution were mixed and stirred uniformly, and then 1 g of ammonium persulfate was added and reacted at 70° C. for 3 h to obtain a modified starch composition B; S10: 26 g of adipic acid dihydrazide was added to 340 mL of modified starch composition A and stirred for 30 min, then 190 mL of modified starch composition B and 10 g of glycerol were added and stirred for 60 min, the pH value was adjusted to 8 with a sodium hydroxide solution having a concentration of 1 mol / L, and finally ultrasonic dispersion was performed for 15 min, the power was 300 W, and the frequency was 40 kHz to obtain a water-resistant modified starch adhesive.

[0026] Comparative Example 1: Compared with Example 1, this comparative example only replaces the "pretreated starch" added in the preparation process of the gelatinized starch solution with "corn starch", and the remaining steps and parameters are the same, which will not be repeated in this comparative example, and finally a water-resistant modified starch adhesive is obtained.

[0027] Comparative Example 2: Compared with Example 1, this comparative example only replaces the "180 mL of gelatinized starch solution" added during the preparation of the modified starch composition A with "pretreated starch solution obtained by uniformly mixing 180 mL of deionized water and 150 g of pretreated starch", and the remaining steps and parameters are the same, which will not be repeated in this comparative example, and finally a water-resistant modified starch adhesive is obtained.

[0028] Comparative Example 3: Compared with Example 1, this comparative example only replaces the "180 mL of gelatinized starch solution" added during the preparation of the modified starch composition A with "corn starch solution obtained by uniformly mixing 180 mL of deionized water and 150 g of corn starch", and the remaining steps and parameters are the same, which will not be repeated in this comparative example, and finally a water-resistant modified starch adhesive is obtained.

[0029] Comparative Example 4: Compared with Example 1, this comparative example only replaces the "pretreated starch" added in the preparation process of the modified starch composition B with "corn starch", and the remaining steps and parameters are the same, which will not be repeated in this comparative example, and finally a water-resistant modified starch adhesive is obtained.

[0030] Comparative Example 5: Compared with Example 1, this comparative example only replaces the step of "adding 23 g of adipic acid dihydrazide to 279 mL of modified starch composition A and stirring for 15 min, then adding 125 mL of modified starch composition B and 5 g of glycerol" in the preparation process of the water-resistant modified starch adhesive of S9 with "adding 23 g of adipic acid dihydrazide to 125 mL of modified starch composition A and stirring for 15 min, then adding 279 mL of modified starch composition B and 5 g of glycerol", and the remaining steps and parameters are the same, and will not be repeated in this comparative example, and finally a water-resistant modified starch adhesive is obtained.

[0031] Comparative Example 6: Compared with Example 1, this comparative example only replaces the "modified starch composition B" added in the preparation process of the water-resistant modified starch adhesive of S9 with the "modified starch composition A", and the remaining steps and parameters are the same, which will not be repeated in this comparative example, and finally a water-resistant modified starch adhesive is obtained.

[0032] Comparative Example 7: Compared with Example 1, this comparative example only does not perform ultrasonic dispersion during the preparation process of the water-resistant modified starch adhesive in S9, and the remaining steps and parameters are the same, which will not be repeated in this comparative example, and finally a water-resistant modified starch adhesive is obtained.

[0033] Performance testing: Bond strength test: According to GB / T17657-2013 "Test Methods for Physical and Chemical Properties of Artificial Boards and Faced Artificial Boards", prepare one poplar wood chip of 50mm×30mm×10mm and one poplar wood chip of 30mm×25mm×10mm respectively, polish them with 600-grit sandpaper, and wipe them with anhydrous ethanol to remove surface impurities. Weigh 0.5g of adhesive and apply it evenly on the bonding surface. Splice the two wood chips into a plywood specimen with an overlap area of ​​30mm×25mm. Use F clamps to apply a pressure of more than 100N to the plywood specimen so that the adhesive can fully infiltrate the overlap surface. Place the specimen at room temperature for more than 48h to fully dry the specimen to obtain the test piece. Then use a universal mechanical testing machine to test the dry shear strength of the test piece; parameter setting: press rate 5mm / min, so that the specimen is destroyed within 1min, test accuracy 10N. The maximum force divided by the bonding area is the shear strength. The bonding strength (MPa) of the water-resistant modified starch adhesives prepared in Examples 1 to 3 and Comparative Examples 1 to 7 was measured according to this method. The test results are shown in Table 1.

[0034] Water resistance test: According to GB / T17657-2013 "Test Methods for Physical and Chemical Properties of Artificial Boards and Faced Artificial Boards", prepare one poplar wood chip of 50mm×30mm×10mm and one poplar wood chip of 30mm×25mm×10mm respectively, polish them with 600-grit sandpaper, and wipe them with anhydrous ethanol to remove surface impurities. Weigh 0.5g of adhesive and apply it evenly on the bonding surface. Splice the two wood chips into a plywood specimen with an overlap area of ​​30mm×25mm. Use F clamps to apply a pressure of more than 100N to the plywood specimen so that the adhesive fully infiltrates the overlap surface. Place the specimen at room temperature for more than 48 hours to fully dry the specimen to obtain the test piece. The test piece was immersed in 35°C water for 1h or 3h, taken out and placed in a 55°C oven for 3h, and then placed in a fixture and tested for wet shear strength using a universal mechanical testing machine; parameter setting: pressing rate 5mm / min, so that the test piece was destroyed within 1min, test accuracy 10N. The maximum force divided by the bonding area gives the shear strength. According to this method, the water resistance (MPa) of the water-resistant modified starch adhesives prepared in Examples 1-3 and Comparative Examples 1-7 was measured, and the test results are shown in Table 1.

[0035] Determination of storage stability: According to GB / T17657-2013 "Test Methods for Physical and Chemical Properties of Artificial Boards and Faced Artificial Boards", prepare poplar wood chips of 50mm×30mm×10mm and 30mm×25mm×10mm respectively, polish them with 600-grit sandpaper, and wipe them with anhydrous ethanol to remove surface impurities. Weigh 0.5g of adhesive stored for 100 days or 300 days after production and evenly apply it on the bonding surface. Splice two wood chips into plywood specimens with an overlap area of ​​30mm×25mm. Use F clamps to apply a pressure of more than 100N to the plywood specimens to fully infiltrate the overlap surface with the adhesive. Place the specimens at room temperature for more than 48 hours to fully dry them and obtain the test pieces. The test piece was immersed in 35°C water for 1 hour, taken out and placed in a 55°C oven for 3 hours, and placed in a fixture to test the wet shear strength of the test piece using a universal mechanical testing machine; parameter setting: pressing rate 5mm / min, so that the test piece is destroyed within 1 minute, test accuracy 10N. The maximum force divided by the bonding area gives the shear strength. According to this method, the storage stability (MPa) of the water-resistant modified starch adhesives prepared in Examples 1-3 and Comparative Examples 1-7 was measured, and the test results are shown in Table 1.

[0036] Table 1: Performance test results of Examples 1 to 3 and Comparative Examples 1 to 7 Data Analysis: It can be seen from Table 1 that the water-resistant modified starch adhesive prepared by the present invention has more excellent bonding strength, water resistance and storage stability.

[0037] This may be due to: (1) The functional groups such as carboxyl introduced during the preparation of pretreated starch can react with diacetone acrylamide, melamine and other substances added later to form more stable chemical bonds and cross-linked structures. These cross-linked structures can effectively prevent the intrusion of water, provide stronger cohesion and adhesion, improve the water resistance of the adhesive, and make the adhesive more secure when bonding objects. In addition, the uniformity, reactivity, structure and performance stability of the starch particles after pretreatment are improved, making the subsequent grafting and cross-linking reactions more sufficient and uniform, and also improving the spreadability and wettability of the adhesive on the surface of the bonded material, further enhancing the water resistance, strong bonding effect and storage stability of the adhesive.

[0038] (2) The acid treatment during the preparation of gelatinized starch liquid shortens the starch molecular chain and increases the fluidity of the starch molecules. The acid treatment and gelatinization process change the structure of the starch molecules and expose more reaction sites, which enables the starch molecules to mix better with other substances in the subsequent processing and react more effectively with grafting monomers such as diacetone acrylamide to form grafted copolymers. At the same time, the viscous network structure formed in the gelatinization process can form a continuous film on the surface of the bonded object, firmly bonding the objects together through the intermolecular force, so that the adhesive has good bonding effect, water resistance, flexibility, etc.

[0039] (3) During the preparation of the modified starch composition A, the gelatinized starch liquid generates free radicals under the action of the initiator, and diacetone acrylamide acts as a grafting monomer to undergo a grafting reaction with the starch, thereby increasing the spatial structure of the starch molecules and the number of active groups, thereby improving the bonding performance of the adhesive. The added adipic acid dihydrazide can also react with diacetone acrylamide under the specific reaction conditions of the present invention to form a cross-linked structure that can effectively prevent the intrusion of moisture, thereby improving the water resistance of the adhesive. Hexadecyltrimethylammonium bromide reduces the surface tension of the system and increases the fluidity of the system, allowing the adhesive to better wet and penetrate the surface of the bonded object, increasing the contact area between the adhesive and the bonded object, and further improving the bonding effect. Ultrasonic oscillation makes the gelatinized starch liquid evenly dispersed, ensuring that the initiator and the grafting liquid can evenly contact and react with the starch molecules, thereby forming a stable chemical structure.

[0040] (4) The melamine in the modified starch composition B can form a three-dimensional network structure with starch and polyvinyl alcohol that contains a large number of active groups and can significantly improve the bonding strength and bonding stability of the adhesive. When it is mixed with a specific proportion of the modified starch composition A, adipic acid dihydrazide will further react with the carbonyl groups on the diacetone acrylamide grafted chains in the composition A. At the same time, the network structure of the composition B itself will intersperse and cross-link with the composition A to form a more complex and dense multiple cross-linked network. This structure can effectively block the intrusion of moisture, greatly improve the water resistance of the adhesive, and enable it to maintain good bonding strength and stability in a humid environment. Moreover, under the action of ultrasonic dispersion, after the composition B is mixed with the specific proportion of the composition A, the composition B can make the components of the adhesive more evenly distributed on the surface of the bonded material, avoiding the situation of insufficient local bonding strength.

[0041] The above is a detailed description of an embodiment of the present invention, but the content is only a preferred embodiment of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A method for preparing a water-resistant modified starch adhesive, characterized in that: The following steps are involved: Step S1: adding pretreated starch and hydrochloric acid solution to deionized water in sequence, then adjusting the pH value to 4-4.5, gelatinizing at 85-90° C. for 1 hour, and cooling to 45-60° C. to obtain gelatinized starch solution; Step S2: After ultrasonically oscillating the gelatinized starch solution, an initiator is added, and the pH value is adjusted to 4-4.

5. After 10-15 minutes, a grafting solution is added dropwise, and the reaction is continued for 0.5-1 hour. The adjusting solution is added, and the reaction is continued for 1-1.5 hours. Then, the pH value is adjusted to 6-7 after reacting at 82.5-85° C. for 1-1.5 hours, and the modified starch composition A is obtained after cooling to 25-35° C.; Step S3: Add adipic acid dihydrazide to the modified starch composition A and stir for 15-30 minutes, then add the modified starch composition B and glycerol and stir for 30-60 minutes, adjust the pH value to 7-8, and obtain a water-resistant modified starch adhesive after ultrasonic dispersion.

2. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: The usage ratio of deionized water, pretreated starch and hydrochloric acid solution in step S1 is 150-200 mL: 150-170 g: 30 mL; The concentration of the hydrochloric acid solution in step S1 is 6 mol / L.

3. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: The method for preparing the pretreated starch in step S1 is as follows: Corn starch is added to deionized water, and after ultrasonic dispersion at 45-60° C., the pH value is adjusted to 8-9 with a sodium hypochlorite solution. After secondary ultrasonication, the pretreated starch is filtered, dried at 50-65° C., ground, and sieved.

4. The method for preparing the water-resistant modified starch adhesive according to claim 3, characterized in that: The amount of deionized water and corn starch used is 1000mL: 200-230g; The ultrasonic dispersion lasts for 25-30 minutes, the power is 280-300W, and the frequency is 35-40kHz; The concentration of the sodium hypochlorite solution is 5 mol / L; The duration of the secondary ultrasonic treatment is 35-50 min, the power is 280-300 W, and the frequency is 35-40 kHz; The sieve used during the sieving process has a mesh size of 200-300.

5. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: The amount of the gelatinized starch solution, initiator, grafting solution, and regulating solution in step S2 is 180-230 mL: 14.3-15 mL: 55-60 mL: 30-35 mL; The duration of the ultrasonic oscillation in step S2 is 10-15 minutes, the power is 280-300W, and the frequency is 35-40kHz.

6. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: The preparation method of the initiator described in step S2 is as follows: Dissolve 5-5.2 g of ammonium cerium nitrate in 14.3-15 mL of deionized water, then add 0.72-0.75 g of nitric acid and stir for 10-15 minutes to obtain an initiator.

7. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: The preparation method of the grafting solution in step S2 is as follows: Dissolve 40-50 g of diacetone acrylamide in 55-60 mL of deionized water to obtain a grafting solution.

8. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: The preparation method of the regulating solution in step S2 is as follows: Dissolve 5 g of hexadecyltrimethylammonium bromide in 30-35 mL of deionized water to obtain a regulating solution.

9. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: In step S3, the usage ratio of the modified starch composition A, adipic acid dihydrazide, the modified starch composition B, and glycerol is 279-340 mL: 23-26 g: 125-190 mL: 5-10 g; The duration of the ultrasonic dispersion in step S3 is 10-15 min, the power is 280-300 W, and the frequency is 35-40 kHz.

10. The method for preparing the water-resistant modified starch adhesive according to claim 1, characterized in that: The preparation method of the modified starch composition B in step S3 is as follows: Step B1: Add 5-10 g of polyvinyl alcohol to 20-30 mL of deionized water, and stir at 80-90° C. for 15-30 min to obtain a polyvinyl alcohol solution; Step B2: Add 2-5 g of melamine to 5-10 mL of deionized water and adjust the pH to 3-5, and stir at 60-80° C. until dissolved; Step B3: 50-60 g of pretreated starch, 100-150 mL of deionized water, 20-30 mL of polyvinyl alcohol solution and 5-10 mL of melamine solution are mixed and stirred evenly, then 0.5-1 g of ammonium persulfate is added and reacted at 50-70° C. for 1-3 h to obtain a modified starch composition B.

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