Water-resistant modified starch adhesive and preparation method thereof

Abstract

The application discloses a kind of water-resistant modified starch adhesives and preparation method thereof, it is related to adhesive technical field.The application includes the following steps: in deionized water sequentially adding pretreatment starch, hydrochloric acid solution, then adjusting pH value and carrying out gelatinization, after cooling, ultrasonic oscillation is carried out and initiator is added, after adjusting pH value, grafting liquid is added dropwise, after reaction, add adjusting liquid, continue to react after heating reaction, then adjust pH value again, after cooling, add adipic acid dihydrazide and stir uniformly, finally add modified starch composition B, glycerol and stir reaction, adjust pH value and obtain water-resistant modified starch adhesive after ultrasonic dispersion.The pretreatment of corn starch of the application effectively improves the uniformity of starch granule, reactivity;The introduction of grafting liquid, modified starch composition B and other components further improves its bonding strength, water resistance and storage stability.Therefore, the adhesive of the application has more extensive application prospect.

Description

TECHNICAL FIELD

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

[0002] Wood adhesive is a soft material that can make different forms of wood tightly glued together under certain conditions. The use of wood adhesive is wide-ranging, including laminated beams, integrated materials, plywood, particle board, fiberboard, composite flooring, decorative veneer, etc. The wood-based panel industry, wood product industry and house decoration industry need to consume a large amount of adhesive. Most of the wood adhesives sold on the market are urea-formaldehyde resin, phenol-formaldehyde resin and melamine-formaldehyde resin synthesized with aldehyde compounds as the main raw material, and are called "three aldehyde adhesives". However, if the process parameters are not properly controlled during the production of three aldehyde adhesives, harmful aldehyde components will inevitably remain in the finished product, causing great harm to the environment and human body. In view of the above, it is urgent to develop new environmentally friendly wood adhesives.

[0003] Starch is a green material widely used in food, medicine, industrial production and other fields. After the structure of starch is opened, the intramolecular cohesion and stability can be improved by starch modification, which is considered as a very potential biomass adhesive raw material. Starch adhesive has the advantages of low raw material cost, wide source and environmental friendliness. Although starch-based adhesives have been applied in some wood processing fields, their limitations in bonding strength, water resistance and storage stability seriously restrict their wide application. In order to solve this technical problem, patent document CN105754516A proposes a starch / phenol-formaldehyde resin composite adhesive and a preparation method thereof, which is composed of the following components 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 the starch adhesive prepared by these existing solutions still need to be improved. SUMMARY

[0004] The purpose of the present application is to provide a water-resistant modified starch adhesive and a preparation method thereof, which solves the following technical problems:

[0005] The existing starch adhesive still has the problem of unsatisfactory bonding strength, water resistance and storage stability.

[0006] The purpose of the present application can be achieved by the following technical solutions:

[0007] A preparation method of a water-resistant modified starch adhesive, comprising the following steps:

[0008] Step S1: sequentially adding pretreated starch and hydrochloric acid solution into deionized water, then adjusting pH value to 4-4.5, and pasting at 85-90℃ for 1h, and then cooling to 45-60℃ to obtain a pasted starch solution;

[0009] Step S2: after ultrasonic oscillation of the pasted starch solution for 10min, adding an initiator, adjusting pH value to 4-4.5, and then adding a grafting solution dropwise after 10-15min, and then adding a regulating solution after 0.5-1h of reaction, and then continuing to react for 1-1.5h, and then adjusting pH value to 6-7 after 1-1.5h of reaction at 82.5-85℃, and then cooling to 25-35℃ to obtain a modified starch composition A;

[0010] Step S3: adding adipic acid dihydrazide into the modified starch composition A and stirring for 15-30min, and then adding a modified starch composition B and glycerol and stirring for 30-60min, and then adjusting pH value to 7-8, and then obtaining a water-resistant modified starch adhesive after ultrasonic dispersion.

[0011] Preferably, the deionized water, pretreated starch and hydrochloric acid solution in step S1 are used in a ratio of 150-200mL: 150-170g: 30mL.

[0012] The concentration of the hydrochloric acid solution in step S1 is 6mol / L.

[0013] Preferably, the pretreated starch in step S1 is prepared as follows:

[0014] adding corn starch into deionized water, ultrasonic dispersing at 45-60℃, adjusting pH value to 8-9 with sodium hypochlorite solution, and then performing secondary ultrasonic treatment, and then performing filtration, drying at 50-65℃, grinding, and sieving to obtain pretreated starch.

[0015] Preferably, the deionized water and corn starch are used in a ratio of 1000mL: 200-230g.

[0016] The ultrasonic dispersing time is 25-30min, the power is 280-300W, and the frequency is 35-40kHz.

[0017] The concentration of the sodium hypochlorite solution is 5mol / L.

[0018] The secondary ultrasonic treatment time is 35-50min, the power is 280-300W, and the frequency is 35-40kHz.

[0019] The sieving is performed using a 200-300mesh sieve.

[0020] Preferably, the dosages of the gelatinized starch solution, initiator, grafting solution, and adjusting solution in step S2 are 180-230 mL, 14.3-15 mL, 55-60 mL, and 30-35 mL, respectively.

[0021] The ultrasonic oscillation in step S2 is performed for 10-15 min at a power of 280-300 W and a frequency of 35-40 kHz.

[0022] Preferably, the initiator in step S2 is prepared as follows:

[0023] 5-5.2 g of cerium ammonium nitrate is dissolved in 14.3-15 mL of deionized water, followed by the addition of 0.72-0.75 g of nitric acid, and stirring for 10-15 min to obtain the initiator.

[0024] Preferably, the grafting solution in step S2 is prepared as follows:

[0025] 40-50 g of diacetone acrylamide is dissolved in 55-60 mL of deionized water to obtain the grafting solution.

[0026] Preferably, the adjusting solution in step S2 is prepared as follows:

[0027] 5 g of cetyltrimethylammonium bromide is dissolved in 30-35 mL of deionized water to obtain the adjusting solution.

[0028] Preferably, the dosages of the modified starch composition A, adipic acid dihydrazide, modified starch composition B, and glycerol in step S3 are 279-340 mL, 23-26 g, 125-190 mL, and 5-10 g, respectively.

[0029] The ultrasonic dispersion in step S3 is performed for 10-15 min at a power of 280-300 W and a frequency of 35-40 kHz.

[0030] Preferably, the modified starch composition B in step S3 is prepared as follows:

[0031] Step B1: 5-10 g of polyvinyl alcohol is added to 20-30 mL of deionized water, and stirred at 80-90 °C for 15-30 min to obtain a polyvinyl alcohol solution;

[0032] Step B2: 2-5 g of melamine is added to 5-10 mL of deionized water, and the pH is adjusted to 3-5, followed by stirring to dissolution at 60-80 °C;

[0033] 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 uniformly, then 0.5-1 g of ammonium persulfate is added and reacted at 50-70°C for 1-3 h to obtain the modified starch composition B.

[0034] As a further aspect of the present application.

[0035] The present application has the following advantages:

[0036] The present application provides a water-resistant modified starch adhesive and a preparation method thereof. The present application effectively improves the bonding strength, water resistance and storage stability of the starch adhesive by the following method.

[0037] (1) The functional groups such as carboxyl groups introduced in the pretreatment starch preparation process can react with the subsequently added substances such as diacetone acrylamide and melamine, forming more stable chemical bonds and crosslinking structures. These crosslinking structures can effectively prevent the intrusion of water, provide stronger cohesion and bonding force, improve the water resistance of the adhesive, and make the adhesive more firm when bonding objects. And the uniformity, reactivity, structure and performance stability of the pretreated starch particles are improved, so that the subsequent grafting and crosslinking reactions are more complete and uniform, and the spreading and wetting properties of the adhesive on the surface of the bonded material are also improved, further enhancing the water resistance, bonding effect and storage stability of the adhesive.

[0038] (2) The acid treatment in the preparation process of the gelatinized starch solution shortens the starch molecular chain and increases the flowability of the starch molecules. The acid treatment and gelatinization process change the structure of the starch molecules, exposing more reaction sites. This allows the starch molecules to better mix with other substances and react more effectively with grafting monomers such as diacetone acrylamide during the subsequent treatment process, forming graft copolymers. At the same time, the viscous network structure formed during the gelatinization process can form a continuous film on the surface of the bonded object, firmly bonding the objects together through intermolecular forces, making the adhesive have good bonding effect, water resistance, flexibility and other properties.

[0039] (3) In the preparation of the modified starch composition A, the gelatinized starch solution generates free radicals under the action of the initiator, and the diacetone acrylamide as the grafting monomer reacts with the starch to increase the spatial structure and the number of active groups of the starch molecules, thereby improving the bonding performance of the adhesive. The adipic acid dihydrazide added can also react with the diacetone acrylamide under the specific reaction conditions of the present application to form a crosslinked structure that can effectively prevent the intrusion of moisture, thereby improving the water resistance of the adhesive. The cetyltrimethylammonium bromide reduces the surface tension of the system and improves the fluidity of the system, so that the adhesive can better wet and penetrate the surface of the bonded object, increase the contact area between the adhesive and the bonded object, and further improve the bonding effect. The ultrasonic oscillation makes the gelatinized starch solution uniformly dispersed, ensuring that the initiator and the grafting solution can uniformly 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 the starch and polyvinyl alcohol, which 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, the adipic acid dihydrazide will further react with the carbonyl groups on the grafting chains of the diacetone acrylamide in composition A, and at the same time, the network structure of composition B itself will also interpenetrate and crosslink with composition A, forming a more complex and dense multiple crosslinked network. This structure can effectively block the intrusion of moisture, greatly improving the water resistance of the adhesive, so that it can still maintain good bonding force and stability in a humid environment. And under the action of ultrasonic dispersion, after composition B is mixed with a specific proportion of composition A, composition B can make the components of the adhesive more uniformly distributed on the surface of the bonded material, avoiding the occurrence of local insufficient bonding force.

[0041] Therefore, the modified starch adhesive prepared by the present application has more excellent bonding strength, water resistance, storage stability, and wide application prospects. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0043] Example 1: A method for preparing a water-resistant modified starch adhesive is as follows:

[0044] S1: 200 g of corn starch was added into 1000 mL of deionized water, and ultrasonic dispersion was performed at 45 °C for 25 min, with a power of 280 W and a frequency of 35 kHz. Then, the pH was adjusted to 8 with a 5 mol / L sodium hypochlorite solution, and secondary ultrasonic dispersion was performed at 45 °C for 35 min, with a power of 280 W and a frequency of 35 kHz. After that, the mixture was subjected to filtration, drying at 50 °C, grinding, and sieving through a 200-mesh screen to obtain pretreated starch;

[0045] S2: 150 g of the pretreated starch, 30 mL of a 6 mol / L hydrochloric acid solution, and 150 mL of deionized water were sequentially added, and then the pH was adjusted to 4 with a 1 mol / L sodium hydroxide solution. After that, the mixture was gelatinized at 85 °C for 1 h, and then cooled to 45 °C to obtain a gelatinized starch solution;

[0046] S3: 5 g of cerium ammonium nitrate was dissolved in 14.3 mL of deionized water, and then 0.72 g of nitric acid was added, and stirred for 10 min to obtain an initiator;

[0047] S4: 40 g of diacetone acrylamide was dissolved in 55 mL of deionized water to obtain a grafting solution;

[0048] S5: 5 g of cetyltrimethylammonium bromide was dissolved in 30 mL of deionized water to obtain an adjusting solution;

[0049] S6: 180 mL of the gelatinized starch solution was subjected to ultrasonic oscillation for 10 min, with a power of 280 W and a frequency of 35 kHz. Then, 14.3 mL of the initiator was added, and the pH was adjusted to 4 with a 1 mol / L sodium hydroxide solution. After 10 min, 55 mL of the grafting solution was added dropwise, and reacted for 0.5 h. Then, 30 mL of the adjusting solution was added, and reacted for 1 h. After that, the mixture was reacted at 82.5 °C for 1 h, and then the pH was adjusted to 6 with a 1 mol / L sodium hydroxide solution. After cooling to 25 °C, a modified starch composition A was obtained;

[0050] S7: 5 g of polyvinyl alcohol was added into 20 mL of deionized water, and stirred at 80 °C for 15 min to obtain a polyvinyl alcohol solution;

[0051] S8: 2 g of melamine was added into 5 mL of deionized water, and the pH was adjusted to 3 with nitric acid. After that, the mixture was stirred at 60 °C until the melamine was dissolved;

[0052] S9: 50 g of pretreated starch, 100 mL of deionized water, 20 mL of the polyvinyl alcohol solution, and 5 mL of the melamine solution were mixed and stirred uniformly. Then, 0.5 g of ammonium persulfate was added, and reacted at 50 °C for 1 h to obtain a modified starch composition B;

[0053] S10: 23 g adipic acid dihydrazide was added into 279 mL modified starch composition A and stirred for 15 min, then 125 mL modified starch composition B, 5 g glycerol were added and stirred for 30 min, the pH value was adjusted to 7 with sodium hydroxide solution with a concentration of 1 mol / L, and finally ultrasonic dispersion was carried out for 10 min at a power of 280 W and a frequency of 35 kHz to obtain a water-resistant modified starch adhesive.

[0054] Example 2: A water-resistant modified starch adhesive was prepared as follows:

[0055] S1: 215 g of corn starch was added into 1000 mL of deionized water, ultrasonic dispersion was carried out for 28 min at a power of 290 W and a frequency of 38 kHz at 50°C, then the pH value was adjusted to 8.5 with sodium hypochlorite solution with a concentration of 5 mol / L, secondary ultrasonic dispersion was carried out for 40 min at a power of 290 W and a frequency of 38 kHz, and then the pretreated starch was obtained by filtration, drying at 55°C, grinding and sieving through a 250 mesh screen;

[0056] S2: 160 g of pretreated starch, 30 mL of hydrochloric acid solution with a concentration of 6 mol / L were sequentially added into 180 mL of deionized water, then the pH value was adjusted to 4.3 with sodium hydroxide solution with a concentration of 1 mol / L, and then the gelatinized starch solution was obtained by gelatinizing at 88°C for 1 h and cooling to 50°C;

[0057] S3: 5.1 g of cerium ammonium nitrate was dissolved in 14.7 mL of deionized water, then 0.74 g of nitric acid was added and stirred for 13 min to obtain an initiator;

[0058] S4: 45 g of diacetone acrylamide was dissolved in 58 mL of deionized water to obtain a grafting solution;

[0059] S5: 5 g of cetyltrimethylammonium bromide was dissolved in 33 mL of deionized water to obtain an adjusting solution;

[0060] 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, then 14.7 mL of initiator was added, the pH value was adjusted to 4.3 with sodium hydroxide solution with a concentration of 1 mol / L, 58 mL of grafting solution was added dropwise after 13 min, the reaction was continued for 0.8 h, then 33 mL of adjusting solution was added and the reaction was continued for 1.3 h, then the pH value was adjusted to 6.5 with sodium hydroxide solution with a concentration of 1 mol / L after the reaction was continued for 1.3 h at 84°C, and the modified starch composition A was obtained after cooling to 30°C;

[0061] S7: 8 g of polyvinyl alcohol was added into 25 mL of deionized water, and stirring was carried out at 85°C for 20 min to obtain a polyvinyl alcohol solution;

[0062] S8: 4 g melamine was added in 8 mL deionized water and adjusted pH to 4 with nitric acid, stirred to dissolve at 70°C;

[0063] S9: 55 g pretreated starch, 125 mL deionized water, 25 mL polyvinyl alcohol solution and 8 mL melamine solution were mixed and stirred uniformly, then 0.8 g ammonium persulfate was added and reacted for 2 h at 60°C to obtain modified starch composition B;

[0064] S10: 25 g adipic acid dihydrazide was added in 315 mL modified starch composition A and stirred for 20 min, then 158 mL modified starch composition B, 8 g glycerol were added and stirred for 45 min, the pH value was adjusted to 7.5 with 1 mol / L sodium hydroxide solution, and finally ultrasonic dispersion was carried out for 13 min at a power of 290 W and a frequency of 38 kHz to obtain a water-resistant modified starch adhesive.

[0065] Example 3: A water-resistant modified starch adhesive was prepared as follows:

[0066] S1: 230 g corn starch was added in 1000 mL deionized water, ultrasonic dispersed for 30 min at a power of 300 W and a frequency of 40 kHz, then the pH was adjusted to 9 with 5 mol / L sodium hypochlorite solution, and then ultrasonic dispersed for 50 min at a power of 300 W and a frequency of 40 kHz, and then filtered, dried at 65°C, ground, and sieved through a 300 mesh screen to obtain pretreated starch;

[0067] S2: 170 g pretreated starch, 30 mL 6 mol / L hydrochloric acid solution were added in 200 mL deionized water in sequence, 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 then cooled to 60°C to obtain a gelatinized starch solution;

[0068] S3: 5.2 g cerium ammonium nitrate was dissolved in 15 mL deionized water, then 0.75 g nitric acid was added, stirred for 15 min to obtain an initiator;

[0069] S4: 50 g diacetone acrylamide was dissolved in 60 mL deionized water to obtain a grafting solution;

[0070] S5: 5 g cetyltrimethylammonium bromide was dissolved in 35 mL deionized water to obtain an adjusting solution;

[0071] S6: 230 mL of the gelatinized starch solution was subjected to ultrasonic oscillation for 15 min at a power of 300 W and a frequency of 40 kHz, 15 mL of the initiator was added, the pH value was adjusted to 4.5 using a sodium hydroxide solution with a concentration of 1 mol / L, 60 mL of the grafting solution was added dropwise after 10-15 min, the reaction was continued for 1 h, 35 mL of the adjusting solution was added, the reaction was continued for 1.5 h, then the reaction was continued at 85 °C for 1.5 h, the pH value was adjusted to 7 using a sodium hydroxide solution with a concentration of 1 mol / L, and the modified starch composition A was obtained after cooling to 35 °C;

[0072] S7: 10 g of polyvinyl alcohol was added to 30 mL of deionized water, and stirred at 90 °C for 30 min to obtain a polyvinyl alcohol solution;

[0073] S8: 5 g of melamine was added to 10 mL of deionized water, and the pH was adjusted to 5 using nitric acid, and stirred to dissolve at 80 °C;

[0074] S9: 60 g of the pretreated starch, 150 mL of deionized water, 30 mL of the polyvinyl alcohol solution and 10 mL of the melamine solution were mixed and stirred uniformly, then 1 g of ammonium persulfate was added and reacted at 70 °C for 3 h to obtain the modified starch composition B;

[0075] S10: 26 g of adipic acid dihydrazide was added to 340 mL of the modified starch composition A and stirred for 30 min, then 190 mL of the modified starch composition B, 10 g of glycerol were added and stirred for 60 min, the pH value was adjusted to 8 using a sodium hydroxide solution with a concentration of 1 mol / L, and finally ultrasonic dispersion was performed for 15 min at a power of 300 W and a frequency of 40 kHz to obtain the water-resistant modified starch adhesive.

[0076] Comparative Example 1:

[0077] This comparative example is compared with Example 1 only by replacing the "pretreated starch" added in the preparation process of the gelatinized starch solution with "corn starch", and the rest of the steps and parameters are the same. This comparative example will not be repeated here. Finally, a water-resistant modified starch adhesive is obtained.

[0078] Comparative Example 2:

[0079] This comparative example is compared with Example 1 only by replacing the "180 mL of the gelatinized starch solution" added in the preparation process of the modified starch composition A with "a pretreated starch solution obtained by uniformly mixing 180 mL of deionized water and 150 g of pretreated starch", and the rest of the steps and parameters are the same. This comparative example will not be repeated here. Finally, a water-resistant modified starch adhesive is obtained.

[0080] Comparative Example 3:

[0081] The comparative example is compared with example 1 only by replacing the "180 mL of gelatinized starch liquid" added in the preparation process of the modified starch composition A with "corn starch liquid obtained by uniformly mixing 180 mL of deionized water and 150 g of corn starch", and the rest of the steps and parameters are the same. The comparative example will not be repeated here. Finally, a water-resistant modified starch adhesive is obtained.

[0082] Comparative example 4:

[0083] The comparative example is compared with example 1 only by replacing the "pre-treated starch" added in the preparation process of the modified starch composition B with "corn starch", and the rest of the steps and parameters are the same. The comparative example will not be repeated here. Finally, a water-resistant modified starch adhesive is obtained.

[0084] Comparative example 5:

[0085] The comparative example is compared with example 1 only by replacing the "23 g of adipic acid dihydrazide is added in 279 mL of modified starch composition A and stirred for 15 min, and then 125 mL of modified starch composition B, 5 g of glycerol" in the preparation process of the water-resistant modified starch adhesive of S9 with "23 g of adipic acid dihydrazide is added in 125 mL of modified starch composition A and stirred for 15 min, and then 279 mL of modified starch composition B, 5 g of glycerol", and the rest of the steps and parameters are the same. The comparative example will not be repeated here. Finally, a water-resistant modified starch adhesive is obtained.

[0086] Comparative example 6:

[0087] The comparative example is compared with example 1 only by replacing the "modified starch composition B" added in the preparation process of the water-resistant modified starch adhesive of S9 with "modified starch composition A", and the rest of the steps and parameters are the same. The comparative example will not be repeated here. Finally, a water-resistant modified starch adhesive is obtained.

[0088] Comparative example 7:

[0089] The comparative example is compared with example 1 only by not performing ultrasonic dispersion in the preparation process of the water-resistant modified starch adhesive of S9, and the rest of the steps and parameters are the same. The comparative example will not be repeated here. Finally, a water-resistant modified starch adhesive is obtained.

[0090] Performance detection:

[0091] Test of bonding strength:

[0092] According to GB / T 17657-2013 "Test methods of physical and mechanical properties for wood-based panels and laminated veneer lumber", one piece of poplar slice with the size of 50mm x 30mm x 10mm and 30mm x 25mm x 10mm was prepared respectively, which was polished smooth with 600 mesh sandpaper and wiped with anhydrous ethanol to remove surface impurities. 0.5g of adhesive was evenly coated on the bonding surface, and two pieces of wood were combined to make plywood test pieces with a lap area of 30mm x 25mm. F clamp was used to apply a pressure of more than 100N to the plywood test piece to make the adhesive fully infiltrate the lap surface. The test piece was placed at room temperature for more than 48h to dry completely, and the test piece was obtained. Then the dry shear strength of the test piece was tested by using a universal mechanical testing machine; parameter setting: the pressing rate was 5mm / min, the test piece was destroyed within 1min, and the test precision was 10N. The shear strength was obtained by dividing the maximum force by the bonding area. The bonding strength (MPa) of the water-resistant modified starch adhesives prepared by the determination of Examples 1-3 and Comparative Examples 1-7 was tested according to this method, and the test results are shown in Table 1.

[0093] Test of water resistance:

[0094] According to GB / T 17657-2013 "Test methods of physical and mechanical properties for wood-based panels and laminated veneer lumber", one piece of poplar slice with the size of 50mm x 30mm x 10mm and 30mm x 25mm x 10mm was prepared respectively, which was polished smooth with 600 mesh sandpaper and wiped with anhydrous ethanol to remove surface impurities. 0.5g of adhesive was evenly coated on the bonding surface, and two pieces of wood were combined to make plywood test pieces with a lap area of 30mm x 25mm. F clamp was used to apply a pressure of more than 100N to the plywood test piece to make the adhesive fully infiltrate the lap surface. The test piece was placed at room temperature for more than 48h to dry completely, and the test piece was obtained. Then the dry shear strength of the test piece was tested by using a universal mechanical testing machine; parameter setting: the pressing rate was 5mm / min, the test piece was destroyed within 1min, and the test precision was 10N. The shear strength was obtained by dividing the maximum force by the bonding area. The bonding strength (MPa) of the water-resistant modified starch adhesives prepared by the determination of Examples 1-3 and Comparative Examples 1-7 was tested according to this method, and the test results are shown in Table 1.

[0095] Determination of storage stability:

[0096] According to GB / T 17657-2013 "Physico-chemical property test method of artificial board and veneered artificial board", one piece of poplar slice with a size of 50mmx30mmx10mm and one piece of poplar slice with a size of 30mmx25mmx10mm were prepared, polished smooth with 600 mesh sandpaper, and wiped with anhydrous ethanol to remove surface impurities. 0.5g of the adhesive stored for 100 days or 300 days after production was uniformly coated on the gluing surface, and the two wood slices were combined to form a plywood test piece with a lap area of 30mmx25mm. A F clamp was used to apply a pressure of 100N or more to the plywood test piece to allow the adhesive to fully infiltrate the lap surface. The test piece was placed in a 55°C oven for 3h after being immersed in water at 35°C for 1h, and then placed in a clamp for wet shear strength testing using a universal mechanical testing machine; parameter settings: a pressing rate of 5mm / min, the test piece was destroyed within 1min, and the test precision was 10N. The maximum force was divided by the bonding area to obtain the shear strength. The storage stability (MPa) of the water-resistant modified starch adhesives prepared from Examples 1-3 and Comparative Examples 1-7 was determined according to this method, and the test results are shown in Table 1.

[0097] Table 1: Performance test results of Examples 1-3 and Comparative Examples 1-7

[0098]

[0099] Data analysis:

[0100] As can be seen from Table 1, the water-resistant modified starch adhesives prepared by the present application have more excellent bonding strength, water resistance and storage stability.

[0101] This may be due to:

[0102] (1) The functional groups such as carboxyl groups introduced during the preparation of the pretreated starch can react with the subsequently added substances such as diacetone acrylamide and melamine, forming more stable chemical bonds and crosslinking structures. These crosslinking structures can effectively prevent the intrusion of water, provide stronger cohesion and bonding force, improve the water resistance of the adhesive, and make the adhesive more firm when bonding objects. And the uniformity, reactivity, structure and performance stability of the pretreated starch particles are improved, so that the subsequent grafting and crosslinking reactions are more complete and uniform, and the spreading and wetting properties of the adhesive on the surface of the bonded material are also improved, further enhancing the water resistance, strong bonding effect and storage stability of the adhesive.

[0103] (2) The acid treatment in the preparation process of the gelatinized starch solution makes the starch molecular chain shorter, and increases the flowability of the starch molecules. The acid treatment and the gelatinization process change the structure of the starch molecules, and more reaction sites are exposed, which makes the starch molecules better mixed with other substances in the subsequent process, and more effectively react with the grafting monomer such as diacetone acrylamide to form a graft copolymer. 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, and firmly bond the object together through the intermolecular force, so that the adhesive has good bonding effect, water resistance, flexibility and the like.

[0104] (3) In the preparation process of the modified starch composition A, the gelatinized starch solution generates free radicals under the action of the initiator, and the diacetone acrylamide as the grafting monomer reacts with the starch to increase the spatial structure and the number of active groups of the starch molecules, thereby improving the bonding performance of the adhesive. The added adipic acid dihydrazide can also react with the diacetone acrylamide under the specific reaction conditions of the present application to form a cross-linked structure that can effectively prevent the intrusion of water, thereby improving the water resistance of the adhesive. The addition of cetyltrimethylammonium bromide reduces the surface tension of the system and improves the flowability of the system, so that the adhesive can better wet and penetrate the surface of the bonded object, increase the contact area between the adhesive and the bonded object, and further improve the bonding effect. Ultrasonic oscillation makes the gelatinized starch solution uniformly dispersed, ensuring that the initiator and the grafting solution can uniformly contact and react with the starch molecules, thereby forming a stable chemical structure.

[0105] (4) The melamine in the modified starch composition B can form a three-dimensional network structure with the starch and polyvinyl alcohol, which 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, the adipic acid dihydrazide will further react with the carbonyl group on the grafting chain of the diacetone acrylamide in the composition A, and at the same time, the network structure of the composition B itself will also interpenetrate and cross-link with the composition A, forming a more complex and dense multiple cross-linked network. This structure can effectively block the intrusion of water, greatly improving the water resistance of the adhesive, so that it can still maintain good bonding force and stability in a humid environment. And under the action of ultrasonic dispersion, after the composition B is mixed with a specific proportion of the composition A, the composition B can make the components of the adhesive more uniformly distributed on the surface of the bonded material, avoiding the situation that the local bonding force is insufficient.

[0106] The above describes one embodiment of the present application in detail, but the content described is only a preferred embodiment of the present application, and cannot be considered as limiting the scope of the present application. Any equivalent changes and improvements made within the scope of the present application should still belong to the patent coverage of the present application.

Claims

1. A method for preparing a water-resistant modified starch adhesive, characterized by, The method comprises the following steps: Step S1: sequentially adding pretreated starch, hydrochloric acid solution into deionized water, then adjusting the pH value to 4-4.5, and then gelatinizing at 85-90℃ for 1h, and then cooling to 45-60℃ to obtain a gelatinized starch solution; Step S2: after ultrasonic oscillation of the gelatinized starch solution, adding an initiator, adjusting the pH value to 4-4.5, and then adding a grafting solution dropwise after 10-15min, and then adding an adjusting solution after 0.5-1h of reaction, and then continuing to react for 1-1.5h, and then adjusting the pH value to 6-7 after 1-1.5h of reaction at 82.5-85℃, and then cooling to 25-35℃ to obtain a modified starch composition A; Step S3: adding adipic acid dihydrazide into the modified starch composition A and stirring for 15-30min, and then adding a modified starch composition B and glycerol and stirring for 30-60min, and then adjusting the pH value to 7-8, and then obtaining a water-resistant modified starch adhesive after ultrasonic dispersion; The preparation method of the pretreated starch in step S1 is as follows: adding corn starch into deionized water, ultrasonic dispersing at 45-60℃, adjusting the pH value to 8-9 with sodium hypochlorite solution, and then performing secondary ultrasonic treatment, and then performing filtration, drying at 50-65℃, grinding, and sieving to obtain the pretreated starch; The preparation method of the modified starch composition B in step S3 is as follows: Step B1: adding 5-10g of polyvinyl alcohol into 20-30mL of deionized water, and then stirring at 80-90℃ for 15-30min to obtain a polyvinyl alcohol solution; Step B2: adding 2-5g of melamine into 5-10mL of deionized water, and then adjusting the pH value to 3-5, and then stirring to dissolve at 60-80℃; Step B3: mixing 50-60g of pretreated starch, 100-150mL of deionized water, 20-30mL of the polyvinyl alcohol solution, and 5-10mL of the melamine solution, and then stirring uniformly, and then adding 0.5-1g of ammonium persulfate and reacting at 50-70℃ for 1-3h to obtain the modified starch composition B.

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

3. The method for preparing the water-resistant modified starch adhesive according to claim 1, wherein: The amount of the deionized water and the corn starch is 1000mL:200-230g; The ultrasonic dispersing time is 25-30min, the power is 280-300W, and the frequency is 35-40kHz; The concentration of the sodium hypochlorite solution is 5mol / L; The secondary ultrasonic treatment time is 35-50min, the power is 280-300W, and the frequency is 35-40kHz; The mesh size of the sieve used in the sieving process is 200-300.

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

5. The method for preparing the water-resistant modified starch adhesive according to claim 1, wherein: The preparation method of the initiator in step S2 is as follows: 5-5.2 g of cerium ammonium nitrate was dissolved in 14.3-15 mL of deionized water, then 0.72-0.75 g of nitric acid was added, stirred for 10-15 min to obtain an initiator.

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

7. The method for preparing the water-resistant modified starch adhesive according to claim 1, wherein: The preparation method of the adjusting solution in step S2 is as follows: 5 g of cetyltrimethylammonium bromide was dissolved in 30-35 mL of deionized water to obtain an adjusting solution.

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

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