Preparation method of cassava starch modified urea-formaldehyde resin

Abstract

The invention relates to the technical field of artificial board manufacturing and the field of adhesive synthesis, and particularly discloses a preparation method of cassava starch modified urea-formaldehyde resin, cassava starch containing more chemical active functional groups (hydroxyl, aryl and the like) is adopted for modification, and the cassava starch modified urea-formaldehyde resin is a natural adhesive and has great development potential; a cross-linked modified cassava starch network has strong stability, so that the stability of the adhesive network can still be kept when the adhesive is subjected to high-temperature hot pressing in the production process of the artificial board; the urea-formaldehyde resin optimized by the cassava starch modified by crosslinking is used for manufacturing the artificial board, so that the manufactured artificial board has more excellent water resistance and stability, and the capability of reducing the content of free formaldehyde in an adhesive is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of synthetic manufacturing of adhesives, and particularly relates to a method for manufacturing modified urea-formaldehyde resin with cassava starch. Background Art

[0003] Consulting the data, at present, the starch used for modified resin in China is mostly corn starch. The proportion of linear molecules, branched molecules, and components such as fat in its composition is quite different from that of cassava starch. At the same time, corn starch is also one of the three major food sources in China. Therefore, after investigation by the research group, it was found that after modification of non-edible cassava starch, it can effectively optimize the general performance of urea-formaldehyde resin for wood-based panels while the physical and chemical properties of its samples do not show obvious decline. Moreover, as the main producing area of cassava starch, Guangxi not only has a great price advantage but also expands the processing and application fields of cassava starch. Summary of the Invention

[0004] The purpose of the present invention is to solve the technical problem of poor performance of starch-modified urea-formaldehyde resin in the prior art, and provide a preparation method for modified urea-formaldehyde resin with cassava starch. The preparation method includes the following steps:

[0005] A preparation method for modified urea-formaldehyde resin with cassava starch, the preparation method includes the following steps:

[0006] (1) Put cassava starch into ammonium persulfate solution, adjust the pH value of the solution so that the reaction system is in a strong acid environment, and react under heating conditions until the solution system reaches a certain viscosity as the first reaction end point to obtain a preliminarily oxidized cassava starch solution;

[0007] (2) Adjust the pH value of the preliminarily oxidized cassava starch solution obtained in step (1) to weakly alkaline, and then add cross-linking small molecules for modification to obtain modified cassava starch;

[0008] (3) Put formaldehyde, urea, melamine, the modified cassava starch obtained in step (2), water, and a modifier into the reaction kettle step by step using the alkali-acid-alkali process for reaction. Each time of feeding, it is necessary to adjust the solution to reach the corresponding pH value, heat to the reaction temperature, and keep heating for reaction until the solution system reaches a certain viscosity as the reaction end point to obtain modified urea-formaldehyde resin with cassava starch.

[0009] Preferably, in step (1), the mass concentration of the ammonium persulfate solution is 10-50%.

[0010] Preferably, in step (1), the viscosity of the system at the first reaction end point is between 50-200 mPa·s.

[0011] Preferably, in step (3), the molar ratio of formaldehyde to urea is 1:1.5.

[0012] Preferably, the crosslinked small molecule is one or more of hydroxypropyl cellulose, polyethylene glycol, chitosan, acrylamide, glycerol, nano-silica, sodium alginate, and the addition amount of the crosslinked small molecule is 1% to 5% of the system mass.

[0013] Preferably, the alkali-acid-alkali process steps are as follows:

[0014] S1: Put formaldehyde and water into the reaction kettle at one time, stir, and add an alkali solution to adjust the pH value to 7.0 - 9.0;

[0015] S2: Add the first urea, the first melamine and the modifier, heat up to 60 - 80 °C and keep the temperature for reaction for 20 - 60 min, add acid to adjust the pH value to 3 - 6, and when the solution viscosity reaches the end point, the first feeding reaction ends;

[0016] S3: Add alkali to adjust the pH value to 4 - 7, add the second urea, the second melamine and the modifier, keep the temperature at 70 - 90 °C for reaction for 20 - 60 min, and when the solution viscosity reaches the end point, the second feeding reaction ends;

[0017] S4: Add alkali to adjust the pH value to 7 - 8.5, add the third urea and the modified cassava starch obtained in step (2), keep the temperature at 70 - 90 °C for reaction for 15 min, and the third feeding reaction ends;

[0018] S5: Cool down to 30 - 60 °C, add the fourth part of urea, and when the adhesive drops to room temperature, it is regarded as the end of the synthesis of the modified urea-formaldehyde resin.

[0019] Preferably, the modifier is polyethylene glycol.

[0020] Preferably, the addition amount of the first melamine and the addition amount of the second melamine are both 1.5% of the total glue amount; the addition amount of the modified cassava starch obtained in step (2) is 2% of the total glue amount, and the total dosage of the modifier is 0.5 - 1.0% of the total glue amount.

[0021] Another object of the present invention also lies in protecting the modified urea-formaldehyde resin of cassava starch prepared by the preparation method of the above-mentioned modified urea-formaldehyde resin of cassava starch.

[0022] Preferably, the cassava starch is non-edible cassava starch.

[0023] Preferably, the purities of the formaldehyde, the urea, and the melamine are all ≥ 95%.

[0024] Compared with the existing technology, the present invention has the following beneficial effects:

[0025] 1. A preparation method of cassava starch modified urea-formaldehyde resin of the present invention oxidatively hydrolyzes raw cassava starch with 10-50% ammonium persulfate solution and distilled water under strong acid conditions at 90-94°C. After the viscosity of the system reaches the required value, the pH of the system is adjusted to 7-8, and the temperature is lowered to 35-40°C. Then, 1%-5% of cross-linked small molecules based on the mass of the system are added, and after stirring evenly, it is stored in the dark at room temperature. Under strong acid and heating conditions, acidic hydrolysis of cassava starch occurs, breaking the α-1,4-glycosidic bonds in the starch molecules to generate oligosaccharides and small molecule sugars with multiple hydroxyl groups, causing partial depolymerization of the starch molecular chains to form short straight-chain molecules. The solubility of the system increases, the viscosity decreases, and the swelling power decreases, avoiding a sharp increase in the molecular weight of the system when modifying urea-formaldehyde resin with modified starch and extending the storage performance of the modified starch. At the same time, under the action of strong oxidants, the hydroxyl groups on the carbon atoms in cassava starch molecules are oxidized to carboxyl groups, which can react with hydroxymethyl and free formaldehyde in the system, reducing the content of free formaldehyde in the finished urea-formaldehyde resin and also improving the water resistance of the resin.

[0026] 2. A preparation method of cassava starch modified urea-formaldehyde resin of the present invention designs the molar ratio of formaldehyde to urea to be 1.5, adopts an alkali-acid-alkali process, and feeds urea 4 times. The dosage of melamine is 1.5% (total glue amount, the same below) and is fed together with the first urea feeding; 2% of modified cassava starch is weighed and fed together with an appropriate amount of the third urea feeding. When the viscosity of the system reaches an appropriate value, the temperature is lowered, and the fourth urea is added. After maintaining the temperature for 20 minutes, the temperature is lowered and the product is discharged. Modified cassava starch is added in the middle and late stages of the polycondensation reaction of formaldehyde, urea, and melamine, introducing a large number of hydroxyl groups, aldehyde groups, and carboxyl groups, which react with the unreacted hydroxymethyl prepolymer, imino group, and free formaldehyde molecules in the system; however, at this time, the number of reactions in the mixed system is much less than that in the initial stage. Therefore, adding modified cassava starch in the third step of urea addition will not have an obvious impact on the properties such as the curing time of the finished resin; in addition, adding an appropriate amount of polyethylene glycol in the early stage makes oxidation starch-polyethylene glycol, oxidation starch-urea-formaldehyde resin, polyethylene glycol-urea-formaldehyde resin, and polyethylene glycol-starch-urea-formaldehyde resin condensation and esterification reactions occur in the system after adding modified cassava starch under weak acid conditions, producing a cross-linked network structure, improving the water resistance and stability of the resin, and reducing the content of free formaldehyde, etc. Specific embodiments

[0027] The following describes the specific embodiments of the present invention in detail, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

[0028] To illustrate the technical effects of the present invention, the present invention uses analytical pure cassava starch to conduct the following experiments. Of course, in specific applications, the following method can be used to treat ordinary cassava starch and then modify it according to the method of the present invention: Wash the ordinary cassava starch to remove impurities and dust therein, and then screen it through a screening device to ensure uniform particle size of the cassava starch. Then, dry the washed and screened cassava starch to remove excess moisture. Usually, hot air drying or vacuum drying methods are used to control the drying temperature and time to avoid thermal degradation of the cassava starch.

[0029] Example 1

[0030] A preparation method of modified urea-formaldehyde resin from cassava starch, the preparation method comprising the following steps:

[0031] (1) Put the cassava starch into an ammonium persulfate solution with a mass concentration of 25%, adjust the pH value of the solution so that the reaction system is in a strong acid environment with a pH value of 3, and react at 90°C under heating conditions until the solution system reaches 200 mPa·s as the first reaction end point to obtain a preliminarily oxidized cassava starch solution;

[0032] (2) Adjust the pH value of the preliminarily oxidized cassava starch solution obtained in step (1) to 8, and then add hydroxypropyl cellulose for modification to obtain modified cassava starch; the addition amount of hydroxypropyl cellulose is 1% of the system mass;

[0033] (3) Put formaldehyde, urea, melamine, the modified cassava starch obtained in step (2), water, and a modifier into the reaction kettle step by step using the alkali-acid-alkali process for reaction. Each time of feeding, it is necessary to adjust the solution to reach the corresponding pH value, heat to the reaction temperature, and keep heating for reaction until the solution system reaches a certain viscosity as the reaction end point to obtain modified urea-formaldehyde resin from cassava starch; wherein, the molar ratio of the formaldehyde to the urea is 1:1.5; the purity of the formaldehyde, the urea, and the melamine is all ≥95%.

[0034] Specifically, the alkali-acid-alkali process steps are as follows:

[0035] S1: Put formaldehyde and water into the reaction kettle at one time, stir, and add an alkali solution to adjust the pH value to 9.0;

[0036] S2: Add the first urea (the addition amount is 1 / 4 of the total urea), the first melamine, and polyethylene glycol, heat up to 80°C and keep the temperature for reaction for 60 min, add acid to adjust the pH value to 6, and when the solution viscosity reaches the end point, the first feeding reaction ends; the addition amount of the first melamine is 1.5% of the total glue amount; the dosage of polyethylene glycol is 0.8% of the total glue amount;

[0037] S3: Adjust the pH value to 7 by adding alkali, add the second portion of urea (the addition amount is 1 / 4 of the total urea), the second portion of melamine and polyethylene glycol, keep the temperature at 90 °C for reaction for 60 min, and when the solution viscosity reaches the end point, the reaction of the second feeding is completed; the addition amount of the second portion of melamine is 1.5% of the total glue amount; the dosage of polyethylene glycol is 0.2% of the total glue amount;

[0038] S4: Adjust the pH value to 8.5 by adding alkali, add the third portion of urea (the addition amount is 1 / 4 of the total urea) and the modified cassava starch obtained in step (2), keep the temperature at 90 °C for reaction for 15 min, and the reaction of the third feeding is completed; the addition amount of the modified cassava starch obtained in step (2) is 2% of the total glue amount;

[0039] S5: Cool down to 60 °C, add the fourth portion of urea (the addition amount is 1 / 4 of the total urea), and when the adhesive drops to room temperature, it is regarded as the end of the synthesis of the modified urea-formaldehyde resin.

[0040] Example 2

[0041] A preparation method of cassava starch modified urea-formaldehyde resin, the preparation method comprises the following steps:

[0042] (1) Put the cassava starch into an ammonium persulfate solution with a mass concentration of 10%, adjust the pH value of the solution so that the reaction system is in a strong acid environment with a pH value of 4, react at 94 °C under heating conditions, and take the solution system reaching 50 mPa·s as the first reaction end point to obtain a preliminarily oxidized cassava starch solution;

[0043] (2) Adjust the pH value of the preliminarily oxidized cassava starch solution obtained in step (1) to 7, and then add nano-silica for modification to obtain modified cassava starch; the addition amount of nano-silica is 1% of the system mass;

[0044] (3) Put formaldehyde, urea, melamine, the modified cassava starch obtained in step (2), water, and modifier into the reaction kettle step by step by the alkali-acid-alkali process for reaction. Each time of feeding, it is necessary to adjust the solution to reach the corresponding pH value, heat to the reaction temperature, keep the heating condition for reaction, and take the solution system reaching a certain viscosity as the reaction end point to obtain cassava starch modified urea-formaldehyde resin; wherein, the molar ratio of the formaldehyde to the urea is 1:1.5; the purity of the formaldehyde, the urea, and the melamine is ≥95%.

[0045] Specifically, the steps of the alkali-acid-alkali process are as follows:

[0046] S1: Put formaldehyde and water into the reaction kettle at one time, stir, and add alkali solution to adjust the pH value to 7.0;

[0047] S2: Add the first portion of urea (dosage: 1 / 5 of the total urea), the first portion of melamine, and polyethylene glycol. Heat up to 60 °C and hold for 20 min. Add acid to adjust the pH value to 3. When the solution viscosity reaches the end point, the first feeding reaction ends; the dosage of the first portion of melamine is 1.5% of the total glue amount; the dosage of the polyethylene glycol is 0.5% of the total glue amount.

[0048] S3: Add alkali to adjust the pH value to 4. Add the second portion of urea (dosage: 1 / 5 of the total urea), the second portion of melamine, and polyethylene glycol. Hold at 70 °C for 20 min. When the solution viscosity reaches the end point, the second feeding reaction ends; the dosage of the second portion of melamine is 1.5% of the total glue amount; the dosage of the polyethylene glycol is 0.5% of the total glue amount.

[0049] S4: Add alkali to adjust the pH value to 7. Add the third portion of urea (dosage: 2 / 5 of the total urea) and the modified cassava starch obtained in step (2). Hold at 70 °C for 15 min. The third feeding reaction ends; the dosage of the modified cassava starch obtained in step (2) is 2% of the total glue amount.

[0050] S5: Cool down to 30 °C. Add the fourth portion of urea (dosage: 1 / 5 of the total urea). When the adhesive cools down to room temperature, it is regarded as the end of the synthesis of the modified urea-formaldehyde resin.

[0051] Example 3

[0052] A preparation method of a cassava starch modified urea-formaldehyde resin, the preparation method comprising the following steps:

[0053] (1) Put the cassava starch into an ammonium persulfate solution with a mass concentration of 50%. Adjust the pH value of the solution so that the reaction system is in a strong acid environment with a pH value of 3.5. React at 92 °C under heating conditions. Take the solution system reaching 150 mPa·s as the first reaction end point to obtain a preliminarily oxidized cassava starch solution.

[0054] (2) Adjust the pH value of the preliminarily oxidized cassava starch solution obtained in step (1) to 7 - 8, and then add sodium alginate for modification to obtain modified cassava starch; the dosage of sodium alginate is 1% of the system mass.

[0055] (3) Put formaldehyde, urea, melamine, the modified cassava starch obtained in step (2), water, and a modifier into the reaction kettle step by step using the alkali-acid-alkali process for reaction. Each time of feeding, it is necessary to adjust the solution to reach the corresponding pH value, heat to the reaction temperature, and keep heating for reaction. Take the solution system reaching a certain viscosity as the reaction end point to obtain the cassava starch modified urea-formaldehyde resin; wherein, the molar ratio of the formaldehyde to the urea is 1:1.5; the purities of the formaldehyde, the urea, and the melamine are all ≥ 95%.

[0056] Specifically, the alkali-acid-alkali process steps are:

[0057] S1: Put formaldehyde and water into the reactor at once, stir, and add alkali solution to adjust the pH value to 8.0;

[0058] S2: adding the first urea (the amount added is 1 / 3 of the total urea), the first melamine and polyethylene glycol, heating to 70°C for 50 minutes, adding acid to adjust the pH value to 4, and when the solution viscosity reaches the end point, the first feeding reaction is completed; the amount of melamine added for the first time is 1.5% of the total amount of glue; the amount of polyethylene glycol used is 0.2% of the total amount of glue;

[0059] S3: adding alkali to adjust the pH value to 6, adding the second urea (the addition amount is 1 / 6 of the total urea), the second melamine and polyethylene glycol, and keeping the temperature at 80°C for 50 minutes. When the solution viscosity reaches the end point, the second feeding reaction is completed; the second melamine addition amount is 1.5% of the total glue amount; the amount of polyethylene glycol is 0.3% of the total glue amount;

[0060] S4: add alkali to adjust the pH value to 8, add the third urea (the amount added is 1 / 3 of the total urea) and the modified cassava starch obtained in step (2), keep the reaction at 80°C for 15 minutes, and the third feeding reaction is completed; the amount of modified cassava starch obtained in step (2) is 2% of the total amount of glue

[0061] S5: Cool down to 50°C, add the fourth portion of urea (the added amount is 1 / 6 of the total urea), and when the adhesive cools to room temperature, the synthesis of the modified urea-formaldehyde resin is considered to be completed.

[0062] Example 4

[0063] The method for preparing a cassava starch-modified urea-formaldehyde resin in Example 4 is substantially the same as that in Example 1, except that chitosan is added in step (2) for modification in an amount of 5% by mass of the system.

[0064] Example 5

[0065] The method for preparing a cassava starch-modified urea-formaldehyde resin in Example 5 is substantially the same as that in Example 1, except that acrylamide is added in step (2) for modification in Example 5, and the amount of acrylamide added is 2% by mass of the system.

[0066] Example 6

[0067] The method for preparing a cassava starch-modified urea-formaldehyde resin in Example 6 is substantially the same as that in Example 1, except that in step (2) of Example 6, equal proportions of glycerol and polyethylene glycol are added for modification, and the added amount of glycerol and polyethylene glycol is 5% of the system mass.

[0068] Control group 1

[0069] A preparation method of modified urea - formaldehyde resin, comprising the following steps:

[0070] S1: Put formaldehyde and water into the reaction kettle at one time, stir, and add alkali solution to adjust the pH value to 9.0;

[0071] S2: Add the first portion of urea (the addition amount is 1 / 4 of the total urea), the first portion of melamine and polyethylene glycol, heat up to 80 °C and keep the temperature for reaction for 60 min, add acid to adjust the pH value to 6, and when the solution viscosity reaches the end point, the reaction of the first feed is completed; the addition amount of the first portion of melamine is 1.5% of the total glue amount; the dosage of polyethylene glycol is 0.8% of the total glue amount;

[0072] S3: Add alkali to adjust the pH value to 7, add the second portion of urea (the addition amount is 1 / 4 of the total urea), the second portion of melamine and polyethylene glycol, keep the temperature at 90 °C for reaction for 60 min, and when the solution viscosity reaches the end point, the reaction of the second feed is completed; the addition amount of the second portion of melamine is 1.5% of the total glue amount; the dosage of polyethylene glycol is 0.2% of the total glue amount;

[0073] S4: Add alkali to adjust the pH value to 8.5, add the third portion of urea (the addition amount is 1 / 4 of the total urea), keep the temperature at 90 °C for reaction for 15 min, and the reaction of the third feed is completed;

[0074] S5: Cool down to 60 °C, add the fourth portion of urea (the addition amount is 1 / 4 of the total urea), and when the adhesive cools down to room temperature, it is regarded as the end of the synthesis of modified urea - formaldehyde resin.

[0075] Control Group 2

[0076] A preparation method of cassava starch - modified urea - formaldehyde resin, the preparation method comprising the following steps:

[0077] Step (1) and step (2) are the same as those in Example 1, the difference is that step (3) is: Put formaldehyde, urea, melamine, the modified cassava starch obtained in step (2), water, and modifier into the reaction kettle step - by - step by the alkali - acid - alkali process for reaction. Each time of feeding needs to adjust the solution to reach the corresponding pH value, heat to the reaction temperature, keep the heating condition for reaction, and take the solution system reaching a certain viscosity as the reaction end point to obtain cassava starch - modified urea - formaldehyde resin; wherein, the molar ratio of the formaldehyde to the urea is 1:1.5; the purities of the formaldehyde, the urea, and the melamine are all ≥95%.

[0078] Specifically:

[0079] S1: Put formaldehyde and water into the reaction kettle at one time, stir, and add alkali solution to adjust the pH value to 9.0;

[0080] S2: Add the first portion of urea (the addition amount is 1 / 4 of the total urea), the first portion of melamine, and the modified cassava starch obtained in step (2), heat up to 80 °C and hold for reaction for 60 min, add acid to adjust the pH value to 6, and when the solution viscosity reaches the end point, the first feeding reaction ends; the addition amount of the first portion of melamine is 1.5% of the total glue amount; the addition amount of the modified cassava starch obtained in step (2) is 2% of the total glue amount;

[0081] S3: Add alkali to adjust the pH value to 7, add the second portion of urea (the addition amount is 1 / 4 of the total urea), the second portion of melamine, and polyethylene glycol, heat up to 90 °C and hold for reaction for 60 min, and when the solution viscosity reaches the end point, the second feeding reaction ends; the addition amount of the second portion of melamine is 1.5% of the total glue amount; the dosage of polyethylene glycol is 1.0% of the total glue amount;

[0082] S4: Add alkali to adjust the pH value to 8.5, add the third portion of urea (the addition amount is 1 / 4 of the total urea), heat up to 90 °C and hold for reaction for 15 min, and the third feeding reaction ends;

[0083] S5: Cool down to 60 °C, add the fourth portion of urea (the addition amount is 1 / 4 of the total urea), and when the adhesive drops to room temperature, it is regarded as the end of the synthesis of the modified urea-formaldehyde resin.

[0084] Control group 3

[0085] A preparation method of a cassava starch modified urea-formaldehyde resin, the preparation method comprising the following steps:

[0086] Steps (1) and (2) are the same as in Example 1, the difference is that step (3) is: put formaldehyde, urea, melamine, the modified cassava starch obtained in step (2), water, and modifier into the reaction kettle step by step using the alkali-acid-alkali process for reaction. Each feeding needs to adjust the solution to reach the corresponding pH value, heat to the reaction temperature, and keep the heating condition for reaction. When the solution system reaches a certain viscosity, it is the end point of the reaction, and a cassava starch modified urea-formaldehyde resin is obtained; wherein, the molar ratio of the formaldehyde to the urea is 1:1.5; the purity of the formaldehyde, the urea, and the melamine is ≥95%.

[0087] Specifically:

[0088] S1: Put formaldehyde and water into the reaction kettle at one time, stir, and add an alkali solution to adjust the pH value to 9.0;

[0089] S2: Add the first portion of urea (the addition amount is 1 / 4 of the total urea), the first portion of melamine, and polyethylene glycol, heat up to 80 °C and hold for reaction for 60 min, add acid to adjust the pH value to 6, and when the solution viscosity reaches the end point, the first feeding reaction ends; the addition amount of the first portion of melamine is 1.5% of the total glue amount; the dosage of polyethylene glycol is 0.8% of the total glue amount;

[0090] S3: Adjust the pH value to 7 by adding alkali, add the second portion of urea (the addition amount is 1 / 4 of the total urea), the second portion of melamine and the modified cassava starch obtained in step (2), keep the temperature at 90 °C for reaction for 60 min, and when the solution viscosity reaches the end point, the reaction of the second feeding is completed; the addition amount of the second portion of melamine is 1.5% of the total glue amount; the addition amount of the modified cassava starch obtained in step (2) is 2% of the total glue amount;

[0091] S4: Adjust the pH value to 8.5 by adding alkali, add the third portion of urea (the addition amount is 1 / 4 of the total urea) and polyethylene glycol, keep the temperature at 90 °C for reaction for 15 min, and the reaction of the third feeding is completed; the dosage of polyethylene glycol is 0.2% of the total glue amount;

[0092] S5: Cool down to 60 °C, add the fourth portion of urea (the addition amount is 1 / 4 of the total urea), and when the adhesive cools down to room temperature, it is regarded as the end of the synthesis of the modified urea-formaldehyde resin.

[0093] Control group 4

[0094] A preparation method of a cassava starch modified urea-formaldehyde resin, comprising the following steps:

[0095] Step (1) and step (2) are the same as in Example 1, the difference is that step (3) is: formaldehyde, urea, melamine, the modified cassava starch obtained in step (2), water, and modifier are added to the reaction kettle step by step by the alkali-acid-alkali process for reaction. Each time of feeding needs to adjust the solution to reach the corresponding pH value, heat to the reaction temperature, keep the heating condition for reaction, and take the solution system reaching a certain viscosity as the end point of the reaction to obtain the cassava starch modified urea-formaldehyde resin; wherein, the molar ratio of the formaldehyde to the urea is 1:1.5; the purity of the formaldehyde, the urea, and the melamine is ≥95%.

[0096] Specifically:

[0097] S1: Add formaldehyde and water into the reaction kettle at one time, stir, and add alkali solution to adjust the pH value to 9.0;

[0098] S2: Add the first portion of urea (the addition amount is 1 / 4 of the total urea), the first portion of melamine, heat up to 80 °C and keep the temperature for reaction for 60 min, add acid to adjust the pH value to 6, and when the solution viscosity reaches the end point, the reaction of the first feeding is completed; the addition amount of the first portion of melamine is 1.5% of the total glue amount;

[0099] S3: Adjust the pH value to 7 by adding alkali, add the second portion of urea (the addition amount is 1 / 4 of the total urea) and the second portion of melamine, keep the temperature at 90 °C for reaction for 60 min, and when the solution viscosity reaches the end point, the reaction of the second feeding is completed; the addition amount of the second portion of melamine is 1.5% of the total glue amount;

[0100] S4: Adjust the pH value to 8.5 by adding alkali. Add the third portion of urea (the addition amount is 1 / 4 of the total urea), the modified cassava starch obtained in step (2), and polyethylene glycol, and keep the temperature at 90 °C for reaction for 15 min. The reaction of the third feeding ends. The addition amount of the modified cassava starch obtained in step (2) is 2% of the total glue amount; the dosage of polyethylene glycol is 1.0% of the total glue amount.

[0101] S5: Cool down to 60 °C, add the fourth portion of urea (the addition amount is 1 / 4 of the total urea). When the adhesive cools down to room temperature, it is regarded as the end of the synthesis of the modified urea-formaldehyde resin.

[0102] Control group 5: It is basically the same as the method of Example 1, except that the preparation method of the modified cassava starch is different. Specifically: (1) Prepare a starch solution with a mass percentage of 70% of cassava starch, and then adjust the pH value of the starch solution to 3.5 in a strong acid environment, and react at 92 °C under heating conditions until the solution system reaches 150 mPa·s as the first reaction end point to obtain a cassava starch solution.

[0103] (2) Adjust the pH value of the cassava starch solution obtained in step (1) to 8, and then add polyethylene glycol for modification to obtain the modified cassava starch; the addition amount of hydroxypropyl cellulose is 1% of the system mass.

[0104] Experimental detection and analysis:

[0105] The modified urea-formaldehyde resins obtained in the above Examples 1 to 6 and Control groups 1 - 5 were respectively detected as follows: Measure the viscosity, free formaldehyde content, solid content, bonding strength, and water resistance. Among them, the viscosity was measured with a No. 4 cup viscometer, and the detection methods of free formaldehyde content, solid content, and bonding strength refer to the national standard GB T14074-2017. The water resistance detection method is: respectively soak the modified urea-formaldehyde resin samples prepared in the above Examples 1 to 6 and Control groups 1 to 5 in water at 25 °C and 60 °C for 3 h, and regularly detect the bonding strength. The results of viscosity, free formaldehyde content, solid content, and bonding strength are shown in Table 1 below, and the water resistance detection results are shown in Table 2 below.

[0106] Table 1

[0107] Test item Viscosity (mPa·s) Free formaldehyde content (%) Solid content (%) Bonding strength (Mpa) Example 1 262 0.02 47.2％ 5.213 Example 2 255 0.03 47.6％ 4.967 Example 3 258 0.01 46.3％ 5.324 Example 4 250 0.02 46.9％ 4.915 Example 5 257 0.01 46.9％ 5.072 Example 6 255 0.03 47.2％ 5.197 Control group 1 198 0.62 45.3％ 3.245 Control group 2 201 0.58 46.2％ 3.892 Control group 3 223 0.50 45.8％ 3.512 Control group 4 247 0.41 46.9％ 3.825 Control group 5 256 0.43 45.7％ 2.856

[0108] Table 2

[0109]

[0110]

[0111] As can be seen from Examples 1 to 6 in Table 1, the modified urea-formaldehyde resin prepared by the present invention has good performance, low formaldehyde content, good water resistance, good heat resistance, high stability, and high bonding strength. From the comparison between Example 1 and Control Group 1, it can be seen that the free formaldehyde content, water resistance, and thermal stability of Example 1 are all higher than those of Control Group 1, indicating that when urea-formaldehyde resin is prepared without using modified cassava starch, its free formaldehyde content is high, and its water resistance and stability are poor. From Example 1 and Control Groups 2 - 4, it can be seen that the free formaldehyde and water resistance of Control Groups 2 - 4 are not as good as those of Example 1, indicating that when modified urea-formaldehyde resin is prepared using the same method to obtain modified cassava starch, with different preparation processes, its free formaldehyde content, water resistance, etc. are not as high as those of the present invention. From the comparison between Example 1 and Control Group 5, it can be seen that the water resistance and free formaldehyde content of Control Group 5 are lower than those of Example 1, indicating that the effects of preparing modified urea-formaldehyde resin with modified cassava starch obtained by different modification methods are different, while the modified urea-formaldehyde resin prepared by the present invention has good water resistance, high stability, and low free formaldehyde content.

[0112] The above description is a detailed description of the preferred and feasible embodiments of the present invention, but the embodiments are not intended to limit the scope of the patent application of the present invention. Any equivalent changes or modifications made under the technical spirit disclosed by the present invention shall fall within the scope of the patent covered by the present invention.

Claims

1. A preparation method of modified urea-formaldehyde resin from cassava starch, characterized in that, The preparation method comprises the following steps: (1) Put tapioca starch into an ammonium persulfate solution, adjust the pH value of the solution so that the reaction system is in a strong acid environment, react under heating conditions, and take the attainment of a certain viscosity of the solution system as the first reaction end point to obtain a preliminarily oxidized tapioca starch solution; (2) Adjust the pH value of the preliminarily oxidized tapioca starch solution obtained in step (1) to weak alkalinity, and then add crosslinking small molecules for modification to obtain modified tapioca starch; (3) Put formaldehyde, urea, melamine, the modified tapioca starch obtained in step (2), water, and a modifier into a reaction kettle step by step using an alkali-acid-alkali process for reaction. Each time of feeding requires adjusting the solution to reach the corresponding pH value, heating to the reaction temperature, and maintaining the heating condition for reaction. Take the attainment of a certain viscosity of the solution system as the reaction end point to obtain tapioca starch modified urea-formaldehyde resin.

2. A method for preparing modified urea-formaldehyde resin from cassava starch according to claim 1, characterized in that, In step (1), the mass concentration of the ammonium persulfate solution is 10-50%.

3. A method for preparing modified urea-formaldehyde resin from cassava starch according to claim 1, characterized in that, In step (1), the viscosity of the system at the first reaction end point is between 50-200 mPa·s.

4. A preparation method of modified urea-formaldehyde resin from cassava starch according to claim 1, characterized in that, In step (3), the molar ratio of formaldehyde to urea is 1:1.

5.

5. A preparation method of modified urea-formaldehyde resin from cassava starch according to claim 1, characterized in that, The crosslinking small molecule is one or several of hydroxypropyl cellulose, polyethylene glycol, chitosan, acrylamide, glycerol, nano-silica, sodium alginate, and the addition amount of the crosslinking small molecule is 1% - 5% of the system mass.

6. A method for preparing a modified urea-formaldehyde resin from cassava starch according to any one of claims 1-6, characterized in that, The alkali-acid-alkali process steps are as follows: S1: Put formaldehyde and water into the reaction kettle at one time, stir, and add an alkali solution to adjust the pH value to 7.0 - 9.0; S2: Add the first urea, the first melamine, and a modifier, heat up to 60 - 80 °C and keep warm for reaction for 20 - 60 min, add acid to adjust the pH value to 3 - 6, and when the solution viscosity reaches the end point, the first feeding reaction ends; S3: Add alkali to adjust the pH value to 4 - 7, add the second urea, the second melamine, and a modifier, keep warm for reaction at 70 - 90 °C for 20 - 60 min, and when the solution viscosity reaches the end point, the second feeding reaction ends; S4: Add alkali to adjust the pH value to 7 - 8.5, add the third urea and the modified tapioca starch obtained in step (2), keep warm for reaction at 70 - 90 °C for 15 min, and the third feeding reaction ends; S5: Cool down to 30 - 60 °C, add the fourth part of urea, and when the adhesive drops to room temperature, it is regarded as the end of the synthesis of the modified urea-formaldehyde resin.

7. A preparation method of modified urea-formaldehyde resin with cassava starch as claimed in claim 1, characterized in that, The modifier is polyethylene glycol.

8. A method for preparing modified urea-formaldehyde resin from cassava starch according to claim 7, characterized in that, The addition amount of the first melamine and the addition amount of the second melamine are both 1.5% of the total glue amount. ； The addition amount of the modified tapioca starch obtained in step (2) is 2% of the total glue amount, and the total dosage of the modifier is 0.5 - 1.0% of the total glue amount.

9. A tapioca starch modified urea-formaldehyde resin prepared by the preparation method of a tapioca starch modified urea-formaldehyde resin according to any one of claims 1 - 8.

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