Modified urea-formaldehyde resin and preparation method thereof

Abstract

The invention discloses a modified urea-formaldehyde resin and a preparation method thereof, an alkali-acid-alkali synthesis process is adopted, in the initial reaction stage of formaldehyde and urea, 5-hydroxymethylfurfural is used as a comonomer to partially replace formaldehyde to participate in the synthesis reaction of the urea-formaldehyde resin, the formaldehyde release amount is reduced, the 5-hydroxymethylfurfural is used as a hydrophobic monomer to be introduced into the urea-formaldehyde resin, and the hydrophobic property of the urea-formaldehyde resin is improved. Due to the existence of part of hydroxyl groups and furan rings in the structure, more activated groups are combined with wood, and the modified urea-formaldehyde resin is low in formaldehyde emission, good in water resistance and enhanced in mechanical property.

Description

[0001] The present invention relates to the technical field of urea-formaldehyde resin modification, and specifically to a modified urea-formaldehyde resin and a preparation method thereof. Background Art

[0002] With the continuous growth of China's economy, the wood-based panel industry has developed rapidly, and the demand for wood adhesives has also increased accordingly. Urea-formaldehyde (UF) resin adhesives have become the most important wood adhesives in China at present due to their low cost and excellent process performance. However, UF resin is formed by the condensation polymerization of formaldehyde, urea, and other modifiers after hydroxymethylation. The unreacted formaldehyde in UF resin and the aminomethylene chains in the cured resin network are reversible, making it easy to release toxic free formaldehyde in a humid environment, which affects people's health. These problems have severely restricted the application of UF resin. To reduce the content of free formaldehyde in UF resin, the most commonly used solutions at present are: reducing the molar ratio of formaldehyde to urea (F / U) in the synthesis of UF resin; adding modifiers such as melamine, polyvinyl alcohol, and phenol; copolymerizing with other adhesives such as MDI. Although these modifications can effectively reduce the free formaldehyde in UF resin, they may lead to a decrease in the bonding performance of the adhesive. Or using chemical modification methods with certain toxicity and pollution makes the modification steps cumbersome and costly, and it is difficult to apply in actual production. Therefore, finding an effective, inexpensive, non-toxic, and renewable method to reduce the free formaldehyde content in UF resin is the research focus. Summary of the Invention

[0003] The purpose of the present invention is to provide a modified urea-formaldehyde resin and a preparation method thereof to overcome the deficiencies existing in the prior art.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] Step (1): Add saccharides, water, a catalyst, and methyl isobutyl ketone (MIBK) into a reaction kettle, stir at 80 °C for 2 - 3 hours, wait until it cools to room temperature, perform solid-liquid separation, extract the reaction product with MIBK, and then recrystallize in MIBK to obtain 5-hydroxymethylfurfural (5-HMF).

[0006] Step (2): Add 5-HMF and paraformaldehyde in different proportions into the reaction kettle, adjust the pH value of the system to 9 - 11 with NaOH (0.1M), heat to 70 - 80 °C and maintain for 30 - 50 minutes to ensure complete dissolution of 5-HMF and paraformaldehyde. Add the first urea, and raise the reaction temperature to 80 - 90 °C, and react for 1 - 2 hours. After the reaction is completed, add glacial acetic acid to adjust the pH value of the system to 3 - 5, then add the second urea, and continue the reaction and polymerization for 2 - 4 hours. Then adjust the pH of the system to 7 - 8 with NaOH solution, add the third urea, keep it warm at 60 - 70 °C for 20 - 40 minutes until all the urea is dissolved, and after cooling and discharging, the modified urea-formaldehyde resin is obtained.

[0007] Preferably, the saccharide used in step (1) is one of fructose, sucrose, glucose or cellulose.

[0008] Preferably, the catalyst in step (1) is one of sulfonic acid resin, Al2O3 or ZSM-5.

[0009] Preferably, the ratio of 5-HMF to paraformaldehyde in step (2) is 0.1 - 1:1 (molar ratio).

[0010] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0011] (1) 5-HMF is derived from carbohydrates such as fructose and glucose, has the advantages of low cost and adjustable structure, and is also a renewable and biodegradable material. Using 5-HMF to partially replace formaldehyde, the obtained modified urea-formaldehyde resin has higher safety and lower volatility.

[0012] (2) Introducing 5-HMF with good thermal stability and hydrolysis stability into the synthesis of urea-formaldehyde resin, there are more hydroxyl groups (such as hydroxymethyl) in the structure of the obtained modified urea-formaldehyde resin, which can interact with the molecules of lignocellulose (i.e., the wood surface), improving the mechanical properties and water resistance of the resin. Specific Embodiments

[0013] Example 1

[0014] (1) Add fructose, water, ZSM-5 catalyst and methyl isobutyl ketone (MIBK) into the reaction kettle, stir at 80 °C for 2 hours, wait until it cools to room temperature, separate the solid and liquid, extract the reaction product with MIBK, and then recrystallize in MIBK to obtain 5-HMF.

[0015] (2) Add 50 g of 5-HMF and 120 g of paraformaldehyde into the reaction kettle, adjust the pH value of the system to 10 with NaOH (0.1 M), heat to 70 °C and maintain for 30 minutes to ensure complete dissolution of 5-HMF and paraformaldehyde. Add 60 g of urea for the first time, and raise the reaction temperature to 80 °C, and react for 1 hour. After the reaction is completed, add glacial acetic acid to adjust the pH value of the system to 3, add 60 g of urea for the second time, and continue the reaction and polymerization for 2 hours. Then adjust the pH of the system to 8 with NaOH solution, add 30 g of urea for the third time, keep warm at 60 °C for 20 minutes until all the urea is dissolved, and after cooling and discharging, the modified urea-formaldehyde resin is obtained.

[0016] Example 2

[0017] (1) Add glucose, water, Al2O3 catalyst and methyl isobutyl ketone (MIBK) into the reaction kettle, stir at 80 °C for 3 hours, wait until it cools to room temperature, carry out solid-liquid separation, extract the reaction product with MIBK, and then recrystallize in MIBK to obtain 5-HMF.

[0018] (2) Add 70 g of 5-HMF and 130 g of paraformaldehyde into the reaction kettle, adjust the pH value of the system to 10 with NaOH (0.1 M), heat to 80 °C and maintain for 40 minutes to ensure complete dissolution of 5-HMF and paraformaldehyde. Add 80 g of urea for the first time, and raise the reaction temperature to 85 °C, and react for 1 hour. After the reaction is completed, add glacial acetic acid to adjust the pH value of the system to 3, add 70 g of urea for the second time, and continue the reaction and polymerization for 2.5 hours. Then adjust the pH of the system to 8 with NaOH solution, add 40 g of urea for the third time, keep warm at 70 °C for 30 minutes until all the urea is dissolved, and after cooling and discharging, the modified urea-formaldehyde resin is obtained.

[0019] Example 3

[0020] (1) Add sucrose, water, Al2O3 catalyst and methyl isobutyl ketone (MIBK) into the reaction kettle, stir at 80 °C for 3 hours, wait until it cools to room temperature, carry out solid-liquid separation, extract the reaction product with MIBK, and then recrystallize in MIBK to obtain 5-HMF.

[0021] (2) Add 100 g of 5-HMF and 100 g of paraformaldehyde into a reaction kettle, adjust the pH value of the system to 10 with NaOH (0.1 M), heat to 70 °C and maintain for 40 minutes to ensure complete dissolution of 5-HMF and paraformaldehyde. Add 90 g of urea for the first time, and increase the reaction temperature to 85 °C, and react for 2 hours. After the reaction is completed, add glacial acetic acid to adjust the pH value of the system to 4, add 75 g of urea for the second time, and continue the reaction and polymerization for 4 hours. Then adjust the pH of the system to 8 with NaOH solution, add 50 g of urea for the third time, keep it warm at 70 °C for 40 minutes until all the urea is dissolved, and after cooling and discharging, a modified urea-formaldehyde resin is obtained.

[0022] Example 4

[0023] (1) Add fructose, water, ZSM-5 catalyst and methyl isobutyl ketone (MIBK) into a reaction kettle, stir at 80 °C for 3 hours, wait until it cools to room temperature, separate the solid and liquid, extract the reaction product with MIBK, and then recrystallize in MIBK to obtain 5-HMF.

[0024] (2) Add 120 g of 5-HMF and 90 g of paraformaldehyde into a reaction kettle, adjust the pH value of the system to 11 with NaOH (0.1 M), heat to 80 °C and maintain for 40 minutes to ensure complete dissolution of 5-HMF and paraformaldehyde. Add 100 g of urea for the first time, and increase the reaction temperature to 90 °C, and react for 2 hours. After the reaction is completed, add glacial acetic acid to adjust the pH value of the system to 4, add 60 g of urea for the second time, and continue the reaction and polymerization for 4 hours. Then adjust the pH of the system to 8 with NaOH solution, add 40 g of urea for the third time, keep it warm at 70 °C for 40 minutes until all the urea is dissolved, and after cooling and discharging, a modified urea-formaldehyde resin is obtained.

[0025] Comparative Example 1

[0026] Add 200 g of paraformaldehyde into a reaction kettle, adjust the pH value of the system to 11 with NaOH (0.1 M), heat to 70 °C and maintain for 40 minutes to ensure complete dissolution of paraformaldehyde. Add 90 g of urea for the first time, and increase the reaction temperature to 80 °C, and react for 1 hour. After the reaction is completed, add glacial acetic acid to adjust the pH value of the system to 4, add 70 g of urea for the second time, and continue the reaction and polymerization for 2 hours. Then adjust the pH of the system to 8 with NaOH solution, add 40 g of urea for the third time, keep it warm at 70 °C for 30 minutes until all the urea is dissolved, and after cooling and discharging, a urea-formaldehyde resin is obtained.

[0027] Effect evaluation method:

[0028] The formaldehyde release amount, modulus of rupture (MOR), modulus of elasticity (MOE), internal bond strength (IB), etc. of the obtained particleboard were determined in accordance with GB / T 17657-2022 "Test Methods for Physical and Chemical Properties of Wood-Based Panels and Overlaid Wood-Based Panels". The results are shown in the following table.

[0029] Table 1 Test Results of Samples

[0030]

[0031] It can be seen from the test results that with the increase in the addition amount of 5-hydroxymethylfurfural, the formaldehyde release amount of the particleboard decreased significantly. This may be because 5-hydroxymethylfurfural replaced formaldehyde to participate in the synthesis reaction, the addition amount of formaldehyde decreased, the ratio of formaldehyde to urea in the reaction process was reduced, resulting in a decrease in the formaldehyde release amount. At the same time, the polymer formed by the reaction of 5-hydroxymethylfurfural and urea helped to stabilize the UF resin network structure and reduced the formaldehyde release amount in the particleboard; the modulus of rupture (MOR) and modulus of elasticity (MOE) can reflect the performance of the board to resist elastic deformation. It can be seen from the test results of Examples 1-4 and Comparative Example 1 that after adding 5-hydroxymethylfurfural, the modulus of rupture and modulus of elasticity of the particleboard were significantly improved as a whole; in addition to the modulus of rupture and modulus of elasticity, compared with Comparative Example 1, the internal bond strength of the particleboard added with 5-hydroxymethylfurfural was also enhanced. The internal bond strength of the particleboard in Example 3 was increased by 23% compared with Comparative Example 1. The internal bond strength of Example 4 was lower than that of Example 3. This may be because the structure of 5-hydroxymethylfurfural is more complex than that of formaldehyde, and its reaction activity is weaker. And during the purification process, compounds such as sugars that are not conducive to the bonding strength were incorporated, resulting in a decrease in the internal bond. However, its internal bond performance is still much greater than that of conventional UF resin (Comparative Example 1); due to the easy hydrolysis and cleavage of its own methylene group, urea-formaldehyde resin has poor water resistance. During the preparation and synthesis process, 5-hydroxymethylfurfural was introduced to modify the urea-formaldehyde resin, introducing more hydrophobic groups, so that the water resistance of the modified urea-formaldehyde resin was greatly improved (Example 4 and Comparative Example 1).

[0032] The above embodiments are some preferred embodiments of the present invention, and are not any form of limitation to the present invention. Any modification, equivalent change, decoration and simplification made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A modified urea-formaldehyde resin and its preparation method, characterized in that, Using 5-hydroxymethylfurfural (5-HMF) synthesized from carbohydrates such as fructose and glucose as a raw material to partially replace formaldehyde in the synthesis process of traditional urea-formaldehyde resin, reducing the harm caused by formaldehyde volatilization. Moreover, due to the adjustable structure of 5-HMF, the mechanical properties and water resistance of the modified urea-formaldehyde resin have been greatly improved.

2. The preparation method of the modified urea-formaldehyde resin comprises the following steps: Add different proportions of 5-HMF and paraformaldehyde into a reaction kettle, adjust the pH value of the system to 9-11 with NaOH (0.1M), heat to 70-80 °C and maintain for 30-50 minutes to ensure complete dissolution of 5-HMF and paraformaldehyde. Add the first urea, and raise the reaction temperature to 80-90 °C and react for 1-2 hours. After the reaction is completed, add glacial acetic acid to adjust the pH value of the system to 3-5, then add the second urea and continue the reaction and polymerization for 2-4 hours. Then adjust the pH of the system to 7-8 with NaOH solution, add the third urea, keep it warm at 60-70 °C for 20-40 minutes until all the urea is dissolved, and after cooling and discharging, the modified urea-formaldehyde resin is obtained.

3. A modified urea-formaldehyde resin and a preparation method thereof according to claim 1, characterized in that, The ratio of 5-HMF to paraformaldehyde is 0.1-1:1 (molar ratio).

4. A modified urea-formaldehyde resin and its preparation method according to claim 1, characterized in that, 5-HMF is prepared by the following steps: Add sugars, water, a catalyst and methyl isobutyl ketone (MIBK) into a reaction kettle, stir at 80 °C for 2-3 hours, wait until it cools to room temperature, separate the solid and liquid, extract the reaction product with MIBK, and then recrystallize in MIBK to obtain a purified product.

5. The preparation of 5-HMF according to claim 3, characterized in that, The sugars used are one of fructose, sucrose, glucose or cellulose, and the catalyst used is one of sulfonic acid resin, Al2O3 or ZSM-5.

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