Method for preparing urea-formaldehyde resin rubber powder by drying method

Abstract

The invention discloses a method for preparing urea-formaldehyde resin rubber powder by a drying method, belongs to the technical field of urea-formaldehyde resin preparation, and is used for solving the problem of resin activity damage caused by a drying process when urea-formaldehyde resin rubber powder is prepared by a spray drying method. The preparation method comprises the following steps: reacting formaldehyde with a first batch of urea under an alkaline condition, adjusting to be acidic after the reaction is completed, carrying out polycondensation, adjusting to be neutral, adding a second batch of urea and melamine, carrying out a reaction, adjusting to be alkaline again, respectively adding a third batch of urea and residual urea, carrying out a reaction, and carrying out dehydration, cooling and discharging. And after discharging, mixing mineral powder filler, drying and crushing to prepare the urea-formaldehyde resin rubber powder. The rubber powder prepared by the method is high in activity after being reduced into glue, the bonding strength, heat resistance and cold resistance are greatly improved, and meanwhile, the content of free formaldehyde is lower than 0.2%.

Description

Technical Field

[0001] This invention relates to the field of urea-formaldehyde resin preparation technology, specifically a method for preparing urea-formaldehyde resin powder by drying. Background Technology

[0002] Urea-formaldehyde (UF) resin has long dominated the wood processing adhesive market due to its readily available raw materials, low cost, fast curing speed, and bonding strength suitable for the needs of indoor engineered wood products. It is widely used in the production of various engineered wood products such as plywood, particleboard, and medium-density fiberboard. Traditionally, urea-formaldehyde resin exists primarily in liquid latex form. Although the preparation process is mature, it has significant drawbacks: liquid urea-formaldehyde resin has a water content of approximately 50%, which greatly increases transportation costs. Therefore, the sales range of urea-formaldehyde resin products produced by adhesive manufacturers is generally limited to within 200-300 kilometers, resulting in a restricted sales area and a short shelf life.

[0003] Urea-formaldehyde resin liquid latex is difficult to dry using traditional methods. To address the storage and transportation challenges of liquid urea-formaldehyde resin, the industry currently widely employs spray drying to prepare liquid urea-formaldehyde resin prepolymer into powder. This process disperses the urea-formaldehyde resin prepolymer into micron-sized droplets through high-pressure atomization, and then rapidly evaporates the moisture in the droplets using a heat medium, thereby obtaining dry urea-formaldehyde resin powder.

[0004] However, urea-formaldehyde resin powder prepared by existing spray drying processes generally suffers from a significant reduction in activity after being reduced with water to form a glue solution. This severely restricts its promotion and practical application in the wood-based panel manufacturing industry, as specifically manifested in the following ways:

[0005] 1. Localized excessive condensation during the drying process leads to decreased activity: Urea-formaldehyde resin prepolymer is a heat-sensitive system. The high temperature used in the spray drying process (the inlet air temperature is usually 180–200℃) can easily cause localized and unexpected subsequent condensation reactions in the prepolymer, resulting in abnormal growth of resin molecular chains, increased branching, decreased water solubility and redispersibility, which in turn leads to reduced reactivity of the reduced adhesive and a slower curing reaction rate.

[0006] 2. Powder structure characteristics hinder reduction and swelling: During spray drying, moisture on the droplet surface evaporates rapidly, and some particles easily form a relatively dense surface structure. Simultaneously, the migration of internal moisture to the surface can cause particle aggregation, forming a microporous structure, and some particles may even agglomerate. These structural characteristics make it difficult for water molecules to quickly and uniformly penetrate into the particles during water reduction, resulting in insufficient swelling and dispersion of the prepolymer. This leads to an unevenly dispersed adhesive system, directly affecting the film-forming properties of the adhesive and its adhesion to the substrate interface.

[0007] 3. Significant Performance Degradation After Reduction: Due to the aforementioned structural changes and decreased activity, the curing activity of the reduced urea-formaldehyde resin powder after spray drying is far lower than that of the original liquid urea-formaldehyde resin of the same type. In practical applications, the curing induction period of the reduced adhesive is significantly prolonged, and the degree of cross-linking curing is insufficient, resulting in a significant reduction in bonding performance. Its performance is difficult to reach the level of the same type of liquid urea-formaldehyde resin, making it difficult to meet the production needs of some engineered wood products that require high bonding strength. Even with optimized curing agent ratios, it is difficult to restore the activity and bonding performance of the reduced adhesive to the level of the liquid resin without increasing formaldehyde release. Summary of the Invention

[0008] To address the problems existing in the preparation process of urea-formaldehyde resin powder, this invention provides a method for preparing urea-formaldehyde resin powder by drying. This method differs from the existing spray drying method. The urea-formaldehyde resin powder prepared by this method has strong activity after reduction, and the bonding strength is greatly improved. At the same time, it is heat-resistant and water-resistant.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A method for preparing urea-formaldehyde resin powder by drying includes the following steps:

[0011] (1) Adjust the pH of formaldehyde to 7.5-8.0, add the first batch of urea, and slowly heat to 90℃-95℃ for heat preservation reaction; the first batch of urea is 50%-55% of the total urea mass, and the mass ratio of total urea to total formaldehyde is 1:1.43-1.63;

[0012] (2) Adjust the pH value to acidic conditions to carry out the condensation reaction. When the reaction solution becomes cloudy, adjust the pH value to neutral or weakly alkaline to terminate the condensation reaction.

[0013] (3) Add a second batch of urea and melamine to continue the reaction;

[0014] (4) Add the third batch of urea to continue the reaction; cool down, and add the remaining urea during the cooling process to continue the reaction;

[0015] (5) Vacuum dehydration to form a viscous colloid, cooling to 50°C and discharging to obtain liquid urea-formaldehyde resin. Add mineral powder to the liquid urea-formaldehyde resin and stir evenly.

[0016] (6) Dry until solid, then pulverize to obtain urea-formaldehyde resin powder.

[0017] The second batch of urea accounts for 7%-12% of the total urea quality.

[0018] The amount of melamine added is 8.0% of the total mass of urea and formaldehyde.

[0019] Step (2) involves adjusting the pH value to acidic conditions, specifically adjusting the pH value to 4.6-5.5.

[0020] The mineral powder is attapulgite powder, and the amount added is 5%-7% of the mass of liquid urea-formaldehyde resin.

[0021] The mineral powder is kaolin or attapulgite powder.

[0022] The specific operation of step (4) is as follows: add the third batch of urea and continue the reaction for 30-40 minutes; then add the remaining urea.

[0023] The third batch of urea is 15%-20% of the total urea quality.

[0024] The beneficial effects obtained by this invention are:

[0025] This invention achieves a low free formaldehyde content urea-formaldehyde resin powder by reacting formaldehyde with urea added in batches, modifying it with melamine, and then combining vacuum dehydration with the addition of mineral powder and drying. The resulting urea-formaldehyde resin powder has a moisture content between 4% and 6%, a free formaldehyde content of less than 0.2%, and a peel strength of 0.83-1.43 MPa after being reduced to liquid latex, which is a very high level.

[0026] This invention uses a drying method to prepare urea-formaldehyde resin powder. Compared with the spray drying method, the preparation process has a lower temperature, causes less damage to the activity of the resin colloid, and results in greater peel strength after reduction and use.

[0027] The high molar ratio between formaldehyde and urea during the reaction process of this invention is beneficial for preparing urea-formaldehyde resin with water resistance and high temperature resistance. The product is in a solid state with low moisture content and has a long shelf life when stored in a sealed container. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of the preparation process in Embodiment 1 of the present invention;

[0029] Figure 2 This is a process flow diagram of the preparation process in Example 2 of the present invention;

[0030] Figure 3 This is a process flow diagram of the preparation process in Embodiment 3 of the present invention. Detailed Implementation

[0031] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0032] The raw materials used in the synthesis of urea-formaldehyde resin in this invention are industrial formaldehyde with a content of 37%, industrial urea with a content of 98%, and melamine with a purity of 99.8% (superior grade). The mineral powder used includes kaolin, attapulgite, and other mineral powders.

[0033] Example 1

[0034] This embodiment is an example of preparing urea-formaldehyde resin powder according to the method of the present invention. In this embodiment, the addition of residual urea is divided into adding the third batch of urea for reaction first, and then adding the remaining urea to continue the reaction, while the vacuum machine is always on. The specific steps are as follows:

[0035] (1) Add a fixed amount of formaldehyde to a three-necked flask, adjust the pH value to 7.5-8.0, and then add 50% of the total urea mass as the first batch of urea. Slowly raise the temperature to 90°C, turn on the vacuum pump, and keep it at 90°C for 30 minutes. The total urea added should be in a ratio of 1:1.43 between the total urea and the total formaldehyde mass.

[0036] (2) Adjust the pH value to 4.6-5.5. When the reaction reaches the point where dense fog appears, adjust the pH value to 7.5-8.0. Add 7% of the total urea mass as the second batch of urea, and add melamine. Turn on the vacuum pump and react for 30 minutes. The amount of melamine added is 8.0% of the sum of the total urea and formaldehyde mass.

[0037] (3) Add 20% of the total urea mass as the third batch of urea, turn on the vacuum pump, react for 30 minutes and then start cooling; add the remaining urea during the cooling process to continue the reaction;

[0038] (4) Vacuum dehydration to a viscous colloid, cooling to 50°C and discharging to obtain a viscous urea-formaldehyde resin; add mineral powder to the urea-formaldehyde resin and stir evenly; the amount of mineral powder added is 5% of the mass of the liquid urea-formaldehyde resin; the mineral powder in this embodiment is kaolin.

[0039] (5) Spread the resin in a tray for a certain period of time to pre-dry some of the moisture, and then put it into a drying net box for several hours to dry. The drying temperature is adjusted to 70℃ to obtain dry urea-formaldehyde resin blocks. Then put the dry blocks into a pulverizer to pulverize them to obtain urea-formaldehyde resin powder. The fineness of the powder is 10-30 mesh.

[0040] Example 2

[0041] This embodiment is an example of preparing urea-formaldehyde resin powder according to the method of the present invention. In this embodiment, the remaining urea is added all at once, and the vacuum machine is only turned on in step (4). The specific steps are as follows:

[0042] (1) Add a fixed amount of formaldehyde to a three-necked flask, adjust the pH value to 7.5-8.0, and then add 53% of the total urea mass as the first batch of urea. Slowly raise the temperature to 95℃ and keep it at 95℃ for 30 minutes. The mass ratio of total urea to total formaldehyde is 1:1.52.

[0043] (2) Adjust the pH value to 4.6-5.5 and react until dense fog appears. Then adjust the pH value to 7.5-8.0. Add 9% of the total urea mass as the second batch of urea and add melamine. React at 90℃ for 40 minutes. The amount of melamine added is 8.0% of the total urea and formaldehyde mass.

[0044] (3) Add 18% of the total urea mass as the third batch of urea and continue the reaction for 35 minutes;

[0045] (4) Add the remaining urea, turn on the vacuum pump, and begin cooling;

[0046] (5) Vacuum dehydration to a viscous colloid, stop dehydration, cool down to 50°C and discharge to obtain viscous liquid urea-formaldehyde resin; add mineral powder to urea-formaldehyde resin and stir evenly; the amount of mineral powder added is 6% of the mass of liquid urea-formaldehyde resin; in this embodiment, the mineral powder is attapulgite.

[0047] (6) Dry for several hours, with the drying temperature adjusted to 75℃, to obtain dry urea-formaldehyde resin granules. Put the dry granules into a pulverizer to pulverize them to obtain urea-formaldehyde resin powder. The fineness of the powder is 10~30 mesh.

[0048] Example 3

[0049] This embodiment is an example of preparing urea-formaldehyde resin powder according to the method of the present invention. In this embodiment, step (3) is optimized: the second batch of urea is added first for reaction, followed by the addition of melamine for reaction, and vacuum dehydration is only performed before discharge. The specific steps are as follows:

[0050] (1) Add a certain amount of formaldehyde to a three-necked flask, adjust the pH value to 7.5-8.0, and then add 55% of the total urea mass as the first batch of urea. Heat to 95°C within 30 minutes and keep warm at 95°C for 30 minutes. The total urea added is in a ratio of 1:1.63 between the total urea and the total formaldehyde mass.

[0051] (2) Adjust the pH value to 4.6-5.5 and react until dense fog appears. Immediately adjust the pH value to 6.5-6.6 and add 12% of the total urea mass as the second batch of urea. React at 90℃ for 50-60 minutes. Adjust the pH value to 7.0-7.1 and add melamine. React at 80℃ for 45 minutes. The amount of melamine added is 8.0% of the sum of the total urea and formaldehyde mass.

[0052] (3) Add 18% of the total urea mass as the third batch of urea;

[0053] (4) Add the remaining urea, turn on the vacuum pump, and continue the reaction;

[0054] (5) Vacuum dehydration to a viscous colloid, stop dehydration, cool down to 50°C and discharge for later use; obtain viscous liquid urea-formaldehyde resin; add mineral powder to urea-formaldehyde resin and stir evenly; the amount of mineral powder added is 7% of the mass of liquid urea-formaldehyde resin; the mineral powder in this embodiment is a mixture of kaolin and attapulgite, and the mass ratio of kaolin to attapulgite is 1:1.

[0055] (6) Then dry for several hours, with the drying temperature adjusted to 80℃, to obtain dry urea-formaldehyde resin granules. Put the dry granules into a pulverizer to pulverize them to obtain urea-formaldehyde resin powder. The fineness of the powder is 10-30 mesh.

[0056] Example 4

[0057] This embodiment is an example of testing the urea-formaldehyde resin powder prepared in Examples 1 to 3, mainly measuring the moisture content, free formaldehyde, and the peel strength of the bond after reduction.

[0058] (1) Detection of free formaldehyde content

[0059] After the adhesive powder is reduced by water, it is hot-pressed into boards, and the free formaldehyde content of the boards is tested. Water is mixed at a ratio of 1:1 (adhesive powder:water) and stirred evenly. Flour is added during the stirring process; the mass of the flour is 8% of the combined mass of the adhesive powder and water.

[0060] After being reduced to liquid urea-formaldehyde resin, the resin is applied at a rate of 150 g / m², with the hot-pressing temperature controlled at 115℃-120℃ and the hot-pressing time at 30 minutes. The amount of free formaldehyde released from the finished boards is then tested.

[0061] The free formaldehyde content was tested using the desiccator method, and the specific steps were carried out in accordance with GB / T 17657-2022 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels".

[0062] After testing, the free formaldehyde content of the boards prepared by the urea-formaldehyde resin powder in Examples 1-3, after being diluted with water and applied, was less than 0.14 mg / L. This demonstrates that even with a high urea to formaldehyde mass ratio, the boards prepared by this invention can achieve extremely low levels of free formaldehyde.

[0063] (2) Bond strength

[0064] The bonding strength was determined by the static shear method specified in GB / T 17657-2022 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels". The specimen size was 50mm×50mm. After equilibration in an environment of (20±2)℃ and (65±5)% RH for 24h, it was loaded at a rate of 10mm / min until failure, and the bonding strength was calculated.

[0065] The adhesive powder prepared in Examples 1-3 was extracted and used to produce boards according to the above method. Multiple board samples were sampled and tested, and the following bonding strengths were obtained: 1.00, 0.87, 1.15, 0.97, 1.00, 0.83, 0.95, 1.02, 1.24, 1.43, 1.11, 0.88, in MPa.

[0066] (3) The peeling performance of plywood impregnation was determined according to the method specified in GB / T 17657-2022. After the specimens were subjected to corresponding temperature and humidity cycling treatment, the peeling of the adhesive layer was checked. The peeling length on each side should not exceed 1 / 3 of the side length of the adhesive layer to be considered qualified. The test results showed that the cumulative peeling length of the same adhesive layer on each side of each specimen did not exceed 25mm, which met the requirements.

Claims

1. A method for preparing urea-formaldehyde resin powder by drying, characterized in that, Includes the following steps: (1) Adjust the pH of formaldehyde to 7.5-8.0, add the first batch of urea, and slowly heat to 90℃-95℃ for heat preservation reaction; the first batch of urea is 50%-55% of the total urea mass, and the mass ratio of total urea to total formaldehyde is 1:1.43-1.63; (2) Adjust the pH value to acidic conditions to carry out the condensation reaction. When the reaction solution becomes cloudy, adjust the pH value to neutral or weakly alkaline to terminate the condensation reaction. (3) Add a second batch of urea and melamine to continue the reaction; (4) Add the third batch of urea to continue the reaction; cool down, and add the remaining urea during the cooling process to continue the reaction; (5) Vacuum dehydration to form a viscous colloid, cooling to 50°C and discharging to obtain liquid urea-formaldehyde resin. Add mineral powder to the liquid urea-formaldehyde resin and stir evenly. (6) Dry until solid, then pulverize to obtain urea-formaldehyde resin powder.

2. The method for preparing urea-formaldehyde resin powder by drying according to claim 1, characterized in that, The second batch of urea accounts for 7%-12% of the total urea quality.

3. The method for preparing urea-formaldehyde resin powder by drying according to claim 1, characterized in that, The amount of melamine added is 8.0% of the total mass of urea and formaldehyde.

4. The method for preparing urea-formaldehyde resin powder by drying according to claim 1, characterized in that, The step (2) of adjusting the pH value to acidic conditions specifically means adjusting the pH value to 4.6-5.

5.

5. The method for preparing urea-formaldehyde resin powder by drying according to claim 1, characterized in that, The amount of mineral powder added is 5%-7% of the mass of the liquid urea-formaldehyde resin.

6. The method for preparing urea-formaldehyde resin powder by drying according to claim 1, characterized in that, The mineral powder is kaolin or attapulgite powder.

7. The method for preparing urea-formaldehyde resin powder by drying according to claim 1, characterized in that, The specific operation of step (4) is as follows: add the third batch of urea and continue the reaction for 30-40 minutes; then add the remaining urea.

8. The method for preparing urea-formaldehyde resin powder by drying according to claim 1, characterized in that, The third batch of urea is 15%-20% of the total urea quality.

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