An ultra-low free formaldehyde urea-formaldehyde resin, its preparation method and application

Abstract

The present invention belongs to the field of wood adhesives, and discloses an ultra-low free formaldehyde urea-formaldehyde resin, its preparation method and application. The ultra-low free formaldehyde urea-formaldehyde resin comprises the following raw materials in parts by mass: 100 parts of aqueous formaldehyde solution (37 wt%), 55 - 80 parts of urea, 0.5 - 2 parts of polyvinyl alcohol, 0.5 - 2 parts of melamine, and 0.5 - 2 parts of formaldehyde scavenger. By adopting a "weak base - weak acid - weak base" synthesis process and using amino-terminated hyperbranched polysiloxane as the formaldehyde scavenger, a urea-formaldehyde resin adhesive with good stability and ultra-low free formaldehyde content (<0.1%) is prepared. The urea-formaldehyde resin adhesive synthesized by the present invention has the advantages of good adhesive performance, good pre-pressing performance, good stability, etc. It uses a green and environmentally friendly formaldehyde scavenger and reduces the dosage of melamine at the same time, and is suitable for the production and processing of artificial boards such as plywood, particleboard and medium density fiberboard.

Description

Technical Field

[0001] The present invention belongs to the field of wood adhesives, and specifically relates to an ultra-low free formaldehyde urea-formaldehyde resin, its preparation method and application. Background Art

[0002] The history of using "adhesives" to bond various things in China can be traced back thousands of years. At that time, the "adhesives" were natural adhesives such as clay, starch, and bone glue. With the development of society, synthetic adhesives emerged and began to replace natural adhesives. Phenolic resin, urea-formaldehyde resin, and melamine-formaldehyde resin basically occupied the entire adhesive market. Among them, urea-formaldehyde resin is widely used in the manufacture of plywood, particleboard, and fiberboard due to its advantages of low-cost and easily available raw materials, high bonding strength, low production cost, and simple and convenient use. However, there is a very big problem in the use of urea-formaldehyde resin - formaldehyde release. Therefore, it is imperative to modify urea-formaldehyde resin to reduce formaldehyde release. The key to low formaldehyde release lies in reducing the free formaldehyde content of urea-formaldehyde resin. Therefore, the present invention has important significance for reducing the free formaldehyde content of urea-formaldehyde resin.

[0003] Patent CN114231226A introduces a preparation method for modifying urea-formaldehyde resin by using hyperbranched amino polymers combined with additives. By adjusting the ratios of formaldehyde, urea, polyvinyl alcohol, and melamine, as well as the types and ratios of hyperbranched amino polymers - additives, a urea-formaldehyde resin adhesive with a formaldehyde release amount of less than 0.030 mg / m 3 was prepared. The urea-formaldehyde resin prepared by this method has high reaction activity, adjustable viscosity, good initial tack, excellent pre-pressing and bonding properties, and less melamine consumption.

[0004] Patent CN106366308A discloses the use of terminal amino hyperbranched polyamide to remove free formaldehyde in urea-formaldehyde resin. When the addition amount of terminal amino hyperbranched polyamide is 2%, a stable urea-formaldehyde resin with a free formaldehyde content of 0.15% and a viscosity of 35.8 mPa·s can be obtained.

[0005] However, there are still some deficiencies in the above patent applications. The preparation method of the hyperbranched polymer is complex, the reaction conditions are not mild, and the free formaldehyde content does not reach below 0.1%. Therefore, it is necessary to find a new formaldehyde scavenger to further reduce the free formaldehyde content and synthesize an ultra-low free formaldehyde urea-formaldehyde resin, which has great application value. Summary of the Invention

[0006] Aiming at some problems existing in traditional formaldehyde scavengers, the present invention introduces terminal amino hyperbranched polysiloxane, solves the problems of small molecular weight and few active groups of traditional formaldehyde scavengers, and greatly reduces the free formaldehyde content in urea-formaldehyde resin.

[0007] The primary object of the present invention is to provide a urea - formaldehyde resin with ultra - low free formaldehyde content.

[0008] Another object of the present invention is to provide a method for preparing a urea - formaldehyde resin with ultra - low free formaldehyde content.

[0009] Still another object of the present invention is to provide the application of the above - mentioned urea - formaldehyde resin with ultra - low free formaldehyde content.

[0010] The objects of the present invention are achieved by the following technical solutions:

[0011] A urea - formaldehyde resin with ultra - low free formaldehyde content, the said urea - formaldehyde resin with ultra - low free formaldehyde content comprising raw materials in the following parts by mass:

[0012] 37wt% aqueous formaldehyde solution 100 parts

[0013] Urea 50 - 80 parts

[0014] Polyvinyl alcohol 0.5 - 2 parts

[0015] Melamine 0.5 - 2 parts

[0016] Formaldehyde scavenger 0.5 - 2 parts.

[0017] Preferably, the formaldehyde scavenger is amino - terminated hyperbranched polysiloxane.

[0018] Preferably, the preparation method of the amino - terminated hyperbranched polysiloxane is as follows:

[0019] Mix 10 - 20 parts of 3 - aminopropyltriethoxysilane (KH - 550) and 15 - 30 parts of deionized water evenly, then adjust the pH value of the system to 5 - 6, the reaction temperature is 50 - 60 °C, react for 6 - 8 hours, and remove the solvents after the reaction. The solvents are water and the ethanol generated in the reaction, and dry to obtain a viscous liquid, which is the amino - terminated hyperbranched polysiloxane; the said parts are all parts by mass.

[0020] Preferably, the drying is vacuum drying, and the vacuum drying conditions are - 0.1 Mpa and the time is 12 - 24 h.

[0021] A method for preparing the above - mentioned urea - formaldehyde resin with ultra - low free formaldehyde content, comprising the following steps:

[0022] (1) Add the aqueous formaldehyde solution, 35 - 45 parts of urea, polyvinyl alcohol and melamine in sequence, adjust the pH value of the system to alkaline with an alkali solution, heat up to 90 - 100 °C, and keep warm for 30 - 60 minutes;

[0023] (2) Adjust the pH value of the system to acidic, keep warm for 60 - 90 minutes. When reaching the cloud point of the system, continue the reaction for another 5 - 10 minutes, then adjust the pH value of the system to 7.5 - 8.5 with an alkali solution, and then add 15 - 20 parts of urea, more preferably 15 - 19 parts of urea, and keep warm at 90 °C for 20 - 40 minutes;

[0024] (3) Cool the reaction system in step (2) to 50 - 60 °C, add the remaining 5 - 15 parts of urea, more preferably 6 - 11 parts of urea, keep warm for 15 - 30 minutes, add a formaldehyde scavenger, naturally cool down to below 40 °C, adjust the pH value of the system to alkaline with an alkali solution, and discharge to obtain an ultra - low free - formaldehyde urea - formaldehyde resin.

[0025] Preferably, the heat - preservation conditions in steps (1) and (2) are oil - bath heat - preservation.

[0026] Preferably, the alkaline pH value in step (1) is 7.5 - 8.5, the acidic pH value in step (2) is 4.5 - 5.5, and the alkaline pH value in step (3) is 7.0 - 9.0.

[0027] Preferably, the alkali solution in steps (1), (2), and (3) is sodium hydroxide or potassium hydroxide, and the concentration is 1 - 2 mol / L;

[0028] The acid solution in step (2) is formic acid, hydrochloric acid or acetic acid aqueous solution, and the concentration is 0.5 - 1 mol / L.

[0029] Preferably, the method for testing the cloud point in step (2) is as follows: Measure once every 2 minutes. After sampling from the system, dropping it into ice water and presenting a white smoky state means reaching the cloud point of the system.

[0030] Preferably, the pH of the amino - terminated hyperbranched polysiloxane is adjusted to 7 - 8 with an aqueous sodium hydroxide solution before adding.

[0031] The application of the above ultra - low free - formaldehyde urea - formaldehyde resin in the production and processing of plywood, particleboard and medium - density fiberboard.

[0032] The present invention has the following advantages and beneficial effects:

[0033] 1. Using amino - terminated hyperbranched polysiloxane as a formaldehyde scavenger, and modifying urea - formaldehyde resin by the combination of melamine and polyvinyl alcohol, the dosage of melamine is reduced, and the structure of urea - formaldehyde resin is further improved, and the performance of the resin is enhanced.

[0034] 2. The amino - terminated hyperbranched polysiloxane has abundant terminal active groups, solving the problems of small molecular weight and few active groups of traditional formaldehyde scavengers, and greatly reducing the free - formaldehyde content in urea - formaldehyde resin, with the free - formaldehyde content < 0.1%.

[0035] 3. The urea-formaldehyde resin adhesive prepared by the present invention has a simple preparation process, is easy to operate, and has excellent bonding performance, reaction activity, initial tack, and pre-pressing performance, and is suitable for the production of wood-based panels such as particleboard, plywood, and medium-density fiberboard. Specific Embodiments

[0036] The present invention will be further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto. For process parameters not specifically noted, conventional techniques can be referred to.

[0037] The polyvinyl alcohol used in the present invention is polyvinyl alcohol 1788.

[0038] Example 1

[0039] (1) Add 100 parts of formaldehyde aqueous solution (concentration 37%), 35 parts of urea, 0.5 part of polyvinyl alcohol, and 0.5 part of melamine to a 500 ml three-necked flask. Adjust the pH of the system to 7.5 with sodium hydroxide aqueous solution (1 mol / L), heat up to 90 °C, and keep warm for 30 minutes.

[0040] (2) Adjust the pH of the system to 4.5 with formic acid aqueous solution (0.5 mol / L), keep warm for 60 minutes. During this period, measure the cloud point every 2 minutes. That is, when the sample is dropped into ice water and presents a white smoke-like state, continue the reaction for 5 minutes, and adjust the pH value of the system to 7.5 with sodium hydroxide aqueous solution (1 mol / L). Then add 15 parts of urea and keep warm at 90 °C for 20 minutes.

[0041] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 6 parts of urea, keep warm for 15 minutes, add 0.5 part of amino-terminated hyperbranched polysiloxane, naturally cool to below 40 °C, adjust the pH value of the system to 7.0 with sodium hydroxide aqueous solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0042] The synthesis method of amino-terminated hyperbranched polysiloxane is as follows:

[0043] Add 10 parts of γ-aminopropyltriethoxysilane (KH-550) and 15 parts of deionized water to the reaction vessel. After mixing evenly, adjust the pH of the system to about 5 with concentrated hydrochloric acid, control the reaction temperature at 50 °C, react for 6 hours. After the reaction, evaporate the solvent and vacuum dry (-0.1 MPa, vacuum dry for 12 - 24 h) to obtain a viscous liquid, that is, amino-terminated hyperbranched polysiloxane. Before the urea-formaldehyde resin synthesis process, adjust the pH to about 8 with 1 mol / L sodium hydroxide aqueous solution.

[0044] The synthesis method of the amino-terminated hyperbranched polysiloxane added in the examples and comparative examples is the same as that of the amino-terminated hyperbranched polysiloxane in Example 1.

[0045] Example 2

[0046] (1) Add 100 parts of aqueous formaldehyde solution (concentration 37%), 35 parts of urea, 1 part of polyvinyl alcohol, and 1 part of melamine into a 500 ml three-necked flask. Adjust the pH of the system to 8.0 with aqueous sodium hydroxide solution (1 mol / L), heat up to 90 °C, and keep warm for 40 minutes.

[0047] (2) Adjust the pH of the system to 5.0 with aqueous formic acid solution (0.5 mol / L), keep warm for 60 minutes. During this period, measure the cloud point every 2 minutes. That is, when the sample drops into ice water and shows a white smoke-like state, continue the reaction for 7 minutes, and adjust the pH value of the system to 7.5 with aqueous sodium hydroxide solution (1 mol / L). Then add 15 parts of urea and keep warm at 90 °C for 20 minutes.

[0048] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 6 parts of urea, keep warm for 20 minutes, add 1 part of amino-terminated hyperbranched polysiloxane, naturally cool down to below 40 °C, adjust the pH value of the system to 8.0 with aqueous sodium hydroxide solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0049] Example 3

[0050] (1) Add 100 parts of aqueous formaldehyde solution (concentration 37%), 45 parts of urea, 1.5 parts of polyvinyl alcohol, and 1.5 parts of melamine into a 500 ml three-necked flask. Adjust the pH of the system to 8.5 with aqueous sodium hydroxide solution (1 mol / L), heat up to 90 °C, and keep warm for 60 minutes.

[0051] (2) Adjust the pH of the system to 5.5 with aqueous formic acid solution (0.5 mol / L), keep warm for 60 minutes. During this period, measure the cloud point every 2 minutes. That is, when the sample drops into ice water and shows a white smoke-like state, continue the reaction for 10 minutes, and adjust the pH value of the system to 8.0 with aqueous sodium hydroxide solution (1 mol / L). Then add 19 parts of urea and keep warm at 90 °C for 40 minutes.

[0052] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 11 parts of urea, keep warm for 20 minutes, add 2 parts of amino-terminated hyperbranched polysiloxane, naturally cool down to below 40 °C, adjust the pH value of the system to 8.0 with aqueous sodium hydroxide solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0053] Example 4

[0054] (1) Add 100 parts of aqueous formaldehyde solution (concentration 37%), 39 parts of urea, 2 parts of polyvinyl alcohol, and 2 parts of melamine into a 500 ml four-necked flask. Adjust the pH of the system to 7.5 with aqueous sodium hydroxide solution (1 mol / L), heat up to 90 °C, and keep the temperature for 50 minutes.

[0055] (2) Adjust the pH of the system to 4.5 with aqueous formic acid solution (0.5 mol / L), keep the temperature for 60 minutes. During this period, measure the cloud point every 2 minutes. That is, when the sample drops into ice water and presents a white smoke-like state, continue the reaction for 7 minutes, and adjust the pH value of the system to 8.5 with aqueous sodium hydroxide solution (1 mol / L). Then add 17 parts of urea and keep the temperature at 90 °C for 30 minutes.

[0056] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 9 parts of urea, keep the temperature for 15 minutes, add 1 part of amino-terminated hyperbranched polysiloxane, naturally cool down to below 40 °C, adjust the pH value of the system to 9.0 with aqueous sodium hydroxide solution (1 mol / L), and discharge the material to obtain the urea-formaldehyde resin adhesive.

[0057] Example 5

[0058] (1) Add 100 parts of aqueous formaldehyde solution (concentration 37%), 45 parts of urea, 0.5 part of polyvinyl alcohol, and 1.5 parts of melamine into a 500 ml four-necked flask. Adjust the pH of the system to 8.0 with aqueous sodium hydroxide solution (1 mol / L), heat up to 90 °C, and keep the temperature for 30 minutes.

[0059] (2) Adjust the pH of the system to 5.0 with aqueous formic acid solution (0.5 mol / L), keep the temperature for 60 minutes. During this period, measure the cloud point every 2 minutes. That is, when the sample drops into ice water and presents a white smoke-like state, continue the reaction for 5 minutes, and adjust the pH value of the system to 8.0 with aqueous sodium hydroxide solution (1 mol / L). Then add 19 parts of urea and keep the temperature at 90 °C for 40 minutes.

[0060] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 9 parts of urea, keep the temperature for 15 minutes, add 1.5 parts of amino-terminated hyperbranched polysiloxane, naturally cool down to below 40 °C, adjust the pH value of the system to 8.0 with aqueous sodium hydroxide solution (1 mol / L), and discharge the material to obtain the urea-formaldehyde resin adhesive.

[0061] Example 6

[0062] (1) Add 100 parts of aqueous formaldehyde solution (concentration 37%), 39 parts of urea, 1 part of polyvinyl alcohol, and 2 parts of melamine into a 500 ml three-necked flask. Adjust the pH of the system to 8.5 with aqueous sodium hydroxide solution (1 mol / L), heat up to 90 °C, and keep the temperature for 60 minutes.

[0063] (2) Adjust the pH of the system to 5.5 with formic acid aqueous solution (0.5 mol / L), keep warm for 60 minutes, measure the cloud point every 2 minutes during this period. That is, when the sample drops into ice water and shows a white smoke-like state, continue the reaction for 10 minutes, and then adjust the pH value of the system to 8.5 with sodium hydroxide aqueous solution (1 mol / L). Then add 17 parts of urea and keep warm at 90 °C for 20 minutes.

[0064] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 11 parts of urea, keep warm for 15 minutes, add 2 parts of amino-terminated hyperbranched polysilane, naturally cool down to below 40 °C, adjust the pH value of the system to 7.0 with sodium hydroxide aqueous solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0065] Comparative Example 1

[0066] (1) Add 100 parts of formaldehyde aqueous solution (concentration 37%), 35 parts of urea, 0.5 part of polyvinyl alcohol, and 0.5 part of melamine into a 500 ml three-necked flask. Adjust the pH of the system to 7.5 with sodium hydroxide aqueous solution (1 mol / L), heat up to 90 °C, and keep warm for 30 minutes.

[0067] (2) Adjust the pH of the system to 4.5 with formic acid aqueous solution (0.5 mol / L), keep warm for 60 minutes, measure the cloud point every 2 minutes during this period. That is, when the sample drops into ice water and shows a white smoke-like state, continue the reaction for 5 minutes, and then adjust the pH value of the system to 7.5 with sodium hydroxide aqueous solution (1 mol / L). Then add 15 parts of urea and keep warm at 90 °C for 20 minutes.

[0068] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 6 parts of urea, keep warm for 15 minutes, naturally cool down to below 40 °C, adjust the pH value of the system to 7.0 with sodium hydroxide aqueous solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0069] Comparative Example 2

[0070] (1) Add 100 parts of formaldehyde aqueous solution (concentration 37%), 39 parts of urea, 1 part of polyvinyl alcohol, and 1 part of melamine into a 500 ml three-necked flask. Adjust the pH of the system to 8.0 with sodium hydroxide aqueous solution (1 mol / L), heat up to 90 °C, and keep warm for 40 minutes.

[0071] (2) Adjust the pH of the system to 5.0 with an aqueous formic acid solution (0.5 mol / L), keep it warm for 60 minutes, measure the cloud point every 2 minutes during this period. That is, when the sample is dropped into ice water and shows a white smoke-like state, continue the reaction for 7 minutes, and then adjust the pH value of the system to 8.0 with an aqueous sodium hydroxide solution (1 mol / L). Then add 17 parts of urea and keep it warm at 90 °C for 30 minutes.

[0072] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 9 parts of urea, keep it warm for 20 minutes, add one part of amino-terminated hyperbranched polysiloxane, naturally cool it to below 40 °C, adjust the pH value of the system to 8.0 with an aqueous sodium hydroxide solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0073] Comparative Example 3

[0074] (1) Add 100 parts of aqueous formaldehyde solution (concentration 37%), 39 parts of urea, 1 part of polyvinyl alcohol, and 2 parts of melamine to a 500 ml three-necked flask. Adjust the pH of the system to 9.0 with an aqueous sodium hydroxide solution (1 mol / L), heat it up to 90 °C, and keep it warm for 60 minutes.

[0075] (2) Adjust the pH of the system to 6.0 with an aqueous formic acid solution (0.5 mol / L), keep it warm for 60 minutes, measure the cloud point every 2 minutes during this period. That is, when the sample is dropped into ice water and shows a white smoke-like state, continue the reaction for 10 minutes, and then adjust the pH value of the system to 8.5 with an aqueous sodium hydroxide solution (1 mol / L). Then add 17 parts of urea and keep it warm at 90 °C for 20 minutes.

[0076] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 11 parts of urea, keep it warm for 15 minutes, add 2 parts of amino-terminated hyperbranched polysiloxane, naturally cool it to below 40 °C, adjust the pH value of the system to 7.0 with an aqueous sodium hydroxide solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0077] Comparative Example 4

[0078] (1) Add 100 parts of aqueous formaldehyde solution (concentration 37%), 35 parts of urea, 1 part of polyvinyl alcohol, and 1 part of melamine to a 500 ml three-necked flask. Adjust the pH of the system to 8.0 with an aqueous sodium hydroxide solution (1 mol / L), heat it up to 90 °C, and keep it warm for 40 minutes.

[0079] (2) Adjust the pH of the system to 5.0 with an aqueous formic acid solution (0.5 mol / L), keep it warm for 60 minutes, measure the cloud point every 2 minutes during this period. That is, when the sample is dropped into ice water and shows a white smoke-like state, continue the reaction for 7 minutes, and adjust the pH value of the system to 7.5 with an aqueous sodium hydroxide solution (1 mol / L). Then add 15 parts of urea and keep it warm at 90 °C for 20 minutes.

[0080] (3) Cool the reaction system in step (2) to 60 °C, add the remaining 6 parts of urea, keep it warm for 20 minutes, add 1 part of amino-terminated hyperbranched polyamide, naturally cool it to below 40 °C, adjust the pH value of the system to 8.0 with an aqueous sodium hydroxide solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0081] Take the urea-formaldehyde resins prepared in Examples 1-6 and Comparative Examples 1-4 and detect the basic properties of the resins such as free formaldehyde content, solid content, viscosity, pot life, and curing time according to the methods specified in the national standard GB / T 14074-2017 (Test Methods for Adhesives and Their Resins for Wood Industry).

[0082] The results are shown in Table 1:

[0083] Table 1 Basic Properties of Urea-Formaldehyde Resins Prepared in Examples 1-6 and Comparative Examples 1-4

[0084]

[0085] Through the comparison between Example 1 and Comparative Example 1, it can be seen that the addition of amino-terminated hyperbranched polysiloxane can effectively reduce the free formaldehyde content of the urea-formaldehyde resin. In the comparison between Example 2 and Comparative Example 2, we can find that when urea is added in three times, different proportions of the first two additions have little effect on the properties of the urea-formaldehyde resin. Therefore, both proportions can be used as the synthesis process data of the modified urea-formaldehyde resin. From the comparison between Example 2 and Comparative Example 4, it can be seen that when adding the same proportion of amino-terminated hyperbranched polysiloxane and amino-terminated hyperbranched polyamide, the free formaldehyde content of the amino-terminated hyperbranched polysiloxane-modified urea-formaldehyde resin is lower. From the comparison between Example 6 and Comparative Example 3, it is found that when the alkalinity in the first alkaline stage increases and the acidity in the second acidic stage weakens, the free formaldehyde content of the modified urea-formaldehyde resin increases, and the solid content and viscosity both decrease to varying degrees. The change of pH will affect the reaction in the polycondensation stage, making the hydroxymethylation incomplete, and the increase in the dosage of the pH regulator will increase the cost.

[0086] The above examples are only explanations of the present invention, but the implementation manners of the present invention are not limited by the above examples. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. An ultra-low free formaldehyde urea-formaldehyde resin, characterized in that, The ultra-low free formaldehyde urea-formaldehyde resin comprises raw materials in the following parts by mass: 37wt% formaldehyde aqueous solution: 100 parts Urea: 50 - 80 parts Polyvinyl alcohol: 0.5 - 2 parts Melamine: 0.5 - 2 parts Formaldehyde scavenger: 0.5 - 2 parts; The preparation method of the ultra-low free formaldehyde urea-formaldehyde resin comprises the following steps: (1) Add the formaldehyde aqueous solution, 35 - 45 parts of urea, polyvinyl alcohol and melamine in sequence. Use an alkali solution to adjust the pH value of the system to alkaline, heat up to 90 - 100 °C, and keep warm for 30 - 60 minutes; (2) Adjust the pH value of the system to acidic, keep warm for 60 - 90 minutes. When the cloud point of the system is reached, continue the reaction for 5 - 10 minutes. Use an alkali solution to adjust the pH value of the system to 7.5 - 8.5, then add 15 - 20 parts of urea, and keep warm at 90 °C for 20 - 40 minutes; (3) Cool the reaction system in step (2) to 50 - 60 °C, add the remaining 5 - 15 parts of urea, keep warm for 15 - 30 minutes, add the formaldehyde scavenger, naturally cool to below 40 °C, use an alkali solution to adjust the pH value of the system to alkaline, and discharge to obtain the ultra-low free formaldehyde urea-formaldehyde resin; In step (1), the alkaline pH value is 7.5 - 8.5; in step (2), the acidic pH value is 4.5 - 5.5; in step (3), the alkaline pH value is 7.0 - 9.0; The formaldehyde scavenger is amino-terminated hyperbranched polysiloxane; The preparation method of the amino-terminated hyperbranched polysiloxane is as follows: Mix 10 - 20 parts of 3-aminopropyltriethoxysilane and 15 - 30 parts of deionized water evenly, adjust the pH of the system to 5 - 6, the reaction temperature is 50 - 60 °C, react for 6 - 8 hours, remove the solvent after the reaction, and dry to obtain a viscous liquid, which is the amino-terminated hyperbranched polysiloxane.

2. A method for preparing the ultra-low free formaldehyde urea-formaldehyde resin according to claim 1, characterized in that, The preparation method comprises the following steps: (1) Add the formaldehyde aqueous solution, 35 - 45 parts of urea, polyvinyl alcohol and melamine in sequence. Use an alkali solution to adjust the pH value of the system to alkaline, heat up to 90 - 100 °C, and keep warm for 30 - 60 minutes; (2) Adjust the pH value of the system to acidic, keep warm for 60 - 90 minutes. When the cloud point of the system is reached, continue the reaction for 5 - 10 minutes. Use an alkali solution to adjust the pH value of the system to 7.5 - 8.5, then add 15 - 20 parts of urea, and keep warm at 90 °C for 20 - 40 minutes; (3) Cool the reaction system in step (2) to 50 - 60 °C, add the remaining 5 - 15 parts of urea, keep warm for 15 - 30 minutes, add the formaldehyde scavenger, naturally cool to below 40 °C, use an alkali solution to adjust the pH value of the system to alkaline, and discharge to obtain the ultra-low free formaldehyde urea-formaldehyde resin; In step (1), the alkaline pH value is 7.5 - 8.5; in step (2), the acidic pH value is 4.5 - 5.5; in step (3), the alkaline pH value is 7.0 - 9.

0.

3. The method for preparing an ultra-low free formaldehyde urea-formaldehyde resin according to claim 2, characterized in that, The heat preservation conditions in steps (1) and (2) are oil bath heat preservation.

4. The method for preparing an ultra-low free formaldehyde urea-formaldehyde resin according to claim 2, characterized in that, The alkali solution in steps (1), (2) and (3) is sodium hydroxide or potassium hydroxide solution, and the concentration is 1 - 2 mol / L; The acidic solution for adjusting the pH value of the adjustment system to acidic in step (2) is formic acid, hydrochloric acid or acetic acid aqueous solution, and the concentration is 0.5 - 1 mol / L.

5. The method for preparing an ultra-low free formaldehyde urea-formaldehyde resin according to claim 2, characterized in that, The cloud point test method described in step (2) is as follows: Measure once every 2 minutes. After sampling from the system and dropping it into ice water, if it presents a white smoke-like state, it means that the cloud point of the system is reached.

6. The method for preparing an ultra-low free formaldehyde urea-formaldehyde resin according to claim 2, characterized in that, The pH of the amino-terminated hyperbranched polysiloxane is adjusted to 7 - 8 with an aqueous sodium hydroxide solution before adding.

7. Application of the ultra-low free formaldehyde urea-formaldehyde resin described in claim 1 in the production and processing of plywood, particleboard and medium density fiberboard.

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