An ultra-low free formaldehyde urea-formaldehyde resin using cysteine as a formaldehyde capturing agent, and a preparation method and application thereof

Abstract

The application belongs to the field of wood adhesives, and discloses a kind of urea-formaldehyde resin with ultra-low free formaldehyde by using cysteine as formaldehyde trapping agent as well as a preparation method and application thereof.The urea-formaldehyde resin with ultra-low free formaldehyde comprises the following raw materials in mass fraction: 30-40 wt% 100-200 parts of formaldehyde aqueous solution, 30-100 parts of urea, 0.1-5 parts of melamine and 0.1-5 parts of cysteine.A urea-formaldehyde resin adhesive with good stability and ultra-low free formaldehyde content is prepared by using cysteine as formaldehyde trapping agent through a synthesis process of "weak base-weak acid-weak base".The urea-formaldehyde resin adhesive synthesized by the application has good adhesive properties, good pre-pressing performance and long storage stability, reduces the amount of melamine used, uses green and environmentally friendly amino acid as formaldehyde trapping agent, reduces production cost, and is suitable for the production and processing of plywood, particle board and medium density fiberboard.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of wood adhesives, and particularly relates to a urea-formaldehyde resin with ultra-low free formaldehyde by using cysteine as a formaldehyde capturing agent, and a preparation method and application thereof. BACKGROUND

[0002] Urea-formaldehyde resin is a thermosetting resin obtained by a series of reactions of urea and formaldehyde. The development of adhesives has experienced thousands of years, from the initial use of bone glue, clay and other adhesives to the current synthetic adhesives such as urea-formaldehyde resin and melamine-formaldehyde resin. Urea-formaldehyde resin is widely used in the fields of building materials industry, packaging materials, coating industry, especially in the manufacturing and processing of plywood, particle board and fiberboard, due to its low-cost raw materials, high initial bonding strength, low production cost, and simple and convenient use. Due to the release of formaldehyde during the use of urea-formaldehyde resin, which poses great harm to human body and causes environmental pollution, researchers have been working on the low-aldehyde modification of urea-formaldehyde resin.

[0003] Patent CN102585130A introduces a preparation method for producing urea-formaldehyde resin by using amino acids as formaldehyde capturing agents, including one or more of glycine, alanine, leucine, valine, glutamic acid, and phenylalanine. By using urea 100 parts, formaldehyde solution 100-120 parts, sodium hydroxide 0.1-0.4 parts, melamine 0.1-0.5 parts, and 2-3 parts of amino acid, the free formaldehyde content of the prepared urea-formaldehyde resin can be reduced to 0.05% to 0.1%, the solid content can be 55% to 60%, and the viscosity can be 60 to 100 mPa·s, meeting the national E1-E0 level environmental protection standard. The condensation reaction between the unstable chemical bonds in the urea-formaldehyde resin and the amino acid eliminates the residual free formaldehyde.

[0004] Patent CN213160713U introduces a preparation device for producing urea-formaldehyde resin adhesive by using amino acids as formaldehyde capturing agents, which can absorb the residual formaldehyde gas during the reaction process, thereby improving the working environment and protecting the health of workers, and has great significance for building a green factory.

[0005] Although the use of amino acids in the above-mentioned patents can reduce the free formaldehyde content of urea-formaldehyde resin to below 0.1%, the use amount of amino acids is high and the types are more, the steps are complex, and the cost is high. Therefore, it is necessary to find a formaldehyde capturing agent to synthesize a urea-formaldehyde resin with ultra-low free formaldehyde, which has great application value. SUMMARY

[0006] The present application solves the environmental pollution problem of some traditional formaldehyde capturing agents, reduces the free formaldehyde content in urea-formaldehyde resin, and is low in cost and green in environmental protection.

[0007] Another object of the present application is to provide a preparation method of the ultra-low free formaldehyde urea-formaldehyde resin.

[0008] Another object of the present application is to provide an application of the ultra-low free formaldehyde urea-formaldehyde resin.

[0009] The object of the present application is achieved by the following technical solutions.

[0010] An ultra-low free formaldehyde urea-formaldehyde resin comprises the following raw materials in mass fraction:

[0011]

[0012]

[0013] Preferably, the ultra-low free formaldehyde urea-formaldehyde resin comprises the following raw materials in mass fraction:

[0014]

[0015] Preferably, the formaldehyde capturing agent is cysteine.

[0016] A method for preparing the ultra-low free formaldehyde urea-formaldehyde resin comprises the following steps:

[0017] (1) formaldehyde aqueous solution and 35-45 parts of urea are sequentially added, the pH value of the system is adjusted to alkaline by adding melamine and an alkali solution, the temperature is increased to 90-100 DEG C, and the system is kept for 30-60 minutes;

[0018] (2) the formaldehyde capturing agent is added, the system is kept for a period of time, the cloud point is measured every 2 minutes during the keeping, when the sample dropped into ice water presents white smoke, the reaction is continued for 5-10 minutes, the pH value of the system is adjusted to 7.5-8.5 by using an alkali solution, 15-19 parts of urea are added, and the system is kept at 90 DEG C for 20-40 minutes;

[0019] (3) cooling the reaction system of step (2) to 50-60℃, adding the remaining 9-11 parts of urea, keeping for 20-40 minutes, naturally cooling to below 40℃, adjusting the pH value of the system to alkaline with an alkali solution, discharging, and obtaining the ultra-low free formaldehyde urea-formaldehyde resin.

[0020] Preferably, step (2) adds the formaldehyde capturing agent to adjust the pH value of the system to acidic.

[0021] Preferably, the acidic pH value is 4.5-5.5.

[0022] Preferably, the alkaline pH value in step (1) is 7.5-8.5, and the alkaline pH value in step (3) is 7.0-9.0.

[0023] Preferably, the alkali solution in steps (1), (2), and (3) is a sodium hydroxide aqueous solution or a potassium hydroxide aqueous solution, and the concentration is 1-2 mol / L.

[0024] Preferably, the keeping condition in steps (1) and (2) is oil bath keeping.

[0025] The above ultra-low free formaldehyde urea-formaldehyde resin is applied in the production and processing of plywood, particle board, and medium density fiberboard.

[0026] The advantages and beneficial effects of the present application are as follows:

[0027] 1. Cysteine is used as the formaldehyde capturing agent, reducing the amount of melamine and the production cost, improving the structure of the urea-formaldehyde resin by participating in the synthesis of the urea-formaldehyde resin, and improving the brittleness of the urea-formaldehyde resin.

[0028] 2. Cysteine serves as both the formaldehyde capturing agent and the pH regulator in the acidic stage, because it contains both the thiol group and the amino group that can react with formaldehyde, and the carboxyl group that can provide acidity, achieving two purposes at once.

[0029] 3. Cysteine as the formaldehyde capturing agent does not cause environmental pollution and toxicity, and is more green and environmentally friendly.

[0030] 4. The urea-formaldehyde resin adhesive prepared by the present application has a simple preparation process, is easy to operate, has excellent initial adhesion and pre-pressing performance, has a long storage stability period, and is suitable for the production of particle board, plywood, and medium density fiberboard. DETAILED DESCRIPTION

[0031] The present application will be further specifically and in detail described below in combination with specific examples, but the implementation manner of the present application is not limited thereto. For the process parameters not specifically indicated, the conventional technology can be referred to.

[0032] Example 1

[0033] (1) Into a 500ml three-necked flask, 100 parts of formaldehyde aqueous solution (concentration 37%), 35 parts of urea, 0.5 part of melamine were added, the pH of the system was adjusted to 7.5 with sodium hydroxide aqueous solution (1 mol / L), and the temperature was raised to 90°C and kept for 30 minutes.

[0034] (2) The pH of the system was adjusted to 5.5 with 0.5 part of cysteine, and the system was kept for a period of time. During the keeping, the cloud point was measured every 2 minutes. When the sample dropped into ice water showed white smoke, the reaction was continued for 5 minutes, the pH of the system was adjusted to 7.5 with sodium hydroxide aqueous solution (1 mol / L), 15 parts of urea was added, and the system was kept at 90°C for 20 minutes.

[0035] (3) The reaction system of step (2) was cooled to 60°C, the remaining 9 parts of urea was added, and the system was kept for 20 minutes. The temperature was naturally lowered to below 40°C, the pH of the system was adjusted to 7.0 with sodium hydroxide aqueous solution (1 mol / L), and the product was discharged to obtain the urea-formaldehyde resin adhesive.

[0036] Example 2

[0037] (1) Into a 500ml three-necked flask, 100 parts of formaldehyde aqueous solution (concentration 37%), 39 parts of urea, 1 part of melamine were added, the pH of the system was adjusted to 7.5 with sodium hydroxide aqueous solution (1 mol / L), and the temperature was raised to 90°C and kept for 40 minutes.

[0038] (2) The pH of the system was adjusted to 5.0 with 1 part of cysteine, and the system was kept for a period of time. During the keeping, the cloud point was measured every 2 minutes. When the sample dropped into ice water showed white smoke, the reaction was continued for 7 minutes, the pH of the system was adjusted to 8.5 with sodium hydroxide aqueous solution (1 mol / L), 17 parts of urea was added, and the system was kept at 90°C for 40 minutes.

[0039] (3) The reaction system of step (2) was cooled to 60°C, the remaining 10 parts of urea was added, and the system was kept for 40 minutes. The temperature was naturally lowered to below 40°C, the pH of the system was adjusted to 9.0 with sodium hydroxide aqueous solution (1 mol / L), and the product was discharged to obtain the urea-formaldehyde resin adhesive.

[0040] Example 3

[0041] (1) Into a 500ml three-necked flask, 100 parts of formaldehyde aqueous solution (concentration 37%), 35 parts of urea, 1 part of melamine were added, the pH of the system was adjusted to 8.0 with sodium hydroxide aqueous solution (1 mol / L), and the temperature was raised to 90°C and kept for 40 minutes.

[0042] (2) Adjust the pH of the system to 5.0 with 1 part of cysteine, and incubate for a period of time. Measure the turbidity every 2 minutes during the incubation period. When the sample turns into white smoke when dropped into ice water, continue the reaction for 10 minutes, and adjust the pH of the system to 8.0 with sodium hydroxide aqueous solution (1 mol / L). Add 17 parts of urea, and incubate at 90°C for 30 minutes.

[0043] (3) Cool the reaction system of step (2) to 60°C, add the remaining 10 parts of urea, and incubate for 30 minutes. Naturally cool to below 40°C, adjust the pH of the system to 8.0 with sodium hydroxide aqueous solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0044] Example 4

[0045] (1) Add 100 parts of formaldehyde aqueous solution (concentration of 37%), 45 parts of urea, and 1.5 parts of melamine to a 500 ml three-necked flask. Adjust the pH of the system to 8.5 with sodium hydroxide aqueous solution (1 mol / L), and warm up to 90°C. Incubate for 60 minutes.

[0046] (2) Adjust the pH of the system to 4.5 with 2 parts of cysteine, and incubate for a period of time. Measure the turbidity every 2 minutes during the incubation period. When the sample turns into white smoke when dropped into ice water, continue the reaction for 5 minutes, and adjust the pH of the system to 8.5 with sodium hydroxide aqueous solution (1 mol / L). Add 19 parts of urea, and incubate at 90°C for 20 minutes.

[0047] (3) Cool the reaction system of step (2) to 60°C, add the remaining 11 parts of urea, and incubate for 20 minutes. Naturally cool to below 40°C, adjust the pH of the system to 8.0 with sodium hydroxide aqueous solution (1 mol / L), and discharge to obtain the urea-formaldehyde resin adhesive.

[0048] Example 5

[0049] (1) Add 100 parts of formaldehyde aqueous solution (concentration of 37%), 39 parts of urea, and 2 parts of melamine to a 500 ml three-necked flask. Adjust the pH of the system to 8.0 with sodium hydroxide aqueous solution (1 mol / L), and warm up to 90°C. Incubate for 30 minutes.

[0050] (2) Adjust the pH of the system to 4.5 with 2 parts of cysteine, and incubate for a period of time. Measure the turbidity every 2 minutes during the incubation period. When the sample turns into white smoke when dropped into ice water, continue the reaction for 7 minutes, and adjust the pH of the system to 7.5 with sodium hydroxide aqueous solution (1 mol / L). Add 19 parts of urea, and incubate at 90°C for 30 minutes.

[0051] (3) The reaction system of step (2) is cooled to 60°C, 9 parts of the remaining urea is added, and the temperature is maintained for 30 minutes. The temperature is naturally lowered to below 40°C, the pH value of the system is adjusted to 7.0 with sodium hydroxide aqueous solution (1 mol / L), and the product is discharged to obtain the urea-formaldehyde resin adhesive.

[0052] Example 6

[0053] (1) 100 parts of formaldehyde aqueous solution (concentration of 37%) and 45 parts of urea are added to a 500 ml three-necked flask, 2 parts of melamine is added, the pH value of the system is adjusted to 8.5 with sodium hydroxide aqueous solution (1 mol / L), the temperature is raised to 90°C, and the temperature is maintained for 60 minutes.

[0054] (2) The pH value of the system is adjusted to 5.0 with 1.5 parts of cysteine, and the temperature is maintained for a period of time. During the temperature maintaining period, the cloud point is measured every 2 minutes. When the sample dropped into ice water presents white smoke, the reaction is continued for 10 minutes, the pH value of the system is adjusted to 8.0 with sodium hydroxide aqueous solution (1 mol / L), and 15 parts of urea is added. The temperature is maintained at 90°C for 40 minutes.

[0055] (3) The reaction system of step (2) is cooled to 60°C, 9 parts of the remaining urea is added, and the temperature is maintained for 30 minutes. The temperature is naturally lowered to below 40°C, the pH value of the system is adjusted to 7.0 with sodium hydroxide aqueous solution (1 mol / L), and the product is discharged to obtain the urea-formaldehyde resin adhesive.

[0056] Comparative Example 1

[0057] (1) 100 parts of formaldehyde aqueous solution (concentration of 37%) and 45 parts of urea are added to a 500 ml three-necked flask, 2 parts of melamine is added, the pH value of the system is adjusted to 8.5 with sodium hydroxide aqueous solution (1 mol / L), the temperature is raised to 90°C, and the temperature is maintained for 60 minutes.

[0058] (2) The pH value of the system is adjusted to 5.0 with 1.5 parts of cysteine, and the temperature is maintained for a period of time. During the temperature maintaining period, the cloud point is measured every 2 minutes. When the sample dropped into ice water presents white smoke, the reaction is continued for 10 minutes, the pH value of the system is adjusted to 8.0 with sodium hydroxide aqueous solution (1 mol / L), and 15 parts of urea is added. The temperature is maintained at 90°C for 40 minutes.

[0059] (3) The reaction system of step (2) is cooled to 60°C, 9 parts of the remaining urea is added, and the temperature is maintained for 30 minutes. The temperature is naturally lowered to below 40°C, the pH value of the system is adjusted to 7.0 with sodium hydroxide aqueous solution (1 mol / L), and the product is discharged to obtain the urea-formaldehyde resin adhesive.

[0060] Comparative Example 2

[0061] (1) Into a 500ml three-necked flask, 100 parts of formaldehyde aqueous solution (concentration 37%), 35 parts of urea, 0.5 parts of melamine were added, the pH of the system was adjusted to 7.5 with sodium hydroxide aqueous solution (1 mol / L), and the temperature was raised to 90°C and kept for 40 minutes.

[0062] (2) The pH of the system was adjusted to 5.5 with formic acid aqueous solution (0.5 mol / L), and the system was kept for a period of time. During the keeping period, the turbidity point was measured every 2 minutes. When the sample dropped into ice water showed white smoke, the reaction was continued for a period of time, the pH of the system was adjusted to 8.0 with sodium hydroxide aqueous solution (1 mol / L), 17 parts of urea was added, and the system was kept at 90°C for 30 minutes.

[0063] (3) The reaction system of step (2) was cooled to 60°C, 10 parts of urea was added, and the system was kept for 40 minutes. The temperature was naturally lowered to below 40°C, 0.5 parts of cysteine was added, the pH of the system was adjusted to 8.0 with sodium hydroxide aqueous solution (1 mol / L), and the product was discharged to obtain the urea-formaldehyde resin adhesive.

[0064] Comparative Example 3

[0065] (1) Into a 500ml three-necked flask, 100 parts of formaldehyde aqueous solution (concentration 37%), 45 parts of urea, 1 part of melamine were added, the pH of the system was adjusted to 8.5 with sodium hydroxide aqueous solution (1 mol / L), and the temperature was raised to 90°C and kept for 60 minutes.

[0066] (2) The pH of the system was adjusted to 4.5 with formic acid aqueous solution (0.5 mol / L), and the system was kept for a period of time. During the keeping period, the turbidity point was measured every 2 minutes. When the sample dropped into ice water showed white smoke, the reaction was continued for a period of time, the pH of the system was adjusted to 8.5 with sodium hydroxide aqueous solution (1 mol / L), 19 parts of urea was added, and the system was kept at 90°C for 40 minutes.

[0067] (3) The reaction system of step (2) was cooled to 60°C, 11 parts of urea was added, and the system was kept for 30 minutes. The temperature was naturally lowered to below 40°C, 1.5 parts of cysteine was added, the pH of the system was adjusted to 9.0 with sodium hydroxide aqueous solution (1 mol / L), and the product was discharged to obtain the urea-formaldehyde resin adhesive.

[0068] The urea-formaldehyde resins prepared in Examples 1-6 and Comparative Examples 1-3 were detected for basic properties such as free formaldehyde content, solid content, viscosity, pot life, and curing time according to the method specified in the national standard GB / T 14074-2017 (Adhesives and resins for wood industry-Test methods).

[0069] Table 1 Basic properties of urea-formaldehyde resins prepared in Examples 1-6 and Comparative Examples 1-2

[0070]

[0071] The table of examples 1-6 and comparative examples 1-3 show that the addition of cysteine, especially when the amount added is 1.5-2 parts, effectively reduces the free formaldehyde content of the urea-formaldehyde resin prepared, to a value <0.1%. By comparing example 1 and comparative example 2, and example 3 and comparative example 3, it can be found that the effect of reducing the free formaldehyde content is not as good when the cysteine is added late, even if the amount added is greater than or equal to the amount added in the middle. When the cysteine is added in the middle of the synthesis of the urea-formaldehyde resin, due to its own thiol and amino groups, the thiol can react with formaldehyde, and the amino group can participate in the condensation between the methylol groups after reacting with formaldehyde, changing the structure of the urea-formaldehyde resin. Although the addition of cysteine after the synthesis reaction of the urea-formaldehyde resin can also effectively reduce the free formaldehyde content of the urea-formaldehyde resin, the effect is not as good as when the cysteine is added in the middle of the reaction, and the addition of cysteine in the middle can also replace the formic acid solution as the pH regulator, reducing the amount of sodium hydroxide solution used for adjusting the pH in the later stage, effectively reducing the cost, and also changing the structure of the urea-formaldehyde resin and improving other properties of the resin.

[0072] The above examples are merely an explanation of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods, and are all included in the protection scope of the present application.

Claims

1. A urea-formaldehyde resin with ultra-low free formaldehyde, characterized in that, The ultra-low free formaldehyde urea-formaldehyde resin comprises the following raw materials in parts by weight: 30-40wt% formaldehyde aqueous solution, 100 parts 59-100 parts urea 1.5-2 parts melamine 1.5-2 parts of formaldehyde scavenging agent; The formaldehyde scavenging agent is cysteine; The preparation method of the ultra-low free formaldehyde urea-formaldehyde resin includes the following steps: (1) Add formaldehyde aqueous solution and 35-45 parts of urea, melamine and alkaline solution in sequence to adjust the pH value of the system to alkaline, heat to 90-100°C and keep warm for 30-60 minutes; (2) Add formaldehyde scavenger, keep warm for a period of time, measure the turbidity point every 2 minutes during the warming period, take a sample and drop it into ice water. When it appears as white smoke, continue the reaction for 5 to 10 minutes, and adjust the pH of the system to 8-8.5 with alkaline solution. Add 15-19 parts of urea and keep warm at 90°C for 20-40 minutes. (3) Cool the reaction system of step (2) to 50-60°C, add the remaining 9-11 parts of urea, keep warm for 20-40 minutes, cool naturally to below 40°C, adjust the pH of the system to alkaline with alkaline solution, discharge the material, and obtain ultra-low free formaldehyde urea-formaldehyde resin. The alkaline pH value in step (1) is 8.0-8.5; in step (2), formaldehyde scavenging agent is added to adjust the pH of the system to 4.5-5.0; the alkaline pH value in step (3) is 7.0-9.

0.

2. A method for preparing the ultra-low free formaldehyde urea-formaldehyde resin according to claim 1, characterized in that, The method includes the following steps: (1) Add formaldehyde aqueous solution and 35-45 parts of urea, melamine and alkaline solution in sequence to adjust the pH value of the system to alkaline, heat to 90-100°C and keep warm for 30-60 minutes; (2) Add formaldehyde scavenger, keep warm for a period of time, measure the turbidity point every 2 minutes during the warming period, take a sample and drop it into ice water. When it appears as white smoke, continue the reaction for 5 to 10 minutes, and adjust the pH of the system to 8-8.5 with alkaline solution. Add 15-19 parts of urea and keep warm at 90°C for 20-40 minutes. (3) Cool the reaction system of step (2) to 50-60°C, add the remaining 9-11 parts of urea, keep warm for 20-40 minutes, cool naturally to below 40°C, adjust the pH of the system to alkaline with alkaline solution, discharge the material, and obtain ultra-low free formaldehyde urea-formaldehyde resin.

3. The method according to claim 2, characterized in that, The alkaline solution mentioned in steps (1), (2), and (3) is an aqueous solution of sodium hydroxide or potassium hydroxide, with a concentration of 1-2 mol / L.

4. The method according to claim 2, characterized in that, The insulation conditions described in steps (1) and (2) are oil bath insulation.

5. The application of the ultra-low free formaldehyde urea-formaldehyde resin according to claim 1 in the production and processing of plywood, particleboard and medium-density fiberboard.

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