CN111333799B - Synthetic method of urea-formaldehyde resin with ultralow formaldehyde release - Google Patents
Synthetic method of urea-formaldehyde resin with ultralow formaldehyde release Download PDFInfo
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- C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
- C08G12/00—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen
- C08G12/02—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen of aldehydes
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- C09J161/00—Adhesives based on condensation polymers of aldehydes or ketones; Adhesives based on derivatives of such polymers
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Abstract
The invention discloses a synthesis method of urea-formaldehyde resin with ultralow formaldehyde release, relates to the field of chemical organic polymer polymerization, and particularly aims at the field of urea-formaldehyde resin adhesive synthesis. The invention comprises four steps: firstly, a high-concentration formaldehyde aqueous solution reacts with urea with the dosage of 25-50% of the formula to synthesize pre-acetal, and then the pre-acetal reacts with urea under weak base condition, and then undergoes condensation reaction under acidic condition, and curing reaction under the action of soy protein hydrolysate. The soybean hydrolyzed protein contains various micromolecular amino acids, amino groups and hydroxyl groups in the amino acids and free formaldehyde undergo polycondensation reaction and acetal reaction to form a polymer with a cross-linked network structure among macromolecules, a large number of hydrophilic groups are blocked, unreacted free formaldehyde is captured, the content of the free formaldehyde in the synthesized urea-formaldehyde resin is lower than the newly issued E1 standard (GB 18580-2017 weather box method), and the ultra-low formaldehyde release is achieved.
Description
Belonging to the field of
The invention relates to the field of organic high molecular polymerization reaction, in particular to the field of synthesis of urea formaldehyde resin adhesive.
Background
The reduction of natural forest resources promotes the artificial board industry in China to enter a rapid development period, and the consumption of wood adhesive for manufacturing artificial boards is huge. Urea formaldehyde resin (UF) has the advantages of easily available raw materials and low manufacturing cost, and has become the most widely used wood adhesive worldwide. Urea Formaldehyde (UF) resins are the largest yielding species in wood adhesives, and are one of the earlier developments and applications for polymeric adhesives. The urea-formaldehyde resin adhesive is widely applied to industries such as wood ware processing, artificial board production, paper adhesion, interior decoration and the like because of the advantages of simple production process, wide raw material sources, low cost, convenient use, high initial viscosity, high adhesive strength and the like, and the use amount of the urea-formaldehyde resin adhesive is about more than 80% of the total consumption amount at present.
At present, the technology of producing the environment-friendly urea-formaldehyde resin adhesive in China is relatively backward, and the main defects are that free formaldehyde is released in the use process, so that the environment is polluted, and the human health is seriously harmed. Therefore, the development of the environment-friendly low-toxicity urea-formaldehyde resin adhesive is always a research hot spot of the wood adhesive industry.
Recently, the national quality inspection administration, the national standards administration and the committee officially release the modified "limit of formaldehyde release in the artificial board for interior decoration materials and its products" (GB 18580-2017), which is formally implemented in the 5 th month and 1 st 2018. The new standard improves the formaldehyde release limit requirement, the formaldehyde release limit value is regulated to be 0.124 mg/cubic meter, and the limit mark is E1; this means that the formaldehyde release limit and detection method in China are connected with the international rail.
Some manufacturers in China use raw materials such as polyvinyl alcohol and melamine to capture free formaldehyde in urea-formaldehyde resin, but because the addition reaction and the polycondensation reaction are reversible equilibrium reactions, the addition of the auxiliary agents only improves the functions of brittleness, strength, water resistance and the like of urea-formaldehyde, has poor effect of eliminating the free formaldehyde, and cannot meet the limit standard of the free formaldehyde.
Urea itself is the most commonly used formaldehyde scavenger, but the reaction activity of urea is relatively low, the input amount of urea must be increased to achieve the ideal formaldehyde eliminating effect, more urea is input to enable more free urea to exist in the resin, and the interval of the urea between resin molecules can cause adverse effects on the adhesive performance of urea-formaldehyde resin, and the adhesive strength, the curing time and the viscosity of the urea-formaldehyde resin are affected.
The research of eliminating free formaldehyde by using vegetable protein in urea-formaldehyde resin adhesive in China is less, the development and starting are late, and the units participating in the research are not more and the strength is weak.
Lanhui (2002) 12 studied to prepare a wood adhesive having a soybean powder content of about 20% by chemically modifying defatted soybean powder with an acylating agent and a crosslinking agent, respectively, freeze-drying the defatted soybean powder, and then preparing a soybean powder-containing wood adhesive with Ca (OH) 2-NaOH-NaSiO3, wherein the acylating agent and the crosslinking agent are used to improve the adhesiveness and water resistance of the soybean adhesive. Zhao Ke (2002) 13) the research on soy protein isolate-polyvinyl alcohol and soy protein isolate-white emulsion composite adhesive provides an environment-friendly adhesive with higher initial adhesion for manufacturing disposable plant fiber snack boxes. Wang Yedong et al (2003) [6] considered that the development of vegetable proteins other than soy proteins as wood adhesives has received attention. In the span (2003) [14], wheat gluten is modified by mercaptoethanol, urea, acetic anhydride, succinic anhydride, sodium dodecyl sulfate (sDS), sodium sulfite and the like, so that a modified gluten-based wood adhesive with high bonding strength (2147-3109 MPa) in a dry state is obtained, and the heat sealing condition of the protein adhesive is studied.
These studies have resulted in the production of adhesives using soy protein alone, rather than modifying urea-formaldehyde resins to remove free formaldehyde. When the product is used, the comprehensive performance indexes such as the joint strength, the initial viscosity, the curing condition and the like are very different from those of urea-formaldehyde resin adhesive in the aspects of raw materials, product price and the like, and the urea-formaldehyde resin adhesive cannot replace the urea-formaldehyde resin adhesive to have great effect and application prospect in the plate market.
Object of the Invention
The invention aims to capture free formaldehyde in a product and carry out end capping treatment by using soy protein hydrolysate in the urea resin synthesis process so as to solve the problem that the free formaldehyde in the urea resin synthesis process exceeds the standard at present, so that the product meets the national E1 mandatory standard requirement.
Technical proposal
The invention comprises four steps: firstly, reacting 50% formaldehyde aqueous solution with a part of urea to synthesize urea formaldehyde pre-shrinking liquid (UFC); then in alkaline environment, urea formaldehyde pre-shrinking liquid (replacing formaldehyde solution) is utilized to carry out addition reaction with the rest urea; only intermolecular polycondensation reaction in acid medium; finally, under alkaline condition, the urea-formaldehyde resin and the soy protein hydrolysate act to absorb the free formaldehyde in the resin.
Firstly, synthesizing urea formaldehyde preshrinking liquid (UFC), using urea formaldehyde preshrinking liquid to replace formaldehyde solution to react with urea to synthesize urea formaldehyde resin adhesive, and can implement synthesis of urea formaldehyde resin by U/F under the condition of low mole ratio, mainly reducing dehydration and reducing free formaldehyde content in product.
The urea formaldehyde pre-shrinking liquid is polymerized with urea to synthesize urea formaldehyde resin, and the urea formaldehyde resin synthesis route adopts weak base-weak acid-weak base; the first step of addition reaction, which is to utilize ammonia water to adjust the alkalinity of the material and to carry out methylolation reaction in a weak base stage; the second step is to adjust the acidity of the reactant by formic acid, and the weak acid stage is to perform polycondensation reaction between methylol urea and urea to generate initial resin; and after the reaction reaches the end, the reactant is returned to be alkalescent by sodium hydroxide alkali liquor, and the urea-formaldehyde resin is used for stopping the reaction under the alkalescent condition.
In the curing reaction stage, soybean protein hydrolysate is used for reacting with urea resin to capture free formaldehyde.
The soybean protein hydrolysate raw material is derived from soybeans, the soybeans are bean products with large usage amount and low price, and the soybeans comprise the following chemical components: 13.12% of moisture, 19.29% of fat, 21.55% of carbohydrate, 2.94% of crude fiber, 38.45% of protein, 4.5% of ash and the like.
70% of the storage proteins in the soybeans are globular proteins, polypeptide chain-shaped substances are formed by polymerization of aglycone, glutamine, alanine, phenylalanine, leucine and the like, and most polar and nonpolar groups in the polypeptide chains form stable multilevel structures through Van der Waals force, hydrogen bond and hydrophobic action, so that compact spheres are formed.
The structural formula is as follows:
the polar and nonpolar amino groups in the spherical soybean protein are blocked, so that the spherical soybean protein is difficult to participate in the reaction of urea formaldehyde resin, and free formaldehyde cannot be effectively removed.
Preparation of soy protein hydrolysate: the soybean protein is hydrolyzed in alkaline water solution at a specific reaction temperature and constant pressure for a certain reaction time to obtain colorless, alkaline, viscous and flowable soybean hydrolyzed protein at room temperature.
Soy protein hydrolysate consists of aglycone, glutamate, lysine, alanine and phenylalaninol acids and other small molecular amino acids.
The chemical structural formula is as follows:
the soybean protein hydrolysate contains various small molecular amino acid components and hydroxyl components, is extremely easy to perform addition reaction and acetal reaction with formaldehyde to form a polymer with a cross-linked network structure among macromolecules, and is used for blocking a large amount of hydrophilic groups and capturing unreacted free formaldehyde, so that the free formaldehyde in the urea formaldehyde resin is removed.
Experimental data shows that: in the curing stage, when the adding amount of the soybean protein hydrolysate is increased by 1%, the free formaldehyde content in the urea-formaldehyde resin can be reduced by 50%.
The synthesis process of urea-formaldehyde resin with ultralow formaldehyde release includes the following steps:
a pre-aldol reaction zone: consists of a formaldehyde storage tank, a pre-aldolization reactor, a urea storage tank and an ammonia water storage tank; adopting 50% concentrated formaldehyde aqueous solution, urea is industrial grade urea, adding 1/2-1/4 of the dosage of the formulation, adjusting the PH value to be alkaline by ammonia water, introducing steam into the interlayer of the reaction kettle for heating, controlling the temperature to be 60-90 ℃, and stirring and reacting for 30-90 min to generate urea formaldehyde fat pre-shrinking liquid.
B addition reaction zone: consists of an addition reaction kettle, an auxiliary agent storage tank, a urea storage tank and an ammonia water storage tank; adding pre-acetal and auxiliary agent into a reaction kettle; adding urea twice according to the reaction process, wherein the dosage of the urea is 1/2-2/5 of the total dosage; ammonia water is used for adjusting the PH value to be alkalescent; the interlayer of the reaction kettle is heated by steam, and the temperature is controlled to be 60-90 ℃; stirring for reaction, and condensing and refluxing generated steam through a reflux condenser; the reaction residence time is 30-90 minutes.
C condensation reaction zone: consists of a formic acid storage tank and a condensation reaction kettle; adding formic acid into the reaction materials, adjusting the pH value of the solution to be acidic 3-6.5, introducing steam into the interlayer of the reaction kettle for heating, and controlling the temperature to be 60-85 ℃; stirring for reaction, and the reaction residence time is 30-90 min, and determining the reaction end point according to a viscosity method or cloud point.
D curing reaction zone: consists of an alkali solution storage tank, a soy protein hydrolysate storage tank, a urea storage tank and a ripening reaction kettle; adjusting the pH value of the material to be alkaline by using alkali liquor, wherein the alkali liquor is 30% sodium hydroxide aqueous solution; adding soybean protein hydrolysate and the rest urea, introducing steam into a jacket, heating, stirring at 60-70 ℃ for reaction, and keeping the reaction time for 30-60 min.
E detection and storage zone: the urea resin storage tank consists of a product detection device and a urea resin storage tank; the product detection device is an online detection system, and the urea resin is fed into a urea resin storage tank for standby after being qualified.
In the pre-aldolization reaction zone, the material of the pre-aldolization reaction kettle is 304/Q235-B, an interlayer and a deceleration stirring device; the other material of the storage tank is 304/Q235-B.
In the pre-aldolization reaction zone, the urea is industrial grade, the sulfate content is less than 0.01 percent, the biuret is less than 0.7 percent, and the free ammonia is less than 0.015 percent.
In the pre-aldolization reaction zone, formaldehyde is of industrial grade, the concentration is more than or equal to 50%, and formic acid is less than 0.1%; methanol less than 1.5% and Fe less than 0.0005%.
In the addition reaction zone, the material of the addition reaction kettle is 304/Q235-B, and the interlayer is provided with a speed-reducing stirring device; the other material of the storage tank is 304/Q235-B.
In the addition reaction zone, the auxiliary agent is polyvinyl alcohol, melamine or a mixture thereof, wherein the polyvinyl alcohol is 50% aqueous solution, and the model is 1799 or 1788; the alkali solution is preferably an ammonia solution; the urea is industrial grade, the sulfate content is less than 0.01%, the biuret is less than 0.7%, and the free ammonia is less than 0.015%.
The material of the condensation reaction kettle is 304/Q235-B, an interlayer and a speed-reducing stirring device; the other material of the storage tank is 304/Q235-B.
In the condensation reaction zone, the acid solution is preferably formic acid aqueous solution, and the concentration is 30-60%.
The material of the curing reaction zone is 304/Q235-B, an interlayer and a speed-reducing stirring device; the other material of the storage tank is 304/Q235-B.
The curing reaction zone is characterized in that the alkali liquor is 30% sodium hydroxide aqueous solution; the soybean protein hydrolysate is a 50% aqueous solution; the urea is industrial grade, the sulfate content is less than 0.01%, the biuret is less than 0.7%, and the free ammonia is less than 0.015%.
A maturation reaction zone, said soy protein hydrolysate quality requirement: the sulfate content is less than 0.015%, the free ammonia is less than 0.005%, the iron content is less than 0.0005%, and the PH value is between 7.5 and 11.5.
The addition amount of the soybean protein hydrolysate is more than or equal to 2 percent (100 percent content), the reaction temperature is 40-70 ℃, and the reaction residence time is 20-60 min.
Advantageous effects
The invention utilizes the soybean protein hydrolysate to reduce the free formaldehyde content in the urea-formaldehyde resin and lock the free precipitation of formaldehyde in the product caused by reversible equilibrium reaction, and simultaneously improves the prepressing performance problem of the urea-formaldehyde resin pressed plywood, the bonding strength of the glued product reaches the national standard, the contradiction between the bonding strength and the formaldehyde release amount is effectively solved, and the plate product becomes a real green product and has strong market prospect.
The urea-formaldehyde resin adhesive prepared by utilizing soy protein hydrolysate has comprehensive performance detected according to GB/T9846-2004 and free formaldehyde index detected by adopting a climatic chamber method of national newly issued standard of artificial board for indoor decoration and decoration materials and formaldehyde release limit in products thereof (GB 18580-2017).
The results of the detection are shown in the table.
UF resin main performance detection index
The performance of the synthesized urea-formaldehyde resin reaches the national standard, wherein the index of free formaldehyde is lower than the national E1 standard.
Drawings
Fig. 1 is a block diagram of a process for synthesizing urea formaldehyde resin with ultra-low formaldehyde emission, which is only a schematic flow chart for explaining the invention, only the necessary equipment for explaining the process is shown, and other obviously needed facilities such as instruments, gas converging equipment, pumps, valves, intermediate tanks and the like are omitted.
Figure 1 reference numerals illustrate:
1. a formaldehyde storage tank; 2. a precondensation aldehyde reactor; 3. a urea storage tank; 4. an ammonia water storage tank; 5. an addition reaction kettle; 6. an auxiliary agent storage tank; 7. a urea storage tank; 8. an ammonia water storage tank; 9. a reflux condenser; 10. a formic acid storage tank; 11. a condensation reaction kettle; 12. an alkali solution storage tank; 13. a soy protein hydrolysate storage tank; 14. a urea storage tank; 15. a curing reaction kettle; 16. a urea resin storage tank; 17. And a product detection device.
The present disclosure is further described with the aid of the accompanying drawings:
(1) Raw materials of formaldehyde, alkali liquor, ammonia water, formic acid, urea and soy protein hydrolysate are respectively sent to corresponding storage tanks of workshops from factory storage tanks, and are metered by a metering device according to a formula and are pumped into a reaction kettle.
(2) Formaldehyde is added into a pre-aldolization reaction kettle through a formaldehyde storage tank and a mass metering pump; stirring is started, ammonia water is added from an ammonia water storage tank, and the PH value is adjusted to be between 6 and 8; urea is added through a urea storage tank; and introducing steam into the interlayer, heating to 80-90 ℃, and controlling the reaction time to synthesize the urea formaldehyde pre-shrinking liquid.
(3) Pumping the reacted urea formaldehyde preshrinking liquid into an addition reaction kettle by a pump, and starting stirring; ammonia water is added from an ammonia water storage tank, and the PH value of the solution is adjusted to 7-9; adding polyvinyl alcohol into an auxiliary agent storage tank; urea is metered and added into a urea storage tank; the jacket is heated by steam, and the temperature is controlled to be 70-90 ℃; and (3) heat exchange is carried out on steam generated by the reaction through a reflux cooler, condensate flows back into the reaction kettle, and the reaction is finished for 30-80 minutes.
(4) Pumping reactants into a condensation reaction kettle by a pump, and starting stirring; adding formic acid into a formic acid storage tank, and regulating and controlling the PH value to be acidic 3-6; introducing steam into the interlayer of the reaction kettle to raise the temperature, and controlling the temperature to be 60-90 ℃; the cloud point method is used for controlling the reaction end point.
(5) Pumping reactants into a curing reaction kettle by a pump, and starting stirring; adding alkali liquor from an alkali liquor storage tank, and regulating the pH value of the solution to 7-9; adding the remaining urea from the urea storage tank; adding soy protein hydrolysate into a soy protein hydrolysate storage tank, introducing steam into a jacket of a reaction kettle to raise the temperature, controlling the temperature to be 60-80 ℃, and cooling and discharging after the reaction is completed.
(6) The product is inspected to be qualified, especially the free formaldehyde is detected according to the formaldehyde release limit value of 'artificial board of interior decoration material and its products' (GB 18580-2017), the formaldehyde release limit value is 0.124 mg/cubic meter (climate box method), and after being qualified, the product is pumped into a urea formaldehyde resin storage tank for storage.
Detailed description of the preferred embodiments (examples)
Raw materials of formaldehyde, alkali liquor, ammonia water, formic acid, urea and soy protein hydrolysate are respectively sent to corresponding storage tanks of workshops from factory storage tanks, and are metered by a metering device according to a formula and pumped into a reaction kettle.
N for whole system 2 Or other inert gas replacement, and detecting that the oxygen content of the system is lower than 10PPM from the exhaust gas.
Formaldehyde enters a pre-aldolization reaction kettle through a formaldehyde storage tank (the formaldehyde content is 37.5 percent) by measuring 810Kg by a mass pump; stirring is started, ammonia water is added into an ammonia water storage tank, and the PH value is adjusted to 7-9.5; adding 135Kg of urea through a urea storage tank; and (3) introducing steam into the interlayer of the reaction kettle for heating, heating to 80-90 ℃, reacting for 60 minutes, and detecting qualified products for later use.
Pumping 337.5Kg of pre-polymerized urea-formaldehyde resin liquid into an addition reaction kettle by a pump, adding ammonia water from an ammonia water storage tank, adjusting the pH value of the solution to 6-9 by ammonia water, adding 25Kg of polyethylene alcohol solution into an auxiliary agent storage tank, heating jacket steam of the kettle, controlling the temperature to 60-90 ℃, stirring until the polyethylene alcohol solution is dissolved, adding 185Kg of urea into the urea storage tank, cooling the reaction product steam by a reflux cooler, and re-entering the cooling liquid into the reaction kettle, wherein the reaction residence time is 60-80 minutes.
Pumping reactants into a condensation reaction kettle by a pump; stirring is started, formic acid is added from a formic acid storage tank, and the PH value is regulated and controlled to be acidic 3-6.5; introducing steam into the interlayer of the reaction kettle to raise the temperature, and controlling the temperature to be 60-90 ℃; the residence time of the polycondensation reaction is 40 to 80 minutes (the end point of the reaction is controlled by a cloud point method).
Pumping the reactant into a curing reaction kettle by a pump; starting stirring, adding alkali liquor into an alkali liquor storage tank, and rapidly adjusting the pH value of the material to 7-9; 10Kg of soybean protein hydrolysate is added (content 50%) from a special storage tank; 17.5Kg of urea is added into a urea storage tank; heating the interlayer steam, and controlling the temperature to be 60-90 ℃; the reaction residence time is 30-50 minutes; cooling to 40 ℃, and pumping the product into a urea resin storage tank for storage after the product is inspected to be qualified.
Claims (7)
1. A synthetic method of urea-formaldehyde resin with ultralow formaldehyde release comprises the following technical processes:
A. precondensation aldehyde reaction zone: consists of a formaldehyde storage tank, a pre-aldolization reactor, an element storage tank and an ammonia water storage tank; adopting 50% concentrated formaldehyde aqueous solution, urea is industrial grade urea, adding 1/2-1/4 of the dosage of the formula, adjusting the pH value to be alkaline by ammonia water, introducing steam into an interlayer of a reaction kettle for heating, controlling the temperature to be 60-90 ℃, and stirring for reacting for 30-90 min to generate urea formaldehyde fat pre-shrinking liquid;
B. addition reaction zone: consists of an addition reaction kettle, an auxiliary agent storage tank, a urea storage tank and an ammonia water storage tank; adding pre-acetal and auxiliary agent into a reaction kettle; the urea is added in two times according to the reaction process, and the dosage of the urea is 1/2 to 2/5 of the total dosage; ammonia water is used for adjusting the pH value to be alkalescent; the interlayer of the reaction kettle is heated by steam, the temperature is controlled at 60-90 ℃, and the generated steam is condensed and reflowed by a reflow condenser; the reaction residence time is 30-90 minutes;
C. condensation reaction zone: consists of a formic acid storage tank and a condensation reaction kettle; adding formic acid into the reaction materials, adjusting the pH value of the solution to be acidic 3-6.5, introducing steam into the interlayer of the reaction kettle for heating, controlling the temperature to be 60-85 ℃, and reacting for 30-90 minutes, and determining the reaction end point according to a viscosity method or cloud point;
D. curing reaction zone: consists of an alkali solution storage tank, a soy protein hydrolysate storage tank, a urea storage tank and a ripening reaction kettle; adjusting the pH value of the material to be alkaline by using alkali liquor, wherein the alkali liquor is 30% sodium hydroxide aqueous solution; adding soybean protein hydrolysate and the rest urea, introducing steam into a jacket, heating, stirring at 60-70 ℃ for reaction, and keeping the reaction time for 30-60 min;
E. detection and storage zone: the urea resin storage tank consists of a product detection device and a urea resin storage tank; the product detection device is an online detection system, and the urea resin is fed into the urea resin storage tank for standby after being qualified.
2. The method for synthesizing the urea-formaldehyde resin with ultralow formaldehyde emission according to claim 1, which is characterized in that: in the pre-aldolization reaction zone, the urea is industrial grade, the sulfate content is less than 0.01 percent, the biuret is less than 0.7 percent, and the free ammonia is less than 0.015 percent.
3. The method for synthesizing the urea-formaldehyde resin with ultralow formaldehyde emission according to claim 1, which is characterized in that: in the pre-aldolization reaction zone, formaldehyde is of industrial grade, the concentration is more than or equal to 50%, and formic acid is less than 0.1%; methanol less than 1.5% and Fe less than 0.0005%.
4. The method for synthesizing the urea-formaldehyde resin with ultralow formaldehyde emission according to claim 1, which is characterized in that: in the addition reaction zone, the auxiliary agent is polyvinyl alcohol, melamine or a mixture thereof, wherein the polyvinyl alcohol is 50% aqueous solution, and the model is 1799 or 1788; the alkali liquor is preferably ammonia water solution; the urea is industrial grade, the sulfate content is less than 0.01%, the biuret is less than 0.7%, and the free ammonia is less than 0.015%.
5. The method for synthesizing the urea-formaldehyde resin with ultralow formaldehyde emission according to claim 1, which is characterized in that: the curing reaction zone is characterized in that the alkali liquor is 30% sodium hydroxide aqueous solution; the soy protein hydrolysate is a 50% aqueous solution; the urea is industrial grade, the sulfate content is less than 0.01%, the biuret is less than 0.7%, and the free ammonia is less than 0.015%.
6. The method for synthesizing the urea-formaldehyde resin with ultralow formaldehyde emission according to claim 1, which is characterized in that: a maturation reaction zone, said soy protein hydrolysate quality requirement: the sulfate content is less than 0.015 percent, the free ammonia is less than 0.005 percent, the iron content is less than 0.0005 percent, and the pH value is between 7.5 and 11.5.
7. The method for synthesizing the urea-formaldehyde resin with ultralow formaldehyde emission according to claim 1, which is characterized in that: the addition amount of the soybean protein hydrolysate is more than or equal to 2%, the reaction temperature is preferably 50-60 ℃, and the reaction residence time is preferably 20-30 min.
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