What is the synthesis process of urea formaldehyde resin?

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The changes in the synthesis process of urea formaldehyde resin are very complex, and the reaction mechanism is still not very clear. There are two existing theories, namely the traditional theory and the sugar formaldehyde theory, which can be used to synthesize urea formaldehyde resins with different structures and properties. Next, the editor will provide a detailed introduction for everyone. Let's quickly follow us to have a detailed understanding.
1、 Application of traditional theory to synthesize urea formaldehyde resin with body structure
According to the traditional theory, the synthesis of urea formaldehyde resin can be divided into two stages: one stage is the formation of Methylol urea, which is the Addition reaction stage; The second stage of resinification is the condensation reaction stage.
1. Addition reaction stage
Urea and formaldehyde undergo Hydroxylation reaction in neutral or weakly alkaline medium (PH 7~8). When the molar ratio (F/U) of formaldehyde to urea is ≤ 1, stable monohydroxymethylurea is generated;
H2N-CO-NH2+CH2O → H2N-CO-NHCH2OH, and then react with formaldehyde to generate dimethylol urea, H2N-CO-NHCH2OH+CH2O → HOH2CHN-CO-NHCH2OH
A small amount of tri Methylol urea and tetra Methylol urea can also be generated, but tetra Methylol urea has not been separated so far. The reaction rate ratio of mono Methylol urea, di hydroxymethyl urea and tri Methylol urea is 9:3:1.
2. Condensation reaction stage
Methylol urea contains active hydroxymethyl (- CH2OH), which can be further condensed to form polymers. Because the polycondensation reaction is very slow under alkaline conditions, only in slightly acidic medium (PH 4-6), the monohydroxymethylurea and di Hydroxycarbamide generated are methylated between unreacted urea, hydroxymethyl and hydroxymethyl of Methylol urea at high temperature to form intermediates of various Condensation polymer. There are basically 5 forms of reactions, and typical reactions include:
A Methylol urea is condensed and dehydrated with hydrogen on the adjacent molecular amine group to form a methylene bond.
H2N-CO-NHCH2OH+H2N-CO-NHCH2OH → H2N-CO-NHCH2NH-CO-NHCH2OH+H2O, the adjacent hydroxyl methyl groups condense to form a dimethyl ether bond and release water. HOCH2NH-CO-NHCH2OH+HOCH2NH-CO-NHCH2OH → HOCH2NH-CO-NHCH2NH-CO-NHCH2OH+H2O, adjacent two molecules undergo dehydration and formaldehyde removal reactions to form a methyl bond: HOCH2NH-CO-NHCH2OH+HOCH2NH-CO-NHCH2OH → HOCH2NH-CO-NHCH2NH-CO-NHCH2OH+H2O
After the formation of the intermediate, it is further condensed to form linear or branched oligomers with methylene and dimethylene as the main body or a small amount connected by ether bonds. It is a mixture of various relative molecular weights, with an average molecular weight of about 700, and is soluble in water. Due to the active groups such as hydroxymethyl, amino, and imine groups in the oligomers of urea formaldehyde resin, they will continue to react over time to form larger molecules. Heating or adding curing agents can accelerate the reaction and form a network structure.
2、 Synthesis of Urea Formaldehyde Resin with Uron Ring Structure Using the Theory of Sugar Formaldehyde
Due to the development of sugar formaldehyde theory, a completely different method from traditional synthesis of urea formaldehyde resin was adopted. Firstly, in a strong acid medium (pH<3.0), urea reacted with formaldehyde to generate a certain number of small molecules with Uron ring structure, and then further polymerized into high molecules containing Uron ring segments. Because the hydrolysis resistance of the Uron ring is better than that of methylene disulfide, the cross-linking density of the synthesized urea formaldehyde resin results in better initial adhesion of the urea formaldehyde resin. However, as the number of Uron rings in the urea formaldehyde resin increases, the curing rate slows down. Therefore, it is advisable to control the content of Uron rings at around 10% during the synthesis of urea formaldehyde resin. The content of Uron rings increases with the extension of reaction time and the increase of temperature. At the same time, the content of hydroxymethyl groups is low, the formaldehyde release is low, and the bonding performance is poor.
Urea formaldehyde resin adhesive is widely used in the production of medium/high density fiberboard due to its good bonding strength, convenient use, easy availability of raw materials, and low cost, becoming an important type of wood adhesive. With the development of Engineered wood industry and furniture industry, enterprises constantly put forward higher requirements for urea formaldehyde resin and its application effect. Developed countries such as Europe, the United States and Japan have strict limits on formaldehyde emission from Engineered wood. In the past 30 years, the research on the synthesis mechanism of urea formaldehyde resin and the formaldehyde emission mechanism of the manufactured artificial board has always been the research hotspot of chemical technicians in various countries, especially those in forest chemical industry. Through these basic research work, we have mastered and obtained resins with excellent performance. These studies involve the formulation design of the resin, the influence of the synthesis process on the molecular structure and the economic evaluation. Based on my years of practice, I would like to discuss my experience in the synthesis technology of urea formaldehyde resin.
Acid synthesis process in strong acid medium:
The use of strong acid synthesis process routes in production practice is still rare in China, which is an unconventional method for synthesizing urea formaldehyde resin. The advantage of the strong acid synthesis process is that after the resin is cured, it has a higher methylene bridge bond compared to the resin synthesized by traditional methods, while the number of methylene ether bonds is less, and a large number of URON ring compounds are found. The reaction process involves the reaction of formaldehyde with urea in a strong acid medium to generate a certain number of URON ring structures, which are then further polymerized into polymers with URON ring links. When introducing heterocyclic derivatives such as URON into the resin, due to the relative reduction of resin crosslinking degree and the increase in molecular length, it is beneficial to improve the water solubility and initial viscosity of the resin. Its application in production is remarkable due to its low hot pressing coefficient and significant improvement in production efficiency. The molar ratio of each stage of its production process has a significant impact on the safety of production and the performance of the adhesive. Our synthesis process under strong acid medium conditions is to add all formaldehyde, adjust the pH of the reaction solution with HCI to be less than 2.5, and add additives to raise the temperature to a certain temperature. Step by step, add U1 to react to the endpoint, and adjust the reaction solution with NaOH to golden yellow. Before adjustment, the appearance is white and transparent, and the transformation to golden yellow is very obvious. Add U2 for secondary polycondensation, and the reaction meets the requirements with a viscosity of 18s-20s (coated in -4 cups); Adjust the pH of the reaction solution to 7-9, add U3 for insulation and reaction for half an hour, then cool down and discharge the material.
In the research of strong acid processes, there are several experiences:
(1) In a strong acid medium, the feeding temperature of urea is an important parameter that determines the reaction of formaldehyde with urea and its products, as well as the post condensation reaction. If not properly controlled, accidents may occur, making the reaction difficult to proceed smoothly.
(2) There is an extreme point for the molar ratio of primary urea and formaldehyde. If it is less than this extreme point, the Intrinsic viscosity of the reaction product will increase significantly, so that it can not be controlled. Due to the experience, the molecular weight of the reaction product can be increased by reducing the molar ratio as reasonably as possible, which can improve the cohesive strength of the adhesive and the wood bonding strength.
(3) The pH value during secondary condensation seriously affects the transparency and storage life of the adhesive. The lower the pH value, the greater the amount of secondary urea added, the more turbid the gel, the poorer its stability, and the shorter its storage period. In our process, the condition is to adjust the reaction solution to golden yellow with NaOH.
(4) The amount of urea added during secondary polycondensation also has an extreme value on the bonding strength of wood. As the urea added in the secondary polycondensation mainly hydroxymethylates with unreacted formaldehyde in the reactant, and continues to polycondensation towards resinification, it is difficult to form the dimethylol urea and its condensate at this stage, so too much urea will reduce the cohesion of the polycondensation product, thus reducing the bonding strength.


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