Urea-formaldehyde resin (UF) is the polycondensation of urea and formaldehyde under the action of a catalyst (alkaline or acidic catalyst) to form an initial urea-formaldehyde resin, and then forms an insoluble and infusible final phase under the action of a curing agent or additive. Thermosetting resin. Cured urea-formaldehyde resin is lighter in color than phenolic resin, translucent, resistant to weak acids and weak alkali, has good insulation properties, excellent wear resistance, and is cheap. It is the most commonly used type of adhesive, especially in the wood processing industry. In the manufacturing of various artificial panels, urea-formaldehyde resin and its modified products account for about 90% of the total adhesive usage. However, urea-formaldehyde resin is easy to decompose when exposed to strong acids and alkalis. It has poor weather resistance, poor initial viscosity, large shrinkage, brittleness, water resistance, and easy aging. Artificial boards produced with urea-formaldehyde resin release formaldehyde during the manufacturing and use process. problem, so it must be modified.
Overview
Urea-formaldehyde resin, also known as urea-formaldehyde resin, referred to as UF, has an average molecular weight of about 10,000. Urea and 37% formaldehyde aqueous solution can be polycondensed under the catalysis of acid or alkali to obtain linear urea-formaldehyde oligomer. Industrially, alkali is used as a catalyst and the reaction is carried out at about 95°C. The molar ratio of formaldehyde/urea is 1.5 to 2.0 to ensure that the resin can be cured.. The first step of the reaction generates mono- and dihydroxymethylurea, and then the hydroxymethyl group and the amino group are further condensed to obtain a soluble resin. If catalyzed by acid,
Causes gel. The product needs to be stored under neutral conditions. Linear urea-formaldehyde resin can be cured at room temperature when ammonium chloride is used as the curing agent. Molding powder is heated and cured at 130 to 160°C, and accelerators such as zinc sulfate, trimethyl phosphate, diethyl oxalate, etc. can accelerate the curing process. Urea-formaldehyde resin is mainly used to make molded plastics, daily necessities and electrical parts. It can also be used as board adhesives, paper and fabric slurries, veneers, building decorative panels, etc. Because of its light color and ease of coloring, the products are often rich in color.
Urea-formaldehyde resin has low cost, light color, high hardness, oil resistance, mildew resistance, good insulation and temperature resistance, but poor weather resistance and water resistance. It is one of the earliest thermosetting resins developed. It was developed by the British Cyanamid Company in 1924 and began to be sold in 1928. In the mid-1930s, the output reached 1,000 tons. In the 1980s, the world’s annual output exceeded 1.5Mt.
The amount of urea-formaldehyde resin used in making plastic products only accounts for about 10% of total production. Urea- formaldehyde resin produced when the molar ratio of formaldehyde to urea is low, together with fillers (pulp, wood powder), colorants, lubricants, curing agents, stabilizers (hexamethylenetetramine, ammonium carbonate), extenders Plasticizer (urea or thiourea) and other components are mixed, and then dried, crushed, ball milled, and sieved to obtain urea-formaldehyde compression molding powder. The temperature for pressing urea-formaldehyde plastic is 140~150℃ and the pressure is 25~35MPa. The pressing time varies according to the thickness of the product, generally 10~60 minutes. Plastic products are mainly electrical lighting equipment and telephone parts.
Urea-formaldehyde resin is generally a water-soluble resin that is easy to cure. The cured resin is non-toxic, colorless, and has good light resistance. It does not discolor after long-term use and does not discolor during thermoforming. Various colorants can be added to produce various bright colors. products.
Urea-formaldehyde resin is hard, scratch-resistant, resistant to weak acids, weak alkali and grease and other media. It is cheap and has certain toughness. However, it is easy to absorb water, so its water resistance and electrical properties are poor, and its heat resistance is not high.
Features
Urea-formaldehyde resin is mainly used to prepare amino alkyd baking paint with alkyd resin to improve the hardness and dryness of the paint film. It can be combined with non-drying alkyd resin to make acid-cured amino paint for varnishing wood furniture. However, the weather resistance, water resistance, and gloss retention are slightly worse. Epoxy resin and alkyd resin can be used to prepare primers and interior paints with different properties. Key features include:
- Cheap price and sufficient raw materials.
- Urea-formaldehyde resin contains polar oxygen atoms in its molecular structure, so it has good adhesion to surfaces. Can be used in primers and mid-coats to improve the bonding force between topcoats.
- Since it can be cured at room temperature when using an acidic catalyst, it can be used in two-component wood coatings.
- The paint film cured with urea-formaldehyde resin has good flexibility.
- Urea-formaldehyde resin has high viscosity, high acid value, and poor storage stability.
Composition
The urea-formaldehyde resin used as the resin for compression molding powder requires the use of a condensation polymer with a shallow degree of reaction. At this time, the resin viscosity is small, which is convenient for impregnating fillers and can ensure proper flow after a long production cycle and drying. In industry, the condensate of urea and formaldehyde at low temperature (a mixture of one and two hydroxymethylureas) is often used. Usually the ratio of urea to formaldehyde is 1:1.5 (molar ratio). After they are completely dissolved at PH=8 (and temperature) 30-35 degrees, then add 0.3%-0.54% oxalic acid and 0.33%-0.33% urea. 0.88% oxalate ester, an exothermic reaction occurs immediately, the temperature rises, the temperature is maintained at 55-60 degrees, and the PH=5.5-6.5 is strictly controlled. The required urea-formaldehyde resin can be obtained in 60-75 minutes. Due to the low degree of polycondensation, it is actually just past the addition reaction stage, and the main polycondensation reaction occurs during the curing process.
Curing agent The curing agent used in compression molding powder is required to have a certain degree of latent potential. Commonly used ones include oxalic acid, phthalic acid, benzoic acid, monochloroacetic acid, etc.
Filler The most commonly used filler is paper pulp, followed by wood powder or inorganic filler (asbestos, glass fiber, mica, etc.). The pulp used is made of wood as raw material, treated with sulfites to dissolve non- cellulose impurities in the wood, and then bleached to obtain pure cellulose. The dosage of filler is 25%-32% of the total material amount. If the dosage is too small, the fluidity of the compression molding powder will be high and the strength of the product will be low. On the contrary, if the dosage is too much, the fluidity of the compression molding powder will be reduced, and the surface of the product will not be smooth and the water resistance will be poor. reduce.
Colorants Colorants can give plastics bright colors. When selecting colorants, attention should be paid to the fact that the colorants used have strong coloring ability, can be dispersed evenly in plastics, and do not discolor under processing temperatures and long-term sunlight exposure. Precipitated from the product. The commonly used colorants are pigments, and dyes are rarely used, and the dosage is 0.01%-0.2% of the material amount.
Lubricant Lubricant can improve the fluidity of the material when pressing the finished product, and can precipitate from the product, forming an isolation film between the product and the mold, making the product less likely to stick to the mold. Commonly used lubricants are metal salts of stearic acid (such as metal salts of zinc, calcium, aluminum, magnesium, etc.) and esters of organic acids (such as cyclohexyl stearate, glyceryl stearate, etc.). The addition amount is 0.1%-1.5% of the material amount. If it is too much, it will pollute the appearance of the product and reduce the gloss; if it is too little, the product will be difficult to demould.
Although the catalyst added to the compression molding powder as stabilizer is a latent catalyst, a small amount of acid will still be released during storage at room temperature, thus affecting the quality of the compression molding powder. Therefore, some alkaline is usually added. The substance is used to absorb the released acid. Commonly used bases are hexamethylenetetramine or ammonium carbonate.
Plasticizers are generally not used in compression molding powder and are only used on special occasions. The purpose is to improve the fluidity of the material and reduce the shrinkage during curing. Available plasticizers are urea and thiourea.
Applications
Used as a cross-linking agent in emulsion adhesives. It can react and cross-link with one OH in polyvinyl acetate emulsion to improve water resistance and heat resistance. The -CH2OH and NH 2 of the urea-formaldehyde resin molecules can condensate and cross-link with the -COOH in the acrylic modified emulsion molecules to a certain extent, reducing the hydrophilic groups, improving the water resistance, and enhancing the anti-freeze ability.
Uses
It can be used in products that do not require high water resistance and dielectric properties, such as power strips, switches, machine handles, instrument casings, knobs, daily necessities, decorations, mahjong tiles, toilet lids, and can also be used in the manufacture of some tableware.
Urea-formaldehyde resin is the most commonly used type of adhesive. Especially in the manufacturing of various artificial boards in the wood processing industry, urea-formaldehyde resin and its modified products account for about 90% of the total adhesive usage.
Modified
Due to the shortcomings of urea-formaldehyde resin such as poor initial viscosity, large shrinkage, high brittleness, water resistance, easy aging, formaldehyde release and curing, it pollutes the environment and harms health. It must be modified to improve performance and expand applications. The modification methods are briefly listed as follows:
Improve initial viscosity
To improve the initial viscosity of urea-formaldehyde resin, modifiers such as polyvinyl alcohol, polyethylene glycol, and hydroxymethylcellulose can be added. However, these substances are more expensive, so starch-based substances can be used, especially starch at the beginning of the synthesis of urea-formaldehyde resin. Just join, the effect will be better. During the synthesis process, starch may be hydrolyzed to produce various dextrins, etc. Since starch has a large relative molecular weight and a high viscosity after dissolution, urea-formaldehyde resin with a high viscosity can be produced by adding a small amount. At the same time, the hydroxyl groups, hydroxymethyl groups and aldehyde groups produced by hydrolysis on the starch molecular chain may participate in the synthesis reaction of urea-formaldehyde resin, which not only improves the initial viscosity, but also improves the bonding strength and storage stability.
Reduce shrinkage
Urea-formaldehyde resin has a large curing shrinkage and is prone to cracks. The adhesive layer produces internal stress, which reduces the viscosity and strength. In order to reduce the shrinkage of urea-formaldehyde resin during curing, some fillers are usually added to the resin glue, such as flour, starch, blood meal and a-cellulose powder, wood flour, soybean flour, etc.
Reduce brittleness
In order to reduce brittleness and improve toughness, polyvinyl alcohol, polyvinyl formal solution, polyvinyl acetate emulsion, VAE solution, etc. can be added, and the initial viscosity and aging resistance can also be improved.
Improve water resistance
When synthesizing urea-formaldehyde resin, adding a small amount of melamine, phenol, resorcinol, alkylamine, furfural, etc. can effectively improve the water resistance of urea-formaldehyde resin. Adding aluminum sulfate, aluminum phosphate, etc. as cross-linking agents can also significantly improve water resistance. Adding fillers such as wood flour, flour, soy flour, iron oxide, and expanded soil during glue mixing can also improve water resistance.
Phenol-modified urea-formaldehyde resin is used, and a benzene ring structure is introduced into the resin to block the water-absorbing group of the resin, significantly improving the water resistance and aging resistance of the resin. Resorcinol or melamine can also be added to the curing agent component, and acrylate copolymer emulsion can be blended with urea-formaldehyde resin to improve water resistance.
Improve bonding strength
The use of multi-complex additives such as polyvinyl alcohol and phenol can improve brittleness, water resistance and bonding strength. Then a neutral-weak acid-weak alkali composite process is used to react at medium temperature. The shear strength of the urea-formaldehyde resin produced is more than 10 times that of the original urea-formaldehyde resin, and the water resistance and boiling resistance are greatly improved.
According to different usage requirements, the performance of urea-formaldehyde resin can be significantly improved by adding modifiers.
Package
Large mouth plastic bucket or iron bucket, net weight 20kg.
Storage and transportation
Store in a cool and ventilated place. The storage time is one month from the production date. If the storage period exceeds, it can still be used after passing the inspection.
Remark
Various brands of urea-formaldehyde resin powder products can be produced according to user requirements, and the products can be stored for a long time.
Urea-formaldehyde resin is the main adhesive in the wood industry. It is widely used due to its high bonding strength, fast curing, good operability, low production cost, abundant and easily available raw materials. However, urea-formaldehyde resin contains free formaldehydeIt is toxic. The lower the free formaldehyde content in the resin, the less toxic it is. There are various ways to reduce the free formaldehyde content in urea-formaldehyde resin. The most effective method is to reduce the molar ratio of formaldehyde to urea, but reduce the formaldehyde content. The dosage of urea-formaldehyde resin will complicate the production process of urea-formaldehyde resin, make it difficult to control the end point, prolong the curing time of the resin, and reduce the bonding strength and storage stability of the resin. Therefore, we are looking for a way to effectively eliminate the low formaldehyde/urea (F/U) molar The method that brings disadvantages to the ratio is very practical. Usually, a low F/U molar ratio is used to synthesize urea-formaldehyde resin. Starting from the principle of resin synthesis, the most suitable number of additions, addition ratio, and addition time are found through experiments, and determined The most suitable pH value, reaction temperature and reaction time during the resin synthesis process are used to prepare urea-formaldehyde resin with low formaldehyde content and good stability.
The production process regulations of urea-formaldehyde resin include the following contents:
1. Inspection of raw materials. The main purpose is to check whether the quality of formaldehyde and urea meets the requirements. Based on the concentration of formaldehyde and the purity of urea, calculate the dosage of formaldehyde and urea in the process formula.
2. Prepare materials. In the production of urea-formaldehyde resin, formaldehyde is usually added first, and the formaldehyde is pumped into a metering tank with a pump, and then sent to the reaction pot after metering. If there is no metering tank, the formaldehyde can be weighed and pumped into the reaction pot with a vacuum pump. Urea is generally weighed using a scale.
3. Stir and heat. After the measured formaldehyde is added to the reaction pot, start the stirrer and add alkali to adjust the pH value to the specified value. At the same time, turn on the steam to raise the temperature. Since the reaction between urea and formaldehyde, the formaldehyde solution is an exothermic reaction, so after adding urea, heat it to a certain After reaching the specified temperature, the steam should be turned off immediately, the temperature should be raised to the specified temperature by self-heating of the reaction, and kept at the specified temperature for a period of time. The temperature at which the steam is turned off during steam heating depends on the equipment materials and the amount of reaction liquid used. Generally, small reaction pots use less amount, have less heat of reaction, and the steam shut-off temperature is slightly higher. In addition, it is also related to the steam pressure used. If the steam pressure is high, the shut-in steam temperature should be lower.
4. The pH value of the reaction liquid medium. The pH value of the reaction liquid medium is a very important condition factor for the synthesis of urea-formaldehyde resin. It should be clearly stipulated in the procedure: what the pH value should be at the beginning of the reaction. After the temperature is raised to the specified temperature, the pH value will be What range should it be in? What should the pH be after the heat preservation is completed? When acid or ammonium chloride is used to adjust the acid, the pH value must also have a certain range.
5. reaction endpoint. The end point of the reaction is the key to the quality of urea-formaldehyde resin. The reaction end point generally uses No. 4 paintThe cup measures the time (s) for the resin liquid to flow out of the cup. It is generally stipulated that the time for the resin liquid to flow out is measured at a certain temperature (with a relatively narrow range); or it is expressed by the turbidity of the resin liquid in the water. For example, it is stipulated that the end point is when the resin droplets taken out become turbid in the water at 20°C. wait.
6. Treatment after reaction endpoint. After the reaction end point is reached, it should be neutralized immediately, that is, alkali should be added immediately after reaching the reaction end point to raise the pH value of the reaction resin liquid to the specified value. At the same time, cooling water should be turned on for cooling treatment.
7. Weighing and storage. After the urea-formaldehyde resin is made, it can be discharged after cooling to the specified temperature. When discharging, it should be weighed and measured, and the output of each pot of glue should be recorded. The finished urea-formaldehyde resin is best stored in a plastic bucket to avoid contact with iron and in a cool place.
Production process
1. Inspection of raw materials. The main purpose is to check whether the quality of formaldehyde and urea meets the requirements. Based on the concentration of formaldehyde and the purity of urea, calculate the dosage of formaldehyde and urea in the process formula.
2. Prepare materials. In the production of urea-formaldehyde resin, formaldehyde is usually added first, and the formaldehyde is pumped into a metering tank with a pump, and then sent to the reaction pot after metering. If there is no metering tank, the formaldehyde can be weighed and pumped into the reaction pot with a vacuum pump. Urea is generally weighed using a scale.
3. Stir and heat. After the measured formaldehyde is added to the reaction pot, start the stirrer and add alkali to adjust the pH value to the specified value. At the same time, turn on the steam to raise the temperature. Since the reaction between urea and formaldehyde, the formaldehyde solution is an exothermic reaction, so after adding urea, heat it to a certain After reaching the specified temperature, the steam should be turned off immediately, the temperature should be raised to the specified temperature by self-heating of the reaction, and kept at the specified temperature for a period of time. The temperature at which the steam is turned off during steam heating depends on the equipment materials and the amount of reaction liquid used. Generally, small reaction pots use less amount, have less heat of reaction, and the steam shut-off temperature is slightly higher. In addition, it is also related to the steam pressure used. If the steam pressure is high, the shut-in steam temperature should be lower.
4. The pH value of the reaction medium. The pH value of the reaction liquid medium is a very important condition factor for the synthesis of urea-formaldehyde resin. The procedures should clearly stipulate: what the pH value should be at the beginning of the reaction, and what range the pH value should be after the temperature is raised to the specified temperature. What should the pH be after the heat preservation is completed? When acid or ammonium chloride is used to adjust the acidity, the pH value must also have a certain range.
5.Reaction endpoint. The end point of the reaction is the key to the quality of urea-formaldehyde resin. The reaction end point is generally measured using the No. 4 paint cup to measure the time (s) for the resin liquid to flow out of the cup. It is generally stipulated that the time for the resin liquid to flow out is measured at a certain temperature (with a relatively narrow range); or it is expressed by the turbidity of the resin liquid in the water. For example, it is stipulated that the end point is when the resin droplets taken out become turbid in the water at 20°C. wait.
6. Treatment after reaction endpoint. After reaching the reaction end point, it should be neutralized immediately, that is, alkali should be added immediately after reaching the reaction end point to raise the pH value of the reaction resin liquid to the specified value. At the same time, cooling water should be turned on for cooling treatment.
7. Weighing and storage. After the urea-formaldehyde resin is made, it can be discharged after cooling to the specified temperature. When discharging, it should be weighed and measured, and the output of each pot of glue should be recorded. The finished urea-formaldehyde resin is best stored in a plastic bucket to avoid contact with iron and in a cool place.
Urea-formaldehyde resin produced from new environmentally friendly formaldehyde is green, environmentally friendly, low-toxic and odorless. Using new environmentally friendly formaldehyde without adding any additives, the urea-formaldehyde resin glue can reach E1 level or E0 level. Urea-formaldehyde resin produced by new environmentally friendly formaldehyde is an upgraded product of ordinary urea-formaldehyde resin.
Raw material ratio
| Raw material | Specification | Dosage (kg) | Remark |
| formaldehyde | 36.5% | 600 | Ordinary formaldehyde |
| New environmentally friendly formaldehyde | 400 | ||
| Urea | Contains ≥N46 | 300 | Commercially available |
| polyvinyl alcohol | Model 2099 or 2299 | Appropriate amount | |
| sodium hydroxide | 30% | Appropriate amount | pressure alkali |
| Ammonium chloride | 20% | Appropriate amount | Adjust acidity |
Production process
1. Add 400 kg of environmentally friendly formaldehyde and 600 kg of ordinary formaldehyde into the reactor and start the stirrer. Add 2 to 4 kg of polyvinyl alcohol (model 2099 or 2299).
2. Add 300 kg of urea and start to heat up.
3. After the insulation reaction is completed, adjust the pH value with ammonium chloride.
4. After gel formation, cool down to 45°C, stop stirring, and then release the material.
The process is simple to make, easy to operate and easy to master.
Resin quality index
| Appearance | Milky white mucus |
| viscosity | 0.25～0.4Pa.s |
| Free formaldehyde content | < 0.05 |
| Curing time | 45～65s |
| pH value | 7.0～8.0 |
| solid content | >50% |
| Storage period (days) | > 20 |
Process characteristics
1. Low cost. The price of environmentally friendly formaldehyde is exactly the same as that of ordinary formaldehyde on the market. The dosage of urea is small, accounting for 30% of the total formaldehyde, saving nearly 50% of urea than ordinary environmentally friendly urea-formaldehyde resin.
2. Environmentally friendly. The resin has very low free formaldehyde content and has very little odor during the manufacturing process. After gelling, there is almost no smell of formaldehyde. The plywood made of this resin has fully reached the national standard E2 and E1 levels after testing and testing by the Technical Supervision Bureau.
3. The production process is simple. Formaldehyde and urea are both fed at once, and there is no need to adjust the pH value in the early stage, making it extremely easy to operate.
