Introduction to furfuryl resin

Thread Content

Introduction to furfuryl resin 1. Overview Furfuryl resin is one of the products in the furan resin series. Furan resins refer to a general term for resins produced from furfuryl alcohol and furfural, which contain furan rings; under the action of strong acids, they solidify into insoluble and non-melting solids. The types include furfuryl alcohol resin, furfural resin, furfurylmethane resin, and furfurylmethane-formaldehyde resin, among others. Furfuryl resin is formed by the polycondensation of furfuryl alcohol with formaldehyde (modified versions also contain urea added). It appears as a dark brown to black liquid or solid, possesses excellent heat and water resistance, as well as high resistance to chemical corrosion; it shows good resistance to acids, bases, salts, and organic solvents, making it an excellent preservative. Furfuryl resin has high strength and is an excellent adhesive for materials such as wood, rubber, metal, and ceramics; it can also be used in the production of coatings. An important use of sugar alcohol resins is as a binder for sand cores in the casting processes of the machinery industry; they are particularly suitable for large-scale, mass-produced machinery, such as that used in the automotive and military industries, as well as in the production of internal combustion engines, diesel engines, sewing machines, etc. When used as a binder for casting sand cores, furfuryl resin has the following characteristics: fast curing speed, low strength at room temperature, and high decomposition temperature ； Depending on the carbon content of the various castings, resins with different nitrogen contents can be selected ； Low gas evolution, high strength at high temperatures, moderate thermal expansion, high brittleness, low tendency to form pores, and high moisture absorption. After urea modification, furfuryl alcohol resins with different nitrogen contents can be produced to meet the requirements of steel casting, iron casting, and other non-ferrous metal casting processes. The production of furfuryl resin in our country began in the 1960s. Relevant institutions conducted extensive and thorough research on the raw materials for the resin, production processes, curing agents, core-making techniques, and production equipment, accumulating a wealth of first-hand data. In China, factories for producing furfuryl alcohol resin were first established in cities such as Guangzhou, Nantong, and Liaoyang. Due to the simplicity of the production processes and equipment, as well as the ease of operation, the production of this resin developed rapidly. There are currently over 50 manufacturers, most of which have relatively low production volumes (around 300–500 tons per year). However, there are also companies with a certain scale and sound management systems; for example, Shandong Shengquan Group carries out integrated production of furfural, furfuryl alcohol, and furfuryl alcohol resin. After the reform and opening up, with the development of the furfural and furfuryl alcohol industries, many towns and individual furfural factories began producing furfuryl alcohol resins through the deep processing of their products, with a total production volume of around 15 kt. With the development of the machinery industry, China’s demand for furfuryl resin is expected to exceed 20 kt/a. At present, a small amount of this resin is exported. If exports of furfuryl resin are used to replace those of furfural and furfuryl itself (China exports around 50 kt to 60 kt of furfural and furfuryl each year, and the vast majority of these exports are used to produce furfuryl resin), the prospects for the production of furfuryl resin become even more promising. Continuously improving product quality, increasing the variety of products, optimizing their performance, expanding their functional capabilities, and raising export volumes will strongly promote the development of China’s furan resin industry. 2 The main raw materials for producing furfuryl alcohol resin are furfuryl alcohol, formaldehyde, and urea; catalysts include sodium hydroxide and acetic acid, while curing agents comprise p-toluenesulfonic acid, xylene sulfonic acid, and benzene sulfonic acid, among others. 2.1 Furfuryl alcohol Furfuryl alcohol is a derivative of furfural, and a considerable portion of the furfural produced around the world is processed into furfuryl alcohol. Furfuryl alcohol is a colorless or pale yellow liquid with a slight aromatic odor; its color deepens when exposed to sunlight and air. Furfuryl alcohol is flammable, with a molecular weight of 98.01, and it is miscible with water. Apart from alkanes, furfuryl alcohol is soluble in most organic solvents, insoluble in petroleum hydrocarbons, and can dissolve fats, resins, acetate fiber, nitrocellulose, etc. When heated, furfuryl alcohol can reduce an ammonia solution of silver nitrate. It is stable to alkalis and can undergo resinification under acidic conditions. The boiling point of furfuryl alcohol is 170°C at 0.098 Mpa; its freezing point (in a stable state) is -14.63°C, and its density is 1.1285 g/cm3. The furfuryl alcohol produced in China has a purity of ≥99.0%. Furfuryl alcohol used in the production of furfuryl resin should comply with the GB/TI4022—92 standard. 2.2 Formaldehyde Formaldehyde is a colorless gas with a strong, distinctive irritant effect; it irritates the eyes and nose. Formaldehyde has a molecular weight of 30, a boiling point of -19.59°C, a freezing point of -92°C, a relative density of 1.067 as a gas, and an explosive limit of 7% to 73% (by volume). Contact of formaldehyde with the skin can cause burns; open-type workshops with natural ventilation should be used at the site of handling. Formaldehyde should be stored in a dry and well-ventilated warehouse with a humidity level of 21–25°C, and it should not be kept there for too long. There are many formaldehyde manufacturers in our country, with production taking place in almost every province and city. The formaldehyde used in the production of furfuryl resin should have a content of ≥37%, and its quality must meet the requirements of GB9009—88. 2.3 Urea Urea, also known as carbamide, is a colorless or white needle-like or rod-shaped crystal; the industrial version appears as white solid particles with a slight reddish tint. It is a commonly used chemical fertilizer and is odorless and tasteless. Urea has a molecular weight of 60.06, a density of 1.335 g/cm3, and a melting point of 132.7°C. It is soluble in water and alcohol, but insoluble in ether and chloroform. Urea solution is weakly alkaline; it reacts with acids to form salts and exhibits hydrolytic properties. At high temperatures, urea can undergo condensation reactions to form biuret, triuret, and cyanuric acid. The urea used in the production of furfuryl resin should be pure, free from impurities, with a nitrogen content of ≥46%, and its quality must meet the requirements of GB2440-81. The above describes the main raw materials used in the production of furfuryl alcohol resin; other raw materials are not listed one by one. 3 Production process and main equipment of furfuryl alcohol resins 3.1 Main equipment: One high-level metering tank for formaldehyde and one for furfuryl alcohol, both of which need to be corrosion-resistant ； One reaction kettle, which should be corrosion-resistant ； Condenser, 1 unit ； One water storage tank ； One buffer tank ； One vacuum pump. 3.2 Production Process of Furfuryl Resin 3.2.1 Formation Mechanism and Structure The mechanism behind the formation of furfuryl resin is quite complex, and it is not yet fully understood. It is generally believed that urea reacts with formaldehyde in a weakly alkaline medium to produce hydroxymethylurea and dimethyloxymethylurea; subsequently, these hydroxymethyl derivatives undergo a condensation reaction with furfuryl alcohol in a weakly acidic medium to form furfuryl resin. This product is a mixture of various molecules with molecular weights ranging from 400 to 600, and its molecular structure is either linear or branched. Furfuryl resin is an oligomer with a very high molecular weight; when subjected to acid and further polycondensation reactions take place, larger insoluble macromolecules can be formed, which is what leads to the curing or setting of the resin. 3.2.2 Feed ratio in furfuryl alcohol resin production During the production of furfuryl alcohol resin, the mol ratio of furfuryl alcohol, formaldehyde, and urea can be determined based on requirements and practical experience; generally, there is no fixed ratio. The mol ratio of furfuryl alcohol, formaldehyde, and urea during production is affected by various factors such as temperature, humidity, the pH value of the reaction mixture, and reaction time. The data presented here are based on practical experience in production: (1) low nitrogen content, with a nitrogen level of less than 4%, at a ratio of 8∶2∶1.3 ； (2) Medium nitrogen: nitrogen content of 4.8%–5%, with a ratio of 8∶2∶1.6 ； (3) High nitrogen content, with a nitrogen level of 7%–8%, and the ratio is 8∶2∶2.6 (for reference only). The higher the nitrogen content, the greater the viscosity of the product; its strength at low temperatures is high while its strength at high temperatures is low, and vice versa. To further enhance the strength of the product, a small amount of silane coupling agent can be added, which is particularly necessary in high-temperature environments. 3.2.3 Production process: First, formaldehyde is added to the reactor; then urea is added while stirring, along with 1/4 of the total amount of furfuryl alcohol. Steam is used for heating, and a sodium hydroxide solution is added to adjust the pH value. Continue to raise the temperature to around 98°C. At this point, backflow liquid can be seen through the scope, indicating the formation of reaction products. Start timing from the moment the reflux liquid is observed; 15 minutes is sufficient. Then, stop the steam supply to cool down the system, and use cold water for cooling. When the temperature drops to 60°C, the remaining 3/4 of the furfuryl alcohol is pumped into the reactor under vacuum. After adding furfuryl alcohol, the pH value is adjusted using dilute acetic acid, so that the pH of the reactants is around 6.5 (6.5±0.2). Then vacuum dehydration is carried out; during this process, the steam pressure should not be too high, and it should not exceed 0.1 MPa. The dehydration time depends on the amount of water to be removed. The theoretical value of the amount of water removed is: weight of formaldehyde × (100–37)% + water contained in the catalyst added. After dehydration, take a sample and ensure that the pH value is at least 6.5; then break the vacuum, stop the steam supply, turn off the heat, allow the temperature to drop below 50°C, and proceed with discharging the material. 3.3 Main factors affecting the performance of furfuryl resin 3.3.1 Raw material ratio The raw material ratio is a key factor influencing the performance of the resin. Practice has shown that when too little formaldehyde is used, the resulting product is mainly hydroxymethylurea; hydroxymethylurea is difficult to convert into resin, which often leads to layering in the product and affects its quality. Only when an appropriate amount of formaldehyde is used to produce a large quantity of dimethyloxymourea at the beginning of the reaction can high-quality resin be obtained during the polycondensation process. If too much formaldehyde is used, there will be an excessive amount of free formaldehyde in the resin, which not only causes environmental pollution and difficulties in production operations, but is also harmful to the health of the workers, as well as leading to waste of materials ； On the other hand, it also reduces the water resistance after curing; the shrinkage of the cured resin increases, making cracking more likely to occur. In production, the weight ratio of urea to formaldehyde (referring to pure form) is approximately 1:(0.8–1). In actual production, due to the volatility of formaldehyde, the actual ratio is around 1:1.1. When the amount of furfuryl alcohol is ≥80%, the nitrogen content in the final product is low, and the resin is suitable for casting steel and cast iron. When the furfuryl alcohol content is ≤60%, the product is suitable only for cast iron, but it has a low cost. 3.3.2 pH value of the reaction solution The reaction of the resin occurs in two stages, namely the addition reaction and the polycondensation reaction, and the requirements for the pH value differ between these two reactions: it is higher in the former and lower in the latter. During the addition reaction stage, urea reacts with formaldehyde to produce hydroxymethylurea and dimethyloxymethylurea; this reaction takes place in a weakly alkaline or neutral environment at a pH of 7–8. If the reaction takes place under strongly alkaline conditions with a pH of ≥9, formaldehyde undergoes side reactions to form formic acid, which consumes excessive amounts of formaldehyde. As a result, the rate of the addition reaction slows down, and the resulting resin tends to separate into layers over time, affecting its strength and service life. If the reaction takes place in an acidic environment with a pH value of ≤5, it is easy to form water-insoluble derivatives of methylene urea, and the lower the pH value, the more of these derivatives are produced. During the polycondensation stage, the pH value primarily affects the reaction rate; a higher pH value results in a slower reaction rate. A slow polycondensation rate of the resin inevitably impacts the production cycle, prolongs the production time, and leads to waste of raw materials. If the pH value is too low, the reaction rate is fast, resulting in the formation of larger molecules instantaneously; this makes it difficult to control the reaction rate, leading to gelation. The product becomes unusable, causing waste and disrupting production. Therefore, to ensure the smooth progress of production, it is extremely important to strictly control the pH value of the solution; this is the key to the success or failure of production, and no carelessness is allowed. 3.3.3 Reaction Temperature and Time The reaction temperature is another important factor that has a significant impact on the reaction rate and product quality; the higher the temperature, the faster the reaction rate. If the reaction temperature is too high, on the one hand, it facilitates the formation of water-insoluble derivatives of methylene urea; on the other hand, it speeds up the reaction rate, leading to gelation and preventing normal production processes from proceeding. On the contrary, if the temperature is too low, the reaction rate becomes too slow, which will **lengthen the production cycle, affect the output volume, increase various costs, raise expenses, and reduce economic efficiency. To this end, it is necessary to strictly control the reaction temperature throughout the entire production process to ensure the smooth progress of production. Reaction time is not an independent factor; it is influenced by factors such as the pH value of the reaction mixture, reaction temperature, and the ratio of raw materials. If all other conditions are strictly controlled, a longer reaction time results in a higher viscosity of the product. However, the reaction time cannot be extended indefinitely; it should be controlled according to the requirements for product quality, in order to obtain the desired and optimal product. 4 Self-hardening furfuryl resin sand process: The self-hardening furfuryl resin sand process is a molding and core-making technique in which, at room temperature, a certain amount of acidic curing agent is added to ensure thorough and uniform mixing with the furfuryl resin and casting sand; thereafter, it hardens on its own within the sand box or core box, without the need for any additional conditions. Its hardening time and speed are related to room temperature, sand temperature, and the ambient temperature. This process is characterized by the ability to undergo self-hardening and shaping at normal temperatures; the molding sand has a certain service life, making it suitable for mass and batch production. The self-hardening furfuryl alcohol resin sand process is widely used in China; it is a molding process with relatively mature technology and extensive practical experience, and most manufacturers in the country employ this process. This process can use furfuryl resin with different nitrogen contents to suit the casting processes of cast steel, cast iron, and other non-ferrous metals. The common curing agents used in this process are: tolsulfonic acid, xylene sulfonic acid, and benzenesulfonic acid. The curing agent is prepared as a water-soluble solution (or a water-ethanol solution) at about 65% concentration; the amount added depends on the curing time required for the casting as well as the environmental conditions at that time (such as room temperature, air temperature, etc.). The amount of furfuryl resin added is generally around 0.8% to 1.5% of the weight of the casting sand. It takes a few minutes to about ten minutes to mix the sand, while the time required to demold can range from several minutes to several hours. The sand should be prepared just before use, as delaying its preparation too long can affect the quality of the sand cores and castings. The use of furan resin in self-hardening sand casting processes in our country began in the 1970s. Relevant research institutions conducted thorough studies on the resin, curing agents, casting sands, and molding processes, after which it was put into use in production. Through years of effort, particularly over the past 20 years when substantial work was carried out in the areas of raw materials, process technology, and the development of complete sets of equipment, remarkable results have been achieved. The successful experiences of many manufacturers were applied in production, and by the 1990s, China had established more than 100 resin sand molding production lines. In our country, as many as 300 kt of castings are produced each year, with the consumption of resin exceeding 15 kt. 5 Issues to Note and Problems to Address In the production of furfuryl resin, although the manufacturing equipment and processes are relatively simple, the production conditions are quite stringent; precise control and monitoring are required. The amounts of materials used must be measured accurately, as must the mixing ratios, and these materials need to be added in sequence. The catalyst must be diluted; it must not be added as concentrated acid or concentrated alkali. The catalyst should be added slowly in order to determine the true pH value of the reaction mixture, and an excess of catalyst should certainly not be used. Reducing the formaldehyde content in resins, improving production and usage conditions, and minimizing pollution are issues that plague enterprises’ production processes and need to be addressed urgently. The level of free formaldehyde in the resin is an important technical parameter for evaluating it. If the amount of free formaldehyde in the resin is too high, it will be released during processes such as sand mixing, molding, and curing, thereby contaminating the environment and posing a risk to the health of those who work with it. Countries have set limits on the content of free formaldehyde in resins, as well as on the maximum level of formaldehyde in the working environment. In the UK, the permitted limit in the working environment is 2 ppm, while in Japan it is 5 ppm. To control the level of free formaldehyde in the work area, it is necessary to regulate its content in resins; therefore, resins with low levels of free formaldehyde have been developed. Currently, the content of free formaldehyde in furan resins produced in developed countries is generally below 0.3%, and many manufacturers in China have also reduced the free formaldehyde content in their products to below 0.3%. To reduce the content of free formaldehyde in the resin, it is necessary to improve the manufacturing process. The main technical improvements include: (1) adjusting the mol ratio of the raw materials, formaldehyde and urea. Reducing the mol ratio of formaldehyde to urea is the most effective and economical method for lowering the amount of free formaldehyde in the resin. However, the content of free formaldehyde in the resin is closely related to the strength of the resin; increasing the formaldehyde content adds polar groups and side chains to the resin molecules, thereby enhancing its cohesive strength and adhesion. Therefore, it is necessary to reduce the mol ratio of formaldehyde to urea appropriately, without compromising the adhesion properties of the resin, in order to decrease the level of free formaldehyde in it. (2) During the reaction process, urea is added in multiple portions. In this way, at the start of the reaction, increasing the mol ratio of formaldehyde to urea facilitates the formation of hydroxymethylurea, allowing urea to react effectively with unreacted formaldehyde, and it also helps to reduce the amount of free formaldehyde in the resin. (3) React at low temperature. Since the reaction between urea and formaldehyde to form hydroxymethylurea is exothermic, carrying out the reaction at low temperatures can promote it to proceed in a direction that favors the formation of hydroxymethylurea, thereby reducing the content of free formaldehyde in the resin. (4) By adding additives, under acidic conditions furfuryl alcohol can undergo polycondensation reactions with a portion of hydroxymethyl urea. The addition of certain aldehyde-reducing agents enables it to react with formaldehyde under certain conditions to form another substance, which also helps to reduce the level of free formaldehyde. Reducing the level of free formaldehyde lowers the strength of the resin. To achieve a reduction in free formaldehyde without affecting the resin’s strength, it is necessary to add reinforcing agents appropriately to compensate for the loss in resin strength. Improving the processing technology of furfuryl alcohol resin and reducing consumption can lower the usage cost of this resin, thereby enhancing the competitiveness of the products. Improving the quality of the resin, adding reinforcing agents, enhancing the condition of the resin coating, and reducing the amount of resin used can all help to lower the cost of castings. It is advisable to use elliptical sand molds, remove sharp edges, reduce the amount of sand used, and lower the sand-to-iron ratio; this can also help reduce the cost of castings. The cost of castings has decreased, and furfuryl resin offers advantages in manufacturing sand cores that no other binder can match; this will expand the market for furfuryl resin and promote the development of its industry. With the development of the national economy and advancements in casting techniques, the demand for furfuryl resin both domestically and internationally will **increase**, which will strongly promote the production and development of furfuryl resin. Curing of furan resin The curing process of furan resin is very complex. It is currently believed that the curing of furan resins is due to the opening of the conjugated double bonds in the furan ring, which leads to cross-linking and the formation of a three-dimensional structure. Furthermore, other active groups in the side chains of furan resins may also participate in the cross-linking reaction during curing. In fact, the curing agents for furan resins are all acidic substances. Curing agents commonly used for phenolic resins can also be used as curing agents for furan resins, such as benzene sulfonyl chloride, p-toluenesulfonyl chloride, ethyl sulfate, phosphoric acid, and p-toluenesulfonic acid. Unlike phenolic resins, furan resins require a higher acidity level from the curing agent; for example, the suitable ratio of ethyl sulfate for furan resins is 98% sulfuric acid : anhydrous ethanol = 2:1. A serious drawback of the aforementioned compounds as curing agents for furan resins is that the reaction between the resin and the curing agent releases a large amount of heat, resulting in a short pot life after formulation and posing operational difficulties. Moreover, the curing reaction is intense, with the release of substantial amounts of water that can lead to the formation of bubbles; this reduces the impermeability of the cured products and increases their brittleness. As a result, it becomes difficult to use glass fiber for reinforcement. Currently, new curing agents for furan resins have been developed, which essentially solve the aforementioned problems. This not only enables furan resin to function like epoxy resin and unsaturated polyester resin. It can be used to manufacture fiberglass, and it also improves the mechanical properties of furan resin products. Generally, these curing agents are used in conjunction with furan resins produced by various manufacturers, or are sold mixed with fillers. Although new curing agents have improved the curing process properties of furan resins, their curing process remains relatively poor compared to epoxy resins and unsaturated polyester resins; for example, the gelation time is longer and the time required for complete curing is greater, which poses difficulties in achieving rapid curing at room temperature. Sometimes, to ensure the quality of the product, it is necessary to keep it at a temperature below 100°C. Properties and applications of furan resin: Heat resistance; Furan resin can withstand temperatures of 180–200°C, making it one of the resins with the best heat resistance among corrosion-resistant resins available today. It is reported that the strength retention rate of furan glass fiber plates remains above 50% at temperatures over 180°C. Stability: The self-polycondensation process of furan resin is slow, so its shelf life is much longer than that of phenol-formaldehyde resin. Furan resin shows little change in viscosity even after being stored at room temperature for one or two years. Flame-retardant properties; furan resin has good flame-retardant properties and produces little smoke when burned. It is reported that the newly synthesized furan resin with good flame-retardant properties does not require the addition of flame retardants, synergists, or fillers like conventional flame-retardant resins. Mechanical properties; the mechanical properties of the improved furan resin have seen a significant improvement. The mechanical properties of fiberglass reinforced with this furan resin (such as flexural strength and impact strength) have reached or come close to those of conventional unsaturated polyester resins. The disadvantage of furan resin is that its curing process is not as convenient as that of epoxy resins and unsaturated resins; to achieve complete curing, it usually requires post-treatment with heat. Furthermore, the vast majority of products made from furan resins are black, which also limits their use in certain applications. Currently, furan resin is widely used in the manufacture of fiberglass-reinforced plastic equipment and pipes. It is particularly suitable for applications such as the recovery of organic chlorinated compounds, pesticides, synthetic fibers, dyes, pulp, and organic solvents, as well as in wastewater treatment systems. Furan resin can also be used as the adhesive for lining acid-resistant brick and tile surfaces, as well as for creating corrosion-resistant flooring. Furthermore, to improve the corrosion resistance and heat resistance of epoxy resins and phenolic resins, furan resins are also often blended with these resins.