Discussion on rigid polyurethane foam
Times: 2020-06-19 | Hit：1507
Abstract: Explains the advantages of rigid polyurethane foam and its application in real life and work, discusses the foaming process and molding method of rigid polyurethane foam, and deeply analyzes the influencing factors of rigid polyurethane foam in foam molding ,for reference.
1 Overview
In recent decades, in various fields of production and life, the output of rigid polyurethane foam has increased rapidly every year. There are a large number of gases and liquids that need to be transported, stored and transported under the conditions of heat preservation and cold preservation. For the heat preservation and cold preservation of these containers, equipment and pipelines, traditional heat preservation and cold preservation materials, such as asbestos, slag wool, perlite, sawdust, asphalt, glass fiber, etc., have many shortcomings. The construction procedures of these heat preservation materials are complicated and the operation is inconvenient. , Some materials are still harmful to the health of construction workers. In addition, these insulation materials have high thermal conductivity, poor thermal insulation effect, serious energy loss, and short service life. In contrast, polyurethane rigid foam has the lowest thermal conductivity and can well eliminate the shortcomings of traditional insulation materials, especially in terms of material insulation performance that other insulation materials cannot compare with. It is mainly used in construction and central heating industries, and secondly used in refrigeration equipment such as refrigerators and freezers, as well as heat insulation materials in the transportation industry.
With the gradual maturity of the production of rigid polyurethane foam, the expansion of market supply, the continuous deepening of research work, the development of new varieties, new technology, and the improvement of new equipment, so that rigid polyurethane foam has been fully developed. In terms of the preparation technology of rigid polyurethane foam products, higher requirements are put forward. Large-scale foaming equipment is generally used to increase labor productivity, reduce costs, and reduce raw material consumption by large-scale, continuous, automated, and serialized. Computers are used to control various parameters such as material temperature, flow, and flow rate to adjust and produce better quality Polyurethane foam.
2 Advantages of rigid polyurethane foam
1) Good insulation effect. Using polyurethane rigid foam and polyisocyanurate rigid foam, not only the insulation layer is thin and light, but also the heat insulation effect is excellent.
2) Polyurethane rigid foam and polyisocyanurate rigid foam have a high closed cell rate, and can produce a dense outer skin layer, which is not easy to seep and has good waterproof effect.
3) Good thermal stability and wide operating temperature range. The new polyurethane rigid foam and polyisocyanuric acid resin rigid foam insulation layer can work for a long time under the working condition of 150 ℃, and the service period can reach 25 years.
4) Good chemical resistance, not only heat and heat preservation, but also good anti-corrosion function.
5) Large-scale mechanized production can be carried out in the factory, with high efficiency and good quality; simple and convenient construction, no damage to the health of the constructors, safe and reliable.
3 Rigid polyurethane foaming process and molding
There are three types of rigid polyurethane foam foaming: prepolymer method, semi-prepolymer method, and one-step method.
1) Prepolymer method: Generally, equal parts of polyether and isocyanate are mixed and reacted to form a low-molecular polymer prepolymer. Then, water and the prepolymer are stirred to form a urea group to form a high polymer. As carbon dioxide is generated during the reaction, chain growth foaming is formed to produce foam.
2) Prepolymer prepared by semi-prepolymer method: It is to mix and react a part of polyether and excess isocyanate to form a low molecular weight polymer with a certain viscosity, and mix the excess polyether with water, catalyst and surfactant at high speed and mix uniformly The rigid or semi-rigid foam formed later.
3) One-step method: the polyether, isocyanate and other components are all mixed at the same time to make foam. The one-step manufacturing process is simple and easy to operate. It can be used directly without processing. It can be carried out at room temperature. The miscibility of the raw materials is good. The foamed product has excellent properties.
The foaming process of rigid polyurethane foam usually mixes polyether or polymeric polyol, isocyanate, water, catalyst, surfactant, etc. in the foaming equipment into a liquid raw material with a certain viscosity and mixing at high speed. After the reaction, the metering pump is mixed at a high speed in the mixing chamber according to the proportion, sprayed into the mold preparation, and cured at a certain temperature to form a polyurethane foam. Nowadays, one-step method is often used in industrial manufacturing to prepare prepolymers of different branch chain types.
The simplest foam molding method for rigid polyurethane foam is manual foaming. The raw materials are placed in a container according to the proportion, mixed evenly at high speed, and injected into the foam filling space. After the chemical reaction is cured, the polyurethane foam is formed. Low raw material utilization rate, suitable for scientific research in the laboratory and temporary construction on site. However, mechanical foaming is used in mass production, including: continuous forming of block foam, pouring foaming, spraying foaming and other processes. The most common molding methods are pouring foaming, spraying foaming, and continuous molding foaming. The properties of foams produced by different foaming processes are also very different, with different shapes and appearances.
4 The influence of rigid polyurethane foam molding
1) Raw materials: Both polyether and polyester are used as components of foam molding. With different ratios of polyether polyol and isocyanate, flexible foam, semi-rigid foam and rigid foam can be produced. The temperature of the raw material directly affects the accuracy of the metering ratio and at the same time affects the viscosity of the material. Once the viscosity changes, the output accuracy of the metering pump will change, which will seriously affect the quality of the product. The polyether polyol component is too large, the resulting foam is soft, and has poor dimensional stability at high temperatures; the isocyanate component is too high, and the foam is hard and brittle.
2) Temperature: During the foam molding process, the material temperature and ambient temperature directly affect the quality of foam plastic products. Long-term storage and temperature changes are also very detrimental to isocyanates. The presence of the dimer in the isocyanate makes the isocyanate change turbid and also interferes with the normal reaction. In production, the temperature is low, the foaming reaction is slow, and the foam curing time is long; the temperature is high, the foaming reaction is fast, and the foam curing time is short.
3) Injection temperature, speed and pressure: Before injection molding, a high-pressure cycle is required, so that the temperature of the material rises due to the high-pressure movement, so the injection temperature and the raw material storage temperature are different. It depends on the cycle time and pressure, and the injection speed has a greater impact on the mixing of raw materials. Generally, the injection speed is fast and the punching is high, and high-quality foam products can be produced.
4) Mold temperature: It is a key factor in foam molding, which can directly affect the milky white, drawing, non-stick, and curing speed of the foam, so it must be strictly controlled. Foam molding is a chemical reaction exothermic process. The heat released vaporizes the foaming agent and rolls and extrudes in the polyurethane filled cavity to form a foam with irregularly sized cells. Mold temperature is a more important process parameter, which not only affects the speed of reaction heat removal, but also affects the molding time, and has an adverse effect on density, surface thickness and dimensional stability. During the foam molding process, the foaming agent reacts to exothermic heat, forming polymers that push, roll, and climb in the filled cavity to contact the mold surface, and the cells rupture and collapse to form a dense layer. If the mold temperature is high, an extremely thin surface layer is formed, which cannot meet the requirements for coloring, and the surface of the part is prone to defects; if the temperature is low, the expansion ratio is small, and the skin is too thick, which will reduce the thermal conductivity.
5) Curing time: After the foam is cured, it is placed at a certain temperature in order to complete the foaming reaction and obtain a good product. When injecting into the mold, the foaming material and the mold should be heated and stored in a higher temperature environment after the foaming reaction is completed. The higher the temperature, the shorter the curing time. The temperature is low, the curing time is long, the curing is insufficient, and the foam strength cannot meet the required requirements. The variety of raw materials and the shape and size of the parts are different, and the required curing time and temperature are also different.
6) Mixing: In foam molding, mixing efficiency is a key factor. According to the characteristics of the raw materials, adjust the metering ratio and material temperature of the foaming equipment to achieve the purpose of full mixing. The mixing is uniform, and the foam cells are fine and uniform; if the mixing is not good, the cells are coarse and uneven, and even insufficient chemical reaction occurs in a local area, which greatly affects the quality of the product.
7) When the mold is foamed and molded, because the polyether and isocyanate react to generate carbon dioxide gas, the polymer expands to generate a certain internal pressure. The mold restricts the development of the foam shape. It should have sufficient strength and smoothness of the inner surface to ensure that the foam is fully filled, and will not cause the foam to crack and hollow in the late foaming stage, so that the foam is evenly distributed in the mold. The injection port and exhaust port should also be opened in a reasonable position, which can reasonably avoid the appearance of local large bubbles and shrinkage.
5 Conclusion
1 Overview
In recent decades, in various fields of production and life, the output of rigid polyurethane foam has increased rapidly every year. There are a large number of gases and liquids that need to be transported, stored and transported under the conditions of heat preservation and cold preservation. For the heat preservation and cold preservation of these containers, equipment and pipelines, traditional heat preservation and cold preservation materials, such as asbestos, slag wool, perlite, sawdust, asphalt, glass fiber, etc., have many shortcomings. The construction procedures of these heat preservation materials are complicated and the operation is inconvenient. , Some materials are still harmful to the health of construction workers. In addition, these insulation materials have high thermal conductivity, poor thermal insulation effect, serious energy loss, and short service life. In contrast, polyurethane rigid foam has the lowest thermal conductivity and can well eliminate the shortcomings of traditional insulation materials, especially in terms of material insulation performance that other insulation materials cannot compare with. It is mainly used in construction and central heating industries, and secondly used in refrigeration equipment such as refrigerators and freezers, as well as heat insulation materials in the transportation industry.
With the gradual maturity of the production of rigid polyurethane foam, the expansion of market supply, the continuous deepening of research work, the development of new varieties, new technology, and the improvement of new equipment, so that rigid polyurethane foam has been fully developed. In terms of the preparation technology of rigid polyurethane foam products, higher requirements are put forward. Large-scale foaming equipment is generally used to increase labor productivity, reduce costs, and reduce raw material consumption by large-scale, continuous, automated, and serialized. Computers are used to control various parameters such as material temperature, flow, and flow rate to adjust and produce better quality Polyurethane foam.
2 Advantages of rigid polyurethane foam
1) Good insulation effect. Using polyurethane rigid foam and polyisocyanurate rigid foam, not only the insulation layer is thin and light, but also the heat insulation effect is excellent.
2) Polyurethane rigid foam and polyisocyanurate rigid foam have a high closed cell rate, and can produce a dense outer skin layer, which is not easy to seep and has good waterproof effect.
3) Good thermal stability and wide operating temperature range. The new polyurethane rigid foam and polyisocyanuric acid resin rigid foam insulation layer can work for a long time under the working condition of 150 ℃, and the service period can reach 25 years.
4) Good chemical resistance, not only heat and heat preservation, but also good anti-corrosion function.
5) Large-scale mechanized production can be carried out in the factory, with high efficiency and good quality; simple and convenient construction, no damage to the health of the constructors, safe and reliable.
3 Rigid polyurethane foaming process and molding
There are three types of rigid polyurethane foam foaming: prepolymer method, semi-prepolymer method, and one-step method.
1) Prepolymer method: Generally, equal parts of polyether and isocyanate are mixed and reacted to form a low-molecular polymer prepolymer. Then, water and the prepolymer are stirred to form a urea group to form a high polymer. As carbon dioxide is generated during the reaction, chain growth foaming is formed to produce foam.
2) Prepolymer prepared by semi-prepolymer method: It is to mix and react a part of polyether and excess isocyanate to form a low molecular weight polymer with a certain viscosity, and mix the excess polyether with water, catalyst and surfactant at high speed and mix uniformly The rigid or semi-rigid foam formed later.
3) One-step method: the polyether, isocyanate and other components are all mixed at the same time to make foam. The one-step manufacturing process is simple and easy to operate. It can be used directly without processing. It can be carried out at room temperature. The miscibility of the raw materials is good. The foamed product has excellent properties.
The foaming process of rigid polyurethane foam usually mixes polyether or polymeric polyol, isocyanate, water, catalyst, surfactant, etc. in the foaming equipment into a liquid raw material with a certain viscosity and mixing at high speed. After the reaction, the metering pump is mixed at a high speed in the mixing chamber according to the proportion, sprayed into the mold preparation, and cured at a certain temperature to form a polyurethane foam. Nowadays, one-step method is often used in industrial manufacturing to prepare prepolymers of different branch chain types.
The simplest foam molding method for rigid polyurethane foam is manual foaming. The raw materials are placed in a container according to the proportion, mixed evenly at high speed, and injected into the foam filling space. After the chemical reaction is cured, the polyurethane foam is formed. Low raw material utilization rate, suitable for scientific research in the laboratory and temporary construction on site. However, mechanical foaming is used in mass production, including: continuous forming of block foam, pouring foaming, spraying foaming and other processes. The most common molding methods are pouring foaming, spraying foaming, and continuous molding foaming. The properties of foams produced by different foaming processes are also very different, with different shapes and appearances.
4 The influence of rigid polyurethane foam molding
1) Raw materials: Both polyether and polyester are used as components of foam molding. With different ratios of polyether polyol and isocyanate, flexible foam, semi-rigid foam and rigid foam can be produced. The temperature of the raw material directly affects the accuracy of the metering ratio and at the same time affects the viscosity of the material. Once the viscosity changes, the output accuracy of the metering pump will change, which will seriously affect the quality of the product. The polyether polyol component is too large, the resulting foam is soft, and has poor dimensional stability at high temperatures; the isocyanate component is too high, and the foam is hard and brittle.
2) Temperature: During the foam molding process, the material temperature and ambient temperature directly affect the quality of foam plastic products. Long-term storage and temperature changes are also very detrimental to isocyanates. The presence of the dimer in the isocyanate makes the isocyanate change turbid and also interferes with the normal reaction. In production, the temperature is low, the foaming reaction is slow, and the foam curing time is long; the temperature is high, the foaming reaction is fast, and the foam curing time is short.
3) Injection temperature, speed and pressure: Before injection molding, a high-pressure cycle is required, so that the temperature of the material rises due to the high-pressure movement, so the injection temperature and the raw material storage temperature are different. It depends on the cycle time and pressure, and the injection speed has a greater impact on the mixing of raw materials. Generally, the injection speed is fast and the punching is high, and high-quality foam products can be produced.
4) Mold temperature: It is a key factor in foam molding, which can directly affect the milky white, drawing, non-stick, and curing speed of the foam, so it must be strictly controlled. Foam molding is a chemical reaction exothermic process. The heat released vaporizes the foaming agent and rolls and extrudes in the polyurethane filled cavity to form a foam with irregularly sized cells. Mold temperature is a more important process parameter, which not only affects the speed of reaction heat removal, but also affects the molding time, and has an adverse effect on density, surface thickness and dimensional stability. During the foam molding process, the foaming agent reacts to exothermic heat, forming polymers that push, roll, and climb in the filled cavity to contact the mold surface, and the cells rupture and collapse to form a dense layer. If the mold temperature is high, an extremely thin surface layer is formed, which cannot meet the requirements for coloring, and the surface of the part is prone to defects; if the temperature is low, the expansion ratio is small, and the skin is too thick, which will reduce the thermal conductivity.
5) Curing time: After the foam is cured, it is placed at a certain temperature in order to complete the foaming reaction and obtain a good product. When injecting into the mold, the foaming material and the mold should be heated and stored in a higher temperature environment after the foaming reaction is completed. The higher the temperature, the shorter the curing time. The temperature is low, the curing time is long, the curing is insufficient, and the foam strength cannot meet the required requirements. The variety of raw materials and the shape and size of the parts are different, and the required curing time and temperature are also different.
6) Mixing: In foam molding, mixing efficiency is a key factor. According to the characteristics of the raw materials, adjust the metering ratio and material temperature of the foaming equipment to achieve the purpose of full mixing. The mixing is uniform, and the foam cells are fine and uniform; if the mixing is not good, the cells are coarse and uneven, and even insufficient chemical reaction occurs in a local area, which greatly affects the quality of the product.
7) When the mold is foamed and molded, because the polyether and isocyanate react to generate carbon dioxide gas, the polymer expands to generate a certain internal pressure. The mold restricts the development of the foam shape. It should have sufficient strength and smoothness of the inner surface to ensure that the foam is fully filled, and will not cause the foam to crack and hollow in the late foaming stage, so that the foam is evenly distributed in the mold. The injection port and exhaust port should also be opened in a reasonable position, which can reasonably avoid the appearance of local large bubbles and shrinkage.
5 Conclusion
The rigid polyurethane foam industry has developed rapidly in different industries. The quality requirements for rigid polyurethane foam are getting higher and higher. Therefore, for different raw materials, by changing the formula, improving the accuracy of the foaming equipment and the foaming molding process, various rigid polyurethane foams with different densities and properties are prepared to meet different applications. plastic products.
Liu Bin