A brief introduction to the basics of isocyanates
Times: 2020-07-22 | Hit：1727
Isocyanate
Like other polyurethane materials, isocyanate is the basic raw material for preparing rigid polyurethane foam. The isocyanate used is mainly TDI and crude MDI (sometimes called polyphenyl polymethylene polyisocyanate PAPI).
Early production of rigid polyurethane foam mainly used TDI, but there were two major problems in its use. One is that the mass percentage of NCO in TDI is too high (48.3%), and the heat released during the reaction is too large, which is not conducive to the control of hard foam production. Second, the high vapor pressure of TDI causes great damage to the production environment and operators during production. Therefore, currently in the production of rigid polyurethane foam, the use of TDI is gradually decreasing. Most of the modified products of TDI are used, that is, the end group formed by the reaction of TDI and polyol is an acropolymer with an NCO group, which reduces the content of an NCO to 25% ~ 35% (mass). At the same time, the molecular weight of isocyanate is increased, its vapor pressure is reduced, and the degree of harm to human health caused by the use of pure TDI is reduced. In addition, in the production of modern polyurethane rigid foam, a large number of crude TDI or a blend of TDI and crude MDI are used to improve the production environment, foaming process and product performance.
Crude TDI is produced during the TDI production process. In the distillation of pure TDI products, 10% ~ 15% of tar-like substances will be produced, which was discarded as production waste liquid and waste residue in the past. After analysis, it is found that the main component is TDI polymer, and contains a certain amount of TDI active ingredients. On the basis of further comprehensive utilization research, it is found that crude TDI composed of adding a certain amount of pure TDI is a good raw material for preparing hard foam. Practice has proved that the toughness of polyurethane rigid foam prepared with crude TDI or TDI prepolymer is even better than that of crude MDI-based rigid foam.
Crude MDI is a product produced during the production of pure MDI. In the condensation reaction of aniline and formaldehyde, two molecules of aniline and one molecule of formaldehyde are condensed to obtain diphenylmethanediamine, which is then phosgenated to generate MDI. However, during the condensation reaction, due to the control of reaction conditions, the A certain amount of chain macromolecules will condense to form polyphenyl polymethylene; amines will react with phosgenation to produce a certain amount of polyphenyl polymethylene polyisocyanates in the production of MDI. It can be seen that the product produced by MDI, before the MDI boutique is distilled out, the system is a mixture, and its composition varies greatly depending on factors such as raw materials, reaction conditions, and process control.
In the production of MDI, after distillation of pure 4,4'-MDI and 2,4'-MDI, the remaining substance is basically a mixture of polyphenyl polymethylene polyisocyanates with a degree of polymerization greater than 1. Depending on the degree of polymerization, its viscosity and NCO content are also different. When using them to prepare rigid polyurethane foams, different types of crude MDI should be selected according to the processing conditions and product performance requirements. For example, when pouring rigid polyurethane foam with complex structure and shape, the viscosity and fluidity of the mixture should be given priority. It is recommended to choose a polymer MDI with lower viscosity and lower functionality; when producing building insulation panels, choose moderate viscosity , Polymeric MDI varieties with higher functionality to shorten the curing time of the product and increase the compressive strength of the foam.
Using polymeric MDI to prepare polyurethane rigid foam has the following advantages compared with TDI. First, because the polyphenyl ring structure is introduced into the polymer molecule, the thermal stability of the foam is improved, and the dimensional stability of the product is improved; second, because the vapor pressure of the polymerized MDI product is low, the volatility is low, and it is in use Among them, the volatilization of harmful gases is small, which is beneficial to the health of operators and the improvement of industrial environmental protection and sanitation. Third, the rigid foam products based on polymer MDI have smooth surface, excellent mold release performance, oil and chemical resistance.
Like other polyurethane materials, isocyanate is the basic raw material for preparing rigid polyurethane foam. The isocyanate used is mainly TDI and crude MDI (sometimes called polyphenyl polymethylene polyisocyanate PAPI).
Early production of rigid polyurethane foam mainly used TDI, but there were two major problems in its use. One is that the mass percentage of NCO in TDI is too high (48.3%), and the heat released during the reaction is too large, which is not conducive to the control of hard foam production. Second, the high vapor pressure of TDI causes great damage to the production environment and operators during production. Therefore, currently in the production of rigid polyurethane foam, the use of TDI is gradually decreasing. Most of the modified products of TDI are used, that is, the end group formed by the reaction of TDI and polyol is an acropolymer with an NCO group, which reduces the content of an NCO to 25% ~ 35% (mass). At the same time, the molecular weight of isocyanate is increased, its vapor pressure is reduced, and the degree of harm to human health caused by the use of pure TDI is reduced. In addition, in the production of modern polyurethane rigid foam, a large number of crude TDI or a blend of TDI and crude MDI are used to improve the production environment, foaming process and product performance.
Crude TDI is produced during the TDI production process. In the distillation of pure TDI products, 10% ~ 15% of tar-like substances will be produced, which was discarded as production waste liquid and waste residue in the past. After analysis, it is found that the main component is TDI polymer, and contains a certain amount of TDI active ingredients. On the basis of further comprehensive utilization research, it is found that crude TDI composed of adding a certain amount of pure TDI is a good raw material for preparing hard foam. Practice has proved that the toughness of polyurethane rigid foam prepared with crude TDI or TDI prepolymer is even better than that of crude MDI-based rigid foam.
Crude MDI is a product produced during the production of pure MDI. In the condensation reaction of aniline and formaldehyde, two molecules of aniline and one molecule of formaldehyde are condensed to obtain diphenylmethanediamine, which is then phosgenated to generate MDI. However, during the condensation reaction, due to the control of reaction conditions, the A certain amount of chain macromolecules will condense to form polyphenyl polymethylene; amines will react with phosgenation to produce a certain amount of polyphenyl polymethylene polyisocyanates in the production of MDI. It can be seen that the product produced by MDI, before the MDI boutique is distilled out, the system is a mixture, and its composition varies greatly depending on factors such as raw materials, reaction conditions, and process control.
In the production of MDI, after distillation of pure 4,4'-MDI and 2,4'-MDI, the remaining substance is basically a mixture of polyphenyl polymethylene polyisocyanates with a degree of polymerization greater than 1. Depending on the degree of polymerization, its viscosity and NCO content are also different. When using them to prepare rigid polyurethane foams, different types of crude MDI should be selected according to the processing conditions and product performance requirements. For example, when pouring rigid polyurethane foam with complex structure and shape, the viscosity and fluidity of the mixture should be given priority. It is recommended to choose a polymer MDI with lower viscosity and lower functionality; when producing building insulation panels, choose moderate viscosity , Polymeric MDI varieties with higher functionality to shorten the curing time of the product and increase the compressive strength of the foam.
Using polymeric MDI to prepare polyurethane rigid foam has the following advantages compared with TDI. First, because the polyphenyl ring structure is introduced into the polymer molecule, the thermal stability of the foam is improved, and the dimensional stability of the product is improved; second, because the vapor pressure of the polymerized MDI product is low, the volatility is low, and it is in use Among them, the volatilization of harmful gases is small, which is beneficial to the health of operators and the improvement of industrial environmental protection and sanitation. Third, the rigid foam products based on polymer MDI have smooth surface, excellent mold release performance, oil and chemical resistance.
Liu Bin