Introduction to MDI and Overview of Production Process

Thread Content

MDI is an umbrella term for diphenylmethane diisocyanate (pure MDI), mixtures containing a certain proportion of pure MDI and polyphenylpolymeric diisocyanates (polymerized MDI), as well as modified versions of pure MDI and polymerized MDI. It is the most important raw material for producing polyurethanes, with small amounts of MDI being used in other applications besides polyurethanes. Polyurethane possesses the elasticity of rubber as well as the strength of plastics, along with excellent processability. It offers advantages in areas such as heat insulation, sound insulation, wear resistance, oil resistance, and elasticity that are unmatched by other synthetic materials. It is the sixth most widely used plastic after polyethylene, polyvinyl chloride, polypropylene, polystyrene, and ABS. It is extensively applied in fields such as national defense, aerospace, light industry, chemicals, petroleum, textiles, transportation, automotive manufacturing, and medicine, and has become an essential emerging material for economic development and people’s livelihoods. 　　MDI and TDI are substitutes for each other, both being raw materials used in the production of polyurethanes. Currently, MDI is slightly more expensive, but it has lower toxicity than TDI; moreover, the polyurethane products produced from MDI exhibit relatively good moldability. 　　Chemical name: Diphenylmethane diisocyanate Product categories: Pure MDI, polymeric MDI, liquefied MDI, modified MDI, etc. 　　Physical properties: Pure MDI is a white to slightly yellow crystal at room temperature. It should be stored at temperatures below 5 degrees Celsius, with a shelf life of three months. It is typically packaged in 250-kilogram iron drums under nitrogen atmosphere (when packaged in tank cars, the shelf life is 10 days). 　　Polymerized MDI: a colorless to brownish liquid that can be stored at room temperature; its shelf life is two years. It is typically packaged in 250-kilogram iron drums under nitrogen atmosphere. 　　Existing technology: The currently widely used MDI production method worldwide is based on aniline as the raw material; phosgene is used in a subsequent step to produce a crude MDI product, which is then separated into pure MDI and polymeric MDI using distillation equipment. 　　Latest technologies: Due to the extreme hazards associated with phosgene, many factories are actively developing new synthesis methods to replace phosgene-based production processes, such as the dimethyl carbonate method. However, these methods have so far only been successfully applied on a pilot scale and cannot be used for large-scale production. 　　Key suppliers:
European and American companies: BASF, Bayer, Huntsman, Dow
Japanese and Korean companies: Nippon Polyurethane, Mitsui, Kumho Mitsui
Domestic company: Yantai Wanhua

Application areas:
Pure MDI: pastes, sole resins, spandex, TPUs, polyurea coatings, etc.
Polymerized MDI: rigid foam, CASE applications

MDI sales models:
Pure MDI: intermediaries (very few), downstream factories
Polymerized MDI: intermediaries (many diverse types), rigid foam compound manufacturers, downstream factories

MDI customer development:
Pure MDI: pastes, sole resins, spandex, and TPUs
Polymerized MDI:
Intermediaries:
Compound manufacturers:
Downstream users in the rigid foam sector: refrigerator and freezer manufacturers, containers, refrigerated trucks, solar water heaters and electric water heaters, disinfection cabinets, wood-look furniture, PU panels, etc. 　　CASE: Adhesives, sealants, coatings, etc. Meaning of MDI in aerosols (pharmaceutical industry): MDI = Meter Dosage Inhaler, which refers to a device for quantitative inhalation. 　　The MDI production process – the phosgene method – involves the condensation of aniline with formaldehyde under acidic conditions; this is a fully developed technical approach used abroad for MDI production. After the reactants are neutralized with a base, distillation is carried out to obtain diphenylmethane diamine (MDA). After dissolving MDA in a solvent, it is subjected to phosgenation to produce polyphenyl polyisocyanates such as 4.4-MDI, 2.4-MDI, 2.2-MDI, or mixed PMDI, which are then distilled and refined to obtain pure MDI. The reaction of free amines with phosgene is the most important method for MDI production. Before the reaction, the amine is dissolved in an inert solvent, and then an excess of phosgene in the same solvent is added gradually at low temperature to form a slurry of carbamoyl chloride and ammonium salt. The mixture is then heated to a high temperature, with additional phosgene being introduced until a clear solution is obtained. This reaction takes place in a solution of about 20%, and to reduce side reactions, phosgene must be used in an excess of more than 50%. Phosgene is highly toxic to humans and lethal; as a result, strict controls are in place at the national level. Moreover, production using phosgene requires large investments, it is difficult to transport and store, the hydrogen chloride produced causes severe corrosion of equipment, the production process poses high risks, and equipment maintenance is challenging. Today, people are actively developing alternative methods to phosgene. The American company Monsanto has filed a patent. TDI and MDI are produced using non-phosgene methods such as amines, carbon dioxide, and dehydrating agents. The reaction first produces a carbamate at near-atmospheric pressure and lower pressures, and then isocyanate is formed through dehydration using phosphorus pentoxide and triethylamine as dehydrating agents. Looking at the entire reaction process, the future main approach is to catalytically synthesize carbamates from carbonyl compounds and then pyrolyze them into MDI. It is reported that the German company BASF has industrial production facilities using the carbamate method in Belgium and the United States. Data reported by Asahi Company show that the production cost is 20% lower using the non-phosgene method compared to the phosgene method. The carbamate method involves first preparing aniline carbamate by combining aniline with a carbamate, then reacting it with *** in the presence of sulfuric acid to form a mixture of MDI, which is subsequently distilled to yield the final product. Aniline first reacts with carbon monoxide, ethanol, and oxygen to produce ethyl anilinoformate (EPC). Then, EPC is condensed with formalin solution to produce ethyl bis(methylene)dianilinecarbamate (MDV); the resulting product is further pyrolyzed to yield MDI and ethanol, which are then recycled for the carbonylation reaction. 　　 During the reaction process, the presence of aniline can reduce the carbonylation reaction of ***, thereby increasing the yield of urethanes. To ensure the reaction proceeds smoothly. Methanol is usually used in excess. The raw material feeding ratio is: methanol: aniline: ***: catalyst = 13.5:1.0:1.0:0.002. The reaction is carried out at a CO pressure of 6.87 Mpa and at 160°C for 3.5 hours to produce EPC. The catalyst used is a new carbonyl compound, and the reaction mixture is quickly discharged and sent to the drum below. 　　 Excess carbon monoxide and the resulting carbon dioxide are fed into a stirred reactor to promote mixing of the solution and organic substances. The reaction is heated using an electric induction coil; it takes place at the interface between the formaldehyde/sulfuric acid layer and the EPC/organic layer. The reaction temperature is maintained at 75°C, and the intermediate products MDV/PMDV are formed under normal pressure. The reactants then enter the organic/solution separator, where the bulk of the H2SO4 catalyst is separated out for reuse. The organic layer was washed with water to remove residual sulfuric acid and unreacted formaldehyde. The reaction mixture contains unreacted EPC, MCV/PMDV, organic solvents and reaction intermediates, as well as the effluent from the organic/solution separator and the liquid catalyst inhibitor; it enters a second concentration reactor where it is converted into MDV/PMDV at 75°C and at atmospheric pressure over a period of about 20 minutes. 　　 The resulting MDV/PMDV is purified and fed into a decomposer, where, in the presence of an inert solvent, the reaction conditions are controlled at 250°C, a pressure of 20 Pa, and a residence time of 1 hour; nitrogen is continuously introduced to remove excess methanol from the reactor. The bottom product is sent to the MDI extraction tower, where MDI and the by-product polyisocyanate are separated. An overview of the MDI production process: I.G. Farben was the first to produce 4,4’-diphenylmethane diisocyanate (MDI) in the 1930s. However, the method of preparing MDI by phosgenation of a homologous mixture obtained through the condensation of aniline/formaldehyde was first developed by Goodyear in 1954. Unfortunately, Goodyear did not commercialize this method. Later, in the 1960s, Carwin/Upjohn in the United States, as well as Bayer and ICI in Europe, independently developed this process. As a result, the method of producing MDI and PMDI by condensing aniline/formaldehyde to obtain a homologous aromatic amine mixture, followed by phosgenation and separation, has become the commonly used approach today. The production process flow diagram for MDI is as follows: http://www.pufair.com/app/pic/mdi_2_clip_image002.gif The basic reaction steps are as follows: 1. Condensation reaction of aniline and formaldehyde – Aniline reacts first with a 25%–35% hydrochloric acid catalyst to form an aniline hydrochloride solution; thereafter, a 37% aqueous formaldehyde solution is added gradually. The condensation reaction takes place at 80°C for 1–2 hours, after which the temperature is raised to around 100°C for another hour to carry out the rearrangement reaction. The solution is then neutralized using an aqueous sodium hydroxide solution, and finally, through steps such as washing, separation, further washing, and distillation, a mixture of diphenylmethane diamine (MDA) with different degrees of condensation is obtained. In the diamine condensate, diphenylmethane diamine accounts for about 70% of the mixture, while the remaining polyphenylmethane polyisocyanate components make up about 30%. Due to the differences in the production process conditions of various manufacturers, the components of their mixtures are not exactly the same. http://www.pufair.com/app/pic/mdi_2_clip_image004.gif http://www.pufair.com/app/pic/mdi_2_clip_image006.gif In the polycondensation reaction between aniline and formaldehyde, the hydrogen atoms on the aniline amino group are quite reactive and readily undergo condensation with formaldehyde at low temperatures; molecular rearrangement then leads to the formation of the corresponding ammonium hydrochloride. During synthesis, depending on the ratios of the raw materials and the process conditions, the product is a mixture with n=0, 1, 2, …; when n=0, MDI is formed after a series of subsequent reactions ； When n>0, polyphenylmethane polyisocyanates (PAPI) with varying degrees of condensation will be generated thereafter. 2. Phosgenation of diamine condensates The phosgenation of diamine condensates is typically carried out industrially in two stages: low-temperature phosgenation and high-temperature phosgenation. During the low-temperature phosgenation stage, diamines react with phosgene and hydrogen chloride to form the corresponding diaminamide salts and hydrochlorides. http://www.pufair.com/app/pic/mdi_2_clip_image008.gif During the high-temperature phosgenation stage, the amide salts and hydrochloride salts of the diamine are primarily converted into the corresponding isocyanates. http://www.pufair.com/app/pic/mdi_2_clip_image010.gif In industrial production, MDI and PMDI products are primarily synthesized from crude MDI through the reaction mentioned above. Then, they must undergo post-treatment processes such as degassing, high-vacuum distillation, purification, and separation to produce pure MDI and PMDI products with different degrees of condensation. 3. Product stability and improvement: The separation and purification of MDI products is an important means to ensure product quality. To ensure that the hydrolyzed chlorine content of the product meets the technical specifications, treatment agents such as acetylpropionylcopper and iron dioxide can be added. Since MDI is a chemical that is highly sensitive to heat, in order to prevent it from undergoing self-polymerization when exposed to heat over extended periods, as well as from exploding due to high-temperature decomposition in distillation vessels operating at temperatures above 250°C, the distillation and purification equipment for MDI must be capable of creating a very high vacuum rapidly. Currently, popular flash evaporation equipment such as thin-film distillation stills are widely used. The advantage of these devices is that the product is exposed to heat for a short time, separation is precise, and the distillation efficiency is high. Additionally, adding certain heat stabilizers can also help improve the thermal stability of the product. Patents indicate that adding a mixture of triphenyl phosphite and 4,4′-thiodibis(6-tert-butyl-3,3′-cresol) to pure MDI products provides good stability for MDI, preventing yellowing during storage. Pure MDI products are white crystals at room temperature, but upon long-term storage at this temperature, reactions such as autopolymerization occur, resulting in the formation of dimers and urea-based insoluble compounds. This causes the color of the product to darken, and the liquid form of the product becomes cloudy due to the presence of insoluble fine particles. Such conditions affect the quality of the product and reduce its performance. To this end, 0.1% to 5% of stabilizers such as triphenyl phosphate, tolsyl isocyanate, and carbonyl isocyanate should be added to pure MDI products before they leave the factory. However, their addition has a certain impact on the mechanical properties and water resistance of polyurethane products. According to Japanese patents, aryl carbonyl isocyanates containing benzene rings have a positive effect on the stability of MDI, without causing any adverse effects on the properties of the resulting products. In addition, tetraethyl orthocarbonate, tetraethyl orthosilicate, tetraphenyl orthosilicate, triethyl aluminate, trialkyl amines, borohydride, and others can also be used as storage stabilizers for MDI. II. Points to consider when selecting MDI 1. Pure MDI is a white to light yellow solid; its main chemical component is 4,4′-MDI, while the other two isomers are 2,4′-MDI and 2,2′-MDI. Depending on the raw material ratios and the synthetic processes used by different manufacturers, the contents of the three isomers in the distilled MDI also vary. As an industrial product, the proportions of the three isomers in MDI produced by distillation are typically maintained at the following levels: 4,4′-MDI 60–99.5%, 2,4′-MDI 0.5–40%, and 2,2′-MDI 0.0–2.0%. 2. Polyphenylmethane polyisocyanate is actually an oligomer of MDI. Abroad*, it is commonly referred to by the name given to it initially by UCC Company, namely PAPI. Currently, many manufacturers and producers both at home and abroad also call it PMDI (Polymeric MDI, polymerized MDI) or C-MDI (Crude MDI, crude MDI). PMDI is a brown, transparent liquid; it is actually a mixture of polyisocyanates with different functional groups. It is generally required that MDI should account for about 50% of the total mixture. Due to the different degrees of condensation of the polyphenylmethane polyisocyanates contained, in addition to pure MDI which accounts for half of the total amount, there are also polyisocyanates with a degree of condensation greater than 1. The structural components of PMDI are shown in the figure below: http://www.pufair.com/app/pic/mdi_2_clip_image012.gif. The average functional degree of polyphenylmethane polyisocyanate products manufactured by most companies is around 2.7, with a viscosity ranging from 100 to 300 mPa·s; their basic physical properties are listed in the table below. Even though the performance metrics of crude MDI provided by different companies appear identical, their internal compositions vary, which is why the performance of the final products they produce also differs. Therefore, a distinction should be made when selecting applications. Performance indicators: NC0 content/%; Amine equivalent; Acidity (as HCl)/%; Hydrolyzed chlorine content/%; Viscosity (25°C)/mPa·S: 31.5, 133.5, 0.11, 0.13, 0.20. Relative density; Flash point (open cup)/°C; Vapor pressure/×10^1 Pa: 1.20 at 10°C, 218 at 25°C; 3.20 at 10°C, 2.13 at 25°C. In actual production, different manufacturers control the reaction conditions according to the intended use of the product and its required performance characteristics, thereby producing a range of PMDI products with varying grades. For example, there are products with a high degree of polymerization containing approximately 35% pure MDI, and their functional group count ranges from 2.9 to 3.1 ； A product with a moderate degree of polymerization, containing approximately 40% pure MDI, with a polymerization degree of around 2.7 ； A low-polymerization product containing about 65% pure MDI, with a functional group degree of 2.2–2.4.