Process for preparing liquid diphenylmethane diisocyanate

Abstract: A process for preparing a liquid MDI, which comprises adding, to MDI, a phosphorin catalyst and a phosphorous acid ester of the formula (I): or the formula (II): wherein R¹, R², R³, R⁴, R⁵, R⁶ and R⁷ each is, same or different, and C₄-C₂₀ alkyl group, and conducting a reaction. EFFECT: The process according to the present invention can produce liquid MDI having stable properties. This liquid MDI has almost no discoloring, even if it is stored for a long time. The present process can commercially provide a polyurethane raw material having an easy handling.

Description: The present invention relates to a liquefaction of diphenylmethane diisocyanate (hereinafter referred to as MDI") widely used as a raw material for a polyurethane, namely a process for preparing liquid MDI. MDI has a very high reactivity and is a very useful compound as a raw material for poyurethane resin prepared by the reaction with a polyol mixture containing an active hydrogen compound, and an additive such as a chain extender and a catalyst. The polyurethane prepared from the raw material of MDI is widely used as a molded article such as a bumper of automobile, a foam such as a sheet cushion, a cast elastomer, a sealing material and the like. MDI is usually comprises 4,4'-isomer together with a small amount of 2,4'-isomer. Both isomers are solid at a room temperature and their melting points are 42°C and 36°C, respectively. MDI often contains another isomer such as 2,2'-isomer. In each case, MDI which is a mixture containing 4,4'-isomer is solid at a room temperature. By the way, when MDI is used for a raw material for polyurethane, it is advantageous that MDI be liquid, in view point of the storing and handling of raw material. For this purpose, there is a method to prepare liquid MDI in which a catalyst is added to MDI to conduct a reaction (Japanese Patent Kokoku Publication No. 7545/1970). However, this method is not always a sufficient method for preparing liquid MDI, since MDI considerably discolors because of a relatively large amount of catalyst and a high reaction temperature and the removal of a solid by-product is necessary. Japanese Patent Kokai Publication No. 100600/1977 discloses a method for preparing liquid MDI having a good storage stability, which comprises adding a phospholine catalyst to MDI to produce a carbodiimide. This method is preferable, since the amount of the added catalyst is small amount such as several ppm to several dozen ppm. The phospholine catalyst has a very high activity so that the amount of the added catalyst is very small, but an impurity such as an acid or hydrolyzable chlorine, which is contained in a very small amount in MDI, gives a strong effect on MDI and the same amounts of the added catalyst do not necessarily give liquid MDI having stable properties. The above matters are remarkable when the composition of the isomers of MDI used for liquefaction is varied and the change of content of the small amount impurity increases. In some cases, the addition of catalyst can give no carbodiimidization so that MDI having the desired properties cannot be obtained. EP-A-0 531 803 discloses a method for preventing coloration of isocyanate compounds by adding a phosphite ester of the formula shown below; in combination with 2,6-di-t-Bu-4-Me-PhOH and triphenyl phosphite. This document only mentions solid aromatic isocyanates and not liquid compositions (only heated liquid). GB-A-1 571 216 provides a process in which the isocyanate is reacted with a phospholine, phospholidine and/or oxide thereof at from RT to 300°C, stopping the reaction with a lewis acid, aromatic carboxylic halide, aromatic sulfonic acid ester and/or alkylsulfate to provide a storage-stable liquid polyisocyanates. EP-A-0 005 233 provides a process wherein isocyanate is reacted at 150-300°C in the presence of 1 ppb to 10 ppm of a phospholin oxide, after reaching the desired cyanate value the mixture is cooled to below 100°C. This provides storage-stable liquid polyisocyanates. US 3 449 256 discloses a process for producing liquid isocyanate composition by adding to MDI a trihydrocarbylphosphate (trialkyl such as triethylphosphate) heating at 160-250°C until the isocyanate content lies in the range of 138-200 and then adding more untreated isocyanate to obtain a liquid isocyanate mixture having an isocyanate equivalent of 127-140 in the final product. The process provides stable liquid polyisocyanate compositions. The inventors of the present application intensively studied a liquefaction of MDI, found that when some kind of phosphorous acid ester which has almost no disadvantage because of addition is used together with phospholine catalyst, the resultant liquid MDI has stable properties and good storage stability and completed the present invention. The present invention relates to a process for preparing liquid diphenylmethane diisocyanate, which comprises adding a phospholine catalyst and a phosphorous acid ester of the formula (I): or the formula (II) wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 are the same or different and each represents a C 4 -C 20 alkyl group, to diphenylmethane diisocyanate, stirring the mixture at a temperature of from 60 to 80°C for a period of from 1 to 10 hours, and adding a deactivating agent to the mixture once the desired degree of carbodiimidization has been reached. MDI is a solid compound at a room temperature which comprises a large amount of 4,4'-isomer, a small amount of 2,4'-isomer and, in some cases, very small amounts of other isomers. MDI is usually called as pure MDI or purified MDI. Specific examples of the phospholine compound which is the catalyst is phospholine, phospholidines, phospholine oxides, phospholidine oxides and derivatives thereof, as described in the above Japanese Patent Kokai Publication No. 100600/1977. 1-Phenyl-3-methylphospholine, 1-ethyl-3-methylphospholidine and 1-ethyl-3-methylphospholidine-1-oxide are preferable and 1-methylphospholine-1-oxide is particularly preferable. The addition amount of the catalyst is varied on the composition of MDI and is usually from 0.0001 to 0.005 parts by weight, preferably from 0.0002 to 0.002 parts by weight, per 100 parts by weight of MDI. The phosphorous acid ester is the compound of the above formula (I) or (II) and is preferably tetratridecyl-4,4'-isopropylidenediphenyldiphosphite (hereinafter referred to as tridecyl compound), tributyl phosphite, tri(2-ethylhexyl) phosphite, tridecyl phosphite and tristearyl phosphite. The phosphorous acid ester is used as an anti-discoloring agent and a stabilizer (cf. Japanese Patent Kokai Publication No. 65264/1993). Liquid MDI containing the phosphorous acid ester can be stably stored for a long time. The addition amount of the phosphorous acid ester is varied depending on the composition of MDI as in the case of the catalyst and is usually from 0.001 to 0.1 parts by weight, preferably from 0.005 to 0.05 parts by weight, per 100 parts by weight of MDI. Namely, the addition amount of the phosphorous acid ester is several dozen times to several hundred times the addition amount of the catalyst. Liquid MDI according to the present invention can be prepared by heating MDI which is solid at a room temperature, to 40-60°C to liquefy MDI; adding the above catalyst and phosphorous acid ester in the given amounts; stirring the mixture at 60-80°C for 1-10 hours, preferably for 3-6 hours: adding a deactivating agent, for example, butyl carbamic acid chloride or acetyl chloride to discontinue the reaction, when a carbodiimidation degree reaches a desired degree. The carbodiimization degree can be monitored by measuring an NCO content of the liquid MDI. In order to obtain liquid MDI having the stable properties, the NCO content is preferably from 29 to 31.5%. EXAMPLES The present invention will be illustrated by the following Examples and Comparatives Examples. Part is part by weight and % is % by weight. Example 1 and Comparative Example 1 Purified MDI (melting point: 38.7°C) was liquefied by heating MDI to 40-50°C. 1-Methylphospholine-1-oxide (3 ppm) was added, then a tridecyl compound (500 ppm) was added and the mixture was stirred at 70°C for 6 hours. In comparison, the mixture without the addition of the tridecyl compound was stirred. The mixture was sample and the NCO content (in %) was measured. After butyl carbamic acid chloride (3 ppm) was added to discontinue the reaction, the mixture was cooled to a room temperature and was stored for one week. When the tridecyl compound was added, MDI kept the liquid state. When the tridecyl compound was not added, MDI was opaque after one day and was fully solid after one week. The NCO content was 30.4% in the case of the addition of the tridecyl compound and 32.8% in the case of no addition of the tridecyl compound. Example 2 Purified MDI (melting point: 38.8°C) was liquefied by heating MDI to 40-50°C. 1-Methylphospholine-1-oxide (3 ppm) was added and the mixture was stirred at 70°C for 3 hours. The mixture was sampled and the NCO content was measured to be 33.5%. Then the tridecyl compound (500 ppm) was added and the mixture was stirred at 70°C for 5 hours. The mixture was sampled and the NCO content was measured to be 30%. Butyl carbamic acid chloride (3 ppm) was added to discontinue the reaction. Then the mixture was stored at a room temperature for one week. The MDI kept the liquid state. Example 3-7 and Comparatives Examples 2-4 In the same manner as in Example 1, 200 ppm of the phoshorous acid ester shown in Tables 1 and 2 was added to prepare liquid MDI. Then the NCO content (in %) was measured. The results are shown in Tables 1 and 2. In comparison, Tables 2 and 3 show the results of no addition of the phosphorous acid ester and the addition of the phosphorous acid ester other than those of the present invention. Examples 3 Examples 4 Examples 5 additive Tridecyl compound Tributyl phosphite Tri(2-ethylhexyl)-phosphite hours for heating and stirring 6 3.1 2.8 NCO content (%) 31.2 30 30 Example 6 Example 7 Comparative Example 2 additive Tridecyl phosphite Tristearyl phosphite not added hours for heating and stirring 3.9 6 6 NCO content (%) 30 31.4 32.8 Comparative Example 3 Comparative Example 4 Comparative Example 5 additive Trinonylphenyl phosphite Tetraphenyldipropylene glycol diphosphate Triphenyl phosphite hours for heating and stirring 6 6 6 NCO content (%) 32.8 32.7 33 Example 8 Two purified MDIs having the different isomer compositions and impurity contents shown in Tables 4 and 5 were used. A phosphorous acid ester was added or was not added in the presence of a catalyst to attempt to liquefy MDI in the same manner as in Example 1 or Comparative Example 1. The results of the state after storage for 10 days are shown in Tables 4 and 5. Purified MDI-A Melting point (°C) 38.7 2,4'-Isomer (%) 1.09 4,4'-Isomer (%) 98.9 2,2'-Isomer (%) 0.01 Acid content (ppm) 3 Hydrolyzable chlorine (ppm) 7 Example 8-1 Comparative Example 8-1 Tridecyl compound (ppm) 200 0 1-Methylphospholine-1-oxide (ppm) 3 3 Hours for heating and stirring 6 6 Liquid MDI O X Viscosity (cps) 26 - NCO content (%) 31.3 33.2 Hue (APHA) 30 - Purified MDI-B Melting point (°C) 38.2 2,4'-Isomer (%) 1.69 4,4'-Isomer (%) 98.31 2,2'-Isomer (%) trace Acid content (ppm) 6 Hydrolyzable chlorine (ppm) 10 Example 8-2 Comparative Example 8-2 Tridecyl compound (ppm) 200 0 1-Methylphospholine-1-oxide (ppm) 3 3 Hours for heating and stirring 5.4 6 Liquid MDI O X Viscosity (cps) 28 - NCO content (%) 30.3 32.2 Hue (APHA) 40 - Note: O: The liquid state was kept after 10 days. X: The solid state was made after 10 days. The process according to the present invention can produce liquid MDI having stable properties. This liquid MDI has almost no discloring, even if it is stored for a long time. The present process can commercially provide a polyurethane raw material having an easy handling.

Claims: A process for preparing liquid diphenylmethane diisocyanate, which comprises adding a phospholine catalyst and a phosphorous acid ester of the formula (I): or the formula (II) wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 and R 7 are the same or different and each represents a C 4 -C 20 alkyl group, stirring the mixture at a temperature of from 60 to 80°C for a period of from 1 to 10 hours, and adding a deactivating agent to the mixture once the desired degree of carbodiimidization has been reached.

Inventors: IMOKAWA TOSHIAKI, KATO YOSHIHARU, IMOKAWA, TOSHIAKI, KATO, YOSHIHARU
Applicants: SUMITOMO BAYER URETHANE CO, SUMITOMO BAYER URETHANE CO., LTD.
Application year: 1995
Main classification: C07C263
IPC codes: C07C267, C07C265, C07F9, C07C263