A process for preparing polyamines and polyisocyanates

Abstract: Disclosed herein is an improved process for preparing a polyamine mixture derived by reacting aniline with formaldehyde in the presence of an acid catalyst the improvement comprising mildly reducing the polyamine mixture by a process step comprising catalytically or chemically hydrogenating the polyamine mixture in the presence of a metal catalyst.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a process for preparing polyamines containing polymeric polyphenyl polymethylene polyamine. More specifically, the present invention relates to polyamines that are useful in preparing lighter color polyisocyanates and foams. Brief Description of the Prior Art: Discoloration of foams made with polymethylene polyphenyl polyisocyanates has, at least, in part been attributed to impurities in the polyisocyanates. Hence, attempts at solving the color problem has entailed treating the polyisocyanates or their precursors so as to remove the impurities therefrom or reduce their adverse effects on color. U.S. 4,792,624 discloses that polymethylene polyphenyl polyisocyanates of improved color can be obtained from certain polyamines which are prepared by the following process. The process comprises the preparation of the corresponding polymethylene polyphenyl polyamine by condensing aniline and formaldehyde in the presence of an acid catalyst which is characterized by adding a minor proportion of a polyamine mixture comprising di(aminophenyl)methanes and oligomeric polymethylene polyphenyl polyamines, (collectively known as polymeric MDA or simply MDA) to an intermediate stage of the condensation reaction where the various intermediately formed aminobenzylamines are present. U.S. 4,465,639 discloses addition of controlled amounts of water to the reaction mixture produced by phosgenation of a mixture of polymethylene polyphenyl polyamines (and the like polyamines produced by condensation of formaldehyde and aromatic amines) prior to complete removal of excess phosgene gives rise to the corresponding polymethylene polyphenyl polyisocyanates having significantly improved properties such color of the polyisocyanates. By the present invention, there is provided polyamines which are useful in the preparation of lighter color polyisocyanates. SUMMARY OF THE INVENTION In accordance with the foregoing, the present invention encompasses a process for preparing a polyamine mixture derived by reacting aniline with formaldehyde in the presence of an acid catalyst comprising reacting a solution of the polyamine mixture with hydrogen at atmospheric pressure (100 kPa) to 28.000 kPa (4000 psig) at temperatures between 0 to 200°C and at reaction times up to 6 hours in the presence of a metal catalyst. It has been found that by subjecting a polyamine mixture containing methylene diphenyl diamine to a mild catalytic hydrogenation to phosgenation, one is able to obtain a polyisocyanate precursor that can be used in the preparation of the corresponding polyisocyanates and foams which are up to 90% lighter in color. DETAILED DESCRIPTION OF THE INVENTION As set forth hereinabove, the polyamine mixtures useful herein can be prepared by reacting aniline with formaldehyde, in the presence of an acid catalyst. The resultant polyamine is then neutralized with an excess of a base such as sodium hydroxide and washed to remove the acid salt which is formed. The polyamine is then purified by distillation to remove unreacted aniline and water. Generally, the method of preparing the polyamine mixture is known. In accordance with the claimed invention, the step of mildly reducing the polyamine mixture comprises catalytically hydrogenating the polyamine mixture by treating the polyamine mixture by say, contacting it with up to 28.000 kPa (4000 psig) of hydrogen pressure in the presence of a catalyst. The term "mildly reducing the polyamine mixture" is used herein to the denote the relatively low degree of hydrogenation of phenyl groups involved in the claimed invention. The catalysts employed herein can be fixed bed or slurry phase catalysts. The fixed bed catalysts useful herein can be pelletized, granular or spherical and can contain a variety of active metals which are known to catalyze hydrogenation reactions. These include but are not limited to the group VIIIA metals such as nickel, palladium, platinum, cobalt, ruthenium and rhodium. Corresponding slurry phase catalysts can also be used. They differ from the fixed bed catalyst primarily in particle size. In the fixed bed process the catalyst is immobile, and the polyamine solution flows through or around the catalyst bed. In the slurry process, the catalyst is suspended in the polyamine solution. Both the slurry phase and fixed bed hydrogenation can be carried out either batch-wise or continously. In the process of this invention, a solution of the polyamine in a hydrogenation solvent is added to the catalyst and the reactor pressurized with hydrogen. A variety of solvents can be used including alcohols, such as methanol, ethanol or t - butanol, and amines such as aniline. Efficient mixing is needed to ensure good contact between the catalyst, the polyamine, and the hydrogen gas. The temperatures used are between 0 to 200°C and the pressures used can range from atmospheric pressure (100 kPa) to 28.000 kPa (4000 psi). Reaction time can range up to 6 hours, as needed to give reduction of the polymethylene polyphenyl polyisocyanate color. The combination of reaction conditions including type of catalyst, temperature, pressure and reaction time should be selected so that the desired color reduction is achieved, without significantly reducing the phenyl rings of the polymethylene polyphenyl polyamine. In accordance with the invention, the polyamine mixture is stripped of substantially any solvent before it is reacted with phosgene. This can be done by distillation. The resultant polyamine can be reacted with a phosgene to produce polyisocyanates. The reaction of phosgene with a polyamine mixture of the claimed invention, corresponding to the desired polyisocyanates is conducted in the presence of an inert solvent such as chlorobenzene. The polyamines are reacted with phosgene in molar ratios of 1.5 to 20 moles and preferably about 2.2 to 4.0 moles of phosgene per amine group. Upon completion of the phosgenation, the excess phosgene and hydrogen chloride formed are separately or simultaneously removed. The phosgenation product which is present after the removal is in the form of a solution and may be separated (e.g., by simple distillation) into a gaseous phase containing volatile compounds having isocyanate groups and a liquid phase which is substantially crude polyisocyanate. The liquid phase can be worked up to produce polyisocyanates in a purer state. In the preparation of foams, the polyisocyanates of the claimed invention are reacted with isocyanate reactive compounds such as polyols in the presence of foaming agents. This and other aspects of the claimed invention are further illustrated by the following examples. Examples A. Hydrogenation of polymethylene polyphenyl polyamines: All of the hydrogenations were performed in a one liter stainless steel autoclave. Polymeric amine samples were obtained from commercial runs of aniline/formaldehyde reactions, followed by neutralization with caustic soda, purification and separation of the polymeric amine. The polymeric amine was liquified using methyl alcohol as a solvent to simplify its handling. 1) Slurry Phase Catalysis: Slurry phase catalytic hydrogenations of MDA were performed under a variety of different conditions. First, a methanol solution of MDA was added to the autoclave, followed by 5-15 g of the catalyst. The autoclave was then sealed and after purging with nitrogen, pressurized with hydrogen to the desired pressure. The reaction mixture was then heated to the desired temperature and held there for the given period of time before cooling. The actual conditions for each experiment are given in Table 1. 2) Fixed Bed Catalysis: The experiments with fixed bed catalysts were run in a one liter Robinson reactor using 30 g of catalyst. The catalysts were supported inside a wire basket inside the reactor. Once the basket was filled, it was placed in the autoclave and the autoclave was sealed. Next a solution of 480 g of MDA in 160 g methanol was added to the autoclave. After flushing with nitrogen, the autoclave was pressurized with hydrogen. The reaction mixture was then heated to 100°C and held there for six hours. B. Solvent Removal: Before phosgenating the MDA, the solvent was removed. This was done by distilling at atmospheric pressure to a temperature of 100°C, followed by a vacuum distillation (< mm Hg) to a temperature of 150°C. The MDA was then cooled to about 100°C and weighed into eight ounce jars. temperature of 150°C. The MDA was then cooled to about 100°C and weighed into eight ounce jars. C. MDA Phosgenation and PMDI Color Determination: 2000 g of chlorbenzene was added to a 3 neck 5 liter flask and cooled to 5°C with an ice bath. Next, 600 g of phosgene was added at a rate of 300 g/hr. The MDA solution, consisting of 150 g of MDA in 1000 g MCB, was then added over 10 minutes under a nitrogen purge. Phosgene addition was then resumed at a rate of 50 g/hr and the mixture was heated. The solution was allowed to reflux at about 130°C for 1 hour while continuing to add phosgene. The phosgene addition was then stopped and the solution was degassed for 15 minutes. Most of the solvent was stripped off at atmospheric pressure and the material cooled to room temperature. The material was then transferred to a 1 liter flask where the remaining solvent was removed under vacuum, taking the pot temperature to 140°C at about 1 mm Hg. The vacuum was broken and the temperature quickly raised to 230°C before cooling under a nitrogen purge. Color determinations were performed by measuring the absorbance at 430 and 520 nm of a 10% solution of the polyisocyanate in chlorobenzene. Phosgenation Color Results for Hydrogenated Reduced Slurry Phase Hydrogenation Catalysts Example No. Value Measured Sample Standard MDA Improvement 1 430 nm 0.073 0.641 88.6% 520 nm 0.018 0.194 90.7% 2 430 nm 0.185 0.641 71.1% 520 nm 0.038 0.194 80.4% 3 430 nm 0.072 0.641 88.8% 520 nm 0.018 0.194 90.7% 4 430 nm 0.072 0.471 84.7% 520 nm 0.018 0.160 88.7% 5 430 nm 0.057 0.471 87.9% 520 nm 0.012 0.160 92.7% 6 430 nm 0.261 0.471 44.6% 520 nm 0.093 0.160 41.7% 7 430 nm 0.040 0.471 91.5% 520 nm 0.008 0.160 95.1% Phosgenation Color Results for Standard MDA and MDA Hydrogenated with Fixed Bed Catalysts Standard MDA Phosgenation Results Example No. Brown Gray 1a 0.601 0.222 1b 0.711 0.278 1c 0.534 0.203 Average 0.615 0.234 Standard Deviation 0.073 0.032 Reduced MDA Phosgenation Results: Example No. Value Measured Sample Improvement 8 430 nm 0.556 9.64% 520 nm 0.169 27.88% 9 430 nm 0.494 19.72% 520 nm 0.171 27.03% 10 430 nm 0.256 58.40% 520 nm 0.083 64,58% 11 430 nm 0.441 28.33% 520 nm 0.168 28.31% 12 430 nm 0.323 47.51% 520 nm 0.086 63.30% 13 430 nm 0.218 64.57% 520 nm 0.063 73.12% Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims: A process for preparing a polyamine mixture derived by reacting aniline with formaldehyde in the presence of an acid catalyst comprising reacting a solution of the polyamine mixture with hydrogene at atmospheric pressure to 28.000 kPa (4000 psig) at temperatures between 0 to 200°C and at reaction times up to 6 hours in the presence of a hydrogen metal catalyst. The process of Claim 1, wherein the step of hydrogenating comprises reacting the polyamine mixture with hydrogen at 786 kPa (100 psig) to 28.000 kPa (4000 psig). A process for preparing a lighter color polyisocyanate comprising reacting phosgene with a polyamine mixture obtained by the process of Claim 1. A process for preparing a foam comprising reacting the polyisocyanate obtained by the process of Claim 3 with an active hydrogen containing material in the presence of a blowing agent.

Inventors: ADKINS RICKY L, BLUE CLARENCE D, SCHILLING STEVEN LYLE, ADKINS, RICKY L., BLUE, CLARENCE D., SCHILLING, STEVEN LYLE
Applicants: BAYER AG, BAYER CORPORATION
Application year: 1992
Main classification: C08G18
IPC codes: C07C263, C08G73, C08G18, C08G101, C08G12