DD227698B1 - PROCESS FOR THE PREPARATION OF ISOCYANATES OF DIISOCYANATODIPHENYLMETHANE SERIES - Google Patents

Description

-2- 226 698

In addition to NaCl, which can form a complex with 4,4'MDA, FeCI ₃ , which is due to corrosion of the equipment due to the hydrochloric acid medium in the system, under these conditions capable of complexing with 4,4'MDA. Even traces of FeCl ₃ and of NaCl cause thermal damage to the MDA / polyamine during the distillative removal of the aniline, as a result of which harmful by-products form and the viscosity of the MDA / polyamine mixture can increase significantly.

A product control with regard to the content of NaCl or FeCl ₃ can be carried out by means of titrimetric determination of the chloride content. If MDA / polyamine mixtures, produced by any large-scale process, do not meet the stated quality criteria, they must be discharged so that a substantially residue-free phosgenation can take place and the share of chlorine-containing compounds or other impurities remains low.

The great advantage of the application of the method according to the invention is that without great technological effort, the term of a large-scale plant for the production of isocyanate is prolonged, since the necessary downtime for cleaning almost completely eliminated and an isocyanate is obtained with high quality. The discharged, not quality-appropriate MDA / polyamine mixture can readily be used for other purposes, in particular it is very suitable as a starting substance for Aminpolyetheralkohole use.

The aromatic amine mixture according to the invention is prepared by acid-catalyzed condensation of aniline and formaldehyde in the molar ratio of aniline to formaldehyde of 1.5 to 2.5: 1 and aniline to acid of 1: 0.5 under conventional conditions, with aniline hydrochloride at a temperature of 45 ⁰ C is homogeneously mixed with an appropriate amount of a 37% aqueous and held at a temperature of 33 ° C formaldehyde solution under high turbulence. The reactor of the initial reaction stage, the reaction mixture leaving is placed in a cooling circuit to a temperature of 55 ⁰ C, 20 min maintained at this temperature, heated to 85 ° C-95 ° C and this temperature was maintained for 120 minutes. The acid reaction mixture is worked up alkaline, washed and unreacted aniline and any dissolved water removed by distillation. The preparation of aromatic isocyanates by phosgenation of the aromatic amine mixture according to the invention with the usual optimized chloride content of less than40ppm, the usual optimized viscosity of 450 mPas maximum and the content of tertiary amines in the form of Dichinverbindungen of less than 4Gew .-% under conventional conditions carried out a single-stage, continuously operating tubular reactor. The mixture of monochlorobenzene solutions of phosgene and amine at about 14O ⁰ C in a ratio of 3: 1 to 4: 1 in a mixing nozzle according to the prior art. The mixed solution reacts at temperatures of 140 ⁰ C to last 190 ⁰ C 'from. Then, the separation of the excess phosgene and the solvent follows in distillation columns, so that thereafter, the purified crude polyisocyanate is present

 The invention is explained in more detail below on 2 comparative examples and 2 embodiments.

Comparative Example 1

For the preparation of MDI / polyisocyanate by phosgenation, an MDA polyamine is used, which is prepared according to Example 1 and has the following analytical characteristics:

4,7Gew .-% Dichin

8ppm chloride

410mPas viscosity / 70 ° C

Residues occur during the phosgenation and during workup, so that after 2 weeks running time the continuous system must be cleaned, the MDI polyisocyanate is dark colored and has significantly higher chlorine values:

0.91% by weight of hydrolyzable chlorine

0,13Gew .-% acidity

235mPasViskosität / 25 ° C

Comparative Example 2

An MDA / polyamine mixture, prepared analogously to Example 1, with

2,0Gew .-% Dichin

52ppm chloride

472 mPas viscosity / 70X

is phosgenated. During the phosgenation as well as during the workup, residues occur which reduce the running time of the plant. In the analytical characterization, the following characteristics are obtained:

0.69 wt% hydrolyzable chlorine

0.089% by weight of acidity

299 mPas viscosity / 25 ⁰ C

Embodiment 1

Hydrochloric acid and water-saturated aniline are used to continuously produce a product stream of an aniline-containing aniline hydrochloride solution in which a molar ratio of aniline to acid of 0.5 is always maintained. This Anilinhydrochloridlösung with a temperature of 45 ° C passes to a mixing device in which under turbulence, a homogeneous mixing with the simultaneously supplied volume flow of 37% aqueous and held at a temperature of 33 ° C formaldehyde solution.

-z- 226 698

The volume flows of the aniline hydrochloride solution and the formaldehyde solution are mixed in a ratio of 3.7: 1. The gas exiting the reactor, the initial reaction stage reaction mixture is quenched in a cooling circuit to a temperature of 55 ⁰ C and held for about 120 minutes at this temperature. Subsequently, the temperature of the reaction mixture is raised to 80 ⁰ C and kept the product in a continuous flow at a temperature of 85 ° C to 95 ⁰ C and a mean residence time of 20 min. After leaving the final reaction zone, the product is cooled to a temperature of 70 ^c C, neutralized with excess sodium hydroxide solution, the organic phase separated and washed with water virtually free of chloride. After removal of residual water and excess aniline by distillation, samples are taken from the product stream and the amine is analytically characterized as follows:

2.2% by weight of dichlorine (by NMR spectroscopy)

16ppm chloride (by titration)

315 mPas viscosity / 70 ° C

This MDA / polyamine mixture is dissolved in monochlorobenzene and phosgenated. In the phosgenation of the process product, a bright chlorobenzene reaction solution is obtained, which is free from deposits. The MDI / polyisocyanate obtained after the work-up by distillation, in which virtually no residues also occur, is characterized by the following characteristics:

0.45% by weight of hydrolyzable chlorine (by means of titration)

0,06Gew .-% acidity

180mPas Viscosity / 25 ⁰ C

Embodiment 2

A prepared according to the procedure of the embodiment 1 MDA / polyamine having the following analytical characteristics:

3.5% by weight of dichin

25ppm chloride

400mPas viscosity / 70 ° C

is dissolved in monochlorobenzene and phosgenated. During the phosgenation and the work-up insignificant amounts of solid residues accumulate, whereby the duration of the continuous plant is not significantly impaired. The analytical examination of the MDI / polyisocyanate mixture gives:

0.75% by weight of hydrolyzable chlorine

0,10Gew .-% acidity

225mPas viscosity / 25 ° C.

Claims (1)

-1- 226 698 Invention claim: Process for the preparation of isocyanates of the diisocyanatodiphenylmethane series by phosgenation of an amine mixture obtained in the condensation of aniline with formaldehyde in a molar ratio of 1.5: 1 to 2.5: 1 and containing less than 40 ppm of chloride and having a maximum viscosity of 450 mPas in that the content of tertiary amines in the feed mixture is less than 4% by weight. Field of application of the invention The invention relates to a process for the preparation of isocyanates of the diisocyanatodiphenylmethane series as raw materials for the production of polyurethane. Characteristic of the known technical solutions For the preparation of di- or higher-functional polyisocyanates, it is known to use methylenedianilines and their higher-nuclear oligomers (referred to below as MDA / polyamine mixtures). In general, the procedure for producing the MDA / polyamine mixture is such that aniline is initially mixed with acidic catalyst, preferably hydrochloric acid, and then formaldehyde is added. The reacted, generally strong hydrochloric acid reaction mixture must be worked up alkaline to obtain the reaction product, washed several times with water and unreacted aniline and dissolved water are removed by distillation. From the MDA / polyamine mixture is prepared by known methods by phosgenation in an inert solvent, such as monochlorobenzene, an MDI / polyisocyanate mixture which can be worked up in a known manner and thereby separated into a polyisocyanate mixture and a mixture containing preferably diisocyanate. In the large-scale process for the preparation of isocyanate by reaction of MDA-polyamine with phosgene residues occur in both the amine production and in particular during and after the Phosgenierungsvorgang whose origin and nature are only partially understood. It is known that NaCl forms a complex formation with 4,4'-MDA, which may be soluble in the crude polyamine mixture or may be suspended, and associated with the thermal load for deposits in distillation units, storage tanks and other equipment containing the MDA / Polyamine mixture passes through, is responsible. By washing the MDA / polyamine mixture at higher temperatures with water, it is possible to destroy this complex for the most part, but leaving residues in the MDA / polyamine. The quality of the isocyanates used for the synthesis of polyurethanes is determined to a large extent already by the conditions in the aniline-formaldehyde reaction. In all hitherto known industrial processes for the preparation of MDA / polyamine, it is not excluded that there may be deviations from the reaction parameters such as aniline-formaldehyde ratio, temperature and mixing in the initial phase of the aniline-formaldehyde condensation and as a result undesired side reactions take place can, whereby secondary and tertiary amines in the form of monomeric N-methyl-MDA or higher-nuclear oligomers and 3,4-dihydro-3-phenylquinazoline (dichlorine) or its derivatives arise and a strong increase in the viscosity can occur. After the phosgenation, chlorine-containing compounds which have a detrimental effect on the reactivity of the isocyanate or substantially impair the effectiveness of catalysts in the polyurethane formation reaction can be formed from the by-products. The reduction or removal of the chlorine-containing compounds is therefore of utmost importance, and there is therefore no lack of attempts, in particular by post-treatment of the isocyanates, to solve this problem. A technically fully satisfactory, applicable without great technological effort process is currently unknown. Object of the invention The aim of the invention is to develop an economical process for the preparation of isocyanates of Diisocyanatodiphenylmethanreihe with little technical effort. Explanation of the essence of the invention The invention has for its object to develop a process for the preparation of isocyanates of üiisocyanatodiphenylmethanreihe, which allows to extend the term of the plant for the production of isocyanates without great technological effort. According to the invention the object is achieved in that an aromatic amine mixture containing tertiary amines in the feed mixture in the form of dichin and its derivatives of less than 4Gew .-% is used. For the preparation of high-quality aromatic isocyanates, the aromatic amine mixture is used for phosgenation in the usual optimum quality with less than 40 ppm of chloride and a maximum viscosity of 450 mPas. It has been found that in particular the content of tertiary amines in the form of dichlorine compounds over 4% by weight contributes substantially to the formation of residues during the phosgenation and deteriorates the quality of the isocyanate with regard to chlorine-containing compounds and color. To determine the content of a process control NMR spectroscopy and / or gas chromatography is performed. Another cause of residue formation during and after phosgenation is too high a viscosity of the MDA / polyamine mixture. To exclude this, only MDA / polyamine mixtures with a viscosity of 450mPas may be used for the phosgenation.