Distillation: processes – separatory – Addition of material to distilland to inhibit or prevent...
Reexamination Certificate
2000-03-22
2002-05-21
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
Addition of material to distilland to inhibit or prevent...
C203S060000, C203S091000, C203S099000, C203S100000, C560S352000
Reexamination Certificate
active
06391161
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a new method for purifying organic isocyanates or mixtures of isocyanates by reducing the content of chlorine compounds. In this method, an isocyanate is treated with a high molecular weight ester of phosphoric acid. The isocyanates which are purified in this manner are preferably used in coatings, polyurethane moldings and as intermediates.
Varying amounts of impurities present in isocyanates, which are due to the isocyanate production process, give rise to variations in the isocyanate activity. Variations in activity have an unfavorable effect on the reproducibility of products produced from these isocyanates, i.e. on the commercial value of these compounds. Thus, aromatic isocyanates such as the known phosgenation products of aniline-formaldehyde condensates, as well as 2,4- and 2,6-diisocyanatotoluene and aliphatic isocyanates such as isophorone diisocyanate contain a number of impurities of this type. These impurities are primarily compounds which contain chlorine and which give rise to variations in activity if these compounds contain “readily mobile” chlorine, or what is termed “hydrolyzable chlorine”. Some of these hydrolyzable chlorine-containing compounds have proven to be relatively stable and they remain in the isocyanate(s) even after distillation. These stable chlorine compounds not only have an unfavorable effect on isocyanate activity but also have an unfavorable effect on the stability of the isocyanate. A uniform, lower content of these impurities, with the consequence of a standardized activity and enhanced ease of further processing of the isocyanate, is therefore important both technically and economically.
There have therefore been a number of investigations aimed at identifying options for the removal of chlorine compounds. There are a large number of patent applications which describe thermal methods for removing such chlorine compounds. For example, it is known that heating an isocyanate, particularly when stripping with an inert gas is employed at the same time, or heating of an isocyanate in an inert solvent under pressure while simultaneously removing volatile compounds under suction, reduces the content of readily separable chlorine compounds. See, e.g., DE-A 1,270,036; DD 271 820; U.S. Pat. No. 3,219,678; GB-A 1,080,717; DE-A 2,237,552; U.S. Pat. No. 3,857,871; U.S. Pat. No. 1,458,223; JP 0 727 808 8 A2; JP 0 634 570 7 A2; and GB-A 1,384,065. JP 6 116 125 OA; JP 0 516 323 1A; DE-A 1,950,101; DE-A 1,938,384; DE-A 2,532,722; DE-A 2,631,168; U.S. Pat. No. 3,853,936; FR-A 1,555,517; DE-A 2,933,601 and U.S. Pat. No. 3,549,504 disclose that isocyanates can be purified by special distillation and crystallization techniques. However, none of these methods achieves effective separation of troublesome chlorine compounds. These known methods are based on what is purely a physical treatment so only readily volatile chlorine compounds are separated. The use of methods of this type is thus restricted to particular isocyanate compounds, which are generally thermally stable and which are useful when a limited reduction in chlorine content is achieved.
Aside from treatment of isocyanate compounds by what are purely thermal techniques, treatments with additives have also been described in the literature. These treatments make it possible to achieve an improved separation of troublesome chlorine compounds. Patent specifications JP 4 501 032 9 B; JP 4 200 413 7 B; JP 5 908 845 2 A; JP 5 910 875 3 A; JP 5 917 245 0 A; U.S. Pat. No. 3,373,182; GB-A 1,111,581; U.S. Pat. No. 3,759,971; U.S. Pat. No. 4,094,894; ZA-A 8,100,606; DE-A 1,138,040; DE-A 1,286,025; U.S. Pat. No. 3,458,558; U.S. Pat. No. 3,264,336; SU-A 806 677 and DE-A 2,210,607 describe additives based on metals or alkali metals, such as metal oxides, metal cyanamides, metal hydrides or metal fatty acid esters for example, in the presence of sterically hindered phenols, metal naphthenates, metal silicates, alkali metal carbonates or organometallic compounds. These additives can only be separated with difficulty from the purified isocyanate, however, and result in unwanted contamination of the corresponding isocyanate product by metals or metal ions. Moreover, almost all metals and metal complexes cause an increase in the formation of byproducts (e.g., trimers, carbodiimides, dimer formation).
Similar restrictions apply to the use of additives such as imidazole (described in GB-A 1,347,647 and JP 0 505 898 2 A), sulfonic acids and esters thereof (described in GB-A 1,458,747), the acidic material described in U.S. Pat. No. 4,996,351, the diethyl sulfate (described in GB-A 1,459,691), the sulfuric acid (also described in GB-A 1,459,691), epoxy compounds (DE-A 2,249,375; JP 0 932 396 8 A2), tetra-substituted ureas (DD 288 598), formic or acetic acid or derivatives thereof (U.S. Pat. No. 3,799,963) and the compounds having trimethylsilyl groups (described in EP-A 524 507). The use of organic bases or acids, particularly when these are employed for the purification of reactive isocyanate compounds, results in unwanted secondary reactions such as trimerization, dimer formation or carbodiimide formation.
Some compounds containing at least one Zerewitinoff-active NH group, such as ureas (DD 285 594), biurets (DD 288 597) or caprolactam (DD 285 593), as well as ammonium salts (DD 288 594), carbodiimides (DD 288 599), salts of primary and secondary amines (DD 288 593), tertiary alcohols and tertiary alkyl carbamates (DD 288 595) are recommended in the prior art for the purification of isocyanates. There are, however, disadvantages to using these materials also. For example, these additives need to be separated from the isocyanate or the usefulness of the isocyanate or distillation residue which contains the additive will be limited. Another particular disadvantage to use of these additives is the considerable reduction in NCO value which sometimes occurs, as well as the increase in viscosity which can result from the formation of biurets when tertiary alcohols are used. The last-mentioned disadvantage applies in particular when water is used to purify the isocyanate (DE-A 1,240,849; DE-A 1,240,849).
Patent Specification DD 288 596 describes low molecular weight alkyl phosphates as additives which are substantially inert to isocyanates and which, at temperatures of at least 200° C. and with the simultaneous use of inert gases, enable an effective reduction in the content of hydrolyzable chlorine to be achieved. In the examples given in this patent, temperatures of 225° C. for several hours are stated to be necessary for an effective reduction of the hydrolyzable chlorine content. These high temperatures, and the periods to which the products are exposed to them, restrict the use of this method to a few, relatively thermally stable isocyanates. Isocyanate compounds, particularly temperature-sensitive, low molecular weight isocyanates, are not stable when subjected to temperature/time conditions of this type, and decompose with the formation of carbodiimides and isocyanurates. This can result in an uncontrolled generation of process heat, which makes the reaction considerably more difficult to control. Moreover, the use of inert gas which is mandatory according to DD 288 596 inevitably results in a highly contaminated off-gas stream which has to be handled. Finally, it is difficult to separate the low molecular weight alkyl phosphates described above from the purified isocyanate, which is also of low molecular weight, by means of distillation or extraction.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a universally applicable method of purifying organic, low molecular weight isocyanates, particularly temperature-sensitive low molecular weight isocyanates, which does not possess the aforementioned shortcomings and which makes it possible to efficiently separate the product isocyanate from the reagents used for reducing the proportion of hydrolyzable chlorine.
These and other objects which will be apparent to those skilled in the art are acc
Brahm Martin
Danielmeier Karsten
Halpaap Reinhard
Hoffman Eric
Mager Dieter
Bayer Aktiengesellschaft
Gil Joseph C.
Manoharan Virginia
Sloane Carolyn M.
Whalen Lyndanne M.
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