Organic compounds -- part of the class 532-570 series – Organic compounds – Isocyanate esters
Patent
1998-03-09
2000-05-23
Geist, Gary
Organic compounds -- part of the class 532-570 series
Organic compounds
Isocyanate esters
422228, C07C27300
Patent
active
060667591
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process and an apparatus for preparing polyisocyanates containing biuret groups from cyclo(aliphatic diisocyanates using steam or a substance capable of splitting off water as reactants which are mixed with one another in a stirred reactor.
(Cyclo)aliphatic polyisocyanates containing biuret groups are used, inter alia, in high-quality light and weather-resistant, two-component PUR surface coatings. Other applications such as adhesives and dispersions are known. An overview of the literature is given in DE 34 03 277.
(Cyclo)aliphatic polyisocyanates containing biuret groups are prepared by reacting the diisocyanates with a certain amount of biuret-forming agent (water or a substance capable of splitting off water) at from 100 to 200.degree. C. Subsequently, the excess monomeric diisocyanate is removed from the crude product thus formed by single-stage or multistage distillation. When t-BuOH (or another substance capable of splitting off water) is used as a biuret-forming agent, the urethane formed is catalytically dissociated into isobutene, carbon dioxide and an isocyanatoamine intermediate. However, this requires high reaction temperatures (>140.degree. C.) and the reaction products take on a yellow discoloration. This is unfavorable since colorless products are required for application reasons, eg. in the case of a clear surface coating. In addition, the use of substances capable of splitting off water (eg. OH-containing molecules) as biuret-forming agents result in formation of by-products which do not have a biuret structure and impair the storage stability of the desired product or cause other process problems. Water is therefore preferred as biuret-forming agent. However, insoluble areas are generally formed during the reaction and the products obtained have a poor storage stability in terms of redissociation into the monomers. As a result, the limit value requiring statutory labeling of 0.5% of free monomeric diisocyanate is quickly exceeded, in particular on storage above room temperature. Proposals have been made for avoiding these disadvantages, (see, for example, EP 259 233 and EP 251 952). These documents describe the use of catalytic amounts of protic acids for avoiding by-product formation in the synthesis of aliphatic and cycloaliphatic polyisocyanates containing biuret groups.
Nevertheless, use of conventional stirred vessels does not succeed in completely reacting the water used as biuret-forming agent with the diisocyanate. Water vapor and diisocyanate escape together with the carbon dioxide formed in the reaction, in accordance with the existing partial pressure. This gas mixture condenses at cold places in the reactor and, in particular in the downstream off-gas cooler. There, the diisocyanate reacts with the water vapor to form polyureas which finally leads to blocking of the off-gas lines and the off-gas condensor. Reliable, long-term operation is not possible in this way.
It is an object of the present invention to provide a reaction procedure and an apparatus suitable for this purpose in which the escaping off-gas is virtually free of water vapor and therefore no measurable polyurea formation occurs in the off-gas system.
We have found that this object is achieved by the reactants being conveyed in countercurrent through a cascade-type stirred reactor comprising at least two stages. This effects a fine distribution of the steam introduced. The ring gas bubbles lose water vapor on their way through the isocyanate solution and become enriched in carbon dioxide.
The multistage nature of the cascade and the baffles installed in the stirred reactor, which baffles prevent the reactants from simply flowing straight through the stirred reactor, considerably increases the residence time of the gas bubbles in the liquid, so that complete absorption of the steam and thus complete reaction can be achieved.
According to a particularly advantageous embodiment of the process of the present invention, the distribution of the gaseous material in the liquid reactant is i
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Bittins Klaus
Bruchmann Bernd
Heider Wolfgang
Wolff Stefan
BASF - Aktiengesellschaft
Geist Gary
Oh Taylor Victor
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