Process for reducing residual isocyanate

Details Process for reducing residual isocyanate Process for reducing residual isocyanate

2001-09-11

2003-12-16

Richter, Johann (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Isocyanate esters

C560S335000

Reexamination Certificate

active

06664414

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for reducing residual monomeric polyisocyanate in a composition comprising a polymer or oligomer bearing isocyanate groups.
BACKGROUND OF THE INVENTION
Polyurethane prepolymers bearing isocyanate groups are commonly used in industry to make a wide variety of polyurethane products. These prepolymers are usually prepared by reacting a polyol with an excess of a monomeric polyisocyanate, for example, a diisocyanate. One of the diisocyanates extensively used industrially is tolylene diisocyanate (TDI). TDI-based prepolymers demonstrate lower viscosities compared to prepolymers formed with other diisocyanates, and are, therefore, particularly desirable.
Disadvantages associated with the process for forming these polyurethane prepolymers, particularly those TDI-based prepolymers, involve the unreacted TDI. For example, a relatively large amount (2-3 wt. %) of unreacted TDI is often present, and its high vapor pressure can lead to industrial hygiene issues.
Methods that have been developed to reduce the quantity of unreacted polyisocyanate contaminating various isocyanate prepolymers include falling film evaporation, wiped film evaporation, distillation, extraction using various solvents, or molecular sieves, or reaction with an organic reactive reagent such as benzyl alcohol. Most of these methods require, however, elevated temperature, reduced pressure, and/or multiple steps.
U.S. Pat. No. 4,061,662 discloses a process for removing unreacted TDI from an isocyanate prepolymer by contacting the prepolymer with molecular sieves. Additional processing steps require additional time to yield the desired prepolymer and increase the cost of the prepolymer.
U.S. Pat. Nos. 3,248,372; 3,384,624; and 3,883,577 describe processes related to removing free isocyanate monomers from prepolymers by solvent extraction techniques. Multiple steps are required in such processes.
It is also possible to distill an isocyanate prepolymer to remove the unreacted diisocyanate according to U.S. Pat. No. 4,385,171. However, it is necessary to use as a chaser a compound that is only partially miscible with the prepolymer and has a higher boiling point than that of the diisocyanate to be removed.
U.S. Pat. Nos. 3,183,112; 4,683,279; 5,051,152; and 5,202,001 describe falling film and/or wiped film evaporation. In U.S. Pat. No. 5,502,001, the residual TDI content can be reduced to less than 0.1 wt % by passing the prepolymer at ~100° C. through a wiped film evaporator, while adding an inert gas, a specially nitrogen, to the distillation process to sweep out the TDI.
It is known that isocyanate compounds react with organic compounds containing active hydrogen atoms such as hydroxy and amino groups. U.S. Pat. No. 3,384,624 disclosed benzyl alcohol as a suitable organic compound to remove excess of monomeric 2,4-TDI. The process requires elevated temperature (over 70° C. for at least 1 hour) and is not suitable for removing 2,6-TDI, which is usually a component of industrial grade TDI.
U.S. Pat. Nos. 3,627,722; 3,632,557; and 4,374,237 disclose silane-containing isocyanate-polyurethane polymers prepared by reacting isocyanate prepolymers with amino silanes. There were no reports of residual monomeric isocyanate compounds. This method was used to introduce additional cross-linking sites from alkoxy silane groups.
SUMMARY OF THE INVENTION
The invention provides a method of reducing the amount of unreacted polyisocyanate, for example, tolylene diisocyanate (TDI) from a polyurethane prepolymer bearing isocyanate groups.
The invention provides a process for reducing the amount of monomeric polyisocyanate in a composition comprising a polymer or oligomer bearing isocyanate groups, the process comprising reacting the composition with a quenching agent, the quenching agent comprising an arylalkyl alcohol and an alkoxysilane bearing a secondary amino group.
Surprisingly, in carrying out the process of the invention, it is generally not necessary to employ elevated temperatures.
DETAILED DESCRIPTION OF THE INVENTION
When an excess of a monomeric polyisocyanate is reacted with a polymer bearing active hydrogen groups, for example, a polyol bearing terminal hydroxyl groups, to form a polymer or oligomer bearing isocyanate groups, a substantial amount of unreacted polyisocyanate, which is toxic, can remain. The process of this invention minimizes the amount of unreacted monomeric polyisocyanate, and thereby reduces toxic risk.
The polymer or oligomer bearing isocyanate groups can be obtained by reacting a polymer bearing groups that are reactive to isocyanate groups, such as amino groups, alcohol groups, or thiol groups, or less preferably carboxyl groups, with an excess of polyisocyanate. The polymer or oligomer bearing isocyanate groups can be prepared, for example, by reacting a monomeric polyisocyanate with a copolymer of polyoxyethylene-propylene polyol using an NCO/OH equivalent ratio of about 5:1 to about 1.05:1, preferably a ratio of about 2.0:1 to 2.5:1. The preparation of isocyanate-terminated prepolymers is disclosed in, for instance, U.S. Pat. Nos. 4,315,703 and 4,476,276 and in references mentioned in those patents. The disclosures of these patents are incorporated herein by reference.
Monomeric polyisocyanates that can be used to prepare the polymer or oligomer bearing isocyanate groups include aliphatic and aromatic polyisocyanates. The preferred polyisocyanates are aromatic polyisocyanates. One of the most useful polyisocyanate compounds that can be used is tolylene diisocyanate, particularly as a blend of 80 weight percent of tolylene-2,4-isocyanate, and 20 weight percent of tolylene-2,6-isocyanate; a 65:35 blend of the 2,4- and 2,6-isomers is also particularly useful. These polyisocyanates are commercially available under the trademark “Hylene”, as Nacconate™ 80, and as Mondur™ RD-80. The tolylene diisocyanates can also be used as a mixture with other monomeric polyisocyanates, for example, methylene diisocyanate. Other polyisocyanate compounds that can be used are other isomers of tolylene diisocyanate, hexamethylene-1,6-diisocyanate, diphenyl-methane-4,4′-diisocyanate, m- or p-phenylene diisocyanate and 1,5-naphthalene diisocyanate. Polymeric polyisocyanates can also be used, such as polymethylene polyphenyl polyisocyanates, such as those sold under the trademarks “Mondur” MRS, and “PAPI”. A list of useful commercially available polyisocyanates is found in Encyclopedia of Chemical Technology by Kirk and Othmer, 2nd Ed., Vol. 12, pages 46, 47, Interscience Pub. (1967).
The invention is here described with reference to TDI as the monomeric polyisocyanate. It should be appreciated, however, that TDI is referred to as an example for the purpose of illustration only, and the invention is also applicable to quenching the amount of other monomeric polyisocyanates. The arylalkyl alcohol or the alkoxysilane that reacts with monomeric TDI desirably will not appreciably react with the terminal isocyanate groups of the polymer or oligomer. For example, TDI is normally used as a mixture of the 2,4- and 2,6-isomers, and the reactivities of the two isocyanate (—NCO) groups of monomeric TDI are different, i.e., the 4-position —NCO group is more reactive than the 2- or 6-position —NCO group. Consequently, reaction of a polyol with 2,4-TDI results in a polymer having terminal isocyanate groups derived mainly from the 2-position of 2,4-TDI, the 4-position —NCO group having been eliminated by reaction with the polyol. Residual 2,4-TDI still contains the more reactive 4-position —NCO group. An arylalkyl alcohol, such as benzyl alcohol can selectively react with the isocyanate group at the 4-position of free residual monomeric TDI with little reaction (usually less than 10%) with the less reactive terminal —NCO groups of the prepolymer, derived from the 2-position of TDI.
In a preferred embodiment, a mixture of residual monomeric 2,4- and 2,6-TDI present in a composition comprising a polymer bearing isocyanate groups is quenched with an arylalkyl alcohol used in conju

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