WO1996033231A1

WO1996033231A1 - Process for preparing a storage-stable mixture of polyisocyanate and phosphate

Info

Publication number: WO1996033231A1
Application number: PCT/US1996/004120
Authority: WO
Inventors: Steven B. Lowenkron; Donald M. Ference; Daniel P. Martone
Original assignee: The Dow Chemical Company
Priority date: 1995-04-17
Filing date: 1996-03-26
Publication date: 1996-10-24
Also published as: CO4410409A1; NO974785L; KR19990007809A; CA2218254A1; NO974785D0; AU5431996A; JPH11503786A; EP0821706A1; BR9608163A; MX9707969A; CN1181765A

Abstract

A process for preparing a liquid, storage-stable composition containing an organic polyisocyanate and an acid phosphate is disclosed. The process includes the steps of (1) heating a mixture of a polyisocyanate and an acid phosphate at a temperature in the range of from 60 °C to 190 °C for a time such that no phase separation occurs upon cooling the reaction mixture to ambient temperature, the mixture containing dissolved gases; and (2) subjecting the mixture to a negative pressure sufficient to remove at least a portion of the dissolved gases therefrom. The negative pressurization significantly slows the rate of carbon dioxide evolution thereafter and therefore enables much longer term storage-stability than is obtainable when the negative pressurization step is not performed, beacause it reduces problems associated with pressurization of containers.

Description

PROCESS FOR PREPARING A STORAGE-STABLE MIXTURE OF POLYISOCYANATE AND PHOSPHATE

This invention relates to the field of polyisocyanates and, more particularly, to the field of storage-stable polyisocyanate/phosphate mixtures.

It is known that mixtures of polyisocyanates and phosphates are useful for various purposes. For example, such can be used as binder resins having releasing properties for wood composite materials. These polyisocyanate compositions are particularly useful as binders in the preparation of particle boards and similar materials. See, for example, U.S. Patents 3,428,592; 3,440,189; 3,557,263; 3,636,199; 3,870,665; 3,919,017; and 3,930,1 10. While some of these mixtures can be made relatively storage-stable in that they do not deposit solids or separate into two liquid phases during relatively short-term storage (see, for example, U.S Patent 4,258,169, Re. 31,703, and JP 618469), they unfortunately still tend to produce carbon dioxide over time which can present problems during longer term storage The carbon dioxide production may be handled via use of release valves for large scale shipment, as for example via railcars, but the problem is more difficult when shipment is on a small scale, such as in drums or smaller containers, which generally are not fitted with relief valves, and which therefore may burst due to the pressurization.

The present invention solves the problem of pressurization of containers. It is an improvement in a process for producing a storage-stable mixture of a polyisocyanate and an acid phosphate, wherein dissolved gases are present, comprising subjecting the mixture to a negative pressure for a time sufficient to remove at least a portion of the dissolved gases therefrom. Preferably the amount of dissolved gases is reduced by at least 10 percent by weight. It has been found that this evacuation, which is preferably carried out at a vacuum pressure of less than 50 mm Hg (2 inches Hg), for a time sufficient to remove at least 90 percent of the dissolved gases in the mixture, more preferably at least 99 percent, results in a mixture which, thereafter, shows markedly decreased gas production. The invention therefore reduces or eliminates problems associated with pressurization of smaller containers used to ship or store the mixture. The polyisocyanate/phosphorus-containing compound mixtures which are treated in the present invention can be prepared from a variety of materials and via a variety of routes. For example, in one embodiment of the present invention the starting material includes one or more phosphorus-containing compounds formed in situ, which function as a release agent when the material is used in formulations which contact certain types of metal surfaces. It is prepared starting with a polyisocyanate and an acid phosphate. This is preferably a mixture of an organic polyisocyanate and from 1 to 20 parts, per 100 parts by weight of the polyisocyanate, of an acid phosphate selected from the group consisting of acid phosphates having the formula:

X X

T T

RX-P-OH and (RX) ₂P-OH

and mixtures of two or more of the acid phosphates, wherein each R is independently selected from the group consisting of alkyl having at least 3 carbon atoms, alkenyl having at least 3 carbon atoms, aryl, aryl substituted by at least one alkyl group, alkyl substituted by at least one acyloxy group, wherein the acyl group is the residue of an aliphatic monocarboxylic acid having at least 2 carbon atoms, and

Rl- ( 0-CH-CHT^

A B

wherein Ri is selected from the group consisting of alkyl, aryl, and aryl substituted by at least one alkyl, one of A and B represents hydrogen and the other is selected from the group consisting of hydrogen, methyl, chloromethyl and 2,2,2-trichloroethyl; X is chalcogen selected from the group consisting of oxygen and sulfur; and m is a number having an average value of 1 to 25.

In defining the starting materials above, the term "alkyl having at least 3 carbon atoms" means a saturated monovalent aliphatic radical, straight chain or branched chain, which has the stated minimum number of carbon atoms in the molecule. Illustrative of such groups are propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, and tridecyl, as well as those having many more carbon atoms such astriacontyl, pentatriacontyl, including isomeric forms thereof. The term "alkyl" when used without the above carbon atom limitation is also inclusive of methyl and ethyl.

The term "alkenyl having at least 3 carbon atoms" means a monovalent straight or branched chain aliphatic radical containing at least one double bond, and having the stated minimum number of carbon atoms in the molecule. Illustrative of such groups are allyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecenyl, heptadecenyl, octadecenyl, nonadecenyl, eicosenyl, heneicosenyl, docosenyl, tricosenyl, pentacosenyl. triacontenyl, pentatria-contenyl, including isomeric forms thereof.

The term "aryl" means the monovalent radical obtained by removing one nuclear hydrogen atom from an aromatic hydrocarbon. Illustrative of aryl are phenyl, naphthyl, biphenyl, triphenyl. The term "aryl substituted by at least one alkyl" means an aryl radical, as above defined, carrying at least one alkyl (as above defined) substituent. Illustrative of such are tolyl, xylyl, butylphenyl, octylphenyl, nonylphenyl, decylphenyl, decyltolyl, octadecylphenyl.

The term "aliphatic monocarboxylic acid having at least 2 carbon atoms" is inclusive of any alkanoic or alkenoic acid having the stated minimum number of carbon atoms. Illustrative of such acids are acetic, propionic, butyric, hexanoic, octanoic, lauric, stearic, oleic, undecylenic, dodecylenic, isocrotonic, palmitic.

Each of the groups R and Ri in the formulae (I) and (II) set forth above can optionally be substituted by one or more inert substituents, i.e., substituents which do not contain active hydrogen atoms and which are therefore unreactive in the presence of the ° polyisocyanate. Illustrative of such inert substituents are alkoxy, dialkylmercapto, alkenyloxy, dialkenylmercapto, chloro, bromo, iodo, fluoro, cyano.

The acid phosphates of the formulae (I) and (II) are, for the most part, well-known in the art, and can be prepared by methods well-known in the art. For example, the acid phosphates (I) and (II) are obtained by reaction of the corresponding alcohol or thiol R-XH, ⁵ wherein R and X are as hereinbefore defined, with phosphorus pentoxide or phosphorus oxysulfide using the procedures described by Kosolapof, Organophosphorus Compounds, John Wiley and Sons, Inc (New York, 1950), pp. 220-221. This reaction gives rise to a mixture of the mono- and di-acid phosphates, which mixture can be separated, if desired, for example by fractional crystallization of the barium and like salts as described in the above cited reference. 0 The individual acid phosphates in the mixture of the mono- and di-acid phosphates obtained in accordance with the above reaction can be employed as starting materials in the process of the invention.

It is possible to convert acid phosphates, having the formula (I) and (II) above, to a product containing the corresponding pyrophosphates and their sulfur-containing analogues, ⁵ such as thiophosphates and pyrophosphothiolates, by heating the acid phosphates in the presence of an organic polyisocyanate, with or without any other reactant such as phosphorus oxychloride, phosgene and the like. If the conditions of heating are carefully controlled as described herein, the resulting product is a homogeneous liquid which can be stored for prolonged periods without any tendency to undergo phase separation. 0 The fact that there are any conditions at all which could lead to the formation of a phase-stable product was, prior to disclosure in, for example, U.S. 4,258,169, Re. 31,703 and JP 618469, surprising to those skilled in the art. It had previously been expected that the reaction of an acid phosphate of the type shown in formulae (I) or (II) with an organic isocyanate would proceed in accordance with the following equation in which R has the significance defined 5 above and R' represents the residue of the organic isocyanate, which latter is shown as monomeric forthe sake of simplicity: O 0 0 t T T ( RO) ₂-P-OH + R ' NCO— ► (R0) ₂-P-0-P (0R) ₂ + C0₂ + R ' NH₂

( III ) ( IV)

The reaction would be expected to result in the formation of the desired pyrophosphate in association with some polyphosphate (if a mono-ester is present). The reaction also would be expected to give rise to the intermediate formation of the amine (IV) corresponding to the starting isocyanate. The amine (I ) would be expected to react immediately with additional isocyanate to form a urea. If a polyisocyanate were used a poly urea would be expected to form, and such should normally be insoluble in the reaction product and separate as a solid either immediately or on standing.

This is indeed what is found to happen when the acid-phosphate (I) or (II) is reacted with the organic polyisocyanate at any temperature below 60°C. However, if the reaction is carried out above that temperature but below 190°C, it was found that it is possible to obtain a product which, on cooling to ambient temperature (15-25°C) and maintaining thereat even for prolonged periods, does not deposit solid material.

However, the reaction temperature is not the only important factor. It is found that the time for which heating is carried out is important and, in general, the higher the reaction temperature, the shorter the period for which the heating can be carried out without consequences which are fatal to the improvement of phase stability in the reaction product. Illustratively, even when the reaction temperature is as low as 60°C, it is found that there is a limited time beyond which further heating causes transformation of the pyrophosphates into what are believed to be higher polyphosphates. When the proportion of the latter in the reaction product reaches a sufficiently high level it is found that on subsequent cooling of the reaction product, the polyphosphates separate generally as a liquid layer immiscible with the polyisocyanate. Further, the higher the reaction temperature, the shorter the period for which the reaction of acid phosphate and polyisocyanate can be allowed to continue without the onset of the above described transformation of the pyrophosphates.

The exact chemical composition of the products which separate, either when operating at a temperature less than the minimum set forth above or when heating for a period longer than that which will give rise to a homogeneous liquid product, is not known precisely and is not believed to be important to an understanding of the invention. The above discussion has been offered by way of explanation only and it is to be understood that the scope and import of the invention is not to be limited in any manner whatsoever by reason of the tentative identification of the by-products set forth above.

The time and the temperature for which the process described hereinabove can be carried out can vary according to the particular acid phosphate and polyisocyanate, as well as the concentration of the acid phosphate, which are employed. The appropriate time in any given instance can be determined readily by a process of trial and error. In general, the reaction times which can be employed vary from several hours at 60°C down to a minute or less at the higher end of the temperature range (190^βQ. As set forth above, the higher the temperature employed, the shorter the reaction time which can be employed without deleterious results.

The manner in which the acid phosphate and the organic polyisocyanate are brought together can also, in certain cases, affect the ability to produce a phase-stable composition in accordance with the invention. It is possible in many instances to bring the two reactants together, in any conventional manner, at ambient temperature, and then to heat the resulting mixture at a temperature within the range set forth for a time which has been determined to give the desired result at the particular reaction temperature chosen. However, it is preferred to preheat the polyisocyanate to the selected reaction temperature and then to add the acid phosphate to the preheated polyisocyanate. When operating a batch type procedure, the addition can be carried out in a single charge or can be carried out slowly over a period of time.

The process of the invention can also be carried out in a continuous manner in which the mixture of polyisocyanate (preferably preheated) and acid phosphate is passed through a heating zone maintained at a temperature within the range set forth above. The rate of flow of mixture through the heating zone is adjusted so that the residence time in the mixing zone corresponds to the selected reaction time. A wide variety of conventional equipment can be employed for this purpose. A particularly useful apparatus is that of the type in which the mixture to be heated is spread in the form of a thin film over the walls of the heating vessel. Atypical example of such apparatus is that set forth in U.S. Pat. No. 2,927,634. In another embodiment the polyisocyanate (preferably preheated) and the acid phosphate are charged continuously, in the appropriate proportions, to a stirred reactor in which the reactants are maintained at the desired temperature. The reaction mixture is withdrawn from the reactor at the same rate as the fresh reactants are added and the rate of addition and withdrawal are such that the residence time of the mixture in the reactor corresponds to the

I selected reaction time. During preparation of the mixture, carbon dioxide is continuously evolved.

Whether the process of the invention is carried out in a batch or continuous manner, it is desirable that the reaction be carried out in the absence of oxygen and moisture, i.e., in the presence of an inert gas such as nitrogen in accordance with the usual practice of handling polyisocyanates. Such may include, for example, a blanket or sparge. Thus, the final mixture is saturated with dissolved gases which include primarily carbon dioxide as evolved, and this is the case whether the reaction is carried out under a nitrogen blanket or under air. However, if a nitrogen sparge is used, the mixture will contain primarily nitrogen, since the nitrogen forces out the carbon dioxide being evolved by the reaction between the polyisocyanate and the acid phosphate. Minor proportions of other gases may also be present in some embodiments.

The proportions in which the polyisocyanate and the acid phosphates (I) and or (II) are employed in the process of the invention can vary over a wide range but advantageously the acid phosphate is employed in an amount corresponding from 1 to 20 parts by weight per 100 parts by weight of polyisocyanate. In a preferred embodiment the amount of acid phosphate employed is such that the polyisocyanate compositions produced in accordance with the invention contain from 0.1 , preferably from 3, to 15, preferably to 10, most preferably to 8, percent by weight of the acid phosphate. The polyisocyanate employed in the process of the invention can be any organic polyisocyanate which contains at least two isocyanate groups per molecule. Illustrative of organic polyisocyanates are diphenylmethane diisocyanate, m- and p-phenylene diisocyanates, chlorophenylene diisocyanate, α,α'-xylylene diisocyanate, 2,4- and 2,6-toluene diisocyanate and the mixtures of these latter two isomers which are available commercially, including triphenylmethane triisocyanates, 4,4-diisocyanatodiphenyl ether, and polymethylene polyphenyl polyisocyanates. The latter polyisocyanates are mixtures containing from 25 to 90 percent by weight of methylenebis(phenyl isocyanate) the remainder of the mixture being polymethylene polyphenyl polyisocyanates of functionality higher than 2.0. Such polyisocyanates and methods for their preparation are well-known in the art; see, for example, U.S. Patent Nos. 2,683,730; 2,950,263; 3,12,008 and 3,097,191.

These latter polyisocyanates are also available in various modified forms. One such form comprises a polymethylene polyphenyl polyisocyanate which has been subjected to heat treatment, generally at temperatures from 150°C to 300^βC until the viscosity (at 25°C) has been increased to a value within the range of 800 to 1500 centipoise (cps). Another modified polymethylene polyphenyl polyisocyanate is one which has been treated with minor amounts of an epoxide to reduce the acidity thereof in accordance with U.S. Patent No. 3,793,362. The polymethylene polyphenyl polyisocyanates can also be employed in the form of prepolymers and quasi-prepolymers, i.e., the products obtained by reacting the polyisocyanate with a minor amount of a polyol, as well as in the form of polyisocyanates which have been partially blocked by reaction with a monohydric alcohol using procedures well-known in the art. Carbodiimide- modified methane diphenyl diisocyanates can also be used.

The polymethylene polyphenyl polyisocyanates are the preferred polyisocyanates for use in the process of the invention. Particularly preferred polymethylene polyphenyl polyisocyanates are those which contain from 35 to 65 percent by weight of methylenebis(phenyl isocyanate). Also preferred are those polyisocyanates having viscosities of less than 500 centipoise (cps), more preferably less than 300 cps, and most preferably less than 200 cps. While any of the acid phosphates of formulae (I) and (II) can be employed in the process of the invention, those acid phosphates wherein R represents alkyl or alkenyl and X represents O, and more particularly, those acid phosphates wherein R represents alkyl or alkenyl having from 8 to 18 carbon atoms and X represents O, exhibit advantages because of ready availability and low cost. Particularly preferred are acid phosphates selected from the group consisting of Cβ to Cιβ alkyl esters of mono- and diester phosphates and mixtures thereof.

The above-described process, which is described in greater detail in U.S. Patent 4,258,169 and Re. 31,703, incorporated herein by reference in its entirety, results in liquid polyisocyanate compositions which are generally more storage-stable as to phase separation than polyisocyanate/phosphate mixtures which have not been subjected to this treatment, but which are still less storage-stable as to carbon dioxide generation than mixtures prepared by the process of the present invention. It is toward this latter aspect of storage stability that the present invention is particularly directed.

The increased stability as to carbon dioxide generation is effected particularly by the second step of the present invention, which involves subjecting the mixture to a negative pressure sufficient to remove at least a portion of the dissolved gases therefrom. In a preferred embodiment, the negative pressure is a vacuum sufficient to remove at least 10 percent of the dissolved gases, more preferably at least 95 percent, and most preferably at least 99 percent. In order to accomplish these higher levels of dissolved gas removal, it is preferred that the negative pressurization (evacuation) be to less than 50 mm Hg (2 inches Hg), and more preferably to less than 10 mm Hg (0.4 inch Hg). The effective time of the evacuation varies, depending on the negative pressure employed as well as the concentration of the acid phosphate in the polyisocyanate. However, preferably the negative pressurization is held for at least 15 minutes, and more preferably at least 30 minutes, and most preferably from 1 to 3 hours. In a preferred embodiment this most preferred period of evacuation is applied while maintaining a pressure reading of less than 10 mm Hg (0.4 inch Hg).

Equipment useful for the negative pressurization includes vacuum pumps, jets, aspirators or other conventional equipment.

The liquid, storage-stable polyisocyanate compositions prepared in accordance with the process of this invention are particularly useful as binder resins for use in the preparation of particle boards in accordance with methods well-known in the art, as described in the "Background" section references cited hereinabove. The compositions useful in the process of this invention possess the advantage of preventing adherence of the particle board to metal surfaces such as caul plates and press plate platens used in the preparation of such a board. For this particular use, i.e., as binder resins for particle board, it is desirable, but not essential, that the polyisocyanate compositions of the invention have a viscosity in the range of from 100 to 3000 centipoise (cps) to facilitate ease of handling in the equipment currently employed in the manufacture of particle board. Viscosities in the above range can be attained readily when employing polymethylene polyphenyl polyisocyanates having an initial viscosity of the order of 25 cps to 1000 cps and subjecting these polyisocyanates to the process of the invention. This represents an additional reason for employing such polyisocyanates in a preferred embodiment of the invention. Where the pol yi socyanate com positi ons of the i n venti on are to be em pi oyed as binder resins in the preparation of particle boards for example, the polyisocyanate composition can be applied to the particle board chips, prior to heating and pressing of the latter, in the form of an aqueous emulsion or dispersion. In order to facilitate the formation of these, it is desirable to employ an emulsifying or dispersing agent. If desired, the agent can be incorporated into the polyisocyanate compositions of the invention so as to enable the particle board manufacturer to prepare the required emulsion or dispersion without the need to employ additional agents. The agent can be any of those known in the art including anionic and nonionic emulsifying and dispersing agents. Among such agents are, for example, polyoxyethylene and polyoxypropylene alcohols and block copolymers of two or more of ethylene oxide, propylene oxide, butylene oxide, and styrene; alkoxylated alkylphenols such as nonylphenoxypoly-(ethyleneoxy)ethanols; alkoxylated aliphatic alcohols such as ethoxylated and propoxylated aliphatic alcohols containing from 4 to 18 carbon atoms; glycerides of saturated and unsaturated fatty acids such as stearic, oleic, and ricinoleic acids; polyoxyalkylene esters of fatty acids such as stearic, lauric, oleic and like acids; fatty acid amides such as the dialkanolamides of fatty acids such as stearic, lauric, oleic and similar acids; and sulfonates, sulfates, carboxyiates and sarcosinates, such as sodium dodecyl enzene sulfonate, calcium dodecyl benzene sulfonate, and sodium lauryl sulfate. A detailed account of such materials is found in Encyclopedia of Chemical Technology, Second Edition. Vol 19, Interscience Publishers (New York 1969), pp. 531-554. As is known to those skilled in the art, however, the long-tern storage stability of the final composition may be compromised in some instances by the presence of certain salts.

Also optionally, a non-active hydrogen containing solvent or diluent can be employed. In a preferred embodiment of the present invention such non-active hydrogen containing materials include esterified polyols and monols. The following examples describe the manner and process of making and using the invention and set forth the best mode contemplated by the inventors of carrying out the invention. However, they are not intended to be, nor should they be construed to be, limiting in any way of the scope of the invention. Example 1

A reaction was carried out using as the acid phosphate a mixture of mono- and di- lauryl acid phosphate (TRYFAC* 5573, commercially available from Henkel Industries) and, as the polyisocyanate, a polymethylene polyphenyl polyisocyanate containing approximately 46.5 percent by weight of methylenebis(phenyl isocyanate) and having an isocyanate equivalent of 134.5 and a viscosity of 25°C of 173 cps (PAPI* 27, commercially available from The Dow

Chemical Company). The polyisocyanate starting amount was 900 pounds, and was charged to a stainless steel reaction vessel equipped with an agitator and nitrogen sparge. It was heated to 80°C with stirring and 32.6 pounds of TRYFAC* 5573 were added over a 5-10 minute interval to control foaming resulting from carbon dioxide evolution. Upon completion of the TRYFAC* addition, the reaction was stirred at 80°C for two hours. Foaming of the reaction mixture was observed to continue. At the end of the reaction the nitrogen flow was shut off, full vacuum was applied (0-50 mm Hg, 0-2 inches Hg) while the temperature was maintained at 80°C, and the product was then cooled to ambient temperature. The vacuum was then discontinued and the product was drummed off. It was stored in an oven at 43-49°C and pressurization recorded as shown in Table 1. The NCO content of the final product was 30.1 percent by weight.

Comparative Example A

A reaction was carried out to illustrate the difference in the rate of drum pressurization between the Example 1 sample and a sample prepared using a similar preparation method but without the step of being subjected to a negative pressure

("Comparative Example A Sample").

To prepare the Comparative Example A Sample, about 900 pounds of PAPI* 27 were heated with 3.5 percent by weight of TRYFAC* 5573 for two hours at 82^CC under nitrogen. No vacuum was applied to the product. The product was cooled to ambient temperature, then loaded into a heavy gauge drum (50 pounds per square inch (psi) rating) and subjected to routine shipping over a period of about 12 days. The drum was equipped with a pressure gauge. It was placed in an oven at 60°C for the pressurization study.

Pressurization was measured over a period of time and recorded in Table 1.

Comparative Example

Day from start Example 1 Sample

A Sample of study psi pressure psi pressure

0 0 0.0

2 — 2.8

6 0 3.5

8 — 4.3

10 — 5.3

14 0 7.5

20 0 —

37 0 —

69 3 —

97 4 —

132 6 —

160 9 —

199 —

^{1 1}

-indicates no data taken.

Claims

1. A process for preparing a liquid, storage-stable polyisocyanate composition comprising the steps of (1) heating a mixture of a polyisocyanate and an acid phosphate at a temperature in the range of from 60°C to 190°C for a time such that no phase separation occurs upon cooling the reaction mixture to ambient temperature, wherein the mixture contains dissolved gases; and (2) subjecting the mixture to a negative pressure sufficient to remove at least a portion of the dissolved gases therefrom.

2. The process of Claim 1 wherein the negative pressure measures less than 50 mm Hg.

3. The process of Clai m 2 wherei n at least 90 percent of the dissolved gases is removed.

4. The process of Claim 1 wherein the negative pressure measures less than 10 mm Hg.

5. The process of Claim 4 wherein at least 99 percent of the dissolved gases is removed.

6. The process of Claim 1 wherein the organic polyisocyanate is polymethylene polyphenyl polyisocyanate, and the acid phosphate is selected from the group consisting of Cg to Ci8 alkyl esters of mono- and diester phosphates and mixtures thereof.

7. The process of Claim 6 wherein the acid phosphate is selected from the group consisting of lauryl acid phosphate, tridecyl phosphate, decyl phosphate and mixtures thereof.

8. The process of Claim 6 wherein the acid phosphate is present in an amount from 3 to 10 percent by weight.

9. The process of Claim 1 wherein the polyisocyanate is a polyphenyl polymethylene polyisocyanate having a viscosity of less than 500 centipoise.

10. In a process for preparing a liquid, storage-stable polyisocyanate composition containing a release agent formed in situ, which includes heating at a temperature in the range of 60°C to 190°C a mixture of an organic polyisocyanate and from 1 to 20 parts, per 100 parts by weight of said polyisocyanate, of a compound of one of the following formulas:

X t

RX-P-OH and ( RX ) ₂P-OH

OH

( I ) ( II )

RI- ⁽O O--CCHH--CCBI TΪΓ

B

wherein R-_\ is selected from the group consisting of alkyl, aryl, and aryl substituted by at least one alkyl, one of A and B represents hydrogen and the other is selected from the group consisting of hydrogen, methyl, chloromethyl and 2,2,2-trichloroethyl; X is chalcogen selected from the group consisting of oxygen and sulfur; and m is a number having an average value of 1 to 25; the heating being carried out for a time such that no phase separation occurs upon cooling the reaction mixture to ambient temperature; the mixture containing dissolved gases; an improvement comprising a further step of subjecting the reaction mixture to a negative pressure for a time sufficient to remove at least a portion of the dissolved gases therefrom.