Method of improving stability of aromatic polycarbodiimides

Abstract: A method of improving the stability of ar-p CDI formed using phosphorous oxide catalysts by deactivating the catalyst by addition to the ar-p CDI of at least one catalyst poison is disclosed.

Description: This invention relates to a method of improving the stability of aromatic polycarbodiimides. More particularly, this invention relates to a method of improving the stability of aromatic polycarbodiimides using catalyst poisons. Aromatic polycarbodiimides (hereinafter referred to as "ar-p CDI") are used as crosslinkers in a variety of applications, including coatings, adhesives, caulks, mastics. Generally, ar-pCDI are made by heating aromatic isocyanates in the presence of phosphorous oxides, usually phospholene oxides such as 3-methyl-1-phenyl-2-phospholene-1-oxide (hereinafter referred to as "MPPO"). The phosphorus oxides catalyze the reaction between 2 -NCO groups to make a carbodiimide (hereinafter referred to as "CDI") and CO 2 : 2 R-NCO → R-N=C=N-R + CO 2 The catalyst reacts with -NCO to form a phosphinimide and carbon dioxide: R'NCO + R 3 P=O → R 3 P=NR' + CO 2 that then reacts with another R'NCO to regenerate the catalyst and make CDI: R 3 P=NR' + R'NCO → R'N=C=NR' + R 3 P=O. After they are made, the ar-pCDI molecules begin to slowly build viscosity until they gel. While not wishing to be bound by theory, it is believed that the residual phosphorous oxide present in ar-pCDI solutions catalyze the reaction between the ar-pCDI and any compounds containing active hydrogens such as water, alcohols, amines, and other materials bearing active hydrogens that may contaminate the ar-pCDI solutions: N=C=N + ROH → N=C(OR)NH Over time, the accumulation of N=C(OR)NH causes the ar-pCDI to build viscosity and eventually gel. This instability limits the usefulness of ar-pCDI because the shelf life is too short, i.e., less than six months, to be commercially viable. Aliphatic pCDI do not suffer the same stability problems as ar-pCDI, but, inter alia, are too reactive to make good crosslinkers for water-borne coatings when compared with ar-pCDI. Thus, ar-pCDI would be preferred if the shelf-life stability problems could be solved. I have discovered that the stability or gel time of ar-p CDI is dependent on the level of phosphorous oxide used to make it and that remains in the ar-p CDI after manufacture; the more phosphorous oxide, the shorter the gel time. I have further discovered a way to deactivate these catalysts. Certain chemicals, hereinafter referred to as "catalyst poisons," may be added to the ar-pCDI after it is made that will react with the catalyst to deactivate it, thus greatly extending the storage stability or shelf-life of the ar-pCDI. These catalyst poisons are not novel. For example in US-A-5,202,358, US-A-4,014,935, and US-A-4,614,785, organic isocyanates containing carbodiimides and/or uretone imine groups are prepared by the partial polymerization of the -NCO groups using phosphorous oxide catalysts. When only partial conversion takes place, the carbodiimides react with further isocyanate groups to give uretone imine groups. To ensure only a partial polymerization of the -NCO groups, US-A-5,202,358 discloses the addition of silylated acid compounds of the formula X-[Si(CH 3 ) 3 ] n to the reaction to terminate the formation of carbodiimide groups. To ensure only a partial polymerization of the -NCO groups, US-A-4,014,935 discloses that the phosphorous oxide catalyst is absorbed onto a substrate or deactivated by halides of hydrogen, tin, or phosphorus, or the oxyhalide of phosphorus or sulfur. To ensure only a partial polymerization of the -NCO groups, US-A-4,614,785 discloses that the phosphorus oxide catalyst is deactivated with sulfonyl isocyanates. In each of the above-referenced patents, the starting material is an isocyanate catalyzed by a phosphorous oxide. Only a partial reaction is desired so that the final product is a mixture. However, the presence of the residual phosphorous oxide leads to an undesired, complete reaction. The deactivation of the catalyst, either by adsorption onto a substrate or by the addition of a catalyst poison, prevents complete conversion to the pCDI. The present invention utilizes the deactivation steps but their purpose is to prevent the residual phosphorous oxide in the completed reaction from gelling the system rather from preventing the completed reaction. Statement of the Invention The present invention is as set out in the accompanying claims. The invention is directed to a method of improving the stability of ar-pCDI formed using a phosphorous oxide catalyst including the step of deactivating the catalyst. Preparation of ar-pCDI using phosphorous oxide catalysts may be by conventional means such as those methods disclosed in US-A-2,853,473, US-A-2,941,966, US-A-2,941,983, US-A-4,487,964 and US-A-5,574,083. On completion of the preparation of ar-pCDI, the phosphorous oxide catalysts is deactivated by addition to the ar-pCDI of at least one catalyst poison. Deactivation may be carried out by addition to the ar-pCDI of at least one catalyst poison. The catalyst poisons useful in the method of invention are silylated acids of the formula X-[Si(CH 3 ) 3 ] n where X represents the neutral acid residue obtained by removal of the acidic hydrogen atoms from an n-basic acid having a pKa value of at most 3, other than a hydrohalic acid, and n is an integer of 1 to 3. Examples of suitable silylated acids of the formula X-[Si(CH 3 ) 3 ) n include silylated sulfonic acids, such as trifluoromethanesulfonic acid trimethyl silyl ester and methanesulfonic acid trimethylsilyl ester; and silylated esters of acids of phosphorous, such as phosphoric acid tris(trimethylsilyl ester) and phosphoric acid diethyl ester trimethylsilyl ester. The catalyst poison may be added to the ar-pCDI at a molar ratio of 0.1:1 to 30:1 (catalyst poison:phosphorus oxide catalyst), preferably, 0.3:1 to 5:1. The catalyst poison may be added neat or as a solution in a suitable solvent. Compositions containing ar-pCDI made by the method of the present invention are more stable to storage at room temperature, especially up to 6 months, and are more thermally stable at elevated temperatures, for example of up to 80-120°C than those compositions containing ar-pCDI made by conventional methods. Some embodiments of the present invention will now be described in detail in the following Examples. Examples Example 1 A 250 ml round bottom flask was outfitted with a magnetic stirrer, N 2 sparge tube, reflux condenser, and thermocouple. The flask was charged with 47.88 g tolylene diisocyanate (mixture of isomers; 80% 2,4-tolylene diisocyanate and 20% 2,6-tolylene diisocyanate), 32.2 g polyethylene glycol monomethyl ether (average molecular weight. = 350 g/mole), and 5.0 g propylene glycol methyl ether acetate. Flask was flushed with N 2 , then sparge rate set to 5 ml/min and flask heated to 80°C for 1 hour. 5.45 g of a 2.04% solution of MPPO in propylene glycol methyl ether acetate and 59.6 g of propylene glycol methyl ether acetate was added and the flask heated to 120°C for 3.25 hours. One hour into heating the flask at 120°C the N 2 sparge was increased to 20 ml/min. The ar-pCDI solution was allowed to cool to room temperature. An IR spectrum taken of the product showed no residual -NCO groups. The ar-pCDI solution was placed in a series of glass vials and catalyst poisons were added as indicated in Table 1. Vials A and B were then placed on a hot plate and heated for 15 minutes on the hot plate's "high" setting. The vials were observed for an increase in viscosity, and the time to gellation was recorded. These results are also given in Table 1. Vial ID Level of ar-pCDI solution (g) Catalyst poison Level of catalyst poison (g) Moles poison:moles MPPO Heated Days to gel A 20.54 chlorosulfonyl isocyanate 0.0229 2.1:1 Yes > 455 B 19.38 phosphoric acid tris(trimethyl-silyl ester) 0.0306 1.3:1 Yes > 455 C 20.40 chlorosulfonyl isocyanate 0.0184 1.7:1 No > 455 D 22.12 chlorosulfonyl isocyanate (33.7% solution in xylene) 0.0137 1.1:1 No > 455 E 21.39 phosphoric acid tris(trimethyl-silyl ester) 0.0326 1.3:1 No > 455 F 21.99 - - 0 No 173 G 10.01 chlorosulfonyl isocyanate (33.7% solution in xylene) 0.0019 0.35:1 No > 455 Those ar-pCDI to which a catalyst poison had been added (Vials A, B, C, D, E and G) showed significant improvement in time to gel compared to the ar-pCDI to which catalyst poison had not been added (Vial F).

Claims: A method of improving the stability of aromatic polycarbodiimide formed using at least one phospholene oxide catalyst: characterized in that , on completion of the preparation of the aromatic polycarbodiimide reaction, the residual phospholene oxide catalyst is deactivated by addition to the aromatic polycarbodiimide of at least one catalyst poison selected from the group consisting of silylated acids of the formula X-[Si(CH 3 ) 3 ] n , where X represents the neutral acid residue obtained by removal of the acidic hydrogen atoms from an n-basic acid having a pKa value of at most 3, other than a hydrohalic acid, and a is an integer of 1 to 3. The method of claim 1 wherein said phospholene oxide catalyst is 3-methyl-1-phenyl-2-phospholene-1-oxide. The method of claim 1 wherein said catalyst poison is added to said ar-pCDI at a molar ratio of 0.1:1 to 30:1 (catalyst poison: phospholene oxide catalyst). The method of claim 1 wherein said catalyst poison is a silylated ester of acids of phosphorous. The method of claim 1 wherein said catalyst poison is phosphoric acid tris(trimethyl-silyl ester).

Inventors: BROWN WARD THOMAS, BROWN, WARD THOMAS
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 2000
Main classification: C08G18
IPC codes: C08G18, C08K5, C08L75, C09K15