I
GEORGE SHKAPENKO, G. T. GMITTER, and E. E. GRUBER Central Research Laboratories, The General Tire & Rubber Co., Akron
9, Ohio
Mechanism of the Water-Isocyanate Reaction Higher temperatures in Foaming and postcuring can probably minimize side reactions with greater stability possible in the, end product
THE
generally accepted theory for reactions of water with isocyanates, which are essential in urethane foam production, assumes that carbamic acid is formed initially. This intermediate then decomposes to yield carbon dioxide and a n amine that can further react with isocyanate to yield urea linkages. However. this simple picture is suspect because the kinetics for reactions of isocyanates with other active hydrogen compounds-alcohols and amines-are extremely complex. Therefore, the mechanism of this reaction was investigated, using water and o-tolyl isocyanate in a 0.02 to 0.01M ratio, respectively, at 80' C. In this compound, reactivity of the isocyanate group is closely related to the o-isocyanate groups in 2,4- and 2,6toluene diisocyanates which are generally used in the preparation of urethane foams. Experimental Because the external expression of all reactions occurring in the hydrolysis of the isocyanate group is a change in the isocyanate concentration and the evolution of carbon dioxide, these parameters were used to follow the course of hydrolysis. The isocyanate content was determined by treating the sample with a n excess of dibutylamine followed by back titration with acid (70). The equipment for determining the amount of carbon dioxide evolved (shown a t right) consists of reaction flask A of 200 ml. capacity which contained the isocyanate in anhydrous $-dioxane to which the water in anhydrous dioxane in flask B was added, after temperature equilibrium had been reached. These flasks were submerged in an oil bath thermostatically controlled to zt0.1' C. The gas buret was used to measure the increase in volume under atmospheric pressure. Samples for isocyanate determinations were obtained in another series of experiments through a sampling pipet used in place of the above gas buret. A positive
Literature Background Subject Determination of NCO content by volumetric titration Reaction of water with isocyanate yields N-substituted carbamic acids Carbamic acid anhydride decomposes to yield additional carbon dioxide Ammonium carbamate on heating decomposes to form water plus urea Synthesis of triethyl ammonium perchlorate Reaction rates of carbanilide with phenyl isocyanate Qualitative color test for free aromatic amines
Ref. (10) (1, 6,7 )
(1,5, 8 ) (9)
( 2 , 4) (6)
(4)
pressure of dry nitrogen was used while sampling. T h e reaction solution was agitated periodically by shaking during reaction and especially before any sampling or readings. The initial concentration of isocyanate in dioxane was generally maintained a t 0.2 mole per liter of solution, and 100 ml. of solution were employed. Triethylamine was used as catalyst in concentrations of 0.036 mole per liter
with the molar ratio of isocyanate to water at 1 to 0.5. Unless noted elsewhere, all of the experimental work was carried out at 80' C. This temperature was selected because it provided the best conditions for following the reaction on the experimental basis used. Urethane foams are usually prepared at room temperature, but the internal temperature rises to 80' C. in a few minutes; hence the temperature used is more realistic in determining the formation of the various intermediates in the early stages of foam preparation. The solubility of carbon dioxide in dioxane was found to be small enough a t room temperature so that it was neglected at
80" C. Discussion Theory and experiment show that the first product of the reaction of water with isocyanate is the N-substituted carbamic acid which decomposes to liberate carbon dioxide (7, 5, 7). A series of experiments was thereupon carried out to favor the formation of iV-o-tolyl carbamic acid through changes in water concentration (Figures 1 and 2). These figures
This drawing shows the apparatus for determining carbon dioxide evolution rates in the hydrolysis reaction of isocyanates
VOL. 52 NO. 7
JULY 1960
605
200
> y
/'/-
0.00
Ieo
8
le0
27
140
N C O : HzO
= I: I I
2
W
2
