Urethane Catalysis

The present invention relates to the preparation of polyurethane foams and is particularly concerned with the use of certain hydrolytically-stable catalysts in premix composition for such foams.

The preparation of cellular polyurethanes by reacting organic isocyanates with polyols containing reactive hydrogen atoms, as determined by the Zerewitinoff reaction, is well-known in the art. These reactions are conventionally carried out in the presence of tertiary amine catalysts or organo-tin catalysts or mixtures of these.

In known formulations particularly for the preparation of flexible polyurethane foams, water is included in the formulation for reaction with part of the isocyanate thereby releasing carbon dioxide gas which acts as a blowing agent. The blowing reaction is promoted by the presence of tertiary amine catalyst. A tin catalyst is also included in the formulation to promote the isocyanate-polyol reaction. Typical various organotin compounds employed or proposed for use as catalysts or co-catalysts in urethane-forming reactions are disclosed, for example, in U.S. Pat. Nos. 3,582,501; 3,836,488; 4,119,585. U.S. Pat. No. 3,392,128 discloses the use of dibutyltin sulfonamide and U.S. Pat. No. 3,980,579 discloses a number of dialkyltin thio carboxylates.

One of the problems encountered in the use of organotin compounds previously employed in polyurethane formulations, is that these organotin compounds hydrolyze in the presence of water with resulting loss in catalytic activity. The problem is particularly evident in commercial formulations wherein a pre-mix composition is prepared and stored, including all of the components except for the isocyanate which is later added to effect the desired polyurethane reaction. This problem has been recognized in U.S. Pat. Nos. 4,032,468 and 4,043,949 wherein the use of certain organotin compounds are proposed which are claimed to be stable against hydrolysis. These patents disclose triorganotin compounds such as methyl or methoxymethyl tin halides and oxides; and certain methyl, allyl, phenyl or benzyl tin compounds in the form of their halides, carboxylates, sulfides, mercaptides, dithiocarbamates or mercaptocarboxylates.

It has now been found, in accordance with the present invention that di- and trialkyl organotin sulfonates are hydrolytically stable and can be employed as catalysts in water-containing premix compositions for the preparation of polyurethane foams.

The organotin compounds employed as polyurethane catalysts in accordance with the invention correspond to the general formula

wherein R is an alkyl radical of 1 to 8 carbon atoms, R' is an alkyl, phenyl or alkyl phenyl radical of 1 to 8 carbon atoms and n is 1 or 2. Preferred compounds include:

dibutyltin bis-methanesulfonate,

dibutyltin bis-benzenesulfonate,

dimethyltin bis-benzenesulfonate,

dioctyltin bis-methanesulfonate,

dibutyltin bis-toluenesulfonate,

tributyltin toluenesulfonate

tributyltin methanesulfonate.

Compounds of the type corresponding to the above formula have been described in the literature but their hydrolytic stability has not been previously noted nor their utilization as catalysts for the urethane reaction previously disclosed.

These di- and tri alkyl tin sulfonates can be prepared in high yield (95%) by the reaction of a dialkyl or trialkyltin oxide with the appropriate organic sulfonic acid in a refluxing hydrocarbon solvent, the water of reaction being removed by azeotropic distillation. Methods for such preparation are described by P. G. Harrison et al in Journal of Organometallic Chemistry, 114 (1976) at pages 47-52.

Except for the particular organotin catalyst employed, the two-part polyurethane foam-forming compositions may be comprised of the usual components heretofore employed. Thus the premix (a) may contain the polyol, cell stabilizer, amine catalyst and organotin catalysts, and water, and (b) the polyisocyanate composition maintained separately for admixture therewith when the polyurethane reaction is to be carried out.

To test the stability of the proposed di- and tri-alkyltin sulfonates, samples of these were refluxed in a 10% water/ethanol solution for 18 hours and recovered quantitatively. Analysis of the refluxed product confirmed that no chemical change in the organotin had occurred. Certain of the samples have been kept in solutions for longer than a month with no evidence of hydrolysis. Other known organotin polyurethane catalysts in contrast to the di- and tri-alkyltin sulfonates, are slowly hydrolyzed in the presence of water. Further, these particular catalysts are stable in a premix containing quaternary amines of all types which are used in formulations to promote a delayed cream time but do not effect the overall cure time of the system.

To evaluate the activity of alkyltin sulfonates as co-catalysts with typical commercial tertiary amine catalysts a series of screening tests were performed. Flexible foams were prepared from formulations containing an alkyltin sulfonate of the invention and compared to similar formulations containing a typical commercial organotin catalyst (stannous octoate). The formulation comprised:

The results are reported in Table 1 below:

The data in Table 1 indicates that dibutyltin bis-methanesulfonate has catalytic activity comparable to the stannous octoate commonly employed in commercial polyurethane foam formulations. Even at 1/4 to 1/2 the amount of the organotin complex comparable activity is observed. These catalysts of the present invention have a considerably larger molecular weight; therefore, the inherent activity of the metal is even greater. It should be noted that these runs were performed by adding the organotin catalyst at the time of mixing before the addition of the isocyanate. Even during this short period the stannous octoate of the prior art systems may be partially deactivated by the water present in the formulation. The physical appearance of the experimental foams with respect to shrinkage, open cells and foam cell appearance were comparable to those obtained with the control catalyst compositions. At the low level of tertiary amine catalyst employed, it would be impossible to produce a foam if the organotin co-catalyst was inactive.

Employing the same formulation as in Example 1, dibutyltin bis-toluenesulfonate (DBTTS) was evaluated, using a solution of this catalyst in dimethylformamide. A portion of this catalyst solution was diluted with 10% water and aged at room temperature for three weeks. From the results reported in Table 2, it is seen that no apparent change in activity occurred in the aged sample. Addition of water to a stannous octoate solution, on the other hand, produces an immediate precipitate of dibutyltin oxide which is relatively inactive as a co-catalyst for urethane production.

The organotin catalysts of the invention find particular use in urethane foam applications where an amine salt or quaternary compound is used as the primary catalyst. These systems are generally employed to produce delayed initiation and gel but do not effect the overall rise time allowing the formulation to completely fill a mold before curing occurs. This example illustrates a practical high density rigid formulation for a premix composition employing an amine salt and organotin sulfonate. Its superior activity at a lower concentration is indicative of an improved delayed action system. The formulation comprised:

The results are reported in Table 3 below.

Although no practical formulations currently exist it is expected that these organotinsulfonates would be excellent catalysts when combind with quaternary amines. In these systems the proposed tin catalysts being stable to water, can be employed in water-containing premix compositions and the premix stored for reasonable time periods as required, without significant loss in catalyst activity. In the case of compositions employing the free tertiary amine catalyst, the organotin sulfonate will form an adduct with the amine on storage, with accompanying loss of activity. These organotin compounds of the invention can be used with free tertiary amine catalysts but only when the resulting mixtures of these are used shortly after being freshly prepared.

Generally neutral salts formed by reaction of a tertiary amine with a carboxylic acid or an anhydride can be employed in this invention. Among the preferred amine salts and quaternary ammonium type polyurethane catalysts suitable for inclusion in these formulations with the organotin sulfonate catalysts are:

(1) carboxylic acid salts of triethylenediamine

(2) carboxylic acid salts of N,N-dimethylpiperazine

(3) N,N,N-trimethyl-N-2-hydroxypropylammonium acetate or other carboxylic acid

(4) N,N,N-trimethyl-N-2-hydroxypropyl ammonium acetate or other carboxcyclic acid

Any of the usual polyisocyanates generally employed in the preparation of polyurethane foams may be used in the practice of the present invention. The polyisocyanate contains at least two isocyanate groups per molecule such as toluene diisocyanate (TDI). It is generally advantageous to employ an undistilled mixture of a technical grade of TDI. Any of the other conventionally employed polyisocyanates such as diisocyanatodiphenylmethane, condensation products providing a plurality of phenyl groups and a plurality of isocyanato groups, hexamethylenediisocyanate, chlorophenyldiisocyanate, bromophenyldiisocyanate, tetraisocyanatodiphenylmethane, and the like may be used.

Likewise, any of the polyols containing reactive hydrogen atoms generally employed in the production of polyurethane foams may be employed in the formulations of the present invention. The polyol contains at least two alkanol groups per molecule. Suitable examples include polyethylene glycol, polypropylene glycol, a linear polyester such as glycolterphthalate, glycolsuccinate, tetramethyleneglycol adipate or other hydroxy terminated linear ester. Also the polyol may be glycerol, a polyethylene ether derivative of glycerol, erythritol, pentaerythritol, manitol, sorbitol, a-methyl glucoside and sucrose. The polyol may be a poly(oxyalkylene)polyol derived from a polyamine such as ethylenediamine or a polyalkylene oxide derivative of a starch. Mixtures of the polyols are satisfactory.