Organotin Catalysts for Isocyanate Reactions

Werner J. Blank d and Edward T. Hesselfi Independent Consultant, Wilton, CT, USA King Industries Inc., Norwalk, CT, USA 

For environmental and health reasons there is an interest in replacing organotin catalysts in certain applications with more benign catalysts. 

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The selection of a snitable catalyst for isocyanate reactions is more complicated than simply finding a compound that accelerates the reaction. Many formulations using organotin catalysts are two-component formulations that are mixed just before application. Depending on end-use, application characteristics of a formulation, such as potlife, and the impact of the catalyst on the final product have to be considered. More recently, environmental considerations have played a major role in the selection of a catalyst. 

Organotin compounds are effective catalysts for the isocyanate-hydroxyl reaction. Tin catalysts have a slight odour, and low amounts are required to achieve a high reaction rate. Examples of organotin catalysts are stannous octoate, stannous oleate, dibutyltin dilaurate and dibutyltin di-2-ethylhexoate. They are often used in conjunction with small concentrations of antioxidants such as tertiary-butyl catechol resorcinol and tartaric acid. 

Complexes of organotin with amidines have been found to be excellent catalysts for the preparation of polyurethane foams, which do not have the disadvantage of any amine odor and, in addition, delayed onset of the isocyanate-hydroxyl reaction An example of a mercapto-delayed organotin catalyst is 2,2,4,4-tetrakis(alkyl)- l,3,2,4-dithia-stannetane.55 Amine salts of amino acids, tertiary amino acids, and tertiary amino acid-nitrile compositions, have been found to be effective as delayed action catalysts for polyurethane synthesis. They are particularly effective when used in combination with an organometallic compound, such as an organotin. ... 

Di-n-butyltin catalysts are being used in the preparation of polyurethane foams. Most polyurethane foams utilize aromatic isocyanates such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) as the isocyanate, and a polyester or polyether polyols as the coreactant. Tertiary amine catalysts are used to accelerate the reaction with water and formation of the carbon dioxide blowing agent. To achieve a controlled rate of reaction with the polyol, an organotin catalyst can be used. Polyurethane foams are not only applied in place, but are also cast in a factory as slabstocks. These foam slabs are then cut for use in car seats, mattresses, or home furnishings. DBTDL is an excellent catalyst in high resiliency slabstock foams. DBTDL shows an excellent reaction profile for this application replacement for DBTDL in such an end-use is difficult and requires a substantial reformulation of the foam. 

For environmental reasons, organotin catalysts are being replaced in a nnmber of applications with more benign catalysts. The replacement of an organotin catalyst often reqnires a complete change of formnlation. Most alternate catalyst systems offer a different reaction profile. Table 6.2.10 shows a partial periodic table and the elements that, according to the literature and our own screening studies,i° are active catalysts for the isocyanate reaction. 

In chiral organotin dibromide- and bistriflate-catalyzed desymmetrization of 2-substituted 1,3-propandiols with phenyl isocyanate the enantiomeric excess of the product was uniquely dependent on the reaction temperature. The chirality of the product was inverted from one enantiomer to anofher upon changing the reaction temperature from 0 to -78 °C (Scheme 12.170) [308]. When, on the ofher hand, this nonracemic organotin dihalide was employed as a catalyst for benzoylation of racemic 1,2-diols, nonenzymatic kinetic resolution was achieved under sophisticated reaction conditions (Scheme 12.171) [309]. 

Chem. Descrip. Organotin Uses Catalyst for PU coatings, adhesives, sealants Features Provides delayed action catalysis of isocyanate/polyol reaction Properties Lt. yel. liq. sol. in alcohols, org. soivs., insol. in water sp.gr. 1.11 vise. 33 cps f.p. -23 C decomp. pt. 255 C flash pt. (PMCC) 130 C 17.5% total tin... 

Polyurethanes. The two major types of catalysts, for the polyol-polyisocyanate reaction to form polyurethanes, are tertiary amines and/or organotin compounds such as dibutyl tin dUaurate. For delayed reactions, the amines and/or the isocyanates can be temporarily blocked by adducts, which are removed and liberated during the cure reaction. 

Metal compounds, and especially organotin compounds, are much more efficient catalysts than the amines, especially for the hydroxyl/isocyanate reaction. This allows the polymer-forming polyol/isocyanate reaction to proceed at a sufficient rate to increase viscosity rapidly to a state where gas is effectively trapped, as well as to develop enough gel strength to present any cell structure from collapsing after gas evolution has ceased. 

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