Preparation of Polyurethane Foams

As described in Sect. 4.2.1 foamed polyurethanes can be obtained especially through the so-called water-crosslinking of suitable polyol/diisocyanate combinations. Carbon dioxide liberated during the reaction partially causes the expansion of the foam. For a number of reasons additional foaming agents (in the form of low-boiling liquids or CO2) are added in practice, i.e., in order to vary the foam density and the pore size. 

The properties of polyurethanes crosslinked in this way depend on the starting materials (polyol and diisocyanate) as well as on the additives and on the intensity of the mixing process. 

Moreover, flexible foams are characterized by utilization of special emulsifiers in their synthesis yielding an open-cell architecture, whereas for rigid foams emulsifiers are chosen that create more closed-cell structures. As diisocyanate for both types, the commercially available mixture of 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate is especially suitable. If foam formation is to take place at room temperature, and especially when hydroxy compounds with secondary hydroxy groups are used [poly(propylene glycol)s], the presence of a catalyst is generally required (see Sect. 4.2.1). 

As previously mentioned, the homogeneity of foamed materials and the proportion of open and closed cells can be influenced by additives such as emulsifiers and stabilizers. Emulsifiers (e.g., sodium, potassium, or zinc salts of long-chain fatty acids) cause a uniform distribution of water in the reaction mixture, ensuring homogeneous foaming, while stabilizers (certain silicone oils) prevent a breakdown of the cell structure at the beginning of the reaction and also act as pore regulators. 

For laboratory experiments, it is sufficient to mix the industrial individual components with vigorous stirring. For the preparation of uniform foams with optimum properties, however, the proportions of polyhydroxy compound, diisocyanate, and additives must be very carefully balanced by using precision feeding and mixing techniques. 

---

Several polyethers are used as intermediates in the preparation of polyurethane foams whilst others such as polyethylene glycol are water soluble. 

Due to low hydrolytic and chemical resistance and to low melting point, aliphatic polyesters have long been considered to be limited to applications such as plasticizers or macromonomers for the preparation of polyurethane foams, coatings, or... 

Crosslinking with water is mainly used in the preparation of polyurethane foams (see Sect. 5.6). 

Toluene diisocyanate is commonly produced as a mixture of the 2,4- and 2,6-iso-niers, that is used as a monomer in the preparation of polyurethane foams, elastomers and coatings, as a cross-linking agent for nylon-6, and as a hardener in polyurethane adhesives and finishes. Polyurethane elastomers made from toluene diisocyanates are used in coated fabrics and clay-pipe seals. Polyurethane coatings made from toluene diisocyanates are used in floor finishes, wood finishes and sealers, and in coatings for aircraft, tank trucks, truck trailers and truck fleets (United States National Library of Medicine, 1997). 

The ingredients for the preparation of polyurethane foam are available from many companies, including the Perlon Corp., Lyons, IL, and American Latex Products, Hawthorne, CA. 

The two most important chemical reactions in the preparation of polyurethane foam are the reaction between isocyanate and hydroxyl compound to form a urethane linkage ... 

Methyl a-D-glucopyranoside is the only product of commercial promise to have thus far emerged from work with protic solvents it has utility in the preparation of polyurethane foams. Mehltretter and coworkers125,126 have described the application of mixtures of D-glucosides obtained by the acid-catalyzed reaction of ethylene glycol, 1,2-propanediol ( propylene glycol ), or glycerol with starch,... 

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. ... 

Hydrolytically stable organotin compounds are also being prepared with sulfonic acids. " These di-alkyltin and trialkyltin sulfonates can be used in combination with t -amines as catalysts in the preparation of polyurethane foams. 

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. 

OTHER COMMENTS used as an intermediate in the preparation of polyurethane foams, elastomeric fibers, and polyamides used in industry as a curing agent for epoxy resins and urethane elastomers, and as a corrosion inhibitor for iron useful in the determination of tungsten and sulfates has also been used in the preparation of azo dyes, as a rubber processing chemical, and as an epoxy resin hardening agent. 

---