Isocyanates polymerization

The polyisocyanates which can be used for preparing isocyanate-based foams are mainly aromatic compounds and some aliphatic or aralkyl polyisocyanates. TDI (toluene diisocyanate) is widely used for flexible foams. Pure MDI (diphenylmethane diisocyanate) is used for elastomers and coatings. Modified TDI and modified MDI are used for high-resilience flexible foams. Polymeric isocyanates (polymeric MDI or oligomeric MDI) are mostly used for preparing rigid urethane and isocyanurate foams, and in part, for preparing flexible and semi-flexible foams. 

Which polymer is formed depends upon the relative rates of subsequent reactions. If chain termination then occurs with loss of X, a cyclic dimer is produced if a third isocyanate molecule is added, followed by loss of X, a cyclic trimer occurs if chain termination is relatively slow, addition of further monomers takes place with formation of a linear polymer. Conditions such as temperature, catalyst concentration, and character contribute to the reaction pattern. The tendency to cyclize no doubt plays a specially large part in isocyanate polymerization. 

Figure 26 shows a comparison of bum-through time between the two foams based on polymeric isocyanate (polymeric MDI) and a TDI-prepolymer (65). In the case of polymeric isocyanate-based foams, a remarkable increase in bum-through time resulted after increasing the NCO/OH ratio, which reached a maximum value at 3.0. 

Weisfield, L.B. "The Estimation of Catalytic Parameters of Metal Acetylacetonates in Isocyanate Polymerization Reaction." Journal of Applied Polymer Science 1961, vol. 5, pp. 424-427. 

In those reactions where the isocyanate enters the cell wall and bulking takes place, the ASE values will be high. If, on the other hand, the isocyanate polymerizes in the lumen and no cell wall bulking takes place, there will be little, if any, ASE as a result of the treatment. Table III shows varying degrees of dimensional stability by reacting southern pine with ethyl, ri-propyl and ji-butyl isocyanate. Less dimensional stability is achieved with phenyl and p-tolyl isocyanates, and 1,6-diisocyanate hexane and none with tolylene-2,4-diisocyanate. 

A number of "crude" isocyanates (polymeric isocyanates), undistilled grades of MDI and TDI, are available in the market. Some of these products, such as various forms of crude MDI, have a functionality varying between 2 and 3. They have a lower reactivity and a lower vapor pressure than the corresponding pure isocyanate. They have found extensive use in one-shot rigid foams. However, they are also employed in coating, sealant, and adhesive applications. The Upjohn Chemical Division has published an excellent bulletin on the use and precautions in handling isocyanates, polyurethanes, and related materials (13A). 

Isocyanates [polymeric MDI (PMDI)] as a fortifier for phenolic resins have only been used in the past in rare cases. Deppe and Ernst [41] reported a precuring reaction between the isocyanate and the phenolic resin, even if both components had been applied separately to the particles. Hse et al. [36] also found good results with an isocyanate and a PF resin added separately to wood particles. Pizzi and Walton [191] reported on the reactions and their mechanisms of PF resins premixed in the glue mix with nonemulsifiable water-based diisocyanate adhesives for exterior plywood. Pizzi et al. [192] reported on the industrial applications of such systems (PF -I- PMDI -I- sometimes tannin accelerator UF + PMDI)... 

Synonyms Diphenylmethane-4,4 -diisocyanate, polymeric 4,4 -Diphen-ylmethane diisocyanate, polymeric Methylenebis (p-phenyl isocyanate), polymeric PMDI... 

Isocyanates polymerize through the carbon-to-nitrogen double bonds by anionic mechanism. Reactions can be catalyzed by sodium or potassium cyanide at-58 °C. N,N -dimethylformamide is a good solvent for this reaction. Other anionic catalysts, ranging from alkali salts of various carboxylic acids to sodium-naphthalene, are also effective. In addition, polymerizations can be carried out by cationic, thermal, and radiation-induced methods. 

Woolley and Fardell conducted comprehensive investigations of flexible and rigid polyurethane foams, including laboratory and full-scale experiments. It was established that a characteristic yellow smoke evolved from the polyurethanes at 200 to 300 °C, especially from the toluylene diisocyanate-based flexible foams. The yellow smoke comprised the total nitrogen content of the original polyurethane and amounted to about 25 per cent m/m of the foam. It was supposed that the thermally released isocyanate polymerized immediately in some way to form a particulate material (the solid part of the yellow smoke). Its apparent decomposition temperature was about 800 C. Mass spectrometry of its decomposition products detected hydrogen cyanide, acetonitrile, pyridine, and benzonitrile. The proportion of HCN increased with temperature. 

Tri-n-butyl phosphine n. (C4H9)3P. A curing agent for epoxy resins, and a catalyst for vinyl and isocyanate polymerizations. 

Monomeric para isocyanate Monomeric ortho isocyanate Polymeric para isocyanate Polymeric ortho isocyanate... 

To explain the adhesion of isocyanate-based adhesives to impervious, apparently unreactive surfaces such as glass and metals, it is considered that isocyanate groups react with the molecular film of tightly absorbed water always present on such surfaces, the reaction of the isocyanate groups with the oxyhydrate layer on metal surfaces and isocyanate polymerization on (alkaline) glass. All of these processes provide intimate contact if not actual chemical bonds between adhesive and adherend. 

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