Diisocyanates isomer ratio

The reactions of several diisocyanates with a large excess of 2-ethyl-hexanol have been reported by Bailey et al. [122]. Reactions were run in benzene with the extent of reaction being followed by loss of infrared absorption at 4.4 pm, characteristic for the isocyanate group. A sharp decrease in the rate of reaction of 2,4-tolylene diisocyanate and the 80 20 isomer ratio of tolylene diisocyanate at approximately 50% reaction was found. In contrast, 2,6-tolylene diisocyanate, 4,4 -diphenyImethane diisocyanate, p-phenylene diisocyanate and m-phenylene diisocyanate showed only a slight decrease in rate as the reaction proceeded. 

The isomer ratio in commercial toluene diisocyanate (that is, the proportion of the 2,4 and 2,6 isomers) affects its reactivity. This ratio may be determined by the infrared method described later. 

Infrared spectroscopy permits the determination of the isomer ratio by suitable IR absorption measurements on several commercially obtainable toluene diisocyanates. It is based on the quantitative measurement of absorption bands due to out-of-plane C—H deformation vibrations of the... 

An ingeniously simple screening method was used by Britain and Gemeinhardt [146] to evaluate catalysts for the isocyanate/hydroxyl reaction. To approximate as closely as possible actual polymerization conditions, the 80 20 ratio of 2,4- and 2,6-tolylene diisocyanate (80 20 TDI) isomers and a polyether triol of 3000 molecular weight were mixed at NCO OH ratio of 1.0. A 10% solution of catalyst in dry dioxane was added, the final catalyst concentration being 1% of the weight of polyether. The time for the mixture to gel at 70°C was noted as an indication of catalytic strength. This technique used the same reactants employed in one-shot flexible polyether-based foam systems, almost completely eliminated solvent, and was used to screen quickly hundreds of possible catalysts. 

The purification is done by fractional distillation. The selectivity to toluene diisocyanates is 97% (based on diamine). In the Mitsubishi process, the overall selectivity to diisocyanates is 81% (based on toluene). In addition to pure 2,4—toluene diisocyanates, two isomeric mixtures are available commercially, with ratios of 2,4— to 2,6—isomer of 80 20 and 65 35. 

Toluene diisocyanate used in flexible polyurethane foam is formed as a number of isomers. The most common TDI mixture is supplied as an 80 20 mix of the 2,4 isomer and the 2,6 isomer. Other ratios are commer-... 

Two types of diisocyanates are employed in polymer preparations aromatic and aliphatic. The most commonly used aromatic diisocyanates are toluene diisocyanate and 4,4 -diphenylmethane diisocyanate. Commercial toluene diisocyanate often comes as a mixture of 2,4 and 2,6 isomers in ratios of 80/20 or 65/35. When the reaction takes place at room temperature, the 4 position is 8 to 10 times more reactive than the 2 position. At elevated temperatures, however, this difference in reactivity decreases, and at 100 °C the reactivity of the isocyanate groups in both positions is approximately equal. 

Toluene diisocyanate has two common isomers the 2,4- and 2,6-TDI. The most commonly supplied TDI is a combination of 80% 2,4- and 20% 2,6-TDI. A 65/35 ratio of the 2,4-/2,6-TDI is also available, as well as a 100% 2,4-TDI product. The reactivity of the two isocyanates on TDI is different. Once the first isocyanate has reacted, the second is roughly eight times less reactive [21], TDI produces lower-viscosity prepolymers than does MDI. Pure 4,4 -MDI has a reactivity difference between the isocyanates of approximately 2/1 [22]. 

Chemistry Polyurethane is produced by the reaction of a polyol with an diisocyanate (or in some instances a polyisocyanate) in the presence of catalysts. The polyols of choice are poly(propylene glycol), block copolymers of ethylene oxide (10-15%) with propylene oxide, or the newer polymer polyols (based on polymers such as polystyrene or styrene-acrylonitrile copolymer). Polyester diols such as polycaprolactone diol can be used in place of the polyether polyol in this reaction. The isocyanate of choice is a mixture of the 2,4 and 2,6 isomers of tolylene di-isocyanate in the ratio of 80 20, generally referred to as 80 20TDI. Other isocyanates such as diphenylmethane di-isocyanate (MDI), hexamethylene di-isocyanate (HMDI), and isophorone di-isocyanate (IPDI) are also used. A tin-based or amine catalyst is used to promote the reaction. Given the wide choice of reactants available, the reaction can yield foams with a range of different mechanical and thermal characteristics. 

Isocyanates (Figure 3.3). Toluene diisocyanate (TDI) is a mixture of mostly 2,4- plus some 2,6-isomer. Two commercial ratios are 80/20 and 65/35. The 4- position is more reactive the 2- and 6- positions are sterically hindered. This gives the processor the ability to make prepolymers (oligomers) and run two-stage reactions. 

Toluene diisocyanates are prepared by direct nitration of toluene to give an 80 20 mixture of the 2,4- and 2,6-dinitro derivatives, followed by hydrogenation to the corresponding diaminotoluenes. Under conditions where the 2-nitrotoluene is first separated and further nitrated, a 65 35 mixture of the dinitro compounds and ultimately of the diamines results. The diamine mixtures are treated with phosgene at temperatures up to 140°C and the derived diisocyanate mixture is isolated and purified by distillation. The two (2,4 and 2,6) isomers differ considerably in reactivity (Table 1.1), so the actual ratio of the two components can be quite important. For many purposes the 80 20 mixture is preferred. The preparation route for the production of TDI is detailed in Fig. 1.7a. 

The precise composition of the mixture depends principally on the aniline to formaldehyde ratio used increasing amounts of aniline favour the formation of diaminodiphenylmethanes. Sometimes the amine mixture is fractionated to give pure 4,4 -diaminodiphenylmethane which is then phosgenated to form diphenylmethane 4,4 -diisocyanate. More commonly, the amine mixture is phosgenated without separation to give crude diphenylmethane diisocyanate which typically contains about 55% diphenylmethane diisocyanates (4,4 - and 2,4 -isomers), 25% triisocyanates and 20% higher polyisocyanates. 

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