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Production of MDI

A simplified block diagram for production of MDI is shown in Figure 2.65. There are many similarities to production of TDI with the essential steps of (i) nitration, (ii) reduction, and (iii) phosgenation being the same. Structurally, the fusing of the aromatic rings by formaldehyde condensation particularly stands out as a difference and has a pronounced effect on eventual urethane properties and optimal applications [142]. The condensation chemistry also produces isocyanate oligomers. The oligomers of MDI actually have more commercial use than the monomer and make it very difficult to treat MDI as a single entity as generally is the case with TDI. Industrially, 1 kg of MDI is produced from 0.8 kg of aniline and 0.9 kg of phosgene producing 0.6 kg of waste HC1.

Innovations in MDI process technology center on incremental improvement in product recovery, reagent minimization, distillation versus crystallization of MDI monomer, plant debottlenecking, etc. Heterogeneous catalysis rather than homogeneous acid catalysis would remove the need for handling caustic for reaction neutralization as

Simplified block diagram for production of monomer, polymeric, and modlfed MDI.

FIGURE 2.65 Simplified block diagram for production of monomer, polymeric, and modlfed MDI.

Structures of 4,4' methylene bisphenyldiamine and the MDA polymer.

FIGURE 2.66 Structures of 4,4' methylene bisphenyldiamine and the MDA polymer.

well as product salt. Nonphosgene routes have been explored for production of MDI as have been for TDI [143]. As with TDI, there is no forecast of these routes finding economic viability. Potential future changes may be to develop plants that flexibly produce monomer MDI, removing the need for distillation steps and serving the higher value monomer markets. Alternatively, construction of plants that exclusively produce pMDI could be envisioned, again removing the need for product separations and maximizing assets to most economically serve the largest volume markets [37].

The diversity of MDI-based products and the means by which producers distinguish their respective products originate in the reaction of aniline with formaldehyde to form MDA and its oligomers (Fig. 2.66). Details of how this reaction is conducted are tightly controlled process variables and industrial secrets.

The mechanism of acid-catalyzed condensation of aniline and formaldehyde does not lead to a single product, but instead to a mixture of MDA positional isomers and oligomers. Control of the final product distribution depends on process variables such as aniline-to-formaldehyde ratios, reaction temperatures and times, and the acid catalyst employed. Figure 2.67 is a simplified pathway for aniline and formaldehyde to reach the final products [ 144,145]. The initial product of reaction is the A'-methylaniline, which loses water rapidly to form the Schiff base. The Schiff base reacts with adventitious aniline to form aminals—one being linear and the other being cyclic. The relative amounts of these forms depend on the ratio of aniline to formaldehyde. The undesired cyclic form is favored when the aniline-to-formaldehyde ratio is low, while the linear form is favored at aniline-to-formaldehyde ratios greater than 2:1.

As mentioned, there is a cyclic product (Fig. 2.68) that can be formed from the Schiff base that is in principle reversible but in practice is avoided by process control. Furthermore, the products and intermediates after formation of the Schiff base conversion to the aminals are not observed except under acidic conditions. The final step to MDA is not reversible, and the reaction with the carbonium ion intermediate is the basis for modern MDI technology and 75% of commercial volume. It should also be clear that the last step to form MDA is not exclusive to formation of the 4,4' products and that the 2,4 product can form and certain process parameters can hinder or enhance its formation.

Process control can influence the product distributions of these reactions. As alluded earlier, the aniline-to-formaldehyde ratio is an obvious variable. Ratios of aniline to formaldehyde are typically 2-5 with higher MDA monomer yields produced at the higher ratios and MDA oligomer produced at lower ratios [146]. Increasing the acid to aniline ratio is also observed to increase MDA monomer composition in the final product. Higher reaction temperatures result in increased oligomer formation as do increases in reaction time and water content (Table 2.11).

Mechanism for production of polymeric and monomelic MDA by reaction of aniline and formaldehyde. Product structure and distribution is largely determined in this step.

FIGURE 2.67 Mechanism for production of polymeric and monomelic MDA by reaction of aniline and formaldehyde. Product structure and distribution is largely determined in this step.

l,3,5tnphenylhexahydritnazine formed by tnmerization of the Schiff base intermediate in the production of MDA.

FIGURE 2.68 l,3,5tnphenylhexahydritnazine formed by tnmerization of the Schiff base intermediate in the production of MDA.

TABLE 2.11 Effect of process variable ratios on relative production of monomeric and polymeric methylene bisphenyldiamine

Process variable

Process direction

MDA

pMDA

Aniline/formaldehyde

Increase

Increase

Decrease

Acid/aniline

Increase

Increase

Decrease

Temperature

Increase

Decrease

Increase

Reaction time

Increase

Decrease

Increase

Water content

Increase

Decrease

Increase

Phosgenation of MDA and pMDA to form MDI and pMDI.

FIGURE 2.69 Phosgenation of MDA and pMDA to form MDI and pMDI.

The phosgenation of MDA and pMDA mixtures follows a similar pathway to that of TDI. A solution of the aromatic anilides is dissolved in a solvent such as monochlorobenzene and then mixed with liquid phosgene to form the carbamoyl chloride. The carbamoyl chlorides are then heated to drive off HC1, which is stripped along with the excess phosgene for recycle back to the production process (Fig. 2.69) [147]. The efficiency of amine conversion to isocyanate in the phosgenation reaction is quite high. About 0.8 kg of MDA produces about 1 kg of MDI (pMDI). The efficiency of the phosgene reaction does not preclude the formation of unwanted side products, some of which are well known and are

Side reaction in MDI production between MDA and MDI to form a urea.

FIGURE 2.70 Side reaction in MDI production between MDA and MDI to form a urea.

quantified in product specifications. One such reaction is the diffusion-limited reaction between amines and the freshly produced isocyanates to form urea functionality (Fig. 2.70). The by-product of Figure 2.70 is of course fully capable of further additional side reactions. Among the reactions the urea by-product can undergo is the phosgene-stimulated hydrolysis of urea to carbodiimide, which can then react with excess isocyanate to form the imino-uretidinedione, which in this context is referred to as "APA" (Fig. 2.71). The addition of another isocyanate to the 4-member ring creates a 6-member triazine ring, another side product of the reaction termed "AP6."

Parameters associated with phosgenation and control of the final products are given in Table 2.12 [148].

Industrially, a portion of pure 4,4' MDI is usually distilled from the pMDI mixture. Historically, the polyurethane industry has consumed as much of this 4,4' component as can be produced, and controlling production has had the effect of maintaining the price. The applications for 4,4' MDI monomer are elastomers, adhesives, and notably spandex fibers. Another reason for manufacturers to limit production is related to the limited shelf life of the pure 4,4' monomer [149]. In the solid state, the 4,4' monomer undergoes a facile dimerization reaction to form the uretidione (Fig. 2.72) [150]. This reaction is intrinsic to the reactivity of isocyanate functionality and the packing of 4,4' MDI in its crystal state [151]. The maximum in the conversion rate is unfortunately around ambient storage temperatures (Fig. 2.73). This warrants normal laboratory storage in freezer conditions. Industrially, 4,4' MDI is maintained in the molten state to facilitate pumping and minimize dimer formation. Regardless, the monomer does not have a long shelf life and must be used in a relatively short time after production.

Formation of APA—a common impurity in pMDI, usually quantified in a producer's certificate of analysis.

FIGURE 2.71 Formation of APA—a common impurity in pMDI, usually quantified in a producer's certificate of analysis.

TABLE 2.12 Effect of process variable ratios on the production of desired MDI and PMDI (the "process") and reactions resulting from numerous potential side reactions

Process parameter

Process direction

Side reactions

Notes

Phosgene/pMDA ratio

Increase

Decrease

Ratio usually 6-8

Concentration pMDA

Decrease

Decrease

-30-35%

in solvent

Residence time in

Increase

Increase

reactor

Reactor temperatures

Increase

Increase

-200°C

Formation of uretidione, a common undesrred side reaction resulting from dimenzation of the isocyanate reaction. The reversion from the dimer to the monomers can occur at temperatures over 100 °C.

FIGURE 2.72 Formation of uretidione, a common undesrred side reaction resulting from dimenzation of the isocyanate reaction. The reversion from the dimer to the monomers can occur at temperatures over 100 °C.

 
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