Synthesis of MDI

Diphenylmethane diisocyanate (MDI) generally exists in three isomeric forms: 4,4’-MDI, 2,4’-MDI, and 2,2’-MDI, with 4,4’-MDI being the predominant form. In polyurethane foam plastics, the most common types are polymeric MDI and modified MDI.

The synthesis of MDI is carried out in two steps: the condensation of diphenylmethane diamine (MDA) and the phosgenation of diamine.

(1) Condensation of Diphenylmethane Diamine

The condensation reaction of aniline with formaldehyde produces two types of intermediates. The first type involves the direct condensation of primary amines on the aniline with formaldehyde, resulting in the formation of methylene. The most feasible method uses nitrobenzene as a raw material, which is prepared into alkoxycarbamate through alkoxy carbonylation, and further reacts to form MDI. The second type involves the condensation of hydrogen atoms on the benzene ring of aniline with formaldehyde to form methylene.

Depending on different reaction conditions such as catalyst type, nature, reaction temperature, and stirring conditions, the proportions of the above two reaction intermediates vary. Generally, the hydrogen atoms on the amino group are more reactive and tend to condense with formaldehyde at lower temperatures. Therefore, after the condensation reaction, the temperature needs to be raised for molecular rearrangement. Additionally, the diamine compounds synthesized with varying raw material ratios contain certain amounts of oligomers, such as trimers and tetramers.

To synthesize compounds with high diamine content, industrial processes mostly use Lewis acids as catalysts, commonly hydrochloric acid. The condensation temperature is controlled between 50-80°C, the rearrangement temperature between 90-150°C, the molar ratio of aniline to formaldehyde is maintained within the range of 2-3, and the mass ratio of aniline to water is adjusted to 1:6.

The batch preparation process of diamine involves reacting aniline with 25%-35% hydrochloric acid to first produce aniline hydrochloride solution, then slowly adding about 37% formaldehyde aqueous solution at 80°C for condensation reaction for 1-2 hours, followed by rearrangement at around 100°C for 1 hour. The product is neutralized with caustic soda solution to make the product slightly alkaline. The mixture is layered, washed with water until neutral, and the oily substance is distilled with steam or under reduced pressure to remove traces of water and residual aniline, yielding a mixture containing a certain amount of polyfunctional diamines.

The reaction between aniline and hydrochloric acid to form aniline hydrochloride is exothermic, with a reaction heat of ΔH = -49.4 kJ/mol HCl. The condensation reaction is also exothermic, with a reaction heat of ΔH = 155.7 kJ/mol formaldehyde.

The continuous production process of diamine is shown in following figure.

In the process, raw aniline and catalyst hydrochloric acid are proportionally introduced into mixer 1 to form aniline hydrochloride. Then, formaldehyde solution is added in mixer 2 at 60-80°C, and the mixture is pumped into the continuous condensation reactor 4, maintaining the temperature between 110-150°C, reaction time from 15 seconds to 5 minutes, and pressure at 70 kPa. The reaction mixture is fed into neutralization tank 6 through discharge valve 5, neutralized with caustic soda solution, and separated in separator 7 to remove the salt water solution. The crude product is further processed in tubular distiller 8 and separator 9 to remove trace water and aniline, resulting in crude MDA.

(2) Phosgenation of Diphenylmethane Diamine

During the synthesis of 4,4’-MDA, there are by-products such as 2,4’-MDA and some oligomers. The quantities of these by-products depend on the synthesis conditions and raw material ratios. Generally, when preparing 4,4’-MDI, diamine compounds are directly used for phosgenation without prior purification. After phosgenation, the products are vacuum distilled to obtain purified 4,4’-MDI.

If partial pure MDI and polyfunctional isocyanates are separated during vacuum distillation, this method is known as the co-production method.

The principle and process of phosgenation of amine compounds have been previously described, so only an example is provided here.

Phosgenation of MDA involves two steps: low-temperature reaction and high-temperature reaction. In the low-temperature reaction stage, the main reactions are:

Diamine (polyamine) reacts with phosgene to form carbamoyl chloride.

Diamine (or polyamine) reacts with hydrogen chloride to form diamine (polyamine) hydrochloride, an exothermic reaction.

The high-temperature reaction is endothermic.

For example, dissolving 200 parts of diamine compounds in 1200 parts of chlorobenzene, slowly adding this amine chlorobenzene solution to a phosgene chlorobenzene solution composed of 350 parts phosgene and 1900 parts chlorobenzene at 20-50°C, under stirring, forms a slurry. The slurry is gradually heated to 110°C within 2 hours, while adding phosgene until the solid in the reaction mixture disappears, requiring about 650 parts of phosgene in total. The remaining phosgene and HCl in the solution are removed by bubbling with N2 or methane. The crude MDI is obtained by vacuum distillation to remove solvents and low-boiling impurities like benzene isocyanate.

For refining pure MDI products, the crude product is further separated and purified using high-vacuum distillation. The boiling point of MDI at different vacuum levels is as follows:

• 160°C / 0.1 kPa
• 180°C / 0.36 kPa
• 190°C / 0.8 kPa
• 160°C / 1.07 kPa

Isocyanate is a thermosensitive compound, so the pot temperature during MDI distillation must not exceed 250°C to prevent polymerization or thermal decomposition, which would reduce the NCO content. Therefore, flash distillation equipment is suitable for separating MDI-PA-PI. Industrially used equipment includes high-vacuum thin-film evaporators such as rising film, falling film, rotary disk, and horizontal or vertical scraper thin-film evaporators.

For synthesizing fiber-grade pure MDI used in spandex, low acidity and hydrolyzed chloride content are required. Additionally, to ensure safety and increase yield, acidic substances, hydrolyzed chloride treatment agents (e.g., acetylacetone copper and iron oxide), and heat stabilizers (e.g., 2,6-di-tert-butyl-4-methylphenol) are added before high-vacuum distillation, each in about 0.1% proportion. Pure MDI can form dimers and other insolubles during storage, leading to darker color and reduced performance, affecting polyurethane product quality. Therefore, storage stabilizers such as triphenyl phosphate, toluenesulfonyl isocyanate, or carbamyl isocyanate compounds are added before final packaging.