Views: 276 Author: Vickey Publish Time: 2023-07-06 Origin: Site
An aromatic diisocyanate is methylene diphenyl diisocyanate (MDI). There are three common isomers: 2,2′-MDI, 2,4′-MDI, and 4,4′-MDI. These are distinguished by the locations of the isocyanate groups around the rings. The most common isomer, commonly known as 4,4′-diphenylmethane diisocyanate, is the 4,4′ isomer. Another name for this isomer is pure MDI. When making polyurethane, MDI interacts with polyols. It is the diisocyanate that is manufactured the most; in 2000, it accounted for 61.3% of the global market.
Over 7.5 million metric tons of MDI and polymeric MDI are produced worldwide each year (as of 2017). The International Isocyanate Institute, whose mission is to promote the safe management of MDI and TDI in the workplace, community, and environment, is comprised of all significant MDI manufacturers.
Using hydrochloric acid as a catalyst, aniline and formaldehyde react to create a mixture of precursors for both polyamines and diamines, which is the initial step in the creation of MDI. The isomer ratio is established by the diamine's isomeric composition, and these diamines are then treated with phosgene to create a mixture of isocyanates.
Both "phosgenations first" and "step-wise phosgenations" are potential response mechanisms for this transition. After distilling the mixture, polymeric MDI—a term for the mixture of oligomeric polyisocyanates—and a mixture of MDI isomers with little 2,4′ isomer content are produced. The MDI isomer combination must be fractionated for further purification.
The isocyanate groups' reactivity depends on where they are located. The two isocyanate groups exhibit very different reactivities in 2,4′-MDI than they do in 4,4′-MDI. Due to steric hindrance, the group at position 4 is roughly four times more reactive than the group at position 2.
The manufacturing of stiff polyurethane is the primary use of 4,4′-MDI. These firm polyurethane foams are utilized in almost all freezers, refrigerators, and buildings across the world because they are effective thermal insulators. Polyethylene adipate, a polyester, and poly(tetramethylene ether) glycol, a polyether, are typical examples of polyols in use. In addition to being utilized as a consumer-friendly, high-strength bottled glue preparation, 4,4′-MDI is also employed as an industrial-grade adhesive.
Polyurethanes are made using methylenediphenyl diisocyanate (MDI), which has a variety of uses, including the creation of hard polyurethane foams for refrigerators and house insulation. Customers who use insulation created with MDI can save energy.
Coatings, adhesives, sealants, and elastomers (CASE), which are found in products like paints, glues, and weatherproof materials, are some other ways that MDI is used in polyurethanes. Numerous styles of footwear, sporting goods, and, to a much lesser extent, certain specialized flexible foams are also made using it. MDI may also be used to make mold cores for the foundry sector and as a binder for wood.
Like the other isocyanates, MDI is a sensitizer and allergen. People who become sensitive to isocyanates may have life-threatening systemic effects from even very tiny doses, such as respiratory failure. Strict technical controls and personal protective equipment are needed while handling MDI. It has the ability to react aggressively with water and other nucleophiles.
