What is isocyanate?
Isocyanate is the functional group with the formula R–N=C=O. It is widely used in “C.A.S.E” applications, i.e., coatings, adhesives, sealants, and elastomers. The most common forms of isocyanates used globally are diisocyanates in the forms of methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), where MDI’s use is 2-3 times higher than that of TDI. MDI/TDI (both aromatics) make up about 96% of the global isocyanate market volume. They react with polyol to form polyurethane, while reacting with polyamine to form polyurea. TDI introduces flexibility while MDI introduces hardness to applications.
The use of TDIs and MDIs in “C.A.S.E” applications reflecting major usage of MDI over TDI:
· Coatings: about 70% of these are MDI-based, the rest TDI-based;
· Adhesives: about 80% of these are MDI-based, the rest TDI-based;
· Sealants: largely MDI-based;
· Elastomers: almost purely MDI-based.
Manufacturing of diisocyanates
- First step is manufacturing of diaminodiphenylmethane (DADPM): reaction of formaldehyde with aniline in the presence of a hydrochloric acid catalyst.
- Second step is phosgenation of DADPM to crude MDI: also termed oligomeric MDI, with a hydrogen chloride by-product.
- Third step is purification of MDI: via fractional distillation, crystallization or sublimation to yield monomeric MDI and Polymeric MDI (PMDI) in separate streams. A typical polymeric MDI consists of approximately 50% MDI, 30% triisocyanate, 10% tetra-isocyanate, 5% penta-isocyanate and 5% higher homologue, with average functionality of 2.7.
- Fourth step is manufacturing of modified/prepolymer MDI: Purified monomeric MDI and/or PMDI can further self-react to form modified MDIs, often called MDI prepolymers or modified MDI.
Toluene is nitrated to dinitrotoluene (DNT) producing an isomer mix that is catalytically hydrogenated to crude toluene diamine (TDA). TDA is purified by fractional distillation to obtain mixes of only 2,4 and 2,6 isomers. The purified TDA is then phosgenated to isocyanates before another bout of distillation to generate a liquid TDI product, typically 65:35 and 80:20 at 2,4:2,6 isomer mixes.
Reactivity of diisocyanates
In both MDI and TDI, the isocyanate groups in the fourth position are the most reactive. In 2,4′-MDI, the reactivity of the group in the 4-position is about four times as great as the group in the 2-position. Therefore, overall, the order of reactivity of MDIs is: 4,4′-MDI > 2,4′-MDI > 2,2′-MDI. Similar to MDI, the 4-positions in TDI is also about four times as reactive as the 2- and 6-positions. Thus 2,4-TDI is more reactive than 2,6-TDI. Lower NCO-prepolymers have a higher viscosity, but give higher elasticity and slower reactivity. Higher NCO-prepolymers are lower in viscosity, but give lower elasticity and faster reactivity. Also, higher NCO-prepolymers will be used if higher surface hardness is needed. Apart from isocyanate reacting with polyol and polyamine, it is also reactive to water/humidity, therefore total quality control of manufacturing, storage, and transportation of isocyanate is of utmost important to ensure quality application for “C.A.S.E”.
Latest market news of isocyanates
Of late, there has been a persistent supply tightness when demand increases. Supply shortages are due to a couple of isocyante plants taken offline by major producers, Germany (BASF) and USA (Covestro, Dow Chemical). The isocyanate material market value will grow to $38,729 million by 2019 driven by increased usage of isocyanate in emerging Asia-Pacific economies.