Isocyanate is one of the raw materials used in the production of polyurethane (PU). Common isocyanates include TDI, MDI, and PAPI.
Toluene diisocyanate (TDI) has two isomers, namely 2,4-TDI and 2,6-TDI. Industrial products on the market are generally T-100, T-80, and T-65. The theoretical NCO content of TDI is 48.234%, with Dow Chemical’s industrial NCO content at 48.2%, and Bayer’s NCO content ≥48%. Additionally, TDI’s isomer ratio, acidity, color, and hydrolyzable chlorine are crucial indicators. According to the national military standard GJB2614-1996 “Toluene Diisocyanate,” the acidity (calculated as HCl) is ≤0.04%, color (APHA) is ≤50, and hydrolyzable chlorine is ≤0.01%.
T-100 refers to TDI with over 98% 2,4-TDI, suitable for synthesizing special prepolymers used in castable polyurethane elastomers and polyurethane coatings. T-80 is a mixture of 80% 2,4-TDI and 20% 2,6-TDI, mainly used in polyurethane flexible foams, elastomers, coatings, and adhesives, making it the most widely used and consumed product in the TDI series. T-65 is a mixture of 65% 2,4-TDI and 35% 2,6-TDI, primarily used in high-load and high-rebound flexible foams.
The reactivity of 2,4-TDI and 2,6-TDI differs significantly, with the reactivity of 2,4-TDI being several times that of 2,6-TDI. This is because the 4-position NCO group in 2,4-TDI is further from -CH3, resulting in almost no steric hindrance, while in 2,6-TDI, the two NCO groups are on either side of -CH3, leading to significant steric hindrance. Additionally, the reactivity of the 4-position NCO group in 2,4-TDI is greater than that of the 2-position, influenced by steric hindrance.
TDI is a colorless (pale yellow) transparent liquid that may yellow upon prolonged exposure to light. It has an irritating odor, and its solidification point depends on the ratio of 2,4-TDI to 2,6-TDI. The melting point is influenced by intermolecular forces, with the molecular structure of 2,6-TDI being more symmetrical, resulting in lower intermolecular forces compared to 2,4-TDI. The higher the proportion of 2,4-TDI, the higher the solidification point. The solidification point of TDI-100 is 20°C, TDI-80 is 13°C, and TDI-65 is 5°C. Therefore, in northern regions, TDI requires insulation to prevent solidification.
