What is Isocyanate?

PU binder

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.

Picture of Sally Zhang

Sally Zhang

CEO of Foshan Alforu Technology Co.,Ltd