1.What are the main types of isocyanates based on their structure?
Answer:
Aliphatic: HDI
Cycloaliphatic: IPDI, HTDI, HMDI
Aromatic: TDI, MDI, PAPI, PPDI, NDI
2.What are the commonly used isocyanates? Write their structural formulas.
Answer:
Toluene diisocyanate (TDI)
Diphenylmethane-4,4′-diisocyanate (MDI)
Polymethylene polyphenyl isocyanate (PAPI)
Liquid MDI
Hexamethylene diisocyanate (HDI)
3.What do TDI-100 and TDI-80 represent?
Answer:
TDI-100: Composed entirely of 2,4-toluene diisocyanate.
TDI-80: A mixture containing 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate.
4.What are the characteristics of TDI and MDI in polyurethane synthesis?
Answer:
The reactivity of 2,4-TDI is significantly higher than that of 2,6-TDI. This is because the NCO group at the 4-position in 2,4-TDI is less sterically hindered, whereas in 2,6-TDI, the NCO groups are affected by steric hindrance from adjacent methyl groups.
MDI has two NCO groups spaced apart with no nearby substituents, resulting in high reactivity for both. Even if one NCO group reacts, the remaining group retains considerable activity, making MDI-based prepolymers more reactive than those derived from TDI.
5.Which isocyanates among HDI, IPDI, MDI, TDI, and NDI have better resistance to yellowing?
Answer:
HDI: Known for its excellent non-yellowing properties as an aliphatic diisocyanate.
IPDI: Produces polyurethane resins with superior optical stability and chemical resistance, commonly used in high-grade, non-yellowing polyurethane applications.
