Basic Concept Q&A on Polyurethane(Part One)

1. What are the different types of commonly used isocyanates structurally classified as?

   Answer: Aliphatic: HDI; Cycloaliphatic: IPDI, HTDI, HMDI; Aromatic: TDI, MDI, PAPI, PPDI, NDI.

2. What are the various types of commonly used isocyanates? Provide their structural formulas.

   Answer: Toluene diisocyanate (TDI), Diphenylmethane-4,4'-diisocyanate (MDI), Polymeric diphenylmethane diisocyanate (PAPI), Liquid MDI, Hexamethylene diisocyanate (HDI).

3. What do TDI-100 and TDI-80 signify?

   Answer: TDI-100 refers to toluene diisocyanate composed entirely of the 2,4 structure; TDI-80 denotes a mixture comprising 80% of the 2,4 structure and 20% of the 2,6 structure.

4. What are the distinguishing characteristics of TDI and MDI in the synthesis of polyurethane materials?

   Answer: In terms of reactivity, 2,4-TDI exhibits several times higher reactivity than 2,6-TDI due to the 4-position NCO being relatively far from the 2-position NCO and methyl, resulting in minimal steric hindrance. In contrast, the reactivity of 2,6-TDI is significantly influenced by steric hindrance from neighboring methyl groups.

   Both NCO groups in MDI are relatively distant from each other and unsubstituted, thus displaying significant reactivity. Even if one NCO participates in a reaction, reducing the activity of the remaining NCO, overall reactivity remains high. Therefore, the reactivity of MDI-based polyurethane prepolymers is greater than that of TDI-based prepolymers.

5. Which types among HDI, IPDI, MDI, TDI, and NDI exhibit better resistance to yellowing?

   Answer: HDI (belonging to non-yellowing aliphatic isocyanates) and IPDI (polyurethane resins made from it exhibit excellent optical stability and chemical resistance, commonly used for manufacturing high-grade non-yellowing polyurethane resins).