Basic Concept Q&A on Polyurethane(Part Three)

1. How does the structure of isocyanate esters affect the reactivity of the NCO group?

   Answer:

   A. Electronegativity of the R group: If the R group is an electron-withdrawing group, the electron cloud density around the C atom in the -NCO group is lowered, making it more susceptible to nucleophilic attack, i.e., more prone to nucleophilic reactions with compounds such as alcohols and amines. If R is an electron-donating group, through electron cloud transfer, the electron cloud density around the C atom in the -NCO group increases, making it less susceptible to nucleophilic attack, resulting in decreased reactivity with compounds containing active hydrogen.

   B. Inductive effect: Aromatic diisocyanates contain two NCO groups. When the first -NCO group participates in a reaction, the unreacted -NCO group acts as an electron-withdrawing group due to the conjugation effect of the aromatic ring, enhancing the reactivity of the first NCO group. This effect is called the inductive effect.

   C. Steric hindrance effect: In aromatic diisocyanates, if both -NCO groups are on the same aromatic ring, the influence of one NCO group on the reactivity of the other NCO group is often significant. However, when the two NCO groups are on different aromatic rings within the same molecule, or when they are separated by alkyl chains or aromatic rings, their mutual influence is minimal, and it decreases with increasing alkyl chain length or number of aromatic rings.

2. Types of compounds containing active hydrogen and their reactivity with NCO:

   Answer: Aliphatic NH2 > Aromatic NH2 > Primary OH > Water > Secondary OH > Phenolic OH > Carboxylate > Substituted urea > Amide > Aminoformate. (If the electron cloud density of the nucleophilic center is higher, its electronegativity is stronger, resulting in higher reactivity with isocyanates and faster reaction rates; conversely, reactivity is lower.)

3. The influence of hydroxyl compounds on their reactivity with isocyanate esters:

   Answer: The reactivity of compounds containing active hydrogen (ROH or RNH2) is related to the nature of R. When R is an electron-withdrawing group (low electronegativity), the transfer of hydrogen atoms is difficult, making the reaction of active hydrogen compounds with NCO relatively difficult; if R is an electron-donating substituent, it can enhance the reactivity of active hydrogen compounds with NCO.

4. What is the purpose of the reaction between isocyanate esters and water?

   Answer: It is one of the fundamental reactions for preparing polyurethane foam plastics. The reaction between them first forms unstable carbamic acid, which then decomposes into CO2 and amine. If there is an excess of isocyanate ester, the generated amine will react with the isocyanate ester to form urea.

5. When preparing polyurethane elastomers, the moisture content of polyol should be strictly controlled.

    Answer: For elastomers, coatings, and fibers where the presence of bubbles is not desired, the moisture content in the raw materials must be strictly controlled, typically requiring it to be below 0.05%.