1.Differences in the Catalytic Effects of Amines and Tin-Based Catalysts on Isocyanate Reactions
Answer: Tertiary amine catalysts are more efficient at catalyzing the reaction between isocyanates and water, while tin-based catalysts are more effective for reactions between isocyanates and hydroxyl groups.
2.Why Can Polyurethane Resins Be Considered Block Copolymers? What Are the Characteristics of Their Chain Structures?
Answer: Polyurethane resins consist of alternating hard and soft segments. The hard segments, formed through reactions involving isocyanates, chain extenders, and cross-linking agents, are characterized by high cohesive energy, larger spatial volume, and rigidity. The soft segments are derived from polyols with carbon-carbon backbones, offering flexibility and forming the elastic parts of the chain.
3.What Factors Influence the Properties of Polyurethane Materials?
Answer: Key factors include cohesive energy of functional groups, hydrogen bonding, crystallinity, cross-link density, molecular weight, and the proportion of hard and soft segments.
4.Sources of Soft and Hard Segments in the Polyurethane Main Chain
Answer: Soft segments originate from oligomeric polyols (e.g., polyester or polyether diols), while hard segments are derived from diisocyanates or their reactions with small-molecule chain extenders.
5.How Do Soft and Hard Segments Affect the Properties of Polyurethane?
Answer:
Soft Segments:
①Molecular Weight: For the same molecular weight of polyurethane, if the soft segment is polyester-based, tensile strength improves with increased molecular weight of the polyester diol. Conversely, for polyether-based soft segments, strength decreases with rising molecular weight, though elongation improves.
②Crystallinity: Contributes significantly to the crystallinity of linear polyurethane, which can enhance performance. However, crystallinity may reduce low-temperature flexibility and cause opacity in the material.
Hard Segments:
Hard segments influence the softening and melting points and high-temperature properties of the material. Polyurethanes made from aromatic isocyanates have rigid aromatic rings in the hard segments, leading to higher cohesive strength and material strength compared to aliphatic isocyanates. However, they are more prone to UV degradation and yellowing, which aliphatic polyurethanes resist.
6.Classification of Polyurethane Foams
Answer:
①Rigid and flexible foams
②High-density and low-density foams
③Polyester-based and polyether-based foams
④TDI-based and MDI-based foams
⑤Polyurethane foams and polyisocyanurate foams
⑥One-shot and prepolymer production methods
⑦Continuous and batch production processes
⑧Block and molded foams
