Basic Concept Q&A on Polyurethane(Part Four)

1. The catalytic effects of amine and tin catalysts on the reaction of isocyanates differ.

   Answer: Tertiary amine catalysts exhibit higher catalytic efficiency in the reaction between isocyanates and water, while tin catalysts exhibit higher catalytic efficiency in the reaction between isocyanates and hydroxyl groups.

2. Why can polyurethane resin be regarded as a block copolymer, and what are the characteristics of its block structure?

   Answer: Polyurethane resin can be considered a block copolymer because it consists of hard and soft segments in its chain structure. Hard segments refer to the segments formed by the reaction of isocyanates, chain extenders, and crosslinkers on the main chain of polyurethane molecules. These groups have higher cohesive energy, larger spatial volume, and greater rigidity. Soft segments refer to the carbon-carbon main chain polymer polyols, which impart flexibility to the polyurethane main chain.

3. What factors influence the properties of polyurethane materials?

   Answer: Factors influencing the properties of polyurethane materials include cohesive energy of the groups, hydrogen bonding, crystallinity, degree of crosslinking, molecular weight, hard segments, and soft segments.

4. What raw materials contribute to the soft and hard segments on the molecular main chain of polyurethane materials?

   Answer: Soft segments are composed of oligomeric polyols (polyesters, polyether diols, etc.), while hard segments are composed of polyisocyanates or their reaction with small molecule chain extenders.

5. How do soft and hard segments affect the properties of polyurethane materials?

   Answer: Soft segments:

   (1) Molecular weight of soft segments: Assuming the molecular weight of polyurethane is the same, the strength of polyurethane increases with increasing molecular weight of polyester diols but decreases with increasing molecular weight of polyether diols, although the elongation at break increases.

   (2) Crystallinity of soft segments: Soft segments contribute significantly to the crystallinity of linear polyurethane segments. Generally, crystallinity is beneficial for improving the properties of polyurethane products, but sometimes it reduces the low-temperature flexibility of the material, and crystalline polymers are often opaque.

   Hard segments: Hard segments typically affect the softening melting temperature and high-temperature performance of the polymer. Polyurethanes prepared from aromatic isocyanates contain rigid aromatic rings in the hard segments, which increase the cohesive strength within the hard segments. These materials generally have higher strength than those made from aliphatic isocyanates but exhibit poorer resistance to UV degradation and are prone to yellowing. Aliphatic polyurethanes do not yellow.

6. Classification of polyurethane foam:

   Answer: (1) Rigid and flexible foam, (2) high-density and low-density foam, (3) polyester-based and polyether-based foam, (4) TDI-type and MDI-type foam, (5) polyurethane foam and polyisocyanurate foam, (6) one-step and prepolymer production methods, (7) continuous and batch production methods, (8) block foam and molded foam.