Basic Concept Q&A on Polyurethane(Part Five)

1. Why can polyurethane resin be considered as a block copolymer, and what are the characteristics of the block structure?

   Answer: Polyurethane resin can be considered as a block copolymer because its chain segments consist of both hard and soft segments. Hard segments refer to the segments formed by the reaction of isocyanates, chain extenders, and cross-linking agents on the main chain of polyurethane molecules. These groups have higher cohesive energy, larger spatial volume, and greater rigidity. Soft segments refer to the polyol segments of the carbon-carbon main chain polymer, which are flexible segments within the main chain of polyurethane.

2. What are the factors affecting the properties of polyurethane materials?

   Answer: The factors affecting the properties of polyurethane materials include cohesive energy of groups, hydrogen bonds, crystallinity, degree of cross-linking, molecular weight, hard segments, and soft segments.

3. What raw materials do the soft and hard segments on the main chain of polyurethane materials come from?

   Answer: The soft segments are composed of oligomeric polyols (such as polyester, polyether diols, etc.), and the hard segments are composed of polyisocyanates or their reaction with small molecule chain extenders.

4. How do the 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 the increase of molecular weight of polyester diols; if the soft segments are polyether, the strength of polyurethane decreases with the increase of molecular weight of polyether diols, but the elongation at break increases. (2) Crystallinity of soft segments: Contributes significantly to the crystallinity of linear polyurethane segments. Generally, crystallinity is advantageous for improving the performance of polyurethane products, but sometimes crystallinity may reduce the low-temperature flexibility of materials, and crystalline polymers are often opaque.

   Hard segments: Hard segments usually affect the softening melting temperature and high-temperature performance of polymers. Polyurethanes prepared from aromatic isocyanates have increased cohesive strength due to the presence of rigid aromatic rings in the hard segments, resulting in generally higher material strength compared to aliphatic isocyanate-based polyurethanes, but they have poor resistance to UV degradation and are prone to yellowing. Aliphatic polyurethanes do not yellow.

5. Classification of polyurethane foam

   Answer: (1) Rigid and flexible foam, (2) high-density and low-density foam, (3) polyester-type and polyether-type 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.

6. Basic reactions for foam preparation

   Answer: Refers to the reaction of -NCO with -OH, -NH2, and H2O. When reacting with polyols, the "gel reaction" during foaming generally refers to the formation reaction of urethane. Because multifunctional raw materials are used in foam raw materials, a cross-linked network is obtained, allowing the foaming system to gel rapidly.

In a foaming system with water present, foaming reactions occur. The term "foaming reaction" generally refers to the reaction where water reacts with isocyanates to form substituted ureas and release CO2.