Basic Concept Q&A on Polyurethane(Part Seven)

1. Nucleation Mechanism of Bubbles

Answer:Gas substances are generated by the reaction of raw materials in a liquid or by the temperature produced during the reaction, causing the gas to evaporate. As the reaction progresses and a significant amount of reaction heat is generated, the production and evaporation of gas substances continuously increase. When the gas concentration exceeds the saturation concentration, bubbles start to form in the solution phase and rise.

2. Role of Foam Stabilizers in the Preparation of Polyurethane Foam

Answer: Foam stabilizers have an emulsifying effect, enhancing the mutual solubility between the components of the foam material. After the addition of organic silicon surfactants, the surface tension γ of the liquid is significantly reduced, decreasing the free energy required for gas dispersion. This makes it easier for air dispersed in the raw materials to nucleate during the mixing process, aiding in the formation of fine bubbles and improving foam stability.

3. Mechanism of Foam Stability

Answer: The addition of appropriate surfactants facilitates the generation of fine bubble dispersions.

4. Formation Mechanisms of Open-Cell and Closed-Cell Foams

Answer:Open-Cell Foam Formation Mechanism: Typically, when the maximum pressure is generated inside the bubbles, the cell wall strength formed by the gel reaction is not high enough to withstand the membrane stretching caused by the increased gas pressure. Consequently, the bubble wall membrane ruptures, and the gas escapes from the rupture, forming open-cell foam.

Closed-Cell Foam Formation Mechanism: For rigid foam systems, the reaction between polyether polyols with high functionality and low molecular weight and polyisocyanates occurs relatively quickly. As a result, the gelation speed is fast enough that the gas inside the cells cannot rupture the cell walls, leading to the formation of predominantly closed-cell foam plastics.

5. Mechanisms of Physical and Chemical Foaming Agents

Answer:Physical Foaming Agents: These agents form foam cells through physical state changes of a substance, such as the expansion of compressed gas, evaporation of liquids, or dissolution of solids.

Chemical Foaming Agents: These are compounds that release gases such as carbon dioxide and nitrogen upon thermal decomposition, forming fine cells within the polymer composition.

6. Preparation Methods for Soft Polyurethane Foam

Answer: There are two methods: the one-step method and the prepolymer method.

Prepolymer Method: This involves first reacting polyether polyols with an excess of TDI to form a prepolymer containing free NCO groups, which is then mixed with water, catalysts, stabilizers, and other components to produce foam.

One-Step Method: All raw materials are directly calculated and fed into the mixing head for direct foam plastic production. This method can be divided into continuous and batch processes.