1.Mechanism of Bubble Nucleation
Answer: Gas substances are generated and vaporized due to chemical reactions within the liquid raw materials or the heat produced by these reactions. As the reaction progresses and the reaction heat increases, the production and evaporation of gas continue to rise. Once the gas concentration exceeds its saturation limit, bubbles begin to form in the solution phase and ascend.
2.Role of Foam Stabilizers in Polyurethane Foam Production
Answer: Foam stabilizers enhance emulsification, improving the mutual solubility of foam components. Adding organosilicon surfactants significantly lowers the liquid’s surface tension (γ), reducing the energy required for gas dispersion. This facilitates bubble nucleation during mixing, promoting the formation of fine bubbles and enhancing foam stability.
3.Mechanism of Foam Stability
Answer: The addition of suitable surfactants supports the formation of finely dispersed bubbles, ensuring better foam stability.
4.Formation Mechanisms of Open-Cell and Closed-Cell Foams
Answer:
Open-Cell Foam Formation: Typically occurs when the bubble walls formed by the gel reaction lack sufficient strength to withstand the increasing internal pressure, causing the walls to rupture and release the gas, resulting in open-cell foam.
Closed-Cell Foam Formation: Common in rigid foam systems where multifunctional, low-molecular-weight polyether polyols react with polyisocyanates. The rapid gelation prevents gas from breaking through the bubble walls, leading to a predominance of closed-cell structures.
5.Foaming Mechanisms of Physical and Chemical Blowing Agents
Answer:
Physical Blowing Agents: These agents create foam through changes in the physical state of substances, such as the expansion of compressed gases, liquid evaporation, or solid dissolution.
Chemical Blowing Agents: These are compounds that release gases like carbon dioxide and nitrogen upon thermal decomposition, forming fine pores within the polymer matrix.
6.Methods for Producing Flexible Polyurethane Foams
Answer:
One-Step Process: All raw materials are calculated and mixed directly in the mixing head to produce foam in one step. This process can be continuous or batch-based.
Prepolymer Method: This involves reacting polyether polyols with an excess of TDI to form a prepolymer containing free NCO groups, which is subsequently mixed with water, catalysts, and stabilizers to produce foam.
