Foam is a type of polyurethane foam plastic, belonging to the category of flexible polyurethane foam plastics. Due to its porous honeycomb structure, it possesses excellent characteristics such as softness, elasticity, water absorption, and water resistance. It is widely used in industries such as sofas, mattresses, clothing, and soft packaging.
Main Raw Materials
1.1 Polyether Polyols
Foam often uses polyether propylene glycol and polyether triol, with low functionality (2-3), low hydroxyl value, and high molecular weight. The molecular formula is:
CH3 − CHO(C3H6O)m(C2H4O) nHCH2O(C3H6O) m (C2H4O) nH
1.2 Isocyanates
The most commonly used isocyanate is toluene diisocyanate, abbreviated as TDI, which has two isomers, namely 2,4-TDI and 2,6-TDI. In foam production, 2,4-TDI accounts for 80%, and 2,6-TDI accounts for 20%.
1.3 Water
Water is indispensable in foam production; it reacts with TDI to release CO2 gas and simultaneously acts as a chain extender.
1.4 Catalysts
Catalysts that promote the reaction between polyether polyols and isocyanates to increase the chain include tin octoate and dibutyltin. Catalysts that promote the cross-linking reaction and enhance the catalysis of CO2 gas released in the reaction between isocyanates and water include triethanolamine, triethylene diamine, and triethylamine.
1.5 Blowing Agents
Commonly used are low-boiling fluorocarbon compounds, such as dichlorofluoromethane (F-12). Due to environmental concerns, F-12 is generally replaced with cyclopentane or dichloromethane, which yields satisfactory results. If producing foam with normal density, the main raw material ratios can be adjusted accordingly, eliminating the need for external blowing agents.
1.6 Foam Stabilizers
Organic silicone foam stabilizers are commonly used, with silicon-carbon bonds (Si-C copolymers) being the main type, typically used at a concentration of 0.5%-5%.
Synthesis Principles of Foam
In the synthesis process of foam, there are chain extension reactions, foaming, and cross-linking processes, all of which are related to the molecular structure, functionality, and molecular weight of the raw materials.
2.1 Chain Extension Reaction
Isocyanates react with di-functional polyether polyols in a chain extension reaction. As isocyanates are in excess by around 5%, the final product of the chain extension is the isocyanate group, promoting rapid chain growth.
2.2 Foaming Reaction Accompanied by Chain Extension
In the foam production process, foaming gas primarily comes from the reaction between TDI and water, generating a large amount of CO2 gas. Simultaneously, newly formed amines react with isocyanates to produce urea-bonded compounds, repeating this process accompanied by chain extension.
2.3 Cross-linking Reaction
Cross-linking reactions are crucial for preparing sponges; occurring too early or too late can lead to a decrease in foam quality or even render it unusable.
2.3.1 Cross-linking of polyfunctional compounds affects the density of the foam. Cross-linking point molecular weight ranges from 2000 to 20000; the smaller the molecular weight, the greater the cross-linking density, resulting in higher foam hardness and decreased softness and elasticity.
2.3.2 Water reacts with isocyanates to form urea-bonded compounds, further reacting with isocyanates to produce triazine structures, cross-linking compounds.
2.3.3 Urea-formaldehyde cross-linking
The hydrogen on the nitrogen atom in the aminoformate reacts with isocyanates to form urea-formaldehyde with a three-way cross-linking structure.
Production Process and Flow
Currently, most foam production uses a one-step box foaming method. Various raw materials are rapidly added to a forming box under high-speed stirring, completing chain extension, foaming, cross-linking, solidification, and other reactions in the forming box to complete foam production. The advantages of this process are a short processing time, low material viscosity, easy control, energy savings, low equipment investment, and suitability for a wide range of densities.
