Polyurethane foam sponges are a variety of soft polyurethane foam plastics known for their porous honeycomb structure, which provides excellent softness, elasticity, water absorption, and water resistance. They find extensive use in industries like furniture, mattresses, clothing, and soft packaging.
1.Primary Raw Materials
1.1 Polyether Polyols
Polyurethane foam sponges are predominantly manufactured from polyether polyols, such as polyether propylene glycol and polyether propylene triol. These polyols have low functionality (2-3), low hydroxyl values, and high molecular weights.
1.2 Isocyanates
The most commonly used isocyanate in PU foam sponge production is toluene diisocyanate (TDI), which exists in two isomeric forms: 2,4-TDI and 2,6-TDI. In the production of PU foam sponges, 2,4-TDI is used 80% of the time, with 2,6-TDI making up the remaining 20%.
1.3 Water
Water plays a crucial role in PU foam sponge production. It reacts with TDI, releasing carbon dioxide (CO2) gas, and also serves as a chain extender.
1.4 Catalysts
Catalysts that facilitate the reaction between polyether polyols and isocyanates for chain extension include stannous octoate and dibutyltin dilaurate. Catalysts that aid cross-linking reactions and the release of CO2 gas during the reaction between isocyanates and water include triethanolamine, triethylene diamine, and triethylamine.
1.5 Blowing Agents
Commonly used blowing agents are low-boiling fluorocarbon compounds, such as dichlorodifluoromethane (F-12). Due to environmental concerns, cyclopentane is often used as a substitute for F-12 or dichloromethane, which provides good results. If producing PU foam sponges with regular density, the proportions of the main raw materials can be adjusted, eliminating the need for external blowing agents.
1.6 Foam Stabilizers
Organic silicon foam stabilizers are commonly employed, with silicon-carbon bond Si-C copolymers being the primary choice, used at concentrations ranging from 0.5% to 5%.
2.Principles of PU Foam Sponge Synthesis
The synthesis of PU foam sponges primarily involves chain extension reactions, foaming, and cross-linking processes. These reactions depend on the molecular structure, functionality, and molecular weight of the raw materials.
2.1 Chain Extension Reaction
The chain extension reaction entails the reaction of isocyanates with polyether polyols, leading to the formation of isocyanate groups. Excess isocyanates (about 5%) in the reaction promote rapid chain extension.
2.2 Foaming Reaction with Chain Extension
In the production of PU foam sponges, foaming gases mainly result from the reaction between TDI and water, generating a significant amount of CO2 gas. Simultaneously, the newly formed amines react with isocyanates to form urea compounds, further promoting chain extension.
2.3 Cross-Linking Reaction
The timing of the cross-linking reaction is critical in PU foam sponge production. If it occurs too early or too late, it can lead to a decrease in the quality of the PU foam sponge.
2.3.1 Cross-Linking of Polyfunctional Compounds
The cross-linking of polyether polyols with isocyanates directly impacts the density of PU foam sponges. Cross-linking points have a molecular weight range of 2000-20000. Smaller molecular weights result in higher cross-linking density, leading to increased foam hardness and reduced softness and elasticity.
2.3.2 Urea Bond Cross-Linking
Water reacts with isocyanates to produce urea compounds, which further react with isocyanates to form triurethane cross-linking compounds.
2.3.3 Urethane Methyl Ester Cross-Linking
The hydrogen on the nitrogen atom in the amino methyl ester group reacts with isocyanates to form triurethane cross-linking structures.
3.Production Process and Flow
Currently, most PU foam sponge production employs a one-step mold foaming process. Various raw materials are rapidly introduced into a forming mold under high-speed stirring, where chain extension, foaming, cross-linking, and curing reactions take place, completing the production of PU foam sponges. This process offers advantages such as a short production cycle, low material viscosity, easy control, energy savings, low equipment investment, and suitability for a wide density range.