What is the One-Step Foaming Process for Ether-Based Flexible Foam?

(1) Raw Materials

The main raw materials include polyether polyols, TDI (typically 80/20 TDI), silicone foam stabilizers, catalysts, water, and other additives such as external blowing agents, fillers, or other additives, depending on the needs.

(2) Foaming Formula

The foaming formula for polyether-based foam is generally similar to that of polyester-based foam. Since polyether has a much lower viscosity than polyester, a highly effective foam stabilizer must be added, typically a polysiloxane-polyoxyalkylene block copolymer. Because the hydroxyl groups at the end of polyether chains are mainly secondary hydroxyl groups, which have lower reactivity, high-efficiency composite catalysts such as tin octoate, dibutyltin dilaurate, and triethylenediamine are generally used.

The typical formula for polyether-based flexible foam in one-step foaming is as follows:

(3) Process Factors

①Polyether Polyol Characteristics

The polyether polyols used in flexible foam typically have a functionality of 2 or more, with a hydroxyl value equivalent of around 1000. The most commonly used are triol polyethers with a molecular weight of about 3000. When the molecular weight of the polyether exceeds 4000, the foam product tends to have larger cell sizes, lower rebound, and increased compression set. Polyethers with primary hydroxyl groups are not used in this case.

As the functionality of the polyether increases, the compression modulus of the foam also increases, but properties such as tear strength, tensile strength, and elongation decrease. Conversely, reducing the functionality results in a decrease in compression modulus, but improves other properties.

To enhance activity, some polyether polyols are end-capped with ethylene oxide to increase the content of primary hydroxyl groups, which reduces curing time and slightly improves compression modulus, tensile strength, tear strength, and elongation.

②TDI Ratio

In one-step flexible foams, TDI 80/20 is commonly used, and the TDI index is typically controlled between 103% and 107%. If the TDI index is too high, it can lead to large pores and closed cells in the foam. If the index is too low (e.g., below 100%), foam strength and rebound will decrease, and small cracks may appear in the foam. A high TDI index also extends curing time, particularly in large blocks of low-density foam, where excessive unreacted TDI can form isocyanurates and urea derivatives, generating heat over several hours and potentially causing charring or even fire.

③Water Content

Increasing the water content, while keeping the TDI index constant, decreases foam density and compressive load. However, after a certain amount, increasing the water content further causes only a small reduction in compressive load, as the increased urea group density in the polymer increases the foam's compressive resistance, offsetting the loss due to reduced foam density.

④External Blowing Agents

External blowing agents help reduce foam density, improve softness, and dissipate reaction heat, preventing charring in large block foams. Commonly used blowing agents include low-toxicity, non-flammable solvents with low boiling points, such as dichloromethane or HFC-245fa. Increasing the amount of external blowing agents reduces foam density and compressive load. However, when more blowing agent is used, the catalyst quantity must also be adjusted to avoid delaying the curing process.

⑤Catalyst Effects

The one-step process requires more active catalysts compared to the prepolymer method. A common catalyst system consists of organotin compounds and tertiary amines. Although dibutyltin dilaurate was historically used, it can cause thermal oxidation and accelerate ether bond cleavage, leading to polymer degradation. Therefore, stannous octoate is now commonly used, which also leads to higher compression strength in the final foam. The right combination of catalysts allows for control over foam porosity, pore size, and compression resistance.

⑥Silicone Foam Stabilizers

Silicone foam stabilizers reduce surface tension, allowing the foam wall to maintain elasticity and prevent rupture. They also help control pore size and uniformity. They are essential in one-step polyether foam production. Early stabilizers were block copolymers of siloxane and polyoxyalkylene, which were prone to hydrolysis. Now, more stable products are available that do not degrade as easily in the foaming process.

⑦Fillers and Other Additives

Fillers such as precipitated calcium carbonate or other inorganic materials can improve the compressive strength of flexible foams. However, this method is less commonly used in practice, as foams containing fillers tend to soften after long-term use due to fatigue. Other additives, such as antioxidants, anti-static agents, flame retardants, UV absorbers, and pigments, can be added depending on the final product requirements.