Polyester-Based Soft Foam One-Step Foaming Process

(1) Raw Materials

Polyester raw materials are generally polyester made from adipic acid and ethylene glycol or other various grades of adipic acid polyester. In some cases, small amounts of aliphatic dimer acid polyester are used. Isocyanates are typically TDI 80/20 and TDI 65/35.

(2) Foaming Formula

Depending on the process and product performance requirements, different raw materials and foaming formulas are used. A typical formula for the one-step foaming process of medium-density polyester-based soft foam is as follows:

• Polyester polyol: 100 parts
• Water: 1.5–3 parts
• Nonionic surfactant: 1.5–3.0 parts
• Tertiary amine catalyst: 0.5–1.5 parts
• TDI 80/20 or 65/35 (NCO index: 1.03–1.07)

During the one-step foaming process, the above formula is generally divided into two or more components, which are then fed from storage tanks by several precision metering pumps into a high-speed mixing head in proportion. The components are rapidly mixed and evenly blended under high-speed stirring and injected onto a belt conveyor (for block foam production) or into a mold (for molded foam production) for foaming. The mixing time is typically 1–5 seconds, and the time for the foam to whiten (i.e., the start of foaming) is 4–6 seconds. The foaming time generally ranges from 40 to 80 seconds. Once the foam has solidified, it is cured for 2 hours at 100°C to obtain the foam plastic product. Alternatively, it can be left at room temperature for a week to achieve the expected strength.

The actual amount of isocyanate in the formula can be calculated using the following formula:

• Reference: 100 parts of polyester (or polyether)

• A = TDI (for polyester or polyether)

           = [Hydroxyl value + Acid value] * [(87 * 100) / (56 * 1000)]

          = (Hydroxyl value + Acid value) * 0.155

• B = TDI (for water substance amount)

           = [Water content in polyester (or polyether) + added free water] * (174 / 18)

           = Total water content * 9.6

• Theoretical total TDI amount = A + B

• Actual TDI consumption = Theoretical TDI amount * TDI index

(3) Process Factors in Foaming

1. Theoretically, the TDI index should be 100%. However, in practical operations, due to side reactions, some TDI is consumed. Considering the reaction rate and the need to improve product performance (especially wet aging performance), the TDI index is generally controlled between 103% and 110%. If it exceeds 110%, it may cause the foam to form a coarse pore structure or cause cracking in the foam product.

2. The isomeric ratio of TDI also affects the foaming process. For the same formula, using 65/35 TDI results in shorter foaming time and higher exothermic temperature (for example, the exothermic temperature of 65/35 TDI is 100–102°C, while that of 100% 2,4-TDI is 87–92°C). 3. This is mainly due to the steric hindrance of 2,4-TDI. An increase in 2,4-TDI ratio also makes the product softer. These factors can be adjusted by the amount of catalyst and the molecular weight of the polyester in the formula.

3. The softness of the foam product can be adjusted by controlling the number of functional groups and molecular weight of the polyester, i.e., adjusting the crosslink density in the polymer molecules.

4. The density of the foam product can be adjusted by the amount of isocyanate, water, or external foaming agent. The more used, the lower the density of the product. For example, the relationship between different amounts of isocyanate and water in the one-step foaming process and the density of the foam product is shown in the following table:

5. External foaming agents can be low-volatility liquids such as dichloromethane, 1-fluoro-3-chloromethane, etc., with 1-fluoro-3-chloromethane being the most ideal. The addition of external foaming agents not only reduces the density of the product but also makes it softer. Additionally, they can remove reaction heat during vaporization, which is particularly useful during the production of large block foam products. The typical usage range is 5%–15%. As the amount of external foaming agent increases, foam stability decreases. Due to the atmospheric pollution caused by CFC-11, alternatives like HFC-141b or HFC-145fa can be used.

6. Catalysts typically used include ethylmorpholine, N-methylmorpholine, triethylenediamine, triethylamine, diethylaminoethanol, tin octoate, dibutyltin, and various metal compounds. Composite catalysts can also be used.

7. The role of long-chain surfactants is mainly to increase the miscibility of the materials, ensuring uniform foam cells and stabilizing the foam to prevent product shrinkage. The typical usage is 1%–2.5% of the polyester content. Excessive amounts may cause coarse foam cells and collapse of the foam.

8. Other additives can also be included in the formula, such as Mobius Company's A-3 additive to improve emulsification performance and prevent shrinkage, A-7 and A-9 to delay foam surface solidification to prevent cracking, or small amounts of mineral oil, such as Kaydol, which is used in the production of micro-porous foam products. The typical usage is 0.05%–0.2% of the polyester content.

9. In mechanical foaming, factors such as the type of mixing head, shear force, stirring speed, residence time, back pressure of the mixing head, and the exit diameter of the mixing head all significantly influence the foam cell size, pore structure, and final product performance. Generally, slow stirring speed or insufficient residence time often leads to coarse cells, hollow sections, or cracks. As stirring speed and residence time increase, the foam cell size becomes finer. However, once these factors exceed a certain limit, coarse cells and collapsed foam may form. The diameter of the mixing head exit also affects foam cell size. A smaller exit diameter can help reduce foam cell size, but if it becomes too small, foam cracking may occur.

10. uring mechanical mixing, small amounts of air entering the mixing head can reduce the foam cell size, but large amounts of air can lead to large cells. Additionally, the presence of dust or contaminants in the foaming system can cause large pores or pinholes.

11. To ensure product performance stability and good repeatability in the process, it is essential to strictly control the material temperature. The recommended material temperature is typically controlled at (24±3)°C, and in practice, it is often maintained within ±1°C.

The above-mentioned process control factors represent general trends. In actual production, depending on raw materials and product performance requirements, formulas and process conditions may be adjusted accordingly to meet the requirements.