How to Design Formulations for Flexible Polyurethane Foam?

To begin designing a flexible polyurethane foam formulation, the first step is to determine the target foam density. Based on the desired density (used to initially estimate the amount of blowing agent), for instance, if producing standard foam with a density of 30 kg/m³, the amount of blowing agent is calculated using an average factor of 95/30 = 3.166666, i.e., approximately 3.17. This should be verified with a lab sample. A basic formulation might look like this:

Polyether 3000: 80 parts (56×0.155×0.8 = 6.944)

 Polyether 2045 (POP): 20 parts (28×0.155×0.2 = 0.868)

 Water: 3.17 parts (3.17×9.667 = 30.644)

 Silicone oil: 1.1 parts

 A33: 0.31 parts

 T9: 0.22 parts

 TDI amount: (6.944 + 0.868 + 30.644) = 38.456 × index = 42.30

This formulation uses only water as the chemical blowing agent. However, the resulting foam may not meet customer requirements in terms of softness or tear strength. Therefore, a physical blowing agent is often added. When the total foaming index is 3.17 and water is used at 1.17, the remaining 2 parts of the index are made up using MC (methylene chloride), calculated as:

MC = 2 × 9 = 18 parts

 Revised formulation:

 Polyether 3000: 80 parts (6.944)

 Polyether 2045 (POP): 20 parts (0.868)

 Water: 1.17 parts (11.3104)

 Silicone oil: 1.1 parts

 A33: 0.31 parts

 T9: 0.22 parts

 MC: 18 parts

 TDI amount: (6.944 + 0.868 + 11.3104) = 19.1224 × index = 21.03

1. Batch foaming:

 Batch foaming is the original method for testing foam properties and reaction behavior. It precedes scale-up to production lines. Compared to machine foaming, batch foaming differs in scale, reactivity, mixing uniformity, and cell distribution. These differences lead to expected variations in foam performance. A control formulation is often used to minimize experimental errors caused by subtle differences in mixing or gas conditions.

Batch foaming Conditions, Procedure, and Precautions:

(1) Raw Materials:

 a. Polyether polyols

 b. Silicone oil

 c. Amine-water mixture (catalyst)

 d. Blowing agents (chemical and physical)

 e. Tin catalyst (stannous octoate + polyol or others)

 f. TDI (80/20)

(2) Preparation:

Prepare the above six components (add coloring agents if needed), and ensure temperature control.

 a. Polyol: Pre-mixed or ready-mixed

 b. Silicone oil: Pre-measured

 c. Amine: Tertiary amine mix (e.g., TEDA + diol in 33:67 ratio)

 d. Blowing agents: Pre-measure MC and water

 e. Tin catalyst: Should be prepared as 5% or 10% solutions due to high activity

 f. TDI (80/20): Accurately measured and preheated

(3) TDI Calculation:

Banual foaming TDI dosage is based on 100 parts of polyol.

TDI(80/20) = [(polyol OH value × wt / 56100) + (water wt / 9)] × 4200 / 48.3 × Index

Example:

Polyol: 100, MC: 5

Silicone oil: 1.3, Tin catalyst: 0.3

H₂O: 4.5, L-580: 0.19, TDI: X, A33: 0.03, Index = 1.1

X = [(56×100/56100 + 4.5/9)×4200/48.3]×1.1 = 57.37

(4) Foaming Conditions:

Keep temperature at 23±1°C and humidity at 50% for stable foaming.

Schematic diagram of batch foam machine

(5) Foaming Procedure:

Add the required amount of polyol to the container, then add and stir silicone oil and amine water for 45 seconds. Adjust temperature. Measure and mix blowing agents; compensate for MC loss due to evaporation. Add tin catalyst and stir for 10 seconds at 3000–4000 rpm. Then add TDI, stir for 7 seconds, and pour into mold. Clean mixer immediately after. Foam expansion occurs rapidly—record rise time. Let foam cure for 24 hours.

The foaming sequence is shown in the figure below

2. Continuous Foaming:

Currently, continuous production line is mainly used in China for producing flexible block foam.

Schematic diagram of continuous foaming machine

3. Foaming Data for MN-3050 Polyether:

Polyether MN-3050 is the most representative polyether for horizontal flexible foam.

Example foaming data table:

Figure (1) shows the relationship between tin catalyst and rise time

Figures (2–4) show the relationship between S.O (X1/100) and airflow (cc/cm²/sec)

From Figures (1) to (4), it is known:

 ①As tin catalyst increases, airflow decreases

②With the same amount of tin catalyst, more blowing agent H2O increases airflow

③With the same amount of tin catalyst, more MC increases airflow

When tin catalyst increases, resinification occurs during gasification, strengthening the foam cell membrane. Gas becomes harder to release, reducing airflow. With more water, gasification improves and air escapes more easily, while foam density also decreases. Thus, both contribute to increased airflow.

Regarding the effect of TDI-80/20 index, it is explained in the index section and omitted here.

Density:

Foam density changes significantly with the amount of water and MC. As the amount increases, density decreases.

The relationship between tin catalyst and foam density is shown as a function of blowing agent amount. In Figures (5) to (10):

Figure (5) MC=3.0 (Water=4.0, 4.5, 5.0)

Figure (6) MC=6.0 (Water=4.0, 4.5, 5.0)

Figure (7) MC=9.0 (Water=4.0, 4.5, 5.0)

Figure (8) Water=4.0 (MC=3.0, 6.0, 9.0)

Figure (9) Water=4.5 (MC=3.0, 6.0, 9.0)

Figure (10) Water=5.0 (MC=3.0, 6.0, 9.0)

From Figures (5) to (10), it is clear:

①As the amount of H2O and MC increases, foam density decreases.

②Increasing tin catalyst also slightly decreases density, showing tin helps gasification even in small amounts.

The relationship between H2O and MC amounts and foam density is shown in Figure (11).

Foam Hardness

Foam hardness is the most important property requirement. It is influenced by various factors. The relationship between foam hardness and H2O, MC, and tin catalyst is detailed in Figures (12)–(17):

Figure (12) H2O=4.0 (MC=3.0, 6.0, 9.0)

Figure (13) H2O=4.5 (MC=3.0, 6.0, 9.0)

Figure (14) H2O=5.0 (MC=3.0, 6.0, 9.0)

Figure (15) MC=3.0 (H2O=4.0, 4.5, 5.0)

Figure (16) MC=6.0 (H2O=4.0, 4.5, 5.0)

Figure (17) MC=9.0 (H2O=4.0, 4.5, 5.0)

From Figures (12)–(17):

 ①Higher tin catalyst increases hardness due to higher crosslinking during foaming. However, due to restrictions on tin usage (to maintain breathability), don’t overly rely on it for hardness.

 ②At fixed water, increasing MC lowers hardness—this is due to lower foam density.

 ③At fixed MC, increasing water doesn't significantly change hardness. This is because although more urea bonds form (increasing hardness), foam density decreases, balancing each other.

The relationship between density and hardness with water as the parameter is shown in Figure (18).

The relationship between MC usage and foam properties (density, hardness) is shown in Figure (19). From this, you can determine the required H2O and MC to achieve the desired properties.

However, these graphs are based on specific conditions. When foaming conditions change, the relationship between blowing agent amount and foam properties also changes.

Elongation

The relationship between tin catalyst and elongation is shown in Figures (20)–(22):

Figure (20) MC=3.0 (H2O=4.0, 4.5, 5.0)

Figure (21) MC=6.0 (H2O=4.0, 4.5, 5.0)

Figure (22) MC=9.0 (H2O=4.0, 4.5, 5.0)

Figures 20–22

From Figures (20)–(22):

As tin catalyst increases, elongation tends to increase due to enhanced resinification. Especially at low tin levels, differences are more obvious.

Water level differences show some variation but not significant.

Even for the same polyether, elongation varies with seasons, which will be explained later. This is attributed to temperature and storage effects of the gas medium.

Tensile Strength

The relationship between tin catalyst and tensile strength is shown in Figure (23) (MC=3). From Figure (23):

 As tin catalyst increases, tensile strength tends to increase.

 Differences caused by water level cannot be concluded based on this result.

Tear Strength

Tear strength is shown in Figure (24).

Tear strength tends to increase with higher H2O levels.

Figures 23–24

Resilience

Elasticity data is shown in Figure (25) (MC=3.0):

①Higher H2O levels reduce resilience.

②Higher tin catalyst also reduces resilience.

Resilience is also related to foam breathability. Generally, good airflow means better elasticity. Foam breathability is easily affected by silicone surfactant type and amount, so selection is important.

Compression Set

The relationship between tin catalyst and compression set is shown in Figure (26) (MC=3):

①As tin catalyst increases, compression set worsens.

②Higher H2O levels also worsen compression set.

Especially with high H2O, compression set deteriorates. So, be cautious when producing low-density foam.