How to Adjust the Tear Strength of Flexible PU Foam by Modifying Formulations?--Part 2

We will explain this from three aspects:

1. Material Structure

2. Reaction Principle

3. Formula

The formula is a product of the combination of material structure and reaction principle.

Factors affecting tear strength include:

1. The Reactants Themselves. For example, polyester groups have very high tear resistance, and their inclusion in the formula significantly improves tear strength.

2. TDI’s Benzene Ring. The benzene ring in TDI is very stable, it can promote crosslinking reactions, thereby increasing tear strength.

3. Crosslinking Reaction Substances. These substances have a trigonal structure, providing strong tear resistance. Additionally, their side chains form a rhombic structure, which also offers tear resistance. The rhombic structure has good heat resistance.

Let’s delve into how we can adjust the formula to modify the tear strength of the sponge. Here is the basic formula:

For instance, if we add 15 parts of halogen flame retardant R, which is acidic, it suppresses the reactions between TDI and urethane and between TDI and urea, promoting the reactions between TDI and water and between TDI and carbamate. However, this can lead to the phenomenon of red core in the sponge. Ideally, promoting the reactions between TDI and water and between TDI and carbamate should increase the reaction heat, which in turn should also enhance the reactions between TDI and urethane and between TDI and urea. But practical tests reveal that adding halogen flame retardant actually results in lower tear strength compared to the original formula. Therefore, we conclude that the halogen flame retardant significantly suppresses the reactions between TDI and urethane and between TDI and urea.

Next, let's consider adding POP (Polyol). This is more complex because POP has long molecular chains, and its structure can affect tear strength. Additionally, its side chains can also influence molecular strength. The effect depends on which force is greater.

Let’s examine a formula with 70 parts PPG (Polypropylene Glycol) and 30 parts POP. We find that this formula requires minimal adjustments, and the resulting sponge is open-celled. However, the reaction temperature within the sponge increases, which in turn increases the hardness of the sponge. As the reaction temperature rises, the strength of the reactions between TDI and urethane and between TDI and urea also increases. The side chains of POP affect the reactions between TDI and water and between TDI and carbamate. Based on our experience, substances that increase the hardness of the sponge foam generally belong to rigid groups, such as TDI. Since TDI is a rigid group and it pairs well with urea, we cannot definitively say it promotes the reaction with urea, but at least it doesn’t suppress it. Therefore, it can be inferred that it increases tear strength.

Finally, let’s see what happens when we add fillers like powders. Adding any of these three powders (CaCO3, BaSO4, or melamine powder) significantly reduces the tear strength of the sponge foam. This indicates that these powders greatly suppress the reactions between TDI and urethane and between TDI and urea, leading to internal cracks in the sponge foam and thus greatly reducing tear strength.