How to Adjust the Formula for PU Foam Burning Inside?--Part Two

This article will discuss how to address sponge core scorching by adjusting the formulation, focusing on external factors such as raw materials(Take the following formulas as an example).

Polyether Activity: Polyether is the main component for foaming. Taking the polyether used in ordinary sponges as an example, with a molecular weight of 3000 and a hydroxyl value of 56, there are two types of ordinary sponge polyether. The first type is entirely PO (propylene oxide), and the second type is partially EO (ethylene oxide). The latter is more active than the fully PO polyether. If the formulation using fully PO polyether is changed to use the more active EO polyether, the amount of T9 (a catalyst) needs to be reduced by 15% due to the higher activity. If T9 is adjusted accordingly, the reaction speed and heat release rate will be similar. Therefore, it may seem that polyether activity has little effect on sponge core scorching because the formulation is unconsciously adjusted.

Adding Stone Powder: Stone powder is an inorganic substance that does not participate in the reaction but blocks it, thereby hindering heat release. Looking at a formulation that uses partially EO polyether, if the formulation uses fully PO polyether, the volume of stone powder needs to be increased by more than 10%. The original formulation does not include stone powder, and for comparison, we kept the formulation ratios mostly unchanged. We can conclude that the internal temperature of the foam in formulation one is certainly lower than that of the original formulation because the addition of stone powder obstructs the reaction. To adjust the reaction rate of formulation one to be similar to the original, the water content can be increased from 4.2 to 4.3, MC can be adjusted to 10, and A33 can be adjusted to 0.26 or 0.27. The ammonia ratio should not be adjusted alone; if adjusted too much, it will lose balance.

Adding Flame Retardants: Flame retardants are acidic and act as catalysts, accelerating the reaction between water and TDI (toluene diisocyanate), which is the main exothermic reaction producing carbon dioxide. In low-density foams, this reaction can account for over 70% of the total heat release. Therefore, when flame retardants accelerate this reaction, the heat release rate increases, and the internal foam temperature rises. Comparing formulation two (which includes flame retardants) with others, we can conclude that the internal foam temperature in formulation two is higher and more prone to scorching. To adjust the formulation, the water content can be changed to 4.1, MC to 12, and A33 to about 0.24.

TDI Index: Analyzing formulation three, when the TDI is 58, the tin content is 0.275; when TDI is 60, the tin content is 0.265. The higher the TDI, the less tin is needed, and vice versa. The higher the TDI index, the more residual TDI there will be, leading to more heat release. However, a higher TDI index suppresses the water-TDI reaction, slowing it down and reducing heat release. Conversely, a lower TDI index speeds up this reaction and increases heat release. As the TDI index increases, the water-TDI reaction slows down and heat release decreases, while the residual TDI reaction increases. This inverse relationship makes it difficult to determine whether the temperature will be higher at a TDI of 58 or 60 for formulation three.