The Influence of MC on the Temperature of Flexible PU Foam

In polyurethane flexible foams, dichloromethane (MC) is often used to adjust the density and hardness of the foam. With a boiling point of only 40.4°C, during foaming, the reaction of water and TDI generates a large amount of heat, causing MC to evaporate into gas, thus expanding the foam body and reducing foam density.

The vaporization of MC consumes a lot of heat, which can affect the foaming process of the foam in some cases. The following two figures show the changes in the maximum foam temperature and the time to reach it after adding different amounts of MC to a specific formula.

From the charts, it can be observed that after adding MC, the maximum foam temperature decreases significantly, and the time to reach the maximum temperature also increases.

These are just changes in data, but how do they manifest during the actual foaming process? To understand this, let's briefly look at the polyurethane reaction process.

The main reaction in polyurethane foaming is the reaction of water and isocyanate to produce carbon dioxide and amine, and the reaction of polyether polyol and isocyanate to produce polyurethane. However, there are many secondary reactions, summarized as reactions generating urethane and reactions generating urea.

Secondary reactions change the molecular structure of the polymer from linear to cross-linked. Due to different reaction conditions and raw materials, the structure of polyurethane can vary greatly. In general, the more secondary reactions, the more complex the cross-linked structure, resulting in increased hardness and improved tear strength. Of course, the resistance to yellowing also improves, but that's another topic. Increasing the foaming index will strengthen secondary reactions.

Having said so much, what does this have to do with MC? Secondary reactions are all endothermic reactions, requiring heat absorption. However, the vaporization of MC also requires a large amount of heat, thus creating a competitive relationship. Adding a large amount of MC will significantly weaken secondary reactions, increasing the proportion of linear structures in the foam, making it softer, and decreasing thermal plasticity.

Even in colder temperatures during winter, attention should be paid to this issue. Properly increasing the water content in the formula to generate more heat helps maintain the physical properties of the foam without significant changes.