Dichloromethane (MC) is frequently added to flexible polyurethane foams to modify their density and rigidity. Given that MC has a boiling point of just 40.4°C, during the foaming process, the heat generated by the reaction between water and TDI causes MC to evaporate, turning into gas and thus expanding the foam, which results in a lower foam density.
The evaporation of MC absorbs a considerable amount of heat, which, in certain situations, can interfere with the foaming process. The following two graphs illustrate how the foam’s peak temperature and the time to reach that temperature change when varying amounts of MC are incorporated into a given formulation.
From these graphs, it is clear that the foam’s maximum temperature drops significantly after adding MC, and the time taken to reach that peak temperature increases as well.
Although these are just data changes, how do they influence the actual foaming process? To understand this, we need to briefly review the polyurethane reaction mechanism.
The primary reaction in polyurethane foam formation is the interaction between water and isocyanate, which generates carbon dioxide and amines, while polyether polyol reacts with isocyanate to form polyurethane. In addition to these, various secondary reactions occur, typically involving the creation of urethane bonds and the formation of urea groups.
These secondary reactions alter the polymer’s molecular structure, transforming it from a linear arrangement to a more intricate cross-linked network. The structure of the polyurethane can vary greatly depending on factors like raw materials and reaction conditions. Generally, the more secondary reactions that take place, the more complex the cross-linked structure becomes, which results in greater hardness and improved tear resistance. There is also an improvement in yellowing resistance, though that will be addressed separately. Increasing the foaming index can enhance these secondary reactions.
Now, how does this relate to MC? Since secondary reactions are endothermic (heat-absorbing), they require heat to proceed. However, the evaporation of MC also consumes significant heat, creating a competition for available thermal energy. When a large amount of MC is added, it reduces the occurrence of secondary reactions, leading to an increase in the proportion of linear structures in the foam. As a result, the foam becomes softer and its thermal plasticity decreases.
This issue is also important to consider in colder weather, especially during winter. In such conditions, increasing the water content in the formula can generate additional heat, helping to maintain the foam’s physical properties and preventing significant changes.
