Mechanism of Slow Rebound Foam Formation

1. Introduction

Since the early 2000s, the slow rebound foam has seen significant development in China, both in terms of technological advancements and raw material supply. Many companies capitalized on this opportunity and reaped substantial profits. However, as the technology for slow rebound foams matured, market demand declined sharply, and these foams are no longer a hot commodity. Consequently, industry attention has decreased, and discussions about slow rebound foam technology have dwindled. Today, we will discuss the technology of slow rebound foams.

2. Characteristics of Slow Rebound Foam

A typical feature of the slow rebound foam is that its deformation process and recovery trajectory do not respond almost instantaneously and synchronously to the application and removal of external forces, unlike ordinary foams. Particularly during recovery, it starts from the outermost edge of the stress core point and gradually recovers inward, with the core point being the last to restore. Utilizing this characteristic, many cushioning products, pads, and craft items have been developed.

This unique feature allows the slow rebound foam to evenly distribute the pressure exerted by irregularly shaped objects, earning it the nickname "zero pressure foam." Additionally, the weak rebound force can maintain the geometric shape formed by the external object pressing on the foam, which is why it is also known as memory foam.

3. Formation Mechanism of Slow Rebound Foam

It is well-known that foams are primarily made from polyether polyol and isocyanate, with necessary additives. Generally, using high molecular weight polyether polyols (e.g., 3000 molecular weight and above) and polymer polyols (e.g., 6000 molecular weight and above) with isocyanate results in fast rebound foams, commonly referred to as ordinary foams.

In contrast, the preparation of slow rebound foams almost always involves combining high molecular weight polyether polyols/polymer polyols with low molecular weight polyether polyols (e.g., molecular weight 700 and 550). Typically, a mix of 3000 molecular weight or higher polyether and 550 or 700 molecular weight polyether is reacted with isocyanate to produce slow rebound foam. The reaction between polyether and isocyanate forms chain segments of varying molecular weights. The substance formed from the reaction of high molecular weight polyether and isocyanate is called a soft segment, while that from the reaction of low molecular weight polyether and isocyanate is called a hard segment. These chain segments combine through chemical and physical bonds, intertwining and accumulating, with slight phase separation due to steric hindrance.

It is understood that high molecular weight polyether has a low hydroxyl value, leading to low crosslinking density with isocyanate, providing larger intramolecular and intermolecular space for movement, making deformation and recovery easier, i.e., high activity in soft segments. Conversely, low molecular weight polyether has a high hydroxyl value, resulting in high crosslinking density with isocyanate, providing smaller intramolecular and intermolecular space for movement, making deformation and recovery more difficult, i.e., low activity in hard segments.

The aforementioned characteristics of the soft and hard segments, coupled with slight phase separation within the polymer, result in the phenomenon where soft and hard segment deformation and recovery are not synchronized during the application and removal of external forces. The core reason is that the deformation and recovery of soft segments are fast, while those of hard segments are slow. This is the formation mechanism of slow rebound foam. Technicians familiar with slow rebound foam formulations know that when the index is fixed, increasing the amount of slow rebound polyether slows down the foam's rebound speed, and decreasing the amount speeds it up, for the reasons described above.

4. Applications

Understanding the formation mechanism of slow rebound foam allows for the design of reasonable formulations based on customer needs. For example, if a customer requires faster rebound speed, the amount of slow rebound polyether can be appropriately reduced, and the amount of ordinary polyether increased. If a customer requires higher hardness, increasing the amount of slow rebound polyether, using POP to replace part of the ordinary polyether, and increasing the TDI index can meet the requirements.

Issues like the tendency of slow rebound foam to form closed cells can also be addressed based on its formation mechanism. Generally, higher foam density leads to easier cell closure; higher TDI index also leads to easier cell closure. When producing high-density slow rebound foam, typically, 550 molecular weight, 306 hydroxyl value polyether is used. This increases the crosslinking density, tightly winding and accumulating chain segments, with slight phase separation, and hard segments dominating, leading to a high cell closure rate. Similarly, a high TDI index increases crosslinking density, concentrating hard segments, thus increasing the tendency for cell closure. This issue can be resolved by increasing the amount of cell-opening agents.