The compression resistance of foam is influenced by various factors, including the structure of the foam’s chain segments, intermolecular chemical bond energy, polymer crystallinity, phase separation degree, isocyanate structure, and the ratio of isocyanates used.
Formation and Characteristics of Slow Rebound Foam
Slow rebound foam is produced by reacting high molecular weight polyols with low molecular weight polyols and isocyanates. High molecular weight polyols create soft segments that are large in volume, have low crosslink density, and high activity, making them easy to compress and quick to recover after the pressure is released. Conversely, low molecular weight polyols form hard segments that are small in volume, have high crosslink density, and low activity, making them difficult to compress and slow to recover. This results in the characteristic slow rebound of the foam, which is fundamental to its manufacturing.
The differences in properties between the soft and hard segments lead to a degree of phase separation. Without phase separation, the foam would behave as a tightly bound whole, compressing and expanding uniformly. However, due to the varied molecular structures and uneven molecular weight distribution of the chain segments, complete phase separation is unavoidable. Slight phase separation causes some hard segments to recover less efficiently, ultimately hindering the soft segments and leading to shrinkage.
Crystallinity of Hard Segments
The higher crystallinity of hard segments compared to soft segments also contributes to poor recovery. In slow rebound foam, the close crosslink points and high crosslink density of hard segments result in smaller molecules that tend to aggregate. The presence of hydrogen bonds increases the crystallinity and cohesion of the material. Compression alters the aggregation state, making it easier for polar groups to fuse. Upon release, the strong cohesive forces of the new aggregation state hinder the return to the original state, causing shrinkage.
Impact of Isocyanate Structure
The structure of isocyanates, such as TDI (toluene diisocyanate), affects the foam’s compression resistance. The two NCO groups in TDI molecules are positioned at 2,4- and 2,6- positions with a certain angle, making them prone to deformation under stress, especially during hot pressing. This deformation and heat loss are particularly evident in bra cup foams, complicating recovery.
Isocyanate Index
The low NCO index in the preparation of slow rebound foam also contributes to poor recovery. While the NCO index of regular foam exceeds 100, it is typically between 85-95 in slow rebound foam, meaning 5-15% of hydroxyl groups do not participate in the reaction. Consequently, the foam lacks a fully integrated network, with many independent chain segments internally.
Solutions to Improve Compression Resistance of Slow Rebound Foam:
- Using High EO Polyether
Replace some slow rebound polyether with high EO (ethylene oxide) polyether, which has a low hydroxyl value and high molecular weight. This reduces phase separation and crystallinity.
High EO polyether’s flexible segments enhance the slow rebound effect and improve low-temperature resistance.
- Adding Polyether-Modified Polyester
Incorporating a small amount of polyether-modified polyester increases internal cohesion due to the ester groups in the polyester segments, enhancing tensile and compressive strength.
- Using High-Functionality Polyether as Crosslinking Agents
Employing high molecular weight polyether as a crosslinking agent and substituting ordinary polyether with high-activity polyether disrupts chain segment distribution, reduces phase separation, increases reaction degree, and decreases crystallinity.
- Incorporating MDI
Using MDI (methylene diphenyl diisocyanate) or adding MDI to TDI improves compression resistance and reduces heat loss. Modified MDI, due to its high branching and intramolecular cyclization, offers better compression resistance compared to pure TDI.
