Causes and Solutions for Slow Rebound Sponge's Poor Compression Resistance

The compressive resistance of a foam is related to many factors such as the structure of various chain segments composing the foam, the chemical bonds between molecules, the crystallinity of polymers, the degree of phase separation, the structure of isocyanates, and the proportion of isocyanates used.

1. Slow rebound foam is formed by the reaction of high molecular weight polyols and low molecular weight polyols with isocyanates. The soft segments formed by high molecular weight polyols have large volumes, low crosslink densities, and high activity. They are easy to compress and quickly recover after pressure is removed. The hard segments formed by low molecular weight polyols have small volumes, high crosslink densities, and low activity. They are difficult to compress and also difficult to recover after external forces are removed. This characteristic gives foams their slow rebound feature and is the basis for manufacturing slow rebound foams.

Because the properties of the soft and hard segments in slow rebound foams are different, there is a certain degree of phase separation between them. If there is no phase separation between the segments, the foam body is a tightly bound whole on a macro scale, leading to the phenomenon of "move one hair and the whole body moves," meaning it shrinks as a whole when compressed and expands when pressure is released. However, the microstructure of the foam determines that this situation cannot be achieved completely. Especially in slow rebound foams, various chain segments have different molecular structures, uneven molecular weight distributions, and unavoidable phase separation. Slight phase separation causes some hard segments, due to their low activity, to have difficulty recovering during the recovery process after external forces are removed. These "escapees" more or less restrain the recovery of soft segments, ultimately leading to shrinking.

2. The crystallinity of hard segments, which is stronger than that of soft segments, is also a reason for poor recovery. Materials have similar compatibilities, which also apply in slow rebound foams. Because the hard segments have closer cross-linking points and higher crosslink densities, the small molecules formed are more likely to aggregate together. Due to the presence of hydrogen bonds, these aggregated hydrogen-containing substances enhance the crystallinity of the material, leading to greater cohesive forces. After compression, external forces change the aggregation state of the chain segments, making it easier for polar groups to fuse together. When the external force is released, the new aggregation state, due to strong cohesive forces, is difficult to return to the pre-stressed state, resulting in shrinkage of slow rebound foams.

3. The structure of isocyanates is also a factor affecting the compression resistance of slow rebound foams. TDI is usually used to produce slow rebound foams. Because the two NCO groups in the TDI molecule are at the 2,4- and 2,6- positions, they have a certain angle between them, making them prone to deformation under stress. Especially under hot pressing conditions, significant deformation and heat loss occur, particularly evident in bra cup foams, making recovery from these deformations difficult.

4. The low NCO index of isocyanates used in the preparation of slow rebound foams is also a reason for poor recovery. The NCO index of ordinary foams is usually above 100, while in slow rebound foams , the NCO index is generally between 85-95. This means that 5-15% of the hydroxyl groups do not participate in the reaction. Therefore, although the surface of the foam appears to be a single entity, internally there is a considerable portion of chain segments that are independent of each other.

Solutions for Improving Compression Resistance of Slow Rebound Foams:

1.Use high EO polyether (so-called blowing agent polyether) to replace some slow rebound polyether.

A. High EO polyether has a low hydroxyl value and a large molecular weight. After reacting with isocyanates, the segments formed have molecular weights greater than or close to those formed when ordinary polyether reacts with isocyanates, reducing the degree of phase separation and crystallinity.

B. High EO content polyether has soft and smooth segments, which can provide good slow rebound effects. Additionally, the addition of high EO polyether can effectively improve the low-temperature resistance of slow rebound foams.

2.Add a small amount of polyether-modified polyester to increase the material's cohesive force.

The polyester segments, due to the presence of ester groups, have high internal cohesive forces and good tensile and compressive properties, significantly improving the compressive resistance of slow rebound foams.

3.Use a small amount of high-functionality and high molecular weight polyether as a crosslinking agent, and replace some ordinary polyether with high-activity polyether for slow rebound.

This disrupts the distribution of chain segments, reduces the degree of phase separation, and increases the reaction degree, reducing crystallinity.

4.Use MDI or add MDI to TDI.

MDI has a different structure from TDI and produces foams with better compression resistance and less heat loss. If using MDI, it is best to use modified MDI (with high branching and easy closure of cells); liquid MDI can also be used, as it is intramolecular cyclization and more resistant to compression. Slow rebound foams made with all MDI have much better compression resistance than pure TDI, and many manufacturers are already using this.