1. Reason
Sponges are thrown goods, and the transportation cost is high. In order to reduce the transportation cost, many factories will compress the package and deliver the goods if the customer does not emphasize it.
For ordinary cotton, after the packaging is removed, it can basically recover to about 95% or more of its original size, and ordinary cotton with high density and good elasticity can almost recover to 99% or even completely.
But for slow-rebound sponges, the situation is different. After almost all specifications of slow-rebound sponges are compressed, packaged, and transported, there will be two situations: one is that they cannot recover temporarily, but as long as the force is no longer applied and the given time is long enough, the sponge can barely recover. to close to the original processing size; in the second case, even after a long enough time, it cannot recover or close to the original processing size, and finally becomes a defective product.
Therefore, the slow-rebound products generally supplied to China are basically not sent in compressed packaging. Although the transportation cost remains high, the product quality is guaranteed, and the supply and demand sides can be at peace.
However, for companies doing export business, the high cost of intercontinental transportation has a great impact on production and operation, so they have to consider the way of compression packaging. However, after compressed packaging and long-distance transportation, some products cannot be restored to the customer's order specifications after unpacking, and are rejected by the customer. This situation will cause certain losses to both the supply and demand sides, and the resulting trade disputes will not Not uncommon.
2. Cause Analysis
The anti-compression ability of the sponge is related to many factors such as the structure of the various segments that make up the sponge, the chemical bond energy between the molecules, the crystallinity of the polymer, the degree of phase separation, the structure of the isocyanate, and the proportion of isocyanate used.
a. The slow rebound sponge is made of high molecular weight polyol and low molecular weight polyol reacted with isocyanate. The soft segment formed by high molecular weight polyol has large volume, low cross-linking density and high activity. The external performance is that it is easy to compress, and easy to recover after the external pressure is removed. The hard segment formed by low molecular weight polyol is small in volume, high in cross-linking density and low in activity. The external performance is that it is difficult to compress and difficult to recover after the external force is removed. This characteristic makes the sponge exhibit the characteristics of slow rebound, and is also the basis for the manufacture of slow rebound sponge.
Because of the different properties of the soft segment and the hard segment in the slow rebound sponge, there is a certain phase separation between the phase segments. If there is no phase separation between the segments, the sponge body is a tightly integrated whole macroscopically, and the phenomenon of "pulling one hair and moving the whole body" will form, that is, the whole body shrinks during compression, and the whole body recovers after the pressure is released. However, the microstructure of the sponge determines that this situation cannot be achieved 100%. In particular, the various segments of the slow-rebound sponge have different molecular structures, uneven molecular weight distribution, and inevitable phase separation. The slight phase separation makes the recovery process after the removal of the external force, some of the hard segments become "slip through the net" due to their low activity and difficulty in recovery. This slip-through fish more or less restrains the recovery of the soft segment, which eventually leads to shrinkage. .
b. The crystallinity of the hard segment is stronger than that of the soft segment, which is also one of the reasons for the poor recovery. The materials have similar compatibility, which also applies to slow-rebound sponges. Due to the close distance between the cross-linking points and the high cross-link density, the hard segments are more likely to aggregate together due to the small molecular weight formed. These aggregated hydrogen-containing substances have enhanced crystallinity due to the existence of hydrogen bonds. Once strong, the cohesion will be great. After compression, the external force changes the aggregation state of the chain segments, making it easier for groups with similar polarities to fuse together (just like when rolling noodles, you should constantly sprinkle cornmeal or sorghum noodles on the rolled dough, otherwise it will roll up When rolling again, the dough will quickly stick together). After the removal of the external force, the newly formed aggregated state is difficult to restore to the state before the force due to the strong cohesion, resulting in the shrinkage of the slow-rebound sponge.
c. The structure of isocyanate is also one of the factors affecting the compressive performance of slow rebound sponge. Usually, TDI is used for the production of slow-rebound sponges. Since the 2 NCOs in the TDI molecule are in the 2,4-position and the 2,6-position, there is a certain angle between them, which is easy to deform under stress, especially in the case of hot pressing, which will produce large Deformation and heat loss, which is especially pronounced on cotton bras, can be difficult to recover from these deformations.
d. The low index of the isocyanate used in the preparation of the slow-resilient sponge is also the reason for the poor recovery of the sponge. The isocyanate index of ordinary sponge is above 100, while in slow rebound sponge, the isocyanate index is generally between 85-95, that is to say, there are 5-15% of hydroxyl groups that do not participate in the reaction. Therefore, there is no overall network in the sponge. The surface looks like a whole, but a considerable part of the chain segments inside are independent.
3. Practical exploration
a. Use high EO polyether (so-called inflatable polyether) to replace part of slow rebound polyether.
The high EO polyether has low hydroxyl value and large molecular weight. The chain segment produced by the reaction with isocyanate has a molecular weight greater than or close to the chain segment produced by the reaction of ordinary polyether and isocyanate, which reduces the degree of phase separation and reduces the crystallinity.
High EO content polyether, the chain segment is soft and can provide slow rebound effect well. In addition, the addition of high EO polyether can effectively improve the low temperature resistance of slow rebound sponge. There are many varieties of such materials in the market, such as Changzhou Zhongya's 5041, Shanghai Dongda's 3500, Jiahua's 1621/1623, Shandong Yinuowei's 8001A, etc.
b. Add a small amount of polyether-modified polyester to improve the cohesion of the material.
Due to the presence of ester groups in the polyester segment, the cohesion is large, and the tensile and tensile effects are good, and the compression resistance of the slow-rebound sponge is significantly improved. The author once used the polyether-modified polyester PM-580 of Zibo Chongmei Company to prepare slow-rebound sponge, and achieved satisfactory pressure resistance effect.
c. Use a small amount of high-functionality and high-molecular-weight polyether as cross-linking agent, and use some high-activity polyether instead of ordinary polyether for slow rebound.
This not only disrupts the segment distribution, but also reduces the degree of phase separation, improves the degree of reaction, and reduces crystallinity. For example, replacing part of common polyether with 330N and adding a small amount of high-functionality polyether from Shanghai Dongda/Jiahua can improve the compression resistance.
d. Use MDI or add MDI to TDI.
Since the structure of MDI is different from that of TDI, the sponge produced with it has good compression resistance and less heat loss. If MDI is to be used, it is best to use modified MDI (coarse MDI with high branching degree and easy to close pores), such as Wanhua's 8019, 8223; Huntsman's 2412, liquefied MDI can also be used, because liquefied MDI is an intramolecular cyclization , more resistant to compression. The pressure resistance of all-MDI slow-rebound sponge is much better than that of pure TDI, and many manufacturers have already used it.
