Adjusting sponge hardness has always been a difficult problem in the production of polyurethane block sponges. Sponge hardness can be adjusted in different ways. Usually, decreasing the hardness of a sponge, i.e. softening the sponge, does not change the other properties of the sponge too much, but it is not possible to increase the hardness of a sponge without adversely affecting the other properties of the sponge.
There are several ways to increase the hardness of a sponge. The first thought is to increase the density of the sponge to increase the hardness of the sponge, sponge density per 1kg/m3 hardness can be increased by about 1.0kPa (CLD 40%), but this will increase the cost; the use of filler materials (CaCO3 and BaSO4) can also increase the hardness, but this method is not permanent, and will reduce the physical properties of the sponge; there is also a common method is to add a polymer polyol, with this method can be used to increase the hardness of the sponge. polymer polyol, with this method can increase the hardness, but the cost of the sponge will be greatly increased; a method to strengthen the three-dimensional structure of the sponge, that is, to increase the number of crosslinks, which can be achieved by increasing the TDI index, however, when the sponge with this method will make the vesicle pore structure change, the permeability decreases, and the number of closed pores increases. In addition, as the reaction temperature increases, the risk of yellowing or even fire within the block increases; another method is to accelerate the gelling reaction, i.e., increase the tin catalyst. This method also causes changes in the structure of the sponge’s vesicles and a decrease in air permeability.
Shinden Chemical has been conducting long-term research on the hardness mechanism of sponges in response to industry needs. As a result of these studies, a new series of hardening additives for sponges has been developed to meet customer needs. The following table shows the list of this series of additives from SHINODEN CHEMICAL:
NT H12 NT H15 NT H18
Type Amine reactive hardener Polyester hardener Polyether hardener
Advantages and disadvantages The product itself has ammonia odor, can participate in the reaction, no VOC emission, can be miscible with the combination of material system, do not need to adjust the formula, the amount of addition is small. Can participate in the reaction, poor compatibility with the combination of material system, can reduce the TDI index to save costs, the price is cheap. Can participate in the reaction, can be miscible with the combination of material system, can reduce the TDI index to save costs, the amount added is larger, the overall price is high.
Take NT H18 as an example, specific experiments to illustrate its performance effect.
Unlike other hardening additives on the market, NT H18 can significantly increase the hardness without affecting the pore structure of the foam. When NT H18 is added to the formulation, there is no need to change the amount of TDI or tin to maintain the open cell structure. The amount used is 1 to 5 pphp, depending on the desired increase in hardness. Depending on the foam density, formulation and production parameters, it is possible to increase the hardness by up to 35% without significantly changing the pore structure.
Table I: General formulations for a laboratory box foam density of 16 kg/m3 sponge are as follows:
Polyol OHN 48 (ppphp) 100-X
Total water (ppphp) 5.2
NT CAT A-33 (ppphp) 0.15
NT CAT T-9 (ppphp) 0.25
Tegostab B8160 (ppphp) 1.3
Methylene Chloride (ppphp) 7.5
NT H18 (ppphp) X
TDI Index 110
As shown in Figure 1, the addition of 3pphp and 5pphp of NT H18 to a sponge with a density of 16kg/m3 increased the hardness of the sponge by 25% and 35% under the laboratory conditions of the reference formulation.
Figure 1: Hardness change curves for NT H18 sponges with 3pphp and 5pphp.
NT H18 can be used not only to increase hardness, but also to reduce the amount of TDI, polymerized polyols or tin in a formulation while maintaining sponge hardness. Therefore, it can be used to reduce costs and facilitate process conditions (reaction temperatures) if desired. As shown in Table 2, the addition of 3 pphp of NT H18 to a sponge with a density of 21 kg/m3 reduces the TDI index from 105 to 100 under laboratory conditions of the reference formulation, the hardness remains almost constant, and the yellowing of the sponge can be significantly improved, as shown in Figure II.
Table II: The general formulation of the sponge with the experimental chamber bubble density of 21 kg/m3 is as follows:
Polyol OHN 48 (ppphp) 100 97
Total water (ppphp) 4.8 4.8
NT CAT A-33 (ppphp) 0.20 0.20
NT CAT T-9 (ppphp) 0.22 0.22
Tegostab B8232 (ppphp) 1.0 1.0
TDCPP (ppphp) 10 10
NT H18 (ppphp) 0 3
TDI Index 110 105
Figure 2: Comparison of yellowing results of sponges obtained from experiments
Summary: Comprehensive results of the above experiments, it can be concluded that NT H18 can significantly improve the hardness of the sponge and other physical properties of the sponge itself basically does not affect, and to a certain extent, can improve the yellowing of the sponge.