Foaming mechanism of PU solution by wet coagulation

The biggest difference between wet solidification and dry solidification is that the membranes formed by wet solidification have porous structure and have great advantages in improving hand feel and sanitation performance. There are three main types of vesicles in the membrane, and their causes are different.

(1) Formation of finger hole: Because PU dissolves in DMF, PU is insoluble in water and DMF is infinitely miscible with water. When the liquid film enters the coagulation bath, the surface of the liquid film first meets water. The concentration of DMF inside and outside the film varies greatly. The diffusion rate of DMF molecules on the surface is much faster than that of water molecules entering the membrane, and the DMF in the membrane can not be replenished. That is, the surface coagulation rate is much faster than the dissolution rate, and the surface PU solidifies rapidly. The dense layer prevents water molecules from entering the membrane. As the concentration of DMF near the surface decreases, the DMF with high concentration in the lower layer moves to the upper layer continuously, which makes the PU solution in the membrane move relatively.

Due to the shrinkage of the surface solid membrane, the shrinkage stress can not be eliminated by the peristalsis of the membrane itself. The tearing of the membrane occurs at the stress concentration and becomes the growth point of finger-shaped holes. Water enters the liquid membrane along the growth point. The first PU solution contacted with water, DMF is extracted immediately. The nearby PU solution forms the difference of DMF concentration. DMF diffuses to this point, and the decrease of DMF content causes coagulation, PU. The dehydration shrinkage makes the finger hole wall shrink again, and then makes the finger hole grow again. Water enters further down the hole. The higher the DMF content of the water solution in the finger hole, the lower the concentration of DMF from inside to outside along the finger hole, resulting in a concentration difference. That is, part of the MDF in the membrane diffuses along the finger hole to the coagulation bath. In this way, DMF continuously diffuses to the coagulation bath with the concentration difference, which drives PU solution to move continuously. Film coagulation shrinkage continuously forms finger hole growth point, and water enters finger hole continuously to grow in. With the continuous occurrence of the process, a large number of finger holes of different sizes are finally obtained.

(2) Formation of sponge pore: The diffusion of water molecules causes some scattered water molecules in the liquid membrane, which aggregate with each other due to some force. When the water molecules reach a certain amount, the DMF solubility in the surrounding PU solution decreases, and finally solidifies around, forming sponge pore. So the formation of sponge hole is the result of slow solidification.

(3) Formation of bottom bubbles: DMF diffuses continuously along the concentration gradient to the coagulation bath during the solidification process of liquid film, which drives PU solution to move continuously to the upper layer. As a result, large bubbles were formed at the bottom edge of the membrane because PU solution could not be replenished.