The impact of TPU film technology on the moisture permeability of the film

The moisture permeability mechanism of the TPU film is that water vapor molecules are first adsorbed on the hydrophilic groups on the surface of the film, then diffused and transferred through the amorphous zone composed of soft segments, and finally desorbed and escaped from the other side. Therefore, any process factors that affect the material microstructure and film thickness will have a significant effect on the moisture permeability performance.

First of all, film thickness is the most direct influencing factor. The amount of moisture permeability is roughly inversely proportional to the thickness. During the casting, film blowing or calendering process, the thickness is controlled by matching the die lip gap, screw speed, traction speed and cooling temperature. The thinner the film, the shorter the diffusion path of water molecules and the better the moisture permeability. However, being too thin will sacrifice mechanical strength and water pressure resistance, which requires a comprehensive trade-off.

Secondly, the processing temperature significantly affects the microphase separation structure of hard segments and soft segments. If the melt temperature is too high, it will destroy the hydrogen bonds and even cause degradation. If the temperature is too low, the plasticization will be uneven. Both of them will disrupt the formation of moisture transmission channels. Only a suitable temperature can form a morphological structure that is conducive to water vapor penetration.

Third, the cooling rate determines the degree of crystallization of the hard segments. Rapid cooling can inhibit crystallization, increase the proportion of amorphous areas, make it easier for water molecules to diffuse in them, and increase moisture permeability. ； Slow cooling makes the crystallization perfect, the structure dense, and the moisture permeability decreases accordingly.

Fourth, the impact of the stretching process is more complex. For dense films, orientation causes the molecular chains to be arranged regularly, the free volume to be reduced, and the moisture permeability to be reduced. ； For microporous membranes that have been biaxially stretched and combined with pore-forming treatment, water vapor can pass directly through the micropores, and the moisture permeability is doubled.

Fifth, post-processing such as annealing will change the degree of crystallinity and phase separation, and a balance needs to be struck between moisture permeability, waterproofing and mechanical properties during production. In addition, polyether-type TPU is superior to polyester-type due to the hydrophilicity of its ether bonds, and has higher moisture permeability. It can further improve the moisture permeability effect when combined with hydrophilic modification methods such as polyethylene glycol. Generally speaking, the core of process control is to regulate the microstructure and thickness of the film to achieve the coordination and unity of moisture permeability and other properties.