The purpose of processing is to make liquid PU resin form solid film with certain structure between fiber spaces. The PU resin, DMF and additives are formulated into size which fully penetrates into the space of nonwoven fabrics by squeezing. Basing on the miscibility of DMF-H2O system and the coagulation of PU in water, the bi-directional diffusion of water and DMF is finished through the coagulation bath. The driving force of diffusion mainly is concentration difference of coagulation bath and DMF in the base. DMF continuously enter into the coagulation bath from the resin solution, at the same time the water enters into the base to make PU gradually becomes viscose state from dissolved state, and finally a solid film is formed. In the film-forming process a large number of micropore are formed because of the application of additives and shrinkage of PU. In coagulation process, PU itself only has the phase transition and its molecular component has no change.
The processing technologies of base mainly include the composite of nonwoven fabrics with PU and the coagulation of PU-DMF-H2O. The functions of main raw materials in the formation of base are as follows.
They are the basic structure of synthetic leather base and they play the role of support and “backbone” in the base.
The coagulation bath is aqueous solution of DMF and its function is to coagulate PU. The coagulation speed can be changed by changing temperature and concentration so that the microporous structure of PU can be adjusted.
It refers to the solution in which PU resin is dissolved, and it is used to dip nonwoven fabrics, as a result, a continuous film is formed between fiber spaces. In this way, the structure of base becomes more integral and the base has good handle, flexibility and physical and chemical properties,
The coating liquid can form a layer film on the surface of dipped base, so that the synthetic leather is provided with good air and moisture permeability.