The production method of wet-process polyurethane synthetic leather is to add DMF solvent and other fillers and additives to the polyurethane wet resin to make a mixed liquid. After degassing by a vacuum machine, it is impregnated or coated on the base fabric, and then placed in water that has affinity with the solvent (DMF) but not with the polyurethane resin. The solvent (DMF) is replaced by water, and the polyurethane resin gradually solidifies to form a porous film, that is, a microporous polyurethane grain layer, which is usually called base (the transliteration of the English word BASE), which means the base material (semi-finished leather). The base can only become a finished polyurethane synthetic leather after dry veneer or surface finishing, such as surface printing, embossing, grinding and other processes. Wet-process polyurethane synthetic leather has good air permeability, moisture permeability, smooth and plump feel, excellent mechanical strength, and is especially similar to natural leather in structure. The production process of wet-process synthetic leather base can be divided into three types: single coating method, impregnation method and impregnation coating method. The base fabrics used are woven fabrics and non-woven fabrics.

　　(I) Single coating polyurethane bass

　　1. Production process

　　The base fabric is unrolled and passed through the fabric storage rack to enter the dipping tank for soaking. Most of the water is squeezed out by the squeezing roller, and the water is removed by the ironing wheel. At the same time, the base fabric is ironed, and then the matching slurry is coated on the coating machine, and then it enters the coagulation tank to form a film, and then it is fully washed, dried, shaped, and cooled into a large roll.

　　2. Main raw materials

　　A. Polyurethane resin: usually ordinary wet resin, special for grinding, special for impregnation and cold-resistant resin, etc. The modulus of the resin (100%) ranges from 2.0MPa to 30.0MPa. According to the softness and hardness of the bass, high and low modulus grades of resin are selected. Single-coated bass has small pores and high density, and a large amount of wood powder and other fillers are often added. Therefore, when the product is used in cold areas, the cold resistance of the product must be fully considered, and resin with good cold resistance should be used.

　　B. Wood powder: Using a certain amount of wood powder in single-coated bass can not only reduce product costs, but also play a role in the solidification process. Wood powder from different manufacturers has different expansion coefficients, so that under the same other materials, the viscosity values are different, which directly affects the quality of the product and the corresponding cost. The fineness of wood powder is generally required to reach 400 mesh or more.

　　C. Anionic surfactant (C-70, C-90): Also known as fast penetrant, it is hydrophilic. It mainly accelerates the exchange rate between DMF and water, increases production speed, and makes the pores finer. Anionic surfactant can generate spherical pore structure, increase resilience, air permeability, and moisture permeability. The general addition amount is between 0.5% and 2.5%. If more is added, the coating is easy to curl and the smoothness decreases.

　　D. Nonionic surfactant (S-80): It is hydrophobic and can delay the solidification speed of the surface, so that the internal DMF and water can be exchanged faster, and a needle-shaped pore structure can be generated. The addition amount is 1%-3%. If it is too large, the production speed will be affected.

　　E. Solvent (DMF): DMF is used to dissolve and dilute polyurethane resin. It is directly mixed with resin to adjust the viscosity of the mixed solution. When the amount of DMF is large, it increases the solidification speed and enlarges the pore structure during the solidification process.

　　F. Color paste: Products with a single solvent DMF system should be used, and the usual addition amount is 5%-8%.

　　G. Base fabric: The base fabric used for single coated bass is mainly based on plain woven fabric and single-sided raised fabric. The amount of cotton in its yarn count directly affects the time it takes to soak in water.

　　3. Key points of production process operation

　　A. Preparation of polyurethane slurry:

　　The slurry viscosity required for single-coating bass is relatively high, generally controlled at 7000-9000Pa.s. The order of adding materials is more important when mixing. Generally, DMF is added to the container first, and then wood powder is added, and the mixture is fully stirred for about 10 minutes with a mixer to ensure that the wood powder is dispersed in the DMF. If DMF and polyurethane are mixed and stirred first, and then wood powder is added, the wood powder will clump and cannot be fully dispersed in the resin solution. Even if filtered, the clumped wood powder is very easy to clog the filter screen, and it is seriously unusable. After the prepared slurry viscosity meets the requirements, it will be degassed by a vacuum degassing machine for 40 minutes. If the slurry viscosity is high, the degassing time can be appropriately extended, and finally filtered with a 60-100 mesh filter screen.

　　B. Water immersion treatment of base fabric:

　　The base fabric is sent to the immersion tank through the fabric storage rack, and then the water in the base fabric is squeezed out by the squeezing roller, and the base fabric is ironed half dry by the ironing wheel. The immersion treatment has two functions. One is to increase the humidity of the fabric to prevent the slurry from penetrating into the base fabric structure, causing the bottom see-through phenomenon and wasting raw materials. The second is that for base fabrics with poor degreasing properties, in addition to clean water, 1% anionic surfactant should be added to the water tank to improve the hydrophilicity of the base fabric and improve the appearance quality of the base fabric.

　　C. Coating solidification:

　　The treated base fabric is evenly coated with the slurry mixture on the base fabric through the coating station using a coating knife. When using a raised fabric as the base, pay attention to the direction of the raised fabric surface. If it is coated along the raised fabric, the base surface will be smooth. If it is coated against the raised fabric, the base surface will be rough. If the coating is too thin, it cannot cover the fabric hair, the base surface will be rough, and the hand feel will be inelastic. If the coating is too thick, it will easily cause uneven bubbles and separation of the surface layer from the base material. The coagulation liquid in the coagulation tank is composed of water and DMF, and the content of DMF is generally 20%-25%.

　　When producing polished leather bass, the DMF content is controlled between 10% and 15%. Too high a content will affect the solidification speed and increase the solid content in the water. Too low a concentration will not only increase the recovery cost of DMF, but also increase the shrinkage rate of the bass surface, causing the curling DMF to migrate quickly into the water, while the water penetrates into the material layer at a slower rate. During the solidification process of the slurry, a certain amount of shrinkage occurs, making the coating thinner and curling to a certain extent. To prevent curling, on the one hand, the DMF concentration in the solidification tank must be ensured, and on the other hand, more S-80 can be added during the batching to delay the solidification of the surface and increase the pores. You can also use a clip to clamp the edge of the cloth and increase the tension roller method to prevent curling.

　　The temperature of the coagulation tank is generally room temperature. It can be heated appropriately in winter, usually controlled at 25°C. Too high a temperature will reduce the fracture strength of the coating, reduce the film modulus, and worsen the surface uniformity, but the microporous structure is uniform. It takes about 8-15 minutes for the coating layer to solidify completely from entering the water to leaving the coagulation tank. This is closely related to the formula, resin brand, DMF concentration in the water, and temperature. Before the material layer is completely solidified, it cannot contact the guide roller, otherwise it will damage the surface layer.

　　D. Washing, drying and coiling:

　　After the polyurethane coating layer is completely solidified, a certain amount of DMF still remains in its cellular layer. These DMF must be forcibly removed in the water washing tank. If the removal is not clean, it will cause defects such as pitting on the surface of the bass after drying. In terms of process control, it is necessary to ensure that the content of DMF in the last water washing tank is below 1%, so that the DMF in the bass can be removed cleanly. In order to speed up the washing speed of DMF and reduce the DMF remaining in the bass, the water washing tank generally needs to be heated. Practice has shown that the temperature of the water washing tank has a great influence on the washing speed of DMF and the amount of DMF remaining in the bass. During the washing process, especially in the second half of the water washing tank, there is very little DMF remaining in the bass. Therefore, the time the bass stays in the washing water has a far less impact on the degree of DMF cleaning than the number of extrusions. After the bass is washed clean, it needs to be dried to remove moisture. The oven temperature should not be too high, generally not exceeding 150°C. At the inlet, the temperature can be higher because the base contains a lot of moisture. As the moisture evaporates, the temperature at the back of the oven can be lower, generally 120°C.