Brief analysis of synthetic leather production: what problems are often encountered in dry leather production
In the production of synthetic leather, it is mainly divided into two production processes: wet method (also known as coagulation coating) and dry method (also known as transfer coating). The dry production process is to apply a coating agent (now mostly polyurethane) on a sheet carrier (release paper), after drying to form a continuous, uniform film, and then apply a binder to the film. It is laminated with the fabric (or wet base) and then dried to remove the release paper from the leather. The coating agent (including the bonding layer) is transferred from the release paper to the fabric (or leather). . The dry process is a relatively complex process involving changes in both chemical and physical properties. Now I will talk about my views and opinions on the problems and solutions that are often encountered in various production processes for peer reference.
Selection and use of release paper
Release paper, also known as transfer paper, accepts a liquid coating agent and carries a coating agent into the oven to allow the coating agent to be smoothly peeled off from the paper after film formation on the paper. Therefore, the dry production has the following types of release paper. Request 1. Resistant to the solvent of the coating agent 2 . The paper itself must have sufficient strength, stiffness and flexibility. The paper surface state (haze, pattern depth and sharpness) is uniform. Release paper is divided into full PU paper, PVC paper and high temperature dual-use paper according to the purpose; according to the surface brightness, there are high light, semi-light, matting, etc.; according to the pattern, there are cowhide, sheepskin, pigskin, cloth, etc.; There are American paper, British paper, Italian paper and Japanese paper. The choice of release paper should be selected according to the product variety and process characteristics, and the price and performance should also be considered. When the release paper is used, it often encounters the following problems and must be solved.
When transporting the release paper, care must be taken during handling to prevent collision and damage. When storing, it must be placed horizontally to prevent moisture.
When the release paper machine is produced, the tension and the parallelism of the guide rollers should be adjusted to prevent the release paper from being deflected and damaging the paper edge. Try to avoid parking during the production process. When using PU paper, the oven temperature should not be too high, preferably below 130 degrees Celsius to prevent the paper surface thermoplastic resin from being deformed due to overheating.
The angle between the leather and the paper should be greater than 90 degrees, preferably 135 degrees, to minimize the peel force. The release paper runs on the machine and is prone to static electricity. A static elimination device must be installed. The simple method is to put a few wet hemp ropes under the paper near the peeling point (be careful not to touch the paper) or pull a few thin copper wires.
The front and back sides of the release paper are different in material, and the shrinkage rate is different when exposed to heat and cooling, which is easy to cause curling. The adjustment method has the ability to adjust the traction force of the equipment on the paper and lower the oven temperature.
For each release of the release paper, it is best to check it once to remove all kinds of defects and eliminate some of the static electricity. It is best to use only one roll per roll during the day of production (especially Warren Paper).
Common problems and solutions in coating production
In the production of coatings, depending on the product requirements, some only apply one surface layer, some coating surface layer and bonding layer, and some coating top layer, surface layer and bonding layer. The method of superimposing with the base fabric (skin) is wet, semi-dry, and dry. Now explain the common problems and solutions in the production process.
Top coat: The top coat is generally made of high modulus resin, the purpose is to increase the touch (smooth, waxy) and wear resistance of the product, or to prevent yellowing, or to produce two colors; gap coating, easy to produce the disease is The coating is uneven, and the color strips and spots are generated. The solution is to reduce the viscosity of the coating agent; use a rubber or sponge with better elasticity under the blade; add the coating and the leveling aid to the coating agent. Agent; choose the appropriate scraper.
Top coat: The top layer is the main body of the dry coating product, which bears the main mechanical function of the coating film. Therefore, the quality is directly reflected in the quality of the final product, and its common disease and solution. As follows: (1) There are pinholes or fine bubbles on the coating film: the pinhole is the whole film (or part) penetrated by the small hole, and the fine bubble is on the surface of the film. The main reason for forming pinholes and fine bubbles is physicochemical properties. On the one hand, there is no reasonable balance between solvent evaporation rate and diffusion rate. Another reason is that in terms of physical properties, there is no defoaming after the seasoning or the small bubbles are not cleaned, or the air is wrapped in the coating or the texture of the paper is very deep, and the coating agent is too late to completely remove the air from the embossed recess. . The solution is as follows: 1 low-boiling and high-boiling solvent in the coating agent should be reasonably proportioned, increase the ratio of high-boiling solvent, 2 oven drying temperature and wind speed segmentation, low-to-high section-by-section adjustment 3, reduction coating gap 4 coating The layering agent must be fully defoamed or reduced in viscosity. 5 Adding the tank and maintaining the storage height 6 Add defoaming and leveling aid. In actual production, the specific cause of pinholes and fine bubbles must be analyzed and one or more solutions selected.
"Fisheye" and "orange peel": The fisheye is a small circular area. There is no coating agent in this area. The orange peel is slightly corrugated on the surface of the coating. The reason for these two diseases is coating. The layering agent does not wet the release paper. The solution is to add a leveling aid or to increase the proportion of the less polar solvent to reduce the surface tension.
Chromatic aberration and color strip: The reason for the color difference and color strip is that the compatibility of the colorant (or pearlescent pigment) with the coating agent is poor or the dispersibility of the colorant is poor or the stirring is uneven. The solution is to add an auxiliary agent to the coating agent. Prevent floating color and increase dispersion. 2 Re-mix the material evenly. 3 Increase the agitation in the storage tank. 4. Reduce the distance between the storage tank baffle and the coating knife and maintain the storage height to increase the rolling speed of the material.
Ideas or slivers: The pips themselves are contaminants. They may come from coating agents that are brought into the coating agent or air-dried on the production line. When the pips are large, they will form a sliver between the applicator and the paper. The solution is 1 Increase the filtration of the coating material 2 Keep the production line and paper clean. 3 Always pay attention to the points stuck on the coating knife and remove them in time.
Pre-peeling and paper adhesion: These two rickets are caused by poor compatibility of the coating agent and the release paper. The adhesion of the coating agent to the paper is too small to cause pre-peeling, i.e., in the tie layer coating agent. Laminated base fabric (skin). During the drying process, the top film has been detached from the paper. The corrective method is to mix some sticky coating agents in the surface layer. The coating agent has too much adhesion to the paper, and when peeling off, the peeling resin on the paper surface is peeled off or the paper is torn and broken. The solution is to add a peeling and anti-sticking aid to the top coating agent, and Choose a coating agent that works well with release paper.
Adhesive layer coating: The choice of binder should consider its compatibility with the top layer film and the bond strength of the bond with the base fabric (skin). The binder has a single and double combination. Generally, manufacturers use a single component, that is, a peeling type. The main problem is that the solvent of the adhesive layer coating agent does not have the compatibility of the surface layer film to produce swelling and penetration. The solution is: 1 increase the thickness of the top layer film 2 increase the solid content of the bonding layer or reduce the coating Volume 3 speeds up the vehicle and reduces the contact time between the adhesive layer solvent and the top film. Another problem is that the solvent resistance of the final product is not good (such as not resistant to sodium or water or toluene or methyl ethyl ketone). The treatment method is to choose the right bond. Add bridging agent and accelerator to the binder.
Laminated base fabric (skin): The basic process is to align the base fabric (skin) with the release paper that has been coated with the adhesive layer coating agent, and then pass through a pair of pressure rollers and then enter the oven to dry. The main process parameters are the viscosity and thickness of the coating adhesive, as well as the gap and pressure of the roll. The gap is generally controlled at 50-70% of the sum of the thickness of the base fabric (skin) and the release paper. The air pressure is generally 3- 4KG/CM2. There are wet, semi-dry and dry stickers for the stacking method. The main problems when superimposing are loose and rotten faces. The solution is to adjust the lamination method or adjust the dry humidity of the adhesive or the gap and pressure of the rolls according to the product and the base fabric (cloth). In addition, the base fabric (skin) is preferably heated and flattened by a preheating roller and then laminated with the coating agent to make the laminate more firm, the surface is flatter, and the hand feel is more.
In short, the dry coating production process is a relatively complicated physical and chemical change process, and the problems in actual production are also ever-changing. When dealing with problems, we must first find out the reasons to determine the solutions and measures, and obtain qualified products. .
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