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Analysis of Common Problems in Dry Coatings in the Process of Waterborne Polyurethane Synthetic Leather
Pinhole on the surface
The causes of pinholes on the surface are: too high viscosity of the surface layer, too thick coating, too high temperature of the first oven, short drying time of the surface layer, excessive bubbles in the prepared material, and air in the coating tank. Join and so on. The situation when the slurry is dried is shown in the figure below. Since the thickness of the release paper is much larger than the slurry, this structure has already been formed before the heat of the heat arrives.
Analysis of Common Problems in Dry Coatings in the Process of Waterborne Polyurethane Synthetic Leather
When the viscosity of the surface layer is too high, the water vapor generated during drying is not easily escaping from the coating, thereby generating a small air bag and causing pinholes. Reducing the viscosity of the top layer mixture can solve the pinholes produced by this cause. The coating is too thick and the water vapor is not easily discharged, which also causes pinholes. Reduce the thickness of the coating when this problem is encountered. If the temperature of the first oven is too high, a large amount of water vapor is instantaneously generated, and this water vapor cannot be effectively discharged, which causes pinholes. Therefore, the temperature of the first oven should not be too high. When the surface layer drying time is short and the surface layer is not completely dried, the slurry of the second layer will affect the discharge of water vapor in the surface layer, and may also cause pinholes. This requires the top layer slurry to be completely dried before it can be applied to the second layer. There are many bubbles in the well-mixed material, which is one of the reasons for pinholes on the surface. The prepared slurry needs to be used after standing defoaming. The addition of air in the coating tank also creates a surface pinhole. In this case, a coating baffle is required to fill the material tank.
The causes of the paper jam are: the gap is too small, there is residue on the paper, the paper joint is too thick or the tape is rolled, the operation is improper, the edge of the scraper is covered by the two coats, the paper jam is accumulated, and the back of the paper is taped. Wait.
When the knife gap is too small, the knife may come into contact with the release paper to jam. Therefore, it is necessary to operate according to the standard process gap. There are residual debris on the paper that will get caught between the paper and the knife, causing paper jams. In this case, the paper examiner should remove the debris from the paper in advance. If the paper connector is too thick or the tape is rolled, it will also cause a paper jam. Therefore, the paper joint must be straight and the tape should be flat and flat. Paper jams may occur when there are tapes and other debris on the back of the paper. Check the surface before use, and pay attention to the back side. If the edge of a coating blade is covered by the two coatings, the coating is applied with a second coating, and the edge material is removed by a feeler at any time. Improper operation is also one of the causes of paper jams, so it is necessary to operate according to the operating procedures.
Surface scratch line
The causes of the scratches on the surface are: the blade edge is defective, the material is dirty, the filter bucket is not clean, the scraper is not clean, the tool is not clean, the filter is not used properly, the number is wrong, and the person is not careful. Dropped in debris, etc.
Defective blade edges can cause uneven loading, too much or too little local material, resulting in scratches. In this case, the scraper should be repaired or replaced. If there is any debris in the material, the filter bucket is not clean, and these impurities will get stuck in the gap of the coating knife to affect the feeding and cause the scratching. Therefore, the filtration should be strengthened and the cleaned bucket can be used. The same equipment such as scraper is not clean, and the use of tools is not clean, which will cause scratches. Equipment should be cleaned and necessary tools should be cleaned. If the filter is used improperly and the number of meshes is wrong, the large particles in the material may get stuck between the gaps of the coating knife, resulting in scratching. Therefore, it is necessary to clarify the scope of use of the filter when filtering, and pay attention to the number of meshes. Of course, accidentally falling into the sundries is also one of the reasons for the scratch line. In this case, the responsibility should be strengthened, and the tow line should be treated immediately with a feeler gauge smaller than the coating gap.
Color strip color difference
The reasons for the color difference of the color strip are: quality problems of raw materials such as color paste, change in coating amount or unstable speed, too low material level, no mixing, poor compatibility of color materials and resin, and no color Strictness, poor coloration, stratification of materials, etc.
If the color of the color strip is caused by quality problems such as color film or color material, it is necessary to strictly control the material. A change in the amount of coating or an unstable speed of the vehicle may result in uneven thickness of the coating, resulting in chromatic aberration. Therefore, the coating gap and the speed must be stable. The material level of the trough is too low, and the unmixed material will cause the unevenness of the slurry to cause color difference of the color strip. It is necessary to keep a certain level of material during the tanning process. Sometimes the quality of the colorant is not problematic, but poor compatibility with the resin used can cause color bars. It is necessary to choose a matching color or resin. If the color difference of the color strip caused by the delamination of the material is to pay attention to the viscosity adjustment, strengthen the stirring and feeding. If the artificial color is not strict and not diligent, the color difference caused by the color difference of the ingredients, the craftsman and the squad leader should strengthen the responsibility and strictly control.
The feel is wrong, the crease is too big
The cause of the hand is wrong, the crease is too large, the resin modulus is wrong, the coating is too thick, the bonding is too wet, and the bonding pressure is not correct.
Modulus is one of the important indicators to measure the feel of the resin. The modulus of the mold is not the same. Therefore, the appropriate resin modulus should be selected. When the modulus is too high, the hardness of the finished leather is large, and creases are likely to occur. At this time, the resin modulus should be lowered. If the coating is too thick, the finished leather is easily crushed and creased. In this case, the amount of coating should be reduced. If the bonding is too wet, a large amount of slurry will penetrate into the substrate, resulting in too hard leather and creases. In this case, the dry humidity should be adjusted. If the bonding pressure is too large, a large amount of slurry will also penetrate into the substrate, resulting in a hard feel.
"Fisheye", "Orange Peel"
“Fisheye” is a small circular area. There is no coating agent in this area. “Orange peel” is a slight ripple on the surface of the coating. The reason for these two drawbacks is that the coating agent is not wetted. For paper, the solution is to add a leveling aid or reduce the viscosity, reduce the surface tension and improve the wettability. If there is an oily hydrophobe on the surface or material of the release paper, it will also cause "fisheye" and "orange peel". When selecting ingredients, choose an additive that does not contain hydrophobic substances, and do not use a batching bucket that has been exposed to oily hydrophobic substances. Do not use PVC release paper on the choice of release paper. In short, the choice of ingredients and release paper should be strictly controlled.
Stump or coating point
The reason for the bright spot or coating point is that there is debris on the back of the paper or that there is debris on the coating roll. These impurities can affect the uniformity of the coating, resulting in a padding or coating point. Therefore, the backside debris should be removed during paper inspection, and the debris on the coating roller should be removed before loading.
Pre-peeling and paper adhesion
These two drawbacks are caused by the poor compatibility of the coating agent and the release paper. The adhesion between the coating agent and the paper is too small to cause pre-peeling, that is, the surface film has been detached from the paper during the drying process of the bonding layer coating agent laminated base fabric (skin), and the corrective method is to mix in the surface layer. Put some sticky coating agent. 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.
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. .
Not fit well
In the dry production of water-based synthetic leather, it is often encountered that the problem of fit, especially the phenomenon of poor fit, which is one of the most headaches in the manufacture of synthetic leather.
There are several reasons for the lack of fit: 1 resin is not properly selected. Generally, the resin used is too soft, which will cause the bonding to be weaker. 2 The amount of sizing of the bonding material or the disintegration of the release paper is serious, resulting in the lack of local sizing, which leads to poor fit. 3 The resin is not wet after the machine is wet. Properly, the adhesive layer resin is too dry, and there is no adhesiveness, resulting in poor bonding; 4 after the bonding, the pressure roller is too loose to cause the bonding is not strong; 5 the base fabric has been treated with anti-adhesive agent or water-repellent agent, etc. The material results in an inability to bond.
In addition, the aqueous resin binder must be sufficiently dried to have adhesive force, otherwise it will be degummed when exposed to water. In particular, the beast sticker is slower than the oily resin under the same conditions. Otherwise, since the adhesive layer resin is not dried, the synthetic leather will be separated by the bottom surface.
Feel too hard
The use of a water-based resin as a bonding layer, especially for a soft synthetic leather, may result in a feeling that the hand feels too hard. The occurrence of such phenomena generally comes from two reasons: one is that the resin selection is too hard; the other is that the osmosis is severely causing the knot phenomenon. On the one hand, the viscosity of the bottom rubber is too thin, which will cause the glue to be bleed. On the other hand, if the pressure is too high, it will cause the glue or the knot, which must be treated differently.