Pinhole defects appear on the surface

 

The causes of pinhole defects on the surface include: excessively high surface viscosity, excessively thick coating, excessively high temperature in the first oven, short surface drying time, excessive bubbles in the prepared material, and air entering the coating tank. The situation of the slurry during drying is shown in the figure below. Because the release paper thickness is much greater than the slurry, this structure is formed before the heat arrives.

 

Analysis of Common Problems in Dry Coating during the Manufacture of Water-Based Polyurethane Synthetic Leather

 

When the surface viscosity is too high, the water vapor generated during drying is difficult to escape from the coating, resulting in small air pockets and pinholes. Reducing the viscosity of the surface mixture can solve the pinholes caused by this reason. If the coating is too thick, water vapor is also difficult to discharge, which will also cause pinholes. When encountering this problem, the coating thickness should be reduced. If the temperature of the first oven is too high, a large amount of water vapor will be generated instantly, and this water vapor cannot be effectively discharged, which will cause pinholes. Therefore, the temperature of the first oven cannot be too high. When the surface drying time is short, and the surface layer is not completely dried, the second layer of slurry will affect the discharge of water vapor from the surface layer, which will also cause pinholes. This requires that the surface slurry must be completely dried before applying the second layer. Too many bubbles in the prepared material are also one of the reasons for surface pinholes. The prepared slurry needs to be allowed to stand and degas before use. Air entering the coating tank will also cause surface pinholes. In this case, a baffle should be added to the coating to fill the tank.

 

Paper Jamming

 

The causes of paper jamming include: too small a gap, residue on the paper, too thick paper joints or curled tape, improper operation, the first coating knife edge being covered by the second coating material accumulation causing paper jamming, and adhesive tape or other debris on the back of the paper.

 

When the coating knife gap is too small, the knife may contact the release paper, causing paper jamming. Therefore, operation should be carried out according to the standard process gap. Residues on the paper will be trapped between the paper and the knife, causing paper jamming. In this case, the paper inspector should remove the debris from the paper in advance. If the paper joint is too thick or the tape is curled, it will also cause paper jamming. Therefore, the paper joint must be straight, and the tape must be firmly adhered and flattened. If there is adhesive tape or other debris on the back of the paper, paper jamming will also occur. Before use, the paper inspector should check the back in addition to the surface. If the edge of the first coating knife is covered by the second coating material accumulation causing paper jamming, the second coating material should be used to cover the first coating material, and the edge material should be removed with a feeler gauge at any time.

 

Surface Scratches

 

The causes of surface scratches include: defects in the blade of the doctor blade, debris in the material, unclean filter bucket, unclean doctor blade and other coating equipment, unclean tools, improper use of the filter screen, incorrect mesh number, and accidental dropping of debris by humans.

 

Defects in the blade of the doctor blade will lead to uneven material application, resulting in too much or too little material locally, causing scratches. In this case, the doctor blade should be repaired or replaced. If there is debris in the material and the filter bucket is not clean, this debris will be stuck in the gap of the coating knife, affecting the material application and causing scratches. Therefore, filtration should be strengthened, and only clean buckets can be used. Similarly, unclean doctor blades and other coating equipment, and unclean tools will also cause scratches. Equipment hygiene should be maintained, and necessary tools should be cleaned. If the filter screen is used improperly or the mesh number is incorrect, large particles in the material may be stuck in the gap between the coating knives, causing scratches. Therefore, when filtering, the scope of use of the filter screen should be clarified, and attention should be paid to the mesh number. Of course, accidentally dropping debris by humans is also one of the reasons for scratches. In this case, responsibility should be strengthened, and any trailing lines should be immediately treated with a feeler gauge smaller than the coating gap.

 

Color Banding and Color Difference

 

The causes of color banding and color difference include: quality problems of color paste, colorants, and other raw materials; changes in coating amount or unstable speed; too low material level in the tank, no stirring; poor compatibility between colorants and resin; lax or infrequent color matching; color difference in mixing; and material stratification.

 

If the color banding and color difference are caused by quality problems of color chips or colorants and other raw materials, the materials must be strictly controlled. Changes in coating amount or unstable speed will cause uneven coating thickness, resulting in color difference. Therefore, the coating gap and speed must be stable. Too low material level in the tank and no stirring will lead to uneven slurry and cause color banding and color difference. Maintaining a certain material level during leather making and continuous stirring by the coating personnel are essential. Sometimes, the quality of the colorant is not a problem, but poor compatibility with the resin used will cause color banding. It is essential to choose matching colorants or resins. If the color banding and color difference are caused by material stratification, attention should be paid to viscosity adjustment and enhanced stirring and material application. If the color banding and color difference are caused by lax or infrequent color matching and color difference in mixing, the technicians and foremen should strengthen their responsibility and strictly control the quality.

 

Incorrect Hand Feel, Excessive Creases

 

The causes of incorrect hand feel and excessive creases include: incorrect resin modulus, excessively thick coating, excessively wet bonding, and incorrect bonding pressure.

 

Modulus is one of the important indicators for measuring the hand feel of resin. If the modulus is incorrect, the hand feel of the finished leather will also be different. Therefore, an appropriate resin modulus should be selected. When the modulus is too high, the finished leather will be hard and prone to creases. In this case, the resin modulus should be reduced. If the coating is too thick, the finished leather will be easily squeezed and creased. In this case, the coating amount should be reduced. If the bonding is too wet, a large amount of slurry will penetrate into the substrate, making the finished leather too hard and prone to creases. In this case, the bonding dryness should be adjusted. If the bonding pressure is too high, it will also cause a large amount of slurry to penetrate into the substrate, resulting in a hard hand feel.

 

Fish Eyes and Orange Peel

 

"Fish eyes" are small circular areas without coating agent, while "orange peel" refers to slight wrinkles on the surface of the coating film. The cause of these two defects is that the coating agent fails to wet the release paper. The solution is to add a leveling agent or reduce the viscosity, reduce the surface tension, and improve the wettability. If there are oily hydrophobic substances on the surface of the release paper or in the material, it will also cause "fish eyes" and "orange peel." When mixing the materials, choose an auxiliary agent that does not contain hydrophobic substances, and do not use mixing buckets that have been in contact with oily hydrophobic substances. When choosing release paper, do not use release paper that has been used for PVC. In short, strict control should be exercised in the selection of materials and release paper.

 

Padding defects or coating spots

 

The reason for these defects or coating spots is the presence of foreign matter on the back of the paper or on the coating roller. This foreign matter affects the uniformity of the coating, resulting in padding defects or coating spots. Therefore, it is necessary to remove foreign matter from the back of the paper during inspection and from the coating roller before feeding.

 

Premature peeling and adhesion to release paper

 

Both of these defects are caused by poor compatibility between the coating agent and the release paper. Poor adhesion between the coating agent and the paper causes premature peeling, i.e., the surface film separates from the paper during the drying process of the bonding layer coating agent and backing fabric (leather). The solution is to mix some adhesive coating agent into the surface layer. If the adhesion between the coating agent and the paper is too strong, peeling will cause the release resin on the paper surface to peel off or tear the paper. The solution is to add a release agent and anti-adhesive agent to the surface coating agent and select a coating agent with better compatibility with the release paper.

 

In short, the dry coating production process is a relatively complex physical and chemical process. The problems encountered in actual production vary widely. When dealing with problems, it is necessary to first find the cause before determining the solution and measures to obtain qualified products.

 

Poor bonding

 

In the dry production of water-based synthetic leather, problems with bonding, especially poor bonding, are frequently encountered. This is one of the most troublesome problems in synthetic leather manufacturing.

 

Poor bonding is generally caused by the following factors: ① Improper resin selection. Generally, the use of resins that are too soft will more likely cause poor bonding; ② The amount of sizing on the bonding material or serious damage to the release paper causes local sizing defects, leading to poor bonding; ③ Improper control of the dry and wet state after the resin is applied, resulting in the bonding layer resin being too dry and lacking adhesion, leading to poor bonding; ④ The pressure roller is too loose after bonding, leading to poor bonding; ⑤ The base fabric has been treated with anti-adhesive agents or water-repellent agents, preventing bonding.

 

In addition, water-based resin adhesives must be fully dried to have adhesion; otherwise, they will delaminate when exposed to water. In particular, for base materials, the speed under the same conditions is more than half slower than that of oil-based resins. Otherwise, because the bonding layer resin is not dried, the synthetic leather will show bottom separation after water rubbing.

 

Too stiff hand feel

 

When using water-based resin as the bonding layer, especially for bonding soft synthetic leather, the hand feel may be too stiff. This phenomenon generally comes from two aspects: one is that the resin selected is too hard; the other is that serious glue penetration leads to consolidation. On the one hand, the viscosity of the base glue resin is too thin, which will cause glue penetration, and on the other hand, too high pressure will also cause glue penetration or consolidation. It is necessary to distinguish and analyze and deal with it comprehensively.