Common shoe upper leather finishing problems generally fall into the following categories.
1. Solvent problem
In shoe production, the solvents commonly used are mainly toluene and acetone. When the coating layer encounters the solvent, it partially swells and softens, and then dissolves and falls off. This usually happens at the front and back parts. Solution:
(1) Select cross-linked or epoxy resin-modified polyurethane or acrylic resin as a film-forming agent. This type of resin has good solvent resistance.
(2) Implement dry filling treatment to enhance the solvent resistance of the coating layer.
(3) Appropriately increase the amount of protein adhesive in the coating liquid to enhance the deep solvent resistance.
(4) Spray cross-linking agent for curing and cross-linking.
2. Wet friction and water resistance
Wet friction and water resistance are very important indicators of upper leather. When wearing leather shoes, you often encounter water environments, so you often encounter wet friction and water resistance problems. The main reasons for the lack of wet friction and water resistance are:
(1) The top coating layer is sensitive to water. The solution is to implement top coating or spray waterproof brightener. When applying the top coating, if casein is used, formaldehyde can be used to fix it; adding a small amount of silicon-containing compounds to the top coating liquid can also enhance its water resistance.
(2) Excessive water-sensitive substances, such as surfactants and resins with poor water resistance, are used in the coating liquid. The solution is to avoid using excessive surfactants and choose resins with better water resistance.
(3) The temperature and pressure of the press plate are too high, and the middle coating agent is not completely attached. The solution is to avoid using excessive wax agents and silicon-containing compounds during the middle coating and reduce the temperature and pressure of the press plate.
(4) Organic pigments and dyes are used. The selected pigments should have good permeability; in the upper coating formula, avoid using excessive dyes.
3. Problems with dry friction and abrasion
When rubbing the leather surface with a dry cloth, the color of the leather surface will be wiped off, indicating that the dry friction resistance of this leather is not good. When walking, the pants often rub against the heels of the shoes, causing the coating film on the surface of the shoes to be wiped off, and the colors of the front and back are inconsistent. There are several reasons for this phenomenon:
(1) The coating layer is too soft. The solution is to use a harder and harder coating agent when coating from the bottom layer to the upper layer.
(2) The pigment is not completely adhered or the adhesion is too poor, because the proportion of pigment in the coating is too large. The solution is to increase the resin ratio and use a penetrant.
(3) The pores on the leather surface are too open and lack wear resistance. The solution is to implement dry filling treatment to increase the wear resistance of the leather and strengthen the fixation of the coating liquid.
4. Leather cracking problem
In areas with dry and cold climates, leather cracking is often encountered. It can be greatly improved by rewetting technology (rewetting the leather before stretching the last). There are now special rewetting equipment.
The main reasons for leather cracking are:
(1) The grain layer of the upper leather is too brittle. The reason is improper neutralization, resulting in uneven penetration of the retanning agent and excessive bonding of the grain layer. The solution is to redesign the water field formula.
(2) The upper leather is loose and of lower grade. The solution is to dry fill the loose leather and add some oil to the filling resin so that the filled leather is not too hard to prevent the upper from cracking during wear. The heavily filled leather should not be left for too long and should not be over-sanded.
(3) The base coating is too hard. The base coating resin is improperly selected or the amount is insufficient. The solution is to increase the proportion of soft resin in the base coating formula.
5. Crack problem
When the leather is bent or stretched hard, the color sometimes becomes lighter, which is usually called astigmatism. In severe cases, the coating layer may crack, which is usually called crack. This is a common problem.
The main reasons are:
(1) The elasticity of the leather is too large (the elongation of the upper leather cannot be greater than 30%), while the elongation of the coating is too small. The solution is to adjust the formula so that the elongation of the coating is close to that of the leather.
(2) The base coating is too hard and the top coating is too hard. The solution is to increase the amount of soft resin, increase the amount of film-forming agent, and reduce the amount of hard resin and pigment paste.
(3) The coating layer is too thin, and the upper layer of oily varnish is sprayed too heavily, which damages the coating layer. In order to solve the problem of wet rubbing resistance of the coating, some factories spray excessive oily varnish. After solving the problem of wet rubbing resistance, the problem of cracking is caused. Therefore, attention must be paid to process balance.
6. The problem of slurry shedding
During the use of shoe upper leather, it must undergo very complex environmental changes. If the coating is not firmly adhered, the coating will often slurry shedding. In severe cases, delamination will occur, which must be given high attention. The main reasons are:
(1) In the bottom coating, the selected resin has weak adhesion. The solution is to increase the proportion of adhesive resin in the bottom coating formula. The adhesion of the resin depends on its chemical properties and the size of the dispersed particles of the emulsion. When the chemical structure of the resin is determined, the adhesion is stronger when the emulsion particles are finer.
(2) Insufficient coating amount. During the coating operation, if the coating amount is insufficient, the resin cannot infiltrate the leather surface in a short time and cannot fully contact the leather, the fastness of the coating will be greatly reduced. At this time, the operation should be adjusted appropriately to ensure sufficient coating amount. Using brush coating instead of spray coating can increase the penetration time of the resin and the adhesion area of the coating agent to the leather.
(3) The influence of the condition of the leather blank on the adhesion fastness of the coating. When the water absorption of the leather blank is very poor or there is oil and dust on the leather surface, the resin cannot penetrate the leather surface as necessary, so the adhesion is insufficient. At this time, the leather surface should be properly treated to increase its water absorption, such as performing a surface cleaning operation, or adding a leveling agent or a penetrant to the formula.
(4) In the coating formula, the ratio of resin, additives and pigments is inappropriate. The solution is to adjust the type and amount of resin and additives and reduce the amount of wax and filler.
7. Heat and pressure resistance issues
The upper leather used in molded and injection molded shoe production must be heat and pressure resistant. Generally, shoe factories often use high-temperature ironing to iron out wrinkles on the leather surface, causing some dyes or organic coatings in the coating to turn black or even become sticky and fall off.
The main reasons are:
(1) The thermoplasticity of the finishing liquid is too high. The solution is to adjust the formula and increase the amount of casein.
(2) Lack of lubricity. The solution is to add a slightly harder wax and a smooth feel agent to help improve the lubricity of the leather.
(3) Dyes and organic coatings are sensitive to heat. The solution is to choose materials that are less sensitive to heat and do not fade.
8. Light resistance problem
After being exposed for a period of time, the surface of the leather becomes darker and yellower, making it unusable. The reasons are:
(1) The discoloration of the leather body is caused by the discoloration of oils, plant tannins or synthetic tannins. The light resistance of light-colored leather is a very important indicator, and oils and tannins with good light resistance should be selected.
(2) Coating discoloration. The solution is that for upper leathers with high light resistance requirements, do not use butadiene resin, aromatic polyurethane resin and nitrocellulose varnish, but use resins, pigments, dye water and varnish with better light resistance.
9. Cold resistance (weather resistance) problem
Poor cold resistance is mainly reflected in the cracking of the coating when the leather encounters low temperature. The main reasons are:
(1) At low temperatures, the coating lacks softness. Resins with better cold resistance such as polyurethane and butadiene should be used, and the amount of film-forming materials with poor cold resistance such as acrylic resin and casein should be reduced.
(2) The proportion of resin in the coating formula is too low. The solution is to increase the amount of resin.
(3) The cold resistance of the upper varnish is poor. Special varnish or ,-varnish can be used to improve the cold resistance of leather, while nitrocellulose varnish has poor cold resistance.
It is very difficult to formulate physical performance indicators for upper leather, and it is not realistic to require shoe factories to purchase completely according to the physical and chemical indicators formulated by the state or enterprises. Shoe factories generally inspect leather according to non-standard methods, so the production of upper leather cannot be isolated. It is necessary to have a better understanding of the basic requirements of the shoemaking and wearing process in order to carry out scientific control during the processing.
Post time: May-11-2024