Production method and Precautions of waterproof leather (PART 2)

Key points of waterproof leather craft design and production process control  

1  Pickling: through pickling, remove the chromium salt which haven’t combined well, residual surfactants in the leather and some water-soluble substances from the blue wet leather.  Rewetting must be deep without any dry hard part.  Pickling should be based on formic acid and oxalic acid, don’t use or use less degreaser as possible. According to the actual soft situation and the state of blue wet leather, we can add a small amount of lipase or acid softening enzyme, organic acid-based deashing agent, aliphatic aldehyde and other materials.  The pH value of rewetting should be controlled at 3.2-3.6.  

2  Chrome retanning: good chrome retanning has positive help to waterproof performance, especially for the blue wet leather which stored for a long time, it must be retanned with chrome to increase the positive charge and more uniform distribution, ensure the chromium content and uniformity of the leather, and then success rate of waterproof leather can be improved.  Add acid t before chromium powder and it can reduce pH value, so that chromium powder absorption is better, but also more uniform, chromium powder dosage should be 2-4%.  

3  Neutralization: neutralization must be deep and thorough, in order to ensure that the subsequent retanning filler materials, dyes and waterproof fatening agent permeate into the interior of the leather.  In general, the higher the degree of neutralization and pH value, the better the waterproof leather.  For firm and thick vamp leather, neutralization should not be too deep, pH value should be controlled in 4.8-5.5; for soft leather and do not need to consider the surface condition, neutralization can be deeper (pH value 5.5-6.5).  Neutralization time must be long (not less than 120 minutes), sometimes overnight neutralization can improve waterproof performance, especially effective for thicker leather.  Neutralizing tannins should be avoided or minimized (less than 2%) to use while neutralizing, as some neutralizing tannins are auxiliary syntanning agents, which contain dispersants and levelling agents that help other materials to penetrate, it will damage the waterproof system of leather.  The neutralizing agent TJ-R819 from Ting Jiang can be used in water proof process, it has fast and uniform penetration and little influence on waterproof.  At the end of neutralization, bromocresol green indicator (B.C.G) was used to check the incision was blue and uniform, the neutralized blue skin grain surface is clear, and the pores are open.  

4  Retanning and filling:  use as little as possible or do not use hydrophilic materials while choosing retanning agent, but all retanning materials such as resin, synthetic tanning agent and tannin extract can affect the waterproof performance of leather, the greater the dose of retanning material use, the worse the waterproof performance of leather.  Therefore, in the production of waterproof leather, on the premise of meeting the performance requirements of waterproof leather, in principle, we should try to use less retanning agent.  Main retanning agents should be acrylic polymer, dicyandiamide, melamine and tannin extract.  

5  Dyeing: the dye used when dyeing will also affect the waterproof performance of leather, because the dye contains neutral salt and other additives in different degrees, such as some dyes look shiny, it may add mirabilite which easy to absorb moisture, these substances have a greater impact on the effect of waterproof.  In addition, the levelling and fixing agents used for levelling and fixing also have adverse effects on waterproofing.  These hydrophilic substances will reduce the waterproof effect of leather.  Therefore, we should choose high-quality dye in the production of waterproof leather so that the waterproof effect can be guaranteed.  

6  Fat liquoring:  fat liquoring of waterproof leather can be carried out in the same bath of filling dyeing bath, also can change the bath to carry out the fat liquoring,  and we can add step by step or can add at a time.  No matter which fat liquoring method is adopted, it is necessary to ensure that the waterproof oil can be deeply permeated and evenly distributed across the whole cross section of the leather.  The penetration depth, bonding fastness and fixation effect of waterproof fat liquor are the key factors for success or failure of waterproofing.  The experiment proves that the waterproof effect of waterproof leather can be more guaranteed after changing bath and fat liquoring.  The total amount of waterproof fat liquoring agent depends on the amount of different product styles and retanning filling materials. The general amount is 8-12%.  Fat liquoring temperature 50-60℃, fat liquoring time 90-120 minutes, sometimes in order to ensure the penetration of waterproof fatting agent, we can also extend the time of fat liquoring appropriately.  

7  Fixation: waterproof leather fixation including formic acid fixation and metal salt fixation.  Formic acid fixation is pre-fixation after the end of fat liquoring. Before acidification, check whether the waterproof oil is well absorbed. Acid fixation is carried out by adding acid for at least three times and slowly reducing the pH value for at least 1 hour.  Metal salt fixation is the last key process in the production of waterproof leather. Metal tanning agent is used to seal the waterproof agent group, so that the waterproof oil in the skin can form insoluble soap and obtain waterproof effect.  Add metal salt twice to avoid coarsening leather grain due to fixation too fast.  Black or dark waterproof leather is fixed with 3-4% chromium powder, while white or light leather is fixed with 2-3% zirconium salt or 5-6% aluminum salt for about 2 hours.  

In addition, after the metal salt is fixed, the leather should be fully washed to remove the residual salt, emulsifier and other hydrophilic substances and reduce the hydrophilic of leather.  When washing, wash with warm water first, and then wash with cold water twice to ensure waterproof effect.  

8  After finish: it is necessary to clean the equipment. The hair felt of the extruding stretching machine should be washed cleanly by hot water with ammonia water, and the vacuum plate should be cleaned.  The crust should be hanged to dry after vacuum drying. The crust also needs a thorough drying at the later stage of drying, and it should be completely dried until there is only 5-8% moisture content left in the leather. Then the moisture is restored and softened, and the waterproof is tested after 2-3 days.  

9  Water washing of waterproof leather: compared with ordinary leather, we conduct more times of water washing for waterproof leather, we should carry out 1-2 times of water washing at the end of each process, it can help remove most of the residual neutral salts, surfactants and other residues in the leather through water washing, it is the foundation for the final waterproof effect.  

Production of waterproof leather is a complex system project, because there are too many factors affecting the waterproofing effect of waterproof leather, our engineers and technicians need to have a comprehensive consideration before we develop the process, and should keep carefully control in the subsequent process of implementation, so that we can ensure the success rate of waterproof leather production.