FINISHING (AFTER TANNAGE)
The finishing of leather is probably the most complicated and least understood phases of the industry. It has been more of an art than science. Finishing leather is not simply a matter of painting the surface to cover up the mistakes of the previous operations or to improve it by concealling scratches; it contributes to the durability and beauty of the leather and must be an integral part of the process. The compatibility of materials, tannage coloring and fatliquoring all play an important role in the character of the leather and the kind of finish it will take.
The finish system is a compromise between conflicting effects. If coverage of defects is the main problem some of the grain beauty will be lost. If resistance to scuffing is desired there is a danger of having a stiff varnished look. Each of these conflicting properties must be balanced in the final finish system.
Adhesion:
When the finish is applied, it must stick; for this reason the leather surface must be “wettable”.A top finish need not adhere to the leather itself, but it must stick to the base coats of the finish to prevent peeling. Leather finishes require extreme flexibility and stretch. Finishes that do not have good adhesion and good flexibility will peel and crack.
Stability:
Leather may be exposed to extreme heat during the manufacture of a shoe. When the shoe is worn in cold weather, extremely low temperatures may be encountered. Thus the film must have a wide range of temperatures over which it is soft and pliable; it must also be hard to maintain the high gloss which is required.
The leather must be able to stand up to a reasonable amount of both wet and dry abrasion and to be refinishable with ordinary shoe polishing methods applied by the consumer.
Coating technology:
Coatings may be classified as:
1) lacquer systems
2) drying oil systems
3) condensation systems
4) latex systems
Lacquer systems: The formation of the film is based on the evaporation of the solvent containing a film-forming material (nitrocellulose dissolved in an organic solvent is an example).
Drying oil system: These are natural drying oils such as lnseed which will undergo polymerization upon drying. This is different from lacquer in that the setting up of the film is not simply a deposition of a high molecular weight material; rather; it is a chemical reaction taking place between the dissolved film-forming materials and atmospheric oxygen. In the drying oils the film forming material(a binder) is an organic chemical having a high degree uf unsaturation.As the oil absorbs oxygen from the air, the unsaturated material is oxidized and reactive portions of the fatty acid molecule develop which can then polymerize with other fat molecules to form a continuous film on the surface.
Condensation Systems: The formation of the film is due to a chemical reaction between the various components of the finish after application.The reaction may form a plastic or polymer in water between two molecules. Such systems are usually heat activated and may be baked, glazed, or hot pressed. Condensation or polymerization is used in the leather industry through protein-aldehyde reactions and with other resin systems.In this kind of finish the reactive components are usually mixed shortly before application, due to the limited pot life of the components.
Latex systems: In this system the binder used is emulsified in water. When the latex is applied the water evaporates, or sinks into the leather and eventually a phase inversion takes place. A continuous layer of the binder spreads on the surface of the material being coated and becomes a continuous film. This film can be diluted with water, enabling thin films to be applied.The latex systems and dispersions of resins in an emulsion form are widely used in preparation of leather finishes.
These products are prepared by first emulsifying a monomer in water.The monomers are usually colorless volatile liquids of quite simple chemical constitution.This emulsion is heated and mixed with a catalyst which causes the monomer molecules to form large polymers.As the polymer size increases, the volatility decreases and becomes more viscous and rigid.Generally films formed from smaller drops give more cohesive films and also penetrate better if the dispersion stability is suitable.
Impregnation:
The break of leather can be greatly improved by filling the spaces between fiber bundles with a resin.This techique was first done by impregnation of the leather with low molecular weight urethane pre-polymers. The low molecular weight allowed penetration of the material in sufficient quantity and to the desired depth.
Urethane finishes:
Urethanes can have a wide range of properties through selection of reacting materials and their ratios. Polyurethanes are synthesized by reacting isocyanates (R-N=C=O) with alcohols (R-OH). The size of the resin is controlled by the ratio of the two reactants. With the variety of isocyanates available and the reactions with amines and acids, the possibilities are tremendous. Urethane laquers, quick dry systems and aqueous base urethane are among most widely used ones.
