Finishing in leathers
The leather finishing is the set of operations and processes that are carried out on the surface of the semi-finished leather, destined to provide it properties suitable for use and pleasant appearance -tactile and visual-.
These processes include several layers, pre-fund, pigment or aniline and lacquers, but also mechanical processing stages such as ironing, engraving or printing of the leather surface.
We could summarize the objective of finishing in two large groups:
• Provide protection and improve physical-mechanical properties, favoring the proper behavior of leather during manufacturing and use; properties that the semi-finished by itself cannot fulfill.
The finish should protect it from mechanical damage, such as rubbing, bending, scratches or scrapes; of dirt and humidity, giving it greater durability and trying to maintain some of the leather's own and special properties such as permeability to air and water vapor, which make hygiene and comfort, especially in leather for footwear.
• Provide attractive and pleasant appearance trying to obtain an item desired by the user, which should "like" and be salable. Therefore, the finish must meet aesthetic requirements according to fashion trends, and grant organoleptic properties, such as soft touch, warmth, softness, etc., trying to preserve the fundamental properties of the semi-finished, to avoid losing its character of natural material.
In addition, finishing of the leather will have to improve the homogeneity of the surface and achieve uniformity between different leathers of a consignment and between different consignment, covering natural defects (barbed wire marks, tick and fly bites, etc.), such as also those originating in conservation and tanning processes.
In short, proper leather finishing will improve the performance of the usable surface that will result in a higher sales value increasing the economic benefit of the company.
The final destination determines the physical-mechanical properties required both for its elaboration and for its use. These are the properties that the finishing of leather for shoes must meet.
• Good adhesion not to peel off during assembly and subsequent use.
• Strength to flexural. Do not crack faced with flexion, accompanying the movement of the leather and the foot when walking.
• Good resistance to dry and damp rubbing, avoiding transfer and loss of color.
• High elasticity to resist elongation during assembly.
• Resistance to humidity and aging.
• High permeability to water vapor.
Some examples of the way in which the finish can influence the resistance to mechanical aggressions, which the leather must withstand during the manufacture of footwear and its use.
• When the shoe upper is mounted on the tip, where force is applied and the leather is stretched to reach the necessary shape, a breakage or cracking of the finish may appear, especially when it comes to women's shoes with a pointed tip. There are several factors that influence in this case, some related to the leather itself (humidity, elasticity of the grain layer), but especially with the finishing film and its anchoring in the grain.
In order to provide resistance to other mechanical aggressions, the top layer is designed harder, less elastic, so it can crack when it supports the effort of the assembly. To avoid the problem in the following leather consignment, it will be necessary to select semi-finished with good resistance and adapt the finish to the requirements of the shoe manufacture. There is a standardized test that allows to evaluate the finished leathers from this point of view (IRAM 8515 Method of determination of the load and distention corresponding to the breakage of the grain and the busting).
• Another case is the resistance of leather against rubbing with textile materials, a situation that may occur during the use of footwear. When rubbing with dry or wet textile, the leather loses color, changes its appearance and the textile can be colored. This damage should be avoided and depends on the conformation of the finishing film, especially its top layer, where a higher strength formulation favors the behavior.
Although in this case, correcting the defect can be try, with the application of a superficial leather treatment, here also the solution will be for the next consignments where the processes will be adjusted. The corresponding standard is IRAM 8516.
As is appreciable, it is difficult to comply with all the requirements simultaneously for a given item, it depends on the raw materials and processes that are applied to achieve an adequate balance, it is necessary to agree between tanners and manufacturers on the specific resistances required.
The technician who performs the superficial treatment, must meet the requirements of the finish without canceling the original properties of the leather and trying not to lose the natural appearance with excessive layers of chemical and auxiliary products.
The amount of superficial covering applied to leather, influences the description of the article as genuine leather, to maintain this condition the applied finish must be less than or equal to 0.15 mm and not exceed 30% of the total thickness of the leather.
The superficial finishing of a leather is composed of several layers and in each one more than one supply can intervene, giving rise to different formulations, in the manner of a different “recipe” that the technician will have to adapt to the required final article.
The products used in the formulations can be grouped according to the function they fulfill.
• Products that form the film when they dry: binders, lacquers and proteins.
• Products that give color: dyes and pigments.
• Auxiliary products: penetrants, waxes, fillers, touch modifiers, etc.
The layers have affinity with each other and, although they differ in properties, they form a film adhered to the grain surface. In the case of footwear, impregnations are applied (which are incorporated into the grain); fund (link fibrous structure of the grain with the film); coverage or color layer (determines film thickness and color, gives resistance properties) and top or starching (it is the final layer, defines mechanical resistances and appearance).
To obtain a final finish that complies with the properties required for use, a good balance of the formulations must be achieved, finding the products that maximize the properties and minimize the problems, accompanied by the mechanical work performed with the appropriate machines.
The machinery used in the finishing process are the same regardless of the final destination of the product, only its dimensions vary. And they can be grouped into:
• Machines for applying paints: blowtorch, curtain or roller.
• Ironing or engraving machines: mechanical or hydraulic presses, irons, satiners.
• Softening machines: staker machine and drum-tumbler.
• Machines for other operations: polishers, brighteners and lapping machine.
There is a diverse number of finishing types that range from the finest, most natural and least covering as aniline, which is applied in leathers of excellent classification where it is not necessary to hide defects, but only to protect; up to the patent leather, with a very covering thermoplastic finishing, of high brightness, which is generally applied to leather of low classification or degrained. Between these two extremes we can find semi-aniline, pigmented, foliated, greased, cracked, metallic, etc.
Currently studies presented at the last IULTCS Congress (International Union of Associations of Chemists and Leather Technologists, in Dresden, Germany, June 2019), tend to find formulations aimed at improving performance and some specific properties.
For example, the development of "nanofinishing", formulations from nano particles to improve flexural strength and the physical combination of mesoporous hollow spheres and polyacrylate to improve water vapor permeability. Or hollow micro capsules in which gaseous, liquid or solid substances are present that release the content under certain conditions (pressure, heat, etc.), achieving a filling effect on the defects and increasing the performance of the leather. Or the investigation of the use of commercial nanoparticles for the multifunctional development of the leather surface, increasing durability, comfort and antimicrobial and self-cleaning properties, processing the leather below the limits of cytotoxicological risk.
RELATED ARTICLE: