Due to the fact that there is a big gap between the supply and demand of high class/grade raw material, tanners need sophisticated systems and products as well as customised procedures to get the maximum returns for their output.
In general, it is most important in modern leather finishing to turn a crust leather, most of the time from different raw hide sources, into a premium leather product by utilising the surface area of the crust to the highest degree. In the automotive sector, this is the so-called cutting yield.
The tanner needs ways to upgrade low grade crust. The result is that tanneries are able to produce profitable leathergoods with a high sales appeal, by using lower grade crust material.
In this field, Clariant offer sophisticated products and systems and tailor-made suggestions for tanneries around the world.
Before finishing crust leather, it is important to know the desired properties of the final article in terms of aesthetics, physicals and, last but not least, costs.
Following is a list of the relevant aspects to consider before starting a finishing process. These points count equally for garment sheepskin leather as well as for high tech automotive leather.
For information about the desired article, it is essential to pick out the best combination of the following points.
Crust
By evaluating the crust, one of the main decisions has to be made. Does the tanner either buff the crust or carry out a process such as a Stucco application (eg using the Melio Stucco range)? The buffing procedure is limited because some leathergoods manufacturers only want full grain leather. Even so, the natural grain can be imitated.
Location/layout
The location of the tannery, and its layout, have an influence on the finishing formulation, due to different climatic conditions around the world. The layout is certainly an important factor in terms of the procedure and process flow of production. Thus, it has to be mentioned that the economic way of manufacturing leather can differ quite a bit.
Comparing the procedures used around the world, it shows that especially in north America, finishing systems that include more than four applications are, in most cases, not acceptable. In some other regions, leather is successfully made with six or seven applications.
In the end, the most important fact is that the tannery makes money from the produced article.
The type of machinery used is also important. Below is an update on what type of machinery/equipment is actually being used in tanneries worldwide.
Machinery/mechanical steps
Application equipment:
1. Rollercoating: today, both ‘synchro’ and ‘reverse’ rollercoaters are commonly used for a wide range of leathers. They are mainly used for furniture and car upholstery, but also for shoe upper leather including splits. In this field, the new reverse rollercoater, which can also be used for soft leather, has been a breakthrough in our industry and has been incorporated into a lot of tanneries in a short period of time.
2. Spraying: in this sector, HVLP or airless systems are state of the art. Atomised spray systems are certainly still used for fine leathers or topcoat applications. However, due to the high loss of chemicals, atomised airguns will be replaced in the future.
3. Padding/tipping: these techniques are mostly applied in countries with low labour costs. Even so, the padding technique for precoats has a positive effect on the final result on certain articles, and is difficult to match with modern technologies.
4. Curtain coating: besides the rollercoating machine, the curtain coater is still commonly used for impregnations and for base coats, for example white sport shoe upper. Alongside the rollercoater, it is unbeatable for the application of patent lacquers (eg Melio PA-System).
Drying equipment:
* Steam heaters. Depending
on the finishing application, lengths of 10-20 metres are mostly used. In some tanneries, catalytic gas dryers with a length of approximately six metres are used. In the case of the steam heaters, a better temperature profile can be achieved. In general, a starting temperature of 80°C and a final temperature of over 100°C are required, especially to achieve high fastness degrees in eg automotive finishing. A cooling zone (circa 3 metres) will improve the piling process.
Conditioning equipment:
* The conditioning equipment as well as the water content of the leather is a very important factor in leather manufacturing. The humidity of the leather should be around 12% throughout the finishing process. Before embossing and during milling especially, the crust should contain this amount of water.
* Staking machines particularly influence the automotive sector by trying to achieve the above mentioned yield figures.
* State-of-the-art at the moment are milling drums which have an automatic regulation for humidity and the possibility of injection of feel agents etc, as well as rotating embossing machines with high throughput rates.
Upgrading finishing possibilities
As mentioned before, the layout and the location of a tannery as well as the machinery options and finally the finisher’s perspective are important in how a tannery produces their leather.
The phrase upgrading has been used and explained many times. Upgrading as a result of chemical products, combined with a perfected finishing procedure. (see machinery/mechanical steps) is essential for good quality leather, and the options for upgrading are discussed now.
1. Stucco technique:
* Stucco application by spatula using, say, Melio Stucco M
* Stucco application by rollercoater using Melio Stucco R, Melio Stucco AR
2. Conventional finishing systems and products. These systems and products include, beside the normal product range:
* Anic or cationic pre-bottoms, based on Melio Ground products for example
* Melio Ground K
* Melio Ground CL
* or pregrounds for fashionable articles, for example for antique look or oiled leathers
* Melio Wax 187, Melio Oil PW 100/110/120
* Formulated binders and top coats, for compact formulations, eg:
* Melio Resin A 820
* Melio Resin A 821
* Melio Resin A 823
* Aqualen Top GC 2030/GC 2031/DC 2040
* Sophisticated upgrading additives, eg:
* Melio 06-H-145 (polishable filling wax)
* Melio Ground PS (improves pigment coverage and feel)
* Complete range of cationic products, such as
* pigments: Catiofin range
* proteins: Melio Top K 201, Melio Top K 202
* fillers, waxes: Melio Ground K, Melio Ground BG, Melio Fettgriff SZ etc
* binders: Melio Promul K 27 etc
3. New upgrading technique:
* Melio Prograde system
With the development of this new finishing technique, Clariant have introduced an efficient and consistent upgrading technology.
The range consists of selected binders which are modified with special additives with very high and consistent upgrading properties.
High physical properties and a superior print retention are further advantages of this new finishing approach.
* Melio Prograde products:
* Melio Prograde C01: pu based product dominantly used for automotive and high tech splits
* Melio Prograde C02: acrylic based product, dominantly used for furniture, sport shoe leather and splits in general
4. Foam finishing:
This milestone in leather finishing – patented years ago by Clariant using the Melio system – has been optimised with time and the results give an upgrading efficiency that cannot be achieved by any other finishing technique.
Nowadays, Melio Foam is state-of-the-art in the finishing of automotive leather but also, for example, for splits and specific shoe upper articles.
5. Oil/waxes:
The wide range of pull-up finishes and fatty/waxy articles are important options for leather upgrading. Clariant offer a complete range of products, from standard oils/ waxes for modern pull-up articles to special waxes and oils for water-resistant crust or leathers.
* Melio PO range, non water mixable, in most cases solvent free
* Melio PW range – oil/wax emulsions
Outlook
The development in leather manufacturing is going towards more automated processes and procedures, alongside which there is a demand for the easy handling of chemical systems and products. This might include finishing systems without external crosslinking action in the future.
Enviromentally friendly products and systems are a relevant issue, which will develop more and more as a real global demand.
The ongoing trend towards low VOC/no VOC products is obvious, even though it will take some time to remove solvents totally.
All of these topics will increase the close collaboration between tanneries and the chemical industry.
It is essential to have a clear and uniform opinion and position towards leather article manufacturers, legislators and users in order to guarantee the image of leather.
