Mechanical drying of hides and skins
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Contents
General description
The overall objective of finishing is to enhance the appearance of the leather and to provide the
performance characteristics expected of the finished leather with respect to:
- colour
- gloss
- handle
- flex
- adhesion
- rub fastness,
as well as other properties as required for the end use, including:
- extensibility
- break
- light and perspiration fastness
- water vapour permeability, and
- water resistance.
Generally, finishing operations can be divided into mechanical finishing processes and coating.
Mechanical finishing processes
A wide range of mechanical finishing processes may be carried out to improve the appearance and the feel of the leather. The following list of operations includes commonly used mechanical finishing processes:
- conditioning (optimising the moisture content in leather for subsequent operations);
- staking (softening and stretching of leather);
- buffing/dedusting (abrading of the leather surface and removing the resulting dust from the leather surface);
- dry milling (mechanical softening);
- polishing;
- plating (flattening);
- embossing a pattern into the leather surface.
These operations may be carried out before or after applying a coat, or between the applications
of coatings. The list is not exhaustive and many other processes exist for special leathers such as
sole leathers, wool-on skins, and special effects leathers
Emerging technologies
Organic solvent-free finishing
Description
The use of organic solvents in top-coats and special effects finishes is still common in Europe. However, the range of organic solvent-free (aqueous-based) and low-solvent finishes is increasing continuously. Whereas the range of aqueous-based and low-solvent systems is considered BAT, top-coat formulations which are completely free of organic solvents are not yet widely available or are only being used for upholstery leather for automotive and furniture use. Acrylates and polyurethanes have been identified as being particularly suitable to create organic solvent-free finishes.
A problem associated with solvent-free finishes is that the finish may form droplets on the leather due to its poor flow properties. Organic solvents reduce the surface tension of water thus giving the finish improved flow properties. Auxiliaries have been developed which improve the properties of the finish, and a number of techniques are emerging allowing for a (near) elimination of solvents in urethane dispersions and acrylic emulsions.
An example of the solvent-free finish formulations is the development of hybrid acrylic polyurethane polymers. These hybrid polymers offer the possibility of totally solvent-free finishing systems.
Whereas several solvent-free finishes are now available from a wide range of chemical suppliers, developments are continuing to improve the technical performance of these finishes.
Achieved environmental benefits
No use of VOCs and no emissions of VOCs.
Cross-media effects
Potentially toxic cross-linking agents are required to improve the performance of the finish.
Status of development
Several formulations are already available on the market. New materials are under development.
Dry abatement of volatile organic compounds
Description
The technique includes the following steps:
- collection of a gaseous stream containing the volatile organic compounds to be abated
- introduction in the same unit of a solid absorbent material suspended in a fluid stream
- treatment of the mix in a centrifugal separator, with separation of the cleaned air
- regeneration of at least some adsorbent material by heating
- recycling of the adsorbent material.
The technique is very flexible and by changing the properties of the solid adsorbent, a wide range of effluents can be treated, regardless of their composition.
Achieved environmental benefits
The emerging technique has been tested on a pilot plant scale (2000 m³/h) in a number of tanneries in Italy. The abated ratio exceeded 85 % for the majority of the volatile compounds treated.
Cross-media effects
The waste production per year is very limited consisting only of the adsorbent which cannot be generated and the desorbed VOCs.
Applicability
The technique is applicable to both new and existing installations but will require an investment.
Driving force for implementation
The driving force is to reduce the discharge of VOCs from the finishing operations and to cope with legislation.
Status of development
The technique has been tested at the pilot scale in various Italian tanneries, with different leather production (shoe upper, upholstery, and clothing). Therefore many substances have been tested such as acetone, alcohols, esters, ethers and aromatic substances (toluene) both as single substrates and as complex mixtures (more than five solvent components). Reference literature [96, Italy 2008].
Other abatement of volatile organic compounds
Two techniques have recently been tested in a study in France [92, Poncet 2006]. The techniques were biological treatment using biofilters, and a combination of adsorption and catalytic oxidation. Zeolite is used for the adsorption and platinum is used for the catalytic oxidation. Both techniques were found suitable for the treatment of air emissions from the leather industry.
Further improvements to spraying techniques
Description
The NESS spray booth uses 3 bars equipped with 100 micropressure airbrush spray guns each. The jets are not rotated. A very even coating pattern is obtained and the air swirl produced by rotating jets is eliminated.
A spraying efficiency in excess of 90 % is obtained.
Achieved environmental benefits
A reduction in the use of coating materials is achieved. VOC emissions per unit of product are reduced. Exhaust scrubbing efficiency is improved. Noise output is reduced.
Applicability
New coating equipment is required.
Economics
Investment in new equipment is required. A reduction in coating use can be achieved.
Status of development
A demonstration project has been completed. Reference literature [134, Sicagroup 2007].
Source: (BAT) Reference Document for the Tanning of Hides and Skins, 2013
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