Coating of hides and skins

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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.


Coating

The purpose of applying a surface coat is:

  • to provide protection from contaminants (water, oil, soiling);
  • to provide colour;
  • to provide modifications to handle and gloss performance;
  • to provide attractive fashion or fancy effects;
  • to meet other customer requirements.


There is a wide range of application methods, each of which has its advantages and disadvantages. A combination of methods can be used to achieve the desired effect on the finished product. In principle, the following types of application methods can be distinguished:


  • padding or brushing the finishing mix onto the leather surface;
  • spray coating;
  • curtain coating, which is passing the leather through a curtain of finishing material;
  • roller coating, which is the application of finishing mix by a roller;
  • transfer coating, which is the transfer of a film/foil onto leather previously treated with an adhesive.


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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