Laser-sharp innovation - multiplexed laser surface enhancement technology

18 July 2016

Dr Francisca Arán Ais, along with other colleagues at INESCOP footwear technology institute in Elda, Spain, delivers a paper on functional leathers by means of multiplexed laser surface enhancement technology, which was also presented at FILK in June.

Leather is used for the production of a wide range of products and, depending on the type of product and its intended use, a different type of leather with specific features is required. In general, traditional finishing systems employed for obtaining functionalised leather such as water resistance, olephobicity, permeability, flame retardancy, antimicrobial properties, abrasion resistance or antistatic properties, imply high energy and water consumption. Besides, many of these treatments require the use of certain chemicals, such as halogenated compounds, biocides or organophosphorous compounds, the use of which is restricted or under consideration by the EU legislation, as is the case of the REACH Regulation and the Biocides Regulation.

For this reason, INESCOP together with several European companies is working on the LIFE TextiLeather project aiming to obtain leather and textiles with flame retardancy, antimicrobial, hydrophobic and oleophobic properties, by means of a technology that helps reduce the use of hazardous substances to human health and the environment.

Leather is not a flammable material in itself but the use of dyes and other finishing products may affect its fire resistance. However, flame retardancy is more and more demanded for certain applications, as in the case of materials for aircraft, train and boat seats, facilities in public buildings or fire-fighters’ footwear. The traditional substances employed in the tanning industry as flame-retardant substances are mainly brominated flame retardants (BFRs) and polybrominated diphenyl ethers (PDBEs). However, the use of these compounds is being restricted in Europe due to their associated risks to human health and the environment. For this reason, in recent years, alternatives have emerged to improve the flame retardancy of leather, such as ammonium bromide and ammonium polyphosphate or silicone polymers, which burn and leave a silica residue that protects the leather beneath.

In the case of antimicrobial properties, the use of biocides in tanneries is limited to preserving hides and skins, with the aim of preventing microbial attack and damage. Studies focusing on the development of leather with antimicrobial properties are new.

However, the TextiLeather project goes one step further and seeks to confer antimicrobial properties on tanned leather, especially for use in the manufacture of footwear as upper, lining or in-sock material.

Case by case

Due to its composition, leather has a hydrophilic nature, but this property is not desirable in certain applications where water-resistant leather is required. In this case, it is necessary to apply waterproofing treatments that usually imply a costly process. Generally, coatings based on waxes, fatty acids, silicones or silanes are used. Their main drawback is that, despite the creation of a hydrophobic film on the leather surface, they may not be sufficient if the film is damaged or deformed upon flexing, which is quite usual when leather is employed for footwear manufacture.

Alternative treatments exist, such as the application of low-pressure plasma. Depending on the nature of the atmosphere in which the treatment is applied, materials can be provided with different properties. Thus, plasma treatment in a He and O2 environment confers hydrophillicity, while the presence of CF4 can confer hydrophobic properties. Nevertheless, the use of greenhouse gases, as is the case of CF4, is restricted by the European Union.

As an alternative to the above-described treatments and drawbacks, the LIFE TextiLeather project proposes the use
of the patented Multiple Laser Surface Enhancement (MLSE) technology for the functionalisation of textiles in a sustainable way, avoiding the use of toxic or hazardous substances. MLSE technology combines plasma and laser treatments and in the presence of non-toxic gases, such as N2O2, allows the surface modification of materials. This treatment produces nanometric scale modifications, which enables the functionalisation of the material without noticeably affecting its appearance. In addition, the MLSE technology consists of a dry, continuous process. Therefore, its application in the tanning process will lead to a significant reduction in the environmental impacts of traditional leather finishing operations, especially in the case of flame-retardant and waterproofing treatments, in terms of greenhouse gas emissions, and water and energy consumption.

Due to its composition, leather has a hydrophilic nature, but this property is not desirable in certain applications where water-resistant leather is required. In this case, it is necessary to apply waterproofing treatments that usually imply a costly process.

Proving the concept

The preliminary studies carried out in the framework of the LIFE TextiLeather project to improve the hydrophobicity of leather by means of the new MLSE technology showed different behaviours for different kinds of leather. The surface treatment by MLSE technology provided promising results with regard to water repellency in static conditions, which could indicate the suitability of this treatment for leathers intended for upholstery or clothing. Besides, it was shown that the MLSE treatment could even yield water repellency properties, in static conditions, much higher than those obtained by conventional processes.

It should be highlighted that the equipment employed in this study and the treatment conditions applied had been optimised for treating textile materials and had never been used for treating leathers before this project. Treatment parameters, laser and plasma power, ambient gas concentration and residence time of the material in the treatment zone are currently being revised. In addition, the substrate feeding system is being modified to allow the treatment of discrete materials, such as leathers. As a result of this project, the MLSE technology is being adapted to treat new materials.

Key parameters for MLSE adaptation

  • Leathers are discrete items. In the current MLSE demonstration plant, feeding systems only allow the treatment of materials supplied in continuous form.
  • Differences in thickness; ranging from 0.56 to 2.50mm. Thickness along the piece might be uneven.
  • In some applications, the combination of two or more functionalities could be desirable.

IPPC Directive (2008/1/EC)

The BREF documents for tanning industries strongly recommend:

  • the reduction of water consumption
  • the reduction of emissions of chemicals into wastewater in the whole production processes, including finishing operations
  • the seeking of alternatives to any harmful chemical, with a special mention to fire-retardant and waterproofing treatments based on halogenated organic compounds.

Water repellency assessment. A: Bovine leather without treatment (Reference B). B: Bovine leather with conventional waterproofing treatment (Reference BWR-0). C: Bovine leather with baseline treatment with MLSE.

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