The traceability of products is vital to all players in the global market (manufacturers, retailers, distributors and consumers). The objectives relating to technical and commercial activities in the leather sector will be the same for other industries. However, in the case of traceability, this is not a matter of regulation or legal obligation.
The function of a traceability system, whose purpose is to be able to identify historic records, source and use of an item or activity, lies in the combination of three components:
• Branding or signature
• Management and exchange of related data
• Definition and legal protection of models and references
The work carried out by CTC focuses on branding. Its main purpose is to secure the continuity of traceability between the raw material and the finished product (ie footwear, leathergoods etc) despite the tanning and cutting operations affecting the material when used.
CTC has developed a discreet branding system which is applicable during leather processing and detectable throughout the life of the material as it is transformed into the finished product.
The French Ministry of Economy, Finance and Industry has contributed to this project with a grant of €68,000 to the project partners.
Which branding for which application?
In previous publications (CTC enterprise no 4, 2004, and no 5, 2005), the requirements, objectives and consequences of implementing traceability in the industry have been defined. The different methods have been presented as well as the constraints and perspectives relating specifically to the leather sector. These combine both the agricultural and industrial worlds.
The practical aspects of implementing the different discreet branding systems have been developed: instruction manual, entry points in the process, system selection based on their use and on the means of detection and analysis of the tracers.
Different solutions have been selected and incorporated into both wet and dry leather processes as follows:
• powder with magnetic resonance
• rare earth salts
• microspheres
• synthetic DNA
In light of the results obtained, CTC decided to develop firstly the microspheres and synthetic DNA applications in the dry stage.
Presentation of the procedures developed
Procedure based on microspheres
This procedure is based on the use of micro-spheres with a polymeric size ranging from 2 to 10 microns.
These microspheres are present in a variety of physical and chemical (nature and coating) properties allowing a wide range of uses in the paper-carton, plastic, glue, ink and varnish and textile industries. These products are inert, chemically stable and relatively resistant to thermal and mechanical constraints.
Finally, the empty micro-envelopes can be filled with a specific fluophore agent but also with synthetic DNA which has been subsequently used in the leather industry.
The microsphere procedure has been the focus of several series of trials given its high potential for use in the first stage of the traceability process. The procedure was designed for wet and dry tannery processes. However, the results obtained at the experimental stage have shown the superiority of the latter process (ie in dry processing).
In this context, a methodology and a standard operation mode have been defined. The use and finishing procedures tested are perfectly comparable to the ones found in the leather industry.
These allow the monitoring of the chemical composition during application, the behaviour of the tracers when using air spraying (application pressure, passage under the spraying valve, distribution on the surface of the leather) and mechanical finishing treatments, such as pressing to modify the grain (pressure effect and thermal effect).
The parameters analysed include the concentrations added, the finishing methods, the size of particles and their positioning in the different layers of finishing.
In conclusion, this procedure gave good reproducibility in terms of detection/ amounts deposited and required only the addition of a relatively light tracer concentration.
There are no risks of contamination and the microspheres are spray- and press- plating resistant. It is also worth noting that the pigment colours are unlikely to interfere in the detection.
Synthetic DNA procedure
Synthetic DNA is the basis of a biological branding system with almost unlimited coding capacity. The procedure is already applicable to paper money, perfumes, and in general, all liquid or solid materials and products. The advantage of DNA is its great safety in this context and it is totally harmless to the consumer.
DNA can be used either in its free state (this is directly incorporated in the marking
environment) or in microsphere capsules, when the environmental conditions are less favourable. This procedure has been assessed according to its use in leather branding.
The method selected is free DNA added in the dry leather process during finishing; this is the most beneficial path in financial terms. The analysis of DNA in the finishing step has been carried out in two stages:
• Chemical compatibility with the finishing components
• Application and detection of DNA in finished leather
During the trials, it was noted that the leather’s natural DNA did not disrupt the procedure. This analysis was a worldwide first as this case has never been discussed.
For each method, compatibility was analysed with the finishing preparations to be applied before and after drying and with each component in these preparations:
• The DNA added to the acrylic polyurethane finishing compounds which were not dried and polymerised was totally detectable and, therefore, compatible
• In proteic finishes, all combinations analysed proved compatible DNA/finishing resins. A specific solvent had to be developed in order to extract a result
• In acrylic-pu finish, compatibility tests on labelled, five-day-old preparations showed DNA stability in a test environment
• Compatibility trials with the DNA/nitrocellulose finish have led to positive results in all cases analysed. These results suggest the possibility of branding an article through several layers.
All the applications above were applied to leather both in semi-aniline and pigmented finishes. In all cases, the concentration factor in the tracer was analysed. The trends obtained were as follows:
• The incorporation of free DNA in the different varieties of leather finish is totally
predictable regardless of the type, ie proteinic, acrylic or polyurethane (pu)
finish. The eventual presence of different solvents or auxiliaries is not a handicap. The most sensitive area is the use of reticulant agents (ie isocyanate) where the concentration and position in formulations could generate problems when
identification takes place at a later stage.
• Branding through free DNA on finishing films will need, for those companies looking to brand their products, a compatibility analysis phase between the components to be used and DNA. Certain mechanical operations on the finished leather, specific to the company, will also be assessed in terms of their effects.
• Traceability on two levels on finished leather is also predictable (combination microspheres, fluorophore and DNA).
Industrial applications on footwear
CTC has already demonstrated that in the laboratory stage (on inert support or on leather samples), luminescent micro-sphere type tracers and synthetic DNA tracers were stable, compatible and detectable in cases of finishes based on proteinic, acrylic and pu resins as well as any combinations of these. The next step to confirm is the effectiveness of these systems in industrial conditions.
The first trial was focused on a calfskin finish to be used for footwear upper leather. The formulation used was based on a pu black pigment base and a pu aqueous fixation applied by a spray finishing machine with eight spraying guns. A reticulant agent was used in both layers. Microsphere marking was added to the bottom layer and the synthetic DNA was added to the base and fixative.
A second trial carried out in another company focused on cow leather with a black pu acrylic finish; in this case only the pu fixation layer had a reticulant agent. On top, an intermediary fixation layer based on a nitrocellulose lacquer was used. The material used on the bottom layer was very different from the previous one as it required a roller coating machine functioning in reverse mode.
The bottom and intermediary fixations were traced with the same agents used in the previous treatment. In each case, the leathers were under thermal pressure via a hot press.
In all cases, the detection and identification of synthetic DNA tracers have been positive when laid on the bottom layer. When marking the fixation layers, there were positive results in the second test with negative results in the first trial. This definitely shows the necessity of adding tracing agents in just the bottom layers or intermediate ones. It can be concluded that the tracers should never be added to the surface or top layers; moreover, the level of reticulant agents must be accounted for in order to adjust the dosage of DNA tracers.
Using direct detection on the bottom layers, 30 - 40% of the tracer was detectable (these percentages are calculated in relation to deposited quantities).
The identification is then guaranteed both in quantity and quality, as long as leather surfaces are traced regularly. The detection rates are probably linked to the presence of black pigments and the reading resource on the ground which is more focused on the more superficial micro-spheres.
A third series of trials was focused on the reaction of tracers when manufacturing and using shoes made with branded leather. CTC checked that leather in the shoes was always identifiable when leaving the factory. It presented identical results to those of the original leather when exiting the tannery. Therefore, the source of the leather was still traceable.
The first results after the trials seem very encouraging and, therefore, confirm the relevance of this technology for commercial applications. Further research is to be carried out in the future to follow up on the results produced in this study.
