Tanning is the chemical process by which putrescible hides and skins are transformed into rot-proof leather. It makes the skin resistant to decomposition, while retaining desirable qualities such as suppleness and durability.

Traditionally, metals such as aluminium, zirconium, iron, titanium and especially chromium have been used in mineral tanning. Basic chromium(III) sulphate is particularly widely used, accounting for around 85% of leather sold. However, chromium(III) can, in certain cases, oxidise to chromium(VI), an allergenic substance whose concentration in leather is restricted to 3 mg/kg (entry 47 of Annex XVII of the European REACH Regulation).

 

 

Chromium(VI) in tanneries: lessons learnt and CTC best practice

CTC has been very actively involved in identifying the phenomena likely to generate chromium(VI) in leather, in order to find and approve possible solutions, whether through the implementation of best practices or the use of prevention agents.

There have been numerous, relevant results. These have been discussed many times in dedicated working groups including French leather manufacturers. In addition to this joint work, CTC takes specific action on an individual basis when tanners are faced with non-conformity problems that they cannot explain or resolve.

 

Hexavalent chromium or chromium(VI)

Entry 47 of Annex XVII of the EC REACH Regulation (n°1907/2006) limits the concentration of chromium(VI) in leather in contact with the skin to 3 mg/kg (ISO 17075-1 and 17075-2). The aim of this restriction is to protect consumers from skin allergies.

This chromium(VI) restriction officially came into force on 1 May 2015. Some brands (leather goods, footwear, clothing, furniture, etc.) go even further: they impose more stringent restrictions by taking into account the ageing of the leather.

 

Diagnosis and analysis of causes

The first stage in providing these services is to study the company's manufacturing process in detail in order to identify the practices that could potentially contribute to the presence of chromium(VI). The manufacturing formulas and application conditions are analysed to identify any deviations that could provide an opportunity for improvement. This is also an opportunity to let the tanner know about the best practices identified through the various experiments previously carried out on the CTC tanning platform or through data obtained from the literature.

 

Action plan on the experimental tanning platform

Once this assessment has been confirmed by the production manager and their teams, CTC's chromium(VI) expert replicates these manufacturing formulas on the tanning platform and varies the parameters in order to identify the source of the problems and possible avenues for improvement. In providing this service over the past two years, CTC has identified a number of causes:

• Inappropriate practices during currying operations, particularly during fatting.
• Use of potential chromium(VI) generating agents in the presence of certain colourants.
• Leather insufficiently protected by protective agents.
• Deviation from standards in wet-work operations.
• Risky storage conditions ...

Once the causes have been confirmed on the experimental platform, the parameters are monitored on the production site for one batch and then for the entire production run.

 

Conditions for the appearance of hexavalent chromium: thermal ageing and photoageing

In many cases, the success of these operations goes beyond simply complying with the regulatory limit of 3 mg/kg. The leather is also artificially aged.

ISO 17075-3 covers thermal ageing (80 °C and a relative humidity of 10% for 24 hours for leathers used in footwear, automobiles and furniture, and 60 °C and a relative humidity of 20% for 24 hours for leathers used in leather goods, gloves and clothing).
By adapting a textile standard, photoageing can be carried out (45°C temperature, UV exposure between 300 and 400 nm at 42 W / m2 for 24 or 48 hours at 40% relative humidity).