From crust to semi-finished using dense pressurised

ABSTRACT In any industrial sector, dyeing is a step which requires large amounts of water. The technology of dense pressurised fluids can create a special solution with very different qualitative properties from traditional ones. One of the novelties of this process is the possibility of continuously modifying the solvating power of a fluid as well as the transport properties (flow rate and mass transfer). Carbon dioxide in supercritical phases accelerates the absorption of molecules with low molecular weight and CTC has studied the potential of this application in leather dyeing. Trials were made after selecting a leather support. Different dyes, additives and dyeing conditions were optimised on a laboratory scale. Leathers showed homogenous superficial dyeing and mass dyeing after treatment using semi-pilot equipment. This project has demonstrated the possibility of achieving a dry way of leather dyeing. The need to work further on the dyestuff chemistry, to improve the affinity of the dyestuff with the leather, and its light stability, is shown. Applying this technology to all steps of leather manufacturing could greatly increase the interest in such an investment. Introduction The study checked the transfer feasibilities in the manufacturing of leather, of the dry dyeing methods already used in other industrial sectors such as textiles. The project consisted of selecting a dyestuff suitable for leather and the conditions of the supercritical CO₂ and for optimising the process parameters. The industrial potential of this technology, in terms of treatment efficiency, fastness of dyeing, reduction of effluent and energy savings were considered. Theory on the supercritical CO₂ The supercritical statement holds simultaneously the density of a liquid and the mobility of a gas, two parameters which are essential in the reaction mechanism on interfaces. CO2, the more commonly used compound, present in supercritical phases, provides very interesting solubility properties of chemicals and improves the swelling of fibres. In certain cases, the use of a recyclable additive is necessary in order to improve product penetration of the treated materials. The properties of supercritical fluids depend upon both the pressure and the temperature. Table 1 shows the different properties of different phases of the CO₂. It underlines the fact that supercritical CO₂ can be as dense as a liquid and less viscous, and that its diffusion coefficient is better than the one in liquid phase. The critical point temperature of CO₂ is 31°C. The technology of supercritical fluids represents a potential solution by proposing new solvating power and impregnation properties. It also reduces production costs by decreasing the amount of water and the elimination of certain post-tanning operations. Among the studies performed on textile dyeing under supercritical CO₂ conditions, several projects have been conducted to develop clean processes for textile dyeing1,2,3. With regard to leather, studies have already been conducted for the degreasing of sheepskins by supercritical CO₂ ^4,5,6,7. One other study from China was carried out over several stages of leather treatment in supercritical CO₂8. Execution modalities This study was conducted in five phases: Phase 1: Establishment of a book of specifications Bibliography and selection of the dyestuffs Phase 2: Study of the parameters that influence the dyeing procedure of leather in super-critical CO₂ Phase 3: Optimisation of the operating conditions of dyeing in supercritical CO₂ Phase 4: Transposition on a semi-pilot scale Phase 5: Pre-industrial evaluation (technical, environmental and economical approach) Results The main parameters studied: Products utilised in the process: * the nature of the leather: the behaviour of three crust leathers (calf, kip, sheep), and the impact of their thickness was evaluated * the nature of the dyestuff: the dyestuff used in the textile industry was tested * the nature of the additive: the current additives used for supercritical CO₂ treatment were assessed. Equipment: * temperature * pressure * evolution during the treatment * duration of the treatment The leather was dyed using laboratory scale and semi-pilot equipment The impact and interactions of these parameters have been assessed through: Physical and mechanical tests: * Measurement of the evolution of the size of the piece. This parameter remains equal before and after treatment * Evolution of the colour with colour matching equipment These values enabled the best covering dyestuff to be chosen. The homogeneity of the colour, by analysing different zones on the leather, was also checked. * The penetration of the dyestuff was assessed by a microscopal study of the cross-section of the leathers * Colour fastness to rubbing (NF G 52301) In dry conditions, the behaviour of the dyestuff is acceptable. When water is added to the rub, the values obtained show a lack of adherence between the leather and the dyestuff, which was not noticed after the labscale treatment (average dry value: 4-5, humid value 3-4). The semi-pilot conditions, which changed a little (use of rotating equipment) could explain this behaviour. The contact with the leather and the reactor during the process could have affected the diffusivity of the dyestuff inside the leather, producing a dyestuff coat on the top of the leather. * Light fastness (NF G 52 302, Xenotest): The average obtained on the blue scale was the value of 1. These very weak values (given on the semi-finished material), show the instability of the dyestuff used * Tear strength (NF G 52 004) The average value obtained is 4.3 daN/mm Chemical analyses The leather used was previously assessed to ascertain that it complied with current environmental requirements. Once coloured, the leather still had the correct properties. The natural stretch of the leather was also checked by hand and is quite similar. The results achieved at semi-pilot scale were transposed to the case of 100kg of leather treatment in order to provide an economical approach to the technology. The main advantages of the dyeing under supercritical CO₂ conditions are the non-use of water, which incurs no effluent treatment, as well as the possibility of recycling the CO₂ and the additive. The duration of the treatment halved in comparison with a traditional dyeing treatment of a crust leather. Regarding energy consumption, the power consumption per kg of leather is 0.95kW. The investment in such equipment is quite important. Conclusion In a context in which the use of water becomes more and more controlled and restricted, finding a means to avoid it is a big challenge. The supercritical treatments belong to possible new clean technologies to minimise the use of water and limit problems connected with water effluent and preservation of natural resources. This project has shown the possibility of realising a uniform dyeing of leather (surface and almost in the cross-section). The use of the semi-pilot equipment changes some of the properties of the leather in comparison with the labscale evaluation. This reinforces the interest to work on dedicated equipment and on the dyestuff chemistry, to improve the affinity of the dyestuff with the support and its light stability. Developing this technology for all steps of leather manufacturing could greatly increase the interest in such an investment. CTC, Lyon¹. Industrial partner: Tanneries Roux², Romans. Scientific partners: CEA VRH-DEN/DT/CD/SPDE/ FSM³; IFTH⁴. Financial partner: Région Rhône Alpes (France).