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Leistungen
The laboratory equipment developed is capable to perform stretching tests of circular leather pieces while immersed in water or any other liquid at controlled temperature.
The system is capable to perform a multidirectional radial stretching of the circular leather pieces holding them at their outer contour by means of a variable number of grippers. The leather is stretched in an homogeneous way, as a difference to other laboratory equipments that stretch the test pieces only in one direction.
The experiment is computer-controlled, so that the experiment can be programmed from the user interface (stretching rates desired for the leather, duration of the experiment, etc.) and different data of the experiment can be monitored and registered for further analysis. Among these data, it is possible to register the tensions in the leather piece during the experiment, by means of waterproof tension sensors (strain gauges) placed at each gripper. The water temperature is also measured and registered during the experiment.
In order to be able to introduce the equipment in a heated water bath, the design is extremely compact and the non waterproof elements have been located above the water level and have been properly insulated.
A rheological model has been developed which underpins the new process and defines the range of conditions (temperature, time, moisture content) within which the new process can be successful for particular leather. The model is based on the way in which the key viscoelastic parameters of storage modulus and loss tangent vary with temperature and moisture content. The temperature profile of loss tangent defines a key transition temperature for a particular leather above which the stress-relaxation profile changes markedly leading to very significant increases in set and area gain.
The stress relaxation process can be mathematically modelled using Maxwell elements and this shows that a marked reduction of the relaxation times of some elements occurs at the transition. The maximum in loss tangent defines the optimum treatment temperature. An upper limit of the treatment temperature is also predicted by the model and is related to a characteristic temperature, which defines the commencement of the time dependent uncoiling of the collagen molecules. This temperature is at least 15 degrees Celsius below the classical shrinkage temperature.
The potential applications of this result are all those leather drying processes where the user is expecting to get a stable leather surface increase. In fact the raw leather is bought by the tanners on the basis of its weight while, after the industrial processes to get the final product, it�s sold on the basis of its surface: therefore, the more yield is got, the bigger economical benefit is achieved.
Especially, the furniture and automotive leather fields are very interested, because big surfaces are used, and of course every surface increase is very important from the economical point of view.
The potential users of this type of plant are all the tanneries using some systems to extend the leather during the drying and are located in every country where leathers are processed.
Most of the potential users (furniture leather manufacturers) are located in Europe, United States, South America, South Africa and Far East.
The main expected benefits of this innovation are time reduction in the extension process, reduction of the workers needed and energy-saving.
A potential barrier to sell this new plant could be the cost of the plant itself, because only a few big tanneries could afford such investment.
Additional engineering studies are necessary to find a compromise between the cost of the plant and the benefits given, so that it can be suitable to most of the possible users.
It�s important not to forget that the tannery field is very variegated for the type of leather (goatskin, sheepskin, pigskin, bovine leather), for the type of final product (shoes, leathergoods, clothes, furniture) and for the type of tannery (big, small and medium). Therefore the proposal for the drying plant must be studied according to the different uses and consequently long time is needed.
A set of conditions of thermal treatments has been studied:
- Temperature;
- Temperature regime (fixed temperature versus gradually increasing);
- Treatment duration;
- Number of drying cycles.
Using hot water and simultaneous multiaxial mechanical stretching have been defined to increase the area yield of three different types of hides:
- Bovine hide;
- Woollen sheepskin;
- Non-woollen sheepskin 40% linear stretching for 15 min and 80 degrees C is the best set of conditions.
The novel treatments have resulted in an area increase of up to 11.7% for bovine hides. Studies at microscopic level suggest the inexistence of different properties in leathers tanned under the new procedure, with respect to the conventional tanning practise. Macroscopic performance tests have been carried out, in order to confirm that physical properties of the leathers are not affected in a negative way.
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