In German called LEFA production LEFA is an abbreviation of the German term Lederfaserwerkstoffe which means leather board material leather board is made from the basic components leather fibers and binders corresponding quantities of felting substances such as synthetic or cotton fibers are sometimes used to obtain special properties the leather fibers consist of shavings of chrome tanned leather vegetable whitening’s and leather waste from trimming thus the function of leather board production is to dispose of the waste produced in the leather industry the main application of leather board material is the shoe industry where it is used to make frames counters insoles middle soles and outsoles for slippers further applications are the bag industry e. g suitcase school bags partitions for briefcases or portfolios and technical products packing’s ,cup leathers etc .
Processes of manufacture
There are two manufacturing procedures
1. Wire box –discontinuous [antiquated now only used occasionally]
2. Fourdrinier machine –continuous [principle of paper machines].
Procedure of manufacture
Both procedures use the same method to make the fibrous pulp they differ only in mechanical processing the composition of the fibers binders add actives and fatliquoring agents vary depending on the intended use. Coarse leather waste is first reduced in size in knife mills and then defibrated in disk mills or also in beaters by adding water. This pulping procedure is continued until there is no more a clot or lumps. Shaving of chrome-tanned and vegetably tanned leathers is only treated in disk mills. The fibers pulp obtained from different types of waste is transferred into separate storage vessels (content of solids of 5-8 %)
The fibrous pulps are then mixed in a mixing vessel often a beater in specified quantities depending on the intended use, and adjusted to a stock density of 1.5 – 3 %. Since the pH is in the acid range (due to the predominance of chrome fibers), a corresponding amount of diluted fat liquoring agents on the desired softness or firmness and on the fat already contained on the fibrous pulp. After thorough mixing the pH value of the float is adjusted to 5 – 7 by weak alkalies. If dyeing of the leather board material is required, simple acid dye less frequently inorganic pigments are added subsequently.
After adequate homogenization and complete absorption of the fat liquoring agents and the dye, if used the binder which has been diluted to 5- 10 % is slowly added while stirring. The quantity used depends on the desired final product and is between 10 – 40 % (calculated in relation to the content of solids)
If the binder has completely dispersed the entire fibrous pulp is precipitated with precipitating agents the end of precipitation is reached if a sample taken from the fibrous pulp and poured onto a filer or a very fine –meshed screen gives A clear filtrate the fibrous pulp obtained is further dewatered and compacted by screen exhaustion and in presses the process can be accelerated by applying a vacuum the fibrous leatherboards are then dried by hot air after a recovery time of 24 hours the leatherboards can be further compacted by means of a hydraulic press or by calendar rollers they are manufactured in thicknesses of 0.15-7 mm and with a specific weight of 0.6-1.0 mm with modern continuous processing on the fourdrinier machine [working widths of 160-300 cm ]it is possible to manufacture leatherboard webs by rolling them up at the end of the machine to obtain a high specific weight of the webs a hydraulic press for intermittent processing is connected to such machines to achieve quick through –feed rates dewatering is an especially important factor
Properties of the substances used
Fibres
The main substance used are chromiferous fibres they give a high tensile strength extension and temperature resistance whereas dewatering and dimensional stability are low vegetably tanned fibres have reversed properties therefore the fibres are applied in mixtures depending on the requirements
Fibrous additives
These are added in corresponding quantities depending on the requirements. Cellulose fires [pulp] increase dimensional stability. Synthetic fibers and cotton fibers improve the tensile strength properties.
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