The leather industry, in spite of using by-products of the meat industry, is also a potential pollutant, as it generates solid and liquid wastes. In the traditional manufacturing process, chromium may be present in a large part of such wastes. A lot of research has been made in order to find ways to valorize these wastes, although for crust and finishing leather trimmings more work is needed to show their potential. The aim of the present investigation is to create useful products through the pyrolysis of waste leather trimmings. The experiments were performed in a vertical semi-batch reactor with ID of 9.7 cm. The influence of operating temperature (490 – 800°C) and heating rate on the composition and distribution of the different phases (solid, liquid and gas) was studied. The gas and the liquid fractions were analyzed by gas chromatography and FTIR, respectively. The char obtained was characterized in terms of its higher heating value and proximate analysis. In addition, the thermal degradation of the leather waste was followed using thermogravimetric analysis.
As the temperature increased, the yields of char and gas phase decreased and increased, respectively. The yield of the liquid phase was almost constant for 490 and 610°C, and decreased for higher temperatures. The effect of heating rate on the yields of pyrolysis products was almost negligible. The amount of chromium oxide in the char tended to increase with increasing temperatures. The FTIR analysis of bio-oils indicated the presence of phenols and alcohols, alkanes, aldehydes, ketones, carboxylic acids and aromatic compounds. GC analysis of the gas phase indicated that CO2 was the most abundant gas at 490°C, while hydrogen presented higher concentrations at bed temperatures of 610, 700 and 800°C.
Different types of leathers with varied end applications can be made from same raw material. Post-tanning is the operation, which imparts the functional properties required for the final leather. The amount of post-tanning auxiliaries such as syntan (S) and fatliquor (F) used in the leather making, make it possible to convert the same animal skin into various leather types. In this work, the sheep wet-blue has been converted into garment (S:F 1:2) and upper (S:F 2:1) leather by varying the S to F ratio. Here, an attempt has been made to study the influence of S to F ratio on the porosity and mechanical properties of garment/upper leather with respect to sheep wet-blue as a control. Thermoporometry results show that the reduction in pore distribution of garment leather with respect to wet-blue leather is about 62.3% and that of upper leather is 83%. Capillary flow porometry results show that the maximum distribution of through pore throat diameter of upper leather is found to be shifted towards 0.1-0.2 µm from 0.3-0.5 µm (wet-blue leather), whereas in the case of garment leather pores are uniformly distributed in the range of 0.1-0.5 µm. The influence of post-tanning auxiliaries on air permeability reduction is higher for upper leather (83%) than garment leather (41%). Morphological analysis using SEM shows that loosely packed arrangement of wet-blue leather becomes cemented fiber structure in case of upper leather and uniform fine fiber splitting has been observed for garment leather. Tensile strength of wet-blue leather is decreased by about 23% when it is converted into upper leather and that of garment leather is increased to about 16%.
The effect of molecular weight of acrylic resin retanning agent on its penetration in leather and the properties of the retanned leather were investigated. After retanning chrome tanned leather by four fluorescent acrylic resins with different weight-average molecular weights (Mw) from 5,000 to 200,000, it was observed that the Mw of acrylic resins was not the main factor influencing their penetration in leather. In fact, the dosage of acrylic resin and the charge of leather were more important factors affecting penetration rate. Additionally, an increase in the Mw of acrylic resin led to an increase in the thickness of leather and a decrease in the softness of leather, which is mainly because the acrylic resins with higher Mw can enter larger gaps between collagen fibers and exhibit a better filling effect. Moreover, when acrylic resins were used together with other anionic retanning agents in retanning process, the penetration and distribution evenness of acrylic resins were remarkably improved, and meanwhile the acrylic resins with different molecular weights presented nearly the same effect on properties of resultant leathers.
In order to study the influence of microwave, especially non-thermal effect, on leather properties, the microwave was used to dry leather together with the oven dried leather as a control. The back, butt and belly of dried leathers were tested for mechanical properties, softness, shrinkage temperature, chrome and fat migration, microstructure and dielectric constant. The results showed that microwave could improve the mechanical properties, softness and shrinkage temperature as a visible non-thermal effect. The non-thermal effect could also promote uniformity of chrome tanning agent and fatliquoring agent in leather and strengthen combination of these materials with collagen. In addition, the fiber weaving of microwave dried leather was more orderly and porous proved by SEM and XRD. Furthermore, the dielectric constant of microwave dried leather was higher than control, showing a direct proof of non-thermal effect for microwave. To sum up, Microwave in leather drying has not only thermal effect but also non-thermal effect on improving leather overall performance.