Applicative Chemistry of Tanning Metallic Heterocomplexes

Leather tanning is one of the oldest human activities, which intelligently exploits an animal by-product, natural leather. Although lost in the mists of history, the evolution of leather processing is closely related to the history of humankind and scientific progress. Types of tanning practiced by mankind are closely related to intuitive knowledge about conservation of organic, putrescible materials and chemical materials which interact with the former. Natural leather processing is presented in conjunction with the evolution of human society and historical epochs. Understanding the leather tanning process is closely related to understanding the interactions between different functional groups of collagen and crosslinkers: vegetable tanning agents, aldehydes or chromium salts. The connection between the value of using leather, its properties and the method of processing is the foundation of understanding natural leather processing technologies and the start of a journey into the fascinating world of alternative ecologic processing and chemistry of mineral tanning agents.

Tanning using trivalent chromium salts underwent an evolution marked by the need to obtain technical performances for the leathers at first and then to limit the penetration of chromium in the environment by means of wastewater or solid waste. The evolution of methods to reduce environmental pollution with trivalent chromium salts included general environmental principles on advanced absorption of chromium, partial replacement of chromium, recycling, recovery and total replacement of chromium. Advanced technologies for exhaustion of chromium from effluents are based on the development of technological conditions favorable to absorption of chromium above the accepted level of concentrations by osmotic balance between the two environments, hide and tanning bath. Partial chromium replacement technologies are probably the most studied methods of reducing the use of chromium as tanning metal. The great diversity of variants proves that there is no imposed variant as of yet in the industry. Recycling and reuse of chromium are methods that require additional investment in facilities for recycling used tanning solutions or for precipitation and dissolution of chromium salts. The advantages consist in reducing environmental pollution and chromium consumption, while disadvantages are related to laborious analytical control and quality of leathers obtained, which is not at the same level as that obtained by using basic salts with a consistent quality. The newest way to reduce environmental pollution by chromium salts is wet-white tanning, which implies a pretanning with chromium-free materials, mechanical processing by splitting and shaving followed by tanning and retanning with chromium salts or syntans. The organic nature of chromium-free wet-white versions is the main advantage, but it also distances itself from the traditional, mineral character of natural leather. In this respect, the approach of tanning metallic hetercomplexes provides both classic features of mineral leathers and reduces environmental pollution.

Developing complex metallic heterocomplexes requires knowledge of synthesis methods based on oxidation-reduction reactions, complexation to boiling point or basification of mixtures of salts of chromium, aluminum, iron, zirconium or titanium, the main tanning metals for natural leather. The purpose of these syntheses is developing heterocomplexes with higher stability to alkali than that of metal salts from which the natural leather tanning properties derive. Chromium-aluminum salts are the only heterocomplexed salts that are currently commercially available, with restricted use to certain types of applications related to maintaining a lighter colour of leather and improved polishing ability. Knowing the possible structure of complex metallic heterocomplexes obtained is a complicated endeavor due to the polydispersity of tanning solutions and the difficulty of separating the tanning heterocomplex in crystal form. IR analyses enabled the formulation of hypotheses on the structure of chromiumzirconium and aluminum-titanium heterocomplexes, while X-ray diffraction of chromezirconium- cerium crystal enabled the identification of the role of sulfate ion of ligand tridentate and not bidentate, as it was previously thought, the identification of oximetallic O-Cr-O bond length of 10Å, identical to the distance between the collagen macromolecules forming the three-macromolecule helix. The close connection between the synthesis method and the stability of tanning metallic heterocomplexes is highlighted by examples of research in the field. The topicality of tanning metallic heterocomplexes is emphasized by their superior properties compared to organic tanning variants, both in terms of economy and technology, and the greater possibility of combining metals or organic ligands to develop structures with different collagen crosslinking properties.

The main synthesis reaction of metallic heterocomplexes is the oxidationreduction reaction in acid medium using glucose as a reducing agent. Understanding the mechanism of synthesis of metallic heterocomplexes with various basicities is related to the stoichiometric mechanism of sulfuric acid release by the aluminum sulphate present in the system, to the use of sulfuric acid by the ferrous sulfate which oxidizes to ferric sulfate. The theoretical study of the synthesis reactions of tanning heterocomplexes reviews all the basicity values which can be obtained and proposes formulas for calculating the amount of sulfuric acid required to obtain a quantity of tanning metallic heterocomplexes containing 100 kg of metal oxides. Monitoring the laboratory scale synthesis of chromium-iron, chromium-aluminum-iron and iron-chromium-zirconium metallic heterocomplexes confirms the stoichiometric mechanism for heterocomplexes with 33% basicity. The influence of aluminum and iron in metallic heterocomplexes consists in destabilizing the complex so that the chromium-iron and the chromium-ironzirconium heterocomplexes are more stable than the chromium-aluminum-iron one. Stability to alkali of the new tanning metallic complexes confirms internal heterocomplexation and the formation of more stable structures than the monometallic salts they derive from. Stability of the three types of tanning metallic heterocomplexes over time is good after 20 days up to 5 months and demonstrates the possibility of using them in leather tanning and retanning under similar conditions to using basic chromium salts. Tanning metallic heterocomplexes enable Cr2O3 offer reduction by 30-50% and therefore reduction of pollution provided that a synergy of interaction with collagen is achieved. Pilot scale synthesis of chromium-iron, chromium-aluminum-iron and chromium-iron-zirconium variants of heterocomplexes with the best stability has validated the theoretic reactions developed and showed the higher stability of tanning metallic heterocomplecxes. Tanning metallic heterocomplexes can also be developed as powder by atomization, and the stability to alkali of solutions obtained from powders is higher in the case of chromium-iron heterocomplexes and remains unchanged for other types of tanning heterocomplexes.

Knowing the structure of tanning metallic heterocomplexes is closely linked to their chemical properties in relation to stability in various environments specific for natural leather tanning and to their ionic nature which influences their penetration in the dermis of natural leathers. This chapter deals with specific analyses for coordination complexes, for the purpose of anticipating the behaviour of tanning metallic heterocomplexes upon the interaction with natural leather. IR and electron spectroscopy have enabled to identify organic ligands, the polynuclearity of complexes by means of sulphate or hydroxyl anions, the metal-oxygen-metal interactions and have eliminated the hypothesis of direct metal-metal interactions. Electron spin resonance spectroscopy allowed identification of structures in predominantly octahedral environments with rare insertions of tetrahedral environments, which provides steric stability to heterocomplex structures. SEM-EDAX analysis determines the existence of tanning metals used and morphological particularities at microscopic level. Analysis of ionic components explains the more pregnant anionic nature compared to that of the best known tanning agent, basic chromium salt, and therefore, the improved efficiency of using tanning metallic heterocomplexes in natural leather tanning.