Abstract:

Toxic chrome salts are used for the production of 70% of today's leather, namely the majority of soft leather. The project proposes to synthesise copolymers of non-toxic vegetable tannins and waste lignocellulosic materials, both derived from the forestry industry, with

• acrylic acid,
• aminoplastic resins, and
• by reacting natural vegetable tannins with small amounts of non-toxic nitroalcohols, and to show that these compounds can substitute toxic chrome salts, completely or at least partially (in retannig operations), to produce acceptable soft leather.

Introduction

The leather tanning industry is one of the most ancient still in operation. Although the technology of leather manufacure has evolved over centuries, and even in recent years, the basic principles for the production of leather have remained the same. Hide proteins, mainly collagen, are rendered insoluble and dimensionally more stable by treatment with chemical products able to fix on them arid render them more resistant to both mechanical wear and less susceptible to biological and other types of attack. The main products used today for leather tanning are as follows:

• Acid salts of trivalent Chrome, or sometime of Aluminum, in particular sulphates, mainly used for the manufacture of soft leathers for shoe uppers and for leather bags.
• Forest-derived, natural vegetable tannins, such as chestnut and flavonoid extracts, mainly used for the manufacture of heavy, rigid and hard leathers for shoe soles, saddles, belts and other implements subject to high wear.
• Aldehydes, in particular formaldehyde and glutaraldehyde
• Sulphonated synthetic polymers such as acid phenol-formaldehyde novolaktype resins
• A number of other synthetic resins (aciylates, oxazolidines, aminoplastic resins, etc.)

Each of the products mentioned above is best suited to the manufacture of certain types of leather. The fact remains however, that the first two on the list account for more than 90% of all the leather manufactured to-day, and that the process based on trivalent Chrome salts accounts by itself for about 70% of the total. Chrome tanning is particularly suited for soft leather as it does not affect hide flexibility and renders the leather very stable, both chemically and physically. The forestry derived vegetable tannins, in contrast, have a strong astringent effect (they fix very effectively on the collagen structure) and give considerable 'body', hardness and toughness to the leather produced with them. Conversely, synthetic resins, in particular the acrylates, give very soft, elastic, light-coloured leathers presenting high resistance to degradation induced by light. These are used for the preparation of clothing articles.

Tanning is carried out in a series of complex, discontinuous operations requiring a considerable amount of labour, and the treatment of waste waters represents one of the major problems in this industry, especially to-thy that European Norms impose ever more stringent effluent limits. The waste-waters are generally treated in biological plus chemical purification plants in particular to abate (never eliminate) residues of chrome and aluminium salts.

It has been amply demonstrated that a natural vegetable tannin is biodegraded with considerable more ease than a synthetic resin, leading consequently to a multifold improvement in effectiveness of existing purification plants. Furthermore, well-defined quality standards as regards these aspects have also been introduced for finished products, for instance in clothing and interior car linings. In this respect two of the tougher requirement limits to comply with are the amount of both the proportion of hexavalent chrome to be used in direct contact with human skin (such as watch straps, shoe uppers, etc) as well as the amount of formaldehyde emission.

Objectives

The main objectives of the project are to obtain

• an equally soft leather as obtainable by chrome tanning but eliminating or completely or partially the need to use chrome as a tanning agent.
• an equally light coloured and light-stable and colour-fast leather as obtainable by chrome tanning but eliminating or completely or partially the need to use chrome as a tanning agent.
• an equally temperature shrinkage-resistant leather as obtainable by chrome tanning but eliminating or completely or partially the need to use chrome as a tanng agent.

Activities As regards the approaches of the proposed project these are centered on three main areas:

• The development of copolymers of acrylic resins with commercial and tannins and other waste lignocellulosic polyphenolic materials,
• The reduction of the photo-oxidation of vegetable tannins by synergy and co-polymerization with synthetic aminoplastic resins.
• The reduction of the temperature induced leather shrinkage by reaction with nonvolatile cross-linking agents other than formaldehyde

Contacts

Coordinator

• Antonio PIZZI / ENSTIB University of Nancy 1 / FRANCE

EC Scientific Officer

• Ciarán MANGAN / European Commission - DG Research Quality of Life Programme - Unit B1.2 / BELGIUM

Participant

• Robert REID / Holmes Halls Tanners Ltd UNITED KINGDOM
• Marco BESSE / Silva Srl ITALY