Tanning Chemistry

The Science of Leather

Anthony D Covington, Michael Redwood, Wolfgang Hummel, Jonathan Cooke

• Éditeur: Royal Society of Chemistry
• Année: 2015
• ISBN:
  9781782626022
• Édition: 1
• Sujet: Technology & Engineering
• Sous-sujet: Industrial Engineering

Even in the 21st Century, the manufacture of leather retains an air of the dark arts, still somewhat shrouded in the mysteries of a millennia old, craft based industry. Despite the best efforts of a few scientists over the last century or so, much of the understanding of the principles of tanning is still based on received wisdom and experience. Leather is made from (usually) the hides and skins of animals - large animals such as cattle have hides, small animals such as sheep have skins. The skin of any animal is largely composed of the protein collagen, so it is the chemistry of this fibrous protein and the properties it confers to the skin with which the tanner is most concerned. In addition, other components of the skin impact on processing, impact on the chemistry of the material and impact on the properties of the product, leather. Therefore, it is useful to understand the relationships between skin structure at the molecular and macro levels, the changes imposed by modifying the chemistry of the material and the eventual properties of the leather. This book aims to contribute to changing the thinking in the industry, to continue building a body of scientific understanding, aimed at enhancing the sustainability of an industry which produces a unique group of materials, derived from a natural source. The Science of Leather is the only current text on tanning science, and addresses the scientific principles which underpin the processes involved in making leather. It is concerned with the chemical modification of collagen, prior to tanning and the tanning reactions in particular. The subject is covered in the following order: collagen chemistry, collagen structure, skin structure, processing to prepare for tanning, the tanning processes and processing after tanning. The aim of the book is to provide leather scientists and technologists with an understanding of how the reactions work, the nature of their outcomes and how the processes can be controlled and changed. The objective is to synthesise a scientific view of leather making and to arrive at an understanding of the nature of tanning - how the wide range of chemistries employed in the art can change the properties of collagen, making leather with different properties, especially conferring different degrees of stabilisation as measured by the hydrothermal stability. Environmental issues are not treated as a separate theme - the impact of leather making on the environment is a thread running through the text, with the assumption that better understanding of the science of leather making will lead to improved processing. The book also reflects on the ways leather technology may develop in the future based on the foundation of understanding the scientific principles which can be exploited. It also includes a subject index, references and a glossary. The book provides the reader with insights into the role science plays in leather technology and provides fundamental understanding, which should be the basis for scientific and technological research and development for the benefit of the global leather industry. The book is aimed at students, leather scientists and technologists, in both academia and industry, in leather production and in chemical supply houses.

CHAPTER 1

Collagen and Skin Structure

1.1 INTRODUCTION

At the heart of the leather making process is the raw material, hides and skins. As the largest organ of the body of mammals, the skin is a complex structure, providing protection against the environment and affording temperature control, but it is also strong enough to retain, for example, the insides of a one tonne cow. Skin is primarily composed of the protein collagen and it is the properties and potential for chemical modification of this protein that offer the tanner the opportunity to make a desirable product from an unappealing starting material. It is part of the tanner’s job and skill to simplify or purify this starting material, allowing it to be converted into a product that is both desirable and useful in modern life.

Collagen is a generic name for a family of at least 28 distinct collagen types, each serving different functions in animals, importantly as connective tissues.^1–4 The major component of skin is type I collagen: so, unless otherwise specified, the term ‘collagen’ will always refer to type I collagen. Other collagens do feature in leather making and their roles are defined later.

Collagens are proteins, i.e. they are made up of amino acids. They can be separated into α-amino acids and β-amino acids (Figure 1.1). Each one features a terminal amino group and a terminal carboxyl group, which become involved in the peptide link (see below), and a sidechain attached to the methylene group in the centre of the molecule. When the amino acids are linked together to form proteins, they create an axis or ‘backbone’ to the polymer, from which the sidechains extend. It is the content and distribution of the sidechains that determine most of the properties of any protein. In the case of collagen, it is the sidechains that largely define its reactivity and its ability to be modified by the stabilising reactions of tanning, when leather is made. In addition, the chemistry of the backbone, defined by the peptide links, offers different reaction sites that can be exploited in some tanning processes.

Figure 1.1 Amino acid structures.

All the common amino acids are found in skin or skin components. There are two notable aspects of the amino acid content of collagen. Hydroxyproline (Figure 1.1) is almost uniquely present in collagen compared to other proteins and, therefore, offers the basis of measuring the collagen content in any skin or skin derivative. Tryptophan (Figure 1.2) is absent, therefore making collagen deficient as a foodstuff.

Figure 1.2 Tryptophan.

In terms of leather making, some amino acids are more important than others, since they play defined roles (Table 1.1): the roles of importance are either in creating the fibrous structure or involvement in the processing reactions for protein modification. Other amino acids, not included in Table 1.1, are important in defining the properties of the collagen, but play less defined roles in the leather making processes.

Table 1.1 Amino acids of importance to leather making.

Name	Symbol	Type	Sidechain (R)	Importance in leather making
Glycine	Gly	α, neutral	–H	Collagen structure
Alanine	Ala	α, neutral	–CH3	Hydrophobic bonding
Valine	Val	α, neutral	–CH(CH3)2	Hydrophobic bonding
Leucine	Leu	α, neutral	–CH2CH(CH3)2	Hydrophobic bonding
Isoleucine	Ileu	α, neutral	CH3CH2CH(CH3)	Hydrophobic bonding
Phenyl-Alanine	Phe	α, neutral	–CH2C6H5	Hydrophobic bonding
Serine	Ser	α, neutral	–CH2OH	Unhairing
Cysteine	CySH	α, neutral, S containing	–CH2SH	Unhairing
Cystine	CyS-SCy	α, neutral, S containing	–CH2SSCH2–	Unhairing
Aspartic Acid	Asp	α, acidic	–CH2CO2H	Isoelectric point (IEP), ^a mineral tanning
Asparagine	Asn	α, neutral	–CH2CONH2	IEP
Glutamic Acid	Glu	α, acidic	–(CH2)2CO2H	IEP, mineral tanning
Glutamine	Gln	α, neutral	–(CH2)2CONH2	IEP
Arginine	Arg	α, basic	–(CH2)3NHC(NH)NH2	IEP
Lysine	Lys	α, basic	–(CH2)4NH2	IEP, aldehydic tanning, dyeing, lubrication
Histidine	His	α, basic		Aldehydic tanning, dyeing, lubrication
Proline	Pro	β, neutral	See Figure 1.1	Collagen structure
Hydroxyproline	Hypro	β, neutral	See Figure 1.1	Collagen structure, hydrogen bonding

^aIn biology, referred to as pI.

Amino acids create macromolecules, proteins such as collagen, by reacting via a condensation process: the amide or peptide link is in bold:

The condensation reaction can be reversed by hydrolysis, by adding the elements of water. Clearly, hydrolysis as set out in this chemical equation cannot be fast, nor does the equilibrium lie to the left, otherwise the protein would be unstable and useless as the basis of life. In contrast, the hydrolysis reaction is catalysed by general acid and general base – importantly for leather making, it is catalysed by H⁺ and OH⁻. The impact on processing can be indicated as follows.

In the earliest stage of processing, hair is usually rem...