Tanning Chemistry

The Science of Leather

Anthony D Covington, William R Wise

• Publisher: Royal Society of Chemistry
• Year: 2019
• ISBN:
  9781788019071
• Edition: 2
• Topic: Technologie et ingénierie
• Subtopic: Ingénierie industrielle

This book offers a state-of-the-art view of leather making, based on the scientific principles underpinning the technology. In particular, it contributes to the understanding of the modern leather industry, allowing practitioners to make judgements about day-to-day problems in the tannery and how change can be applied in a predictable way. Major themes running through the book are the economics and environmental impact of leather making and how these will ensure the sustainability of the industry.

This second edition of Tony Covington's Tanning Chemistry is a revision, update and extension in collaboration with a new co-author, Will Wise. The update reflects the advances made in the past decade, including a discussion of the impact of new information concerning the chemistry of sulfide. The original chapters have been re-organised and new chapters on novel modes of reagent delivery and the principles of finishing are now included. Enzymology is addressed as a separate topic, as are environmental impact and the future of leather.

The book will be useful to all those involved in the supply chain, from farm, through students, chemical suppliers and tanners, to leather goods brands. Leather science is the key to understanding leather technology, to make it work, to make it work better and to keep it ahead of the competition.

1Collagen and Skin Structure

In order to understand the principles that underpin the conversion of hide or skin to leather, it is necessary to know the fundamental structure of the raw material and how that structure might be modified chemically. The chemistry of collagen defines not only the sequencing of its amino acid constituents but also the physical nature of its structure and how it creates levels of structure or a hierarchy. This depends on the chains creating a triple helix as the basic unit of structure. The chemical properties of collagen are defined by the sidechains on the helices, which may be charged, depending on the pH; in this way, collagen can undergo a wide range of covalent or electrostatic reactions, which are the basis for tanning processes. At the heart of the chemistry of collagen is the relationship with water, which is an integral feature of structure: the supramolecular matrix of water around the triple helices provides the region for chemical modification, leading to tanning technology.

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 1 tonne cow. Skin is primarily composed of the protein collagen and it is the inherent 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 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 – hence, unless specified otherwise, here the term ‘collagen’ will always refer to type I collagen. Other collagens do feature in leather making, however, and their roles are defined later.

Collagens are proteins, that is, they are made up of amino acids. They can be separated into the α-amino acids and the β-amino acids, as shown in Figure 1.1. Each one features a terminal amino group and a terminal carboxyl group, which become involved in the peptide link (see later), 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. 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, α and β.

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, represented in Figure 1.1, is almost uniquely present in collagen compared with other proteins, therefore offering the basis of measuring the collagen content in any skin or skin derivative. Tryptophan, shown in 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, set out in 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 the table, 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 in leather making

Name	Abbreviation	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
Phenylalanine	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 as shown in the following equation, where the amide or peptide link is in bold:

The condensation reaction, removing the elements of water, can be reversed by hydrolysis, by adding the elements of water. Clearly, hydrolysis, as set out in this 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. On the other hand, 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 process...