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.
|Publisher:||Royal Society of Chemistry, The|
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About the Author
Anthony D Covington is Emeritus Professor of Leather Science at the University of Northampton, UK. The book is the culmination of a 40 year career in chemistry and the leather industry which started with a graduateship of the Royal Institute of Chemistry (now the Royal Society of Chemistry) at Teesside Polytechnic (now the University of Teesside). He continued his chemistry experience with doctoral studies in physical organic chemistry at Stirling University, followed by post doctoral research in physical chemistry at the University of Newcastle upon Tyne. Professor Covington then joined the British Leather Manufacturers' Association (later to become BLC The Leather Technology Centre of Northampton, UK), where he spent 18 years engaged in research and development, industrial consultancy and problem solving in the UK and overseas. From 1995-7, he was President of the International Union of Leather Technologists and Chemists Societies (IULTCS) and since 1995 he has been with the British School of Leather Technology of the University of Northampton. Professor Covington has 21 doctoral completions and 230 technical publications to his name.
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The Science of Leather
By Tony Covington
The Royal Society of ChemistryCopyright © 2009 Tony Covington
All rights reserved.
Collagen and Skin Structure
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. 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.
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.
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.
Amino acids create macromolecules, proteins such as collagen, by reacting via a condensation process: the amide or peptide link is in bold:
H2N-CHR-CO2H + H2N-CHR-CO2H = H2N-CHR-CO-HN-CHR-CO2H + H2O
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 removed and at the same time the skin is given a prolonged alkali treatment, typically conducted over about 18 hours for cattle hides, often in aqueous lime solution, Ca(OH)2; longer treatment may result in detectable damage to the fibre structure. In saturated lime solution, [OH-] = ~10-2 molar. Conversely, pickling in brine solution with acid is routinely used as a preservation technique for more vulnerable sheep skins, enabling them to be transported across the world between Europe and Australia and New Zealand over a period of several months: here [H+] = ~10-2 molar. Hence, by this practical comparison, in which pickled pelt remains undamaged for months, but limed pelt shows damage after a few hours, hydroxyl ion produces a much faster reaction than hydrogen ion – at least 100 × faster.
An important feature of the peptide link is that it is partially charged. The link can be expressed in two forms. The charged structure makes chemical sense, but nature does not favour charge separation in this way. However, the electronegativity difference between the oxygen and the nitrogen means that the structure can be set out in a slightly different way, shown in Figure 1.3.
The two parts of the peptide link each carry only a partial charge, but this still allows the peptide link to play significant roles in the interaction between the protein and water and in the fixation of reagents in the leather making processes, most important in post tanning, when the leather is dyed and lubricated (Chapters 16 and 17).
1.2 HIERARCHY OF COLLAGEN STRUCTURE
It is a feature of the properties of collagen that it has 'layers' of structure, collectively known as the 'hierarchy' of collagen structure, which combine to allow the formation of fibres. These can be defined as follows, starting with the most fundamental element of structure.
1.2.1 Amino Acid Sequence
Collagens are characterised by a repeating triplet of amino acids: -(Gly-X-Y)n-. Therefore, one-third of the amino acid residues in collagen are glycine. Furthermore, X is often proline and Y is often hydroxyproline: 12% of the triplets are -Gly-Pro-Hypro-, 44% are -Gly-Pro-Y- or Gly-X-Hypro- and 44% are -Gly-X-Y- where X and Y are not defined. In this way, the helical shape of the molecule is determined (see below).
The amino acid compositions of bovine skin proteins are compared in Table 1.7. In leather making terms, the amino acid sequence plays only a minor role; indeed, the technologies for making leather are essentially the same for all animal skins: variations in technologies are much more dependent on the macrostructure of the skin and its age rather than the details of the chemistry of the protein. The amino acid content determines the reactivity of the protein towards reagents such as tanning compounds and the sequence influences the formation of electrostatic links, which is important for protein stability but which is also exploited in leather making: the amino acid content determines the isoelectric point (see below), which together with pH controls the charge on the protein. Therefore, these features of the protein influence its affinity for different tanning reagents, but there is little difference between animal species in terms of the outcomes of stabilising reactions. Here, stability or more commonly the hydrothermal stability (resistance to wet heat) is conventionally measured by the temperature at which the protein loses its natural structure: for collagen this is referred to as the 'shrinkage temperature', T,s or sometimes as the melting or denaturation temperature.
The only discernible difference in hydrothermal stabilities of collagens is observed in extreme variations in skin source, where there can be big differences in shrinkage temperature, depending on the natural environment of the animals concerned (see Table 1.4 below). When there are differences in shrinkage temperatures between collagen sources, the difference is also reflected in the shrinkage temperatures acquired from stabilising chemistries (tanning options).
1.2.2 The α-Helix
The presence of a high content of ß-amino acid causes the chain of amino acids to twist, due to the fixed tetrahedral angles, locking the twist in place (Figure 1.4). Notably, the twist is left-handed, i.e. anti clockwise, because of the natural L-conformation of the molecules.
1.2.3 The Triple Helix
The notion of the triple helix structure of collagen was first proposed by Ramachandran. In type I collagen, the monomeric molecule, protocollagen, contains three chains, designated αI(I) and αI(2): there are two αI(I) chains and one αI(2) chain, which differ only in the details of the amino acid sequence. These three chains twist about each other in a right-handed or clockwise triple helix: this is only possible because of the high glycine content, which has the smallest α-carbon sidechain, a hydrogen atom, so that glycine is always situated in the centre of the triple helix (Figure 1.5).
Each α-chain is about 1050 amino acids long, so the triple helix takes the form of a rod about 300 nm long, with a diameter of 1.5 nm: this is the monomeric unit from which the polymeric fibrous structure is created. In the production of extracellular matrix from the fibroblast cells of the skin, the triple helix is synthesised as the monomer procollagen and then the monomers self assemble into a fibrous form. Before it becomes fibrous, soluble collagen can be isolated as triple helices and it is in this form it can be extracted from immature skin under acid conditions.
In the triple helix there is an inside and an outside of the structure. The minimum size of the glycine sidechain and its occurrence every third residue in the alpha helix allows it to fit into the inside part of the structure. If the sidechain was bigger, the triple helix could not form.
At each end of the triple helices there are regions that are not helical, consisting of about 20 amino acids, called the telopeptide regions. If soluble collagen is isolated with the aid of pepsin, a proteolytic enzyme, triple helices are obtained without the associated telopeptide regions. In the case of acid extraction of soluble collagen, individual triple helices are obtained with the telopeptide regions intact (Figure 1.6).
In the context of leather making, the importance of the telopeptide regions lies in their role in bonding, to hold the collagen macromolecule together: the covalent bonds holding the triple helices together link the helical region of one triple helix to the non helical region of another triple helix, indicated in Figure 1.6. Modelling studies have shown that, because of their flexibility, the telopeptide chains are capable of their own interactions between a-helices and triple helices. In tanning terms, the telopeptide regions probably do not play a significant role in the preparative processes or in the stabilising reactions: in the latter case, the stability arises from the structure created between the triple helices, based on the high degree of structure already in place in the helical region, but which is not present in the more random telopeptide chains.
1.3 ISOELECTRIC POINT
Importantly, the triple helix is also held together by electrostatic bonds, as follows:
P-CO-2 ... +H3N-P
These are the so-called 'salt links', formed by electrostatic reaction between acidic and basic sidechains of the protein, which together determine the isoelectric point of collagen. The IEP is an important parameter, because it controls the charge on the protein at any given pH: since the reactions in leather making are usually dependent on charge, they are in turn dependent on the IEP and the way the value of the IEP varies during processing. The isoelectric point is defined in several ways, but the most fundamental definition is that it refers to the point on the pH scale at which the net charge on the protein is zero, illustrated as follows:
[FORMULA EXPRESSION NOT REPRODUCIBLE IN ASCII]
Because the charge on the protein can be adjusted with acid or alkali, to make the protein positively or negatively charged, respectively, there must be a point on the pH scale when the net charge passes through zero. Therefore, the IEP depends on the relative availability of groups that can participate in reactions with acid and alkali, the carboxyl and amino groups. So IEP can be defined as follows:
[FORMULA EXPRESSION NOT REPRODUCIBLE IN ASCII]
where: f indicates some function of concentration and T indicates total concentration.
The second, expanded form of the equation indicates that it is only the total availability of a species that is important, its form is unimportant, i.e. the IEP does not change with pH.
More strictly, the definition should incorporate the role of the pKa or pKb of these groups, which would require that each specific pH active group would be treated separately, exemplified as follows:
[FORMULA EXPRESSION NOT REPRODUCIBLE IN ASCII]
In this way, all i and j species provide individual contributions to the isoelectric point. Unfortunately, the nature of the functions f is not known, so calculation of the precise value of the IEP is not yet possible. However, this is clearly not an intractable problem.
The equations are usefully presented in this form, with the amino function as numerator and the carboxyl function as denominator, because they then predict how varying the functions influences the IEP, in terms of the direction of change (Table 1.2).
At the isoelectric point, the following phenomena occur, each of which might be used as the definition, as a whole or in part.
net charge is zero
content of intramolecular salt links is maximised
swelling is at a minimum
shrinkage temperature (and other aspects of hydrothermal stability) is maximum
There are two points relating to isoelectric point and its relevance to leather making that are very important and worth emphasising:
1. IEP is a point on the pH scale, so it does not change with changing pH of the system. The IEP of collagen is the same whether is in the alkaline, limed state or in the acidic, pickled state.
The importance of this point is that the isoelectric point can only be changed if there is a chemical change that alters the availability of active groups; this can occur in the beamhouse processes, in the tanning processes and in the post tanning processes.
2. The charge on collagen is determined by the relative values of the IEP and the pH. If the pH is higher than the IEP, the collagen is negatively charged, and if the pH is lower than the IEP, the collagen is positively charged. Moreover, the further the pH is from the IEP, the greater is the charge, although it is limited by the availability of amino and carboxyl groups.
The importance of this concept relates to the application of charged reagents, particularly post tanning reagents and their interaction with the charged leather substrate.
It can be assumed that at the starting point of processing for collagen, prior to raw pelt going into liming (alkaline treatment for unhairing and modification of the skin's fibre structure), the IEP is at physiological pH, about 7.4.
1.4 COLLAGEN AND WATER
An important part of the structure of collagen is the role of water, which is an integral part of the structure of collagen and hence of its chemically modified derivatives (Table 1.3).
Gustavson observed that the shrinkage temperature of raw skin depends on the pyrrolidine content, i.e. proline and hydroxyproline (Table 1.4); this observation was extended by Privalov, who demonstrated that the relationship relies more on the hydroxyproline content than on the proline content. The entropy loss for the secondary amino acids in the denatured state is less than for other residues, because the ring component of their structures restricts steric conformations; therefore, this could contribute to the relationship between the higher stability and higher proline and hydroxyproline content of the protein. Clearly, the hydrothermal stability of the collagen is also a reflection of the environment in which the animal lives.
Berg and Prockop extracted protocollagen, a non-hydroxylated version of collagen, and demonstrated that its structure is the same as collagen, based on the dimensions of the polypeptides and optical rotatory properties. It was found that the shrinkage temperature is 15 °C lower than hydroxylated collagen, indicating the importance of hydroxyproline to the stability of collagen.
Privalov believed that the hydrogen bonding by water at hydroxyproline is important in stabilising collagen, but thought that the Ramachandran model could not solely explain the high denaturation energy – rather the stabilisation probably included wider layers of water. He stated:
having in mind the tendency of water molecules to cooperate with their neighbours, it does not seem improbable that the hydroxyprolyl can serve as an initiator to an extensive network of hydrogen bonds. This envelopes the collagen molecule and might be responsible for the exceptional thermodynamic properties of collagen.
In this way, he regarded the water bound to the triple helix as constituting a matrix structure. Model studies have confirmed that the triple helix is indeed surrounded by such a water structure, nucleated at hydroxyproline, as indicated in Figure 1.7. The crystal structure gives experimental proof of the existence of water bridges, whereby the triple helices are surrounded by a cylinder of hydration and the hydroxyproline residues appear to act as the 'keystones', connecting the water molecules to the polypeptide. It can be seen that the grooves in the triple helix are filled with solvent molecules directly bonded to the anchoring groups on the peptide or connected to those waters in the first hydrogen bonded shell. The third shell of water molecules completes the cylinder of hydration, which is supramolecular solvation. While it was realised that without the hydroxyproline residues there were only localised regions of water structure, Berman later suggested that the hydroxyproline acts as a nucleus for water bridges, extending the network far beyond what might be expected and that is critical for the lateral assembly and the supermolecular structure of collagen. Engel et al. demonstrated that, compared with peptides of the same length, those containing Hypro residues had higher thermal stability than those without Hypro.
Excerpted from Tanning Chemistry by Tony Covington. Copyright © 2009 Tony Covington. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
Chapter 1: Collagen and skin structure; Chapter 2: Skin and its Components; Chapter 3: Curing and preservation; Chapter 4: Soaking; Chapter 5: Unhairing; Chapter 6: Liming; Chapter 7: Deliming; Chapter 8: Bating; Chapter 9: Pickling; Chapter 10: Tanning; Chapter 11: Chrome tanning; Chapter 12: Mineral tanning; Chapter 13: Vegetable tanning; Chapter 14: Other tannages; Chapter 15: Post tanning; Chapter 16: Dyeing; Chapter 17: Fatliquoring; Chapter 18: Drying; Chapter 19: Theory of tanning; Chapter 20: Future technologies