No description available.

The deformation behaviour of chrome tanned crust leather (with or without fatliquor) and finished leather has been measured in three-point bend, tensile and compression tests. Relations between the experimental results and the perceived softness of the leather were investigated. The instrumentally assessed softness of a set of leathers was represented by parameters obtained from these tests and compared with a quantified subjective assessment. Differential Scanning Calorimetry (DSC) was used to analyse the water status in crust leathers. The influence of water content prior to staking on its softening effect was investigated. An orthogonal experiment was applied to investigate the influence of various factors (e.g. amplitude and frequency) on the softening effect of mechanical action to obtain an optimum combination of these factors. The bending force, F, increased in a linear way with increasing sample width, w, in accordance with the standard theory of beam bending. However the change in F with increasing sample thickness, t, departed (markedly in some cases) from the standard theory. The theory of laminated materials offered a quantitative description of the dependency provided that the grain and corium layers were regarded as the laminae. F was approximately proportional to the actual bending length, 1, raised to a power between -2 and -3. An empirical equation based on the results of the bend testing was developed. A parameter, Se/t’, can be used to rank the softness of leather, where Se is equivalent to the bending stiffness in standard bending theory, and ii is the approximate index in the power function describing the relationship of the bending force F to the sample thickness t (F t’1) for a particular type of leather. A model describing the compression behaviour of crust leather is proposed which involves four stages of compression: (i) hair-like fibres being bent flat onto the surface and buckling of vertical fibre bundles, (ii) closing up of voids between fibre bundles, (iii) closing up of voids between fibres, and (iv) compressing of solid fibres. DSC results indicate that water has a stronger interaction with collagen molecules in an unfatliquored leather than in a fatliquored one. Fatliquor appears to function as a shield between water and collagen. The influence of water content prior to staking on the resulting softening effect was clearly reflected in bending and tensile tests, but was not nearly so evident in a compression test. It is shown that a maximum softening effect can be obtained in the staking process when moisture content is about 30 - 40%

This thesis mainly reports the dynamic mechanical behaviour of leather within the temperature region of -100°C and 300°C, where three major viscoelastic transition were identified, termed alpha, beta and gamma. The beta transition peak represents the glass transition temperature of the amorphous region of collagen molecules. It was also shown that tanning agents act as plasticisers and depress the glass transition temperature to a lower temperature. Thus the tanning process itself may be viewed as a plasticisation of the collagen molecule. In this event, tanning molecules interpose themselves between the collagen chains, thus reducing the forces holding the chain together. Different tanning agents show differing degrees of plasticisation. The effect of water on the viscoelastic transitions of leather was also investigated. It was shown that leather remains in a transitional viscoelastic region between -50 and 70°C regardless of the moisture content of a sample. This imparts unique properties to leather. Initially, the absorbed water molecules act as a plasticiser and depress viscoelastic transitions to a lower temperature region. Depending on the leather type, above a certain percentage of absorbed water splitting of the glass transition peak is observed. This may be due to a preferential hydration of certain hydrophilic amino acid residues leading to separation of the transitions due to hydrophobic and hydrophobic amino acid residues. It was demonstrated that the rate of stress relaxation is temperature dependent and the stress relaxation property of leather above and below the glass transition differs greatly. Two critical temperatures related to heat setting were identified, which may be termed the critical and the optimum temperature. The critical temperature is the temperature above which the set increases markedly and has been positively identified as the glass transition temperature. Finally, changes in the dynamic modulus during the drying of leather revealed information concerning the nature of the moisture-leather relationship at the critical stages accompanying drying. It was concluded that leather undergoes three different phases during drying where only the final phase is related to the final stiffness of the leather.

One of the major problems facing the meat and leather industries is dung cladding on hides. Dung dried onto hides is difficult to remove during the early stages of processing, resulting in the necessity for prolonging the soaking step, with the consequence of risking bacterial damage to the hides. Microscopic examination of dung on hide reveals that adhesion is between the dung and the hair alone, with no sticking of the dung to the epidermis. This accounts for the difficulty in removing dry dung; the matrix of the hair within the dung creates a strong composite material. Analysis shows that major components of the composition of dung are lignocellulosic materials (lignin, hemicellulose and cellulose). This project concerns solubilising the dung faster, to reduce soaking time by targeting these components with enzyme technology, using hemicellulases, cellulases, and laccases from commercial products and a range of fungal organisms (Coriolus versicolor and Aspergillus niger). Removal of dung from hide pieces is affected by the action of cellulase, xylanase and laccase separately over 20-24 hours. Mixture of these enzymes removes dung in 6-8 hours demonstrating that synergistic activity is more effective than separate enzyme treatments. When commercial enzyme mixture or Coriolus versicolor or Aspergillus niger cultural broths were used to effect dung removal in tannery drums in a larger scale experiment, in some cases the removal of dung was observed within 1 hour by the help of the mechanical action and nonionic surfactant. The enzyme treatment did not affect the components of hide, such as collagen, hyaluronic acid and dermatan sulphate and it was found that enzyme treated leathers and control samples showed no differences in terms of their physical characteristics. Investigations in the mechanism of dung removal by spectrophotometry, chromatography and chemical techniques show that the enzyme treatments remove the dung from hides without changing the fundamental structure of the substrate. The enzymes probably attack the structural parts in lignocellulose, which are most susceptible to solubilising with enzymes: these attacks make small changes in the carbohydrate and lignin contents, but these changes are powerful enough for effective dung removal

To elucidate the reaction between collagen, condensed tannins and oxazolidine, two prodeiphinidin tannins have been studied: extracts from pecan nutshell pith and myrica esculenta bark. The tannin chemistry study showed pecan tannins are copolymers of catechin and gallocatechin with a ratio 1:6 and myrica tannins are prodeiphinidins with 40% of the structural units gallated. The average molecular weight was measured by GPC and the number average molecular weight of tannins was found to be 25 00-3000. The molecular structure of oxazolidine, 1 -aza-5-ethyl- 1,3-dioxacyc1o[3 .3.0] octane, was confirmed by IR, GC-MS and ‘H-NMR spectra. From NMR, the ratio between cis and trans conformations is nearly 1:1. Complete hydrolysis of oxazolidine is slow under weakly acidic or neutral conditions. The reaction of polyphenols with oxazolidine was modelled using simple phenols phioroglucinol, pyrogallol and resorcinol; the process was followed by chromatographic and NMR methods. The results show the reaction is second order: phloroglucinol has the highest reactivity. By using catechin and gallocatechin, it was shown that the cross-linking reaction of gallocatechins can happen between an A- ring and a B-ring, while for catechin, the reaction is only at the A-ring. Polyphenol polymer studies also support this result; more oxazolidine can be reacted with prodeiphinidins. The reaction of collagen with oxazolidine was studied; the reaction sites were determined using modified collagen. Few cross-linking reactions were found, which is supported by hydrothermal isometric tension (HIT) results. The reaction between polyphenol, collagen and oxazolidine was studied by HIT and it was found that the cross-linking is between collagen and tannins, which is responsible for the high stability of treated collagen. Based on this, organic combination tanning can be explained as an effect of covalent bonding, hydrogen bonding and hydrophobic bonding together, but the high shrinkage temperature is mainly due to the covalent bonding forming a complex matrix structure around the collagen fibres

A review across the periodic table of tanning effects of simple inorganic compounds reveals that many elements are capable of being used to make leather. But, if the practical criteria of effectiveness, availability, toxicity and cost are applied, the useflul options reduce to chromium (III) the benchmark orto titanium (IV) zucomum (V) iron (II/III) and aluminium (II) For mainly environmental reasons, alternative tanning agents to chromium are needed, However. none so far investigated match the all round properties achieved with chromium (III) salts. In tanning terms, the chemistry of titanium (IV) is dominated by the titanyl ion, TiO2, limiting its reactivity with collagen. An alternative approach to titanium tanning, using metastable titanium (III) salts as possible tanning agents, was investigated in this study. A method to determine the titanium content in leathers was developed. Complexes of Ti (III) were investigated using various spectroscopic techniques, including uv/vis spectroscopy, electron spin resonance, electron nuclear double resonance and nuclear magnetic resonance dispersion spectroscopies. The complexes used in subsequent tanning studies were based on titanium (III) citrate, gluconate and tartrate. However, in comparative trials with chromium (III) salts, the study highlighted that titanium (III) salts were not suitable for tanning collagen. Titanium (III) saks were found to be useflul in the production of semi- metal tamlages. The research was extended to investigate the hydrothermal shrinking of tanned collagen. Techniques such as extended x-ray absorption — fine structure and x-ray absorption near structure were used to investigate the interaction of chromium (III) tanning salts and titanium (III) complexes with collagen. The research demonstrated that the shrinking reaction is independent of the tannage. It involves instead the breaking of hydrogen bonds, rather than the breakdown of the tannin-collagen molecule. From this work, and a reappraisal of older work on the shrinking reaction, a new theory of tanning has been formulated based on co-operating units and the role of crystallinity in stabilising the collagen structure. The influence of the solvent is also discussed

Necin bark (1zadirach1a indica) contains a mixture of complex chemical components identified as: One procyanidin trirner Four dimers Cl Epicatechin-[4a-8j-epicatechin epicatechin. B1 Epicatechin- [4[3-8]-catechin, B2 Epicatechin- [43-8]-epicatechin, B3 Catechin-[4ct-81-catechin, B4 Catechin-[4a-81-epicatechin. Four well known monomers (+)-Catechin, (-)-Epicatechin, (+)-Gallocatechin, (-)-Epigallocatechin. Other compounds Quercetin, Kaempferol, Fisetin and Gallic acid These compounds were characterised by chromatographic and spectroscopic techniques, including by thiolytic degradation with toluene-a-thiol and using acetylated and methyl derivatives. Structural elucidation of the cleavage products was by ‘H-NMR. The backbone of the isolated compounds consisted mainly of a mixture of flavan-3-ol units with 2R- configuration. The purity of monomers, dimers and trimer was checked by HPLC on Lichrosorb RP8 column. The molecular weight distribution ranges between 250 and 5500 and the average molecular weight distribution was found to be 1550 by gel permeation chromatography. Gas chromatography-mass spectroscopy demonstrated the complex nature of NBT molecular masses consist of monomeric, dimeric and trimeric units of procyanidin B and C types respectively. IR and FT-IR spectroscopy results were consistent with the rest of the techniques used. The hydrothermal stability of neem bark tannin tanned and crosslinked hide powder and leathers was found to be paralleled to mimosa tannin extract. Studies of the physical properties of tanned and retanned leathers showed corn pai.ahility of micem bark tannins with mimosa tannin extract

Kraft lignin degradation by a biomimetic system was investigated, using haemin and hydrogen peroxide, which mimics the catalytic mechanism of lignin peroxidase to produce phenolic compounds. The degradation products were identified using spectroscopic methods and gas chromatography-mass spectrometry. 2-methoxyphenol, 4-hydroxybenzaldehyde, vanillin and vanillic acid were produced and their formaldehyde polymerisation products were used for tanning collagen. The denaturation (shrinkage) temperature of collagen was raised to 80°C through hydrogen bonding interactions between the polymers and the protein. For dyeing of hide powder, the lignin degradation products were reacted with laccase (a polyphenol oxidase); 2-methoxyphenol gave the darkest colour. These products have potential to be used as raw materials for tanning and dyeing of animal skins. Therefore, this can add value to the industrial byproduct (Kraft lignin) and reduce its environmental impact. Polyphenols are the basic building blocks of natural skin and hair pigments: they can be polymerised to create versions of natural black, brown and orange melanins. These reactions can be catalysed by laccase; not only do they create colour, but also if they are conducted in the presence of collagen, the resulting pigment is bound to the leather in a covalent tanning manner. Therefore, the leather becomes coloured by a lightfast pigment, which is firmly fixed. It appears to be a feature of this type of stabilising mechanism for collagen, the creation of a polyphenol matrix around the triple helix, that it has the added benefit of actually strengthening the leather. Since every traditional process step effectively weakens collagen, especially tanning, this is the first chemical reaction that has been shown to reverse that effect. Leather tanning reactions between collagen and dihydroxynaphthalenes (DHNs) and oxazolidine have been investigated, using hide powder and sheepskin pickled pelt. This investigation showed that some DHNs have a tanning effect on collagen. The measurement of combined and cross-linked DHNs on collagen showed that 30-40% of 1,6- and 2,6-DHNs were fixed through covalent bonding. Shrinkage temperature of the leather changed little after the non combined DHNs had been removed from the leather, indicating that the high stability of the combination tanned leather comes from the covalent bonding formed between DHNs and collagen through oxazolidine.

The aim of this research was to understand better the rheological behaviour of part processed leather in order to improve area yield. Rheological behaviour was characterised in uniaxial tests by measuring the stress-strain relationship, the immediate and recovered set and stress relaxation. The influence of moisture content, time under strain and applied strain were determined. The effect of drying under strain was also explored. Results were interpreted using rheological and structural models. It was found that for wet leather the relationship between applied strain and long term set is nonlinear. Moreover, below a critical strain no set is imparted. At this critical strain there is a change from a bending to a tensile mode of fibre deformation. Long term set only occurs in wet leather when collagen fibres are subject to tension. In wet leather, recovery occurs because collagen fibres return to their initial bent configuration. The long term set of partially processed leather at low strains increases with decreasing moisture content because collagen fibres permanently deform due to the rupture of intra-fibre adhesions. Inter-fibre adhesions formed on drying result in a yield point on the stress-strain curve. At the yield point the adhesions rupture and the mode of fibre deformation changes from stretching between adhesions to bending. A model, which views the structure of leather as adhesion nodes, joined by segments of fibres, has been used to predict increases in the initial modulus caused by drying under strain, in terms of the recruitment of straight fibre segments between nodes. When material is held under strain, there is a proportionate decrease in stress and increase in the immediate set, which is shown to be consistent with the Maxwell model. The two stage recovery of set is fitted to a multiple Voigt model. A novel way of achieving area yield without loss of tensile strength or softness is suggested in which the leather’s structure is relaxed whilst remaining wet and is then fixed by drying rapidly under strain

Chrome tanned bovine leather comprises two principal strata, the grain and the corium. The tensile strength and specific work of fracture of these two materials was investigated using uniaxial tensile tests and trouser tear tests respectively. Corium material was observed to be intrinsically stronger and tougher than grain material. The greater resistance of corium material to fracture is attributed to the processes of fibre debonding and pull out. The viscoelastic nature of grain and corium material was investigated by examining differences in strength and toughness over a range of deformation rates. The ultimate tensile properties and the specific work of fracture of both grain and corium material are rate dependent. Energy dissipation during a strain cycle was measured over a range of strain rates and strain levels to determine the bulk hysteresis of grain and corium specimens. The influences of specimen orientation and fatliquor (oil) on strength and toughness are also considered. The notch sensitivity of grain and corium materials has been scrutinised. Corium material is highly notch insensitive, whereas the fracture of grain material is notably sensitive to the presence of notches. The mechanism of fibre orientation and the phenomenon of fibre independence (or fibre autonomy) are responsible for the notch insensitive fracture behaviour of corium material. Strain distribution was measured in grain and corium single edge notch specimens. The radius of curvature of the notch was assessed throughout deformation / fracture and local strains at two dimensional levels were measured. Local strains ahead of the advancing crack and the radius of curvature of the crack are considerably higher with corium specimens than with grain specimens