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

This study investigated the modification of leather rupture properties following impregnation with polyurethane. It was hypothesised that a better polymer - leather interaction would increase strength. Surface properties of leather fibres were altered (using a hydrophobic agent based on a phosphate ester) to more closely match those of the polymer. This resulted in a more even distribution of polymer on the leather fibrils. Application of the hydrophobic agent alone was investigated and optimum offers of the treatments determined. An untreated sample and one impregnated without surface treatment were included as controls. Trouser tear testing was used to assess rupture resistance as well as a stratigraphic tear test which was used to evaluate any changes in peeling properties. An approximately linear relationship between peeling strength and sample split depth was observed which indicated that the grain - corium junction is a gradually changing interface. The surface modification of fibres led to increases in tear strength. Applying polymer to leather with surface modified fibres resulted in further increases in tear strength. Investigation of single fibres determined that no treatments influenced the fibre strength. There was, however, a decrease in the interfacial bond strength between fibre and polymer when surface modification was used. This was related to an increased fibre pull out length for treated samples. Leathers of 11% hydrophobing agent and 10% polyurethane gave the best combination of tear strength and softness. It was concluded that polymer impregnation of hydrophobic leather resulted in a bonded fibre structure. When stress is applied to such leather, fibres debond from the network and disentangle from the fibre structure. Work must be done to overcome surface friction between fibre and polymer. However, there is a balance between (a) interfacial bond strength and friction, and (b) fibre strength. This balance can be manipulated to obtain optimum overall strengthening

The effects of ultrasound (38 kHz, 1.3 W cm2) on the dyeing, fatliquoring and tanning of leather have been investigated and the mechanisms whereby ultrasound influences these processes were elucidated. Compared with a conventional process, ultrasonic dyeing can either shorten the dyeing time by 40-70% or facilitate low temperature dyeing. This remarkable enhancing effect has been attributed mainly to an increased diffusion coefficient (D) of dyestuff in the presence of ultrasound. It was found that sonication is more effective in the initial phases than in the late phases of the dyeing process. Application of ultrasound during the fatliquoring process or simply in the preparation of fatliquors resulted in an increase of leather fat contents (up to 40%), especially in the inner corium layer, indicating an improved penetration. This can be partly attributed to a reduction of particle size by 20-30%. In contrast to dyeing, ultrasound was found to be more effective later rather than earlier in the fatliquoring process. Chromium and aldehyde tanning processes were accelerated only marginally (1 0%) but the mimosa tanning process was speeded up significantly (by up to 100%) by using ultrasound. Leathers tanned in the presence of ultrasound had shrinkage temperatures 3-5°C higher than conventionally processed controls. A more even chromium distribution and less chromium leaching were obtained after using ultrasound. The results showed that ultrasound can increase the dispersion rate and the available tannin content (by 7%) of mimosa, as well as reducing its particle size by 50%. It was also found that ultrasonic treatment can prevent mould from growing on mimosa tanned leathers. It is concluded that ultrasound is more effective in a process which involves a colloidal rather than a true solution system. The prevailing effects of ultrasound on the former processes are to increase the diffusion coefficient and reduce the aggregation. This is due to cavitation

Fundamental elements of chrome-collagen interactions and the crosslinking mechanism at the molecular level have been investigated to gain detailed understanding that can be translated into improvements in the commercial process. Amide group hydrolysis during conventional liming proceeds by two mechanisms: direct hydrolysis with hydroxyl catalysis or intramolecular catalysed hydrolysis that can be measured by the degree of racemisation at L- asparagine. Gas chromatography mass spectrometry analysis of limed collagen demonstrated that there was no initial rise in the amount of D-aspartic acid during the first 24 hours of liming, indicating that, within the conventional period of liming, collagen remained intact. Circular dichroism has demonstrated that beamhouse processing has dispersive and relaxing effects on the collagen triple helices, but without disruption. Attenuated total reflection Fourier transform infrared spectroscopy analysis provided information regarding changes in the secondary structure of collagen during tanning. Complete breakdown of chrome tanned collagen was achieved by the use of a cocktail of proteolytic enzymes: this allowed an analysis of the isolated crosslinks, which indicated preferred reaction at aspartate sidechain carboxyls. This is supported by chrome tanning studies following treatment of collagen by amidase enzymes. Geometric details of chromium(lll) species involved in the tanning reaction were elucidated by extended X-ray absorption fine structure analysis. Linear tetrameric chromium species were found to be predominant in the tanning process. Direct measurements of the mechanism were made using electron spin resonance spectroscopy. A two-stage reaction mechanism is postulated: physical interaction and chemical complexation, affected by reaction conditions such as pH, temperature and the presence of neutral salts. Nuclear magnetic relaxation dispersion measurements confirmed changes in the speciation of chromium(lll) under tanning conditions.

The optimum cutting of various parts of a shoe, prior to shoe manufacture requires knowledge of the topographical variation of what are termed “lines of tightness”. Currently the cutting operation for shoe parts is guided by a general assumption about the pattern of the lines of tightness. There is a need to have available a system which can determine, in a non-destructive way, the lines of tightness in an indvidual piece of leather. Initially an image analysis system was developed to investigate the uni-axial deformation behaviour of leather. This technique provided more information about the stress-strain behaviour of a leather sample along the gauge length than a conventional mechanical test and it was possible to accurately measure the strain distribution along the gauge length. A system was developed which could determine the relative displacement of marked spots along the gauge length of the sample using images captured during a uni-axial, bi-axial or multi-axial tensile test. The separation of the marked spots along the direction of applied stroke allowed the determination of longitudinal strain while contraction across the width was also measured in some cases, which was useful in calculating the Poisson’s ratio of leather for which a great variation was observed between different locations (Butt, Belly, Neck etc). Various approaches were investigated to determine the lines of tightness. Firstly, the local Poisson’s ratio was observed since a higher value of this parameter was associated with these lines of tightness. Secondly, biaxial stretching of leather by a series of actuators for each axis indicated the lines of tightness along the actuator with lower strain values. Thirdly, the strain was measured when the leather was stretched along number of known axes. This latter technique appeared to be the best approach and mathematical modeling was investigated to provide further refinement. A mechatronics-based device by industrial application of the third approach was also proposed. The software was written using a graphical programming system (LabVJ EW)

A novel powder coating binder system based on UV-curable, semi-crystalline polyurethane acrylates has been developed for heat-sensitive and highly flexible substrates, such as leather. The developed powder coating film had good abrasion and adhesion properties. The concepts applied in this work were based on the adjustment and control of the degree of crystallinity of hinders by three ways: (i) formulating different molecular weight monomers for random copolymerization, (ii) introducing functional monomer with pendant groups and (iii) cross-linking during curing. Amongst the three approaches explored in this work, a combination of branching and cross-linking was found most successful. A crystalline binder with cross-linkable pendant groups was employed as the powder coating main binder; this is the major difference from other work on powder coatings where amorphous resin alone or partially blended with a crystalline component are used as binders. The preparation of UV curable powder coating binder resins and their properties, such as crystallinity, viscosity and tensile modulus and elongation to break have been studied by DSC, SEM, FTIR-ATR and DMTA methods. It was found that the polyurethane acrylate binders (made from DHBA, BEP, DEG and HDI etc.) are potentially useful for formulating novel powder coatings. The properties of the developed powder coating finished leather have been tested. Hot plating processes and an EMB application method for leather finishing have been discussed as well. Nano-scale silica particles surface modified with acrylate has been studied, and their properties or properties of powder coatings formulated with the particles have been characterized. The powder coating rub fastness was improved by formulation with surface modified nano-scale silica particles

Thesis submitted in fulfilment of the requirements for the degree: Doctor of Technology: DESIGN In the Faculty of Informatics and Design At the Cape Peninsula University of Technology / Currently, the South African leather tanning industry is dominated by chromium tanning which has been identified as highly polluting. Vegetable tanning is considered less environmentally hazardous. Yet, there have been no plans to promote it in South Africa. Vegetable tanning process utilises vegetable matter for converting animal skins or hides into pliable material that is known as leather. South Africa has a variety of plants that produce tannins. However, the focus of this study is on the mimosa locally known as black wattle (Arcacia miernsii). Furthermore, the South African vegetable tanning sector has been left behind in terms of research and development despite the country being endowed with the Mimosa plant that is widely used in the vegetable tanning process. The country is also endowed with surplus labour which can be absorbed by the leather industry which is labour intensive.

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.