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Thermal stability of hide and leather at different moisture contents
... Tanning agents used to preserve skin/hide (mostly as chromium (III) sulphate) [3] interact both with the triple helix of collagen and its supramolecular water layer [3,18] by Collagen aggregates into microfibrils and then into fibrils. These structures are bonded together to create fibers organized in a complex network (tropocollagen) [3,16]. ...
... Tanning agents used to preserve skin/hide (mostly as chromium (III) sulphate) [3] interact both with the triple helix of collagen and its supramolecular water layer [3,18] by different interactions, including covalent, hydrogen, and Wan der Waals bonding, involving both the acid and basic group of the amino acids ( Figure 3) [3,19]. Therefore, the crosslinking between the tanning agents and the triple helix of the collagen is responsible for some of leather's peculiarity and, above all, for its stability [3,20]. ...
Tanneries generate large amounts of solid and liquid wastes, which contain harmful chemical compounds in the environment, such as chromium, that is used in the tanning process. Until now, they have been almost completely dumped in landfills. Thus, finding eco-sustainable and innovative alternatives for the management and disposal of these wastes is becoming a huge challenge for tanneries and researchers around the world. In particular, the scientific and industrial communities have started using wastes to produce new materials exploiting the characteristics of leather, which are strongly connected with the macromolecular structure of its main component, collagen. None of the reviews on leather waste management actually present in the scientific literature report in detail the use of leather to make composite materials and the mechanical properties of the materials obtained, which are of fundamental importance for an effective industrial exploitation of leather scraps. This comprehensive review reports for the first time the state of the art of the strategies related to the recovery and valorization of both hydrolyzed collagen and leather waste for the realization of composite materials, reporting in detail the properties and the industrial applications of the materials obtained. In the conclusion section, the authors provide practical implications for industry in relation to sustainability and identify research gaps that can guide future authors and industries in their work.
... Although the value seems to be lower than the chrome-tanned leather, it is comparable to that of conventional vegetable-tanned leather (35). However, it has been shown that the dry heat shrinkage temperature of the native collagen is 221°C at 0% moisture content (36). Hence, in principle, a skin matrix tanned using a tanning agent, capable of providing adequate shrinkage temperature as well as desired properties, would suffice for its application in dry condition (36,37). ...
... However, it has been shown that the dry heat shrinkage temperature of the native collagen is 221°C at 0% moisture content (36). Hence, in principle, a skin matrix tanned using a tanning agent, capable of providing adequate shrinkage temperature as well as desired properties, would suffice for its application in dry condition (36,37). To substantiate the above argument, differential scanning calorimetric analysis was performed for the control and experimental crust leathers samples. ...
Globally, the leather industry is currently undergoing radical transformation due to pollution and discharge legislations. Thus, the leather industry is pressurized to look for cleaner options for processing the raw hides and skins. Conventional methods of pre-tanning, tanning and post-tanning processes are known to contribute more than 98% of the total pollution load from the leather processing. The conventional method of the tanning process involves the "do-undo" principle. Furthermore, the conventional methods employed in leather processing subject the skin/ hide to a wide variation in pH (2.8-13.0). This results in the emission of huge amounts of pollution loads such as BOD, COD, TDS, TS, sulfates, chlorides and chromium. In the approach illustrated here, the hair and flesh removal as well as fiber opening have been achieved using biocatalysts at pH 8.0, pickle-free natural tanning employing vegetable tannins, and post-tanning using environmentally friendly chemicals. Hence, this process involves dehairing, fiber opening, and pickle-free natural tanning followed by ecofriendly post-tanning. It has been found that the extent of hair removal and opening up of fiber bundles is comparable to that of conventionally processed leathers. This has been substantiated through scanning electron microscopic analysis and softness measurements. Performance of the leathers is shown to be on par with conventionally chrome-tanned leathers through physical and hand evaluation. The process also exhibits zero metal (chromium) discharge and significant reduction in BOD, COD, TDS, and TS loads by 83, 69, 96, and 96%, respectively. Furthermore, the developed process seems to be economically viable.
... Schroepfer investigated the thermal stability of untanned and tanned bovine collagen with different hydration degrees by using differential scanning calorimetry (DSC) and confirmed that crosslinking degrees and drying increased the denaturation temperature of bovine hide collagen [4]. Komanowsky found no significant difference in the thermal denaturation temperature between raw and tanned leather when the moisture content was lower than 30% [5]. Tang investigated the dry heat resistance of hide and leather by using hot stage microscopy and found that the drying process was beneficial to increase the dry heat stability of hide powder [6]. ...
Effect of retanning on the thermal stability of leather is eliciting increasing attention. However, the relationship between the hydrophilicity of retanning agents and the heat resistance of leather and the corresponding mechanism remain unclear. Herein, phenolic formaldehyde syntans (PFSs) were selected as models to explore the effect of the hydrophilicity of retanning agents on the thermal stability of retanned leather. The thermal stability of leather was closely correlated to the hydrophilic group content (sulfonation degree) of PFSs. As the sulfonation degree decreased, the water absorption rate of PFSs and their retanned leathers decreased, whereas the thermal stability of leather increased. Molecular dynamics simulation results proved that the introduction of PFSs could reduce the binding ability of collagen molecules with water and thus decreased the water molecules around the PFS-treated collagen. These results may provide guidance for the tanners to select retanning agents reasonably to improve the thermal stability of leather.
Graphical Abstract
... In the DSC configuration used for this study, the powdered hide is well hydrated, and the heating rate of 1.5°C per min in these experiments is comparable to the heating rate for hide strips. 16 Our results suggest that pilot scale sulfide dehairing may be more damaging to hide structure than pilot scale oxidative dehairing or sulfide dehairing on an industrial scale. ...
The US meat industry currently produces approximately 35 million cattle hides annually as its most valuable coproduct. These hides serve as raw material, first for the leather industry, and then for the gelatin, and biomaterials industries. The conversion of animal hides into leather is a multistep process that has evolved more as art form than as science. Economic or environmental issues typically dictate changes in beam-house processes that prepare the hide for tanning. The tanner evaluates these changes, in terms of impact on tannery costs and quality of leather produced. Thus far, the effects of beam-house processes on the molecular characteristics of collagen have received little attention. The basis for tanning and most biomaterials applications is the stabilization of the collagen matrix, thus any changes to the molecular characteristics of hide collagen may be expected to impact these applications. This study showed that while the effects of different dehairing processes on the structure and stability of monomeric collagen were similar, the effects on the collagen fiber structure were distinct. These results are anticipated to assist the tanner as well as the manufactures of collagen-based biomaterials and gelatin to better understand their substrate and changes to it that may occur when beamhouse processes are altered.
... Thus, the lower the water content, the higher the T d . Komanowsky [5] attributes this phenomenon to the fact that on drying, heat stabilization is enhanced by formation of strong interand intramolecular and ionic bonds between acidic and basic groups. But the conventional shrinkage phenomenon is measured in an aqueous medium, and for this reason, the T d measurements are carried out in excess of water. ...
Differential scanning calorimetry (DSC) allows to study the enthalpy changes which are associated with the denaturation of collagen, of which shrinkage is the macroscopic manifestation. Damage to skin, leather or parchment may be expected to manifest as a decrease of either the temperature of denaturation and/or the enthalpy changes. These modifications are associated with those of other chemical and mechanical characteristics.DSC is a very fine tool to follow in an easy way the reaction of leather or parchment to an ageing or a conservation treatment. While for leather ΔH seems to have less than Td, for parchment, it must be looked at very carefully since an unchanged Td value can be accompanied by a fall of ΔH. The case of archeological waterlogged leather is particular since the state of deterioration is apparently not correlated with the hydrothermal stability.
This paper reviews the efficacy and risks from using thermal control methods for the protection of cultural property. Thermal methods are attractive as they leave no residues, are inexpensive, and applicable to many scales of problem. For many organic materials and assemblies, the risks from thermal techniques are eliminated by the simple act of enclosing objects in a vapour barrier prior to treatment. The magnitude of the risks and possible mitigation procedures for common materials are discussed. A procedure developed for applying solar heat for insect extermination in textiles is given as an illustration.
This present study reports the effect of Aloe barbadensis miller and carrageenan on the physical properties of crusted leather. Tensile strength, tear strength, elongation at break and distensions for control samples at all processes of crusting operations were not significantly different from those of treated samples (p= 0.0972, 0.1324, 0.1565 and 0.040741), respectively. The trend showed that values for treated samples were slightly smaller than those for control samples, although still within the standard recommended range for quality leather. The prospects of using the two eco benign products to improve the organoleptic (sensorial) characteristics of leather are valid. The study recommends the innovative application of the products to leather industry. More studies to determine the ideal volume fraction and particle sizes of carrageenan and Aloe barbadensis miller are needful. Better results can be obtained if appropriate wetting mechanism is adopted to lower contact angle and increase adhesion.
In this paper, the dry heat resistance of hide and leather is studied by means of thermal platform microscope. Such factors as drying, tanning, types of tanning agents and tannage degree that may affect the dry heat resistance of hide and leather are included. Changes and mechanisms of samples when dry heated are analyzed and discussed. We use KCrSO4, CrCl3, Cr(NO3)3 and Sugar reduced chrome liquor as tanning agents to study tanning effects on the dry heat resistance of hide fiber. It can be found that chrome tanning may reduce the initial shrinkage temperature, and increase the final shrinkage temperature. It is to say that chrome tanning may broaden the shrinkage range of hide fiber. Dry heat resistance of hide fibers with different degrees of tanning is also studied. Shrinkage ratio of hide fiber tanned with KCrSO4 decreases with increasing KCrSO4 concentration in the tanning liquors. Hide fiber tanned with sugar-reduced chrone liquor posseses the best dry heat resistance in all the four kinds of tanned hide fiber samples studied.
Genipin, a naturally occurring protein crosslinking agent, isolated from the fruit of Gardenia jasmindides Ellis, is beginning to replace glutaraldehyde as a fixative for biological tissues. Earlier research in this laboratory demonstrated that the apparent shrinkage temperature of hide powder could be increased from 60°C to 79°C by treating the powdered hide with 5% genipin at pH - 7 and 35°C for 24 hr, a significant improvement, but not enough for most leather uses. Typical tannages proposed to reduce the use of chromium are combinations of vegetable tannins or aldehydes with each other or with minerals. In this study, the tanning effect on bovine hide powder of genipin in combination with aluminum or vegetable tannins has been investigated. In terms of thermal stability, vegetable tannins in combinations with genipin appear to offer little advantage over either component alone, suggesting little if any synergy. Likewise, aluminum tanning over a genipin pretannage appears to provide little advantage. However, when hide powder was first tanned with 8% aluminum and then retanned with genipin the thermal stability increased linearly with increasing concentrations of genipin (2% to 10%). The apparent shrinkage temperature was > 100°C for aluminum post-tanned with 8%-10% genipin, suggesting the possibility that a practical combination tannage based on genipin could be designed.
The bark of Acacia mangium tree was smashed and extracted by acetone solution, and the extraction was then treated by petroleum ether. the solution was extracted by diethyl ether and ethyl acetate successively. So, the tanin in the bark of Acacia mangium was divided as diethyl ether fraction, ethyl acetate fraction and water fraction respectively. The GPC was used to measure the molecular weight of these fractions, and the particle sizes were examined by zetasizer ZS instrument. The results showed that the ether fraction consisted of the smallest size molecule with molecular weight of 415Da, possessing weaker tanning ability but the highest penetration rate. The Molecular weight of ethyl acetate fraction was 1788Da, owing a significant tanning ability. For water fraction, the molecular weight was 2808Da, and the biggest particle size was observed in water fraction, which had the highest tanning ability. The order of thermal stability of the collagen treated by those fractions was: water fraction> ethyl acetate fraction> diethyl ether fraction; the order of penetration rate was: diethyl ether fraction > ethyl acetate fraction> water fraction. This result could provide a valuable reference for the Acacia mangium tannin fractions.
Genipin, a derivative of the iridoid glycoside geniposide in the fruit of Gardenia jasmindides Ellis, is a naturally occurring protein crosslinking agent, which is not cytotoxic, and is beginning to replace glutaraldehyde as a fixative for biological tissues. This study was undertaken to evaluate the potential of iridoid compounds, in particular genipin, for use as tanning agents. The tanning effect of genipin on bovine hide powder was investigated. Parameters including the dosage of genipin, reaction time, pH and temperature of the tanning process were varied. The peak temperature (Tp) obtained by differential scanning calorimetry (DSC) of treated hide powder was used to evaluate the tanning effect. The results indicated that hide powder which had been treated with 5% genipin at pH - 7 and 35 °C for 24 h had a Tp of 79 °C. A secondary observation was that the tanned hide powder exhibited a dark blue color characteristic of the reaction of primary amines with genipin. A likely mechanism for genipin tanning is discussed. These results provide a new potential approach to chromium-free tanning for the leather industry.
Thermal analysis of chrome bovine leather was performed by means of thermogravimetry (TGA) and differential scanning calorimetry (DSC). The glass-transition, the crystallisation and the denaturation temperatures of the collagen molecule within the leather matrix were measured. At low moisture content one single endothermic peak which corresponds to the denaturation process of leather and the evaporation of water was detected. At higher moisture contents two endothermic peaks were detected, the first one corresponding to denaturation and the second one to evaporation of water. The temperature of the low-temperature peak decreased as the water content increased. At much higher moisture content the low temperature peak maintained its temperature being equal to the shrinkage temperature of leather determined by the IUP-16 standard method. The glass transition of room-conditioned leather determined by DSC in sealed pans was approx. 45°C.
The interaction between collagen and its water of hydration is of special importance since the mechanical properties and disorders of cartilage, tendons, and ligaments are dependent upon this interaction. The present paper describes the influence of the crosslinking phenomenon on the hydration structure of collagen fibers. NMR and dielectric measurements reveal the influence of different chemical treatments on the transverse relaxation time and polarization of the collagen fibers. The frequency dependence of dielectric constant of collagen fibers displays an induction behaviour on low frequencies. Bound water constrained in collagen fibers seems to provide signatures for changes induced by crosslinking agents on the pore diameter and distribution in collagen fibers. A correlation of transverse relaxation time of water in dry and wet states presented in this study presents an experimental tool for examining the differences in efficacy of crosslinking agents. Changes in the dielectric relaxation, dynamics of water structure and hydroporometric structure of collagen are dependent on the nature of crosslinking material. The changes brought about in the pore structure of crosslinked collagen matrices have been studied using mercury intrusion porosimetry and scanning electron microscopy. Understanding the role of water in stabilization process of collagen has resulted in development of chrome free tanning systems.
The influence of solvents on combination tanned leather was established with observation of thermal stability, and the microstructure of the leather sample before and after shrinkage. Thermal stability of leather tanned with MI (mimosa), GA (glutaraldehyde), OZ (oxazolidine) and Al (aluminum sulphate) were reported using the shrinkage temperature for helix and random coil transition after soaking in 0.5, 1.0 and 2.0 mol/L urea solution, (pH4.0, 6.0 and 10.0) water, colza oil, glycerin and engine oil. The shrinkage temperature(Ts) of all leather samples was decreased by soaking in these solvents. The concentration of urea solution had a very obvious effect on GA-MI and MI-OZ-tanned leather, whose Ts reduced to 84.2° C and 84.6° C from 94.8° C and 100.9° C respectively. The influence of acidity and basicity of the solution also cannot be ignored, and OZ-MI leather was the most susceptible one whose Ts reduced to 85.3° C from 97.7° C The thermal stability of leather involving MI and Al can be affected remarkably by glycerin. All of the leather samples showed a more obvious Ts decrease when it was soaked from the dry state than in moist state. Scanning electron microscope (SEM) studies revealed that the microstructure of collagen is destroyed when it shrinks and that the degree of destruction depends on the cross-linking agents.
The comminuted bark of Acacia mangium was extracted with acetone solution, and the extract was then degreased with petroleum ether. The degreased solution was extracted by diethyl ether and ethyl acetate successively. In this way, the tannin of Acacia mangium was divided into the diethyl ether fraction, the ethyl acetate fraction, and the water fraction, respectively. The tannin molecular weight of these fractions was measured by gel permeation chromatography (GPC) and the particle sizes of tannin were examined by a Zetasizer ZS instrument. The results showed that the ether fraction consisted of the smallest molecules with an average molecular weight of 415Da possessing weak tanning ability but having a fast penetration rate. The molecular weight of the ethyl acetate fraction was 1788Da which showed significant tanning ability. The molecular weight of the water fraction was 2808Da with better tanning ability, and the biggest particle size was shown by this tannin. The thermal stability of the hide powder and cowhide tanned by these tannins was as follows: water fraction > ethyl acetate fraction > diethyl ether fraction; the order of penetration was: diethyl ether fraction > ethyl acetate fraction > water fraction. These results could provide a valuable reference for the use of the Acacia mangium tannin.
Chrome tanned cattlehide collagen fibers were retanned and fatliquored by different chemicals. The samples were immersed in artificial sweat for 3 to 15 days. The sweat-soaked collagen fibers were then heated in air. The shrinkage temperature (Ts) and shrinkage ratio were recorded to characterize the thermal stability of the collagen fiber samples. It was found that the shrinkage ratio decreased after the processes of retanning and fatliquoring. After sweat-soaking treatment, the shrinkage temperature of the retanned and fatliquored collagen fibers was decreased and shrinkage ratio was increased, indicating an impaired thermal stability. The Ts decreased with the increase of the sweat soaking time. The chrome retanning liquor and gluteraldehyde (GTA) retanned collagen fibers exhibit slow decrease of shrinkage temperature with the increase of sweat-soaking time, showing great resistance to sweat. Compared with that fatliquored by other fatliquoring agents studied, collagen fibers fatliquored by neat's-foot oil has better sweat-resistant property.
Several important attributes of microwave drying under vacuum render it feasible and attractive. Unlike during other forms of drying, during microwave drying the moisture content is the same throughout the hide, especially if the hides are uniformly exposed to microwave energy by tumbling inside mixers. This permits drying of unhaired, delimed and wrung hides to a uniform moisture content at which they can be accurately split without delay. By carrying out the drying operation under vacuum (about 36 torr absolute pressure), denaturation of collagen is prevented. Furthermore, unlike warm air drying, microwave drying is quick enough to limit microbial contamination of the product. Costing $14.60 per hide, the drying process is expensive; yet it is economical because the flesh split is clean and can be sold as edible-grade collagen. If splitting is conducted at a moisture content below 38% on a moisture free basis, growth of bacteria is insignificant. At 21% moisture content on a moisture free basis all microbial growth ceases. By marketing the products at the latter moisture content wrapped in plastic to prevent rehydration, their storage life is expected to be very long.
The pickled goat skins were crosslinked by chromium, aluminum, wattle extract and chestnut extract, respectively. The samples were thermal treated in a container held at different temperature for 5 h for different cycles. The xerothermic stability, water vapor permeability and dimensional stability of the samples were characterized before and after thermal treatment. The effect of thermocycling on the properties of tanned samples was discussed. It was found that the thermocycling at moderate temperature (50 °C) exerted little influence on the xerothermic shrinkage temperature (Txs) of the tanned collagen fibers. The treatment at higher upper limit temperature (100 °C and 150 °C) decreases the Txs and results in an impaired thermal stability. The water vapor transmission rate was decreased after the thermal treatment, likely due to the reduction of the amount of hydrophilic groups of collagen macromolecules as well as the decrease of the pore ratio of leather. Treatment at high temperature damaged the dimensional stability of the specimens.
In case of leather objects, degradation usually occurs by a combination of factors such as temperature and relative humidity, light and insect and fungi. Because chemical composition differs on the types of leather materials, leather objects affect differently even in the same environment. According to UV degradation, the overall color and gloss difference appeared severe in turn of the cowskin, sheepskin and pigskin specimens. In addition, despite short-term period of RH degradation, leather materials showed stable result on high RH circumstances. Nevertheless, if the leather sustained for a long time on the high RH, the environment can be the cause of mold or microorganisms. This study is to understand the leather objects and the future conservation and then to establish the conservational management of leather object for the future.
To obtain acceptable dry heat resistance of leather by using proper retaining agents, the effect of typical retaining agents on dry heat resistance was investigated. The results indicated that the leathers retanned by different retanning agents have varying dry heat resistance, and the dry heat resistance is not always positively correlated with hydrothermal stability. Heating leather at 150 degrees C for 30 mm in the dry state led to minor damage to the crosslinking of collagen fibers, such as destruction of hydrogen bonds or other weak crosslinks. The increase of water content in leather exhibits negative effect on the dry heat resistance. Therefore, it was observed that the retanning agent that has lower hydrophility lead to higher dry heat resistance of leather. In addition, it seems that the retaining agents with good filling property and thermal stability are effective in achieving high dry heat resistance of leather.
Synthetic Na-zeolites (NaA, NaX, NaAX) were investigated as tanning agents in leather production from sheepskin and calfskin pelts. It was found in laboratory scale testing that the combined use of zeolite and chrome sulphate results in both higher float exhaustion and higher shrinkage temperatures in shorter time than in conventional chrome tannage. The best results were obtained with zeolite NaA. The operating conditions were optimized with respect to chrome and zeolite concentration, tanning bath pH and pelt weight/bath volume ratio. The analysed Si/Al ratio in the leather is about 1.1, similar to that of NaA zeolite.The promoting effect of zeolite A in chrome tanning has been confirmed on pilot scale: higher tanning rate as well as higher float exhaustion were found. Mechanical properties of the finished leathers, such as tensile strength, elongation, tearing strength and ball bursting are similar or better when compared with chrome usually tanned leather.
Leather, a collagen structure material, usually requires the addition of fatliquoring agents that have been recently found to cause instability during heating in critical manufacturing processes. The effect of fatliquoring agent concentration and drying temperature on the leather properties was investigated. Leather shrinkage, leather absorption of water and water vapour, and water vapour permeability were measured under defined conditions. The samples of bovine leather obtained after commercial liming and chrome tanning processes were used. The samples were fatliquored with 3, 6, 9 and 12% solutions of anionic synthetic fatliquoring agent and dried at different temperatures. The results of the investigation are corroborated by scanning electron microscope (SEM) study. It was shown how the concentration of fatliquoring agent and drying temperatures influence the leather properties.
Chemical modifications of collagen, of the type known in the leather tanning industry, can raise the denaturation temperature, from 60°C in its natural state, up to 130°C. There are only a few chemical reactions known to be capable of achieving the highest values and these have long been assumed to be unrelated. Here, we show for the first time that all the stabilising mechanisms are fundamentally the same, regardless of chemical type. Any single component of a stabilising reaction has the effect of linking part of the collagen structure into the surrounding matrix of water: the outcome is always to confer moderate hydrothermal stability, up to 85°C. The effect is merely to hinder the shrinking/denaturation transition, so no single component reaction can exceed this moderate result. However, in addition, a second reaction component can be applied in the process, which may have the ability to lock the linked structure together, creating a macromolecular structure around the triple helices. The effect of the concerted interaction with collagen is to prevent more effectively the unravelling of the triple helices and thereby to raise the hydrothermal stability to much higher values of denaturation temperature. This new proposed ‘link- lock’ mechanism opens up the possibility of achieving high collagen stability in new ways, which will contribute to the development of new collagenic biomaterials.
The conversion of animal hides into leather, the most valuable coproduct of the US meat industry, is a multistep process that has evolved more as art form than as science. A variety of dehairing and other hide preparation processes have been adopted without an understanding of how they affect the chemical and physical properties of the resulting leather. To develop a basis for designing or selecting effective sustainable tanning agents, a suitable model system must include an evaluation of the effects of pretanning steps. A protocol for the production of powdered hide is described. Moisture, ash, total protein as collagen, hydrothermal stability, collagenase resistance, proteoglycan content and molecular weight distribution were compared for powdered hide prepared from bated and not-bated hide. This research provides a basis for assessing the effects of different process steps, and represents an initial step in the development of well-characterized model systems for comparing research from different laboratories on tanning mechanisms.
We report the dynamic mechanical behaviour of leather within the temperature range - 100 to 300°C, where three major viscoelastic transitions were identified, termed α, β and γ. The β transition peak represents the glass transition temperature of the amorphous region of collagen molecules. It also demonstrates 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: the tanning molecules interpose themselves between the collagen chains, thus reducing the forces holding the chains together. Different tanning agents show differing degrees of plasticisation.
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