Collagen structure changes during chrome tanning in propylene carbonate
Green solvents, such as propylene carbonate (PC), can be used in leather processing to improve the efficiency of chrome tanning and reduce wastewater. Here we report on a combined small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) study on PC and its efficacy as a carrier medium during chrome tanning. SAXS analysis on the collagen structure of chrome tanned leather using PC, compared to conventionally tanned leather using water, showed an increase in Cr uptake in addition to the more uniform penetration of Cr through the leather cross-section. The increased binding of Cr to the collagen matrix drives the decreased hydration environment of the collagen triple helix. Furthermore, DSC studies show a uniform hydrothermal stability for the PC samples due to the more even distribution of Cr through the collagen matrix. Understanding the mechanisms by which chrome tanning occurs in non-aqueous solvents can guide us towards a more sustainable future for the leather industry.
Journal of Leather Science and Engineering 2019
Particle Size Evolution of Chrome Tanning Agent on Tanning Effect
In a traditional method of chrome tanning, the particle size (PS) of tanning agents is critical for its penetration and the performance of resultant leather. The temperature, pH value and concentration of chrome solution are the important parameters to influence the size scale of the chromium complex. In present study, the PS evolution of a chrome tanning agent (CTA) with pH value and temperature changes was investigated firstly, then the influence of the evolution of PS on the tanning process and hydrothermal stability of crust leather was also investigated. The results indicated that when the temperature varied from 25°C to 30°C and 35°C, the pH of the chrome solution increased from 2.5 to 3.8, the PS of CTA increased from 982 nm to 2899 nm, 1265 nm to 3384 nm and 1289 nm to 3630 nm, respectively, showing a rapid increasing tendency. Correspondingly, when the PS of CTA increased, the chromium absorption rate increased from 31.0% to 53.6% at 25°C, 33.8% to 55.9% at 30°C and 34.5% to 56.4% at 35°C, whilst the uniformity of the chromium distribution in crust leather was gradually reduced. At the same initial tanning temperature, the shrinkage temperature (Ts) of wet blue tended to increase first and then decrease with the increase of the CTA’s PS, and reached above 100°C when the PS was in a range from 2000 nm-2700 nm. As the PS continues to increase, the inner layer was insufficiently tanned due to excessive tanning of surface, leading to a decrease in hydrothermal stability of crust leather.
JALCA August 2019
Microwave Irradiation: An Innovative Routine to Promote Goat Skin Chrome Tanning Process
The chemical reaction with microwave assistance has faster rate, higher yield and milder condition. In order to study the influence of microwave on chrome tanning process, the chrome tanning with microwave heating (MW) was carried out at a different initial pH and terminal pH regraded as experimental samples and with water bath heating (WB) at corresponding condition was the control in the study. The shrinkage temperature, chrome content and chrome distribution of wet blue were determined to obtain the influence of microwave on tanning effect. In addition, the pickled skins were modified by esterification and deamination and then the modified skins were tanned under microwave to know whether microwaving would affect the combination sites between collagen and chromium complex or not. The results showed that microwaving had a positive effect on the chrome tanning process, showed a higher shrinkage temperature and chrome content as well as more uniform chrome distribution for leather. Especially, the shrinkage temperature of MW leather which exceeded 100°C when the tanning terminal ending pH was only 3.3, indicating the tanning with microwave assistance could complete under a lower pH than the usual. Moreover, the combination sites between collagen and chromium of MW leather was the same as WB, indicating the effect of microwave could not change the combination sites. To sum up, the microwave could promote chrome tanning and make it finish at a milder condition, which might innovate the tanning technology and provide a new chrome tanning method in the future.
JALCA August 2019
Organic-inorganic Hybrid Coatings via Sol-gel Route for Leather Finishing
Despite its popularity in other industries, including textiles, for the preparation of inorganic and hybrid coatings, the use of sol-gel for leather finishing is a barely explored field. To exploit its unique and versatile chemistry, we have here evaluated the potential of sol-gel route for preparing protective coatings and achieve, at the same time, water- repellency on treated leather. To this aim, tetra-ethyl-ortho-silicate (TEOS) and functionalized Si-alkoxides, containing either alkyl or fluorinated groups were selected and compared.
The results of characterization indicate that, for the chosen formulations, alcohol-based sols were patently more effective than their water-based counterparts. Very high contact-angles were obtained for both alkyl- and fluorinated alkoxides, but, not surprisingly, fluorinated coatings provided a longer-lasting hydro-repellency. Sols well preserved leather grain, with no patent accumulation on pores or ridges, while only minor changes in colour was observed in most formulations.
Overall, our results suggest that a careful optimization of sol-gel formulation can offer unexplored, versatile and very effective solutions in leather finishing.
JALCA August 2019
Insight into Understanding Incorporation of Glycidoxypropyltrimethoxysilane for Improving Hydrothermal Stability and Porous Structure of Silicic Acid Tanned Leather
Tanning with silicic acid, a kind of silicon-containing material, is considered as a more environmentally benign process; however, the obtained leather would become stiff and brittle during storage in spite of possessing acceptable shrinkage temperature (Ts). Herein, we incorporated organosilicon material attaching silicon atoms with T linkage structure (e.g. glycidoxypropyltrimethoxysilane, GPTMS) in the leather-making process to investigate hydrothermal stability and porous structure, in particular their variation along with different storage time, through Ts determination, DSC analysis, SEM observation and SEM-EDS test. The results confirmed that the introduction of GPTMS stabilized properties of the resultant leather during accelerated ageing process, which may be ascribed to that GPTMS could prevent silicon hydroxyl groups existing in collagen fibre matrix from further condensation. Besides, the tanning mechanism of chrome-free tanning agent based on silicon materials such as silicic acid and GPTMS was investigated preliminarily by means of FT-IR and XPS analysis, indicating hydrogen bonds between silicon hydroxyl groups and amino groups in collagen molecules, and the covalent bonding formed by epoxy group of GPTMS and carboxyl groups of collagen fibres are beneficial to improvement of the properties of silicon-based materials tanned leather. These findings provide an effective and promising strategy to control condensation of silicon hydroxyl groups, and are of great significance to the development of a chrome-free tanning technology based on silicon materials.
JALCA August 2019
Soy protein isolate reinforced yak skin collagen edible films for ultraviolet barring function
In this study, collagen extraction from yakskin was conducted through an acid-pepsin hydrolysis process. The molecular weight (Mw) of yakskin collagen was 51825 Da. An amino acid analysis showed that yakskin collagen is rich in nutrients and contains essential amino acids. The effects of SPI on the mechanical properties (tensile strength (TS) and elongation at break (EAB)), light transmittance, water vapour permeability (WVP) levels, FTIR, X-ray diffraction (XRD) patterns, thermal properties, water contact angles and yakskin collagen-based filmsʼ cross section morphologies were studied. The results showed that excellent ultraviolet barring functions were achieved from edible films with the incorporation of SPI. The edible films produced by SPI present better properties than films composed of only yakskin collagen. Yakskin collagen/SPI composite film could be more widely used in edible food packaging film. As plasticiser, sorbitol can effectively replace glycerol. It opens up a new direction for the effective utilisation of yakskin resources in Qinghai province.
SLTC Journal July/August 2019
A rural African story of hide sourcing
Before the end of Apartheid many hides from indigenous rural farmers would be dried and could be carried on the roof of green Putco (non-white passengers) buses in South Africa. These hides and skins entered the leather supply and were being sold to merchants in country towns. They would be baled and then enter the supply chain. With the decline of the Putco bus service and the rise of smaller Kombis (minibuses), the transporting of these hides became more difficult for the indigenous farmer from outlying places. The Kombis didnʼt want hides on their rooves. I ran a bovine hide-sourcing company in Southern Africa for some years. The company was called Tholanizikhumba Trading cc. which translated from the IsiZulu language means ʻcome let us go fetch hides/leatherʼ (In IsiZulu, it is the same word ʻskhumbaʼ for both leather and hide).
SLTC Journal July/August 2019
Synthesis and application of a novel cationic surfactant based on quaternary ammonium salt for leather fatliquoring
A fatliquor is an indispensable agent for leather, giving softness, fullness and elasticity. A quaternary ammonium salt cationic surfactant (cetyl-dimethyl-(3-methacrylamide) propyl bromide (DMPC16)) was synthesised from N-(3-dimethylaminopropyl) methacrylamide (DMAPMA) and hexadecane bromide by nucleophilic substitution reaction. The structure and properties of quaternary ammonium cationic surfactant were characterised by FTIR, 1 H-NMR, elemental analyser, TGA and surface tension analyser. Further, it was applied in the fatliquoring process of wet-white (Clariant EasyWhiteTan F-90) tanning system to test the physical and mechanical properties, softness, air permeability and absorptivity of the fatliquoring leather. The results showed that quaternary ammonium salt cationic surfactant was successfully prepared, and its surface tension was 31.87mN/m. The application tests showed that the absorption rate of fatliquor reached 97.8%. At the same time, the softness and physical and mechanical properties of the treated leather were improved, including softness increased by 109%, tensile strength increased by 17.2%, tear strength increased by 50.1%, and air permeability increased by 891.21L/(cm2•h). The results showed that quaternary ammonium cationic surfactant had good fatliquoring properties in F-90 tanning system, which can improve the softness, tensile strength and tear strength of leather.
SLTC Journal July/August 2019
Low salt preservation of Australian and New Zealand sheepskins
The current and long established methods for preserving skins in Australia and New Zealand are described. These methods use large amounts of sodium chloride plus combinations of up to three biocides which are not registered in the REACH regulations. This paper describes work carried out to develop a method of low salt preservation (LSP) with auxiliaries used widely in the food and beverage industries and this was achieved.
SLTC Journal July/August 2019
Development of nanocomposites with flame retardant effect for leather and fabric
The main objective of this work is to develop new nanocomposites to apply on the substrates of the leather and textile industry for seats of spaces and public vehicles. Specifically, we want to obtain a flame retardant effect using nanostructured coatings that confer thermal stability and fire resistance. The different nanomaterials should be included in a polymeric matrix; the hybrid organic-inorganic polymers will act as carriers of the nanomaterials, with the ability to interact with the nanoparticles and walls of the nanocapsules. In addition, they must show affinity for the collagen, in order to be introduced in the three-dimensional structure of leather and fixing them on its surface, as well as interacting with the textile substrate. The flame retardant capacity of the two synthesised products has been achieved according to regulations UNE-EN 1021: 06 part 1 and 2 and standard UNE 23723: 1990.
SLTC Journal July/August 2019
Organic fluorosilicone modified polyacrylate emulsion finishing agent
In order to improve the hydrophobicity and applied properties of leather products, 3- (Trimethoxysilyl)propyl methacrylate (KH-570) and Dodecafluoroheptyl methacrylate (G04) were used
as modified monomers preparing a fluorosilicone polyacrylate leather finishing agent by semicontinuous seed emulsion polymerisation. The structure of the prepared latex film was characterised and analysed by FTIR and XPS. The effects of the addition of silicone monomer on the latex film and the application properties were investigated by SEM and TG. The results showed that: both the organic fluorine and the silicone monomers participated in the polymerisation, and the particle size of the emulsion was 50-70nm. XPS showed that the fluorine and silicon elements in the film-air interface were 1.74% and 3.12% higher than the theoretical values. SEM showed that the surface microstructure of the latex film of the fluorosilicone copolymer emulsion was relatively rough, and the roughness was helpful to further improve the hydrophobicity and antifouling property of the finished leather. TG showed that, after the addition of silicone monomer, the heat resistance of the film was obviously improved. Compared with fluorinated polyacrylate emulsion, the contact angle of the leather sample finished by organo fluorosilicone polyacrylate emulsion to water increased about 7°. The applied properties of the finished leather were also improved.
SLTC Journal July/August 2019
Preparation and application of modified corn straw as a biosorbent for Cr(VI) adsorption
Modified corn straw, MCS, was prepared by pretreatment and esterification modification technology. The optimisation and mechanism of Cr(VI) adsorption performance was critically investigated via orthogonal experimental design and single-factor comparison experiments. Experimental results indicated that the MCS dosage and pH value had the greatest effect on the Cr(VI) adsorption process. Moreover, the Cr(VI) adsorption rate (CrAR) and maximum adsorption capacity (CrAC) were 96.8 % and 121mg/g respectively under the optimal condition (MCS dosage: 0.04g, adsorption time: 45 minutes, pH3, adsorption temperature: 40°C, Cr(VI) initial concentration: 10mg/L), which was an increase of 10.3 times compared to unmodified corn straw. FTIR analysis testified that carboxyl and ester groups were introduced into the MCS. Meanwhile, it turned out that the cellulose structure of MCS was found to be more orderly by SEM analysis than that of CS. The MCS might show great potential for application in the adsorption of Cr(VI) in tannery wastewater as a new type of biosorbent.
SLTC Journal July/August 2019
Cleaner chrome tanning: technology of low-chrome tanning without salt, pickling and short procedure
Tannery effluent with high salinity and chromium have a serious environmental impact. The traditional chrome tannage that involved the use of sodium chloride, acid and chromium is one of the main origins of salt and chromium pollution. In this study, a non-pickling, low-chrome tanning technology was developed. The novel Chrome-free agent SL can be directly employed to tan bated cattle pelts and the wet white was obtained. Then the shaved wet white was pre-treated by Poly-carboxylate auxiliary agent and tanned by chrome powder. It was tested that the shrinkage temperature of the wet white, the initial pH of chrome tanning, the consumption of chrome powder, the shrinkage temperature of the chrome-tanned leather, the content of Cr2O3 in effluent, the absorption of chromium and the other properties of the chrome-tanned leather. It was found that the shrinkage temperature of the wet white tanned by SL reached over 80oC, the optimal consumption of Poly-carboxylate auxiliary agent was 2wt% based on the weight of the shaved wet white, the better low-chrome tanning conditions were that the wet white was tanned by 4wt% chromium powder for 150 – 180min at room temperature when the initial pH value was 3.5. The next processes were the same as traditional chrome tannage. Meanwhile, the shrinkage temperature of the leather tanned by the low-chrome tannage reached more than 95oC, the absorption of chromium was 96%, the content of Cr2O3 in the effluent was under 200mg/L. For the low-chrome tanned leather, the absorption of dyestuff, fat-liquor reached 99.5%, 82.5% respectively. Compared with the traditional chrome tanned process, not only was the conventional pickling process eliminated, the process was shortened and reduced the pollution of sodium chloride, but it can reduce 50% of the consumption of chrome powder, improve the absorption of chromium and can reduce content of Cr2O3 in effluent.
XXXV IULTCS Congress Proceedings 25-28 June 2019
Sulfide unhairing: rethinking the received wisdom
The removal of hair from a hide or skin by dissolving it with a mixture of lime and sulfide is a fundamentally understood feature of leather technology. Or is it? For a long time, it has been accepted within the leather literature that, in water, sulfide may be present as either hydrogen sulfide (H2S), hydrosulfide (HS-) or sulfide (S2-), depending on the pH. pH < 6 6 < pH < 12 pH > 12 H2S(aq) (equation is reversable) HS-(aq) (equation is reversable) S2-(aq) The generally accepted mechanism of hair burning is sulfide attack at the cystine disulfide linkages in keratin. Also, it is believed that the unhairing reaction only proceeds at an appreciable rate in the presence of the dianionic S2- species, because that fits with the technological observation that unhairing reactions only proceed at pH greater than 12. However, recent publications have provided substantive proof that the S2- species does not exist in aqueous media at any pH: researchers were unable to observe any evidence of the S2- species in a solution of Na2S dissolved in hyper-concentrated NaOH and CsOH using Raman spectroscopy. The assigned second pKa for removal of the second proton has now been estimated to be 19, making the concentration of S2-(see below) vanishingly small. HS- (equation is reversable) S2- + H+ There is a clear contradiction between the currently accepted mechanism for sulfide unhairing with the evidenced speciation of sulfide species in aqueous environment. Here the implications for this important process are discussed and possible alternative mechanisms postulated that fit with the new knowledge.
XXXV IULTCS Congress Proceedings 25-28 June 2019