Thermal degradation of collagen-based materials that are supports of cultural and historical objects

The thermal analysis methods (TG, DTG and DTA) were used for the investigation of the thermal degradation of some recent manufactured tanned leathers, leathers that are supports of cultural or historical objects (leather from book covers (XVII-XIX centuries); leather from an Austrian belt (Franz Joseph period), Cordoba leather (XVII century), lining leathers), recent and patrimonial parchments and recent extracted collagen (sorts of collagen obtained from bovine leather at different pH-values, bovine collagen with different hydration degree). At progressive heating, all investigated materials exhibit two main successive processes, associated with the dehydration and thermo-oxidative degradation. Each analyzed material has a characteristic thermal analysis curve (TG, DTG and DTA) that can be considered a material 'fingerprint'. This result suggests the use of the thermal analysis methods to identify of leathers from which the patrimonial objects are manufactured. The rate of thermo-oxidation of recent manufactured tanned leathers is substantially higher than the rate of the same process corresponding to naturally aged leathers that exhibit values of the thermo-oxidation rate appropriate to those obtained for parchments and collagens. The rate of thermo-oxidation of leather can thus be used as a criterion to distinguish between recent manufactured leather and patrimonial one. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of collagen-based materials that are supports of cultural and historical objects

The TG, DTG, DTA methods were used for investigation of the thermal degradation in static air atmosphere of some collagen-based materials (some sorts of collagen, recent manufactured parchments and tanned leathers, patrimonial (historical) leathers). At the progressive heating, all investigated materials exhibit two main successive processes, associated with the dehydration and thermo-oxidative degradation. The patrimonial leathers were divided in two groups, namely: a group containing the majority of the analyzed materials, for which the rates of the thermo-oxidation process are substantially lower than those corresponding to the recent manufactured leathers, and a group for which the rates of thermo-oxidation process are closed to those corresponding to the recent manufactured leathers. Consequently, if by the thermal analysis in air atmosphere of a leather sample, a value of the rate of the thermo-oxidation process smaller than that corresponding to the recent manufactured leathers is obtained, then the analyzed leather is a patrimonial one. The reciprocal statement of this qualitative criterion for distinction between recent manufactured leather and patrimonial leather is not valid. The DSC analyses of collagen-based materials were performed in air (DSC(air)) and in water (DSC(water)). The denaturation process takes place at lower temperatures in water than in air. Unlike recent manufactured leathers and parchments, each patrimonial leather exhibits on DSC(water) plot 2-4 peaks. Consequently, the number of peaks from DSC(water) curve could be another qualitative criterion for distinction between a recently manufactured leather and a patrimonial leather. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of K–H3O jarosite as thermal witness material

The scope of this work was the assessment of thermo-oxidative deterioration, hydrothermal stability, and crystalline zone deterioration of some bookbinding leathers from some religious books published in XVIII century stored in Romanian libraries. In this purpose, the following thermal analysis methods were employed: thermogravimetry/derivative thermogravimetry (TG/DTG), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The thermo-oxidative damage of investigated leathers was characterized by the rate of the first thermo-oxidation process put in evidence in TG/DTG curves recorded in static air atmosphere. The hydrothermal stability was characterized by shrinkage temperature determined by DSC analysis of leathers in water excess. The damage of the crystalline zone of leathers was determined by DSC in nitrogen flow and DMA analyses. The qualitative damage for each leather and each kind of degradation was evaluated using the criteria resulted by thermal analysis of a large number of collagen-based materials (pure collagens, new and old parchments and leathers). The obtained results could be used for finding the best possible methods for preservation and/or restoration of the investigated bookbinding leathers.     

13C solid-state NMR complemented by ATR-FTIR and micro-DSC to study modern collagen-based material and historical leather

Ancient vegetable tanned leathers and parchments are very complex materials in which both different manufacturing and deterioration processes make their study and chemical characterisation difficult. In this research, solid-state nuclear magnetic resonance (NMR) spectroscopy was applied to identify different tannin families (condensed and hydrolysable) in historical leather objects such as bookbindings, wall upholsters, footwear and accessories, and military apparel. Furthermore, leather deterioration with special focus on collagen gelatinisation was investigated. A comparison with Fourier transform infrared (FTIR) spectroscopy and micro-differential scanning calorimetry (micro-DSC) was also performed to support the ¹³C CP-MAS NMR findings and to point out the advantages and limitations of solid-state NMR in analysing historical and archaeological leathers. A wide database of NMR and FTIR spectra of commercial tannins compounds was also collected in order to characterise historical and archaeological leathers.     

Application of ATR–FTIR spectroscopy to the analysis of tannins in historic leathers: The case study of the upholstery from the 19th century Portuguese Royal Train

Tanning materials of historic leather samples collected from the 19th century Portuguese Royal train were analyzed by attenuated total reflection–Fourier transform infrared (ATR–FTIR) spectroscopy. Studied leathers were visually identified as morocco leathers, one of the most valued types of vegetable tanned leathers. In technical and historic literature, morocco leathers are described as a distinctive type of vegetable tanned leather, with a typical grain surface pattern, made from goat skins and sumac (Rhus coriaria) leaves.ATR–FTIR spectra of the Royal train leathers were investigated and compared with 10 reference tanning materials obtained from different plants in use in the 19th century, here described. Two different types of vegetable tanned leathers were identified. The obtained spectra allowed to confirm the presence of morocco leathers as well as to detect a different type of vegetable tanned leather, probably applied as a restoration material in a past intervention. This study shows the usefulness of ATR–FTIR to distinguish different types of historic leathers based in the spectroscopic characteristic IR bands of vegetable tannins used for leather production, which can be of great assistance for conservation condition assessments.     

Effect of accelerated thermal ageing on the thermal behaviour of the recently made parchments

To reveal the fire injuring of parchment, the changes in the thermal behaviour of some goat parchments, obtained from skins originating from different animals, as a result of thermal aging were determined by thermal analysis methods (DSC; simultaneous TG/DTG, DSC; micro hot table (MHT)). Thermal aging of parchments was revealed to bring about the decrease in shrinkage temperature, absolute value of enthalpy of denaturation in water and some changes in non-isothermal parameters characteristic for dehydration process in static air atmosphere. The results obtained by DSC analysis performed in N₂ and O₂ flows as well as those obtained by simultaneous TG/DTG, DSC analyses have shown that both softening (melting) process parameters and parameters of thermo-oxidative processes have not been changed by thermal ageing. The results obtained by thermal analysis methods were correlated with those obtained by microscopic investigation of parchment samples immersed in water and scanning electron microscopy (SEM). The application of these microscopic techniques has revealed the morphology changes in the investigated parchments as a result of thermal degradation.     

Tannin fingerprinting in vegetable tanned leather by solid state NMR spectroscopy and comparison with leathers tanned by other processes

Solid state 13C-NMR spectra of pure tannin powders from four different sources--mimosa, quebracho, chestnut and tara--are readily distinguishable from each other, both in pure commercial powder form, and in leather which they have been used to tan. Groups of signals indicative of the source, and type (condensed vs. hydrolyzable) of tannin used in the manufacture are well resolved in the spectra of the finished leathers. These fingerprints are compared with those arising from leathers tanned with other common tanning agents. Paramagnetic chromium (III) tanning causes widespread but selective disappearance of signals from the spectrum of leather collagen, including resonances from acidic aspartyl and glutamyl residues, likely bound to Cr (III) structures. Aluminium (III) and glutaraldehyde tanning both cause considerable leather collagen signal sharpening suggesting some increase in molecular structural ordering. The 2?Al-NMR signal from the former material is consistent with an octahedral coordination by oxygen ligands. Solid state NMR thus provides easily recognisable reagent specific spectral fingerprints of the products of vegetable and some other common tanning processes. Because spectra are related to molecular properties, NMR is potentially a powerful tool in leather process enhancement and quality or provenance assurance.     

The use of thermal analysis methods for authentication and conservation state determination of historical and/or cultural objects manufactured from leather

Leather products have been useful materials since the dawn of human history. Many leathers objects are valuable treasures due to the history they represent, and their preservation challenges museum custodians and private collectors alike. In this article, the applications of thermal analysis methods (Micro Hot Table (MHT), thermogravimetry/derivative thermogravimetry (TG/DTG), dynamic scanning calorimetry (DSC), differential thermal analysis (DTA), and dynamic mechanical analysis (DMA)) for the characterization of the recently manufactured and old leathers are presented. These methods can be used for the assessment of deterioration degree of leathers, and, therefore, the data obtained by these techniques could be useful for achievement of the suitable preservation procedures as well as the effects of conservation treatments. In addition, it was pointed out that these methods are suitable for qualitative distinction between the recently manufactured leathers and heritage items.     

Characterisation and evaluation of the environmental impact on historical parchments by differential scanning calorimetry

Our recent developments concerning the assessment of parchments deterioration using DSC are reported. Measurements performed on samples in excess water conditions, in static air and gas flow provided qualitative and quantitative information on parchment ageing and deterioration at microscopic and mesoscopic level, when assembly of fibres/fibrils is weakened, partially and eventually completely lost, and at molecular level, when triple helix uncoiling occurs. A damage ranking scale based on a large collection of DSC parameters obtained by investigating artificially aged samples was set up. Deconvolution of the DSC thermal denaturation peaks in excess water enabled evaluating and discriminating stability of parchments with similar damage levels. Further experimental evidences such as softening of the crystalline fraction of collagen, thermal-oxidation and collagen gelatinisation were detected by DSC measurements in gas flow and static air, and related to specific deterioration patterns. DSC measurement of wet samples provided an objective and reliable method for evaluating parchment shrinkage temperature overcoming the limitations of conventional methods.     

Synthesis of an amino-terminated waterborne polyurethane-based polymeric dye for high-performance dyeing of biomass-derived aldehyde-tanned chrome-free leather

An amino-terminated waterborne polyurethane-based polymeric dye (AWPUD) was synthesized for the high-performance dyeing of chrome-free leather tanned by biomass-derived aldehyde (BAT). Fourier transform infrared spectroscopy (FTIR) and ¹H NMR spectra illustrated the successful synthesis of AWPUD via incorporating small-molecule dyes (SMDs) (C.I. Acid Red 14) and sodium 2,4-diaminobenzenesulfonate into the polyurethane skeleton. The amino group content of AWPUD was tested to be 1.34 wt% (based on dry weight), which included 0.65 wt% of terminal amino groups. The characteristic absorption peaks of AWPUD were observed at 538 nm and 573 nm, similar to those of SMDs. AWPUD-dyed crust leathers exhibited better dyeing performances than SMD-dyed crust leather in terms of comparable dye penetration degree, lower chroma of wastewater, and higher filler-dye uptake ratio. In addition, AWPUD-dyed crust leathers had higher dyeing uniformity, dry-wet rubbing resistance, and thermal stability. Meanwhile, AWPUD-dyed crust leathers had better comprehensive physical properties in terms of higher mechanical strengths, comparable softness, and smooth grain surface. These results suggest that AWPUD has the great potential to be used in the high-performance dyeing of BAT-tanned leather.     

The influence of sodium chloride on the melting temperature of collagen crystalline region in parchments

In order to observe the influence of sodium chloride on the melting temperature of collagen crystalline region in three new parchments, samples were soaked in water (blanks) and NaCl solutions of different concentrations, then removed, dried in air and measured by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The melting temperature of crystalline region of collagen, T _m, was determined as the minimum of the endothermal peak in the range 200–250 °C and as the inflection point of the decrease of storage modulus, respectively. There was observed a decrease in melting temperature of the salt-treated parchments compared to the samples soaked in water, sometimes significant (~20 °C) at certain concentrations of NaCl. Simultaneous TG/DTG/DSC thermal analysis (STA) was also applied for the determination of the amount of sodium chloride in salt-treated parchments, by calculating the mass loss due to the vaporization of NaCl, which occurs above 800 °C. By plotting T _m determined by DSC and DMA versus the NaCl content of the samples, an apparent minimum is observed. Additional information regarding the structural features was also obtained through X-ray diffraction (XRD) and attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR). XRD data put in evidence the preservation of collagen crystalline region in all salt-treated samples, while FTIR measurements did not showed significant modification of collagen. By removing the sodium chloride from the salt-treated parchments through washing with water, there is a return of the melting temperatures to the values of blank samples, demonstrating the reversibility of this phenomenon.     

Assessment of damage in old parchments by DSC and SEM</o:p>

Summary Environmental impact on parchment was investigated by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Parchments subjected to accelerated ageing and old parchments were compared to evaluate quality and extent of deterioration. Stability of fibrillar collagen within parchment was determined from the changes in thermodynamic parameters associated with thermal denaturation. Parchment surface was characterised, and specific morphological criteria were selected for damage assessment. The thermodynamic and morphological changes of collagen induced by deterioration are discussed, and their correlations are proposed as a means of ranking damage in old parchments.</o:p>     

Influence of containing moisture on hydrothermal stability of modified collagen thermal characteristics analysis by DSC

The thermal stability of sheepskin collagen cross-linked with chrome sulfate and mimosa (MI)–oxazolidine (OZ), respectively, had been researched in this experiment. All samples’ shrinkage temperatures (T _s) are determined by a special T _s-testing-apparatus and denaturation temperatures (T _d) are determined by the differential scanning calorimetry. The relations between the modified collagens containing moisture and their hydrothermal stability, T _s or T _d, were studied. The results show that the cross linking agents can enhance the thermal stability of modified collagen whose T _s are 109.8 and 110.6 °C for collagen treated with chrome and MI–OZ, respectively. When the samples contain 25–71.9% moisture for chrome leather and 20–71.1% for leather treated with MI–OZ, the hydrothermal stability will decrease with the increase of moisture. It was found that the difference between T _s and T _d of collagen modified by chrome is more obvious than that of collagen modified with MI–OZ. And when the moisture of chrome leather exceeds 55%, T _d cannot express thermal stability of modified collagen as a substitute for T _s, and the moisture is 40% for leather tanned with MI–OZ.     

Thermal analysis on parchments I: DSC and TGA combined approach for heat damage assessment

Ancient, new and artificially aged parchments were investigated with both differential scanning calorimetry (DSC) and thermogravimetry (TGA). Criteria to define a quantitative ranking of the damage experienced by the bulk collagen of historical parchments were assessed. A damage-related correlation was found between the collagen denaturation temperature and the moisture content of the parchment. Qualitative rules for the evaluation of the damage at the nano-and mesoscopic level were achieved on the basis of peculiarities of the shape and width of the DSC signals and confirmed by small angle X-ray scattering patterns.     

Study of the influence of NaOH treatment on the pyrolysis of different leather tanned using thermogravimetric analysis and Py/GC-MS system

This paper presents a study of the influence of a NaOH treatment on leather samples tanned using different tanning agents. Thermogravimetric analysis (TG/DTG) and flash pyrolysis (Py/GC-MS) was carried out on treated samples, and the results were compared with those obtained with the untreated samples. Treatment with sodium hydroxide seemed to decrease the temperature of maximum leather decomposition rate between 4 and 27 °C, depending on the tanning agent, and increased the range of temperatures where leather degradation occurs. From flash pyrolysis experiments, it was found that the volatile products obtained from NaOH-treated samples contain higher amounts of nitrogen compounds than those obtained from untreated samples. The results of a multivariate analysis applied to pyrolytic products obtained in the flash pyrolysis of samples showed greater similarity between the results obtained in all treated samples than between treated and untreated samples produced with the same tanning process.     

Chemikalien für die Lederherstellung. Vom Kollagen tierischer Häute zum Werkstoff Leder

Leather is a valuable material manufactured from collagen network of hides and skins by suitable chemical and physical processes. The article describes structure and reactivity of the most important leather chemicals, necessary to convert collagen into leather.     

Improvement of hydrothermal stability and chrome uptake of leather: Amino-terminated hyper-branched polymer as high exhausted tanning auxiliary

Carboxyl-terminated hyper-branched polyester is extensively used as high exhaustion chrome tanning auxiliary owing to its strong coordination ability with trivalent chrome. Owing to lower electronegativity of N than O, the coordination ability with trivalent chrome for hyper-branched polyamide is stronger than polyester. Herein, in this paper, an amino-terminated hyper-branched polyamide (HBPN) is synthesized and used as high exhaustion chrome in tanning of pickling pigskin (HBPN–Cr). The effects of utilization of HBPN in denaturation temperature, shrinkage temperature, chrome uptake, and microstructure structure of wet-blue leather are studied. Moreover, the anti-oxidation stability of wet-blue leather is preliminary studied by heat treatment. The results show that incorporation of 1% HBPN in pickling process can significantly improve chrome uptake to 83.32%. The hydrothermal stability of wet-blue leather is improved due to the formation of higher crosslinking degree of collagen fibers. In addition, for the wet-blue leather without unsaturated fatliquoring agent, through heat treatment, hexavalent chrome does not appear after 24 ​h heat treatment. On the contrary, the chrome complex changes from hydroxyl copolymerization to more stable oxygen copolymerization or chrome oxide for C–Cr and HBPN-Cr tanned leather, respectively. These results will build a data support for the further study of hyper-branched polymer chrome tanning auxiliaries.     

Effect of venous wall immobilization on the thermal degradation of collagen

The results from a comparative study of the thermal denaturation of collagen in the venous walls of reference samples and samples with varicose disease are presented. Changes in the organization of collagen network of the tissue matrix are detected via thermal analysis and multiphoton microscopy with recording of the second harmonic generation (SHG). It is established that the collagen network of venous walls degrades in varicose disease. It is shown that the disordering of the tertiary structure of collagen molecules is reflected in a 40% drop in the enthalpy of protein denaturation compared to reference (ΔH _D = 12.4 ± 4.9 J/g dry residue). The disorganization of fiber structures is recorded on SHG images. It is shown that upon the hydrothermal heating of sequestered samples of venous walls, the complete degradation of the tissue network occurs at 75°C. However, it is noted that upon the mechanical immobilization of samples of both types, the stability of collagen increases and complete denaturation is observed at temperatures above 84°C. It is suggested that the number of available conformations of polypeptide chains in the random coil state falls under tension, lowering ΔS _D and raising the temperature of the denaturation of protein.     

Study on thermal degradation of cattlehide collagen fibers by simultaneous TG–MS–FTIR

Leather was useful materials since dawn of human history for excellent properties, but thermal degradation mechanism was not very clear yet. In this paper, much progress has been made in elucidating the thermal stability and thermal degradation mechanism by thermoanalytical study in argon. Thermogravimetric analysis simultaneously coupled with mass spectrometry and Fourier transform infrared spectrometry was employed to study the thermal degradation of cattlehide collagen fibers through in-depth analysis of the evolved gas. Thermogravimetry analyses carried out on sample, deprived from any residual catalyst and highlighted a two-step thermal degradation. New evidence demonstrates that the process during temperature range from 373 to 513 K was phase transformation. Photographs of polarizing microscope confirmed the conclusion. The decomposition of cattlehide collagen fibers starts at about 523 K. The cattlehide collagen fibers may undergo the process of melting, oxidation and decomposition. In decomposition, more than three steps take place. The mass spectra and Fourier transform infrared spectrometry stated clearly that double bond of carbon to oxygen, carbon to sulfur and carbon to nitrogen were destroyed firstly because the carbon dioxide, carbon monoxide and ammonia evolved simultaneously. The second peak of carbon monoxide in mass spectra indicated that some organic fragments were decomposed above 1073 K which confirmed that thermal degradation of leather is more than three steps.     

The Effect of multi-reprocessing on the structure and characteristics of thermoplastic elastomers based on recycled polymers

The effect of multi-reprocessing on the phase structure and characteristics is studied for thermoplastic elastomers based on recycled high-density polyethylene (HDPE), ethylene-propylene-diene rubber (ternary copolymer of ethylene, propylene, and 5-ethylidene-2-norbornene) (EPDM), and recycled ground tire rubber (RGTR). Analysis of the viscous flow characteristics of thermoplastic elastomers shows that, independently of the number of processing cycles, all samples are characterized by the required flow characteristics at elevated temperatures. Processing of thermoplastic elastomers is accompanied by the competing processes of crosslinking and degradation of macromolecules in a polymer mixture. The results of DSC study and dynamic mechanical thermal analysis show that, as the number of processing cycles is increased, phase separation between amorphous and crystalline phases in thermoplastic elastomers decreases. Insignificant intermolecular crosslinking induced by the processing of thermoplastic elastomers appears to have almost no effect on the physicomechanical characteristics of the final material.