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Protein Hydrolysates from Biogenic Waste as an Ecological Flame Retarder and Binder for Fiberboards

Cite this: ACS Omega 2020, 5, 50, 32227–32233
Publication Date (Web):December 8, 2020
https://doi.org/10.1021/acsomega.0c03819

Copyright © 2020 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

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The increasing demand for sustainable building materials requires alternative flame retarders, which have superior sustainability to those previously used. In this respect, we present our initial results with protein hydrolysates made from poultry-feather waste for the preparation of flame-retardant fiberboards. Impregnated wood fibers show a significantly decreased decomposition rate in the region between 300 and 450 °C, as measured by thermogravimetric analysis. Final combustion of the impregnated fibers is shifted up by 50 °C to the interval 450–500 °C and occurs stepwise rather than instantaneously as for untreated wood. At a total protein content of approx. 10 wt %, plates produced in the “wet” process are self-extinguishing and show very little subsequent smouldering. In three-point bending tests, these fiberboard prototypes were able to withstand stresses of up to 15 N/mm2, the threshold required by DIN EN 622 for commercial, formaldehyde-bound MBH fiberboards. This indicates that the upcycled protein hydrolysates not only have an impressive flame-retarding effect but also can be used as a fully sustainable binder for a new generation of ecological fiberboards. As these boards are based solely on natural materials, they can be shredded and composted at the end of their life cycle.

Introduction

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The recent rise in the number of large and severe fires such as in the Grenfell Tower (London 2017), the entertainment centre Simnjaja wischnja (Kemerowo 2018), the Brazil National Museum (Rio de Janeiro 2018), Notre Dame (Paris 2019), and the Abbco Tower (Schardscha, UAE, 2020) has brought the importance of flame retarders back into public awareness. Although there is a large number of trusted and reliable substances available on the market, the re-evaluation of toxicological aspects, for example, as brought forward by the regulation (EG) no. 1907/2006 of the European Union known as REACh, and increased environmental awareness has led to a number of proven fire retarders such as antimony or brominated compounds being banned or viewed critically. (1) For example, the previously common flame retarder hexabromocyclododecane (HBCE) was classified as a persistent, bioaccumulative, and toxic substance and restricted by means of a sales ban. (2) Other substances such as antimony(III) oxide are listed as possible carcinogens by the International Agency for Research on Cancer and are still under investigation within the REACh process. Therefore, environmentally friendly and “green” fire retarders are the subject of a number of current research efforts. (3,4) In addition, the quest for increased sustainability has led to wood becoming increasingly popular again, not only for interior fittings but also as a building material for multistorey buildings. Bringing these two recent trends together in order to provide large quantities of sustainable, 100% biological, and nontoxic materials for fire-protected wood buildings requires novel approaches.
There have been some attempts to use proteins as flame retarders. (5) Of these, casein is very promising as it naturally contains high amounts of phosphorus; (3) however, casein intervenes with the food chain and is, therefore, unsuitable for large-scale technical applications. Another interesting protein is hydrophobin, which consists of 100 amino acids with 8 cysteine units and is similarly found to reduce the burning rate of cotton. (6) The composition of hydrophobin is similar to that of the very abundant protein keratin, which is found in beaks, horns, hoofs, hair, and feathers. (7) However, these are usually considered as waste, as they have very few value-added uses. This is despite the fact that keratin has some remarkable thermal properties such as a high decomposition temperature. In this context, we have also recently shown the beneficial effects of shredded poultry feathers on the thermal stability of biobased, polycondensation-type thermoset composites. (8) Moreover, keratin does not burn—hence, keratin waste cannot be disposed of by incineration—and as a result, wool keratin was tested as flame-retardant coating on cotton (9) and poultry feathers in polypropylene. (10,11)
Developing this property of keratin further led us to explore methods to transform keratin waste into flame-retardant impregnations for wood building materials. Because feather keratin is highly inter- and intramolecularly cross-linked by disulphide bridges, the protein needs to be hydrolyzed to obtain impregnation solutions to be used in the “wet process” of manufacturing fiberboards. We chose an alkaline hydrolysis because it can be run in an eco-friendly manner at room temperature. Such protein solutions greatly improve the thermal stability of wood-based materials, as determined by thermogravimetric analysis (TGA), and their flammability. In addition, the strength of our hot-pressed fiberboards matches that of commercial ones, bound with formaldehyde resins.

Materials and Methods

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Goose feathers (Figure S1A) were provided by Treude & Metz GmbH, Bad Laasphe, Germany, as a random mixture of tail and chest feathers as well as downs. The mixture was cut to a uniform size on a Retsch SM cutting mill fitted with a 200 μm sieve (Figure S1B). Dust-free beech wood fibers with a size of 0.5–1 mm (RG HB 500/1000) were obtained from J. Rettenmaier & Söhne GmbH + Co KG, Germany (Figure S1B). Analytical-grade NaOH and Na3P3O9 (STMP) were from VWR, Germany, and used as received.

Alkaline Hydrolysis of Feathers and Phosphorylation

In a typical experiment, 10 g of shredded feathers was suspended in 100 mL of 1 M NaOH solution and shaken at room temperature for 24 h. The resulting turbid solution was centrifuged to obtain a clear, yellow protein solution and a viscous phase. After decanting, the solution is acidified with 1 M HCl to pH = 5 and the precipitated protein is filtered, washed with distilled water, and finally dried by lyophilization. Phosphorylation of the protein hydrolysate followed a published procedure. (12) For that, 2 g of the dried keratin hydrolysate was mixed with 50 mL of water and the pH was adjusted to 12 with 2 M NaOH. A total of 1.5 g of STMP was added to the reaction mixture and stirred for 3 h at 50 °C. The pH is checked and readjusted to pH 12 every 15 min during the reaction.

Wood Impregnation

The impregnation solutions were prepared by dissolving 70 mg/mL of the dried protein hydrolysate in 0.2 M NaOH and mixed with wood fibers in ratios of 0.5, 1, and 1.5 mL/g for 5 min. The impregnated fibers were finally dried at 50 °C to constant weight.

Amino Acid Analysis

The amino acid analysis was performed according to the method of Spackmann, Moore, and Stein. (13) A mixture of 20 mg of the dry sample, 5 mL of 6 N hydrochloric acid, and a few drops of 0.1% aqueous phenol solution (antioxidant) was heated at 110 °C for 24 h. After cooling, the sample was transferred quantitatively into a 10 mL volumetric flask. The excess of hydrochloric acid was removed on a rotatory evaporator at 50 °C. The residue was washed several times with distilled water and finally dried in a stream of dry nitrogen before being dissolved in a lithium acetate buffer at pH 2.2. The amino acid analysis was performed by ion exchange chromatography after column derivatization with ninhydrin. Because of the acidic conditions in the first step, glutamine and asparagine cannot be detected as they are hydrolyzed to glutamic and aspartic acid; tryptophane is completely destroyed in the process.

ATR–FTIR Spectroscopy

Infrared IR spectra were recorded using a PerkinElmer Spectrum Two UATR FTIR spectrometer equipped with a diamond ATR (attenuated total reflection) window. All spectra were recorded in the spectral range of 4000–400 cm–1 with 12 scans at a spectral resolution of 4 cm–1. Before each measurement, the diamond ATR crystal was cleaned with isopropanol.

Thermogravimetric Analyses

Measurements were performed on a PerkinElmer TGA 4000 instrument heating from 30 to 850 °C at a rate of 10 K/min under a continuous nitrogen or oxygen flow rate of 20 mL/min. Samples were stored in a standardized climate (RH 55%; T = 21 °C) prior to the measurements. The sample weight was set to 100% at 150 °C, where residual water was completely evaporated, to allow better comparability between samples.

Horizontally Burning and Smouldering Tests

Tests were performed on a horizontal specimen following the UL94 (14) standard. For sample preparation, 1 g of treated or untreated wood fibers was evenly distributed in a rectangular mould of 7 × 1 cm2 and pressed at 150 °C and 10 bar/cm2 machine pressure for 10 min into approximately 1.2 mm-thick plates. The plates were marked at intervals of 5 mm over a length of 50 mm, mounted horizontally at one end, and lit at the other. The experiments were filmed and evaluated with an image-processing software.

Mechanical Testing

A total of 1.5 g of treated or untreated dry wood fibers was evenly distributed in a rectangular mould of 7 × 1 cm2 and the surface was sprayed evenly with distilled water to obtain a moisture content of approx. 15 wt %. The samples were pressed at 150 °C and 10 bar/cm2 machine pressure for 10 min into approx. 2 mm-thick plates. The three point bending test was performed on an Instron 5566 universal testing machine with a free length of 35 mm, an initial load of 1 N, and a strain rate of 1 mm/min.

Results and Discussion

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The feathers were received both washed and greasy, but the fat content was not found to have any influence on the hydrolysis as the same results, such as, for example, the time–concentration profile shown in Figure 1A, were obtained with both types. Alkaline hydrolysis of the shredded feathers produces a turbid solution than can be separated by centrifugation into a clear solution, containing soluble protein fragments, and a viscous phase, which consists of still partially cross-linked protein chains. The following work focuses on the clear protein solution, although the viscous phase can also be transformed into value-added products (this is beyond the scope of this article). A typical time–conversion curve for the alkaline hydrolysis of the shredded feathers at room temperature is shown in Figure 1A. It should be noted that the feathers contain approx. 8% of water under ambient conditions. Thus, the maximum amount of dissolved keratin under the present experimental conditions is 0.092 g/mL. Within 50 h, approx. 80% of the feathers are solubilized, but such extended reaction times also lead to the formation of large amounts of very small and undesired protein fragments, as observed using gel electrophoresis (Figure 1B). In order to obtain longer chains for the following investigations, the hydrolysis is usually stopped after 24 h (approx. 70% solubilization) at the expense of producing more of the viscous phase.

Figure 1

Figure 1. Progress of the alkaline hydrolysis of shredded feathers using 0.1 g of feathers per mL of solution (A) and development of the molecular weight of the fragments over time as determined by SDS-PAGE (B). The black boxes in B indicate the most strongly colored areas.

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Alkaline hydrolysis of the feathers not only decreases the length of the protein chain but also eliminates the disulphide bridges that cross-link the protein. This improves the solubility and allows higher protein concentrations in the impregnation solution but also causes a change in amino acid composition (Table S1). The most notable change is the overall decrease in cysteine by about 78%, which is most likely caused by β-elimination from cystine in the alkaline environment to form dehydroalanine and a persulfide (Figure 2). (15−17) Both persulfide and dehydroalanine are rather reactive and can undergo subsequent reactions, for example, with other amino acids, which may account for some of the differences in the amino composition of the feathers and the hydrolysate. In addition, some of the more water-soluble amino acids might be lost during work-up of the hydrolysate so that more hydrophobic ones such as leucine (+14%) and proline (+18%) might be overestimated. The observed decrease in sulfur-containing amino acids is beneficial for the application as a flame retarder as the amount of harmful sulfur oxides formed in the case of a fire is decreased.

Figure 2

Figure 2. β-Elimination cleaves a disulfide bridge into dehydroalanine and a persulfide. The stars denote the continuation of the protein chains.

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Because phosphate derivatives are known to improve the flame-retardant properties of materials, the protein hydrolysates were reacted with sodium trimetaphosphate (STMP) in alkaline solution following an established procedure. (12) STMP is known to react with both serine and lysine, but based on the amino acid analysis in Table S1, serine with a content of 10.8% (as compared to lysine with 0.6%) appears to be the prevalent reaction partner in the present case. Infrared spectra of the product show a new absorption band at 1083 cm–1, which is in accordance with the literature and indicates the presence of phosphate groups. (18) However, the amount of serine does not decrease significantly after phosphorylation (Table S1) and the literature is divided on whether or not the phosphate group is really covalently bound or only physically adsorbed on the protein surface. (19)
For TGA, an enriched oxygen atmosphere (approx. 35 vol % O2) was used to simulate the conditions during a fire because TGA under a nitrogen atmosphere more resembles pyrolysis. All four materials under investigation show a rather similar decomposition behavior up to a temperature of approx. 300 °C (Figure 3). Beyond 300 °C, the decomposition of beech wood accelerates and between 300 and 330 °C, the residual weight decreases from 75 to 40% of the initial weight. This was previously associated with the decomposition of the hemicellulose. (20) The beech wood is fully decomposed at 450 °C. For the three keratin-based materials, the mass loss above 300 °C proceeds at a lower rate compared to beech wood (approx. 60% of the initial weight at 330 °C) and both feathers and hydrolysate are only fully decomposed at approx. 600 °C. The thermal behavior of the hydrolysates was found not to depend much on the time of hydrolysis beyond 8 h of the reaction. Shorter times (<8 h) produced materials with slightly faster decomposition (data not shown). The behavior of the phosphorylated hydrolysate does not differ substantially from that of the feathers or the pure hydrolysate in the region up to 500 °C. However, decomposition of the phosphorylated keratin leaves a residual mass of approx. 7%, which is most likely because of the phosphate content and not a sign of an improved thermal stability. Because the additional phosphorylation step reduces the sustainability of the presented approach, the phosphorylated keratin hydrolysate is not considered in the following experiments.

Figure 3

Figure 3. TGA (A) and dTG (B) of feathers (black solid line), beech wood fibers, protein hydrolysate, and phosphorylated hydrolysate under an oxygen atmosphere.

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The solutions used for the impregnation of wood are prepared by dissolving 70 mg/mL of the isolated hydrolysate in 0.2 M NaOH solution. This concentration was found to be the solubility limit at that pH. Using more concentrated NaOH solutions in order to obtain more concentrated impregnation solutions is possible, but it was considered impracticable in view of potential future applications. Higher concentrations of keratin inevitably increase the solution viscosity, whereas the solutions should have a low viscosity to allow complete impregnation of the wood fibers. Within this study, the impregnation solutions were prepared from the isolated protein hydrolysate in order to achieve standardized conditions, but the crude hydrolyzed mixtures could also be used after adjusting the protein concentrations.
The wood fibers were impregnated with these solutions in ratios of 0.5, 1, and 1.5 mL/g. The latter is about as much as the wood can absorb without appearing wet. The uptake of the keratin solution by the wood fibers proceeds in two steps, an initial, rather quick filling of the wood pores followed by the slower swelling of the wood matrix. ATR–Fourier-transform infrared (FTIR) spectroscopy of the impregnated wood shows the typical bands of beech wood, for example, at about 1030 cm–1, and the amide-I and -II bands of keratin, for example, at about 1630 and 1516 cm–1 (Figure 4).

Figure 4

Figure 4. ATR–FTIR of feathers, beech wood fibers, and beech fibers treated with keratin.

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The treated and thoroughly dried wood fibers were subject to TGA under nitrogen (not shown) to determine the char residue at 850 °C. This correlates with the limiting oxygen index (LOI) that indicates the volume concentration of oxygen needed for combustion to occur. (21) Impregnation with the protein solution increases the LOI from 23 (char residue 14%) to 26 (char residue 21%), and this is not due to the sodium content in the impregnation solution, as this constitutes only 0.7% of the total mass. However, TGA under nitrogen resembles pyrolysis more than a fire event, so the analyses were repeated in an enriched (approx. 35 vol %) oxygen atmosphere (Figure 5).

Figure 5

Figure 5. TGA (A) and dTG (B) of untreated wood fibers and keratin-impregnated wood fibers in an oxygen atmosphere.

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The degradation behavior is similar for all samples up to approx. 300 °C with a mass loss of approx. 27% (Figure 5A). As already shown in Figure 3, the untreated wood then undergoes rapid decomposition and is fully decomposed at 450 °C. The wood sample treated with 0.5 mL/g is rather similar to pure wood; however, increasing the amount of keratin continuously (to 1.0 mL/g and 1.5 mL/g respectively) slows down the degradation rate in the temperature region between 300 and 450 °C and the degradation behavior of the treated wood gradually approaches that of feathers. This is in accordance with previous observations using urea or guanidine carbonate on cotton and was addressed to a change in the reaction energetics from endothermic to slightly exothermic. (22) At 450 °C, the wood sample treated with 1.5 mL/g shows a residual weight of approx. 31%, while the untreated wood is fully decomposed. It should be noted that the absolute amount of keratin in this sample is only 10.5 wt % and, yet, the thermal profile matches that of pure feathers in this temperature region. Starting at 455 °C, the samples treated with 1 and 1.5 mL/g begin decomposing, which ends slightly above 500 °C with a residual mass of approximately 3%. However, the final combustion of the sample treated with 1.5 mL/g appears to be stepwise as seen in the dTG curves (Figure 5B) indicating that the remaining protein still interferes with the decomposition mechanism.
With these promising TGA results at hand, the treated and untreated wood fibers were pressed into 7 × 1 cm2 plates to test their flammability and to verify the flame-retarding effects of the keratin hydrolysate indicated in the TGA measurements. For a proof-of-principle experiment, a horizontal setup following the UL 94 standard was chosen to evaluate the burning distance, speed, and time. All plates made from untreated wood burn with a large shining flame and burn down the complete distance of 50 mm. In contrast, the impregnated plates burn with a less bright flame and extinguish after several millimeters (Figure 6).

Figure 6

Figure 6. Horizontal flammability test shown for untreated and keratin-impregnated plates that closely resemble the average values given in Table 1. The distance between the black marks at the bottom edge of the plates is 5 mm.

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The results of the flammability tests are summarized in Table 1. Besides the self-extinguishing behavior, the keratin-impregnated plates burn at about half the rate of the untreated ones.
Table 1. Evaluation of the Flammability Tests on Horizontal Plates
 treatment of wood fibers
parameteruntreated (n = 8)keratin-impregnateda (n = 7)change (%)
distance/mm50 ± 0b14.1 ± 9.4c–72
time/s116.5 ± 7.948.4 ± 27.6–59
rate/mm·s–10.43 ± 0.030.28 ± 0.07–44
a

Impregnated with 1.5 mL/g of a 70 mg/mL hydrolysate solution in 0.2 M NaOH.

b

The sample burned down the complete distance under investigation.

c

The fire ended with self-extinguishing.

In certain wall constructions such as concealed wood fiber insulations, smouldering is an even greater danger than the actual fire. To evaluate the smouldering properties, the samples were continued to be monitored after being extinguished with a CO2 extinguisher (untreated plates) or self-extinguishing (treated plates). The untreated samples showed a massive smouldering for up to 5 min during which most of the unburnt sample was consumed. In contrast, the impregnated samples showed only a slight smouldering after self-extinguishing, which did not consume any unburned material and subsided within a minute.
Three point bending tests were used to evaluate the material properties. Because no glue is used to prepare the plates, it was of particular interest to assess the effect of the keratin impregnation on the fiber–fiber adhesion, which directly reflects the strength of the plates. Samples prepared from untreated wood fail at 5 N/mm2, while those prepared from keratin-impregnated wood can withstand up to 15 N/mm2 (Figure 7). This is exactly the strength prescribed in DIN EN 622 (23) for medium-density fiberboards prepared from the “wet” process (type MBH) with a thickness of <10 mm. The bending modulus of untreated boards is 455 ± 118 N/mm2 and of keratin-treated boards is 1398 ± 307 N/mm2.

Figure 7

Figure 7. Evaluation of mechanical parameters of treated and untreated keratin plates.

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Conclusions

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Protein hydrolysates are efficient flame retarders for wood fibers and fiberboards. The hydrolysates can be made in an eco-friendly process at room temperature using biogenic protein waste as a resource. Data provided by TGA suggest that the protein hydrolysate slows down the hemicellulose decomposition leading to a reduced overall decomposition rate between 300 and 450 °C. For wood-based materials, these changes in the TGA curves could be used as an indicative tool to test for an increased degree of flame retardation. In a flammability test following the UL 94 standard, plates pressed from the treated wood fibers were self-extinguishing and showed significantly reduced burning distances and rates. In a real fire event, flames would spread less quickly across keratin-treated fiberboards providing more time for evacuation or fire-fighting operations. The protein hydrolysate also significantly improved the smouldering behavior, an important safety feature for wood fiber insulations. In addition, plates pressed from protein-treated wood fibers showed bending strengths similar to commercial fiberboards bound with formaldehyde resins, the latter having the major drawback of being volatile organic compound (VOC) releasers. Added keratin can also be expected to adsorb VOCs, as already shown for wool, (24) and this will be the subject of further studies. The possibility of making flame-retarding binders from biogenic waste creates added value and is a true upcycling that paves the way for a more sustainable future.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03819.

  • Details on the amino acid composition of the used feathers and hydrolysate as well as optical appearance of the crude feathers, shredded feathers, and beech wood fibers (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Author
    • Markus Brenner - Institute of Building Materials Research, RWTH Aachen University, Schinkelstraße 3, Aachen 52062, Germany
  • Funding

    This work was funded by the Federal Ministry of Food and Agriculture through the Fachagentur Nachwachsende Rohstoffe e. V. under grant no. 2009617.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors thank Prof. Dr. C. Popescu and Dr. P.-F. Metz for discussions. Technical assistance from Lutz Burow and Markus Haag is greatly appreciated.

References

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    A ubiquitous biol. material, keratin represents a group of insol., usually high-sulfur content and filament-forming proteins, constituting the bulk of epidermal appendages such as hair, nails, claws, turtle scutes, horns, whale baleen, beaks, and feathers. These keratinous materials are formed by cells filled with keratin and are considered 'dead tissues'. Nevertheless, they are among the toughest biol. materials, serving as a wide variety of interesting functions, e.g. scales to armor body, horns to combat aggressors, hagfish slime as defense against predators, nails and claws to increase prehension, hair and fur to protect against the environment. The vivid inspiring examples can offer useful solns. to design new structural and functional materials.Keratins can be classified as α- and β-types. Both show a characteristic filament-matrix structure: 7 nm diam. intermediate filaments for α-keratin, and 3 nm diam. filaments for β-keratin. Both are embedded in an amorphous keratin matrix. The mol. unit of intermediate filaments is a coiled-coil heterodimer and that of β-keratin filament is a pleated sheet. The mech. response of α-keratin has been extensively studied and shows linear Hookean, yield and post-yield regions, and in some cases, a high reversible elastic deformation. Thus, they can be also be considered 'biopolymers'. On the other hand, β-keratin has not been investigated as comprehensively. Keratinous materials are strain-rate sensitive, and the effect of hydration is significant.Keratinous materials exhibit a complex hierarchical structure: polypeptide chains and filament-matrix structures at the nanoscale, organization of keratinized cells into lamellar, tubular-intertubular, fiber or layered structures at the microscale, and solid, compact sheaths over porous core, sandwich or threads at the macroscale. These produce a wide range of mech. properties: the Young's modulus ranges from 10 MPa in stratum corneum to about 2.5 GPa in feathers, and the tensile strength varies from 2 MPa in stratum corneum to 530 MPa in dry hagfish slime threads. Therefore, they are able to serve various functions including diffusion barrier, buffering external attack, energy-absorption, impact-resistance, piercing opponents, withstanding repeated stress and aerodynamic forces, and resisting buckling and penetration.A fascinating part of the new frontier of materials study is the development of bioinspired materials and designs. A comprehensive understanding of the biochem., structure and mech. properties of keratins and keratinous materials is of great importance for keratin-based bioinspired materials and designs. Current bioinspired efforts including the manufg. of quill-inspired aluminum composites, animal horn-inspired SiC composites, and feather-inspired interlayered composites are presented and novel avenues for research are discussed. The first inroads into mol.-based biomimicry are being currently made, and it is hoped that this approach will yield novel biopolymers through recombinant DNA and self-assembly. We also identify areas of research where knowledge development is still needed to elucidate structures and deformation/failure mechanisms.
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    Brenner, M.; Popescu, C.; Weichold, O. Anti-Frothing Effect of Poultry Feathers in Bio-Based, Polycondensation-Type Thermoset Composites. Appl. Sci. 2020, 10, 2150,  DOI: 10.3390/app10062150
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    Anti-frothing effect of poultry feathers in bio-based,polycondensation-type thermoset composites
    Brenner, Markus; Popescu, Crisan; Weichold, Oliver
    Applied Sciences (2020), 10 (6), 2150CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
    The formation of polycondensation-type thermoset resins from natural reactants such as citric and glutaric acid, as well as 1,3-propanediol and glycerol, was studied. Monitoring the mass loss by thermogravimetric anal. (TGA) allowed the rate consts. of the esterification to be calcd., which were in the order of 7·10-5 s -1 for glutaric acid and approx. twice as high for citric acid. However, the combination citric acid/glycerol was previously reported to froth up at high conversions, giving rise to foams, which makes the prepn. of compact engineering composites challenging. In light of this, we obsd. that shredded poultry feathers not only increased the conversion and the reaction rate of the combination citric acid/glycerol, but increasing the amt. of feathers continuously decreased the no. of visible bubbles. The addn. of 20 wt% of feathers completely prevented the previously reported frothing and gave rise to compact materials that were macroscopically free of defects. Besides this, the addn. of feathers also improved the fire-retardant properties. The tensile properties of the first specimens are still rather low (σ = 11.6 N/mm2, E = 750 N/mm2), but the addn. of poultry feathers opens a new path for green thermoset resins.
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    Wang, X.; Lu, C.; Chen, C. Effect of chicken-feather protein-based flame retardant on flame retarding performance of cotton fabric. J. Appl. Polym. Sci. 2014, 131, 40584,  DOI: 10.1002/app.40584
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    Effect of chicken-feather protein-based flame retardant on flame retarding performance of cotton fabric
    Wang, Xueyan; Lu, Changqin; Chen, Chenxiao
    Journal of Applied Polymer Science (2014), 131 (15), 40584/1-40584/8CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
    A new kind of eco-friendly chicken-feather protein-based phosphorus-nitrogen-contg. flame retardant was synthesized successfully with chicken-feather protein, melamine, sodium pyrophosphate, and glyoxal. And its structure was characterized by Fourier transform IR spectroscopy, and the thermogravimetry of the agent was analyzed. Then the flame retarding performances of the chicken-feather protein-based flame retardant and in combination with the borax and boric acid in application to a woven cotton fabric were investigated by the vertical flammability test and limited oxygen index test. In addn., the surface morphologies of the treated and untreated fabrics were conducted by the scanning electron micrographs (SEM), and the thermogravimetric analyses of the treated and untreated cotton were explored, and the surface morphologies of char areas of the treated and untreated fabrics after burnt were tested by the SEM. The results showed that the flame retardancy of the cotton fabric treated by the chicken-feather protein-based flame retardant in combination with borax and boric acid was improved further, and the combination of the chicken-feather protein-based flame retardant and borax and boric acid could facilitate to form a homogeneous and compact intumescing char layer, and the combination of them plays a good synergistic effect in the improvement of the flame retardancy of the treated cotton fabric. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40584.
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    Jung, D.; Bhattacharyya, D. Keratinous Fiber Based Intumescent Flame Retardant with Controllable Functional Compound Loading. ACS Sustainable Chem. Eng. 2018, 6, 13177– 13184,  DOI: 10.1021/acssuschemeng.8b02756
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    Keratinous Fiber Based Intumescent Flame Retardant with Controllable Functional Compound Loading
    Jung, Daeseung; Bhattacharyya, Debes
    ACS Sustainable Chemistry & Engineering (2018), 6 (10), 13177-13184CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    An intumescent flame retardant working with various combinations of acid source, blowing agent, and char former promises high performance and low toxicity. However, the research for more potent functional constituents and their best combination is still extremely important. Here we report on a novel way to use keratinous fibers as the host material for creating an effective flame retardant. A simple soln.-based treatment to implant amine phosphate and phosphoric acid in the fiber through sequential monomer infiltration is found to be significantly effective for applying flame retardancy and reducing the flammability of polymeric materials. After the flame-retardant fiber modification, polypropylene (PP) shows significantly improved flame retardancy to achieve V-0 grade and >70% reduced peak heat release rate in vertical burning and cone-calorimeter tests, resp. We expect this strategy of converting the low-grade keratinous fiber to valuable flame-retardant material to become a novel and attractive soln. for achieving fire safety, value addn., and ecofriendly recycling goals at the same time.
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    Jung, D.; Persi, I.; Bhattacharyya, D. Synergistic Effects of Feather Fibers and Phosphorus Compound on Chemically Modified Chicken Feather/Polypropylene Composites. ACS Sustainable Chem. Eng. 2019, 7, 19072– 19080,  DOI: 10.1021/acssuschemeng.9b04894
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    Synergistic Effects of Feather Fibers and Phosphorus Compound on Chemically Modified Chicken Feather/Polypropylene Composites
    Jung, Daeseung; Persi, Ilenia; Bhattacharyya, Debes
    ACS Sustainable Chemistry & Engineering (2019), 7 (23), 19072-19080CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    We have developed a method of converting chicken feather to high-performance flame retardant through a soln.-based chem. treatment. The synergistic effect of the chicken feather fiber and loaded ethylenediamine phosphate (EDAP) in the flame retardant chicken feather (FR CFF) allows the modified polypropylene (PP) composite to achieve the highest fire performance grade, V-0, in the vertical burning test and ∼80% reduced peak heat release rate in the cone calorimeter test. Furthermore, FR CFF/PP shows a significantly lower drop in tensile strength compared to that of a conventional IFR/PP composite. Better char formation and improved interfacial bonding, ascribed to the FR CFF, are the main reasons for the simultaneous enhancements in flame retardancy and mech. performance. The overall material properties of FR CFFs and the modified PP composites were investigated through mass spectrometry, IR spectroscopy, SEM, thermal anal., vertical burning tests, cone-calorimeter characterizations, and tensile property evaluation. Flame retardant chicken feather modification makes the polypropylene composite achieve simultaneously improved flame retardancy and mech. properties.
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    Improvement of the functionalities of soy protein isolate through chemical phosphorylation
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    A method of chem. phosphorylation was developed to modify soybean protein to improve its functional properties. The reaction was carried out by incubating soybean protein isolate and cyclic Na trimetaphosphate in an aq. soln. at pH 11.5 and 35° for ∼3 h. The phosphoesterification of serine residues and the phosphoramidation of lysine residues in soybean protein were obsd. The phosphorylated soybean protein isolate prepd. therefrom exhibited improved functional properties in terms of aq. soly., water-holding capacity, emulsifiability, and whippability. The nutritive bioavailability of soybean protein isolate was not impaired by phosphorylation.
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    Moore, S.; Spackman, D. H.; Stein, W. H. Chromatography of amino acids on sulfonated polystyrene resins. An improved system. Anal. Chem. 1958, 30, 1185– 1190,  DOI: 10.1021/ac60139a005
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    cf. C.A. 49, 3289b. Improved procedures were developed for the chromatographic detn. of amino acids on columns of finely pulverized 8% cross-linked sulfonated polystyrene resins. Complete analyses of protein hydrolyzates can be performed in 48 hrs., with fraction collectors and in 24 hrs. with automatic recording equipment (C.A. 52, 15142g). The system may also be used to det. amino acids and related compds. in blood plasma, urine, and animal tissues. Complete directions are given for prepg. the resin, packing the columns, and the use of the columns. Analyses of known mixts. gave recoveries of 100 ± 3%, except methionine for which the recovery is 95%. Samples contg. glutathione (I) and glutamic acid (II) are treated with Na2SO3 to convert I to the S-sulfonate: the resulting material can be analyzed for II without interference from I.
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    Alkali treatment of SS-contg. proteins with different structures and properties resulted in the formation of similar type products, but with different energies of activation. The principal protein studied was lysozyme, with comparative studies on bovine pancreatic RNase, bovine α-lactalbumin, bovine serum albumin, chicken ovotransferrin, and several avian ovomucoids. Alkali treatment of proteins (10-5M protein in 0.1M NaOH at 50° for 24 h) resulted in the loss of cystine and lysine and the formation of new amino acids. Alkali treatment was accompanied by an increase in absorbance at 241 nm with time until it reached a max. at which time it started decreasing and finally plateaued. The rate of increase in absorbance at 241 nm was a function of both base and SS concn. The mechanism of action appeared to involve a β-elimination of the disulfides resulting in the intermediate, dehydroalanine. Michael-type nucleophilic addns. of the ε-amino groups of lysine, the S of cysteine, and the N of ammonia to the double bond of the dehydroalanine lead to the formation of lysinoalanine, lanthionine, and β-aminoalanine, resp. The energy of activation (Ea) for several SS-contg. proteins was in the range of 14.2 kcal/mol for Golden-Amherst pheasant cross ovomucoid to a high of 23.8 kcal/mol for lysozyme, whereas the change in free energy ΔF*, was essentially the same (20.2 kcal/mol) for all proteins.
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    Helmerhorst, E.; Stokes, G. B. Generation of an acid-stable and protein-bound persulfide-like residue in alkali or sulfhydryl-treated insulin by a mechanism consonant with the beta.-elimination hypothesis of disulfide bond lysis. Biochemistry 1983, 22, 69– 75,  DOI: 10.1021/bi00270a010
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    Generation of an acid-stable and protein-bound persulfide-like residue in alkali or sulfhydryl-treated insulin by a mechanism consonant with the β-elimination hypothesis of disulfide bond lysis
    Helmerhorst, E.; Stokes, G. B.
    Biochemistry (1983), 22 (1), 69-75CODEN: BICHAW; ISSN:0006-2960.
    According to the β-elimination mechanism for alkali-disulfide bond lysis in proteins, persulfide (RSSH) and dehydroalanine (RCH=CH2) are generated whereas thiol (RSH) and sulfinic acid (RSO2H) are the products of the alternate alk. hydrolysis mechanism. insulin [9004-10-8] Is a small protein consisting of 2 polypeptides contg. 1 intrachain and 2 interchain SS bonds. The lysis of the disulfide bonds of insulin in alkali (pH 13, 22°) or by sulfhydryl compds. (pH 9, 22°) apparently proceeds by the β-elimination mechanism. For each disulfide bond lysed, 1 OH ion was consumed and 1 SH was generated as predicted by the β-elimination mechanism. The intensity of a chromophore generated at 240 nm (extinction coeff. = 16,000 M-1 cm-1) is consistent with the generation of persulfide and dehydroalanine and greater than could be accounted for by sulfinic acid and thiol; simple model persulfides are reported to absorb weakly at 335 nm. A chromophore, reported by others as a broad shoulder on the tyrosyl absorption band, was estd. from 1st deriv. difference spectra to be centered at 323 nm (extinction coeff. = 670 M-1 cm-1); the chromophore at 323 nm was abolished by cyanide with the formation of thiocyanate. Two mol of SH were generated/mol of insulin and at least 1.5 mol were cyanolyzable; half a mol of the cyanolyzable S was acid labile and formed H2S and ligation products as predicted. Contrary to the predicted reactivity of persulfide in acid, 1 mol of cyanolyzable S was stabl/mol of acidified insulin. This acid-stable and cyanolyzable S residue eluted with the protein fraction on Sephadex G-25. This persulfide was also generated by thiolate ions. The alkali-treated insulin was cleaved into smaller components by lysis of the 2 interchain SS bonds. The intrachain SS bond was unreactive in alkali except under denaturing conditions.
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    Is the alkaline cleavage of disulfide bonds in peptides an α-β elimination reaction or a hydrolysis?
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    International Journal of Peptide & Protein Research (1977), 10 (3), 185-9CODEN: IJPPC3; ISSN:0367-8377.
    The cleavage of oxidized glutathione by alkali was studied as representative of the cleavage of protein disulfides. This process is quite different when studied in 10-4N or 2 × 10-1N NaOH. At low alkali concn., no spectral changes are noted; at higher hydroxyl concn. the appearance of persulfide groups (followed at 335 nm), the formation of thiocyanate (arising from cold cyanolysis of persulfide groups), and the absorbance at 240 nm follow the same kinetics. The amt. of half-cystine, recovered as cysteic acid after a 3-h reaction, is significantly lower than calcd. Thus, oxidized glutathione undergoes β-elimination at high pH values, and persulfide groups absorb not only at 335 nm (as already known) but also at 240 nm where the contribution of other absorbing species is not very high.
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    A review with 47 refs. on the use of various reagents to phosphorylate proteins. Attention is also given to covalent attachment of low-mol.-wt. organophosphorus compds. to proteins. The nature of the phosphate linkages involved and the effects of phosphorylation on the functional properties, as well as on the in vitro and in vivo digestion of the proteins, are discussed. Of the phosphorylating reagents tested so far, only POCl3 and Na trimetaphosphate might prove economical and practical reagents for large-scale application.
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  1. Huo Xinsheng, Guochao Yang, Qiuhui Zhang. Application status and development prospects of bio-based flame retardants in packaging materials. European Journal of Wood and Wood Products 2023, 81 (6) , 1337-1357. https://doi.org/10.1007/s00107-023-01977-w
  2. Arunee Kongdee Aldred, Prapaipat Klungsupya, Wasin Charerntantanakul, Oliver Weichold, Panwad Sillapawattana. Preparation of Keratin-Metal Complexes Derived from Different Treatments of Chicken Feather Waste. Waste and Biomass Valorization 2023, 9 https://doi.org/10.1007/s12649-023-02154-z
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  • Abstract

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    Figure 1

    Figure 1. Progress of the alkaline hydrolysis of shredded feathers using 0.1 g of feathers per mL of solution (A) and development of the molecular weight of the fragments over time as determined by SDS-PAGE (B). The black boxes in B indicate the most strongly colored areas.

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    Figure 2

    Figure 2. β-Elimination cleaves a disulfide bridge into dehydroalanine and a persulfide. The stars denote the continuation of the protein chains.

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    Figure 3

    Figure 3. TGA (A) and dTG (B) of feathers (black solid line), beech wood fibers, protein hydrolysate, and phosphorylated hydrolysate under an oxygen atmosphere.

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    Figure 4

    Figure 4. ATR–FTIR of feathers, beech wood fibers, and beech fibers treated with keratin.

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    Figure 5

    Figure 5. TGA (A) and dTG (B) of untreated wood fibers and keratin-impregnated wood fibers in an oxygen atmosphere.

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    Figure 6

    Figure 6. Horizontal flammability test shown for untreated and keratin-impregnated plates that closely resemble the average values given in Table 1. The distance between the black marks at the bottom edge of the plates is 5 mm.

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    Figure 7

    Figure 7. Evaluation of mechanical parameters of treated and untreated keratin plates.

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  • References

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    This article references 24 other publications.

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      A ubiquitous biol. material, keratin represents a group of insol., usually high-sulfur content and filament-forming proteins, constituting the bulk of epidermal appendages such as hair, nails, claws, turtle scutes, horns, whale baleen, beaks, and feathers. These keratinous materials are formed by cells filled with keratin and are considered 'dead tissues'. Nevertheless, they are among the toughest biol. materials, serving as a wide variety of interesting functions, e.g. scales to armor body, horns to combat aggressors, hagfish slime as defense against predators, nails and claws to increase prehension, hair and fur to protect against the environment. The vivid inspiring examples can offer useful solns. to design new structural and functional materials.Keratins can be classified as α- and β-types. Both show a characteristic filament-matrix structure: 7 nm diam. intermediate filaments for α-keratin, and 3 nm diam. filaments for β-keratin. Both are embedded in an amorphous keratin matrix. The mol. unit of intermediate filaments is a coiled-coil heterodimer and that of β-keratin filament is a pleated sheet. The mech. response of α-keratin has been extensively studied and shows linear Hookean, yield and post-yield regions, and in some cases, a high reversible elastic deformation. Thus, they can be also be considered 'biopolymers'. On the other hand, β-keratin has not been investigated as comprehensively. Keratinous materials are strain-rate sensitive, and the effect of hydration is significant.Keratinous materials exhibit a complex hierarchical structure: polypeptide chains and filament-matrix structures at the nanoscale, organization of keratinized cells into lamellar, tubular-intertubular, fiber or layered structures at the microscale, and solid, compact sheaths over porous core, sandwich or threads at the macroscale. These produce a wide range of mech. properties: the Young's modulus ranges from 10 MPa in stratum corneum to about 2.5 GPa in feathers, and the tensile strength varies from 2 MPa in stratum corneum to 530 MPa in dry hagfish slime threads. Therefore, they are able to serve various functions including diffusion barrier, buffering external attack, energy-absorption, impact-resistance, piercing opponents, withstanding repeated stress and aerodynamic forces, and resisting buckling and penetration.A fascinating part of the new frontier of materials study is the development of bioinspired materials and designs. A comprehensive understanding of the biochem., structure and mech. properties of keratins and keratinous materials is of great importance for keratin-based bioinspired materials and designs. Current bioinspired efforts including the manufg. of quill-inspired aluminum composites, animal horn-inspired SiC composites, and feather-inspired interlayered composites are presented and novel avenues for research are discussed. The first inroads into mol.-based biomimicry are being currently made, and it is hoped that this approach will yield novel biopolymers through recombinant DNA and self-assembly. We also identify areas of research where knowledge development is still needed to elucidate structures and deformation/failure mechanisms.
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      The formation of polycondensation-type thermoset resins from natural reactants such as citric and glutaric acid, as well as 1,3-propanediol and glycerol, was studied. Monitoring the mass loss by thermogravimetric anal. (TGA) allowed the rate consts. of the esterification to be calcd., which were in the order of 7·10-5 s -1 for glutaric acid and approx. twice as high for citric acid. However, the combination citric acid/glycerol was previously reported to froth up at high conversions, giving rise to foams, which makes the prepn. of compact engineering composites challenging. In light of this, we obsd. that shredded poultry feathers not only increased the conversion and the reaction rate of the combination citric acid/glycerol, but increasing the amt. of feathers continuously decreased the no. of visible bubbles. The addn. of 20 wt% of feathers completely prevented the previously reported frothing and gave rise to compact materials that were macroscopically free of defects. Besides this, the addn. of feathers also improved the fire-retardant properties. The tensile properties of the first specimens are still rather low (σ = 11.6 N/mm2, E = 750 N/mm2), but the addn. of poultry feathers opens a new path for green thermoset resins.
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      A new kind of eco-friendly chicken-feather protein-based phosphorus-nitrogen-contg. flame retardant was synthesized successfully with chicken-feather protein, melamine, sodium pyrophosphate, and glyoxal. And its structure was characterized by Fourier transform IR spectroscopy, and the thermogravimetry of the agent was analyzed. Then the flame retarding performances of the chicken-feather protein-based flame retardant and in combination with the borax and boric acid in application to a woven cotton fabric were investigated by the vertical flammability test and limited oxygen index test. In addn., the surface morphologies of the treated and untreated fabrics were conducted by the scanning electron micrographs (SEM), and the thermogravimetric analyses of the treated and untreated cotton were explored, and the surface morphologies of char areas of the treated and untreated fabrics after burnt were tested by the SEM. The results showed that the flame retardancy of the cotton fabric treated by the chicken-feather protein-based flame retardant in combination with borax and boric acid was improved further, and the combination of the chicken-feather protein-based flame retardant and borax and boric acid could facilitate to form a homogeneous and compact intumescing char layer, and the combination of them plays a good synergistic effect in the improvement of the flame retardancy of the treated cotton fabric. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40584.
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      An intumescent flame retardant working with various combinations of acid source, blowing agent, and char former promises high performance and low toxicity. However, the research for more potent functional constituents and their best combination is still extremely important. Here we report on a novel way to use keratinous fibers as the host material for creating an effective flame retardant. A simple soln.-based treatment to implant amine phosphate and phosphoric acid in the fiber through sequential monomer infiltration is found to be significantly effective for applying flame retardancy and reducing the flammability of polymeric materials. After the flame-retardant fiber modification, polypropylene (PP) shows significantly improved flame retardancy to achieve V-0 grade and >70% reduced peak heat release rate in vertical burning and cone-calorimeter tests, resp. We expect this strategy of converting the low-grade keratinous fiber to valuable flame-retardant material to become a novel and attractive soln. for achieving fire safety, value addn., and ecofriendly recycling goals at the same time.
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      A review with 47 refs. on the use of various reagents to phosphorylate proteins. Attention is also given to covalent attachment of low-mol.-wt. organophosphorus compds. to proteins. The nature of the phosphate linkages involved and the effects of phosphorylation on the functional properties, as well as on the in vitro and in vivo digestion of the proteins, are discussed. Of the phosphorylating reagents tested so far, only POCl3 and Na trimetaphosphate might prove economical and practical reagents for large-scale application.
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      The pyrolysis characteristics of three main components (hemicellulose, cellulose and lignin) of biomass were investigated using, resp., a thermogravimetric analyzer (TGA) with differential scanning calorimetry (DSC) detector and a pack bed. The releasing of main gas products from biomass pyrolysis in TGA was online measured using Fourier transform IR (FTIR) spectroscopy. In thermal anal., the pyrolysis of hemicellulose and cellulose occurred quickly, with the wt. loss of hemicellulose mainly happened at 220-315° and that of cellulose at 315-400°. However, lignin was more difficult to decomp., as its wt. loss happened in a wide temp. range (from 160 to 900°) and the generated solid residue was high (∼40%). From the viewpoint of energy consumption in the course of pyrolysis, cellulose behaved differently from hemicellulose and lignin; the pyrolysis of the former was endothermic while that of the latter was exothermic. The main gas products from pyrolyzing the three components were similar, including CO2, CO, CH4 and some orgs. The releasing behaviors of H2 and the total gas yield were measured using Micro-GC when pyrolyzing the three components in a packed bed. Hemicellulose had higher CO2 yield, cellulose generated higher CO yield, and lignin owned higher H2 and CH4 yield. A better understanding to the gas products releasing from biomass pyrolysis could be achieved based on this in-depth investigation on three main biomass components.
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