Chemical Finishing of Bast Fibers and Woods Using Hydrolyzed Keratin from Waste Wool or Down

Chemical Finishing of Bast Fibers and Woods Using Hydrolyzed Keratin from Waste Wool or Down

Kawahara, Yutaka

ABSTRACT

Chemical treatments using hydrolyzed keratin from waste wool or down are effective for enhancing the tensile properties of bast fibers. Treatment with hydrolyzed down keratin is also useful for suppressing shrinkage in the course of drying waterlogged archaeological wood excavated from sites. This novel treatment shows some degree of probable use for conservation treatments of such waterlogged woods.

Extensive studies have reported on enhancing the physical properties of cotton fibers, but little attention has focused on the chemical modification of jute fibers. Recently, jute fibers were used as reinforced composite materials in Germany. Gassan and Bledzki [1] treated jute fibers with a dense caustic soda solution under isometric conditions to enhance their tensile properties. However, using dense caustic soda is unsound environmentally, and controlling isometric conditions with dense caustic soda is not easy. In this paper, we propose a novel chemical modification of jute fibers with a neutral keratin solution under slack conditions. The processing is convenient compared with the dense caustic soda treatment.

We have used this chemical modification as a conservation treatment for archaeological wood excavated from sites, since a dense neutral stable keratin solution can be easily prepared. Such archaeological wood is usually found in a biodegraded and waterlogged condition. Biodegradation is due to microorganic activities in the soil, which thin and degrade the cell walls of wood, creating a porous structure. Water in the soil penetrates into the pores, and then the wood becomes waterlogged and fragile. However, wood that has just been excavated from an archaeological site is no different than normal wood because the water in the pores keeps the wood’s appearance unchanged. As the excavated wood dries, however, extensive shrinkage leads to the occurrence of numerous cracks on the surface. Accordingly, excavated wood is always stored in water until it can be treated with a suitable chemical for conservation.

The key process in conservation treatment is the exchange of water in the pores of the wood for a chemical that will not further attack the biodegraded cell walls. Conservation treatment with polyethylene glycol (PEG) is most popular in Japan. PEG shows high accessibility to water and can be substituted for water in wood by increasing the concentration of PEG in the treatment bath. PEG-treated woods have high stability against shrinkage. From an environmental point of view, however, the PEG method has some problems, which must be overcome: the treatment is accompanied by the production of a harmful organic acid [2]. In addition, there is the cost of the waste PEG solution.

This article reports a novel chemical finishing treatment of bast fibers for enhancing tensile properties using keratin from waste wool or down as the biomass resource. This treatment is also useful for conserving waterlogged archaeological wood.

Two kinds of bast fibers were prepared-commercial jute (Corchorus olitorius) fibers and the fibers from Japanese wisteria (Wisteria floribunda). (Jute fibers were provided by Professor A. K. Bledzki at Kassel University in Germany.) The wisteria fibers were retted with caustic soda solution, and the bast fibers were dewaxed in ethanol-benzene (1:1) for 24 hours.

Waste duck down was dissolved in 1N NaOH at 70°C for 3 hours, then neutralized with acetic acid. A commercial keratin solution from wool (55% concentration, 25,000-35,000 Mw) was purchased from Ichimaru Pharcos Co. Ltd. The bast fibers were immersed in a keratin solution at 40-45°C for 20 minutes, then rinsed with water.

Waterlogged specimens were first impregnated with a 10% keratin solution prepared from waste down at 60°C for 3 days. The concentration of the keratin solution was increased from 10 to 40%, and a similar impregnation procedure was repeated at the same temperature and for the same period. Wood was also treated with a conventional PEG treatment (4000 Mw). The specimens were first impregnated with a 30% PEG solution at 60°C for 3 days, then the PEG concentration was increased by 20 or 30% to 100%, and a similar impregnation procedure was repeated at the same temperature and for the same period. The treated specimens were rinsed with water and dried at ambient temperature, shaded against light.

Tensile tests were made of the bast fibers at 20°C and 65% RH using an automatic Tensilon tester (A&D, type STA-1150). The gauge length was 20 mm and the cross-head speed was 1 mm/min. Values were the average of 20-30 tests.

Treated wood specimens dried at ambient temperature for 40 days were irradiated with ultraviolet rays from fluorescent lamps (20 W × 2, [lambda] = 253.7 nm) located over 30 mm from their surfaces.

Results and Discussion

The tensile properties of keratin-treated bast fibers are listed in Table I. Jute fibers were treated with a 10% keratin solution from down, and the fibers from Japanese wisteria were treated with a 55% keratin solution from wool. The tensile properties were enhanced for both fibers. T-tests revealed that the differences in tensile strength are significant at a confidence level of 99%. Gassan and Bledzki [1] treated jute fibers with dense caustic soda solution under isometric conditions to enhance their tensile properties. The treatment we have developed in this study can be performed with a neutral keratin solution under slack conditions. The processing is more convenient compared with the dense caustic soda treatment, but the effect of the treatment on the tensile properties of jute fibers is marked for the dense caustic soda treatment under isometric conditions.

When chemical modifications are made to bast fibers, the cell structure should be taken into account: that is, the molecular weight of keratin will influence the mechanisms of attachment to plant fiber cells. It is well known that PEG with a molecular weight larger than 3000 cannot penetrate into the cell walls of wood [8]. The molecular weight of chicken down keratin is no more than about 10,000 [5], so it is probable that the down keratin used to modify the jute fibers becomes small enough molecularly through hydrolysis to penetrate into the cell walls. As for wisteria fibers, the molecular weight of wool keratin used for modification is in the range of 25,000-35,000, so it seems that only the wool keratin coats the cell walls of the wisteria fibers. Therefore, we can conclude at least that the reinforcing mechanism of keratin will differ according to molecular weight.

Ikuta et al. [3] used keratin from wool to reinforce the interfacial strength between glass fibers and resin in glass fiber composites, and reported that keratin showed positive adhesion to a thermoset-type epoxy and unsaturated polyester resins. If the keratin-treated jute fibers are used to produce natural fiber composites with those resins, marked enhancement in tensile properties can be expected compared with jute fiber composites without keratin treatment. However, further investigations, including the chemical reaction of keratin to cellulosic materials, are needed and will be reported elsewhere.

The keratin from down prepared through the hydrolysis procedure reinforces the jute fiber cells. Hence, when wood is treated with the solution of keratin from down, a bulking effect is expected. Furthermore, the solution is stable even though the concentration reaches 40%. The impregnating effect is also promising. We used this chemical modification for the conservation treatment of archaeological wood excavated from sites.

From several preliminary experiments, Figure 1 shows the typical drying behavior of the wood specimens at ambient temperature. The weights of untreated specimens drastically decrease in no more than 10 days. However, for the specimens treated with keratin, drying proceeds slowly and the weights become constant in more than 40 days. Rapid drying causes numerous cracks on the surfaces of specimens, and the hydrophilic nature of keratin suppresses rapid drying.

Table II shows shrinkage measured for the specimens in Figure 1. There is extensive shrinkage for the untreated specimens, especially in the tangential direction, but for the treated specimens, there are only slight expansions in both directions. In order to investigate the influence of the composition of protein on shrinkage, we prepared a 10% silk fibroin solution using a similar procedure. We did not measure the molecular weight of hydrolyzed fibroin but the molecular weight of silk fibroin decreased to lower than 200 when silk fibroin was hydrolyzed with hydrochloric acid [6]. Thus, it seems that the molecular weight of silk fibroin becomes small enough through the hydrolysis procedure and probably penetrates into the cell walls. We also expected a bulking effect. Waterlogged specimens from Japanese evergreen oak were used, and the shrinkage was as follows: in the tangential direction, 54.8% for the untreated and 30.7% for the treated, and in the radial direction, 19.2% for the untreated and 2.1% for the treated. Silk fibroin also suppressed the shrinkage of waterlogged wood. However, silk fibroin cannot be used for conservation treatments. For such treatments, it is necessary to prepare a stable, high concentration solution of chemicals for impregnation. Dissolved silk fibroin tends to settle when the solution concentration exceeds 10%, but the solution of keratin from down is stable even though it reaches 40%. The difference in the stability of the solutions from these two proteins is probably due to the fact that more than 75% of silk fibroin consists of glycine and alanine [4], while more than 20% of down consists of polar amino acids such as glutamic acid, aspartic acid, and arginine, which are less in silk fibroin [7].

For conservation treatments, we must use chemicals that cannot be attacked by mold. The wood specimens treated with the keratin solution have not been attacked by mold for more than 1 year. The keratin solution has also been stable and has never been attacked by mold. Furthermore, the hue of the wood specimens is not affected by the conservation treatment with keratin.

Archaeological wood subjected to the conservation treatment will be exhibited in museums, where the wood in showcases is lighted by various light sources. Few investigations, however, have been made of photodegradation during a long-term exhibition. Therefore, we observed the photodegradation behavior of treated archaeological wood against uv rays. Figure 2 shows the surfaces of treated wood specimens before and after UV irradiation. The surface of the PEG-treated wood (Figure 2a) was covered with PEG, which seeped through the wood due to absorption of vaporized water in the humid air because PEG has high accessibility to water. The conventional conservation method with PEG was not effective at protecting wood against UV rays at all (see Figure 2b). There was serious destruction of the wood’s cellular structure. For the surfaces of wood specimens treated with keratin, however, the cellular structure was almost completely retained after the treatment and was hardly touched by UV irradiation (see Figures 2c and d).

As mentioned before, we have confirmed that treatment with the aqueous solution of keratin from down developed in this study has some degree of probable use for conservation treatment of waterlogged archaeological wood excavated from sites.

Conclusions

Chemical treatments with hydrolyzed keratin from waste wool or down are effective at enhancing the tensile properties of bast fibers. The treatment with hydrolyzed down keratin is also useful for suppressing shrinkage in the course of drying of waterlogged archaeological wood excavated from sites. This novel treatment can probably be used for conservation treatment of such wood.

ACKNOWLEDGMENTS

We thank professor A. K. Bledzki at Kassel university (Germany) for providing jute fibers. This work was supported by the TOSTEM Foundation for Construction Materials Industry Promotion in Japan.

Literature Cited

1. Gassan, J., and Bledzki, A. K., Alkali Treatment of Jute Fibers: Relationship between Structure and Mechanical Properties, J. Appl. Polym. Sci. 71, 623-629 (1999).

2. Glastrup, J., Degradation of PEG-A Review, in “Proc. 6th ICOM Group on Wet Organic Archaeological Materials Conference,” York, U.K., 1996, pp. 377-382.

3. Ikuta, N., Onishi, A., and Yanagawa, A., Uses of Keratin Protein as a Film Former for Glass Fiber Composites, in “Proc. 3rd Joint Canada-Japan Workshop on Composites,” Kyoto, 2000, pp. 55-59.

4. Kirimura, J., Studies on Amino Acid Composition and Chemical Structure of Silk Protein by Microbiological Determination, Bull. Sericul. Exp. Sta. 17,447-514 (1962).

5. Koga, J., Feather Keratin, Its Science and Utilozation, Selkatsu Zoukei 43, 18-26 (1998).

6. Lu, A., Aral, M., and Hirabayashi, K., Production of Tussah Silk Powder by Hydrochloric Acid Hydrolysis, J. Serie. Sci. Jpn. 65, 392-394 (1996).

7. Schroeder, W. A., and Kay, L. M., The Amino Acid Composition of Certain Morphologically Distinct Parts of White Turkey Feathers, and of Goose Feather Barbs and Goose Down, J. Am. Chem. Soc. 77, 3901-3908 (1955).

8. Yamaguchi, T., Ishimaru, Y., and Urakami, H., Effect of Temperature on the Dimensional Stability of Wood with Polyethylene Glycol, 1: Bulking Effect, Mokuzai Gakkaishi 45, 434-440 (1999).

Manuscript received February 11, 2002; occupied April 12, 2002.

YUTAKA KAWAHARA,1 RIE ENDO, AND TERUO KIMURA

Division of Advanced Fibro-Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585 Japan

1 To whom correspondence should be addressed: email: kawahara@ipc.kit.ac.jp

Copyright Textile Research Institute Feb 2004

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