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A new consolidation system for aged silk fabrics: Effect of reactive epoxide-ethylene glycol diglycidyl ether
Reactive & Functional Polymers 73 (2013) 168–174
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A new consolidation system for aged silk fabrics: Effect of reactive epoxide-ethylene glycol diglycidyl ether Dun Huang a, Zhiqin Peng a,b,⇑, Zhiwen Hu a,b,⇑, Shuo Zhang a, Jun He a, Lifen Cao a, Yang Zhou c,d, Feng Zhao c,d a
Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, China Research Laboratory of Cultural Relic Conservation Materials, Zhejiang Sci-Tech University, Hangzhou 310018, China c China National Silk Museum, Hangzhou 310002, China d Key Scientific Research Base of Textile Conservation, State Administration for Cultural Heritage, Hangzhou 310002, China b
a r t i c l e
i n f o
Article history: Received 11 June 2012 Received in revised form 21 August 2012 Accepted 28 August 2012 Available online 5 September 2012 Keywords: Ethylene glycol diglycidyl ether Aged silk fabric Consolidation Fibroin Reactive epoxide
a b s t r a c t A new consolidation system for fragile ancient silk fabrics by fibroin with the support of ethylene glycol diglycidyl ether (EGDE) was developed in our group. To figure out the mechanism and the effect of EGDE in the system, aged silk fabrics treated with EGDE have been investigated in this paper. Silk fabrics were artificially aged in sodium hydroxide aqueous solution to simulate fragile ancient silk fabrics. The aged silk fabrics were treated with EGDE aqueous solution by spraying. The resultant silk fabrics were systematically investigated by tensile test, thermogravimetric analysis (TGA), thermal ageing resistance test, attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR), solid-state 13C cross polarization/magic angle spinning nuclear magnet resonance (13C CP/MAS NMR) and amino acid analysis (AAA), etc. Results indicate that the breaking stress and strain of the treated silk fabrics increase more than four and two times, respectively. The maximum decomposition temperature of the treated silk fabrics is much higher than that of the aged silk fabrics. The treated silk fabrics exhibit a better thermal ageing resistance than the aged silk fabrics. Chemical interactions occurred between EGDE and silk fibroin molecules in silk fabrics. This work provides useful information for the protection of historic silk fabrics. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Historic silk fabric is one of the most precious Chinese cultural heritages and of great value in science, art and history. Due to its protein composition and the existence of amorphous region in its structure [1], silk fabric is easily deteriorated by water, light, heat and microorganisms [2–7]. So the consolidation of the ancient silk fabrics is of great importance for further display and research. Methods including weave, mount, silk screen, resin, adhesive, Parylene C and graft copolymerization [8–12] are adopted commonly and efficiently to consolidate historical silks. But the applications are limited by three unresolved problems in reality. One is inadequate consolidation effect: it is found that silk fabrics consolidated by Parylene C became even more aged after undergoing light over the years [13]. The other one is limited performing conditions and potential harm to silks in the long run. Bacteria cellulose film con-
⇑ Corresponding authors. Address: Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, China. Tel.: +86 134 29196526; fax: +86 571 87321350 (Z. Peng), tel.: +86 136 00518471 (Z. Hu). E-mail addresses: [email protected] (Z. Peng), [email protected] (Z. Hu). 1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.reactfunctpolym.2012.08.019
solidation developed recently by Wu et al. [14] shows great reinforcement on ancient silk fabrics, with tensile strength of the consolidated silk fabric improved about two times. However, target bacteria should be cultured and perished under strict conditions in their method, which are inconvenient to perform and need long time to be done. In addition, the improper bacteria or incomplete removal of bacteria will bring potential harm to historic silk as time elapses, considering the different compositions of silk with cellulose. The last one is inacceptable appearance and handle change. In the case of Parylene C consolidation, the consolidated silk fabrics even suffered from too high fabric stiffness, color difference or yellowing. Therefore, we have developed a new consolidation system for those precious silk cultural heritages, which solves the above problems [15–19]. Fibroin, which is homologous and compatible with silk, was used to consolidate aged silk with the support of ethylene glycol diglycidyl ether (EGDE). This new consolidation system was named as silk fibroin/EGDE (SF/EGDE) system in our study. The consolidation was done by spraying at room temperature, which is easy to perform and even suitable to reinforce ancient silk fabrics in excavation place. The tensile strength and elongation at breaking of the SF/EGDE system consolidated silk fabrics could be increased to about ten times and four times, respectively; while
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the color difference and fabric stiffness of the consolidated silk fabric changed little, compared with those of the aged silk fabric [18]. This significant improvement of the mechanical property attracts great attention in the field of cultural heritage protection. The improvement probably results from the triadic interactions of EGDE, fibroin and silk fabric. However, the detailed mechanism is unclear. Therefore, as a beginning work to figure out the consolidation mechanism, this paper aims to focus on the role of EGDE on the mechanical property improvement of the aged silk fabrics. EGDE is an epoxide with bifunctional groups, which has high reactivity with amines, alcohols, phenols, carboxylic acids and thiols [20]. Thus, EGDE can be used to crosslink methacrylic acid, pyrazole, triazole and imidazole to prepare three-dimensional polymers [21]. Due to its reactivity with amino groups, EGDE is used to prepare DNA network gels [22–25]. EGDE also has an application in the modification of polyrotaxane fibers [26], silk fibers [27,28] and silk fabrics [20,29] to improve their tenacity, elongation at break, thermal stability, moisture retygain, crease recovery, etc. Up to now, no reports are focused on the effect of EGDE on the consolidation of aged silk fabrics to our knowledge. Therefore, the collateral purpose of this paper is to figure out whether EGDE has the reactivity with aged silk fabric and the possible interactions. 2. Experimental 2.1. Materials Silk fabrics was provided by Zhejiang Misai Silk Co. Ltd., China; sodium hydroxide (AR) was provided by Hangzhou Gaojing Fine Chemical Co. Ltd., China; ethylene glycol diglycidyl ether (EGDE) was obtained from Nagase ChemteX Co. Ltd., Japan. All the water used in the experiment is distilled water. 2.2. Sample preparation 2.2.1. Preparation of artificial aged silk samples Silk fabrics (20 cm 5 cm) were immersed in 50 g/L sodium hydroxide aqueous solution with a bath ratio of 50:1 at 35 °C for 9 h. Then the fabrics were washed to neutral pH and dried at room temperature. Artificial aged silk samples were then obtained. The artificial aged samples were called aged samples for short in the following context. 2.2.2. Preparation of sample treated with EGDE The aged silk fabrics were smoothly put on a piece of textile, such as silk fabric, and then the aged sample and textile were supported by a net frame. EGDE solution with the concentration of 2.5 wt% was gently sprayed on the aged silk fabrics at ratio of piece/10 mL by a sprayer at 25 °C. After 2 days’ natural drying, the treated samples were dipped into water to remove unreacted EGDE by exchanging water every 4 h in 2d. Then the samples were dried at room temperature and ready for tests.
measured by a Konica Minolta Spectrophotometer (CM-700d, Japan) with setting aged sample as standard color. The fabric stiffness of the samples (15 cm 2.5 cm) was measured with an electronic stiffness tester (LLY-01, China) at 20 °C and 65%RH. Thermal properties were carried out under nitrogen by thermogravimetric analysis (TGA) (Pyris 1, America) programmed under isothermal conditions with the temperature from 50 °C to 800 °C at the rate of 20 °C/min. The morphologies of surface and fracture interface of the samples were observed using a field emission scanning electron microscopy (FESEM) instrument (S4800, Japan). The samples were mounted on a copper plate and sputter-coated with gold layer for 30 s. Fourier transform infrared spectroscopy (FTIR) spectra of EGDE and Attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR) spectra of silk fabrics were obtained by a spectrometer (Nicolet 5700, America) in the range of 750– 4000 cm 1. The EGDE liquid was coated on potassium bromide (KBr) pellet prior to data collection. The silk fabrics were put on the spectrometer directly after folding several times to get better information. 13C nuclear magnet resonance(13C NMR) spectrum of EGDE and solid-state 13C cross polarization/magic angle spinning nuclear magnet resonance (13C CP/MAS NMR) spectra of silk fabrics were got by a 400 MHz spectrometer (AVANCE AV, Switzerland) with the range of 0–250 ppm. The silk fabrics were prepared in the same way as that of made for ATR–FTIR test. The silk fabrics were ground into powders and EGDE was dissolved into D2O before testing. Amino acid analysis (AAA) was used to study the kinds and contents of amino acids involved in the reaction. The samples were prepared after being immerged in EGDE aqueous solution for 6, 24, and 48 h, then washed and dried for tests. The dried samples were hydrolyzed with 6 M hydrochloric acid aqueous solution for 24 h at 110 °C. The hydrated solution was dried by nitrogen. Then the hydrolyzate together with internal standard substance were dissolved in derivative liquid and detected by AAA equipment (Waters 2695, America).
3. Results and discussion 3.1. Physical properties The stress–strain curves of the aged sample and sample treated with EGDE are shown in Fig. 1. The breaking stress and breaking strain of aged sample are about 3.6 MPa and 11.2%, respectively; while those of treated sample are about 17.9 MPa and 30.1%, respectively. Clearly, the strength and toughness of the treated sample are much higher than those of the aged sample, with the
2.2.3. Preparation of samples for thermal ageing resistance test The aged silk fabrics and silk fabrics treated with EGDE were exposed to 100 °C heating for 18 days in an oven. Then the samples with thermal treatment were ready for test. 2.3. Measurement Mechanical properties of the samples were tested by a multifunction tensile tester (KES-G1 Type, Japan) at 20 °C and 65%RH with the clamping length of 40 mm and elongation rate of 3 mm/ min. The samples with the size of 5 0.5 cm were glued on a paper frame and then mounted on the tensile tester. Color difference was
169
Fig. 1. Typical stress–strain curves of (a) aged and (b) treated samples.
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Table 1 Physical properties of aged and treated samples. Breaking strain (MPa) Aged sample Treated sample
Breaking stress (%)
3.6 ± 0.3
11.2 ± 0.3
17.9 ± 1.5
30.1 ± 0.6
Color differences /
Fabric stiffness (mNcm) 0.43 ± 0.08
0.24 ± 0.12
0.46 ± 0.06
breaking stress and strain increased more than four and two times, respectively. The consolidation effect of EGDE is quite obvious on the aged samples but not as good as that of the SF/EGDE system, which shows nine times increasing of the strength [18]. This indicates that EGDE plays a remarkable role in improving the mechanical properties of the treated silk fabrics. But the strength of the silk fabrics treated with EGDE is lower than that of the silk fabrics treated with SF/EGDE system, which illustrates that EGDE is not the only factor to consolidate the aged silk fabric in the SF/EGDE consolidation system. As shown in Table 1, the color difference between the treated and aged sample is about 0.24. The color difference is less than 1.00 and it is believed to be acceptable in the conservation of historic textiles [14]. It also shows that the fabric stiffnesses of aged and treated sample are about 0.43 mNcm and 0.46 mNcm, respectively, indicating almost no change in handle with the EGDE treatment. Therefore, similar with the SF/EGDE system, the EGDE treatment also does not change the appearance and handle of the silk fabrics. 3.2. FESEM observation The fractural morphologies of the aged and treated sample are shown in Fig. 2. The integral fractural morphology of aged sample (Fig. 2a) shows that the distribution of fibers’ breaking lengths lies in a narrow range. The magnifying morphology of aged fiber
(Fig. 2b) shows a smooth fracture plane, seemed to be brittle rupture. It indicates that the inner structure of the sample is relatively uniform, with same endurance to external force. The fractural morphology of treated sample is quite different from that of aged sample, as shown in Fig. 2c and d. The fractural lengths of treated sample are in a large range (Fig. 2c) and the fracture plane of treated fiber is out of flatness (Fig. 2d), seemed to be ductile rupture. This fractural morphology is a sign that the inner structure of the treated sample is not uniform, with different capability to endure external force. Generally, material with ductile rupture possesses high strength and toughness than that with brittle rupture in the same fiber material. The different fracture morphologies illustrated from Fig. 2 agree with the results of tensile test. 3.3. Thermal property The thermal behavior of aged and treated samples are shown in Fig. 3 and the decomposition temperature range, maximum decomposition temperature and percentages of residual mass are given in Table 2. As shown in Fig. 3, the thermal decompositions of both the aged and treated samples are all taken place in three stages. All the TGA and DTG curves show a small mass loss under 100 °C (the first stage) attributed to the removal of absorbed water. In the second stage (200 °C–500 °C), both the aged and treated sample go through a distinct weight loss. The decomposition starts at 257 °C and finishes at 480 °C for the aged sample, which increases to 264 °C and 499 °C, respectively, for the treated sample. The maximum decomposition rate appears at 360.42 °C for the aged sample, but it increases to 375.55 °C for the treated sample, which is 15 °C higher than that of the aged sample. In the third stage, decomposition of the aged sample starts at 498 °C and finishes at 700 °C, and the maximum decomposition rate appears at 617.66 °C. In contrast, decomposition of the treated sample starts at 499 °C and finishes at 725 °C. The maximum decomposition rate
Fig. 2. FESEM of (a and b) aged and (c and d) treated samples.
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Fig. 3. TGA and DTA curves of (a) aged and (b) treated samples. Table 2 Thermal analysis data of aged and treated samples.
Aged sample Treated sample a
IIa III II III
Decomposition temperature range (°C)
Maximum decomposition temperature (°C)
Residual mass (%)
257–480 498–700 264–499 499–725
360.42 617.66 375.55 639.41
40.6 1.1 40.8 0.3
II, III refers to the second and third stage, respectively.
appears at 639.41 °C, which is about 22 °C higher than that of aged sample. All the above mentioned data show that the treated sample has better thermal properties than the aged sample. In addition, it can also be found from Fig. 3 that the DTG peaks of the treated sample are wider than those of the aged sample in the second and third stages. This indicates that the decomposition rate changes slower in the treated sample than that of the aged sample, which also proves that the treated sample has higher thermal stability.
3.4. Thermal ageing resistance Physical properties of the aged and treated sample before and after thermal ageing were tested to evaluate their properties of thermal ageing resistance. Typical stress–strain curves are shown in Fig. 4. The breaking stress, color difference and fabric stiffness before and after thermal ageing are listed in Table 3. It was found that the breaking stress of aged samples decreases from 3.6 MPa to 1.9 MPa; while that of treated samples decreases from 17.9 MPa to
13.8 MPa, after thermal treatment. The treated samples seem to have a better thermal ageing resistance in stress loss with the decrement of 22.9%, which is lower than that of the aged sample (47.9%). The color difference values of the aged and treated sample after thermal treatment are about 5.29 and 4.21, respectively. It indicates that the treated sample has a better thermal ageing resistance in color change than the aged sample after thermal treatment. The values of fabrics stiffness of aged and treated sample are about 0.54 mNcm and 0.55 mNcm, respectively. It indicates that the thermal treatment has same influence on the aged and treated sample.
3.5. Interaction between silk fabrics and EGDE Fourier transform infrared spectroscopy (FTIR) has been widely used to detect the molecular reaction and conformation transition of silk fibroin [30–32]. The FTIR spectrum of EGDE is shown in Fig. 5. The absorption at 1101 cm 1 attributes to the ether bond stretching vibration. The bands at 910 cm 1 and 856 cm 1 arise from the stretching vibrations of the epoxide groups in EGDE. Fig. 6 shows the ATR–FTIR spectra of the aged and treated samples. The peak at 1070 cm 1 is higher in treated sample (Fig. 6b) than that in aged sample (Fig. 6a). This higher absorption should result from the influence of the absorption of 1101 cm 1 in EGDE. It is a sign that there is ether bond in the treated sample. However, the bands at 910 cm 1 and 856 cm 1 representing the stretching vibrations of epoxide groups in EGDE are not shown in the spectrum of EGDE treated sample. Herein, it could be concluded that there is ether bond of EGDE in the treated sample, but the epoxide groups
Fig. 4. Typical stress–strain curves of (A) aged and (B) treated samples. (a) sample before thermal ageing and (b) sample after thermal ageing.
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Table 3 Physical properties of the aged and treated sample before and after thermal ageing. Breaking strain (MPa) Aged sample Aged sample after thermal ageing Treated sample Treated sample after thermal ageing
Color differences
Fabric stiffness (mNcm)
3.6 ± 0.3 1.9 ± 0.3
/ 5.29 ± 0.61
0.43 ± 0.08 0.54 ± 0.12
17.9 ± 1.5 13.8 ± 1.6
0.24 ± 0.12 4.21 ± 0.53
0.46 ± 0.06 0.55 ± 0.09
Fig. 7.
13
C NMR spectrum of EGDE.
Fig. 5. FTIR spectrum of EGDE.
Fig. 8.
13
C CP/MAS NMR spectrum of (a) aged and (b) treated samples.
Table 4 Amino acid composition (mol%) of aged and treated samples.
Fig. 6. ATR–FTIR spectrum of (a) aged and (b) treated samples.
were gone, which means the epoxide groups should be reacted with the silk fabric. Moreover, no physically attached EGDE was left, otherwise its absorptions at 910 cm 1 and 856 cm 1 would be shown. Amide I can be used to detect whether there is a conformation transition in silk fibroin molecules [32] after the silk fabrics treated with EGDE. The aged and treated sample display the same peak at 1623 cm 1, which is assigned to b-sheet conformation. No difference is shown between the peaks. It seems that the crystallinity degree of the fabrics before and after EGDE crosslinking was not changed; or even if it changed, the change could not be tested by ATR–FTIR. This indicates that the treatment did not have much influence on the conformation of b-sheet in the silk fibers, which means that the treatment did no harm to the inner structure of the silk fabrics. 13 C CP/MAS NMR was also used to verify the interaction between EGDE and silk. The 13C NMR spectrum of EGDE is shown in Fig. 7 and the spectra of aged and treated sample are shown in
Amino acid
Aged sample
Treated for 6h
Treated for 24 h
Treated for 48 h
Asp Ser Glu Gly Arg Thr Ala Pro Cys Tyr Val Met Lys Ile Leu Phe
0.80 ± 0.06 12.41 ± 0.1 0.77 ± 0.04 51.02 ± 1.99 0.42 ± 0.02 0.71 ± 0.02 24.22 ± 0.11 0.51 ± 0.01 0.27 ± 0.03 6.59 ± 0.2 2.71 ± 0.03 0.03 ± 0.01 0.18 ± 0.01 0.60 ± 0.04 0.44 ± 0.04 0.65 ± 0.02
0.71 ± 0.03 12.43 ± 0.13 0.75 ± 0.07 52.73 ± 0.27 0.40 ± 0.01 0.72 ± 0.01 24.34 ± 0.45 0.49 ± 0.01 0.25 ± 0.04 2.74 ± 0.19 2.82 ± 0.04 0.06 ± 0.01 0.15 ± 0.01 0.58 ± 0.01 0.50 ± 0.03 0.65 ± 0.01
0.75 ± 0.04 12.32 ± 0.08 0.71 ± 0.02 53.05 ± 0.26 0.41 ± 0.02 0.69 ± 0.01 24.26 ± 0.28 0.50 ± 0.00 0.32 ± 0.05 1.69 ± 0.15 2.82 ± 0.02 0.05 ± 0.02 0.11 ± 0.02 0.62 ± 0.05 0.50 ± 0.04 0.66 ± 0.03
0.73 ± 0.06 12.62 ± 0.16 0.73 ± 0.03 51.37 ± 0.63 0.37 ± 0.05 0.71 ± 0.02 25.68 ± 0.79 0.47 ± 0.03 0.19 ± 0.05 1.01 ± 0.02 2.81 ± 0.06 0.05 ± 0.01 0.05 ± 0.01 0.62 ± 0.01 0.47 ± 0.03 0.53 ± 0.05
Fig. 8. It shows in Fig. 7 that there is a peak at 70.2 ppm attributing to the absorption of ether band. It can be seen from Fig. 8 that a new absorption at 69.5 ppm appears in the treated sample which presents the same absorption shown at 70.2 ppm in EGDE. It demonstrates that EGDE reacts with silk fabrics, because the sample is the same as that of made for ATR–FTIR test, which confirms the removal of physically attached EGDE. Amino acid analysis (AAA) can detect the content change of amino acids if they are reacted and the reaction products are hard
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Fig. 9. Schematic reaction of (a) Tyr and (b) Lys with EGDE.
to be hydrolyzed. So it can be used to detect which amino acid participates in the reaction. The contents of amino acids of aged sample and samples with different treating times are listed in Table 4. It shows that most of the contents of the amino acids are similar regardless of the treatment except tyrosine and lysine. The molar content of tyrosine in aged sample is about 6.59%, which decrease to 2.74% (treated for 6 h), 1.69% (treated for 24 h) and 1.01% (treated for 48 h). The molar content of lysine has the similar trend which decreases from 0.18% to 0.15% (treated for 6 h), 0.11% (treated for 24 h) and 0.05% (treated for 48 h). This changing tendency indicates that the contents of tyrosine and lysine are decreased with the increase of treating time, because of their participation in the reaction with EGDE in the process of consolidation. Silk fiber has a tendency of yellowing after exposed to sunlight and heat. It attributes to the presence of tyrosine and tryptophan residues in the fiber, which tend to form yellow chromophores with the effect of light or heat [33]. The decreased content of tyrosine residues in treated sample may result in fewer sites to be affected by thermal treatment. Therefore, the content change of tyrosine also provides a reason for the lower color change of treated sample than that of aged sample. EGDE consists of several methylenes with two epoxide groups at either end. The epoxide group is cyclic ether with three ring
atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Therefore, EGDE was easily reacted with phenolic hydroxyl group of the tyrosine and/or amino group of the lysine in its either end by opening of the strained rings during the treatment even at room temperature (25 °C). In this way, the degraded fibroin fibrils, keys to endure external force including heating, were cross-linked by the methylene ether bonding groups. Thus, the mechanical and thermal properties of the treated samples are greatly improved. The schematic reactions were presented in Fig. 9. On the other hand, these two amino acids locate mainly in the amorphous region of silk fibroin where degradation takes place preferentially [1,7,34]. So they tend to be exposed to reaction in the treatment rather than other amino acids such as alanine and glycine which locate in inside and crystalline region of the silk fibroin [1,35]. Therefore, stable chemical bonds are formed between silk fibroin molecules in silk fibers by the crosslink of EGDE in the amorphous region, which results in more stable inner structure of the treated sample than that of the aged sample, and thus it is responsible for the higher strength, toughness, thermal stability and thermal ageing resistance in the treated samples as discussed before.
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4. Conclusions Artificial aged silk fabrics treated with EGDE exhibit great improvements in mechanical, thermal properties and thermal ageing resistance without color and flexibility change. So, EGDE plays a positive role in the consolidation of aged silk fabrics. Chemical interactions between EGDE and silk fabric were verified and responsible for the mechanical property improvement in this study. EGDE has so high reactivity that it can be reacted with silk fabrics at room temperature by simple spraying, which can avoid heating and operational damages compared with other consolidations usually done at higher temperature with complicated operations. Therefore, EGDE could be selected as a potential for treatment of very fragile historic silk fabrics even at the excavating scene. Acknowledgments This research was financially supported by Special Funds for the Protection of Cultural Heritage in Zhejiang Province (No. 2011-202) and Nation Natural Science Foundation of China (No. 51102214). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
A new consolidation system for aged silk fabrics: Effect of reactive epoxide-ethylene glycol diglycidyl ether Dun Huang a, Zhiqin Peng a,b,⇑, Zhiwen Hu a,b,⇑, Shuo Zhang a, Jun He a, Lifen Cao a, Yang Zhou c,d, Feng Zhao c,d a
Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, China Research Laboratory of Cultural Relic Conservation Materials, Zhejiang Sci-Tech University, Hangzhou 310018, China c China National Silk Museum, Hangzhou 310002, China d Key Scientific Research Base of Textile Conservation, State Administration for Cultural Heritage, Hangzhou 310002, China b
a r t i c l e
i n f o
Article history: Received 11 June 2012 Received in revised form 21 August 2012 Accepted 28 August 2012 Available online 5 September 2012 Keywords: Ethylene glycol diglycidyl ether Aged silk fabric Consolidation Fibroin Reactive epoxide
a b s t r a c t A new consolidation system for fragile ancient silk fabrics by fibroin with the support of ethylene glycol diglycidyl ether (EGDE) was developed in our group. To figure out the mechanism and the effect of EGDE in the system, aged silk fabrics treated with EGDE have been investigated in this paper. Silk fabrics were artificially aged in sodium hydroxide aqueous solution to simulate fragile ancient silk fabrics. The aged silk fabrics were treated with EGDE aqueous solution by spraying. The resultant silk fabrics were systematically investigated by tensile test, thermogravimetric analysis (TGA), thermal ageing resistance test, attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR), solid-state 13C cross polarization/magic angle spinning nuclear magnet resonance (13C CP/MAS NMR) and amino acid analysis (AAA), etc. Results indicate that the breaking stress and strain of the treated silk fabrics increase more than four and two times, respectively. The maximum decomposition temperature of the treated silk fabrics is much higher than that of the aged silk fabrics. The treated silk fabrics exhibit a better thermal ageing resistance than the aged silk fabrics. Chemical interactions occurred between EGDE and silk fibroin molecules in silk fabrics. This work provides useful information for the protection of historic silk fabrics. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Historic silk fabric is one of the most precious Chinese cultural heritages and of great value in science, art and history. Due to its protein composition and the existence of amorphous region in its structure [1], silk fabric is easily deteriorated by water, light, heat and microorganisms [2–7]. So the consolidation of the ancient silk fabrics is of great importance for further display and research. Methods including weave, mount, silk screen, resin, adhesive, Parylene C and graft copolymerization [8–12] are adopted commonly and efficiently to consolidate historical silks. But the applications are limited by three unresolved problems in reality. One is inadequate consolidation effect: it is found that silk fabrics consolidated by Parylene C became even more aged after undergoing light over the years [13]. The other one is limited performing conditions and potential harm to silks in the long run. Bacteria cellulose film con-
⇑ Corresponding authors. Address: Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, China. Tel.: +86 134 29196526; fax: +86 571 87321350 (Z. Peng), tel.: +86 136 00518471 (Z. Hu). E-mail addresses: [email protected] (Z. Peng), [email protected] (Z. Hu). 1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.reactfunctpolym.2012.08.019
solidation developed recently by Wu et al. [14] shows great reinforcement on ancient silk fabrics, with tensile strength of the consolidated silk fabric improved about two times. However, target bacteria should be cultured and perished under strict conditions in their method, which are inconvenient to perform and need long time to be done. In addition, the improper bacteria or incomplete removal of bacteria will bring potential harm to historic silk as time elapses, considering the different compositions of silk with cellulose. The last one is inacceptable appearance and handle change. In the case of Parylene C consolidation, the consolidated silk fabrics even suffered from too high fabric stiffness, color difference or yellowing. Therefore, we have developed a new consolidation system for those precious silk cultural heritages, which solves the above problems [15–19]. Fibroin, which is homologous and compatible with silk, was used to consolidate aged silk with the support of ethylene glycol diglycidyl ether (EGDE). This new consolidation system was named as silk fibroin/EGDE (SF/EGDE) system in our study. The consolidation was done by spraying at room temperature, which is easy to perform and even suitable to reinforce ancient silk fabrics in excavation place. The tensile strength and elongation at breaking of the SF/EGDE system consolidated silk fabrics could be increased to about ten times and four times, respectively; while
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the color difference and fabric stiffness of the consolidated silk fabric changed little, compared with those of the aged silk fabric [18]. This significant improvement of the mechanical property attracts great attention in the field of cultural heritage protection. The improvement probably results from the triadic interactions of EGDE, fibroin and silk fabric. However, the detailed mechanism is unclear. Therefore, as a beginning work to figure out the consolidation mechanism, this paper aims to focus on the role of EGDE on the mechanical property improvement of the aged silk fabrics. EGDE is an epoxide with bifunctional groups, which has high reactivity with amines, alcohols, phenols, carboxylic acids and thiols [20]. Thus, EGDE can be used to crosslink methacrylic acid, pyrazole, triazole and imidazole to prepare three-dimensional polymers [21]. Due to its reactivity with amino groups, EGDE is used to prepare DNA network gels [22–25]. EGDE also has an application in the modification of polyrotaxane fibers [26], silk fibers [27,28] and silk fabrics [20,29] to improve their tenacity, elongation at break, thermal stability, moisture retygain, crease recovery, etc. Up to now, no reports are focused on the effect of EGDE on the consolidation of aged silk fabrics to our knowledge. Therefore, the collateral purpose of this paper is to figure out whether EGDE has the reactivity with aged silk fabric and the possible interactions. 2. Experimental 2.1. Materials Silk fabrics was provided by Zhejiang Misai Silk Co. Ltd., China; sodium hydroxide (AR) was provided by Hangzhou Gaojing Fine Chemical Co. Ltd., China; ethylene glycol diglycidyl ether (EGDE) was obtained from Nagase ChemteX Co. Ltd., Japan. All the water used in the experiment is distilled water. 2.2. Sample preparation 2.2.1. Preparation of artificial aged silk samples Silk fabrics (20 cm 5 cm) were immersed in 50 g/L sodium hydroxide aqueous solution with a bath ratio of 50:1 at 35 °C for 9 h. Then the fabrics were washed to neutral pH and dried at room temperature. Artificial aged silk samples were then obtained. The artificial aged samples were called aged samples for short in the following context. 2.2.2. Preparation of sample treated with EGDE The aged silk fabrics were smoothly put on a piece of textile, such as silk fabric, and then the aged sample and textile were supported by a net frame. EGDE solution with the concentration of 2.5 wt% was gently sprayed on the aged silk fabrics at ratio of piece/10 mL by a sprayer at 25 °C. After 2 days’ natural drying, the treated samples were dipped into water to remove unreacted EGDE by exchanging water every 4 h in 2d. Then the samples were dried at room temperature and ready for tests.
measured by a Konica Minolta Spectrophotometer (CM-700d, Japan) with setting aged sample as standard color. The fabric stiffness of the samples (15 cm 2.5 cm) was measured with an electronic stiffness tester (LLY-01, China) at 20 °C and 65%RH. Thermal properties were carried out under nitrogen by thermogravimetric analysis (TGA) (Pyris 1, America) programmed under isothermal conditions with the temperature from 50 °C to 800 °C at the rate of 20 °C/min. The morphologies of surface and fracture interface of the samples were observed using a field emission scanning electron microscopy (FESEM) instrument (S4800, Japan). The samples were mounted on a copper plate and sputter-coated with gold layer for 30 s. Fourier transform infrared spectroscopy (FTIR) spectra of EGDE and Attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR) spectra of silk fabrics were obtained by a spectrometer (Nicolet 5700, America) in the range of 750– 4000 cm 1. The EGDE liquid was coated on potassium bromide (KBr) pellet prior to data collection. The silk fabrics were put on the spectrometer directly after folding several times to get better information. 13C nuclear magnet resonance(13C NMR) spectrum of EGDE and solid-state 13C cross polarization/magic angle spinning nuclear magnet resonance (13C CP/MAS NMR) spectra of silk fabrics were got by a 400 MHz spectrometer (AVANCE AV, Switzerland) with the range of 0–250 ppm. The silk fabrics were prepared in the same way as that of made for ATR–FTIR test. The silk fabrics were ground into powders and EGDE was dissolved into D2O before testing. Amino acid analysis (AAA) was used to study the kinds and contents of amino acids involved in the reaction. The samples were prepared after being immerged in EGDE aqueous solution for 6, 24, and 48 h, then washed and dried for tests. The dried samples were hydrolyzed with 6 M hydrochloric acid aqueous solution for 24 h at 110 °C. The hydrated solution was dried by nitrogen. Then the hydrolyzate together with internal standard substance were dissolved in derivative liquid and detected by AAA equipment (Waters 2695, America).
3. Results and discussion 3.1. Physical properties The stress–strain curves of the aged sample and sample treated with EGDE are shown in Fig. 1. The breaking stress and breaking strain of aged sample are about 3.6 MPa and 11.2%, respectively; while those of treated sample are about 17.9 MPa and 30.1%, respectively. Clearly, the strength and toughness of the treated sample are much higher than those of the aged sample, with the
2.2.3. Preparation of samples for thermal ageing resistance test The aged silk fabrics and silk fabrics treated with EGDE were exposed to 100 °C heating for 18 days in an oven. Then the samples with thermal treatment were ready for test. 2.3. Measurement Mechanical properties of the samples were tested by a multifunction tensile tester (KES-G1 Type, Japan) at 20 °C and 65%RH with the clamping length of 40 mm and elongation rate of 3 mm/ min. The samples with the size of 5 0.5 cm were glued on a paper frame and then mounted on the tensile tester. Color difference was
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Fig. 1. Typical stress–strain curves of (a) aged and (b) treated samples.
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Table 1 Physical properties of aged and treated samples. Breaking strain (MPa) Aged sample Treated sample
Breaking stress (%)
3.6 ± 0.3
11.2 ± 0.3
17.9 ± 1.5
30.1 ± 0.6
Color differences /
Fabric stiffness (mNcm) 0.43 ± 0.08
0.24 ± 0.12
0.46 ± 0.06
breaking stress and strain increased more than four and two times, respectively. The consolidation effect of EGDE is quite obvious on the aged samples but not as good as that of the SF/EGDE system, which shows nine times increasing of the strength [18]. This indicates that EGDE plays a remarkable role in improving the mechanical properties of the treated silk fabrics. But the strength of the silk fabrics treated with EGDE is lower than that of the silk fabrics treated with SF/EGDE system, which illustrates that EGDE is not the only factor to consolidate the aged silk fabric in the SF/EGDE consolidation system. As shown in Table 1, the color difference between the treated and aged sample is about 0.24. The color difference is less than 1.00 and it is believed to be acceptable in the conservation of historic textiles [14]. It also shows that the fabric stiffnesses of aged and treated sample are about 0.43 mNcm and 0.46 mNcm, respectively, indicating almost no change in handle with the EGDE treatment. Therefore, similar with the SF/EGDE system, the EGDE treatment also does not change the appearance and handle of the silk fabrics. 3.2. FESEM observation The fractural morphologies of the aged and treated sample are shown in Fig. 2. The integral fractural morphology of aged sample (Fig. 2a) shows that the distribution of fibers’ breaking lengths lies in a narrow range. The magnifying morphology of aged fiber
(Fig. 2b) shows a smooth fracture plane, seemed to be brittle rupture. It indicates that the inner structure of the sample is relatively uniform, with same endurance to external force. The fractural morphology of treated sample is quite different from that of aged sample, as shown in Fig. 2c and d. The fractural lengths of treated sample are in a large range (Fig. 2c) and the fracture plane of treated fiber is out of flatness (Fig. 2d), seemed to be ductile rupture. This fractural morphology is a sign that the inner structure of the treated sample is not uniform, with different capability to endure external force. Generally, material with ductile rupture possesses high strength and toughness than that with brittle rupture in the same fiber material. The different fracture morphologies illustrated from Fig. 2 agree with the results of tensile test. 3.3. Thermal property The thermal behavior of aged and treated samples are shown in Fig. 3 and the decomposition temperature range, maximum decomposition temperature and percentages of residual mass are given in Table 2. As shown in Fig. 3, the thermal decompositions of both the aged and treated samples are all taken place in three stages. All the TGA and DTG curves show a small mass loss under 100 °C (the first stage) attributed to the removal of absorbed water. In the second stage (200 °C–500 °C), both the aged and treated sample go through a distinct weight loss. The decomposition starts at 257 °C and finishes at 480 °C for the aged sample, which increases to 264 °C and 499 °C, respectively, for the treated sample. The maximum decomposition rate appears at 360.42 °C for the aged sample, but it increases to 375.55 °C for the treated sample, which is 15 °C higher than that of the aged sample. In the third stage, decomposition of the aged sample starts at 498 °C and finishes at 700 °C, and the maximum decomposition rate appears at 617.66 °C. In contrast, decomposition of the treated sample starts at 499 °C and finishes at 725 °C. The maximum decomposition rate
Fig. 2. FESEM of (a and b) aged and (c and d) treated samples.
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Fig. 3. TGA and DTA curves of (a) aged and (b) treated samples. Table 2 Thermal analysis data of aged and treated samples.
Aged sample Treated sample a
IIa III II III
Decomposition temperature range (°C)
Maximum decomposition temperature (°C)
Residual mass (%)
257–480 498–700 264–499 499–725
360.42 617.66 375.55 639.41
40.6 1.1 40.8 0.3
II, III refers to the second and third stage, respectively.
appears at 639.41 °C, which is about 22 °C higher than that of aged sample. All the above mentioned data show that the treated sample has better thermal properties than the aged sample. In addition, it can also be found from Fig. 3 that the DTG peaks of the treated sample are wider than those of the aged sample in the second and third stages. This indicates that the decomposition rate changes slower in the treated sample than that of the aged sample, which also proves that the treated sample has higher thermal stability.
3.4. Thermal ageing resistance Physical properties of the aged and treated sample before and after thermal ageing were tested to evaluate their properties of thermal ageing resistance. Typical stress–strain curves are shown in Fig. 4. The breaking stress, color difference and fabric stiffness before and after thermal ageing are listed in Table 3. It was found that the breaking stress of aged samples decreases from 3.6 MPa to 1.9 MPa; while that of treated samples decreases from 17.9 MPa to
13.8 MPa, after thermal treatment. The treated samples seem to have a better thermal ageing resistance in stress loss with the decrement of 22.9%, which is lower than that of the aged sample (47.9%). The color difference values of the aged and treated sample after thermal treatment are about 5.29 and 4.21, respectively. It indicates that the treated sample has a better thermal ageing resistance in color change than the aged sample after thermal treatment. The values of fabrics stiffness of aged and treated sample are about 0.54 mNcm and 0.55 mNcm, respectively. It indicates that the thermal treatment has same influence on the aged and treated sample.
3.5. Interaction between silk fabrics and EGDE Fourier transform infrared spectroscopy (FTIR) has been widely used to detect the molecular reaction and conformation transition of silk fibroin [30–32]. The FTIR spectrum of EGDE is shown in Fig. 5. The absorption at 1101 cm 1 attributes to the ether bond stretching vibration. The bands at 910 cm 1 and 856 cm 1 arise from the stretching vibrations of the epoxide groups in EGDE. Fig. 6 shows the ATR–FTIR spectra of the aged and treated samples. The peak at 1070 cm 1 is higher in treated sample (Fig. 6b) than that in aged sample (Fig. 6a). This higher absorption should result from the influence of the absorption of 1101 cm 1 in EGDE. It is a sign that there is ether bond in the treated sample. However, the bands at 910 cm 1 and 856 cm 1 representing the stretching vibrations of epoxide groups in EGDE are not shown in the spectrum of EGDE treated sample. Herein, it could be concluded that there is ether bond of EGDE in the treated sample, but the epoxide groups
Fig. 4. Typical stress–strain curves of (A) aged and (B) treated samples. (a) sample before thermal ageing and (b) sample after thermal ageing.
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Table 3 Physical properties of the aged and treated sample before and after thermal ageing. Breaking strain (MPa) Aged sample Aged sample after thermal ageing Treated sample Treated sample after thermal ageing
Color differences
Fabric stiffness (mNcm)
3.6 ± 0.3 1.9 ± 0.3
/ 5.29 ± 0.61
0.43 ± 0.08 0.54 ± 0.12
17.9 ± 1.5 13.8 ± 1.6
0.24 ± 0.12 4.21 ± 0.53
0.46 ± 0.06 0.55 ± 0.09
Fig. 7.
13
C NMR spectrum of EGDE.
Fig. 5. FTIR spectrum of EGDE.
Fig. 8.
13
C CP/MAS NMR spectrum of (a) aged and (b) treated samples.
Table 4 Amino acid composition (mol%) of aged and treated samples.
Fig. 6. ATR–FTIR spectrum of (a) aged and (b) treated samples.
were gone, which means the epoxide groups should be reacted with the silk fabric. Moreover, no physically attached EGDE was left, otherwise its absorptions at 910 cm 1 and 856 cm 1 would be shown. Amide I can be used to detect whether there is a conformation transition in silk fibroin molecules [32] after the silk fabrics treated with EGDE. The aged and treated sample display the same peak at 1623 cm 1, which is assigned to b-sheet conformation. No difference is shown between the peaks. It seems that the crystallinity degree of the fabrics before and after EGDE crosslinking was not changed; or even if it changed, the change could not be tested by ATR–FTIR. This indicates that the treatment did not have much influence on the conformation of b-sheet in the silk fibers, which means that the treatment did no harm to the inner structure of the silk fabrics. 13 C CP/MAS NMR was also used to verify the interaction between EGDE and silk. The 13C NMR spectrum of EGDE is shown in Fig. 7 and the spectra of aged and treated sample are shown in
Amino acid
Aged sample
Treated for 6h
Treated for 24 h
Treated for 48 h
Asp Ser Glu Gly Arg Thr Ala Pro Cys Tyr Val Met Lys Ile Leu Phe
0.80 ± 0.06 12.41 ± 0.1 0.77 ± 0.04 51.02 ± 1.99 0.42 ± 0.02 0.71 ± 0.02 24.22 ± 0.11 0.51 ± 0.01 0.27 ± 0.03 6.59 ± 0.2 2.71 ± 0.03 0.03 ± 0.01 0.18 ± 0.01 0.60 ± 0.04 0.44 ± 0.04 0.65 ± 0.02
0.71 ± 0.03 12.43 ± 0.13 0.75 ± 0.07 52.73 ± 0.27 0.40 ± 0.01 0.72 ± 0.01 24.34 ± 0.45 0.49 ± 0.01 0.25 ± 0.04 2.74 ± 0.19 2.82 ± 0.04 0.06 ± 0.01 0.15 ± 0.01 0.58 ± 0.01 0.50 ± 0.03 0.65 ± 0.01
0.75 ± 0.04 12.32 ± 0.08 0.71 ± 0.02 53.05 ± 0.26 0.41 ± 0.02 0.69 ± 0.01 24.26 ± 0.28 0.50 ± 0.00 0.32 ± 0.05 1.69 ± 0.15 2.82 ± 0.02 0.05 ± 0.02 0.11 ± 0.02 0.62 ± 0.05 0.50 ± 0.04 0.66 ± 0.03
0.73 ± 0.06 12.62 ± 0.16 0.73 ± 0.03 51.37 ± 0.63 0.37 ± 0.05 0.71 ± 0.02 25.68 ± 0.79 0.47 ± 0.03 0.19 ± 0.05 1.01 ± 0.02 2.81 ± 0.06 0.05 ± 0.01 0.05 ± 0.01 0.62 ± 0.01 0.47 ± 0.03 0.53 ± 0.05
Fig. 8. It shows in Fig. 7 that there is a peak at 70.2 ppm attributing to the absorption of ether band. It can be seen from Fig. 8 that a new absorption at 69.5 ppm appears in the treated sample which presents the same absorption shown at 70.2 ppm in EGDE. It demonstrates that EGDE reacts with silk fabrics, because the sample is the same as that of made for ATR–FTIR test, which confirms the removal of physically attached EGDE. Amino acid analysis (AAA) can detect the content change of amino acids if they are reacted and the reaction products are hard
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Fig. 9. Schematic reaction of (a) Tyr and (b) Lys with EGDE.
to be hydrolyzed. So it can be used to detect which amino acid participates in the reaction. The contents of amino acids of aged sample and samples with different treating times are listed in Table 4. It shows that most of the contents of the amino acids are similar regardless of the treatment except tyrosine and lysine. The molar content of tyrosine in aged sample is about 6.59%, which decrease to 2.74% (treated for 6 h), 1.69% (treated for 24 h) and 1.01% (treated for 48 h). The molar content of lysine has the similar trend which decreases from 0.18% to 0.15% (treated for 6 h), 0.11% (treated for 24 h) and 0.05% (treated for 48 h). This changing tendency indicates that the contents of tyrosine and lysine are decreased with the increase of treating time, because of their participation in the reaction with EGDE in the process of consolidation. Silk fiber has a tendency of yellowing after exposed to sunlight and heat. It attributes to the presence of tyrosine and tryptophan residues in the fiber, which tend to form yellow chromophores with the effect of light or heat [33]. The decreased content of tyrosine residues in treated sample may result in fewer sites to be affected by thermal treatment. Therefore, the content change of tyrosine also provides a reason for the lower color change of treated sample than that of aged sample. EGDE consists of several methylenes with two epoxide groups at either end. The epoxide group is cyclic ether with three ring
atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Therefore, EGDE was easily reacted with phenolic hydroxyl group of the tyrosine and/or amino group of the lysine in its either end by opening of the strained rings during the treatment even at room temperature (25 °C). In this way, the degraded fibroin fibrils, keys to endure external force including heating, were cross-linked by the methylene ether bonding groups. Thus, the mechanical and thermal properties of the treated samples are greatly improved. The schematic reactions were presented in Fig. 9. On the other hand, these two amino acids locate mainly in the amorphous region of silk fibroin where degradation takes place preferentially [1,7,34]. So they tend to be exposed to reaction in the treatment rather than other amino acids such as alanine and glycine which locate in inside and crystalline region of the silk fibroin [1,35]. Therefore, stable chemical bonds are formed between silk fibroin molecules in silk fibers by the crosslink of EGDE in the amorphous region, which results in more stable inner structure of the treated sample than that of the aged sample, and thus it is responsible for the higher strength, toughness, thermal stability and thermal ageing resistance in the treated samples as discussed before.
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4. Conclusions Artificial aged silk fabrics treated with EGDE exhibit great improvements in mechanical, thermal properties and thermal ageing resistance without color and flexibility change. So, EGDE plays a positive role in the consolidation of aged silk fabrics. Chemical interactions between EGDE and silk fabric were verified and responsible for the mechanical property improvement in this study. EGDE has so high reactivity that it can be reacted with silk fabrics at room temperature by simple spraying, which can avoid heating and operational damages compared with other consolidations usually done at higher temperature with complicated operations. Therefore, EGDE could be selected as a potential for treatment of very fragile historic silk fabrics even at the excavating scene. Acknowledgments This research was financially supported by Special Funds for the Protection of Cultural Heritage in Zhejiang Province (No. 2011-202) and Nation Natural Science Foundation of China (No. 51102214). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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