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Journal of Cultural Heritage 15 (2014) 159–164 Available online at www.sciencedirect.com Original article Deacidification of paper relics by plasma technology Qinglian Li a , Sancai Xi b , Xiwen Zhang a,b,a Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b Research Institute of Cultural Heritage, Zhejiang University, Hangzhou 310027, China a r t i c l e i n f o Article history: Received 14 December 2012 Accepted 12 March 2013 Available online 22 April 2013 Keywords: Plasma treatment Deacidification Paper Tensile strength a b s t r a c t With the acidification of paper and paper-containing relics becoming increasingly serious, a convenient, effective and harmless method for deacidification has become an urgent necessity in the protection of paper relics. In this research, a novel method for reducing the acidity of paper by plasma technology is presented, which can be used simply at room temperature and atmospheric pressure. The pH of the paper rises to alkalescence rapidly after treatment and remains stable with no color change, with a slight accompanying increase in the mechanical properties of the paper. © 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction Paper as a medium for written information is extremely impor- tant for transmitting and preserving knowledge and cultural history. However, with aging, more and more precious paper relics have become yellowed and brittle because of the acidification of paper. The main structure of paper consists of cellulose fibers. Cellu- lose is a polymer consisting of linear (1-4)D-glucopyranosyl units which will hydrolyze in acidic conditions. Acidity from the absorp- tion of atmospheric pollutants, oxidation of lignin and additives from the papermaking process can all act as catalysts in the process of acid hydrolysis of paper [1]. These hydrolysis catalysts penetrate the paper fiber and cause glucosidic bond scission, which results in the degradation of cellulose. Furthermore, the hydrolysate contains additional acidic substances, which make this process a vicious cycle which ultimately results in a significant decrease in the mechanical strength of paper. Therefore, the removal or prevention of acidity is one of the most significant problems to be addressed in paper reinforcement and conservation. There are currently several methods for paper deacidification being applied in conservation and restoration [2], but they have several unavoidable drawbacks [1,3]. For example, direct contact with chemical deacidification reagents which are mostly based on non-environmentally friendly solvents [4,5], will lead to paper crin- kle, and visual appearance altering. Some other methods have very rigid deacidification conditions [6]. Thus, an efficient, convenient Corresponding author. Department of Materials Science and Engineering, Zhe- jiang University, Hangzhou 310027, China. E-mail address: [email protected] (X. Zhang). and environmentally friendly method for deacidification of paper is urgently needed. Non-equilibrium plasma chemistry (often known as cold plasma chemistry) which has been used to modify macromolecular sur- faces via various high-energy ions, electrons, free radicals and photons, is a dry and clean process without environmental con- cerns, such as use of hazardous reagents and solvents. It has been successfully applied in a number of processes, such as plasma clean- ing, etching and coating. One of the main advantages of the plasma approach is that any resulting modifications are limited only to the material surface, leaving unaffected the bulk properties [7]. Thus, for the specific case of paper relics, treatment by the plasma method should guarantee the paper protection against surface damage. When considering alkaline reagents for deacidification, the water-soluble inorganic compounds are much better than com- plex metallo-organic compounds, due to their lower toxicity and polluting effects [3], as well as cost. When comparing some alka- line inorganic salts, calcium compounds, in general, show better performance than the other commonly used reagents like sodium and magnesium compounds which are considered to be significant in the yellowing of the treated paper [8–10]. The neutralization reaction between an acid substance and OH from the Ca(OH) 2 can occur directly in the environmental humidity. As a result, some of the free calcium ions deep in the fiber can bond to the carboxyl anions in the oxidized cellulose and decelerate the degeneration rate of the cellulose, while other calcium ions on the surface could transform into CaCO 3 for further protection [11,12]. Since many ancient papers with higher calcium carbonate content exhibit a longer-term stability, the use of Ca(OH) 2 would be a better choice of alkaline reagent for their conservation. In this paper, a cold plasma system has been used with a deacidification reagent consisting of saturated Ca(OH) 2 solution 1296-2074/$ see front matter © 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.culher.2013.03.004

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Page 1: Deacidification of paper relics by plasma technology

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Journal of Cultural Heritage 15 (2014) 159–164

Available online at

www.sciencedirect.com

riginal article

eacidification of paper relics by plasma technology

inglian Lia, Sancai Xib, Xiwen Zhanga,b,∗

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, ChinaResearch Institute of Cultural Heritage, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

rticle history:eceived 14 December 2012ccepted 12 March 2013

a b s t r a c t

With the acidification of paper and paper-containing relics becoming increasingly serious, a convenient,effective and harmless method for deacidification has become an urgent necessity in the protection ofpaper relics. In this research, a novel method for reducing the acidity of paper by plasma technology

vailable online 22 April 2013

eywords:lasma treatmenteacidification

is presented, which can be used simply at room temperature and atmospheric pressure. The pH of thepaper rises to alkalescence rapidly after treatment and remains stable with no color change, with a slightaccompanying increase in the mechanical properties of the paper.

© 2013 Elsevier Masson SAS. All rights reserved.

aperensile strength

. Introduction

Paper as a medium for written information is extremely impor-ant for transmitting and preserving knowledge and culturalistory. However, with aging, more and more precious paper relicsave become yellowed and brittle because of the acidification ofaper. The main structure of paper consists of cellulose fibers. Cellu-

ose is a polymer consisting of linear �(1-4)D-glucopyranosyl unitshich will hydrolyze in acidic conditions. Acidity from the absorp-

ion of atmospheric pollutants, oxidation of lignin and additivesrom the papermaking process can all act as catalysts in the processf acid hydrolysis of paper [1]. These hydrolysis catalysts penetratehe paper fiber and cause glucosidic bond scission, which results inhe degradation of cellulose. Furthermore, the hydrolysate containsdditional acidic substances, which make this process a viciousycle which ultimately results in a significant decrease in theechanical strength of paper. Therefore, the removal or prevention

f acidity is one of the most significant problems to be addressedn paper reinforcement and conservation.

There are currently several methods for paper deacidificationeing applied in conservation and restoration [2], but they haveeveral unavoidable drawbacks [1,3]. For example, direct contactith chemical deacidification reagents which are mostly based onon-environmentally friendly solvents [4,5], will lead to paper crin-

le, and visual appearance altering. Some other methods have veryigid deacidification conditions [6]. Thus, an efficient, convenient

∗ Corresponding author. Department of Materials Science and Engineering, Zhe-iang University, Hangzhou 310027, China.

E-mail address: [email protected] (X. Zhang).

296-2074/$ – see front matter © 2013 Elsevier Masson SAS. All rights reserved.ttp://dx.doi.org/10.1016/j.culher.2013.03.004

and environmentally friendly method for deacidification of paperis urgently needed.

Non-equilibrium plasma chemistry (often known as cold plasmachemistry) which has been used to modify macromolecular sur-faces via various high-energy ions, electrons, free radicals andphotons, is a dry and clean process without environmental con-cerns, such as use of hazardous reagents and solvents. It has beensuccessfully applied in a number of processes, such as plasma clean-ing, etching and coating. One of the main advantages of the plasmaapproach is that any resulting modifications are limited only to thematerial surface, leaving unaffected the bulk properties [7]. Thus,for the specific case of paper relics, treatment by the plasma methodshould guarantee the paper protection against surface damage.

When considering alkaline reagents for deacidification, thewater-soluble inorganic compounds are much better than com-plex metallo-organic compounds, due to their lower toxicity andpolluting effects [3], as well as cost. When comparing some alka-line inorganic salts, calcium compounds, in general, show betterperformance than the other commonly used reagents like sodiumand magnesium compounds which are considered to be significantin the yellowing of the treated paper [8–10]. The neutralizationreaction between an acid substance and OH− from the Ca(OH)2 canoccur directly in the environmental humidity. As a result, some ofthe free calcium ions deep in the fiber can bond to the carboxylanions in the oxidized cellulose and decelerate the degenerationrate of the cellulose, while other calcium ions on the surface couldtransform into CaCO3 for further protection [11,12]. Since manyancient papers with higher calcium carbonate content exhibit a

longer-term stability, the use of Ca(OH)2 would be a better choiceof alkaline reagent for their conservation.

In this paper, a cold plasma system has been used with adeacidification reagent consisting of saturated Ca(OH)2 solution

Page 2: Deacidification of paper relics by plasma technology

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60 Q. Li et al. / Journal of Cultu

o impregnate alkaline groups into the paper fibers and to modifyhe fiber surfaces. Parallel investigations using traditional chemicaleacidification treatments were carried out, using the same appa-atus but without activating the plasma system, so that paper wasnly treated by Ca(OH)2, similar to the traditional aqueous deacid-fication method. Measurements of pH and tensile strength of theaper samples were also made in each case. Moreover, differentinds of colored paper were treated to measure the correspondingolor change after plasma treatment. In addition, scanning electronicroscopy was used to analyze the fiber surfaces of both treated

nd untreated samples, and the elemental content of the samplesas measured by energy-dispersive X-ray spectroscopy [1].

. Experimental

.1. Preparation of paper samples

Machine-made paper manufactured in different years, from the920’s to 1990’s, eight kinds of hand-made paper which are Chinesert paper from Jiajiang, bamboo paper from Liangping, raw, mixedoso bamboo paper (#15), raw, mixed bitter bamboo paper (#30),

linker, mixed bitter bamboo paper (#42), clinker, pure bitter bam-oo paper (#48), pelure paper from Wenzhou, straw paper werehosen, six kinds of colored machine-made paper (blue, red, yellow,ink, light green, green) and A4 paper samples covered with differ-nt pigments (vermilion, Chinese ink, printing ink, malachite, eosin,hthalocyanine blue) were chosen as deacidification test samples.ach sample was cut into pieces of dimensions 130 mm × 150 mm,nd maintained under room conditions of 23 ± 1 ◦C, 50 ± 2% humid-ty for at least one day prior to testing.

.2. Plasma processing

The experimental apparatus for plasma processing consists of anrc plasma gun, RF power supply (120–260 W, 0–20 kHz), a conicalask to provide alkaline reagent, gas with valve and a flow meter,hich is depicted in schematic form in Fig. 1. The saturated Ca(OH)2

olution firstly fills the plasma gun in vapor form, together withrgon. The intensity of the plasma can be controlled by adjustinghe flow rate of gas and the power of the generator. The arc plasmaun is connected to a computer numerical control electromotor inrder to allow it to move on a designated route at a certain speed,o that every part of the paper placed in the attainable region of the

lasma gun can be treated directly in the plasma zone for a certaineriod of time.

The experiments were carried out under the following workingonditions: temperature 20–25 ◦C, atmospheric pressure, output

Fig. 1. Schematic of the specially designed plasma treatment apparatus.

ritage 15 (2014) 159–164

voltage 75 V, arc power 100 W. The gas used was argon with therate of 4 L/min, pH of alkaline agent was 12.48, the longitudinalspeed of the plasma gun was 35 mm/s and the transverse distancewas 2 mm each time.

2.3. Traditional parallel test method

A parallel test simulating a traditional aqueous deacidificationmethod was made using the same experimental apparatus withoutactivating the plasma system, so that the alkaline agent saturatedCa(OH)2 solution sprayed directly onto the paper samples from theinactive arc plasma gun. In this case, the samples were treated onlyby alkaline agent with all the other factors the same as for theplasma treatment process.

2.4. Accelerated aging procedure

The stability of the deacidification samples was investigatedby moisture-heat-accelerated aging using an aging oven. Artificialaging was performed in the aging oven at a temperature of 80 ◦Cand humidity of 65% for 72 hours, which corresponds to 25 years ofnatural aging [1].

2.5. Tensile tests

Tensile strength tests on paper sheets were performed on acomputer controlled tensile testing machine with an extensometergauge of 25 mm and a test speed of 5 mm/min. At least ten speci-mens, which were 100 mm long and 15 mm wide, were tested foreach type of paper sheet in order to check for repeatability.

2.6. pH tests

The current surface method of evaluating the pH of paper usinga flat electrode is known to be flawed, as it is really the pH of thesolution that moistens the paper surface [13]. A more accurate butdestructive method is cold extraction measurement [14]. For thismethod, paper samples are cut into pieces and dispersed in colddeionized water. Accordingly, 1 g of the paper samples was addedto 40 ml of cold, deionized water for 1 hour and the pH of the waterafter the extraction time was deemed to be the pH of paper. For thepresent study, we used the non-destructive surface pH measure-ment technique to measure the pH of the paper relics, and the coldextraction method for ordinary samples.

2.7. Scanning electron microscope (SEM) and energy-dispersiveX-ray spectroscopy (EDS) analysis

The surfaces of both untreated and plasma-treated sampleswere coated by evaporation with gold for 80 seconds before exami-nation. A scanning electron microscope (Hitachi-TM3000) was usedto investigate the surface morphologies and determine the contentof the specified elements at the same time.

3. Results and discussion

3.1. Measurement of properties before and after plasmadeacidification

3.1.1. pH and tensile strength variationAfter the deacidification treatment by plasma, both machine-

made (Table 1) and hand-made (Table 2) paper samples

experienced an increase in pH. Since there will be a long-termconsumption of the alkaline compound, the pH achieved after treat-ment should be above neutral but not high enough to cause alkalinedepolymerization. The optimal value was in the 8.0 range, as the
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Q. Li et al. / Journal of Cultural Heritage 15 (2014) 159–164 161

Table 1pH and tensile strength of machine-made paper before and after plasma treatment.

Sample date pH Tensile strength (kN/m)

Untreated Treated Untreated Treated

1990s 6.324 7.619 1.941 ± 0.078 1.907 ± 0.0441980s 6.537 7.945 1.720 ± 0.064 1.703 ± 0.1391970s 6.192 7.471 1.613 ± 0.062 1.908 ± 0.0831960s 6.395 8.063 1.801 ± 0.068 1.838 ± 0.1071950s 5.672 7.589 0.942 ± 0.051 0.983 ± 0.0591940s 6.022 8.105 1.039 ± 0.051 1.014 ± 0.0351930s 5.545 7.752 1.374 ± 0.118 1.364 ± 0.1091920s 6.446 7.914 1.352 ± 0.087 1.467 ± 0.082

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Table 3Color change of various hand-made paper samples.

Type of paper �E 1 �E 2 �E 3

Chinese art paper from Jiajiang 0.63 0.17 1.44#30 bitter bamboo paper 1.66 0.73 2.12Bamboo paper from Liangping 2.52 0.74 3.17Pelure paper from Wenzhou 0.92 0.50 1.03Straw paper 0.27 0.78 1.34

Tp

ig. 2. Results of tensile strength measurements before and after aging for thequivalent of 50 years for different plasma-treated and untreated samples.

xperimental results showed. Furthermore, we can see that the ten-ile strength increased, especially for the hand-made paper, whichas raised nearly 20%. This phenomenon is likely due to the effect of

he plasma, since the high energy could change the structures of thebers and accordingly improve the mechanical properties. Com-ared with the hand-made paper samples, machine-made paperontains large amounts of additives from the pulping process [15],hich may influence the modification effects on the fiber surface.

.1.2. Tensile strength before and after agingIt is known that during the paper aging process, cellulose can

epolymerize in the presence of acid, leading to acidification andragility. The longer paper is kept over time, the more potentialhere is for it to be seriously affected by acidic substances. In thisesearch, four typical kinds of Chinese paper samples were sub-ected to deacidification by plasma, and then aged artificially in the

ging oven, imitating 25 years of aging under natural conditions,nd then their tensile strength was measured.

The results of the aging study, presented in Fig. 2, show thatfter aging, the tensile strength of the blank samples was reduced

able 2H and tensile strength of hand-made paper before and after plasma treatment.

Type of hand-made paper pH

Untreated T

Chinese art paper from Jiajiang 6.324 7Bamboo paper from Liangping 6.518 8#15 moso bamboo paper 6.473 7#30 bitter bamboo paper 6.966 8#42 bitter bamboo paper 7.049 8#48 bitter bamboo paper 6.755 8Pelure paper from Wenzhou 6.983 8Straw paper 6.126 7

�E 1: color change between aged and un-aged states of the same treated sample;�E 2: color change between treated and untreated sample before aging; �E 3: colorchange between treated and untreated sample after aging.

sharply to nearly 70%, while the plasma-treated samples still main-tained more than 85% of their tensile strength. If we compare theaging measurements, the treated samples still have much bettertensile strength, which means that the plasma treatment improvesthe mechanical properties, together with deacidification.

3.1.3. Color changesMolecular modifications in the cellulose polymers in paper

caused by degradation reactions can be manifest in color changes.The evaluation of the color changes was measured by the CIE L*a*b*system [11]. The three parameters (L*, a*, b*) in the model repre-sent the lightness (L*) of the color (the smallest value indicatingblack), a* its position between red and green (the smallest valueindicating green) and b* its position between yellow and blue (thesmallest value indicating blue) respectively. After the evaluationof parameters L*, a*, b*, the total color change �E*

ab, before andafter the plasma deacidification treatment, was determined, withthe following equation:

�E∗ab =

[(�L∗)2 + (�a∗)2 +

(�b∗)2

]1⁄2(1)

In Eq. (1), the smaller the value of �E*ab, the less difference there

is between paper samples. In general, a �E*ab value of less than 1.5

is deemed to be undetectable by the human eye.In the present study, three kinds of color changes were mea-

sured for every sample. The color change of the same treated samplebefore and after deacidification by plasma is given by �E 1. �E 2represents the color change between treated and untreated sam-ples before aging, while �E 3 is for samples after aging.

The color changes for five different types of hand-made papersamples are presented in Table 3. These results indicate that all thesamples show little color change between treated and untreatedsamples, proving that plasma treatment does not change the colorof the paper [16]. The artificial aging revealed a relatively large dif-ference between treated and untreated samples according to�E 3compared with the variation between aged and un-aged samplesafter plasma treated in �E 2. Since the yellowing and darkening

of cellulose is related to the acid hydrolysis and oxidation of thepolymer chains, and after aging the treated samples showed a sim-ilar color, it is evident that the plasma treatment can decrease thespeed of cellulose degradation to some extent.

Tensile strength (kN/m)

reated Untreated Treated

.514 0.968 ± 0.056 1.167 ± 0.049

.032 0.493 ± 0.070 0.536 ± 0.028

.723 0.838 ± 0.054 0.949 ± 0.039

.287 0.833 ± 0.040 1.333 ± 0.082

.615 1.001 ± 0.029 1.195 ± 0.033

.125 1.306 ± 0.058 1.448 ± 0.032

.860 1.118 ± 0.046 1.161 ± 0.057

.906 0.845 ± 0.025 0.898 ± 0.029

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162 Q. Li et al. / Journal of Cultural Heritage 15 (2014) 159–164

Table 4Color change of different colored papers and pigments.

Type of pigment �E 1 �E 2 �E 3

A4 paper with vermilion 0.33 1.62 1.05Chinese art paper with Chinese ink 0.43 1.09 1.79A4 paper with Chinese ink 0.11 0.66 1.52A4 paper with printing ink 0.21 0.51 1.07A4 paper with malachite 0.13 0.53 0.59Chinese art paper with malachite 0.03 0.87 0.89A4 paper with eosin 4.43 3.87 3.66A4 paper with phthalocyanine blue 1.00 0.77 2.71Blue machine-made paper 0.84 0.86 0.42Red machine-made paper 1.84 1.07 1.94Yellow machine-made paper 0.03 0.54 2.96Pink machine-made paper 0.69 0.77 0.15

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Light green machine-made paper 0.58 0.40 1.12Green machine-made paper 0.38 0.63 0.69

Different kinds of pigments and colored papers were treated andhe corresponding �E*ab values before and after deacidificationere measured (Table 4).

For most of the pigments and colored papers, the color changesere small and random, so that we can consider that no real color

hange takes place after deacidification treatment, with the excep-ion of red pigment. Since the carbonyl groups in the cellulosend pigment are chromophores [17], the observation of little colorhange means that plasma treatment does not change the structuref the polymer chains or accelerate the hydrolysis reaction. For theosin, whose main component is tetrabromofluorescein, the phe-olic groups would easily be oxidized during the aging process,hich leads to the large observed changes of color.

.2. Plasma vs. chemical treatment

Most tradition deacidification methods using chemical reagentsuch as Ca(OH)2 directly show a significant effect immediately uponpplication. In the present study, we use both chemical and plasmareatments for deacidification in order to compare the methods andbserve any differences between them (Table 5).

The pH of all types of samples increased substantially if mea-ured immediately after deacidification with both chemical andlasma treatments. However, we kept the treated samples in alosed environment for one and half a months and measured the

H again. After 45 days, we obtained an interesting result, wherebyhe pH of the sample, which was treated by the chemical method,howed an obvious decrease, while those treated by the plasmaethod maintained a pH similar to that of the previous 45 days.

0 10 20 30 40 50 60 70 80 90

Time(day)

Chemical tre atmentpH

5.5

6.0

6.5

7.0

7.5

8.0

Plasma tre atmentUntreated

ig. 3. pH variation of paper treated by different deacidification methods over 90ays’ post-treatment time.

Fig. 4. SEM micrographs of the surface of (a) untreated paper sample; (b) treatedby chemical method; (c) treated by plasma method. Optical micrographs of eachsample are shown in the upper left-hand corner.

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Q. Li et al. / Journal of Cultural Heritage 15 (2014) 159–164 163

Table 5pH comparison between chemical and plasma treatments of paper samples.

Measured immediately after treatment Measured 45 days later

Untreated Chemical Plasma Untreated Chemical Plasma

30′ newspaper 5.369 7.336 7.164 5.437 6.753 7.00980′ newspaper 5.995 7.824 7.871 6.142 7.437 7.752Magazine paper 6.603 8.734 8.867 6.568 8.247 8.785Chinese art paper 6.324 7.515 7.541 6.414 7.033 7.515Bamboo paper 5.668 8.576 8.841 5.337 8.109 8.816Pelure paper 6.828 8.911 8.937 6.589 8.879 8.971Straw paper 5.804 8.316 8.210 5.773 8.156 8.274

(b) a

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Fig. 5. pH of paper relics (a) before and

his result illustrates that the effect of deacidification lasts longerhen using the plasma method compared to the traditional chem-

cal deacidification treatment.In order to study the process of pH change, a large piece of

940’s newspaper was cut into equal halves, with one half treatedy the chemical method and the other treated by plasma. Both ofhe paper samples were kept in a closed environment and the pHas measured every few days by the cold extraction method to

btain the accurate pH. From the results shown in Fig. 3, the pH

f the chemically treated sample dropped gradually, while that ofhe sample treated by plasma remained unchanged. After 45 days,he pH trend of both samples remained stable, with the alkalinityf the plasma- treated sample being much stronger, while the pH

Fig. 6. Photograph of paper relics (a) before and

fter plasma deacidification treatment.

of the other sample was nearly the same as that at the beginningof the study.

3.3. Elemental analysis and surface morphology

Energy-dispersive X-ray spectroscopy (EDS) was used to analyzethe elemental composition of the fibers of the paper surfaces, withthe results given in Table 6. We found that the content of both C andO changed with increasing the content of Ca2+ after deacidification

by both methods. However, for the composition ratio of O/C, theplasma-treated sample showed a notable difference. While boththe untreated and chemically treated samples had the same ratioof 1.17, the ratio of O/C rose to 1.23 for the plasma-treated sample,

(b) after plasma deacidification treatment.

Page 6: Deacidification of paper relics by plasma technology

164 Q. Li et al. / Journal of Cultural He

Table 6Elemental composition of fibers obtained from energy-dispersive X-ray spec-troscopy (EDS).

Treatment condition Composition ratio Elemental composition (%)

O/C C O Ca

Untreated 1.17 46.1 53.8 0.1Chemical 1.17 45.6 53.4 1.0Plasma 1.23 44.6 54.8 0.7

Table 7Color change for untreated and treated paper relics.

Untreated Treated �E

L* a* b* L* a* b*

1 57.35 10.39 28.65 58.86 10.27 28.27 0.692 66.86 7.54 28.22 66.97 7.66 28.47 0.303 67.09 9.13 30.05 66.22 8.48 30.00 1.09

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4 65.23 8.27 28.87 65.01 8.45 28.48 0.485 66.75 7.70 28.29 66.98 7.85 28.84 0.61

hich means that the chemical structure of the fiber surface wasnfluenced by the plasma treatment, with some new oxygen groupsreated [7].

Fig. 4 shows the optical microscopy results and scanning elec-ron micrographs of the fiber surfaces of untreated, chemicallyreated and plasma treated paper samples. The main structure ofhe fibers treated by plasma has changed very little compared withhe untreated sample under the optical microscope. However, its clearly seen that the surface of the fibers became rough afterreatment by plasma while the surface of the other two samplesemained smooth. There were many fiber branches, similar to theevillicate formed during the paper pulping process, which meanshat the high-energy plasma does not damage the polymer chains;nstead, it modifies the surface. The surface of the fibers was acti-ated and split into many tiny fibers, which increases the specificurface area and exposes more hydroxyl groups, allowing them toorm more hydrogen bonds with neighboring fibers. Meanwhile,he plasma process may also produce new oxygenated species asndicated by the EDS result, which can also form more hydrogenonds. Thus, the number of connection points between fibers would

ncrease, thereby enhancing the bonding forces between fibers inhe paper, ultimately resulting in improved mechanical behavior inhe paper in macroscopic terms.

.4. Plasma technology applied to paper relics

An acidified page of an authentic book from the Ming dynastyabout 1600 A.D.) was chosen as the test subject for this procedure,nd pH measurements the CIE L*a*b* system were used to deter-ine the efficiency of plasma deacidification on this example of a

aper relic.After application of the plasma deacidification technique, the

H of the page rose from 4.478 to 7.737 (Fig. 5), demonstrating thathe paper changed to alkalescency after treatment, without anyther apparent changes to the paper (Fig. 6). The L*a*b* chromatic

berration analysis for the same point before and after treatmentresented in Table 7, shows that the color change for the ancientage was around 1.0, which confirms that the plasma deacidifi-ation method applied to paper relics only lowers the degree of

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ritage 15 (2014) 159–164

acidification, and does not influence the appearance of the paperor the pigment and writing on it.

4. Conclusions

This report presents a novel deacidification method for paperrelics using atmospheric plasma technology with advantages overthe present wet-chemical-based deacidification methods, includ-ing shortening the deacidification duration from several hoursto just a few minutes, avoiding paper crinkle, color change andvisual appearance altering caused by direct contact of paperwith solution-stated deacidification reagents, and prolonging themaintaining effect for paper deacidification. The proposed plasmatechnology could strengthen the paper since part of shallow sur-face fibers might be activated and split into tiny fibers to form morehydrogen bonds with neighboring fibers, while the original appear-ance of paper could be preserved due to the short plasma processingperiod that avoids the degradation of cellulose.

Acknowledgements

This work was supported by grants from the Foundation ofState Administration of Cultural Heritage of China (No. 20110105),and from the Foundation of Cultural Heritage Conservation Scienceand Technology Project of Zhejiang in 2012 (Application of plasmadeacidification key technology in deacidification of neoteric andmodern paper relics).

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