Ethylene Oxide for the Mass Deacidication of Paper Relics

In order to solve the international problem of deacidication protection at scale for the whole book and archives, this paper designs the equipment and technology of bulk deacidication treatment with ethylene oxide gas phase. the procedure is simple and the investment is small. In this process, a mixture of ethylene oxide and argon gas is added with the aid of medium vacuum, and the large-scale deacidication of acidication books and archives can be realized by controlling temperature and humidity. This technology does not need to screen acidic and alkaline paper. After the overall treatment, the paper pH value remains at 7.0 to 9.5, which has no effect on handwriting, no discoloration, and has a certain strengthening effect on the paper. The accelerated aging test shows that the pH value remains stable after deacidication and the durability is enhanced. It is a new deacidication technology of paper cultural relics with high eciency, safety, good compatibility and scale.

folds and deforms, books expand and thicken, and archival handwriting, calligraphy and painting fade. As the most widely used method for deacidi cation of paper cultural relics in the world, Bookkeeper also has some defects. When the paper porosity is low, MgO particles can not fully penetrate into the ber interior and may cause the paper white. On the other hand, the acid removal solution adds surfactants whose long-term effects on the paper are unclear. [18] Some methods, such as Paper save, Bookkeeper, also provide methods for deacidi cation of the whole Book, but they need the assistance of high vacuum and large equipment size [16] . Books need to be soaked in the acid solution, and with a brush to spread out each page to achieve uniform immersion. This requires books to have a certain strength, ancient books and other serious acidi cation of cultural relics, calligraphy and painting, and other scroll cultural relics without re-mounted can't be deacidi ed.
For a long time, many countries have carried out a lot of work on the batch deacidi cation protection of the whole book and the whole volume of archives. The traditional solution deacidi cation method the formation of powdery depositions and the bleeding of inks and colours in some occasions [16] . Gas phase deacidi cation requires the cooperation of high vacuum environment, and the equipment requirements are very demanding [19] . Therefore, it is urgent to study the new e cient and practical theory and technology of deacidi cation which can be processed on a large scale.
The gas phase batch deacidi cation equipment of ethylene oxide is designed in this paper. The mechanical strength, FT-IR, high resolution X-ray diffraction was used to characterize the simulated samples of untreated and EO-Ar treated paper samples before and after arti cial accelerated aging. It was found that the degree of polymerization of treated samples was higher than that of untreated samples. The characterization of by GC-MS, FT-IR, SEM shows that epoxy ethane has esteri cation reaction with acid substances in the paper, produces ester substances, which plays a certain role in strengthening the paper.

Paper samples
In this paper, acidi ed books from the 1950s to the 1980s were selected as the simulation samples of the whole book, and Ingrain paper from the 1940s (provided by the archives bureau of shaanxi province), Xuan paper made with traditional Chinese techniques (provided by the archives bureau of shaanxi province) were selected as paper simulation samples (Table 1).

Deacidi cation treatment equipment and deacidi cation process
The equipment consists of deacidi cation box, in ation system, control system, vacuum system, exhaust degradation system, safety system and other parts. The schematic diagram of EO-Ar deacidi cation equipment is shown in Fig. 1.Ethylene oxide gas needs to be gasi ed in the gasi cation chamber and then lled into the chamber. The argon inlet system is divided into dry argon and humidi ed argon. The purpose of lling water vapor with argon as carrier gas is to control the relative humidity of the chamber and achieve better deacidi cation effect.Deacidi cation temperature was controlled at 40℃±1℃.

FE-SEM
During ber section observation, the paper samples were rst soaked with water and then frozen in liquid nitrogen for 1 min. The frozen paper sample is broken along the length perpendicular to the paper. The frozen paper was broken along the longitudinal direction perpendicular to the paper and dried in a vacuum drying oven (60℃) for 2 h.
The sample to be tested is pasted on the special sample table with conductive adhesive, and the surface of the sample is not damaged. An ion sputtering instrument was used to spray gold 60 s uniformly on the surface of the sample to increase the conductivity of the sample. SU-8020 eld emission scanning electron microscope (Hitachi hi-tech co., LTD.) was used for the test, and the test condition was: acceleration voltage 1 kV. The spectrometer is equipped with a microscope and an ATR lens containing germanium crystals.

FT-IR analysis of paper extract
Weigh 300 mg of dried KBr, put them in agate mortar, mix well and grind in one direction for 2 min. Pour the ground KBr into the tablet mold, and pressurize it under 20 MPa pressure for 3 min to obtain the fully transparent potassium bromide tablet. Drop a drop of methanol extract from paper on the pressed potassium bromide tablet, and the infrared absorption spectrum of methanol was measured after volatilization.

GC-MS
Chromatographic column: hp-5 ms capillary column (30.0 m, 0.32 mm, 0.25 ms);The temperature of the column box is 80℃, the temperature of the injection port is 250℃, and the heating procedure is maintained at 80℃ for 2 min and 5℃/min for 230℃ and 3 min. With the carrier gas He, the purge ow rate is 3.0 mL/min, and the column ow rate is 1.97 mL/min.

Deacidi cation process
In this paper, simulated paper samples and discarded acidi cation books were deacidi ed with ethylene oxide for reinforcement. The EO-Ar treated and untreated blank control samples were tested and analyzed to demonstrate the bene cial effects of ethylene oxide on paper archives and cultural relics.
The deacidi cation process is carried out in the deacidi cation equipment independently developed and designed ( Fig. 1). Place the samples to be processed in the deacidi cation vehicle neatly and push them into the box, close the door and seal them. After the rst vacuumization reaches below 10 mmHg, stop the vacuumization and ll argon gas to normal pressure. Control certain temperature and humidity conditions for 24 h pretreatment. After the pretreatment, vacuumize below 10 mmHg and ll with ethylene oxide and argon at a certain proportion and speed. Control a certain temperature and appropriate time for static treatment. Finally, the water ring vacuum pump and light medium are opened to degrade the exhaust gas. After the pressure drops to 100 mmHg, nitrogen is charged to the normal pressure to degrade the exhaust gas again. After reciprocating for three times, open the air valve to return the pressure inside the box to normal pressure, and open the box to take out the sample.
There are four main factors that affect the deacidi cation effect of epoxy ethane -argon system on paper, namely the processing temperature, processing time, pretreatment humidity and gas ratio. The optimum process conditions for deacidi cation of epoxy ethane -argon gas system were found by orthogonal test. The pH value of sample 1 after deacidi cation was taken as the test index. In this experiment, the interaction between factors is not considered. Let's use the L 9 (3 4 ) orthogonal table. Factor levels are shown in Table 2.
The range analysis process of the test results is shown in Table 3. The pH value of acidi ed wood pulp paper with pH 3.8 was taken as the test index.  According to the direct comparison of Table 3, among the 9 experimental results, the deacidi cation effect of experiment 8 is the best, and it can be seen that the optimal combination of all factors in this experiment is A 3 , B 2 , C 2 and D 3 . The value of R indicates that there is a signi cant order of factors in this experiment, and the primary and secondary relationship is A B D C. In other words, the pretreatment humidity and the ratio of ethylene oxide to argon are the main factors affecting the deacidi cation effect. The second is the processing time and temperature. Therefore, it is concluded by orthogonal test that the pretreatment relative humidity and gas ratio are the most in uential factors in deacidi cation of ethylene oxide -argon gas system. The best condition for deacidi cation is that the deacidi cation item is pretreated for 24 h under the condition of 80%RH and 40℃, vacuumed to less than 10 mmHg. The ethylene oxide and argon (volume ratio 7:3) were lled into the tank at a rate of 2 mmHg/min to atmospheric pressure. Keep the temperature at 40 for 10 h. In addition, in the three levels of factor D in the table, D 2 and D 3 are very little different. Considering the deacidi cation e ciency, D 2 is selected in the actual deacidi cation to improve the mass deacidi cation e ciency.

pH variation
In order to prove the effect of EO-Ar mass deacidi cation process on the deacidi cation effect of paper. In this paper, the pH values of EO-Ar treated and untreated paper samples were tested.
After epoxy ethane treatment, the pH value of the samples can reach 7-9 [22] , which is the safe pH range for the durable preservation of paper cultural relics.
Sample 1 is Ground-wood printing paper, produced in the 1980s, which is the representative of acid paper archives. Before treatment, the pH value was about 5.57, and after treatment, the safe pH value was 8.74. Sample 2 is an ancient bamboo ber paper sample, which is the representative of weak acid paper archives. Before treatment, the pH value was about 5.57, and after treatment, the safe pH value was 8.74. Sample 3 is handmade rice paper, a representative of alkaline paper cultural relics. You can see that there's not much change in pH after treatment, only a slight increase. It indicates that epoxy ethane treatment will not cause too high pH of alkaline paper as traditional acid-base neutralization reaction deacidi cation, and prevent alkaline degradation of alkaline paper due to deacidi cation. To sum up, the pH of paper cultural relics treated with ethylene oxide can reach the optimal range of 7-9, no matter it is acidic paper or alkaline paper, and it will not cause too high pH of alkaline paper in the same volume or book.
Samp. 4, Samp. 5 is the book sample from the 1940s to the 1990s. The acidi cation is extremely serious. Before treatment, the pH is between 3-4, and after treatment, the pH can reach between 7-8, which is the most suitable range for paper durable preservation. Sample 6 is an ancient book. It is representative of the severely acidic Ancient books. The pH value was about 3.25 before deacidi cation, and increased to 7.12 after EO-Ar treatment.
Sample 5 is a sample of a thick book, 4.5 cm thick, with 765 pages in total. This book is a small dictionary of modern Chinese published by People's Education Press in 1980. We did a pH test every 100 pages from the rst page of the book to study the effect of the book's thickness on deacidi cation. As can be seen from Fig. 2(b), the pH value of both the middle page and the rst page of the book reaches a neutral alkalinity range of more than 7. The deacidi cation method is not affected by the thickness of the book, and each page can be effectively deacidi ed.

Effect on thermal stability of paper
Studies show that the natural degradation of paper is divided into acid catalytic degradation [7] and thermal oxidative decomposition [23] . It can be seen from pH values the EO-Ar treated and untreated of Samp.1, Samp.2, and Samp.3 that the EO-Ar treatment can effectively remove the acid in the paper and eliminate the hidden danger of catalytic hydrolysis of paper acid. However, whether EO-Ar treatment can effectively alleviate the thermal decomposition and aging of paper bers, we conducted thermogravimetric curve analysis of EO-Ar treated and untreated Sample. Figure 3 shows the TG and DTG curves of EO-Ar treated and untreated Sample. This curve was obtained at room temperature to 600℃, 20℃/min. It can be seen that there are three peaks on the DTG curve, indicating that thermal decomposition can be divided into three steps.
The rst step corresponds to the loss of adsorbed water weight within the paper ber. The second step corresponds to the mass loss of small molecules such as lignin and hemicellulose in paper. The third step corresponds to the thermal decomposition of cellulose in paper.
TG curves and DTG curves of untreated and EO-Ar treated acid paper are shown in Fig. 3a and Fig. 3b. It is obvious that the thermal decomposition of acid paper is divided into three stages. But different from alkaline and weak acid paper, it has the greatest loss of mass for the second stage. This is because sample 1 has been seriously acidi ed degradation, the paper ber crystal area has been destroyed. Due to the β-(1,4)-glycosidic bonds fracture in cellulose molecules, most of the cellulose has been broken down into small molecular bers. The thermal decomposition of small molecular cellulose coincides with that of lignin and hemicellulose. In the TG curve, the mass loss in the second step is the largest, the initial decomposition temperature decreases, and the thermal stability of the paper decreases. However, after EO-Ar treatment, the paper thermal decomposition temperature increased to 256.3℃. It shows that EO-Ar treatment can effectively improve the thermal decomposition temperature and thermal stability of paper.
The TG curves of sample 2 and sample 3 are basically the same. The only difference is that the TG curve of sample 2 has a signi cant mass loss around 269℃. This is because sample 2 is slightly acidic Ingrain paper, containing a small amount of lignin pectin and other small molecules not removed during the papermaking process. The initial extrapolation decomposition temperature of the untreated sample 2 was 297.49℃, reached the peak at 337.25℃, and the thermal decomposition ended at about 353.75℃ in Fig. 3c. The initial extrapolation decomposition temperature of the EO-Ar treated sample 2 was 296.96℃, reached the peak at 344.71℃, and the thermal decomposition ended at about 358.69℃ in Fig. 3d. It is speculated that ethylene oxide and paper carboxylic acid esteri cation reaction or it is condensed with its hydroxyl group to form esteri ed oligo -glycol. The paper ber thermal stability is improved to prevent paper thermal degradation.  Fig. 3e. The initial extrapolation decomposition temperature of the EO-Ar treated sample 3 was 331.75℃, reached the peak at 357.73℃, and the thermal decomposition ended at about 372.88℃ in Fig. 3d. It can be seen that the decomposition temperature of sample 3 also increased to a certain extent after EO-Ar treatment. It shows that EO-Ar treatment can also improve the thermal stability of alkaline paper. In addition, the weightlessness composition of alkaline paper treated with EO-Ar is slightly higher than that of untreated control samples, indicating that EO has a protective effect on inorganic materials such as calcium carbonate in alkaline paper.

Effect on mechanical strength of paper
The mechanical strength of paper is the basis for the use of books, archives and other paper cultural relics [24] . In order to analyze the in uence of EO-Ar treatment on paper strength, the tensile force, folding endurance and tearing resistance of raw and EO-Ar paper samples were tested.
As can be seen from Fig. 4, the mechanical strength of the paper samples after EO-Ar treatment has been improved to a certain extent. Although the increase in strength is extremely small, it also indicates that the EO-Ar deacidi cation process will not adversely affect the mechanical properties of paper. As can be seen from Fig. 4a and Fig. 4b, the tensile force of sample 1 is very high, but its folding resistance and tearing resistance are low. In particular, the tearing resistance is the smallest of the three samples. Although sample 1 has a certain tensile strength, but the folding and tearing resistance is very low, indicating that the paper is very fragile, breaking at a touch. Sample 2 is Bamboo paper, the basic weight of the paper is very small, the thickness of the paper is very uneven. The paper has the place of shade grain ber is less, resulting in folding endurance can not be tested, the error is very large. However, it can also be seen from Fig. 4c and Fig. 4d that the tensile force and tearing resistance are slightly increased both machine direction and cross-direction.
Sample 3 is handmade Xuan paper with alkaline pH. It can be seen from Fig. 3 that the machine direction and cross-direction tensile force, folding endurance and tearing resistance are relatively high. Rice paper ber is relatively long, paper is very soft. After EO-Ar treatment, the mechanical properties of the paper are improved, which makes the paper more durable. It shows that EO-Ar mass deacidi cation process will not cause damage to alkaline paper.
Whether the EO-Ar mass deacidi cation process is really conducive to long-term paper preservation requires further accelerated aging tests [4] .

Effect on the aging resistance of paper
It can be seen that EO-Ar treatment has obvious deacidi cation effect on acidi ed paper, and on the macro level, EO-Ar treatment does not cause direct loss of mechanical strength of paper, but increases to some extent. Whether EO-Ar treatment is bene cial to the durable preservation of paper, the following experiment is done in this paper. EO-Ar treated and untreated acidi ed paper samples and acidi ed books were put into the aging box at the same time for manual accelerated aging test, aging 240 h ± 1 h under 105℃ conditions. Mechanical strength conservation rate and crystallinity index test was carried out on the samples after aging.

Mechanical strength retention rate
The tensile strength, exural strength and tear strength of the untreated paper samples and the EO-Ar mass deacidi cation paper samples were tested before and after aging. The mechanical strength retention rate after aging was calculated, and the in uence of EO-Ar treatment on paper durability was further analyzed.
As can be seen from Fig. 5, the mechanical strength retention rate of the paper after EO-Ar deacidi cation reinforcement treatment is higher than that of the untreated samples, indicating that the EO-Ar treatment slows down the aging of the paper to a certain extent. Figure 5a and 5b show the retention rates of longitudinal and transverse tensile strength of paper samples, respectively. It can be seen that the strength retention rate of EO-Ar treated paper after aging is much higher than that of untreated samples, indicating that EO-Ar treatment can signi cantly improve the aging resistance of paper. Figure 5c is the longitudinal folding endurance retention rate, and Fig. 5d is the lateral folding endurance retention rate. It can be seen that, whether the paper longitudinal or transverse, after the EO-Ar deacidi cation treatment, the paper folding endurance retention rate is much higher than the untreated samples, and even some of the untreated samples are extremely fragile after aging, folding endurance cannot be measured at all, while the EO-Ar treated samples still maintain 40-50% folding endurance. It is speculated that the reaction products attached to the paper ber network structure, on the one hand directly increased the strength of the paper ber and the binding force between the ber bundles, on the other hand to reduce the loss of the paper binding water, so that the paper still maintain a certain degree of toughness.

Crystallinity index Cr.I based on XRD
In order to prove the bene cial effect of EO-Ar mass deacidi cation on paper durability, this paper conducted dry heat treatment arti cial ageing for both the EO-Ar treated and untreated samples conducted XRD test for the samples before and after aging. The XRD crystallinity index Cr.I of the ber is calculated according to the diffraction intensity [25,26] .
Where, I 002 is the XRD peak strength of cellulose crystallization zone, with 2θ between 22° and 23°. I am is the amorphous region of cellulose with an XRD peak intensity 2θ of about 18°.
It can be seen from Fig. 6 that after arti cial accelerated aging, the crystallinity index of paper treated by EO-Ar is higher than that of the untreated blank control group. It indicates that EO-Ar treatment has a certain protective effect on the crystallization area of paper, which is conducive to the durability of paper.
It is worth noting that several sharp strong peaks in FIG. 6a correspond to X-ray diffraction peaks of talc powder packed in the paper. For printing paper, talc powder and calcium carbonate will be added to improve the paper's printability.

Mechanism of deacidi cation reaction
There are two main reasons for acidi cation degradation of paper cultural relics. On the one hand, acidic and oxidizing gases (CO 2 , SO 2 , HS, NO, NO 2 , etc.) [3] in the external environment are enriched in paper. These acidic and oxidizing gases lead to paper cellulose acid catalytic degradation and oxidative decomposition [27] .
Especially lignin and hemicelluloses [28] , chemical properties are not stable, easy to be oxidized to form acid.
On the other hand is the papermaking process, due to bleaching, sizing and other introduced acid substances.

FT-IR spectral analysis
In order to study the deacidi cation mechanism of EO-Ar mass deacidi cation process, this paper tested the Untreated and EO-Ar treated paper by ATR-FTIR spectroscopy [29,30] . This method can be used to study whether the ber structure changes caused by the EO-Ar treated of paper, as shown in Fig. 7. The FT-IR spectra of the EO-Ar treated and untreated samples were compared and some changes were found.
In the infrared spectrum of paper samples, the wide and strong absorption peak of about 3330 cm − 1 is the associative hydroxyl O-H stretching vibration peak [31] . Because the hydroxyl group is a strong polar group, the paper cellulose molecules adsorbed water formed hydrogen bonds between each other, so only wide and strong associated hydroxyl absorption peak can be observed. The peak around 2900 cm − 1 is C-H and CH 2 stretching vibration [32] . The peak around 1640 cm − 1 is attributed to the bending vibration peak of O-H [33] . The peak around 1429 cm − 1 is the bending vibration peak of C-H [34] . The plane bending vibration of O-H is at 1370 cm − 1 . The peak at 1160 cm − 1 was attributed to the stretching vibration peak of C-O-C, which was related to the cellulose β-(1-4) glycoside bond. About 1105 cm − 1 is the asymmetric tensile vibration of glucosidecyclic ring. The stretching vibration peak of primary alcohol C-O is at 1028 cm − 1 , and the stretching vibration peak of secondary alcohol C-O is around 1055 cm − 1 [35] .
The characteristic peak of cellulose is located in the spectral range of 850-1500cm − 1 [36] . Among them, the 1650 cm − 1 -1450 cm − 1 region is the framework vibration of aromatic ring in lignin. In the infrared spectra of Fig. 7a, 1509 cm − 1 and 1602 cm − 1 represent the p-phenyl ring vibration, and 811 cm − 1 or so represents the vibration absorption peak outside the benzene ring surface. This indicates that the acid paper contains more lignin. In Fig. 7b and 7c, there was no absorption of 1509 cm − 1 , 1602 cm − 1 , and 811 cm − 1 , indicating that lignin content in basic Xuan paper and Weak acid bamboo paper was very low. This is also the rice paper is not easy to acidify yellow, one of the reasons for the long preservation life. Different from rice paper (Fig. 7c) and bamboo paper (Fig. 7b), acid paper (Fig. 7a) has absorption peaks around 1733 cm − 1 , 1718 cm − 1 and 1647 cm − 1 . The literature suggests that 1718 cm − 1 is the stretching vibration absorption peak of ketone and carboxylic acid carbonyl C = O, and 1647 cm − 1 is the stretching vibration absorption peak of aldehyde carbonyl [37,38] . Therefore, Fig. 7a 1718 cm − 1 is attributed to the stretching vibration absorption peak of carboxylic acid carbonyl C = O. After EO-Ar treatment, the peak position shifted slightly to the high wave number, and it was speculated that the carbonyl group of aldehyde [39] , ketone and carboxylic acid [40] reacted with ethylene oxide to form ester carbonyl group. The formation of the carbonyl ester can be veri ed by the absorption peak of C-O-C asymmetric stretching vibration of the 1200 cm − 1 ester.
However, this peak is associated with asymmetric stretching vibration of C-O-C of cellulose β-(1,4) glycoside bond. It is impossible to determine whether EO-Ar treated acid paper actually has esters, which needs further study.
As can be seen from Fig. 7d, after the EO-Ar deacidi cation treatment, ester group-related peaks at 1730 cm − 1 were signi cantly enhanced, while peaks related to carboxyl and aldehyde groups at 1666 cm − 1 were signi cantly weakened. This indicates that ester substances are indeed formed after EO-Ar treatment. It is speculated that with the participation of water, ethylene oxide after ring opening reacts with carboxyaldehyde group produced after paper ber degradation and ester is formed. In Fig. 7d, the -OH peak at 3379 cm − 1 is strong, because the paper extraction agent is methanol solution, which is not considered.
What happens to ethylene oxide during the deacidi cation process that increases the pH value of the paper and makes it more exible and mechanically stronger. In this paper, the methanol extract of paper treated with ethylene oxide was analyzed by GC-MS to determine the reaction product, and then the mechanism of ethylene oxide acting on the acidi ed paper was speculated.  The results showed that the severely aged paper contained a large number of acidic substances in Table 3.
The structure of cellulose was a long linear polymer based on the repeated dimer anhydrous ber disaccharide unit. The hydroxyl groups on the anhydrous glucose unit form carbonyl and carboxyl groups during oxidation or photooxidation [41] . The formation of aldehydes and ketones on carbon atoms C-2 and C-3 in anhydrous glucose unit is the cause of yellowing of cellulose during the aging process [42] . Acid catalyzed cellulose hydrolysis, resulting in cellulose β-1, 4-glycoside bond fracture, and then the formation of more acidic substances, which is the main reason for the loss of mechanical strength of the paper.
A large number of polyethylene glycol and esters with different molecular weights were detected in the paper treated with ethylene oxide. From the point of view of the structure of ester substances, it is produced by the esteri cation reaction between polyethylene glycol with different molecular weight and acid. During the treatment, the gas ethylene oxide was ring-opening polymerized into polyethylene glycol with different molecular weights. The main reaction of ethylene oxide deacidi cation is polyethylene glycol with acid dehydration condensation to form ester. Ester substances are formed on the surface of cellulose containing a certain amount of free water and acid substances, which is the main reaction of ethylene oxide deacidi cation (Fig. 10). The presence of polyethylene glycol (PEG) formed by ring-opening polymerization of ethylene oxide increases the toughness of paper bers. Figure 8 shows the forensic identi cation of bodily secretions published by People's Publishing House in 1979(Sample 4). The cover is color printing on offset paper, and the inside page is printed on ordinary mechanical offset paper. The book is cut into two parts, the lower part is strengthened by EO-Ar deacidi cation, and the upper part is not treated. After the treatment, EO-Ar treatment and untreated parts were put into moist heat aging box for arti cial accelerated aging for 240 h.

Macroscopic feature
As can be seen from Fig. 8, the color printed pages such as the cover and back cover of the untreated samples and the inner pages are seriously yellowed and brittle after aging, breaking at a touch. The EO-Ar treated book paper retains its original color and strength after accelerated aging. The contrast between the untreated paper samples and the EO-Ar treated paper samples is very obvious, indicating that the EO-Ar treatment has a strong protective effect on coated paper. EO-Ar mass deacidi cation can increase the paper toughness, so that the paper has a better aging resistance.
Book covers and other color printing paper generally for coated paper. This is to get better color when printing, the paper added talc kaolin and other llers to improve the whiteness of the paper and printability. Compared with the untreated samples, the coated paper after EO-Ar treatment has a higher whiteness and physical strength after aging, which indicates that EO-Ar treatment not only has a certain protection effect on the paper ber itself, but also on the paper llers.
After the accelerated aging of the untreated samples, in addition to the yellowing and fragility of the paper, the color printing ink has also changed color to a certain extent, with a dark color, as shown in Fig. 8a. This in addition to paper yellowing and darkening affect the ink color, the ink itself has changed the color. However, EO-Ar processing of the bottom half of the book in the aging ink color is still bright. It shows that EO-Ar treatment can protect the paper and increase the durability of color printing inks.

SEM analyses
The ber and porous structure from nano-to micrometer range can be observed by scanning electron microscopy (SEM). In order to study the effect of EO-Ar treatment on the microstructure of paper bers, eld emission scanning electron microscopy tests were carried out for Sample 4(Acidic paper), which were EO-Ar treatment and untreated treatment.
The paper after dry and aging, EO-Ar processing paper ber sections a full, round texture Fig. 9a, And untreated paper ber dry shrinkage. Fold, crisp, fracture, powder, peeling Fig. 9b. Look at Fig. 9b can be found that the ber surface with a layer of thin lm, analyzes its possible the formation of ethylene oxide polymerization by polyethylene glycol (PEG) deposited on the ber surface (Fig. 9b), retained the ber moisture content. hence EO-Ar ber by dry heat aging does not appear shrink after processing, it also from microstructure proved that EO-Ar aging after processing paper high mechanical strength retention than untreated samples. In conclusion, EO-Ar treatment has a certain strengthening effect on alkaline paper.
As can be seen from Fig. 9, after Sample 4 is aged by dry heat, the EO-Ar treated paper ber is smooth and mellow, while the untreated paper ber is crisp,breakage peeling.
It indicates that EO-Ar treatment consumes free acid in paper and prevents autocatalytic acid hydrolysis and oxidation β-elimination decomposition of paper ber. However, the presence of free acid and high temperature conditions in untreated paper, acid hydrolysis and β-elimination decomposition of the combined effect of the paper ber degradation. EO-Ar treatment forms a lm on the surface of acidic mechanical paper ber, protecting the paper ber and preventing the oxidation β-elimination decomposition of the ber, which is also the microscopic performance of the increase in mechanical strength of paper after EO-Ar deacidi cation. In conclusion, EO-Ar treatment has the effect of deacidifying and strengthening acid paper.
As can be seen from the paper ber section in Fig. 9c and 9d, there are signi cant differences in the inner wall of the ber cavity after accelerated aging. The inner wall of the untreated paper ber is cracked and tilted with ake shedding, while the inner wall of the paper ber strengthened by EO-Ar treated is smooth and round. In addition, the ber wall thickness of the EO-Ar treated paper was about 2.4 µm, and the ber wall thickness of the untreated paper was about 1.5 µm. This indicates that in the aging process, the untreated paper loses water, and the glucose chain recombines due to water loss, making the paper brittle. However, the EO-Ar treated paper retains some water, while the glucose chain is not recombined, thus maintaining better softness. It further shows that EO-Ar deacidi cation reinforcement can thoroughly and effectively remove the acid inside the paper ber, improve the aging resistance of the paper ber, and extend the life of paper cultural relics.
Based on this, we speculated that EO-Ar scale deacidi cation mechanism is shown in Fig. 10. In the whole deacidi cation system, argon plays a role in promoting the penetration of ethylene oxide in paper, adjusting humidity and ensuring the safety of deacidi cation system [43] . Ethylene oxide as a deacidi cation agent can produce nucleophilic substitution reaction with free acid organic acid in acidi ed paper and eliminate a large number of acidic substances [44] . Part of ethylene oxide self-ring-opening polymerization to form polyethylene glycol, increase the toughness of paper. Ethylene oxide can also nucleophilic substitution reaction with alkaline substances, will not lead to non-acidic paper alkaline too strong. Some small molecular weight polyethylene glycol can be condensed with hydroxyl group in cellulose to form branched oligopolyeglycol and enhance the antioxidant function of paper.

Conclusions
Using the mixture of argon and ethylene oxide as deacidi cation agent, the paper cultural relic mass deacidi cation research was carried out by using independently developed deacidi cation equipment and technology. There is no need to sort out the deacidi cation items, only need to be put into the box, vacuum pumping, gas distribution, aeration, static reaction and other simple operations, making the large-scale deacidi cation and reinforcement process extremely simple. At the same time, the environmental protection and safety facilities, such as zero emission of photomedia degradation, have been developed. Argon has fast diffusibility and no speci c adsorption, which makes ethylene oxide diffuses quickly and the microscopic reaction is uniform and deep. It overcomes the disadvantages of mechanical dispersion and liquid immersion in the international existing technology, such as low e ciency and damage to the paper of deterioration, and lays a foundation for the e cient operation of the whole mass deacidi cation and reinforcement.
The study revealed that ethylene oxide gas could nucleophilic substitution reaction with free acids in acidi ed waste paper and esteri cation reaction with organic acids in situ, and established a stable system for removing free acids and organic acids. At the same time, with alkaline paper in the free alkali, organic alkali can also occur nucleophilic substitution reaction, so that its pH basically unchanged, keep appropriate. Ethylene oxide in aging paper after the reaction to open loop self-polymerization to form polyethylene glycol, or cellulose hydroxyl condensation to form branched oligopoly glycol, enhanced the anti-oxidation function of paper, protect the ber crystal area, improve the durability of paper. It overcomes the weakness of alkaline materials in international technology, such as incomplete reaction to organic acids, easy acid re ux and lack of reinforcement. Discard it will make alkaline paper alkali retention excessive lead to corrosion, degradation of the fatal weakness. In particular, the above functions and advantages accord with the fact that archives, books and ancient books are mixed with acid and alkali, which lays a theoretical foundation for the mass deacidi cation and reinforcement.

Declarations
Availability of data and material The datasets analysed during the current study are not publicly available but are available from the corresponding author on reasonable request.   Mechanical strength retention rate before and after aging, machine direction tensile strength retention rate (a), cross-direction tensile strength retention rate (b), machine direction folding endurance retention rate (c), crossdirection folding endurance retention rate (d), machine direction tearing resistance retention rate (e), crossdirection tearing resistance retention rate (f) SEM images of Sample 4 after aging The ber surface Untreated ×20k a The ber surface EO-Ar treated ×20k b The inner wall of the untreated broblast cavity ×3.5k c The inner wall of the EO-Ar treated broblast cavity ×3.5k d Figure 10 Protective effect of EO-Ar treatment on paper bers

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