Deterioration of Contemporary Kaihua Handmade Paper: Evolution of Molecular, Supermolecular and Macroscopic Structures


 Valued for its toughness and durability, Kaihua paper has been famous in China since the Qing Dynasty and is in the focus of the study on paper life extension. Understanding of the complex degradation behavior of handmade paper under various aging conditions is an essential precondition for preparing contemporary long-life handmade paper. However, reactions sequence and quantitative analysis of microstructure of cellulose and its relation to macroscopic deterioration of handmade paper still are difficult to interpret. Herein, different types of Kaihua handmade papers, produced by various raw materials and crafts, were artificially aged to study the different evolution of their multi-scale structures in dry-heat (DH) and wet-heat (WH) conditions systematically. Two-dimensional correlation spectroscopy (2D-COS) distinguished possible carbonyl vibrations involving hydrolysis and oxidation of cellulose and gave sequential changes of various carbonyl groups, illustrating different evolution behaviors of molecular structure of papers in DH and WH aging process. The energy and distance of hydrogen bond, crystal size and microfiber accessibility were quantitatively calculated during the degradation of cellulose. The dual roles of water molecules, as plasticizers for fibers and as carriers of protons, continuously promoting the hydrolysis of cellulose at molecular and supramolecular scales were elucidated further. In addition, correlations between microstructural evolution and macroscopic deterioration of handmade papers involving reduced mechanical properties and yellowing were revealed.


Introduction
Traditional handmade paper, as an important carrier of inheriting human civilization, is featured excellent performance of its durability ( great reputation as the king of paper that could last for more than 1000 years (Luo et al. 2021; Tang et al. 2017). Kaihua paper is a kind of legendary handmade paper in China, which was used exclusively by the imperial palace in the Qing Dynasty. It was considered that the craft of Kaihua paper has reached the pinnacle of ancient papermaking, which has great culture value. In recent years, the Research Institute of Kaihua Paper has been established in Kaihua County to restore Kaihua handmade paper, which represents one of the highest levels in China. Although modern papermaking techniques have enhanced the production e ciency, the resultant loss of quality in handmade paper, especially long-term stability, calls for a united effort on this common decline. (Luo et al. 2021; Baty et al. 2010). Hence, it is of fundamental importance to understand the dynamic degradation of paper in multi-scale structures, and nd out how an accumulation of microscopic variation of cellulose structure results in the macroscopic deterioration of handmade paper involving discoloration and loss of mechanical resistance, which will enable ultimately access to preparing long-life handmade paper and possessing the great signi cance to the restoration of ancient books and cultural relics.
Presently, degradation of handmade paper is controlled by mixed hydrolytic and oxidative mechanisms at molecular scale, which is accelerated autocatalytically by protons and active oxygen species. It has been characterized by IR, UV/Vis, Raman and NMR spectroscopic method due to the nondestructive characteristics of these testing methods ( Carbonyl groups of various degrees of freedom as good candidates, which exhibit high activity in FTIR spectra, are able to trace the changes of molecular structure of cellulose involving mixed hydrolytic and oxidative mechanism (Abidi et al. 2014;Celino et al. 2014; Liu et al. 2017). J. Lojewska de ned the oxidation index in FTIR to follow the degradation of cellulose and demonstrated a parallel-consecutive mechanism of cellulose oxidation by tting the spectra with multiple Gaussian-Lorentzian functions (Lojewska et al. 2005(Lojewska et al. , 2006. However, the band assignment remains unsatis ed since the peaks at rang of 1600 to 1800 cm − 1 are weak and overlapping. Moreover, the attempt to distinguish the sequence of reactions and their interactions is much more di cult. Two-dimensional correlation spectroscopy (2D-COS) offers a powerful tool to explore the reaction process at the molecular scale in complex systems and evaluate the differences appearing during an Nevertheless, paper degradation is a complex system, especially for handmade paper with various raw materials and papermaking process. Recent studies offer little information on the dynamic changes of microstructure systematically from molecule to supermolecule for various handmade papers and aging conditions. Furthermore, the relationships between multiscale microstructure and macro properties are still unclear so far.
Herein, we focus on the evolution of the multi-scale structure of four typical handmade papers (with various raw materials and papermaking process produced in Kaihua, China) in DH and WH accelerated aging conditions. 2D-COS analysis has been applied to distinguish carbonyl groups of various degrees of freedom from the hydrolytic and oxidative path and trace their evolution sequence. Moreover, the dynamic changes of multiscale structure for various handmade materials are investigated to offer quantitative analysis during the aging process systematically in terms of groups, molecular chain, crystal structure, hydrogen bonding networks and micro bril morphology under different external conditions (relative humidity, temperature). In this way, the multi-scale structural evolution of various handmade papers from molecule to supermolecule is traced, which can be associated with the deterioration of macroscopic characteristics, and make a better understanding of various degradation mechanism of handmade papers.

Accelerated aging of samples
For dry-heat (DH) aging, all samples were placed into a temperature controlled dry-heat oven at 120 ± 2 ℃ for 1, 3, 5, 7, 14, 21, 28, 42, 56 and 70 days. This temperature is chosen in agreement with the conditions of accelerated aging for the cultural durable paper giving by the standards ISO 5630/4-1986 and GB/T 464. For wet-heat (WH) aging, considering more profound changes occurred in higher humidity, all samples were placed into a temperature and relative humidity (RH) controlled oven at 70 ℃ and 80% RH.

Group content measurement
The content of reducing carbonyl group was determined according to the colorimetric method proposed by Szabolcs and Matija (Kocar et al. 2004). The content of carboxylic acid group was measured using conductometric titration methodology (Saito and Isogai 2004). More details can be found in SI.3 from supplementary data.
Crystalline-amorphous structure measurement The crystallinity, crystal size and spacing were determined by X-ray diffractometer (D8 Advance, Bruker). The incident X-ray radiation was the Cu Kα with a power of 30 kV and 10 mA. The crystalline index, crystal size and d-spacing of samples were calculated through the peak area method (Eq. 1), Scherrer equation (Eq. 2) and Bragg equation (Eq. 3) respectively in SI.4.

Degree of polymerization measurement
Intrinsic viscosities of the samples were obtained by a Nordic viscometer using 0.5 M of copper ethylenediamine as the solvent, and were calculated to degrees of polymerization (DP) by the reported method (De Silva and Byrne 2017).
ATR-FTIR spectroscopy and Two-dimensional correlation spectroscopy (2D-COS) analysis FTIR spectra were collected using the FTIR spectrometer (Frontier, Perkin Elmer) with a sampling accessory diamond window. All spectra were recorded on ATR mode at 4 cm − 1 resolution in the region of 600-4000 cm − 1 and 16 scans. Three points were tested on each sample. For the analysis of carbonyl bands, all papers were placed in the oven at 100℃ for 30 min before experiments to desorb free water whose bending vibrations mask the carbonyl bands. Additionally, collecting the spectra at the oven lamp to prevent the samples from re-absorbing water. 2D-COS analysis was carried out from ART-FTIR spectra as input data for generating correlation maps via 2Dshige software. As for the analysis of hydrogen bond, the DH and WH weathered samples were carefully re-conditioned at 23℃ and 50% RH before measurements. The area of the CH vibration band (2900 cm − 1 ) of the initial spectrum was the normalization factor for absorbance values. The hydrogen bonds peaks were obtained by deconvolution and peak-tting. The energy and distance of hydrogen bond can be calculated by Eq. 4 and Eq. 5 in SI.5.

Morphology measurement
Scanning electron microscopy (SEM) images were taken by a microscope (FEI FP Quanta 250) at the voltage of 15 kV and a working distance of 7 mm.

Tensile measurement
The tearing strength of the sample was determined by using the falling pendulum (elmendorfs) apparatus (Thwing-albert) according to PN-EN ISO 13937: 2002 test standard. The sample was balanced and measured at 23 ℃ and 50% RH with dimensions of 60 mm length and 60 mm width. The tearing strength test was repeated to get the average value of three specimens.

Color measurement
Color measurements of sample were recorded using a spectrometer (Lorentzen-wetter). Three replicates were measured at three locations on each specimen. The change in color △E was calculated using the following equation: △L, △a and △b are the differences between the initial and nal values of L, a and b, respectively (Fabiyi et al. 2008).

Results And Discussion
The experimental objects of this work were four typical handmade papers produced in Kaihua in Zhejiang Province, where the production of Wikstroemia plants accounted for more than 90% in China during the 1960s. Thick bast and thin ber of Wikstrokea skin meet the requirement of making high-quality handmade paper, leading to the good toughness and excellent durability of some traditional handmade paper such as Dongba paper and Zang paper in China, and Gampi paper in Japan. Kaihua, where might produce imperial paper in Qing Dynasty, has developed a number of excellent traditional handmade papers. Herein, we rst carried out the characterization of basic physical and chemical properties in terms of four handmade papers (Table S1 and Fig.S1).
Our work concentrates on the multi-scale analysis of the microstructure evolution and degradation of various Kaihua handmade papers at different aging conditions. The correlation between micro-structure with macroscopic properties is highlighted, whereby the microstructural dependence in terms of molecule and supermolecule is the key issue.
Evolution of molecular structure of Kaihua handmade paper in DH and WH aging process The samples are evaluated through 2D-COS from the time-dependent infrared spectra of the aged handmade papers (Fig. 1). All samples including 1#-4# handmade papers conform to this behavior, illustrating basically similar changes of molecular structure for various raw materials and technics.
Taking 1# handmade paper as an example, the regions from 1800 to 1580 cm − 1 of the 2D-COS, where has proved to follow the degradation of aged cellulose under various conditions, are shown in Fig. 1a-h. Considering the complex reaction stage in aging process and to pick up useful local features of the correlation pro les, we evaluate 2D maps for two treatment time ranges of 0-7 days and 7-70 days.
In the synchronous spectrum of 1# in DH condition (Fig. 1a), important auto-peaks at 1595, 1695, 1715, 1745 cm − 1 were evidenced. The presence of these peaks is assigned to C = C stretching from lignin, conjugated carbonyl, aldehydic groups and ester from cellulose, respectively, which indicates that these peaks undergo signi cant changes. More information can be obtained from the asynchronous correlation spectrum. As shown in Fig. 1b Fig. 1c and d), changes of peaks at 1735 and 1760 cm − 1 assigned to carboxylic group (Lojewska et al. 2005) and gamma lactone (Fabiyi et al. 2008) are identi ed in addition to the peaks at 1595, 1695, 1715, 1745 cm − 1 . The assignment of bands supports the consecutive character of the reaction mechanism and the band around 1735-1745 cm − 1 reveals a nal oxidation stage of carbon atoms. We nally obtain the following sequence of the spectral change after 7 days: 1745 1735 1715, 1760 cm − 1 . This order of reaction means that C = C from lignin changes rst, followed by conjugated carbonyl, aldehydic groups and ester from cellulose. In the second exposure time range, ester exhibits a profound change and carboxylic acids occur as a nal oxidation stage of carbon atoms. We can infer that 1# handmade paper mainly experiences a complex oxidation reaction pattern in DH aging process, starting from ketones turning into conjugated diketones and from aldehydes into carboxylic acid. Simultaneously, intramolecular esteri cation between acid and alcohol gives rise to lactone.
The spectral region from 1800 to 1580 cm − 1 of 1# in WH condition, shows some different characteristics of the 2D-correlation IR spectra compared with DH aged samples (Fig. 1e-h). In the synchronous 2Dcorrelation IR spectrum of WH aging, one broad auto-peak at 1715 is observed throughout the aging process. This auto-peak can be considered to be generated from hydrolysis of hemiacetal bonds and eventually produce aldehyde groups by opening the terminal rings, which is supported by one reaction path that converts carbon atoms from glucopyranose into aldehydic group based on a simple hydrolysis model (Lojewska et al. 2006). Different from dry-heat aging process, the predominant reaction is hydrolysis in the presence of water vapor.
According to the analysis of synchronous and asynchronous spectra, the following sequence of the spectral changes is established: 1595 > 1695 > 1715 > 1735 > 1745 cm − 1 in the range of 0-7 days and 1715 > 1695 > 1735 > 1745 cm − 1 in the range of 7-70 days, indicating that modi cation occurs in C = C from lignin at the beginning of aging, followed by the conjugated carbonyl, aldehydic groups, carboxylic group and ester from cellulose. For longer treatment time, an increase of the extent of modi cation of aldehydic groups continues to taking place and prior to carboxylic group and ester. Therefore, the moment of absorbed water and C = C bonds of substituted aromatic ring coupled with conjugated C-O bonds in lignin is changing rst, followed by carbohydrates. Although the oxidation and hydrolysis reactions occur in both DH and WH aging process, different behaviors are showed, especially after 7 aging days. The increase of carbonyl and carboxyl groups in the rst seven days proves that oxidation and hydrolysis occur simultaneously. The water molecules accelerate the production of carboxyl groups, which further promote the occurrence of hydrolysis. As the aging time increases, the content of carboxyl group increases rapidly in DH condition because of a series of consecutive oxidative reactions ( Fig. S3a and b). While the content of aldehyde increases rapidly in WH condition, indicating that hydrolysis process still plays a main role and generates the mainly formation of aldehydic groups on terminal rings (Fig. S3c and d).
We further veri ed the aging process under different conditions through the changes of breakage rate of cellulose molecular chain and integral area dynamics curve from infrared spectrum. The increase in number of chain scissions is the essential mechanism of cellulose degradation. As shown in Fig. 2a, the number of molecular chain breaks show a different trend in DH and WH condition. In WH condition, the function ts well to the data and the kinetic curve is closed to 1st order rate law, indicating the predominating reaction is hydrolysis, which is in accordance with the Ekenstam model. In DH condition, an important feature of curve is that it exhibits acceleration followed by chain breakage number stabilization. The kinetic curve in DH seems complicated and a single exponential function does not t it, re ecting a complex reaction. All this information can be consistent with our previous two-dimensional infrared analysis results. Oxidation and hydrolysis occur in the initial stage in DH. After seven days, oxidation predominates and precedes hydrolysis. The main reaction is hydrolysis in WH condition and water molecules catalyze the hydrolysis of glycosidic bonds.
As mentioned above from 2D-COS, the carbonyl groups with different degrees of freedom at the region of 1640 ~ 1800 cm − 1 in FTIR reveal a good ability to trace the hydrolysis and oxidation of handmade paper.
Therefore, we build a model to re ect the aging process of various handmade papers based on the change of the integral area from 1680-1800 cm − 1 in the normalized infrared spectrum. The integral quantization of carbonyl bands in infrared spectra has been proved to be feasible (Lojewska et al. 2005). In this work, we narrowed the integral region and concentrated on the more sensitive parts. As shown in Fig. 2b, the integral area kinetic curve is similar to the kinetic curves based on DP measurements, except that the gap between DH and WH is reduced after 7 days. This shows that the infrared spectrum re ects well on the occurrence of oxidation reactions in this region. The mixing mechanism of hydrolysis and oxidation of handmade paper is effectively represented during the aging process, which indicates the feasibility of non-destructive monitoring of aging behavior via infrared spectroscopy. We calculate the exact content of reducing carbonyl and carboxyl by colorimetry and titration methodology, respectively. The total amount of them is in accordance with the curve of infrared integration, which further con rms the reasonableness of the aging process that we inferred (Fig. 2c).

Evolution of supramolecular structure of Kaihua handmade paper in DH and WH aging process
The supramolecular structure of cellulose plays a crucial role in determining the rate of degradation process, as a high supramolecular order of the polymer chain usually impedes degradation. We carry out an analysis of ultra-structure to illustrate evolution of the hydrogen bond and crystalline-amorphous region in greater detail using peak tting method.
We trace the change of hydrogen bond for 1# sample by FTIR and calculate the energy and distance of hydrogen bond. As shown in Fig. 3a  basically unchanged and no distortion occurs after accelerated aging for 70 days. We obtain the crystallinity, crystal size and interplanar spacing via SI.4. As shown in Fig. 3c and d, no signi cant change of crystallinity in aging process is observed even after 70 days, suggesting that most of the aging degradation takes place in the amorphous zone. The crystal size in DH aging process does not change too much, while crystal size gradually increases in the early stage of WH aging and then becomes constant in the latter stage. It is also found that interplanar crystal spacing gradually increased with the process of aging both in DH or WH condition, indicating that the accessibility is increased due to the molecular chain breaking and more loose structure in amorphous region.
Those results re ect that the degradation mainly occurs in the amorphous zone, and decreased DP also causes slight imperfections in the crystalline structure, which is more obvious in the system with water vapor. Based on the evolution of molecular and supramolecular structures, we propose a mechanism diagram to illustrate the difference of the ne structure of paper cellulose in DH and WH aging process (Fig. 3e). In DH condition, the removal of bond water contributes to the increase of hydrogen bond energy, causing free hydroxyl group on the cellulose chain to recombine and form new intramolecular and intermolecular hydrogen bonds, which results in the increase of hydrogen bond forces and the decrease of hydrogen bond spacing. Hydrolysis and oxidation reactions occur after dehydrated, which cause the breaking of molecular chains and retard continuous formation of intermolecular hydrogen bond. Therefore, the loss of bond water is an important reason that accelerates aging process. In WH condition, water molecules enter the amorphous regions and react with different cellulose-surface hydroxyl groups. A fraction of intermolecular hydrogen bonds of cellulose chains is lost and compensated by hydrogen bonds with water molecules. As a result, the cellulose chain structure is slightly loose, leading to the decreased energy and increased distance of intermolecular hydrogen bond (Wang et al. 2014). At a same time, a swelling of the cellulose chain structure generates a release of some strain that is originally in place in the cellulose chains, leading to few imperfections, expanded d-spacing and increased accessibility between micro bers in crystalline region. Therefore, long-term effects of water have much more harmful to handmade paper due to water molecules as a dual function: as a plasticizer for cellulose and as carriers of protons, catalyzing the hydrolysis of hemiacetal bonds.
These two different processes can also be veri ed on SEM images in Fig. 4. As for the DH aging, the bers of Wikstroemia monnula ber (1#) begin to be slightly tapered and peeled after 7 days. As the aging time further increases, reduced diameter and peeling can be observed resulting from dehydration and broken bonds. Different from DH aging, the bers show obvious swelling behavior during WH aging process. The ber starts to appear ake and organize into bundles of aligned bers once the aging reaction occurs. The longer the aging days are, the higher the bers swell. This swelling behavior is probably linked to the in ltration of water molecular in handmade paper matrix.
The effect of craft and raw materials on aging degradation of Kaihua handmade paper Furthermore, the effects of craft and raw materials on aging degradation of handmade paper are studied. As shown in the Fig. 5a, the rate of chain scission increases as a two stages model, similar to infrared integral area curve in Fig. 5b. Speci cally, the degradation rates of 1# to 4# handmade papers in early stages are 6.97×10 − 5 , 9.63×10 − 5 , 6.34×10 − 5 , 4.91×10 − 5 in DH and 6.95×10 − 5 , 5.81×10 − 5 , 2.48×10 − 5 , 1.48×10 − 5 in WH respectively, exhibiting that degradation seriously depends on the cooking and bleaching process: the chain scission rate of 4# handmade paper with traditional CaO cooking and plant ash bleaching process is approximate ve times lower than that of the Na 2 CO 3 cooking and sodium hypochlorite bleaching process (1#) in WH. In addition, the aging rate of handmade paper with hybrid materials is lower than that of single wikstromia and green sandalwood.
Since the dominance of amorphous degradation and very slow degradation in the crystallization zone, an equilibrium is reached when the fast consumption of the glycosidic bonds in the amorphous region.
Combined with the changes of DP, crystal structure and hydrogen bond of four handmade papers during degradation, we found that the aging process is relevant to the initial amount of glycosidic bonds and stability of weak link in the amorphous regions, and is also affected by the changes of pH level. Although the raw material and initial crystallinity of 4# handmade paper is similar to that of 1#, 4# handmade paper remains more glycosidic bonds in the amorphous region by traditional processing, making the chain breaking rate slow down ( Fig. S4 and Fig. S5). As shown in Table S2, 4# handmade paper also shows a more stable hydrogen bond network involving energy and accessibility under all conditions. Moreover, we note that the SEM-EDS analysis indicates that 4# paper contains micron-sized particles with a high level of calcium that are spread in the handmade paper, which could be a product of the traditional pulp technology (Fig. 5d). Although other types of handmade papers contain calcium content, their content is much lower than that of 4#. Those micro-sized and calcium-rich particles in 4# handmade paper, presumably calcium oxide or calcium hydroxide, are effective in neutralizing acidic products of the hydrolysis of paper. The slowest rate of pH change of 4# handmade paper in four handmade papers proves it (Fig. 5c).
Relationship between macroscopic degradation and microstructural change These microscopic changes that we have discussed ultimately affect macroscopic performance. Finally, we test the chromatic aberration and tearing degree of handmade paper with various aging conditions. As shown in Fig. 6a and b, the tearing strength resistance of four handmade papers decreases by 36.5-47.9% in DH condition and by 37.2-49.6 % in WH condition after 70 days. The trend of tearing strength resistance ( Fig. 6a and b) is highly consistent with that of the degree of polymerization (Fig. 5a). The decline rate of the rst stage ranks as: 1#>2#>3#>4#, indicating that the decline rate of mechanical properties of handmade paper is positively related to the molecular chain breaking rate of handmade papers. Interestingly, the results show that the formation of chromatic aberration has little to do with the amount of lignin, but strongly depends on the number of carbonyl groups in the form of aldehydes and conjugated diketones. The initial carbonyl content determines the rate of chromatic aberration, and the nal chromatic aberration value is positively correlated with the carbonyl content generated during aging process (Fig. 6c-f). The bleaching method of hypochlorite acid introduced more ketone groups and aldehyde groups, so the handmade paper (1#-3#) prepared by this process has more initial carbonyl group content and a higher yellowing rate than the handmade paper (4#) prepared by the traditional process. Kyujin Ahn et al reported that the carboxyl group did not give rise to chromophore generation independent whether present as a free acid or in their protected form of a lactone. However, the carboxyl group promotes and degenerates the color condensation reaction of carbonyl groups (Ahn et al. 2019).
This can explain that 4# handmade paper still shows the slowest coloration rate due to its lower carbonyl content, although with higher lignin and carboxyl content. Also, the WH aging process appears more pronounced yellowing compared with DH aging, which because the continuous generation of new reductive terminal carbonyl groups are produced from hydrolysis when water molecules are present, making the chromatic aberration value increases rapidly.

Conclusions
In summary, a systematic study of multi-scale structure evolution of four typical Kaihua handmade papers in DH and WH accelerated aging process is reported. The different degradation mechanism of Kaihua handmade paper in DH and WH aging process is revealed. 2D-COS distinguishes possible carbonyl vibrations involving hydrolysis and oxidation of cellulose and gives different reaction paths of molecular structure of paper in DH and WH conditions. The energy and distance of hydrogen bond, crystal size and micro ber accessibility are quantitatively calculated during the degradation of cellulose. A dual function of water molecules is further recognized: as a plasticizer for ber and as carriers of protons, continuously promoting the hydrolysis of cellulose and will be more harmful to the lifetime of handmade paper in the long run. The degradation behavior of different types of papers also indicates that the handmade paper produced by traditional craft exhibits more excellent durability due to the more initial glycosidic bonds, relatively stable hydrogen bond networks and higher pH value in it.
In addition, correlations between microstructure evolution and macroscopic deterioration of paper are explained. The yellowing of handmade paper is determined by the content of aldehydes and conjugated diketones carbonyl groups, while the decreasing rate of mechanical properties is positively correlated with the fracturing rate of molecular chain of handmade paper. All the information helps to elaborate the complex degradation mechanism of handmade paper under different aging conditions, and will be the startup for the design of more durable handmade paper. Synchronous and asynchronous 2D-COS (a-h) and FTIR (I and j) of 1# handmade paper in 1800-1580 cm-1 region. The dry-heat and wet-heat treatment time range of 0-7d and 7-70d.        Tearing strength retention (a and b), chromatic aberration (c and d) and carbonyl group content (e and f) of four typical handmade papers at various aging days in DH and WH condition.