Correlation of wood properties with chemical composition and microstructure of wood components

doi:10.21203/rs.3.rs-2611726/v1

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Correlation of wood properties with chemical composition and microstructure of wood components

https://doi.org/10.21203/rs.3.rs-2611726/v1

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In order to investigate the nature of the change of wood properties, to understand and master the deterioration mechanism of wood properties, this paper takes the wooden components replaced during the renovation of the Guhuaxuan in the Imperial Palace as the research object, nondestructive testing technology, chemical composition analysis and electron microscope scanning(SEM) analysis were adopted to study the changes of the chemical composition and microstructure of the wooden components, so as to establish the correlation between the macroscopic performance of wood and its chemical composition and microstructure. The results showed that cellulose, hemicellulose and lignin degraded to a certain extent after the wood was used for a period of time under the influence of visible light, temperature and water, and the components and degrees of degradation were different at different locations. The correlation between heterocellulose, α-cellulose, lignin and wood component density and modulus of elasticity (E_d) are good, and the order was α-cellulose > heterocellulose > lignin. The density and crystallinity of wood components are positively correlated with E_d.

Wood components

Wood property

Chemical Components

Microstructure

Correlation

Wood components are subject to a decrease in detection accuracy due to cracks, decay and insect infestation, which are inextricably linked to changes in the chemical composition and microstructure of wood. As the basic structural and functional unit of wood, the cell wall structure influences the strength of the wood fibers and is the main load-bearing structure of the wood. The cell wall of wood is composed of chemical components such as lignin, cellulose and hemicellulose. Changes in cell wall structure are usually assessed by chemical composition, microscopic morphology, and cellulose crystallinity. Recognition of such mechanistic endogenous factors can help in damage analysis, restoration decisions, and targeted conservation studies of ancient buildings.

For ancient timberwork buildings in China, Wang Xueliang (2008) considered the effects of insect infestation, decay, long-term load effects and dry shrinkage cracks on the long-term resistance of wood and proposed a multi-factor stochastic time-varying resistance model for wooden structures of historical buildings based on the Gerhards damage accumulation model. Qin et al. (2017) established a strength degradation model for old wood members that can consider the time-varying properties of old wood members based on the cellular characteristics and load-bearing of wood, considering the phenomenon that the performance data of old wood have large dispersion and poor correlation. Most of these studies have focused on the determination of wood deterioration and the effect on the physical and mechanical properties of wood. The wood properties of wood depend mainly on its cell wall material (Ge 1985), and its macroscopic damage of all types and reduction of mechanical properties are attributed to the decay of physical and chemical properties of wood at the microscopic scale (Mi 2021). Liu Wenbin (2006) determined the changes in the chemical composition and microscopic situation of the wooden components of Wuyingdian in the Imperial Palace and found that the mechanical properties and chemical composition were significantly changed, and the main chemical components of the cell walls were degraded. Wang Chuangui et al. (2012) explored the effect of hemicellulose and lignin on the mechanical properties of wood and found that lignin had a significant effect on the elastic modulus of the cell wall. Li et al. (2019) explored the aging of the load-bearing wooden elements of Changchungong in the Imperial Palace and found changes in the relative crystallinity of the wooden elements as well as in the chemical groups in the components. Wang et al. (2014) found that the increase in crystallinity was one of the most important factors leading to the increase in the longitudinal elastic modulus of the cell wall. Han et al. (2019) found that the morphological, chemical and physical properties of the cell walls of wood components occurred with the growth of use, and degradation of hemicellulose and lignin occurred.

As can be seen, a large number of studies have been conducted on the physical and mechanical properties of ancient timberwork buildings. Most of the studies on wood chemical composition and microstructure at home and abroad focus on the following. There is a correlation between changes in wood chemical composition and its mechanical properties; wood cell walls are related to its physical, mechanical and chemical properties; and wood microstructure is closely related to wood wood properties. However, they rarely consider the specificity of ancient architectural values, combining chemical composition and microstructure to interpret them. Macroscopic damage is often caused by the decay of the wood cell wall structure, and an in-depth study at the microscopic scale is a fundamental requirement for its deterioration analysis. In the conservation of ancient buildings, direct macroscopic studies using a large number of old wooden elements are more difficult to achieve, and traditional characterization tools for wood studies are difficult to reach the cellular level. With the use of new analytical principles, methods, and techniques in recent years, optical and wave spectroscopy techniques are widely used in various disciplines with many advantages such as fast analysis speed and high accuracy of results (Anna et al. 2015; Romagnoli et al. 2018 ; Yuan et al. 2020). These analytical methods are more suitable for microscopic analysis of wooden elements of ancient buildings because they require very few samples, are nondestructive, and can provide in-depth analysis of wood preservation from the perspective of its main chemical composition and crystalline structure.

This paper will start from chemical composition and microstructure analysis, using chemical composition analysis, infrared spectroscopy analysis, electron microscopy scanning analysis and X-ray diffraction analysis, etc., to study the chemical composition and microstructure changes of Guhuaxuan wooden components, and establish the correlation between them. This can understand the changes in the timber properties and the damage of the old wooden elements after years of service, and strive to provide a scientific basis for the reasonable restoration and protection of ancient wooden buildings and the maximum protection of their original value, with a view to providing an important reference value for the protection, inheritance, research and utilization of ancient wooden buildings in China.

Materials

The material for Guhuaxuan repair to replace a eave purlin, such as Fig. 1 (a) D5-D6. This wood component was located outdoors, in direct contact with light, heat, moisture, etc. and subject to natural aging effects such as continuous loading, and was a typical natural aging wood component. The diameter of the wood component is about 30 cm, and the total length is 300 cm. Through on-site investigation and species identification, the wood component is hardwood pine. The wood was divided into 6 sections according to 50cm-100cm-150cm-200cm-250cm-300cm, numbered A, B, C, D, E, F, G in order, and nondestructive testing was performed in order. The wood component is shown in Fig. 1. The specimens were divided into 10 groups according to the actual depth, as shown in Table 1, and the longitudinal section grouping and height ratio are shown in Table 2. A number of outer and inner specimens were taken from the three positions of number A, D and G. The inner side of all three positions was marked as 1 and the outer side was marked as 2, as shown in Fig. 2. The samples taken were analyzed for chemical composition and infrared spectroscopy, and the remaining samples were analyzed for microstructure, and the sampling method is shown in Fig. 3.

Table 1

Cross-section Grouping and Depth Ratio
Name	Cross-section Depth and Ratio
Name	Depth(mm)	Ratio(d)
Group 1	15	1/20
Group 2	45	3/20
Group 3	75	1/4
Group 4	105	7/20
Group 5	135	9/20
Group 6	165	11/20
Group 7	195	13/20
Group 8	225	3/4
Group 9	255	17/20
Group 10	285	19/20
Note: The method for determining the depth ratio (d) of the cross distance from the bark depth is the distance from the detection position to the bark divided by the diameter of the wood.

Table 2

Vertical Section Grouping and Height Ratio
Name	A-B	B-C	C-D	D-E	E-F	F-G
Height (cm)	50	100	150	200	250	300
Ratio (h)	1/6	1/3	1/2	2/3	5/6	1
Note: The method for determining the height ratio (h) of the vertical distance from the base of the tree is the distance of the detection position from the base of the tree divided by 400 cm.

Methods

1. Nondestructive testing. (1) Use the micro-drilling resistance instrument to test the positions in sequence as shown in the figure. (2) The residual damage of the wooden elements was determined using a FAKOPP multi-point stress wave meter. (3) The longitudinal propagation velocity of the wood members was measured with a FAKOPP two-pin stress wave meter. (4) Samples were taken at positions A, B, C, D, E, F and G. Three specimens for moisture content determination and density determination were taken at each position, each including one core position and two sapwood positions.

2. Chemical composition analysis. Samples of wood components at three locations A, D and G were taken, totaling six. The appropriate amount of wood chips were cleaned, crushed, ground and sieved, and the wood powder samples with 40 ~ 60 mesh were taken for chemical composition determination. The lignin was determined with reference to GB/T2677.8-1994 (1994) ; the heterocellulose was determined with reference to GB/T2677.10-1995 (1995); the α-cellulose was determined with reference to GB / T 744–1989 (1989).

3. Infrared spectral analysis. A portion of the material from each of the six sampled samples was analyzed by IR spectroscopy, and each specimen was scanned three times.

4. Wood identification (species identification)。According to GB /T 29894 − 2013 (2013), the samples were made into sections with 3 faces, transverse, axial and radial, and observed under a color optical microscopic imaging system. The mapping was compared with the Chinese Wood Journal by feature extraction.

5. Scanning electron microscope analysis。Sections up to 5 mm × 5 mm × 3 mm were cut, taped to the experimental table, gold sprayed on the cut surface, evacuated, scanned and photographed, and observed.

6. X-ray diffraction analysis。Appropriate amount of wood chips were crushed and ground at each sampling point to make 80 mesh wood powder, and the chips were pressed at room temperature. CRD patterns were collected from the above specimens using an X-ray diffractometer.

Calculation of the crystallinity of different specimens according to the empirical method of Segal et al. (1959)

Crl=(I₀₀₂-I_am) / I₀₀₂×100%

where: Crl——relative crystallinity, %

I ₀₀₂—(002)extremely high intensity of lattice diffraction angle of wood, representing the crystalline and amorphous regions;

I _am—(2θ = 18° minimum intensity in the vicinity)scattering intensity of wood non-lattice background diffraction, I₀₀₂ is the same as I_am .

1. Analysis of experimental results

(1) Chemical composition analysis

Table 3 and Fig. 4 show the results of the chemical composition analysis of the wood components. Where P is the chemical composition of modern hardwood pine. The heterocellulose and alpha cellulose of all wood component samples were reduced compared to modern healthy lumber, and this difference arose due to the degradation of the main chemical components (lignin and heterocellulose) in the old wood components. The lignin of D1 and D2 was higher than that of healthy wood, probably because lignin is more solid and less prone to degradation, and the wood quality is better retained at this sampling location. From the comparison at different locations of old wood, the heterocellulose and cellulose of A1, D1 and G1 were higher than the values of A2, D2 and G2. The main reason for this is that the specimens in the A2, D2, and G2 are in direct contact with the atmosphere in the outer layer, which can be affected by environmental factors such as visible light, temperature, and moisture, resulting in the degradation of hemicellulose and cellulose. As for the lignin, the A1, D1 and G1 are lower than the values of A2, D2 and G2. It indicates that partial degradation of lignin occurred at the outer. The most obvious of the lignin degradation is the oxidation reaction and the opening of the aromatic ring, and these changes will have corresponding characteristic peaks in the IR spectra.

Table 3

table of chemical composition content of wood components
Number	Heterocellulose	α-cellulose	Lignin	Ash
P	68.37%	46.36%	27.76%	-
A1	42.06%	27.56%	21.16%	0.29%
A2	44.69%	31.62%	23.74%	0.34%
D1	25.64%	21.87%	30.29%	0.43%
D2	23.62%	19.21%	32.68%	0.77%
G1	43.49%	30.72%	21.08%	1.55%
G2	40.23%	25.83%	22.50%	1.12%

(2) Infrared spectral analysis

Figure 5 shows the IR spectra of the six samples. Figure 6 shows the infrared spectra of the six samples in the region of the fingerprint region from 800 to 1800 cm^− 1 of the infrared spectrum. The absorption peak located near 1730 cm^− 1 came from the C = O stretching vibration on the acetyl group, and this peak position is the characteristic absorption peak of hemicellulose distinguishing it from other components. Comparing the spectra of six samples, the peak could be detected, indicating that hemicellulose was not degraded. However, the absorption peaks near 1600 cm^− 1 and 1500 cm^− 1 are lignin benzene ring skeleton stretching vibration absorption peaks, especially the absorption peak near 1500 cm^− 1, which is attributed to the pure absorption band of lignin benzene ring skeleton stretching vibration. An absorption peak near 1500 cm^− 1 was detected in all six samples, indicating that the lignin in this old wood component was not completely degraded. The absorption peak near 1260 cm^− 1 was attributed to the C-O stretching vibration in lignin, which was detected in all of the old wood components, indicating that lignin had not undergone complete degradation. Absorption peaks generated by C-O-C deformation vibrations in cellulose and hemicellulose near 1158 cm^− 1 are detectable at positions A1 and D1, and not detected at other positions. 895 cm^− 1 was the characteristic absorption peak of cellulose, and there were no other strong absorption peaks nearby, which were less influenced by other components, but the peak was not detected in any of the six samples, indicating that the degradation of cellulose occurred in the old wood components. The results of infrared spectroscopy were consistent with the results of the main chemical composition analysis, further demonstrating that the degradation of heterocellulose in this old wood component was more significant, while the lignin was better retained.

(3) Sample appearance and degree of decay

Figure 7 shows the appearance of the specimens. Through the appearance of observation, it is found that the outermost wood has become flimsy and can be easily peeled off by hand. Some feel soft and will produce indentation when gently pressed by hand. Some have even cotton wool-like and lost the shape of the wood itself. However, there are some positions where the wood basically remains intact, only the color has changed, and the wood is still hard and firm.

(4) Macrostructure identification

Upon observation of the sample sections, the microscopic anatomical structure characteristics of the wood obtained were all the same, and the specific macroscopic characteristics of the samples were as follows, as shown in Fig. 8.

(1) The microstructure is characterized by obvious growth whorl, uniform width, dark color of latewood band between whorls, width of latewood band accounts for 1/4 ~ 1/2 of the width of growth whorl, yellowish brown wood, no special odor, straight grain, fine structure.

(2) Axial tubercles are characterized by rectangular, hexagonal or irregular polygons in cross-sectional earlywood tubercles, single row and rarely two rows of rimmed pores in the diameter wall, and round and oval rimmed pores; oval and polygonal in cross-sectional latewood tubercles. The transition from earlywood to latewood is sharp, and the width of the latewood band accounts for 1/4 ~ 1/2 of the growth whorl width.

(3) The wood rays are characterized by fusiform wood rays and homogeneous monoclinic wood rays, with transverse resin channels visible in the middle of the fusiform wood rays; homogeneous monoclinic wood rays consist of ray thin-walled cells and ray tube cells, 3 ~ 15 cells high; ray tube cells are located within 1 ~ 3 cell widths of the upper and lower edges of the rays, with deep serrations on the inner wall and marginal striations resembling the striations on the wall of the tube cells. The type of cross-field striae is window pane type, 1 ~ 3.

(4) The resin tracts are characterized by axial resin tracts distributed in the latewood zone and radial resin tracts present in the fusiform wood rays.

Based on the above microscopic anatomical features, and after comparing with the wood classification features recorded in the "Chinese Wood Journal", it is known that the wood member is hardwood pine.

(5) SEM analysis

The results of the SEM analysis of the three sections are shown in Fig. 9. Figure 9(a) shows the scanning electron microscopy results of the new material of Pinus sylvestris. As seen in the cross section, the canal cells are round and oval, the threaded fissures on the canal cell wall are obvious, and more cell gaps are seen at the corner of the canal cells, and the cells are arranged slightly neatly between each other. As seen in the diameter section, the inner wall of the tubular cell is smooth, with rimmed pores in a single row, rarely in two rows, cross-field pores are windowpane, and serrated thickening in the ray tubular cell is obvious. From the chordal section, it can be seen that the resin tract structure is relatively complete, the intercellular bonding is tight, a few ray canal cells are permeable, a large number of rimmed pores exist on the canal wall, and the structure is relatively complete.

Figure 9(b) and Fig. 9(c) show the results of three sections of the wood at locations A and D of the old wood members. As seen in the cross-sections, the cell wall structures are all present, but the internal structure has been destroyed, and some of the S₂ and S₁ layers are separated, and the deformation of the cell wall is obvious, and the cell wall shrinks and becomes thinner, indicating that the degradation of the cell wall of the old wood members has occurred. From the diameter section, most of the grain holes and nearby structures were intact, a few had signs of cracking and unidentified material appeared around them, indicating that slight degradation had occurred. From the chordal section, most of the wood ray skeleton structure, ray tube cells and thin-walled tissues were intact, while some tube cells of the skeleton structure had been crushed, and the cell wall had been twisted and squeezed, indicating that the cell wall had been damaged.

(6) X-ray diffraction analysis

Figure 10 shows the XRD plots of the six specimens, and Fig. 11 and Table 4 show the relative crystallinity of the six specimens. As can be seen from the figure, the crystalline diffraction peaks appearing near 17°, 22.5° and 35° of the X-ray diffraction angle (2θ) represent the crystalline diffraction intensity of the wood cellulose (101), (002) and (040) surfaces, respectively. Where 2θ is 22.5°, reflecting the crystallographic diffraction peak on the cellulose (002) face. The positions of the cellulose diffraction peaks of the six samples remained consistent, indicating that the crystalline structure did not change in the wood component samples. The highest absorption intensity of cellulose crystalline diffraction peak was found in A1 sample, and the lowest absorption intensity was found in D2 crystalline diffraction peak. The difference between the values of A and G at the two ends of the wood member was not significant, and the value of D at the middle position was lower, indicating that the relative crystalline content of the cellulose crystalline region at the middle position of the wood member was greatly reduced. The intensity of the diffraction peaks at A and G was significantly reduced, indicating that, in addition to the degradation of lignin, the hemicellulose and cellulose amorphous regions decreased much more than that at D, resulting in stronger diffraction peaks and higher relative crystallinity of cellulose. While the intensity of the diffraction peaks at D was significantly reduced, indicating that the degradation of lignin in wood components at D was accompanied by partial crystallization of cellulose crystalline regions under the long-term influence of light and ambient humidity (moisture), which led to the transformation of some crystalline regions into amorphous states and the decrease of relative crystallinity. It can also be seen from the figure that the trough, which appears near 2θ of 18°, is the scattered intensity of the diffraction from the amorphous region in the wood fiber. The lower intensity of sample D indicates that the degradation of the amorphous region in sample D is obvious. Similarly, the diffraction intensity of the A2 amorphous region was the lowest among the six samples, indicating that the degradation of the amorphous region also occurred during the transition from the cellulose crystalline region to the amorphous region in the badly decayed wood components, which led to a decrease in the diffraction intensity of the amorphous region along with a significant decrease in the relative crystallinity in their wood components. In addition, by calculating the relative crystallinity in the wood, it was found that the relative crystallinity of A2 was the highest, reaching 35.1%, and the decrease in the amorphous region in hemicellulose or cellulose led to an increase in the proportion of cellulose crystalline region in the wood as a whole, and the D2 sample decreased to 19.34%, and the chemical composition analysis results showed that the cellulose content in D2 was lower than that in A2 and G2, indicating that its cellulose and hemicellulose were more seriously degraded, and the The reason may be due to the degradation of hemicellulose to remove the acetyl group to generate acetic acid, which destroyed the structure of cellulose and made the crystallinity of cellulose reduced.

Table 4

crystallinity of samples at different positions
Samples	A1	A2	D1	D2	G1	G2
Crystallinity	34.23%	35.10%	28.20%	19.34%	30.57%	27.54%

2. Correlation of wood properties with chemical composition and microstructure of wood components

(1) Correlation with chemical composition

The experimentally measured heterocellulose, alpha cellulose and lignin were correlated with the density and modulus of elasticity of the wood members. As shown in Fig. 12 to Fig. 14 and Table 5. The results showed that the correlation coefficients R² of heterocellulose and α-cellulose with density and elastic modulus were greater than 0.6, and the correlations were good. The correlation coefficients R² of lignin with density and elastic modulus were all less than 0.6, and the correlations were average. From the above analysis, it can be seen that heterocellulose, α-cellulose and lignin all have some correlation with the density and modulus of elasticity of wood members, and the order of their correlation is α-cellulose > heterocellulose > lignin. The joint equation established for heterocellulose, α-cellulose and lignin with density and modulus of elasticity has a higher correlation coefficient as seen from the established equations.

Table 5

the relationship between the density and MOE of hemicellulose, α-cellulose and lignin
Name	Relationship formula	R²
hemicellulose y与density x	y = 0.7053x + 0.1546	0.6588
hemicellulose y与E_d x	y = 13.122x + 0.6012	0.742
hemicellulose z与densityx和E_d y	z = 8.72x + 5.192y + 4.949	0.7429
α-cellulose y与density x	y = 1.4329x + 0.0384	0.7301
α-cellulose y与E_d x	y = 26.615x-1.5488	0.8196
α-cellulose z与densityx和E_d y	z = 5.421x + 2.792y + 8.8010	0.8207
lignin y与density x	y=-1.2362x + 0.7249	0.567
lignin y与E_d x	y=-20.773x + 10.65	0.5209
lignin z与densityx和E_d y	z=-36.97x-0.5456y + 43.46	0.5703

(2) Correlation with microstructure

a. relationship between density and crystallinity

As seen in Fig. 15 and Table 6 from the test results of the wood components, the density of wood and crystallinity are positively correlated. The density of wood increases with the increase of crystallinity. In wood with higher crystallinity, the areas where cellulose molecular chains are densely arranged are larger and the areas where they are disordered are less, and more solid material is gathered per unit volume, thus increasing the density of the wood.

Table 6

ensity and crystallinity of wood
Name	A1	A2	D1	D2	G1	G2
Crystallinity(%)	34.23	35.10	25.54	19.34	30.47	28.20
Density (g/cm³)	0.4989	0.5011	0.3806	0.29	0.4353	0.3712

b. Relationship between E_d and crystallinity

From the test results of the wood components, Fig. 16 and Table 7 show that the E_d and crystallinity of wood are positively correlated. This is because the cellulose molecular chains in the crystalline region are neatly arranged and connected by glycosidic bonds with higher forces. The smaller the strain under the same stress, the greater the E_d (Ding et al. 2012; Fan et al. 2015; Guo et al. 2019).

Table 7

E_d and crystallinity of wood
Name	A1	A2	D1	D2	G1	G2
Crystallinity(%)	34.23	35.10	25.54	19.34	30.47	28.20
E_d (GPa)	6.2	7.62	4.41	3.49	5.6	5.12

1. After a period of time after the use of wood components, the hemicellulose content and cellulose content of A1, D1, and G1 were higher than A2, D2, and G2 from different locations, while the lignin content, A1, D1, and G1 were lower than A2, D2, and G2. It indicates that the specimens in the areas of A2, D2 and G2 are in direct contact with the atmosphere in the outer layer, which will be affected by environmental factors such as visible light, temperature and moisture, thus leading to the degradation of hemicellulose and cellulose, while partial degradation of lignin has also occurred.

2. The diffraction intensity of the A2 amorphous region was the lowest among the six samples, indicating that during the transition from the cellulose crystalline region to the amorphous region in the heavily decayed wood components, the amorphous region was also degraded, which led to a decrease in the diffraction intensity of the amorphous region and a significant decrease in the relative crystallinity of the wood components.

3. The correlation between heterocellulose, α-cellulose, lignin and wood component density and modulus of elasticity are good, and the order of correlation is α-cellulose > heterocellulose > lignin.

4. The density and crystallinity of wood components are positively correlated with the modulus of elasticity.

ACKNOWLEDGMENTS

This work was supported by the National Key R&D Program of China (2019YFC1520900 and 2020YFC1522402) and the Beijing Municipal Commission of Education-Municipal Natural Science Joint Foundation (Grant No. KZ202010005012). And the authors are grateful for a special grant from the Forbidden City Cultural Heritage Conservation Foundation and the Vanke Foundation.

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No competing interests reported.

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Correlation of wood properties with chemical composition and microstructure of wood components

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