A method for analyzing the intrafiber bonding strength of chemimechanical pulp based on two-dimensional correlation near-infrared spectroscopy

Abstract

This invention discloses a method for analyzing the internal bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy. The method includes the following steps: preparing chemimechanical pulp fiber samples with different internal bonding strengths and manipulating them into hand-made sheets; equilibrating the moisture content of the hand-made sheets under different relative humidity conditions; acquiring the near-infrared dynamic spectrum of the hand-made sheets in order of increasing relative humidity; performing two-dimensional correlation analysis on the dynamic spectrum to obtain the autocorrelation spectrum; and calculating the ratio of the signal intensity at wavelengths of 1394 nm and 1465 nm as the relative intensity I. 1394 / 1465 Test the bonding strength within the handwritten sheet according to standard methods, and calculate the test value and I. 1394 / 1465 The relationship curve of values; obtain I for the sample to be tested according to the above steps. 1393 / 1465 After obtaining the value, the fiber bond strength can be calculated by substituting it into the relationship curve. This invention enables efficient and non-destructive analysis of the strength performance indicators of pulp samples.

Description

Technical Field

[0001] This invention relates to the field of spectral analysis, specifically to a method for analyzing the internal bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy. Background Technology

[0002] Paper sheets are formed by the interweaving and bonding of fibers. The internal bond strength of paper and paperboard refers to the strength generated by the hydrogen bonds between fibers, and is an important indicator of pulp strength performance. The traditional Scott internal bond strength test method is cumbersome and destructive to the sample; therefore, it is necessary to develop a convenient, efficient, and non-destructive method for analyzing the internal bond strength of paper fibers. Near-infrared spectroscopy (780–2500 nm) mainly reflects the overtone and combination frequency absorption information of the vibrations of hydrogen-containing groups (CH, OH, NH, SH) in molecules. It has good absorption responses to the main chemical components of wood fibers, such as cellulose, hemicellulose, lignin, and intermolecular hydrogen bonds. Furthermore, it has advantages such as low analytical cost, no sample pretreatment required, and simple and flexible measurement methods. Therefore, near-infrared spectroscopy is suitable for non-destructive testing and process analysis of pulp fiber materials. However, due to the characteristics of near-infrared spectroscopy, such as weak signal intensity and severe peak overlap, the correspondence between spectral signals and analytical indicators is quite complex. Usually, chemometric algorithms are used to establish models for qualitative and quantitative analysis. Moreover, there are high requirements for the number of samples and preparation methods for modeling, which increases the difficulty of applying this analytical method.

[0003] According to the theory of two-dimensional correlation near-infrared spectroscopy, applying external perturbations (such as moisture, temperature, and pressure) to a sample induces changes in the vibrations of molecular groups and their cooperative effects, resulting in changes in the spectral response. By collecting a series of spectral signals from the sample under external perturbation and then mathematically transforming the one-dimensional spectral signals onto a two-dimensional scale, a two-dimensional correlation spectrum can be formed. Because different molecular groups exhibit different spectral responses to perturbations, overlapping information in the original spectrum is separated in the two-dimensional correlation spectrum, significantly improving the resolution of the spectral signal. This facilitates the analysis and extraction of near-infrared spectral characteristics, reducing the difficulty of analysis. Summary of the Invention

[0004] The internal bonding strength of chemimechanical pulp fibers is mainly determined by the hydrogen bonding between hydrogen-containing functional groups such as active hydroxyl groups on the fiber surface. When the water molecule environment of the active hydroxyl groups changes, a near-infrared spectral response change occurs at their characteristic absorption wavelength (1394 nm). The higher the content of active hydroxyl groups, the stronger the spectral response. Furthermore, the characteristic absorption at 1465 nm in the cellulose crystalline region shows relatively stable spectral changes in response to moisture disturbance and can be used as an internal standard signal. Utilizing these characteristics, this invention establishes a moisture disturbance-two-dimensional correlation near-infrared spectral analysis method to extract the two-dimensional correlation spectral characteristic signals of hydrogen-containing groups such as active hydroxyl groups on the fiber surface and establish their correspondence with internal bonding strength, achieving efficient and non-destructive analysis of the strength performance indicators of pulp samples.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for analyzing the internal bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy includes the following steps:

[0007] (1) Prepare no fewer than 10 chemimechanical pulp fiber samples with different internal bonding strengths by changing the chemical impregnation conditions and refining intensity, and process the chemimechanical pulp fibers into a basis weight of 60-120 g / m³ according to the method described in the national standard GB / T 24324-2009 (Preparation of laboratory paper for physical testing of pulp by conventional sheet forming method). 2 Handwritten slips;

[0008] (2) Place the hand-made sheet in a series of environments with different relative humidity to equilibrate the moisture, and the equilibration time shall not be less than 3 hours, while maintaining the ambient temperature at 23±2℃. The relative humidity range of the different environments is 11~98%RH, which can be adjusted by different saturated salt solutions;

[0009] (3) Collect near-infrared spectral signals of the hand-made sheet after moisture equilibration in order of relative humidity from low to high to form a dynamic spectrum;

[0010] (4) Perform two-dimensional correlation analysis on the dynamic spectrum to obtain the autocorrelation spectrum, and calculate the ratio of the signal intensity at wavelengths of 1394 nm and 1465 nm as the relative intensity I. 1394 / 1465 ;

[0011] (5) Test the internal bond strength of the chemimechanical pulp fiber samples according to the national standard GB / T 26203-2010 (Determination of internal bond strength of paper and paperboard), and calculate I. 1394 / 1465 Linear regression curve of the relationship between the binding strength and the internal binding strength;

[0012] (6) Obtain the relative intensity I of the autocorrelation spectrum of the sample to be tested according to steps (1) to (4). 1394 / 1465 The relative intensity I of the autocorrelation spectrum1394 / 1465 The internal bonding strength of the sample can be calculated by substituting the linear regression curve from step (5).

[0013] in:

[0014] The chemimechanical pulp fiber in step (1) refers to pulp fiber prepared by chemical mechanical method, that is, the wood chips are first chemically impregnated and pretreated, and then the wood chip fibers are dissociated into pulp by the action of disc milling. According to the pulping process, it can be divided into bleached chemical thermal-mechanical pulp (BCTMP), alkaline peroxide mechanical pulp (APMP), and pre-conditioning refiner chemical alkaline peroxide mechanical pulp (PRC APMP), etc.

[0015] In step (2), the following different saturated salt solutions are used to control relative humidity: lithium chloride saturated solution (11% RH), potassium acetate saturated solution (23% RH), magnesium chloride saturated solution (34% RH), potassium carbonate saturated solution (44% RH), sodium bromide saturated solution (58% RH), potassium iodide saturated solution (68% RH), sodium chloride saturated solution (76% RH), potassium chloride saturated solution (86% RH), sodium carbonate saturated solution (92% RH), and disodium hydrogen phosphate dodecahydrate saturated solution (98% RH).

[0016] In step (3), the near-infrared wavelength acquisition range is set to 1330-1500nm, the signal integration time is 60ms, and the single-point signal acquisition scan is performed 32 times. Near-infrared diffuse reflectance signals are collected at 10 random measurement points on both sides of the hand-copied sheet. The average value is taken and converted into diffuse reflectance absorbance using the following formula:

[0017]

[0018] In the formula:

[0019] x(w) — Diffuse reflectance absorbance of the near-infrared spectrum of the handwritten sheet at wavelength w;

[0020] S(w) — The diffuse reflectance signal of the near-infrared spectrum of the handwritten sheet at wavelength w;

[0021] D(w) — Dark noise signal collected by obstructing the light source;

[0022] R(w) — Reference signal acquired using a standard whiteboard (polytetrafluoroethylene material).

[0023] In step (4), the original dynamic spectrum is first preprocessed with standard normal transformation and second derivative, and then the synchronous two-dimensional correlation spectrum intensity is calculated. The autocorrelation peak on its diagonal is extracted as the autocorrelation spectrum, as follows:

[0024] (a) Calculate the moisture disturbance-dynamic spectrum using the following formula:

[0025] M min ≤m≤M max W min ≤w≤W max

[0026] In the formula:

[0027] y(w, m) — Moisture perturbation-dynamic spectral signal intensity at wavelength w in the near-infrared spectrum of the hand-copied sheet when the relative humidity is m;

[0028] x(w, m) — The diffuse reflectance absorbance of the near-infrared spectrum of the handmade sheet at wavelength w when the relative humidity is m;

[0029] —The average diffuse reflectance absorbance of the near-infrared spectrum of handmade sheets at wavelength w under different relative humidity conditions;

[0030] M min —The minimum relative humidity, according to this invention, is 11±1%;

[0031] M max —The maximum relative humidity, according to this invention, is 98±1%;

[0032] W min —The minimum wavelength, in this invention, is 1330nm;

[0033] W max --Maximum wavelength, 1500nm in this invention.

[0034] (b) Calculate the synchronous two-dimensional correlation spectrum using the following formula:

[0035]

[0036] y(w) T =[y(w,m1),y(w,m2),...,y(w,m n )]

[0037] In the formula:

[0038] Φ(w1, w2) — Simultaneous two-dimensional correlation spectral intensity of moisture perturbation-dynamic spectrum at wavelengths w1 and w2.

[0039] n — the number of relative humidity conditions set.

[0040] Let w1 = w2, then the autocorrelation spectrum I(w) = Φ(w1, w2);

[0041] The ratio of the signal intensity at wavelengths of 1394 nm and 1465 nm in the autocorrelation spectrum is calculated as the relative intensity.

[0042] In step (5), the least squares method is used to calculate I. 1394 / 1465 The linear regression curve of the relationship between the binding strength and the internal binding strength is B = a + kI. 1393 / 1465 Calculate parameters k and a using the following formula.

[0043]

[0044]

[0045] In the formula:

[0046] —I of the i-th chemical pulp sample 1394 / 1465 value;

[0047] —g chemical pulp sample I 1394 / 1465 The average value;

[0048] --The average value of the internal bonding strength test results of g chemical mechanical pulp samples;

[0049] B i —Test value of internal bond strength of chemical mechanical pulp sample;

[0050] g — Number of chemical pulp samples used to establish the relationship curve.

[0051] The beneficial effects of this invention are as follows:

[0052] Near-infrared spectroscopy can accurately reflect the distribution and content of active hydroxyl groups on the surface of pulp fibers, making it suitable for quantitative analysis of the internal bonding strength of pulp. However, due to severe spectral signal overlap and low peak resolution, near-infrared analysis usually requires the establishment of nonlinear analysis models using chemometric algorithms, and a large number of modeling samples are needed to obtain stable modeling results. This invention proposes a method for analyzing the internal bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy. Utilizing the high sensitivity of near-infrared signals containing hydrogen groups to changes in the water molecule environment, high-resolution two-dimensional correlation spectral characteristic signals of active hydroxyl groups are extracted. These characteristic signals exhibit a significant linear correlation with the internal bonding strength, allowing for the establishment of an accurate relationship curve model with fewer modeling samples. This method offers advantages such as convenience, efficiency, and non-destructive analysis. Attached Figure Description

[0053] Figure 1 The effect of free degree in chemimechanical pulp fibers prepared by different processes on the intrafiber bonding strength;

[0054] Figure 2 Dynamic spectra of handwritten sheets under different relative humidity conditions;

[0055] Figure 3 Synchronous two-dimensional correlation spectrum;

[0056] Figure 4 Autocorrelation spectra of different types of organic pulp samples;

[0057] Figure 5 Relative intensity I of autocorrelation spectrum 1394 / 1465 The curve showing the relationship between the internal bonding strength B and the internal bonding strength. Detailed Implementation

[0058] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0059] A method for analyzing the internal bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy includes the following steps:

[0060] Sample preparation:

[0061] This example uses the PRC-APMP process to prepare poplar chemimechanical pulp fiber samples. Poplar wood chips were washed with hot water and then steamed in a steam chamber (105℃, 15 min). The steamed wood chips were then extruded using a twin-screw extruder. The extruded wood chips were mixed evenly with impregnation chemicals and then refined using a high-consistency refiner. The refined pulp was then kept at 95℃ for 1 h. After the heat treatment, the pulp underwent a second refinement, and chemimechanical pulp samples with different free degrees of fiber were prepared by controlling the refinement intensity. The samples were then acidified, washed, and de-latented before being stored for later use. This example uses four chemical impregnation processes (see Table 1), with each process preparing four chemimechanical pulp fiber samples with free degrees ranging from 100-600 mL CSF, for a total of 16 samples.

[0062] Table 1 PRC-APMP Pulping Process Conditions

[0063]

[0064] Two-dimensional correlation near-infrared spectroscopy analysis:

[0065] The chemical mechanical pulp fiber sample was manipulated to a basis weight of 100 g / m³ according to the method described in the national standard GB / T 24324-2009 (Preparation of laboratory paper for physical testing of pulp - conventional paper forming device method). 2The handwritten sheet. Relative humidity was controlled using the following saturated salt solutions: lithium chloride saturated solution (11% RH), potassium acetate saturated solution (23% RH), magnesium chloride saturated solution (34% RH), potassium carbonate saturated solution (44% RH), sodium bromide saturated solution (58% RH), potassium iodide saturated solution (68% RH), sodium chloride saturated solution (76% RH), potassium chloride saturated solution (86% RH), sodium carbonate saturated solution (92% RH), and disodium hydrogen phosphate dodecahydrate saturated solution (98% RH).

[0066] Hand-copied slides were placed under different relative humidity conditions for 3 hours to equilibrate moisture, with the ambient temperature maintained at 23±2℃. Then, near-infrared spectral signals were acquired sequentially from low to high relative humidity, with the wavelength acquisition range set to 1330-1500 nm, signal integration time 60 ms, and single-point signal acquisition scans performed 32 times. After standard normal transformation and second derivative processing, two-dimensional correlation analysis was performed to obtain synchronous two-dimensional correlation spectra, autocorrelation spectra, and relative intensity Ig. 1394 / 1465 .

[0067] Internal bond strength relationship curve:

[0068] The internal bond strength (B) of chemimechanical pulp fiber samples was tested according to the national standard GB / T 26203-2010 (Determination of Internal Bond Strength of Paper and Paperboard). Twelve samples from processes P1, P2, and P3 were used as a calibration set to establish the relationship between B and I. 1394 / 1465 The linear regression curve B = kI 1393 / 1465 +a. Four samples from the P4 process were used as a test set to verify the prediction accuracy of the relationship curve.

[0069] Results and Discussion:

[0070] Chemi-mechanical pulp fiber samples with different degrees of freeness were prepared by changing the chemical impregnation conditions and pulping intensity. The effect of freeness on the intrafiber bonding strength in chemi-mechanical pulp samples prepared by different processes was investigated. Figure 1 List. From Figure 1 It can be seen that, under the same chemical impregnation process conditions, the intrafiber bond strength gradually increases as the degree of freeness decreases. This is because, with the increase in pulping intensity (decreased degree of freeness), more fibers are dissociated and fibrillated, thus exposing more hydroxyl groups on the surface and enhancing the hydrogen bonding between fibers. Increasing the amount of chemicals used in chemical impregnation allows the wood fibers to swell and soften more fully, improving fiber disintegration efficiency in the subsequent pulping process. Therefore, under the same degree of freeness, the intrafiber bond strength increases significantly with the increase of chemical dosage.

[0071] Near-infrared dynamic spectra of hand-made chemimechanical pulp fiber sheets under different relative humidity conditions Figure 2The characteristic absorption peaks are listed below. After second-derivative processing, the characteristic absorption peaks appear as negative peaks in the spectrum. Specifically, 1360 nm is the absorption peak of CH in hemicellulose, 1394 nm is the absorption peak of OH in the amorphous region of cellulose, 1438 nm is the absorption peak of bound water adsorbed in the fiber, and 1465 nm is the absorption peak of OH in the crystalline region of cellulose. With increasing relative humidity, more water molecules are adsorbed in the fiber, and the broad water peaks interfere with the OH absorption peaks. Two-dimensional correlation analysis of the dynamic spectrum can significantly improve the peak resolution. Figure 3 This is a synchronous two-dimensional correlation spectrum, reflecting the similarity of spectral signal changes at different wavelengths as a result of moisture disturbance. Significant autocorrelation peaks appear at 1360 nm, 1394 nm, 1438 nm, and 1465 nm, indicating that the near-infrared characteristic absorption signals at these locations are highly sensitive to moisture disturbance.

[0072] Furthermore, cross-peaks appeared between different wavelengths, with positive cross-peaks indicating spectral changes in the same direction at the two wavelengths, and negative cross-peaks indicating spectral changes in opposite directions at the two wavelengths. Figure 3 It can be seen that as the fiber adsorbs more water molecules, the absorption peak of bound water at 1438 nm gradually strengthens, while the absorption peak of OH in the amorphous region of cellulose (1394 nm), which easily adsorbs water molecules, also strengthens simultaneously. However, the absorption peaks at 1360 nm and 1465 nm gradually weaken, mainly due to the change in relative refractive index. As the equilibrium moisture content of the fiber increases, the propagation medium of near-infrared light in the fiber gradually changes from air-fiber to water-fiber. The refractive indices of water, air, and wood fiber are 1.0, 1.33, and 1.5, respectively. The greater the difference in refractive index between the two media, the stronger the interface reflection signal, which reduces the detection depth of near-infrared light inside the fiber. This reduces the characteristic absorption of near-infrared light by fiber structures that do not have direct contact with water molecules, thus weakening the spectral signal.

[0073] Autocorrelation spectra can be obtained by extracting the autocorrelation peaks on the diagonal of the synchronous two-dimensional correlation spectrum. Autocorrelation spectra of chemimechanical pulp fiber samples with different internal binding strengths are shown below. Figure 4 As shown, with the increase of internal binding strength, the intensity of the autocorrelation peak at 1394 nm gradually increases, while the intensity of the autocorrelation peak at 1465 nm remains relatively stable. The relative intensity I of the autocorrelation peaks at the two wavelengths is considered. 1394 / 1465 A linear regression was performed on the binding strength B, and the results are as follows: Figure 5 As shown, the relationship curve is B = 528.89I. 1394 / 1465 -336.78, correlation coefficient R 2 =0.9883. This indicates good fitting accuracy.

[0074] The relative intensities of the autocorrelation peaks of four chemimechanical pulp samples from the P4 process were substituted into the relationship curve to calculate the predicted internal bond strength, which was then compared with the measured values. The results are shown in Table 2. The average relative error of the relationship curve prediction results was 2.72%, indicating that the internal bond strength relationship curve established using two-dimensional correlation spectral characteristics has good prediction accuracy and can be used for efficient and non-destructive analysis of the strength properties of chemimechanical pulp.

[0075] Table 2 Prediction results of internal bond strength

[0076] <![CDATA[Predicted value (J / m 2 )]]> <![CDATA[Measured value (J / m 2 )]]> <![CDATA[Deviation (J / m 2 )]]> Relative error (%) 96.90 99.34 2.43 2.45 128.64 124.5 -4.14 3.33 167.57 161.7 -5.87 3.63 213.15 210 -3.15 1.5

[0077] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for analyzing the internal bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy, characterized in that: Includes the following steps: (1) Prepare chemical mechanical pulp fiber samples with different internal bonding strengths, and manipulate the chemical mechanical pulp fiber samples into hand-made sheets; (2) Place the handmade sheets in a series of environments with different relative humidity to balance the moisture; (3) Collect near-infrared spectral signals of the hand-made sheet after moisture equilibration in order of increasing relative humidity to form a dynamic spectrum; (4) The dynamic spectrum is subjected to two-dimensional correlation analysis to obtain an autocorrelation spectrum, and the signal intensity ratio of the autocorrelation spectrum at wavelengths of 1394 nm and 1465 nm is calculated as a relative intensity I 1394 / 1465 ; (5) The internal bond strength of the chemi-mechanical pulp fiber sample is tested according to the national standard, and I is calculated 1394 / 1465 The linear regression relationship curve of the internal bond strength; (6) Obtain the relative intensity I of the autocorrelation spectrum of the sample to be tested according to steps (1) to (4). 1394 / 1465 The relative intensity I of the autocorrelation spectrum 1394 / 1465 The internal binding strength of the sample can be calculated by substituting the linear regression curve from step (5). The method for obtaining the autocorrelation spectrum by performing two-dimensional correlation analysis on the dynamic spectrum in step (4) is as follows: First, the original dynamic spectrum is preprocessed by standard normal transformation and second derivative, and then two-dimensional correlation analysis is performed. Then, the synchronous two-dimensional correlation spectrum is calculated, and the autocorrelation peak on its diagonal is extracted as the autocorrelation spectrum. Specifically, the following steps are included: (a) Calculate the moisture disturbance-dynamic spectrum using the following formula: In the formula: y(w,m) —Moisture perturbation-dynamic spectral signal intensity at wavelength w in the near-infrared spectrum of the hand-copied sheet at a relative humidity of m; x(w,m) —The diffuse reflectance absorbance of the near-infrared spectrum of the handmade sheet at wavelength w when the relative humidity is m; —The average diffuse reflectance absorbance of the near-infrared spectrum of handmade sheets at wavelength w under different relative humidity conditions; M min —Minimum relative humidity; M max —Maximum relative humidity; W min —Minimum wavelength; W max —Maximum wavelength; (b) Calculate the synchronous two-dimensional correlation spectrum using the following formula: In the formula: Φ(w 1 ,w 2 ) —Simultaneous two-dimensional correlation spectral intensity of moisture perturbation-dynamic spectrum at wavelengths w1 and w2; n —Number of relative humidity conditions set; make w 1 =w 2. Autocorrelation spectrum I(w) = Φ(w) 1 ,w 2 ) ; Step (5) uses the least squares method for calculation. I 1394 / 1465 Linear regression curve of binding strength B = a + k I 1393 / 1465 Calculate the parameters using the following formula. k and a : In the formula: ——No. i Individualized mechanical pulp samples I 1394 / 1465 value; —— g Individualized pulp samples I 1394 / 1465 The average value; —— g The average value of the internal bonding strength test results of each individual mechanical pulp sample; B i —Test value of internal bond strength of chemical mechanical pulp sample; g —Number of chemical pulp samples used to establish the relationship curve.

2. The method for analyzing the intrafiber bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy according to claim 1, characterized in that: The chemimechanical pulp fiber sample in step (1) refers to pulp fiber prepared by the chemimechanical method.

3. The method for analyzing the intrafiber bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy according to claim 1, characterized in that: In step (1), there should be no fewer than 10 samples of chemimechanical pulp fibers with different internal bonding strengths.

4. The method for analyzing the intrafiber bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy according to claim 1, characterized in that: The quantitative amount of the hand-made sheets produced in step (1) is 60-120 g / m³. 2 .

5. The method for analyzing the intrafiber bonding strength of chemimechanical pulp based on two-dimensional correlation near-infrared spectroscopy according to claim 1, characterized in that: In step (2), the relative humidity range in different relative humidity environments is 11~98%RH, which is controlled by different saturated salt solutions; the different saturated salt solutions include lithium chloride saturated solution, potassium acetate saturated solution, magnesium chloride saturated solution, potassium carbonate saturated solution, sodium bromide saturated solution, potassium iodide saturated solution, sodium chloride saturated solution, potassium chloride saturated solution, sodium carbonate saturated solution, and disodium hydrogen phosphate dodecahydrate saturated solution.

6. The method for analyzing the intrafiber bonding strength of chemimechanical pulp based on two-dimensional correlation near-infrared spectroscopy according to claim 1 or 5, characterized in that: In step (2), the time for the hand-made sheet to balance the moisture in different relative humidity environments shall not be less than 3 hours, and the ambient temperature shall be maintained at 23±2℃.

7. The method for analyzing the intrafiber bonding strength of chemimechanical pulp fibers based on two-dimensional correlation near-infrared spectroscopy according to claim 1, characterized in that: In step (3), the near-infrared wavelength acquisition range is set to 1330-1500nm, the signal integration time is 60ms, and the single-point signal acquisition scan is performed 32 times. Ten measurement points are randomly selected on both sides of the hand-copied sheet to obtain near-infrared diffuse reflectance signals. The average value is taken and converted into diffuse reflectance absorbance using the following formula: x(w) —The diffuse reflectance absorbance of the near-infrared spectrum of the handwritten sheet at wavelength w; S(w) —The diffuse reflectance signal of the near-infrared spectrum of the handwritten sheet at wavelength w; D(w) —Dark noise signals collected by obstructing the light source; R(w) —Reference signal acquired using a standard whiteboard.

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