CN115452645B - A method for characterizing wood cell wall stiffness based on low-field nuclear magnetic resonance - Google Patents
A method for characterizing wood cell wall stiffness based on low-field nuclear magnetic resonance Download PDFInfo
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Abstract
The invention discloses a method for characterizing the rigidity of a wood cell wall based on low-field nuclear magnetic resonance, which comprises the steps of taking out a wood sample at regular time when the wood sample is dried, weighing the wood sample, carrying out low-field nuclear magnetic resonance scanning, collecting the full relaxation signal intensity of the wood sample by adopting an MSE-CPMG pulse sequence, obtaining the moisture content and free induction attenuation curve of the wood sample, and calculating the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample by a data inversion algorithm combining Gaussian fitting and exponential fitting. The characterization method is lossless and continuous and has high measurement precision.
Description
Technical Field
The invention relates to the field of wood cell wall rigidity characterization, in particular to a low-field nuclear magnetic resonance-based wood cell wall rigidity characterization method.
Background
The water-soluble resin is a low molecular weight resin containing a large amount of hydrophilic groups, has the characteristics of low viscosity, good fluidity, small molecular weight range, excellent water mixing ratio and the like, and is suitable for the impregnation enhancement modification treatment of wood and bamboo. The method comprises the steps of impregnating water-soluble low molecular weight resin into cell cavities and cell walls of biomass fiber materials such as wood, bamboo and the like by using a vacuum-pressurizing method, continuously changing the phase state of liquid resin which enters micro-nano pores of the cell cavities in the subsequent drying stage, gradually solidifying to form a solid macromolecular structure, filling the solid macromolecular structure into the cell cavities, and possibly carrying out chemical crosslinking reaction with cell wall components, thereby enhancing the mechanical properties of the cell walls. Therefore, by characterizing the drying process and the real-time response of the cell wall stiffness of the modified material after the drying is completed, the reinforcement modification mechanism of the resin impregnation material can be better interpreted.
The characterization method of the stiffness of the resin impregnation modified material comprises macroscopic mechanical property characterization and microscopic mechanical property characterization. Wherein, the universal mechanical testing machine is commonly used for measuring the macroscopic mechanical property of the resin impregnation modified treatment material. By applying a constant deformation or load to the test specimen, the corresponding load or deformation is measured, and the modulus of elasticity (MOE) and flexural strength (MOR) of the test specimen are calculated by the formula. However, the test process of the method forms irreversible damage to the sample, the rigidity change of the sample in the whole drying process cannot be represented by one sample, and the test error caused by the natural variability of the wood can exist when a plurality of samples are used for continuous test. The method for characterizing the micromechanics properties of the modified material mainly adopts a nanoindentation technology, and obtains the rigidity value of the cell wall of the sample by applying continuous load to the cell wall of the sample and testing the indentation depth of the nanometer scale and fitting. The method can accurately measure the cell wall rigidity, is applied to representing the mechanical properties of the cell wall of wood, bamboo and modified treatment materials, has high sample preparation and test requirements, is a destructive test, and can only measure the mechanical properties of the cell wall in a certain water content state.
Therefore, the continuous and nondestructive testing method for the cell wall rigidity of the modified material is provided, data support is provided for water-soluble low molecular weight resin impregnation modification and subsequent drying and curing research of biomass materials such as wood, bamboo and the like, and the method has important significance.
Disclosure of Invention
The invention aims to solve the technical problem of overcoming the defects of the prior art and providing a wood cell wall rigidity characterization method which is nondestructive and continuous and has high measurement accuracy and is based on low-field nuclear magnetic resonance.
In order to solve the technical problems, the invention adopts the following technical scheme:
A method for characterizing wood cell wall stiffness based on low-field nuclear magnetic resonance, comprising the following steps:
S1, drying a wood sample, taking out the wood sample at regular time, weighing the wood sample, and performing low-field nuclear magnetic resonance scanning, wherein the full relaxation signal intensity of the wood sample is acquired by adopting an MSE-CPMG pulse sequence during the low-field nuclear magnetic resonance scanning;
S2, calculating to obtain the water content of the wood sample according to the weighing quality, and establishing a free induction attenuation curve of the wood sample according to the intensity of the complete relaxation signal;
And S3, calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample through a data inversion algorithm combining Gaussian fitting and exponential fitting according to the moisture content and the free induction decay curve of the wood sample.
As a further improvement to the above technical solution:
The step S3 specifically comprises the following steps:
s301, judging that the water content of the wood sample is larger than a preset threshold, executing a step S302 when the water content of the wood sample is lower than the preset threshold, otherwise, executing a step S303;
S302, calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample by adopting a fitting function shown in the formula (1) according to the free induction decay curve of the wood sample;
S303, calculating to obtain the transverse relaxation time T 21 and the integral peak area A 1 of the transverse relaxation time according to the free induction decay curve of the wood by a data inversion algorithm by adopting a fitting function shown in the formula (2);
In the formulas (1) and (2), T 21、T22、T23 is the transverse relaxation time of the component with different degrees of freedom, a 1、A2、A3 is the integral peak area of the transverse relaxation time spectrum of the component with different degrees of freedom, T is the decay time, n 1＝2,n2＝n3 =1, and c is a constant.
In the present invention, the preset threshold is 30%.
The MSE-CPMG pulse sequence has the parameters of 2000-12000 of echo number, 8-64 of scanning times and 0.1-0.25 ms of echo time.
The wood sample is a wood sample which is subjected to resin modification treatment or not subjected to resin modification treatment.
The wood sample is a wood sample subjected to end-capping treatment.
The resin modification treatment specifically comprises the following steps of immersing a wood sample with the water content of 0% in a resin solution with the mass concentration of 10% -50% to obtain a wood sample subjected to resin impregnation modification treatment with the water content of more than 30%.
The impregnation is vacuum pressurized impregnation, and specifically comprises the steps of placing a wood sample with the water content of 0% in a resin solution with the mass concentration of 10% -50%, keeping the wood sample for 30-50 min under the pressure of 0.06-0.08 MPa, keeping the wood sample for 5-10 h under the pressure of 0.5-0.8 MPa, decompressing and taking out the wood sample, and obtaining the wood sample subjected to resin impregnation modification treatment with the water content of more than 30%.
As a general inventive concept, the present invention also provides a wood cell wall stiffness characterization apparatus based on low field nuclear magnetic resonance, comprising:
The first weighing scanning module is used for drying the wood sample, taking out the wood sample at regular time, weighing the wood sample, and performing low-field nuclear magnetic resonance scanning, wherein the full relaxation signal intensity of the wood sample is acquired by adopting an MSE-CPMG pulse sequence during the low-field nuclear magnetic resonance scanning;
the second data processing module is used for calculating the water content of the wood sample according to the weighing quality and establishing a free induction decay curve of the wood sample according to the intensity of the complete relaxation signal;
The third data processing module is used for calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample through a data inversion algorithm combining Gaussian fitting and exponential fitting according to the moisture content and the free induction decay curve of the wood sample.
As one general inventive concept, the present invention also provides a low field nmr-based wood cell wall stiffness characterization apparatus, comprising a microprocessor and a memory, interconnected, the microprocessor programmed or configured to perform the steps of the low field nmr-based wood cell wall stiffness characterization method described above.
As one general inventive concept, the present invention also provides a computer-readable storage medium having stored therein a computer program programmed or configured to perform the foregoing low field nuclear magnetic resonance-based method of characterization of wood cell wall stiffness.
The principle of the invention is as follows:
In long-term experiments, the applicant finds that the natural variability exists when the wood and bamboo are used as a natural biomass material, and the cell wall rigidity of samples obtained from different parts is obviously different. At present, the characterization methods of the cell wall rigidity of the macroscopic or microscopic layers of the modified wood and the unmodified wood are almost the destructive methods, the structural damage to the test sample itself exists in the test process, the rigidity characterization of a sample at different moisture content stages cannot be realized, and the moisture content is an important influencing factor of the cell wall rigidity of the wood. Therefore, the applicant measures the hydrogen proton content (H proton) in the water molecules through a specific sequence (MSE-CPMG pulse sequence) to obtain the transverse relaxation time T 21 (used for representing the rigidity of the wood cell wall) and the transverse relaxation time integral peak area A 1 (used for representing the rigidity component content of the wood cell wall) of the sample, the sample is not destroyed in the test process, the real-time response of the rigidity of the cell wall can be obtained through the relaxation signal changes of different water content stages of the same sample, and the test result is more accurate.
Compared with the prior art, the invention has the advantages that:
1. Compared with the commonly used pulse sequences such as CPMG (Carr-Purcell-Meiboom-Gill) and SE-SPI (Spin Echo Single Point Imaging), the MSE-CPMG pulse sequence used in the method breaks through the limitation of the instrument dead time of the low-field nuclear magnetic equipment, namely, not only can capture signals of wood cell walls, common bound water and all free water with attenuation time longer than the instrument dead time, but also can capture nuclear magnetic signals of cell walls and the most tightly bound water of the wood with attenuation time shorter than the instrument dead time (such as 0.042ms of a certain equipment), and other sequences can only capture signals of partial cell wall substances, partial common bound water and all free water of the wood, so that the MSE-CPMG pulse sequence is more accurate in capturing of solid matter relaxation information such as the wood cell walls and resin with a certain curing degree, and has higher signal to noise ratio.
2. According to the method for characterizing the rigidity of the wood cell wall based on low-field nuclear magnetic resonance, a data inversion algorithm combining Gaussian fitting and exponential fitting is adopted to calculate and obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample, and compared with the traditional data inversion algorithm such as CONTIN, SIRT (simultaneous iterative reconstruction technique), the data inversion algorithm combining Gaussian fitting and exponential fitting is adopted, different fitting functions are selected according to the components and content changes of solids and liquids in a sample, so that the relaxation information change from the solids to the liquids (extremely short transverse relaxation time to long transverse relaxation time) is obtained through more accurate inversion.
Drawings
Fig. 1 is a flow chart of the present invention.
FIG. 2 is a graph showing the MOE and MOR changes of a sample of unmodified material during the drying process in example 1 of the present invention.
FIG. 3 is a schematic representation of the variation of A 1、T21 of the unmodified material sample during the drying process in example 1 of the present invention.
FIG. 4 is a graph showing the MOE and MOR changes of the modified material samples during the drying process in example 2 of the present invention.
FIG. 5 is a schematic representation of the variation of A 1、T21 of the drying process modification sample in example 2 of the present invention.
FIG. 6 is a schematic diagram showing the variation of T 21 in the drying process of the sample before and after modification in example 3 of the present invention.
FIG. 7 is a schematic diagram showing the change in A 1 in the drying process of the sample before and after modification in example 3 of the present invention.
Detailed Description
The present invention will be described in further detail below. The instruments or materials used in the present invention are commercially available unless otherwise specified.
As shown in fig. 1, the method for characterizing the rigidity of the wood cell wall based on low-field nuclear magnetic resonance comprises the following steps:
S1, drying a wood sample, taking out the wood sample at regular time, weighing the wood sample, and performing low-field nuclear magnetic resonance scanning, wherein the full relaxation signal intensity of the wood sample is acquired by adopting an MSE-CPMG pulse sequence during the low-field nuclear magnetic resonance scanning;
S2, calculating to obtain the water content of the wood sample according to the weighing quality, and establishing a free induction attenuation curve of the wood sample according to the intensity of the complete relaxation signal;
And S3, calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample through a data inversion algorithm combining Gaussian fitting and exponential fitting according to the moisture content and the free induction decay curve of the wood sample.
The step S3 specifically comprises the following steps:
s301, judging whether the water content of the wood sample is larger than a preset threshold (the preset threshold is a fiber saturation point, and the water content is 30%), executing a step S302 when the water content of the wood sample is lower than the preset threshold, otherwise, executing a step S303;
S302, calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample by adopting a fitting function shown in the formula (1) according to the free induction decay curve of the wood sample;
S303, calculating to obtain the transverse relaxation time T 21 and the integral peak area A 1 of the transverse relaxation time according to the free induction decay curve of the wood by a data inversion algorithm by adopting a fitting function shown in the formula (2);
In the formulas (1) and (2), T 21、T22、T23 is the transverse relaxation time of the component with different degrees of freedom, a 1、A2、A3 is the integral peak area of the transverse relaxation time spectrum of the component with different degrees of freedom, T is the decay time, n 1＝2,n2＝n3 =1, and c is a constant. The different degree of freedom components include, but are not limited to, wood cell walls, free water in different tissue cell cavities, bound water in cell walls, cured resin (or a step cured resin (from liquid to solid)).
The data inversion algorithm adopts a method of combining Gaussian fitting (n 1 =2) and exponential fitting (n 2＝n3 =1), wherein the Gaussian fitting is used for interpreting a fast attenuation interval of an FID attenuation (free induction attenuation) curve, signals belonging to a short relaxation substance (such as a cell wall signal and a tightly bound water signal), the exponential fitting is used for interpreting a slow attenuation interval of the FID attenuation curve and signals belonging to a long relaxation substance (such as a free water signal), and the FID attenuation curve can be interpreted more accurately by combining two fitting formulas, so that more accurate data can be obtained.
Example 1 cell wall characterization method of unmodified Material sample, comparison of macroscopic mechanical Properties and microscopic mechanical Properties of unmodified Material sample
The method for characterizing the cell wall rigidity of the wood based on the low-field nuclear magnetic resonance of the embodiment does not carry out modification treatment on the resin for the wood, takes out the resin at regular time when drying, carries out a low-field nuclear magnetic resonance scanning test and a macroscopic mechanical property test, carries out calculation fitting on the result of the low-field nuclear magnetic resonance scanning test, and specifically comprises the following steps:
(1) Sample preparation 20 samples of 20mm (R). Times.20 mm (T). Times.300 mm (L) and 1 sample of 7mm (R). Times.7 mm (T). Times.20 mm (L) were sawn from fresh sawn aspen, and the initial water content was about 100%.
(2) And (3) drying, namely sealing the end surfaces of the samples with two sizes by using epoxy resin, and recording the quality of each sample after sealing. The samples were placed in an 80 ℃ oven, taken out at intervals and their mass recorded, and then subjected to macroscopic mechanical property testing and low field nuclear magnetic resonance scanning.
(3) Macroscopic mechanical property test, namely taking out 1 sample of 20mm (R) multiplied by 20mm (T) multiplied by 300mm (L) periodically in a drying process, weighing the sample, calculating the real-time water content, and then testing the MOR and MOE change of the unmodified wood according to national standards of GB/T1936.1-2009 method for testing the flexural strength of wood and GB/T1936.2-2009 method for testing the flexural modulus of elasticity of wood. In this embodiment, the macro mechanical property test is mainly used to verify the feasibility of the characterization method of the present invention, and in the actual characterization method, this step is not a necessary step.
(4) And (3) carrying out low-field nuclear magnetic resonance test, namely taking out an unmodified material sample with the mass of 7mm (R) multiplied by 7mm (T) multiplied by 20mm (L) periodically in the drying process, weighing the sample, calculating the real-time water content, carrying out low-field nuclear magnetic resonance scanning, and then placing the sample back into an oven for continuous drying after scanning, and repeating the steps. The sequence is MSE-CPMG pulse sequence, the pulse sequence parameter is 2000 echo number, 8 scanning times and 0.1ms echo time.
In this example, the water content was calculated using the following formula:
Wherein MC is the water content of the sample, M 1 is the mass of the sample when the sample is taken out in a drying manner, and M 0 is the absolute dry mass of the sample.
(5) And (3) data processing, namely obtaining a Free Induction Decay (FID) curve of the sample by using the MSE-CPMG pulse sequence, and obtaining the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the Free Induction Decay (FID) curve by using a fitting algorithm. The fitting algorithm is a data inversion algorithm combining Gaussian fitting and exponential fitting, the fitting function is shown as formulas (1) and (2), wherein the formula (1) is two-component fitting and is suitable for a state with low water content, and the formula (2) is three-component fitting and is suitable for a state with high water content, and the fitting function is determined according to the water content of a sample.
Where T 21、T22、T23 is the transverse relaxation time of the different degree of freedom component, a 1、A2、A3 is the total peak area of the transverse relaxation time spectrum of the different degree of freedom component, T is the decay time, n 1＝2,n2＝n3 =1, and c is a constant. In this embodiment, the degree of freedom component refers to cell wall, free water in different tissue cell cavities, bound water in cell walls, such as wood cell wall rigid polymer component, wood fiber cell cavity and wood ray cell cavity free water, and wood duct cell cavity water. A 1、A2、A3 corresponds to the peak area of the relaxation time appearing on the T 2 spectrum, and the unit is the intensity of signals, such as the content of cell wall rigid polymer, the content of bound water in the cell wall, the content of free water in the cell cavity of wood fiber and the cell cavity of wood ray, and the content of moisture in the cell cavity of wood duct.
And obtaining MOR and MOE changes of the samples at different drying stages through macroscopic mechanical property test. The transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the samples in different drying stages are obtained through formulas (1) and (2), and are shown in figures 2 and 3 respectively.
As can be seen from FIG. 2, the water content is above the saturation point of the fiber, and the macroscopic mechanical properties MOE and MOR of the unmodified material hardly change with the decrease of the water content. When the water content is below the saturation point of the fiber, the unmodified MOE and MOR gradually increase as the water content decreases.
Fig. 3 shows the change of the cell wall rigidity response of the unmodified material in the drying process, including the transverse relaxation time T 21 and the transverse relaxation time integral peak area a 1, and the trend of the macroscopic mechanical property and the microscopic mechanical property of the unmodified material can be found to be the same, which indicates that the characterization method of the embodiment has feasibility for characterizing the microscopic mechanical property of the cell wall of the material.
Example 2 cell wall characterization method of modified Material, comparison of macroscopic mechanical Properties and micromechanics Properties of modified Material
The method for characterizing the cell wall rigidity of the wood based on the low-field nuclear magnetic resonance of the embodiment comprises the steps of modifying the resin for the wood, taking out the resin at regular time in a drying process, performing a low-field nuclear magnetic resonance scanning test and a macroscopic mechanical property test, and performing calculation fitting on a low-field nuclear magnetic resonance scanning test result, and specifically comprises the following steps:
(1) The modifier is prepared by measuring the solid content of a water-soluble melamine formaldehyde resin (MUF) solution, and adjusting the concentration of the solution to 30% by mass ratio by using distilled water according to the modification requirement.
(2) Sample preparation 1 sample of 20mm (R). Times.20 mm (T). Times.300 mm (L) 20, 7mm (R). Times.7 mm (T). Times.20 mm (L) was sawn from a sufficiently air-dried aspen log, and the moisture content was dried to 0% in an absolute dry state. Vacuum-pressure impregnation treatment with 30% strength resin solution was employed. The vacuum-pressurizing impregnation process comprises the steps of firstly maintaining for 30min under the pressure of 0.08MPa, then maintaining for 5h under the pressure of 0.8MPa, finally slowly decompressing and taking out the sample, wherein the initial water content is about 90%.
(3) And (3) drying, namely sealing the end face of the sample by using epoxy resin, and recording the quality of each sample after sealing. The samples were placed in an 80 ℃ oven, taken out at intervals and their mass recorded, and then subjected to macroscopic mechanical property testing and low field nuclear magnetic resonance scanning.
(4) And (3) testing macroscopic mechanical properties, namely taking out 1 sample of 20mm (R) multiplied by 20mm (T) multiplied by 300mm (L) periodically in a drying process, weighing the sample, calculating the real-time water content, and testing the MOR and MOE change of the modified material sample according to national standards of GB/T1936.1-2009 method for testing the flexural strength of wood and GB/T1936.2-2009 method for testing the flexural modulus of wood. In this embodiment, the macro mechanical property test is mainly used to verify the feasibility of the characterization method of the present invention, and in the actual characterization method, this step is not a necessary step.
In this example, the water content was calculated using the following formula:
Wherein MC is the water content of the sample, M 1 is the mass of the sample when the sample is taken out in a drying manner, and M 0 is the absolute dry mass of the sample.
(5) And (3) carrying out low-field nuclear magnetic resonance test, namely periodically taking out 7mm (R) multiplied by 7mm (T) multiplied by 20mm (L) modified material samples in a drying process, weighing the mass, calculating the real-time water content, carrying out low-field nuclear magnetic resonance scanning, and then placing the samples back into an oven for continuous drying after scanning, and repeating the steps. The sequence is MSE-CPMG pulse sequence, the pulse sequence parameter is 2000 echo number, 8 scanning times and 0.1ms echo time.
(6) And (3) data processing, namely obtaining a Free Induction Decay (FID) curve of the sample by using the MSE-CPMG pulse sequence, and obtaining the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the Free Induction Decay (FID) curve by using a fitting algorithm. The fitting algorithm is a data inversion algorithm combining Gaussian fitting and exponential fitting, the fitting function is shown as a formula (1) and a formula (2), wherein the formula (1) is two-component fitting and is suitable for a state with low water content, the formula (2) is three-component fitting and is suitable for a state with high water content, and the specific fitting algorithm depends on the water content of a sample.
Wherein T 21、T22、T23 is the transverse relaxation time of the different degree of freedom component, A1, A2, A3 are the spectrum integration peak areas of the different degree of freedom component transverse relaxation time, T is the decay time, n 1＝2,n2＝n3 =1, and c is a constant. In this embodiment, the degree of freedom component refers to cell wall, free water in different tissue cell cavities, bound water in cell wall, cured resin (or resin which is gradually cured (from liquid state to solid state)), such as wood cell wall rigid polymer component, wood fiber cell cavity and wood ray cell cavity free water, and wood duct cell cavity water.
And obtaining MOR and MOE changes of the modified material samples at different drying stages through macroscopic mechanical property test. The transverse relaxation times T 21 and the transverse relaxation time integral peak areas A 1 of the samples in different drying stages are obtained through formulas (1) and (2), and are shown in figures 4 and 5 respectively.
FIG. 4 shows the macroscopic mechanical property changes of MOE and MOR of the modified material samples during drying. As can be seen from the graph, the resin is gradually solidified and filled in the pore structure of each layer of wood in the drying process of the modified material, so that the macroscopic mechanical property change of the modified material shows a linear increasing change trend in the whole water content stage above and below the saturation point of the water content fiber.
Fig. 5 shows the change in the cell wall stiffness response of the modified material during drying, including the transverse relaxation time T 21 and the transverse relaxation time integral peak area a 1, which were found to have the same trend. However, compared with the macro mechanical property test, the micro mechanical property test sample is always the same sample, so that the fitting effect of A 1 and T 21 is better, and the data is more accurate.
Example 3 cell wall characterization method of modified Material, cell wall stiffness comparison before and after Wood modification
The method for characterizing the cell wall rigidity of the wood based on the low-field nuclear magnetic resonance in the embodiment comprises the following steps of performing a low-field nuclear magnetic resonance test on the wood before the wood is modified, performing a low-field nuclear magnetic resonance scanning test on the wood after the wood is modified, and respectively performing calculation fitting on the results of the low-field nuclear magnetic resonance scanning test:
(1) The modifier is prepared by measuring the solid content of a water-soluble melamine formaldehyde resin (MUF) solution, and adjusting the concentration of the solution to 50% by using distilled water according to the requirement of wood impregnation modification.
(2) Sample preparation A sample of 7mm (R). Times.7 mm (T). Times.20 mm (L) was sawn from fresh wood sawn, and the initial water content was about 90%.
(3) And (3) drying, namely sealing the end face of the sample by using epoxy resin, and recording the quality of each sample after sealing. The samples were placed in a 60 ℃ oven, weighed and the mass recorded at intervals, and then subjected to low field nmr scanning.
(4) The low-field nuclear magnetic resonance test comprises the steps of using an MSE-CPMG pulse sequence as a sequence for low-field nuclear magnetic resonance scanning, wherein the pulse sequence parameters comprise the number of echoes of 12000, the scanning times of 64 and the echo time of 0.25ms. After the test, the water content is dried to 0% of absolute dry state and is used for the low molecular weight resin impregnation modification treatment.
(5) And (3) modifying, namely carrying out vacuum-pressurizing impregnation treatment on the unmodified material by adopting a resin solution with the mass concentration of 50%. The vacuum-pressurizing impregnation process is that the vacuum-pressurizing impregnation process is firstly kept for 50min under the pressure of 0.06MPa, then kept for 5h under the pressure of 0.5MPa, and finally the sample is slowly decompressed and taken out, and the initial water content is about 90%.
(6) And (3) testing the low-field nuclear magnetic resonance of the modified material, namely periodically taking out a modified material sample with the size of 7mm (R) x 7mm (T) x 20mm (L) in a drying process, weighing the mass, calculating the real-time water content, and then carrying out low-field nuclear magnetic resonance scanning. The sequence used is MSE-CPMG, the sequence parameters are that the number of echoes is 12000, the scanning times is 64, and the echo time is 0.25ms.
In this example, the water content was calculated using the following formula:
Wherein MC is the water content of the sample, M 1 is the mass of the sample when the sample is taken out in a drying manner, and M 0 is the absolute dry mass of the sample.
(7) And (3) data processing, namely obtaining a Free Induction Decay (FID) curve of the sample by using the MSE-CPMG pulse sequence, and obtaining the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the Free Induction Decay (FID) curve by using a fitting algorithm. The fitting algorithm is a data inversion algorithm combining Gaussian fitting and exponential fitting, the fitting function is shown as a formula (1) and a formula (2), wherein the formula (1) is two-component fitting and is suitable for a state with low water content, the formula (1) is three-component fitting and is suitable for a state with high water content, and the specific fitting algorithm depends on the water content of a sample.
Where T 21、T22、T23 is the transverse relaxation time of the different degree of freedom component, a 1、A2、A3 is the total peak area of the transverse relaxation time spectrum of the different degree of freedom component, T is the decay time, n 1＝2,n2＝n3 =1, and c is a constant. In this embodiment, the degree of freedom component refers to cell wall, free water in different tissue cell cavities, bound water in cell wall, cured resin (or resin which is gradually cured (from liquid state to solid state)), such as wood cell wall rigid polymer component, wood fiber cell cavity and wood ray cell cavity free water, and wood duct cell cavity water.
According to the embodiment, the change of the cell wall rigidity of the same sample before and after the resin impregnation modification treatment is characterized, so that the resin impregnation modification effect is accurately interpreted, and a scientific reference basis is provided for optimizing the wood resin impregnation parameters and controlling the process. Namely, firstly taking out unmodified wood at regular time in the drying process to perform low-field nuclear magnetic resonance scanning, then performing low-molecular-weight resin impregnation modification treatment, and then taking out modified wood at regular time in the drying process to perform low-field nuclear magnetic resonance scanning, wherein under the condition of the same water content, the T 21 and the A 1 of the same sample before and after modification are changed.
Fig. 6 and 7 show the changes in the transverse relaxation time T 21 and the transverse relaxation time integral peak area a 1 before and after modification of the wood during drying, respectively. The higher the cell wall stiffness, the less freedom of hydrogen proton movement and the shorter the transverse relaxation time T 21. As can be seen from fig. 6 and 7, the rigidity and the content of the rigidity components of the wood before and after modification are obviously improved, and the modification treatment effectively enhances the mechanical strength of the wood cell wall. The characterization method provided by the invention eliminates the defects of the traditional method, and can be used for in-situ and accurate comparison of the cell wall intensities before and after sample modification.
In addition, the invention also provides a wood cell wall rigidity characterization device based on low-field nuclear magnetic resonance, which comprises the following components:
The first weighing scanning module is used for drying the wood sample, taking out the wood sample at regular time, weighing the wood sample, and performing low-field nuclear magnetic resonance scanning, wherein the full relaxation signal intensity of the wood sample is acquired by adopting an MSE-CPMG pulse sequence during the low-field nuclear magnetic resonance scanning;
the second data processing module is used for calculating the water content of the wood sample according to the weighing quality and establishing a free induction decay curve of the wood sample according to the intensity of the complete relaxation signal;
The third data processing module is used for calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample through a data inversion algorithm combining Gaussian fitting and exponential fitting according to the moisture content and the free induction decay curve of the wood sample.
In addition, the invention also provides a wood cell wall stiffness characterization device based on low-field nuclear magnetic resonance, which comprises a microprocessor and a memory which are connected with each other, wherein the microprocessor is programmed or configured to execute the steps of the wood cell wall stiffness characterization method based on low-field nuclear magnetic resonance.
Furthermore, the invention provides a computer readable storage medium having stored therein a computer program programmed or configured to perform the aforementioned low field nuclear magnetic resonance-based method of characterization of wood cell wall stiffness.
It will be appreciated by those skilled in the art that embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-readable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein. The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks. These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
While the invention has been described in terms of preferred embodiments, it is not intended to be limiting. Many possible variations and modifications of the disclosed technology can be made by anyone skilled in the art, or equivalent embodiments with equivalent variations can be made, without departing from the scope of the invention. Therefore, any simple modification, equivalent variation and modification of the above embodiments according to the technical substance of the present invention shall fall within the scope of the technical solution of the present invention.
Claims (8)
1. A method for characterizing the rigidity of wood cell walls based on low-field nuclear magnetic resonance is characterized by comprising the following steps:
S1, drying a wood sample, taking out the wood sample at regular time, weighing the wood sample, and performing low-field nuclear magnetic resonance scanning, wherein the full relaxation signal intensity of the wood sample is acquired by adopting an MSE-CPMG pulse sequence during the low-field nuclear magnetic resonance scanning;
S2, calculating to obtain the water content of the wood sample according to the weighing quality, and establishing a free induction attenuation curve of the wood sample according to the intensity of the complete relaxation signal;
S3, calculating to obtain transverse relaxation time T 21 and transverse relaxation time integral peak area A 1 of the wood sample through a data inversion algorithm combining Gaussian fitting and exponential fitting according to the moisture content and free induction decay curve of the wood sample;
The step S3 specifically comprises the following steps:
s301, judging that the water content of the wood sample is larger than a preset threshold, executing a step S302 when the water content of the wood sample is lower than the preset threshold, otherwise, executing a step S303;
S302, calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample by adopting a fitting function shown in the formula (1) according to the free induction decay curve of the wood sample;
(1)
S303, calculating to obtain the transverse relaxation time T 21 and the integral peak area A 1 of the transverse relaxation time according to the free induction decay curve of the wood by a data inversion algorithm by adopting a fitting function shown in the formula (2);
(2)
In the formulas (1) and (2), T 21、T22、T23 is the transverse relaxation time of the components with different degrees of freedom, a 1、A2、A3 is the integral peak area of the transverse relaxation time spectrum of the components with different degrees of freedom, T is the decay time, n 1=2,n2=n3 =1, and c is a constant;
The MSE-CPMG pulse sequence has the parameters of 2000-12000 of echo number, 8-64 of scanning times and 0.1-0.25 ms of echo time.
2. The method for characterizing cell wall stiffness of wood according to claim 1, wherein the wood sample is a wood sample subjected to or not subjected to a resin modification treatment.
3. The method for characterizing cell wall stiffness of wood according to claim 2, wherein the wood sample is a wood sample subjected to end capping treatment.
4. The method for characterizing cell wall stiffness of wood according to claim 2, wherein the resin-modified treatment comprises immersing a wood sample having a water content of 0% in a resin solution having a mass concentration of 10% -50% to obtain a resin-immersed modified wood sample.
5. The method for characterizing the rigidity of a wood cell wall according to claim 4, wherein the impregnation is vacuum pressurized impregnation, and specifically comprises the steps of placing a wood sample with the water content of 0% in a resin solution with the mass concentration of 10% -50% for 30-50 min under the pressure of 0.06-0.08 MPa, then for 5-10 h under the pressure of 0.5-0.8 MPa, decompressing and taking out the wood sample, and obtaining the wood sample subjected to the resin impregnation modification treatment.
6. An apparatus for implementing the low field nuclear magnetic resonance-based wood cell wall stiffness characterization method according to any one of claims 1-5, characterized by comprising:
The first weighing scanning module is used for drying the wood sample, taking out the wood sample at regular time, weighing the wood sample, and performing low-field nuclear magnetic resonance scanning, wherein the full relaxation signal intensity of the wood sample is acquired by adopting an MSE-CPMG pulse sequence during the low-field nuclear magnetic resonance scanning;
the second data processing module is used for calculating the water content of the wood sample according to the weighing quality and establishing a free induction decay curve of the wood sample according to the intensity of the complete relaxation signal;
The third data processing module is used for calculating to obtain the transverse relaxation time T 21 and the transverse relaxation time integral peak area A 1 of the wood sample through a data inversion algorithm combining Gaussian fitting and exponential fitting according to the moisture content and the free induction decay curve of the wood sample.
7. A low-field nuclear magnetic resonance-based wood cell wall stiffness characterization device comprising a microprocessor and a memory, wherein the microprocessor is programmed or configured to perform the steps of the low-field nuclear magnetic resonance-based wood cell wall stiffness characterization method according to any one of claims 1 to 5.
8. A computer readable storage medium having stored therein a computer program programmed or configured to perform the low field nuclear magnetic resonance-based method of characterizing wood cell wall stiffness of any one of claims 1-5.
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| CN113030148A (en) * | 2021-03-24 | 2021-06-25 | 浙江省林业科学研究院 | Microscopic on-line detection method for phase state change of water-soluble low-molecular-weight resin |
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