CN115873145B - Preparation method of carboxymethyl chitosan - Google Patents
Preparation method of carboxymethyl chitosan Download PDFInfo
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- CN115873145B CN115873145B CN202310212835.2A CN202310212835A CN115873145B CN 115873145 B CN115873145 B CN 115873145B CN 202310212835 A CN202310212835 A CN 202310212835A CN 115873145 B CN115873145 B CN 115873145B
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Abstract
The invention provides a preparation method of carboxymethyl chitosan, and belongs to the field of carboxymethyl chitosan. The preparation method of the carboxymethyl chitosan comprises the following steps: primary alkalization, secondary alkalization, primary carboxylation, secondary carboxylation and post-treatment. According to the preparation method of the carboxymethyl chitosan, aiming at high molecular weight chitosan, ideal substitution effect can be obtained, effective substitution can be realized on the 2-primary amino of the chitosan, and substitution degree is effectively improved; meanwhile, the adverse effect caused by microwave radiation in the carboxylation process of the prior art is overcome, the problem that the microwave radiation can cause chain breakage and degradation of high molecular weight chitosan is avoided, and the viscosity average molecular weight of the prepared carboxymethyl chitosan is (23.8-26.9) multiplied by 10 4 The total substitution degree is 1.81-1.87, wherein the substitution degree of the O position is 1.40-1.44, and the substitution degree of the N position is 0.41-0.43.
Description
Technical Field
The invention relates to the field of carboxymethyl chitosan preparation, in particular to a preparation method of carboxymethyl chitosan.
Background
Chitin, also known as chitin, is a polysaccharide substance extracted from shell of marine crustacean, and has chemical formula (C) 8 H 13 O 5 N) n . Chitin is a nontoxic and odorless white or off-white semitransparent solid, and can be dissolved in concentrated hydrochloric acid, phosphoric acid, sulfuric acid and acetic acid, and is insoluble in alkali and other organic solvents, and insoluble in water. The chitosan (chitosan) can be prepared by removing more than 55% of N-acetyl groups of chitin by concentrated alkali treatment, and is insoluble in water and soluble in partial dilute acid. Chitin and chitosan have many unique properties such as biodegradability, cell affinity and biological effect, especially chitosan containing free amino group, are the only alkaline polysaccharide in natural polysaccharide, and can be widely applied to foodThe product, environmental protection, beauty and health care, antibacterial agent, medical fiber, medical dressing, artificial tissue material, drug slow release material, gene transduction carrier, biomedical field, medical absorbable material and other fields.
Carboxymethyl chitosan is a chitosan derivative modified by carboxymethyl, has good water solubility, permeability, moisture retention and antibacterial property, and can promote the growth of epithelial cells, delay cell aging and promote the healing of skin ulcer wounds. The method is applied to aspects such as medicine, beauty, fruit and vegetable fresh-keeping and the like. Meanwhile, the carboxymethyl chitosan can also be used as flocculating agent, metal ion chelating agent and the like in water treatment. In recent years, carboxymethyl chitosan has become one of the most studied and used chitosan derivatives.
In the prior art, the preparation method of carboxymethyl chitosan mainly comprises the step of carrying out carboxymethyl substitution reaction on chitosan and chloroacetic acid and the like in an alkaline environment. The carboxymethyl chitosan is prepared through the process of substituting carboxymethyl with 2-position primary amino group, 3-position hydroxyl group and 6-position hydroxyl group of chitosan. The substitution sequence is 6-hydroxyl, 3-hydroxyl and 2-primary amino, and the substitution degree is obviously increased along with the reduction of the molecular weight of chitosan. This is because, as the molecular weight increases, chitosan acts as a long-chain polymer compound, its steric hindrance and strong intramolecular and intermolecular hydrogen bonding effects, its structure changes from an disordered structure to an ordered structure, which seriously hinders NaOH and CICH 2 The permeation of small molecules such as COOH and the like can prevent carboxymethyl substitution reaction from proceeding, and finally, the ideal substitution effect of the high molecular weight chitosan can not be obtained, particularly, the effective substitution on the 2-primary amino can not be carried out, the substitution degree can not be effectively improved, and the further expansion of the application of the carboxymethyl chitosan in various fields is limited.
Further, in the prior art, a microwave radiation mode is generally adopted to improve the substitution effect of carboxymethyl. However, in the actual operation of preparing carboxymethyl chitosan by adopting high molecular weight chitosan, the microwave radiation may cause phenomena of chain breakage, degradation and the like of the high molecular weight chitosan, which not only causes the reduction of part of the molecular weight of the chitosan, but also causes the blending of the carboxymethyl chitosan with high and low molecular weight; the quality of the prepared carboxymethyl chitosan product is unstable, and stable and uniform moisturizing performance cannot be realized in the later application process.
Disclosure of Invention
In order to solve the technical problems in the prior art, the invention provides a preparation method of carboxymethyl chitosan, aiming at high molecular weight chitosan, ideal substitution effect can be obtained, effective substitution can be realized on the 2-primary amino of chitosan, and substitution degree is effectively improved; meanwhile, the adverse effect caused by microwave radiation in the carboxylation process of the prior art is overcome, the problems that the high molecular weight chitosan is broken and degraded due to the microwave radiation are avoided, the quality of the prepared carboxymethyl chitosan product is stable, and stable and uniform moisturizing performance is realized in the later application process.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
a preparation method of carboxymethyl chitosan, which comprises the following steps: primary alkalization, secondary alkalization, primary carboxylation, secondary carboxylation and post-treatment.
The primary alkalization method comprises the steps of adding sodium hydroxide into an ethanol water solution with the volume concentration of 50-55% under the stirring condition, and adjusting the concentration of the sodium hydroxide to 15-20wt% to prepare alkalization liquid; under the stirring condition, adding chitosan into the alkalization liquid, and adjusting the concentration of the chitosan in the alkalization liquid to 5-7wt% to prepare chitosan dispersion liquid; performing first ultrasonic treatment on the chitosan dispersion liquid for 10-30min; then cooling to minus 30 ℃ to minus 35 ℃ at a cooling rate of 0.2-0.5 ℃/min, and carrying out heat preservation for 3-5h to prepare a primary alkalization substance;
in the primary alkalization, the viscosity average molecular weight Meta of the chitosan is (35-40) multiplied by 10 4 The deacetylation degree is 75-85%, and the grain diameter is 0.1-0.3mm;
in the primary alkalization, the frequency of the first ultrasonic treatment is 30-33kHz, and the ultrasonic power is 400-500W.
The secondary alkalization method comprises the steps of adding saturated sodium hydroxide aqueous solution under stirring when the primary alkalization substance is naturally rewarmed to 3-8 ℃, adjusting the concentration of sodium hydroxide to 35-40wt%, and performing secondary ultrasonic treatment for 10-20min to obtain a secondary alkalization substance;
in the secondary alkalization, the frequency of the second ultrasonic treatment is 33-35kHz, and the ultrasonic power is 400-500W.
The primary carboxylation method is to put a predetermined amount of carboxylation agent and catalyst into the secondary alkalization substance, heat up to 40-50 ℃ and then keep the temperature, and carry out primary carboxylation reaction for 1-2h to obtain the primary carboxylation substance. In the primary carboxylation reaction process, ultraviolet light with the wavelength of 385-395nm is adopted for radiation treatment.
In the primary carboxylation, the carboxylation agent is a mixture of chloroacetic acid and trichloroacetic acid, and the weight ratio of the chloroacetic acid to the trichloroacetic acid is 1:3-4;
in the primary carboxylation, the addition amount of the carboxylation agent is 3-6 times of the weight of chitosan in the primary alkalization.
In the primary carboxylation, the catalyst is a mixture of lauryl dimethyl amine oxide and dodecyl trimethyl ammonium chloride, and the weight ratio of the lauryl dimethyl amine oxide to the dodecyl trimethyl ammonium chloride is 2-3:1;
in the primary carboxylation, the addition amount of the catalyst is 0.2-0.5% of the weight of chitosan in the primary alkalization.
The secondary carboxylation method is that the primary carboxylation is continuously heated to 50-60 ℃ and then is kept at the temperature, and the secondary carboxylation reaction is carried out for 30-60min, so as to prepare the secondary carboxylation. In the secondary carboxylation process, ultraviolet light with the wavelength of 390-400nm is adopted for radiation treatment.
The post-treatment method comprises the steps of naturally cooling the secondary carboxylate to normal temperature, putting the secondary carboxylate into absolute ethyl alcohol with the volume of 3-5 times, then adopting acetic acid with the concentration of 30-40wt% to adjust the pH value to 6.8-7.2, and standing until no solid matters are separated out; and then adopting a centrifugal rotating speed of 12000-15000rpm to centrifugally separate out solid matters, adopting a sufficient amount of ethanol solution with the concentration of 70-80% to wash the solid matters until no chloride ions exist, then adopting 2-3 times of absolute ethanol for leaching once, and then placing the solid matters in a vacuum environment, and drying at the temperature of 60-70 ℃ until the weight is unchanged, thus obtaining the carboxymethyl chitosan.
Compared with the prior art, the invention has the beneficial effects that:
(1) According to the preparation method of carboxymethyl chitosan, the chitosan is subjected to alkalization and swelling treatment in a secondary alkalization mode in a specific ethanol solution; in the secondary alkalization process, ultrasonic treatment with specific technological parameters (such as ultrasonic frequency and the like) is set, and adverse effects of ultrasonic on high molecular weight chitosan are avoided while the alkalization and swelling of the chitosan are promoted; realizes good alkalization and swelling effects, reduces OH - The difficulty of penetrating into the chitosan is improved, the alkalization active center is easier to form, the reaction effect of subsequent carboxylation is promoted, the substitution degree is effectively improved, and particularly, the substitution degree of carboxymethyl on 2-primary amino is improved.
(2) According to the preparation method of carboxymethyl chitosan, the specific carboxylation agent, the catalyst and the secondary alkalization substance are used for carrying out secondary carboxylation, and ultraviolet light with specific wavelength is arranged for carrying out radiation treatment in the secondary carboxylation process, so that the adverse effect of ultraviolet light on high molecular weight chitosan is avoided while the carboxylation reaction of the alkalized chitosan is promoted, the problems of chain breakage and degradation which possibly occur are avoided, the substitution degree is further improved, and particularly the substitution degree of carboxymethyl on 2-primary amino is improved. The carboxymethyl chitosan has viscosity average molecular weight (Meta) of (23.8-26.9) x 10 4 The total substitution degree is 1.81-1.87, wherein the substitution degree of the O position is 1.40-1.44, and the substitution degree of the N position is 0.41-0.43.
(3) According to the preparation method of the carboxymethyl chitosan, the quality of the prepared carboxymethyl chitosan product is stable, and the moisture absorption rate of 24 hours is 36.8-37.3% and the moisture absorption rate of 48 hours is 40.7-41.6% under the condition of RH=43% through detection; under the condition of RH=81%, the moisture absorption rate for 24 hours is 50.5-51.4%, and the moisture absorption rate for 48 hours is 55.8-57.5%; under the condition of RH=43%, the 24-hour moisture retention rate is 356.1-357.2%, and the 48-hour moisture retention rate is 411.9-412.8%; under the condition of RH=81%, the 24-hour moisture retention rate is 434.0-435.5%, and the 48-hour moisture retention rate is 471.1-472.3%.
(4) The preparation method of carboxymethyl chitosan has the advantages of simple process method, easy control of the preparation process and suitability for large-scale industrial production.
Detailed Description
Specific embodiments of the present invention will now be described in order to provide a clearer understanding of the technical features, objects and effects of the present invention.
Example 1
A preparation method of carboxymethyl chitosan, which comprises the following steps: primary alkalization, secondary alkalization, primary carboxylation, secondary carboxylation and post-treatment. The method comprises the following steps:
1. primary alkalization
Adding sodium hydroxide into an ethanol water solution with the volume concentration of 50% under the stirring condition, and adjusting the concentration of the sodium hydroxide to 15wt% to prepare an alkalization solution; under the stirring condition, adding chitosan into the alkalization liquid, and adjusting the concentration of the chitosan in the alkalization liquid to 5 weight percent to prepare chitosan dispersion liquid; performing first ultrasonic treatment on the chitosan dispersion liquid for 10min; then cooling to minus 30 ℃ at a cooling rate of 0.2 ℃/min, and carrying out heat preservation treatment for 3 hours to obtain the primary alkalization product.
In one alkalization, the viscosity average molecular weight (Meta) of chitosan is 35 multiplied by 10 4 The degree of deacetylation was 85% and the particle size was 0.1mm.
In one alkalization, the frequency of the first sonication was 30kHz and the ultrasonic power was 400W.
2. Secondary alkalization
And (3) when the primary alkalization substance is naturally rewarmed to 3 ℃, adding a saturated sodium hydroxide aqueous solution under the stirring condition, adjusting the concentration of sodium hydroxide to 35 weight percent, and performing secondary ultrasonic treatment for 10min to obtain a secondary alkalization substance.
In the secondary alkalization, the frequency of the second ultrasonic treatment is 33kHz and the ultrasonic power is 400W.
3. Primary carboxylation
Adding a predetermined amount of carboxylation agent and catalyst into the secondary alkalization product, heating to 40 ℃, preserving heat, and performing primary carboxylation reaction for 1h to obtain primary carboxylation product. In the primary carboxylation reaction, ultraviolet light with the wavelength of 385nm is adopted for radiation treatment.
The carboxylation agent is a mixture of chloroacetic acid and trichloroacetic acid, and the weight ratio of the chloroacetic acid to the trichloroacetic acid is 1:3.
The addition amount of the carboxylation agent is 3 times of the weight of the chitosan.
The catalyst is a mixture of lauryl dimethyl amine oxide and dodecyl trimethyl ammonium chloride, and the weight ratio of the lauryl dimethyl amine oxide to the dodecyl trimethyl ammonium chloride is 2:1.
The addition amount of the catalyst is 0.2% of the weight of the chitosan.
4. Secondary carboxylation
And (3) continuously heating the primary carboxylate to 50 ℃, preserving heat, and carrying out secondary carboxylation reaction for 30min to obtain the secondary carboxylate. In the secondary carboxylation process, ultraviolet light with the wavelength of 390nm is adopted for radiation treatment.
5. Post-treatment
Naturally cooling the secondary carboxylate to normal temperature, adding the secondary carboxylate into absolute ethyl alcohol with the volume being 3 times that of the secondary carboxylate, then adopting acetic acid with the concentration of 30wt% to adjust the pH value to be 6.8, and standing until no solid is separated out; and then adopting a centrifugal rotating speed of 12000rpm to centrifugally separate out solid matters, adopting a sufficient amount of ethanol solution with the concentration of 70% to wash the solid matters until no chloride ions exist, then adopting 2 times of absolute ethanol for leaching once, and then placing the solid matters in a vacuum environment, and drying at the temperature of 60 ℃ until the weight is unchanged, thus obtaining the carboxymethyl chitosan.
Example 2
A preparation method of carboxymethyl chitosan, which comprises the following steps: primary alkalization, secondary alkalization, primary carboxylation, secondary carboxylation and post-treatment. The method comprises the following steps:
1. primary alkalization
Adding sodium hydroxide into an ethanol water solution with the volume concentration of 52% under the stirring condition, and adjusting the concentration of the sodium hydroxide to 18wt% to prepare an alkalization solution; under the stirring condition, adding chitosan into the alkalization liquid, and adjusting the concentration of the chitosan in the alkalization liquid to 6 weight percent to prepare chitosan dispersion liquid; performing first ultrasonic treatment on the chitosan dispersion liquid for 20min; then cooling to minus 32 ℃ at a cooling rate of 0.3 ℃/min, and carrying out heat preservation treatment for 4 hours to obtain the primary alkalization product.
In one alkalization, the viscosity average molecular weight (Meta)) Is 38X 10 4 The degree of deacetylation was 80% and the particle size was 0.2mm.
In one alkalization, the frequency of the first sonication was 31kHz and the ultrasonic power was 450W.
2. Secondary alkalization
And (3) when the primary alkalization substance is naturally rewarmed to 5 ℃, adding a saturated sodium hydroxide aqueous solution under the stirring condition, adjusting the concentration of sodium hydroxide to 38 weight percent, and performing secondary ultrasonic treatment for 15min to obtain a secondary alkalization substance.
In the secondary alkalization, the frequency of the second ultrasonic treatment is 34kHz and the ultrasonic power is 450W.
3. Primary carboxylation
And (3) adding a predetermined amount of carboxylation agent and catalyst into the secondary alkalization product, heating to 45 ℃, preserving heat, and carrying out primary carboxylation reaction for 1.5h to obtain the primary carboxylation product. In the primary carboxylation reaction, ultraviolet light with the wavelength of 390nm is adopted for radiation treatment.
The carboxylation agent is a mixture of chloroacetic acid and trichloroacetic acid, and the weight ratio of the chloroacetic acid to the trichloroacetic acid is 1:3.5.
The addition amount of the carboxylation agent is 5 times of the weight of the chitosan.
The catalyst is a mixture of lauryl dimethyl amine oxide and dodecyl trimethyl ammonium chloride, and the weight ratio of the lauryl dimethyl amine oxide to the dodecyl trimethyl ammonium chloride is 2.5:1.
The addition amount of the catalyst is 0.3 percent of the weight of the chitosan.
4. Secondary carboxylation
And (3) continuously heating the primary carboxylate to 55 ℃, preserving heat, and carrying out secondary carboxylation reaction for 40min to obtain the secondary carboxylate. In the secondary carboxylation process, ultraviolet light with wavelength of 395nm is adopted for radiation treatment.
5. Post-treatment
Naturally cooling the secondary carboxylate to normal temperature, adding the secondary carboxylate into absolute ethyl alcohol with the volume of 4 times, then adopting acetic acid with the concentration of 35wt% to adjust the pH value to 7.0, and standing until no solid is separated out; and centrifuging at 14000rpm to separate solid, washing the solid with 75% concentration ethanol solution to no chloride ion, leaching with 2.5 times volume of anhydrous ethanol, and drying at 65deg.C in vacuum until the weight is unchanged to obtain carboxymethyl chitosan.
Example 3
A preparation method of carboxymethyl chitosan, which comprises the following steps: primary alkalization, secondary alkalization, primary carboxylation, secondary carboxylation and post-treatment. The method comprises the following steps:
1. primary alkalization
Adding sodium hydroxide into an ethanol water solution with the volume concentration of 55% under the stirring condition, and adjusting the concentration of the sodium hydroxide to 20wt% to prepare an alkalization solution; under the stirring condition, adding chitosan into the alkalization liquid, and adjusting the concentration of the chitosan in the alkalization liquid to 7 weight percent to prepare chitosan dispersion liquid; performing first ultrasonic treatment on the chitosan dispersion liquid for 30min; then cooling to minus 35 ℃ at a cooling rate of 0.5 ℃/min, and carrying out heat preservation treatment for 5 hours to obtain the primary alkalization product.
In one alkalization, the viscosity average molecular weight (Meta) of chitosan is 40×10 4 The degree of deacetylation was 75% and the particle size was 0.3mm.
In one alkalization, the first sonication was at a frequency of 33kHz and an ultrasonic power of 500W.
2. Secondary alkalization
And (3) when the primary alkalization substance is naturally rewarmed to 8 ℃, adding a saturated sodium hydroxide aqueous solution under the stirring condition, adjusting the concentration of sodium hydroxide to 40 weight percent, and performing secondary ultrasonic treatment for 20min to obtain a secondary alkalization substance.
In the secondary alkalization, the frequency of the second ultrasonic treatment is 35kHz and the ultrasonic power is 500W.
3. Primary carboxylation
And (3) adding a predetermined amount of carboxylation agent and catalyst into the secondary alkalization product, heating to 50 ℃, preserving heat, and carrying out primary carboxylation reaction for 2 hours to obtain the primary carboxylation product. In the primary carboxylation reaction, ultraviolet light with wavelength of 395nm is adopted for radiation treatment.
The carboxylation agent is a mixture of chloroacetic acid and trichloroacetic acid, and the weight ratio of the chloroacetic acid to the trichloroacetic acid is 1:4.
The addition amount of the carboxylation agent is 6 times of the weight of the chitosan.
The catalyst is a mixture of lauryl dimethyl amine oxide and dodecyl trimethyl ammonium chloride, and the weight ratio of the lauryl dimethyl amine oxide to the dodecyl trimethyl ammonium chloride is 3:1.
The addition amount of the catalyst is 0.5% of the weight of the chitosan.
4. Secondary carboxylation
And (3) continuously heating the primary carboxylate to 60 ℃, preserving heat, and carrying out secondary carboxylation reaction for 60min to obtain the secondary carboxylate. In the secondary carboxylation process, ultraviolet light with the wavelength of 400nm is adopted for radiation treatment.
5. Post-treatment
Naturally cooling the secondary carboxylate to normal temperature, adding the secondary carboxylate into absolute ethyl alcohol with the volume of 5 times, then adopting acetic acid with the concentration of 40wt% to adjust the pH value to 7.2, and standing until no solid is separated out; and then adopting a centrifugal rotating speed of 15000rpm to centrifugally separate out solid matters, adopting a sufficient amount of ethanol solution with the concentration of 80% to wash the solid matters until no chloride ions exist, then adopting absolute ethanol with the volume of 3 times to wash the solid matters once, and then placing the solid matters in a vacuum environment, and drying at 70 ℃ until the weight is unchanged, thus obtaining the carboxymethyl chitosan.
Comparative example 1
The technical scheme of the embodiment 2 is adopted, and the difference is that: 1) In the primary alkalization and secondary alkalization steps, the first ultrasonic treatment and the second ultrasonic treatment are omitted. 2) In the primary alkalization step, deionized water is used for replacing the 52% ethanol water solution.
Comparative example 2
The technical scheme of the embodiment 2 is adopted, and the difference is that: 1) In the primary carboxylation step, heating and heat preservation are carried out by adopting microwave radiation so as to assist the carboxylation reaction; wherein the frequency of the microwave radiation is 3000MHz and the microwave power is 600W. 2) The secondary carboxylation step was omitted.
The carboxymethyl chitosan prepared in examples 1-3 and comparative examples 1-2 was tested for the indexes related to viscosity average molecular weight, total substitution degree, substitution degree at O-position and substitution degree at N-position, and the specific results are shown in Table 1:
TABLE 1 carboxymethyl chitosan molecular weight and substitution degree detection results summary table
Further, the carboxymethyl chitosan prepared in examples 1 to 3 and comparative examples 1 to 2 was examined for its moisture absorption and retention properties. Specifically, the moisture absorption properties of the materials were measured under humidity conditions in which the Relative Humidity (RH) was 43% and 81%, respectively; the moisturizing properties were measured under conditions of a Relative Humidity (RH) of 43% and 81% and drying, respectively.
The method for detecting the hygroscopicity of the carboxymethyl chitosan comprises the following steps: respectively weigh W 0 Carboxymethyl chitosan prepared in examples 1-3 and comparative examples 1-2, which were each 0.5g, were placed in a desiccator for moisture absorption with a saturated aqueous ammonium sulfate solution to maintain relative humidity rh=43% and relative humidity rh=81%, and were weighed once at 24 hours and 48 hours, respectively, to obtain W 1 Numerical values. And the moisture absorption rate was calculated.
The method for calculating the moisture absorption rate comprises the following steps: moisture absorption (%) = [ (W) 1- -W 0 ）/W 0 ]×100%
The method for detecting the moisture retention of the carboxymethyl chitosan comprises the following steps: the carboxymethyl chitosan prepared in examples 1-3 and comparative examples 1-2 has water content of H 0 10wt% and then placing in a relative humidity RH=43% and a relative humidity RH=81% and a silica gel dryer, respectively, and weighing once at 24H and 48H to obtain H 1 Numerical values. And the moisture residual rate was calculated.
The method for calculating the moisture residual rate comprises the following steps: moisture residual ratio (%) = (H) 1 /H 0 ）×100%
The specific test results are shown in Table 2:
TABLE 2 carboxymethyl chitosan moisture absorption rate and moisture retention rate test result summary table
The percentages used in the present invention are mass percentages unless otherwise indicated.
Finally, it should be noted that: the foregoing description is only a preferred embodiment of the present invention, and the present invention is not limited thereto, but it is to be understood that modifications and equivalents of some of the technical features described in the foregoing embodiments may be made by those skilled in the art, although the present invention has been described in detail with reference to the foregoing embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (2)
1. The preparation method of carboxymethyl chitosan is characterized by comprising the following steps: primary alkalization, secondary alkalization, primary carboxylation, secondary carboxylation and post-treatment;
the primary alkalization method comprises the steps of adding sodium hydroxide into an ethanol water solution with the volume concentration of 52% under the stirring condition, and adjusting the concentration of the sodium hydroxide to 18wt% to prepare alkalization liquid; under the stirring condition, adding chitosan into the alkalization liquid, and adjusting the concentration of the chitosan in the alkalization liquid to 6 weight percent to prepare chitosan dispersion liquid; performing first ultrasonic treatment on the chitosan dispersion liquid for 20min; then cooling to minus 32 ℃ at a cooling rate of 0.3 ℃/min, and carrying out heat preservation treatment for 4 hours to obtain a primary alkalization product;
in the primary alkalization, the frequency of the first ultrasonic treatment is 31kHz, and the ultrasonic power is 450W;
in the primary alkalization, the viscosity average molecular weight of the chitosan is 38 multiplied by 10 4 The deacetylation degree is 80%, and the particle size is 0.2mm;
the secondary alkalization method is that when the primary alkalization substance is naturally rewarmed to 5 ℃, saturated sodium hydroxide aqueous solution is added under the stirring condition, the concentration of sodium hydroxide is regulated to 38 weight percent, and secondary ultrasonic treatment is carried out for 15min, so as to obtain the secondary alkalization substance;
in the secondary alkalization, the frequency of the second ultrasonic treatment is 34kHz, and the ultrasonic power is 450W;
the primary carboxylation method comprises the steps of adding a predetermined amount of carboxylation agent and catalyst into a secondary alkalization substance, heating to 45 ℃, preserving heat, and carrying out primary carboxylation reaction for 1.5h to obtain a primary carboxylation substance; in the primary carboxylation reaction process, ultraviolet light with the wavelength of 390nm is adopted for radiation treatment;
in the primary carboxylation, the carboxylation agent is a mixture of chloroacetic acid and trichloroacetic acid, and the weight ratio of the chloroacetic acid to the trichloroacetic acid is 1:3.5;
the addition amount of the carboxylation agent is 5 times of the weight of the chitosan in one-time alkalization;
in the primary carboxylation, the catalyst is a mixture of lauryl dimethyl amine oxide and dodecyl trimethyl ammonium chloride, and the weight ratio of the lauryl dimethyl amine oxide to the dodecyl trimethyl ammonium chloride is 2.5:1;
the addition amount of the catalyst is 0.3% of the weight of chitosan in primary alkalization;
the secondary carboxylation method is that the primary carboxylation is continuously heated to 55 ℃ and then is kept at the temperature, and the secondary carboxylation reaction is carried out for 40min to prepare the secondary carboxylation; in the secondary carboxylation process, ultraviolet light with wavelength of 395nm is adopted for radiation treatment;
the post-treatment method comprises the steps of cooling, regulating pH value and precipitating the secondary carboxylate, separating out solid matters, washing and drying to obtain carboxymethyl chitosan.
2. The method for preparing carboxymethyl chitosan according to claim 1, wherein the post-treatment method is that the secondary carboxylate is naturally cooled to normal temperature, put into absolute ethyl alcohol with 3-5 times of volume, then the pH value is adjusted to 6.8-7.2 by adopting acetic acid, and the mixture is stood until no solid matters are separated out; and then centrifugally separating out solid matters, washing the solid matters by adopting a sufficient amount of ethanol solution until no chloride ions exist, and then leaching the solid matters once by adopting 2-3 times of absolute ethanol, and drying the solid matters in vacuum until the weight is unchanged, thus obtaining the carboxymethyl chitosan.
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