CN110128591B - A kind of method for improving and maintaining high degree of substitution carboxymethyl starch viscosity - Google Patents

Abstract

The invention belongs to the technical field of modified starch, and discloses a method for improving and maintaining the viscosity of carboxymethyl starch with a high degree of substitution. Method: 1) Mix carboxymethyl starch with water to obtain starch paste; 2) Mix monomer with starch paste in the form of solution, adjust the pH of the system to 7.5-9.5, and then add initiator under inert atmosphere , react at 50-80° C. for 3-8 hours, adjust the pH of the reaction product to neutrality, and perform subsequent processing to obtain composite modified carboxymethyl starch; the monomer is a kind of acrylamide or acrylic acid. The method of the invention improves the viscosity of the carboxymethyl starch, reaches 5-24 times of the original carboxymethyl starch with different degrees of substitution, and solves the problem that the viscosity of the carboxymethyl starch decreases with the increase of the degree of substitution. With the increase of the degree of substitution, the composite modified carboxymethyl starch obtained by the method of the present invention can maintain its viscosity at a high level all the time, and has good viscosity stability.

Description

A kind of method for improving and maintaining high degree of substitution carboxymethyl starch viscosity

technical field

The invention belongs to the technical field of modified starch, and in particular relates to a method for improving and maintaining the viscosity of carboxymethyl starch, especially the viscosity of carboxymethyl starch with a high degree of substitution, by performing graft compound modification on carboxymethyl starch.

Background technique

Carboxymethyl starch is an anionic modified starch obtained by carboxymethyl etherification and modification of starch, and has a very wide range of applications. In terms of performance, when the etherification substitution degree of carboxymethyl starch reaches 0.1, it can be swelled into a uniform starch paste in cold water, and the paste liquid has excellent performance and is very convenient in application.

However, when the degree of substitution by etherification of carboxymethyl starch reaches about 0.2, continuing to increase the degree of substitution will not only not increase the viscosity, but will have the disadvantage that the viscosity of the paste will continue to decrease as the degree of substitution increases, and the degree of substitution will The higher the viscosity, the lower the viscosity.

The high degree of substitution of carboxymethyl starch has the defect that with the increase of the degree of substitution, the viscosity of the paste liquid decreases rapidly, which greatly weakens its thickening performance and directly affects its freeze-thaw stability, flocculation and adsorption performance. and other properties, which greatly limits the application of carboxymethyl starch in the fields of sizing agent, flocculant and textile size. How to overcome the above defects and prepare carboxymethyl modified starch with high degree of substitution and high viscosity at the same time has always been an important scientific and technological research topic in this technical field.

In order to improve and enhance the viscosity and viscosity stability of carboxymethyl starch with high degree of substitution, there have been many research reports in China, such as the use of cross-linking agents such as epichlorohydrin, sodium trimetaphosphate, sodium tripolyphosphate, hexametaphosphate Sodium phosphate is used for secondary modification to prepare cross-linked composite modified carboxymethyl starch, etc. The purpose is to increase its viscosity by cross-linking. In the patent application "Preparation method of high-viscosity cross-linked carboxymethyl starch" (CN 104311681 A), use epichlorohydrin as a cross-linking agent to prepare cross-linked carboxymethyl starch; in the patent application "Ultra-high viscosity carboxymethyl starch" Preparation method of sodium starch base" (CN 105859895 A), using sodium trimetaphosphate as a cross-linking agent to prepare cross-linked sodium carboxymethyl starch and the like. The above modification method can improve and help the viscosity and stability of carboxymethyl starch to a certain extent, but it still does not meet the requirements.

The present invention performs grafting compound modification on carboxymethyl starch, and successfully prepares a compound modified carboxymethyl starch with high degree of substitution and high viscosity at the same time, and the viscosity is not significantly decreased due to the increase of the degree of substitution. base starch. The viscosity of the composite modified carboxymethyl starch of the present invention is close to or even exceeds 20000 mPa·s, and reaches 5-24 times of the original carboxymethyl starch with different degrees of substitution. The composite modified carboxymethyl starch prepared by the present invention not only has the viscosity greatly increased, but also remains stable with the increase of the substitution degree.

SUMMARY OF THE INVENTION

In order to overcome the shortcomings and deficiencies of the prior art, the purpose of the present invention is to provide a method for improving and maintaining the viscosity of carboxymethyl starch with high degree of substitution. The invention adopts monomers to carry out graft modification on carboxymethyl starch, so as to improve and maintain the viscosity of carboxymethyl starch, especially carboxymethyl starch with high degree of substitution.

The object of the present invention is achieved through the following technical solutions:

A method for improving and maintaining the viscosity of carboxymethyl starch with a high degree of substitution, comprising the following steps:

1) carboxymethyl starch is mixed with water to obtain starch paste;

2) Mix the monomer with starch paste in the form of solution, adjust the pH of the system to 7.5-9.5, then add an initiator under an inert atmosphere, react at 50-80 °C for 3-8 hours, and adjust the pH of the reaction product to neutrality, followed by treatment to obtain compound modified carboxymethyl starch.

The mass-volume ratio of carboxymethyl starch to water described in step 1) is (10-100) g: (200-800) mL;

The carboxymethyl starch described in step 1) includes more than one in the carboxymethyl starch made from corn starch, tapioca starch, potato starch, wheat starch, rice starch and/or waxy (waxy) corn starch, and the carboxymethyl starch is The degree of carboxymethyl substitution in methyl starch is 0.1 to 2.0.

The monomer described in step 2) is one of acrylamide or acrylic acid; when the monomer is acrylamide, the solution is acrylamide aqueous solution; the mass ratio of acrylamide to water in the acrylamide aqueous solution=1:1～1:3 When the monomer is acrylic acid, the solution is the sodium hydroxide solution of acrylic acid; in the sodium hydroxide solution of acrylic acid, the mass ratio of acrylic acid to sodium hydroxide solution=1:1～1:4; the mass concentration of sodium hydroxide solution is 20%～40%;

The mass ratio of the monomer described in step 2) to the carboxymethyl starch in the starch paste is 0.5:1 to 2:1;

In step 2), the pH of the adjustment system is 7.5-9.5, which means that the sodium hydroxide solution is used for adjustment; the mass concentration of the sodium hydroxide solution is 10%-30%;

In step 2), the inert atmosphere is nitrogen atmosphere.

In step 2), the initiator is potassium persulfate; the initiator is added in the form of an aqueous solution, and the mass ratio of the initiator to water in the aqueous solution is (5-12):100.

In step 2), adjusting the pH of the reaction product to neutrality refers to adjusting with a sulfuric acid solution.

The consumption of the initiator described in step 2) is 0.6%～6% of the quality of carboxymethyl starch;

The follow-up treatment in step 2) refers to sedimentation and filtration with a volume fraction of 95% ethanol aqueous solution, washing with a volume fraction of 90% ethanol aqueous solution, drying, pulverization, and sieving.

The method of the invention can improve the viscosity of carboxymethyl starch, and with the increase of the degree of substitution, the viscosity can be kept stable, avoiding the phenomenon that the viscosity of the high-substituted carboxymethyl starch paste is low at present. In addition, the prepared carboxymethyl starch also Has other better properties such as higher clarity and better water retention.

Compared with the prior art, the present invention has the following advantages:

(1) In the present invention, polyacrylamide and polyacrylic acid branched chains are introduced in the carboxymethyl starch molecule by means of graft copolymerization, which can comprehensively improve various performance indicators of the original carboxymethyl starch.

(2) The composite modified carboxymethyl starch prepared by the method of the present invention can greatly increase the viscosity of the carboxymethyl starch paste, and the increase range is 5-24 times that of carboxymethyl starch with different degrees of substitution.

(3) With the increase of the substitution degree of the compound modified carboxymethyl starch prepared by the method of the present invention, the viscosity of the carboxymethyl starch paste liquid can be maintained at a high level all the time, and has good viscosity stability.

(4) The composite modified carboxymethyl starch prepared by the method of the present invention also has the advantages of higher transparency and better water retention.

Description of drawings

Fig. 1 is the viscosity curve of compound modified carboxymethyl starch prepared by the present invention and unmodified carboxymethyl starch (original carboxymethyl starch) under different degrees of substitution; wherein curve (1) is compound modified carboxymethyl starch Base starch, curve (2) is the original carboxymethyl starch.

Detailed ways

The present invention will be further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

Example 1

(1) 40g of carboxymethyl cornstarch with a degree of substitution of 0.6 and 500mL of water were mixed to obtain starch paste;

(2) 40g acrylamide was mixed with 40g water, stirred until completely dissolved, the solution was added to the starch paste liquid, and stirred evenly; then dropwise added a sodium hydroxide solution with a mass concentration of 10% to adjust the pH=7.5 of the reaction system;

(3) Slowly fill with nitrogen for 30min;

(4) mix 0.24g potassium persulfate with 4.8g water, stir until completely dissolved, add the initiator solution to the starch paste with adjusted pH and mix well;

(5) 50 ℃ of heat preservation reaction 5h, obtain reaction product;

(6) after the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use a volume fraction of 95% ethanol aqueous solution to settle, filter, wash with a volume fraction of 90% ethanol aqueous solution, dry at 55 ° C, pulverize, sieve, The compound modified carboxymethyl starch was obtained.

The original carboxymethyl starch and the prepared compound modified carboxymethyl starch were made into a paste with a mass fraction of 3%. After the gelatinization was complete, the viscosity was measured by a rotational viscometer to 3090mPa·s and 24700mPa·s, respectively. The magnification is about 8 times; the transparency of the two pastes tested by UV spectrophotometer is 18.40% and 19.97%, respectively, and the increase rate is about 1.1 times; The volume of the precipitated water was 5.9 mL and 1.8 mL, respectively, and the water retention rate increased by about 2.00 times.

Example 2

(1) mix 60g of carboxymethyl tapioca starch with a degree of substitution of 0.1 and 800mL of water to obtain starch paste;

(2) Mix 90g acrylamide with 140g water, stir until completely dissolved, add the solution to the starch paste liquid, and stir evenly; under stirring conditions, dropwise add 30% of the starch paste liquid with a mass concentration to the starch paste liquid added to the monomer. Sodium hydroxide solution, adjust the pH=8.5 of the reaction system;

(3) Slowly fill with nitrogen for 30min;

(4) mix 0.6g potassium persulfate with 5.4g water, stir until completely dissolved, add the solution to the starch paste with adjusted pH and mix well;

(5) 60 ℃ heat preservation reaction 3h, obtain reaction product;

(6) After the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution for sedimentation, filtration, 90% ethanol aqueous solution washing, drying at 55°C, pulverizing, and sieving to obtain composite modified carboxymethyl base starch.

Example 3

(1) mix 20g of carboxymethyl wheat starch with a degree of substitution of 1.2 and 600mL of water to obtain starch paste;

(2) mix 10g acrylamide with 30g water, stir until completely dissolved, add the solution to the starch paste, and stir evenly;

(3) under stirring condition, drip the sodium hydroxide solution that mass concentration is 20% in starch paste liquid, adjust the pH=9 of reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) mix 1.2g potassium persulfate with 12g water, stir until completely dissolved, add the solution to the starch paste with adjusted pH value and mix well;

(6) 70 ℃ heat preservation reaction 3h, obtain reaction product;

(7) after the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution for sedimentation, filtration, 90% ethanol aqueous solution washing, drying at 55°C, pulverizing, and sieving to obtain composite modified carboxymethyl base starch.

Example 4

(1) mix 30g of carboxymethyl potato starch with a degree of substitution of 0.7 and 800mL of water to obtain starch paste;

(2) mixing 60g acrylic acid with 60g mass concentration of 20% sodium hydroxide solution, stirring to uniform mixing, adding the solution to the starch paste liquid, stirring uniformly;

(3) under stirring conditions, dropwise add the sodium hydroxide solution that the mass concentration is 15% in the starch paste liquid that adds the monomer, adjust the pH=7.5 of the reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) Mix 0.2g potassium persulfate with 2.5g water, stir until completely dissolved, add the solution to the starch paste with adjusted pH and mix well;

(6) 50 ℃ heat preservation reaction 5h, obtain reaction product;

(7) after the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution for sedimentation, filtration, 90% ethanol aqueous solution washing, drying at 55°C, pulverizing, and sieving to obtain composite modified carboxymethyl base starch.

Example 5

(1) 70g of carboxymethyl waxy cornstarch with a degree of substitution of 1.0 and 600mL of water were mixed to obtain starch paste;

(2) mix 140g acrylamide with 280g water, stir until completely dissolved, add the solution to the starch paste, and stir evenly;

(3) under stirring condition, dropwise add the sodium hydroxide solution that mass concentration is 15% in the starch paste liquid that adds monomer, adjust the pH=8.5 of reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) mix 0.5g potassium persulfate with 8g water, stir to dissolve completely, add the solution to the starch paste with adjusted pH and mix well;

(6) 70 ℃ heat preservation reaction 4h, obtain reaction product;

(7) after the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution for sedimentation, filtration, 90% ethanol aqueous solution washing, drying at 55°C, pulverizing, and sieving to obtain composite modified carboxymethyl base starch.

The original carboxymethyl starch and the prepared compound modified carboxymethyl starch were made into a paste with a mass fraction of 3%. After the gelatinization was complete, the viscosity was measured using a rotational viscometer, and the results were 1056mPa·s and 25050mPa·s respectively. ; The viscosity increase rate is about 24 times; the transparency of the two paste liquids tested by UV spectrophotometer is 16.30% and 21.70%, and the increase rate is about 1.33 times; 10mL of the two paste liquids are taken and centrifuged at 2500r/min. After 1 h, the volume of the precipitated water was 6.6 mL and 3.6 mL, respectively, and the water retention rate increased by about 1.80 times.

Example 6

(1) mixing 60g of carboxymethyl tapioca starch with a degree of substitution of 0.5 and 700mL of water to obtain starch paste;

(2) mix 75g acrylamide with 120g water, stir until completely dissolved, add the solution to the starch paste, and stir evenly;

(3) under stirring conditions, dropwise add the sodium hydroxide solution that the mass concentration is 10% in the starch paste liquid that adds the monomer, adjust the pH=7.5 of the reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) 0.36g potassium persulfate was mixed with 6g water, stirred until completely dissolved, the solution was added to the starch paste with adjusted pH value and mixed well;

(6) 80 ℃ heat preservation reaction 3h, obtain reaction product;

(7) after the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution for sedimentation, filtration, 90% ethanol aqueous solution washing, drying at 55°C, pulverizing, and sieving to obtain composite modified carboxymethyl base starch.

Example 7

(1) mixing 10 g of carboxymethyl rice starch with a degree of substitution of 1.8 and 200 mL of water to obtain a starch paste;

(2) mix 15g acrylamide with 45g water, stir until completely dissolved, add the solution to the starch paste, and stir evenly;

(3) under stirring condition, slowly add the sodium hydroxide solution that mass concentration is 25% in the starch system that adds monomer, adjust the pH=9 of reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) mix 0.6g potassium persulfate with 5.4g water, stir until completely dissolved, add the solution to the starch paste with adjusted pH and mix well;

(6) 50 ℃ heat preservation reaction 7h, obtain reaction product;

(7) After the reaction, adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution to settle, filter, wash with 90% ethanol aqueous solution, dry at 55°C, pulverize and sieve to obtain composite modified carboxymethyl starch.

Example 8

(1) 100 g of carboxymethyl corn starch with a degree of substitution of 0.8 and 800 mL of water were mixed to obtain starch paste;

(2) mix 100g of acrylamide with 100g of water, stir until completely dissolved, add the solution to the starch paste, and stir evenly;

(3) under stirring condition, drip the sodium hydroxide solution that mass concentration is 20% in the starch paste liquid that adds monomer, adjust the pH=8 of reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) mix 0.6g potassium persulfate with 6g water, stir until completely dissolved, add the solution to the starch paste with adjusted pH value and mix well;

(6) 60 ℃ heat preservation reaction 4h, obtain reaction product;

(7) after the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution for sedimentation, filtration, 90% ethanol aqueous solution washing, drying at 55°C, pulverizing, and sieving to obtain composite modified carboxymethyl base starch.

The original carboxymethyl starch and the prepared compound modified carboxymethyl starch were made into a paste with a mass fraction of 3%. After the gelatinization was complete, the viscosity was measured using a rotational viscometer, and the results were 1162 mPa s and 18700 mPa s respectively. , the viscosity increase rate is about 16 times; the transparency of the two paste liquids are 18.73% and 21.3% respectively by using UV spectrophotometer, and the increase rate is about 1.13 times; 10mL of the two paste liquids are centrifuged at 2500r/min. After 1 h, the volume of precipitated water was 6.3 mL and 2.8 mL, respectively, and the water retention rate increased by about 1.88 times.

Example 9

(1) mixing 80g of carboxymethyl rice starch with a degree of substitution of 0.4 and 400mL of water to obtain starch paste;

(2) mix 60g acrylamide with 90g water, stir until completely dissolved, add the solution to the starch paste, and stir evenly;

(3) under stirring condition, dropwise add the sodium hydroxide solution that mass concentration is 10% in the starch paste liquid that adds monomer, adjust the pH=8.5 of reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) 0.48g potassium persulfate was mixed with 6g water, stirred until completely dissolved, the solution was added to the starch paste with adjusted pH value and mixed well;

(6) 50 ℃ heat preservation reaction 5h, obtain reaction product;

(7) After the reaction, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution to settle, filter, wash with 90% ethanol aqueous solution, dry at 55°C, pulverize, and sieve to finally obtain compound modified carboxylate methyl starch.

The original carboxymethyl starch and the prepared compound modified carboxymethyl starch were made into a paste with a mass fraction of 3%. After the gelatinization was complete, the viscosity was tested by a rotational viscometer, and the results were 2975mPa·s and 21100mPa·s respectively. , the viscosity increase rate is about 7 times; the transparency of the two pastes is 15.10% and 17.40% respectively by using a UV spectrophotometer, and the increase rate is about 1.15 times; 10mL of the two pastes were centrifuged at 2500r/min. After 1 h, the volumes of precipitated water were 6.1 mL and 2.3 mL, respectively, and the water retention rate increased by about 1.97 times.

Example 10

(1) 50g of carboxymethyl waxy cornstarch with a degree of substitution of 2.0 and 500mL of water were mixed to obtain starch paste;

(2) mix 80g acrylamide with 80g water, stir until completely dissolved, add the solution to the starch paste, and stir evenly;

(3) under stirring condition, dropwise add the sodium hydroxide solution that mass concentration is 15% in the starch paste liquid that adds monomer, adjust the pH=9.5 of reaction system;

(4) Slowly fill with nitrogen for 30min;

(5) mix 0.4g potassium persulfate with 4g water, stir to dissolve completely, add the solution to the starch paste with adjusted pH and mix well;

(6) 70 ℃ heat preservation reaction 3h, obtain reaction product;

(7) After the reaction, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use 95% ethanol aqueous solution for sedimentation, filtration, 90% ethanol aqueous solution washing, drying at 55°C, pulverizing, and sieving to obtain composite modified carboxymethyl base starch.

Example 11

(1) 30g of carboxymethyl cornstarch with a degree of substitution of 0.2 and 500mL of water were mixed to obtain starch paste;

(2) 30g of acrylamide is mixed with 30g of water, stirred until completely dissolved, the solution is added to the starch paste liquid, and stirred evenly; then dropwise add a sodium hydroxide solution with a mass concentration of 10% to adjust the pH=7.5 of the reaction system;

(3) Slowly fill with nitrogen for 30min;

(4) mix 0.18g potassium persulfate with 3.6g water, stir until completely dissolved, add the initiator solution to the starch paste with adjusted pH and mix well;

(5) 50 ℃ of heat preservation reaction 5h, obtain reaction product;

(6) after the reaction is completed, add sulfuric acid solution dropwise to adjust the pH of the reaction product to neutrality, use volume fraction of 95% ethanol aqueous solution for sedimentation, filtration, volume fraction for 90% ethanol aqueous solution washing, drying at 55 ° C, pulverization, sieving, The compound modified carboxymethyl starch was obtained.

The original carboxymethyl starch and the prepared compound modified carboxymethyl starch were made into a paste with a mass fraction of 3%. After the gelatinization was complete, the viscosity was measured by a rotational viscometer to 4870mPa·s and 26000mPa·s, respectively. The magnification is about 5 times; the transparency of the two pastes is 14.21% and 62.73% respectively by using a UV spectrophotometer, and the increase rate is about 4.4 times; 10mL of the two pastes were taken and centrifuged at 2500r/min for 1h. , the precipitation water was 6.2mL and 2.6mL respectively, and the water retention rate increased by about 1.94 times.

Performance test: The effect of substitution degree on viscosity:

Preparation of compound modified carboxymethyl starch:

Mix 40 g of carboxymethyl starch with different degrees of substitution (the degrees of substitution are 0.2, 0.4, 0.6, 0.8, 1.0) and 600 g of distilled water evenly (warm up to 55°C and fully gelatinize for 50 min) to obtain starch paste; weigh acrylamide The solid (mass ratio of acrylamide solid and carboxymethyl starch is 1:1) is mixed with distilled water (mass ratio of acrylamide and distilled water is 1:1), stirred until it is completely dissolved, and the acrylamide solution is added to the starch paste, stirred 10min, adjusted to the pH=7.5 of the system, then filled with nitrogen, time 30min; Weigh potassium persulfate solid (potassium persulfate and acrylamide mass ratio 0.006:1), add distilled water (the mass ratio of potassium persulfate and distilled water is 0.06:1), stir until the dissolution is complete, add potassium persulfate solution to the starch paste containing monomers, keep stirring at 55 ° C, take out after 4 hours of reaction, and use 95% ethanol aqueous solution for precipitation to obtain flocculent precipitates , filtered, washed with a volume fraction of 90% ethanol aqueous solution, and dried at 55° C. for 24 hours to obtain carboxymethyl starch acrylamide graft copolymer (ie, composite modified starch).

The composite modified carboxymethyl starch prepared above and the carboxymethyl starch with different degrees of substitution were respectively made into 3% paste and their viscosity was tested. The test results are shown in Figure 1. Fig. 1 is the viscosity curve of compound modified carboxymethyl starch prepared by the present invention and unmodified carboxymethyl starch (original carboxymethyl starch) under different degrees of substitution; wherein curve (1) is compound modified carboxymethyl starch Base starch, curve (2) is the original carboxymethyl starch.

The viscosity of the composite modified carboxymethyl starch of the present invention is close to or even exceeds 20000 mPa·s, which is more than 5-24 times that of the original carboxymethyl starch with different degrees of substitution. In contrast, the viscosity of unmodified carboxymethyl starch gradually decreased with the increase of the degree of substitution, as shown by curve (2) in Figure 1. The comparison of the viscosity of the carboxymethyl starch and the composite modified carboxymethyl starch shows that the composite modified carboxymethyl starch prepared by the method of the present invention not only greatly increases the viscosity, but also does not significantly decrease with the increase of the degree of substitution. These advantages will help to expand the application of high degree of substitution carboxymethyl starch in different fields and increase the value of carboxymethyl starch.

Claims (6)

1. A method for improving and maintaining the viscosity of carboxymethyl starch with high degree of substitution is characterized in that: the method comprises the following steps:

1) mixing carboxymethyl starch with water to obtain starch paste;

2) uniformly mixing a monomer with the starch paste in a solution form, adjusting the pH of a system to 7.5-9.5, then adding an initiator in an inert atmosphere, reacting at 50-80 ℃ for 3-8 h, adjusting the pH of a reaction product to be neutral, and performing subsequent treatment to obtain the composite modified carboxymethyl starch;

the monomer in the step 2) is one of acrylamide or acrylic acid;

the mass ratio of the monomer to the carboxymethyl starch in the starch paste in the step 2) is 0.5: 1-2: 1;

the carboxymethyl substitution degree in the carboxymethyl starch in the step 1) is 0.1-2;

the initiator in the step 2) is potassium persulfate; the using amount of the initiator in the step 2) is 0.6-6% of the mass of the carboxymethyl starch;

the monomer in the step 2) is one of acrylamide or acrylic acid; when the monomer is acrylamide, the solution is acrylamide aqueous solution; when the monomer is acrylic acid, the solution is sodium hydroxide solution of acrylic acid; the mass ratio of acrylamide to water in the acrylamide aqueous solution is =1: 1-1: 3; the mass ratio of acrylic acid to the sodium hydroxide solution in the sodium hydroxide solution of acrylic acid is =1: 1-1: 4; the mass concentration of the sodium hydroxide solution is 20-40%.

2. The method for increasing and maintaining the viscosity of carboxymethyl starch with high degree of substitution according to claim 1, characterized in that: the mass volume ratio of the carboxymethyl starch to the water in the step 1) is (10-100) g: (200-800) mL.

3. The method for increasing and maintaining the viscosity of carboxymethyl starch with high degree of substitution according to claim 1, characterized in that: the initiator is added in the step 2) in the form of aqueous solution; the pH value of the adjusting system in the step 2) is 7.5-9.5, which means that a sodium hydroxide solution is adopted for adjusting.

4. The method for increasing and maintaining the viscosity of carboxymethyl starch with high degree of substitution according to claim 3, characterized in that: in the step 2), the initiator is added in the form of an aqueous solution, and the mass ratio of the initiator to water in the aqueous solution is (5-12): 100, respectively;

the pH value of the adjusting system in the step 2) is 7.5-9.5, which means that a sodium hydroxide solution is adopted for adjusting; the mass concentration of the sodium hydroxide solution is 10-30%.

5. The method for increasing and maintaining the viscosity of carboxymethyl starch with high degree of substitution according to claim 1, characterized in that: adjusting the pH of the reaction product to be neutral in the step 2) refers to adjusting by using a sulfuric acid solution;

the subsequent treatment in the step 2) is settling by adopting an ethanol water solution with the volume fraction of 95%, filtering, washing by adopting an ethanol water solution with the volume fraction of 90%, and drying.

6. The composite modified carboxymethyl starch is characterized in that: obtained by the process according to any one of claims 1 to 5.

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