Preparation method of hydroxypropyl modified starch

Abstract

The invention relates to the technical field of starch modification, and provides a preparation method of hydroxypropyl modified starch, which comprises the following steps: acidizing starch to obtain acidized starch milk, and adjusting the pH value of the acidized starch milk to 9-11 to obtain starch milk; the preparation method comprises the following steps: uniformly mixing starch milk, a catalyst and an epoxypropane solution, and stirring at 35-45 DEG C to obtain a reactant; performing alcohol precipitation, washing, filtering and drying on reactants to obtain hydroxypropyl modified starch; wherein the catalyst comprises tetraethylammonium hydroxide, sodium carbonate and tetrabutylammonium bromide. Through the technical scheme, the problem of low substitution degree of hydroxypropyl modified starch in related technologies is solved.

Description

Technical Field

[0001] This invention relates to the field of starch modification technology, specifically to a method for preparing hydroxypropyl modified starch. Background Technology

[0002] Starch, as a naturally abundant and inexpensive polymer, has been widely used in food processing, papermaking, and textile printing and dyeing due to its excellent biocompatibility, biodegradability, and basic thickening and binding functions. However, the strong hydrogen bonds between glucose units in natural starch molecules make it difficult to meet the performance requirements of complex applications. Hydroxypropyl modification, as a mainstream starch modification method, introduces hydroxypropyl groups into the hydroxyl groups of starch glucose units to break intermolecular hydrogen bonds. By controlling the degree of hydroxypropyl substitution (DS), a comprehensive upgrade of starch performance can be achieved.

[0003] Among them, highly substituted hydroxypropyl starch plays an irreplaceable role in a variety of fields due to its strong hydrophilicity and stable structure: in pharmaceuticals, it can be used as a matrix for sustained-release tablets or as a raw material for capsules to achieve long-acting drug release with good biocompatibility; in oil extraction, it is used as a drilling fluid filtration reducer, and its temperature and salt resistance helps protect oil and gas reservoirs; in high-end coatings, it can be adapted to water-based systems to improve the smoothness and scrub resistance of the coating film; it can also be used as a sizing agent for specialty paper to enhance paper performance, or as a thickening and stabilizing agent in cosmetics, etc. These needs all rely on the unique properties of highly substituted hydroxypropyl starch, which is difficult to replace with low-substituted hydroxypropyl starch.

[0004] However, existing industrial technologies mainly focus on low-substituted hydroxypropyl starch. Due to limitations in the preparation process, it is difficult to prepare high-substituted hydroxypropyl starch, which has long relied on imports and is expensive. This makes it difficult to meet the needs of self-sufficiency in high-end papermaking, pharmaceuticals, oil extraction and other fields. Therefore, it is necessary to propose a method for preparing hydroxypropyl modified starch with high substitution degree. Summary of the Invention

[0005] This invention proposes a method for preparing hydroxypropyl modified starch, which solves the problem of low substitution degree of hydroxypropyl modified starch in related technologies.

[0006] The technical solution of the present invention is as follows: This invention proposes a method for preparing hydroxypropyl-modified starch, comprising the following steps: S1. After acidifying the starch, acidified starch milk is obtained. Then, the pH of the acidified starch milk is adjusted to 9-11 to obtain starch milk. S2. Mix starch milk, catalyst and propylene oxide solution evenly and stir at 35~45℃ to obtain reactants; S3. The reactants were subjected to alcohol precipitation, washing, filtration, drying, and pulverization to obtain hydroxypropyl modified starch; The catalyst comprises tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide.

[0007] As a further technical solution, the mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide in the catalyst is 4~6:9:1, preferably 5:9:1.

[0008] In the preparation method of hydroxypropyl modified starch of this invention, when the mass ratio of tetraethylammonium hydroxide in the catalyst is greater than 6:9:1, the strong alkalinity of tetraethylammonium hydroxide will significantly increase the pH of the system, accelerate the breaking of glycosidic bonds in the starch molecular chain, leading to degradation and a decrease in molecular weight. Simultaneously, it exacerbates the hydrolysis side reaction of propylene oxide, generating a large number of impurities and reducing etherification selectivity and the degree of substitution of the target product. When the mass ratio of tetraethylammonium hydroxide in the catalyst is less than 4:9:1, it cannot provide sufficient activation kinetics for the starch hydroxyl groups, resulting in insufficient active sites for the etherification reaction, a significant decrease in reaction efficiency, and a low degree of substitution in the hydroxypropyl modified starch. When the mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide in the catalyst is 4~6:9:1, hydroxypropyl modified starch with a higher degree of substitution can be obtained.

[0009] As a further technical solution, the total mass of the catalyst is 0.5% to 1.5% of the starch mass, preferably 0.9%.

[0010] As a further technical solution, in step S2, the mixing involves first mixing starch milk, tetraethylammonium hydroxide and sodium carbonate for 20-40 minutes, then uniformly mixing with tetrabutylammonium bromide for 8-12 minutes, and finally adding propylene oxide solution for mixing.

[0011] In this invention, a method for preparing hydroxypropyl modified starch involves first mixing starch slurry with tetraethylammonium hydroxide and sodium carbonate for 20-40 minutes, then adding tetrabutylammonium bromide and mixing for 8-12 minutes before adding propylene oxide. This method allows the two alkaline catalysts, tetraethylammonium hydroxide and sodium carbonate, to fully interact and gradually establish a stable alkaline environment. This avoids excessive local alkalinity that could lead to starch gelatinization or degradation, while also allowing the starch particles to swell appropriately and activate the hydroxyl groups. Subsequently, the phase transfer catalyst tetrabutylammonium bromide is added and uniformly dispersed within 8-12 minutes. This improves the contact probability and reaction efficiency between propylene oxide and the activated hydroxyl groups, thereby significantly increasing the degree of hydroxypropyl substitution in the product.

[0012] As a further technical solution, the volume-to-mass ratio of the propylene oxide solution to starch is 1~3mL:1g.

[0013] As a further technical solution, the preparation method of propylene oxide solution includes the following steps: dissolving propylene oxide in a solvent to obtain propylene oxide solution.

[0014] As a further technical solution, the solvent includes one of methanol, ethanol, alkyl ketone, and isopropanol, preferably isopropanol.

[0015] As a further technical solution, the volume ratio of propylene oxide to solvent is 1:15~20.

[0016] As a further technical solution, the starch includes at least one of cassava starch, potato starch, corn starch, sweet potato starch, and wheat starch.

[0017] As a further technical solution, in step S1, the acidification treatment is to add a 10% acetic acid aqueous solution to the starch, wherein the mass of the acetic acid aqueous solution is 15-40% of the mass of the starch, and the acidification time is 1.5-4 hours.

[0018] The working principle and beneficial effects of this invention are as follows: This invention uses tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide as catalysts to enhance the degree of substitution of hydroxypropyl-modified starch through their synergistic effect. Tetraethylammonium hydroxide can rapidly abstract hydrogen protons from starch hydroxyl groups to generate highly reactive oxygen anions, providing sufficient active sites for the etherification reaction; however, its strong alkalinity easily triggers starch chain degradation and propylene oxide hydrolysis. At this point, sodium carbonate, through CO32-... 2- / HCO3 - Ionization equilibrium acts as a buffer, stabilizing the system pH within the weakly alkaline range. This prevents strong bases from damaging starch molecules while maintaining the active state of hydroxyl anions. Based on this, tetrabutylammonium bromide, acting as a phase-transfer catalyst, can transfer active hydroxyl anions from the aqueous phase to the organic phase containing propylene oxide via ion-pair interactions. This solves the problem of poor miscibility between starch slurry and propylene oxide solution, significantly improving the collision reaction efficiency between propylene oxide and active hydroxyl groups. The synergistic effect of these three factors ultimately achieves the efficient preparation of highly substituted hydroxypropyl modified starch. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 A method for preparing hydroxypropyl-modified starch includes the following steps: S1. Place 100g of corn starch in a reactor, add 15mL of 10% acetic acid aqueous solution, acidify for 4h to obtain acidified starch milk, then add sodium hydroxide to adjust the pH to 10 to obtain starch milk. S2. Mix starch milk and tetrabutylammonium bromide evenly for 10 min, then add tetraethylammonium hydroxide and sodium carbonate, mix and stir for 30 min, then add 200 mL of propylene oxide solution, and stir at 40 °C for 2 h to obtain the reactant. S3. After the reactants are cooled to room temperature, 1500 mL of ethanol is added. After standing and separating into layers, the mixture is washed. This process is repeated 3 times. After filtration and drying, hydroxypropyl modified starch is obtained. The mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide is 4:9:1, and the sum of the masses of tetrabutylammonium bromide, tetraethylammonium hydroxide, and sodium carbonate is 0.5% of the mass of cassava starch. The preparation method of propylene oxide solution includes the following steps: dissolving 11 mL of propylene oxide in 187 mL of methanol to obtain propylene oxide solution.

[0021] Example 2 A method for preparing hydroxypropyl-modified starch includes the following steps: S1. Place 100g of cassava starch in a reactor, add 30mL of 10% acetic acid aqueous solution, acidify for 3h to obtain acidified starch milk, then add sodium hydroxide to adjust the pH to 10 to obtain starch milk. S2. Mix starch milk and tetrabutylammonium bromide evenly for 10 min, then add tetraethylammonium hydroxide and sodium carbonate, mix and stir for 30 min, then add 200 mL of propylene oxide solution, and stir at 40 °C for 2 h to obtain the reactant. S3. After the reactants are cooled to room temperature, 1500 mL of ethanol is added. After standing and separating into layers, the mixture is washed. This process is repeated 3 times. After filtration and drying, hydroxypropyl modified starch is obtained. The mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide is 4:9:1, and the sum of the masses of tetrabutylammonium bromide, tetraethylammonium hydroxide, and sodium carbonate is 0.9% of the mass of cassava starch. The preparation method of propylene oxide solution includes the following steps: dissolving 11 mL of propylene oxide in 187 mL of isopropanol to obtain propylene oxide solution.

[0022] Example 3 A method for preparing hydroxypropyl-modified starch includes the following steps: S1. Place 100g of wheat starch in a reactor, add 40mL of 10% acetic acid aqueous solution, acidify for 1.5h to obtain acidified starch milk, then add sodium hydroxide to adjust the pH to 10 to obtain starch milk. S2. Mix starch milk and tetrabutylammonium bromide evenly for 10 min, then add tetraethylammonium hydroxide and sodium carbonate, mix and stir for 30 min, then add 200 mL of propylene oxide solution, and stir at 40 °C for 2 h to obtain the reactant. S3. After the reactants are cooled to room temperature, 1500 mL of ethanol is added. After standing and separating into layers, the mixture is washed. This process is repeated 3 times. After filtration and drying, hydroxypropyl modified starch is obtained. The mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide is 4:9:1, and the sum of the masses of tetrabutylammonium bromide, tetraethylammonium hydroxide, and sodium carbonate is 1.5% of the mass of cassava starch. The preparation method of propylene oxide solution includes the following steps: dissolving 11 mL of propylene oxide in 187 mL of ethanol to obtain propylene oxide solution.

[0023] Example 4 Compared with Example 2, the only difference in Example 4 is that the mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide in this example is 5:9:1.

[0024] Example 5 Compared with Example 2, the only difference in Example 5 is that the mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide in this example is 6:9:1.

[0025] Example 6 Compared with Example 2, the only difference in Example 6 is that in this example, step S2 is as follows: starch milk is mixed evenly with tetrabutylammonium bromide, tetraethylammonium hydroxide and sodium carbonate, stirred for 30 min, and then 198 mL of propylene oxide solution is added, and stirred at 40 °C for 2 h to obtain the reactant.

[0026] Comparative Example 1 Compared with Example 6, the only difference in Comparative Example 1 is that tetraethylammonium hydroxide is replaced with an equal amount of sodium hydroxide in the catalyst of this comparative example.

[0027] Comparative Example 2 Compared with Example 6, the only difference in Comparative Example 2 is that the catalyst in this comparative example is composed of tetraethylammonium hydroxide and sodium carbonate in a mass ratio of 4:9.

[0028] Comparative Example 3 Compared with Example 6, the only difference in Comparative Example 3 is that the catalyst in this comparative example is composed of tetraethylammonium hydroxide and tetrabutylammonium bromide in a mass ratio of 4:1.

[0029] Comparative Example 4 Compared with Example 6, the only difference in Comparative Example 4 is that the catalyst in this comparative example is only tetraethylammonium hydroxide.

[0030] The hydroxypropyl modified starches prepared in Examples 1-6 and Comparative Examples 1-4 were tested according to the following methods: 1. Test method for degree of substitution of hydroxypropyl modified starch: Refer to the experimental method specified in standard GB / T 20376-2006 "Determination of hydroxypropyl content in modified starch by proton nuclear magnetic resonance spectroscopy" to determine the hydroxypropyl group content in hydroxypropyl modified starch, denoted as W; calculate the degree of substitution (i.e., hydroxypropyl / glucosyl) according to the following formula: DS=162W / [(100-W)×58]; In the formula: W is the hydroxypropyl group content (%); 162 is the relative molecular mass of dehydrated glucose; 58 is the relative molecular mass of propylene oxide.

[0031] The test results are shown in Table 1: Table 1. Results of the degree of substitution test for hydroxypropyl modified starch.

[0032] As can be seen from the data in Table 1, compared with Comparative Examples 1-4, the hydroxypropyl modified starch prepared in Examples 1-6 has a higher hydroxypropyl content and degree of substitution, indicating that the addition of the catalyst composed of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide can significantly improve the degree of substitution of hydroxypropyl modified starch.

[0033] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing hydroxypropyl-modified starch, characterized in that, Includes the following steps: S1. After acidifying the starch, acidified starch milk is obtained. The pH of the acidified starch milk is then adjusted to 9-11 to obtain starch milk. S2. Mix starch milk, catalyst and propylene oxide solution evenly and stir at 35~45℃ to obtain reactants; S3. The reactants were subjected to alcohol precipitation, washing, filtration and drying to obtain hydroxypropyl modified starch; The catalyst comprises tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide.

2. The method for preparing hydroxypropyl modified starch according to claim 1, characterized in that, The mass ratio of tetraethylammonium hydroxide, sodium carbonate, and tetrabutylammonium bromide in the catalyst is 4~6:9:

1.

3. The method for preparing hydroxypropyl modified starch according to claim 1, characterized in that, The total mass of the catalyst is 0.5% to 1.5% of the starch mass.

4. The method for preparing hydroxypropyl modified starch according to claim 1, characterized in that, In step S2, the mixing process involves first mixing starch milk, tetraethylammonium hydroxide, and sodium carbonate for 20-40 minutes, then uniformly mixing with tetrabutylammonium bromide for 8-12 minutes, and finally adding propylene oxide solution for further mixing.

5. The method for preparing hydroxypropyl modified starch according to claim 1, characterized in that, The volume-to-mass ratio of the propylene oxide solution to starch is 1-3 mL: 1 g.

6. The method for preparing hydroxypropyl modified starch according to claim 1, characterized in that, The preparation method of propylene oxide solution includes the following steps: dissolving propylene oxide in a solvent to obtain propylene oxide solution.

7. The method for preparing hydroxypropyl modified starch according to claim 6, characterized in that, The solvent includes one of methanol, ethanol, alkyl ketone, and isopropanol.

8. The method for preparing hydroxypropyl modified starch according to claim 6, characterized in that, The volume ratio of propylene oxide to solvent is 1:15~20.

9. The method for preparing hydroxypropyl modified starch according to claim 1, characterized in that, The starch includes at least one of cassava starch, potato starch, corn starch, sweet potato starch, and wheat starch.

10. The method for preparing hydroxypropyl modified starch according to claim 1, characterized in that, In step S1, the acidification treatment involves adding a 10% (w / w) aqueous acetic acid solution to the starch, wherein the mass of the aqueous acetic acid solution is 15-40% of the starch mass, and the acidification time is 1.5-4 hours.

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