A method for preparing cross-linked starch nanoparticles

Abstract

A method for preparing cross-linked starch nanoparticles is disclosed. The method comprises four steps: (1) starch dissolution and pretreatment, (2) antisolvent granulation, (3) freeze-drying, and (4) cross-linking heat treatment. This invention addresses the practical problems of applying starch to biodegradable materials by providing an industrially feasible process for preparing cross-linked starch nanoparticles. The size effect of the nanoparticles compensates for the shortcomings in the mechanical properties of starch, and appropriate chemical cross-linking endows the nanoparticles with insoluble and infusible properties and good water resistance, thus laying the foundation for manufacturing starch-based biodegradable materials with superior performance.

Description

Technical Field

[0001] This invention relates to a method for preparing cross-linked nano-starch particles. Background Technology

[0002] Starch is the most abundant natural polymer besides cellulose. It is a major component of grains and an important chemical raw material, with wide applications in food, medicine, cosmetics, adhesives, printing and dyeing, and biodegradable materials. In 2021, the Tianjin Institute of Industrial Biotechnology, Chinese Academy of Sciences, achieved a major breakthrough in the artificial synthesis of starch, realizing the world's first de novo synthesis of starch from carbon dioxide in the laboratory. This breakthrough further alleviates the severe constraint of starch's use as an industrial raw material on food supply, paving the way for the large-scale application of starch in industry, especially in the materials field. Introducing starch as a filler into biodegradable resin materials can fully utilize starch's characteristics as a nutrient source to promote biodegradation, while also significantly reducing the cost of biodegradable resins. Therefore, starch-based biodegradable resins have become an important type of biodegradable material with the best market prospects. However, starch has insufficient water resistance, poor interfacial compatibility with resins due to its hydrophilic and lipophilic properties, and poor mechanical properties, failing to provide reinforcement when used as a filler. These shortcomings result in serious performance limitations of starch-based biodegradable materials, thus restricting their performance and application areas. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides an industrially scalable process for preparing cross-linked starch nanoparticles. By leveraging the size effect of nanoparticles, the invention compensates for the deficiencies in the mechanical properties of starch. Furthermore, appropriate chemical cross-linking imparts insoluble and infusible properties and good water resistance to the starch nanoparticles, thus laying the foundation for the manufacture of starch-based biodegradable materials with superior performance.

[0004] The present invention discloses a method for preparing cross-linked starch nanoparticles, the method comprising the following steps:

[0005] Step 1: Dissolving and Pretreatment of Starch

[0006] (1) Dissolve 0.2 to 2 parts by weight of phosphorus pentoxide in 500 to 2000 parts by weight of distilled water, place it in a reaction vessel, add 100 parts by weight of starch, heat to boiling while stirring, and reflux for 45 to 180 min.

[0007] (2) Dissolve 0.1 to 0.5 parts by weight of itaconic acid in water, add hydroquinone and mix well to use as a polymerization inhibitor;

[0008] (3) Inject the polymerization inhibitor into the reactor of step (1) and maintain the system boiling and reflux for 15-30 minutes; take a sample and test the pH value. After the pH test paper turns red, stop stirring and lower the temperature of the system to 90-100℃ to obtain starch hydrosol.

[0009] Step 2: Antisolvent granulation

[0010] Under conditions of -50 to -30°C, 5000 to 10000 rpm, and ultrasonic assistance, 3000 to 5000 parts by weight of ethanol were used as the cooling zone, and 500 to 2000 parts by weight of starch hydrosol from step one were injected into the cooling zone in batches to obtain a mixture containing nano starch particles.

[0011] Step 3: Freeze-drying

[0012] The mixture containing nano starch particles was centrifuged to collect the solid phase. The solid phase was added to distilled water at a mass ratio of 1:1 to 5, ultrasonically dispersed, and then rapidly pre-frozen in a low temperature environment of -50 to -30℃. The solid phase was crushed, spread out, and placed in a freeze dryer. It was then freeze-dried at -60℃ and a pressure of less than 3Pa for 24 to 48 hours to obtain dried nano starch particles.

[0013] Step 4: Crosslinking heat treatment

[0014] The nano-starch particles obtained by freeze-drying in step three are placed in a vacuum heating chamber and heated for 1 to 3 hours at a temperature of 130 to 150°C and a pressure of less than 3 Pa to obtain the cross-linked nano-starch particles.

[0015] Furthermore, the starch is corn starch, potato starch, or tapioca starch.

[0016] Further, in step (2), hydroquinone is added, and the mass fraction of hydroquinone after addition is 0.05-0.5%.

[0017] Further, the mass fraction of itaconic acid in step (2) is 0.05% to 0.5%.

[0018] Furthermore, in step two, the power density of the ultrasound-assisted treatment is 0.05–0.5 W / cm². 2 .

[0019] Furthermore, the volume fraction of ethanol in step two is 70%–90%.

[0020] Furthermore, the centrifugation conditions in step three are: centrifugation at 2000–8000g for 3–10 minutes.

[0021] Furthermore, the liquid phase processed by centrifugation in step two is dehydrated using a 4A molecular sieve.

[0022] Furthermore, in step two, under conditions of -50 to -30°C, 5000 to 10000 rpm, and ultrasonic assistance, 4000 to 5000 parts by weight of ethanol is used as the cooling zone, and 800 to 1500 parts by weight of the starch hydrosol from step one is injected in batches into the cooling zone to obtain a mixture containing nano-starch particles.

[0023] Further, in step three, the mixture containing nano-starch particles is centrifuged to collect the solid phase. The solid phase is then added to distilled water at a mass ratio of 1:2 to 4, ultrasonically dispersed, and then rapidly pre-frozen in a low-temperature environment of -50 to -30°C. The solid phase is then crushed, spread out, and placed in a freeze dryer. It is then freeze-dried at -60°C and a pressure of less than 3 Pa for 24 to 48 hours to obtain dried nano-starch particles.

[0024] In this invention, during the dissolution and pretreatment of starch, samples are taken, and the pH value of the mixed product is detected using pH test paper. When the pH test paper directly in contact with the mixed solution turns red, while the test paper wetted by the liquid diffusion remains unchanged, it can be considered that all the acidic centers in the mixed system have been chemically bonded to the molecular chains of the starch sol, thus the starch sol is acidic, while the liquid phase diffusing from the sol is neutral. This phenomenon is used as an indication that the pretreatment is complete. After the pretreatment is completed, stirring is stopped, and the temperature of the system is lowered to 90-100°C. At this temperature, the original multilayer structure of natural starch is destroyed, and phosphoric acid (generated by dissolving P2O5 in water) and itaconic acid both form chemical bonds with the starch molecular chains through ester bonds. However, due to the presence of water and polymerization inhibitors, no further molecular chain cross-linking is formed.

[0025] In the antisolvent granulation process, the added starch hydrosol rapidly forms tiny ice crystals in the cooling zone. These ice crystals then quickly dissolve, generating relatively uniform nano-starch particles, which are then evenly dispersed in the system under the combined action of ultrasound and high-speed shearing. During this process, the low temperature and the dissolution of water in ethanol provide the driving force for starch crystallization. The low temperature of the cooling zone causes the injected aqueous phase to freeze rapidly, inhibiting the movement and migration of starch molecular chains. High-speed shearing ensures the fastest possible heat transfer (hydrosol cooling) and mass transfer (water and ethanol miscibility). Ultrasonic input prevents the generated nano-starch particles from agglomerating due to collisions. These four process conditions complement each other, thus promoting the formation of nano-starch particles.

[0026] In the freeze-drying process, during the re-dispersion of nano-starch particles with distilled water, a suitable amount of surfactant can be added to prevent nano-particle aggregation (not essential). The purpose of adding distilled water is to replace and dilute residual ethanol to meet the necessary conditions for freeze-drying. The separated liquid phase mainly consists of ethanol (more than 70% by mass), water (10-30% by mass), low molecular weight starch micelles, a small amount of oligosaccharides, the polymerization inhibitor hydroquinone (which still has good solubility in cold ethanol), and a small amount of impurities. After low-temperature filtration, the above liquid phase is dehydrated using a 4A molecular sieve and then recycled. The 4A molecular sieve, after absorbing water, can also be recycled after being dehydrated by heating to 380°C.

[0027] During the cross-linking heat treatment, the acidic molecular sheets formed by the cross-linking agents phosphoric acid and itaconic acid in the nano-starch particles through esterification continue to react with the hydroxyl groups on the starch molecular chains, resulting in dehydration and cross-linking to form ester bonds. Simultaneously, the double bonds in itaconic acid undergo thermal polymerization to form C-C bonds. Insufficiently cross-linked nano-starch particles still contain acidic molecular sheets, while fully cross-linked nano-starch particles are neutral and do not react with cold NaOH solution. Therefore, samples are taken during the cross-linking heat treatment process, swollen with cold NaOH solution, and titrated to determine whether the NaOH has been consumed, thus indicating whether the cross-linking reaction is complete. Phosphoric acid is chosen as the cross-linking agent because it is a moderately strong acid, providing sufficient acidity during pretreatment to promote starch hydrolysis, disrupt the original multi-level structure of starch, and facilitate the formation of starch hydrogels. Itaconic acid is chosen as the cross-linking agent because, in addition to containing two carboxyl groups that can react with hydroxyl groups, the double bonds in its molecular structure are located on one side of the molecule, resulting in less steric hindrance during thermal polymerization and thus superior cross-linking performance. Introducing a crosslinking agent system composed of phosphoric acid and itaconic acid into starch can effectively form chemical crosslinks of molecular chains through the two methods mentioned above, thereby improving the water resistance, wear resistance, and hardness of nano-starch particles. At the same time, it endows the crosslinked nano-starch particles with infusible and insoluble properties, which are more conducive to the application of nano-starch particles as reinforcements in the field of biodegradable materials.

[0028] This invention uses phosphoric acid and itaconic acid as crosslinking agents and starch hydrolysis catalysts, thus avoiding the generation of acidic waste gas and wastewater. The process does not use organic solvents other than ethanol; ethanol can be recycled, and both the antisolvent and liquid-phase recovery processes are carried out at low temperatures, avoiding ethanol volatilization loss. The polymerization inhibitor hydroquinone is used in extremely low amounts, making it relatively safe. It is separated from the product starch particles by dissolving in a large amount of ethanol, therefore not affecting the safety of the crosslinked nano-starch particles.

[0029] The present invention has the following beneficial effects:

[0030] This invention addresses the practical problems of applying starch in the field of biodegradable materials by providing an industrially scalable process for preparing cross-linked starch nanoparticles. The method compensates for the shortcomings of starch's mechanical properties by utilizing the size effect of nanoparticles, and imparts insoluble and infusible properties and good water resistance to the nanoparticles through appropriate chemical cross-linking, thereby laying the foundation for manufacturing starch-based biodegradable materials with superior performance. Attached Figure Description

[0031] Figure 1 Particle size analysis of nano starch particles;

[0032] Figure 2 Particle size analysis of nano starch particles;

[0033] Figure 3 Photographs and electron microscope images of nano starch particles. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the spirit of the contents disclosed in the present invention will be described in detail below. After understanding the embodiments of the present invention, any person skilled in the art can make changes and modifications based on the technology taught in the present invention without departing from the spirit and scope of the present invention.

[0035] The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0036] Example 1

[0037] The preparation method of cross-linked nano-starch particles in this embodiment is as follows:

[0038] Dissolve 0.5g of phosphorus pentoxide in 1000ml of distilled water, add 100g of corn starch, place in a reaction vessel, and heat to boiling while stirring at high speed. Reflux and continue the reaction for 90 minutes. Take samples for testing. When pH test paper in direct contact with the mixture turns red, while the paper moistened by the diffusion of the liquid remains unchanged, it indicates that all acidic centers in the mixture have been chemically bonded to the starch sol molecular chains. Stop stirring and lower the temperature of the system to 90℃. At this point, the original multilayer structure of natural starch is destroyed, and phosphate forms chemical bonds with the starch molecular chains through ester bonds, but no further molecular chain cross-linking occurs.

[0039] Using 2000 ml of ethanol at -30°C as the cooling zone, approximately 1100 ml of starch hydrosol was rapidly injected into the cooling zone in 10 portions under high-speed shear (5000 rpm) and ultrasonic input. Cooling was performed simultaneously with the injection to maintain the temperature of the cooling zone below -20°C. Throughout the process, the starch hydrosol was observed to rapidly form fine ice crystals in the cooling zone, which then dissolved.

[0040] A suspension containing nano-starch particles was placed in a large-capacity centrifuge and centrifuged at 5000g for 5 minutes to separate the solid and liquid phases. The separated liquid-containing solid phase was mixed with distilled water at a mass ratio of 1:2 and rapidly dispersed under ultrasonic input. The dispersion was poured into a flat pan and rapidly pre-frozen at -30℃. The pre-frozen solid phase was crushed, spread out, and placed in a freeze dryer for freeze-drying at -60℃ and a pressure less than 3Pa for 48 hours to obtain dried nano-starch particles. The liquid phase separated by centrifugation was filtered at low temperature and then dehydrated using a 4A molecular sieve. The water-absorbed 4A molecular sieve was then heated to 380℃ for further dehydration.

[0041] The freeze-dried nano-starch particles were placed in a vacuum heating chamber and heated for 2 hours at 135℃ and a pressure less than 3Pa. During this time, the acidic molecular sheets formed by the esterification reaction of the cross-linking agent phosphate in the nano-starch particles continued to react with the hydroxyl groups on the starch molecular chains, resulting in dehydration and cross-linking to form ester bonds. After heat treatment, samples were taken and swollen with 0.5 mol / L NaOH solution. The amount of NaOH consumed was determined by potentiometric titration. The results showed that the nano-starch particles prepared by this process were completely cross-linked and neutral. The particle size analysis of the prepared cross-linked nano-starch particles is as follows: Figure 1 As shown, its average particle size D50 is 333.1 nm.

[0042] Example 2

[0043] The preparation method of cross-linked nano-starch particles in this embodiment is as follows:

[0044] Dissolve 0.3g of phosphorus pentoxide in 1000ml of distilled water, add 100g of potato starch, place in a reaction vessel, and heat to boiling while stirring at high speed. Reflux and continue the reaction for 120min. Dissolve 0.2g of itaconic acid in an appropriate amount of warm water beforehand, and add a small amount of hydroquinone as a polymerization inhibitor. Quickly inject the itaconic acid aqueous solution into the reaction vessel, maintain the system at boiling and reflux, and continue the reaction for 30min. Take samples for testing. When the pH test paper directly in contact with the mixture turns red, while the test paper wetted by the liquid diffusion does not change color, it is determined that all the acidic centers in the mixture have been chemically bonded to the molecular chains of the starch sol. Stop stirring and lower the temperature of the system to 95℃. At this point, the original multilayer structure of natural starch is destroyed, and phosphoric acid and itaconic acid form chemical bonds with the starch molecular chains through ester bonds, but no further molecular chain cross-linking occurs.

[0045] Using 1500 ml of ethanol at -50°C as the cooling zone, approximately 1100 ml of starch hydrosol was rapidly injected into the cooling zone in 10 portions under high-speed shear (8000 rpm) and ultrasonic input. Cooling was performed simultaneously during injection to maintain the temperature of the cooling zone below -35°C. Throughout the process, the starch hydrosol was observed to rapidly form fine ice crystals within the cooling zone, which then dissolved.

[0046] A suspension containing nano-starch particles was placed in a large-capacity centrifuge and centrifuged at 8000g for 3 minutes to separate the solid and liquid phases. The separated liquid-containing solid phase was mixed with distilled water at a mass ratio of 1:1.5 and rapidly dispersed under ultrasonic input. The dispersion was poured into a flat pan and rapidly pre-frozen at -30℃. The pre-frozen solid phase was crushed, spread out, and placed in a freeze dryer for freeze-drying at -60℃ and a pressure less than 3Pa for 36 hours to obtain dried nano-starch particles. The liquid phase separated by centrifugation was filtered at low temperature and then dehydrated using a 4A molecular sieve. The water-absorbed 4A molecular sieve was then heated to 380℃ for further dehydration.

[0047] The freeze-dried nano-starch particles were placed in a vacuum heating chamber and heated for 1 hour at 150°C and a pressure less than 3 Pa. During this process, the acidic molecular sheets formed by the esterification reaction of the cross-linking agent phosphate in the nano-starch particles continued to react with the hydroxyl groups on the starch molecular chains, resulting in dehydration and cross-linking to form ester bonds. Simultaneously, the double bonds in itaconic acid underwent thermal polymerization to form C-C bonds. After heat treatment, samples were taken and swollen with 0.5 mol / L NaOH solution. The amount of NaOH consumed was determined by potentiometric titration. The results showed that the nano-starch particles prepared by this process achieved sufficient cross-linking, and the product was neutral. The particle size analysis of the cross-linked nano-starch particles prepared under the above process conditions is as follows. Figure 2 As shown, its average particle size D50 is 302.1 nm.

[0048] Example 3

[0049] The preparation method of cross-linked nano-starch particles in this embodiment is as follows:

[0050] Dissolve 1g of phosphorus pentoxide in 1000ml of distilled water, add 100g of tapioca starch, place in a reaction vessel, and heat to boiling while stirring at high speed. Reflux and continue the reaction for 60 minutes. Dissolve 0.5g of itaconic acid in an appropriate amount of warm water beforehand, and add a small amount of hydroquinone as a polymerization inhibitor. Quickly inject the itaconic acid aqueous solution into the reaction vessel, maintain the system at boiling and reflux, and continue the reaction for 30 minutes. Take samples for testing. When the pH test paper directly in contact with the mixture turns red, while the paper wetted by the liquid diffusion remains unchanged, it is determined that all acidic centers in the mixture have been chemically bonded to the starch sol molecular chains. Stop stirring and lower the system temperature to 90℃. At this point, the original multilayer structure of natural starch is destroyed, and phosphoric acid and itaconic acid form chemical bonds with the starch molecular chains through ester bonds, but no further molecular chain cross-linking occurs.

[0051] Using 2000 ml of ethanol at -50°C as the cooling zone, approximately 1100 ml of starch hydrosol was rapidly injected into the cooling zone in 15 portions under high-speed shear (10000 rpm) and ultrasonic input. Cooling was performed simultaneously during injection to maintain the temperature of the cooling zone below -40°C. Throughout the process, the starch hydrosol was observed to rapidly form fine ice crystals within the cooling zone, which then dissolved.

[0052] A suspension containing nano-starch particles was placed in a large-capacity centrifuge and centrifuged at 3000g for 10 minutes to separate the solid and liquid phases. The separated liquid-containing solid phase was mixed with distilled water at a mass ratio of 1:3, along with a small amount of Tween 60, and thoroughly dispersed under ultrasonic input. The dispersion was poured into a flat dish and pre-frozen at -50℃. The pre-frozen solid phase was crushed, spread out, and placed in a freeze dryer for freeze-drying at -60℃ and a pressure less than 3Pa for 48 hours to obtain dried nano-starch particles. The centrifuged liquid phase was filtered at low temperature and then dehydrated using a 4A molecular sieve. The water-absorbed 4A molecular sieve was then heated to 380℃ for further dehydration.

[0053] The freeze-dried nano-starch particles were placed in a vacuum heating chamber and heated for 1 hour at 150°C and a pressure less than 3 Pa. During this process, the acidic molecular sheets formed by the esterification reaction of the cross-linking agent phosphate in the nano-starch particles continued to react with the hydroxyl groups on the starch molecular chains, resulting in dehydration and cross-linking to form ester bonds. Simultaneously, the double bonds in itaconic acid underwent thermal polymerization to form C-C bonds. After heat treatment, samples were taken and swollen with 0.5 mol / L NaOH solution. The amount of NaOH consumed was determined by potentiometric titration. The results showed that the nano-starch particles prepared by this process achieved sufficient cross-linking, and the product was neutral. The product was insoluble in boiling water, and the water absorption swelling rate was less than 17%. The particle size analysis report of the cross-linked nano-starch particles prepared under the above process conditions is as follows. Figure 3 As shown, its average particle size D50 is 280 nm. With an average sphericity of 0.838, the above-mentioned cross-linked nano-starch particles are nearly spherical short-diameter particles.

Claims

1. A method for preparing cross-linked starch nanoparticles, characterized in that... The method is performed according to the following steps: Step 1: Starch Dissolution and Pretreatment (1) Dissolve 0.2 to 2 parts by weight of phosphorus pentoxide in 500 to 2000 parts by weight of distilled water, place it in a reaction vessel, add 100 parts by weight of starch, heat to boiling while stirring, and reflux for 45 to 180 min. (2) Dissolve 0.1 to 0.5 parts by weight of itaconic acid in water, add hydroquinone and mix well to use as a polymerization inhibitor; (3) Inject the polymerization inhibitor into the reactor of step (1) and maintain the system boiling and reflux for 15-30 minutes; take a sample and test the pH value. After the pH test paper turns red, stop stirring and lower the temperature of the system to 90-100℃, but not 100℃, to obtain starch hydrosol. Step 2: Antisolvent granulation Between -50 and -30 o Under the conditions of temperature C, 5000~10000 rpm, and ultrasonic assistance, 3000~5000 parts by weight of ethanol was used as the cooling zone, and 500~2000 parts by weight of starch hydrosol from step one was injected into the cooling zone in batches to obtain a mixture containing nano starch particles. Step 3: Freeze-drying The mixture containing nano-starch particles was centrifuged, and the solid phase was collected. Distilled water was added to the solid phase at a mass ratio of 1:1 to 5, and the mixture was ultrasonically dispersed. Then, the dispersion was carried out at -50 to -30°C. o Rapidly pre-freeze at a low temperature of C, crush, spread out, and place in a freeze dryer at -60°C. o C. Freeze-dry for 24-48 hours under a pressure of less than 3 Pa to obtain dried nano-starch particles; Step 4: Crosslinking heat treatment The nano-starch particles obtained from freeze-drying in step three were placed in a vacuum heating chamber at a temperature of 130-150°C. o C. Heating for 1-3 hours under a pressure of less than 3 Pa yields the cross-linked nano-starch particles.

2. The method for preparing cross-linked starch nanoparticles according to claim 1, characterized in that... The starch mentioned is corn starch, potato starch, or cassava starch.

3. The method for preparing cross-linked nano-starch particles according to claim 1, characterized in that... The addition of hydroquinone in step (2) results in a hydroquinone mass fraction of 0.05-0.5%.

4. The method for preparing cross-linked starch nanoparticles according to claim 1, characterized in that... The mass fraction of itaconic acid in step (2) is 0.05~0.5%.

5. The method for preparing cross-linked nano-starch particles according to claim 1, characterized in that... In step two, the power density of ultrasound-assisted treatment is 0.05~0.5 W / cm². 2 .

6. The method for preparing cross-linked nano-starch particles according to claim 1, characterized in that... The volume fraction of ethanol in step two is 70%~90%.

7. The method for preparing cross-linked nano-starch particles according to claim 1, characterized in that... Centrifugation conditions in step three: centrifuge at 2000~8000g for 3~10min.

8. The method for preparing cross-linked nano-starch particles according to claim 1, characterized in that... In step two, the liquid phase processed by centrifugation is dehydrated using a 4A molecular sieve.

9. The method for preparing cross-linked nano-starch particles according to claim 1, characterized in that... The step two mentioned is in the range of -50 to -30. o Under conditions of temperature C, 5000~10000 rpm, and ultrasonic assistance, 4000~5000 parts by weight of ethanol was used as the cooling zone, and 800~1500 parts by weight of starch hydrosol from step one was injected into the cooling zone in batches to obtain a mixture containing nano starch particles.

10. The method for preparing cross-linked nano-starch particles according to claim 1, characterized in that... In step three, the mixture containing nano-starch particles is centrifuged, the solid phase is collected, and distilled water is added to the solid phase at a mass ratio of 1:2~4. The mixture is then ultrasonically dispersed and then cooled to -50~-30°C. o Rapidly pre-freeze at a low temperature of C, crush, spread out, and place in a freeze dryer at -60°C. o C. Freeze-dry under a pressure of less than 3 Pa for 24-48 hours to obtain dried nano-starch particles.

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