Thermoplastic starch having high ductility and anti-retrogradation and a method for preparing the same

Abstract

The application belongs to the technical field of high polymer materials, and provides a preparation method of thermoplastic starch with high ductility and anti-regeneration, comprising the following steps: wet ball milling starch, drying to obtain ball-milled starch; gelatinizing the ball-milled starch, adding esterifying agent and fatty acid lubricant, fully blending with gelatinized starch, heating to react, taking out and drying after the reaction is completed, removing unreacted esterifying agent, and drying again to obtain the thermoplastic starch; the esterifying agent is maleic anhydride or acetic anhydride; and the fatty acid lubricant is stearic acid. The thermoplastic starch has not only high ductility with an elongation at break of more than 1500%, but also excellent anti-regeneration characteristics with good ductility (more than 1000%) still maintained after 6 months.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a thermoplastic starch with high extensibility and resistance to retrogradation and its preparation method. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Petroleum-based plastics take hundreds of years to degrade naturally, and their huge demand exacerbates "white pollution," seriously affecting the ecological balance and endangering human health. Therefore, in recent years, a large number of bio-based materials with good degradation properties have been developed to replace petroleum-based plastics. Among them, starch has become a hot topic due to its diverse sources, abundant reserves, and high chemical activity. Furthermore, statistics show that huge amounts of grain are lost annually due to improper storage. As a major component of most grains, starch urgently needs further effective utilization. However, starch's naturally occurring polyhydroxyl and multi-branched structure gives it complex crystallization characteristics, further resulting in its brittleness, difficulty in melting, and tendency to revert to its original state, severely hindering its use as a plastic substitute.

[0004] Currently, there are two main methods for preparing thermoplastic starch: adding small-molecule plasticizers to increase the intermolecular spacing of starch, or replacing hydroxyl groups through esterification to weaken intermolecular hydrogen bonding, thereby enhancing the mobility of starch chains to achieve melting. Plasticized starch often has good extensibility; however, plasticizers are mostly polar small molecules that easily precipitate and accumulate between starch chains (CN1303154 All-starch type biodegradable plastics), leading to starch retrogradation. Esterified starch can only improve the thermoplasticity of starch but cannot achieve effective extensibility gains.

[0005] Stearic acid is widely found in plants and animals and is a good lubricant. It is often used in combination with plasticizers to enhance the properties of starch-based materials, and is not used alone (CN103819754B, A high-toughness starch-based thermoplastic and its preparation method). However, its effect on the extensibility of starch-based materials is currently unclear. Wu (Wu X, Chen Y, Lv X, et al. Effect of stearic acid and sodium stearate on cast cornstarch films[J]. Journal of Applied Polymer Science, 2012, 124(5): 3782-3791.) et al. pointed out its effect in their studies, while Rafael (Rafael C, Guadalupe MG, Eduardo HM, et al. Optimization of acetylated starch films from purple sweet potato: effect of glycerol, carboxymethylcellulose, and stearic acid[J]. Materials Research Express, 2021, 8(11): 115101.) et al. pointed out its destructive effect on extensibility.

[0006] Patent CN103435709A discloses a mechanically activated solid-phase oxidative esterification composite modified starch and its preparation method. The starch is first pretreated by drying, and then all materials (starch, oxidizing agent, esterifying agent (acetic anhydride, stearic acid, palmitic acid, citric acid, succinic anhydride, maleic anhydride, lauric acid, stearoyl chloride, or a mixture thereof), and additives) are placed in a ball mill, causing the mixture to undergo simultaneous oxidation and esterification reactions to obtain oxidative esterification composite modified starch. However, the esterifying agent is mixed under anhydrous conditions, resulting in a low degree of esterification substitution. Furthermore, the starch cannot form a complex with stearic acid; therefore, the starch's extensibility and resistance to retrogradation still need improvement. Summary of the Invention

[0007] To address the shortcomings of the existing technologies, this invention develops a thermoplastic starch with high extensibility and resistance to retrogradation, along with its preparation method. Under aqueous conditions, this invention simultaneously mixes an esterifying agent and a lubricant with starch, allowing the starch to undergo esterification while simultaneously forming a complex with the lubricant. The complexation reaction is completed concurrently with the starch reaching a fusible degree of esterification substitution. The resulting thermoplastic starch not only possesses high extensibility with an elongation at break exceeding 1500%, but also exhibits excellent resistance to retrogradation, maintaining good extensibility (exceeding 1000%) even after 6 months.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A first aspect of the present invention provides a method for preparing a thermoplastic starch with high extensibility and resistance to retrogradation, comprising:

[0010] Starch is wet-milled and then dried to obtain ball-milled starch;

[0011] The ball-milled starch is gelatinized, and then an esterifying agent, a fatty acid lubricant, and gelatinized starch with a water content of 40%-150% are added and thoroughly mixed. The mixture is heated to 75℃-95℃ for reaction. After the reaction is completed, the starch is removed, dried, and any unreacted esterifying agent is removed. The mixture is then dried again to obtain the final product.

[0012] Esterification and the chain-breaking hydrolysis of starch under the action of esterifying agents require the participation of water. However, excessive water will over-dilute the esterifying agent and reduce the esterification effect. The subsequent removal of water will also consume a lot of time and energy. Therefore, this application must control the moisture content (water relative to the weight of starch) between 40% and 150%.

[0013] Meanwhile, starch and fatty acid lubricants need to reach a molten state between 75℃ and 95℃ to form a good complex, thereby effectively improving the extensibility and anti-retrogradation properties of thermoplastic starch.

[0014] In some embodiments, the esterifying agent is maleic anhydride or acetic anhydride; alternatively, an esterifying agent with an acidity comparable to maleic anhydride and a melting point below 75°C may be used to ensure that the starch chain can be hydrolyzed and broken down during esterification.

[0015] In some embodiments, the fatty acid lubricant is stearic acid, or other fatty acids with similar physicochemical properties, such as palmitic acid.

[0016] In this invention, starch cannot be ball-milled in an aqueous environment as it will gelatinize and the starch granules cannot be broken down. Therefore, in some embodiments, the solvent for wet ball milling is anhydrous ethanol.

[0017] In some embodiments, starch and anhydrous ethanol are mixed in a mass ratio of 1:1 to 1:3.

[0018] In some embodiments, the ball milling conditions are: revolution speed 10-30 r / min, rotation speed 100-300 r / min, and milling time 20-60 min.

[0019] In some embodiments, the gelatinization conditions are gelatinization at 85°C to 90°C for 10-30 minutes.

[0020] In some embodiments, the mass ratio of ball-milled starch to water is 50-70:30-60.

[0021] In some embodiments, the mass ratio of gelatinized starch, esterifying agent, and lubricant is 60-90:7-25:3-15. More specifically, too little esterifying agent will not achieve the degree of esterification substitution necessary to melt the starch, while too much esterifying agent will hinder the complexation of starch and stearic acid. Therefore, the present invention must control the esterifying agent content (relative to the weight of gelatinized starch) between 15% and 60%.

[0022] Too little stearic acid results in insufficient complexation, while too much stearic acid remains free within the starch, both of which negatively impact the material's extensibility. Therefore, this application requires controlling the stearic acid (lubricant) content (relative to the weight of gelatinized starch) between 5% and 30%.

[0023] In some embodiments, the reaction is carried out under heating conditions of 75°C to 90°C for 6-12 hours.

[0024] In some embodiments, after drying, the sample is frozen with liquid nitrogen and ground into powder with a particle size of less than 200 mesh. Then, acetone is added to wash the powder to remove unreacted excess esterifying agent.

[0025] More specifically, including:

[0026] Starch and anhydrous ethanol are added to a ball mill jar for ball milling to disrupt its integrity and increase its specific surface area, providing a larger reaction surface for further modification and making the modification more thorough (ball milling can be omitted, but the resulting thermoplastic starch will have slightly poorer extensibility). A small amount of water is added to the dried ball-milled starch, and gelatinization is completed at 85°C. The esterifying agent, fatty acid lubricant, and gelatinized ball-milled starch are then thoroughly mixed and placed in an 85°C oven. During this process, the esterifying agent esterifies and breaks down the starch chains, while the fatty acid lubricant forms a complex with the starch. After a period of time, the mixture is removed and dried to remove unreacted esterifying agent, resulting in a thermoplastic starch with high extensibility and resistance to retrogradation.

[0027] In a second aspect, the present invention provides a thermoplastic starch with high extensibility and resistance to retrogradation prepared by the above-described method.

[0028] A third aspect of the present invention provides the application of the above-described thermoplastic starch in the preparation of bio-based materials.

[0029] Beneficial effects of the present invention

[0030] (1) This invention solves the problems of starch being difficult to heat process and having high brittleness, and realizes the preparation of thermoplastic starch and its brittle-tough transformation. The obtained starch exhibits super extensibility and has an elongation at break of more than 1500%.

[0031] (2) The present invention solves the problem of easy retrogradation of thermoplastic starch. Stearic acid and esterified starch form a stable complex that can maintain good extensibility (more than 1000%) after being placed in an indoor environment without any protection for 6 months. In addition, the thermoplastic starch has a very low water content (<3%), and water does not play an obvious plasticizing role. Therefore, there is no retrogradation problem caused by water evaporation.

[0032] (3) The thermoplastic starch prepared by the present invention can also achieve a self-repair function of brittle-toughness within a certain temperature range. It becomes brittle at temperatures above 80°C and below 120°C, but can achieve self-repair from brittle to toughness in an indoor environment without taking any environmental change measures.

[0033] (4) This invention proposes a new preparation method for tough thermoplastic starch. Attached Figure Description

[0034] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. Exemplary embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0035] Figure 1 This is a tensile stress-strain curve of the thermoplastic starch prepared in Example 1 of the present invention;

[0036] Figure 2 This is a photograph of the thermoplastic starch stretching part prepared in Example 1 of the present invention after being stretched;

[0037] Figure 3 These are DSC curves of the thermoplastic starch prepared in Example 1 of this invention before and after modification;

[0038] Figure 4 This is a crystallinity curve of the thermoplastic starch prepared in Example 1 of the present invention before and after modification;

[0039] Figure 5 These are polarized light microscope images of the thermoplastic starch prepared in Example 1 of this invention before and after modification;

[0040] Figure 6 The tensile stress-strain curves of the thermoplastic starch prepared in Example 1 of this invention after being placed in an indoor environment without any protection for 1, 3, and 6 months are shown.

[0041] Figure 7 This is a physical image showing the changes in weight of the thermoplastic starch prepared in Example 1 of this invention, which became brittle at 85°C and then self-repaired before becoming tough.

[0042] Figure 8 This is a tensile stress-strain curve of the thermoplastic starch prepared in Example 1 of the present invention after it becomes brittle and self-heals at 85°C;

[0043] Figure 9 This is a graph showing the weight change of the thermoplastic starch material prepared in Example 1 of the present invention before and after drying. Detailed Implementation

[0044] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0045] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the specific embodiments are explanations of the present invention and not limitations thereof.

[0046] Example 1

[0047] Starch and anhydrous ethanol were placed in a ball mill jar at a mass ratio of 1:2. The ball mill was set to an orbital speed of 10 r / min and a rotational speed of 200 r / min for 40 min. After ball milling, the starch was removed and dried to obtain ball-milled starch. (Ball milling can also be omitted, but the resulting thermoplastic starch will have slightly poorer extensibility.)

[0048] Using mass as the unit of measurement, 40 parts of water were added to 60 parts of ball-milled starch, mixed thoroughly, sealed, and gelatinized at 85℃ for 20 minutes. 70 parts of gelatinized starch, 20 parts of esterifying agent, and 10 parts of lubricant were mixed thoroughly, sealed, and reacted at 85℃ for 8 hours. Maleic anhydride was chosen as the esterifying agent, and stearic acid was chosen as the lubricant. After removal, the sample was dried at 100℃ to remove excess moisture, then frozen with liquid nitrogen and ground into powder with a particle size of less than 200 mesh. Acetone was added to wash the powder to remove unreacted excess esterifying agent. After washing, the sample was dried at 60℃ to obtain a thermoplastic starch with high extensibility and resistance to retrogradation. This starch was then molded into standard samples for various tests at 60℃ and 10 MPa for performance characterization.

[0049] Example 2

[0050] Starch and anhydrous ethanol were placed in a ball mill jar at a mass ratio of 1:3. The ball mill was set to an orbital speed of 10 r / min and a rotational speed of 300 r / min for 20 min. After ball milling, the starch was removed and dried to obtain ball-milled starch. (Ball milling can also be omitted, but the resulting thermoplastic starch will have slightly poorer extensibility.)

[0051] Using mass as the unit of measurement, 30 parts of water were added to 70 parts of ball-milled starch, mixed thoroughly, sealed, and gelatinized at 85℃ for 30 minutes. 60 parts of gelatinized starch, 25 parts of esterifying agent, and 15 parts of lubricant were mixed thoroughly, sealed, and reacted at 85℃ for 12 hours. Maleic anhydride was chosen as the esterifying agent, and stearic acid as the lubricant. After removal, the sample was dried at 100℃ to remove excess moisture, then frozen with liquid nitrogen and ground into powder with a particle size of less than 200 mesh. Acetone was added to wash the powder to remove unreacted excess esterifying agent. After washing, the sample was dried at 60℃ to obtain a thermoplastic starch with high extensibility and resistance to retrogradation. This starch was then molded into standard samples for various tests at 60℃ and 10 MPa for performance characterization.

[0052] Example 3

[0053] Starch and anhydrous ethanol were placed in a ball mill jar at a mass ratio of 1:1. The ball mill was set to an orbital speed of 10 r / min and a rotational speed of 100 r / min for 60 min. After ball milling, the starch was removed and dried to obtain ball-milled starch. (Ball milling can also be omitted, but the resulting thermoplastic starch will have slightly poorer extensibility.)

[0054] Using mass as the unit of measurement, 60 parts of water were added to 40 parts of ball-milled starch, mixed thoroughly, sealed, and gelatinized at 85℃ for 10 min. 90 parts of gelatinized starch, 7 parts of esterifying agent, and 3 parts of lubricant were mixed thoroughly, sealed, and reacted at 85℃ for 6 h. Acetic anhydride was chosen as the esterifying agent, and palmitic acid as the lubricant. After removal, the sample was dried at 100℃ to remove excess moisture, then frozen with liquid nitrogen and ground into powder with a particle size of less than 200 mesh. Acetone was added to wash the powder to remove unreacted excess esterifying agent. After washing, the sample was dried at 60℃ to obtain a thermoplastic starch with high extensibility and resistance to retrogradation. This starch was then molded into standard samples for various tests at 60℃ and 10 MPa for performance characterization.

[0055] Example 4

[0056] The properties of the thermoplastic starch from Example 1 were tested. The tensile stress-strain curve of the thermoplastic starch showed that its elongation at break exceeded 1500%. The DSC curve reflected the formation of its starch-stearic acid complex and the presence of free stearic acid, as well as the thermal properties of each component. XRD reflected the changes in its crystal structure, showing the formation of V-shaped crystals, and the presence of diffraction peaks of both the starch-stearic acid complex and free stearic acid. Polarizing microscopy revealed its crystal morphology, including unreacted spherical starch and rod-shaped crystals of the formed starch-stearic acid complex. Figure 6 It can be seen that after being placed in an indoor environment without any protection for 1, 3, and 6 months, thermoplastic starch still maintains good extensibility, exceeding 1000%.

[0057] For the experiment on the transformation of thermoplastic starch from brittleness to toughness at 85℃, samples were first placed in disposable petri dishes and then baked in an 85℃ oven for 1 hour. The brittle starch was then ground into powder and left to stand in an indoor environment without any environmental changes for 24 hours. The starch regained its toughness during this process. The sample quality did not change significantly; the main losses were due to moisture absorption, evaporation, and minor losses during kneading. This process indicates that the starch had a low moisture content.

[0058] The weight change of thermoplastic starch material before and after drying was tested. The moisture content of the thermoplastic starch was <3%. The results are as follows: Figure 9 As shown.

[0059] Comparative Example 1

[0060] The difference from Example 1 is that the gelatinized starch, esterifying agent, and lubricant were dry ball-milled in a ball mill and stirred at 85°C for 8 hours.

[0061] Comparative Example 2

[0062] The difference from Example 1 is that the mixture was sealed and reacted at 50 / 60 / 70°C for 8 hours (the starch did not gelatinize). Specifically, at 50°C, the starch did not gelatinize, and neither maleic anhydride nor stearic acid could melt; at 60°C, the starch did not gelatinize, and neither could stearic acid melt; when the temperature was increased to 70°C, maleic anhydride and stearic acid melted, but the starch did not gelatinize.

[0063] Comparative Example 3

[0064] The difference from Example 1 is that the mixture was sealed and reacted at 120°C for 8 hours.

[0065] Comparative Example 4

[0066] The difference from Example 1 is that only the esterifying agent is added, and no lubricant is added.

[0067] Comparative Example 5

[0068] The difference from Example 1 is that only a lubricant is added, without an esterifying agent.

[0069] Comparative Example 6

[0070] The difference from Example 1 is that maleic anhydride is added first for esterification, and stearic acid is added for complexation after esterification is completed.

[0071] Table 1

[0072]

[0073]

[0074] As can be seen from the comparison between Example 1 and Comparative Example 6, esterification followed by lubrication will increase steric hindrance due to the substitution of ester groups, which will limit the complexation of stearic acid and starch to a certain extent.

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a thermoplastic starch with high extensibility and resistance to retrogradation, characterized in that, include: Starch is wet-milled and then dried to obtain ball-milled starch; The ball-milled starch is gelatinized, and then an esterifying agent, a fatty acid lubricant, and gelatinized starch with a water content of 40%-150% are added and thoroughly mixed. The mixture is heated to 75℃-95℃ for reaction. After the reaction is completed, the starch is removed, dried, and any unreacted esterifying agent is removed. The mixture is then dried again to obtain the final product. The mass ratio of ball-milled starch to water is 40-70:30-60; The esterifying agent is maleic anhydride or acetic anhydride; the fatty acid lubricant is stearic acid or palmitic acid; The mass ratio of gelatinized starch, esterifying agent, and lubricant is 60-90:7-25:3-15.

2. The method for preparing thermoplastic starch with high extensibility and anti-retrogradation properties as described in claim 1, characterized in that, The solvent used in the wet ball milling process is anhydrous ethanol; Alternatively, starch and anhydrous ethanol can be mixed in a mass ratio of 1:1 to 1:

3.

3. The method for preparing thermoplastic starch with high extensibility and anti-retrogradation properties as described in claim 1, characterized in that, The ball milling conditions are: revolution speed 10-30 r / min, rotation speed 100-300 r / min, and milling time 20-60 min.

4. The method for preparing thermoplastic starch with high extensibility and anti-retrogradation properties as described in claim 1, characterized in that, The gelatinization conditions are as follows: gelatinize at 75℃~90℃ for 10-30 minutes.

5. The method for preparing thermoplastic starch with high extensibility and anti-retrogradation properties as described in claim 1, characterized in that, The reaction is carried out under heating conditions of 85℃~90℃ for 6-12 hours.

6. The method for preparing thermoplastic starch with high extensibility and anti-retrogradation properties as described in claim 1, characterized in that, After drying, the sample is frozen with liquid nitrogen and ground into powder with a particle size of less than 200 mesh. Then, acetone is added to wash the powder to remove unreacted excess esterifying agent.

7. The thermoplastic starch with high extensibility and resistance to retrogradation prepared by the method according to any one of claims 1-6.

8. The application of the thermoplastic starch according to claim 7 in the preparation of bio-based materials.

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