Preparation method of starch plastic with low surface energy and high hydrophobicity

Abstract

The invention discloses a preparation method of starch plastic with low surface energy and high hydrophobicity. The thermoplastic starch plastic comprises the following components in percentage by weight: 73% of starch, 21.8-24.5% of lactic acid and 2.7-5.5% of bleached shellac. The preparation method comprises the following steps: uniformly mixing starch, lactic acid and bleached shellac according to a certain ratio, and preparing the thermoplastic starch plastic by a reactive extrusion (single-screw or double-screw extruder) method. The thermoplastic starch plastic has excellent water resistance, the lowest surface energy of the thermoplastic starch plastic can reach 38.41 mJ / m < 2 >, the maximum surface water contact angle of the thermoplastic starch plastic can reach 90.7 degrees, and the purpose of hydrophobicity of the thermoplastic starch plastic is achieved; in addition, compared with traditional glycerin plasticized thermoplastic starch plastic, the tensile strength can be improved by 3-7 times, certain elongation at break can be kept, and the mechanical property is also remarkably improved.

Description

Technical Field

[0001] The invention belongs to the field of biodegradable plastics and relates to a method for preparing starch plastic with low surface energy and high hydrophobicity. Background Art

[0002] As the pollution problem caused by petroleum-based plastics becomes increasingly serious, the development and application of biodegradable plastics is an effective strategy to reduce the environmental pollution caused by polyolefin plastics. Starch, as a polysaccharide bio-based polymer material widely distributed in nature, is widely found in the seeds and rhizomes of plants such as corn, wheat, and potatoes. It has many advantages such as abundant sources, low price, biodegradability and renewable nature, making it one of the best choices for biodegradable plastic raw materials. However, due to the presence of a large number of hydroxyl groups on the starch macromolecular chain, hydrogen bonds can be formed within and between molecules, and a certain crystalline structure is formed. As a result, the melting temperature of natural starch is higher than its decomposition temperature, and plastic processing like general plastics cannot be achieved. This greatly limits the application of starch and prevents its biodegradability from being realized. For this reason, starch is usually blended with a plasticizer under high temperature and high shear conditions. The interaction between the plasticizer and starch destroys the original hydrogen bond system of the starch, reduces the crystallinity of the starch, and gives it plasticity. This type of starch is called thermoplastic starch and is mainly used in food, medicine, health, agriculture and other fields.

[0003] Traditional plasticizers generally use polyols, such as glycerol, ethylene glycol, etc., although these substances can effectively improve the processing properties and performance of starch, but have limited effect on the water resistance and mechanical properties of thermoplastic starch, especially the poor water resistance caused by the hydrophilicity of polyol plasticizers themselves seriously restricts the application range of materials. In recent years, improving the water resistance of thermoplastic starch has become a research focus. At present, the method for improving the water resistance of thermoplastic starch mainly includes methods such as blending modification, surface modification, esterification modification, and plasticizer modification. Patent CN105566690B reports a method for preparing a starch-based degradable bioplastic, which is hydrophobized by starch to obtain a high-hydrophobicity starchy material, and then the raw material components are mixed in proportion, and a masterbatch is made with a twin-screw extruder. After the masterbatch is mixed with a general-purpose polymer plastic, injection molding, pressing, blow molding, and hollow molding are made into starch-based degradable bioplastic. Although the hydrophobicity of the bioplastic is significantly improved, the addition of general-purpose plastics has an impact on its biodegradability and the preparation process is complicated. Patent CN110277673B reports a method for preparing a hydrophobic thermoplastic starch modified with a polyionic liquid. The method first dissolves corn starch and a polyionic liquid in dimethyl sulfoxide, and then undergoes drying, heating, melting, and pressing into a film. Although the water contact angle of the resulting film is significantly improved, the preparation process is complicated and involves solvent treatment, which is not conducive to industrial application. Patent CN103044719B discloses a method for preparing a highly hydrophobic thermoplastic starch plastic, which is formed by blending oxidized starch with elastic particles, centrifuging, washing, drying, and crushing, and then melt-extruded with a plasticizer and a lubricant. Although the water contact angle of the resulting material is significantly improved to 108°, the complexity of its multi-step process and the introduction of elastic particles result in a decrease in the degradability of the material. Therefore, the development of a degradable thermoplastic starch material with a simple process and excellent hydrophobic properties remains a key research direction in this field.

[0004] Currently, plasticizers used in thermoplastic starch plastics include polyols and amines such as glycerol, ethylene glycol, and urea, as well as organic acids such as lactic acid. While exploring the preparation of starch plastics, the inventors unexpectedly discovered that bleached shellac, a natural resin, possesses excellent hydrophobicity and has broad development potential in the field of biodegradable films. Consequently, they developed a method for preparing thermoplastic starch plastics with low surface energy and high hydrophobicity. Specifically, when lactic acid and bleached shellac are used simultaneously in reactive extrusion to prepare the starch plastic, both the water resistance and mechanical properties of the starch plastic can be significantly improved. Based on these findings, the present invention was completed.

[0005] Therefore, the present invention provides a method for preparing a starch plastic with low surface energy and high hydrophobicity.

[0006] Specifically, the highly hydrophobic thermoplastic starch plastic of the present invention is composed of the following components by weight:

[0007] Starch 80 (73%)

[0008] Lactic acid 24-27 (21.8-24.5%)

[0009] Bleached shellac 3-6 (2.7-5.5%)

[0010] The specific method adopted by the present invention is:

[0011] 80 parts of starch, 24-27 parts of lactic acid and 3-6 parts of bleached shellac are uniformly mixed in a container; the mixture is extruded into granules in a single-screw or twin-screw extruder, with the temperature in each temperature zone of the extruder being 110° C. and the rotation speed being 25-35 r / min; and the mixture is hot-pressed in a hot press at a temperature of 100° C. to obtain a highly hydrophobic thermoplastic starch plastic.

[0012] The advantages of the present invention are:

[0013] 1) Starch, lactic acid and bleached shellac are widely available, non-toxic and pollution-free, and have low cost;

[0014] 2) The water resistance and mechanical properties of starch plastics prepared with lactic acid and bleached shellac were significantly improved;

[0015] 3) The starch plastics prepared from starch, lactic acid and bleached shellac maintain complete biodegradability as a whole. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Water contact angle diagram of the surfaces of various embodiments and comparative examples

[0017] Figure 2 Atomic force microscope images of the surfaces of Example 3 and Comparative Example 1 DETAILED DESCRIPTION

[0018] In order to make the technical solution of the present invention more clearly understood by those skilled in the art, the following examples are given for illustration. It should be noted that the following examples do not limit the scope of protection claimed by the present invention.

[0019] In the following examples and comparative examples, the starch plastic film obtained by the hot press was cut into standard samples of 40×10×0.6 mm and measured using an E44.304 electronic universal testing machine produced by MTS Industrial Systems (China) Co., Ltd. at a tensile rate of 50 mm / min.

[0020] Example 1:

[0021] 80 parts of starch, 27 parts of lactic acid and 3 parts of bleached shellac are uniformly mixed in a container; the mixture is extruded into granules in a single-screw or twin-screw extruder, with the temperature in each temperature zone of the extruder being 110° C. and the rotation speed being 25-35 r / min; the mixture is hot-pressed in a hot press at a temperature of 100° C. to obtain a thermoplastic starch plastic.

[0022] Example 2:

[0023] 80 parts of starch, 25.5 parts of lactic acid and 4.5 parts of bleached shellac are uniformly mixed in a container; extruded into granules in a single-screw or twin-screw extruder, with the temperature of each temperature zone of the extruder being 110° C. and the rotation speed being 25-35 r / min; and hot pressed in a hot press at a temperature of 100° C. to obtain a thermoplastic starch plastic.

[0024] Example 3:

[0025] 80 parts of starch, 24 parts of lactic acid and 6 parts of bleached shellac are uniformly mixed in a container; extruded into granules in a single-screw or twin-screw extruder, with the temperature of each temperature zone of the extruder being 110°C and the rotation speed being 25-35 r / min; and hot pressed in a hot press at a temperature of 100°C to obtain a thermoplastic starch plastic.

[0026] Comparative Example 1:

[0027] 80 parts of starch and 30 parts of lactic acid are uniformly mixed in a container; extruded into granules in a single-screw or twin-screw extruder, with the temperature in each temperature zone of the extruder being 110° C. and the rotation speed being 25-35 r / min; and hot-pressed in a hot press at a temperature of 100° C. to obtain thermoplastic starch plastic.

[0028] Comparative Example 2:

[0029] 80 parts of starch and 30 parts of glycerin are uniformly mixed in a container; extruded into granules in a single-screw or twin-screw extruder, with the temperature in each temperature zone of the extruder being 110° C. and the rotation speed being 25-35 r / min; and hot-pressed in a hot press at a temperature of 100° C. to obtain thermoplastic starch plastic.

[0030] Table 1

[0031] Mechanical properties Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Tensile strength (MPa) 3.00 4.34 6.16 1.73 0.85 Elongation at break (%) 149 106 99.3 197 126

[0032] Table 2

[0033]

[0034] Table 1 shows the mechanical properties of various embodiments and comparative examples. The test data show that the mechanical properties of the starch plastic prepared by using lactic acid and bleached shellac simultaneously in the present invention are significantly improved, especially under the premise of maintaining a certain fracture toughness, the tensile strength is significantly improved. Compared with the comparative example 2 of conventional glycerol-plasticized thermoplastic starch, the tensile strength can be increased by 3-7 times; Table 2 shows the water contact angle and surface energy data of various embodiments and comparative examples. The test data show that the use of lactic acid and bleached shellac to co-plasticize thermoplastic starch can significantly improve the water surface contact angle of the material, reduce its surface energy, and exhibit excellent water resistance. Figure 1 As shown in FIG. 3 , it can be observed that the contact angle of Example 3 reaches 90.7°, which indicates the best hydrophobic effect. This change is related to the distribution of bleached shellac on the surface of the material. Figure 2 As shown, the surface atomic force microscopy images of Example 3 and Comparative Example 1 show a clear contrast in the three-dimensional surface morphology. The surface of the sample of Example 3 with the addition of bleached shellac has a surface morphology of alternating microscopic concave and convex surfaces, which is one of the main factors for its low surface energy and high hydrophobicity.

Claims

1. A method for preparing a starch plastic with low surface energy and high hydrophobicity, characterized in that: The low surface energy and highly hydrophobic thermoplastic starch plastic is obtained by following the steps below: 80 parts starch, 24-27 parts lactic acid, and 3-6 parts bleached shellac are uniformly mixed in a container; extruded into granules in a single-screw or twin-screw extruder, with each temperature zone of the extruder at 110°C and a rotation speed of 25-35 rpm; and then hot-pressed at 100°C in a hot press to obtain a thermoplastic starch plastic with low surface energy and high hydrophobicity. Among them, the thermoplastic starch plastic made from 80 parts of starch, 24 parts of lactic acid and 6 parts of bleached shellac has the best water resistance, and its surface energy reaches the lowest 38.41mJ / m 2 , the surface water contact angle reaches a maximum of 90.7°.

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