Full starch plastic mainly refers to thermoplastic starch. Thermoplastic starch was developed on the basis of the full starch concept proposed in the field of international degradable materials in the late 20th century. In full starch plastic, no traditional petroleum-based plastic is added, starch is the main material, the starch content is high, and other added components can be degraded.

Introduction

Thermoplastic starch is also called "unstructured starch". The starch structure is disordered by a certain method to make it thermoplastic. Starch molecules are polysaccharide molecular structures and contain a large number of hydroxyl groups. Due to the effect of hydrogen bonds between molecules and within molecules, their melting temperature is relatively high, and their decomposition temperature is lower than their melting temperature. Therefore, during thermal processing, starch molecules decompose before melting. Traditional plastic mechanical processing methods mostly use thermal processing molding. Therefore, to obtain starch-based full starch plastics, it is necessary to make natural starch thermoplastic. This thermoplasticity can be achieved by changing the internal crystal structure of starch molecules. Destroying the intramolecular and intermolecular hydrogen bonds and disrupting the double helix crystal structure of starch molecules will reduce the melting temperature of starch and make it thermoplastic.

Process

The preparation process of thermoplastic starch mostly adopts extrusion, injection and molding, and the plasticizers used are generally water, glycerin, etc. vanSoest of Utrecht University in the Netherlands studied the mechanical properties of thermoplastic starch using water as a plasticizer. The amount of water added should be between 5% and 15%. When it is less than 5%, the material is very brittle and cannot be measured. When the amount added is about 15%, the material becomes softer and more difficult to shape. When the water content is between 5% and 7%, the material performance is similar to that of brittle materials, and the yield point cannot be observed. Stepto et al. of the University of Manchester in the UK used water as a plasticizer to modify potato starch and analyzed its mechanical properties. They selected three levels of plasticizer addition: 9.5%, 10.8%, and 13.5%. By analyzing the stress-strain curve, it can be seen that the initial modulus of the sample is close to that of HDPE and PP, which is 1.5MPa; the yield strength of the sample is inversely proportional to the plasticizer content. The yield strength of the sample at 9.5% water content is 68N/mm2, and when the water content increases to 13.5%, its yield strength drops to 42N/mm2. Robbert et al. from the University of Groningen in the Netherlands analyzed a variety of different starches using glycerol as a plasticizer. The glass transition temperature (Tg) of starch also affects the mechanical properties of the sample. When Tg is low, the tensile strength, modulus, elongation at break and impact strength of the experiment increase, while the Tg of starch with high amylose content is relatively low. Therefore, the higher the amylose content in starch, the softer the starch product. According to Robbert's experiment, the tensile strength of waxy corn containing 25% plasticizer is close to 10MPa, and the elongation at break is 110%, which is the best comprehensive performance among the selected starches. Yosbii et al. from Peking University and Japan Atomic Energy Research Institute studied the use of electron beam irradiation to produce starch-based plastics with glycerol and polyethylene glycol as plasticizers. They successfully produced starch-based films, and found that irradiation can cause chemical reactions in the molecules of each component to form a complete network structure and enhance the tensile properties of the film.

From the above research, it can be seen that starch can be modified to obtain thermoplastic starch, and the properties of thermoplastic starch can be improved by changing the processing method and the type of plasticizer.

Due to the shortcomings of thermoplastic starch such as poor mechanical properties and strong water absorption, researchers began to consider using fibers as a reinforcing agent and adding them to the thermoplastic starch matrix to improve the properties of the material. Natural fibers and starch have the same polysaccharide molecular structure. Blending fibers with thermoplastic starch can achieve a better reinforcing effect.

Curvelo et al. from the Sao Carlos Institute of Chemistry in Brazil used giant tail fiber as a reinforcing agent to improve the mechanical properties of thermoplastic starch. Compared with unreinforced thermoplastic starch, the tensile strength of reinforced thermoplastic starch increased by 100% and the elastic modulus increased by 50%. And it was concluded that the water absorption of the material decreased with the increase of fiber content.

Gaspar et al. of Budapest University, Hungary added cellulose, hemicellulose and zein to thermoplastic corn starch with glycerol as plasticizer. The study found that the mechanical strength of thermoplastic starch reinforced with hemicellulose and zein was better (10.4MP and 11.5MPa). Brazilian researchers Guimaraes et al. compared the reinforcing effect of sugarcane fiber and banana fiber on thermoplastic starch. It was found that the tensile properties of the reinforced samples were significantly enhanced, and the surface bonding of sugarcane fiber and thermoplastic starch was better than that of banana fiber.

Prachayawarakorn et al. of King Mongkut's Institute of Technology Ladkrabang, Thailand, studied thermoplastic rice starch reinforced with cotton fiber and found that the tensile properties of the material increased and the water absorption decreased after adding cotton fiber. By comparison, it was found that when the same content (10%) of cotton fiber or low-density polyethylene was added, the mechanical properties, thermal stability, water absorption and biodegradability of the sample with cotton fiber were better.

Sreekumar et al. from the University of Rouen in France studied the effect of sisal fiber on thermoplastic wheat flour and found that sisal fiber can enhance the tensile properties of thermoplastic wheat flour, but its fluidity will decrease.

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