All-starch plastic mainly refers to thermoplastic starch. Thermoplastic starch was developed on the basis of the concept of all-starch proposed in the field of international degradable materials in the late 20th century. In all-starch plastics, traditional petroleum-based plastics are not added, starch is the main material, the starch content is high, and other added components can be degraded.
table of Contents
1 Introduction
2 Craft
Introduction
Thermoplastic starch is also known as "unstructured starch". The starch structure is disordered by a certain method to make it thermoplastic. The starch molecule is a polysaccharide molecular structure and contains a large number of hydroxyl groups. Due to the intermolecular and intramolecular hydrogen bonds, its melting temperature is higher, and its decomposition temperature is lower than its melting temperature. Therefore, during thermal processing, starch molecules Decompose first without melting. Traditional plastic machining methods mostly use hot processing molding, so to make starch-based all-starch plastics requires natural starch to have thermoplastic properties. This thermoplasticity can be achieved by changing the internal crystal structure of the starch molecule. It breaks the intramolecular and intermolecular hydrogen bonds, and disrupts the double helical crystal structure of the starch molecule. This will reduce the melting temperature of the starch and make it thermoplastic.
Craftsmanship
The preparation process of thermoplastic starch mostly uses extrusion, injection and molding, etc. The plasticizer used is generally water, glycerin and so on. Van Soest of Utrecht University in the Netherlands used water as a plasticizer to study the mechanical properties of thermoplastic starch. The amount of water added should be between 5% and 15%. Below 5%, the material is very brittle and cannot be carried out. According to the measurement, when the added amount is about 15%, the material becomes softer and it is more difficult to form. When the water content is between 5% and 7%, the material performance is similar to that of a brittle material, and no yield point is observed. The University of Manchester, Stepto et al. Used water as a plasticizer to modify potato starch and analyzed its mechanical properties. Their plasticizers were added at three levels of 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 HDPE and PP, which is 1.5MPa; the yield strength of the sample is inversely proportional to the plasticizer content, and the yield strength of the sample is 68 N when it contains 9.5% water / mm2, when the water content increases to 13.5%, its yield strength drops to 42 N / mm2. Robbert of the University of Groningen in the Netherlands used glycerin as a plasticizer to analyze a variety of different starches. The glass transition temperature (Tg) of starch also has an effect on the mechanical properties of the sample. When the Tg is low, the tensile strength, modulus, elongation at break and impact strength of the experiment increase, while the starch with a high amylose content has a relatively low Tg. Therefore, the higher the amylose content in starch, the softer the starch product. According to the Robbert experiment, the tensile strength of waxy corn with 25% plasticizer is close to 10 MPa, and the elongation at break is 110%, which is the best comprehensive performance among the selected starches. Yosbii of Peking University and Japan Institute of Atomic Energy studied starch-based plastics with glycerin and polyethylene glycol as plasticizers under electron beam irradiation. A starch-based film was successfully prepared, and it was found that irradiation can cause chemical reactions of various component molecules to form a complete network structure and enhance the tensile properties of the film.
From the above research, it can be seen that the starch can be modified to obtain thermoplastic starch, and the performance of the thermoplastic starch can be improved by changing the processing method and the type of plasticizer.
Because thermoplastic starch has the disadvantages of poor mechanical properties and strong water absorption, researchers began to consider using fiber as a reinforcing agent to be added to the thermoplastic starch matrix to improve the performance of the material. Natural fiber and starch have the same polysaccharide molecular structure. Blending fiber and thermoplastic starch can get better reinforcement effect.
Brazil's San Carlos Institute of Chemistry, Curvelo et al. Used giant tail fiber as a reinforcing agent to improve the mechanical properties of thermoplastic starch. Compared with unreinforced thermoplastic starch, reinforced thermoplastic starch has increased tensile strength by 100% and elastic modulus by 50%. And it is concluded that the water absorption of the material decreases with increasing fiber content.
Budapest University of Hungary Gaspar et al. Added cellulose, hemicellulose and zein to thermoplastic corn starch using glycerin as a plasticizer. Studies have found that hemicellulose and zein-reinforced thermoplastic starch have better mechanical strength (10. 4 MP and 11. 5 MPa). Brazilian researcher Guimaraes and others compared the strengthening 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 adhesion of sugarcane fiber and thermoplastic starch was better than that of banana fiber.
Prachayawarakorn and other research institutes of the Royal Emperor Technical College of Rakabang in Thailand conducted a study on cotton fiber-reinforced thermoplastic rice starch. It was found that after adding cotton fiber, the tensile properties of the material increased and the water absorption decreased. By comparison, when adding the same content (10%) of cotton fiber or low density polyethylene, the mechanical properties, thermal stability, water absorption and biodegradability of the samples added with cotton fiber are superior.
Sreekumar and others 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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