Whole starch plastic
Whole starch plastic mainly refers to thermoplastic starch. Thermoplastic starch was developed in the late 20th century on the basis of the concept of whole starch, which was proposed in the field of international degradable materials. In the whole starch plastic, the traditional petroleum-based plastic is not added, starch is the main material, the starch content is high, and other added components can be degraded.
Thermoplastic starch is also called "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 bonding, the melting temperature is higher, and the decomposition temperature is lower than the melting temperature. Therefore, during thermal processing, starch molecules Decomposes without melting. Traditional plastic mechanical processing methods mostly use thermoforming, so to make starch-based whole starch plastics, natural starch must be made thermoplastic. This thermoplasticity can be achieved by changing the crystalline structure inside the starch molecule. It destroys the intra- and intermolecular hydrogen bonds and disrupts the double helix crystal structure of starch molecules. This will reduce the melting temperature of starch and make it thermoplastic.
The preparation of thermoplastic starch mostly uses extrusion, injection, molding, etc. The plasticizers used are generally water, glycerin, and the like. Van Soest of Utrecht University in the Netherlands has studied the mechanical properties of thermoplastic starch with water as a plasticizer. The amount of water added should be between 5% and 15%. Below 5%, the material is very brittle and cannot be carried out. It is determined that when the content is about 15%, the material becomes soft and difficult to form. When the water content is between 5% and 7%, the material performance is similar to brittle materials, and no yield point is observed. Stepto et al., University of Manchester, UK, 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 known that the initial modulus of the sample is close to HDPE and PP, which is 1.5 MPa; the yield strength of the sample is inversely proportional to the plasticizer content, and the yield strength of the sample when the water content is 9.5% is 68 N / mm2, when the water content increases to 13.5%, its yield strength drops to 42 N / mm2. Robbert et al. From 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 affects the mechanical properties of the sample. The Tg is low, and the tensile strength, modulus, elongation at break and impact strength of the experiment are increased, while the Tg in the starch with high amylose content is relatively low. So the higher the amylose content in the starch, the softer the starch product. According to Robbert's experiments, the tensile strength of waxy corn containing 25% plasticizer is close to 10 MPa, and the elongation at break is 110%, which is the best comprehensive performance of starch. Peking University and Yosbii of the Japan Atomic Energy Research Institute studied starch-based plastics using glycerin and polyethylene glycol as plasticizers for electron beam irradiation. The starch-based film was successfully prepared, and it was found that irradiation can cause chemical reactions of each component molecule to form a complete network structure and enhance the tensile properties of the film.
It can be known from the above studies that starch can be modified to obtain thermoplastic starch, and the performance of thermoplastic starch can be improved by changing processing methods, types of plasticizers and other means.
Because thermoplastic starch has the disadvantages of poor mechanical properties and strong water absorption, researchers have begun to consider using fiber as a reinforcing agent and adding it to the thermoplastic starch matrix to improve the performance of the material. Both natural fiber and starch have a polysaccharide molecular structure. Blending fibers with thermoplastic starch can obtain a better strengthening effect.
Curvelo, Brazil's San Carlos Chemical Research Institute, etc., uses giant tail fiber as a reinforcing agent to improve the mechanical properties of thermoplastic starch. Compared with unreinforced thermoplastic starch, reinforced thermoplastic starch has a 100% increase in tensile strength and a 50% increase in elastic modulus. And it is concluded that the water absorption of the material decreases with the increase of the fiber content.
Gaspar et al. From Budapest University in Hungary added cellulose, hemicellulose, and zein to thermoplastic corn starch using glycerin as a plasticizer. Studies have found that the mechanical strength of hemicellulose and zein-reinforced thermoplastic starch is better (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 strengthened samples were significantly enhanced, and the surface bonding between sugarcane fiber and thermoplastic starch was better than banana fiber.
Prachayawarakorn and other technical institutes in Thailand ’s Lakabang Sangha Technical College have conducted research on cotton fiber-reinforced thermoplastic rice starch, and found that the tensile properties and water absorption of the material decrease after adding cotton fibers. By comparison, when the same content (10%) of cotton fiber or low density polyethylene is added, the mechanical properties, thermal stability, water absorption and biodegradability of the cotton fiber sample are better.
Sreekumar of Rouen University in France and other researchers 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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