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Posts sent on: 2015-12-27

27 Dec 2015 
»¿Starch-Based Plastic Foams From Various Starch Sources1 Corn, tapioca, wheat, rice, and potato starches were extruded with grafted as was confirmed by X-ray diffraction patterns of extrudates and 30% polystyrene and 5% magnesium silicate or 1% polycarbonate or water solubility indexes. The expansion, unit density and water solubility 0.5% azodicarbonamide in a single screw extruder. Brabender laboratory index of the extrudates depended on source of starch and the type of extruder at 140'C barrel temperature, 140 rpm screw speed, and 16% additive. In general, tapioca and corn starches gave the best products starch moisture content on dry weight basis. The foams did not show with any of the additives. Plastic foams are used extensively as cushioning materials for the protection of fragile products during transportation and handling. Although useful and desirable for many purposes, the indestructibility of petroleum-based plastics is a growing concern because of their accumulation in the environment. Efforts have been made to provide suitable alternatives to replace these plastics with biodegradable polymers. These biodegradable polymers can degrade in the environment by action of microorganisms in the presence of moisture. Research on the development of biodegradable polymers derived from renewable resources has gained considerable momentum in recent years and it has been estimated that worldwide demand for degradable plastics will reach 3.2 billion pounds by the year 2000. Agricultural crops provide ample sources of biopolymers which can be readily used to make biodegradable plastics. Commercially available starch foams have successfully replaced polystyrene foams. However, these foams disintegrate immediately when exposed to moisture, limiting their use in environments that are not moisture-free. Bhatnagar and Hanna developed a process to make starch-based foams replacing as much as 70-80% of the plastic with starch without compromising the functional properties of the end product (Bhatnagar and Hanna 1995a,b). These foams had much better water resistance than did starch foams made from hydroxypropylated starch and polyvinyl alcohol. In this study, we made packaging foams from various starches and compared their physical and mechanical properties. from Dow Chemical, Midland, MI. All formulations were based on a starch to polystyrene ratio of 70:30 on a dry weight basis. Three additives were tried: 1) azodicarbonamide, 2) magnesium silicate and 3) polycarbonate. Azodicarbonamide, magnesium silicate, and polycarbonate were added separately at rates of 0.5, 5, and 1%, respectively, based on dry weight of starch and polystyrene mixture. The percentages were based on earlier research (Bhatnagar and Hanna 1993; Bhatnagar and Hanna 1995a-c). Starch moisture content was adjusted to 16% dry basis and then blended with polystyrene and other additives in a Hobart mixer for 2 min. The samples were stored overnight in plastic jars to equilibrate before extrusion. Extrusion was performed in a single-screw laboratory extruder with a 1.9-cm barrel diameter, a 20:1 barrel length to diameter ratio, and a 3-mm cylindrical die nozzle. Barrel temperatures of feed, compression, metering and mixing, and die sections were maintained at 60, 140, and 140��C, respectively. The screw speed was 140 rpm. Product temperature and pressure were monitored using a transducer. Process variables of pressure, barrel and melt temperatures, screw speed, and torque were recorded by a computer interface and controller unit using a Programloader version 1.9.5 software. The 3-mm die nozzle used gave continuous cylindrical rope-like extrudates that were cooled at room temperature and broken into finite lengths for testing purposes. Spring index refers to the ability of a material to recover its original shape after it has been deformed. The force required to initially compress the sample and the force required to recompress the same sample after 1 min of releasing the initial load were determined using an Instron universal plastic recycling machine testing machine. A 6-mm diameter cylindrical probe was used to compress a sample to achieve a deformation of 2 mm at a loading rate of 30 mm/min. The initial gauge length of the sample was 20 mm. Recovery of the sample was determined by dividing the recompression force after 1 min by the initial compression force. An ideal elastic body has a spring index of 1. Higher percent recoveries correspond to materials that are more elastic or resilient. Compressibility of a sample, i.e., the force necessary to achieve a deformation of 2 mm, was determined using a UTM employing the conditions noted above in measuring spring index. A high value was attributed to a sample that was relatively hard, i.e., less compressible, while a lower value was attributed to a sample that was easily compressed. Water Solubility Index Water solubility index was used to describe water solubility of the extrudates with some modifications. Water solubility of solid peanuts as well as crushed peanuts was determined. Sample were crushed because it was difficult to grind the extrudates to an exact size due to their plastic content. For water solubility of solid peanuts, extrudates were cut into 2.5- cm long pieces, while crushed samples were prepared by crushing the extrudates for 1 min. Crushed or solid extrudate was dispersed in 100 ml of water in a centrifuge tube. The dispersed samples were held for 30 min with periodic stirring and were centrifuged at 5,000 x g for 10 min. The supernatants were carefully decanted and solute concentrations were determined by phenol sulfuric acid rather than by oven-dry method suggested by Anderson et al. Water solubility indices were determined at room temperature and expressed as percent on a dry matter basis.
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