Introduction In recent years the research and development on biodegradable polymers has - PDF document

more renewable resources has grown, because ofmore renewable resources has grown, because ofpolyesters, which are petroleum based, but they arebiodegradable. The other group contains polymersfrom renewable resources, like thermoplastic starchor poly-lactic-acid (PLA). Both TPS and PLA canbe produced from common, natural resource, andthey are readily biodegradable.Starch can be found in different plants like wheat,maize, potato, rice. This material is in abundanceall over the world. Starch has a granular structure,which is built by two main carbohydrate polymers:the linear amylose and the highly branched amy-lopectin. According to the botanical source theamylose-amylopectin ratio can be different. The 804 © BME-PT and GTE set-ups and storing conditions were used to analyse the effects on the properties of thermoplastic starch. Investigated Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2007.111 Starch has unfortunately a hydrophilic character,which also retards its widespread industrial appli-cations. While in contact with water, pure starchproducts swell and then dissolve. Another factor isthe low mechanical properties of starch and thehigh shrinkage values when injection moulded.Although starch possesses these drawbacks, it is apotential material of biodegradable products in thenear future. To overcome these drawbacks, a greatnumber of experiments were taken. Many authorsreported the behaviour ofstarch reinforced withstarch reinforced withproperties of starch improved, while the ageing, thehydrophilic character, and the shrinkage valuesdecreased. Many authors also reported the behav-iour of a starch-based blend. In this case starch wasassociated with another biopolymer, often withbiodegradable polyesters, like polycaprolactone(PCL), polyesteramide (PEA), or with the starchbased polylacticacid (PLA) [16–21]. The blendingof starch with biodegradable polyesters alsodecreased the hydrophilic behaviour and theshrinkage values, but it had no effect on the ageingof starch. The mechanical properties of blends varywith the starch-biodegradable polyester content,but unfortunately only some of them are compati-ble with starch. In recent years, the researches con-centrated on the cross linking of starch, and theassociation of starch with nano-particles [22–27].With the help of cross-linking or nano-particles, theageing, and strong hydrophilic character of thermo-plastic starch can be reduced.Starch plays the role of the matrix material in allthese biocomposites, constituting the majoritycomponent of these blends. When injectionmoulded, the different injection moulding parame-ters could affect the long-term behaviour of TPS.This paper focuses on the effect of different injec-tion moulding set-ups on the time dependentbehaviour of TPS.2. ExperimentalMaize starch was used for the experiments.According to the manufacturer, the amylose-amy-lopectin ratio is 25:75. Maize starch was plasticizedwith glycerol (99% purity) and distilled water. Thestarch:glycerol:water ratio was 70:16:14 and glyc-erol-mono-stearate was added as a lubricant. A spe-cial silicone based screw cleaner material was usedto clean the screw of the extruder and injectionmoulding machine from any contamination (pro-vided by Szilor Ltd., Hungary). The TPS was pro-duced in a two step process. First, the materialswere weighted and mixed, than the mixture wasprocessed with a Brabender Plasti-Corder PL 2100double screw extruder (diameter 25mm screw,=20) with zone temperatures of 90-130-120-relative humidity for 4weeks) before injectionwere injection moulded with an Arburg 320600-diameter 35mm screw. Table1 contains the maininjection moulding pressure was 1400bars, and theholding pressure was varied from 600 to 1000barsthe time dependence of TPS (Table2). The injec-(Table2). Tábi and Kovács – eXPRESS Polymer Letters Vol.1, No.12 (2007) 804–809 Table 1.The main injection moulding parameters used 44 ] 12 ] 50 20 15 1. Zone 110 2. Zone 115 3. Zone 120 4. Zone 125 5. Zone 130 25 seen in Figure5 and6.modulus (Figure7) in time.72hours after the ejection, the holding pressuretensile modulus. 168hours after the demoulding,researched in following papers. The effect of themonitored on Figure8. Although the shrinkage ofstarch (Figure9), which is almost totally disap-peared after the injection moulding (Figure10) or 807 Tábi and Kovács – eXPRESS Polymer Letters Vol.1, No.12 (2007) 804–809 Figure 5.The effect of ageing time on the tensile stress The effect of ageing time on the tensile modulus The effect of ageing time on the yield strain Linear shrinkage as a function of the ageing TPS surface after the first extrusion. Nativestarch pellets still can be found the second extrusion process (Figure11). Althoughcan be detected as a function of ageing (Figure12).for 72hours. After this time period the holding[1]Zhang M. Q.: Polymeric materials from natural Tábi and Kovács – eXPRESS Polymer Letters Vol.1, No.12 (2007) 804–809 Figure 10.TPS surface after injection moulding. Nativestarch granules totally destructur TPS surface after a second extrusion process.Native starch granules totally destructured Aged TPS fracture surface. 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