The composites of polystyrene with plant fibers and mineral powders
Ryszard Steller, Danuta Żuchowska, Wanda Meissner Politechnika Wrocławska, Instytut Technologii Organicznej i Tworzyw Sztucznych, ul. Wybrzeże Wyspiańskiego 27, 50-370 Wrocław
Annals 1 No. 2, 2001 pages 242-245
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abstract Polystyrene and its copolymers are widely used in industry. Their contribution both to polymer production and polymer waste exceeds 10%. A relatively small part of this contribution (2%) falls to expandable polystyrene, but due to the very low density its waste becomes comparable with the total volume of other polymer waste. Additionally, the expandable polystyrene has a lower molecular weight, and hence the worse mechanical properties, in comparison with typical polystyrenes. It results in a number of serious problems connected with the management, e.g. recycling, of the waste. One of the possible ways of expandable polystyrene recycling is its modification. In this study the expanded polystyrene waste after densification with the two-roll-mill at ca. 120oC during 2 minutes and grinding was blended with hemp or flax fibers or with Al2O3 powder. The process was also carried out with the two-roll-mill at temperature 160oC during 10 min. The fiber in amount of 0÷30% or the powder in amount of 0÷20% was added to the molten polystyrene. The components were blended until the homogeneous blend was obtained. The samples for mechanical testing were prepared from grind by injection molding at 190oC. The rheological properties were measured using the grind. The relatively low temperatures and shear rates (shear stresses) on every processing stage were indispensable to prevent the polystyrene and plant fibers from strong thermomechanical degradation. The mechanical properties and melt flow index of waste polystyrene-plant fibers blends are presented in Table 1. The same properties of a standard polystyrene are given for comparison. It is evident from Table 1 that there are no significant differences in properties due to the kind of fibers. Both hemp and flax increase the static mechanical properties, especially the tensile and flexural strengths. The impact strength (dynamic property) shows a minimum at 10% fiber content. The impact strength is generally lower than that for pure polystyrene. The lack of dynamical toughening can be attributed to the loss of continuity of polystyrene phase and a weak adhesion between polymer and fiber. The rheological measurements have shown that the flow curves of polystyrene-fiber systems at 170, 190 and 210oC can be adequately described by the power law. The constants K and n of the power law determined with a capillary rheometer are summarized in Table 2. Figures 1 and 2 present examples of the flow curves of polystyrene-hemp and polystyrene-flax compositions at 170oC. It can be seen that with increasing fiber content the constant K increases and the constant n decreases. It means that the fiber addition rises the viscosity of the system and simultaneously causes its rheological behavior more non-Newtonian. The qualitatively similar effect brings the temperature lowering. Table 3 presents the comparison of mechanical and rheological properties of waste and standard polystyrenes filled with Al2O3 powder. It can be seen that the powder addition increases the tensile strength (static property) and decreases the impact strength (dynamic property) and the flowability of the system in both cases. This behavior is similar to the systems with fibers discussed previously. However, for waste polystyrene blends a stronger tensile strength rise and a weaker impact strength fall than that for the standard polystyrene can be observed. It testifies probably to a better adhesion between polymer and powder in blends with waste polystyrene, which is due to its lower molecular weight. It follows from Tables 1 and 3 that the plant fibers are more efficient toughening agents for the waste polystyrene than the Al2O3 powder. A further improvement of their efficiency is possible by the simultaneous use of other modifications such as the addition of thermoplastic elastomers and preparation of the fiber surface with suitable adhesives.