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Thermoplastic starch (TPS) was blended in a twin screw mixer with 5 and 10 mass % of poly(ethylene-co-acrylic acid) (EAA), poly(ethylene-cu-vinyl acetate) (EVA) or cellulose acetate (CA) in order to improve the TPS properties. The influence of such low amounts of thermoplastic polymer (TPP) on the processability, mechanical properties and water resistance of TPS was investigated. The specimens for mechanical properties determinaiton were conditioned at 45, 67 and 85% relative humidity (RH) before measurements. Processability, characterized by viscosity v.s. shear rate dependence, was determined by capillary rheometry. The obtained results showed that the TPS blends behaved as pseudoplastic polymers and that even 10 mass% of EAA or EVA significantly decreased the TPS viscosity, whereas addition of the ;ame amount of CA had no influence on the TPS viscosity. Tensile measurements showed that the RH had a much larger influence on the tensile properties than the additior of the investigated TPP. Tensile strength and tensile modulus values for TPS and its blends, after conditioning at 45% RH were an order of magnitude larger than the values for the corresponding samples kept at 85% RH. whereas elongation at break values were nearly ten times smaller. Dynamic mechanical analysis of the TPS and its blends was performed using a mechanical spectrometer. The specimens were subjected to dynamic torsion at a constant frequency in the temperature range from 30 to 170°C. The glass transition temperature, determined from the temperature dependence of tan δ, was influenced much more by relative humidity than by the type of TPP used. The swelling of the TPS and the investigated blends was determined gravimetrically. It was found that the TPS/EAA blends had the largest water resistance.