Interrelationship between Interfacial Tension and Rheological Properties of LDPE/PS Blends.

Abstract

The relationships between the interfacial tension and rheological properties in LDPE/PS blend system have been investigated. The block copolymer SEBS has been used as a compatibilizer. The interfacial tension was measured by the breaking thread method. Its value of LDPE/PS was 5.52dyne/cm and it is decreased rapidly with SEBS contents to 1wt% and then level off to a saturation value of 3.1dyne/cm. From the oscillatory shear deformation test, G' of the LDPE/PS blend increased with the LDPE contents at low frequencies due to the interfacial tension while G" showed similar behavior with that of pure LDPE and PS. The emulsion model considering the interfacial tension and droplet size in blends could predict the dynamic storage and loss modulus in reasonable range. Gramespacher and Meissner's emulsion model was used in this study. When the SEBS 1wt% was added to the LDPE/PS blend as a compatiblizer, there was a slight drop of storage modulus at low frequency ranges. As the SEBS contents increased over 3wt%, however, there was a dramatic increase of storage modulus. It was because of the saturation of the interfacial and micelle formation. As the SEBS content increased, the droplet size of LDPE decreased and the amount of micelle increased at the LDPE/PS interface.(PS) blends. Breaking thread method was used to measure the interfacial tension of blends. Also, the effects of compatibilizer on the morphology and rheological properties have been investigated. While many researches mentioned about the mechanical properties improvement due to the addition of copolymer as compatibilizer, relatively few reported about the rheologi ca l char ac ter isti cs. 1 3-15 ) Styrene-ethylenebutadiene-styrene block-co-polymer (SEBS) was used as a compatibilizer 16) , and an experimental study was conducted to investigate the effect of co-polymer concentration on the rheological properties of LDPE/PS blends. EXPERIMENTSMaterials LDPE, PS, and SEBS were used in this experiment. LDPE was from LG Chemical (Tg 107.43°C, MI 3.0 g/10min, density 0.920 g/cm 3 ), PS also from LG Chemical (Tg 97.11°C, MI 3.3 g/10min, density 1.05 g/cm 3 ), and SEBS was Kraton G block-co-polymer from Shell company (density 0.9 g/cm 3 , MI 65 g/10min, EB block 70%, Styrene block 30%). CompoundingPrior to mixing operation, LDPE and PS were dried over 24 hours at 80°C, and SEBS over 12 hours at 60°C in an air circulating oven before being compounded to prevent the hydrolytic degradation during processing. The blend compositions of LDPE/PS were 5/95, 10/90, 15/85, 20/80, and the effect of SEBS copolymer was studied with 20/80 LDPE/ PS blends. The copolymer compositions investigated were 1, 2, 3, 5, 7, 10wt% relative to the total weight of the blend.