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SYNOPSISThe melt rheology of blends of natural rubber ( N R ) and ethylene-vinyl acetate copolymer (EVA) has been studied with reference to the effects of blend ratio, cross-linking systems, shear stress, and temperature. When EVA formed the dispersed phase, the viscosity of the blends was found to be a nonadditive function of the viscosities of the component polymers at lower shear region, i.e., a positive deviation was observed. This behavior has been explained based on structural buildup of dispersed EVA domains in the continuous NR matrix. The effect of the addition of silica filler on the flow characteristics of the blends has been investigated. The melt elasticity parameters such as die swell, principal normal stress difference, recoverable shear strain, and elastic shear modulus of NR-EVA blends were also evaluated. 0 1993 John Wiley & Sons, Inc. I NTRO DU CTI 0 NIn recent years, blending of two or more polymers to produce the required combination of properties for specific end uses is gaining industrial importance. Knowledge of flow behavior of polymer blends is of paramount importance to optimize processing operations. In polymer blends, the viscosity of individual components plays a critical role in the flow and forming of compounds. Several investigations have been made to understand the complicated rheological behavior of polymers in view of its relevance to processing.'-4 The melt flow behavior of polymer blends and that of polymers containing cross-linked particles have been studied by different research group^.^-" Danesi and Porter l 1 studied the rheological behavior of polypropylene and ethylene propylene rubber. They explained how the state of dispersion is affected by conditions of blend preparation and extrusion. Utracki and Samut l 2 studied the flow properties of various polyethylene blends. Kuriakose and De13 and Thomas et al.14 studied the * To whom all correspondence should be addressed. Recently, Varughese l5 studied the rheological behavior of poly (vinyl chloride) /epoxidized natural rubber (PVC/ENR) blends. All these studies indicated that the dispersed phase in an immiscible blend is deformable and may give rise to wide range of sizes and shapes during processing. This, in turn, determines the morphology of the system. The melt rheology is particularly sensitive to morphology. Blends of ethylene-vinyl acetate copolymer (EVA) and natural rubber ( NR) are now being used in many applications such as in cables and footwear. These materials combine the superior mechanical properties of NR and the excellent aging properties of EVA. Recently, we have examined the miscibility, morphology, mechanical properties, and aging characteristics of these However, the rheological behavior of these blends has not been studied. This paper examines the rheological characteristics of NR-EVA blends by capillary rheometry. The effects of blend ratio, cross-linking system, and silica on the viscosity of the blends have been studied. In addition, the morphology of extrudate has been examined to understand the...

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