Rheological breakdown in the continuous shearing of plasticized poly(vinyl chloride)

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Abstract

The melt behavior under continuous simple laminar shearing of two poly(vinyl chloride) compositions plasticized with different proportions of di-2-ethylhexyl phthalate, has been studied in a cone-and-plate rheometer. In tests at constant shear rates between 0.09 and 9.8 sec.−1, tangential stress was measured as a function of shear over a range of temperatures which was extended by the application of hydrostatic pressure to prevent break-up of the sample. When no hydrostatic pressure was applied, the normal stress difference p11p22 was also determined, and shear recovery was measured. In tests at constant tangential stress in the range 0.4–34 g./cm.2, shear was measured as a function of time. During constant-rate shearing, the melts—in common with other polymers—generally showed a reversible reduction in stress and recoverable strain (rheological breakdown) which increased with the stress. At sufficiently low stresses, however, the stress and strain recovery increased with shearing, and this was attributed to recovery from rheological breakdown suffered during the original milling. It is considered that shearing at first disrupts the network formed by secondary cross linkages between the molecules, and then progressively reduces the molecular entanglements to an equilibrium level determined by the conditions. There is an intervening stage in which the decline in shear recovery is temporarily arrested, for which no entirely satisfactory explanation can be offered. At moderate and high shears the strain recovery decreases with increasing shear rate.

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