ISSN:
0021-8995
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
Notes:
A modification of the piston head of the Instron rheometer has enabled its use in studying rheological properties of SBR elastomers over a shear rate range of 30-2500 sec.-1. In place of the usual piston head, a Teflon plug with a cavitated bottom was used. This eliminates leakage between the piston and barrel and results in uniform force traces that can be extrapolated to zero barrel length to yield the total force of extrusion. Hence, it gives a practical means to study, on sound theoretical basis, the rheological properties which are all important in the prediction of factory processing behavior and factors which influence it. Extrusions of SBR 1712 and 1712-HAF compound were made through capillaries having 180° entrance angles, diameters of 0.033, 0.043, 0.052, and 0.070 in. and length/diameter ratios near 1, 2, 4, 6, 8, and 10 at temperatures of 80, 90, 110, and 130°C. and crosshead speeds of 1/2, 1, 2, and 5 in./min. Linear fits were found between length/diameter (L/D) ratio and the force of extrusion (at zero barrel length). The slopes of these lines give values of true shear stress which fit one relationship to Newtonian shear rates at each of the four temperatures independent of die dimensions. Plots of log true shear stress versus log true shear rate show curvilinear behavior for clear SBR 1712 elastomer over the entire shear rate range of 45-3900 sec.-1 at all four temperatures. The same plots for the HAF compound of SBR 1712 showed curvilinear behavior below shear rates of 100 sec.-1 and linear behavior above with a power law exponent of 6.1. There is a great difference in slopes between lower and higher shear rates at all four temperatures. This indicates that extrapolation from low shear rate instruments cannot be made to predict behavior at higher rates common to factory equipment. This observation holds for both clear SBR 1712 elastomer and its HAF compound. True shear stresses at corresponding true shear rates (and therefore viscosities) were considerably higher for the compound than for the clear elastomer, as was expected. However, the addition of black reduces the extent of deviation from Newtonian behavior. Even though uncorrected for recoverable shear strain, viscosities were calculated and used to calculate activation energies according to the Arrhenius equation. Straight lines were obtained covering temperatures at 90, 110, and 130°C. at all shear rates with activation energies of 1.5 kcal./mole of both the clear elastomer and its compound. Log viscosity at 80°C. in all cases was above the straight lines through the log viscosities at the other three temperatures. This indicates a higher activation energy at temperatures below 90°C. Addition of black apparently does not affect substantially the identity of the rheological unit. Extrusion die swell was found to decrease with increasing temperature and increasing L/D ratio at all rates of shear. At a given temperature it increases rate of shear. Swell was anisotropic for the clear elastomer but not for the compound.
Additional Material:
13 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/app.1964.070080316
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