Processing Elastomeric Compositions Twin screw extruder
A twin screw extruder for mixing carbon black with rubber and with a first mixing zone with single screw extruder elements for heating rubber to about 130°-220° C., and a second port for adding carbon black and a third port for adding oil.
Upon running the recipes through each test, the test results included the following:
1. Traditional stress/strain mechanical properties. This measured the overall quality of mixing process and distributive uniformity.
2. Thermal gravimetric analysis. This measured mass percent, and variance of major recipe components, that is polymer, oil, carbon black and mineral fillers.
3. The Monsanto oscillating disc rheometer. This measured the chemical kinetics of vulcanization chemistry and dynamic shear modulus.
4. Capillary theology. This evaluated the degree of polymer blending and dispersive mixing by measuring shear stress and viscosity as a function of shear rate and temperature.
5. Dynamic torque modulus. This measured the degree of dispersive mixing.
6. Transmission electron microscopy. This provided visual evaluation of filler dispersion.
All of the testing in this and the following examples were made with the Table III recipes and were evaluated relative to a statistically appropriate batch process mixed control of the identical recipe.
The process of the present sheet production extruder was carried out in these tests utilizing a co-rotating twin screw device of Berstorff and Werner-Pfleiderer types. For this Example II, the process utilized a two mixing zone approach, wherein the polymers, reinforcement agents and processing aids were all admixed together in the first zone, and the vulcanizing agents were then admixed in the second zone. While good to excellent dispersion of the reinforcing fillers was accomplished, process temperatures were unacceptably high with unacceptable production rates. Using recipes 1 and 3, process exit temperatures of 160°-200° C. were measured, and premature vulcanization and polymer degradation was observed at production rates of approximately 310 kg/s/m 3 . In this instance, the mixing extruder length to diameter ratio ranged from 12:1 to 20:1 with residence times of 45-130 seconds.
In a single screw extruder for mixing carbon black with rubber and of the type with a pair of cylinders with feed ports for adding materials to be mixed, a pair of extrusion screws positioned within the cylinders and with elements for shearing, admixing, moving and extruding materials through the twin screw extruder.