Recently a new system called “SmartBlender” has been developed. This system uses a principle of fluid dynamics known as chaotic advection to fold a masterbatch or other components into a matrix polymer. Layering or folding materials together form a variety of controlled and repeatable polymer morphologies; from layers, ribbons and platelets to spongy interpenetrating structures. Unusual properties are said to result.

Among these novelties is “directional conductivity,” in which blends of LLDPE with a carbon-black masterbatch show electrical conductivity in the machine direction, but not the transverse direction, when extruded into film, sheet, tube or rods. Chaotic advection creates linear striations of carbon black particles in the machine direction, which act like tiny wires. The striations can also be cross-connected to give conductivity in the transverse direction or through the film thickness. Computer simulations show that these striations are really spirals with projecting filaments, the spirals being arranged parallel to the axis of the “SmartBlender”.
Clemson’s striated blends reportedly achieve a much higher level of conductivity with less carbon black than conventional compounds. Tests show that as little as 0.5 wt% carbon black mixed via chaotic advection can achieve a level of conductivity comparable to about 3% carbon black with conventional twin-screw compounding. Because a twin-screw disperses additives evenly, it takes a lot more carbon black for the particles to be close enough to touch each other and create a continuous electrical path.

Chaotic advection creates the spongy morphologies by stretching and folding thinner and thinner layers of LDPE in the PP matrix. After repeated layering, the LDPE layers become so thin they eventually rupture, letting the PP flow through the holes in the LDPE. Holes also form in the PP layers, creating a fibrous spongy structure out of the stiffer PP. Other potential applications for chaotic advection include materials with improved tensile and barrier properties and selective permeability.

The SmartBlender is fed by two 22mm diam. single-screw extruders, each of which has a metering pump to give precise control of the ingredient ratio. The two melt streams enter a crosshead die from opposite sides, then pass into a distribution head. The matrix material passes through a single central port, while the secondary resin or masterbatch goes through nine small ports arranged in a circle around the central one.
From the distribution head, the material enters the cylindrical blending chamber, which is round at both ends and oval in the middle and externally heated with multiple zones. The chamber contains two 22-mm-diameter stirring rods, which are slightly offset (15 mm) from the chamber’s central axis. The rods are turned independently by stepper motors with computer control of the direction, speed, and number of rotations.
According to one mixing recipe, the rods corotate, but one of them spins three times faster than the other for a specific number of turns. Then it slows down and the opposite rod turns faster for a specific number of turns. Changing the rod rotation protocol can produce blends of differing morphologies without any equipment modification.

(Developed by the Dept. of Mechanical Engineering at Clemson University in Clemson, S.C. USA with National Science Foundation support)