To compound polymers with hollow glass microspheres, we recommends the use of a compounding extruder, such as a twin-screw extruder or a Buss Kneader, with a downstream feedport for adding the hollow glass microspheres to the polymer melt. The extruder screw flights beyond this point should be set up to impart a minimum amount of shear stress. Distributive mixing elements, such as gear mixers, are preferred. Aggressive dispersive mixing elements, such as reverse flight elements and kneading blocks, are not recommended. Single screw extruders are generally not recommended because they typically do not have a downstream port and often contain “barrier” designs or other narrow tolerance/high shear features. A system designed for adding chopped fiberglass to a polymer would be a suitable starting point when developing a low shear extrusion system for hollow glass microspheres.
In order to minimize hollow glass microsphere breakage, it is preferable to add the hollow glass microspheres downstream from where the polymer is melted, rather than into the feedport with the polymer pellets. An auger-driven side feeder at the downstream port, rather than a simple open hopper, will provide the most consistent feeding behavior. Although not as critical as feed location or method, it is further advisable to minimize extruder screw speed and to minimize extruder outlet pressure.
Hollow glass microspheres should be fed with an automated feeder. Although a volumetric feeder should produce satisfactory results, a gravimetric feeder – ideally one equipped with a twin shaft – is preferred. It is recommended that the microspheres be supplied to the feeder automatically; for example, using an automated vacuum feed system with a pre-hopper.
Like other fillers, hollow glass microspheres can entrain some air into the polymer during feeding and compounding. If entrained air in the final compounded pellets is undesirable, use a vacuum vent downstream of the hollow glass microsphere feed port.
Any pelletizing method suitable for the polymer of choice is suitable for a system containing hollow glass microspheres. For plastics highly-filled with hollow glass microspheres, it might be advisable to use an under-water strand pelletizer or water slide pelletizer.
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