Ultra spherical granulation of thermoplastics and compounds by a vibrational dropping process
Thorsten Brandau and Egbert Brandau
Trends in plastic development and processing have recently been moving toward increasingly high quality and free flowing particles. The well approved industrial processes do not always meet the exacting standards which modern manufacturing demands of them due to their varying size distribution and odd shapes. These properties are detrimental to efficient processing and lead to agglomeration, inexact dosage, abrading with loss of material or low reproducibility of castings. The use of small and perfectly round microspheres with exactly the same size circumvents all of the disadvantages that are encountered while using powders and granulates.
The recent development describes a method for producing plastic particles with tailored properties and with a uniform spherical geometry and a narrow grain size distribution.
The liquid plastic is gently pumped through a vibrating nozzle system where upon exiting the fluid stream breaks up into uniform droplets. The surface tension of these droplets molds them into perfect spheres in which solidification is induced during a short period of free fall. Solidification can be induced in a gaseous medium through cooling or drying and/or in a liquid medium through cooling or chemical reaction. Amplitude and frequency of the nozzle oscillation or the liquid oscillation are held constant to attain a monodisperse grain size distribution.
To ensure that the droplets are not flattened on entry into the cooling liquid, i.e. undergo a geometric change, it is provided that the droplets enter at an accurate angle or tangential a liquid layer having one laminar flow proportion. The flow direction should be the same as the fall direction of the droplets in ball form. The cooling liquid has a temperature that is in the range of the Vicat temperature of the plastic droplets.
It is possible to dispose a mushroom-shaped liquid distribution unit underneath the nozzle head, along the surface of which a liquid layer is formed. The mushroom-like arrangement has a diameter such that the droplets enter the liquid layer in the substantially vertical area thereof. An appropriate liquid layer can also be obtained by a funnel-like arrangement with a convex design of the wall faces in relation to the drop distance.
Vibration can be induced via an elastic membrane directly to the small quantity of liquid plastic just before exiting the nozzles. This has the big advantage that only small vibrators are able to vibrate the laminar fluid stream. The operation of a nozzle system of about 300 nozzles needs a magnetic vibrator with a sine force of 20N.
If the nozzle plate has to be vibrated itself it can be fabricated out of Titanium to avoid very heavy permanent magnet vibrators. For high temperature operation and corrosive materials special materials like ceramics or graphite can be used as nozzle plate material.
Types of Production Facilities
Microsphere production units can be designed and constructed from the laboratory scale up to full size production plants. The throughput and the price of the production units vary with the size of the microspheres and the complexity involved in the solidification process.
Based on a sphere diameter of 1 mm lab installations have a plastic throughput of about 20 kg/h, pilot plants about 100 to 200 kg/h, and production units can be installed up to 2 t/h. Microsphere production units have a minimal space requirement (15 to 40 sq. Feet), the energy consumption is very low and they are noiseless during operation. These units operate at atmospheric pressure or slightly above and can be designed to be explosion proof according to the GLP/GMP guide lines. Microsphere production units need practically no maintenance, therefore only a minimal staff is required. Units with fully automated controls and remote monitoring can be delivered as an option.
Applications and Types of Microspheres
Microspheres produced from molten organics and polymers can be used for dosing, proportioning, compounding, coloring and light stabilization. Microspheres with dissolved or embedded active agents, with or without coating or coloring are used for numerous plastic, pharmaceutical and cosmetic products. Plastic materials like polyethylene, polypropylene, polymethacrylates, polyesters, i.e. most of the thermoplastics and the non curing components of thermosetting like novolak and epoxy resins can be granulated to microspheres. Using special mixtures of organic and aqueous solutions polyamides, polystyrene and others or compounds can be transferred to spheres. Microspheres produced by this unique process in the rang of 100 µm and 5000 µm have a monodisperse grain size distribution, are free flowing and roll with practically no friction that means there is no abrasion guaranteeing a dust free environment.