This article presents just a few examples of specialty mixing equipment used in plastics processing. Mixer selection is always formulation-dependent so partner with a reliable mixer supplier offering testing resources to validate a particular mixing strategy. For established processes, a careful re-evaluation of the mixing procedure will often reveal achievable steps for reducing not only cycle time and energy consumption but also waste/rejects, labor cost, cleaning and maintenance.
Polyvinyl alcohol solutions
Polyvinyl alcohol (PVOH, also referred to as PVA) is a versatile polymer that plays multiple roles in the manufacture of adhesives, sealants, cosmetics, paper, textiles and many other products.
Multi-Shaft Mixers are widely used in the preparation of PVOH solutions. Equipped with two or more independently-driven agitators working in tandem, Multi-Shaft Mixers deliver a robust combination of high shear agitation and laminar bulk flow within a wide viscosity range: from water-like consistency to several hundred thousand centipoise (cP).
The most economical design is the Dual-Shaft Mixer which features an anchor agitator and a high speed disperser. Turning at tip speeds around 5,000 ft/min, the saw-tooth disperser blade creates a vigorous vortex that quickly wets out dry PVOH powders or pellets as they are added into warm or room-temperature water (a defoamer may be pre-blended into the water). The sweeping action of the low-speed anchor agitator enhances the exchange of materials within the vessel, continuously “feeding” the disperser blade with fresh product.
For added versatility and shear capability, a rotor/stator assembly may be supplied in addition to the disperser blade and anchor agitator. This Triple-Shaft Mixer design is particularly ideal for applications where other raw materials are dispersed into the PVOH solution and a very tight particle size distribution is required in the finished product.
Formulated for a wide variety of coating, molding and screen printing applications, plastisols are dispersions of polyvinyl chloride (PVC) resin in a liquid plasticizer. Fillers, additives and colorants are also typical components of plastisols.
Multi-Shaft Mixers are well-proven equipment in the manufacture of plastisols. But formulations that undergo very high viscosity peaks (above 1 million cP) are better prepared in Planetary Dispersers and Double Planetary Mixers. These machines consist of two or more blades which rotate on their respective axes as they revolve around the mix vessel. Unlike the agitators in a Multi-Shaft Mixer which rotate from a fixed axis, the blades in a planetary-style mixer continually advance into the batch and are thus able to continuously contact fresh product.
Polymers and additives
Polymer additives such as lubricants, UV and heat stabilizers, colorants, preservatives, flame retardants and impact modifiers are commonly supplied as dry powders or pellets which are blended with virgin plastic feedstock prior to compounding. Thorough and accurate mixing is necessary to achieve a consistent finished product.