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Innovative blending technology that can replace complex feedblocks and multi-layer dies |
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A structured fluid dynamic mixing process known as chaotic advection causes unusual micro scale and nano scale phase morphologies. This process has been applied for melt blend two or more polymers. The process creates unusual micro-scale and nano-scale phase morphologies that potentially can optimize extruded film properties using fewer materials and extruders than with highly layered co-extrusion. The technology can thus replace complex feedblocks and multi-layer dies and eliminate need for costly tie-layer resins. It can change from one extruded structure to another simply by adjusting the rotation of �stir rods� in the barrel. It is also possible to create films or other extrusions with special properties such as electrical conductivity or additive-release characteristics.
The mixing systems are built by Smart Blending Technologies LLC in USA. The melt streams to be combined are each fed by their respective extruders into the �smart blender,� a barrel or cylinder containing two or more independently rotating stir rods that cause the chaotic advection motion and create a defined melt structure rather than a homogeneous blend. The smooth stir rods rotate at different speeds in specific periodic sequences. The rods� rotation stretches and folds two or more melt streams into a �marble cake� pattern, but on a micro scale. The rods are oriented lengthwise in the barrel and taper at the downstream end to leave space so the structured melt can flow out without disturbing the pattern.
The rotating rods create micro- or nano-layers. A specific rod rotation sequence can create morphologies with hundreds or even thousands of layers. A variety of melt structures can also be derived from the layers, formed by repeated stretching and folding, which break up the layers to form derivative shapes like ribbons, long fibers, and thin platelets, as well as the more conventional morphology of droplets of one material dispersed in the other.
A sponge-like interpenetrating material can result from simultaneous ruptures in a multi-layer melt. Spongy material blends can also be made in a twin-screw extruder, but without as much variety or control over end properties.
Smart Blending�s first two commercial machines each use a different blending-chamber shape and rod configuration, depending on whether the end product is tubular or flat film. The first machine was for cast film, delivered in December 2007 to a maker of packaging film. It consists of two independently driven rods in an oval barrel that is placed between two or more extruders and a modified conventional flat die. The die was built by Premier Dies Corp., USA with a wider than normal in feed and a longer, more streamlined body to preserve the blend morphology created by the stir rods.
The latest and most complex device so far is for blown film, which was delivered in 2008 to another film producer for product development. In this case, the blending device is installed vertically and replaces a blown film die, so the structured blend is extruded directly through an air ring and blown into a bubble.
The device consists of a section to distribute polymer melts from two (or more) extruders into an annular blending chamber. The chamber has six rods mounted between two concentric cylinders. Each rod is independently driven by a variable-speed motor. The rods are rotated in a periodic sequence, determined by computer simulation. From the chaotic advection chamber, the structured blend enters a streamlined flow path leading to the exit lip, where melt is formed into the bubble. This first device has a 4-in. opening with a 4:1 BUR to produce a 16-in. diameter bubble.
Potential polymer combinations include pairs of immiscible polymers or polymers with different surface tension, typically in 80/20 or 85/15 blend ratios. Combinations can include LDPE/EVOH, PP/EPDM, PS/PP, nylon 6/EVOH, or LDPE/PS. Smart blending can also combine similar polymers like HDPE and LDPE into a film 500 microns thick with over 20,000 nano layers, each less than 12 nm thick. More than two materials can be combined into a blown film containing sub-micron-thick alternating layers of nylon 6, nylon 6 plus nano clay, and EVOH. Such a film exhibits barrier properties never before achieved due to platelet alignment and localization of platelets within the numerous layers. All the industrial machines built so far are believed to be under 225kg/hr capacity.
(Source: Plasticstechnology) |
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