For three decades, microlayer films gave an iridescent shimmer to decorative packaging. But in the past three to five years , these films with several layers are designed for gas barrier, UV blocking, electronic displays and high strength window glass laminates. Commercial microlayer films are cast coextrusions with anywhere from 30 to 1000 layers, each layer from 0.02 to 5 mi­crons thick. The seemingly improbable feat of assembling all those layers is accomplished in a coextrusion feedblock by splitting and stacking a small number of input melt streams, sometimes no more than two or three.

Microlayer films divide broadly into optical films that selectively filter or reflect particular wavelengths of light and barrier films with remarkable strength and flexibility. Some products combine light control and high strength.
Optical films typically use fewer polymers, and more and thinner layers�from 100 to 1000 or more. Barrier films use fewer, thicker microlayers and more polymers, typically using seven to 11 extruders.

There are two basic commercial feedblock technologies for microlayer films. Dow Chemical Co. invented the first in the 1960s; and die maker Cloren Inc. introduced the second in the early 1990s. More recently, Black Clawson has also developed its own microlayer feedblocks, but so far only for R&D uses. Clemson University's new melt-folding technology, called �chaotic advection,� can create a repeatable stack of up to 1000 semi-continuous microlayers in a feedblock-like device called a SmartBlender. There is also ongoing microlayer research at Case Western Reserve University, Cleveland, and the Univ. of Minnesota, Minneapolis.
Dow's feedblock technology, termed an interfacial generator or layer multiplier, splits and stacks microlayers. Layer multiplication is produced sequentially, and Dow can multiply a given layer stack almost indefinitely. Dow's newer versions are said to produce more uniform layers by means of flow-path equalization.
Dow is still active in microlayer feedblock developments. It has a new license agreement with EDI to offer Dow's third-generation technology for microlayer packaging. EDI has a three-extruder microlayer film lab line set up for trials of up to 48 layers. It has run trials with up to 32 layers so far. EDI is exploring potential for material cost savings by using less expensive materials in combinations that perform like a more expensive material.

Cloeren's microlayer feedblocks split the melt flow, and then realign the flows into packets, each typically containing 17 or 34 continuous layers. Packets are then stacked together. Cloeren has made up to 452 layers this way. Cloeren developed an approach different from Dow's layer-stacking technology. A recent Cloeren patent application describes a method for splitting melt streams, creating a microlayer structure in the feedblock. Each stream can be split multiple times. The multiple streams are then aligned vertically and thicker surface layers are brought together around the microlayer composite core at the end of the feedblock. The whole package is then extruded through a conventional cast film die.
Cloeren has sold more than half a dozen microlayer feedblocks for films 2.5 to 3.5 meters wide. Three units went to companies producing EVOH-based films that reportedly have more than twice the gas barrier of nine-layer blown film containing nylon or EVOH. The big advantages beside higher barrier are increased strength and flexibility. Cloeren has retrofitted existing lines to produce microlayers. Retrofitting isn't always possible because these lines usually extrude downward through a vertical feedblock. Microlayer feedblocks are longer than conventional ones, so the extruders are typically placed up on a mezzanine to get the required �drop height.�