Vacuum Web Coating

This-film technology that attracted my attention uses the layering extrusion technique to produce a series of thin barrier layers that can then be incorporated in the centre of a standard film material. 

Barrier materials can have advantages when used as separate layers. If there is an easy diffusion path through one layer it does not usually align with the easy path in another layer and the closer these layers are the more effective these layers will become. Hence increasing the number of layers and thinning them down can have the advantage that for the same amount of material the barrier performance can be significantly increased.

The system described gave the options of using nanometer fillers such as clay flakes, or an antimicrobial, or an oxygen scavenger to be incorporated into one of the layers and linked by an adhesive tie layer to a high barrier material such as EVOH.  This would be extruded as a 5 layer  active layer / tie layer / EVOH / tie layer / active layer  multilayer extrusion that was passed through a die that divided the extrusion and layered it so that it could become a 20 layer version of the same materials.  This multilayer material then was coextruded as the core between two layers of polypropylene to become the barrier sheet.

The clay flakes can vary in type, shape and size. When they are extruded the flakes naturally align themselves along the easy axis and so will be aligned parallel to the film surface. To produce a good barrier with these flakes they need to overlap each other so that anything wanting to diffuse through the film has a tortuous path. To get many layers of flakes to give a more tortuous path the flakes need to be very thin whilst maintaining a high aspect ratio. Ideally they also need to be transparent to be able to maintain the film clarity.

Although this technology looks to offer some potential advantages I do have the concern that in an age where there is a big push towards recycling of polymers this is providing a solution that makes recycling more difficult.  Many of these multilayer extrusion solutions prevent any recycling because of the difficulty or cost of separating or chemically breaking down the multilayer structure.  In some respects this can be seen as an easy solution rather than the best solution to the problem.