European process makes sturdy posts from `dirty' plastic
STURDY plastic posts driven into the sand form barriers along West Germany's north coast, in an experiment designed to arrest accelerating beach erosion there. If successful (and early indications are that they will be), many similar barriers will be placed along the coast - and a mountain of mixed, dirty plastic will be removed from the German waste scene.Skip to next paragraph
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In Switzerland, vine stakes made of such waste are being tried. If acceptable, they will help that almost squeaky-clean nation to become cleaner still.
What is significant about both the erosion barriers and the stakes is that they shouldn't even exist, according to a long-popular belief about plastics. These products were made from mixed post-consumer plastic - unsorted, dirty waste that contains some paper and glass as well as soft metals like aluminum and copper. Until recently, blending these diverse wastes was considered impossible.
The standard reasoning was that - apart from undesirable contaminants like oils, paints, traces of congealed milk, and so on - the differing chemical components of various plastics salvaged from the waste stream made them incompatible. Only by costly separation of the types would effective plastics recycling be possible.
But Mike Baele (pronounced Bahl), an engineer at a Swiss-Belgian company with an English name, Advanced Recycling Technology Ltd., refused to accept the negative view.
``Where is the [scientific] literature showing where someone tried and failed?'' he would ask. No one could show him any, other than the facts that various resins melt at different temperatures and react differently when heat is applied.
This meant that conventional melting of the plastic would not be acceptable. So his company came up with an alternative heating system considered the key to mixed-plastics recycling: A screw-type feeder uses friction to heat the waste just enough to homogenize it before it enters the extruder.
No one is certain what happens to the finely ground plastic in the feeder, but the theory is that softer, film-type plastics melt completely, while harder types soften almost to the point of becoming liquid, so that all the resins combine to form a solid piece when extruded.
``We don't yet know how the resins bond together, all we know is that they do,'' Mr. Baele says. Extrusions can be made in various profiles - rectangular, round, even star-shaped.
Items with critical structural requirements could not be made from a mix that might vary from batch to batch. But a range of products, from fence posts and stakes to pier poles and bench planks, is possible.
Some of the first fence posts and lumber fabricated show no sign of degradation after more than five years in use. In effect new plastic with a short life span is being recycled into plastic with an indefinite life span.
The product can be nailed, sawed, screwed, and glued just the way wood is. Coming as it does from mixed plastic, it is not quite as smooth and good looking as the lumber being made from pure, high-density polyethylene (milk-jug plastic) in Chicago (see Ideas page, Oct. 4). But it is cheaper to make and usable where appearance is less critical. In lumber-short Europe, agricultural applications are thought to be particularly appropriate for the plastic extrusions.