Of purity, danger and extrusion: The importance of screens in plastics recycling


sorting plastic waste for recycling

Purity and Danger is the title of a book by British anthropologist Mary Douglas, who shows the basic instinct of people to trust the definable, the unmixed, the pure, as they see anything else as a threat to the limited predictability of life. This might account for the wearing of white in religious rituals and the repetition of certain words and even foods as comforting habits, distinguishing Us from Them. Douglas is mainly concerned with human cultures, but I extend this to explain the worldwide negative image of plastic, arising from its synthetic, artificial—hence unknown and unpredictable—nature.

Last week, the plastics recycling industry met in Washington, DC, for two days of presentations and an exhibition. The program, exhibitor list and some presentations are available from the sponsor, Plastics Recycling. Since there is an extruder at the end of most recycling lines to make pellets or sometimes finished products, extrusion plays a major role, but it is really a link in a complex chain of operations: Collection, baling, separation, washing, grinding and shredding (not the same thing), sometimes mixing with other ingredients, and finally feeding into an extruder.

There were several makers of all-in-one recycling lines, where the items to be recycled are fed into one end and pellets come out the other. Like all in-line systems, the production rate is limited by the slowest process. The design of the components must take that into consideration, which requires some knowledge of how much material is to be processed (in pounds or kilograms per hour) and its form and status (particle size, cleanliness).

And here is where purity comes in, as there is always an issue of contamination with recycling, especially post-consumer recycling (PCR), the primary focus of this conference and trade show. Even if the resin is not degraded by heat or, in the case of PET, PC and PA, moisture, its performance can be diminished by embedded particles, which can act as stress concentrators (such as in pressure pipe), or just cosmetically in high-end packaging. These particles can be removed by filtration (screening) and the technology is well ahead of most of our needs, thanks to the textile industry. The hard part is estimating the commercial significance of the contamination in order to justify the cost of finer screening.  

Such technology includes laser-drilled plates instead of screens (some with conical rather than straight-through holes); sintered metal fibers; pleated or 3-D screens, which greatly increase screen surface (fewer stops for screen changing); self-cleaning devices, which continuously scrape off or blow back the contaminants to the outside; and other continuous devices that that replace the contaminated screen with a new, clean screen.

The screening decision is also important because of the relation to melt temperature and resin degradation, which may affect color, physicals or both. The finer the filtration, the higher the back pressure in the extruder, the more power is needed from the motor to push through, and the hotter the melt becomes from the dissipated motor energy. The barrel can be cooled, but this is not very efficient because cooling makes the melt more viscous at the barrel wall, which means more work for the motor and, thus, more heat added.

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