Extrusion: From Properties to Contamination, a Call for Clarity

Allan Griff | Mar 20, 2023

The real old year has ended. November was number 9, December was number 10, January was 11, and Februa(ry) was 12. March is the next moon cycle, full to gone and back to full in a moonth, or month, if you prefer.

The sap is flowing up from roots to the leaf buds, the petals fall down, the wind blows sideways. St. Patrick ends the winter — in Ireland, not Montana — and Monday, March 20, is the spring equinox, when the length of the day (sunrise to sunset) is equal to the length of the night (sunset to sunrise). That means warmth is approaching: Get your seeds in the ground soon or you won't have any food by the winter solstice in December, when the days are shortest. So, who needs month names? The ones we use today were set by the Romans, because the moon did it for us.

But we run our extruders all year because we can. We know how to make electric light and motors, which work day and night, and batteries for power storage, and how to connect atoms of carbon, oxygen, hydrogen, and sometimes nitrogen into long chains. We know how to get useful stuff, like plastics, from these atoms as found in petroleum and natural gas and water and air and (for PVC) salt.

Trees and other plants run the reaction the other way. They combine water and the carbon dioxide that we and all animals breathe out, with a little help from chlorophyll — the green stuff in leaves — and the sun to make surplus oxygen for us animals to breathe in, and new leaves and seeds and fruit, many that we call food, which we can digest to fuel our own growth and activity. Plastics are indigestible but have plenty of other uses.

Chlorophyll contains no chlorine, but each molecule has one magnesium atom, similar to the iron atom in hemoglobin in our blood. Interesting example of how simple isn't always better, and how chemistry is just a code language and not an object of fear and awe. This month I’ll review some of the basics to help understand the unmagic that goes on in our extrusion factories.

Plastics are not "forever," as the molecules will break apart and weaken if they stay too hot for too long. They may discolor, too, especially if they are exposed to daylight for a long time. We can't see ultraviolet (UV) rays, but the plastics can. That's because there is energy holding them all together, and we know how much for each type of bond — C-C, C=C, C-O, etc. Don't ignore this energy when judging a process, as is common among advocates of breaking down molecules as a method of recycling.

Running too fast may stress the cooling system and lead to overheating, thus degrading the resin. What can you do with such a product without losing money or customers?

What is meant by physical properties? Sometimes we don't know and have to deal with professional avoiders who can't say, "I don't know." They may guess right or wrong, or reply with another general term like, "Not strong enough" or "That's what their buyer said." Numbers, please.

These are common but still too-general terms, and need test numbers and acceptable values. Some sellers/buyers prefer fuzziness, as it allows making deals even if the numbers aren't quite right. Such people are seldom in production, where a volt is a volt and a degree is a degree, no matter what you may want. OK, you can confuse Celsius and Fahrenheit, but not very well. In 1776 it was different.

If there is a physical problem, it is easy and often necessary to separate stiffness/rigidity from tensile elongation and impact strength. What are the limits to product thickness/weight? They all can be managed with formulation, which is resin selection plus additives — which ones, in what form, and how much — but the formulator should know the desired result. Imagine playing football without goal lines. Maxing one might be minimizing others. And if you’ve got the material already, what can you do with what isn't used and what are the costs involved? It is not enough to angelize recycling by saying, "As long as it's reused, that's OK." That needs test numbers and conditions as well as results. ASTM and ISO tests are often similar, but if you’re working international you may need to deal with both, as well as other standards that may apply, like NSF or anything unique to a sales contract like +/- limits. Sometimes it makes no difference in service.

This is another incomplete word. A simple microscope may identify it, which may tell you how to reduce or eliminate it. There are many sources, such as insects in the silos, napkins thrown in scrap bins, and degraded bits from overheated melt in the head. Seldom is the extruder barrel involved, as most contaminants can be stopped by the screens at the end of the barrel. With a lot of solid contamination, continuous and self-cleaning screeners may be worth their cost if they allow use of more-contaminated but cheaper resin (recycled ag film, for example). Undispersed additive can be seen in the microscope, too, and particle size and shape matter. Color changes will go through screens, maybe due to overheating, and sometimes are impossible to fix.

This is the fuzziest word of all. There is no established index, and although we think we know what it means, we can't talk about more or less unless we agree on the relative importance of biobased, compostable, degradable (to what?), and quantitative toxicity of plastics and some of the precursors and additives. Too much to get into here, but basic to much plastics promotion today.

About the author

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house, and now in his virtual version. He wrote Plastics Extrusion Technology, the first practical extrusion book in the United States, as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].

No live seminars planned in the near future, or maybe ever, as his virtual audiovisual seminar is even better than live, says Griff. No travel, no waiting for live dates, same PowerPoint slides but with audio explanations and a written guide. Watch at your own pace; group attendance is offered for a single price, including the right to ask questions and get thorough answers by e-mail. Call 301/758-7788 or e-mail [email protected] for more info.

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