Extrusion basics: Would you buy used resin from this man?

The image of “used” carries a lot of baggage. “Would you buy a used car from this man?” is a way of saying that this person isn’t honest. Used needles give you AIDS, and in some cultures a prior divorce is a stigma to remarriage or political position.

Used car salesman

So, it was no surprise when I learned about a buyer of plastic products demanding the use of virgin material only—no scrap. This bothered me, as I’m a techie engineer and not an image-follower. I deal with properties and performance and, ultimately, money, in the form of cost to do a job. That means we have to know enough to define the job—just insisting on “virgin” is avoiding such knowledge.

It’s deceptive, too: Unless the spec states a supplier and product number, the processor is free to buy what he pleases, wherever he pleases, and that isn’t always good. Also, even the same number is no guarantee, as all “virgin” has a range of properties. When I worked in tech service for a resin supplier, our specs for nominal 2.0 melt index LDPE were really 1.7 to 2.4. Any processor who knows his melt index should be aware that a 1.7 will not run like a 2.4. Now if you were buying enough, we might keep a closer eye on the melt index and ship only between 1.85 and 2.15, but that was neither official nor guaranteed. The buyer had to be able to deal with variation no matter what the spec.

What are the virgin-ians really worried about? Three things stand out.

Pre-processing. No matter whether the feed includes off-spec product—sheet and forming trim, pipe that is too thick or thin—or compound with color or other ingredients added by a compounder, that portion of the feed has gone through another heat history, and some chains will be broken and toughness and tensile strength of the product may be lower. But that isn’t always bad. More—like faster and earlier—isn’t always better. Know how much tensile and impact you need. Maybe even a drop in these properties will still make a good product, so you don’t have to pay more to get what you don’t need.

There is other good news, too. This degradation is sometimes avoidable or reversible. Chains can be lengthened by crosslinking, or the breakage discouraged by using more antioxidants or lower processing temperatures. In some cases, the material can even get stronger just by heat processing, if cross-linking occurs faster than chain breakage. (I proved this once with HDPE in a law case, where the other side was accusing us of using “scrap.”)

With condensation polymers, such as PET, polycarbonate and the nylons (polyamides), moisture will break the chains at processing temperatures, so super-drying (not just hot air) is needed. There is a lot of knowledge available in this area for anyone processing these materials. There are additives that can join chains to regain viscosity and strength, and vented barrels that may eliminate separate drying, but for PET, recrystallization (“cooking” in a hopper-type vessel) may still be needed. Don’t confuse crystallizing with solid-stating. Solid-stating is also a cooking of the PET but at different temperatures and times, connecting the broken chains and thus regaining viscosity/strength. The added equipment and operating cost may be justified by allowing purchase of cheaper resin with shorter chains (the scrap), and no need to buy additives.

Color and contamination. The more pre-processing, the more the color will shift from clear or white toward yellow and brown. This may matter in a color-matched product for a vehicle or appliance, but not in irrigation pipe or ag film. As for contamination, the same appearance needs apply, plus the worry that a contaminant particle will be a stress concentrator and cause early failure. Where this matters, such as pipe under pressure, make sure screening is adequate (see my PlasticsToday column of March 2019) and keep scrap bins well-marked, clean and covered.

Material brought in from outside, whether processed by a compounder (check what screens they use) or post-industrial scrap or PCR (post-consumer resin). There is an old Russian proverb—trust but verify—made famous by Ronald Reagan in his dealings with the Soviet Union. Actually, this is a basis for science, too: Believe what you see, not the other way around. Watch out for molding materials (high flow). They won’t behave well in an extruder (and vice versa), but usually can be run (try a lower melt temperature), especially if the price justifies the inconvenience and the product does its expected job.

“Verify” means test what comes in. It may help to tell the source what you will test for and what you expect. Often a melt indexer and a density column are enough. Infrared and thermal (DSC) are the next step up. You may also want tests relevant to your product, like chem resistance or tensile. A little lab extruder can sometimes replace or supplement a melt indexer; it’s not ASTM/ISO, but it does give processing data like melt temp and pressure and shows consistency. Test enough samples to confirm the result, typically five, a minimum of three. And use the accumulated test data to establish precedent and range (see last month’s column on fishiness).

So, the moral of this story is to buy performance, not image.

And in case you forgot since last month, plastics are not— none of them—toxic. They have the image of a “necessary evil” because they are synthetic (man-made, it’s us we don’t trust) and based on chemistry, a science that challenges our need to believe the impossible. I’ve been saying this for a long time, getting into serious discussions with strangers, already boring to my family, but still waiting for our industry to look more deeply into these origins, especially the “impossible” part.

Image: www.freund-foto.de/Adobe Stock

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. He wrote the first practical extrusion book back in the 1960s 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].

Griff conducts live seminars across the country. The next ones are scheduled for the fall in Atlanta, Toronto and Indianapolis. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen any time, any where. E-mail Griff at the address listed above for more information.

His recent webinar, What All Extruders Should Know, is now available on demand. Watch the free webinar at your convenience by clicking here.

 

Comments (0)

Please log in or to post comments.
  • Oldest First
  • Newest First
Loading Comments...