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Forklift with waste for recycling

On Extrusion, Recycling, and the Purity Test

A high potential for contamination in the recyclate makes melt screening, or filtration, critical. Fortunately, there are a lot of options.

I attended the Plastics Recycling Conference in Nashville about a week before tornadoes tore through Tennessee and COVID-19 had scared everyone away from airports and airplanes. I am a driven recycler — I just spent an hour yesterday at a city recycling center to see how the system is working in California, now that the cost of virgin resin has dropped below some confirmed recycle. The recycle sells because buyers want to say — or are obliged to say — that their products contain it.

I have no problem with this. If people are impressed enough to pay more for it, or to vote for people who reflect their fears, that’s the market and the representative system at work. My problem is when I hear that recycling — any recycling — is always better for our health or environment. Maybe yes, maybe no. Science is seldom so absolute, even if people often are. I need a metric, and energy is all I can find. If we have reliable life-cycle energy numbers (not so easy), I find some recycling, such as two-liter PET bottles and most cans, saves energy. Glass, however, is indefensible: It’s heavier, needs cases for protection, requires more fuel to transport, represents a smaller load per truck, and must be reheated and re-melted to be recycled.

Also debatable is chemical recycling, which is now called “advanced recycling” and involves breaking the bonds that hold the monomers together and making new polymer chains from them. This has to be energy-negative, as the same bonds that are broken must be reformed, which is an energy break-even. Then, we have to add the energy costs of collection, cleaning and separation. Pyrolysis, where some, but not all, of the chain bonds are broken, is a compromise, but there is so much desire for this “green” option that no one talks about energy.

As for extrusion, that is needed for almost all recycling, but it’s the easy part. Many companies offer appropriate extrusion equipment, and you have to know what the feed looks like and deal with washing, drying, and grinding. Those are the easy—but also dirty and sometimes too expensive—parts. The hardest parts are collection and separation, consistency of feed, and, of course, selling the product or reusing it in-house.

Image: Franz12/Adobe Stock

All recycle is not the same before the law, even if the “law” is what your buyer puts on its product or whether it is legally mandated. Is edge trim from your sheet production recycle, and does it count in the percentage you declare? It does have a second heat history, which may or may not matter in the end use. If deterioration in properties occurs, is it still all recycle if we use helper additives like antioxidants, crosslinkers, or chain extenders to restore or even exceed original strength? Or if we use purple dye to mask yellowing? Or blend it with compatible virgin, or even incompatible virgin plus compatibilizer?

Then there is the difference between post-industrial and post-consumer recycle. Known post-industrial should have more value if consistent in viscosity as well as appearance. Purchase should have such specifications, which is easy to say, but also easy to forget in the face of a lower price. Buyers should test incoming materials, anyway, so they can see problems ahead and deal with them, if they can.

With such a high potential for contamination, melt screening (filtration) becomes critical. Fortunately, there are a lot of options, ranging from no screening at all — clean feed, less critical end use — to very fine screening, a way of adding more value to the material. The finer the screening, the faster the screens will clog, and the more often they will need to be changed.

Some screeners continuously sweep the screen and remove contaminants; others have two parallel paths so one can stay in use while the other is changed; some are completely manual while others have hydraulic or pneumatic assist or quick change; and some run a continuous strip of screen or ring of screens that move around so resistance (thus back pressure) remains more or the less the same. Screening also requires attention to screen material, weave (square, Dutch), wire thickness (gauge), mesh (wires per inch or centimeter), and use of a screen pack (a sandwich of several screens of different or the same mesh and wire sizes). The heat stability and viscosity of the plastic matter, too: If the plastic can take more heat, the screens can be left in longer. Greater contamination buildup increases back pressure in the extruder and draws more power from the motor, which may generate more frictional heat in the screw.

With good screening, conventional pelletizing systems work, and it may even be possible to feed the melt into a finished-product die (pipe, sheet) and avoid another heat history. Beware of differing line speeds before committing to this, as sometimes it is better to make pellets on a big dedicated line, and then use them later on smaller lines, or pack them for sale.

All thermoplastics are recyclable, and all are non-toxic and, therefore, non-polluting, so it doesn’t matter if they last 1000 years or six weeks. In fact, I distrust degradables, as the decomposition products are unpredictable, even if they do end up as water and carbon dioxide. However, people are still plastophobic because of plastic’s connection to fossil fuels (as are all materials), its synthetic origin (natural isn’t always good for you, e.g., viruses), and its basis in science, which must deny the miracles we need for functional sanity. I know you’ve heard this before — like every month — but it still is way below mainstream. If you agree, talk to others. If you have questions, ask. And if you don’t agree, even more reason to write or call, as maybe we can learn from each other.

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 algriff@griffex.com.

Griff conducts live seminars across the country, both public and private. None are scheduled for the next few months, but they will resume once the risks of travel have subsided. In the meantime, Griff offers a Virtual Seminar, which can be seen any time, any where. The Virtual Seminar covers the same material as the live seminars and it can be shared with people within the same company. The content can be adapted to specific needs. E-mail Griff at the address listed above for more information.

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