On Extrusion, Recycling, and the Purity Test


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.

Forklift with waste for recycling
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.

Original Source


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.