Why can’t we recycle plastic in the same way we do for metal? Melt it and remold it?
In: Technology
Plastic processing engineer here.
Plastic parts can be ground and reused (regrind) but options for doing so are limited. Plastic has a heat history, the more you melt and reheat plastic, the more it degrades. As the plastic degrades you lose its physical properties. This means you have to use virgin material (no regrind) for many applications, such as automotove or medical parts.
Regrind also can be more difficult to process, as it is more prone to part defects. Visible burns in the final product or brittleness would be a few examples. Regrind typically needs to be mixed with virgin material during processing to avoid these defects.
The size of the regrind also matters. With virgin material the plastic comes in little plastic pellets, all the same size. When these are melted down they all melt at relatively the same time. With regrind you get bigger chunks and dust. The dust tends to burn while the bigger chunks may not melt evenly. There is equipment to remove dust and more evenly chop parts, but then there is now additional cost to reusing materials.
Then you get into contamination issues. Typically when doing regrind you don’t reuse consumer products, instead you are taking the scrap at the plant and regrinding it there. Even keeping it at the plant level, the chances for contamination is high. If a part is dropped on the floor and picks up dirt, now its in your regrind.
There are many different types of plastic and each may require different processing temperatures. In many cases you cannot even mix different grades of the same material. I may have 5 different types of polypropylene at the plant. each with different fillers or additives that should not be mixed. This creates a logistical problem at the plant, because now you need 5 different grinders and 5 different storage containers for you materials. All it takes is one person putting the wrong part in the wrong bin or grinder and now you have a lot of scrap.
Taking this a step further, this contamination could create machine damage in some cases. Incompatibile materials such as polycarbonate (melts at round 400F) and polypropylene (melts around 300F) mixed together could chew up your machines by running unmelted material through. Mixing materials such as PVC and Acetal can create explosive chemical reactions.
Many common plastics are thermoplastics and can be remelted. There is another group of materials called thermoset. These materials undergo a chemical reaction to harden them. (example epoxy) These materials cannot be reverted back to their original form and recycled.
TLDR: Some plastics can be reused, but process problems and costs make reusing plastics infeasible in many cases.
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