...but sometimes you can. I was reminded this afternoon of something strange that I saw many years ago. This company was running polypropylene and wanted it just a little bit softer, more like polyethylene. Basically, they wanted a PP copolymer (propylene polymerized with a few percent ethylene), but they were too cheap to pay for it. So they did something that I would never had thought to do: added some PE pellets in along with the PP.
This is where every cell in my brain started screaming "You can't do that!" The two polymers are not compatible, they have different crystallization patterns and cells, there is no reason that anything good should have come from it, but somehow it did. I still do not understand it to this day, other than to assume that the added PE was in such a small amount to not cause a problem. Somehow it worked. The tensile properties were what they wanted. They were happy.
But that's not all that had challenged my ability to bite my tongue. There was one other offensive item: THEY DID THIS ON A SINGLE SCREW EXTRUDER!
A single screw extruder is a piece of equipment known for poor mixing capabilities. A twin-screw extruder would have been a much wiser choice, as the intermeshing screws are able to open and close volumes and thereby mix materials. For an analogy, think about using your hands to mix spices or herbs into ground meat. You can maybe get by if you use just one hand, but it will be so much more effective if you use two. And so it goes with screws in an extruder.
Again, somehow this all worked for them. Don't ask me how.