So I have a PhD in mtls science, and I studied a bit of plastics thermo. However I never worked on the recycling problem.
Plastics, or polymers that aren’t cross linked, are in a sense easy to recycle - Just melt them and recast the part. 3D printing hobbyists even recycle their filament.
The problem is that any melted plastic doesn’t mix with any plastic other than itself. So if you have 99% polyethylene and 1% polypropylene and melt them together they won’t mix. But they won’t separate perfectly either (so you can’t skim the PP put).
Instead you get inter-penetrating phases (a spinodal). These phases have surfaces that don’t stick well together so the end result is that any parts made of this are exceptionally weak.
So this is the dilemma: if you have even 1% of different polymer in your mix, the plastic will suck. But we use at least 4 or 5 different plastics in everyday life all chucked into the same bins, all looking more less the same.
Solutions?
1. One solution is to heat all the plastics so that they break apart and therefore use them for feedstock. But this is very expensive, and the feedstocks aren’t very pure.
2 Make cat fuel. Cheaper then above, since the purity requirements aren’t as strong, but it’s basically the same as number 1. This doesnt increase CO2 emissions since the plastic is displacing oil
3. Burn it for energy. This needs far less strict requirements than 2 and lower CO2 emissions (since you don’t have to waste energy getting oil out). However, you’d have to basically ban public use of perfluorinated polymers or make them very expensive, or make them easy to identify (Teflon, gore-Tex.)
Frankly I’d ban perfluorinates except for use in critical industries. That chemistry is ecologically nasty.
4. Force companies to color their containers according to plastic type. I.e no more clear Pepsi or Coke bottles - all PE products are color, say green. All PP colors are red, etc.