I have always wondered if the plastic stuff seals off more gas than cardboard wads or loose materials. Made some amazing long shots over the years with them.
In my testing, plastic wads shoot slower than cardboard wads. It wasn't a gigantic difference, but more than I expected. Does that mean one seals better than the other, I don't know. I just know that a card wad will shoot faster than a lead shot wad. Maybe a steel shot wad is a different story, I've not tried them over a chronograph.
I can't believe how many people willingly use scrunched up trash in their guns. I'm not saying it won't work, but you are leaving a TON of performance on the table. If I ever forget wads, you can bet I'll be using a bunch of leaves to make it work, and very likely getting inconsistent patterns. I'll use leaves way before I ever use newspaper or toilet paper, which are not suitable for shotgun wads at all. If everything you shoot is inside of 20 yards, then have at it. There's a ridiculous amount of things that make really good wads out there that you can get for free, to suggest such a practice to a newbie. The guy is already going to be cutting wads from cereal boxes, why would you suggest someone uses leaves, or newspaper, which will not work as well as using the wads he already will be making.
Any kind of cardstock, cereal box, solid boxes, slip sheets etc. all work phenomenally well. Just be sure to use enough so the total stack is at least 1/8" thick.
Leather wads, they work really, really well. Even a single cow leather wad works really well. 2 even better.
Felt works great, but you need to use enough. 1/2" provides a good seal with a dense felt.
Plastic wads do work, they aren't fantastic, but they do work.
Even corrugated cardboard, if you use enough to make a stack 1/8" thick when they are compressed works well enough.
Tow or rope fibers work, as long as you use a sufficiently large ball
What all of these things have in common is they form a solid wad that seals to the bore. Crumpled newspaper does not have this property. Logic says that if you use enough of it, eventually you should get a good seal, but it's not worth the work. Here is what I would do if SHTF, and I had no wads. I would take a bunch of smaller leaves from a tree, and carefully stack them so that they made a stack at least 1/2", maybe 3/4" when compressed. I would then carefully stuff them into muzzle, trying to keep the layers horizontal to the bore. Then finally ram it down. This way does work, but it's not that much fun.