Gøðn has too much time on his hands.
A short experiment in melting various kinds of plastic from soda bottles, with an eye toward later casting.

Legend:
(A) toaster oven
(B) water pot
(C) ceramic saucer
(D) cut-off bottle containing melted bottle cap
(E) two friction-fit cut-off bottle containing nails.
(F) Altoids tin. Curiously strong. Curiously Useful.
(G) Stick
(H) Soda Bottle Caps, Red and Yellow
(I) Gasket from bottle cap
(J) remains of 2-liter bottle
(K) milk-bottle-cap
(L) Melted (K)
(M) Melted yellow cap
(N) Melted gaskets
(O) Melted red cap
(P) Melted bottle body
(Z) Leather utility gloves (not shown)

Um... Why? It's always sort of bugged me to have to throw away soda bottles, or at best, send them off in a recycling bin or ship them across state lines to a place where they'll trade them for a nickle. partly because it's a pain, but mostly because anything that I have that much of ought to be good for SOMETHING. So recently, my usual death-cult1 meeting having been canceled, I found myself staring at a trashcan full of empty bottles, and decided to experiment, to see whether any of the plastic involved could be melted down into a useable form, without special equipment.

Protocol: Separate out the assorted bits of plastic, chop up the big peices to manageble sizes, stick them in a small container, and bake in the oven for 3-4 minutes, starting at 250 degrees, and increasing in 25 degree increments until something useful happens. I also tried boiling the bits in water. Bottlecaps and liners soften enough to deform if you then squeeze them, but not enough to be useful. When the plastic starts to collapse and consolidate, mash it into a lump with a stick, try pressing the lump between two boards to see if it will take a shape, then quench the lump in water and see what you end up with.

If you examine your average soda bottle, you will discover that there are four kinds of plastic involved. The bottle itself, the label, the bottle cap, and, inside the bottle cap, a soft plastic gasket that may or may not come out easily. I peeled off the labels and threw them away, for three reasons: (1) there appear to be a lot of coloring agents, and that means ink or dies, and that generally means more than usually toxic fumes. (2) The label is so thin that I fear that it will burst into flames. (3) There's not enough bulk to be intersting anyway. Just for fun, I tried the soft blue plastic caps from a typical gallon water/milk jug, too.

Now, my toaster oven is not an exemplar of thermal precision, and one of the things I noticed is that the "temperatures" things melt at are typically about 24 degrees(F) higher if the thing has been running for 20 minutes or so. The numbers below are the settings AFTER the oven has heated itself. Anyway, these are the results I got:
Clear soda-bottle Colored soda bottle caps Gasket material Milk-bottle-lids
425°F 350°F 300°F 325°F
Polyethylene terphthalate (PET) polypropylene? Low Density Polyethelyne? (LDPE)

Better pictures:

Clear plastic bottle: This plastic outgasses before it starts melting. It's the only one of the four types that produced a smell strong enough to be bothersome, Probably that means it's doing something bad to the air. The plastic isn't clear when it's done, it winds up a frothy white, with carmelized brown streaks running through it. I dunno if the brown streaks are water contamination, leftover plastic/glue from where the label was stuck on, or what. The resulting mass is really sticky, and messy to work with, leaving long strands sort of like melted cheese between the plastic, whatever you're sticking it to, and whatever you're moving it with. It takes a really smooth finish when you press it against glass or ceramic, but the resulting plastic lump is hard, brittle, and tends to have sharp edges and points. It's usable, but dissappointing. Whatever you tru to make with it is likely to need some machining afterwards. The good news is, a big lump of this globbed onto a stick is really hot, so when you quench it in water, it first steams dramaticaly, and then, because the outside hardens first and then the inside, the lump screams as it cools. (Note to self, if my phone is capable of emailing a photo to me, it's probably capable of emailing a .WAV file or something) While I'm on the subject of noise: This plastic left a fair amount stuck to the ceramic saucer I melted it in. I subsequently switched to an altoids tin because I have more of those than disposable ceramic saucers, but the other plastics popped off the metal after they cooled, because I could flex the metal.

In general, I think that the clear plastic could better be used in a really small vacume-forming oven, or just cut up and used as is. (window-panes for dollhouses?) I use a lot of them with the tops cut of and friction fit-together as containers for nails, screws, etc. I go through a lot more soda bottles than peanut butter.

Later: I tried re-melting the clear plastic again, this time in the altoids container. This doesn't work worth a damn. It took forever, and I had to go all the way up to 475°F to get it to melt at all, and it hardened into a hot, gluey, immovable mass before I could transfer a blob anywhere, the high heat cooked the finish on the altoids tin, and I can't get the resulting blob of plastic out, even after cooling. I am incresingly convinced that re-melting PET in a home lab environment is more of a pain than it's worth.

Soda Bottle Caps: I was quite pleased with these. They don't smell as obnoxiously as the clear bottle-body, and they soften at a lower temperature. The resulting mass is stiff and sticky, but not anywhere near as sticky as the clear plastic. The lump stays in lump form without tacky strands, forms well, and peels off the tin easily when cool. This is the cheap-plastic-toy plastic of my childhood. Be warned, however, that if you use the bottlecaps without peeling the gaskets out first, you end up with streaks of slightly softer, translucent blue plastic boogers running through your resulting lump. This is not esthetically pleasing, but it doesn't seem to hurt anything.

I like this stuff, and am soon going to try press-molding it with a putty-knife.

Gasket material: This stuff softens at a lower temperature than the rest of the bottle cap, but doesn't start fuming at the temperatures required for the latter. YOu could use the same settings for both. Melted separately, the gasket material is stickier and a little harder to work with than the bottlecaps, but nowhere near as much so as the clear bottle material. It forms a translucent blue marbled material that's really quite pretty. It's soft as long as it's warm, but turns harder as it cools. (All the plastics end up harder and more brittle after re-melting)

This produces a useable plastic that's fun to look at, but I can't offhand think of anything particular that it's specially good for.

Milk Bottle Lids: This is stickier than soda-bottles, but less so than the gaskets. It forms well, and is opaque, and is a perfectly good lump of plastic. I don't like it as well as I do the soda-bottle lids, but I can't say why. Probably it's because the colors aren't as vivid.

Swiped the following table from Nasa:

Footnote:1: Really, there is no death cult. And anyone who say's there is, isn't in it.

-------More later-------


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