HDPE Bottle Cap Fun Facts. Here I am doing a quick research project using the HDPE bottle caps people have been bringing me to recycle. Meet your average soft drink bottle lid...
Firstly.
All HDPE will have roughly the same specifications. It has a Specific Gravity (SD) of 0.95 meaning if you had a cubic centimetre block of it vs a cubic centimetre (cc) of water, the HDPE would weigh 0.95g and the water, 1g.
That means that the HDPE will float in water, I tested it with my sample - and yes it does.
- I used my flat sandwich press which limits the outer dimensions of the piece I can make in it to around 25cm x 15cm.
- I could place a grid of 8 bottlecaps across, 4 bottlecaps deep. I placed one of those BBQ protector sheets on the bottom first, left the long edge of it leaning up against the lid of the sandwich press, lining up the 32 bottle caps as close together as I could.
- Then I folded the BBQ protector sheet over them, closed the press, and put about a kilo of weight on top.
- Switched the unit on and left it for 20 minutes, then did not open the press or remove the weights but left it to sit for another 40 minutes to cool down.
- That last bit's important.
- For reason A) the plastic is tacky and despite all the non-stick claims for the BBQ sheets, it'll stick and get pulled like toffee if you try to take a peek and you'll end up with a warped sheet.
- Reason B) is that even if you don't try to peek between the sheets, as the plastic cools down it'll shrink at different rates depending where it's getting cooled the most, and this will always put a bow in it. (If you don't believe me, make a small thin patch of plastic sheet, peel it off the BBQ sheet as soon as you're able to, and drop it in a bucket of cold water. Scrunch!)
- By 40 minutes it was still warm but my plan was to put a second layer of bottle caps on, so it was all going back for a second heating anyway. I just eased the BBQ sheet off the top of the plastic (using cotton gloves because it wasn't really all that hot by then - but I recommend good silicone heat-resistant gloves anyway) and added another 32 caps in the same grid pattern.
- I then proceeded as for step 3 again but set my timer for 30 minutes as there's a bit more plastic to heat up.
- At the end of that time I again switched the unit off and then walked away - but for at least an hour this time.
My setup looked like this:
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| That's around 1kg of lead on top. |
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| And by this time I was trying another experiment with other caps. |
In the case of the lower image, the bottle caps are off the larger juice bottles but they're still all type 2 (number 2 in a recycle triangle symbol) or HDPE. I could have used all LDPE (4) but it's more flexible and softens at a lower temperature than HDPE.
As I said, I left the first experiment for over an hour to cool, because it's a bit warm (34C) here so the whole setup was still warm to touch - but also, luckily, the plastic had set solid enough to no longer bow during the remaining cooling.
And the first experiment result was this:
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| As you can see, burnt sugars, and you can see each bottle cap still. |
The result didn't need to be fantastic for my purposes for it so I decided not to waste too much time on it. Washing it using a reasonable washing system I don't yet have, using hand shears to painstakingly cut it all into smaller more uniform chips (and there's another tool I need, a decent shredder...) was deemed to be too much mucking around for a sheet of plastic I was just going to use for in internal panel that will have stuff mounted on it, out of sight..
The dirty looking marks are where there was dried drink on the lids, and because I didn't really care about appearance or strength too much, I left them unwashed. Also, because I figured it out all by myself when my first ever experiment had that same "grotty" look. Drinks (and milk) have sugars in them, the sugar when put in the press and heated gets burnt and "toasted" and also melted into the plastic itself. And that's a problem.
You'll appreciate that whereas with a sheet of clean plastic, I can return all the shavings and trimmings back into the tub to go round again, but once you've got some toasted sugars baked into the sheet, all trimmings will be contaminated with crud you can't get out by washing.
For my project, this is a sub-panel behind a control panel and will never be seen, there's plenty of room to place it with only minimal material needing to be taken off - and I wanted to show you one of my first "gotchas" I came across. You literally *must* wash every piece with water and maybe a small amount of surfactant (detergent) if you want a good clean output. Don't depend on your collectors to do it...
This shows the importance of having some means to wash and dry your plastic well, why a better press than a sandwich press is eventually going to be on your wishlist too, and for the sake of a consistent output, you need something that will shred or flake the plastic to a small consistent size that will form a uniform product.
I've got the materials for a decent wash system, and when I build it the documentation will be here. I'm hoping to find a way to shred/grind plastics after washing and drying, and again, if either I get enough in donations to buy one or build one, the build and use will appear here.
That second experiment
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| ... this seemed like a much better idea in theory ... |
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| ... until you consider the wildly dofferent expansion ratios of plastic versus steel ... |
The idea was to put some fine wire mesh in between, but at this scale it does not work ... The steel mesh is fine when everything's melting, it's still sort of fine as everything's cooling down between the plates of the press, but as soon as the pressure comes off . . . sproing!
It was a first class lesson in first class stuff-ups thanks to my first-class lack of thinking things through.
What I learned from that
This would probably have worked if there was a 10mm - 20mm sheet of plastic involved, but you need a modified Precious Plastic sheetpress and cooling press set A) that I can't afford and B) that would wipe out all the spare space in my workshop and C) would need a separate mains power line in the workshop. My little sandwich press can barely handle 6mm thicknesses before it can't supply heat fast enough, so the contraction of iron mesh even as thin as that mesh was, will always result in a topographical map.
The idea for putting reinfocing inside a panel came to me because I've previously ironed plastic into cotton cloth, which works a treat and gives the plastic greater strength - just like the glass cloth in fibreglass. Because cloth (or fibreglass cloth) doesn't really expand with heat, and will always let the plastic win the shrinkage contest..
As I said, one could possibly stretch a wire mesh across a Precious Plastic sheetpress halfway up, but you'd need a special split edge form with space for the mesh. And it'd leak plastic, resulting in the top side always having sag.
The reason I thought of the wire mesh was twofold, A) as observed above, the strength factor, but B) also the fact that mesh as fine as I was using can also help to screen things from electrical interference. It'd be a great Faraday cage if one could connect the edges together and to earth.
Anyhow - this has to wait until I can test it with a tee shirt press converted to mini sheetpress that can make consistent products. I might be able to make 8mm thick sheets on that with a bit of care.
Re-stocking
We give our 10c Container Return Scheme plastic bottles to a friend's kids, and we keep the bottlecaps because HDPE. But now we've told the kids that if they collect bottlecaps of their other bottles before recycling them for the 10c, I'll pay them a few more bucks. I know, I'm a soft touch. But the kids are saving to buy themselves things and start savings accounts, and I'm happy to help them get some business nous. Plus - I'm slowly getting a 20litre tub filling up with hdpe.
My hope is to one day be able to put collection bins around the place and collect a useful amount of material. But that means buying or making lockable bins, and driving around town checking and emptying bins regularly. And once I have a system, I can donate it to the local Community House and they can use it to make money for their good works. But I won't be doing it with my pension income.
Anyway - like share bookmark donate - I'll take all the help I can get to get to my next goal of getting a tee shirt press I can make into a sheetpress. Your donations make it possible for me to keep my online stuff online and to keep experimenting and presenting my results for others..
