Building a bottle top pyramid

This is from the archives from paul regarding the bonding of recycled plastic bottle tops (300,000+ to break the world record) for the largest pyramid
Starting at 100 x 100 , then on top goes 99 X 99 , then 98 X 98 ... you get the drift

"Your project sounds exciting, but does have some technical challenges. Polyethylene (numbers 2 and 4) and Polypropylene (number 5) are both very non-polar solids (ie they like oil/don't like water) and as such are very hard to make adhere to anything (but their non-polar nature makes them great for packaging of household and industrial chemicals).

I did a Google search on "polyethylene to polyethylene bonding” and did find some useful information, including some companies advertising possible adhesive solutions, which involve pre-treating the surface (which would increase polarity), but the general conclusion is that welding would be best (and this would also not affect recyclability).

My suggestion would be to do some experiments with basic spot welding ie heat up small sections of two caps such that they melt (using a very localised heat source) and then push them together."
Thanks Paul

Other peoples thoughts , Was also looking at the idea of the worlds highest single stack of recycled plastic bottle caps.

I have found bonding the caps extremely challenging. Having attempted it using numerous glues. the results of which are outlined below:
Super glue- no
Aradlite - 2 part epoxy - No
Hard as nails - No (moderate but not good enough).
tarzan grip- no
Gell bond- no
PVA- No
Silicone- Minor success but no.
Xylene (painted)- No
Xylene + heat (painted + butane blow torch) - Yes (but only a flakey bond).

Welding as stated above works extremely well and can be done using a very small copper blow torch pipe (similar to brazing) from a butane lighter or a soldering iron. A soldering iron although a bit tedious works and we have bonded numerous caps with success.
-Ian @ Thomastown.