# How to determine if your plastic is HDPE



## dracb

I went looking this afternoon for HDPE in the form of cutting boards at "Good Will" stores, Dollar stores and similar. There was very little plastic that was marked with the triangle around a "2" and most of that was in cases for used tools. There was an abundance of white plastic cutting boards from China but no real indication of what polymer they were made of. They were cheap enough that I bought a couple with the thought of trying to weld them together to get the thickness I need. On the way home I started thinking of how I might quickly and safely prove the polymer these boards are made of. The scheme I came up with was simple, very quick and safer than burning tests.

Common polymers have a wide range of densities. So by dropping a chip of the polymer in question into a glass of water one can separate HDPE (density 0.95-0.96), LDPE (0.92-0.94)and PP (,0.92) which will float from the denser polymers which will sink in water. Recovering the chip of questionable polymer drop it into a glass of rubbing alcohol (70% isopropol which has a density of 0.92) . If the chip floats it is PP. recover your chip of plastic and next drop it into a solution of 2 parts rubbing alcohol to 1 part water to get a solution with a density of 0.946666666666666666666666667) if the chip sinks it is most likely HDPE (density >0.95) where as LDPE (density0.92 to 0.94) will float. If you are still unsure change the rubbing alcohol/water solution to a 1:1 ratio to get a density of 0.96 and the plastic chip should float if it is HDPE.

With small chips this can be accomplished very easily starting with:

a. a table spoon full of water for the first test

b. two table spoons of rubbing alcohol for the second test

c. Mix the water and alcohol together for the third test

d. Add another spoon full of water to the mix if you want to run the forth test to see the HDPE float proving a density between 0.946 and 0.96.


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## Urban Fisher

This is awesome info! Not sure how to confirm it unless I run each one through the same test...but I'm way too lazy for that. So I guess I believe you. 

But a good thing is that I am also super cheap! So I don't buy my HDPE. If it's not clearly marked, then I don't want it!


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## dracb

The HDPE sheets available at the local plastics distributor are either white or black. One could get bored after a while with just three colour choices. But! There are lots of pretty coloured plastic materials out there in manufactured products that do not have any markings on it to show which variety the plastic is. Most of the new China sourced plastics when marked are PP. Howerver second hand stores seem to have an abundance of tool cases of various colours some of which are marked as HDPE but again the brightly coloured ones seem to be unmarked. Coleman coolers come in some nice bright colours some of which are marked HDPE and others not. Some of the richest and most unique colours are found in bottle caps. Who knows what those are made of or even if the manufacturer changes the source plastic periodically. I would be loath to spend hours making up a blank from odd materials thought to be HDPE only to have the crafted piece fail due to some of the scrap not being HDPE.

With the suggested system each and every bottle cap can be checked in seconds to dramatically reduce the chance of mixing non-HDPE plastic in your blank. Literally it is snip snip with wire cutters to clip out a triangular piece of plastic with out any fuzz that might trap an air bubble. drop into the 2:1 alcohol mix and if HDPE it should slowly sink. Recover the chip and drop in the 1:1 mixture and it should barely float. It can almost be done in the time it took you to read the last two sentences. You have now proved your plastic has a density between 0.9466666666666666666666666667 and 0.96 which is quite a narrow range and it means you may discard some very low density HDPE while ensuring that you do not include LDPE in your batch of plastic. With the exception of LDPE the other polymers are either less than density 0.92 or greater than 1.2. These are substantial and significant differences. No system except chemical analysis is perfect and safe but short of park benches and such products manufactured from recycled mixtures of polymers this system should minimize the chance of ruining your batch of scavenged HDPE.

As an aside I did find large sheets of an unknown plastic being sold as cutting boards at the local restaurant supply. These come in six or seven different colours and various thicknesses. If these prove to be HDPE this will dramatically improves the availability of coloured HDPE for making sling blanks


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