View Full Version : Density: Why Does PET Rise to Surface of Tap Water

10-08-2012, 04:51 PM
Scraps of what are believed to be made of polyethylene terephthalate (PET)
rise to the surface of tap water (at about 20C/68F) here.

PET has a density in the area of 1.38 g/cm, while the maximum density of
pure water is 1.0 (at 4C). Even salt water still has lower density.

Is the only explanation that the scraps must be something other than PET
possessing a density of less than local tap water?

10-08-2012, 04:56 PM
I suppose, If the surface is torn and fractured by machining etc. it could be holding onto enough gas to lower the apparent density ???

10-08-2012, 05:04 PM
The scraps are cuttings taken from molded bottles - perhaps 25 mm.

I push them down into a basin of water, release them and they rise
back to the surface. There doesn't appear any bubbles attached
to the surfaces - I suppose that the sample is small enough and the
delta between the densities close enough that it wouldn't take much
gas to induce bouyancy.


10-08-2012, 05:22 PM
If they are floating (just barely) on water they are made of Polypropylene (SG ~.97). Soda bottles are usualy made from PET and a lot of other food contact items are made from PP

10-08-2012, 05:25 PM
It is probably not virgin PET. When they recycle they are allowed to have a certain percentage of mix of compatible plastics. Low density Polyethylene is a likely suspect with a density of about 0.9. I know from experience that bottles do not act the same as pure PET when they are melted.

Mike Burdick
10-08-2012, 05:26 PM
Could it be...

(material) (density)
HDPE 0.952 to 0.965
LDPE 0.917 to 0.940

10-08-2012, 05:37 PM
ckalley & Evan, Thank you for the insights.

PP or defiled PET - either material would explain why the observations are at odds with
what I expected to happen.


10-08-2012, 05:54 PM
Could it be...The characters 'AP28' appear embossed on the material, but this
may be a vendor SKU ID. A brief search didn't turn up any meaningful
material identification.

At the heart of it, the speculation already provided is sufficient.
The inquiry is non-critical and it is enough to know that the material
is suspect, not my reasoning or methodology.


Rich Carlstedt
10-08-2012, 10:53 PM
You didn't say what kind of bottle it was ?
As Evan pointed out, they now recycle bottles and your water bottle is not virgin PET.
The cut or shear edges hold micro bubbles, so burnish them to assure that is not the reason.
You could also add some surfacant to the water to further eliminate air contamination.

Jaakko Fagerlund
10-09-2012, 12:53 AM
And you can always do a burn test to see what plastic it (mainly) is.

Grind Hard
10-09-2012, 02:39 AM
HDPE and PET are not compatable from a melt processing standpoint. They are to a point but bottles are blow-molded and from what I know of the process anything that interferes with the melt-strength or integrity of the parrison (read: blank) results in severe issues such as blown out ends and ruptured sidewalls. I polished and repaired some stretch-blowmolding rods a long time ago for a small-time custom bottling company here... I went to the plant to watch the machine run afterwards.

PET is kept seperate and recycled with it's own kind. Now where it gets interesting... there are hundreds of "brands" of PET and dozens of sub-formulas of each type of material, with their attendant additives, clairifiers and colorants.

Most plastics don't alloy very well, which is why recycling of the materials is a daunting challenge. Then you get into multi-layer co-extruded films and bottles made from multiple layers.... It's a real mess.

I'll stick with metals and leave plastic to my brother. Metal is far simpler to work and understand from my POV. :)