25.2 Distinguishing Between Polyolefins and Poly(Vinyl Chloride)

Chemical Concepts Demonstrated: low density and high density polyethylene, differences between polyethylene and poly(vinyl chloride), differences between polystyrene and poly(vinyl chloride)


Distinguish between the following:

Polyethylenes Small pieces of polyethylene plastics are dropped into a 50/50 mixture of CH3OH and water.
Poly (vinyl chloride)--PVC A hot coppoer wire is poked through a PVC plastic and then reinserted into a flame.
Polystyrene Polystyrene drinking glass and PVC bottle are stepped on.

Observations and Explanations:

    Polyethylenes sink if they are high density and float if they are low density.  Polyethylene produced in fairly harsh conditions (high temperature, high pressure, free radical polymerization) tends to have more branching, resulting in a lower density (~.92 g/mL).  Polyethylene produced at low temperatures with the aid of the Ziegler/Natta catalyst branch less, and they tend to have a higher density (~.96 g/mL).   The 50/50 mixture of CH3OH and water has a density of .94 g/mL.

    If plastics contain chlorine (as PVC plastics do), the test above (called the Beilstein test) will result in a green flame coming off of the copper wire.   This test also works for other halogens.

    Polystyrene shatters, while the PVC bottle does not.   Polystyrene tends to be glassy and amorphous, while PVC polymers make excellent squeeze bottles.