Monday June 17, 2013 | 08:13 AM
Posted by Harold Johnson

Following on from Thursday’s post on plastics that sink vs. plastics that float... why do some plastics sink while others float?

It’s all about density. Density is just how much “stuff” is in a material divided by how much room that “stuff” takes up. Pure water has a density of ~1.00. (Seawater ranges but is roughly 1.025. The saltier the water, the denser the water. In the hyper-salty Dead Sea, a 14-lb bowling ball floats!)

Consumer plastics walk a line on either side of seawater’s density. #2, #4, and #5 are less dense. They float. (As long as sediment and algae doesn’t weigh them down.) #1, #3, and #6 are more dense. They sink. (As long as no trapped air bubbles buoy them.)

Here’s a great little home science experiment you can do to test plastic for yourself. All you need is 5 clear skinny cups or test tubes; long tweezers; vegetable oil, castor oil, tap water, glycerin, & light corn syrup; and some scraps of plastic containers/toys.

Fill each cup / test tube about ¾ full with a different liquid, in the following order: (1) vegetable oil, (2) castor oil, (3) water, (4) glycerin, (5) light corn syrup.

Take a sample of plastic, cut a small scrap from it, and plunk it into tube #3, water. Make extra sure there are no air bubbles stuck to it. Then see whether it sinks or floats.

If your sample floats in water:

Take another scrap of the same plastic and place it in the castor oil. If it still floats, take a third piece and place it in the vegetable oil.

If your sample sinks in water:

Take another scrap of the same plastic and place it in the glycerin. If it still sinks, take a third piece and place it in the corn syrup.

So, what did your plastic do?

Here are a couple experiments of mine. First, the blue plastic:

The chart says this should be low-density polyethylene. And it was. (The #4 plastic cap to a glass milk bottle.)

A second experiment shows a little white & brown chip floating even in vegetable oil:

That points to polypropylene. And it was too. (This was #5 plastic from a yogurt tub.)

For those who like the science of it, here are the densities of the various liquids:

  * Vegetable Oil - 0.92

  * Castor Oil - 0.96

  * Water - 1.00

  * Glycerin - 1.26

  * Light corn syrup - 1.33

And approximate densities of the numbered plastics:

  * Polypropylene - 0.90

  * Low-Density Polyethylene - 0.93

  * High-Density Polyethylene - 0.97

  * Polystyrene - 1.06

  * Flexible PVC - 1.16 - 1.35

  * PET - 1.38 - 1.39

  * Rigid PVC - 1.3 - 1.45

Of course the above list doesn’t include polyester fiber, nylon, polycarbonate, polyurethane, acrylic, etc. (A nice overview of common plastics can be found here.) Still, it covers the vast majority of the plastic things we use today.

So, in your household trash/recycling, what things would float if they got into the ocean? What would sink?

How much of what we’re doing to our seas is out-of-sight/out-of-mind?

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About the Author

Visiting a Maine beach in March 2010, Harold Johnson was shocked by the ocean-borne debris left by recent storms. He grabbed a garbage bag and a camera, and hasn’t looked back.

Since then he has spent most of his free time studying marine pollution, coastal ecosystems, and the mysteries and science of ocean and shore.

Copyeditor and writer by trade, historian and archaeologist at heart, Johnson’s philosophy is simple: Dig below the surface, travel the currents, make the connections, learn. Then share what you learn. He lives in Saco with his wife and young daughter. Follow on Twitter @FlotsamDiaries.

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