Only one type of plastic does not float *in salt water at least*, and that is type I PETE plastic, the hard clear kind that drink bottles are made of. It is the most abundantly manufactured kind, and it does not float.
Types of Plastic:
1 PET
2 HDFE
3 PVC (rafts)
4 CDPE (bags)
5 PP
6 PS (polystyrene)
7 Other
I posted once about the
Great Pacific Waste Dump, basically just parroting media hype. It turns out the plastic in the ocean is mostly in tiny bits instead of in a big island of capped bottles. It is thickest in the five GYRES on the planet, which appear to me to be doldrums where there are no tradewinds or strong currents. The most directly alarming thing about the litter of plastic bits is that it is covered in life that is migrating in a whole new way. Barnacles, biofilm and plankton all hitch a ride or get tangled in the mess. We had NO IDEA what this is going to mean in the long run. A new name has been coined for all the microorganisms on the polypropylene and polyethlene in the ocean: the Plastisphere. The only organism named by Emelia DeForce PhD in last night's Science Pub talk was Vibrio, the same genus as cholera. I was dying to ask if MRSA was on the plastic around Hawaii but we left because the line was long and we were done. All the factoids in this post are courtesy of Dr DeForce.
Plastic is made from crude oil into nerdles (sp?), which are small balls of hard petroleum product. Those can then be shipped to the manufacturers who combine them with additives and make their product.