Why does polypropylene float and repel water?

Mainly because of three reasons:

1. Surface tension:

- polypropylene: 29 dynes / cm2
- water: 72 dynes / cm2

There are several scientific characteristics of meltblown polypropylene that cause it to repel water and float. First there is the surface energy of water (72 dynes/cm2) versus polypropylene (29 dynes/cm2). A dyne is a metric unit of force. This large difference is what causes polypropylene to be water repellent or hydrophobic. The surface energy of salt water is between 60-65 dynes/ cm2 and will make the meltblown polypropylene slightly less repellent.

2. Density:

- polypropylene: specific gravity of 0.9
- water: specific gravity of 1
- sea water: specific gravity of  > 1

Then there is the specific gravity as defined as the density of polypropylene relative to the density of fresh water. The specific gravity of fresh water is 1.0 and the specific gravity of polypropylene is 0.9. This simply means that polypropylene is lighter than water and will float. Salt water's density is higher than the density of fresh water. In salt water the meltblown polypropylene will be even more buoyant.

3. The amount of air between the fibers:

The "pronounced buoyancy" of meltblown polypropylene is a result not only of the surface energy and the specific gravity but also of the air that is trapped between the fibers of the sorbent. The fiber packing is so dense that the surface energy of the sorbent will not allow water to replace the air that naturally exists between the fibers, thus the sorbent will not become water-logged. The net result is that an oil sorbent will float indefinitely.

The only thing that may cause polypropylene to float lower than the water line is the presence of a contaminant such as a surfactant or emulsifier in the water. Such contaminants might include soap, detergent, alcohol or chemical dispersant. The presence of these surfactant materials would cause the oil to be at least partially soluble in the surrounding water and would change the surface energy enough to allow water to penetrate between the fibers. The polypropylene will not sink (as to the bottom) but it can hold some water in the presence of the surfactants.

For the polypropylene to sink to the bottom, it would need to be weighted (sand, rock, debris, or heavier than water type liquid) to offset the buoyancy of polypropylene. In open water conditions, polypropylene will float indefinitely, even if fully saturated with oil.