Consequently, pure hydrocarbons are the only group of
polymers potentially lighter than water. Although polystyrene
(PS) is a pure hydrocarbon, it is slightly heavier than
water. This is because PS contains benzene rings in its
macromolecule, which generate substantial interaction
forces making PS relatively densely packed. PS comes
with densities of 1040 kg/m³ or slightly higher. Since the
density of seawater lies in the range of 1030 to 1045 kg/
m³ for all open seas and oceans, PS just floats or just
sinks in seawater. Therefore, PS has to be considered as
possibly present in floating marine litter.
Polyolefins are also pure hydrocarbons, but unlike PS,
they are relatively loosely packed due to their linear but
branchy molecules and the absence of benzene rings. As
a result, all pure polyolefins are lighter than water (1000
kg/m³) and therefore float on seawater as well. Exceptions
are polyolefin materials in which fillers are used,
such as chalk or glass fiber. Such filled materials may be
heavier than water depending on the filler content.
Polyolefins are the largest group of plastics, with a share
of approximately 40% of global production, and can be divided
into the following major sub-groups: polypropylene
(PP), low-density polyethylene (LDPE), linear low-density
polyethylene (LLDPE) and high-density polyethylene
(HDPE). The density of pure polyolefins lies in the range
of approx. 900 – 960 kg/m³. As polyolefins are essentially
built of linear chains of C and H only (i.e. –CH2– and –CH3
groups), differences in density within this group are a result
of differences in their spatial structures, as shown
in Figure 9.2.