Synthetic polymers can be divided into thermoplastics
and thermosets. Thermoplastics are referred to as plastics.
Plastics consist of large polymeric molecules, see
Figure 9.1. At room temperatures, the individual molecules
are condensed to a solid phase due to intermolecular
cohesion forces. By increasing the temperature beyond
a critical value – which lies in the range of 180–260°
C for most plastic types – plastics will melt. This property
makes them suitable for recycling into new plastic products.
Examples of major plastic types are polyethylene
(PE) and polypropylene (PP), polystyrene (PS), polyethylene
terephthalate (PET) and polyvinyl chloride (PVC).
Objects made of thermoset material can be regarded
as a single giant molecule, see Figure 9.1. This three-dimensional
network of covalent bonds breaks down into
smaller molecules at elevated temperatures, i.e., heating
will destroy the original thermoset material without
melting first. Consequently, recycling options of thermosets
are limited to energy recovery or to use as filler in
new polymer products after grinding to powder. Examples
of thermosets are polyurethane (PUR) and most types of
rubber (e.g. car tyres). All types of thermosets are heavier
than water, except for foamed materials, such as PUR
foam, which therefore may occur in floating marine litter.
The density of polymers depends on both the atomic
weight of the constituting atoms – e.g. carbon (C), hydrogen
(H) and oxygen (O) – and the spatial structure of
the macromolecules, i.e. how densely the molecules are
packed in the material. The latter mainly depends on the
strength of the interaction forces between the macromolecules
and their shape (i.e. linear or branched). Since
heavy elements in the macromolecules also tend to generate
strong interaction forces that pull the molecules
closer together, all common polymer types containing elements
other than carbon and hydrogen are heavier than
water.