Types of Plastic
While many types of plastic are made and are possible, the vast
majority of industrial plastic production falls into six main types: polyethylene (PE),
polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PUR), PE terephthalate (PET),
and polystyrene (PS). We explore the alphabet soup of plastic goops below. All numbers
shown are approximate, and there can be ranges for melt temperatures based on stabilizers,
impurities, and other variables in the making of the plastic.
The major polymer produced worldwide, PE is generally categorized as High Density
Polyethylene (HDPE) or Low Density Polyethylene (LDPE), or also Linear Low Density
Polyethylene (LLDPE). Plastic bags and wrap are common uses of PE. PE is a low-strength
plastic, with a low melting point (80 °C or 176 °F), but somewhat higher melting points
are common for medium and high density PE, as high as 120 to 180 °C or 248 to 356 °F, all
within the range of household heating devices. PE is typically white, unless dyed. PE is
resistant to absorbing water.
HDPE has a density or specific gravity of 0.93 to 0.97 g/cm³,
which means HDPE will float in freshwater or seawater.
LDPE has a density or specific gravity of 0.857 to 0.925 g/cm³, which means LDPE
will float in freshwater or seawater.
PP, also known as polypropene, is the second most produced polymer world-wide. It is
strong and flexible, so often used to make plastic
rope. However, pure PP can become brittle below 0 °C or 32 °F. The melting point of PP
ranges from 130 °C or 266 °F to 171 °C or 340 °F.
PP has a density or specific gravity between 0.895 and 0.92 g/cm³, which means PP
will float in freshwater or seawater.
Polyvinyl Chloride (PVC)
PVC is the third-most produced plastic polymer world-wide. Typical uses include plumbing pipe and toy bricks.
PVC has a density or specific gravity of 1.1 to 1.45 g/cm³ (flexible PVC is typically
less dense (lighter) than rigid PVC). Therefore, unless air is trapped in the PVC, PVC will not float
in freshwater, and generally will not float in seawater. Still, there are regular reports
of plastic toy bricks and related figures washing up on ocean beaches, so wave and current
turbulence may be enough to keep this material near the ocean surface.
Polyethylene Terephtalate (PET, also PETE, PETP, PET-P, polyester, Dacron, Terylene)
PET is the fourth highest production plastic polymer worldwide. The density or specific
gravity of PET is 1.38 g/cm³, which means it should sink in freshwater and seawater.
However, as much of the PET plastic which ends up in water is in the form of bottles,
and if the forms capture even a little air, they tend to float. This is why photos of
floating plastic garbage tend to include recognizable plastic bottles.
The melting point of PET is 260 C or 490 F.
Typical uses for PET include thin-wall (single-use) plastic drink bottles, especially
transparent bottles. A variant which includes stronger composition (e.g. Mylar) is used
where more strength is required.
Polystyrene (PS, Styrofoam)
PS is the fifth highest production plastic polymer worldwide.
The density or specific gravity of PS ranges from 0.96 to 1.04 g/cm³. At the higher end
of the range, I would expect PS to sink in freshwater. However, I have never experienced
a polystyrene product which sank in freshwater (expanded beadboard, extruded board, drink cup, packing
'peanuts', take-out food 'clamshells', etc.). In fact, large blocks of extruded PS are sold
specifically to provide buoyancy for rafts and floating docks. The melting point of PS is
170 to 280 C or 340 to 535 F .
Polyurethane (PUR, PU)
Polyurethane is not used in many single-use consumer products. A generic density figure for
polyurethane is meaningless, as many applications are foams with high degrees of air
entrainment. E.g. one low density closed cell foam has a listed density of 0.160 g/cm³, which would
be extremely buoyant in water. Most polyurethanes do not melt, as they are thermosetting polymers.
(Thermoplastic polyurethanes do exist but are uncommon in consumer plastic products.)
The density of seawater is typically around 1.02 to 1.03 at the surface, depending on temperature.
For more on plastics identification and properties, Life Without Plastic
presents a fairly complete, while succinct, guide.
In short, a lot of single-use consumer-market plastics float in water, at the interface
between water and air, and there is a lot of life at the interface to ingest that
floating plastic pollution.