Why Do Some Plastics Float While Others Sink?

The claim that “plastic is denser than water” is not true of all plastics. Density varies by polymer and by the particular formulation. Whether an object floats also depends on its shape and whether it contains air or other materials.

The basic rule is simple: an object floats if it can displace enough water to support its weight. That rule applies to a solid plastic pellet and to a hollow bottle, but you have to use the right density for each case.

The buoyancy rule

Water pushes upward on an immersed object with a force equal to the weight of the water the object displaces. This is the buoyant force, described by Archimedes’ principle.

A freely floating object settles at a depth where the upward force balances its weight. If the object’s average density is less than the water’s density, it can displace enough water while still partly above the surface. If its average density is greater than the water’s density, it must be fully submerged and still weighs more than the displaced water; it sinks unless something else supports it.

Fresh water has a density close to $1.0\,\text{g/cm}^3$ at ordinary temperatures. Seawater is somewhat denser, typically around $1.02$ to $1.03\,\text{g/cm}^3$, so an object may float in seawater even if it sinks in fresh water.

Different plastics have different densities

“Plastic” refers to a broad family of materials, not one substance with one density. Typical values vary with grade, additives, and manufacturing, but these examples show the difference:

Plastic	Approximate density	Typical result in fresh water
Polypropylene (PP)	$0.90\,\text{g/cm}^3$	Floats
Polyethylene (PE)	$0.91$–$0.97\,\text{g/cm}^3$	Usually floats
Polystyrene (PS)	About $1.04$–$1.06\,\text{g/cm}^3$	Usually sinks when solid and unfoamed
PET	About $1.3$–$1.4\,\text{g/cm}^3$	Usually sinks
PVC	About $1.3$–$1.4\,\text{g/cm}^3$	Usually sinks

These are representative ranges, not guaranteed values for every product. Foamed polystyrene, for example, contains many small gas-filled cells and has a much lower overall density than solid polystyrene.

For a homogeneous solid that floats freely, the fraction of its volume below the surface is approximately its density divided by the water’s density. A polypropylene piece with density $0.90\,\text{g/cm}^3$ would therefore be about 90% submerged in fresh water, ignoring small effects such as surface tension.

Why a hollow bottle can float

A bottle’s plastic walls may be denser than water, but the bottle is not a solid block of plastic. If it is sealed and mostly full of air, its mass is spread over the volume enclosed by its outside surface. That average density can be far below the density of water.

For example, suppose an empty sealed bottle has a mass of 20 g and displaces 500 cm³ of water when fully submerged. Its average density, including the air inside, is:

$$\frac{20\,\text{g}}{500\,\text{cm}^3}=0.04\,\text{g/cm}^3.$$

The 500 cm³ of displaced fresh water has a mass of about 500 g, corresponding to a weight of about 4.9 N. The bottle needs to displace only enough water for the displaced water’s weight to balance the bottle’s weight; that amount of water has a mass of about 20 g. In an idealized rigid bottle, adding cargo increases the weight; it will sink once the bottle cannot displace enough water to support the bottle and cargo together.

If water gets inside, the object’s weight increases. A bottle that is open, damaged, or filled with water may therefore float lower or sink. Trapped air, sealed cavities, and foam can all increase buoyancy by keeping the object’s average density low.

Common mistakes

• Treating all plastics as one material. Some common plastics are less dense than water; others are denser.
• Using the wall material’s density for a hollow object. For a bottle or toy, compare the object’s total mass with the volume of water it displaces.
• Assuming anything made of dense plastic must sink. A hollow shape or trapped air can make the complete object float.
• Assuming floating means the object is weightless. A floating object still has weight; the displaced water provides an equal upward force.
• Blaming surface tension for large objects floating. Surface tension can matter for tiny objects, but ordinary plastic bottles and pieces float mainly because of buoyancy and their average density.

So, some plastics float because the polymer itself is less dense than water; some plastic objects float because their shape encloses air; and some plastics or plastic objects do sink. The material and the whole object must be considered separately.