Density of plastics

Density measures the mass per unit volume. It is denoted by the Greek word ρ (rho). The formula of density is: 

 

where ρ is the density, m is the mass, and V is the volume. 


 

Units of density

The SI unit of density is kg/m³. The density of the various states of matter is given by:
 

• g/cm³ – for solids
• g/mL – for liquids
• g/L – for gases

For plastics, it is normally expressed as g/cm³. 


 

Definition of bulk density

Bulk plastic density is defined as a substance's weight based on a particular volume. It is an important parameter that affects the:
 

• Storage of plastic materials in a container.
• Sizing of the drying equipment.
• Flow of the material.

Each plastic substance has a different bulk density. There are three ways to express bulk density:
 

• Pounds per cubic foot (lb/cu ft)
• Grams per cubic centimeter (g/cc)
• Kilograms per liter (kg/l)

Specific gravity or relative density is a dimensionless quantity. It is the ratio of the density of a material to the density of a standard material. It simplifies the comparison of density across different systems of units.
 

 The density of a plastic sample may change according to the following factors:

 

Crystallinity

The density of the plastics depends on the crystalline structure of the polymers.
 

• Amorphous polymers have a random molecular structure. This results in lower density due to the disorganized arrangement of the chains.
• Crystalline polymers have a systematic and compact arrangement of polymer chains and crystals. Their higher crystalline structure results in high density.

Temperature

Density is indirectly proportional to temperature. As the temperature increases, most materials tend to expand. This increases the volume of the material which in turn decreases in density.

 

Source: Research Gate

Other factors like loss of plasticizers and absorption of solvent may change the density of plastics.
 

To determine the exact density, it is important to:
 

• Identify the material,
• Follow the physical changes in the sample,
• Indicate the uniformity among different sampling specimens,
• Indicate the average density of a large item, and
• Calculate the strength weight and cost-weight ratios.

Test methods to determine plastics density

• ASTM D1505: This method determines the material's density by the density-gradient technique. A material is placed in a liquid column of variable density with standard floats. These floats are usually glass beads of known density. The density of the material is then calculated. This is based on its relative position to the floats in the column and the densities of the glass beads.

• ASTM D792 - 98: This method determines the specific gravity and density of solid plastics. Solid plastics are in forms such as sheets, rods, tubes, or molded items. This test involves test methods such as:
   

  • Test Method A – For testing solid plastics in water, and
  • Test Method B – For testing solid plastics in liquids other than water.

  
   

• ISO 1183-2: The method specifies a gradient column method. It determines the density of non-cellular molded or extruded plastics in void-free form. Density gradient columns are columns containing a mixture of two liquids. The density in the column increases uniformly from top to bottom.