How to Measure Specific Gravity of Plastic?

The specific gravity of plastic is determined by comparing the density of the plastic material to the density of water.

Understanding Specific Gravity

Specific gravity is a dimensionless measure that indicates how dense a substance is compared to a reference substance, which in the case of plastics, is water. It's a fundamental property in material science that helps us understand how plastics behave. According to the provided reference, specific gravity is calculated as the density of the plastic divided by the density of water.

The Calculation

The formula to calculate the specific gravity (SG) of plastic is:

SG = Density of Plastic / Density of Water

Since specific gravity is a ratio of two densities, it has no units.

Measuring Specific Gravity of Plastic: Practical Approaches

Here are methods to determine the specific gravity of plastic:

1. Density Measurement and Calculation

Determine the Density of the Plastic:
- Method A (Volume Displacement):
  - Weigh a sample of the plastic using a scale.
  - Using a graduated cylinder partially filled with water, record the initial volume of water.
  - Submerge the plastic sample in the water and record the new volume.
  - Subtract the initial volume from the new volume to get the volume of the plastic.
  - Calculate density by dividing the mass of the plastic by its volume (Density = Mass/Volume).
- Method B (Direct Measurement): Some laboratory equipment can directly measure the density of solid samples, which would simplify the first step.
Use Density of Water as Reference: The density of water is approximately 1 g/cm³ or 1000 kg/m³ at standard temperature and pressure.
Calculate Specific Gravity: Divide the density of the plastic by the density of water, according to the reference provided.

Example:

Let's say you measure the density of a plastic sample as 1.2 g/cm³. The specific gravity would be:

SG = 1.2 g/cm³ / 1 g/cm³ = 1.2

2. Using a Density Gradient Column

This more advanced method involves a tube filled with liquids of varying densities. A plastic sample is placed in the column. The specific gravity is then determined by the sample's settling point based on its density, compared to the liquid gradient. This is especially useful when measuring small quantities and density is close to that of water. This is not covered by the reference provided.

3. Using a Pycnometer

A pycnometer is a specialized flask to measure the density of a liquid or solid. First measure the weight of an empty dry pycnometer. Then measure the weight of a known volume of water, and record this weight difference. Then place your plastic sample in the flask and add water to fill. Measure the weight of the flask, sample, and water, recording this weight difference. Using those values, the density of the plastic can be determined and ultimately used to find specific gravity using the density of water as reference. This is not covered by the reference provided.

Importance of Specific Gravity

Material Identification: Specific gravity can help identify the type of plastic.
Quality Control: It ensures consistent material properties in manufacturing.
Design and Engineering: It's used in engineering to predict material behavior under different conditions.
Recycling: Sorting plastics for recycling can be made simpler by knowing their specific gravity

Key Takeaways

Specific gravity is a ratio comparing a plastic's density to water's density.
It is a dimensionless value.
It can be determined by either direct density measurement followed by calculation, or the use of specialized instruments such as density gradient columns, and pycnometers.
Knowing specific gravity is vital for identifying, quality-controlling and processing plastic materials.

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