The standard outlines a test procedure based on the Archimedes principle, which states that the buoyant force on a submerged object is equal to the weight of the fluid that the object displaces. By measuring the weight of a specimen in air and then its apparent weight when submerged in a liquid of known density, you can calculate its volume and, consequently, its density and specific gravity.
ASTM D792 is broken down into two main test methods, which differ primarily in the immersion liquid used.
Method A: For Testing Solid Plastics in Water
Objective: To determine the density and specific gravity of solid plastics that are unaffected by water and heavier than water. This is the most common procedure and is used for a wide range of plastic materials.
Procedure:
A specimen of the plastic is weighed in air.
It is then fully immersed in distilled water at a standard temperature of 23°C.
The specimen’s apparent weight while submerged in the water is recorded. If the specimen is lighter than water and would float, a “sinker” (a heavy object of known weight) is used to hold it down.
Using the weights in air and in water, and the known density of water at 23°C, the density and specific gravity of the plastic are calculated.
Method B: For Testing Solid Plastics in Liquids Other Than Water
Objective: To determine the density and specific gravity of solid plastics that are lighter than water or are affected by water (e.g., they absorb water and swell, or they dissolve).
Procedure:
The procedure is the same as Method A, but the immersion liquid is a different fluid that will not react with the plastic and whose density is known.
Common liquids used for Method B include alcohols like ethanol or isopropanol, or hydrocarbons, which are less likely to be absorbed by certain plastics.
The specific gravity of the plastic is calculated using its weight in air and its apparent weight in the chosen liquid, and factoring in the known density of that liquid. This is an important step to ensure the final result is comparable to a water-based density.
