Plastic Material: Identification by Buoyancy Test

The buoyancy test is one of the ways to measure the density of a substance. It can be used to test both solids and liquids. According to Archimedes' principle, when an object is completely immersed in a liquid, the volume of the object is equal to the volume of the displaced (overflowed) liquid, the weight of an object reduced in a liquid is equal to the weight of the liquid displaced after the object putting into, that is, the buoyancy it experiences. Therefore, whether it is a sinking object or a floating object, the buoyancy it experiences in the liquid = the weight of the liquid displaced by the object.

**Ji-Horng Plastic Co., Ltd. is a professional plastic injection molding factory in central Taiwan. Our main products including various plastic buckles and luggage accessories. If you have mold tooling requirements, welcome to contact us! **

**This article is for academic reference only and we are not capible of selling plastic raw materials**

Contact number: +886-4-8355057 / Email: service@ji-horng.com.tw

In our another article: Plastic Material: Density & Specific Gravity, it is mentioned that the density is calculated by dividing the weight by the volume. The density obtained by dividing the unit weight of water (grams or kilograms) by the unit volume (cubic centimeters or cubic meters) is exactly Close to or equal to 1; therefore, "water" becomes a very good reference material in measuring the density of different substances.

How to Apply Buoyancy Test on Plastics?

To put it simply, when measuring the density of an irregularly shaped object, it is difficult to accurately calculate the volume of the object. Therefore, we can use water to make a preliminary judgment through the buoyancy method. This principle can also be applied to the density measurement of plastics. :

When the density of plastic is greater than the density of water (D>1), the object sinks to the bottom.
When the density of plastic is less than that of water (D<1), the object floats on the water.

Test Result of Common Plastic Material:

*Remarks: PLA is an environmentally friendly material made from corn starch and can be naturally decomposed in a composting environment.

From the above table, it shows that PP, LDPE, and HDPE float on the surface because their density is smaller than water. Therefore, we can simply distinguish the plastic materials. If we'd like to distinguish more accurately, it is necessary to use burn test and identify the smell and melting state. For detailed information, please refer to the extended reading.