Determining Polyethylene Specific Gravity Thresholds for Tariff Entry
Polyethylene tariff entry requires density testing per ISO 1183-1 Method A on annealed plaques, where crossing 0.940 g/cm³ shifts classification from 3901.10 to 3901.20.
Pycnometry
Meaning
Material measurement methods calculate the density of solid materials by determining their volume through fluid displacement. The technique of pycnometry is used to measure the specific gravity of polymers, powders, and compounded resins. This method provides a rapid and accurate measurement of density in a laboratory setting.
Instrument Operation
Gas pycnometers use helium or nitrogen to measure the volume of a specimen chamber. In pycnometry, the pressure change of the gas is used to calculate the volume of the polymer specimen. Because gas can enter tiny pores, the method measures the true density of the polymer.
This density is calculated by dividing the mass of the specimen by its measured volume.
Specimen Preparation
Surface cleanliness is critical to prevent errors in volume measurement. Specimen preparation for pycnometry requires the removal of any oils or surface contamination. The specimen must be completely dry because moisture can interfere with the gas pressure measurements.
If the specimen is contaminated, the measured volume will be incorrect, leading to errors in the density calculation.
Dynamic Compression
Laboratory technicians use this method to audit the density of compounded resins. Because compounding adds heavy fillers like glass or minerals to the polymer, the density of the compound is much higher than the base resin. Pycnometry allows the technician to verify the filler loading of the compound quickly, ensuring that the material meets the product specification.
This test prevents parts with incorrect mechanical properties from being produced.