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.
Micro-Bubble Error
Meaning
Measurement defect types degrade the accuracy of laboratory analysis by introducing systemic bias to the test results. A micro-bubble error occurs when tiny air bubbles attach to the surface of a polymer specimen during density testing. These bubbles alter the buoyancy of the specimen, leading to incorrect density measurements.
Physical Mechanism
Surface tension forces cause air bubbles to cling to the hydrophobic polymer specimens. During a gradient column or pycnometry test, the presence of a micro-bubble error reduces the effective weight of the specimen in the liquid. This reduction in weight causes the specimen to appear less dense than it actually is.
This can lead to the false rejection of resin batches.
Specific Gravity
Calculated density values are skewed downward by the trapped air. Even a tiny bubble can cause a significant micro-bubble error in the calculated specific gravity. For a polymer with a density of 0.950, a trapped bubble can make the density appear to be 0.940.
This shifts the classification of the resin from high density to medium density, disrupting quality control records.
Procedural Prevention
Washing steps are required to remove bubbles from the specimens before testing. Technicians prevent micro-bubble error by using wetting agents or by degassing the specimens in a vacuum chamber. Adding a surfactant to the column liquid reduces surface tension and prevents bubbles from sticking to the specimen.
This step ensures that the density measurements are accurate and consistent across runs.