True Density
Density measurement using gas displacement reveals the volume of solid polymer material in a sample without including the volume of internal pores or voids. This technique of helium pycnometry is employed in yarn laboratories to measure the crystallinity of synthetic fibers. Since helium atoms are small, they penetrate the smallest pores to give an accurate measurement of the skeletal volume of the fiber.
The test is limited to dry samples because moisture in the fiber pores blocks the access of the helium gas.
Fiber Crystallinity
High crystallinity in polyester and nylon fibers results in higher density values during testing. By measuring the true density with helium pycnometry, the lab determines the ratio of crystalline to amorphous regions in the fiber. This ratio explains the yarn performance during dyeing and heat setting.
Porosity Calculation
Combining this skeletal density with the envelope density of the yarn allows researchers to calculate the internal porosity of technical fabrics. When a fabric is heavily coated, the helium gas can only reach the exposed surface, which modifies the measured volume. Therefore, helium pycnometry provides a clear benchmark for evaluating the effectiveness of yarn coatings and finishes.
Moisture Interference
Residual moisture in the fiber structure must be removed by drying the sample before running the test. Any trapped water molecules alter the displacement of the gas and lead to incorrect density readings.
