Paperboard Moisture Sorption Dynamics and Water Vapor Permeability Fundamentals
Paperboard moisture sorption and vapor permeability govern pack structural integrity and shelf life through fiber swelling, stiffness loss, and mass flux.
Gravimetric Cup Test
Permeability Measurement
Water vapor transmission rate evaluation is performed by measuring the change in weight of a sealed container containing a desiccant or water and covered by a test specimen. Conduct of the gravimetric cup test remains the standard method for determining the barrier properties of paper, board, and coated substrates under controlled environmental conditions. This physical measurement determines the rate at which moisture passes through the material.
Experimental Procedure
Assembly of the test cell requires sealing the paperboard specimen over the open mouth of a shallow cup, which contains a dry desiccant to create a zero percent relative humidity environment inside. The sealed cup is then placed in a climate-controlled chamber maintained at a constant high humidity and temperature, establishing a stable moisture gradient across the sample. Over time, the cup is weighed at regular intervals to track the steady-state increase in mass as water vapor penetrates the board.
This change in mass is plotted against time to calculate the hourly rate of moisture transmission.
Barrier Calibration
Calibrating this test method involves strict control of the air velocity and temperature inside the test chamber. Even small fluctuations in airflow can disrupt the boundary layer of air over the cup, leading to inaccurate results. This technique provides the baseline data needed to compare different polymer coatings and surface treatments.