TAPPI T 523 Dynamic Measurement of Water Vapor Transfer from Sheet Materials

The standard “TAPPI T 523 Dynamic measurement of water vapor transfer from sheet materials”, developed by the Technical Association of the Pulp, Paper and Converting Industries (TAPPI), describes a procedure using a closed cell to rapidly evaluate the water vapor transfer rate (WVTR) of sheet packaging materials, particularly coated paper barrier films, at any normal or elevated temperature.

This procedure involves sandwiching a sample sheet between a high-humidity chamber (90 percent relative humidity) and a dry chamber (5 percent or less relative humidity) and determining the rate of change of humidity in the dry chamber. By means of a calibration curve, these dynamic test results can be converted to grams of moisture per square meter-day.

This procedure is a general method that allows the use of any dynamic measurement device. This method is not intended for any environmental, Food and Drug Administration (FDA) or other regulatory applications.

The climatic conditions to which packaging materials are exposed vary greatly, so water vapour transmission testing by any procedure serves to compare different materials rather than to predict their actual performance in the field. Although some barrier materials undergo changes in their solid state below 38°C, this elevated temperature is often used to accelerate gravimetric water transmission testing (TAPPI T 464 Water vapour transmission rate of paper and board at elevated temperature and humidity). Dynamic procedures are sufficiently sensitive that neither dangerously high temperatures nor a large humidity gradient are required.

The rate of moisture transfer from a relatively humid atmosphere through a barrier to a drier atmosphere changes with time until equilibrium is reached. The length of the transition period varies with the capacity of the test material to dissolve and retain water. If the test material swells, there will also be a change in the diffusion coefficient. For example, with a hydrophobic plastic film, equilibrium is normally reached in less than an hour. With papers that have barrier films on both sides, equilibrium may take days.

However, with a dynamic test procedure, the rate of transmission of water vapour through the barrier material during the transition period can be used to assess the barrier quality of the material. The values ​​thus measured can be compared with those obtained under steady-state conditions or, when a suitable correlation is provided, can be used directly for control purposes. The essential requirement for dynamic testing is that the conditions governing the flow are constant.

The material sample for testing is taken in accordance with the standard “TAPPI T 400 Sampling and acceptance of a single sheet of paper, paperboard, containerboard or related product”. From each test unit, triplicate test samples should be selected that are free from defects, pinholes or other defects that are not normal to the material and of a suitable size to suit the equipment used.

In terms of conditioning, completely hydrophobic test samples are not sensitive to water and satisfactory stability is quickly achieved in a normal test cycle.

The dynamic measurement device components used in the tests are:

• A permeation cell designed so that a defined area of ​​the test specimen is sandwiched between two chambers. One chamber contains a dry atmosphere, the other chamber contains an atmosphere of high relative humidity.
• Compression arrangement to permit rapid insertion and removal of the test specimen when the cell is open and suitably sealed to prevent significant leakage of water vapor between the surrounding atmosphere and the dry chamber when the cell is closed.
• The humidity chamber includes provisions for maintaining humidity within plus or minus 2 percent of the desired level. By using liquid water or a saturated sponge, a substantially saturated atmosphere can be achieved within the chamber with a small distance between the liquid and the sample, but without direct contact between the liquid and the sample. Lower humidities can be achieved by using saturated salt solutions instead of water or by using a gas stream of accurately controlled humidity.
• The dry chamber, which has connections to a purge gas source such as nitrogen or clean air, is dried to 5 percent relative humidity or less as required by the instrument. The chamber is purged with dry gas, isolated with valves, and the moisture accumulation is measured. It is essential that the surfaces of the dry chamber do not absorb significant amounts of moisture.
• Humidity sensor and indicator that responds quickly and is capable of measuring relative humidity changes of 0,05 percent or less. The sensor measures the humidity change within a dry chamber for testing. The sensor may be an infrared radiation beam, an electrical resistance element, or an electrolytic cell. The rate of change of humidity in a dry cell is determined by the time it takes for the concentration to change between two selected levels or by its change over a specified period of time.
• The temperature control system is required to maintain the permeability cell and test sample at a constant, known temperature within plus/minus 0,2 degrees.

Among the numerous testing, measurement, analysis and evaluation studies carried out for businesses in various sectors, our organization also provides testing services defined in the “TAPPI T 523 Dynamic measurement of water vapor transfer from sheet materials” standard with its trained and expert staff and advanced technological equipment.