Gas transmission rate

Three general test procedures used to measure the permeability of plastic films are the absolute pressure method, the isostatic method, and the quasi-isostatic method. The absolute pressure method (ASTM D 1434, Gas Transmission Rate of Plastic Film and Sheeting) is used when no gas other than the permeant in question is present. Between the two chambers a pressure differential provides the driving force for permeation. Here the change in pressure on the volume of the low-pressure chamber measures the permeation rate. 

ISO 2556, Plastics - Determination of the gas transmission rate of films and thin sheets under atmospheric pressure - Manometric method, 1974. 

ISO 1663 1999 Rigid cellular plastics - Determination of water vapour transmission properties ISO 2556 1974 Plastics - Determination of the gas transmission rate of films and thin sheets under atmospheric pressure - Manometric method ISO 6179 1998 Rubber, vulcanized or thermoplastic - Rubber sheets and rubber-coated fabrics - Determination of transmission rate of volatile liquids (gravimetric technique)... 

ISO 15105-1 2002 Plastics - Film and sheeting - Determination of gas-transmission rate -Part 1 Differential-pressure method... 

Testing of plastics - determination of gas transmissions rate - Part 1 Volumetrical method for testing of plastic films, DIN Standard DIN 53380, Beuth Verlag, Berlin, 2000. 

ASTM D3985, 2002. Standard test method for oxygen gas transmission rate through plastic film and sheeting using a coulometric sensor. 

Standard Reference Material 1470 is a 23 micrometer thick polyester film whose gas transmission characteristics with respect to helium, carbon dioxide, oxygen, and nitrogen have been carefully measured. A completely computerized manometric permeation measuring facility developed at NBS was used for the measurements. The steps taken to characterize the gas transmission rate of this material over the range of pressures from 67.5 kPa to 135 kPa and over the range of temperatures from 18 °C to 31 °C are described. The results obtained in these measurements are compared with those in the literature. The role of Standard Reference Material 1470 in improving the repeatability and reproducibility of gas transmission measurements employing other instrumentation is discussed. 

NBS s involvment in SRM s for gas transmission studies arose out of activities within ASTM Committees D-20 (Plastics) and F-2 (Flexible Barrier Materials) that were aimed at standardizing a new instrumental method employing a coulometric oxygen detector for the measurement of oxygen gas transmission rates in materials used for various kinds of packaging (4 ). A subsequent reevaluation of the precision data for the classical manometric and volumetric methods (5) for gas transmission measurements showed that there are serious problems with the reproducibility and repeatability these methods. The use of SRM 1470 to verify the calibration is currently being written into ASTM standards D-3985 and D-1434. 

ASTM Standard D-3985, "Standard Test Method for Oxygen Gas Transmission Rate through Plastic Film and Sheeting Using a Coulometric Sensor" in Part 35 of Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, Revised Annually. 

We have chosen to express our results as permeances rather than permeabilities because we are presently unable to justify the assumption that the properties of the material are uniform throughout the film. We suspect that normalizing the results to unit thickness would introduce additional scatter. The SRM certificate gives instructions for calculating gas transmission rates, which are the quantities that are measured directly. 

Permeability of Thermoplastic Containers to Packaged Reagents or Proprietary Products Oxygen Gas Transmission Rate Through Plastic Film and Sheet using a Coulometric Sensor Water Permeability of Geotextiles by Permittivity Permeability of Leather to Water Vapor... 

F1927-98el Determination of Oxygen Gas Transmission Rate, Permeability and Permeance at Controlled Relative Flumidity... 

In addition to the permeability coefficient, other parameters are also used to express the barrier characteristics of plastic materials. These include permeance (R), gas transmission rate (GTR), water vapor transmission rate (WVTR), and thickness normalized flow. The relationship between these parameters is shown in Fig. 14.3. 

The gas transmission rate, GTR, is the quantity of permeant flowing per unit area per unit time. The water vapor transmission rate, WVTR, is a GTR defined specifically for water vapor. The two together are permeant transmission rates, identified as F in Fig. 14.3. 

The relationship between the permeability coefficient and gas transmission rate is defined analogously. Thus to convert a measured WVTR or GTR to P, all that is necessary is to multiply by the film thickness that was used, and divide by the partial pressure difference used to make the measurement of WVTR or GTR. It should be noted that, in practice, WVTR values, rather than being defined as shown in Fig. 14.3, often have already been normalized to unit film thickness. In that case, WVTR is converted to P simply by dividing by the partial pressure difference at which the measurement was made, Ap. 

If (2 changes with pressure, Eq. 7 would give an average value for the permeability. If there is any doubt as to the homogeneity of the material it is usual to calculate the gas transmission rate, or permeance, for the sample under test rather than to calculate a permeabilit>. The gas transmission rate (GTR) is the flux per unit pressure drop... 

In this section well-established techniques used for measuring permeability and gas transmission rate are described. 

From Eq. 8 it can be shown that the instantaneous gas transmission rate is given by... 

BS 2782 uses Eq. 12 to measure the gas transmission rate at a single point chosen on the steady-state portion of the plot of mercury displacement against time. ASTM D 14. 4 uses an approximation to the integrated form of Eq. 12 for an extended time interval. 

ASTM D1434 [25] provides for a second method of measuring the gas transmission rate of plastics, namely that at constant pressure. This technique also forms the basis of a rubber test standard [29], In this case the pressure is kept constant in the low-pressure chamber and the volumetric change in the permeated gas is measured. Equation 11 reduces to... 

The volumetric displacement is plotted again.st time, and the slope of the linear steady-state portion is used in the calculation of gas transmission rate and permeability coefficient. 

The ASTM standards adopt definitions that are consistent w ith the equivalent definitions for gas transmission. Water vapor transmission rate is the mass transfer rate of water vapor per unit area (g nr 24h). Permeance is the ratio of the water vapor transmission rate to the difference in vapor pressure between the surfaces of the test piece measured in mm of mercury this unit is known as the metric perm (g nr 24h mmHg). This is equivalent to the gas transmission rate. Permeability is the product of the permeance and the thickness of the test piece, assuming that the permeance is inversely proportional to thickness for homogeneous materials this unit is known as the perm-centimetre (g cm nr 24h mmHg). Since the adoption of SI units, the water vapor permeability may also be expressed in the units of microgram meter per newton hour (pgm N h or pgm m Pa h ). 

Organic polymers as such cannot serve the necessary low oxygen transntission rates to ensure an acceptable lifetime. However, special techniques of deposition can reduce the gas transmission rates. 

Specially constructed cells are used to measure the gas transmission rate. After a film sample has been clamped into a cell, test gas is flushed through chambers on both sides of the sample. Test gas is admitted to one side of the sample the test chamber on the other side is evacuated, and gas is allowed to permeate through the film sample into the evacuated chamber for a measured length of time. Using the geometry of the cell and film sample, with the measured pressure and temperature of the test gas which permeated the sample, the GTR can be calculated. (See Fig. 

Gas transmission rate is vital in vacuum and gas packaging and for packaging fresh produce items that must breathe.f... 

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