ASTM Standard: How to Test Film for Water Vapour Permeation Rate

Introduction

The films applied in packaging, construction, medical devices, and industrial applications sometimes include moisture barriers. To establish whether a film is good enough to guard a product or structure, its ability to resist the transfer of water vapour is tested in a standardized manner. ASTM standards provide the practical answer to the question of how one can test a film in terms of the water vapour permeation rate, specifying the way the specimen is to be prepared, the conditions in which the test is supposed to be conducted, the testing procedure, and the calculation and interpretation of test results.

Which ASTM Standards Are Used to Test Water Vapour Permeation of Films

ASTM establishes two main standards for evaluating the vapour permeation of water in films. ASTM E96/E96M is a gravimetric approach that is used on films, sheets, papers, and membranes, whereas ASTM F1249 is an instrumental approach particularly used on plastic films and sheeting. The selection of standards varies depending on film type, film thickness, and the sensitivity needed.

How ASTM E96/E96M Tests Film for Water Vapour Permeation

In ASTM E96/E96M, the film is cut to size and then covered over the open mouth of a test cup. Based on the chosen procedure, the cup may either have a desiccant (dry cup method) or distilled water (wet cup method). The sealed assembly is then put in a controlled environment with a given temperature and relative humidity. When water vapour passes through the film by the humidity gradient, the weight of the test cup varies. The cup is then weighed at regular intervals till the mass change attains a constant rate. The rate at which water vapour permeates is determined by the mass change rate, the size of the exposed film, and the time of the test. The test inquiry with the use of this method uses direct measurement of the increase or decrease in moisture using the film.

How ASTM F1249 Tests Film for Water Vapour Permeation

ASTM F1249 is an automated method that is particularly appropriate when working with thin plastic films having low permeability. The specimen film is placed inside a test cell that has a dry gas stream and a humidified gas stream in the cell. The dry gas takes the water vapour through the film to a modulated infrared sensor. The sensor records the volume of vapour that has been permeated continuously, and this enables direct calculation of WVTR without having to weigh periodically. It is easier, quicker, more sensitive, and has better repeatability, which is ideal in high-barrier packaging films.

Test Conditions Defined by ASTM

Both ASTM E96/E96M and ASTM F1249 give test temperature and relative humidity levels, usually 23°C or 38°C, with predefined humidity levels. Such controlled conditions are used to make the results of various laboratories comparable and applicable to specific use.

Why ASTM Water Vapour Permeation Testing Is Important

ASTM standards offer an assurance of data on moisture barriers by giving precise guidelines on how to test the film to determine the rate of permeation through the water vapour. The methods are used to aid in the selection of materials, prediction of shelf life, confirmation of quality, and compliance with regulatory requirements. Application of ASTM-approved procedures provides a secure scientific basis for film performance claims and recognition in the world.