The Bendtsen method determines the air permeance of paper and cardboard by measuring the airflow through a unit area of the material under a specified pressure difference. The sample is securely clamped in a cylindrical holder to prevent leakage, and a constant pressure difference is created across the material's surface. The airflow rate is then measured using a manometer.
The main equipment used in this method is the Bendtsen apparatus. This apparatus is specially designed to securely clamp the paper or cardboard and create a pressure difference across it. The apparatus consists of a cylindrical sample holder, a means of creating a constant pressure difference (such as a vacuum pump), and a manometer for measuring the airflow rate.
The test specimen should be prepared according to ISO 186, which provides guidance on sampling to determine the average quality of paper and cardboard. The test specimen should then be conditioned according to ISO 187, which specifies a standard atmosphere for conditioning and testing paper, cardboard, and pulps, as well as procedures for monitoring the atmosphere and conditioning samples.
Once the sample has been prepared and conditioned, it is clamped into the Bendtsen apparatus. The pressure difference is then applied, and the airflow rate through the material is measured.
The air permeance is calculated by dividing the measured airflow rate by the pressure difference across the material. This value is expressed in micrometers per second per pascal (µm/(Pa·s)).
Repeatability and reproducibility limits should be established for each type of paper and cardboard being tested. These limits will depend on the nature of the material, the test conditions, and the precision of the measurement apparatus.
The quality of the test results can be assured by carefully following the procedures specified in this document, regularly calibrating the Bendtsen apparatus, and conducting repeat tests to check for consistency. Regularly scheduled maintenance of the Bendtsen apparatus, as well as routine training of personnel, are also key aspects of quality assurance.