BS ISO 5636 — Paper and Board: Determination of Air Permeance (Bendtsen, Sheffield, Gurley)

📄 BS ISO 5636 is the British Standard implementing the ISO 5636 series, which specifies methods for determining the air permeance of paper and board. Air permeance reflects the porosity of the fibre structure and is directly related to the breathability and filtration function of bags, cigarette paper, filter paper and packaging paper; it is also used to assess formation and structural uniformity. Parts 3 to 6 of the series give several methods based on different principles for the medium range of air permeance.

1. Series Overview and Scope 📌

The BS ISO 5636 series provides several methods for medium-range air permeance determination:

• Part 3 (Bendtsen method): a constant-pressure air stream passes through a defined sample area; permeance is expressed as flow rate (mL/min).
• Part 4 (Sheffield method): measures air flow rate at a defined pressure difference and expresses permeance in Sheffield units.
• Part 5 (Gurley method): measures the time for a defined volume of air to pass through the sample (Gurley seconds).

The methods differ in principle, apparatus and result expression, and suit different permeance levels of paper grades.

2. Series Structure and Key Points 📋

The series comprises four documents; the core readable parts are ISO 5636-3, 4 and 5, while Part 6 is a scanned version summarised here from public domain knowledge.

3. Key Technical Requirements 🔬

3.1 Principle and apparatus

Each method specifies the measuring principle, apparatus construction (pressure head, sample clamp, flow/time measuring device) and key dimensions. The Bendtsen method measures flow at constant pressure; the Sheffield method measures flow at a fixed pressure difference; the Gurley method records the time for a fixed air volume.

3.2 Sampling, conditioning and specimen preparation

The Standard specifies sampling, conditioning (standard temperature/humidity) and specimen preparation so specimens are in a consistent state. Paper moisture affects fibre porosity and permeance, so conditioning must be unified.

3.3 Result expression and conversion

Each method gives calculation and expression of results (flow, Sheffield units or Gurley seconds), with calibration and reference samples to ensure instrument accuracy. Conversion between methods requires the standard or reference samples.

4. Engineering Practice and Application 🛠️

• 🔍 Method matching: cigarette paper and filter paper (high permeance) often use the Gurley or Bendtsen method; packaging paper and light board often use Sheffield or Bendtsen. Choose by paper grade and supply agreement.
• 🔧 Quality control: permeance is a sensitive indicator of formation and structural uniformity, used for online and dispatch quality monitoring; abnormal variation suggests stock or forming problems.
• 📊 Functional evaluation: for filter, packaging and printing papers, combine permeance with pore size and ink absorption to evaluate function.
• 📋 Traceability: calibrate regularly with traceable reference samples, and retain instrument calibration records and test data.

4.4 Calibration, interlaboratory comparison and data management

Each permeance method is sensitive to instrument condition: pressure sensors, flow meters and timers drift with time, so calibrate regularly with traceable reference samples and join interlaboratory comparisons. Calibration reveals systematic deviation; comparison verifies consistent execution of the method. For production control, keep the method, operators and conditions fixed for longitudinal comparability; in release records note the method, conditioning and environment so abnormal variation can be traced. For exacting grades such as filter and cigarette paper, combine permeance with pore size and thickness rather than relying on a single indicator.

5. Frequently Asked Questions ❓