Overview
ISO 4638:1984 specifies a laboratory method for determination of air flow permeability of flexible cellular polymeric materials (flexible foams). The procedure applies to test pieces cut from finished products and describes the principle (Darcy’s law), required apparatus (measurement cell, flow source, flowmeter, differential pressure device), test-piece preparation, test conditions and calculation/expressions of results. The standard is intended both for materials characterization and for quality assurance of cellular polymeric products.
Key topics and technical requirements
- Scope: Test pieces of cylindrical or parallelepiped shape; applicable to flexible cellular polymeric materials.
- Principle: Permeability K is calculated from Darcy’s law (K = qv · μ · L / (A · Δp) - see standard for symbols and units). Specific air flow resistance Rm and air flow resistance R are related to K and thickness.
- Apparatus:
- Measurement cell (cylindrical diameter 50–120 mm or square side 45–105 mm), sealed around edges to avoid leaks.
- Stable air source (vacuum or pressure) with fine flow control; stability such that instantaneous flow is estimated to better than ±2.5% and variation ≤1% per minute.
- Volumetric flowmeter placed close to test piece with ±2% full-scale accuracy; flowmeter readable between 20–100% scale.
- Differential pressure measurement capable of resolving ~1 Pa (inclined manometer or equivalent).
- Thickness gauge/support with perforated support ≥70% open area.
- Test pieces:
- Lateral tolerance ±1 mm; thickness typically 50–100 mm (multiple pieces may be stacked if thin).
- Remove skins for permeability characterization; weigh and record dimensions in situ.
- Test procedure: Measure Δp for at least five flow velocities in the laminar regime; derive K vs. velocity and report representative K (two significant figures) or R values. Report test conditions, density (ISO 845), orientation and any deviations.
- Reference values and notes: Dynamic viscosity of air at 23 °C listed in the standard; guidance on laminar flow limits and measurement cell sizing included.
Applications and users
ISO 4638 is used by:
- Foam manufacturers and process engineers for production quality control and batch acceptance.
- Materials scientists and R&D labs for characterizing foam microstructure and correlating structure to mechanical or acoustic properties.
- Acoustic engineers using specific resistance Rm for sound absorption modeling.
- Test laboratories performing standardized permeability testing for specifications and compliance.
Related standards
- ISO 845 - Determination of apparent density of cellular rubbers and plastics.
- ISO 471 - Standard temperatures, humidities and times for conditioning test pieces.
- ISO 7231 - (Related QA method based on constant pressure drop).
Keywords: ISO 4638, air flow permeability, flexible cellular polymeric materials, permeability testing, specific air flow resistance, Darcy’s law, foam quality assurance.
