Gurley Seconds: Paper and Film Air Permeability Unit
Gurley seconds measure the time required for 100 cubic centimetres of air to pass through a defined area of paper, film, or nonwoven material under a standard pressure differential. The unit captures how densely packed fibres impede airflow, providing a critical specification for packaging, filtration, and battery separator applications.
Developed alongside the Gurley densometer, the measurement has become entrenched in TAPPI, ISO, and ASTM standards. This article examines the testing methodology, historical context, and how Gurley values inform design choices across industries. For additional pore-scale insights, compare with the specific surface area discussion.
Use the paper roll calculator to tie permeability metrics to production planning.
Definition and Apparatus
One Gurley second equals one second required for 100 cm³ of air to traverse the specimen. The standard apparatus consists of a vertically oriented cylinder containing a falling piston. As the piston descends, it forces air through a clamped sample with a circular test area, typically 1.0 in² (6.45 cm²). The elapsed time between start and finish marks provides the Gurley reading.
ISO 5636 and TAPPI T460 specify acceptable tolerances for piston mass, cylinder diameter, and sealing gaskets, ensuring reproducibility across laboratories. ISO 5636-5 further defines low-pressure Gurley methods while ISO 5636-2 covers Bendtsen air leakage for comparison. Modern automatic densometers include optical sensors to timestamp piston travel and store digital data for statistical analysis.
Historical Development
The Gurley method originated in the 1930s through the W. & L.E. Gurley Company, responding to papermakers’ needs to quantify porosity beyond subjective inspection. Early instruments used mechanical timers; later versions integrated photoelectric cells and automated lifting mechanisms. The unit has endured because it correlates well with end-use performance in packaging permeability and cigarette paper burn rates.
Industry bodies harmonised competing permeability metrics by adopting Gurley seconds as a reference, while also publishing conversion tables to Frazier air permeability (ft³/min·ft²) and Bendtsen roughness values. Such crosswalks enable engineers to translate between time-based and volumetric flow measurements.
Interpreting Values and Limitations
Higher Gurley numbers indicate lower permeability. Dense battery separators may exceed 3000 Gurley seconds, while breathable tissue papers fall below 10. Because the measurement is influenced by temperature, humidity, and sample conditioning, standards prescribe equilibrium preconditioning at 23 °C and 50% relative humidity. Edge leakage and sample deformation can skew results, necessitating careful specimen cutting and clamp pressure control.
Gurley seconds capture through-plane resistance but not in-plane diffusion or tortuosity. Complementary methods—such as mercury porosimetry, capillary flow porometry, or gas permeability testing—provide a fuller picture of pore architecture when required. Converting Gurley seconds to permeability coefficients uses relationships like k = (V × μ × L)/(A × ΔP × t), highlighting the dependence on viscosity μ, specimen thickness L, and test area A.
Data Quality and Automation
Automated Gurley systems integrate barcode scanning and laboratory information management systems to associate readings with roll numbers, fibre blends, and coating formulations. Control charts using moving range or exponentially weighted moving average (EWMA) statistics reveal subtle permeability drift before customer complaints arise. Routine verification with glass capillary standards or leak-free blanks validates instrument tightness, safeguarding test integrity.
Applications and Decision-Making
Packaging engineers specify Gurley ranges to balance product protection with degassing needs in food and pharmaceutical wraps. In energy storage, lithium-ion separator films rely on tight Gurley tolerances to maintain electrolyte retention while preventing dendrite penetration. Filtration media designers correlate Gurley seconds with particle retention and ventilation targets.
Specialty papers, such as banknotes and security documents, use Gurley values to control ink absorption and anti-counterfeiting features. Consumer goods like tea bags leverage specified permeability to ensure consistent brewing characteristics.
Quality Control and Importance
Gurley measurements support statistical process control by signalling fibre furnish changes, refining conditions, or coating defects. Integration with automated data systems allows mills to track trends and implement corrective actions before products fall out of specification. Regulatory regimes—such as those governing breathable medical packaging—cite Gurley limits to safeguard sterility and performance.
By translating microstructural porosity into actionable time units, Gurley seconds remain a cornerstone metric for porous web materials, enabling efficient collaboration between material scientists, converters, and equipment suppliers.