Newsprint Air Permeability Testing: Standards, Methods, and Quality Control

Newsprint air permeability testing is a critical quality control procedure that measures how readily air passes through the porous structure of newsprint paper, directly affecting ink absorption, drying speed, and print quality. This technical article provides a comprehensive overview of internationally recognized testing standards such as ISO 5636, TAPPI, and GB methods, along with sample preparation requirements, testing procedures, and practical applications in paper manufacturing and printing operations.

Technical Overview

Newsprint is a lightweight, uncoated printing paper manufactured primarily from mechanical pulp, typically with a small proportion of chemical pulp added for strength. With basis weights generally ranging from 40 to 49 g/m², newsprint is engineered for high-speed printing processes such as coldset web offset and flexographic newspaper printing. The performance of newsprint on modern printing presses depends heavily on its internal pore structure, which is quantified through air permeability testing.

Air permeability, also referred to as air permeance or air resistance depending on the measurement convention, describes the rate at which air flows through a paper sheet under a defined pressure differential. Because newsprint relies on absorption and penetration of the ink vehicle into the paper structure rather than evaporation for ink setting, permeability is one of the most influential properties governing printability. Insufficient permeability can cause slow ink setting, smudging, and set-off, while excessive permeability may lead to strike-through, reduced print density, and degraded print sharpness.

The measurement principle common to all recognized methods involves clamping a test piece over a defined measurement area, applying a standardized air pressure differential across the sheet, and measuring either the volume of air passing through per unit time or the time required for a fixed volume of air to pass. Results are expressed either as air permeance in SI units of micrometers per pascal second (μm/(Pa·s)) or in method-specific units such as milliliters per minute (Bendtsen) or seconds per 100 mL (Gurley).

Testing Samples

Sample Types and Sources

Samples submitted for newsprint air permeability testing typically include the following forms:

• Standard newsprint sheets cut from production rolls or delivered paper bales
• Recycled-content newsprint produced from deinked pulp streams
• Premium or improved newsprint grades used for commercial inserts and color sections
• Laboratory handsheets prepared during pulp and process development studies
• Finished printed samples evaluated for post-print structural characterization

Sampling and Conditioning Requirements

Sampling should be performed in accordance with ISO 186, which governs the selection of representative test pieces from lots of paper and board. Test pieces must be free from watermarks, wrinkles, folds, pinholes, and visible defects, and should be taken no closer than 15 mm to the paper edge to avoid edge effects. Both sides of the sheet should be identified, as slight differences may exist between the wire side and felt side.

Before measurement, all specimens must be conditioned to equilibrium in the standard atmosphere specified in ISO 187, namely 23 °C ± 1 °C and 50 % ± 2 % relative humidity. Conditioning is essential because moisture content significantly affects fiber swelling, pore dimensions, and consequently the measured permeability. A minimum of ten valid specimens is generally recommended so that a statistically meaningful mean value, maximum and minimum values, and standard deviation can be reported.

Testing Items

The core testing item for this evaluation is the air permeability of newsprint, which may be characterized through several internationally standardized determinations:

• Air permeance by the Bendtsen method, expressed in mL/min
• Air resistance by the Gurley method, expressed in seconds per 100 mL
• Air permeance by the Schopper-Riegler method
• Air permeance by the Sheffield method for highly porous grades
• Conversion of results to unified air permeance values in μm/(Pa·s) where specified by the customer

In comprehensive quality assessments, air permeability testing is frequently bundled with related physical and printability properties of newsprint, including basis weight (ISO 536), thickness and bulk (ISO 534), Bendtsen roughness (ISO 8791-2), water absorptiveness by the Cobb method (ISO 535), ash content (ISO 2144), and tensile strength properties (ISO 1924-2). This combined testing provides a complete picture of sheet structure and printing behavior.

Testing Methods

Bendtsen Method

The Bendtsen method, standardized in ISO 5636-3 and TAPPI T 536, measures the volume of air in milliliters per minute passing through a clamped test area of 10 cm² under a constant pressure differential of 1.47 kPa. Modern instruments use electronic flow measurement and offer selectable pressure levels to extend the measuring range for highly porous papers such as newsprint. Higher Bendtsen values indicate greater permeability. This method is well suited for routine production control because of its speed and repeatability.

Gurley Method

The Gurley method, covered by ISO 5636-5, TAPPI T 460, and the corresponding Chinese national standard GB/T 5402, determines the time required for 100 mL of air to pass through a test area of 6.42 cm² under the pressure of a falling column of oil. Results are reported as Gurley seconds, where higher values correspond to lower permeability. Because newsprint is a relatively open, porous grade, Gurley values for newsprint are typically low, and the method is most informative when comparing subtle differences between furnishes or process conditions.

Schopper-Riegler Method

The Schopper-Riegler method, specified in ISO 5636-2 and the Chinese national standard GB/T 458, measures air flow through a defined clamped area under a fixed hydrostatic pressure head. The instrument records either the volume of air transmitted per unit time or the time required for a specified air volume to pass through the specimen. This method has long been established in paper mills using Chinese and European national standards and remains a common reference point for newsprint specifications in many procurement contracts.

Sheffield Method

The Sheffield method, standardized in ISO 5636-4 and TAPPI T 547, measures the rate of air flow through the paper under a prescribed pressure differential and reports results in Sheffield units. It is particularly applicable to very porous sheets where other methods may exceed their calibrated range, making it a useful complementary technique for lightweight mechanical grades.

Instrument Calibration and Reporting

All instruments must be verified against calibrated reference orifices or certified reference materials traceable to national metrology institutes. Test reports should clearly state the method applied, the applicable standard, the number of test pieces, the mean value, the range, the standard deviation, the conditioning atmosphere, and any deviations from the standard procedure. Since results from different methods are not directly interchangeable without conversion, the test report must always identify the method used.

Applications

Air permeability data for newsprint serve a wide range of practical purposes across the paper and printing value chain:

• Ink setting and drying control: In coldset web offset printing, ink dries primarily by penetration into the porous sheet. Permeability testing allows mills to tune absorbency so that ink sets quickly without excessive strike-through or show-through.
• Press runnability optimization: Sheet structure characteristics reflected in permeability influence print-through behavior, marking, and overall performance on high-speed press lines.
• Manufacturing process control: Mills correlate permeability with refining intensity, furnish composition, filler loading, pressing, and calendering conditions, enabling rapid feedback for process adjustment.
• Recycled fiber evaluation: As deinked pulp content in newsprint increases, permeability testing helps characterize how secondary fibers alter sheet density and pore structure.
• Product specification and trade compliance: Publishers, printers, and paper merchants specify permeability limits in purchase agreements, and accredited third-party laboratories provide impartial verification of conformity.
• Research and development: Development of lightweight newsprint and hybrid printing grades relies on permeability data to balance strength, opacity, and absorbency targets.

FAQ

What is the typical air permeability of newsprint?

Newsprint is a relatively open, porous grade compared with fine papers. Exact values depend on basis weight, furnish, and calendering, so specifications should always be agreed upon with reference to a defined test method. In general, newsprint exhibits high Bendtsen permeability values and correspondingly short Gurley times, reflecting its open structure engineered for rapid ink vehicle absorption.

Why do different test methods give different numbers for the same paper?

Each method applies a different pressure differential, test area, and measurement convention. Bendtsen reports flow rate at constant pressure, while Gurley reports the time for a fixed air volume. Although values can be converted approximately to unified air permeance units, direct comparison is only valid when the same standard and conditions are used.

How many specimens are required for a reliable result?

A minimum of ten valid test pieces per sample is recommended by the relevant standards. Because paper is an anisotropic, inherently variable material, reporting the mean together with the range and standard deviation provides a more complete characterization than a single measurement.

What factors can influence test results?

Key influencing factors include moisture content and conditioning history, refining level of the pulp, filler and ash content, calendering intensity, basis weight, sheet formation uniformity, and the presence of any physical defects in the test area. Strict adherence to conditioning requirements in ISO 187 is essential for reproducible results.

Can permeability testing predict print quality directly?

Permeability is a strong indicator of ink vehicle absorption behavior, but print quality also depends on roughness, compressibility, opacity, and ink formulation. For this reason, permeability testing is most valuable when performed as part of a comprehensive printability assessment combining multiple standardized physical tests.

How should samples be submitted to a laboratory?

Samples should be protected from moisture, excessive heat, direct sunlight, and physical damage during transport. Sufficient material should be provided to allow cutting of at least ten defect-free test pieces, and the required test method or target specification should be clearly communicated to the laboratory before testing begins.