Bursting strength testing of secondary fiber corrugated paper is a core quality control procedure that measures the ability of recycled-content packaging materials to resist rupture under uniform pressure during filling, stacking, and transportation. This article provides a technical overview of the testing principle, applicable standards such as ISO 2758, ISO 2759, ASTM D774, and GB/T 454/1539, sample preparation requirements, test procedures, and practical guidance for manufacturers and accredited third-party laboratories working with recycled fibers.
Technical Overview
Secondary fiber corrugated paper is a paper and paperboard product manufactured predominantly from recovered paper grades such as old corrugated containers (OCC), mixed waste paper, and deinked printed stock. Because recycled fibers undergo repeated pulping, drying, and re-wetting cycles, they are typically shorter, more hornified, and less able to form strong inter-fiber bonds than virgin fibers. As a result, the mechanical performance of secondary fiber corrugated paper must be carefully evaluated, and bursting strength is one of the most widely specified and measured properties for this purpose.
What Is Bursting Strength
Bursting strength is the maximum uniformly distributed hydraulic pressure that a paper or board specimen can withstand before rupturing when pressure is applied perpendicular to its surface through an elastic diaphragm. It is expressed in kilopascals (kPa) and reflects the combined contribution of tensile strength and stretchability of the sheet in all directions. For corrugated products, bursting strength serves as a practical, single-value indicator of the overall toughness of the board structure, including the combined effect of the liners and the corrugating medium.
Why Recycled Fibers Affect Burst Performance
The use of secondary fibers introduces several variables that directly influence burst values, which is why dedicated testing of recycled corrugated paper is essential:
- Fiber shortening and damage during repulping and screening reduce the average fiber length available for load-bearing bonds.
- Hornification, the irreversible stiffening of cell walls after repeated drying cycles, reduces fiber swelling and bonding potential.
- Higher fines content increases density but may reduce stretch, lowering the rupture energy absorbed by the sheet.
- Contaminants such as residual adhesives, stickies, fillers, and wet-strength broke can create weak points that initiate premature rupture.
- Variability in recovered paper furnish composition leads to batch-to-batch fluctuation in burst values, requiring frequent statistical monitoring.
Relevant Standards
Bursting strength testing of secondary fiber corrugated paper is typically performed in accordance with internationally and nationally recognized standards, including:
- ISO 2758, which specifies the determination of bursting strength of paper using the hydraulic pressure increase method.
- ISO 2759, which specifies the determination of bursting strength of board, including corrugated board.
- ASTM D774/D774M, the standard test method for bursting strength of paper.
- GB/T 454 and GB/T 1539, the Chinese national standards for bursting strength of paper and board respectively, widely applied to domestic corrugated products.
- GB/T 10739 and ISO 187, which define the standard conditioning atmosphere for paper and board samples prior to testing.
- JIS P 8112, the Japanese industrial standard covering bursting strength determination of paper and board.
Testing Samples
Finished Corrugated Board
Samples are taken from converted corrugated board, such as single-wall, double-wall, or triple-wall structures produced with recycled liners and recycled medium. Specimens must be flat, free from visible defects, creases, or crush damage, and must not include seam areas, printing-heavy zones, or regions affected by die-cutting unless these are specifically the object of evaluation. For board testing under ISO 2759 or GB/T 1539, each specimen should be large enough to cover the full clamping annulus with an overhang of sufficient margin to prevent slippage.
Component Papers: Linerboard and Corrugating Medium
In addition to finished board, the individual plies are frequently tested to characterize the contribution of each layer. Recycled linerboard (testliner) and recycled corrugating medium samples are tested as plain sheets under ISO 2758 or GB/T 454, allowing mills and converters to verify that incoming rolls meet grade specifications before production.
Sampling and Conditioning
Specimens should be cut from positions distributed across the machine direction and cross direction of the sheet or board to capture structural variability. A minimum of 10 valid specimens, and preferably 20, should be prepared for each determination. All samples must be conditioned in the standard atmosphere of 23 °C ± 1 °C and 50 % ± 2 % relative humidity for at least 24 hours, or until moisture equilibrium is reached, because moisture content has a strong and well-documented effect on burst values. Testing should be carried out in the same conditioned atmosphere whenever possible.
Testing Items
The principal test item is the bursting strength of the secondary fiber corrugated paper or board, reported in kPa. From this measurement, several derived parameters and evaluations are commonly required:
- Absolute bursting strength (kPa), the maximum rupture pressure recorded for each specimen.
- Burst index (kPa·m²/g), calculated by dividing the bursting strength by the grammage of the sample, enabling fair comparison between papers of different basis weights.
- Statistical evaluation, including the arithmetic mean, standard deviation, coefficient of variation, and the maximum and minimum values of the specimen set.
- Compliance assessment against grade specifications, customer agreements, or product standards that define minimum burst values for recycled board grades of a given flute profile and grammage.
- Comparative analysis between virgin and recycled furnish formulations, or between different recovered paper sources and processing conditions.
Testing Methods
Testing Principle
The test is based on the hydraulic diaphragm bursting principle. A conditioned specimen is securely clamped between two concentric annular clamps above an elastic diaphragm. Fluid is then delivered beneath the diaphragm at a controlled, steady flow rate, causing the diaphragm to deform and apply an increasingly uniform pressure to the specimen. The pressure at the moment the specimen ruptures is captured by a calibrated pressure transducer and recorded as the bursting strength.
Apparatus Requirements
A bursting strength tester suitable for this application should meet the following requirements:
- A clamping system with a lower clamp aperture of 31.5 mm ± 0.05 mm for board testing under ISO 2759, and 30.5 mm ± 0.05 mm for paper testing under ISO 2758, with concentricity and surface finish controlled to prevent slippage or premature edge failure.
- An elastic diaphragm of specified material and thickness, capable of protruding through the clamp aperture by the amount defined in the standard, and replaced at regular intervals or whenever signs of fatigue appear.
- A hydraulic pumping system delivering a constant, ripple-free flow rate within the tolerance band specified by the applicable standard.
- A calibrated pressure measurement and indication device accurate to the resolution required by the standard, with traceable calibration and routine verification against a certified reference gauge.
- A rigid, level mounting surface and vibration-free environment to ensure repeatability.
Test Procedure
The general procedure for testing secondary fiber corrugated paper is as follows:
- Verify the instrument calibration status and inspect the diaphragm and clamp faces for wear, contamination, or damage.
- Condition the specimens to equilibrium at 23 °C ± 1 °C and 50 % ± 2 % relative humidity.
- Determine and record the grammage of the sample, which is required for calculating the burst index.
- Clamp the first specimen securely, ensuring it lies flat without pre-stretching or wrinkling, and apply the clamping force specified by the instrument or standard.
- Start the hydraulic flow and increase the pressure at the steady rate defined by the standard until the specimen ruptures.
- Record the maximum pressure indicated at rupture, release the clamp, and remove the specimen.
- Repeat the test for the required number of specimens, distributing the sampling positions across the sheet.
- Discard and replace results from specimens that slip in the clamp, rupture at the clamp edge, or fail for clearly abnormal reasons, and note any exclusions.
Calculation and Expression of Results
The mean bursting strength is calculated from all valid individual results. The burst index is then obtained by dividing the mean bursting strength in kPa by the grammage in g/m², and is expressed in kPa·m²/g. The test report should include the mean, standard deviation, and coefficient of variation, the number of tests performed and any discarded results, the standard atmosphere used for conditioning, the applicable test standard, and full identification of the sample including furnish description, grammage, and board construction.
Factors Influencing Test Results
Several factors must be controlled to obtain meaningful results for recycled corrugated paper:
- Moisture content: elevated humidity softens the fiber network and typically reduces burst values, making strict conditioning essential.
- Clamping pressure: insufficient clamping force causes specimen slippage and artificially low readings, while excessive force can initiate edge damage.
- Diaphragm condition and fluid temperature: aged diaphragms or cold hydraulic fluid alter the pressure transfer characteristics and bias results.
- Instrument calibration: regular gauge verification and flow-rate checks are necessary to maintain accuracy over time.
- Board structure: flute type, adhesive application, and crush damage from the corrugating or converting process all influence the measured burst of finished board.
- Furnish variability: heterogeneous recovered paper streams require larger specimen populations to achieve representative statistics.
Applications
Bursting strength testing of secondary fiber corrugated paper supports a wide range of industrial and commercial activities:
- Mill quality control, where continuous burst monitoring verifies that recycled linerboard and medium rolls conform to grade specifications before shipment.
- Converter incoming inspection, allowing box plants to qualify incoming recycled board lots and detect furnish degradation or process drift.
- Product design and development, supporting the optimization of furnish blends, layer structures, refining strategies, and chemical additives such as starch-based strengthening agents.
- Compliance and certification, providing objective evidence that recycled packaging meets the minimum performance requirements referenced in customer specifications and regulatory frameworks.
- Sustainability programs, documenting that board produced with a high secondary fiber content still achieves the mechanical performance required for protective packaging.
- Failure analysis, helping to determine whether package damage in the field is attributable to insufficient board strength, moisture exposure, or converting defects.
- Trade acceptance and dispute resolution, where results from accredited third-party laboratories provide a neutral technical basis for supplier and buyer agreement.
FAQ
Which standard should be used for corrugated board made from recycled fibers?
Finished corrugated board, regardless of whether it contains recycled or virgin fibers, is normally tested under ISO 2759 or GB/T 1539, which are the board bursting standards. The individual component papers, such as recycled linerboard or corrugating medium, are tested under ISO 2758, GB/T 454, or ASTM D774 as paper samples. Selecting the correct standard matters because the clamp aperture, flow rate, and pressure range differ between the paper and board methods.
What is the burst index and why is it useful for recycled paper?
The burst index is the bursting strength divided by grammage, expressed in kPa·m²/g. Because secondary fiber products are often engineered at higher basis weights to compensate for weaker recycled fibers, the burst index allows performance to be compared on an equal-mass basis, making it a fairer indicator of furnish quality than absolute burst alone.
How many specimens are required for a valid determination?
A minimum of 10 valid specimens is generally required, with 20 preferred, particularly for recycled furnishes known to exhibit higher variability. Specimens that slip in the clamp or fail at the clamp edge should be excluded, documented, and replaced, and the final report should state the number of tests and any exclusions.
Why do recycled corrugated papers show lower bursting strength than virgin grades?
Repeated recycling cycles shorten fibers and cause hornification of the cell walls, which reduces inter-fiber bonding and stretchability. Manufacturers can recover performance by increasing basis weight, blending a proportion of virgin long fiber, applying strengthening chemicals, or optimizing refining and pressing, all of which should be verified through systematic burst testing.
How does bursting strength relate to other corrugated board tests?
Bursting strength measures the combined tensile and stretch resistance of the sheet under multi-directional pressure, and it correlates broadly with overall board toughness. It complements, but does not replace, edge crush test (ECT) results for stacking strength, flat crush test results for flute integrity, and puncture resistance for rough handling, so a complete board specification typically combines several of these measurements.
Can burst testing be used to certify recycled packaging for export markets?
Yes. Bursting strength data generated under recognized standards such as ISO 2759, ASTM D774, or GB/T 1539, preferably verified by accredited third-party laboratories, is widely accepted in supplier qualification, trade documentation, and compliance with packaging performance requirements in international markets.
