Burst strength testing is a fundamental quality control method used to evaluate the resistance of paper, paperboard, and corrugated packaging materials to rupture under applied hydraulic or pneumatic pressure. This article provides a comprehensive technical overview of packaging material burst strength testing, covering applicable international standards, sample types, test items, testing procedures, calculation methods, and typical industrial applications, helping manufacturers verify packaging durability and achieve regulatory compliance.
Technical Overview
Burst strength (also referred to as bursting strength) is defined as the maximum uniformly distributed pressure, applied perpendicular to the surface of a test specimen, that the material can withstand before rupturing. It is one of the most widely used indicators of the structural integrity of paper-based packaging materials, reflecting the combined tensile strength and toughness of the sheet as well as its resistance to internal and external stress during handling, stacking, and transportation.
In a standard burst strength test, a circular specimen is securely clamped between two annular clamps. Pressure is then applied through a fluid or gas medium behind an elastic rubber diaphragm, causing the specimen to bulge until it fails. The peak pressure recorded at the moment of rupture, typically expressed in kilopascals (kPa) or pounds per square inch (psi), is reported as the bursting strength of the material. To enable fair comparison between materials of different grammages, the burst index—calculated as bursting strength divided by grammage—is frequently reported alongside the absolute value.
Burst strength testing of packaging materials is governed by several recognized international and national standards, including ISO 2758 for paper, ISO 2759 for board, ASTM D3786 for flexible sheet materials, TAPPI T 403 and TAPPI T 810 for paper and board, GB/T 6545 and GB/T 1539 in the Chinese national standard system, and JIS P 8112. Accredited third-party laboratories routinely perform these tests as part of packaging quality verification and compliance programs.
Testing Samples
Kraft Paper and Wrapping Paper
Uncoated and coated kraft papers used for wrapping, protective interleaving, and paper bag manufacturing are commonly tested to verify their bursting resistance. These materials typically exhibit bursting strengths ranging from approximately 150 kPa to 600 kPa depending on grammage and fiber composition.
Linerboard and Corrugating Medium
The facings and fluted medium used in corrugated fiberboard production are evaluated individually to characterize the contribution of each component layer to the overall strength of the finished board.
Corrugated Fiberboard
Single-wall, double-wall, and triple-wall corrugated fiberboard are primary test subjects for burst strength evaluation. The test result supports downstream box performance predictions, including correlations with box compression strength and the traditional basis for box classification systems such as the 200 lb, 275 lb, and 350 lb classes used in commerce.
Solid Fiberboard and Heavy Board Grades
Dense solid boards used for heavy-duty packaging, tubes, cores, and industrial components are tested in accordance with board-specific procedures, generally following ISO 2759 or equivalent standards.
Paper Sacks and Industrial Bags
Multi-wall paper sacks for cement, chemicals, flour, and other bulk materials require verified burst resistance to prevent catastrophic failure during filling, dropping, and palletizing operations.
Composite and Coated Packaging Materials
Laminated structures combining paper with polyethylene, aluminum foil, or other barrier layers may be assessed for burst performance when rupture resistance is a critical functional requirement of the packaging system.
Testing Items
A complete burst strength evaluation typically includes the following test items and reported parameters:
- Bursting strength: the maximum pressure at rupture, reported in kPa or psi, calculated as the arithmetic mean of all valid specimen results.
- Burst index: bursting strength divided by grammage, expressed in kPa·m²/g, which normalizes results for materials of different basis weights.
- Grammage (basis weight) determination: measured gravimetrically in accordance with ISO 536 or equivalent standards to support burst index calculation.
- Statistical evaluation: standard deviation and coefficient of variation of the specimen population, which indicate the uniformity and reliability of the material.
- Directional assessment: paper is essentially isotropic with respect to burst, but testing specimens with each face toward the diaphragm is required to detect two-sidedness in the sheet.
- Conditioning verification: documentation that specimens were conditioned in a standard atmosphere of 23 °C ± 1 °C and 50 % ± 2 % relative humidity in accordance with ISO 187 prior to testing.
Testing Methods
Apparatus
The test is performed using a bursting strength tester comprising a clamping system with an upper and lower annular clamp, a pressure chamber filled with glycerol or a similar transmission fluid, an elastic rubber diaphragm, a motor-driven piston or pneumatic pressure source, and a calibrated pressure transducer or Bourdon gauge for peak pressure capture. The clamping surface configuration and diaphragm inflation characteristics are precisely specified in the governing standards to ensure interlaboratory reproducibility.
Specimen Preparation
Specimens must be free from creases, wrinkles, watermarks, visible defects, and areas containing tape or printing anomalies that could influence results. Each specimen should be large enough to cover the full clamping area with an overlap of at least 10 mm beyond the clamp circumference, typically a minimum of 70 mm × 70 mm. Depending on the applicable standard, a minimum of 10 to 20 specimens is tested, with successive specimens alternately positioned so that first one side and then the other faces the diaphragm.
Test Procedure
- Condition the specimens for the prescribed duration in the standard atmosphere until equilibrium moisture content is reached.
- Verify and, if necessary, replace the rubber diaphragm, and confirm the calibration status of the pressure measurement system.
- Insert the specimen between the clamps and apply the specified clamping force, taking care not to allow the specimen to slip or be damaged during clamping.
- Apply pressure at the constant rate specified in the standard until the specimen ruptures, as indicated by a pressure drop or automatic detection.
- Record the maximum pressure achieved as the bursting strength for that specimen.
- Repeat the test for all specimens and calculate the mean value, standard deviation, and, where grammage data are available, the burst index.
Hydraulic Versus Pneumatic Methods
Hydraulic (Mullen-type) instruments remain the reference method described in ISO 2758, ISO 2759, and TAPPI T 403, and are suitable for both paper and heavy board. Pneumatic instruments are occasionally used for specific flexible materials in accordance with ASTM D3786; however, results obtained by different pressure media are not directly interchangeable, and the method used must always be stated in the test report.
Applications
Burst strength testing serves a wide range of practical purposes across the packaging value chain:
- Quality control in corrugated box plants, where burst values are used as acceptance criteria for incoming linerboard and medium and as routine process verification for finished board.
- Box classification and commercial specification, since many buyer specifications, transport packaging rules, and carrier requirements still reference minimum bursting strength values for fiberboard boxes.
- Performance correlation, providing input data for estimating box compression behavior and overall distribution performance of shipping containers.
- Supplier evaluation and incoming inspection programs that compare material lots and verify conformance to purchase agreements.
- Research and development of new fiber formulations, recycled fiber blends, and lightweighting strategies that must maintain strength while reducing grammage.
- Regulatory compliance and certification support, including test reports issued by accredited third-party laboratories for export packaging, dangerous goods packaging qualification, and customer audits.
- Failure analysis, where unexpectedly low burst values help diagnose furnish changes, refining issues, or moisture damage responsible for field failures.
FAQ
What is the difference between bursting strength and edge crush strength?
Bursting strength measures resistance to rupture under multi-directional pressure applied to the face of the material, and is historically associated with the ability of a box wall to withstand rough handling and impacts. Edge crush strength (ECT) measures the compressive resistance of the vertical edge of corrugated board and correlates more directly with stacking strength. Both are complementary indicators used in modern corrugated board specification.
Why is the burst index used instead of absolute bursting strength alone?
The burst index normalizes bursting strength by grammage, allowing meaningful comparison between lightweight and heavyweight materials. Two boards may show the same absolute burst value while using very different amounts of fiber, and the burst index reveals which material delivers strength more efficiently.
How many specimens are required for a valid test?
Under ISO 2758 and ISO 2759, at least 10 specimens are normally tested, with successive specimens turned over so that each face is alternately exposed to the diaphragm. Testing a greater number of specimens improves the statistical confidence of the reported mean value.
Does moisture content affect burst strength results?
Yes. Paper and board strength properties are strongly influenced by moisture content. Specimens must therefore be conditioned at 23 °C and 50 % relative humidity before testing, and all results are only valid for material equilibrated to this standard atmosphere.
How often should a bursting strength tester be calibrated?
The pressure measurement system should be calibrated at regular intervals against certified reference gauges or transducers in accordance with the instrument manufacturer's recommendations and the laboratory's quality management system, and after any diaphragm replacement or major maintenance. Routine daily verification using check specimens is also recommended before production testing begins.
Can burst strength testing be performed on plastic films and laminates?
Hydraulic bursting tests are primarily designed for paper and board structures. For flexible plastic films, other methods such as tensile, seal strength, and puncture resistance testing are generally more appropriate, although bursting-type methods described in ASTM D3786 exist for certain flexible sheet materials.