Burst strength (Mullen test)
The hydraulic pressure that punctures a sheet. Why corrugated liner grades are specified by burst index, and what the numbers actually mean.
Burst strength is the hydrostatic pressure required to rupture a paper sheet clamped over a circular orifice. It is the primary mechanical specification for containerboard grades — liner and fluting — because it correlates with how well a corrugated box resists internal pressure, puncture, and stacking loads.
The Mullen test (ISO 2758)
The test apparatus clamps a paper sample over a 30.5 mm diameter rubber diaphragm. Hydraulic pressure is applied at a constant rate until the sheet ruptures. The result is reported in kPa (kilopascals).
ISO 2758 is the international standard. TAPPI T403 is the US equivalent; the two give comparable results. Both require conditioning at 23 °C / 50% RH for at least four hours before testing.
Burst index: the normalized figure
Raw burst pressure scales with gsm — heavier boards rupture at higher pressures simply because there is more fiber in the cross-section. To compare grades of different weights fairly, the burst index normalizes by gsm:
Burst index (kPa·m²/g) = burst pressure (kPa) / grammage (g/m²)
A liner at 150 g/m² achieving 900 kPa has a burst index of 6.0 kPa·m²/g. A liner at 200 g/m² achieving 900 kPa has an index of only 4.5 — the heavier board is weaker per unit weight.
Burst index is what buyers actually specify for containerboard. Market minimums for virgin kraft liner are typically 3.0–4.5 kPa·m²/g; testliner (recycled) runs 2.0–3.5 kPa·m²/g.
Burst index ranges by liner type
| Grade | Basis (g/m²) | Burst index (kPa·m²/g) | Notes | |---|---|---|---| | Kraft liner (top) | 125–200 | 4.0–5.5 | Virgin fiber; highest structural integrity | | Kraft liner (inner) | 100–150 | 3.5–4.5 | Lower cost than top liner | | Testliner | 100–200 | 2.5–3.5 | Recycled OCC; economy grade | | Fluting (corrugating medium) | 80–175 | 1.8–3.0 | Flat crush matters more than burst here | | Sack kraft | 70–90 | 4.0–6.5 | Extensibility also specified alongside burst |
| Attribute | Test Liner WPI-g-000863 | Fluting WPI-g-000864 |
|---|---|---|
| Paper type | industrial_packaging | industrial_packaging |
| GSM | 80–400 g/m² | 80–400 g/m² |
| Fiber | Recycled OCC | Recycled OCC |
| Confidence | 41% | 41% |
| Source | wpi_split | wpi_split |
The testliner and fluting grades above (WPI-g-000863, WPI-g-000864) are the split grades from the recent canonical data-quality pass — previously combined, now distinct entries. Their wide gsm ranges (80–400 g/m²) reflect the market breadth captured in the raw data; actual trade specifications narrow the range considerably.
Burst vs ring crush
Burst index is easy to measure but is a poor predictor of box compression strength (BCT). Ring crush test (RCT, ISO 12192) and short span compression test (SCT, ISO 9895) correlate better with edge compression and stacking performance.
The industry has largely moved toward SCT for corrugated liner specifications because it directly tests the in-plane compression resistance — the load mode that determines how many boxes you can stack. Burst remains specified because:
- Buyers inherited it from decades of trading practice
- It is fast and cheap to run
- It catches gross quality problems (contaminated furnish, wet damage) reliably
WPI editorial note: If you're qualifying a new containerboard supplier and they offer only burst data, ask for SCT or RCT alongside it. A board can pass burst and still fail under stacking loads due to poor fiber alignment or calendering issues.
For sack kraft — a related structural grade — burst and extensibility (stretch at break) are co-specified. Sack kraft at 70–90 g/m² (WPI-g-000018) is designed to absorb energy on drop impact; burst tells you puncture resistance, extensibility tells you how much deformation it can absorb before failure.