Structural Quotient
Paper strength characteristics represent a ratio derived from the force required to rupture a specimen divided by its grammage. Burst index quantifies this capacity through the Mullen test, providing a normalized value that allows for the comparison of papers differing in basis weight. Calculations proceed by taking the burst pressure in kilopascals and dividing the result by the grammage expressed in grams per square meter.
Manufacturers utilize this value to assess the internal bonding strength of the sheet when fibre orientation varies across the machine direction and cross direction. Precise control over this value prevents web breaks during high speed printing or converting operations.
Mechanical Constraint
Fibre composition dictates the physical limits of a substrate under pressure. Softwood pulps produce longer, interlocked structures that improve the burst index compared to short fibre hardwood pulps. Refining intensity during the papermaking process increases the surface area for bonding, which improves the index until fibre damage occurs.
Excess refining leads to a brittle sheet that fails prematurely under stress. Recycled fibre content often contains fillers or fines that reduce the bonding density, resulting in lower scores for the finished product.
Performance Expectation
Industrial standards define the baseline requirements for packaging grades such as linerboard and corrugated medium. Laminating processes or coating applications can alter the surface tension and moisture content of the stock, causing variations in the measured result. Environmental humidity impacts the equilibrium moisture content of the fibres, shifting the index during climate controlled testing conditions.
Consistent measurement ensures the substrate maintains structural integrity throughout the supply chain. Paper stocks with a higher burst index provide superior resistance to puncturing and tearing during transit.
