ISO, Cobb and TAPPI: the paper tests every industrial buyer should understand

Every roll of industrial paper ships with a Certificate of Analysis (COA), a one-page summary of half a dozen lab tests. Most procurement teams sign for the delivery without reading it. That is a mistake — the COA is the only objective evidence that the paper you paid for is the paper you received.

The five tests that matter most

1. Grammage (ISO 536)

Grams per square metre. Allowed tolerance is typically ±2.5%. A 150 g/m² liner delivered at 145 g/m² is technically out of spec and will under-perform on tensile and stiffness.

2. Tensile strength MD/CD (ISO 1924-2)

Force required to break a strip in machine direction (MD) and cross direction (CD), in kN/m. The MD/CD ratio reveals fibre orientation — a value above 2.5 means the paper will tear easily across the web.

3. Cobb 60 (ISO 535)

Grams of water absorbed by 1 m² in 60 seconds. Critical for PBL bonding and for any printed bag. Lower Cobb = more sized, more water-resistant surface.

4. Air permeability (ISO 5636 / Gurley)

How fast air passes through the sheet. Matters for sack filling, for PBL drying, and for any vacuum-pickup converting line.

5. Burst strength (ISO 2758)

kPa of pressure before rupture. Standard for sack kraft and shopping bag base paper.

ISO vs TAPPI: do they give different numbers?

Yes — slightly. ISO is the European standard, TAPPI the North American one. Tensile, tear and burst values can differ by 3–8% between the two methods on the same sample. Always demand ISO results when sourcing from European mills.

"A COA without the standard reference next to each value is not a COA — it's a marketing leaflet."

How WeePaper verifies

We retain a control sample from every shipment, cross-test critical values in an independent ISEGA-recognised lab on first order, and provide full traceability back to the mill batch number. If your COA ever drifts, you'll know before the roll goes on the line.