Cobb value in gypsum liner paper: how much water absorption a board line can tolerate
Cobb60 is the single most misread number on a liner datasheet. What it measures, what a gypsum line actually needs, and why 'lower is better' is wrong.
Every plasterboard liner datasheet carries a Cobb value, and almost every buyer reads it as a quality score: lower means less water absorption, therefore better paper. In a gypsum board line the opposite is closer to the truth. The liner must absorb a controlled amount of water from the stucco slurry — enough to key the paper to the core, not so much that it saturates, delaminates or slows the dryer.
What Cobb60 actually measures
ISO 535 (and its TAPPI T441 equivalent) clamps a 100 cm² ring onto the sheet, floods it with water for 60 seconds, blots and reweighs. The result, in g/m², is the mass of water the surface took up in that minute. It is a surface sizing test, not a porosity test and not a strength test — two liners with identical Cobb can behave completely differently on the line if their internal bond differs.
Target ranges by liner position
| Liner | Typical Cobb60 (top side) | Typical Cobb60 (wire side) | Why |
|---|---|---|---|
| Face liner (ivory) | 20–35 g/m² | 45–70 g/m² | Sized top side for jointing and paint; open wire side to key into the core. |
| Back liner (grey) | 30–50 g/m² | 50–80 g/m² | No decorative duty; absorption is tuned purely for bond and dryer load. |
| Moisture-resistant (MR) face | < 25 g/m² | 40–60 g/m² | Heavier surface size plus wet-strength resin for EN 520 type H boards. |
Cobb too low: the failure nobody expects
- Poor paper-to-core bond, showing up as blisters or paper lift after the dryer.
- Edge cracking on scoring and snapping, because the liner never keyed into the gypsum crystal network.
- Slurry riding on the surface, giving an uneven core density profile across the web.
Cobb too high: throughput and cost
Excess absorption pulls free water out of the slurry and into the paper, where it has to be evaporated again. On a line running 100 m/min every additional 10 g/m² of absorbed water across a 1.2 m web is roughly 72 kg of water per hour that the dryer must remove. That is gas, dwell time and, eventually, line speed.
How to specify it in a purchase order
- State Cobb60 per side (top and wire) with a tolerance band, not a single maximum.
- Name the standard: ISO 535 conditioned at 23 °C / 50 % RH per ISO 187.
- Ask the mill for reel-level COA data, not a nominal grade value.
WeePaper ships PBL with per-reel certificates of analysis covering grammage, Cobb60 both sides, tensile MD/CD and moisture. If you are troubleshooting bond or dryer load, send us your current COA range and target line speed and we will match a mill position against it.
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