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Effects of free and dissolved gas on Sizing and Coating
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Reduces AKD, ASA and rosin sizing retention and consistency
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Creates uneven coat weight, gloss and print mottle
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Increases deposits, machine contamination and chemical consumption
Excess gas in the wet end, whether dissolved, entrained, or present as visible foam, can reduce sizing efficiency and disrupt the process stability needed for a uniform base sheet and consistent coating performance.
Why gas control matters
Internal sizing agents such as AKD, ASA, and rosin depend on being retained and distributed uniformly throughout the sheet. Excess gas can concentrate sizing particles in foam instead of on fibers and fillers, reducing sizing efficiency, increasing losses to the white-water system, and raising sizing chemical consumption.
The result can be inconsistent water resistance, variable Cobb values, and localized absorbency differences – or, on print and coated grades, uneven ink penetration and print density variation. In severe cases, this can contribute to edge-wicking or other converting and end-use performance issues.
Gas variability also affects retention, drainage, and stock flow stability, increasing the risk of deposits and an inconsistent base sheet reaching the coater. Among common sizing systems, ASA tends to be the most sensitive to air and foam-related losses, while AKD and rosin are typically more tolerant.
Coating can’t fix what’s already wrong underneath
Because coating can’t fully mask an uneven base sheet, this often shows up downstream as coat weight instability, gloss variation, and print mottle — even when the coating recipe itself is stable and unchanged.
Measuring Free and Dissolved Gas
Monitoring both free (entrained) and dissolved gas provides visibility into an often-overlooked source of wet-end variability. Online measurement helps operators identify changing process conditions early, enabling more effective control of retention, drainage, sizing, and overall wet-end stability.
Typical symptoms of excess / variable gas content
- Varying Cobb values despite constant size dosage
- Increased sizing chemical consumption
- More white water foam and deposits
- Lower first-pass retention
- Print mottle and gloss variation
- Coat weight instability and variable holdout
- Brightness and opacity swings
- Quality variability despite stable recipes
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