Does FAS cause brightness reversion?

Direct Answer

Direct Answer Not more than other reducing agents, but reversion is a multi-factor problem and the causes need to be separated.

Where reversion comes from

Brightness reversion in paper comes mainly from three mechanisms:

• Carbonyl and phenolic hydroxyl groups in lignin forming new chromophores under light and oxygen
• Metal ions, especially iron and copper, catalysing oxidation
• Residual reducing species in the stock being oxidised in air and leaving new colour bodies behind

How formamidinesulfinic acid (FAS) relates to it

Reductive bleaching converts chromophores into non-coloured forms, and that conversion is reversible: on exposure to air and light some structures re-oxidise, which shows up as brightness that will not hold. This is inherent to reductive bleaching, not specific to FAS, and sodium dithionite behaves the same way.

How to reduce it

• Wash thoroughly after bleaching to remove residual reducing agent and dissolved material
• Control metal ions, adding a chelant where necessary
• Where long-term brightness stability is required, add a light oxidative stage or a brightness stabiliser after the reductive stage
• Avoid prolonged high-temperature drying and strong light exposure

How to judge it

Measure reversion with an accelerated ageing test, for example several hours in an oven at 105 C or xenon-arc exposure, and compare side by side with a dithionite route to see whether the difference is acceptable. Comparative data settles the question better than arguing about mechanism.

Bottom line

Brightness from reductive bleaching is inherently reversible and FAS is no worse than other reducing agents; controlling metal ions and washing out residual liquor are the keys to limiting reversion.

Sources

1. The effect of bleaching reagents on bleachabilities of DIP and environmental loads，KTAPPI 47(1), 2015，DOI 10.7584/ktappi.2015.47.1.066