Will FAS affect pulp strength?

Direct Answer

Direct Answer Under recommended conditions the effect is small. If conditions run out of range, strength does fall.

Why the effect is small

Reductive bleaching does not delignify. It acts mainly on chromophoric groups and attacks cellulose far less than oxidative bleaching does. That is precisely why reductive bleaching is widely used on mechanical and high-yield pulps.

When strength does drop

• Temperature too high, particularly above 90 C for a long dwell
• Alkali charge too high, accelerating alkaline hydrolysis and lowering the degree of polymerisation of cellulose
• Dose far above requirement, leaving excess reduction in the system
• Insufficient intermediate washing between the oxidative and reductive stages, so residual oxidant keeps working during the later hot stage

What to measure

In bench trials measure tensile index, tear index, burst index and freeness together. Under recommended conditions these normally move within measurement error. If tensile index falls by more than 5%, go back and check temperature and alkali charge.

One point worth noting

If strength drops, check first whether it is caused by carryover from the oxidative stage rather than attributing it to formamidinesulfinic acid (FAS). Intermediate washing and residual peroxide control are often the real cause.

Bottom line

Little effect on pulp strength under recommended conditions; loss of strength is mainly driven by excessive temperature, excessive alkali and oxidant carryover.

Sources

1. US5958184A, Process for producing thiourea dioxide（还原当量质量比、碱中水解式、木浆与脱墨浆等当量漂白实施例），https://patents.google.com/patent/US5958184A/en
2. The effect of bleaching reagents on bleachabilities of DIP and environmental loads，KTAPPI 47(1), 2015，DOI 10.7584/ktappi.2015.47.1.066