Can FAS be used together with an optical brightening agent?

Direct answer

FAS and optical brightening agents (OBA) can be used together, but never in the same bath. The reason is straightforward: most OBAs depend on their structure for the optical effect, and a reducing environment damages part of that structure, while the OBA system's own components can consume FAS. The correct order is reductive bleaching first, OBA afterwards, with a wash in between.

1. Why they must be separated

2. Recommended stage sequence

1. Oxidative stage (for example peroxide)

2. Intermediate wash: remove residual oxidant

3. Reduction stage (FAS): remove residual chromophores and lift brightness

4. Wash

5. OBA stage: run at the pH and temperature the OBA requires

6. Later stages: sizing and other auxiliaries as needed

3. Four operating points

1. Do not skip the intermediate wash: residual oxidant consumes FAS, and residual FAS affects the OBA

2. Follow the OBA supplier's pH and temperature, not the reduction-stage conditions

3. Run a bench comparison: measure brightness for reduction only, OBA only, and the combination, to confirm the combination really wins

4. Watch reversion: re-test the combination after drying or ageing

4. Two common mistakes

• Adding OBA to the reduction bath for a one-step process: brightness may look acceptable briefly, but the OBA is already damaged and cannot be recovered
• Comparing initial brightness only: the value of the combination lies in stable brightness, not the reading straight out of the bath

Sources

The stage sequence is standard papermaking and textile practice. Compatibility between a specific OBA and FAS must be confirmed from the product data sheet and a bench trial.