Color loss/variation

1. Loss of color on wire side can, in certain specialty grades run at lower speeds, be corrected by slowing down and thus reducing table roll suction.
— similarly, foils and wet suction boxes cause less two-sidedness.
— fines stripping from wire side leads to two-sidedness.
2. The normal method of correcting color loss is to dye-up wire side at size press.
3. A change to dyes (direct) with less pronounced two-sidedness should be considered.
4. Two-sidedness may be reduced by use of suitable dye mordants/dispersants, e.g., napthalene sulfonic acid derivatives.
5. For direct dyes on a bleached pulp, addition of clay will tone down top side and more dye can then be added.
6. If pigments are used, lack of aluminum ions is probably the reason for color loss, as in a closed system it is possible to maintain pH but lose aluminum ions.
— this tendency can be compensated for by the addition of sodium aluminate.
7. Beater addition of oxidized starch will reduce pigment and filler retention.
— oxidized starch from broke system or recycled fibers can also have negative influence on retention.
8. The system pH must be carefully controlled.
— most dyes are chemical indicators and will yield different colors in different pH ranges.
— the first thing to do is check the system pH which means testing and double checking rather than relying on machine meters.
9. Check for changing broke addition (whether colored or not) as this can affect color.
10. Maintain the correct levels of fiber development.
— changing refining will affect the final color with most dyestuffs.
11. Check for residual bleach in broke systems.
Related information can be found in PH varying.