Sizing agents

1. A variety of sizing agents is used to provide control over the moisture receptivity of the sheet.
2. The addition of too low a level of sizing reduces the water resistance of the sheet and can lead to excess starch pick-up at the size press.
3. Too much sizing agent in the system can contribute to foaming at the wet end, increased alum demand (for rosin), reduced drainage and sheet strength, along with a greater tendency toward deposition on felts, wires, dewatering elements, and suction boxes.
4. High stock temperatures hurt sizing efficiency.
— partially offset by additional alum, lower pH, and low stock consistency.
5. Monitor water quality, especially hardness.
— calcium and magnesium ions interfere with rosin/alum sizing.
6. The order and points of addition of alum and rosin are very important to obtain the best sizing efficiency.
— seems to be mill and machine specific so use local experience to guide you.
— it is important to maintain the same order and level of addition through any run.
7. The pH must be controlled.
— optimum pH for rosin/alum is in range of 4.2 – 4.5, but this varies with type of pulp, amount of alum, water quality, and temperature.
— water from different mill sources can have similar pH values but very different total acidity which will influence sizing efficiency.
8. Fillers generally have a negative charge that competes with the fibers for the positively charged rosin/alum complex.
— more sizing agent is needed with highly filled sheets.
9. Additives, such as wet-strength agents and cationic starches generally help sizing efficiency.
10. Good first-pass retention aids internal sizing.
— the fines preferentially absorb the rosin/alum particles, so fines retention is critical.
11. Small amounts of defoamer can benefit sizing by removing entrained air.
— excessive defoamer is very detrimental to good sizing.
12. The sizing reaction takes place during drying.
— dryer temperatures should be graduated so that the rosin/alum precipitate is properly fused with the moist heat.
— too rapid drying (too hot too early) severely impairs neutral and alkaline sizing.
— gradient drying is usually best for all additives.