Rosin/alum sizing

1. Unbleached pulps are easier to size than bleached; kraft easier than sulfite.
2. Calcium and magnesium ions in the incoming water can interfere with rosin/alum sizing.
3. The optimum PH is generally in the range of 4.2 – 4.5 but can vary with the type of pulp, amount of alum, water quality and temperature.
4. Most fillers have a negative charge and compete with the negatively charged fibers for the positively charged rosin/alum complex.
— wet strength agents and cationic starches may improve sizing efficiency.
5. The order and points of addition of rosin and alum need to be controlled to obtain the best sizing efficiency.
— the optimum order and points of addition vary from mill to mill and even from machine to machine; usually determined by trial and error.
6. Foam in the wet end can reduce the retention of the rosin/alum particles.
— small amounts of defoamer may improve sizing, however, larger levels of defoamer addition act as wetting agents and impair sizing..
7. Moderate refining improves sizing efficiency due to better formation and retention.
— rosin size generally performs better when added after refining.
8. Retention aids usually improve sizing efficiency.
— the rosin/alum particles are attracted to the fines (large surface area) making fines retention essential to good sizing.
9. High stock temperatures generally hurt sizing.
— additional alum and lower stock consistencies can offset the effect of high stock temperatures.
10. Dryers should be graduated in temperature and well controlled.
¾ the rosin/alum precipitate fuses when exposed to moist heat so proper drying is an essential part of the sizing mechanism.