Starch pick-up, sheet properties

Note: To obtain the desired pick-up rate at the size press, many variables, some of which are interrelated, must be closely controlled.

1. The basis weight uniformity of the sheet being surface-sized is important since the amount of size being picked up and the penetration of the sizing agent into the sheet are practically independent of sheet weight.
— the ratio of surface size to fiber varies as the weight of the sheet changes, assuming that the sizing solution is not changed.
— a lower ratio of size to fiber results on heavier sheets, and correspondingly, there will be less improvement in paper properties such as burst and tensile strength.

2. The sheet moisture entering the size press must be closely controlled.
— if the sheet is too dry or too wet, it will not absorb size properly.
— it is generally accepted that between 4 – 12 % moisture is the optimum range for best sizing.
— the higher the moisture, usually the greater the pick-up.

3. The amount of internal sizing and how the sheet is handled are also important factors.
— the more internal sizing present in the sheet, the less the surface coating picked up at the size press.
— the manner in which the internal size migrates in the sheet may have the effect of increasing the size concentration in certain areas.
— this is particularly true if the size migrates to the surface of the sheet due to vacuum pull or squeezing in the press section.

4. As the sheet density increases, the sheet absorbency is reduced.
— as density increases, operational adjustments must be made, such as modification of the time of exposure to the sizing agent, or improvement in the ability of the size to penetrate.

5. The surface smoothness of the paper is somewhat related to the density, since frequently an attempt to improve the surface finish will result in an appreciable reduction in caliper and a corresponding increase in density.
— sheets which have a smoother surface will pick up less sizing agent if other factors are held constant.