Surface sizing with starch solutions at high solids contents

Abstract

Surface sizing at high at solids contents from 8% to 30% yields up to 75% savings in afterdrying capacity. Decreasing the wetting of the sheet will also increase paper machine efficiency by reducing the number of web breaks. Also, the same afterdrying capacity can be used for both MSP coating and surface sizing at size solids contents of 20%-30%.

Surface sizing solids contents of up to 30% have not been used due to the insufficient stability of starch at elevated solids contents and maintaining the binding power of the starch. Also the ability to maintain the amount of starch at the MSP application at very high solids contents has been lacking. Here, both the chemistry of the starch solutions and the application technology used needed to be developed in order to produce the desired applied starch amount at starch solids contents of up to 30% with desired binding properties. The features of the base paper also needed to be adjusted to fit the features of high solids surface sizing.

The findings of this study prove that it is possible to use a wide variety of starch viscosities and molecular weights even at increased solids contents. Increasing starch solids contents with more stable starches is therefore not restricted to highly degraded starches only with their lower binding power. When the solids content of the starch solution is increased from 8% to 30%, less starch penetrates into the sheet and more starch remains on the paper surface. The concentration of starch on the surface of paper decreases sheet porosity, oil absorption and internal strength, as well as increases its surface strength and bending stiffness. At very high solids contents internal strength behavior also suggests that starch may locally penetrate into the larger sheet pores. This observation is supported by pore size distribution analysis. The base paper for high solids surface sizing can also be optimized by adjusting the press to dryer web draw. Here, the aim could be to minimize the press draw below 2% when internal strength or air permeability are critical.

A method for obtaining more informative starch distribution curves was developed to provide a helpful tool in characterizing the effect of various process parameters on the z-distribution of starch in the sheet. An additional feature of this method is the definition of a dimensionless penetration number Qtot that can be used to quantitatively compare the penetration of surface sizing starch in different paper samples.

A novel interpretation is presented of the role of surface sizing starch with respect to the elastic modulus and bending stiffness properties of the sheet. When increasing the elastic modulus or bending stiffness of the sheet, starch may merely promote shrinkage potential instead, which leads – when shrinkage is not allowed – to increased drying stress that, in turn, leads to an increase in the elastic modulus of the sheet and a further increase in bending stiffness.