The influence of dextran molecular weight on the dry strength of dextran-impregnated paper.

Application: This study looks at the role of dry strength additives.

**********

Papermakers have two fundamental choices when it comes to strengthening paper with polymers--wet-end addition and size press/coating. New surface coating/sizing technologies give printing paper producers the opportunity to replace wet-end addition with surface treatments. These offer the advantage of high polymer loadings, particularly at the surface, while simplifying the wet-end chemistry. However, we were concerned that surface treatment would not be as effective as wet-end treatment for strength.

From a scientific perspective, one might expect these two strategies to yield different results. Polymers such as cationic starch, when added to the wet end, tend to adsorb onto all available surfaces before sheet formation. Thus, fiber/fiber bonds are formed between polymer-coated surfaces. By contrast, size press or coater application of water-borne polymers is essentially an impregnation operation, where the solution wicks into preformed paper, it is not clear to what extent polymers penetrate into existing fiber/fiber bonds. The goals of this work was to look at the role of the size (molecular weight) of the strength-enhancing polymer and to compare the effectiveness of wet-end addition to surface treatment.

We report results from a series of laboratory experiments in which we impregnated unbeaten bleached kraft pulp handsheets with dextran solution as a model strength-enhancing polymer. Two major conclusions resulted. First, when compared at the same polymer contents, wet-end and surface treatment gave similar strength improvements. Second, the ability of dextran to strengthen paper was sensitive to molecular weight. Paper impregnated with dextran with a molecular weight of 2 million daltons had almost twice the tensile index of paper treated with 77,000 D. It seems that more of the higher molecular weight dextran migrated to the fiber-fiber junctions. S!

Pelton, Zhang, Chen, and Moghaddamzadeh are with McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7; email Pelton at [e-mail]