How polymers strengthen filled papers

We compared two strategies for strengthening filled papers. One used polymers to increase cellulose-cellulose adhesion, the other used polymers to increase cellulose-calcium carbonate adhesion. We made two-ply laminates from wet, filler-free handsheets with a sparse coating of precipitated calcium carbonate (PCC) in the ply-ply interface. After those had dried, we measured the force required to delaminate them. Polymer treatment of one or both plies increased the strength of laminates with low PCC contents due to enhanced cellulose-cellulose adhesion. However, at intermediate filler contents this strategy failed. Treatment of PCC with polymer to strengthen cellulose-PCC adhesion gave modest strength improvements. By far the strongest laminates were obtained when cellulose fines were present in the PCC layer.