Durability of the Sizing Degree of AKD and ASA Sized Papers Investigated by Contact Angle Measurements and ToF-SIMS

The influence of storage conditions on the sizing degree of AKD and ASA sized pilot papers was evaluated. A number of pilot papers sized with AKD or ASA were prepared from ECF bleached pulp fibers, unfilled and filled with 20% PCC, respectively and investigated in terms of sizing degree over a period of several months. The papers were stored at 23C and 50% RH either wrapped in aluminium foil or as separate sheets exposed to open atmosphere. The unfilled papers stored protected from ambient atmosphere after papermaking showed only a marginal reduction in sizing during prolonged storage. Only the paper having the lowest AKD-dosage suffered from reduced water-resistance, the Cobb60-value changed from 34 to 79g/m2. The PCC filled papers stored in the same conditions lost some of their sizing, to a higher extent in the case of AKD than for ASA sized papers. This was attributed to the further hydrolysis of the size catalyzed by PCC. In comparison, the sizing of the papers stored as separate sheets dropped significantly even after a few weeks in storage. In the end of the storage the AKD papers, particularly the unfilled ones had lost their sizing efficiency to a clearly higher extent than the ASA papers. The reduction in the sizing level occurred mainly during the first five weeks for the unfilled ASA and AKD papers, after which the process continued at a slower rate. The ToF-SIMS analysis revealed that both AKD and hydrolyzed AKD, the latter being the major portion, were present at the outermost surface of the unfilled AKD sized papers, but in significantly lower levels than in the case of the corresponding protected papers. In other words, a significant loss of AKD mass had occurred for the papers exposed to an open atmosphere. This was attributed to migration of AKD. The results demonstrated that ASA sized papers also suffered from size loss. The ToF-SIMS results showed no signal for active, nonbonded ASA and instead clear signals were observed for hydrolyzed ASA and for calcium and aluminium. As in the papers wrapped in aluminium foil, ASA was mainly in its hydrolyzed form. Although to a markedly lower extent, the reason for sizing loss in the case of ASA was the same as for AKD.