On the Internal Sizing Mechanisms of Paper with AKD and ASA Related to Surface Chemistry, Wettability and Friction
Sammanfattning: Paper and board are hydrophobized (sized) to control the spreading and absorption of waterbased inks and retard the absorption and edge penetration of liquid packaging by aqueous liquids. Alkenyl ketene dimers (AKD) and alkenyl succinic anhydride (ASA) are synthetic sizing agents that are generally used under neutral or slightly alkaline papermaking conditions.The overall objective of this thesis is to improve understanding of the internal sizing of paper and board by AKD and ASA by establishing a link between the sizing mechanism on one hand, and properties of sized papers, such as surface chemistry, wettability and friction, on the other. Fundamental research has been conducted in parallel with more applied research on laboratory and pilot papers. Significant effort has been expended to study the spreading behavior of AKD. The main instrument to characterize the surface chemical composition of AKD and ASA sized papers was X-ray photoelectron spectroscopy (XPS). By combination with time-of-flight secondary ion mass spectrometry (ToF-SIMS) we have been able to determine the lateral distribution and the chemical state of the sizing agent at the paper surface. Combined with contact angle measurements using liquids with different surface tensions, and other methods to analyze the amounts of size in paper, this has enabled us to obtain a deeper knowledge of the sizing mechanisms of AKD and ASA.The results indicate a definitive relationship between the redistribution of AKD at the surface of pilot papers and the drying profile used during papermaking. However, the spreading was not complete, as also seen on a model surface. Further spreading of AKD was shown to occur via surface diffusion in the form of an autophobic monolayer precursor. The spreading rate increased linearly with temperature and showed an inverse proportionality with respect to the melting point of the AKD. This monolayer spreading is relatively slow the diffusion coefficient being of the order 10-11 m2/s. AKD spreading was not hindered by hydrolyzed AKD (ketone) that spread as well. Moreover, AKD spread on the surface of crystalline calcium carbonate. In laboratory papers, the extractives present on CTMP fiber surfaces appeared to have enhanced the spreading of AKD when the fibers were in water.In spite of a slightly lower retention, ASA covered the surface of unfilled and PCC-filled pilot papers to a significantly higher extent than AKD. The ASA sized papers, however demonstrated slightly lower resistance to water. This was attributed to formation of hydrolyzed ASA products. The results obtained confirm the proposed sizing mechanism for ASA, where the hydrolyzed ASA plays a key role. The sizing level of these papers stored wrapped in aluminium foil at 23 °C and 5O %RH was nearly unchanged over prolonged storage time. In contrast, the papers exposed to ambient conditions suffered from sizing loss, most likely due to hydrolysis and migration. The reduction of the sizing degree was higher for the AKD than ASA sized unfilled papers and the catalytic effect of PCC contributed to the hydrolysis of AKD in PCC-filled papers.As expected, the use of sizing agent reduced the surface energy of paper. The higher the sizing degree of paper the lower the surface energy, and thus the higher the resistance to wetting. This was particularly seen in the contact angles with ethylene glycol having a lower surface tension than water.AKD significantly decreased the friction between unfilled papers, whereas ASA had no impact. This difference was attributed to surface chemical composition. Friction reduction for the AKD sized papers started at the AKD coverage normally found in paper produced for low water absorption. As expected, PCC filler increased paper-to-paper friction.
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