Contact angle and IGC measurements for probing surface-chemical changes in the recycling of wood pulp fibers

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The objective of this study was to use dynamic contact angle (DCA) analysis and inverse gas chromatography (IGC) to probe the surface-chemical changes in wood pulp fibers during recycling. A simplified wet-dry-rewet cycle was performed on hardwood bleached kraft fibers to represent the recycling process. The DCA measurements revealed that the overall effect of recycling was an increase in the non-polar (dispersive) component and a corresponding decrease in the polar component of the surface free energy, hence resulting in a total surface free energy that remained essentially unaltered. The DCA experiment also showed that virgin fibers lost both their electron-accepting (γS+) and their electron-donating (γS-) characteristics when converted to paper. Upon rehydration, the fibers recovered some surface acidity (γS+) but surface basicity (γS-) continued to decrease. The changes in polar surface free energy correlate well with the changes in hydroxyl number determined independently using the acetylation method. IGC could not detect changes in the dispersive component of the surface free energy induced by recycling. The acid-base (KA and KB) changes in the IGC measurements were also indistinguishable between virgin fibers and recycled fibers. This research concludes that DCA analysis is preferable to IGC in better reflecting the surface changes in fiber recycling, and γS- can at least be treated as an empirical parameter to complement the hydroxyl availability data in distinguishing among virgin, paper, and recycled fibers.

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Journal of Adhesion Science and Technology