Wet strength improvement of paper via crosslinking of cellulose using polymeric carboxylic acids and aldehydes
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The wet-strength resins currently used in papermaking industry are under environmental scrutiny for emission of carcinogenic formaldehyde or adsorbable organic halides. In this research, polycarboxylic acids, dialdehydes, and the combination of polycarboxylic acid and dialdehydes with poly(vinyl alcohol) (PVA) were investigated as potential environmentally friendly alternatives. Kraft paper was treated by "pad-drycure" process and evaluated for its wet strength, dry strength, Z-direction tensile strength, stretch, breaking energy, folding endurance, swelling, and water sorption. Two polycarboxylic acids, low molecular weight (MW) poly(maleic acid) (PMA) and high MW poly(methyl vinyl ether-maleic acid) (PMMA), demonstrated similar efficiency for improving wet strength. However, PMA caused paper embrittlement and seriously reduced folding endurance, while PMMA significantly improved dry strength and folding endurance. Scanning electron microscopic examination revealed that the fibers in PMA-treated paper were less swollen when soaked in water than those in PMMA-treated one. Dry performance of PMA-treated paper was greatly improved by combining PVA into the crosslinking system. Dialdehydes combined with PVA were also investigated as wet-strength agents. Glyoxal alone provided excellent temporary wet strength without the need of catalyst and exposure to elevated temperature, while glutaraldehyde alone imparted durable wet strength at the expense of folding endurance, particularly at high level of wet strength. Combining PVA as co-crosslinker significantly improved wet strength, dry strength, folding endurance, and water sorption of paper crosslinked by glutaraldehyde. The glutaraldehyde/PVA system shows high efficiency at low curing temperature around 110oC and pH close to neutral. The location of crosslinks was vital to the properties of treated paper. The different behavior of wet-strength resins originated from the different distribution of crosslinks in the intrafiber and interfiber areas. Low MW crosslinkers were able to penetrate into the fiber interior to form inter-lamellae and inter-fibrillar crosslinks, while high MW resins tended to stay on the fiber surface and crossing areas to produce interfiber crosslinks. PVA reacted with glutaraldehyde, and PVA/glutaraldehyde promoted the formation of interfiber crosslinks. Compared with intrafiber crosslinks, interfiber crosslinks provided not only higher efficiency for improving wet strength but also extra benefit for improving dry properties of paper.