Abstract
Poly(ethylene-alt-maleic acid), PEMAc, is a linear polymer that, along with its isomer polyacrylic acid, has the highest carboxylic acid content of any polymer. The goal of this work was to elucidate the mechanisms that control the amount of PEMAc that is permanently fixed on pulp fibers after the impregnation of dry pulp with a dilute PEMAc solution followed by drying/heating (curing). Two mechanisms by which PEMAc is fixed to cellulose fibers were discovered, chemical, and physical fixation. With room temperature drying only physical fixation is operative. Evidence supports the explanation that physical fixation is a consequence of the slow swelling and dissolution of thick dried PEMAc deposits on fiber surfaces. Chemical fixation includes grafting to cellulose plus enhanced cohesion within thick PEMAc layers, possibly due to interchain crosslinking. The pH of the PEMAc impregnation solution determines the fixation mechanism for curing temperatures above 100 °C. Physical fixation dominates when pH > 8 whereas chemical fixation dominates for impregnation pH values ≤ 7, suggesting the curing reactions require partial or complete protonation of the succinic acid moieties. The yield of impregnated polymer fixed to the fibers after washing depends upon the fixation mechanism. When chemical fixation dominates, the yields for low and high molecular weight PEMAc doses less than 0.1 meq/g (6.3 kg PEMA/tonne dry pulp) were close to or equaled 100%. By contrast, when the primary mechanism is physical fixation, yields are ~50% for high molecular weight PEMAc and 0–20% for low MW PEMAc. These results show that high PEMAc fixation yields can be achieved under curing conditions that could be implemented in pulp drying machines producing dry market pulp.
Highlights
Softwood kraft pulps are known to be one of the strongest wood pulps for paper products, the literature contains many examples of efforts to improve the mechanical properties of pulp by increasing the density of charged groups on fiber surfaces
Evidence supports the explanation that physical fixation is a consequence of the slow swelling and dissolution of thick dried PEMAc deposits on fiber surfaces
By contrast, when the primary mechanism is physical fixation, yields are ~50% for high molecular weight PEMAc and 0-20% for low MW PEMAc. These results show that high PEMAc fixation yields can be achieved under curing conditions that could be implemented in pulp drying machines producing dry market pulp
Summary
Softwood kraft pulps are known to be one of the strongest wood pulps for paper products, the literature contains many examples of efforts to improve the mechanical properties of pulp by increasing the density of charged groups on fiber surfaces. (Belgacem and Gandini 2005) (Laine et al 2000) (Laine et al 2002) Surface charge enhancement can give stronger fiber/fiber joints, increased ion-exchange capacities, increased water absorbency, and increased functional groups for subsequent surface modification. Despite these potential advantages, kraft market pulps with enhanced surface properties are not widely marketed because bleached cellulose fibers are barren, unreactive surfaces that are difficult to chemically modify under the aqueous conditions in a pulp mill. Succinic anhydride moieties on the copolymers can covalently couple to cellulose by forming ester linkages. In a second step the anhydrides react with cellulosic alcohols to form ester linkages
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