Determination of Degree of Polymerization of Cellulose in Ligneous Papers
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Removing the undissolved lignin from cadoxen solutions does not necessarily make the DP values more reliable. The dissolved lignin affects the viscosity of solutions of ligneous papers, which in turn affects the calculated DP values of cellulose. In the past, numerous attempts have been made to determine the DP of cellulose in ligneous pulps or papers. The usual approach has been to delignify ligneous materials prior to DP measurements. The delignification procedures caused depolymerization of cellulose to various degrees, thus introducing substantial errors in the DP values obtained [7]. EXPERIMENTAL Delignification The procedure has been developed from a method employed by the Pulp and Paper Research Institute of Canada for chlorite delignification of wood and mechanical pulps [8]. The chlorite solution is prepared by mixing the following reagents in the specified order and proportion: 40 ml of deionized water, 0.125 ml of acetic acid and 0.375 g of sodium chlorite (NaC10 2). Four grams of paper strips are placed in an Erlenmeyer flask and 160 ml of freshly prepared chlorite solution is added. The paper should be completely immersed in the solution. The flask is covered with Parafilm and placed in a water bath at 700C. The period for delignification is determined individually for each paper, as discussed below. As delignification proceeds, the contents of the flask are agitated at 15 minutes intervals. The reaction is stopped by placing the flask in an ice bath and cooling to below 100C. Samples retaining the form of strips are washed with deionized water until neutral. Samples in the form of a pulp suspension are filtered using a sintered glass crucible (coarse) and washed with deionized water until neutral. Before further treatment, the samples are dried for at least 3 days between sheets of blotting paper at room temperature. Viscosity measurements The following procedure is a modification of the method used by Doty [9] and Burgess [10]. Paper samples are treated with sodium borohydride to stabilize oxidizing groups [111, air dried, and have their moisture content determined according to TAPPI Standard T 412 Om-88. The paper is then cut into uniformly small pieces (about 2 x 2 mm). Twenty millilitres of 100% cadoxen reagent are added to the weighed sample. The weight of sample, usually between 50 and 300 mg, depends on the required efflux time (between 2 and 3 minutes for all dilutions). The suspension of paper in cadoxen is stirred with a magnetic stirrer at room temperature until the paper is dissolved (at least 60 minutes for non-ligneous paper, overnight for ligneous paper). The solution is diluted with 20 ml of water and centrifuged for 15 min at 3000 rpm at room temperature. The decanted supernatant (stock solution) is used for viscosity measurements. For viscosity measurements, dilutions of the stock solution in concentrations of 50, 67 and 100% are prepared. The efflux time for each dilution is measured at 30_+0.1 0 C using Canon-Fenske viscometer and the intrinsic viscosity rii of the cellulose soluti
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