Determination of the Degree of Oxidation in Dialdehyde Cellulose Using Near Infrared Spectroscopy

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1 Determination of the Degree of Oxidation in Dialdehyde Cellulose Using Near Infrared Spectroscopy Bestämning av oxidationsgraden i dialdehydcellulosa med nära infraröd spektroskopi Carl-Magnus Brandén Faculty of Health, Science and Technology Chemical Engineering 30 credits Supervisors: Lars Renman, Karlstad University Mari-Ann Norborg and Adrianna Svensson, Stora Enso Examiner: Lars Järnström

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9 Mw AGU Mw OU x y m m = Mw OU x + Mw AGU y y = m Mw OU x Mw AGU DO( ) = x x + y 100

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13 2 2.1 Experimental Materials Sodium periodate (Fluka Analytical) 99, 8% Hydroxylamine hydrochloride (Fluka Analytical) 99, 0% Sodium hydroxide pellets (Sigma-Aldrich) 97, 0% Never dried softwood Kraft pulp from Imatra mill (milled to 30 SR) Never dried hardwood Dissolving pulp from Enocell mill (not milled) (Carbohydrate analysis results of the pulps can be found in Appendix Table 11.) 2.2 Oxidation of cellulose In the production of samples the reaction conditions surrounding the oxidation were kept constant except for a variation in reaction times. A 2 % pulp solution at 50 C with a NaIO4 /cellulose ratio of 1.5 (w/w). With an unadjusted ph, which initially was around ph 4.9. In the blank samples the NaIO4 was not used. Much of the method was inspired by the previous work of E. Höglund, 2015 [22]. A typical oxidation proceeded as follows: 1.2 g pulp (dry weight) was added into a 100 ml round-bottom flask and preheated deionized water was added so that the total weight was 58.2 g. The flask was then placed in a heated polyethylene glycol (PEG) bath on a magnetic stirrer. When the solution temperature stabilized at 50 C, 1.8 g of NaIO4 was added. The flask was then covered with aluminium foil to avoid the photo decomposition of periodate(see Figure 6). After a set reaction time, the flask was removed from the PEG bath and swiftly placed in a cool water bath. The product was filtered off using a Buchner funnel with an extra wire mesh(pore size mm2 ). The product was then washed five times using 30 ml of deionized water each time. The product was then removed from the mesh and placed in a zip-bag. Spectra were taken the same day and then the sample was stored in a refrigerator (5 C) until the DO analysis could be performed the following day. Figure 6: The setup for the oxidation process. 9

14 2.3 Determining the degree of oxidation To determine the degree of oxidation, two measurements were made for each sample. The resulting DO for a sample was taken as the mean value of the two measurements. Before the reaction a 0.25 M hydroxylamine hydrochloride solution was ph adjusted to ph 4 using 0.1 M Sodium hydroxide and a Mettler Toledo Seven2Go ph-meter. For each measurement 0.1 g (dry weight) of sample (torn into smaller pieces (Fig 7)) and a magnetic-bar was placed in the reaction vessel. 25 ml of the hydroxylamine hydrochloride solution was then added and left to react for 2h 30 min at ca 200 r.p.m. The product was then filtered off to a pre-weighed Munktell filter (Grade 3) using a Buchner funnel under suction. Small amounts of deionized water were used to rinse the flask and pieces of product on the filter. The product was left to dry in an oven (105 C) for at least 1 hour and then put in a desiccator to cool down prior to weighing. Samples where weighed on a Mettler AE160 scale (standard deviation 0.1 mg). The filtrate was transferred into a beaker and titrated back to ph 4 using 0.1 M Sodium hydroxide and the volume needed was noted. The DO could then be calculated using Equations 1 and 2 (see subsection 1.1.3). Figure 7: Samples torn into smaller pieces. 2.4 Dry content The dry content was determined using between g of moist sample in a Mettler Toledo HR73 halogen moisture analyzer (Figure 8). The sample size used for the dry content analysis was very small and as the measurement has an standard deviation of 1 mg, the results reported might deviate with ±0.66% (±2σ), just from the measurement deviation alone if the wet sample weighs 0.3 g. Dry content might vary even more within the sample and only one analysis for the dry content was performed on each sample. Therefore the dry content presented in this work is not an exact value, but rather a pointer, due to the possibility of large variations. Besides the dry content analysis, the moisture analyser was also used to pre-weigh the filters used in the degree of oxidation determination. Figure 8: Moisture analyser. 10

15 2.5 Collecting spectra The software Vision 3.4 and a Perstorp NIRS System 6500 ( nm) with a reflectance module was used for the collection of spectra. The samples were torn into smaller pieces and then manually packed into a 4 mm thick round sample cup with a quartz window(see Figure 9). For each sample eight spectra of thirty-two scans were collected from different orientations. This was done since the instrument did not rotate the sample by itself. A spectrum can be affected by several variables, such as moisture differences within the sample or voids created during the manual packing. By creating an averaged spectrum from the collection of spectra, a more representative spectrum can be acquired. The sample pellet created during the manual packing was then stored in the refrigerator for later experiments. For six of the samples used in the training set a second run of spectra was collected after adding some drops of water to the pellet. This was done to see if a larger variance in the moisture content could be introduced. For four of those samples the pellet weight was measured before and after the addition of water. The same procedure was applied on the samples of the prediction set (see section 2.6.3). Samples which had been stored in the refrigerator were tested after more then two weeks to see how well the model performed on older samples. It has been reported that reactivity decreases within this time [19] and it would be interesting to see if this change appeared in the model; the same sample pellet as in the first sampling was used. NIR-spectra are temperature sensitive, therefore the stored samples where removed from the refrigerator a few hours before spectra was collected, so that the samples would have time to acquire room temperature. Self-diagnostic tests of the instrument were performed prior to the collection of spectra in order to verify the reliability. It should be noted that the instrument did not pass the test in the region of nm. The parameter that did not pass the tests were the instruments noise RMS value, which was 1.5 times the diagnostic tests threshold. Figure 9: The NIR instrument used for the collection of spectra and a filled sample cup with an aluminium insert used to reduce the amount of sample needed. To complement the interpretation of the NIR spectra, mid-ir spectra were collected from three stored samples with different degree of oxidation, using Omnic 9.2 software and a Thermo Scientific Nicolet is10 FT-IR Spectrometer with a SmartOrbit ATR accessory. 11

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47 N = 18 t 2.1 DO avg ± t s pooled / n = DO avg ± / 2 = DO avg ± 1.02

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