The pulp properties were evaluated with different techniques summarized in Figure 13.
Figure 13: Summary of the different pulp properties and the characterization tests evaluated in this thesis.
2.1. Materials and chemicals
Screened and hand-picked softwood chips from BillerudKorsnäs Skärblacka Mill (a mixture of 70 % Spruce (Picea abies) and 30 % of Pine (Pinus sylvestris) were used in this study. For section 3.5, where pulps were analyzed by RX-techniques, softwood chips of only Pine were also included.
The white liquor, for the cooking trials, was prepared from stock solutions of sodium hydroxide (NaOH) and hydrogen sulphide (Na2S) to obtain the desired effective alkali charge and sulfidity in the kraft cooking. For NaOH, pastilles of puriss grade NaOH (VWR International AB, Radnor, PA, USA) were dissolved in deionized water to a concentration of approximately 10 M. Technical grade Na2S flakes (VWR International AB) were dissolved in deionized water to a concentration of approximately 1.5 M NaSH and 1.3 M NaOH.
The raw materials and main topics studied in the different manuscripts are summarized in Table 1.
Table 1: Summary of the pulps and topics studied in the papers and manuscripts.
Paper I Paper II Paper III Paper IV Paper V Paper VI Raw
material
70 % Spruce; 30 % Pine x x x x x x
100 % Pine x
Study subjects
High alkali impregnation x x
Kraft cooking x x x x x x
Oxygen delignification x x x x x
Fiber charges x x x x x
Fiber morphology x x x x x
Tensile strength x x x x
Bleaching x
Supramolecular structure x x
2.2. Methods
2.2.1. Kraft cooking (with standard impregnation)
Kraft cook trials were performed either in 2.5 dm3 steel autoclaves or in a 2 kg recirculated digester. The trials were performed with an effective alkali (EA) of 22 - 23 %, a sulfidity of 30 %, a liquor/water ratio of 4.5 l/kg and a temperature of 160
°C.
For small trials 250 g (o.d.) of wood chips were placed into autoclaves under vacuum and subsequently injected with 5 bar of nitrogen. The autoclaves were placed in a steam-heated glycol bath with rotation and slight inclination, at 100 °C for 30 min for the impregnation step, and at 160 °C for the cooking step. At the end of the cooking, the autoclaves were cooled down in a water bath. For larger quantities of pulp, the recirculated digester was used. The kraft cooking trials were stopped at different H-factors (cooking times) depending on the desired kappa number.
After the cooking step the spent liquor was drained off the chips and collected for analysis. The cooked chips were washed in deionized water for 10 h in self-emptying metal cylinders and then defibrated and screened in the NAF water jet defibrator.
The shives were collected, dried at 105 °C, and weighed.
2.2.2. Kraft cooking (with high alkali impregnation)
The trials with high alkali impregnation were performed using the recirculated digester but with a slightly different procedure. The trials were done with an effective alkali (EA) of 31 %, a sulfidity of 52 %, a liquor/water ratio of 4.5 l/kg and a temperature of 160 °C. The high sulfidity in the impregnation was chosen so that the hydrosulfide ion concentration in the cooking stage would be similar to the hydrosulfide concentration after REF impregnation. The impregnation was done at a temperature of 120 °C. After the impregnation, a black liquor sample was collected
and the residual alkali was measured to ensure that the EA concentration was ca. 0.9 M. Approximately 4 l of free black liquor was removed from the digester, and approximately 4 l, corresponding to the entrapped liquor, remained in the digester.
The alkali concentration in the cooking stage was adjusted to 0.5 M by dilution with deionized water and the temperature was raised to 160 °C at a rate of 3 °C/min. As in the standard kraft cooking, the trials were stopped at different H-factors.
2.2.3. Oxygen delignification
Pulp for oxygen delignification (O) was prepared in polyethylene bags filled with 20 to 60 g of pulp (o.d) with NaOH (between 1 – 4 %), 0.5 % of MgSO4, and water to a consistency of 12 %. The bags were closed by heat-welding, kneaded by hand initially, and then placed in a vibrational paint-shaker for uniform mixing of the chemicals.
After the mixing, the pulps were placed in pressurized steel autoclaves coated with Teflon. The autoclaves were closed, pressurized with 7 O2 bar, and placed in a steam- heated glycol bath at 100 °C, with rotation and a slight inclination. After the delignification, the pulps were washed with filtrate and deionized water.
2.2.4. Bleaching
The bleaching sequences used were OQ(Op)DED and OQ(Op)PP for both cooking experiments with standard and with high alkali impregnation. All the bleaching trials and chelating stages (Q) were conducted at a consistency of 10 % in strong polyethylene plastic bags, double sealed and kneaded in a vibrational paint-shaker for uniform mixing. After mixing, the bags were placed in a water bath at the chosen temperature. The extraction stage (OP) was performed in pressurized autoclaves.
The residual amount of the chemicals after bleaching was determined with a thiosulfate titration. The chemical charges for each sequence are shown in Table 2.
Table 2: Conditions for the chelating "Q" and extraction "Op" stage, followed by the bleaching conditions for chlorine dioxide stages (D0 and D1), alkaline extraction (E) and hydrogen peroxide stages (P1 and P2).
Q Op D0 E D1 P1 P2
Charge (%) 0.27 % DTPA-
0.6 % H2O2;
5bar O2 2.5 0.6 2.5 3 3
Temperature (°C) 90 90 75 70 75 75 90
Time (min) 60 70 70 60 120 120 110
Final pH 5 – 6.2 10.5 – 11.5 3.5 – 4 11–11.5 3.5 – 4 10.5 –11.5 10.5 –11.5
2.2.5. PFI- refining and laboratory handsheets
The pulp samples were refined in a PFI-mill according to ISO 5264-2, and handsheets were prepared according to ISO 5269-1 with deionized water to a grammage of 60 g/m2.
2.3. Analysis
2.3.1. Kappa number
The kappa number was determined according to the ISO 302:2004 standard and it reflects the amount of lignin remaining in the pulp and is also indicative of the degree of delignification.
2.3.2. Carbohydrate composition
The carbohydrate composition was determined according to SCAN-CM 71:09. The xylan, glucomannan and cellulose contents were calculated from the monosaccharides according to Janson (1974).
2.3.3. Total fiber charge measurement
The total fiber charge of the pulp was determined by conductometric titration according to Katz et al. (1984). Before the measurement, the pulps were washed with 0.01 M HCl at a pH of 2 for 30 min, then filtrated and washed with deionized water until the filtrate conductivity was below 5 µS/cm. Subsequently, the pulps were subjected to another wash with 0.001 M NaHCO3 at pH of 9 for 30 min and washed with deionized water until the conductivity was below 5 µS/cm. The final wash was once again performed with 0.01 M HCl at a pH of 2 for 30 min and deionized water, to convert the pulp into H+ proton-form.
For the measurement, the pulps were dispersed in 500 ml of deionized water with 0.01 M HCl and 0.01 M NaCl. The conductometric titration was performed with 0.1 M NaOH, in a microprocessor-controlled titrator (Metrohm – Titrino 702SM).
2.3.4. Schopper-Riegler degree
The Schopper-Riegler degree provides information about the drainability of the pulp and was determined according to ISO 5267–1.
2.3.5. Water retention value
The water retention value (WRV) quantifies how much water the pulp fibers can hold, also known as the fiber swelling ability. WRV measurements were performed according to SCAN-C 62:00, where a wet pulp sample is subjected to centrifugation forces and then is measured after centrifugation and after drying. The remaining water after centrifugation corresponds to the swelling of the fibers and the fiber internal fibrillation. The WRV can be seen as a sum of the water inside the fiber wall (W), the water in the surface (S) and the water inside the lumen (L) - Figure 14.
𝑊𝑅𝑉 = 𝑊 + 𝑆 + 𝐿
2.3.6. Fiber saturation point
The fiber saturation point (FSP) is a solute exclusion test that measures the water inside the fiber wall (W) that is inaccessible to a dextran solution (Figure 14).
Approximately 1 g (o.d.) of pulp was immersed in the dextran solution of a high
molecular weight (2·106 kDa), at a concentration of 1 % for at least 3 days. This measurement was made according to (Stone and Scallan 1967).
Figure 14: Illustration of the fiber cavities and fiber cell wall for the WRV and FSP test.
2.3.7. Physical and mechanical characterization of laboratory handsheets The laboratory handsheets were characterized by grammage (ISO 536), structural thickness (SCAN-P 88:01), and mechanical properties including tensile index, stiffness index, and strain-at-break (ISO 1924-3). All the measurements were made in a controlled environment at 23 °C and 50 % relative humidity.
2.3.8. Fiber morphology
The fiber morphology was evaluated in a Lorentzen & Wettre (L&W) Fiber Tester, where the fibers in an aqueous suspension are transported by a strong flow, sufficient to orientate them in two dimensions without causing deformations. A digital imaging system acquires and analyses the images taken of the fibers and the physical parameters such as fiber length, fiber width, shape factor (defined as straightness of the fiber) and number of kinks are calculated from the software. The curl index was calculated from the shape factor, related to the highest percentage of fibers with lengths between 1.5 and 3 mm, according to Page et al. (1985) by the equation [1]:
𝐶𝑢𝑟𝑙 𝑖𝑛𝑑𝑒𝑥 (%) = 1
𝑠ℎ𝑎𝑝𝑒 𝑓𝑎𝑐𝑡𝑜𝑟− 1, [1]
The measurements were made in duplicate for each pulp sample at different refining levels.