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8TH INTERNATIONAL PAPER AND COATING CHEMISTRY SYMPOSIUM, 2012
Nordic Pulp and Paper Research Journal Vol 28 no 1/2013 6
Topo-chemical modification of fibres by grafting of carboxymethyl cellulose in pilot scale Mikael Ankerfors, Elisabeth Duker and Tom Lindstrm
KEYWORDS: Carboxymethyl cellulose, CMC, grafting, Carboxymethylation, Process integration, Sheet formation, Fibre flocculation SUMMARY: The aim of this study was to graft carboxymethyl cellulose (CMC) on to bleached softwood kraft pulp at temperatures below 100C and to do a pilot paper machine trial in order to examine the influence of the CMC on dewatering, sheet formation and mechanical properties.
During the pilot trial, one CMC grafted pulp was compared to a pulp with 3 different refining degrees.
It was shown that CMC-grafting improves the mechanical properties of paper with only a minor effect on the sheet density. It was also shown that the CMC grafting is less detrimental to dewatering than refining and at a certain tensile index a higher dry content after pressing could be reached. The formation number of the paper produced in the FEX trial was not significantly affected by the addition of CMC. ADDRESSES OF THE AUTHORS: Mikael Ankerfors (email@example.com), Tom Lindstrm (firstname.lastname@example.org), Innventia AB, Box 5604, SE-114 86 Stockholm, and Elisabeth Duker (email@example.com), BillerudKorsns AB, SE-718 80 Frvi (previously an employee of Innventia). Corresponding author: Mikael Ankerfors There are several parameters affecting the strength properties of paper such as fibre-fibre bonded area (relative bonded area), specific bond strength, fibre length, sheet formation and stress concentrations. Traditionally, strength improvements are achieved by refining the fibres or by the addition of chemical additives. Both of these strategies have downsides as well. An increased amount of refining, increases the flexibility of fibres, swells them, and fibrillates their surfaces, which in turn facilitate the development of strong intermolecular forces by sheet consolidation during drying of the web. Unfortunately, refining also leads to an enhanced paper sheet density, which in turn has a negative impact on bending stiffness and on the light scattering coefficient of the sheet. Additionally, refining of pulp requires energy and will, due to fibre swelling, lead to an increased dewatering resistance in the forming and pressing operations.
Chemical dry strength additives such as modified starches or gums are not considered to affect the fibre flexibility, instead a dry strength agent may work by three different mechanisms (Lindstrm et al. 2005): i) by consolidating the sheet, ii) by increasing the specific bond strength or, as was shown by Lindstrm et al. (1985), iii) by decreasing the local stress concentrations in the sheet. Today, many of the most frequently used
additives are consolidation agents and in addition to an increased dry strength their presence will lead to an increased sheet density.
One method (developed at Innventia) to improve the paper sheet strength properties, without influencing sheet consolidation, is the use of bipolar activators, e.g., carboxymethyl cellulose (CMC). It was shown that CMC, under certain conditions, can be irreversibly attached to cellulose fibres and increase the charge density of fibres (e.g. Laine et al. 2000). Due to strong repulsion between the negatively charged fibres and the negatively charged CMC, a sufficiently high electrolyte concentration is required in order to attach CMC to the fibre. In addition, the attached amount of CMC increases with increasing temperature and exposure time (Laine et al. 2000). Provided that the CMC used has a sufficiently high molecular weight, the attachment occurs selectively on the fibre surface. The exact mechanism of interaction between the fibre surface and the CMC molecule is not yet fully understood but a co-crystallisation mechanism (Yllner, Enstrm 1957, Laine et al. 2000) has been suggested. The irreversible attachment process of CMC on the fibre surface is referred to as surface car-boxymethylation or CMC physical grafting.
Grafting of CMC is known to increase the water retention value (WRV) due to the water holding capacity of the CMC layer (Laine et al. 2002). However, laboratory experiments showed that this increased water retention did not affect the dewatering (Mei 2002). Another advantage with CMC grafting is a decreased fibre flocculation due to a decreased inter-fibre friction (Yan et al. 2006a).
CMC grafting has, up to now, only been performed and studied in a laboratory environment, but the advantages mentioned above brought about a trial on Innventias pilot paper machine, FEX. The main objective of the study was to demonstrate that CMC could be grafted onto pulp in a larger scale and to quantify paper improvements (paper sheet strength properties, runnability and formation) on a pilot paper machine.
Experimental The experiments were done in two sets of trials. One pilot paper machine trial on Innventias FEX machine and one fibre flocculation trial using the pulps from the FEX-trial (Trial 1) and one fibre flocculation trial studying the influence of conductivity using lab-grafted pulps (Trial 2).
Material The pulp used in all experiments was an industrial Elementary Chlorine Free (ECF) bleached (bleaching sequence was OD(EOP)DED) and dried softwood kraft pulp from Sdra Tofte mill, Sdra Cell, Sweden with the trade name Sdra Black R.
8TH INTERNATIONAL PAPER AND COATING CHEMISTRY SYMPOSIUM, 2012
7 Nordic Pulp and Paper Research Journal Vol 28 no 1/2013
The carboxymethyl cellulose (CMC) used was sodium-CMC from Hercules, Sweden with trade name Aquasorb A-500. The CMC had a DS of approximately 0.4 and a high molecular weight of 700-1000 kDa (values from the supplier). The CMC was dissolved in tap water (Trial 1: 6.7 kg CMC/m3, Trial 2: 4.4 kg CMC/m3) and the CMC-solution was stirred overnight before use.
Trial 1: Pilot trial at the FEX pilot paper machine Pulp treatment The dried pulp was reslushed in a pilot scale pulper for six minutes before use.
Since the idea was to compare CMC-grafting with refining the pulp was also refined to different levels. The refining was done in a Twin Flow disc refiner from Voith, Germany (TF2-E, refining disc diameter = 26 inch) with a refining energy of 50, 100 and 150 kWh/tonne (see Table 1 for details). The CMC grafted pulp was also refined with 50 kWh/tonne before the grafting treatment. CMC grafting In earlier work by Laine et al (2000), CMC grafting was carried out in laboratory scale at high temperatures (up to 120 C) and electrolyte concentrations (0.05 M CaCl2). In order to prevent high conductivity on the paper machine the electrolyte concentration should preferably be lowered, also in order to be able to use an atmospheric reaction vessel, the temperature needed to be reduced. The grafting conditions used in this work are shown in Table 2.
The CMC grafting was carried out in a 20 m3 stirred vessel and heating was done by steam in an external heat exchanger, which the pulp was pumped through. Due to the volumes, the heating to the target temperature 95C, took some time (see Table 3). When the pulp had a temperature of 85C, the CMC solution was added. The pH was adjusted to approximately 8 using NaOH pellets and the pre-heating was continued until the temperature reached 95C. This temperature was then held for 1.5 h in the presence of 0.01 M CaCl2. The total contact time, including pre-heating, was 420 min. However, since CMC grafting took place at a higher temperature, this time cannot be directly said to be the reaction time. After the grafting process, the pulp was cooled down to a temperature of approximately 70C and left to stand overnight.
For comparison the pulps that were not CMC grafted were also heat treated in the same way but without the CMC addition. Due to different starting temperatures, the total time was a bit shorter than for the CMC pulp (approximately 370 min in total). Another reason for
Table 1. Refining conditions.
Pulp Refining energy
[kWh/tonne] Specific edge load
[Ws/m] CMC pulp 50 0.6 Reference pulp 50 0.6 Reference pulp 100 1.2 Reference pulp 150 1.8
Table 2. Modified conditions for CMC grafting. Process parameters CMC addition 10 kg CMC/tonne fibres Final temperature 95C Reaction time (at final temperature) 1.5 h pH 8 Electrolyte concentration (CaCl2) 0.01 M Pulp concentration Around 4%
heat treatment without CMC, was that also sheet formation was studied and the paper making temperature is known to affect sheet formation (Hourani 1988) because the collision frequency increases with temperature as this drag/inertia decreases when the viscosity decreases, resulting in deteriorated sheet formation (Yan et al. 2006a). Therefore, the different pulps had to be produced at the same temperature on the paper machine to enable a fair comparison. For practical reasons, all pulps were therefore heated up and cooled down in the same way.
For a more detailed description of the temperature change during heating up and the reaction conditions, see Table 3.
Pulp samples, for the determination of total charge density, surface charge density and water retention value, were taken out for all four different pulps, when the digestion was completed. Pulp was also taken out for further flocculation analysis in a flow loop system. Papermaking on the FEX pilot paper machine Papermaking, using CMC grafted pulp and three differently refined reference pulps, was performed on Innventias pilot paper machine FEX. In this trial a twin-wire roll former was used. The machine speed was 400 m/min and the paper produced had an average gramm