Good bonding for low-energy HT-CTMP by press drying

Transcription

1 Good bonding for low-energy HT-CTMP by press drying By T. Pynnönen, E. Hiltunen, J. Paltakari, J.E. Laine and H. Paulapuro Abstract: The effect of press drying on the sheet properties of HT-CTMP (high-temperature CTMP) was determined. In the HT-CTMP process, preheating and refining are carried out at high temperature (above 165 C). The energy consumption is very low, but the resulting fibres have poor bonding ability due to the lignin coating on the fibre surface. One potential solution to improve bonding could be press drying, which is carried out at elevated temperature under pressure. The elevated temperature promotes softening and presumably the flow of wood polymers on the fibre surface, which leads to enhanced bonding. Laboratory-scale studies showed that press drying significantly improves the bonding of the lignin-coated HT-CTMP fibres. I T ISwell known that the energy consumption of the wood-refining process is dramatically reduced if the chips are preheated and refined at elevated temperature (above 165 C). The energy consumption is approximately kwh/t, which is only a fraction of the required energy consumption of the conventional TMP process. The resulting low-energy pulp is known as hightemperature thermomechanical pulp (HTMP). HTMP has also been referred to as Asplund pulp. It is being used commercially to manufacture medium-density fibreboard (MDF); therefore, the designation as MDF-pulp is also in use. It is generally assumed that the essentially lower energy consumption is a consequence of the fact that the glass transition temperature of lignin is exceeded at high refining temperatures. When lignin is in soft state, the fibre separation occurs along the lignin-rich middle lamella. Consequently, the energy required to separate the fibres from each other is very low, but the resulting fibres are coated with lignin. The lignin-covered fibre surface is smooth, unfibrillated and hydrophobic in nature. The fibres are stiff and have poor bonding ability. However, because HTMP contains mostly whole fibres, the average fibre length is high, whereas the fines content is very low. Unfortunately, high temperature strongly darkens the fibres. Because of the poor bonding ability and low brightness, high-temperature fibres have found limited use in the paper industry, though some studies have been made concerning their use. Allison [1] treated HTMP (Pinus radiata) with ozone and refined it further in a laboratory PFI mill. Ozonation and secondary refining were found to improve the strength properties substantially. After ozonation, it was possible to fibrillate the modified fibre surface by PFI refining. According to Allison [1], ozonation modifies the lignin on the fibre surface, so HTMP fibres can be further refined. Kibblewhite et al. [2] suggest that ozonation selectively modifies the surfaces of HTMP fibres and fines, which increases the bonding intensity rather than the bonded area. Ozonation of HTMP had only a slight influence on brightness, but a substantial increase in bright- ness was obtained by peroxide bleaching. Unfortunately, the ozone treatment was found to reduce the yield. Allison [3] also combined chemical treatment of wood chips with high-temperature refining. Pinus radiata chips were impregnated with sodium sulphite (Na 2 SO 3 ) and refined at elevated temperature. After refining, the resulting HT-CTMP (high-temperature chemithermomechanical pulp) was treated with ozone and refined further in a PFI mill. The total energy consumption was estimated at 1,000-1,300 kwh/t. Compared with conventional TMP and CTMP, the strength properties were generally better, but the optical properties were poorer. Brightness and yield suffered from the high temperature. However, sulphite impregnation of chips was found to produce a significant improvement in brightness at high refining temperature. Sulphonation also improved the T. PYNNÖNEN Tuomo.Pynnonen@hut.fi J. PALTAKARI E. HILTUNEN J.E. LAINE H. PAULAPURO Pulp & Paper Canada T :3 (2004) 33

2 Metal Wire Hotplate Blotters Wet sheet Coldplate FIG. 1. Effect of moisture content on glass transition temperature [8]. FIG. 2. Laboratory-scale press drying equipment. FIG. 3. Bauer McNett fractions of bleached pulps. FIG. 4. Effect of hot plate temperature on sheet density. Press drying pressure was 1,300 kpa. yield at high temperature. Similar findings were also made by Höglund et al. [4], who studied the MTMP-process (modified thermo-mechanical pulp). Wood chips were impregnated with differing amounts of sodium sulphite and preheated to 130 or 170 C. After preheating, the chips were refined at atmospheric pressure. The high preheating temperature was found to lower the yield. However, the loss in yield decreased significantly when the sodium sulphite content of chips was 2-12%. Sodium sulphite is assumed to act as a buffer, reducing the dissolution of wood polymers. The darkening of the pulp at high temperature was also detected, but it diminished considerably as the sodium sulphite content increased. The energy consumption of the HT- CTMP process is low [5-7]. The HT-CTMP fibre surface is smooth and unfibrillated, but sulphonation has reduced the stiffness of the long fibres [5]. HT-CTMP has a high fraction of long fibres, which may closely resemble chemical pulp [5]. Consequently, the average fibre length is high, and the fines content is low. Long fibres have the potential to generate high tear strength and fracture toughness. Because the long-fibre fraction is high and the shive content is low, HT-CTMP has been TABLE I. Some properties of bleached pulps used in the study. HT-CTMP 450,kWh/t HT-CTMP 700,kWh/t CTMP 1150,kWh/t Sodium sulphite dose (%) Refining overpressure (kpa) Energy consumption excluding reject treatment (kwh/t) ,150 Peroxide dose (%) Freeness (ml CSF) Length-weighted fibre length (mm) found to be able to suit for the manufacture of fluff and tissue [6]. One potential solution to expand the papermaking potential of HT-CTMP could be press drying. In press drying, the wet paper web is dried at elevated temperature under z-directional pressure. The temperature is high enough to promote softening of lignin and hemicelluloses. However, a requisite for the softening of lignin is an adequate moisture content during press drying, Fig. 1. As a consequence of the softening of lignin, the fibres become more flexible. This leads to densification of the sheet and enhanced bonding. In addition, it is assumed that lignin and hemicelluloses also flow to some extent on the fibre surface, which improves the bonding between fibres [9]. From the above, it is reasonable to expect that the lignin-coated HT-CTMP fibres lend themselves very well to press drying. The Condebelt drying process, which is based on drying at elevated temperature under z-directional pressure, has been in commercial use for several years [10]. There are some published results on the press drying of high-temperature pulps. Philips [11] introduced press drying of ozonated HTMP. In addition, press drying of spruce HTMP sheets has been found to improve bonding [8]. Höglund [12] pressed and dried HT-CTMP sheets simultaneously, reporting that the tensile index increases substantially when the :3 (2004) T 58 Pulp & Paper Canada

3 FIG. 5. Effect of hot plate temperature on tensile index. Error bars show 95% confidence intervals. FIG. 6. Relationship between sheet density and tensile index, when press drying was carried out at pressures of 500, 1,300 and 2,000 kpa. Hot plate temperature was 125 C. FIG. 7. Effect of press drying on tensile index. Tensile index is plotted against sheet density. temperature is raised from 20 to 60 C. However, there appears to be little experimental data on the press drying of HT-CTMP. The objective of the present study was to determine the effect of press drying on the sheet properties of high-temperature chemithermomechanical pulp (HT-CTMP). EXPERIMENTAL Materials: Altogether five pulps were used in the study. Three peroxide-bleached pulps were obtained from a paper mill, Table I, and two non-bleached pulps were originally defibrated in a pilot plant using chips with two different sodium sulphite contents. All pulps were refined from spruce chips (Picea abies). The non-bleached pulps were refined at an overpressure of 600 kpa. The chemically bound sodium sulphite contents of chips were 0.3 and 0.65%. The energy consumptions were: HT-CTMP 0.3% 400 kwh/t and HT-CTMP 0.65% 300 kwh/t. All pulps were obtained as dried pulps, which were hot-disintegrated according to SCAN-M 10:77. Fibre fractionation was carried out in a Bauer McNett apparatus (SCAN-M 6:69). Handsheet Preparation: Handsheets were formed with whitewater circulation; 80 g/m 2 handsheets were made from the bleached pulps and 100 g/m 2 handsheets from non-bleached pulps. To evaluate the properties of conventional handsheets, wet pressing and drying were carried out according to the SCAN-C 26:76 standard method. In addition, some handsheets made from bleached pulps were heavily wet-pressed (9 min at 1,500 kpa FIG. 8. Effect of press drying on Scott bond. FIG. 9. Effect of sodium sulphite content on Scott bond after press drying. Scott bond plotted as a function of sheet density. and 16 min at 2,000 kpa), and finally dried conventionally on plates. Handsheets for the press drying experiments were wetpressed at a pressure of 460 kpa; the wet pressing time was 4 min for the bleached pulps and 7 min for the non-bleached pulps. Standard methods were used to evaluate the handsheet properties: sheet density (SCAN-P 7:96), tensile strength (SCAN-P Pulp & Paper Canada T :3 (2004) 35

4 TABLE III. Some sheet properties of conventionally dried sheets. Bleached pulps Non-bleached pulps HT-CTMP 450,kWh/t HT-CTMP 700,kWh/t CTMP 1150,kWh/t HT-CTMP 0.3% HT-CTMP 0.65% Density (kg/m 3 ) Tensile index (Nm/g) ISO-brightness (%) Light scattering coefficient (m 2 /kg) TABLE II. Press drying conditions and pressing times of bleached pulps. Hot plate Pressure Press drying time (s) temperature ( C) (kpa) HT-CTMP 450,kWh/t HT-CTMP 700,kWh/t CTMP 1150,kWh/t 75 1, , , , , :80), Scott bond (TAPPI T 596 pm-00) and optical properties (SCAN-P 3:93 and SCAN-P 8:93). Press Drying Equipment: Press drying of handsheets was carried out with a static laboratory plate press. The configuration during press drying is shown in Fig. 2. Handsheets were press-dried until a dryness of 90-95% was reached. The temperature of the cold plate was 10 C and the temperature was kept constant by running water through the plate. Press Drying of Bleached Pulps: The initial moisture contents of the handsheets were: HT-CTMP 450,kWh/t 62%, HT-CTMP 700,kWh/t 62.5% and CTMP 1150,kWh/t 66%. Press drying was performed under various conditions, which led to different press drying times, Table II. Press Drying of Non-Bleached Pulps: Press drying of non-bleached handsheets was also carried out with the plate press, Fig. 2. However, an additional blotter was placed between the wet handsheet and the hot plate. The initial moisture contents were: HT-CTMP 0.3% 47.5% and HT-CTMP 0.65% 47%. The moisture content after press drying was 8%. Press drying was carried out at different hot plate temperatures at a constant pressure of 1,800 kpa. The hot plate temperatures were 80, 100, 120, 140, 160, 185 and 210 C, and the press drying times were 140, 70, 35, 19, 13, 7.5 and 4.5 s, respectively. RESULTS, DISCUSSION The fibre length fractions determined with the Bauer McNett apparatus, Fig. 3, confirmed that the used bleached HT-CTMP pulps had a high fraction of long fibres and a low content of fines. The average fibre lengths of the bleached HT-CTMP pulps were also very high (Table I). The combination of a high amount long fibres and a low content of fines resulted in high bulk, when sheets were made conventionally, Table III. The four HT-CTMP pulps had low sheet densities and tensile strengths. In general, compared with HTMP pulps [1,8], the peroxide-bleached HT-CTMP pulps had a high level of brightness. The brightness levels of the non-bleached HT-CTMP pulps were also reasonable. Effect of Press Drying of Bleached HT- CTMP: Press drying of bleached HT-CTMP handsheets was carried out at different hot plate temperatures (75, 100, 125 and 150 C). Press drying led predictably to high sheet density, Fig. 4, regardless of the used hot plate temperature. At high temperature, lignin softens, which makes the originally stiff fibres flexible. Because of the z-directional pressure applied in press drying, the softened fibres will consequently flatten and conform to each other, which leads to densification of the sheet. However, it must be kept in mind that a given hot plate temperature is not necessarily the same as the actual sheet temperature during press drying. For example, when the hot plate temperature is 75 C, the wet sheet will most probably reach that temperature at the press drying time of s. But the wet sheet will hardly reach the hot plate temperature of 150 C, when the press drying time is below 2 s. Depending on the press drying times, the drying intensities can thus be quite different. The effect of the hot plate temperature on the tensile index is illustrated in Fig. 5, which shows that already at 75 C a relatively high tensile strength level is reached. According to Unkila el al. [12], a wet web temperature of 120 C during Condebelt drying is a prerequisite for enhanced bonding between fibres. However, it is well known that sulphonation significantly decreases the softening temperature of wood [14,15]. Therefore, it should be possible to press dry CTMP or HT-CTMP at lower temperatures than pure mechanical pulps for the same tensile strength. The effect of pressure (500, 1,300 and 2,000 kpa) during press drying was also determined at a hot plate temperature of 125 C (Fig. 6). When the press drying pressure is raised, both sheet density and tensile strength increase, as can be expected. The effect of press drying on the tensile index is illustrated in Fig. 7, in which tensile indexes of all test points (Table II) are plotted as a function of sheet density. The normally dried handsheets were heavily wet-pressed to obtain a high density level, and then finally dried conventionally on plates. Compared with a normal drying at the same density level, press drying improved the tensile index significantly. Higher tensile strength is most likely related to increased interfibre bonding. Similarly, press drying also clearly enhances Scott bond, Fig. 8, which is an indication of higher interfibre bonding. At elevated temperature, the wood polymers can be assumed to soften and flow on the fibre surface, leading to increased bonding between fibres. The wood polymers probably function as a glue between fibres. The relatively biggest improvements in strength properties were obtained with the HT-CTMP 450,kWh/t pulp: the tensile index rose by more than 50% and Scott bond increased by %. Regardless of the very low level of fines in the HT-CTMP 450,kWh/t pulp, good bonding was achieved. Effect of Sulphonation Rate: The effect of the sulphonation rate of wood on press drying was studied with two HT-CTMP pulps that were not bleached. The chemically bound sodium sulphite contents of chips were 0.3 and 0.65%. Press drying was carried out at different hot plate temperatures ( C) at constant pressure (1,800 kpa). The relationship between Scott bond and sheet density is illustrated in Fig. 9. In general, press drying led to high sheet density. Both HT-CTMP 0.3% and HT-CTMP 0.65% had low densities and practically the same tensile strengths when the properties of conventionally dried sheets were evaluated (Table III). However, when the sheets are press-dried, a higher sulphonation rate leads to clearly better Scott bond at the same density level. Because of their increased flexibility, more sulphonated :3 (2004) T 60 Pulp & Paper Canada

5 fibres probably give greater advantages in press drying. More sulphonated fibres have also lower softening temperature. Furthermore, it can also be speculated that sulphonation modifies the fibre surface, which increases the bonding intensity in press drying. CONCLUSIONS The HT-CTMP process is a CTMP process in which wood chips are preheated and refined at high temperature. The energy consumption is very low, but the resulting fibres have poor bonding ability because of the lignin coating on the fibre surface. HT- CTMP has a large fraction of long fibres and a low amount of fines. In general, high temperature has a darkening effect on fibres, but HT-CTMP can be manufactured to a reasonable brightness level. One potential solution to expand the use of HT-CTMP could be press drying. Press drying is carried out at high temperature under z-directional pressure, causing wood polymers on the fibre surface to soften. It is generally assumed that the wood polymers also flow on the fibre surface, which leads to enhanced bonding. Therefore, press drying should be very advantageous for the lignin-coated HT-CTMP fibres. Laboratory-scale studies showed that press drying significantly improved the strength properties of HT-CTMP handsheets. Compared with conventionally wetpressed and dried HT-CTMP handsheets, the tensile index of press-dried handsheets was 15-75% higher at the same density level. Scott bond also improved considerably, which is probably an indication of greater interfibre bonding. An increased in the sulphonation rate of HT-CTMP resulted in enhanced Scott bond. Because of its lower softening temperature of sulphonated pulp, sulphonated pulps can probably be press-dried at lower temperatures than other mechanical pulps. Press drying leads to high sheet density, which may impede more widespread use of the process. However, the technology needed for this is commercially available at present. ACKNOWLEDGEMENTS The authors thank Mr. Teemu Kara and Mr. Lionel Clerc for their technical assistance. LITERATURE 1. ALLISON, R.W. Effect of Ozone on High-temperature Thermomechanical pulp. Appita J. 32(4): (1979). 2. KIBBLEWHITE, R.P., BROOKES, D., ALLISON R.W. Effect of Ozone on the Fiber Characteristics of Thermomechanical Pulps. Tappi J. 63(4): (1980). 3. ALLISON, R.W. Low Energy Pulping Through Ozone Modification. Appita J. 34(3): (1980). 4. HÖGLUND, H., BODIN, O. Modified Thermomechanical Pulp. Svensk Papperstidn. 79(11): (1976). 5. HÖGLUND, H., WILHELMSSON, K. The Product Must Determine the Choice of Wood Type in Mechanical Pulping. Proc., Intl. Mech. Pulping Conf., Oslo, Norway, 1-22 (1993). 6. HÖGLUND, H., BÄCK, R., DANIELSON, O., FALK, B. CTMP-Process, Mölnlycke Ab (Sweden). U.S. Patent (Issued Mar. 4, 1997). 7. HÖGLUND, H. Mechanical Pulp Fibres for New and Improved Paper Grades. Proc. 7th Intl. Conf. on New Available Technol., Stockholm, Sweden, (2002). 8. PYNNÖNEN, T., PALTAKARI, J., HILTUNEN, E., LAINE, J.E., PAULAPURO, H. Effect of Press Drying on Sheet Properties of High-temperature Thermomechanical pulp (HTMP). Appita J. 55(3): (2002). 9. KUNNAS, L., PAULAPURO, H., LEHTINEN, J., KIVIRANTA, A. The Effect of Condebelt Drying on the Structure of Fiber Bonds. Proc., Papermakers Conf., Atlanta, (1993). 10. RETULAINEN, E., HÄMÄLÄINEN, A. Three Years of Condebelt Drying at Stora Enso s Pankakoski Mill. Tappi J. 83(5):84 (2000). 11. PHILLIPS, R.B. Low Energy TMP Furnish of Improved Strength by Ozonation and Press Drying. International Paper Company (USA). U.S. Patent (Issued Jul. 5, 1983). 12. HÖGLUND, H. Tomorrow s Challenge for Mechanical Pulps. Summing up of the IMPC Das Papier 51(7-8): (1997). 13. UNKILA, K., LEHTINEN, J., JUNTUNEN, T. An Assessment of the Quality Aspects of Condebelt Dried Board and Paper. Proc., Helsinki Symp. of Alt. Meth. of Pulp and Paper Drying, Helsinki,, (1991). 14. ATACK, D., HEITNER, C. Dynamic Mechanical Properties of Sulphonated Eastern Black Spruce. Trans. TS, CPPA 5(4):TR99-TR108 (1979). 15. HTUN, M., ENGSTRAND, P., SALMEN, L. The Implication of Lignin Softening on Latency Removal of Mechanical Chemimechanical Pulps. JPPS 14(5):J109-J113 (1988). Résumé: La consommation d énergie du procédé PCTM-HT (PCTM haute température) est très faible, mais les fibres produites possèdent une faible capacité de cohésion en raison de la lignine adhérant à la surface de fibres. Le pressage-séchage pourrait se révéler une solution possible permettant d améliorer la cohésion. La température élevée lors du pressage-séchage favorise le ramollissement des polymères du bois sur la surface de la feuille, ce qui permet d améliorer la cohésion. Des études en laboratoire ont démontré que le pressage-séchage améliore substantiellement la cohésion des fibres de PCTM-HT. Reference: PYNNÖNEN, T, HILTUNEN, E., PALTAKARI, J., LAINE, J.E., PAULAPURO, H. Good bonding for low-energy HT-CTMP by press drying. Pulp & Paper Canada 105(3): T57-61 (March, 2004). Paper presented at the 89th Annual Meeting in Montreal, QC, on January 28 to Not to be reproduced without permission of PAPTAC. Manuscript received on May 28, Revised manuscript approved for publication by the Review Panel on August 8, Keywords: PRESS DRYING, TEMPERATURE, FIBER BONDING, CHEMITHERMOMECH- ANICAL PULPS, ENERGY CONSUMPTION. Pulp & Paper Canada T :3 (2004) 37