Energy Efficient High Quality CTMP for Paperboard

Energy efficient high quality CTMP for paperboard

Niklas Klinga (1, 2), Hans Höglund (1) and Christer Sandberg (2) (1) Mid Sweden University, FSCN, SE-851 70 Sundsvall, Sweden (2) Holmen Paper Development Centre, Braviken Paper mill, SE-601 88 Norrköping, Sweden E-mail: [email protected]

ABSTRACT This paper discusses the relationship between bulk and internal bond strength in paper sheets and their dependency on fibre length, fibre flexibility and fibre surface properties. It also discusses an interesting process concept for manufacturing of energy efficient high quality CTMP for paperboard. Post-refining pilot trials of spruce HTCTMP with an initial freeness of 740 ml were carried out at Metso Paper R&D in Sundsvall, Sweden. Both gentle high consistency and severe low consistency post-refining were performed. High consistency post-refining, at high energy input, gave freeness levels below 70 ml and still preserved the fibre length. These fibres were characterised by a very high flexibility giving sheets with a tensile index as high as 64 kNm/kg. Long fibres can however cause formation problems on a board machine which in turn can lead to poor surface properties, hence shorter fibres are from that perspective desirable. The low consistency post-refining resulted in a rapid drop in freeness due to fibre cutting. This was achieved at an extremely low specific energy input, which probably preserved most of the original fibre stiffness. In spite of this low energy input it was possible to reach the same Z-strength at a given bulk, as for the high consistency post-refined pulp. This implied that high bulk at certain internal bond strength could be achieved with stiff fibres even though the content of long fibres was low. Energy efficient low consistency post- refining of spruce HTCTMP yields high quality pulp at a total energy input of ~800 kWh/admt and is an interesting process concept for production of pulps intended for paperboard.

INTRODUCTION Refiner mechanical pulps have been used as a middle layer in paperboard for over thirty years as a replacement for groundwood pulps. In the beginning refiner mechanical pulp, RMP, or thermomechanical pulp, TMP, was used. The use of chemithermomechanical pulp, CTMP, in this application has however increased in the northern part of Europe over the years [1]. Mechanical and chemithermomechanical pulps are interesting due to their high bulk, which makes it possible to construct board with high stiffness at low grammage compared to chemical pulps. For paperboard, high bending stiffness is usually built up by having outer plies with high tensile stiffness and one or several bulky plies in between, so that the outer plies are placed at as high distance as possible from each other. These materials are designed to an optimized bending stiffness [2-6]. Suitable pulp properties for the paperboard middle ply are high bulk at a certain internal bond strength. A high bulk is a prerequisite for stiffness, whereas a certain internal bond strength is required to prevent board delamination. The main advantage of using mechanical pulp fibres in paperboard is their inherent stiffness. A high bulk can be obtained by an increased amount of long fibres, an increased amount of stiff fibres or both [2, 4].

In the CTMP process wood chips are impregnated with a weak solution of sodium sulphite at alkaline pH prior to pressurized refining. One effect from this treatment is softening of lignin and the fibre rupture during refining will therefore be concentrated to the lignin rich middle lamella. This results in higher amounts of long fibres and lower amount of fines and shives at a certain energy input compared to TMP [7]. A high content of long fibres is important for all products where high bulk is desired, this makes CTMP specially suitable for such products. Since the shive content is low in CTMP, long fibre content can be kept high by producing pulps at high freeness levels [8]. The energy input can be lowered even more if preheating is performed at a temperature (T~170°C) well above the softening temperature of lignin, which is the situation in production of HTCTMP [2]. Such process conditions yield pulps with long, smooth and well preserved fibres as well as low contents of shives and fines. The energy input to low shive contents is approximately 500 kWh/admt and the average fibre length is higher and the fines content is lower compared to CTMP [2, 8]. Long fibres can however cause formation problems on a board machine which in turn can lead to poor surface properties, hence shorter fibres are from that perspective desirable. Benefits from short and stiff fibres can be obtained by using high density hardwood CTMP, e.g. birch [2, 9], or by cutting the long fibres in softwood CTMP with low consistency refining. Pulps from high temperature processes (e.g. HTMP and HTCTMP) are, due to their high content of long and stiff fibres with lignin coated surfaces, considered to have poor bonding ability [10]. It is also generally considered that enormous amounts of energy would be required in further refining to improve bonding properties. A lot of effort has been put on finding ways to improve the strength properties of high temperature pulps; e.g. by adding fines [11], by the use of press drying at high temperature and high pressure [12-14] and by the use of ozone [15]. In reference [12] it is indicated that lignin coated fibres have a high bonding potential in a sheet structure when the effect of fibre stiffness is reduced.

For the purpose of this paper, post-refining pilot trials of HTCTMP from spruce ( PiceaAbies ) were performed with the aim of evaluating the relationship between bulk and internal bond strength in paper sheets and their dependency on fibre length, fibre flexibility and fibre surface properties. It also discusses an interesting process concept for manufacturing of energy efficient high quality CTMP for paperboard.

EXPERIMENT HTCTMP is manufactured with chemical impregnation of the wood chips prior to preheating and refining at a temperature well above the softening temperature of lignin. The HTCTMP used for the purpose of this trial was provided by the SCA Östrand mill outside Sundsvall, Sweden. The pulp was a fluff pulp, with a freeness of 740 ml, intended for the use as absorbing material in hygiene products. Post-refining pilot trials were carried out at Metso Paper R&D in Sundsvall, Sweden. Both gentle high consistency and severe low consistency refining were performed. The aim with the high consistency refining was to gently knead the fibres to increase fibre flexibility but preserve fibre length and fibre surface characteristics. The high consistency refining was performed in an atmospheric single disc refiner in steps of an approximate energy input of 500 kWh/admt. The aim with the low consistency refining, which was performed in a CONFLO refiner run at high intensity, was to reduce fibre length but preserve fibre stiffness. The high and low consistency refining trials are from here on referred to as the HC and LC refining trials respectively. The technical data are presented in table I.

TABLE I. TECHNICAL DATA OF PILOT TRIALS

Equipment HC refining LC refining Refiner type ROP-20 JC-00 Size 20” - Rotational speed 1500 rpm 1200 rpm Load 32-42 kW 43-94 kW Segments 5811 AAD Consistency ~25% ~3% Production rate 0.9-1.2 kg/min 4.2-4.5 kg/min

Pulp evaluation The evolution of the pulp properties during the refining trials was evaluated in accordance with ISO and TAPPI standards. Optical measurements on fibres were performed in Fiberlab. The physical properties bulk, tensile index and Z-strength were measured for standard TAPPI sheets, formed with circulated white water.

Some of the pulps were fractionated on a 50 mesh wire in a fractionation device similar to, but larger than, a Britt Dynamic Drainage Jar. These long fibre pulps are distinguished from the whole pulps by the addendum R50 in figure legends. Comparisons between long fibre pulps and whole pulps were performed in order to study the influence of fines on physical properties of laboratory sheets made according to both the TAPPI and the Rapid Köthen methods [16]. The main difference between these methods is that TAPPI sheets are wet pressed at 400 kPa at room temperature and dried at 50% R.H. and 23 °C whereas Rapid Köthen sheets are pressed at 100 kPa at 93 °C until the sheets are dry.

Acoustic Emission monitoring Acoustic Emission (AE) monitoring was used to detect the onset and evolution of the damage process during tensile strength measurements. This testing was carried out on some of the whole pulps from the refining trials but also on their respective long fibre pulps. This way to evaluate bonding ability and pulp properties was inspired by the work initiated by the authors in [17]. More in-depth theory on AE can be found in the literature [18-19], the following should be considered as an attempt to summarise how AE monitoring is performed in paper contexts and what some of the possible outcomes are. AE can be detected with a piezoelectric resonance frequency sensor attached to the paper specimen during tensile strength tests, figure 1.

Figure 1. The sensor attached to a paper strip (left). The sensor is fixed to the strip with a magnet (right)

When recording the AE during a tensile strength test and plotting load and cumulative sum of acoustic events as a function of displacement, the plot will have the typical appearance shown in figure 2.

Load Acoustic events

N f

F c Load Load (N) Cumulative sum of acousticof sum Cumulative events 0,1 N f

δ Displacement (m) δ i f

Figure 2. Typical AE curve for a paper material

In figure 2, Nf denotes the total number of acoustic events at failure and Fc is the load at 10% of Nf. The parameters δi and δf are the displacement at 10% of Nf and displacement at Nf respectively. Literature has shown that damage starts, i.e. fibres or inter fibre bonds start to break, when the elastic strain energy density, W, reaches a critical value, Wc. The critical elastic strain energy density, Wc, is thereby a measure of how resistant the paper material is to damage and can be calculated from the AE curve by the use of equation 1 [19].

F 2 W = c (1) c 2BwkL Here, Fc is the load (N) when the number of acoustic events equals 10% of the total number of acoustic events, B is the specimen width (m), w is the grammage (kg/m 2), k is the slope (N/m) of the linear part of the load-displacement curve and L is the gauge length (m).

Additional information on the evolution of damage can be obtained by comparing the strain at break with the strain at 10% of the total number of acoustic events, Nf. This was done by defining the parameter η, equation 2.

ε − ε η = f i , where (2) ε f

δ δ ε = f and ε = i (3, 4) f L i L

Tensile specimens with a width of 15 mm and a gauge length of 100 mm were loaded at a constant strain rate of 1% per minute, i.e. 1 mm per minute. For each sample, 10 specimens were tested.

RESULTS AND DISCUSSION Post-refining trials of spruce HTCTMP were performed with the aim of comparing gentle high consistency refining and severe low consistency refining. The HC refining was performed in order to gently knead the fibres to increase fibre flexibility, whereas the LC refining was performed at high intensity with the intention to cut the fibres. Figure 3 shows freeness, keeping in the mind that freeness does not say much about fibre properties but still is illustrative, as a function of specific energy input for the HC and LC refining.

800 Start pulp 700 Gentle High Consistency refining 600 Severe Low Consistency refining

500

400

300

200

100 Canadian Standard Canadian Freeness (ml) 0 0 500 1000 1500 2000 2500 3000 3500 4000 Specific energy input (kWh/admt)

Figure 3. Freeness development during post-refining trials

The HC refining gave a freeness reduction down to 70 ml at very high energy input, whereas the LC refining gave a freeness reduction down to 200 ml at very low energy input compared to the HC refining. The fibres were however affected very differently by the two refining mechanisms, as can be seen in figure 4.

Figure 4. Light microscope fibre pictures of start pulp (left), HC refined pulp at 3500 kWh/admt (top right) and LC refined pulp at 250 kWh/admt (bottom right). Real picture sizes 640x480 m.

Figure 4 illustrates that the fibres have become more flexible, but that fibre length probably was preserved during HC refining. The top right picture also illustrates that a lot of fibrillar fines, still attached to the fibres, were created during refining. The LC refining, on the other hand, seem to have cut the fibres, bottom right picture. These findings were verified by optical measurements of the average fibre length, left picture in figure 5.

2,5 100 Fines 90

80 2,0 70 BMcN <200 60 BMcN 100-200 1,5 50 BMcN 30-100 BMcN 16-30 40 BMcN >16 30 1,0 Start pulp Gentle High Consistency refining 20 Severe Low Consistency refining

Fiberlab fibre length (length weighted, mm) weighted, (length Fiberlab length fibre 10

0,5 eachin fraction(%) BMcN Amount Long fibres 0 500 1000 1500 2000 2500 3000 3500 4000 0 Start pulp HC 2000 HC 2500 LC 140 LC 250 Specific energy input (kWh/admt) kWh/admt kWh/admt kWh/admt kWh/admt

Figure 5. Fiberlab fibre length as a function of specific energy input (left) and Bauer McNett fractions (right)

The optical measurements in Fiberlab show that fibre length was preserved throughout the HC refining, the small increase in fibre length between the start pulp and the first point on the HC curve is believed to be an effect of fibre straightening. The left picture in figure 5 also reveals that fibres indeed were cut during LC refining. Pulp evaluation according to Bauer McNett, right picture in figure 5, shows that the long fibre content has decreased during HC refining. This fact implies that fibres were made flexible, hence passing the wires more easily. The Bauer McNett evaluation also shows that the fractions were affected most during LC refining. A lot of fibres from the two most coarse fractions were cut, resulting in a large middle fraction. The freeness reductions were thus achieved by two completely different mechanisms, kneading or cutting. The pulps were also evaluated by means of measuring physical properties of standard TAPPI sheets, figure 6.

4,5 4,5 Start pulp Start pulp

4,0 Gentle High Consistency refining 4,0 Gentle High Consistency refining Severe Low Consistency refining Severe Low Consistency refining

3,5 3,5 /g) /g) 3 3

3,0 3,0 Bulk(cm 2,5 Bulk(cm 2,5

2,0 2,0

1,5 1,5 0 500 1000 1500 2000 2500 3000 3500 4000 0,5 1,0 1,5 2,0 2,5 Specific energy input (kWh/admt) Fiberlab fibre length (length weighted, mm)

Figure 6. Bulk of TAPPI sheets as a function of specific energy input (left) and bulk of TAPPI sheets as a function of fibre length (right)

The reduction in bulk was achieved by either making the fibres flexible or cutting the fibres. The bulk reached the same level regardless if the long fibres were kept or cut. The small increase in fibre length between the start pulp and the first point on the HC curve is believed to be an effect of fibre straightening. Figures 7 and 8 show tensile index and Z-strength as a function of specific energy input and bulk respectively.

70 450

400 60 350 ) 50 2 300

40 250

30 200

150 20 Z-strength Z-strength (kN/m Gentle High Consistency refining 100

Tensile index (kNm/kg) Tensile Gentle High Consistency refining 10 Severe Low Consistency refining 50 Severe Low Consistency refining Start pulp Start pulp 0 0 0 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 4000 Specific energy input (kWh/admt) Specific energy input (kWh/admt)

Figure 7. Tensile index (left) and Z-strength (right) of TAPPI sheets as a function of specific energy input

70 450 Gentle High Consistency refining Severe Low Consistency refining 400 60 Severe Low Consistency refining Gentle High Consistency refining Start pulp 350 Start pulp ) 50 2 300

40 250

30 200

150 20 Z-strength Z-strength (kN/m 100 Tensile index (kNm/kg) Tensile 10 50

0 0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 1,5 2,0 2,5 3,0 3,5 4,0 4,5 3 3 Bulk (cm /g) Bulk (cm /g)

Figure 8. Tensile index (left) and Z-strength (right) as a function of bulk (TAPPI sheets)

HC refining did, at high energy input, yield pulps with fibres characterised by a very high flexibility giving sheets with both a very high tensile index and a high Z-strength. The very high strength values were achieved in spite of the low content of fines (figure 5, right picture) and a high coverage of lignin on the fibre surfaces, which is a characteristic of fibres from the HTCTMP process. This shows that the fibre flexibility in this case controlled the bonding in the sheet structure to a greater extent than the surface characteristics. Strong bonds were formed when the sulphonated lignin surfaces were in contact. The tensile index did not reach as high values for the LC refined pulps, which probably can be explained by the fact that the fibres were cut. The Z-strength, on the other hand, reached remarkably high values in spite of the extremely low energy input. Also, the bulk of the LC refined pulps was as good as or even better than the HC refined pulps when comparing at a given Z-strength.

Long fibre fraction pulps (>50 mesh) Some of the pulps were fractionated on a 50 mesh wire in a fractionation device similar to, but larger than, a Britt Dynamic Drainage Jar. The pulps fractionated were the start pulp (~88% long fibres), two HC refined pulps (2000 and 2500 kWh/admt, ~78 and ~70% long fibres respectively) and two LC refined pulps (140 kWh/admt and 250 kWh/admt, ~84 and ~65% long fibres respectively). The long fibre pulps are distinguished from the whole pulps by the addendum R50 in figure legends. Comparisons between long fibre pulps and whole pulps were performed in order to study the influence of fines on physical properties of laboratory sheets made according to both the TAPPI and the Rapid Köthen methods, figures 9 and 10.

6 TAPPI sheets Rapid Köthen sheets

/g) 5 3 y = x

2 Bulk,(cm whole pulps

1 1 2 3 4 5 6 Bulk, R50 pulps (cm 3/g)

Figure 9. Bulk for whole pulps as a function of bulk for long fibre pulps

70 500

TAPPI sheets ) 2 60 Rapid Köthen sheets y = x 400 50

300 40

30 200

20 TAPPI sheets 100 10 Rapid Köthen sheets Z-strength, whole pulps pulps Z-strength, (kN/m whole

Tensile index,Tensile whole (kNm/kg) pulps y = x 0 0 0 10 20 30 40 50 60 70 0 100 200 300 400 500 Tensile index, R50 pulps (kNm/kg) 2 Z-strength, R50 pulps (kN/m )

Figure 10. Tensile index for whole pulps as a function of tensile index for long fibre pulps (left) and Z-strength for whole pulps as a function of Z-strength for long fibre pulps (right)

There was a clear correlation, for both bulk and strength properties, between long fibre pulps and whole pulps. This indicates that the properties of the long fibres were important for the whole pulp properties. The presence of fines, i.e. in the whole pulps, affected the TAPPI sheet properties more than the Rapid Köthen sheets. This fact can be explained by the difference in pressing and drying principle between the methods. The TAPPI sheets were only wet pressed at room temperature, which meant that the fibres were allowed to spring back once pressure was released. The presence of fines in the whole pulp then led to a larger effect on the degree of bonding, this was most clearly visualised for Z-strength in the right picture in figure 10. The Rapid Köthen sheets were on the other hand pressed at a high temperature and until the sheets were fully dried, which meant that the fibres were locked into their positions. The presence of fines in the whole pulps then also led to a higher degree of bonding, but to a much smaller extent than for the TAPPI sheets. This was most clearly visualised in figure 9 where there was a one-to-one relationship between whole pulp bulk and long fibre bulk when measuring on Rapid Köthen sheets. This implies that the importance of fines diminished when fibres were locked into their positions during drying, it was possible to get a good bonding anyway when the effect of fibre stiffness was reduced.

Acoustic Emission The results above illustrate how the physical sheet properties were affected by either making the fibres flexible or by cutting the fibres. In order to study the bond characteristics in more detail, Acoustic Emission (AE) monitoring was used to detect the onset and evolution of the damage process during tensile strength measurements. This testing was carried out on some of the whole pulps from the refining trials but also on their respective long fibre pulps. The critical elastic strain energy density, Wc, was calculated from equation 1 and is a measure of how resistant the paper material is to damage, figure 11.

250 TAPPI sheets, R50 (J/kg) c TAPPI sheets 200 Rapid Köthen sheets, R50 Rapid Köthen sheets 150

100

50 Criticalelastic strain energy density,W 0 0 10 20 30 40 50 60 70 Tensile index (kNm/kg)

Figure 11. Critical elastic strain energy density as a function of tensile index

There was a distinct relationship between the parameter Wc and the tensile index, this relationship was valid from very weak to very strong pulps and even when the sheets were formed according to different methods. There was no discrepancy between HC refined and LC refined pulps. When looking at Wc as a function of Z-strength, here shown for Rapid Köthen sheets, there was a clear discrepancy between HC and LC refined pulps, figure 12.

250 Start pulp, R50 Start pulp (J/kg) c 200 HC refining, R50 HC refining LC refining, R50 LC refining

150

100

50 Criticalelastic strain energy density,W 0 150 200 250 300 350 400 450 500 Z-strength (kN/m 2)

Figure 12. Critical elastic strain energy density as a function of Z-strength (Rapid Köthen sheets) Figure 12 illustrates that a high Wc was not a prerequisite for a high Z-strength, the LC refined pulps reached high Z- strength values even though Wc was lower than for the HC refined pulps. This finding can probably be explained by the difference in types of fracture mechanisms between tensile strength tests and Z-strength tests. When sheets made from mechanical pulps are brought to failure, it is predominantly the bonds between fibres that are breaking, not the fibres themselves [19]. In a tensile strength test, inter fibre bonds are locally exposed to shear stress whereas in a Z- strength test the bonds are locally exposed to tensile stress. The critical elastic strain energy density, Wc, is calculated based on AE data from specimens loaded in tension, i.e. with inter fibre bonds loaded in tension. The fact that the points in figure 12 do not fall on the same line seem to indicate a fundamental difference in mechanisms regarding shear and tensile loading of the bonds. The fundamental reason for the difference in behaviour between the HC and LC refined pulps is not fully understood, but will be further investigated.

Additional information on the evolution of damages was obtained by comparing the strain at break with the strain at

10% of the total number of acoustic events, Nf. This was done by defining the parameter η, figure 13.

0,5

0,4

0,3 ηηη 0,2

0,1 Start pulp, R50 Start pulp HC refining, R50 HC refining LC refining, R50 LC refining 0,0 0,5 1,0 1,5 2,0 2,5 Strain at break (%)

Figure 13. The parameter η as a function of strain at break (Rapid Köthen sheets)

The parameter η can by definition vary between 0 and 1. An η close to 0 indicates that the majority of the acoustic events occur close to final failure whereas an η close to 1 indicates that acoustic events occur over a wider deformation interval. Both HC and LC refining resulted in a decrease in η, this might imply that the average bond strength did not only increase but that the bond strength distribution probably also was narrowed when the pulps were refined, figure 14. Work on developing a method to be able to measure bonding ability distribution of fibres in mechanical pulps of all grades is already in progress by the authors in [17].

Figure 14. Schematic sketch of how fibre bond strength distribution might be affected during refining

If this proposition is true, then the measure of the parameter η during AE monitoring could be considered as an attempt to describe how pulp fibre bond distributions are affected during refining. The narrowing of the fibre bond distribution was achieved by increasing fibre flexibility during HC refining. In LC refining, the narrowing of the fibre bond distribution was achieved by cutting the fibres.

CONCLUSIONS For the purpose of this paper, post-refining pilot trials of HTCTMP from spruce ( PiceaAbies ) were performed with the aim of evaluating the relationship between bulk and internal bond strength in paper sheets and their dependency on fibre length, fibre flexibility and fibre surface properties. The essence of the results are presented below.

• HTCTMP fibre bonding properties were improved significantly by either performing HC or LC refining. • Densification was achieved by two different mechanisms: by making fibres flexible or by fibre cutting. • Good bonding was achieved even though the fibres had lignin coated surfaces. • Good bulk at a certain Z-strength was achieved with LC refining even though the fibres were cut. • A very high Z-strength was achieved with LC refining at extremely low energy input. • The influence of fines on sheet properties diminished when sheets were pressed at high temperature and until they were fully dried, compared to wet pressing. • Lignin coated stiff fibres formed strong bonds as long as they were forced together at high temperature and locked into that position until the sheets were fully dried. • Acoustic Emission monitoring during tensile strength tests indicated a fundamental difference in mechanisms regarding shear and tensile loading of the bonds. • The parameter η was used as an attempt to describe how pulp fibre bond distributions are affected during refining. Finally, LC refining of spruce HTCTMP yields high quality pulp at a very low total energy input of ~800 kWh/admt and is certainly an interesting process concept for production of pulps intended for paperboard.

ACKNOWLEDGEMENTS The authors would like to thank SCA Östrand mill for providing pulp and Metso Paper R&D for performing the pilot refining trials. Per Gradin, Staffan Nyström and Stefan Lindström at Mid Sweden University are gratefully acknowledged for invaluable contributions during Acoustic Emission measurements and evaluations, and Lorna Casson at Iggesund Paperboard Workington, Holmen AB, for linguistic revision. Holmen AB and the Knowledge Foundation are acknowledged for financial support through the FSCN Industrial Mechanical Pulp Research College at Mid Sweden University.

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