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Energy Efficient High Quality CTMP for Paperboard

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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 ( Picea Abies ) 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.
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