Abstract
Elongation at failure is an important but underrated functional property of paper. Traditionally, elongation has been of specific importance for sack and bag paper grades. Mechanical treatments at high consistency are known to induce fibre deformations that contribute to the elongation of paper. However, it is not clear to what extent different fibre deformations can improve the elongation of paper. The aim of this work was to investigate the influence of three mechanical treatments on fibre and paper properties. The wing defibrator, the E-compactor, and the Valley beater were used for treating chemical softwood pulp. It was found that the type and intensity of mechanical treatments significantly affect the formation of fibre deformations, and thus the resulting properties of paper. The combination of high-consistency wing defibrator treatment and subsequent low-consistency valley beating provided paper with high elongation potential and good strength properties without impairing the dewatering properties.
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The Elongation Potential of Paper – How Should Fibres be Deformed to Make Paper Extensible?
Xiling Zeng,a,* Alexey Vishtal,b Elias Retulainen,b Eino Sivonen,c and Shiyu Fu a
Elongation at failure is an important but underrated functional property of paper. Traditionally, elongation has been of specific importance for sack and bag paper grades. Mechanical treatments at high consistency are known to induce fibre deformations that contribute to the elongation of paper. However, it is not clear to what extent different fibre deformations can improve the elongation of paper. The aim of this work was to investigate the influence of three mechanical treatments on fibre and paper properties. The wing defibrator, the E-compactor, and the Valley beater were used for treating chemical softwood pulp. It was found that the type and intensity of mechanical treatments significantly affect the formation of fibre deformations, and thus the resulting properties of paper. The combination of high-consistency wing defibrator treatment and subsequent low-consistency valley beating provided paper with high elongation potential and good strength properties without impairing the dewatering properties.
Keywords: Elongation; Fibre deformations; Microcompressions; Shrinkage; Tensile strength
Contact information: a: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, No.381 Wushan Road, Tianhe District, Guangzhou 510640, China, b: VTT Technical Research Centre of Finland, P.O. Box 1603, Koivurannantie 1, Jyväskylä, 40101 Finland, c: VTT Technical Research Centre of Finland, P.O. Box 1300, Sinitaival 6, Tampere 33101 Finland; *Corresponding author: zeng.xiling@gmail.com
INTRODUCTION
The mechanical properties of paper have been the subject of research since the rise of modern papermaking. Conventionally, tensile strength has been the primary target of improvement, while the other properties have been somewhat underrated. The elongation of paper is one of these underrated properties. In principle, elongation is the ability of material to increase its linear length under the action of external mechanical forces; the increase in the linear length is attributed to elastic and plastic deformations (Levlin and Söderhjelm 1999). This property increases the tensile energy absorption (TEA) potential of paper, which is defined as the integral of the tensile force and specimen elongation up to the point of failure, and is important for runnability of the web on the paper machine and in the printing house (Hristopulos and Uesaka 2002; Uesaka 2005; Deng et al. 2007), and for converting operations of paper and paperboard (Gärd 2002; Post et al. 2011; Östlund et al. 2011). Elongation of paper is also one of the central components of formability of paper-based materials (Vishtal and Retulainen 2012). The elongation potential of paper relies on three principal factors: properties of single fibres, the character of interfibre bonds between them, and the structure of the fibre network formed in the papermaking and converting processes (Dumbleton 1972; Page et al. 1985; Seth 1996; Welsh 1965). However, when the fibres have sufficient bonding, the elongation of a typical fibre network is primarily dependent on the single-fibre properties (Seth 2005). Mechanical treatments of fibres at high consistency are known to induce fibre deformations that contribute to the elongation potential of the paper (Page et al. 1985, Seth 2005). Gentle laboratory beating at low consistency tends to straighten and lengthen fibres, reducing fibre deformation (Mohlin et al 1996; Seth 2005; Seth 2006). The combination of the high- and low-consistency refining is also known as a method to improve the elongation and tensile energy absorption of the paper at low airflow resistance (Sjöberg and Höglund, 2005 and 2007). This approach is used for production of sack and bag grades of paper in the industry. Also, compressive treatment at high consistency has been shown to be a potential fibre treatment to improve strength properties of paper. It causes different types of deformations in axial and transverse dimensions of fibres (Hartman 1985; El-Sharkawy 2008). However, there is still a lack of information in scientific literature on the subject of how mechanical treatments of different type and intensity affect the fibre deformations and the stress-strain properties of paper, and how the other essential paper properties are affected.
The influence of the mechanical treatments with three different devices, the wing defibrator (high consistency, HC), a compressive E-compactor (HC), and a Valley beater (low consistency, LC), on fibre and paper properties was investigated in this work. The combination of HC wing defibrator treatment and subsequent LC Valley beating was also studied. The effect of these treatments on fibre properties and zero-span tensile strength has been reported in a recent publication (Zeng et al. 2012).
EXPERIMENTAL
Materials
The fibre raw material used in the study was first-thinning bleached pine kraft pulp obtained as pulp sheets from the Pietarsaari mill of UPM-Kymmene. The chemical composition and fibre properties of the first-thinning pulp are quite close to those of conventional once-dried softwood market kraft pulps.
Methods
Mechanical treatments
Three different mechanical devices were used to treat the fibres: the wing defibrator (HC), the E-compactor (HC), and the Valley beater (LC). Figure 1 shows schematic illustrations of the wing defibrator and E-compactor devices.
Fig. 1. Schematic illustrations of the wing defibrator (A) (Sundström et al. 1993) and the E-compactor (B)
Wing defibrator treatment
The wing defibrator is a high-intensity mixer fitted with four rotating blades; its primary utilization is in the preparation of mechanical pulp (Sundström et al. 1993). Each batch of 150 g (oven-dry) pulp was adjusted to a consistency of approximately 35% before the mechanical treatment. The rotation speed was 750 rpm, and the gap between the blades and the stator bars was 1 mm. There was a 20-minute pre-heating period. The jacket and chamber of the wing defibrator were heated by direct streaming. The condensate from the heating steam decreased the consistency of the pulp during the pre-heating phase. The conditions of the wing defibrator treatment are summarized in Table 1. The pure heating treatment without a mechanical treatment took place in an oil bath under similar conditions
Table 1. Conditions in the Wing Defibrator Treatment
E-compactor treatment
The E-compactor (Fig. 1B) is a device with two rotating cogwheels that employs both compressive and hydraulic forces to press fibres through conical holes. It was developed at VTT Tampere. The fibres were treated at 30% consistency by passing them once or twice through the E-compactor with conical holes of 2 mm in diameter.
Valley beater treatment
The applied beating procedure was in accordance with the SCAN-C 25:76 method. Pulp measuring 360 g (oven-dried weight) was used for one batch in Valley beater and was diluted to the 23 L of total volume, giving a consistency of 1.57 g/L. The pulp was disintegrated in the beater without a load for 30 minutes. The disintegrated sample was defined as the “untreated fibres” sample. Then, pulp samples of 2 L were taken after 15, 30, 45, and 60 min of beating.
Combined high consistency (HC) and low consistency (LC) treatment
The combined HC and LC treatment included a wing defibrator treatment (WD) followed by a Valley beating (VB). The refining conditions for HC treatment are shown in Table 2, and LC beating was performed according to the SCAN-C 25:76. The SR number after the Valley beating was 23. The conditions used in the HC-refining are shown in the Table 2.
Table 2. The Conditions for HC Wing Defibrator Treatment
Fibre and pulp analysis
Fibre samples (approx. 0.1 g oven-dry mass) were taken for the automated fibre analysis with the STFI Fibermaster by Lorentzen & Wettre. Fibre parameters, including fibre length, fibre width, fibre curl, kinks, and fines content were analysed. The dewatering properties were characterised by the Schopper Riegler number (SR number) and the water retention value (WRV) in accordance with the ISO 5267-1 and SCAN-C 62:00 standards, respectively.
The fibre samples were unstained and observed under a light microscope (Olympus BX50) using transmitted light. The polarized mode was used for analysis, in which the angle positions of polarizer and analyser were varied.
A scanning electron microscope (SEM, LEO DSM 982 Gemini FEG-SEM, NORAN Instruments, Inc.) was used for inspecting the surface structure of the handsheets. Paper samples (ca. 10 mm×10 mm) were attached on carbon adhesive discs (12 mm) pressed on 12.5 mm aluminium stubs. A thin layer (ca. 10 nm) of platinum was sputter coated on each sample surface prior to analysis. The SEM analyses of the samples were conducted using an acceleration voltage of 1.0 keV or 2.0 keV.
Handsheet preparation
Handsheets were prepared according to SCAN-C 26. In addition to standard plate drying, a second set of handsheets were air-dried between two wire fabrics that had a gap of around 1 to 3 mm, which allows free shrinkage of the handsheets without excessive cockling or curling.
Paper properties analysis
The strength properties of the handsheets, including tensile strength, elongation, tensile energy absorption (TEA), and tensile stiffness, were determined using a universal material-testing device (LR10K, LLOYD Instruments) in accordance with SCAN-P38. Light scattering coefficient of handsheets was determined in accordance with ISO-9416.
The procedure applied for shrinkage potential measurement can be described as follows: Handsheets were marked after wet pressing by making holes using a square plate with awls at each corner. The four punched holes defined a square with a known perimeter. After this, the handsheets were allowed to dry and shrink freely. The extent of shrinkage was calculated from the change of the perimeter using a high-resolution scanner and special software. Equation (1) was used for the calculation of the shrinkage,
(1)
where Lw is the perimeter of the rectangular perimeter in the handsheet before drying and Ld is the perimeter of the dried handsheet.
RESULTS AND DISCUSSION
Fibre Properties
The effects of the wing defibrator and Valley beater treatment on the fibre properties and dewatering properties of pulp can be found in a previous publication (Zeng et al. 2012). The treatment of pulp using the E-compactor device brought about drastic changes in the fibre structure, which can be seen in Table 3.
Table 3. Fibre Parameters, Water Retention Value, and Drainage Properties of E-Compactor Treated Pulp
E-compactor results in Table 3 show that fibre length was reduced by as much as 40% even though the fibres were passed through the E-compactor only once. Fibre width was increased because of the flattening and collapse of fibres. Kinks were induced to fibres during the E-compactor treatment. It can be concluded that E-compactor treatment caused severe fibre deformations and damage, such as fibre flattening, squashing, and fibre cutting.
The influence of HC wing defibrator treatment and subsequent LC valley beating on the fibre properties can be seen in the Table 4.
Table 4. The Effect of HC Wing Defibrator Treatment and Subsequent LC Valley Beating on Fibre Parameters and Drainage Resistance
In Table 4 it can be clearly seen that the HC wing defibrator treatment created deformations (curl and kinks) in the fibres, while the subsequent LC beating straightened the fibres, released fibre curl and kinks, and increased the swelling of the fibres. Fines content was increased by both of the mechanical treatments.
Polarized light microscopy was used for the evaluation of the fibre deformations caused by the different types of mechanical treatment. The emphasis in this study was on the identification and characterization of microcompressions in fibres. The polarized images allow for better observation of the changes in the fibre structure caused by the mechanical treatments. Figure 2 shows polarized images of the untreated, HC wing defibrator treated fibres, and combined HC wing defibrator and LC valley beater treated fibres.