Technologies Ltd, Ashford, Kent, UK). Observations were made in a Philips XL 30 ESEM (FEI Company,. Eindhoven, Netherlands), operated at 10 kV with.
to investigate if the fines fractions contribute differently when they are added to different base fibre fractions a comparison was also made with cross-wise mixing of fibre fraction and fines from one CTMP and one PGW pulp.
MECHANICAL PULPING PROCESS IMPACT ON FINES PROPERTIES AND SIGNIFICANCE FOR STRENGTHDENSITY RELATIONSHIPS OF BOARD CENTRE LAYERS 1
METHODS AND MATERIALS
1
Anders Moberg , Kristian Goldszer , Carl-Henrik Ljungqvist1, Frank Peng1, Jonas Hafrén2, Dinesh Fernando2, Geoffrey Daniel2 1)
2)
Pulp samples Fines material was isolated from five different commercial high-freeness mechanical pulps with high bulk and wide spread in character (Table I).
Renewable Packaging R&DI, Stora Enso, Karlstad, SWEDEN Department of Forest Products, SLU, Uppsala, SWEDEN
TABLE I. FIBRE CHARACTERISTICS OF THE UNFRACTIONATED PULPS CTMP 1 CTMP 2 HT-CTMP 1 HT-CTMP 2 Fibre length* (mm) Fibre width* (um) Coarseness FiMa (ug/m) Bauer McNett Bauer McNett Bauer McNett Bauer McNett Bauer McNett Bauer McNett
ABSTRACT The impact of fines character on sheet strength and density was studied by isolating fines from five commercial mechanical pulps, adding them to the same base fibre fraction and making hand sheets. A threefold difference in strength contribution was found depending on fines character differences such as flake or fibrillar content and degree of decomposition. The fines impact on sheet properties were quantified with fast characterization of shape factor and size distribution with a flow cytometer. Varying lignin distribution in different size classes of fines could be detected with a high-throughput automated fluorescence microscope.
>12 mesh (%) 12-30 mesh (%) 30-50 mesh (%) 50-100 mesh (%) 100-200 mesh (%) 0.20 mm
Four of the pulps were produced from spruce and one from birch wood (HT-CTMP2). The five mechanical pulps were fractionated into fines and fibre fractions over a 100 mesh wire using a large scale Britt Dynamic Drainage Jar (BDDJ). The fibre fractions remaining on the wire were stored in a cooling room in plastic bags at 6 oC. The water with fines passing through the wire was stored in large tanks for sedimentation in several steps and the concentrated fines were then stored in a cooling room at 6 oC. It was found that the remaining fibre fractions after this large scale BDDJ fractionation with a 100 mesh wire had similar Bauer McNett distributions as the corresponding fibre fractions from standard Bauer McNett fractionation with a 200 mesh wire.
INTRODUCTION High freeness mechanical pulps are used in centre layer(s) of multiply board due to their high bulk and contribution to high bending stiffness. Depending on conditions, more than 20% of the wood material may be degraded into fine particles (i.e. “fines”) during the process of refining. In earlier investigations it was proposed that fines may bridge, fill or block inter-fibre networks (1). The quantity and quality of fines differ significantly depending on process conditions and/or the raw materials and thereby strongly affect the properties (2). Depending on size and shape, the fines add more to sheet density and strength properties (fibrillar fines) or light scattering (flake-like fines) (3).
The CTMP1 and PGW pulps were the same as used by Ljungqvist et al (4). Sheet making Hand sheets of basis weight 100 g/m2 were made in tap water from the pure CTMP1 fibre fractions and with addition of 15% and 30% of fines from the five different mechanical pulps. The PGW base fibre fraction was also used for sheet making with addition of 0, 15 and 30 % of its own PGW fines and CTMP1 fines. A formette sheet former was used with retention aid addition (0.5 kg/t CPAM + 1,5 kg/t Bentonite) to ensure that the fines stayed in the sheets. Nozzle type 2510 was used with 2,5 bar pressure. Rotational speed was 1050 rpm and dewatering time 2,5 minutes. The sheets were wet pressed between blotting papers in a 10 cm diameter roll press in three steps: 1. double felted two passages at 1 bar, 2. double felted two passages at 5 bar, 3. only blotting papers without felts with single passage at 5 bar. New blotting papers were used in each pressing step and drying was done between the blotting papers from last pressing step in a bow dryer at 90 oC.
The current study was focussed on the use of high freeness pulps for board and involved determination of the impact of wood raw material and process conditions on fines properties and their effects on the resulting strength-density relationship. These aspects are of major importance for board stiffness, convertibility and functionality of the final package. A variety of both established and novel analytical techniques were used to understand the impact from fibre separation processes on fines quality and resulting sheet properties. Refining and process conditions affect both the fibre fraction characteristics and the fines properties. In order to separate the influence from fines quality and fibre fractions, hand sheets were prepared from the same spruce CTMP long fibre fraction with addition of varying amounts of five different fines types. In order
1
Pulp and Paper testing Pulp and paper testing was made according to SCAN M6 (Bauer McNett), ISO5267-2 (Freeness), ISO 536 (Basis weight), SCAN-P 88:01 (Structural density), ISO1924-3 (Tensile index, Tensile energy abs. index, Tensile stiffness. index), ISO9895 (SCT index), ISO 9416:2009 (Light scattering coefficient), TAPPI UM 403 1991 (Scott bond strength), ISO15754 (Z directional strength).
SCAN-CM 60:02 (Total acidic groups in pulp). Surface charges were measured according to (8, 9). Specific Sedimentation Volume, SSV Specific sedimentation volumes were measured on dilute fines suspensions with 2 g/l consistency. Prior to measuring sedimentation, the fines suspensions were de-aerated with vacuum for 10 minutes in an ultrasonic bath. Sedimentation volumes were measured in 100 ml measuring cylinders that were turned upside down gently three times before starting sedimentation. The volume of sedimented dry matter substance was measured after 24 hours and specific sedimentation volume was reported as cm3/g dry matter.
Fines Analysis Scanning Electron Microscopy, SEM Ca 20 ml of fines suspension from each pulp sample was pipetted and all fines particles collected by filtering. Each fines sample was then dehydrated separately, as described by Fernando and Daniel (5). Samples were then dried in an Agar E3000 Critical Point Dryer (Agar Scientific Ltd, Stansted, UK) with carbon dioxide as the drying agent, and subsequently coated with gold using an Emitech K550X sputter device (Quorum Technologies Ltd, Ashford, Kent, UK). Observations were made in a Philips XL 30 ESEM (FEI Company, Eindhoven, Netherlands), operated at 10 kV with images recorded digitally.
Flow Cytometry, FCM Flow cytometry is a newly adopted technique for fines characterization based on the principle that every type of particle has its own characteristic in light scattering (10). In the flow cytometer the sample to be analyzed is fed into the system where a sheath of fluid focuses the particles so that they pass the laser one by one. When the particles are hit by light emitted from a laser source, light is scattered and the intensity of scattered light is monitored in forward- and side direction with use of detectors. The intensity of scattered light corresponds to the size of particles.
Autofluorescence Microscopy For the automated fluorescence microscopy analysis, fines were suspended in water and a droplet (~ 50 l) of the suspension added into each well of a multi-well plate. The plate was inserted in an ImageXpress Micro cellular imaging system from Molecular Devices (Sunnyvale, CA, U.S.). The imageXpress microscope allows for high-throughput automated and consecutive image acquisition of several wells of several multi-well plates. Between 4-6 wells were used for each sample, in each run, and from each well between 25 to 100 wide field fluorescence images were automatically collected at excitation wavelength 352-402 nm and emission = 417-477 nm (i.e blue). Enough images were collected to ensure that > 5000 separate fine particles were identified in each sample. The fines were visualized and identified exclusively by intrinsic lignocellulose autofluorescence. All raw image data were collected as tiff files and thereafter analyzed using the image processing and analyses software, Cell profiler (6, 7). In Cellprofiler a designated “pipeline” was created for wood-pulp fines that allowed for automated segmentation, identification and analysis of the fines based on their intrinsic autofluorescence. The output data were collected in excel files as tables of surface area, perimeter, length, width, form factor and fluorescence properties for each fine particle.
Considering the shape of fines, a hypothesis was formulated in order to distinguish between flakes and fibrils based on the following assumptions: - Flakes can have different light scattering intensities (pulse height) but the time they are exposed to the laser (pulse width) is similar for all particles - Fibrils are longer compared to flakes meaning that the majority of the fibril particles will have higher pulse width while the pulse height can vary within a wide range. RESULTS AND DISCUSSION Paper sheet properties Fines addition during sheet making increased both density and strength considerably, but the effects varied greatly depending on fines type and origin, as shown for z-strength in Figure 1.
Chemical Analysis and Charged Groups Measurements The chemical composition of the fines was analyzed according to SCAN-CM 71:09 (Carbohydrate composition in pulp), TAPPI T 222 (Acid insoluble lignin “Klason lignin”), TAPPI UM 250 (Acid soluble lignin), SCAN-CM 49:09 (Acetone soluble matter). The total charges of fines were measured according to
Figure 1. Z-strength vs density with addition of 15 and 30% fines to CTMP1 fibre fraction
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CTMP1 fines gave almost three times higher strength than birch HT-CTMP2 fines, which increased density with only little improvement of strength. The trends were also similar for the other measured strength properties, but with varying magnitude. The smallest differences were observed for short span compression strength and tensile stiffness index, which are probably determined more by the fibre fraction than fines properties. Scott bond, tensile index and tensile energy absorption showed high dependence of fines type and were similar in magnitude as z-strength, or higher.
The CTMP2 fines, however, seemed to have higher proportion of cut segments in addition to frequently observed other flake-like particles such as broken/unbroken parenchyma cells (Fig. 3c).
a V
Pa
Results from cross-wise mixing indicated that the effect of fines type was similar regardless of fibre fraction type, as shown for tensile energy absorption in Figure 2.
V
b
Figure 2. Tensile energy absorption index vs density for CTMP1 and PGW fibre fractions cross-wise mixed with 0, 15 and 30% of each other’s fines.
c
Pa
FF
CTMP1 gave superior strength improvement compared with PGW. Both the fines and fibre fraction of PGW pulp gave lower strength and higher light scattering at a given density, which reflects a harsher fibre separation compared with CTMP (4). FF
Fines Analysis Ultrastructural characterization of the fines (Figure 3), could qualitatively explain many of the observed sheet properties.
FF
BW
Pa
d
The HT-CTMP2 birch fines contained a large proportion of flake-like particles such as vessel elements, various types of parenchyma cells (e.g. long and short brick-like etc.) and broken fibre wall fragments primarily from outer wall layers (Fig. 3a), which may be expected to contribute to density but not so much to bond strength between the fibres.
FE
FF
The spruce fines had more thread- and ribbon fibrillar structures that can contribute more to strength properties. CTMP1 fines that gave best strength were composed predominantly of long ribbon-type fibrils from the inner secondary S2 layer (Fig. 3b) and contained more broad and band-like ribbons (inset bottom right, Fig 3b). These broad cellulose fibrillar structures are good binders in a pulp and substantially contribute to the strength properties of a final product (11).
FF
Figure 3a-d. SEM micrographs showing morphological ultrastructure of fines material produced during different processes indicating differences in the quality of fines: (a) HT-CTMP2 birch; (b) CTMP1; (c) CTMP2; (d) PGW. BW=broken wall materials; FE=fibre ends;
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FF=fibre fragments; Pa=parenchyma cells; V=vessel elements. Bars: a, 30 μm; b, 10 μm; c,d 40 μm. The PGW fines also had many flake like broken fibre wall fragments (Fig. 3d) and long ribbon fibrils were also observed frequently (inset top right, Fig 3d). In addition it was commonly seen very small parenchyma cells that were torn apart (arrows in inset bottom right, Fig 3d) indicating a harsh grinding action.
CTMP 1 Lignin acidsoluble (205 nm) (vikt-%)
1,3
0,6
5,0
39
36
38
28
26
Total anhydro carbohydrates (vikt-%)
55,1
57,7
56,3
60,5
56,6
Glucose anhydro (vikt-%)
34,8
36,7
34,3
25,6
37,6
Galactose anhydro (vikt-%)
3,9
2,9
3,9
1,2
3,2
Arabinose anhydro (vikt-%)
2,1
1,8
2,5
1,1
1,8
Xylose anhydro (vikt-%)
6,0
7,9
6,9
31,0
6,2
Mannose anhydro (vikt-%)
8,3
8,4
8,7
1,5
7,7
Acetone extract (vikt-%)
0,17
0,13
0,15
0,20
1,63
Lignin Klason (vikt-%)
The ultrastructural observations could be quantified using both SSV and rapid measurements using flow cytometry (Figure 4).
CTMP 2 HT-CTMP 1 HT-CTMP 2 PGW
0,54
0,91
Surface charge (negative) (mekv/kg)
52
30
Total acidic group content (mmol/kg)
183
89
Among the spruce pulps, there was a higher lignin content in the CTMP fines, probably as an effect of sulphonation giving cleaner crack propagation between the fibres and more lignin rich middle lamella fragments compared with PGW fines. The higher number of charged groups in CTMP, coming from sulphonation presumably also contributes to strength. Autofluorescence microscopy revealed a difference in relative distribution of lignin in different size classes of fines (Figure 6).
Higher strength contribution
Figure 4. FCM shape factor vs SSV for the different fines types (high shape factor means more circular appearance). The fines types contributing less to strength had a more circular shape and packed closer after sedimentation. The fines giving low sheet strength also shifted towards smaller size distribution (Figure 5). Figure 6. Lignin distribution from the automated autofluorescence microscopy Notably the HT-CTMP fines had about 50 % of the lignin associated with the larger fines, which may be related to enhanced lignin softening with the high temperature during pre-heating before refining. CONCLUSIONS An increased sheet density with low strength improvement was observed for birch CTMP fines that contained many flake like vessel cells, fibre wall fragments and various types of parenchyma cells , while more thread- and ribbon fibrillar structures found in spruce fines gave up to three times higher strength. The fines type giving best strength was from a CTMP process and contained more broad- and band-like ribbons. PGW fines had fibrillar character but were of smaller size and with even small disrupted parenchyma cells, indicating a harsher fibre separation that may explain the lower strength contribution.
Figure 5. Size distribution of fines according to FCM, normalized by number of particles The chemical composition and number of charged groups in the fines (Table II) varied a lot with a high amount of xylose and low amount of mannose found in the birch fines, which reflects another hemicellulose composition. TABLE II. CHEMICAL COMPOSITION AND CHARGED GROUPS
The fines impact on sheet properties could be quantified with fast characterization of shape factor and size distribution using flow cytometry. Process related differences in lignin distribution in different size classes of fines could be detected with a high-throughput automated fluorescence microscope combined with image analysis.
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cellulosic fibers”, Nordic Pulp and Paper Res. Journal 2, p 71-79 (1989)
ACKNOWLEDGEMENTS The work was carried out within the framework of the "Branschforskningsprogram för skogs- och träindustrin" and program "Process and product developments through unique knowledge of wood fiber ultrastructure" (2007-03230). The program was financed by VINNOVA, six Forest based industries (Eka Chemicals, Holmen, SCA, Smurfit Kappa Kraftliner, StoraEnso, Södra Cell) and involves collaborative work between SLU, Innventia, KTH and MiUn.
10. GOLDSZER K. AND WAHLSTRÖM T., “Characterization of fines with use of flow cytometry”, International Mechanical Pulping Conference, Xi’an, P.R of China , p 58-61 (2011) 11. HAFRÈN J., FERNANDO D., GORSKI D. AND DANIEL G., “Fibre and fine fractionsderived effects on pulp quality as a result of mechanical pulp refining consistency”, J Wood Sci Technol. (In Press) (2014)
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