The Deformation Behavior of Wet Lignocellulosic Fibers

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1 The Deformation Behavior of Wet Lignocellulosic Fibers Rob Lowe Art J. Ragauskas Institute of Paper Science and Technology Georgia Institute of Technology

2 Wet Fiber Deformability Review previous methods. Describe essential elements and modifications. Explanation of the optical setup. Experimental details. Images Results and Discussion

3 Flexibility, Conformability, Pliability compactability, collapsibility, deformability Wet fiber flexibility is linked to the ability of pulp fibers to consolidate into a paper sheet. Deformability is a very important paper property Apparent Density Tensile fiber-fiber bond formation

4 Measuring Flexibility Many ways to measure it cantilever Seborg and Simmonds 1 Robertson 2 Mohlin 3 Tam Doo and Kerekes 4 Kuhn 5 1. Seborg, C.O. and F.A. Simmonds, Measurement of the Stiffness in Bending of Single Fibers. Paper Trade Jounal, (17): p Robertson, A.A., E. Meindersma, and S.G. Mason, The measurement of fibre flexibility. Pulp and Paper Magazine of Canada, (1): p. T3-T Mohlin, U.-B., Cellulose fibre bonding: Part 5. Conformability of pulp fibers. Svensk Papperstidning, (11): p Tam Doo, P.A. and R.J. Kerekes, A method to measure wet fiber flexibility. TAPPI Journal, (3): p Kuhn, D., et al., A dynamic wet fibre flexibility measurement device. Journal of Pulp and Paper Science, (10): p. J337-J342.

5 Measuring Flexibility The most relevant and straightforward method was developed by Steadman and Luner 6 25 µm stainless steel wire Fiber Wire Side View L Glass Slide Bottom View L 6. Steadman, R. and P. Luner. The effect of wet fibre flexibility on sheet apparent density. in Transactions of the Eight Fundamental Research Symposium: Papermaking Raw Materials: Their Interaction with the Production Process and their effect on Paper Properties Oxford, United Kingdom.

6 Measuring Flexibility The region of optical contact is observed fibers with regions closer to the wire are said to be more flexible Steadman, R. and P. Luner. The effect of wet fibre flexibility on sheet apparent density. in Transactions of the Eight Fundamental Research Symposium: Papermaking Raw Materials: Their Interaction with the Production Process and their effect on Paper Properties Oxford, United Kingdom.

7 Measuring Flexibility The Steadman method is outside the realm of paper. Collapsed pulp fibers provide a stepheight of 1-10 µm 25 µm 2-5 µm

8 A New Method Measure deformability using actual fiber crossings. Lower unbonded span length Scale closer to what actually occurs in a paper sheet

9 A New Method Higher Imaging Demands Magnification 40X Leica Objective with correction collar Contrast Mica as a quarter-wave plate Chlorazol Black dyed fibers Glass Slide with Fibers Mica Sheet Objective Micrograph

10 Equipment Key pieces of equipment: Leica DM-IRM inverted, reflected light microscope Hamamatsu ORCA-ER digital camera 50 watt metal halide lamp Handsheet mold Press PFI Refiner

11 Method Slides were made by draining a dilute suspension of pulp fibers onto a filter paper. Filter Paper Glass Slides Wet Blotters

12 Method The fibers/filter paper was the pressed onto glass slides in a handsheet press for 2 minutes and 50 psi. Blotter Coupon 4 Glass Slides Press

13 Observing fiber Intersections h L L

14 Interference Fringes Interference fringes were found to occur interference A B

15 Measuring Deformability The wavelength of light is used as a ruler. Monochromatic light λ = 547 ± 10 nm Each fringe occurs at ½ λ H = n λ 2 Where n is the order of the black fringe, λ is the wavelength of light, and H is the local height of the air wedge

16 Experimental Mill produced, northern Canadian softwood Unbleached (kappa # = 20), Never dried Mill produced, Finnish hardwood Unbleached (kappa # = 15), Never dried Bleached (ISO Bright = 84), Never dried Bleached (ISO Bright = 84), Pulp dried PFI refined 300 revolutions 1000 revolutions 2000 revolutions 4000 revolutions

17 Experimental Slides were made as described. Intersections were observed. Freespans were measured using an image analysis software package. Stepheights were deduced by analyzing interference fringe patterns.

18 300 revs

19 1000 revs

20 2000 revs

21 4000 revs

22 Measuring Deformability Generally measured along the central axis. Area 1 Freespan = 17.3µm Stepheight = 1.19µm Area 2 7 fringes Freespan = 13.0µm Stepheight = 1.19µm 7 fringes

23 Stepheight versus Freespan A Lightly Refined Unbleached SW Pulp Revs Trendline y = 9.29x R 2 = Revolutions 35 Freespan ( µm) Average Stepheight = 2.22 µm Stepheight (µm)

24 Stepheight versus Freespan A Lightly Refined Unbleached SW Pulp The effect of fiber collapse A) Uncollapsed Fiber B) Collapsed Fiber Freespan Freespan

25 Stepheight versus Freespan Refined Unbleached SW Pulps Increased flexibility Refining only serves to increase the flexibility of fibers Reducing freespan at a given stepheight Freespan Unbeaten Stepheight Beaten

26 Stepheight versus Freespan Refined Unbleached SW Pulps Increased fiber collapse Refining only serves to increase fiber collapse Reducing stepheight and, thereby, freespan Freespan Unbeaten Beaten Stepheight

27 Stepheight versus Freespan Refined Unbleached SW Pulps Freespan (µm) Stepheight (µm) y = 9.29x R 2 = Revs 1000 Revs 2000 Revs 4000 Revs Trendline Trendline Trendline Trendline y = 9.19x R 2 = 0.46 y = 9.76x R 2 = 0.70 y = 9.36x R 2 = 0.77

28 Stepheight versus Freespan Refined Unbleached SW Pulps Refining reduces stepheight Freespan ( µ m) Grand Average (All Data) Trendline y = 9.61x R 2 = Stepheight (µm)

29 Conformability and Deformability Average stepheight falls to 1.1µm Black spruce cell wall thickness 1µm How can the average stepheight be less than 2 µm? Some images have essentially a 0 µm stepheight

30 4000 revs

31 Lens Shaped Fibers The cell wall deforms more readily than previously thought Increased with PFI refining Consistent with other work Fiber skirts (Nanko and Ohsawa 1989) (Page, Sargent and Nelson 1965) Documented in micrographs of paper cross-sections

32 Unbleached HW Pulp 70 Unbleached Hardwood Freespan (µm) Stepheight (µm) 300 Revs 1000 Revs 2000 Revs 4000 Revs 300 Regression 1000 Regression 2000 Regression 4000 Regression y = 9.32x R 2 = 0.45 y = 9.99x R 2 = 0.56 y = 9.54x R 2 = 0.82 y = 9.07x R 2 = 0.74

33 Unbleached HW and SW Pulp Unbleached Never Dried Hardwood and Softwood Unbleached SW SW Regression y = 9.22x R 2 = 0.95 Unbleached HW HW Regression y = 9.77x R 2 = 0.98 Freespan (µm) Stepheight (µm)

34 Never Dried and Pulp Dried HW 40 Never Dried and Market Bleached Hardwood Never Dried Market Never Dried Regression y = 8.70x R 2 = 0.98 Market Regression y = 8.27x R 2 = 0.97 Freespan (µm) Stepheight (µm)

35 Wet Pressing Unbleached HW Wet Pressing for Unbleached Hardwood Unbleached HW Wet Pressing Regression y = 9.22x R 2 = Freespan (µm) Stepheight (µm)

36 Unbleached and Bleached HW Pulp Bleached and Unbleached Never Dried Hardwood Bleached Bleached Regression y = 8.70x R 2 = 0.98 Unbleached Unbleached Regression y = 9.77x R 2 = 0.98 Freespan (µm) Stepheight (µm)

37 The Constant Ratio Species, pulp drying, wet pressing all result in a constant stepheight/freespan ratio. Flexibility of the overlying fiber is largely irrelevant Sheets are densified by pressing or drying via a change in the stepheight of the underlying fiber

38 A New Deformation Mechanism Bending has been dismissed d = 4 ql f 72EI s For a shear controlled mechanism, stepheight should be proportional to L 2 d = ql 2 f s 2GA A new deformation mechanism?

39 Changing Freespan Could adhesion between the glass and the fiber be more important than flexibility? Freespan (µm) Bleached and Unbleached Never Dried Hardwood Bleached Bleached Regression y = 8.70x R 2 = 0.98 Unbleached Unbleached Regression y = 9.77x R 2 = 0.98 Bleached fibers have lower surface charge Stepheight (µm)

40 Changing Freespan High M W CMC grafted to fiber surface. 40 Surface Surface CMC Grafting Stepheight remained constant. Freespan increased by 30% Freespan ( µm) mg/g 12 mg/g 0 mg/g A change in the adhesion properties could change the freespan Stepheight (µm)

41 Conclusions The main effect of refining is to reduce the stepheight in the fiber crossing for both unbleached and bleached pulps Increases the tendency of the cell wall to collapse and deform The freespan of a fiber intersection is not controlled by the flexibility of the overlying fiber. The ratio of stepheight to freespan remains relatively constant for most treatments. Neither bending or shear based models can explain the behavior suggesting a new mechanism is required Could fiber-glass adhesion be important?

42 Thank You!