Benefits of microfibrillated cellulose in Paperboard

Jonathan Phipps
Robyn Hill

PRESENTED BY:
Jonathan Phipps
Principal Scientist
FiberLean Technologies Ltd.

Introduction

• Microfibrillated cellulose (MFC) is well established as an additive in graphic papers
  • Increased filler content
  • Increased wet web, tensile and Z direction strength
  • Reduced porosity
• Packaging grades present new challenges
  • Complex, multi-layered structures
  • Laboratory evaluation methods
  • Delamination resistance
  • Bending stiffness

Impact of MFC on Graphical paper physical properties

Lab study Mesmer recirculating hand sheets (12 sheets) 70% Eucalyptus, 30% NBSK, 550 CSF Intracarb 60 filler

• It is possible to increase filler content by 10% or more and suffer no strength loss.
• Wet web strength is also increased – typically by more than dry strength
• Increasing fibre bonding and filler content decreases bulk

Overcoming bulk loss with filler increase

• Full scale trial, copy paper machine
• MFC has a positive effect on formation, retention, porosity and smoothness
• Calender loading can be reduced to restore bulk and stiffness
• Large filler increase still possible

Bending Stiffness and Elastic properties

• Bending a sheet of material stretches its outer surface and compresses its inner surface
• Resistance to bending is therefore directly related to the elastic modulus of the material
• Modulus (E) = Force / (x-section area x strain)
• For paper, x-section area depends on pressing and calendering, so instead we define tensile stiffness index (TSI): –
  TSI = Force/ (width x gsm x strain)

Stiffness calculated from tensile properties agrees well with experimental values

Calculated stiffness typically shows less variability than measured values, because grammage variations are eliminated

For a single layer, Bending stiffness = (E x t³ ) /12 = (TSI x gsm x t²) /12

Stiffness is very sensitive to thickness

Corrugated grades – White Top Liner

I-beam

White-top liner board

• White Top Liner is a 2-layer sheet
  • Typically a filler-containing bleached short fibre pulp layer is laminated on top of an unbleached long fibre Kraft layer
• The white layer hides the brown layer and provides a surface for printing
  • Minimum grammage is used to achieve optical coverage
  • Maximum possible filler content for opacity
• Final corrugated stiffness is high from the I-beam effect of the fluting
  • Liner bulk/bending stiffness are not required
  • Liner needs high tensile stiffness index from white and brown layers, but it is often not a specification
  • Main specifications are brightness, burst strength, Short-span compression strength, surface/Z-direction strength & delamination resistance
  • Filler loading is limited by its effect on Z-direction and burst strength (wet web strength is not important)
  • Specifications could be obtained with a range of grammages, but grades are still typically defined by basis weight

Use of MFC for filler increase and top layer weight reduction

• Pilot papermachine trial, 100% Eucalyptus 500 CSF, GCC filler, 70gsm
• Brightness calculation based on 80% ISO target on 25% ISO base
• Replacing top layer weight with unbleached Kraft base maintains burst and reduces costs

White Top Liner full scale example

Simultaneous filler increase and gsm reduction 2.5% MFC in white layer +9% filler, 10gsm of top layer replaced by brown layer Conditions chosen to maintain brightness Machine speed, filler retention and runnability maintained	Specification maintained at reduced cost Minor changes in strength properties Large reduction in air permeability 40% increase in Scott Bond Potential for higher filler increase and brightness

Folding Boxboard (FBB)

• Folding boxboard is a multilayer product
  • Typically FBB middle layers are made from bulky but weak TMP, outer layers from chemical pulp
• The outer layers hide the middle layers and also contribute strongly to bending stiffness
  • Bulk of middle layer separates the outer layers to enhance the ‘I-beam’ effect
  • Outer layers require high tensile stiffness to resist stretching and enhance overall stiffness
  • Outer layers may contain filler but loading may be limited by effect on stiffness
  • Z-direction strength and resistance to delamination are critical
• Stiffness of complex structures can in principle be calculated
  • Elastic modulus and thickness of each layer required
  • Contributions of each layer are calculated and added together: –

where

Making 3-layer handsheets

• Top/bottom layer weight and thickness from trimmed excess
• Total weight and thickness from 3-ply sheet
• Middle weight and thickness from difference

Measured vs. calculated stiffness for 3-ply sheets

Filler and MFC in top and bottom layers Calculated stiffness is slightly lower than measured stiffness Trends with filler and MFC are more consistent in calculated values

Effects of mfc in Folding Boxboard outer layers

Calculations of 3 ply bending stiffness with bulky CTMP mid-ply 45% ISO brightness Bulk 2.7 cm3 g-1 Tensile Stiffness Index 2.0 kN m g-1 180 gsm total – 40 gsm top & bottom, 100 gsm mid-ply Outer ply properties taken from handsheet study

Adjusting the outer layer weights

		180 gsm total Brightness 70 ISO Stiffness 9.5mN m
Stiffness passes through maximum with outer layer weight Outer layers have high tensile stiffness but low bulk ‘I-beam’ effect is reduced at high outer layer weights Low stiffness targets can be achieved without filler or MFC MFC allows use of high filler content and low outer layer weight		180 gsm total Brightness 75 ISO Stiffness 9.5 mN m

Adjusting the outer layer weights

		180 gsm total Brightness 70 ISO Stiffness 12.0 mN m
Stiffness passes through maximum with outer layer weight Outer layers have high tensile stiffness but low bulk ‘I-beam’ effect is reduced at high outer layer weights Higher Stiffness and Brightness targets can only be achieved with use of MFC and filler		180 gsm total Brightness 75 ISO Stiffness 12.0 mN m

Folding boxboard full scale example

Simultaneous filler increase and gsm reduction in both outer layers 2.25% MFC in each +10% filler in each, 4gsm of top layer and 3gsm of bottom replaced by middle layer Conditions chosen to maintain stiffness Machine speed, filler retention and runnability maintained	Specification maintained despite outer layer gsm reductions Bending stiffness and brightness unchanged Significant increase in smoothness

Conclusions

Single layer products

• MFC allows bulk and stiffness to be maintained with large increases in filler content
• Improved smoothness and formation
• Reduced calender load

White Top Liner

• MFC allows large filler increases in the white layer
• Increased delamination resistance
• Increased burst strength
• Reduced white layer / increased brown layer grammage

Folding Boxboard

• MFC in outer layers can enable improved stiffness and brightness specifications
• Increased delamination resistance and tensile stiffness
• Increased filler content
• Reduced outer-ply / increased mid-ply grammage