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Interfaces in composites based on wood and other lignocellulosic fiber Mark Hughes Department of Forest Products Technology Helsinki University of Technology Finnish-Japanese Workshop on Functional Materials Espoo & Helsinki, Finland 25

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25 May 2009 Department of Forest Products Technology

Interfaces in composites based

  • n wood and other

lignocellulosic fiber

Mark Hughes

Department of Forest Products Technology Helsinki University of Technology Finnish-Japanese Workshop on Functional Materials Espoo & Helsinki, Finland 25 & 26 May 2009

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25 May 2009 Department of Forest Products Technology

Contents

  • Overview of research activities in the

Wood Materials Technology group

  • Wood veneer surfaces in relation to

bonding

  • Interfaces in lignocellulosic fibre reinforced

polymer matrix composites

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25 May 2009 Department of Forest Products Technology

Research & teaching groups at the department

  • Chemical Pulping and Wood Refinery
  • Clean Technologies
  • Forest Biorefinery
  • Forest Products Surface Chemistry
  • Paper Converting and Packaging
  • Paper Technology
  • Printing Technology
  • Wood Chemistry
  • Wood Material Technology
  • Wood Product Technology
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25 May 2009 Department of Forest Products Technology

Current research themes in Wood Materials Technology

  • Wood fracture (particularly in relation to its use as

a structural material)

  • Mechanics of wood and wood-based composites
  • Physics of the veneer peeling process
  • Non-destructive evaluation of veneer
  • Wood modification (thermal, chemical,

impregnation)

  • Surface properties in wood (veneer)
  • Wood and non-wood fibre reinforced polymer

matrix composites, including “nanocomposites” (particularly interfaces)

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25 May 2009 Department of Forest Products Technology

Interfaces in wood based materials

  • Composite materials based on wood or other

lignocellulosic fiber offer many opportunities for the development of new materials

  • Interfaces (or interphases) created during the

formation of wood and natural fibre-based composites largely control both the short and long term performance of these materials

  • Interfaces are influenced by the physical and

chemical properties of the substrate (fibre), the properties of the adhesive (matrix) and the interaction between the two in the formed system (micromechanics).

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25 May 2009 Department of Forest Products Technology

Veneer surfaces

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25 May 2009 Department of Forest Products Technology

Peeled veneer production for plywood

  • Industrially logs are soaked at

temperatures of up to 70 oC before peeling

  • r slicing into veneer
  • This is known to affect the colour of the

veneer

  • But how does this affect the surface in

relation to the formation of the adhesive bond with a liquid resin?

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25 May 2009 Department of Forest Products Technology

Effect of soaking on color

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25 May 2009 Department of Forest Products Technology

Effect of soaking temperature

  • n wetting behavior of veneer

20 40 60 80 100 120 140 20 40 60 80 Time [s] Contact angle [º] 20 ºC 40 ºC 50 ºC 70 ºC

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25 May 2009 Department of Forest Products Technology

Automated Bonding Evaluation System

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25 May 2009 Department of Forest Products Technology

Effect of soaking temperature on bond development

2 4 6 8 10 12 100 200 300 400 500 Time [s] Bond strength [N/mm

2]

50 ºC 70 ºC 20 ºC

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25 May 2009 Department of Forest Products Technology

Conclusions

  • History and processing influence the surface of

the peeled veneer

  • This in turn influences adhesive bond formation
  • Implications for all wood-adhesive joints
  • Current and future research is focussing on

mechanisms behind these changes in wood (chemistry) and the impact on adhesion

  • Subject of a forthcoming Tekes & industry funded

3 year project (starting in June 2009)

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25 May 2009 Department of Forest Products Technology

Interfaces in natural fibre reinforced composites

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25 May 2009 Department of Forest Products Technology

Bast fibre

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25 May 2009 Department of Forest Products Technology

Bast vs. glass…

  • The reported properties of many natural fibres

make them potentially suitable as reinforcement in high performance composite materials

Fibre type Density (g cm-3) Young’s modulus (GPa) Tensile strength (MPa) E-glass 2.56 76 2000 Flax 1.4-1.5 50-70 500-900 Hemp 1.48 30-60 310-750 Jute 1.4 20-55 200-450

(Sources: Hull & Clyne, 1996; Ivens et al, 1997)

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25 May 2009 Department of Forest Products Technology

Structural properties

  • For thermosetting polymer matrix

composites:

  • Good stiffness (comparable with GFRP)
  • Adequate strength
  • Poor fracture properties (order of

magnitude lower work of fracture)

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25 May 2009 Department of Forest Products Technology

Reinforcement efficacy

  • For good reinforcement fibres of high

aspect ratio are required

  • Aspect ratio of e.g. flax ultimate fibres

around 1200. Potentially good reinforcement

  • But fibre damage may play a significant

role

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25 May 2009 Department of Forest Products Technology

Fibre properties

  • Bast (and wood fibre) fail in compression

through the formation of kink bands

  • In 1998, Davies and Bruce published a

paper showing that the Young’s modulus and tensile strength of flax and nettle fibre are negatively affected by the presence of these so called micro-compressive defects

  • r kink bands….
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25 May 2009 Department of Forest Products Technology

Polarised light

Unprocessed hemp fibre Mechanically processed hemp fibre

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25 May 2009 Department of Forest Products Technology

Fibre structure

Failure through the formation of kink bands

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25 May 2009 Department of Forest Products Technology

Effect on the interface

  • Micro tensile specimen
  • Half fringe photoelasticity system
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25 May 2009 Department of Forest Products Technology

Stress concentrations

Shear stress distribution in an epoxy matrix adjacent to a defect in a strained specimen at small deformation

(Source: Hughes et al, 2000)

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25 May 2009 Department of Forest Products Technology

Matrix plastic deformation

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25 May 2009 Department of Forest Products Technology

Interface behaviour

  • 2

2 4 6 8 1 1 2 1 4 1 6 8 1 2 1 6 2

microcompressive defects

C B A

"interface" principal stress difference composite tensile stress far-field matrix principal stress difference principal stress difference (M N m-2) distance along fibre (fibre diameters)

(Eichhorn et al, 2001)

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25 May 2009 Department of Forest Products Technology

Fibre-matrix debonding

Failed single filament composites showing fibre-matrix debonding in regions of high shear stress concentration adjacent to fibre defects (and fracture)

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25 May 2009 Department of Forest Products Technology

Matrix cracking

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25 May 2009 Department of Forest Products Technology

Deformation behaviour

(UD composite of ca. 55% volume fraction)

0.0 0.5 1.0 1.5 2.0 100 200 300 400

Tensile stress (MPa) Strain (%) E D C B A

A - Initial linear region B - Yield point C - Reduced stiffness D - Strain hardening E - Failure

(Hughes et al, 2007)

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25 May 2009 Department of Forest Products Technology

Fibre model

  • Continuous fibre acts as a series of shorter fibres
  • r segments
  • Kink bands act as the loci of microstructural

failure

– fibre fracture – fibre-matrix debonding – matrix cracking

  • Affects composite macroscopic behaviour
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25 May 2009 Department of Forest Products Technology

Summary

  • Composite properties are influenced by the

fibre properties and particularly the presence of micro-compressive defects

  • Micro-compressive can be removed, but not

practicable in reality

  • Alter the fibre architecture to improve

properties

  • “Deconstruct” the cell wall and isolate the

microfibrils – use these as reinforcement

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25 May 2009 Department of Forest Products Technology

  • Manipulation of the “fibre architecture” at

macroscopic and microscopic levels, as well as “interface engineering” to improve composite performance

  • “Fibre architecture” includes:

– fibre geometry (aspect ratio) – fibre orientation – packing arrangement – fibre volume fraction (Vf)

Fibre architecture & interface engineering

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25 May 2009 Department of Forest Products Technology

Fibre modification

  • Pectinolytic enzymes were used to preferentially

remove the inter-cellular binding substances and degrade any extraneous adhering tissue

  • Chelating agents for calcium (EDTA), which forms

part of the pectin structure, was used to remove pectin

  • Combinations of chelating agents and enzymes

were employed, applied sequentially and together

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25 May 2009 Department of Forest Products Technology

Tensile strength

1 2 3 4 5 6 10 20 30 40 50 60 70 80

Treatment Tensile strength, MPa

Untreated Water control EDTA Enzyme 1 stage 2 stage

(Stuart et al, 2005)

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25 May 2009 Department of Forest Products Technology

Ongoing research

  • Continue to develop an understanding of the

effect of fibre defects and other structural features

  • n interface behaviour

– Wood and non-wood fibre – Micro-fibrillated cellulose

  • Interface engineering

– Bulk and surface modification

  • Development of NF textiles for composite

applications

– Bi- and multi-axial fibre structures; woven and non- woven

  • Nanocomposites
  • The above mentioned research is the subject of

several ongoing research projects funded by: The Academy of Finland, the European Commission and Helsinki University of Technology

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25 May 2009 Department of Forest Products Technology

References

  • Davies, G.C. and Bruce, D.M. (1998). Effect of Environmental Relative Humidity and

Damage on the Tensile Properties of Flax and Nettle Fibers. Textile Res. J., 68(9): 623-629

  • Eichhorn, S.J., Baillie, C. A. and Zafeiropoulos, N., Mwaikambo, L.Y. and Ansell, M.P.,

Dufresne, A., Entwistle, M., Herrera-Franco P.J., and Escamilla, G.C., Groom, L., Hughes M. and Hill, C., Rials, T.G., Wild P.M. (2001). Current International Research into Cellulosic fibres and Composites. J. Mat. Sci. 36: 2107-2131

  • Hughes, M., Carpenter, J. and Hill, C. (2007). Deformation and Fracture Behaviour of

Flax Fibre Reinforced Thermosetting Polymer Matrix Composites. J. Mat. Sci. 42(7):2499-2511

  • Hughes, M., Hill, C.A.S., Sèbe, G., Hague, J., Spear, M. and Mott, L. (2000). An

Investigation into the Effects of Microcompressive Defects on Interphase Behaviour in Hemp-Epoxy Composites Using Half Fringe Photoelasticity. Composite Interfaces 7(1): 13-29

  • Hull, D. and Clyne, T.W. (1996). An Introduction to Composite Materials. Cambridge

University Press, Cambridge, UK

  • Ivens, J., Bos, H. and Verpoest, I. (1997). The Applicability of Natural Fibres as

Reinforcement for Polymer Composites. In: Renewable Biproducts: Industrial Outlets and Research for the 21st Century. June 24-25, 1997, EC-symposium at the International Agricultural Center (IAC), Wageningen, The Netherlands

  • Stuart, T., Liu, Q., Hughes, M., McCall, R.D., Sharma, S. and Norton, A. (2005)

Structural Biocomposites from Flax – Part I: Effect of Bio-technical Fibre Modification

  • n Composite Properties. Compos Part A-Appl S 37(3): 393-404 2006