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25 May 2009Department of Forest Products Technology
Interfaces in composites based
on wood and other
lignocellulosic fiber
Mark HughesDepartment 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 2009Department 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 2009Department 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 2009Department 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 2009Department 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 2009Department of Forest Products Technology
Veneer surfaces
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25 May 2009Department of Forest Products Technology
Peeled veneer production for
plywood
• Industrially logs are soaked at
temperatures of up to 70 oC before peeling
or 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 2009Department of Forest Products Technology
Effect of soaking on color
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25 May 2009Department of Forest Products Technology
Effect of soaking temperature
on wetting behavior of veneer
0
20
40
60
80
100
120
140
0 20 40 60 80
Time [s]
Co
nta
ct
an
gle
[º]
20 ºC 40 ºC 50 ºC 70 ºC
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25 May 2009Department of Forest Products Technology
Automated Bonding Evaluation
System
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25 May 2009Department of Forest Products Technology
Effect of soaking temperature on
bond development
0
2
4
6
8
10
12
0 100 200 300 400 500
Time [s]
Bo
nd
str
en
gth
[N
/mm
2]
50 ºC 70 ºC 20 ºC
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25 May 2009Department 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 2009Department of Forest Products Technology
Interfaces in natural fibre
reinforced composites
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25 May 2009Department of Forest Products Technology
Bast fibre
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25 May 2009Department 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
Fibretype
Density(g cm-3)
Young’smodulus
(GPa)
Tensilestrength(MPa)
E-glass 2.56 76 2000
Flax 1.4-1.5 50-70 500-900Hemp 1.48 30-60 310-750Jute 1.4 20-55 200-450
(Sources: Hull & Clyne, 1996; Ivens et al, 1997)
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25 May 2009Department 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 2009Department 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 2009Department 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 or kink bands….
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25 May 2009Department of Forest Products Technology
Polarised light
Unprocessed
hemp fibre
Mechanically processed
hemp fibre
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25 May 2009Department of Forest Products Technology
Fibre structure
Failure through the formation of kink bands
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25 May 2009Department of Forest Products Technology
Effect on the interface
•Micro tensile specimen
•Half fringe photoelasticity system
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25 May 2009Department 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 2009Department of Forest Products Technology
Matrix plastic deformation
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25 May 2009Department of Forest Products Technology
Interface behaviour
-2 0 2 4 6 8 10 12 14 16
8
12
16
20
microcompressive defects
C
BA
"interface" principal stress difference
composite tensile stress
far-field matrix principal stress difference
princi
pal s
tress d
iffere
nce
(M
N m
-2)
distance along fibre (fibre diameters)
(Eichhorn et al, 2001)
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25 May 2009Department 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 2009Department of Forest Products Technology
Matrix cracking
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25 May 2009Department of Forest Products Technology
Deformation behaviour(UD composite of ca. 55% volume fraction)
0.0 0.5 1.0 1.5 2.0
0
100
200
300
400T
en
sile
str
ess
(M
Pa)
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 2009Department of Forest Products Technology
Fibre model
• Continuous fibre acts as a series of shorter fibres or 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 2009Department 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 2009Department 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 2009Department 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 2009Department of Forest Products Technology
Tensile strength
1 2 3 4 5 60
10
20
30
40
50
60
70
80
Treatment
T
en
sil
e s
tre
ng
th,
MP
a
Untreated Water control EDTAEnzyme 1 stage 2 stage
(Stuart et al, 2005)
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25 May 2009Department of Forest Products Technology
Ongoing research• Continue to develop an understanding of the
effect of fibre defects and other structural features on 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 2009Department 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 Interfaces7(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 on Composite Properties. Compos Part A-Appl S 37(3): 393-404 2006