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Flax fibres as a reinforcement for epoxy composites

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Title: Flax fibres as a reinforcement for epoxy composites


1
Flax fibres as a reinforcement for epoxy
composites
  • Isabel Van de Weyenberg

Promoter Prof. Dr. Ir. I. Verpoest
2
Contents
  • Introduction
  • What are flax fibres?
  • Why flax fibres?
  • Problem Statement
  • Influence of fibre processing on UD composite
    properties
  • Focus on strength
  • The alkalisation of flax fibres
  • Micromechanical characterisation of the interface
    strength
  • Conclusions
  • Suggestions for future research

3
What are flax fibres?
Introduction
  • From flax plant, which is grown for its
  • fibres (? textile industry linen) or
  • seeds (? linseed oil)
  • Annual renewable
  • First proofs of flax culture date from 5000 B.C.
  • Flanders had a prosperous flax industry, until
    the rise of man-made fibres (mid 20th century)

4
Structure of flax fibres
Introduction
5
Chemical composition
Introduction
  • Cellulose-based (65 85 ) with high
    crystallinity (70 75 )
  • Mechanical properties (strength and stiffness)
  • Low solvent solubility
  • Other important constituents
  • Hemicelluloses (10 - 18 ) ? hydrophilicity
  • Pectins (1 3 ) ? cementing material
  • Lignins (1 4 ) ? hydrophobicity

6
Processing of flax fibres
Introduction
Recovering of shorter fibres
Removal of wooden particles
Carding
Slivers
Scutched Tow(10 - 30 cm)
Hackled Tow(10 - 30 cm)
Scutching
Green flax
Retting
Long Flax(gt 30 cm)
Hackling
Slivers
Combingand aligning
Removal of pectins
Yarns
Spinning
Refining
7
Why use flax fibres in composite applications?
Introduction
  • Compared to glass fibres
  • CO2-neutral life cycle
  • no waste problems fully combustible and
    biodegradable
  • less abrasive
  • thermal and acoustic insulation
  • low density
  • reasonable properties
  • variable properties
  • hydrophilic
  • Compared to other natural fibres
  • best mechanical properties

8
Problem statement
Problem Statement
  • Natural fibres display good mechanical fibre
    properties, BUT
  • As reinforcement in polymer composites, these
    properties are not transferred and the intended
    composite strength is not achieved
  • Structural applications are not feasible
  • Why and how to solve this?
  • Natural character of plant fibres play a role
  • Variability in properties
  • Composition of fibre surface ? adhesion with resin

9
Goals
Problem Statement
  • Which fibre grade gives best composite?
  • Influence of fibre processing
  • How to optimise the adhesion between fibres and
    matrix?
  • Alkali treatment of the fibres
  • Silane treatment of the fibres
  • How to measure interface quality?
  • Direct method (microdroplet)
  • Indirect method (transverse three point bending)

10
Research scheme
Problem Statement
  • UD composites
  • matrix epoxy

11
Contents
  • Introduction
  • What are flax fibres?
  • Why flax fibres?
  • Problem Statement
  • Influence of fibre processing on UD composite
    properties
  • Focus on strength
  • The alkalisation of flax fibres
  • Micromechanical characterisation of the interface
  • Conclusions
  • Suggestions for future research

12
Fibre processing
UD composites
3 retting degrees 4 mech. stages ? 12
materials
CST-
Carding
Slivers
CHT-
Scutched Tow(10 - 30 cm)
Hackled Tow(10 - 30 cm)
Scutching
Green flax
Retting
Long Flax(gt 30 cm)
Hackling
Slivers
Green -GHalf-retted -HNormal retted -N
SLF-
HS-
Yarns
Spinning
Refining
13
UD composite production
UD composites
  • Unidirectional composites
  • Epoxy resin film HM 533 (Hexcel Composites S.A.)
  • Vf 40 vol
  • Autoclave curing (1h at 125 C, 3 bar pressure)

14
Longitudinal propertiestensile strength
UD composites
15
Transverse propertiestensile strength
UD composites
16
Partial conclusions
UD composites
  • Retting degree
  • Avoid green fibres
  • Preferably normal retted fibres
  • Mechanical processing
  • Increasing cost price with further processing
  • Carded scutched tow gives optimal
    performance/cost
  • Interface optimisation is necessary!
  • Only 45 of the fibre strength is efficiently
    transferred into the composite!

17
Contents
  • Introduction
  • What are flax fibres?
  • Why flax fibres?
  • Problem Statement
  • Influence of fibre processing on UD composite
    properties
  • The alkalisation of flax fibres
  • Micromechanical characterisation of the interface
  • Conclusions
  • Suggestions for future research

18
Alkalisation of flax fibres
Alkalisation
  • Treatment with NaOH affects the fibres
  • Swelling reaction
  • Removal of surface impurities (e.g. waxes)
  • Partial removal of hemicelluloses pectins ?
    fibrillation
  • Re-organisation of the fibre structure
  • Roughening of the fibre surface
  • Effect on the composite properties
  • Better adhesion with matrix due to roughening
  • Effective interfacial area increases due to
    fibrillation
  • Improved wettability of the fibres by the resin

19
Batch
Alkalisation
  • UD composites, HM 533 epoxy matrix, 40 vol flax
    slivers
  • Fibre treatment with NaOH
  • Concentration 1, 2 and 3
  • Time 20 minutes
  • Rinse several times in distilled water and
    acidified water
  • Drying oven at 80 C during 8 hours
  • Autoclave curing (1h, 125 C, 3 bar pressure)
  • Three point bending properties
  • Longitudinal direction
  • Transverse direction

20
Residual fibre strength after alkalisation
Alkalisation
Removal of the cementing constituents? poorer
stress transfer inside the fibre
21
Longitudinal propertiesflexural strength
Alkalisation
30
Cannot be due to increased fibre strength?
better interface quality
22
Transverse propertiesflexural strength
Alkalisation
Due to enhanced interface
23
Batch conclusions
Alkalisation
  • Mild alkali treatments enhance the composite
    properties, both in longitudinal and transverse
    direction
  • Effects are due to improved interface quality
    between fibres and matrix, due to
  • Roughening of fibre surface
  • Removal of waxy substances, impurities and pectins
  • Interesting to investigate continuous process
  • Time should be reduced
  • Flax roving instead of slivers

24
Continuous set-updrumwinder
Alkalisation
25
Continuous set-uptreatment section
Alkalisation
Time 45 sec
26
Continuous set-updrying section
Alkalisation
27
Low NaOH concentrations
Alkalisation
  • Mild treatments do not affect the fibre
    properties
  • No effect on the longitudinal composite
    properties

28
Low NaOH concentrations (2)
Alkalisation
  • The removal of impurities promotes the adhesion
    between fibre and matrix
  • Improved transverse composite performance due to
    better interface!

Treatment time of 45 sec is too short
29
Higher NaOH concentrations
Alkalisation
Composite properties are comparable to matrix
properties !!!
30
Residual alkali
Alkalisation
31
Partial conclusions
Alkalisation
  • Alkalisation
  • Positive effect (cfr. batch treatments)
  • Simple and interesting for industrial
    applications
  • Continuous set-up encounters bottlenecks
  • Treatment time should be elongated
  • Residual alkali should be removed by improved
    rinsing (e.g. diluted acid)
  • Roving should be dried thoroughly before resin is
    impregnated (e.g. squeezing out excess moisture)

32
Contents
  • Introduction
  • What are flax fibres?
  • Why flax fibres?
  • Problem Statement
  • Influence of fibre processing on UD composite
    properties
  • The alkalisation of flax fibres
  • Micromechanical characterisation of the interface
  • Conclusions
  • Suggestions for future research

33
Interface quality in flax/epoxy
Interface Characterisation
  • Interface between untreated flax and epoxy is
    poor
  • How to quantify this?
  • Silanisation of the fibre might give improvement
  • Measurement methods
  • Macromechanical e.g. transverse three point
    bending
  • On UD composite samples ? real material
  • Voids, misalignments
  • Fast test
  • Micromechanical e.g. microdroplet test
  • Single fibre and droplet ? local test, focused on
    interface
  • Time consuming

34
Microdroplet test set-up
Interface Characterisation
35
Microdroplet test outcome
Interface Characterisation
36
Materials
Interface Characterisation
  • Carded flax slivers, i.e. non-twisted fibre
    bundles of shorter flax tow average fibre
    length ? 20 cm
  • Epoxy LMB 6305/HY 5021 BD (Vantico)
  • Aminoethyl-aminopropyl-trimethoxysilane (Dow
    Corning Z-6020 Silane) performed at U.Gent by B.
    De Corte
  • Principle X3Si R H2O ? R Si(OH)3 3 HX

37
Silanisation parameters
Interface Characterisation
  • Reference treatment
  • Degreasing of fibres with hexane/ethylacetate
  • 0.20 silane/acetone/water solution
  • Catalyst for hydrolysis dibutyltindilaurate
  • 3h treatment time _at_ RT
  • Curing _at_ 105 C for 2h
  • Several parameters can be changed
  • Treatment temp. (-5 C 55 C) higher T
    better hydrolysis
  • Silane curing temperature (decrease to 50 C)
  • Treatment time (elongation to 24h)
  • Solvent ethanol instead of acetone (less
    damaging to fibre)
  • Autocatalytic
  • ...

38
Apparent interfacial shear strength
Interface Characterisation
39
Detailed analysis
Interface Characterisation
  • Calculated bond strengths ?d show similar
    tendencies as ?apparent
  • Most reliable results are based on experimentally
    measured values for ?friction
  • Remark
  • Thermal stresses, ?T, can become significant
  • Friction after silanisation is decreased
  • Smoothening of the surface
  • Standard deviation on the results is large!
  • Natural character of the fibres
  • Used model is not optimal for natural fibre
    systems

40
ESEM on low performance composites
Interface Characterisation
41
ESEM on high performance composites
Interface Characterisation
Resin
Rougher fibre surfaces
42
Partial conclusions
Interface Characterisation
  • Silane fibre treatments
  • Hydrolysis is important
  • The examined parameter combinations are not
    optimal
  • Microdroplet tests
  • Time consuming
  • Results should be considered with care
  • Analysis
  • Average interfacial strength gives limited
    information
  • Detailed analysis provides estimate for bond
    strength and frictional stress
  • ESEM
  • besides fibre/matrix adhesion also internal fibre
    adhesion should be optimised

43
Contents
  • Introduction
  • What are flax fibres?
  • Why flax fibres?
  • Problem Statement
  • Influence of fibre processing on UD composite
    properties
  • The alkalisation of flax fibres
  • Micromechanical characterisation of the interface
  • Conclusions
  • Suggestions for future research

44
Conclusions
Conclusions
  • Carded slivers from scutched tow
  • Optimum combination of price and properties
  • Fibre treatments are essential to improve the
    interface quality
  • Alkalisation is efficient, IF
  • Fibres are rinsed thoroughly
  • Fibres are dried optimally
  • Silanisation results in minor effect on composite
    properties
  • Hydrolysis step should be promoted
  • Mild treatment parameters are beneficial
  • Microdroplet tests illustrate that the frictional
    and thermal stresses cannot be neglected compared
    to bond strength

45
Realisations
Conclusions
  • Optimal fibre processing degree is determined
  • Alkalisation
  • First steps towards continuous process for
    manufacturing unidirectional alkalised flax/epoxy
    composites
  • Bottlenecks are identified in this continuous
    process
  • It is shown that the internal adhesion inside the
    fibre should be enhanced as well as the
    fibre/matrix bonding
  • First impulse towards detailed micromechanical
    analysis of the flax/epoxy interface is made!

46
Can flax compete with glass?
Conclusions
430
Elementary fibres
331
47
Suggestions for future research
Conclusions
  • Internal adhesion should be improved
  • Remove and replace weak substances
  • Keep alignment of the cellulose regions
  • Limit variability of the fibres
  • Processing and handling with care
  • Cultivation methods
  • Optimise fibre/matrix adhesion
  • e.g. alkalisation
  • Other innovative treatments
  • Biodegradability attractive cost price should
    be maintained
  • Micromechanical characterisation of the
    interface adaptation of the model is necessary
    for natural fibre composites

48
Thank you!
49
Cellulose
Linear molecules from linked glucose units
Inter- and intramolecular hydrogen bonding
Highly ordered cellulose microfibrils (? high
crystallinity)
50
Hemicelluloses example
Glucuronoarabinoxylan with a xylan backbone and
side chains containing arabinose and glucuronic
acid (taken from Taiz L. and Zeiger E.)
Short, branched, amorphous, low molecular
weight Low resistance to chemical attack
Hydrophilic In between the cellulose
microfibrils
51
Pectins example
Homogalacturonan (pectic acid) (taken from Taiz
L. and Zeiger E.)
Complex, large, branched structures of
polysaccharides Cement for the cell walls
52
Lignin example
Partial structure of lignin molecule from beech
wood (taken from Taiz L. and Zeiger E.)
Amorphous High molecular wt Complex, 3D
network Rigidity
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