This seminar report discusses chemical modifications of natural fibres for composite applications. It introduces composites and their constituents such as reinforcement, matrix and interface. It then classifies fibre reinforced composites and discusses market trends for natural fibre reinforced composites in automotive and construction industries. The report examines advantages and disadvantages of natural plant fibres such as flax, hemp and jute compared to conventional fibres. It describes the structure of plant fibres and various physical, biological, nanotechnology and chemical modification techniques to improve mechanical properties and interface bonding of natural fibres in composites. Specific chemical treatments including alkali, silane, acetylation and permanganate treatments are explained along with reaction mechanisms. Case studies on jute
Chemical modifications of natural fibres for composite applications
1. Seminar Report Presentation on:
Chemical Modifications of
Natural Fibres for Composite
Applications
-
KETKI SURESH CHAVAN
Final Year B.Tech.- F.T.P.T.
201003021052
2. Introduction to Composites…
Heterogeneous nature
created by the assembly of two or more components with fillers
or reinforcing fibres and a compactable matrix
Constituents of a Composite material are:
Reinforcement: Discontinuous, Stiffer, Stronger.
Matrix: Continuous, Less Stiff, Weaker
Interface: A third phase exists between reinforcement and the matrix
because of chemical interactions or other processing effects
plays an important role in controlling failure mechanisms, fracture
toughness
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3. Classification of FRCs.
Ref.:Agrawal B D & Broutman L J, 1980
Fibre Reinforced
Composites
Single Layer
(same orientation
& properties in
each layer)
Continuous
fibre
Reinforcement
Unidirection
Reinforcement
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Bi directional
Reinforcement
(woven fabric)
Multi layer
(angle ply)
Discontinuous
fibre
Reinforcement
Random
Orientation
Preferred
orientation
Laminates
Hybrids
4. Market Trends for NFRCs
Automotive & Construction were largest
segment among all natural composite
applications.
Several automobile models, first in
Europe and then in North America,
featured natural reinforced thermosets and
thermoplastics in door panels, package
trays, seat backs and trunk liners.
Dräxlmaier Group and Faurecia supply
interior parts such as headliners, side and
back walls, seat backs, and rear deck trays
to GM, Audi, and Volvo among others.
Bast fibre composites for Automotive &
Wood plastic composites for
Construction & Building.
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Ref:[http://www.researchandmarkets.com/repo
rts/1933235]
5. The Reinforcement
Their Major Disadvantages:
Conventional Fibres:
High Densities
Carbon
Non Renewable
Glass
Non Recyclabable
Aramids
High Energy Consumption
during Manufacture
Others include: Boron,
Alumina, Silicon Carbide,
Quartz fibres.
Not CO2 Neutral
Non biodegradable
Expensive
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7. The Natural Plant Fibres
Classification:
Advantages:
Leaf (pineapple, sisal,
banana)
Abundantly available
Renewable resources
Seed (cotton, milkweed)
Relatively less costly
Bast (hemp, flax, jute)
Biodegradable
Fruit (coir, kapok, oil palm)
Flexible for processing
Grass (bagasse, bamboo)
No health hazards during
manufacture
Stalk (rice straw)
Wood fibres (soft & hard
wood)
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Desirable aspect ratio, low
density and relatively good
tensile and flexural modulus.
8. Structure of Plant Fibres
Natural plant fibres are
constitutes of cellulose
fibres, consisting of
helically wound cellulose
micro - fibrils, bound
together by an amorphous
lignin matrix.
Lignin keeps the water in
the fibre; acts as a
protection against
biological attack and as a
stiffener to give stem its
resistance against gravity
forces and wind.
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Hemicellulose found in the
natural fibres is believed
to be a compatibilizer
between cellulose and
lignin.
10. Disadvantages of Natural fibres for
applications in composites.
Enormous Variability in Properties
Lack of FIBRE – MATRIX adhesion
Poor Moisture resistance
Poor Fire resistance
Lower durability
Limited Maximum Processing Temperatures
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These problems are being dealt with today by carrying out various
modifications & treatments. These have different efficiencies for
improving the mechanical properties of fibres, the adhesion
between matrix and fibre result in the improvement of various
properties of final products.
11. Physical modifications
Thermo treatment followed by Calendaring & Stretching:
Softening the lignin & hemicellulose, bringing it to surface &
forming of Water resistant surface (Hydrophobic layer)
Plasma Treatment:
Two types: Corona Discharge at Atm. Press.
High Frequency Cold Plasma
This treatment does not at all affect the bulk properties of the
natural fibres.
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12. Biological Modifications
Involves the use of naturally occurring microorganisms, namely
bacteria and fungi.
Retting is the controlled degradation of plant stems to free the
bast fibres from their bundles, as well as to separate them from
the woody core and epidermis. During the retting process,
bacteria (predominantly Clostridia species) and fungi, release
enzymes to degrade pectic and hemicellulosic compounds in the
middle lamella between the individual cells.
Separation of pectic & hemicellulosic substances helps the main
fibres to become clean & get exposed to the matrix effectively
for better interfacial adhesion.
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Process is time consuming, water polluting & the quality of
fibres obtained is very much dependent on quality of water used.
13. Nanotechnology
Nanotechnology can be used to modify natural fibres to
introduce new function onto the surface of fibres and enhance
the performance of final natural fibre – based products. It is
believed that the application of NT to modify natural fibres
offers high economic potential for the development of natural
fibre – based industry.
Layer-by-Layer Deposition and Sol-Gel processes are the main
approaches which have commonly been employed.
A combination of Biological treatment & NT has also been
studied on Hemp & Sisal fibres by using bacteria:
Gluconacetobacter xylinus treatment on the fibres & then
fabricated this treated cellulose on the surface of natural fibres.
This helped increase the strength of the Bio composites made
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from them.
14. Chemical Modifications
Chemical modification utilizes chemical agents to modify the
surface of fibres or the whole fibre throughout. The chemical
treatment of fibre is aimed at:
improving the adhesion between the fibre surface and the
polymer matrix.
not only modify the fibre surface but also increase fibre strength.
Reducing water absorption by composites (increasing moisture
resistance) & improving mechanical properties of the composite
materials.
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17. ALKALINE TREATMENT
Also known as Mercerisation
The important modification done by alkaline treatment is the disruption
of hydrogen bonding in the network structure, thereby increasing surface
roughness.
This treatment removes a certain amount of lignin, wax and oils covering
the external surface of the fibre cell wall, depolymerizes cellulose and
exposes the short length crystallites. The treatment changes the
orientation of the highly packed crystalline cellulose order, forming an
amorphous region.
It is reported that alkaline treatment has two effects on the fiber:
(1) It increases surface roughness resulting in better mechanical interlocking;
(2) It increases the amount of cellulose exposed on the fiber surface, thus
increasing the number of possible reaction sites.
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18. ACETYLATION TREATMENT
A reaction introducing an acetyl functional group (CH3COO–)
Acetylation of natural fibres is a well-known esterification
method causing plasticization of cellulosic fibres.
Chemical modification with acetic anhydride (CH3-C(=O)-OC(=O)-CH3) substitutes the polymer hydroxyl groups of the cell
wall with acetyl groups, modifying the properties of these
polymers so that they become hydrophobic.
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19. BENZOYLATION TREATMENT
Benzoyl chloride is most often used in fibre treatment. Benzoyl
chloride includes benzoyl (C6H5C=O) which is attributed to the
decreased hydrophilic nature of the treated fibre and improved
interaction with the hydrophobic matrix.
Benzoylation of fiber improves fiber matrix adhesion, thereby
considerably increasing the strength of composite, decreasing its
water absorption and improving its thermal stability.
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20. ACRYLATION & ACRYLONITRILE
GRAPHTING
Acrylation reaction is initiated by free radicals of the cellulose
molecule. Cellulose can be treated with high energy radiation to
generate radicals together with chain scission. Acrylic acid
(CH2=CHCOOH) can be graft polymerized to modify natural
fibres.
Acrylonitrile (AN, (CH2=CH–C≡N)) is also used to modify
fibres. The reaction of Acrylonitrile with fibre Hydroxyl groups
occurs in the following manner:
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21. SILANE TREATMENT
Silane is a chemical compound with chemical formula SiH4. Silanes are
used as coupling agents to let natural fibres adhere to a polymer matrix,
stabilizing the composite material. Silane coupling agents may reduce the
number of cellulose hydroxyl groups in the fibre – matrix interface.
In the presence of moisture, hydrolysable alkoxy group leads to the
formation of silanols. The silanol then reacts with the hydroxyl group of
the fibre, forming stable covalent bonds to the cell wall that are
chemisorbed onto the fibre surface.
Therefore, the hydrocarbon chains provided by the application of silane
restrain the swelling of the fibre by creating a crosslinked network due to
covalent bonding between the matrix and the fibre.
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22. COUPLING AGENTS
Maleated coupling agents are widely used to strengthen composites
containing fillers and fibre reinforcements.
Maleic anhydride is not only used to modify fibre surface but also the PP
matrix to achieve better interfacial bonding and mechanical properties in
composites. The PP chain permits maleic anhydride to be cohesive and
produce maleic anhydride grafted polypropylene (MAPP). Then the
treatment of cellulose fibres with hot MAPP copolymers provides covalent
bonds across the interface.
The mechanism of reaction of maleic anhydride with PP and fibre can be
explained as the activation of the copolymer by heating (170 C) before
fibre treatment and then the esterification of cellulose fibre.
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24. ISOCYANATE TREATMENT
The isocyanate group is highly susceptible to reaction with the
hydroxyl groups of cellulose and lignin in fibres. Isocyanate is
reported to work as a coupling agent used in fibre-reinforced
composites.
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25. PERMANGANATE TREATMENT
Permanganate is a compound that contains permanganate group
MnO4- . Permanganate treatment leads to the formation of
cellulose radical through MnO3- ion formation. Then, highly
reactive Mn3+ ions are responsible for initiating graft
copolymerization.
Most permanganate treatments are conducted by using
potassium permanganate (KMnO4) solution (in acetone) in
different concentrations with soaking duration from 1 to 3 min
after alkaline pre-treatment.
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26. PEROXIDE TREATMENT
Peroxide treatment of cellulose fibre has attracted the attention
of various researchers due to easy processing ability and
improvement in mechanical properties. Organic peroxides tend
to decompose easily to free radicals (RO∙), which further react
with the hydrogen group of the matrix and cellulose fibres.
Benzoyl peroxide (BP (C6H5CO)2) and Dicumyl peroxide (DCP
(C6H5C(CH3)2O)2) are chemicals in the organic peroxide family
that are used in natural fibre surface modifications. In peroxide
treatment, fibres are coated with BP or DCP in acetone solution
for about 30 min after alkali pre-treatment.
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28. SODIUM CHLORITE TREATMENT
This treatment involves the bleaching of natural fibres with
sodium chlorite which cleans the fibres thoroughly but makes
them rough. This roughness is responsible for better Fibre –
Matrix adhesion which is possible because of the interlocking of
the rough fibre surface & the matrix polymer chains.
The bleaching treatment involves the use of an Activating Agent
which has a function to decompose Sodium chlorite to liberate
Nascent Oxygen & not Chlorine Dioxide which is responsible
for the bleaching action.
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30. JUTE FIBRE COMPOSITES
1. Alkaline Treatment with 5% NaOH solution for 2h, 4h & 8h at R.T.
Result: Mechanical properties of fibres improved due to increase in Crystallinity
Composite material: treated and untreated jute (15 wt%) reinforced unsaturated
polyester (UPE).
Result:
DSC analysis it was found that thermal stability enhanced due to the
resistance offered by the closely packed cellulose chain in combination with
the resin.
Flexural strength of the composite prepared with 2 h and 4 h alkali treated
fibre were found to increase by 3⋅16% and 9⋅5%, respectively.
8 h treated fibre exhibited maximum strength properties, the composite
prepared with them showed lower strength value.
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31. JUTE FIBRE COMPOSITES
2. Comparison of Alkaline & Coupling agent Treatment:
Composite material: jute/polybutylene succinate (PBS) biocomposites with fibre
content of 20%(by wt.)
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37. FLAX FIBRE COMPOSITES
1.
Alkali & Bleaching agent treatment with & without Compatibilizer
Treatment: Alkaline treatment followed by Bleaching Treatment
Resin: Polypropylene; Compatibilizer : MAPP (5 % by wt. of composite)
Results:
Without Compatibilizer, Tensile properties of composites were inferior than
those of Pure PP.
With Compatibilizer, favourable increase in Tensile strength of Composites
was observed. Also, continuously increased trend of composite modulus can
be found in all cases (untreated, bleached and treated) and reached a
maximum value at 65/5/30 (% wt PP/MAPP/ fibre loading).
MAPP helped to improve both tensile strength and Young’s modulus of the
composites compared to those without MAPP.
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Ref: The Canadian Society for Bioengineering, 2008, Paper no: 084364, pp.
39. FLAX FIBRE COMPOSITES
2. Comparison of Silane & Styrene Treatments
Silane (Si) and styrene (S) treatments were applied on flax fibres in order to
improve their adhesion with a polyester resin and to increase their moisture
resistance.
In the case of (S) treatment, the presence of styrene increased the moisture
resistance of the treated fibres and made compatible the fibres and the
matrix.
In the case of (Si) treatment, a good hydric fibre/matrix interface was
obtained due to crosslinking reactions and hydrogen bonding between
water molecules and free hydroxyl groups of (Si) treated fibres.
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40. 40
Ref: Journal of Composites Science & Technology; Vol 71 (6); April
2011;pages 893 - 899
42. SISAL FIBRE COMPOSITES
1.
Alkaline treatment
The effect of NaOH concentration (0.5, 1, 2, 4 and 10%) for treating sisal fibre
– reinforced composites and concluded that maximum tensile strength resulted
from the 4% NaOH treatment at room temperature. Thereafter, the tensile
strength of the composites decreased, as the increase in concentration of NaOH
caused excess delignification resulting in weaker & damaged fibres & thus less
strong composites.
2. Silane treatment
Solution used: 2% aminosilane in 95% alcohol
pH : 4.5 to 5.5; Duration for soaking: 5 mins
The treatment was followed by air drying of the fibres for 30 mins which
Hydrolysed the Silane coupling agent.
Results: Increased fibre – matrix interfacial adhesion.
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43. SISAL FIBRE COMPOSITES
3. Acetylation treatment of Raw Sisal
Pre – treatment: 18% NaOH solution
Treatment: Glacial Acetic Acid treatment followed by Acetic Anhydride
(containing 2 drops of conc.H2SO4) for a period of 1 hour.
Result: Treated surface of sisal fibre reportedly became very rough and had a
number of voids that provided better mechanical interlocking with the
polystyrene (PS) matrix.
4.
Permanganate Treatment
Alkaline pre – treated sisal fibres were used
Permanganate solutions in acetone were prepared of concentrations 0.033,
0.0625 & 0.125% & the fibres were dipped in them for 1 min each.
Results: Reduced hydrophilicity of fibres thus moisture resistance of composites
increased.
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44. SISAL FIBRE COMPOSITES
5. Grapht Copolymerisation of Acrylonitrile
Study was carried out using combination of NaIO4 and CuSO4 as initiator in an
aqueous medium at temperatures between 50 and 70 C.
Results:
It was found that untreated fibres absorbed the most water and 25% AN-grafted
sisal fibres absorbed the least water, suggesting that changes in chemistry of the
fibre surface reduced the affinity of fibres to moisture.
It was also found that grafting of chemically modified fibres with 5% AN
brought a higher increase in tensile strength and Young’s modulus of fibres than
grafting with 10 and 25% AN.
The explanation for this was that grafting at low concentration of AN may create
orderly arrangement of polyacrylonitrile units.
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Ref: J Polym Environ (2007) 15:25–33
46. BAMBOO FIBRE COMPOSITES
5 treatments are compared:
1. Maleic Anhydride Treatment: alkali treated fibres soaked in 5% MA in Xylene
for 24 hours, with fibres to solvent ratio 1:10 (wt./vol)
2. Permanganate Treatment: 5% NaOH treated fibres soaked in 1% KMnO4 in
acetone solution for 20 mins
3. Benzoyl Chloride Treatment: 5% NaOH treated fibres soaked in a solution of
150ml Benzoyl chloride dissolved in 2litres of 5%NaOH, for 15 mins
4. Benzyl Chloride Treatment: 5% NaOH treated fibre mat further treated with a
solution of 250gm Benzyl chloride in same 5% NaOH solution as used before
for 6 hours at 80 C.
5. Pre – impregnation with Epoxy resin: alkali treated samples pre – impregnated
with solution of 2% epoxy dissolved in Acetone for 2 hours at R.T.
Matrix: Epoxy or Polyester resins
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47. BAMBOO FIBRE COMPOSITES
Results:
Flexural properties of maleic anhydride treated bamboo polyester
composites improved by nearly 50%.
The tensile strength and modulus of permanganate-treated bamboo polyester
composite increased by 58% and 118%.The tensile strength and modulus of
benzoylated bamboo fibre polyester composite improved by 71% and 118%,
respectively.
After benzoylation of the bamboo fibre, the water absorption is 16% for the
bamboo-epoxy composite compared to 41% by untreated bamboo-epoxy
composite. The water absorption by benzylated bamboo reinforced polyester
composite was 16.8% compared to 51% by untreated bamboo-polyester
composite.
Pre-impregnation improved the mechanical and water resistant properties of
both epoxy and polyester-based composites.
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52. SUMMARY
This seminar report was a brief study of the various chemical
treatments & modifications which are being done today on Natural
fibres to increase the Fibre – Matrix adhesion in the composites in
which they are being used widely. Also the study referred to some
specific Natural fibres which are being used widely in composites &
the treatments done on them which are prevalent. A greater
understanding of extracted fibre properties, treated fibre properties,
and their respective composites’ properties is achieved.
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