Chemical modifications of natural fibres for composite applications

 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..  A combi

Chemical Modifications of

Natural Fibres for Composite Applications

Final Year B.Tech.- F.T.P.T

201003021052

Seminar Report Presentation on:

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 effectsplays an important role in controlling failure mechanisms, fracture toughness

Classification of FRCs.

Fibre Reinforced Composites

Single Layer (same orientation

& properties in each layer)

Continuous fibre Reinforcement

Unidirection

Reinforcement

Bi directional Reinforcement (woven fabric)

Discontinuous fibre Reinforcement

Random Orientation orientationPreferred

Multi layer (angle ply)

Laminates Hybrids

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Ref.:Agrawal B D & Broutman L J, 1980

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.

 Others include: Boron,

Alumina, Silicon Carbide,

The Natural Fibres

The Natural Plant Fibres

Classification:

 Leaf (pineapple, sisal,

banana)

 Seed (cotton, milkweed)

 Bast (hemp, flax, jute)

 Fruit (coir, kapok, oil palm)

 Grass (bagasse, bamboo)

 Stalk (rice straw)

 Wood fibres (soft & hard

wood)

Advantages:

 Abundantly available Renewable resources

 Relatively less costly

 Biodegradable

 Flexible for processing

 No health hazards during manufacture

 Desirable aspect ratio, low density and relatively good tensile and flexural modulus

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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.

 Hemicellulose found in the

natural fibres is believed to

be a compatibilizer between

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Mechanical properties of Natural fibres

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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

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

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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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

 Process is time consuming, water polluting & the quality of

fibres obtained is very much dependent on quality of water used

 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 from them

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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

Chemical Compositions of Natural fibres

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Various Chemical Treatments

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 interloc king;

(2) It increases the amount of cellulose exposed on the fiber surface, thus

increasing the number of possible reaction sites 17

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 (CH3C(=O)-CH3) substitutes the polymer hydroxyl groups of the cell wall with acetyl groups, modifying the properties of these

-C(=O)-O-polymers so that they become hydrophobic

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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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:

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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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

Reaction Mechanism

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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

 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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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

Free Radical Reaction

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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

Natural fibre Case StudiesJUTE, HEMP, FLAX, SISAL & BAMBOO fibre Composite Materials

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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 CrystallinityComposite 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 ⋅16% and 9⋅5%, respectively ⋅16% and 9⋅5%, respectively

 8 h treated fibre exhibited maximum strength properties, the composite

prepared with them showed lower strength value

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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JUTE FIBRE COMPOSITES

3 Sodium Chlorite Treatment

MLR = 1:50; pH = 4; Temp = 98°C; Time = 2 hours

Results: 75% lignin removal achieved; colour change to slivery white

Composite material : 60%(by wt.) treated fibres in Low viscosity Unsaturated Polyester resin

Results:

Increased Flexural modulus, Shear modulus & Toughness but slight decrease in the Tensile modulus High Flexural modulus of about 18.84 GPa

SEM Images show swollen jute fibres:

(left: untreated; right: treated)

(mag.: -500)

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Ref: POLYMER COMPOSITES,

FEBRUARY 1999, Vol 20, No 1

HEMP FIBRE COMPOSITES

1 Bleaching with Sodium Chlorite:

Composite material:0 to 30% fibre loading; 1-pentene/ polypropylene copolymer matrix

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Ref: Journal of Reinforced Plastics and Composites, 2008, Vol 27,

pp.1533-HEMP FIBRE COMPOSITES

Applications of hemp

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)

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

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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Ref: Journal of Composites Science & Technology; Vol 71 (6); April

Applications of flax

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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

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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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 createorderly arrangement of polyacrylonitrile units

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