Chemical modifications of natural fibres for composite applications

Chemical Modifications of Natural Fibres for Composite Applications- KETKI SURESH CHAVAN

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

201003021052

Seminar Report Presentation on:

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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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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

Preferred orientation

Multi layer(angle ply)

Laminates Hybrids

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

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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. Ref:[http://www.researchandmarkets.com/

reports/1933235]

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The Reinforcement

Conventional Fibres: Carbon Glass Aramids Others include: Boron,

Alumina, Silicon Carbide, Quartz fibres.

Their Major Disadvantages: High Densities Non Renewable Non Recyclabable High Energy Consumption

during Manufacture Not CO2 Neutral Non biodegradable Expensive

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The Natural Fibres

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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 cellulose and lignin.

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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.

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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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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.

Process is time consuming, water polluting & the quality of fibres obtained is very much dependent on quality of water used.

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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 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

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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Chemical Compositions of Natural fibres

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Various Chemical Treatments Alkaline treatment Silane treatment Acetylation Benzoylation Acrylation & Acrylonitrile Graphting Coupling agents Isocyanate treatment Permanganate treatment Peroxide treatment Sodium Chlorite treatment

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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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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)-O-

C(=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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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:

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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.

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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.

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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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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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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.

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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 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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JUTE FIBRE COMPOSITES2. Comparison of Alkaline & Coupling agent Treatment:

Composite material: jute/polybutylene succinate (PBS) biocomposites with fibre content of 20%(by wt.)

32 Ref: Composites Part A, 2009, Vol. 40, pp. 669-674.

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JUTE FIBRE COMPOSITES3. 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)

Ref: POLYMER COMPOSITES, FEBRUARY 1999, Vol. 20, No. 1

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HEMP FIBRE COMPOSITES1. Bleaching with Sodium Chlorite:

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

Ref: Journal of Reinforced Plastics and Composites, 2008, Vol. 27, pp.1533-1544.

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HEMP FIBRE COMPOSITES2. Alkaline Treatment

Composite material: 40% (by volume) fibre loading; PLA matrix

Ref: Journal of Composite Materials, 2007, Vol. 41, pp.1655-1669.

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Applications of hemp

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FLAX FIBRE COMPOSITES1. 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.

Ref: The Canadian Society for Bioengineering, 2008, Paper no: 084364, pp.

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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.

40Ref: Journal of Composites Science & Technology; Vol 71 (6); April 2011;pages 893 - 899

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Applications of flax

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SISAL FIBRE COMPOSITES1. 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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SISAL FIBRE COMPOSITES3. 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 COMPOSITES5. 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.

Ref: J Polym Environ (2007) 15:25–33

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Applications of sisal

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BAMBOO FIBRE COMPOSITES5 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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BAMBOO FIBRE COMPOSITESResults: 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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BAMBOO FIBRE COMPOSITESMechanical properties of Bamboo/Epoxy composites

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BAMBOO FIBRE COMPOSITESMechanical properties of Bamboo/Polyester composites

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BAMBOO FIBRE COMPOSITESImpact strength & Water Absorption of Composites

Ref: Journal of Reinforced Plastics and Composites 30(1) 73–85;2011

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Applications of Bamboo

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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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