unit 3 study material

8/7/2019 Unit 3 Study Material

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COMPOSITE MATERIALS

Compiled by

Dr. R. Vijayaraghavan

Dr. D. Thirumalai

Dr. S. Sasikumar


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COMPOSITE MATERIALS

Composite material is a material composed of two or more distinct phases (matrix phase and

dispersed phase (filler)) and having bulk properties significantly different form those of any of the

constituents.

Matrix phase

The primary phase, having a continuous character, is called matrix. Matrix is usually more ductile

and less hard phase.

The functions and requirements of the matrix are to:

1.

Keep the fibers in place in the structure;

2.

Help to distribute or transfer loads;

3. Protect the filaments, both in the structure and before and during fabrication;4.

Control the electrical and chemical properties of the composite;

5.

Carry interlaminar shear.

Dispersed (reinforcing) phase

The second phase (or phases) is embedded in the matrix in a discontinuous form. This secondary

phase is called dispersed phase. Dispersed phase is usually stronger than the matrix, therefore it is

sometimes called reinforcing phase.

The needs or desired properties of the matrix that depend on the purpose of the structure

are:

1.

Minimize moisture absorption and have low shrinkage;

2.

Low coefficient of thermal expansion;

3.

Must flow to penetrate the fiber bundles completely and eliminate voids during the

compacting/curing process; have reasonable strength, modulus and elongation (elongation

should be greater than fiber);

4. Must be elastic to transfer load to fibers;5.

Have strength at elevated temperature (depending on application);

6.

Have low temperature capability (depending on application);

7.

Have excellent chemical resistance (depending on application);

8.

Be easily processable into the final composite shape;

9.

Have dimensional stability (maintain its shape).


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Factors considered for Selection of Matrix

In selecting matrix material, following factors may be taken into consideration:

1.

The matrix must have a mechanical strength commensurate with that of the reinforcement i.e.

both should be compatible. Thus, if a high strength fibre is used as the reinforcement, there is

no point using a low strength matrix, which will not transmit stresses efficiently to the

reinforcement.

2.

The matrix must stand up to the service conditions, viz., temperature, humidity, exposure to

ultra-violet environment, exposure to chemic3l atmosphere, abrasion by dust particles, etc.

3.

The matrix must be easy to use in the selected fabrication process and life expectancy.

4.

The resultant composite should be cost effective.

The fibres are saturated with a liquid resin before it cures to a solid. The solid resin is then said to

be the matrix for the fibres.

Classification of Composites

Composites is classified according to the nature of the matrix. Thus, composites can be classified

as metal, ceramic or polymer based. Metal matrices of iron, Nickel, Tungsten, Titanium,

Aluminium and Magnesium are used for high temperature usage in oxidizing environment.

Ceramic matrices are often used with carbon, metal and glass fibers, and are used in rocket engine

parts and protective shields. Glass matrices are mostly reinforced with carbon and metal oxidefibers. Heat resistant parts of engine, exhausts and electrical components are their primary

applications.


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Composites can be classified with respect to different parameters. The important ones are

described below. The composite material can be classified broadly by their constituent

components. There are mainly three categories of composites:

(a) Natural composite materials: These include wood, bone, bamboo, muscle and other tissues.

(b) Micro composite materials: These comprise of metallic alloys, toughened thermoplastics,

Sheet molded compounds and reinforced thermoplastics.

(c) Macro composite (Engineering materials): These include galvanized steel, reinforced concrete

beams, helicopter blades etc.

The polymeric composites are mainly micro-composites. They are further classified according to

the reinforcement forms into particulate reinforced, fiber reinforced and laminar composites.

(i) Particulate reinforced composites: these include materials reinforced by spheres, rods, beads,

flakes and many other shapes of roughly equal axes. The examples of polymeric materials

incorporating fillers such as glass spheres or finely divided powders, polymers with rubber

particles etc.

(ii) Fiber reinforced composites: These contain reinforcements having much greater strength than

their cross-sectional dimensions. e.g.: glass fiber reinforced plastics, carbon fibers in epoxy resinsetc.

(iii) Laminar composites: These are composed of two or more layers held together by the matrix

binder. These have two of their dimensions much larger than the third. e.g.-wooden laminates,

glasses, plastics etc.

Another classification of particulate composites is based on the particle size of the dispersed

phase. More recently, with advances in synthetic techniques and the ability to readily characterizematerials on an atomic scale has lead to interest in nano-meter size materials. Since nanometre -

size grains, fibers and plates have dramatically increased surface area compared to their

conventional-size materials, the chemistry of these nanosized materials is altered compared to

conventional materials. This can be micro composite, nanocomposites and molecular composites.


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ADVANTAGES AND DISADVANTAGE OF COMPOSITES

Advantages

Summary of the advantages exhibited by composite materials, which are of significant use in

aerospace industry are as follows:

1. High resistance to fatigue and corrosion degradation.2.

High strength or stiffness to weight ratio. As enumerated above, weight savings are

significant ranging from 25-45% of the weight of conventional metallic designs.

3.

Directional tailoring capabilities to meet the design requirements. The fibre pattern can be

laid in a manner that will tailor the structure to efficiently sustain the applied loads.

4.

Composites offer improved torsional stiffness. This implies high whirling speeds, reduced

number of intermediate bearings and supporting structural elements. The overall part count

and manufacturing & assembly costs are thus reduced.

5. High resistance to impact damage.6.

Composites are dimensionally stable i.e. they have low thermal conductivity and low

coefficient of thermal expansion. Composite materials can be tailored to comply with a broad

range of thermal expansion design requirements and to minimise thermal stresses.

7.

The improved weatherability of composites in a marine environment as well as their

corrosion resistance and durability reduce the down time for maintenance.

8.

Material is reduced because composite parts and structures are frequently built to shape rather

than machined to the required configuration, as is common with metals.

9. Excellent heat sink properties of composites, especially Carbon-Carbon, combined with theirlightweight have extended their use for aircraft brakes.

10. Improved friction and wear properties.

The above advantages translate not only into airplane, but also into common implements and

equipment such as a graphite racquet that has inherent damping, and causes less fatigue and

pain to the user.

Disadvantage of Composites

Some of the associated disadvantages of advanced composites are as follows:

1. High cost of raw materials and fabrication.2.

Transverse properties may be weak.

3.

Reuse and disposal may be difficult.

4.

Difficult to attach.

5.

Hot curing is necessary in many cases requiring special tooling.

6.

Hot or cold curing takes time and analysis is difficult.


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7.

Matrix is subject to environmental degradation.

However, proper design and material selection can circumvent many of the above disadvantages.

New technology has provided a variety of reinforcing fibres and matrices those can be combined

to form composites having a wide range of exceptional properties. Since the advanced composites

are capable of providing structural efficiency at lower weights as compared to equivalent metallic

structures, they have emerged as the primary materials for future use.

Morphology of Polymer Composites

Morphology is the study of shape, size, texture and phase distribution of physical

objects. Schematic illustration of clay and CNTs morphology in chitosan nanocomposites

is shown in following figure . In the composites based on chitosan/CNTs containing 0.4

wt % CNTs, nanotubes can be well dispersed in chitosan, but no filler network could be

formed due to its low concentration (Figure 1 a). In the composites based on

chitosan/clay containing 3 wt % clay, formation of 2D clay platelets network is possible

(Figure 1b). In chitosan/clay-CNTs ternary nanocomposites, ID CNTs are confined in 2D

clay platelets network, which results in a much jammed and conjugated 3D clay-CNTs

network (Figure 1c). The interactions and networks in the system can be divided into: (1)

clay-clay network, (2) clay-CNTs network, (3) CNTs-polymer-clay bridging, (4)

polymer-polymer network. The formation of different networks and interactions could bethe main reason for the observed synergistic reinforcement of CNT and clay on chitosan,

as they are in favor of the stress transfer of chitosan onto clay and CNTs.

Fig. 1 : Schematic illustration of morphology of clay and CNTs in chitosan nanocomposites: (a)

chitosan/0.4% CNTs; (b) chitosan/3% clay; (c) chitosan/3%clay/0.4% CNTs. The interaction and

networks in the system could include: (1) clay-clay network; (2) clay-CNTs network; (3) CNTs-

polymer-clay bridging; (4) polymer-polymer network.


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Examples

Ti-alloy metal matrix composite reinforced with diamond-coated Textron (SiC) fibres.

SEM view of C/SiC composite

SEM images of porous microspheres/ nanofibers composite film and water contact angle

of such film.

This shows a transmission electron microscopy (TEM) micrograph of a block copolymer-nanoparticle composite with an onion-like morphology.


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Properties of the composites

The numerous features of composite materials have led to the widespread adoption and use

through many different industries. It is because of these unique features of composites that people

benefit. Below are some of the most important features of composites, and the benefits they

provide

Lightweight

Composites are incredibly lightweight, especially in comparison to materials like concrete, metal,

and wood. Often a composite structure will weigh 1/4 that of a steel structure with the same

strength. That means, a car made from composites can weigh 1/4 that of a car made from steel.

This equates to serious fuel savings.

High Strength

Composite materials are extremely strong, especially per unit of weight. An example of this are

the high tenacity structural fibers used in composites such as aramid and S-Glass, which are

widely used in body armor. Due to high strength composites, soldiers are well protected from

blast and ballistic threats.

Corrosion and Chemical Resistance

Composites are highly resistant to chemicals and will never rust or corrode. This is why themarine industry was one of the first to adopt the use of composites. Boats made with fiberglass

can stay in the highly corrosive salt water without rusting.

Elastic

Fiber reinforced composites have excellent elastic properties. When one bends metal, it will yield

or dent. However, when composites are bent, they want to naturally snap back into place. This

feature is ideal for springs, and is why composites are used in car leaf springs and in the limbs of

archery bows.

Non-Conductive

Certain composites, such as composite made with fiberglass, are non-conductive. This is

important because often a structure is needed that is strong, yet will not conduct electricity. An


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example of this are ladders. Aluminum ladders can be an electrocution hazard, while ladders

made with fiberglass are not a risk if the ladder was to cross a power line.

Polymer matrix composites:

Resin systems such as epoxies and polyesters have limited use for the manufacture of

structures on their own, since their mechanical properties are not very high when compared to, for

example, most metals. However, they have desirable properties, most notably their ability to be

easily formed into complex shapes. Materials such as glass, aramid and boron have extremely

high tensile and compressive strength but in solid form these properties are not readily apparent.

This is due to the fact that when stressed, random surface flaws will cause each material to crack

and fail well below its theoretical breaking point. To overcome this problem, the material is

produced in fibre form, so that, although the same number of random flaws will occur, they will

be restricted to a small number of fibres with the remainder exhibiting the materials theoretical

strength. Therefore a bundle of fibres will reflect more accurately the optimum performance of

the material. However, fibres alone can only exhibit tensile properties along the fibres length, in

the same way as fibres in a rope.

It is when the resin systems are combined with reinforcing fibres such as glass, carbon and

aramid that exceptional properties can be obtained. The resin matrix spreads the load applied to

the composite between each of the individual fibres and also protects the fibres from damage

caused by abrasion and impact. High strengths and stiffnesses, ease of moulding complex shapes,

high environmental resistance all coupled with low densities, make the resultant composite


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superior to metals for many applications. Since PMCs combine a resin system and reinforcing

fibres, the properties of the resulting composite material will combine something of the properties

of the resin on its own with that of the fibres on their own, as surmised in Figure 1.

Fig.1: Combined effect on modulus of the addition of fibers to a resin matrix

Properties of polymer matrix composites:

The properties of the composite are determined by

a)

properties of the fiber

b) properties of the resin

c)

ratio of fibre to resin in the composite and

d) geometry and orientation of the fibers in the composite.

The higher the fiber volume fraction, the better will be the mechanical properties of the resultantcomposite. However, the fibers need to be fully coated in resin to be effective. The inclusion of fiber in the manufacturing process leads to imperfections and air inclusions.

E.g.. a) In boat- building industry fiber level will be 30 40 %.

b) In aerospace industry precise process are used to manufacture materials having

70% of fiber.

The geometry of the fibers in a composite is important since fibers have their highest mechanical

properties along their length than across width. This leads to the highly anisotropic properties of

composites. This is very advantageous since it is only necessary to put material where loads will

be applied and thus redundant material is avoided.The manufacturing processes, which are

employed have critical part to play in determining the performance of the resultant structure.


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Loading characteristics of polymer matrix composites :

There are 4 main direct loads that any material in a structure has to withstand. They are

tension, compression, shear and flexure.

a) Tension

Figure 2 shows a tensile load applied to a composite. The response of a composite

material to tensile loads is very dependent on the tensile stiffness and strength properties of the

reinforcement fibers and these are far higher than the resin system on its own.

Figure 2 Illustrates the tensile load applied to a composite body.

b) Compression

Figure 3 shows a composite under a compressive load. Here, the adhesive and stiffness properties

of the resin system are crucial, as it is the role of the resin to maintain the fibres as straight

columns and to prevent them from buckling.

Figure 3 - Illustrates the compression load applied to a composite body.

c) Shear

Figure 4 shows a composite experiencing a shear load. This load is trying to slide adjacent layers

of fibres over each other. Under shear loads the resin plays the major role, transferring the

stresses across the composite. For the composite to perform well under shear loads the resin

element must not only exhibit good mechanical properties but must also have high adhesion to

the reinforcement fibre. The interlaminar shear strength (ILSS) of a composite is often used to

indicate this property in a multiplayer composite (laminate).

Figure 4 - Illustrates the shear load applied to a composite body.


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d) Flexure

Flexural loads are really a combination of tensile, compression and shear loads. When loaded as

shown (Figure 5), the upper face is put into compression, the lower face into tension and the

central portion of the laminate experiences shear.

Figure 5 - Illustrates the loading due to flexure on a composite body.

Metal Matrix Composites :

Metal matrix composites, at present though generating a wide interest in research fraternity, are

not as widely in use as their plastic counterparts. High strength, fracture toughness and stiffness

are offered by metal matrices than those offered by their polymer counterparts. They can

withstand elevated temperature in corrosive environment than polymer composites. Most metals

and alloys could be used as matrices and they require reinforcement materials which need to be

stable over a range of temperature and non-reactive too. However the guiding aspect for the

choice depends essentially on the matrix material. Light metals form the matrix for temperatureapplication and the reinforcements in addition to the aforementioned reasons are characterized by

high moduli.

Most metals and alloys make good matrices. However, practically, the choices for low

temperature applications are not many. Only light metals are responsive, with their low density

proving an advantage. Titanium, Aluminium and magnesium are the popular matrix metals

currently in vogue, which are particularly useful for aircraft applications. If metallic matrix

materials have to offer high strength, they require high modulus reinforcements. The strength-to-

weight ratios of resulting composites can be higher than most alloys.

The melting point, physical and mechanical properties of the composite at various temperatures

determine the service temperature of composites. Most metals, ceramics and compounds can be

used with matrices of low melting point alloys. The choice of reinforcements becomes more

stunted with increase in the melting temperature of matrix materials.


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Metal Matrix Composites are composed of a metallic matrix (Al,Mg,Fe,Cu etc) and a

dispersed ceramic (oxide, carbides) or metallic phase( Pb,Mo,W etc). Ceramic

reinforcement may be silicon carbide, boron, alumina, silicon nitride, boron carbide,

boron nitride etc. whereas Metallic Reinforcement may be tungsten, beryllium etc .

MMCs are used for Space Shuttle, commercial airliners, electronic substrates, bicycles,

automobiles, golf clubs and a variety of other applications. From a material point of view,

when compared to polymer matrix composites, the advantages of MMCs lie in their

retention of strength and stiffness at elevated temperature, good abrasion and creep

resistance properties. Most MMCs are still in the development stage or the early stages of

production and are not so widely established as polymer matrix composites. The biggest

disadvantages of MMCs are their high costs of fabrication, which has placed limitationson their actual applications . There are also advantages in some of the physical attributes

of MMCs such as no significant moisture absorption properties, non-inflammability, low

electrical and thermal conductivities and resistance to most radiations. MMCs have

existed for the past 30 years and a wide range of MMCs have been studied.

Compared to monolithic metals, MMCs have:

1.

Higher strength-to-density ratios

2.

Higher stiffness-to-density ratios

3.

Better fatigue resistance

4.

Better elevated temperature properties

5.

Higher strength

6.

Lower creep rate

7.

Lower coefficients of thermal expansion

8.

Better wear resistance

The advantages of MMCs over polymer matrix composites are:

1. Higher temperature capability2.

Fire resistance

3.

Higher transverse stiffness and strength

4.

No moisture absorption

5.

Higher electrical and thermal conductivities


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6.

Better radiation resistance

7.

No out gassing

8.

Fabric ability of whisker and particulate-reinforced MMCs with conventional

metal working equipment.

Some of the disadvantages of MMCs compared to monolithic metals and polymer matrixcomposites are:

1.

Higher cost of some material systems

2.

Relatively immature technology

3.

Complex fabrication methods for fiber-reinforced systems (except for casting)

4.

Limited service experience

Numerous combinations of matrices and reinforcements have been tried since work on

MMC began in the late 1950s. However, MMC technology is still in the early stages of development, and other important systems undoubtedly will emerge. Numerous metals

have been used as matrices. The most important have been aluminum, titanium,

magnesium, and copper alloys and superalloys.

The most important MMC systems are:

1.

Aluminum matrix

2.

Continuous fibers: boron, silicon carbide, alumina, graphite

3.

Discontinuous fibers: alumina, alumina-silica

4.

Whiskers: silicon carbide

5.

Particulates: silicon carbide, boron carbide

6.

Magnesium matrix

7.

Continuous fibers: graphite, alumina

8.

Whiskers: silicon carbide

9.

Particulates: silicon carbide, boron carbide

10.

Titanium matrix

11. Continuous fibers: silicon carbide, coated boron

12. Particulates: titanium carbide

13. Copper matrix

14. Continuous fibers: graphite, silicon carbide

15. Wires: niobium-titanium, niobium-tin


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16. Particulates: silicon carbide, boron carbide, titanium carbide.

17. Superalloy matrices

Stir Casting Method Of Fabrication Of MMCs

Liquid state fabrication of Metal Matrix Composites involves incorporation of dispersed

phase into a molten matrix metal, followed by its Solidification. In order to provide high

level of mechanical properties of the composite, good interfacial bonding (wetting)

between the dispersed phase and the liquid matrix should be obtained. Wetting

improvement may be achieved by coating the dispersed phase particles (fibers). Proper

coating not only reduces interfacial energy, but also prevents chemical interaction

between the dispersed phase and the matrix. The simplest and the most cost effective

method of liquid state fabrication is Stir Casting.

Stir Casting

Stir Casting is a liquid state method of composite materials fabrication, in which a

dispersed phase (ceramic particles, short fibers) is mixed with a molten matrix metal by

means of mechanical stirring. The liquid composite material is then cast by conventional

casting methods and may also be processed by conventional Metal forming technologies.

Stir Casting is characterized by the following features:

1.

Content of dispersed phase is limited (usually not more than 30 vol. %).

2.

Distribution of dispersed phase throughout the matrix is not perfectly

homogeneous:

There are local clouds (clusters) of the dispersed particles (fibers);

There may be gravity segregation of the dispersed phase due to a

difference in the densities of the dispersed and matrix phase.

The technology is relatively simple and low cost.

Distribution of dispersed phase may be improved if the matrix is in semi-solid condition.

The method using stirring metal composite materials in semi-solid state is called

rheocasting. High viscosity of the semi-solid matrix material enables better mixing of the

dispersed phase.


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Ceramic Matrix Composites:

Ceramic-matrix composite development has continued to focus on achieving useful

structural and environmental properties at the highest operating temperatures. The high

risk associated with this task foreshadows the relatively small number of commercial

products. However, development of CMCs for other uses has also been pursued, and

significant commercial products now exist.

Ceramics can be described as solid materials which exhibit very strong ionic bonding in

general and in few cases covalent bonding. High melting points, good corrosion

resistance, stability at elevated temperatures and high compressive strength, render

ceramic-based matrix materials a favourite for applications requiring a structural materialthat doesnt give way at temperatures above 1500C. Naturally, ceramic matrices are the

obvious choice for high temperature applications.

High modulus of elasticity and low tensile strain, which most ceramics posses, have

combined to cause the failure of attempts to add reinforcements to obtain strength

improvement. This is because at the stress levels at which ceramics rupture, there is

insufficient elongation of the matrix which keeps composite from transferring an

effective quantum of load to the reinforcement and the composite may fail unless the

percentage of fiber volume is high enough. A material is reinforcement to utilize the

higher tensile strength of the fiber, to produce an increase in load bearing capacity of the

matrix. Addition of high-strength fiber to a weaker ceramic has not always been

successful and often the resultant composite has proved to be weaker.

The use of reinforcement with high modulus of elasticity may take care of the problem to

some extent and presents pre-stressing of the fiber in the ceramic matrix is being

increasingly resorted to as an option.

When ceramics have a higher thermal expansion coefficient than reinforcement materials,

the resultant composite is unlikely to have a superior level of strength. In that case, the


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composite will develop strength within ceramic at the time of cooling resulting in

microcracks extending from fiber to fiber within the matrix. Microcracking can result in a

composite with tensile strength lower than that of the matrix.

Ceramic-Matrix Composite Applications

Ceramic-matrix composites have successfully entered service as exhaust nozzle flaps and seals in

the F414 engine, now used in the Navy F-18 E/F (Fig. 9). The exhaust temperature of the F414 is

over 80 0C (145 0F) higher than for the F404 engine used in the previous version of the F-18. As a

result, the metal flaps and seals were failing in tens of hours. The CMC parts consist of a Nicalon

(Dow Corning Corp.) fiber with an inhibited carbon matrix. A thick SiC overcoat and glaze

provide protection from oxidation. There are 12 flaps and 12 seals per engine, and the seals are

attached to metal backing plates with metal rivets and a zirconia overcoat. The seals are subjected

to the highest temperatures, and the flaps must support the largest mechanical loads. Further, theflaps must survive a high thermal gradient, and the CMC is subjected to rubbing with the back

face of the seal. Insertion of the CMC flaps and seals has produced a weight savings of nearly 1

kg (2 lb) per engine relative to the metal parts. Because this mass is at the very back of the

aircraft, additional weight savings can be obtained by removing ballast to shift the center of

gravity of the aircraft. The CMC flaps have a useful life that is at least double the design

requirement of 500 hours.

Ceramic-matrix composites are now also commercially available as brake rotors for automobiles.Short carbon fibers and carbon powder are pressed and sintered into a porous green compact,

which is then easily machined to shape. This part is then reheated and infiltrated with molten

silicon, which reacts with the carbon to form SiC. The resulting disc is 50% lighter than

conventional discs, yielding a 20 kg (44 lb) weight saving in the Porsche 911 Turbo. Since the

rotor weight is unsprung, improved handling also results. The wear rate is half that of

conventional metal rotors, and a service life of 300,000 km (185,000 miles) is reported. The new

Porsche braking system uses anMMCbrake pad. Ceramic-matrix composite brake rotors have also

been demonstrated for the Inter-City Express high-speed trains in Germany, where a total weightsavings of 5.5 metric tons is obtained per trainset.

Polymer Nano Composites: Preparation, Properties And Applications

By a strict definition of nanocomposites, i.e., any filler submicron in size, there already are


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significant volumes of nanocomposites being produced. These amount to more than 20 million

pounds. However, since these fillers are on the upper end of nanocomposites size range, most

sources have excluded them from consideration. The field of material science has become quite

popular and pragmatic with a tremendous lust for composite materials that exhibit the positive

characteristics of both the components. World-wide, there has been a new desire to tailor the

structure and composition of materials on sizes of the nanometer. This resulted in the generation

of nanocomposites. Polymer nanocomposites are polymers that have been reinforced with small

quantities (less than10%) of nanosized filler particles. Nanocomposites have been found to

exemplify even more positive attributes than the predecessors do and thus we are trying to

understand what occurs when nanocomposites of a polymer and inorganic components are

produced.

Although particle filled polymer composites have been extensively studied because of their widespread applications in the automobile, household and electrical industries, recently

nanocomposites generate much interest among the various scientists principally, because of their

potential they offer for applications in high performance coatings, catalysis, electronics, magnetic

and biomedical materials. These nanocomposites are a new class of matrix filled with nanosize

fillers. This study is based on some of the advantages of the nanocomposites over the

conventional composites. Several advantages of these nanocomposites have been identified. They

include efficient reinforcement with minimal loss of ductility and impact strength, heat stability,

flame retardance, improved abrasion resistance, reduced shrinkage and residual stress and alteredelectronic and optical properties. The decrease in size of the domain to less than 100 nm enables

good optical transparency. e.g., ultrafine TiO2 produces pearlscent effects. High surface area in

comparison with small pore size can be used as catalysts for a wide variety of chemical reactions.

For example, porous silica by pyrolysis of polymer hybrid. In addition to this Lithium, Calcium

and Zinc salts can also be used to form homogeneous metal containing polymer hybrids for

interesting ion conductive properties. Thirdly, molecular aggregates and boundary structure

differs for nanosized particles as compared to conventional ones. The number of grain

boundaries, pore density and the boundary energies are high for nanocrystals and hence exhibitnovel electrical, magnetic and improved mechanical behavior. e.g.; ferric oxide and cadmium

sulphide.

Another example for nanocomposite in nature is the natural bone bone consists of approximately

30% matrix material and 70% nanosized mineral. Here the matrix material is collagen fibers


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(polymer) and the mineral is hydroxyapatite crystals 50nmx25nmx3nm (ceramic). The high

mechanical properties of bone are supposed to be due to the nanocomposite material.

In polymer nanocomposites research, the primary goal is to enhance the strength and toughness of

polymeric components using molecular or nanoscale fillers. Composites that exhibit a change in

composition and structure over a nanometre scale have shown remarkable property enhancements

relative to conventional composites. Most notable are increased modulus, increased gas barrier,

increased heat distortion temperature, resistance to small molecule permeation, improved ablative

resistance, increase in atomic oxygen resistance and retention of impact strength etc.

Interestingly, these performance improvements are achieved without increasing the density of the

base polymer, without degrading its optical qualities and without making it any less recyclable.

It is a remarkable fact that in addition to the profound changes in physical properties, whichmaterials display when they are nanometer in scale, the chemical behavior is profoundly altered

as well. When an inorganic solid is composed of only a few thousands of atoms, it has a great

deal of surface area. By binding an appropriate organic molecule to this inorganic surface, it is

possible to make nanocrystals behave chemically just like an organic macromolecule. Typically

an inorganic nanocrystal will be coated with a monolayer of surfactant, rendering the nanocrystals

hydrophobic. In this configuration the nanocrystals are soluble in non-polar solvents. If the

solvent is removed the nanocrystals aggregate but not fuse, since a layer of surfactant separates

them. These nanocrystals can be redissolved. Further the surfactant can be exchanged of withanother organic molecule, enabling the nanocrystals to be placed in almost any chemical

environment.

Classification

Nanocomposites are classified into thermoplastic and thermoset nanocomposites.

1. Thermoplastic nanocomposites: these materials are divided into two major categories, i.e.,commodity resins and engineering resins. Thermoplastics filled with nanometer-size materials

have different properties than thermoplastics filled with conventional materials. Some of the

properties of nanocomposites, such as increased tensile strength, may be achieved by using higher

conventional filler loading at the expense of increased weight and decreased gloss. Other

properties of nanocomposites such as clarity or improved barrier properties cannot be duplicated


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by filled resins at any loading.

Polymer nanocomposites were developed in the late 1980s in both commercial research

organizations and academic laboratories. The first company to commercialize these

nanocomposites was Toyota, which used nanocomposites parts in one of its popular car models

for several years. Most commercial interest has focussed on thermoplastics. Thermoplastics can

be broken into two groups: less expensive commodity resins and more expensive (and higher

performance) engineering resins. One of the goals of nanocomposites was to allow substitution of

more expensive engineering resins with a less expensive commodity resin nanocomposite.

Substituting a nanocomposite commodity resin with equivalent performance, as a more expensive

engineering resin should yield overall cost savings.

2. Thermoset nanocomposites: these have received less commercial interest in theirdevelopment than thermoplastic nanocomposites, but these materials may be relatively

straightforward to bring into production. Furthermore, thermoset nanocomposites can offer some

significant advantages over conventional thermosets. At this point of time, there has been much

less commercial interest in thermoset nanocomposites compared to thermoplastics. This neglect

may not continue much longer since thermoset nanocomposites have some distinct advantages

over neat thermoset resins.

Nanocomposites can also be classified based on the filler into three, viz., clay (silica) based,inorganic-polymer layered and inorganic-polymer hybrids. In the clay variety considerable work

was done in the recent years. The filler particles are the individual layers of a lamellar compound,

most typically clay. Since a single clay layer is only 10 A thick, it has a very large aspect ratio,

usually in the range of 200-2000. This makes it possible to use very small amounts (i.e., a few

weight percent) of clay to interrupt the structure of a polymer matrix on a nanometer length scale.

The resulting nanocomposites can exhibit dramatically altered physical properties relative to the

pristine polymer. The key to forming such novel materials is understanding and manipulating the

guest-host intercalation chemistry occurring between the polymer and the layered compounds.

Giannelis and co-workers did a lot of work on polymer layered silicate nanocomposites. The

static and dynamic properties of these systems are thoroughly investigated. Despite the

topological constraints imposed by the host lattice, mass transport of the polymer, when entering

the galleries defined by adjacent silicate layers, is quite rapid and the polymer chains exhibit


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mobilities similar to or faster than polymer self-diffusion. However, both the local and global

dynamics of the polymer in these nanoscopically-confined galleries are dramatically different

from those in the bulk. On a local scale, intercalated polymers exhibit simultaneously a fast and a

slow mode of relaxation for a wide range of temperatures, with a marked suppression of co-

operative dynamics typically associated with the glass transition. On a global scale, relaxation of

polymer chains either tethered to or in close proximity ((1nm as in intercalated hybrids) to the

host surface are also dramatically altered.

In the third category the focus is on the nanocomposites formed from inorganic fillers in polymer

matrix. These are materials in which nanoscopic inorganic particles, typically 10-100 angstrom in

atleast one dimension, are dispersed in an organic polymer matrix in order to improve

dramatically the performance properties of the polymer. In this process first we have to prepare

the nanosized particles of inorganic moiety and then to incorporate it in the matrix. One of theprimary objectives of the various synthesis techniques is to control the particle size either by

spatial conditions, such as size of pores and entities in the media, or by reaction kinetics.

Stabilising nanosize metal or semiconductor particles are critical. Several advantages have been

reported for the usage of polymer as the matrix.

Preparation of Nanocomposites.

A polymeric particle/ polymer nanocomposite contains a rigid polymer component dispersedwithin a flexible polymer matrix on a nanoscale level. The rigid polymer, with high modulus and

high strengths, usually has high melting temperature, is insoluble in organic solvents, and

combining it with the flexible polymer is thermodynamically unfavorable. Therefore it is very

difficult to prepare a nanocomposite, and phases may undergo segregation during processing and

end use. Hydrodynamic effects and physi- or chemisorption of matrix at filler surface governs the

reinforcement.

Nanocomposites are prepared mainly by three methods:

i) Sol- gel process, This includes two approaches: hydrolysis of the metal alkoxides and then

polycondensation of the hydrolyzed intermediates. This process provides a method for the

preparation of inorganic metal oxides under mild conditions starting from organic metal

alkoxides, halides, esters etc .


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ii) in-situ intercalative polymerization, which is a good method for the preparation of

polymer/clay mineral hybrids. A novel class of fillers is anisotropic layered silicates of the

montmorillonite type, which can be modified by cation exchange with organic ammonium salts,

thus producing organophilic clays, further called organoclays. Organophilic modification affords

compatibility between filler and polymer. Different methods have been introduced to achieve

matrix-filler compatibilization: melt or solution intercalation of organoclay with polymers, cation

exchange of montmorillonite with polymers bearing quartenary ammonium groups, or cation

exchange and subsequent polymerization with monomers containing quaternary ammonium

groups. These compatibilisation techniques account for improved interfacial adhesion and

effective dispersion of either intercalated silicate layer aggregates or even individual exfoliated

silicate layers. Such nanocomposites exhibit superior stiffness, impact, strength and heat

distortion temperature. In this method the mostly used clay is montmorillonite (MMT) because of the large surface area (about 750m2/g) and large aspect ratio (greater than 50), with a platelet

thickness of 10 A.

iii) In situ polymerization, which is a method where nanometer scale inorganic fillers or

reinforcements are dispersed in the monomer first; then this mixture is polymerized using a

technique similar to bulk polymerization.

Properties of nanocompositesNanocomposites offer much different properties than conventional composites. The most

important ones are enhanced mechanical strength, optical transparency, improved thermal

stability, improved barrier properties, improved flexibility, novel electrical properties etc.

Physical properties

According to the linear mixture equation the density of a composite, re, is a linear combination of

densities of the matrix and filler and their respective volume fractions. Petrovic et al (73) studied

the effect of filler concentration on density of polyurethane filled with microsilica and nanosilica,which is given in the Figure 13. It was found that the density of the samples increased with filler

concentration in both series, but more so in the series with microsilica. The increase in density is

attributed to the increase in volume of the polymer matrix on incorporation of nanoparticles. In

the same system swelling studies were done. The degree of swelling for two series is given in

figure 14. Since a lower degree of swelling indicates better curing, it is obvious that the sample


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with 50% nanosilica stands out as less cured. Also the glass transition temperature increased with

increased filler concentrations both in nanocomposites and microcomposites. It seems that

although there was a strong interaction between the matrix and the filler (which should have

increased Tg), an opposing effect came from incomplete curing of the matrix.

Mechanical properties

Novak reported that there exists an increase in hardness and scratch resistance with the addition

of nanoparticles to polymer matrix. But in polyurethane nanosilica composites first there is an

increase and then it decreases. From the tensile graph the nanosilica shows a 600% improvement

in elongation at break. The tensile strength of the composites with nanosilica and microsilica,

shown in indicated that up to a 20% filler concentration there was not muchdifference between

the nanosilica and microsilica effect. But experiments with nanosilica in PDMS Elastomers

showed that the elongation decreased and strength increased with increasing filler concentration.On the other hand, nanotitania filled PDMS networks showed partial increase in elongation at

break with increasing filler content that is no regular pattern can be said to be emerging.

Exfoliated polymer/silicate systems have been found to exhibit superior mechanical properties

than the conventionally filled systems. The mechanical properties of PVA/Na+ Montmorillonite

nanocomposites were studied for low silicate loadings, and Youngs modulus was found to

increase by 300% for 5-wt% silicate, with only a 20% decrease in toughness, and no sacrifice of

the stress at break compared to the case of neat PVA. In addition, for these low loadings, thermalstability from TGA measurements was shown to be slightly enhanced, and high optical purity was

retained.

Barrier properties

With the dispersion of the ultra thin inorganic layers throughout the polymer matrix, the barrier

properties of the nanocomposites are expected to enhance strongly compared to the respective

polymer. In PVA/Na+Montmorillonite nanocomposites the water vapor transmission rates weremeasured for the pure polymer and several of its low MMT nanocomposites. The permeabilities

decreased to about 40% of the pure water vapor transmission values for silicate loadings of only

4-6-wt%. This decrease is attributed to the increased path tortuosity of the penetrant molecules

and to the enhanced modulus of the polymer matrix .


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Thermal properties

The DSC studies in poly(vinyl alcohol)/Na+Montmorillonite nanocomposites established that

there is a suppression of the thermal transitions (Tg and Tm) for the purely intercalated systems.

Bulk PVA has a Tg at 70 0C and a melting transition at 225 0C. But for the fully intercalated

hybrids DSC does not detect any transitions between 35 and 250 0C . For these neatly intercalated

nanocomposites both Tg and Tm are too weak and/or too broad to measure, or they are

suppressed due to the polymer confinement. Although the physical origin of this behaviour is still

under debate, this absence of thermal events is in agreement with the general behavior of

polymers intercalated in clays and synthetic silicates.

Optical properties

Petrovic et al. studied the optical properties of polyurethane-nanosilica composites in detail. They

observed that at all filler concentrations the composites were transparent, while those of micro

silica were not. UV/VIS spectra of 1mm thick samples showed total absorption below 320 nm

and high transmission between 450 and 900nm in all samples with nanosilica.

Conjugated polymers show good optoelectronic properties. Poly(p-phenylenevinylene) (PPV) and

its derivatives are used for this purpose. For the improvement of optical and electronic properties

of PPV several attempts were made. A feasible way to improve the optical properties is tocombine PPV with inorganic nanoparticles. Incorporation of Cadmium Selinide nanoparticles

made PPV a blue light emitter and showed enhanced luminescence. Blends of TiO2 nanoparticles

with PPV got improved photovoltaic properties.

Rheological properties

The rheological properties of in-situ polymerized nanocomposites with end tethered polymer

chains were first described by Giannelis et al.. They found that the flow behavior of poly((e-

caprolactone) and polyamide-6 nanocomposites differed extremely from that of the neat matrices,whereas the thermorheological properties (Arrhenius activation energy of flow) of the composites

were entirely determined by that behavior of the matrix. The slope of the storage modulus G' and

the loss modulus G'' versus the frequency (in the terminal region was smaller than 2 and 1

respectively. Values of 1 and 2 are expected for melts of linear monodisperse polymers and the

large deviation, especially for small amounts of silicate loading in the percentage range may be


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due to network formation. However, such nanocomposites based on the in-situ polymerization

technique exhibit fairly broad molar mass distribution of the polymer matrix, which hides the

structure relevant information and impedes the interpretation of the results.

Flame retardancy

Research in the area of condensed flame retardants for polymers usually builds upon existing

technologies. They are metal hydroxides (alumina, magnesium hydroxide) or phosphorus based

materials. However, these materials tend to weaken mechanical properties while improving

flammability resistance. No new major flame retardant technology has emerged in this area for

quite some time. Polymer-clay nanocomposites have generated a great deal of interest primarily

due to improved mechanical and thermal properties. Also, they have improved flammability

resistance while maintaining good mechanical properties, a key advantage over existingcondensed phase flame-retardants. Morgan and co-workers did extensive work on this aspect.

They have shown that polymer-clay nanocomposites have greatly reduced heat release rates.

Also, they have observed polymers, which normally do not char, or leave any carbonaceous

residue upon burning, produce char in the presence of clay.

The most important difficulty in the development of clay/polymer nanocomposites with the

purpose of enhancing fire retardancy is that the most efficient structure for the enhancement of

fire retardancy may not result in the best mechanical properties. The enhancement of fireretardancy in layered silicate/polymer nanocomposites is achieved essentially via the formation of

torturous passways to inhibit the evolution of flammable volatile pyrolysis species. This may

become less effective when the silicate layers separating apart over a certain distance to cause the

collapse of the torturous passways.

Polymer-layered silicate (PLS) nanocomposites offer effective flame retardancy without creating

environmental problems in terms of combustion, recycling and disposal of the end products. This

is the most successful approach developed so far to produce environmental-friendly flameretarding polymers.

Dielectric properties

Dielectric spectroscopy (DEA) is a powerful tool in studying relaxation phenomenon in polymers


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and composites. It provides information about the location and activation energy of relaxation

transitions, the dipole moments of the subunits involved, concentration and mobility of charge

carriers and so on. Dielectric measurements in polyurethane-nanosilica composites showed that

both the nonfilled and the filled composites exhibit an overlapping transition consisting of two

subrelaxations, which become resolved at the highest frequencies only. Wei et al. studied the

effect of poling on the dielectric properties of the PT/PEK-C nanocomposite films. The difference

between the dielectric constants of the components is very large. The figure shows the

temperature dependence of dielectric constants of compacted sample of PT ultra fine particles and

PEK-C polymer. From the figure Tg of the nanocomposite thin film is about 2000C,because of

plasticization. Plasticization should be considered inorder to determine the poling temperature of

the nanocomposite thin films. Besides the viscosity of polymer descends, and the alignment of Pt

ultrafine particles is easy at high temperature. But the conductivity of PT/PEK-C composite thin

films increases fast with temperature, i.e., the thin films is broken down easily at hightemperature.

Applications of Nanocomposites

In the forgoing discussion, it has been observed that nanocomposites have set the current trend in

the novel materials drawing considerable interest due to the unusual properties displayed by them.

Several authors have adopted various techniques to prepare nanocomposites. However, the

techniques they utilized are very cumbersome which require careful control of various parameters

such as pH, moisture, temperature etc.

In recent years significant progress has been achieved in the synthesis of various types polymer-

nanocomposites and in the understanding of the basic principles, which determine their optical,

electronic and magnetic properties. As a result nanocomposite-based devices, such as light

emitting diodes, photodiodes, photovoltaic solar cells and gas sensors, have been developed, often

using chemically oriented synthetic methods such as soft lithography, lamination, spin-coating or

solution casting. Milestones on the way in the development of nanocomposite-based devices were

the discovery of the possibility of filling conductive polymer matrices, such as polyaniline,substituted poly(paraphenylenevinylenes) or poly(thiophenes), with semiconducting

nanoparticles: CdS, CdSe, CuS, ZnS, Fe3O4 or Fullerenes, and the opportunity to fill the polymer

matrix with nanoparticles of both n- and p- conductivity types, thus providing access to peculiar

morphologies, such as interpenetrating networks, p-n nanojunctins or fractal p-n interfaces, not

achievable by traditional microelectronics technology.


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The peculiarities in the conduction mechanism through a network of semiconductor nanoparticle

chains provide the basis for the manufacture of highly sensitive gas and vapor sensors. These

sensors combine the properties of the polymer matrix with those of the nanoparticles. It allows

the fabrication of sensor devices selective to some definite components in mixtures of gases or

vapors. Magnetic phenomena, such as superparamagnetism, observed in polymer-nanocomposites

containing Fe3O4 nanoparticles in some range of concentrations, particle sizes, shapes and

temperatures, provide a way to determine the limits to magnetic media storage density.

Over the last decades, the polymer nanocomposites application have gained their commercial

footing, due in large part to the efforts of resin manufacturers, compounding and master batch

producers who now offer user friendly products. Nanocomposites differ from traditional plastic

composites in that they provide these properties with minimal impact on articles weight and theydo so without providing penalties. Lastly in packaging nanocomposites deliver with good clarity,

a combination not possible using traditional composites approaches.

unit 3 study material

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