Composite and nanocomposite materials

Constituents of composite o Matrix Continuous phase : Continuous or bulk material o Reinforcement Dispersed Phase : Added primarily to increase the strength and stiffness of matrix o

Composite and nanocomposite

materials

Constituents of composite

o Matrix (Continuous phase) : Continuous or bulk material

o Reinforcement (Dispersed Phase) : Added primarily to increase the strength and stiffness of matrix

o The reinforcement is generally can be in the form of fibres, particles, whiskers or flakes

The most common man made composites can be divided into three main groups based on the matrix

Matrix

Matrix materials

Role of matrix

• It binds the fibres together and acts as

medium

• It transfer the stress to the dispersed phase

• It protect the individual fibres from damage

Dispersed phase (reinforcement)

• Metallic, ceramic, organic materials

• Reinforcement form can be fibers, short fibers,

Whiskers, flakes, particulates

• Depending on the form, volume fraction of reinforcing phase the composite will have isotropic, quasi-

isotropic and anisotropic properties.

• Metals, metaloxides, C, B, Mo, W,, SiC, SiO2 Al2O3,

TiO2 , ZnO, steel, Talc, fibers of glass,

o The dispersed phase can be any material in the form of fibres, particles, whiskers or flakes

Arrangement of dispersed phase in composites

Composite material with at least two

constituent parts, one being a metal.

The other material may be a different metal or another material such as a ceramic or organic compound

o Carbide drills are often made from a tough cobalt matrix with hard tungsten carbide particles inside.

o Modern high-performance sport cars, such as those built by Porsche, use rotors made of carbon fiber within a silicon carbide matrix.

o Ford offers a Metal Matrix Composite (MMC) driveshaft

o The F-16 Fighting Falcon uses monofilament silicon carbide fibres in a titanium matrix for a structural component of the jet's landing gear

o MMCs are nearly always more expensive than the more conventional materials they are replacing.

o As a result, they are found where improved properties and performance can justify the added cost

o Today these applications are found most often in aircraft components, space systems and high-end or "boutique" sports equipment

o Ceramic Matrix composite (CMC)

o A given ceramic matrix can be reinforced with either discontinuous reinforcements, such as particles, whiskers

or chopped fibres, particulates having compositions of Si3N4, SiC, AlN, titanium diboride, boron carbide, and boron nitride or with continuous fibres

o The desirable characteristics of CMC include high-temperature stability, high thermal shock resistance, high hardness, high corrosion, resistance, light weight, nonmagnetic and nonconductive properties and versatility in providing unique engineering solutions.

Applications:

 CMCs find promising applications

in the area of cutting tools and in heat engines where the components should withstand aggressive

environments

 In Aircraft engines- use of stater

vanes formed of CMC in the hot section of the F136 turbofan engine

is under consideration

Polymer matrix Composites

o Polymers constitute the most important matrix materials and are used

in more than 95% of the composite products in use today

Polymer

Polymer matrix composites

 Thermoplastic polymer matrices-

- Thermoplastics are incorporated in the composite system by melting and solidifying by cooling.

- The physical reaction being reversible in nature.

- Thermoplastics have low creep resistance and low thermal stability compared to thermosetting resins

 Thermoset polymer matrices-

- Thermosetting resins are more common for the development of composite systems.

- Solidification from the liquid phase takes place by the action of an irreversible chemical linking reaction, generally in the presence of heat and pressure

cross- Elastomer based composites-

- The greater extensibility and high-energy storing capacity make them a suitable continuous phase for composite materials

- Unlike plastics, a wide variety of flexible products can be made using elastomers as the matrix phase

- They offer elastic strain higher than that of metals and can be stretched rapidly, even under small loads.

Polymer matrix composites

properties of easily forming into complex shapes

and compressive strength but on application of stress random surface flaws will cause the material to crack even below breaking point

the fiber form since these flaws can be reduced

exceptional properties are obtained

between each of the individual fibers and also protects the fibers from damage caused by abrasive

and high environmental resistance and low densities make these composites superior to even metals for many applications

Fig.: The combined effect on modulus of the addition

of fibers to resin matrix

Applications of polymer matrix composites

Properties of composites

o The properties of the composite are determined by

a) properties of the fiber b) properties of the binder c) ratio of fibre to resin in the composite and d) geometry and orientation of the fibers in the composite

o The higher the fiber volume fraction, the better will be the mechanical properties of the resultant composite

- 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

Properties of composites

o The geometry of the fibers in a composite is important since fibers have their highest mechanical properties along their length than across width

o This leads to the highly anisotropic properties of composites

o This is very advantageous since it is only necessary to put material where loads will be applied and thus redundant material is avoided.

o The manufacturing processes, which are employed have critical part

to play in determining the performance of the resultant structure.

Loading Characteristics of composites

material in a structure has to withstand

a) tension

d) flexure

o The inter-laminar shear strength of a composite is often used to indicate these properties in a multiplayer composite (laminate)

Fig : The shear load applied to a composite body

Flexure

o Flexural loads are a combination of tensile, compressive and shear loads

o In the figure shown,

- the upper face experiences compression,

- the lower face experiences tension and

- central portion of the laminate experiences shear.

Fig :The loading due to flexure on a composite body

Comparison with other structural materials

o The composite properties can vary by a factor of 10 with a) the range of fiber contents and

b) orientation of the fibre commonly achieved

o The lowest properties for each material are associated with simple manufacturing processes and material forms.

o The higher properties are associated with higher technology manufacturing like aerospace industry

Nanocomposites

nanoparticles and super-critical fluid forming technology may lead to a new class of materials that are light weight, high strength and multifunctional.

constructions and electronic industries because of their improved mechanical properties and physical properties over pure polymers

Types of polymer nanocomposites

o Polymer nanocomposites are divided into two general types:

a) Intercalated nanocomposites consisting of a regular penetration of the polymer in between the clay layers.

b) Delaminated/exfoliated nanocomposites where thick layers of the nanofillers are dispersed in the matrix forming a monolithic structure on the microscale

INTERCALATED EXFOLIATED

Types of nanocomposites

o Nanomaterials are usually divided as:

a) Platelet like nano structure (clay) b) Nanotubes & nanofibers (CNF) c) Spherical nanoparticles (ceramics, metals, block copolymers)

o All three types of nanoparticles have been used in Nanocomposite synthesis and processing, the following nanoparticles have attracted much attention:

a) plate-like clay nanoparticles b) carbon nanofibers and

c) carbon nanotubes

Polymer foams

• Polymer foams are widely used in insulations, cushions, as absorbents and weight bearing structures

• Foams of high porosity with interconnected pores have also been used as tissue engineering scaffolds for cell attachment and growth

• The different polymers used for foam applications are:

polyurethane, polystyrene, polypropylene etc.

• Depending on the composition, cell morphology and physical properties, polymer foams can be categorized as

a) rigid foams and b) flexible foams.

• Rigid foams are widely used in building insulating appliances, transportation, packaging, furniture, floatation and cushion.

• Flexible foams are used for bedding, textiles, gaskets, sports applications, shock and sound attenuation and shoes.

• Polymer foams can be classified as macro cellulose (> 100 μm), micro cellulose (1 - 100 μm), ultra micro cellulose (0.1 - 1 μm) and nano cellulose ( 0.1 – 100 nm)

Synthesis of nanocomposite foams

o The aim of synthesis of nanocomposites is to achieve controlled nanoparticle dispersion and distribution in a polymer matrix.

- The nanoparticle should have a large surface area and high aspect ratio to achieve improved mechanical properties, thermal stability.

- Separation and distribution of nanoparticles throughout polymer matrices should be ensured.

o Other methods to accomplish good dispersion of nanoparticles are:

a) ultrasonication, b) high shear mixing, c) surfactant addition and

d) functionalization

- Binding of surfactants into the surface of nanoparticles increases the hydrophilicity and compatibility through ion exchange reactions with the polymer matrices

- Large amount of solvent required and product cost is high

- The nanoparticles may re-agglomerate

o Inorganic layered silicates are able to exfoliate in water and form colloidal particles

o Several polymer nanocomposites, including polyethylene oxide, polyvinyl alcohol, polyacrylic acid are prepared using solution blending.

Synthesis of nanocomposites

b) Melt blending:

o Direct mixing of nanoparticles with a molten polymer

o Process eliminates the use of solvent

o Economically attractive route in fabricating polymer nanocomposites.

- Nylon 6, polystyrene and polypropylene composites are manufactured

by this method.

o This melt intercalation gives a simple way of preparing nanocomposites.

o Polar interactions of polymer and clay surface play a critical role in achieving particle dispersion

o For non polar polymers (polypropylene) a compatibilizer such as maleic anhydride modified (PP-MA) is commonly added to improve the compatibility

of polypropylene and clay.

o Polymers and carbon nanofibers, nanocomposites are also synthesized through this method

o Shear stress is needs to be controlled at an appropriate level to disintegrate and disperse nanoparticles

o Carbon nanotubes and nanofibers have also been synthesized via in situ polymerization 10 wt% of polystyrene was added into the mixture of styrene carbon nanofibers to achieve a higher initial viscosity and consequently a more stable fiber suspension

o Polystyrene, polyvinyl chloride and polyolefins are three primary thermoplastics used in polymer foams.

Synthesis of nanocomposites

During in situ polymerization,

o Reactive groups containing carbon-carbon double bonds were introduced

to the clay surface to increase the clay exfoliation

o A nanoclay was prepared by the ion exchange of a reactive cationic surfactant 2-methacryloxyethyl hexadecyldimethyl ammonium bromide (MHAB) with cations on the montmorillonite surface

o Closite is a clay containing non polar aliphatic chain with the anchored organic surfactant with polymerizable groups on MHAB provides an additional kinetic driving force for layer separation

o Complex exfoliation was reported for polystyrene nanocomposites synthesized with this reactive nanoclay at a clay concentration of 20 wt %

Synthesis of PS nanocomposites

o Polystyrene clay nanocomposites were synthesized in both intercalated and exfoliated structures.

o To prepare the nanocomposites, organo-nanoclay particles are pre-mixed with

PS and then mechanically blended in single or twin screw extruders

o The formation of nanocomposites depends on the penetration of polymer chains into the interlayer regions to separate the layers

o In situ polymerization has also been used to prepare PS nanocomposites

o By using reactive surfactants, the copolymerization of the interlayer surfactant and styrene monomer provides the driving force for delamination of clay crystallite.

Synthesis of PVC nanocomposites 1) By melt blending:

o Used to prepare exfoliated nanocomposites of PVC.

o Particles used include clay, calcium carbonate hydrosulphite, copper and antimony trioxide

o The polar nature of the C-Cl bond makes it possible to form exfoliated nanocomposites of PVC in melt blending

o A plasticizer like dioctylphthalate may serve as a co-intercalate to increase clay dispersion in PVC

Synthesis of PVC nanocomposites

o Highly exfoliated PVC clay nanocomposites can also be produced by flocculating a mixture

of polymer and clay mineral dispersion.

(or)

o By solution blending.

- Organoclay tends to induce the degradation of PVC because of its low thermal stability.

o To reduce the degradation of PVC one of the following approaches is used:

i) Co-intercalate dioctylphthalate into organoclay and then compound the mixture with PVC Dioctylphthalate covers the quaternary amine groups preventing a contact between amine and active chlorine atoms

(or)

ii) Intercalate or exfoliate nanoclay in a polymer such as epoxy or polycaprolactum which has good miscibility with PVC, by in situ polymerization to get a layer of epoxy or polycaprolactum which prevents the direct contact of organoclay with PVC in melt blending, inhibiting its degradation

Surface modification of dispersed phases

Surface modification of dispersed phase

• The dispersed phase (reinforcement) can be chemically functionalized for better dispersion in the matrix.

• Functionalization improves the compatibility of the dispersed phase with the matrix materials

• This also helps in size reduction and stabilization of particles in the

matrix by prevention of agglomeration

• Colloidal particles with similar surface charge are well dispersed