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PLASTICS AND NANOTECHNOLOGY

BY

ALUKO EBENEZER OLANREWAJU

A Physical Chemistry of Polymer Term Paper

Department of Chemistry,

Federal University of Technology, Akure, Nigeria.

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1.0 INTRODUCTION

Plastics are an integral part of our every-day-use in our lives. It is very difficult to realise how

important plastics have become to our everyday lives: in our clothing, the pen that we write

with, the chair that we sit on or the wrapping of the food that we eat. Plastics give us the

possibility of manufacturing well-designed, beautiful products from the very many different

types of plastics materials that are commonly available today. Within manufacturing

technology there is a very high degree of technological understanding of plastics and a range

of sophisticated technological processes that enable us to make them and shape them in

numerous ways. A recent of such is Nanotechnology.

Nanotechnology is an exciting technological advancement that has the potential to contribute

significantly to the future of plastic. Through nanotechnology, special nanocomposites can be

created that will be more dent, heat, and scratch resistant. Yet, the resins used to create the

plastic can be processed with the same equipment currently used to process resins.

Nanotechnologies can be perceived as the design, characterization, production, and

application of structures, devices, and systems by controlling shape and size of material

particles on nanometer scale, with one nanometer being equivalent to one-billionth of a

meter. As a point of reference, a sheet of paper is about 100,000 nanometers thick. With

nanotechnology, an object can be imaged, measured, modeled, and manipulated right down to

each nanometer. At this very tiny level, the chemical, physical, and biological properties of

materials are different from when in their bulk form. Some are better at conducting electricity

or heat, some are stronger, some have different magnetic properties while some reflect light

better or change colour as their size is changed. These new properties, therefore, can be used

in different ways such as to enhance the properties of plastics.

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2.0 PLASTICS

The word plastic itself comes from the Greek word plasticos, which means to be able to be

shaped or moulded by heat. As we will see, shaping plastics by using heat is a basic part of

nearly all plastics manufacturing processes.

Polymers or Plastics?

„Polymers‟ is a general term for all plastic materials and means that they are organic, carbon

based compounds whose molecules are linked together in long chain patterns. When we talk

about plastics in general we will call them polymers, and when we talk about specific plastic

materials we will give them their real names, such as nylon or polythene.

2.1 TYPES OF PLASTICS

Plastics can be divided into categories:

2.1.1 Natural plastics

These are naturally occurring materials that can be said to be plastics because they can be

shaped and moulded by heat. An example of this is amber, which is a form of fossilised pine

tree resin and is often used in jewellery manufacture.

2.1.2 Semi synthetic plastics

These are made from naturally occurring materials that have been modified or changed but

mixing other materials with them. An example of this is cellulose acetate, which is a reaction

of cellulose fibre and acetic acid and is used to make cinema film.

2.1.3 Synthetic plastics

These are materials that are derived from breaking down, or ‟cracking‟ carbon based

materials, usually crude oil, coal or gas, so that their molecular structure changes. This is

generally done in petrochemical refineries under heat and pressure, and is the first of the

manufacturing processes that is required to produce most of our present day, commonly

occurring plastics.

Synthetic and semi synthetic plastics can be further divided into two other categories. These

two categories are defined by the ways in which different plastics react when heated.

2.1.3.1 Thermoplastics

These are plastics that can be softened and formed using heat, and when cool, will take up the

shape that they have been formed into. But if heat is reapplied they will soften again.

Examples of thermoplastics are acrylic and styrene, probably the most common plastics

found in school workshops.

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2.1.3.2 Thermosetting plastics

These are plastics that soften when heated, and can be moulded when soft, and when cool

they will set into the moulded shape. But if heat is reapplied they will not soften again, they

are permanently in the shape that they have been moulded into. Examples of thermosetting

plastics are polyester resins used in glass reinforced plastics work, and melamine

formaldehyde used in the manufacture of Formica for kitchen work surfaces.

2.2 PLASTICS PRODUCTION

Most modern plastics are derived from natural materials such as oil, coal and natural gas with

crude oil remaining the most important raw material for their production. The starting point

for the production process is the distillation, in petrochemical refineries, of the raw material

into fractions (different parts). The heavy fractions give us lubrication oils and the heavy oils

used for heating fuels. The lighter fractions give us gas, petrol, paraffin and naphtha. The

chemical building blocks for making plastics come mainly from naphtha.

Fig. 2.1: Distillation of Crude Oil

The start of making plastics is to subject naphtha to a cracking process in which complex

organic chemical compounds are separated into smaller molecules, dependent on their

molecular weight. These smaller molecules include ethylene, propylene, butene and other

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hydrocarbons. The compounds produced through the cracking process are then further refined

to produce the base plastic materials.

Fig 2.2: Cracking of Naphtha

2.3 THE STRUCTURE OF POLYMERS

To understand how plastics are made, and why certain plastics are suitable for some uses, and

others not, you have to understand a little about the structure of polymers. Polymers are large

molecules made up of many smaller molecules. 'Poly' means many and 'mer' means units.

These smaller units are called monomers (mono = one, mer = unit) and are joined together

through polymerisation to form polymers. A polymer contains hundreds of thousands of

monomers.

Polymerisation, which means the linking of monomers to form polymers results from two

kinds of chemical reaction called condensation and addition. Polymers fall into two distinct

groups, thermosetting plastics and thermoplastics.

Thermosetting polymers are converted into their final form by heat and once set cannot be

softened by further heating. Thermoplastics however are softened and become fused or

'plastic' by moderate heating and then harden again on cooling. This process can be repeated

many times without radically altering the thermoplastic properties.

In a monomer atoms are joined by double bonds, and these must be broken and new bonds

created between adjacent atoms to form the long chain molecules of a polymer through

polymerisation.

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Fig. 2.3: Polymerisation

If the chains run parallel to each other the structure is said to be crystalline (made of crystals).

This contrasts with the disorder of tangled chains in an amorphous (shapeless) structure.

Many polymers have both crystalline and amorphous regions, and the proportion of

crystalline and amorphous regions in a polymer depends on its chemical composition,

molecular arrangement and how it has been processed. Crystallisation is one of the two

principles that have been used to produce strong, stiff polymers (e.g. polythene and nylon).

The other is the formation of strong bonds between the chains which is a process known as

cross linking. Polymers such as bakelite and urea formaldehyde have many strong cross links

which do not soften with increased heat, but set once and for all after their initial moulding.

This is how we get the „set‟ in thermosetting. These plastics remain comparatively strong

until over heating leads to a breakdown of the cross links and chemical decomposition. By

contrast, in thermoplastic polymers only weak forces hold the chains together and these

materials can be softened by heating and if necessary remoulded. On cooling they recover

their original properties and retain any new shape.

2.4 MODIFYING POLYMERS

Polymers are initially produced as powders, granules or laces (like straws). However they are

seldom used in their pure form but are changed by adding other materials to give them their

special properties. For example, they may need to be resistant to sunlight, or very flexible or

cheap. By using different types of additive the properties of the base polymer may be

modified in very many different ways so that the range of use of the base polymer is

extended. Some of the possible additives are:

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2.4.1 Blowing Agents

Many plastics products are cellular in structure. These expanded or foamed products can be

made from base polymers by the addition of what are known as 'blowing agents'. This is done

in the moulding process by adding filler, which foams throughout the hot polymer filling it

with bubbles. A good example of this is expanded polystyrene.

2.4.2 Colourants

Plastics can be coloured by using either dyes or pigments. Dyes give transparent colours and

pigments give opaque ones. A very wide range of colours can be obtained from clear to

opaque black with a naturally clear material such as acrylic.

2.4.3 Fillers

The term „filler‟ is a name given to a range of materials that are added to polymers in order to

modify their properties. They may also be added to a base polymer to lower the

manufacturing cost of a product made from it. A good example of this is one of the first

examples of the use of fillers, adding wood flour to phenol formaldehyde.

2.4.4Protective Agents

Many plastics are degraded by heat and light, especially from the ultra violet light in sunlight.

A special form of polyvinyl chloride is called UPVC, which is used in the manufacture of

drainpipes so that the pipes do not degrade in the sunshine.

2.4.5 Impact Modifiers

Some plastics are designed to have great impact strength; the ABS used in the manufacture of

car bumpers for example. Impact modifiers are polymers that when added to other polymers

improve the impact resistance of a product. Impact modifiers, based on acrylic polymers, are

used in conjunction with PVC in order to get clear rigid bottles with good impact strength.

2.4.6 Lubricants

These additives are widely used to help plastics flow in moulds. They are usually waxy

materials that act by reducing the stickiness of the plastic on the mould surface.

2.4.7 Plasticisers

Plasticisers, such as dioctylphthalate are added to polymers in order to make the resulting

materials more flexible. They make plastics more plastic. Plasticisers can change

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unplasticised polyvinyl chloride (UPVC) from a hard, rigid material suitable for drainpipes

into a soft, flexible material suitable for upholstery.

2.5 PLASTICS PROCESSING

Because of the properties of polymers it is possible to mould them and change their shape

using a number of different repetitious manufacturing processes. The most important of these

are extrusion, injection moulding, blow moulding, vacuum forming, extrusion blow

moulding, rotational moulding, calendaring, foaming and compression moulding.

2.5.1 Extrusion

Extrusion is a process that can be compared to squeezing toothpaste out of a tube.

Thermoplastic granules are forced through a heated barrel and the fused polymer is then

squeezed through a die that is the profile of the extruded component.

Fig. 2.4: Extruding a Pipe

The extrusion is cooled by water or air as it leaves the die and is finally cut to the required

length. The shape of the die can be varied from a simple hole with a centrally supported core

to produce tubes such as pipes, to very complex sections for curtain tracks or hollow window

frames.

2.5.2 Blow Moulding

Blow moulding is a simple process where compressed air is introduced underneath a warmed

sheet of thermoplastic material forcing the material into a mould cavity, or allowing it to

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expand freely into the shape of a hemisphere. It is a good way of forming large domes, which

when made out of clear acrylic sheet is often used in shop displays.

Fig 2.4: Blow Moulding

2.5.3 Vacuum Forming

This is a very common manufacturing process used, for example, to make a range of plastics

packaging. Think of the boxes sandwiches come in, or the inner in a chocolate box, or your

acrylic bath. It is really the opposite of blow moulding. Instead of the warmed plastic sheet

being forced into a mould by air pressure, in vacuum forming the air is drawn out from under

the softened plastic sheet, so it is forced over or into a mould by atmospheric pressure.

Vacuum forming is a very common and effective way of producing complex shapes in

thermoplastic sheeting.

Fig. 2.5: Vacuum Forming

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2.5.4 Extrusion Blow Moulding

This is a combination of extrusion and blow moulding and is often used where the article to

be made has a narrow neck, such as a bottle. The plastic material is first extruded as a tube

shape into an open die. The die is then closed to seal the ends of the tube and air is blown in

forcing the plastic tube to take up the shape of the die cavity. As the material is extruded first

and then blow moulded, the process is known as extrusion blow moulding.

Fig. 2.6: Extrusion Blow Moulding

2.5.5 Injection Moulding

This process is one of the most common of all plastics manufacturing processes. The

polymer, in granule form, is heated until fused and forced into a closed mould. Because of the

viscous (thick, syrupy) nature of the fused polymer, very high pressures are needed to make it

flow, which means that the machine and mould have to be very strong to withstand the forces

involved.

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Fig. 2.7: Injection Moulding

A typical industrial injection moulding machine uses a screw to force the granules along a

heated barrel, and when the granules become fused the screw is used as a plunger to force the

polymer into the mould. The moulds are usually made from high-grade steel to withstand the

forces involved and must also be highly polished to produce a very good finish on the

product, as any scratches will show up in the moulded plastic surface. Because of the ability

of the plastic to show even the smallest of marks very fine detail can be cut into the surface of

the mould, for example in the form of trademarks, lettering or textures.

2.5.6 Rotational Moulding

Rotational moulding is used to produce hollow thermoplastic products such as drums, storage

tanks and litterbins. A carefully calculated amount of plastic is placed in a closed mould that

is heated in an oven and rotated slowly around both a vertical and horizontal axes. The plastic

material fuses and sticks to the hot mould surface, building up the required thickness. The

mould is then gradually cooled by air or water while still rotating. The mould is opened, the

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finished product removed and the mould reloaded and closed for the next cycle. The time it

takes to make one of the products is known as the product‟s cycle time.

Fig 2.8: Rotational Moulding

2.5.7 Foaming

Plastic foams used in packaging and the upholstery industry can be produced by using

blowing agents that are mixed with the base polymer. When heated these agents release gas

which form bubbles in the plastic. Another method is to inject compressed nitrogen gas into

molten plastics during the moulding process. A third method is to freeze a gas within the

plastics granules which then expands due in the heat of the moulding process.

2.5.8 Calendering

Calendering is used to produce plastic sheeting and products such as floor tiles, coated fabrics

and coverings for car interiors. Fused thermoplastic is extruded on to heated rotating rollers

that squeeze the material into a continuous sheet or film. The film is cooled by jets of air or

water, before being cut to suitable lengths or loaded onto rolls.

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Fig. 2.9: Calendering

2.5.9 Compression Moulding

Compression moulding is one of the oldest manufacturing technologies associated with

plastics and was used in 1854, for example, by Samuel Peck to make picture frames from

shellac mixed with wood flour. The process is almost always used with thermosetting

plastics. A carefully weighed amount of thermosetting polymer is placed into a preheated

lower mould cavity. The mould is then closed by the placing of the upper half and subjected

to further heat, and pressure provided by a press, often of several hundred tons capacity. The

pressure and heat causes polymerisation and the flow of the plasticised material within the

mould.

Fig. 2.1.0: Compression Moulding

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Raw thermoplastic materials containing fillers may be compressed whilst cold into small

blocks of predetermined weight called preforms. Using preforms in compression moulds

saves having to weigh out powdered material each time the mould is filled and decreases the

cycle time, as the preforms may be preheated.

A development of compression moulding is transfer moulding. In this process the

thermosetting polymer is first loaded into a heating chamber above the mould cavity and

allowed to polymerise. It is then squeezed through channels into the mould cavity by the

action of a powerful press. Transfer moulding is used when complex mouldings are required

and the polymer needs to flow quickly around the mould cavity.

Many plastic articles have metal parts included within them during the moulding process.

These metal parts are called inserts, and may, for example, be in the form of captive nuts used

in conjunction with bolts to hold other parts of the final product assembly. The inserts are

placed in recesses in the lower mould either by hand or by using loading jigs before the

polymer is introduced into the mould. Compression and transfer moulding are manufacturing

techniques that lend themselves to the inclusion of moulded inserts. When inserts are used the

technique is often termed insert moulding.

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3.0 NANOTECHNOLOGY

The word nano in nanotechnology has origin in Greek word nanos meaning dwarf (small).

Prefix nano means one-billionth part, i.e. 10-9

, for example one nanometre is one-billionth

part of a meter. How large is one nanometre! Perhaps it can be imagined by the diameter of

single human hair which is approximately 100,000 times greater than one nanometre.

Nanoparticles are atomic assembly that exhibits outstandingly different behaviour than the

bulk of material. An example may be seen in ceramics which are known as brittle and rigid

materials.

Nanotechnologies can be perceived as the design, characterization, production, and

application of structures, devices, and systems by controlling shape and size of material

particles on nanometre scale. The origin of nanotechnology can be traced to a talk given by

Richard Feynman on 29th

December 1959 at the annual meeting of the American Physical

Society at the California Institute of Technology, U.S.A., on the topic of “There is Plenty of

Room at the Bottom”. Further, the term nanotechnology was introduced by Norio Taniguchi,

Tokyo University, almost 15 years after in 1974. The concept was expanded by Eric Dreklor,

MIT, USA in 1986 through his book Engines of Creation: “the Coming Era of

nanotechnology”. Subsequently, nanoscience then became the study of phenomena and

manipulation of materials at atomic (0.2nm approx.), molecular or macromolecular scales

(around 100nm).

The term nanotechnology describes a range of technologies performed on a nanometre scale

with widespread applications as an enabling technology in various industries.

Nanotechnology encompasses the production and application of physical, chemical, and

biological systems at scales ranging from individual atoms or molecules to around 100

nanometres, as well as the integration of the resulting nanostructures into larger systems.

What is different about materials on a nanoscale compared to the same materials in larger

form is that, because of their relatively larger surface-area-to-mass ratio, they can become

more chemically reactive and change their strength or other properties. Moreover, below 50

nm, the laws of classical physics give way to quantum effects, provoking different optical,

electrical and magnetic behaviours. A ceramic material can be made deformable when their

constituent grain size is reduced to low nanometer range. Also, a small amount of

nanoparticles of a substance when included in a polymer matrix having similar size range the

resulting system exhibits an exceptional performance level.

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There is no single branch of science and technology or industry that is not affected by

nanotechnology. The development and innovations in the next 10 years or so would even be

showing stronger influence of nanotechnology on most materials we are surrounded. To date,

it has accommodated the multiplication of applications in material manufacturing (more

importantly in polymer modification and synthesis), and computer/electronic chips, medical

diagnosis, health care, finishing formulations, energy, automations, biotechnology,

packaging, space, aircrafts, protection and security.

Nanoscale materials have been used for decades in applications ranging from window glass

and sunglasses to car bumpers and paints. Now, however, the convergence of scientific

disciplines (chemistry, biology, electronics, physics, engineering etc.) is leading to a

multiplication of applications in materials manufacturing, computer chips, medical diagnosis

and health care, energy, biotechnology, space exploration, security and so on. Hence,

nanotechnology is expected to have a significant impact on our economy and society within

the next 10 to 15 years, growing in importance over the longer term as further scientific and

technology breakthroughs are achieved. Sales of emerging nanotechnology products have

been estimated by private research to rise from less than 0.1 % of global manufacturing

output today to 15 % in 2014. These figures refer however to products” incorporating

nanotechnology” or” manufactured using nanotechnology”. In many cases nanotechnology

might only be a minor – but sometimes decisive - contribution to the final product.

3.1 PLASTICS AND NANOTECHNOLOGY

Nanotechnology is an exciting technological advancement that has the potential to contribute

significantly to the future of plastic. Through nanotechnology, special nanocomposites can be

created that will be more dent, heat, and scratch resistant. Yet, the thermoplastic resins used

to create the plastic can still be processed with the same equipment currently used to process

resins.

Through the use of nanotechnology, the plastics industry hopes to achieve several amazing

new accomplishments. For example, it may be possible to create auto body paints that are

completely scratch-resistant. Or, many materials and products currently in use can be reduced

further in size while improving efficiency. Memory chips the size of a postage stamp, yet

capable of holding the data equivalent to 25 DVD's, is also on the horizon through the use of

nanotechnology. Similarly, solar panels that can be manufactured at a much lower cost then

they currently are may be able to be produced with nanotechnology.

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By mimicking structures found in seashells, scientists have created a transparent plastic that

is as strong as steel. For years scientists have tried to build sturdy materials for larger

products from ultrastrong nano-size building blocks, such as nanotubes, nanosheets and

nanorods, only to have the larger structures turn out comparatively weak. It involves using a

specially designed machine to dip a piece of glass about the size of a stick of gum alternately

into a gluelike polymer solution and a dispersion of clay nanosheets. These materials form

cooperative hydrogen bonds with each other across the layers, which give rise to a "Velcro

effect". This effect, coupled with the arrangement of the nanosheets in a brick-and-mortar

structure, make the final product (as thick as a piece of plastic wrap) very strong.

Fig. 3.1: A Photomicrograph

Shown above is a photomicrograph which shows a cross-section of the layered plastic

material, built up from a gluelike polymer solution and a dispersion of clay nanosheets. The

arrows indicate the thickness of the material: about 1.5 microns, or less than a tenth the width

of a a human hair.

3.2 NANOCOMPOSITES

A composite is any material made of more than one component. Composite materials are

solid multiphase materials formed through the combination of materials with different

structural, physical and chemical properties. This makes composites different from the other

multi component systems such as blends and alloys. Composites are widely used in such

diverse as transportation, construction and consumer products (Giannelis, 1996).

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Composites offer unusual combinations of component materials properties such as weight,

strength, stiffness, permeability, electrical, biodegradability and optical properties that is

difficult to attain separately by individual components. A composite material can be custom

tailored to have specific properties that will meet special requirements. The optimum design

of composite component materials and manufacturing process to meet the target properties

for specific engineering application is very important.

In a composite, one phase is continuous and is called matrix, while the other is filler material

which make the dispread phase. Composite materials based on the nature of matrix phase can

be divided into polymeric, ceramic and metallic composites. Usually the filler phase is

embedded to the host matrix phase to make a composite which has properties far from either

phase alone. Polymers often have advantages over other materials such as metals and

ceramics. They are widely used in various technical applications because of their unique

advantages such as ease of production, light weight and ductility. However they have lower

mechanical, modulus and strength properties compared to that of metals and ceramics. The

commercial importance of polymers and their increasing use, results to the continuous

demand for improvement in their properties to meet the necessary conditions. By the

composite technology, polymer properties are improved while maintaining their light weight

and ductile nature (Jordan et al., 2005).

3.2.1 Polymer Composites and Nanocomposites

A polymer composite is made by the combination of a polymer and synthetic or natural

inorganic filler. Fillers are employed to improve the desired properties of the polymer or

simply reduce the cost. Nowadays, the application of polymer composites as engineering

materials has become state of the art. Polymer composites with improved mechanical,

thermal, barrier and fire retardancy properties are widely used in very large quantities in

variety of applications. However by the application of conventional fillers such as talc,

calcium carbonate, fibres, etc., it often requires to use a large amount of filler in the polymer

matrix to have significant improvements in the composite properties which may result to

some other undesired properties such as brittleness or loss of opacity (Zhang & Fridrich,

2003).

The final properties of reinforced polymers (composites) are influenced by the nature,

properties and content of components, dimensions of components and micro structure of

composite and interfacial interactions between matrix and dispersed phase. The efficiency of

properties improvements depends strongly on the properties (mechanical) of the filler, the

adhesion between matrix and filler and especially on the aspect ratio of the filler. The aspect

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ratio of the filler is very important and crucial for many properties in composite such as

electrical (Al-Saleh & Sundararaj, 2010; Grossiord et al., 2008), mechanical (Zhang et al.,

2007) and thermal (Meneghetti & Qutubuddin, 2006) properties.

Polymer composites with the high aspect ratio of nano-fillers such as platelet clays, carbon

nanotubes and nanofibers are receiving considerable attention due to their unique

multifunctional and highly enhanced properties. Combination of filler nanoscale dimension

and high aspect ratio with its nanoscale dispersion within polymer matrix leads to the

significant improvements in the polymer properties at very low filler volume fractions. As a

result of lower filler using, the macroscopic homogeneity and low density of primary polymer

retains as well as its opacity in the final nanocomposite system. Polymer nanocomposites are

the new class of hybrid materials in this category (Mittal, 2009).

A nanocomposite is defined as a composite material in which at least one dimensions of at

least one component is in the nanometer size scale (< 100 nm). Although the term

nanocomposite represent a new and exciting field in material science and technology, the

nanocomposites have actually been used for centuries by the nature. Using natural materials

and polymers such as carbohydrates, lipids and proteins, nature makes strong nanocomposites

such as bones, shells and wood (Hussain et al., 2006). However in recent years the

characterization and control of structures at the nanoscale have been studied, investigated and

exploited by the learning from the natural surroundings.

Consequently the nanocomposite technology has emerged as an efficient and powerful

strategy to upgrade the structural and functional properties of synthetic polymers. This is the

new nanocomposite science, so referred recently in nanotechnology, and was started by the

Toyota report (Usuki et al., 1993) on the superior improvement in the properties of nylon-6

by the preparation of exfoliated nylon-6/clay nanocomposites and has been continued by

more recent studies with carbon nanotubes, carbon nanofillers, exfoliated graphite,

nanocrystaline metals and fibers modified with inorganic fillers used in polymeric composites

(Paul & Robeson, 2008).

Polymer nanocomposites have attracted great attention worldwide academically and

industrially due to the exhibition of superior properties such as modulus, strength, toughness

and barrier far from those of conventional micro composites and comparable with those of

metals. However polymer nanocomposites have added advantage of lower density and ease

of processability. In polymer nanocomposites, the filler has at least one dimension in the

nanometer scale and its nanoscale dispersion within the polymer matrix leads to the

tremendous interfacial contacts between the polymer and inorganic filler which causes to the

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superior properties than those of bulk polymer phase. When the dimensions of filler particles

are decreased to the nanoscale, their properties change also significantly (Kumar et al., 2009).

This is well-known as nano-effect. In the other word the effect of nanofillers on the polymer

properties is different from that of predicted by using the thermodynamical studies for the

reduced particle size fillers (Gacitua et al., 2005).

Studies and modelings using continuum mechanics reveal that the enhanced properties of

nanocomposites are strongly dependent on the particular features of nanofiller system, in

particular, its content, aspect ratio and the ratio of filler mechanical properties to those of the

matrix (Sheng et al., 2004). The nanoscale is considered where the dimensions of filler

particles (diameter), platelets (thickness) or fibers (diameter) are in the size range of 1-100

nm.

A broad spectrum of polymer properties can be improved by nanocomposite technology such

as mechanical, thermal, barrier, durability, chemical stability, flame retardancy, scratch/wear

resistance, biodegradability as well as optical, magnetic and electrical properties (Armentano

et al., 2010; Cosoli et al., 2008; Ma et al., 2006; Pandey et al., 2005).

The final properties of nanocomposites are determined by the component properties,

composition, micro-structure and interfacial interactions. However it has been established

that the properties of nanocomposites are strongly influenced by the dimensions and micro

structure of filler phase (Sheng et al., 2004). In other word the filler nature has a main effect

on the final morphology and properties of the polymer nanocomposite. Clays are one group

of nano-fillers which have been widely used for the preparation of polymer nanocomposites.

Recently there has been a growing interest for the development of polymer/clay

nanocomposites due to their dramatic improved properties compared to the conventional

filled polymers in a very low fraction of filler addition (LeBaron et al., 1999; Thostenson et

al., 2005).

Polymer/clay nanocomposites have received intense attention and research interest driven by

the unique properties which can never be obtained by micro size fillers or especially by other

nanofillers. The value added properties enhanced without the sacrificing of pure polymer

processability, mechanical properties and light weight, make the clays more and more

important in modern polymer industry. Clay minerals are belonging to a main group of

silicates with layered structure known as layered silicates.

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3.2.2 Structure of Clays

Clays have layered structure. The layers are built from tetrahedral sheets in which a silicon

atom is surrounded by four oxygen atoms and octahedral sheets in which a metal like

aluminium or magnesium is surrounded by eight oxygen atoms. The tetrahedral (T) and

octahedral (O) sheets are fused together by sharing the oxygen atoms. Unshared oxygen

atoms are present in hydroxyl form. Two main arrangements of tetrahedral and octahedral

sheets fused together make the one layer structure of clay. One tetrahedral fused to one

octahedral (1:1) is known as kaolin group with the general composition of Al2Si2O5(OH)5

and the layer thickness of ~ 0.7 nm. The crystal lattice consisted of one octahedral sheet

sandwiched between two tetrahedral sheets (2:1) with the total thickness of 0.94 nm is well

known as phyllosilicates. The 2:1 phyllosilicate layers can be electrostatically neutral with no

inter layer ion present known as pyrophyllite. Due to not presence of inter layer ions, the

layers do not expand in water. When silicon in tetrahedral sheets is substituted by aluminium,

the 2:1 structure is called mica. The negative charge induced by this substitution is balanced

by the insertion of potassium cations between layers. Due to the equal size of potassium

cation and the hole created by Si/Al tetrahedral sheets, the presence of potassium cation

makes no inter layer spacing. Consequently the 2:1 layers are held together strongly and the

swelling or exfoliation of layers is not possible. When in neutral pyrophyllite, the aluminum

cations in the octahedral layers are partially substituted by divalent magnesium or iron

cations the smectite clay group is formed. The negative charge created by this substation is

balanced by inter layer counter sodium or calcium ions. The charge created on the layers is

not locally constant and varies from layer to layer. An average charge value is considered for

the layers which can be determined by cation exchange capacity (CEC) measurement.

Due to the unequal size of inter layer cations with the holes of tetrahedral sheets, the presence

of inter layer cations causes to an inter-layer spacing. The layers stay near together with a

regular gap between them. The gap is called as inter layer or gallery. The basal spacing of

clays can be measured or calculated from their X-ray diffraction patterns. The inter layer

dimension is also dependent to the clay nature and swelling or hydration degree of inter layer

cations. The inter layer distance is about 1 nm for montmorillonite. The electrostatic and van

der waals forces holding the layers together are relatively weak and inter layer distance varies

depending on the charge density on the layers, inter layer cation radius and its degree of

hydration. Because of the inter layer spacing and weak inter-layer forces, the cations present

between the layer can be hydrated in aqueous solutions well known as clay swelling. The

22

swelling causes to the more increasing of inter layer space. The charge density on the clay

layers is different in various clay groups with different clay structures (Mittal, 2009).

Due to the inter layer spacing and weak forces between layers, especially in the hydrated

form, other molecules also can be intercalate between layers, leads to the expanding of

layered lattice and finally may be caused to the separation of individual layers. The unique

intercalation/exfoliation behaviour of smectite clay minerals which is responsible to the high

aspect ratio of this clay type makes them very important and powerful as reinforcing filler for

polymers.

3.2.3 Preparation of Polymer/Clay Nanocomposites

Many efforts have been made for the preparation of intercalated and exfoliated polymer/clay

nanocomposites with improved properties. A variety of polymer characteristics including

polarity, molecular weight, hydrophobicity, reactive groups as well as clay characteristics

such as charge density and its modified structure and polarity are affective on the

intercalation of polymer chains within the clay galleries. Therefore different synthetic

approaches have been used for the preparation of polymer/clay nanocomposites.

In general there are four preparation methods including insitu template synthesis, solution

intercalation, insitu intercalative polymerization and melt intercalation.

3.2.3.1 Insitu Template Synthesis

In this method the clay layers are synthesized insitu in the presence of polymer chains. The

polymer and clay primary materials are dissolved in an aqueous solution. Typically

magnesium hydroxide, silica and lithium fluoride as clay building blocks are mixed with

polymer in a solvent. The gel or slurry is refluxed usually at high temperatures followed by

washing and drying. The nucleation and growth of clay layers are take place on the polymer

chains and the polymer chains are trapped in the clay inter layers. Although the clay layers

may be well dispersed within the polymer matrix without the modification of clay by onium

cations, however this method has serious disadvantages. The high temperature applied for the

synthesis of clay layers causes to the decomposition of polymers. Only hectorite clay is

synthesized at the lower temperatures. Also the synthesized clay crystals, generated by the

self-assembly process, have tendency to aggregate. Therefore this method is not widely used

and only a few nanocomposites using hectorite clay and poly (vinyl alcohol), polyaniline and

polyacrylonitrile have been synthesized by this method (Alexandre & Dubois, 2000; Lagaly,

1999; Zanetti et al., 2000).

23

3.2.3.2 Solution Intercalation

In this method the polymer or prepolymer is dissolved in a solvent and the clay is dispersed

in the same solution. The clay is swollen in the solvent and the polymer chains intercalate

between the layers. The intercalated nanocomposite is obtained by solvent removal through

vaporization or precipitation. Clays can be swollen easily in solvents such as water, acetone,

chloroform and toluene. The polymer chains are absorbed onto the delaminated sheets.

During the solvent evaporation the entropy gained by the exit of solvent molecules from the

interlayer spacing, allows the polymer chains to diffuse between the layers and sandwiching

depending on the interactions between polymers. This technique is used for the preparation of

epoxy/clay nanocomposites. However due to the need for use solvent, this technique cannot

be applied in industry. The nanocomposite preparation by emulsion polymerization, with the

clays dispersed in the aqueous solution, is also categorized as solvent intercalation method

(Rehab & Salahuddin, 2005). Toyota group has been used this technique to produce

polyimide/clay nanocomposites (Yano et al.,1993).

Fig. 3.2: Intercalation Process

24

3.2.3.4 Insitu Intercalative Polymerization

This technique was the first method used for the preparation of polymer/clay nanocomposite

by Toyota research group in the preparation of Nylon-6 nanocomposite from caprolactam

monomer (Usuki et al., 1993). In this method the organoclay is swollen in monomer liquid or

monomer solution. The monomers diffused into the inter layer spacing are polymerized by

the heat or radiation, by the diffusion of an initiator or by the organic initiator present on the

organic modifier of clay (Hussain et al., 2006). The polymerization is carried out within the

clay galleries as well as extra galleries. The growth of polymer chains results to the

exfoliation and formation of disordered structure.

This method is suitable for the preparation of thermoset/clay nanocomposites and has been

widely used for the epoxies and styrenic polymer nanocomposites (Lan et al., 1995). The

polarity of monomer and clay layers determines the diffusion rate and equilibrium

concentration of monomer within the clay galleries. Consequently the exfoliation and

dispersion of clay layers can be tailored by the clay and monomer chemistry (Pavlidou &

Papaspyrides, 2008). Polyaniline/MMT nanocomposites have been prepared by the insitu

polymerization of aniline in the presence of MMT (Olad & Rashidzadeh, 2008). Both

unmodified and organomodified MMT were used in the preparation of nanocomposites. Due

to the using of acidic aqueous solutions for the polymerization of aniline, the polar organic

anilinium cations can be interact with both unmodified and modified clays.

3.2.3.5 Melt intercalation

Clay is mixed within the polymer matrix in molten temperature. The conventional methods

such as extrusion and injection molding are used for dispersion of clay layers within the

polymer matrix. This method is effective technique for the preparation of thermoplastic

nanocomposites (Kornmann et al., 2001). The polymer chains are intercalated or exfoliated

into the galleries. Clays are organically modified and polymer chains are surface modified

with more polar functional groups to enhance their compatibility and therefore promote the

exfoliation. In melt intercalation method no solvent is required and it has many advantages

for the preparation of nanocomposites and is a popular method for industry (Ray et al., 2003).

3.2.4 Properties of Polymer/Clay Nanocomposites

The aim for the addition of clay minerals to the polymers is to improve the polymer

properties and to produce the polymer/clay nanocomposites with demand characteristics for

desired applications. Almost it is wanted to obtain a nanocomposite with demand properties

25

and characters or overcome the drawbacks of polymers while remaining the intrinsic

advantages of primary polymer matrix. Because of the low price, availability, high aspect

ratio as well as desirable nanostructure and interfacial interactions, clays can provide

dramatic and adjustable improved properties at very lower loadings which help to the more

remaining of polymer original useful properties. The nature and properties of components as

well as preparation methodology and conditions affects on the final properties of

polymer/clay nanocomposite (Ray, 2010). In this section the various improved properties of

polymer/clay nanocomposites as well as the mechanism and effective parameters are

discussed.

3.2.4.1 Mechanical Properties

The first and important goal in the application of fillers to the polymer is improvement in the

mechanical properties and therefore fillers are commonly called as reinforcement agents

(Jiang et al., 2005). The mechanism of the reinforcement is based on the higher resistance of

rigid filler materials against straining due to their higher module. When rigid filler is added to

the soft polymer matrix, it will carry the major portion of applied load to the polymer matrix

under stress conditions, if the interfacial interactions between filler and matrix are adequate

(Tortora et al., 2002; Gorrasi et al., 2003).

Therefore the larger the interface between filler and matrix, and also more strong interfacial

interactions, the great the reinforcement effect will be. Also the modulus increases by

increasing the filler ratio in the composite or nanocomposite composition. Because of the

rigid structure of clay layers and their high aspect ratio, they have proven to be very effective

in the increasing of modulus and stiffness of the polymer matrix in well dispersed condition.

In fact due to the very higher aspect ratio of nano-clay fillers compared to that of regular

fillers such as glass fibers, dramatic improvements in the mechanical properties of

nanocomposites are achieved even at very lower nanoclay additions (LeBaron et al., 1999).

In another approach, polymer chains, adhesived by strong physisorption forces onto the rigid

clay mono layers, become equally a portion of rigid material and dramatically exhibit high

modulus. Considering the very large interfacial area in well dispersed nanocomposite

structures, significant improvement in the modulus can be expected. However any

enhancement in the polymer-clay interfacial contact leads to the better stress transfer in the

nanocomposite. It has been well demonstrated that the enhancement in the interfacial

adhesion properties between clay and polymer by surface modification of polymer chains

using a suitable polar compatibilizer causes to the increasing in the mechanical properties of

26

nanocomposite (Osman et al., 2005; Mittal, 2008). It should be noticed that in higher loadings

of compatibilzers, they may cause to the some degree of plasticization due to their lower

molecular weight, which negatively affect on the modulus of nanocomposite (Mittal, 2008).

3.4.2 Thermal Properties

Thermal stability of polymer/clay nanocomposites have been analyzed and compared with

that of pure polymer under various oxidative (air) or non-oxidative (inert gases) conditions.

Thermogravimetric analysis (TGA) has been usually used to evaluate the thermal stability of

polymer and their nanocomposites. In this technique, the weight loss of the material due to

the formation of volatile compounds under degradation because of the heating and

temperature rising is monitored. In general, it has been reported that the polymer/clay

nanocomposites are thermally more stable than pure polymers (Ray & Bousima, 2005;

Becker et al., 2004; Zanetti et al., 2004; Wang et al., 2002). That is the incorporation of clay

fillers into the polymer matrices results to the improvement of their thermal stability. The

effect of clay layers has been more explained as superior insulation and mass transport barrier

against the volatile compounds generated during the decomposition of polymer under thermal

conditions (Zhu et al., 2001).

Also it has been reported that clays assist in the formation of layered carbonaceous char

during degradation of polymer/clay nanocomposite (Ray et al., 2003). Clay minerals are

inorganic materials and are almost stable in the temperature ranges that organic polymers are

degraded in to volatile compounds. Therefore in TGA experiments the clay content of

nanocomposites is remain as residue after heating program (Gilman, 1999).

Blumstein (1965) first reported the improved thermal stability of PMMA/MMT

nanocomposite. TGA showed that PMMA intercalated into the Na-MMT has 40-50 °C higher

decomposition temperature. Vyazovkin et al. (2004) reported the thermal stability of

polystyrene/clay nano-composite compared to pure PS. They have showed that PS/clay

nanocomposites have 30-40 °C higher degradation temperature compared to pure PS under

nitrogen and air heating degradation conditions. Other studies have been showed that the

nature of the clay modifier can affect on the thermal stability of modified clay and related

nanocomposites (Mittal, 2007).

In some other studies the thermal decomposition behaviour of nanocomposites and pure

polymers under air (oxidative decomposition) and inert gas (non-oxidative decomposition)

have been compared (Berta et al., 2006; Zhao et al., 2005; Beyer et al., 2002). Results

showed that the formation of insulator layered char in oxidative degradation is achieved

27

better than non-oxidative degradation heating program using some inert gases such as helium

(Zhao et al., 2005; Beyer et al., 2002).

3.4.3 Flame Retardancy

Because of the large use of polymers especially in domestic applications, there is need to

reduce their potential for ignition or burn in order to make them more safer in applications.

Conventionally chemical additives as flame retardants are used to retardant the ignition and

control burn.

3.4.4 Barrier Properties

One of the most considerable effects of clays in the polymer matrix properties is the dramatic

improvement of barrier properties of polymers. Clay sheets are naturally impermeable. Clays

increase the barriers properties of polymers by creating a maze or tortuous path that retards

the diffusion of gas molecules through the polymer matrix (Pavlidou & Papaspyrides, 2008;

Yano et al., 1993).

The degree of enhancement in the barrier properties depends on the degree of tortuosity

created by clay layers in the diffusion way of molecules trough the polymer film. The

tortuous factor is determined by the ratio of actual distance which diffusive molecule is

walked to the shortest distance to diffuse (polymer film thickness). This factor is affected by

the aspect ratio of clay dispersed in the matrix.

Increasing the lateral length of clay sheet as well as increasing of exfoliation or dispersion

degree cause to the more barrier enhancement in the polymer matrix. Many studies have

reported the barrier properties of polymer/clay nanocomposites against the diffusion of gases

and vapors (Tortora et al., 2002; Ray et al., 2003; Lange & Wyser, 2003; Giannelis, 1996;

Koh et al., 2008).

3.4.5 Anticorrosive Properties

Polymers are widely used as anticorrosive coatings on metals to prevent the corrosion.

Primarily polymeric coatings act as physical barrier against the diffusion of aggressive

species to the metal surface. However most polymer coatings show some degree of

permeations especially in long time period of contact with aggressive species. Various

techniques have been applied to improve the barrier effect of polymeric coatings.

One of the effective methods is based on the preparation of polymer based composite or

nanocomposite coatings by the addition of proper fillers to the polymer coating matrix. It has

been shown that the addition of platelet fillers like as layered silicates, effectively improves

the anticorrosive barrier effect of polymer coatings by increasing the length of the diffusion

28

pathways for aggressive species. Clay materials due to their platelet structure and high aspect

ratio, in well dispersed state, decrease the permeability of polymer coating films by

increasing the diffusion pathways.

The nanocomposite of various polymeric materials such as polystyrene, poly (styrene-co-

acrylonitrile), polyaniline, polypyrrole, polysulfone, polyacryltes, polyimide and epoxy with

unmodified and organically modified clays have been investigated as anticorrosive coatings

on metals (Yeh et al., 2008; Olad & Rashidzadeh, 2008).

29

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