Engineering MaterialsChapter 6

AdhesivesThe material or substance which is capable of

uniting or bonding two other materials

(adherends) together by adhering strongly to

the surface of both.

Example: Glue, gum and cement

Characteristics of an ideal adhesive

Should give a strong bond.

Should not be affected by air, heat or moisture.

Should resist chemicals, acids and bases.

Can be easily spread over the surface.

Quality of an adhesive is evaluated by

Persistence

Strength of bond setting upon drying

Quickness of bonding

Degree of stickiness

Adhesive ActionAdhesive action resulting in the bond between adherends by physical or chemical forces.

I. Specific Adhesion:

Physical or Chemical forces are responsible for the adhesion between the

surfaces.

Permanent bond is formed when the interfacial boundary energy of the

adhesive and adherent surfaces is lower than the sum of the surface

energies of the adhesive and the surfaces of adherends.

II. Mechanical Adhesion:

When the adhesive simply fills the voids or pores in the surfaces of the adherends and hold them by an interlocking action.

III. Fusion Adhesion:

When the adhesive or its solvent dissolves partly the surfaces and hold them together.

CLASSIFICATION OF ADHESIVES

AdhesivesBased on the composition of the principal components

Based on Mechanism of adhesion

Natural Synthetic

1. Solvent Responsive

2. Heat sealing

3. Pressure sensitive

4. Chemically reactive

Starch Glues

• Blood albumin• Casein

Epoxy Phenol

Formaldehyde Melamine

• Animal • Vegetable

Phenol formaldehyde

Classification of Adhesives Adhesives based on the composition of principal components

1. Adhesive based on naturally occurring materials

Ex. Vegetable gums, starches

Animal glues.

2. Adhesive based on synthetic materials

Ex. Phenol formaldehyde, epoxy,

Polyvinyl acetate(Fevicol)

Classification of Adhesives

Adhesives based on their mechanism of adhesion

1. Solvent Responsive Adhesive – Used in the form of solvent

Main component of these adhesives are:

Adhesive bases

Volatile liquid carriers

Plasticizer

Adhesion augmenting resins

Ex. Vinyl polymers, alkaloid resins,

natural resins and vegetable gums

2. Heat sealing adhesives

Inactive solids at room temperature.

They can work at high temperature.

Bonding strength depends on temperature, pressure and time.

Ex. Waxes, cellulose esters, polyvinyl resins, rubber, and its

derivatives.

3. Pressure sensitive adhesive

Used in the form of adhesive tape.

Able to provide instantaneous adhesion by applied small pressure and

similarly can be removed from the surface using a small energy.

Main Components:

a. Tape backing b. Adhesive c. Tackifying resins

d. Plasticisers e. Minerals f. Fillers

g. Antioxidants

.

4. Chemically reactive adhesivesThese type of adhesives undergo chemical changes

during the formation of the bond, chemical changes

like crosslinking, condensation or polymerization.

Ex: Phenol formaldehyde,

melamine formaldehyde, urea,

epoxy resin etc.

Nano-MaterialsNanoscienceNanotechnologiesNanomaterials

Important Definitions

Nanoscience Nanotechnology NanomaterialsStudy of phenomena and manipulation of materials at atomic,molecular and macromolecular scales, where properties differ significantly from those at larger scale.

It is design, characterization, production and application of structures, designs and systems by controlling shape and size at nanometer scale.

Materials with at least one dimension less than 100 nm.

E.g. thin film,nanowire, nanotube, colloids, quantum dots, etc.

Properties of Nanomaterial

Relatively Larger Surface areaMake material more chemically reactive.Also affect its strength and chemical

properties.

Quantum Effect Affects the optical, electrical andmagnetic behaviour of material.

Synthesis of Nanomaterials

Applications of Nanomaterial Sunscreens and cosmetics

Composites

Coatings and Surfaces

Tougher and Harder cutting tools

Fuel Cells

Displays

Medical Implants

Paints

Catalysts

Magnetic material

Military Battle Suits

Machinable Ceramics

FULLERENES

The first fullerene was discovered in 1985 by Sir Harold W. Kroto,

Richard E. Smalley and Robert F. Curl.

Fullerene, also called buckminsterfullerene.

A closed spherical cage shape – Buckyballs

A cylindrical shape - Carbon Nanotubes

A cage like molecules composed of 60 carbon atoms (C60) joined together

by single and double bonds to form a hollow sphere with 12 pentagonal

and 20 hexagonal faces.

Each C-atom on the cage surface is bonded to three other C-atoms. So,

each C-atom is sp2 hybridized.

Preparation of Fullerenes

Methods:Arc Vaporization of Graphite (Kratcshmer-Huffmann

apparatus) Laser Ablation MethodOther methods includes:

i. Hydrocarbon combustionii. Low pressure helium sputteringiii. Electron beam evaporationiv. Inductively coupled RF evaporation of graphite

Structure of Fullerene (C60) It has Truncated Icosahedron structure. Soccer ball shape An icosahedron is a polygon with 60 vertices and 32

faces (12 – pentagonal & 20 hexagonal) 90 covalent bonds between them, 60 single bonds and 30 double bonds.

C-atom is present at each vertex of this structure. Aromatic and has resonating structures. C60 is also known as bulky ball because it is a spherical

cluster of C-atoms arranged in series of 5- & 6-membered rings)

Properties of Fullerene Aromaticity

Has π-bonding electrons which are free to localize or delocalize in different chemical situations.

C60 in water tends to pick up two more electrons and become an anion.

Chemistry

The reactive characteristics of fullerenes is electrophilic at double bonding, which reduces angle stress by changing hybridized orbit carbons into sp3 hybridized one.

This decrease in hybridization permits the bonds to bend lesser on closing the tube making the fullerene molecule more stable.

SolubilitySoluble in Solvents

Common solvent used is Carbon disulfide

Solutions of pure fullerene have deep purple and violet colour.

Properties of Fullerene

Applications of Fullerene

Used as a lubricant

As a superconductor when mixed with alkali metals

As a soft Ferromagnets

Used in electronic and microelectronic devices

Non-linear optical devices

In Composites

In Drug delivery

Carbon Nanotubes(CNTs)

A nanotube is a nanometer-scale tube-like structure.

They are like nanowires, in terms of aspect ratio.

But unlike nanowires, nanotubes are hollow.

It’s diameter is in the order of few nanometers.

It’s length is in order of few milimeters.

Exhibit extraordinary strength and unique electrical properties and are efficient conductors.

Applications: Nanotechnology, electronics, optics and material science.

Structures of Nanotubes

Structure of single walled nanotube(SWNT)

A. Zig-Zag nanotubes B. Armchair nanotubes C. Chiral nanotubesApex of Hexagon Parallel to length Apex perpendicular to length All other orientations of Hexagons

Structures of Nanotubes

Structure of multi walled nanotube (MWNT)Russian Doll model

Sheet arranged in concentric cylinders.

Parchment ModelSingle sheet is rolled in around itself.(Like

rolled paper)

Distance between graphene layers is 3.3 Å

Properties of Nanotubes

Nanotubes have the highest strength to weight ratio of any known material.

Can easily penetrate membranes such as cell walls.

Can function like needle at cellular level.

Their electrical resistance changes significantly when other molecules attach themselves to carbon atoms.(used to develop sensors to detect chemical vapour)

Nanowires are 1D nanostructures which generally havediameters of the order of tens of nanometers, withunconstrained length scales! The length to diameter ratiomay be as much as 1000.

Nanorods are also 1D nanostructures where each of theirdimensions range from 1–100 nm. Standard aspect ratios(length divided by width) are 3-5.

Nanowires

Liquid Crystals The intermediate state exists between the crystalline solid state and the

liquid state. Condensed fluid phases with spontaneous anisotropy. First observed in Cholesterol myristate.

Molecules in this state are oblong and rigid rod-shaped. In the cell wall of living organism, molecules are in liquid crystal state.

Temperature State of substance<71 0C Solid crystalline71 0C Cloudy Liquid Crystal State86 0C Clear Liquid State

Liquid Crystals can defined as condensed fluid phases with Spontaneous anisotropy.

Liquid Crystals

Liquid Crystals

Classification of Liquid Crystals

Thermotropic Liquid Crystal – The organic compounds which

exhibit liquid crystalline phases as the temperature varies.

Example: p-Azoxyanisole.

Lyotropic Liquid Crystal – It consist of two or more components

that exhibit liquid crystalline properties in certain concentration

ranges.

Example: Soap

Molecular Arrangement in Different MesophaseNematic Phase: The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point in the same direction (along the director).

Smectic A PhaseMolecules in this phase show a positional order.The increased order means that the smectic state is more “solid-like”.

Molecules are arranged as in the smectic-A mesophase, but the director is at a constant tilt angle measured normally to the smectic plane.

Smectic C Phase

Cholesteric Phases:• Composed of nematic mesogenic molecules containing chiral

center which produces intermolecular forces.• This favour alignment between molecules at a slight angle to

one another.• Can be visualised as a slack of very thin 2D nematic-like

layers with the director.

Columnar Phases: • Shaped like disks instead of long rod.• It is characterized by stacked columns of molecules.• Columns are packed together to form a 2D-crystalline array.

Applications of Liquid CrystalsLiquid Crystal Displays(LCDs)Thermographic BehaviourOptical imaging and recordingUsed for non Destructive mechanical testing of

material under stress.Visualisation of Radio Frequency(RF) wavesErasable optical DisksLight modulator ,etc.

Organic Electronic Materials Called “Organic” because materials consist of polymers

and small molecules

Used in electronic and optoelectronic devices.

OEM Generally split into to groups:

1. Small polymers

Deposited by vapour method and have a well

defined molecular mass.

2. Polymers

Must be processed from solution and have molecule weight distribution.

These OEM has lead to the synthesis of new class of polymers known as Conducting polymers.

Organic Electronic Materials The most successful application of OEM is OLED(Organic

light emitting device).

This is generally used in long lived and highly efficient colour displays.

-Shivam Padmani (19K051)