CHEMISTRY OF ENGINEERING MATERIALS

Dr.A.Geetha

Associate Professor & Head

Department of Chemistry

Kongu Engineering College

Perundurai-638 060

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UNIT - V

Lubricants - functions – requirements - classification with examples –properties: viscosity, viscosity index, flash and fire point, cloud and pour point, oiliness, aniline point and carbon residue – Explosives – requirements – classification – manufacture of important explosives (TNT, GTN and RDX) – Rocket propellants – properties and classification –

Lubricants

Lubricants – A substance, which when applied between two moving parts reduces friction by introducing a slippery film between the two surfaces.

Lubrication - process of reducing friction and wear between two moving surfaces by applying lubricant in between the moving parts.

Characteristics of Good Lubricant

• Should possess good thermal stability
• Should have high boiling point and low freezing point
• Should have high flash and fire point than the operating temp.
• Should have high oiliness, viscosity index and aniline point
• Should show high resistance towards oxidation and corrosion
• Should have low cloud and pour points than the operating temp.
• Should have higher aniline point
• Should have good detergent quality

Functions of Lubricants

Functions of Lubricants

• To keep moving parts apart - The thin layer of lubricant acts as physical barrier between the moving parts thereby reducing friction, surface fatigue and also reduces heat generation, operating noise and vibrations. This is termed as hydrodynamic lubrication.
• To reduce friction – reduced friction results in reduced heat generation and reduced formation of wear particles which in turn will results in improved efficiency. Additives known as friction modifiers are added to lubricants to reduce surface friction.
• To protect against wear - lubricants prevent wear by keeping the moving parts apart. Lubricants may have anti wear or extreme pressure additives to enhance their performance against wear and fatigue

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• To transfer heat – Gaseous lubricant transfers heat, liquid lubricants transfer heat as well as reduces thermal stress.
• To carry away contaminants and debris – lubricants have the capacity of carrying away internally generated debris and contaminants introduced into it.
• To prevent corrosion – lubricants having additives form chemical bonds with surfaces to prevent corrosion and rust.
• To seal gases- lubricants seal the space between moving parts through capillary force. This is known as seal pistons and shafts

Classification of Lubricants

classified based on their physical state as

1.Gaseous Lubricants

• Have lower viscosity and higher compressibility. Ex: air, steam

2.Liquid Lubricants

• Ex: Water, mineral oil-naphthenics, PAO, lanolin (also

acts as corrosion inhibitor), vegetable,oil-palm, castor,

sunflower oil & whale oil.

3.Solid Lubricants

• Used at high temp, because of their lamellar structure they are effective in the form dry powder
• Graphite- In an oxidative atmosphere, graphite is effective at high temp around 450°C. For good lubrication water vapour is necessary & its not effective in vacuum.

Refer text book

• Molybdenum disulphide (MoS₂): has hexagonal crystal str. It works better than graphite and effective in vacuum. It oxidizes beyond 400°C hence it can be used up to 400°C.
• Boron nitride (BN): The hexagonal form of BN (Ceramic powder) has lubricating property, can be used at high temperatures up to 1200°C and in oxidizing atm.
• Polytetrafluoroethylene (PTFE): Dispersions of PTFE in oil and water show lubricating action. Performs up to 260°C. Unlike other solid lubricants PTFE does not have layered structure.
• Mixture of lubricants like MoS₂, PTFE and anti-friction & anti-wear additives with polymers and sintered ceramic powders is called Self lubricating composites

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4. Semisolid Lubricants

• Contains thickening agents like soaps of Na,Ca,Li or Al mixed with liquid lubricant.

Ex. Greases- 80% mineral oil + 10% soaps + 10% additives (inhibitors, antioxidants, antiwear)

• Lithium based greases- water, dust and coal resistant. Used in the temp range 15-130°C
• Calcium based greases (cup greases)- water resistant, can

be used up to 70°C, at high temp mineral oil and soap

separates out.

• Aluminium based greases-have highest resistance to water and acids. Can be used up to 80°C.

• Sodium based greases-slightly soluble in water hence cannot be used in wet conditions. Can be Used up to 120°C.
• Non-soap greases-lubricating material dispersed in matrix of fine clay. These are also called as high performance greases due to their broad range of temp performance.

5. Metals/Alloys Lubricants

• Used in sliding surfaces and bearings. Ex. Pb, Sn, Zn alloys are used in sliding bearings.

Properties of Lubricants

1.Viscosity

• It is the property of any liquid which is a measure of its resistance to flow. If viscosity of oil is low, excessive wear takes place. If viscosity is too high, friction between layer increases.
• Viscosity of lubricating oil should be consistent over a wide range of temperatures.

2.Viscosity Index (VI)

• It is an arbitrary scale, used to determine the variation of viscosity of a fluid with temperature.
• Higher viscosity index - change of viscosity with temp is smaller.
• Low viscosity index - viscosity of lubricating oil changes rapidly.
• Good lubricant should have high VI.

3. Flash and Fire Point

• Flash and fire point - determine the volatility, fire resistance and uses of lubricant at high operating conditions.
• Flash point is important in determining shipping, storage and safety.
• Variation in flash and fire indicates the contamination of lubricant.
• Flash point – lowest temp at which the lubricating oil gives off enough vapours to ignite but not burn when small flame is brought near to it.

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• Fire point – lowest temp at which the vapours of oil burn continuously for at least 5s, when flame is brought near to it. Good lubricant should have flash point above the temp at which it is to be used. Fire points are 8-10% higher than the flash point.

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4.Cloud and Pour Points

• Cloud point – The temp at which lubricating oil becomes cloudy or hazy in appearance when it is slowly cooled.
• Pour point – the temp at which lubricating oil ceases to flow or pour when it is cooled slowly.
• Cloud and pour point indicate the suitability of lubricant in cold conditions as well as identifying the source of oil.
• Lubricant should possess low pour point to avoid solidification in low temp applications.

5. Oiliness

• Ability to stick on to the surface of machine parts, under pressure or load. Lubricants with good oiliness stay on between the lubricated surfaces.
• Poor oiliness of lubricant has a tendency to squeeze out when subjected to load or pressure.
• Mineral oil possess low oiliness, can be improved by adding additives like vegetable oil.

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6. Aniline Point

• Aniline point – indicates the possible deterioration of oil in contact with rubber used in packing.

6. Aniline Point

Higher aniline point means lubricant contains higher percentage of paraffinic hydrocarbons and a lower percentage of aromatic hydrocarbons. Aromatic hydrocarbons have a tendency to dissolve certain types of rubber hence their low level is desirable in lubricants.

7. Carbon Residue

• Lubricants are carbon rich compounds and on heating to high temp, these get deposited as carbon residue.
• Carbon residues deposits are harmful for IC engines and air-compressors.
• Good lubricant should have low-carbon residue.

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Explosives

• An explosive is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of  light, heat, sound and pressure.
• Explosive may be a pure single compound or mixture of compounds.

Requirements of good explosives

• Should be inexpensive and stable under normal condition.
• It must have at least one chemical bond that can be easily broken or a molecule having low energy of dissociation.
• The rate of decomposition should be fast to produce large volume of gaseous products exothermally
• Should have a positive oxygen balance.

Oxygen balance -

It indicates the oxygen contained in the molecule that is used to express the degree to which an explosive can be oxidized. The molecule is said to have a positive oxygen balance if it contains more oxygen than is needed to oxidize C and H to CO₂ and H₂O respectively then the combustion will be complete.

Classifications of Explosives

• Explosives are classified on the basis of their sensitivity, velocity & physical forms
• Primary explosives
• Relatively small amount of energy is required for initiation
• Explosive is extremely sensitive to impact, friction, heat, static electricity and electromagnetic radiation

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• They are used to initiate the explosion of large quantities of less sensitive explosives
• Ex. Lead azide, Mercury fulminate, Tetracene and Diazodinitro phenol.

b. Secondary explosives

• Less sensitive than primary explosive. Require substantially more energy to initiate.
• They are safe to handle and store. Ex. TNT, Picric acid and RDX.

c. Tertiary explosives (blasting agents)

• Insensitive to shock and cannot be reliably detonated by primary explosives, need secondary explosives for detonation.
• Used in mining, construction and terrorism. ex. Ammonium nitrate/ fuel

oil

b. High Explosives

• Materials that detonate with explosive velocity rates ranging from 3,000 to 9,000 m/s.
• High explosives used in military are pure substances whereas industrial explosives are mixture of pure explosive with non-explosive materials
• High explosives are further classified into

i. Single compound explosives- contain only one chemical compound.

• Ex. Ammonium nitrate- stable, non-toxic & cheap.
• Picric acid- used as a booster to detonate another less sensitive explosive like TNT

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iii. Plastic explosives- combination of explosives in plastic state and are hand-moulded or press-loaded Into various shapes. Ex.PETN (Pentaerythritol tetranitrate )

ii. Binary explosives- it is a mixture of TNT with other explosive. It is heated to liquid state and poured into the container then allowed to solidify. Ex. Pentotile- mixture of 50% TNT and 50% PETN.

Pentaerythritol tetranitrate

iv Dynamites- Contains nitroglycerine. It is an oily liquid, which detonates by pressure, shock, temp above 50°C. Different types of dynamites are

• Straight dynamites - contains 15% to 60% nitroglycerine in carbonaceous fuel with sodium nitrate, used for blasting hard rocks, coal and mineral demolitions
• Gelatin dynamites- contain nitroglycerine partly gelatinized by nitrocotton
• Gun cotton - obtained by soaking cotton for 30 min in a cooled mixture of H₂SO₄, HNO₃ then dried, explodes rapidly when exposed to fire.

Gelatin

Classification based on physical form

based on physical form explosives classified as

1. Pressings, b. Castings, c. Polymer bonded explosives, d. Rubberized,

e. Extrudable, f. Binary, g. Slurries & gels, h. Blasting agents

Preparation of 2,4,6-trinitrotoluene (TNT)

TNT is synthesized in a three-step process. First toluene is nitrated using mixture of acids to mono-

nitrotoluene. Then its re-nitrated to trinitrotoluene using anhydrous mixture of nitric acid and oleum.

Preparation of Glyceryl Trinitrate (GTN)

• It is prepared by mixing pure glycerol with 50:50

mixture of conc.H₂SO₄ & HNO₃

• It is used as a major component in many explosives.
• Dynamite is a mixture of 75% GTN and 25% Kielselguhr.
• Gelatin dynamite - 8% nitrocellulose & 92% GTN

Preparation of RDX

• It is a first nitramine explosive & as powerful as PETN & GTN
• Less sensitive & used in military
• Developed from hexahydro-1,3,5-trinitro-1,3,5-triazine

War department in Woolwich,UK named it as RDX

• Prepared by treating hexamine with NH₄NO₃ & fuming nitric acid.

Then the mixture is warmed & treated with cold water to get RDX.

Rocket Propellants

• It is a mixture of fuel and oxidizer (Fuel is a substance that burns when combined with oxygen.

an oxidizer is an agent that releases oxygen for combination with a fuel).

• Combustion takes place quickly in a controlled manner with huge emission of hot gases which exist through a small jet or nozzle at high velocities.
• Fuels used - H₂, hydrocarbon and hydrogen containing compounds like aniline, hydrazine etc..
• Oxidizer - liquid oxygen, ozone, ammonium perchlorate, etc..

Properties of a good propellant

• Should burn at a slow and steady rate.
• Should catch fire in the presence of an oxidizer in a short time.
• Should be safe to handle and store
• Should not leave any solid residue on burning, should produce products like H₂, CO₂, CO, N₂.
• Should be stable, non-corrosive and non-hydroscopic.

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Classifications propellants

Propellants are classified into solid, liquid, gaseous and hybrid

Solid rocket propellant

• Solid propellant also called as grain or stick. It’s a mixture of fuel and oxidizer.
• It is used in launching projectiles from guns, rockets and missile systems.
• Homogeneous solid propellants - mixture of propellants are thoroughly mixed in a colloidal state.
• Heterogeneous solid propellants – dispersion of oxidizing agent in a fuel mass. Ex. Gun powder
• Single-base propellant – single propellant is used. Ex. Nitrocellulose.
• Double-base propellant – solid propellant contains two materials. Ex. Ballislite has nitrocellulose

and nitroglycerine mixture.

Liquid rocket propellants

• More versatile and the engine using them can be checked and calibrated easily.
• Monopropellant - homogeneous system has fuel as well as oxidizer. Ex. H₂O₂, nitromethane etc.,
• Bipropellants - liquid fuel & oxidizer injected in the combustion chamber separately.

Gaseous rocket propellants

• In gaseous rocket propellants, the compressed gas is used in engine

Hybrid rocket propellants

• Mixture of solid and liquid fuel is used.

Few rocket propellants currently used are

• Liquid O₂ & liquid H₂- space shuttle’s main engine & in upper stage of GSLV
• Liquid O₂ & liquid CH₄- development of raptor (Space X) BE-4 engines.
• Liquid O₂ & ethanol – used in German world war II
• Inhibited red fuming nitric acid & hydrazine- Soviet Scud-C, aks SS-1.

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ABRASIVES

The word is derived from Latin Abrasio and it means - Scraping down

Definition

It may be defined as a hard material that can be used for Polishing, Smoothening, and Grinding the surface of other materials(objects).

• Usually, Abrasives are very hard substances ranging from naturally occurring Sand to the hardest material Diamond.
• Selection/choice of abrasive material is based on the object.

For Example

If the object to be polished is soft, then the abrasive material for the object is also soft.

Wood is an object, then the abrasive material is Sand paper

https://www.youtube.com/watch?v=oFWImpoctts

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Different Forms of Abrasives

There are three forms of abrasives

1. Grit (Loose, granular or powdered particles)
2. Bonded materials (Particles are bonded into wheels, segments or stick shapes)
3. Coated materials (Particles are bonded to paper, plastic, cloth or metal)

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• They are used in both powdered form(Grit) and block form (bonded /Coated materials).
• For using in powdered form, the material is

crushed and ground to different grades and sizes

and then this powder is used as such /and also

after bonding it on cloth or paper.

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Emery SiC

The block form (bonded/coated material) are pieces cut in suitable shapes from the abrasives material and they are used as grindstones, whetstones, millstone, and cutting edges.

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Examples:

Abrasives in such a form which are used in Dental applications are shown here.

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Properties of Abrasives

The following are the important properties of abrasive materials

1. Hardness
2. Toughness
3. Refractoriness
4. Chemical Reactivity
5. Thermal Conductivity
6. Brittleness

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1. Hardness
1. The hardness of an abrasive is the most important quality and is defined as the capacity of an abrasive material to grind another substance.
2. Harder the abrasive, better will be its capacity to Grind the surface of the other substance.
3. It is measured on Moh’s scale.

Hardness of the common abrasives in increasing order is given below in the Table.1

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Talc is the Softest and

Diamond is the hardest Abrasive material

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https://www.youtube.com/watch?v=vCWS9w9dk1U

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2. Toughness

An abrasive’s toughness is often measured and expressed as the degree of friability, the ability of an abrasive grit to withstand impact without cracking, spalling or shattering.

3.Refractoriness (Melting Temperature)

Instantaneous grinding temperatures may exceed 3500°C at the interface between an abrasive and the work piece being ground. Hence, melting temperature is an important property and it should be high for a good abrasive.

4. Chemical Reactivity

Any chemical interaction between abrasive grains and the material being abraded affects the abrasion process. A good abrasive must not react with the object being abraded.

5. Thermal Conductivity

Abrasive materials may transfer heat from the cutting point of abrasive to the work piece. The cooler the cutting point, the harder it is.

6. Brittleness

An abrasive material should be brittle so that it can function effectively.

The other important character of an abrasive is its ability to withstand high temperature.

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Classification of Abrasive

Abrasives are broadly classified into two types.

1. Natural

2. Artificial (or) Synthetic

Natural abrasives are further classified and are given below.

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1. Natural Abrasives
1. The Natural abrasives occur in natural deposits as minerals or rocks in the crust of the earth. It can be mined and processed for use with little alteration.

Examples: 1. Quartz

2. Garnet Siliceous Abrasives

3. Diamond

4. Emery Non-Siliceous Abrasives

5. Corundum

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2. Artificial (or) Synthetic

The Artificial or Synthetic abrasive group includes a number of materials possessing very high hardness.

Examples: 1. Silicon Carbide (SiC) or Carborundum

2. Boron Carbide (B₄C)

3. Aluminum Oxide (Al₂O₃)

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Natural Abrasives-Siliceous Abrasives

1. Quartz

• It is a crystalline silica.
• It has a hardness of 7 on Moh’s scale.
• It is used as scouring powders (Scrubbing) and soaps.
• Also it is used for grinding floor, pigments, ores etc.

2. Garnet

• The garnets are trisilicates of alumina, magnesia and ferrous oxide.
• Hardness of garnets ranges from 6 to 7.5 on Moh’s scale.
• These are too soft for grinding steel and iron, but �when glued to paper or cloth, they are used for finishing �hard woods.
• They are also used for bearing pivots in watches, glass �grinding and polishing metals.

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Natural Abrasives-Non-Siliceous Abrasives.

Most of the abrasives used in industries

are belong to a synthetic group, except diamond.

1. Diamond

• It is the most popular abrasive.
• It consists of crystallized carbon
• It is a natural abrasive and purest form of carbon (C). 
• It has a great hardness ( H=10 ).
• In the Gem variety, Diamond is perfectly colorless.
• Being the hardest substance, it has a very long life compared to other materials.
• It gets destroyed only by impacts.
• They are used for cutting, grinding and polishing surfaces in rock drillers and as a cutting saw.

https://www.youtube.com/watch?v=eMjzCgoIwjU

(Diamond impregnated wheel)

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2. Emery

• It is a mixture of corundum and black magnetite

(iron oxide, Fe₃O₄).

• It has high scratching or rubbing capacity and

is used on the tips of cutting, drilling tools etc.

• Artificial emery is used for buffing and polishing.

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3. Corundum (Al₂O₃)

• It is a crystallized aluminium oxide.
• It is the second hardest substance.
• Its hardness is 9 on Moh’s scale
• It finds use in grinding glasses, gems, lenses,

metals and cutting metal.

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Artificial / Synthetic Abrasives

1. Carborundum (SiC)
1. It belongs to a synthetic or artificial group.
2. In composition, it is silicon carbide.
3. It is brittle and its hardness ranges between

9 and 9.5 on Moh’s scale.

• It is the hardest abrasive material after diamond.
• It was first manufactured in 1890 in an attempt

to produce artificial diamonds.

Preparation

• It is manufactured by heating a mixture of high purity Si (Sand) and Carbon (coke) together with some salts and saw dust in an oven to a very high temperature (3000°C).

SiO₂ + C SiC + 2CO

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Common applications of carborundum includes in the making of

1. grinding wheels which are used to grind glass, granite, carbides, rubber, chilled iron, porcelain, bronze, brass, finishing leather etc.
2. abrasive papers.
3. abrasives cloth and tool sharpening blocks.

2. Boron Carbide(B₄C)

• It is also known as “Norbide”
• It is harder than carborundum (SiC).
• Its hardness varied from 9.5 to 9.7 on Moh’s scale.
• It is chemically inert.

Preparation

• It is prepared by heating coke with boron Oxide, B₂O₃ to 25000°C in an electric furnace.

2B₂O₃ + 7C B₄C + 6CO

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Uses

• It is used for cutting steel, tungsten carbide, and other hard materials.

https://www.youtube.com/watch?v=vCWS9w9dk1U

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Application of Abrasives

Abrasives find applications in domestic, medical and industrial fields.

1. For removing scales from the surfaces of metals.
2. Sharpening of cutting tools.
3. For polishing wood, metals and plastic surfaces.
4. Abrasive powders are used by dentists for cleaning teeth.
5. For making grinding wheels, abrasive papers and cloth.
6. For obtaining fine finish on metal surfaces (Cu,steel), hard abrasives (hardness>4) cannot be used. For this purpose , soft abrasives (hardness 1-4) are preferred. For Example, diamond and metal oxides such as Fe₂O₃ (rouge), Cr₂O₃ are soft abrasives used for metal finishing.

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ADHESIVES

Definition

“An adhesive is a substance which is used

for holding two surface together”. (or)

“An Adhesive is a substance which is used to joint

two or more parts so as to form a single unit”.

• The process to adhere on the surface is called the adhesion. According to Adhesive Sealant Council in America defined the adhesives as:

“A material used for bonding that exhibits flow at the time of application”.

Requirements for Adhesives

A good adhesive possesses the following qualities:

• It must “Wet” the surface.
• It must “Adhere” to the surface (i.e., the degree of stickiness should be high).
• Bonding should takes less time and “Develop Strength”.
• It Remains “Stable” (i.e., Its durability should be high).

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Classification of Adhesives

1. Natural and synthetic adhesives.
2. Organic and inorganic adhesives.
3. Structural and non structural adhesives.
4. Difference in chemical composition.
5. Difference in physical form.
6. Classification by application.

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1.Natural and synthetic adhesives.

Natural: If binding constituent of the adhesive is derived from natural source is called natural adhesives.

Eg., pitch, collagen, rubber etc.

Synthetic: Active part of synthetic is made by synthesis of chemical substances.

Eg., thermosetting plastic, thermoplastic materials and all structural adhesives other than cement are made synthetically.

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 2. Organic and inorganic adhesives.

Organic: If active agent of the adhesive is based on organic molecule it is called organic adhesive.

Eg., epoxies, acrylics, urethanes, phonemics, silicones etc.

Inorganic: Inorganic adhesives are consist of metallic (Al, Mg, Fe) and non metallic (Si, Ca) compounds.

Eg., Cement of different kind, clays, lime etc.

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3. Structural and non structural adhesives.

Structural: Non structural:

• Sustain load • Holding components

• Tolerate environment • Sealing against fluids

• Provide needed service life • Impact absorption

•Electrical, water, or

thermal insulations

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4. Based on difference in chemical composition.

Thermosetting

It can not be heated or softened once they cured.

Eg., acrylics, epoxies, polyester, polyimides, silicones, urea and melamine formaldehyde

etc. These are used where

• good shear strength from room temperature to 260ºC,
• good resistance to heat with little elastic or creep deformation under load above room temperature,
• good resistance to organic and inorganic solvents. Only fair peel strength compared to thermoplastics.

Thermoplastic

• It can be softened and formed again and again.

Eg. acrylic, polyvinyl alcohol, nylon, cellulose acetate, polyamide etc.

• Service limited to below 65-90ºC, poor creep strength, poor resistance to organic solvents.

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Elastomers: It is made of natural and synthetic materials got superior elongation and toughness properties.

Eg., natural rubber, neoprene, polyisobutylene, polysulfide, reclaimed rubber, silicone etc.

It has high flexibility and superior peel strength use temperature limited to 70 to 200 ºC.

Uses:

• Mostly used for non-structural applications such as vibration damping, impact absorption, sealing and accommodating mismatched thermal expansion coefficients.
• These are available as solvent solutions, dispersions, pastes (cements) and pressure sensitive tapes.

Adhesive alloys:

• These are combinations or alloys of resins from two or more chemical groups from thermosetting, thermoplastic, and elastomeric types.
• Adhesive alloys are excellent for joining dissimilar materials to one another, such as metals, ceramics, glasses, and thermosetting and thermoplastic polymers.

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Some common varieties of adhesive alloys include

1. Epoxy – phenolics (a thermosetting alloy),
2. Epoxy – polysulfone (a thermosetting alloy),
3. Epoxy – Nylon (a thermosetting – thermoplastic alloy),
4. Neoprene – Phenolic (an elastomeric – thermosetting alloy)
5. Vinyl – Phenolic (a thermoplastic – thermosetting alloy)

5. Classification on the basis of Physical form:

• Liquid adhesive.
• Paste adhesives.
• Dry powder adhesive.
• Tap and film adhesive.

6. Classification on the basis of Application:

• Spray-able.
• Brush-able.
• Trowel-able.
• Extrude-able. https://www.youtube.com/watch?v=0RGtizlasaw

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Advantages

• Application of adhesives is a very simple process.
• Bonding between surfaces occurs easily and quickly.
• Adhesives can be applied to the surfaces of any material, such as glass and metal, metal and metal, metal and plastic, plastic and plastic, and ceramic and ceramic.
• Adhesive joints are leak-proof for gases and liquids.
• High load carrying and has high strength to weight ratio.
• Suitable for thin and thick structures.
• Little or no change in chemistry.
• Absorb shock and vibration loads.
• Suitable for dissimilar materials.
• Reduce galvanic and crevice corrosion.
• Sealant, electrical and thermal insulators.
• High heat is not required for bonding.

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