Engineering Materials Chapter 6

Adhesive The 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 ACTION Adhesive action resulting in the bond between adherends by physical or chemical forces. Three ways: 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 Based on the composition of the principal components Based on Mechanism of adhesion Solvent Responsive Heat sealing Pressure sensitive Chemically reactive Epoxy Phenol Formaldehyde Malamine Natural Synthetic Starch Glues Vegetable Animal Casein Blood albumin

Classification of Adhesives Adhesives based on the composition of principal components Adhesive based on naturally occurring materials Ex. Vegetable gums, starches and animal glues. Adhesive based on synthetic materials Ex. Phenol formaldehyde, epoxy, polyvinyl acetate(Fevicol)

Classification of Adhesives Adhesives based on their mechanism of adhesion 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

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: Tape backing b. Adhesive c.Tackifying resins d. Plasticisers e. Minerals f. Fillers g. Antioxidants Examples: Rubbers, acrylates and silicones.

4. Chemically reactive adhesives These type of adhesives undergo chemical changes during the formation of the bond, chemical changes like crosslinking, condensation or polymerization. Ex: Phenol formaldehyde, malamine formaldehyde, urea, epoxy resin etc.

FULLERENES

The first fullerene was discovered in 1985 by Sir Harold W 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 in Kratcshmer-Huffmann apparatus. Laser Ablation Method Other methods include: Hydrocarbon combustion Low pressure helium sputtering Electron beam evaporation 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)

Applications of Fullerene Use as a lubricant. As a superconductor when mixed with alkali metals. As a soft Ferro magnets. Use in electronic and microelectronic devices. Non-linear optical devices. Composites. Drug delivery.

Nanomaterials(Nanowires and Nanorods) Nanotechnology involve designing and producing objects or structures at a very small scale, on the level of 100 nanometres (100 millionth of a millimetre) or less. Nanomaterials are one of the main products of nanotechnologies– as nano-scale particles, tubes, rods, or fibres. Nanoparticles are normally defined as being smaller that 100 nanometres in at least one dimension. Applications in healthcare, electronics, cosmetics, textiles, information technology and environmental protection. Self Study Topic

Liquid Crystals The intermediate state exists between the crystalline solid state and the liquid state. Condensed fluid phases with spontaneous anisotropy. This state was first observed by Friedrich Reinitzer (1888) in cholesterol myristate. Cholesterol myristate Below 71 0C , Solid crystalline At 71 0C, Cloudy Liquid Crystal State. At 86 0C, liquid State Molecules in this state are oblong and rigid rod-shaped. In the cell wall of living organism, molecules are in liquid crystal state.

Classification of Liquid Crystals Thermotropic liquid Crystal – The organic compounds which exhibit liquid crystalline phases as the temperature varies. For 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 Mesophases Nematic Phases: 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 Phase and Smectic C Phase Smectic phase –A: Molecules in this phase show a positional order. The increased order means that the smectic state is more "solid-like“. In the smectic-C mesophase, molecules are arranged as in the smectic-A mesophase, but the director is at a constant tilt angle measured normally to the smectic plane.

Applications of Liquid Crystals Liquid Crystal Displays: Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline molecules in the presence or absence of an electric field. Thermographic Behaviour: Cholesteric liquid crystals are used in thermography for measuring surface temperatures and to indicate temperature variation. This is because when the temperature of a cholesteric liquid crystal is changed over a particular range, they exhibit colour changes over the entire colour scale from red to violet. The change is reversible and the colours occur in the reverse sequence as the temperature comes down.

Some of the specific uses are; In the thermography of human skin In the thermal mapping of solid state electronic devices For surface thermography in aerodynamic testing To determine the efficiency of heat exchangers In the testing of radiations in those devices which convert radiation energy into heat energy.

They are used for non-destructive mechanical testing of materials under stress. This technique is also used for the visualization of RF (radio frequency) waves in wave-guides. Low molar mass (LMM) liquid crystals have applications including erasable optical disks, full color "electronic slides" for computer-aided drawing (CAD), and light modulators for color electronic imaging. Some of the liquid crystals are used in hydraulic break/clutch system due to their high viscosity values. An application of liquid crystals that is now being explored is optical imaging and recording.

Definition, types, characteristics, and applications of Refractories Glass Organic Electronic materials

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