UNIT 2 A
Unit -2
2 marks Questions.
1. Differentiate Thermoplasts and Thermosets polymer?
A.
2. What are the
synthetic polymers?
A. Synthetic
polymers are human-made polymers. From the utility point of view they can be
classified into four main categories: thermoplastics, thermosets, elastomers
and synthetic fibers. They are found commonly in a variety of consumer products
such as money, super glue, etc.
A wide variety of synthetic polymers are available with
variations in main chain as well as side chains. The back bones of common
synthetic polymers such as polythene, polystyrene and poly acrylates are made
up of carbon-carbon bonds, whereas hetero chain polymers such as polyamides,
polyesters, polyurethanes, polysulfides and polycarbonates have other elements
(e.g. oxygen, sulfur, nitrogen) inserted along the backbone. Also silicon forms
similar materials without the need of carbon atoms, such as silicones through
siloxane linkages; these compounds are thus said to be inorganic polymers.
Coordination polymers may contain a range of metals in the backbone, with
non-covalent bonding present.
3. What are advantages of FRP?
A. There
are following :
·
Infinite
potential
·
Low
weight
·
Mechanical
strength
·
High
impact strength
·
Resiliance
·
Formability
·
Chemical
resistance
·
Corrosion
resistance
·
Weather
proof
·
Electrically
insulating
·
Thermally
insulating
·
Fire
resistance
·
Low
thermal expansion
·
Anti
magnetic, no spark
·
Durable
custom colours
·
Long
life
4. What is the difference between Addition and condensation
polymerization?
A.
Addition
polymerization
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Condensation
polymerization
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The process of synthesizing addition polymers is known as
addition polymerization. There should be multiple bonded monomers to initiate
such reaction. This is a chain reaction; therefore, any number of monomers
can join into a polymer. There are three steps to a chain reaction, they are
initiation, propagation and termination. For an example, we will take the
synthesis of polyethylene, which is an additional polymer used to make
products like garbage bags, food wrap, jugs, etc. The monomer for
polyethylene is ethene (CH2=CH2). Its repeating unit is –CH2-. In the initiation
step, peroxide radical is generated. This radical attacks the monomer to
activate it and produce a monomer radical. During the propagation phase, the
chain grows. Activated monomer attacks another double bonded monomer and
attaches together. Ultimately the reaction stops when two radicals join
together and form a stable bond. Chemists can control the length of the
polymer chain, reaction times and other factors to obtain the required
polymer.
|
Any condensation process, which results in the formation
of polymers, is known as condensation polymerization. A small molecule like
water or HCl is released as a by-product during the condensation
polymerization. The monomer should have functional groups in ends, which can
react together to continue the polymerization. For example, if the joining
ends of two molecules have a –OH group and a –COOH group, a water molecule
will be released and an ester bond formed. Polyester is an example for a
condensation polymer like that. In the synthesis of polypeptides, nucleic acids
or polysaccharides, condensation polymerization takes place within biological
systems.
|
5. Write an example
for composite material? What are the different types of composites?
A.
• Examples:
Cemented carbides (WC
with Co binder)
Plastic molding compounds containing
fillers
Rubber mixed with
carbon black
Wood (a natural
composite as distinguished
from a synthesized composite)
Types of Composite Matrix Materials
There are three main types of composite matrix materials:
• Metal Matrix Composites
• Ceramic Matrix Composites
• Polymer Matrix
Composites
6. What is matrix and
reinforcement?
A. Matrix
The matrix
is the monolithic material into which the reinforcement is embedded, and is
completely continuous. This means that there is a path through the matrix to
any point in the material, unlike two materials sandwiched together. In
structural applications, the matrix is usually a lighter metal such as
aluminium, magnesium, or titanium, and provides a compliant support for the
reinforcement. In high-temperature applications, cobalt and cobalt–nickel alloy
matrices are common.
Reinforcement
The
reinforcement material is embedded into a matrix. The reinforcement does not
always serve a purely structural task (reinforcing the compound), but is also
used to change physical properties such as wear resistance, friction
coefficient, or thermal conductivity. The reinforcement can be either
continuous, or discontinuous. Discontinuous MMCs can be isotropic, and can be
worked with standard metalworking techniques, such as extrusion, forging, or
rolling. In addition, they may be machined using conventional techniques, but
commonly would need the use of polycrystalline diamond tooling (PCD).
Continuous
reinforcement uses monofilament wires or fibres such as carbon fiber or silicon
carbide. Because the fibres are embedded into the matrix in a certain
direction, the result is an anisotropic structure in which the alignment of the
material affects its strength. One of the first MMCs used boron filament as
reinforcement. Discontinuous reinforcement uses "whiskers", short
fibres, or particles. The most common reinforcing materials in this category
are alumina and silicon carbide.
7. Give the example for metal- matrix composites and
ceramic-matrix composites?
A.
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Metal matrix composites
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Ceramic matrix composites
|
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For example, carbon fibers are
commonly used in aluminum matrix to synthesize composites showing low
density and high strength. However, carbon reacts with aluminum to generate a
brittle and water-soluble compound Al4C3 on the surface of the fiber. To prevent this
reaction, the carbon fibers are coated with nickel or titanium boride.
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The important
commercially available CMCs are C/C, C/Sic, Sic/Sic and Al2O3/Al2O3.
silicon
carbide (Sic), alumina (Al2O3) and mullite (Al2O3–SiO2)
fibers
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