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Saturday, 5 October 2019

Classification of Thermoplastic

Plastic is an organic material and the word Resin is generally used for Plastics. It is classified into two main groups.


(a) Thermo Plastic
(b) Thermo Setting

Perspex, Polyethylene, PVC course under thermo plastic while Bakelite, Silicon resin, Epoxy resin come under thermo setting. Thermo plastic is those materials which soften on the application of heat with or without pressure and on cooling it sets into shape. Thermo setting resin requires heat and pressure to mould them into shape.

Plastic Materials:

These are organic substances which resemble the natural resins in properties such as plasticity and heat resistance, but prepared artificially by the recurring chemical reaction of a simple organic compound. During the reaction, the molecules of the simple organic compound combine together to form a high molecular weight composition. This reaction is called polymerisation. Polymerization can take place in two ways : Additional polymerisation and Condensation polymerisation.
In the additional polymerisation, the reacting organic materials combine with each other without the evolution of a by-product and the molecular weight of the product is multiple of that of the original reacting molecules e.g., the formation of Polystyrene and ethylene. In the condensation polymerisation the reacting molecules combine in such a way that there is evolution of a by-product of low molecular weight like water e.g., the formation of polyster resins.

Classification of Thermoplastic resins:

These materials soften on the application of heat, with or without pressure but require cooling to bring them into shape. When it is heated to about 140°C, it starts softening and then it can be easily moulded without any change in its chemical structure. These are the products of organic substances like oil and coal. To achieve the pre-desired properties fillers like soot, mica, glass fibres, wood etc., are used. The three types of thermoplastic resins are : Perspex, Polyethylene, PVC.

Perspex (Cellulose Acetate):

It is obtained by controlled hydrolysis of triacetate solution at a temperature around 60°C. It is fairly hard but susceptable to oxidation at high temperatures. It has a low softening temperature and low inflammability. It has good electrical properties but is very unstable in the presence of ozone and hence cannot be used as insulation in high voltage apparatus. It is highly hygroscopic and shrinks as a result of ageing. It has a good mouldability, toughness and good impact strength. It is not repairable, as it is a brittle material having less weight. It gives good appearance. It is used as insulation in low voltage capacitors, as impregnating material and as coating on insulated paper. It is also used in the manufacture of switches, lamp holders, plug sockets, and instrument mountings.


Polyethylene Thermoplastic:

It is obtained by polymerisation of ethylene. Its molecular weight ranges between 1500 and 80000. It has good electrical and mechanical properties, less hygroscopic, poor solubility in various solvents but dissolves at higher temperatures in benzene, and petroleum solvents, frost resistant and brittle at low temperatures. Its dielectric constant and power factor show remarkable consistancy in wide frequency and temperature range. It is the lightest of all plastics, but is brittle at low temperatures and softens at 93°C. Earlier the process of manufacturing this resins was done at high pressure of about 1050 to 2100 kg / cm2 and at temperature of 100°C to 300°C for polymerisation but the present method makes use of catalysts which enables its manufacture at atmospheric pressure and temperature around 100°C.

It is used in large quantities for general purpose insulation of wires, conductors, television and communication circuits, high frequency cables etc., as a jacketing material for different types of cables, as a containers for chemicals, corrosion resistant coatings, electrical insulators, pipes and tubing. Some of the important properties of Polyethylene are shown in Table. Polyethylene is not fit for use in power cables at high voltage because of low melting point, susceptibility to cracking in atmosphere. As insulation it is used upto 90°C. Cross linked polyethylene which overcomes the problem of cracking is used for high voltage upto 132 kV.

Properties of Low and High Density Polyethylene

Properties
Low density Polyethylene
High density Polyethylene
Dielectric strength (kV / mm)
20 - 160
20 - 60
Dielectric constant (at 10 Hz)
2.28 - 2.32
2.25 - 2.32
Power factor (at 60 Hz)
1 - 5 x 10-4
2 - 6 x 10-4
Tensile strength (kg / cm2 x 10-3)
0.09 - 0.16
0.16 - 0.365
Surface resistance (ohm)
1014 - 1017
1014 - 1015
Volume resistivity (ohm - cm)
1017- 1019
10-15 – 10-16
Water absorption (%)
less than 0.01
less than 0.01
Elongation (%)
110 - 160
40 - 300
Softening temperature (°C)
94 - 110
--
Crystalline melting point (°C)
110 - 116
126 - 136
Specific heat
0.55
--
Relative density
0.91 - 0.925
0.95 - 0.970


It is obtained either in viscous, liquid form, gum or tough flexible solid suitable for moulding. It is wax like appearance, translucent, odourless and light. It is flexible over a wide range of / temperatures and chemically resistant. It does not absorb moisture and has a high resistivity and dielectric strength. Its dielectric constant is low. It is cheap and freely available.

Poly Vinyl Chloride (P.V.C.) :

It is obtained by the combination of acetylene and hydrogen chloride in the presence of a catalyst at a temperature of 50°C. It can also be produced by the polymerisation of ethylene dichloride and sodium hydroxide in the present of catalyst. These resins are hard, little flexible, very slightly hygroscopic, rigid and insoluble in most of the industrial solvents. It is obtained either in the opaque or coloured condition. By the addition of different types of materials the properties like flexibility, mouldability, hygroscopicity, electrical properties are affected. Some important properties of P.V.C is shown in Table.

Properties of P.V.C

Insulation resistance (ohm - cm)
1012 - 1013
Specific gravity
1.03
Temperature of use (°C)
60 - 90
Brittle temperature (°C)
30
Softening temperature (°C)
120
Dielectric constant
5 - 6
Dielectric strength (kV / mm)
30

Effect of Fillers on P.V.C. :

Filler like wood, flour, cotton, flocks, asbestos, glass, fibres, powdered mica, talc, flour spar quartz flour etc., when added will make the PVC cheap and greatly influence its properties depending on the filler used.

Effects of Stabilizers on P.V.C. :

Stabilizers like metal, soaps, lead, stearate, calcium carbonate, calcium oxide etc., are used which prevents the PVC from degradation. PVC when exposed to sunlight or heated to high temperature evolves hydrogen chloride which affects many insulation properties. It corrodes the parts of the apparatus which conies in contact with it. But the stabilizer prevents the evolution of hydrogen chloride and hence the PVC remains safe.

Effects of Plasticizers on P.V.C. :

Plasticizers like tricresyl phosphate, dibutyl phthalate are used to control the flexibility of the PVC, so that moulding becomes easy. With the use of plasticizers, PVC can be easily moulded and extruded.

Effect of Additives on P.V.C. :

In the manufacture of electrical cables, it is necessary to use different colours to distinguish the different cores of the table as red, yellow, blue etc. Colouring pigments are added to give the desired colour. Lubricating additives help in extrusion and moulding process.


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