4. Non metallic engineering Material By Mr. S.M. Patange
Plastic It is synthetic organic material having high molecular weight which can be moulded into any desired form when subjected to heat and pressure in presence of catalyst. Polymerization: The process of joining or linking together a large number of small molecule (monomers) to form a large molecule (polymer) under specific condition of temperature, pressure and catalyst is known as polymerization. The term polymer is derived from Greek word poly – many and mer – parts. Polymer: It is high molecular weight substance containing one or more kinds of structural units joined to each other by co-valent bonds to give linear or three dimensional crosslinked networkis called as polymer. Formation of plastic: There are two methods, i) Addition polymerization ii) Condensation polymerization A) Addition polymerization: It is a process in which the monomers of same type undergo repeated addition resulting in the formation of long chain polymer without elimination of simple molecules like H2O, HCl, NH3 etc. Exemple: Polyethylene (Polythene), PVC etc. Formation of polyethylene from ethylene: The free radical (monomer) is formed by breaking of one bond of ethylene molecule.This monomer joined to each other at the radical end to form polyethylene.
B) Condensation polymerization: It is a process in which the monomers of different types joined together by the condensation forming a large polymer with the elimination of simple molecules like H2O, HCl, CH3OH etc. Formation of Bakelite: It is prepared by condensing phenol & formaldehyde in presence of acidic / alkaline catalyst.
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Compounding of plastic: Generally plastic are compounded with other substances during their formation to improve their properties. The substances used are resin, fillers, plasticizer, accelerators and pigment. Write four constituents of plastic. State one example and one function of each constituent. Constituents: 1) Resins (or binders) :Function: It holds the different constituents together. e.g.: PVC, bakelite, polystyrene, polyethylene, nylon, urea-formaldehyde, polyester etc. 2) Fillers: - Function: a) It increases hardness, tensile strength, opacity, finish and workability of plastic. b) It r e d u c e s the cost , shrinkage on setting and brittleness of plastics. c) Asbestos provides heat and corrosion resistance to the plastic material. Examples: Quartz, mica, asbestos, paper pulp, cotton, graphite etc. 3) Plasticizers:Function: It increases plasticity & flexibility of plastics. Examples: Camphor, tributy phosphate, oleic acid, stearic acid etc. 4) Accelerators / Catalysts :Function: These are used in moulding of thermosetting plastics. It decreases the time required for moulding. Examples: H2O2 (hydrogen peroxide), benzoyl peroxide etc. 5) Colouring Matter (or Pigments) :Function: The colouring matter used in plastics to impart beautiful shade of colours.
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Examples: organic dyestuffs & inorganic pigments like red lead, cobalt blue, chrome green etc.
Properties
& Applications of plastics:
Types of plastics: There are two types of plastics: a) Thermosoftening plastics b) Thermosetting plastics Difference between thermoplastics and thermosetting:
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Rubber Rubber is also known as elsatomers. It shows elasticity property. It elongated 5 to 10 times of its original length by small stretching force. It retains its original length after removing stretching force. It can be easily compressed. Elsatomers are coiled or spring shape. Types of rubber: Natural rubber: Natural rubber is a linear polymer of an unsaturated hydrocarbon called isoprene (2methyl butadiene). There may be as many as 11,000 to 20,000 isoprene units in a polymer chain of natural rubber. Natural rubber is produced from a milky emulsion known as „latex‟. This is obtained by cutting narrow strips from the bark of rubber tree. Eg. Gutta-percha, balata, Hevea, Guayule. The latex contains 30 to 40% solid rubber and 55 to 60% water, depending age of tree.
Synthetic rubber: These are high polymer possessing some physical properties of natural rubber. It is vulcanized rubber. It can be stretched 300 % to its length and retain its original length and dimension quickly. Synthetic rubbers are made by the polymerization .These rubbers are tougher, more flexible and more durable than natural rubbers. The following are the main types of synthetic rubber i) Buna-S ii) Buna-N iii) Neoprene iv) Thiokol
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v)
Butyl rubber
1. It is obtained from latex of rubber tree. 2. It is non-resistant of oxidation. 3. It is soft & sticky at higher temperature. 4. It is soluble in organic solvent. 5. Tack property is high. 6. It has capacity to absorb large quantity of water.
1. These are rubber like products obtained by chemical 2. It has oxidation resistance. 3. It does not become soft & sticky at higher temperature. 4. It is insoluble in organic solvent. 5. Tack property is low. 6. It is water resistant.
Vulcanization of natural rubber: 1. Process:- “The process which involves addition of sulphur or H2S to crude (raw) natural rubber at high temperature & pressure to improve properties of crude natural rubber is called vulcanization.” 2. Most of all the processes of vulcanization is addition of “sulphur”. 3. Heating the raw rubber with sulphur to a high temperature, sulphur, and combines chemically at double bonds in the rubber molecule of different rubber springs vulcunisation brings. 4. Reaction:-
A tyre rubber contains 3 to 5% sulphur, while a battery case rubber contains 30% sulphur. Ebonite rubber is hard and non- elastic, contains 52% sulphur. Important Properties of Synthetic Rubber:- (consider any three) 1) Elasticity:- “Elasticity is the property by which a material undergoes deformation under stress. & regains its original shape on the removal of the stress.” e. g. Rubber bands tubes for bicycles, automobiles & aeroplanes etc. 2) Tack:- Tack is the property of rubber by which two or more surfaces can stick to each other. e. g. Manufacture composite articles like tyre. 3) Rebound:- Ability to absorb energy & return without permanent deformation of a synthetic rubber is rebound. e. g. Rubber ball will bounce when dropped use in shock absorbers. 4) Hardness:- “Hardness is ability of the rubber to with stand wear & abrasion & resists penetration. e. g. shock absorber, gaskets etc. 5) Stress – Strain Properties (Tensile Strength):- A dumb bell shaped piece of synthetic rubber is cut with a die on narrow portion, make two marks on inch port as shown in figure.
It is then placed in the machine & stretched until it breaks. The load at which it breaks is its tensile strength. e. g. Property used for V-belts & conveyor belts. 6) Abrasion Resistance:- The property of resistance to wearing away of a surface by friction is known as abrasion resistance. e. g. Making shoe heels & soles, shock absorber making for heavy machineries & automobile parts. Applications of rubber:
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Thermal insulating materials: The materials or substances which are used to prevent the flow of heat are known thermal insulators. The substances having extremely low conductivity which prevent the loss of heat by conduction or radiation are called Thermal insulating materials. Characteristics of good thermal insulating material : 1. Its thermal conductivity should be low. 2. It should be fire proof. iii. It should be cheap. 3. Its density should be low. v. It should be water proof. 4. It should be chemically inert to water, surrounding atmosphere and high temperature. 5. It should be odourless during use. 6. It should withstand the effect of shock and vibrations. 7. It should be capable of bearing load in working operation. 8. It should be physically & mechanically stable at working Factors affecting thermal conductivity of insulators: i) Pores: The thermal conductivity materials are pores in nature. Its thermal conductivity is quite low because of entrapped air or gases in pores.These prevent the flow of heat. A large number of fine pores is preferred than few large pores. Thus good thermal insulator should highly pores. ii) Moisture: The presence of moisture in the pores increases thermal conductivity, because air in the pores is replaced by highly conductivity water. Hence surface of thermal insulator should be water proof. The pores on the surface should be as close as possible. So that moisture cannot be enter them. Classification of thermal insulating material:
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Thermal insulators are of two types a) Organic thermal insulators: These are suitable for temperature below 1500C. Generally these materials have low density and large number of small air packets. Eg. Wool, Cotton, Silk, Paper etc. b) Inorganic thermal insulators: These are suitable for temperature above 1500C. Eg. Asbestos, glass, glass wool etc. Thermocole Preparation: “Thermocole is a foamed plastic obtained by blowing compressed air into molten polystyrene or polyurethane is known as “thermocole” Properties : 1. It has low density. 2. It has low thermal conductivity & electrical conductivity. 3. It is quite shock - proof. 4. It is quite strong, though extremely light. 5. It is chemically inert & resists ageing. 6. It can be used up to 55 0C Applications: 1. It is used as heat insulator in refrigeration, cold-storage, ice-boxes, air conditioning etc. 2. It is used as ideal packing material for delicate electrical & electronic equipment. 3. It is used for decorative purposes. 4. It is used for protecting screen in radars. 5. It is used for floats in water Glass wool Preparation: “Glass filaments are obtained by forcing molten mass of alkali-free glass through sieve holes having the average diameter of 0.0005 cm. The filaments so obtained are thrown over a rapidly revolving drum to get the material in wool-like form.” Properties
Application
1. It has low thermal conductivity 2
(0.034 kcal/m /0C/hr.)
1. Hence it is widely used as thermal insulating material in motors, ovens, refrigerators, walls and roofs of houses
2. It has fibrous wool like structure .
2. So it is used in air filters as dust filtering material.
3. It is resistant to the chemicals and does not absorb moisture .
3. So it is used as a filtering material for corrosive liquids like acids and acidic solutions in industry.
4. It has low electrical conductivity .
4. So it is used in electrical insulation.
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