PLASTICS. Disadvantages: Expensive Combustible Poor ductility High softness Deformation under load Low heat resistance.

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1 PLASTICS Plastics: An organic substance which changes its shape by the application of heat and pressure is called Plastics. This is done in presence of catalyst. Properties: These are usually light in weight. These will have low thermal and electrical conductance. Corrosion resistant, insect resistant. Have low fabrication cost. Easily moldable. Easily workable. Chemically inert. Transparent Possess good shock absorption capacity. Impermeable to water. Have good strength and toughness. Possess dimensional stability. Have low maintenance cost. Disadvantages: Expensive Combustible Poor ductility High softness Deformation under load Low heat resistance. Classification of Plastics: Plastics are classified into two types viz., Thermoplastics and Thermosetting Plastics according to the manner of setting. Thermo plastics: Thermoplastics are those, which can be softened on heating and hardened on cooling. Their hardness is a temporary property, subject to change with temperature. Repeating of heating or cooling donot alter the chemical nature of T.P. because the changes involved are purely of physical nature. T.Ps are usually soft, weak and less brittle and can be reclaimed from the wastes. These are formed by addition polymerization and consists of long chain linear polymers with little or no cross linking. These resins are soluble in some organic solvents. Eg: Polythene, P.V.C., Poly Styrene etc., Thermosetting Plastics: Thermosetting plastics are those, which change irreversibly into hard and rigid materials on heating. These are permanent setting resins

2 and during moulding, acquire 3-dimensional cross linked structure with strong covalent bonds. The bonds donot break, even on heating. Thermosetting resins are usually harder, stronger and more brittle than termoplastics and they cannot be reclaimed from wasters. The resins are formed by condensation polymerization and consists of 3D network structure joined by strong covalent bonds which makes them insoluble in almost all organic solvents. Eg:- Phenol Formaldehyde (Bakelite), Epoxy plastics, Silicone plastics etc., Differences between Thermoplastics and Thermosetting Plastics: THERMOPLASTICS 1) These are products of addition polymerization. 2) They have either linear or branched structures. 3) They softened on heating and hardened on cooling. 4) Adjacent polymer chains are held by either vanderwalls forces or by dipole-dipole forces. 5) Soluble in organic solvents. 6) They are soft, weak, less brittle. 7) They can be remoulded, reshaped and reused. 8) They can be reclaimed from waste i.e., they can be recycled. THERMOSETTING PLASTICS 1) These are the products of Condensation polymerization. 2) They have 3-dimensional, cross linked network structure. 3) They donot soften on heating. 4) Adjacent polymer chains are held together by strong covalent bonds called crosslinks. 5) Insoluble in organic solvents. 6) They are hard, strong, more brittle. 7) They can t be remoulded and hence can t be reused. 8) They cannot be reclaimed from waste and hence cannot be recycled. COMPOUNDING OF PLASTICS (OR) MOULDING CONSTITUENTS OF PLASTICS: Compounding of plastic is the process of addition of some external material to give specific properties to the product. These external materials not only give the required property to the plastic but also makes the plastic economical. The components of compounding of plastics are as follows:

3 1. Resins: The product of polymerization is called resin and this forms the major portion of the body of plastics. This acts as a binder which holds the different constituents together. Eg:- Thermoplastic and Thermosetting Plastic Resins. 2. Plasticisers: Plasticizers increase plasticity and flexibility of the polymer. Plasticisers neutralize the intermolecular forces of attraction between polymer chains. Thus imparts greater freedom of movement between the polymeric molecules. Eg:- Vegetable oils, Camphor, Tri Cresyl Phosphate, esters of steric acid etc., 3. Fillers: Fillers give better hardness, tensile strength, opacity, finish and workability to the plastics. It reduce the cost of polymers. They decrease the shrinkage of the polymer on setting. Fillers also reduce the brittleness of the polymer. Some special types of fillers are added to impact special properties to polymer like Barium Salts make polymers impermeable to x-rays and Asbestos provides heat and corrosion resistance to polymers, Carborandum, Quarts and Mica gives hardness to the polymers. 4. Lubricants: Lubricants make moulding of plastic easier. They impart flawless, glossy finish to the products. Lubricants prevent moulded article from sticking to the fabrication equipment. Eg:- Waxes, oils, stearates, oleates and Soaps. 5. Catalysts (or) Accelerators: They accelerate the polymerization of fusible resin during moulding operation into cross linked infusible form for thermosetting resins. Eg:- Benzoyl peroxide, hydrogen peroxide, acetyl sulphuric acid etc., 6. Stabilisers: Stabilisers improve thermal stability during polymerization. Eg:- White lead, lead chromate, red lead, stearates of lead, Cadmium and Barium. 7. Colouring Materials: These are added to impart desired colour to the final product. The decorative colours are provided by both organic dye stuffs and inorganic salts. Eg:- Black Colour - Carbon Black. White Colour - ZnO (or) CaCO3. Red Colour Ferrix Oxide. Green Colour Chromium Oxide. Crimson Colour Antimony Sulphide.

4 SUMMARY OF COMPOUNDING OF PLASTICS: Stabilisers increase Thermal Stability. Eg: Lead Salts, Ca, Ba. Colouring Materials gives desired colour to plastics. Eg: ZnO, Fe2O3 etc., Catalysts accelerates the crosslinking of Resins. Eg: H 2O 2, Cu, Ag, Pb. COMPOUNDING OF PLASTICS Resin forms body of the plastics and binder for different ingredients. Eg: TP, TSP. Lubricants makes moulding easier, gives flawless, glossy finish. Eg: Waxes, oils, Stearates and oleates etc., Fillers gives hardness, tensile strength, opacity, finish, workability, reduce cost. Eg: Ba Salts, Quarts, Mica etc., Plasticisers give plasticity and flexibility. Eg: Veg. Oils, Waxes, tri cresyl phosphate, oils etc., FABRICATION OF PLASTICS (OR) MOULDING OF PLASTICS: The method by which plastics can be fabricated, depends on the thermal behavior of the plastics i.e., whether it is thermoplastic or thermosetting plastic and also on the shape of the finished product. The following are the methods employed for the fabrication of Plastics: 1) Compression Moulding: This method can be adopted for both Thermosetting and Thermoplastic Resins. The quantity of the resin, filler and other ingredients placed in the mould should be such that the volume of the compact mass is slightly more than the volume of the cavity of the mould. The mould is usually preheated. It is closed after placing the ingredients and heat and pressure are applied, wherein the ingredients soften and fill the cavity of the mould. For T.P. resins, the mould is cooled before releasing the pressure, otherwise the object will distort. The thermosetting resin has to be kept in the mould for a period of time for the completion of

5 the formation of permanent shape (Curing Time i.e., the time required for the polymer to set in the mould is called curing ). 2) Injection Moulding: This method is widely used for the fabrication of Thermoplastic materials. In this method, the Thermoplastic material is softened by heating, and the hot plastic is injected into a mould where it sets and takes the shape of the mould. The mould is kept at cold temperature. It is a modification of compression moulding. The advantage of this process is that it is rapid. Advantages: 1) This method is characterized by high speed production and low mould cost. 2) Loss of material will be less. The disadvantage of this is a large number of cavities cannot be filled simulateneously, so there is a limitation to the designed articles. 3) Transfer Moulding: Transfer moulding is a method which was the principle of injection moulding for T.S. Plastics. The powdered compounded plastic

6 material is in a chamber applied with minimum temperature and high pressure till it begins to become soft and semisolid. Then it is injected into a mould by a plunger working at high pressure, due to high friction developed at the nozzle, the soft plastics material is ejected from the orifice into the mould which is heated upto the curing temperature required for setting the fabricated article is then ejected out mechanically. Advantages: 1) Since the plasticized mix flows very slowly into the mould, very delicate articles with intricate shapes can be produced. 2) The articles are free from flow marks. 3) Mould cost is less. 4) Extrusion Moulding: Extrusion molding is used for moulding of Thermo Plastic materials into articles of uniform cross section like tubes, rods, sheets, wires, cables etc., The Thermo Plastic ingredients are heated to plastic state and then pushed by means of a screw conveyor into a die, having the required outershape of the article to be fabricated. The extruded article gets cooled due to atmospheric exposure or artificially by air jets, or by water sprayer in a long conveyor which carries away the cooled product.