Reinforcement. Fillers. There are two important classes of Plastics

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1 MATERIALS SCIENCE- MECH PLASTICS. The term plastics has been applied to those synthetic non-metallic materials that can be made sufficiently fluid to be shaped readily by casting, molding or extruding and that may be subsequently hardened to preserve the desired shape. POLYMERIC MATERIALS: Synthetic large molecules are made by joining together thousands of small molecular units known as monomers. Addition polymerization Condensation polymerization Examples of main additives: Antistatic agents. Pigments Coupling agents Plasticisers Extenders Reinforcement. Fillers Stabilizers Flame retardants. Catalysts. Lubricants There are two important classes of Plastics (a) THERMOPLASTIC MATERIALS e.g of thermoplastics :Acrylics, ABS, Polyethylene,PVC, Polystyrene, Nylon, Cellulose acetate, Acetal, Polycarbonate, Poly methyl mathacrylate (PMMA), PTFE, Polyamides, Polyesters, Polyimides, Polysulfones and Polypropylene. S. Venkannah Faculty of Engineering 1

2 Thermoplastic are subdivided into crystalline (ordered) or amorphous (random) structure. (b)thermosetting PLASTICS A thermosetting plastic is produced by a chemical reaction which has two stages. Thermosets are quite rigid materials and their mechanical properties are not heat sensitive. Examples are Alkyds, Aminos, Silicones, Phenol formadehyde, Melamine formaldehyde, Urea formaldehyde, Epoxies and some Polyesters. PLASTICS AVAILABLE TO ENGINEERS, Engineering plastics Structural foam Thermosets Elastomers Thermoplastics Polymer alloys Composites Liquid crystal polymers Main properties of plastics: S. Venkannah Faculty of Engineering 2

3 Light weight Wide range of colors Resistance to deterioration by moisture Fairly good resistance to attack by inorganic acids, salts and bases Plastics in many cases are non-flammable, self extinguishing, and burn very slowly Can be transparent, translucent or opaque Good dimensional stability Good sound absorption properties Good thermal and electrical insulator Good tensile strength Can be easily be fixed in position Good resistance to peeling Quite durable Easy maintenance of plastic surfaces Factors affecting the properties of plastics: Branching : Copolymerization: Compounding polymers:. Blending of plastics: Commercial forms available: Plastic sheets Plastic panels Plastic tiles Plastic pipes Plastic laminates S. Venkannah Faculty of Engineering 3

4 Building Blocks used for certain desired properties: Material Property Enhancement Polycarbonate, Acrylic, Impact strength ABS, Ethylene-propylene rubber Polyphenylene oxide, Heat resistance, tensile polycarbonate, strength, stiffness polysulfone, polyimide PVC, chlorinated Flame retardancy polyethylene PVC, Nylon, Polyester Chemical resistance (PET and PBT) Nylon, Ethylene vinyl Barrier properties alcohol Polystyrene, ABS, Processability Acrylic SELECTION OF PLASTICS: (i) Mechanical properties (ii) Corrosion susceptibility and degradation. (iii) Wear resistance and frictional properties (iv) Special properties, e.g, thermal, electrical, optical and magnetic properties, damping capacity. (v) molding and/or other methods of fabrication. (vi) Total cost attributable to the selected materials and manufacturing route. S. Venkannah Faculty of Engineering 4

5 Degradation: thermal aging Stress whitening: Crazing Physical or Chemical attack: Oxidation Environmental stress cracking: SPECIAL PROPERTIES: Thermal properties-plastics tend to be rigid and brittle at low temps. and soft and flexible at high temps. Impact resistance is improved at elevated temps., while mechanical properties are reduced and destructive actions of external agents to which they are sensitive are accelerated. In general moderate temps. are required for storage of plastics over long periods; low temps. are to be avoided because of the low temp. brittleness of most of the plastics; and high temps. should be avoided because of rapid loss of mechanical properties,volatization of plasticisers and the susceptibility of a large number of plastics to distortion. Electrical properties: Traditionally plastics have established themselves in applications which require electrical insulation. S. Venkannah Faculty of Engineering 5

6 The thermal and electrical properties of some plastics e.g. polyacetylene, Polyaniline, Polypynole, Polythiophene, can be increased by doping. The electrical conductivity also increases with absorption of moisture. Effects of light: Prolonged exposure to sunlight will affect adversely all plastics with the exception of tetrafluoroethylene(teflon). Flammability: Critical Oxygen Index(COI) is defined as the minimum concentration of Oxygen, expressed as volume per cent, in a mixture of Oxygen and Nitrogen that will just support combustion under the conditions of test. Effects of Oxygen: Organic plastics are nearly all subject to oxidation when exposed to the atmosphere. The process is accelerated by high temps. and light but over long periods of time, oxidative deterioration may take place at room temp. Effects of moisture:some polymers such as nylon are able to absorb water and makes the polymer more plastic. S. Venkannah Faculty of Engineering 6

7 ΠΡΟΧΕΣΣΙΝΓ ΟΦ ΠΛΑΣΤΙΧΣ: EXTRUSION INJECTION MOLDING THERMOFORMING CALENDERING EXTRUSION STRETCH BLOW MOULDING: ROTOMOLDING: EXPANDABLE BEAD MOLDING: REACTION INJECTION MOLDING COMPRESSION MOLDING TRANSFER MOULDING. BONDING OF THERMOPLASTICS: Fusion bonding Solvent bonding Friction bonding Induction bonding Ultrasonic bonding Vibration bonding MOLDING REINFORCED THERMOSETS. HAND LAYUP MATCHED DIE MOLDING RIGIDIZED VACUUM FORMING PULTRUSION FILAMENT WELDING. S. Venkannah Faculty of Engineering 7

8 CASTING: a) Conventional casting method b) Centrifugal casting c) Potting and encapsulation d) Foam molding and casting COLD FORMING: The important considerations are: The material must be sufficiently ductile at room temperature The materials deformation must be non recoverable SOLID PHASE FORMING: RECYCLING OF PLASTICS BIODEGRABLE PLASTICS: S. Venkannah Faculty of Engineering 8