Slide 1: POLYMERS Slide 2: A polymer has a repeating structure, usually based on a
carbon backbone. The repeating structure results in large
chainlike molecules. In each polymer molecule, the atoms are bound together by covalent bonds. Polymers
Poly = many; Mer = unit -> Polymer = many units. Slide 3: Covalent Chain Configurations and Strength Increase in Strength Linear Polymers: Polyethylene, polyvinyl chloride (PVC), polystyrene, polymethyl methacrylate (plexiglass), nylon, fluorocarbons (teflon) Branched Polymers: Many elastomers or polymeric rubbers Cross-linked Polymers: Many elastomers or polymeric rubbers are cross-linked (vulcanization process); most thermosetting polymers Network Polymers: Epoxies, phenol-formaldehydes. Slide 4: Physical Properties of Polymers
Composed of very large molecules
Low modulus of elasticity (low stiffness)
Low tensile and compressive strengths
Can be crystalline or semi-crystalline structure
Deformation is very sensitive to temperature
Low thermal and electrical conductivity(good insulator)
Creep at room temperatures
Low temperatures make plastics brittle
Plastic deformation Slide 6: Thermoplastics
Soften when heated and harden when cooled
Varying degree of ductility
Can be recycled
Can not withstand high temperatures Thermosets
Become permanently hard after the initial heating-cooling cycle
Can not be recycled
Can withstand high temperatures Slide 7: Industrially Important Polymers
About 85% of the world plastics consumption consists of four polymers. These polymers are produced in high volume at very low cost. They are all thermoplastics.
Polyethylene (PE) electrical wire insulation, flexible tubing, squeeze bottles
Polypropylene (PP) carpet fibers, ropes, liquid containers (cups, buckets, tanks), pipes
Polystyrene (PS) packaging foams, egg cartons, lighting panels, electrical appliance components
Polyvinyl chloride (PVC) bottles, hoses, pipes, valves, electrical wire insulation, toys, raincoats Slide 9: Polymers are formed by many low temperature processes.
Reaction Injection Molding Polymer Processing Methods Slide 10: The polymer is heated to the liquid state and forced through a die under pressure resulting in an endless product of constant cross section. 60% of polymers are prepared in this way.
Examples: tubing, pipes, window frames, sheet, insulated wire. Extrusion .
Using the same method as extrusion the material coming out of the die is blown into a film.
An example is plastic wrap. Film Blowing Slide 11: Similar to extrusion, the polymeris heated to the liquid state, but it is prepared in metered amounts, and the melt is forced into a mold to create the part. It is not a continuous process. Many toys are made by injection molding. Injection Molding The melted polymer is put into a mold, then compressed air is used to spread the polymer into the mold. It is used to make many containers such as plastic soda containers and milk
jugs. Blow Molding Slide 12: Solid polymer is placed in a mold, the mold is heated and puts pressure on the polymer to form the part. Liquid monomers are placed in the mold avoiding the need to use temperature to melt the polymer or pressure to inject it. The monomers polymerize in the mold forming the part. Compression Molding Reaction Injection Molding Slide 13: Elastomers
Elastomers are long polymer chains above their glass transition temperature. Elastomers are usually lightly crosslinked and are easily deformed. Common elastomers include:
polybutadiene (used in shoe soles)
polyisobutylene (used in automobile tires
polyisoprene (natural rubber). Slide 14: Elastomeric polymer chains can be crosslinked, or connected by covalent bonds. This process is sometimes called vulcanization.
Crosslinking is initiated by heat, light, or the addition of chemicals.
Crosslinking makes elastomers reversibly stretchable for small deformations. When stretched, the polymer chains become elongated and ordered along the deformation direction. When no longer stretched, the chains randomize again. The crosslinks guide the elastomer back to its original shape. Slide 15: Crosslinking makes elastomers reversibly stretchable for small deformations. Stretched Returned to randomization Slide 16: Back to Table of Contents