Mechanical Engineering CHAPTER 6 Plastics (Polymers) The word plastics is from the Greek word Plastikos, meaning “able to be shaped and molded” Ken Youssefi Mechanical Engineering

Polymer Poly mer many repeat unit (building blocks) H Polyethylene (PE) Cl C H Poly(vinyl chloride) (PVC) H Polypropylene (PP) C CH3 Carbon chain backbone 2 2

Mechanical Engineering Polymers The earliest synthetic polymer was developed in 1906, called Bakelite. The development of modern plastics started in 1920s using raw material extracted from coal and petroleum products (Ethylene). Ethylene is called a building block. Polymers are long-chain molecules and are formed by polymerization process, linking and cross linking a particular building block (monomer, a unit cell). The term polymer means many units repeated many times in a chainlike structure. Most monomers are organic materials, atoms are joined in covalent bonds (electron-sharing) with other atoms such as oxygen, nitrogen, hydrogen, sulfur, chlorine,…. Ken Youssefi Mechanical Engineering

Ancient Polymers Naturally occurring polymers (those derived from plants and animals) have been used for centuries. Wood – Rubber Cotton – Wool Leather – Silk Oldest known uses Rubber balls used by Incas

The structure of polymers Ken Youssefi Mechanical Engineering

Addition (Chain) Polymerization Initiation Propagation Termination 6 6 6 6

Chemistry and Structure of Polyethylene Tetrahedral arrangement of C-H Polyethylene is a long-chain hydrocarbon. Top figure shows repeat unit and chain structures. Other figure shows zigzag backbone structure. 7 7

Some Common Addition Polymers

Condensation (Step) Polymerization 9 9 9 9

Some Condensation Polymers

Classification of plastics There are two major classifications of polymers Thermoplastics As the temperature is raised above the melting point, the secondary bonds weaken, making it easier to form the plastic into any desired shape. When polymer is cooled, it returns to its original strength and hardness. The process is reversible. Polymers that show this behavior are known as thermoplastics. Thermosetting Plastics (thermosets) Thermosetting plastics are cured into permanent shape. Cannot be re-melted to the flowable state that existed before curing, continued heating for a long time leads to degradation or decomposition. This curing (cross-linked) reaction is irreversible. Thermosets generally have better mechanical, thermal and chemical properties. They also have better electrical resistance and dimensional stability than do thermoplastics. Ken Youssefi Mechanical Engineering

Examples of Thermoplastics

Examples of Thermoplastics Polymer http://www2.dupont.com/Teflon/en_US/index.html http://en.wikipedia.org/wiki/Teflon

Thermoset data

Properties of Plastics Corrosion resistance Low electrical and thermal conductivity, insulator Easily formed into complex shapes, can be formed, casted and joined. Wide choice of appearance, colors and transparencies Ken Youssefi Mechanical Engineering

Applications of Thermoplastics Design requirement: strength Applications: Valves, gears, cams, pistons, fan blades, … Plastics: nylon, acetal (delrin), polycarbonate, phenolic Design requirement: wear resistance Applications: bearings, gears, bushings, wheels, …. Plastics: nylon, acetal (delrin), polyurethane, phenolic, polymide Ken Youssefi Mechanical Engineering

Applications of Thermoplastics Design requirement: functional and decorative Applications: knobs, handles, cases, moldings, pipe fittings, … Plastics: ABS, acrylic, polyethylene, phenolic, polypropylene, polystyrene Design requirement: functional and transparent Applications: lens, goggles, signs, food processing equipment, … Plastics: acrylic, polycarbonate, polystyrene, polysulfone Design requirement: hollow shapes and housings Applications: pumps, helmets, power tools, cases, … Plastics: ABS, polyethylene, phenolic, polypropylene, polystyrene, polycarbonate Ken Youssefi Mechanical Engineering

Applications of Thermosetting Plastics Epoxies Properties: good dimensional stability, excellent mechanical and electrical properties, good resistance to heat and chemicals Applications: electrical components requiring strength, tools and dies, fiber reinforced epoxies are used in structural components, tanks, pressure vessels, rocket motor casing Phenolics Properties: good dimensional stability, rigid, high resistance to heat, water, electricity, and chemicals Applications: laminated panels, handles, knobs, electrical components; connectors, insulators Ken Youssefi Mechanical Engineering

Applications of Thermosetting Plastics Polyesters (thermosetting, reinforced with glass fibers) Properties: good mechanical, electrical, and chemical properties, good resistance to heat and chemicals Applications: boats, luggage, swimming pools, automotive bodies, chairs Silicones Properties: excellent electrical properties over a wide rang of temperature and humidity, good heat and chemical properties Applications: electrical components requiring strength at high temp., waterproof materials, heat seals Ken Youssefi Mechanical Engineering

Thermoplastic vs Thermoset • Thermoplastics: --little cross linking --ductile --soften with heating --polyethylene polypropylene polycarbonate polystyrene • Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NOT soften with heating --vulcanized rubber, epoxies, polyester resin, phenolic resin 3

Mechanical Engineering Popular Plastics Polyethylene (LDPE (low density) and HDPE (high density) Properties: good chemical and electrical properties, strength depends on composition Applications: bottles, garbage cans, housewares, bumpers, toys, luggage Acetal (Delrin) Properties: good strength, good stiffness, good resistance to heat, moisture, abrasion and chemicals Applications: mechanical components; gears, bearings, valves, rollers, bushings, housings ABS Properties: dimensionally stable, good strength, impact and toughness properties, good resistance to abrasion and chemicals Applications: automotive components, helmets, tool handles, appliances, boat hulls, luggage, decorative panels Ken Youssefi Mechanical Engineering

Mechanical Engineering Popular Plastics Polycarbonates Properties: very versatile and has dimensional stability, good mechanical and electrical properties, high resistance to impact and chemicals Applications: optical lenses, food processing equipments, electrical components and insulators, medical equipments, windshields, signs, machine components Nylons Properties: good mechanical and abrasion resistance property, self-lubricating, resistant to most chemicals but it absorbs water, increase in dimension is undesirable Applications: mechanical components; gears, bearings, rollers, bushings, fasteners, guides, zippers, surgical equipments, Ken Youssefi Mechanical Engineering

Disadvantages of using Plastics Low strength Low useful temperature range (up to 600 oF) Less dimensional stability over period of time (creep effect) Aging effect, hardens and become brittle over time Sensitive to environment, moisture and chemicals Poor machinability Ken Youssefi Mechanical Engineering

Mechanical Engineering Ken Youssefi Mechanical Engineering

Mechanical Engineering Ken Youssefi Mechanical Engineering