1. 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

2. 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

3. 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

4. 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

5. The structure of polymers
Ken Youssefi
Mechanical Engineering

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

7. 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

8. Some Common Addition Polymers

9. Condensation (Step) Polymerization9 9 9 9

10. Some Condensation Polymers

11. 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

12. Examples of Thermoplastics

13. Examples of Thermoplastics
Polymer

14. Thermoset data

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. Mechanical Engineering
Ken Youssefi
Mechanical Engineering

25. Mechanical Engineering
Ken Youssefi
Mechanical Engineering