1. THERMOPLASTIC POLYMERS

2. INTRODUCTION Thermoplastic polymers are also known as Thermoplasts
Thermoplasts soften when heated (and eventually liquefy) and harden when cooled processes are totally reversible and may be repeated
On a molecular level, as the temperature is raised, secondary bonding forces are diminished so that the relative movement of adjacent chains is facilitated when a stress is applied
Irreversible degradation results when the temperature of a molten thermoplastic polymer is raised to the point at which molecular vibrations become violent enough to break the primary covalent bonds.

3. Thermoplasts are relatively soft
Most linear polymers and those having some branched structures with flexible chains are thermoplastic
These materials are normally fabricated by the simultaneous application of heat and pressure

4. Polymer’s Structures Linear polymers Branched polymers
Bonding – monomers are linked together by covalent bonds, forming a polymer chain (primary bonds). The polymer chains are held together by secondary bonds. The strength of polymers comes in part from the length of polymer chains. The longer the chain, the stronger the polymer. More energy is needed to overcome the secondary bonds.
Linear polymers
A sequential structure resulting in thermoplastics like nylon, acrylic, polyethylene. A linear polymer may contain some branched and cross-linked chains resulting in change in properties.
Branched polymers
Side branch chains are attached to the main chain which interferes with the relative movement of the molecular chains. This results in an increase in strength, deformation resistance and stress cracking resistance. Lower density than linear chain polymers.

5. 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

6. 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

7. 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

8. Popular Plastics Polycarbonates Nylons
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,

9. Commodity Thermoplastics
Polyethylene (PE),
Polystyrene (PS),
Polypropylene (PP),
Polyvinyl Chloride (PVC or vinyl) – 80% of all thermoplastics!!
Also Styrene Acrylonitrile (SAN)
Flows at elevated temperatures.
Has a glass transition temperature.
Long polymer chains
Can be remelted and recycled.

10. Engineering Thermoplastics
7.2: Basic Types of Polymers:
Engineering Plastics:
Polycarbonate (PC),
Acrylonitrile-butadiene-styrene (ABS),
Polyamide (Nylons, PA)
Engineered plastics account for about 10% of all plastic usage.
Generally have higher tensile strength and elongation than commodity plastics

11. Thermoplastic vs. Thermoset - Properties
Chemical Resistance
Thermoplastic – Some have good chemical resistance, but most can be dissolved or weakened by at least some chemical compounds
Thermoset – Better chemical resistance than any thermoplastic
Dimensional Stability
Thermoplastic – Because they respond to heat, their dimensional stability is related to their service temperature and their glass transition temperature.
Thermoset – Excellent dimensional stability
Creep (cold flow)
Thermoplastic – Although it can be improved by adding fillers and reinforcements, they are not as good as Thermosets
Thermoset – Much better than thermoplastics
Molded-in-stresses (war page)
Thermoplastic – caused by uneven cooling, part design, mold design, and process parameters
Thermoset – relatively low stresses which yield less distortion.

12. Thermoplastic vs. Thermoset - Properties
Toughness
Thermoplastic – Inherently tough, although some types can be brittle
Thermoset – not tough, brittle unless reinforced – fiberglass
Coloration
Thermoplastic – Easily colorable and color is maintained
Thermoset – Limited options and colors tend to fade or discolor over time.
Clarity
Thermoplastic – Many clear polymers are available
Thermoset – Very few options for clarity
Shrinkage
Thermoplastic – Varies based on type of
polymer (Semi-crystalline/Amorphous)
Thermoset – Varies by process

13. Thermoplastic vs. Thermoset - Properties
Long term properties
Thermoplastic – Need to be estimated based on short term data.
Thermoset – Can be predicted based on experimental data
Cost
Thermoplastic – Less expensive and easier to process
Thermoset – Need more skill and produce more scrap which cannot be reprocessed.
Tool Wear
Thermoplastic – Less tool wear with unreinforced polymers
Thermoset – More prevalent mold wear and greater potential for mold damage. Most Thermosets are filled.

14. ACRYLONITRILE BUTADIENE STYRENE (ABS)
CHARACTERISTICS:
Good strength and toughness
Resistant to heat distortion
Good electric properties
Soluble in organic solvents
APPLICATION:
Refrigerator linings, lawn and garden equipment, toys, highway safety devices

15. ACRYLICS- PMMA CHARACTERISTICS: APPLICATION:
Outstanding light transmission
resistance to weathering
Fair mechanical properties
APPLICATION:
Lenses, transparent aircraft enclosures, drafting equipment, outdoor signs

16. POLYTETRAFLUOROETHYLENE-TEFLON
CHARACTERISTICS:
Chemically inert in all environments
Excellent electrical properties
Low coefficient of friction
May be used to 260oC
Poor cold-flow properties
APPLICATION:
Anticorrosive seals, chemical pipes and valves, bearings, anti adhesive coatings, high temperature electronic parts

17. POLYAMIDES-NYLON (PA)
CHARACTERISTICS:
Good mechanical strength
Abrasion resistance and high toughness
Low coefficient of friction
Absorbs water and some other liquids
APPLICATION:
Bearings, gears, cams, bushings, handles and jacketing for wires and cables

18. POLYETHYLENE (PE) CHARACTERISTICS: APPLICATION:
Chemically resistant and electrically insulating
Tough and relatively low coefficient of friction
Low strength and poor resistance to weathering
APPLICATION:
Flexible bottles, toys, tumblers, battery parts, ice trays, film wrapping materials

19. PVC CHARACTERISTICS: APPLICATION: Low cost, general purpose materials
Rigid, can be made flexible with plasticizers
Susceptible to heat distortion
APPLICATION:
Pipes, valves, fittings, floor tiles, wire insulation, toys, phonograph records, safety glass inter layers

20. POLYPROPYLENE (PP) CHARACTERISTICS: APPLICATION:
Resistant to heat distortion
Excellent electrical properties and fatigue strength
Chemically inert
Inexpensive and poor resistance to UV light
APPLICATION:
Sterlizable bottles, packing film, TV cabinets, luggage

21. POLYSTYRENE-STYRON (PS)
CHARACTERISTICS:
Excellent mold ability
Good dimensional stability
Good electrical, heat and strain resistance
Poor chemical and corrosion resistance
Susceptible to ultra violet lights
APPLICATION:
Packaging and insulation foams, lighting panels, appliance components