THERMOPLASTIC POLYMERS

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.

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

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.

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

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

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

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,

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.

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

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.

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

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.

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

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

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

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

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

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

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

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