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Polymers IME 340/240 Plastics and Polymers • Polymers first used as a word in 1866 for natural organic polymers such as cellulose (for photographic ...
Author: Jocelyn Price
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Polymers IME 340/240

Plastics and Polymers • Polymers first used as a word in 1866 for natural organic polymers such as cellulose (for photographic film, packaging, textile fibers) • Plastics first used as a noun in 1909 (used as synonym) • The first synthetic polymer was a thermoset (phenolformaldehyde) Bakelite was developed in 1906 • Extremely large molecules (macromolecules) • Wide variety of uses developed throughout 20th century • From Greek work plastikos – able to be molded and shaped • Plastics can be machined, cast, formed, and joined with relative ease requiring little post-processing or surfacefinish operations

Plastics Pros and Cons • Advantages over metals • • • • • • • •

Corrosion resistance and resistance to chemicals Low electrical and thermal conductivity Low density (good for lightweight components) High strength-to-weight ratio, particularly when reinforced Noise reduction Wide choice of colors and transparencies Ease of manufacturing and complexity of design possibilities Relatively low cost

• Possible disadvantages • • • • •

Low strength and stiffness High coefficient of thermal expansion Low useful temperature range (up to 350°C, 660°F) Less dimensional stability in service over time Water absorption

Plastics and Polymers

Structure of Polymers • Mainly hydrocarbons • Monomer – basic building block • Long-chain molecules formed by polymerization, the linking and cross-linking of different monomers with secondary bonds (van der Waals, hydrogen, ionic) Figure 7.4 (a) Linear structure--thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures. (b) Branched structure, such as in polyethylene. (c) Cross-linked structure--many rubbers or elastomers have this structure, and the vulcanization of rubber produces this structure. (d) Network structure, which is basically highly cross-linked--examples are thermosetting plastics, such as epoxies and phenolics.

Linked with primary covalent bonds Lower density, because branches interfere with packing efficiency

Thermosets

Thermoplastics treated with UV, x-rays, or electron beams

Structure of Polymers

Figure 7.2 Basic structure of polymer molecules: (a) ethylene molecule; (b) polyethylene, a linear chain of many ethylene molecules; (c) molecular structure of various polymers. These are examples of the basic building blocks for plastics

Molecular Weight and DP • The higher the molecular weight of a polymer, the greater the average chain length • Commercial polymers usually have molecular weight between 10,000 and 10,000,000 • Degree of polymerization (DP) is the ratio of molecular weight of the polymer to the molecular weight of the repeating unit – the higher the DP, the higher the polymer’s viscosity and thus higher processing costs. Also equals average number of mers in the molecule.

Strength Properties of Plastics

Strength Properties

Crystallinity

Figure 7.6 Amorphous and crystalline regions in a polymer. The crystalline region (crystallite) has an orderly arrangement of molecules. The higher the crystallinity, the harder, stiffer, and less ductile the polymer.

Effects of temperature, crystallinity, and cross-linking

Effects of Crystallinity Polyethylene type

Low Density (LDPE)

High Density (HDPE)

Degree of crystallinity

55%

92%

Specific gravity

0.92

0.96

Modulus of elasticity

20,000 psi

1000,000 psi

Melting temperature

239°F (115°C)

275°F (135°C)

High Temperature Behavior • Amorphous polymers do not have a specific melting point, but do have a glass transition temperature • Below Tg they are hard, rigid, brittle, and glassy • At high temperatures they are rubbery or leathery • Partly crystalline TABLE 7.2 Material T (°C) T (°C) polymers do have Nylon 6,6 57 265 Polycarbonate 150 265 a melting point Tm Polyester 73 265 Polyethylene • Repeating heating High density –90 137 Low density –110 115 Polymethylmethacrylate 105 — and cooling can Polypropylene –14 176 Polystyrene 100 239 cause degradation Polytetrafluoroethylene –90 327 Polyvinyl chloride 87 212 or thermal aging Rubber –73 — of thermoplastics g

m

Melting Points and Glass Transition Temperatures

Effect of Temperature on Strength Properties

Types of Plastics • Thermoplastics • Become easier to form or mold above the glass transition temperature or melting point • Increased temperature weakens secondary bonds • Reversible process • Can become anisotropic as chains align during stretching • Crazing and stress whitening occurs under tensile stress or bending

• Thermosets • Cross-linking and three dimensional arrangements • Irreversible curing process

• Elastomers (Rubber)

Additives • Plasticizers • Low molecular weight solvents with high boiling points that impart flexibility and softness by lowering the glass-transition temperatures • Often used in PVC and to make thin sheets, films, tubing, shower curtains, and clothing materials

• Carbon black (soot) • To protect against ultraviolet radiation degradation

• Fillers • To reduce the cost of the polymer, or possibly improve properties • Wood flour, silica flour, clay, talc, fibers of glass/cellulose/asbestos

• Colorants – organic dyes or inorganic pigments • Flame retardants • Lubricants • To reduce friction during processing or prevent sticking to molds

Thermoplastics • Acetals • good strength, stiffness, and resistance to creep, abrasion, moisture, heat, and chemicals • bearings, cams, shower heads

• Acrylics (PMMA) • moderate strength, good optical properties (often transparent, can be opaque, and resistance to weather, chemicals, electricity • Lenses, lighted signs, skylights, windshields, lighting fixtures, (Lucite), (Plexiglas)

• ABS (acrylonitrile-butadiene-styrene) • Dimensionally stable and rigid, good strength and toughness, good resistance for impacts, abrasion, chemicals, and electricity • Legos, helmets, luggage, refrigerator liners, telephones

• Cellulosics • Rigid, strong, tough, but weather poorly, affected by heat, chemicals • Pens, knobs, eyeglass frames, safety goggles, toys, machine guards

Thermoplastics • Fluorocarbons • Good resistance to high temperature (Tm = 327°C) • Teflon cookware, chemical-process equipment, electrical insulation for high temperature wire and cable, gaskets, seals

• Polyamides – Nylons • Good mechanical properties, abrasion resistant, self-lubricating • Fasteners, zippers, tubing, surgical equipment, gears, bearings

• Polyamides – Aramids • High tensile strength and stiffness • Kevlar bulletproof vests, radial tires, fibers for reinforced plastics

• Polycarbonates • Good mechanical and electrical properties, high impact resistance • Food-processing equipment, bottles, bullet-resistant window glazing, optical lenses, safety helmets, machine guards (Lexan), medical aparatus

Thermoplastics • Polyesters (also thermosets) • Good mechanical, electrical, chemical properties • Gears, cams, load-bearing members, pumps, (Mylar)

• Polyethylenes – low density (LDPE), high density (HDPE) • Good electrical and chemical properties • Bumpers, housewares, garbage cans, bottles, toys, packaging

• Polyethylenes – ultra high molecular weight (UHMWPE) • High impact toughness and resistance to abrasive wear • Artificial knee and hip joints

• Polypropylenes • Good mechanical, electrical, chemical props., resistance to tearing • Milk & juice containers, weather stripping, auto trim & components

Thermoplastics • Polystyrenes (Styrofoam) • Inexpensive, average properties, somewhat brittle • Disposable packaging for meat, cookies, and candy, insulation

• Polysulfones • Excellent resistance to heat, water, and steam, highly resistant to chemicals but are attacked by organic solvents • Steam irons, coffeemakers, medical equipment that is sterilized, aircraft cabin interiors, power tool & appliance housings, insulators

• Polyvinyl chloride (PVC) • Wide range of properties, inexpensive, rigid or flexible • Signs, construction pipes and conduits • Wire and cable coatings, flexible tubing, footwear, imitation leather, upholstery, records, gaskets, seals, films and coatings, (Saran)

• Polyimides – • structure of thermoplastic but nonmelting characteristic of thermoset

Thermosets • Polyimides • Structure of thermoplastic but nonmelting characteristic of thermoset • Pump components, electrical connectors for high-temperature use, sports equipment, safety vests, aerospace parts

• Alkyds • Good electrical insulating properties, impact resistant, low water absoption, dimensional stability • Electrical components

• Aminos (Urea and Melamine) • Hard and rigid, resistant to creep, abrasion, electrical composition • Countertops, small appliance housings, handles, dinnerware

• Epoxies • Excellent mechanical and electrical properties, good dimensional stability, strong adhesive properties, can be fiber-reinforced

Thermosets • Polyesters • Often reinforced with glass (or other fibers) • Available as casting resins • Boats, luggage, chairs, automotive bodies, swimming pools

• Phenolics • Brittle, but rigid and dimensionally stable, high resistance to heat, water, electricity, chemicals • Knobs, handles, telephones, bond material to hold abrasive grains together in grinding wheels, electrical devices, insulators

• Silicones • Weather well and excellent electrical properties over a wide range of humidity and temperature, resist chemical and heat • Oven gaskets, heat seals, waterproof materials

Elastomers • • • • •

Amorphous polymers with low Tg Highly kinked, twister, and curled polymer structure Stretch, but return to original shape after load is removed Soft with low elastic modulus Natural and synthetic rubber, silicones, polyurethane • Also cross-linked structure, as formed through vulcanization Figure 7.12 Typical load-elongation curve for rubbers. The clockwise lop, indicating the loading and the unloading paths, displays the hysteresis loss. Hysteresis gives rubbers the capacity to dissipate energy, damp vibration, and absorb shock loading, as is necessary in automobile tires and in vibration dampers placed under machinery.

Environmental Considerations • • • •

Plastics contribute 10% of municipal solid waste High volume, relative to their weight 1/3 of plastic production is for disposable products Most plastics are made from synthetic polymers that are derived from nonrenewable natural resources (coal, petroleum, etc), not biodegradable, and difficult to recycle • 3 biodegradable plastics have been developed thus far but more research is needed • Material conservation efforts are best • Thermoplastics are recycled by remelting them and reforming them into other products – watch for symbols! • 1 –Polyester (PETE), 2 –Polyethylene -HDPE, 3 – vinyl (PVC), 4 – Polyethylene -LDPE, 5 – Polypropylene (PP), 6 – Polystyrene (PS), 7 – other