polymers - university of rhode island 240/polymers_general.pdf · structure of polymers • mainly...

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 (phenol-

formaldehyde) 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 surface-

finish 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.

Thermosets

Lower density,

because branches

interfere with

packing efficiency

Thermo-

plastics

treated

with UV,

x-rays, or

electron

beams

Linked with primary

covalent bonds

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

polymers do have

a melting point Tm

• Repeating heating

and cooling can

cause degradation

or thermal aging

of thermoplastics

TABLE 7.2

Material Tg (°C) Tm (°C)

Nylon 6,6

Polycarbonate

Polyester

Polyethylene

High density

Low density

Polymethylmethacrylate

Polypropylene

Polystyrene

Polytetrafluoroethylene

Polyvinyl chloride

Rubber

57

150

73

–90

–110

105

–14

100

–90

87

–73

265

265

265

137

115

—

176

239

327

212

—

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