Category Materials and the Environment: Eco-Informed Material Choice

Polyethylene (PE)

The material. Polyethylene, (—CH2—)n, first synthesized in 1933, looks like the simplest of molecules, but the number of ways in which the—CH2 units can be linked is large. It is the first of the polyolefins, the bulk thermoplastic polymers that account for a dominant fraction of all polymer consumption. Polyethylene is inert, and extremely resistant to fresh and saltwater, food, and most water-based solutions. For this reason it is widely used in house­hold products, food containers, and chopping boards. Polyethylene is cheap and particularly easy to mold and fabricate. It accepts a wide range of colors, can be transparent, translucent or opaque, has a pleasant, slightly waxy feel, can be textured or metal coated, but is difficult to print on.

Composition

(—CH2—CH2—)n

P...

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Polypropylene (PP)

The material. Polypropylene, or PP, first produced commercially in 1958, is the younger brother of polyethylene, a very similar molecule with similar price, processing methods, and application. Like PE it is produced in very large quantities (more than 30 million tons per year in 2000), growing at nearly 10% per year, and like PE its molecule lengths and side branches can be tailored by clever catalysis, giving precise control of impact strength, and of the properties that influence molding and drawing. In its pure form poly­propylene is flammable and degrades in sunlight. Fire retardants make it slow to burn and stabilizers give it extreme stability, both to UV radiation and to fresh and saltwater and most aqueous solutions.

Composition

(CH2-CH(CHs))n

General properties

Density

8...

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Polyamides (Nylons, PA)

The material. Back in 1945, the war in Europe just ended, the two most prized luxuries were cigarettes and stockings made of nylon. Nylon (PA) can be drawn to fibers as fine as silk and was widely used as a substitute for it. Today, newer fibers have eroded its dominance in garment design, but nylon-fiber ropes and nylon as reinforcement for rubber (in car tires) and other polymers (PTFE, for roofs) remain important. Nylon is used in product design for tough casings, frames and handles, and, reinforced with glass, as bearings gears and other load-bearing parts. There are many grades (Nylon 6, Nylon 66, Nylon 11, etc.), each with slightly different properties.

Composition

(NH(CH2)5C0)n

General properties

Density

1120 –

1140

kg/m3

Price

*3.55 –

3.91

USD/kg

Mechanic...

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Acrylonitrile butadiene styrene (ABS)

The material. Acrylonitrile butadiene styrene, or ABS, is tough, resilient, and easily molded. It is usually opaque, although some grades can now be transparent, and it can be given vivid colors. ABS-PVC alloys are tougher than standard ABS and, in self-extinguishing grades, are used for the cas­ings of power tools.

Composition

(CH2-CH-C6H4L

General properties

Density

1010

– 1210

kg/m3

Price

2.3

– 2.6

USD/kg

Mechanical properties

Young’s modulus

1.1

– 2.9

GPa

Yield strength (elastic limit)

18.5

– 51

MPa

Tensile strength

27.6

– 55.2

MPa

Elongation

1.5

– 100

%

Hardness—Vickers

5.6

– 15.3

HV

Fatigue strength at 107 cycles

11

– 22.1

MPa

Fracture toughness

1.19

– 4.29

MPa. m1/2

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Polymers

Polymers are the chemist’s contribution to the materials world. The fact that most are derived from oil (a nonrenewable resource) and the difficulty of disposing of them at the end of their life (they don’t easily degrade) has led to a view that polymers are environmental villains. There is some truth in this, but the present problems are soluble. Using oil to make polymers is a better primary use than just burning it for heat; the heat can still be recov­ered from the polymer at the end of its life. There are alternatives to oil; polymer feed stocks can be synthesized from agricultural products (notably starch and sugar, via methanol and ethanol). And thermoplastics—provided they are not contaminated—can be (and, to some extent, are) recycled.

Thermoplastics soften when heated and ha...

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Cast iron, ductile (nodular)

The material. The foundations of modern industrial society are set, so to speak, in cast iron: it is the material that made the Industrial Revolution possible. Today it holds a second honor: that of being the cheapest of all engineering metals. Cast iron contains at least 2% carbon; most have 3-4%—and from 1-3% silicon. The carbon makes the iron very fluid when molten, allowing it to be cast to intricate shapes. There are five classes of cast iron: gray, white, ductile (or nodular), malleable, and alloy. The two that are most used are gray and ductile. This record is for ductile cast iron.

Composition

Fe/3.2-4.1% C/1.8-2.8% Si/ < 0.8% Mn/ <0.1% P/ <0.03% S

General properties

Density

7050 – 7250 kg/m3

Price

*0.7 – 0.8 USD/kg

Mechanical properties

Young’s modulus

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Stainless steel

The material. Stainless steels are alloys of iron with chromium, nickel, and often four or five other elements. The alloying transmutes plain carbon steel that rusts and is prone to brittleness below room temperature into a material that does neither. Indeed, most stainless steels resist corrosion in most normal environments, and those that are austenitic (like AISI 302, 304, and 316) remain ductile to the lowest of temperatures.

Composition

Fe/ < 0.25C/16-30Cr/3.5-37Ni/ < General properties

Density

Price

Mechanical properties

Young’s modulus

Yield strength (elastic limit)

Tensile strength Elongation Hardness—Vickers Fatigue strength at 107 cycles Fracture toughness

Thermal properties

Melting point

Maximum service temperature Thermal conductor or insulator? Thermal conductivity Specific ...

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Low alloy steel

The material. Addition of manganese (Mn), nickel (Ni), molybdenum (Mo), or chromium (Cr) to steel lowers the critical quench rate and comes to cre­ate martensite, allowing thick sections to be hardened and then tempered. Adding some vanadium, V, as well creates a dispersion of carbides, giving strength while retaining toughness and ductility. Chrome-molybdenum steels such as AIS 4140 are used for aircraft tubing and other high-strength parts. Chrome-vanadium steels are used for crank and propeller shafts and high-quality tools. Steels alloyed for this purpose are called low alloy steels, and the property they have is called hardenability.

Composition

Fe/ < 1.0 C/ < 2.5 Cr/ < 2.5 Ni/ < 2.5 Mo/ < 2.5 V/

General properties

Density Price

Mechanical properties

Young’s modulus Yield strength (e...

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Low carbon steel

The material. Think of steel and you think of railroads, oil rigs, tankers, and skyscrapers. And what you are thinking of is not just steel, it is car­bon steel. That is the metal that made them possible; nothing else is at the same time so strong, so tough, so easily formed, and so cheap. Carbon steels are alloys of iron with carbon and often a little manganese, nickel, and silicon. Low carbon or "mild" steels have the least carbon—less than 0.25%. They are relatively soft, easily rolled to plate, I-sections or rod (for reinforcing concrete), and are the cheapest of all structural metals; it is these that are used on a huge scale for reinforcement, steel-framed build­ings, ship plate, and the like.

Composition

Fe/0.02-0.3C.

General properties

Density

7800 –

7900

kg/m3

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Nickel-based superalloys

The material. With a name like superalloy there has to be something spe­cial here. There is. Superalloy is a name applied to nickel-based, iron-based, and cobalt-based alloys that combine exceptional high-temperature strength with excellent corrosion and oxidation resistance. Without them, jet engines would not be practical: they can carry load continuously at temper­atures up to 1200°C. The nickel-based superalloys are the ultimate metal­lic cocktail: nickel with a good slug of chromium and lesser shots of cobalt, aluminum, titanium, molybdenum, zirconium, and iron. The data in this record spans the range of high-performance, nickel-based superalloys.

Composition

Ni + 10 to 25% Cr + Ti, Al, Co, Mo, Zr, B, and Fe in varying proportions. General properties

Density

7750

– 8650

k...

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