Category Materials and the Environment: Eco-Informed Material Choice

Natural rubber (NR)

The material. Natural rubber was known to the natives of Peru many cen­turies ago and is now one of Malaysia’s main exports. It made the fortune of Giles Macintosh who, in 1825, devised the rubber-coated waterproof coat that still bears his name. Latex, the sap of the rubber tree, is cross-linked (vulcanized) by heating with sulfur; the amount of cross-linking determines the properties. It is the most widely used of all elastomers—more than 50% of all produced.

Composition

(CH2—C(CH3)—CH—CH2)n

General properties

Density

Price

Natural rubber is used in medical equipment, fashion items, tubing, and tires.

Typical uses. Gloves, car tires, seals, belts, antivibration mounts, electrical insulation, tubing, rubber lining pipes, and pumps.

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Phenolics

The material. Bakelite, commercialized in 1909, triggered a revolution in product design. It was stiff, fairly strong, could (to a muted degree) be col­ored, and, above all, was easy to mold. Earlier products that were hand­crafted from woods, metals, or exotics such as ivory, could now be molded quickly and cheaply. At one time the production of phenolics exceeded that of PE, PS, and PVC combined. Now, although the ration has changed, phe – nolics still have a unique value. They are stiff, are chemically stable, have good electrical properties, are fire-resistant, and are easy to mold—and they are cheap. Thermosetting phenolics are recyclable but by a different means than that for thermoplastics. Molded phenolic, ground into a fine powder, can be added to the raw material stream...

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Polyester

The material. Polyesters can be thermosets, thermoplastics, or elastomers. The unsaturated polyester resins are thermosets. Most polyester thermo­sets are used in glass fiber/polyester composites. They are less stiff and strong than epoxies, but they are considerably cheaper. This record is for thermosetting polyester. It cannot be recycled.

Composition

(OOC—C6H4—COO—C6H10)n General properties

Density

1040 –

1400

kg/m3

Price

*4.07 –

4.47

USD/kg

Mechanical properties

Young’s modulus

2.07 –

4.41

GPa

Yield strength (elastic limit)

*33 –

40

MPa

Tensile strength

41.4 –

89.6

MPa

Compressive strength

*36.3 –

44

MPa

Elongation

2-

2.6

%

Hardness—Vickers

9.9 –

21.5

HV

Fatigue strength at 107 cycles

*16...

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Epoxies

The material. Epoxies are thermosetting polymers with excellent mechani­cal, electrical, and adhesive properties and good resistance to heat and chemical attack. They are used for adhesives (Araldite), surface coatings, and, when filled with other materials such as glass or carbon fibers, as matrix resins in composite materials. Typically, as adhesives, epoxies are used for high-strength bonding of dissimilar materials; as coatings, they are used to encapsulate electrical coils and electronic components; when filled, they are used for tooling fixtures for low-volume molding of thermoplastics.

Composition

(O-C6H4-CH3-C-CH3-C6H4L

Epoxy paints are exceptionally stable and protective and take color well...

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Polyhydroxyalkanoates (PHA, PHB)

The material. PHAs are linear polyesters produced in nature by bacterial fermentation of sugar or lipids derived from soybean oil, corn oil, or palm oil. They are fully biodegradable. More than 100 different monomers can be combined within this family to give materials with a wide range of prop­erties, from stiff and brittle thermoplastics to flexible elastomers. The most common type of PHA is PHB (poly-3-hydroxybutyrate), with properties similar to those of PP though it is stiffer and more brittle. The following data is for PHB.

Composition

(CH(CHs)—CH2—CO—O)n General properties

Density

1230

– 1250

kg/m3

Price

3.2

– 4

USD/kg

Mechanical properties

Young’s modulus

0.8

– 4

GPa

Yield strength (elastic limit)

35

– 40

MPa

Tensile strength

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Polylactide (PLA)

The material. Polylactide (PLA) is a biodegradable thermoplastic derived from natural lactic acid from corn, maize, or milk. It resembles clear poly­styrene and provides good aesthetics (gloss and clarity), but it is stiff and brittle and needs modification using plasticizers for most practical applica­tions. It can be processed like most thermoplastics into fibers, films, ther – moformed, or injection molded.

Cargill Dow polylactide food packaging.

Ecoproperties: material

Embodied energy, primary production

52

– 54

MJ/kg

CO2 footprint, primary production

*2.3

– 2.43

kg/kg

Water usage

*100

– 300

l/kg

Ecoproperties: processing

Polymer molding energy

*8.61

– 9.49

MJ/kg

Polymer molding CO2

*0.68

– 0.75

kg/kg

Polymer extrusion energ...

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Polystyrene (PS)

The material. Polystyrene is an optically clear, cheap, easily molded poly­mer, familiar as the standard "jewel" CD case. In its simplest form PS is brittle. Its mechanical properties are dramatically improved by blending with polybutadiene but with a loss of optical transparency. High-impact PS (10% polybutadiene) is much stronger, even at low temperatures (meaning strength down to -12°C). The single largest use of PS is a foam packaging.

Composition

(CH(C6Hs)-CH2)n

General properties

Density

1040

– 1050 kg/m3

Price

1.81

– 1.99 USD/kg

Mechanical properties

Young’s modulus

1.2

– 2.6

GPa

Yield strength (elastic limit)

28.7

– 56.2

MPa

Tensile strength

35.9

– 56.5

MPa

Compressive strength

31.6

– 61.8

MPa

Elongation

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Polyvinylchloride (tpPVC)

The material. PVC (vinyl) is one of the cheapest, most versatile, and, with polyethylene, the most widely used of polymers and epitomizes their multi­faceted character. In its pure form—as a thermoplastic, tpPVC—it is rigid and not very tough; its low price makes it a cost-effective engineering plas­tic where extremes of service are not encountered. Incorporating plasti­cizers creates flexible PVC, elPVC, a material with leather – or rubberlike properties and used a substitute for both. By contrast, reinforcement with glass fibers gives a material that is sufficiently stiff, strong, and tough to be used for roofs, flooring, and building panels.

Composition

General properties

Density

Price

Mechanical properties

Young’s modulus Yield strength (elastic limit) Tensile strength Compre...

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Polyethylene terephthalate (PET)

The material. The name polyester derives from a combination of Polymerization and esterification. Saturated polyesters are thermoplastic; examples are PET and PBT, which have good mechanical properties to tem­peratures as high as 175°C. PET is crystal clear and impervious to water and CO2, but a little oxygen does get through. It is tough, strong, and easy to shape, join, and sterilize, allowing reuse. Unsaturated polyesters are thermosets; they are used as the matrix material in glass fiber/polyester composites.

Composition

(CO-(C6H4)-CO-O-(CH2)2-O)n

General properties

Density

1290

– 1400

kg/m3

Price

*1.63

– 1.79

USD/kg

Mechanical properties

Young’s modulus

2.76

– 4.14

GPa

Yield strength (elastic limit)

56.5

– 62.3

MPa

Tensile str...

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Polycarbonate (PC)

The material. PC is one of the "engineering" thermoplastics, meaning that they have better mechanical properties than the cheaper "commodity" polymers. The benzene ring and the —OCOO-carbonate group combine in pure PC to give it its unique characteristics of optical transparency and good toughness and rigidity, even at relatively high temperatures. These properties make PC a good choice for applications such as compact disks, safety hard hats, and housings for power tools.

Composition

(O—(C6H4)—C(CH3)2—(C6H4)—CO)n

General properties

Density

Price

Mechanical properties

Young’s modulus Yield strength (elastic limit) Tensile strength Compressive strength Elongation Hardness—Vickers Fatigue strength at 107 cycles Fracture toughness

Ecoproperties: processing

Polymer molding energy P...

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