Materials Handbook 15th ed - G. Brady_ H. Clauser_ J. Vaccari (McGraw-Hill_ 2002) Episode 10 ppsx

PC-ABS alloys extend the exceptionally high impact strength of car-bonate plastics to section thicknesses over 0.0625 in 0.16 cm.Tradenames of PC-ABS alloys include Cycoloy of General E

species is also called buttonwood and buttonball It is a yellowish,

compact wood with a fine, open grain The density is about 40 lb/ft3(641 kg/m3) It resembles maple and gives a beautiful grain whenquartered It is employed in cabinetwork

PLASTIC ALLOYS AND BLENDS. Alloys consisting of two tics compounded into a single resin The two polymers must be melt-compatible Some polymers are naturally compatible; others requirethe use of compatibilizing agents The purpose of alloying polymers

thermoplas-is to achieve a combination of properties not available in any singleresin Acrylonitrile butadiene styrene (ABS) is a common alloy

ingredient, alloys including polycarbonate-ABS (PC-ABS),

ABS-polyurethane (ABS-PUR), ABS-nylon, ABS-polysulfone, and ABS-polyvinyl chloride (ABS-PVC) Besides PC-ABS there are polycarbonate-polybutylene terephthalate (PC-PBT), polycar- bonate-acrylonitrile styrene acrylate (PC-ASA), and polycar- bonate acrylic (PC-Acrylic) and polyetherimide-polycarbonate

(PEI-PC) alloys And besides ABS-PVC, there are polyvinyl

chlo-ride-acrylic, polyvinyl chloride-chlorinated polyethylene

(PVC-CPE), and acrylic-polyvinyl chloride alloys There are also

polyethylene alloys, polyphenylene oxide-polystyrene, and polyphenylene oxide-polybutylene terephthalate (PPO-PBT)

alloys

PC-ABS alloys extend the exceptionally high impact strength of

car-bonate plastics to section thicknesses over 0.0625 in (0.16 cm).Tradenames of PC-ABS alloys include Cycoloy of General ElectricPlastics; Pulse of Dow Plastics; Bayblend of Bayer.; and Celstran of

Polymer Composites Inc Cycoloy C2950 HF features a 30% greater flow rate than Cycoloy C2950 and a UL94 V-O flame retardance at a

thickness of 0.06 in (1.5 mm) and a UL94 5VB at 0.10 in (2.5 mm).Cycoloy LG9000 is a low-gloss, or matte-finish, grade for unpainted,interior auto parts It provides 60% less gloss than standard grades andhas a tensile strength of 7,900 lb/in2 (54 MPa), a flexural strength of12,300 lb/in2 (85 MPa), a flexural modulus of 335,000 lb/in2 (2,310MPa), a heat deflection temperature of 225°F (107°C) at 264 lb/in2 (1.8MPa), and a notched Izod impact strength of 10 ft lb/in (534 J/m) The

Dow grades include Pulse 830, 1725, 1735, and 1745 Pulse 1725 is a

brominated (flame-retardant), ignition-resistant grade; Pulse 1735 is anonbrominated, ignition-resistant grade Pulse 1745 is brominated, but

it does not contain polybrominated diphenyl ether, which may generateenvironmentally harmful dibenzodioxin and dibenzofuran This alloyand antimony-, bromine-, and chlorine-free, flame-retardant Bayblend

FR2000 and FR 2010 are intended for computer and business-machine

housings Both have a specific gravity of 1.18, a tensile strength at yield

720 PLASTIC ALLOYS AND BLENDS

of 8,700 lb/in2(60 MPa), a flexural strength at 5% strain of 13,800 lb/in2(95 MPa), and a flexural modulus of 390,000 lb/in2 (2,689 MPa) Thenotched Izod impact strength is 8 and 10 ft.lb/in (430 and 530 J/m),respectively, and the 264-lb/in2(1.8-MPa) deflection temperature is 180and 200°F (82 and 93°C).

Celstran PC-ABS alloys are reinforced with 25 or 40% long, glass

fibers The RTP 1800 A PC-acrylic alloys, from RTP Co., are tougher

than polycarbonate, having an unnotched impact strength of 39ft.lb/in (2,081 J/m) and a notched Izod impact strength of 16 ft.lb/in(854 J/m) and the processing ease of acrylics Tensile strength is 7500lb/in (52 MPa) and the flexural strength is 12,000 lb/in2 (83 MPa).They are white in color and can be modified for coloring, flame retar-dance, abrasion resistance, electromagnetic shielding, and antistaticcharacteristics

Triax Nylon 6-ABS, from Bayer, are semicrystalline

injection-mold-ing and extrusion alloys that combine good chemical and fatigue tance with excellent abrasion resistance General-purpose 1120 and

resis-1180 grades have, respectively, a specific gravity of 1.06 and 1.07, and,dry, as-molded, a tensile strength at yield of 6,300 and 5,800 lb/in2 (43and 40 MPa), a flexural modulus of 300,000 and 250,000 lb/in2 (2,069and 1,724 MPa), a notched Izod impact strength at 73°F (23°C) of 14 to

15 and 15 to 16 ft.lb/in2(747 to 801 and 801 to 854 J/m), and a lb/in2 (1.8-MPa) deflection temperature of 149 and 165°F (65 and74°C) and 129 and 142°F (54 and 61°C) The alloys absorb mois-ture and should be dried to 0.15 and 0.35% moisture before pro-cessing

264-Electrafil ABS-1200/SD, of DSM Engineering Plastics, combines

ABS with a static-dissipative polymer for protection against static discharge At 15% relative humidity, the static decay rate is lessthan 2 s compared with 99 s or more for unmodified ABS Althoughthe alloy’s flexural modulus is reduced considerably—270,000 lb/in2(1,862 MPa) versus 340,000 lb/in2 (2,344 MPa)—tensile strength andheat-deflection temperature are only slightly less, and impactstrength is unaffected ABS-PUR alloys combine the excellent abra-sion resistance and toughness of the urethanes with the lower costand rigidity of ABS

electro-ABS-PVC alloys are available commercially in several grades One

of the established grades provides self-extinguishing properties, thuseliminating the need for intumescent (nonburning) coatings in ABSapplications, such as power tool housings, where self-extinguishingmaterials are required A second grade possesses an impact strengthabout 30% higher than that of general-purpose ABS Also ABS-PVCalloys can be produced in sheet form The sheet materials haveimproved hot strength, which allows deeper draws than are possible

PLASTIC ALLOYS AND BLENDS 721

with standard rubber-modified PVC base sheet They also are ging when exposed to the heat of sunlight Some properties of ABS-PVC alloys are lower than those of the base resins Rigidity, ingeneral, is somewhat lower, and tensile strength is more or less depen-dent on the type and amount of ABS in the alloy.

nonfog-Another sheet material, an alloy of about 80% PVC and the restacrylic plastic, combines the nonburning properties, chemical resistance,and toughness of vinyl plastics with the rigidity and deep-drawing mer-

its of the acrylics The PVC-acrylic alloy approaches some metals in its

ability to withstand repeated blows Because of its unusually high ity, sheets ranging in thickness from 0.60 to 0.187 in (1.5 to 0.5 cm) can

rigid-be formed into thin-walled, deeply drawn parts Kydex 200, of Kleerdex

Co., is an acrylic-PVC alloy in the form of sheet 0.04 to 0.25 in (1 to 6.4mm) thick in various surface textures and colors The tensile strength is6,000 lb/in2 (41 MPa), flexural modulus is 350,000 lb/in2 (2,413 MPa),and the notched Izod impact strength is 4 to 6 ft lb/in (214 to 320 J/m).PVC is also alloyed with chlorinated polyethylene (CPE) to gainmaterials with improved outdoor weathering or to obtain better

low-temperature flexibility The PVC-CPE alloy applications include

wire and cable jacketing, extruded and molded shapes, and film

sheeting Acrylic-base alloys with a polybutadiene additive have

also been developed, chiefly for blow-molded products The acryliccontent can range from 50 to 95%, depending on the application.Besides blow-molded bottles, the alloys are suitable for thermoformedproducts such as tubs, trays, and blister pods The material is rigidand tough and has good heat-distortion resistance up to 180°F (82°C).Polyphenylene oxide (PPO) can be blended with polystyrene to pro-

duce a PPO-polystyrene alloy having improved processing traits

and lower costs than nonalloyed PPO The addition of polystyrenereduces tensile strength and heat-deflection temperature somewhat

and increases thermal expansion Gemax is a PPO-PBT alloy of General Electric Plastics GTX alloys, also of this company, combine

PPO and nylon and have heat-deflection temperatures up to 302°F(150°C) and good stiffness without glass or mineral fillers Variations

of this alloy combine various levels of stiffness, impact strength, andheat resistance GTX 990EP is nanotube-filled and has been used for

auto mirror housings and fuel filler caps Xenoy is a PC-PBT alloy of

the same company, for, among other uses, auto-bumper impact beams,

and Ultem LTX alloys are PC-ASA alloys LTX100A and 100B are

aimed at thick-wall connectors and extrusions; LTX300A and 300B

provide greater heat resistance Ultem ATX alloys, from General

Electric Plastics, combine polyetherimide and polycarbonate for

impact strength and heat resistance Du Pont’s “low-warp” PBT-ASA

722 PLASTIC ALLOYS AND BLENDS

alloys, Crastin LW9020 and LW9030, with 20 and 30% glass filling,

respectively, are intended mainly for electrical housings and

connec-tors Also for electrical applications is Bayer Corp.’s Makroblend

DP4-1389, an alloy of polyethylene terephthalate (PET) and

polycar-bonate Ultradur S, of BASF Corp., is a glass-filled PBT, ASA, and

PET alloy It is slightly lighter in weight than PBT, has better flow, isless prone to distortion and moisture absorption, and provides a bet-

ter surface appearance GE Plastics’ Geloy XP4034 PC-ASA alloy

fea-tures high impact strength and heat resistance as well as lowmoisture absorption An alloy of polypropylene modified with

non-cross-linked ethylene-propylene-diene monomer and Surlyn

ionomer provides “soft-touch” auto instrument panels

Hostalloy 731, of Hoechst Celanese, is an alloy of several

polyethyl-enes of different molecular weights It has a density of 0.034 lb/in3(941kg/m3), a tensile yield strength of 5,000 lb/in2 (35 MPa), and a flexuralmodulus of 170,000 lb/in2(1,170 MPa) The alloy approaches the abrasionresistance and impact strength of ultrahigh-molecular-weight polyethyl-

ene but is more readily processed Hivalloy, of Montell North America, is

a line of propylene-based copolymer and terpolymer alloys that combinethe strength of the semicrystalline monomer with the attributes of vari-ous amorphous monomers They are intended to compete with amor-phous plastics such as acrylonitrile butadiene styrene and polycarbonate

as well as with semicrystalline acetal, nylon, and thermoplastic polyester.The styrene-propylene G series has a specific gravity as low as 0.92, amelt flow as high as 0.055 lb/10 min (25 g/10 min), a Gardner impactstrength of up to 27 ft.lb (36 J), and a flexural modulus as high as 1 

106 lb/in2 (6,900 MPa) Toughened G-series alloys include ethylenepropylene and styrene-ethylene-butadiene-styrene modifiers to boostimpact strength, especially at low temperatures (22°F, 30°C).Hivalloy W alloys are of polypropylene and acrylic

PLASTIC BRONZE. A name once applied by makers of bearing bronzes tocopper alloys that are sufficiently pliable to assume the shape of theshaft and make a good bearing by running in These bronzes have a vari-ety of compositions, but the plasticity is always obtained by the addition

of lead, which in turn weakens the bearing In some cases the lead tent is so high, and the tin content so low, that the alloy is not a bronze

con-These copper-lead alloys have been referred to as red metals The

plas-tic bronze ingot marketed by one large foundry for journal bearings

con-tains 65 to 75% copper, 5 to 7 tin, and the balance lead Semiplastic

bronze usually contains above 75% copper and not more than 15 lead ASTM alloy No 7 has about 10% lead, 10 tin, 1 zinc, 1 antimony, and

78 copper The compressive strength is 12,500 lb/in2(85 MPa)

PLASTIC LAMINATES. Resin-impregnated paper or fabric, produced

under heat and high pressure Also referred to as high-pressure

plastic laminates Two major categories are decorative

thermoset-ting laminates and industrial thermosetthermoset-ting laminates Most of the

decorative thermosetting laminates are a paper base and are

known generically as papreg Decorative laminates are usually

com-posed of a combination of phenolic- and melamine-impregnated sheets

of paper The final properties of the laminate are related directly tothe properties of the paper from which the laminate is made

Early laminates were designated by trade names, such as Bakelite

of Union Carbide, Textolite of General Electric, Micarta of Westinghouse, Phenolite, Condensite, Dilecto, Haveg,

Spauldite, and Synthane Formica, of Formica Corp., designated

various types of laminates with a decorative facing layer for such uses

as tabletops Trade names now usually include a number or symbol todescribe the type and grade Textolite, for example, embraces morethan 70 categories of laminates subdivided into use-specification

grades, all produced in many sizes and thicknesses Textolite 11711

is an electronic laminate for such uses as multilayer circuit boards It

is made with polyphenylene oxide resin and may have a copper oraluminum cladding The tensile strength is up to 10,000 lb/in2 (69MPa), and the dielectric strength is 400 V/mil (16106 V/m)

Phenolyte Y240 is a paper-base laminate bonded with a

polyester-modified melamine which gives high dielectric strength and arc tance together with good punching or blanking characteristics in

resis-thicknesses up to 0.125 in (0.32 cm) Doryl H17511, of Westinghouse,

has glass fabric laminations bonded with a modified phenolic resinbased on diphenyl oxide and polyphenyl ether This laminate has aflexural strength of 27,400 lb/in2 (184 MPa) at 480°F (249°C)

Luxwood, of Formica Corp., for furniture, is a 0.0625-in (0.16-cm)

laminate with photographic reproductions of wood grains on the face,

while Beautywood is this material in thicker sizes for wall panels.

Industrial thermosetting laminates are availabile in the form of

sheet, rod, and rolled or molded tubing Impregnating resins commonlyused are phenolic, polyester, melamine, epoxy, and silicone The basematerial, or reinforcement, is usually one of the following: paper, wovencotton or linen, asbestos, glass cloth, or glass mat NEMA (NationalElectrical Manufacturing Association) has published standards coveringover 25 standard grades of these laminates Each manufacturer, inaddition to these, usually produces a range of special grades

Laminating resins may be marketed under one trade name by theresin producer and other names by the molders of the laminate

Paraplex P resins, of Rohm & Haas, for example, comprise a series

of polyester solutions in monomeric styrene which can be blended

with other resins to give varied qualities But Panelyte, of St Regis

Paper Co., refers to the laminates which are made with phenolic,melamine, silicone, or other resin, for a variety of applications

PLASTIC POWDER COATINGS. Although many different plastic ders can be applied as coatings, vinyl, epoxy, and nylon are most oftenused Vinyl and epoxy provide good corrosion and weather resistance

pow-as well pow-as good electrical insulation Nylon is used chiefly for its standing wear and abrasion resistance Other plastics frequentlyused in powder coating include chlorinated polyethers, polycarbon-ates, acetals, cellulosics, acrylics, and fluorocarbons

out-Several different methods have been developed to apply these

coat-ings In the most popular process, fluidized bed, parts are preheated

and then immersed in a tank of finely divided plastic powders, whichare held in a suspended state by a rising current of air When thepowder particles contact the heated part, they fuse and adhere to thesurface, forming a continuous, uniform coating Another process,

electrostatic spraying, works on the principle that oppositely

charged materials attract each other Powder is fed through a gun,which applies an electrostatic charge opposite to that applied to thepart to be coated When the charged particles leave the gun, they areattracted to the part, where they cling until fused together as a plas-tic coating Other powder application methods include flock and flowcoating, flame and plasma spraying, and a cloud-chamber technique

PLASTICS. A major group of materials that are primarily talline hydrocarbon substances composed of large molecular chains

noncrys-whose major element is carbon The three terms—plastics,

poly-mers, and resins—are sometimes used interchangeably to identify

these materials However, the term plastics has now come to be the

commonly used designation

The first commercial plastic, Celluloid, was developed in 1868 to replace ivory for billiard balls Phenolic plastics, developed by

Baekeland and named Bakelite after him, were introduced around the

turn of the twentieth century A plastic material, as defined by the

Society of the Plastics Industry, is “any one of a large group of materialsconsisting wholly or in part of combinations of carbon with oxygen,hydrogen, nitrogen, and other organic and inorganic elements which,while solid in the finished state, at some stage in its manufacture ismade liquid, and thus capable of being formed into various shapes, mostusually through the application, either singly or together, of heat andpressure.”

There are two basic types of plastics based on intermolecular

bond-ing Thermoplastics, because of little or no cross-bonding between

molecules, soften when heated and harden when cooled, no matter how

often the process is repeated Thermosets, on the other hand, have

strong intermolecular bonding Therefore, once the plastic is set intopermanent shape under heat and pressure, reheating will not soften it.Within these major classes, plastics are commonly classified on the

basis of base monomers There are over two dozen such monomer

families or groups Plastics are also sometimes classified roughly intothree stiffness categories: rigid, flexible, and elastic Another method

of classification is by the “level” of performance or the general area ofapplication, using such categories as engineering, general-purpose,and specialty plastics, or the two broad categories of engineering andcommodity plastics

In general, plastics are lightweight, are dielectric, and provide lowthermal conductivity They are resistant to many environments, havelow softening temperatures, and can be formed into complex shapes.They are also viscoelastic—that is, after an applied load is removed,plastics tend to continue to exhibit strain or deformation with time.Plastics can be built of one, two, or even three different monomers,

and are termed homopolymers, copolymers, and terpolymers,

respectively Their geometric form can be linear or branched Linear

or unbranched polymers are composed of monomers linked end to end

to form a molecular chain that is like a simple string of beads or apiece of spaghetti Branched polymers have side chains of moleculesattached to the main linear polymer These branches can be composedeither of the basic linear monomer or of a different one If the side

molecules are arranged randomly, the polymer is atactic; if they

branch out on one side of the linear chain in the same plane, the

poly-mer is isotactic; and if they alternate from one side to the other, the polymer is syndiotactic.

Plastics are produced in a variety of different forms Most common are

plastic moldings, which range in size from less than 1 in to several

feet (2 cm to several meters) Thermoplastics, such as polyvinylchloride

and polyethylene, are widely used in the form of plastic film and

plas-tic sheeting The term film is used for thicknesses up to and including

10 mils (0.25 cm), while sheeting refers to thicknesses over that

Rigid-rod polymers are high-performance thermoplastics characterized by

long, stiff molecules and high strength and stiffness Until the 1992development by Maxdem Inc of injection and compression moldable and

extrudable Poly-X grades, they could only be spun into fibers or cast as

film from acid solutions The moldable grades, described as substitutedchains of benezene rings called polyparaphenylene, have tensile moduli

of 0.9  106 lb/in2 to 2.6  106 lb/in2 (6,205 to 17,927 MPa) Potentialuses include printed-circuit boards, electronic connectors, scratch-resis-tant windows, lenses, bearings, bushings, and valves

Both thermosetting and thermoplastic materials are used as

plas-tic coatings on metal, wood, paper, fabric, leather, glass, concrete,

ceramics, or other plastics There are many coating processes, ing knife or spread coating, spraying, roller coating, dipping, brush-ing, calendering, and the fluidized-bed process Thermosetting

includ-plastics are used in high-pressure laminates to hold together the

reinforcing materials that comprise the body of the finished product.The reinforcing materials may be cloth, paper, wood, or glass fibers.The end product may be plain, flat sheets, or decorative sheets as incountertops, rods, tubes, or formed shapes

PLASTICS ADDITIVES. Almost all plastics contain one or more tive materials to improve their physical properties and processingcharacteristics or to reduce costs There are a wide range of additivesfor use with plastics, including antimicrobials, antistatic agents, clar-ifiers, colorants, fillers, flame retardants, foaming agents, heat stabi-lizers, impact modifiers, light stabilizers, lubricants, mold-releaseagents, odorants, plasticizers, reinforcements, and smoke retardants

addi-Fillers are probably the most common of the additives They are

usually used to either provide bulk or modify certain properties.Generally, they are inert and thus do not react chemically with theresin during processing The fillers are often cheap and serve to reduce

costs by increasing bulk For example, wood flour, a common low-cost

filler, sometimes makes up 50% of a plastic compound Other typicalfillers are chopped fabrics, asbestos, calcium carbonate, talc, gypsum,and milled glass Besides lowering costs, fillers can improve properties.For example, asbestos increases heat resistance, and cotton fibersimprove toughness

Plasticizers are added to plastics compounds either to improve

flow during processing by reducing the glass transition temperature

or to improve properties such as flexibility Plasticizers are usuallyliquids that have high boiling points Polyvinyl chloride accounts for

about 80%, but phthalates and polyesters are also used Stabilizers

are added to plastics to help prevent breakdown or deteriorationduring molding or when the polymer is exposed to sunlight, heat,oxygen, ozone, or combinations of these Thus there are a widerange of compounds, each designated for a specific function.Stabilizers can be metal compounds, based on tin, lead, cadmium,barium, and others And phenols and amines are added antioxidantsthat protect the plastic by diverting the oxidation reactions to them-selves The trend, however, is away from toxic lead and cadmium forenvironmental reasons For heat stabilizers, the lead can be diluted

by adding calcium-zinc compounds These compounds are intendedmainly for use with polyvinyl chloride for window profiles and pipe

Some are also alternatives for barium-zinc compounds, which arealternatives for cadmium heat stabilizers.

Catalysts, by controlling the rate and extent of the polymerization

process in the resin, allow the curing cycle to be tailored to the cessing requirements of the application Catalysts also affect the shelflife of the plastics Both metallic and organic chemical compounds are

pro-used as catalysts Colorants, added to plastics for decorative

pur-poses, come in a wide variety of pigments and dyestuffs Colorantshave been metal-base pigments such as cadmium, lead, and selenium,but here, too, environmental concerns have led to a trend away fromthese metals More recently, liquid colorants, composed of dispersions

of pigments in a liquid, have been developed Titanium dioxide, themost common pigment, is usually surface treated (coated) to aid dis-persion in processing and reduce power in mixing Alumina is themost common coating; silica and zirconia are also used Organic coat-ings include polyols, the most common, plus amines, siloxanes, andphosphated fatty acids

Clarifiers are used to impart clarity to plastics for packaging and

other applications Sorbitol, for example, is used for polypropylene in food packaging and other uses Coupling agents based on

organometallic titanates, zirconates, and aluminates bond to the face of glass, carbon, aramid, and other reinforcements for plastics,

sur-making them more dispersible Silane coupling agents, for example, are used for both thermosets and thermoplastics Flame retardants

are added to plastic products that must meet fire-retardant ments, because polymer resins are generally flammable, except forsuch notable exceptions as polyvinyl chloride In general, the function

require-of fire retardants is limited to the spread require-of fire They do not normallyincrease heat resistance or prevent the plastic from charring or melt-ing Some fire-retardant additives include compounds containingchlorine or bromine, phosphate-ester compounds, antimony thrioxide,alumina trihydrate, magnesium hydroxide, ammonium phosphatesand melamine amyl phosphate, and zinc borate Halogen compoundshave long dominated but have lost favor in recent years due to con-cern over generating toxic combustion products, such as bromides,hydrochloric acid, and brominated dioxins and furans, on burning.Thus, the use of halogens has been reduced (the most widely usedhaving contained as much as 80%) or eliminated

Reinforcement materials in plastics are not normally considered

additives Usually in fiber or mat form, they are used primarily toimprove mechanical properties, particularly strength Althoughasbestos and some other materials are used, glass fibers are the pre-dominant reinforcement for plastics

PLATINUM. A whitish-gray metal, symbol Pt It is more ductile thansilver, gold, or copper and is heavier than gold The melting point is3217°F (1769°C), and the specific gravity is 21.45 The Brinell hard-ness of the annealed metal is 45, and its tensile strength is 17,000lb/in2 (117 MPa); when hard-rolled, the Brinell hardness is 97 andtensile strength 34,000 lb/in2 (234 MPa) Electrical conductivity isabout 16% that of copper The metal has a face-centered-cubic latticestructure, and it is very ductile and malleable It is resistant to acidsand alkalies, but dissolves in aqua regia Platinum is widely used injewelry, but because of its heat resistance and chemical resistance it

is also valued for electrical contacts and resistance wire, ples, standard weights, and laboratory dishes Generally too soft foruse alone, it is almost always alloyed with harder metals of the samegroup, such as osmium, rhodium, and iridium An important use of

thermocou-the metal, in thermocou-the form of gauze, is as a catalyst Platinum gauze is

of high purity in standard meshes of 45 to 80 per inch (18 to 31 percentimeter), with wire from 0.0078 to 0.003 in (0.020 to 0.008 cm) in

diameter Dental foil is 99.99% pure and of maximum softness.

Platinum foil for other uses is made as thin as 0.0002 in (0.0005

cm) Platinum powder comes in fine submesh particle size It is

made by chemical reduction and is at least 99.9% pure, with phous particles 12 to 138
in (0.3 to 3.5
m) in diameter Atomizedpowder has spherical particles of 50 to 200 mesh, and is 99.9% pure

amor-and free-flowing Platinum flake has the powder particles in the

form of tiny laminar platelets which overlap in the coating film The

particles in Platinum flake No 22 have an average diameter of 118

in (3
m) and thickness of 3.9
in (0.1
m)

Because of the high resistance of the metal to atmospheric

corro-sion even in sulfur environments, platinum coatings are used on

springs and other functioning parts of instruments and electronicdevices where precise operation is essential Electroplating may be

done with an electrolyte bath of platinum dichloride, PtCl2, or

platinum tetrachloride, PtCl4 Hard plates may be produced with

an alkaline bath of platinum diamine nitrite, Pt(NH3)2(NO2)2.Coatings are also produced by vapor deposition of platinum com-pounds; thin coatings, 0.0002 in (0.0005 cm) or less, are made bypainting the surface with a solution of platinum powder in an organicvehicle and then firing to drive off the organic material, leaving anadherent coating of platinum metal

Platinum occurs in small, flat grains or in pebbles usually in alluvialsands, and the native metal generally contains other metals of the plat-inum group The largest nugget ever found came from South America andweighed 2 lb (0.9 kg) The chief sources of the metal are Russia andColombia, with smaller amounts from Alaska, Canada, and South Africa

Some platinum is obtained from the copper-nickel ores of Canada andSouth Africa There are no commercial ores of the metal, but the rare

mineral sperrylite is found in Wyoming and in Ontario It is a platinum

arsenide, PtAs2, found in small grains of a tin-white metallic luster The

only other known natural compound is the rare mineral cupperite, which is a platinum sulfide, PtS The Russian platinum is 99.8 to 99.9%

pure, with some iridium Platinum is sold by the troy ounce (0.03 kg), 1

in3(16 cm3) of the metal weighing 11.28 troy oz (0.34 kg)

PLATINUM ALLOYS Platinum-iridium alloys are employed for

instruments, magneto contacts, and jewelry The alloys are hard,tough, and noncorrosive An alloy of 95% platinum and 5 iridium,when hard-worked, has a Brinell hardness of 170; an alloy with 30%iridium has a Brinell hardness of 400 The 5 and 10% alloys are usedfor jewelry manufacture; the 25 and 30% alloys are employed formaking surgical instruments An alloy of 80% platinum and 20 irid-ium is used for magneto contact points, and the 90–10 alloy is widelyused for electrical contacts in industrial control devices The addition

of iridium does not alter the color of the platinum The 5% alloy solves readily in aqua regia; the 30% alloy dissolves slowly

dis-Platinum-cobalt alloys, with about 23% cobalt, are used for

per-manent magnets Platinum-nickel alloys, with as much as 20%

nickel, are noted for high strength With 5% nickel, for example, sile strength of the annealed alloy is about 90,000 lb/in2 (621 MPa),and with 15% it increases to 130,000 lb/in2 (896 MPa) Strengthalmost doubles with appreciable cold work

ten-Platinum-rhodium alloys are used for thermocouples for

temper-atures above 1100°C (2012°F) The standard thermocouple is platinumversus platinum–10% rhodium Other thermocouples for higher oper-ating temperatures use platinum-rhodium alloys in both elements.The alloys of platinum-rhodium are widely used in the glass industry,particularly as glass-fiber extrusion bushings Rhodium increases thehigh-temperature strength of platinum without reducing its resistance

to oxidation Platinum-rhodium gauze for use as a catalyst in

pro-ducing nitric acid from ammonium contains 90% platinum and 10

rhodium Platinum-rhenium alloys are efficient catalysts for

re-forming operations on aromatic compounds The platinum alloyshave lower electrical conductivity than pure platinum, but are gener-ally harder and more wear-resistant and have high melting points Aplatinum-rhenium alloy with 10% rhenium has an electrical conduc-tivity of only 5.5% that of copper compared with 16% for pure plat-inum Its melting point is 3362°F (1850°C), and the Rockwell Thardness of the cold-rolled metal is 91 compared with 78 for cold-rolled

platinum Platinum-ruthenium alloy, with 10% ruthenium, has a

melting point of 3272°F (1800°C) and an electrical conductivity 4%that of copper.

Platinum-tungsten alloys, with 2 to 8% tungsten, have been

used for aircraft-engine spark plug electrodes, radar-tube grids,strain gages, glow wires, switches, and heating elements The tung-sten markedly increases electrical resistivity while decreasing thetemperature coefficient of resistivity It also substantially increasestensile strength—to about 130,000 lb/in2(896 MPa) for platinum (8%tungsten alloy in the annealed condition)—and tensile strength morethan doubles with appreciable cold work

PLYWOOD. A laminated wood made up of thin sheets of wood gluedtogether with the grains of successive layers set at right angles togive strength in both directions Plywood is an outgrowth of the lami-

nated wood known as veneer, which consists of an outside sheet of

hardwood glued to a base of lower-cost wood The term veneer

actu-ally refers only to the facing layer of selected wood used for artisticeffect, or for economy in the use of expensive woods Veneers are gen-erally marketed in strip form in thicknesses of less than 0.125 in(0.32 cm) in mahogany, oak, cedar, and other woods The usual pur-pose of plywood now is not aesthetic but to obtain high strength with

low weight The term laminated wood generally means heavier

lami-nates for special purposes, and such lamilami-nates usually contain aheavy impregnation of bonding resin which gives them more of thecharacteristics of the resin than of the wood

Plywood usually comes in 4- by 8-ft (1.2- by 2.4-m) panels and isalways built up with an odd number of layers The cross-ply construc-tion gives strength in both directions and also gives symmetricshrinkage stresses A three-ply softwood panel of equal ply thick-nesses may shrink about 0.080 in (0.203 cm) in width and 0.100 in(0.254 cm) in length, but increasing the thickness of the core ply canequalize the shrinkage; or equalization may be obtained by increasingthe number of plies The odd number of plies gives a symmetry of con-struction about the core ply Low-cost plywoods may be bonded withstarch pastes, animal glues, or casein, and they are not water-resistant,but are useful for boxes and for interior work Waterproof plywood forpaneling and general construction is now bonded with synthetic resins,but when the plies are heavily impregnated with the resin and the

whole is cured into a solid sheet, the material is known as a hardboard

or as a laminated plastic rather than a plywood.

For construction purposes, where plywood is employed because of itsunit strength and nonwarping characteristics, the plies may be of asingle type of wood and without a hardwood face The Douglas FirPlywood Association sets up four classes of construction plywood

under general trade names Plywall is plywood in wallboard grade;

Plypanel is plywood in three standard grades for general uses; Plyscord is unsanded plywood with defects plugged and patched on

one side; and Plyform is plywood in a grade for use in concrete forms.

The bulk of commercial plywood comes within these classes, thevariations being in the type of wood used, the type of bonding adhe-

sive, or the finish Etch wood, for example, is a paneling plywood

with the face wire-brushed to remove the soft fibers and leave the

hard grain for two-tone finish Paneling plywoods with faces of

mahogany, walnut, or other expensive wood have cores of lower-costwoods, but woods with good physical qualities are usually chosen Thetensile strength of a white ash three-ply plywood parallel to the grain

of the faces is about 6,200 lb/in2 (43 MPa), that of a mahogany wood is 6,400 lb/in2 (44 MPa), and that of a walnut plywood is 8,200lb/in2 (57 MPa) Stabilite, of Georgia Pacific Corp., is a wood lami-

ply-nate with the veneers impregply-nated with phenolic resin and bondedtogether with the grain of the plies parallel It has the density ofhardwood and the workability of cherry wood

The K-Veneer used during the Second World War as a substitute

for plywood was a 0.1875-in (0.478-cm) fir or hemlock sheet bonded

to a heavy kraft paper Welchboard is 0.375-in (0.953-cm) plywood

with a smooth, grainless surface produced by curing on one sideunder heat and pressure a mixture of wood pulp and synthetic resin

A great variety of trade-name plywoods are marketed for paneling,but they do not always have the typical characteristics of plywood

and are often paneling boards rather than plywoods Some have

metal faces, or they may have special-purpose cores or backings

Fybr-Tech, developed by Technical Plywoods for aircraft paneling,

has a 0.0156-in (0.038-cm) walnut veneer on a 0.25-in (0.635-cm)balsa wood core, with a 0.005-in (0.013-cm) vulcanized fiber back.The weight is 0.2 lb/ft2 (0.98 kg/m2) One of the earliest aluminum-

faced plywoods was the English Plymax of Venesta, Ltd

Metal-faced plywoods are strong and can be riveted Metalite,

developed by United Aircraft Corp., has thin sheets of strong minum alloy bonded to both sides of a balsa wood core, the grain of

alu-the wood being perpendicular to alu-the metal faces Siding-panel 15,

of Weyerhauser Co., has a Douglas fir plywood core and a facing of0.01-in (0.025-cm) embossed aluminum with a vinyl resin finish

Flexwood consists of very thin sheets of veneer glued under heat

and pressure to cotton sheeting, used as an ornamental covering for

walls Algonite is Masonite faced with fancy veneers Protekwood,

designed for protection against vermin attack, has a sheet of wood between two sheets impregnated with asphalt and resin andbonded with a urea-formaldehyde resin to a total thickness of 0.156

in (0.396 cm) Parkwood is a flexible, woven veneer made with

thin strips of mahogany or other fancy wood pressed between sheets

of transparent cellulose acetate or other plastic Novoply, for panels,

furniture, and structural parts, has a core of resin-impregnated woodchips bonded between hardwood veneers Sheets are up to 0.75 in(1.91 cm) thick and have high strength

POISON GASES. Substances employed in chemical warfare for abling people, and in some cases used industrially as fumigants Theyare all popularly called gases, but many are liquids or solids Normally,information on military gases is kept secret, but the tear gases used bypolice are also poisonous, often causing serious damage to eyes, throat,

dis-and lungs Anesthetic gases have not been used so far in chemical warfare, but are used in medicine One of the simplest of these, nitrous

oxide, N2O, called laughing gas, produces a deep sleep Fluorthane,

or ethyl fluoride, is a volatile liquid like ether, but is nonexplosive, and

it is used to replace ether in surgery Cyclopropane is a potent

anes-thetic It is not less than 99% by volume of C3H6

Poison gases are classified according to their main effect on thehuman system, but one gas may have several effects They are grouped

as follows: lethal gases, intended to kill, such as phosgene;

lachryma-tors, or tear gases, which have a powerful irritating effect on the eyes,

causing temporary blindness and swelling of the eyes with a copious

flow of tears; vesicants, or skin blisterers, such as lewisite and tard gas; sternutatory gases, which induce sneezing; and camou-

mus-flage gases, which are harmless, but cause soldiers to suffer the

inconvenience of wearing gas masks and thus reduce their morale.Some of the gases have a sour, irritating odor and are also classified as

harassing agents Gases are also sometimes designated as casualty agents and further subdivided into persistent and nonpersistent A systemic gas is one that interferes with one phase of the system, such

as carbon monoxide, which paralyzes the respiratory function of the

blood A laryrinthic gas is one that affects an organ of the body, such

as dichlormethyl ether, which affects the ears.

Effects of persistent gases, such as mustard, remain over theground for as long as 7 days, but phosgene is quickly decomposed by

dampness Obscuring agents, such as white phosphorus, and the

toxic smokes, such as diphenylaminochloroarsine, are also classified

as war gases Dusts of materials having catalytic properties, but notpoisonous themselves, may be used to penetrate gas masks and createpoisons, such as carbon monoxide, within the mask Carbon and oilsmokes may be used to choke the filters of gas masks and cause theirremoval Absorbents used in gas masks are usually activated charcoaland soda lime These will absorb or disassociate most of the toxic

gases, but will not stop carbon monoxide A mixture of powdered

oxides of copper, manganese, silver, and cobalt, called Hopcalite, is

used as a catalyst to oxidize carbon monoxide

Lethal gases are divided into four classes: actual poisons, such as

hydrocyanic acid, which kill with little pain; asphyxiating gases,

such as phosgene, diphosgene, and chloropicrin, which affect themembranes of lungs, destroying them and allowing blood to fill theair sacs; poisons which destroy the lining of air passages and blockthe passages to lung tissues, such as mustard gas and ethyl-dichloroarsine; and poisons which affect the nose and throat, causinggreat pain, headache, and vomiting, such as diphenylchloroarsine

Mustard gas, (CH2ClCH2)2S, also known as blister gas, yperite, and yellow cross, is an oily liquid which boils at 410°F (210°C) and

vaporizes easily in the air It destroys the cornea of the eyes, blistersskin, affects the lungs, and causes discharge from the nose and vomit-ing One part in 14 million parts air is toxic and cannot be detected in

dilutions by smell Another powerful vesicant is Bromlost, which is

dibromethyl sulfide, (CH2BrCH2)2S It is a solid melting at 70°F

(21°C) Sulvanite is ethylsulfuryl chloride, ClSO3C2H5 It is a orless liquid boiling at 275°F (135°C)

col-Lewisite, CHCL:CH AsCL2, is a liquid boiling at 374°F (190°C)

It is a powerful vesicant, causing painful blisters on skin, pain in theeyes, nose irritation, permanent impairment of eyesight, and arsenic

poisoning It forms a heavy mist and has been called dew of death.

Chloropicrin, called aquinite, klop, and nitrochloroform, is nitrotrichloromethane, CCl3NO2 It is a persistent lachrymatoryand lethal poison It is a colorless liquid boiling at 234°F (112°C), with

a specific gravity of 1.692 It is used as a soil fumigant to control

insects and fungi Tonite is chloroacetone, CH3COCH2Cl, a clearliquid vaporizing at 246°F (119°C) It is a powerful lachrymator andskin blisterer As it is very reactive, it is also used in the synthesis ofpharmaceuticals, dyes, and organic chemicals Also used in largequantities as chemical intermediaries are the phosphorus chemicals,

phosphorus oxychloride and phosphorus trichloride Among

the toxic cyanides of industrial importance are hydrogen cyanide and cyanogen chloride Thiodiglycol, which is structurally close

to mustard gas, is a raw material used widely in industry

Bromoacetone is a colorless liquid of composition CH2BrCOCH3,with a specific gravity of 1.631 and boiling point 259°F (126°C) It isthrown in bombs or shells and disseminated as a mist which attacks the

eyes Bromobenzyl cyanide is a solid of composition BrC6H4CH2CN,with a melting point of 77°F (25°C); when impure, it is a liquid, but it isnot purified, as it is easily decomposed It is very persistent and because

of its odor is classified as a harassing agent It was called camite by the French Another tear gas, chloroacetophenone, C6H5COCH2Cl, is a

white, crystalline solid, specific gravity 1.321, and melting point 138°F(59°C), which, when thrown as a vapor, has a sweet, locustlike odor butproduces pains in the eyes and temporary blindness The tear gas called

Mace, used for riot control, contains this substance In contact with the

skin, it causes severe dermatitis and often permanent damage to the

eyes The French gas fraissite is benzyl iodide, C6H5CH2I, a liquid

boiling at 439°F (226°C) The gas papite is acrolein with stannic ride Calderite is benzyl bromide with stannic chloride Xylyl bro-

chlo-mide, CH3C6H4CH2Br, is a colorless liquid boiling at 421°F (216°C).When disseminated as a mist from explosive bombs, it causes a copious

flow of tears The tear gas known as CS gas is chlorobenzol

mal-ononitrile and is destructive to nasal and lung tissues Benzyl chloride

also has been used as a tear gas

Martonite, a powerful lachrymator, is a mixture of 20%

chloroace-tone and 80 bromoacechloroace-tone Bretonite is iodoacechloroace-tone, CH3COCH2I,

a brownish liquid boiling at 216°F (102°C), mixed with stannic

chlo-ride as a lachrymator Manguinite is cyanogen chlochlo-ride, CNCl,

which boils at 55°F (13°C) and is a lachrymator Mixed with arsenic

trichloride to make it more toxic, it was used under the name of

vit-rite Campillit is cyanogen bromide, CNBr, a white solid melting

at 126°F (52°C) and vaporizing at 142°F (61.3°C) The fumes are

highly toxic, paralyzing the nerve centers Diphenylchloroarsine,

or blue cross, (C6H5)2AsCl, is a sneezing gas which penetrates gas

masks, forcing their removal It affects chiefly the nose and throat,

but is used with other more violent gases Adamsite is a greenish,

granular solid of composition (C6H4)2NHAsCl which has a pleasantodor but burns the nose and throat Many of the lachrymators have

important industrial uses Phenyl isocyanate, C6H5NCO, is awater-white liquid of specific gravity 1.101 and boiling point 324°F(162°C), used for the production of alkyd resins, ureas, urethanes,

and other chemicals Decontaminants, used for combating the

effects of poison gases, are neutralizing chemicals The

decontami-nant known as STB, or supertropical bleach, is a mixture of

chlori-nated lime and ground quicklime

Another recent poison-gas category is for supertoxic lethal

weapons, defined as chemicals with a median lethal dose less than or

equal to 6.4  106oz/lb (0.5 mg/kg) by subcutaneous administration, or0.156 lb/(min ft3) [2,000 mg/(min m3)] by inhalation Among these are

Tabrun GA, or ethyl N,N-dimethylphosphoramidocyanidate; Sarin

GB, isopropyl methylphosphonofluoridate; Soman GD,

1,2,2-trimethyl-propyl methylphosphonofluoridate; BZ, bis 2-chloroethyl sulfide; VX, ethyl S-2-diisopropylamino-ethyl methylphosphonothiolate; pinacolyl

alcohol, 3,3-dimethylbutanol-2; Saxitoxin; mustard gas; and

methylphosphonylfluoride

POLONIUM. A rare metallic element, symbol Po, belonging to thegroup of radioactive metals, but emitting only alpha rays The melt-ing point of the metal is about 489°F (254°C) It is used in meteorolog-ical stations for measuring the electrical potential of air.

Polonium-plated metal in strip and rod forms has been employed

as a static dissipator in textile-coating machines The alpha rays ize the air near the strip, making it a conductor and drawing off static

ion-electric charges Polonium 210 is obtained by irradiating bismuth,

100 lb (45 kg) yielding 0.04 oz (1 g) of polonium 210 It is used as aheat source for emergency auxiliary power such as in spacecraft Themetal is expensive, but can be produced in quantity from bismuth

POLYARYLATES. These high-heat-resistant thermoplastics arederived from aromatic dicarboxylic acids and diphenols When molded,they become amorphous, providing a combination of toughness,dimensional stability, high dielectric properties, and uv stability.Polyarylates have heat-deflection temperatures up to 345°F (653°C) at

264 lb/in2(1.8 MPa) The resins can be injection-molded, extruded, andblow-molded, and sheet can be thermoformed These resins also areblended with other engineering thermoplastics and reinforcements

POLYARYLENE ETHER BENZIMIDAZOLES. PAEBI polymers, developed

at NASA-Langley in 1991, combine high-gloss transition temperatureand optical transmission with inherent resistance to high-energy par-ticles, especially atomic oxygen, thus being suitable for thermal con-trol in harsh space environments, such as low earth orbit Thoughreacting similarly to other plastics on initial exposure to atomic oxy-gen, PAEBI incorporates a phosphorus-oxygen linkage that subse-quently forms an in situ protective coating, making the plastic morethan 15 times as resistant as Kapton polyimide to erosion by atomic

oxygen The polymer is produced in the form of TOR and TOR-LM

castable films by Triton Systems Inc and, for thermal-control

appli-cations, can be metallized with aluminum, silver, or Inconel

POLYCARBONATE RESINS. Traditionally made by reacting bisphenol A(BPA) and phosgene or by reacting a polyphenol with methylene chlo-ride and phosgene The monomer may be OC6H4C(CH3)2C6H4OC:O In

a continuous, solventless, melt-phase process recently developed byGeneral Electric Plastics Japan and Japan’s Mitsui PetrochemicalIndustries, polycarbonate is made by reacting diphenylcarbonate withBPA at 480 to 570°F (250 to 300°C) in the presence of a proprietary cat-alyst This approach eliminates the need for toxic phosgene gas withBPA dissolved in an excess of the chloride The molecular structure is

in double-linked zigzag chains that give high rigidity The resin is moplastic It is crystalline with rhombic crystals

Polycarbonate is a linear, low-crystalline, transparent, lar-weight plastic It is generally considered to be the toughest of allplastics In thin sections, up to about 0.1875 in (0.478 cm), its impactstrength is as high as 16 ft lb (22 J) In addition, polycarbonate is one

high-molecu-of the hardest plastics It has good strength and rigidity and, because high-molecu-ofits high modulus of elasticity, is resistant to creep These properties,along with its excellent electrical resistivity, are maintained over a tem-perature range of about 275 to 250°F (170 to 121°C) It has negligi-ble moisture absorption, but it also has poor solvent resistance and, in astressed condition, will craze or crack when exposed to some chemicals

It is generally unaffected by greases, oils, and acids Polycarbonateplastics are easily processed by extrusion; by injection, blow, and rota-tional molding; and by vacuum forming They have very low and uni-form mold shrinkage With a white light transmission of almost 90%and high impact resistance, they are good glazing materials They havemore than 30 times the impact resistance of safety glass Other typicalapplications are safety shields and lenses Besides glazing, polycarbon-ate’s high impact strength makes it useful for air-conditioner housings,filter bowls, portable tool housings, marine propellers, and housings forsmall appliances and food-dispensing machines

Lexan, of General Electric Plastics, is the most common trade

name Others include Apec and Makrolan of Bayer, Calibre of Dow Chemical, Inpilon of Mitsubishi Gas, and Panlite of Teijin Chemical Lexan F-8000 is a polycarbonate sheet that can be lami- nated with polyvinyl fluoride film, such as Du Pont’s Tedlar, and

thermoformed Among other applications, it is intended for aircraft

cabin windows Lexan Nu-View is a glazing laminate intended to keep mass-transit windows free of graffiti It comprises Lexan MR5

sheet with a 0.10-in (2.54-mm)- thick abrasion-resistant film oneither face or both faces If the film is marked, it can be easily

removed, leaving a clean surface Krystaflex, of Morton

International, is a laminate of transparent polyurethane between twopolycarbonate sheets It is intended as a glass replacement in banks,

prisons, and other facilities Panlite LN-2250, of Japan’s Teijin

Chemical, is a low-bromine, flame-retardant polycarbonate

Calibre 1080 DVD, for digital video disks and other optical media

products, features purity, clarity, and dimensional stability for tion and toughness for damage resistance Birefringence (measure oflight propagation through the polycarbonate medium) is low, reducingnoise and signal errors Light transmission is 91%, the refractiveindex 1.583%, ultimate tensile strength 7,000 lb/in2 (48 MPa), and

replica-Izod impact strength is 5 ft.lb/in (267 J/m) Forex-EPC, from

Alusuisse Airex AG of Switzerland, is expanded foam sheet of closed,homogeneous cell structure It weighs only one-half as solid sheet, is

impact resistant to 148°F (100°C) and heat resistant to 284°F(140°C), and can be vacuum-formed to crisp shapes.

POLYESTER THERMOPLASTIC RESINS. There are several types ofmelt-processible thermoplastics, including polybutylene terephtha-late, polyethylene terephthalate, polyethylene naphthalate, and aro-matic copolyesters

Polybutylene terephthalate (PBT) is made by the

transesterfi-cation of dimethyl terephthalate with butanediol through a catalyzedmelt polycondensation These molding and extrusion resins have goodresistance to chemicals, low moisture absorption, relatively high con-tinuous-use temperature, and good electrical properties (track resis-tance and dielectric strength) PBT resin is sensitive to alkalies,oxidizing acids, aromatics, and strong bases Various additives, fillers,and fiber reinforcements are used with PBT resins, in particularflame retardants, mineral fillers, and glass fibers PBT resins andcompounds are used extensively in automotive, electrical and elec-tronic, appliance, military, communications, and consumer product

applications Valox is the trade name of General Electric Plastics’

PBT polyester There are many other producers, including BASFCorp., DSM Engineering Plastics, and Hoechst Celanese

Polyethylene terephthalate (PET) is a widely used

thermoplas-tic packaging material Beverage bottles and food trays for microwaveand convection oven use are the most prominent applications PETresins are made from ethylene glycol and either terephthalic acid orthe dimethyl ester of terephthalic acid Most uses for PET require thematerial’s molecular structure to be oriented Orientation of PET sig-nificantly increases tensile strength and reduces gas permeabilityand water vapor transmission For packaging uses, PET is processed

by blow molding and sheet extrusion Typical trade names are

Cleartuf and Traytuf (Goodyear Tire & Rubber Co.), Tenite and Kodapak (Eastman Chemical Products), Melinar (ICI Americas),

and Selar (Du Pont) Crisper, a synthetic paper of Toyobo of Sanyo

Corp of America, is pearlescent PET film having excellent

printabil-ity and good wear and ultraviolet resistance Spectar PETG, of

Eastman Chemical Products, is an extrudable copolyester for thick [to0.5-in (13-mm)] sheet Its impact strength is superior to that ofacrylic, approaching that of polycarbonate Color quality is equivalent

to that of polycarbonate but less than that of acrylic, which is alsomore scratch-resistant Injection-molding grades of PET, with fillersand/or reinforcements, also are available for making industrial prod-ucts Producers include Allied Signal, DSM Engineering Plastics, DuPont, Eastman Chemical Products, General Electric Plastics, and

Hoechst Celanese Rynite CR grades, of Du Pont, contain

nonmetal-lic conductive ingredients for superior electrical and thermal

conduc-738 POLYESTER THERMOPLASTIC RESINS

tivity, heat dissipation, and electromagnetic shielding Ertalyte, of

Polymer Corp., comes in rod form for machined parts

Polyethylene naphthalate (PEN), of Amoco Chemicals Co., is

based on the naphthalate monomer dimethyl 2,6-naphthalate It isintended for copolymers and blends in packaging applications, such

as stretch-blow-molded bottles and, as a homopolymer, in thin andthick film for electrical and electronic applications Relative to PET,PEN bottles have demonstrated similar crystallinity, much betteroxygen and moisture barrier resistance, higher hot-fill temperature,and better appearance and dimensional stability after caustic wash-ing Replacing glass bottles in applications too demanding for PET is

a potential application Microwavable packaging is a potential use forthick film Magnetic tape is a current thin-film application.Candidate electrical and electronic applications for thick film includeprinted-circuit boards, capacitors, and membrane switches

Ektar FB, of Eastman Chemical Products, is a family of poly

1,4-cyclohexylenedimethylene terephthalate (PCT) polymers They arehigh-temperature, semicrystalline, thermoplastic polyesters with amelting point of 545°F (285°C) and service temperatures up to 340°F(171°C) Tensile strengths range up to 22,000 lb/in2 (152 MPa), withflexural moduli up to 19  105lb/in2(13,100 MPa) and heat-deflectiontemperatures at 264 lb/in2 (1.8 MPa) up to 523°F (273°C) They areintended for mechanical and electrical parts

Polyester film is generally made by the condensation of terephthalicacid and ethylene glycol The extremely thin film, 0.00025 to 0.0005 in(0.00063 to 0.0013 cm), used for capacitors and for insulation of motorsand transformers, has a high dielectric strength, up to 6,000 V/mil(236106V/m) It has a tensile strength of 20,000 lb/in2(138 MPa) withelongation of 70% It is highly resistant to chemicals and has low waterabsorption The material is thermoplastic, with a melting point of about

490°F (254°C) Polyester fibers are widely used in clothing fabrics The textile fiber produced from dimethyl terephthalate is known as Dacron.

For magnetic sound-recording tape, polyester tape has the cules oriented by stretching to give high strength The 0.005-in (0.013-cm) tape has a breaking strength of 120 oz (3.4 kg) per 0.25 in (0.64cm) of width Electronic tape may also have a magnetic-powder coat-ing on the polyester But where high temperatures may be encoun-tered, as in spacecraft, the magnetic coating is applied to metal tapes

mole-Aromatic polyesters (liquid crystals) have high mechanical

prop-erties and heat resistance Commercial grades are aromatic polyesters,which have a highly ordered or liquid-crystal structure in solution andmolten states A high degree of molecular orientation develops duringprocessing and, hence, anistropy in properties Typically, these melt-processible resins can be molded or extruded to form products capable

of use at temperatures over 500°F (260°C) Tensile strengths up to

POLYESTER THERMOPLASTIC RESINS 739

35,000 lb/in2 (241 MPa) and flexural moduli up to 4.6  106 lb/in2(31,717 MPa) are reported for liquid-crystal polymers Chemical resis-tance also is excellent Trade names for liquid-crystal polymers include

Vectra, of Hoechst Celanese, Xydar, of Amoco Performance Products,

and Granlar, of Granmont Applications are in chemical processing

and electronic, medical, and auto components

POLYESTER THERMOSETTING RESINS. A large group of syntheticresins produced by condensation of acids such as maleic, phthalic, oritaconic with an alcohol or glycol such as allyl alcohol or ethylene gly-col to form an unsaturated polyester which, when polymerized, willgive a cross-linked, three-dimensional molecular structure, which inturn will copolymerize with an unsaturated hydrocarbon, such asstyrene or cyclopentadiene, to form a copolymer of complex structure

of several monomers linked and cross-linked At least one of the acids

or alcohols of the first reaction must be unsaturated The polyestersmade with saturated acids and saturated hydroxy compounds are

called alkyd resins, and these are largely limited to the production

of protective coatings and are not copolymerized

The resins undergo polymerization during cure without liberation

of water and do not require high pressure for curing Through thesecondary stage of modification with hydrocarbons, a very widerange of characteristics can be obtained The most important use ofthe polyesters is as laminating and molding materials, especiallyfor glass-fiber-reinforced plastic products The resins have highstrength, good chemical resistance, high adhesion, and capacity totake bright colors They are also used, without fillers, as castingresins, for filling and strengthening porous materials, such asceramics and plaster of paris articles, and for sealing the pores inmetal castings Some of the resins have great toughness and areused to produce textile fibers and thin plastic sheet and film Otherresins are used with fillers to produce molding powders that cure atlow pressure of 500 to 900 lb/in2 (3 to 6 MPa) with fast operatingcycles

Polyester laminates are usually made with a high proportion

of glass-fiber mat or glass fabric, and high-strength reinforcedmoldings may contain a high proportion of filler A resin slurrymay contain as high as 70% calcium carbonate or calcium sulfate,with only about 11% of glass fiber added, giving an impactstrength of 24,000 lb/in2(165 MPa) in the cured material Bars andstructural shapes of glass-fiber-reinforced polyester resins of hightensile and flexural strengths are made by having the glass fibersparallel in the direction of the extrusion Rods and tubes are made

by having the glass-fiber rovings carded under tension, then ing through an impregnating tank, an extruding die, and a heat-

pass-740 POLYESTER THERMOSETTING RESINS

curing die The rods contain 65% glass fiber and 35 resin Theyhave a flexural strength of 64,000 lb/in2 (441 MPa) and a Rockwell

M hardness of 65

Physical properties of polyester moldings vary with the type ofraw materials used and the type of reinforcing agents A standardglass-fiber-filled molding may have a specific gravity from 1.7 to2.0, a tensile strength of 4,000 to 10,000 lb/in2 (28 to 69 MPa) withelongation of 16 to 20%, a flexural strength to 30,000 lb/in2 (207MPa), a dielectric strength to about 400 V/mil (16  106V/m), and aheat distortion temperature of 350 to 400°F (177 to 204°C) Themoldings have good acid and alkali resistance But since an almostunlimited number of fatty-type acids are available from naturalfatty oils or by synthesis from petroleum, and the possibilities ofvariation by combination with alcohols, glycols, and other materialsare also unlimited, the polyesters form an ever-expanding group ofplastics

Het acid, of Hooker Chemical, is a complex chlorinated phthalic

acid produced by hydrolyzing the product of the condensation ofmaleic anhydride with hexachlorocyclopentadiene made from pen-tane This acid is reacted with glycols and maleic anhydride to give

a hard polyester resin which is then cross-linked with styrene to

give the liquid Hetron resin, which will cure with heat and a

cata-lyst to an insoluble solid The resin contains 30% chlorine It is usedfor making laminated or reinforced plastics Another chlorinated

polyester resin is FR resin It is flame-resistant, cures at normal

temperatures, and is used for such layup lamination work as boatbuilding and tank construction

Some of the polyester-type resins have rubberlike properties, withhigher tensile strengths than the rubbers and superior resistance tooxidation These resins have higher wear resistance and chemicalresistance than GRS rubber They are made by reacting adipic acidwith ethylene glycol and propylene glycol and then adding diiso-cyanate to control the solidifying action They can be processed asrubber, but solidify more rapidly

An extensive group of polyesters and alkyd resins with good heat

stability can be made from pyromellitic dianhydride, C10H2O6, a

benzene tetracarboxylic dianhydride PMDA is a white powder

with melting point of 549°F (287°C) It reacts with alcohols,

ben-zene, and other hydrocarbons It is produced from mellitic acid,

C6(COOH)6, which has a melting point of 550°F (288°C) It occurs in

brown coal, peat, and the mineral mellite, or honeystone, which

is hydrous aluminum mellate Tetrahydrophthalic anhydride is

easier to combine with styrene than phthalic or maleic anhydrideand gives coating resins that are flexible and have quicker curewith high gloss

POLYESTER THERMOSETTING RESINS 741

POLYETHYLENES. Polyethylene thermoplastic resins include low-density polyethylenes (LDPEs), linear low-density polyethylenes(LLDPEs), high-density polyethylenes (HDPEs), and ethylene copolymers, such as ethylene-vinyl acetate (EVA) and ethylene-ethyl-acrylate (EEA), and ultra-high-molecular-weight polyethylenes(UHMWPEs) In general, the advantages gained with polyethylenesare light weight, outstanding chemical resistance, good toughness,excellent dielectric properties, and relatively low cost compared toother plastics The basic properties of polyethylenes can be modifiedwith a broad range of fillers, reinforcements, and chemical modifiers,such as thermal stabilizers, colorants, flame retardants, and blowingagents Further, polyethylenes are considered to be very easy to pro-cess by such means as injection molding, sheet extrusion, film extru-sion, extrusion coating, wire and cable extrusion coating, blowmolding, rotational molding, fiber extrusion, pipe and tubing extru-sion, and powder coating Major application areas for polyethylenesare packaging, industrial containers, automotive, materials han-dling, consumer products, medical, wire and cable insulation, furni-ture, housewares, toys, and novelties Quantum Chemical’s

carbon-black-filled Petrothene PR92941 LLDPE permits

laser-printing, which is more durable than traditional ink-jet printing

Glidestar compounds, from EM Corp for oil-filled, self-lubricated

products, include the polyethylene-based 400 Series

The basic building blocks for polyethylenes are hydrogen and bon atoms These atoms are combined to form the ethylene monomer,

car-C2H4, that is, two carbon atoms and four hydrogen atoms In the merization process, the double bond connecting the carbon atoms isbroken Under the right conditions, these bonds re-form with otherethylene molecules to form long molecular chains Ethylene copoly-

poly-mers ethylene-vinyl acetate (EVA) and ethylene-ethyl acrylate

(EEA) are made by the polymerization of ethylene units with

ran-domly distributed comonomer groups, such as vinyl acetate (VA) and ethyl acrylate (EA).

Three basic molecular properties affect most polyethylene ties: crystallinity (density), average molecular weight, and molecularweight distribution

proper-Molecular chains in polyethylenes in crystalline areas are arrangedsomewhat parallel to one another In amorphous areas, they are ran-

domly arranged High-density polyethylene resins have molecular

chains with comparatively few side chain branches Therefore, thechains are packed more closely together The result is crystallinity up

to 95% Chevron Chemical’s recent hexene-based HDPEs for blowmolded and pipe provide better performance than its butene-basedproducts, having specific gravities of 0.27 to 0.32, tensile strengths at

yield of 3,600 to 4,300 lb/in2(25 to 30 MPa), and elongations at breakexceeding 500%.

Low-density polyethylene (LDPE) resins have crystallinity

from 60 to 75% Linear low-density polyethylene (LLDPE)

resins have crystallinity from 60 to 85% For polyethylenes, thehigher the degree of crystallinity, the higher the resin density.Higher density, in turn, influences numerous properties Withincreasing density, heat-softening point, resistance to gas and mois-ture vapor permeation, and stiffness increase However, increaseddensity results in a reduction of stress cracking resistance and low-temperature toughness LLDPE resins have a specific gravity of0.915 to 0.940; LDPE resins range from 0.910 to 0.930; and HDPE

from 0.941 to 0.965 Dowlex NG 3347A LLDPE, an ethylene-octene

copolymer of Dow Plastics for stretch-film applications, provides up

to 225% stretch with flow and puncture resistance Dowlex NG 1P

grades are intended for injection-molded containers High-strengthfilm grades of LLDPE, based on Union Carbide’s (now Dow Plastics)and Unipol gas-phase polymerization technology, are said to provide

much higher dart-impact strength than previous Unipol copolymers.

NTX grades, of Mobil Polymers, are hexene LLDPE copolymers.

Novacor Chemicals refers to its Novapol LLDPE as “super-hexene” grades Others since are from Equistar Chemical, its Petrothene

Select providing 8,500 lb/in2(59 MPa) tensile strength at break and525% elongation Sumitomo Bakelite of Japan combines axially ori-ented Nylon 6 with ethylene-vinyl alcohol for flexible, stand-upwater and juice pouches

Dow Plastics’ and Exxon’s “single-site” metallocene is used for bothLDPE and HDPE, and is making LLDPE increasingly competitive inLDPE markets Metallocene-catalyzed polyethylene features narrowmolecular-weight and composition distribution It’s stronger and

tougher than the conventional and thus can be thinner Exxon’s Exceed

metallocene-LLDPE film grades for heavy-duty bags, liners, and trashbags have tensile strengths at yield of 1,260 to 1,340 lb/in2 (8.7 to 9

MPa) and ultimate elongations of 475 to 630% Plastomer refers to

polyethylene resins having greater elasticity than typical of plastics butless than that of elastomers Such plastics include certain Dow andExxon metallocene-catalyzed, low-density linear copolymers and more

recently Eastman Chemical’s Mxsten CV linear hexene copolymers for

blown and cast films in food and nonfood packaging applications.Besides low density (specific gravities of about 0.91), high stiffness, andgood Dart impact strength, they offer greater tack low-heat-sealingqualities

Atoms of different elements, such as carbon, hydrogen, etc., havedifferent atomic weights For carbon, the atomic weight is 12, and for

hydrogen it is 1 Thus, the molecular weight of the ethylene unit isthe sum of its six atoms (two carbon plus four hydrogen), or 28 Everypolyethylene resin consists of a mixture of large and small chains, i.e.,chains of high and low molecular weights The molecular weight ofthe polymer chain generally is in the thousands The average of these

is called the average molecular weight As average molecular weight

increases, resin toughness increases The same holds true for tensilestrength and environmental stress cracking resistance (crackingbrought on when molded parts are subjected to stresses in the pres-ence of liquids such as solvents, oils, detergents, etc.) Because ofincreasing melt viscosity, polyethylene resins become more difficult toprocess as the average molecular weight increases Melt index is ameasure of the polymer’s melt viscosity under standard conditions oftemperature and pressure Melt index is inversely related to theresin’s average molecular weight: As average molecular weightincreases, melt index decreases Generally, a polyethylene resin withhigh molecular weight has a low melt index, and vice versa Melt vis-cosity is an extremely important property since it affects the flow ofthe molten polymer The resin’s flow when melted increases withincreasing melt index

Molecular weight distribution is the relative distribution of large,medium, and small molecular chains in the polyethylene resin Whenthe distribution is made up of chains close to the average length, the

resin is said to have a narrow molecular weight distribution Broad

molecular weight distribution polyethylenes are those resins with a

wider variety of chain lengths In general, resins with narrow lar weight distributions will have good low-temperature impactstrength and low warpage Resins with broad molecular weight (MW)distributions generally will have greater stress cracking resistanceand greater ease of processing

molecu-High-molecular-weight low-density polyethylene resins are

used to extrude high-clarity, tough film used in shrink packaging and

for making heavy-duty bags High-molecular-weight high-density

polyethylene resins (MW 200,000 to 500,000) have excellent

envi-ronmental stress cracking resistance, toughness, high moisture rier properties, and high strength and stiffness The HMW-HDPEresins are used in film, pressure pipe, large blow moldings, and sheet

bar-for thermobar-forming Ultra-high-molecular-weight polyethylenes

generally are considered to be those resins with molecular weightsgreater than 2  106(materials with MW up to 6  106are available).These resins have excellent abrasion resistance, stress cracking resis-tance, and toughness They are, however, much more difficult toprocess than standard polyethylenes and require special formingtechniques UHMWPE resins are used in applications requiring high

wear resistance, chemical resistance, and low coefficient of friction.

Nonwoven fabric of UHMWPE Spectra fibers of Allied Signal is

used in a thermosetting matrix for soldiers’ helmets Made by CGSGallet SA of France, they are more ballistic-resistant than conven-tional steel helmets

Polyethylene of low molecular weight is used for extending andmodifying waxes, and in coating compounds especially to add tough-ness, gloss, and heat-sealing properties Such materials are called

polyethylene wax, but they are not chemical waxes They can be

made emulsifiable by oxidation, and they can be given additional

properties by copolymerization with other plastics The

polymethyl-ene waxes are microcrystalline and have sharper melting points

than the ethylene waxes They are more costly, but have high luster

and durability Polybutylene plastics are rubberlike polyolefins

with superior resistance to creep and stress cracking Films of thisresin have high tear resistance, toughness, and flexibility and areused widely for industrial refuse bags Chemical and electrical prop-erties are similar to those of polyethylene and polypropylene plastics

Polymethyl pentene is a moderately crystalline, polyolefin plastic

resin that is transparent even in thick sections Almost optically clear,

it has a light transmission value of 90% Parts molded of this plasticare hard and shiny with good impact strength down to 20°F(29°C) Specific gravity (0.83) is the lowest of any commercial solidplastic A major use is for molded food containers for quick-frozenfoods that are later heated by the consumer

POLYMERIC TRANSFLECTIVE MATERIALS. These materials consist ofhundreds or more of alternating coextruded layers of two dissimilar,optically clear thermoplastics, such as acrylic and polycarbonate, forlight management and decorative effects Developed by Dow Plastics,

they are referred to as transflective materials because they

simul-taneously transmit and reflect light The composite structure vides a reflective surface ranging from diffuse to spectral ormirrorlike Changes to the front and back layers create variouseffects, such as pearlescence

pro-POLYPHENYLENE OXIDE. A plastic that is notable for its highstrength and broad temperature resistance There are two major

types: polyphenylene oxide (PPO) and modified PPO (General Electric Plastics’ Noryl) These materials have a deflection tempera-

ture ranging from 212 to 345°F (100 to 174°C) at 264 lb/in2(1.8 MPa).Their coefficients of linear thermal expansion are among the lowestfor thermoplastics Room-temperature strength and modulus of elas-ticity are high, and creep is low In addition, they have good electrical

resistivity Their ability to withstand steam sterilization and theirhydrolytic stability make them suitable for medical instruments, elec-tric dishwashers, and food dispensers They are also used in the elec-trical and electronic fields and for business-machine housings.

Tensile strength and modulus of phenylene oxides rank high amongthermoplastics They are processed by injection-molding, extrusion,and thermoforming techniques Structural-foam grades, with theirhigh rigidity, are suitable for large structural parts Because of gooddimensional stability at high temperatures and under moisture condi-tions, these plastics are readily plated without blistering

POLYPHTHALAMIDE PPA is a family of semicrystalline

thermoplas-tics of Amoco Performance Products, with the trade name Amodel,

having advantages over conventional nylons in resistance to ity, auto fuels, water/glycol mixtures, and heat The melting tempera-ture is 590°F (310°C), and unreinforced, the resin has a specificgravity of 1.17, 0.81% 24-h water absorption, a tensile yield strength

humid-of 15,100 lb/in2(104 MPa), a flexural modulus of 475,000 lb/in2 (3,275MPa), a heat-deflection temperature of 248°F (120°C) at 264 lb/in2(1.8 MPa), and Izod notched impact strength of 1 ft lb/in (53 J/m) Anextra-tough grade is slightly lighter in weight, less water-absorbent(0.65%), and considerably less strong and rigid, but has an impactstrength of 20 ft lb/in (1,068 J/m) With 45% glass reinforcement, thespecific gravity is 1.56, tensile strength (at break) 38,000 lb/in2 (262MPa), flexural modulus 2.1  106 lb/in2(14,480 MPa), heat-deflectiontemperature 549°F (287°C), and impact strength 2.5 ft lb/in (133

J/m) Thermocomp UF-1006, of LNP Engineering Plastics, is a 30%

glass-reinforced grade based on Amodel Its continuous-use ture is 545°F (285°C) versus 480°F (249°C) for similarly reinforcedNylon 6/6 RTP Co makes a line of lubricated compounds based onPPA for gears and bearings Polytetrafluoroethylene and/or siliconeserve as lubricants, and glass, carbon, and/or aramid fibers as rein-

tempera-forcements An RTP 4000 grade with 15% aramid fiber, 14 PTFE,

and 2 silicone is the most wear-resistant

POLYPROPYLENE PLASTICS. An important group of synthetic tics employed for molding resins, film, and texture fibers Developed

plas-in 1957 plas-in Italy and Germany, they are produced as polypropylene

by catalytic polymerization of propylene, or they may be copolymers

with ethylene or other material Propylene is a methyl ethylene,

CH3CH:CH2, produced in the cracking of petroleum It belongs to the

class of unsaturated hydrocarbons known as olefins, which are

desig-nated by the word ending -ene Thus propylene is known as propene

as distinct from propane, the corresponding saturated compound of

the group of alkanes from petroleum and natural gas These

unsatu-rated hydrocarbons tend to polymerize and form gums, and are thusnot used in fuels although they have antiknock properties

In isotactic polypropylene, the most common, the carbon atoms

linked in the molecular chain between the CH2units each have a CH3and an H attached as side links, with the bulky side groups spiraledregularly around the closely packed chain The crystalline structureimparts rigidity, and the material has a melting point of 329°F(165°C) Strength and rigidity can be increased further by nucleation

or by broadening the molecular weight distribution and increasing

isotacticity, resulting in resins referred to as high-crystallinity

poly-propylenes Atactic polypropylene has a random molecular

struc-ture and lacks the crystallinity and rigidity required for manyapplications It is used as an adhesive or is mixed with other materi-

als, such as asphalt, for roofing applications Syndiotactic

polypropylene, whose side groups are arranged in an alternating

symmetric pattern, is a recent development based on the use of allocene catalysts The material is crystalline and has a lower meltingpoint than that of the isotactic type but better impact strength andclarity It has been produced by Mitsui Toatsu Chemicals of Japanand is suitable for film and fibers Metallocene-catalyzed polypropy-lene features narrow molecular-weight, tacticity, and composition dis-

met-tribution Metocene polypropylene X50081 is a high-flow grade

from Targor of Germany

Metocene X50149, a film grade having good heat-sealing tics and oxygen and moisture barrier resistance, combines the stiffness

characteris-of a homopolymer with the optical clarity characteris-of a random copolymer

Adding Exxon’s metallocene-catalyzed Exact 3024 plastomer to a

nucleated and clarified polypropylene stabilized for gamma sterilizationprecludes radiation embrittlement while maintaining clarity and heatresistance, performance qualities traditionally difficult to combine.Isotactic polypropylene includes homopolymers, copolymers, andrandom copolymers The homopolymers are used mainly for fibersand filaments, oriented and cast films, blow-molded bottles, andinjection moldings Fiber and filament uses include textiles and car-pets, bags, netting, and decorative ribbons In film applications theyprovide high tensile and tear strengths, stiffness, clarity, gloss, andheat-sealing characteristics Unfilled and unreinforced injectionmoldings have a density of about 0.033 lb/in3 (913 kg/m3) and adielectric strength of 600 V/mil (23.6  106V/m) Tensile strength is4,500 to 6,000 lb/in2 (31 to 41 MPa), flexural strength is 6,000 to8,000 lb/in2 (41 to 55 MPa), flexural modulus is 170,000 to 250,000lb/in2(1,170 to 1,720 MPa), Izod notched impact strength is 0.4 to 1.4

ft lb/in (21 to 75 J/m), and the heat-deflection temperature at 264

POLYPROPYLENE PLASTICS 747

lb/in2 (1.8 MPa) is 120 to 140°F (49 to 60°C) Talc, a common filler,appreciably increases rigidity, and glass fibers, the common rein-forcement, markedly increase strength and rigidity Homopolymersalso provide the “living hinge” effect, that is, the ability to withstand

repeated flexing RCW-280 and RCY-280 polypropylenes, of

Solvay (Belgium), are highly rigid, isotactic block copolymers

Reflex, a line of flexible polypropylenes of Rexene Corp., combines

about 80% atactic material for flexibility and 20% isotactic materialfor strength and chemical resistance There are several homopoly-mers and copolymers, with tensile modulus ranging from 5,000 to55,000 lb/in2 (34 to 379 MPa) and elongation to 1000% and more.They are intended to replace plasticized polyvinyl chloride in auto-clavable health care products and other plastics in extruded profiles

and injection-molded parts Petrothene random copolymers from

Quantum Chemical are based on BASF’s gas-phase polymerizationprocess and markedly increase optical quality and, having melt flowrates (MFRs) up to 0.0075 lb/min (34 g/10 min), speed cycle time

Heterophasic copolymers, formed by the polymerization of a

rub-ber phase such as ethylene propylene, increase impact strength.Random copolymers, made by the random addition of ethylene, havegreater clarity, a lower but broader melting range, and lower flexuralmodulus Their impact strength is greater than that of the homopoly-

mers but less than that of the heterophasic copolymers The Accutuf

line of Amoco polymers provides impact strengths ranging to 13 ftlb/in (694 J/m) with flexural modulus up to 205,000 lb/in2(1,410 MPa).The company’s Accpro line optimizes rigidity: to 350,000 lb/in2 (2410MPa)

Reactor-made polypropylene copolymers combine high melt flowrates with enhanced stiffness and toughness, and better color match,taste, and odor performance, promoting uses for thin-wall food contain-

ers Montell’s 100-MFR Pro-fax Ultra has a tensile strength at yield of

3,400 lb/in2(23 MPa), a flexural modulus of 180,000 lb/in2(1,240 MPa),and a notched Izod impact strength of 0.8 ft lb/in (43 J/m) A 55-MFRgrade has somewhat lower strength and stiffness, but toughness more

than doubles: to 2 ft lb/in (107 J/m) Reactor-made Newstren, from

Chisso Corp of Japan, features as much as 10 times the melt strength

of conventional polypropylene, increasing sag resistance and improvingcontrol in sheet-making operations Resulting grades are used for ther-moforming applications Another high-melt-strength (HMS) grade,

Newfoamer, is for foamed sheet and board Montell markets HMS polypropylene for multiple processes, including thermoforming,

extrusion, and blow molding

Fiberfil J-69, of DSM Engineering Plastics, is based on a highly

crystalline polypropylene that is chemically coupled to glass to

pro-748 POLYPROPYLENE PLASTICS

vide mechanical properties greater than traditionally glass-reinforcedmaterial With 30% glass, tensile strength is 14,200 lb/in2 (98 MPa),flexural modulus exceeds 900,000 lb/in2 (6,200 MPa), and notchedIzod impact strength is 1.9 ft lb/in (101 J/m) Glass reinforcement

ranges from 10 to 40% Comtex polypropylene, from Vantage

Polymers of England, comprises layers of hot-pressed woven fabric ofdrawn polypropylene fibers and features high mechanical properties:12,300 lb/in2 (85 MPa) tensile strength, 350,000 to 500,000 lb/in2(2,410 to 3,450 MPa) flexural modulus, 56 ft lb/in (2,990 J/m) notchedIzod impact strength, 264-lb/in2 (1.8-MPa) heat-deflection tempera-ture of 290°F (143°C), and low thermal expansion It is intended forproducing hot-pressed shapes

Biaxially oriented polypropylene (BOPP) film (film stretched

in two ways) has greatly improved moisture resistance, clarity, andstiffness It is used for packaging tobacco products, snack foods,baked goods, and pharmaceuticals Metallized grades also are avail-able for packaging designed for extended shelf life

Foamed polypropylene includes expandable polypropylene (EPP) bead and injection-molded structural foam EPP provides

greater energy absorption and flexibility than expandable polystyreneand nearly the same insulative quality Many auto bumper cores usethe beads to meet impact requirements Structural PP foam moldingsconsist of solid outer skin and foam core They are used to achievegreater stiffness in larger, lightweight parts (strength-to-weight ratiosare 3 to 4 times greater than those for solid parts)

Polypropylene fiber was originally produced in Italy under the

name of Merkalon Unless modified, it is more brittle at low

temper-atures and has less light stability than polyethylene, but it has abouttwice the strength of high-density linear polyethylene Monofilamentfibers are used for filter fabrics, and have high abrasion resistanceand a melting point of 310°F (154°C) Multifilament yarns are used

for textiles and rope Polypropylene rope is used for marine

hawsers, floats on water, and does not absorb water as Manila ropedoes It has a permanent elongation, or set, of 20%, compared with19% for nylon and 11% for Manila rope, but the working elasticity is16%, compared with 25% for nylon and 8% for Manila The tensilestrength of the rope is 59,000 lb/in2 (407 MPa) Fine-denier multifila-

ment polypropylene yarn for weaving and knitting dyes easily and comes in many colors Chlorinated polypropylene is used in coat-

ings, paper sizing, and adhesives It has good heat and light stability,high abrasion resistance, and high chemical resistance

POLYSTYRENES. A thermoplastic resin used for molding, in lacquers,and for coatings, polystyrene is formed by the polymerization of

monomeric styrene, which is a colorless liquid of composition

C6H5CH:CH2, specific gravity 0.906, and boiling point 293°F (145°C)

It is made from ethylene, and is ethylene with one of the hydrogen

atoms replaced by a phenyl group It is also called phenyl ethylene and vinyl benzene As it can be made by heating cinnamic acid,

C6H5CH: CHCO2H, an acid found in natural balsams and resins, it is

also called cinnamene In the form of vinyl toluene, which consists

of mixed isomers of methyl styrene, the material is reacted with ing oils to form alkyd resins for paints and coatings

dry-The polymerized resin is a transparent solid very light in weightwith a specific gravity of 1.054 to 1.070 The tensile strength is 4,000

to 10,000 lb/in2(28 to 69 MPa), compressive strength 12,000 to 17,000lb/in2 (83 to 117 MPa), and dielectric strength 450 to 600 V/mil (18

106to 24  106V/m) Polystyrene is notable for water resistance andhigh dimensional stability It is also tougher and stronger at low tem-peratures than most other plastics It is valued as an electrical insu-lating material, and the films are used for cable wrapping

When produced from methyl styrene, parts have a Rockwell M

hardness to 83, with tensile strengths to 8,900 lb/in2 (61 MPa), andhave a stiffness that makes them suitable for such products as cabi-nets and housings Dielectric strength is also high, above 800 V/mil(32106 V/m), and the resin is thus used for electronic parts Theheat distortion temperature is 215°F (101°C)

Styrenes are subject to creep Therefore, the long-term bearingstrength (over 2 weeks) is only about one-third the short-time tensilestrength Since their maximum useful service temperature is about160°F (71°C), their use is restricted chiefly to room-temperatureapplications Because of their low cost and ease of processing, poly-styrenes are widely used for consumer products The impact gradesand glass-filled types are used quite widely for engineering parts andsemistructural applications Because of good processing characteris-tics, polystyrenes are produced in a wide range of forms They can beextruded; injection-, compression-, and blow-molded; and thermo-formed They are also available as film sheet and foam

Polystyrenes can be divided into several major types pose grades, the lowest in cost, are characterized by clarity, colorabil-ity, and rigidity They are applicable where appearance and rigidity,but not toughness, are required Impact grades of polystyrenes areproduced by physically blending styrene and rubber Grades are gener-ally specified as medium, high, and extra-high As impact strengthincreases, rigidity decreases Medium-impact grades are used where acombination of moderate toughness and translucency is desired High-impact polystyrenes have improved heat resistance and surfacegloss The extra-high-impact grades are quite low in stiffness, and

their use is limited to parts subject to high-speed loading High-impact

grades include Chevron Chemicals’ Valtra polystyrene, Dow Plastics’ ES, BX, and XB polystyrene, and Huntsman Chemicals’ XI

and XR polystyrene These grades have Gardner impact strength

ranging from about 230 to 320 ft lb (312 to 434 J) and 95% 60°

Gardner gloss Dow’s Styron A-1120 polystyrene is a

medium-impact strength, high-gloss grade for injection molding Its notchedIzod impact strength is about 1.1 ft lb/in (59 J/m) and gloss is 90%

BASF’s Avantra 166H polystyrene is a high-molecular-weight

high-clarity, tough grade with a tensile strength at break of 6000 lb/in2(41 MPa) and a tensile modulus of 350,000 lb/in2(2,400 MPa)

Highly crystalline syndiotactic polystyrene, based on

metal-locene catalysts, was developed by Idemitsu Petrochemical of Japan

in a joint effort with Dow Plastics It has a melting point of 518°F(270°C), which is similar to that of Nylon 6/6 and about 3 times that

of conventional, or amorphous, polystyrene Tensile and flexuralstrengths are 5,100 lb/in2(35 MPa) and 9,200 lb/in2 (64 MPa), respec-tively, and, with 30% glass reinforcement, 17,000 lb/in2(117 MPa) and26,800 lb/in2(185 MPa)

Styrene can be polymerized with butadiene, acrylonitrile, and other

resins The terpolymer, acrylonitrile-butadiene-styrene ated ABS), is one of the common combinations Styrene-acryloni-

(abbrevi-trile (SAN) has excellent resistance to acids, bases, salts, and some

solvents It also is among the stiffest of the thermoplastics, with atensile modulus of 400,000 to 550,000 lb/in2 (2,758 to 3,792 MPa)

Acrylate-styrene-acrylonitrile (ASA) has very good weathering

resistance (nonyellowing), good toughness, and stress cracking

resis-tance Styrene-butylene resins are copolymers that mold easily

and produce thermoplastic products of low water absorption and goodelectrical properties They have strength equal to that of the vinyls

with greater elongation Kraton DI401P, from Shell Chemicals, is a

clear, rigid, thermoplastic high-styrene (75% by weight)-butadieneblock copolymer supplied in pellets for injection molding food andnonfood packaging products, toys, purchase displays, hangers andother applications Typical properties are 65 Shore D hardness, 1.01specific gravity, 3,600 lb/in2 (25 MPa) tensile yield strength, 2,500lb/in2 (17 MPa) tensile break strength, 120% elongation at break,5,400 lb/in2 (37 MPa) flexural strength, 0.5 ft lb/in (27 J/m) notchedIzod impact strength, and 2% optical haze

Foamed polystyrene is used in many forms, including extruded

sheet (which is then thermoformed, e.g., egg cartons, trays); able polystyrene (EPS) beads, which contain a blow agent (usuallypentane) and which are processed into low-density [0.7 to 10 lb/ft3(11

expand-to 160 kg/m3)] foamed products, such as hot and cold drink cups and

protective packaging; and block and heavy sheet, used for thermalinsulation.

PONTIANAK. A gum from the trees Dyera costulata and D laxifolia of

Borneo and Malaya The commercial pontianak is a grayish-whitemass like burned lime and contains 60% water, with only 10 to 25%rubberlike materials It is a rubber, but has a high content of resinsimilar to balata and gutta percha, and is classified with the lowerguttas It is used in the friction compounds employed for coatingtransmission belting, in insulations and varnishes, for mixing withgutta percha, and also to adulterate or replace chicle for chewing

gums It is also called jelutong Pontianak copal is from varieties

of Agathis trees of Borneo Its peculiar turpentinelike qualities come

from the method of tapping It is valued for varnishes

POPLAR. The wood of several species of the tree Populus The black

poplar, or English poplar, P Nigra, of Europe, is a large tree with

blackish bark The wood is yellowish white with a fine, open grain It

is soft and easy to work The density is about 25 lb/ft3(400 kg/m3) It

is used for paneling, inlaying, packing cases, carpentry, and paper

pulp Lombardy poplar is a hybrid variety of this species It is a

tall, columnar tree that is male only and can be propagated only fromrootstocks It is grown in the United States for shelter belts, but insome countries is grown in fruit districts as a wood for packing boxes

White poplar, P alba, is a larger tree native to the United States.

The wood is similar to that of the black poplar Cottonwood is

another species of poplar Gray poplar is from the tree P canescens,

of Europe The color of the wood is light yellow It has a tough, closetexture somewhat resembling that of maple It is used for carpentry

and flooring The wood of the canary whitewood is called Virginia

poplar, or simply poplar, but belongs to a different family of trees.

Aspen is also called poplar

PORCELAIN Porcelains and stoneware are highly vitrified ceramics that are widely used in chemical and electrical products Electrical

porcelains, which are basically classical clay-type ceramics, are

con-ventionally divided into low- and high-voltage types The high-voltagegrades are suitable for 500 V and higher and are capable of with-

standing extremes of climatic conditions Chemical porcelains and

stoneware are produced from blends of clay, quartz, fledspar, kaolin,

and certain other materials Vita porcelain, a feldspathic type, is

used for dental restorations Porcelain is more vitrified thanstoneware and is white A hard glaze is generally applied Stonewarescan be classified into two types: a dense, vitrified body for use with

corrosive liquids, and a less dense body for use in contact with

corro-sive fumes Chemical stoneware may range from 30 to 70% clay, 5

to 25 feldspar, and 30 to 60 silica The vitrified and glazed productwill have a tensile strength up to 2,500 lb/in2 (17 MPa) and a com-pressive strength up to 80,000 lb/in2 (552 MPa) Industrial stone-

ware is made from specially selected or blended clays to give desired

properties

Both chemical porcelains and stoneware resist all acids excepthydrofluoric Strong, hot, caustic alkalies mildly attack the surface.These ceramics generally show low thermal-shock resistance and ten-sile strength Their universal chemical resistance explains their wideuse in the chemical and processing industries for tanks, reactorchambers, condensers, pipes, cooling coils, fittings, pumps, ducts,blenders, filters, and so on

Ceratherm, of U.S Stoneware Co., is an acid-resistant and

heat-shock-resistant ceramic having a base of high-alumina clay It isstrong and nonporous and is used for pump and chemical-equipmentlinings The chemical stoneware of General Ceramics Inc., made with

a mullite-zircon body, is white, dense, strong, and

thermal-shock-resis-tant The ceramic marketed under the name of Prestite by

Westinghouse Electric Corp for insulators and molded electrical parts

is blended of flint, feldspar, kaolin, and ball clay It is nonporous andmoistureproof without a glaze, has high dielectric strength, has a ten-sile strength of 5,000 lb/in2(34 MPa), and has a compressive strength

of 48,000 lb/in2 (331 MPa) The ceramic produced by Rostone Corp

under the name of Rosite, for molded electrical parts and panels, is a

calcium-aluminum-silicate mixed with asbestos It withstands atures to 900°F (482°C), is resistant to alkalies, and has a compressivestrength up to 15,000 lb/in2(103 MPa)

temper-POROUS METALS. Metals with uniformly distributed controlled poresizes, in the form of sheets, tubes, and shapes, used for filtering liquidsand gases They are commonly made by powder metallurgy, and thepore size and density are controlled by the particle size and the pressureused Stainless steel, nickel, bronze, silver, and other metal powders areused, depending on the corrosion resistance required of the filter Poresizes can be as small as 7.9
in (0.2
m), but the most generally usedfilters have pores of 157, 315, 472, and 984
in (4, 8, 12, and 25
m).Pore sizes have a uniformity within 10% The density range is from 40

to 50% of the theoretical density of the metal Standard filter sheet is

0.30 to 0.60 in (0.76 to 1.52 cm), but thinner sheets are available Sheets

as thin as 0.004 in (0.010 cm), and with void fractions as high as 90%,

have been made for fuel cells and catalytic reactors Porous steel is

made from 18–8 stainless steel, with pore openings from 787 to 2,559

in (20 to 65
m) The fine-pore sheet has a minimum tensile strength

of 10,000 lb/in2(69 MPa), and the coarse has a strength of 7,000 lb/in2

(48 MPa) Felted metal, developed by the Armour Research

Foundation, is porous sheet made by felting metal fibers, pressing, andsintering It gives a high strength-to-porosity ratio, and the porosity can

be controlled over a wide range In this type of porous metal the poresmay be from 0.001 to 0.015 in (0.003 to 0.038 cm) in diameter, and ofany metal to suit the filtering conditions A felted fiber filter of 430stainless steel with 25% porosity has a tensile strength of 25,000 lb/in2(172 MPa)

Gasar materials are porous metals, ceramics, and glasses

pro-duced in the Ukraine by melting in a furnace filled with hydrogen orother gases Controlling gas pressure and cooling time governs poresize, shape, orientation, and volume Pore size can range from 197

in (5
m) to 0.4 in (10 mm) and volume from 5 to 75% Developed in

1960 at the Ukraine’s Dnepropetrovsk Metallurgical Institute, cations include rocket kerosene-fuel atomizers, water-purificationoxygenators, bronze bearings, magnesium aerospace panels, nickelchemical filters, and high-temperature ceramic catalyst supports inrocket and jet engines

appli-POTASH Also called pearl ash A white, alkaline, granular powder, which is a potassium carbonate, K2CO3or K2CO3:H2O, used in softsoaps, for wool washing, and in glass manufacture It is producedfrom natural deposits in Russia and Germany and also produced fromwood and plant ashes The U.S production is largely from potashsalts of New Mexico, from the brines of Searles Lake, California, andfrom solar evaporation in Utah, and usually contains iron, clay, and

salt as impurities Kalium is a high-purity grade that results from an

evaporation-crystallization step; it has about 62.4% K2O and lessthan 1 salt The material is marketed by Kalium Chemicals Thematerial as produced by Hooker Chemical Co is a free-flowing, whitepowder of 91 to 94% K2CO3, or is the hydrate at 84%, or calcined at99% purity The specific gravity of potash is 2.33 and melting point

1668°F (909°C) Hartsalz, mined in the Carpathian Mountains and

used for producing potash, is a mixture of sodium chloride, potassiumchloride, and magnesium sulfate It is also a source of magnesium.The extensive potassium mineral deposits at Strassfurt and

Mülhausen contain sylvite, KCl; carnallite, KCl MgCl2 6H2O;

kainite, K2SO4  MgSO4  MgCl2  6H2O; and leonite, K2SO4 MgSO4 4H2O There are at least 1  109tons (9  108metric tons) of

the potash mineral Wyomingite in the deposits near Green River,

Wyoming It is a complex mineral containing leucite, phlogopite, side, kataphorite, and apatite It has 11.4% K2O, with sodium oxide,

magnesium oxide, phosphorus pentoxide, and other oxides Thesylvite ore mined at Carlsbad, New Mexico, contains KCl and NaCl It

is electrically refined to 99.95% KCl, and is used to produce causticpotash Electrolysis of the chloride solution yields caustic potash

POTASSIUM. An elementary metal, symbol K, atomic weight 39.1,

also known as kalium It is silvery white, but oxidizes rapidly in air

and must be kept submerged in ether or kerosene It has a low ing point, 145°F (63°C), and a boiling point of 1392°F (756°C) Thespecific gravity is 0.855 at 68°F (20°C) It is soluble in alcohol and inacids It decomposes water with great violence Potassium is obtained

melt-by the electrolysis of potassium chloride Potassium metal is used incombination with sodium as a heat-exchange fluid in atomic reactors

and high-temperature processing equipment A potassium-sodium

alloy contains 78% potassium and 22 sodium It has a melting point

of 12°F (11°C) and a boiling point of 1393°F (756°C) and is a silvery,

mobile liquid Cesium-potassium-sodium alloys are called BZ

Alloys Potassium hydride is used for the photosensitive deposit on

the cathode of some photoelectric cells It is extremely sensitive andwill emit electrons under a flash so weak and so rapid as to be imper-

ceptible to the eye Potassium diphosphate, KH2PO4, a colorless,crystalline, or white powder soluble in water, is used as a lubricantfor wool fibers to replace olive oil in spinning wool It has theadvantages that it does not become rancid as oil does and can beremoved without scouring Potassium, like sodium, has a broadrange of use in its compounds, giving strong bonds Metallurgically

it is listed as having a body-centered-cubic structure, but the atomsarrange themselves in pairs in the metal as K2, and the structure iscryptocrystalline

POTASSIUM CHLORATE Also known as chlorate of potash and

potassium oxymuriate A white, crystalline powder, or lustrous,

crystalline substance, of composition KClO3, employed in explosives,chiefly as a source of oxygen It is also used as an oxidizing agent inthe chemical industry, as a cardiac stimulant in medicine, and intoothpaste It melts at 675°F (357°C) and decomposes at 752°F(400°C) with the rapid evolution of oxygen It is odorless but has aslightly bitter, saline taste The specific gravity is 2.337 It is nothygroscopic, but is soluble in water It imparts a violet color to theflame in pyrotechnic compositions

Potassium chloride is a colorless or white, crystalline compound

of composition KCl, used for molten salt baths for the heat treatment

of steels Trona is a fertilizer- and explosives-grade material from Kerr-McGee Chemical Corp Morton is a pharmaceutical-quality

potassium chloride that has no iodine contamination It is sold byMorton Salt The specific gravity is 1.987 A bath composed of threeparts potassium chloride and two parts barium chloride is used forhardening carbon-steel drills and other tools Steel tools heated inthis bath and quenched in a 3% sulfuric acid solution have a verybright surface A common bath is made up of potassium chloride andcommon salt and can be used for temperatures up to 1652°F (900°C).

POTASSIUM CYANIDE. A white, amorphous or crystalline solid of position KCN, employed for carbonizing steel for case hardening andfor electroplating The specific gravity is 1.52, and it melts at about1550°F (843°C) It is soluble in water and is extremely poisonous, giv-ing off the deadly hydrocyanic acid gas For cyaniding steel the latter

com-is immersed in a bath of molten cyanide and then quenched in water,

or the cyanide is rubbed on the red-hot steel For this use, however,sodium cyanide is usually preferred, because of its lower cost and thehigher content of CN in the latter Commercial potassium cyanide is

likely to contain a proportion of sodium cyanide Potassium

ferro-cyanide, or yellow prussiate of potash, can also be used for

case-hardening steel It has composition K4Fe(CN)6 and comes in yellowcrystals or powder The nitrogen as well as the carbon enters the steel

to form the hard case Potassium ferricyanide, or red prussiate

of potash, is a bright-red, granular powder of composition

K3Fe(CN)6, used in photographic reducing solutions, in etching

solu-tions, in blueprint paper, and in silvering mirrors Redsol crystals,

of American Cyanamid Co., is the name of this chemical for use as areducer and mild oxidizing agent, or toner, for photography

Potassium cyanate, KCNO, is a white, crystalline solid used for

the production of organic chemicals and drugs It melts at 590°F

(310°C) The potassium silver cyanide used for silver plating

comes in white, water-soluble crystals of composition KAg(CN)2

Sel-Rex, of Bart-Messing Corp., is this material Potassium gold cyanide has a similar function in gold plating Platina, of Platina

Laboratories, comes as colorless tablets that are soluble in bothwater and alcohol

POTASSIUM NITRATE Also called niter and saltpeter, although

these usually refer to the native mineral A substance of compositionKNO3, it is used in explosives, for bluing steel, and in fertilizers Amixture of potassium nitrate and sodium nitrate is used for steel-tempering baths The mixture melts at 482°F (250°C).Potassium nitrate is made by the action of potassium chloride on

sodium nitrate, or Chile saltpeter It occurs in colorless, prismatic

crystals, or as a crystalline, white powder It has a sharp saline taste

and is soluble in water The specific gravity is 2.1, and the meltingpoint is 639°F (337°C) It is found in nature in limited quantities inthe alkali region of the western United States Potassium nitrate con-tains a large percentage of oxygen which is readily given up and iswell adapted for pyrotechnic compounds It gives a beautiful, violet

flame in burning It is used in flares and signal rockets Potassium

nitrite is a solid of composition KNO2used as a rust inhibitor, for theregeneration of heat-transfer salts, and for the manufacture of dyes

POTATO. The bulbous tubers of the roots of the annual plant

Solanum tuberosum, native to Peru but now grown in many parts of

the world It is used chiefly as a direct food, but is also employed formaking starch and alcohol The potato was brought to Europe in

1580, and received the name of Irish potato when brought to New

England in 1719 by Irish immigrants The plant is hardy and has ashort growing cycle, making it adaptable to many climates The tubercontains about 78% water, 18 starch with some sugar, 2 proteins, 1potash, and only about 0.1 fats The average water loss in storage isabout 11% There are more than 500 varieties of potato cultivated

Dehydrated potato, produced as powder, flake, and porous

gran-ules, is widely used in restaurants and institutions and is marketedunder various trade names for home use

The sweet potato is the root bulb of the trailing perennial vine

Ipomoea batata, native to tropical America, but now grown

exten-sively in the southern United States and in warm climates In South

America it is known by the Carib name batata Like the white potato,

the sweet potato has a high water content, but is rich in sugars Thereare many varieties and two general types: one with a dry, mealy, yel-low flesh and the other with a soft, gelatinous flesh higher in sugars

The latter type is called yam in the United States, but the true yam is

a larger tuber from the climbing plant Diascorea alata, grown widely

in the West Indies The sweet potato is used as a direct food, but largeamounts are also employed for making preserves, starch, and flour for

confectionery Alamalt is cooked and toasted sweet potato ground to a

powder for use in confectionery It adds flavor as well as sugar to the

confectionery Sweet-potato flake, used in foodstuffs, is cooked and

dehydrated sweet potato in orange-colored flakes with the flavor ofcandied sweet potato It is reconstituted with milk or water

The taro is the root tuber of the large, leafy plant Colocasia

escu-lenta, which constitutes one of the chief foods of southeast Asia and

Polynesia There are more than 300 varieties grown The tuber is high

in starch, has more proteins than the potato, but has an acrid taste

until cooked The pasty starch food known as poi in the Pacific Islands

is made from taro In Micronesia the taro is called jaua, and the

mwang plant, Cyrtosperma chamissonis, is called taro This plant is

larger, and the rootstock weighs as much as 50 lb (23 kg), while thetaro does not exceed 5 lb (2 kg) Taro matures in 6 months, while the

mwang requires 2 years The dasheen is a variety of taro grown in the southern United States The yautia, grown in the West Indies,

resembles the taro, but is from the large plant Xanthosma

sagitti-folium It is high in starch and has greater food value than the potato.

PRECIOUS METALS. The metals gold, silver, and platinum, which areused for coinage, jewelry, and ornaments, and also for industrialapplications Expense or rarity alone is not the determining factor;rather, a value is set by law, with the coinage having an intrinsicmetal value as distinct from a copper coin, which is merely a token

with little metal value The term noble metal is not synonymous,

although a metal may be both precious and noble, such as platinum.Although platinum was once used in Russia for coinage, only gold andsilver fulfill the three requisites for coinage metals Platinum does

not have the necessary wide distribution of source The noble metals

are gold, platinum, iridium, rhodium, osmium, and ruthenium.Unalloyed, they are highly resistant to acids and corrosion Radiumand certain other metals are more expensive than platinum but arenot classified as precious metals Because of the expense of the plat-inum noble metals, they may be alloyed with gold for use in chemical

crucibles Platino is an alloy of 89% gold and 11 platinum Palau is the name of an alloy of gold and palladium, and rhotanium is a

rhodium-gold alloy

PREFINISHED METALS. Sheet metals that are precoated or treated

at the mill so as to eliminate or minimize final finishing by theuser The metals are made in a ready-to-use form with a decorativeand/or functional finish already applied A large number and vari-

ety of prefinished metals have been developed Prepainted metals

are produced using various organic coatings on many common rous and nonferrous metals Extra durability or special decorative

fer-effects are provided by plastic-metal laminates Polyvinyl

chlo-ride, polyvinyl fluochlo-ride, and polyester are the plastics commonlyused

Black-coated steel is used to give a high thermal emittance in

electronic equipment The base metal is aluminum-deoxidized steelcontaining 0.13% carbon, 0.45 manganese, 0.04 maximum phospho-rus, and 0.05 maximum sulfur The steel is coated with a 5% byweight layer of nickel oxide, which is reduced in a hydrogen furnace

to form a spongy layer of nickel This sponge is impregnated with acarbon slurry to form a black, carbonized surface

Preplated metals consist of a thin, electrodeposited plate of one

metal or alloy on a base, or substrate, usually sheet, of another metal.Steel is the most common base metal, and it is commonly plated withbrass, chromium, copper, nickel, nickel-zinc, or zinc Other commonpreplated metals are chromium-plated brass, copper, nickel, or zinc;and nickel-plated brass, copper, or zinc The surface of the plate may

be mirror-bright, satin, bright satin, embossed, antique, or black, ormay have some other finish The plate, usually 0.001 to 0.005 in(0.025 to 0.127 mm) thick, is sufficiently ductile to withstand shear-ing, bending, drawing, and stamping operations Common joiningmethods include lock-seaming, stud welding, adhesive bonding, andspot welding One of the earliest groups of preplated metals included

Brassoid, Nickeloid, and Chromaloid of American Nickeloid,

which were brass-, nickel-, and chromium-plated zinc sheet Otherproducers of preplated metals have included Apollo Metals, NationalSheet Metal, and Thomas Steel Strip

Prefinished metals are now available with almost any metal plated

or bonded to almost any other metal, or single metals may be had finished in colors and patterns They come in bright or matte finishesand usually have a thin paper coating on the polished side, which iseasily stripped off before or after forming The metals are sold under

pre-a vpre-ariety of trpre-ade npre-ames pre-and pre-are used for decorpre-ative pre-articles, pre-ances, advertising displays, panels, and mechanical parts

appli-PRESERVATIVES. Chemicals used to prevent oxidation, fermentation,

or other deterioration of foodstuffs The antioxidants, inhibitors, andstabilizers used to retard deterioration of industrial chemicals are notusually called preservatives The most usual function of a preserva-tive is to kill bacteria, and this may be accomplished by an acid, analcohol, an aldehyde, or a salt A legal requirement under the Foodand Drug Act is that a preservative be nontoxic in the quantities per-

mitted Sugar is the most commonly used preservative for fruit ucts Sodium chloride is used for protein foods Sodium nitrate is

prod-reduced to sodium nitrite in curing meats, and the nitrite has aninhibitory action on bacterial growth, the effect being greatest in acid

flesh Potassium sorbate, KOCOCH:CH:CHCH3, a white, uble powder, inhibits the growth of many molds, yeasts, and bacteriawhich cause food deterioration, and it is used in cheese, syrups, pick-les, and other prepared foods

water-sol-The inhibitory effect of organic acids is due chiefly to the

undissoci-ated molecule Acetic acid is normally more toxic to bacteria than

lactic acid; but when sugar is present, the reverse is true, and citricacid then has little toxicity The inhibitory action of inorganic acids isdue mainly to the pH change which they produce The antimicrobe