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products of chemistry
George B. Kaunman CaliforniaState University,Fresno Fresno, CA 93740
Identifying Polymers through Combustion and Density Avrom A. Blumberg DePaul University, Chicago, IL 60614
Analytical chemistry is an important subject both because it enables us to know how much of what substances there are in a material and because it offers practical experience with some characteristic chemical reactions of these substances. Polymer chemists use many techniques to characterize and identify polymers: among them are osmometry, light scattering, and ultracentrifugation to establish molecular weights; infrared and nuclear magnetic spectrometry; differential calorimetry; liquid chromatography. AU of these methods involve costly and specialized instruments. This note describes two simole. inexoensive and instructive methods to distinguish amokg thecommon polymers: combustion and ovmlvsis as examoles of destructive analvsis. and for nond&tru&ve testingSLdensityThese methods are best for simple, unblended polymers in common objects such as textile fibers and packaging. Combustion and Pyrolysis Because some polymers produce toxic combustion products such as hydrogen chloride, carbon monoxide, and hydrogen cyanide, it is essential laboratory practice always to use very small specimens and in a good exhaust hood. (These toxic combustion gases also are important factors in domestic and commercial fues.) Combustion in air can be studied by moving a thin strip or a few fibers, held by a clamp or tongs, in and out of a flame. Pyrolvsis. in a closed crucible with limited air suoolv. .. ?.mav p&duce a distillate to examine after the crucible is coolei. And densities are estimated bv observincr flotation or sinking in a series of common liquids and skutions. Data for the polymers discussed below are given in the table. In combustion, we observe carefully how the thin strip or fibers act near and in a clean gas or alcohol flame.
Dues the matenal become son when it i~ hot?
'Does it seem w burn or only to blister and char7 1)oes it shnnk back from the flame? How does the hot edge of the polymer look? Are smoke and fumes produced? Does it continue to bum with its awn flame when withdrawn from the gas or alcohol flame? Does it drip molten polymer as it bums? What sort of residue is left after the combustion stops? In pyrolysis we also can examine any liquid and solid residues. The Polyolefins: Polyethylene and Polypropylene These pure hydrocarbons,with repeat units CHzCHzand CH2CH(CH3),are, along with the natural and synthetic rubbers and cellulosic materials, the polymers produced in the greatest amount. Examples of products made from polyethylene are squeezable bottles, electrical insulation, shrinkable packaging film, and food containers and wrap-
ping. Polypropylene is the substance in the common Herculon carpet fiber, rope, netting, hospital ware, and plastic pipe. These polymers burn both when in a flame and when withdrawn. They drip like a candle, and the odor resembles that of a candle. his is expected because these hvdrocarbon oolvolefms have the same chemical comwsition as paraffin. kthough these polymers are often coiored with opaque pigments, they are naturally translucent a t rwm temperature (because of both amorphous and crystalline domains) and become transparent when hot since the crystalline regions disappear. Translucency is restored on cooling. The density of polyethylene varies fmm 0.91-0.98 g/cm3 and that of polypropylene from 0.89-0.92. Branched polymers tend to have lower densities than linear polymers. The Chlorinated Polyolefins: Polyvinyl Chloride and Polyvinylidene Chloride These have the repeat units CHzCHCl and CHzCClz Polyvinyl chloride (PVC)is the binyl'in vinyl garden hose, floor tiles, phonograph records, rainwear, simulated leather, insulation, drain pipes, and shower curtains. Polyvinylidene chloride is used in pipes, textiles for awnings and canvas seats, and insect screening. The copolymer saran is mostly polyvinylidene chloride, but also contains some PVC. At mom temperature polyvinylidene chloride is flexible, and PVC is brittle. For this reason PVC is treated with a softening agent or plasticizer to make it soft and pliant. The characteristic odor of shower curtains and simulated leather upholstery is due to the plasticizer and not to W C . These polymers burn only with difficulty and are self-extinguishing when withdrawn from the flame. Poor flammability is typical of halogenated polymers. The flame may be sooty, and the fumes are acidic from the hydrogen chloride produced and will turn litmus paper red. The stmng odor of HCI is noticed. In pyrolysis a dark charred mass may remain. If there is a liquid, it will be acidic. The old Beilstein test (6) for chlorine, bromine, or iodine is also useful for these polymers. A heavy copper wire, held either firmly by tongs or pliers or by one end pushed into a cork, is heated in a Bunsen flame until the green copper flame color disappears. Then the red hot wire is plunged into a polymer sample and returned to the flame. A green flame indicates that the hot copper has reacted with one of these three halogens and that its surface contains volatile copper salts. Chlorinated polymers are more dense than most other polymers, consistent with the relatively high atomic weight of chlorine. PVC has a density in the range 1.351.42 and polyvinylidene chloride 1.67 to about 1.85 g/cm3. Volume 70 Number 5 May 1993
399
Polymer Data Name
Densiy (@cm )
Combustion
Name
Density (u~L)
Combustion
polyethylene, I
0.91-0.98
like a wax candle
nylon 6 ,I6
polypropylene, 2
0.8W.92
like a wax candle
cellulose. 17
1.5
polyvinyl chloride. 3
1.351.42
self-extinguishing
like paper, wood, marshmallows
polyvinylidene chloride, 4
1.67-1.85
self-extinguishing
proteins, 18
1.3&1.35
material does not soften; when removedfrom flame self-extinguished; usually
polytetrafluoroethylene, 5
2.2
decomposes; acid fumes; char residue
polystyrene, 6
1.04-1.06
polyacrylic acid, 7
1.2
sooty flame
1
polymethyl acryiate, 8 hard to ignite; slow burning; formaldehyde odor; lingering smoke when extinguished
polymethacrylic acid, 9 polymethyl methacrylate, 10
1
polyacryionitrile, 11
1.2
polyethylene terephthalate, 12
1.35
0.92; up to 1.3 if vulcanized
softens and bums; sunurous odor when vulcanized
smokeless flame
polychloroprene (neoprene), 24
1.2
self-extinguishing; acid fumes
epoxies. 25
1.2
urea-formaldehyde,
melt and darken; white ammoniacal smoke; easily extinguished when removed from flame
Structures of Comwunds ListedAbove
COOH
COOCH,
I
-[CHCbl-
I ICLL
-[ccHJ-
8
7
400
natural rubber, 21
0.93
0.97 - 1.I5
CY 10
soften, drip near flame; char; many self-extinguishing; fumes may be acidic or basic
butyl rubber, 23
nylon 4,7,15
I -1CCHdI
1.1-1.3
smoky flame; men and char; mildly acidic fumes
1.25
COOCH,
polyurethanes, 20
smoky flame
polycarbonate, 14
I
0.96-1.5
butadiene rubber, 22 0.9
melt; neutral fumes; may selfextinguish when removed from flame
----fCHCHJ-
silicones, 19
sample withdraws from flame; odor similar to burning hair
from ethylene glycol HOCH2CH20H and terephthalic acid, 13
MX)H
dark ash left; odor of burning hair or feathers noticeable smoke; silica residue
-'
9
'a
CN
I
-[CHCH2]-
0 11
Journal of Chemical Education
12
26 melamineformaldehyde, 27
:::1
very smoky, alkaline flame char and crack in flame; not easily ignited; amine odor in flame; alkaline fumes
A Fluorinated Polymer: Teflon With a repeat unit CF2CF2,this smooth-surface polymer is used in nonsticking cooking vessels and for low-friction high-temperature gaskets and joint packing. Teflon does not bum in air but decomposes slowly with the production of toxic acidic fumes, leaving a char residue. Like polyethylene it is translucent and becomes clear when hot. The density is about 2.2 g/cm3. Polystyrene This common polymer has a benzene ring on alternating main polymer chain carbons, that is., its repeat unit is CH2CHC6H5.It is most often seen in the common disposable. foamed. insulated hot beverage CUDS. It also is used in liiht-diffu8ing panels; packagingmate'rial for breakable -mods: some hard. brittle, and clear medicine vials: and acoustical tile. Polystyrene bums readily and, because of the aromatic benzene rings, with a very sooty flame. The characteristic odor of styrene can sometimes be detected. The density of clear, unfoamed polystyrene is 1.04-1.06 g/cm3. Polymethyl Methacrylate Polymethyl methacrylate is marketed as Rohm & Haas Plexiglas and Dupont Lucite and is a clear glassy polymer, used in place of window glass, as lenses, bubble windows, and in fiber optics. The repeat unit is CH&(CH3)COOCH3. PMMA does not ignite easily and burns slowly and often with a smokeless flame with a n inner blue color, verv much like a candle. When extinguished, it does not have alingering smoke. When very hot, the odor of formaldehyde can be detected. The combustion and pyrolysis is similar to that of the polymethacrylics, CHzC(CHJCOOH, used in adhesives, and polymethyl acrylates, CH2CHCOOCH3, in varnishes and paints. Their densities are all about 1.2 g/cm3. Polymethyl methacrylate is a good example of a glassy polymer or a glass. Aglass is a rigid amorphous or noncrystalline substance, usually made by melting a crystalline solid and then cooling it rapidly enough so that there is insufficient time for the atoms and molecules in the melt to align in the regular manner of a crystal; the randomness of the melt is frozen in. Glasses are typically transparent, and when they fracture it is not alonp a flat plane. It also i s possible to m n k r a glass by preparinga concentrated solution or syrup of sucrose in boiling water and cooling it to room temperiture by pouring the hot liquid on a m&al or stone slab. With coloring and flavor, simple candies are made this way. Pure PMMAis a &ass below 105 *Cand pliant above. For polystyrene the corresponding temperature is 100 OC, and for polyvinyl chloride the glass transition temperature is 81 'C. Polyethylene has a low glass transition temperature, about -100 *C.To form a glass from molten polyethylene it is necessary to lower very rapidly the polymer temperature from above 140 'C to below -100 'C. Quartz (silicon dioxide, SiOz)has a different time scale for glass formation. If crystalline quartz is heated above its melting point, 1723 'C, it will become amorphous. Unless it is cooled a t a geologically slow rate, it will form a glass a t lower temperatures.
-
Polvacrvlonitrile . With repeat unit CH2CHCN,polyacrylonitrile could also be called vinyl cyanide, but not without worrying the public unnecessarily. A s the substance used to make Orlou, Acrilan, and Creslan, it is widely used as a wool substitute. Fibers contract and withdraw from a flame and when ignited burn like cotton but with an odor something like burning hair. The vapors contain both ammonia (NH3) and hydrogen cyanide (HCN) and are both alkaline and toxic.
Careful work in a good exhaust hood is obviously essential in much of chemistry. The density is slightly less than 1.2 gicm3. Polyesters
These have a wide variety of uses. Alkyd resins made from polyfunctional acids and alcohols are highly crosslinked and rigid after curing and are used in paints, linoleum, tableware, and adhesives. Polyethylene terephthalate. O(CH7kOCOC~HAC0. marketed as Dacron. Temlene. and ~ilar,Lsused for textile and tire fibers and is the bas; for Dhotoaa~hicfilm. dis~osablesoft beverage containers. and ma&& tape' ~ d l ~ c a r b o n a t esuch s as OC6H4C(CH3)~C~H40C0, transparent and strong, are used for safety glass, in bulletproofwindows, light fixtures, and electric popcorn makers. Unsaturated polyesters are used in fiber-glass reinforced panels for roofing, boats, and duct work. These burn with a smoky flame. C r o s s - l i e d alkyd resins, like all cross-linked polymers, do not soften but burn, usually with smoking. The second set of polyesters, such as dacron, melt and char and have mildly acidic fumes. The film mylar shrinks from the flame and has a clean huming edge. The third set of ~olvesters.such as Lexan. melts, has neLtra1 fumes, and maynot always bum outside the applied flame. In pyrolysis the distillate is neutral. The initially unsaturated polyesters, cross-linked after curing, char without softening, burn with a hissing or popping sound, and produce acidic fumes. The density of the alkyds range from 1.0 to 1.4; of fiber polyesters about 1.35; of polycarbouate about 1.25; and the last group about 1.2, but combined with glass fiber the overall density is about 1.8 g/cm3.
exa an;
Polyamides: The Nylons
The repeat units of the nylons include the amide linkage -NHCO-. Nylons are made in several different ways: from a single monomer with both a carboxylic acid and an amine group; from a lactam; which is the ring-molecule amide made from this; and from a dicarboxylic acid and a diamine. In addition to forming fibers for textiles, nylon is made into rope, tire cord, fish line, bolts and nuts, and carnets. Nvlon swcimens melt and darken and emit a white hkaline amiie or ammonia smoke. Their flames are easily extinmished when removed from the flame. The various nylons have densities from 0.97-1.15 g/cm3. Cellulosic Polymers
These are obtained from natural cellulose. some with chemical modification. Rayon and cellophane are dissolved d cellulose acetates are used for and r e ~ r e c i ~ i t a t ecellulose: fibers,-pho~ographicfilm, eyeglass frames, lacquers, and clear packaging sheets; and cellulose nitrate is used as a resin in lacquers as well as smokeless gunpowder. It formerly was in use as Celluloid. Cellulose does not soften when near or in a flame, but it b u m with that characteristic sweet odor that we associate with burninp or bumed wood or paper, as the cellulose, a polyglucose, c&wnelizes. Cellulose acetate melts when hot burns with a feeble smoky flame, and a strip of this material forms beads a t the edge touching the flame. Both the smoke and the pyrolysis distillate are acidic, from the acetic acid released. Cellulose nitrate, as one might expect, melts and bums vigorously and also releases a n acidic fume. Cellulose has a density of about 1.5, the acetate about 1.3, and the nitrate about 1.4 g/cm3-Like starch, cellulose forms a blue color with iodine. Volume 70 Number5 May 1993
401
The Protein Polymers: Silk and Wool Like the nylons or polyamides, these also have -NHCOor peptide (the term used with proteins and polypeptides) linkages. Their use is widespread in clothing, carpets, ete. These fibers do not soften when in or near a flame, and when removed from the flame they stop burning. Usually a dark char ash is left. In pyrolysis wool fibers shrink and then char; silk chars without noticeable shrinkage. The odor is that of burning hair or feathers, which are also proteins. An important difference between wool and silk is that silk dissolves in concentrated hydrochloric acid, but wool does not. The densities are about 1.30-1.35 g/cm3; density varies because these proteins pick up about half their mass in water under humid conditions, with a concomitant lowering of density. Silicones These have Si(RR')O repeating units and are commonly found in leather water~roofinaliauids. antistick soravs. " , and in silly putty. usuafiy bum'with a noticeable smoke but, unlike carbon-backbone polymers, leave a silica residue. Depending on the side groups R and R', the density is usually in the range of 0.96-1.5 and sometimes even as high as 1.7 g/cm3.
he^
.
Polyurethanes These have the repeat unit RNHCOO and are used in elastic fibers, foams for mattresses, footwear, and varnishes. They soften when in or near a flame where they drip and char. Many polyurethanes stop burning when removed from a flame. Vapors may be acidic or alkaline. Densities range from 1.1-1.3 g/cm3. Rubbers and Elastomers There is a wide variety of rubbers, used in tires, rubber bands, adhesives, tubing, shoe soles and heels, mats, athletic equipment, safety wear, waterproof sheets, footwear and gloves, and gaskets. Natural rubber is a hydrocarbon and can be stretched to about 10 times its initial length. Rubber is usually vulcanized or cross-linked with sulfur compounds. Surgeons'gloves, with only a few percent sulfur, are soft and easilv extended. As the sulfur content is increased the vulcar&ed rubber becomes harder, more dense, and less extensible. The verv hard ebonite. used in combs, for example, contains more than 30'i sulfur content and has a density of 1.1 to 1.3 a~cm".com~aredto the dcnsity of natural Gbber, about 032 g/&n3. +he more heavily vulcanized rubbers are almost always blended with other materials which affect burning char&teristics and density. While natural rubber, with cis-isoprene, CH2C(CH3)=CHCH2 repeat units, soRens and bums readily with smoke, crosslinked or vulcanized rubber has a characteristic odor of sulfur-containing hydrocarbons when, for example, tires are burned. The sulfur links one chain to another at the unsaturated double bonds of isoprene. Unvulcanized rubber com~risesindividual polv (cis-isoprene) chains that separate when the rubber is heated and melts. Vulcanization lessens the motion of the isoprene chains, and charring is observed rather than softening and melting. In pyrolysis the distillate of natural rubber is neutral. The h e a d y vulcanized ebonite ignites but not readily and produces both hydrogen sulfide I H2S,and sulfurdioxidc (SO2), .. leaving a char residue. Butadiene rubber, with CH2CH=CHCH2 repeat unitsand a density of0.9 g/cm3,burns with a smoky flame. Styrene-butadiene rubber has the characteristic
402
Journal of Chemical Education
sooty flame of styrene polymers and a density in the range 0 S 1 . 0 g/cm3. Butyl rubber, with CHZC(CH~)~ units and density about 0.93 g/cm3, bums without smoke. Chlomprene rubber, with CH2CCI=CHCH2 units, is self-extinguishing when removed from a flame and has a higher density, about 1.2 g/cm3 or more. Unlike other rubbers, it has a strongly acidic pyrolysis distillate. Thermoset or Cross-Linked Polvmers These include, in addition to the alkyd resin polyesters, the epoxv resins, urea-formaldehvde, melamin+formaldehyde, bhenol-formaldehyde, and the common vulcanized rubbers. They do not soften or melt but decompose by blistering and charring with or without an obvious flame. Identification through combustion and densitv is not as simple for these thinnoset polymers as for those which soften, the thennoplastic polymers. The epoxies, used as adhesives, coatings, and in f;ber-reinforced goods, bum with a very smoky flame. After pyrolysis the dark oil-like liquid is alkaline and has an amine odor. The epoxies have densities of about 1.2 g/cm3. The urea- and melamineformaldehyde copolymers are used for laminating wood, foam insulation, electric plugs, buttons, and plastic dinilame nerware. Thev char and develoo stress cracks in a -- but are not easily ignited. The s&oke and the pyrolysis residue have fishlike amine odors and are alkaline. The density is about 1.5 g/cm3. Phenol-formaldehyde resins are the original Bakelite and are used for early molded telephones, electrical fixtures. the handles for wokine ~ o t sand . the familiar -- for- mica surfaces. They char to lib;& both phenol and formaldehyde. Their densities are about 1.2-1.35 g/cm3. ~
Density Unlike most other substances, polymers have densities often given as a range of values. Polymer density depends on whether the polymer is linear or branched, the extent of crystallinity, the molecular weight, and past thermal history. To determine densities a series of simple liquids or solutions is required. The polymer densities range here from 0.88 to 2.3 g/cm3. Liquids should be nonsolvents; aqueous or alcohol mixtures are the most convenient. Isopropyl alwhol has a density of 0.786 g/cm3. Several alwhol and water mixtures can be prepared and their densities determined by weighing known pipet-delivered volumes. From a prepared graph of density as a function of composition, com~ositionscorres~ondinato densities of 0.86.0.88. 0.90, . . . 0.98 and 1.00 &an3 c& be prepared. For den& ties greater than 1.00 g/cm3, the following pure liquid and solutions are useful for preparing graduated series of liquids: pure glycerine, 1.26; 26% MgSOl(aq), 1.30; 89% sucrose (prepared from boiling water and cooled to room temperature a b r solution). 1.473: 60% BaL. -. 1.97: 75% ZnI,.-. 2.39 g/cm3. In determining the density of a small fiber one should be aware that very small fibers will float on a less dense liquid because of surface tension. Fibers should always be gently pushed beneath the liquid surface to avoid this.
;
Summary Interest in the reaction of substances a t h i ~ htemmoeraturcs precedes modem chemistry. Early work& spdke of driving off the essence or spirit of a material b.y heat and thought of a material as cmsisting of both a>esidue or body and a spirit or essence (e.g., phlogiston). It was not until Lavoisier's time that calcination was seen as a gain of oxygen rather than as a loss of an essence. A major part ofehcmistry is the understandingofobservable properties in terms of molecular comoosltion. The several combustions, pyrolyses, c h h n g s , and densities men-
~
~
tioned a b v e show a clear dependence on molecular structure. Convincing arguments can be made about conducting these exercises in good exhaust hoods, and a fuller understanding can be had about the kind of toxic materials present in fires. Chemistry continually evolves in the way that investigations are conducted. We think of today's chemists using elaborate and expensive equipment to~probemore finely into substances. Friedrich Konrad Beilstein (1836-1906) extended the role of analvsis in oreanic chemistw and was the f r s t modern encyclopedist ofchemistry. he first edition of his Handbuch der organischen Chemie (1881) contained the chemical and physical properties for 15,000 comoounds. It has to be eratifvinn that so old and s i m ~ l ae toolis the Beilstein t e s c s stih useful.
Sources The formulas for monomer repeat units are found in the Merriam-Webster Unabridged Dictionary ( I ) and in many lesser dictionaries. Polymer densities are found in the CRC handbooks (2)through the 63rd edition (1928-1983) but
not in later editions. More recent CRC handbooks contain glass transition temperatures but not densities. Both old and newer CRC handbooks include polymer names and formulas. Although not as widely available, the two polymer handbooks (3,4) and the encyclopedia (5) contain extensive data on molecular eompositi6n, density; for combustion and pyrolysis, Brandrup and Immergut (3) is especially useful. Avery large number of texts on polymer chemistry and polymer science contain molecular formulas for the polymers mentioned here. Literature Cited 1. Gove, P E., Ed. WebsrmrS ThirdNem InfernofloMl Didio-; Marim-Wwbek SpwirngReld, MA, 1981. 2. Weaat, Robert C. Handkmk of ChPmisfry and Physics, 63d ed.; CRC Press: Boes Raton. FL.1882:oo C727-Cl36. 3. ~ ~ ~ ~J.i rdm m & e r, ~ tE. , H..Ed*. Polympr Hondbd, 3rd ed.; Why:New York, 1989.
.-, . ., .. 6.Kmschwita, J. I., Ed. Encyclopedia ofPi1ymr Science ondEngi-n'ng. Why:New York, 1986;Vol. 4,p 487. 6. Vogel, A. I. A TPrlbwk ofPmctlml Organic Chrmiahy; 3rd ed;h g m a n r : h d m , 1972; pp 289-210.
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