chemiAry on #tamp/ edited by:
JAMES 0.SCHRECK University 01 Normern Colorado Gredey. CO 80839
C. MARVIN LANG University of Wisconsin Stevens Point. WI 54481
Chemigry of Postqe Stamps: Dyes, Phosphors, Adhesives John 6. Sharkey Pace University, New York, NY 10038 Philately, the study of postage stamps, can serve as the introduction to a variety of interesting chemical topics not normallv covered in the undereradnate chemistry curriculum. rim a historical standpoi&, numerous chemists and a varietv of chemical discoveries have been depicted on postage stamps ( I ) . Avariety of man-made chemicals have found their way into the production of postage stamps: dyes and pigments, phosphors, and polyvinyl adhesives. The colors of various dyes and pigments can be elucidated by an understanding of ligand field theory, charge transfer, molecular orbital theory, and band theory. The fluorescent and phosohorescent nrooerties of certain comoounds have been used in the pape; k d ink used to produce stamps. The production of adhesives such as dextrin and polyvinyl alcohol often involves acid hydrolysis, polymerization and the use of preservatives and flavoring agents. In brief. a wealth of chemical the production of theory and technology~is~ncorporated'in modern postage stamps. Dyes and Pigments
The history of philately shows a gradual increase in the use of color in the inks used to produce postage stamps. The use of gray and grayish colors from carhon black was common in early stamps. Low chroma reds and brown were introduced through the use of natural plant dyes, such as indigo, and animal dyes, such as cochineal. Today, many synthetic organic dyes and pigments give rise to much stronger stamp colors. are the The oiements and dves used in st am^ . .. . ~roduction . most impvrtant ingredients in colored inks; however, the oils. fillers. resins. and eums in the ink can alsoaffect the way a stamp appears to the-eye. Due to lack of record keeping & for reasons of security, there is little history on the dyes and pigments used in early U.S. stamps. Investigators (2),using reflection spectrophotometry, have provided much information about these materials. For the philatelist, the precise pigment or dye composition of the printing ink is important because it establishes a "fingerprint" of the stamp and thereby helps to uncover forgeries. Many 19th century colorants were mineral in nature: for exam~le.vellow lead nitrate (Ph(N01j2),redor black iron oxide (F'e2d;j, white lead oxide IPbO). and ), . . blue ferriferrocvanite I F ~ ~ I F ~ ( C N ) & H ? O green chromium o x i d e 7 ( ~ r 2 0 &he& m&ri&, when mixed in a suitable oil without dissolving to form a uniform dispersion, are called pigments. A striking example of an early pigment is ultramarine (CaNa~Al&i~02&SO1, approximate), which was originally obtained from the blue
stone lapis lazuli. It was used in the 1893 40 Columbian stamp depicted in Figure 1. Also used were colorants which were vegetable or animal in origin, such as indigo and red cochineal. These materials are generally referred to as dyes, since they are soluble in the oil or vehicle of the ink. Finally, a limited number of synthetic dyes were produced from coal tar. A number of other chemicals had to be added to these dyes or pigments before they could he used as inks in printing postage stamps. Linseed oil or refined petroleum oil was used as a carrier for the colorant. Inert chemicals such as aluminum hydrate or calcium carbonate were added as fillers to impart opacity to the ink. A drying agent, containing elements such as manganese and cohalt, as well as other resins, solvents, waxes, and plasticizers weie frequently added to the ink to obtain desired flow and hardening- characteristics. Table 1lists the various dyes and pigments that are currentlv heine used bv the United States Postal Service for the printing of postage stamps. This information was provided bv the Bureau of Enmavine and Printine. which is res~onsibie for printing mostof the-current ~ S - p o s t a ~stamis e (3). Many of the toxic inorganic materials (PhCrOa. CdS.. HeS) - . havebeen replaced by synthetic organi'c dyes. ~ n i l i n edyes, for example, may be almost any color, depending on the
Figure 1. The blue pigment ultramarinewas used in the printing ink of this 1893 stamp.
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sequences of chemical treatments applied to the parent aniline molecule. The synthesis of seveiil of the pigments mentioned in this paper was the subject of a recent article in this Journal (4). The Orlglns of Color To the student of chemistry, it is highly informative to investieate the orieins of the color of these dves and niementson an atomicor mt,lecular level. Kurt Nassau ( 5 0 )lists 13 causes of color. involvinr five broad enerev schemes. The three tabulated in Table 2 a r e important i&nderstanding the origins of colors in dves and pigments. Also listed are some examples of dyes and pigmekcthat have been used or are currently being used to produce color on postage stamps. These transitions are arbitrary, and in fact many now consider ligand field theory and energy bands to he special cases of molecular orbital theory. There does, however, seem to be more than one structural characteristic capable of giving rise to the colors exhibited by these compounds. For example, although most of the transition metals form highly colored compounds, some nontransition elements such as lead and antimony do likewise. One thing seems clear, substances exhibit color because thev absorb selected wavelengths of white light and transmit other wavelengths. Indeed, the absorption is not limited to the visible reeion. An in-depth~discussionof the electronic transkons involved in the dyes and pigments that have been used in the production of postage stamps is beyond the scope of this paper. The interested reader is referred to reference 5 and to a highly informative series of articles in this Journal (6). However, an example of eachof the transitions is depicted in Table 2. Chromium oxide green, or chromic oxide (Cr203), has been used as an artist's color since about 1860 (7). . . It is one of the pigments that exhibit color due to crystal field transitions. Chromium in this compound is in the 3+ oxidation state, which leaves three unpaired electrons in the five 3d orbitals of the ion. In the isolated Cr(II1) ion. these five orbitals are degenerate. However, in c r 2 o 3the chromium ion is in the distorted octahedral environment of six oxygen
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Dyes and Plgments Used in the Printing of Current U.S. Postage Stamps
Table 1.
Yellow Pigments a. Diarylide yeilow-AAA b. Hansa Yellow G c , lron Oxide yellow
type, sometimes called dichlarob&ridene
Orange Plgmem a. Dinitraaniline orange b. Dianisidine orange Red Pigments a. Red Lake "C"-calcium salt b. Lithor Rubine-calcium salt C. Red 20-calcium and barium salts Blue Pioments a. Copper Phtblocyanne B ue-GS and RS b Alka Bl~e-RS ano GS tr phenylmetbne type c. lron B ue (Mi or, 8 1 4
atoms. The strength of the ligand field alters the energy levels of the chromium ion. The energy level diagram of Cr(II1) in a distorted octahedral ligand field and associated electronic transitions are shown in Figure 2 (5b).Although the figure depicts the chromium ion as an impurity in emerald, beryllium aluminum silicate, the environment of chromium is exactly the same as in Cr203. TWOtransitions are important: the 4A1 4T1transition occurs a t about 2.8 eV in the violet, and the (A2 -f 4T2transition occurs a t 2.0 eV in the yellow-red portion of the visible spectrum. The result is astrong hlue-green transmission a t about 2.4 eV, whichgives emerald and Cr90qthe characteristic ereen color. Thus. for chromium as well as other transition metal ions, the enkrgy field created by the ligand removes the energy degeneracy of the d suhlevels, resulting in levels with different energies. Electronic transitions between these and other nearby energy levels give rise to the beautiful colors of the transition metal compounds. I t was seen above that a compound must absorb some electromagnetic radiation in the visible region of the spectrum in order to have color. In the case of colorless oreanic " molecules, the energy difference between the molecular orbitals reauires the ereater enerev of an ultraviolet nhoton to cause promotion ofelectrons tohigher energy orbi'tals. This is for saturated compounds and unsaturated compounds containing only a few double bonds. However, it is observed that, as the degree of conjugation in the compound increases, the energy difference hetween the levels decreases and less energy will be required to cause absorption. This effect is known as a bathochromic red shift. For noncyclic conjugated polymers, CH3(-CH=CH-).CH3, if n is a t least 8, the compound will absorb in the visible and hence exhibit color. Hence, the compound 2,4,6,8-decatetraene is yellow because some absorption occurs a t the violet end of the spectrum. A second class of conjugated organic compounds that exhibit color are cyclic, nonbenzenoid compounds. Examples of these are the blue and green phthalocyanines, which are currently used in the inks of postage stamps. The vast majority of organic dyes, however, contain an extended conjugated system to which are attached electron donating and accepting groups. These include benzenoid conjugated systems such as dinitroaniline orange and dianisidine orange. For these compounds the conjugated system is increased in size by adding electron acceptors (such as nitrates), by direct conjugation, or by involving nonbonding p orbitals that can interact strongly with the pi system of the molecule. Furthermore. the donor-accentor interaction denends not onlv on the size of the conjugate system, but also on their relative positions in the rine svstems. Conseauentlv. - . there is a ereat Gariety of colored compounds using aniline as the parent chromophore.
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Table 2.
Causes of Color In Dyes and Plgments
Transitions lnvolvlng Llgmd Field Effects Transition metal compounds and impurities Cr,09, Chromium oxide green Fe.0~. Black iron oxide
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Green Pigments a. Phthalocyanine Green b. Malachite Green
Dam-acceptor colorants Dinitraaniline orange (2,Bdlnitroaniline) Dianisidine orange (3.3'dimemoxybenzidine)
Violet Pigments a. Methyl violet b. Carbomle violet Black Pigments a. Carbon Blacks-oil b, Iron oxide black
and furnace blacks
White Pigments a. Titanium dioxide--opaque b. Calcium carbonate-transparent c. Barite (barium sulfate)-transparent
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Transitions Between Molecular Orbitals Conjugated organic compounds Hansa yellow Copper phthalocyanine blue
Journal of Chemical Education
Charge-transfer co orants PoCrO,. Cnrome ye1 ow CaNa,A &02rSrS0.. Jltramarine blue Transitions hvolvhrg Energy Bands Semiconductors HgS, Vermillion CdS. Cadmium vellaw
\RED FLUORESCENCE
LIOAND FIELD
(aV)
Figure 2. The term diagram of Cr(l1i) in a distorted octahedral ligand field (A), the energy levels and transitions In emerald (61,and the resulting abswptlon spectrum and fluorescence of emerald (C). The green color of the pigment Cr.0. is due to identicalIransitlons. Reprinted wilh permission 01 John Wiley 8 Sons,Inc. (5).
Fiaure 3.Same US. alrmail stamus with a ohosohorercem coatino containina ca1el.m solcate Tne 8c stamp. Scon U64.*as prmlea m 1963 m a was the llrsl a m a d stamp lo be coated w tn a pnosphorercenr taggang compouno
ne slamps depicted m Figure 3 aher exposure to shooFgue 4 uavelengln .Itravolet radiation Tne orange-rea phospnore9cenceis due lo a tagging compound containing calcium silicate.
r w r e 0 . Iwa us. last-class sramps, expos- In VlslDle ilght, coatea wrm the tagging compound zinc orthosilicate. The 1963 5Q City h i 1 Delivery stamp was m% first stamp to be entirely tagged with a phosphorescentcompound.
trgure e. me same stamps shown in Figure 5 after expasure to shortwavelength ultraviolet radiation. The coating on lhs stamp, zinc otlhosiiicate, shows phosphorescencein the green region of me visible region.
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Another group of compounds whose transitions involve molecular orbitals, are those that exhibit color due to charge transfer. Charge transfer occurs when two different ions with different oxidation states are close enough to permit overlapping of their orbitals. Absorption in the &iblecauses an electron to be promoted from the ion having the lower oxidation number to the one having the higher oxidation number. For example, in chrome yellow (PbCr04), Cr(V1) has a strong attraction for electrons and will be more stable if an electron moves from the oxygen to the chromium. This is an example of ligand-to-metal charge transfer and requires the absorption of blue light. Hence, the resulting yellow color of lead chromate. The pigment ultramarine, the use of which is illustrated in Figure 1, also employs charge transfer. Transitions amone the molecular orbitals of the Sa- ion cause a strong absorption band a t 2.1 eV in the yellow, leading to the blue color of this pigment. This is an example of anion-anion charge transfer:Ii is very possible that the same charge transfer mechanism is a t work in the donoracceptor colorants menrioned previously. The third type of electronic transiuon involving dyes and pigments are~thoseinvolving energy bands. pigments such as vermillion (HgS) and cadmium yellow (CdS) do not appear to have any of the structural characteristics of the dyes and pigments already mentioned; hence the origins of their colors must involve another type of electronic transition. The properties of these semiconductors can be understood through a discussion of "hand theory". A crystal is viewed as having two sets of energy levels resulting from the combination of the valence orbitals of all the atoms. These sets of energy levels are analogous to bonding and antibonding orbitals when orbitals from just two atoms combine. The lower set of energy levels is called the valence band and is occupied by the valence electrons of the atoms. The higher set is called the conduction band, where electrons are free to move throughout the entire crystal. Metals are excellent conductors of electricity because there is no energy gap between the valence band and the conduction baud. In semiconductors, however. there is a hand eao. .. and enerev -. is reauired to promote electrons from the valence hand to the conduction band. If the band eau corres~ondsto the visible region. electrons may he from a lower band to a hl'ghe; band and the material will exhibit color. For example, cadmium sulfide, the pigment cadmium yellow, has a band gap of 2.6 eV. This band gap energy permits absorption of violet and some blue, hut none of the other colors, leading to a yellow color for this material. A smaller band gap, as in the pigment vermillion (cinnabar, HgS) with a hand gap of 2.0 eV, results in all energies but the red being absorbed and thus leading to a red color. When the hand gap is less than 1.77 eV, the limit of the visible spectrum, all light is absorbed and compounds such as galena (PbS) are black.
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large amount of mail that must be handled each day. Each letter must be checked to ensure postage is affixed; each must be canceled and sent on its way. In the early '60's the Post Office Department began using facerlcanceler machines, which could sort and cancel letters a t a rate of up to 30,000 pieces per hour. The method employs a stamp overprinted with a nearly invisible phosphorescent material that, when excited by split-second exposure to short-wavelength ultraviolet radiation, emits a brief afterglow. Mail with stamps that glow different colors is diverted into separate channels. The phosphorescent material can be applied to the basic paper coating of the stamp, in the printing inks, or as a special phosphor coating that can be applied in lines, blocks, or overall. The process by which some phosphor substance is incorporated in stamp production is known as tagging. Tagging was first tried in Ohio in 1963 on an 86 airmail stamp (Scott #64) (8). The material used for tagging was a calcium silicate compound, CaSiOa, which, when exposed to short-wavelength UV, glows orange-red. The appearance of the stamp in visible and ultraviolet light is illustrated in Figures 3 and 4. T o distinguish first-class stamps from airmail stamps, anorhtr tnggin~compound was chosen. zinc orrhosilicate. %n-SO4,u h ~ c hglows pale green. This is illustrated in Figures 5 and 6. From a chemical standpoint, two important questions arise. What is the origin of the phosphorescence in these materials, and why were materials chosen that phosphoresce rather than fluoresce? I t is interesting that the answer to the first question lies not with the chemical composition of the two compounds but in the impurities that are present in each. If a small amount of chromium impurity (activator) is present in calcium silicate, as in the mineral wollastonite, it glows orange-red when subjected t o short-wavelength UV radiation. If zinc orthosilicate has small amounts of manganese activator present, as in the mineral willemite, it glows green under the same conditions. The origin of phosphorescence is thus due to crystal field transitions involving Cr(II1) or Mn(I1). The Mn(I1) ion, for example, in a tetrahedral site
The luminescent properties of certain papers and inks have been used in two important ~hilatelica ~ ~ l i c a t i o nThe s. first application is the use of phosphors on the surfaces of stamps, so that facerlcanceler machines can locate and cancel the stamps. Phosphors are materials that give off visible light when exposed to ultraviolet radiation. Another philatelic application is in the detection of fakes and forgeries. A major problem encountered by the Postal Service is the
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When certain materials are subjected to short-wave radiation, they will emit radiation of longer wavelength, often in the visible spectrum. This phenomenon of induced light emission is called luminescence. Fluorescence and phosphorescence are two types. Fluorescence is a glowing effect produced by subjecting certain substances to ultraviolet light, in which the glow characteristics are limited to the duration of the UV exposure; there is no afterglow. Phosphorescence is similar to fluorescence, but the glow characteristics continue, not only during the UV exposure but also afterwards. 198
Journal of Chemical Education
Energy-levelschemed adye shovrngaosorptm A fluorescenceF. laser IranZltlOn L phosphore~cenceP lnlerna conversoon IC, ana n t e r s y r tern crorslngz SC R e v nlad r lh permlss on of .oh" W ey &Sons nc (9 Figure 7
B
produces 'TI-%I emission as green light on exposure to UV light. In addition to inorganic materials, a number of organic dves also . nhosnhoresce. The seouence of stens leadine to . phosphorescence is shown in ~ i i u r e7 (5c). l his figureUdepicts the energy level scheme of a dye showing absorption, fluorescence, a laser transition, phosphorescence, internal conversion. and intersvstem crossine. Ahsorntion of radiation causes the electrons in a mole&le to be promoted to excited singlet states (S, or 52). Another excited state is called the triplet state, which differs from the singlet state in that the spin of two of its electrons are arranged so that they are parallkl rather than opposed. In some cases, it is possible for the molecule to change states, i.e., change from the singlet to the triplet state. The singlet-to-triplet change is called intersystem crossing. This can happen, for example, when the molecule contains heavy atoms that can reverse the relative orientations of pairs of electrons because of its strong spin-orbit interaction. The molecule can lose energy to its environment via internal conversion and finally revert hack to the eround state via ohosnhorescence. Since the . . radiation is ghen off slowly, the emission may last long after the original excited state is formed. Why are the phosphorescent chemicals used rather than chemicals that fluoresce? The answer is that the paper . . used in theprodurtion of stampsand envelopesas well as printing inks contains chemicali that tluoresce. In 1959, paper manufarturers started to recyrle paper. Chemicals were added to remove old printing inks, and these chemicals reacted to the U\' lieht in nronortion to the auantitv wed. In addition. paper-manufaciurers experimented );y adding different chemicals (optical bleaches) that gave paper a whiter appearance without the use of harsh bleaches, which could break down pulps. Although the Postal Service no longer uses fluorescent paper, envelope manufacturers still do, hence the need for tagging compounds that phosphoresce rather than fluoresce. The Postal Service stopped using fluorescent paper when tagging with phosphorescent materials began. However, in the transition period, 1962-1964, a number of hybrid stamps appeared-stamps with both fluorescent (stamp paper) and phosphorescent (tagging compound) properties (for example, the 1963 and 1964 Christmas stamps). The elimination of domestic U.S. air mail led to uniformity in tagging in 1980-all stamps are now tagged with comnounds that nhosnhoresce ereen. Furthermore. to . . distinguish tagging compounds from papers and inks that fluoresce under lone-waveleneth UV. onlv taeeine" compounds are used thatiespond to short-kavelkngth UV radiation. The nature and concentration of phosphors currently used by the Postal Service is not known. The reason for this, uu
Table 3. T V O ~oSf Adhesives
Protein or protein derivatives Starch. celluose, and their derivatives Thermopla~ticsynthetic resins Thermosetting synthetic resins Natural resins and bitumens Natuml and synlhetic rubbers Inorganic adhesives
Adhesives Examoles
Glues made from hides, bones, etc., fish glues, and protein glues Dexlrins, gum arabic, ghatti. tragacanth Polymerized material such as polyvinyl acetate and polyvinyl sicoh01 Phenol aldehydes, urea aldehydes, polyurethane resins Prcducts made from asphalts. shella~s Rubber latex, natural and synthetic Sodium silicate, plaster of Paris, cement
in view of the accessibility of sophisticated color copying machines. is to discouraee counterfeitine. A second philatelic application of luminescent chemicals is in the detection of fakes and forgeries. Since UV will reveal many differences in materials not seen in visible light, it can frequently be used to detect repairs, alterations, and forgeries. The reason is that material used to repair stamps may have different fluorescent . nroperties from -genuine copies of . the issue. For example, cancellation removals, color removal, erasures, cleaning, gum changes, and reperforation all leave fluorescent evidence. Long-wave UV is most useful for examining the fluorescent properties of paper. Adheslves Adhesives are materials or compositions that enahle two surfaces to become united. Adhesives can be broadly classified as oreanic and inoreanic. The oreanic adhesives mav be of animal; vegetable, orsynthetic origin. I t is more informative, however, to classify adhesives based in their chemicalnature (9)as shown in Table 3. Three of these adhesives have found use in postage stamps pro(lt~crdin thr llnited States: gum arahir, dextrin, and pulwinyl alra,hd ,PVA). These nw water remoiitenilble ndhesives in that they are dry hut hecome activated hy moisture when used on envelopes.'l'he earliest gums.gum arabicand potato stnrch~lrrivutivcs,werecrt~deat 1)est'l'hey tended todeteriorate hy cracking, peeling, and yellowing. Many 19th century stamps are now found in a state 01 deterioration. Modern adheiives are expected to ha\.e n much h g e r lifetime. Dry gum, or I'VA gum is a modem replarement for natural glue made from drxtrosr or thr earlier eum arahic. It wni dr\.eloped because dextrin gum tended-to ahsorh humidity and become sticky before its intended use. I t was first used in the 1971 Eisenhower booklet stamps (Scott #1395c). I t is now used on all booklets and most single-color sheet and coil definitive stamps. Each of these three adhesives is briefly discussed in the following paragraphs. Gum arabic is a direct exudate from the acacia tree found in various tropical areas. Its composition is a complex mixture of Ca, Mg, and K salts of arahic acid, a complex branched polysaccharide that contains ealactose, rham. . mwe. glucuronic acid, and arahinose residues. Reported molecular weights are in the range of 1,60,00U to 1,160,IJUO.Used as a stabilizer and thickener in foods, its main nonfood uses are in the formulation of inks and adhesives. It is highly soluble in water and slightly acidic. Dextrin, also called British gum, is a water-soluble, gummv substance comnosed of a mixture of low-molecularweight pdysuccharides. It is prepared from food grain corn starch hv acid hvdrolssis . . followed h\. oolvmerization of the hydrolyzed fragments. In addition t i i & adhesive properties, it is used as a thickening agent in nrintina inks and food. Dextrin applied to haikiof posiage stamps tends to deteriorate with age. This is dearly shown in the scanning rlr1.tnm photnmirrogmph (Sl.:hli shown in Figure 8. In this 1975 stamp the layer of dextrin is seen and can he distinguished fromthe underlying layer of cellulose of the paper. Cracks in the dextrin layer, invisible to the human eye, appear as deep crevices under the SEM. In time. the crack will deenen and cause damage to the paper. (To the philatelist, what is on the back of a stamp is almost as important as what is depicted on the front. Gum deterioration or disturbance of any kind will lessen the monetary value of the stamp. I t is not surprising, therefore, that regumming of stamps has become a common practice. It is not an easv matter to distinguish remmmed stamps from the originai(lO)). PVA is a colorless, water-soluble resin of formulation (-CHzCHOH-1,. I t is used chiefly as an adhesive and as a sizer in the manufacturer of textiles, paper, and plastics. Since it does not ahsorh moisture, it is ideal for tropical use and for suchapplications asstamp booklets, which may sit in outdoor vending machines for weeks before sale. Its method Volume 64 Number 3 March 1987
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Figure 8. A SEM photomicrograph (SOOX magnification) showing theadhesive dextrin on the back of a postage stamp. The photo shows a cracked adhesive layer and the underlying layers of cellulose of the stamp paper. The stamp is the 11Qprinting press stamp. Scon #1593.
of preparation should he of interest to students of organic chemistry. Polyvinyl alcohol cannot be made by the polymerization of vinyl alcohol, since this compound does not exist. A simplified version of the synthesis is described: S t e p 1. Acetylene reacts with acetic acid to form vinyl acetate, using HgS04as a catalyst. This monomer is a flammable liquid, moderately toxic, with a sharp, irritating odor. S t e p 2. The vinyl acetate is then polymerized to give polyvinyl acetate, a polymer that is neutral, tasteless, odorless, and nontoa-
ie. Step 3. Finally the polyvinyl acetate reacts with methanol, in the presence of acid to give methyl acetate and the polyvinyl alcohol. The reaction mixture is held at 57-59 "C until the lowest boiling
component, methyl acetate, distills and drives the reaction to completion. This aeid-catalyzed reaction, called transesterification, is quite complex, involving up to six steps (11). PVA is applied to the paper while it is being manufactured. The gum, which actively impregnates the paper, exhibits some texture or grain, as is clearly shown in the SEM photograph in Figure 9. Its appearance under the SEM is clearly distinguishable from that of dextrin. Formulations used in the adhesives on current postage stamps can vary in order to impart properties to the adhesives. For example, blends of PVA and polyvinyl acetate, as well as blends of dextrin and polyvinyl acetate, have been used. A number of other ingredients may he added to the adhesive before it is applied to the paper in order to impart specific properties. Modifying ingredients are added for flexihility, spreading quality, ease of re-wetting, increased tackiness and for exposure to heat, cold, dampness, and
200
Journal of Chemical Education
Figure 9. A SEM photomicrograph (1000X magnification) showing the adhesive PVA used on the back of Scon # I 6 1 3 The dull gum shows completely different characteristics compared to the shining dentrin layer shown in Figure 8.
dryness. Among these are glycerin, corn syrup, glycols, urea, sodium silicate, and emulsified waxes. Preservatives such as sodium benzoate, quaternary ammonium salts, and phenol derivatives may also he added. Finally, scenting or flavoring agents may he added, such as oils of wintergreen, lemon, grape, or anise. It is apparent that there is a wealth of chemistry underlying the use of adhesives on postage stamps. ~ d h e s i v e s in , general, represent an important and rapidly expanding field, one worthy of coverage in the undergraduate chemistry curriculum. Acknowledgment I wish to thank Bonnie M. Westhrook, External Affairs Officer of the Department of Treasury, Bureau of Engraving and Printing, for providing information on the dyes and pigments currently used in postage stamps. I also thank Seymour Z. Lewin, Professor of Chemistry, New York University, who took the photomicrographs with the scanning electron microscope. Literature Cited
.. , ,. , . .... 6 . Olna. M.V. J. Cham. Educ 1980,57,256.264.267. 7. Laurie, A . P. New Liphl on Old M~lstws:Sheld~n:London. 1935; p U . 8. Scollr Sperialiled Collrlopllr of United Stales Stamps: Scott:Now York: 1916:p 13. 9. Van Nosliondb Scianfilic Encyrlopedio. 5th ed.: Van Nosfrsnd: New York. p 39. 10. Apofelhaum. E.P. Philately with Ezpeib; pamphlet on regumins free from E. P. Apelelbaum Inc.. 1W6 Walnut Street. Philadelphia, PA 19108. 11. Murrison, R. T.; Boyd, R. N . 01ponic Chemistry, 2nd ed.: Allyn and Bsmn: Boston,
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