real world of indwt rial chemift ry
edited by
W. C. FERNELlUs Kent State University Kent. OH 44242 HAROLD WITTCOFF Chem Systems. Inc.. 303 South Broadway Tarrytown. NY 10591
Pearl Essences: A Specialty Chemical Product Yoshio Morita 34-29 3-chome Honamanuma, Sugunami-ku, Tokyo 167, Japan Buttons have been associated with mankind since prehistoric days. They h w e been made from a varietv ofrnaterials. Is there anyhi& new in such an old subject‘! Sea, modern chemical t e c h n d o ~ yhas made significant contributions to the making of the-blanks (sheets) from which buttons are cut. Let us consider here only the common buttons found on most shirts. Some of the ~rinciolesinvolved in the relevant technology are: refractive index, multiple reflection, control of crvstal size and habit, oolvmer theory (in choosine-. orooer vehicles), and dispersionand orientation of a solid in a liquid (the oiement in the vehicle). N& that the button is not opaque but reflects light in a characteristic way with a faint display of colors. Pearls and shells of certain molluscs possess this same property. In fact, shirt buttons for a long time were cut from oyster shells (e.g., Petria maxima). However, there were difficulties associated with the dependence of one manufactured article on the uncertain raw material supply from nature. Suppose the growth of the shells is much retarded by had weather or other natural conditions. or the fishine costs soar. How could thrse difficulties be circumvented~ T h r aooearance uf a hutton deoends on the oresence of small, thin platelike crystals (verishort in one axis as com~ a r e to d the other two). In pearl and oyster shells there are planes consisting of minute crystalline platelets of calcium carbonate stacked in parallel orientation wit,h cementing protein layers in between. At each interface the incident light is reflected partly and the rest transmitted downward. Due to the multitude of interfaces, the reflections and transmissions are repeated until the intensity of light has become infinitesimal. In this orocess a substantial nortion of the incident light is visible as reflected light, from uarious depths. This is distinct from the mirror reflection of a metal surface. A reasonably obvious way to produce a material analoeous to ovster shell would he to obtain either a natural or synihetic essence and suspend i t in a material (monomer or semipolymer) that can "set" (polymerize) to a solid. T o understand some of the problems involved it is necessary to consider some aspects of crystallo~raphv polymer- - - and of . . ization. A crystal of a given compound is characterized by a constancy of angles hetween like faces. This does not mean, however, that all individual faces of that crystal are alike. Sunoression of erowth on soecific faces results in avarietv of .. formsso that the habit varies from specimen t o specimen. In producing the thin platelets suitable for pearl essence one must first find a material which exhibits the platelike habit and then control the conditions of orecioitation, . . . etc... so that a product of high quality can be produced in each reaction
batch. The choice of materials is further limited in that the platelets must have a high refractive index (minimum of 1.8).1 When incorporating the pearl essence into the polymerizing medium, it is necessary to insure a parallel orientation of the platelets. The key is the applications of a shear force during the polymeri~ationor curing of the resin a t a highly viscous stage. The crystals will arrange themselves so that they offer the minimum resistance to flow-aligned so that the flat surfaces are parallel to the direction of flow. The ~olvmerizahle material (acrvlic and unsaturated nolvester . . . . revins are the polymers of chbire) must he rhosen so that the final omduct w~llhave a sufficicntlv- hirh softeninr -uuint to withstand laundering and pressing. Acrylic monomer (or semi~olvmer) . . or unsaturated oolvester resin containine peati essence is cast in a mold, &hi& may he a pair of d a t e s sealed a t the perioheries with a easket, or a closed (or sometimes open) c3inder that can sp&. In the former case the force is generated by certain oscillations along the plates, and in the latter case hy the gravitational flow against the spinning cylinder wall. The condition of oscillation or spinnine is i m ~ o r t a n tas well as the . nolvmerization or curine " conditions. What actuallv are such olatelets? Their develooment has taken place in stages called generations.
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First Generation Pearl Essence The historv ( I ) of the first pearl essence eves back to the 17th century, starting from the application-of fish silver t o simulate a pearl. Fish silver is a collection of guanine-hypoxauthene crystals obtained from the silvery layer on the skin surface of fish (e.~.,herrina). Crystals extracted from fish scales are rectified and made into a concentrate paste by mixing with a vehicle (predominantly nitrocellulose). This natural product was costly and its supply depended on the availahility of the fish. Naturally a substitute material less expensive and more readilv available was needed. While unsuccessful trials were in progress, studies of optics revealed the conditions that such a material must satisfy: The datelets have a refractive index ~ienificantlvereater than that thr medium ,greater than I . 8 1 ' k d ade(u& chemical stability \agoinst hear, light, etr.,.
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Journal of Chemical Education
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The refractive index of most plastic materials is about 1.5. With the same index for the platelets dispersed, the plastic remains transparent and the platelets are not visible. A s the index difference is greater, so is the reflection from the interfaces.
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The platcleti have extremely muc,th, flat surfacesand an optical thirkuuis (dctual dimenaiwal t l ~ i c k w i stimes 1he refrartiw indell o.clusc lo 1J8 nm aspoasible tfor the nlmimum rrfl~ctivity~. Within the limit that no platelet be discernible, planar dimensions be as great as possible.
Based on these findings, the most important second generation material was developed. The Second Generation Although lead arsenate and bismuth oxychloride were used for limited applications, these were of little importance. Basic lead carbonate, on the other hand, following the pioneering work of S. Hachisu (2, 3, 7) proved to have the necessary properties and gradually superseded the natural pearless&e. 1t can be made in virtuall$ any desired platelet size, has excellent light stability and reasonable heat stabilitv. The crvstalline olatelets are reeular hexaronal. a ~ n r o x i i a t e l y 10 to 20 fi iniide-to-side wi&h and 5cto 70n;o'thick with a refractive index, n, near 2.0. Colored crystals developed later (4, 5) that produce the color by interference of liaht had an "o~ticalthickness" raneine from 200 to about 650 nm. This synthetic pearl essence wasprovided mostly by two manufacturers, one in the US.and the other in Japan. The manufacturing process used in Japan was different from that in the US.(6), and the former will be summarized here (7). The commercial process must satisfy the following: a substantial yield per operation without concomitant formation of small crystals which spoil the optical effect; a uniform crvstal size (es~eciallv thickness): . . .. and the retention of the integrity of the crystals until the completion of the process. Both fragmentation and agglomeration of crystals are undesirable. To meet these requirements, controlled nucleation, growth under dynamic equilibrium of the reaction system and the "flushing" process are indispensable. (The term "flushing" means a process of mixing an aqueous suspension of pigment with an organic medium and then removing water.) The actual process consists of two steps: reaction and finishing. Reaction Process A lead acetate solution and a potassium carbonate solution are mixed for precipitation. A very small amount of the resultant ~ r e c i ~ i t a consistine te of nuclei each weiehine as little as 16-15g k diluted with a\uantity of basic l e a i a c z a t e solution. This dilution results in the disappearance of the precipitate even though i t is not dissolved. First, the diluted material is added to the mother liauor to a concentration of 10" to 1012nuclei per liter. ~ e r a t i oisi started to provide the agitation and after a few seconds. when weak. silkv streaks are visible, a flow of carbon dioxide is started a n d also the feed of basic lead acetate solution. The rates of addition are slow at the heginning and are gradually inrreared commensurate with the crvsral arowth. If the addition exceeds the growth capability; additional nuclei are formed and small crystals result. Before establishing this procedure, there were many empirical trials, and one fortuitous consequence was the observation that appropriate addition of a nitrate solution can accelerate the growth of thickness, which enabled the production of colored crystals. Whereas most impurities hindered normal crvstal erowth. this imouritv worked positively like those i n t h e s~miconductorindustry, where a minute amount of an i m.~ u r i.t vis . ~ u r.~ o s eadded l v to the silicon (doping process) to make a semicondukor material. Finishing Process The crystals are allowed to settle and are decanted and washed with pure water. The crystals are hydrophobic but not lyophilic, so that the aqueous dispersion is unstable and transfer into an organic phase is difficult. Depletion of dis-
sulvrd lmd by washing results in irreversihlr ngglom~mtion of crystals, and d q i n g of the aqueous suspension brings about aconcretion ,frlle mass. It was found hjrchance that nitrocellulose was a good flushing agent. For wider applications, however, new flushing agents were necessary. Consequently, various synthetic resins and surfactants were dissolved in appropriate solvents and examined. None was free from agglomeration. I t was in 1956 when the author thought, from the above experience, that dissolved lead might be responsible for the stability of the aqueous dispersion; hence, the organic substance containing intramolecular lead might be a good flushing agent. A surfactant manufacturer suggested that conventional sodium salt detergents could be readilv"nrecioitated as lead salts. Their use nroved . . successful, providing fast transfer without fragmentation or agglomeration. Addition of this material, lead alkylhenzene sulfonate, to various resin solutions previously tried, eliminated agglomerations. Shortly afterward, barium was substituted for lead for better heat stability, and a way to broader applications was opened. During the 1960's substantial quantities were exported to the U.S (An American patent (6) covering the product and the process was filed in 1957. Japanese patents (7), however, were earlier, i.e., filed in 1951. This priority limited the enforceability of the former.) everth hi less, the second generation pearlessence had several drawbacks, such as limited heat stability, low acid resistance. ~ o t e n t i alead l toxicitv and related environmental pollution problems. I t was destined to give way t o a third generation product: coated mica flakes. The Third Generation The origin of this product is as old as 1963 (8,9). Titanium dioxide is an ideal material for pearl pigment with its very high refractive index (about 2.51, excellent chemical stabilitv. nontoxicitv. and availabilitv of the raw material at a r&sonable price, if i t can he made in a platelet form. Unfortunately, i t does not crystallize readily in platelets. There is, however, a way to make this substance in platelet particles: the use of flat substrate surfaces. Several existing flat surfaces including glass plates, were tried as the substrate, but mica (muscovite) flake is the only successful material so far. I t took almost one decade, howiver, until a quality acceptable as pearl essence finally appeared on the market. Since then, most of the application fields of the second generation have been occupied by the third generation, but buttons are still made from the former. The reason is (as the author learned from his old colleague who now works a t a German manufacturer of third generation pearl pigment) that processability (cutting) of button blanks made from this oearl is noor due to the hardness of mica. This German firm. as well as the American manufacturer mentioned before, are active in the US. and a number of improvement patents (10-12) have been issued for those companies, but this time the Japanese manufacturer had no such priority as it had for the second generation. Its whole production was abandoned before the earliest American patents (8, 9) expired, due to pearl essence. Now rapid decline of the second the German firm has production facilities in Japan. Postscript
A careful observer will note that nacreous piaments are not confined to buttons or to basic lead caibonate- and titania-coated mica flakes. Such piaments of a variety of compositions are found in thermo&tics, coatings, nailpolishes, hand lotions, soaps, etc. About titanium dioxide, a recent (1985) newspaper article reported development of "flake titanium white" (for face powder, etc.) by a Japanese cosmetics manufacturer using fine ceramics technology. If the dimensions can he controlled, this might mean a harbinger of a fourth generation pearl pigment, i.e., titania pearl pigment without the micaceous carrier. Volume 62 Number 12 December 1985
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Acknowledgment
The author wishes to express his appreciation to James E. Armstrong I11 for helpful suggestions and his deepest gratitude to W. C. Fernelius whose intensive support and advice have made completion of this article possible. Literature Clted (1) Docker, W. E., Metal Finishing, Mar.. Apr. and M a y 1963. (2) Hachisu, S.,Sci. Light. 6[1] (1957). (3) Haehisu, S.,J. Color, 32131 (1959). (4) Greenstein, L. M.. "Nacreous Pigment and Their Rope&d'. Proe. Sci. Scetion, No. 45, May 1966. For an explanation of why colorlas materials such as basic lead
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(5) (6) (7) (8) (9) (10) (11) (12)
carbanafeortitanivmdiaxidecan~rduodmiorbyintedcnneeofi~~t,t,. Nassau, K.. "The PDmies and Chemistry of Color,).John Wilcvsnd Son, Ine., 1983. ~ i l l e rH. , A,, et d.,USP 3,125,485, ~ a r17,1964, . "colored optical ~ ~ ~ m c n t s . " ~ i l l e rH. , A., end Greenatein, L. M., USP ~ , % o . ~ ~ I , A DAP,. P I . LO,195%c h e m . ~ b . , 56.1554f (1962). JspanesePstients 1952, No.4493,filedApr.4,1951,and1953,No. 2738filed Sept 11, 1952. N - E I I ~ I ~versions S~ am available, Klenke. E. F.. Jr.. and Stratton. A. .1 USP 3,087,827, Apr. 30, 163, cf. Belg. 619,446. ch&.~bs.,58;12772~(1963). Linter, H. R., USP 3,087,828-829, Apr. 30, 1963, cf. Belg. 619,447, Chem. Abe., 58, 12772H (1963). Esselharn,R.,Bemard,H.Ger.Offen.2,522,572,Dec.6,1976,Chom.Abs.,S6,5M166e (1977). Merck,Gor.2.628.353,Ckm.Ab.,88,75381s(1978). kmanini,L.,USP4,192,291,Mar 11,1980, Chem.Aba.,9.3,96mp (1980).
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