Chemistry Everyday for Everyone edited by
Products of Chemistry
George B. Kauffman
The Chemistry behind Carbonless Copy Paper
California State University Fresno, CA 93740
Mary Anne White Department of Chemistry, Dalhousie University, Halifax, NS B3H 4J3, Canada
We have all done it—filled out a form by completing the top page of carbonless copy paper with a pen, making replicas on the lower copies. When we do this we may not appreciate that the annual worldwide consumption of carbonless copy paper is of the order of 109 kg, and, given its ease of use, carbonless copy paper has almost eliminated the use of carbon paper in multiple forms. Carbonless copy paper provides an excellent illustration of many chemical processes and principles. The basis of carbonless paper is illustrated in Figure 1 for the preparation of a single replica. The top page, on which one writes, is undercoated with microencapsulated dye precursor. The pressure of the pen causes the microcapsules to break, exposing the dye precursor to a reagent that is coated on the top of the lower sheet; the ensuing reaction leads to a colored product. For multiple copies, intermediate sheets are coated on both top (reagent to react with the dye precursor on the sheet above) and bottom (microcapsules of dye precursor to react with the reagent on the sheet below). Let us look in more detail at the components of carbonless copy paper and important chemical considerations.
Figure 1. Carbonless copy paper. Writing on the top copy bursts the microcapsules containing dye precursor, allowing them to react with the reagent layer to form a colored replica of the path of the pen.
Microencapsulation of the Dye Precursor Microcapsules, of diameter 3 to 8 µm (1), are used to contain the dye precursor until application of pressure causes them to burst. This allows the dye precursor to react with the reagent in the coating of the adjacent paper. Early microcapsules for this application were prepared by precipitation of water-soluble polymers (gelatin, gum arabic, esterified cellulose) onto finely dispersed emulsion droplets containing organic solvent and the dye precursor. This precipitation was controlled by pH and temperature. Once precipitated in a colloidal form, the polymers of the capsule wall were cross-linked for additional mechanical stability (2). Synthetic polymers are now used in microcapsules to allow more control over the properties. For example, from monomers dissolved separately in organic and aqueous phases, polyurea and polyamides polymerize in the emulsion that forms at the two-phase interface. Other polymers such as melamine/ formaldehyde or urea/formaldehyde (monomers dissolved in the aqueous phase) precipitate around dye precursor droplets at the interface (2). Dye Precursors Known in the trade as color formers, the dye precursor is generally a colorless organic compound (leuco dye, meaning literally “white dye”) that becomes colored on reaction with H+ from an acid. This reaction is shown in Figure 2 for the original color former, crystal violet lactone. Change in hybridization from sp3 to sp2 and concomitant charge delocalization throughout the planar aromatic structure of the
Figure 2. The reaction of crystal violet lactone. Crystal violet lactone is a leuco dye. When protonated, it transforms from colorless to red-black.
Figure 3. Fluoran structure. Like crystal violet lactone (Fig. 2), fluorans are leuco dyes that become colored with acid-induced cleavage of the phthalide bond.
cationic product lowers the electronic excited states from the UV for the dye precursor to the visible range for the protonated product. The reaction product for crystal violet lactone is a red-black chromophore. Fluorans (see Fig. 3) can also be used as color formers. The mechanism of their color production is analogous to that of crystal violet lactone, that is, acid-induced cleavage of the phthalide bond.
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The intrinsic color of the product can be considerably changed by chemical modification of the dye precursor. In addition, the nature of the reagent layer can play an important role in the color of the final copy (3). Carbonless copy papers are available to give copies in black, red, orange, blue, green, and violet, depending on the chemical composition of the dye precursor(s) and the reagent layer. Reagent Layer Typical reagent layers (3) include clays such as bentonite, layered materials with compositions such as [(Mg 3 ᎑z Li z)(Si 4᎑u Alu )O10(OH)2 ]M+z+u
and
[Al x Fey3+Mg z)2(Si4 ᎑ (u+v)Fev3+ Alu )O 10(OH)2]M+u+ v+ zⴢ) (4). Organic reagent layers are also possible, with phenolformaldehyde resins dominant (3). Zinc silicylates are also used (3). Either intrinsically or through prior acid treatment, the reagent layer is a proton donor for the reaction with the dye precursor (3). Achievement of Usable Carbonless Copy Paper Although the principles of carbonless copy paper are straightforward to a chemist, a number of other matters must be considered to produce a marketable product. For example, although crystal violet lactone is the prototypical color former and is still a major dye precursor today, its colored product shows poor light-fastness on clays. This problem has been resolved using a combination of color formers that enhance the color after crystal violet lactone has faded and resin reagent layers that slow the fading process (3). Factors to consider for the reagent layer include the components’ uses as coatings (poor rheological properties make this a problem for many clays), yellowing on aging (resins), and color-fastness (depends on color former and reagent) (3). The original solvents used for the dye precursors in carbonless copy paper were PCBs. A combination of organic solvents is usually preferred now, and factors to be considered are the solvent color (should be colorless), the solvent power
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for the dye precursor, inertness of the solvent with respect to the dye precursor, low vapor pressure (to withstand the paper drying process in production without rupture of the microcapsules), low viscosity (to allow rapid reaction of the dye precursor and the reagent), surface tension (less than that of the reagent-coated paper for satisfactory wetting), absence of dye bleeding, favorable interaction with the reagent layer, the effect of the solvent on the color intensity of the final product, and the stability of the microcapsule walls with respect to the solvent (2, 3). As in all aspects of carbonless copy paper production, safety and cost of materials are important factors. Diisopropylnaphthalene and phenylethyltetralin are two of the major solvents used currently. Conclusion From the original invention in 1957 by the U.S. company National Cash Register (5) to an important product in our daily lives, carbonless paper is based on mechanically initiated chemical reactions. The development of these products has relied upon research in coatings, organic synthesis, polymers, clays and resins, microencapsulation, photochemistry, colloids, and solution chemistry. Research in this area is ongoing (6). Acknowledgments Useful correspondence with P. Chauvigné (Ciba-Geigy) is gratefully acknowledged. This work was supported by the Killam Trusts. Literature Cited 1. Drechsler, G. W. Coating 1986, 9, 310. 2. Stadelhofer, J. W.; Zellerhoff, R. B. Chem. Ind. (London) 1989, No. 7, 208. 3. Petitpierre, J. C. In 1983 Coating Conference, San Francisco, 15–18 May 1983; TAPPI Proceedings; TAPPI: Atlanta, 1983; pp 157–165. 4. Grim, R. E.; Güven, N. Bentonites; Elsevier: Amsterdam, 1978. 5. Green, B. K. Microcapsules Containing Oil; U.S. Patent 2800458, 1957; Green, B. K.; Schieicher, L. Microcapsules Containing Oil U.S. Patent 2800457, 1957. 6. See, for example, Katritzky, A. R.; Zhang, Z.; Lang, H.; Jubran, N.; Leichter, L. M.; Sweeny, N. J. Mater. Chem. 1997, 7, 1399.
Journal of Chemical Education • Vol. 75 No. 9 September 1998 • JChemEd.chem.wisc.edu
