The Diazo Copying Process An Example and Demonstration of Applied Organic Chemistry for the Undergraduate Student B N C ~Osterby University of Wisconsin-La Crosse, La Crosse. WI 54601 The diazo copying process is an organic-based reprographic method that is used extensively for the duplication As depicted in of microfilm and engineering Figure 1,the copying process works by passing light through nonimage areas of the master, thus producing a latent image on the diazo film or paper, which is then developed with ammonia. The process is a positive-working reprographic system. The diazo copyingprocess uses reactions and principles found in organic chemistry textbooks and is an excellent example and demonstration of applied organic chemistry.
Aromatic Diazonium Salts The chemistry of the copying process is based on two properties of aromatic diazonium salts; (1)they decompose in the presence of light (eq I), and (2) they react with couplers t o form azo dyes (eq 2). Y Y
-
*
x-
coupler
Latent image
Master Diozofilm or paper
DEVELOPING STEP
Figure 1. Light passes through the nonimage areas of the transparent master prcducing a latent image. The latent image is developed with ammonia.
Et,-
1I
qy qO" N:
111
PRINTING STEP
The Chemistry ol the Dlazo Copying Process
+
Light
N
I.,
azo dye
Most of the diazonium salts used commercially are yellow, orange-yellow, or orange powders; thus, diazo paper and film have a characteristic yellow color. The diazoniums are most often available as the tetrafluoroborate and tetrachlorozincate salts, and in some cases as the trichlorozincate, hexafluorophosphate, or sulfate salts. Some typical diazonium salts which can be used in the diazo process are shown below?
Presented in part at the 10th Biennial Conference on Chemical Education, West Lafayette, IN, July 31-August.4, 1988. Kosar, J. Light-Sensitive Systems; Wiley: New York, 1965; pp 194-307. Brinckman. E.; Delzenne. G.: Pwt. A,; Willems. J. Unconventiorr a1 Imaging Processes: Focal: London, 1978, pp 47-56. Murray, M. D. In Photography andfleprography, 7th ed; Sturge,J. M., Ed.: Van Nostrand Reinhold: New York, 1977; pp 427-430. Dinaburg. M. S. Photosensitive Diazo Compounds and Their Uses; F. m. a-k London. -.. . , 1967. 5 ........................... Informetinn nwtainino to the and couolers was . .diazonium salts -~ obtained from bulletins or booklets from Andrews Paper and Chemical Co.. Pon Washington. NY; ABM Chemicams Ltd., Stockpoll. England: East Shore Chemical Co., Muskegon, MI; and Fairmount Chemical Co., Newark, NJ.
'
'
~
1026
Journal of Chemical Education
~
~
~
~~~~
Couplers and Acid Stabilizers
The couplers are weak organic acids such as phenols, naphthols, and compounds with an active methylene. They are chosen such that coupling takes place when the coupler is in its conjugate base form hut not in its molecular form (eq 3). T o inhibit conjugate base formation, and thus precoupling, an acid stabilizer is added. The acid stabilizer is often an organic compound such as 5-sulfosalicylic acid, p-toluenesulfonic acid, or citric acid.
(negligible coupling)
(rapid coupling)
Dye color, in general, is controlled by the coupler. Naphthols usually give a blue dye, phenols a brown, red-brown, or yellow-brown dye, and compounds with an active methylene, a yellow dye. Often two or more couplers are used to get the desired color. For example, a black image area is formed when a blue, yellow, and brown dye are formed in the proper amounts. Some typical couplers and their dye colors are shown helow:5 naphthols
Printing Step
In the printing step the aromatic diazonium salt is decomposed by light in the nonimage areas, thus losing its dyeforming property. The rate a t which the diazonium salt photochemically decomposes controls the printing speed. For reprographic purposes this rate is slow, thus intense light sources are required. Typically a mercury vapor lamp of 2,000-10,000 W is used in diazo copiers. Commercial diazo paper or film is usually most sensitive in the 350-450 nm range. Developing Step
The latent image is developed when ammonia is introduced. The ammonia neutralizes the acid stabilizer and oromotes conjugate base formation of the coupler(s). The cou~ l e r ( sand ) diazonium salt react formine azo dves. The ooacity of the image areas is determine; by t i e amount of diazonium salt and coupler(s), and also the azo dye formed. Overdevelopment does not occur. ~~
~~
.~
Demonstrating the Diazo Copying Method Using Commercial Diazo Film or Paper
NHR
Y
OH
b
S03Na (violet)
phenols
(brown)
(yellow-brown)
CI
(red-brown)
Diazo paper can be easily printed using an overhead projector or sunlamp and a master (e.g., an overhead projector transparency). In a typical demonstration, an overhead transparency master and the diazo paper are placed together on the overhead projector. The lamp is turned on, and in approximately 2 min the nonimage areas are bleached out leaving a distinct yellow latent image. The latent image is developed hy simply suspending the diazo paper in a large jar containing a small amount of concentrated ammonium hydroxide. Development is instantaneous. The overhead projector demonstrations were done on a Bell and Howell (Model 301) with a 650-W light source. Other overhead projectors may require a longer print time. Diazo film can be printed in a similar manner but will require a longer print time (several minutes) using an overhead projector. Development is also a bit slower than with diazo paper. One advantage of using diazo film, however, is that the yellow diazonium color is visible with an overhead projector and its d i s a ~ ~ e a r a ncan c e be followed. - One disadvantage ofusing commercial diazo film or paper is that the exact chemical comDonents are now known. Thus. the diazo copying process is nnt nearly as amenable to classroom discussion as a film with known composition. Making Your Own Diazo Film
Chemicals acetone ethanol celluloee propionate (medium molecular weight) 5-sulfosalicylicacid naphthol AS (2-hydroxy-3-naphthoic acid phenylamide) or other couoler(s)
compounds with an active methylene
(all are yellow)
CH2Nf
Equipment transparency film for a plain paper copier (e.g., Scotch 501) 6.5- X 12.0-in.arction of window glass (or similar flat surface) %-in. stainlesssteel rod (length,15-16 in.) 22-gauge galvanized steel wire heat gun tape Volume 66
Number 12
December 1989
1027
The Diazo Solution Procedure Prepare a diazo solution by dissolving 1.40 g cellulose propionate (medium molecular weight), 0.10 g 5-sulfosalicylicacid, 0.10 g naphthol AS (blue coupler), and 0.10 g 4-diazo-Nfl-diethylanilinefluorohorate in 25 mL of 50:50 (v:v) acetoneletbanol. The cellulose propionate, 5-sulfasalicylic acid, naphthol AS, and 4-diazo-N,N-diethylaniline fluoroborate were obtained from Aldrich Chemical Company, Milwaukee, WI. The cellulose propionate should be added well in advance of the other solid components as it takes several hours to dissolve. Naphthol AS is not completely soluble in the solution at room temoerature. Gentle heatine will dissolve it. Other couoiers tested were.reaorcinol ired-brown &el. 2-12-hvdroxveth. . . ,~ org)phenol (yellow-brown dye), and 2,4-dihydrory-N-(2-hydrox. yethgllhenzamide (brown dye). These were nlsu obtnrned fnrm Aldrich Chemical Company. The diazo to coupler ratios were apprmimately 1eq diazo2 eq coupler. ~~
Coating the Film with the Diazo Solution Procedure
A drawdown bar (wire bar) is made by tightly wrapping galvanized steel wire (22 gauge) about a %-in. stainless steel rod as depicted in Figure 2. The ends of the wire are taped down to the bar. The length of coiled wire on the bar, not including the tape, should exceed 6.5 in. (the width of the glass plate). As shown in Figure 3 a half sheet of the transparency film (5.5 X 8.0 in.) is taped to the window glass. Approximately 2.5-3.0 mL of the diazo solution is applied in a line across the top of the transparency film. The drawdown bar is pulled down the transparency film, carrying the diazo solution, and coating the film. The coated film is then dried with a heat gun. If the coated film is not heat treated, the coating hecomes cloudy instead of transparent. The mated film can be printed and develuped in therame manner as the commercial film. Usine the Bell and Huwell overhead onkctor, the printing time was about 2.5 min. Development w& rapid with ammonia vapors from concentrated ammonium hydroxide.
Ffgure 2. Galvanized steel wire (22 gauge) is tigmly wrapped around a %-in. stainless steel bar.
1028
Journal of Chemical Education
The diazo copying process is an estahlished, organic-based repro~raphic . - . techniaue. T h e chemistry of the process illustrates (and demonstrates) some impo-rtant concepts in organic chemistry. The use of acid t o inhibit coupling followed by base to promote coupling illustrates acid-base equilibria and the relative activating influences of -OH vs. -0-in electrophilic aromatic substitutions. T h e directing influences of substituents on the phenol and naphthol couplers can be used to predict dye structures. The Camino, 4-thio, and 2-alkoxy substitution pattern enhances the thermal stability of the diazonium salts and thus demonstrates resonance effects. The relationship between dye color and coupler can be e x ~ l a i n e dby coniunction effects. The diazo copying processcan be used as & example of applied organic chemistry, an illustration of chemical principles and reactions, an-organic chemistry classroom demo&ration, or an experiment in organic lab. Acknowledgment The author is erateful to Northern Eneravine Coro.. . .Soar. ta, WI, for the lopportunity to researcc the &azo copying Drocess. I am also indebted t o Jack Bulloff for ~rovidinz? invaluable information and direction. I would &o like to thank Steve Rosso of Target Systems, Inc., Salt Lake City, UT, for supplying diazo paper and information on diazo copiers and diazo papers.
-
Figure 3. Coatlng the film with the diazo solution
