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I n d . Eng. Chem. Res. 1987, 26, 2144-2148
ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 13. Sacco, A.; Caulmare, J. C. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 9. Shah, Y . T.; Stuart, E. B.; Sheth, K. D. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 518. Siegel, R.; Howell, J. R. Thermal Radiation Heat Transfer; McGraw-Hill: Washington, DC, 1981. Suzuki, G.; Uchida, M.; Ohsaki, K.; Onodera, T.; Umemura, T.; Sundaram, K. M. Presented a t the Spring National Meeting, American Institute of Chemical Engineers New Orleans, April, 1986.
Trimm, D. L. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 9. Tsai, C. H.; Albright, L. F. In Industrial and Laboratory Pyrolysis; Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32; American Chemical Society: Washington, DC, 1976; Chapter 16. Zimmerman, G.; Kopinke, F. D.; Nowak, S. Presented at the Spring National Meeting American Institute of Chemical Engineers, New Orleans, April 1986.
Received for review December 24, 1986 Revised manuscript received June 8, 1987 Accepted July 15, 1987
A New Photoimaging System Based on Singlet Oxygen David S. Breslow,* David A. Simpson, Brian D. Kramer, Robert J. Schwarz, a n d Norman R. Newburg+ Hercules Research Center, Wilmington, Delaware 19894
Singlet oxygen chemistry has been used to develop a highly photosensitive lithographic plate which can be imaged by projection or in a camera and which shows no reciprocity failure when laser imaged. The system consists of a polymeric film former, a monomer capable of free-radical polymerization, a singlet oxygen acceptor, a singlet oxygen sensitizer, and a redox metal catalyst. Exposure is carried out in air, the plate is processed in the absence of oxygen to cross-link the exposed areas, and an aqueous alkaline developer is used to dissolve the uncross-linked areas. Plate sensitivity is about 0.2 m J cm-?-, 2-3 orders of magnitude more sensitive than commercial wipe-on plates. Commercial feasibility has been demonstrated. Photopolymerization represents an excellent approach
to amplifying the effect of a single photon. Walker et al. (1970) estimated that quantum yields for photopolymerization systems can range from IO3 to IO6 double bonds consumed per photon. High efficiency can only be obtained under optimum conditions. The classic problem with photopolymerization systems is inhibition by oxygen. Oxygen can quench photosensitizers and can interact with free radicals to inhibit chain growth. We decided to use the normally deleterious quenching reaction, which can generate singlet oxygen, to form a novel latent image. Activation of this latent image in the absence of oxygen results in polymerization, allowing eventual image development. Our ultimate goal was to develop a photopolymerization system which would be sufficiently sensitive to be imaged by projection, in a camera, or by a low-power laser. For these imaging methods, certain requirements must be met. (1)The system must be sensitive to low light levels and not show reciprocity failure during laser imaging. (2) It must be sensitive to visible light. Ultraviolet light would require quartz optics in a projector or camera, which would be very expensive. In addition, inexpensive, reliable lasers emit only in the long-wavelength region. (3) A projection plate must be negative working, Le., the exposed areas must be ink receptive and printing. A camera plate would have to be positive working, although there are methods which allow reversal. Laser imaging is compatible with negative or positive working systems. Although various possibilities exist for utilization of singlet oxygen chemistry, we decided to investigate lithography first. Lithography is a printing process based on the ability of a surface to accept ink or water. A li*Present address: Breslow Associates, Wilmington, DE 19803. Deceased.
thographic printing plate is prepared by a photographic process in which the ink-receptive, or oleophilic, area is usually a cross-linked polymer (2-10 pm thick); the hydrophilic area, usually aluminum, rejects ink when wet by water. During printing, the press applies water and ink alternately; the ink adhering to the oleophilic areas is transferred to a rubber roller and then to paper. Because of this intermediate ink transfer, the process is often referred to as “offset” printing.
Discussion Ground-state oxygen is a triplet, possessing two unpaired electrons with parallel spins; this accounts for its radical-like activity. Singlet oxygen, however, has its electrons paired; as a result, its chemistry is quite different. Only 22 kcal(92 kJ)/mol is required to convert triplet oxygen to the first excited singlet state, ‘Ag. Thus, sensitizers activated by visible light have sufficient energy to effect the conversion. The ‘Ag state is a relatively long-lived species, allowing for efficient chemical reaction in solution (Adams and Wilkinson, 1972). Singlet oxygen reacts with olefins possessing an allylic hydrogen by an “ene-type”reaction to form an unsaturated hydroperoxide; the reaction is favored by alkyl groups on the double bond. For example, 2,3-dimethyl-3-hydroperoxy-1-butene is formed from 2,3-dimethyl-2-butene, eq 1 (Foote et al., 1968). 1,2-Dimethylcyclohexene,since it (1)
contains two different types of allylic hydrogen, yields a mixture of two hydroperoxides in an approximately 9: 1 ratio, eq 2 (Foote, 1968). The relative reactivities of 1,2dimethylcyclohexene and 2,3-dimethyl-2-buteneto ‘02are 0.53:l (Kopecky and Reich, 1965). 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26,No. 10, 1987 2145
4-
used (eq 5). For long-term storage of lithographic plates HHco0c2H5 CH3 CHSDCOOC~H~
+ /
11%
8 9%
The chemistry of this new photoimaging system is generalized in eq 3. The system consists of the following ingredients: (1)polymeric film former, (2)monomer capable of free-radical polymerization, (3) singlet oxygen acceptor, (4) singlet oxygen sensitizer, (5) redox metal catalyst.
HsCzOOC
H
CH,
CH,
"COOC2H5
5A
,
,COCI
CH,
CH3-
exposure ( l a t e n t image formation)
-
bisphenol A
hr
302
sensitizer
'02
58
(5)
processing ( p o l y m e r i z a t i o n )
-
+ Mn R O . + monomer
ROOH
RO-
development unexposed areas removed
+
Mn+'
+
OH-
X - l i n k e d polymer
solvent
finished plate
(3)
Polymeric Film Former. Almost any film-forming polymer can be used as a binder (Breslow and Simpson, 1981). However, to form a coating insensitive to inks and solvents, it was considered desirable to use a binder which could be cross-linked or copolymerized by radicals in the presence of a monomer. Good results were obtained using a polyester prepared from a mixture of diethylene glycol and propylene glycol reacted with a mixture of fumaric and adipic acids, combined with a phenoxy resin (a polymer bisphenol A-epichlorohydrin condensate) modified with ethyl fumaryl groups to make the polymer reactive. The polyester could also be combined with a bisphenol Apropylene oxide condensate esterified with fumaric acid or with a partially hydrolyzed poly(viny1 acetate) reacted with ethyl fumaryl chloride. To allow development with aqueous base, the phenoxy resin was modified with maleic anhydride (eq 4) instead of with ethyl fumaryl chloride.
I
COCH=CHCOOH
(4)
It was then found that the polyester could be eliminated and properties were considerably improved, probably because of improved adhesion of the coating to the aluminum substrate. Increasing the extent of maleic substitution increased the photospeed, but above about 50% substitution, problems were encountered because of some cross-linking in unexposed areas. Polymerizable Monomer. Although plates could be prepared without polymerizable monomers, photospeeds were dramatically increased upon the addition of monomers, especially if they were polyfunctional. Pentaerythritol tri- or tetracrylate gave excellent results, as did a bisphenol A-epichlorohydrin resin end-capped with acrylic acid. Singlet Oxygen Acceptor. Initially it was felt that it would be advantageous to have the acceptor part of the base resin. This was accomplished by carrying out a Diels-Alder reaction of a fumarate polyester with 2,3-dimethyl-lJ-butadiene. Subsequent work indicated no particular advantage of this approach, and diethyl trans4,5-dimethyl-4-cyclohexene-1,2-dicarboxylate (5A) was
the less volatile bisphenol diester of 3,4-dimethyl-3cyclohexene-1-carboxylicacid (5B)was substituted. Singlet Oxygen Sensitizers. Dyes with triplet energies greater than 22 kcal/mol and high quantum yields for triplet formation make excellent sensitizers, e.g., rose bengal, erythrosin. However, most of our work was done with zinc tetraphenylporphyrin, because of its good solubility and absorption properties. Good light absorption is a necessity for very thin films, such as those used on lithographic plates. Metal Catalysts. Although various metal redox catalysts are operable, the best results were obtained with vanadyl naphthenate or vanadyl acetylacetonate. On the assumption that a metal catalyst would decompose hydroperoxides as soon as they formed, we initially sprayed exposed plates with vanadyl acetylacetonate, immediately placed them in an inert atmosphere, and heated them to develop the image. Much to our surprise, we discovered that the reaction between the vanadyl chelate and the hydroperoxide formed on exposure was very slow at room temperature, perhaps because the formulation was below its glass transition temperature. Exposed plates incorporating vanadyl acetylacetonate with the other ingredients could be stored in air at ambient temperature for several hours after exposure without losing the image. Photospeed. Plate sensitivity is rarely a problem in contact exposure through a negative, since intense lighting can be used. In projection or camera imaging, however, the amount of light available is limited. To get some idea of what kind of plate sensitivity was required, we measured the light intensity at the film plane of a commerical graphic arts camera. Under practical conditions, this was found to be 2.0 pW cm-2. For a 60-s exposure (the maximum considered practical), this would equate to an energy of 1.2 X lo-' J cm-2,which would mean that, with a quantum yield of 1,the maximum concentration of hydroperoxide which could be formed in a 10-pm film would be about 5 X lo4 M. Scouting experiments showed that it was possible to cross-link a 10-pm film comprised of a mixture of ethyl fumaryl modified phenoxy resin, diethylene glycolpropylene glycol-adipic acid-fumaric acid polyester, pentaerythritol triacrylate, vanadyl acetylacetonate, and the hydroperoxide synthesized from the reaction of singlet oxygen with diethyl trans-4,5-dimethyl-4-cyclohexene1,2-dicarboxylate. As little as 1 x lo-* M hydroperoxide gave good cross-linking, demonstrating that the system had the potential for a camera-speed lithographic plate. Since more light is available by projection, a projection plate is also feasible. We then determined the quantum yield of hydroper-
2146
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
oxide formation under typical operating conditions. It had been demonstrated that the quantum yield of product formation was equal to the quantum yield of triplet sensitizer formation, using erythrosin or rose bengal in methanol solution, provided that the oxygen and singlet oxygen acceptor levels were above about IOw3M (Gollnick, 1968); the product in this work was the cyclic peroxide formed from singlet oxygen and 2,5-dimethylfuran. 2,5Dimethylfuran is reported to have about 5 times the reactivity of 1,2-dimethylcyclohexene as a singlet oxygen acceptor (Foote, 1968). By use of a representative film containing zinc tetraphenylporphyrin as sensitizer and a dimethylcyclohexene ester as acceptor, a quantum yield of hydroperoxide formation of 0.43 f 0.07 was found, the large experimental error being caused by the very low hydroperoxide levels being measured. The quantum yield was not decreased in the presence of vanadyl acetylacetonate. The quantum yield of triplet sensitizer formation for zinc tetraphenylporphyrin has been reported to be 0.8 (Dzhagarov, 1970; Petsold et al., 1973);there have been other reports of lowered quantum yields in solid films (Bourdon and Schnuriger, 1966; Dubosc et al., 1971; Meyer. 1970). Laser Exposure. With the present trend toward digital handling of data, a printing plate which could be laser imaged would be highly desirable. Most available plates are sensitive only to ultraviolet light, whereas the more reliable and less expensive lasers emit predominantly in the visible or in the near-infrared. For practical purposes, the laser beam must scan at very high rates, and the necessity of deliverying high-intensity light in extremely short-exposure times may lead to reciprocity failure. A high photon flux can result in quenching between excited states of molecules, and if free-radical initiation of POlymerization is involved, a high flux will lead to high instantaneous radical concentrations with poor initiating efficiency because of radical-radical recombination. Using an argon ion laser emitting a t 514.5 nm, it was demonstrated that the sensitivity of a singlet oxygen POlymerization system was about 2 X 10" J cm z. Since the energy required for imaging by projection with white light was shown to be in this same range, no reciprocity failure occurs. This is undoubtedly due to the very rapid quenching of excited dye by triplet oxygen to generate singlet oxygen, which reacts with the olefin acceptor to form hydroperoxide. The intermediate format ion of hydroperoxides precludes the generation of high instantaneous radical concentrations. The demonstrated reactivity is more than adequate for direct imaging of newspaper plates with low-power visible lasers. Lithographic Plates. Plates were prepared by coating aluminum sheet with solutions containing the essential ingredients to yield a 5-l0-pm coating after drying; red safelights were used, since zinc tetraphenylporphyrin has little absorption in this wavelength region. Care was taken to minimize hydroperoxide formation in the preparation of the ingredients, as well as during the coating and drying steps; since only about 1 x lo4 M hydroperoxide is formed on exposure, extraneous hydroperoxide would lead to cross-linking in unexposed areas. After exposure, plates were placed in a vacuum chamber and heated, after which they were washed with aqueous sodium bicarbonate to remove unexposed areas. A dark purple-brown image was obtained, which allowed visual inspection of plate quality. There are many problems involved in preparing a useful lithographic plate. The primary concern, of course, is to obtain an image with the light available. With line copy, this is ordinarily not a problem; one increases either the
light intensity or the exposure time. With halftone art work, however, it is not so simple. If one increases exposure in order to hold halftone dots in light areas, all too frequently the dark areas will fill in completely. By careful control of conditions, it was possible to prepare, with projection or laser imaging, 160-lineplates holding 5% dots in light areas while keeping 90% dots open in the dark areas. For comparison, newspapers generally use 65 lines, and high-quality magazines use 150-200 lines. Plates prepared by this approach were run on a standard weboffset press for 25000 copies with no deterioration in plate or print qualit]. Since plates produced by this approach can be developed several hours after exposure, they have the added advantage of being applicable to the step-and-repeat exposure process used frequently for book printing. In this case, as many as 32 pages are printed from 1 plate, so it is imperative that the initial exposure be stable for 30 min or more, the length of time required to expose the entire plate before it can be developed.
Experimental Section Maleic-Modified Phenoxy Resin. To an 8-02 pop bottle was added 90 mL of reagent-grade dimethylformamide (dried over 4A molecular sieves), 28.4 g (0.10 equiv of OH) of phenoxy resin PKHC (Union Carbide, MW 30000, dried in vacuo), and 0.28 g of 2,6-di-tert-butyl-4methylphenol (BHT). To the solution was added 9.8 g (0.10 mol) of powdered maleic anhydride; the bottle was capped, purged with argon, and heated in an oil bath a t 100 "C for 16 h. The light-tan liquid was poured slowly into 3 L of water in a Waring blender; the white, rubbery powder was collected by suction filtration and washed several times in 3 L of 50/50 methanol/water. The polymer was again collected and dried in vacuo to constant weight: acid number 76.5, or a substitution of 45 mol %. Diethyl 4,5-Dimethyl-trans-4-cyclohexene-1,2-dicarboxylate. A solution of 22.1 g (0.27 mol) of 2,3-dimethylbutadiene, 43.0 g (0.25 mol) of diethyl fumarate, and 0.4 g of BHT in 50 mL of dry xylene was heated in an 8-02 pop bottle at 100 "C for 24 h. The volatiles were stripped on a rotary evaporator at aspirator pressure and the residue distilled through a 10-in. Vigreux column under vacuum to give the desired compound, bp 85-90 "C (0.025 mm). The material was redistilled and stored in sealed tubes under argon. NMR (CDC1,) T 5.91 (q,3.74 H, J = 7 Hz), 7.3 (m, 1.8 H), 7.86 (m, 4 H), 8.38 (s, 6 H), 8.80 (t, 6.1 H, J = 7 Hz). 3,4-Dimethyl-3-cyclohexenecarbonyl Chloride. To a stirred mixture of 32.8 g (0.40 mol) of 2,3-dimethylbutadiene and 0.10 g of copper powder in 50 mL of benzene under nitrogen was added dropwise over a 2-h period 36.2 g (0.40 mol) of acrylyl chloride. The reaction mixture was allowed to stand overnight, and the volatiles were stripped on the rotary evaporator at aspirator pressure. The residue was fractionally distilled to give 38.5 g (56%) of product, bp 88 "C (4.5 mm). Reaction of 3,4-Dimethyl-3-cyclohexenecarbonyl Chloride with Bisphenol A. To a solution of 11.4 g (0.05 mol) of bisphenol A and 8.7 g (0.11 mol) of pyridine in 125 mL of anhydrous ether was added dropwise with stirring 17.3 g (0.10 mol) of 3,4-dimethyl-3-cyclohexenecarbonyl chloride. The solution was allowed to stand overnight and filtered to remove solids. The filtrate was washed successively with 100-mL portions of water, 2% sodium carbonate, water, and saturated sodium chloride solution. The ether layer was dried over anhydrous magnesium sulfate and stripped to dryness to yield a waxy solid. The solid was recrystallized from methanol to give white
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2147 crystals, mp 103.5-104.5 "C. NMR (CDClJ
T
2.9 (q,8 H,
J = 8 Hz), 7.1-8.1 (m, 13.4 H), 8.33 (s, 17.6 H). Zinc Tetraphenylporphyrin. A mixture of 16.7 g (0.076 mol) of zinc acetate, 41.8 mL (0.052 mol) of pyridine, and 835 mL of reagent-grade dimethylformamide was heated to reflux and 27.4 g (0.045 mol) of meso-tetraphenylporphyrin (Aldrich) added. The resulting solution was refluxed 5 min and cooled to room temperature. The volume of the solution was reduced to about 300 mL on the rotary evaporator and 1.25 L of chloroform added. The solution was extracted with distilled water, the organic layer was filtered through anhydrous sodium sulfate, and the solvent stripped under reduced pressure to give 33.7 g of purple crystals after drying for 48 h at room temperature in vacuo. Light Intensity at Image Plane of a Commercial Camera. The light intensity at the focal plane of a Future Foto horizontal process camera was measured with a silicon detector radiometer (Optronic Laboratories Inc., Silver Springs, MD, Model 730). The camera was equipped with a 24-in., f 11 Artar lens and was set at 1:l magnification; a piece of plain white paper on the copyboard was used as target. This was illuminated with a pulsed xenon unit (Macbeth Arc Lamp Co., Philadelphia, PA) consisting of eight 1.5-kW lamps having a color temperature of about 5900 K. The detector was fitted with filters to pass light from 590 to 700 nm, the flat portion of the spectral response curve for the detector. From the readings obtained, it was calculated that the light intensity between 400 and 710 nm was 2.0 pW cm-2. For an exposure of 60 s, the estimated maximum allowable, this would be equivalent to 1.2 X lo4 J cm-2, or 5 X 1O'O einstein cm-2 assuming an average photon energy of 3.6 X J (the energy at 550 nm). Therefore, for a quantum yield of 1, a 10-pm film would contain at most a 5 X M concentration of hydroperoxide. Quantum Yield of Hydroperoxide Formation. By use of the formulation described under plate preparation, a 5.8-pm coating was deposited on Mylar film by whirl coating. The exposure unit consisted of a lamp housing containing a 600-W GE Quartzline DY S lamp mounted on an optical bench. The light was passed through an infrared filter (Schott Optical Glass UG-l), a double convex focusing lens, and a 546-nm interference filter (Detric Optics). A photometer (Optronics Model 730) was used to determine the light intensity at the film surface, the light reflected ( l l % ) , and the light transmitted (16%); thus, 73% of the incident light was absorbed. After the f i i was exposed for 30 min (the long exposure was required to form a measurable amount of hydroperoxide), 4-6 3-cm disks were cut from the film, placed in a nitrogen stream for 10 min to remove unreacted oxygen, added to a degassed solution of equal volumes of chloroform and 1% sodium iodide in isopropyl alcohol, and shaken for 45 min. Under these conditions, the films completely dissolve. Control experiments using 1-methyl-1-hydroperoxy-Zmethylenecyclohexane as a model showed that this reaction time was sufficient for quantitative conversion. The solutions were then measured in a Cary spectrophotometer set at 358 nm, at which wavelength Is- was determined to have a molar absorptivity of 2.56 x lo4L mol-' cm-'. Unexposed film, treated similarly, was used as a blank for each run, since zinc tetraphenylporphyrin has an absorptivity of 1.7 X lo4 L mol-' cm-' at this wavelength. Hydroperoxide concentrations found in the film were in the 0.02 M range. Under a variety of conditions, the quantum yield for hydroperoxide formation was 0.43, with a standard deviation of 0.07 for 10 experiments. Halving the sensitizer concentration
or eliminating the metal catalyst had little effect on the quantum yield. Lithographic Plate Preparation. A representative formulation consisted of 100 g of maleic-modified phenoxy resin, 33 g of pentaerythritol acrylate HF (Union Carbide, a mixture of tri- and tetracrylate), 13 g of bis(dimethy1cyclohexenylcarboxy)-4,4'-isopropylidenediphenolate,10 g of zinc tetraphenylporphyrin, and 0.08 g of vanadyl acetylacetonate made up in 1 L of chloroform/ethanol (95/5). For large-scale work, this was modified to 100 parts resin, 40 parts acrylate, 10 parts singlet oxygen acceptor, 4.2 parts porphyrin, 0.42 part vanadyl chelate, and 3.3 parts wetting agent (Monsanto's Multiflow) in 333 parts Cellosolve acetate (Union Carbide). For laboratory work, grained aluminum plates (Lith-Kem-KoWipe-0 0.012-in. offset plates, Lith-Kem Corp., Lynbrook, NY) were coated with the first formulation on a Tasope face-up whirl coater (Tasope Co., Aurora, MO) spinning at 150 rpm and then air dried; 5-10-pm coatings were formed. All operations were carried out in a dark room illuminated by a Kodak No. 7 filtered safelight. On a large scale, the second formulation was applied to 25 '/2-in. X 38-in. X 0.012-in. plates by roller coating and dried by infrared heat Gyrex Model 633 roller coater and Model 833-2 double-section infrared drying line (Gyrex Corp., Santa Barbara, CA) to give coatings averaging 9-10 pm in thickness. Plate Processing. After exposure, the plate must be heated in the absence of oxygen. Initially this was done in a glovebag under nitrogen a t about 40 "C for 30 min. For commercial plates, a vacuum chamber equipped with an electric heater was constructed. The plate was placed in the chamber, with the heater at room temperature, the chamber was evacuated, and the heater turned on. In about 45 s the temperature would reach the setting and then overshoot by 10-15 deg. A typical cycle was 3-min pumping, 3 min at 60 "C; it was found later, however, that 2 min at 80 "C would double the plate sensitivity. Better results were realized by substituting an infrared heater for the hot plate; temperature was reached more rapidly and heating was more uniform, a 2-3 fold increase in sensitivity being obtained with a 60-s heat cycle. After heating, the unexposed polymer was removed by spraying with aqueous 2 % sodium bicarbonate containing 1% wetting agent (Renex 30, IC1 Americas) and 5% butyl Cellosolve (Union Carbide). Etching times of 60-90 s were sufficient by using commercial equipment (Merigraph wash-out unit, Hercules). Protection Imaging. Plates were imaged on a full-size imposition camera (Opti-Copy projector, Opti-Copy, Inc., Kansas City, MO). The imposer was fitted with 6000 W of pulsed xenon backlighting and was used for projecting negative transparencies at f 11 (1:l magnification) onto the litho plate. By use of 133-line copy negatives, excellent plates were produced with 30-9 exposures and processing at 60 "C for 3 min under vacuum. With a 15-s exposure through a Bychrome test negative (150-line, 5-90% screen range, Bychrome Co., Columbus OH), the halftone dots could be held through the entire range by processing at 80 "C for 2 min. Laser Imaging. An argon laser (Coherent Radiation, Palo Alto, CA, Model CR-8) was used for exposure. The system employed a single facet 45" spinning mirror and a drum sample holder mounted on a single axis translation stage. Linear scanning speed was controlled by the mirror spinning rate and the line-scan spacing by the translation rate of the drum. In a typical exposure through a 21-step neutral density wedge (Stauffer Sensitivity Guide), using the 514.5-nm wavelength of the laser with 1.7 W delivered
I n d . E n g . Chem. Res. 1987, 26, 2148-2151
2148
to the drum, a mirror rotation rate of 217.4 rev sT1 and a linear scanning rate of 2.3 X lo-* cm s-l (translation frequency of 8696 Hz), led to 9.5 X W delivered to step 10 of the sensitivity guide, the last solid step held after heating in vacuo for 30 min at 40 "C. This solid step was calculated as a sensitivity of 2 x J cm-2, essentially equivalent to the sensitivity found by projection imaging with white light. Thus, there was no reciprocity failure. Large plates (16-in. X 221/2-in. X 0.009-in.) were used with a Laser Pagefax plate system (Image Information Inc., Danbury, CT). This system uses a He-Ne laser to scan the copy, the information being transferred to a 4-W argon laser, which was used multiline (488.0/514.5 nm) to expose the plate. It was found that with 40 mW of power on the plate, optimum printing conditions were realized when the fifth to sixth steps on a Stauffer Sensitivity guide was held, a sensitivity of 6.8 X J Under these conditions, a full range, 5-909'0, of 65-line halftone dots could be held (at lower exposures, the 5% dots were lost during printing), and a full-size newspaper plate could be imaged in 60 s. A press run on a commercial offset press (Dual-Lith, Davidson Corp., Brooklyn, NY) was halted after 25 000 copies with no evidence of plate wear.
Acknowledgment We are indebted to L. Kangas for development of the hydroperoxide analysis and to B. H. Gingrich, M. J. Centuolo, and J. J. Dolan for technical assistance. We also thank the Hercules Printing Plant, the Gyrex Corporation for assistance in developing a coating system for the plates, Image Information Inc. for the use of their laser system,
Kingsport Press for the use of their Opti-Copy projector and facilities, and Photocolor Inc., Newark, DE, for use of their camera. Registry No. 5B,68892-29-5; diethyl 4,5-dimethyl-trans-4cyclohexene-1,2-dicarboxylate, 68658-27-5; 3,4-dimethyl-3-cyclohexenecarbonyl chloride, 69815-57-2; acrylyl chloride, 814-68-6; 2,3-dimethylbutadiene, 513-81-5; bisphenol A, 80-05-7; zinc tetraphenylporphyrin, 14074-80-7;vanadyl acetylacetonate, 315326-2; pentaerythritol triacrylate, 3524-68-3; pentaerythritol tetraacrylate, 4986-89-4; oxygen, 7782-44-7.
Literature Cited Adams, D. R.; Wilkinson, F. J. Chem. SOC.,Faraday Trans. 1 1972, 11, 586. Breslow, D.S.; Simpson, D. A. U.S. Patents 4 271 259 and 4 272 610, 1981. Bourdon, J.; Schnuriger, B. Photochem. Photobiol. 1966,5, 507. Dubosc, J. P.; Mercier, C.; Bourdon, J. Bull. SOC.Chim. Fr. 1971, 3286. Dzhagarov, B. M. Opt. Spectrosc. 1970,28,33. Foote, C. S. Acc. Chem. Res. 1968,1, 104. Foote, C. S.; Wexler, S.; Ando, W.; Higgins, R. J . A m . Chem. SOC. 1968,90, 975. Gollnick, K. Adv. Photochem. 1968, 6, 1. Kopecky, K. R.; Reich, H. J. Can. J. Chem. 1965,43, 2265. Meyer, G. Bull. SOC.Chim. Fr. 1970,702. Petsold. 0.M.;. Bvteva. I. M.: Guronovich. G. P. ODt. . Spectrosc. _ 1973,'34, 343. Walker, P.; Webus, Z. J.; Thommes, G. A. J . Photogr. SEI. 1970,I8, 150.
Received for review January 16, 1987 Revised manuscript received June 23, 1987 Accepted July 11, 1987
Some Interesting Kinetic Observations on the Aqueous Permanganate Solutions Zhi-Xin Lin D e p a r t m e n t of Chemistry, W u h a n University, W u h a n , China
Thomas T.-S. Huang* Department of Chemistry, E a s t Tennessee S t a t e University, Johnson City, Tennessee 37614
Photochemical decomposition products in the UV+sible region and reaction products with hydrogen peroxide of aqueous permanganate solutions have been studied under neutral, basic, and acidic conditions. Although the product distributions of the reactions are different at different pH levels, the two types of reactions, i.e., the photochemical reactions and the reduction reaction by hydrogen peroxide, basically follow the same reaction path. This suggests that both types of reactions start with the same elementary step (probably a charge-transfer step) and then proceed to give different products according to the environmental condition of the solutions surrounding the permanganate ions. The industrial use of permanganate solutions will continue to increase as long as new processes are developed, utilizing its high oxidation potential in the field of water and air pollution control (Obuchi et al., 1974), as in the sterilization (Hamilton, 1974) and deodorization (Prosselt and Reidies, 1965) of water and air. The decomposition and oxidation-reduction reactions of permanganate solutions are thus an important subject of research studies. Photochemical decomposition of aqueous permanganate ion was studied (Zimmerman, 1955) under a variety of conditions in 1955. This study revealed that (a) the quantum yield, +, is small and decreases with increasing wavelength, (b) is independent of the pH and the concentrations of reactants and products, (c) at longer
+
wavelengths there is a small positive temperature coefficient of 4, decreasing with decreasing temperature, and (d) all of the atoms of the photochemically produced O2 come from Mn0,- ions. It concluded that the photochemical decomposition was a simple dissociation of Mn04into two fragments and that no appreciable electron exchange between the dissociated fragments destined to become O2and either H,O or OH- occurred. The mechanism of the oxidation-reduction reaction of permanganate ion reported in the literature was, however, much more complicated. Different paths were proposed for different situations (Wilberg and Gear, 1966; Symons, 1953). Generally speaking, these dealt with the kinetic explanation of product distribution by proposing various
0888-588518712626-2148$01.50/0 1987 American Chemical Society
