Medium-Pressure Lamps

The most frequently used lamp for UV curing processes is medium-pressure mercury lamp. Its emission spectrum can be used to excite the commonly used photoinitiators. Moreover, this type of lamp has a relatively simple design, is inexpensive, can be easily retrofitted to a production line, and is available in lengths up to 8 ft (2.5 m). Power levels in common use are in the range 40 to 240 W/cm, and even higher levels are available for special applications.  

The spectral output of the medium-pressure lamp can be altered by doping, i.e., adding a small amount of metal halide to the fill material, with mercury. Commonly doped lamps are iron and gallium lamps. Medium-pressure lamps have long lifetimes (typically in excess of 3,000 h), although the intensity of the emitted light and the relative intensity of the spectral lines change with time. This affects their performance to the point that 

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Photolytic. Major products reported from the photooxidation of isopropylbenzene with nitrogen oxides include nitric acid and lienzaldehyde (Altshuller, 1983). A / -hexane solution containing isopropylbenzene and spread as a thin film (4 mm) on cold water (10 °C) was irradiated by a mercury medium pressure lamp. In 3 h, 22% of the applied isopropylbenzene photooxidized into a,a-dimethylbenzyl alcohol, 2-phenylpropionaldehyde, and allylbenzene (Moza and Feicht, 1989). 

Photolytic. A n-hexane solution containing /n-xylene and spread as a thin film (4 mm) on cold water (10 °C) was irradiated by a mercury medium pressure lamp. In 3 h, 18.5% of the p-xylene photooxidized into p-methylbenzaldehyde, p-benzyl alcohol, p-benzoic acid, and p-methylacetophenone (Moza and Feicht, 1989). Glyoxal and methylglyoxal were produced from the photooxidation of p-xylene by OH radicals in air at 25 °C (Tuazon et al., 1986a). The rate constant for the reaction of p-xylene and OH radicals at room temperature was 1.22 x lO " cmVmolecule-sec (Hansen et al., 1975). A rate constant of 7.45 x 10 L/molecule-sec was reported for the reaction of p-xylene with OH radicals in the gas phase (Darnall et al, 1976). Similarly, a room temperature rate constant of 1.41 x 10 " cm /molecule-sec was reported for the vapor-phase reaction of p-xylene with OH radicals (Atkinson, 1985). At 25 °C, a rate constant of 1.29 x lO " cmVmolecule-sec was reported for the same reaction (Ohta and Ohyama, 1985). 

Part of this solution is now spread on a glass plate with a spatula and irradiated with UV light (Hg medium-pressure lamp), until a hard crosslinked and insoluble film is obtained (5 min).The distance between the source of radiation and the substrate should be about 20 cm.This Is an example of a photo-cured coating. 

Photolysis of fused 1,3-thiazine 5-oxides (240) by a 125 W medium pressure lamp leads to stable 1,2,4-oxathiazolidines (241) presumably through a diradical intermediate (Equation (28)) <83CC199>. 

High-pressure mercury lamps operate at pressures about 10 atmospheres and essentially two types are used in industrial applications. The point source lamp focuses on a small-diameter spot, thus delivering an intense radiation to that spot. The capillary lamp is used for narrow webs up to approximately 20 cm (8 in.) wide.4 They are capable of producing a wider spectrum than the medium-pressure lamps and operate with higher power (150-2880 W/cm). Their disadvantage is a relatively short operating life, typically hundreds of hours. 

Pcntacarbonyl[(methoxy)(methyl)carbene]chromium(0) (16, R1 = R2 = Me 475 mg, 1.9 mmol) and the chiral ene carbamate (see above equation) (718 mg, 3.8 mmol) in CH2Ci2 (20 mL) were placed in a Fischer Porter pressure tube and saturated with CO (3 eycles to 6.33 atm of CO) and then irradiated (450-W Conrad-Hanovia 7825 medium-pressure lamp, Pyrex well) under 6.33 atm CO overnight. The yellow-green solution was filtered through a pad of Cclitc, and the solvent evaporated. The crude mixture was placed in a sublimation apparatus and heated at 30-50°C (1 Torr) until no further Cr(CO)6 could be obtained. The residue was preabsorbed on silica gel and flash chromatographed yield 319 mg (61 %) mp 133-134"C. 

Medium Pressure Lamp.—A lamp filled with mercury vapor and operated at a pressure of about 1 atmosphere. The total intensity in the near ultraviolet and visible is lower than that of high pressure lamps, but photochemically useful light at wavelengths less than 3000 A. is produced. The ultraviolet spectrum consists of reasonably narrow lines with only a weak continuum. Hence, in conjunction with a filter or monochromator it is a good source for monochromatic radiation including reversed 2537-A. radiation (see definition below). 

Resonance Lamp.—Such lamps (sometimes called low pressure lamps) are often used as line sources in photochemical studies. These usually contain a small amount of a metal vapor (e.g., mercury, cadmium, zinc, etc.) and several mm pressure of a rare gas. They operate at relatively low current (ca. 100 ma.) and high voltages (several thousand volts). This is in contrast to a typical medium pressure lamp which may operate off a 110-220 v. power supply delivering ca. 3-5 amp. The most common example in photochemistry is the mercury resonance lamp which has strong emission of the unreversed resonance lines at 2537 A. and 1849 A. (ca. 90% or more of the total) along with other, much weaker lines ( resonance lines are those which appear both in absorption and emission). There is little continuum. Sources of this type are widely used for photosensitized reactions. 

Depending on the concentration of the substrate(s) the reaction time required may be of the order of days rather than hours. This can often of course be shortened by irradiating with a higher-intensity lamp 500-watt medium-pressure lamps are commonly used. 

Metal hydride reduction of methyl dehydroabietate (64b) afforded alcohol (65) whose tosyl derivative on heating with sodium iodide and zinc in dimethylformamide yielded (66). The use of hexamethylphosphoramide gave an inferior yield of (66). Regio and stereoselective acetoxylation with Pb(OAc)4 in acetic acid at 100°C gave only 30% yield of (67) but the same experiment realized with Pb(OAc)4 using a Hg medium-pressure lamp at room temperature yielded (67) in 74%. The H NMR spectrum of (67) showed that the OAc group was introduced in 7a-position. This on subjection to acid-catalyzed 13-elimination (EtOH/10% HC1) produced (68) in quantitative yield. 

The cafalyst was used in aqueous suspension well mixed by means of a magnefic sfirrer. The reacting mixture was illuminated by a mercury medium-pressure lamp (t)q5e B, Helios Italquartz, Milan, Italy) coaxial with the photoreactor. A Pyrex thimble surrounding the lamp allowed the circulation of distilled water in order to cool the lamp and cutoff infrared radiation in fhese conditions fhe reactor temperature was of 295 + 2K. Lamps of 125, 500, or 1,000 W electric power were used average irradiances... 

For 60 we observe an AB- rons system, which is very close to an A2-system, corresponding to the protons at C-7 and C-8, while for 61, these same protons form an A2 system and appear as a single signal. Replacement of the ester group by an alcohol function makes the protons almost indistinguishable. The irradiation of the alcohols is performed in a quartz vessel by means of a 400-watt medium pressure lamp in solution in diethyl ether. 

Vinylcyclopropenes have been found to rearrange photochemically to cyclopen-tadienes. Thus, irradiation of l,2,3-triphenyl-3-vinylcyclopropene (233) in t-BuOH using a 450 W medium pressure lamp and a Pyrex filter for 3.5 h afforded 1,2,3-triphenylcyclopen-tadiene (234) in 87% yield (equation 153) . 

Irradiations were conducted at 366 nm (Corning 7-83 filter combination) in a merry-go-round apparatus in which the samples rotated about a stationary 450-Watt Hanovia medium pressure lamp for constant light exposure. The samples consisted of degassed 5 ml solutions of the photoinitiators in neat MMA contained in Pyrex tubes. Two initiator concentrations were utilized 1.05 x 10and 4.11 x 10 which corresponded to 16 and > 99% light absorption, respectively. Samples were irradiated to about 7% monomer conversions which required 15 min for the optically dense solutions and 30 min for the tubes with low initiator concentration. The resulting poly-... 

The most common sources of radiation for kinetic photochemical studies are mercury lamps of which there are three types. The low-pressure lamp is used mainly for mercury-sensitised studies. For general photochemical studies the most useful lamp is the medium-pressure lamp having several lines of reasonable... 

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