CA1116005A - Photochromic aziridine recording media - Google Patents

Abstract

Abstract Photochromic azinidines can be applied to a substrate to provide a deposit thereon, preferably followed by application of a barrier coating which is substantially oxygen-impermeable, which will provide a thermally stable optically erasable imaging medium.

Description

~lile 913,771 3a3~71D~i P~OTOCHROMIC AZIRIDINE RRCORDING MEDIA

This invention relates to media which can be imaged by exposure to actinic radlatiorl. More partlcularly, the invention relates to articles capable of being used f'or irnaging or recording bas~d on photochromic aziridines, which allow the image-formin~ operatlon t;o be reversible~ i.e., the recorded image may be erased.
Many conventional imaglng materials undergo lrreversible changes when exposed to actinic radiation. Thus, erasure is impossible without physical destruction of the image itself. Additionally, in many instances~ the ima~e formed is latent, and subsequent development ls necessary.
Photochromes are compounds which change color reversibly on exposure to actinic radiation. Such dlrect-developing photochromic materials, however~ traditionally suffer the limitation that images produced from systems con-taining these materials have very little stability, l.e., the image will ~ade spontaneously within a few minutes or hours at room temperature.
It has now been ascertained that particularly defined photochromic aæiridine compounds can be utilized for imaging wherein the recorded image may be erased and informa-tion may be added, the film may be re-used, etc.
~ urthermore, when the compounds are vapor coated on a substrate, such can be utilized for high resolu-tion, long term recording. These films can be utilized indata recording applications such as video disc or mlcrofiche, especially ln conJunction with high densit~ data recording.

Besides being direct developing, such medla can be updated or erased. The films can further be used as intermedlates in photographic and/or copylng processes and as prooring materials.
The articles can also be used, in con~unctlon with spectral fllters, to monitor exposure to actlnic radiatlon~
In Schleigh et al, U.S. Patent No. 3,~9LI,874, there is described the use of aziridines in photoreductive imaging. A reducible, image-forming compound is comblned wlth the photochromic aziridine ln a binder on a substrate to form a radiation-sensitive layer. Upon exposure to actlnic radiation, followed by heating, an image may be obtained.
Furthermore, partly crystalline and partly crystallagraphlcally aligned photochromic aziridines and oxiranes are dlsclosed as having utility in windshields, sunglassesg and light switching devlces in U.S. Patent No. 3,964,823.
It has now been found that by applying herein- ~
after-define~ photochromic aziridines onto a substrate, and by ~ -utilizing an oxygen barrier material to cover the aziridine

2~ coating, the lifetime Or the image formed by exposure to actinic radiatlon can be increased at least a thousand times more than that of the photochromic aziridine in oxygen or air.
In accordance with the invention, there is pro-vided a thermally stable optically erasable imaging or record- ;
ing medium comprising a substrate having on at least one surface thereof coating or film of at least one photochromic ~;-aziridine o~ the formula . ~

9~jj ~ N02 N N

Rl R2 whereln Rl and R2 separately are hydrogen, lower alkyl,phenyl~
or ortho or para lower alkyl or lower alkoxy substikuted phenyl, and together are alkylene having 4 to 7 carbon atoms inclusive 3 and overlying said coating, or in some instances integral therewith, a substantially oxygen~lmpermeable barrier coating which is reasonably transparent to actinic radiation.
The medium can be imaged by exposure to actinic radiation, optically erased, and reimaged, with the image being substantially resistant to thermal bleaching.
: 15 The photochromic azlridines utilized in this invention are 2Rl,2R2-6tp-nitrophenyl)-4-phenyl-1,3-diazabi- ;;
cycloC3.1.0]hex-3-enes, which can be structually designated:

:~ ~ H / -~ :
20 ~ -H

N N ~ ::

~: Rl 2 -wherein Rl and R2 taken separately are hydrogen~ lower alkyl;
phenyl, or ortho or para lower alkyl or lower alkoxy substi- .
, tuted phenyl (wherein the term "lower" designates ~rom 1 to 4 carbon atoms)~ and wherein Rl and R2 taken tog~ther are alkylene having 4 to 7 carbon atom~ inclusive. These compound~
can be synthesized by the method dlsclosed by Heine et al in J. Org. Chem. 32, 2708-10 (1967) and in U.S. Patent No.

3,609,165. The most preferrad compound ~or this invention is the dimethyl derivatlve~ 2,2'-dimethyl-6(p-nitrophenyl)-ll-phenyl-1,3-dlazabicYclo[3.l.o]hex-3-ene. Pr~ferred alkylene derivatives include the cyclohexyl and cyclopentyl deriva-tives.
These aziridines are colorless prior to exposure to an electron b~am or ultraviolet radiatlon, but upon exposure, the compounds turn various shades o~ blue depending upon the Rl and R2 groups contalned t~lerein. If such compounds are place~ in the dark, they agaln become colorless; hence the color change ls reversible. Additionally, they may be bleached thermally or by exposure to visible radiation, i.e. 3 they will revert to their colorless condition by use of such methods.
Therefore, it is known that the colorless form of the photo-chromic azirldine can be converted to the colored form uPon exposure to electron beam or ultraviolet radiation, and the reversible reaction baok to the colorless form can occur upon exposure of the aziridine to visible light~ or when placed in ;
the dark, or thermally.
This can be deplcted by the following: ;
A _ l B
dark --.

wherein A represents the colorless form, B the colored ~orm~
vl is a radiation frequency limiked to the ultravlolet, ancl V2 is a radlation frequency limited to the visible.
It has now been ascertalned that in the absence of oxygen~ the thermal bleaching reaction can be substantially dimlnished~ and the photochromic material can be rendered dark-stable, i.e.

A ~ B
hv ~ ?~ ~
To illustrate this phenomenon, a vapor-deposited film of the dlmethyl derivative was coated on a quartz substrate and allowed to stand in air for 24 hours to become slightly turbid, and was then irradiated to yield the deep blue color.
The sample ~las placed in an optical cell in whlch the atmos-sphere could be controlled.
Under a nitrogen atmosphere at rsorn temperature,the transmissive optical denslty ~measured at 620 nm) oP the colored film remained essentially constant for seven days;
however, upon introduction of oxygen, the color bleached rapidly. These results are shown in Figure 1.
Alternatively, using the same set-up with the film in an air atmosphere, but at 60C, exposure to ultraviolet radiation caused the ~ilm-to color practically instantaneously, and then thermally bleach very rapidly. The coloration-bleach cycle was repeated six times, and then the atmospherewas changed from air to nitrogen, still maintaining the temperature at 60C. Exposure to ultraviolet with the film in the nitrogen atmosphere provided a stable colored form which showed no bleaching over 180 minutes. These startling results are illustrated ln Figure 2.

' ., To ~urther illustrat0 the results o~ an oxygen atmosphere on the photochromic compoundsg strips of rllter paper were saturated with benzene solutions of various aziri-dines of the above formula and dried. The strlps were then irradiated with ultraviolet to generate the blue form. One set o~ these irradiated strips was kept in a nitrogen atmos-phere in the dark, and another set was kept in an oxy~en atmosphere. Both sets were malntained at room temperature in the dark. The time required to bleach to one-half of the original optical density was estimated visually and i8 recorded in Table I. Considerable care was taken to minimize exposure of the samples to light during the periodic readout.

Table I

Time Required to Bleach to _ One-ha_~ Optical De _ity Aziridine Derivative 2 Atmosphere N? Atmosphere 1 R2 CH3 40 mlnutes >1 year (Rl ~ R2) = cyclopentyl3 hours>3 months (Rl + R2) - cyclohexyl10 hours~2 months ~ ;
Rl = CH3, R2 C6~510 minutes~3 months Rl = H, R2 C3 7 10 hours 1 month Rl = R2 = C2H520 minutes 3 days Rl = CH35 R2 = CH(CH3)210 minutes3 weeks Rl = H, R2 C6H5~10 minutes 2 days Rl = H, R2 = o-methoxyphenyl<10 minutes 15 hours It has been further determined~ however, that even in an inert atmosphereg such as nitrogen, initially opti- ;~

cally clear thin vapor deposited films of the aziridine com-pounds become turbid or cloudy after a period of tlme, e.g., ,, 24 hours. It is believed that such a condition ls due to crystal growth of the compound.
Tnitlally optically clear films were examined within one hour after deposition with a scanning electron microscope. The resulting micrographs revealed a uniform array of non-crystalline hillocks, less than one mlcrometer in the longest dimension o~ the base of the hillocks. A
similar sample was stored in nitro~en for 24 hours and under-went a vlsible change from a transparent fllm to a turbid ~ilm.
Scanning ~lectron micrographs o~ the turbid ~ilm revealed that the hlllocks had grown substantially and ba~es o~ the hillocks were irregular. Many of the hillocks had ~oined together to ~orm three or more peaks. The regular arra~ had essentially disappeared. Therefore~ maintaining a nitrogen, argon, neon, etc. inert atmosphere around the photochromic coating will serve to inhibit thermal bleaching o~ the colored form, but will not prevent the inltlally clear film from becoming turbid, which will decrease image resolution. While such a condition is not preferred, even such turbid films have utility as an imaging medlum.
It has been found that by utilizing a ~ilm-~orming barrier coating over the thin aziridine fllm, which is substantially impermeable to oxygen, the image stability, i.e.
resistance to thermal bleaching, and the ~ilm transparency, i.e., image resolution capability, can be ef~ectively main-tained. Exemplary and preferred materials which are substan-tially impermeable to oxygen and inert toward the aziridine, i.e., crystal growth inhibiting~ such as polyvinyl alcohol or gelatin, applied as thin ~ilms over the photochromic ~ilm, can effectively prevent crystal growth and also act as an , .
. ~ . .

oxygen barrler to minlmize bleaching. Imaged fil~ coated wlth polyvlnyl alcohol, for example, have maintained their clarity and image density for periods exceeding one year.
In the case where the azlrldlne is simply coated from a solvent solutlon, the dried coatlng will be mlcrocrystalline ln nature. Obviously~ the turbidity, crystal growth, etc. is unlmportant ln that instance.
The barrier coating should o~ course be reason-ably transparent to actinic radiatlon. To mlnlmize crystalli-zation of the photochromlc azirldine and therefore turbidity,the barrler coating should be applled as soon a~ter azlridlne deposition as po~sible and before imaging.
Dyes can be added to the barrier coat to select wavelengths that cause coloration of the aziridine. ~or example, A]izarine Yellow dye can be added to the polyvinyl alcohol coating to minimize backgroulld coloration from incan-descent room lighting, but still allow imaging with, for example, the 325 nm llne from a helium-cadmium laser.
Since moisture wlll adversely effect the oxy~en-barrier properties Or polyvlnyl alcohol~ lt would be desirable in a practical recording device to incorporate a radiation transparent moisture barrier elther in intimate contact with the article or surrounding it, e.g., films of a copolymer vinylidene chloride and vinyl chloride.
In the case of vapor deposition ~or high resol-ution imaging, the temperature of the recelving substrate, for condensation of the photochromic aziridine thereon, ls critical~
If the substrate is either too cold or too warm, a non-uniform coating deposlts and the thln aziridine film will appear to be turbid and/or blotchy. In contrast, a transparent, uniform 8~

,, : ; ~ , , , homogeneous coating is attained lf the temperature of the receiving substrate is about ~120 to -140C.
Scanning electron microscopic examination of the blotchy films at lOOX magnification revealed that the aziridine depo~its in the form of islands. The clear, trans-parent films and the turbid films were further examined with scanning electron mlcroscope at 7000X to lO,OOOX magnlrication.
The micrographs of clear9 transparent films illustrated a uniform array of non-particulate hillocks. The valleys between the hillocks were of minimal area compared to the area of the hillocks; and the longest dimenslon of the hillocks was less than one ~m. The thickness of the film was about o.6 ~m.
Although the hillocks had rounded tops, the bases of the hillocks were not necessarlly round. Some appeared to be oval, ellipsoidal and somewhat irregular, although the~ were predominantly clrcular.
The scanning electron micrographs of the turbid films revealed a similar hillock structure, but the hillocks grew together to show an unmistakably dendrltic structure.
The tree-like structure would have branches of ten to twenty micrometers or more.
The turbid and/or blotchy visual appearance provides light scattering and hence a severe reduction in the resolution of images produced with such films. In contrast, the clear, transparent films provide minimal llght scattering and high resolution of images.
The following tables are included for the indl-cated sublimation conditions, wherein material was deposited on quartz microscope slides. In Table II~the dimethyl aziri dine derivative, i.e.~ wherein Rl = R2 = CH3 was utili~ed.

~6~V~i For Table III, an aziridlne wher~ln Rl - H and R2 = n-C3H7 was utillz~d. In Table IV~ the aziridlne wherein Rl ~ R2 =
cyclohexyl was utillzed.

Table II
5Quartz Receptor Time of Bath Tem~. ~ ~ Comments 140C 25C 2.5 min. Blotchy~ non-unl~orm films visible; at lOOX, islands of' deposits are indicated;
at lO,OOOX, d~ndritlc effect i~ clearly shown.
140C -75C 2,5 minO Vlsibly turbid ~ilm; at 7000X, dendrltic effect predominates.
140C-l2goc 2.5 min. Vlsibly transparent, clear film; at lO,OOOX, regular array of hillocks less than one ~m dlameter; film thickness 0.6 ~m.
140C-153C 2.5 min. Visibly turbld film; at lO,OOOX extensive dendritic effect.
140C-168C 2.5 min. Visibly non-uni~orm~
turbid film; at lO,OOOX, predomlnant dendritic structure with somewhat larger hillocks.
~, Table III
Quartz Recep~or Time of Temp~ Comren'~
102C-130C 3 min. No coating 111C132C 8 min. No coating 128C-130C 5 min. Light unlform ~ilm 132C-132C 6.5 mln. Clear, kransparent ~llm 131C-169C 6.5 min. Turbid film with spots 131C25~C 6.5 mln. Non-uniform, turbid coatlng .:

Table IV
__ Quartz Receptor l'ime of Bath Temp. ~ De~osltlon Comments 131C -133C 6O5 min. Light~ uniform film 148C 133C 6.5 mln. Clear, transparent ~llm 150C -166C 605 min. Somewhat turbld ~ilm with poor adherence to substrate 150C -75C 6.5 min. Fair film, but poor adherence to substrate 150C 25C 6.5 min. F:Llm does not adhere to substrate From the foregoing tables~ it can be seen that the intermediake condensation temperature would appear to be preferably in the range of about -120C to about -140C, at least when the dimethyl derivatiYe is utilized. This conden-sation temperature, as is indicated in Tables III and IV, may vary sQmewhat depending on the particular aziridine utilized to form the film.
Where vapor depositlon is unnecessary in pre-paring the recording medium~ the aziridines can be simply dissolved in an organic solvent, e.g.~ benzene, at a concen-tration sufficient to provlde a uniform microcrystalllne coat-lng, applied to the substrate surface, and dried. In this instance it ls preferred to use saturated solutions to maxi-mize coloration on porous substrates, e.g., paper. Following this~ the film-formlng oxygen barrier material can be coated over the microcrystalline aziridine layer ln a single coating operation or, preferably as multiple coatings to maximize lmage stability.

Alternatively, the aziridines can be coated from dispersions with a film~forming blnder material, such as cellulose nitrate, polyacrylonitrile3 polyvinyl alcohol, etc.

, ." , . ., , ,~ .
., It is imperative that 9 in this lnstance, the aziridlne~ be in microcrystalline form on the substrate to ~unctlon ln khe inventlon, and therefore binder compound~ in which the aziridines are ~oluble should be avoided. Concentration of the particulate azlrldlnes ln the dlspersion should be su~flcient to provide a uniform mlcrocrystalline aziridine coating on the substrate.
In this latter case, lt is preferred to utillze rllm-forming binder compounds which in themselves are sub stantially impermeable to oxy~en, e.g., polyvlnyl alcohol as a separate oxyg~n barrier overcoat may become unnecessary.
To maximlze image stability~ however, again it is pre~erred to utilize one or more subsequent barrier coatings of a sub-stantially oxygen impermeable material.
The receptor ~ubstrates utilized ln the present invention may be ~lexible or rigid, and may be reflectlve, opaque, or transparent, and when slmply solution coated, porous as well. High quality images can be produced on azirldine-coated substrates such as glass, quartz, polycarbod-limide-primed polyester film, tin oxlde-coated quartz and glass, and polyester which has been vapor coated with alumi~
num. The physlcal properties o~ the substrate surface will, of course, affect the structure of the thin photochromlc ~ilm.
If, for example, vapor depositlon is undertaken on a quartz ;;
substrate having rough pollshing marks, the striations due to such polishlng can be clearly observed on the ~ilm Readable lmages may be produce~ by relatively low inkensity ultraviolet exposure. For example~ when the dimethyl derivative is utilized, exposure in the range of about 10 to about 20 milli~oyles/square centimeter ~at 325 nm) will produce images having excellent resolution. Readable images can be obtained at an exposure as low as 5 milli~oules per square centimeter.
Samples of the photochromic recording medla have been made utillzing as substrates Kodak KTFR photore~lsts and electron beam resists, e.g., epoxidlzed polybutadiene and polymethylmethacrylate. These substrates were slmply vapor coated with photochromlc aziridine and subsequently overcoatecl with polyvinyl alcohol. Thls construction allows for instant "read-after-write" checking of in~ormatlon ln making such article~ as video disc masters, wh~re it is desired to record error-correction codes. The constructlon is also use~ul in checking image quality in lithographic and printing plates prior to development o~ the flnal relie~ image. The aziri-dlne and polyvinyl alcohol layers can be simply removed alongwith the unpolymerized resist material following the checking procedure.
This aspect o~ the lnvention will now be more speciflcally described with the aid Or th~ following non-limiting examples~ wherein all parts are by weight unless otherwise specified. In all cases, preparation of the photo-chromic film was carried out in a laboratory equipped with yellow sa~e lights to eliminate extraneous ultraviolet radia-tlon.
Sublimations were carried out in a glass labor-atory apparatus. The apparatus consists of a heavy-walled outer chamber that can be connected through a three-way stop-cock to a vacuum pump or dry nitrogen or air line. Inside this chamber is a detachable cold ~inger which can be cooled by passing cold dry nitrogen through its lnner walls. The ~- -13-temperature of th~ cold finger was det2rmined with a th~rmo-couple. The sample to be sublimed was placed in th~ bottom of the outer chamber and the substrate was attached in intimate contact to the cold finger so as to maximlze thermal contact.

A small amount tO.25 gm) Or 2,2'-dime~,hyl-6(p~
nitrophenyl)-4-phenyl-1,3-diazabicyclo[3.1.0]hex-3-ene was placed in the bottom of the subllmation apparatus. A clean, 2 mm thick quartz substrate was firmly attached to the flat cold finger o~ the sublimaklon tube with conductive adheslve tape. The apparatus was assembled and evacuated to about 0.15 Torr. Cold dry nitrogen was passed through the ~acket of the cold flnger until the temperature o~ the exit gas was 15steady at about -130C. At this point, a hot oil bath (140C) was utilized to warm the aziridine and sublime it onto the quartz substrate.
Under these conditions, a transparent aziridine ~ -film of about o.6 micron thickness was deposited in two minutes.
The oil bath was removed and the cold nitrogen line replaced by a room temperature compressed air line to warm the cold finger rapldly to room temperature. At that point, air was admitted to the chamber and the aziridine-f~lmed substrate -;
removed. It was optically clear. The substrate was then dipped immediately into a 4 percent by weight aqueous solution of polyvinyl alcohol to provide the oxygen barrier and to pre-vent further crystal growth o~ the photochromic aziridine.
The substrate wa~ then dried and dipped again to lnsure a uni~orm coatlng o~ the entlre area.

Electron micrographs kaken o~ ~reshly deposlted aziridine ~llms (prior to coatin~ wlth polyvinyl alcohol~ show these to be typically about o.6 micron thick, and having unl~orm structure. Films that have been allowed to stand in alr or nitrogen (without a polyvinyl alcohol overcoat) change ~rom optically clear to turbld within 24 hours.
A clear, sharp blue image on a colorless trans-parent background was obtained using 15 mllliJoules/square B centimeter from a Sylvanla F4T5/BLB black llght when the sample was contact prlnted using a photographic negative. Following exposure, the sample was stored in the dark in air at room temperature. Over a one year period, the lmage ~delity has substantially remained. Only on the sample edges, where the polyvinyl alcohol coating thickness decreases, or in blemished coatlng spots, has the lmage bleached.
A similar aziridine-filmed substrate was imaged as described above. Thèn the imaged substrate was placed in a light-tight box which had a window made ~rom an ultraviolet cutof~-visible transmitting filter (Kodak CS-3-6~). The box, 20 containlng ~he plate, was set out ln sunlight ~or about one- -half hour. The box was ~ubsequently opened in a dark room under yellow light. The image had been completely blaached, i.e., erased. The plate was reexposed with ultraviolet as descrlbed before and a clear~ sharp image was agaln obtained.
No ghosts were detectable. This process could be repeated over and over with~ut visible loss in quality o~ the resulting lmage.
Resolutlon of the aziridine-filmed quartz plate was determlned by recording and then reading out a standing-wave grating. Under yellow safe lights, a fringe pattern f . ' . , ` ~ , . . ~

D~

was formed by interferrlng two expanded beam~ ~rom the 325 nmline of a helium~cadmium laser, The sample was exposed to 17 milliJoules per square centlmeter. An image o~ 1020 line pairs per millimeter was recorded. The ~act that this grating had been recorded was confirmed by passing ~ helium-neon laser beam (633 nm) through the imaged sample and obser,ving spots dlffracted from the zero order beam~ The ratio o~ intensity of the first ord~r beam to that o~ the ~ero order beam was 0.4 percent.
Re5ponse time o~ the polyvinyl alcohol-coated azirldine-filmed quartz plate was determined by measuring the change in its transmittance at 633 nm as a functlon o~ time after exposure to a ten nanosecond UV-nitrogen laser pulse (345 nm). The response time to a one mllli~oule laser pulse (25 milli~oules per square centlmeter) was less than the 200 nanosecond response time of the photomultiplier tube used to make the measurement. The sample was found to color in le~s than 200 nanoseconds. , Thermal stability of the exposed blue form was ~o determined in two experiments. In the first, the azirldlne-fllmed substrate was placed in a special optical sample chamber and kept in 52C in air. After ultravlolet irradiation with a Xenon source, absorbance of the 620 nm peak was monitored as a function of time. The sample showed no detectable change in 60 mlnukes. Films with no polyvinyl alcohol barrier coat-l~g bleached completely withln 30 minutes.
In a second experiment, one o~ the samples was imaged and then stored in the dark at room temperature for one year. Only at the edge o~ the sample, where the barrier layer was thin, was there any noticeable thermal bleaching.

Measurements were made of the rate of optical bleaching of an exposed polyvinyl alcohol-coated photochromic aziridine film by the bleachlng action o~ red (633 nm) light.
The previously exposed (blue) sample was placed ln the path of an expanded beam ~rom a holium-neon laser. The lntensity o~ that beam at the sample plane was measured with a Gamma Scientific 820A photometer. Small portions o~ the beam lnci-dent on the ~ilm and the beam exlttlng from the ~ilm were samples by means of beam splitters. Thus, the change ~n tran~-mlttance o~ that beam could be ~ollowed as the sample bleached.An exposure Or approximately 350 milli~oules per square centi~
meter was required to bleach the sample from a transmissive optlcal density of 0.65 to one o~ 0.325.

~ .:
Two clean, 2 mm thlck quartz substrates were coated with 2,2'dimethyl-6 (p-nitrophenyl) 4-phenyl-1,3-diaza- -bicyclo[3.1.0]hex-3-ene as described in Example 1. The coated `~ ;
substrates were immediately dipped into a 4 percent by weight aqueous solution of polyvinyl alcohol and dried. The poly 20 vinyl alcohol coating procedure was repeated several times to ~ ~ -provide dry polyvinyl alcohol coatlngs of about 2 ~m thlckness.
One sample was exposed (in vacuo) to an electron beam for 0.232 seconds having an accelerating potential of 26 kilovolts, a beam current o~ 10 microamps and a beam area of 2.88 square centimeters (21 milli~oules per square centimeter).
The sample developed an erasable image which had a trans-missive optical density o~ about 0.5. The second sample was given a 2 second 0xposure (in vacuo) to an electron beam having an accelerating potentlal o~ 165 kilovolts, a beam current of 5 milliamps and beam area o~ 225 square centimeters .

. ~.

(7.3 ~oules per square centlmeter). A hlgh quality image was ob~ained whlch was erasable by exposure to vi~ible light.

Strips o~ filter paper were dipped ln a sa~ur-ated benzene solution o~ 2,2' dimethyl-6(p-nltrophenyl)-4-phenyl-1,3-diazabicyclo[3.1.0]hex-3-ene and dried. The resulting strlps were subsequently dlp-coated in a 4 percent by weight aqueous polyvinyl alcohol solution and dried with a heat gun. One strip wa8 dipped in the polyvinyl alcohol and dried once, another three times and another five times. These strips and an uncoated strip were then exposed to ultraviolet radiation from a Sylvanla F4T5/BLB black light to bring them to a reflective optical density of about 0.90 and then stored in air in the dark in a 50C oven. A second set was stored in air in the dark in a 0C refrigerator. At various inter-vals the samples were removed and re~lective optical density measurements were made. These measurements are illustrated in Tables V and VI below.

Table V - Darlc Storage in Air at 50C
:
Re~lective Optical Density No. of Coatings 0 200 400 600 800 1000 Polyvinyl Alcohol min. min. min. min. min. min.
O . 9 0 . 1 ---- ~
1 0.9 0O47 0.4 0O3~ .3 0.25 ~ 0.90.73 o.68 o.65 0.63 0.62 0.9 n.78 0.75 0.7ll 0.73 0O73 :
:''' ' ~ , , .:

-Table VI - Dark ~tora~ in Air at 0~C

Reflective Optical Den~ity No~ of Coatings 0 200 400 600 800 looo Polvvinyl Alcohol min. min. min~ min. min. mln.
~ _ _ _ _ _ _ 0 0.9 o.63 0 55 0.48 0.44 o.36 1 0.9 0.~ 0 77 0.71l 0.72 0.69 3 o.g o.36 0,.~5 o.84 0.83 0.82 0.9 0.~8 0.87 0.8~ 0.85 o.84 At the end of this experiment the samples were bleached optically by a one-half hour exposure to a 100 watt yellow General ~lectric "Rug Lite" at a distance of slx inchesO
The samples were reimaged and there was no apparent loss in sensitivity, nor were ghost patterns preserlt.

Example 4 A 1.0 gram sample of 2,2'-dimethyl-6(p-nltro-phenyl)-4-phenyl-1,3-diazabicyclo[3.1.0]hex-3-ene was ground to a fine particle size in a mortar and pestle. Ten grams of a 4 percent by weight aqueous polyvinyl alcohol solution were added to the aziridine and the-mixture was then ~round for a 20 few minutes to achieve a uniform dispersion. The dispersion -:
was applied by brush to white cardboard and dried with a heat gun. The entire substrate was then dipped in a 4 percent by weight aqueous polyvinyl alcohol solution and dried with a heat gun. Dipping and drying was repeated three more times to insure a complete seal of the aziridine from air.

This medium was then contact-printed from a negative by exposing it for five seconds to the mercury lamp in a "Colite'1 exposure unit. A clear, sharp blue image on white background was produced. The image was bleached out by exposing the sample for one-half hour to a yellow incandescent ... ., . . , . ., ~

L6(D~

General Electric "Bug Lite" at a distance of slx inches.
When it was reimaged, there were no ghost images or apparent 105s 0~ sensiti~ity. This lmaged sample was stored in the dark with no image deterloration over a perlod of one week.
A second porkion o~ this sample was dip-coated and dried twice from a solutlon of 0.15 gram Alizarine Yellow dye in 25 milliliters of a ll percent by weight aqueous poly-vinyl alcohol solution. This sample was exposed for 60 seconds to the l'Collte'l unit through a ne~ative. A sharp, green image on yellow background was obtained. The back-ground in thls image came up only slightly a~tar 30 minutes exposure to overhead IlCool White" ~luorescent lights.
When the aziridines are utlllzed to monitor radiation, ~ilt~ means are disposed between the aziridine compound and the source o~ actlnic radiation to allow only radiation of the character being monitored to reach the aziri~
dine compound. At least one color standard is provided wlth which to compare the color developed by the aziridine compound. ~`
In the preferred embodiment, the color standard ls separate from the aziridine comopund, e.g., as the background color of the substrate. When the color developed by the aæiridine compound matches the appropriate color standard, the user is informed that the preselected amount of radiation exposure has been reached.
The amount of radiation causing the specific aziridine to develop the color of the color standard may be accurately determined by measuring the lntensity of the lamp output with a photometer at the sample plane and the time of exposure. The product of the intensity and exposure time gives the exposure (e.g., Joules/cm2)O The visual match between -2n-the color of the patch and the color standard 1Y deflned as the "end point". Several di~ferent patches of one speclfic aziridine may be us0d wlth a given reference color. Each patch may be covered by a dlfferent filter to attenuate the amount o~ radiation to which the aziridlne compound is exposed.
Then3 each patch is calibrated so t;hat a user wlll be informed as to the amount of radiation required to color match each patch with the reference color. Alternatively, a speci~ic azlridine may be color matched wlth several different stan-dards. Again, each color match wlll represent a differentprecalibrated exposure. However, it is preferable to use a single color standard.
Where more than one discrete patch of azirldine compound are present and the patches have different sensitiv-ities to the actinic radiation being monitored (by overlaylngthe patches with attenuating layers of differing strengths), a separate color standard is not required~ In such a monitor, the aziridine patches themselves can serve as a means of com-parison to alert the user that the predetermlned amount of radiation has been reached. Since optical density at satur-ation (deepest blue color possible~ of each photochromic aziridine is relatively constant, a saturated patch may be used as a color standard for another unsaturated patch.
The patch with the filter which attenuates the least w111 reach saturation first, and the filter which atten-uates the most will reach saturation last. Thus, with photo-metric calibration, the exposure for saturation may be deter~
mined for each patch. Hence, it is possible to use a monitor containing a series of patches, which encompass at some lnter-mediate level a predetermined critical level of exposure. For ..:: , :: .
s , :: , : ., example~ if the permissible eight hours dose is 3.0 milli-joules/cm , a monitor havlng patches which reach saturation at 1.0, 2.0, 3.0, 4.0 and ~.0 milli~oules/cm2 could be used.
The user would note that the first patch colors and reache.s saturation first~ and then the second will follow suit. At this point, the user has been exposed to two MilliJoules/cm2.
The third patch is becoming blue and the fourth patch is lighter blue. Scme time prior to the third patch becoming the same color as the first and second patches, the user should leave the actinic radiation area to avoid becominp expost~d to the permisslble limit.
The monitor of this aspect of the present invention is especially useful in the treatment of psoriasis, where a patient receives a photoactive drug and is periodically exposed to ultraviolet radiation. An article by Parrish et a]
in the l~ew ~n~land Journal of rledicine 291, 1207-11 (1974) described psoriasis photochemotherapy. The patient is first treated with a photoactive drugr (e.g.~ 8-methoxypsoralen) by oral or topical adrninistration, followed by exposure to specific actinic radiation. Generally, the near UV radiation in the range of 320 to 390 nanometers is utilized. l'he quan-tity of radiation required varies between about 1 and 20 joules/cm2 and is dependent upon patient tolerance and response to the drug. The procedure is expected to involve an initial intensive treatment phase L ollo~Jed by a long term maintenance program. Each patient is initially "titrated" to deterrnine tolerance and effectiveness of treatment. Since the amount of UV exposure utilized in the treatment is critical to both the efficacy of the photoactive drug and the minimization of toxic reactlons such as er~thema, an accurate monitoring of radia~ion , : .. , :
, . . ~ :.

6~ S

exposure is important.
Currently, expensive electronic integratln~
devices are bein~ utili~ed and developed to monitor the radiation dosage. The inex~ensive, easlly rabrlcated, reli-able, accurate and reusable monitor of the present lnventlonis well suited ~or use ln this treatment method.
The monltor can also be utilized as a sun exposure monitor, although for this use it ls desirable to add an additlonal filter to prevent color bleachlng from the large amount o~ vislble radiation present ln sunlight. The device may be used by a sunbather or those concenred about sensitivity to the solar radiation, and the approPriate chan~e ln color of the aziridine compound indicates the amount o~
exposure to erythemal radiation. The monitor may also be used to lndicate necessary adJustments for solar collector panels by comparlng the rate of change of the color wlth panel posi-tion.
Since the photochromic a~lridinesused in this invention respond to actinic radiation as far out as about 450 nanometers; the device, with appropriate filters, ma~ be used to monltor the exposure received by plant life. Cumulative light exposure around 435 nanometers ~blue) is indicated by the change ~rom colorless to blue 9 and alternatively exposure around h75 nanometers (red) can be correlated with the extent of optical bleaching ~rom blue to colorless. Thus, light exposure which af~ects processes such as chlorophyll forma-~ . ~
tion, photomorphogenesis, phototropism, etc. can be monitored.

The device may also be used to monltor ultra-violet therapy given to infants undergoing treatment for jaundice. Another use for the device is in monitoring exposure by industrial workers to ultraviolet light and electron beam radiation used in various manu~acturing processes.

Example 5 _ _ Psoriasis Treatment Monitor The aziridlne compound, 2 a Z ~ -dirnethyl-~(p-nitrophenyl)~4-phenyl-1,3-diazabicyclo[3.1,0]hex-3-~ne ~0.25 g), was ground to a flne particle slze in a mortar and pestle.
Aqueous polyvinyl alcohol (PVA) solution (3 g o~ solution) was added to the azlridine compound and the mixture was ground rO~ a ~ew mlnutes to achleve a uniform dispersion. The disper-sion was applied by brush to bond paper and drled with a heat gun. The entlre substrate was then dipped in an a~ueous 4%
PVA solution and dried with a heat gun. Dipping and drying were repea~ed three more times to completely seal the aziridine -compound from oxygen.
Patches o~ the aziridine coated substrate (about 0.25 cm2) were attached to a color standard sheet. The color standard sheet was bond paper coated with blue Ben,~amln Moore Formulation 9-31 Flat Latex paint~ This paint was s01ected because of the visual match between it ~nd the colored form of the aziridine dispersion at a reflective optlcal density of 0.8ll when viewed throu~h the ~ilter s~stem (described below).
Five patches of aziridine coated substrate were attached to strips o~ the color standard sheet about 1.5 cm x 7.5 cm so that the patches were completely surrounded by the colored background. The entire strip was then coated with a 4%
aqueous soIution of PVA.
The attenuating filters were prepared by rirst making a master mix and then successively dlluting it to achieve various concentrations. The master mix was prepared -2~-as ~ollows: ~lylene WS (53.5 g - DuPont 4,4~ methylenebls-cyclohexyllsocyanate) was charged to a 250 ml three~neck flask nd stirred and heated to 50C. Polycaprolactone polyol (83.o g - molecular weight about 530) was added and the temperature rose to 84~C. A~ter two hours of stirrlng~ the temperature had decreased to 65C. Dibutyltin dilaurate (0.24 g) was then added. Hydroxyethyl methacrylate (HEMA, 37.4 ~) was slowly added to the stirred mixture and allowed to react ror about 45 minutes. The resulting syrupy mixture was designated 10 Oligomer "A"~ Oligomer "A" (5.05 g) was mixed with 5.05 g of V-Pyrol~ monomer (N~vlnyl-2-yyrrolldone ~rom GAF Corp.). To this mixture, anisoin ethyl ether (0.5 g) and ~,a-diethoxy-acetophenone (0.1 g) were added. To the resulting solution, Genacryl Yellow 3G (C.I. #4~055, 2.0 ~), 2,4 dihydroxybenæo-15 phenone (0.424 g) and Alizarine Yellow 5GS (C.I. #14055, o.o386 g) were added sequentially and stirred until dissolved.
This solution was addad to a mixture o~ Oligomer 7'A" (43.3 g) HEMA (43.3 g) and a,a-diethoxyacetophenone (o.86~ g) to form the ~aster mixO The master mix was dlluted with various amounts of a diluent made o~ 1:1 Oligom~r l'A":HEMA ~ 1 wt %
a,a-dlethoxyacetophenone as shown in Table VII.

Table VII

19.9997 210.0002 o.5798 37.9999 1.2223 47.0002 1.7830 -5S.9999 2.8716 ~ ;
Each of these solutions was coated onto Mylar with a ~44 Meyer bar and exposed under a nitrogen atmosphere to a bank o~ Sylvania F15T 8/BL lamps rOr 2.5 minutes on the ~ ~J~
~25-,: , . ~

.6~

coated side, 10 minutes on the back side, and then an addi tional 5 minutes on the coated side, The resultlng cured films provlde a relatively ~lat spectral response to near UV
radiation.
The cured films are 0asily removed from the Mylar and cut into the appropriate size to fit over the photo-chromic azlridine patches. The most concentrated mixture fllm is conveniently attached to the patch at one end of the sub-strate, wlth a systematlc decrease in concentratlon over the other patches, The spectral response shaping ~ilter was ~re pared by dissolving the followlng ingredients in 4~.5 g of a 1:1 mixture of Oligomer "A" and HEMA:
0.750 g Genacryl Yellow 3G (C.I, #48055);
0.0782 g dihydroxybenzophenone;
0.213 g phenylsallcYlate;
0.252 g anisoln ethyl ether; and 0.260 g ~,a-diethoxYacetophenone.
The resulting solutlon was coated on Mylar with a R44 Meyer bar and cured in a nitrogen atmosphere with a bank of Sylvania F15T 8/BL lamps. The coated side was exposed for ten minutes and the backside for five minutes, The resulting film was peeled from the Mylar and was o.o68 mm thick, A portion of thls film 1.5 cm x 7.5 cm was attached to the attenuating filters. The entire assembly was covered by a layer of Mylar to form a monitor, The monitor was exposed to a pair of Sylvania FR 40 BL-235 lamps. These lamps have an emission spectrum similar to the lamps used in treat~ng psoriasis. As the monltor was exposed to the radiation~ the photochromic aziridine chan~ed to a blue color. The patch under the attenuatlng ~ilter 5 ~irst matched the color o~ the color standard followed by 4 through l as the length Or time of exposure increased. By the kime that the patch under filter 1 matched the color o~ the standard, the other patches were darkened 80 thak lt was ver~ easy to visually determlne that their "end-points" had been passed.
At the point where the color o~ the photo-chromic azlrldine patch matched the color standard, the reflec-tlve optical denslty (herelnafter re!ferred to as "optical density") was o.88 through the monltor's ~ilters as measured by a MacBeth RD~519 Vensitometer with cyan ~ilter.
A monitor- was exposed to the Sylvania FR40BL-235 lamps until the color of all patches hadpassed the "end-point".
The optlcal density of each patch was determlned and the moni-tor was then placed in the dark for 72 hours. At the end ofthls time, the optical density was measured agaln. The data is recorded in Table VII. There was little change in optlcal density following 72 hours in the dark. The monitor was then exposed to a yellow GoE~ ~'Bug Llte" at a distance of 15 cm for two hours. The optlcal density was determined. The results (also shown in Table VIII) lndicate that the bleaching under visible radiation is substantial, and that the monitors ma~
be reused.

Table VIII

Optical_Density Sample 1 ? 3 4 5 ::
Immedlately a~ter exposure to UV loO0 1~12 1.34 1032 1.39 After 72 hours in dark o.961.09 1.29 1.28 1. 39 A~ter exposure to visible light 0.26 0.26 0.2G 0.26 0.26 . .
,.

6~

Comparlson of the spectral response of this device and the phyRiological response of human skin following ingestion of 8-methoxypsoralen indlcates that it would be a sllperior monitor for determining the radiatlon exposure in psoriasis treatment.

~ ' ' The aziridine compound, 2,2'-dimethyl~6(p-nitrophenyl)-4-phenyl-1,3-diazabicycloC3.1.0]hex-3-ene was dispersed in an aqueous polyvlnyl alcohol solution. 'rhe solution was applied to white cardboard and completely sealed ln PVA as descrlbed in Example 5.
The monitor was then prepared as follows:
The entire light sensltive substrate was covered with masking tape. The masking tape was cut and removed in areas where the color standard is to be applied. The entire surface was covered with Ben~amin Moore 9-31 Flat Latex paint.
The masking tape (in areas of light-sensltive patches) was then removed. -The entire substrate was then dip-coated in 4%
PV~ and dried (repeated three tlmes).
Filters (combining the spectral response shap-ing filter and the attenuating filter into one) to be placed over the individual patches were prepared in the following manner.
A master dye mixture was prepared ~y addin8 the following ingred~ents to 1324 g of 20.5% mod. cell. acetate in acetone solution:
14.8391 g Genacryl Yellow (Berncolors~ Inc. -Bernacryl Yellow, 4G);

.

1.4298 g Allzarine Yellow (Berncolors, Inc. -Bernachrome Yellow 6G);
2.3421 g dihydroxybenzophenone, and 2.0992 g phenyl salicylate.
The mixture was warmed and stirred t;o dlssolve the dyes and then passed through a pressure filter.
A serie~ o~ dilutlons of this master mlxture was made by adding various amounts of 20.5% mod. cell. ac~tate/
acetone solution. Samples of these solutions were knife-coated at wet thicknesses of 3.65 mm and then air dried in an oven at 80C (0.0457 mm dry). Samples were selected that gave repre sentative endpoints of each film when placed over the aziri-dine dispersion and exposed with theFR40BL-23~ lamps. The dllutions u~ed in this example are given below.

Amount of Amount of20.5% Mod. Cell.
Sample # Master Mix (~Acetate in ~cetone (~) 2 93.5 8.7 3 85.o 15.0

4 76.o 2l~.0 85.0 33 Samples of these films were cut to size and ;;
fixed to the aziridine patches with Dow Corning SII,ASTIC 732 B 25 RTV adheslve. A layer of 2 mil Mylar was then glued over all five filters with the SILASTIC adhesive, and the entire assembly was allowed to cure under pressure for 24 hours.
The monitor was exposed to a palr of FR 40 BL-235 lamps at the rate of 2.08 milliwatts/cm2 (measured with a Gamma Scientific 820A Photometer) until each of the endpoints had been reached. The sample was then optically bleached with J~ "~

.

o~

a G.E. "~ug Lite". Thl~ cycle was repeated two more time~.
On ths fourth cycle, the optlcal density Or each of the patches was measured periodically (throu~h the filters) with an RD-100 densitometer. The sample was sub-sequently bleached optically and a ~l~th cycle of UV exposuregiven. At the end of the fi~th exposure 3 the sample was placed in the dark and left for 69 hours at room temperature.
Optical density readings were then taken. These data are summarized below.
The underllned readings, ln each sample, were the prevlGusly ~udged visual "endpoints". Thus, in these ten determinations the visual endpoints varied with optlcal denslty between 0.89 and o.g2 t< 3~), and the extent of bleaching in the dark was minimal~

4th Exposure Cycle Exposure (J!cm2) Optical Density of Sample #

O 28 .27 .30 .28 .26 ;~
~196 .45 D 39 .36 32 .31 .595 ~62 ~56 .46 .40 .36 -1.2~~83 .73 ~55 .48 .42 1.58 .92 .80 .60 .52 .~6 2.18 .92 .68 .62 .52 4.05 .85 .76 .67 ; 1~-75 ~91 o~l .70

5.74 .87 .76

6~72 ^89 .77 9.87 t90 . .

Expos~re Optica ~
_ 2 3 4 5_ 0 .29 .2~ .31 .28 .28 .196 .1~4 .37 .34 .34 o 29 .595 .62 .53 .42 .41 .34 1.49 .91 .78 .5~ ~52 .46 2.18 .92 .70 .61 .52 3.96 .88 .78 .68 4.45 .~9 .79 .~8 5.75 .86 .73 6.72 .9~ .78 -8.90 .~5 10.6 . go Optical Density :
A~ter Standing in Dark 69 Hours .86 .88 .88 .82 .86 Example 7 20Psorlasis Treatment Monitor - :;
The radiation sensitive.substrate was prepared by coating an aziridine-PVA dispersion to a thickness o~
approximately 0~ 025 mm on phenyl-terminated polycarbodiimide- .
::: primed polyester and subsequently coating with PVA as described .
~ 25 in Example 5. A color standard background was prepared by .
, : coating primed polyester with the latex paint described in :~
Example 5. Ten holes (about 0O 25 cm2) were punched in ~his : reference strip, and transparent double-coated pressure sensi- ~:
tive adhesive tape was attached to the uncoated side over the hole!s. Patches of the light sensitive substrate were then pressed into the holes in the reference-strip and held in .~ .;
position with the pressure sensitive adhesive. A strip of tracing paper (Crane - 100% cotton from American Pad and -Paper Co.) (1.5 cm x 7.5 cm) was placed over the painted sur-face.

:

. . . , , , .. ~ ;; :

rllhe ~ilter s,ystem for the exposure side con-sisted of a layer Or t,he spectral response shapin~ ~llter o~
Example 5 and a neutral density step wed~e rllter (Stouffer #95). The neutral density step wedge filter was placed over the spectral response shaping filter wlth each step covering a ~ifrerent light sensitlve patch. The readout side was covered with visible transmitting UV-absorbing filters made up o~ the spectral response shaping ~llter plus a filter of the cur~d master mix o~ Example 5 and held in place by the double-coated pressure sensitive adheslve tape, The assembled monitor was exposed for 16 minutesto th0 FR 40 ~L-235 lamps of Example 5 at the rate of 1.8 x 10 3 watts/cm2 and the difference in the color intensity of the various segments could be easily distinguished visually.
Reflectance O.D. measurements were subsequently made through the filters of the readout side using a MacBeth RD-51~ dens~to-meter with a cyan filter. The results are shown in Table IX.

Table IX

Optical Density l 1.10 2 1.12 3 1.02 4 1.00 .88 6 .82 ;

7 .79 ~ .7l 9 .6~
.64 ~, ~ackground1.03 ": .
.

~ 6~ S

~ Monitor The Yellow dYe, Setoflavin T (C.I ~49005, 0.0225 g), was dissolved in 7 cc o~ a 4% aqueous PVA solution.
This dyed PVA solutlon was then mlxed with the azlridine com-pound (().25 g) to make the light-sensitlve substrate as described in Example 5.
Alizarine Yellow (C.I. #14055, 0.0345 ~) was dissolved in one ml o~ 1:1 ethanol:acetone. This solution was mixed with 2.83 g of 39% mod cell. acetate in acetone and then coated onto a Mylar web usin~ a #44 Meyer Bar (drled film thlckness - 0.01 mm).
Films o~ the Aliæarine Yellow/mod. cell. acetate were then laid over patches o~ the Seto~lavin-aziridine sub-strates. The flrst patch was covered by one ~llm, and thesecond by two. A CS-7-54 (Corning ~lass Works - Corning Glass #9363) UV-transmitting, visible-blocking filter was placed over all o~ the segments.
This assembly was exposed outside to ~ull (~innesota winter) sunlight. After 25 minuteæ exposure, the light sensitlve patch under a single ~ilm was considerably darker, but only slightly darker under two film~. Reflectance O.D. readings were made through the Ali7arine Yellow (C.I.
#14055)-mod. cell. acetate ~ilm using a MacBeth RD-519 densito-meter (cyan ~ilter). The data is recorded in Table X.

Table X

Time O.D.
1 Film 2 Fllms Start .23 .23 ~xpose 2:lQ-2:35 p.m. 1.40 .29 Expose 2:50-4:20 p.m. 1.51 54 . .

O~

Transmittance measurements as a runct,ion o~
wavelength were made of a combination of two layers of the 3.1% Alizarine Yellow-mod. cell, acetate and a 0,015 mm film of 3.2% Seto~lavin/PVA. These tran~mlttance values and the azirldine-PVA response values were multlplied to get the approximate device spectral response.

~9 Three dispersions of aziridine compound ln PVA
were prepared as in Example 5, each havlng a different aziridine/PVA ratio. These were coated to approximately 0.05 mm l;hickness onto cardboard. A spectral response shaping filter (~xample 5) was placed over samples of each of the dispersions. They were then exposed to the blacklights of Example 5 and optical density readings were taken. The read- -ings (Table XI~ illustrate that it is possible to obtain a spread of endpoints by using a common ~ilter9 but different loadings of aziridine in the light sensitlve substrate, and these differences can be observed visually.

Table XI
-- .
Optical Densit~
llelght__ Weight Ratio Aziridine 4% PVA Aziridine~PVA 0 10 m~/cm2 40 rn~/cm2 .0186 g .178 g 2.6.12 .26 .88 .oo70 g .17~ g 9~ o12 .22 .59 25.0018 g .178 g .25.12 .15 .27 In carrying out the above measurements, the spectral response shaping filter was removed prior to deter-minin~ the optical density.

.

. : . . ,; .
"~
. , ~:

~1~6~
s Plant Lighting Indlcator ~or Red Llght Respon~e .__ (~ ) Filter paper was dlpped in a saturated benzene solutlon Or the Rl = R2 = C~f3 derivative and the solvent evap-orated. This strip was coated with 4% PVA and dried wlth a heat gun. The PVA coatin~ was repeated three more tlmes. The sample was irradlated to an optical denslty o~ .70 (RD 100 densitometer).
Strips of 3M brand infrared transparency ~ilm Type ~577 were stacked to varlous thicknesses over the irradi- ;
ated aziridine to makè a step wedge attenuating filter for red llght. A CS 3-69 UV cut-of'f, visible transmitting filter was placed over this wedge. The entire assembly was taped to a cardboard backing.
One o~ these indicators was placed in a window with northern exposure and one with a southern exposure (both inclined 45). They were exposed from sunrlse to sunset on a cloudy-bright January day. There was significantly more bleach-ing in the sample given southern exposure.
The experiment was repeated on a totally cloudy January day. ~oth samples showed equal bleaching, but consid-erably less bleaching than that ~ound in the monltors exposed on the cloudy-brlght day.
The experiment was repeated on a totally sunny day. For the sample given a southern exposure, complete bleachin~ occurred at all positions having up to lO layers of the blue transparency material over them. For the sample le~t in the northern exposure, only a trace o~ bleachlng occurred under the area with 4 layers. No noticeable bleaching occurred in areas co~ered with more than 4 layers.

., - , - ..
, , : , " ., .
. ",, :
- . - -~

~601~

Exam~
Plant Ll~htin~ Indlcakor for Blu~ Llr~ 9C~e A substrate was prepared as described in Exam~le 6. Alizarine Yellow (4.0 g~ of` Bernchrome Yellow) and 4.0 g Or tetrahydroxybenzophenone were dissolved ln a solution of 14.5~ mod. cell, acetate in acet;one. Thls served as the master solution used to make ~he combined spectral shaping and attenuating filters. Thls solutiorl was diluted with 14.5% mod.
cell. acetate to malce representative samples. These samples were coated to .38 mm wet (.o56 mm dry) by a knife coating and dried two hours ln an 80C oven. From these films, two samples were selected.
Baslc Blue 7 (C.I. ~42595, 0.362 g) was dissolved ln 159.4 g of 24.2% mod. cell. acetate in acetone and coated wlth a knife coater to 0.305 mm wet~ 0.046 mm dry). This f`ilter, which is to be removed during readout, attenuates visible light in the red region of the spectrum where the azlridine is most easily optically bleached. (It is, however, essentially transmitting in the near- W and far visible region.) Samples of the Alizarine Yellow/tetrahydroxy-benzophenone in mod. cell. acetate fllters were placed over the indicators and then a layer o~ 0.05 mm Mylar placed over that. A layer o~ the Basic Blue 7 fllm was laid temporarlly over the entire assembly. These lndicators were then placed 7 cm f`rom a pair of G.E. #40CW Cool White Fluorescent lamps (3.24 milliwatts/cm2 total output at sample plane). Reflective optical density readings of the aziridine/PVA layer were taken periodically (through the filters) with a MacBeth RD-100 densi-tometer (yellow f`ilter position). These readings are givenin Table XII.

Table XII
__ Optlcal Densit,y After # Grams~ GramsTotal Exposure of Master14.5% Mod.Cell. (Joul~s/cm2) 5Sam~le # MixtureAcetate/Acetone 0 9.72 23.3 65 2 l 100 0 .44 .82 1.00 1.15 2 80 20 ,52 .96 1.12 1.32 The Basic Blue 7 Filter was removecl and the sample was bleached with a G.E. "Bug Lite".

~, ~lectron Beam Radiation Monitor The a~lridine, 2,2'-dimethyl-6(p-nitrophenyl)-4-phenyl~1,3-diazabicyclo[3.1.~hex-3-ene (0.25 ~m), was ground to a fine particle size in a mortar and pestle. Aqueous poly-vinyl alcohol (PVA) solution (3 gm of 4% solution) was added to the aziridine and the mixture was then ground for a few minutes ;
to achieve a uni~orm dispersion. The dispersion was applied by brush to bond paper and dried with a heat gun. The entire substrate was then dipped in an aqueous 4% PVA solution and dried with a heat gun. Dipplng and drying was repeated three more tlmes to completely seal the aziridine from alr. Patches ~ ~
of this material were attached to the comparison sheet ln the ~' manner Or Example 5 and the entire substrate was dip-coated ln 4~ PVA solution and dried. The entire monitor substrate was ;
2~ covered with a single layer of the UV-attenuatin~, visible transmlttlng film prepared for the psoriasls treatment monitor of Example 5, Sample l (.o68 mm) to prevent coloration by room light. Dif~erent thicknesses of Mylar were then stacked over each of the light sensitive patches to provide step-wise atten-~30 uation of the electron beam.

,:
'': :' -37- ' Thls a~sembly was ~ixed to a cardboard backing and passed through the purged exposure chamber of an Energy Sciences Electrocurtaln Systems hlgh energy electron beam radiatlon curing unit (acceleratlng potential 175 Kev, beam current - 4.32 x lO 6 amps/cm2, l.ll second exposure (.84 Joules/cm2). After exposure, reflective optical densities were measured for each Or the patches of the monitor (through the filters) using an AD-~19 densitometer (blue filter position).
The sample was then re-exposed under the same conditions (cumulative electron beam exposure l. 68 Joules/
cm2). The e~fects of these exposures on the optical densities of the various segments is shown in Table XIII.

Table XIII

Total Thickness of Beam Attenuating Layers 0 D O.D-. Af~er Electron and Mylar Before Segment Filter)~xposure .84 Joules/cm2 1.68 Joules/cm2 ~ .
l .094 mm.18 1,28 --2 .129 mm. 221.03 1.23 25 3 .170 ~m.24 .34 .40 4 .221 mm. 34 .34 .33 .246 mm.38 .38 .37 6 .272 mm. 36 .36 .36 7 .297 mm. 39 .42 .42 30 8 .322 mm. 35 .39 .36 , This sample was optically bleached with a ~.E. "Bug Lite" and re-~maged. The sample showed little or no loss of sensiti~ity.
In addition to the above appllcation of the monltor as an indicator of cumulative exPosure, the monitor 35 substrate could be used as a beam penetration indicator.

- 3~ -

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A thermally stable, optically erasable recording medium comprising a substrate having on at least one surface thereof a first coating of at least one photochromic aziridine of the formula wherein R1 and R2 separately are hydrogen phenyl, lower alkyl, or ortho or para lower alkyl or lower alkoxy-substituted phenyl or together are alkylene having 4 to 7 carbon atoms; and overlying said first coating, at least one substantially oxygen-impermeable barrier coating said barrier coating being, reasonably transparent to actinic radiation.

2. The recording medium of claim 1 wherein said first coating is a thin, homogeneous, vapor-deposited film.

3. The recording medium of claim 1 wherein said first coating is microcrystalline,

4. The recording medium of claim 2 wherein said film is transparent and non-dendritic and said barrier coating is crystal growth inhibiting.

5. The recording medium of claim 1 wherein R1 and R2 are methyl groups.

6. The recording medium of claim 1 wherein said barrier coating com-prises polyvinyl alcohol.

7. The recording medium of claim 1 wherein said barrier coating comprises gelatin.

8. The recording medium of claim 1 wherein said alkylene is selected from the group consisting of cyclohexyl and cyclopentyl.

9. The recording medium of claim 1 wherein said substrate is porous.

10. The recording medium of claim 9 wherein said substrate is paper.

11. The recording medium of claim 1 wherein said first coating additionally contains a film-forming binder.

12. The recording medium of claim 11 wherein said binder is polyvinyl alcohol.