Abstract:
Rapid changes in absorption are made within the volume of a photosensitive transparent member that absorbs incident radiation to prevent the photocopy of a document, the original transparency being restored after discontinuance of the incident radiation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to radiation blocking systems and in particular to methods and materials used as coatings to prevent document reproduction. 
     2. Description of Related Art 
     Photochromic materials, in particular the spiropyrans, are candidates for many light controlling applications such as, sun glasses, optical switches, Q switches, photochromic images, optical data processing and fast response filters to prevent the reproduction of documents. In the latter case the response speed of the filter should be faster than the time required to reach the threshold of detectability in the photosensitive element of the reproduction (e.g. photocopy) machine. The response times of certain spiropyrans are typically in the nano or pico second range when activated in a solution of a suitable solvent, but their conditioning into films or coatings typically requires the presence of a plastic binder which decreases considerably the reaction speed and prevents the reversibility of the phenomenon. The conditionment into film is otherwise necessary to the use of spiropyrans for preventing the reproduction of a document. 
     Accordingly, it is an objective of this invention to address at least some of the above noted difficulties. 
     Another objective of this invention is to develop radiation blocking materials that are photosensitive to the radiations utilized for the copying and/or reproduction of documents. 
     Still another objective of the present invention is to devise coatings or films capable of generating a blocking filter for said radiations in a time less than the time threshold of the recording photosensor. 
     A further objective of the present invention is to develop coatings or films capable of restoring original viewing transparency after the filtering action preventing the reproduction of a document. 
     Yet another objective of this invention is to develop coatings or films capable of blocking the radiations generated by conventional xerographic techniques to prevent the reproduction of a document without altering the readability of the document under normal viewing conditions. 
     A specific objective of this invention is the fabrication of coatings or films to prevent the reproduction of a document. 
     SUMMARY OF THE INVENTION 
     These and other objectives are addressed by this invention with a photosensitive member fabricated by crosslinking diacetylene monomers utilizing spiropyrans as crosslinks and a photosensitizing dye. The dye may enable the polydiacetylene to respond to radiations within the visible spectrum used for the reproduction of a document, thereby causing the spiropyrans to open into their colored form. At substantially the same time, the spiropyran itself may absorb the ultraviolet part of the spectrum, also providing ring opening in its colored form. The two concurrent actions may result in a deep coloration of the film, which substantially prevents the recording of the optical pattern representing the document. 
     Once the radiation ceases to exist, the spiropyran may return to its closed form and the film may revert to transparency. 
     The interaction of the diacetylene chains in the polymer with the spiropyran links may create conditions for selective response to a specific spectral distribution of the printing light as well as a given rise time of the light pulse. These conditions can be adjusted by the length and composition of the link containing the spiropyran, thus creating various degrees of conjugation between the pi-orbitals of the spiropyran and of the monomer chains. The introduction of a carbonyl group (See FIG. 6) in the link may provide a continuous intramolecular overlap of pi-orbitals extending from the spiropyran to the diacetylene chains. Stabilized orbital delocalization corresponding to specific electronic states can be made to match a given spectral distribution in the printing light and allow the system to differentiate from ambient reading light. 
     Another element of differentiation is related to the kinetics of ring opening of the spiropyran in the presence of other active groups in the link itself or in the monomer chains. Multistable states can relate to the rise time of the printing pulse. 
     In view of these considerations, crosslinked polymer structures which will respond specifically to the printing light while remaining unaffected by ambient reading light may be designed in accordance with this invention. The specificity may be determined by a number of factors, such as the choice of the diacetylene monomer, of the spiropyran, the length and composition of the link containing the spiropyran, the absorption spectrum of the spiropyran and that of the photosensitizer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents the molecular structure of a spiropyran. 
     FIG. 2 shows a number of spiropyrans preferred in the practice of the present invention. 
     FIG. 3 illustrates various structures of polydiacetylenes. 
     FIG. 4 shows a polydiacetylene crosslinked by a benzothiazol spiropyran. 
     FIG. 5 represents a polydiacetylene crosslinked by a benzothiazol spiropyran and two CH2 groups. 
     FIG. 6 represents a polydiacetylene crosslinked by a benzothiazol spiropyran and two carbonyl groups. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The photosensitive element of the present invention may be formed by simultaneous polymerization and crosslinking of a diacetylene monomer in the presence of a modified spiropyran and a dye sensitizer. 
     The polymer structure generally comprises three elements, namely the spiropyran, the diacetylene monomer and the dye sensitizer. 
     The Spiropyrans 
     The spiropyrans are organic molecules which can be represented as follows (R1 represents a hydrocarbon group, aliphatic or aromatic): ##STR1## 
     The n identical or different groups represent groups such as NO 2 , CH 2  Cl, alkoxy (preferably OCH 3 ) or a ring, for example the benzene ring when n=2. 
     The benzopyran part with various substitutions is always included. Part (A) represents various heterocyclic systems, those most frequently encountered being shown in Table 1. 
     
                       TABLE 1______________________________________ ##STR2##        ##STR3##       X = OY = N benzoxazoline       X = NY = N benzimidazoline       X = SeY = N benzoselenazoline       X = CHCHY = N quinoline (or iso)       X = SY = Sbenzodithiol       X = SY = O benzoxathiol       X = CHCHY = O benzopyran ##STR4##   X = SY = Sdithiol X = SY = O oxathiol ##STR5##   X = SY = N thiazolidine X =O Y = N oxazolidine X = CH.sub.2 Y       = N pyrrole ##STR6##   X = OY = N 1,3-oxazine X = SY = N 1,3-thiazine X = CH.sub.2 Y       = N piperidine ##STR7##        1,4-thiazine ##STR8##        phenanthridine ##STR9##        xanthene ##STR10##       acridine______________________________________ 
    
     The properties of these photochromic compounds have been described in many books and publications. An exhaustive study is presented in G. H. Brown&#39;s book &#34;Photochromism&#34; (Wiley, Interscience). Spiropyrans are typically uncolored in their closed form, but can be reversibly opened and transformed in colored mecrocyanines under ultraviolet irradiation. The merocyanine can be reverted into spiropyran either thermally or photochemically, by irradiation in the absorption band of the merocyanine, which is in the visible. 
     Preferred spiropyrans for use in this invention are the benzothiazoline where X=S and Y=N (See Table 1) because of high response speed (t picosecond). 
     When spiropyrans enter into solid solution in a conventional binder to form a film, the response time may be considerably reduced due to hinderance from the binder molecules, but when they become part of a polymer structure as in the present invention the speed is generally unaffected. 
     In a preferred embodiment, the polymer matrix comprises a coating having thickness from about 1 to 10 microns. 
     The Diacetylene Monomer 
     A variety of diacetylene monomers can be used to build up the polymer according to this invention. Some of them are listed in FIG. 3. A preferred one is the bisphenol A dipropargyl ether: ##STR11## 
     The Dye Sensitizer 
     Preferably, the dye sensitizer includes erythrosin, auramine, or ethyl violet. 
     Various preferred embodiments as described may provide systems employing the use of a crosslinked polymer structure containing a fast response photosensitive element in the crosslink which interacts with photoconductive polymer chains to provide a bi-stable filter. The polymer in the form of a coating may be used to substantially prevent the reproduction of a document by darkening at a speed superior to the time delay necessary to trigger the activation of the photosensitive element in a photocopier. Such polymer may be designed to react only to a specific spectral distribution of the light source used in the photocopier, thus substantially preventing filtering action under normal reading light. 
     EXAMPLE 
     Preparation of the diacetylene monomer: 
     The monomer may be obtained by reaction of 4,4&#39;isopropylenediphenol (Bisphenol A) and propargyl bromide in acetone in the presence of potassium carbonate. The mixture is refluxed for 72 hours. 
     Polymerization and Crosslinking 
     A methyl alcohol solution containing the monomer and a spiropyran in stoichiometric proportions, a dye sensitizer and small quantities of benzophenone may be spread and polymerized on the surface of a document by exposure to ultraviolet radiations for five minutes. 
     Although the foregoing description, drawings, and example present the invention in terms of specific methods, materials and applications, it will be apparent to those skilled in the art that other materials may be substituted, other manufacturers followed and other uses found. All such substitutions and changes consistent with polydiacetylenes crosslinked by spiropyran compounds are within the scope of the present invention.