Depositing latent fingerprints and development thereof
This is a division of application Ser. No. 822,066 filed Aug. 5, 1977 now U.S. Pat. No. 4,174,409. 
    
    
     This invention relates to a method for providing latent fingerprints having prolonged latency on documents as a means for identification and/or endorsement and to compositions therefor. 
     Because fingerprints are considered to be unique to an individual they provide a means of identification which is substantially infallible. Employment of fingerprints for identification or authentication purposes requires deposition of a fingerprint which can be compared either with known standards for purposes of verification or can be used as a means of identifying the individual who originally deposited the fingerprints if such identification becomes necessary. This constitutes a form of endorsement. 
     The method of fingerprinting widely used by law enforcement agencies in which the fingers are coated with ink and then applied to suitable forms is entirely unacceptable for identification on documents for various reasons. First the soiling of the hands antagonizes the individual whose fingerprints are being deposited. Further the fingerprints applied, for example, to a check can be smudged in handling or cause soiling of other documents. 
     The alternative procedure is to apply latent fingerprints to the document and subsequently develop them by some suitable procedure. Latent fingerprints from normal skin oils are well-known to be of only limited permanency, probably only a few days at most when applied to paper, and to have a tendency to smudging or disappearance as oils are transported in the fibers of the paper by capillarity throughout the paper. Because of these factors it is difficult to recover fingerprints from paper after more than a short period and the prints recovered are more likely to be those of the last persons to handle the paper and not those applied for identification purposes. 
     It is an object of this invention to provide invisible latent fingerprints on documents for purposes of identification and/or endorsement which possess prolonged latency and are susceptible to development substantially without interference from fingerprints arising from normal handling. Other objects will become evident herein elsewhere. 
     In accordance with the above and other objects of the invention it has been found that fingerprints formed after fingers are moistened by compositions comprising certain substantially non-volatile higher fatty polybasic acids surprisingly possess the desired permanency, are not dissipated or smudged by capillarity in a paper substrate and are readily developed by simple application of toner particles of various types even after periods as much as 1 to 8 or more weeks. Many other oily materials are found not to possess these unusual properties. The higher fatty polybasic acids are substantive to paper and to toner particles and are of the general formula (C n  H 2n-x  CO 2  H) y  where n is from 10 to 20 and preferably 18 and x is a small odd integer and y is 2 to 4, preferably x and y are 3. Such preferred acids are often known as &#34;trimer acids&#34; because they are formed by trimerization of diunsaturated aliphatic acids. The structures may include branches and rings. Substantivity to paper is believed to be associated with the presence of three carboxyl groups. These higher fatty polybasic acids, in addition to being substantive to paper, are oleophilic and therefore are adherent to toner particles comprising an oleophilic matrix. The acids are further non-hardening and substantially non-volatilizable when exposed to air and are non-hygroscopic and non-irritating and provide invisible latent fingerprints. 
     The preferred polybasic acids have structures which are not readily shown by formula and are obtained as noted above by the polymerization of unsaturated fatty acids, e.g., linoleic acid. Trimeric acids are present in various proportions in many commercially available polymeric acids such as those sold under the Trademark name, Hystrene® from Humko Products, Chemical Division, Memphis, Tenn. and Empol® from Emery Industries Inc., Cincinnati, Ohio. Other sources of supply are available. The preferred trimeric polybasic acids tend to be oily or waxy materials of relatively high viscosity which can be described as honey-like. They may be used directly or by dilution in an oily base such as mineral oil, trioctyl phosphate, dialkyl phthalate or other neutral relatively non-volatile solvent. In general the composition should have an oily consistency without tackiness in order to be conveniently used. Properties of a number of such commercial products are tabulated in Table I. 
     
                       TABLE I                                                     
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           % Acid Composition                                             
                           Viscosity                                      
Commercial Name                                                           
             Monobasic Dimer   Trimer                                     
                                     25° C. (cSt)                  
______________________________________                                    
Empol®                                                                
1010         0         97       3    5,200                                
1012          8-10     85-88   4-5   5,200                                
1014         1         95       4    5,600                                
1016         trace     87      13    5,300                                
1018         trace     83      17    7,500                                
1022         3         75      22    6,200                                
1024         trace     75      25    9,500                                
1028         6-8       72-77   17-20 6,100                                
1031         14-16     65-70   16-19 3,200                                
1040         0         20      80    60,000                               
1041         0         10      90    18,000                               
1054-A       1         71      28    ˜11,000*                       
1056-A       2         79      19    ˜15,000*                       
Hystrene®                                                             
3695         1         95       4    6,800                                
3680         trace     83      17    8,000                                
3675         trace     75      25    9,000                                
5460         trace     40      60    30,000                               
3680 S       1         84      15    14,000                               
3675 X       1         87      12    9,300                                
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 *estimated                                                               
 
    
     The composition selected is conveniently impregnated into an absorbent pad, such as an uninked stamp pad as could be used for rubber stamps, but, of course, free from colored ink. Other means of dispensing will be evident to those of skill in the art such as: individual strips of paper containing a small amount of the composition, sponges, felts, leathers, synthetic fabrics, etc. The finger or fingers (which terms herein include the thumb) which are desired to provide identification are moistened from the pad or paper strip and applied to a portion of the document or paper intended to carry the identification or endorsement. Greatest clarity will result by careful imprinting with slight rolling motion, but in the interest of convenience contacting, e.g., by merely grasping the document or paper will provide substantial identificatory markings. The imprinted document or paper, e.g. a check, is then handled in normal business procedures. When question arises as to the identity of the person who imprinted the document even 1 to 8 or more weeks later, the fingerprints are developed by dusting with toning powder such as those described in U.S. Pat. Nos. 3,377,286  and 3,639,245. A particularly convenient procedure is to make a brush of magnetic toning particles on a magnetized wand and draw the brush across the latent print. The latent print has sufficient affinity for the toner particles to remove them from the brush and unused particles are retained. This results in relatively little or no soiling from excess toner. Particularly preferred toner particles are colored black by powdered magnetite in a fusible base and the developed print can then be rendered permanent by warming to fuse the base and affix the particles to the fibers of the paper substrate. It will be evident that the present procedure and compositions provide a convenient tracing method for identification of passers of checks and in other situations, e.g. authentication of documents and the like. Such latent markings on the document are sometimes referred to as endorsements. 
    
    
     EXAMPLE 1 
     Pads of absorbent material, or artificial leather, are impregnated with each of several polybasic fatty acids and fingerprints are then imposed on various bond or security papers using light pressure. The papers are Hammermill Sentry Safety paper available from Hammermill Paper Co., Erie, Penn.; La Monte Safety Design paper available from La Monte Division of Georgia-Pacific Corporation, Warwick, N.Y.; Nekoosa white bond paper available from Nekoosa-Edwards Paper Co., Port Edwards, Wis.; and Timberline® Safety Criss Cross from Boise Cascade Papers, Portland, Ore. Several prints are made of each material. Prints are developed at successive intervals using black magnetite-containing toner particles available commercially as &#34;VHS&#34; and &#34;VQC®&#34; imaging powders (3M Company) and fixed by a suitable method, e.g. warming briefly. Comparison of the prints shows satisfactory legibility extending to periods of latency of up to 11 weeks. Such data are summarized in Table II. 
     
                       TABLE II                                                    
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                                Minimum                                   
Fingerprinting                  Lifetime                                  
Substance   Paper               (weeks)                                   
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Empol®1054A                                                           
            Hammermill Sentry Safety                                      
                                5                                         
&#34;           La Monte Safety Design                                        
                                11                                        
&#34;           Nekoosa White Bond  11                                        
Empol®1056A                                                           
            Hammermill Sentry Safety                                      
                                1                                         
Empol®1040                                                            
            La Monte Safety Design                                        
                                6                                         
&#34;           Nekoosa White Bond  6                                         
Empol®1015                                                            
            Hammermill Sentry Safety                                      
                                1                                         
Empol®1016                                                            
            La Monte Safety Design                                        
                                1                                         
Empol®1010                                                            
            La Monte Safety Design                                        
                                1                                         
Hystrene®3695                                                         
            La Monte Safety Design                                        
                                3                                         
Empol®1054A                                                           
            Timberline®Safety Criss Cross                             
                                3                                         
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     EXAMPLE 2 
     Pads of filter paper are impregnated with Empol® 1054A polybasic acids in concentrations of 100%, 90% diluted with 10% trioctyl phosphate and 75% diluted with 25% light mineral oil. In each case fingerprints are imposed on bond paper, or Safety paper and then developed by application of toner powder after 1 week. The prints are found to be clear and recognizable.