Patent Publication Number: US-2004055720-A1

Title: Paper compositions, imaging methods and methods for manufacturing paper

Description:
REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims the benefit of prior, co-pending provisional application serial No. 60/412,132, filed Sep. 19, 2002. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This application relates to a novel paper composition and, more particularly, to paper which is suitable for use in electrophotographic copying and printing methods using dry or liquid toners as well as to methods for forming images on the paper and a method for manufacturing the paper.  
       BACKGROUND OF THE INVENTION  
       [0003] In the well known art of electrophotography a latent electrostatic image is initially formed on a photoconductive surface, typically by depositing a substantially uniform electrostatic charge on the photoconductive surface and exposing the charged surface to an imagewise pattern of radiation which corresponds to an image to be reproduced thereby discharging the photoconductive surface in an imagewise pattern. The latent electrostatic image is then developed by applying to it a composition of charged colored particles, which, depending upon the charge on the colored particles, that is, negative or positive, can be arranged to adhere to areas of the photoconductive surface having the higher potential or lower potential, respectively. The image thus formed on the photoconductive surface can then be transferred to a receiver material, typically paper, and adhered thereto so as to provide the desired reproduction. The development of the latent electrostatic image can be either by a “dry” process wherein a dry composition of colored particles is used or by a “wet” process wherein colored particles are dispersed in a liquid vehicle, typically an insulating, nonpolar liquid such as mineral oil or the like.  
       [0004] The developer composition, which is utilized to form the visible image, includes particles of the image-forming material, commonly referred to as “toner”, such as, for example, carbon black, or other colored pigments, or dyes, and a thermoplastic polymeric binder material. The thermoplastic polymeric binder materials together with other charge control agents and the colored pigments, also referred to hereinafter as pigmented polymer particles, are chosen so as to impart the desired charge triboelectrically to the image-forming material, as well as to provide an adequate degree of plasticity either at the temperature of the transferring surface or, where a specific fusing step is used to bind the image to the receiver surface, at the temperatures of the fusing step. The plasticity is necessary to fuse the pigmented toner particles together (cohesive strength), and to the paper (adhesive strength).  
       [0005] As mentioned previously, the visible image formed on the photoconductive surface is transferred to the receiver material. Such transfer can be made directly to a receiver material to form the final hard copy image. There are also known electrophotographic imaging methods in which the image formed on a photoconductive surface is first transferred to an intermediate transfer surface, also referred to hereinafter as ITS, and transferred from that surface to a final receiver material. Methods of this type are commonly referred to as “digital offset printing”. A method of this type, using a modulated laser beam to write the image on the photoconductor is described in U.S. Pat. No. 4,708,460.  
       [0006] According to the method described in U.S. Pat. No. 4,708,460, a photoconductive drum is charged electrostatically, exposed imagewise by means of a laser, and the resulting latent image developed by applying pigmented polymer particles in a liquid suspension, or emulsion, to the drum. The image formed on the drum is transferred to an ITS, whereupon the liquid vehicle, typically mineral oil or the like, is heated and a significant amount is driven off and the pigmented polymer particles are caused to melt or soften. Subsequently the image is transferred to a final receiver sheet and adhered thereto. In monochrome printing a single color image is formed on the receiver material. In multicolor printing two or more separate monochrome images are formed on the drum in registration and transferred to the receiver sheet.  
       [0007] The receiver materials, which are useful in electrophotographic copying and printing, including digital offset printing, are required to have a number of characteristics. The receiver must be able to rapidly bond-the pigmented polymer particles in the short contact time between the receiver and the transferring surface, or during the short duration of a receiver image fusing step. Hereinafter, any reference to dwell time refers to the duration of either the image transfer step or the fusing step. The rapid bonding will result in strong adhesion of the image-forming material to the receiver surface, which in turn will provide maximum retention of the pigmented polymer particles on the receiver surface, thereby resulting in high color saturation and image contrast. Also, where the printed image is strongly adhered to the receiver surface, the image is afforded more protection from scratching, scuffing, or marring during subsequent handling and processing.  
       [0008] With strong image adhesion to the receiver surface during the transfer step, complete or substantially complete transfer of the pigmented polymer particles can take place without leaving any appreciable image residue on the transferring surface. In instances where there is incomplete transfer of the image to the receiver surface, and repeated printing of the same image is carried out, a significant residual image can be built up on the transferring surface, which can cause a ghost or spurious image to be seen when a different image is then formed on the transferring surface and subsequently transferred to the receiver. Additionally, for electrophotographic printing and copying using a liquid developer composition, the paper must be able to accept the liquid carrier for the pigmented polymer particles so as to not only create good adhesive strength but also create good cohesive strength.  
       [0009] Receiver materials should also have a high surface strength so as to prevent unprinted area ghosting, or spurious images appearing on the receiver surface. When the surface strength of the receiver is not sufficiently strong at the temperatures and pressures of the transfer step, material can transfer from the receiver to the transferring surface, and with repeated printing of the same image, a significant deposit can be built up on the transfer surface in non imaged areas. This build up can then create ghost or spurious images upon subsequent printing of a different image. When paper is used as a receiver, and given the presence of fillers (clay, calcium carbonate, titanium dioxide etc), and fibers, typically used in papermaking, when such fillers and fibers are inadequately adhered to the surface, a deposit of such materials can build up on the transferring surface, particularly when higher temperatures and pressures are used during image transfer, and as described above, cause ghost or spurious images.  
       [0010] As the state of the art advances, there is a continuing need for new and improved materials for use as final receiver materials in such imaging methods.  
       SUMMARY OF THE INVENTION  
       [0011] It is therefore an object of this invention to provide a novel paper composition.  
       [0012] It is another object of the invention to provide a paper composition that is useful as a receiver material for images formed by imaging methods.  
       [0013] It is another object of the invention to provide a paper composition which is useful as a receiver material for images formed by electrophotographic imaging methods, including dry and wet copying and printing methods.  
       [0014] Still another object of the invention is to provide a paper composition which is useful as a receiver material for images formed by electrophotographic imaging methods wherein the image is formed by a liquid developer composition, and the image is either transferred to a receiver and fused thereto or transferred to an intermediate transfer surface prior to being transferred to the receiver.  
       [0015] A further object is to provide an imaging method wherein the paper composition of the invention is utilized as the receiver material.  
       [0016] Yet another object is to provide electrophotographic printing methods including digital offset printing methods wherein a paper composition according to the invention is utilized as the receiver material.  
       [0017] Still another object of the invention is to provide a method for manufacturing the paper of the invention.  
       [0018] In one aspect of the invention there is provided a paper composition, which may be bleached, which comprises at least one anionic polymeric material and at least one binder material and which does not include more than about 20% by weight of mechanical fiber; and preferably not more than about 10%. In a preferred embodiment the paper includes from about 0.1 to about 18.0 lbs/3300 ft 2  of finished paper of at least one anionic polymeric material and from about 0.25 to about 10.0 lbs/3300 ft 2  of finished paper of at least one binder material and particularly preferably, from about 0.20 to about 5.0 lbs/3300 ft 2  of finished paper of at least one anionic polymeric material and from about 1.0 to about 5.0 lbs/3300 ft 2  of finished paper of at least one binder material.  
       [0019] As is known by those skilled in the art, “mechanical fiber” refers to groundwood pulp and thermomechanical pulp. Groundwood pulp is defined as a mechanical wood pulp produced by pressing a barked log against a pulpstone and reducing the wood to a mass of relatively short fibers. Thermomechanical pulp is defined as a high-yield pulp produced by a thermomechanical process in which wood particles are softened by pre-heating under pressure prior to a pressurized primary refining stage. This type of pulp replaces or reduces the chemical pulp component in newsprint or groundwood papers. See The Dictionary Of Paper, Fourth Ed., American Paper Institute, Inc., New York, N.Y. 1980, pages 205 and 416.  
       [0020] The paper composition of the invention may be of any type, including paper typically used in dry and wet electrophotographic copying and printing methods, paperboard, or poster board, and packaging paper upon which images may be formed by various image-forming techniques.  
       [0021] In another aspect of the invention there are provided imaging methods including electrophotographic imaging methods, including dry and wet methods, and including both direct and indirect methods (offset) of image transfer, which utilize, as the receiver for the images formed, paper comprising at least one anionic polymeric material and at least one binder material  
       [0022] According to another aspect of the invention there is provided a method for manufacturing paper of the invention which comprises adding the anionic polymeric material and the binder material, individually or in combination, at any point during the paper manufacturing method or at any point up to the formation of an image on the paper.  
       [0023] The anionic polymeric materials utilized according to the invention contain repeat functional units capable of forming anionic salts such as, for example, various polymeric carboxylic acids, sulfonic acids and phosphonic acids, which on reacting with bases can form the corresponding salts. The anionic polymeric materials can be either homopolymers or copolymers. The copolymers may be of any type including graft and block copolymers.  
       [0024] Any suitable binder materials may be utilized according to the invention including for example, starches such as non-ionic starches, latexes, proteins, alginates, vegetable gums and cellulose derivatives such as, for example, carboxymethylcellulose, hydroxymethylcellulose and the like.  
       [0025] The anionic polymeric and binder materials can be applied to one or both sides of the paper and can be applied either in the form of solutions, emulsions or dispersions of the polymers or copolymers or as combinations thereof. Hereinafter, when reference is made to a polymer “mixture”, it should be understood that any such form is included. The anionic polymeric materials and the binder materials may be applied in combination or separately.  
       [0026] Typical suitable anionic polymeric materials which are useful in accordance with the invention include, for example, homopolymers of acrylic acid, methacrylic acid, maleic acid, phosphonic acid, sulfonic acid and copolymers thereof with monomers such as ethylene, styrene, acrylamide, and acrylonitrile, including anionic polyacrylamide, that is, polyacrylamide containing carboxylic acid functionality from either acrylic acid or methacrylic acid. Further, copolymers of maleic anhydride with ethylene or styrene can also be used.  
       [0027] Salts of the homopolymeric and copolymeric anionic materials may also be used including monovalent and polyvalent salts. Typical suitable monovalent salts include ammonium salts and salts of alkali metals such as sodium salts. Typical suitable polyvalent salts include salts of metals such as zinc and aluminum. If the anionic polymeric material is to be added during the papermaking process, then selection of the metal cation should be made to avoid undesirable interactions with other paper making materials. Further, the anionic polymeric materials may be at least partially esterified, that is, some or all of the repeating functional units can be ester groups.  
       [0028] It has been found that the effectiveness of the paper in strongly adhering the pigmented polymer particles to the paper surface is a function of a number of factors including the plasticity, or mobility, of the anionic polymeric material, that is, its ability to rapidly come in contact with the pigmented polymeric toner particles at the receiver temperature during image transfer or fusing. The plasticity, or mobility, of the anionic polymeric material is a function of the softening temperature of the material. This property of the anionic polymeric materials will be discussed in relation to their Vicat softening temperature. (See ASTM Test D1525-00 Standard Test Method For Vicat Softening Temperature of Plastics). Preferably, the Vicat softening temperature of the anionic polymeric material should be less than the receiver surface temperature during image transfer or fusing step. Further, the shorter the dwell time of the image transfer or fusing step, it is preferred that the Vicat softening temperature should be lower than the receiver surface temperature by a greater extent. In a particularly preferred embodiment, the anionic polymeric material should have a Vicat softening temperature of from about 10° C. to about 100° C. below the receiver surface temperature for dwell times in the range of 1500 to 250 milliseconds.  
       [0029] As will be described later, for a preferred embodiment, the receiver surface temperature, when in contact with an intermediate transfer surface at a temperature in the vicinity of 125° C. (low end of ITS surface temperature range) for dwell times of 1000 milliseconds, may be in the vicinity of about 90° C. For such a preferred embodiment, Vicat softening temperatures equal to, or less than about 90° C. are preferred. The receiver surface temperature during an image fusing step, which can be practiced in dry or wet electrophotographic methods, and which is generally present in dry electrophotographic methods, can be higher. Fusing temperatures deployed typically range from about 100° C. to about 250° C. In these embodiments of the image-forming methods of the invention, the Vicat softening temperature of the anionic polymeric material could be up to about 180° C.  
       [0030] The Vicat softening temperature of the anionic polymeric materials is dependent upon a number of factors. Such factors include the type of anionic polymeric material, i.e., whether a homopolymer or a copolymer, and the particular chemical type of the repeat functional units. For example, homopolymers of polyacrylic acids or polymethacrylic acids typically have lower Vicat softening temperatures than styrenesulfonic acids. Polymaleic acids, being dicarboxyliic acids, typically have a much higher softening point than either polyacrylic acid or polymethacrylic acid. The copolymer type and ratio also typically have a significant effect on Vicat softening temperatures. Copolymers with ethylene or styrene typically have higher Vicat softening temperatures than the comparable anionic homopolymers. In general, the salts of these acids typically have higher Vicat softening temperatures compared to the acid form. Generally, the higher the degree of salt formation, the higher will be the Vicat softening temperature.  
       [0031] Image adhesion is also a strong function of the retention of the anionic polymer at or near the paper surface, which is dependent, in part, on the viscosity of the anionic polymer mixture and the method by which it is applied to the paper. In general, the higher the viscosity, with the upper limit being dependent on the application method selected, the lower will be the penetration of the anionic polymer into the paper, and the higher the concentration of the anionic polymer at or near the paper surface. Other factors which influence the viscosity of the mixture and hence the retention of the anionic polymer material at or near the paper surface include the molecular weight of the polymer, the degree of salt formation, the type of counter ion, and the pH of the mixture.  
       [0032] Generally, the anionic polymer material should be compatible with the pigmented polymer materials so as to ensure rapid bonding. For the case of liquid developer electrophotography, the anionic polymeric material should also be compatible with the pigmented polymer carrier fluid, so as to ensure absorption of the carrier fluid into the paper for both good cohesive and adhesive strength of the image.  
       [0033] The paper of the invention provides very good surface adhesion to pigmented polymer particles thus providing complete or at least substantially complete transfer of pigmented polymer particles used to form images in various electrophotographic imaging methods. Substantial transfer of the pigmented polymer particles to the paper surface significantly reduces or essentially eliminates any “ghost” images that can result from any image residue remaining on the transfer surface. The paper also rapidly absorbs liquids, typically used as the vehicle for liquid developers, such as, for example, insulating non-polar fluids such as aliphatic hydrocarbons used in certain electrophotographic imaging methods, to provide good cohesive strength of the receiver image.  
       [0034] The paper of the invention also has a hard surface with strongly adhered filler materials and paper fibers which is particularly advantageous in a preferred digital offset printing method of the invention for the conditions of the image transfer from the intermediate transfer surface to the paper as will be described in detail below herein. The hard surface of the printing paper significantly reduces or substantially eliminates any intermediate transfer surface memory, or ghosting, which can result in undesired “ghost” or spurious images on the receiver from material transferred to the transfer surface in non image areas. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0035] For a better understanding of the invention as well as other objects and advantages and further features thereof, reference is made to the following detailed description of various preferred embodiments thereof taken in conjunction with the accompanying drawing wherein:  
     [0036]FIG. 1 is a diagram showing the paper path in one particular commercial printing machine, the HP/Indigo 1000 TurboStream digital offset printing machine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0037] The paper composition of the invention may be of any type including paperboard, or poster board, packaging paper and papers typically used in copying and printing methods and comprises at least one anionic polymeric material and at least one binder material and does not include more than about 20% by weight mechanical fiber and preferably not more than 10% by weight. In a preferred embodiment the paper includes from about 0.1 to about 18.0 lbs/3300 ft 2  of finished paper of at least one anionic polymeric material and from about 0.25 to about 10.0 lbs/3300ft 2  of at least one binder material and particularly preferably from about 0.2 to about 5.0 lbs/3300 ft 2  of finished paper of at least one anionic polymeric material and from about 1.0 to about 5.0 lbs/3300 ft 2  of finished paper of at least one binder material. The paper may have any basis weight. Preferably, the basis weight suitable for paper used as the receiver in electrophotographic copying and printing is in the range of from about 20 to about 400 pounds based on 500 sheets of 25″ by 38″. Further, the paper composition may have any desired gurly stiffness, measured according to standard TAPPI specification T-543 (Bending Resistance of Paper). In a preferred embodiment the paper has gurly stiffness in the machine direction of about 25 to about 6000 grams.  
     [0038] As discussed previously, the anionic polymeric materials include repeating units, which are capable of forming anionic salts. The anionic polymer-may be a homopolymer or a copolymer. The homopolymer or copolymer can be either in the acid form, or partially, or wholly, in the salt form. As stated earlier, the Vicat softening temperature of an anionic polymer is affected by the degree of salt formation, the type of counter ion present, the valence of the counter ion, the molecular weight, copolymer type, copolymer ratio, pH of the polymer mixture from which the material is applied to the paper and the degree of esterification of the repeating functional units of the polymer.  
     [0039] In a particularly preferred embodiment, the Vicat softening temperature is from about 10° C. to about 100° C. lower than the receiver surface temperature for dwell times in the range of 1500 to 250 milliseconds. It is also preferred to apply the anionic polymeric material to the paper from a polymer mixture, which has a viscosity sufficiently high to ensure maximum retention of the anionic polymer at or near the surface of the paper.  
     [0040] Selection of a specific anionic polymer or polymers for a particular paper composition and the optimum amount(s) can be carried out by standard experimental test practices. The selection can be greatly simplified by the use of a test method which simulates the environment of either image transfer or image fusion to the receiver. While the use of such a method can be fairly general and cover a broad range of electrophotographic methods, the specific ranges of the variables will depend upon the specific electrophotographic method. A suitable method for initially testing anionic polymeric materials will now be described by way of an example directed to the digital offset liquid electrophotographic methods carried out using suitable electrophotographic printing machines. By way of example only and not as a limitation, we refer to one family of machines, the HP/Indigo (Hewlett Packard) electrophotographic printing machine models 1000 through 4000, all of which employ an intermediate transfer surface (ITS). Those skilled in the art will understand that other such machines can be used to practice the invention.  
     [0041] A specific test apparatus is a transfer press such as, for example, an AW-3000 Transfer Press made by Airwave Inc., Cincinnati, Ohio. Similar devices made by other manufacturers are commercially available and may be used for this purpose. The press consists of a heated platen with a lever that can serve as the base for the ITS material. Once the ITS material is affixed to the platen, it can be used to apply pigmented polymer to the paper surface under heat and pressure. The temperature of the platen is regulated to approximately simulate the receiver surface temperatures typically encountered in the HP/Indigo digital offset printing machines mentioned above. The HP/Indigo digital printing machines typically have ITS surface temperatures of from about 125° C. to about 180° C., resulting in receiver surface temperatures of about 90° C. for the lower end of the ITS range. Although not mandatory, it is desirable to use an intermediate transfer surface material similar to the one which is used in the actual printing machine. For the HP/Indigo printing machines mentioned above, an identical ITS material, that is, HP/Indigo product designation MPS 2177-42 was selected. Further, the surface temperature of the ITS in the test apparatus was set at 105° C. for a majority of the testing so as to achieve a paper surface temperature in the vicinity of 90° C. for 1000 millisecond dwell time. As stated above, this temperature simulates the lower end of the temperature range of the intermediate transfer surface in the HP/Indigo machines. The lower end of the range was selected so as to increase the sensitivity.  
     [0042] As stated earlier, the anionic polymer should be compatible with both the pigmented polymer and the carrier fluid in which it is dispersed. Since the composition of the specific pigmented polymer toner particles used in any commercial electrophotographic printing or copying machine is typically not in the public domain, it is preferable to use the pigmented polymer particles actually used in the machine of interest. Thus, the black pigmented polymer available from HP/Indigo having the product designation MPS 2131-42 was used. The same test can be repeated for other color pigmented polymers. Generally, for this practice of liquid electrophotography, it has been found that when the anionic polymer is a good bonding agent for the chosen black pigmented polymer, it will also satisfactorily bond to the pigmented polymers of other colors.  
     [0043] In operation, the black pigment was diluted with mineral oil, specifically that available from HP/Indigo with a product designation MPS 2017-43, the pigmented polymer dispersant, and applied to the ITS, which was affixed to the platen of the transfer press. In general, the higher the coverage of the pigmented polymer on the ITS surface, the greater is the test sensitivity. Consequently, the coverage of the black pigmented particles to be applied to the ITS, was established by applying enough pigmented polymer particles so as to achieve an image density of about 1.40 or higher on the paper surface.  
     [0044] The transfer press platen was then brought in contact with the paper receiver containing the anionic polymer being tested. It is also important that the dwell time of the test, that is, the duration for which the heated ITS with the applied black pigmented polymer is in contact with the paper, be similar to that present in the actual printing machine; in this instance approximately in the range of 300 to 1000 milliseconds. The tests described below have been carried out at dwell times of both 250 and 1000 milliseconds, which adequately span the range in actual practice.  
     [0045] The paper samples with the transferred black-pigmented polymer were then tested for adhesion efficacy via either cellophane tape, that is, Highland® Clear 6200, or Scotch Drafting Tape® Brand 230, available commercially from 3M Corporation. The tape was applied uniformly to the printed surface and a 1 Kg weight roller was applied to the paper surface twice to get good tape adhesion to the pigmented polymer on the paper surface. The tape was then pulled away from the printed surface. Subsequently, the test sample was scanned with an Expression 1600 scanner (Epson. Corp.), and the scanned sample analyzed for the percentage of the material removed by the tape.  
     [0046] In Table 1 the test sensitivity is shown as a function of the type of tape used. It can be seen that the cellophane tape gives poorer results and is therefore more discriminating.  
     [0047] In Table 2, the adhesion test data are presented as a function of dwell time. The dwell time data comparisons clearly indicate that, with all other variables being the same, adhesion is poorer at 250 milliseconds dwell time compared to 1000 milliseconds dwell time. It can be further seen that at lower coverages there is a higher sensitivity to dwell time. A majority of the testing was carried out at both the high (1000) and low (250) dwell times.  
     [0048] Table 3 shows the results obtained with commercially available digital offset printing papers. It can be seen that all of the papers that were tested, exhibited very poor adhesion.  
     [0049] In Table 4 the adhesion data are presented as a function of anionic polymer coverage. It is seen that at higher coverages the adhesion is stronger. Thus, for polyethyleneacrylic acid at a coverage of 0.34 lb/3300 ft 2 , there is a loss of about 14% from the tape pull. When the coverage was increased to 1.42 lbs/3300 ft 2 , the loss was about 4%. Generally, adhesion is seen to have improved as the coverage increased. In testing different anionic polymers, a majority of the comparative evaluations were undertaken at lower anionic polymer coverages so as to increase selectivity.  
     [0050] Table 5 shows the adhesion data as a function of viscosity of the polymer mixture from which the anionic polymer was applied to the paper. For the same applied coverage of approximately 1.46 lbs/3300 ft 2 , it can be seen that adhesion performance at the lower viscosity is significantly poorer. At the lower viscosity the results indicate that there was much greater penetration of the anionic polymer into the paper with correspondingly lower surface retention, and hence, poorer adhesion.  
     [0051] Table 6 shows adhesion data as a function of molecular weight for polyacrylic acid and polystyrenesulfonic acid homopolymers. The adhesion results obtained with the acidic form of polyacrylic acid show that as the molecular weight, and hence the polymer mixture viscosity, is increased from 5,000 to 50,000, the adhesion results improve from about 32% to about 7%, that is, only about 7% of the image-forming particles were removed from the paper which had the highest molecular weight anionic polymer. This was so even though the lowest molecular weight polymer was present at a significantly higher amount of 1.48 lbs/3300 ft 2  compared to 0.93 lb/ft 2  for the higher molecular weight polymer. Comparing the adhesion performance of the 10,000 molecular weight polymer versus the 50,000 molecular weight polymer at a similar coverage of about 0.9 lb/3300 ft 2 , it is seen that the adhesion performance improved from a loss of about 15% to about 7%.  
     [0052] The data for polystyrenesulfonic acid polymers, at molecular weights of 70,000 and 400,000 (in the salt form) show again that at the higher molecular weight the adhesion results are better, about a 12% loss for the higher molecular weight polymer compared to an approximately 20% loss for the lower molecular weight material, even though the coverage for the former was somewhat lower at 0.26 lb/3300 ft 2  versus 0.34 lb/3300 ft 2 .  
     [0053] Thus, it can be seen that higher molecular weight anionic polymers, which can increase polymer mixture viscosity and minimize anionic polymer penetration into the paper, thereby increasing surface retention, can provide significantly improved adhesion performance.  
     [0054] In Table 7, the adhesion data are presented as a function of the salt or acid form of the anionic polymer at dwell times of 1000 and 250 milliseconds The data for the 70,000 molecular weight polystyrenesulfonic acid in the acidic form, at both dwell times, show undesirably low adhesion as indicated by the approximately 41% and 59% losses, respectively. These data would indicate that due to the low viscosity of the acid form of the polymer there was lower surface retention of the polymer and consequently poorer adhesion. The Na salt form of the polymer, which increases polymer mixture viscosity, at both dwell times exhibited significantly better adhesion performance as seen by the approximately 20% and 18% losses, respectively.  
     [0055] As stated above, to increase selectivity, testing was undertaken at relatively low coverages of the anionic polymers. Consequently, the adhesion performance of the polymers can be further improved by increasing the coverage of the polymers.  
     [0056] It is important to note however that although an anionic polymer of specific molecular weight, degree of neutralization, anionic polymer cation, such as Li + , Na + , K + , NR 4   + , valency of the cation and polymer mixture pH can be selected to create the requisite anionic polymer mixture viscosity for maximizing retention at or near the surface, and achieve good adhesion performance, nevertheless the Vicat softening temperature of the anionic polymeric material is also a very important factor in the adhesion performance of the paper of the invention. Thus, as stated above, it is preferred that the Vicat softening temperature be below the paper surface temperature during the transfer or fusing step. At the dwell time of 1000 milliseconds, and with the ITS at 105° C., the paper surface temperature during transfer of the image from the ITS is estimated to be about 90° C. Although, the Vicat softening temperature of the 400,000 molecular weight polystyrene sulfonate, at about 80° C. (Table 6) is higher than that of the polyethyleneacrylic acid, at about 40° C. (Table 4), it is nevertheless still below the estimated paper surface temperature of 90° C. and its adhesion performance is comparable to that of polyethyleneacrylic acid at approximately similar coverage (See Table 4). Thus, the data shown in Table 6 and Table 7 indicate that, in accordance with the preferred embodiment of the invention where the Vicat softening temperature is about 10° C. or more below the paper surface temperature during the transfer or fusing steps, the viscosity of the polymer mixture will have a significant effect on adhesion performance, and desirable results can be achieved by selecting a sufficiently high viscosity mixture consistent with the specific application technique selected.  
     [0057] In Tables 8 and 9 adhesion data are presented for anionic polymers having a broad range of Vicat softening temperatures at dwell times of 1000 and 250 milliseconds respectively. It can be seen from the data in Table 8 that both polyethylene acrylic acid and polymethacrylic acid, which have Vicat softening temperatures of about 40° C. or below, which are considerably lower than the paper surface temperature of 90° C., have approximately the same tape pull loss of about 13% at the low coverage of about 0.34 lb/3300 ft 2  and 0.42 lb/3300 ft 2 , respectively. However, for polymaleic acid, with a Vicat softening temperature of about 110° C. which exceeds the paper surface temperature of 90° C., the adhesion performance dropped to an approximately 25% loss even though the coverage was higher at 0.6 lb/3300 ft 2 . The results for polyethylenemaleic anhydride, with an even higher Vicat softening temperature showed an even larger deterioration of the adhesion performance, an approximately 47% loss at similar coverage. The data in Table 9, obtained at the lower dwell time of 250 milliseconds, show even higher degradation for the polymaleic acid and polyethylenemaleic anhydride.  
     [0058] It should be noted that both polymaleic acid and polyethylenemaleic anhydride are in the acid form, and the data clearly indicate that as the Vicat softening temperature exceeds that of the paper surface temperature, the adhesion results degraded, and exhibited higher sensitivity to dwell times. It should be further noted that for tests with the polystyrenesulfonic acid and polystyrene sulfonate (salt form), based on the surface pH measurement of the paper surface, it was determined that the anionic polymer was less than 25% in the salt form after being applied to the paper. For such cases, the Vicat softening temperature would not be significantly different from that of the acid form, and consequently the stated Vicat softening temperature is that of the acid form. See, for example the data shown in Tables 6 and 7.  
     [0059] The invention will now be further described with respect to the types of bonding which can occur, it being understood that this discussion is for the purpose of assisting those skilled in the art to better understand and practice the invention, and there is no intention to be bound to any theoretical mechanism by which the advantageous results provided in accordance with the invention can be obtained since such results have been demonstrated by actual experimentation.  
     [0060] It is believed that hydrogen bonding can occur to effectively bond the anionic polymer to both the paper materials such as fibers and binders (starch, etc.) as well as to the pigmented polymer particles. Typically, paper has a preponderance of sites capable of hydrogen bonding. Where the pigmented polymer also contains an adequate percentage of polymeric anionic material, then hydrogen, or hydrophilic, bonding mechanisms can prevail to not only bind the anionic material to components in the paper but also to the pigmented polymer particles, which form the image transferred to the paper, and be effective in providing good adhesion. However, where the pigmented polymer particles include copolymers, which may be likely, and the copolymer includes a hydrophobic moiety such as ethylene or styrene, employing a copolymer as the anionic polymer may be advantageous.  
     [0061] It can be seen from Table 10 that polyethyleneacrylic acid, a copolymer, and polyacrylic acid, a homopolymer, at coverages of approximately 0.95 lb/3300 ft 2  have comparable adhesion data. Similarly, polyethyleneacrylic acid a copolymer, and polymethacrylic acid, a homopolymer, at coverages of approximately 0.38 lb/3300 ft 2 , have comparable adhesion data. Further, comparing the adhesion performance of the sample having polyethyleneacrylic acid with 20% acrylic acid with that of the sample having polyethyleneacrylic acid with 10% acrylic acid, that is, with a lower carboxylic acid content polymer, it can be again seen that the adhesion performances are comparable. These results indicate that for the liquid electrophotographic methods carried out in the HP/Indigo digital offset printing machines either hydrophilic or hydrophobic bonding would be quite effective in providing good adhesion.  
     [0062] Where stronger bonds are desired to provide extremely high degree of adhesion strength, use of salts of homopolymers or copolymers with multivalent metal ions can be considered. Polyvalent metal ions have the potential to cross link polymer molecules and hence create stronger adhesion. Table 11 shows the adhesion performance data obtained with polyethyleneacrylic acid in the acid form and in the salt form with polyvalent aluminum as the counter ion. It can be seen that the sample with the aluminum salt form exhibited better adhesion performance.  
     [0063] Based on an analysis of the data presented in Table 4 through Table 11, it can be seen that the above test procedure has identified a number of suitable anionic polymeric materials for incorporation in the paper of the invention to be used as the receiver for the liquid electrophotographic methods carried out in the HP/Indigo digital offset printing machines. The preferred anionic polymeric materials for incorporation in paper for use with these printing machines are: polyacrylic acid having a molecular weight of about 50,000; polymethacrylic acid having a molecular weight of about 15,000; polystyrenesulfonate (Na Salt) having a molecular weight of about 400,000; and polyethyleneacrylic acid at either the 10% or the 20% acrylic acid. Also, the data show that some of the lower molecular weight anionic polymers such as polyacrylic acid having a molecular weight of about 10,000 or polystyrenesulfonate (Na Salt) having a molecular weight of about 70,000 could be suitable depending upon the application technique, the composition of the polymeric mixture, that is, the presence of other viscosity building species such as starch or, by increasing anionic polymer solubility and hence viscosity via an increase in polymer mixture pH. Further, the data show that other anionic polymers such as polymaleic acid could be suitable at higher transfer or fusing temperatures.  
     [0064] Since there is high dependence of paper surface temperature on a number of operational factors such as, for example, the basis weight of the paper used, with the higher basis weight providing a greater heat sink and lower surface temperature, ITS surface temperature, dwell time etc., for greater robustness in performance, that is, an ability to provide the desired results over a broader range of operational conditions, it would be desirable to select the anionic polymer Vicat softening temperature, which is lower than the paper surface temperature by a larger amount, that is, which is more towards the higher end of the preferred temperature difference range of from about 10° C. to about 100° C. Hence, the particularly preferred anionic polymeric materials for incorporation in the paper receiver for these applications are the polyacrylic acid having a molecular weight of about 50000, polyethyleneacrylic acid at either the 10% acrylic acid or the 20% acrylic acid levels and the polymethacrylic acid, (salt form).  
     [0065] As stated earlier, the anionic polymeric material should bond with both the paper materials as well as the pigmented polymer particles so that they are retained on the paper surface. The adhesion of the anionic polymer to the paper is primarily through hydrogen bonding and, as discussed earlier, bonding to the pigmented polymer particles can also be primarily, or partially, through hydrogen bonding. Thus, the activity of the anionic polymer to adhere to the paper and in some cases to the pigmented polymer particles is directly related to the degree of the anionic polymer in the acid form. The pH of the anionic polymer mixture from which the polymer is applied to the paper can be used to regulate the amount of polymer in the active, or acid, form and that in the inactive, or dissociated, form. Generally, the higher the pH, the greater is the inactive form. This effect can be seen from the data in Table 12. For example, comparing polyethyleneacrylic acid applied from polymer mixtures at pH 6.9 and 7.3, the adhesion results are seen to degrade from 4% to 8% at the higher pH for the dwell time of 1000 milliseconds and from 9% to 18% at the shorter 250 milliseconds dwell time. Hereinafter, any reference to the active form of the anionic polymer implies the acidic form of the polymer.  
     [0066] It was stated earlier that the paper receiver material should also have a high surface strength so as to prevent unprinted area ghosting, or spurious images appearing on the receiver surface. A primary-requirement of the binder material then is to strongly bind typical paper additives such as calcium carbonate, clay, titanium dioxide and short, medium and long fibers in order to provide a hard paper surface, which is substantially free from loose particles and fibers. Additionally, it was also stated that the paper surface should preferably remain hard at the temperatures and pressures encountered during image transfer so as to prevent unprinted area ghosting, or spurious images appearing on the paper surface. The binder material should have sufficient binding strength to the typical paper additives so as to minimize or substantially eliminate abrasion and transfer of such paper additives and fibers during image transfer. Thus, the binder material should be substantially unaffected with respect to its binding strength with respect to paper fibers, fillers, etc. at the temperature of the paper during transfer.  
     [0067] Additionally, for the case of liquid electrophotographic printing methods, as in the preferred embodiment, the binder material also should be compatible with the carrier fluid or dispersant for the pigmented polymer particles. Upon application of the carrier fluid containing the pigmented polymer particles, the fluid should wet the paper and drain from the surface.  
     [0068] The binder materials may be anionic, cationic or neutral. Typical suitable binder materials which are useful in accordance with the invention are starches, such as non-ionic starches, latexes, proteins, alginates, vegetable gums, and cellulose derivatives such as, for example, carboxymethylcellulose, hydroxyethylcellulose and the like. The binder materials may be present in the paper individually or in combination. As stated earlier, for a preferred embodiment, the binder material is present in the paper in an amount of from about 0.25 to about 10.0 lbs/3300 ft 2 , and particularly preferably from about 1.0 lbs to about 5.0 lbs/3300 ft 2  of finished paper.  
     [0069] Cost and other considerations in paper making often dictate the selection of materials that may not provide optimum results with respect to the primary requirements of either the anionic polymeric materials or binder materials. For example, some naturally occurring or modified naturally occurring materials such as starch are very inexpensive and therefore can be advantageously used as binder materials according to the invention because of cost considerations. However, some-of these starches and other materials may not create a sufficient surface hardness and thereby may create a significant intermediate surface memory or ghosting phenomenon. In a preferred embodiment, because of cost considerations, Ko-Film 280 starch, available commercially from Hercules Corp., is the binder material. In such cases, it then becomes advantageous to use materials that significantly enhance the fulfillment of the primary requirement of either the anionic polymeric material or the binder material or both. Although test results indicate that Ko-Film 280 starch binder material does not optimally fulfill the paper surface strength requirements at the temperatures of image transfer, it has been found that the performance of this binder material can be enhanced through the use of other materials.  
     [0070] Also, many of the starches and other binder materials, and paper fibers have cationic sites, which can preferentially bind to the anionic polymer so as to markedly reduce the amount of anionic polymer available to bind the image-forming pigments. Hence, during manufacture of the paper containing the anionic polymer material, and particularly where the anionic polymer material and the binder are introduced simultaneously, as is the case in a preferred embodiment, the anionic polymer material may interact with other components such as cationic groups in starches, as is the case with Ko-Film 280 starch. The data shown in Table 13 illustrate this phenomenon. It should be noted that the higher the percentage of the inactive form of the anionic polymer the lower the interaction with anionically reactive materials such as Ko-Film 280 starch.  
     [0071] Where the anionic polymer material has a high activity, unless high coverages of the anionic polymer material are used, the functional anionic units of the anionic polymer can get partially or wholly tied up with the other paper materials including binder, thereby reducing the efficacy of the anionic polymer material to provide the desired adhesion results. It was shown earlier (see Table 11) that the activity of the anionic polymer can be controlled with pH, that is, the higher the pH the more inactive the anionic polymer. Thus, a particularly preferred anionic polymer for use in conjunction with Ko-Film 280 starch binder is the salt of polyethyleneacrylic acid with a fugitive ammonium counter ion, commercially available from Michelman Corporation with product designation MP 4990R. The ammonium salt form of the anionic polymer, depending upon the degree of conversion, which can be determined by the pH of the anionic polymer mixture, can be significantly inactivated at pH 7 and above. It was also shown earlier that the pH of the polymeric mixture could be adjusted upward to achieve a lower degree of activation by the addition of NH 4 OH. The key advantage obtained by reducing the activity of the anionic polymer mixture is that it can minimize the percentage of the anionic polymer interacting with components in the paper as it is being applied to the paper. This in turn will allow more of the anionic polymer to be available to bind the toner particles to the paper surface. The significant advantage of having a fugitive counter ion is that it can be driven off as NH 3  from the paper during drying, and given adequate temperatures and residence time in the dryer, the paper can essentially be completely active as it comes out of the dryer. When using the NH 4  salt, the dependency on the dryer to drive off the ammonia is illustrated in Table 12. With the other variables being the same, it can be seen that as the temperature of the drying is reduced, a lower amount of the polymer reverts back to the active form and hence the adhesion performance degrades. Of course, the drying duration can also have a similar effect.  
     [0072] Previously reference was made to the use of materials that can significantly enhance the fulfillment of the primary requirement of either the anionic material or the binder material or both. Two or more anionic polymeric materials may be used in combination, as can two or more binder materials. The specific anionic polymeric material(s) and the binder material(s) in any specific paper composition according to the invention-should be selected with respect to each other. The selection criteria for any combination of anionic polymeric material(s) and binder material(s) include the selection of materials which exhibit very good properties for one of the requirements of the respective materials and which also can provide at least some benefit with respect to another requirement. For, example, experiments have shown that when anionic polyacrylamide, which has carboxylic acid functionality, commercially available from Hercules Corp. under product designation M 1343, is used in conjunction with the ammonium salt of polyethyleneacrylic acid and Ko-Film 280 starch, dramatically improved surface strength of the paper at the temperatures of image transfer can be obtained and thereby substantially lessen the presence of ghosting or spurious image from build up in non imaged areas. Additionally, the strongly anionic polyacrylamide, although useful in accordance with the invention but not as efficacious as polyethyleneacrylic acid, can also improve the efficacy of the primary anionic material, that is, polyethyleneacrylic acid. It is believed that the strongly anionic polyacrylamide preferentially binds to the cationic sites of a binder material such as Ko-Film 280 starch thereby preventing such cationic sites from binding to the primary anionic polymer and thereby increasing its efficacy while also providing the additional benefit of substantially reducing ghosting by cross-linking to the binder material. Thus, the use of anionic polyacrylamide can provide the multiple functions of binding to the cationic sites of a binder material, cross-linking the binder material and binding to the image-forming material.  
     [0073] The respective amounts of the anionic polymeric and the binder materials, which are utilized in any specific paper composition, are determined in part by the number of functional groups in the molecule in relation to the overall size of the molecule. The amounts of the anionic polymeric material and binder material in the paper composition are also a function of the surface finish of the paper. The optimum amounts of anionic polymer(s) and binder material(s) in any specific paper designed to be used with any specific printing or copying machine can be determined by routine scoping experiments.  
     [0074] As discussed previously, for good image quality there must be maximum toner particle transfer to the paper receiver material and maximum toner particle retention by the paper surface. The smoothness of the paper surface has a significant impact on the adhesion of the toner particles to the paper surface. A rougher surface typically has better adhesion to the toner particles. However, a rougher surface typically is also more susceptible to “ghosting.  
     [0075] The Sheffield method, described in TAPPI Test T-538, OM-96, which is listed in TAPPI Test Methods (1996-1997), is a commonly accepted technique for measuring the surface smoothness of paper. The paper smoothness is inversely proportional to the Sheffield number, i.e., the higher the Sheffield numbers the rougher the paper surface. Generally, the Sheffield smoothness of the paper of the invention is from about 20 to about 400.  
     [0076] A preferred printing paper of the invention comprises from about 0.20 to about 5.0 lbs. of an ammonium salt of polyethyleneacrylic acid, (Michelman product designation MP 49990R), about 0.10 to about 1.0 lb of anionic polyacrylamide, (Hercules product designation M1343) and about from about 1.0 to about 5.0 lbs of Ko-Film 280 starch, each based on 3300 ft 2  of finished paper.  
     [0077] The electrophotographic printing methods provided according to the invention include those where the pigmented polymer toner particles are applied to the latent electrostatic image in a dry or wet composition with direct or indirect (offset) image transfer to receiver, and wherein an image is formed on a paper receiver material that includes at least one anionic polymeric binder material and at least one binder material. In a preferred embodiment the paper used in these printing methods does not have more than about 20% by weight of mechanical fiber and particularly preferably not more than about 10% by weight. In another preferred embodiment the paper used in these printing methods includes from about 0.1 to about 18.0 lbs/3300 ft 2  of finished paper of at least one anionic polymeric material and from about 0.25 to about 10.0 lbs/3300 ft 2  of finished paper of at least one binder material and particularly preferably from about 0.2 to about 5.0/3300 ft 2  of anionic polymeric material and from about 1.0 to about 5.0 lbs/3300 ft 2  of at least one binder material.  
     [0078] Preferred electrophotographic methods are those digital offset printing methods wherein an electrostatic latent image formed on a photoconductive surface, typically by applying a substantially uniform electrostatic charge to the photoconductive surface and irradiating the charged surface with image-modulated laser beam(s), is rendered visible with a liquid toner composition, transferred to a heated intermediate transfer surface and transferred from the latter to the final paper receiver material. Digital offset printing methods are well known in the art and therefore extensive discussion of such methods is not required here.  
     [0079] An imaging method of this type is described in U.S. Pat. No. 4,708,460. There is described in the &#39;460 patent an apparatus wherein an image initially formed on a photoconductive surface by development with a liquid developer composition is transferred to an intermediate member positioned closely to the photoconductive member. The image is subsequently simultaneously transferred and fused to a copy sheet. The printing paper of the invention is useful as the receiver sheet according to this method.  
     [0080] The paper of the invention may be used as the receiver for images formed by any suitable electrophotographic printing machine. FIG. 1 shows one particular printing machine, the Indigo TurboStream 1000 digital offset printing machine, having a developer drum that attracts excess non-image ink while repelling image ink, a PIP drum, which carries the image, an ITM drum on which a transfer blank is located, and an impression drum that forms a printing nip with the ITM drum. There are other commercially available printing machines with the same or different configurations, which carry out either digital offset printing or direct transfer printing  
     [0081] Electrophotographic printing apparatus and methods can be used to form monochromatic and polychromatic images. Polychromatic, or multicolor, images can be formed by two general methods. In one such method monochromatic color separation images, e.g., magenta, yellow and cyan, are formed successively and each is transferred, in registration, to the receiver. Digital offset printing machines that carry out this method include the HP/Indigo 1000 and 3000 printing machines in which each individual color separation image is formed, transferred to the intermediate transfer surface and then to the paper receiver in registration. In another such method, each color separation image is formed, transferred to an intermediate transfer surface in registration to form the multicolor image on the intermediate transfer surface and the multicolor image is then transferred to the paper receiver. Digital offset printing machines, which carry out this method, include the HP/Indigo 2000 and 4000 printing machines. Where the printing paper of the invention is used as the receiver for multicolor images formed according to the latter method it is preferred to utilize higher concentrations of the anionic polymeric materials and binders since the contact time of the paper with the heated intermediate transfer surface is less than in the former method where the receiver is brought into contact with the heated intermediate transfer surface more than one time, e.g., three or four times.  
     [0082] The paper of the invention may be produced by any conventional method that converts fiber slurry into paper, and may be bleached. Further, the anionic polymeric material and the binder material may be applied to the paper of the invention, either individually or in combination, at any point during the paper manufacture or they can be applied to the paper at any point after the paper manufacture process and before the formation of an image on the paper. The anionic polymeric material and the binder material may be mixed with the pulp fiber slurry, which is made into a paper sheet. The pulp fiber may be mainly composed of wood pulp and may contain additionally a fibrous material such as a synthetic pulp, synthetic fiber, glass fiber or the like. The anionic polymer and binder materials may be applied to paper by means of an air knife coater, a roll coater, a Champlex coater, a gravure coater, etc to a plain paper sheet or a coated sheet. Further, a plain paper or coated sheet may be immersed in a mixture of the materials, which may be a solution, dispersion, emulsion or combinations thereof, excess fluid removed and the paper dried.  
     [0083] In a preferred embodiment, both the anionic polymer and the binder are applied to the paper at a size press addition station during manufacture of the paper. Simultaneous addition of these materials at a size press addition station confers significant cost advantages. However, there may be other situations where it is advantageous to apply the anionic polymer and/or the binder to the paper other than at the size press addition station, including addition at any point after the paper manufacturing process and before the formation of the image on the paper.  
     [0084] The rheology of the anionic polymer and binder mixture at the size press addition station should be optimized for the chosen application method. That is, the viscosity of the anionic polymer mixture, under the conditions of being applied to the paper, as discussed earlier, should be sufficiently high so as to maximize retention of the anionic polymer and binder materials at or very near the paper surface. For a specific anionic polymer and binder, maximizing the percent solids of the anionic polymer mixture can also favorably impact the viscosity. It should be further noted that raising the pH to keep the activity of the anionic polymer low would also increase the solubility of the salt, and hence increase the viscosity. However, as stated earlier, the maximum viscosity at the time of application to the paper has to be kept below the allowable maximum for the chosen application method. Additionally, it is also important to keep the temperature of the size solution low so that NH 3  is not prematurely driven off. The temperature of the paper coming in to the size press station will have a large impact on the temperature of the size press polymer mixture. The temperature of the paper at the size press addition station can be maintained at a desired low level by having cooling capability either for the size station polymer mixture, or the incoming paper, or both. For a preferred embodiment, the pH of the polymeric mixture is from about 6 to 8.  
     EXAMPLES  
     [0085] The invention will now be further described in detail with respect to various preferred embodiments by way of examples, it being understood that these are intended to be illustrative only and the invention is not limited to the materials, processes, or compositions recited therein. The papers according to the invention, described below, were printed in The HP/ Indigo TurboStream 1000 digital offset printing machine, available from HP/Indigo, N.V., SM Veldhoven, Netherlands. All parts and percentages recited in the examples are by weight unless otherwise stated. All coverages are shown in lbs/3300 ft 2  of finished paper unless otherwise stated.  
     Example I  
     [0086] This example illustrates the pigment adhesion results obtained from a digital offset printing method utilizing a printing paper according to the invention having a paper surface finish of Sheffield 140 and varying amounts of an anionic polymeric material, namely an ammonium salt of polyethyleneacrylic acid, (Michelman MP 4990R) The binder material was Ko-Film 280 starch present at about 2.1 lbs. A black pigment was used to form the images on the paper  
                   TABLE I                       Amount of polyethyleneacrylic acid   Adhesion Of Black Pigment                  0.45   65%         0.57   85.8%       0.97   93.6%                  
 
     [0087] The adhesion values were obtained by applying a piece of Scotch Drafting Tape® Brand 230 tape to the image areas of the copy sheet fifteen minutes after printing the paper, and then peeling the tape from the paper. The image-forming pigment, which adhered to the tape was then measured using a scanner. The percent adhesion values shown in Table I are the percentages of pigment remaining on the paper. It can be seen that the adhesion of the image-forming pigment to the paper improved as the amount of polyethyleneacrylic acid present in the paper increased.  
     Example II  
     [0088] This example illustrates the pigment adhesion results obtained from a digital offset printing method utilizing a printing paper according to the invention having a paper surface finish of Sheffield 140 and varying amounts of the same anionic polymeric material described in Example I, in combination with a second anionic polymeric material, namely, anionic polyacrylamide, (Hercules product designation M1343) present in an amount of about 0.33 lb/3300 ft 2  of finished paper. The binder material was Ko-Film 280 starch present at about 2.75 lbs/3300 ft 2  of finished paper. A cyan pigment was used to form the images on the paper.  
                   TABLE II                       Amount of polyethyleneacrylic acid   Adhesion Of Cyan Pigment                  0.45   93.8%       0.50   98.3%                  
 
     [0089] It can be seen from a comparison of the results shown in Tables I and II that the presence of the anionic polyacrylamide in combination with the ammonium salt of polyethyleneacrylic acid provided improved image pigment adhesion to the paper receiver.  
     Example III  
     [0090] This example illustrates the “ghosting” results obtained from a digital offset printing method utilizing a printing paper according to the invention having a paper surface finish of Sheffield 140. The severity of the ghosting phenomenon can be judged by the number of cleaning sheets of paper required to remove from the ITS the items which cause the ghost images on the printed paper. This example illustrates the results obtained with varying amounts of binder material. The binder material was Ko-Film 280 starch. The anionic polymeric material comprised a combination of the ammonium salt of polyethyleneacrylic acid described in Example I, present in an amount of from about 0.45 to about 0.50 lb/3300 ft 2  of finished paper and the anionic polyacrylamide described in Example II, present in the amount of about 0.33 lb/3300 ft 2  of finished paper.  
     [0091] 2000 Image cycles, of successive images were printed, that is, successive transfers of 2000 images from the intermediate transfer surface to the paper receiver material. After each 2000 image cycle six cleaning sheets were printed. The cleaning sheets were paper sheets according to the invention, which were processed in the digital offset printing machine identified above. The cleaning sheets did not have an image printed on them but rather had a yellow pigment uniformly distributed across their surfaces. The cleaning sheets were examined for the presence of the ghost image or image from the previous cycle. The number of cleaning sheets shown is that required to remove from the intermediate transfer surface the residual material retained thereon after three 2000 image cycles, described above, that is, a cumulative transfer of 6000 images from the intermediate transfer surface to the paper receiver material.  
                           TABLE III                                   Amount of Binder Material   Number Of Cleaning Sheets                                                    2.28   16           2.45   8           2.79   2                      
 
     [0092] It can be seen that the severity of the ghosting phenomenon, as evidenced by the number of cleaning sheets required to remove from the intermediate transfer surface the materials causing the ghosting, decreased significantly as the amount of binder material was increased.  
     [0093] Although the invention has been described with respect to various preferred embodiments, it is not intended to be limited thereto, but rather those skilled in the art will recognize that variations and modifications may be made therein which are within the spirit of the invention and the scope of the appended claims.  
               TABLE 1                          TEST SENSITIVITY AS A FUNCTION OF TAPE TYPE                                     COATED   POLYMERIC   MOLECULAR       PRODUCT   VICAT SOFTENING                                         COMPOUND   TYPE   EIGHT MW   ACID/SALT   VENDOR   DESIGNATION   TEMPERATURE ° C.               Poly Ethylene   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R   40° C.       Acrylic Acid   ETHYLENE/           ACRYLIC ACID       Poly Ethylene   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R   40° C.       Acrylic Acid   ETHYLENE/           ACRYLIC ACID       Poly   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R   40° C.       Ethylene   ETHYLENE/       Acrylic Acid   ACRYLIC ACID       Poly   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R   40° C.       Ethylene   ETHYLENE/       Acrylic Acid   ACRYLIC ACID                                             COATED COVG IN       DWELL TIME MILLI-                                             #/REAM   ITS TEMP ° C.   TAPE USED   SECONDS   % LOSS                                                     1.04   105° C.   CELLOPHANE   1000   5.63                   TAPE                   HIGHLAND 6200                   CLEAR           1.04   105° C.   SCOTCH   1000   3.14                   DRAFTING                   TAPE           1.04    90° C.   CELLOPHANE   1000   1.90                   TAPE                   HIGHLAND 6200                   CLEAR           1.04    90° C.   SCOTCH   1000   0                   DRAFTING                   TAPE                      
 
     [0094]               TABLE 2                          TEST SENSITIVITY AS A FUNCTION OF DWELL TIME                                         COATED   POLYMERIC   MOLECULAR       PRODUCT                                             COMPOUND   TYPE   EIGHT MW   ACID/SALT   VENDOR   DESIGNATION                       POLY ETHYLENE   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           ACRYLIC ACID   ETHYLENE/               ACRYLIC               ACID           POLY ETHYLENE   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           ACRYLIC ACID   ETHYLENE/               ACRYLIC               ACID           POLY ETHYLENE   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           ACRYLIC ACID   ETHYLENE/               ACRYLIC               ACID           POLY ETHYLENE   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           ACRYLIC ACID   ETHYLENE/               ACRYLIC               ACID           POLY STYRENE   Homo Polymer   70,000   ACID   POLY           SULFONIC ACID               SCIENCES           POLY STYRENE   Homo Polymer   70,000   ACID   POLY           SULFONIC ACID               SCIENCES                                             VICAT SOFTENING   COATED COVG IN       DWELL TIME MILLI-                                         TEMPERATURE ° C.   #/REAM   ITS TEMP ° C.   TAPE USED   SECONDS   % LOSS                                             40° C.   0.98   105° C.   CELLOPHANE   1,000   6.34                   TAPE HIGHLAND                   6200 CLEAR       40° C.   0.98   105° C.   CELLOPHANE   250   9.09                   TAPE HIGHLAND                   6200 CLEAR       40° C.   0.34   105° C.   CELLOPHANE   1,000   13.78                   TAPE HIGHLAND                   6200 CLEAR       40° C.   0.34   105° C.   CELLOPHANE   250   19.66                   TAPE HIGHLAND                   6200 CLEAR       ˜70° C.   0.38   105° C.   CELLOPHANE   1,000   41.09                   TAPE HIGHLAND                   6200 CLEAR       ˜70° C.   0.38   105° C.   CELLOPHANE   250   59.44                   TAPE HIGHLAND                   6200 CLEAR                    
     [0095]               TABLE 3                          COMMERCIALLY AVAILABLE PAPER       PRODUCT ADHESION PERFORMANCE                         COATED   DWELL TIME MILLI-                                     COMPOUND   ITMSTEMP ° C.   TAPE USED   SECONDS   % LOSS                                         Eastern Opaque   105° C.   CELLOPHANE   1000   29.35%               TAPE               HIGHLAND 6200               CLEAR       Xerox Color   105° C.   CELLOPHANE   1000   26.29%       Xpressions       TAPE               HIGHLAND 6200               CLEAR       Hammermill Color   105° C.   CELLOPHANE   250   28.81%       Copy       TAPE               HIGHLAND 6200               CLEAR       Georgia Pacific   105° C.   CELLOPHANE   250   55.86%       Microprint       TAPE               HIGHLAND 6200               CLEAR       Xerox Color   105° C.   CELLOPHANE   250   29.46%       Xpressions       TAPE               HIGHLAND 6200               CLEAR                    
     [0096]               TABLE 4                          IMPACT OF ANIONIC POLYMER COVERAGE ON ADHESION                                         COATED   POLYMERIC   MOLECULAR       PRODUCT                                             COMPOUND   TYPE   EIGHT MW   ACID/SALT   VENDOR   DESIGNATION                       Poly Ethylene   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           Acrylic Acid   ETHYLENE/               ACRYLIC               ACID           Poly Ethylene   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           Acrylic Acid   ETHYLENE/               ACRYLIC               ACID           Poly Ethylene   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           Acrylic Acid   ETHYLENE/               ACRYLIC               ACID           Poly Ethylene   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           Acrylic Acid   ETHYLENE/               ACRYLIC               ACID           Poly Ethylene   80/20   17,000   NH 4 SALT   MICHELMAN   MP4990R           Acrylic Acid   ETHYLENE/               ACRYLIC               ACID                                             VICAT SOFTENING   COATED COVG IN       DWELL TIME MILLI-                                         TEMPERATURE ° C.   #/REAM   ITS TEMP ° C.   TAPE USED   SECONDS   % LOSS                                             40° C.   1.42   105° C.   CELLOPHANE   1,000   4.11                   TAPE                   HIGHLAND 6200                   CLEAR       40° C.   1.04   105° C.   CELLOPHANE   1,000   5.63                   TAPE                   HIGHLAND 6200                   CLEAR       40° C.   0.98   105° C.   CELLOPHANE   1,000   6.34                   TAPE                   HIGHLAND 6200                   CLEAR       40° C.   0.72   105° C.   CELLOPHANE   1,000   7.50                   TAPE                   HIGHLAND 6200                   CLEAR       40° C.   0.34   105° C.   CELLOPHANE   1,000   13.78                   TAPE                   HIGHLAND 6200                   CLEAR                    
     [0097]               TABLE 5                          IMPACT OF ANIONIC POLYMER MIXTURE VISCOSITY ON ADHESION                         SOLUTION/Dis-           persion           VilSCOSITY                                     COATED   POLYMERIC   MOL WEIGHT       CENTI   PRODUCT                                         COMPOUND   TYPE   M W     ACID/SALT   POISE @2   VENDOR   DESIGNATION               Poly Ethylene   80/20   17,000   NH 4     447   MICHELMAN   MP4990R       Acrylic Acid   ETHYLENE/       SALT       35%   ACRYLIC       DISPERSION   ACID       50% DILUTION   80/20   17,000   NH 4      8*   MICHELMAN   MP4990R           ETHYLENE/       SALT           ACRYLIC           ACID                                         VICAT SOFTENING   COATED COVG IN       DWELL TIME MILLI-                                         TEMPERATURE ° C.   #/REAM   ITS TEMP ° C.   TAPE USED   SECONDS   % LOSS                                             40° C.   1.42   105° C.   CELLOPHANE   1,000   4.11                   TAPE HIGHLAND                   6200 CLEAR       40° C.   1.51   105° C.   CELLOPHANE   1,000   12.25                   TAPE HIGHLAND                   6200 CLEAR                                    
     [0098]               TABLE 6                          IMPACT OF ANIONIC POLYMER MOLECULAR WEIGHT ON ADHESION                                 COATED   POLYMERIC   MOL WEIGHT       PRODUCT                                     COMPOUND   TYPE   M W     ACID/SALT   VENDOR   DESIGNATION                                             POLY ACRYLIC   HOMO   5,000   Acid   ALDRICH   CAS #       ACID   POLYMER               9003-01-4       POLY ACRYLIC   HOMO   10,000   Acid   ALDRICH   CAS #       ACID   POLYMER               9003-01-4       POLY ACRYLIC   HOMO   50,000   Acid   ALDRICH   CAS #       ACID   POLYMER               9003-01-4       POLY STYRENE   HOMO   70,000   Acid   POLY   CAS #       SULFONIC ACID   POLYMER           SCIENCES   28210-41-5       POLY STYRENE   HOMO   70,000   Na   POLY   CAS #       SULFONATE   POLYMER       SALT   SCIENCES   2695-37-6       POLY STYRENE   HOMO   400,000   Na   POLY   CAS #       SULFONATE   POLYMER       SALT   SCIENCES   2695-37-6                                 TAPE USED           CELLOPHANE                                     VICAT SOFTENING   COATED COVG IN       TAPE HIGHLAND   DWELL TIME MILLI-           TEMPERATURE ° C.   #/REAM   ITS TEMP   6200 CLEAR   SECONDS   % LOSS                                             &lt;40° C.   1.48   105° C.   CELLOPHANE   1000   31.75%                   TAPE       &lt;40° C.   0.9   105° C.   CELLOPHANE   1000   14.91%                   TAPE       &lt;40° C.   0.93   105° C.   CELLOPHANE   1000   6.50%                   TAPE       ˜70° C.   0.38   105° C.   CELLOPHANE   1000   41.09%                   TAPE       ˜70° C.*   0.34   105° C.   CELLOPHANE   1000   20.09%                   TAPE       ˜80° C.*   0.26   105° C.   CELLOPHANE   1000   12.31%                   TAPE                            
     [0099]               TABLE 7                          IMPACT OF ACID &amp; SALT FORM OF ANIONIC POLYMER ON ADHESION                                         COATED   POLYMERIC   MOL WEIGHT       PRODUCT                                             COMPOUND   TYPE   M W     ACID/SALT   VENDOR   DESIGNATION                       POLY STYRENE   HOMO   70,000   SULFONIC   POLY   CAS #           SULFONIC ACID   POLYMER       ACID   SCIENCES   28210-41-5           POLY STYRENE   HOMO   70,000   Na SALT   POLY   CAS #           SULFONATE   POLYMER           SCIENCES   2695-37-6           POLY STYRENE   HOMO   70,000   SULFONIC   POLY   CAS #           SULFONIC ACID   POLYMER       ACID   SCIENCES   28210-41-5           POLY STYRENE   HOMO   70,000   Na SALT   POLY   CAS #           SULFONATE   POLYMER           SCIENCES   2695-37-6                                     TAPE USED           CELLOPHANE                                     VICAT SOFTENING   COATED COVG IN       TAPE HIGHLAND   DWELL TIME MILLI-           TEMPERATURE ° C.   #/REAM   ITS TEMP ° C.   6200 CLEAR   SECONDS   % LOSS                                             ˜70° C.   0.38   105° C.   CELLOPHANE   1,000   41.09                   TAPE       ˜70° C.   0.34   105° C.   CELLOPHANE   1,000   20.09                   TAPE       ˜70° C.   0.38   105° C.   CELLOPHANE   250   59.44                   TAPE       ˜70° C.*   0.34   105° C.   CELLOPHANE   250   18.43                   TAPE                            
     [0100]               TABLE 8                          IMPACT OF VICAT SOFTENING TEMPERATURES ON ADHESION                                 COATED   POLYMERIC   MOL WEIGHT       PRODUCT                                     COMPOUND   TYPE   M W     ACID/SALT   VENDOR   DESIGNATION                                             Poly Ethylene   80/20   17,000   NH 4     MICHELMAN   MP4990R       Acrylic Acid   ETHYLENE/       SALT           ACRYLIC ACID       Poly Methacrylic   HOMO   15,000   Na   POLY   CAS #       Acid   POLYMER       SALT   SCIENCES   25087-26-7       POLY MALEIC   HOMO   1,000   ACID   POLY   CAS #       ACID   POLYMER           SCIENCES   26099-09-2       POLY ETHYLENE   HOMO       ACID   POLY   CAS #       MALEIC   POLYMER           SCIENCES   9006-26-2       ANHYDRIDE                                 TAPE USED           CELLOPHANE                                     VICAT SOFTENING   COATED COVG IN       TAPE HIGHLAND   DWELL TIME MILLI-           TEMPERATURE ° C.   #/REAM   ITS TEMP ° C.   6200 CLEAR   SECONDS   % LOSS                                             40° C.   0.34   105° C.   CELLOPHANE   1,000   13.78                   TAPE       &lt;40° C.   0.42   105° C.   CELLOPHANE   1,000   13.25                   TAPE       ˜110° C.   0.6   105° C.   CELLOPHANE   1,000   25.34                   TAPE       ˜120° C.   0.31   105° C.   CELLOPHANE   1,000   47.28                   TAPE                    
     [0101]               TABLE 9                          IMPACT OF VICAT SOFTENING TEMPERATURES ON ADHESION                                         COATED   POLYMERIC   MOL WEIGHT       PRODUCT                                             COMPOUND   TYPE   M W     ACID/SALT   VENDOR   DESIGNATION                       Poly Ethylene   80/20   17,000   NH 4     MICHELMAN   MP4990R           Acrylic Acid   ETHYLENE/       SALT               ACRYLIC ACID           Poly Methacrylic   HOMO   15,000   Na   POLY   CAS #           Acid   POLYMER       Salt   SCIENCES   25087-26-7           POLY MALEIC   HOMO   1500   ACID   POLY   CAS #           ACID   POLYMER           SCIENCES   26099-09-2           POLY   HOMO       ACID   POLY   CAS #           ETHYLENE   POLYMER           SCIENCES   9006-26-2           MALEIC           ANHYDRIDE                                     TAPE USED           CELLOPHANE                                     VICAT SOFTENING   COATED COVG IN       TAPE HIGHLAND   DWELL TIME MILLI-           TEMPERATURE ° C.   #/REAM   ITMSTEMP ° C.   6200 CLEAR   SECONDS   % LOSS                                             40° C.   0.34   105° C.   CELLOPHANE   250   19.66                   TAPE       &lt;40° C.   0.42   105° C.   CELLOPHANE   250   14.33                   TAPE       ˜110° C.   0.6   105° C.   CELLOPHANE   250   31.76                   TAPE       ˜140° C.   0.31   105° C.   CELLOPHANE   250   57.00                   TAPE                    
     [0102]               TABLE 10                          HYDROPHILLIC AND HYDROPHOBIC BONDING IMPACT ON ADHESION                             COATED   POLYMERIC   MOL WEIGHT   PRODUCT                                     COMPOUND   TYPE   M W     ACID/SALT   VENDOR   DESIGNATION               Poly Ethylene   80/20   17,000   NH 4     MICHELMAN   MP4990R       Acrylic Acid   ETHYLENE/       SALT           ACRYLIC           ACID       POLY   0/100   50,000   ACID   ALDRICH       ACRYLIC ACID   HOMOPOLYMER       Poly   80/20   17,000   NH 4     MICHELMAN   MP4990R       Ethylene   ETHYLENE/       SALT       Acrylic Acid   ACRYLIC           ACID       POLY   HOMO   15,000   Na   ALDRICH   CAS #       Methacrylic   POLYMER       Salt       25087-26-7       ACID       Poly   80/20     7000*   NH 4     MICHELMAN   MP4990R       Ethylene   ETHYLENE/       SALT       Acrylic Acid   ACRYLIC           ACID       Poly   90/10    13000*   NH 4     MICHELMAN   MP2960       Ethylene   ETHYLENE/       SALT       Acrylic Acid   ACRYLIC           ACID                                         VICAT SOFTENING   COATED COVG IN       DWELL TIME MILLI-           TEMPERATURE ° C.   #/REAM   ITM TEMP ° C.   SECONDS   % LOSS                                         40° C.   0.98   105° C.   1,000   6.34       &lt;&lt;40° C.   0.93   105° C.   1,000   6.50       40° C.   0.34   105° C.   1,000   13.78       ˜50° C.   0.42   105° C.   1,000   14.33       40° C.   1.42   105° C.   1,000   4.11       72° C.   1.53   105° C.   1,000   3.72                                    
     [0103]               TABLE 11                          IMPACT OFPOLYVALENT METAL ION ON ADHESION                                 COATED   POLYMERIC   MOL WEIGHT       PRODUCT                                     COMPOUND   TYPE   M W     ACID/SALT   VENDOR   DESIGNATION               Poly Ethylene   80/20   17,000   NH 4     MICHELMAN   MP4990R       Acrylic Acid   ETHYLENE/       SALT           ACRYLIC           ACID       Poly Ethylene   80/20   17,000   NH 4         Acrylic Acid   ETHYLENE/       SALT       (Above) + ALUM   ACRYLIC           ACID       Poly Ethylene   80/20   17,000   NH 4     MICHELMAN   MP4990R       Acrylic Acid   ETHYLENE/       SALT           ACRYLIC           ACID       Poly Ethylene   80/20   17,000   NH 4         Acrylic Acid   ETHYLENE/       SALT       (Above) + ALUM   ACRYLIC           ACID                                 TAPE USED           CELLOPHANE                                     VICAT SOFTENING   COATED COVG IN       TAPE HIGHLAND   DWELL TIME MILLI-           TEMPERATURE ° C.   #/REAM   ITS TEMP ° C.   6200 CLEAR   SECONDS   % LOSS                                             40° C.   1.04   105° C.   CELLOPHANE   1,000   5.03                   TAPE           1.04 + 0.3   105° C.   CELLOPHANE   1,000   4.39                   TAPE       40° C.   1.04   90° C.   CELLOPHANE   1,000   1.90                   TAPE           1.04 + 0.3   90° C.   CELLOPHANE   1,000   0.30                   TAPE                    
     [0104]               TABLE 12                          IMPACT OFANIONIC POLYMER ACTIVITY ON ADHESION                                     COATED   MOL WEIGHT       Anionic Polymer       PRODUCT                                         COMPOUND   POLYMERIC TYPE   M W     ACID/SALT   Mixture Ph   VENDOR   DESIGNATION               Poly Ethylene   80/20 POLY   17,000   NH 4     6.9   MICHELMAN   MP 4990R       Acrylic Acid   ETHYLENE/ACRYLIC       SALT           ACID       Poly Ethylene   80/20   17,000   NH 4     6.9   MICHELMAN   MP 4990R       Acrylic Acid   ETHYLENE/ACRYLIC       SALT           ACID       Poly Ethylene   80/20   17,000   NH 4     6.9   MICHELMAN   MP 4990R       Acrylic Acid   POLYETHYLENE/       SALT           ACRYLIC ACID       Poly Ethylene   80/20   17,000   NH 4     6.9   MICHELMAN   MP4900R       Acrylic Acid   POLYETHYLENE/       SALT           ACRYLIC ACID       Poly Ethylene   80/20   17,000   NH 4     7.3   MICHELMAN   MP4990R       Acrylic Acid   POLYETHYLENE/       SALT       (Above) +   ACRYLIC ACID       NH 4 OH       Poly Ethylene   80/20   17,000   NH 4     6.9   MICHELMAN   MP 4990R       Acrylic Acid   POLYETHYLENE/       SALT           ACRYLIC ACID       Poly Ethylene   80/20   17,000   NH 4     7.3   MICHELMAN   MP4990R       Acrylic Acid   POLYETHYLENE/       SALT       (Above) +   ACRYLIC ACID       NH 4 OH                                             VICAT SOFTENING   COATED COVG IN       DRYING DURATION   DWELL TIME MILLI-           TEMPERATURE ° C.   #/REAM   DRYING TEMP   MIN   SECONDS   % LOSS                                             40° C.   0.98   153° C.   2   1,000   6.34               (305° F.)       40° C.   1.02   121° C.   2   1,000   8.43               (250° F.)       40° C.   0.88   107° C.   2   1,000   14.94               (225° F.)       40° C.   1.42   153° C.   2   1,000   4.11               (305° F.)       40° C.   1.32   153° C.   2   1,000   8.00               (305° F.)       40° C.   0.98   153° C.   2   250   9.09               (305° F.)       40° C.   1.01   153° C.   2   250   18.04               (305° F.)                            
     [0105]               TABLE 13                          IMPACT OF STARCH ANIONIC POLYMER MIXTURES ON ADHESION                                 COATED   MOL WEIGHT       PRODUCT   VICAT SOFTENING                                         COMPOUND   POLYMERIC TYPE   M W     ACID/SALT   VENDOR   DESIGNATION   TEMPERATURE ° C.               Poly Ethylene   80/20   17,000   NH 4     MICHELMAN   MP 4990R   40° C.       Acrylic Acid   POLYETHYLENE/       SALT           ACRYLIC ACID       Poly Ethylene   80/20   17,000   NH 4     MICHELMAN   MP 4990R   40° C.       Acrylic Acid   ETHYLENE/       SALT           ACRYLIC ACID                                     ANIONIC POLYMER                                         STARCH COVG IN   COVG IN Lbs/3300       DRYING DURATION   DWELL TIME MILLI-           Lbs/3300 Sq Ft   Sq Ft   DRYING TEMP   MIN   SECONDS   % LOSS                                             0   0.34   153° C.   2   1,000   13.78               (305° F.)       2.99   0.48   121° C.   2   1,000   24.15               (250° F.)