Electrophotographic printing media.

<u style="single">Background and Field of the Invention</u>

0001The present invention relates to media used in electrophotographic printing and more particularly to a plastic sheet or film for use in electrophotographic printing comprised of a transparent or opaque polymeric substrate coated with a polymeric coating having particular hardness and glass transition temperature parameters.

0002Laser electrophotography is an important electronic non-impact printing technology. It has several advantages over traditional mechanical impact printing techniques, such as high resolution, low noise level and high speed. However, currently available receptor media for laser printers, particularly for desktop laser printers, do not provide satisfactory image quality. They are frequently deficient in toner adhesion and resolution and in providing uniformly dense characters. The present invention overcomes these problems in laser electrophotography. While laser electrophotographic printing is specifically discussed in the present specification, the invention is equally applicable to other exposing radiation such as light emitting diode (LED), liquid crystal shutter (LCS) and the like techniques.

0003The laser electrophotographic process normally creates images on a coated polymeric substrate in five steps: charging, imaging, developing, image transfer and fixing. The individual steps of the process generally include the following: <ul id="ul0001" list-style="none"><li>(1) The electrophotographic process begins when a uniform electric charge is deposited onto a photoconductor drum in the dark;</li><li>(2) An electrostatic latent image is then created on the photoconductor by exposing the photoconductor to an oscillating narrow laser beam that is turned on and off digitally;</li><li>(3) The photoconductor is then exposed to toner particles, wherein toner particles having the correct polarity adhere to the exposed latent image;</li><li>(4) The medium to be printed is then passed between the photoconductor and a transfer corona to cause the toner particles to transfer from the photoconductor to the medium; and</li><li>(5) The transferred toner particles are then fixed to the medium by one of various procedures known in the art.</li></ul>

0004The last two steps in the imaging process represent difficult problems in the electrophotographic printing process. Although transfer of toner particles to the receptor layer is primarily driven by electrostatic forces, suitable transfer and fixing of toner particles to the receptor layer depends substantially upon the properties of the receptor medium. First, in order to insure the fidelity of the image transfer, toner particles must interact weakly with the photoconductor and strongly with the medium. Then, the receptor layer must be able to receive the toner particles completely in order to insure good image resolution. Finally, in the fixing process which follows image transfer, the toner particles must have a good affinity to the receptor layer in order to achieve image bonding.

0005The receptor sheet used in the process must also meet various important criteria. Most importantly, the receptor sheet surfaces must have suitable surface properties to insure reliable transport through the printer.

0006One of the most common problems in electrophotographic printing involves stoppage or delays resulting from jams due to inappropriate surface properties of the medium. In fact, if the imaging medium does not pass through the printer, none of the other qualities is relevant. Other factors include resistance to tearing and sufficient thermal stability to avoid buckling and loss of planarity.

0007Although various recording media have been proposed for use with laser electrophotographic printers, none of them has satisfied the substantial need in the art, particularly for laser printers such as the HP LaserJet and Apple LaserWriter. Current commercial laser media have two main drawbacks. First, the toner cannot be transferred fully from the photoconductor to the receptor layer due to poor toner affinity to this layer. Such incomplete toner transfer creates hollow characters and poor image resolution, both of which are considered as being serious quality defects. Secondly, images can also abrade or flake off from the medium because of poor toner adhesion. Other problems relating to imaging, medium handling and aesthetics are also encountered.

<u style="single">Summary of the Invention</u>

0008An object of the present invention is to provide a medium which overcomes the above-mentioned drawbacks. Specifically, the invention provides a medium with improved image quality and toner adhesion while retaining reliable transport qualities. These improvements are attained according to the invention by selecting polymers having particular hardness and glass transition temperature parameters. In particular, the polymeric coating or matrix should be designed to have a Tukon hardness in the range of about 0.5 to about 5.0, preferably 1.0 to 4.0, and a glass transition temperature (Tg) in the range of about 5° to about 45°C., preferably 15° to 40°C. Since an increase in the free volume, i.e., the molecular mobility of a polymer, is inversely proportional to the glass transition temperature, a lower glass transition temperature enhances the interaction of toner particles and the surface of the receptor sheet. Reliable feed through the laser printer is also essential and is achieved principally by means of the suitable selection of pigments. Other factors affecting feed reliability are the matrix binder and the conductive materials used as anti-static agents. Suitable solvent selection for the polymeric mixture and use of a surface active agent are important for the formation of uniform coatings free of optical defects, such as streaks, reticulation and mottle.

0009In a preferred embodiment of the invention, a substrate base is provided with a coating which contains at least one polymer, preferably an acrylic resin, a polyolefin pigment, a styrene-type conductive agent and a hydrocarbon surface active agent.

<u style="single">Detailed Description of the Invention</u>

0010The media for electrophotographic printing according to the invention generally comprise a plastic film substrate (a polymeric substrate) having a coating composition on one or both sides which enhances uniform and efficient image transfer and which promotes the adhesion of toner particles. The coating composition comprises one or more polymers dispersed or dissolved in a suitable vehicle, one or more pigments, and/or an antistatic agent, and/or surface active agent.

0011Use of pigments to control the surface properties is essential to the design by reducing static, avoiding blocking and promoting slip, while providing suitable friction to help propel the receptor sheet through the printer.

0012The base or substrate for the media of the invention is a suitable polymeric material base film having suitable transparent and physical characteristics so as to be resistant to tearing and resistant to damage by heat encountered in a printer, particularly in the fixing unit. Suitable polymeric materials for use as the base film substrate are thermoplastic polymers, including polyesters, polysulfones, poly(vinylchloride), poly(vinyl acecate), polycarbonates, polymethylmethacrylate, cellulose esters and others. A polyethylene terephthalate polyester film is a particularly preferred film base. The thickness of the base film is not particularly restricted, but should generally be in the range of about 2 to 10 mils, preferably about 3.0 to about 5.0 mils. The polymeric base or substrate nay be pretreated to enhance adhesion of the polymeric coating thereto.

0013The coating on the base film, according to the invention, normally has the following characteristics: <ul id="ul0002" list-style="none"><li>1. High toner receptivity (chemically and physically).</li><li>2. Relatively soft and flexible at the working temperature of the printer.</li><li>3. Excellent light and heat stability.</li><li>4. Capable of forming an optically uniform, non-tacky and smooth film.</li><li>5. Compatibility with antistatic agents.</li><li>6. Compatibility with particulate or pigment systems.</li></ul>

0014Coatings formed from the coating composition of the invention should have a Tukon hardness of about 0.5 to about 5.0, preferably from about 1.0 to about 4.0 and a glass transition temperature of about 5° to about 45°C, preferably from about 15° to about 40°C.

0015Tukon hardness is measured on a Tukon Hardness Tester, Model 300 (Page-Wilson Corporation, Bridgeport, Connecticut). Detailed information concerning the test procedures is available in "Tentative Method of Test for Use of the Tukon Hardness Tester, Method No. F5-52", July 11, 1952 (Rohm &amp; Haas Company, Spring House, Pennsylvania).

0016For Tukon hardness measurements, the coating solutions are coated onto Bonderik 1000 and dried at 50°C. for 45 minutes. The thickness of the resulting film is approximately 0.8 mils.

0017The glass transition temperature (Tg) is measured by differential scanning colorimetry (DSC) using a DuPont 910 DSC thermal analyzer, calibrated with appropriate standards. The reading and baseline errors from replicate DSC experiments lead to a typical accuracy in Tg of about 2°C. Measurements of heat flow versus temperature are made upon heating in the range Of 0° to 200°C. at a heating rate of 20°C./minute. The sample chamber is purged with dry nitrogen. Film-like samples are encapsulated in aluminum DSC cells. The mid-point method, i.e., identification of the maximum in the derivative of heat flow versus temperature curve, is used to obtain Tg data from the measured DSC curves.

0018The polymers employed in the coating according to the invention can be thermoplastic or thermosetting resins, and are preferably aqueous acrylic emulsions. However, many other polymers or copolymers can be used as long as they meet the above-mentioned criteria. The coating composition should preferably contain from 10% to 35% by weight of the polymer (solids content). The coating composition is applied to the base film in an amount to provide a final dry coat weight of preferably about 1 to 4 grams per square meter of coating, although suitable coatings may be achieved with lesser or greater amounts of coating weight. This provides a dry coating thickness of about 0.05 to 0.5 mils.

0019Preferred acrylic emulsions useful in the coating composition of the invention are the acrylic resins sold by Rohm &amp; Haas Company under the trademarks Rhoplex, particularly Rhoplex AC-73, HA-12, HA-16, B-15J.

0020According to a preferred embodiment of the invention, the polymeric coating comprises about 0.1 to about 10% by weight of pigment based on the weight of dry coating.

0021Pigments that can be used in the coating composition to modify the surface properties of the medium include calcium carbonate, kaolin, aluminum hydroxide, crystalline polyolefins such as polyethylene or polypropylene, polytetrafluoroethylene, silica, and other organic or inorganic pigments. The pigments primarily provide increased abrasion resistance, slip and anti-blocking characteristics.

0022The surface properties resulting from the addition of pigment are characterized in terms of the coefficient of friction. The coefficients of static friction and of dynamic friction of the receptor layer surface-to-backing (or surface-to-surface when the polymeric coating is on both sides of the substrate) should be in the range of from about 0.20 to 0.80 and from about 0.10 to 0.40, respectively.

0023The coating composition according to the invention may optionally contain conductive agents as anti-static additives. Preferred examples of conductive or anti-static agents used in the invention include sulfonated polystyrene, copolymers of dimethyl diallyl ammonium chloride and diacetone acrylamide, poly(dimethyl diallyl ammonium chloride), quaternary cellulose acetate, quaternary acrylics, copolymers of dimethyl diallyl ammonium chloride and N-methyl acrylamide and other conductive materials known in the art. These conductive agents should be contained in an amount of about 0.1 to about 5% by weight of dry coating such that the surface resistivity of the receiving sheet is usually about 1 x 10⁷ to 1 x 10¹⁴ ohms/sq. at 50% relative humidity and 20°C.

0024Surface active agents, such as wetting agents, dispersing agents, defoaming agents and anti-foaming agents, may be incorporated in the coating to improve surface properties and coatability. Both hydrocarbon type and fluorocarbon type surface active agents can be used. Preferred surface active agents are, for example, FC-430 (3M) and Surfynol 104 (Air Products &amp; Chemicals, Inc.).

0025A curing agent may be used in the coating composition if a crosslinkable resin is employed. Inclusion of a crosslinking agent will improve the strength of the coatings and the heat stability. Selection of an appropriate crosslinking agent depends on the type of resin to be utilized in the coating composition, and suitable crosslinking agents useful with particular resins are <u style="single">per se</u> known in the art. For example, an active nitrogen-containing compound may be used as a crosslinking agent such as the use of methylated melamine with a polyester resin. If included in the composition, the crosslinking agent is generally used in an amount of from about 0.5 to about 30 wt. % of the dried coating.

0026The side of the receptor sheet which does not bear the toner-receptive coating may need a backing material in order to reduce electrostatic charging and to reduce sheet-to-sheet friction and sticking. The backing may either be a polymeric coating, polymeric film or paper. The coefficient of static friction of the receptor layer surface-to-backing should be in the range of about 0.20 to 0.80, and the coefficient of dynamic friction should be in the range of about 0.10 to 0.40. The coefficient of friction is determined in accordance with ASTM D1894-78.

0027The coating formulation, which is used for the formation of the polymer coating on the polymeric substrate, generally is an aqueous coating formulation, but an organic solvent such as methanol, ethanol, cellosolve solvent, etc., can be employed in combination with water as the vehicle, if desired. A coalescing agent may be used therein to improve leveling, scrub resistance, gloss, adhesion, and enamel holdout. An organic solvent soluble formulation may also be devised which performs similarly to the aqueous-based systems.

0028Any of a number of coating methods may be employed to coat the coating composition onto the film base, such as roller coating, wire-bar coating, dip-coating, air-knife coating, slide coating, curtain coating, doctor coating or gravure coating. Such techniques are well known in the art.

0029Although the film is designed primarily for electrophotographic printers, it may be employed in electrophotographic copiers with similar advantages.

0030The following Examples are given merely as illustrative of the invention and are not to be considered as limiting.

Example I

0031A coating composition having the following formulation is prepared: <tables id="tabl0001" num="0001"><img file="EP0442567A2_D0001.tif" /></tables>

0032Rhoplex HA-12 and Rhoplex B-15J resins are added to a drum containing water and mixed for 10 minutes. The styrene-type conductive agent (Versa-TL 125) is then added to the drum with agitation.

0033Dispersing solvent (Cellosolve), hydrocarbon surfactant (Surfynol 104) and polyolefin pigment (Shamrock S-395) are added to a pail and mixed for 30 minutes with a high speed Cowles mixer.

0034The pigment dispersion is added slowly to the resin mixture with agitation and the resulting coating solution is applied to a poly(ethylene terephthalate) film (ICI United States Inc.). The coating is dried at 120°C for 2 minutes.

Example II

0035A coating composition having the following formulation is prepared: <tables id="tabl0002" num="0002"><img file="EP0442567A2_D0002.tif" /></tables>

0036Rhoplex AC-73 and Rhoplex B-15J resins are added to a drum containing water and mixed for 10 minutes. The styrene-type conductive agent (Versa-TL 125) is then added to the drum with agitation.

0037Cellosolve solvent, hydrocarbon surfactant (Surfynol 104) and polyolefin pigment (Shamrock S-395) are added to a pail and mixed for 30 minutes with a high speed Cowles mixer.

0038The pigment dispersion is added slowly to the mixture containing the resins with agitation. The resulting coating solution is applied to a poly(ethylene terephthalate) film (ICI United States Inc.). The coating is dried at 120°C for 2 minutes.

Example III

0039A coating composition having the following formulation is prepared: <tables id="tabl0003" num="0003"><img file="EP0442567A2_D0003.tif" /></tables>

0040Rhoplex HA-16 and Rhoplex B-15J resins are added to a drum containing water and mixed for 10 minutes. The styrene-type conductive agent (Versa-TL 125) is then added to the drum with agitation.

0041Dispersing solvent (Cellosolve), hydrocarbon surfactant (Surfynol 104) and polyolefin pigment (Shamrock S-395) are added to a pail and mixed for 30 minutes with a high speed Cowles mixer.

0042The pigment dispersion is added slowly to the resin mixture with agitation and the resulting coating solution is applied to a poly(ethylene terephthalate) film (ICI United States Inc.). The coating is dried at 120°C for 2 minutes.

0043The films produced in accordance with the foregoing Examples have the following glass transition temperatures and Tukon hardness values. <tables id="tabl0004" num="0004"><img file="EP0442567A2_D0004.tif" /></tables>

0044All of the coated films of Examples I-III have excellent image quality and toner adhesion when used in an electrophotographic printer. In contrast, a number of currently available commercial films for electrophotographic printing can be used as comparative examples. These films give poor image quality and toner adhesion. Examples of these films are: <tables id="tabl0005" num="0005"><img file="EP0442567A2_D0005.tif" /></tables>

0045The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.