WO2001084247A2

WO2001084247A2 - Apparatus, method and wax coatings for improved durability and visual appearance of printed images

Info

Publication number: WO2001084247A2
Application number: PCT/US2000/021552
Authority: WO
Inventors: James A. Baker; Julie Y. Qian; Michael R. Bury; Brad B. Rassatt
Original assignee: Imation Corp.
Priority date: 2000-03-30
Filing date: 2000-08-07
Publication date: 2001-11-08
Also published as: WO2001084247A3

Abstract

An apparatus and a method suitable for applying a thin protective wax overcoat on an ink-bearing surface of a receptor in which the coating thickness is precisely controlled. The thin overcoats can be applied uniformly over the imaged surface. Alternatively, non-uniformities can be selectively introduced, e.g., to produce decorative effects. Useful waxes include crystalline organic waxes such as organosilicone waxes having a silicone backbone to which are covalently bonded side-chain crystallizable, aliphatic, hydrocarbon side chains having at least 14 carbon atoms, and waxes in the form of graft copolymer organosols having side-chain or main-chain crystallizing polymeric moieties that independently and reversibly crystallize at a temperature greater than or equal to 22 °C. The invention also features novel graft copolymer organosol compositions.

Description

APPARATUS , METHOD AND WAX COATINGS FOR IMPROVED DURABILITY AND VISUAL APPEARANCE OF PRINTED IMAGES

BACKGROUND

This invention relates to protecting an ink-bearing surface of a receptor using a wax coating.

Images printed on receptor surfaces such as paper or polyester are susceptible to damage that can distort the image. Examples include scratching, blocking, and smearing.

Waxes have been used to enhance the damage resistance of printed images. One approach involves adding the wax directly to the liquid toner or ink used to prepare the printed image. Another approach involves applying the wax as an overcoating on the surface of the image. Typically, the wax is applied in the form of a dispersion or emulsion using techniques such as bar coating. Neither of these approaches, however, is capable of applying very thin, protective wax coatings uniformly over the imaged surface. Nevertheless, such coatings would be desirable both from an economic and aesthetic point of view.

SUMMARY

The invention, in one aspect, relates to an apparatus and a method suitable for applying a thin protective wax overcoat on an ink-bearing surface of a receptor in which the coating thickness is precisely controlled. The ability to provide a thin protective coating results in low cost per page. The thin wax overcoat can be written on using water- and solvent-based inks and pencil. In addition, it is possible to erase pencil marking from the thin overcoat surface.

The thin overcoats can be applied uniformly over the imaged surface. Alternatively, non-uniformities can be selectively introduced, e.g., to produce decorative effects.

Examples of useful waxes include crystalline organic waxes such as organosilicone waxes having a silicone backbone to which are covalently bonded side-chain crystallizable, aliphatic, hydrocarbon side chains having at least 14 carbon atoms, and waxes in the form of graft copolymer organosols having side- chain or main-chain crystallizing polymeric moieties that independently and reversibly crystallize at a temperature greater than or equal to 22°C. The invention also features novel graft copolymer organosol compositions.

The wax coatings improve image durability by providing blocking, erasure, and scratch resistance. They are particularly useful for improving the blocking and erasure durability of images printed using liquid electrophotography. The wax coating may also be used to improve water resistance and lightfastness of images, notably ink jet printed images, printed using inks based on fugitive dye-based colorants, and to control or enhance the gloss of any printed image. The wax coating can be applied to images printed on both sides of a receptor (duplex printing) without offsetting or image damage. In addition, the wax coating does not interfere with document feeding when reproducing images using conventional photocopiers equipped with recirculating document handlers. Other features and advantages of the invention will be apparent from the following description of preferred embodiments thereof, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an apparatus for applying a thin, protective wax coating to a printed receptor surface that includes a reservoir in which wax is melted and applied to a metering roller partially submerged in the reservoir. FIG. 2 is a side view of the apparatus shown in FIG. 1.

FIG. 3 is a perspective view of another apparatus for applying a thin, protective wax coating to a printed receptor surface in which the wax is provided in the form of a cylindrical roller.

FIG. 4 is a perspective view of another apparatus for applying a thin, protective wax coating to a printed receptor surface in which the wax is provided in the form of a bar.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRD?TION

Thin, uniform wax coatings are applied over an image printed on the surface of a receptor such as a sheet or continuous web in order to protect the printed image. The image may be formed using any known printing technique, including offset printing, dye sublimation printing, piezoelectric ink jet printing, thermal ink jet printing, dry or liquid electrophotography, etc.; preferably, the images are printed using liquid electrographic (electrostatic, ionographic, and electrophotographic) printing methods. The receptors are commonly made of paper or card stock, but may include printed plastic sheets, particularly printed polyester overhead projection (OUP) films.

The thickness of the protective wax overcoat (measured in terms of coat weight per unit area of receptor surface) is selected such that it is sufficient to provide good image protection, while minimizing visual coating defects such as void, ribs, and the like. In addition, cost considerations dictate that these objectives be realized using a minimum amount of wax. In general, suitable wax coating weights are in the range of 0.8-7 g/m²of receptor surface, more preferably 1-3.3 g m², and most preferably 1.2-1.7 g/m². Any number of commercially available organic waxes may be used as the protective overcoat. Examples of suitable waxes are listed in "Using Waxes and Polymers to Improve Coatings," J. R. Carroll, R. M. Bradley and A.I. Kalmikoff, _ Coatings Technol.. 10, 435 (1994). Examples of suitable neat (100% solids) waxes include natural waxes such as vegetable waxes (e.g. Carnauba and Montan) and aliphatic hydrocarbon waxes (paraffins), and synthetic waxes such as microcrystalline waxes, Fischer-Tropsch waxes, crystalline polyolefins (e.g., polyethylenes and polypropylenes), poly(tetrafluoroethylenes), and silicone- functional paraffinic copolymers. The waxes may also be used as solutions in suitable (e.g. paraffinic or isoparaffinic) solvents, as dispersions in suitable volatile solvents, and as dispersions or emulsions (e.g. in water).

A number of wax characteristics influence the ability of the wax to provide good protective coating performance. One of these characteristics is the crystallization temperature (melting point) of the wax. Generally, any wax with a melting point above the normal use and storage temperature of the coated image will be suitable as a protective coating; however, a melting point above 38°C is generally desired in order to meet the minimum blocking durability requirements for printed paper as specified in ASTM test method Dl 146-88. On the other hand, too high a melting point will lead to high energy consumption in order to melt the wax (latent heat of fusion) and maintain the wax in a molten state. Preferably, the normal melting temperature of the wax is in the range 38-70°C, more preferably 43-65°C. It is also desirable that the wax have a low coefficient of friction to allow the wax to impart good abrasion, rub, and mar resistance to the overcoated image, particularly when tested for erasability according to the modified Crocking test carried out according to ASTM test method F1319-90 (modified to use a number two pencil eraser instead of a linen cloth). It is also desirable to that the wax be clear and resist yellowing to avoid distorting or tinting the underlying printed image, and that wax have good chemical resistance properties (e.g., to water, alkali, and solvents) to maximize durability.

The viscosity and surface tension of the liquid wax, whether molten or dissolved/dispersed in a carrier solvent, influence the ability of the wax to be applied as a thin, uniform coating. Generally, too low a wax viscosity at coating conditions can result in wicking or imbibition of the wax into the receptor, causing "oil spots." Too high a viscosity, on the other hand, can lead to excessive coating thickness and coating defects (e.g. ribbing). With respect to surface tension, it is generally desirable to minimize surface tension at the coating conditions of the wax because this results in a high spreading coefficient, making it easier to produce thin, uniform coatings.

Particularly useful waxes applying a thin, uniform protective coating to printed images are the synthetic silicone-functional waxes manufactured by Genesee Polymers Corporation (Flint, ME) under the trade names EXP-58 and GP- 533. EXP-58 is a 100% active organosilicone wax copolymer. It consists of a silicone backbone onto which are covalently bonded side chain crystallizable aliphatic hydrocarbon chains having at least 14 carbon atoms. The approximate molecular weight of EXP-58 is 12,000 Da, and the melting point lies in the range 38-50°C, with a mean softening temperature of 43°C. This wax provides improved blocking and erasure durability at temperatures up to the mean softening temperature, or 43°C. GP-533 is a 100% active organosilicone wax copolymer. It consists of a silicone backbone onto which are covalently bonded side chain crystallizable aliphatic hydrocarbon chains, as shown schematically by the following structure:

CH₃

I

[CH₃]₃Si-O-— [Si-O]4o— Si[CH₃]₃

The approximate (theoretical) molecular weight of GP-533 is 19,400 Da, and the melting point lies in the range 58-70°C, with a mean softening temperature of 65°C. This wax provides improved blocking and erasure durability at temperatures up to the mean softening temperature, or 65°C. Also suitable for forming thin, protective wax coatings are graft copolymers dispersed in an organic solvent in which the graft copolymer is prepared by reacting a wax graft stabilizer, e.g., a functionalized silicone wax or an acrylate such as behenyl acrylate, with monomers such as acrylic or methacrylic esters. These graft copolymer dispersions, known as organosols, are preferably dispersed in a volatile aliphatic or isoparaffinic hydrocarbon solvent and applied to the metering roller in a fluid state. Organosols have as a principle advantage the ability to maintain the low fluid viscosities required for coating even at relatively high weight percent loadings in the carrier liquid.

The wax coating may be applied to the imaged receptor surface using a number of coating methods, including reverse roll coating, forward roll coating, gravure roll coating, extrusion die coating, slide coating, knife coating, and the like. Reverse roll coating is particularly preferred. One example of a useful reverse roll coating apparatus for applying a thin, protective wax coating is shown in Figs. 1 and 2. Referring to Figs. 1 and 2, there is shown a wax coating apparatus 10 designed to permit the application of a thin and uniform protective wax coating onto the imaged side of a pre-printed receptor such as paper or plastic film. Apparatus 10 includes a driven metering roller 12, an optional doctor blade 14, a drive roller 16, and a heated/pressurized nip 18 defined by the surfaces of metering roller 12 and drive roller 16. Drive roller 16 rotates in a reverse direction relative to metering roller 12 to feed the imaged receptor into nip 18. Metering roller 12 is partially immersed in a heated reservoir 20 that holds a supply of molten wax. The wax is supplied in a fluid (molten) state from reservoir 20 to metering roller 12 and subsequently applied in molten form to the imaged side of the receptor at nip 18. The uniform protective coating on the imaged receptor surface is obtained by controlling the temperature and pressure at nip 18, as well as by careful selection of the chemical and physical properties of the wax, as described above. A nip load of 15 to 20 lbs is preferred and may be obtained by use of torsional springs 22. Alternatively, compression springs, weights, etc. may be used. The preferred method to obtain the desired nip temperature is to preheat the wax in reservoir 20 to effect liquefaction, and then apply the liquid wax to the surface of the image using a heated metering roller 12 while optionally heating the image receptor from the back side using a heated drive roller 16.

Melting of the wax in reservoir 20 may be achieved by heating the wax above its crystalline melting temperature using any well known direct heating means, such as bar heaters, resistance heaters, heat lamps, and the like. In the case of drive roller 16 and metering roller 12, either internal (e.g. heating lamps 24 and 26, respectively, as shown in FIG. 2) or external (e.g. a heating roller) heat sources may be used. The drive roller and metering nip temperatures are generally greater than the melting point of the wax, and preferably in the range 65-75°C.

Drive roller 16 drives the imaged receptor through the nip. In general, it has a diameter between 1 and 2 inches. Preferably, the surface of drive roller 16 is covered by a compliant elastomeric coating 28 to facilitate transfer of the molten wax from metering roller 12 to the imaged side of the imaging receptor. The preferred speed of the drive roller is 3 inches per second.

Metering roller 12 provides the surface from which the imaged receptor is coated. The surface finish and thermal properties of metering roller 12 are optimized to obtain uniform, defect-free coatings. A smooth, aluminum metering roller is preferably used because of its desirable thermal properties, and a hard coat anodized metering roller is preferable because of the excellent wear resistance of anodized rollers.

Metering roller 12 is preferably driven. Its speed relative to drive roller 16 largely determines the coating thickness obtained. To obtain a desirable coating thickness of 0.0002 to 0.0003 inches, for example, the speed of metering roller 12 is preferably in the range 5-7 inches per second, more preferably 6 inches per second. Thus, metering roller 12 is preferably overdriven relative to drive roller 16.

To minimize coating defects introduced from vibration or discontinuous motion of drive roller 16, the drive train is preferably driven indirectly using a V- belt or, as shown in FIGS. 1 and 2, a timing belt arrangement 30. To provide a coating of wax on the image side only, drive roller 16 must engage and disengage metering roller 12 as the imaged receptor enters and exits the nip. A timing cam 36 is used to prevent leading and trailing edge wax coating defects as drive roller 16 engages and disengages metering roller 12. The speed rate is controlled through the timing cam. The imaged receptor may be lifted as drive roller 16 is disengaged.

A trailing edge doctor blade 14 is preferably positioned in operative engagement with metering roller 12 in order to obtain a smooth, pre-metered, uniformly thin wax layer on metering roller 12 prior to transfer of the molten wax to the imaged receptor surface. The angle defined between doctor blade 12 and the plane defined by the imaged receptor as it passes through nip 18 influences the wax flow across the blade and thus stability and uniformity of the coating thickness. Doctor blade angles greater than 50 degrees are preferable for minimizing coating defects resulting from flaws in the edge of the doctor blade, roller runout, and roller surface defects. Accordingly, operating at a doctor blade orientation of 50 degrees or greater generally provides for more consistent and more uniform coatings of the wax on the imaged receptor. It also renders the doctor blade self-cleaning. Doctor blade 14 is preferably operated in contact with metering roller 12, as shown in FIGS. 1 and 2, in order to obtain more consistently uniform wax coatings without coating defects due to doctor blade chatter. However, a gap may be set between doctor blade 14 and metering roller 12, in which case the gap is preferably less than 0.0005 inches.

FIG. 3 illustrates a second embodiment of a wax coater suitable for applying a thin, uniform wax coating to the surface of an imaged receptor. In this embodiment, the protective wax coating is applied to metering roller 12 by a cylindrical wax applicator roller 32 operatively engaged with the surface of metering roller 12 and driven by frictional contact with metering roller 12. Cylindrical wax applicator roller 32 features solid wax cylindrically molded around a solid core or shaft. It is preferably externally heated such that wax at the surface of wax applicator roller 32 is in a substantially molten state when contacting metering roller 12. Alternatively, shear heating between wax applicator roller 32 and metering roller 12 may be used to produce a thin layer of molten wax at the surface of wax applicator roller 32.

In a variation of the embodiment shown in FIG. 3 (not shown), metering roller 12 may be dispensed with entirely, and cylindrical wax applicator roller 32 may be directly engaged to the imaged surface of the imaging receptor. In this variation, wax applicator roller 32 performs the dual role of a metering roller and a source of wax.

Non-uniform pressure may cause cylindrical wax applicator roller 32 to wear in a non-uniform manner, leading to non-uniform application of the wax onto metering roller 12. In a preferred embodiment, this problem is overcome by applying a doctor blade (not shown) directly to the molded wax roller surface or by operative engagement of a doctor blade to the surface of metering roller 12 downstream of the wax applicator roller 32 but upstream of nip 18. Alternatively, the wax may be fed onto the metering roller from a solid wax bar applicator 34 loaded against metering roller 12, as shown in FIG 4. Loading is preferably achieved using compressional springs (not shown) positioned at each end of bar 34 to urge the wax against the surface of metering roller 12.

Both sides of a duplexed (two sided) imaged receptor may be sequentially coated using any of the above-described coating devices by alternately feeding each printed side of the imaged receptor through nip 18 (or the nip formed between cylindrical wax applicator 32 and drive roller 16, if no metering roller is used). Preferably, however, both sides of a duplexed image are coated simultaneously using the previously described coating apparatus wherein the drive roller from a first wax coating sub-system (designated the lower wax coating sub-system) is also the metering roller in a second wax coating sub-system (designated the top side coater) oriented above the lower wax coating sub-system. Either wax coating subsystem may include any of the individual wax coating apparatus embodiments described previously.

The above-described methods and apparatus may be used to apply smooth, uniform wax coatings. However, it is also possible to achieve decorative effects by applying the wax overcoat in a non-uniform manner such that coating instabilities (longitudinal ribs) are observed. Under conditions where ribbing instabilities are observed with a roll coating wax coating apparatus, the wax coating takes on a reticulated surface of regular striations such that the images take on a textured appearance. Such ribbing instabilities are described in E. Cohen and E. Gutoff, "Modern Coating and Drying Technology," (VCH Press: NY, 1992) and as noted therein, will result when the wax coater is operated at conditions of very high Capillary Number:

Nc_a = u σ where

V = Coating roller velocity η = shear viscosity of the coating fluid σ = surface tension of the coating fluid

Thus, the decorative textured appearance can be readily obtained by coating the wax at high speed, or by coating the wax at low temperature (high viscosity), or by coating waxes having low surface tensions, such as silicone or fluorocarbon waxes.

An alternative non-uniform coating that produces a decorative pattern may be obtained when the wax coater is operated in a manner such that Marangoni interfacial instabilities driven by cellular convection (Benard cells) are observed. The mechanism of cellular convection driven by either density gradients or by surface tension gradients caused by temperature gradients in a coating is described inE. Cohen and E. Gutoff, "Modern Coating and Drying Technology," (VCH Press: NY, 1992), pp. 132-94 and in Velarde andNormand, "Convection," Scientific American, 243, 92 (1980).

Under conditions where Marangoni instabilities are observed, the wax coating takes on a surface of irregular hexagonal patterns such that the images take on a canvas-like appearance similar to oil paintings. Such instabilities will result when the wax coater is operated at conditions of high Marangoni Number, typically greater than 80:

N_Ma = ( dT (dTidy)h² > 80

where (dσ/dT) = variation of coating fluid surface tension with temperature

(dT/dy) = variation of temperature across the coating thickness h = wet coating film thickness η = shear viscosity of the coating fluid k= thermal conductivity of the coating fluid p = density of the coating fluid C_p = liquid heat capacity at constant pressure of the coating fluid. Thus, the decorative canvas appearance driven by Marangoni cellular convection can be readily obtained, e.g., by coating the wax at high temperature (low viscosity) or by using low surface tension waxes such as the aforementioned silicone waxes that exhibit higher dσ/dT.

EXAMPLES Example 1

Image durability performance was determined on solid monochrome images of yellow, magenta, cyan, and black ink that were printed using the drum type liquid electrophotographic printing apparatus described in U.S. 5,650,253 (Fig. 1). The gel organosol liquid inks used to make these images were prepared according to the methods described in Examples 37-40 of U.S. 5,698,616. Two distinct gel organosols were used to make inks. One gel organosol was prepared at a core/shell ratio of 8 using a 1/3 weight ratio of methyl methacrylate/ethyl acrylate in the organosol core as described in Example 23 of U.S. 5,698,616. This organosol had a calculated core glass transition temperature of -1°C, and yielded printed images that were essentially tack-free to the touch. This ink will be referred to as the "-1°C Tg ink." Another gel organosol was prepared in manner similar to Example 23 of U.S. 5,698,616 at a core/shell ratio of 8 using approximately a 1/5 weight ratio of methyl methacrylate/ethyl acrylate in the organosol core. This organosol had a calculated core glass transition temperature of -10°C, and yielded printed images that were quite tacky to the touch. This ink will be referred to as the "-10°C Tg ink." Single-sided monochrome images prepared using either -1°C Tg or -10°C

Tg inks were coated using the wax coating apparatus illustrated in Figs. 1 and 2, configured with a molten wax tank, a metering roller, and a doctor blade. The images were coated with wax at various coating weights between 0.04-0.20 g/page (calculated thicknesses of 0.78-3.90 microns using a bulk wax specific gravity of 0.85 g/ml). The coated images, as well as an uncoated control image, were subjected to adhesive (ink to paper) and cohesive (ink to ink) blocking tests according to ASTM test method Dl 146-88. Test conditions of 43°C @ 50% relative humidity, 58°C @ 75% relative humidity, and 63°C @ 75% relative humidity were used. The following ratings were used for blocking resistance, and to rate adhesive and cohesive failure:

NO = No blocking (no image adhesion);

1 = 1^st degree blocking (image adhesion, but no image damage);

2 = 2^nd degree blocking (image adhesion and some image damage); VS = Very slight; S = Slight;

M = Medium; H = High; VH = Very high.

Image Erasure Resistance (EER) was also determined at 23 °C using the modified Crocking test based on ASTM test method F 1319-90, modified by substituting a standard #2 pencil eraser for the standard linen cloth called for in the test method.

The results are summarized in Tables I and π, and indicate that both the relatively tacky -10°C Tg inks and the lower tack -1°C Tg inks showed improved durability to blocking at temperatures up to just below the softening point of the GP-533 wax (58°C) when the coating weight was in the range 0.066-0.15 g/page. This is a significant improvement relative to the uncoated images, which both fail the adhesive and cohesive blocking tests at the minimum test condition (38°C and 50% relative humidity) called for in the ASTM test method. The minimum coating weight for blocking performance corresponds to a calculated wax coating thickness of 1.2 microns (approximately 0.05 mil). Image Erasure Resistance was also improved for the wax coated images relative to the uncoated controls, rising to the 96-100% range for coating weights above 0.066 g/page, compared to values between 80-92% for the uncoated controls.

Table 1: Effect of Wax Coating Weight on Blocking and Erasure Resistance of Black Ink

Ink: Tg=-10C ι Core/Shell = 8; EHMA HEMA-TMI/MMA EA (97/3-4.7//25 75); Monarch 120 Carbon Black;

Organosol/Pigment=6

Table II: Effect of Wax Coating Weight on Blocking and Erasure Resistance of Colored Inks

Ink: Tg=-10C @ Core/SheIl=8; EHMA HEMA-TMI/ MMA EA (97/3-4.7//25/75)

Example 2

Full color images were printed on Georgia Pacific Laser 1000 paper using the drum type liquid electrophotographic printing apparatus and -1°C Tg inks described in Example 1. Some of these images were coated with the GP-533 wax using the wax coating apparatus shown in Figs. 1 and 2 and equipped with a metering roller and doctor blade. The wax coating weight was measured as 0.06 g/page. The wax coated images and uncoated control images were measured for colorimetric parameters (difference in reflectance optical density and L* a* b*) using a Greteg Spectrolina spectrophotometer; gloss and gloss difference were also measured at 60° using a tri-gloss glossmeter. The data, summarized in Table HI, show a 0.1 ROD unit increase in D,^, a general increase in chroma for the chromatic colors (CMYRGB), and a 1-5 unit increase in gloss due to the wax overcoat.

Table 111 Colorimetry and Gloss Evaluation for Wax-Coated Images

Example 3

This example describes the application of a protective wax coating using a solution of wax dissolved in a volatile hydrocarbon liquid. Genesee Polymers Corp. (Flint, MI) GP-533 silicone wax copolymer was dissolved inNORPAR 12 solvent (Exxon Corp., Houston, TX) at 25% w/w by first heating the wax to a temperature above its melting point, then pouring it slowly into the hydrocarbon solvent with stirring. The resulting solution was applied to printed images (printed using -1°C Tg inks described in Example 1) using the wax coating apparatus shown in Figs 1 and 2. The heater in the wax tank was not used; however, both the metering roller and drive roller were heated to approximately 75-85°C.

Table TV summarizes the blocking and erasure test results as a function of calculated wax coating thickness. The data demonstrate that cohesive blocking resistance and image erasure resistance increase with wax coating thickness, and that a coating thickness of 0.15 mil (approximately 0.19 g/page coating weight) achieved acceptable blocking performance at a test temperature of 43°C and 50% relative humidity.

Table IV: Effect of Solvent-Coated Wax Thickness on Blocking and Erasure Resistance of Black Inc.

25% (w/w/) Genesee GP-533 Silicone Wax Copolymer in Solvent*

Ink: Tg=-l°C @ Core/Shell=8; EHMA/HEMA-TMMMMA/EA (97/3-4.7//2S/75); Monarch 120 Carbon Black; Organosol/Pigment=6; 0.100 g Zirconium HEXCHEM/gPi

"Organic solvent commercially available under the trade designation NORPAR 12 Example 4

This example describes the use of a controlled crystallinity organosol based on a side chain crystallizable graft copolymer as a protective coating material. The copolymer was prepared according to the methods and procedures disclosed in U.S. 5,886,067. The first step involved the preparation of a graft stabilizer based upon behenyl acrylate (mean crystallization temperature 58°C) as follows. A 32 ounce, narrow-mouth glass bottle was charged with 439 g of NORPAR 12 solvent, 146 g of Behenyl Acrylate (BHA), 5 g of 96% hydroxy ethyl methacrylate (HEMA), and 1.5 g of AIBN initiator. The bottle was purged with dry nitrogen for 1 minute at a rate of approximately 1.5 liters/minute, then sealed with a screw cap fitted with a Teflon liner. The cap was secured in place using electrical tape. The sealed bottle was then inserted into a metal cage assembly and installed on the agitator assembly of an Atlas Launder-Ometer (Atlas Electric Devices Company, Chicago, IL). The Launder-Ometer was operated at its fixed agitation speed of 42 rpm with a water bath temperature of 70°C. The mixture was allowed to react for approximately 16-18 hours, at which time the conversion of monomer to polymer was quantitative. The mixture was heated to 90°C for 1 hour to destroy any residual AEBN, then was cooled to room temperature. The bottle was then opened and 2.3 g of 95% dibutyl tin dilaurate

(DBTDL) and 7.1 g of meta-dimethyl isopropenyl benzyl isocyanate (TMI) were added to the cooled mixture. The bottle was purged for 1 minute with dry nitrogen at a rate of approximately 1.5 liter/minute, then sealed with screw cap fitted with a Teflon liner. The cap was secured in place using electrical tape. The sealed bottle was then inserted into a metal cage assembly and installed on the agitator assembly of the Atlas Launder-Ometer. The Launder-Ometer was operated at its fixed agitator speed of 42 rpm with a water bath temperature of 70°C. The mixture was allowed to react for 6 hours, at which time the conversion was quantitative. The resulting graft stabilizer product was a transparent liquid containing no visible insoluble mater when it was heated above 65°C and formed a clear solid when it was cooled to room temperature.

The percent solids of the graft stabilizer product was determined to be 26.32% using a gravimetric infrared drying oven attachment to a precision analytical balance (Mettler Instruments Inc., Highstown, NJ). Approximately two grams were sample were used in each determination of percent solids using this gravimetric method. The graft stabilizer product was a copolymer of BHA and HEMA containing random side chains of TMI and is designated herein as BHA HEMA- TMI (97/3-4.7% w/w). It was converted into a -1°C Tg controlled crystallinity graft copolymer organosol as follows. A 5000 ml 3 -necked round bottom flask equipped with a condenser, a thermocouple connected to a digital temperature controller, a nitrogen inlet tube connected to a source of dry nitrogen, and a magnetic stirrer was charged with a mixture of 2855.35 g of NORPAR 12 solvent, 319.15 g of ethyl acrylate (EA), 67.20 g of methyl methacrylate (MMA), 319.15 g of the behenyl acrylate graft stabilizer at 26.32% polymer solids, and 6.30 g of AIBN initiator. While magnetically stirring the mixture, the reaction flask was purged with dry nitrogen for 30 minutes at a flow rate of approximately 2 liters/min. A hollow glass stopper was then inserted into the open end of the condenser and the nitrogen flow rate was reduced to approximately 0.5 liters/min. The mixture was heated to 70°C with stirring, and the mixture was allowed to polymerize at 70°C for 16 hours. The conversion was quantitative.

The resulting organosol product was an opaque white dispersion when it was heated above 65°C and formed an opaque solid when it was cooled to room temperature. This organosol is designated BHA/HEMA-TMI //MMA/EA (97/3- 4.7//25/75/5% w/w). The percent solids of this organosol dispersion was determined to be 12.70% using the gravimetric infrared drying method.

The behenyl acrylate organosol was vacuum stripped to a graft copolymer weight loading of 28% w/w in NORPAR 12 solvent. This organosol was then coated onto full color images using multiple passes through the wax coating apparatus shown in Figs 1 and 2. It was necessary to heat the wax organosol in order to convert it to a fluid state in the wax reservoir.

The uncoated images, which had been previously printed on Georgia Pacific Laser 1000 paper using the drum type liquid electrophotographic printing apparatus and -10°C Tg inks as described in Example 1, were extremely tacky to touch. The images coated with the controlled crystallinity organosol, however, were essentially tack-free at room temperature. These images would be expected to remain tack-free, and therefore resist blocking, at temperatures up to the average melting point of the behenyl acrylate (approximately 58°C). Example 5

This example describes use of another controlled crystallinity organosol based on a side chain crystallizable graft copolymer as a protective coating material. The copolymer was prepared according to the methods and procedures disclosed in U.S. 5,886,067. The first step involved the preparation of a graft stabilizer based upon an amino functional silicone organosilicone wax copolymer (GP-628) having a mean softening temperature of 65°C, as follows. A 32 ounce, narrow-mouth glass bottle was charged with 654.96 g of

NORPAR 12 solvent, 30 g of a terminal amino-functional silicone wax copolymer (GP-628 from Genesee Polymer Corp., Flint, MI), and 5.04 g of TMI. The bottle was sealed with a screw cap fitted with a Teflon liner. The cap was secured in place using electrical tape. The sealed bottle was then inserted into a metal cage assembly and installed on the agitator assembly of an Atlas Launder-Ometer (Atlas Electric Devices Company, Chicago, IL). The Launder-Ometer was operated at its fixed agitation speed of 42 rpm with a water bath temperature of 70°C. The mixture was allowed to react for approximately 6 hours at which time the conversion was quantitative. The percent solids of the graft stabilizer product was determined to be

5.20% using the gravimetric infrared drying method. The product was a copolymer of GP-628 amino-functional silicone wax copolymer and TMI in the form of a transparent liquid containing no visible insoluble matter. It was converted into a -1°C Tg controlled crystallinity graft copolymer organosol as follows

264.40 g of NORPAR 12 solvent, 43.20 g of EA, 14.40 g of MMA, 276.92 g of the graft stabilizer at 5.20% polymer solids, and 1.08 g of AIBN were combined in a 32 ounce bottle. The bottle was purged with dry nitrogen for 3 minutes at a rate of approximately 1.5 liters/minute, then sealed with a screw cap fitted with a Teflon liner. The cap was secured in place using electrical tape. The sealed bottle was then inserted into a metal cage assembly and installed on the agitator assembly of an Atlas Launder-Ometer. The Launder-Ometer was operated at its fixed agitation speed of 42 rpm with a water bath temperature of 70°C. The mixture was allowed to react for approximately 16-18 hours, at which time the resulting organosol was cooled to room temperature. The cooled mixture was an opaque white dispersion. The percent solids of the organosol dispersion was determined to be

11.83% using the gravimetric infrared drying method. Organosol particle size was determined by dynamic light scattering on a diluted toner sample (typically less than 0.0001 g/ml) using a Malvern Zetasizer HI Photon Correlation Spectrometer (Malvern Instruments Inc., Southborough, MA). The dilute samples were ultrasonicated for one minute at 100 watts and 20 kHz prior to measurement. The organosol had a z-average diameter of 83.9 nm with a standard deviation of 3.1 nm.

This organosol is designated GP-628/TMI/ MMA EA (86/14-25/75% w/w). It was then vacuum stripped to a graft copolymer weight loading of 44.5%. Single-sided monochrome images printed on paper were prepared using the liquid electrophotographic printing apparatus described in U.S. 5,650,253 (Figs. 3 and 4). The inks used in this printing apparatus were prepared using a -10°C core Tg organosol, as described in Example 1. Printed paper sheets were coated using the wax coating apparatus illustrated in Figs. 1 and 2, configured with a molten wax tank, a metering roller, and a doctor blade. It was not necessary to heat the silicone wax organosol to convert it to a fluid state in the reservoir. The pre- imaged sheets required multiple coating passes in order to obtain a uniformly thick coating. The sheets were dried in an oven maintained at 80°C for five to ten minutes between each coating pass. The coated images obtained with different numbers of coating passes ranging between 1 and 6, as well as an uncoated control image, were subjected to adhesive (ink to paper) and cohesive (ink to ink) blocking tests according to ASTM test method Dl 146-88. Test conditions of 58°C @ 75% relative humidity were used. Image Erasure Resistance (TER) was also determined at 23 °C using the modified Crocking test based on ASTM test method F 1319-90, modified by substituting a standard #2 pencil eraser for the standard linen cloth called for in the test method. The results of these experiments demonstrated that images coated with more than five coating passes passed both adhesive and cohesive blocking tests under the test conditions. The wax organosol coated images also exhibited improved Image Erasure Resistance (TER) relative to the uncoated control image. Image erasure resistance for the images with six coating passes ranged from 91- 98%.

Example 6

A full color image was printed on standard Georgia Pacific Laser 1000 paper using a commercially available thermal inkjet printer (HP 820 Cse, Hewlett- Packard Corp, Palo Alto, CA). The yellow, magenta, and cyan inks used in this printer are known to be based on fugitive dyes with poor water fastness. The full color inkjet image was coated with the GP-533 wax using the wax coating apparatus illustrated in Figs. 1 and 2, configured with a molten wax tank, a metering roller, and a doctor blade. The wax coating weight was measured as 0.11-0.2 g/page.

A tissue (Kim- Wipe, Kimberly Clark Corp, Neenah, WI) was dipped into normal tap water and squeezed dry. The tissue was then wiped quickly over the wax coated inkjet image and the uncoated control image. The uncoated control showed significant smearing and bleeding of the inks wherever the wet tissue contacted the image. On the other hand, only slight ink smearing or bleeding was observed for the wax-coated image.

Other embodiments are within the following claims.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for providing an ink-bearing surface of a planar receptor with a protective wax coating, said apparatus comprising: (a) a source of crystalline, organic wax; and

(b) a nip formed between a pair of rollers adapted to receive a planar receptor having an ink-bearing surface and apply a controlled amount of molten wax to the ink-bearing surface of the receptor, wherein said rollers rotate in opposite directions.

2. An apparatus according to claim 1 wherein said pair of rollers comprises a drive roller and a metering roller, and said source of wax comprises a heatable reservoir into which a wax sample is placed, wherein said metering roller and said reservoir are positioned relative to each other such that molten wax is delivered from said reservoir to said metering roller for application to the ink-bearing surface of a planar receptor at said nip.

3. An apparatus according to claim 1 wherein said pair of rollers comprises a drive roller and a metering roller, and said source of wax comprises a wax application roller that includes a core and a wax layer surrounding said core, wherein said metering roller and said wax application roller are positioned relative to each other such that molten wax is delivered from said wax application roller to said metering roller for application to the ink-bearing surface of the receptor at said nip.

4. An apparatus according to claim 1 wherein said pair of rollers comprises a drive roller and a metering roller, and said source of wax comprises a wax layer surrounding said metering roller.

5. An apparatus according to claims 1-4 wherein said wax has a melting temperature ranging from about 38-70°C.

6. An apparatus according to claims 1-5 wherein said wax comprises an organosilicone wax.

7. An apparatus according to claim 6 wherein said organosilicone wax comprises a silicone backbone to which are covalently bonded side chain crystallizable, aliphatic, hydrocarbon chains having at least 14 carbon atoms.

8. A method for providing an ink-bearing surface of a planar receptor with a protective wax coating comprising:

(a) heating a crystalline organic wax to a temperature at or above its melting temperature to form a molten wax composition; (b) applying said molten wax composition to an ink-bearing surface of a planar receptor to form a molten wax coating overlying said ink- bearing surface at a coat weight of between about 0.8 and 7 g/m² of said ink- bearing surface; and

(c) cooling said coating to form a protective wax coating overlying said ink-bearing surface.

9. A method according to claim 8 comprising applying said molten wax composition under conditions selected to create ribbing instabilities in the wax coating.

10. A method according to claim 8 comprising applying said molten wax composition under conditions selected to create Marangoni instabilities in the wax coating.

11. A graft copolymer organosol composition comprising a graft (co) polymeric steric stabilizer covalently bonded to a thermoplastic (co) polymeric core, wherein said graft (co) polymeric steric stabilizer comprises a side-chain or main-chain crystallizing polymeric moiety comprising behenyl acrylate.

12. A graft copolymer organosol composition comprising a graft (co) polymeric steric stabilizer covalently bonded to a thermoplastic (co) polymeric core, wherein said graft (co) polymeric steric stabilizer comprises a silicone backbone to which are covalently bonded side chain crystallizable, aliphatic, hydrocarbon chains that independently and reversibly crystallize at a temperature greater than or equal to 22°C.

13. A composition according to claims 11-12 wherein said graft (co) polymeric steric stabilizer comprises a (behenyl acrylate)-(hydroxyethyl methacrylate)-(meta-dimethyl isopropenyl benzyl isocyanate) terpolymer and said core comprises an ethyl acrylate-methyl methacrylate copolymer.

14. A composition according to claims 11-12 wherein said graft (co) polymeric steric stabilizer comprises an organosilicone wax-(meta-dimethyl isopropenyl benzyl isocyanate) copolymer in which said organosilicone wax comprises an amino-functional silicone backbone to which are covalently bonded side chain crystallizable, aliphatic, hydrocarbon chains having at least 14 carbon atoms.