CA1338134C

CA1338134C - Transparent coatings for graphics application

Info

Publication number: CA1338134C
Application number: CA000612080A
Authority: CA
Inventors: Armin James Paff; Mohammad Iqbal
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1988-10-21
Filing date: 1989-09-20
Publication date: 1996-03-12
Anticipated expiration: 2013-03-12
Also published as: EP0365307A3; AU630110B2; DE68917075D1; DE68917075T2; JP2987158B2; AU4175489A; US4935307A; EP0365307A2; JPH02208366A; EP0365307B1; KR900006459A

Abstract

This invention relates to transparent films for use in graphics applications, such as, for example, visual transparencies for overhead projectors.
One disadvantage of presently available ink-receptive layers for transparent films for pen plotters and ink jet printers is that their high level of sorption capacity leads to poor dimensional stability, which, in turn, leads to curling of the film whenever moisture content changes, either due to imaging, or due simply to changes in humidity or to drying out due to the heat encountered on the stage of an overhead projector.
Curl can be reduced by applying the same image receptive layer to both sides of the film, so that any expansion or contraction will be applied symmetrically to both sides of the film. This adds to manufacturing cost, and still may not completely eliminate curl, especially in cases where moisture content differs on opposite sides of the film.
Accordingly, it would be desirable to provide an ink-receptive layer for transparent films that has good dimensional stability.
This invention provides a hydrophilic, polymeric blend which provides improved durability and reduced curl when used as an image-receptive layer on graphic arts films. The blend comprises at least one water-absorbing, hydrophilic polymeric material, at least one hydrophobic polymeric material having acid functionality, and at least one polyethylene glycol. The layer formed from the blend provides high sorption capacity combined with good physical integrity even when wet, along with dimensional stability.
The invention also provides sheets suitable for preparing visual transparencies having a backing bearing on at least one major surface thereof an image-receptive layer formed from the aforementioned blend.

Description

133813~
TRANSPARENT COATINGS
FOR GRAPHICS APPLICATIONS

Background of the Invention It is well-known that transparencies for use with overhead projectors can be produced by imagewise deposition of liquid ink of various colors onto thin, flexible, transparent polymeric substrates. Such imagewise ink deposition can be performed by such apparatus as pen plotters and ink jet printers of the type normally used for applying images to paper. In the imaging of paper by such ink deposition methods, the porous nature of the paper is an important factor in the drying of the ink, as well as in the final image quality. In contrast to paper, however, the non-porous nature of transparent, polymeric substrates makes depositing of ink from pen plotters or ink jet printers thereon very difficult, with the result that poor quality images of low durability are formed.
A common characteristic of inks used in pen plotters and ink jet printers is that they are exposed to open air, prior to imaging, for long periods of time. Yet, even after long term exposure to open air, the ink must function in a satisfactory manner, without deterioration, particularly without loss of solvent. In order to meet this requirement, ink formulations typically utilize solvents of very low volatility, such as water, ethylene glycol, propylene glycol, and other like solvents. Because of the low volatility of the solvents in these ink formulations, solvent evaporation as a mechanism for drying the ink after imaging is very limited.
In the case of printing on paper by means of ink jet printers, images are composed of small dots, with the spacing between the dots being of such a magnitude as to allow lateral diffusion of the ink on the substrate to just fill areas between adjacent dots. In the case of printing ` -2- 133813~
on film by means of ink jet printers, the situation is quite different, with little or no ink spreading, and with a tendency toward irregular beading up of the ink into droplets of various sizes, with the droplet sizes being unrelated to the intended dot size. When such beading occurs, image quality is diminished.
Problems of ink beading and failure to dry arise because most transparent films suitable for graphics applications, in addition to being non-porous, are also, in many cases, hydrophobic. Imageability of transparent, polymeric film with inks containing solvents of low volatility has been improved by applying hydrophilic, water absorbent, ink-receptive layers to transparent, polymeric film substrates. In addition to absorbing the low volatility solvents found in ink formulations, these layers must exhibit ink-receptivity, durability, non-tackiness, and the image quality required of commonly accepted graphic arts materials.
It is desirable that water-absorbing polymeric compositions retain some degree of physical durability, dryness to the touch, and non-tackiness even after absorbing significant amounts of water or aqueous ink, as would occur in the imaging process. secause polymeric materials have a strong tendency to be softened or even dissolved by the absorption of liquids, the absorption of significant quantities~of liquids is inconsistent with retention of physical and handling properties desired in graphic arts materials. Attempts at resolving these apparently conflicting goals, while at the same time meeting the additional requirement of retaining transparency, have often involved the use of polymer blends.
In U.S. Patent No. 4,503,111, coatings comprising blends of polyvinyl pyrrolidone and either polyvinyl alcohol or gelatin are disclosed. The materials disclosed in U.S.
Patent No. 4,503,111 exhibit good ink-absorption when used in ink jet printers, but tend to be tacky in imaged areas, and at high humidity, exhibit tackiness even in non-imaged - 13~813~

areas.
Another undeslrable feature of the polymer blends dlsclosed ln that patent ls dlmenslonal sensltlvlty to molsture content, resultlng ln a tendency for the fllm to curl when sub~ected to heatlng or changes ln humldlty.
Polymerlc blends that are less tacky and molsture-sensltlve, but whlch stlll maintaln water absorptlvlty, are dlsclosed ln U.S. Patent Nos. 4,300,820 and 4,369,229, whereln blends of hydrophlllc polymers wlth terpolymers havlng hydrophoblc and hydrophillc parts, along wlth acld functlonalitles, are dlsclosed. Although these patents do not dlsclose lnk-receptlve coatlngs, the polymerlc blends dlsclosed thereln have been found useful as water absorbent layers capable of retalnlng a degree of durablllty and non-tacklness even after absorblng slgnlflcant quantltles of water. However, U.S. Patent Nos. 4,300,820 and 4,369,229 do not mentlon elther graphlc arts materlals or the varlous propertles characterlzlng lmage quallty whlch are lmportant ln such materlals.
European Patent Appllcatlon EP 0 233 703 (publlshed August 26, 1987), dlscloses blends of acryllc polymers wlth polyvlnyl-pyrrolldone (PVP) for use ln lnk-receptlve layers, whereln compatlbillty between the hydrophlllc PVP and the hydrophoblc acryllc polymer ls achleved by lncorporatlng carboxyllc acld groups lnto the acrylic polymer. It does not appear posslble to ad~ust the formulatlons dlsclosed thereln to slmultaneously provlde adequate drylng, low tack, and X

813~
- 3a -acceptable lateral mlgratlon of lnk ln many of the commerclally avallable lnk ~et prlnters.
In addltlon to pre-lmaglng optlcal requlrements, such as clarlty, polymerlc blends used ln transparent lnk-receptlve layers must also exhlblt satlsfactory post-lmaglng quallty. It ls deslrable that the lnk dry, tack-free, ln a reasonably short tlme. A typlcal requlrement for drylng may be that the lmaged sheet be sufflclently dry and tack-free to allow stacklng ln the tlme requlred to lmage the next sheet.
Addltlonally, conslderable spreadlng of the lnk dots is desirable to achieve adequate color density. At the same time, any spreading of ink dots should be adequately controlled so as to produce sharp, smooth lines of demarcation between contrasting colors. It has been found 5 that this combination of features is not readily attainable in a single formulation. ~or this reason, multi-layer coatings have been found useful.
U.S. Patent Nos. 4,225,652, 4,301,195, and 4,379,804, disclose ink-receptive materials having multiple 10 layers, with the first, or underlayer, being highly ink-absorbent, and the second, or topcoat, layer, being ink-permeable, while at the same time being quite durable, even after the application of ink. Because the topcoat layer need not retain any significant amount of ink, but merely 15 allow ink to pass through to the underlayer, many of the softening effects resulting from high levels of solvent retention can be avoided in the topcoat layer. Further, since the underlayer, which is softened by the absorption of ink, is protected by the topcoat layer, the overall image is 20 more durable and less tacky. It has been found, however, that when highly water-absorbent materials, such as, for example, polyvinyl pyrrolidone, are used for the ink-absorbent underlayer, the degree of softening which occurs is sufficient to cause severe reduction in the 25 durability of the overall imaged layer, and that topcoating is not sufficient to overcome this reduction in durability.
When materials which are less susceptible to softening due to ink-absorption are used for the underlayer, the ink-absorbing ability of the system is likely to be unacceptably 30 low.
Another disadvantage of presently available ink-receptive layers is that their high level of sorption capacity leads to poor dimensional stability, which, in turn, leads to curling of the film whenever moisture content 35 changes, either due to imaging, or due simply to changes in humidity or to drying out due to the heat encountered on the stage of the overhead projector.

133813~

Curl can be reduced by applylng the same lmage receptlve layer to both sldes of the fllm backlng, so that any expanslon or contractlon ls applled symmetrlcally to both sldes of the sheet.
Thls adds to manufacturlng cost, and stlll may not completely ellmlnate curl, especlally ln cases where molsture content dlffers on opposlte sldes of the fllm.
Other methods of lmpartlng dlmenslonal stablllty to molsture sensltlve materlals lnclude chemlcal cross-llnklng, as ls done ln photographlc gelatin coatlngs, and permanent swelllng, as occurs when humectants are added to wood, paper, or other flbrous celluloslc materlals. It ls well-known, however, that chemlcal cross-llnklng reduces molsture absorptlon, and ls therefore not a deslrable method for stablllzlng lnk-receptlve coatlngs.
In the case of lnk absorblng layers, however, the addl-tlon of humectants to provlde permanent swelllng due to molsture saturatlon ls not an acceptable means for lmpartlng dlmenslonal stablllty, slnce such saturatlon greatly reduces the ablllty of such a layer to absorb aqueous solvent.
The present lnventlon provldes lmproved materlals for use ln transparent lnk-receptlve layers. The lmprovements dls-closed hereln result ln faster drylng, lower tack of lmaged sheets and less curl of the sheets.
summarY of the Inventlon The lnventlon provldes composltlons for preparlng dlmen-slonally stable, lnk-receptlve layers for transparent graphlcal materlals, sultable for lmaglng by lnk ~et prlnters. Accordlng to 13~8134 5a 60557-3740 the present lnventlon there ls provlded a transparent sheet sult-able for maklng vlsual transparencles comprlslng a thln, trans-parent backlng bearlng on at least one ma~or surface thereof an lnk-receptlve layer comprlslng a transparent composltlon comprlslng a blend of (a) at least one water-absorblng, hydro-phlllc, polymerlc materlal, (b) at least one hydrophoblc polymerlc materlal havlng acld functlonallty, and (c) at least one polyethy-lene glycol, sald composltlon havlng a haze value equal to or less than about 15%, as measured ln accordance wlth ASTM D1003-61 ~Reapproved 1977).

-E~

133813~'1 In a preferred embodlment, the lnk-receptlve layer ls overcoated wlth an lnk-permeable protectlve layer, whlch protectlve layer preferably contalns a partlculate materlal, e.g. starch.
The transparent sheet of thls lnventlon exhlblts good dry tlme, good tack tlme, good lmage denslty, and a low level of fllm curl. Accordlngly, the sheet ls excellent for preparlng transparencles for overhead pro~ectlon by means of lnk ~et prlnters.
Brlef Descrlptlon of the Drawlnq The drawlng ls a cross-sectlonal vlew of a sheet sultable for vlsual transparencles accordlng to the present lnventlon.
Detalled DescrlPtlon of the Inventlon Referrlng now to the drawlng, a sheet 10 sultable for preparlng vlsual transparencles comprlses a backlng 12 bearlng on at least one ma~or surface thereof an lnk-receptlve layer 14. In the preferred embodlment, an lnk-permeable protectlve layer 16 overlles lnk-receptlve layer 14.
Contalned ln protectlve layer 16 ls partlculate materlal 18.
Ink-receptive layer 14 comprlses a polymerlc blend havlng as component (a) at least one water absorblng, hydrophlllc, polymerlc materlal, (b) at least one hydrophoblc polymerlc materlal lncorporatlng acld functlonal groups, and (c) at least one polyethylene glycol.
The term `~hydrophlllc", as used hereln, ls used to descrlbe a materlal that ls generally receptlve to water, elther ln the sense that lts surface ls wettable by water or 133~13i - 6a -ln the sense that the bulk of the materlal ls able to absorb slgnlflcant quantltles of water. More speclflcally, materlals that exhlblt surface wettablllty by water are sald to have hydrophlllc surfaces, whlle materlals that have surfaces not wettable by water wlll be sald to have ~ 7_ 133813~

hydrophobic surfaces. The term "water-absorbing hydrophilic materials' as used herein, is used to describe materials that are capable of absorbing significant quantities of water, including those that are water-soluble. When molecular structures are being discussed, monomeric units will be referred to as hydrophilic units if they have a water sorption capacity of at least one mole of water per mole of monomeric unit. Sorption capacities of various monomeric units are given, for example, in D.W. Van Krevelin, with the collaboration of P. J. Hoftyzer, Properties Of Polymers: Correlations With Chemical Structure, Elsevier Publishing Company (Amsterdam, London, New York: 1972) pages 294-296. Monomeric units will be referred to as hydrophobic if they form water-insoluble polymers capable of absorbing only small amounts of water when polymerized by themselves.
In cases where the blend is to be prepared by forming a solution containing components (a), (b), and (c), it is further desired that all three of components (a), (b), and (c) be soluble in a single solvent or blend of solvents.
Solvents or blends of solvents useful for this purpose will hereinafter be called common solvents for the blend of components (a), (b), and (c). Solutions prepared by dissolving components (a), (b), and (c) in a common solvent will hereinafter be called solution blends of components (a), (b), and (c). Eecause the compositions for preparing ink-receptive layer 14 are preferably applied to backing 12 as liquid coatings that are subsequently dried to form ink-receptive layer 14, it is desired to form solution blends of components (a), (b), and (c).
Component (a), the water-absorbing hydrophilic polymeric material of ink-receptive layer 14, comprises polymerized combinations of monomeric units selected from the following:

(1) vinyl lactams having the repeat group structure:

133813~

, (CH2 ),~

CH2~ ~ C=O

~CH2-C

wherein n can be 2 or 3;

(2) alkyl tertiary amino alkyl acrylates or methacrylates having the structure:

~ CH2-C ~

o~ o ( CH2 ) m ~N\

wherein m can be 1 or 2, Rl represents hydrogen or -CH3, R2 represents hydrogen or an alkyl qroup having up to 10 carbon atoms, preferably 1 to 4 carbon atoms, and R3 represents an alkyl group having up to 10 carbon atoms, preferably 1 to 4 carbon atoms;

(3) alkyl quaternary amino alkyl acrylates or methacrylates having the structure:

Rl tCH2 -C ~
C--O

(CH2)p R2_ IN - R X

` -9- 133813~

wherein p can be 1 or 2, Rl, R2, and R3 are as defined above, and R4 represents an alkyl group having up to 10 carbon atoms, preferably 1 to 4 carbon atoms, and X~
represents a halide ion, R25O4-, R3So4-~ or R4 S04-;

(4) 2-vinylpyridine; and (5) 4-vinylpyridine.

Polymerization of the foregoing monomers to form component (a) can be conducted by free-radical techniques, with time, temperature, proportions of monomeric units, and other conditions of polymerization adjusted to obtain the desired properties of the final product. Component (a) can be a homopolymer or a copolymer.
Component (b), the hydrophobic polymeric material of ink-receptive layer 14, is preferably derived from combinations of acrylic or other hydrophobic ethylenically unsaturated monomeric units copolymerized with monomeric units having acid functionality. Hydrophobic monomeric units suitable for preparing component (b) have the following characteristics:
(i) they would form water-insoluble polymers if polymerized by themselves;
(ii) they contain no pendant alkyl groups having more than 10 carbon atoms, preferably no more than 4 carbon atoms;
and (iii) they are capable of being copolymerized with at least one species of acid-functional monomeric unit.
The hydrophobic monomeric units suitable for preparing component (b) are preferably selected from the following:

(1) acrylates and methacrylates having the structure -lo- 1338131 ~CH2 C ~
~--o R

wherein R5 represents hydrogen or -CH3, and R6 represents an alkyl group having up to 4 carbon atoms, preferably 1 or 2 carbon atoms, a cycloaliphatic group having up to 9 carbon atoms in the cyclic ring, such as, for example, isobornyl, a substituted or unsubstituted aromatic group such as, for example, benzyl, and an oxygen-containing heterocyclic group having up to 10 carbon atoms, such as, for example, tetrahydrofurfuryl;

(2) acrylonitrile;

(3) styrene or -methylstyrene; and (4) vinyl acetate.

If R6 is an aromatic group, it preferably contains only one aromatic ring. If R6 is a substituted aromatic group, the substituents are preferably alkyl groups or substituted alkyl groups, preferably having up to four carbon atoms. Preferred substituents for the alkyl groups are halogens, such as, for example, bromine or chlorine.
Acid functional monomeric units suitable for preparing component (b) are preferably selected from acrylic acid and methacrylic acid, but other carboxylic acids capable of being copolymerized with the hydrophobic monomeric units described previously can also be used.
When acrylic or methacrylic acid is used, the weight fraction of these acid units in the polymer chain can range from about 2% to about 20%, with the weight fraction of hydrophobic monomeric units ranging from about 98% to about 80%, with the preferred weight fraction for the acid functional groups ranging from 5% to 12%, with the l- 1338134 preferred weight fraction for hydrophobic monomeric units ranging from 95% to 88%.
It is preferred that the glass transition temperature of component (b) be above about 20C, and that the solubility properties of component (b) be such that a solution blend of components (a), (b), and (c) can be prepared in the same solvent or blend of solvents.
The polymeric materials comprising component (b) can be prepared by emulsion or solution polymerization of the foregoing monomers, using, for example, a free-radical initiator, and conventional means of extraction and drying.
Component (b) can be a homopolymer or a copolymer.
Component (c), the polyethylene glycol of ink-receptive layer 14, is included for the purpose of curl reduction. It has been found that lower molecular weight polyethylene glycols are more effective than higher molecular weight polyethylene glycols for reducing curl and maintaining a low level of haze. For example, it was found that polyethylene glycol having a molecular weight of about 600 reduces curl to an acceptable level when present in an amount of only about 10% of the concentration of component (a) (when coated as a solution and dried for two minutes at 85C) whereas when polyethylene glycol having a molecular weight of about 4000 is used in the same polymeric blend and coated and dried under the same conditions, it must be added in the amount of 20% of the concentration of component (a) and that this higher amount of high molecular weight polyethylene glycol brings about noticeable haze in layer 14. Accordingly, it is preferred that the polyethylene glycol have a molecular weight of less than 4000. If the drying conditions are more severe, more polyethylene glycol may be needed to reduce curl.
The composition for forming ink-receptive layer 14 can be prepared by dissolving components (a), (b), and (c) in a common solvent, in the appropriate proportions.
Well-known methods for selecting a common solvent often make use of Hansen parameters, which are quantities that characterlze the lndlvldual contrlbutlons to coheslve energy denslty made by the lntermolecular dlspersion forces, dlpole forces, and hydrogen bondlng forces of a partlcular compound.
Soluble solld materlals tend to be more readlly dlssolved by llqulds havlng Hansen parameters wlthln a speclfled range of thelr own, thls range belng called the solublllty envelope, and less readlly dlssolved by llqulds havlng Hansen parameters outslde of the solublllty envelope. Because of thls tendency, Hansen parameters can be used as a basls for selectlng slngle solvents or for formulatlng solvent blends capable of dlssolvlng a partlcular solld materlal or comblnatlon of solld materlals.
Experlmentally determlned Hansen parameters and solublllty envelopes for a varlety of solld materlals, along wlth Hansen parameters for many commonly used solvents, as well as formulas for mathematlcally estlmatlng Hansen parameters of materlals not llsted, can be found ln Barton, A.F.M. CRC Handbook of Solublllty Parameters and Other Coheslon Parameters, CRC Press, Inc., (Boca Raton: 1983).
Systematlc methods for utlllzlng Hansen parameters to compute solvent blend formulatlon capable of dlssolvlng partlcular comblnatlons of solld materlals ln more compllcated systems may be found ln Teas, J.P., "Solublllty Parameters", Treatlse on Coatlngs, R.R. Myers and J.S. Long, ed., Vol. 2, Part II, Marcel Dekker (New York: 1976), p. 413-448. Because such computatlonal methods often result ln more than one posslble solvent formulatlon, addltlonal experlmental evaluatlon may be needed to arrlve at an optlmal formulatlon.

1~38134 - 12a -Addltlonal constraints upon the cholce of solvents may arlse from practlcal requlrements lnvolvlng coating, drylng, toxlclty, and other conslderatlons, as would be expected by one of ordlnary sklll ln the art.
In the case of polymerlc blends, polymers are _ -13- 13381~

generally said to be compatible if the composition resulting from the blending process appears homogeneous, the test for and definition of homogeneity depending upon the intended application of the blend. In the case of a transparent ink-receptive layer comprising polymeric blends, polymers forming the blend are said to be compatible if the resulting layer is optically clear, with little or no haze, e.g. equal to or less than about 15%, as measured in accordance with ASTM D1003-61 (Reapproved 1977) and if images printed thereon appear uniform, without the appearance of undesirable patterns due to inhomogeneities in the ink-receptive layer.
Combinations of polymeric materials that are compatible upon blending include those that are miscible in one another due to a favorable free energy of mixing, as well as those which form association complexes, by, for example, hydrogen bonding between the different polymers.
In these cases, compatibility means the absence of domains of differing refractive indices of sufficient magnitude to cause visible haze. When polymers having refractive indices which are of nearly equal value are blended, the presence of separate domains may not manifest itself as haze, because if the refractive indices of the two polymers are nearly equal, the variations in refractive index due to inhomogeneity will be low. After imaging, however, the presence of separate, phase-separated domains may manifest itself by uneven edges, non-uniform lateral ink migration, or non-uniform image appearance in areas of solid color.
Solution blends of components (a), (b), and (c) can be conveniently applied to backing by conventional coating techniques, such as a Mayer bar coating, knife coating, reverse roll coating, rotogravure coating, or other suitable coating methods. Drying of the coated solution is preferably conducted with heated air.
In embodiments of the present invention wherein an ink-permeable protective layer 16 is applied, the preferred material for layer 16 is polyvinyl alcohol.

13~813-~

Layer 16 can also lnclude partlculate materlal for the purpose of lmprovlng handllng and flexlblllty, the preferred partlculate materlal belng starch. Other materlals sultable for layer 16 of thls lnventlon are dlsclosed ln U.S. Patent No. 4,225,652, U.S. Patent No. 4,301,195, and ln U.S. Patent No. 4,379,804.
The composltlon for formlng the protectlve layer 16 ls preferably prepared by dlsperslng flnely dlvlded polyvlnyl alcohol ln cold water, agltatlng the dlsperslon rlgorously, and then gradually heatlng the dlsperslon by an external source or by direct in~ection of steam, as described, for example, in Monsanto Technical Bulletin No. 6082F, incorporated herein by reference. After cooling the dispersion to room temperature, particulate material 18, e.g.
starch, can be added to the dispersion with mixing. Mixing can be conducted with a conventional propeller type power-driven mixing apparatus. The preferred mean particle diameter for particulate material 18 is in the range of 5 to 25 micrometers, provided that at least 25% of the particles have a diameter of 15 micrometers or greater. The amount of particulate material 18 that can be present ls llmited by the requirement that the flnal coating be transparent, with a haze level of equal to or less than about 15~, as measured according to ASTM D1003-61 (Reapproved 1977). Starch can be elther unmodlfied natural starch or modified starch, as described in Modified Starches: Properties and Uses, O. B.
Wurzburg, ed., CRC Press, Inc. (Boca Raton: 1986) provided 133813~

- 14a -that the partlcles remain lnsoluble ln the coating solutlon at the temperatures used ln the coatlng process.
Addltlves can be lncorporated lnto the coatlng solutlon for layer 16 for the purpose of lmprovlng coatablllty and other processlng features. In partlcular, thlckeners, such as xanthan gum, can be added to the solutlon to ald ln the dlsperslon of the partlculate materlal and to lmprove coatablllty.

133813~
Protective layer 16 can be applied in solution form by means of such coating methods as Mayer bar coating, knife coating, reverse roll coating, rotogravure coating, or other suitable coating means. Drying of the coated solution is preferably conducted by heated air.
Suitable materials for backing 12 are preferably transparent and flexible. Suitable polymeric materials include polyethylene terephthalate (PET), cellulose acetates, polystyrene, polyethylene, polypropylene, polycarbonate, and other like materials, with PET being preferred. It is preferred that backing be provided as a film having a caliper ranging from about 50 micrometers to about 125 micrometers. Film backings having a caliper less than 50 micrometers are difficult to handle in the manner customary for graphical materials, and film backings having calipers over 125 micrometers are so stiff that they present feeding difficulties in some commercially available ink jet printing machines.
In order to improve adhesion of ink-receptive layer 14 to backing 12, a priming layer (not shown) can be interposed between these two layers. It is preferred that the material of priming layer not be sensitive to water.
Alternatively, a surface modification, such as corona treatment or plasma treatment, can be applied to one or both major surfaces of backing 12, for the purpose of improving adhesion of ink-receptive 14 layer to backing 12.
Sheets 10 of the present invention are typically tested for the following characteristics: dry time, tack time, image density, and film curl. Methods of testing for these characteristics are described below.

Dry Time An image printed by an ink jet printer may be considered to be dry when, upon pressing a sheet of bond paper against the imaged area, ink does not transfer to the paper. Samples for testing of the time required for drying (dry time) may be prepared by first preconditioning the _ -16- 1338131 samples to be tested by placing them in an environment having a temperature of 21C and 50% relative humidity for 24 hours, then printing onto the sample, by means of a commercially available color ink jet printer, a test pattern that includes large areas of solid color as well as narrower lines, in all of the colors printable by the printer. A suitable test for dry time may then be performed by placing the imaged sample on a flat surface large enough to completely support it, placing a 0.5 inch wide strip of bond paper in contact with the area to be tested, such that the paper contacts areas containing all of the colors printed by the printer, and pressing the strip into intimate contact with the imaged sheet by means of a rubber roller having a width of about 6 inches, a diameter of about 2 inches, a Shore hardness of about 50 on the durometer A scale, and weight of about 600 to 700 grams. After one pass of the roll, the strip of paper is removed and examined for ink transfer. If no ink has transferred to the paper, the image is considered to be dry. If ink transfer to the paper is detected, the test is repeated 60 seconds later, and at subsequent 60 second intervals until no transfer is detected. Dry time is reported as the total time interval after imaging at which no ink transfer to the paper is detected. If ink transfer is still detected 5 minutes after imaging, the test may be terminated and the dry time simply reported as greater than 5 minutes, since dry times of more than 5 minutes are unacceptable.

Tack Time Tack time, as used herein, is a measure of the time required for the image to dry to a point where unprimed polyethylene terephthalate (PET) film, upon being pressed against the imaged area, will no longer adhere to it. It has been found that the test for tack time may be applied to the sample prepared and tested for dry time described hereinabove. After dry time has been determined, 1~3813 1 _ -17-a strip of 100 micrometer caliper unprimed PET film having dimensions of 3 inch X 9 inch is placed in contact with the imaged surface. The PET strip is pressed into intimate contact with the film by means of the roller described previously. If the resulting adhesion of the test strip to the image surface is sufficiently low to allow the test strip to be lifted from the imaged sheet without having to physically hold down the imaged sheet, the tack is considered to be acceptably low. The tack test is performed at regular time intervals after completion of the dry time test, and tack time is reported as the total time interval from the time of imaging to the time at which tack becomes acceptably low.

Image Density Image density can be measured by means of the Macbeth TD 903 densitometer, or equivalent instrument, using the status A filters, as is common practice when testing color positive films. Samples may be prepared for testing by first preconditioning at 21C and 50% relative humidity for 24 hours, then imaging solid areas of cyan, magenta, yellow, and black. Densities are measured using the green filter for the magenta area, the red filter for the cyan area, the blue filter for the yellow area, and the red filter for the black area, according to the procedure given in the instructions accompanying the densitometer.
Because image density has been found to change with time, densitometer readings are normally taken at times of 10 minutes, 30 minutes, and 24 hours after imaging.

Film Curl Film curl is commonly measured by preparing a sample of the film to be tested having dimensions of 8.5 inch X 11 inch and placing this sample upon a flat surface of a size sufficiently large to completely support the film sample, in such a way that when curl occurs, the edges of the film lift up. Curl, measured in millimeters, is then deflned to be the maxlmum helght above the flat surface reached by any polnt along the perlphery of the fllm as a result of curllng. Thls test ls commonly run at 21C and a relatlve humldlty of 50%.
Other aspects of lmage quallty may be evaluated by prlntlng approprlate test patterns on the sample sheet belng tested and ~udglng such propertles as lmage unlformlty, bleedlng of one color lnto a sharply deflned area of another color, flngerprlntlng, and marklng or blemlshlng produced by the lmaging process ltself or by the normal handllng encountered ln the use of graphlc materlals. Such ~udgments can be made by dlrect vlewlng or by vlewlng ln a pro~ectlon mode, by means, for example, of an overhead pro~ector.
The followlng, non-llmltlng examples further lllustrate the present lnventlon.
Bxample 1 Component (b), the hydrophoblc polymerlc materlal for lnk-receptlve layer 14, was prepared by comblnlng 91 parts by welght of methyl methacrylate, 9 parts by welght of acryllc acld, 0.15 part by welght of azo-lsobutyro-nltrlte ("Vazo"~, avallable from E. I. DuPont de Nemours & Company), and 200 parts by welght of ethyl acetate ln a one-plnt brown bottle.
The mlxture was purged wlth dry nltrogen gas for flve mlnutes and then lmmersed ln a constant temperature bath malntalned at 60C for 18 to 24 hours. The resultlng methyl methacrylate/acryllc acld copolymer was coagulated ln methanol, flltered, and drled ln a vacuum oven at 50C for 18 X

hours. Comparlson of the flnal polymer welght wlth the welght of the startlng materlals showed a yleld of better than 97%.
A coatlng solutlon for the preparatlon of lnk-receptlve layer 14 was prepared by comblnlng 270 parts by welght of ethanol and 630 parts by welght of ethyl acetate, to whlch was added, wlth agltatlon at room temperature, 65 parts by welght of component (a), l.e. poly-N-vlnyl pyrrolldone (K90~, avallable from GAF Chemlcals Corporatlon), 6.25 parts by welght of component (c), l.e. polyethylene glycol (~Carbowax~ 600", avallable from Union Carbide Corporatlon), and 28.15 parts of component (b), prepared as described previously. Agitation was contlnued untll a clear solution was obtained.
A layer of this solutlon havlng 75 mlcrometer wet thlckness was coated onto a sheet of polyvlnylldene chlorlde primed polyethylene terephthalate ("Scotchpar"D, available from Minnesota Mining and Manufacturing Company) having a caliper of 100 micrometers by means of a knife coater. Drylng of the layer was conducted by means of heated air at a temperature of 85C for two minutes. Samples of coated film havlng dlmenslons 8.5 lnches X 11 lnches were cut from thls sheet and condltloned at 21C and 50% relatlve humldlty for 24 hours.
The condltloned sheets were lmaged wlth a Hewlett-Packard Palnt~et lnk ~et prlnter, uslng the standard cyan, magenta, yellow, and black lnk supplled therewlth, and the lmage evaluatlon test pattern descrlbed prevlously. The lmaged sheets were tested for dry tlme, tack tlme, lmage denslty, fllm curl, flngerprlntlng, lmage unlformlty, and lmage bleed. Fllm curl was found to be zero, and drylng was nearly complete after 60 seconds. However, the lmaged sheet remalned tacky even after 24 hours, and was easlly flngerprlnted. Image denslty was wlthln the acceptable range, and lmage unlformlty was excellent.
ComParatlve ~xamPle A
A coatlng solutlon for the preparatlon of lnk-receptlve layer 14 was prepared as ln Example 1, but wlthout polyethylene glycol. A sample of coated fllm was prepared wlth thls polyethylene glycol-free coatlng solutlon, accordlng to the procedure of Example 1. Measurement of fllm curl showed the hlghest polnt on the fllm to be elevated 15 mllllmeters above the flat surface. Thls was consldered to be a very severe curl and the fllm would not be acceptable for use as a vlsual transparency for overhead pro~ectlon. Thls comparatlve example lllustrates the effectlveness of polyethylene glycol ln the preventlon of curl ln coated fllms.
~xamPle 2 A coatlng solutlon for protectlve layer 16 for a vlsual transparency accordlng to the present lnventlon was prepared by dlsperslng 2.9 parts by welght of polyvlnyl alcohol ("Vlnol~ 540", available from Alr Products and Chemlcals) ln about 40.0 parts by welght of room temperature delonlzed water, and then bubbllng steam through thls mlxture untll a clear solutlon was obtalned. Addltlonal room temperature delonlzed water as needed was then added to the solutlon to brlng the total water content up to 65.8 parts by - 13~13~

- 20a -welght. Thls solutlon was further dlluted by the addltlon of 30.8 parts by welght of ethanol. Flnally, 0.15 part by weight of xanthan gum thlckener ("Keltrol ~ TF", avallable from Kelco Dlvlslon of Merck and Company) and 0.40 part by welght of catlonlc starch ("Lok Slze~ 30", avallable from A.E. Staley Manufacturlng Company) were added and mlxed untll the xanthan gum was dlssolved and the starch was unlformly dlspersed.
Thls solutlon was applled over lnk-receptlve layer 14 of sheet 10 prepared accordlng to Example 1, so as to form an lnk-permeable protectlve layer 16 over ink-receptlve layer 14 formed accordlng to Example 1. Appllcatlon of protectlve layer 16 was carrled out by means of a knlfe coater, ad~usted so as to apply the coatlng at a rate sufflclent to result ln a coatlng welght, after drylng, of 0.05 to 0.07 gram per square foot. Drylng was carrled out by heated alr at a temperature of 85C for two mlnutes. Samples were prepared for evaluatlon by cuttlng the coated fllm lnto sheets havlng dlmenslons of 8.5 lnches X 11 lnches and condltlonlng these sheets at 21C
and 50% relatlve humldlty for 24 hours.

-21- 133813 i The conditioned sheets were imaged using a Hewlett-Packard Paintjet ink jet printer, using the image evaluation test pattern described previously, and tested in the manner described in Example 1. Dry time and tack time were both 60 seconds, which was considered to be excellent.
Slight fingerprinting was noted, but was considered to be within acceptable limits. Image density, image uniformity, and image bleeding were found to be within acceptable limits.
Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims

1. A transparent sheet suitable for making visual transparencies comprising a thin, transparent backing bearing on at least one major surface thereof an ink-receptive layer com-prising a transparent composition comprising a blend of (a) at least one water-absorbing, hydrophilic, polymeric material, (b) at least one hydrophobic polymeric material having acid function-ality, and (c) at least one polyethylene glycol, said composition having a haze value equal to or less than about 15%, as measured in accordance with ASTM D1003-61 (Reapproved 1977).

2. The transparent sheet of claim 1 wherein said water-absorbing polymeric material is derived from monomeric units selected from the group consisting of:
(1) vinyl lactams having the structure wherein n can be 2 or 3;
(2) alkyl tertiary amino alkyl acrylates having the structure wherein m can be 1 or 2, R1 represents hydrogen or -CH3, R2 represents hydrogen or an alkyl group having up to 10 carbon atoms, and R3 represents an alkyl group having up to 10 carbon atoms;
(3) alkyl quaternary alkyl acrylates having the structure wherein p can be 1 or 2, R1, R2 and R3 are as defined above, R4 represents an alkyl group having up to 10 carbon atoms, and X-represents a halide ion, R2SO4-, R3SO4-, or R4SO4-;

(4) 2-vinylpyridine; and (5) 4-vinylpyridine.

3. The transparent sheet of claim 2 wherein R2 represents hydrogen or an alkyl group having up to 4 carbon atoms.

4. The transparent sheet of claim 2 wherein R3 represents an alkyl group having up to 4 carbon atoms.

5. The transparent sheet according to claim 2 wherein R4 represents an alkyl group having up to 4 carbon atoms.

6. The transparent sheet of claim 1 wherein said hydrophobic monomeric units are selected from the group consisting of (1) acrylates and methacrylates having the structure wherein R5 represents hydrogen or -CH3, and R6 represents an alkyl group having up to 4 carbon atoms, a cycloaliphatic group having up to 9 carbon atoms in the cyclic ring, an aromatic group having a single aromatic ring and unsubstituted or substituted by alkyl or haloalkyl having up to 4 carbon atoms, and an oxygen-containing heterocyclic group having up to 10 carbon atoms;
(2) acrylonitrile;
(3) styrene;
(4) .alpha.-methylstyrene; and (5) vinyl acetate.

7. The transparent sheet of claim 6 wherein R6 represents an alkyl group having 1 or 2 carbon atoms.

8. The transparent sheet of claim 6 wherein R6 represents an isobornyl group.

9. The transparent sheet of claim 6 wherein R6 represents a benzyl group.

10. The transparent sheet of claim 6 wherein R6 represents a tetrahydrofurfuryl group.

11. The transparent sheet of claim 1 wherein said at least one hydrophobic polymeric material having acid functionality is a copolymer prepared from hydrophobic monomeric units and acrylic acid or methacrylic acid monomeric units, said acid monomeric units comprising from about 2% by weight to about 20% by weight of said copolymer.

12. The transparent sheet of claim 1 wherein said at least one hydrophobic polymeric material having acid functionality is a copolymer prepared from hydrophobic monomeric units and acrylic acid or methacrylic acid monomeric units, said acid monomeric units comprising from about 5% by weight to about 12% by weight of said copolymer.

13. The transparent sheet of claim 1 wherein said polyethy-lene glycol has a molecular weight of less than 4000.

14. The transparent sheet of claim 1 wherein overlying said ink-receptive layer is an ink-permeable protective layer.

15. The transparent sheet of claim 14 wherein said protec-tive layer contains particulate material.

16. The transparent sheet of claim 14 wherein said partic-ulate material is starch.