Anti-blocking clear ink receiving sheet

The present invention is directed to an ink-receiving sheet having anti-blocking properties, containing (A) a polymer substrate; (B) an ink-receptive coating disposed on at least one layer which having a water-soluble component; and (C) particulates dispersed in the ink-receptive coating, having an average particle size of from 15 um to about 50 um, a particle size span is equal to or smaller than 1.0, and a refractive index of from about 1.2 to about 2.4. The present invention is also directed to the ink receptive coating per se, and to methods of ink jet printing using the above ink-receiving sheet.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an ink receiving sheet and, more 
particularly, to a transparent ink receiving sheet having anti-blocking 
properties for use with ink jet printers. 
2. Description of the Related Arts 
In order to achieve high color density and fidelity during ink jet printing 
on an ink receiving sheet, the laydown of the ink receiving sheet is 
usually high. However, current commercial ink receiving sheets, in 
particular transparent ink receiving sheets, do not allow high ink laydown 
because of blocking between image that is formed on the ink receiving 
sheet and any materials that may come into contact with the image. In 
other words, because of the nature of the ink and the ink receiving sheet, 
ink undesirably transfers from the ink receiving sheet to materials in 
contact with the ink receiving sheet. The blocking has become one of the 
major problems in the field, particularly with high speed ink jet 
printers. 
There have been many attempts to improve anti-blocking performance of ink 
receiving sheets. A number of designs have been proposed for use in 
various ink receiving sheets. Iqbal et al., U.S. Pat. No. 4,935,307, 
discloses an ink permeable protective layer containing a particulate 
material; Desjarlais, U.S. Pat. No. 4,775,594, discloses the use of silica 
as an anti-blocking agent; Light, U.S. Pat. No. 5,084,338, discusses inert 
particles having a particle size of 25 um or less; Bedell, U.S. Pat. No. 
4,547,405, also discusses use of particles such as glass beads in the ink 
receiving sheet. Although these proposals disclose the use of particles, 
none of them have specified three key functional parameters: particle size 
distribution, particle size limitation and refractive index. Desired 
anti-blocking property and clarity only can be achieved when the particle 
size, particle size distribution and refractive index are optimized. When 
the particle size is too small, the particles do not protrude through the 
ink receiving coating and anti-blocking property is poor. When the 
particles are too large, the particles will be projected when the ink 
receiving sheet is used as a transparency for presentation. In addition, 
the difference in refractive indices between the particle and the ink 
receiving coating affects the clarity and projection quality. Obviously, 
the solutions proposed in the prior art do not solve the problems in the 
field. These designs have to compromise anti-blocking properties and 
clarity. As a result, an undesirable compromise must be made between ink 
laydown and anti-blocking property. 
The present invention discloses an optimized design that offers both 
excellent anti-blocking property and high clarity of the ink receiving 
sheet. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a transparent 
ink-receiving sheet which will avoid the blocking problems associated with 
prior art ink receiving sheets, while still maintaining high ink laydown 
and clarity. 
Another object of the present invention is to provide an ink-receptive 
coating for an ink receiving sheet which will impart anti-blocking 
properties without the need for a separate ink-permeable protective 
coating, while still maintaining high ink laydown and good clarity. 
A further object of the present invention is to provide an improved ink jet 
printing process for printing images on transparent ink jet receiving 
sheets, which avoids the problems associated with prior art processes. 
These and other objects and advantages are obtained by the present 
invention, which presents a solution to the need for an anti-blocking 
clear ink receiving sheet. The improvements in anti-blocking property and 
clarity are attained, according to the invention, by using specific 
particulates as a spacer in the ink receiving sheet. 
More particularly, the objects and advantages of the present invention are 
obtained by an ink-receiving sheet having anti-blocking properties, 
comprising (A) a polymer substrate; (B) an ink receptive coating disposed 
on at least one side of the substrate, and comprising at lease one layer 
which comprises a water-soluble component; (C) particulates dispersed in 
said ink receptive coating, having an average particle size of from about 
15 um to about 50 um, preferably from about 20 um to 40 um and a particle 
size span equal to or smaller than 1.0, preferably &lt;0.8, and (D) 
particulates dispersed in said ink receptive coating having a refractive 
index of from about 1.2 to about 2.4, wherein the ink receptive coating 
has a surface through which said particulates are exposed. 
The objects and advantages of the present invention are also obtained by an 
ink receptive coating for an ink receiving sheet, comprising (1) at least 
one layer comprising a water-soluble component; and (2) particulates 
dispersed therein having an average particle size of from 15 um to about 
50 um, a particle size span equal to or smaller than 1.0 and a refractive 
index of from 1.2 to about 2.4, wherein said coating has a surface through 
which the particles are exposed. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while they may indicate preferred embodiments of the invention, are given 
by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
DETAILED DESCRIPTION OF THE INVENTION 
Examples of suitable substrates for the ink receiving sheet include 
transparent plastics, such as poly(ethylene terephthalate), polycarbonate, 
polystyrene, cellulose esters, poly(vinyl acetate), and others. The 
thickness of the substrate is not particularly restricted, but should be 
in the range of about 1.5 to about 10 mils, preferably about 2.0 to about 
5.0 mils. The substrates may be pretreated to enhance adhesion of the 
coatings thereto. The ink receptive coating, Which is disposed on at least 
one side of the polymer substrate, contains at least one layer comprising 
at least one water-soluble component. The ink receptive coating may have a 
single layer structure, or may have multiple layers. When multiple layers 
are present, the particulates can reside in any of these layers, as long 
as the particulates are exposed on the surface of the ink receptive 
coating. 
The ink receptive coating may contain both water-soluble and 
water-insoluble components, as long as the ink receptive coating functions 
to receive ink. Examples of water-soluble components include poly(vinyl 
alcohol), poly(vinyl acetate), poly(vinyl pyrrolidone), poly(acrylic 
acid), cellulose esters, gelatins, proteins, poly(ethylene oxide), 
alginates, poly(ethylene glycol) and water-soluble gums. Examples of 
water-insoluble components include methyl methacrylate, styrene, urethane, 
butadiene, 2-hydroxyethyl acrylate, ethyl acrylate, N-hydroxyethyl 
acylamide, N-hydroxymethyl acrylamide, and ethylene terephthalate. These 
water-soluble and water-insoluble components may be incorporated as the 
component of a homopolymer, a copolymer, or a polymer blend. The coating 
weight of the ink receptive coating may be from about 2 g/m.sup.2 to about 
30 g/m.sup.2 and preferably, from about 4 g/m.sup.2 to about 20 g/m.sup.2. 
The particulates disclosed in this invention have an average particle size 
of from about 15 um to about 50 um, preferably from about 20 um to about 
40 um; a particle size span is equal to or smaller than 1.0, preferably 
&lt;0.8; and a refractive index of from about 1.2 to about 2.4. Examples of 
the particulates include glass beads, poly(methyl methacrylate), 
polystyrene, starch, silica, polyurethane, calcium carbonate and other 
organic and inorganic particles having specified particle size, particle 
size span and refractive index. 
The concentration of the particulates in the ink receiving sheet may be 
from about 0.5% to about 10% (weight percentage based on coating solid 
content), depending on the particle size, the particle size distribution 
and ink laydown. Usually, a low concentration is required when large 
particulates having small particle size span are used. 
The smoothness of the ink receiving sheet disclosed in this invention may 
be from about 200 to about 400 Sheffield units, preferably from about 240 
to about 360 Sheffield units. The haze of the ink receiving sheet is less 
than about 8%. The Sheffield smoothness was measured on Paper Smoothness 
Tester, model 538 (Hagerty Technologies). The haze was measured on Haze 
Guard System, XL-211 (BKY Gardner). The average particle size and the 
particle size distribution were measured on MasterSizer, MS-20 (Malvern 
Instruments). The average particle size is defined by the mean particle 
size or D50. The particle size distribution is expressed by the particle 
size span, which is defined as: 
EQU Particle Size Span=(D90-D10)/(D50) 
where D90 is the 90th percentile diameter, D10 is the 10th percentile 
diameter, and D50 is the 50th percentile diameter. 
When the ink receptive coating is on one side of the substrate, the side of 
the substrate which is not covered with ink receptive coating may be 
attached to a backing material in order to reduce electrostatic charge and 
to reduce sheet-to-sheet friction and sticking. The backing material may 
be either a polymer coating, an ink receptive coating, a polymer film, or 
paper, in accordance with what is known in the art, and is not 
particularly limited. To prevent stacking blocking, the particles 
disclosed in this invention can also be added in the backing materials. 
Any of a number of art recognized coating methods may be employed to coat 
the ink receptive coating onto the polymer substrate, such as roller 
coating, wire-bar coating, dip coating, extrusion coating, air knife 
coating, curtain coating, slide coating, doctor coating, or gravure 
coating. Such techniques are well known in the art.

The following Examples are merely illustrative of the invention and are not 
to be construed as limiting the invention. 
EXAMPLE 1 
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Underlayer PVP-K90.sup.1 12.0 parts 
Copolymer A.sup.2 
7.5 parts 
Particulate I.sup.3 
0.3 parts 
Dowanol PM.sup.4 17.3 parts 
MEK 61.4 parts 
Surface layer 
Hydroxyethyl Cellulose.sup.5 
1.8 parts 
Water 97.7 parts 
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.sup.1 Poly(vinyl pyrrolidone), GAF Corporation. 
.sup.2 A copolymer of methyl methacrylate and hydroxyethyl methacrylate, 
40% solid. 
.sup.3 Glass bead, the average particle size is about 22 um, the particle 
size span is about 0.72 and the refractive index is about 1.65 (from the 
supplier). 
.sup.4 Propylene glycol monomethyl ether, Dow Chemical Corporation. 
.sup.5 Hydroxyethyl cellulose, Union Carbide. 
The underlayer coating was coated on the polyester base using a No. 36 
Meyer rod. After drying the underlayer coating at 120.degree. C. for about 
2 minutes, the surface layer coating was coated using No. 8 Meyer rod 
under the same conditions. The dry coat weight of the ink receptive 
coating is about 10 g/m.sup.2. 
EXAMPLE 2 
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Underlayer PVP-K90 9.6 parts 
Copolymer A 6.0 parts 
Quaternary copolymer.sup.1 
8.6 parts 
Particulate I 0.3 parts 
Dowanol PM 16.3 parts 
MEK 57.7 parts 
Surface layer 
Hydroxyethyl Cellulose 
1.8 parts 
Water 97.7 parts 
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.sup.1 Quaternary copolymer of methyl methacrylate and dimethylaminoethyl 
methacrylate, 35% solid. 
The underlayer coating was coated on the polyester base using a No. 36 
Meyer rod. After drying the underlayer coating at 120.degree. C. for about 
2 minutes, the surface layer coating was coated using No. 8 Meyer rod 
under the same conditions. The dry coat weight of the ink receptive 
coating is about 10 g/m.sup.2. 
EXAMPLE 3 
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Underlayer PVP-K90 12.0 parts 
Copolymer A 7.5 parts 
Particulate II.sup.1 
0.3 parts 
Dowanol PM 17.3 parts 
MEK 61.4 parts 
Surface layer 
Hydroxyethyl Cellulose 
1.8 parts 
Water 97.7 parts 
______________________________________ 
.sup.1 Poly(methyl methacrylate), the average particle size is about 28 
um, the particle size span is about 0.65 and the refractive index is abou 
1.49 (from J. Brandrup & E. H. Immergut, Polymer Handbook, third edition, 
John Wiley & Sons, 1989). 
The underlayer coating was coated on the polyester base using No. 38 Meyer 
rod. After drying the underlayer coating at 120.degree. C. for about 2 
minutes, the surface layer coating was coated using a No. 8 Meyer rod 
under the same conditions. The dry coat weight of the ink receptive 
coating is about 10 g/m.sup.2. 
EXAMPLE 4 
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Underlayer PVP-K90 8.4 parts 
Copolymer B.sup.1 
8.4 parts 
Quaternary copolymer 
9.8 parts 
Particulate III.sup.2 
0.2 parts 
Dowanol PM 13.5 parts 
MEK 58.1 parts 
Surface layer 
Hydroxyethyl Cellulose 
1.8 parts 
Water 97.7 parts 
______________________________________ 
.sup.1 A graft copolymer of methylmethacrylate and hydroxyethyl 
methacrylate, 25% solid. 
.sup.2 Glass bead, the average particle size is about 41 um, the particle 
size span is about 0.3, and the refractive index is about 1.51 (from the 
supplier). 
The underlayer coating was coated on the polyester base using a No. 46 
Meyer rod. After drying the underlayer coating at 120.degree. C. for about 
2 minutes, the surface layer coating was coated using No. 8 Meyer rod 
under the same conditions. The dry coat weight of the ink receptive 
coating is about 10 g/m.sup.2. 
COMATIVE EXAMPLE 1 
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Underlayer PVP-K90 8.67 parts 
Copolymer A 5.42 parts 
Particulate IV.sup.1 
0.2 parts 
Quaternary copolymer 
10.1 parts 
Dowanol PM 20.7 parts 
MEK 53.5 parts 
Surface layer 
Hydroxyethyl Cellulose 
0.5 parts 
Particulate IV 0.14 parts 
Water 98.4 parts 
______________________________________ 
.sup.1 Poly(methyl methacrylate), the average particle size is about 18 
um, the particle size span is about 1.19 and the refractive index is abou 
1.49. 
The underlayer coating was coated on the polyester base using a No. 46 
Meyer rod. After drying the underlayer coating at 120.degree. C. for about 
2 minutes, the surface layer coating was coated using a No. 16 Meyer rod 
under the same conditions. The dry coat weight of the ink receptive 
coating is about 10 g/m.sup.2. 
COMATIVE EXAMPLE 2 
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Underlayer PVP-K90 8.7 parts 
Copolymer B 8.7 parts 
Quaternary copolymer 
10.1 parts 
Particulate V.sup.1 
0.4 parts 
Dowanol PM 20.7 parts 
MEK 50.0 parts 
Surface layer 
Hydroxyethyl Cellulose 
1.8 parts 
Water 97.7 parts 
______________________________________ 
.sup.1 Corn starch, the average particle size is about 15 um, the particl 
size span is about 1.05 and the refractive index is about 1.52 (from 
KirkOthmer Encyclopedia of Chemical Technology, second edition, Volume 18 
John Wiley & Sons, 1969). 
The underlayer coating was coated on the polyester base using a No. 46 
Meyer rod. After drying the underlayer coating at 120.degree. C. for about 
2 minutes, the surface layer coating was coated using a No. 8 Meyer rod 
under the same conditions. The dry coat weight of the ink receptive 
coating is about 10 g/m.sup.2. 
Samples prepared according to the above Examples and Comparative Examples 
were printed on a Hewlett-Packard ink jet printer with a color ink 
cartridge at 50% RH and 22.degree. C. The samples were allowed to dry for 
about 15 minutes and then were placed in a plastic sleeve. The samples 
were stored in the plastic sleeve at 80% RH and 30.degree. C. for 72 
hours. Blocking was judged by examining the size of the contact areas 
between the image and the sleeve and assigning a scaled score thereto (a 
score of 5 being the best and a score of 0 being the worst). The results 
are summarized in Table 1. 
TABLE I 
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Performance Comparisons 
Smoothness 
Haze (%) (Sheffield Units) 
Blocking 
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Example 1 2.5 336 5 
Example 2 2.7 341 5 
Example 3 3.7 330 5 
Example 4 1.7 373 5 
Comparative 
10.5 273 3 
Example 1 
Comparative 
8.9 193 0 
Example 2 
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The 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.