WO2000032407A1

WO2000032407A1 - Ink-jet recording sheet

Info

Publication number: WO2000032407A1
Application number: PCT/US1999/009302
Authority: WO
Inventors: Susan K. Yarmey; Michael L. Steiner
Original assignee: Imation Corp.
Priority date: 1998-11-30
Filing date: 1999-04-29
Publication date: 2000-06-08

Abstract

The thick porous ink-jet recording sheet having a recording layer comprised of packed bed of colloidal size nonporous pigment bound together with a latex binder wherein the pigment concentration is greater than the latex critical pigment volume concentration. An ink-jet recording sheet comprising: a) a paper substrate having first and second major surfaces, and b) a recording layer having thickness of at least 30$G(m) on at least one of the major surfaces of the paper substrate, and comprising at least: i) a latex binder, and ii) a nonporous pigment having an average particle size of less than 500nm in an amount effective to exceed the latex critical pigment volume concentration so as to provide pores within the recording layer.

Description

INK-JET RECORDING SHEET

FIELD OF THE INVENTION

The invention broadly relates to image receiving elements. More specifically, the invention relates to ink jet receptors, particularly porous ink jet receptors.

BACKGROTJ ) OF THE INVENTION

Inkjet printing is a printing technique in which images (e.g., graphs, pictures, symbols, text, etc.) are produced by the ejection of uniformly shaped droplets of ink onto the receptor surface of a recording sheet. This printing technique is widely used in the personal and small office markets. Other applications include low end proofing, and medical referral markets.

The maximum image resolution and throughput of an ink jet printer are primarily determined by the size of the jetted drop and the rate of drop ejection. Several factors limit ink jet printers from attaining the maximum image resolution capable of being achieved by the printer. One of these limiting factors is the nature of the recording sheet receiving the jetted drops.

Investigators have found image quality to be directly related to the ink absorption rate of the receptor surface of the recording sheet, with image quality increasing as absorption rate increases. See, for example, S. J. Bares and K. D.

Rennels, "Paper Compatibility With Next Generation Ink- Jet Printers" Tappi Journal 123 (1990); M. B. Lyne and J. S. Aspler, "Paper For Ink Jet Printing" Tappi Journal 68, 106 (1985); and M. Shioya, K. Iwata, S. Matsui, and T. Ohta, "Ink And Paper For Excellent Ink Jet Printing- Factors Controlling Print Ouality" Journal of Imaging Technology 15, 217 (1989). As a general matter, investigators have found that when the ink droplets are not absorbed quickly enough, the ink tends to spread and interact with neighboring droplets of ink, resulting in such defects as feathering, pooling, or bleeding. Such problems are exacerbated as the droplet ejection frequency is increased for purposes of increasing throughput.

Ink receptor surfaces can be divided into two basic types: continuous phase systems and discontinuous phase systems. Continuous phase systems generally function by swelling to absorb water or ink deposited onto the receptor surface, with the rate of absorption determined by the chemical nature of the polymer used in the system. Typical polymers used in continuous phase systems include gelatins, polyvinyl alcohol, and cellulose. Exemplary continuous phase systems are discussed in United States Patents Nos. 3,889,270, 4,503,111, and 5,141,599.

While generally effective as an ink jet receptor surface, most polymeric continuous phase systems are water soluble, thereby reducing the waterfastness of the receptor. Some polymeric continuous phase systems have overcome the waterfastness issue by mixing an insoluble cross-linked polymer into the system (e.g., forming a semi-interpenetrating network as described in United States Patents Nos. 5,342,688 and 5,389,723). However, the introduction of an insoluble cross-linked polymer into the system intrinsically reduces the absorption rate of the system.

Discontinuous phase systems function by providing pores within the receptor surface capable of absorbing ink by capillary forces. Discontinuous phase systems are generally preferred over continuous phase systems as they absorb ink considerably faster than continuous phase systems.

Discontinuous phase systems are divided into two basic types. A first type of a discontinuous phase system, known as a "porous discontinuous phase system", utilizes micron sized porous pigment particles in the recording layer for purposes of absorbing ink jetted onto the recording layer into the particles through a multitude of tiny interconnected pores in each particle. Ink recording sheets having a porous, discontinuous phase system recording layer are disclosed in United States Patent Nos. 5,165,973; 5,270,103; 5,397,619; 5,478,631; International Published Application WO 97/01448.

A second alternative type of discontinuous phase system, known as a nonporous discontinuous phase system" utilizes nonporous pigment particles held together by a polymeric binder in such a manner that interstitial voids are created between the pigment particles capable of absorbing ink jetted onto the receptor surface.

Inkjet recording sheets with a recording layer of the porous discontinuous phase system type generally provide good ink absorptivity and superior ink capacity, in exchange for some loss in the glossy appearance of the recording sheet. Alternatively, ink jet recording sheets with a recording layer of the nonporous discontinuous phase system type provide superior ink absorptivity and a glossy appearance in exchange for a limited ink capacity due to practical limitations upon the thickness of the coating.

One effort to produce an ink recording sheet providing both superior ink receptivity and superior gloss is described in United States Patent No. 5,478,631 wherein a substrate is coated with both (i) an intermediate ink jet recording layer comprised of porous μm sized particles in a water-soluble binder for purposes of providing effective ink absorptivity, and (ii) an upper glossy layer comprised of colloidal nm sized particles in a latex binder for purposes of providing a glossy appearance. It is noted that the patent specifically discloses that an ink recording sheet having a single recording layer comprised of the colloidal nm sized particles in a latex binder possesses unacceptable ink capacity (i.e., results in overflow). Accordingly, a substantial need exists for a simple, high gloss, crack resistant ink jet recording sheet having superior ink absorption rate and ink absorption capacity.

SUMMARY OF THE INVENTION

An ink jet recording sheet wherein the recording layer on the substrate has a thickness of at least 30 μm and includes a latex binder and a nonporous pigment having an average particle size of less than 500 nm. The concentration of pigment in the recording layer exceeds the latex critical pigment volume concentration so as to provide pores within the recording layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an enlarged side view of an end of one embodiment of the invention.

Figure 2 is a graph demonstrating the time required to absorb a drop of ink into a discontinuous nonporous recording layer relative to the diameter of the particles contained within the recording layer based upon equations [2] - [6] set forth herein.

DETAILED DESCRIPTION OF THE INVENTION

10 Ink Jet Recording Sheet

20 Substrate

30 Subbing Layer

40 Recording Layer

50 Anti-curling Layer Definitions

As utilized herein, including the claims, the term "absorptivity", refers to the rate at which a drop of ink can be absorbed into the ink jet recording sheet (volume/time).

As utilized herein, including the claims, the term "capacity", refers to the amount of ink that can be absorbed into the ink jet recording sheet (volume).

As utilized herein, including the claims, the term "diameter", when used in connection with a particle, means the longest measurement across the particle.

As utilized herein, including the claims, the phrase "essentially neutral charge", when used in connection with the recording layer, means that the recording layer does not include more than a trace amount of an intentionally added ionically charged constituent.

As utilized herein, including the claims, the phrase "latex binder" means an aqueous dispersion of water-insoluble polymeric particles.

As utilized herein, including the claims, the phrase "latex critical pigment volume concentration" refers to the highest pigment concentration in a polymer film before the onset of voids or pores in the film, (i.e., that point at which the concentration of latex binder is no longer sufficient to completely occupy the interstitial sites between pigment particles and a void volume is created within the layer). As utilized herein, including the claims, the phrase "nonporous pigment" refers to a pigment particle which is essentially impermeable to ink (i.e., ink is not appreciably absorbed into the pigment particle).

As utilized herein, including the claims, the term "thickness", when used in connection with the recording layer, means the thickness of the recording layer on a dry basis.

As utilized herein, including the claims, "wt%" is based upon the solids content of the composition (i.e., calculated on a dry basis).

Performance

Performance of the inkjet recording sheet 10 (i.e., absorption and capacity) is dependent upon a number of variables, but is predominantly controlled by particle size and particle size distribution within the inkjet recording layer 40. As capillary theory dictates, the larger the size of the pores within the inkjet recording layer 40 the higher the absorption rate. Hence, absorption rate can be easily manipulated by altering particle size.

Absorption of ink into porous media is governed by capillary forces. Most models of capillary penetration into porous media are based upon the Washburn equation:

where h is the height of liquid in the capillary, R is the capillary radius, μ the liquid viscosity, t time, γι_v the liquid surface tension, and θ the three phase contact angle the liquid makes with the capillary surface. The Washburn equation has been incorporated into an analytical model for capillary penetration of small drops into porous media. For a drop of constant radius on a porous receptor, the time required to absorb the drop T_CDA is given by,

^τ -{$ ^[2]

where N₀ is the volume of the drop, x& is the diameter of the drop-receptor contact area, and:

where p_p is the pore area density and R, μ, γι_v, and θ are as previously defined.

Thus, for purposes of designing an inkjet recording sheet, there are three variables that can increase absorption of ink into the porous receptor: R, θ, and Pp. The radius of the pores (R) influences the rate of absorption to the greatest extent (t^'.e., as the pore radius is increased the time to absorb a droplet decreases by the fifth power). Accordingly, the rate of absorption can be dramatically increased by a modest increase in the radius of the pores.

The contact angle the liquid makes with the porous material (θ) is also important (i.e., as the contact angle is increased the rate of absorption is decreased). Hence, it is desirable for the droplet to completely wet the porous surface of the inkjet recording sheet.

Finally, the pore area density (p_p) linearly changes absorption rate, with an increase in the number of pores increasing the rate of absorption. o

The other variables in this equation cannot easily be controlled by receptor properties. For example, investigators have shown that the radius of a drop impacted onto the receptor surface, equivalent to ra, has little to do with the surface properties of the receptor for the scales involved in inkjet printing.

In summary, to maximize ink absorption by the inkjet recording sheet 10, the recording layer 40 should have a high density of large diameter pores and provide good wetting of the ink on the surface of the recording layer 40.

The inkjet recording layer 40 is a porous film essentially formed by packing together many similarly size spherical particles into a packed bed (i.e., pigment particles and latex binder particles). For simplicity, the balance of this analysis will assume that the particles are spherical and of equal size (i.e., monodisperse).

A monodisperse bed of spheres when packed as tightly as possible will yield a void fraction of 26%. Thus, 26% of the receptor volume is available for ink absorption. The pore area density (p_p) at the surface of such a packed bed is represented by the formula:

P_P = [4]

where r is the radius of the particles in the bed.

Pores within a packed bed of spheres are unlike the capillary tubes modeled above. Rather, pores formed by a bed of spheres can be approximated by a capillary tube of the same radius as a sphere that just passes through the opening formed by three neighboring spheres. This radius, known as the Haines in-sphere radius (H_r), is calculated as: where r is as previously defined.

Computer simulations of capillary flow in packed beds have further refined the Haines in-sphere radius approximation. Data from such experiments has yielded a good approximation of a capillary radius as:

Using equations [2] - [6], the time (TC_DA) required to absorb an ink droplet on the surface of a packed bed of spheres can be estimated. As graphically shown in Figure 2, the time required to absorb the ink drop is significantly reduced as the diameter of the particles increases. It is specifically noted that absorption rate increases as particle size decreases from 500 to 100 nm, with a dramatic increase in absorption rate as the particle size decreases from 100 nm.

Construction

The discontinuous inkjet recording sheet 10 includes a thick coating of a discontinuous recording layer 40 on a suitable substrate 20, wherein the recording layer 40 provides excellent ink absorptivity and capacity with good integrity. SUBSTRATE

The substrate 20 may be any of the typical materials used in the construction of ink jet recording sheets capable of providing the necessary visual appearance and structural support for the recording layer(s) 40. Examples of suitable substrates include paper, cloth, polymers, metals, and glass. Thin flexible sheets are generally preferred, with paper the substrate of choice when an opaque support is desired, and polymeric films used when a translucent or transparent appearance is desired. The thickness of the substrate 20 is preferably in the range of about 0.05 to 1.0 mm.

SUBBING LAYER

The major surface of the substrate 20 to be coated with the recording layer 40 may optionally be treated with a subbing layer 30, such as a primer or an antistatic layer, before the recording layer 40 is coated onto the substrate 20.

RECORDING LAYER

The recording layer 40 is comprised of a nonporous pigment held together by a latex binder. The recording layer 40 should be at least about 30 μm thick in order to provide sufficient capacity. Recording layers 40 possessing acceptable appearance can be formed up to a thickness of about 100 μm, with a thickness of between about 35 to 85 μm preferred.

Pigment

Pigments suitable for use include any of the conventional nonporous pigments used in the production of coated paper, including specifically, but not exclusively, nonporous types of (i) inorganic pigments such as alumina, aluminum hydroxide, aluminum oxide, aluminum silicate, barium sulfate, calcium carbonate, calcium silicate, calcium sulfate, kaolin, magnesium silicate, silica, silicic acid, sodium silicate, talc, titania, titanium dioxide zinc carbonate, zinc oxide, and mixtures thereof, (ii) organic pigments such as plastic pigments, urea resin pigments, and mixtures thereof, and (iii) mixtures of inorganic and organic pigments.

Pigments having an average particle size of less than 500 nm are capable of producing a recording layer 40 having the desired appearance and performance. Pigment particles having an average particle size of between about 10 - 500 nm are generally preferred, with pigment particles having an average particle size of between about 50 -300 nm desired and pigment particles having an average particle size of between about 50 -100 nm favored.

The pigment particles can be of substantially any desired shape, with symmetrical particles, particularly spherical particles, generally preferred as they enhance the performance characteristics of the recording layer 40.

Latex Binder

The nonporous pigment particles are held together by a latex binder.

The latex binder is provided in an amount sufficient to hold the pigment particles together and provide an acceptable appearance, while producing a pigment volume concentration above the latex critical pigment volume concentration so as to leave pores (i.e., interconnected interstitial voids) within the recording layer 40 and thereby enhance performance. Ink jetted onto a recording layer 40 having a pigment concentration above the latex critical pigment volume concentration will be channeled into and stored within the pores in the layer 40 through capillary action.

The polymer of the latex binder can be a homopolymer or copolymer. The copolymer may be random, block, graft, or alternating polymers of two or more monomers. Preferred polymers are those having a glass transition temperature of less than about 30°C and capable of forming a continuous film by coalescence. Specific polymers suitable for use as the latex binder including specifically, but not exclusively, polyacrylates, styrene-butadiene copolymers, ethylene-vinyl acetate copolymers, nitrile rubbers, poly (vinyl chloride), poly (vinyl acetate), ethylene- acrylate copolymers, and vinyl acetate-acrylate copolymers. Such latex binders are widely available from a number of sources.

When a charged pigment and/or latex is used, it is generally preferred to match the charge of the pigment and latex used in the recording layer (e.g., combining an anionic latex with an a ion pigment) to facilitate stability of the dispersion.

Relative Concentrations of Pigment Particles and Latex Binder

The relationship between the volume concentration of pigment particles in a polymeric film (PNC) and the critical pigment volume concentration (CPVC) can be represented by:

VC Λ = -T^Γ^ [7]

CPVC

where A is the reduced pigment particle concentration. By definition, discontinuous films will have a A > 1, while continuous films will have a A < 1.

The critical pigment volume concentration in a pigment latex film (LCPNC), behaves differently from the CPVC discussed above. Briefly, LCPNC is dependent upon pigment and latex particle geometry in addition to the interactive pigment - polymer chemistry. Assuming spherical pigment and latex particles, LCPVC can be described by the following relationship:

where ni is the number of latex particles at the LCPVC, rip is the number of pigment particles at the LCPNC, d_p is the pigment particle diameter, and dt is the latex particle diameter. Equation [8] can be rearranged to relate the volume of pigment needed to reach the LCPNC:

where N_p is the pigment volume concentration at the LCPNC and Vi is the latex volume concentration at the LCPNC. Hence, for a given pigment and latex chemistry, equation [9] describes the effect of particle geometry on LCPNC or the minimum pigment concentration required to form a porous film. It is noted that as pigment particles become larger more pigment is required to form a porous film. Conversely, as latex particles become larger, less pigment is needed to produce a porous film. It is generally preferred that the latex particles used in the recording layer 40 have a diameter at least as large as the diameter of the pigment particles, with latex particles having a diameter at least VΛ times greater than the diameter of the pigment particles most preferred. This geometrical dependency can be used to manipulate the porosity of a pigment latex film. By choosing a pigment particle size, latex particle size and latex volume, the absorption requirements can be met.

Other properties of the latex can effect LCPNC, such as T_g. A latex with a high T_g is not very soft and tends to coalesce slowly, retarding movement of the latex into the pores and yielding a lower LCPNC. Similarly, a latex that does not wet the pigment well will not move readily into the pores and will yield a lower LCPVC.

The addition of latex binder to the pigment particles increases a number of desired properties and characteristics of the recording layer 40, such as increasing integrity and wear resistance, reducing cracking, improving adhesion to the substrate 20. However, the addition of binder causes A to move towards 1, signaling a decrease in porosity and a decrease in performance. Hence, a careful consideration of binder concentration is necessary to create a suitable inkjet recording layer 40 having a proper balance of performance characteristics. As a general matter, a weight ratio of pigment to latex of about 5: 1 to 10: 1, preferably about 6:1 to 8:1, provides acceptable balancing of the competing performance characteristics.

Additives

The typical additives such as mordants, surfactants, plasticizers, antistatic agents, buffers, coating aids, matting agents, particulates for managing mechanical processing of the inkjet recording sheet, hardeners, colorants, viscosity modifiers, preservatives, and the like may optionally be incorporated into the inkjet recording layer as desired.

ANΉ-CURLING LAYER

An anti-curl layer 50 may optionally be coated on the back side of the substrate 20. Method of Manufacture

BLENDING OF CONSTΓTUENTS

Substantially any convenient method may be employed to blend the constituents together to form the recording layer composition. Exemplary procedures include, (i) dispersing the pigment in water and then adding the aqueous latex binder into the aqueous dispersion of pigment, (ii) dispersing the pigment in water and then adding the aqueous dispersion of pigment into the aqueous latex binder, (iii) simultaneously charging the pigment and the latex binder into a suitable mixer such as an agitator.

COATING AND DRYING OF RECORDING LAYER

The recording layer may be coated by any of the conventional techniques for coating such materials, including specifically, but not exclusively, extrusion coating, direct and indirect gravure coating, knife coating, Mayer rod coating, roll coating, etc.

Similarly, the coated recording layer may be dried by any of the conventional techniques for drying such coated recording layers.

OPTIONAL PROCESSING

The recording layer 40 can be calendared to improve gloss, with the calendar rolls heated or unheated and rotating concurrent or countercurrent with respect to movement of the inkjet recording sheet 10. Care must be taken to avoid excessively compressing the film so as not to decrease the porosity and thereby the performance of the recording layer 40. EXAMPLES

Glossary

Airflex 500 A nonionic latex of ethylene vinyl acetate copolymer having an average particle size of 170 nm and a T_g of 5 ° C available from Air Products of Allentown, Pennsylvania.

Airflex 510 A nonionic latex of ethylene vinyl acetate copolymer having an average particle size of 500 nm and a T_g of 6 ° C available from Air Products of Allentown, Pennsylvania.

Airflex 4500 An anionic latex of ethylene vinyl chloride copolymer having an average particle size of 120 nm and a T_g of 3 ° C available from Air Products of Allentown, Pennsylvania.

Airflex 4514 An anionic latex of ethylene vinyl chloride copolymer having an average particle size of 120 nm and a T_g of 12 ° C available from Air Products of Allentown, Pennsylvania.

Airvol 165 A high molecular weight and highly hydrolyzed polyvinyl alcohol available from Air Products of Allentown, Pennsylvania. Flexbond 227 An anionic latex of styrene acrylic acid copolymer having an average particle size of 80 nm and a T_g of 24 ^* C available from Air Products of Allentown, Pennsylvania.

Flexbond 381 A slightly anionic latex of vinyl acetate acrylate copolymer having an average particle size of 300 nm and a T_g of 13 ° C available from Air Products of Allentown, Pennsylvania.

Flexbond 325 A nonionic latex of vinyl acetate acrylate copolymer having an average particle size of 300 nm and a T_g of 19 ° C available from Air Products of Allentown, Pennsylvania.

Gohsenol Polyvinyl alcohol available under the mark Gohsenol KL-03™ available from Nippon Synthetic Chemical Industry Co., Ltd. of Osaka, Japan.

MP1040 Particulate silica having a diameter of 100±30 nm available from Nissan Chemical Industries, Ltd. of Tokyo, Japan.

MP3040 Particulate silica having a diameter of 300±30 nm available from Nissan Chemical Industries, Ltd. of Tokyo, Japan.

MP4540 Particulate silica having a diameter of 450±30 nm available from Nissan Chemical Industries, Ltd. of Tokyo, Japan. P:B Ratio of Particle to Binder.

Snowtex N Particulate silica having a diameter ranging from 11-14 nm available from Nissan Chemical Industries, Ltd. of Tokyo, Japan.

Snowtex ZL Particulate silica having a diameter ranging from 70- 100 nm available from Nissan Chemical Industries, Ltd. of Tokyo, Japan.

Syloid 74 Irregularly shaped particulate silica having a diameter ranging from 4.8μm to 25.3 μm with a median diameter of 12.2 μm, a pore volume of 1.15 c³/g, a median pore diameter of 146 A, and a surface area of 315 m²/g, available from Grace Davison of Baltimore, Maryland.

Glass transition temperature.

Vinac 885 A slightly anionic latex of vinyl acetate having an average particle size of 150 nm and a T_g of 32 ° C available from Air Products of Allentown, Pennsylvania.

Testing Procedures

ABSORPTION RATE

A single droplet of Jetfill™ magenta ink is jetted from an Epson print head onto the recording layer. Ink absorption is viewed through a microscope, aimed parallel to the surface of the recording layer, giving a side view of the ink drop being absorbed into the layer. The microscope was attached to a video camera and frame- grabber capable of permitting computer analysis and storage of the images. Computer analysis of the images yields drop volume with respect to time. The number of seconds required for the drop of ink to completely disappear into the recording layer is measured, with the average of two readings reported as the absorption rate for the sample. When ink was still present after 0.33 seconds, the absorption rate is recorded as ">0.33" followed by the percentage of initial drop volume remaining on the surface of the recording layer at that time since absoφtion time is well beyond the desired values of less than about a tenth of a second.

ABSORPTION CAPACITY

An actual test chart with boxes of primary and secondary colors is printed on an Epson 800 inkjet printer using Jetfill™ ink. The printed test charts were visually observed an given a value of 1 to 7 with 1 indicating a severely blurred imaged caused by actual pooling of the ink on the surface of the receptor: 5 indicating an acceptable image with minimal spreading of the dye from one box to another; and 7 indicating a clear image with no detectable spreading of the dye from one box to another. THICKNESS OF RECORDING LAYER (Calculated)

The dry thicknesses of recording layers having no interstitial voids was calculated in accordance with equation [10] set forth below.

ThicknessDry ⁼ (Thicknesswet)(Densitysoiution)(% Solids)/(Densitys₀iidβ) [10]

The dry thicknesses of recording layers having interstitial voids was calculated in accordance with equation [11] set forth below.

ThicknessDry = (Thicknesswgt)(DensitVsni„t;nn % Solids) [11]

(Density_Soiids)(0.63 + gramsbmde gramSb-nd-T + gramspigment))

THICKNESS OF RECORDING LAYER

(Measured)

The diy thicknesses of the recording layers was physically measured with a micrometer, with an average of 3 readings recorded as the coating thickness.

Standard Sample Construction

Recording films containing the particulate and binder materials as set forth in Table One were knife coated onto the substrate identified in Table One and dried for 5 minutes at 80°C unless otherwise noted. COMPARATIVE EXAMPLES l AND 2

(Commercially Available Continuous Phase Paper)

Imation™ Photograde Quality Paper for Inkjet Printers and Kodak™ Inkjet Photoweight Premium Glossy Paper were tested for ink absorption rate in accordance with the ink absoφtion rate testing protocol set forth herein, except that six drops of ink forming a single droplet on the surface of the paper were used rather than a single drop. Test results are reported in Table One.

Both recording sheets exhibited slow absoφtion rates of greater than

0.33 seconds.

COMPARATIVE EXAMPLES 3-8

(Λ < 1 (nonporous): Porous Particles: H₂0 Soluble Binder)

Recording layers having a Λ < 1 (i.e., concentration of pigment particles below the critical pigment volume concentration) were constructed in accordance with the Standard Sample Construction Procedure using the particle, binder, and P:B set forth in Table One. The binder was mixed with the particles as a 12.5% solution in de-ionized water, and the resultant composition coated at a wet thickness of 5 mils. The thickness of the dried recording film was calculated and the absoφtion rate of the dried recording film tested using 6 drops of ink to form a single droplet. The results of the testing were recorded, and are set forth in Table One.

As shown in Table One, an increase in absoφtion rate was observed as the silica to binder ratio was increased. Without intending to be unduly limited thereby, we believe that this increase in absoφtion rate is due to an increase in the number of pores within the particles available to take-up the ink. Unfortunately, we also observed an unacceptable loss in film integrity (i.e., the film cracked and easily flaked off the substrate) as the silica to binder ratio was increased, suggesting that at least some of the increase in absoφtion rate may be due to ink up-take into these relatively large cracks in the film.

COMPARATIVE EXAMPLES 9-12 (Λ > 1 (porous): Nonporous Particles: H₂0 Soluble Binder)

Recording layers having a A > 1 (i.e., concentration of pigment particles above the critical pigment volume concentration) were constructed in accordance with the Standard Sample Construction Procedure using the particle, binder, and P:B set forth in Table One. The binder was mixed dropwise with the particles at room temperature as a 10% solution in de-ionized water, and the resultant composition coated using a 16 Meyer rod at a wet thickness of 1.44 mils. The thickness of the dried recording film was calculated and the absoφtion rate of the dried recording film tested and recorded. Test results are set forth in Table One.

As shown in Table One, an increase in absoφtion rate was observed as pigment particle size increased. Without intending to be unduly limited thereby, we believe that this increase in absoφtion rate is due to an increase in the size of the pores between particles available to take-up the ink.

All recording layers exhibited poor film integrity (i.e., the film cracked and easily flaked off the substrate) which tended to worsen as the size of the pigment particle decreased. Without intending to be unduly limited thereby, it is believed that cracking of the recording layer will tend to increase as the weight ratio of pigment to binder increases, particles size decreases, film thickness increases, and drying rate increases. EXAMPLES 13-55

(A > 1 (porous): Nonporous Particles: Latex Binder)

Recording layers having a A > 1 (i.e., concentration of pigment particles above the critical pigment volume concentration) were constructed in accordance with the Standard Sample Construction Procedure using the particle, binder, and P:B set forth in Table One. The latex binder was diluted to 20 wt% solids with de-ionized water and added dropwise to the monodisperse colloidal particles at room temperature, and the resultant composition knife coated at a wet thickness of 3 mils. The thickness of the dried recording film was calculated and the absoφtion rate of the dried recording film tested and recorded. Test results are set forth in Table One.

Several observations and conclusions can be drawn from the test results:

(1) An increase in absoφtion rate was observed as pigment particle size increased. Without intending to be unduly limited thereby, we believe that this increase in absoφtion rate is due to an increase in the size of the pores between particles available to take-up the ink.

(2) An increase in absoφtion rate was observed as the pigment to binder weight ratio (P:B) increased, once the LCPVC was achieved for the recording layer - achieved at about 4:1 for the tested recording layers.

(3) The LCPVC decreases as the size of the latex particles increases relative to the size of the pigment particles. Hence, less pigment was needed to produce a porous film, and an increase in absoφtion rate was observed at a given silica concentration as the size of the latex particles increased relative to the size of the pigment particles. (4) An increase in absoφtion rate was observed as the glass transition temperature was increased. Without intending to be unduly limited thereby, we believe that this effect was due to the slower coalescence of higher T_g latexes, causing the latex to fill fewer pores in the receptor film (i.e., decreasing the LCPVC). However, we further observed that an inherent consequence of using a latex with a higher T_g was a decrease in film integrity, as demonstrated by the "cracking" notation in Table One when Vinac 885 (a latex having a T_g of 32 ° C) was used.

All recording layers - except for those using Vinac 885 as the latex binder - exhibited good film integrity (i.e., no visually detectable cracking or flaking).

EXAMPLES 56-60 (A > 1 (porous): Nonporous Particles: Latex Binder)

Recording layers having a A > 1 (i.e., concentration of pigment particles above the critical pigment volume concentration) were constructed in accordance with the Standard Sample Construction Procedure using the particle, binder, and P:B set forth in Table Two. The latex binder was diluted to 20 wt% solids with de-ionized water and added dropwise to the monodisperse colloidal particles at room temperature, and the resultant composition extrusion coated at gaps of 1.5 mil, 2.0 mil, 2.5 mil, 3.0 mil, and 3.8 mil. The thickness of the dried recording film was measured and the absoφtion capacity of the dried recording films tested and recorded. Test results are set forth in Table Two.

As shown in Table Two, for the tested recording layer, a recording layer of between about 23-33 μm is necessary to achieve acceptable results, while superior visual results are first observed at a thickness somewhere between about 33 and 41μm. TABLE ONE

(ABSORPTION RATE)

Claims

We claim:

1. An inkjet recording sheet comprising: a) a paper substrate having first and second major surfaces, and b) a recording layer having a thickness of at least 30 μm on at least one of the major surfaces of the paper substrate, and comprising at least: i) a latex binder, and ii) a nonporous pigment having an average particle size of less than 500 nm in an amount effective to exceed the latex critical pigment volume concentration so as to provide pores within the recording layer.

2. The inkjet recording sheet of claim 1 wherein the recording layer has a thickness of between about 30 to 100 μm.

3. The inkjet recording sheet of claim 1 wherein the recording layer has a thickness of between about 35 to 85 μm.

4. The inkjet recording sheet of claim 1 wherein the pigment is essentially symmetrical with an average particle size of between about 50 to 300 nm.

5. The inkjet recording sheet of claim 1 wherein the pigment is spherical with an average particle size of between about 50 to 100 nm.

6. The inkjet recording sheet of claim 1 wherein (i) the latex binder includes polymeric particles, and (ii) the average diameter of the polymeric particles is greater than the average diameter of the pigment particles.

7. The inkjet recording sheet of claim 6 wherein the average diameter of the polymeric particles is at least IV-. times greater than the average diameter of the pigment particles.

8. The inkjet recording sheet of claim 1 wherein the latex has a glass transition temperature of less than 30°C.

9. The inkjet recording sheet of claim 1 wherein the latex is selected from the group consisting of vinyl acetate latex, ethylene- vinyl chloride copolymer latex, vinyl acetate-ethylene copolymer latex, and vinyl acetate-acrylate copolymer latex.

10. The inkjet recording sheet of claim 1 wherein the pigment is silica.

11. The inkjet recording sheet of claim 1 wherein the weight ratio of pigment to latex is at least 5:1.

12. The inkjet recording sheet of claim 1 wherein the weight ratio of pigment to latex is at least 6:1.

13. The inkjet recording sheet of claim 1 wherein the weight ratio of pigment to latex is between 5:1 to 10:1.

14. The inkjet recording sheet of claim 1 wherein the weight ratio of pigment to latex is between 6: 1 to 8 : 1.

15. The inkjet recording sheet of claim 1 wherein the latex possesses an essentially neutral charge.