WO2009051577A1

WO2009051577A1 - Paper coating composition

Info

Publication number: WO2009051577A1
Application number: PCT/US2007/021975
Authority: WO
Inventors: E. Craig Mitchell; Gerald D. Miller; Kien Van Phung
Original assignee: Celanese International Corporation
Priority date: 2007-10-15
Filing date: 2007-10-15
Publication date: 2009-04-23

Abstract

Coating compositions, methods of applying the coating compositions, and articles incorporating the coating compositions are provided. The coating compositions are useful for coating a variety of substrates to reduce blocking between adjacent layers of the substrates on which the compositions are applied. The coating compositions incorporate a pigment, a binder, and hydrophobically modified polyvinyl alcohol.

Description

PAPER COATING COMPOSITION

FIELD OF THE DISCLOSURE

[0001] This disclosure relates to paper coating compositions and methods of making and using such compositions. The coating compositions incorporate a binder system and amounts of hydrophobically modified polyvinyl alcohol.

BACKGROUND INFORMATION

[0002] Paper coating compositions, or coating colors, are used by the paper industry to impart the desired moisture resistance, physical properties and appearance to certain grades of finished paper. Generally, the coating composition is an aqueous dispersion consisting mainly of mineral pigments like clay, calcium carbonate or titanium dioxide, and pigment binders of natural protein, starch or synthetic polymer emulsions. Styrene-butadienes and polyvinyl acetates are examples of such synthetic emulsion binders. Coating compositions may also contain additives, such as thickeners, humectants and lubricants.

[0003] The coating is generally applied to a paper substrate to improve the appearance of the substrate. An important purpose of a coating is to provide a smooth substrate surface that is substantially free of indentations or valleys. A smooth surface is necessary for printing good quality images on substrates and for effective transfer of ink, or other coloring agent, to the substrate. A smooth substrate surface is obtained by coating the substrate surface with a pigment coating composition.

[0004] Coatings are also applied to paper stock to prevent adjacent layers of the paper stock (including paperboard) from sticking together. This condition, known as blocking, not only hinders subsequent processing or packaging of the paper stock, but can also damage the paper product. Blocking can be especially troublesome when the stock is spooled onto a roll or drum. Often the spooling step results in pressure within the paper reel, thereby pushing adjacent layers against one another. Additionally, the amalgamation of multiple layers of the paper stock typically produces an increased temperature within the rolled up paper stock. This combination of increased pressure and temperature, along with the inherent moisture within the paper stock, are the primary contributing factors leading to a blocking situation.

[0005] These coating compositions usually comprise an aqueous dispersion including mainly mineral pigments such as clay, calcium carbonate, or titanium oxide, and pigment binders of natural protein, for example, casein or soy protein, starch, or synthetic polymers such as emulsions. Coating compositions are usually applied to a continuous web of the substrate material by high speed coating machines having a fluid application device, such as blade coaters, air knife coaters, rod coaters and roll coaters. The machines used in modifying the media, typically include a reservoir in fluid communication with a reservoir. The flow properties of coating color compositions for substrates are of significant importance with regard to the runnability (or flow) of the color during the coating operation. These flow properties are often controlled by a thickener. It is sometimes advantageous to use faster coaters to increase productivity and to use higher solids coating compositions to decrease drying costs and improve binder distribution.

BRIEF DESCRIPTION OF THE DISCLOSURE

[0006] This disclosure relates to coating compositions, methods of applying the coating compositions, and articles of commerce, such as paper stock, incorporating the coating compositions. The coating compositions described herein are useful as an anti-blocking agent for coating a variety of substrates. The coating compositions incorporate a pigment system that may be selected from a variety of suitable systems, including clay and calcium carbonate, and a binder. The binder may include latex that can optionally be mixed with polyvinyl alcohol, or may be entirely comprised of polyvinyl alcohol. The composition may further include hydrophobically modified polyvinyl alcohol. [0007] In one embodiment, the composition comprises a pigment, a binder, and hydrophobically modified polyvinyl alcohol. In another embodiment, the coating composition comprises to about 10% by weight of hydrophobically modified polyvinyl alcohol. In another embodiment, the composition comprises from about 0.5 % by weight to about 8% by weight of hydrophobically modified polyvinyl alcohol. In another embodiment, the composition comprises from about 1% by weight to about 6% by weight of hydrophobically modified polyvinyl alcohol. In another embodiment, the composition comprises from about 2% by weight to about 5% by weight of hydrophobically modified polyvinyl alcohol. In another embodiment, the composition comprises from about 3% by weight to about 4% by weight of hydrophobically modified polyvinyl alcohol. Optionally, the hydrophobically modified polyvinyl alcohol may be fully hydrophobic. Optionally, the hydrophobically modified polyvinyl alcohol may be partially hydrophobic.

[0008] The coating compositions may also include a variety of additional components such as lubricants, defoamers, and other conventional additives. DETAILED DESCRIPTION OF THE INVENTION

[0009] This disclosure relates to coating compositions and methods of applying and using substrate coating compositions. The coating compositions provide an anti-blocking function when applied to a substrate. Accordingly, also included within this disclosure are the substrates on which the coating composition has been applied. The compositions are generally aqueous based incorporating dissolved or suspended solids that include a pigment, binder, and optionally other coating composition additives. Optionally, the present composition may also include a lubricant for enhancing its flowability. [0010] The composition may optionally include auxiliary additives such as dispersing agents, protective colloids, solvents for the colloids, sequestering agents and the like, none of which change the basic and novel characteristics of the concentrates. Auxiliaries also include minor amounts of binder, plasticizer, filler, water retention aids such as carboxymethyl cellulose and so forth. Optical brighteners may also be included with the composition herein described. [0011] The pigment may be selected from any pigment useful for substrate coating compositions. Exemplary suitable pigments are aqueous dispersions of coating grade clays, such as kaolin clays, titanium dioxide, calcium carbonate, barium sulfate, talc, zinc sulfate, aluminum sulfate, calcium oxide reaction products, lithopone, zinc sulfide, other coating pigments, other similar materials, and mixtures thereof. In one embodiment, the pigment system comprises at least 90 wt.% of clay. In another embodiment, the pigment system comprises at least 90 wt.% of calcium carbonate. Additionally, the composition may also contain other additives, such as zinc oxide and/or a small amount of a dispersing or stabilizing agent, such as tetra-sodium pyrophosphate.

[0012] Ingredient units may be expressed as dry pts/100 pigment as well as wet parts/100 pigment. In terms of raw materials, pigments are typically delivered in dry or in water pre-dispersed form generally ranging from about 70% to 76% solids. When weight percentages are discussed herein they are synonymous with dry part classification, thus when cited herein a 5% weight percent is substantially equal to 5 dry parts per 100 of the total composition. [0013] The binder may be selected from any suitable binder useful for binding the pigment particles and other solids in the coating composition. Exemplary binders are natural protein materials such as casein or soy protein, starch, or synthetic polymer emulsions such as a latex material incorporating a styrene butadiene, acrylic latex, vinyl acetate latex, or styrene acrylic copolymers, or natural occurring latex materials, a polyvinyl alcohol polymer material. Latexes are typically delivered at about 50% solids.

[0014] As noted above, optical brighteners that may possibly be used with the composition described herein includes concentrates having the tradenames UVITEX® and TINOP AL® from Ciba Specialty Chemicals, BLANKOPHOR® from Lanxess, other optical brighteners include HOSTALUX® and LEUCOPHOR® from Clariant. In one embodiment, the composition may include up to 1 part of optical brightener for every 100 parts of pigment. The optical brightener concentrate may further comprise polyethylene glycol in some cases and the optical brightener active ingredient comprises a stilbene compound. The stilbene compound may be a sulfonated stilbene compound may be a stilbene compound as noted above. OBs for coating may be delivered in liquid form at about 17% active ingredient, and can be of the tetra or hexa sulfonated variety.

[0015] Suitable lubricants for use with the present composition can include calcium stearate, polyethylene emulsions, and polyglyceride. In one embodiment, the composition may include up to 1 part of lubricant for every 100 parts of pigment.

[0016] In one embodiment, the coating composition may comprise from about 75 parts to about 125 parts of pigment and from about 4 to about 15 parts of binder. In another embodiment, the paper coating composition may comprise from about 80 parts to about 1 10 parts of pigment and from about 5 parts to about 13 parts of binder. In yet another embodiment, the paper coating composition may comprise from about 80 parts to about 105 parts of pigment and from about 5.5 parts to about 12 parts of binder. In yet one additional embodiment, the paper coating composition may comprise substantially 100 parts of pigment and from about 6 parts to about 11 parts of binder.

[0017] In one embodiment, when the binder for use with the composition comprises latex (such as styrene butadiene or a polyvinyl acetate emulsion), it may comprise up to about 15 parts for every 100 parts of pigment. Optionally, the latex binder may comprise up to about 12 parts for every 100 parts of pigment. Alternatively, the latex binder may comprise up to about 10 parts for every 100 parts of pigment. Yet still optionally, the latex binder may comprise up to about 8 parts for every 100 parts of pigment. In another embodiment, the latex binder may comprise up to about 6 parts for every 100 parts of pigment.

[0018] Hydrophobically modified polyvinyl alcohol (HM PVOH) can be included in the composition in an amount for every 100 parts of pigment, this amount may be any range workable for a coating when applied to a substrate such as paper stock, the substrate will then have improved anti-blocking performance.

[0019] One example of a Hydrophobic modified PVOH suitable for use with the present disclosure can be synthesized by reacting vinyl acetate monomer (VAM) with other monomers bearing a hydrolysis stable and hydrophobic group. The monomers of this example include vinyl neoalkanoates (commonly known as Veova®), alkyl vinyl ethers and alkyl acrylamides. The range of carbon unit in alkyl can range from 6 to 12. In a general procedure of synthesis, VAM can be mixed with a hydrophobic monomer and polymerized in the presence of a radical initiator. Methanol is a suitable solvent for such polymerization. After polymerization, residual monomer can be removed by distillation and saponified in the presence of an alkaline catalyst to yield HM-PVOH. In this example, the hydrophobic monomer should comprise about 1.0 to 5.0 of the percent total weight fraction. The molecular weight of the HM-PVOH may be controlled by the concentration of total monomers and the level of the initiator.

[0020] HM-PVOH can be characterized by lower surface tension. For example, the solution of a fully hydrolyzed HM-PVOH has a surface tension of 48 dynes/cm in comparison to 67 dynes/cm of a fully hydrolyzed PVOH. Such low surface tension makes the coating color easy to coat and more compatible with other sizing agents, which are hydrophobic in nature. [0021] HM PVOH may be delivered in solid form to be cooked into solution prior to using, or in liquid form at 5% to 10% solids. [0022] In one embodiment, the HM PVOH content of the composition may have a weight percent (of the total composition) of at least about 0.5, at least about 1, at least about 1.5, at least about 2, at least about 2.5, at least about 3, at least about 3.5, at least about 4 at least about 4.5 and at least about 5. The upper limits of the HM PVOH content include a weight percent of at least about 20, at least about 10, at least about 9, at least about 8, at least about 7, at least about 6 and at least about 5.5. It should be pointed out that the HM PVOH ranges for the compositional mixtures can include combinations of any of the lower ranges with any of the upper ranges of the HM PVOH.

[0023] In one embodiment, the polyvinyl alcohol polymers useful in accordance with the compositions, products, and methods described herein have the following characteristics: a degree of polymerization of about 300 to about 4,000, a weight average molecular weight of about 10,000 to about 200,000, and is from about 85 mole % to about 99.9 mole % hydrolyzed.

[0024] It should be pointed out that scope of the present disclosure includes the application of the composition herein described to any substrate comprising a print media (i.e. a surface onto which something can be printed). Examples of suitable substrates include paper, bright white inkjet paper (bond), translucent bond, paperboard, cardboard, natural tracing paper, vellum, clear film, matte film, coated paper, heavyweight coated paper, high-gloss photo paper, semi-gloss photo paper, paper-based semi-gloss, satin poster paper, studio canvas, banners with TYVEK®, colorfast adhesive vinyl; scanner: non-abrasive paper, vellum, mylar, sepia, blueprints, plastic film, plastic laminate, foam board, and cardboard, to name but a few.

Experimental Evaluations:

[0025] A series of coating solutions were prepared where all but one (the control solution) comprised varying amounts of HM PVOH. The constituents for each of these solutions are listed in Table 1 ; also shown are the respective amounts for each constituent. The amounts shown are in weight parts and values are provided for both dry and wet.

Table 1

[0027] The clay used was Englehard Lustra #1 and one part of clay was high-shear mixed with one part of deoinized water for 30 minutes. After mixing, each composition was stirred for 30 minutes. A microwave solids, viscosity, and pH test was conducted on each sample. [0028] These compositions were applied as a topcoat to a commercial board sample

(James River/ St. Francisville, B-105/STFVL board). The solutions were applied to the board with a number 6 Meyer rod. The target coat weight was 4 pounds per 1000 square feet of applied surface area. Each sample was dried in a forced air oven at 250° F for a period of about 30 seconds. The coated boards were cut into 1" strips with a Lako® tensil die cutter and heat sealed with a Lako® SL- 10 hot tack/seal tester at 200° F, 20 psi for 15 seconds (50% and 100% humidity) and 27O⁰F, 20 psi for 15 seconds (50% and 100% humidity). The seal strength was tested with a 180-degree peel test method on an Instron® Model 5542 machine. The peel rate was 200mm/min and the test results were in Newton-force.

[0029] Five test samples were prepared for each of the different compositions. Peel tests were performed on each of the test samples, the peel tests were conducted at 200° F at 50% humidity, 200⁰ F at 100% humidity, 27O⁰F at 50% humidity, and 27O⁰F at 100% humidity. The amount of force required to peel the adjacent test samples was recorded. Of the four test conditions, only the test performed at 270° F 100% humidity yielded peel test strength results recordable by the Instron® device. The results of that test are shown in Table 2 below.

Table 2

[0030] Based on the test results of Table 2, no blocking was detected in the test samples coated with compositions having 1% HM PVOH and 4% HM PVOH. The test samples having compositions of the remaining percentages of HM PVOH (2%, 3%, 5% and 6%) exhibited generally reduced peel strength values. Accordingly, coating substrates with compositions having HM PVOH decreases blocking between adjacent layers of substrate.

[0031] Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations could be made without departing from the spirit and scope of the invention as defined by the following claims.

Claims

What is claimed is:

1. A media composition comprising: a substrate; and a coating on the substrate, wherein the coating comprises a pigment, a binder, and hydrophobically modified polyvinyl alcohol.

2. The media composition of Claim 1, wherein the hydrophobically modified polyvinyl alcohol comprises to about 10% by weight of the coating.

3. The media composition of Claim 1, wherein the hydrophobically modified polyvinyl alcohol comprises from about 0.5 % by weight to about 8% by weight of the coating.

4. The media composition of Claim 1, wherein the hydrophobically modified polyvinyl alcohol comprises from about 1% by weight to about 6% by weight of the coating.

5. The media composition of Claim 1, wherein the hydrophobically modified polyvinyl alcohol is fully hydrophobic.

6. The media composition of Claim 1, wherein the hydrophobically modified polyvinyl alcohol is partially hydrophobic.

7. The media composition of Claim 1, wherein the pigment is selected from the group consisting of aqueous dispersions of coating grade clays, titanium dioxide, calcium carbonate, barium sulfate, talc, zinc sulfate, aluminum sulfate, calcium oxide reaction products, lithopone, zinc sulfide, other coating pigments, other similar materials, and mixtures thereof.

8. The media composition of Claim 1, wherein the pigment comprises at least 90 wt.% of the component selected from the list consisting of clay and calcium carbonate.

9. The media composition of Claim 1, wherein the binder is selected from the group consisting of casein, soy protein, starch, synthetic polymer emulsions, styrene butadiene, acrylic latex, vinyl acetate latex, styrene acrylic copolymers, and mixtures thereof.

10. A method of coating a media substrate comprising: applying a composition to a layer of a media substrate, wherein the composition comprises, a pigment, a binder, and hydrophobically modified polyvinyl alcohol..

11. The method of Claim 10 further comprising printing on the coated surface of the substrate.

12. The method of Claim 10, wherein the hydrophobically modified polyvinyl alcohol comprises to about 10% by weight of the coating.

13. The method of Claim 10, wherein the hydrophobically modified polyvinyl alcohol comprises from about 0.5 % by weight to about 8% by weight of the coating.

14. The method of Claim 10, wherein the hydrophobically modified polyvinyl alcohol comprises from about 1% by weight to about 6% by weight of the coating.

15. The method of Claim 10, wherein the hydrophobically modified polyvinyl alcohol is fully hydrophobic.

16. The method of Claim 10, wherein the hydrophobically modified polyvinyl alcohol is partially hydrophobic.

17. The method of Claim 10, wherein the pigment is selected from the group consisting of aqueous dispersions of coating grade clays, titanium dioxide, calcium carbonate, barium sulfate, talc, zinc sulfate, aluminum sulfate, calcium oxide reaction products, lithopone, zinc sulfide, other coating pigments, other similar materials, and mixtures thereof.

18. The method of Claim 10, wherein the pigment comprises at least 90 wt.% of the component selected from the list consisting of clay and calcium carbonate.

19. The method of Claim 10, wherein the binder is selected from the group consisting of casein, soy protein, starch, synthetic polymer emulsions, styrene butadiene, acrylic latex, vinyl acetate latex, styrene acrylic copolymers, and mixtures thereof.

20. The method of Claim 10, wherein the step of coating a media substrate is performed with a media modifying system.