Patent Grant
10195888
The field of the invention relates to the improvement of image quality and ink adhesion on polymer films, particularly with water-based printing inks.
Digital printing—particularly inkjet and laser printing—is the fastest growing area of the printing market. While conventional printing processes such as offset lithography, rotogravure, and flexography are at present growing at the rate of (at most) 1% per year, digital printing is growing at the rate of more than 7% per year.
Advantages of digital printing over “conventional” printing processes include:
1. The ability to produce high quality but low volume products more economically than traditional printing processes.
2. The ability to produce personalized products by “Variable Data Printing” (VDP). Using VDP, each item in a print run can be varied to suit each customer, if desired.
Printing on plastic films and metal or metalized foils is a key part of the printing and packaging markets. These products are used as packaging, outdoor display, or as personalized information-carrying cards (drivers' licenses, credit cards, debit cards, etc.).
The plastic film industry is vast, comprising many end-uses (packaging, display, laminates, and others). According to the 2007 Economic Census released by the U.S. Census Bureau, the plastic bag and pouch industry (2007 NAICS code 326111) consumed more than $3 billion (US) of raw materials. A similar amount of raw material was consumed by other areas of plastic film usage (2007 NAICS code 326112). At a conservative estimate of $2/kg for raw materials, this shows that several million tonnes at least of raw plastics are used by the plastic film and sheet industry in the US alone.
Printing with water-based flexographic inks on smooth, impermeable materials such as polymer films presents a challenge. In particular, increasing environmental pressures on printers will force the greater use of water-based inks. However, printing with water-based inks presents serious problems on non-porous and frequently water-repellent polymers. Water-based inks will transfer and spread unevenly on such surfaces. Furthermore, ink adhesion to such surfaces is frequently poor. For this reason, pre-treatment of polymer surfaces with a high voltage electrical corona is commonplace before printing and converting. However, digital print quality—particularly water-based inkjet print quality—is poor on corona treated polymer film.
The Prior Art contains several examples of nanofibrillated cellulose (NFC) as a component of conventional paper coatings. For example, Hamada and Bousfield coated papers with different amounts of NFC, followed by printing with water-based flexographic inks, and with water-based ink jet inks (both dye-based and pigmented). The visual quality of pigmented inks was greatly improved. In a related study, Hamada et al. combined prepared heavier coatings in which NFC acted either as coating pigment (preponderance of NFC in the coating), or as coating binder (preponderance of clay blended with a small amount of NFC). NFC as binder in a clay coating did greatly improve the ink holdout, but there was no advantage of the NFC over the much less expensive polyvinyl alcohol) binder.
Zou also showed that solid CNC films (thickness of approximately 100 μm) can be printed by inkjet printing, and give superior print quality to specialty silica coated inkjet papers.
The Prior Art does contain many examples of patents in the narrow field of coatings specifically for the inkjet printing of plastic transparencies for overhead projectors. In general, the coatings described in such patents depend on water-receptive polymers sometimes combined with small amounts of filler. Nevertheless, no publications or patents have been found covering the topic of printing of film coatings containing cellulose nanocrystals coated onto polymer substrates.
The Prior Art contains many examples of patents in which soluble cellulosic derivatives such as methyl cellulose, hydroxypropyl cellulose, and others are used as film forming polymers for coatings. A recent patent described the addition of “oligofructose fibers” (i.e., soluble cellulose dietary fiber material such as inulin) to these coatings, possibly as reinforcing agents.
Plastic packaging materials for food require resistance to penetration of both water vapour and oxygen gas. It is well-known in the Prior Art that polymers such as polyethylene and polypropylene provide good protection against water vapour, but poor protection against oxygen gas. Conversely, it is also well-known that cellulosic films provide good protection against oxygen gas but poor protection against water vapour.
The object of this concept disclosure is to provide an improved recording medium for digital printing, particularly by the inkjet and laser printing, as well as for the flexographic (flexo) and other printing processes as well. This is achieved by means of applying thin coatings of cellulose nanocrystals (CNC) blended with polyvinyl alcohol) (PVOH) or other polymeric materials onto polymer films, to help trap the ink, to prevent the ink from spreading and bleeding on the polymer film surface, and to improve the ink adhesion. The hydrophilic nature of CNC and the high surface area and small pores of CNC film create a substrate layer to provide a fast ink setting while retaining the ink dye or pigments on the surface. Controlled adhesion of the ink/toner and of the CNC to the polymer film is achieved by blending polyvinyl alcohol (PVOH), ethylene vinyl acetate copolymer (EAA) or carboxylated latexes with the CNC, and by pre-treating the polymer film with an electrical corona discharge before coating with CNC. In particular, adhesion of the ink film in the presence of water is achieved by substituting ethylene vinyl acetate copolymer (EAA) or carboxylated latexes for the PVOH in the blend. A higher ratio of CNC to EAA gives faster ink drying, but poorer ink wet rub resistance. A lower ratio of CNC to EAA gives slower ink drying, but better ink wet rub resistance. Therefore, ink drying rate vs. wet rub resistance can be balanced according to customer need by varying the ratio of CNC to EAA in the coating. Thin coatings of CNC/polymer blends are also effective at reducing the oxygen transmission rate (OTR) through plastic films. Although the coatings described in this work were applied by hand, the materials are fully compatible with industrial coating equipment.
In accordance with one aspect of the present invention, there is provided a polymer recording medium for digital printing, at least one layer of a coating on the medium, the coating comprising a cellulose nanocrystals (CNC), and a polymeric material compatible with CNC.
In accordance with another aspect of the present invention, there is provided the recording medium herein described, wherein the polymeric material is selected from a polyvinyl alcohol, an ethylene vinyl acetate (EVA) copolymer, an ethylene acrylic acid (EAA) copolymer, a carboxylated latex and combinations thereof.
In accordance with another aspect of the present invention, there is provided the recording medium herein described, wherein the coating has a weight ratio of polymeric material (PM) to CNC is from 0.1% to 95.0% PM/CNC.
In accordance with another aspect of the present invention, there is provided the recording medium herein described, wherein the coating has a weight ratio of polymeric material (PM) to CNC is from 20% to 80% PM/CNC.
In accordance with another aspect of the present invention, there is provided a coating composition for a polymer recording medium, the coating comprising a cellulose nanocrystals (CNC), and a polymeric material compatible with CNC.
In accordance with another aspect of the present invention, there is provided the composition herein described, wherein the polymeric material is selected from a polyvinyl alcohol, an ethylene vinyl acetate (EVA) copolymer, an ethylene acrylic acid (EAA) copolymer, a carboxylated latex and combinations thereof.
In accordance with another aspect of the present invention, there is provided the composition herein described, wherein the coating has a weight ratio of polymeric material (PM) to CNC is from 0.1% to 95.0% PM/CNC.
In accordance with another aspect of the present invention, there is provided the composition herein described, wherein the coating has a weight ratio of polymeric material (PM) to CNC is from 20% to 80% PM/CNC.
In accordance with another aspect of the present invention, there is provided a method of producing a polymer recording medium for digital printing comprising preparing an aqueous suspension comprising a cellulose nanocrystals (CNC), and a polymeric material compatible with CNC. providing the recording medium; treating the medium in a corona discharge; preparing a suspension of a cellulose nanocrystals (CNC), and a polymeric material compatible with CNC, and applying at least one layer of the suspension onto the medium.
In accordance with another aspect of the present invention, there is provided the method herein described, wherein the polymeric material is selected from a polyvinyl alcohol, an ethylene vinyl acetate (EVA) copolymer, an ethylene acrylic acid (EAA) copolymer, a carboxylated latex and combinations thereof.
In accordance with another aspect of the present invention, there is provided the recording medium herein described, wherein the coating has a weight ratio of polymeric material (PM) to CNC is from 0.1% to 95.0% PM/CNC.
In accordance with another aspect of the present invention, there is provided the method herein described, wherein the coating has a weight ratio of polymeric material (PM) to CNC is from 20% to 80% PM/CNC.
In accordance with another aspect of the present invention, there is provided the method herein described, wherein the corona discharge is in a range from 5 to 25 W min/m2.
The present invention is aimed at improving the print quality and the adhesion of printing inks and digital inks to plastic films. Print quality is defined in terms of improved colour reproduction, greater print density (i.e., a “blacker” black print), more uniform printed areas, better definition (less raggedness) of lines and printed characters, and improved ink adhesion. The fundamental concept of a polymer recording medium 100 for digital printing including a thin coating 1 of CNC on a flexible plastic film 2 is shown in
As already stated, digital printing processes are the fastest growing processes today. However, while traditional “conventional” printing processes such as offset, rotogravure (gravure), and flexo are growing very slowly (if at all), they still dominate both publication and package printing.
In general, flexo is the process of choice for low/medium quality package printing, offset is the process for medium/high quality package printing, and gravure is the process of choice for high quality package printing, especially when a long production run is required. Since water-based rotogravure inks are chemically similar to water-based flexo inks, we will use only water-based flexo inks as examples of this class of water-based printing inks.
Problems with water-based inkjet inks in general include
The present invention has been evaluated on three different types of polymer films. The three polymers were chosen as they cover a wide range in both cost and (of importance for this invention) a wide range of surface free energy. The surface free energy of a material is a measure of its ability to be wet by water and other fluids. A higher surface free energy is more easily wet by water. An example of this is a clean glass surface. A lower surface free energy is more difficult to wet by water. An example of this is a wax paper or other water-repellent material.
The three polymers and their surface free energies are:
Water based inks have the advantages of being cheaper and more environmentally friendly than solvent-based inks. Although water-based flexo inks have supplanted solvent-based inks on paper-based packaging, solvent-based inks are still important in the printing of plastic films. The reason is that for many applications, water-based inks on plastic films cannot match the print quality or the drying rate of solvent-based inks, particularly for multi-colour process printing.
Plastic films (with and without thin CNC coatings) were printed with water-based and solvent-based black flexo inks specifically formulated for use on plastic films.
Plastic films (with and without thin CNC coatings) were printed with water-based inkjet inks on desktop printers made by different equipment manufacturers.
Print density or print optical density is a measure of the darkness of an ink film. For a black ink film, a higher print density indicates a “blacker” film. For commercial printing, print densities of at least 1.2 and as high as 1.7 are desirable, depending on the requirements of the particular print job. Printed ink films must also be resistant to removal, for example, by rubbing, by peeling, and by other actions.
Print quality was determined in terms of the optical print density of the black inks.
The resistance of the printed ink films to rub was measured using a Sutherland ink rub tester, U.S. Pat. No. 2,734,375, Canadian Pat. 532,864, manufactured by the Brown Company, Kalamazoo, Mich.
The resistance of the printed ink films to rub in the presence of water (“wet rub Resistance”) as measured as above, after the addition of 0.07 mL of water to the surface of the print.
The resistance of the printed ink films to peel (i.e., ink removal by forces perpendicular to the printed surface) was determined by the technique described by Skowronski and Bichard, using the Instron 5867 Tester. The basic illustration of a peel test measurement schematic is presented in
The drying of freshly-printed flexo ink was measured as the setoff of the wet ink film onto a reference paper. The schematic of the setoff test 200 is shown in
The setoff includes an engraved anilox roll 10 that prints ink directly onto the CNC-coated film. At position 20, CNC-coated plastic film receives ink, and wet ink may remain on the CNC-coated plastic film. As the roll 10 rotates it transfers this remaining wet ink from the CNC-coating to the reference paper as a measure of ink drying at position 30. In a preferred embodiment, the setoff test 200 apparatus also includes a corona discharge 40, adjacent the engraved anilox roll 10, where the CNC plastic film can be treated.
The rate of transmission of oxygen gas through the coated and uncoated plastic films was measured on the OpTech O2 Platinum instrument, made by Mocon Inc., Minneapolis, Minn., USA. Measurements were made at a temperature of 23° C. and a relative humidity of 50%.
Typical Procedure to Prepare CNC-Based Coatings
Typically, one starts with an aqueous stock suspension of approximately 4% (by weight) of CNC. A concentrated (approximately 30% by weight) solution of polyvinyl alcohol) is added dropwise until the final proportion of 9 parts (by weight) of CNC and 1 part (by weight) of PVOH is reached. Sodium sulfate is added to achieve a concentration of 0.005 molar Na2SO4. The mixture is then dispersed for several hours until uniform. The addition of a small amount of sodium sulfate is useful for viscosity control, but is not an essential part of the invention.
Both CNC in its acidic form (suspension pH of approximately 2.5) and its neutralized form (pH approximately 6.5) were used. Unless otherwise noted, in this document, the neutralized form is referred to.
Typical Procedure to Apply a Thin Coating of CNC Onto a Polymer Film Surface
Immediately before coating, a sheet of PET or other polymer film is treated under a commercial corona discharge, typically (although not exclusively) within the range 5 to 25 W·min/m2. In this document, “corona”, “corona treatment” or “corona power” are always understood as being given in units of W·min/m2.
The corona-treated polymer sheet is immediately taped to a uniform glass sheet. The CNC/PVOH mixture was is distributed on the Mylar™ film using a drawdown “bird” or blade suitable to give a dry coat weight of approximately 3 g/m2.
CNC coatings were made containing PVOH in the ratio of 0.1% to 95% by weight of PVOH in the coating. CNC coatings were also made in the ratio of 20 parts by weight of CNC with 80 parts of ethylene acrylic acid copolymer or 80 parts of latex polymer; in the ratio of 20 parts by weight of CNC with 40 parts of ethylene acrylic acid copolymer and 40 parts of PVOH; and in the ratio of 20 parts by weight of CNC with 40 parts of latex polymer and 40 parts of PVOH.
A coating containing 90% by weight of CNC and 10% by weight of polyvinyl alcohol) (PVOH) was applied to a sheet of PET film.
Vertical and horizontal lines of nominal 0.4 mm thickness were printed on a commercial inkjet transparency film, on pure PET film, and on pure PET film that had been treated with corona power of 10 immediately before printing. The same lines were also printed onto PET that had been coated by a mixture of CNC/PVOH immediately after corona treatment.
Visual examples have already been provided in
As shown in Table 1, the line width on the commercial inkjet transparency film was close to the nominal width of 0.4 mm, and the raggedness of the line was minimal. The line width on the control untreated PET films was nearly triple the nominal value, and the line raggedness was severe. In contrast, the line width on the PET films that had received a thin coating of CNC/PVOH were all close to the nominal value, and the line raggedness was minimal. We note that the degree of line broadening may depend upon the chemistry of the inkjet ink that is used.
The degree of undesirable colour-to-colour bleeding was greatly reduced on a thin coating of CNC/PVOH on PET sheets compared to untreated PET alone, as shown quantitatively in Table 2. Visual examples have already been presented in
As shown in Table 3, undesirable colour to colour bleeding in inkjet printing on polyethylene and polystyrene films is greatly reduced by the presence of a thin coating of CNC. Visual examples have already been provided in
Ink removal by rubbing action on inkjet prints depends on the type of plastic, on the level of corona pre-treatment of the plastic before coating with CNC, and on the amount of PVOH in the CNC mixture.
For all three plastic films (PET, polyethylene, and polystyrene) the lowest amount of ink removed by dry rubbing is achieved by corona pre-treatment of 25 watts power, before coating with 20% of PVOH by weight in the CNC blend. We note in Table 4 that on the polyethylene film, corona pre-treatment alone gives a certain amount of resistance to rubbing, but without the superior print quality of the CNC coating.
The ink peel strength (i.e., removal of ink by forces perpendicular to the printed surface) is greatly increased by the presence of a thin coating of CNC. As shown in Table 5, the peel strength of inkjet ink on unmodified PET plastic is poor. A small amount (0.3% by weight) of PVOH in an CNC coating increases the peel strength somewhat. A PVOH content of more than 2.5% or more by weight increases the peel strength to a maximum, beyond which there is no statistically significant benefit to additional PVOH addition.
As shown in Table 6, line quality in laser xerographic printing of PET film is improved with a thin coating of CNC film compared to the uncoated PET film.
A thin coating of CNC on PET, polyethylene, and polystyrene films was printed with water-based and solvent-based flexographic inks formulated specifically for use on plastic films. The optical print density (blackness) of the printed ink film is a common measure of the print quality, and is a common quality target that printers are required to meet. Table 7 shows that on plain plastic films without CNC coatings, the optical print density of the water-based ink is inferior to the optical print density obtained with solvent-based inks. Table 7 also shows that with a thin CNC coating on the plastic surfaces, print density is greatly improved and is equivalent to the print density obtained with solvent-based ink. We further note that not only are water-based inks more environmentally friendly than solvent-based inks, but water-based are less expensive, providing another potential benefit to printers.
The improved dry adhesion and the improved print quality of the prints is maintained with a proportion of as little as 20 parts of CNC to 80 parts of PVOH. Other proportions may be optimal depending on the ink chemistry.
As shown in Table 8, the acid form of the CNC imparts better rub resistance to printed ink films than does the neutral form. The inks themselves are alkaline, and it is known that an acidic substrate can more easily set or immobilize an alkaline ink.
A thin coating of CNC on PET, polyethylene, and polystyrene films was printed with water-based and solvent-based flexographic inks formulated specifically for use on plastic films. The printed samples were rubbed in a standard dry ink rub test. The results in Table 9 show that on plain plastic films without CNC coatings, a very large percentage of the ink is removed. However, the resistance to removal of ink by rub is greatly improved when the ink is printed onto a thin coating of CNC rather than onto the plastic surface. This reduction in dry ink rub has also been illustrated in
As shown in Table 10, the wet rub resistance of a film of 20 parts CNC and 80 parts PVOH is poor. In this Example, the substitution of 80 parts of ethylene acrylic acid copolymer in place of 80 parts of PVOH or the substitution of 80 parts of styrene-acrylonitrile latex in place of 80 parts of PVOH maintained the print density improvement and the dry rub resistance, but at the same time gave a greatly improved wet rub resistance.
A thin coating of CNC plus EAA reduces the oxygen transmission rate (OTR) of a polyethylene film. The benefits largely derive from the CNC content in the thin coating. As shown in Table 13, the OTR of the polyethylene film is somewhat reduced by a thin containing 20% CNC and 80% EAA. The OTR is much further reduced by a coating containing 80% CNC and 20% EAA. As also shown in Table 13, the normalized OTR is far lower with 80% CNC in the coating, compared to the coating containing 20% CNC or to the untreated polymer film.
This application is a National Phase entry of PCT/CA2015/050389 filed May 5, 2015, in which the United States of America was designated and elected, and which remains pending in the International phase until Nov. 9, 2016, which application in turn claims priority under 35 USC 119(e) from U.S. Provisional Application Ser. No. 61/991,042 filed May 9, 2014.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2015/050389 | 5/5/2015 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2015/168784 | 11/12/2015 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4474850 | Burwasser | Oct 1984 | A |
| 4547405 | Bedell et al. | Oct 1985 | A |
| 5126194 | Light | Jun 1992 | A |
| 20120009365 | Simpson et al. | Jan 2012 | A1 |
| 20150140237 | Schmidt | May 2015 | A1 |
| Number | Date | Country |
|---|---|---|
| 2010124378 | Nov 2010 | WO |
| 2012150383 | Nov 2012 | WO |
| 2013060934 | May 2013 | WO |
| 2013186367 | Dec 2013 | WO |
| 2014044870 | Mar 2014 | WO |
| 2011112618 | Sep 2015 | WO |
| Entry |
|---|
| Supplementary European Search Report dated Dec. 19, 2017 from corresponding EP 15789566.5. |
| Hamada, H. and Bousfield, D.W., Nano-fibrillated cellulose as a coating agent to improve print quality on synthetic fiber sheet, 11th TAPPI Advanced Coating Fundamentals Symposium, Munich, Oct. 2010. |
| Hamada, H., Beckvermit, J., and Bousfield, D.W., Nanofibrillated Cellulose with Fine Clay as a Coating Agent to Improve Print Quality, Paper presented at Papercon 2010, Tappi Press Atlanta 2010. |
| Zou, X. Production of nanocrystalline cellulose and its potential applications in specialty papers, Presented at Specialty Papers 2010, Chicago, Nov. 10-11, 2010. PIRA, Leatherhead, UK. |
| http://www.surface-tension.de/solid-surface-energy.htm; accessed Sep. 28, 2011. |
| http://www.rubtester.com/index.html, Accessed Nov. 4, 2011. |
| Skowronski, J. and Bichard, W., Fiber-to-Fiber Bonds in Paper. (1). Measurement of Bond Strength and Specific Bond Strength, J. Pulp Paper Sci. 13:J165 (1987). |
| Aspler, J.S., Newsprint requirements for water-based flexography, Part III: Influence of newsprint properties on flexo ink holdout, J. Pulp Paper Sci. 14:J66 (1988). |
| PCT International Search Report from corresponding PCT/CA2015/050389 filed May 5, 2015. |
| http://www.census.gov/, accessed Apr. 27, 2011. |
| Number | Date | Country | |
|---|---|---|---|
| 20170267009 A1 | Sep 2017 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61991042 | May 2014 | US |
