FIELD OF THE INVENTION

[0001] The present invention relates generally to the field of using ink jet printing to decorate substrates of consumer electronic goods. Specifically, the invention pertains to decorating articles of metal or plastics in nature that are exposed to varying environmental conditions and are subject to repetitive physical contact by a user. In another aspect, the present invention is directed to a method for producing wear-resistant high quality images on rigid, non-porous surfaces.

BACKGROUND OF THE INVENTION

[0002] Inkjet printing is emerging as the digital printing method of choice because of its ability to produce high resolution images, its flexibility, high speed, and the

affordability of this printing method. Inkjet printers operate by ejecting, onto a receiving substrate, controlled patterns of closely spaced and sometimes overlapping ink droplets. By selectively controlling the pattern of ink droplets, ink jet printers can produce a wide variety of printed features, including text, graphics, and holograms.

[0003] The non-contact printing methods rely on depositing individual ink droplets in a controlled manner onto the surface to be decorated. In order to achieve good image quality, the ink drops need to spread, join together, to a degree that is appropriate. This process requires a low advancing contact angle between the ink and the substrate.

[0004] Accordingly, ink/substrate combinations that result in good image quality when printed with contact methods such as screen printing, often exhibit insufficient wetting and poor image quality when used with a non-contact printing method such as ink jet printing. Inks used in ink jet printers vary in the chemical makeup and thus do not adhere to all substrates. Ink compositions have to be modified, therefore, to optimize adhesion on the surface of interest. For example, insufficient wetting results in low radial diffusion of the individual ink drops on the surface of the substrate (also referred to as "dot gain"), low color density, and banding effects (e.g. gaps between rows of drops). Thus, image quality is controlled by the wetting and flow properties of the ink used. [0005] Water-based and solvent-based inks that typically contain about 90% of an organic and/or an aqueous solvent are the most commonly used inks especially for applying onto porous substrates like paper. Water-based inks require porous substrates or substrates with special coatings that absorb water, while solvent-based inks also demands the use of a porous substrate or coatings and are prone to emitting organic solvents during drying of the ink.

[0006] For application directly onto rigid non-porous substrates such as plastic, metal sheet, wood etc a radiation-curable ink composition comprising polymerizable ingredients can be used. The polymerizable ingredients readily react upon exposure to a suitable radiation source (e.g. ultraviolet light, electron beam) to form a cross linked polymer network.

[0007] A general problem with forming good quality images of ink jet inks is that ink compositions do not uniformly adhere to all substrates.

SUMMARY OF THE INVENTION

[0008] The present invention relates to a method for decorating and protecting surfaces of rigid or semi-rigid substrates.

[0009] The method comprises the steps of (a) providing a surface; (b) applying a radiation curable or heat curable ink receptive coating to the substrate surface; (c) depositing an ink composition on to the ink receptive coating after or simultaneously with applying the ink receptive coating; (d) radiation cure or heat cure the resulting coating.

[0010] The ink for ink jet printing images can comprise organic solvents or be water diluted. Radiation curable inks also exist and they can also comprise organic solvents or water.

[0011] The ink receptive coating can have a composition that is either heat curable or radiation curable. Both types of compositions can comprise organic solvents or be water based. It is important that the inks having organic solvents are combined with ink receptive coating also having organic solvent and the water based inks must be combined with water based receptive coatings.

[0012] The radiation curable ink receptive coating composition comprises: • about 20% to about 80% by weight of a suitable radiation curable oligomer;

• about 15% to about 75% by weight of a suitable radiation curable monomer;

• about 1% to about 5% by weight of a suitable photo-initiator; and

• optional amount of an organic solvent or mixture of solvents if solvent based inks are to be used or of water if water based inks are to be used.

[0013] The heat curable ink receptive coating composition comprises:

Pack (A)

• about 40% to about 90% by weight of a hydroxyl functionalized polyacrylic resin, or a hydroxyl functionalized polyester resin;

• about 5%> to about 15% by weight of a suitable additive; and

• about 5% to about 20% by weight of a an organic solvent or mixture of solvents if solvent based inks are to be used or of water if water based inks are to be used;

Pack (B)

• about 30%) to about 90%> by weight of a polyisocyanate or and mixtures of

polyisocyantes; and

• about 5% to about 20% by weight of an organic solvent or mixture of solvents if solvent based inks are to be used or of water if water based inks are to be used; and

Pack (C) of an organic solvent or mixture of solvents if solvent based inks are to be used or of water if water based inks are to be used to form the ink receptive coating composition, and wherein

the components of packs (A), (B) and (C) are blended together prior to the application of the ink receptive coating composition; and the mole ratio of isocyanate to hydroxyl functional group is in the range from about 1.0 to about 1.5 or a mole ratio in the range from about 1.0 to about 1.3.

[0014] The ink receptive coating allows the deposited ink to adhere to the surface so that a good quality image that meets the physical and chemical requirements demanded by common consumer products is achieved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention provides a method for decorating and protecting surfaces of rigid or semi-rigid substrates. The method comprises the use of a quality ink and an inkjet printer. Specifically, the substrates to be decorated or protected are various plastics or metal sheets that come in contact with harsh environmental conditions, or have surfaces subjected to repetitive contact, such as a mobile telephone, computer, automotive interiors, household machines or cosmetic or electronic packages, or are durable for outdoor usage. "Durable for outdoor usage" refers to the ability of the article to withstand temperature extremes, exposure to moisture ranging from dew to rainstorms, and colorfast stability under sunlight's ultraviolet radiation. The threshold of durability is dependent upon the conditions to which the article is likely to be exposed and thus can vary. At minimum, however, the articles of the present invention do not delaminate or deteriorate when submersed in ambient temperature (25 °C.) water for 24 hours, nor when exposed to temperatures (wet or dry) ranging from about -40 °C to about 60 °C.

[0016] The ink jet printer may include design modifications in order to effectively print on a rigid or semi-rigid substrate compared to its normal use for printing on paper sheets, but the basic principle of color distribution from the printing head of a piezo ink jet printer is maintained.

[0017] To obtain a high resolution decorative surface on a rigid or semi-rigid substrate, the inventive method relies on the use of an ink receptive coating. The application of the ink receptive coating to the substrate before jet printing the ink allows for an improved quality of the printed image. According to the present invention certain coating compositions exhibit a surprisingly good balance between ink uptake and the color density of the image obtained. It is also surprising that the ink receptive coating composition can be substantially free of filler.

[0018] The ink receptive coating composition can be heat cured or radiation cured. The composition can be water based or include an organic solvent or mixture of solvents. In order for the receptive coating to be compatible with the ink, a water based ink should be used with a water based receptive coating and an organic solvent based ink with an organic solvent based receptive coating.

Ink Receptive Coating

[0019] To achieve good image quality during ink jet printing the printed ink drops must spread to within an acceptable range to provide a complete solid fill of the image. It was found that the use of an improperly formulated coating as the ink receptive layer can result in an image of poor quality. With ascribing to any particular theory, the inventor believes that an improperly formulated ink receptive coating will hinder or prevent the proper spread of the ink drops, which will result in an unfilled background that can contribute to reduced color density and banding defects (i.e. gaps between the rows of ink drops). Alternatively, if the ink drops spread too much, it would result in an overall loss of resolution in the image, poor edge acuity, and inter-color bleeding.

[0020] The inventive ink receptive layer described herein exhibits a good balance of ink uptake and color density. To maintain the quality of the image, it is important that the size of an inkjet dot remains substantially the same after it is dropped on the receptive layer at least until the drop dries and becomes immobilized. The image quality can be

quantitatively expressed with reference to color density and with regard to the final ink dot diameter, as described in U.S. Pat. No. 4,914,451.

[0021] The ink receptive coating compositions for use in the invention may be solvent- based or water-based depending on the solvent used in the ink composition of choice and the level of resolution desired in the final image.

[0022] In one embodiment, therefore, is provided an ink receptive coating composition that comprises about 20%-80% of a suitable radiation curable oligomer, about 15%-75% of a suitable radiation curable monomer, about l%-5% of a suitable photoinitiator as well as possibly other additives. Optional amount of organic solvent or solvent mixtures or water when water based inks are to be used, is added.

[0023] Alternatively, the ink receptive composition is a heat curable composition. The composition is formed by mixing the contents of two separate packs (A) and (B) prior to the application of the composition on to a surface. Thus, pack (A) comprises from about 40% to about 90% w/w of a hydroxyl functionalized polyacrylic resin and/or a hydroxyl functionalized polyester resin with a hydroxyl functional group content of l%-5% w/w on solids. Also present are suitable additives in an amount from about 5%> to about 15 > w/w and about 5% to about 20% w/w of a suitable organic solvent or mixture of organic solvents or water when water based inks are to be used.

[0024] Included in Pack B is about 30%> - 90%>w/w of a suitable polyisocyanate or a mixture of polyisocyanates and about 10%> - 70%> w/w of solvent, or mixture of solvents, or water when water based inks are to be used. [0025] The composition for the ink receptive layer can also comprise an additional amount of organic solvent or solvent mixture, or water when water based inks are to be used for mixing the components of packs (A) and (B). In a preferred embodiment, the components provided in packs (A) and (B) are blended together prior to the ink receptive composition is to be applied to a surface. The mole ratio of hydroxyl functional group to isocyanate in the ink receptive composition is in the range from about 1.0 to about 1.5 and more preferably a mole ratio in the range from about 1.0 to about 1.3. The blending of the components should be made in a relatively short time before applying it to the surface. Normally a storage time of the blended composition should not exceed four hours.

[0026] In an embodiment of the inventive method, the surface of the substrate to be decorated is directly coated with the ink receptive composition so as to form an ink receptive coating. In another aspect of the invention, however, the surface to be decorated is first primed using a primer followed by deposition of the ink receptive coating. A primer can be used for achieving a suitable color background for the image to be printed and for enhancing the adhesion of the ink receptive coating to the substrate.

[0027] The choice of components for the ink receptive coating is important because the ability of the ink to adhere to the ink receptive layer will depend on the chemical makeup of the ink receptive layer. Thus, a particular composition of the ink receptive coating capable of adhering one particular type decorating ink (that is, solvent based, water based or radiation curable ink), may not be suitable for a another type of ink. Thus, if the ink particles diffuse too quickly in the medium of the ink receptive layer, ink from the ink jet printer can run or cause "sagging" of the ink layer which can result in poor image quality.

[0028] To prevent ink running and sagging, the solubility parameters of the ink, ink receptive layer and the corresponding base polymer of the primer composition are matched. Further, the primer is chosen such that it exhibits good adhesion to the substrate, and to the ink receptive layer. It also serves as a way to bring the necessary background color if needed. Good adhesion is needed as measured according to ASTM D 3359-95A.

[0029] The ink is deposited using a piezo ink jet printing mechanism in order to create an ink jetted image. As used herein "ink jetted image" and "ink jet printed" both refer to an image created with an ink jet printing process employing a water based or organic solvent borne ink. The ink can also be a radiation curable ink comprising water or organic solvent. The image may be text, graphics, coding (e.g. bar coding), etc., being comprised of a single color, multi-colored or being unapparent in the visible light spectrum. The article whose surface has been decorated can optionally comprise additional coatings, such as a top coat that overlays the imaged ink receptive layer. In another embodiment, the primer and imaged ink receptive layers, as well as the entire decorated article, is durable for outdoor usage, preferably, without the use of additional protective layers.

[0030] The ink can be printed in a separate step or be printed simultaneously with the ink receptive coating.

[0031] After printing the ink, the coating is cured using a suitable radiation curing method or heat cured. Heat curing can be made at 50 - 80°C and the heating can take between 30 to 60 minutes.

[0032] Energy source used for achieving cross linking of the radiation curable functionality may be actinic (e.g., radiation having a wavelength in the ultraviolet (UV) or visible region of the spectrum), accelerated particles (e.g., electron beam (EB) radiation), or the like with UV and EB being preferred. Suitable sources of actinic radiation include mercury lamps, xenon lamps, carbon arc lamps, tungsten filament lamps, lasers, electron beam energy, and sunlight.

[0033] The surface of the article to be decorated should be smooth and uniform to maximize the clarity and detail of the printed image. In one embodiment, the smoothness is less that 40%, or less than 25%, or less than 15% Sheffield roughness. The brightness of the medium to be imaged should be greater than 90%, or even greater than 95%. In another embodiment, the ink jet medium has an opacity of greater than 95%.

Primers

[0034] The selection of primer is based on the intended ink receptive composition, ink composition as well as the chosen substrate. The primer alters the surface properties of the substrate such that ink receptive layer and the ink jet ink are able to interact with the surface to be decorated in a consistent manner, resulting in good image quality. The primer is chosen such that the main liquid component of the intended ink receptive composition (e.g. the UV curable composition), is able to interact and adhere to the surface of the primer. [0035] Primer compositions in accordance with the present invention comprise a base polymer. The base polymer may be a single polymer or a blend of polymers. The blend of polymers can form a homogeneous mixture or be multiphase, exhibiting two or more distinct peaks when analyzed via differential scanning calorimetry (DSC). Further, the primer composition may comprise an interpenetrating network of the base polymer in an insoluble matrix or vice-versa. The primer compositions for use in the invention include solvent-based primer compositions and water-based primer compositions, and can be cured by heat or radiation.

[0036] Solvent-based primer compositions comprise the base polymer admixed with a solvent. The solvent may be a single solvent or a blend of solvents. The solvent-based primer composition preferably contains about 5 to about 60 parts by weight of the base polymer, more preferably about 10 to about 40 parts base polymer and most preferably about 10 to about 30 parts base polymer, with the remainder of the primer composition being solvent and optional additives.

[0037] Alternatively, in another embodiment, a water-based primer can be used. Water- based primers are preferably emulsions or dispersions that are substantially free of water soluble base polymers.

Ink Compositions

[0038] High quality ink compositions are often used with drop-on-demand (impulse) printhead, especially in a piezoelectric drop-on-demand print head. The surface tension of piezo inkjet compositions is moderate to low. Preferred formulations have a surface tension in the range of from about 20 mN/m to about 50 mN/m and more preferably in the range of from about 22 mN/m to about 40 mN/m at the print head operating temperature. Further, piezo ink compositions typically have Newtonian or substantially Newtonian viscosity properties. In the context of the invention "a Newtonian fluid" refers to a fluid whose viscosity is at least substantially independent of shear rate. Newtonian fluids have a power law index of 0.95 or greater and is calculated using the expression η = my ^11"1 , wherein η. is the shear viscosity, γ is the shear rate in s ^"1 , m is a constant, and n is the power law index. See, for example, C. W. Macosko, Rheology: Principles,

Measurements, and Applications, ISBN #1-56081-579-5, p. 85. [0039] Suitable piezo inks for use in the invention include ink compositions

commercially available from Epson worldwide under the trade designations "Epson Stylus Series Inks" and ink compositions available from Zhongheng Company in Guangzhou, China.

[0040] The phrase "solvent based ink composition" refers to non-aqueous inks. Either a single solvent or a blend of solvents can be used in the solvent based ink composition. Suitable solvents used for such ink compositions include without limitation alcohols such as isopropyl alcohol (IP A) or ethanol; ketones such as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), diisobutyl ketone (DIBK); cyclohexanone, or acetone; aromatic hydrocarbons such as toluene; isophorone; butyrolactone; N-methylpyrrolidone; tetrahydrofuran; esters such as lactates, acetates, including propylene glycol monomethyl ether acetate, 2-butoxyethyl acetate, diethylene glycol ethyl ether acetate (DE acetate), ethylene glycol butyl ether acetate (EB acetate), dipropylene glycol monomethyl ether acetate (DPMA), iso-alkyl esters such as isohexyl acetate, isoheptyl acetate, isooctyl acetate, isononyl acetate, isodecyl acetate, isododecyl acetate, isotridecyl acetate or combinations of these.

[0041] Because organic solvent based compositions tend to dry more rapidly than aqueous based compositions, the former are preferred for ink jet imaging. Further, organic solvents suitable for use in accordance with an embodiment of the inventive method, typically have a boiling point of less than 250 °C. and a vapor pressure of greater than 5 mm of mercury at 200 °F. (93 °C). Highly volatile solvents, such as MEK and acetone, tend to be avoided, as such solvents dry too quickly resulting in nozzle clogging at the print heads. Further, a suitable viscosity range for piezo ink compositions is about 3 centipoise to about 30 centipoise at the operating temperature of the printhead.

Example 1

[0042] Onto a white PC/ABS substrate was applied an ink receptive coating composed of 41.36 g of U0857 resin from Lihua Resin company, 13.79 g of SR-351 resin from

Sartomer Co., 6.89g of resin Dimensions UA-747 from Zhongshan DSM, 3.91 g of

Miramer M202 from Miwon Specialty Chemical Co., 1.64 g of Laromer 8996 from

BASF, 12.4 g of M890 solution from Degussa, 8.73g of Ethyl Acetate, 5.00 g of TPO photoinitiator from BASF, 0.55 g of Tego Glide 410, 6.22 g of Dowanol PM acetate, and

2.08 g of Butyl Cellosolve. The receptive coating can be applied to the substrate using common application methods such as spray, dip, or curtain coating etc. After the coating is applied, the substrate is loaded in a piezo digital printer retro-fitted to accept a flat substrate. The desired image is then printed non-contactedly onto the receptive coating. After the printing, the substrate is moved and irradiated in a Fusion UV curing flatbed system equipped with mercury lamp light source. The typical energy required to reach complete cure is about 600 mini-joule/cm 2 at 300 mini-watt/cm 2.

[0043] The resulted image is high in quality with appropriate dot gain to ensure complete translation of information as in the original picture file onto the decorated substrate.

[0044] They system is now completely cured and can be subjected to performance test as detailed below.

Example 2

[0045] Onto a white PC/ABS substrate was applied an ink receptive coating composed of 65.00 g of Macrynal SM515-70 resin from DSM, 13.79 g of SR-351 resin from

Sartomer Co., 6.89g of resin Dimensions UA-747 from Zhongshan DSM, 3.91 g of Miramer M202 from Miwon Specialty Chemical Co., 1.64 g of Laromer 8996 from BASF, 12.4 g of M890 solution from Degussa, 8.73g of Ethyl Acetate, 5.00 g of TPO photoinitiator from BASF, 0.55 g of Tego Glide 410, 6.22 g of Dowanol PM acetate, and 2.08 g of Butyl Cellosolve. The receptive coating can be applied to the substrate using common application methods such as spray, dip, or curtain coating etc. After the coating is applied, the substrate is loaded in a piezo digital printer retro-fitted to accept a flat substrate. The desired image is then printed non-contactedly onto the receptive coating. After the printing, the substrate is moved and irradiated in a Fusion UV curing flatbed system equipped with mercury lamp light source. The typical energy required to reach complete cure is about 600 mini-joule/cm 2 at 300 mini-watt/cm 2.

[0046] They system is now completely cured and can be subjected to performance test as detailed later.

Example 3- An optional primer

[0047] Onto a grey PC/ABS substrate was applied a primer coating composed of 39.00 g of U-1908R resin from A&P Resin, 21.00 g of Setal 90173 resin from Nuplux, 3.00 g of CAB 531-1 from Eastman Chemical Co., 5.00 g of Dowanol PM Actetate, 7.50 g of n- Butyl Acetate, 7.50 g of xylene, 20.00 g of 844-0061 tint paste from Degussa. The primer coating can be applied to the substrate using common application methods such as spray, dip, or curtain coating etc. Appropriate solvents can be used to dilute the formulation prior to application. After the coating is applied, the substrate is further subjected to the applying of ink receptive coating as detailed above.

[0048] Typical Performance tests:

[0049] Thermal shock durability: Test method developed based on IEC 60068-2-14: Environmental testing - Part 2: Tests. Test N: Change of temperature, and IEC 60068-2- 33: Environmental testing - Part 2: Tests. Guidance on change of temperature tests. Detail test requirements: Temperature change '-40° / +85 °C; Temperature change time 3 min; 5 cycles, 1 cycle = 2h/2h; After 2 h recovery in the laboratory environment; Visible decomposition are not allowed; Mechanical decomposition are not allowed

[0050] Humidity endurance: requirement: 3x6 or 18 days; cyclic 25/55°C; Relative humidity shall be between 93...96 %; Visible decomposition are not allowed; Mechanical decomposition are not allowed.

[0051] Chemical endurance: Resistance to various of chemical reagents such as Hand cream, sun lotion, lip stick, make-up cream, insect repellent, cooking oil etc.

[0052] UV Radiation endurance: Product materials shall have no mechanical and visual changes, mechanical parts shall be unbroken and have no deformation after the test.