METHOD OF DETERMINING INK RECEPTIVITY ON SURFACES

Abstract: A method for determining ink receptivity on surfaces is disclosed. The method may be used to predictably and reliably screen suitable substrate coating compositions (and/or substrates) or alternately suitable ink compositions in printing applications. The method relies on determining the response of a surface to ink(s) in terms of the force needed to immerse and remove a sample of the surface from an ink. Preferably, the force is expressed in terms of a contact angle of the ink with respect to the surface. By adjusting the composition of the substrate coating composition (and/or substrate) or of the ink to result in the inks behaving in a similar fashion to the coated or uncoated substrate, predictable printed image characteristics may be obtained relating to such properties as image density and definition.

Description: METHOD OF DETERMINING INK RECEPTIVITY ON SURFACES BACKGROUND OF THE INVENTION 1.1 Field of the Invention The present invention is generally directed toward improved methods for determining the receptivity of ink to substrates based on the force needed to extract a sample of the substrate in contact with an ink. More particularly, the present invention provides a method for predicting multicolor ink jet printability on surfaces of substrates by contact angle measurement alone without having to print on the surface. 1.2 Related Art A problem in evaluating compositions suitable as coating surfaces for printing is the necessity of having to coat large samples of paper (or other substrates), actually print on the samples and subsequently evaluate the quality of the printed samples for print resolution and density. While the foregoing is a straightforward and reliable approach in selecting suitable coating compositions, it is somewhat time consuming, particularly with regard to sample preparation and analysis of the image after it has been printed. U.S. Patent No. 4,446,174 ('174 patent) describes a method for producing a recorded image on an image-receiving sheet with a jet of aqueous ink. The ink jet is projected onto the sheet comprising a surface layer containing a pigment. The surface layer adsorbs a coloring component present in the ink. The invention purports that when the surface layer contains a pigment and adhesive with an R f value (ratio of travelling distance in the pigment coating of dye to that of the solvent in the aqueous ink) of 0.59, a suitable combination of pigment coating and aqueous ink has been found. The principle under which this method operates is that of paper chromatography. -2- However, a disadvantage of the method described in the '174 patent is that to get an R f number, the coating has to be applied to a glass plate. This means that paper variables are not accounted for in the test. Thus, it is possible that the method of the '174 patent will predict a coating having an acceptable R f number but when applied to a piece of paper or media the results are different. In contrast, the present invention offers an alternate method for determining the proper pigment coating and/or substrate compositions for use with multi-colored aqueous inks as hereinafter described which actually tests the ink's receptivity with the coating in conjunction with the substrate or the substrate alone. SUMMARY OF THE INVENTION The present invention relates to methods for determining a surface's receptivity to ink comprising the steps of: (a) providing a sample of the surface; (b) immersing and removing the sample with the ink; and (c) measuring the force during the immersion and removal of the sample from the ink. In another embodiment, the invention is concerned with a method for determining a suitable surface coating composition for receiving multicolored inks comprising the steps of: (a) providing a sample of a surface coated with the composition; (b) immersing and removing individual portions of the sample with more than one color of ink; (c) measuring the force during the immersion and removal of the sample portions from each of the inks; (d) comparing the force measured for each of the inks; (e) adjusting the coating composition if the measured force for each of the ink vary by more than a preset deviation. -3- ln yet another embodiment, the invention relates to a method for determining a suitable ink composition in response to a surface comprising the steps of: (a) providing a sample of a surface coated with the composition; (b) immersing and removing the individual portions of the sample with more than one color of ink; (c) measuring the force during the immersion and removal of the sample portions from each of the inks; (d) comparing the force measured for each of the inks; and (e) adjusting one or more of the ink compositions if the measured force between each of inks and surface vary by more than a preset deviation. Another embodiment of the invention relates to a method for determining a suitable substrate composition for receiving multicolored inks comprising the steps of: (a) providing a sample of the substrate; (b) immersing and removing the individual portions of the sample with more than one color of ink; (c) measuring the force during the immersion and removal of the sample portions from each of the inks; (d) comparing the force measured for each of the inks; and (e) adjusting the substrate's composition if the measured force for each of the inks vary by more than a preset deviation. Advantages of the methods of this invention include providing a quick, and reliable method for screening coating compositions suitable for use in printing applications. Alternately, the method may be used to adjust the composition of a dye or ink in response to a surface or as a method of determining a suitable substrate composition in response to a dye or ink. -4. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic of a device useful in the practice of this invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The methods of this invention relate to an improved tool for quickly and reliably screening suitable printing surfaces and printing inks which result in exceptional printed image characteristics of high density and high definition. The present invention will become more apparent from the following definitions and accompanying discussion. The following definitions are helpful in understanding the invention. Substrate - refers to media capable of being printed upon, such as paper, cardboard, fabric, canvas, polymers such as vinyl, acetate, and mylar. Surface - refers to coated or uncoated substrate surfaces. Contact Angle - refers to the tangent angle of a liquid to a surface. Depending on the affinity of a liquid to a surface, the liquid can spread across the surface (low contact angle) or be repelled by the surface (high contact angle). Contact angles may be calculated from dynamic contact angles measured during immersion and removal of a surface from a liquid during a measured time period. Too much deviation in contact angle will impart either loss of density or excessive bleed. Additionally, it is desirable to control the value of the contact angle between 60° and 110°, preferably between 70° and 100°, most preferably between 70° and 90°. Contact angles less than 70° will tend to have poorer image density. Contact angles greater than 100° will tend to have greater bleed. The intent is to have similar contact angle -5- responses for all the inks to achieve an excellent balance of color density and definition. If any one or any combination of the four inks respond to the coating outside these ranges, the performance will be impacted. It will either have bleed of colors generated with inks that respond with greater than 100° or poor density of colors generated with less than 70°. Preset Deviation - refers to an acceptable variation in the measured or calculated force between the ink and surface which gives the desired printed quality in terms of color density and color definition. Thus, in some applications, the required variation may be quite small in order to achieve a balance of high density and high resolution. Typically acceptable printed quality is achieved when the deviation in terms of contact angle is not more than ±10, preferably not more than ±5, most preferably not more than ±3 degrees from the numeric average of the measured contact angles. In its fullest embodiment, the present invention is based on identifying coating compositions or inks which respond equally in terms of the individual ink's affinity to a surface. For example, finding a coating composition that responds equally to the different surface tensions (a measure of the force or affinity of the ink to a surface) of the four colors used in ink jet printing would result in exceptional image density and definition. Too much deviation in surface tension among the inks in response to the coating surface will impart either a loss of image density or excessive bleed (loss of definition). Thus, it is desirable to have the various ink surface tensions near the same value in order to achieve a similar affinity to the coating composition. In a preferred embodiment, the affinity of the ink(s) to the surface is expressed in terms of the contact angle between the ink(s) and the surface. As noted above, the contact angles may be determined from measurement of a dynamic contact angle. Commercially available dynamic contact angle measurement devices may be used such as the Cahn DCA 315 available from Cahn Instruments, Madison, Wl. This device contains the instrument software "WinDCA" copyrighted 1996 which calculates a contact angle based on the immersion and removal of a surface sample into a liquid a distance of 4mm in 25 seconds from the point of contact. These parameters can be changed. Figure 1 depicts a simplified schematic of the Cahn DCA 315 apparatus and a short description of its operation follows. A 1" x 1" square SAMPLE (1) is secured to an ARM (2) which in turn is connected to a FORCE MEASUREMENT DEVICE (3). A BEAKER (4) containing the TEST INK (5) is elevated manually until the TEST INK (5) is in close proximity to the SAMPLE (1). Typically, this distance is less than 0.5 cm. The instrument program in the device is run which raises the TEST INK (5) in order to contact and immerse the SAMPLE (1). Once contact is made between the SAMPLE (1) and the surface of the TEST INK (5), the change in force associated with the contact is made. The change in force over a specified distance indicates the affinity or repulsion of the TEST INK (5) for the SAMPLE (1). The program then measures the force during retraction of the SAMPLE (1) from the TEST INK (5). All of the measurements are then used to calculate the dynamic contact angle for the TEST INK (5). While applicants have demonstrated the invention through sample/ink affinity as determined by the Cahn device, other means to gauge the level of sample/ink affinity may be used. Such means include a goniometer, reflection techniques and other conventional ways to measure affinity through surface tension techniques. EXAMPLES The procedure using the Cahn DCA 315 device described above is demonstrated here. Four test inks, representative of inks used in in jet printing applications were prepared from paper dyes available from Clariant Corporation. The paper dyes used were Brilliant Blue GNS Liquid (Cyan color), Red A-4G New Liquid (Magenta color), Cartasol Yellow 3GF Liquid (Yellow color), and Ink Black RAS Liquid Purified 075 (Black color). Each of -7- these colors were made into test inks by diluting 100 microliters of the color with a sufficient amount of deionized water to achieve a total test ink volume of 40 milliliters. The measured surface tension of each of these inks are as follows: Ink Surface Tension (dynes/cm) Cyan 56.8 Magenta 61.6 Yellow 60.8 Black 63.7 The affinity of these prepared colors to three surfaces were determined. The surfaces tested were a uncoated piece of bond paper (Base Sheet), a commercially available inkjet premium coated sheet (Premium Sheet) and an experimental coated sheet (Experimental Sheet). Following the procedure described above using the Cahn DCA 315 device, the following contact angles were determined for the prepared inks and test sheets and are reported in TABLE 1. TABLE 1 CONTACT ANGLE (Deαreesϊ SHEET CYAN(dev V MAGENTA(dev ) 1 YELLOWCdev.V BLACK(dev V AVG BASE 111.2(+17.35) 88.3(-5.55) 89 1(-4.75) 86.8(-7.05) 93.85 PREMIUM 79 1(+2.28) 80.2(+3.38) 79.1 (+2.28) 68.9(-7.92) 76.82 EXPERIMENTAL 81.3(+0.05) 80 3(-0.95) 81.5(+0.25) 81 9(+0.65) 81.25 1 dev = deviation in contact angle from AVG = individual ink contact angle - AVG Referring to TABLE 1 , one sees that for the uncoated BASE SHEET, the contact angle for the Cyan color deviates quite substantially compared to the deviation in contact angles of the Magenta, Yellow and Black colors. Referring to the PREMIUM SHEET, one sees that the contact angle for the BLACK color deviates quite substantially from those of the Cyan, Magenta, and Yellow colors. Referring to the contact angle results of the -8- EXPERIMENTAL SHEET, one sees little deviation. The effectiveness of the present invention in predicting and correlating printing characteristics, such as color density and color definition based on contact angles, are shown below based on evaluation of printed samples using a HP 560 C Deskjet. Description of Color Density and Definition Ratings The DOS based HP Deskjet printer Dietzgen test pattern generates areas for analysis for both color density and definition. Color densities are measured with a COSAR 200 densitometer with a polarized filter. Five readings are taken for each color from the approximately 1 " x 1 " color block generated by the printers. The color density readings are taken at four corners and the center of the color square, averaged and then recorded. For color definition, the solid color blocks are used for all the primary and combination colors. Ratings are determined by "Good", "Slight", and "Moderate". The distinction for each relates to the color block edge sharpness, non-printed line sharpness (within the color blocks), in both horizontal and verticle directions. However, the test pattern has additional areas to determine black color definition. It has blocks for tone comparison. Black blocks of 3 , 34, and full tone also indicate definition. The better the tone distinction the better the definition. Finally, the area of the pattern for fine definition is such that eight lines are produced, both horizontally and vertically, from a common point. They separate approximately 1 degree from each other over a distance of 1.70 inches. Therefore, the closer to the focal point each line is identifiable, the better the definition. The ratings are comprised of each of these areas and given an overall definition rating. The color density and color definition results are summarized in Table 2. TABLE 2 COLOR DENSITY AND COLOR DEFINITION RESULTS Sheet Cyan Magenta Yellow Black Color Density BASE 1.32 0.97 0.77 1.75 PREMIUM 1.65 1.10 0.85 2.50 EXPERIMENTAL 1.51 1.07 0.83 1.88 Color Definition BASE SLIGHT SLIGHT GOOD SLIGHT PREMIUM GOOD GOOD GOOD MODERATE EXPERIMENTAL GOOD GOOD GOOD SLIGHT Referring to the results of Table 1 and Table 2, one see that the method of this invention is capable of predicting both color density and definition. For example, the contact angle results of Table 1 order the SHEETS in the following order from lowest to highest contact angle for the color Cyan: PREMIUM (79.1), EXPERIMENTAL (81.3) and BASE (111.2). The color density results of TABLE 2 for these SHEETS have the corresponding order for color density from lowest to highest color density: BASE (1.32), EXPERIMENTAL (1.51 ), and PREMIUM (1.65). Thus, one see that the higher the contact angle the lower the color density. Thus, by knowing the relative contact angle relationship, the relative color density relationship can be determined. All the other inks exhibited a corresponding correlation between contact angle and color density. Likewise, the color definition results are correlatable by comparing the contact angle deviations of Table 1 to the observed color definition results of Table 2. For example, the color Cyan has the following order in terms of measured contact angle deviations from lowest to highest: EXPERIMENTAL (+0.05), PREMIUM (+2.28), and BASE (+17.35). Correspondingly, the color definition results report EXPERIMENTAL and PREMIUM as GOOD and the BASE as SLIGHT. Therefore, one observes that color definition is adversely affected when contact angle deviations are large. The results for all the other colors correspondingly demonstrate this trend. -10- Thus, observations made concerning contact angle values and contact angle deviations can be used to predict and correlate color density and color definition results. The principles, preferred embodiments, and modes of operating of this invention have been described in the foregoing specification. However, the invention which is intended to be protected herein is not to be construed as limited to the particular forms disclosed, since they are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims: -11- What is claimed is: 1. A method for determining a surface's receptivity to ink comprising the steps of: (a) providing a sample of the surface; (b) immersing and removing the sample with the ink; and (c) measuring the force during the immersion and removal of the sample from the ink. 2. The method of claim 1 , wherein the force is expressed in terms of surface tension. 3. The method of claim 1 , wherein the force is expressed as a contact angle of the ink with respect to the surface. 4. The method of claim 3, wherein the contact angle is between 60 and 110. 5. The method of claim 4, wherein the contact angle is between 70 and 100. 6. A method for determining a suitable surface coating composition for receiving multicolored inks comprising the steps of: (a) providing a sample of a surface coated with the composition; (b) immersing and removing individual portions of the sample with more than one color of ink; (c) measuring the force during the immersion and removal of the sample portions from each of the inks; (d) comparing the force measured for each of the inks; (e) adjusting the coating composition if the measured force for each of the ink vary by more than a preset deviation. -12- 7. The method of claim 6, wherein the force is measured in terms of contact angle. 8. The method of claim 7, wherein the value of the measured contact angles are between 60 and 110 degrees. 9. The method of claim 8, wherein the preset deviation in each of the contact angles is no more than + 10 degrees from the numeric average of all the measured contact angles. 10. The method of claim 9, wherein the preset deviation is ± 5 degrees. 11. The method of claim 10, wherein the preset deviation is ± 3 degrees. 12. A method for determining a suitable ink composition in response to a surface comprising the steps of: (a) providing a sample of a surface coated with the composition; (b) immersing and removing individual portions of the sample with more than one color of ink; (c) measuring the force during the immersion and removal of the sample portions from each of the inks; (d) comparing the force measured for each of the inks; and (e) adjusting one or more of the ink compositions if the measured force between each of inks and surface vary by more than a preset deviation. 13. The method of claim 12, wherein the force is measured in terms of contact angle. -13- 14. The method of claim 13, wherein the value of the measured contact angles are between 60 and 110 degrees. 15. The method of claim 14, wherein the preset deviation in each of the contact angles is no more than + 10 degrees from the numeric average of all the measured contact angles. 16. The method of claim 15, wherein the preset deviation is + 5 degrees. 17. The method of claim 16, wherein the preset deviation is ± 3 degrees. 18. A method for determining a suitable substrate composition for receiving multicolored inks comprising the steps of: (a) providing a sample of the substrate; (b) immersing and removing the individual portions of the sample with more than one color of ink; (c) measuring the force during the immersion and removal of the sample portions from each of the inks; (d) comparing the force measured for each of the inks; and (e) adjusting the substrate's composition if the measured force for each of the inks vary by more than a preset deviation. 19. The method of claim 18, wherein the force is measured in terms of contact angle. 20. The method of claim 19, wherein the value of the measured contact angles are between 60 and 110 degrees. -14- 21. The method of claim 20, wherein the preset deviation in each of the contact angles is no more than + 10 degrees from the numeric average of all the measured contact angles. 22. The method of claim 21 , wherein the preset deviation is ± 5 degrees. 23. The method of claim 22, wherein the preset deviation is + 3 degrees.

Inventors: LONDO MICHAEL G, LONDO, MICHAEL, G.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1999
Main classification: G01N11
IPC codes: G01N13, G01N33