CA2099777A1

CA2099777A1 - Oil based ink composition

Info

Publication number: CA2099777A1
Application number: CA002099777A
Authority: CA
Inventors: Thomas M. Fritz
Original assignee: Individual
Current assignee: Beidel Printing House Inc
Priority date: 1991-11-22
Filing date: 1992-11-20
Publication date: 1993-05-23
Also published as: WO1993010199A1; AU3148293A

Abstract

A non-aqueous coating for use on printable substrate is disclosed. A polyurethane/varnish coating is used to impart improved gloss and abrasion resistance to the printed substrate. The non-aqueous coating, furthermore, imparts waterproofing and color enrichment properties to the substrate. A polyurethane-containing ink experiencing improved dry-back and faster drying is also disclosed.
The ink may be a soybean oil-based ink, a rubber-based ink, or a petroleum oil-based ink.

Description

W093/10199 PCT/US92/lOt8~
209~ ~7 OIL-BASED INK COMPOSITION

This is a continuation-in-part of U.S. Patent Application Serial No. 07/628,568, filed December 17, l990.
FIELD OF THE INVENTION
The present invention relates to ink compositions applied to substrates, and in particular to inks comprised of a mixture of polyurethane and an air-curable, oil-based ink applied directly to a substrate.
BACKGROUND OF THE INVENTION
A coating is a clear over-print applied to a press sheet to improve the overall glossiness and provide abrasion resistance to the printed image. Today's printing industry demands high gloss, high abrasion resistant coatings, for example as dust jackets for books, for advertising brochures, and food packaging.
Coatings are applied in-line on-press as an extra press unit, off-line on coating or varnishing machines, or in-line followed by off-line to achieve the greatest effect. Coatings are applied locally to selected image areas with conventional offset lithographic techni~ues on press, or with blanket cut-outs on coating machines.
Coatings may a~so be applied over the entire sheet with offset blankets either on-press or on a coating machine.
The term "varnishing" is still used to define in-line coating using printing units to apply a coating to a localized area.
Occasionally, the term is also used to differentiate creative, special-effect coating from production coating. Varnishing originally referred to coating with a boiled linseed oil varnish, applied on-press as if the varnish were a clear ink. Although varnish increased gloss and rub-resistance, anti-setoff powder used in printing decreased the gloss and rub-resistance. In some cases, varnish actually decreased gloss.
In general, there are two types of coa~ings presently used by the printing industry; "aqueous" coatings and "W " coatings, neither of which has proven wholly satisfactory. Aqueous (acrylic) and ultraviolet ( W ) coatings have over the years, however, come to replace varnish as the primary coating mediums for over-, . ~

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WO93/10199 PCTIUS92/1018;_ 2 ~ 7 7 printing.
An aqueous (acrylic) coating is a water-soluble, thermoplastic polymer coating that requires heat and air to dry or harden the coating. Aqueous coatings are 60% solvent, (water and alcohol), much of which escapes in the coating and drying process, along with some o the gloss. Once applied and dried under infrared heat, aqueous coatings harden instantly, but because they are thermoplastîcs they can be softened with reapplication of heat.
A W coating is a monomer coating that requires ultra violet energy to dry or harden the coating. UY coatings have a photo-initiator which start the ultraviolet hardening process. UV
coatings are solventless, which means that onca applied and dried under ultraviolet energy UV coatings harden instantly and are not susceptible to softening.
Although aqueous and UV coatings have improved ~he level of gloss and abrasion resistance over previously used coatings, acrylic and W coatings are not without problems. For example, aqueous coatings will not adhere to a previous UV coating. Also aqueous coatings are less glossy and less rub-resistant than UV
coatings. A~ueous coatings have a tendency to "reactivate" on the sheet, for example, when stored in a warm warehouse, which can cause the sheets to stick together and ruin the job.
W coatings are more expensive than aqueous coatings, requiring special expensive equipment and ultraviolet lights, which require workers to wear protective clothing and goggles. W
coatings have a tendency to crack when a printed shee-t is folded, and cannot be printed over once applied to a substrate. Also, W
coatingS typically require sp~cial catalysts and other additives to impart an acceptable coating.
In general, the prior art coatings are not especially useful in many different applications for which coatings are required, such as hot- and cold-food packaging. Often, coatings of the prior art exhibit low gloss, poor scratch resistance, poor waterproofing, heat and cold sensitivity, and adherence problems relative to the substrate. Frequently the coatings of the prior art do not significantly minimize the tendency for ink colors to fade. Also, .

. .:: . '- , . . ' ' W093/10199 PCT/US92/1018~
2 ~ r~ 7 7 these prior art coatings do little to improve the otherwise dull colors of ink printed on recycled paper. Accordingly, it would be useful to develop a coating for printed substrates that eliminated or reduced most or all of the above-noted problems experienced with the prlor art coatings.
In general, inks comprise a mixture of pigment, vehicle, and additives. The pigment is what gives ink its color. The vehicle typically comprises a rosin-based resin, an oil, and/or a petroleum distillate. The oils may be vegetable, petroleum or rubber-based.
The additives include driers, anti-skinning agents and waxes.
Most recently, soybean oil, a vegetable oil, has been gaining acceptance as an alternative to petroleum oils in the manufacture of printing inks. Soybean oil-based printing inks have been used for both black and color printing since about 1987. Black soy inks are typically 70-80% soybean oil and color soy inks generally have a lower oi~ content by weight, including a larger proportion of pigment than the black inks.
Soybean oil-based printing inks are consi~ered to be environmentally advantageous relative to petroleum-based inks. Soy ink produces less vapors, known as "volatile organic compounds" or "V.O.C.'s", which are regulated by the EPA. Petroleum-based inks often have a 25-40~ V.O.C. rating whereas soy ink V.O.C. ratings are typically l~ss than 10%, often in the 2-4% range, and in some cases even 0~. Soy ink is also biodegradable than petroleum-based inks.
The quantity of soybean oil used in inks varies, depending upon manufacturer and the end use of the ink. Newspaper inks generally use greater than 50% by weight soybean oil, sheet-fed printers use about 20-40~ soybean oil, magazine inks use 10-15%
soybean oil and business forms are typically printed with inks containing about 40% soybean oil by weight. These percentages are based on the non-pigment portion of the ink.
Despite its growing acceptability, soybean oil-based inks have not yet realized their full potential as a replacement for petroleum-based inks. One of the concerns of soybean oil-based inks is the tendency for these inks to dry more slowly ~han the ,. ~.

W093/10199 PCT/US92/1018~
~ZOOJ9~I7rI

petroleum-based inks. This tendency to dry more slowly makes soy inks less attractive to commercial printers with high volume heat-set presses. The soybean oil-based links also tend to have inferior dry-back characteristics, adversely affecting print quality, as the ink tends to dry back into the sheet, being absorbed by the sheet as it dries. Soy inks also frequently exhibit inferior scratch resistance. Accordingly, a significant advance in the art could be realized if a soybean oil-based ink could be developed that experienced faster drying than conventional soybean oil-based inks and achieved better dry-back and scratch resistance than conventional soybean oil-based inks.
It is an object of the present invention to provide a non-aqueous coating that is waterproof and highly abrasion resistant.
It is a further object of the present invention to provide a coating that imparts improved gloss to a substrate.
It is yet anothsr object of the present inventlon to provide a coating which protects against fading of colors over which the coating is applied.
It is another ob;ect of the present invention to provide a coating which is insensitive to hot and cold temperature extremes.
It is still another object of the present invention to provide a coating which brightens ink or other pigments over which the coating is applied.
It is a further object of the present invention to allow coating and/or printing over surfaces which normally cannot be adhered to.
It is another object of the present invention to provide a coating which is quick-drying.
It is still another object of the invention to provide an oil-based ink that experiences improved drying properties, including aster drying and improved dry-back.
It is yet another object of the invention to provide an oil-based ink that exhibits increased yield.
It is a furthes object of the invention to provide an oil-based ink that yields improved scratch resistance.
SUMMAF<Y OF T~IE I~VENTION

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WO93/10199 PCT/US92/1018~
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According to the present invention, it has unexpectedly been found that a coating for printed and other substrates comprising a mixture of polyurethane and a clear-coat varnish, without any catalysts or additives, produces a waterproof coating ~hat is stable in both hot and cold environments, and imparts greater scratch resistance and gloss relative to prior art coatings. The coating dries relatively rapidly and produces improved color enhancement and fade resistant relative to prior art coatings.
In a preferred embodiment of the invention, polyurethane and clear-coat varnish are present in a weight ratio of about l:16 to l:l0 and most preferably about l:12. Because the coating is non-aqueous, it imparts waterproof properties that are not available from the aqueous coatings. The coating does not, however, require ths expensive ultraviolet equipment required for the W coatings.
In another embodiment of the invention polyurethane is mixed directly with oil-based ink, imparting improved properties to the ink.
In yet another embodiment of the invention, a coating of polyurethane/varnish is applied to the printed substrate printed with the polyurethane/ink composition.
Still another embodiment of the present invention incorporates polyurethane into an air-curable oil or rubber-based ink composition. In one preferred embodiment, a soybean oil or petroleum oil-based ink composition includes from 1% to 50~
polyurethane by weight based on the total weight of the ink prior to polyurethane addition. The ink experiences improved dry-back even at levels as low as l~ polyurethane by weight.
BRIEF DESCRIPTION OF THE FIGURES
Figure l illustrates an infrared spectrograph for varnish alone.
Figure 2 illustrates an infrared spectrograph for polyurethane alone.
Figure 3 illustrates an infrared spectrograph for varnish with polyurethane accordin~ to a preferred embodiment of the present invention.
Figure 4 illustrates an infrared spectrograph of process . .,, '' , ~., ~:
, ~ . : ~ : , ~ 6 yellow ink with polyurethane according to a preferred embodiment of the present invention.
Figure 5a-f illustrates dry-back results of preferred embodiments of the tnvention, comparing ink containing no polyurethane to ink ~ontaining polyurethane.

DETAILED DESCRIPTION OF T~E INV~NTION
One embodiment of -the present invention comprises a non-aqueous composition for use as a waterproof coating, which is stable in both hot and cold environments. Preferably, ~he composition comprises a mixture of a polyurethane and a clear-coat varnish. As used herein, the term "polyurethane" re~ers to any of the varlous polymeric materials known as polyurethane. By way of example, but not limitation, a polyurethane material that has been used with success in the present invention comprises a product by the name ZAR~LOSS, manufactured by United Gilsonite Laboratories, of Scranton, Pennsylvania. Another suitable polyurethane is manufactured by Minwax, of Montvale, New Jersey, and is sold as clear gloss polyurethane.
The ZAR~GLOSS product comprises approximately 45% by weight mineral spirits and 50% by weight modified polyurethane resin, and less than about 5% kerosene.
It is, of course, contemplated to be within the scope of the invention that it would be possible to mix polvurethane resin directly with the varnish, or the ink, or to use other polyurethane mixtures, either with or without a mineral spirit carrier, as would be appreciated by those skilled in the ar~.
As used herein, the term "varnish" refers to any oleoresinous varnish that may be used in the printing industry. Varnishes suitable for coatings are known, and include, for example those described in U.S. Patent No. 2,4l2,592, incorporated by reference herein. "Clear coat varnish" is a well-known ~erm used and understood by those skilled in the printing industry. By way of example, but not limitation, a varnish that has proven useful in connection with the present invention includes that known by the name ALGLOSS OVERVARNIS~, product code P017, manufaotured by Algan, ,.

~093/10199 RCT/US92/1018~
Q ~ ~ 3 rl ~ 7 Inc. of Chagrin Falls, Ohio. ~lso, overprint varnish sold by Cal, Inc. of Denver, CO has proven effective in composi~ions of the present invention.
In a preferred embodiment of the invention, approximately one part of polyurethane, by weight, is mixed with approximately lO to 16 parts of varnish by weight to produce a non-aqueous coating which may be applied directly to the inked sheet. Most preferably, one part by weight polyùrethane to twelve parts by weight varnish is used. If the ratio of varnish to polyurethane goes much below lO, the polyurethane tends to form a layer which interferes with providing an acceptable coating. If the ratio goes much above l6, the improved results of the present invention becomes less pronounced.
Surprisingly, it has been found that when this coating is applied and dried, the resultin~ coated substrate exhibits superior gloss, abrasion resistance, color enhancement and fade resistance relative to prior art coatings.
The use of polyurethane generally in connection with coatings is not new. ~ille, et al. U.S. Patent No. 4,496,675 discloses an aqueous dispersion based on a polyurethane synthetic resin. This dispersion includes a hydrolyzed shellac as a second binder component. However, this coating is aqueous, and must be manufactured by hydrolyzing the shellac, by heating the composition in water in the presence of bases to prepare the aqueous solution.
Such a coating would not hold up to the waterproof requirements of the present invention. Haas et al~, U.S. Patent No. 4,775,558, discloses polyurethanes combined with other materials, such as linseed oil. However, the art has failed to recognize the unexpectedly superior coating properties achieved using the critical limitations taught by the present invention, wherein only l part by weight polyurethane is used with lO to 16, and most preferably 12, parts by weight of varnish as a waterproof coating composition exhibiting high gloss and scratch resistance.
Example l A coating of the present invention was prepared by thoroughly mixing one part ZAR@~,LOSS polyurethane by weight with l2 parts by ~,, . . .. .~ - , - . . . ,,, ~, .
:; : , .: ,., ~ : -, WO 93/10199 PCT/US92/1018'_ q ~9rl1~ 8 weight of ALGLOSS OVERVARNISH. A light reflection test wasconducted using paper substrate printed with dense black ink. The same ink batch was used for all test pieces, and the light reflectivity test was conducted according to industry accepted practice, which includes illuminating the test piece at a 60 angle and measuring the amount of light reflected from the test piece using a gloss meter. In each case, the coating applied ~o the test piece was allowed to dry before testing.
Table I below discloses the comparative light reflection results of uncoated paper (Sample A) a coating ~f the prior art (varnish only, Sample B) and the coating of the present invention (Sample C). As illustrated, the coating of the present invention provides far superior light reflective properties than either the.
uncoated or varnished surfaces. This is particularly surprising, in that only about 8% (wt.) of the Sample C composition by weight comprised polyurethane (only 4% on polyurethane solids basis), yet Sample C exhibited over 58% better gloss than the varnish coating alone, Sample B.
Table I
Liqht Reflection Test - Polvurethane/Varnish Coatinq Sample % Liqht Reflected A Dense Black Only 48.5%
B Dense Black with Varnish 48.5%
C Dense Black Wi~h Polyurethane/Varnish 77.5%
It has been found that the coating of the present invention produces a high-gloss coating which dries in approximately 2 hours, requiring non heated dryers. However, if heated driers are used, it would be possible to apply a thicker coating, resulting in even higher gloss. The coating produced is scratch resistant, and requires no special coater or tower-type baking oven. The coating can be run on a small duplicator offset press, a large offset press or a letter press.
The coating of the invention is applied in the same manner as ink except the entire surface is coated. No special mixing is required, but the polyurethane and varnish should be thoroughly , .

W093/10199 PCT/US92/1~18~
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mixed prior to application.
The coating of the present invention protects ayainst ultraviolet rays, enriches colors of printed materials, bonds to surfaces that normally cannot be overprinted, such as uv coatings, and brightens ink on recycled paper. Significantly, the coating o the present invention imparts waterproof properties to the substrate, an important consideration for food packaging.
Additionally, the coating on the present invention has been found not to be affected by heat or cold.
Example 2 A sunli~ht test was used to determine the present invention's ability to prevent ultraviolet fading. The test pieces were pr~nted with a yellow-green ink, which has a high tendency to fade, ~nd set in a sunny window for approximately three weeks. Each of the test pieces was partially covered to provide a control color for comparison. The three test pieces included an uncoated sheet, a sheet coated with a W coating, and a sheet coated with a coating of polyurethane/varnish as described in Example 1. Upon inspection at the conclusion of the test, the uncoated sheet experienced almost complete color loss, while the W coated sheet exhibited some fading, as clearly indicated by a strong contrast line separating the exposed and unexposed portions of the sheet. The sample coated with the polyurethane/varnish coating of the present invention exhibited the least amount of fading, as the line separating the exposed surfaces from the unexposed surfaces was barely discernible.
Example 3 Cardstoek used for food packaging was coated wlth the polyurethane/varnish coating of Example 1 and placed in a freezer for 7 weeks. No frost penetration or water damage to the underlying substrate was visible ater this time period. When this type of coated sheet is placed under running tap water, no water penetration to the substrate occurs, and water rolls quickly off the surface.
Although the precise mechanism whereby the polyurethane and ., , ~: '' . , : .,:, W093/10199 PCT/US92/1018~_ varnish interact, when used in the preferred ratios, is not precisely understood, it is believed that the polyurethane and varnish combine chemically in some way, producing a coating yielding the superior properties of the present invention. An IR
spectrum has been run on the coating of tha present invention, indicating that the combined polyurethane and varnish comprise a product which is different from the polyurethane and varnish individually. See Figures 1-4.
In another embodiment of the invention polyurethane is added directly to an oil-based ink in an amoun~ of l:lOO to l:l parts by weight polyurethane to oil-based ink. As used herein, the term "oil-based ink" is intended to embrace inks con~aining vegetable oil, petroleum oil and/or rubber-based inks of all kinds. It has unexpectedly been found that by adding the polyurethane to the oil-based ink in this way the ink adheres to a previously coated job.
The polyurethane also acts as a tack reducer, and enables reds, blues and violets to withstand heat application without reactivation (fading) or bronzing. The polyurethane in turn brightens the ink pigment and imparts higher gloss to the finished print. Furthermore, it has been found that the ink dries harder and more quickly, with less setoff, and better release than the same ink without polyurethane. Also, the polyurethane and ink mixture produces a smoother solid with less ink (higher yield) and protects against fading.
ExamPle 4 The same polyurethane/varnish coating as usad in E~ample l was employed, except in this example, one part of polyurethane per lO
parts of oil-based dense black ink, by weight, (from the same ink batch used in Example l) was used. Three test pieces were run, Sample D, which was the polyurethane/ink alone, Sample E, which was the polyurethane/ink overcoated with varnish only, and Sample F, which was the, polyurethane/ink overcoated with the polyurethane/varnish coating of Example l. The results are shown in Table II, which shows that the polyurethane, when added to the ink, increases reflectivity slightly over the ink without .

WO93/10199 2 D 9 ~ 7 ~ 7 PCT/U592/1~18~ ~

polyurethane shown in Sample A of Table I. Also as shown in Table II, when the test piece was varnished, (Sample E) reflectivity actually dropped, but was improved significantly (35.8%) by overcoating with the polyurethane/varnish coating of the present invention (Sample F).
It has been found that when the ink contains polyurethane, it is possible to print on substrates that will not normally accept ink. For example, we have unexpectedly f ound that the polyurethane, when added to rubber-based inks, permits these inks to print on surfaces, such as enamel, to which such links normally do not adhere.
Table II
Liqht Reflectivi~Y Test - Polyurethane in_Ink Sample ~ Liaht Reflected D Dense Black Only 50.4%
E Dense Blac~ With Varnish 49.2 F Dense Black With Polyurethane/Varnish 66.8%
Example 5 A sheet of paper having a W coating was printed with the same type of ink as described in Sample F of Example 4 above. The ink remained on the surface, and after drying, adhered to the surface and could not be rubbed off. The same W coated paper was printed with a standard ink, that is, not containing polyurethane, and was allowed to dry. The resulting ink smeared off the sheet when dry with little hand pressure.
In a preferred method of practicing the present invention the lnk to which polyurethane has been added, preferably in quantities of about one to fifty parts polyurethane per one-hundred parts ink, by weight, is printed on a substrate, followed by the application of a polyurethane/varnish coating (of the type described in Example l). After the coating is applied, the printed substrate is allowed to cure in conventional manner, for example by air-drying or through heated drying if desired. However, it has been surprisingly found that drying times for the present invention have .~
'',:,~ ~. " '`,, W093/10l99 PCT/US92/1018 been reduced from typically 24-48 hours to only 2-1/2 hours.
The coatings of the present invention are useful on a variety o printable substrates including paper, cardboard, liner-board, card-stock, wood, metal, mineral substrates, enamel, plastics, cellophane and foil.
ExamPle 6 A Southerland Rubber Tester was used to test the abrasion resistance of the coatings of th~ present invention. A coating similar to that described in Example 1 was applied to various substrates under various conditions as set forth in Table III, indicating extremely good abrasion resistance.
Table III
Substrate Pound TestNo. Strokes Abrasion 100# Gloss Enamel 2 100 No Visible Scuff Marks 100# Gloss Enamel 4 80 Visible Scuff Marks 10 Pt. Coated 2 50 Very Light Board Stock Scuff Marks 10 Pt. Coated 4 50 Light Scuff Marks Board Stock Cambell Board Stock 4 40 Visible Scuff Marks Poly Board Stock 4 40 Visible Scuff Marks 1266 Board Stock 4 40 Visible Scuff Marks 1061 ~oard Stock 4 40 Visible Scuff Marks Recycled Board 4 40 Visible Scuff Marks It is preferred in practicing the present invention that oil-based inks be used, taking advantage of the non-aqueous nature of the coating.
Optionally, following the curing of the coating used according .

W093/10199 PCT/U~92/1018~
2~777 to the present invention, it is possible to coat the pri~ted substrate with a W coating, using conventional methods.
Example 7 In another embodiment of the invention, an air-curable ink composition consisting of a mixture of an oil-based ink and polyurethane in a weight ratio of polyurethane to ink of about l:lOO to about l:l was prepared. Figures 5a-f show the results of (a) dry-back tests run using an ink containing petroleum oil and no polyurethane, which had an 8 point dry-back; (b) an ink containing one part polyurethane by weight to 70 parts petroleum oll-based ink having a 4 point dry-back; (c) an ink containing one part polyurethane to 40 parts petroleum oil-based ink having a 4O7 polnt dry-back; (d) an ink containing one part polyurethane to 4 parts petroleum oil-based ink having a 5 point dry-back; (e) an ink containing one part polyurethane to two parts petroleum oil-based ink having a 3 point dry-back; and (f) an ink containing one part polyurethane to one part petroleum oil-based ink having a 1.7 point dry-back. All parts in this example are on a weight basis, and the same oil-based black ink was used in all examples illustrated in Figures 5a-f. As Figures 5a-f and this E~ample 7 indicates, adding as little as about l.4% by weight polyurethane to an oil-based ink composition can improve the dry-back dramatically, in this case, by 50%.
A normal, or generally acceptable dry-back measure is 5-lO
points. It is highly preferred, however, to have less dry-back, and zero dry-back is especially desirable. In general, the higher the dry-back, the lower the density of the printed ink, when dry, and the more ink needed to bring the density up to desired levels.
WQ have unexpectedly found, as evidenced by visual comparison, tha~
the polyurethane-containing inks of the invention can experience zero dry-back, and in some cases actually experience eqative dry back, that is, a hiaher density when dry than when wet on the substrate.
The inks tested in Figures 5a-f proved to have better rub characteristics than the same inks in which no polyurethane was :, .. .. : , ~

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added. These results further establish the advantages of the present invention.
In addition to petroleum-based inks, the invention may also be used with vegetable oil-based inks, such as soy ink. A bonafide soy ink must use soybean oil as its only vegetable oil ingredient, except for trace amounts of tung oil, chinawood oil, etc.
The minimum acceptable content o~ soybean oil in soy inks, as set by the American Soybean Association, varies according to the type of ink:
Minimum Soybean T~pe Oil Content*
soy news inks 55 percent soy sheet-fed inks 20 percent soy ~orms inks 40 percent soy heat-set inks lO percent *of the non-pigment portion of the ink, by weight.
We have found that the benefits of the inven~ion, including improved dry-back, (better density), faster drying, and reduced ink consumption (higher-yield) can be realized by mixing polyurethane with soybean oil-based inks, having at least 10% by weight soybean oil exclusive of pigment in the ink prior to the addition of polyurethane. Soybean oils containing about 10-90% by weight soybean oil exclusive of the pigment can be advantageously used according to the present invention simply by mixing a quantity of polyurethane with the soybean ink prior to use. We have found that a lower limit on the advantages of the invention is reached at about l~ by weight polyurethane based on the oil-based ink. The upper limit, if one exists, appears to be dictated more by viscosity concerns; the example of Figures 5e and 5f were found to lose viscosity to the point that body-gum additives would need to be added in order to use those inks on a sheet-fed press. However, these viscosity limitations are primarily economic considerations.
A typical, commercially available soybean oil-based ink contains the iollowing components: soybean oil, pigment, resins, and waxes.

W093/10199 PCT/US92/1018~
2a~7~7 The air-curable ink compositions of the present invention are advantageous in that they do not require the significant equipment expenditure required for the radiation-curable (UV) ink compositions. As usad herein, the term "air-curable" is intended to embrace inks and coatings which may be dried or cured without aid of radiation, such as ultraviolet radiation. Additionally, the use of polyurethane reduces drying times and produces better dry-back than is possible in oil-based inks not containing the polyurethane. While the precise mechanism is not fully understood, it is believed that the polyurethane gives better penetration to the substrate, allowing a thinner ink film to be applied, producing quicker drying and reduced consumption relative to inks not incorporating the polyurethane.
ExamPle 8 Sheet ~ed oil-based ink was mixed with polyurethane in a ratio of 1 part polyurethane to 16 parts, ink, by weight, based on the total weight of the ink prior to mixing. The following Table IV
summarizes the results. The ink with polyurethane experienced no "bronzing", while the ink without polyu~ethane experienced bronzing, which is a frequent problem with red and blue inks. The ink containing polyurethane exhibited less setoff and better drying time than the ink without polyurethane. The ink used in Table IV
was Warm Red #82/660, National Printing Ink, Marietta, Georgia.

Table IV
, . . .~ = . _ _ _ _ Tack Stability Viscosity Yield PU Conductivity InX (1200 rpm) ~ _ No Poly-1 Pt Poly-15.5 172 143064% 2400 urethane to 16 Pts.
Ink. 21.5 192 23316C~ 2200 I . . _ _ _ _ == ~ =
As Table IV illustrates, the ink containing polyurethane exhibited a 63~ higher yield then the same ink with no .",~ .:: ~,, ,.,. , ~

~ 16 polyurethane. The other data in Table IV, including PU (% water pickup) demonstrate that the ink composition of the present invention exhibits properties generally recognized as acceptable by ink users and manufacturers. The lower conductivity of the invention relative to the ink with no polyurethane, is generally recognized as preferred by ink manufacturers.
Example 9 Soy ink sold under the name SOYA, by national Printing Ink, Marietta, Georgia, was mixed with polyurethane in a ratio of one r part polyurethane to 16 parts soy ink, by weight. Four ink colors, black, blue, yellow, and red, were mixed with polyurethane. The resulting ink compositions were tested for dry-back. The results are shown in Table V. Each of the figures in Table v represents a densitometer average reading for ten samples for each color.

Table V
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SOYA r- ¦ SOYA SOYA
Ink _ _ 81ack BlueYellow Red Densitometer 173.78 123.75 89.90 138 Reading Wet _ _ . _ _ Densitometer 174.67 123.25 91.90 140 Reading Dry , _ _ _, The densitometer readings indicate that the addition of polyurethane to soy ink unexpectedly results in kink compositions having a higher density when dry than when wet. Dry-back is determined by subtracting the dry densitometer reading from the wet densitometer reading. Thus, the present invention not only produces ink with near zero dry-back, (blue), but actually negative dry-back, in the cases of the black, yellow, and red inks.
Exam~le 10 Rub tests have shown that, in addition to having better dry-back, the soy inks containing polyurethane are more scratch resistant than soy inks without polyurethane. A black, sheet-fed ~093/lOt99 PCT/US92/1~18~
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process soybean ink was rub tested against the same ink containing 1 part polyurethane to 16 parts ink by weight, ~sing a Southerland Rub Tester. Although both inks showed scratching, the ink containing polyurethane showed better scratch resistance than the ink containing no polyurethane. These same inks were tested for dry-back. The results are shown in Table VI. As these results indicate, the soybean ink containing polyurethane experienced less dry-back than the ink without polyurethane.
Table VI
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Densitometer Densitometer Readinq~ Readinq Dry 81ack Soybean Ink - No Polyurethane 164.00 158.50 I _ _ _ Black Soybean Ink - 1 Part Polyurethane to 16 Parts Ink 165.33 164.00 _ _ _ . - __ -The invention has been described herein in terms of particular embodiment, which are representative and illustrative only. The particular examples serve ~o illustrate my invention, but not limit same, the invention being defined more generally by the following claims and their equivalents. While many objects and advantages of the invention have been set forth, it is understood that the invention is defined by the scope of the following claims, not by the objects and advantages.

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Claims

I claim:

1. An air-curable ink composition comprising a mixture of an oil-based ink and polyurethane in a weight ratio of polyurethane to said oil-based ink of about 1:100 to about 1:1.

2. The ink composition of claim 1 wherein said oil-based ink comprises a soybean oil-based ink.

3. The ink composition of claim 2 wherein said soybean oil-based ink contains at least 10% by weight soybean oil, exclusive of pigment.

4. The ink composition of claim 3 wherein said soybean oil-based ink contains about 10-90% by weight soybean oil, exclusive of pigment.

5. The ink composition of claim 1 wherein said oil-based ink comprises a petroleum oil-based ink.

6. The ink composition of claim 1 wherein said oil-based ink comprises a rubber-based ink.

7. The ink composition of claim 1 wherein said polyurethane is present in said composition in a ratio of about one part polyurethane to about 10 to 16 parts oil-based ink, by weight.

8. The ink composition of claim 1 wherein said oil-based ink is red, blue, yellow, or black.