GB2134533A - Screen printing ink and printing press therefor - Google Patents

Abstract

A screen printing ink comprises an acrylic monomer, a vinyl resin, a pigment grind comprising pigment and plasticisers and a catalyst comprising a paste of benzoyl peroxide and phthalate plasticiser. It may be used in high-speed printing operations having a two-phase drying system where one phase quickly forms a protective coating over the printed image and a second phase cures or polymerizes over a period of time. A printing press for handling the ink is compact in nature and includes a heated plate over which the entering web picks up heat before passing over the impression cylinder where it receives a printed image together with a plurality of heated plates beyond the impression cylinder which heat the printed web sufficiently to cause setting or gellation of the ink so that it is thereafter stackable.

Description

SPECIFICATION Screen printing ink and printing press therefor This invention relates in general to a new and improved screen printing ink and a printing press for handling the printing ofthe ink onto a web, and more particularlyto a new and improved screen printing ink which incorporates a two-phase drying system so that high-speed printing can be accomplished, and still more particularly to an improved ink incorporating a gel phase that quickly sets up a protective coating or film on the ink so that it is stackable and a polymerization phase for completely curing the ink, and still more particularly to a printing press for handling the two-phase ink.

Heretofore, considerable difficulties have been encountered in drying ink in high-speed screen printing operations. Since it is necessary to have high-speed drying in order to accomplish high-speed printing, the printing speed has been limited bythe speed of drying ofthe inks used. Particularly, in screen printing operations,thedrying problem is caused bythe thickness of ink printed which is necessaryto obtain quality screen printing products. In screen printing operations it is typical to print from 1/2 to 3 mils of ink (wet thickness) which is about five to fifteen times thickerthan other printing processes. This thickness is dictated bythe need to havethickopaque printing.

The most common drying inks known for screen printing have been solvent based, such as when they are printed and the solvent is evaporated, the ink is dry. In such systems, the highest printing speed is achieved only where highly volatile solvents are used.

Such solvents usually give off an undesirable odor as well as polluting the air around the printing press.

Some are also highly toxic. Highly volatile ink systems also cause the inkto quickly dry in the mesh ofthe printing screen. Thus, if a press stops for a short period oftime, drying ofthe ink in the mesh occurs, which requires a shutdown to clean the screen before printing can be resumed. Use of some inks even causes drying in the screen when the press is still running whereveryfine detailed images are being printed. Thus, it is extremely important to prevent drying in the screen.

More recent high-speed drying has been accomplished by irradiating certain types of resins with ultraviolet light emanating from sources such as high intensity mercuryvapor lamps, or other suitable sources. Since screen printing inks are desirable because oftheir opacity, the difficulty of penetrating these opaque colors with ultraviolet light rays has been a negative factor. The light sources are also rather costly, and the inks also are costly. Such inks do not dry in the screen, but these negative factors are overcome with this new ink system of our invention.

Otherknown inksystems use epoxies or urethanes for obtaining quick drying characteristics. However, their pot life in the printing press would be of such a shortdurationthattheywould haveto be changed quite often in the press in order to continue quality printing operations.

It also has been heretofore known to provide an ink system using an acrylic monomer and vinyl resins which would quickly cure upon being subjectto a high temperature thatwould require applying heat to the face ofthe printed ink. This type of ink is disclosed in our earlier U.S. Patent 3,170,393. The monomer used forthis system was rather highly toxic and emitted a noxious odor. It is not always practical or even possible to use an ink system requiring a high temperature curing step. Moreover, it was necessary for curing the wet imprinted ink ofthe above patentto apply heat directly to the ink. That was accomplished by either the use of a roller, belt or plate surface which had been heated. Further, this patentwasconcerned with applying a particular surface finish to the ink.

The present invention provides a screen printing ink having the characteristic of being quickly dried on its exposed surface to prevent blocking which comprises a premix, a pigment grind and a catalyst, wherein said premix includes an acrylic monomer mixed with a vinyl resin, said pigment grind includes a mixture of plasticizers and pigment, and said catalyst is a paste of benzoyl peroxide and a phthalate type plasticizer or dispersing agent.

The ink system of the present invention, at least in preferred embodiments, overcomes the difficulties heretofore encountered where it is desired to have high-speed screen printing oeprations since it provides an ink drying system enhancing high-speed printing operation by incorporating high-speed ink drying to allow stacking. The drying system includes a non-sticky gel phase, wherein the acrylic monomer becomes a gelation agentforthevinyl powder, removing the liquid phase, and a curing or polymerization phase which cures the inkovera given period of time. The gel phase enables the immediate stacking ofthe printed product by subjecting the inkto a given amount of heat.

Thetwo-phasesystem ink includes as the main ingredients an acrylic monomer which polymerizes in the presence of a catalyst and a low-temperature vinyl resin which solvates in the acrylic monomer. A higher temperature vinyl resin can be used where higher drying temperatures are permissible. Typically, the low-temperature vinyl gels and becomesasolid at about 1 55 F, after which it will not block or stick together when stacked. While the ink is still soft, it may be safely handled. In about 48 hours the acrylic phase polymerizes, thereby completing the drying process ofthe ink. The acrylic monomer has good adhesion to a variety of substrates as well as a low shrinkage factor upon curing which enhances the adhesion character- istics.

Formulation of the two-phase ink system of the invention involves preparation of a premixformula, a pigment grind vehicle into which pigmentsmay be ground to form a pigment grind, and a catalyst. The premixformula is mixed with the pigmentgrind and catalyst in orderto produce the printing ink. The pigmentswill betypically ground in a vehicle consist- ing of a monomeric plasticizer, such as Monsanto 160, and a polymeric plasticizer, such as Rohm and Haas G54.The pigment grinds are very concentrated and are added to the premix formula in amounts from one to twenty percent depending upon the resu Its needed.

Further, solvent-like materials such as butyl carbitol may be added and optionally cured with vinyl powder.

The butyl carbitol acts like a solvating plasticizer ratherthan a solvent and serves to help printing characteristics and in some cases adhesion to the substrate.

Additives of various kinds can be used to modify the basic formula, to achieve various printing rhelogies, orto achieve better adhesion to certain substrates, but in all cases they must be only those chemicals which donotsolvatethevinyl resins at room temperature.

The preferred ink ofthe present invention is non-polluting when going through the drying process.

Further, it has a low odor and toxicity, togetherwith a high flash point. Moreover, the ink has a high opacity characteristic. Itwill not dry in the screen if left for extended periods of time, such as ten minutes or more. If left longer, such as around eight hours, the screen may be easily conditioned again for printing by wetting it with a damp solvent cloth. When the ink has been printed, it completes the gel phase quickly while being subjected to a low-temperature heat. Once the gel phase produces the protective coating,the ink is non-sticking and therefore is immediately stackable.

For more rapid polymerization and in some cases to develop the maximum chemical resistance ofthe final printed film, the printed web may be passed after gelation over a heated roller, inkagainstthe heated surface, at a temperature of about 260"F. This can only be done with substrates which can tolerate the extra heat. Such heating for half a second to a second is sufficientto complete polymerization.

The printing press ofthe invention, which is especially suitable for handling printing operations with the ink of the present invention, includes the standard rotary press unit such as the type shown in U.S. Patent 3,155,034 in conjunction with a substrate pre-heat station for raising thetemperature ofthe substrate prior to application ofthe ink and a drying station orsection downstream from the printing operation for assisting the gel phase ofthe inkto produce the protective coating which permits the printed substrate to be stackable. The preheat and drying or post-heat stations are arranged to coact with the printing station in orderto provide a compact printing press.

The single figure is a schematic view ofthe printing press according to the invention and is particularly useful for handling the ink system ofthe invention.

The ink ofthe present invention embodies a two-phase system. The ingredients permitthe ink to have a non-sticky gel phase and a curing or polymerizing phase so that it is especially useful in high-speed screen printing operations, which atthe same time overcomes many ofthe disadvantages in many heretofore known quick drying inks. Its primary ingredients are an acrylic monomer and a solvating vinyl resin which together form a premixthat is then mixed with a pigment grind and a catalyst atthe time or nearthe time of use. The pigment grind is composed of a pigment grind vehicle and one or more suitable pigments to give the desired colorand opacity.The pigment grinds are added to the premix in amounts from one to twenty percent, while the catalyst is added in an amount of about two percent of the acrylic monomer by weight. Pigments will be typically ground in a vehicle consisting of a system with good grinding or dispersing characteristics, Atypical formula forthe premixwould include alboutone pound of liquid isobornyl acrylic monomer (room and HaasCo.) thoroughly dispersed by a- high-speed mixer witch a low-temperature solvatitg vinyl resin inthe amount of about 2-1f4 pounds of powdered geo vinyl dispersion resin 25 (B. R Goodrich-Chemical Co.y.The acrylic monomer becomes a gelation agentforthevinyl powder.

AtypicalLpigmventgrind vehicle wouIdncl ude proportionally byweightabouttwo pounds of Monsanto Chemical Co. 160 monomeric plasticizer such as butyl benzylphthalate type m ixed with about one pound of Roh-m and HaasCo. G54 polyrneric plasticizer of the epoxidized saybean oil type. Pigments such as titanium dioxide are around in the vehicle in various percentages depending on the characteristics ofthe pigment In the case oftitanium dioxide, two pounds of pigment are used to one pound of the vehicle.

Preparation ofthefinal printing ink in a liquid phase would typically involve mixing about eight pounds of premixwith about one pound ofthe pigment grind with about one ounce or one percent by weight of a catalyst such as the benzoyl peroxide pasteANS 55 made by Lucidol. This paste consists generally of about fifty percent benzoyl peroxide and fifty percent of a phthalate type plasticizer or dispersing agent.

These ingredients should be thoroughly mixed fairly close to thetimethe ink is to be used and preferably within a few days as the catalyst otherwise loses its strength even though the ink will not gel at room temperature. It should be appreciated that other additives can be used within theframeworkofthe premix, pigment grind and benzoyl peroxide paste. To improve printing characteristics and sometime adhesion, butyl carbitol thickened with a vinyl powder may be added, which acts like a solvating plasticizer rather-- than a solvent, since it has no solvating action at room temperature.

While the premix is primarily useful as a base for-pigmented ink-systems, it also may be used without pigmentation as an adhesive, particularlyforgluing together multiple layersofpapersuch as in making honeycomb products or as a laminating adhesivefor plastic materials. Itwill be appreciated that allinks must have adhesive qualities as they must adhere to the-substrate onwhich they are printed. Thefinal adhesive mixture would include the premixtogether w.iLth the catalyst, again, in the amount of abouttwo percent ofthe acrylic monomer. For example, about one ounce of catalyst would be thoroughly mixedwith about eight pounds of premix. When usihg the premix and catalyst as an adhesive, it wou Id be preferableto promptly complete polymerization by-subjecting the adhesive to high-temperature curing such as about 2600F., although this step would not be necessary if gelling werefirst accomplished bysubjecting itto about 1 55 F. heat and the product using the adhesive could be stored for a period of about 48 hours before usage.

An example of the type of printing press especially suitablefor handling the ink ofthe present invention is illustrated inthesinglefigurewherein the printing of ink on a substrate and the initial drying ofthe ink to take it th rough the gel phase is achieved. This printing press includes a printing station 10 and a drying station 11.

The printing station 10 comprises a rotary screen printing press of the general type illustrated in my U.S.

Patent 3,155,034, wherein it includes a printing drum 14 coacting with an impression cylinder 15. The printing drum 14 is ofthe usual type which comprises a metal printing screen of the type shown in any of my earlier U.S. Patents 3,482,300; 3,696,741; 3,759,799 and 3,759,800. Moreover, it could include a metal printing screen that is otherwisecommerciallyavail- able. Suitable means is used to rotate the drum 14 and a squeegee 16 is mounted internally ofthe screen to force the inkthrough the image on the screen onto the substrate which passes by the screen and overthe impression roller 15. The impression roller likewise is suitably driven in orderto maintain it atthe same speedasthedrum 14.

The drying station 11 includes a preheat station 20 for preheating the web prior to the printing operation accomplished by the printer 10 and a post-heat or drying station 21 for continuing the application of heat to the substrate following the printing operation. The substrate being printed is in the form of a web 22 of any suitable or desired material on which printing isto be accomplished. This web enters the printing press and passes over a guide roller 23 to the preheat station which includes an elongated curvate heated plate 24 that extends from the roller 23 on the input side of the printer 10 and onto the impression cylinder 15.From the impression roller 15 the web is carried through the post-heat station 21 ofthe drying unitfirst over an elongated heated curvate plate 25 downwardly and around a guide roller 26 and onto a second heated curvate and elongated plate 27. Each of the heated plates 24,25 and 27 is respectively heated by infrared panels 24a, 25a and 27a, although it may be appreciated that they could be heated by any suitable source.

It will be appreciated that the surface ofthe paper onto which the printing isto be accomplished is not engaged directly by the heated plate in the preheat station 20, nor is the wet imprinted ink engaged by the heated plates 25 and 27 in the post-heat station 21.

This arrangement considerably simplifies the handling ofthe web as it is not necessaryto have complex and power driven heating elements for engaging the wet ink. It is not until afterthe web with the inked image has traversed both heated plates 25 and 27 that it is touched by a roller or other element. At that time the web is passed over reversing roller 30 and downwardly around a guide roller31 and out of the printing press. Roller30 may be used as a heated roller to achieve the more rapid cure of the acrylic monomer, as indicated above.

The infrared panels are provided with a proportional type of control to maintain the heated plates at about 1750 to 185 F. on the surface over which the paper traverses and with sufficient output to heat the mass of paper being passed overthe plates. The preheat station functionsto preheat the paper and assist in stabilizing its temperature and getting it up to a temperature before the printing operation.The paper traversing the heated plate 24 in the preheat station 20 will reach about 1 20" depending upon the weight of the paper.Some heat will be transmitted from the paperto the impression cylinder 15 warming the cylinder up to perhaps 11 00F. The web of paper or other material then passes overthe heated plates 25 and 27 before the face of the printed stock is touched by the reversing roller 13. This roller is coated with Teflon or some other non-stick material to guard against any adherence between the roller and the web orthe printed ink. Bythe time the web reaches the reversing roller, the protective film or covering on ink will have been established. Actually, when the ink temperature reaches about 1600F.,the setting or gellation ofthe ink occurs and the web is then ready to be handled and rewound.The web speed will be between 100 and 200 feet per minute which is considered high-speed screen printing. After the web leaves the printing press or as it leaves the last roller, if it is desired to chill the stock, a chill roll may be utilized.

Forexample,the roller 31 could be a chill roll if desired.

While a printing press is disclosed which is of a relatively compact nature for handling the printing of the ink ofthe present invention, it should be appreci ated that the ink could be used with other types of printing presses. The paper may be heated in ways otherthan that illustrated.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention, but it is understood that this application is to be limited only by the scope ofthe appended claims.

Claims (23)

1. Ascreen printing ink having the characteristic of being quickly dried on its exposed surface to prevent blocking which comprises a premix, a pigment grind and a catalyst, wherein said premixincludesan acrylic monomer mixed with a vinyl resin, said pigment grind includes a mixture of plasticizers and pigment, and said catalyst is a paste of benzoyl peroxide and a phthalate type plasticizer or dispersing agent.

2. The ink of Claim 1,wherein said premix includes by weight about 2-1/4 parts ofvinyl resin to about one part of monomer.

3. TheinkofClaim 2,whereinsaid pigmentgrind includes a pigment grind vehicle having about two parts monomeric plasticizer ofthe butyl benzyl phthalate type to about one part polymeric plasticizer of the epoxidized soybean oil type.

4. The ink of Claim 3,wherein said catalyst includes fifty percentbenzoyl peroxide and fifty percent plasticizer of a phthalate type.

5. The ink of Claim 4, wherein the pigment grind is mixed with the premix in any amounts, typically one to twenty percent.

6. The ink of Claim 4, wherein about one percent byweight of catalyst is mixed with about one part of pigment grind and about eight parts of premix.

7. A screen printing ink having a two phase system for curing or drying which comprises an acrylic monomer phase that polymerizes by the addition of a catalystoverabouta48-hourperiod of time and a vinyl dispersion resin phase that solvates in the acrylic monomer and gels upon being subjected to a given level of heat to provide a protective coating over the ink immediately following application to a substrate.

8. The ink of Claim 7, wherein the resin is of a low-temperature type which will gel at about 1 55"F.

9. A rotary screen printing pressforhigh-speed web printing comprising a rotatable printing drum, an impression cylinder coacting with the drum, a preheat station for preheating the web ahead of the printing drum, and a post-heat station for heating the web following the printing of ink thereon to dry the ink.

10. The press of Claim 9, wherein said preheat station includes means for heating the non-ink receiving surface ofthe web.

11. The press of claim 10, wherein said post-heat station includes means for heating the non-printed side of the web.

12. The press of Claim 11, wherein said heating means of both the preheat and post-heat stations include elongated plates heated by infrared panels.

13. The press of Claim 12, wherein said heating means of said post-heat station includes two elongated metal plates and infrared panels for heating said plates.

14. ThepressofClaim 13, which includes a reversing non-stick coated rollerfollowing said plates on which the ink side of the web engages.

15. The press of Claim 14, wherein the reversing roller is heatedto about 260"F. for more rapid polymerization ofthe acrylic monomer.

16. The press of Claim 14, wherein said metal plates of said preheat and post-heat stations extend upright and generally below the printing drum and impression cylinder.

17. A rotary screen printing press for high-speed web printing of a two-phase ink, wherein the ink includes an acrylic phase that polymerizes over a period of about 48 hours and a gel phase that sets upon being subjected to a given amount of heat, wherein said press comprises a rotatable printing drum, an impression cylinder coating with the drum, a preheat station for preheating the web ahead ofthe printing drum, and a post-heat station for heating the web following the printing of ink thereon to drythe ink.

18. The press of Claim 17, wherein said ink comprises an acrylic monomer phase that polymerizes by the addition of a catalyst over about a 48-hour period of time and a vinyl dispersion resin phase that solvates in the acrylic monomer and gels upon being subjected to a given level of heatto provide a protective coating over the ink immediately following application to a substrate.

19. A method of screen printing and drying a two-phase ink system on a web, wherein the ink system includes a non-sticky gel phase which sets up upon being subjected to heat to produce a protective coating on the ink and an acrylic phase that cures a monomer in the presence of a catalyst over about a 48-hour period, which method comprises the step of preheating the web, imprinting the two-phase ink system ontotheweb, applying heattothe printed web to setthe gel phase, and stacking the web for the acrylic phase.

20. An adhesive having the characteristic of being dried upon being subjected to a predetermined level of heat which comprises a premix and a catalyst, wherein said premix includes an acrylic monomer mixed with a low-temperature vinyl resin and said catalyst is a paste of benzoyl peroxide and a phthalate type plasticizer or dispersing agent.

21. The adhesive of Claim 20, wherein said premix includes by weight about 2-1/4 parts of vinyl resin to about one part of monomer.

22. The adhesive of Claim 21,wherein said catalyst includes fifty percent benzoyl peroxide and fifty percent plasticizer of a phthalate type.

23. The adhesive of Claim 22, wherein about one part by weight of catalyst is mixed with about eight parts of premix.