CA1044087A

CA1044087A - Method of using resin powders to cure solvent-free inks

Info

Publication number: CA1044087A
Application number: CA222,839A
Authority: CA
Inventors: William B. Neuberg
Original assignee: Shamrock Chemicals Corp
Current assignee: Shamrock Chemicals Corp
Priority date: 1974-03-19
Filing date: 1975-03-19
Publication date: 1978-12-12
Also published as: JPS50133011A; CH599860A5; GB1492878A; US3911160A; FR2264670A1; IT1032332B; JPS5916951B2; DE2508972A1; FR2264670B3

Abstract

ABSTRACT OF THE DISCLOSURE

This invention is concerned with a method of curing solvent-free inks by application of a powdered resin to a freshly printed surface. Powder, which does not adhere to the ink, is removed and the printed surface then is passed through a heating tunnel where the resin melts, thereby curing the ink. Optionally, the printed surface may be leveled mechanically following curing. Such leveling produces a higher gloss. Air pollution which normally is caused by solvent fumes in conventional processes is eliminated by this method. Moreover, heat energy is con-served as the need to vaporize a solvent and to carry-off the heated vapors is eliminated.

Description

~J~ 7 It is known that powdered resins can be used in the drying of printing inks. Among these are anti-offset powders which coat the entire printed surface and thereby provide spacing between adjacent printed sheets (U.S. Patent No. 2,110,219).
Another use of resin powders is in thermographic printing processes. A resin powder, having a particle size in the 30 to 200 mesh range, is applied to the ink on the `~ -freshly printed surface and melted to create a raised effect which simulates engraving. The particle size of the powder controls the thickness or degree of raise of the printing.
This thickness is generally between 0.1 and 0.2 mm. (100 and 200 microns~ and it is necessary that the particle size of the powder be controlled to a close degree of uniformity.
The particle size is limited by the size of type used in printing since particles, which are too big, will overflow the sides of the letters and cause the edges to be uneven.
Particles, which are too small, will not produce a proper degree of raise, and will also produce an area of mottled "
appearance on areas of heavy ink coverage. Thermographic `
inks may or may not contain solvent.
Another printing process, described in U.S.
Patent No. 2,317,372 utilizes a finer resin powder, having an average particle size of about 16 to 60 microns, to coat the freshly printed ink. This process does not create a raised effect in contrast to the thermographic process.
However, the ink in this process must contain a solvent to ;~
solubilize the resin powder. The presence of solvent has two major drawbacks. Firstly, sufficient heat to vaporize the solvent must be provided and the heated vapors then must be re~oved from the heating area; this results in a waste ~L , r7 of heat energy. Secondly, air pollution is caused by the expelled solvent vapors.
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Methods of printing with solvent-free ink also are known. However, past methods of printing with non-poliuting solvent-free inks have several disadvantages (John W. Vanderhoff "De-inking -- the Ink Industries Position"
American Ink Maker, April, 1973, pp 42-46). Such inks require several hours to dry by oxidation and have been overcoated with a thin, fast-drying, transparent coating that protects the ink film until it dries. These coatings often are alcoholic soluble propionate resins which are permeable to oxygen. Thus, there is solvent effluent from the coating resin solution which creates air pollution.
It has now been discovered that solvent-free inks can be rapidly and economically cured with powdered resins.
In a preferred embodiment of the present invention, there is provided in a printing process comprising the step of applying a solvent-free oil or liquid ink to a surface, the improvement comprising:
(a) applying a powder resin having an average particle size between about 5-10 microns to the inked surface, and (b) curing the resinated ink by heating to provide a non-raised printed surface with a thickness below about 7 microns.
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In greater detail, a solvent-free ink is conventionally applied to a surface. Powdered resin in which 95% of the particles have a siæe between about 1 and 26 microns and an average particle size of between c about 5 and 10 microns is then applied to the surface and it adheres to the wet ink. Subsequently, any excess `--powder is removed. The printed surface is passed through ~ `
a heating tunnel where the resin is melted. At this stage, ;`~
the melted resin combines with the liquid ink. This combination cures when cooled to ambient temperature if a thermoplastic resin is used or cures by polymerization if a thermosetting resin is used thus creating a non-raised, printed surface with a thickness below about 7 microns e.g., about 0.5 to 5 microns.
It is critical in the present invention to utilize a solvent-free ink. As defined herein, solvent-free inks fall into the following categories, among others: `

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1. Drying Oil Vehicle Inks: These inks are formulated and manufactured from linseed, tung, soya, safflower, dehydrated castor, poppyseed and oiticia oils and may be modified with drying catalysts, such as metal soap dryers.
2. Drying Oil Alkyds: These inks are made from the above~defined drying oil vehicle inks by additions of glycerine and isophthalic acid in sufficient amounts to achieve a desired working viscosity for the inks.

3. Resin Modified Drying Oils and Dryin~ Oil Alkyds: These inks are made from phenolic or maleic resin modified drying oils or drying oil alkyds.

4. Gum Rosin and Tall Oil Rosin: These inks are made from either unreacted gum rosin or tall oil rosins.

5. Non-Drying Oils: Inks made from mineral oils or petroleum oils, for example, newspaper inks.

6. Epoxy, Polyurethane and Polyamide ~odified Inks: Although some of these in~s may contain solvents, inks of this type without solvents may be used. Selection of the ink is at the discretion of the printer who must decide which ink is best for a particular surface. If the surface is paper, the printer may choose one ink; if the surface is metal, he may choose another ink.
The resin that is used in the process of the present invention must be ground to a particle size finer than about 325 mesh. The optimum particle size (95%~ is between about 1 and 26 microns. ~owever, it is critical to have an average particle size between about 5 and 10 microns. These powders may be produced by grinding, cryogenic grinding, air milling, air classifying, spray drying, crystallization, or by combinations of these processes or -,. . .

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procedures. A typical powder has the following particle size count as measured microscopically using Martin's Diameter:
d n (diameter ~number ;
microns~ coun~ted~ nd 5-8 115 575- 920 `

17-20 21 357~ 420 ~, 33-36 2 66- 72 ' ! . ' .
37~40 3 110- 120 Average particle size equals the sum of nd divided by the sum of n, i.e., 2612/485 to 4068/485 or 5.38 to 8.38 microns.
Any suitable resin may be employed in this invention provided that it can be powdered to the aforementioned required particle size, melted at the appropriate temperature, fused to a compatible composition with the ink and solidified on cooling to form a tack-free, immobile surface. This therefore, includes both thermoplastic and thermosetting resins as well as resins ~hich become thermosetting when `~
combined with the ink or additives used in the ink. The following resins, among others, are suitable: rosin, rosin modified with maleic, fumaric, phenolic or inorganic components , .

rosin, petroleum, polyamide, natural, alkyd, epoxy, acr~lic urethane, amino, wax-modified, pigment- or filler-modified, ;
plasticizer-modified and inorganics.
Wax may be added optionally to the powdered resin in an amount ranging from about 1/2 to 3~ by weight in order to increase rub resistance. It is necessary, however, that the wax have a particle size comparable to that of the powdered resin. The following waxes, among others, may be used: paraffin, microcrystalline, natural, synthetic, compounded, polyethylene, and polytetrafluoroethylene. ' Similarly, other materials may be used as additives, including colorants, cure'promoters, leveling agents, and functional additives among others.
The powdered resin must be applied to the freshly -printed surface by a method which will give a reIativeIy uniform coating. The quantity of powdered resin used is usually kept to the minimum required to cure the'ink. The preferred method of powdered resin application is by electro-static powder coating. Thus, in one embodiment of the present invention, electrostatic guns are used to release a precisely controlled powder distribution by means of controlling an electrostatic charge on the resin particles and an opposite charge on the freshIy printed surface.
The eLectrostatic guns are housed in a ch'amber having entry and exit openings to permit passage of the freshly printed surface therethrough. This chamber confines the'powder to the area of application so that the powder ' will not pollute the air. ' ' In other embodiments, the powder may be applied ~ '-with an air gun. A fluidized bed also may be used wherein the freshly printed surface is passed through an atmosphere .

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containing the powder, or an electrodynamic device may be employed.
Powder, which does not adhere to the ink, may be removed prior to curing. In a preferred embodiment, an air knife us used for this purpose. ~n air knife is an appara-tus ha~ing a slit or a series o~ holes throu~h which air is ~`
foxced. Suitable air pressures are in the range of between about 5 and 50 pounds per square inch. The air stream is directed against the powder coated surface and blows the 10 powder o~f the areas which are not printed with ink. The `
air used for this purpose may be ionized to promote dissipation of the electrostatic charges. This apparatus may be housed in its own chamber or in the same chamber as the electro- ;
static guns and downstream of the area where the powder is applied. It is preferable to recycle the powder which is removed.
Curing proceeds by heating in a tunneI to effect a sur~ace temperature about 50 to 150F. above the meLting point of the resin. Direct flame or infrared heating ~;
20 tunnels among others may be used ~or this curing step.
The printing process being continuous and dynamic by its nature requires the adjustment of inking rate, fountain solution, and other variables to arrive at the v desired product on the delivery end of the press. Like-wise, the rate of powder application is controlled by the positioninq of the powder ~uns, choice of voltages, feed rates, and other controls must be adjusted to be compatible with the press. Generally, it is desired to use only that - -amount of powder which is required to cure the ink. This 30 will vary from one printing application to the next depending on the amount of coverage, the stock, and the quality of work desired.

' ' When the powder application rate is cut back for greatest economy, there results a slight reduction in gloss.
Further reduction of powder results in incomplete curing;
therefore, the powder must be then increased. Taking a proof signature at this stage of adjustment reveals an irregular surface when viewed under magnific:ation. Using a low power microscope, it can be seen that resin particles tend to have fused with the individual dots in the printing half-tone structure.
It has been found that increased gloss can be achieved by leveling the surface machanically prior to allow-ing it to cool or in the process of cooling. This can be accomplished by means of the chill rolls normally employed to complete the curing of heat-set printing or it can be done by means of additional rolls. There must be sufficient pressure or drag to level the resin coating, but not so much as to smear the printing. The resulting surface has a glass~ appearance when viewed under a low power micro-scope. The thickness is reduced to a range of less than one micron. Furthermore, still lower rates o~ powder appli-cation can now be utilized and sti11 maintain complete curing.
Thus, in accordance with this invention, it is now-possible to cure a conventionally applied solvent-free x~;
printing ink rapidly, in a high speed process, without r~
producing raised print. The problem of air pollution caused by heated solvent vapors or the solvent containing protective ~
overcoatings utilized in prior art processes is eIiminated ~ ;
bv the present invention. Moreover, as compared with prior art processes that utilize heat to dry the ink, energy is conserved by the present invention as it is not necessary to provide heated air to carry off solvent vapors.

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A typical apparatus for carrying out the present invention is illustrated in the attached schematic drawing.
A roll 1 of paper continuously feeds paper 2 to a conventional printing press 3 where the solvent-~ree ink is applied. A
static eliminator or precharging device 4 controls the - `
electrostatic charge on the freshIy printed paper 2 which ~ ;
then enters chamber 5 where electrostatic guns 6 discharge electrostatically charged powder. The powder covered paper `
2 then enters air knife 7 where powder which has not adhered to the ink is removed. As the paper passes through heating ``
tunnel 8, the resin melts and fuses with the ink. Chill ;
rolls ~ provide a drive for the paPer 2 and effect a cooling and leveling of the printed surface, which renders the printing cured.
The following examples axe submitted to illustrate ' but not to limit this invention. The apparatus used to generate`these examples is the same as that described in the attached schematic drawing.
Ex mple I
A non-drying process blue offset ink was used to print a detailed picture having some letter characters and a wide range of half-tone density. The paper was eIectro-statically charged and a powder was used having an average particle size of about 7 microns and ha~ing the following compositions:
Composition % ~.P.
Polyamide Resin 97.0 250F.
Polyethylene Wax 2~5 215F. `~
Polytetrafluoroethylene 0,5 620F.
This powder was fed at a rate of 5 grams~minute. T~e powder was then charged eIectrostatically with a polarity opposite .. . . . . .

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to that of the paper. Excess powder was not removed. The printed and powdered paper was passed through an infrared heating tunnel having a temperature of 550F. The surface emerged at a temperature of 350F. On cooling a cured glossy print was produced having a thickness of about 5 microns.
Example II
The procedure of Example I was repeated e~cept that the air knife was used to remove excess powder. A
cured glossy image was produced having a thickness of about 5 microns.
- Example III
The procedure of Example II was repeated except that the rate at which powder was applied was reduced by one half. A cured, semi-gloss image was produced having a thick-ness of about 4 1~2 microns.
EXample IV
The procedure of Example III was repeated except ;
that the chill roll was used to leveI the print. A cured, `'~
glossy image was produced having a thickness of about 3 1/2 `microns.
Having set forth the general nature and preferred embodiment of the present invention, the scope is now ;~
particularly pointed cut in the eppended claim~.

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Claims

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. In a printing process comprising the step of applying a solvent-free oil or liquid ink to a surface, the improvement comprising:
(a) applying a powder resin having an average particle size between about 5-10 microns to the inked surface;
and (b) curing the resinated ink by heating to pro-vide a non-raised printed surface with a thickness below about 7 microns.

2. The process of claim 1 comprising the additional step of providing said resin with about one-half to three percent powder by weight of a powdered wax having a particle size comparable to the particle size of the resin.

3. The process of claim 1 or 2 in which said step of applying said resin to said surface is accomplished electro-statically.

4. The process of claim 1 or 2 in which said step of applying said resin to said surface is accomplished electro-dynamically.

5. The process of claim 1 or 2 comprising the additional step of removing resin which does not adhere to the ink by blowing air across said surface.

6. A process of claim 1 in which said resin is thermoplastic and said curing step is accomplished by heating said surface to a temperature of about 50°F. to 150°F. above the melting point of said resin.

7. The process of claim 1, 2 or 6 in which said step of heating is accomplished by passing said surface through a heating tunnel.

8. The process of claim 1, 2 or 6 in which said heating step is accomplished by direct-flame heating.

9. The process of claim 1, 2 or 6 in which said heating step is accomplished by infrared heating.

10. The process of claim 1, 2 or 6 comprising the additional step of leveling.

11. The process of claim 1, 2 or 6 in which said resin is thermosetting and said curing step is accomplished by polymerization.