Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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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, . " . : ! . ' . ' ' . ,. .; ' . ~ . , . , . . ,.,. '
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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
1-4 251 251-1004
5-8 115 575- 920 `
9-12 46 414- 552
13-16 27 351- 432
17-20 21 357~ 420
21-24 10 210- 240
25-28 6 150- 168
29-32 3 87- 96
~,
33-36 2 66- 72 ' ! . ' .
37~40 3 110- 120
41-44 1 41- 44
485 2612-4068
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
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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.
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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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