FIELD OF THE INVENTION
[0001] The present invention relates to a method for making a flexographic printing master
comprising an aerosol jet printing step.
BACKGROUND OF THE INVENTION
[0002] Flexography is a form of printing process which utilizes a flexible relief plate,
the flexographic printing master. It is basically an updated version of letterpress
that can be used for printing on almost any type of substrate including plastic, metallic
films, cellophane, and paper. Flexography is widely used for printing on packaging
material, for example food packaging. Also, with flexography continuous patterns,
such as for gift wrap and wall paper, can be printed.
[0003] Today flexographic printing masters are prepared by both analogue and digital imaging
techniques. Analogue imaging typically uses a film mask through which a flexographic
printing precursor is exposed. Digital imaging techniques include:
- Direct laser engraving as disclosed in e.g. EP-As 1710093 and 1936438;
- UV exposure through a LAMS mask wherein LAMS stands for Laser Ablative Mask System
as disclosed in e.g. EP-A 1170121;
- Direct UV or violet exposure by laser or LED as disclosed in e.g. US6806018; and
- Inkjet printing as disclosed in e.g. EP-As 1428666 and 1637322.
[0004] The major advantage of an inkjet method for preparing a flexographic printing master
is an improved sustainability due to the absence of any processing step and the consumption
of no more material as necessary to form a suitable relief image (i.e. removal of
material in the non printing areas is no longer required).
[0005] EP-A 641648 discloses a method of making a photopolymer relief-type printing plate wherein a
positive or negative image is formed on a substrate by inkjet printing with a photopolymeric
ink and subjecting the resulting printed substrate to UV radiation, thereby curing
the ink composition forming the image.
[0006] US6520084 discloses a method of preparing flexographic printing masters by inkjet wherein a
removable filler material is used to support the relief image being printed and wherein
the relief image is grown in inverted orientation on a substrate. Disadvantages of
this method are the removal of the filler material and the release of the relief image
from the substrate.
[0007] EP-A 1428666 discloses a method of making a flexographic printing master by means of jetting subsequent
layers of a curable fluid on a flexographic support. Before jetting the following
layer, the previous layer is immobilized by a curing step.
[0008] In
US7036430 a flexographic printing master is prepared by inkjet wherein each layer of ink is
first jetted and partially cured on a blanket whereupon each such layer is then transferred
to a substrate having an elastomeric floor, thereby building up the relief image layer
by layer. A similar method is disclosed in
EP-A 1449648 wherein a lithographic printing plate is used to transfer such layers of ink to a
substrate.
[0009] US2008/0053326 discloses a method of making a flexographic printing master by inkjet wherein successive
layers of a polymer are applied to a specific optimized substrate. In
US2009/0197013, also disclosing an inkjet method of making a flexographic printing master, curing
means are provided to additionally cure, for example, the side surfaces of the image
relief being formed. In
EP-A 2223803 a UV curable hot melt ink is used. Each of the deposited layers of ink is gelled
before a subsequent layer is deposited. After a printing master with sufficient thickness
is formed, a curing step is carried out.
[0010] EP-As 1637926 and
1637322 disclose a specific curable jettable fluid for making flexographic printing masters
comprising a photo-initiator, a monofunctional monomer, a polyfunctional monomer or
oligomer and at least 5 wt. of a plasticizer. The presence of the plasticizer is necessary
to obtain a flexographic printing master having the necessary flexibility. Also in
EP-A 2033778, the curable jettable fluid for making a relief image by inkjet on a sleeve body
contains a plasticizer.
[0011] A flexographic printing master formed on a support by an inkjet method typically
comprises an elastomeric floor, an optional mesa relief and an image relief as disclosed
in
EP-A 2199082.
[0012] To realize a high resolution flexographic printing master with such inkjet methods
it is advantageous to use a print head with small nozzle diameters, for example producing
3 pl fluid droplets. Print heads with such small nozzle diameters however require
low viscosity fluids. The requirement for such a low viscosity however imposes constraints
on the choice of the ingredients of the fluid. For example it limits the amount of
monomers having a high viscosity or the amount of plasticizers, while often a high
amount of such ingredients is preferred to prepare flexographic printing masters with
optimal physical properties.
[0013] It would therefore be advantageous to realize a method for preparing high resolution
flexographic printing masters having optimal physical properties, the method having
the same advantages as the inkjet method described in the prior art and using fluids
of which the viscosity does not have to be very low.
SUMMARY OF THE INVENTION
[0014] It is an object of the present invention to provide a method for preparing a flexographic
printing master having a high resolution and good physical properties.
[0015] The object of the present invention is realised with by the method for preparing
a flexographic printing master as defined in claim 1.
[0016] Preferred embodiments are defined in the dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
Figure 1 gives a schematic presentation of an aerosol jet printing system.
Figure 2 illustrates a difference between inkjet printing and aerosol jet printing.
Figure 3 gives a schematic presentation of a preferred embodiment of an apparatus
to carry out the method of the present invention.
Figure 4 gives a schematic presentation of a preferred embodiment a flexographic printing
master.
DETAILED DESCRIPTION OF THE INVENTION
Aerosol Jet Printing
[0018] The present invention relates to a method for preparing a flexographic printing master
comprising an aerosol jet printing step.
[0019] Aerosol Jet Printing, which has been developed by Optomec, preserves most of the
advantages of inkjet printing, while reducing many of its limitations. The technique
is developed for use in the field of printed electronics. The technique is disclosed
in for example
US2003/0048314,
US2003/0020768,
US2003/0228124 and
WO2009/049072. An Aerosol Jet Print Engine is commercially available from Optomec, for example
the Aerosol Jet Printer OPTOMEC AJ 300 CE. More details on the Aerosol Jet Printing
technique and engine are found on the Optomec website www.optomec.com.
[0020] Virtually any liquid having a viscosity less than 5000 mPa.s can be deposited using
the Aerosol Jet Printing technique while inkjet printing requires fluids having a
viscosity of less than 20 mPa.s.
[0021] As already mentioned above, using higher viscous fluids may be advantageous when
preparing flexographic printing masters having optimal physical properties.
[0022] In Aerosol Jet Printing a collimated beam of material is dispensed on a substrate.
This allows the resolution to be maintained over a wide range of standoff (head to
substrate) distances. This enables larger standoff distances to be used than are possible
with inkjet printing. The differences between inkjet and Aerosol Jet Printing are
schematically shown in Figure 2. In inkjet, the drops are typically of low density.
The standoff (500) between the nozzle and the substrate is usually about 1 mm. The
droplets spread out upon leaving the nozzle, and the system is normally optimized
to achieve optimum results at the fixed standoff distance. In Aerosol Jet Printing,
a collimated beam of material is formed which is then deposited on the substrate.
Therefore, standoff distances from 1 to 5 mm can be used without loss of resolution.
This feature is very important for printing features over an existing topology as
found in many electrical devices and circuits, and enables conformal printing, which
is the application for which the Aerosol Jet Printing technology was originally developed.
The thickness of the total relief image (floor - mesa relief and image relief) of
a flexographic printing master may be up to several mm. When using inkjet printing
and when the print head does not move in the z direction, the standoff distance between
the print head and the lower layers of the printing master may be several mm resulting
in a different resolution of the lower an upper layers making up the printing master.
However, by using such a collimated beam of material, the resolution of the lower
and the upper layers will almost not vary when using Aerosol Jet Printing.
[0023] In an Aerosol Jet Printing system, rather than producing individual droplets of ink,
an aerosol is produced, focused and directed toward the substrate. A schematic presentation
of such an Aerosol Jet
[0024] Printing system is given in Figure 1. The basic system consists of two key components:
a first module (200) for forming an aerosol (210) from a fluid (220) and a second
module (300) focussing the aerosol (210) and depositing the aerosol droplets on a
substrate (400). Similar to continuous inkjet, this aerosol stream can be shuttered
to interrupt the stream.
[0025] As described above, one of the key advantages of the Aerosol Jet Printing system
is its large materials window. Aerosols can be formed from fluids as viscous as 5000
mPa.s. In Aerosol Jet Printing printing, two different ways of forming an aerosol
can be employed, depending on the characteristics of the material to be deposited.
An ultrasonic transducer can be used for nebulizing low viscosity fluids (0.7 to 10
mPa.s). Here, a piezoelectric transducer produces high frequency pressure waves, which
are transmitted through a coupling fluid (typically water) into the deposition fluid.
This atomization technique works best for suspended particles of less than 50 nm.
For higher viscosity fluids (10 to 5000 mPa.s) or larger suspended particles (< 0.5
µm) pneumatic atomization is used. In this technique, a high velocity gas stream is
used to shear the liquid stream into droplets.
[0026] In both of these atomization techniques, a dense aerosol jet is produced. One of
the most unique features of the Aerosol Jet Printing System is that the aerosol stream
can be focused to a fine, collimated (the cross sectional diameter does not vary as
a function of the distance from the nozzle) beam. Here, the focusing gas (300) surrounds
the aerosol completely so that droplets do not touch the inner walls of the nozzle,
eliminating clogging and other problems. This aerosol jet focusing gives rise to a
jet diameter which is much smaller than that of the nozzle orifice. A variety of line
widths can be produced, from about 10 to 150 µm.
Method of preparing a flexographic printing master
[0028] A preferred flexographic printing master according to the present invention is disclosed
in
EP-A 2199082. It typically comprises on a substrate (1000), preferably a sleeve body, in this
order, a floor (600), a mesa relief (700) and an image relief (800).
EP-A 2199082 disclosed a method for preparing such a flexographic printing master with inkjet.
[0029] In one embodiment of the present invention, the above mentioned methods of preparing
a flexographic printing master are modified in that all layers are now deposited by
the aerosol jet printing technique instead of the inkjet printing technique.
[0030] However, it may be advantageous to combine both the inkjet printing and aerosol printing
technique. For example, to form the floor and the mesa relief, where resolution is
not an issue, inkjet printing may be used to optimize the throughput of the method,
for example by using large fluid droplets. The image relief, where resolution must
be as high as possible, may then be deposited using the aerosol jet printing technique.
[0031] In another embodiment, only the upper most layer (the top layer of the image relief)
is deposited using aerosol jet printing, while all other layers are deposited using
inkjet printing. High viscous fluids may then be used to deposit the top layer to
optimize the properties of the printing areas of the flexographic printing master.
To further optimize the flexographic printing master, more then one top layer, for
example two, three or more, may be applied using aerosol jet printing.
[0032] In still another embodiment, the upper most layer(s) of the floor and the mesa relief
may be applied using aerosol jet printing while the other layers are applied using
inkjet.
[0033] To further optimize the throughput of the method, the floor may be precoated on a
sleeve body by conventional coating techniques. The mesa relief and the image relief
are then applied on the precoated floor by aersol jet printing, or the combination
aerosol jet printing - inkjet printing. Also, the image relief may be directly applied
on the floor.
Curable fluid
[0034] As described above, the method according to the present invention may use aerosol
jet printing only, or a combination of aerosol jet printing and inkjet printing.
[0035] The same fluid may be used in both the aerosol jet printing step and the inkjet printing
step but preferably different fluids are used, optimized towards the printing technique,
hereinafter referred to as the curable aerosol jet fluid and the curable inkjet fluid.
For example, for the aerosol jet printing step high viscous fluids are preferably
used to optimize the properties of the obtained flexographic printing master.
[0036] Typical ingredients for both types of fluids are preferably selected from the group
consisting of a monofunctional (meth)acrylate monomer, a difunctional (meth)acrylate
monomer, a multifunctional (meth)acrylate monomer or oligomer, a low viscous monofunctional
urethane acrylate oligomer (especially for curable inkjet fluid), a higher viscous
mono-or multifunctional urethane acrylate (especially for the curable aerosol jet
fluid), an initiator, a plasticizer, an inhibitor, an elastomeric binder, a surfactant,
a colorant, a solvent, an humectants, a biocide.
Monofunctional (meth)acrylate monomer
[0037] The curable fluid may comprises a monofunctional (meth)acrylate monomer. Any monofunctional
(meth)acrylate monomer, such as disclosed for example in
EP-A 1637322, paragraph [0055], may be used.
[0038] However, the curable fluid preferably comprises a cyclic monofuntional (meth)acrylate
monomer. Examples of such cyclic monofunctional (meth)acrylates are isobornyl acrylate
(SR506D from Sartomer), tetrahydrofurfuryl methacrylate (SR203 from Sartomer), 4-t.butylcyclohexyl
arylate (Laromer TBCH from BASF), dicyclopentadienyl acrylate (Laromer DCPA from BASF),
dioxalane functional acrylates (CHDOL10 and MEDOL10 from San Esters Corporation),
cyclic trimethylolpropane formal acrylate (SR531 from Sartomer), 2-phenoxyethyl acrylate
(SR339C from Sartomer), 2-phenoxyethyl methacrylate (SR340 from Sartomer), tetrahydrofurfuryl
acrylate (SR285 from Sartomer), 3,3,5-trimethyl cyclohexyl acrylate (CD420 from Sartomer).
[0039] Particularly preferred cyclic monofunctional (meth)acrylates monomers are isobornyl
acrylate (IBOA) and 4-t.butylcyclohexyl arylate (Laromer TBCH from BASF).
[0040] The amount of the cyclic monofunctional (meth)acrylate monomer is preferably at least
25 wt.%, more preferably at least 30 wt.%, relative to the total weight of the curable
fluid.
Difunctional (meth)acrylate monomer
[0041] A preferred difunctional (meth)acrylate monomer is a polyalkylene glycol di(meth)acrylate.
Such compounds have two acrylate or methacrylate groups attached by an ester linkage
at the opposite ends of a hydrophilic polyalkylene glycol. Typically, the longer the
length of the polyalkylene chain, the softer and more flexible the obtained layer
after curing.
[0042] Examples of such polyalkylene glycol di(meth)acrylates include: 1,3-butylene glycol
diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene
glycol dimethacrylate, dipropylene glycol diacrylate, ethylene glycol dimethacrylate,
polyethylene glycol (200) diacrylate, polyethylene glycol (400) diacrylate, polyethylene
glycol (400) dimethacrylate, polyethylene glycol (600) diacrylate, polyethylene glycol
(600) dimethacrylate, polyethylene glycol dimethacrylate, polypropylene glycol (400)
dimethacrylate, tetraethylene glycol diacrylate, tetraethylene glycol dimethacrylate,
triethylene glycol diacrylate, triethylene glycol dimethacrylate, tripropylene glycol
diacrylate, tripropylene glycol diacrylate, and combinations thereof. The number between
brackets in the above list refers to the Molecular Weight (MW) of the polyalkylene
chain.
[0043] Highly preferred polyalkylene glycol diacrylates are polyethylene glycol diacrylates.
Specific examples of commercially available polyethylene glycol diacrylate monomers
include SR259 [polyethylene glycol (200) diacrylate], SR344 [polyethylene glycol (400)
diacrylate], SR603 [polyethylene glycol (400) dimethacrylate], SR610 [polyethylene
glycol (600) diacrylate], SR252 [polyethylene glycol (600) dimethacrylate], all Sartomer
products; EBECRYL 11 [poly ethylene glycol diacrylate from Cytec; Genomer 1251 [polyethylene
glycol 400 diacrylate] from Rahn. Polyethylene glycol (600) diacrylate, available
as SR610 from Sartomer, is particularly preferred.
[0044] Other preferred difunctional acrylate or methacrylate monomers are e.g. butane diol
diacrylate, alkoxylated hexanediol diacrylate, alkoxylated neopentyl glycol diacrylate
and alkoxylated hexanediol dimethacrylate.
[0045] The amount of the difunctional (meth)acrylate monomer is preferably at least 10 wt.%
of the total monomer content.
[0046] Particularly preferred difunctional (meth)acrylate monomers are those according to
Formula I or II,
wherein
k and m in Formula I is an integer ranging from O to 5,
1 in Formula I is an integer ranging from 1 to 20
n in Formula II is 1, 2, 3 or 4,
R is H or CH
3, and
R' is H or an alkyl group.
[0047] Difunctional (meth)acrylate monomers according to Formula I are typically derived
from diols containing an -(CH
2)- backbone. Preferred compounds according to Formula I are polyoxytetramethylene
diacrylate (Blemmer ADT250); 1,9 nonanediol diacrylate; 1,6 hexanediol diacrylate
(SR238); 1,6 hexanediol dimethacrylate (SR239); 1,4 butanediol diacrylate (SR213);
1,2 ethanediol dimethacrylate (SR206); 1,4 butanediol dimethacrylate (SR214); ethoxylated
1,6 hexanediol diacrylate (Miramer M202)
[0048] Difunctional (meth)acrylate monomers according to Formula II are typically derived
from diols containing a glycol ether backbone. The R' group in Formula II is preferably
H or methyl. Preferred compounds according to Formula II are dipropyleneglycol diacrylate
(DPGDA, SR508), diethylene glycol diacrylate (SR230), triethyleneglycol diacrylate
(SR272), 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene
glycol diacrylate, diethylene glycol dimethacrylate, dipropylene glycol diacrylate,
ethylene glycol dimethacrylate, tetraethylene glycol diacrylate, tetraethylene glycol
dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate,
tripropylene glycol diacrylate, tripropylene glycol diacrylate, and combinations thereof.
[0049] The amount of the difunctional acrylate monomer according to Formula I or II is at
least 1 wt.%, preferably at least 5 wt.%, more preferably at least 7.5 wt.%, relative
to the total weight of the curable fluid.
Multi functional (meth)acrylate monomer
[0050] The curable fluid further comprises a tri-, tetra- or penta-functional (meth)acrylate
monomer. It has been observed that the hardness of the cured layer obtained from the
curable fluid becomes too high when too much tri-, tetra- or penta-functional (meth)acrylate
monomer is present in the fluid. The Shore A hardness of the cured layer must be kept
below 80, to ensure good physical properties of the flexographic printing master.
It has been observed that the maximum concentration of the tri-, tetra- or penta-functional
(meth)acrylate monomer to ensure a proper hardness depends on their functionality.
Typically, the higher their functionality, the lower their maximum allowable concentration
to ensure a Shore A hardness below 80. In addition to its effect on the hardness,
the functionality of the tri-, tetra- or penta-functional (meth)acrylate monomers
also influences their viscosity, and thus also the viscosity of the curable fluid.
Typically, the higher their functionality, the higher their viscosity. As the viscosity
of the curable inkjet fluid, measured at jetting temperature, is preferably below
15 mPa.s, this also limits the maximum concentration of the tri- tetra- or penta-functional
(meth)acrylate monomer in the jettable fluid.
[0051] Preferably, the maximum concentration of the tri-, tetra- or penta-functional (meth)acrylate
monomer, dependent on their viscosity, is as depicted in the following table.
visco (mPa.s) |
< 100 |
100 - 250 |
250 - 5000 |
> 5000 |
functionality |
3 |
20 wt.% |
17.5 wt.% |
15 wt.% |
10 wt.% |
4 |
15 wt.% |
12.5 wt.% |
10 wt.% |
7.5 wt.% |
5 |
10 wt.% |
8 wt.% |
6 wt.% |
4 wt.% |
[0052] The minimum concentration is preferably 0.5 wt.%, more preferably 1 wt.%).
[0053] For the curable aerosol jet fluid, the higher viscosities are allowable as described
above. Therefore, higher concentrations of multifunctional (meth)acrylate monomers
may be used.
[0054] Preferred examples are ditrimethylol propane tetraacrylate (DTMPTA), glycerol triacrylate
and their alkoxylated, i.e. ethoxylated or propoxylated, derivatives.
[0055] Specific compounds are trimethylol propane tetraacrylate (TMPTA), commercially available
as Miramer M300; propoxylated TMPTA, commercially available as SR492; ethoxylated
TMPTA, commercially available as Miramer M3130; DTMPTA, commercially available as
SR355; propoxylated glyceryl triacrylate, commercially available as SR9021 and SR9020.
[0056] Other specific compounds are dipentaerythritol pentaacrylate (DIPEPA), commercially
available as SR399LV; tri-acrylate esters of pentaerythritol, such as pentaerythritol
triacrylate (PETIA); tetraacrylate esters of pentaerythritol, such as PETRA, commercially
available as SR295; ethoxylated PETRA, commercially available as SR494; alkoxylated
PETRA, commercially available as Ebecryl 40.
Urethane acrylate oligomer
[0057] The curable fluid, especially the curable inkjet fluid, may further contain monofunctional
urethane acrylate oligomers.
[0058] Urethane acrylates oligomers are well known and are prepared by reacting polyisocyanates
with hydroxyl alkyl acrylates, usually in the presence of a polyol compound. Their
functionality (i.e. number of acrylate groups) varies from 1 to 6. A lower functionality
results in lower reactivity, better flexibility and a lower viscosity. The polyol
compound forms the backbone of the urethane acrylate. Typically the polyol compounds
are polyether or polyester compounds with a functionality (hydroxyl groups) ranging
from two to four. Polyether urethane acrylates are generally more flexible, provide
lower cost, and have a slightly lower viscosity and are therefore preferred.
[0059] Commercially available urethane (meth)acrylates are e.g. CN9170, CN910A70, CN966H90,
CN962, CN965, CN9290 and CN981 from SARTOMER; BR-3741B, BR-403, BR-7432, BR-7432G,
BR-3042, BR-3071 from BOMAR SPECIALTIES CO.; NK Oligo U-15HA from SHIN-NAKAMURA CHEMICAL
CO. Ltd.; Actilane 200, Actilane SP061, Actilane 276, Actilane SP063 from AKZO-NOBEL;
Ebecryl 8462, Ebecryl 270, Ebecryl 8200, Ebecryl 285, Ebecryl 4858, Ebecryl 210, Ebecryl
220, Ebecryl 1039, Ebecryl 1259 and IRR160 from CYTEC; Genomer 1122 and Genomer 4215
from RAHN A.G. and VERBATIM HR50 an urethane acrylate containing liquid photopolymer
from CHEMENCE.
[0060] The curable inkjet fluid preferably comprises monofunctional urethane acrylate oligomers,
more preferably monofunctional aliphatic urethane acrylates, having a very low viscosity
of 100 mPa.s or lower at 25°C, like for example Genomer 1122 (2-acrylic acid 2-{[(butylamino)
carbonyl]oxy}ethyl ester, available from Rahn AG) and Ebecryl 1039 (available from
Cytec Industries Inc.).
[0061] The total amount of the monofunctional urethane acrylate oligomer is preferably at
least 5 wt.%, more preferably at least 7.5 wt.%, relative to the total weight of the
curable fluid.
[0062] Because viscosity of the aerosol jet fluids is not that restricted as for the inkjet
fluids, the aerosol jet fluid may contain both mono- and multifunctional urethane
acrylates
Other monomers or oligomers
[0063] Additional mono- or multifunctional monomers or oligomers may be used to further
optimize the properties of the curable fluid.
Initiators
[0064] The curable fluid comprises an initiator which, upon exposure to radiation or heat,
initiates the curing, i.e. polymerization, of the jetted droplets.
[0065] However, it is also possible to carry out the curing by electron beam radiation where
the presence of an initiator is not mandatory.
[0066] Preferably a photo-initiator is used which upon absorption of actinic radiation,
preferably UV-radiation, forms high-energy species (for example radicals) inducing
polymerization and crosslinking of the monomers and oligomers of the jetted droplets.
[0067] A combination of two or more photo-initiators may be used. A photo-initiator system,
comprising a photo-initiator and a co-initiator, may also be used. A suitable photo-initiator
system comprises a photo-initiator, which upon absorption of actinic radiation forms
free radicals by hydrogen abstraction or electron extraction from a second compound,
the co-initiator. The co-initiator becomes the actual initiating free radical.
[0068] Irradiation with actinic radiation may be realized in two steps, each step using
actinic radiation having a different wavelength and/or intensity. In such cases it
is preferred to use 2 types of photo-initiators, chosen in function of the different
actinic radiation used.
[0069] Suitable photo-initiators are disclosed in
EP-A 1637926 paragraph [0077] to [0079].
[0070] To avoid extraction of the photo-initiator out of the flexographic printing master
during printing, copolymerizable photo-initiators (and/or co-initiators) such as disclosed
in the unpublished
EP-A 10195896.5 (filed on 2010-12-20) may be used.
[0071] A preferred total amount of initiator is 1 to 10 wt.%, more preferably 2.5 to 7.5
wt.%, of the total curable fluid weight.
Plasticizer
[0072] A plasticizer, as disclosed in for example
EP-A 1637926 ([0085] - [0091]) may be added to the curable fluid. Such a plasticizer is typically
a substance which, when added to a flexographic printing master, increases the softness
and flexibility of that printing master. However, as mentioned above, such plasticizers
may migrate to the surface of the relief image or may be extracted out of the relief
image by the flexo printing ink during printing. For that reason, it is preferred
to use a copolymerizable plasticizing monomer such as a low Tg monomer of which the
corresponding homopolymer has a glass transition temperature below -15°C or diallylphthalate,
as disclosed in the unpublished
EP-A 10195895.7 (filed on 2010-12-20).
Inhibitors
[0073] Suitable polymerization inhibitors include phenol type antioxidants, hindered amine
light stabilizers, phosphor type antioxidants, hydroquinone monomethyl ether commonly
used in (meth)acrylate monomers, and hydroquinone, methylhydroquinone, t-butylcatechol,
pyrogallol may also be used. Of these, a phenol compound having a double bond in molecules
derived from acrylic acid is particularly preferred due to its having a polymerization-restraining
effect even when heated in a closed, oxygen-free environment. Suitable inhibitors
are, for example, Sumilizer
® GA-80, Sumilizer GM and Sumilizer
® GS produced by Sumitomo Chemical Co., Ltd. Since excessive addition of these polymerization
inhibitors will lower the sensitivity to curing of the curable jettable liquid, it
is preferred that the amount capable of preventing polymerization be determined prior
to blending. The amount of a polymerization inhibitor is generally between 200 and
20 000 ppm of the total curable fluid weight.
Oxygen inhibition
[0074] Suitable combinations of compounds which decrease oxygen polymerization inhibition
with radical polymerization inhibitors are: 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butane-1
and 1-hydroxy-cyclohexyl-phenyl-ketone; 1-hydroxy-cyclohexyl-phenyl-ketone and benzophenone;
2-methyl-1[4-(methylthio)phenyl]-2-morpholinopropane-1-on and diethylthioxanthone
or isopropylthioxanthone; and benzophenone and acrylate derivatives having a tertiary
amino group, and addition of tertiary amines. An amine compound is commonly employed
to decrease an oxygen polymerization inhibition or to increase sensitivity. However,
when an amine compound is used in combination with a high acid value compound, the
storage stability at high temperature tends to be decreased. Therefore, specifically,
the use of an amine compound with a high acid value compound in inkjet printing should
be avoided.
Synergist additives may be used to improve the curing quality and to diminish the
influence of the oxygen inhibition. Such additives include, but are not limited to
ACTILANE
® 800 and ACTILANE
® 725 available from AKZO NOBEL, Ebecryl
® P115 and Ebecryl
® 350 available from UCB CHEMICALS and CD 1012, Craynor CN 386 (amine modified acrylate)
and Craynor CN 501 (amine modified ethoxylated trimethylolpropane triacrylate) available
from CRAY VALLEY.
[0075] The content of the synergist additive is in the range of 0 to 20 wt.%, preferably
in the range of 5 to 15 wt.%, based on the total weight of the curable fluid.
Elastomeric binder
[0076] The elastomeric binder may be a single binder or a mixture of various binders. The
elastomeric binder is an elastomeric copolymer of a conjugated diene-type monomer
and a polyene monomer having at least two non-conjugated double bonds, or an elastomeric
copolymer of a conjugated diene-type monomer, a polyene monomer having at least two
non-conjugated double bonds and a vinyl monomer copolymerizable with these monomers.
Preferred elastomeric binders are disclosed in
EP-A 1637926 paragraph [0092] and [0093].
[0077] Due to their high molecular weight, the addition of elastomeric binders may cause
an increase in viscosity of the curable fluid. Therefore, the amount of elastomeric
binder is preferably less than 5 wt.% for the curable inkjet fluid. In a particular
preferred embodiment, no elastomeric binder is added to the curable inkjet fluid.
As viscosity is not an issue, more elastomeric binder, preferably more than 5 wt.%,
more preferably more than 10 wt.%, may be used for the curable aerosol jet fluid.
Surfactants
[0078] The surfactant(s) may be anionic, cationic, non-ionic, or zwitterionic and are usually
added in a total amount below 20 wt.%, more preferably in a total amount below 10
wt.%, each based on the total curable fluid weight.
[0079] Fluorinated or silicone compounds are preferably used as a surfactant, however, a
potential drawback is bleed-out after image formation because the surfactant does
not cross-link. It is therefore preferred to use a copolymerizable monomer having
surface-active effects, for example, silicone-modified acrylates, silicone modified
methacrylates, fluorinated acrylates, and fluorinated methacrylates.
Colorants
[0080] Colorants may be dyes or pigments or a combination thereof. Organic and/or inorganic
pigments may be used. Suitable dyes include direct dyes, acidic dyes, basic dyes and
reactive dyes. Suitable pigments are disclosed in
EP-A 1637926 paragraphs [0098] to [0100].
[0081] The pigment is present in the range of 0.01 to 10 wt.%, preferably in the range of
0.1 to 5 wt.%, each based on the total weight of curable fluid.
Solvents
[0082] The curable fluid preferably does not contain an evaporable component, but sometimes,
it can be advantageous to incorporate an extremely small amount of a solvent to improve
adhesion to the ink-receiver surface after UV curing. In this case, the added solvent
may be any amount in the range of 0.1 to 10.0 wt.%, preferably in the range of 0.1
to 5.0 wt.%, each based on the total weight of curable fluid.
Humectants
[0083] When a solvent is used in the curable liquid, a humectant may be added to prevent
the clogging of the nozzle, due to its ability to slow down the evaporation rate of
curable fluid.
[0084] Suitable humectants are disclosed in
EP-A 1637926 paragraph [0105]. A humectant is preferably added to the curable jettable liquid
formulation in an amount of 0.01 to 20 wt.% of the formulation, more preferably in
an amount of 0.1 to 10 wt.% of the formulation.
Biocides
[0085] Suitable biocides include sodium dihydroacetate, 2-phenoxyethanol, sodium benzoate,
sodium pyridinethion-1-oxide, ethyl p-hydroxybenzoate and 1,2-benzisothiazolin-3-one
and salts thereof. A preferred biocide is Proxel
® GXL available from ZENECA COLOURS.
[0086] A biocide is preferably added in an amount of 0.001 to 3 wt.%, more preferably in
an amount of 0.01 to 1.00 wt.%, each based on the total weight of the curable fluid.
Preparation of a curable jettable fluid
[0087] The curable fluids may be prepared as known in the art by mixing or dispersing the
ingredients together, optionally followed by milling, as described for example in
EP-A 1637322 paragraph [0108]and [0109].
Viscosity of the curable fluids
[0088] As described above, the method according to the present invention may use the aerosol
jet printing only, or a combination of aerosol jet printing and inkjet printing.
[0089] The curable fluids to be used for inkjet printing have a viscosity at jetting temperature
of less than 15 mPa.s, preferably of less than 12 mPa.s and more preferably of less
than 10 mPa.s.
[0090] The curable fluids to be used for aerosol jet printing have a viscosity of less than
5000 mPa.s at jetting temperature, preferably of less than 2500 mPa.s and more preferably
of less than 1000 mPa.s.
Flexographic printing support
[0091] Two forms of flexographic printing supports may be used: a plate form and a cylindrical
form, the latter commonly referred to as a sleeve. If the print master is created
as a plate form, the mounting of the plate form on a printing cylinder may introduce
mechanical distortions resulting in so-called anamorphic distortion in the printed
image. Such a distortion may be compensated by an anamorphic pre-compensation in an
image processing step prior to halftoning.
[0092] Creating the print master directly on a sheet form mounted on a print cylinder or
directly on a sleeve avoids the problem of geometric distortion altogether.
[0093] Using a sleeve as support provides improved registration accuracy and faster change
over time on press. Furthermore, sleeves may be well-suited for mounting on an inkjet
printer having a rotating drum, as shown in Figure 1. This also makes it possible
to create seamless flexographic printing sleeves, which have applications in printing
continuous designs such as in wallpaper, decoration, gift wrapping paper and packaging.
[0094] The term "flexographic printing support", often encompasses two types of support:
- a support without elastomeric layers on its surface; and
- a support with one or more elastomeric layers on its surface. The one or more elastomeric
layers form the so-called elastomeric floor.
[0095] In the method of the present invention, the flexographic printing support referred
to is a support, preferably a sleeve, without one or more elastomeric layers forming
an elastomeric floor. Such a sleeve is also referred to as a basic sleeve or a sleeve
base. Basic sleeves typically consist of composites, such as epoxy or polyester resins
reinforced with glass fibre or carbon fibre mesh. Metals, such as steel, aluminium,
copper and nickel, and hard polyurethane surfaces (e.g. durometer 75 Shore D) can
also be used. The basic sleeve may be formed from a single layer or multiple layers
of flexible material, as for example disclosed by
US2002466668. Flexible basic sleeves made of polymeric films can be transparent to ultraviolet
radiation and thereby accommodate backflash exposure for building a floor in the cylindrical
printing element. Multiple layered basic sleeves may include an adhesive layer or
tape between the layers of flexible material. Preferred is a multiple layered basic
sleeve as disclosed in
US5301610. The basic sleeve may also be made of non-transparent, actinic radiation blocking
materials, such as nickel or glass epoxy. The basic sleeve typically has a thickness
from 0.1 to 1.5 mm for thin sleeves and from 2 mm to as high as 100 mm for other sleeves.
For thick sleeves often combinations of a hard polyurethane surface with a low-density
polyurethane foam as an intermediate layer combined with a fibreglass reinforced composite
core are used as well as sleeves with a highly compressible surface present on a sleeve
base. Depending upon the specific application, sleeve bases may be conical or cylindrical.
Cylindrical sleeve bases are used primarily in flexographic printing.
[0096] The basic sleeve or flexographic printing sleeve is stabilized by fitting it over
a steel roll core known as an air mandrel or air cylinder. Air mandrels are hollow
steel cores which can be pressurized with compressed air through a threaded inlet
in the end plate wall. Small holes drilled in the cylindrical wall serve as air outlets.
The introduction of air under high pressure permits to float the sleeve into position
over an air cushion. Certain thin sleeves are also expanded slightly by the compressed
air application, thereby facilitating the gliding movement of the sleeve over the
roll core. Foamed adapter or bridge sleeves are used to "bridge" the difference in
diameter between the air-cylinder and a flexographic printing sleeve containing the
printing relief. The diameter of a sleeve depends upon the required repeat length
of the printing job.
Apparatus for creating the flexographic printing master
[0097] Various embodiments of an apparatus for creating the flexographic printing master
by the method according to the present invention may be used. In principle a flat
bed printing device may be used, for both the inkjet printing and the aerosol jet
printing steps, however, a drum based printing device is preferred. A particularly
preferred drum based printing device (100) using a sleeve body as flexographic support
is shown in Figure 3.
[0098] The sleeve body 130 is mounted on a drum 140. The drum 140 rotates in at a certain
speed in the X-direction around axis 110. A printing device 160 moves in the Y-direction.
The printing device 160 may be an aerosol jet printing device (module 200 in Figure
1) when only aerosol jet printing is used to prepare the flexographic printing master
or a both such an aerosol jet printing device and a conventional inkjet print head
when both aerosol jet printing and inkjet printing are used to prepare the flexographic
printing master.
[0099] A curing means may be arranged in combination with the printing device, travelling
therewith so that the curable fluid is exposed to curing radiation very shortly after
been jetted (see Figure 3, curing means 150, printing device 160). It may be difficult
to provide a small enough radiation source connected to and travelling with the printing
device. Therefore, a static fixed radiation source may be employed, e.g. a source
of UV-light, which is then connected to the printing device by means of flexible radiation
conductive means such as a fibre optic bundle or an internally reflective flexible
tube.
[0100] Alternatively, a source of radiation arranged not to move with the printing device,
may be an elongated radiation source extending transversely across the flexographic
printing support surface to be cured and parallel with the slow scan direction of
the print head (see Figure 3, curing means 170). With such an arrangement, each applied
fluid droplet is cured when it passes beneath the curing means 170. The time between
jetting and curing depends on the distance between the printhead and the curing means
170 and the rotational speed of the rotating drum 140.
[0101] A combination of both curing means 150 and 170 can also be used as depicted in Figure
3.
Printing device
[0102] The printing device for aerosol jet printing has already been described.
[0103] For inkjet printing, conventional print heads may be used. The means for inkjet printing
includes any device capable of coating a surface by breaking up a radiation curable
fluid into small droplets which are then directed onto the surface. In the most preferred
embodiment the radiation curable fluids are jetted by one or more printing heads ejecting
small droplets in a controlled manner through nozzles onto a flexographic printing
support, which is moving relative to the printing head(s). A preferred printing head
for the inkjet printing system is a piezoelectric head. Piezoelectric inkjet printing
is based on the movement of a piezoelectric ceramic transducer when a voltage is applied
thereto. The application of a voltage changes the shape of the piezoelectric ceramic
transducer in the printing head creating a void, which is then filled with radiation
curable fluid. When the voltage is again removed, the ceramic returns to its original
shape, ejecting a drop of fluid from the print head. However the inkjet printing method
is not restricted to piezoelectric inkjet printing. Other inkjet printing heads can
be used and include various types, such as a continuous type and thermal, electrostatic
and acoustic drop on demand types. At high printing speeds, the radiation curable
fluids must be ejected readily from the printing heads, which puts a number of constraints
on the physical properties of the fluid, e.g. a low viscosity at the jetting temperature,
which may vary from 25°C to 110°C and a surface energy such that the printing head
nozzle can form the necessary small droplets.
[0104] An example of a printhead according to the current invention is capable to eject
droplets having a volume between 0.1 and 100 picoliter (pl) and preferably between
1 and 30 pl. Even more preferably the droplet volume is in a range between 1 pl and
8 pl. Even more preferably the droplet volume is only 2 or 3 pl.
[0105] The unpublished EP-A's 10173533.0 and 10173538.9 (both filed 2010-08-20) and 10158421.7
(filed 2010-03-30) preferred constellations of multiple printheads, preferably back
to back printheads, are disclosed.
Curing
[0106] Typically for each layer of the relief image, immediately after the deposition of
a fluid droplet by the printing device the fluid droplet is exposed by a curing source.
This provides immobilization and prevents the droplets to run out, which would deteriorate
the quality of the print master. Such curing of applied fluid drops is often referred
to as "pinning".
[0107] Curing can be "partial" or "full". The terms "partial curing" and "full curing" refer
to the degree of curing, i.e. the percentage of converted functional groups, and may
be determined by, for example, RT-FTIR (Real-Time Fourier Transform Infra-Red Spectroscopy)
which is a method well known to the one skilled in the art of curable formulations.
Partial curing is defined as a degree of curing wherein at least 5 %, preferably 10
%, of the functional groups in the coated formulation or the fluid droplet is converted.
Full curing is defined as a degree of curing wherein the increase in the percentage
of converted functional groups with increased exposure to radiation (time and/or dose)
is negligible. Full curing corresponds with a conversion percentage that is within
10 %, preferably 5 %, from the maximum conversion percentage. The maximum conversion
percentage is typically determined by the horizontal asymptote in a graph representing
the percentage conversion versus curing energy or curing time. When in the present
application the term "no curing" is used, this means that less than 5 %, preferably
less than 2.5 %, most preferably less than 1 %, of the functional groups in the coated
formulation or the fluid droplet are converted. In the method according to the present
invention, applied fluid droplets which are not cured are allowed to spread or coalesce
with adjacent applied fluid droplets.
[0108] Curing may be performed by heating (thermal curing), by exposing to actinic radiation
(e.g. UV curing) or by electron beam curing. Preferably the curing process is performed
by UV radiation.
[0109] The curing means may be arranged in combination with the printing device, travelling
therewith so that the curable fluid is exposed to curing radiation very shortly after
been jetted (see Figure 3, curing means 150, printing device 160). It may be difficult
to provide a small enough radiation source connected to and travelling with the printing
device. Therefore, a static fixed radiation source may be employed, e.g. a source
of UV-light, which is then connected to the printing device by means of flexible radiation
conductive means such as a fibre optic bundle or an internally reflective flexible
tube.
[0110] Alternatively, a source of radiation arranged not to move with the printing device,
may be an elongated radiation source extending transversely across the flexographic
printing support surface to be cured and parallel with the slow scan direction of
the print head (see Figure 3, curing means 170). With such an arrangement, each applied
fluid droplet is cured when it passes beneath the curing means 170. The time between
jetting and curing depends on the distance between the printing device and the curing
means 170 and the rotational speed of the rotating drum 140.
[0111] A combination of both curing means 150 and 170 can also be used as depicted in Figure
3.
[0112] Any UV light source, as long as part of the emitted light can be absorbed by the
photo-initiator or photo-initiator system of the fluid droplets, may be employed as
a radiation source, such as, a high or low pressure mercury lamp, a cold cathode tube,
a black light, an ultraviolet LED, an ultraviolet laser, and a flash light. For curing
the inkjet printed radiation curable fluid, the imaging apparatus preferably has a
plurality of UV light emitting diodes. The advantage of using UV LEDs is that it allows
a more compact design of the imaging apparatus.
[0113] UV radiation is generally classified as UV-A, UV-B, and UV-C as follows:
- UV-A: 400 nm to 320 nm
- UV-B: 320 nm to 290 nm
- UV-C: 290 nm to 100 nm
[0114] The most important parameters when selecting a curing source are the spectrum and
the intensity of the UV-light. Both parameters affect the speed of the curing. Short
wavelength UV radiation, such as UV-C radiation, has poor penetration capabilities
and enables to cure droplets primarily on the outside. A typical UV-C light source
is low pressure mercury vapour electrical discharge bulb. Such a source has a small
spectral distribution of energy, with only a strong peak in the short wavelength region
of the UV spectrum.
[0115] Long wavelength UV radiation, such as UV-A radiation, has better penetration properties.
A typical UV-A source is a medium or high pressure mercury vapour electrical discharge
bulb. Recently UV-LEDs have become commercially available which also emit in the UV-A
spectrum and that have the potential to replace gas discharge bulb UV sources. By
doping the mercury gas in the discharge bulb with iron or gallium, an emission can
be obtained that covers both the UV-A and UV-C spectrum. The intensity of a curing
source has a direct effect on curing speed. A high intensity results in higher curing
speeds.
[0116] The curing speed should be sufficiently high to avoid oxygen inhibition of free radicals
that propagate during curing. Such inhibition not only decreases curing speed, but
also negatively affects the conversion ratio of monomer into polymer. To minimize
such oxygen inhibition, the imaging apparatus preferably includes one or more oxygen
depletion units. The oxygen depletion units place a blanket of nitrogen or other relatively
inert gas (e.g. CO
2), with adjustable position and adjustable inert gas concentration, in order to reduce
the oxygen concentration in the curing environment. Residual oxygen levels are usually
maintained as low as 200 ppm, but are generally in the range of 200 ppm to 1200 ppm.
[0117] Another way to prevent oxygen inhibition is the performance of a low intensity pre-exposure
before the actual curing.
[0118] A partially cured fluid droplet is solidified but still contains residual monomer.
This approach improves the adhesion properties between the layers that are subsequently
printed on top of each other. Partial intermediate curing is possible with UV-C radiation,
UV-A radiation or with broad spectrum UV radiation. As mentioned above, UV-C radiation
cures the outer skin of a fluid droplet and therefore a UV-C partially cured fluid
droplet will have a reduced availability of monomer in the outer skin and this negatively
affects the adhesion between neighbouring layers of the relief image. It is therefore
preferred to perform the partial curing with UV-A radiation.
[0119] A final post curing however is often realized with UV-C light or with broad spectrum
UV light. Final curing with UV-C light has the property that the outside skin of the
print master is fully hardened.