[0001] The invention relates to a process for coating a board- or paper-like substrate with
a powder paint composition and use of the process in the application of overprint
varnishes.
[0002] In industrial applications varnishing of, for example, cellulose based sheets and
webs is usually carried out using solvent or water-based varnishes. The solvent-based
varnishes have as a disadvantage the evaporation of solvents into the atmosphere.
Because of environmental reasons emission of solvents has to be reduced drastically.
The water-based varnishes have as a disadvantage that they need much energy for drying.
To achieve the highest possible gloss the use of calanders is suitable. This separate
step is costly and limits the process speed.
[0003] Alternatively, liquid UV-curable varnishes are used to overcome these difficulties.
However, this group of materials is often rejected because of their sensibilizing
and irritating character due to the applied monomers, which are the main components
of these coating liquids. Another disadvantage of this coating technology can be that
the smell makes the coating unsuitable for food packaging.
[0004] GB-A-2056885 discloses a process for coating a cellulose fibre substrate like cardboard
with a powder paint composition and curing by radiant heat. Generally a binder system
for a powder paint composition consists of a resin and a crosslinker. A disadvantage
of the process according to GB-A-2056885 is the limited capacity of the applied powder
paint spray guns which imposes a restriction on the rate of application of powder
paint to the substrate. Another disadvantage is that it is very difficult to achieve
coatings having a layer thickness below 15µm.
[0005] A further disadvantage is the major thermal load imposed on the heat-sensitive substrate
during curing of the powder paint composition.
[0006] It is the object of the present invention to provide a process for coating a board
or paper-like substrate (which substrate can already be precoated) at process speeds
of substrates between, for example, 1 and 400 meter substrate/minute, giving a high
quality coating with a thickness between for example 1 µm and 100 µm.
[0007] The invention is characterized in that powder paint particles are first charged by
friction or induction in the presence of magnetic or non-magnetic particles, are next
transported and then applied to the substrate, or applied to a transfer medium and
are then transferred to the substrate, by means of an electric field between the substrate
respectively the transfer medium and the means of transport, whereafter the powder
paint composition is cured or fused into a powder coating wherein the powder paint
particles are based on solid particles of a radiation curable binder system.
[0008] Suitable radiation curable binder systems include for example a polymer having unsaturated
groups and a cationic curable system based on, for example epoxy resins.
[0009] Preferably, the radiation curable binder system comprises a polymer having unsaturated
groups.
[0010] Preferably, said polymer having unsaturated groups has a degree of unsaturation between
about 200 and about 1800 grams of polymer per mole of unsaturated group.
[0011] Charging of the powder paint particles by friction or induction, transport or conveying
and application to the substrates can be effected using processes commonly known in
photocopying technology or laser printer technology (which processes are elucidated
in, for example, L.B. Schein, Electrography and Development Physics, pages 32-244,
Volume 14, Springer Series in Electrophysics 1988; the disclosure of which is incorporated
herein by reference).
[0012] According to a preferred embodiment of the invention the powder paint particles are
mixed with magnetic or non-magnetic carrier particles to obtain friction charging
whereupon the mixture is transported and the powder paint particles are applied to
the substrate or a transfer medium, and subsequently transfereed to the substrate,
by means of an electric field between the substrate or the transfer medium and the
means of transport.
[0013] Depending on the conductivity of the substrate the electric field between the substrate
and the transfer medium or means of transport can be applied with suitable methods
like for instance a corona discharge or a moving or fixed counterelectrode (see for
instance Schein, pp. 36-37 and 47).
[0014] In case a transfer medium is used, the powder paint particles are first applied to
the transfer medium by means of an electric field, transported to the substrate by
the transfer medium and then applied to the substrate by, for example, electrical,
electrostatic or mechanical forces. Thermal processes can also be used in this application
step.
[0015] The present process allows powder paint particles to be applied to coated or uncoated
board- or paper-like substrates such as, for example, cardboard, hardboard, fibre
board for example MDF, particle board, chipboard, plywood, veneer, block board (which
board products can be paperfaced), wood, timber and paper at rates up to, for instance,
400 meters of substrate/minute with a film thickness of the cured film of between,
for example, 1 µm and 100 µm.
[0016] Preferably, the polymer having unsaturated groups has a molecular weight (Mn) between
about 800 and about 6800 grams/mole and a viscosity between 1 dPas and 800 dPas (measured
with an Emila rheometer at 165°C).
[0017] Preferred polymers are unsaturated polyesters, unsaturated polyacrylates and acrylated
polyesters.
[0018] Preferably, the unsaturated polymer has a degree of unsaturation between 300 and
1800 grams per mole of unsaturated group (WPU), a molecular weight (Mn) between about
800 and about 6800 and a viscosity between 1 dPas and 400 dPas. The preferred degree
of unsaturation for an unsaturated polyester is between 500 and 1500 grams per mole
of unsaturated group.
[0019] The molecular weight (Mn) is preferably between about 2500 and about 4500 grams/mole.
[0020] The unsaturated polyester can be (semi)crystalline or amorphous.
[0021] In general, the glass transition temperature (Tg) of the amorphous resin is higher
than 40°C. Semi-crystalline resins have a Tg and a melting point (Tm), and Tg is lower
than Tm.
[0022] The melting point of the (semi)crystalline unsaturated polyester is between about
40°C and about 180°C, and preferably, is between about 60°C and about 140°C. The melting
point and the melting range are discussed at pages 36, 286 and 295 of Misev, Powder
Coatings, Chemistry and Technology Wiley 1991.
[0023] Thermosetting powder coatings that are based on unsaturated polyester are described
in Misev, Powder Coatings, Chemistry and Technology, at pages 167-170.
[0024] Preparation of the unsaturated polyester can be carried out in a single-step process
in which unsaturated polyfunctional carboxylic acids and glycols are heated to a temperature,
for example, between about 180°C to about 230°C for between about 6 hours to about
15 hours.
[0025] Preparation of the unsaturated polyester can also be carried out in a two-step process
in which saturated glycols and saturated polyfunctional carboxylic acids are esterified
in a first step at, for example, between about 230°C to about 250°C for between about
2 hours to about 8 hours. In a second step, saturated or unsaturated glycols and acids,
or mixtures thereof, are esterified at, for example between about 180°C and 220°C
for between about 2 to about 8 hours.
[0026] In general, the unsaturated polyester is obtainable from the condensation of one
or more aliphatic or cycloaliphatic mono-, di- or polyfunctional alcohols, or mixtures
thereof, one or more aliphatic, cycloaliphatic, or aromatic di- or polyfunctional
carboxylic acids, or mixtures thereof, and if desired, a monofunctional carboxylic
acid or the corresponding ester of this monofunctional carboxylic acid.
[0027] Examples of suitable alcohols and glycols include, among others, benzyl alcohol,
ethylene glycol, propylene glycol, neopentylglycol, butanediol, hexanediol, dimethylol
cyclohexane, diethylene glycol, glycerol, trimethylol propane, pentaerytritol, dipentaerythritol,
or mixtures thereof. Instead of an alcohol or glycol, or together with an alcohol
or glycol, one or more epoxy compounds such as, for example, ethylene oxide, propylene
oxide, allyl glycidyl ether, or mixtures thereof, can be used.
[0028] Examples of suitable di- or polyfunctional carboxylic acids include maleic acid,
fumaric acid, itaconic acid, citraconic acid, malonic acid, succinic acid, glutaric
acid, adipic acid, sebacic acid, 1,4-cyclohexane dicarboxylic acid, hexahydrophthalic
acid, hexachloroendomethylenetetrahydrophthalic acid, dichlorophthalic acid, isophthalic
acid, terephthalic acid, trimellitic acid, or mixtures thereof.
[0029] Fumaric acid, maleic acid, or a mixture thereof, is preferred.
[0030] The carboxylic acids can also be applied in the corresponding anhydride form, so
that, for example, tetrahydrophthalic anhydride, maleic anhydride, phthalic anhydride,
or mixtures thereof can be used.
[0031] If desired, the unsaturated polyester can also be obtained from saturated or unsaturated
monofunctional carboxylic acids, or mixtures thereof. These monofunctional carboxylic
acids include, for example, synthetic or natural fatty acids having 2 to 36 carbon
atoms. Corresponding esters of monofunctional alcohols such as glycerol are used for
esterification. Examples of suitable monofunctional carboxylic acids include, among
others, lauric, stearic, oleic, linoleic, benzoic, acrylic, methacrylic acid, or mixtures
thereof. The unsaturated polyester can also contain dicyclopentadiene.
[0032] If the unsaturated polyester is hydroxyl-functionalized, then the hydroxyl number
of the polyester is usually between about 18 mg KOH/gram of resin and about 75 mg
KOH/gram of resin.
[0033] If the unsaturated polyester is acid-functionalized, the acid number is usually between
about 18 mg KOH/gram of resin and about 75 mg KOH/gram of resin.
[0034] Suitable acrylate polymers include, for instance, acrylate polymers having several
side chains containing unsaturated groups.
[0035] These polymers can be obtained by means of a two-step process. In the first step
an acrylate polymer is prepared by a conventional polymerization process, in which
also a certain proportion of functional monomer is copolymerized to obtain a functionalized
acrylate polymer. This functional monomer, which usually is present in amounts of
between 3 and 60 wt. %, can be, for example, an epoxy functionalized monomer such
as glycidyl (meth)acrylate; an acid functionalized monomer such as, for instance,
(meth)acrylic acid, a hydroxy functionalized monomer such as, for instance hydroxyethyl(meth)acrylate;
or an isocyanate functionalized monomer such as, for instance, TMI (meta-isopropenyl-α-α-dimethylbenzyl-isocyanate).
[0036] In the second step for the acrylate synthesis, an addition reaction, is carried out
between the functional groups of the acrylate polymer which is prepared in the first
step and a compound having both a functional group capable of reacting with the functional
groups and also an unsaturated group in the side chain.
[0037] For the addition reaction of this second step, the functionalized acrylate resin
can be dissolved in a solvent such as for example toluene, xylene and/or butyl acetate.
The compound having the unsaturated group, is added at temperatures between, for example,
50°C and 150°C and next, stirring takes place for some hours. Progress of the reaction
can be monitored by means of titrations of, for instance, acid groups or isocyanate
groups. Addition reactions of this second step are, for instance, between a compound
having an acid group and an epoxy functional acrylate polymer, a compound having a
hydroxyl group and an isocyanate functionalized acrylate polymer, a compound having
an isocyanate group and a hydroxyl functionalized acrylate polymer, a compound having
an anhydride group and a hydroxyl functionalized acrylate polymer or a compound having
an epoxy group and an acid functionalized acrylate polymer.
[0038] It is preferred to allow an epoxy functionalized acrylate polymer to react with (meth)acrylic
acid.
[0039] The unsaturated side chain can have, for example, methacrylate ester, allyl, vinyl,
vinyl ether, or anhydride groups, or mixtures thereof. Suitable compounds that are
allowed to react with the functionalized acrylate copolymer are, for example, methacrylic
acid, glycidyl methacrylate, TMI, allylglycidyl ether, hydroxybutylvinyl ether, maleic
anhydride, or mixtures thereof.
[0040] The unsaturated acrylate polymer can also be obtained by means of a multi-step process.
For example, one of the isocyanate groups of a diisocyanate compound such as, for
example, isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI) or methylenebiscyclohexylisocyanate
(hydrogenated MDI) is allowed to react with a methacrylate monomer having the hydroxyl
functionality. The resulting urethane compound, which contains isocyanate and methacrylate
groups for further reaction, can subsequently be allowed to react with, for example,
an acrylate copolymer having the hydroxyl functionality. This acrylate copolymer can
be prepared using, for example, hydroxyethyl methacrylate as a functionalized monomer.
[0041] Depending on the binder system a crosslinker has to be applied or can optionally
be applied.
[0042] A preferred crosslinking agent for the radiation curable binder composition according
to the present invention is an oligomer or polymer having one or more vinyl ether,
allyl, vinyl ester, or (meth)acrylate groups. More preferably an oligomer or polymer
having at least two vinylether groups is applied.
[0043] The crosslinking agent having vinyl ether groups is preferably made of a divinyl
ether functionalized urethane based on, for example, a diisocyanate and hydroxybutyl
or hydroxyethyl vinyl ether.
[0044] Suitable crosslinking agent having (meth)acrylic groups include for example dimethacrylate
functionalized urethane based on, for example, a diisocyanate and hydroxyethyl methacrylate
or polymethacrylate functionalised urethanes.
[0045] Suitable difunctional isocyanates used in the preparation of the crosslinking agent
include isophorone diisocyanate, or methylene diisocyanate, or methylenebiscyclohexylisocyanate.
Functionalized oligomers derived from these diisocyanates such as isocyanurates, uretdiones,
and biurets can, for example, also be used.
[0046] Preparation of vinyl ether functionalized diurethanes can be carried out by means
of the process described in, for example, EP-A-505 803.
[0047] Preparation of suitable vinyl ether terminated ester oligomers for use as crosslinking
agent is described in WO-A-89/10.346, the complete disclosure of which is incorporated
herein by reference. A suitable vinyl ester functional compound is, for example, divinyl
adipate.
[0048] The crosslinking agent preferably contains between about 1 to 10 vinyl groups, and
more preferably, contains at least two vinyl groups per molecule.
[0049] The crosslinking agent can be a liquid or a solid. Solid crosslinking agents are
preferred because good powder stability is more easily achieved, after combination
of the crosslinking agent with the unsaturated polymer.
[0050] The crosslinker having allyl groups can for example have 2-12 allyl groups. The crosslinker
can have allylether groups and further ester or urethane groups. The crosslinker can
also have vinylether and allylether groups. It is also possible to use vinylether
and allylether containing crosslinkers.
[0051] Very suitable crosslinkers include, for example, the ester of trimethylolpropane
diallyl ether or pentaerythritol triallyl ether and a polycarboxylic acid (anhydride).
Also oligomers containing allyl groups, for example, allyl alcohol, trimethylolpropane
diallyl ether, allyl glycidyl ethers and pentaerythritol triallyl ether. Examples
of suitable isocyanates are isophorone diisocyanate, toluene diisocyanate, hexamethylene
diisocyanate and methylene biscyclohexylisocyanate.
[0052] The relative amounts of polymer having unsaturated groups and crosslinking agent
in the binder composition depend in part on the choice of crosslinking agent. In general,
if the crosslinking agent is vinyl ether functionalized, the equivalent ratio of polymer
unsaturation to crosslinking agent unsaturation is between 1:2 and about 2:1, and
preferably, about 1:1. In general, if the crosslinking agent is a methacrylate or
a vinyl ester functionalized compound, the equivalent ratio of polymer unsaturation
to crosslinking agent unsaturation is between about 1:2 and about 2:1, and preferably,
about 1:1.3.
[0053] A suitable cationic curable system based on epoxy resins is disclosed in EP-A-667381.
[0054] Radiation curing is preferably carried out via ultraviolet (UV) or electron beam
(EB) curing. These techniques are described in, for example, the article, "UV and
EB-curing", by S.J. Bett et al. in Jocca 1990 (11), pgs. 446-453. Most preferably
UV-curing is applied. Very suitable UV-lamps for curing thin layers or to obtain high
speeds are excimer lamps (as described for example at pages 48-52 of the Conference
Proceedings of RAD TECH Europe September 25, 1995, in Maastricht).
[0055] For UV-radiation curing of the powder paint formulation a photoinitiator is mixed
with the binder composition at a temperature between, for example, about 50°C and
about 150°C. Mixing can take place in a solvent or in the melt with use of an extruder,
wherein the latter is preferred. Pigments, flow promotors, other additives, or mixtures
thereof, can also be used.
[0056] Another advantage of the present invention is the fast cure at lower temperatures,
in comparison with the prior art processes related to thermocuring systems, resulting
in a lower thermal load on the substrate which is very relevant for the present substrates.
[0057] An important advantage of the radiation curing according to the present invention
is that flow of the powder paint on exposure to heat is fully separated from the curing
reaction on exposure to UV or electron beam radiation, which enables powder coatings
to be made that exhibit excellent flow.
[0058] Common additives such as, for example, pigments, fillers, flow promoters, tribo additives,
stabilizers, or mixtures thereof can be added to the binder system. Suitable pigments
include, for example, inorganic pigments such as titanium dioxide, zinc sulphide or
iron and chromium oxide, and organic pigments such as, for example, azo compounds.
[0059] Suitable fillers include, for example, metal oxides, silicates, carbonates, sulfates,
or mixtures thereof, as known to those skilled in the art.
[0060] The powder paint compositions can, for example, be prepared and characterized as
described in, for example, the aforementioned Misev on pages 224-283. The selection
of grinding, classifying and sieving equipment is important for obtaining the desired
particle size of the powder paint particles. For this purpose target mills, fluidised
bed mills, static classifiers and rotating classifiers may be used.
[0061] Upon curing or fusing of a powder paint composition a powder coating is obtained.
[0062] The median particle size (by volume) of the powder paint particles X
50,3 (as defined according to the description and notation at pages 12-14 of Mechanische
Verfahrenstechnik by Prof. Rumpf (Carl Hansen Verlag, 1975)) can be, for example,
below about 200 µm, and preferably, is between about 5 and about 60 µm.
[0063] The selection of the particle size depends on, for example, the desired final coating
thickness for a given application.
[0064] The particle size distribution can be as broad as it is in conventional powder paint
technology.
[0065] Preferably, the particle size distribution is relatively narrow. More preferably,
the ratio X
75,3:X
25,3 < 3 (according to the definition in the aforementioned Rumpf), since the efficiency
of the development step may vary with the particle size.
[0066] It is one of the advantages of the process according to the present invention, that
it is possible to apply particles having median particle sizes between about 5-30
µm. These particle sizes are very suitable to be applied for a clear varnish. It is
very difficult to apply particles of such a size with conventional spray guns.
[0067] Carrier particles can be either magnetic or non-magnetic. Preferably, the carrier
particles are magnetic particles.
[0068] Suitable magnetic carrier particles have a core of, for example, iron, steel, nickel,
magnetite, γ-Fe
2O
3, or certain ferrites such as for example CuZn, NiZn, MnZn and barium ferrites. These
particles can be of various shapes, for example, irregular or regular shape.
[0069] Generally, these carrier particles have a median particle size between 20 and 700
µm. Preferably, the carrier particle size distribution is narrow and more preferably
the ratio X
75,3:X
25,3 < 2.
[0070] Exemplary non-magnetic carrier particles include glass, non-magnetic metal, polymer
and ceramic material.
[0071] Non-magnetic and magnetic carrier particles can have similar particle size.
[0072] Preferably the carrier core particles are coated or surface treated with diverse
organic or inorganic materials to obtain, for example, desirable electrical, triboelectrical
and/or mechanical properties. Inorganic materials are described in for example US-A-4925762
and US-A-5039587. Organic coating materials include, for example, polymers having
fluor-, silicone-, acrylic-, styrene-acrylic, melamine- or urethane-group. Mixtures
of these polymers can also be used. Preferably a fluor containing polymer is used
as coating.
[0073] For coating the carrier particles, any suitable method to coat a thin or thick layer
on a powder surface, such as, for example spray coating in a fluidized bed or dry
coating in a rotary kiln, can be used.
[0074] The carrier coatings can comprise suitable fillers or additives to control for example,
triboelectrical, electrical or mechanical properties of the carrier coating. For example,
conductive materials such as, carbon black and metals powder, or charge controlling
material and flow improving materials can be used.
[0075] The carrier particles may be conductive (as described in for example US-A-4076857)
or non-conductive.
[0076] Preferably, carrier particles having high voltage break through are used so that
high electric fields can be used between the transport means and the substrate or
the transfer medium to achieve a thick powder coating layer.
[0077] A developer comprises a mixture of powder paint particles and carrier particles.
A development method is a way of developing and a development unit is a complete system
comprising of, for example, a developer roller (transport medium), mixing screw(s),
a supply device, blades, detectors and the like. Suitable examples are described in,
for example, GB-A-2097701, US-A-4147127 and US-A-4131081.
[0078] In the resent invention the development method can be either one-component or two-component.
According to a preferred embodiment of the invention the two-component development
method, in which the carrier particles are mixed with the powder paint particles,
is used. Alternatively, also all kinds of monocomponent development like, for example,
magnetic conductive, magnetic insulative or non magnetic development as disclosed
on pages 203 ff of the above mentioned Schein or binary developers consisting of e.g.
magnetic toner and carrier as described in US-A-5516613 can be used.
[0079] Preferably, a combination of powder paint particles having a X
50,3 below 80 µm and a X
95,3 below 120 µm and carrier particles having a X
50,3 below 180 µm and a X
95,3 below 200 µm is used.
[0080] More preferably, a combination of powder paint particles having a X
50,3 below 30 µm and above 5 µm and a X
95,3 below 50 µm and carrier particles having a X
50,3 below 180 µm and above 5 µm and a X
95,3 below 200 µm is used.
[0081] In the two-component developer the amount of powder paint particles can be, for example,
between about 1 and about 50 wt.% and preferably between about 5 and about 25 wt.%
(relative to the amount of developer). It is an advantage of the process according
to the invention that it is possible to use powder paint concentrations well in excess
of 10 wt.%, which is usually not possible in printing applications. Consequently,
the amount of carrier particles can be between about 50 and about 99% by weight (relative
to the amount of developer) and preferably is between about 75 wt.% and about 95 wt.%.
[0082] The powder paint concentration can be externally or internally (in the development
unit) controlled. External control can be effected by measurement of layer thickness
of uncured or cured powder by, for example, optical, photothermical or dielectrical
means. Internal control can be carried out in the developer station by means of powder
paint concentration control by any suitable means like inductive control (see, for
example, US-A-4147127 and US-A-4131081) or volume control.
[0083] In a two-component development method the powder paint particles are triboelectrically
charged by intensive mixing and friction with the carrier particles.
[0084] In the process according to the present invention it is also possible to use a one
component development method with the carrier particles being incorporated in the
powder paint particles as disclosed in, for example, US-A-4803143 and US-A-4543312.
[0085] In a one-component development method the particles are charged by induction or friction,
depending on the selection of the powder paint particles.
[0086] Both one- and two-component developers can be transported by magnetic, electric and/or
mechanical transport.
[0087] Preferably, the means of transport is a conveying method.
[0088] The mixture is preferably conveyed by means of a magnetic roller as described in
for example US-A-4460266. In addition to a magnetic brush apparatus also useful in
the present process are, for example, non-magnetic cascade development (see page 94
of the hereinbefore incorporated reference of L.B. Schein) and magnetic cascade development
(as described in for example "The 9th International Congress on Advances in Non-Impact
Printing Technologies/Japan Hardcopy '93, pages 133-140). In addition also transport
by air, for example, powder cloud development can be used. An exemplary process is
described in for example US-A-2725304. Also jumping (projection) development (see
for example Electrophotography, Fourth International Conference, pages 207-228, Nov.
16-18, 1981) can be carried out.
[0089] When two-component developers are used, the parameters which are relevant for the
process (such as, for example, powder paint concentration, development potential and
machine parameters) can be chosen depending on the application. This may lead to batch
developer replacement, e.g. after certain time intervals or if certain parameters
are out of a control range. Preferably, continuous developer material replenishment,
as described in, for example, US-A-4614165, can be used to avoid process interruption
for batch replenishment.
[0090] The substrate can have any desired shape such as band or sheet, i.e. continuous or
discontinuous.
[0091] If a transfer medium is used between the transport means and the substrate any suitable
transfer medium like, for example, metallic drums, dielectric drums, foil from metals
or polymers like silicone rubber, as described in "Proceeding of IS&T's Seventh International
Congress on Advances in Non Impact Printing Technologies", Vol. II, page 453-462,
a web as disclosed in EP-A-332223, or composite materials can be used.
[0092] Alternatively a two step transfer may be used in which a layer of powder paint is
developed on a first transfer medium and then transferred to a second transfer medium
and finally transferred to the substrate.
[0093] The present coating process can be advantageously used in the application of varnishes.
Surprisingly, it was found that fully closed layers having a thickness of about 5
µm can easily be obtained with the process according to the invention whereas it is
very difficult to obtain such layers with spraying techniques.
[0094] Generally, more layers can be applied with or without intermediate fusing separately
on the substrate or separately on the transfer medium and jointly transferred to the
substrate.
[0095] A preferred process according to the present invention thus comprises charging of
the powder paint particles by intensive mixing and friction with magnetic carrier
particles, transport of carrier particles and powder paint particles with the aid
of a magnetic roller and subsequent application of the powder paint particles to a
substrate or a transfer medium, and subsequently transferred to the substrate, by
means of an electric field between the substrate or the transfer medium and the magnetic
roller, whereafter the powder paint composition is cured or fused to a powder coating
using techniques known in the art.
[0096] For melting, the applied powder paint can be transferred into an oven or can be exposed
to IR radiation, or a combination of both. Alternatively all other suitable fusing
methods e.g. as disclosed on page 47-49 of the above mentioned Schein can be used.
The applied powder paint is molten at temperatures between, for example, about 60°C
and about 140°C to form a continuous, smooth, softened or molten coating with coating
thickness between, for example, about 1 µm and about 100 µm. Then, the hot panel is
cured with use of a UV light source. Post-heating can also be carried out. The powder
coating adheres to the substrate.
[0097] It is possible to apply the coating on printed substrates produced with common printing
techniques (offset, gravure, flexo, digital printing either electrographic or inkjet).
The printing process and the coating process can be carried out in two separate steps
(off-line) or as well in one production step (on-line).
[0098] The coating may cover the full area of the substrate or only part of it (spot coating).
In case of spot coating any image creating technology as described in the above mentioned
Schein may be used to create an image on a suitable transfer medium or as first transfer
medium in case of two step transfer as described before.
[0099] The carrier particles may be metered to a mixing arrangement in which, one or more
means of intensive mixing such as, for example, worm wheels are present along with
a magnetic roller. Suitable mixing arrangements are described in, for example, "Proceeding
of IS&T's Seventh International Congress on "Advances in Non Impact Printing Technologies",
Vol. 1, pages 259-265.
[0100] Next, an amount of powder paint particle is fed into the mixing arrangement which
is selected so as to obtain a powder paint particles concentration of, for example,
about 5-25 wt.% powder paint relative to the amount of carrier particles. In this
way, a developer is formed.
[0101] During the intensive mixing and friction in the mixing arrangement, due to the action
of the worm weels, the carrier particles and the powder paint particles become oppositely
(tribo) electrically charged. A layer of electrically charged powder paint particles
covers the carrier particles. The carrier particles subsequently act as carrier for
the charged powder paint particles. Next, the developer is fed to the magnetic roller,
on which a brush-like structure, also known as the magnetic brush, is formed. The
magnetic roller transports the brush to the contact area with the substrate or transfer
medium. In this way, powder paint particles, as well as carrier particles, become
available at the contact area with the substrate or transfer medium. By applying a
sufficiently strong electric field between the magnetic roller and the substrate or
transfer medium the powder paint particles can be drawn from the brush to the substrate
or transfer medium, onto which they adhere electrostatically. In the case a transfer
medium is used, the powder paint particles are subsequently transported to the contact
area of the transfer medium and the substrate and next transferred to the substrate
by means of one of the aforementioned transfer processes.
[0102] Finally, the magnetic brush is scraped off the magnetic roller as it is returned
into the mixing bin.
[0103] As a result, a layer of powder paint particles forms on the substrate, which layer
is substantially free from carrier particles, and can then be cured. Eventually, an
apparatus which can catch carrier particles can be introduced such as for example
a catching equipment for residual carrier, as described in "Proceeding of IS&T Eighth
International Congress on Advances in Non Impact Printing Technologies", pages 391-393.
The thickness of said layer of powder paint particles can, for example, be controlled
via the electric field strength between the magnetic roller and the substrate, or
the transfer medium, the magnitude of the charge on the powder paint particles (e.g.
by varying the concentration and the mixing time) and the roller speed.
[0104] The process according to the invention can advantageously be applied in equipment
for small-scale application of overprint varnishes. These machines can be used online
or offline in connection with black and white, two colour or full colour copy machines
or computer output printers to produce paper or cardboard sheets with high gloss and
high resistance e.g. against fingerprints, oil and acids. The process may be used
as an alternative to lamination.
[0105] The process can also be used in-line or off-line in connection with printing machines
of any speed and any technology (for example large scale) like offset, gravure, flexo
or digital printing to produce overprint varnishes e.g. for cover sheets of booklets
and printed cardboard for packaging.
[0106] The process may be used for varnishes, which may be applied below or above a tonerbased
image to improve adhesion of the toner to the paper and to improve protection against,
for instance, falsification of a document.
[0107] The development station with toner hopper and optional the transfer means and the
fusing and curing means may be designed in a way that it can be easily interchanged,
for instance as a cartridge, by any (unskilled) person, for example, for use in a
copying machine or laser printer. Consequently, these small scale overprint varnish
application machines can be used, for example, in printing shops, copy shops and offices.
[0108] The article "An electrostatic process for applying dry coatings on paper" by Reif
(TAPPI, October 1955, Vol 38, no 10, pages 607-609) discloses an electrostatic process
for applying dry coatings on paper. The applied coating unit and process are different
from the present application because amongst others the coating material is suspended
as a dust cloud in air, the charging is carried out with a corona discharge, the powder
paint particles are not charged in the presence of magnetic or non-magnetic particles
and there is no reference to a radiation curable binder system.
[0109] The invention will be further described based on the following non-limiting examples.
Examples
Example I
Preparation of powder paint composition I
[0110] A powder paint composition was prepared by premixing 830 parts by weight of a polyester
resin (Uralac XP3125 of DSM Resins), 170 parts by weight of a crosslinker (Uralac
ZW 3307 of DSM Resins), 10 parts by weight of photoinitiator (Irgacure 184™) and 6
parts by weight of flow agent (Byk 361™) in a Henschel batch mixer and next melt kneaded
in a Buss-Ko-Kneader PLK 46.
[0111] The cooled extrudate was milled first in a hammer mill to a particle size < 3mm and
then fed into a target mill (NPK Labojet) obtaining a powder paint with a medium particle
size of 10.8 µm and a d
75,3/d
25,3 ratio of 3.3.
Example II
Preparation of a carrier
[0112] 998 parts by weight Cu-Zn-ferrite powder, having a median particle size of 81 µm
and a ratio X
75,3/X
25,3 of 1,32 (both measured with the laser granulometer Cilas HR 850), were dry coated
with 2 parts by weight polyvinylidenedifluoride (Kynar 301F™) by mixing both materials
in a Lödige mixer and coating the polymer on the surface of the ferrite in a rotary
kiln at 200°C under N
2-atmosphere to obtain a carrier with a medium size of 80 µm, a ratio X
75,3/X
25,3 of 1.32, a resistance of 1.1∗10
10 Ohm at a potential of 10V and a break-through voltage above 1,000V (both measured
in a c-meter of Epping GmbH).
Example III
Preparation of developer I
[0113] 16% by weight of the powder paint composition according to Example I and 84% by weight
of the carrier according to Example II were mixed at room temperature intensively
in a "Skandex" paint shaker for 5 minutes to obtain a developer. The charge distribution
of the developer was measured in a q-meter of Epping GmbH showing a narrow charge
over diameter (q/d)-distribution with a median of 2.2 fC/10 µm, a standard deviation
of 1.7 fC/10 µm and 5.2% oppositely charged particles.
Example IV
Coating on a substrate
[0114] The developer according to Example III was filled in a magnetic brush unit mounted
in a distance of 2.5 mm of a rotating metal drum available as the ld-tester (developer
life time tester) from Epping GmbH. On the rotating drum a cardboard substrate of
0.3 mm thickness preprinted with red offset printing ink was mounted. The rotation
speed of the drum (i.e. coating speed) was 30 m/minute, and the speed of the magnetic
brush was 78 meters/minute in the same direction as the drum. The doctor blade of
the magnetic brush was adjusted to a distance of 1.5 mm to the magnetic roller. The
magnetic pole was adjusted -10 degrees compared to the line between both rotating
axes. The development potential of the drum against the developer roller was set to
the following values (Table I):
TABLE I
development voltage |
average coating layer thickness |
coating properties |
-1000 V |
7.5 µm |
high gloss (97%), closed film, oil-and tensid tight |
-800 V |
3.5 µm |
high gloss (94%), closed film, oil-and tensid tight |
-600 V |
2.5 µm |
medium gloss (75%) |
-400 V |
1.5 µm |
low gloss (38%) |
-200 V |
1 µm |
slight gloss (19%) |
After application on the cardboard substrate, the powder was heated at 70°C by means
of IR to obtain a good flowing film, followed by a UV radiation treatment (1000mJ/cm
2). The result in the examples of the first two blocks of Table I is a chemical resistant,
hard and scratch resistant coating.
[0115] The gloss was measured with a Reflectometer supplied by Dr. Lange at 60° detection
angle (according to DIN 67530).
Example V
Preparation of powder paint II
[0116] 99.5 parts by weight of the powder paint composition of Example I were mixed with
0.5 parts of weight of siliciumdioxide (Aerosil R504™) in a Henschel batch mixer at
maximum speed for 3 minutes to achieve a powder paint II with excellent powder flow
behaviour.
Example VI
Preparation of powder paint III
[0117] 99.5 parts by weight of the powder paint composition of Example I were mixed with
0.5 parts of weight of titaniumdioxide (Aerosil P25™) in a Henschel batch mixer at
maximum speed for 3 minutes to achieve powder paint III with excellent powder flow
behaviour.
Example VII
Preparation of developer II
[0118] 11 parts by weight of the powder paint composition II according to Example V and
89 parts by weight of the carrier according to Example II were mixed in a "Skandex"
paint shaker for 5 minutes to obtain a non-dusting developer II.
Example VIII
Preparation of developer III
[0119] 15 parts by weight of the powder paint composition III according to Example VI and
85% by weight of the carrier according to example II were mixed in a "Skandex" paint
shaker for 5 minutes to obtain a non-dusting developer III.
Example IX
Coating a substrate
[0120] Analogous to Example IV the developer II according to Example VII and the developer
III according Example VIII were used to produce a chemical resistant, hard and scratch
resistant coating on 0.3 mm thick cardboard preprinted with red offset ink (see Table
II).
TABLE II
|
development voltage |
average coating layer thickness |
coating properties |
developer II |
-1000V |
7.6 µm |
medium gloss (55-60%), closed film, oil and tensid tight |
developer III |
-1000V |
9.7 µm |
high gloss (80-85%), closed film, oil and tensid tight |
Example X
Preparation of a powderpaint IV
[0121] The cooled extrudate from Example I was milled first in a hammer mill to a particle
size <3 mm and then fed into a fluidized bed mill (Condux CFS8), having a nozzle diameter
of 4 mm. The material was milled with 6 bar overpressure at 3800 r.p.m. of the classifier
wheel incorporated in the mill and after that fed into a separate classifier (Condux
CGS16) at 8000 r.p.m. of the classifier wheel to reduce the fraction of fine particles
below 5 µm in the powder obtaining a powder with a median particle size of 13.7 µm
and a ratio X
75.3/X
25.3 ratio of 2.3. 99.5 parts by weight of this powder were mixed with 0.5 parts of weight
of titaniumdioxide (Aerosil P25™) in a Henschel batch mixer at maximum speed for 3
minutes to achieve powder paint IV with excellent powder flow behaviour.
Example XI
Preparation of developer IV
[0122] 10 parts by weight of the powder paint according to Example X and 90% by weight of
the carrier according to example II were mixed in a "Skandex" paint shaker for 5 minutes
to obtain a non-dusting developer IV.
Example XII
Coating a substrate
[0123] Analogous to example IX the developer IV according to Example XI was used to produce
a 7.5 µm thick coating on a 90 g/m
2 paper preprinted in a commercially available dry toner based 4 colour copier/printer.
The coating was proven to be hard and chemical, scratch and fingerprint resistant.
The coated sheets were medium glossy (60-70%).
The protection of prints against falsification were improved as any damage of the
surface can easily be detected by the change of gloss of the surface.