Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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22
Ink Jet Printing
Naomi
Luft Cameron
Datek
Infonncctiorl
Services, Newton\~ille,
Massachusetts
1
.O
INTRODUCTION
Ink jet printing refers to any system in which droplets of ink are ejected onto
a
printing
surface to form characters, codes, or other graphic patterns. The ink jet concept dates from
the 1860s, when Lord Kelvin developed the first practical jet for pattern generation. Early
commercialization was in the oscillographic recorder area in the 1950s. Since the 1960s,
ink jet developments have focused on computer output, with major contributions made
by such scientists
as
Hellmuth Hertz in Europe (Lund Institute, Sweden) and Richard
Sweet (Stanford University) and Steven Zoltan (Brush Instruments) in the United States.
Current commercial products range from printers for direct coding of packages, to
high speed-low resolution direct mail printers (from Diconix), to graphic
arts
quality color
plotters (from Iris Graphics), to graphic arts quality color plotters (from Hewlett Packard).
To
address this range
of
applications, several variants
of
the technology have been devel-
oped. Each approach involves tradeoffs among cost, speed, reliability, and print quality,
determined by interactions between hardware and supplies.
Ink jet printing functions include:
creation of an ink stream or droplets under pressure
ejection of ink from a nozzle orifice
control of drop size and uniformity
control
of
which drops reach the paper
placement of drops on the recording surface
Control of these processes depends on several design variables, such as nozzle size,
firing rates, drop deflection methods, and ink viscosity. Changing any variable typically
requires adjustments to other system variables, making
R&D
advances slow and expensive.
Ink jet printers fall into two basic categories: continuous jet (synchronous) and
impulse jet (drop-on-demand). Most early development took place in the continuous jet
203
204
CAMERON
arena, but recent emphasis has shifted to the less complex (and therefore less costly) drop-
on-demand approaches.
2.0
CONTINUOUS
JET
PRINTING
Continuous ink jet systems operate by forcing pressurized ink in a cylinder through nozzles
in a continuous stream. Nozzle diameters range from
3
to
0.5
mil: the smallest nozzles
can require up to
600
psi of pressure to eject the ink. The ink stream is unstable, breaking
into individual droplets either naturally or through some applied stimulation such as ultra-
sonic vibration. Electrostatic deflection is used
to
control the droplets, which either reach
the page in the desired pattern or are deflected into a “gutter” or “catcher” (see Fig.
1).
Deflection can either be binary or variable.
Continuous jet printers either recycle or discard the unused ink, which can account
for up to
98%
of
the generated droplets. Although preferable
in
terms of supply costs,
recycling adds substantially to system complexity because of the need for pumps and ink
purity safeguards (such as filters and solvent balance controls).
Continuous ink jet systems are capable
of
very high speed printing, with drop fre-
quency rates sometimes in excess of
100
kHz. At these rates, it is difficult to control
individual droplets: their tendency to combine into larger drops leads to print quality
concerns. Ideally, however (i.e., when controlled), this tendency can yield enhanced image
quality: because each printed pixel may be composed of multiple droplets, it is possible
to
control the pixel size, resulting in halftoning capabilities. High resolution halftoning
cuts the speed
of
continuous ink jet systems considerably, however.
Head
Assembly
Charge Electrode
High
Voltage
Deflection
Plater
Paper
Figure
1
Continuous ink jet print system.
(From:
R.
G.
Sweet et
al.,
U.S.
Patent 3,373,437.)
INK JET PRINTING
205
-
.
(ELECTRIC TERMINALS
FOR
TRANSDUCER)
PIEZOELECTRIC TRANSDUCE
iR
Figure
2
Piezoelectric impulse jet nozzle.
(From:
S.
Zoltan,
U.S.
Patent 3,683,212.)
3.0
IMPULSE JET (DROP-ON-DEMAND) PRINTING
In contrast to continuous jet printers, drop-on-demand ink delivery systems create drops
only
as
needed, thereby eliminating the need to control excess droplets. These systems
are inherently binary; a drop either is ejected for placement on the receiver sheet or it's
not.
Most
of
the system complexity of an impulse jet printer is in the printhead, since
no recycling mechanism is required. Typically an impulse jet printhead has a pressurized
reservoir
of
ink held directly behind a nozzle or orifice. When activated by electrical
pulses, drops
of
ink are ejected and directed to page.
There are two basic methods
of
activating the ink droplets. The earliest impulse jet
models used
piezoelectric
transducers, which squeeze the ink chamber or impulse the
chamber at one end. Figure
2
shows a schematic of the piezoelectric approach. The alterna-
tive approach,
thermal activation
of ink drops, is gaining popularity as a result
of
develop-
ments by Hewlett Packard and Canon.
A
heater creates a bubble of ink vapor, which
forces ink drops from the nozzle (Fig.
3).
Most newer ink jet printers use impulse jet technologies to address general-purpose
printing applications. Advantages include mechanical simplicity, low hardware cost, and
Rapid Heating
Ink Vaporization
Bubble
Expansion and
Drop Ejection
Bubble
Contraction and
Drop Formation
Bubble
Collapse
206
CAMERON
simplified logic. However, there are disadvantages as well: drop-on-demand printers are
more sensitive to shock and vibration and have slower dot ejection rates (sometimes as
low as
3
kHz). In addition, market acceptance has been slow, partly because
of
early
reliability problems due to nozzle clogging from dried ink or paper dust.
4.0
INK
JET
INKS
Ink jet printer design requires close matching of mechanical components. imaging inks,
and. in many cases, the receiver materials. Ink chemistry is a critical link, determining
diverse attributes such as viscosity, drop flight, corrosive properties, surface tension, drying
time, dot shape, optical density and edge acuity, fade resistance, and compatibility with
printing surfaces.
Traditionally inks have been liquid, with a water
or
solvent base. Continuous jet
inks are usually based on one of a wide range of solvents, which permit fast drying
on
both porous and nonporous substrates. Impulse jet printers require high boiling point inks,
usually water-based. Several recently commercialized or wax materials that are melted
for ejection but solidify immediately on the receiver. Proponents claim that solid inks
solve print quality problems from undesirable wicking of ink into paper fibers, although
at the cost of considerable system complexity and embossed output, which is objectionable
to some.
Ink formulation involves a series of tradeoffs, depending on type of printing system
and the requirements of target applications. For instance, in impulse jet systems, where
danger of nozzle clogging exists, inks must not dry within the nozzle; yet once ejected,
they must dry fast enough on the paper to minimize feathering in the paper fibers. The
difficulty of this undertaking is evident from the fact that few current
ink
jet printers
offer true plain paper printing; most systems require special clay-coated papers to prevent
wicking and this is a drawback in office environments.
Coloring agents in ink jet inks
also
present
a
challenge. Because pigment particles
could make the ink too viscous and might cause clogging or undue wear, dyes are used
instead. Resulting problems include lack of optical density (a grayish hue) and archival
problems due to
a
tendency to fade.
The success of ink jet printing depends on developments that match systems' capabil-
ities to printing requirements. Ink jet products have been highly successful for years in
environments where speed and surface independence have been more important than image
quality (e.g., for package coding and direct mail). Recent improvements
in
reliability,
resolution. paper tolerance, and coloring agents could ultimately bring high quality office
printing and color applications within reach.
Ink jet has been among the most challenging output technologies to perfect, with
enormous
R&D
outlays and a long history of failed products. However. recent progress
suggests that the allure of ink jet
as
a potentially elegant, low cost printing solution will
finally be justified.
BIBLIOGRAPHY
"Ath~crr~ce.s
in
rml-btlpttct
prirltir~g
tr~~hrlologiesfor-
cornputer.
mtl
ofice
ctpplicntinrls,"
in Proceed-
ings from the First International Congress on Non-impact Printing, Venice, June
1981,
Joseph
Gaynor, Ed. New York: Van Nostrand Reinhold,
1982.
Ink Jet Printing-Its Role in
Word
Processing and Future Printer Markets. Newtonville, MA: Datek
Information Services. August 1988.
Solid Ink Jet Color Imaging Technology. Newtonville, MA: Datek Information Services, August
1988.
23
Electrodeposition
of
Polymers
1
.O
INTRODUCTION
The electrodeposition of polymers is an extension of painting techniques into the field of
plating and, like plating, is
a
dip coating process. The
art
of metal plating utilizes the fact
that metal ions, usually Ni’+ or C$+, can be discharged on the cathode to give well-
adhering deposits
of
metallic nickel, copper, etc. The chemical process of deposition can
be described
as
1/2 Me7+
+
I
F
(or 96,500 coulombs) of electrons gives 1/2 Me”. In the
case of electrodeposition
of
ionizable polymers, the deposition reaction is described
as
R3NH+OH-
+
1
F
-
R3N
+
H20
or the conversion of water-dispersed, ammonium-
type ions into ammonia-type, water-insoluble polymers known
as
cathodic deposition.
Alternatively, a large number of installations utilize the anodic deposition process RCOO-
+
H+
less
1
F
-
RCOOH. It should be mentioned that
“R”
symbolizes any of the widely
used polymers (acrylics, epoxies, alkyds, etc.).
The electrodeposition process is defined
as
the utilization of “synthetic, water dis-
persed, electrodepositable macro-ions.”
2.0
ADVANTAGES
Metal ions, typically 1/2 Ni’+ show an electrical equivalent weight 1/2 Ni‘+ equal to
approximately 29.5 g, while the polymeric ions typically used for electrodeposition exhibit
a
gram equivalent weight
(GEW)
of approximately 1600. Thus,
1
F
plates out of 30 g of
nickel and deposits 1600
g
of macroions. If we consider the thickness
of
the deposit, the
advantage is even larger, since nickel has
a
specific gravity of 9, whereas pigmented
organic coatings have
a
specific gravity of approximately
1.4.
Another advantage, and probably the reason for the rapid and still continuing indus-
trial growth
of
the polymeric electrodeposition process, is the formation of uniformly thick
coats on all surfaces of a formed workpiece, including such extreme recesses
as
the insides
of car doors. This ability to extend coats into recesses is known as throwing power.
207
208
BREWER
Still another advantage is the extremely small emission
of
vapors of volatile organic
compounds (VOC), ranking electrodeposition with powder coating and radiation cure as
the least polluting coating processes. Finally. the cost
of
operating an electrocoating tank
is lower than that of any other painting method.
3.0
HISTORY
The earliest observation of migration in an electrical field was made in Moscow
in
1808,
by Reuss, who observed colloidal clay in water moving toward the anode. In 186
1
Quinke
in Berlin referred to migration observations made by Faraday and described the anodic
migration of
30
substances in water and cathodic migration in turpentine. The first observa-
tion of an electrodeposition was made in London
in
1905, by Picton and Linder, who
found that ferric hydroxide in the presence of alcohol forms “hornlike” deposits on the
cathode.
An electrophoretic separation
of
toxins and antitoxins was observed by Field and
Teague, also
in
London, in 1907. The use of electrophoresis
as
an analytical tool culminated
in 1948 when Tyselius received
a
Nobel prize.
The earliest industrial application for electrophoresis seems to have been made in
1919 when Davis, at General Electric, in Schenectady, New York, received
a
patent for
the electrodeposition of bitumenous matter on wires. In 1923 Klein of London received
a patent for the electrodeposition of rubber. A number of patents were granted after 1936
to Clayton, at Crosse
&
Blackwell, London for the deposition of waxes from emulsions.
All of these processes utilized naturally occurring substances and none
of
them met with
lasting success.
In 1965 Bogart, Burnside, and Brewer reported on the Ford electrocoating system
which, by that time, had produced several million automotive wheels, hundreds
of
complete
car bodies, and large numbers of samples of automotive, appliances, and general sample
pieces.
‘
It is currently estimated that
1500
electrodeposition coating lines are
in
operation
in the United States and that another
1500
lines operate overseas.
4.0
PROCESS
Spray painting and dip coating, use
a
carefully prepared and thinned batch
of
paint
in
a
pot, agitated to ensure uniformity. Then the paint is transferred onto the workpiece
until
the pot is empty. Opposed
to
this, electrodeposition essentially transfers
only
the paint
solids onto the workpiece. Thus, after the painting is done, the tank is still full
of
the
aqueous portion of the paint, except for the paint bath increments that have been lifted
out.
These volumes of bath cannot be rinsed back into the tank by the use
of
water, because
this would cause the tank to overflow. A fraction of the bath is, therefore, passed through
an
ultrafilter, which retains the paint solids, while the aqueous phase passes through filtra-
tion
or
permeation. The ultrafiltrate is then used
to
rinse the freshly coated workpieces
and returns the lifted paint to the tank.
For
the same reason, paint of comparatively high
concentration is used to replace the coated-out solids. The chemistry
of
the paint dispersion
and deposition can be symbolized by the following equations:
ELECTRODEPOSITION
OF
POLYMERS
209
RCOOH
+
BOH
-
RCOO-+ B+
+
H20
solubilization
(1)
deposition
anodic solubilizer dispersed base
paint in- external anions
soluble
solubilization
R3N
+
HX
7
R3NH+
+
X-HzO
(2)
deposition
cathodic typically
paint lactic
acid
dispersed acids
cations
These two processes use bases or acids as external solubilizers. and the equations
show that the bulk of the solubilizer stays
in
the bath. Thus, insufficiently solubilized
paint has to be used as feed material, while the bath provides its excess solubilizer to
balance the requirement.
A
promising invention uses self-solubilizing cathodic materials called sulfonium
bases.
@
R>s++
OH-
deposition
@H+
R>
R
so
R'
macro
cation
paint sulfoxide
4.1
Throwing Power
Throwing power is measured as the depth of deposited paint
in
a standard cavity formed
by test panels and narrow spacers. Indeed, most important feature
of
the electrocoating
process is the ability
to
extend durable paint films into extreme recesses, for instance,
automobile doors.
While throwing power is
a
property of the paint, it can be increased through higher
applied voltage and higher bath conductivity (which helps transport electricity into re-
cesses) lower coulomb per gram paint requirement (more efficient use of the electricity)
larger openings for entrance into
a
cavity, and
a
larger perimeter of the opening.
In
other
words, slits are more effective than round holes.
4.2
Maintaining a Steady State
Unlike other paint processes, the operation of an electrocoating tank, is similar
to
elec-
troplating in that
a
balance of incoming and outgoing materials is necessary. Paint solids
and evaporated water
as
well as other incidentally lost materials must be replaced. More
precisely, the original tank
fill
is formulated
to
give one or more test pieces, meeting
all
the required solidities. For instance,
a
certain combination
of
a certain combination of
resin, pigments, solvents, cross-linkers. and other materials gives the required deposited
coating. Yet the coating may contain these components in somewhat different proportions.
Typically, a tank fill of
70
wt
%
resin plus
30
wt
8
pigment may produce
a
deposited
coating of
68
wt8 resin plus
32
wt% pigment. If
so,
the paint replenishment (feed) must
be richer
in
pigment than the original composition
(fill). In
general terms, the feed must
21 0
BREWER
replace all the materials leaving the tank. This is merely an analytical problem, but it can
be cumbersome until solved.
4.3
Rupture Voltage
Of the factors that increase throwing power, increased voltage is the easiest to apply.
There is, however, a limit called rupture voltage. It seems to be a dielectric breakthrough
of the forming film and it depends on the surface of the metal being coated. At any rate,
when voltages higher than the rupture voltage are applied, blemished (pockmarked) films
and lower solidities (which reduce saltspray resistance, etc.) result.
5.0
EQUIPMENT
Figure
I
represents
a
typical electrocoating installation.
5.1
Conveyors
High volume production uses overhead conveyors entering the tank at a
30"
angle. To
save energy and space, free and power conveyors providing
90"
vertical entry sometimes
are combined with bulk-coating.
5.2
Metal Preparation
Most electrocoating installations use seven-to nine-stage zinc phosphate. Iron phosphate
is also widely used. For nonferrous metals, special pretreatments are applied.
5.3
Tank Enclosures
Safety is provided by a tank enclosure, which is interlocked with the power source and
conveyor. Any entry de-energizes the system.
CONVEYOR
-
Figure
1
Schematic diagram
of
electrodeposition
process;
D.I.W.
=
deionized water.
ELECTRODEPOSITION OF POLYMERS
21
1
5.4
Dip Tanks
Tanks are operated on
a
continuous basis (conveyor or coil stock) or batch-type entry,
in
the latter Case using “power and free” conveyors. The workplace is usually completely
submerged
15
cm below the surface with
15
cm clearance on
all
sides. Tanks provide
a
workplace residence of approximately
3
minutes, though coating of cans and coils is done
in seconds.
5.5
Rectifiers
Direct current power sources of less than
5%
ripple factor over the
full
range are employed.
5.6 Counterelectrodes
Most electrocoating tanks are lined with an epoxy coat approximately
IO
mils thick. In
this case, the tank, the conveyor, the merchandise, and
so
on are all grounded: only the
counterelectrodes, inserted below the surface of the paint, are of opposite (hot) polarity.
The inserted electrodes are protected by plastic grates. If it is required to collect the
counterions (acid or base groups), then the counterelectrodes are separated from the tank
fill by membranes through which counterions pass by
a
process called electrodialysis. The
tank itself can be used
as
counterelectrode unless separation of counterelectrode fluid is
desired.
5.7
Agitation
Electrocoating baths
of
up to
500,000
liter volume contain from 10-20 wt
%
of solids
and approximate the viscosity of water. Ejector nozzles and other agitating devices are
used to move the entire bath volume in
6-30
minutes
to
prevent the solids from settling.
The dwell time in the bath is described
as
“turnover rate.” For instance, the
fill
of
an average tank of 100,000 liter volume contains about
10,000
kg of paint solids. One
turnover is reached when
10,000
kg of solids (feed) has been added, which takes from
10
working days up to
IO
months depending on the configuration of the merchandise and
the production rate. At this time, by the law of probability, much of the original fill material
still dwells in the tank. Considerable pumping stability is, therefore, required.
5.8 Temperature Control
Most tanks operate between
75”
and 95°F. Practically
all
the required electrical and agita-
tional energy is converted into heat and must be removed by use
of
chillers.
5.9
Ultrafilter
Paint savings and removal
of
dissolved impurities are accomplished by ultrafiltration,
using membranes permeable to water, salts, and substances of less than
300
molecular
weight. The ultrafiltrate (permeate) is used for usually three spray rinses and sometimes
one dip rinse, followed by
a
deionized water rinse.
5.10
Paint Filters
Filter bags, wound filters, and indexing filters
5-50
km
pore size are used.
5.11 Paint Makeup
Symbolized in Equations
1
and 2, paint makeup is actually the last step in the manufacture
of water-borne paints and is accomplished by mixing tank contents with
all
the needed
paint components.