Molecular sieve abstract
A method and apparatus for developing an electrostatic latent image
on an electrographic element with a liquid developer by contacting
the liquid developer with a molecular sieve zeolite.
Molecular sieve claims
I claim:
1. In a method of developing an electrostatic charge pattern on
the surface of an electrographic element with a liquid developer,
the improvement which comprises contacting the liquid developer
with a molecular sieve zeolite.
2. The method of claim 1 wherein the liquid developer is passed
through a zone containing molecular sieve zeolite particles.
3. The method of claim 2 wherein the molecular sieve zeolite particles
are restrained within the zone.
4. The method of claim 1 wherein the liquid developer is continuously
passed through the zone containing molecular sieve zeolite particles.
5. The method of claim 1 wherein the liquid developer is intermittently
passed through the zone containing molecular sieve zeolite particles.
6. The method of claim 1 wherein the molecular sieve zeolite is
selected from the group consisting of Type A and Type X molecular
sieve zeolites.
7. The method of claim 6 wherein the molecular sieve zeolite is
a Type A molecular sieve zeolite.
8. The method of claim 6 wherein the molecular sieve zeolite is
a Type X molecular sieve zeolite.
9. The method of claim 1 wherein the liquid developer comprises
an insulating carrier liquid.
10. The method of claim 1 wherein the liquid developer includes
an insulating carrier liquid, a binder, a colorant and a charge
control agent.
11. The method of claim 1 wherein the liquid developer available
for use in developing the electrostatic charge pattern is recirculated
through a zone of molecular sieve zeolite particles.
12. The method of claim 1 wherein the molecular sieve zeolite particles
are bonded agglomerates of 1000 to 5000.mu..
13. An apparatus for developing an electrostatic latent image on
an electrographic element which comprises a sump for housing a quantity
of liquid developer, an applicator head for applying the liquid
developer to the surface of the electrographic element, a means
for delivering the liquid developer from the sump to the applicator
head and a means for contacting the liquid developer with a molecular
sieve zeolite.
14. The apparatus of claim 13 wherein the means for passing the
liquid developer over a molecular sieve zeolite includes a pervious
bag containing molecular sieve zeolite particles, said bag disposed
within the sump.
15. The apparatus of claim 13 wherein the means for passing the
liquid developer over a molecular sieve zeolite includes a liquid
developer recirculation means at least a portion of which contains
molecular sieve zeolite particles.
16. The apparatus of claim 13 wherein the means for contacting
the liquid developer with a molecular sieve zeolite is located above
the lowest level of the liquid development system.
17. The apparatus of claim 13 wherein the means for contacting
the liquid developer with a molecular sieve zeolite is located substantially
near the top of the sump.
Molecular sieve description
FIELD OF THE INVENTION
This invention relates to electrography and more particularly to
a method of development utilizing a liquid developer.
BACKGROUND OF THE INVENTION
Electrographic imaging and development processes have been extensively
described in both the patent and other literature. Generally, these
processes include the common steps of forming an electrostatic charge
image, often called an electrostatic latent image, on an insulating
surface, such as a photoconductive insulating layer coated on a
conductive support. The electrostatic latent image is then rendered
visible by a development step in which the charge image-bearing
surface is brought into contact with a suitable elctrostatic image
developer composition which deposits toner particles on either the
charge or uncharged image areas. After development, the visible
image is either fixed directly to the electrostatic charge-bearing
surface or it is transferred to another surface, such as paper,
where it is there fixed.
Liquid developer compositions of the type described, for example,
in Metcalfe et al, U.S. Pat. No. 2907674 issued Oct. 6 1959
have been used to develop latent electrostatic images. Such developers
usually comprise a stable dispersion of charged particles, known
as toner particles, comprising a pigment, such as carbon black,
generally associated with the resinous binder, such as, an alkyd
resin, dispersed in an electrically insulating liquid which serves
as a carrier. A charge control agent is sometimes included to stabilize
the magnitude and polarity of the charge on the toner particles.
In some cases, the binder itself serves as a charge control agent.
U.S. Pat. No. 3939087 and No. 4019911 issued to Vijayendran
et al recognize high humidity as a problem in obtaining good prints
in electrophotography. These patents also teach the addition of
highly hydrophobic agents, such as silane treated fumed silica to
the liquid developer to help solve this problem as the silanes are
not sensitive to humidity.
U.S. Pat. No. 4058470 issued to Moschovis et al also teaches
the addition of hydrophobic colloidal silica to a liquid developer
to increase the oleophilicity of the liquid developer.
Mayer U.S. Pat. No. 2877133 and No. 2890174 teach a liquid
developer having silica aerogel mixed therein as a suspending agent
for the opaque marking particles.
While high humidity conditions are detrimental to liquid electrographic
developers generally in that the density of the developed image
is decreased, more serious problems can be encountered as the developers
become more complex. For example, in U.S. Pat. No. 3788995 issued
to Stahly et al copolymers are described that stabilize both the
dispersion of the toner particles in the carrier liquid and the
charge. These copolymers contain hydrophilic polar groups which
can ionize in the presence of ambient moisture to form ionic species
that compete with charge toner particles for latent image sites,
thus reducing image density.
In addition, certain colorants when added to liquid developers
to impart a desired color to a finally developed image can increase
the difficulties with regard to image densities under high humidity
conditions. It is believed that these colorants contain a large
number of hydrophilic groups which, increases the hydrophilicity
of the medium in the presence of water, thus causing increased ionization
of the hydrophilic polar groups present in the copolymers as previously
discussed.
Regardless of the mechanism by which degradation of the finally
developed image occurs, it is essential that the particular developer
employed produce uniform images over extremely broad relative humidity
conditions.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for developing
over a wide range of humidity conditions, an electrostatic charge
pattern of an electrographic element with a liquid developer which
is subjected to the action of a molecular sieve zeolite.
In accordance with the method of this invention, a body of molecular
sieve zeolite particles or pellets are positioned in such a fashion
that the liquid developer is subjected to the surface of the molecular
sieve zeolite particles during the electrostatographic process.
This is accomplished by establishing a zone wherein the zeolite
is constrained, the zone being located such that the liquid developer
is passed over the surface of the molecular sieve zeolite. In electrographic
apparatus using a liquid development method, the developer is generally
in a state of agitation. This condition can be taken advantage of
in practicing the method of this invention by strategically locating
the zone containing the molecular sieve zeolite where contact of
the liquid developer and the molecular sieve particles will occur
naturally as a matter of course when the apparatus is in operation.
In this case, the action of the zeolite on the developer is on a
continuous basis. The contact can also be accomplished on a repeated
or intermittent basis by disposing the zone such that the liquid
developer passes over molecular sieve zeolite particles only at
certain times or under certain conditions.
DESCRIPTION OF THE DRAWINGS
The method and apparatus in accordance with this invention is now
made with reference to the accompanying drawings wherein like reference
numerals designate like parts and wherein:
FIG. 1 is a schematic representation showing one embodiment in
accordance with this invention; and
FIG. 2 is a schematic representation of a second embodiment in
accordance with this invention.
DETAILED DESCRIPTION OF THE INVENTION
With reference to FIG. 1 sheet or web 10 is an imaging element
such as, for example, a photoconductive member which moves by two
pairs of spaced rollers 11 12 and 13 14 into closely spaced relationship
relative to an applicator head generally shown as numeral 15. This
head 15 can be a multiple slit applicator head as described and
claimed in U.S. Pat. No. 3407786 issued to Beyer and York Oct.
29 1968. The imaging member 10 faces the applicator 15 and can
be maintained in closely spaced relationship thereto by means of
guide plate 16. The imaging member 10 is moved relative to applicator
head 15 by means of the rollers shown or any other suitable method,
in the direction shown by the arrow.
The applicator head 15 is positioned within the sump 17 which contains
a liquid developer composition. Applicator head 15 is supplied with
liquid developer by means of pump 20 through conduit 19. Positioned
within the sump 17 and extending below the surface of the liquid
developer is a container 18 in which a suitable molecular sieve
zeolite 24 is disposed. Container 18 defines a zone through which
liquid developer continuously passes due to the motion thereof caused
by the action of pumps 20 and 23. The container 18 can be of any
suitable material that will restrain the molecular sieve particles
while permitting passage of the liquid developer. A nylon mesh bag
having openings smaller than the particle size of the sieve is particularly
convenient, however, any material that permits the flow of and is
not subject to attack by the developer is satisfactory. Sump 17
is equipped with a recirculation system comprising conduit 22 which
connects the sump to a filter 21. The filter 21 is for the purpose
of filtering out large oversized particles from the liquid developer
while permitting the components of the liquid developer including
the toner particles contained therein to pass therethrough. Filter
21 is connected by means of conduit 25 to recirculation pump 23
which returns the developer to the sump 17 by conduit 26. The action
of pumps 20 and 23 maintain the liquid developer in continuous motion
and portions thereof are continuously passing through the zoner
18 of the molecular sieve zeolite.
FIG. 2 is similar to FIG. 1 except that in place of the molecular
sieve being disposed in container 18 it is incorporated into the
recirculation passage in container 27 that is, as shown, located
substantially at the highest point of the recirculation system.
As the liquid developer circulates from the sump 17 and back again
by way of pump 23 and conduit 26 it passes through container 27
and is subjected to the action of the molecular sieve zeolite contained
therein. The recirculation system may be operated continuously during
operation of the apparatus or on an intermittent basis. For example,
pump 23 may be cycled on and off for various time periods. Container
27 may be positioned at any location within the recirculation system,
such as before or after filter 21 or after pump 23. However, it
is preferred that container 27 be located above the lowest point
in the recirculation system to prevent clogging of the pores of
the molecular sieve particles with toner particles when the apparatus
is not operating. Container means 27 may also be disposed in conduit
19 between pump 20 and applicator head 15. In this regard, the liquid
developer contacts the molecular sieve zeolite particles only during
the development step of the electrographic process. In fact, the
molecular sieve zone may be positioned in any fashion within the
development system as long as the liquid developer is moved past
the molecular sieve thus being exposed to the action thereof.
Container 27 is provided with a means for restraining the molecular
sieve therein while permitting the liquid developer, including the
toner particles to pass through, such as, a fine screen having opening
therein smaller than the particle size of the sieve material but
large enough to permit passage by the developer.
Any suitable molecular sieve zeolite such as, for example, Type
A, Type L, Type X, Type Y and mixtures of these zeolites may be
used in this invention. The molecular sieve materials are crystalline,
hydrated metal allumino-silicates which are either made synthetically
or naturally occurring minerals. Such materials are described in
U.S. Pat. Nos. 2882243 2882244 3078636 3140235 and 4094652
all of which are incorporated herein by reference. In the practice
of this invention the two types, A and X are preferred. Molecular
sieve, zeolites contain in each crystal interconnecting cavities
of uniform size, separated by narrower openings, or pores, of equal
uniformity. When formed, this crystalline network is full of water,
but with moderate heating, the moisture can be driven from the cavities
without changing the crystalline structure. This leaves the cavities
with their combined surface area and pore volume available for absorption
of water or other materials. The process of evacuation and refilling
the cavities may be repeated indefinitely, under favorable conditions.
With molecular sieves close process control is possible because
the pores of the crystalline network are uniform rather than of
varied dimensions, as is the case with other adsorbents. With this
large surface area and pore volume, molecular sieves can make separations
of molecules, utilizing pore uniformity, to differentiate on the
basis of molecular size and configuration.
Molecular sieves are crystalline, metal aluminosilicates with three
dimensional network structures of silica and alumina tetrahedra.
This very uniform crystalline structure imparts to the molecular
sieves properties which make them excellent desiccants, with a high
capacity even at elevated temperatures. Some molecular sieves, in
addition to this high adsorptive capacity, have the ability to indicate
relative humidity by a change in color, which can be utilized to
determine the point where reactivation is required. This feature
is useful in the practice of this invention as the molecular sieve
can be replaced when color change first becomes apparent. It may
even be desirable to make the container 27 of a transparent material
to permit viewing of the sieve.
The crystalline metal alumino-silicates have a three-dimensional
interconnecting network structure of silica and alumina tetrahedra.
The tetrahedra are formed by four oxygen atoms surrounding a silicon
or aluminum atom. Each oxygen has two negative charges and each
silicon has four positive charges. This structure permits a sharing
arrangement, building tetrahedra uniformly in four directions. The
trivalency of aluminum causes the alumina tetrahedron to be negatively
charged, requiring an additional cation to balance the system. Thus,
the final structure has sodium, potassium, calcium or other cations
in the network. These charge balancing cations are the exchangeable
ions of the zeolite structure.
In the crystalline structure, up to half of the quadrivalent silicon
atoms can be replaced by trivalent aluminum atoms. Zeolites containing
different ratios of silicon to aluminum ions are available, as well
as different crystal structures containing various cations.
In the most common commercial zeolite, Type A, the tetrahedra are
grouped to form a truncated octahedron with a silica or alumina
tetrahedron at each point. This structure is known as a sodalite
cage.
When soldalite cages are stacked in simple cubic forms, the result
is a network of cavities approximately 11.5 A in size, accessible
through openings on all six sides. These openings are surrounded
by eight oxygen ions. One or more exchangeable cations also partially
block the face area. In the sodium form, this ring of oxygen ions
provides an opening of 4.2 A in diameter into the interior of the
structure. This crystalline structure is represented chemically
by the following formula:
The water of hydration which fills the cavities during crystallization
is loosely bound and can be removed by moderate heating. The voids
formerly occupied by this water can be refilled by adsorbing a variety
of gases and liquids. The number of water molecules in the structure
(the value of X) can be as great as 27.
The sodium ions which are associated with the aluminum tetrahedra,
tend to block the openings, or conversely may assist the passage
of slightly oversized molecules by their electrical charge. As a
result, this sodium form of the molecular sieve, which is commercially
called 4 A, can be regarded as having uniform openings of approximately
4 A diameter.
Because of their base exchange properties, zeolites can be readily
produced with other metals substituting for a portion of the sodium.
Among the synthetic zeolites, two modifications have been found
particularly useful in industry. By replacing a large fraction of
the sodium with potassium ions, the 3 A molecular sieve is formed
(with openings of about 3 A). Similarly, when calcium ions are used
for exchange, the 5 A (with approximately 5 A openings) is formed.
The crystal structure of the Type X zeolite is built up by arranging
the basic sodalite cages in a tetrahedral stacking (diamond structure)
with bridging across the six-membered oxygen atom ring. These rings
provide opening 9-10 A in diameter into the interior of the structure.
The overall electrical charge is balanced by positively charged
cation(s), as in the Type A structure. The chemical formula that
represents the unit cell of Type X molecular sieve in the soda form
is shown below:
As in the case of the Type A crystals, water of hydration can be
removed by moderate heating and the voids thus created can be refilled
with other liquids or gases. The value of X can be as great as 276.
A prime requisite for any adsorbent is the possession of a large
surface area per unit volume. In addition, the surface must be chemically
inert and available to the required adsorbate(s). From a purely
theoretical point of view, the rate at which molecules may be adsorbed,
other factors being equal, will depend on the rate at which they
contact the surface of adsorbent particles and the speed with which
they diffuse into partices after contact. One or the other of these
factors may be controlling in any given situation. One way to speed
the mass transfer, in either case, is to reduce the size of the
adsorbent particles.
While the synthetic crystals of zeolites are relatively small,
e.g., 0.1 .mu.m to 10 .mu.m, these smaller particles may be bonded
or agglomerated into larger shapes. Typical commercial spherical
particles have an average bonded particle size of 1000 .mu.m to
5000 .mu.m (4 to 12 mesh). Other molecular sieve shapes, such as
pellets (1-3 mm diameter), Rashig rings, saddles, etc., are useful.
Any of the particular shapes set forth above are useful in accordance
with this invention, however, those bonded shapes such as pellets,
Rashig rings, saddles and the like are preferred as they are less
inclined to becoming clogged with the toner particles present in
the liquid developers. Also, they are more readily constrained within
the zone, thus preventing the contamination of the liquid developer.
Type 4 A molecular sieve zeolite bond agglomerates prepared in the
shape of pellets having a particles size of 1000 to 5000 .mu.m are
most preferred in conducting the process in accordance with this
invention.
As indicated above, the method in accordance with this invention
is useful regardless of the particular liquid developer employed.
In their most elementary form, liquid developers are made up of
an insulating carrier liquid and a binder.
Suitable liquid carrier vehicles may be selected from a wide variety
of materials which are electrically insulating and have a fairly
low dielectric constant. The dielectric constant should be less
than about 3 while the volume resistivity should be greater than
about 10.sup.10 ohm/cm. Suitable carrier liquids include halogenated
hydrocarbon solvents, for example, fluorinated lower alkanes, such
as trichloromonofluoromethane, trichlorotrifluoroethane, etc., having
a boiling range typically from about 2.degree. to about 55.degree.
C. Other hydrocarbon solvents are useful, such as isoparaffinic
hydrocarbons having a boiling range of from about 145.degree. to
about 185.degree. C., such as Isopar G (a trademark of the Exxon
Corporation) or cyclohydrocarbons, such as cyclohexane and cyclopentane.
Additional carrier liquids may also be useful in liquid developer
compositions including high polysiloxanes, odorless mineral spirits,
n-hexane, octane and the like.
Any suitable binder material known in the liquid developer art
may be employed in the process in accordance with this invention,
such as, a wide range of materials including resins of the following
type: polyethylene, phenol-formaldehyde, rosin modified phenol-formaldehyde,
maleic rosin polyesters, soya-modified alkyd resins, modified pentaerythritol
ester of rosin, maleic alkyl modified rosin esters, modified phenolic
resins, soya oil and linseed oil modified alkyds, methylphenol-formaldehyde,
xylenol-formaldehyde and any other resinous material set forth in
U.S. Pat. No. 3779924 issued to Chechak. In addition, the binder
materials may include any of those prepared in accordance with U.S.
Pat. No. 4052325 to Santilli granted Oct. 4 1977 which discloses
certain polyester type resins for use as binders and also U.S. Pat.
No. 4202785 to Merrill et al directed to a class of polyester
ionomers as binders, all the above-mentioned patents being incorporated
herein by reference as well as the compositions set forth therein.
A particularly useful binder in the preparation of the liquid developers
are those described and claimed in U.S. Pat. No. 3788995 issued
to Stahly et al. This patent discloses random copolymers prepared
from monomers having a predetermined relationship with the carrier
liquid to be utilized in the liquid developer. At least one of the
monomeric moieties of the copolymer is a polar moiety and at least
one other of the monomeric moieties of the copolymer is one that
is soluble in the carrier liquid of the developer. The copolymers
employed are made up of at least two monomer units and may contain
as many as 4 or even more different monomeric units. Examples of
suitable copolymers include poly(styrene-co-lauryl methacrylate-co-sodium
acrylate), poly(styrene-co-lauryl methacrylate-co-2-sulfoethyl methacrylate),
poly(styrene-co-lauryl methacrylate-co-3-sulfopropyl methacrylate,
sodium salt), poly(butyl methacrylate-co-lauryl methacrylate-co-2-sulfoethyl
methacrylate), poly(ethyl methacrylate-co-lauryl methacrylate-co-2-sulfoethyl
methacrylate), poly(t-butylstyrene-co-styrene-co-lithium sulfoethyl
methacrylate), poly(t-butylstyrene-co-lithium methacrylate), poly(vinyltoluene-co-lauryl
methacrylate-co-lithium methacrylate-co-methacrylic acid) and the
like. These polymers are in many cases useful as binders as well
as dispersing agents and charge-control agents to maintain the particles
in suspension and to maintain constant charge on the particles,
respectively. They may be used solely as the binder material, however,
are generally utilized together with other polymers to impart desired
characteristics.
Polyesters, such as those disclosed in U.S. Pat. Nos. 4052325
and 4507377 all of which are incorporated herein by reference
are useful as binders in accordance with this invention.
As indicated previously, in their most elementary form, liquid
developers comprise a liquid carrier vehicle together with a binder.
However, as a practical matter, various additional materials are
generally incorporated in present day commercial liquid developers.
While colorless images may be utilized for the preparation of a
hydrophobic image for use in a lithographic printing process, generally
a colorant is added to the binder to form toner particles.
When visible images are desirable, useful results are obtained
from virtually any of the wide variety of known dyes or pigment
materials. Particularly good results are obtained by using various
kinds of carbon black pigments. A representative list of colorants
may be found, for example, in Research Disclosure, Vol. No. 109
May, 1973 in an article entitled "Electrophotographic Elements,
Materials and Processes".
Various charge control agents may also be incorporated in the liquid
developer. In addition to the charge control polymers described
in U.S. Pat. No. 3788995 other suitable charge control agents
may be incorporated in the liquid developer, such as, for example,
phosphonate materials described in U.S. Pat. No. 4170563 quaternary
ammonium polymers described in U.S. Pat. No. 4229513 and metal
salts as described in U.S. Pat. No. 3417019.
Waxes and dispersing agents for the wax are also included as optional
disperse components in liquid developers. Suitable waxes and dispersing
agents include those described in European Patent Application Publication
Number 0 062 482 filed Mar. 30 1982 which describes liquid electrographic
developers comprising an electrically insulating organic carrier
liquid containing dispersed constituents and dissolved constituents,
the dissolved constituents comprising an electrically insulating
organic dispersing liquid that forms a solution with the carrier
liquid. The dispersed constituents include at least one thermoplastic
resin and wax particles and a dispersing agent for the wax particles
that is insoluble in the solution of carrier liquid and dispersing
liquid, but soluble in the dispersing liquid alone. The wax particles
are polyolefin wax, carnauba wax and ester wax or an amide wax.
The dispersing liquid used with these waxes is preferably an alkylated
aromatic liquid when the carrier liquid is a isoparaffinic hydrocarbon,
such as Isopar G. Other suitable dispersing agents include copolymers
of ethylene and vinylacetate marketed under the trademark Elvax
by DuPont Company.
The invention is further illustrated by the following examples
in which parts are by weight unless otherwise specified:
EXAMPLE 1
About 50 parts by weight of poly[neopentyl-4-methylcyclohexene-12-dicarbarboxylate-co-terephthalate-c
o-5-sodiosulfophthalate] 53/43/4 25 parts of Bonadur Red, C. I.
Pigment Red 200 (Sun Chemical Company) about 12.5 parts of a polyethylene
wax (Epolene E-12.TM.) Eastman Kodak Company) and about 12.5 parts
of an ethylenevinylacetate copolymer 72/28 (Elvax 210) DuPont Company
were first melt blended pulverized then dispersed in a sand mill
in the presence of, about 50 parts of poly(t-butylstyrene-co-styrene-co-lithium
sulfoethylmethacrylate) 72/24/4 and about 350 parts of poly Isopar
G. After 6 hours milling 8 parts of (t-butylstyrene-lithium methacrylate)
and about 282 parts of Isopar G.RTM. were added to the mill and
milling continued for 1 hour after which the dispersion was diluted
with sufficient Isopar G to form a liquid developer containing 8
grams of dry toner per liter of developer.
EXAMPLE 3
It can be seen from this table that at high relative humidity the
density of the image produced seriously drops off without the presence
of the molecular sieve. The quality of the images at high relative
humidity does not suffer in the presence of molecular sieve zeolite
when compared with images produced with or without molecular sieve
zeolite at normal room conditions of relative humidity.
EXAMPLE 4
A liquid developer was prepared as in Example 1 except that 25
parts of Rangoon Yellow (Sun Chemical Company) were used in place
of 25 parts of Bonadur Red and final dilution was to 2.0 g of dry
toner per liter of Isopar G. The water content of the developer
after 24 hours at ambient RH was 48.3 parts per million and the
transmission density 1.05. When 400 parts by volume of liquid developer
were subjected to 10 parts of type 4 A molecular sieve zeolite the
water content was 24 parts per million and the density 1.13. The
same developer after 72 hours at 72% relative humidity with and
without being subjected to the molecular sieve were 147.2 parts
per million density 0.63 and 74.5 parts per million density 0.95
respectively.
It can readily be seen that the density of images at ambient and
at 72% relative humidity when the developer is subjected to a type
4 A molecular sieve are comparable while that at high humidity without
the use of the sieve is seriously degraded.
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