U.S. patent number 3,916,065 [Application Number 05/315,958] was granted by the patent office on 1975-10-28 for electrostatographic carrier particles.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Louis H. Burrows, Jr., Joseph H. Moriconi.
United States Patent |
3,916,065 |
Moriconi , et al. |
October 28, 1975 |
Electrostatographic carrier particles
Abstract
Coated electrostatographic carrier particles comprising a core
coated with a thermoset, crosslinked acrylic resin to form a thin
continuous film of the resin on the core, the acrylic resin
comprising esters of acrylic and methacrylic acids.
Inventors: |
Moriconi; Joseph H. (Rochester,
NY), Burrows, Jr.; Louis H. (Rochester, NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23226839 |
Appl.
No.: |
05/315,958 |
Filed: |
December 18, 1972 |
Current U.S.
Class: |
430/111.1;
428/403; 428/407; 428/442; 428/406; 428/460; 428/463; 428/436 |
Current CPC
Class: |
G03G
9/1137 (20130101); Y10T 428/31649 (20150401); Y10T
428/2996 (20150115); Y10T 428/2998 (20150115); Y10T
428/31699 (20150401); Y10T 428/2991 (20150115); Y10T
428/31627 (20150401); Y10T 428/31688 (20150401) |
Current International
Class: |
G03G
9/113 (20060101); G03G 009/02 (); G03G
013/08 () |
Field of
Search: |
;117/26,1B,1M,1S,1C
;252/62.1P ;260/37N,37R,37EP,71 ;427/14,403
;428/406,407,436,442,460,463 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sofocleous; Michael
Claims
What is claimed is:
1. An electrostatographic coated carrier particle having an average
particle diameter between about 50 microns and about 1,000 microns,
said carrier particle comprising a metallic core coated with a
thermoset crosslinked acrylic resin to form an adherent durable
film of said resin on said core, said acrylic resin comprising
esters of acrylic and methacrylic acids.
2. An electrostatographic coated carrier particle according to
claim 1 wherein said thermoset acrylic resin is a
hydroxyl-functional resin crosslinked with nitrogen resins.
3. An electrostatographic coated carrier particle according to
claim 1 wherein said thermoset acrylic resin is a
carboxyl-containing polymer crosslinked with epoxy resins.
4. An electrostatographic coated carrier particle according to
claim 3 wherein said carboxyl-containing polymer has been cured
with melamine-formaldehyde resins.
5. An electrostatographic coated carrier particle according to
claim 1 wherein said acrylic resin is self-curing at a temperature
of about 180.degree.F.
6. An electrostatographic coated carrier particle according to
claim 1 wherein said acrylic resin is curable with catalysts or
co-resins at a temperature of about 180.degree.F.
Description
BACKGROUND OF THE INVENTION
This invention relates, in general, to electrostatographic imaging
systems, and, in particular, to improved developer materials and
their uses.
The formation and development of images on the surface of
photoconductive materials by electrostatic means is well-known. The
basic electrophotographic process, as taught by C. F. Carlson in
U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic
charge on a photoconductive insulating layer, exposing the layer to
a light and shadow image to dissipate the charge on the areas of
the layer exposed to the light and developing the resulting
electrostatic latent image by depositing on the image a finely
divided electroscopic material referred to in the art as "toner".
The toner will normally be attracted to those areas of the layer
which retain a charge, thereby forming a toner image corresponding
to the electrostatic latent image. This powder image may then be
transferred to a support surface such as paper. The transferred
image may subsequently be permanently affixed to the support
surface as by heat. Instead of latent image formation by uniformly
charging the photoconductive layer and then exposing the layer to a
light and shadow image, one may form the latent image by directly
charging the layer in image configuration. The powder image may be
fixed to the photoconductive layer if elimination of the powder
image transfer step is desired. Other suitable fixing means such as
solvent or overcoating treatment may be substituted for the
foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to
the electrostatic latent image to be developed. One development
method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is
well-known as "cascade" development. In this method, a developer
material comprising relatively large carrier particles having
finely-divided toner particles electrostatically clinging to the
surface of the carrier particles is conveyed to and rolled or
cascaded across the electrostatic latent image bearing surface. The
composition of the toner particles is so chosen as to have a
triboelectric polarity opposite that of the carrier particles. As
the mixture cascades or rolls across the image bearing surface, the
toner particles are electrostatically deposited and secured to the
charged portion of the latent image and are not deposited on the
uncharged or background portions of the image. Most of the toner
particles accidentally dsposited in the background are removed by
the rolling carrier, due apparently, to the greater electrostatic
attraction between the toner and the carrier than between the toner
and the discharged background. The carrier particles and unused
toner particles are then recycled. The technique is extremely good
for the development of line copy images. The cascade development
process is the most widely used commercial electrostatographic
development technique. A general purpose office copying machine
incorporating this technique is described in U.S. Pat. No.
3,099,943.
Another technique for developing electrostatic latent images is the
"magnetic brush" process as disclosed, for example, in U.S. Pat.
No. 2,874,063. In this method, a developer material containing
toner and magnetic carrier particles is carried by a magnet. The
magnetic field of the magnet causes alignment of the magnetic
carriers in a brush-like configuration. This magnetic brush is
engaged with an electrostatic latent image bearing surface and the
toner particles are drawn from the brush to the electrostatic
latent image by electrostatic attraction.
Another technique for developing electrostatic latent images is the
"touchdown" process as disclosed, for example, in U.S. Pat. Nos.
2,895,847 and 3,245,823 to Mayo. In this method, a developer
material is carried to a latent image bearing surface by a support
layer such as a web or sheet and is deposited thereon in conformity
with said image.
Carrier particles are made from or coated with materials having
appropriate triboelectric properties as well as certain other
physical characteristics. Thus, the materials employed as the
carrier particles or the coatings thereon should have a
triboelectric value commensurate with the triboelectric value of
the toner to enable electrostatic adhesion of the toner to the
carrier particles and subsequent transfer of the toner from the
carrier particles to the image on the plate without excessive power
requirements. Furthermore, the triboelectric properties of all the
carrier particles should be relatively uniform to permit uniform
pick-up and subsequent deposition of toner. The materials employed
in the carrier particles should have an intermediate hardness so as
not to scratch the plate or drum surface upon which the
electrostatic image is initially placed while being sufficiently
hard to withstand the forces to which they are subjected during
recycle. The carrier particles as well as the surface thereof also
should not be comprised of materials which are so brittle as to
cause either flaking of the surface or particle break-up under the
forces exerted on the particles during recycle. The flaking causes
undesirable effects in that the relatively small flaked particles
will eventually be transferred to the copy surface thereby
interfering with the deposited toner and causing imperfections in
the copy image. Furthermore, flaking of the carrier particle
surface will cause the resultant carrier particles to have
non-uniform triboelectric properties when the carrier particle is
composed of a core material different from the surface coating
thereon. This results in undesirable nonuniform pick-up of toner by
the carrier particles and non-uniform deposition of toner on the
image. In addition, when the carrier particle size is reduced, the
removal of the resultant small particles from the plate becomes
increasingly difficult. Thus, the type of materials useful for
making carrier particles or for coating carrier particles, although
having the appropriate triboelectric properties, are limited
because other physical properties which they possess may cause the
undesirable results discussed above.
It is highly desirable to alter triboelectric properties of the
carrier cores to accommodate the use of desirable toner
compositions while retaining the other desirable physical
characteristics of the carrier particle. The alteration of the
triboelectric properties of carrier particles by applying a surface
coating thereon is a particularly desirable technique. With this
technique, not only is it possible to alter the triboelectric
properties of carrier particles made from materials having
desirable physical characteristics, it is also possible to employ
materials previously not suitable as carrier particles. Thus, for
example, carrier particles having desirable physical properties
with the exception of hardness, can be coated with a material
having desirable hardness as well as other physical properties,
rendering the resultant product useful as carrier particles.
While ordinarily capable of producing good quality images,
conventional developing materials suffer serious deficiencies in
certain areas. The developing materials must flow freely to
facilitate accurate metering and even distribution during the
development and developer recycling phases of the
electrostatographic process. Some developer materials, though
possessing desirable properties such as proper triboelectric
characteristics, are unsuitable because they tend to cake, bridge
and agglomerate during handling and storage. Adherence of carrier
particles to reusable electrostatographic imaging surfaces causes
the formation of undesirable scratches on the surfaces during image
transfer and surface cleaning steps. The tendency of carrier
particles to adhere to imaging surfaces is aggravated when the
carrier surfaces are rough and irregular. The coatings of most
carrier particles deteriorate rapidly when employed in continuous
processes which require the recycling of carrier particles by
bucket conveyors partially submerged in the developer supply such
as disclosed in U.S. Pat. No. 3,099,943. Deterioration occurs when
portions of or the entire coating separates from the carrier core.
The separation may be in the form of chips, flakes or entire layers
and is primarily caused by fragile, poorly adhering coating
materials which fail upon impact and abrasive contact with machine
parts and other carrier particles. Carriers having coatings which
tend to chip and otherwise separate from the carrier core must be
frequently replaced thereby increasing expense and loss of
productive time. Print deletion and poor print quality occur when
carrier particles having damaged coatings are not replaced. Fines
and grit formed from carrier disintegration tend to drift and form
undesirable and damaging deposits on critical machine parts. Many
carrier coatings having high compressive and tensile strength
either do not adhere well to the carrier core or do not possess the
desired triboelectric characteristics. The triboelectric and flow
characteristics of many carriers are adversely affected when
relative humidity is high. For example, the triboelectric values of
some carrier coatings fluctuate with changes in relative humidity
and are not desirable for employment in electrostatographic
systems, particularly in automatic machines which require carriers
having stable and predictable triboelectric values. Another factor
affecting the stability of carrier triboelectric properties is the
susceptibility of carrier coatings to "toner impaction". When
carrier particles are employed in automatic machines and recycled
through many cycles, the many collisions which occur between the
carrier particles and other surfaces in the machine cause the toner
particles carried on the surface of the carrier particles to be
welded or otherwise forced into the carrier coatings. The gradual
accumulation of permanently attached toner material on the surface
of the carrier particles causes a change in the triboelectric value
of the carrier particles and directly contributes to the
degradation of copy quality by eventual destruction of the toner
carrying capacity of the carrier. Thus, there is a continuing need
for a better developer material for developing electrostatic latent
images.
It is therefore an object of this invention to provide developers
which overcome the above-noted deficiencies and are suitable for
use in electrostatographic reproduction processes.
It is another object of this invention to provide carrier particles
which possess improved electrostatic and physical properties for
efficient and prolonged use in electrostatographic reproduction
processes.
It is a further object of this invention to provide carrier
particles having a hard and tough coating which tenaciously adheres
to the carrier core whereby the carrier particles are more
resistant to toner impaction, chipping and flaking.
It is another object of this invention to provide developing
materials which flow more freely.
It is yet another object of this invention to provide carrier
coatings having more stable triboelectric values.
It is a further object of this invention to provide carrier
coatings having higher tensile and compressive strength.
It is yet another object of this invention to provide carrier
coatings having greater resistance to disintegration.
It is still another object of this invention to provide more toner
impaction resistant carrier coatings.
It is another object of this invention to provide developers having
physical and chemical properties superior to those of known
developer materials.
The above objects and others are accomplished, generally speaking,
by providing a carrier for electrostatographic developer mixtures,
said carrier comprising a core coated with a thermosetting acrylic
resin. The thermosetting acrylic resin electrostatographic carrier
coatings of this invention comprise esters of acrylic and
methacrylic acids. Further, the thermosetting acrylic resin
electrostatographic carrier coatings of this invention may be
generally subdivided into two groups according to their mechanism
of crosslinking. One of these groups is the hydroxyl-functional
type of thermosetting acrylic resins which are normally crosslinked
with nitrogen resins. The other group is the carboxyl type of
thermosetting acrylic resins which are normally crosslinked with
epoxy resins. In general, the carriers of the present invention are
prepared by coating a granular carrier material consisting of a
core, base or substrate composed of any selected material which may
be of high specific gravity such as glass or steel beads, covered
with a thermosetting acrylic resin coating suitable to impart the
desired electrostatographic properties to the carrier material so
that it will properly charge an electroscopic powder when mixed
therewith, while maintaining such a relative specific gravity as to
insure against adherence of the carrier material to an
electrostatographic imaging surface.
Such a carrier material may be produced by adhering an outer
coating comprising a thermosetting acrylic resin to a core, base or
substrate carrier material by adding a thermosetting acrylic resin
that is self-curing or made curable with catalysts or co-resins to
the carrier material and curing the thermosetting acrylic resin
thereon. The coated core, base or substrate carrier material is
then mixed with an electroscopic powder and employed in developing
an electrostatic latent image.
Acrylic resins are either thermoplastic, that is, remeltable, or
thermosetting. A few acrylic resins lie in between in that they are
thermoplastic under certain conditions and thermosetting under
other conditions. The thermosetting acrylic resins of this
invention are those that solidify or set on heating or curing and
cannot be remelted. The thermosetting acrylic resins of this may be
prepared from acrylic acid, CH.sub.2 =CHCOOH, or from a derivative
of acrylic acid. The acrylic monomers that may be employed to
prepare these thermosetting acrylic resins are a suborder of the
parent group of vinyl monomers. The vinyl grouping that is common
to all these monomers is CH.sub.2 =C<. When an acid or carboxyl
group is joined to the vinyl radical together with a hydrogen atom
or methyl group, the products are acrylic and methacrylic acids.
These are the starting points for all acrylic resins.
Esterification of the acrylic acids with various alcohol
substituents gives a large group of acrylic esters with a wide
range of properties dependent upon the chain length of the alcohol
used. By halogen substitution, another large group of useful
acrylates is obtained. Further, nitrogen substitution also yields
another useful group of acrylics which may be exemplified by
acrylonitrile. The thermosetting acrylic resins of this invention
may be condensation polymers formed through the reaction of their
functional groups with the possible elimination of water and
similar by-products. The reactivity of the starting compounds of
this invention, in terms of the number of functional groups
involved in the reaction, determines the type or structure of
polymer formed. In order for a polymer to be formed, the reaction
components should each have at least two reactive functional
groups, that is, have bifunctional groups. For the preparation of
the thermosetting acrylic crosslinked resin polymers of this
invention, it is necessary that at least one of the reactants be
trifunctional, and may be tetrafunctional to provide the three or
more points of attachment to the molecule for the network structure
of the thermosetting polymer. The thermosetting property of the
acrylic resins of this invention thus is usually associated with a
crosslinking reaction which forms a three-dimensional network of
polymer molecules. It has been found that crosslinking the resins
improves their craze resistance and thermal stability when they are
employed as carrier coatings for electrostatographic developer
materials.
Coating of the carrier materials is accomplished with thermosetting
acrylic resin polymer solutions which upon curing give rise to
crosslinked thermoset acrylic resins. Curing may be defined as
changing the physical properties of a material by chemical
reaction, usually to a harder or more permanent form, and the term
is sometimes synonymous with set. During the curing cycle, complete
polymerization takes place and the completely crosslinked acrylic
polymer is formed thus making it thermosetting or non-fusible. The
intermediate thermoplastic polymer is sometimes referred to as the
"B-stage" resin, and simpler modifications of the early reaction
mixture are sometimes called "A-stage" resins. The "C-stage" is
usually the final stage in the reactions of a thermosetting acrylic
resin, that is, a fully cured or set stage. Thermosetting acrylic
resins generally cannot be reshaped once they have been fully cured
and this property has been found to be extremely advantageous for
electrostatographic developer materials.
Any suitable polymer of a self-curing or curable thermosetting
acrylic resin may be employed as the carrier coating for the coated
carriers of this invention. Typical thermosetting acrylic polymers
include acrylic and methacrylic esters such as methyl acrylate,
ethyl acrylate, butyl acrylate, tertbutyl acrylate, 2-ethylhexyl
acrylate, neopentyl acrylate, methyl chloroacrylate, isobornyl
acrylate, cyclohexyl acrylate, dodecyl acrylate, hexyldecyl
acrylate, isopropyl acrylate, tetradecyl acrylate, secbutyl
acrylate, methyl methacrylate, ethyl methacrylate, propyl
methacrylate, isopropyl methacrylate, isobutyl methacrylate, butyl
methacrylate, n-butyl methacrylate, pentyl methacrylate, hexyl
methacrylate, cyclohexyl methacrylate, decyl-octyl methacrylate,
lauryl methacrylate, stearyl methacrylate, 1,3-butylene
dimethacrylate, 2-n-tert-butylaminoethyl methacrylate, 2-butyl
methacrylate, glycidyl methacrylate, 2-chlorethyl methacrylate,
3,3-dimethylbutyl methacrylate, 2-ethylhexyl methacrylate,
2-methoxyethyl methacrylate and mixtures thereof. The polymers
derived from these acrylic and methacrylic esters vary from soft,
elastomeric, film-forming materials to hard resins. Many of these
material are commercially available and are self-curing or made
curable with catalysts or co-resins. This latter property allows
for control over the desired electrostatographic properties of the
coated carrier. For example, the curable thermosetting acrylic
resin carrier coating materials of this invention may generally be
carboxyl containing polymers which react with other resins such as
epoxies and with curing agents such as hexamethoxymethylmelamine.
These curable thermosetting acrylic resins exhibit hardness,
adhesion and thermal properties similar to the self-curing resins.
In addition, these curable thermosetting resins provide a further
advantage in enabling control of the triboelectric properties of a
developer composition giving them a versatility not usually found
with the heat reactive resins. Further, the curable thermosetting
acrylic resin carrier coating materials of this invention may be
crosslinked with a wide variety of amino or epoxy resins to provide
coating compositions possessing maximum electrostatographic
properties. Depending on the crosslinking modifier employed, these
resins may be formulated to produce carrier coatings having a good
combination of flexibility and hardness, excellent chip resistance,
one coat adhesion, exterior durability and good resistance to
humidity.
Thus, any suitable crosslinking modifier may be employed with the
curable thermosetting acrylic resin carrier coating materials of
this invention. Typically, a wide variety of urea and
melamine-formaldehyde resins are suitable crosslinking additives
for these resins. For example, a melamine resin of the methylated
methylol type usually provides the best low temperature bake
performance as well as good short cycle high temperature bake
properties. The butylated resins generally provide more economical,
mar-resistant carrier coatings but flexibility suffers slightly. In
the case of curable thermosetting acrylic resin - amino resin
carrier coating compositions for low temperature curing in the
range of about 180.degree.F. to about 220.degree.F., a small
addition of an acid catalyst is desirable for optimum crosslinking.
Similarly, a small amount of catalyst may be used to shorten the
curing time at high temperatures. Some of the most commonly used
catalysts include p-toluene sulfonic acid, phenyl and butyl
phosphoric acids, and they are generally employed in amounts of
about 0.3 to about 1.5 parts by weight per 100 parts by weight of
total resin solids. In the case of curable thermosetting acrylic
resin-melamine-formaldehyde compositions modified with epoxy
resins, the addition of about 10 to about 30 parts total solids of
epoxy resin, and about 15 to 25 parts total solids of melamine
resin per 100 parts total solids of the acrylic resin are preferred
for optimum carrier coating properties.
Any suitable vinyl monomer may be employed in the carrier coating
materials of this invention. Vinyl monomers may be defined as those
monomers containing the characteristic ethylenically unsaturated
structure: >C=C<, and capable of undergoing addition
polymerization. Typical vinyl monomers include: esters of saturated
alcohols with mono and polybasic unsaturated acids such as alkyl
acrylates and methacrylates, haloacrylates, diethyl maleate, and
mixtures thereof; vinyl and vinylidene halides such as vinyl
chloride, vinyl fluoride, vinylidene chloride, vinylidene fluoride,
tetrafluoroethylene, chlorotrifluoroethylene and mixtures thereof;
vinyl esters such as vinyl acetate, unsaturated aromatic compounds
such as styrene and various alkyl styrenes, parachlorostyrene,
parabromostyrene, 2,4, dichlorostyrene, vinyl napthalene,
paramethoxystyrene and mixtures thereof; unsaturated amides such as
acrylamide, methacrylamide and mixtures thereof; unsaturated
nitriles such as acrylonitrile, methacrylonitrile,
haloacrylonitrile, phenylacrylonitrile, vinylidene cyanide, and
mixtures thereof; N-substituted unsaturated amides such as
N,N-dimethyl acrylamide, N-methyl acrylamide and mixtures thereof;
conjugated butadienes such as butadiene, isoprene and mixtures
thereof; unsaturated ethers such as divinyl ether, diallyl ether,
vinyl alkyl ether and mixtures thereof; unsaturated ketones such as
divinyl ketone, vinyl alkyl ketone and mixtures thereof;
unsaturated aldehydes and acetals such as acrolein and its acetals,
methacrolein and its acetals, and mixtures thereof; unsaturated
heterocyclic compounds such as vinyl pyridine, vinyl furan, vinyl
coumarone, N-vinyl carbazole, and mixtures thereof; unsaturated
alicyclic compounds such as vinyl-cyclopentane, vinyl-cyclohexane
and mixtures thereof; unsaturated thio compounds such as vinyl
thioethers; unsaturated hydrocarbons such as ethylene, propylene,
coumarone, indene, terpene, polymerizable hydrocarbon fractions,
isobutylene and mixtures thereof; allyl compounds such as allyl
alcohol, allyl esters, diallyl phthalate, triallylcyanurate and the
like. Any suitable homopolymer, copolymer or terpolymer of the
above materials may be used in the carrier coating materials of
this invention. Polymers of the types above include polyvinyl
butyral, copolymers of methacrylic acid with methylmethacrylate,
with acrylonitrile or with styrene, copolymers of vinyl acetate
with maleic anhydride, copolymers of nitrostyrene with
diethylmaleate, copolymers of styrene with acrylic and methacrylic
acids and esters, and the like. Included in the class of acrylics
are the polymethacrylates, polyacrylates and copolymers of
acrylonitrile. There are many variations in this class, mainly
concerned with combinations of methacrylate esters and acrylate
esters, as well as acrylonitrile used in making these polymers.
These esters range from methyl to stearyl with just about every
possible member of the homologous series in between. Although the
methyl and ethyl esters of acrylic and methacrylic acids are most
prevalent, 2-ethylhexyl and n-butyl acrylates are typical of some
of the other esters used in combination with methacrylate esters to
achieve the properties desired in the copolymers. These acrylic
monomers are supplied in the form of clear, water-white liquids and
can be mixed together in varying proportions and polymerized to a
hard, clear, transparent solid by the addition of catalysts such as
acid, ether peroxides, or diazo compounds, and heat. Because of the
possibility that slight traces of contaminants, including air and
ultraviolet light, may initiate polymerization and cause a
spontaneous explosion, an inhibitor such as hydroquinone is usually
added to the monomers for safe storage and shipping.
The carrier coating thermosetting acrylic resin materials of this
invention are made by polymerization of acrylic and methacrylic
derivatives, chiefly from the esters of acrylic acid and
methacrylic acids, ethyl acrylate and methyl acrylate. The resins
may be polymerized by conventional bulk, suspension, sovlent, and
emulsion techniques. Depending upon the consitution of the
monomers, the method and conditions of polymerization, the resins
may range from soft, sticky semi-solids to hard, tough solids. They
may also be copolymerized with a large number of other monomers to
contribute to internal plasticization, increased polymerization
rate, greater toughness, improved compatibility with other resins,
better adhesion to substrate, and better resistance to heat, light,
and weathering. For example, dimethylaminoethyl methacrylate and
t-butylaminoethyl methacrylate provide means for introducing into
copolymers pendant amino groups which can promote adhesion to many
substrate and provide anchoring sites for dyes and pigments. The
amino groups can also serve as reactive sites for secondary
crosslinking. Hydroxyethyl methacrylate and hydroxypropyl
methacrylate permit the introduction of reactive hydroxyl groups
into copolymers and thus possibilities for subsequent crosslinking
and for increased adhesion to certain substrate, particularly for
thermosetting coatings. Allyl methacrylate may be employed as a
crosslinking agent for other resins to raise their softening point
and increase their hardness. Generally, emulsion and solution
techniques are used to yield polymers in forms convenient for
coatings. Thus, depending on the monomers selected and the method
of polymerization, acrylic polymers having a wide range of physical
properties may be obtained. However, all the acrylics have an
unusual degree of resistance to the effects of long exposures to
sunlight, heat and weathering. In this respect, they are much
superior to related materials such as styrene, vinyl chloride and
vinyl acetate.
Under various conditions of polymerization the end product may have
a low or high molecular weight. In general, the lower the molecular
weight the softer the material and the higher the molecular weight
the tougher the material. Usually, the physical nature of the
polymer depends on the monomer or combination of monomers used. In
general, the lower acrylic esters are softer materials such as
2-ethyl-hexyl acrylate and n-butyl acrylate and range to the tough
but flexible methyl acrylate. The methacrylic esters are somewhat
harder.
When the polymer formed consists of a straight chain of molecules
it is usually thermoplastic. However, the addition of varying
amounts of crosslinking agents during polymerization that will form
random bridges from chain to chain causes the polymer to become
less thermoplastic and more thermosetting. Crosslinking agents are
di-functional materials such as dimethacrylates and diallylic
compounds and the like. Partially crosslinked polymers are
generally used to improve heat resistance.
Acrylics are well-known and some of the better known commercial
acrylic resins are solution polymers in an organic solvent
available from Rohm & Haas Company, Philadelphia, Pa. under the
name of "Acryloid". Acrylics are also available from Union Carbide
Corporation under the name of "Acrylic Resin". The "Acryloid"
resins of Rohm & Haas are acrylic copolymer solid resins and
the "Acrysol" resins are solutions for coatings. Acryloid A-10,
used for coatings and finishes, is a solution of styrene-acrylic in
Cellosolve acetate, and acryloid B-7 is a solution in ethylene
dichloride. The best all-around resin properties are usually
obtained from copolymers rather than the homopolymers. Although
most of the compositions of the Acryloid resins are not given in
the literature, it may be safe to assume that many of them are
copolymers of the various acrylic and methacrylic esters.
One of the important features that makes acrylic polymers desirable
as coating materials is that generally no plasticizer is needed to
produce a soft film. Acrylics are referred to as being internally
plasticized by use, in desirable proportion, of the normally soft
monomers in their preparation. Plasticizer is efficient as a
softening agent, but tends to migrate to the surface, giving the
plastic a greasy feel, and eventually loss of plasticizer in
coatings causes embrittlement that leads to failure. Internally
plasticized acrylics will retain their softness indefinitely.
Although homopolymers are used in some cases, most acrylics are
copolymers of 2, 3 and 4 monomers. Most vinyls and their sub-order,
acrylics, are classified as thermoplastic materials that can be
repeatedly softened by heating. However, heat softening is often a
feature that is undesirable in the final application as in
electrostatographic development processes. Therefore, the
thermosetting property of acrylics is highly desirable in
electrostatography.
The polymerizable monomers employed to form the carrier coating
materials of this invention may be mixed with any suitable
free-radical initiator or catalyst capable of polymerizing the
monomers or prepolymers. By a "free-radical initiator or catalyst"
is meant a compound which is capable of producing free-radicals
under the polymerization conditions employed, such as compounds
having and --O--O-- or an --N=N-- linkage. Examples of the more
commonly employed free radical initiators or catalysts include:
alkyl peroxides, such as tert-butyl hydroperoxide, and
di-tert-butyl peroxide; acyl and aroyl peroxides, such as dibenzoyl
peroxide, perbenzoic acid, dilauroyl peroxide, perlauric acid, and
acetyl benzoyl peroxide, azo compounds, such as
azo-bis-isobutyronitrile, dimethylazo-diisobutyrate,
azo-bis-1-phenylethane and alkali metal azodisulfonates; and the
like. In general, the free radical initiators or catalysts are
employed in an amount from about 0.0001 to about 5.0 percent based
on the combined weight of the polymerizable ingredients.
The polymerization temperature to be employed is generally
dependent on the batch size, the amount of initiator or catalyst
present, the molecular weight to be attained, and the activation
energy of the polymerization reaction. The rate of polymerization
increases with an increase in temperature. Because greater
exothermic reactions occur at high temperatures and increase the
danger of uncontrollable reactions, high temperatures are
preferably employed where the heat of polymerization may be removed
under controlled conditions, e.g., in jacketed tubes through which
the polymerizable or partially polymerized material is continuously
passed, or in stirred kettles. The polymerization reaction is
conducted at a temperature that is at or above the activation
temperature of the particular free radical catalyst employed but
below the boiling points of the monomers present at the pressures
used. The polymerization temperature employed is usually within the
range of about 60.degree.C. to about the reflux temperatures of the
monomer mixture at atmospheric pressure. Reaction times ranging
from about 6 to about 48 or more hours are usually employed at
atomospheric pressure in batch type operations. However, economy
and operating conditions such as the use of pressure or a vacuum
may determine the use of higher or lower temperatures.
Polymerization may be effectuated by suitable methods such as by
bulk or solvent polymerization techniques. If a solvent is
employed, it can be any suitable true organic solvent, i.e., a
liquid unreactive to the system but capable of dissolving the
reactive components. Typical well-known solvents include the
chlorinated, ketone, ester and hydrocarbon solvents such as for
example, xylene, benzene, toluene, hexane, cyclopentane,
1,1,1-trichloroethylene, ethyl acetate, methyl ethyl ketone,
dioxane, 1,1,2-trichloroethane and the like.
The degree of polymerization may be determined by periodic
molecular weight tests of samples taken from the reaction mixture.
When the weight-average molecular weight of the polymer is
sufficient, as controlled, by the reaction conditions including
time, temperature, catalyst and type of monomer, the polymer may,
if necessary, be dissolved in any suitable solvent and applied to
carrier substrates by conventional coating methods, e.g., spraying,
dipping, or fluidized bed coating. Typical solvents for the
polymers include the solvents described immediately above.
Any suitable coating thickness may be employed. However, a coating
having a thickness at least sufficient to form a thin continuous
film is preferred because the carrier coating will then possess
sufficient thickness to resist abrasion and prevent pinholes which
adversely affect the triboelectric properties of the coated carrier
particles. If a partially polymerized linear or crosslinked
prepolymer is to be used as the coating material, polymerization is
completed in situ on the surface of the carrier by futher
application of heat. To achieve further variation in the properties
of the final resinous product, well-known non-reactive additives
such as plasticizers, resins, dyes, pigments, wetting agents and
mixtures thereof may be mixed with the resin.
Any suitable well-known coated or uncoated carrier material may be
employed as the substrate of the carriers of this invention.
Typical carrier core materials include sodium chloride, ammonium
chloride, aluminum potassium chloride, Rochelle salt, sodium
nitrate, potassium chlorate, granular zircon, granular silicon,
methyl methacrylate, glass, silicon dioxide, flintshot, iron,
steel, ferrite, nickel carborundum and mixtures thereof. Typical
carrier substrates for "touchdown" donor surfaces include cloth,
metal-backed paper, cellophane, aluminum foil, resins such as
polyethylene terephthalate and polyvinyl resins, cellulosic
derivatives, protein materials, and combinations thereof. Many of
the foregoing and other typical carrier materials are described by
L. E. Walkup in U.S. Pat. No. 2,618,551; L. E. Walkup et al. in
U.S. Pat. No. 2,638,416; E. N. Wise in U.S. Pat. No. 2,618,552; and
C. R. Mayo in U.S. Pat. Nos. 2,805,847 and 3,245,823. An ultimate
coated carrier particle having an average diameter between about 50
microns to about 1000 microns is preferred in cascade systems
because the carrier particle then possesses sufficient density and
inertia to avoid adherence to the electrostatic images during the
cascade development process. Adherence of carrier beads to an
electrostatographic drum is undesirable because of the formation of
deep scratches on the drum surface during the image transfer and
drum cleaning steps, particularly where cleaning is accomplished by
a web cleaner such as the web disclosed by W. P. Graff, Jr. et al.,
in U.S. Pat. No. 3,186,838.
Any suitable finely-divided toner material may be employed with the
coated carriers of this invention. Typical toner materials include
gum copal, gum sandarac, rosin, cumaroneindene resin, asphaltum,
gilsonite, phenolformaldehyde resins, rosin modified
phenolformaldehyde resins, methacrylic resins, polystyrene resins,
epoxy resins, polyethylene resins and mixtures thereof. The
particular toner material to be employed obviously depends upon the
separation of the toner particles from the coated carrier beads in
the triboelectric series. Among the patents describing
electroscopic toner compositions are U.S. Pat. No. 2,659,670 to
Copley; U.S. Pat. No. 2,753,308 to Landrigan; U.S. Pat. No.
3,079,342 to Insalaco; U.S. Pat. No Re. 25,136 to Carlson and U.S.
Pat. No. 2,788,288 to Rheinfrank et al. These toners generally have
an average particle diameter between about 1 and about 30
microns.
Any suitable pigment or dye may be employed as the colorant for the
toner particles. Toner colorants are well known and include, for
example, carbon black, nigrosine dye, aniline blue, Calco Oil Blue,
chrome yellow, ultra marine blue, Quinoline Yellow, methylene blue
chloride, Monastral Blue, Malachite Green Oxalate, lampblack, Rose
Bengal, Monastral Red, Sudan Black BN, and mixtures thereof. The
pigment or dye should be present in the toner in a sufficient
quantity to render it highly colored so that it will form a clearly
visible image on a recording member.
Any suitable conventional toner concentration may be employed with
the coated carriers of this invention. Typical toner concentrations
include about 1 part toner with about 10 to 200 parts by weight of
carrier.
Any suitable well-known electrophotosensitive material may be
employed as the photoreceptor with the coated carriers of this
invention. Well-known photoconductive materials include vitreous
selenium, organic or inorganic photoconductors embedded in a
non-photoconductive matrix, orgnaic or inorganic photoconductors
embedded in a photoconductive matrix, or the like. Representative
patents in which photoconductive materials are disclosed include
U.S. Pat. No. 2,803,542 to Ullrich, U.S. Pat. No. 2,970,906 to
Bixby, U.S. Pat. No. 3,121,006 to Middleton, U.S. Pat. No.
3,121,007 to Middleton, and U.S. Pat. No. 3,151,982 to Corrsin.
The surprisingly better results obtained with the thermosetting
acrylic resin coating materials of this invention may be due to
many factors. For example, the marked durability of the coating
material may be due to the fact that these resins provide improved
abrasion resistance with the substrates tested. Greatly improved
adhesion over conventional coating materials is obtained when the
thermosetting acrylic resin coating materials of this invention are
applied to glass, steel or similar metallic particles. Coatings
prepared from the polymers of this invention possess smooth outer
surfaces which are highly resistant to cracking, chipping, and
flaking. The smooth tough surface enhances the rolling action of
the carrier particles across the electrostatographic surfaces and
reduces the tendency of the carrier particles to adhere to the
electrostatographic surfaces. When these thermosetting acrylic
resin polymers are employed in coatings for electrostatographic
carriers, carrier life is unexpectedly extended particularly with
respect to carrier coating durability. Additionally, the
hydrophobic properties of the resins of this invention appear to
contribute in some unknown manner to the stability of the
triboelectric properties of the coated carrier over a range of
relative humidity values. The carrier coatings are easily prepared
and exhibit improved stability during extended periods of storage.
Control of molecular weight of the coating materials is better
resulting in improved desirable carrier coating properties. The
carrier coatings employed in the present invention are non-tacky
and have sufficient hardness at normal operating temperatures to
prevent impaction; form strong adhesive coatings which do not flake
under normal operating conditions; have triboelectric values such
that they can be used with a wide variety of presently available
toners in present electrostatographic processes and are hydrophobic
so that they retain a constant triboelectric value. Thus, the
coated carrier particles of this invention have desirable
properties which permit their wide use in presently available
electrostatographic processes.
The thermosetting acrylic resin carrier coating materials of this
invention are further characterized by high impact strength, good
resistance to weathering and to most chemicals, and good
formability. When used as electrostatographic carrier coatings,
these acrylic resins provide coatings having excellent durability
when employed in continuous electrostatographic development
processes which require the recycling of carrier particles by
bucket conveyors partially submerged in the developer material
supply. In addition, when these acrylic resins are applied to a
carrier material from a solvent system, they are easily applied and
dry quickly. Further, these acrylic resins have good heat and
chemical resistance which is desirable when employed as carrier
coatings in the presence of various conventional electroscopic
toner materials and at the conditions encountered in
electrostatographic machines.
The following examples further define, describe and compare
preferred methods of preparing the coated carriers of the present
invention and of utilizing them in electrostatographic
applications. Parts and percentages are by weight unless otherwise
indicated.
EXAMPLE I
About 10 pounds of 600 micron glass beads having a density of about
2.5 were placed in a Vibratub (available from Vibraslide, Inc.,
Binghamton, N.Y.) and heated to about 80.degree.C. with agitation.
The Vibratub is stopped and a coating solution of about 454.8 grams
of about a 10.0 percent by weight solution of Acryloid AT-50
(available from Rohm & Haas, Philadelphia, Pa.) containing
about 1.8 grams of DuPont Oil Red Dye is added to the Vibratub. The
coating solution and the heated beads were mixed for about 2 hours
duirng which time the temperature was increased to about
182.degree.C. and the coating dried and cured on the glass
beads.
EXAMPLE II
About 10 pounds of 600 micron glass beads having a density of about
2.5 were placed in a Vibratub (available from Vibraslide, Inc.,
Binghamton, N.Y.) and heated to about 80.degree.C. with agitation.
The Vibratub is stopped and a coating solution of about 454.8 grams
of about a 10.0 percent by weight solution of Acrylic Resin 100
(available from Union Carbide Corporation, New York, N.Y.)
containing about 1.8 grams of DuPont Oil Red Dye is added to the
Vibratub. The coating solution and the heated beads were mixed for
about 2 hours during which time the temperature was increased to
about 180.degree.C. and the coating dried and cured on the glass
beads.
EXAMPLE III
About 10 pounds of 100 micron steel carrier cores were placed in a
Vibratub (available from Vibraslide, Inc., Binghamton, N.Y.) at
room temperature of about 72.degree.F. A coating solution of about
36.3 grams of about a 50.0 percent by weight solids solution of
Acryloid AT-50 (available from Rohm & Haas, Philadelphia, Pa.)
and about 145.2 grams of acetone is added to the Vibratub. The
Vibratub is started and the coated carrier cores are heated to
dryness. The temperature is increased to about 300.degree.F. and
maintained for about one-half hour to promote crosslinking of the
coating.
EXAMPLE IV
About 10 pounds of 250 micron steel carrier cores were placed in a
Vibratub (available from Vibraslide, Inc., Binghamton, N.Y.) at
room temperature of about 70.degree.F. A coating solution of about
30.5 grams of about a 60.0 percent by weight solids solution of
Acrylic Resin 100 (available from Union Carbide Corporation, New
York, N.Y.) and about 161.3 grams of acetone is added to the
Vibratub. The Vibratub is started and the coated carrier cores are
heated to dryess. The temperature is increased to about
180.degree.C. and maintained for about one-half hour to cure the
coating.
EXAMPLE V
About 5 pounds of 600 micron glass beads having a density of about
2.5 were placed in a Vibratub (available from Vibraslide, Inc.
Binghamton, N.Y.) at room temperature of about 74.degree.F. A
coating solution of about 45.4 grams of about a 50.0 percent by
weight solids solution of Acryloid AT-70 (available from Rohm &
Haas, Philadelphia, Pa.), about 3.4 grams of
hexamethoxymethylmelamine, and about 182.0 grams of xylene is added
to the Vibratub. The Vibratub is started and the glass beads with
the coating solution are heated to about 180.degree.C. for about 1
hour to cure the coating.
EXAMPLE VI
About 5 pounds of 250 micron steel carrier cores were placed in a
Vibratub (available from Vibraslide, Inc., Binghamton, N.Y.) at
room temperature of about 73.degree.F. A coating solution of about
11.3 grams of about a 50.0 percent by weight solids solution of
Acryloid AT-70 (available from Rohm & Haas Philadelphia, Pa.),
about 11.3 grams of Bisphenol A epoxy resin (Epon 1001, available
from Shell Chemical Company, New York, N.Y.), and about 182.0 grams
of a solvent mixture comprising about 137.0 grams of toluene and
about 45.0 grams of cellosolve acetate is added to the Vibratub.
The Vibratub is started and the steel cores with the coating
solution are heated to about 180.degree.C. for about 0.5 hour to
cure the coating.
EXAMPLE VII
About 5 pounds of 600 micron glass beads having a density of about
2.5 were placed in a Vibratub (available from Vibraslide, Inc.,
Binghamton, N.Y.) at room temperature of about 71.degree.F. A
coating solution of about 45.4 grams of about a 60.0 percent by
weight solids solution of Acrylic Resin 120 (available from Union
Carbide Corporation, New York, N.Y.), about 3.4 grams of
hexamethoxymethylmelamine, and about 182.0 grams of xylene is added
to the Vibratub. The Vibratub is started and the glass beads with
the coating solution are heated to about 180.degree.C. for about
1.5 hours to cure the coatings.
EXAMPLE VIII
About 5 pounds of 250 micron steel carrier cores were placed in a
Vibratub (available from Vibraslide, Inc., Binghamton, N.Y.) at
room temperature of about 72.degree.F. A coating solution of about
11.4 grams of about a 60.0 percent by weight solids solution of
Acrylic Resin 120 (available from Union Carbide Corporation, New
York, N.Y.), about 9.2 grams of Bisphenol A epoxy resin (Epon 1001,
available from Shell Chemical Company, New York, N.Y.), and about
182.0 grams of a solvent mixture comprising about 137.0 grams of
toluene and about 45.0 grams of Cellosolve acetate is added to the
Vibratub. The Vibratub is started and the steel cores with the
coating solution are heated to about 180.degree.C. for about 0.5
hour to cure the coating.
In the following Examples, IX through XVI, the relative
triboelectric values generated by contact of carrier beads with
toner particles is measured by means of a Faraday Cage. The device
comprises a brass cylinder having a diameter of about 1 inch and a
length of about 1 inch. A 100-mesh screen is positioned at each end
of the cylinder. The cylinder is weighed, charged with about a 0.5
gram mixture of carrier and toner particles and connected to ground
through a capacitor and an electrometer connected in parallel. Dry
compressed air is then blown through the brass cylinder to drive
all the toner from the carrier. The charge on the capacitor is then
read on the electrometer. Next, the chamber is reweighed to
determine the weight loss. The resulting data is used to calculate
the toner concentration and the charge in micro-coulombs per gram
of toner. Since the triboelectric measurements are relative, the
measurements should, for comparative purposes, be conducted under
substantially identical conditions. Thus, a toner comprising a
styrene-n-butyl methacrylate copolymer and carbon black as
disclosed by M. A. Insalaco in U.S. Pat. No. 3,079,342 is used as a
contact triboelectrification standard. Obviously, other suitable
toners such as those listed above may be substituted for the toner
used in the Examples.
EXAMPLE IX
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example I. The relative
triboelectric value of the carrier measured by means of a Faraday
Cage is about 21 micro-coulombs per gram of toner. In machine life
tests employing cascade development of a positively charged
reusable imaging surface and developing charged image areas, the
developer performs well and print quality is good throughout the
test. Substantially no toner impaction or carrier abrasion is
observed.
EXAMPLE X
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example II. The
relative triboelectric value of the carrier measured by means of a
Faraday Cage is about 28 micro-coulombs per gram of toner. In
machine life tests as in Example IX, the developer performs well
and print quality is good throughout the test. Substantially no
toner impaction or carrier abrasion is observed.
EXAMPLE XI
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example III. The
relative triboelectric value of the carrier measured by means of a
Faraday Cage is about 24 micro-coulombs per gram of toner. In
machine life tests employing magnetic brush development, the
developer performs well and print quality is good throughout the
test. Substantially no toner impaction or carrier abrasion is
observed.
EXAMPLE XII
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example IV. The
relative triboelectric value of the carrier measured by means of a
Faraday Cage is about 20 micro-coulombs per gram of toner. In
machine life tests as in Example XI, the developer performs well
and print quality is good throughout the test. Substantially no
toner impaction or carrier abrasion is observed.
EXAMPLE XIII
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example V. The relative
triboelectric value of the carrier measured by means of a Faraday
Cage is about 25 micro-coulombs per gram of toner. In machine life
tests as in Example IX, the developer performs well and print
quality is good throughout the test. Substantially no toner
impaction or carrier abrasion is observed.
EXAMPLE XIV
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example VI. The
relative triboelectric value of the carrier measured by means of a
Faraday Cage is about 32 micro-coulombs per gram of toner. In
machine life tests as in Example XI, the developer performs well
and print quality is good throughout the test. Substantially no
toner impaction or carrier abrasion is observed.
EXAMPLE XV
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example VII. The
relative triboelectric value of the carrier measured by means of a
Faraday Cage is about 22 micro-coulombs per gram of toner. In
machine life tests as in Example IX, the developer performs well
and print quality is good throughout the test. Substantially no
toner impaction or carrier abrasion is observed.
EXAMPLE XVI
A developer sample is produced by mixing about 1 part colored
styrene-n-butyl methacrylate copolymer toner particles with about
99 parts of the coated carrier particles of Example VIII. The
relative triboelectric value of the carrier measured by means of a
Faraday Cage is about 29 micro-coulombs per gram of toner. In
machine life tests as in Example XI, the developer performs well
and print quality is good throughout the test. Substantially no
toner impaction or carrier abrasion is observed.
Although specific materials and conditions were set forth in the
above examples for making and using the developer materials of this
invention, these are merely intended as illustration of the present
invention. Various other toners, carrier cores, substituents and
processes such as those listed above may be substituted for those
in the examples with similar results.
Other modifications of the present invention will occur to those
skilled in the art upon a reading of the present disclosure. These
are intended to be included within the scope of this invention.
* * * * *