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The surface science of xerography
Charles B. Duke a,*, Jaan Noolandi b, Tracy Thieret c
a Xerox Corporation, Wilson Center for Research and Technology, 800 Phillips Road 114-38D, Webster, NY 14580, USAb Xerox Corporation, Xerox Research Center Canada, 2660 Speakman Drive, Mississauga, Ont., Canada L5K 2L1
c Xerox Corporation, Wilson Center for Research and Technology, 800 Phillips Road 114-41D, Webster, NY 14580, USA
Received 22 May 2000; accepted for publication 19 April 2001
Abstract
Over the past four decades xerography, the dry ink marking process developed by the photocopy industry, has grown
from nothing into a $170 billion industry worldwide. This amazing commercial success is due to the fact that during this
period, xerographic technology experienced constant and often-dramatic improvement created by sustained indus-
trywide research and development. Indeed, the development of the xerographic copying and printing industry is one of
the great applied surface science successes of all time. In this article we outline the story of the advances in xerographic
technology during the past four decades, describe the profound dependence on these advances of the control of surface
and interface properties of increasingly sophisticated multi-component materials systems, and indicate the potential
impact on the industry of the continuing development of the surface and interface science of the multi-component
materials packages used in xerographic technology. 2001 Elsevier Science B.V. All rights reserved.
Keywords: Adhesion; Dielectric phenomena; Electrical transport (conductivity, resistivity, mobility, etc.); Photoconductivity; Surface
electronic phenomena (work function, surface potential, surface states, etc.); Surface energy; Surface melting
1. Introduction
The practice of xerography began in 1938 with
Chester Carlsons first xerographic print [1,2].
That humble beginning in an apartment in New
York City spawned the biggest change in workpractices in the history of the office. The first
demonstration of the system contained the essen-
tial elements of the modern electrophotographic
copier or printer. The xerographic process consists
of creating an electrostatic image on a photocon-
ducting drum or belt, developing that image with a
pigmented charged powder called toner, transfer-
ring that image to a substrate, typically paper, and
then melting the toner to fuse it onto the sub-
strate. When in 1959 this process was embedded
into the legendary 914 copier, the world changedforever. Hand copying and carbon paper were
banished; access to personal information was de-
mocratized; and the era of personal publishing
began. Most readers of this article have never
known a world without the convenience of per-
sonal copying and printing. 1959 marked the dawn
of the modern information era.
From these modest beginnings, printing and
copying using the xerographic process is an industry
that has been generated by over four decades
Surface Science 500 (2002) 10051023
www.elsevier.com/locate/susc
* Corresponding author. Tel.: +1-716-4222106; fax: +1-716-
2655080.
E-mail address: [email protected] (C.B. Duke).
0039-6028/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.
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of continual improvement of the implementation of
the xerographic process steps. Many of the im-
provements are the result of sophisticated materials
packages whose design and operation rely heavilyon the understanding and control of surface and
interface properties. Thus, xerography is a tech-
nology that has been generated and advanced by
the control and utilization of interfacial pheno-
mena. The story of the development of the xero-
graphic copier/printer industry as a case history
in value of basic research in US industry has been
summarized by the Committee for Economic De-
velopment [3]. In this article we describe the xero-
graphic process, indicate some of the major material
challenges in designing a modern, commercial
xerographic marking engine, and develop several
illustrative examples of the role of surface and
interface science in overcoming these challenges.
This might be a footnote in history of minor in-
terest to the intended audience for this volume, first
year graduate students in a wide variety of scientific
and engineering disciplines, but for one thing:
During the past decade the world has changed
profoundly and with it the practice of both science
and engineering. The world has entered an era
of unprecedented pace in the generation of new
knowledge and its commercial application in globalmarkets. The search for new knowledge is increas-
ingly pursued in a context in which its application
to generate economic value is valued even more
highly than the novelty and impact of the knowl-
edge itself. Few of todays first year graduate stu-
dents will pursue their careers without repeatedly
feeling the need to solve practical problems under
tight time constraints. The story of the development
of xerographic technology by solving difficult,
practical surface science problems is a harbinger of
the future. By appreciating the whys and hows ofthe development of xerography, you set forth the
elements of a blueprint for your own professional
success in this new world.
2. Industry overview
Xerographic printing and copying is big busi-
ness. The scope of the industry is monitored by
several consulting firms, one of which is CAP
ventures from whose services we take the data
noted below [4]. From the humble beginnings of
Chester Carlsons kitchen experiments in 1938 andthe first automatic copier in 1959, the worldwide
market served by xerographic marking engines
and the services built around them has exploded to
become $167 B in 1998 growing at 12% per year.
This corresponds to 1.2 trillion pages made by
xerographic printers, copiers and fax machines.
For comparison, the corresponding number of
pages made on desktop ink jet marking engines
is 160 billion, i.e., nearly 10 times smaller. As in-
dicated in Ref. [3], this entire industry and its
amazing growth is an outcome of an aggressive
research and development (R&D) program carried
out over six decades. Much of this R&D was de-
voted to solving surface and interface technical
problems needed to produce better image quality,
improved reliability and lower cost of xerographic
products.
An important aspect of the industry is that
its business model derives an important portion
of its profits from selling consumables, specifi-
cally the dry ink and replacements for some of the
parts that wear out over the life of the engine (like
tires on an automobile). Dry ink is the blackpowder that you get all over you when it is not
adequately fused to the paper or when you try
to change the bottle of dry ink and spill some. In
1998 the dry ink market alone was over $8 B
growing at 10% per year, and shared by over
12 multi-national firms, with the largest shares
being held by Xerox and Canon. One of the
important replacement parts is the photorecep-
tor, the worldwide market for which in 1998
was approximately $10 B. As we see below, both
photoreceptors and the dry ink are complex,multi-component organic composites designed
and tested using sophisticated surface science
concepts and instrumentation. Thus, through the
vehicle of xerographic consumables, surface and
interface science and technology are exerting a
decisive influence on over $20 B of cash flow every
year: a number that is comparable to the entire US
annual expenditure for basic research in recent
years [5].
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3. The xerographic process: description of process
steps
As indicated above, the xerographic processconsists of five process steps that begin by charging
a photoconductive belt or drum; generating a la-
tent image on this photoconductor by imagewise
exposure to light; development of this latent image
with charged toner; transfer of the toner image to
a substrate; and fusing of the image to that sub-
strate. A sixth process step, the cleaning of residual
charge off the photoreceptor, is needed in com-
mercial devices to reuse the same photoreceptor
for subsequent imaging. We begin this section with
an explanation of the nature and operation of the
subsystems that together comprise a monochrome
xerographic marking engine, as illustrated in Fig. 1
[1,2,610].
3.1. Photoreceptor
The photoreceptor is the central element of the
xerographic process. This devices fundamental
responsibility is the transport of the page images of
a document in their various forms through the
steps of the process. While performing this func-
tion the photoreceptor also serves as the image-generating surface. Specifically, charge deposited
on the photoreceptor is discharged imagewise by
the photogeneration in a charge-generator layer of
holes that migrate to the negatively charged upper
surface to discharge this surface as shown in Fig. 2
[1,2]. The device is a loop that recycles through the
process steps. It may be realized either as a rigid
drum or as a belt, as shown in Fig. 1. A detailed
description of the operation of multi-layer photo-
receptors is given in Refs. [2,6]. Designing and
fabricating a multi-layer photoreceptor that isstable for hundreds of thousands of impressions
and supports resolutions of up to 1200 spots per
inch uniformly across its page-size surface is a
challenging technical problem which involves the
solution of multiple interface science problems as
indicated in the caption to Fig. 2. For contribu-
tions to this feat for the belt photoreceptors used
in the Xerox 10 series and subsequent products
Damodar Pai, Jack Yanus, and Milan Stolka of
Xerox received the Heroes of Chemistry 2000
award sponsored by the Industry Relations Officeof the American Chemical Society. This illustrates
a first lesson for prospective surface scientists:
Outstanding applied surface science is recognized
not only by ones sponsors, but also by ones peers.
3.2. Charging
In most xerographic architectures the charging
subsystem is presented with a clean (of toner) and
erased (of static electric charge) photoreceptor as
indicated in Fig. 3 [1]. Its output is a uniformlycharged photoreceptor surface at a defined volt-
age. The charging device is typically a fine tungsten
wire operated at a few hundred volts DC and a few
thousand volts AC. This current produces an ion-
ization of the air in its vicinity. These ions are the
source of a corona wind that charges the pho-
toreceptor. A corotron-charging device is built by
surrounding the wire with a grounded aluminum
housing on three sides [1]. In modern charging
Fig. 1. Schematic diagram of the monochrome xerographic
process. A photoreceptor belt is uniformly charged in step (1).
An image is written on this belt by a laser in step (2), thereby
generating a charge image on the belt. This charge image is
converted into a powder image of toner on the belt in the de-
velopment step (3). This powder image is transferred to a sheet
of paper in the transfer step (4) and subsequently fused to the
paper in step (5). Residual toner on the photoreceptor is cleaned
off in step (6) and the process repeats.
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devices, called scorotrons, a biased mesh grid is
interposed between the wire and the photoreceptor
[1]. This grid enforces a charge limit. As the device
charges toward the grid potential, the electric field
between the grid and the photoreceptor surface
drops to zero, leading to a known uniform voltage
at the exit from the charging station. This is a goodexample of the use of passive elements operated
using physical principles to stabilize the xero-
graphic process.
3.3. Exposure/illuminator
The exposure station receives a photoreceptor
with a uniform charge spread two dimensionally
across its surface. In modern digital xerographic
marking engines the photoreceptor is discharged
imagewise by a laser beam so that the output is a
latent image in which the charged and discharged
areas of the photoreceptor are a reflection of the
image that ultimately appears on the output me-
dium. Such a station is indicated schematically in
Fig. 4 [1,2]. Usually the discharged areas corre-
spond to the black areas in a monochrome image.Gray areas are described by halftone dot patterns
[11]. Typical resolutions are 6001200 dots per
inch (dpi).
The desired imaging property of the photore-
ceptor is that in the dark, the device functions as
a capacitor, maintaining an electrostatic voltage
between the upper surface and the ground plane
on the lower surface. When exposed to light,
however, the device becomes a conductor. The volt-
age drops to near zero. Because there is little lateral
Fig. 2. Successive blow-up diagrams of the structure and operation of a multi-layer photoreceptor belt. The multiple interface,
photogenerator and transport layers are coated on a polyethylene terephthalate (PET) belt. The operation of the photoreceptor in a
xerographic marking engine is shown in the diagram in the left-hand panel. The physical processes that occur to discharge the pho-
toreceptor upon exposure to light are indicated in the diagram in the lower right-hand corner. The light penetrates through the charge
transport layer and is absorbed in the charge generation layer where it generates an electronhole pair. The hole migrates to the front
of the photoreceptor where it discharges the charge pattern on the surface. The electron migrates to the grounded electrode and flows
to ground. Designing multi-layer photoreceptors involves the solution of sophisticated interface science problems associated withcharge injection across the various interfaces, assuring that charge does not get trapped either in the layers of the photoreceptor or their
interfaces, and guaranteeing that charge does not spread as it is generated and transported across photoreceptor so that resolutions of
up to 1200 dpi may be achieved uniformly over page-size images.
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conduction, a small spot of light produces a lo-
calized area of voltage difference. These properties
are achieved by the multi-layer photoreceptor ar-
chitecture indicated in Fig. 2. Residual, trapped
charges prohibit the exposed voltage from achiev-
ing the ground potential. Aging of the photore-ceptor includes increases in both the dark decay,
where the device voltage decreases with time even
in the dark, and the residual potential, the min-
imum voltage achievable upon exposure. Reduc-
ing aging and dark decay to allowable limits is a
challenging problem involving considerable use of
surface science techniques and tools to identify and
mitigate the root causes of these phenomena.
A modulated or diode laser provides a conve-
nient source of light to expose the photoreceptor in
an imagewise way. The light is scanned laterally
using a polygon mirror rotated at a constant an-
gular velocity. The photoreceptor motion provides
the second dimension linear variation so that a
raster scan is produced on the photoreceptor sur-
face much in the same way that a television picture
is generated. The resolution is typically limited by
practical design considerations (e.g., laser power,
photoreceptor sensitivity, cost of the optics) rather
than fundamental physical phenomena (e.g.,
charge spreading on the surface of the photore-
ceptor). More detailed descriptions of the illumi-
nator subsystem that exposes the photoreceptormay be found in Refs. [1,2]. The design and fab-
rication of the diode lasers for use in illuminators
is a challenging technical problem in its own
right. Don Scifres, Robert Burhnam and William
Streifer developed the pioneering technology for
one of the early generations of semiconductor
diode lasers at the Xerox Palo Alto Research
Center (PARC). Scifres spun out the design and
manufacturing technology from PARC as the firm
SDL to manufacture these lasers for Xerox. Sub-
sequently SDL has become a major supplier oflasers to the entire optical communications
industry, and Scifres has remained its chief exec-
utive officer (CEO). Scifres and Burnham were
elected to the US National Academy of Engi-
neering for their contributions to this technology.
Scifres also was awarded the 1997 George E. Pake
Prize of the American Physical Society for his role
in commercializing the technology and thereby
generating jobs for young scientists and engineers.
This illustrates a second lesson for prospective
Fig. 3. Schematic diagram of a corotron charging subsystem. A
corotron wire at high voltage generates negative ions that flow
onto the photoreceptor belt to give it a uniform charge as it
passes below an orifice in the grounded shield surrounding the
wire. Sometime a grid is added below the orifice to bettercontrol the potential on the photoreceptor. In this case the
charging subsystem is called a scorotron. In recent years com-
pletely solid-state versions of these devices have been built and
tested, but have seen limited deployment in the industry. The
perfection of such charging bars remains a challenge for the
next generation.
Fig. 4. Schematic diagram of the exposure (illuminator)
subsystem. A laser beam is swept across the surface of the
photoreceptor by a rotating polygon of mirrors. The beam is
turned on and off imagewise either by a modulator or by the
current through a laser diode. Everywhere that the beam strikes
the photoreceptor, it is discharged as indicated by the absenceof a charge pattern. Many technical challenges face the designer
of a commercially successful diode-laser-based illuminator in-
cluding selecting a combination of laser life, laser power, laser
spot size, laser pulse shape, photoconductor sensitivity, optical
and polygon design that satisfy the cost, stability and com-
pactness requirements of a modern xerographic marking
engine.
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surface scientists: Such are the rewards awaiting
successful young scientists and engineers in the
new world of global commerce.
3.4. Registration
In color-printing systems the photoreceptor
must be imaged separately for each of four (cyan,
magenta, yellow and black or CMYK) color
separations. One way to do this, the multi-pass
cyclic architecture, is shown in Fig. 5. The four
color panels must be registered relative to one
another. Mis-registration results in undesirable
color shifts, image blur, unimaged areas, and other
defects. The timing of the individual exposure
steps is regulated such that the images overlay one
another within a few 10s of microns. In the color-
printing architecture shown in Fig. 5, there is
an additional registration requirement. When the
individual color separations are built up on the
output media, these transfer steps must occur with
the same level of precision. A description of the
various architectures used to implement color xe-
rography may be found in Refs. [1,6]. The solution
of the color registration problems is an active area
of research in modern control system design. The
use of novel sensors on the photoreceptor and
feedback loops to the motion control of the pho-
toreceptor or the illuminator is enhancing the
precision of the registration from the order of 80
100 lm to the order of 10 lm, at which point thehuman eye can no longer detect mis-registration
defects. The integration of these sensors into the
photoreceptor involves the solution of a host of
surface and interface problems new to the practice
of xerography, thereby generating challenges for
the next generation of xerographers.
3.5. Development
The development subsystem is presented with a
latent image on the photoreceptor. Upon exit the
toned image is visible on its surface. A schematic
diagram of a magnetic brush development subsys-
tem is shown in panel (a) of Fig. 6 [1,6], although
many other types of subsystems are used com-
mercially [1,6]. Within the magnetic brush devel-
opment subsystem there are two major materials
components collectively called developer and il-
lustrated in panel (b) of Fig. 6. The first is the toner,
or dry ink. Toners are complex multi-component
composite materials packages the composition of
which is indicated in panel (c) of Fig. 6. Toner is the
most challenging materials package in the system
Fig. 5. Schematic diagram of the cyclic color xerographic process. In this process one color is imaged, developed and transferred in
each pass of the photoreceptor belt. The developers for the other colors are cammed out as indicated by the arrows in the diagram. The
four-color image is built up on the paper as each of the four developed powder images is transferred sequentially. Then, the final four-
color image is fused as it exits the transfer zone.
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because there are so many properties it must
exhibit. These include powder and viscous flow,
charging, melting properties, color, toxicity, size,
and adhesion to the various surfaces with which it
comes in contact during the xerographic process.
Many of these properties deal with how the
toner interacts with surfaces. Powder flow governs
the capability of the toner to be dispensed from the
hopper to the developer housing and once there, to
mix well with the contents. Additives are required
Fig. 6. Successive blow-ups of a development subsystem (panel (a)), development zone and carrier bead (panel (b)), and toner particle
(panel (c)). Key parameters and features of each of these are indicated in the figure.
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to generate suitable performance. Viscous flow
determines the ability of the toner to wet the sur-
face of the output medium when melted by the
fusing subsystem. Melt properties are constrainedby a desire to reduce the temperature required to
fuse the toner to the output media, and hence
energy consumption, on the one hand, and on the
other hand by requirements for storage in hot
environments. There are stories of toner bottles
turning into big crayon containers after transport
in trucks across the desert because their blocking
temperatures (i.e., the temperature at which the
toner spontaneously aggregates in the bottle) were
too low. Size distribution is a determinant in image
quality. Large toners make sharp edges in images
look blurry. Small toners are difficult to contain
and can become airborne contaminants. Pigments
may not be chosen arbitrarily. They must pass
toxicity testing and also must produce the desired
colors. The loading of the pigments into the toner
polymer base (see panel (c) of Fig. 6) must not be
excessive or the pigment will dominate the other
properties. Release properties come into play
when, during fusing, the toner must adhere to the
output media rather than to the fuser roll (see
Fig. 8) to be deposited on a subsequent sheet in a
process called hot offset. With all these constraints,the challenge of constructing a material that sat-
isfies them is quite a challenge. The design and
fabrication of the sophisticated multi-component
toner materials packages are discussed in Ref. [8].
They remain one of the major challenges for sur-
face scientists in xerography.
As shown in panel (b) of Fig. 6, there is com-
monly a second component in the developer
housing. The carrier is typically a ferrite of about
100 lm diameter. These carriers form brushes
which are rotated by the magnetic fields in thedeveloper roll as indicated in panels (a) and (b) of
Fig. 6. The toner, when agitated against the carrier,
develops a triboelectric charge and adheres to it.
Typically this requires the ferrite core of the
carrier to be coated with an organic polymer that
yields the desired charge exchange. Together this
mixture is referred to as developer. Developer
aging occurs when the toners impact the surfaces
of the carrier particles, diminishing the carrier sur-
face area available for tribocharging. One of the
responsibilities of the development system is to use
the electric field generated by the photoreceptor
image charge to influence the charged toners to
migrate to the photoreceptor. Thus, the two forcesholding the toner to the carrier must be overcome.
Both electrostatic forces and the adhesion forces
are commonly broken by agitation of the developer
in or near the nip created by the development roll
and the photoreceptor surface.
A bias is applied to the housing to generate the
electric fields for development. The value of this
bias voltage lies between the photoreceptor volt-
ages for the charged and discharged areas. Thus, a
potential is generated in the nip that produces a
force, the product of the toner charge and the
external field that for the toned regions points
toward the photoreceptor. The same bias produces
a similar force in the untoned regions of the image
but in the opposite direction, thereby driving the
toners away from the photoreceptor toward the
developer roll. The development subsystem de-
scribed in Figs. 1 and 5 is called discharged area
development in which the negatively charged
toners migrate toward the uncharged (i.e. exposed)
areas of the photoreceptor [1,2]. Unlike lightlens
copiers, in which the charged areas are developed,
this is the common mode of exposure for digitalprinters and copiers, selected primarily in order to
extend the life of the laser by reducing its duty
cycle.
As is the case for the other subsystems, the de-
sign of new developer subsystems is an active area
of research. The essential physics problem to be
solved is to insure the reliable transport of toner
from the bottle that you insert into your copier or
printer onto the photoreceptor at precisely the
right place for hundreds of thousands of copies
completely uniform across the page. One mightthink of this as applied soft condensed matter
physics. In the early 1980s a new system design
called highly agitated zone (HAZE) development
was invented which provided both excellent broad
area and image detail development with hard-
ware that was smaller and lower cost than its
predecessors. This design was first incorporated in
a 62-copies-per-minute Xerox product, the Xerox
1065 Marathon copier, introduced in 1987. Over
the past 13 years, this product family has produced
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more than 100 000 machines in the US. Further-
more, the HAZE design was subsequently incor-
porated in several additional major Xerox product
platforms for copiers and printers. The combinedrevenue from all of the products embodying this
development system was nearly half of Xerox
total revenue in 1996. In recognition of the con-
tribution of the HAZE development process to
the success of three generations of Xerox current
products, the inventor of the process, Dan Hays
of the Xerox Wilson Center for Research and
Technology, was awarded the 1997 American In-
stitute of Physics Prize for Industrial Applications
of Physics. More innovations like this are in the
pipeline, especially with the decreasing size of
marking engines and the move to color. Lots of
surface and interface problems must be solved to
move little 5 lm toner particles around over 10 in.
swaths, so there is ample opportunity for surface
and interface scientists in this area. This illustrates
a third lesson for prospective surface scientists:
Opportunity abounds, and fame as well as fortune
awaits those who generate important new inven-
tions.
3.6. Transfer
The transfer step begins with a toned photo-
receptor image and a sheet of media. The toner is
transferred from the photoreceptor to the media
with a minimum of residual mass remaining on the
photoreceptor as indicated in Fig. 7. This process
uses electrostatic forces on the toner charge to
encourage it to move to the paper. Corotrons or
biased rolls are used to manipulate the toners
electrostatically. When printing color in the nor-
mal way by mixing together a set of primaries,
variations in the transfer efficiency alter theamount of transferred toner from each constituent
and result in noticeable differences in color re-
production. Transfer efficiencies in excess of 98%
are typically achieved in modern xerographic
marking engines, but even at this figure efficient,
stable transfer remains one of the major research
problems limiting the performance of modern xe-
rographic marking engines. This is a promising
area for a prospective surface scientist to establish
her or his fame and fortune.
3.7. Media handling
In order to position the media to receive the
xerographic image at the transfer station, addi-
tional processes must be considered. Sheets of
paper (or vugraphs) must be removed from a stack,
transported to the transfer station, and presented
to it at the precise time such that the arrival of the
photoreceptor image and the media may be syn-
chronized within an accuracy of a few tenths of
millimeters. The most common type of media is cut
sheets, although the use of continuous webs issometimes employed in high-throughput printing
systems. The properties of media vary greatly from
one type to another and sometimes even from
sheet to sheet (e.g., the use of different weights and
finishes of paper in a single document). In some
printing systems the media are escorted through
the system with gripper bars or tacked to an elec-
trostatically charged transport medium. In most
electrophotographic systems, however, the media
are passed between subsystems and, after marking,
sent to the finishing station, by mechanical devicesthat rely on frictional forces. The development of
gentle media handling systems based on distributed
sensors and actuators is a major frontier of modern
printing systems research, due to the high impor-
tance of handling a wide range of media in color
printing. This is a huge opportunity for the devel-
opment and application of unique microelectro-
mechanical (MEMS) devices and technology to the
printing business by the next generation of applied
physicists.
Fig. 7. Schematic diagram of a transfer subsystem. A corotron
biases the incoming paper positively so that it attracts the
negatively charged toner to transfer from the photoreceptor.
The geometries shown in this figure are highly idealized.
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3.8. Fusing
Most readers of this paper are probably aware
of the fuser in their xerographic copier or printerbecause documents in the output tray are warm to
the touch. Simply put, the fuser takes the toner
powder transferred to the paper by the transfer
subsystem and fixes it to the paper by melting it,
usually by applying heat and pressure, so that it
flows into the fibers of typical office paper or into a
specially prepared surface layer on coated stock or
transparencies. Once the toner is on the media as it
exits the transfer subsystem prior to fusing, it may
be readily smudged (or even blown off) because the
only forces holding it are the electrostatic forces
remaining from the transfer step. Fusing melts the
layers of toner on the media to achieve fixing the
image as indicated in Fig. 8 [8,9]. The heating may
take place using a radiant source or more com-
monly a heated roll, which assists the fixing of the
toner to the media by pushing it into the media.
These rolls are sometimes coated with a functional
oil (called a release agent) that forms covalent
bonds with the roll surface in order to keep the
toner from adhering to it. Roll fusers are com-
plicated multi-component materials systems, em-
bodying multiple layers of composite materials.
Their design and fabrication are discussed in Refs.
[9,10]. When an electrophotographic printer/copier
is first turned on, the warm up time is consumed
in heating the fuser. Indeed, during warm-up, allthe power available to the device is used to heat the
fuser as rapidly as possible. As xerographic devices
become faster, image quality requirements become
higher, and power constraints become tighter, re-
search is moving toward belt fusers, that can warm
up much faster and use a lot less power than roll
fusers. This is an arena ripe with opportunity for
the applied surface scientist and the control engi-
neer to collaborate on designing smart fusers and
associated toners that precisely fuse the images on
each sheet, at minimal power, image by image,
without a smudge or smear to be seen.
3.9. Cleaning and erase
The photoreceptor cycle is completed in the
cleaning and erase stations. Input to these sub-
systems is a surface that has untransferred residual
toner and some remaining charge. The erase sta-
tion, not shown in Figs. 1 and 5, consists of a
flood exposure of the photoreceptor performed
by a suitable light source. This step is designed to
discharge the photoreceptor and residual toner ascompletely as possible so that the toner may be
removed more easily from the photoreceptor and a
consistent initial state may be prepared for the
charging subsystem.
Cleaning, illustrated schematically in Fig. 9,
is performed using one or more of a number of
technologies. It is common in many printers to
use an elastomer blade to scrape the toners off
without damaging the surface of the photorecep-
tor. Biased cleaning brushes rotating in contact
with the photoreceptor belt also are used fre-quently. The collected toner is placed into a waste
toner bottle, to be recycled later. Perhaps the
highest goal for cleaning subsystems is to eliminate
them entirely by designing transfer subsystems
that are sufficiently efficient, thus enabling a lower
machine price by reducing both the cost and
number of required subsystems. This is currently
one of the greatest technical challenges facing
a prospective surface scientist working on xero-
graphy.
Fig. 8. Schematic diagram of a fusing subsystem based on us-
ing a release agent. The release agent coats the heated fuser roll
so that the toner does not stick to the roll. Under the influence
of heat and pressure in the fuser, the toner particles are fused
together (i.e., the toner powder becomes a thin polymer film)
and fixed to the paper as described in Fig. 11.
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4. Material challenges in xerography: the surface
connection
As noted above, much of the functional per-
formance of the xerographic process is generated
by the use of sophisticated materials packages,
typically multi-component organic composites,
which must satisfy many constraints. Due to their
composite nature, interfacial properties play a
dominant role in their performance and stability.
The specifics vary with the nature of the materials
package, with multi-layer systems, film compo-
sites, and multi-component powders being three
broad material classes exhibiting distinct design
rules and criteria.Photoreceptors (Fig. 2) and fuser rolls (Fig. 8)
are two prominent examples of multi-layer mostly
organic composites. Both embody sophisticated
boundary layer treatments to ensure the mechan-
ical integrity of the device under hostile operating
conditions (e.g., heat, corona charging, media con-
tact and release and repeated flexion and tension).
Wear is a central concern for bothdue primarily
to media contact for the fuser rolls but also due to
the transfer and cleaning subsystems for the pho-
toreceptor. Moreover both must exhibit good re-lease properties: the photoreceptor to release toner
powder in the transfer step and the fuser roll to
release melted toner on the media upon exit from
the fuser. Finally, the electrical, optical, and ther-
mal properties of these devices must be designed
carefully. Fuser rolls need adequate thermal con-
ductivity. Charges generated in the charge gener-
ation layer (CGL) of a photoreceptor must cross
multiple interfaces and move through the device
to discharge the receptor upon exposure. All of
these functional requirements require exquisitely
designed interfacial properties both between the
various layers of the photoreceptors and fuser rolls
and between the base polymer and its loadingmaterial for the CGL.
Developer and toner powders (Fig. 6) have
different design criteria. The toner and carrier
coating must exhibit the correct magnitude and
kinetics of contact charge exchange: very delicate
surface properties. They must flow well in powder
form for the developer subsystem to work. The
toner must exhibit viscoelastic flow when subjected
to heat and pressure in order to get a good fix to
the paper, a suitable gloss on the image, and the
reduction of light scattering within the fused toner
layer to acceptable levels for transparencies. The
humidity sensitivity (i.e., dependence on adsorbed
water) of the toner powder flow and contact
charge exchange must be controlled. All of these
characteristics result from controlling the surface
and interface properties of the external and inter-
nal surfaces in the toners and carriers. We develop
this theme in the following section by considering
three specific examples in more detail.
5. Microscopic control of organic composites
5.1. Multi-layer photoreceptors
The dual-layer photoreceptor design shown in
panel (a) of Fig. 2 is based on separating the
functions of photogeneration and charge carrier
transport. This allows the flexibility to select photo-
generator materials optimized for different wave-
lengths of the light sources used in xerography
(e.g. visible LED image bars, IR diode lasers)
while designing the rest of the photoreceptor tosatisfy other constraints on its behavior (e.g., wear,
speed, stability). The current trend is toward the
use of organic photoconductors. The substrate can
be a metal drum or a flexible metalized polyester
belt, as shown in Fig. 2. The substrate is coated
with an undercoat (blocking) layer which serves
to prevent the injection of charge carriers in the
dark from the grounded electrode. The CGL
consists of pigment particles dispersed in a poly-
mer binder. The pigment is selected and optimized
Fig. 9. Schematic diagram of a brush cleaning subsystem. By
either mechanical or electrostatic forces, the residual toner is
stripped of the photoreceptor and collected in a waste bottle
(not shown).
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for the specific application. Some of the pigments
used include perylenes, phthalocyanines and azo
compounds [2,6]. The function of the thin CGL
(about 0.22 lm) is to absorb the incident lightand photogenerate charge carriers efficiently. The
photogenerated charge carrier must be injected
and transported through the next layer to reach
the surface of the photoreceptor. The charge
transport layer (CTL) is coated on top of the CGL
and consists of a film (1530 lm thick) of insu-
lating polymer doped with charge-transporting
molecules. Most of the active molecules, which
include aryl amines and hydrazone, transport only
holes and the photoreceptor surface is charged
negative. The CTL should be transparent to the
incident light and the layer is designed to be clear
and amorphous. Additional layers are included in
practical photoreceptors, e.g., an adhesive layer on
the metalized polyester to improve adhesion of belt
photoreceptors and polymer overcoats on top of
the CTL to extend the life of the photoreceptor.
The basic phenomena in the operation of the or-
ganic photoreceptors are the photogeneration, in-
jection and transport of charge carriers. Detailed
descriptions of the design and operation of these
devices have been given in Ref. [2].
Surface and interface science plays a key rolein the design of practical photoreceptors. The
CGL is typically a pigment dispersed in a binder,
which requires control of the pigment/binder in-
terfaces for effective photogeneration. The injec-
tion of holes into the CTL and electrons into the
ground electrode requires engineering of the elec-
tronic properties of the CGL interfaces. The ad-
hesion of the CTL, CGL, and backplane layers
typically is achieved by special interfacial chemi-
cal treatments. Polyesters, polyamides, poly(vinyl
butyral), poly(vinyl alcohol), polyurethane, andpolyacrylonitrile have been used as adhesives in
contact with the supporting substrate. The adhe-
sive layer is of a thickness from about 0.001 lm
to about 1 lm. This layer may also contain con-
ductive and nonconductive particles, such as zinc
oxide, titanium dioxide, silicon nitride, and car-
bon black to provide desirable electrical and op-
tical properties [12]. Often the top surface of
the photoreceptor is coated with an overcoat de-
signed to reduce the wear on the photoreceptor
by the charging, transfer, and cleaning subsystems
and to increase the charge acceptance from the
charging subsystem. Thus, the solution of practi-
cal surface and interface science problems lies atthe foundation of the technical issues associated
with fabricating cost-effective, long-life commer-
cial multi-layer photoreceptors.
5.2. Dry ink: conventional and chemical toner
Conventional toners are fabricated by dispers-
ing pigments in a polymer base, grinding and
classifying. Descriptions of their design and pro-
cessing are available in the literature [7]. Chemical
toner represents a major advance over conven-
tional processing. This new chemical technology
involves molecular design and micro-fabrication
allowing precision manufacturing of custom toner
particles on the micrometer scale. Particle size
distribution and surface characteristics are strong
drivers of image quality. Narrow size distributions
and spherical particles have more uniform charge
per unit mass Q=m and hence, more predictablebehavior in the development subsystem. Con-
ventional toner manufacture yields a wide distri-
bution of particle sizes and a particle morphology
characteristic of broken glass with jagged edges.Chemical toner technology permits the manufac-
turer to specify a narrow distribution of nearly
spherical particles produced by the process, as well
as a wide range of materials compositions.
A number of chemical toner processes have been
commercialized. Among these are suspension po-
lymerization, emulsion aggregation (EA), solvent
dispersion, and encapsulation. We discuss here EA
as practiced by the Xerox group, Nippon Carbide
and Konica [1315]. The EA process is a powerful
technology to produce monodispersed compositeparticles from the submicron to the micrometer
range. Controlled aggregation of submicron poly-
mer latex particles, together with pigment parti-
cles, is an important process in the development
and production of toners as indicated in Fig. 10.
Starting from a latex suspension, larger aggregates
of well defined size and narrow size distribution
form under moderate shear with various additives.
The final aggregates are heated above their glass
transition temperature to coalesce, followed by
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washing and drying for use as toners. The particle
properties, as well as the sizes and size distribu-tions, depend critically on the aggregation process,
as controlled by surfactants and additives that
modify their interfacial properties.
The observed dependencies of the final aggregate
size on process parameters such as initial latex
particle size, ionic strength, and ratio of cationic to
anionic surfactants can be explained by taking into
account the kinetics of aggregation, and using a
charged liquid droplet model for the aggregate
energetics [16]. The morphology of EA toner par-
ticles depends in a complex way on the conditionsused during the coalescence of the latex aggregates.
The coalescence portion of the EA toner making
process involves converting the tightly bound
aggregates of latex and pigments into particles that
can be employed in the xerographic process. The
coalescence of latex particles in a drying paint film
is perhaps the phenomenon having most in com-
mon with the EA latex coalescence process. Pri-
mary aggregates approximately 1 lm in diameter
can contain between 100 and 200 latex particles for
a latex 200 nm in diameter. A toner-sized second-
ary aggregate of diameter 5 lm would containabout 100 primary aggregates or 15 000 latex par-
ticles.
The advantages of the EA process to make
chemical toner are small toner size, narrow parti-
cle size distributions, tunable morphology, wide
materials design latitude, and low cost. The par-
ticles are finer and more uniform in size and shape
compared with the crushed multi-component
toner particles. These qualities are important for
obtaining more effective development and image
transfer. Therefore they yield superior imagequality in the xerographic process. The controlled
and adjustable modification of the interfacial
properties of these specially designed materials
using surfactants and additives makes this quality
improvement possible.
5.3. Fuser components
Roll fusing involves the melting, coalescence,
and spreading of toner particles, as well as the
Fig. 10. Schematic diagram comparing fabrication of conventional toner to that of chemical toner. The various steps of both fab-
rication processes are indicated in the figure. The differences in the morphologies of the resulting toner particles are evident in the
pictures at the bottom of the figure.
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adhesion of toner to paper and the cohesive
strength of toner, relative to its adhesion to the
fuser roll as indicated in Fig. 11 [2]. The important
variables in the melting, coalescence, and spread-
ing of toners are temperature, time, pressure, vis-
cosity, particle size, and the surface tension of
toner [2,9,10,17]. In particular, adhesion of an
image to a substrate depends on the surface ener-
gies, the contact area between the surfaces, thetoner modulus, the hardness and the brittleness of
the toner. In general, coalescence and toner fusion
is increased proportional to the pressure, surface
tension and dwell time and inversely proportional
to the viscosity and particle size. One important
variable affecting coalescence is the melt visco-
elasticity. External additives on the fuser roll, such
as silicone oil, assist in releasing the toner. These
additives can, however, increase the coalescence
temperature in some toners, which can cause toner
aggregation and hence grainy images: not a de-sirable outcome. The effect of additives such as
waxes in the toner can also enhance toner release
from the fuser roll and reduce or eliminate the
necessity of using oil in the fusing subsystem. In
general, materials screening for fusing involves
initially selecting the viscoelastic properties of
toner resins for optimum fusing performance, and
then studying the effects of various additives such
as surface charge control agents on fusing. Inter-
face science provides the foundation for designing
the treatments of the surface layers of the fuser
rolls for release, for studying the chemical reac-
tions of release agents with the fuser roll and toner
surfaces, and for providing the specialty adhesivesneeded to hold the layers of the fuser and donor
rolls together under adverse operating conditions.
Detailed examples of several applications of in-
terface science studies for the design of multi-
player organic components for fuser and transfer
subsystems are given by Badesha and Swift [18].
As noted earlier, this is an arena of major oppor-
tunity for the next generation of applied surface
scientists.
6. Semiconductor processing connections
While most xerographic components involve
organic composites, the revolution in the size and
performance of semiconductors also is affecting
xerographic marking engines. The most important
of these influences are the use of laser diodes in the
exposure subsystem, the use of ink jet arrays to
mark directly on the output media, and the in-
creased use of active feedback in xerographic
process control. In this section we explore some of
the impacts of the semiconductor process revolu-tion on xerographic marking engines.
6.1. Semiconductor diode lasers
In high-speed printing tradeoffs are required
between high resolution, high speed, and manu-
facturing tolerances. Fig. 4 shows a raster output
scanning illuminator subsystem with a polygon
scanner photoreceptor and laser system along with
scanning and correction optics. High resolution
requires large polygon facets, on the other hand,whereas high speed is advantaged by having many
facets. The use of multiple beam diode lasers is
able to circumvent part of this tradeoff by allowing
higher speeds to be attained with fewer facets.
In addition, as higher resolution becomes more
important, it is advantageous to go to shorter
wavelengths, so that higher resolution can be
achieved with small polygon facets and good depth
of focus. Alternatively, short wavelengths can be
used at todays resolution of 600 dpi to increase
Fig. 11. Diagram of the physical processes involved in fusing.
These are shown as they occur after the toner on the paperenters the fuser as indicated in Fig. 8 (adapted from Ref. [2]).
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the speed of a printing system by using a large
number of smaller polygon facets. A Moores law
type assessment reveals that print resolution for
office printers is doubling about every 12 years andthat 1200 dpi is upon us. A blue laser print engine
operating at a resolution 1200 dpi enables offset
quality xerographic printing at high speeds pro-
vided that a photoreceptor with suitable spectral
sensitivity becomes available commercially.
IIIV compound semiconductor materials for
semiconductor diode lasers are prepared by metal
organic chemical vapor deposition (MOCVD). In
this technique the group 3 species (aluminum,
gallium, indium) are transported into a growth
chamber as the vapors of organo-metallic precur-
sors e.g. trimethylgallium. The group 5 species,
however, are transported as hydrides for example
arsine, phosphine, or ammonia. The development
of this technique was a triumph of surface science
in the 1970s [19,20].
Laser diodes are fabricated from materials of
exceptional structural and optoelectronic perfec-
tion. Structural defects (such as dislocations) or
impurities can seriously degrade the luminescence
efficiency of semiconductor materials. Thus, the
materials from which laser diodes are made start
with single crystal substrates to ensure that thedeposited films are as structurally perfect as pos-
sible. As indicated in Fig. 12, red and near-infrared
lasers are grown on gallium arsenide single crystal
substrate wafers, fiber optic devices are deposited
over indium phosphate substrates, and nitride
lasers are deposited on sapphire substrates. Nit-ride laser structures differ from infrared or red
laser diodes in a number of important respects. IR
and red lasers, used today in xerographic marking
engines, are grown perfectly lattice matched, on
gallium arsenide substrates [21]. Therefore the
dislocation density is very small, representing the
defect density of the substrate seed crystal. On
the other hand, nitride laser structures are most
often deposited on sapphire substrates, corre-
sponding to a 14% lattice mis-match, which pro-
duces an enormous defect density [22]. Such a
large concentration of defects would render IR
or red emitting materials optically inactive. The
nitrides apparently do not suffer from these dis-
locations, however, a surprising fact that is still
under investigation. The ongoing development
of nitride lasers is a frontier in modern materials
and interface science, which directly impacts
xerographic marking technology [23].
6.2. Microelectromechanical systems (MEMS)
markers
The field of MEMS, deals with micron scale
machining, mechanical functionality, and low
Fig. 12. Schematic diagram of the MOCVD fabricated blue (left-hand panel) and red (right-hand panel) laser diodes for xerographic
exposure systems. The emission of light from the active region is indicated by a green arrow. The different layers of these diode lasers
are indicated. The geometry is simplified relative to that of a production laser. The structures are comprized of epitaxial layers of
single-crystal material. Different active layers are required to generate light in different spectral regions: GaAs for IR and InGaN for
blue. Different substrates are required in order to get sufficiently accurate lattice matching for adequate epitaxial growth.
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manufacturing cost for a large number of identical
parts, as well as enabling high-bandwidth me-
chanical behavior [24]. An example is a thermal
ink jet (TIJ) head that integrates electronics andsensor actuator functionality, and uses polymers
in a large area application. MEMS technology
already has exerted a profound impact on the
printing industry by virtue of its use to generate an
unprecedented cost/performance ratio for TIJ
products that currently dominate the desktop
color-printing market [25].
The two main performance qualifiers for print-
ing products are image quality and print speed.
One approach to increasing print speed is through
an increasing number ejectors per print head,
economically enabled through a high degree of
modularity and integration, including on-chip
addressing logic. In terms of image quality, a high
integration level enables ever-increasing image re-
solution. The implementation of this strategy relies
on MEMS technology to combine fluid flow
pathways, ejector nozzles, fluid reservoirs, heater
elements, MOS power drivers and addressing logic
onto one silicon die. In the Xerox process [26] TIJ
die are fabricated by wafer level bonding of a bulk
micromachined silicon channel wafer to an
MOS heater wafer with an intermediate polyi-mide spacer layer. The wafer bonding is based on a
thermal cure adhesive process. The channel wafer
contains the fluid flow channels and local link
reservoirs, micromachined using wet anisotropic
etching of crystalline silicon. The front face is
subsequently coated with hydrophobic film to
avoid flooding by the ink. The heater wafer is a
MOS wafer that contains 128 polysilicone heater
elements (one per channel), addressing logic, dri-
ver circuitry and power switches. The integration
of these elements with on-board fluidics requiresseveral microelectronic process and device design
tradeoffs.
The adoption of batch fabrication technology
to produce disposable print heads has changed
the rules of the desktop color-printing market,
launching low end printing products onto cost
and performance trajectories that are analogous
to semiconductor industry trajectories. The tech-
nology consists of a unique blend of large-scale
integration (LSI), microfluidics and thermody-
namics. This integration requires design compro-
mises to be made across various energy domains
(e.g., electrical, thermal, mechanical, chemical),and various technologies (e.g., MOS, microma-
chining, ink, and paper). Yet the efficiency of the
batch fabrication techniques is sufficiently high for
disposable print heads to be economically viable.
The design, manufacture and commercial intro-
duction of such disposable heads illustrate well the
complexity of the processes and devices to which
modern applied surface scientists are making major
contributions.
6.3. Sensing and control
Even with all the emphasis on reproducibility of
the process and the materials components, external
disturbances still cause the quality of the printed
output to vary. Variations in temperature, humi-
dity, toner consumption, and media composition
all lead to corresponding variations in the ap-
pearance of the output prints. For monochrome
printing, image quality stabilization was resolved
using simple algorithms and a few sensors. In color
printing the subsystem latitudes that produce
predictable color images are more restricted, andoutput variations much less tolerable. The prod-
ucts of semiconductor processing have been uti-
lized to mitigate this problem in two specific ways.
Firstly, the sensors that are used to detect vari-
ations in the intermediate and final process outputs
require increasing levels of integration. Their
functions include sensing, calibration, networking,
timing, and recently, considerable computation.
The sensor device is self-contained including the
sensing element, drive electronics, communications,
computation, and memory all coresident. Theselevels of integration may be obtained using as-
semblies but are increasingly implemented using
MEMS processing for the sensing element and
compatible CMOS processes for the other com-
ponents. A preliminary example of this type of
integration is the spectrophotometer produced by
MicroParts [27]. It has an optical input and inte-
grates a MEMS fabricated diffraction grating
coupled to a CCD array for spectral discrimination
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and detection. The output of the sensor is an array
of numbers corresponding to the visible spectrum
of the light at the optical input.
Secondly, with inexpensive sensors providingfrequent, accurate, and calibrated readings and the
performance of semiconductor chips constantly
increasing, sophisticated process control algo-
rithms running in standard commercial semicon-
ductor processors become feasible. Ten years ago
with 8 bit devices even elementary arithmetic op-
erations (e.g. floating point division) occupied 10s
of milliseconds (ms). The dwell time for sensed
features on the photoreceptor is less than 5 ms for
even moderate speed printers. Thus, the feature
could pass under the sensor without being detected
because the processor was busy doing division.
Real-time sensing and computation was impos-
sible. Now matrix inversion, Fast Fourier Trans-
form, and other complex computations may be
performed in less than 1 ms and may be inter-
rupted by real-time events in multi-tasking envi-
ronments. This capability, thanks to the results
of Moores law, has been combined with the last
three decades of advances in automatic control
theory to provide both the algorithms and the
computational environment to compute process
adjustments in real time. These adjustments alterthe behavior of the xerographic system to provide
disturbance rejection resulting in consistent, pre-
dictable image quality from a very complex color-
printing system [28].
This trend of using MEMS devices as sensors,
actuators, and mechanically functional units plus
the application of the computational consequences
of Moores law will continue with the result that
more commercial semiconductor devices will be
integrated into future photocopy machines and
printers.
7. Direct marking and the future of xerography
Perhaps the greatest competitive challenge to
xerography in the digital era is its replacement by
smaller, simpler and hence less expensive direct
marking devices. Direct marking involves replacing
the charging, exposure and development subsys-
tems shown in Fig. 1 with a direct marking head
that writes on a consumable such as paper, or an
intermediate belt or drum from which the image is
transferred to the consumable medium. Commer-cially successful direct marking engines have fewer
parts, less weight, smaller footprints, and simpler
architectural design than their xerographic coun-
terparts. This technology also offers a potential
cost advantage of direct marking inks with respect
to xerographic developer, one for achieving liquid
ink/lithographic document appearance and image
quality, and the prospect of easier maintenance
with fewer replacement parts. For the time being,
however, they operate at slower speeds and with
generally lower image quality than xerographic
printers. One approach to improving the image
quality and extending the range of media for both
xerographic and direct marking is by exploring
marking engines in which the marker produces an
image on an intermediate belt or drum from which
it is subsequently transferred and fused to the final
medium in a single transfuse step. In the Oce
printing scheme [29], an intermediate transfer me-
dium can be preheated to the toner melting tem-
perature at the print fuser, and in one belt
revolution the page panel scrolls through the high-
pressure fuser, handing the image off to the con-sumable media (e.g., paper or transparency). In this
technology, the imaging of the toner exiting the
direct marking head is always carried out on the
intermediate transfer medium eliminating substrate
variability (e.g., different papers, transparencies).
After the toner is imaged on the intermediate
transfer medium, it is fused and transferred at the
same time to the substrate. Hence the substrates
can encompass a wide range of paper weights,
finishes, and coatings. This type of subsystem, in
which transfer and fusing are thus combined, is acurrent frontier in xerographic marking. The belt
can be heated because it does not come into contact
with a temperature-sensitive element like a photo-
receptor. An addition, the subsystem can deliver a
very thin layer of a release agent to the belt [30]
before it comes in contact with the imaging mate-
rial (toner), enabling efficient release of the toner
from the intermediate transfer medium, which can
be in the form of an endless belt [29]. Obviously, the
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design of the toner, the intermediate medium and
the transfuse subsystem process depend sensitively
on the surface and interface properties of the toner
and medium, and their dependence on tempera-ture. It is evident, therefore, that the solution of
surface and interface science problems plays a
major role in the design and implementation of
such transfuse subsystems.
Historically, xerography rose to commercial
prominence because of its unique suitability to
make black and white (monochrome) copies via
light-lens exposure. The digital era has funda-
mentally changed its competitive positioning. First,
color, unachievable at adequate image quality with
lightlens exposure, is enabled by the use of digital
scanners and color-correction software. Second,
the use of digital originals in printers and copiers
has obviated the competitive advantages of light-
lens xerography and rendered direct marking
engines competitive with xerographic marking
engines. At the desktop, where acquisition cost is
king, slow speeds are tolerable, and image quality
can be good enough, color TIJ marking directly
on paper have captured the market. In the high
volume, multiple-copy markets (e.g., magazines,
newspapers) lithography has been king for many
years. Thus, color xerography is sandwiched in themiddle, advantaged relative to each by special
features (e.g., speed and image quality relative to
TIJ and short-run cost and variable data capability
relative to lithography) but fighting for its share of
the short-run digital color-printer/copier market.
The winners and losers in this competition will
be determined largely by the speed and efficacy of
the solution of sophisticated surface and interface
problems associated with the toners or inks and
their interactions with both the image bearing
media and the various surfaces with which theycome in contact during the marking process. Long
life, low cost and high image quality require so-
phisticated consumables that reduce the complex-
ity of the marking engine by assuming added
complexity in the materials themselves. In each
market space the race will be won by the marking
technology that yields solutions most quickly to
the shortfalls in areas of high customer value. For
direct marking devices, key issues include image
quality, image permanence, and range of output
media. For xerography they are cost, media lati-
tude, and in some markets image appearance
relative to lithography. For lithography they are
short-run cost, automation of setup and main-tenance of high image quality, and suitability for
office environments. Progress on all of these rests
on surface and interface science because in all
cases the results are dominated by the conse-
quences of the interactions of the ink or toner with
the surfaces that they touch during the course of
the marking process. For direct marking, the ink
media interaction is the key to image quality and
permanence. For xerography, lowering the cost
and improving the reliability of the process re-
quires ever more sophisticated multi-component
photoreceptor, toner, carrier and fuser materials
packages. For lithography, process control via
sensors and electronics and effluent control are
key issues. All of these are dominated by media-
materials interactions driven by surface and in-
terface phenomena. Thus, we can confidently
anticipate that in the future, as in the past [1,2,
610], the solution of challenging applied prob-
lems in surface and interface science by talented
and creative surface scientists, will enable the in-
exorable march of color-marking engines, xero-
graphic as well as others, to better, faster, cheaper.
8. Synopsis
The amazing development of the xerographic
copier and printer industries since the 1950s has
been a direct consequence of sustained investment
in basic research underlying the Xerographic pro-
cess and associated materials [3]. It resulted from
the diligent, systematic application of physics,
chemistry and surface science, leading to wealth forinvestors and excellent careers, sometimes even
recognition and fame, for the scientists and engi-
neers who pioneered the major advances. The ad-
vent of the digital, networked era has changed
the calculus relative to the stand-alone lightlens
copier era (195585), rendering other marking
technologies more competitive in certain market
segments. Commercial success in this new era de-
pends even more upon exploitation of the fruits
of research in surface and interface science, than
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in the copier era. There is a continuing big payoff
for applications of surface and interface science in
the printer/copier industry and a generous supply
of as yet unsolved problems to challenge the nextgeneration of creative applied scientists and engi-
neers.
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