cbduke ss 500 1005 2002 the surface science of xerography

8/3/2019 CBDuke SS 500 1005 2002 the Surface Science of Xerography

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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.

P II: S0 0 3 9 -6 0 2 8 (0 1 )0 1 5 2 7 -8


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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].

1006 C.B. Duke et al. / Surface Science 500 (2002) 10051023


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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.

C.B. Duke et al. / Surface Science 500 (2002) 10051023 1007


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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.



10/19

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.



11/19

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).



12/19

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



13/19

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.



14/19

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]).



15/19

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.



16/19

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



17/19

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



18/19

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



19/19

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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[2] D.M. Pai, B.E. Springett, Physics of electrophotography,

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[3] J.S. Weston (Ed.), Xerographic Process Research, Amer-

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[4] J.E. Shane, USA and European Marking Materials Fore-

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[5] Science and Engineering Indicators, National Science

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[6] L.B. Schein, Electrophotography and Development Phys-

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