Pira printing guide series
`
`MARK BOHAN
`
`PLEASE DO NOT REMOVE
`
`FAST FELT 2012, pg. 1
`Owens Corning v. Fast Felt
`IPR2015-00650
`
`

`
`A Pira International printing guide
`
`Printing Materials: Science
`and Technology
`
`Bob Thompson
`
`Pira International
`Randalls Road
`Leatherhead
`Surrey
`KT22 7RU
`UK
`Tel: (+44) (0) 1372 802080
`Fax: (+44) (0) 1372 802079
`E-mail: publications@pira.co.uk
`http://www.pira.co.uk/
`
`FAST FELT 2012, pg. 2
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`

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`The facts set out in this publication are obtained from sources which we believe to be
`reliable. However, we accept no legal liability of any kind for the publication contents,
`nor for the information contained therein, nor conclusions drawn by any part3’ from it.
`
`No part of this publication may be reproduced, stored in a retrieval system, or transmit-
`ted, in an3’ form or by any means, electronic, mechanical, photocopying, recording or
`otherwise without the prior permission of the Copyright owner.
`
`Copyright Pira International 1998
`
`ISBNs 1 85802 150 2 (hardback)
`1 85802 168 5 (paperback)
`
`Typeset in the UK by Pantek Arts, Maidstone, Kent
`Printed and bound in the UK by Redwood Books, Trowbridge, Wiltshire
`
`To Chris and Children.
`Thank you for your patience and tolerance
`
`FAST FELT 2012, pg. 3
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`

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`Contents
`
`List of Tables
`List of Figures
`Acknowledgements
`
`Principles and concepts
`
`Chapter 1 Atoms and molectfles
`Some basic ideas and concepts
`Atoms and elements
`Molecules and compounds
`Atomic structure and quantum theory
`Atomic structure
`Atomic number
`Atomic mass
`Isotopes
`Isotopic abundance and relative atomic mass
`Bohr atom
`Energy levels within the atom
`Quantum theory of the atom
`Light and wave-particle duality
`Wave behaviour
`Particle behaviour -- quantum effects
`Excitation of electrons
`Energy levels and electron orbits
`Further quantum concepts
`Molecular excitation
`Excitation of molecules
`Band theory of solids
`Extrinsic semiconductors
`Diodes
`Transistors
`Charge coupled devices
`
`Chapter 2 Applications of quantum effects in printing
`Light sources and optoelectxonic devices
`Production of light
`Lasers
`Laser applications
`General principles of laser action
`Three-level laser
`Four-level laser
`Solid-state lasers
`Gas lasers
`Chemical and excimer lasers
`Dye lasers -- tunable lasers
`Light emitting diodes and semiconductor lasers
`Photoconduction
`Luminescence, fluorescence and phosphorescence
`Singlet and triplet states
`Phosphorescence and the triplet state
`
`xiv
`xv
`xxiii
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`3
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`Printing materials: science and tedmology
`
`Contents
`
`Phosphorescent inks and postal coding
`Luminescent materials
`Television monitors
`Electrochromic materials
`Photoemission and photoelectric cells
`Photoelectric cell
`Photomultiplier
`Photoconductive cell
`Photovoltaic or barrier layer cell
`Applications
`Charge coupled device
`
`Chapter 3 Chemical bonding
`Quantum numbers
`Orbital or magnetic quantum number M
`Spin quantum number S
`Energy levels in the atom
`Periodicity and the Periodic Table of the elements
`Electron configurations
`Chemical bonding: chemical reactivity and valency
`Valence
`Types of chemical reaction
`Covalent bonding
`Molecular orbitals
`Nonbonding orbitals
`Antibonding orbitals
`Physical bonds
`Hydrogen bonding
`Dipole-induced-dipole interactions
`
`Chapter 4 Organic chemistry and printing
`Introduction
`Hybridization of atomic orbitals
`sp3 hybridization of carbon
`sp2 hybridization of carbon
`sp hybridization of carbon
`Electrons in delocalized orbitals -- conjugation
`Benzene ring
`Aliphatic hydrocarbons and their derivatives
`Hydrocarbons
`Structural isomers
`Chain isomers
`Derivatives of the aliphatic hydrocarbons
`Functional groups
`Homologous series of aliphatic alcohols
`Homologous series of carboxylic acids
`Structural isomers
`Polyhydric substitution
`Esters
`Aromatic hydrocarbons and their derivatives
`Substitution products
`Condensed (fused) aromatic hydrocarbons
`Diazotization, coupling and the synthesis of dyes and pigments
`
`58
`60
`61
`63
`64
`64
`64
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`69
`69
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`80
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`92
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`94
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`96
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`100
`101
`101
`101
`104
`104
`106
`106
`106
`106
`106
`108
`109
`110
`111
`114
`115
`
`Chapter 5 Polymers and their applications in printing
`Introduction
`Definitions
`Polymerization process
`Thermoplastic polymers
`Thermosetting polymers
`Polymerization processes
`Addition polymerization
`Examples of addition polymers
`Condensation polymers
`Metallized films
`Polyisocyanates and polyurethanes
`Packaging materials requirements
`Coefficient of friction
`Sealability of materials
`Surface treatment
`Barrier properties
`General physical properties
`Photopolymers
`
`Chapter 6 Basic optics for printing
`Introduction
`Types of wave
`Velocity, wavelength, frequency and amplitude
`Electromagnetic waves
`Reflection and refraction
`Reflection
`Refraction
`Optical fibres and light guides
`Prisms and lenses
`Prisms
`Lenses
`Interference and diffraction of light
`Diffraction gratings
`Iridescence
`Light scattering
`Measurement of light
`Luminous efficacy
`Radiometric and photometric quantities
`Characterizing light sources
`Radiant and luminous intensities
`Luminous intensity profiles of light sources
`Inverse square law
`Illumination levels for printing applications
`
`Chapter 7 Miscellaneous topics
`Physical concepts
`Units
`Mass, weight and force
`Energ3b work and power
`Temperature
`Surface effects
`Surface tension and surface energy
`
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`132
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`wii
`
`Printing materials: science and technolog~v
`
`Surface energy considerations in printing processes
`The lithographic principle
`Acids, alkalis and pH
`Strong and ~veak acids and alkalis
`Measuring acidity and alkalinity
`
`Substrates
`
`Chapter 8 Paper manufacture
`Introduction
`Paper fibres and pulps
`Paper selection
`Visual appeal
`Fibre structure
`Cellulose, hemicelluloses and lignin
`Paper manufacture
`Stage 1 -- pulp preparation
`Chemical processes
`Bleaching
`Stage 2 -- stock preparation
`Stage 3 -- beating or refining the pulp
`Paper manufacturing process
`Wet-end
`Wire section
`Press and drier sections
`Calendering and finishing
`Calendering
`Supercalendering
`Machine glazing (MG)
`Paper coatings (coated art papers and boards)
`
`Chapter 9 Recycled paper
`Introduction
`Recycling process
`Furnish
`Fibre preparation
`Screening
`Centrifugal cleaning
`Flotation
`Washing
`Deinking plant function
`Deinking chemistry
`Pulper
`Pulper chemicals
`Bleaches
`Chelating agents
`Sodium silicate
`Agglomerating chemicals
`Surfactants
`Dispersants and the principles of wash deinking
`Collector chemicals and flotation
`Displectors
`Difficult to deink papers
`
`179
`181
`181
`182
`183
`
`193
`193
`194
`196
`197
`198
`198
`201
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`207
`212
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`228
`228
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`235
`235
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`238
`240
`240
`240
`244
`245
`246
`247
`248
`250
`251
`252
`252
`
`Clarification
`Stickles
`
`Chapter 10 Paper properties and printing problems
`Introduction
`Surface and directional properties of paper and board
`Substance, caliper and bulk
`Compressibility
`Surface smoothness/roughness
`Air permeance (porosity)
`Static and dynamic friction
`Absorbency
`Surface strength and internal bond strength
`Picking
`Fluffing
`Splitting
`Testing techniques
`Strength properties
`Stiffness
`Folding endurance
`Burst strength
`Tear resistance
`Optical properties
`Gloss
`Brightness
`Whiteness, yellowness and tint indices
`Fluorescence
`Opacit3’
`
`253
`253
`
`257
`257
`258
`258
`260
`260
`264
`265
`265
`268
`269
`269
`270
`270
`270
`271
`276
`277
`278
`278
`279
`280
`282
`283
`283
`
`Chapter 11 Influences of moisture and relative humidity on paper and board 285
`Relative humidity and moisture
`285
`Hygroscopic, deliquescent and efflorescent substances
`285
`Relative humidity
`286
`Effect of relative humidity on paper and board properties
`287
`Moisture content
`289
`Measurement and testing of relative humidity and moisture content
`293
`Relative humidity
`293
`Moisture content
`294
`Temperature
`295
`
`Chapter 12 Adhesion and adhesives
`Introduction
`Principles of adhesives
`Operating parameters for adhesives
`Open time
`Wet tack
`Compression
`Solidification
`Types of adhesive
`Drying adhesives
`Hot melt adhesives
`Curing adhesives
`Radiation curing
`Ultraviolet curing pressure-sensitive adhesives
`
`297
`297
`297
`300
`300
`300
`300
`301
`301
`301
`302
`304
`305
`305
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`FAST FELT 2012, pg. 6
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`Pm~tttng matertals: sctence attd technology
`
`Cotttent~
`
`Styrenic block copolymers
`Comparison of the stages involved in bond formation
`Adhesive classes and properties
`Acrylics
`Animal glues
`Casein
`Starch
`Dextrins
`Pregelatinized starch
`Ethylene-vinyl acetate copolymer hot melts
`Polyamide hot melts
`Polyester hot melts
`Rosin hot melts
`Natural rubber-latex adhesives
`Polyurethanes
`Polyvinyl acetate (PVA)
`Polyvinyl alcohol
`Polyvinylidene chloride
`SBS and SIS block copolymers
`Styrene-butadiene rubber
`Vinyl acetate copolymers
`Vinyl acetate-ethylene copol3 mers for liquid applications
`Theories of adhesion
`Mechanical adhesion
`Chemical adhesion
`Theories of chemical adhesion
`Contact angle and wettability
`Surface modification
`
`Inks and coatings
`
`Chapter 13 Printing inks -- the basics
`Introduction
`Pigments
`Types of pigments
`Vehicles
`Printing processes
`Ink requirements for printing processes
`Ink vehicles and drying mechanisms
`Vehicles for liquid inks
`Vehicles for paste inks
`Resins
`UV curing vehicles
`Drying mechanisms
`Physical drying mechanisms
`Chemical drying systems
`Radiation drying and curing
`Optical properties of ink films
`Gloss and opacity
`
`Chapter 14 Rheology and ink transfer requirements
`Introduction
`Viscosity
`
`307
`308
`309
`309
`310
`311
`311
`311
`312
`312
`312
`312
`313
`313
`313
`314
`314
`314
`315
`315
`315
`315
`315
`316
`316
`317
`319
`320
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`325
`325
`325
`326
`32"7
`328
`329
`33~
`338
`338
`342
`343
`344
`344
`34~
`348
`351
`354
`
`356
`356
`358
`
`Newtonian flow
`Units of viscosity
`Non-Newtonian behaviour
`Measurement of non-Newtonian flow
`Ink tack
`Wet-on-wet printing
`Ink distribution and transfer on the press
`
`Chapter 15 Organic pigment technology
`Introduction
`Pigments for multicolour printing
`Azo pigments
`Physical characteristics of organic pigments
`Pigment crystallinity
`Effects of anistropy on pigment crystals
`Surface polarity and pigment dispersion
`Effect of particle size on print quality factors
`Flocculation of pigment dispersions
`
`Chapter 16 Chemistry and physics ofcolour
`Types of colourants
`Dyes
`Dye classes
`Pigments
`Lakes
`Colour Index
`What gives colourants their colour?
`Colour vision
`Selective absorption of light to give reflected colour
`Colour reproduction
`Absorption of electromagnetic radiation by molecules
`How materials modify light
`Absorption of electromagnetic radiation by molecules
`Principles of spectroscopy and densitometry
`Spectroscopy
`Colour chemistry
`Mechanisms of colour in organic molecules
`
`Chapter 17 Measurement and control of the colour of inks and ink films
`Introduction
`Densitometry
`Polarizing filters
`Ink film thicknesses
`Measurement and control of ink colour
`Dot gain
`Colour and colour difference measurement
`Tristimulus colorimeter
`Spectrophotometer
`Colour difference
`Colour space equations
`
`Chapter 18 Radiation curing and water-based ink systems
`Introduction
`Radiation curing inks and varnishes
`
`358
`360
`362
`365
`366
`367
`368
`
`371
`371
`371
`372
`374
`374
`375
`376
`376
`379
`
`381
`381
`381
`382
`385
`386
`386
`388
`388
`390
`391
`392
`394
`398
`400
`401
`403
`404
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`410
`410
`411
`413
`414
`415
`418
`420
`421
`422
`428
`431
`
`432
`432
`432
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`FAST FELT 2012, pg. 7
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`

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`Organic photopolymers
`Non fight-sensitive systems
`Thermal imaging systems
`Thermal cross-linking plates
`Optical devices: imagesetters and platesetters
`External drum scanning
`Internal drum scanner
`Flatbed scanning (flat field)
`
`Chapter 22 Photographic systems
`Introduction
`Requirements for a light-sensitive system
`Silver halide emulsions
`Stages in producing a photographic image
`Emulsion variables and sensitometry
`Sensitometry and the characteristic curve
`Contrast or gamma of the emulsion
`Colour film
`Photographic systems for prepress
`Lith process
`Rapid access
`Nonsilver graphic arts film
`VSX film
`V-IX film
`Environmental management of waste solutions
`
`Appendix I: List of chemical elements
`
`Appendix 2: Periodic Table of the elements
`
`Appendix 3: Further reading
`
`Index
`
`x~
`
`501
`503
`503
`506
`509
`509
`509
`509
`
`511
`511
`511
`512
`513
`517
`517
`519
`524
`526
`527
`533
`535
`536
`537
`538
`
`541
`
`542
`
`543
`
`551
`
`xff
`
`Printing materials: sczence attd technology
`
`Ink cure considerations
`Chemistry of UV curing
`Cationic curing
`Electron beam curing
`Water-based inks
`Impervious substrates
`Packaging applications
`
`Imaging Systems
`
`Chapter 19 Digital imaging techniques and materials
`Introduction
`Digital proofing
`Digital printing
`Digital presses
`AGFA Chromapress
`Xeicon Press
`Indigo E Print 1000 Press
`Scitex Spontane
`Heidelberg Quickmaster DI-46-4 Press
`Digital photography
`Toner printing systems
`Electrophotography
`Toners
`Other toner-based printing systems
`Ink jet printing
`Ink jet technologies
`Thermal transfer printing
`Thermal wax transfer
`Dye sublimation printing
`Dye diffusion thermal transfer
`Laser ablative technology
`
`Chapter 20 Photopolymer plates
`Introduction
`Negative and positive working plates
`Lithographic plates
`Positive working plates
`Negative working plates
`Flexographic plates
`Plate exposure
`Processing
`Plates for waterless offset
`Plate types
`Plate exposure and processing
`
`Chapter 21 New plate technologies -- materials for digital platemaking
`systems
`Computer to plate and computer to press systems
`Types of platemaking systems
`Energy considerations
`Silver halide imaging media
`Electrophotographic plates
`
`433
`436
`441
`442
`442
`443
`447
`
`451
`451
`451
`452
`452
`453
`453
`453
`453
`453
`454
`455
`455
`459
`460
`462
`462
`467
`467
`468
`469
`472
`
`474
`474
`475
`476
`477
`478
`481
`484
`484
`485
`487
`489
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`491
`491
`493
`493
`494
`499
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`

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`19
`Digital imaging techniques
`and materials
`
`Introduction
`
`The science and technolog3, surrounding the materials used in digital systems are, gener-
`ally speaking, not new. Electrophotography has been used in photocopiers since the
`1940s (Xerox). Ink jet and thermal transfer printing have been used to code mail by the
`British Post Office since the 1960s, and dye sublimation has been used to print T shirts
`from screen printed transfers, for some years.
`What is new is the introduction of digital control systems that have extended the use-
`fulness and versatility of applications of the materials in both existing and new printing
`technologies. The driving force behind the development of these materials came from
`the prepress end of the industry where demands for quicker, cheaper origination and
`colour proofing systems were made.
`The need to convert soft proofs from a colour monitor into hard copy that might be
`acceptable as a contract proof has been one influence. The advent of digital photogra-
`phy, where images are captured and manipulated electronically before being printed as
`hard cop3; has been another. Third, digital printing is now hailed as a serious prospective
`contender to the conventional printing processes. If all three are put together, the print-
`ing process, from the capture of live or still originals, proofing and printing can all be
`done electronically without the need for conventional film, plate, proof or printing press.
`Before discussing the processes and their materials requirements, it would be useful to
`look at the applications a little more closely.
`
`Digital proofing
`
`Digital proofing systems have become a very convenient way of converting computer-
`manipulated images into hard copy without the need for intermediate films. For example,
`DuPont have introduced the Digital CromalinX~l alongside their highly successful and long
`established analogue system. The analogue CromalinrM has established a reputation as a
`reliable contract proof and any innovations in the digital field will have at least to match it
`to gain credibility.
`The requirements for digital systems are the same as for any other: consistency from
`one proof to the next; reliability of the proofing engine; compatibility with existing
`proofing systems; ease of process control and simplicity of effective colour calibration;
`image quality acceptable to modem screening technolog3,; inks with the same CMYK
`colourspace; last, but not least, cost -- although individual digital proofs are cheap, the
`capital investment is high.
`Digital proofing systems make use of various imaging technologies that are
`described more fully, shortly. Of these, Agfa, QMSrM, Seiko and 3M produce sys-
`tems that use thermal transfer. Canon produce a laser system in which dry toner is
`
`45t
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`
`454
`
`Pnnttng materials: sclcn~’e arid technology
`
`Digital imaging techniques arid materials
`
`455
`
`option looks economically more attractive for longer runs. Ho\vever, toner-based and ink
`jet presses have the advantage of variable imaging. Since there is no fixed printing plate,
`the image can be changed for successive prints by changing the digital information sup-
`plied to the engine from the front end.
`
`photoconductors, may also be associated with certain coloured molecules. Their colour
`is only incidental to the colour of the print. The printing technologies that follow can be
`grouped under the heading of nonimpact printing.
`
`Digital photography
`
`Toner printing systems
`
`Digital photography enables the capture of live or still images electronically. There
`are t~vo ways in which this is currently done. In the first, the scene is scanned in a
`raster fashion similar to a conventional television camera. The light image is turned
`into electrical signals by the camera photocells. The resulting image is one of high
`resolution and quality but, because it is relatively slow to record, it is currently best
`suited to still life scenes. The second method uses an array of charge coupled devices
`(CCDs) \vhich are actually phototransistors that turn light into electrical energy.
`With both technologies the electrical images can then be edited, manipulated and
`montaged to suit the photographers wishes.
`An example of the first type is the Celsius still-life camera. It has a resolution equiva-
`lent to 150 lines on an 8 x 10in2 picture. The scan time is 17s and it can be used with
`high frequency fluorescent or tungsten studio lighting.
`The Celsius 160 is a CCD single shot-camera giving an image equivalent to 150 lines
`on a 4 × 3 in2 picture. The resolution is inevitably poorer than for a scanning system,
`because it is determined by the number of transistors in the CCD array; these are cur-
`rently limited by the technology.
`The single main disadvantage at present is the cost of cameras and ancillary computer
`equipment. The cameras are currently priced at around £20000 which far outstrips that
`of a conventional 35 mm film emulsion camera.
`The advantages of digital photography are, however, potentially great. The costs of
`conventional film, processing, and electronic scanning into colour sets is relatively
`expensive. Several shots are usually required to get it right. With digital photography, the
`first image is the final image because it can be observed and corrected as it is created.
`The scanning is done simultaneously because the colour information is digitized by the
`camera, whichever type is used. This, in turn, reduces running costs and lead times, and
`gives the photographer complete command over creativity by enabling him to manipu-
`late and edit at will. The photographer has control over the complete image-capture
`process. Furthermore, the images can be instantaneously transmitted to customers using
`ISDN for remote art direction and approval.
`A factor of great importance today is that the process is environmentally friendly.
`There is no silver halide film invoh, ed, with its associated disposal problems of spent
`developer and silver-rich fixing solutions.
`The major technologies employed in digital imaging engines are thermal, including
`dye sublimation and dye diffusion thermal transfer (D2T2), ink iet and electrophoto-
`graphic. Because new colourants are expensive to develop, little attempt had been made
`to do so until the current wave of interest in digital printing techniques became signifi-
`cant. Colourants fall into two categories, dyes and pigments, depending upon their mode
`of use.
`Colourants for electronic printers may take the form of either dyes or pigments. Dyes
`may be used in ink iet and thermal transfer systems, whereas pigments are the basis for
`the coloured toners used in electrophotographic printing. There are also occasions when
`coloured molecules are used, not specifically for their colour, but rather for effects that
`coloured molecules may induce. For example, the electrostatic effects exhibited by some
`charge control agents in toners, and the photoconductive effects exhibited in organic
`
`There are five main types of toner printing systems. They can all be computer driven
`and they use optical or electrical techniques in order to form the latent images to which
`toner may be attracted. These are electrophotography, ion deposition, electrostatic, mag-
`netographic and electrographic. The most important and widely used of these is electro-
`photography.
`
`Electrophotography
`
`The subject of electrophotography introduced in Chapter 2 can now be extended to
`laser printers using organic photoconductor systems rather than selenium coated drums.
`The operation of a laser printer is shown in Figure 19.1.
`
`Laser
`exposure
`
`Chargtng
`
`-e,-- Paper / film
`
`Z
`
`(a)
`
`Negative charge
`toner
`
`/
`
`/
`
`Fusing
`system
`
`~-,.i ~JTOner hO’per
`Substrate
`
`~
`~ ~
`[ Transfer
`scototron
`
`1
`
`Charge ...........................
`
`Photoconductor
`
`2
`
`Image w~th laser/LED ...... ~’" ......... "~’" .....
`
`Latent electrostatic ir
`
`3
`
`Develop
`
`_~ _
`¯ ~¯ .....
`
`¯ ............ ¯ ....
`
`4 Transfer toner by paper
`
`¯
`
`¯
`
`5 Fix toner by heat
`fus=on
`
`(b)
`
`Figure 19.1: The principles ofelectrophotography: (a) arrangement for a laser printer working
`in the opposite sense to a white-light photocopier; (b) basic process in electrophotography;
`(c) schematic diagram of the electrophotographic process
`
`The stages of operation shown in Figure 19.1 (c) are as follows:
`
`¯ Stage 1. A uniform electrostatic charge is applied to the photoconducting
`layer on the printing drum.
`
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`Stage 2. This is the image step in which a laser or LED, driven by an imaging
`computer, writes the digitized light information onto the photocon-
`ductor. The incident light renders the photoconductor conducting
`and charge corresponding to the shape of the light image leaks away,
`leaving a latent electrostatic image of the background area and an
`uncharged image area.
`
`¯ Stage 3. This is the development stage in which toner particles given the same
`triboelectric (i.e. induced electrostatic) charge as the background are
`repelled into uncharged image areas on the drum.
`
`¯ Stage 4. The toner is transferred onto the final substrate. A second corona or
`scorotron is required to give the substrate an opposite charge to that
`of the toner in order to assist its transfer from the OPC drum.
`
`¯ Stage 5. The toner is heat fused onto the substrate to give the visible image.
`
`The key ingredients of electrophotographic systems are the organic photoconductor
`coated onto the exposure drum, and the toner itself.
`
`Organic photoconductor (OPC)
`Organic photoconductors have now replaced the semiconducting selenium materials for
`many applications, although the new Mitsubishi digital press uses amorphous selenium
`rather than OPC.
`The organic photoconductor is a dual layer device having a thin charge generat-
`ing layer (CGL) on top of which is coated a thicker charge transport layer (CTL)
`(Figure 19.2). This dual layer is deposited onto an aluminized polyester substrate
`that is earthed.
`Light from the imaging source passes through the electrostatically charged surface of
`the charge transport layer and strikes the charge generating layer. The charge generat-
`ing layer contains a pigment that absorbs light and photodissociates to produce an ion-
`pair complex, thus generating a positive hole P+ and an electron e-. The electron
`escapes to earth through the aluminized backing and the positive hole migrates to the
`CGL/CTL interface. The successful transportation of the holes relies on a high degree
`of crystallinity of the pigment aggregates. Crystal defects serve to trap the positive holes
`and stop the process.
`The charge transport layer (CTL) contains an electron-rich compound. This is a
`donor compound that will readily release and donate electrons to a positive hole. The pos-
`itive hole is thus neutralized but the donor molecule D in turn becomes a positive hole.
`
`D->D+ +e-
`
`P++e-
`
`>P
`
`overall
`
`D + P+
`
`> D+
`
`The next donor molecule in the layer above then donates an electron to D+, then the
`next one above and so on:
`
`DI+ + D2
`
`> D1 + D2+
`
`D2+ + D3
`
`> D2 + D3+ and so on
`
`Electron
`layer"k bght
`
`CC
`
`i
`
`~
`P+
`
`~
`e-,
`
`Ion-pa~r
`complex
`
`Charge transport layer
`
`Charge generation layer
`
`Figure 19.2: Mode of action of an organic photoconductor
`
`This progressive hopping mechanism transports
`the hole up through the charge transport layer until
`it reaches the surface of the CTL which is covered
`with electrons. The positive hole then combines
`with a surface electron, leaving a neutralized area
`where the photon of light first entered the OPC. A
`latent charge image is thus formed which can subse-
`quently be fixed and made visible by the application
`of a charged toner medium. A further consideration
`of the charge generation and charge transport mate-
`rials can now be made.
`
`Charge generation materials
`Molecules used in the charge generation layer must
`be capable of absorbing light that falls onto them and
`then, by dissociating into an ion-pair complex, pro-
`duce electrical charges proportional to the amount of
`incident light. The absorption frequencies for the
`charge generating material (CGM) will depend upon
`the type of device in which it is being used. For white
`light copiers the material must respond to wave-
`lengths between 400 and 700nm, in order to be able
`to record different coloured originals. The pigments
`used in the CGM are therefore necessarily coloured
`and Figure 19.3 (a) shows the absorption characteris-
`tics for the pigment dibromoanthanthrone used in
`white light copiers. It absorbs right across the spec-
`trum, making it suitable for recording all colours of
`the spectrum. Since its absorption values peak in the
`red regions, its colour appearance is one of a green-
`yellow pigment. Figure 19.3(b) shows the molecular
`structure for dibromoanthanthrone which clearly
`demonstrates the extensively delocalized n system
`
`400
`
`500 600 700
`Wavelength (nm)
`
`(a)
`
`0
`
`Br
`
`Br
`
`(b)
`
`Figure 19.3: Spectral sensitivity
`range of charge generation materials
`for white light copiers: (a) absorption
`characteristic for pigments for a white
`light copier; (b) molecular structure of
`the pigment dibromoanthanthrone
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`400
`
`700 800 nm
`
`associated with the coloured molecules dis-
`cussed in Chapter 16.
`In laser printers, digital proofing and press
`systems, the images are written onto the
`OPC using red or near infrared (800nm)
`lasers or LEDs. The pigment absorption
`characteristics required for these are shown
`in Figure 19.4 together with the chemical
`structure of a typical cyan coloured pigment,
`the u-crystalline form of copper-free phtha-
`locyanine. (There are txvo crystalline forms
`of cyan pigments, the hue varying from the
`red shade of the a-crystalline form to the
`green shade of the [3-crystalline form. The
`hues of the 458copper-free types are gener-
`ally greener (Chapter 15).
`For the CGM pigments to be effective in
`generating charge, they must be highly crys-
`talline and pure to prevent trapping of the
`positive holes on their way to the CTL
`interface. Their particle size range should be
`well defined, giving uniformly sized parti-
`cles of very small size to ensure high resolu-
`tion of the electrostatic image.
`
`Figure 19.4: Spectxal sensitivity range for
`charge generating materials for laser imaging:
`(a) absorption specmam for IR laser/LED
`CGM imaging pigment; Co) ~x-crystalline
`form of copper free phthalocyanine pigment in
`which the copper atom has been replaced by
`two hydrogen atoms
`
`Charge transport materials
`The purpose of the charge transport mater-
`ial (CTM) is to transport positive holes
`from the CTL, where they are generated by
`incident light, to the surface of the OPC
`layer where they neutralize electrons. The
`molecules for this task must be very willing
`to donate electrons to the arriving positive
`holes. They must therefore have low ioniza-
`tion potentials, which means that they
`are easily oxidized by losing electrons. Fur-
`ther, if they are rich in electrons capable of
`undergoing transfer, the efficiency of the
`donor molecules will be high. Typically, such
`electron-rich molecules will be conjugated
`molecules, such as those having a large
`number of benzene rings. They will also
`contain groups that are known to be power-
`ful electron donors, such as the alkyl and
`arylamino groups. The role of chemistry
`has been to identify the chemical types
`likely to offer these qualities. They can be
`narrowed down to oxadiazoles, pyrazolines,
`leucotriphenylmethanes, hydrazones and
`arylamines; the last two are probably the
`most important.
`The structure for leucotriphenylmethane is shown in Figure 19.5. The leuco com-
`pounds are the basis for an important group of dyes and pigments in their own rights.
`
`G2HsNH\ CH] H3
`
`CH
`
`Figure 19.5: Leucotriphenylrnethane
`
`Leuco dyes are used in carbonless office papers. The coloured image is generated by a
`pressure-induced chemical reaction between the leuco dye, enclosed in microcapsules
`40-80 ~-n in diameter, and an acidic developer. The leuco compound is colourless but is
`readily oxidized (giving up electrons) into a coloured dye form.
`An important consideration with all pigment and dye systems is that they should be
`toxicologically safe. Unfortunately, aromatic compounds are often carcinogenic and
`cannot be used. This places a restriction on the choice of chemical types at our disposal.
`
`Toners consist essentially of pigment (soot or colourant) dispersed in thermoplastic res-
`ins. Unlike conventional inks, toner resins exhibit a glass transition temperature of
`around 65-70°C, at which point they soften and flow. The thermoplastic resins are typi-
`cally styrene-acrylate copolymers.
`The complete toner system contains two types of molecule, the first of which is obvi-
`ously the colourant which may be black, cyan, yellow or magenta. The second is the
`charge control agent (CCA) which has the function of imparting and controlling the
`electrostatic (triboelectric) charge necessary to direct the toner particles to the image
`areas on the photoconductive drum (Figure 19.2).
`Toner particle sizes dictate the resolution of detail that can be printed; the smaller the
`particles, the higher the resolution attainable. For dry toners the minimum size that can
`be handl