Thedesignofcolortonerspart1 by Recycling Times Media Corporation

THE DESIGN OF

COLOR

TONERS AN INTRODUCTION Dr. Edul N. Dalal, Independent Consultant

COLOR TONER

Electrophotography (also known as xerography) is a dry photocopying process invented by Chester Carlson in 1938. The process was commercialized by the Haloid Company, which later became Xerox Corporation. The Xerox 914, launched in 1959, was the first automatic photocopier. Xerox later developed a printing technology based on the original photocopying process by replacing the optical system with a laser imager, and launched the Xerox 9700 laser printer in 1977. More recently, LED imagers have been introduced to replace the scanning laser beam with a fixed, page-wide array of light emitting diodes. Color printers eventually followed the original monochrome printers. There are two very different types of color printing systems: Process Color Printing, wherein a wide range of colors is produced from a relatively small set of primary color toners; and Spot Color Printing, wherein each output color corresponds to one of the color toners. Spot color systems have a limited range of application, such as rendering specific company logo colors in documents. Toner is the dry ink, in fine powder form, used in electrophotographic printing. The majority of process color printing systems use a set of four subtractive primary color toners—cyan, magenta, yellow and black—although systems have been developed with from three to six or more primaries. The color gamut—the

This article is the first in a multipart series meant to serve as a Tutorial on color toners with its primary focus on colorimetric design. Part 1 is an introduction to electrophotography. Subsequent Parts review relevant color science topics and discuss materials, manufacturing, functional requirements and design parameters.

COLOR TONER

Work Function (V)

Material

3.4

Polyvinylpyridine

3.7

Nylon

4.0

PMMA, Epoxy

4.8

Polyester

5.0

Teflon (PTFE)

range of colors that can be produced by the printerâ&#x20AC;&#x201D;depends critically on selection of the primary toners. Toner Charging In order for the electrophotographic printing process to work, the toner has to be capable of being charged to a desired level. Moreover, this charge has to satisfy many requirements for uniformity across particles, stability with age and environmental conditions, etc. Various methods of charging toner will be briefly reviewed in this section, while the charge requirements will be discussed in a future article in this Series. Most toners are charged triboelectrically although other methods exist, such as inductive charging. Triboelectric charging occurs when dissimilar materials come into contact. The sign and magnitude of the resulting charge can be estimated from the Triboelectric Series (Table 1), which is a ranking of various materials according to their work function. When two dissimilar materials are rubbed together, the one with the lower work function charges positively relative to the other material, and vice versa. Thus, nitrogen-containing polymers, such as Nylon, charge positively relative to halogen-containing polymers, such as Teflon. Two-component charging systems utilize a carrier, which is a coarse powder. The carrier is mixed with toner to charge it, and

Table 1: Triboelectric series, showing work function (V) of some common polymers. Materials at the top of the series (low work function) charge positively relative to materials further down.

typically contains magnetic particles, such as iron beads, coated with an appropriate material for triboelectric charging. The magnetic property allows the carrier to be held back magnetically while the toner is developed under the influence of an electric field. Carrier bead diameters are typically an order of magnitude larger than toner particle diameters. Single-component systems do not use carriers. Instead, the toner is charged by induction, or by rubbing it between a blade and a donor roll. Six Basic Steps The electrophotographic printing process consists of six basic steps (Fig. 1): 1. Charging the photoreceptor to produce a uniform electrostatic charge. The photoreceptor consists of a conductive layer coated with a photoconductive material such as selenium, or an organic film containing photoconductive pigments such as phthalocyanines or squaraines. The photoreceptor is typically charged using a high-voltage corona charging device, such as a corotron or scorotron. 2. Exposing the photoreceptor to produce the latent electrostatic image. This is typically done by laser or LED illumination. The photoreceptor is normally an insulator, but where exposed to light it becomes

conductive, thereby dissipating the charge to a ground plane. Foreground image areas may be discharged while background areas are left charged, or vice versa. 3. Developing the latent image with charged toner. The toner attaches only to appropriately charged areas of the photoreceptor and is repelled by areas of the same-sign charge as the toner. There are several methods of applying toner to the charged photoreceptor. Mag brush development involves a toner-carrier mixture held on a donor roll by magnetic fields to form a "brush"; this has the advantage of bringing the toner into contact with the charged latent image for ease of development, but tends to scratch the developed image. Jumping development requires the toner to jump across a gap to the charged latent image using high voltage electrical fields; this avoids problems with the mag brush, but tends to produce allor-nothing development due to an avalanche effect, making it difficult to control. Hybrid development systems (HJD, HSD) attempt to avoid these problems by using a high-frequency AC field to dislodge toner from the donor roll, together with a DC field for development. 4. Transferring the developed image to a substrate, such as paper. This

COLOR TONER

Dr. Edul N. Dalal has over 30 years of experience in corporate research at Xerox Corporation, where he held several engineering, management and strategic leadership positions. He Fig. 1: Schematic representation of the electrophotographic printing process

retired from Xerox at the end of 2013, having served as Technical Manager, Principal Scientist, and Research

is done by applying a bias voltage which drives the toner away from the photoreceptor and toward the paper. Transfer may be done directly from photoreceptor to paper, or via a drum or belt. The intermediate member is elastic and provides more uniform contact and therefore better transfer, but at the cost of two transfer steps instead of one. 5. Fusing the toner image onto the substrate to make it permanent. Fusers use heat to soften the toner and cause it to adhere to the substrate, either by contact with a heated roller, or by non-contact radiation heaters. Roll fusers are sometimes replaced with belt fusers to get a longer dwell time. Contact fuser design has to address issues such as, poor release (i.e., toner sticking to the roll/belt surface) and fuser roll/belt life reduction at high temperature. Non-contact radiant or flash fusers avoid these problems, but place challenging demands on toner design, which currently cannot be fully met. 6. Cleaning the photoreceptor in preparation for the next cycle. Incomplete cleaning leads to dirty background areas in the printed image. Also, in color printing, toner supply of one color can get contaminated with toner of other colors. An electric field alone is inadequate for cleaning since

uncharged or wrong-sign toner is often present. Consequently mechanical assistance is usually utilized, such as with a rubber scraper blade or counter-rotating fur brush roll. After cleaning, the photoreceptor is ready to be charged again, and the cycle is repeated as needed. Color Electrophotography In the case of color printing, the cycle is repeated four times for each image, with cyan, magenta, yellow and black toners. It can be done sequentially on a single photoreceptor, but this requires four passes thereby slowing the color system to a quarter of the speed of an equivalent monochrome system. Numerous single-pass permutations have been developed for overcoming this speed limitation. In one embodiment, the cycle is repeated near-simultaneously on four separate photoreceptors. The four toned images are then transferred onto the substrate, either directly or via an intermediate belt. In an alternative embodiment, known as image-on-image, a single photoreceptor is used with four exposure and development stations. This involves developing the four color images on top of each other on the photoreceptor, and requires the use of hybrid scavengeless development (HSD), instead of mag brush or jumping development, to avoid disturbing the image. â&#x2013;

Fellow. He has extensive experience in many aspects of electrophotographic color printing, with a special focus on color science and image quality. His areas of specialization include: design and formulation of color toners; gloss and color specification and tolerances; and instrumental, perceptual and preference aspects of image quality quantification and optimization. Dr. Dalal has focused on research, innovation and intellectual property throughout his career, with 84 U.S. patents issued and 26 additional patent applications pending, as well as over 40 technical publications. Dr. Dalal is available for consultation in the areas of his expertise. Dr. Dalal can be contacted by email at: edalal.tech@gmail.com