Photocopying and laser printing have become commonplace, but if the information being copied or printed is for long-term retention, issues of image stability need to be considered.
This advice explains the technologies and their suitability for creating records for long-term retention.
Xerography, the most common photocopying process, uses the basic principles of static electricity to create an image on a sensitised surface and then transfer it to paper, as described below.
The laser printing process is very similar to the xerographic photocopying process. The major difference is that the exposure step for laser printers employs a laser beam to draw the image onto the sensitised surface to form the latent electrostatic image.
Both single-colour photocopying and single-colour laser printing employ the same technology as dry toner black photocopying and laser printing.
Full-colour photocopying uses the same principles as the equivalent dry toner single-colour xerography technology except that the process is repeated once for each colour – yellow, magenta and cyan, and then black. That is, each colour is separately laid onto the paper.
A typical dry toner is predominantly composed of a heat-sensitive polymer, acrylic or styrene, or both. The next major ingredient is a pigment such as carbon black. Carriers are an active ingredient in a toner. They are used to generate a charge on the toner particles. The carrier is typically an iron or steel powder, which may be coated with resin. The presence of carriers in the image formed on paper may adversely affect the stability and longevity of documents, particularly those created by the liquid process. Dry toners mostly remain on the surface with little penetration of the paper fibres, therefore the effect of carriers is lessened.
Some toners do not contain carriers. These toners, known as single-component toners, employ a magnetised roller to charge the toner particles.
Liquid toners are composed of a suspension of pigmented or dyed resin particles in an insulating liquid. The liquid process involves washing or spraying the electrostatic image with this dispersion.
Liquid toner particles are significantly smaller than dry toner particles and usually contain acrylic resin combined with carbon black.
Images formed from liquid toners penetrate and colour the paper fibres, unlike dry toners, which adhere to the paper surface. Liquid toner images cannot easily be removed from the surface of the paper.
Liquid toner has some limitations. Even though the liquid carrier is fast drying, it does not evaporate instantly. Hence liquid-based systems work at about half the speed of the equivalent dry toner system. Images formed from the liquid process are also prone to fading.
Colour toners are commonly based on organic dyes, which are subject to fading and known to change colour. Xerographed documents that are to be retained long term should be made using dry toners containing pigments of known stability and on archival quality papers.
A recent study showed that various colour toners perform differently under identical irradiation with ultraviolet (UV) light sources. Colour toners that had pigment impregnated within a polyester resin showed better fusion with the page and the printed images were less subject to fading. This is because polyester is a good absorber of UV light. This theory is supported by the finding that photographs placed in polyester sleeves are less subject to fading.
Choice of toner depends largely on its particular characteristics and suitability for different applications. When choosing a toner for the creation of long-term records, degradation of the toner should be considered. Toner materials may degrade because of the following factors:
Carbon black, used in most black toner formulations, is a light blocker and absorber, which helps prevent light-related degradation reactions.
The physical durability of a xerographed copy depends primarily on how well the toner adheres to the paper. Time, temperature and pressure appear to be the most important factors, but adhesion may also be affected by the surface finish and porosity of the paper. It is more difficult for dry toners to penetrate coated papers and papers with small pore sizes. Moisture content of the paper can be critical for the toner-to-paper transfer process and may also affect the fixing process.
Full-colour images are thicker than black toner images because they are composed of four layers of toner. Consequently, they do not fully adhere to the paper and are more prone to flaking during flexing or folding.
The durability of photocopied and laser-printed documents depends largely on the quality of the paper used, as well as the degree to which the paper and toner adhere after processing.
Please contact the Agency Service Centre if you need further advice on photocopied or laser-printed documents.
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