Volume 2, Number 3
Aug 1989

Paper Supplies Requirements for Electrophotographic Printing

An excerpt from "Supplies Requirements for Electrophotographic Printing," a paper given at the 1988 International Printing and Graphic Arts Conference, Washington, DC, October 1988. Reprinted with permission from the TAPPI Proceedings of this conference.

Among the non-impact printing technologies, electrophotographic printing has been fastest growing. Cut sheets and continuous form printers are now available that can reach processing speeds up to 80 cm/sec. Using roll feeding, a high speed forms printer processes more than 80 kg of standard paper per hour. This imposes demands on the technical characteristics of paper supplies.

Plain Paper Requirements

Generally, manufacturers of electrophotographic printers specify plain, bond-type paper as the data medium recommended for their products. This is for both economic and technical reasons. The low cost of bond-grade paper makes high-speed printing affordable. Suitable functional properties include traditional ones such as strength and stiffness and more novel properties such as heat resistance, curl and electrical characteristics (1,2).

Printing of cut sheets has been based on the usage of xerographic paper brands originally developed for electrophotographic copying. Printing of continuous forms (business forms) has been based on the register bond grade, manufactured to provide improved handling characteristics at high printing speeds (2)(see Table 1). In both cases somewhat nonspecific paper definitions have created uncertainties regarding the performance of specific paper brands in specific printers:

Table 1. Effect of Paper Properties on Handling Characteristics (from Ref. 2)

Xerocopy Paper. A general term for any grade of paper suitable for copying by the xerographic process. However, commercial xerocopy papers are usually modified bond grades made from chemical wood pulps in basis weights ranging from 16 to 24 pounds (17 x 22 - 500), and characterized by a smooth finish, heat stability, non-curling qualities and good aesthetic properties such as color, brightness and cleanliness. [from The Dictionary of Paper., 4th ed. American Paper Institute, 1980.]

This has been particularly true for chemical modifications of existing paper brands. The present trend towards neutral to alkaline papermaking has raised concern over two specific actions: a) treatment with novel chemicals (specifically sizing agents) and b) loading with increased filler amounts.

Synthetic Sizing

Concern has been raised over the effect on electrophotographic printing of paper not made by traditional methods in which moderate levels of rosin size were added to the furnish under acid conditions. Specifically, synthetic sizing under alkaline conditions has been found to influence the print quality, that is, the adhesion between toner and paper (fusing grade) (2) (Table 2). In continuous forms printing, a gradual decrease in fusing grade occurred during long runs, due to fuser roll contamination by organic, heat-unstable paper additives, as shown in Figure 1. In such cases, a change to rosin-sized paper would gradually restore the fusing grade and even bring it above the initial grade obtained for uncontaminated roll processing of the synthetic-sized forms.

Table 2. Fusing Grade Perception

Figure 1. Fusing grade variation for synthetic- and rosin- sized papers (from Ref. 2). Fuse-stroke value is a measure of the fusing grade.

For commercially manufactured xerographic papers of adequate to good fusing quality, the combined effect of internal and surface sizing has been found to be very similar for rosin- and alkyl ketene dimer-sized papers (5). Oxygen-to-carbon ratios as well as hydrophobicity variations were similar (Table 4). These data indicate that for both synthetic- and rosin-sized paper the sizing agents should not be allowed to reduce the oxygen-to-carbon ratio below 0.40-0.45, or to raise the hydrophobicity level above 35-40%

Since these results were published, it has become clear that a considerable variation in fusing grade can be obtained for both rosin- and synthetic-sized papers. The internal sizing additives increase the hydrophobicity of the paper surface. For excessive amounts of sizing additives, this creates a low-energy surface that prevents the spreading and adhesion of the thermoplastic toner particles (3). Using ESCA, (Electron Spectroscopy for Chemical Analysis) to study the surface chemistry of the paper, work done in our laboratory has established a correlation between the fusing grade and the hydrophobicity or oxygen-to-carbon ratio at the surface of xerographic bond-type papers (4,5). Table 3 shows the effect of hydrophobicity on the fusing quality of a number of similar experimentally made papers (rows A-F) and a commercially manufactured paper of similar furnish (row G). The rosin additive reduced the relative fusing grade to 57% of that of unsized paper at a substantial increase in hydrophobicity. The surface sizing (oxidized starch) increased the fusing grade to the level generally found for commercially available xerographic bond-type papers (raw G). This latter increase in fusing grade, with starch surface sizing, occurred with a moderate decrease in hydrophobicity (45% versus 54%). Light refining (row D) reduced the fusing grade due to a poorly bonded paper surface. Neither hard refining nor filler omission (rows E and F) significantly reduced hydrophobicity or fusing grade from control values.

Filler Loading

Typically, xerocopy papers with good aesthetic properties, as specified in the API's definition, have contained up to 10-12%. filler added as clay. Now calcium carbonate is increasingly used as a filler, and changes in paper technology, particularly in the European marketplace, have increased the filler level drastically. In work done by IBM Corporation, fillers in 204 European papers used for electrophotographic printing and copying were tested. The most common filler level was 15-20% for both clay- and calcium carbonate-filled sheets, and many of the carbonate-containing samples showed filler levels above 20%. (Figure 2). Several samples contained other inorganics (for example, talc) in varying amounts.

Figure 2. Distributions of filler content in 204 European clay- and carbonate-containing papers.

Concern over a high filler level in electrophotographic printing pertains to the possibility of contamination (dusting) and wear. Medium to high speed electrophotographic printers may process more than 100 km of paper per month. The handling and processing of such large paper quantities lead to the contamination and wear of contacting surfaces.

The general belief has been that paper wear relates to contaminants associated with the filler particles in the paper (6). However, except for specific cases of high wear rate, it has been difficult to assess the wear of typical papers used for electrophotographic printing, or to detect any correlation between the wear rate and nature and amount of filler.

Therefore, 14 selected papers were wear analyzed in the recently described Roshon drum tester (7). The paper set contained papers from the same mill but of different filler loadings. The wear volume of material removed from the wear specimen (52100 steel ball) was measured after two hours (107 meters) sliding. The data, shown in Figure 3, indicate approximately the same wear rate for all the carbonate-filled papers, regardless of filler level, whereas the clay-filled papers produced either a very low wear rate or a rate somewhat higher than that for the carbonate-filled papers. Element analysis of other constituents failed to explain the three-level wear variation.

Figure 3. Wear levels created by clay- and carbonate-containing papers. o=Clay, 0=carbonate.

While these results do not help clarify our rather incomplete understanding of the paper wear process, they do indicate that increased filler loading is not associated with increased wear. Also, they provide design parameters and criteria for realistic machine wear predictions.

Conclusions

The examples given above demonstrate the kinds of materials analysis which can be carried out for plain paper supplies to assure their functionality in electrophotographic printing. The rapidly expanding technology and its requirements for paper supplies call for increased investigation of the properties required for adequate performance. Perhaps it is noteworthy that the analysis described above included non-traditional paper testing (wear), sophisticated analytical equipment (ESCA), and the consideration of non-conventional paper characteristics (surface energetics). This illustrates the novel requirements that non-impact printing imposes on materials characterization.

Literature Cited

1. C.J. Green, Jr., Tappi J., "Functional Paper Properties in Xerography," 64 (5): 79 (1981).

2. J. Borch and R.G. Svendsen, IBM J. Res. Dev., "Paper Material Considerations for System Printers," 28 (3): 285 (1984).

3. J. Borch, Paper Technol. Inc., "Sizing and its Effect on Paper-Polymer Adhesion," 26 (8): 388 (December 1985/January 1986).

4. M.M. Farrow, A.G. Miller, and A.M. Walsh, Colloids and Surfaces in Reprographic Technology, M. Hair and M.D. Croucher, Editors, ACS Symposium Series No. 20, American Chemical Society, Washington, DC, 1982, P. 455.

5. J. Borch and A.G. Miller, Proceedings of the Tenth Cellulose Conference, Syracuse, N.Y., to be published in Appl. Polymer Sci. Symposium No. 43.

6. R.G. Bayer, Wear, "Wear by Paper and Ribbon," 49: 147 (1978).

7. R.G. Bayer, Tappi J., "Abrasiveness of Electrosensitive Papers," 67 (10 - 76 (1984).

Acknowledgments

The author wishes to thank B. Cederblad, A.G. Miller, I and J.Z. Raski for obtaining the data described.