Background of the Invention

This invention relates to a dielectric printing paper, of the type used to selectively attract toner particles by use of differential electrostatic potential on the surface of the paper, and to an improved and versatile process for making such a paper.

Electrostatic printing papers, or dielectric papers suitable for electrostatic printing as they are more properly described, are well known to the art. These papers are to be distinguished from the photosensitive papers which are commonly used with office copying equipment.

Dielectric printing is based on forming a charged area on a dielectric surface by electron-beam, or some other such selective surface charging ^' means. The charged area is then directly contacted with a toner selectively attracted to the areas of the paper made electrically receptive to it. There is no intermediate light-caused discharging process, and photoconductive materials are not generally useful in dielectric printing processes using liquid toners and wnerein, for example, a print speed of 18,000 lines per minute is typical. In general, dielectric copy sheets are used in high-speed copying processes. Papers heretofore used in such processes tend to be expensive because of their utilization of expensive organic conductivity-imparting additives, of relatively expensive coating substrates, and of relatively expensive dielectric coating procedures. Summary of the Invention

It is a principal object of the invention to provide a dielectric printing paper of improved feel and excellent imaging characte istics.

It is a further object of the invention to provide a novel, versatile, process for making a dielectric printing pape .

Other objects of the invention are to provide a novel two-sided dielectric printing paper suitable for operation at high printing rates, as when it is fed from rolls and pre folded continuous assemblies of paper, and a process for making such paper.

Still another object of the invention is to provide a relatively inexpensive dielectric copy sheet which has a dielectric coating characterized by an excellent combination of good opacity, gloss, charge-retention, respo se-speed, contrast, and image resolution.

A further object of the invention is to provide a dielectric copy sheet, and process for making the same, wherein a ground wood paper product, is utilized as a substrate for a dielectric coating. Other objects of the invention wilj be obvious to those skilled in the act on their reading this disclosure.

The above objects have been substantially achieved by the development of a dielectric printing sheet characterized by use of a dielectric coating on a relatively conductive substrate. A coating is discontinuous and to be contrasted with cast or solvent-coated coatings wherein the coating forms a continuous film over very substantial areas of tne printing sheet. The coating of the invention is comprised of inorganic fillers in a dielectric thermoplastic matrix. The fillers are non-photoconductive, and are carefully selected to provide a good combination of opacity, gloss, charge retention, response-speed, contract and image resolution characteristics without compromising the objective of obtaining a low-cost product. The fillers are carried onto the substrate coated within a thermoplastic matrix polymer having suitable dielectric properties. In the dry-coating process of the invention, a discontinuous coating is believed to contribute a good "nand" to the paper and also to the excellent imaging characteristics. Indeed, the paper of the invention has sufficient discontinuities in its coating that it is

susceptible to graying by toner when it is processed at speeds substantially slower than the state-of-art printing speeds. At the higher speeds utilized in the art, the imaged paper has an excellent background, the toner not

5 having the contact time required to penetrate and reside in the coating.

The surface resistivity between (a) the salt- impregnated portion of the sheet and (b) the dielectric surface should differ by at least four, but preferably about

1° five or more orders of magnitude.

It has been discovered that particularly favorable results can be achieved when a substantial part of the coating, preferably at least about 40% by weight, but most advantageously 50% or more by weight, is inorganic filler.

15 Barium sulfate advantageously comprises 50% or more of the filler and preferably 30% or more of the coating as a whole.

Other fillers which can be used, preferably in small quantities, are titanium dioxide and zinc oxide. None of these materials, however, is as desirable for use as barium

^* ^° sulfate which, although relativly inexpensive, contributes excellent image-receiving properties. The coating weight is normally between 5 to 11 lbs per 3,000 square feet of coated paper.

Polyolefins, including olefinic copolymers, are among

2 ~ ^" the polymers useful in the practice of the invention. Polyethylene is a highly adequate polymeric carrier for the fillers of the invention. A particular polyethylene, or any other polymer applied by the preferred coating procedures, is usually selected with attention to the flow

30 characteristics of the polymer. Thus a low density, i.e. low melting and low crystallinity polymer is often most suitable. Polyethylenes sold under the trade designation DYLT by Union Carbide Corp. or Na250 and tfa212 by U.S. I. Chemicals are suitable. However, even this material is

³ 5 beneficially modified with adhesion promoting and flow modifying resins such as, for example, polymerized olefins

and diolefins and sold under the trade designations "Wingtack 95" by Goodyear, a hard, synthetic, high melting point wax consisting essentially of a mixture of high molecular weight, saturated, straight chain, paraffin hydrocarbons, and a minor proportion of branched chain, paraffin hydrocarbons, e.g. those sold under the trade designation Paraflint H---1 by Moore and Munger.

In general, the critical physical properties of the polymer, insofar as the product is concerned, are its high resistivity and ability to contribute good dielectric characteristics to the coating. A large number of thermoplastic polymers can meet this criteria. In practice however, there have been practical limitations for wet- coating processes based on the need to find an effective solvent system for the polymers to be used. Tnis process the invention by-passes such a limitation and also allows discontinuous coating to be formed.

A particular advantage of the invention is the capability of constructing a valuable dielectric print she using a ground wood paper substrate. Thus, the economic advantage of the process of the invention inherent in avoiding solvent-coating procedures and using inexpensive conductors is increased by an ability to avoid the use of calendered substrate. Calendered paper surfaces are disrupted when wet by either water or an organic solvent a "wild fibers" stand up on the surface due to tne disruptio

Using the process of the invention the surface is not disrupted but rather is actually improved by mechanically passing through the nip between the blade and the backing roll. It is not necessary to calender the stock before coating by this process. Even if the aqueous solution of magnesium chloride is applied first, it does not adversely affect the surface smoothness of the subsequently applied dry coating. This permits a lower weight of dry coating t be able to give a smooth surface on non-calendered sheets than is possible with solvent (either aqueous or organic)

systems. Even groundwood type substrates need not be calendered.

Other advantages of the process are its ability to withstand high speed operation, e.g. speeds of up to 1,500

⁵ to 4,000 feet per minute, its ability to be used with moisture bearing substrates. Indeed, there appears to be no reason that the coating step could not be an adjunct to the hign rate apparatus used in commercial paper making processes.

10 The conductive salt is selected from any of a number of soluble salts which serve as a means to impart conductivity to the sheet and also as a humectant, thereby preserving the conductivity over a wide range of temperatures and levels of humidity. Magnesium chloride is wholly satisfactory for

^•**•* --' this purpose. Similar salts would be operable. The 100- volt surface resistance of the coated sheet is normally at least 10 ¹³ ohms at 50% relative humidity and 70°F.

As will be clear to those skilled in the art, tne product of the invention are usually sold in roll form or in

20 the form of pre-folάed, perforated assemblies. Illustrative Embodiments of the Invention

In this application and accompanying drawings there is shown and described a preferred embodiment of the invention and suggested various alternatives and modifications thereof, but it is to be understood that these, are not 5 intended to be exhaustive and that other changes and modifications can be made within the scope of tne invention.

These suggestions herein are selected and included for purposes of illustration in order that others skilled in the art will more fully understand the invention and the 0 principles thereof and will be able to modify it and embody it in a variety of forms, each as may be best suited in the condition of a particular case. In the Drawings

Figure 1 illustrates, schematically, a dielectric printing sheet of the invention.

Figure 2 illustrates, schematically, a double-coated printing sheet of the invention.

Figure 1 illustrates a conventional dielectric printing sheet 10 according to the invention wherein a dielectric coating 12 is coated on a salt-impregnated substrate 14. Coating 12 comprises 50% by weight of inorganic filler 16. Substrate 14, a ground wood-type paper, comprises a magnesium-chloride impregnant.

Figure 2 is a dielectric sheet 18 similar to that of Figure 1 excepting sheet 18 is coated on both sides with a dielectric coating of the invention.

Example 1 A base paper, bleached k aft, of a weight 33 IDS. per 3,000 square feet (e.g. about 50 grams per square meter), is impregnated with an aqueous solution of magnesium chloride. The application is carried out to assume about 0.6 lbs. of the salt and is distributed throughout each 3,000 square feet of paper, e.g. about 1.5% of the weight of tne impregnated paper.

A dielectric coating material is prepared from the following ingredients:

% by Height ^*

Ti0 (rutile) 20%

BaS0 ₄ 30%

Polyethylene

(Na212 from USI Chemicals) 40%

ingtack 95 5%

Paraflint H-l 5%

The primary polyethylene is a low density , e.g. low- crystalline material. This coating material, when applied, exhibits an excellent combination of whiteness, electrical resistivity, receptivity to commercial liquid toners, dry toners, and low gloss. Aesthetically, a paper coated therewith compares well with untreated bond paper and is an improvement over more expensive, commercially-accepted, dielectric papers.

The coating is applied at about 6 lbs. per 3,000 (square) feet (about 10 grams per square meter) by conventional dry coating procedures, e.g. that process described in U.S. Patents Nos. 3,690,297 and 3,723,169. The material is applied at 1,200 feet per minute at a temperature of 400°F. in general, this procedure provides for the direct coating of the formulation by melting and without use of ancilliary. solvent carriers. The resulting coating is discontinuous and it is believed that the excellent feel of the resulting paper is at least partially assignable to this fact.

The resultant dielectric paper exhibits surface resistivities as follows:

Applied Potential: 100 volts 500 volts Dielectric Side: 5x10 ¹³ ohms/sq 2.8xl0 ¹² ohms/sq Conductive Side: 7.5xl0 ⁷ ohms/sq 5.2xl0 ⁷ ohms/sq

The conductivity characteristics of the paper remain acceptable when the paper is stored at relative humidities of from 20- to 70%, and indeed from 10 to 90%, at temperatures from 20°F to 120°F. The resultant sheet was used successfully in conjunction with a commercial printing machine (Honeywell PPS printer) at a rate of 18,000 lines per minute.

Example 2 Example 1 is repeated excepting that the dielectric coating was carried out before the aqueous salt solution impregnation.

Example 3 Example 1 is repeated and, thereafter, a second dielectric coat of the same material is placed on the secon side of the previously impregnated and coated paper. The resulting paper is of excellent hand and performs well in electrostatic printing of both sides.

It will be understood that in one-side printing papers the reverse (conductive) side is grounded and the electrostatic charge is placed on, and held in the localize imaging areas, i.e. areas to which toner is attracted. In two-sided embodiments, the grounding electrode is coupled t the conductive inner zone of the sheet.

Example 4 The following formula was utilized to prepare the dielectric coating: zinc Oxide ' 10%

Polyethylene 40%

Paraflint H-l 5% wingtack 95 5%

The zinc oxide was that available from New Jersey Zinc unde the trade designation Kadox 15. It is not a photosensitive grade.

The coating was applied to a conductive substrate, as described in Example 1.

Surface resistivities of the paper were as follows: Applied Potential 100 Volts 500 Volts

Dielectric Side 5 x 10 13 ohm/sq 5 x ohm/sq Conductive Side 2 x 10 ohm/sq 1.6 x 10 ⁷ ohm/sq

•This paper also performed well on a 1,800-line per minute dielectric printer. it is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all

statements of the scope of the invention which might be said to fall therebetween .