Dielectric product

An improved dielectric printing paper and process for making the same, characterized by excellent resolution, contrast, and feel. The paper incorporates an inorganic salt such as magnesium chloride as a conductivity-providing ingredient. The dielectric coating is formed with a high loading of inorganic fillers and is applied by a dry process to form a discontinuous dielectric coating on the conductive paper substrate.

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 versatile 
process for making such a paper. 
Electrostatic printing paper, 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 wherein, 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. 
It has long been a problem to provide electrostatic printing papers having 
a high-filler content. For example, U.S. Pat. No. 3,847,661 is typical of 
a coating laid down from a liquid medium. There are a number of problems 
caused by such processes. If the liquid medium is a solvent for a 
polymeric matrix, then there is substantial contamination of the surfaces 
of filler product with the polymer as evaporation of the solvent takes 
place. This interferes with ink-receptively of the filler reducing 
substantially any absorbency of said filler for eventual use in imaging. 
If the liquid medium is water, then there is an increased chance of 
excessive filler polymer segregation and, moreover, there is often a 
tendency to disrupt the water-laid fibers of the paper being coated. 
Another problem with aqueous coating of the dielectric layer is the fact 
that one must, from a practical point of view, limit selection of the 
electrolyte, used to impart a degree of conductivity to the substrate 
paper, to one which has relatively low water solubility. Even with such a 
limitation made, the use of aqueous coating procedures in manufacturing 
operations can result in unwanted contamination of the dielectric coating 
with the substrate electrolyte. The process of U.S. Pat. No. 3,847,661 is 
characterized by a substantially continuous polymer layer and is limited 
to low pigment levels. U.S. Pat. No. 3,956,562 to Shibata discloses a 
process for increasing the filler content of coatings by pre-coating the 
particular with a plastic envelope which remains on the surface of the 
particle in the coating and, to that extent, interferes with the imaging 
performance contributed by the inorganic filler present on the surface of 
the paper. Even with the pretreatment, however, total particle content of 
the coating is limited. 
In the above discussion, the term liquid coating systems relates to those 
using volatile organic solvents or water as coating vehicles of low 
viscosity. 
It should be noted that this discussion of the prior art is made with 
knowledge of the present invention and after having an opportunity to 
evaluate the advantages of the present invention and the probable reasons 
for those advantages, in light of drawbacks of the prior art processes. It 
is not to be inferred that the disadvantages of the prior art or the 
reasons for such disadvantages were realized by prior artisans before the 
present invention was made. 
SUMMARY OF THE INVENTION 
It is a principal object of the invention to provide a dielectric printing 
paper of improved feel and excellent imaging characteristics. 
It is a further object of the invention to provide a novel, versatile, 
process for making a dielectric printing paper. 
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, response-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. 
Another object of the invention is to provide a process for making a 
dielectric print sheet wherein a better definition is maintained between 
conductive cellulosic substrate and the dielectric coating. 
Still another object of the invention is to provide a print sheet of 
improved ink absorptivity on the inorganic filler particles coated 
thereon. 
Other objects of the invention will be obvious to those skilled in the art 
of 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 which are very limited in 
filler-bearing capacity and wherein the coating forms a continuous film 
over very substantial areas of the printing sheet including, often, only a 
partial coating of the filler near the surface of the 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 comprising 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. The dry-coating 
process of the invention is believed to contribute a good "hand" to the 
paper and also to the excellent imaging characteristics because of the 
increased population of particles at the surface. It is particularly 
surprising that such a concentration of particles does not cause excessive 
electroconductivity of the coating. Indeed, the coated paper of the 
invention has sufficient toner compatibility 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 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 five or more orders of magnitude. 
It has been discovered that particularly favorable results can be achieved 
when a substantial volume of the coating is inorganic filler. Preferably, 
the amount of filler used will be at least about 40% by weight, but most 
advantageously 50% or more by weight, of the coating. Barium sulfate 
advantageously comprises 50% or more of the filler and preferably 30% or 
more of the weight 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 relatively inexpensive, contributes excellent 
image-receiving properties. The coating weight is normally between 5 and 
11 lbs per 3,000 square feet of coated paper. 
Polyolefins, including olefinic copolymers, are among 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 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 Na212 by U.S.I. 
Chemicals are suitable. However, even this material is 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 of 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. This 
process of the invention by-passes such a limitation and also allows a 
discontinuous coating to be formed, allows superior surface exposure of 
the filler, and a better mechanical and electrical definition at the 
interface between paper substrate and dielectric coating. 
By discontinuous coating is meant one wherein the particles are not in a 
such particle-to-particle contact which allows them to contribute 
excessive conductivity to the sheet and, on the other hand, the polymeric 
matrix is not in the form of a substantially continuous film of the type 
which dominates the surface characteristics of the paper by coating, and 
interfering with the absorbency of, the filler particles. 
A particular advantage of the invention is the capability of constructing a 
valuable dielectric print sheet 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 a calendered substrate. 
Calendered paper surfaces are disrupted when wet by either water or an 
organic solvent and "wild fibers" stand up on the surface due to the 
disruption. 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 to 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, although it may prove desirable to do so depending on the 
particulars. 
Other advantages of the process are its ability to withstand high speed 
operation, e.g. speeds of up to 1,500 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 high 
rate apparatus used in commercial paper making processes. 
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.sup.13 Ohms at 50% relative humidity and 70.degree. F. 
As will be clear to those skilled in the art, the product of the invention 
is usually sold in roll form or in the form of pre-folded, 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 
intended to be exhaustive and that other changes and modifications can be 
made within the scope of the 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 
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.

FIG. 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. 
FIG. 2 is a dielectric sheet 18 similar to that of FIG. 1 excepting sheet 
18 is coated on both sides with a dielectric coating of the invention. 
It is particularly advantageous, in the process of the invention, that a 
suitable degree of surface smoothness can be achieved even without 
calendering operations. Sheffield surface smoothness values, known in the 
art, can be readily tailored in the range of 150-240 without calendering. 
If smoother surfaces are desired, it is usually convenient to use a 
calendering step in which case smoothness values as low as 100 can be 
achieved. Usually smoothness values in the range of 125-225 will be 
acceptable, the optimum value depending upon the precise nature of the 
imaging process in which the paper is to serve as a substrate. 
For many applications, it has been found desirable to use relatively 
tougher, i.e., relatively attrition-resistant compositions. Polystyrene, 
amorphous or non-crystalline polyesters, polyamides, thermoplastic 
polyurethane, mixtures of various extrusion grade polymers inclusive of 
block coploymer compatibilizing agents as known to the art, and the like 
are suitable as are many other polymers now utilized in more conventional 
extrusion coating and extrusion processes. 
EXAMPLE 1 
A base paper, bleached kraft, of a weight 33 lbs. 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 assure 
about 0.6 lbs. of the salt is distributed throughout each 3,000 square 
feet of paper, e.g. about 1.5% of the weight of the impregnated paper. 
A dielectric coating material is prepared from the following ingredients: 
______________________________________ 
% by Weight 
______________________________________ 
TiO.sub.2 (rutile) 20% 
BaSO.sub.4 30% 
Polyethylene 40% 
(Na212 from USI Chemical) 
Wingtack 95 5% 
Paraflint H-1 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. Pat. Nos. 3,690,297 and 3,723,169. The material 
is applied at 1,200 feet per minute at a temperature of 400.degree. 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: 
5 .times. 10.sup.13 ohms/sq 
2.8 .times. 10.sup.12 ohms/sq 
Conductive Side: 
7.5 .times. 10.sup.7 ohms/sq 
5.2 .times. 10.sup.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.degree. F. to 120.degree. 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 second 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 localized imaging areas, i.e. areas to which toner is 
attracted. In two-sided embodiments, the grounding electrode is coupled to 
the conductive inner zone of the sheet. 
EXAMPLE 4 
The following formula was utilized to prepare the dielectric coating: 
______________________________________ 
BaSO.sub.4 40% 
Zinc Oxide 10% 
Polyethylene 40% 
Paraflint H-1 5% 
Wingtack 95 5% 
______________________________________ 
The zinc oxide was that available from New Jersey Zinc under 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 .times. 10.sup.13 ohm/sq 
5 .times. 10.sup.12 ohm/sq 
Conductive Side 
2 .times. 10.sup.7 ohm/sq 
1.6 10.sup.7 ohm/sq 
______________________________________ 
This paper also performed well on a 18,000-line per minute dielectric 
printer. 
In general, coating compositions of Examples 5-7 have better mechanical 
strength than do the coatings based on a polyolefins matrix. This may be 
important in some applications. Also, it is found that better image 
resolution is achieved with the polystyrene coatings. The coatings are 
conveniently 9 lbs. per 3,000 square feet: 
EXAMPLE 5 
35 lbs. Polystyrene molding powder sold by Amoco under the trade 
designation G3-F1. 
5 lbs. Polymeric viscosity modifier sold by U.S.I. Chemicals under the 
trade designation NA-250. 
50 lbs. Barium sulfate powder (less than 5 micron average particle size). 
5 lbs. Titanium dioxide sold by N.L. Industries under the trade designation 
Titanox 2071. 
5 lbs. of polyolefin sold by Goodyear under the trade designation Wingtack 
95. 
EXAMPLE 6 
40 lbs. of polystyrene sold by Amoco under the trade designation G3-CO. 
5 lbs. of Wingtack 95. 
50 lbs. of barium sulfate. 
5 lbs. of titanium dioxide. 
EXAMPLE 7 
50 lbs. of polystyrene sold by Amoco under the trade designation G3-F1. 
5 lbs. of Wingtack 95. 
40 lbs. of barium sulfate. 
5 lbs. of titanium dioxide. 
It is particularly noted that the demarcation between the 
electrolyte-bearing substrate paper and the coating is excellent in papers 
coated according to the process of the invention. Under a magnification of 
650 times, this is manifested by a substantially well-defined line which 
is free of incursions of coating material into the substrate and free of 
fiber disruption of the substrate at the interface. 
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.