1. Technical Field of the Invention
The present invention relates to transparent receptor components or substrates comprising a polyester base for electrostatic photocopying, notably for the production of overhead transparencies.
2. Description of the Prior Art
It is known to this art that photocopies that can be projected onto a screen by means of an overhead projector are easily obtained from a source document by electrostatic photocopying same onto a transparent base. According to this process a uniform, positive or negative electrostatic charge is applied to a photoconductive surface preserving this charge when it is maintained sheltered from light, and the image of a subject to be copied (text, drawing and the like) is formed on said surface by means of an optical system and of a source of light. In the regions where light impinges on the charged surface, the electrical charge is dissipated by grounding, the other regions then forming or defining the electrostatic image of the source document; particles of an electrostatic ink in powder form (hereinafter simply "toner") of opposite charge to that of the electrostatic surface are then deposited onto the latter via electrostatic attraction. The image thus obtained is then intimately contacted with a base, for example a transparent base substrate, to which an opposite electrostatic charge is applied, in order to transfer the toner from the electrostatic surface onto the base. The image thus obtained is fixed onto the base by heat treatment and/or by pressure.
The transparent bases employed for producing projectable electrostatic photocopies must satisfy various requirements to obtain projected images of excellent quality, in particular, sharp images. They must exhibit, particularly, a transparency, a dimensional stability and a high slip, a low capacity for accumulating static electricity charges and good adhesion to the image printing material. It too is known that these bases must exhibit, more particularly, a transparency such that the percentage of light scattered by the passage of the light rays through the thickness thereof, or haze, is less than or equal to 7%. It is also known that when these bases are prepared from a biaxially oriented film, especially a polyester, they must exhibit a dimensional stability such that their shrinkage at 150.degree.-170.degree. C., namely, under the temperature conditions required for fixing the toner, is less than 1% in the directions of drawing.
By reason of their excellent mechanical, physical and chemical properties, polyester films constitute a material of choice for producing transparent bases for projectable electrostatic photocopies. However, their low slip, their great tendency to accumulate static electricity by rubbing or induction and their relative chemical inertness, which is reflected in a low adhesiveness of the toner to the base, present problems which have required solutions complicating the manufacture of transparent bases and making it more costly.
Thus, the drawback of the slip of polyester films cannot be resolved by creating a surface roughness using particles of a filler dispersed within the polyester. Indeed, because of the thickness of the films destined for use as a base far electrostatic photocopying, which ranges from 50 to 200 .mu.m, the presence of filler throughout their thickness imparts a substantial haze which is incompatible with such intended use. The transparency can be obtained only at the expense of a decrease in the filler content, which compromises the slip of the bases and consequently the ability of the bases to slide over one another in the reams employed in photocopying machines, or to slide over the metal surfaces of said machines.
The low affinity of the polyester films for the toner is reflected in easy removal of the toner when the photocopies are being handled and by a gradual deterioration of the image. In its turn, the ability of the polyester base to accumulate static electricity interferes both with the uniform deposition of the toner onto the base during photocopying, which is detrimental to image quality, and with the sheet-by-sheet feed of the bases from a ream source.
To avoid the problems presented by the adhesion of the toner to the base, the transparency, the slip and the tendency to accumulate electrical charges, it has been proposed to deposit, by spreading onto at least one face surface of a filler-free transparent polyester film, a coating comprising a polymeric binder which has at least a good adhesion to the toner and which contains fillers creating a sufficient roughness on the base to impart the necessary slip thereto, and, if appropriate, an antistatic agent. In an alternative embodiment of this solution, a filled adhesive or primer coating is deposited onto one face surface of the polyester film and an antistatic coating on the other face surface.
Thus, U.S. Pat. No. 4,526,847 describes depositing onto a filler-free polyester film a coating of a composition comprising a solution or nitrocellulose in an organic solvent (esters, ketone) containing a plasticizer, a filler which has a particle size of 0.3 to 10 .mu.m (for example colloidal silica) and an antistatic agent.
European Patent Application EP-A-332,183 describes transparent components for electrostatic photocopying, namely, a filler-free polyester base coated with a layer of an acrylic binder containing a filler and an antistatic agent. However, it remains necessary to interpose between the acrylic coating and the polyester a primer facilitating adhesion of the is layer destined to receive the toner to the polyester.
And European Patent Application EP-A-104,074 describes depositing onto one face surface of a polyester base film an acrylic coating containing a filler and providing the adhesion to the toner and, on the other face surface of the polyester base, a coating containing an electrically conductive polymer; placing of a primer coating between the polyester base and the layer receiving the toner is also recommended.
The solutions proposed by the aforesaid prior art are only partially satisfactory. Indeed, the use of compositions based on organic solvents presents health and safety concerns. Certain of the solutions adopted entail increasing the number or layers of coatings: (a) application of a primer which exhibits good adhesiveness of the polyester base to the latter receiving the toner; (b) deposition of a receiving layer providing the adhesiveness of the toner, good slip and, where appropriate, good electrical conductivity; and (c) optionally deposition or an antistatic coating onto the face surface of the base film which is opposite to that receiving the toner. It has also been found that the introduction or an antistatic agent into the adhesiveness layer destined to receive the toner may, depending on the nature of said agent, decrease the effectiveness of the adhesion of the toner to the receptor layer. The presence of fillers in the layer receiving the toner also presents the disadvantage of requiring a thickness of said layer which is greater than that required merely from the standpoint of enhancing the adhesion of the toner to the base.