FR2735141A1 - Photographic or magnetic antistatic transparent layer compsn. - Google Patents

Abstract

Aq. compsn. for an antistatic layer comprises: (i) vanadium pentoxide; and (ii) a polyurethane; where: (1) the polyurethane includes no sulphonated gps. and is stable in an acidic medium. Also claimed are: (I) a method for obtaining a transparent antistatic abrasion-resistant film; and (II) an antistatic film.

Description

ANTISTATIC LAYER COMPOSITION AND FILM COMPRISING
THIS LAYER
The present invention relates to an aqueous composition for antistatic layer, and a film comprising this layer, useful in the fields of printing, electrophotography, photoengraving, screen printing. More particularly, the invention relates to an antistatic composition based on vanadium pentoxide and polyurethane which gives transparent layers, practically colorless and resistant to abrasion.

 A known surface treatment for rendering an antistatic film consists in depositing on this film a layer based on polyvinyl alcohol and calcium chloride. Patent FR 2 273 642 describes a polyester film usable in printing processes, carrying an antistatic layer which consists of a polyvinyl alcohol or a polyvinyl acetate, and calcium chloride. The antistatic properties are not destroyed when these films are subjected to stretching and heat-stabilization, on the other hand, they depend on the ambient humidity.

 Mineral antistatic agents are known which make it possible to obtain an antistatic layer whose antistatic properties are independent of humidity.

 Plastic films which can be used as a support for electrophotography have been described, for example by US Pat. No. 4,571,361. These films consist of a support of polyethylene terephthalate on which there is a conductive layer of fine particles of metal oxides of Zn, Ti, Sn, Sb, etc. The antistatic properties of such films remain good even at low humidity. On the other hand, the transparency is far from satisfactory since the turbidity is around 10%.

 It is known to use antistatic layers containing vanadium pentoxide, because these layers are very transparent and their antistatic properties do not depend on variations in humidity.

 Patent application WO 91/02289 describes a photographic support comprising a support of polyester, cellulose acetate or of paper coated with resin covered with an antistatic layer containing vanadium pentoxide, and with a barrier overlay containing a latex polymer having hydrophilic functionality. To improve the adhesion of the coating compositions to the polyester film supports, a polymeric substrate layer is applied. The composition for forming the antistatic layer consists of V205 and a polymeric binder such as a latex of acrylonitrile terpolymer, vinylidene chloride and acrylic acid.

 The document EP 599 741 describes a photographic support similar to the previous one except that a barrier layer of heat-thickening acrylamide is placed on the antistatic layer of V205.

 In these two documents, the barrier layer prevents vanadium pentoxide from diffusing out of the antistatic layer, which makes it possible to have permanent antistatic protection. In addition, the barrier layer allows excellent adhesion of the antistatic layer, the emulsion layer, or the layer controlling the transverse curvature.

 Vanadium pentoxide is also used to obtain antistatic layers for plates for printing by "flexography", that is to say printing by pad. Document EP 573 365 describes such a layer applied to the film of a printing plate by "flexography". The film comprises a flexible support covered with a photocurable layer of polyurethane and an antistatic layer of vanadium pentoxide. The layer of vanadium pentoxide can also be applied to a flexible support used as an overlay and which can be coated. In this case, the antistatic effect is transferred to the photocurable layer after lamination.

 Patent application WO 93/24322 describes an antistatic composition consisting of vanadium pentoxide and a sulfonated polymer, such as a sulfopolyester or a sulfopolyurethane. Example 18 describes the preparation of a polyethylene terephthalate film coated with a layer of sulfopolyester and vanadium pentoxide, useful for transparencies for photocopying. There is no example in this patent application of a film useful in the printing or electrophotography field which comprises an antistatic layer based on vanadium pentoxide and sulfopolyurethane.

 The abrasion resistance of the antistatic layers of vanadium pentoxide can be improved by adding a protective layer on the antistatic layer. Such layers are described in US Pat. No. 5,366,855 in which a coalesced protective layer consisting of film-forming polymer colloidal particles, such as polyurethanes, and non-film-forming layers is applied to the antistatic layer of vanadium pentoxide. The layers obtained are resistant to abrasion but have an average transparency.

 Thus it would be desirable to have a composition which makes it possible to obtain an antistatic layer, both very transparent, abrasion resistant and practically colorless, which can be used in the fields of printing, electrophotography , photogravure or serigraphy.

 This problem is solved with the aqueous composition for antistatic layer according to the present invention, comprising as main constituents between 0.010 and 0.100% of vanadium pentoxide, and between 0.025 and 0.300% of polyurethane having no sulfonated groups and stable in acid medium, the percentages being expressed by weight relative to the total weight of the composition. The aqueous composition can also contain between 0.001 and 0.200% of wax and / or between 0.001 and 0.020% of matting agent depending on the final applications.

 The invention also relates to a process for obtaining a transparent abrasion-resistant antistatic film comprising applying in order on a support, a substrate layer, then the antistatic layer obtained from the aqueous composition described above.

 Finally, the invention relates to an antistatic film consisting of a support, a substrate layer, and the antistatic layer obtained from the aqueous composition described above.

 The composition according to the invention contains vanadium pentoxide, which makes the antistatic properties of the layer obtained independent of humidity. This vanadium pentoxide can be obtained in the form of a colloidal solution or "gel" by the various known methods.

 One can for example prepare the gel of V205 by hydrolysis of oxoalkoxides with a molar excess of deionized water. In this case, the vanadium pentoxide is advantageously prepared in situ from a precursor and an alcohol.

 Preferably, the vanadium pentoxide is prepared by the method described in US Pat. No. 4,203,769 in which the vanadium pentoxide is brought to a temperature at least 1000C above its melting point and quenching is carried out by immersing it suddenly in water. The vanadium pentoxide obtained by this method is presented as a gel of colloidal particles of V205 in the form of filaments.

 Vanadium pentoxide can be doped with lithium, chromium, zinc, niobium or preferably silver.

 The presence of polyurethane in the composition makes it possible to obtain a very hard layer having a permanent antistatic power. The polyurethane must be compatible with the acid pH of the vanadium pentoxide gel, that is to say chemically inert in an acid medium.

A polyurethane containing no sulfonated groups and stable in an acid medium is used in the composition according to the invention.

 Indeed, if the V205 gel was mixed with a conventional polyurethane having a pH greater than 7, the fibers of the V205 gel would be destroyed at this pH and there would be a loss of antistatic properties. If this polyurethane were acidified to obtain a composition which can be used in an acid medium, there would be a risk of flocculation of the polyurethane in the presence of the V205 gel.

 A preferred polyurethane is a linear polyurethane based on mixed ester units of hexamethylene diisocyanate, sold by Bayer under the name of polyurethane dispersion v. This product is an anionic aqueous dispersion at 40% by weight of polyurethane, free of organic solvents. The polyurethane-dispersion Vs is miscible with water in any proportion and stable at pH values from 1 to 13.

 The composition according to the invention can also contain wax, which makes it possible to obtain a slippery surface, less sensitive to scratches. The abrasion resistance of the layer is then reinforced. Preferably, the wax used is Carnauba wax dispersed in water. Any other natural or synthetic wax compatible with the composition and the intended purpose can be used, for example beeswax, polyethylene wax, paraffin. The amount of wax used is chosen according to the slip coefficient that one wants to obtain.

The antistatic composition according to the invention can also contain a matting agent useful for making the surface of the antistatic layer slightly rough. The matting agents can be chosen from those mentioned in
Research Disclosure 36544, IX, D, September 1994, for example polymers or copolymers of ethylenically unsaturated mono- and dicarboxylic acids in a and ss, their esters, half-esters and corresponding sulfonic acids, styrene, acrylonitrile and fluorinated ethylenes, or finely divided inorganic particles such as various forms of silica and calcium sulfate, barium or zinc. Preferably, polymethyl methacrylate beads or silica in the form of a powder or colloid are used.

 The matting agent must be able to disperse easily in the solution without flocculating. It must also be chemically inert. The size of the matting agent particles is between 1 and 10 μm. Preferably, particles of matting agent of two different sizes are used, the weight ratio of which is between 1: 5 and 1:10.

 Finally, the antistatic composition according to the invention can comprise a surfactant to promote the coating operation, the amount of which is adjusted according to the coating system used. Known surfactants are described in Research Disclosure 36544. IX. A. September 1994.

Such an antistatic composition can be applied to a transparent polymer support, to constitute a film according to the invention, usable in the fields of printing, electrophotography, photoengraving, serigraphy, etc. Such supports are described in
Research Disclosure 36544. XV, September 1994.

The support is preferably a polyester support, such as polyethylene terephthalate (PET), advantageously coated with a substrate layer to improve the adhesion of the antistatic layer. A preferred substrate layer is obtained from a dispersion containing a polymer latex such as a terpolymer of acrylonitrile, vinylidene chloride and acrylic acid and resorcinol, in an amount preferably between 0.1 to 1% by weight, making it possible to create bonds between the terpolymer and the
FART.

 The coating method used to apply the antistatic composition is preferably air knife coating, but any other suitable method can also be used, for example a coating die, an engraved cylinder or a Mayer bar.

 In the film according to the invention, the thickness of the antistatic layer obtained is between 0.005 and 0.05 μm, and the thickness of the substrate layer is between 0.02 and 0.20 μm.

 When the film according to the invention is used as a mounting support for the graphic arts, it must be very resistant to abrasion, since the user tends to rub it before each use. This is why the presence of wax is essential to ensure a sufficiently smooth and resistant surface. On the other hand, it is undesirable to introduce into the composition a matting agent which would unnecessarily increase the roughness.

 When the film according to the invention is used as a transparent for rear projection, the presence of the wax is not essential. However, the matting agent is very important. Indeed, the matting agent is used to obtain on the one hand, a surface roughness necessary for the drive of the transparency in the photocopier and, on the other hand, avoids optical bonding from turn to turn when two transparencies are superposed . This is particularly the case for automatic feed photocopiers where transparencies are stacked.

 The surface resistivity of the antistatic layer of transparencies must not be less than 5.109 Q / O, otherwise the toner would not be attracted and the image obtained would be of very poor quality. (The resistivity in ohms per square (Q / O) represents the electrical resistance of a square of thin film of substance measured in the plane of the substance between the opposite sides of the square. Its value does not depend on the size of the square) .

Example 1: Application as a mounting bracket
A V205 gel doped with silver with 4% V205 is prepared according to the method of US Pat. No. 4,203,769. The V205 gel has a pH of 3.4. 24.7 g of this gel are diluted in 400 ml of deionized water, then 400 g of an aqueous dispersion of polyurethane at 0.5% obtained by dilution of the polyurethane dispersion v, 2 g of p-nonyl phenoxy polyglycol are added. commercially available under the name Olin 10G Surfactants, and 16 g of a dispersion of Carnauba wax at 13.40% by weight, at room temperature. An aqueous composition is thus obtained containing 0.021% by weight of V205 and 0.05% of polyurethane, having a pH 3.5.

 This composition is applied by coating using an overflow cuvette and an air knife, on each side of a support of polyethylene terephthalate (PET) substrate with a layer consisting of a dispersion aqueous containing 4% by weight of acrylonitrile terpolymer, vinylidene chloride and acrylic acid, 0.5% resorcinol and 0.3% saponin.

 The antistatic film obtained is then dried at 1150C.

 The thickness of the substrate layer is 0.03 zm and the thickness of the antistatic layer is 0.015 zm.

 The film obtained in this example is subjected to tests relating to antistatic properties, resistance to scratches and fingerprints, transparency and coloring.

 The antistatic properties are tested according to the method used for the mounting supports by rubbing three times the surface of the film using a cotton handkerchief, then by placing the film above the ashes. If the ash adheres to the surface of the film, the antistatic layer is not effective enough. After having made three rubs on the film of this example ten times, the ash does not adhere to the surface of the film, which shows that the antistatic properties are excellent.

 The surface resistivity measured by charge kinetics is approximately 107 Q / O.

 The abrasion resistance is assessed by visual observation of scratches resulting from rubbing the surface of the film with a handkerchief or from the regular application of a fine sandpaper in movement weighted with a weight. There are very few scratches.

 When handling the mounting bracket, no fingerprints are observed.

 The turbidity measurement is done according to the AFNOR method by measuring the amount of light scattered through the support compared to the amount of light transmitted when it receives a light beam at 90. The turbidity of the film of the example is very low, of the order of 2%.

 Color measurements made in the C.I.E.

LAB gave the following results
L * = 95.2
a * = - 0.8
b * = 1.5.

 These results show that the coloration is very low and that the visual clarity of the color (L) is very high.

Example 2: Application as a transparent antistatic film
An antistatic composition is prepared as in Example 1, except that polymethyl methacrylate beads of two different sizes are added as matting agent, ie 0.010% by weight of 4 μm beads and 0.001% 8 µm beads, and no wax is applied. The thickness of the substrate layer is also 0.03 Zm while the thickness of the antistatic layer is here 0.010 zm.

 The surface resistivity measured by film loading kinetics is 1010 Q / O.

 The ash and scratch resistance tests give results similar to those obtained in Example 1. Again, the antistatic properties and the abrasion resistance are excellent.

 In general, the turbidity of commercial transparencies is at best 5%. In the case of the transparent film according to the invention, a turbidity of less than 1% has been measured.

Finally, the color measurements made in the system
CIE LAB gave results similar to those of Example 1.

 Additional tests were carried out to observe the quality of the print, in particular that of the solid areas obtained on the transparency. The solid areas obtained have a maximum optical density of approximately 1.15, while for commercial transparencies it is between 0.9 and 1.15.

 The resistance of these flat areas was then estimated. On a conventional support, when you scrape the flat with a nail, it tears off very quickly, that is to say after having scratched once, or at most five times. The flat obtained on the film of the present invention does not tear off even after ten scrapes.

 The width of a fold on the flat was also measured using a thread counter. For a commercial transparency, the width of the fold goes up to 0.05 mm. In the case of the present invention, the folding leaves no trace.

 The transparent films according to the invention have all the qualities desired for a transparency for photocopying.

Claims (13)

1 - Aqueous composition for antistatic layer  transparent, practically colorless and resistant to  abrasion, characterized in that it includes as  main constituents between 0.010 and 0.100% of  vanadium pentoxide and between 0.025 and 0.300% of a  polyurethane not containing sulfonated groups and  stable in acidic medium, the percentages being  expressed in weight relative to the total weight of the  composition. 2 - Aqueous composition for antistatic layer according to the  claim 1 further containing between 0.001 and  0.200% wax. 3 - Aqueous composition for antistatic layer according to the  claim 1 further containing between 0.001 and  0.020% matting agent. 4 - Aqueous composition for antistatic layer according to the  claim 1 wherein the vanadium pentoxide  is in the form of a colloidal solution. 5 - Aqueous composition for antistatic layer according to the  claim 1, wherein the polyurethane is a  polyurethane based on mixed ester patterns  of hexamethylene diisocyanate. 6 - Aqueous composition for antistatic layer according to the  claim 5, wherein the polyurethane is  used in the form of an aqueous dispersion  anionic. 7 - Aqueous composition for antistatic layer according to the  claim 2, wherein the wax is wax  of Carnauba. 8 - Aqueous composition for antistatic layer according to the  claim 3 wherein the matting agent is  polymethyl methacrylate or silica powder  or colloidal. 9 - Process for obtaining a transparent antistatic film  abrasion resistant consisting of  1) apply a layer on a polymer support  substrate,  2) cover the substrate layer with the composition  aqueous according to claims 1 to 8. 10 - Transparent and practically colorless antistatic film  having a resistivity between 107 and  1010 Q / O, turbidity less than 2%, characterized  in that it consists of a polymer support, a  substrate layer, and an antistatic layer  obtained from the aqueous composition according to the  claims 1 to 8. 11 - Antistatic film according to claim 10, in  which the substrate layer is obtained from  a dispersion comprising a polymer latex and  resorcinol. 12 - Antistatic film according to claim 11, in  which the polymer latex is a terpolymer latex  acrylonitrile, vinylidene chloride and acid  acrylic. 13 - Antistatic film according to any one of  claims 10 to 12, wherein the thickness of the  antistatic layer is between 0.005 and  0.05 clam, and the thickness of the substrate layer is  between 0.02 and 0.20 Fm.