FR2473957A1 - Mat polyester film for instantaneously erasable slate - coated with resin and pigment, giving sharp trace - Google Patents

Abstract

Polyester film for producing a slate which can be instantaneously erased comprises a film support of the polymer coated with a matting layer contg. a resin and a pigment. The film support is 25-125 microns thick and the matting layer 4-15 microns thick and has a gloss value of 40-300 (60-200). The matting layer is coated with an antiadherent translucid or transparent layer 0.1-2 microns thick. The obtd. film has an opacity of 15-50%. Also claimed is a slate comprising a casing in which are stacked a coloured support coated with a paraffin layer and a polyester film as above, the coated layer facing the paraffin layer, and a device for erasing traces from the slate. The slate forms a trace when a stylet is pressed on the top surface, by local adhesion of the polyester film to the paraffin layer. The use of the above polyester film gives a sharp trace and no creasing of the film or paraffin transfer to the film occurs after repeated erasing.

Description

 The present invention relates to slates with instant erasure or "magic" slates. It relates more particularly to an instantaneous erasing slate film, comprising a polyester support film coated with a matting layer comprising in particular a resin and a pigment.

Slate erasers have been known for many years and have replaced, advantageously, the traditional slate on which the writing was made with a chalk.

Generally, these slates, instantaneous erasure, consist of a sheet of colored cardboard coated with a thin sheet of translucent paper and a transparent sheet of protectionvempilées inside a flat housing, whose anterior face is pierced by a large window surrounded by a narrow frame. When a pressure is applied by means of a stylet on the transparent protective sheet, the gluing of the translucent sheet on the colored cardboard is locally caused, a trace of the color of said cardboard appearing locally.

The instantaneous erasure of this trace is performed using a movable strip, inserted between the cardboard and the sheet of translucent paper.

Such slates, instantaneous erasure, however, have the disadvantage of being fragile: in fact, the sheet of translucent paper used being very thin, erasure by translation of the movable strip quickly causes a crumpling of said sheet of paper , which makes the slate unusable.

To overcome this drawback, it has been suggested in French Patent No. 2 189 213 to replace the polyester protective sheet and the translucent sheet with a single sheet of matted polyester, the matting layer being placed against the paraffin colored cardboard.

However, the use of a single sheet has given rise, to this day, to any commercial achievement, because the matt polyester films, generally available commercially, do not have the qualities required to obtain a trace of good quality. It is generally found by using the commercially available matt polyester sheets disadvantages due to the use thereof such as insufficient contrast of the trace on the bottom, insufficient adhesion of the trace on paraffin, transfer of paraffin on the matting layer, etc.

There is also, in certain cases, too great flexibility of the polyester support, which causes a permanent wound of the paraffin layer during writing, or on the contrary too great rigidity of the polyester support, not allowing to obtain a sufficiently fine and durable trace.

To avoid these disadvantages, the Applicant has developed a matt polyester film for the realization of instantaneous erasing slates (or similar device), characterized in that the polyester support has a thickness of between 25 microns and 125 microns , while the matting layer has a thickness between LI microns and 12 microns and a surface state giving a saturation value between 40 and 300, said matting layer being coated with a translucent or transparent layer, anti-adhesive thickness between 0.1 micron and 2 microns, the opacity of the film thus obtained being between 15% and 500.

As polyester support, it is possible to choose any type of transparent polyester such as polyethylene terephthalate, ethylene polynaphthalenate, etc.
When the thickness thereof is less than 25 microns, there is generally a permanent wound of the paraffin layer, while a thickness greater than 125 microns does not provide a good quality trace, combining good finesse and good adhesion. Preferably, a polyester support having a thickness of between 50 microns and 90 microns will be used.

The support film will undergo a mechanical, electrical or chemical anchoring treatment before coating the matting layer. As an anchoring treatment well known to those skilled in the art, mention may be made, in a non-limiting manner, of mechanical embossing, corona discharge treatment, or chemical treatment such as etching of the surface layer of the film by an acid, used alone, or with other constituents, or the coating of a layer of polyester resin, acrylic resin, polyvinylidene chloride, etc.
On this film thus treated is coated a matting layer, the purpose of which is to generally make the receptive medium ink and pencil. The matting layer according to the invention comprises at least one coating resin and a mineral or organic pigment.

Many resins are particularly suitable for this application.

Mention may be made, in a nonlimiting manner, of cellulose esters (acetate, aceto-propionate, acetobutyrate, propionate, cellulose butyrate), polyvinyl esters, polyvinyl formals, butyrals, acrylic resins, polyesters and polyurethanes. , ... For more details, one can for example refer to the French patent No. 2,217,160.

 As organic pigment which may be suitable, mention may be made in a nonlimiting manner, of silicon dioxide, titanium dioxide, hydrated or non-hydrated alumina, calcium carbonate, magnesium carbonate, mixed silicates or a mixture of these various pigments.

However, to obtain, with these pigments, a matting layer according to the invention, it is necessary that the roughness of the layer is between 0.1 micron and 3 microns, thus obtaining saturation values between 40 and 300.

The satin value of a matte layer is defined according to the French standard AFNOR 903-012 (determination of smoothness - Bekh method).

The principle of measurement is as follows: Bekh smoothness is the time required for the flow of a determined volume of air from the ambient atmosphere under a determined pressure difference, between the surface of the paper and a polished surface. ring shape, under fixed contact conditions. The Bekh smoothness depends on the shape, the total volume and the distribution of the voids which lie between the surface of the material and an ideal theoretical plane, under the fixed contact conditions. The resistance against the passage of air between the test piece and the polished surface will be greater as the material will be smoother.

It has been found that the surface condition of the matting layer conditions the quality of the pattern. A surface that is too smooth (average roughness less than 0.1 micron and / or quantity of pigment that is too small relative to the quantity of resin) does not allow very close traces to be made.

The Applicant believes that in this case, there is no flow of air between the different traces, which leads to the formation of a "bubble" which under the pressure of the writing pen flushes the air on the sides and erases the adjacent trace.

A surface that is too rough (average roughness greater than 3 microns and / or too much pigment) does not give the permanence of the line.

The weight ratio between pigment and resin in such layers having a satin value, as defined above, should vary between 10/1 and 1/10. However, we find that the best results are obtained when this ratio varies between 4/1 and 1/1.

Within these limits, the satin value is generally between 60 and 200, which makes a good compromise between the desired qualities.
When the values of the parameters mean roughness and saturation value of the matting layer are respected, it is also necessary to respect a third parameter which is the thickness of the matting layer. This must not, indeed, have a thickness less than 4 microns and greater than 15 microns. When the thickness of the layer is less than 4 microns, said layer is not enough "absorbent". When it is greater than 15 microns, said layer becomes too rigid. In both cases, the contact of the matting layer and the paraffin layer is of poor quality, which gives an insufficiently clear and durable trace. In particular, it is found in use, a rapid transfer of the paraffin layer on the matting layer where the pressure of the writing pen is exerted.

On this matting layer is coated an anti-adhesive layer of thickness between 0.1 micron and 2 microns, the total opacity of the matting layer and the anti-adhesive layer being between 15% and 50%. Such a layer makes it possible to obtain a trace of excellent quality, the film thus obtained having all the qualities required for the realization of a good "magic" slate. Such an anti-adhesive layer may be transparent or translucent, but its opacity, added to that of the matting layer, must not be less than 15% or greater than 50%.

When it is less than 15%, the resulting pattern lacks contrast, whereas an opacity higher than 50% requires too much writing pressure to obtain total transparency at the level of the trace, which limits considerably longer the life of the slate.

Silicone resins, polyethylene resins, natural or synthetic waxes, fatty acid chromium salts having a carbon chain length ranging from 14 to 22 carbon atoms, and fluorohydrocarbon resins can be used as anti-adhesive layers. , such as polytetrafluoroethylene, polyvinylidene chlorides or copolymers thereof, or a mixture of one or more of the resins or waxes mentioned below.

These resins or waxes must, in order to be usable on the film, according to the invention, have good film-forming properties and give a surface wetting tension of less than 35 dynes-cm. In other words, such surfaces can be wetted only by liquids having a surface tension of less than 35 dynes-cm. In order to evaluate this essential parameter, it suffices to use the known method of depositing on a surface thin liquid films of calibrated surface tension (for example using the Dognon-Abribat apparatus). The commercially available matt films commonly have a surface wetting tension of between 40 and 72 dynes-cm. After treatment, using a release layer, this wetting tension of the surfaces must necessarily be less than 35 dynes-cm. so that the bonding forces between the paraffin layer and this anti-adhesive layer are optimal in the context of the present invention.

This anti-adhesive layer may be applied by any of the known coating techniques, such as, for example, air blade scraper, reverse-roll cylinders, engraved rolls or rods or rods. coiled threads.

According to a variant of the invention, the mat film can also be obtained by coextrusion according to US Pat. No. 3,515,626, insofar as the parameters necessary for producing the film according to the invention are respected. In this case, a film having two layers will be chosen: one made of transparent polyester, unfilled, the other of filled polyester, as described in this patent.

The opacity of the film, coated with matting and anti-adhesive layers, is measured using a reflectometer, according to the AFNOR 903-006 standard. This measurement is the ratio of the diffuse photometric reflection factor of the material placed on a black background to the diffuse photometric reflection factor of the same support placed on a white background. This ratio, in particular, increases at the same time as the opacity from a very low value for a transparent support, up to 100% for a perfectly opaque material. The measuring apparatus used must meet certain geometrical, spectral and photometric characteristics: with the electric cameras, these conditions are obtained by using a green tristimulus filter, established for the apparatus in question and for the standard source C.

The film thus obtained, the paraffin-coated colored cardboard sheet and its housing are then assembled, for example, in the manner described in French Patent No. 2 189 218.

The invention will be better understood with the aid of the following exemplary embodiments, given in a nonlimiting manner, together with the single figure which represents a sectional view of a "magic" slate, obtained with a film according to the invention. .

In this figure, are successively stacked in the housing 1, a colored sheet of cardboard (or other material) coated with a thin layer 3 of paraffin, on which is placed a transparent polyester film 7, coated with a layer of matting 6 and a release layer 5. When it wants to erase a trace, a strip 4 which can move relative to the lateral ends of the housing 1, allows to take off the release layer 5 of the paraffin layer 3, where the stylus 8 has made said trace.

EXAMPLE 1
On a bi-oriented ethylene polyethylene terephthalate film (TERPHANE brand) 75 microns thick, the following anchoring solution is deposited Polymethyl methacrylate chlorine 2 g.

 Methylene chloride 0.1 l.

This layer is then dried at 1000 ° C. for approximately 15 minutes. The following matting solution is then deposited. Methacrylate copolymer, acrylate
thyle and amide acrylic (in rap
molar port 80/10/10) 2.4 g.

 Silica Gasil 64 6.0 g.

 Methyl ethyl ketone 45.0 g.

 . Toluene 18.0 g.

 . Ethyl lactate 7.0 g.

After drying, a release layer consisting of a dispersion of polyethylene resin sold by Hoechst under the name of
HARDOMER RA 02 (weight deposited: 2 g / m2).

A film having a wetting tension of 30 dynes cm is obtained. After mounting this film in a housing, as described above, we obtain a magic slate giving excellent results: there is no paraffin transfer on the film and traces made, even close to each other are perfectly net.

EXAMPLE 2
On a 50 micron thick polyester sheet, having undergone an anchoring treatment based on trichloroacetic acid and polyvinyl alcohol, the following matting layer is deposited. Water 60 g.

Partially polyvinyl acetate
hydrolyzed 15 g.

 . Methanol 30 g.

 . Hydroxyethylcellulose 5 g.

 Glxal 10 g.

 Titanium dioxide 1 g.

 . Aluminum oxide 12 g.

 . Methylglolcol 6 g.

After drying, an anti-adhesive layer is coated with an alcoholic solution of a chromium salt of fatty acid, sold under the trade name
QUILON (Du Pont de Nemours). The wetting tension of the film is 32 dynescm. After mounting in a magic slate, we obtain the same results as in Example 1.

EXAMPLE 3
On a polyester film 75 microns thick, having undergone an anchoring treatment based on trichloroacetic acid and silica, is deposited the following matting layer Nitrocellulose 9.5 g.

 . Methyl methacrylate 12.0 g.

. Denatured alcohol 40.0 g.

. Methyl ethyl ketone 67.0 g.

 Silica GASIL 64 6> 0 6.0 g.

. Xylene 23.0 g.

After drying for 8 minutes at 10 ° C., an aqueous-alcoholic silicone resin solution 631 (RHONE-POULENC) is applied at a rate of 3 g / m 2. This layer is dried for 4 minutes at 140 ° C. The wetting tension of the film is 32 dynescm.

After mounting in a magic slate case, we obtain the same results as in Example 1.

Claims (1)

 I - Coated polyester film for the production of instant-erasing slates, comprising a polyester support film coated with a matting layer containing a resin and a pigment, characterized in that the support film, with a thickness of between 25.degree. microns and 125 microns, comprises a matting layer of thickness between 4 and 15 microns and having a surface state giving a saturation value of between 40 and 300, said matting layer being coated with a translucent or transparent antiadherent layer , with a thickness of between 0.1 micron and 2 microns, the opacity of the film thus obtained being between 15% and 50%.  II - Film according to claim 1, characterized in that the matting layer is obtained by coating with a resin and a pigment, whose weight ratio varies between 10/1 and 1/10 and preferably between 4 / 1 and 1/1.  III - Film according to claim 11, characterized in that the resin is chosen from cellulose esters, polyvinyls, polyvinyl formals and butyrals, acrylic resins, polyester resins, polyurethanes, said resins being used alone or in admixture.  IV - Film according to claim I, characterized in that the matte polyester film is obtained by coextrusion and comprises a pigment-free layer and a layer containing organic or inorganic pigments.  V - Film according to one of claims I to IV, characterized in that the roughness of the matting layer is between 0.1 micron and 3 microns.  VI - Film according to one of claims I to V, characterized in that the pigment is selected from silica, titanium dioxide, calcium carbonate, magnesium carbonate, silicates, aluminas, hydrated aluminosilicates or no, mica or glass powder.  VII - Film according to one of claims I to VI, characterized in that the matte film has a satin value of between 60 and 200.  VIII - Film according to one of claims I to VII, characterized in that the release layer consists of a homopolymer resin or copolymer based on silicone, polyethylene, natural or synthetic waxes, chromium salts of acid carbon-chain length fatty acids varying from 14 to 32 carbon atoms, fluorohydrocarbon resins or polyvinylidene polychorides, said resins or waxes being used alone or in mixtures and having a superficial wetting tension after coating and drying on the matt film, less than 35 dynes-cm.  IX - Instant-dry slate-type apparatus comprising a housing in which are stacked a colored support coated with a layer of paraffin and a polyester film coated on one side facing the paraffin layer, and means for erasing the trace on said slate, characterized in that the coated polyester film is in accordance with one of claims I to VIII.