Antistatic sheet.

Translation of description of equivalent WO 9320280 A1

STATIC SHEET

The invention relates to a paper sheet having antistatic properties.

Such a sheet can be used in various fields.

It can be used to make items for which stnécessaire dissipate static charges generated during use or for items whose primary or secondary function of dissipating electrostatic charges forming and can be dangerous in a given environment.

the flexible abrasive is known consisting of a carrier sheet to which are bonded with an adhesive abrasive grains for which it is necessary to dissipate static charges created during use. Indeed, if the abrasive is not treated to dissipate the charges, the dust formed during the abrasion of an object settles and clogs the abrasive grains; it decreases the yield abrasion. Furthermore workers can suffer electric shocks that make them react with uncontrolled actions endanger them during their work.

There are also known the laminates that are commonly used for the manufacture of furniture, working tables, wall panels and other. The resulting products are used primarily in operating rooms, clean rooms and computer rooms. In these rooms must prevent dust is especially chosen because of the attraction of electrostatic charges and we must also avoid sudden electrostatic discharge, it is necessary that the surfaces being in the room are treated to dissipate regularly static electricity.

In the field of abrasive product is treated with a conductive support or the adhesive layer or the surface of the grains abrasives. As products conductors were used quaternary ammonium salts, carbon black, metal powders or fibers, metal alloys, metal salts, doped conductive polymers or inorganic pigments made conductive by coating with an electroconductive layer metal oxide.

It is known to use carbon black mass or surface to provide a conductive surface.

In EP-A-414494 is rendered conductive abrasive incorporating carbon black in the adhesive used to bond the abrasive grains.

In patent -JS-A-3942959 the abrasive is rendered conductive by a layer of a conductive compound which may be a metal, metal alloy, metal pigment, a salt or a metal complex, this layer being disposed between two insulating layers. The conductive substance may be the back of the medium on the support surface (below the adhesive), mixed with the adhesive or on the grains.

In FR-A-2276144 abrasive driver is rendered by arranging the abrasive grains over a conductive layer, the conductive material is particularly graphite.

In application EP-A-408 943 is rendered conductive abrasive by treating the surface of the layer containing the abrasive grains with a solution containing a quaternary ammonium salt.

In application EP-A-398 580 is rendered conductive abrasive by treating the surface of the layer containing the abrasive grains by a doped conjugated polymer such as polythiophene, polyaniline, polypyrrole.

In the field of laminates are treated sheets of paper constituting the product by a driver.

Let us first recall how are manufactured in general stratified by distinguishing between two types of existing laminates Currently, high pressure and low pressure laminates called the so-called laminates.

Producing said high pressure laminates from a core consisting of a stack of sheets, generally of kraft paper impregnated with a thermosetting resin, especially a phenolic resin.

Once the sheets of kraft paper impregnated with resin, the dried, cut and then stacked on each other; the number of stacked sheets depends on the applications, it generally varies between three and nine.

Subsequently placed on the stack of sheets forming the core, a decorative sheet that can be united with printed patterns or presented an iridescent or metallic appearance and being impregnated with a thermosetting resin does not darken when heated (eg resin melamine-formaldehyde). Sometimes placed on top of the decorative sheet, a protective covering sheet, called "overlay", also impregnated with a resin, devoid of pattern and transparent in the final laminate. The stack of the various types of impregnated sheets is placed in a press provided with a metal sheet giving the surface appearance; laminated to the assembly under heat and pressure; a unitary structure extremely hard and having a decorative effect is obtained.

Producing said laminated low pressure similarly to that of high pressure laminates, but is carried out the lamination of the decorative sheet directly to a panel of wood particles or other base support.

There is a third type of product, the finished sheet, which also belongs to the category of decorative papers. This sheet of paper which is pre- or post-impregnated (usually a mixture of latex and melamine-formaldehyde resin) is intended to be glued onto a particle board or other substrate.

In FR-A-2540041 a laminate is made conductive by the fact that part of the sheets forming the core is conductive by incorporation into each sheet of material electrically conductive such as carbon black, a metal or etal or conductive fibers salts.

In FR-A-2557167 is recommended to disperse conductive fibers in a paper for laminate conductor. These fibers are carbon fibers, metal fibers or fibers coated with a metal.

In JP-A-58034861 describes as an antistatic agent for plastics, a conductive pigment obtained by deposition of tin or indium on non-conductive inorganic base pigment followed by heating in an oxygen atmosphere for form their oxide. The metal oxide is not doped.

As pigment used, many pigments are cited without specifying any benefits linked to a particular pigment or a geometrical form of a family of pigments.

In US-A-5071676 describes a conductive pigment usable for imparting antistatic properties to paper and cardboard. This pigment is constituted by a non-conductive substrate coated with an electroconductive layer of tin oxide doped with antimony, itself covered with a layer imparting to the pigment an isoelectric point between 5 and 9 to facilitate its dispersion. The substrate can be any, it is not critical for the invention.

EP-A-415478 describes a colored pigment and driver usable in laminated paper. This pigment is constituted by a base pigment of rutile titanium dioxide and mixed phase coated with a layer of tin oxide doped with antimony. Rutile has a spherical geometry, this pigment is spherical.

All these cities conductive products have various disadvantages.

The major disadvantage of carbon black is that it necessarily gets dirty products, which can be annoying a aesthetic point of view and so that 1'on can not print these products as desired.

A disadvantage of certain products such as doped polymers is their high price. A disadvantage of certain products such as quarternary ammonium salts is that it gives the articles too low conductivity to have a good flow of electrostatic charges. Another disadvantage of conductive salts is that the level of conductivity of products containing them will vary with the relative humidity.

A disadvantage of some conductive products such as aluminum is the water sensitivity; in the presence of water there is a dangerous evolution of hydrogen. We can not use them easily in aqueous media.

Disadvantages associated with the use of conductive fibers are firstly 1 * mottled appearance gave the paper particular with carbon fibers and secondly the reduction of the physical characteristics of the paper. Some mineral pigments made conductive by a conductive metal oxide layer can give a too low conductivity for the safe dissipation of electrostatic charges especially when implemented in a paper application.

The applicant proposes to solve the drawbacks mentioned,

The object of the invention is to provide a paper sheet which has sufficient electrical conductivity level conferring antistatic properties.

The skilled person knows from experience that for an item effectively dissipate electrostatic charges, it is preferable that the surface resistivity not greater than about 10⁷ ohms measurement made by ASTM 257-66 standard.

A second object is to provide a sheet having antistatic properties which can be carried out entirely in aqueous medium. A third object is to provide a sheet having properties antistatic that do not vary with the relative humidity. A fourth object is to provide a sheet having antistatic properties which has a neutral appearance esthéthique that is to say that the product that will make the conductive sheet should not or slightly change the appearance of the sheet.

Another object is to provide a sheet having antistatic properties, the mechanical properties are good. Another object is to provide a sheet having antistatic properties which has a low cost price.

The Applicant has found that the objects of the invention are achieved by producing a sheet which includes conductive pigments having a lamellar-type basic structure and provided with an electrically conductive layer of doped metal oxide.

As mentioned above inorganic pigments coated with doped metal oxides are known for their electrically conductive properties, however, the Applicant has found that for pigments having an electroconductive layer based on the same oxide and for the same dopant, thus having a priori comparable intrinsic conductivity levels, the sheets of paper containing these pigments have very different final conductivities according to the base structure (geometry) of the pigment support the layer- such results are shown in appended table 1, the metal oxide chosen doped being doped tin oxide with antimony, this doped oxide is deposited on the pigments supports of different geometric shapes. The conductive pigments were coated in an aqueous medium under the same conditions using the same binder (PVA polyvinyl alcohol) and in the same ratio 1: 1, on the surface of a sheet of paper.

The sheet surface resistivity was measured according to ASTM 257-66 for a relative humidity of 50% (the conductivity of sheets that can be obtained by taking 1'inverse resistivity).

From this study it appears that the pigment in the base structure has an influence on the final conductivity of the sheet. The desirable resistivity level for good dissipation of electrostatic charges (less than 10⁷ ohms) is achieved with the pigments having a lamellar-type basic structure (and therefore geometric flat shape); the resistivity is in fact of the order of 10⁵ ohms.

The invention therefore provides a paper sheet having antistatic properties characterized in that it includes conductive pigments having a lamellar-type basic structure and having at least one electrically conductive layer of doped metal oxide.

Preferably the doped metal oxide is a tin oxide doped with antimony.

According to the invention, pigments having a lamellar structure can be selected for example from micas, talc, kaolin, bentonites, montmorillionites or glass particles. In a particular case of 1'invention the conductive pigment is a mica coated with a layer of tin oxide doped with antimony.

The mica pigments coated with an antimony doped tin oxide layer has good transparency to light, they do not affect the aesthetic appearance of the paper that includes the.

In another particular case of 1'invention the conductive pigment is a mica coated with a titanium oxide layer, optionally a layer of silica, and coated with a tin oxide layer doped with the 'antimony. These pigments have a_. some iridescence but little affect the appearance of the paper with them. It may be interesting to use in areas where the decorative effect of 1 • iridescence is sought for example in the field of laminates.

The conductive pigments may be incorporated in mass during the manufacture of the sheet on the paper machine or be deposited on the surface of the sheet by impregnation in a size press or by any coating means or by impression. Preferably the conductive pigments are provided in aqueous medium.

Preferably the sheet is characterized in that it bears, on at least one side a layer containing at least the said conductive pigments and at least one binder.

The binder is usually a binder used in papermaking, such as water-soluble binders, latex.

It may be advantageous to use a water-soluble binder, such as polyvinyl alcohols or starches to obtain an easily repulpable sheet.

The layer may optionally contain other adjuvants commonly used in papermaking, such as viscosity regulating agents, for example carboxymethyl cellulose, antifoams, etc ... Preferably the amount of conductive pigments deposited on the surface of the sheet is between 1 and 10 g / m² By dry weight.

The treated sheet is based on cellulose fibers, it may include other organic fibers (polyethylene fibers, polypropylene, polyester ...) or mineral fibers such as glass fibers. It may also comprise other additives used in papermaking such as fillers, sizing agents, binders, wet strength agents, retention agents, anti-foaming agents, viscosity regulating agents, agents regulating pH etc ...

The invention also provides a flexible abrasive product having antistatic properties, characterized by the fact that it for supporting said sheet to antistatic properties.

The abrasive can be obviously in sheet form but also in other forms such as continuous strip, disc etc. ....

Preferably the used sheet comprises the conductive pigments surface. The conductive pigments may be on the back of the abrasive or the surface bearing the grain below the adhesive.

As it is known to use a conductive product in the adhesive or grains, can consider such use of conductive pigments.

The invention also relates to a decorative sheet obtained from said sheet in antistatic properties. The invention also relates to a laminate having antistatic properties which is characterized in that it comprises at least one sheet as said sheet to antistatic properties. Said sheet can be used as part of the soul of the laminate or as decorative sheet or possibly as overlay.

Said sheet can also be a finished sheet.

The following examples show possible embodiments of a leaf according to the invention; they also show that the surface resistivity (and hence conductivity) does not or very little varies with the relative humidity.

EXAMPLE 1

On a sheet of paper one layer, using a Meyer bar, an aqueous composition of conductive pigments of the type mica coated with tin oxide doped with antimony, marketed by Merck, and starch as binder. The pigment-binder ratio is 5: 1.

On another sheet layer is a similar composition but the binder is a PVA.

Is carried samples having weights of different layer of conductive pigments (expressed in dry weight in the table below).

Is measured each sample surface resistivity according to ASTM 257-66 standard and for relative humidity (denoted

RH) of 50 and 20%. The results in the table below show that the resistivity of the sheet does not vary with the relative humidity.

The color of the paper obtained according 1'invention is not changed. <img id="imgf000012_0001" he="53" wi="160" file="imgf000012_0001.tif" img-format="tif" img-content="table" orientation="portrait" inline="no" />

EXAMPLE 2

samples were carried out as in Example 1 but using as a conductive pigment of mica coated with a titanium oxide layer and a silica layer and finally coated with a tin oxide layer doped antimony, tin ratio / antimony being 85:15. This pigment is commercially available from

MERCK.

According to the results of the table below shows that the relative humidity has no influence on the level of conductivity of the sheets obtained according to the invention.

The leaves have a slight iridescence that has little aesthetic appeal.

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TABLE 1