STRUCTURE THEREFOR

The present invention relates to a dissipation panel, especially for use as a floor for dissipation of static electricity.

Static electricity constitutes a considerable and well-known problem, in particular on premises where use is made of computers and other electronic equipment. Electrostatic discharge (ESD) is a well-known concept in computer technology, and a material apt to be used as an appropriate floor covering providing adequate dis¬ charge, is in general demand.

Electronic components are differently susceptible to electrostatic discharge, the highly susceptible ones being liable to be knocked out as early as at a voltage of a few dozen volts.

It is above all people working on the premises in question that transmit the static electricity, and they become electrostatically charged by moving about in the room. Static electricity of up to 10 kV can be generated in a few seconds when a person walks on an ordinary floor covering. Static, electricity is liable to erase memories, distort data, damage sensitive components or produce la- tent defects which do not immediately appear, but which may eventually result in a malfunction of the components. Moreover, these defects are usually very difficult to de¬ tect.

The damage caused by ESD often costs vast sums of money.

As increasingly sensitive components are being deve¬ loped, the computer rooms, and especially the floors on which people are walking more or less continuously, have to meet higher standards. Efforts have been made to produce a floor covering dissipating static electricity, but without complete suc¬ cess so far.

The floor covering should produce a uniform dissipa¬ tion across its entire surface and be time-invariant. The floor should also be easy to lay, as well as easily adaptable to different types of substrates. It must function also on uneven substrates, and on conductive as well as insulating substrates.

Tests have shown that a great many of the semiσonduc- tive plastic floors on the market are of a very unequal quality as regards.surface resistance and volume resist- ance.

Considering the manufacturing technique, this is not surprising.

Different carbon mixtures constitute the usual addi¬ tive apt to make the floors conductive, but chemical addi- tives are also used. In present-day production engineer¬ ing, it is difficult to provide a homogeneous distribution of the conductive constituents, and this means that the test results may vary considerably.

Different methods are used in order to dissipate the static electricity from the floor materials.

It is usual to lay conductive copper strips under the wearing layer. This often means that considerable parts of the semiconductive/antistatic floor will not be earthed, which results in a static charge. Furthermore, copper strips are expensive and time-consuming to apply.

It is also previously known to spread a mixture of carbon particles or conductive chemical additives and, for example, a varnish or a two-component adhesive on a semi- conductive surface coating. See Swedish Patent 8600069-2. This, however, involves the problem that the dissipa¬ tion capacity varies with time. The carbon particles in the adhesive will slowly sink down so that the conduc¬ tivity gradually disappears. Chemical substances in the adhesive react with plasticisers in the floor covering and impair the qualities of the floor structure, as has been shown in tests involving a large number of floors.

It is also known to produce antistatic floor cover¬ ings of rubber mixed with pulverised carbon. It is, how¬ ever, difficult to distribute the carbon homogeneously in the mass, and it is impossible to admix as much pulverised carbon as is needed for the dissipation without impairing or eliminating the original qualities of the rubber mate¬ rial.

An inhomogeneous mixture only establishes so-called "conductive islands", giving an unexceptable and irregular result.

The mass of plastic or rubber also has a tendency to function as an insulating cover.

One object of the present invention is to provide a panel dissipating static electricity, which is especially intended to be used as a floor covering, but which may also serve as a table top or the like.

Another object of the invention is to provide a panel dissipating static electricity, which has a satisfactory and uniform dissipation of static electricity, irrespec- tive of the substrate, the time, and the surroundings. A further object of the invention is to provide a base structure intended to form a part of a panel, or to be used separately for dissipation of static electricity. These objects are achieved by means of a panel for dissipation of static electricity, which is characterised in that it comprises a surface coating of conductive plas¬ tic, rubber or the like, and a base structure including natural fibres, mineral fibres, pulverised carbon, one or more minerals, and, optionally, suitable additives. A base structure is also provided and is character¬ ised in that it comprises natural fibres, mineral fibres, pulverised carbon, one or more minerals, and one or more suitable additives.

The panel according to the invention can have any shape or size. For example, a number of small panels can

together form a continuous floor covering, or the floor covering can be in the form of a single panel. The panel can be delivered in rolled-up condition.

The panel according to the invention comprises a se- miconductive surface coating of a known type, suitably a plastic coating in which carbon particles have been dis¬ persed, and a base structure including natural fibres, such as cellulose, mineral fibres, for example glass fibres or mineral wool fibres, pulverised carbon, such as carbon black, and one or more minerals, for instance kao¬ lin, bentonite, talc or silicon.

The base structure may also comprise different addi¬ tives, such as flocculating agents, surface-treating agents, and pH-adjusting agents, all well known in the art.

Suitable proportions of the included materials in percent by weight of the total mixture, are as follows:

* MMMF = Man Made Mineral Fibre

The conductive base structure is joined with a semi- conductive surface coating of a known type, either by glu¬ ing or by being placed loosely against said coating.

An ordinary flooring glue may be used, but the best results are had when one uses a flooring glue intermixed with a conductive additive. The composition of the base structure according to the invention can be varied, in order to render the mate¬ rial more or less pliant, depending on its application. If

the substrate is very uneven, more clay mineral is ad¬ mixed, so that the base structure more easily conforms to the sμbstrate.

Because of the composition of the base structure ac- cording to the invention, a great deal of carbon can be admixed, producing a satisfactory and uniform dissipation of static electricity, which is fully sufficient, even if the substrate is not completely even. There is no migra¬ tion of carbon particles as time goes on, and the par- tides are homogeneously dispersed in the mass.

In humid surroundings, a cellulose fibre normally ab¬ sorbs moisture and expands, while it shrinks when dried up. This creates undesired stresses in the material, a problem which we have succeeded in minimising by the pre- sent composition. The mineral fibres form a network enabl¬ ing the cellulose to expand. An addition of bentonite fur¬ ther strengthens the stability of the base structure. The bentonite, which easily absorbs water during the mixing step, imparts to the base structure a certain porosity after drying so that moisture can be absorbed without the base structure changing its shape.

The base structure can be manufactured in any shape: as a continuous floor covering, a floor covering on a roll, or as smaller square-shaped panels. The base structure according to the invention is schematically produced as follows:

The ingredients are step by step mixed with water, the pH-value of the mixture being regularly adjusted. Subsequently, the mixture is conducted to a perforated cylinder for dehydration and from there, via a take-up roll, to a press and an oven.

The invention is further illustrated by the following Examples. Example 1 This Example describes the manufacture of a base structure having a preferred composition.

65%

10%

15%

pH-adjusting agents - Alum - Sulphuric acid Surface-treating agents - Polyethylene oxides Surface-active agents - Flocculating agents

1+2+4 are mixed with water in a mixer until an ap- prox. 3% mixture is obtained, whereupon the pH-value which may be 4.5-10, usually 7-8, is adjusted to 5.

Then, the mixture is pumped to a vat, where the con¬ ductive carbon particles, and thereafter the surface- treating agents and the surface-active agents, are added. After a further downward adjustment of the pH-value, the stock is screened, whereupon the mixture is diluted and pumped to the board machine.

Just before the outlet to the machine, the pH-value of the material is adjusted to 4.2-4.5, whereupon addi¬ tional surface-active agents and surface-treating agents are added.

After the flocculation, the diluted stock which has a dry solids content of 0.3-0.7% by weight, is put on a ro¬ tating perforated cylinder, whereupon the mixture is car¬ ried to a take-up roll via a continuous conveyor. During the transport, the water content of the mixture is adjust¬ ed to 60-80% by means of vacuum suction.

The wet sheet is rolled up on a cylinder and automa¬ tically cut when the right thickness has been achieved. The wet sheet is thereafter brought to a flat press where its water content is reduced to about 50%, and the gram age of the sheet is adjusted.

The sheet is then dried in a continuous hot-air oven. Example 2

A floor panel (A) representative of the floor cover¬ ings on the market, was compared with a floor panel (B) according to the invention.

Different floor panels were compared with respect to their maximum charge in kilovolt while being walked upon. A semiconductive floor panel (A) made of vinyl and of a known type, was compared with a floor panel (B) according to the invention. The floor panel (B) comprises a 2.5 mm thick surface coating of vinyl mixed with pulverised carbon, and a 1 mm thick base structure according to Example 1, glued to said coating. The two floor panels were tested on an insulating substrate. The test person walked on the floor panel both at a normal walking pace and with a shuffling gait, and wore soles made of different materials, i.e. conductive rubber, leather or PVC. The tests were made at room temperature, the air having an average moisture content of 25%. The results are apparent from Table 1.

Table 1

Maximum charge in kilovolt with:

Normal walking pace Shuffling gait

Conductive Leather PVC Conductive Leather PVC rubber soles soles rubber soles soles soles soles

The tests show that the panel (B) according to the invention gave very low charges of the person in question, far below the limit of what is "perceptible", i.e. normal¬ ly below 1.0 kV, whereas the panel (A) gave very high va¬ lues, especially at a shuffling gait. Example 3

A conventional antistatic floor panel (II) was com¬ pared with a floor panel (I) according to the invention.

The diagram below illustrates the body potential at

10 different types of movement, and makes it abundantly clear that the ability of the floor according to the invention to dissipate static electricity is superior.

The panel according to the invention is in particular intended to serve as a floor or floor covering, but may as well be used as a table top, a chair cover or suchlike, i.e. every type of surface or covering where one wishes to 35 avoid static electricity.