GB2123745A - Laminate for dissipating static electricity - Google Patents

Abstract

A laminate for the controlled dissipation of static electricity comprises a base of electrically non- conductive material and a surface sheet bonded thereto which includes a finely divided expanded layer silicate, said sheet having a surface resistivity in the range 10<4>-10<11> ohms per square. The laminate is useful as a bench top on which to assemble, dismantle or pack static-sensitive devices or to cover the walls of compartments in which static-sensitive electronic equipment is worked with.

Description

SPECIFICATION Laminate This invention relates to a laminate, and more particularly to a laminate for the controlled dissipation of static electricity.

The utility of such a laminate can be illustrated with reference to static-sensitive devices i.e. those whose special electrically conductive properties are at risk of irreparable breakdown owing to the rapid discharge, via their terminals, of relatively low electrostatic potentials of the order (e.g. 50 volts) generated by routine handling. A laminate such as the invention provides may form a bench-top on which to assemble, dismantle or pack static-sensitive devices. Again, such a laminate may be used for covering the walls of rooms or other compartments in which static-sensitive electronic equipment is housed or worked with.

The invention has arisen from our discovery that a finely divided expanded layer silicate in sheet form, particularly vermiculite or hydrobiotite, is not - as might be supposed by analogy with the closely related mineral mica - a good electrical insulator, but has on the contrary a small but significant ability to conduct electricity. Furthermore, we have discovered that this conductivity is not destroyed if, to improve the abrasion resistance of the sheet of finely divided silicate, organic polymeric material is included in it.

According to the present invention a laminate for the controlled dissipation of static electricity comprises a base of electrically non-conductive material and a surface sheet bonded thereto which includes a finely divided expanded layer silicate, said sheet having a surface resistivity in the range 1041011 ohms per square.

Preferably the surface resistivity of the surface sheet is in the range 10'109 ohms per square.

The base of electrically non-conductive material will ordinarily have a flexural strength of at least 1 MPa and preferably at least 10 MPa. Suitable materials for the base are: fibre-reinforced organic polymeric material, such as asbestos- or glass-fibre-reinforced plastics materials, e.g. polyvinyl chloride; and materials having a basis of wood fibre, such as plywood, blockboard, chip board, particle board or hardboard.

The expanded layer silicate employed is preferably vermiculite which has been exfoliated chemically rather than thermally, as described for example in British Specifications 1 016 385, 1 9 305, 1 593 382 and 1 593 383. The resistance to abrasion of a sheet consisting entirely of finely divided expanded layer silicate is low, and we prefer to include in the sheet an amount of organic polymeric material which forms from 0.1 to 95% by weight of the sheet, and particularly from 5 to 90% by weight; we have found that this improves abrasion resistance. Suitable organic polymeric materials are: natural rubber; acrylonitrile-butadiene copolymers; polyacrylates; acrylonitrile-butadiene-styrene polymers; and expoxy resins.

The sheet of finely divided layer silicate can be made in a flexible form suitable for bonding to the electrically non-conductive base by evaporating an aqueous suspension of finely divided layer silicate, but it is preferably one that has been made by submitting to electrophoresis an aqueous suspension of the expanded layer silicate in which the aforesaid organic polymeric material is dispersed, to deposit on a support a coating which contains from 0.1 to 95% by dry weight of the polymeric material, and then stripping the electrophoretically deposited coating from the support.

Preferably the thickness of the dry sheet is in the range 0.05-0.5 mm.

The most effective static-dissipating work surfaces at present in use are black, owing to the presence in them of granular (non-fibrous) carbon to provide the required level of conductivity. The present invention makes it possible to produce effective non-black static dissipating work surfaces, because there is no need to use carbon. Moreover, it something different from the colour of the finely divided silicate material is desired, it is possible to provide it by including pigments or dyes in the electrophoresis medium from which the flexible surfacing sheet is prepared.

Examples 1 and 2 iater in this specification illustrate the preparation of suitable flexible sheet incorporating finely divided expanded layer silicate. The table below summarises the properties of sheets obtained following generally the procedure of those Examples, but with the specific differences in the electrophoresis medium that are listed in columns 2 and 3 of the table. Surface resistivity was measured after the sheet material had come to equilibrium with the atmosphere of the laboratory.

Composition of electrophoresis medium ~ . % wt. of Organic Vol. % organic Surface polymer Vol. % Vermiculite polymer resistivity latex latex slurry in sheet ohm/square Acrylic latex 5 95 43.5 2,3 x 106 (see Example 1) 10 90 53.6 3.5 x 1010 25 75 80.7 2.0 x 109 50 50 88.6 1.8 x109 Nitrile latex 5 1 95 26.4 2.2 x 108 (see Example 2) 10 90 80.7 2.5 x 108 25 75 90.1 2.3 x 1010 50 50 90.8 1.0 x 10 75 25 89.4 1 1.0 x 10

To prepare the laminate from the base material and the flexible sheet of finely divided expanded silicate, one of two methods of bonding may be employed: hot pressing, or use of adhesive at ambient temperature.

In hot pressing, a foil of polyvinyl chloride, polyethylene or like plastics material is laid between the base and the flexible sheet, and the assembly is subjected to elevated temperature e.g. 150--1700C.

and elevated pressure e.g. 100--600 psi (690-4137 kPa). Of course if the base employed is itself of thermoplastics material, no foil is necessary in hot pressing. By use of a patterned mould, or insertion of a press blanket between the assembly and the platen of the press, a decorative textured finish can be imparted to the laminate that is formed.

Alternatively, the flexible sheet of finely divided silicate is bonded to the base by a suitable adhesive e.g. a polyvinyl acetate or polyvinyl alcohol glue, contact adhesive or rubber solution.

The invention is further illustrated by the Examples herein and with reference to the accompanying drawing, which is a diagram showin an electrophoresis cell whose anode receives a deposit of material which is later stripped from the anode in the form of flexible sheet.

EXAMPLE 1 a. A rectangular sheet (50 x 25 mm) was cut from a highly polished sheet 1 mm thick of pure zinc, and made the anode 1 in an electrophoresis cell 2 containing a slurry 3 made by mixing 90 parts by volume of a slurry of exfoliated vermiculite with 10 parts by volume of an acrylic polymer latex. The slurry of exfoliated vermiculite contained 6.5% by weight of vermiculite of particle size less than 50 ym, and had been prepared following generally the procedure of example 1(A) in British Patent Specification 1 593 382. The acrylic polymer latex was a commercially available material sold under the name Revacryl A275 (an anionic emulsion,45.5% solids content, of a self-linking acrylic polymer curable at 1200C; pH 4; particle size 0.2 yam).

The cathode 4 of the cell was of perforated tin-plate.

The anode 1 had on the side remote from cathode 4, and on its edges, a coating 5 of polyurethane lacquer.

b. Electrophoresis was carried out for 2 minutes at a voltage gradient of 1 volt/mm to form an electrophoretic deposit 6 on the anode 1.

c. The anode 1 was then removed from the electrophoresis cell, rinsed with water, dried at 800C for 1 hour and then heated at 1 200C for 3 minutes to cure the acrylic polymer present in the coating deposited on it. The coating 6 was then stripped from the anode. The highly flexible sheet thus obtained was 0.1 mm thick and contained 53% by weight of acrylic polymer, the rest being exfoliated vermiculite.

Its surface resistivity was 3.5 x 10'0 ohms per square.

EXAMPLE 2 Following generally the procedure of Example 1 a coating 6 was formed by electrophoretic deposition from a slurry obtained by mixing 90 parts by volume of a slurry of exfoliated vermiculite with 10 parts by volume of a commercially available butadiene-acrylonitrile latex (solids content 41%; pH 10.3; particle size 0.13cm).

The flexible sheet obtained contained 81S/o by weight of butadiene-acrylonitrile copolymer. Its surface resistivity was 2.5 x 108 ohms per square.

EXAMPLE 3 A base of the structural cladding material sold under the name Duraglas (glass-fibre-reinforced polyvinyl chloride; thickness 3 mm) was overlaid with a sheet prepared according to Example 2 which had been dried in an oven at 1 1 OOC for -21-hour to remove superficial moisture, and the assembly was put between the platens of a conventional steam-heated press. Pressing was carried out at 300 psi for 20 mins; platen temperature was 1 600C.

The platens were then cooled and the pressure was released. The laminate formed was removed and allowed to come to equilibrium with the atmosphere of the laboratory (temperature 200C; relative humidity 60%). The surface resistivity of the layer silicate surface of the laminate was not substantially different from that of the flexible sheet material before lamination.

Abrasion resistance of the sheet, as measured by a procedure which recorded the number of sweeps of a loaded hardened polished steel stylus over the surface before penetration occurred, was 150, compared to only 20 for a laminate made with a surface of electrophoretically deposited pure vermiculite.

EXAMPLE 4 A laminate was prepared following generally the procedure of Example 3 from the sheet material of Example 2 and a base of plywood 6.5 mm thick, using a foil of polyvinyl chloride 0.1 mm thick to bond the sheet to the base.

Claims (7)

1. Laminate for the controlled dissipation of static electricity, comprising a base of electrically nonconductive material and a surface sheet bonded thereto which includes a finely divided expanded layer silicate, said sheet having a surface resistivity in the range 1 owe10" ohms per square.

2. Laminate according to Claim 1, in which said surface resistivity is in the range 107--109 ohms per square.

3. Laminate according to Claim 1 or 2 in which said surface sheet includes an amount of organic polymeric material forming from 0.1 to 95% by weight of the sheet.

4. Laminate according to Claim 1 or 2 in which said surface sheet includes an amount of organic polymeric material forming from 5 to 90% by weight of the sheet.

5. Laminate according to any one of Claims 1 to 4 in which the base is a fibre-reinforced organic polymeric material.

6. Laminate according to any one of Claims 1 to 4, in which the base has a basis of wood fibre.

7. Laminate according to any preceding Claim in which the base has a flexural strength of at least 1 MPa.