GB2101559A

GB2101559A - Electrically conductive container

Info

Publication number: GB2101559A
Application number: GB08219759A
Authority: GB
Inventors: Patrick Brooks
Original assignee: HUMBER FABRICS Ltd
Current assignee: HUMBER FABRICS Ltd
Priority date: 1981-07-09
Filing date: 1982-07-08
Publication date: 1983-01-19
Also published as: GB2101559B

Abstract

In an electrically conductive container for powdered or granular materials a major part of the surface is electrically interconnected to ensure that the resistivity between that part and an electrically conductive and earthable member is below a safe value, i.e. 10<7> ohms, thereby minimising the risk of dust explosion due to static electricity. This is achieved by using electrically conducting thread in forming the container from the sheet material or the woven fabric which embodies electrically conductive yarns, and/or by utilising strips of electrically conductive material secured into the container.

Description

SPECIFICATION Electrically conductive container This invention relates to an electrically conductive container, and more particularly to a container such as a sack or bag for transporting a powder or granular material.

One problem that is encountered with such containers, particularly when made of a plastics material, is that of static electricity which can be hazardous, particularly where the container contents produce a dust explosion risk and/or the contents are being emptied from the container into a flammable atmosphere. To overcome this problem it has been proposed to make the container from a fabric having electrically conductive yarns woven, knitted, or otherwise incorporated therein. By this means it has been found that the generation of incendive sparks by discharge of static electricity from the surface of the fabric is reduced.

In order to protect the contents of the container from contamination it is generally desirable to employ a container that is substantially impermeable to water. To this end, the fabric has been formed from an impervious material or, when formed from woven yarns, has been given a thin impervious coating, for example, of a suitable plastics material, on one or both surfaces thereof.

The generation of incendive sparks by discharge of static electricity from the impervious material or the coated surface of the woven fabric can be reduced by having electrically conductive yarns embedded in material or in the coating as well as, or as an alternative to, conductive yarns woven, knitted or otherwise incorporated in the fabric. However, it has been found that such embedded conductive yarns are susceptible to damage during handling of the container. Such damage may result in loss of the protection against static induced ignition.

As an alternative to embedding yarns in the above-mentioned impervious coating it has also been proposed to use a coating composition which is electrically conductive. However, difficulties may arise in reliably distinguishing between a container having an electrically conductive coating and one having a nonelectrically conductive coating.

By ensuring sufficient electric connection of the electrically conductive yarns incorporated in the material or the fabric, to a member adapted to be connected to earth, that the electrical resistance between a substantial majority of the electrically conductive yarns and said member is less than 107 ohms, we have found that the risks of static induced ignition can be minimised.

Accordingly we provide a container formed from water impermeable sheet material or a woven fabric having a water impermeable coating on one surface thereof, having electrically conductive yarns incorporated therein and exposed from one surface thereof, characterised in that said sheet material or woven fabric includes electrically conductive means to provide sufficient electrical connection, between said conductive yarns and an electrically conductive member adapted to be connected to earth, that the resistance between a substantial majority of said conductive yarns and said member is below 1 07 ohms.

By the term a substantial majority we means at least about 75%. Thus, although preferred, it is not essential that every conductive yarn has a resistance to the electrically conductive member, hereinafter termed the earthing member, below 107 ohms; some conductive yarns may be electrically isolated, i.e. have a resistance to the earthing member above 107 ohms, but this will not significantly increase the risk of static induced ignition provided that the electrically isolated yarns are disposed randomly amongst nonisolated conductive yarns over the container surface.Where the sheet material or woven fabric contains adjacent conductive yarns that are electrically isolated from the earthing member, each area containing adjacent conductive yarns that are electrically isolated from the earthing member should not form more than about 5%, and preferably represents less than2%, of the surface area of the container. Preferably in any area containing adjacent electrically isolated conductive yarns not more than 3 adjacent conductive yarns are electrically isolated from the earthing member.

In particular no part of the container surface should include an area larger than about 12 cm x 12 cm that is free of non-isolated conductive yarns.

The earthing member may be an electrically conductive member fastened to the container and to which a separate earthing clip may be fastened by the container handles. Alternatively the earthing member may be provided by means, e.g.

straps or slings for lifting the container. In this case the earthing member is considered to be that part of the lifting means with which the lifting apparatus makes an electrical connection, so that, when the container is lifted, by said lifting means, using an earthed lifting apparatus, for example an earthed crane or fork-lift truck, said earthing member in the form of said lifting means is connected to earth via said lifting apparatus.

Where the lifting means are straps or slings which provide the earthing member, they may be made of fabric containing electrically conductive yarns extending along the length of the straps or slings.

In the present invention it is necessary to make provision for electrical connection between a substantial majority of the electrically conductive yarns in the fabric and the earthing member.

The containers when formed from woven fabric may be made by stitching one or more pieces of the coated woven fabric containing the electrically conductive yarns into the desired shape. We have found that, in many cases, sufficient electrical connection can be made between the electrically conductive yarns in the fabric and the earthing member simply by stitching the fabric into the desired container shape using an electrically conductive thread.

Therefore according to a further aspect of the invention we provide a container formed from a fabric having electrically conductive yarns incorporated therein and having a water impermeable coating on one side thereof stitched into the desired shape by means of an electrically conductive thread.

The thread may be a metal thread or a thread of a natural or synthetic fibrous material coated or impregnated with a conductive material. For example a metal coated thread may be employed.

Alternatively, where a synthetic fibrous material is employed, the thread may be rendered conductive by the incorporation of a conductive filler, e.g.

graphite or a metal powder, into the material from which the thread is formed. Alternatively and preferably, the thread comprises a composite thread spun from natural or synthetic fibre yarns and metal, e.g. stainless steel yarns.

It will be appreciated that such electrically conductive thread need only be used for any particular line of stitching or seam where that line of stitching or seam is relied upon to obtain the electrical connection between the electrically conductive yarns in the fabric and the earthing member. Thus it is not essential that a line of stitching or seam is made using electrically conductive thread if electrical connection between the electrically conductive yarns and the earthing member is achieved via another route over the container surface.

While the use of electrically conductive thread for any particular line of stitching where electrical connection is required may give electrical connection when the pieces of fabric have their coated sides adjacent or coated and uncoated sides adjacent, it is preffered to arrange seams where electrical connection is required between pieces of fabric such that uncoated sides of the fabric pieces are adjacent.

Since it is possible that an electrically conductive thread may not make electrical contact with each electrically conductive yarn appearing along any given line of stitching in a piece of fabric, the chances of electrical connection between those electrically conductive yarns in that piece of fabric may be increased by means of a strip of electrically conductive material, e.g. metal foil or a felt formed from electrically conductive fibres, disposed along the line of stitching against the uncoated side of that piece of fabric.

Where a strip of electrically conductive material is employed interposed between the uncoated sides of adjacent pieces of fabric, electrical connection may be obtained without the use of electrically conductive thread. Even so, the use of electrically conductive thread in conjunction with a strip of electrically conductive material may be advantageous.

The fabric may be woven from a natural or synthetic fibrous or ribbon-like material, e.g.

fibrillated polypropylene, with tondi strive yarns, e.g. metal, for example stainless steel, yarns incorporated into the weave, e.g. as some of the warp yarns, or knitted from a mixture of conductive and non-conductive yarns.

In a preferred fabric the conductive yarns are warp yarns spaced apart by not more than 3 cm; preferably there is a distance of 0.5 to 2.5 cm between adjacent conductive yarns.

The fabric has an impermeable coating of, for example, a plastics material such as polyethylene or polypropylene, on one side thereof. In order that static on the coated side is readily dissipated, the coating preferably has a thickness below 500 ym.

Although liable to damage, as mentioned hereinbefore, conductive yarns may also be embedded in the coating.

The container is preferably in the form of a bag made by stitching up the conductive-yarn containing fabric. Alternatively the fabric may be tubular and simply stitched at the bottom (and at the top after filing) to form a sack).

Where the container is intended for a .substantial amount of product, e.g. to contain 400 kg or more and/or to have a capacity of e.g. 400 litres or more, i.e. an intermediate bulk container, straps or loops are preferably secured to the bag to assist lifting thereof by mechanical handling equipment such as cranes. Preferably the container has a capacity not exceeding 3m3.

If the straps or loops are formed from a conductive material such as a woven fabric containing conductive yarns and are fastened to the bag e.g. by stitching using the conductive thread so as to provide an electrical connection between the conductive part of the straps or loops and the conductive yarns in the fabric from which the bag is constructed, the straps or loops may provide the earthing member.

As an example a rectangular bag suitable for packaging 1 tonne of pentaerythritol and having dimensions 91 cm x 91 cm x 125 cm was fabricated from a fabric comprising woven polypropylene in fabrillated or tape form having stainless steel warp threads spaced at2 cm intervals and having a coating of polypropylene of thickness 30 ym on one side. The bag was formed by stitching the fabric with a conductive composite thread spun from polyethylene terephthalate and stainless steel yarns.

The bag was made by laying two similarly sized rectangular pieces of the fabric, coated side down, one upon the other to produce a cruciform. The two pieces of fabric were then stitched together round the perimeter of the square where the fabric pieces overlapped one another. Each arm of the cruciform was folded upwards and stitched to two adjacent arms to form the sides of the bag. A further piece of the fabric, having a filling sleeve, formed from yet another piece of the fabric, sewn therein, was stitched to the upper edges of the bag sides to form the top.

A hole was cut in the bottom of the bag and an emptying spout, formed from another piece of the same fabric, was inserted into the hole and stitched, by means of the conductive thread, to the bottom of the bag around the perimeter of the hole.

Two slings made of a woven polyester fabric containing some stainless steel warp yarns were stitched to the bag, each sling passing down one pair of adjacent corners of the bag and underneath the bottom thereof.

The bag was suspended, by the slings, from an earthed metal hook and a probe was passed round the six interior surfaces of the bag, and round the interior surfaces of the filling and emptying spouts. The resistance between each stainless steel warp thread encountered and earth was measured. Over 95% of the stainless steel warp threads had a resistance to earth of less than 1 O5ohms. No area containing adjacent electrically isolated (i.e. having a resistance to earth above 1 07 ohms) conductive yarns formed more than 1% of the total surface area of the bag. In no area of the container surface were more than three adjacent yarns isolated.

Samples of the fabric employed for the construction of the body of the bag were tested by charging an unearthed sample by rubbing with a dry cloth. The maximum charge density that could be achieved was about 1.5 x 10-5Cm-2. This charge density was achieved whether the coated or the uncoated side of the fabric was rubbed. The charge transfer from the unearthed charged fabric to an adjacent earthed hemisphere was about 1 x 1 0 > 7C from the coated side and 1.3 x 1 0-7C from the uncoated side: in the latter case, the discharge was of the corona discharge type. The ability of the discharges from the coated and uncoated sides of the fabric to ignite flammable vapour/air mixtures was tested. Discharges from the coated side of the fabric ignited a vapour/air mixture having an ignition energy of 02 mJ in, on average, 6 out of every 1 0 tests (i.e. a 60% probability of ignition). Discharges from the uncoated side in one of the tests ignited the mixture with ignition energy 02 mJ and in, on average, 1 out of every 20 tests, ignited a mixture having an ignition energy of 0.04 mJ (i.e. a 5% probability of ignition).

When the stainless steel wires in the fabric were earthed, either directly or via a resistance of 107 ohms, insufficient charge could be applied to either side of the fabric to permit spark discharges therefrom.

A charge of 6 x 10-7 Cm-2 could be generated on the coated side but no measurable transfer to an earthed hemisphere occurred and no ignition of a mixture of ignition energy 0.04 mJ occurred. No measurable charge could be generated on the uncoated side.

Claims

1. A container formed from water impermeable sheet material ora woven fabric having a water impermeable coating on one surface thereof, having electrically conductive yarns incorporated therein and exposed from one surface thereof, characterised in that said sheet material or woven fabric includes electrically conductive means to provide sufficient electrical connection, between said conductive yarns and an electrically conductive member adapted to be connected to earth, that the resistance between a substantial majority of said conductive yarns and said member is below 107 ohms.

2. A container as claimed in claim 1, wherein any area of the container containing adjacent conductive yarns that are electrically isolated from said electrically conductive member forms less than2% of the surface area of the container.

3. A container as claimed in claim 1 or claim 2, wherein said electrically conductive member is provided by slings or straps, adapted for lifting the container, having electrically conductive yarns extending along their length.

4. A container as claimed in any one of claims 1 to 3, formed by stitching said material or fabric into the desired shape and said electrically conductive means comprises electrically conductive thread used for at least some of said stitching.

5. A container as claimed in any one of claims 1 to 4, wherein a woven fabric is employed and is formed by stitching said fabric into the desired shape, and wherein electrical connection between the conductive yarns of adjacent pieces of fabric is faciiitated by forming the seam between said adjacent pieces of fabric with the uncoated sides of the fabric adjacent one another.

6. A container as claimed in any one of claims 1 to 5, wherein a woven fabric is employed and is formed by stitching said fabric into the desired shape, and wherein electrical connection between the conductive yarns appearing along a line of stitching in a piece of the fabric is facilitated by providing a strip of an electrically conductive material disposed, along that line of stitching, against the uncoated side of that piece of fabric.

7. A container as claimed in any one of claims 1 to 6, wherein a woven fabric is employed and electrical connection between conductive yarns of adjacent pieces of fabric is achieved by positioning the uncoated sides of said pieces of fabric adjacent to one another and interposing a strip of electrically conductive material between said uncoated sides.

8. A container as claimed in any one of claims 1 to 7, wherein the conductive yarns in the material or fabric are warp yarns spaced apart by not more than 3 cm.

9. A container as claimed in any one of claims 1 to 8, wherein a woven fabric is employed and the impermeable coating has a thickness of less than 500 cm.

10. A container formed from a water impermeable sheet material or a woven fabric having a water impermeable coating on one side thereof, having electrically conductive yarns incorporated therein and being stitched into the desired shape by means of electrically conductive thread.

1 1. A container substantially as hereinbefore described.