DE29823441U1 - Flame retardant electrically conductive fabric - Google Patents

Description

EUP10A Tl.doc

Al

Applicant: eurea Verpackungs GmbH & Co. KG Industriestrasse 55/57 48432 Rheine

15 titles:

Flame retardant electrically conductive fabric

30

Representative: Patent Attorney Dr. Helmut Hoffmeister Goldstraße 36 48147 Münster

EUPIOJZdQC

Flame retardant, electrically conductive fabric

The invention relates to an electrically conductive fabric made of
Plastic fibres, plastic ribbons or plastic threads for
Manufacture of high-strength packaging or lining webs, which has electrically non-conductive and electrically conductive threads, as well as products made from this fabric

Bulk material containers and linings, especially for shafts in civil engineering and mining.

For synthetic fabrics, when used in a dry environment
electrostatic charges, especially due to friction,

which, when in contact with earthed objects,

and/or persons. The high energy of the arc can lead to the ignition of explosive air/gas or air/dust mixtures. In a
In this environment there is also an increased risk of fire

drive.

Fabrics are known from DE-AS 19 28 330 which, in order to avoid electrostatic charging, consist of two different fiber materials, of which one fiber material is permeated by an electrically conductive soot dispersed throughout the entire fiber and the other fiber material is free of soot.

The disadvantage of these fabrics is that they result in a fabric with reduced strength and elasticity because they contain threads in which the carbon black is dispersed throughout the entire fiber if the carbon black is contained in the fiber in such an amount that sufficient electrical conductivity is achieved. It is pointed out that sufficient electrical conductivity cannot be achieved if the amount of carbon black contained in the fiber is too small. Flame retardant properties of the fabric are also not disclosed.

The applicant's patent specification DE 39 38 414 C2 discloses a bulk material container made of an electrically conductive fabric that consists of synthetic fibers or plastic threads and has electrically non-conductive and electrically conductive threads, the electrically conductive threads consisting of a polyolefin and containing dispersed carbon black and/or graphite and being woven into both the warp and the weft.

A fabric of this type is well suited to the strong mechanical stresses that occur when the fabric is used for a flexible bulk material container or for a lining sheet in mining, and the woven-in electrically conductive threads ensure that electrostatic charges are safely dissipated. The advantage is that the elastic modulus of the electrically conductive threads is smaller than that of the rest of the thread material woven into the warp and weft. This prevents the electrically conductive threads of the fabric from breaking under a lower load than the threads of the non-conductive base fabric, thus avoiding interruptions in the conductive grid.

The disadvantage is that the electrically conductive thread is highly flammable due to its high carbon content

and does not go out even in the vicinity of difficult-to-burn, non-electrically conductive threads of the base fabric, but rather spreads through the fabric like a burning wick, destroying the neighboring areas.

In plastics technology, it is generally known (cf. SAECHTLING: Kunststoff-Taschenbuch, p. 58, 25th edition, Carl Hanser Verlag 1992) to add flame-retardant additives to plastics, which extinguish a flame by, in the case of organic compounds containing chlorine or bromine, making it more difficult for oxygen to reach the base substance when exposed to flame, or - for example in the case of aluminum hydroxide - releasing water and/or carbon dioxide at high temperatures.

The disadvantage is that an additional filling of a plastic filled with conductive carbon black or graphite with a flame-retardant additive greatly reduces the strength values of the thread made from such a plastic and makes it unsuitable for use in fabrics for the production of high-strength packaging or linings.

Furthermore, GB 21 01 559 Al discloses a bulk material container that is made from a fabric into which metal threads are incorporated, via which electrostatic charges of the fabric can be discharged.

The disadvantage of this solution is that these threads drawn from solid metal are only incorporated into the fabric as warp threads, which impairs the overall conductivity. Above all, the stretching behavior of the metal fibers or threads differs greatly from the stretching behavior of the rest of the fabric. This easily leads to the metal threads breaking and thus to an interruption of the conduction. In the case of static electricity , such interruption points

ladling greatly increases the risk of sparks and explosions. Nor have any properties of the base fabric been revealed that would make it suitable for use in areas at risk of fire.

The invention is based on the task of further developing a fabric of the type mentioned at the beginning that is electrically conductive and at the same time flame-retardant and can therefore be used in areas at risk of explosion and fire. The fabric should also be suitable for the production of high-strength packaging and linings.

According to the invention, the object is achieved in that

- that the electrically non-conductive threads are equipped, coated or filled with a flame-retardant additive,

- that the electrically conductive threads consist of a thermoplastic material which is equipped with at least one additive which increases the conductivity or is filled in finely dispersed form, or that the electrically conductive threads are coated with at least one additive which increases the conductivity and

- that at least the electrically conductive threads can be shrunk in length by means of heat, whereby the shrinkage of the electrically conductive threads is greater than the shrinkage of the electrically non-conductive threads.

"Flame retardant" refers here to the fire behaviour of a fabric in which the ignition of the fabric is prevented or delayed when exposed to a flame and in which the flame extinguishes itself within a short period of time after ignition.

A fabric is referred to as "electrically conductive" if its surface resistance and earth resistance measured according to DIN 53482 are at least smaller than the volume resistance of air, i.e. smaller than Ω ⁸ Ω, so that a discharge of electrostatic electricity is possible.

The term "threads" here refers to all spun or extruded plastic fibers, plastic ribbons cut from films or other plastic threads that can be processed using weaving technology.

The "shrinkage" of plastics is defined as the absolute difference in length change caused by relaxation or retardation processes, relative to the initial length.

Surprisingly, a fabric according to the present invention has flame-retardant properties, although the electrically conductive threads are made of flammable plastics and do not contain any flame-retardant additives, or the additives are contained at most in a small amount, which alone is not sufficient to extinguish a flame.

The electrically conductive threads of the fabric according to the invention are able to safely discharge an electrostatic charge to an earth conductor via the applied metal layer or the metal particles contained therein.

When a flame is applied to the fabric according to the invention, the plastic melts in the flame-exposed area. While the additives in the non-conductive threads prevent ignition or cause them to extinguish quickly, the ignition of the electrically conductive threads is prevented by

that they contract strongly under the influence of heat and thus shrink away from prolonged exposure to the flame. The introduction of heat from neighboring areas of non-conductive, almost incombustible threads leads to rapid shrinkage of the electrically conductive threads, so that they can escape the endangered area even before the immediate exposure to flames. The shrinkage occurs extremely quickly.

In order to achieve this effect, according to the invention at least the electrically conductive threads are shrinkable under the influence of heat. If such threads are woven into a non-shrinkable base fabric, this thread contracts when exposed to flames and extends out of the fabric as far as the influence of the heat introduced by the flame reaches. Outside the heat-affected zone, the thread retains its mechanical and electrical properties.

The invention provides that the shrinkage of the electrically conductive threads is always greater than the shrinkage of the electrically non-conductive threads. It is therefore also possible that a heat-shrinkable thread is selected for the non-conductive base fabric. The entire fabric thus also escapes the effects of flames by shrinking. The higher degree of shrinkage of the conductive threads relative to the shrinkage of the base fabric ensures that the latter can quickly escape the effects of flames.

Good results in the flammability test are achieved when the shrinkage of the electrically conductive threads is 1.2 to 4 times, preferably 2 times, the shrinkage of the electrically non-conductive threads.

The electrically non-conductive threads in an advantageous embodiment consist of a polypropylene which is coated with a

filled and/or coated with a flame-retardant halogen-containing additive. This material is cost-effective and is therefore particularly suitable for the production of large-area fabric panels.

The electrically conductive threads can be made of a plastic in which the conductivity is increased by filling it with particles of metals or metal compounds or with carbon-containing particles such as soot or graphite. Such a filling can be achieved cost-effectively by adding the conductivity-increasing particles to the plastic raw material.

However, it is also possible to use a plastic that is already electrically conductive to produce the thread, whereby a homogeneous thread is obtained. Polyaniline is well suited as a conductive plastic for the present invention. The conductive plastic can be added to the thread material as an additive or the thread can consist entirely of a conductive plastic.

The conductive threads can be vapor-coated with silver. The good electrical conductivity and the good adhesion properties of silver to the plastic enable electrostatic conduction over the outside of the thread with a small layer thickness. Vapor-coating of the outside of the thread can also be used on a thread that is already equipped or filled with conductivity-enhancing additives, which further improves the conductivity.

A cost-effective form of training involves vapor-coating a monofilament, for example an extruded polyamide thread, with metal or equipping it with conductive particles.

The flexibility of the thread is increased in a further embodiment in which the electrically conductive threads are multifilaments. Such a thread can consist of several fibers, of which at least one fiber is filled and/or coated with conductive particles. Fibers of the same thickness can be twisted together, and it is also conceivable to arrange a sheath of thinner fibers around a thicker, conductive fiber as a core, with which the tear resistance is increased.

In order to be able to absorb high forces and ensure electrostatic discharge, the electrically conductive threads are made of a polyester, preferably polyethylene terephthalate (PET).

A significant advantage of the invention with regard to the electrical properties is that the electrically conductive threads can be woven into the warp and/or the weft. If electrically conductive threads are woven into both the warp and the weft threads, a FARADAY cage is created when producing physical structures from the fabric according to the invention. Electrostatic charges can thus be discharged two-dimensionally and increase safety when using the fabric in a hazardous atmosphere. The fabric can be loaded in any direction due to the similar strength values of electrically conductive and non-conductive threads without the conductive threads breaking.

In order to improve the removal of the shrinking thread when exposed to flame, it is advantageous for the cross-section of the electrically non-conductive threads to be 2 to 10 times the cross-section of the electrically conductive threads. The shrinking, electrically conductive fiber is thus not

Base tissue is blocked and can escape unhindered from the flame-exposed area.

In a further embodiment of the invention, every tenth to eightieth warp or weft thread is an electrically conductive thread, whereby a balanced combination is achieved from the point of view of production costs, the optical appearance of the fabric and the electrostatic properties.

In order to avoid the formation of "islands" of electrostatically insulating fabric sections in the middle of the grid of electrically conductive threads, the area of the non-conductive fabric between each two warp and weft threads enclosing a rectangle is at most 100 cm ² . It has proven advantageous both for the electrical properties and for the manufacture and mechanical strength of the fabric if the distance between the electrically conductive threads in the warp direction (3) is 1 to 5 cm, preferably 2.5 cm, and in the weft direction is 10 cm to 60 cm, preferably 40 cm.

Environmental influences such as air humidity can influence the required surface resistance and the earth resistance at which charge dissipation is achieved. The resistances should be as small as possible in order to ensure reliable dissipation of electrostatic charges in all cases. It is therefore a significant advantage of the invention that, due to the good conductivity of the electrically conductive threads and the suitable arrangement of the threads in the fabric, a surface resistance and an earth resistance (according to DIN 53 482) of less than Ω ⁴ Ω can be achieved - with simultaneous flame-retardant properties of the fabric and good mechanical strength values.

In order to be suitable for the production of high-strength packaging or lining sheets, the fabric advantageously has a breaking strength of 50 N/mm ² to 250 N/mm ² with an elongation at break of 10% to 50%.

It is advantageous that the elastic modulus of the electrically conductive threads is smaller than that of the non-conductive threads. This prevents the electrically conductive threads of the fabric from breaking under a lower load than the threads of the non-conductive base fabric. Interruptions in the conductive grid are therefore avoided in any case.

The invention is explained in more detail using an example which is shown in the drawing. The figures show in detail:

Fig. 1 a fabric according to the invention after a flame test in plan view,

Fig. 2 a fabric according to the prior art according to a

Flame test as a comparison example in top view,

Fig. 3 shows a bulk container made from the fabric according to the invention in perspective view,

and ■"· ■ ■

Fig. 4 shows a shaft in underground mining lined with the fabric according to the invention in a schematic sectional view.

Fig. 1 shows a section of a fabric 100 produced according to the invention. Both the warp thread 3 and the weft thread 4 are ribbons made of a thermoplastic material. Such ribbons are obtained in a simple manner by producing a film made of a plastic, which is then cut into ribbons by a knife in the direction of the web. Since standard plastics, in particular polypropylene, are also suitable and the ribbons are less

textile yarns have a large width of approx. 0.5 to 5 mm, large-area fabrics can be produced inexpensively. Electrically conductive threads 2.1, 2.2 are woven into the fabric 100, which are shown schematically here as a double line for better illustration; in a preferred embodiment, the electrically conductive threads have a smaller cross-section than the threads of the base fabric. The conductive threads 2.1, 2.2 are woven in both the warp and the weft, so that electrostatic electricity can be dissipated in all directions of the surface.

The electrically conductive threads 2.1, 2.2 of the preferred embodiment are twisted yarns that are textured from several polyester fibers, of which at least one fiber, preferably 4 to 6 fibers, is vapor-coated with silver. The electrically conductive threads 2.1, 2.2 are monoaxially stretched. The stretching is brought about to 4 to 10 times the initial length, with a stretch ratio of 5:1 to 6:1 being favorable for most types of plastic. Stretching is carried out after heating the plastic to a temperature above the glass transition temperature of an amorphous thermoplastic or above the crystallite melting temperature of a semi-crystalline thermoplastic and is maintained during cooling. The plastic molecules are aligned by stretching. The configuration of molecules aligned in the stretching direction, frozen by cooling, does not correspond to the thermodynamic equilibrium state, so that an internal stress state arises. Reheating, which makes the molecules mobile, leads to a reduction in these stresses and thus to at least a partial reversal of the imposed deformation. The effect is also known as the "memory effect".

If the electrically conductive threads 2.1, 2.2 are exposed to strong heating, for example by an open flame or by the heat of melt on neighboring threads 3, 4 in the fabric, the conductive fibers 2.1, 2.2 contract and their length in the heated area is only a fraction of the original length.

Example (see Fig. 1) .

The lower area of the fabric 100 in Fig. 1 shows the fabric changes after a test to determine the fire behavior. The test criteria were selected according to DIN/EN/ISO 6941 "Measurement of the flame spread properties of vertically arranged samples", which enables an assessment to be carried out in accordance with DIN 66083.

A burner flame was directed at an angle of 30° against the lower edge 5 of a fabric sample 100 in an area 6. In the area 6 directly exposed to the flame, the thermoplastic warp and weft threads (3, 4) melt due to the high heat and form a crust 7 made of plastic melt. Since they are provided with flame-retardant additives, the flamed threads 3, 4 extinguish approximately one to two seconds after the burner flame is removed.

The electrically conductive threads 2.1, 2.2 have shrunk considerably in the heat-affected zone. The heat supplied by the burner flame and the adjacent melting ribbons made the molecular chains of the silver-vaporized polyester thread mobile and the forced stretching state was reduced.

Since the electrically non-conductive threads 3, 4 of the fabric 100 shown in this example do not shrink, the shrinkage of the conductive threads 2.1, 2.2 causes a relative movement

tension, with which the conductive threads are pulled away from the danger point and out of the base fabric. Due to the shrinkage of the conductive thread 2.1, for example, a gap 8.1 is created in the fabric 100 in which a single warp thread is missing. However, the weft threads 4 are all present in the area 8.1 of the gap, so that the strength of the fabric is not significantly reduced.

The flame therefore does not spread further in the fabric and the damaged area in the fabric remains limited to the area 6 of direct flame exposure; in particular, outside the area directly exposed to the flame, both the mechanical and electrostatic properties of the fabric are retained.

Surprisingly, it has been shown that the fabric 100 according to the invention is flame-retardant, although the electrically conductive thread 2.1, 2.2 is not filled with a content of additives that could alone cause the extinguishing of a flame, as is the case with the warp and weft threads 3,4.

Comparative example (see Fig. 2)

The test to determine the fire behavior is repeated on a sample of a fabric 200 known from the prior art. The fabric 200 consists of the same warp threads 3 and weft threads 4 that were used in the fabric 100; the plastic of the threads also contains flame-retardant additives.

The electrically conductive threads 2.1', 2.2' are monofilaments that have a high proportion of carbon black, which ensures good conductivity. Flame-retardant additives

cannot be added, as the strength of the threads would be significantly reduced.

The sample is exposed to flame in the area 6' in the same test setup as in the example described above. Due to the high carbon content, rapid ignition and wick-like burning of the electrically conductive threads 2.1', 2.2' was observed. The flame moved along the lattice-like woven threads 2.1', 2.2' through the fabric 200, whereby the threads 3, 4 of the base fabric were also destroyed despite the flame-retardant additives contained therein. The test had to be stopped at the level of destruction of the fabric 200 shown in Fig. 2, as the flame did not extinguish itself.

With the demonstration of flame-retardant properties of the electrically conductive fabric 100 according to the invention, it is suitable for applications in explosive and/or fire-prone environments, such as those typically found in solvent processing plants and in mining due to coal dust and methane gas in the underground air.

Figure 3 shows a flexible bulk material container 10 made from the fabric 100, which consists of a carrying bag 15 with a carrying strap designed as transport loops 17, 17'. In its lid area 14, the carrying bag 15 has a filling nozzle 18 and in its base area 11 an outlet nozzle 19. The carrying bag 5 is made from the high-strength, electrically conductive and flame-retardant synthetic fiber fabric 100. In the collar area 16, in the lid area and in the area of the filling nozzle 18 and outlet nozzle 19, a compaction of the grid 12 made of electrically conductive threads 2.1, 2.2 can be provided to optimize the conduction behavior. Conductive threads 2.1, 2.2 can also be incorporated into the material for the carrying loops 17, 17' to ensure conduction.

• · · · · ft

material. It is important for safety that the container is completely grounded during filling and emptying, so that any electrostatic charges can be dissipated.

Fig. 4 shows a cross section through a shaft 30 underground. In horizontally driven tunnels and shafts with a large cross section, the overburden 31 must be supported, especially if these are intended for the transport of people and/or materials. For this purpose, supports 33 are pressed against the rock 31 with hydraulic rams 32. In order to prevent smaller rock fragments 34 breaking out of the rock 31 from falling between the supports 33, large-area lining sheets or mats are pressed onto the supports 33. These can be wire mats, which are, however, heavy and require a correspondingly large transport capacity. When a fabric sheet 100 according to the invention is used to line the shaft 30, a material that is significantly lighter and easy to cut is available. Since the fabric 100 is very strong, it does not tear even when pressed against protruding rock fragments 34. Electrostatic charges can be discharged to the ground via the woven grid of electrically conductive threads 2.1, 2.2. The possible spread of a fire along the lining is prevented by the flame-retardant properties of the fabric 100.

Claims (21)

Al EUP10A_A2.doc 1. Electrically conductive fabric (100) made of plastic fibers, plastic tapes or plastic threads for the production of high-strength packaging or lining webs, which has electrically non-conductive (3,4) and electrically conductive threads (2.1, 2.2), characterized, - that the electrically non-conductive threads (3,4) are equipped, coated or filled with a flame-retardant additive, - that the electrically conductive threads (2.1, 2.2) consist of a thermoplastic material which is equipped with at least one additive which increases the conductivity or is filled in finely dispersed form, or that the electrically conductive threads (2.1, 2.2) are coated with at least one additive which increases the conductivity and - that at least the electrically conductive threads (2.1, 2.2) can be shrunk in length by means of heat, whereby the shrinkage of the electrically conductive threads (2.1, 2.2) is greater than the shrinkage of the electrically non-conductive threads (3, 4). 2. Fabric (100) according to claim 1, characterized in that when exposed to heat, the shrinkage of the electrically conductive threads (2.1, 2.2) is 1.2 to 4 times, preferably 2 times, the shrinkage of the electrically non-conductive threads (3, 4). A2 3. Fabric (100) according to claim 1 or 2, characterized in that the additive increasing the conductivity is a metal or a metal compound, preferably in particle form. 4. Fabric (100) according to claim 1 or 2, characterized in that the conductivity-increasing additive is carbon in the form of soot and/or graphite or an inorganic conductivity-increasing substance. 5. Fabric (100) according to claim 1 or 2, characterized in that the conductivity-increasing additive is an intrinsic polymer, preferably polyaniline. 6. Fabric (100) according to at least one of claims 1 to 5, characterized in that the electrically conductive threads (2.1, 2.2) are coated with silver. 7. Fabric (100) according to at least one of claims 1 to 6, characterized in that the electrically conductive threads (2.1, 2.2) are monofilaments. 8. Fabric (100) according to at least one of claims 1 to 6, characterized in that the electrically conductive threads (2.1, 2.2) are multifilaments, of which at least one fiber is equipped with additives that increase the conductivity. 9. Fabric (100) according to at least one of claims 1 to 8, characterized in that the electrically conductive threads (2.1, 2.2) are made of a polyester, preferably of polyethylene terephthalate (PET) ₇ . 10. Fabric (100) according to at least one of claims 1 to 9, characterized in that the electrically conductive threads (2.1, 2.2) are woven into the warp (3) and/or the weft (4). A3 11. Fabric (100) according to at least one of claims 1 to 10, characterized in that the electrically non-conductive threads (3, 4) consist of a polypropylene which is filled and/or coated with a flame-retardant halogen-containing additive. 12. Fabric (100) according to at least one of claims 1 to 11, characterized in that the cross section of the electrically non-conductive threads (3, 4) is 2 to 10 times the cross section of the electrically conductive threads (2.1, 2.2). 13. Fabric (100) according to at least one of claims 1 to 12, characterized in that the elastic modulus of the electrically conductive threads (2.1, 2.2) is smaller than the elastic modulus of the remaining thread material woven into the warp (3) and weft (4). 14. Fabric (100) according to at least one of claims 1 to 13, characterized in that the distance between the electrically conductive threads (2.1, 2.2) in the warp direction (3) is 1 to 5 cm, preferably 2.5 cm, and in the weft direction (4) is 10 cm to 60 cm, preferably 40 cm. ~· 15. Fabric (100) according to at least one of claims 1 to 16, characterized in that every tenth to eightieth warp (3) or weft thread (4) is an electrically conductive thread (2.1, 2.2). 16. Fabric (100) according to at least one of claims 1 to 17, characterized in that the surface resistance and/or the earth resistance of the fabric (according to DIN 53 482) is less than λΩ ⁴ Ω. A4 17. Fabric (100) according to at least one of claims 1 to 16, characterized in that the breaking strength of the fabric (100) is 50 N/mm ² to 250 N/mm ² at an elongation at break of 10% to 50%. 18. Bulk goods container (10) which consists of a flexible carrying bag and carrying devices attached thereto (carrying loop (17, 17'), eyelet, belt or similar) and in which at least the carrying bag has been made of a high-strength fabric (100) according to at least one of claims 2 to 18. 19. Bulk goods container (10) according to claim 18, characterized in that the bulk goods container has an increased number of electrically conductive threads in its lid (14) and collar area (16) compared to the remaining fabric of the carrying bag. 20. Bulk goods container (10) according to claim 18 or 19, characterized in that when using carrying loops (17, 17'), these are also made of a high-strength fabric (100) according to at least one of claims 1 to 18. 21. Lining (30) for mining and underground shafts, which is made from a high-strength fabric (100) according to at least one of claims 2 to 18.