Abstract:
Antistatic fabric for flexible containers for bulk material that includes electrically non-conducting threads and static dissipative, special permanent antistatic threads. The static dissipative, special permanent antistatic threads are made of a thermoplastic synthetic with an additive mixed in that increases the conductivity. The static dissipative, special permanent antistatic thread is shaped like a small band or tape with an approximately rectangular cross-section or a multifilament of very thin filaments.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a static dissipative fabric for flexible containers for bulk material. 
     When in use, a separation of charge occurs on fabrics made of non-polar synthetics, especially due to friction, such that electrostatic charges collect on the surface of the fabric and increases static charge in a finite area if they cannot dissipate via the air due to a dry environment with low air humidity. Upon contact with grounded objects and/or persons, these charges can suddenly discharge, whereby a high-energy spark can flash across that may be capable of igniting a dust/air mixture or a gas/air mixture and of triggering an explosion. 
     In addition, there is the risk of charge accumulation due to electrostatic induction. With this physical phenomenon, an electrical field can be formed between two bodies, where a non-contact charge transfer occurs. Thus, electrical charges that are generated when filling a container made of a synthetic fabric can be transferred to adjacent bodies with greater capacities, e.g., non-grounded metal barrels on wood pallets. In this manner, an explosion hazard may exist even in the surrounding area of a container made of synthetic fabric, because of spark generation. 
     The German Patent No. DE 39 38 414 C2 of the applicant discloses a container for bulk material made of an electrically conducting fabric that consists of synthetic fibers or synthetic threads and that includes electrically non-conducting as well as electrically conducting threads, where the electrically conducting threads are made of a polyolefin and contain dispersed carbon black and/or graphite and that are woven into both warp and weft. 
     A fabric of such kind is well suited for the strong mechanical strain that occurs when using the fabric for a flexible container As, for bulk material, and a carrier to dissipate the electrostatic charge is ensured through the electrically conducting threads woven into the fabric. 
     A fabric considered “electrically conducting” exhibits a dissipation resistance to ground of less than 10 8  Ω. Such a dissipation resistance is generally required for explosion protection measures based on various technical safety regulations, and also for flexible containers for bulk material made of Type “C” polypropylene fabric according to the classification of the German industrial research group “Brennbare Stäube/Elektrostatik”. This classification has become accepted worldwide. 
     However, it has been observed that paradoxically such a low dissipation resistance of the fabric entails an adverse effect: due to its low resistance, charges can move rapidly and with a high charge density across the entire surface of the fabric and can suddenly discharge at a point where contact occurs with a charge carrier of an opposite charge for example, a grounded person. Thus, a ground connection always needs to be established prior to the filling procedures that could cause a charge separation, to ensure that if a charge comes into existence it can flow from the fabric immediately to ground. 
     However, this ground connection has proven to be an impediment, because, for example, prior to filling, a container for bulk material has to be individually and manually grounded using a metal clip and a metal cable, and thereafter, the ground connection has to be manually removed. Furthermore, there is the risk of forgetting to make the ground connection due to carelessness. 
     Known from the British Patent No. GB 21 01 559 A1 is a container for bulk material that is manufactured of a fabric that has metal threads woven into it, where said threads are capable of discharging the electrostatic charge of the fabric. 
     The disadvantage of this solution is that the stretching behavior of metal fibers or threads deviates significantly from that of the remaining fabric. This can easily lead to breakage of the metal fibers and thus to an interruption in the discharge. 
     An additional risk is that the metal threads that are made of, e.g., copper, or iron, or alloys thereof, corrode in air. Due to such interruption points, the risk of a spark generation and explosion is increased significantly in case of a static charge. 
     Also known are fabrics that have an antistatic agent applied, such that the finished prefabricated fabric can discharge electrical charges. 
     However, a fabric manufactured in this manner only meets the requirements with regard to fire and explosion hazards in its new condition. The state of the art static dissipative coating, which is capable of discharging Charges, is not durable and has a limited useful life. Equipping a container with an applied static dissipative coating has proven unsuitable in such applications, where the fabric is subject to strong mechanical abrasion. Bulk material containers are subjected to mechanical abrasion in handling, truck loading or unloading, transit and/or stacking. This great expansion of the highly loaded synthetic fabric can cause the coating to tear or separate from the fabric. Such containers are used repeatedly in multiple trip applications. A particular risk exists, when the loss of electrical conductivity caused by the abrasion is not recognized during the container&#39;s multiple trip use and the user assumes protective conditions when they no longer exist. 
     Known from the U.S. Pat. No. 5,679,449 and the U.S. Pat. No. 6,112,772 are flexible containers for bulk material, so-called flexible intermediate bulk containers (FIBC), that are made of a material that contains conductible threads that are metallized. In the issued U.S. patents, the effect of the so-called corona discharge is described. Corona discharge occurs on the very small curvature radius of the woven, metallized carrier threads or tips. The corona discharge is a very weak discharge to the air that is limited to the immediate surroundings of the tips and occurs continuously over a long period such that manual grounding via a grounding cable is not required. 
     However, the conductivity of the fabric is still large enough that a quick transportation of the charge and a related sudden discharge with spark generation can occur upon contact with a large downward sinking charge. 
     Another disadvantage is that the static dissipative, conductible threads in the known fabric are difficult to manufacture and fabricate. Even the application of a metallic surface on a core made of synthetic polymers is involved and expensive. The antistatic sheathing is subject to mechanical wear as has been described above for the full-surface coating. 
     Additionally, the antistatic thread has a cross-sectional geometric design that deviates from the typical fabric weave used for containers for bulk materials and therefore causes problems with regard to process ability. 
     For reasons of process ability and mechanical toughness, the diameter of the coated thread cannot be kept as small as would be desirable for utilizing the corona discharge effect over the entire length of the thread and not only at its ends. Thus, localized charge fields that cannot dissipate through the corona discharge but dissipate suddenly can still occur on the surface of the fabric under unfavorable conditions. 
     Known from the International Patent Publication No. WO 96/09629 is an antistatic additive for thermoplastic synthetics, where said additive consists of a thermoplastic polymer mass that contains an electrically conductible web of non-metallic, microcrystalline pins. This web can be fused such that it can be produced using methods that are common in the synthetics industry and crystallizes out when the synthetics melt cools down. The microcrystalline web is embedded in the polymer mass, and is thus, wear resistant, because it cannot be removed from the surface of a component by abrasion. Furthermore, the embedding of the microcrystalline web in the thermoplastic polymer mass will not separate, migrate or dilute from the originally processed properties and/or state. 
     SUMMARY OF THE INVENTION 
     It is the objective of the invention to develop a fabric of the type mentioned above that exhibits permanent static dissipative properties and that can, therefore, be used in explosion and fire hazard zones and that especially does not need to be grounded in all applications. 
     According to the invention, this objective is achieved with a static dissipative fabric for flexible containers for bulk materials, where said fabric includes electrically non-conducting and static dissipative, called antistatic threads, where the antistatic threads are made of a thermoplastic synthetic with a special permanent antistatic additive mixed in that reduces the electrostatic resistivity, and where the static dissipative thread is designed either in the shape of small band called tape with an almost rectangular cross-section or as a thin thread with a substantially round cross-section. 
     Here, the term “antistatic” refers to a fabric whose static discharge resistance of the surface according to German Standard (Deutsche Industrie Norm) DIN 53482 is greater than 10 8  Ω and less than 10 11  Ω. With such a resistance, the flow of electrons is strongly inhibited and controlled yet still possible. 
     The term“small bands” refers to small synthetic bands that can be produced from extruded tape, multi filament yarn, monofilament and/or cut from foil, that have in their cross-section a greater width in relation to their thickness and that can be converted to fabrics through weaving. 
     Mixing the conducting additives into the polymer mass of the threads accomplishes on the one hand that the electron conduction within the thread is enabled, and on the other hand that the specific electrical resistance of the resultant thermoplastic mixture is so great that the electron flow is possible only at a very slow and controlled rate. This ensures that a continuous electron flow and a constant dissipation of the charge to the surrounding area is possible within a short period of time so that not enough charge can be stored in the bulk container that could lead to a sudden high-energy discharge generating an igniting spark. 
     Very narrow, sharp edges are present due to the geometry of the permanent antistatic band-shaped thread. Along the entire length of the small tape, a corona discharge can occur at the edges enabling a continuous and controlled dissipation of the charge from the surface of the fabric to the surrounding area. In addition to the advantage of the corona discharge at the narrow edges, another advantage exists due to the relatively wide small tape resulting in a large surface that exhibits a capacity to take up electrical charges. The charges are distributed across the large surface of the fabric, thus avoiding local charge concentrations that could lead to sudden high-energy discharge. 
     To avoid the creation of “islands” or small areas of electro statically insulating fabric sections amidst the grid of electrically conducting threads, the distance of one thread to the next is not less than 1 cm and not greater than 5 cm. Preferably, a distance of 3 cm is selected. 
     It is advantageous that the fabric includes a special permanent antistatic synthetic coating covering the warp threads, the weft Hi threads and the static dissipative, permanent antistatic threads, where said coating is made of a thermoplastic synthetic with an additive mixed in that increases the conductivity. This coating results in a large-area distribution of the charge across the fabric surface and thus in dissipating local charge peaks. 
     For the reasons mentioned above, one can in most applications avoid using manually connected grounding clips and cables when filling, handling, or discharging a flexible container for bulk material. A flexible container for bulk materials consists of a flexible carrying bag with attached carrying devices, where at least the carrying bag is made of fabric with static dissipative properties subject to the invention, so that a contact-free dissipation of the electrical charge to the surrounding area is made possible. 
     Especially with a flexible container for bulk material of the kind mentioned above, where at least the carrying bag is made of the static dissipative fabric subject to the invention and where a permanent antistatic synthetic coating is applied on the surface of the fabric, it is advantageous that the ability to establish manual grounding remains. For this purpose, a grounding clip only needs to be attached to a fold of the fabric. In this case, the antistatic coating not only ensures a good charge distribution across the entire surface but also that the ground clip is electrically connected with the static dissipative threads in the warp and/or weft. 
     Additional advantageous designs become apparent from the sub-claims and the following description of an exemplary embodiment. 
     For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a perspective view of a container for bulk material made of the fabric subject to the invention. 
     FIG. 2 shows a greatly magnified overhead view of a section of the surface of a static dissipative, special permanent antistatic. 
     FIG. 3 shows an overhead view of a fabric subject to the invention. 
     FIG. 4 shows a schematic discharge plot for the fabric subject to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will now be described with reference to FIGS. 1-4 of the drawings. Identical elements in the various figures are designated with the same reference numerals. 
     FIG. 1 shows a flexible container  10  for bulk material made of the fabric  100 , where said container  10  consists of a carrying bag  15  with a carrying belt designed as transport loops  17 ,  17 ′. In its lid section  14 , the carrying bag  15  includes a filler spout  18  and in its bottom section  11  an outlet spout  19 . The carrying bag  15  is made of the antistatic fabric  100  subject to the invention. In the collar section  16 , in the lid section  14  as well as in the area of the filler spout  18  and the outlet spout  19 , a more dense grid  12  of the static dissipative, permanent antistatic threads can be provided to optimize the discharge behavior. Conduction material is also integrated in the material for the carrying loops  17 ,  17 ′ to ensure the discharge, 
     FIG. 2 is a schematic view of a static dissipative, special permanent antistatic warp thread  4  or weft thread  5  as it can be seen under a microscope. With this advantageous embodiment, 20 percent in mass of an additive as described in WO 96/09629 is mixed into a base polymer, here polypropylene. 
     Numerous microcrystalline pins  6  are embedded in a matrix  8  of the base polymer, where said pins  6  are meltable and crystallize upon cooling. In this manner, the mixture can be extruded, injected or processed using another synthetics processing method. 
     The pins  6  are arranged in the matrix  8  with such a density that they are in contact with one another or that they overlap. Thus, using the example of the section presented here, numerous current paths  7  are formed by the microcrystalline pins  6  between a random point 7.1 and another point 7.2 at the other end of the thread section, where one such path is shown as an example as a thick drawn line. An inhibited charge transport is possible along such a current path  7 . 
     The density of the microcrystalline pins  6 , and therefore the number of generated current paths  7 , is adjusted by metering the mass portion of the additive between 5% and 30% versus the polypropylene matrix, which in turn influences the overall electrostatic resistivity of the polymer mixture. Furthermore, it can be provided that in addition to the additive, pin-shaped metal particles of macroscopic dimensions that are of about 0.1 to 2 mm in length are embedded in the matrix  8 . These metal particles protrude in the shape of fine tips from the surface of the static dissipative threads; a corona spray discharge can occur at these tips. 
     FIG. 3 shows a section of a fabric  100  made according to the invention. Both warp threads  3  and weft threads  4  are small bands) tapes made of a thermoplastic synthetic. Such small tapes are easily obtained in that a synthetic foil is made that is then cut into small tapes with a knife in the direction of the web; the bands are then stretched. Because standard synthetics, especially polypropylene, are suitable and the small bands are relatively wide about 0.5 to 5 mm, when compared to textile threads, large-area fabrics can be made cost-effectively. Woven into the fabric  100  are static dissipative, special permanent antistatic threads  4  that are drawn schematically as thick double-lines for a clearer presentation. 
     Preferably, the static dissipative, special permanent antistatic threads  4  are woven into the warp at a distance of 3 cm to one another. To ensure a dissipation of the electrical charge, especially in case of breakage of the static dissipative, special permanent antistatic threads  5  can also be woven into the weft at greater distances, preferably of 30 cm. In case of an interruption of a antistatic warp thread  4 , a bypass of the electrical charge flow to the next intact warp thread  4  can be accomplished via these weft thread  5 . 
     Whether the static dissipative, special permanent antistatic threads  4  are mainly woven as weft threads or as warp threads has no influence on the electrical properties of the fabric subject to the invention, and can be selected according to the Am requirements of the weaver. 
     Because the geometry of the non-conducting threads  2 ,  3  and of the static dissipative, special permanent antistatic threads  4 ,  5  is preferably the same, there are no difficulties in weaving if the static dissipative, special permanent antistatic threads are woven in the weft. 
     However, round threads with mixed-in special permanent antistatic additives can be used as well if they are very thin and thus provide the possibility of a good corona discharge. Round threads can be grouped as so-called multifilaments to form a larger thread. 
     FIG. 4 shows the discharge of the fabric. The voltage drop is plotted over time. Starting with a high potential UH, the charge supply is interrupted at the time t1. As the solid top line shows, the charge dissipates continuously through continuous corona discharges at the edges of the small bands, the ends of the threads, and eventually at the macroscopic metal tips that may have been mixed in, until it reaches a minimum at a level of UL1 at the time t2. The fabric was not grounded in the FIG. 4 example. 
     If the same fabric is grounded using a grounding cable, the charge dissipates faster and to an even lower level UL2—as indicated by the broken line; however, the levels of both remaining potentials UL1 and UL2 are low enough, such that no igniting spark occurs through contact with a person or a metallic object, etc. 
     There has thus been shown and described a novel static dissipative fabric for flexible containers for bulk material which fulfills all the object and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is to be limited only by the claims which follow.