Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2001-05-11
2003-06-03
Pyon, Harold (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C383S024000, C383S109000, C383S113000, C383S116000, C383S117000, C139S00100C, C442S110000, C428S368000, C428S374000, C428S397000
Reexamination Certificate
active
06572942
ABSTRACT:
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
&OHgr;. 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'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
Boyd Bruce A.
Grewe Andreas
Grewe Stephan
Hartmann Siegfried
Wurr Egon
Egan Brian P
Eurea Verpackungs GmbH & CO. KG
Milde & Hoffberg LLP
Pyon Harold
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