DE9417151U1 - Electrically conductive multilayer tape - Google Patents

Description

ELECTRICALLY CONDUCTIVE MULTILAYER TAPE

The invention relates to electrically conductive multilayer tapes comprising thermoplastic resin and electrically conductive particles, as well as fabrics made from such tapes.

Bulk containers made from woven tapes of plastic materials such as polyolefins and polyesters are widely used for storing, handling and transporting bulk particulate materials such as fertilizers, cement, chemicals, agricultural commodities and minerals. The use of such containers is favored by the high strength and durability, low cost, chemical inertness, good yield and recyclability and ease of manufacture of the tapes and fabrics. A disadvantage of such containers, however, is their tendency to build up electrostatic charges due to friction which occurs during loading, unloading and movement of the contents. In the case of containers made from woven polypropylene tapes, the surface resistivity is typically in the range of about 1 × 10 -1 to 1 × 10 -1 ohms. However, when flammable gases or particle/air mixtures are present in the environment, resistivities of 1 × 10 -1 to 1 × 10 -1 are acceptable. ⁸ or below is necessary to prevent the build-up of an electrostatic charge that may pose an ignition risk.

In mining, for example, bulk containers have been made from fabrics in which metal threads or carbon fibres have been incorporated in order to prevent explosions due to electrostatic electricity when filling or emptying bulk containers in areas where flammable gases are present and to dissipate the electrostatic charge. Such fabrics in which carbon fibres

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interwoven with polyolefin filaments are described, for example, in CA-PS-I 143 673. A disadvantage is that the metal threads and carbon fibers have a lower elongation than the other filaments or yarns of the fabric, which leads to a break in the conductive threads. This break in turn causes an interruption in the conductivity and in practice can increase the risk of sparking and explosion if an electrostatic charge occurs. It is also known to use synthetic fabrics that have been made conductive or non-electrically chargeable by chemical treatments. However, these treatments increase the cost and complexity of the fabric manufacturing processes and often lose their effectiveness over time. Longer-lasting treatments impair the recyclability of the fabrics.

Another solution was to produce fabrics from ribbons or other yarns containing conductive particles, e.g. metallic particles or conductive carbon black.

WO 93/01110 discloses flexible bulk containers made from fabrics in which bands with incorporated conductive particles are present in the warp and/or weft of the fabric at specific intervals to impart sufficient conductivity to dissipate the surface charge. Container fabrics in which conductive threads of synthetic fibers containing conductive carbon black are interwoven with non-conductive threads in both the warp and weft to effect dissipation of the electrostatic charge are described in US-PS-5 092 683.

Although these containers and fabrics provide protection against electrostatic charging, they have disadvantages. Co-weaving conductive filaments or threads with tape yarns typically used in container fabrics usually requires separate beaming and separate tension control of the different yarn types

due to their different dimensions, cross-sections and strengths. This can lead to an increase in the cost and complexity of the weaving machines and processes. However, without such modifications, filament breakage and interruption of the weaving process due to overlap of the filaments with the wider flat ribbons often occur. However, even with separate beams and separate tension control, weaving conductive filaments with ribbons often results in fabrics in which the typically smaller, substantially round cross-section filaments are overlapped or covered by the wider flatter ribbons. This results in the conductive filaments being buried in the fabric for part of their length. Although this burial of the filaments does not normally affect the actual conductivity or discharge capability of the fabrics, their market and utility value can be limited because the fabrics are often not recognized as conductive by customers and consumers. Other disadvantages of the conductive filaments and tapes according to WO 93/01110 are their strength, elongation and manufacturing. Loadings of conductive particles high enough to impart adequate electrical conductivity complicate melt spinning of the filaments, extruding the films and slitting the tapes. When melt spinning the filaments, high particle loadings increase shear and therefore polymer degradation, reducing filament strength. The presence of the conductive particles also reduces strength, which can lead to breakage during weaving. The use of higher molecular weight polymers to compensate for polymer degradation is ineffective because such polymers are usually too viscous in the molten state to obtain good dispersion of the relatively high loadings of conductive particles required for sufficient conductivity. Problems encountered when melt spinning highly filled filaments,

and a modified process for spinning and partially orienting such filaments are described in US Patent 5,091,130. High loadings of conductive particles also cause difficulties in the extrusion of films and slitting into tapes. Often extruded films are formed with thin or weak points, so that the films or tapes can break during drawing and weaving. Gaps can also occur in the extruded film, which affect product quality and processability. Furthermore, as with threads or filaments filled with conductive particles, the strength of the tapes can be reduced due to the conductive particles. The presence of conductive particles in the films also affects their slitting into tapes, since the particles abrade the slitting knives. The quality of the tape suffers as a result, unless additional costs are incurred for more frequent knife replacement.

The electrostatic charge on synthetic fabrics is also detrimental in other applications and environments. Examples include the clinging of clothing, the electric shock often felt when touching a grounded object while standing on a carpeted floor, and the damage to sensitive electronic circuits due to the accumulation of charge in carpets.

The invention relates to an electrically conductive multilayer tape, which is characterized by at least one layer which comprises a thermoplastic polymer and electrically conductive particles contained therein, forms an outer surface of the tape and is connected to at least one layer of a thermoplastic polymer composition, which is at least substantially free of electrically conductive particles, at the interface between the layers.

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In a specific embodiment, the invention relates to an electrically conductive, substantially flat, multilayer tape of substantially rectangular cross-section comprising at least one layer constituting a substantially flat outer surface of the tape and comprising at least one thermoplastic resin containing an effective amount of electrically conductive particles, said layer being bonded to at least one other layer comprising a thermoplastic resin composition substantially free of conductive particles.

The invention further relates to fabrics comprising such conductive tapes, as well as bags, containers, carpet backings and other products made from such fabrics.

An advantage of the tapes of the invention is that the layer(s) comprising the thermoplastic resin and electrically conductive particles contain sufficient conductive material to impart conductivity, while the layer(s) which are substantially free of conductive particles impart strength. Thus, the disadvantages of known conductive filaments and tapes are avoided, in which strength is sacrificed at the expense of conductivity or vice versa.

Another advantage of the tapes of the invention is that they can be easily manufactured in dimensions that match those of non-conductive tapes used to weave various fabrics, thus avoiding irregularities in the fabric and "burying" of the conductive filaments. This improves weaving performance and when weaving the tapes of the invention with pigmented or so-called "natural" colored tapes, the conductive nature of the resulting fabrics and articles is visually apparent.

A further advantage of the tapes according to the invention is that they remain conductive even after repeated use, unlike products with conductive surface treatments which lose their effectiveness over time. A further advantage, particularly over fabrics containing conductive metal filaments or carbon fibers, is that the tapes, fabrics and fabric products according to the invention can be recycled without separating incompatible filaments.

The electrically conductive multilayer tapes of the present invention can be prepared, for example, by quenching a film comprising at least one layer of a thermoplastic resin containing electrically conductive particles and at least one layer of a thermoplastic resin composition substantially free of electrically conductive particles; slitting the quenched film longitudinally into tapes; and orienting the film at least longitudinally by stretching the tapes.

Advantages of this method of production compared to processes for producing single-layer conductive tapes or filaments are the cost advantages resulting from the lower use of conductive particles and the better product quality. According to the invention, only a portion of the tapes are filled with conductive particles and the use of conductive particles is therefore considerably lower than in the production of single-layer conductive tapes or filaments. When coextruding a multilayer film, the essentially non-conductive layer(s) supports the adjacent conductive layer(s), so that a more uniform thickness of the extruded film is achieved and gaps in the film are essentially avoided. Compared to the production of single-layer tapes, higher

Extrusion speeds and throughputs and better product quality can be achieved. The tapes of the invention are less prone to breakage during drawing than single-layer tapes, so that higher production rates can be achieved because the layer free of conductive particles provides greater strength than a single layer filled with conductive particles. In addition, slitting is more efficient and less expensive because the layer free of conductive particles makes slitting into tapes easier and the slitting knives are less damaged than in the case of single-layer tapes containing conductive particles.

When weaving fabrics, the conductive tapes of the invention also provide advantages over conductive filaments. As mentioned above, conductive filaments normally require separate beaming of the cut warp tapes to avoid burring in the fabrics and breakage due to increased tension caused by the smaller circumference of the filament yarns. In contrast, the tapes of the invention can be beamed simultaneously with other warp tapes to be woven into the fabrics, since the former are manufactured in dimensions and linear densities compatible with the other tapes for weaving purposes.

The tapes of the invention are readily woven or knitted into fabrics suitable for use in the manufacture of a wide variety of finished goods, such as bulk containers, industrial and agricultural bags, carpet backings, and mining fabrics.

In the drawing show:

Fig. 1 a side view of a two-layer

electrically conductive tape according to the invention;

Fig. 2 is a cross-section through the tape of Fig. 1; Fig. 3 is a side view of a three-layer

electrically conductive tape according to

Invention;
Fig. 4 an electrically conductive fabric according to the

Invention.

The electrically conductive tapes of the present invention are substantially flat structures having a substantially rectangular cross-section. The tapes have a multilayer construction in which at least one layer comprises a thermoplastic polymer composition which is substantially free of electrically conductive particles and at least one other layer, which provides a substantially flat outer surface of the tape, comprises a thermoplastic resin composition in which electrically conductive particles are dispersed in at least one thermoplastic resin in an amount effective to impart electrical conductivity. The term "at least substantially free of electrically conductive particles" characterizes compositions which are either free of such particles or contain them in such a small amount that neither the conductivity nor the strength of the base composition are appreciably affected. The individual layers of the multilayer tapes of the invention extend substantially equally along their width and length, with a surface of each layer being connected to a surface of an adjacent layer at an intermediate interface such that the total thickness is composed of the thicknesses of the individual layers. This laminar structure of continuous layers includes both strength-imparting and conductive layers over virtually the entire length and width of the tapes. It will be apparent to those skilled in the art that the structure of the tapes distinguishes them from filaments both structurally and functionally. The

The essentially flat nature and the essentially rectangular cross-section of the tapes makes them particularly suitable for weaving into flat fabrics such as those used to make bags and containers. When densely woven, these fabrics give good coverage and weaving efficiency, expressed as the number of yarns required to cover a given area. The flat nature of the tapes also allows good contact between the warp and weft tapes at the crossing points of the fabric, thus promoting the conductivity of the fabrics. In contrast, filaments when woven with tapes are not well suited to making flat fabrics, nor do they allow the same level of coverage efficiency.

Features and properties of the tapes of the invention are illustrated in the drawing. As shown in Figures 1 and 2, the tape 1 has a laminar construction with a conductive layer 3 comprising a thermoplastic resin composition having electrically conductive particles dispersed therein bonded to a non-conductive layer 5 comprising a thermoplastic resin substantially free of conductive particles, with an interface 7 between the layers. Figure 2 also illustrates the generally flat nature of the tapes of the invention and their substantially rectangular cross-section.

The tapes according to the invention can be manufactured in such a way 0 that they have a strength and conductivity tailored to the respective application. In general, tapes with specific resistances

of up to about 1 x 10 ohms and tensile strengths of at least about 1.8 cN/dtex in the longitudinal direction provide a combination of conductivity and strength suitable for a wide variety of uses. For most

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In many applications there is little need for resistivities below about 1 x 10 ⁴ ohms and the loadings of conductive particles required to achieve such low resistances can be so high as to complicate manufacture of the tapes. The strength of the tapes of the invention must also be related to elongation. The tapes should be sufficiently strong to resist breakage during weaving and also have sufficient elongation so that stresses to which fabrics woven therefrom are subjected do not cause breakage. However, if the elongation is too high this can be at the expense of conductivity since stretching can cause a break in the contact between the conductive particles in the conductive layers. In general an elongation of about 5 to 30% is suitable and about 10 to 25% is preferred to ensure conductivity even in the stretched state. In a preferred embodiment of the invention, wherein the belts are used in fabrics for bulk intermediate containers of the type used for transporting solid bulk chemicals or other solid particulate materials, the belts preferably have resistivities of from about 1 x 10 ⁵ to about 1 x 10 ⁷ ohms, tenacity of at least about 2 cN/dtex in the longitudinal direction, and elongations of from about 15 to 25%.

Preferred configurations of the tapes of the invention are two- and three-layer structures. Examples of these are shown in Figures 1 to 3. In a two-layer configuration, the conductive layer comprising a thermoplastic resin composition containing conductive particles and the layer comprising a thermoplastic resin composition substantially free of conductive particles are bonded in laminar contact, with one surface of each layer bonded to the other at an interface and the other surface

each layer provides a substantially flat outer surface. A three-layer conductive tape, most preferred according to the invention, is shown in Fig. 3. The tape 9 has two conductive layers 11, 13, each comprising a thermoplastic resin having conductive particles dispersed therein, bonded to the substantially non-conductive layer 15 such that the latter is sandwiched between the two conductive layers. This construction not only provides conductivity and strength, but when the tapes thus constructed are used in the warp and weft of fabrics, they also promote good surface conductivity and dissipation of electrostatic charge because good contact of the conductive surfaces of warp and weft tapes at their intersection points is ensured by the conductive nature of the layers forming both outer surfaces of the tapes.

Although two- and three-layer constructions are preferred, the invention optionally includes additional layers. For example, additional layers of thermoplastic resin compositions may be provided to impart special properties, e.g., flame resistance, additional strength, antimicrobial properties, or other properties to the tapes and fabrics made therefrom. Regardless of the number of layers, the most preferred tapes in making conductive fabrics capable of dissipating electrical charge are those constructed so that the electrically conductive layer(s) comprise at least a portion of both outer surfaces of the tapes. In this regard, three-layer tapes are most preferred from the standpoint of cost, manufacturability, and application properties.

As a thermoplastic resin of the layers of the

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Any polymers that can be processed into films and subsequently into tapes can be used in the tapes of the invention. The polymers used in the adjacent layers can be the same or different and should be compatible in the sense that they can be bonded together by heat, pressure, ultrasonic treatment, adhesives, combinations of these or other suitable bonding agents. Examples of such polymers are polyamides, linear polyesters and polymers of substituted or unsubstituted olefin monomers such as polyvinyl chloride, polyacrylamide, polyacrylonitrile, polyvinyl acetate, polyacrylic acid, polyvinyl methyl ether, polyethylene, polypropylene, poly(1-hexene), poly(4-methyl-1-pentene), poly(1-butene), poly(3-methyl-1-butene), poly(3-phenyl-1-propene) and poly(vinylcyclohexane). Homopolymers and copolymers can be used, as well as blends of such polymers with one or more other thermoplastic polymers.

Preferred are homo- and copolymers based on alpha-olefin monomers having 2 to about 12 carbon atoms and mixtures thereof, such as polyethylene, polypropylene, ethylene-propylene copolymers, polyisobutene, poly(4-methyl-1-pentene), poly(1-butene), poly(1-hexene), poly(5-methyl-1-hexene) and the like. Particularly preferred alpha-olefin resins are polyethylenes, polypropylenes and propylene-dominated copolymers of high or low density or linear polymers of this type of low density. Propylene polymer resins are most preferred due to their cost, processability and application properties.

The propylene polymer resins most preferred for making the tapes of the invention comprise essentially crystalline homopolymers of propylene or copolymers of propylene with minor amounts, e.g. up to about

3 0 mole percent of one or more other copolymerizable alpha-olefins, such as ethylene, butene-1 and pentene-1, as well as blends of propylene polymers with minor amounts, e.g. up to about 20 weight percent, of another polyolefin, e.g. LDPE or LLDPE. Such propylene polymers are known and commercially available. A particularly preferred propylene polymer resin is homopolymeric polypropylene.

The propylene polymer resin into which the electrically conductive particles are incorporated preferably has a flowability of about 1.5 to 20 g/10 min to obtain film and tape properties suitable for the intended use while facilitating dispersion of conductive particles at a sufficiently high loading to achieve useful electrical conductivity. Polypropylene resins with higher melt viscosities, e.g., flowabilities below about 1.5 g/10 min, are less suitable because electrically conductive particles are difficult to disperse. Degradation of the polypropylene polymer during processing can cause the flowability to increase to about twice that of the starting material and also result in lower melt strength during extrusion and weak films and tapes with thin patches and uneven density. Accordingly, propylene polymers with low melt viscosity, e.g. flowabilities above about 20 g/10 min, are not preferred.

Preferably, the propylene polymer has a flowability of about 2 to 15 g/10 min in order to achieve good dispersion of the electrically conductive particles and to ensure the formation of films with a substantially uniform thickness. When a propylene polymer is used as the thermoplastic polymer to produce the layer substantially free of conductive particles, the flowability of this polymer is usually in the range of

about 2 to 20 g/10 min, preferably about 2.5 to 15 g/10 min. When the film is prepared by coextrusion, the flowability of the composition comprising conductive particles and thermoplastic polymer used for the conductive surface layer(s) is at most equal to and preferably slightly lower than the flowability of the composition used for the substantially non-conductive layer, so that the greater melt strength of the former compensates for the strength-reducing effect of the conductive particles, while the greater flowability of the latter simultaneously promotes smooth extrusion of the more viscous filled resin composition. When producing the tapes according to the invention by coextrusion of a multilayer film, it is particularly preferred if a composition comprising a propylene polymer having a flowability of about 2 to 5 g/10 min, electrically conductive particles and optionally up to about 10% by weight of LDPE or LLDPE is used for the conductive layer(s) and a composition comprising a propylene polymer resin having a flowability of about 3 to 8 g/10 min, which is substantially free of conductive particles, is used for the substantially non-conductive layer(s). Particularly good results are obtained in coextrusion if the ratio of the flowability of the composition used for the conductive layer(s) to the flowability of the propylene polymer resin composition used for the substantially non-conductive layer(s) is about 0.1:1 to 0.5:1, in particular about 0.2:1 to 0.4:1. Unless otherwise stated, the flow properties given here are determined according to ASTM D-1238 at 230 ⁰ C and a filling quantity of 2.16 kg.

Electrically conductive particles that can be used according to the invention are, for example, metallic powders, particles and whiskers as well as

electrically conductive carbon black. Metals that can be used include iron, aluminum, silver, and copper. The particles should be fine enough to be dispersible in the thermoplastic polymer used for the conductive layer(s). In general, average particle sizes of less than about 25 jtxm give good results, although for a particular material, thermoplastic resin, and tape manufacturing process, larger particle sizes may be advantageous. Carbon black is a preferred conductive material due to its relatively low cost, good dispersibility in thermoplastic resins, and chemical inertness. It is also more suitable for plastics recycling than metallic particles or powders.

Depending on the properties, carbon blacks are capable of imparting high electrical conductivity or, on the other hand, extremely high resistance. According to the invention, electrically conductive carbon blacks are used to impart conductivity so that electrons can flow through the layer(s) of the tape containing the carbon black dispersed therein. The electrical conductivity of conductive carbon black is related to its particle size, structure and content of volatile components. The conductivity achieved according to the invention by using carbon black is based on the dispersion of electrically conductive carbon black particles in the thermoplastic resin matrix or continuous phase, the carbon black particles being in particle-to-particle contact within the conductive layer(s). For the purposes of the invention, the carbon black should therefore have a fairly complex structure and be easily dispersible in the thermoplastic resin used to form the conductive layer(s). Carbon blacks with so-called medium and high structure, which generally comprise voluminous, more or less irregularly shaped primary aggregates of relatively low density and consist of numerous primary particles with

Carbon blacks consisting of branching and chain formation are well suited for the tapes of the invention because of their high conductivity and good dispersibility. Less highly structured carbon blacks may also be useful but require higher loading levels than high or medium structure carbon blacks with associated losses in strength and elongation to achieve good conductivity. Conveniently, the average particle size of the carbon black is about 15 to 35 nm, preferably about 20 to 30 nm. Examples of preferred carbon blacks are Vulcan ^(R) XC72R and P, both high structure materials from Cabot Corporation. Typical properties are a nitrogen surface area of 250 and 140 m /g, respectively, an average particle size of 30 and 20 nm, a volatile content of 1.5% and 1.4%, respectively, and a density of 0.096 g/cm ³ and 0.224 g/cm ³ . Combinations of different carbon blacks may be used where appropriate to utilize the advantages of each component.

0 The conductivity also depends on the degree of dispersion of the carbon black or other conductive particles in the conductive layer(s) of the tape. The concentration of the conductive particles on one or both surface layers of the tape according to the invention favors an effective use of the conductive particles.

Conveniently, about 10 to 40 weight percent of the electrically conductive layer is conductive particles. Below about 10 weight percent, conductivity may be insufficient, while above about 40 weight percent, extrusion of conductive layers of uniform thickness is difficult and the strength and elongation of the resulting tapes may be too low to be used in weaving or in bag and container fabrics. Using too much conductive carbon black may result in reduced conductivity due to shearing and destruction of the carbon black particles. Preferably, about 25 to 35 weight percent conductive carbon black is used to provide conductivity.

combined with good strength and elongation without complicating the film extrusion. In the three-layer tape construction preferred according to the invention, each of the outer layers filled with conductive carbon black preferably contains about 25 to 35% by weight of conductive carbon black. It is particularly preferred if each such layer contains approximately equal amounts of conductive particles so that the surface resistance of each layer is approximately the same. Of course, different loading levels can be used in the layers if this is desired in view of the respective application purposes.

The multilayer tapes have a construction in which the layer(s) of thermoplastic polymer composition containing electrically conductive material constitute a portion of the total thickness of the tapes and the layer(s) of thermoplastic polymer composition substantially free of conductive material constitute a portion of the thickness. Generally, the layer(s) containing conductive particles constitute about 10 to 90% of the thickness, preferably about 30 to 60%, to achieve a good balance of strength and conductivity. In the particularly preferred three-layer tape having an inner layer of thermoplastic polymer composition substantially free of conductive particles and two outer layers of conductive polymer composition, the inner layer preferably constitutes about 40 to 60% of the thickness and each of the outer layers constitutes about 20 to 30% of the thickness. For most applications, the thickness of the outer layers in this construction is about the same. The total thickness of the multilayer tapes is preferably in the range of about 30 to 200 μm, although thicker tapes are also suitable for some applications.

If necessary, one or more layers of the

The tapes of the invention may contain additives to impart other properties, provided that the additives do not affect the strength and electrical conductivity of the tapes or their manufacture. Examples of additives that can be used are antioxidants, antistatic agents, lubricants, UV absorbents, pigments such as titanium dioxide and non-conductive carbon black, matting agents, heat, light and oxidation stabilizers, opacifiers such as chalk and calcium carbonate, antimicrobial agents such as 2,4,4'-trichloro-2'-hydroxydiphenyl ether, flame retardants and various fillers such as talc, calcium carbonate, gypsum, kaolin, silica and diatomaceous earth.

The average particle diameter of these fillers should generally be no greater than about 5 μηι, preferably about 1 to 3 μνα. When fillers are used in one or more layers, each such layer preferably contains no more than about 10% by weight of filler, more preferably about 0.5 to 6% by weight, based on the weight of the polymer component. Higher filler levels may impair the processability and dispersion of the conductive particles.

The electrically conductive tapes of the present invention can be prepared by a process comprising: preparing a quenched film comprising at least one layer of a thermoplastic resin composition containing an effective amount of electrically conductive particles and at least one layer of a thermoplastic resin composition substantially free of conductive particles; slitting the quenched film longitudinally; and orienting the quenched film. The slitting and orientation can be performed in any order. Preferably, the quenched film is slit longitudinally into a plurality of tapes and the tapes are then oriented.

However, suitable tapes can also be obtained by first orienting the quenched foil and then slitting the oriented foil into tapes.

Any method may be used to incorporate the electrically conductive particles into the thermoplastic resin. Melt blending, e.g. in an extruder, usually gives a more uniform dispersion of the conductive particles than dry blending. However, dry blending is also applicable and, if carried out prior to melt blending, can facilitate the latter and give a more uniform dispersion of the conductive particles. Carbon black or other conductive particles may also be incorporated into the polymer material by means of Banbury mixers or continuous blending techniques. Batch processing on heated two-roll mills is also applicable. Concentrates of carbon black or other conductive particles in thermoplastic resins, preferably the same as the resin of the conductive layer or a resin compatible therewith in the sense that it is readily dispersible and melt processable under similar conditions and forms a single phase system, may also be used with good results.

The multilayer films can be produced by any known method, e.g. by extrusion coating, extrusion lamination or other laminating processes, coextrusion and bonding of 0 separate film layers by thermal means or with adhesives. Single layer films produced by various processes, e.g. by calendering, extrusion or casting, can be laminated with other films with adhesives or by the application of heat and pressure or they can be coated to form multilayer films. The blow extrusion and slot casting or T-slot extrusion processes are

particularly preferred for the production of both single- and multi-layer films.

In the blown extrusion process, a tube of polymer melt is extruded from an annular die, inflated with air to a size determined by the desired film and tape properties, cooled with chilled or normal temperature air, folded into a flat tube, and wound onto reels for further processing. For coextruded multilayer films, the polymer melt tube is formed in multiple layers including at least one outer surface layer having conductive particles therein and at least one other layer substantially free of conductive particles.

In the slot cast extrusion process, the polymer melt is extruded from a slot die into a quenching device, such as a water bath or chill roll, which rapidly cools the polymer melt into a quenched film. A flat die lip structure is preferably used for extruding films, although profiled dies can also be used. In these processes, the quenching rate is usually increased relative to the rate of polymer melt flow from the die to effect stripping.

To produce coextruded films comprising a thermoplastic polymer composition substantially free of conductive particles as one or more layers and a thermoplastic polymer composition comprising electrically conductive particles as one or more other layers, an extruder is used to extrude the film of the thermoplastic polymer composition substantially free of conductive material. One or more

other extruders may be used to extrude a film of a polymer composition containing conductive particles from the same or a different slot die. For example, the films may be brought into contact at a gap between two rolls. If a layer of conductive polymer composition on both sides of the composition that is substantially free of conductive particles is desired, the polymer melt containing conductive particles may be split between the two slot dies, with one of the films coming into contact with the substantially non-conductive film at a first gap and a second film coming into contact with the other side of the substantially non-conductive film at a gap between the second pair of rolls. Alternatively, multiple extruders may be used to feed a polymer melt to a coextrusion die that allows two or more different layers to be coextruded from a single die.

0 The polymer composition containing conductive particles can also be supplied as a molten film which can be brought into contact with a film layer of a thermoplastic polymer composition which is essentially free of conductive particles at or immediately before the gap between the two rollers rotating against each other. Separate films of the thermoplastic resin composition with or essentially without conductive particles can also be combined to form a multilayer film, for example by the action of heat and pressure or with suitable adhesives.

After manufacture, the film is quenched. Quenching is carried out to cool the film and to solidify the resin(s) of which it is composed. Quenching can be carried out by any known method. Examples of suitable quenching techniques are passing the film over a

chill roll or by a water bath. Quenching times and temperatures will vary depending on the operating speeds and the particular resin used, but can be readily determined by one skilled in the art. It is important that quenching of the molten film be carried out after sufficient draw down so that the film is stabilized for further processing. Sufficient draw down of the molten extruded film can be achieved, for example, by appropriately selecting the extrusion speed, the speed at which the molten film is fed to the quenching device, or both speeds so that quenching is carried out at a higher speed relative to the extrusion speed.

Slitting of the quenched foil can be carried out by any known method. Usually, a series of parallel cutting blades or knives is used over which the foil is passed under moderate tension. Slitting can be carried out after or, preferably, before orienting the foil. If carried out after orientation, the foil is cut into tapes of a width that makes them suitable for weaving or knitting. Normal widths are in the range of about 0.1 to 8 mm for typical weaving applications and equipment, although wider tapes can be made if necessary and can be woven in the folded or fibrillated state even on conventional looms. If slitting is carried out before orientation, wider widths are obtained due to the constriction of up to 50% that can occur as a result of orientation of the tapes.

The films or tapes can be oriented using any known method. It is important that the tapes are oriented at least longitudinally. Although this

is not required, biaxial orientation in both the longitudinal and transverse directions can also be carried out. The orientation results in improved tensile strength of the films or tapes in the orientation direction and improves their processability. The orientation is normally carried out at a sufficiently high temperature to soften the films or tapes and cause relaxation of the polymer molecules. The orientation in the longitudinal direction is preferably carried out by passing the tapes over a series of rollers which rotate at increasing speeds. Preferably, the roller speeds are such that the speed ratio of the downstream roller to the upstream roller is about 2.5:1 to 8:1. This ratio is also referred to as the stretch ratio. The person skilled in the art will know that the entire tension can be exerted by one roller or roller assembly on a subsequent roller or roller assembly or, if necessary, in several stages over several rollers or roller assemblies. At stretch ratios below about 2.5:1, the stretch of the bands may be insufficient to provide them with sufficient strength, while at stretch ratios above about 8:1 5 the risk of breakage increases.

In a preferred embodiment of the invention, polypropylene tapes are made from films extruded at temperatures of about 200 to 290 ^° C.

Particularly preferred are multilayer films in which at least one substantially non-conductive layer of polypropylene, which is substantially free of conductive particles, is co-extruded in a sandwich configuration between two conductive outer surface layers comprising a polypropylene resin and containing about 20 to 40% by weight of conductive carbon black. The best results are achieved when the polypropylene from which the

substantially non-conductive layer is extruded has a flowability of about 3 to 8 g/10 min and the polypropylene resin from which both conductive layers are extruded has a flowability of about 2 to 5 g/10 min, the ratio of the flowability of the resin composition filled with conductive particles to the flowability of the substantially non-conductive resin composition being about 0.2:1 to 0.4:1. Such flowability and flowability ratios promote the smooth extrusion of films with very uniform thickness and thereby provide good tape strength and conductivity.

In these preferred embodiments, the film widths and thicknesses can be suitably selected in view of the processing equipment, the economics and the desired purpose of the tapes. Preferably, the film width is about 0.15 to 2 m and the film thickness about 50 to 500 μηη. The extruded film is drawn off and then quenched, preferably by contact with a cool roller having a surface temperature of about 10 to 50 ⁰ C or by immersion in a water bath of about 15 to 45 ° C. The quenched film is then dried by heat or in a stream of air.

After quenching, the film is preferably slit into a number of ribbons using knives arranged side by side at suitable distances. The ribbons are then oriented by passing them over rotating rollers into a heating zone, e.g. an oven, and stretching or drawing them to effect orientation. Preferred temperatures are in the range of about 120 to 195°C. The degree of stretch required to orientate and to achieve ribbons with a good balance of strength and elongation is achieved by drawing at a stretch ratio preferably in the range of

about 4:1 to 8:1. The resulting tapes preferably have strengths of at least 1.8 cN/detex and elongations of about 10 to 25%, determined according to DIN 53857.

Following orientation, the tapes may optionally be heat treated to reduce shrinkage. The tapes are then collected and individual tapes are wound onto separate spools or fed directly to the weaving or knitting machine. In a preferred embodiment, the tapes are obtained as monoaxially oriented, substantially flat multilayer structures having a thickness of about 25 to 220 μια and a width of about 1 to 5 mm.

Optionally, the ribbons may be fibrillated during or after their manufacture. By "fibrillation" is meant the discontinuous slitting or cutting of a ribbon substantially lengthwise to form a plurality of substantially parallel rows of slits, the rows of which are preferably arranged side by side. The longitudinal region of the ribbon having the slits represents the fibrillated region of the ribbon. The longitudinal region of the ribbon between the rows of slits represents the non-fibrillated region. The percentage obtained by dividing the fibrillated length by the sum of the fibrillated and non-fibrillated lengths and multiplying the quotient by 100% is referred to as the fibrillation ratio. When twisted, the fibrillated ribbons assume a substantially filamentary configuration. Such fibrillated ribbons offer numerous advantages over filaments filled with conductive particles in terms of strength, elongation and efficiency of use of the conductive particles. As such, they can be used not only for the production of fabrics, but also, for example,

can also be used as conductive threads for darning and sewing.

Fibrillation of the conductive tapes of the invention can be carried out within a wide range of fibrillation ratios to meet the requirements of the particular application. In multi-layer tapes, fibrillation can also provide improved conductivity due to better contact of the conductive surfaces of fibrillated warp and weft tapes at their intersection points in the fabric, so that even two-layer tapes are highly effective for charge dissipation. However, fibrillation can also reduce the strength-enhancing effect of the non-conductive layer(s) of the tape and therefore a balance of conductivity and strength should be sought.

Fibrillation can be carried out using any suitable device which provides substantially parallel rows of slits. Examples are given in GB Patents 1,073,741 and 1,262,853 and in US Patent 3,427,912. Preferably, fibrillation is carried out on a standard needle roller fibrillator using needle bars having about 8 to 60, preferably about 10 to 40, needles per centimetre. The angle between the radius of the needle roller and the needle can affect the length of the slit created in the tape. According to the invention, good results are achieved when the angle is about 15 to 45°. The needles can be arranged straight or staggered on the needle bar.

Conventional looms, warp knitting and stitchbonding machines or other devices suitable for producing fabrics from tapes can be used to make fabrics from tapes. A variety of constructions, e.g. plain weave (basic weave), rib weave, check weave, twill weave, satin weave, chain stitch and tricot weave, can be used.

Depending on the intended use of the fabric and the desired level of electrostatic charge dissipation, electrically conductive tapes having linear densities of about 200 to 2000 deniers can be woven with non-conductive yarns so that the conductive tapes are spaced about 1 to 40 cm, preferably about 1 to 4 cm, in the warp direction and about 2 to 30 cm in the weft direction. The use of such tapes at such spacing will typically result in fabrics having resistivities of up to about 1 x 10 ohms, preferably about 1 x 10 ⁵ to 1 x 10 ⁷ ohms. An advantage of the tapes of the invention is that they can be readily manufactured in dimensions and linear densities that match those of standard tapes so that no special measures are required for the manufacture of the fabrics. Although the dimensions and linear density of the tapes of the invention are preferably approximately equal to those of the non-conductive tapes or yarns in the fabric, the invention also encompasses fabrics in which some or all of the conductive tapes are different in dimension and/or linear density from the other yarns of the fabric.

Fabrics comprising combinations of electrically conductive tapes and conventional thermoplastic tapes suitable as fabrics for finished products capable of dissipating electrostatic charge are preferably woven in a plain weave. These fabrics generally have a construction with warp tapes in the range of about 10 to 40 ends per inch and weft tapes in the range of about 2 to 40 ends per inch. Linear densities are usually in the range of about 200 to 1500 denier. Depending on the application of the fabric and the degree of electrostatic charge dissipation required, electrically conductive tapes of about 200 to 2000 denier and a spacing of about 1 to 40 cm,

preferably 2 to 4 cm between the conductive tapes in the warp direction and about 2 to 30 cm between the conductive tapes in the weft direction. An example of such a fabric is shown in Fig. 4, wherein the fabric 17 comprises conventional warp and weft tapes 19 and 21 interwoven with electrically conductive warp and weft tapes 23 and 25 according to the invention.

Fabrics having a combination of electrically conductive and conventional thermoplastic tapes suitable as primary carpet backings with improved conductivity and static resistance are also preferably woven in a plain weave. Such fabrics typically have a construction with both warp and weft tapes in the range of about 11 to 28 tapes per inch and linear densities in the range of about 100 to 1500 denier. The warp tapes are preferably in the range of about 200 to 600 denier and the weft tapes are preferably in the range of about 300 to 1000 denier. Preferably, the carpet backing fabric has warp tapes in the range of about 20 to 28 ends per inch and linear densities of about 250 to 55 denier and weft tapes in the range of about 12 to 32 ends per inch and linear densities of about 400 to 900 denier.

In this construction, electrically conductive bands of preferably about 200 to 800 denier are used at a distance of about 1 to 30 cm, with a spacing of the conductive bands of about 1 to 3 cm in the warp or about 20 to 30 cm in the weft being particularly preferred.

Fabrics comprising combinations of electrically conductive tapes and conventional tapes suitable for containers and bags, e.g. bulk intermediate containers, and having improved conductivity and static resistance are also preferably woven in a plain weave. Preferably, such fabrics have about 6 to 30 ends per inch in warp and weft and the tapes have

linear densities of about 800 to 3000 denier. Such fabrics preferably have a construction with warp bands in the range of about 10 to 25 ends per inch and linear densities of about 1000 to 2200 denier and weft bands in the range of about 10 to 20 ends per inch and linear densities of about 1200 to 2300 denier. The electrically conductive bands of these fabrics preferably have a thickness of about 30 to 180 μηα, linear densities of about 2 to 2500 denier and are used at a spacing of about 2 to 40 cm per conductive band in the warp and about 2 to 40 cm per conductive band in the weft.

In a particularly preferred embodiment, the conductive tapes used in such fabrics are three-layer tapes in which about 40 to 60% of the total thickness of the tapes is made up of a middle layer of thermoplastic resin, preferably polypropylene, which is substantially free of conductive particles, and two outer surface layers adhered to either side of the middle layer, each of these layers comprising about 15 to 35% of the total thickness of the tape, and the surface layers are made up of polypropylene with medium or high structure carbon black incorporated therein. In such tapes, the carbon black comprises about 25 to 35% by weight of each of the conductive layers. Particularly preferred conductive tapes for these applications have a non-conductive core comprising up to about 50% of the film thickness and conductive surface layers each comprising up to about 25% of the thickness.

Bags and bulk containers can be made from these fabrics in any suitable configuration. In general, such bags and containers comprise a body portion of usually substantially rectangular shape when made from flat woven fabrics or of substantially cylindrical shape when made from

made of round-woven fabrics. The interior of the
Body area is usually connected to the outside via
one or more pouring spouts or tubular
Sections connected to end plates of the
body area. Furthermore, lifting loops

or handles attached to the body area. The tissue of the
Pouring spouts are usually a lighter fabric than
the body tissue, while the lifting handles are often made of heavier tissue. -The tissues are usually

sewn together. For optimal drainage of

electrostatic charge, the different fabrics are joined together so that the conductive bands of one fabric type are in contact with those of the other fabric type. The
Fabrics can be joined together in any way. To improve the contact of the conductive yarns,
sewn with a conductive thread. The containers can be
if necessary, also be lined with a lining.
This is usually made of thermoplastic resin and may contain conductive particles.

Claims (7)

PROTECTION CLAIMS Electrically conductive multilayer tape, characterized by at least one layer (3,11,13) comprising a thermoplastic polymer composition containing electrically conductive particles dispersed in at least one thermoplastic polymer, said layer (3,11,13) forming an outer surface of the tape and being connected to at least one layer (5,15) of a thermoplastic polymer composition which is at least substantially free of electrically conductive particles at an interface (7) between the layers. 2. Electrically conductive tape according to claim 1, characterized in that the thermoplastic polymer is a propylene polymer. 3. Electrically conductive tape according to claim 2, characterized in that the thermoplastic polymer is a crystalline polypropylene homopolymer. 4. Electrically conductive tape according to one of claims to 3, characterized in that the electrically conductive particles comprise conductive carbon black. 5. Electrically conductive tape according to one of claims to 3, characterized in that it has a strength of at least about 1.8 cN/dtex, an elongation of about 5 to 30% and a specific resistance of up to 1 x 10 ⁸ ohm. . Electrically conductive tape according to one of claims to 5, characterized in that it has been fibrillated. 7. A fabric comprising an electrically conductive tape according to any one of claims 1 to 6.