SE452280C - ELECTRIC LEADING PLASTIC ARTICLES AND PROCEDURES AND RESOURCES FOR PRODUCING THEREOF - Google Patents

Description

25 30 35 452 280 2 covers to provide protection against electromagnetic radiation. However, a disadvantage of such coatings is that they are not durable. In most cases, these coatings also require special and expensive treatment and application procedures.

Attempts to impart electrical conductivity to the plastic itself (so that it protects against electromagnetic waves) have also been made by introducing or atomizing relatively large quantities of conductive fillers. Such conductive fillers include carbon black, aluminum shavings, wire ends, metal-coated glass fibers, wire mesh, and carbon wire. However, there are certain disadvantages associated with these conductive fillers. Some fillers do not allow sufficient atomization in the plastic matrix and clump or break down excessively into very small particles so that their shielding effect is greatly reduced. This breakdown makes it necessary to add a larger amount of conductive particles, which makes uniform atomization even more difficult with a negative effect on the mechanical properties of the material.

Finally, it is known that to obtain effective protection against electromagnetic radiation, the conductive particles in the plastic matrix must include a significant aspect ratio, i.e. length/diameter ratio (L/D).

These particles must form as much as possible a continuous conductive circuit in the matrix to increase conductivity without, however, significantly altering the physical and mechanical properties of the plastic matrix.

Disclosure of the Invention An object of the present invention is to make plastic articles with less than about 0.5 volume percent fine electrically conductive fibers, which are randomly and substantially evenly distributed, so that the distributed fibers provide adequate conductivity in each direction in the articles for use, for example, as electrical interference (EMI) shielding.

The fibers may be uniformly distributed throughout the body of the article, for example in a plate or disc, or otherwise precisely within predetermined areas thereof, for example near one or both or parts of its outer or planar surfaces. The fine fibers preferably have an equivalent diameter of less than about 0.015 mm and greater than about 0.002 mm.

Another object is to provide means and methods for manufacturing flat or sheet-like plastic articles with effective protection against electromagnetic radiation of at least 24 dB within a given frequency range (for example between 0.1 and 10 GHz and especially at 1 GHz) while maintaining their normal properties. Flat and sheet-like articles then mean strips, non-uniform cross-sectional profiles, foils, thin films, tubes, caps, bags, cloths and other containers.

For this purpose, electrically conductive fibers are finely divided into plastic articles with a length and "equivalent" diameter ratio (D/L) varying from about 0.0005 to about 0.008 for a large part of the fibers. These fibers can be, for example, metal fibers with an average length L and between 0.5 and 5 mm. The term "equivalent" diameter D refers to the square root of the ratio of the fiber cross-sectional area divided by W. The average length L refers to the total sum of the lengths of the constituent fibers divided by the number of fibers. At an average length of L = 0.5 mm, fibers with a length shorter than 0.5 mm are certainly present. However, a majority of the fibers have a length that is approximately the average length. According to the invention, these dimensional limits for the fibers satisfy the above-mentioned protection requirements at the exceptionally low volume concentration c (%) of conductive filler, namely at between about. 0.05 volume percent and about 0.5 volume percent. Furthermore, when the plate or sheet thickness is less than 3 mm, C2 becomes 1.4 D/L -0.90052 and for plate thicknesses between 3 and 6 mm, CE becomes. D/L-0.0013_ These low concentrations exert almost no influence whatsoever on the dimensional ratios of the plastic articles. It has further been discovered that antistatic plastic articles can be produced by mixing electrically conductive fibers into the plastic at low concentrations (less than approximately 0.5 volume percent) and wherein the concentration C with respect to the fiber dimensions in the antistatic plastic also satisfies the ratio C2D/L-0.0013_ The fibers should then occur at least closest to the outer surface of the article. 452 280 10 15 20 25 30 35 4 It is thus possible according to the invention to manufacture composite plastic articles with such a low content of conductive fibers and to randomly and evenly distribute the fibers in the plastic so that the article has a predetermined level of conductivity. The concentration of conductive fibers can then vary between approximately 0.03 volume percent and approximately 0.5 volume percent.

In addition, optimal filling limitations can be achieved by adding fibers during the industrial production of L, D/L limits also satisfy the following equation: c§3.34 D/L - 'o.ooo41. i Since the contact between the fibers must be as good as possible in the plastic matrix to stimulate conductivity, it has proven important to have a relatively smooth surface. This means that roughness on the fiber surface should not project above or extend below the average level of the fiber surface by more than approximately 1 m. In this way, it is statistically most likely that there is an optimal number of contact surfaces between adjacent fibers, which contact surfaces also have optimal dimensions.

Stainless steel fibers, manufactured according to a process of bundle drawing described in, for example, U.S. Patents 2,050,298 and 3,379,000, have shown particularly suitable internal conductivity conditions for this application. This can possibly be attributed to the fact that they are less prone to forming a more or less insulating oxide layer on their surfaces in contrast to, for example, aluminum or copper fibers. This means that the contact resistance at the fiber contact points remains low. They are also usually more inert than Al and Cu towards most plastics. Other fibers, such as HASTELLOY-X (varum,), INCONEL (varum.) Ti and Ni, are also useful. A suitable specific conductivity is equal to or greater than 0.5% of the normal conductivity of the fibers.

In principle, the invention is applicable to most plastics, preferably of the thermoplastic type, using conventional molding techniques such as casting, drawing, injection molding, compression molding and foam molding. 10 15 20 25 30 35 452 280 5 The articles may accordingly be of a flexible, rigid or elastic nature. However, the invention is very readily applicable to thermoplastic resins and their conventional molding techniques, such as drawing and injection molding, using plastic pellets as starting material.

In practice, it is therefore recommended to add conductive fibers in one way or another to used plastic pellets or to mix them into these pellets, so that their compatibility with the plastic is not affected and an optimally uniform distribution of the conductive fibers in the plastic is achieved during conventional molding processes.

According to an important aspect of the invention, the uniform distribution is achieved by using plastic granules as an intermediate product for the manufacture of the articles, the granules being at least about 0.4 cm long and loaded with conductive fibers. The average length of the fibers in the granules will slightly exceed that of the fibers in the final articles since there is always a breakage of a number of fibers during the molding process. However, measures according to the invention will be provided further in the description to counteract this tendency to breakage.

Furthermore, the volume concentration of conductive fibers in the granules will always be greater than the required final concentration in the molded articles. For example, if it is desired to manufacture an article containing 100% of the above-described granules with a final concentration of 0.3% by volume of metal fibers in the article, then the average volume concentration of the metal fibers in the granules must be at least 0.33%. However, if it is desired to manufacture an article with the same final concentration of metal fibers (0.3% by volume) based on a mixture of 67% by volume of pure plastic pellets and 33% by volume of plastic granules loaded with metal fibers, then the average volume concentration of metal fibers in these granules is preferably at least 0.99%.

In general, the methods for manufacturing plastic articles with predetermined conductive parts according to the invention comprise the following steps: A fiber/plastic mixture is formed which has a content of conductive fibers in the range of 20-70 volume percent and supplying a substantially parallel fiber arrangement. This mixture is mixed with a predetermined amount of substantially pure plastic material and the mixture is fed into, for example, the magazine of a draw mixer. In such an apparatus, the plastic material is heated to softening and processed (kneaded) for uniform distribution of the fibers therein. Small shear forces are therefore introduced to avoid major fiber breakage. However, the shear forces must remain of sufficient magnitude to obtain an even distribution of the fibers within the plastic. To form the article, the thus processed viscous mass may be further fed by means of a feed screw through suitable pipes, channels or slots for molding or may be directly and continuously drawn into rods, tubes, sheets, films or plates or injection molded.

When a mixture of pure plastic pellets and composite granules, which include fibers as described above, is used, cylindrical granules are selected with a diameter at least equal to the average thickness of the pure pellets. This dimension usually reduces the risk of the embedded conductive fibers breaking during the heat treatment and kneading of the granule/pellet mixture before the appropriate molding. The length of the admixture granules should preferably be between 0.4 and 1.2 cm.

For practical purposes, it is convenient to use plastic granules of standard size and concentration which can be easily mixed and treated with conventional plastic pellets in the desired proportion to obtain a predetermined volume concentration of conductive fibers in the final product. Obviously, the main raw material of these granules is preferably the same resin as the material of the articles to be formed. Furthermore, the cross-sectional area of the mixture granules will be at least as large as that of the pure resin pellets. For example, a volume percentage of metal fibers in the mixture granules of 1% has been found to be suitable. The metal fiber content of the granules can be selected between approximately 0.5 and 2 volume percent.

However, the mixture granules may also contain other plastic materials than those from which the articles are to be made. The softening and melting point of the resin in the mixture granules must, however, be lower than that of the plastic from which the article is to be made in order for the mixture granules to be easily dispersed and mixed with the main raw material of the article during the manufacture thereof at high temperatures, thereby enabling a distribution of the conductive fibers therein under minimal shear forces.

The main raw material must also be compatible with the blend granule resin for other reasons. For example, this resin must not decompose or react with the main raw material when the latter is heated to its processing and casting temperature.

The most suitable base product for the conductive fibers to be introduced is a wire bundle, although other fiber bundles, such as fiber strips and cellulose wool yarn, can be used.

The fiber strips should then have sufficient thread count or density (titer) and the fiber lengths should be long enough to form truly homogeneous bundles with sufficient tensile strength for treatment and processing. Average fiber lengths of 7 cm and approximately 2000 fibers per strip cross-section are suitable. Usually the fiber bundles are embedded in a plastic matrix so that the fiber content therein is between 20 and 70 volume percent. The impregnated fiber bundle is allowed to solidify (for example by cooling) to produce a so-called thread with a cross-sectional area that is preferably not less than and approximately equal to the cross-sectional size of the plastic pellets included in the main raw material.

The trees can be round or have different cross-sectional shapes, such as oval, flattened or rectangular shapes, to facilitate winding and cutting into pieces.

The thread may comprise 35-000 adjacent sub-threads (or fibres) across its cross-section, but a smaller number (at least about 1000 sub-threads) is advisable. 10 15 20 25 30 35 452 280 8 It is often recommended to enclose the impregnated bundle with a casing of either the same plastic as the main thread material or the same or a different plastic with which the bundle was impregnated. This stimulates the gradual dissolution of the cut bundle and the uniform fibre distribution in the plastic matrix during mixing at high temperatures. The thread is cut into predetermined lengths, hereinafter referred to as granules, with such a length size being between about 0.4 and 1.5 cm.

It is obvious that the plastic materials with which the fiber bundle was impregnated and enclosed must be compatible with the main raw material of the article to be formed. For example, where this raw material is a thermoplastic material, the impregnating material preferably comprises a low molecular weight thermoplastic polymer, such as polyethylene, polypropylene, polyester, polyacrylate, polymethacrylate, polystyrene, PVC and PVC copolymer.

The thermoplastic granules with the conductive fibers distributed therein are prepared by manufacturing a dry mixture of pure plastic pellets (the main raw material) and a number of granules in which a suitable amount of parallel fibers are embedded, which fibers have approximately and essentially the same length as the granules. This mixture is then kneaded in a drawing mixer under elevated temperature and under the application of a small shear force to finely distribute the conductive fibers in the plastic material. The resulting mass is then drawn into one or more threads of suitable cross-section and cooled. Finally, the threads are cut into granules with a length of at least 0.4 cm.

Preferred embodiments The invention will now be further described in connection with a number of embodiments shown in the accompanying drawings, where Fig. 1 shows in perspective the forming and final stages of a wire formed from an impregnated and enclosed bundle of conductive fibers and a granule encapsulated from this wire, Fig. 1A shows in perspective a wire similar to Fig. 1 but with a flat cross-section, Fig. 2 shows a plastic granule containing distributed conductive fibers, Fig. 3 shows graphically the relationship between the wave frequency E (f) of electromagnetic radiation and the shielding effect (SE) of a 3 mm thick plastic sheet containing conductive filler material and Fig. 4 shows graphically the optimal working range for the invention through the relationship fiber concentration in relation to the D/L value.

Example 1 Fig. 1 shows a substantially round, untwisted bundle of 20,400 stainless steel wires AISI 3l6L (marketed by the applicant under the trademark BEKINOX) with an equivalent wire diameter of 0.008 mm. This was passed through a bath containing a solution of 20% by weight of linear polyester with a relatively low molecular weight (approximately 14,000) of the type Dynapol 5 L850 (Dynamite Nobel) in trichloroethylene. After leaving the bath, the bundle was drawn through a round drawing channel having a diameter of ü.8 mm and dried. The dry bundle thus comprises 6.2% by weight of resin (which is equal to 70% by volume of metal fibers). Such an impregnated bundle was enclosed in a wire drawing device (Maillefer type with fixed centering) with the same polyester Dynapol L850. The round drawing die had a diameter of 2 mm. After the thus drawn wire 2 had cooled down, it was cut into cylindrical granules 3 with a length of 1 cm. The granules consisted of approximately 13% by weight of resin, which is equal to approximately 52% by volume of metal fibers. When the bundle was cut, almost no fiber ends were pulled out of the bundle and hooking and flattening of the fiber ends were avoided. This was important to ensure correct dosage and fluid dispersion. The granules were then dry-mixed by tumble mixing technique with the usual thermoplastic pellets of various resin types in the proportion of 9.75% by weight of granules and 90.25% pure plastic pellets and drawn into a substantially round wire with a diameter of 1.. .__._ 10 15 20 25 30 35 452 280 10 4 mm and a metal fiber content of approximately B% by weight.

After cooling, this drawn material was cut again into granules 4 (Fig. 2) with a length of 1 cm. In these granules the metal fibres were found to be evenly distributed with a volume content of approximately 1.1 percent. The shear forces acting during drawing were kept sufficiently low so that further fibre breakage was avoided. One of the measures taken to keep the shear forces to a minimum included the removal of the filter plates in the nozzle inlet. The temperature in the nozzle of the single screw extruder was 260°C when NORYL-SE90 (a modified polyphenylene oxide from General) was used. When Cycolac AM 1000AS (an ABS resin from Borg Warner) was used, the drawing temperature of the nozzle was 2200°. When Lexan L13848-141R-111 (a polycarbonate from General Electric) was used, it was 225°C. The extruder was of the Samafor 45 type with a length/diameter ratio of the screw equal to 25. The feed channel in the head closest to the drawing channel was an annular space between a tapered outer surface of a core and the concentrically arranged conical inner surface of the die head. The channel was thus aligned with the drawing channel and the shear was thereby increased somewhat, resulting in better fiber dispersion, with the fibers being more or less oriented in the direction of drawing.

The mixture granules thus obtained were dry-mixed with an equal weight percent of clean plastic pellets and fed to an Ankerwerk V24/20 injection molding machine with a screw, to which a mold was connected for molding sheets with a thickness of 2.3 mm, a length of 30 cm and a width of 25 cm. The temperatures were 25o°C, 21o°C and 29o°C for the Noryl, Cycolac and Lexan resins, respectively, and the temperature of the mold was set at 80°C, 50°C and 90°C, respectively. The screw rotated the screw chamber at 44 rpm. The nozzle opening had a diameter of approximately 1 cm. The Noryl, Cycolac and Lexan sheets had smooth surfaces and the fiber distribution was uniform throughout the sheets. The metal fiber concentration was 4 weight percent or 0.5 volume percent. Stainless steel fibers of 10 l5 20 25 30 35 452 280 l Bekinox brand have a specific conductivity of approximately 2% of the copper standard.

Example 2 Under similar conditions as in Example 1, injection molded plates were made from the thermoplastic resins mentioned above.

However, a flat bundle of 20,400 Bekinox wires, which were closely packed and had a diameter of 0.008 mm, was used, as shown in Fig. 1A. As in Example 1, the flat bundle was impregnated with a solution of Dynapol L850 and passed through a rectangular 5 x 0.5 mm peeling channel. The dried bundle comprised 6.4 weight percent resin in a drawing opening at 160°C. The dimensions of the rectangular drawing die were 5 x 0.6 mm and the resulting cooled strand comprised 23 weight percent resin, which is approximately equal to 32 volume percent metal fibers. The flat wire was cut into 1 cm lengths, whereby hooking and flattening of the fiber ends were absolutely absent. Fixing the fibers in a flat bundle in the resin matrix for proper cutting of granules proved to be very effective. The flat granules obtained were then dry-mixed without difficulty with pure plastic pellets in ratios between 10.66 and 89.33 weight percent and drawn into a substantially round wire with a diameter of 4 mm (see Example 1). The metal fiber content was approximately 8 weight percent, which corresponds to approximately 1.1 volume percent. Composite granules with a length of 1 cm were cut from this wire. After dry-mixing these composite granules with an equal weight of pure plastic pellets and injection molding the mixture, as described above, a uniform dispersion could be observed. The average fiber length was estimated to be approximately 1.5 mm and the final concentration was again 0.5 volume percent. See area A in Fig. 4.

The shielding ability of the injection-molded plates against electromagnetic radiation was tested. As is known, the shielding ability of a plastic material with conductive filler material can be determined in proportion to the plate thickness by comparing the reflection R (5) measured at a radiation frequency (for example 10 GHz) with the reflection (100%) of a reference material such as a metal plate. If the electrical properties of the material are sufficiently homogeneous and the conductive filler material in the plastic forms a mesh with a sufficiently fine mesh (for example of the order of magnitude smaller than the wavelength of the radiation to be shielded), then the shielding frequency range can be determined. In addition, it is known that for a large area of application, the shielding performance is extrapolated to whole numbers of applications for electrically conductive plastics, the shielding requirements are met when a shielding efficiency (SE) of 25 dB is achieved at a frequency of l GHz. It has also been found that the SE value for electric fields and for materials with a specific resistance between 0.01 - 100 cm is always minimal in the vicinity of 0.4-5 GHz for plate thicknesses between l and 6 mm and with a distance of approximately l - l0 cm between the wave source and the plastic plate. The relationship between the shielding efficiency SE and the wave frequency f is shown in Fig. 3 for a plate thickness of 3 mm and a distance between the source and the plate of l cm. Curve l refers to the relationship for the reflection value R = 99 % measured at 10 GHz while curve 2 refers to the relationship for R = 70 % measured at 10 GHz. For example, if the reflection R is measured to be 80% at 10 GHz (source/plate distance is 1 cm) for a conductive plastic plate with a thickness of 3 mm, then it can be obtained from Fig. 3 that the SE value will be 70% and at 1 GHz at least 35 dB at all frequencies, when R = SE becomes 5 38 dB.

Analogously, the following values are obtained for other plate thicknesses when measuring at a distance of 1 cm between source and plate.

Thickness 10 GHz l GHz (mm) R (z) sr (de) R (x) sE (dB) 4 70 35 70 41 85 35 70 34 1 95 35 70 27 From the shielding theory (Schultz) it can further be obtained that the specific resistance f(1cm), for homogeneous conductive plastic plates and independent of the thickness of the plate, has the following values corresponding to the following reflection values (R - %): R (%) ?(ßcm) 99 0.11 ss 0.53 90 1.1 80 2.2 7o 3.3 Consequently, it can be obtained from the data that a thicker plate can exhibit a lower specific conductivity (l/Q) and a lower reflection value to achieve the same shielding efficiency (SE) at a given frequency (for example at 1 GHz). The D/L value of the fibers can thus be greater in a thicker plate than in a thinner plate for the same fiber concentration or in other words, the fiber concentration in a thicker plate can be less than in a thinner plate when D/L is the same in both plates. The transmission, reflection and: esisticity measurements were carried out on injection-molded plates. The transmission and reflection measurements were made at 10 GHz. For these measurements, the plates were placed between a wave transmitter (an oscillator), to which a first horn antenna was connected via a circulator and a second horn antenna which was connected to a second detector. The energy generated by the oscillator was transmitted to the plate via the first antenna and the transmitted energy was recorded via the second antenna by the second detector connected thereto. The reflected energy was returned to the detector connected thereto. The amount of energy reflected is expressed to the first antenna and recorded by a first, as a percentage (R-value) of the amount of energy (100%) reflected by a metal plate under the same conditions. When the amount of energy emitted is equal to zero, for the purpose of measuring and recording the reflection, a movement of the plate at a constant speed between and near the first antenna to the second antenna at a distance of 22 cm takes place. This movement starts at a minimum distance of 14.5 cm from the circulator.

This dynamic method enables the measurement of any errors that could occur in static measurements when the position of different plates in relation to the circulator is not exactly the same during successive measurements.

The measured reflection signal is indeed always the result of successive reflections and the reflections between the plate samples and the metal (circulator, antenna). This gives a standing wave pattern as a function of the distance between the sample and the transmitter. In the dynamic method, the average value of the generated standing wave pattern is determined by a microcomputer.

To measure the specific resistance (resistivity), the plates or discs are connected near their opposing edges between clamps in an electrical circuit. To obtain good conduction between these clamps and the conductive fibers in the clamped plate edges, the latter are degreased and coated with silver paint.

The measurement results were as follows (average values): Reflection Radiation Resistivity (%) (%) (flflm) Noryl 65 0 2 Lexan 71 0 3 Cycolac 65.5 0 4 This shows that the extruded sheets with a thickness of 2.3 mm were on the border between insufficient and sufficient shielding efficiency (35 dB) for certain areas of use. See area A in Fig. 4.

Example 3 A similar resin-impregnated flat wire bundle (thread) as in Example 2 was cut into granules of 1 cm length and, as in Example 2, mixed with pure resin pellets (Cycolac) in the desired proportion. These resin pellets had the usual dimensions (approximately 0.5 cm long, 0.5 cm wide and 0.2 cm thick). The mixture was drawn into a round wire and cut to form composite granules with approximately 1.1 Volume percent metal fibers (see Example 2). The composite granules were dry-mixed with pure plastic pellets in a 50/50 proportion and fed into a Maurer-type injection molding machine, the nozzle of which has a diameter of 0.75 cm. The same temperatures were used as in Example 2. If the shielding characteristic is also to be sufficient in the immediate vicinity of the nozzle, the injection should preferably take place at a slow speed and/or with an applied after-pressure at the end of the injection process which is kept as low as possible. The injection-molded plates were 5 mm thick. The average length L of the fibers was determined by cutting thin slices from these plates, after which the resin was dissolved from these slices and an analysis was made of the remaining fiber network under a microscope. Area B in Fig. 4 corresponds to the fiber length distribution thus determined.

Screening and conductivity measurements were made as described above. The results are summarized in the following table.

Reflection Radiation Resistivity (%) (flcm) Cycolac 68 0 4 Example 4 Flat granules with 20,400 parallel stainless steel N fibers with a diameter of 8/um and a length of 3 mm, embedded in 8% by weight of acrylic resin K 70 (from Kontakt Chemie) were added directly to a 45% solution of thermosetting polyester resin Derakene 411 in styrene with thorough stirring. The fibers of the granules were evenly and randomly distributed in the resin and the usual accelerator was added as a catalyst. The relatively liquid mass was cast into 30x30x0.3 cm plates and aerated. The mold was closed and rotated during the cold hardening process to prevent the metal fibers from sinking to the bottom of the ingot. The hardened plate contained 0.5% by volume of metal fibers. In Fig. 4, this mixture composition corresponds to point G. Measured reflectance was 92% at a resistivity of 0.43J\cm and an emissivity of 0%.

Similar plates (same dimensions) were manufactured with the compositions shown below. Reflection, radiance and resistivity were measured. 452 280 ä 10 15 20 25 30 35 16 D L C R Resistivity, Emissivity, Point in Fig. 4 l _ (mm) (mm) (2) (24) (ß-cm) (%) 0.000 3 0.25 70 1.44 o c 0.004 3 0.25 87 1.68 0 n § 0.004 3 0.50 84 3.11 0 E 0.004 3 0.12 70 15.1 0 F From these examples and results, limits were obtained for the volume concentration of the fibers (c %) as a function of the D/L ratio of the fibers. The straight line 1 in Fig. 4 corresponds to C Z 1.4 D/L-^QJw082 while the straight line 2 represents the equation CIÉ 3.34 D/L -0.00041. According to the invention, the area between the two straight lines 1 and 2 determines the optimum ratios of C, D and L to provide sufficient shielding efficiency for plates with a thickness of less than 3 mm. For plate-like or disc-like articles with a thickness between 3 and 6 mm, the straight line 3 in Fig. 4 constitutes the lower limit for obtaining sufficient cutting. This straight line corresponds to the equation Q¿D/L-070013_ Example 5 A substantially round, untwisted bundle of approximately 10,000 Bekinox stainless steel wires AISI 3l6L with an equivalent wire diameter of 0.004 mm was impregnated and encapsulated with, for example, a Dynapol L850 solution as explained in example 1 to form a strand. Granules of 0.5 cm length were cut from this strand and dry-mixed in the appropriate proportion with Cyclac-KJB pellets to form granules. The granules were again manufactured by drawing in a Samafor 45 drawing device (example 1) and comprised approximately 0.5 volume percent fibers. Their length was chosen to be 1 cm. After dry-blending these granules with an equal weight of Cycolac-KJB pellets, the mixture was fed to the extrusion press used in Example 1 to press a plate of 2.3 mm thickness. A uniform dispersion of approximately 0.23% by volume of fibers was achieved in the plate and the average fiber length was estimated to be approximately 0.7 mm. This result is indicated by line H in Fig. 4. The antistatic property of this plate was evaluated by rubbing the plate with a textile pad to generate an electric charge on its surface.

The plate was then brought near a certain amount of fine cigarette ash lying on a table. There was no particular tendency for the ash to be lifted from the table and coat the underside of the plate. However, when the same antistatic dust test was repeated with a pure Cycolac-DJB resin plate without any metal fibers, the ash was immediately attracted to the plate.

Example 6 Approximately 10,000 Bekinox stainless steel fibers in strip form with an equivalent fiber diameter of 0.0074 mm were impregnated with and provided with a casing of a Dynapol L850 resin as explained in connection with example 1. The strand had a metal fiber content of approximately 25 volume percent. Granules of 0.6 and 0.3 cm length were cut from this strand and dry-drum-mixed with plastic pellets of the Cycolac-KJB type (gray) to obtain a composition of 0.5 volume percent metal fibers and balancing resin. The mixture was fed directly into the magazine of an injection molding machine of the Stubbe S150/235 type (working pressure 130 kg/cm2, injection pressure 30 kg/cm2, after-pressure 30 kg/cm2). The temperature in the spray channel was 205°C and the spray time 4 seconds for a sprayed plate 30x30 cm and with a thickness of 3 mm. The metal fibers were distributed mainly evenly in the plastic. The electrical properties are shown in the table below (average values).

Fiber length Reflection Radiation Resistivity in granule (flmü (%) (%) Laem) 3 70 0 7 6 67 O ll The reflection value at a metal fiber content of 0.5% in the plastic still results in a shielding efficiency of more than 25 dB.

According to experience, one can expect a sufficient shielding capacity (25 dB) with smaller stainless steel fibers having approximately 0.0065 mm in diameter (D) and with a direct feed to the injection molding machine of a mixture containing granules of approximately 3-5 mm length and a metal fiber content in the granules amounting to approximately 65 volume percent, for example at approximately 10,000 fibers per granule.

This experiment thus proves that good screening results can be obtained with a direct input of granules at the extrusion stage and thus the intermediate stage of manufacturing grains is eliminated.

To manufacture articles of thermoplastic foam material in molds, a predetermined mixture of pure plastic pellets containing a suitable amount of blowing agent can be used, as described above. It is also possible to mix the blowing agent in powder form with pure plastic pellets and with a suitable amount of composite grains.

For example, said pellets can be moistened so that the powder adhering to them can be spread sufficiently evenly over them.

Afterwards, the mixture can be fed into the injection molding machine in the usual way.

For the preparation of thermoplastic elastomer articles (e.g. comprising the elastomer polyester Hytrel), elastomer pellets mixed with a suitable proportion of composite grains or granules can be used, which granules are prepared on the same elastomer base. However, the shear forces must be particularly low during the kneading and pressing processes.

For sheet pressing of prepregs, it is possible to pre-disperse the conductive fibers in liquid resin at a suitable concentration.

To mass press viscous mixtures of resins and fibers, the conductive fibers can be dispersed in the mass in a similar manner.

More specifically, it is possible to preliminarily mix the conductive fibers with other fibers, for example with reinforcing fibers such as glass, carbon, polyaramid fibers, and to distribute this fiber mixture in the same way in the resin.

For processing in thermoplastic resins, it is possible to replace the previously described thread of conductive fibers embedded in plastic with a thread comprising a mixture of glass fibers and conductive fibers in the desired proportion. It is also possible to impregnate glass fiber bundles in a side-by-side position with bundles of conductive fibers to form the thread. Finally, it may be preferable to mix threads comprising reinforcing fibers and cut them into granules with threads comprising conductive fibers and cut into granules in a suitable weight proportion and to feed them to the injection molding machine while, if desired, adding a suitable quantity of pure plastic pellets (main raw material).

An advantageous method for distributing a very small portion of conductive fibers, such as metal fibers, in plastic consists of starting from a strip mixture comprising thermoplastic textile fibers with a relatively low melting point mixed with a desired portion of such metal fibers.

The strip mixture is then impregnated or impregnated and coated with, for example, a polymer with a relatively low molecular weight to obtain a thread which, after solidification, is cut into granules again. When the granules are added to the plastic pellets and the mixture is heat-treated, the thermoplastic textile fibers in the granules soften and disappear into the plastic matrix. The step of pre-mixing metal fibers among the textile fibers allows for better separation of the metal fibers in the plate and eliminates any occurrence of metal fiber clumps during the heat-kneading process prior to injection molding.

Certain other additives in the plastic may also provide benefits with respect to shielding properties either by improving the electrical conductivity of the plastic due to its suitable electrical properties or by facilitating the distribution of the conductive fibers during the process or by both. Certain flame retardant additives added during the mixing of the raw plastic material have improved shielding properties in combination with the addition of stainless steel fibers in the plastic as described above.

The invention has been particularly described with reference to its application to shielding radio and high-frequency waves. In cases with a considerable p/D ratio of the thin conductive fibers in the plastic matrix, electromagnetic waves in the radar frequency range can be absorbed to a large extent. The volume concentration of the fibers can in this case be very small since good conductivity is not required for camouflage against radar waves. Here, the surface resistivity of the plastic plates provided with finely divided conductive fibers will preferably be greater than 100JL/sq. but usually it will be about 40-50%. A reflectance value of 10% is sufficient, the ratio between the fiber concentration and D/L will in most cases correspond to a point in the area to the left of the straight line 2 in Fig. 4 at concentrations lower than 0.25 volume percent.

Stainless steel fibers were used in the examples. Other electrically conductive fibers are in principle also usable, for example glass fibers with metal coating, provided that the distribution process in the plastic matrix can take place under sufficiently low shear forces to avoid the tendency of the fibers to break. It may also be necessary to adapt the injection molding conditions: the rheology of the plastics during injection molding and the injection speed. The diameter of the drawing channel will be at least twice the thickness of the plate to be formed.

In addition to the polymers described in the examples, several other resins may be used in the manufacture of the final product comprising conductive fibers. These include, but are not limited to, polycarbonates, polyacetates, polyarylates, polyvinyl chloride, fluoropolymers such as polyvinylidene fluoride, polyolefins, polyacetals, polystyrene, etc. Although the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the embodiments disclosed but is intended to cover various modifications and equivalent arrangements as encompassed by the inventive concept defined by the claims.

Claims (1)

1. 452 280 21 PATFNIKRAV _________ Plastic article which is electrically conductive in any direction, at least within certain predetermined parts, and comprises a plastic material with electrically conductive fibers, characterized in that the fibers exhibit a specific conductivity of at least 0.5% of the copper standard and that they are randomly but substantially evenly distributed within said parts, which fibers exhibit a length L and an equivalent diameter D which varies between approximately 0.002 and approximately 0.015 mm, so that the ratio D/t varies from approximately 0.000§ to approximately 0.008 for the majority of the fibers and in that the volume concentration (C%) of the fibers within said parts varies between approximately 0.05 'Jin- ntli Lingefšii' Û,b 9.3. Article according to claim 1, characterized in that it is designed as a plate or disc. Article according to claim 2, characterized in that when the thickness of the plate or disc is less than approximately 3 mm, the volume concentration C of the conductive fibers in the plate or disc satisfies the ratio C21.& D/L-0.ÜOÛ82 0Cn when the thickness of the plate or disc is between 3 and approximately 6 mm, the volume concentration (C) satisfies the ratio C 2 D/L-0.Ü0l}. Q. Article according to claim 5. characterized in that C i }.3fi D/t-0.000ü1. Article according to claim 1. characterized in that the conductive fibers have relatively smooth surfaces. All ikel eiiligt iiatetfl claim 1 ellivr S, k ä ti n ri t ti c k- n a d av that the conductive fihrnrnu ar of rnstsfritt steel. Article according to putentkrnv 1, k à n n e t e C k n n d of ntt plnuten konsistema en termnhdrdande hnrts_ tl. Article according to fiatenlkiwiv 1, k ä n n e t P c k n :i d :iv :ill iila-.itteri ar nn teruiiiplzist i:;k ttnrts. Article according to pzitentkrzix ti, k ii n n n t e c k n a tl av that it ür is manufactured by nprulpressning. lll. Article according to nnti-iilkrzivi 7 or Il, k ä n u e. t e e k- ii zi d :iv :itt den titgtiif: av skiimplzifzt. . ...- ...__-nn 10 20 SU 452 280 22 Article according to claim 7 or 8, characterized in that the resin is an elastomer. Article according to claim 10, characterized in that the foam plastic is an elastomer. Article according to claim b, characterized in that it also includes other fibers. 1&. Article according to claim 12, the others characterized in that at least some of the fibers are reinforcing fibers. Article characterized in that according to any of the preceding claims, by a shielding efficiency against electromagnetic radiation of at least about 25 dB within a frequency range of about 0.1 to about 10 GHz. Plastic granule having an average length of between 0.4 and 1.2 cm comprising plastic material and electrically conductive fibers distributed in the plastic material, characterized in that the content of conductive fibers is on average greater than the final concentration of fibers in the article and in that the fibers are on average longer in the granule than in the article. Plastic granule according to patent claim 16, characterized in that the volume concentration of the fibers is between approximately 0.5% and approximately 2%. Plastic granule according to patent claim 16 or 17, characterized in that it additionally comprises other fibers. Wire comprising a bundle of conductive fibers embedded in plastic, characterized in that the fiber content is between 20 and 70 volume percent and that the average diameter of the fibers is at most 0.015 mm. Wire according to claim 19, characterized in that it has a flattened cross-sectional surface. A wire according to claim 19 or 20, characterized in that the filament (in its two-dimensional structure) comprises between about 100 and 550 adjacent fibers. A wire according to claim 21, characterized in that the plastic in the bundle is embedded in a thermoplastic polymer with a relatively low molecular weight, characterized in that: 10 20 50 452 280 23 A method for forming plastic articles having at least predetermined conductive parts, characterized in that the steps a) providing a fiber/plastic mixture with a content of conductive fibers of about 20 to about 70 volume percent and in which the fibers lie substantially parallel, b) mixing this fiber/plastic mixture in a predetermined a fixed volume of plastic material and C) heating this mixture and processing the heated mixture while maintaining low shear conditions to avoid fiber breakage, but with sufficient shear to obtain uniform distribution of the fibers in the pellet. 2A. method according to claim 25. k 5 n n e c e n t i o n that the plastic material according to step b) consists of plastic pellets. method according to claim 23. c a n n e c e c k- n a t of a further step of forming the article by “ drawing the processed mixture through a die. method according to claim 23, c a n n e c e c k- n a t of that the further step of forming the article is by injection molding the processed mixture. method according to claim 25, c a n n e c e c k- n a t of that the volume of pure plastic material is regulated so that a drawn wire is obtained with a conductive rubber content of between about 0.5 and about 2 volume percent. method according to claim 27, c a n n e c e C k- n a t of a further step where the drawn wire is cut into granules with a length of about 0.5 to 1.2 cm. method according to claim 28. c a n n e c e C k- n a t of the further step of mixing the granules with a a predetermined amount of plastic material to form a mixture in which about 0.05 to about 0.5 volume fraction of fibers are substantially uniformly distributed, which mixture is formed into a sheet article in which the D/I ratio varies from about 1.0 to 0.000 for the majority of the fibers. 452 280 24 A method according to claim 29, characterized in that the plastic in the granules has a softening or melting point that does not exceed that of the plastic material in which they are incorporated. 5 31. a method according to claim 29, characterized in that the article is formed by extrusion molding. a method according to claim 29, characterized in that the article is formed by injection molding.