CA1194688A

CA1194688A - Plastic article containing electrically conductive fibers

Info

Publication number: CA1194688A
Application number: CA000418669A
Authority: CA
Inventors: Lode Soens
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1981-12-30
Filing date: 1982-12-29
Publication date: 1985-10-08
Also published as: GB2150936B; AU562698B2; HK90286A; GB2150936A; AU9189882A; DE3248658C2; FR2519180B1; CH659724A5; GB2112796B; FR2519180A1; SE8207260D0; LU84554A1; SE452280B; GB8501468D0; CH654970A5; IT8249746A0; SG65686G; CH659723A5; SE8207260L; IT1189446B

Abstract

ABSTRACT

The present invention relates to articles, particularly plate or sheet-like articles made of plastic in which very low contents of fine elec-trically conductive fibers are uniformly dispersed so as to make the articles conductive. It also relates to specific intermediate plastic products, referred to as grains, threads and granules, and the processes for manufacturing each of these products as well as the final conductive articles. The articles can be used as a suitable shielding against radio-frequency and high-frequency electromagnetic radiation or as antistatic plastic articles.

Description

PLASTIC ARTICLE CONTAINING ELECTRICALLY CONDUCTIVE FIBERS
.

The invention relates to articles, particularly plate-or sheet-like articles made of plastics with a very low content of fine electrically conductive fibres which are dis-persed in the plastic matrix. It also relates to specific plastics grains and granules as intermediate products and processes for the manufacture of these articles, as well as applieations for these articles such as, for example, articles with a suitable shielding capacity against radio frequency and high-frequency electromagnetic radiation or antistatic plastic articles.
The incorporation of electrically conductive fibers in plastics is well known, as, for example, for reinforcement purposes and/or for improving their electrical and/or thermal conductivity.
However r for some time authorities, as for example in the United States, have been showing concern for environ-mental hazards of various kinds of electromagnetic radia-tion, in particular those with high frequencies such as radar waves, microwaves and those produced by signals used in electronic circuits, e.g. in digital devices. The use of radio-frequency and high-frequency electromagnetic radiation will grow in the future as a consequence of the widespread application of microprocessors, digital calculators and weighing scales for cash registers, electronic typewriters, and other personal and business computers with associated peripherals, electronic toys and games, military equipment, etc.
When such devices are housed in metal boxes, they are sufficiently protected against emission of radio-frequency and high frequency radiation by the metal itself which reflects the emi,tted radiation towards the box inside.
Interference with and disturbance of radio, television or other electronic waves are thus avoided.
However, there is a trend to replace metallic boxes by plastic housings. So far, it has been customary to apply electrically cond~ctive coatings on th~se plastic housings to provide a shield against the emission of electro-magnetic radiation. But a drawbaLck of such coatings is that they are not very durable. Moreover, in most cases, these coatings require special and expensive processing and application methods.
Attempts at imparting electrical conductivity to the plastics themselves (so that they shield against electro-magnetic waves) have also been suggested by the incorpor-ation and dispersion of relatively big quantities ofconductive fillers. Such conductive fillers include carbon black, aluminium flakes, cut wire, metal coated glass fibers, wire meshes and carbon fibers. However, some drawbacks are associated with these conductive fillers. Some fillers do not permit sufficient dispersion in the plastic matrix and clog together or break exces-sively and degrade to very small particles so that their shielding effect is strongly reduced. This degradation makes it necessary to add a greater amount of conductive particles which renders a uniform dispersion even more difficult while having a negative impact on the mechanical properties of the material.
Finally, it is known that for effective shielding against electromagnetic radiation the conductive particles in the plastic matrix must possess a considerable aspect ratio, i.e~ length-to-diameter (L/D) ratio; these particles must form as much as possible a continuous conductive net-work in the matrix in order to increase the conductivity without, however, substantially changing the physical and mechanical properties of the plastic matrix~

3 ~

It is an object of the pre~ent invention to make pla~tic articles with leB~ than about 0.5 ~ volume of fine electrically conductive fibers, which are randomly and substantially evenly distributed such that the distributed fiber~ provide a ~uitable conductivity in any direction in the articles for use, for example, as electro-magnetic interference (EMI) 6hielding. ~he fiber~ can be evenly di~tributed throughout the body of the article 9 e.g.
a plate or sheet, or otherwise just in certain predeter-mined area6 thereof, e.g. next to one or both or part oflts exterior or plane surfaces. The fine fiber~ have preferably an equivalent diameter of le~s than about 0~015 mm and more than about 0O002 mm.

It is another object of the invention to provide mea~6 and measures to manufacture plate- or ~heet-like pla~tic article~ with a ~hielding effec-tiveness again~t electromagnatic radiation of at lea~t about 25 dB within a wide frequency range(e.g. between 0.1 and 10 G~z and in particular at 1 GHz) while main-taining their normal mechanical prDpertie~. Plate- and 3heet-like articles are understood to compri~e lats, various shaped profile cross-section~, foils 9 thin films, tubes 9 hou~ings, bag~, cover~, or other containers.

2~
For this puxpose 9 electrically conductive fibers are dispexsed in the plastic article having a length and an "equivalent" diameter ratio (D/L) which ~aries from about 0.0005 to about 0.008 for a ma~or part of the fibers. ~hese fibers may, for example9 be metal fibers with an a~erage length L between 0.5 mm and 5 mm.

8~

The term "equival~nt'l diameter D means the square root of the quotient of the surface of the fiber cross-section divided by ~, ~he average length L means the total sum of the lengths of the incorporated fibers divided by the number of fibers. At an average length of L = 0.5 mm, there will certainly be fibers with a length shorter than 0~5 mm.
However, a major part of the fibers has a length approximat-ing the average length. According to the invention these fiber dimension limits meet the above-mentioned shielding requirements at an exceptionally low volume concentration C
(%) of conductive fillers, namely between approximately 0.05 volume percent and about 0.5 volume percent. Moreover, when the plate or sheet thickness is smaller than 3 mm, then C ~
1.4 D/L - 0.00082 and for plate thicknesses between 3 mm and 6 mm C ~ D/L - 000013. These low concentrations exert almost no influence on the aspect of the plastic articles. I have further discovered that I can produce antistatic plastic articles by dispersing electrically conductive fibers in the plastic, at low concentrations (less than about 0.5~ volume3 and wherein the concentration C with respect to the fiber dimensions in said antistatic plastic may even meet the relationship C < D/L - 0.0013. The fibers should then be present at least next to the outer surface of the articles.

It is thus possible according to the invention to make plastic composite articles with such low conductive fiber content therein and to randomly and uniformly distribute said fibers in the plastic so that the article has a predetermined level of conductivity. The conductive fiber concentration can thereby vary between about 0.03%
vol~ and about 0.5 % vol.

Furthermore, optimal D/L limits can be reached by adding the fibers during the industrial manufacture of plastic articlesr within the ahovementioned L, D and C
limits. These D~L limits then also satisfy the following equation : C ~ 3.34 D/L 0~000410 ( Si~ce, i~ the plastic matrix, the contact be~.een the fib~rs mu~t be as good a~ pos~ible to stimulate the oondu¢-tivity, it h~s appeared to be important that they po~e~
a relati~rely smooth ~urface. Thi~ ths,t rough~a~e~ o~
5 the iber surIace should pro~ect abollr3 or ~:lct~d ~uld~r the average level of the fiber ~urface less th~ about 1 ,um.
In this ~ay it; 18 Eitatil~ltiCRlly moBt likely that there ~ill be an optlm~l nwnber of oontact ~urfacan bat~eeIl n~i~hborine fibera, which contact surfaGe~ mor~oYer ha~s opt~al dl~e~a-10 ~io~.

Stainle~a steel fiber~ / msnufactured b~ a methodof bu~dle dra~ing 311 de~oribed e.g. i~ the IJoSo Pat~D~t ~o~ 2.050~298 or l~o. 3.379.000, show~d particularl~ suitabl~
intrln~ic conduotiva propart~e~ for this applicatio~. Pro~
bably this i~ attributable to the faot that they are le~
prone to form a moro or less i~ulating o~ide layer o~ their ~urfac~a in contra~t with, for exampl~, aluminium or copper fibers. ~his mea~s that the contact resi~tance in th~ fiber contact point~ remains low~ ~ually, they are al~o more inert than ~1 or Cu towQrds most pla~tic~. Other fibers such a~
~a~telloy*X, I~concl* ~i or Ni are usable a~ wall. A suitable speGific sonducti~ity of tha fibsrs i~ at lea~t 0.5 ~ of the copper standard.
In principle, the invention i8 applicable to most pla~ticc, pref4rably thermopla~tlc typsa9 under the appli-cation of the usual ~hapi~g techniques ~uch ~B ca~ting, e~-tru~ion, ~nJectiQ~ molding, press moldi~g and foamingO
3o ~ ccordingly9 th0 articles ~ay ha~a a fle~ible, rigid or elastomario ~ature~ Ho~sr, the invention is very easily applic~ble to ther~opla~tic re~ ~nd to their conrentional ~h~ping technique~ ~uch a~ e~tru~ion aad lnJactio~ molding * Trade Mark ~' by u~ing pla~tic pelleta a~ a ~tarting mat0rial 0 ~herefore ~
in practice, it l~ r~com~nded tc add the condu~tiYe flberD
in ona way or ~other to the pla~tic pellet~ sr to inGOrpO-~xate thsm lnto ths~e pelleta ~o that -th~ir co~patib~lit;r ~ith 5 the pla~tic~ i~ not thr~atened and a~ optlm~ r ~io~ di~
persion of the conductive fibcra ln the plastlo~ i0 rea¢hed during con~entional shaping proce~e~ O

~ccording to an importa3~t aspect of th~ ention"
10 the uniform disp0rcion i8 obtaincd b;y u~ing plastic gra:Lne a~ an intermediate product for th~ fabrication of the srti~l~
~hereby the grain~ ara at lea~t about O.4 s~m long and loaded with conductlYe fibers~ ~he average longth of the flber~ in the grain~ will slightly ex¢eed that of the fib3rs in the final articl~ ~ince during the molding procsss, a number o.f fiber~ alway~ ~et broken. Further on 9 inventiv~ msasure~ are deocribed to counteract thi~ pronene~ to fibsr breakag~

Moreover, the volume concentration of conduc-tiYe fiber~ in the grains will always be greater th~n the required end concentration in the molded article. If, for ~ample, it i~ de~ired to ma~ufacture an article co~prising 100 percent of the abo~e described grain~ and with an end concentration of 0. 3 percent by ~olume of metal fib~rs in the ar-ticle 7 thRn the a~erage vOlUm8 cona0ntration of motal fibers in the grains will be at lea~t 0.33 percent. If 9 ho~ever, it 1~ de~lrad to make an article with thc ~ame end concentration of metal fiber~ (0.3 volum~ percent) ~ the ba~is of ~ mi~ture of 67 percent b~ volume of pure pla~ti~ pellets and 33 parcent by ~olume of pla~tic grains loadad ~ith metal fibers, then the a~erage ~olume concentration of metal fibcr0 ~n the~e grain~ will pref0rably be at least 0.99 percant~

In g~neral9 proce~e3 for mak~ng pl~stic articl~
having predetermin~d conducti~e portions th~rein a~oordin~ to

3~

the invention include the following steps. A fiber/plastics composite is provided having a conductive fiber content rang-ing from about 20% to 70% vol. and presenting a substantially parallel fiber arrangement therein. This composite is S admixed with a predetermined amount of substantially pure plastic material and the blend is introduced in the hopper of for example an extrusion mixer. ~n such apparatus the plastîc material is heated to soften it and worked (kneaded) to evenly disperse the fibers therein. Low shear forces are thereby applied to avoid excessive breakage of the fibers, however the shear forces must remain of a sufficient high level to evenly distribute the fibers within the plastic.
To form the article, the so worked viscous mass can then be further forwareed by an extruder screw through suitable orifices, channels or slots to a mould or it can be directly and continuously extruded to rods, tubes, sheets, films or plates or injection molded.

When using a mixture of pure plastic pellets and composite grains which include fibers as described above, then cylindrical composite grains will be chosen with a diameter at least equal to the average thickness of the pure pellets. This measure usually reduces the proneness of the embedded conductive fibers to break during the hot mixing and kneading of the grain pellet mix preliminary to the actual molding. The length of the composite grain will, preferably, be between about 0.4 cm and about 1.2 cm.

For practical considerations, it is useful to provide plastic grain with standard dimensions and standard concen-tration and which can easily be mixed and processed with conventional plastic pellets in the desired proportion for obtaining a predetermined volume concentration of conductive fibers in the ~nd produot. Ob~iou~l~, th~ main ra~ m~terial of t~e~e grains will preferably be the same resin as that of the article to be formed. The cross-sectional surface of the composite grains will, moreover, at least be equal to that of the pure resin pellets. For example, a metal fiber volume precentage in the composite grain~ of 1 pexce~t has proYed to be suit~ble. ~h~ metal fibax oonts~t in the grai~ c~ be Ghose~ betwee~ about O.5 ~ ~ol~ and about 2 ~ vol.
However, the composite grains may also contain plastic material different from that of the article to be madeO The softening and melting point of the resin in the composite grains must, however, be lower than that of the plastic from which the article will be made to enable the composite grains, during the manufacture of the article, to spread easily and mix with the rnain raw material used for the article, at high temperatures, to thereby allow the conduc-tive fibers to disperse therein under minimal shear forces.
The main raw material must also, for other reasons, be compatible with the resin of the composite grains. For example, this resin may not disintegrate or react with the main raw material when the latter is heated to its processing and molding temperature~
The most suitable basic product Eor the conductive fibers to be incorporated is a filament bundle, although other fiber bundles such as fiber slivers and staple fiber yarns are also applicable.
The fiber slivers then shall possess a sufficient yarn number or tex (titre) and the fiber lengths shall be sufficiently long to form a properly coherent bundle ~ith sufficient tensile strength for handling and processing. Average fiber lengths of 7 cm and approximately 2,000 fibers per sliver cross-section are suitableO Generally, the fiber bundles are embedded in a plastic matrix so that the fiber content therein is between 20 volume percent and 70 volume percent. The impregnated fiber bundle is allowed to stiffen (e.g. by cooling) in order to produce a so-called thread having a cross-sectional sueface, prefcrably not smaller than and about equal to the cross-sectional size of the plastic pellets of the main raw material.
The thread may be eound or have a variety of other cross-section shapes, e.g. oval, flattened, or rectangular, to facilitate winding up and chopping into particlesr The thread may comprise 35,000 ad~acent filame~ts (or fib~r3~ 8 oro~c sectio~, but a lo~or numb~r (at leaet about 1,000 filame~tc) i~ rcco~mendableO
It is often recommended to envelop the impregnated bundle with a sheath made of either the same plastic as the main raw material, or the same or another plastic as that with which the bundle was impregnated. This stimulates the gradual disintegra-tion of the cut bundle and the uniform dispersion of the fibers in the plastic matrix while mixing at high temperatures. The thread is chopped into predeter-mined lengths, referred nerein after as granules, with such lengths ranging from at least about 0.4 cm and at most about l~S cm.
It is evident that the plastic material with which the fiber bundle was impregnated and sheathed 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 resin is preferably a relatively Low molecular weight thermoplastic polymer ~uch a~ a polyethylene, polypro-68~

pylene, polyester, polyacrylate, polymethacrylate,polystyrene, POV.C. and P.V.C~-copolymers.
~ he thermoplastic grains with the conductive fibers dispersed therein are prepared by making a dry mix of pure plastic pellets (the main raw material~
and a number of granules in which an appropriate quantity of parallel fibers are embedded, which fibers possess approximately or predominantly the same lengths as the granules. Tnis mix is subsequently kneaded in an extrusion mixer under elevated temperature and under the application of low shear forces in order to disperse the conductive fibers in the plastic material. Thereafter, the soft mass is extruded into one or more threads with suitable cross-sections and cooled down. Finally, the threads aretransversally chopped into grains with lengths of at least about 0.4 cm.

The invention will now be further described by means of a few embodiments and with reference to the accompany drawings, in which:

FIGURE 1 is a partial perspective view of the formative and finished stages of a thread formed from an impregnated and sheathed bundle of conductive fibers and a granule cut from this thread;

FIGURE lA is a partial perspective view of a thread as in FIGURE l but having a flattened cross-section;

FIGTJRE 2 is a drawing o~ a plastic grain containing dispersed conductive fibers;

FIGURE 3 is a graphic repeesentation of the relationship between the wave ~requency (f~ of electromagnetc radiation and the shielding effec-tiveness (~E) of a 3mm-thick plastic plate containing conductive ilLers; and FIGURE 4 is a graphic repres~ntation of the optimal field of operation for the invention in terms of ~iber concentrations and D/L ratios~

Example l With reference first to Figure l, a substan-tially round, not twisted, bundle 1 of 20,400 stainless steel filaments, AISI 316L of the type BEKINOX~ (trademark of applicantj with an equivalent filament diameter of 0~008 mm, was passed through a bath containing a solution of 20% by weight of a relatively low molecular weight linear polyester (M.W. circa 14,000~ of the type Dynapol L850 (Dynamit Nobel) in trichloroethylene.
After leaving the bath, the hundle was pulled through a round stripping orifice, with a diameter of 1.8 mm, and dried. The dr;ed bundle thus comprised ~.2 percent by weight of resin ~which equals 70 percent by volume of metal fibers).
Such impregenated bundle was enveloped in a wire sheath ~ ,, -~ * Trade Mark extruder (~ype Maille~er with fixed centering) with the same polyester Dynapol ~850. The round extrusion noæzle had a diameter of 2 mm. After the thus extruded thread 2 had cooled down, it was chopped into cylindrical granules 3 with a length of 1 cmO The granules comprise~ approximately 13 percent by weight of resin which equals approximately 52 percent by volume of metal fibers. When cutting the bundle almost no metal fiber ends were pulled out of it and hook formation and flattening of the fiber ends were avoided. T'nis was important to assure a reli~ble dosage and fluent dispersion. Then the granules were dry mixed by tumble blending technique~ with the usual thermoplastic pellets of various kinds of resins in the proportion of 9.75 percent by weight of granules to 90.25 percent pure plastic pellets and extruded into a substantially round thread with a diameter of 4mm and a metal fiber content of approximately 8 weight percent. After cooling, this extruded thread was cut ayain into grains 4 (Figure 2) with a length of 1 cm. In these grains, the metal fibers appeared to be evenly dispersed with a volume content of about 1.1 percent. The shear Eorces encountered during extrusion were held sufficiently low so that excessive fiber breakage was avoided. One of the measures applied to keep the shear forces down to a minimal ievel involved the removal of the filter plates at the inlet of the nozzle. The temperature at the nozzle of the single-screw extruder was 260 degrees centi- !
grade when NORYL*-SE90 (a modified polyphenyleneoxide of General Electric) was used. When using Cycolac* AMlOOOAs (an ABS resin of Borg Warner) the extrusion temperature at the nozzle was 220 degrees centigrade~ When using Lexan*
L13848-141R-lll (a polycarbonate of General Electric) *Trade Mark it was 225 degrees centigrade. The extruder was of the type Samafor 45 with a length-to-diamete~ ratio of the screw equal to 25. The feeding channel in the head next to the extrusion ori~ice was a ring like space between a tapering outer surface of a mandrel and the concentrically arranged conical inner surface of the nozzle headO The channel was thereby confininq towards the extrusion orifice and shear was thereby somewhat increased and this resulted in a ~etter fiber dispersion whereby the fibers were more or le89 oriented in th~ extrusion direction.

The thus obtained composite grains were dry mixed with an equal weight quantity of pure plastic pellets and fed to an injection molding machine of the Ankerwerk V24/20 type ~ith a screw to which a mold was connected for molding plaques with a thickness of 2.3 mm, a length of 30 c~, and a width of 25 cm. The temperatures in the screw chamber were respectively 250 degrees centigrade, 210 degrees centigrade and 290 degrees centigr~de, respectively for the Noryl, Cycolac, and Lexan resins, and the temperature of the molds was set at respectively 80 degrees centigrade, 50 degrees centigrade and 90 degrees centigrade. The screw rotated at 44 revolutions per minute. The nozzle opening had a diameter of approximately l cm.
The Noryl-, Cycolac- and Lexan-plates had smooth surfaces and the fiber dispersion or distribution throughout the plates was even. The concentration of metal fibers amounted to 4 weiqht percent or 0.5 volume percent. The Bekino ~ stainless steel fibers have a specific conductivity of about 2 % of the copper standard.

/

-Exam~le 2 Under similar conditions as in Example 1, injection molded ylates were made o~ the thermoplastic resins mentioned above. However, a flat bundle of 20,400 adjacent Bekinox~ filaments with a diameter of 0.008 mm was used as is shown in FIGURE lA. As in Example 1, the flat bundle was again impregnated with a Dynapol L850 solution and stripped through a rectangular 5 mm x O.5 mm orifice. The dried bundle comprised 6.4 percent by weight of resin and was enveloped with the same poiyester resin in a slot extruder at 160 degrees centiyrade. The dimensions of the rectangular extrusion nozzle were S mm x 0.6 mm and the obtained cooled strand comprised 23 percent by weignt of resin which equals approximately 39 percent by volume o~ metal fiber. The flat thread was chopped in 1 cm lengths whereby hook formation and flattening o~ the fiber ends were absoluteLy avoided. Clamping of the ibers in a ~lat bundle in the resin matrix for the 8ak~ of accurately cutting the granules proved to be very efective. The obtained flat granules were then, without any difficulty, dry mixed with pure plastic pellets in a ratio between 10.66 and 89.33 weight percent and extruded into a substantially round thread with a diameter of 4 mm (see Example 1)~ The metal fiber content amounted to approximately 8 weight percent which corresponds to approximate]y 1~1 volume percentO Composite grains with a length of 1 cm were cut ~rom tnis threadO After dry mixing these composite grains with an equal weight of pure plastic pellets and injection molding of the mixture as described above, an even dispersion was observed. The average fiber length was estimated at approximately 10 lo 5 mm and the end concen-tration again amounted to 0.5 volume percent. See area A in Figure 4.
The shieLding behavior against electro-magnetic radiation of the injection molded plates was tested. As known, the shielding behavior of a plastic material loaded with conductive filler can be determined in proportion to the plate thickness by comparing the reflection R (%) measured at one radiation frequency (e.g. 10 GHz) with the reflection (100%) on a reference material such as a metal plate.
If the electrical properties o the material are sufficiently homogeneous and the conductive filler in the plastic forms a network with a sufficiently small mesh size (e.g. of an order of magnitude smaller than the wave length of the radiation to be shielded), then the shielding behavior can be extropolated Eor the full frequency range. Moreover, it is known that fo~
a great number of application for electrically conductive plastics, the shielding requirements are met when a shielding effectiveness (SE) of 25 dB is obtained at a frequency of 1 GHz. It was also found that the SE value for electric fields and for materials with a specific resistance between O.OLr-cn and lOO_f~cm always is minimal in the vicinity of O.4 to 5 GHZ for plate thicknesses between 1 and 6 mm and with a distance of approximately 1 cm to 10 cm between the wave source and the plastic plate. A relationship between the shielding effectiveness SE and the wave frequency f is shown in Figure 3 Eor a plate thickness of 3 mm and a distance between the source and the plate of 1 cm. Curve 1 refers to the relationship for reflection values R = 99%t measured at 10 GHæ, whereas curve 2 shows the relationship for R = 70%, again at 10 GHz. If, for example, for a conductive plastic plate with a thickness of 3 mm, a reflection R is measured of 80% at 10 GHz (source-to-plate distance is 1 cm), then it can be derived from Figure 3 that the SE value will be at least 35 dB at any frequency.
When R= 70% and 1 GHz, then SE =38 dB.
Analogously, the following values hold for other plate thicknesses and measured at a distance o~
1 cm between source and plate:

ThicknesslOGHz ! 1 GHz R (~) SE (dB) ll R~%) SE (d~)

4 70 35 70 41 1 g5 35 70 27 ~rom the shielding theo~y (Schultz) it oan further be derived that the specific resistance ~ (JfL cm), for homogeneously conductive plastic plates and indepen-dently of the plate thickness, shows the following values corresponding with the following reflection values ~R - ~). See table:

R (%) ~l~cm) 99 Ooll ~)~ 53 1.1 Hence, it can be derived from the data that a thicker plate may possess a lower speciEic conduc-tivity ll/i) and a lower reflection value to reach the same shielding effectiveness (SE! at a given frequency (e.g. at 1 GHz). Thus, the ~/L value of the fibers may at a same fiber concentration be higher ln a thicker plate than in a thinner plate, or, in other words, the ~iber concentration in a thicker plate may be smaller than in a thinner plate when D/L is equal in both plates.
Transmission, re1ection and resistivity measurements were conducted on the injection-molded plates . The transmission and reflection measurements were made at 10 GHz. For these measurements the plates were placed between a wave emitter 'an oscillator) to which, via a circulator, a irst horn antenna was connected and a second horn antenna which is connected to a second detector. The energy generated by the oscillator is sent to the plate via the first antenna and the transmitted energy is, via the second antenna, registered by the second detector connected thereto. The reflected energy is returned to the first antenna and registered by a first detector connected thereto. This amount o reflected energy i5 expressed in percent (R-value) of the amount of energy (l00%) which is reflected by a metal plate in the same circumstances. When the amount of transmitted energy is equal to zero, then, for the purpose of reflection measurement and registration, the plate is reciprocated at constant speed between and from near the first antenna to the second antenna over a distance of 22 cm. This move-ment starts at least 14.5 cm away from the circulator.
~his dynamic method enables the avoidance o~ measuring errors which might occur in static measurements when the position of the various plates relative to the cieculator is not exactly the same during the succes-sive measurements. Indeed, the measured reflection signal is always the result of successive reflections and rereflections between the plate specimen and the metal (circulator, antenna). This produces a standing wave pattern as a function of the distance between specimen and emitter. In the dynamic method, -the average value of the registered standing wave pattern is determined by a microprocessor.
For the measurement of the specific resistance (resi~tivity), the plates or sheets are connected near their oppo3ite edges between clamps in an electric circuit. To obtain good conductive contact between these clamps and the conductive fibers in the clamped plate edges, the latter are ~coured and coated with silver paint.
The measuring results were as follows (average values):

19 ~

Refl~tion rransmission Specific r~istance (%) (~ (~ ~m) Noryl 6S 0 2 Lexan 71 0 3 Cycolæ 1 65.5 O _ _ This shows that the injection-moided plates with a thickness of 2.3 mm were on the limit between insufficient and sufficient shielding effectiveness (35 dB) for certain applications. See area A in Figure 4.

Example 3 A similar resin-impregnated flat filament bundle (thread) as in Example 2 was chopped into granules of 1 cm length an~ as in Example 2 mixed with pure resin pellets (Cycolac) in the desired propor-tion. These resin pellets had the usual dimensions (approximately 0.5 cm long, On5 cm wide, and 0.2 cm .hick). The mixtuee was extruded into a round thread and cut to form composite grains containing approximately l.lr ~ercent by volume of metal fibers (see Example 2). The composite grains were then dry mixed with pure plastic pellets in a 50/50 proportion and were fed to an injection molding machine of the Maurer type with a nozzle orifice having a diameter of 0~95 cm. The same temperatures as in Example 2 were applied. If also the shiel~ling characteristics must be sufficient in the immediate vicinity of the nozzle, the injection will preferably take place at a slow pace and/or an after-pressure will be applied at the ~o end of the injection process, which is kept as low as possible. The injection molded plates were 5 mm thick. The average fiber lengith L was determined by cutting very thin slices from these ~lates, and subsequently dissolving the resin from these slices and analysing the remaining fiber nettin~ under a microscope. Area B in Figure 4 corresponds with the thus determined fiber length distribution. The .shielding and conductivity measurements were conducted as described above. The results are summarized in the table below:

ReflectionTransmissionSpecific r~istance (~) (%~ (f~ cm) Cycolæ68 4 Example 4 Flat granules comprising 20,400 paraLlel stainless steel fibers with a diameter of 8 ~m and a length of 3 mm embedded in 8 percent by weight of acrylate resin K70 (from the company Kontakt Chemie) were, under careful stirring, directly added to a 45%
solution of a thermo-hardening polyester resin ~erakene 411 in styrene. The fibers from the gran~les were evenly and randomly di~persed in the resin and the u~ual accelerators were added, as well as a catalyzer. The relatively liquid ~ass was molded into 30 cm x 30 cm x 3 mm plat~s and de-aerated. The mold was closed and rotated during the cold hardening process to prevent the metal fibers from settling to the bottom of the mold. The hardened plate comprised 0.5 percent by volume of metal Eibers. In Figure 4 this mix composition corresponds with point G. The measueed re~lection amounted to 92% at a specific resistance o~
0.43~cm and at a transmission of 0%.
Similar plates (same dimensions) were made with compositions as mentioned hereunder. Reflection~
transmission and specific resistance were measured.

D (mm) I L (mm) C (~) R (~) specif. trans~ssion point in , res st. (%)Figure 4 _ ~ ) 0.008 3 0.~ 70 1.44 0 C
0.0~ 30.25 37 1.68 0 D
0.004 3 O.S0 84 3.11 0 E
0.0~ 3L0.12 70 15.1 F

From the examples and results limits were derived for the volume concentration of the fibers (C%) as a function of the D/L ratio of the fibers. Th~
straight line 1 in Figure 4 corresponds to C = 1.4 D/L -0.00082 where2s the straight line 2 represents the equation C = 3.34 D/L - 0~00041. According to the invention, the area between the two straight lines 1 and 2 determines the optimal conditions for C, D and L to provide sufficient shielding effectiveness for plates with a thickness smaller than 3 mm. For plate-or-sheet-like articles with a thick-ness between 3 mm and 6 mm, the straight line 3 in Figure 4 will be the lower limit for providing sufficient shielding.
This straight line corresponds with the equation C = D/L -0.0013.

~ i~

Example 5 A substantially round, non-twisted bundle of about 10,000 Bekino ~ stainless steel filaments AISI 316L
with an equivalent filament diameter of 0.004 mm was impregnated and sheathed, for example, with a Dynapol 1.850 solution as explained in Example l to form a strand.
Granules of 0.5 cm in length were cut from this strand and dry blended in the appropriate proportion with CYCOLAC-KJB-pellets to make grains. The grains were again made by extrusion on the Samafor 45 extruder (Example l~ and com-prised about 0.5 % volume of the fibers. Their lenqth was chosen at l cm. After dry blending again these grains with an equal amount in weight of Cycolac KJB pellets, the mixture was fed to the injection molding machine used in Example 1 to mold a plate of 2.3 mm thick. An even disper-sion of about 0.23~ volume of fibers was realized in the plate and the average fiber length was estimated at about 0.7 mm. This result is indicated by line H in Figure 4.
The antistatic performance of this plate was estimated by rubbing the plate with a textile pad so as to generate an electrical charge on its surface. The plate was then brought in the vicinity of a certain quantity of fine cigarette ash dust laying on a table. There was no significant tendency for the ash dust to lift from the table and to deposit itself on the underside of the plate.
~owever, when repeating the same antistatic dust test with a pure CYCOLAC-KJB-resin plate, devoid of metal fibers, the ash dust was immediately attracted to the plate.
Example 6 About 10,000 BEKINOX stainless steel fibers in sliver form with an equivalent fiber diameter of 0.0074 mm was impregnated and sheathed with a Dynapol L850 resin as explained in Example lo The strand had a metal fiber content of about 25 % vol. Granules of 0.6 cm resp. 0.3 cm in length were cut from this strand and dry tumble blended with plastic pellets of Cycolac KJB (grey) to obtain a composition of 0.5 % vol. metal fibers and the balance resin. The blend was directly fed into the hopper of an injection molding machine of the type Stubbe S150/235 (operating pressure 130 kg/cm2, injection pressure 30 kg/cm2, after pressure 30 kg/cm2). The temperature at the injection orifice was 205 degrees Centigrade and the injection time 4 sec. for a molded plate of 30 cm by 30 cm and with a thickness of 3 mm. The metal fibers were substantially evenly distributed in the plastic~ The electrical properties are given in the table below (average values).

Fiber length Reflection Transmission Specific in granule Resistance ~MM) % % OHM CM
-The reflection value at a metal fiber content of 0.5 %
in the plastic still results in a shielding effectiveness more than 25 dB.
According to our experience we can expect a sufficient shielding performance ~25 dB) with less stainless steel fibers havir.g about 0.0065 mm in diameter (D) and with a direct feed at the injection molding machine of a mixture containing granules of about 3 to 5 mm in length and a metal fiber content in the granules of about 65 % vol.
e.g. at about 10,000 fibers per granule.
This experiment thus proves that good shielding results are achievable with a direct introduction of granules at the injection molding stage and thus deleting the inter-mediate step of making grainsO
To manufacture articles of thermoplastic foam material in molds, one may use, as described hereabove, a predeter-mined mix of pure plastic pellets containing an appropriate amount of blowing agent. It is also possible to mix the blowing agent in powder form with pure plastic pellets and with a suitable amount of composite grains.

23a For example, the pellets can be moistened so that the powder sticking to them can spread sufficiently evenly over themO Afterwards, the mixture can be fed to the injection molding machine in the usual manner.
For the preparation of thermoplastic elastomer articles (e.g. comprising an elastomeric polyester Hytrel), elasto mer pellets can be used mixed with a suitable proportion of composite grains prepared on the basis of the same elastomer. However, the shear forces must be particularly low during the kneading and molding processes.

For shcet molding~ pre-impregnated ~iber sheets (prepregs) it is possible t~ disperse the conductiv~ ~ibers preliminarily in the liqu.d resin in an appropriate concentr~tion. For bulk molding viscous mixt~res of resin and fibers, the conductive ~ibers can ~e dispersed in the mass in a similar way.
In particular, it is possible to mix the conductive fibers preliminarily with other fibers, e.g. reinforcing fibers such as glass ~ibers, carbon fibers, polyar~mid ~ibers, and ~o disperse this fiber mix in some way in the resin. F~r processing into thermoplastic resins~ it is possible to replace the aforedescribed thread of conductive fibers embedded in plastics by a thread comprising a mixture of glass ~ibers and conductive fibers in the desired proportion. It is also possible to impregnate glass fiber bundLes in a side-to-side disposition with bundles of conductive fibers to Eorm the thread.
Finally, it may be preferre~ to mix threads comprisin~
rein~orcement fibers and cut into granules with threads compr ising conductive fibers and cut into ~ranules in an appropriated weight proportion and to Feed them to the molding machine, while adding, if ~ desir~d/ a suitable quantity of pure plastic pelLets (main raw material).

~ n advantageous method of distributing in the plastic a very low percentage of conductive fibers such as mstal fibers, consists of starting with a blended sliver comprising thermoplastic textile fibera with a rela-tively low melting point intermingled with a desired percentage of such metal fibers. The blended sliver i~ then impreg-nated, or impregnated and coated with e.g~ a relatively low moleculalr weight polymer to obtain a thread which, after solidification, is further chopped to granules~

~hen adding the granules to the plastic pellet~ and hot working the mixture, the thermoplaætic textile fibe~ in the granules are ~oftened and di appear in the pla~tic matrix. The step of preblending -the metal fibers among ~aid textile fibers enables a better ~eparation of the metal fib~rs in the plastic and eliTninates any occu~rence of metal fiber clusters during the hot kneading proces6 prior to molding.
Certain other additives in the plastic may favor also the shielding properties either by improving the electrical conductivity of the p1astic due to its proper electrical properties or by facilitating the dispersion of the conductive fibers during pr essing or by both. Some flame retardant agents added during compounding of the raw plastic material have improved the shielding behavior in combination with the incorporation of stainless st~el fiber~ in pla~tio~ as de~oribed abo~eD
The invention has particularly bee~ described in the light of its application of shielding a3ainst radio frequency and high frequency waves. In case of a considerable L/D ratio of the thin conductive fibers in the plastic matrix, electromagnet;c w~ves in the radar frequency range can be greatly absorbed. The volume concentration of fibers may in this case be very low since good conductivity is no requirement for camoufla~e against radar waves. Here, the surface resistivity of the plastic plates containing dispersed 3 conductive fibers will preferably be 'nigher than loo~nL/sq. A reflection value of 10% is sufficient, but generally it will be approximately 40 - 50%. The relationship between fiber concentration and D/L wi~l in most cases correspond to a point in the area to the left of the straight line 2 in Figure 4 at concentra--tions lower than 0.25 volume percent.

Stainless steel fibers were used in the examples. Other electrically conductive fibers are, in principle, also applicable, e.g. glass fibers witn a metal coating in 50 far as the dispersion process in the plastic matrix can take place under suficiently low shear forces in order to counteract the proneness or tendency of the fibers to break. Possibly, it will also be necessary to adapt the injection molding conditions: rheology of the plas~ics during injection molding and in~ection speed. The diameter of the extrusion orifice will at least be twice the thickness of the plate to be molded.
Besides the polymers described in the examples numerous other resins can be used in producing the finished product which incorporates conductive fibers. These include but are not limited to polycarbonates, polyacetates, polyarylates, polyvinylchloride, fluoro polymers such as polyYinylidene-~uorida, polyolefins, polyacetal~ 9 poly~tyrene 9 etc~
While the invention has been described inconnection ~ith what is presently considered to be the mo~st practical and pre~erred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments ~ut on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation 50 as to encompass all such modifications and equivalent structures.

Claims

Claims:

1. A plastic article, which is electrically conductive in any direction at least in certain predetermined parts, comprising a plastic material including electrically conductive fibers which are randomly and substantially evenly distributed in said parts, said fibers having a specific conductivity of at least 0.5% of the copper standard and having a length L
and an equivalent diameter D varying between about 0.002 mm and about 0.015 mm so that the ratio D/L varies from about 0.0005 to about 0.008 for a major part of said fibers and wherein the volume concentration (C%) of said fibers in said parts varies between about 0.05% and about 0.5%.

2. An article according to claim 1 in the form of a plate or sheet.

3. An article according to claim 2, wherein when the thickness of said plate or sheet is less than about 3 mm, the volume concentration C of conductive fibers in the plate or sheet meets the relationship C ? 1.4 D/L - 0.00082 and when the thickness of said plate or sheet is between about 3 mm and about 6 mm the volume concentration (C) meets the relationship C ? D/L - 0.0013.

4. An article according to claim 3, whereby C ? 3.34 D/L - 0.00041.

5. An article according to claim 1, wherein the conductive fibers have relatively smooth surfaces.

6. An article according to claim 1 or 5, wherein the conductive fibers are stainless steel fibers.

7. An article according to claim 1, wherein the plastic is a thermosetting resin.

8. An article according to claim 1, wherein the plastic is a thermoplastic resin.

9. An article according to claim 8, whereby it is made by injection molding.

10. An article according to claim 7, wherein it is a foamed resin.

11. An article according to claims 7 or 8, wherein the resin is an elastomer.

12. An article according to claim 10, wherein the foamed resin is an elastomer.

13. An article according to claim 1, wherein it comprises still other fibers.

14. An article according to claim 13, wherein at least part of the other fibers are reinforcing fibers.

15. An article according to claim 1, 2 or 3, having a shielding effectiveness against electromagnetic radiation of at least about 25 dB within a frequency range of about 0.1 to about 10 GHz.

16. A plastic grain having a length between about 0.4 cm and 1.2 cm including plastic material and electrically conductive fibers distributed therein, whereby the conductive fiber content therein is, on an average, higher than the end concentration of fibers in the article and whereby, on an average, the fibers are longer in the grains than in the article.

17. A plastic grain according to claim 16, wherein the volume concentration of the fibers is between about 0.5%
and about 2%.

18. A plastic grain according to claim 16 or 17 whereby they further include other fibers.

19. A thread comprising a bundle of conductive fibers embedded in plastic, wherein the fiber content therein is between 20 volume percent and 70 volume percent and wherein the fiber diameter is at most about 0.015 mm.

20. A thread according to claim 19, whereby it has a flattened cross-sectional surface.

21. A thread according to claim 19, whereby the fiber bundle contains in its cross-section between about 1,000 and 35,000 adjacent fibers.

22. A thread according to claim 21, whereby the plastic in which the bundle is embedded comprises a thermoplastic polymer of relatively low-molecular weight.

23. A plastic composite article comprised of a plastic material containing less than about 0.5% volume concentration (C%) of electrically conductive fibers having a specific conductivity of at least 0.5% of the copper standard and having a D/L ratio ranging from about 0.0005 to about 0.008 for a major part of the fibers and which are randomly and substantially uniformly distributed so that the article has a predetermined level of conductivity.

24. A plastic composite as in claim 23, wherein the concentration of the conductive fibers varies from about 0.03% to about 0.5%.

25. A plastic composite as in claim 23, wherein the conductive fibers have an equivalent diameter (D) ranging from about 0.002 mm to about 0.015 mm and an average length (L) varying from about 0.5 mm to about 5.0 mm.

26. A plastic composite as in claim 25, wherein the article is a plate or sheet.

27. A plastic composite as in claim 26, wherein the thickness of the plate or sheet is less than 3 mm and C ? 1.4 D/L - 0.00082.

28. A plastic composite as in claim 26, wherein the thickness of the plate or sheet varies from 3 mm to about 6 mm and C ? D/L - 0.0013.

29. A plastic article formed from a plastic material having at least predetermined portions through which conductive fibers are randomly and substantially uniformly distributed, said conductive fibers having a specific conductivity of at least 0.5% of the copper standard and being present in said article at a concentration C of less than about 0.5% volume, with a major part of the fibers having a D/L ratio which varies from about 0.0005 to about 0.008.

30. A process for forming plastic articles having at least predetermined conductive portions including the steps of:
(a) providing a fiber/plastic composite having a conductive fibers content ranging from about 20% to about 70% vol. and having a substantially parallel fiber arrangement therein, (b) admixing this fiber/plastic composite from step (a) with a predetermined volume of substantially pure plastic material, and (c) heating said mixture and working the heated mixture while maintaining low shear conditions to avoid excessive breakage of the fibers, but with sufficient shear to evenly distribute the fibers within the plastic.

31. A process according to claim 30, wherein the pure plastic material of step (b) is comprised of plastic pellets.

32. A process according to claim 30 including the further step of forming the article by extrusion through a die of the worked mixture.

33. A process according to claim 30 including the further step of forming the article by injection molding the worked mixture.

34. A process according to claim 32, wherein the volume of the pure plastic material is adjusted so as to obtain an extruded thread with a conductive fiber content ranging between about 0.5% and about 2% vol.

35. A process as in claim 34 including the further step of chopping the extruded thread into grains having a length from about 0.4 cm to about 1.2 cm.

36. A process according to claim 35 including the further step of admixing the grains with a predetermined volume of substantially pure plastic material so as to provide a mixture wherein from about 0.05% to about 0.5%
vol. of fibers are substantially evenly distributed, which mixture is formed to a plastic article wherein the D/L
ratio varies from about 0.0005 to 0.008 for a major part of the fibers.

37. A process as in claim 36, wherein the plastic in the grains has a softening and melting point, respectively, at most equal to that of the plastic material with which they are admixed.

38. A process as in claim 36, wherein the article is formed by extrusion through a die.

39. A process as in claim 36, wherein the article is formed by injection molding.