Note: Descriptions are shown in the official language in which they were submitted.
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
12
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
13
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.
14
/
-
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
2 8S 35 70 34
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:
17
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
18
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
2 1
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~
22
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
23
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
-
3 70 0 7
6 67 0 11
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.
24
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.
26
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.
