Conducting reinforced plastics

The invention relates to the use of an assembly comprising at least one non-conductive or substantially non-conductive carrier material and at least one conductive fiber web which has been provided on at least one side of the carrier material, with fibers of the conductive web having been brought into electrically conductive contact, through the carrier material, with the other side of the carrier.

The present invention relates to the lowering of the electrical resistance 
of fibre-reinforced plastics by incorporating conductive fibres in these 
plastics. 
The use of plastic materials for storage tanks for inflammable materials 
and for housings of electronic apparatus has boomed, inter alia on account 
of the good corrosion resistance and the minor weight of plastics. 
However, a problem that is often encountered here is that electrostatic 
charging can occur owing to the high electrical resistance. 
Discharge can be accompanied by sparks, which involves the risk of 
explosion. The surface resistivity of (fibre-reinforced) plastics is 
generally at least 10.sup.14 .OMEGA.#. No electrostatic charging is to be 
expected if the surface resistivity is .ltoreq.10.sup.9 .OMEGA.# (measured 
according to VDE 0303 part 3) and the volume resistivity is 
.ltoreq.10.sup.6 .OMEGA..multidot.cm (DIN 51953). 
Combustible gas, vapour and dry matter/air mixtures can be ignited by the 
discharge phenomena (sparkover) of electrostatic charges. The minimum 
required ignition energy determines which class the explosive mixtures are 
classified into. Three classes are distinguished, viz.: 
Class 1--ignition energy .gtoreq.0.30 mWs. 
Dry matter/air mixtures require between 10 and 100 mWs. 
Class 2--ignition energy 0.025-0.30 mWs. 
Vapours of solvents and petrol. 
Class 3--ignition energy .ltoreq.0.025 mWs. 
Hydrogen, acetylene. 
The possibility of rendering plastics electrically conductive by adding 
conductive additives is generally known. By adding to the plastic such 
conductive additives as graphite, carbon, aluminum, silver and copper 
powder, the desired conductive properties are obtained. The use of short 
metal fibres and short metallized fibres is also known. An article in 
"Polymer Engineering and Science", December 1977, Vol. 17, No. 12, titled 
"Conductive Polymeric Compositions" mentions the use of conductive fibres. 
In that article it is indicated that fibrous conductors are significantly 
better than powders, flakes and beads. The article is limited to 
conductive particles and fibres having a maximum aspect ratio of 35:1. The 
term `aspect ratio` refers to the ratio between the length and the 
diameter of a particle. 
The use of short conductive fibres, however, is an expensive affair in 
connection with the amount which is required to obtain the desired effect, 
viz. obtaining a conductive network. This requires that up to about 40 wt. 
% of material be added to the plastics. The same applies to the use of 
metal powders. Moreover, during the injection of resin in which conductive 
additives have been dispersed (RTM technique), the filtering out of these 
materials by the reinforcement presents a problem. Also, the mechanical 
properties of the finished product are influenced negatively by these 
"fillers". In spite of the fact that electrically conductive plastics have 
long been known, the use thereof for many applications has been limited by 
the negative economic, mechanical and processing aspects. 
Housings of electronic apparatus are typically required to shield any 
electromagnetic radiation. In practice, this shield is often obtained by 
arranging a shielding material in the interior of the housing. The use of 
a plastics material for the housing that has inherently shielding 
properties typically meets with the objection that obtaining the shielding 
properties renders the material so expensive that the use thereof is 
prohibitive or the properties thereof are so poor that the material does 
not enable proper use. 
One object of the present invention is to provide a system for providing 
plastics with electrically conductive properties, so that they are 
suitable for use in the fields described hereinabove. 
The invention relates to the use of an assembly comprising at least one 
non-conductive or substantially non-conductive carrier material and at 
least one conductive fibre web which has been provided on at least one 
side of the carrier material, with fibres of the conductive web having 
been brought into electrically conductive contact, through the carrier 
material, with the other side of the carrier material, for making a 
reinforced plastics material. 
The invention also relates to a plastics article comprising a matrix resin 
reinforced with an assembly comprising at least one non-conductive or 
substantially non-conductive carrier material and at least one fibre web 
which has been provided on at least one side of the carrier material, with 
fibres of the conductive web having been brought into electrically 
conductive contact, through the carrier material, with the other side of 
the carrier material. 
Surprisingly, it has been found that by adapting the existing reinforcement 
of plastics and more in particular of thermosetting plastics, such as 
polyester resins and epoxy resins, a sufficient conduction can be obtained 
while maintaining the good material properties. This adjustment of the 
reinforcement amounts to the provision of a conductive fibre web on the 
normally used carrier material, with the fibres being arranged through the 
base material, for instance by needling. In this way a conduction is 
obtained throughout the entire material. The reinforced plastics material 
so produced thereby acquires a so-called continuity conductivity. 
The present invention, therefore, provides a system that has such an effect 
on the plastics materials that they can be used in the three classes of 
explosive mixtures referred to above. Moreover, it is possible to produce 
shielding properties in plastics with the aid of this reinforcement 
material. 
An important advantage of the invention is that, using a plurality of 
layers of such a carrier material, a laminate of sufficiently low 
continuity resistivity and volume resistivity can be formed. Surprisingly, 
it has been found that the comparatively small quantity of conductive 
fibres which has been passed through the carrier material gives sufficient 
conductivity to the laminate. This applies not only when the layers of the 
carrier material have been arranged in such a manner that the "bottom 
side", i.e., the side with few conductive fibres is in contact with the 
conductive web, but also when two `bottom sides` have been positioned 
against each other. 
The conductive fibre web to be used is a web of conductive fibres. Examples 
include webs of metallized fibres, metal fibres, or of fibres which have 
been provided with conductive additives. If the fibre web consists partly 
or wholly of metal fibres, the metals for the fibres may have been chosen 
from the conductive metals and alloys thereof. Examples of suitable metals 
are steel, copper and nickle. When using metallized fibres, it is 
preferred to use fibres which have been metallized with nickle, copper or 
silver, with alloys based on one or more of these metals, or consecutively 
with two or more of these metals. A suitable type of fibre is an acrylic 
fibre which has been metallized first with copper and then with nickle. 
The conductive fibre web may consist exclusively of conductive fibres, but 
it is also possible to use a combination of conductive and non-conductive 
fibres in the web. For a good conductivity to be obtained, the length of 
the conductive fibres is preferably 40-70 mm. The web can be bonded 
thermally, chemically or mechanically. If so desired, it is also possible 
to use a woven or knitted fabric. The amount of conductive fibres must be 
sufficient for providing the desired conductivity. This can be determined 
by means of simple experiments. Generally, the amount of conductive fibres 
in the fibre web will be between 5 and 100 wt. %. Preferably, this amount 
is 5-25 wt. %. 
As a carrier material according to the invention, all kinds of materials 
can be used. The carrier material may or may not be a reinforcing carrier 
material. It is also possible to use a core material as a carrier 
material. Such a core material generally does not have a reinforcing 
function, although it is possible to use a reinforcing core material. 
Finally, it is observed that it is also possible for a carrier material, 
which may or may not be reinforcing, having a conductive web provided 
thereon, to be combined with a core material that may or may not be 
reinforcing. 
According to the invention, all kinds of materials can be used as carrier 
materials. Examples of such carrier materials include foam plastics, 
honeycomb materials, foils which may or may not be perforated, and in 
particular fibre webs which may or may not be woven. 
Examples of carrier materials which are preferably used are glass mats, 
glass wovens, carbon wovens, woven fabrics, knitted fabrics and mats of 
other types of fibres such as aromatic polyamide. 
Core materials such as fibre webs which have been provided with expanded 
microbeads can also be advantageously rendered electrically conductive in 
accordance with the invention. 
The conductive properties are achieved by providing the carrier material 
with conductive fibres which are placed/strewn onto the carrier material 
(reinforcement material). The conductive fibres preferably have an aspect 
ratio of 500 or more. Typically, it will be in the neighbourhood of 
4000-5000. A practical method with an even fibre distribution is for 
instance a card web. The fibre mat is subsequently anchored mechanically 
in and by the reinforcement material. This mechanical anchoring can for 
instance be obtained using needle machines or hydro-entanglement 
installations. The reinforcement could also be rendered electrically 
conductive by stitching electrically conductive yarns/wires/filaments 
through the reinforcement material. All anchoring methods whereby 
conductive fibres extend vertically through the reinforcement material are 
suitable. It is also possible to use a knitting or weaving technique, 
whereby a so-called two-and-a-half or three-dimensional knitted or woven 
fabric is obtained, provided that in the vertical direction an 
electrically conductive wire or yarn is used. 
The formation of the conductive web can be realized by all known techniques 
for making a fibre web, more particularly a non-woven. By mixing the 
conductive fibres with other fibres, substantially any desired low dose 
can be distributed evenly and set accurately. 
By stacking the conductive webs so obtained, as is conventional in the 
production of plastics laminates, a material is formed which is conductive 
through and through (volume conductivity). This is a property which is 
required for the use of these materials in mining, for instance. 
It is observed that by providing the reinforcement material which has been 
rendered conductive exclusively in the top layer, it is possible to obtain 
only a surface conduction. In a number of cases this may be sufficient, 
while yet the advantage of the much simpler operation and handling of the 
assembly has been obtained. In fact, one of the advantages of the 
invention is that fewer operations are necessary for lamination and 
impregnation when using an assembly according to the invention. This is 
also advantageous for obtaining improved reproducibility of the properties 
of the final material. 
It is possible to provide an electrically conductive layer at any desired 
point in the laminate. Placing the assembly with the conductive fibre web 
at the outside of a laminate moreover yields a smoother conductive surface 
of the laminate. The contour of the reinforcement material is compensated 
(cushioning effect). Inasmuch as only a small amount of (electrically 
conductive) fibres has been added to the reinforcement material, the 
processability (impregnation) of the reinforcement material and the 
mechanical properties of the final article are hardly, if at all, affected 
negatively. 
The article according to the invention can be made from a thermoplastic or 
a thermosetting plastic. Examples of thermosetting plastics are phenol 
resins, epoxy resins, polyester resins and polyurethane resins. 
Thermoplasts which are eligible for use include the various `engineering 
plastics` such as polypropene, ABS and related styrene polymers, 
polycarbonate, polyetherimide, polyphenylene oxide, polyphenylene sulfide 
and mixtures of these plastics. These plastics may also be reinforced with 
fibres. 
The fibres to be used for the conductive fibre web, and also for the 
carrier material are, in particular, acrylic fibres, polyester fibres, 
glass fibres, carbon fibres and aramid fibres. Of course, the choice of 
the fibres is partly determined by the temperatures and the mechanical 
load which the material must be capable of resisting during manufacture 
and use. 
The articles according to the invention can be made in different ways, 
depending on the materials to be used. In such systems, generally a closed 
mould is used. The assembly provided with the conductive fibre web or a 
combination of two or more of such assemblies is introduced into a mould, 
optionally with other materials which can serve as reinforcement of the 
plastics, for instance for making a fibre-reinforced laminate. Suitable 
methods for use within the framework of the present invention are resin 
transfer moulding (RTM), vacuum-injection, cold pressing, hand lay-up, 
spray-up, pulltrusion and GTM (Glass Mat Thermoplastics). 
In RTM and vacuum-injection, a liquid resin, such as a thermosetting 
polyester resin, is injected into a closed mould in which are already 
placed one or more assemblies such as described hereinabove. Cold pressing 
is based on the same technique as RTM, with this difference that the resin 
is not injected but is pressed into the assembly during the closure of the 
mould. 
Hand lay-up and spray-up are techniques in which the laminate is built up 
layer by layer (assembly and resin). In pulltrusion, the assembly, with 
resin added under pressure, is drawn through a die, followed by curing. 
In GTM a fibre web, for instance a glass web, which has been impregnated 
with a thermoplastic resin, is deformed in a mould so as to form a 
laminate. The starting material may be one or more of such webs, in 
combination with at least one assembly according to the invention. 
However, it is also possible to deform in this manner an assembly which 
has been impregnated with a thermoplastic resin. 
When working with a closed mould system, in combination with a liquid 
resin, it is preferable to provide the assembly to be used with wetters 
and/or breathers so as to improve the quality of the material surface. In 
this manner an air-bubble free smooth plastics surface can be obtained. 
Finally, the invention also relates to an assembly consisting at least of a 
conductive fibre web which has been needled onto a glass mat. 
The invention will hereinafter be illustrated in and by the following 
example, without being limited thereto.

EXAMPLE 
On glass mats of about 450 g/m.sup.2, a fibre web of about 50 g/m.sup.2 was 
placed. This fibre web contained 10 wt. % of metallized (first coppered 
and then nickled) fibre and 90 wt. % of polyester fibre. Using a needling 
machine, this web was bonded to the corresponding glass mat. The 
metallized fibres accordingly extend through the two surfaces. 
Using these glass mats provided with metallized fibre, a laminate was made 
(RTM). 
Four layers (stacked, i.e., the conductive web against the glass mat) of 
this electrically conductive reinforcement material were processed into 
this laminate: 
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Glass mat: about 1800 g/m.sup.2 (23.6%) 
Fibre web: about 200 g/m.sup.2 (2.6%) 
Polyester resin: about 5630 g/m.sup.2 (73.8%) 
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Electrical and mechanical properties of this laminate which contains only 
0.26 wt. % of metal fibre, are 
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Surface resistivity: .ltoreq.30 
.OMEGA.# 
Continuity resistivity: 
1.2 k.OMEGA..cm 
E-modulus: 5890 N/mm.sup.2 
Bending strength: 141 N/mm.sup.2 
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In tests where no conductive web was used, comparable values for E-modulus 
and bending strength were obtained. The electrical properties, however, 
were poor.