Composite article and method for making same

A composite material and a method for making same are disclosed. A composite article including an electrically and/or thermally insulating substrate and protective layers on each side thereof is particularly disclosed. Said material includes protective layers consisting of fibers of a heat-stable material flocked onto the insulating substrate, and heat-stable coating resin. The resulting protective layer has improved protective properties, particularly moisture-proofness. The composite material may also be used as a metal layer carrier for forming flexible printed electrical circuits.

The present invention relates to a composite article and to a process for 
its manufacture. 
It relates more particularly to a composite article including an 
electrically and/or thermally insulating substrate and protective layers 
for this substrate which are placed on each face thereof. 
In the field of electrical technology electrical motors are generally 
insulated using flexible materials such as films made of plastic, like 
polyester films. However, these films can be sensitive to the environment, 
especially to moisture, or can have insufficient mechanical and 
heat-resistance properties. To overcome these disadvantages it has been 
proposed to form a composite material including an electrically insulating 
film sandwiched between two protective layers made of heat-resistant 
paper. This composite material requires, on the one hand, the manufacture 
of papers consisting, for example, of chopped aramid fibres and of 
fibrids, also of aramid, and, on the other hand, an application of the 
paper onto the electrically insulating support in a continuous manner in 
order to have good adhesiveness between the layers. This process for the 
manufacture of the composite material is relatively complex. In addition, 
the leaktightness of the protective paper layers, especially in relation 
to moisture, is not perfect. 
Furthermore, electrically insulating substrates, especially electrically 
insulating flexible films, are also employed for the manufacture of 
flexible electrical printed circuits. These articles are obtained by 
metallizing one face, preferably both faces, of the film and then by 
producing electrical circuits by, for example, processes of chemical 
etching of the metallized layer. Electronic components such as 
transistors, diodes or capacitors are soldered onto these circuits, for 
example by tinning. 
The support films must have good dimensional stability, especially at the 
soldering temperature. For this reason polyester films are increasingly 
replaced by films made of plastic which have improved heat resistance and 
dimensional stability, such as polyimide or polyaramid resins. However, 
the cost of manufacture of these plastics is much higher. 
One of the objectives of the present invention is to overcome these 
disadvantages by proposing a thermally and electrically insulating 
composite material including protective layers which are leakproof, 
especially to moisture, and which can be obtained by a simple application 
process. This material can be employed as an electrically insulating 
substrate for the production of flexible electrical printed circuits after 
deposition of a metal layer on at least one of the faces of the composite 
material by the processes for application of a film of copper or for 
metallizing. 
To this end the invention proposes a composite article including a 
continuous support layer and protective layers placed on at least one face 
of the support or substrate layer. 
According to the invention at least one of the protective layers is formed 
by a heat-resistant resin including reinforcing fibres made of 
heat-resistant material which are flocked onto the surface of the support 
or substrate. 
According to a preferred embodiment of the invention the support or 
substrate layer is an electrically and/or thermally insulating continuous 
layer. 
Films made of electrically and/or thermally insulating synthetic material 
such as polyester, polyolefins, polycarbonates, polyphenylene sulphide, 
polyimides, thermotropic polyesters and the like, may be mentioned, by way 
of example, as support or substrate layer which is suitable for the 
invention. 
Fibres made of heat-resistant material which are suitable for the invention 
are, for example, fibres made of polyimide, polyaramid and 
polyamide-imide. 
Polyimide, polyamide-imide, polyaramid, bismaleimide, epoxy and triazine 
resins or a mixture thereof may be mentioned by way of example of 
heat-resistant resin which is suitable for the invention. 
More advantageously, resins which are suitable for the invention are resins 
which can be deposited onto the layer of flocked fibres in the form of 
solution, the resin being precipitated either by exchange of the solvent 
with a third solvent, according to a coagulation process, or by 
precipitation, for example, in gel form, or by evaporation of the solvent. 
Preferred resins which may be mentioned in particular are polyamide-imides, 
and especially those obtained by reaction of a diisocyanate such as 
toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, diphenyl ether 
4,4,'-diisocyanate, meta-phenylene diisocyanate, 1,6-hexamethylene 
diisocyanate, biscyclohexyl diisocyanate or the like with an acid 
anhydride such as trimellitic anhydride or a dianhydride in the presence 
or absence of aromatic diacids such as isophthalic acid or terephthalic 
acid, and 1,3-dicarboxybenzenesulphonate. 
According to another characteristic of the invention the flocked fibres are 
placed on the support or substrate layer according to the conventional 
flocking techniques, that is to say by coating the surface to be flocked 
with an adhesive an depositing a layer of fibres which are kept 
substantially parallel to each other and perpendicular to the surface to 
be flocked by, for example, an electrical field. 
Suitable adhesives for the invention which may be mentioned are 
heat-resistant adhesives, for example the two-component adhesives of 
polyester/polyurethane type which are marketed by the company Henkel. 
In one embodiment of the invention the fibres may be of different or 
identical lengths. The length of the fibres and the density of the flocked 
layer are thus determined as a function of the properties and of the use 
of the composite structure, for example as a function of the desired 
thickness and surface appearance of the protective layers. 
The protective layer thus formed is continuous and exhibits good mechanical 
properties, especially due to the presence of the flocked fibres which, 
under the effect of the coagulation or precipitation of the coating resin, 
can be flattened down towards the support layer and can form a reinforced 
protective layer. 
In another embodiment of the invention the composite material thus formed 
may be subjected to a finishing treatment which consists, in particular, 
in applying pressure to the surface of the protective layer in order to 
configure the latter as a function of the desired utilizations. This 
treatment may also be a form of calendering which will make it possible to 
obtain uniformity of the surface of the protective layer(s) and of the 
thickness of the composite material. 
The material of the invention exhibits good cohesion, due especially to the 
bonding of the coating resin to the face of the support layer, this bond 
being improved and reinforced by the presence of the flocked fibres. 
In addition, the cohesion of the protective layers makes it possible to 
improve the aging resistance of the composite material by limiting the 
exposure of the support layer, such as the polyester film, to the 
surrounding atmosphere, such as moisture, radiation and the like. The 
insulating properties of the support layer are thus maintained at an 
acceptable level for a longer period. 
Furthermore, the use of a coating resin allows numerous additives to be 
added to the flocked protective surface(s), such as water-repelling 
compounds, lubricants, colorants or the like. These additives make it 
possible to improve the properties and the configuration of the composite 
material for the desired utilization. 
Thus, in the case where the composite material of the invention is intended 
to be employed as support for a metallic layer, especially for the 
manufacture of flexible electrical printed circuits, the coating resin may 
include a filler such as a metal oxide like, for example, cuprous oxide, 
allowing the surface of the protective layer to be metallized. 
One of the subjects of the present invention is especially a composite 
material of structure as described above and including a metal layer at 
least on one of its faces. This metal layer may be deposited by any known 
techniques for depositing metal on the faces of a plastic film. An example 
which may be mentioned--no limitation being implied--is the technique of 
applying a sheet of metal, for example of copper, by adhesive bonding to 
the face of the composite material or, preferably, the formation of a 
metallic layer by electrolytic and/or electrochemical metallizing 
processes. In fact, owing to the possibility of introducing a metallizing 
precursor additive into the coating resin, the composite structure of the 
invention permits an easy application of a metallizing process, like that 
described in French Patent No. 2518126. 
Briefly, the metallization process described in this French Patent consists 
in introducing into the substrate to be metallized a filler capable of 
giving rise at the surface to metallic priming sites for the 
metallization. Suitable fillers are especially nonconductive metal oxides 
such as copper oxide. The surface of the substrate is then subjected to 
the action of a reducing agent, such as a borohydride, capable of reducing 
the abovementioned metal oxide to conductive metal. The reduced metal 
forms metallic priming sites which are compatible with direct metallizing 
by an electrolytic and/or electrochemical route. 
The metallized composite material of the invention is ideally suitable for 
the production of a flexible electrical printed circuit because the whole 
unit has good dimensional stability even at soldering temperatures, 
whatever the nature of the support film. In fact, as the protective layers 
are produced using heat-resistant materials, the shrinkage of the support 
film is prevented. This advantage makes it possible to employ a support 
film which does not necessarily have good dimensional stability in 
relation to temperature, like polyester films. 
Furthermore, the structure of the invention exhibits better aging 
resistance allowing the printed circuits to be employed for a long time or 
in a hostile environment. Finally, the electrical insulation between the 
two faces of the substrate is improved. 
Another subject of the invention is a process for the manufacture of a 
composite material as described above, which consists: 
in depositing a layer of flocked fibres on at least one face of an 
electrically and/or thermally insulating support layer, consisting of 
heat-resistant fibres, and 
in coating the flock with a heat-resistant resin to form a protective layer 
for the insulating support layer. 
According to a characteristic of the invention the coating of the flocked 
layer is carried out with a solution of resin, the resin being 
precipitated onto and/or into the layer of flocked fibres in order thus to 
obtain a layer of fibre-reinforced resin. 
Precipitation of the resin is to be understood especially as the technique 
which makes it possible to deposit a solution of resin onto the flocked 
layer and to bring this layer into contact with a liquid which is a 
nonsolvent for the resin, for example by immersing the structure in the 
nonsolvent liquid; this technique is known by the name of coagulation. 
However, other techniques causing the precipitation or gelling of the resin 
may be employed without departing thereby from the scope of the invention. 
The composite structure is next subjected to drying to remove the solvent 
or the nonsolvent. 
The coating of the flocked fibres by coagulation of a resin makes it 
possible, during the coagulation, to replace the solvent of the resin, 
which generally has a high boiling point, with another compound which is a 
nonsolvent for the resin, such as water, which has an appreciably lower 
boiling point. The drying of the composite structure can thus be carried 
out at a lower temperature, reducing the risks of damage to the support 
film. 
According to an embodiment of the invention the coating solution has a 
resin weight content of between 5% and 50%. 
Suitable solvents for the invention are, for example, N-methylpyrrolidone, 
dialkylureas such as dimethylethyleneurea (DMEU) or dimethylpropyleneurea, 
dimethylacetamide, dimethylformamide, ketones, dimethyl sulphoxide or a 
mixture of these with other inert solvents such as xylene, dimethyl ether 
or di- or triethylene glycol. 
The coagulation of the resin is generally carried out by bringing the 
flocked layer coated with the resin solution into contact with water. 
The resin solution may contain additives which make it possible to improve 
its ability to wet the flocked fibres. However, to improve the wetting of 
the fibres in the resin, they may be treated with a finishing agent which 
modifies the surface tension. 
After drying, the composite structures of the invention exhibit good 
cohesion. 
Other advantages and details of the invention will appear more clearly in 
the light of the examples which are given below solely by way of 
illustration.

EXAMPLES 
Preparation of a composite material with uncoated flocked layers 
A two-component adhesive of polyester/polyurethane type, marketed by the 
company Henkel, is deposited with a gravure roll onto a 2GT polyester film 
of 125 .mu.m thickness at a rate of 50 g/m.sup.2. 
This adhesive-coated polyester film travels, resting on an apron, under a 
feed hopper for polyamide-imide flock fibres marketed under trademark 
Kermel (count 2.2 dtex--length 0.3 mm). 
Between the feed hopper and the film a device enables an electrical field 
to be established in which the fibres delivered by the hopper are 
accelerated and oriented, and this causes them to be planted in the 
adhesive (the electrical field is established between a grid at the hopper 
outlet and the film-support apron). 
At the exit from the deposition of the flocked fibres, beater rolls 
followed by a suction exhaust remove the fibres which have not been 
secured by the adhesive. 
Thus flocked, the film undergoes a heat treatment to perform the drying and 
the crosslinking of the adhesive. 
In the case of two-face flocking the same treatment is then performed on 
the other face. 
The material thus obtained, completely symmetrical, has a weight per unit 
area of 330 g/m.sup.2 (including 175 g of polyester film and 155 g of 
adhesive+flocked fibres). 
Coating of the flocked layers 
A polyamide-imide resulting from the polycondensation of trimellitic 
anhydride and of 2,4-toluene diisocyanate is prepared in 
dimethylethyleneurea (solids content: 27%-viscosity 280 poises at 
25.degree. C.), according to the process described in European Patent No. 
0360707. 
Two solutions of the above resin in dimethylethyleneurea are prepared: 
Solution A1: viscosity 85 poises at 30.degree. C. 
Solution A2: viscosity 3.2 poises at 30.degree. C. 
The flocked layers of the composite material obtained above are coated with 
the solutions A1 and A2. 
The coating is carried out by immersing the flocked film in a trough 
containing the solution and then wringing the coated material between 2 
rollers pressed against one another by adjustable springs. 
On leaving the wringing the coated material is immersed in water, causing 
the coagulation of the solution of polyamide-imide, and is then washed 
under a light stream of water to extract the residue of solvent retained 
by the resin. 
At this stage the material treated by coating-coagulation has a dry and 
solid feel but actually contains traces of solvent and a certain 
percentage of water. These components (solvent and water) ensure some 
plasticization of the polyamide-imide. 
In this plasticized state densification of the product can be carried out 
by pressing or calendering. 
It is also possible to obtain this densification merely by drying the 
material at a temperature close to 100.degree. C. to remove the trapped 
water. This water removal causes a shrinkage of the coating resin in the 
direction of thickness. 
The characteristics of the composite materials thus obtained are given in 
Table I below: 
TABLE 1 
______________________________________ 
Pass Thickness 
Test No. Solution pressure .mu.m 
______________________________________ 
1 A2 not pressed 
0.98 
2 A1 not pressed 
1.06 
3 A2 70 bars 0.56 
4 A1 70 bars 0.78 
5 A2 250 bars 0.43 
6 A1 250 bars 0.55 
______________________________________ 
The quantities of resin which are retained are 
230 g/m.sup.2 with solution A1 
185 g/m.sup.2 with solution A2. 
A coating coagulation was carried out with the resin of the preceding test 
but after a dilution to obtain a solution which had a viscosity of 34 
poises at 30.degree. C. (solution C1). 
Starting with a polyimide-amide resin resulting from the condensation of 
100 moles of 2,4-toluene diisocyanate, 20 moles of trimellitic anhydride, 
76 moles of terephthalic acid and 4 moles of sodium 
1,3-dicarboxybenzenesulphonate, in DMEU, a solution is prepared by 
dilution in DMEU to obtain a viscosity of 25 poises at 30.degree. C. 
(solution C2). 
The 330 g/m.sup.2 flocked film employed previously is treated by coating 
coagulation as already described with the above solutions C1 and C2. 
The characteristics of the products obtained are collated in Table II 
below: 
TABLE II 
______________________________________ 
Pass Weight per 
pressure Thickness 
unit area 
Test No. Solution 
(bar) .mu.m g/m.sup.2 
______________________________________ 
7 C1 70 610 860 
8 C2 70 540-560 
525 
9 C1 250 530 560 
10 C2 250 480-520 
530 
11 C1 250 460 525 
12 C2 250 480 525 
______________________________________ 
In these examples each impregnation was performed separately, resulting in 
the appearance of different resin contents as a function of the operating 
conditions. 
The following examples illustrate an advantage of the invention, namely the 
possibility of incorporating additives into the impregnating solutions. 
By way of example, the manufacture of a composite material will be 
described in which the surfaces of the protective layers are metallized in 
order thus to obtain a flexible electrical printed circuit. 
Cu.sub.2 O (cuprous oxide) marketed by the company Norddeutsche Affinerie 
is introduced into solutions C1 and C2 in a proportion of 30% by weight 
relative to the dry resin contained in the solutions. 
These filled solutions are employed according to a process which is 
identical with that described in the preceding examples, to obtain 
composite structures whose characteristics are collated in Table III 
below: 
TABLE III 
______________________________________ 
Weight per 
Test Cu.sub.2 O/dry Thickness 
unit area 
No. Solution resin Pressing 
.mu.m g/m.sup.2 
______________________________________ 
13 C1 30% 250 bars 
470 555 
14 C2 30% 250 bars 
450 555 
______________________________________ 
After coating coagulation the material is dark red. 
To remove the film of pure resin at the surface and to reveal the Cu.sub.2 
O particles, the substrate is treated by wet sandblasting under Vapor 
Blast (trade name) equipment with an AVB 90 (trademark) abrasive and a 
blasting pressure of 4 bars at a distance of 15-20 cm. The substrate is 
then carefully rinsed and dried under a stream of warm air. The surface 
roughness shows the coefficients Ra&gt;3 and Rt&gt;28. 
The surfaces of the flocked layers are next subjected to a metallization 
process. 
The Cu.sub.2 O reduction bath is an aqueous solution including: 
0.7 g of Rhodopol.RTM. (heteropolysaccharide) 
5 g of potassium hydroxide 
20 g of Na Kc double tartrate 
20 g of sodium iodide and 
50 g of NaBH.sub.4. 
The material to be metallized is immersed in the bath and immediately 
withdrawn, at a speed of approximately 1 cm/s in order that the formation 
of the film of reactant at the surface of the substrate should be 
homogeneous. The substrate withdrawn from the reduction bath is left in 
the open air and then washed and rinsed with water. 
The rinsed, undried substrate is immersed in a bath of chemical type EC 580 
(trade reference) copper from the company Kemifar, at 40.degree. C., for 
15 min. 
After rinsing, the copper-coated substrate is passivated to prevent 
oxidation, by immersion in a passivating solution at a concentration of 3% 
in water of KEM 1044 (trade name) from the company Kemifar. 
Whatever the nature of the solution C1 or C2, the thickness of the chemical 
copper deposit is from 1.5 to 1.8 .mu.m. 
This metallized composite material can be treated by known processes to 
produce a flexible electrical printed circuit.