Cascade arc plasma torch and a process for plasma polymerization

The present invention is directed to an arrangement of cascade arc plasma torches in a process for plasma polymerization. The arrangement is utilized in low temperature plasma polymerization coating and utilizes a plasma reactor with concentric electroconductive rings separated by insulator rings. The rings are positioned between electrode connectors and form a central passage through the plasma reactor. A voltage supply source provides a voltage across the plasma reactor so as to initiate and continue the polymerization coating process. Downstream of the plasma reactor, an arrangement is provided by which monomeric gas is fed into a passage at a downstream end of the reactor so as to enable plasma polymerization.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a cascade arc plasma torch for use in low 
temperature plasma polymerization coating. The invention also extends to a 
process for plasma polymerization at low pressure and at low temperature. 
Plasma polymerization is a technique which emanates from the early 1960's. 
In the early stage of development of this technique it was a widely held 
impression that plasma polymerization was a highly exotic method used to 
form an ultra-thin layer of a polymer. The method was of interest mainly 
as a curiosity in the methodology of polymerization, since the resulting 
plasma polymer was believed to be identical to a conventional polymer 
derived from the same monomer. During the more than two and a half decades 
that have elapsed since the earliest development took place it has become 
apparent that polymers produced by this technique have characteristic 
properties differing from those of polymers produced by more conventional 
means. Today it is more appropriate to describe plasma polymerization as a 
unique method which can be used to prepare special materials. 
Today plasma polymerization is a technique which has left the stage of 
laboratory curiosity. Although it is difficult to estimate the extent to 
which highly proprietary industrial operations are presently involved in 
plasma polymerization activities, it is known that some large scale 
industrial applications of plasma vacuum deposition polymerization are 
currently used. As examples there may be mentioned that the head light 
reflectors used in most European cars are manufactured by Bosch using a 
large scale vacuum deposition process. In this practical application the 
vacuum deposition of metal for the reflecting surface and the deposition 
of plasma polymer onto the metal surface to protect it from corrosion and 
tarnishing are combined into one continuous process. 3M Company produce 
optical storage discs by vacuum deposition of various components including 
methyl methacrylate. The indicated industrial processes use plasma 
polymerization to an extent which verifies that it has achieved an 
importance far beyond that of a mere laboratory curiosity. 
However, the techniques presently used for deposition by a plasma 
polymerization are associated with severe drawbacks among which the 
following may be mentioned. 
In conventional plasma polymerization the total substrate surface area that 
can be coated evenly is limited by the total volume of plasma because the 
substrate surface must be immersed in the plasma volume. 
The yield of deposited polymer in relation to the monomer used is very low 
in conventional plasma polymerization due to the fact that the plasma 
volume accommodating the substrate contains such monomer throughout its 
whole volume. 
Excessive fragmentation of monomer molecules takes place due to ionization 
resulting in even more impaired yield of deposited polymer. 
The present invention has for a main object to provide new techniques based 
on the use of a cascade arc plasma torch operated at low temperature for 
plasma polymerization coating while eliminating or at least greatly 
reducing the drawbacks associated with the prior art plasma polymerization 
techniques. 
Another object of the invention is to provide a cascade arc plasma torch 
for use in low temperature plasma polymerization. 
Yet another object of the invention is to provide a process for plasma 
polymerization at low pressure and at low temperature using a cascade arc 
generator. 
The present invention is based on an entirely new concept whereby a monomer 
or a mixture of monomers will be injected into a plasma torch or plasma 
jet and not into a vacuum chamber as in the conventional art. Due to the 
high velocity of gas in the low temperature plasma torch generated in a 
cascade arc generator the back diffusion of monomer molecules to the 
energy input zone where ionization occurs is practically nil. This means 
that the monomer introduced into the cascade arc torch will not be 
subjected to the ionization process, which by some investigators is 
believed to be an essential step for plasma polymerization. Although the 
invention is not to be construed to be limited to any specific mechanism 
or operational principle it has been found quite surprisingly that plasma 
polymerization takes place without the ionization of monomers which up to 
now has been believed to be necessary. 
Accordingly, the present invention provides for a cascade arc plasma torch 
apparatus for use in low temperature plasma polymerization coating, said 
apparatus comprising means for vacuum generation, a plasma reactor 
including concentric electroconductive rings separated by insulator rings, 
said rings being arranged between electrode connectors and forming a 
central passage through said plasma reactor, a voltage supply source 
providing a voltage across said plasma reactor, and supply means for 
introducing an inert gas into said passage. Such apparatus is 
characterized by first inlet means positioned at the downstream end of the 
plasma reactor for feeding monomeric gas enabling plasma polymerization 
into said passage at the downstream end thereof. 
Such first inlet means for the supply of monomeric gas is preferably 
constituted by inlet parts or nozzles distributed around the periphery of 
the central passage of the apparatus. 
According to another aspect of the invention the apparatus can be provided 
with powder inlet means positioned adjacent to said first inlet means, 
preferably downstream thereof, which enable introduction of powder into 
the generated plasma for plasma polymerization processing of powder. Such 
powder inlet means can likewise be constituted by inlet parts or nozzles 
distributed around the periphery of the central passage. 
Furthermore, the invention provides a process for plasma polymerization at 
low pressure and at low temperature, said process comprising the steps: 
(a) creating a plasma in a cascade arc generator to form a plasma torch 
which is directed into a low pressure zone; 
(b) injecting a monomeric gas into said plasma torch; and 
(c) directing the plasma torch resulting from step (b) onto a substrate to 
form a film thereon by plasma polymerization. 
The pressure of the low pressure zone is preferably less than about 100 
Torr. The monomeric gas supplied into the plasma zone preferably contains 
monomers selected from hydrocarbons, halogenated hydrocarbons, silanes and 
organosilanes optionally together with hydrogen. 
According to a special feature of the invention the substrate to be coated 
by plasma polymerization is a powder, such as a metallurgical powder, 
which is introduced into the plasma torch downstream of the site of 
injection of the monomeric gas. 
According to yet another aspect of the process of the invention the 
substrate is an elongated member such as a wire, a tubing, a band or a 
filament, etc., introduced in the direction of its length into the process 
before the monomeric gas injection site, whereby the pretreatment of 
substrate surface and a deposition of polymer could be achieved in a 
single uniform process. By such pretreatment undesirable contamination on 
the substrate can be easily removed. 
The present invention provides new techniques, whereby a high yield of 
deposited polymer is obtained and a well defined localized polymer 
deposition is made possible. In order to obtain uniform coating of a wide 
area the plasma torch can be arranged to scan the location of deposition 
in a predetermined pattern. An important aspect in this context is the 
fact that the invention provides freedom from the limitation based on 
plasma volume/surface area which is encountered in the conventional plasma 
polymerization using glow discharge. 
Another advantage of the low temperature plasma torch technique described 
herein is the fact that the coating operation is less disturbed by other 
external factors, such as magnetic field and the presence of Faraday Cage 
effect and, consequently, the process of the invention can be applied to 
the coating of the inner surface of a Faraday Cage, such as the inner 
surface of an automobile body. 
The present invention also enables powder processing by a plasma 
polymerization. The main object of this aspect of the invention is to 
alter the surface characteristics of powder by applying an ultra-thin 
layer of plasma polymer coating. Such applications include sintering 
powders in general, such as ceramic powders and metal powders for powder 
metallurgy. Furthermore, the invention is useful for protective coating of 
metallic pigments, such as aluminum powder used in automotive paints, and 
coloured pigments and fillers used in paints and plastics. In this context 
other gases, such as oxygen, for example for anodization of aluminum 
powder, and methane can be fed through the arc system by mixing with an 
inert gas, such as argon. More complex monomers could be fed through the 
monomer inlet located downstream of the central passage of the plasma 
torch apparatus. 
By injecting a monomeric gas into the plasma torch created by the cascade 
arc at the downstream end of the plasma reactor passage chemical reaction 
of the monomers with the excited species in the plasma can be utilized to 
perform polymeric deposition without subjecting the monomer molecules to 
ionization. This is an advantage since the ionization of organic molecules 
causes extensive fragmentation of the original molecular structure thus 
reducing the yield of deposited polymer and also altering its properties. 
In most plasma polymerization procedures using glow discharge an energy 
input of 0.1 to 15 GJ/kg monomer is generally found to be necessary to 
form plasma polymers. However, this high energy level also causes 
excessive fragmentation of the original monomer structure. Using the 
technique of the present invention a low pressure cascade arc can be 
generated at an energy input of 1 to 10 MJ/kg monomer. This is several 
orders of magnitude lower in energy input than that used in conventional 
art. Therefore, the low pressure cascade arc of this invention offers the 
possibility of forming polymeric films whose chemical structure can be 
controlled primarily by the selection of monomer structure and only 
secondarily by energy input level. 
In a cascade arc reactor of this invention the monomeric gas injected into 
the reactor cannot diffuse back into the cascade arc generator because of 
the high forward flux of the plasma. This results in a surprisingly high 
deposition rate of polymer despite the relatively small monomer flow rate 
compared to that of the inert gas, such as argon, entering the reaction 
chamber. This means that the rate of the chemical reaction in such system 
is high and that most of the monomer or mixture of monomers injected into 
the reactor are quickly and efficiently consumed by the flame and 
deposited on the substrate. 
The invention will now be further illustrated by non-limiting embodiments 
and specific examples. The embodiments will be described in conjunction 
with the appended drawing, wherein:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The cascade arc reactor is a kind of a new version of a plasma torch useful 
in local deposition of plasma polymers. The reactor or generator shown 
diagrammatically in FIGS. 1 to 3 and generally designated 1 is built up 
from a series of concentric metallic rings 3 separated by insulator rings 
5. The metallic rings 3 float electrically between cathodes 13 which are 
located in the end cap 7, but insulated from the cap, and an anode ring 8. 
A circular sleeve 10 is attached to the downstream end of the anode 8 via 
an insulator ring. A relatively low DC voltage power supply 11 is 
connected between the cathodes 13 and the anode 8 as shown in FIG. 1. A 
suitable voltage is about 0.2 to 2 kV. The system of metallic rings 3 and 
insulator rings 5 forms a central passage 15, and in the downstream 
extension of said section 15 a plasma torch 17 will be formed under 
operation. Argon is injected into the system at end cap 7 and provides a 
high gas flux into the vacuum at the outlet of the reactor. Torch 17 is 
directed into a vacuum chamber (not shown in the drawing), and the plasma 
is provided by field emission between multiple cathodes 13 in the end cap 
and the anode ring 8. The torch operates typically in a vacuum of about 10 
Torr. 
Injector ring 9 is shown more in detail and by enlarged views in FIGS. 2 
and 3. It is provided with inlets 19 for monomeric gas opening into a 
ring-shaped channel 21 which in turn is connected to radial channels 23 
opening into the central passage 15. 
The embodiment shown in FIGS. 4 and 5 corresponds largely to that shown in 
FIGS. 1 to 3 but its injector ring 29 has been modified to include powder 
inlets 31 for feeding powder for processing into the central passage 15 
via radial channels 33. This arrangement for feeding powder into the 
central passage 15 is situated downstream of the site of introduction of 
monomeric gas. 
The embodiment shown in FIG. 6 corresponds in essential parts to that shown 
in FIG. 1. However, the torch generator of FIG. 6 has been modified by 
providing an extension of the system of metallic rings 3 and insulator 
rings 5 and the anode ring 8 in the form of a sleeve 35 provided with an 
end wall 37 with a central hole 39. Injector ring 9 of the same 
construction as shown in FIGS. 2 and 3 is concentrically attached to the 
outside of end wall 37 for the introduction of a desired monomer into the 
central passage of the generator. 
Sleeve 35 is provided with a slanted inlet aperture 41 in its wall intended 
for the introduction of a wire or filament 45. The wire or filament 45 is 
passed around a guide member 43 and continues through the central hole 39 
and the injector ring 9 out into torch 17 for coating by plasma 
polymerization. The coated wire or filament 45 then continues around 
another guide member 47 and is wound up onto a storage member 49. It is of 
course possible, in order to save costs and increase capacity to arrange a 
plurality of inlet apertures for simultaneous treatment of a plurality of 
elongated members such as wires, tubings, bands, filaments, etc. In such 
case the inlet apertures are suitably arranged evenly distributed around 
the sleeve wall. 
When passing through the inner space of sleeve 35 wire or filament 45 will 
be subjected to surface cleaning before passing through injector ring 9 
out into torch 17. This pretreatment of the wire or filament 45 is useful 
for obtaining good adhesion of the coating to be deposited onto same. 
The embodiment shown in FIGS. 7 and 8 is similar to that of FIG. 6; however 
a plurality of cascade arc plasma torches are shown at reference numerals 
50a, 50b, 50c and 50d (FIG. 8). In this embodiment, the plasma arc torches 
are arrayed substantially circularly around a principal axis X. Due to an 
inclined mounting arrangement of the torch generators 52, the axis of each 
plasma torch 50a, 50b, 50c and 50d will converge to a focal point on the 
principal axis X. 
Accordingly, each of the torches will provide an extended zone of flames 54 
through which the elongated members, such as wires, tubings, bands, 
monofilaments, etc., 45 can be passed. The embodiment of FIGS. 7 and 8 
permits the elongated members to be treated, to have a straight path, 
along the principal axis X, through the zone 54 provided by the plurality 
of torches. Further, this embodiment permits a large number of wires or 
monofilaments, etc., to be simultaneously treated. FIG. 7 shows such an 
arrangement in which storage spools 49 unwind and wind, for example, a 
wire or monofilament 45 as it passes over guide members 47 in proceeding 
from an uncoated state upstream of the torches to a coated state 
downstream of the zone 54 in the direction of the arrows. This type of 
multi-torch arrangement permits the coating of a plurality of wires or 
monofilaments 45, as schematically shown by storage spools 49a and 49b. 
While the invention has been described in terms of coating wires or 
monofilaments, it is also suitable for multifilament yarns and other 
elongated members having a variety of configurations. 
The arrangement of FIG. 9 shows a device for the treatment of powders in 
which the powder is preferably fed vertically upwardly by an injector 
system of the type shown in FIG. 7. Therein, the torch generators 52 are 
of the same type as that shown in FIG. 7. Accordingly, two cascade arc 
plasma torches are shown at 50a and 50b. A gas 56 is introduced in the 
direction of the principle axis X. The gas is introduced into a tube 58. 
Downstream from the introduction of the gas into tube 58, conduits 60 are 
provided for the introduction of powders 62 into the tube 58. By the 
introduction of the powder 62 in this manner, the powder is blown, as 
shown in FIG. 9, in a vertically upward direction, by the gas 56. 
The powders are treated in the zone 54 which extends partially into a 
housing 64 defining a chamber 66 therein. The chamber 64 is provided with 
openings 68 so as to permit the treated powders 62 to fall downwardly from 
the zone 54 and into a suitable chamber (not shown) for collection. 
It is apparent from the foregoing description that other modifications of 
the embodiments are possible. For example, the embodiment described in 
conjunction with FIG. 7 could be utilized in which a single torch is used 
and is positioned to be obliquely arranged relative to the path of the 
filament to be treated. An additional arrangement which is contemplated is 
a two-staged treatment process by which the embodiment of FIG. 6 is 
utilized for the treatment of a filament by the use of a single cascade 
arc plasma torch. Subsequently, a plurality of cascade arc plasma torches 
as contemplated by the arrangement of FIG. 7 could be arranged downstream 
of the single torch. This arrangement would permit a second stage of 
treatment in which several torches are arrayed substantially circularly 
around a principle axis X and downstream of a single torch. 
It is also contemplated that a plurality of torches could be circularly 
arrayed as shown in FIG. 7 and 8 and the powder to be treated could be fed 
by a screw injector or other such arrangement downstream of the arrayed 
torches. Such an arrangement would also be utilized in conjunction with a 
single torch as in the embodiment of FIG. 6. 
The invention will now be described by specific examples, wherein the 
embodiments of the apparatus of this invention shown in the drawings are 
used. 
EXAMPLE 1 
A cascade arc reactor of the type shown in FIG. 1 of the drawings and 
containing a cascade arc system has a 2 mm diameter pathway and a monomer 
inlet system located at the downstream side of the anode. The vacuum 
chamber is evacuated to a system pressure of less than 1 mtorr, and argon 
is then introduced at the end cap 7 on the cathode side of the reactor at 
a flow rate of approximately 2000 sccm/min. An arc is generated by 
applying a voltage of 700 volt, and tetrafluoroethylene is introduced into 
the monomer inlet at a flow rate of approximately 12 sccm/min. A cold 
drawn steel sheet, 3 cm.times.3 cm, is placed at the tip of the 
flame-shaped arc discharge. Plasma polymer of tetrafluoroethylene is 
deposited on the substrate at a rate of approximately 100 nm/min. The 
atomic ratio of fluorine to carbon is approximately 1.8. 
EXAMPLE 2 
The same reactor as described in Example 1 and the same conditions are used 
except for the monomer, which in the instant example is methane and 
approximately 6 sccm/min. of methane is introduced into the cathode cap. 
Plasma polymer of methane is deposited on the substrate at a rate of 
approximately 400 nm/min. The atomic ratio hydrogen to carbon is at most 
about 1.0. 
EXAMPLE 3 
A powder feeding system as described in FIGS. 4 and 5 is provided on the 
cascade arc reactor as described previously having a 4 mm diameter pathway 
at the downstream side of the monomer inlet. Argon is added at a flow rate 
of approximately 600 sccm/min., and methane is added at a rate of 
approximately 6 sccm/min. from a monomer inlet located immediately after 
the anode ring 8. A plasma polymer forming arc is created by applying 600 
volt, and steel powder for use in powder metallurgy is fed into the arc 
plasma via the powder inlet by a gear pump at a rate of approximately 2 
ccm/min. Plasma polymer coated powder is collected in a trap located 
within the vacuum chamber. 
Plasma polymer coated powder and uncoated powder behave differently in 
regard to flow, in that untreated powder has a flow of approximately 30 
secs. per 50 g, whereas plasma polymer coated powder has a flow of 
approximately 26 secs. per 50 g. Apparent density and compressibility are 
substantially unchanged. 
EXAMPLE 4 
An extension tube made of machinable ceramic with an inner diameter 
approximately the same as the pathway of the cascade arc reactor, i.e., 4 
mm, and a length of 50 mm is arranged on the cascade arc reactor as 
depicted in FIG. 6. This extension tube is provided with a slanted small 
hole having a diameter of approximately 1 mm, through which a wire or 
filament is passed into the centerpart of the tube. At the downstream end 
of the extension tube monomer inlet ring of the type described in FIGS. 2 
and 3 is arranged. Plasma polymerization coating of a Nylon 66 filament is 
carried out using the same conditions as described in Example 1, the 
filament being fed through the extension tube and through the flame 17. 
The filament is a monofilament having a diameter of 0.3 mm, and the plasma 
polymer coated filament shows highly hydrophobic surface characteristics 
similar to those of Teflon. 
EXAMPLE 5 
The same apparatus as described in Example 4 is used in this example, 
wherein a guide wire of spring steel having a diameter of 0.5 mm is plasma 
polymer coated. While maintaining the essential properties of the material 
of the guide wire its friction vis-a-vis for example plastics and living 
tissue is substantially reduced. This is of practical significance in 
using the guide wire for the insertion of for example catheters into the 
body. 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing application. The invention 
which is intended to be protected herein should not, however, be construed 
as limited to the particular forms disclosed, as these are to be regarded 
as illustrative rather than restrictive. Variations and changes may be 
made by those skilled in the art without departing from the spirit of the 
present invention. Accordingly, the foregoing detailed description should 
be considered exemplary in nature and not limited to the scope and spirit 
of the invention as set forth in the appended claims.