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BACKGROUND OF THE INVENTION
The present invention relates to vapor permeation curable coatings and more
particularly to the spray guns and allied equipment used therefor.
Vapor permeation curable coating traditionally are a class of coatings
formulated from aromatic hydroxyl-functional polymers and polyisocyanate
cross-linking agents wherein an applied film thereof is cured by exposure
to a vaporous tertiary amine catalyst. Originally, in order to contain and
handle the vaporous tertiary amine catalyst economically and safely,
curing chambers were developed. Curing chambers typically are
substantially empty boxes through which a conveyor bearing the coated
substrate passes and in which the vaporous tertiary amine, normally borne
by an inert gas carrier, contacts such coated substrate.
Such traditional vapor permeation curable coatings requirements have been
altered by the vaporous amine catalyst spray method disclosed by Blegen in
U.S. Pat. No. 4,517,222. Such vaporous catalyst spray method relies on the
concurrent generation of an atomizate of a coating composition and a
carrier gas bearing a catalytic amount of a vaporous tertiary amine
catalyst. Such generating atomizate and vaporous catalytic amine-bearing
carrier gas flow are admixed and directed onto a substrate to form a film
thereof. Curing is rapid even at room temperature and the use of a curing
chamber is not required.
The vaporous spray catalyst method necessarily employs a variety of spray
guns and allied equipment therefor. Such spray guns can be hand operated
or can be automated for robotic implementation thereof. Additionally, the
vaporous spray catalyst method quite readily has been adapted to both
conventional air atomizing and air-assisted spray guns as well as
electrostatic spray guns. Despite the universal application of the
vaporous spray catalyst method, certain problems have developed in
adapting such technology to existing commerical coatings lines. One
problem is that when the spray gun is not in constant use, the temperature
of the gun will equilibrate to the ambient temperature. When the "cold"
spray gun then is activated, the vaporous amine in the vaporous amine gas
flow can condense within the amine spray path within the gun and
especially at the spray cap within the gun. Such amine condensation then
can be carried with the atomized paint flow and can cause spotting on the
substrate being coated. In electrostatic spray guns, and especially when
the vaporous amine line to the electrostatic spray gun is not properly
insulated, a path of liquid amine droplets can form from the air cap of
the spray gun down through the line towards the vaporous amine generator.
If the electrical path to the electrostatic or corona-inducing needle is
not adequately insulated from the vaporous amine gas flow, then an
electrostatic charge can be conducted down the liquid amine path which
results in the safety mechanism within the electrostatic spray gun system
shutting off the gun, otherwise the electrostatic charge keeps draining
away. On an automated assembly line, it will be appreciated that such an
occurrence results in unacceptable down time of the assembly line.
While some conventional spray guns have adequate provision for sealing the
electrical line path from the vaporous amine gas path, many conventional
spray guns are not so-designed since conventional technology employs only
air in such lines so that condensation of an electrical carrier is not a
problem confronted by spray gun manufacturers. The present invention is
addressed to the foregoing concerns which have arisen in implementation of
the vaporous spray catalyst method as disclosed in the '222 patent cited
above.
BROAD STATEMENT OF THE INVENTION
The present invention broadly has applicability to any spray gun which is
adapted for use in the vaporous spray catalyst method. In particular, the
present invention broadly is directed to a spray gun for spraying atomized
liquid with the assistance of a vaporous amine flow via an amine flow path
established in said gun. The improvement of the present invention
comprises means actuable in the absence of said vaporous amine flow to
cause a heated non-amine gas, preferably air, to flow through and purge
the amine flow path within the gun for maintaining a desired flow path
temperature adequate for suppressing amine condensation in said flow path.
The spray gun can be a hand spray gun, an automated spray gun, an air
atomizing spray gun, an air-assisted airless spray gun, an electrostatic
spray gun, or any variation thereof.
One embodiment of the present invention is adapted for a spray gun, which
can be an electrostatic spray gun, of the type having a pneumatic line
connected to gun actuation means wherein gas flows in said pneumatic line
only when said spray gun is spraying. This embodiment of the present
invention comprises two-way pilot actuated valve means interposed in the
line supplying the vaporous amine flow to the gun wherein the pneumatic
line is the pilot therefor whereby said pilot valve means is open only
when said spray line has gas flow therethrough. Pressure regulator means
are established in the vaporous amine line prior to said pilot valve means
to establish a pressure, P.sub.a, of vaporous amine flow in said amine
line. A heated gas flow line having pressure regulator means for
establishing a pressure, P.sub.p, means for heating said gas flow, and
unidirectional gas flow valve means immediately prior to said heated gas
flow line connection to said vaporous amine line after said pilot valve
means are established and adjusted so that P.sub.a is set above P.sub.p.
Such arrangement is effective for permitting the heated gas flow purge to
flow through the amine gas flow line when the gun is not in operation for
maintaining the desired flow path temperature adequate for suppressing
amine condensation in the flow path in the gun.
In another embodiment of the present invention wherein the gun is actuated
by non-penumatic means, e.g. hand operated, electrically operated, or the
like, the present invention comprises pressure regulation means followed
by unidirectional flow valve means interposed in the amine line connected
to the spray gun. Adjustable reference pressure sensing control means are
connected with the amine line after said amine line pressure regulation
means to sense the pressure in the amine line. Pressure regulation means
are disposed in a control gas flow line which line is connected with said
sensing control means as the reference pressure and which control gas flow
is withdrawn from said sensing control means so long as the reference
pressure in said amine line is greater than the pressure in the control
gas flow into said sensing control means. Two-way pilot actuated valve
means is interposed in a heated gas flow line which is at a pressure
greater than the pressure in said amine line. The control gas flow is
withdrawn from the sensing control means and is the pilot therefor whereby
said pilot valve means is open only wnen said pilot control gas flow has
gas flow therethrough. Unidirectional gas flow means is interposed in the
heated gas flow line which runs from said pilot valve means to the amine
line at a point after said amine line unidirectional flow valve means. The
control gas pressure regulation means is adjusted so that the control gas
pressure is greater than the pressure in said amine line when the gun is
spraying and there is flow therein, but the control gas pressure is less
than the pressure in said amine line when the spray gun is idle and the
pressure in said amine line increases because there is no gas flow
therein. The gun is modified so that a minor purge gas flow passes
therethrough when said gun is idle, i.e. not spraying.
Another embodiment of the present invention is directed to a spray gun
which is known as an air-assisted airless electrostatic spray gun wherein
the atomized liquid is atomized with the assistance of a vaporous amine
atomizing flow in an amine flow path in the gun and with the assistance of
a fan gas flow via a gas flow path in the gun. This embodiment of the
present invention comprises means actuatable in the absence of said
vaporous amine flow to cause a heated non-amine purge gas to flow through
and purge one or both of said amine flow path or said gas flow path for
maintaining a desired flow path temperature adequate for suppressing amine
condensation in said amine flow path in said gun.
Advantages of the present invention include the ability to virtually
eliminate vaporous amine from condensing in the spray gun when the spray
gun is idle. Another advantage is that the ability to maintain the spray
gun warm for suppressing amine condensation is accomplished economically
and efficiently without the need or expensive automated control equipment.
Another advantage is that the invention can be readily adapted to
conventional spray coatings lines. Yet another advantage is the adaptation
of the invention to virtually any spray gun be it conventional air
atomized, electrostatic, air-assisted airless electrostatic, or any
combination thereof. These and other advantages will be readily apparent
to those skilled in the art based upon the disclosure containing herein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial cross-sectional view through the air cap and spray end
of a conventional air-assisted airless electrostatic spray gun;
FIG. 2 is a diagrammatic representation of the hot gas purge technique
applied to a pneumatically-actuated robotic spray gun;
FIG. 3 is a diagrammatic representation of the hot gas purge technique
applied to a hand-actuated spray gun;
FIG. 4 is a partial cross-sectional elevational view of the hand spray gun
of FIG. 3 which spray gun has been modified for implementation of the hot
gas purge technique;
FIG. 5 is a partial cross-sectional view through the barrel of another
electrostatic spray gun;
FIG. 6 is a sectional view of the air cap taken along line 6--6 of FIG. 5;
and
FIG. 7 is another cross-sectional view of the air cap of the gun in FIG. 5.
The drawings will be described in detail in connection with the description
of the invention which follows.
DETAILED DESCRIPTION OF THE INVENTION
The problems which can be encountered in adapting the vaporous spray
catalyst method to electrostatic spray gun use can be appreciated by
referring to FIG. 1 which is a partial cross-section elevational view of
the air cap and end assembly of an electrostatic spray gun such as
disclosed in U.S. Pats. Nos. 3,907,202 and 4,055,300. Body 10 of the spray
gun has central passage 12 through which the paint is hydrostatically
delivered through nozzle 14 of air cap assembly 16. Air cap assembly 16 is
screwed onto the end of body 10 to form annular chamber 18 therein.
Electrical line 20 runs through body 10 to annular electrical ring 22. Cap
16 bears springbiased electrical connector assembly 24 which contacts
electrical connector ring 22 regardless of the position at which cap 16
ultimately is positioned when screwed onto the end of body 10. Electrical
connector assembly 24 has protruding contact pin 26 which connects to
annular connector ring 28 which also is connected to needle 30. Needle 30
is a corona discharge needle which charges the hydrostatically delivered
paint particles as they exit orifice 14. Needle 30 is retained within horn
32, there being opposedly-disposed horn 34 also provided at the end of cap
16.
Gas passageway 36 is in flow communication with manifold 38 which is
connection with chamber 18 formed by air cap 16 and body 10. In
conventional utilization of the electrostatic spray gun, air is intended
to be delivered to the spray gun via line 36. In accordance with the
vaporous spray catalyst method, line 36 is fed with a vaporous tertiary
amine gas flow, normally containing a carrier gas. It will be observed
that the vaporous catalytic mine in chamber 18 can contact electrical
contact pin 26 directly. While such contact passes no substantial threat
to the operation of the spray gun when air is delivered into chamber 18,
problems can develop when amine condenses within chamber 18. Completing
the amine flow path within the spray gun, passageway 40 disposed in horn
32 and passageway 42 disposed within horn 34 are in flow communication
with chamber 18 and deliver the vaporous catalytic amine for contact with
the atomized liquid particles exiting orifice 14. The flow exiting
passageways 40 and 42 normally are called the fan air or shroud air in
conventional use of the air-assisted airless electrostatic spray gun. It
is such fan or shroud air delivery systems which now bear the vaporous
catalytic amine.
It will be appreciated that when the initial flow of vaporous catalytic
amine enters chamber 18 after a period of non-use of the spray gun, i.e.
an idle condition of the spray gun, amine can condense on the colder
surfaces which form chamber 18. Such liquid amine condensate can be
carried out of passageways 40 and 42 causing dripping problems from the
nozzle of the spray gun as well as forming surface imperfections on the
substrate being coated. Moreover, if the liquid amine is in sufficient
quantity to run back down the amine line via line 36, a direct electrical
path for shorting out or voltage reduction of the spray gun has been
established. The present invention addresses such amine condensation
phenomena by providing means actuable in the absence of vaporous amine
flow in line 36 and through the amine flow path established within the
spray gun wherein such means causes a heated non-amine gas (preferably
heated air) to flow through and purge the amine flow path for maintaining
the amine flow path at a desired temperature for suppressing amine
condensation in the flow path. When the operator commences actuation of
the idle gun, passageway 35, manifold 38, chamber 18, and passageways 40
and 42 will be at a sufficiently warm temperature so that the incoming
vaporous amine will not condense. Should any condensation occur during use
of the gun, however, when the gun becomes idle, the heated gas flow again
will purge the amine flow path for vaporizing any amine condensate
collected therein.
In terms of temperature, flow rate, and time, the purge flow is adequate to
vaporize any amine condensate in the amine flow path and suppress amine
from condensing when the idle gun is first put into use. The appropriate
temperature to focus on, then, is the amine flow path temperature which
should be adequate to vaporize any amine condensate thereon. Practically
speaking, the amine flow path temperature should be established between
about 50.degree. and 65.degree. C., depending on the particular amine
used. Since the flow rate and time of the purge are variables, the purge
flow temperature typically can range from about 30.degree. to 95.degree.
C. and preferably from about 50.degree. to 82.degree. C. Flow rates
typically can range from about 1-4 liters per second, though this range
can vary significantly as those skilled in the art will appreciate. Then,
also, the time of the purge, in addition to the purge flow rate and purge
flow temperature, is a variable which can be used in designing the system.
It will be appreciated that higher air flows mean that lower temperatures
can be used and that higher purge flow temperatures mean that a lower flow
rate can be used. For that matter, the purge may be intermittent, rather
than continuous in order to accomplish the vaporization of any amine
condensate and maintenance of desired flow path temperature.
In automated assembly lines wherein robotically-controlled spray guns are
disposed, a variety of means of actuation of the spray gun are possible.
One prevelant technique employs a pneumatic line for actuating the spray
gun. Pneumatic lines also can be used to a drive a generator disposed
within the spray gun for creating the voltage or stepping up low voltage
fed to the spray gun to a sufficiently high potential for use in some
types of spray guns (e.g. the gun of FIGS. 5-7) in electrostatic spraying.
One embodiment of the present invention for controlling a
pneumatically-actuated spray gun, whether electrostatic or otherwise, is
set forth at FIG. 2. Spray gun 50 is fed by paint line 52 and can be an
electrostatic spray gun, such as described in connection with FIG. 1, or
can be a conventional air-atomized spray gun. Regardless of the particular
design of the spray gun, pneumatic line 54 is connected to spray gun 50
for actuating the spray gun by permitting air or other gas to flow
therethrough. In accordance with the vaporous spray catalyst technique,
amine generator 56 (e.g. as disclosed in U.S. Pat. No. 4,540,531) provides
a flow of gas-borne vaporous tertiary amine catalyst via line 58 through
pressure regulator 60 and via line 62 into two-way pilot actuated valve
means 64. Pneumatic line 54 acts as the pilot for pilot actuated valve 64
which operates in conventional fashion to permit vaporous amine flow in
line 62 to pass therethrough and be withdrawn via line 66 so long as gas
is flowing in the trigger line, i.e. pneumatic line 54. A suitable pilot
actuated valve for use in the present invention includes PR-2 valve
supplied by Kremlin, Incorporated (Addison, Illinois). Further on pilot
actuated valves can be found, for example, In Chapter 22 of Chemical
Engineers' Handbook, Perry and Chilton Editors, Fifth Edition, McGraw-Hill
Book Company (New York, New York, 1973). It will be appreciated that the
hook-up of pilot actuated valve 64 means that the amine gas flow will be
contemporaneous with gas flow in pneumatic line 54, i.e. amine will flow
when the gun is being actuated and will cease when the gun is idle.
The purge gas scheme commences with line 68 which provides air or other
purge gas to flow via pressure regulator 70 and thence via line 72 through
heating means 74 which can be any suitable direct or indirect heating
means including electrical heating, steam heating, or the like. The heated
pilot gas flow is withdrawn from heating means 74 via line 76 and passed
through unidirectional valve means 78 which suitably can be check or other
one-way flow valve. The heated purge gas then is passed via line 80 into
amine gas flow line 66. Pressure regulators 60 and 70 are adjusted so that
the pressure of the amine gas fow, P.sub.a, is set above the pressure of
the pilot gas flow, P.sub.p. This pressure differential is needed so that
when pilot actuated valve 64 is in its normal operating or open
configuration, the amine gas flow will preferentially flow through line 82
into gun 50 rather than the purge air via line 80 into line 82.
Unidirectional valve 78 prevents any amine gas flow from flowing
backthrough the purge gas flow system.
When gas is flowing via pneumatic line 54, pilot actuated valve 64 is in
its normal open position and vaporous tertiary amine gas flow enters gun
50 via line 82. When the flow of gas in pneumatic line 54 is ceased, pilot
actuated valve 64 closes which permits purge gas via line 80 to flow into
line 82 and thence into gun 50 for heating and purging the amine flow path
within the gun, i.e., evaporate any condensed amine therein. The purge gas
does not flow back through the vaporous amine lines because pilot actuated
valve 64 is in a closed position. Thus, it will be appreciated that an
efficient and economic method for purging the spray gun has been provided
and is operated based on pneumatic line 54 which controls spray gun 50. It
should be appreciated that spray gun 50 is unconventional in the sense
that external access to the air cap assembly is provided separate from the
pneumatic trigger. Such dual external access capability enables a unique,
simplified control circuit to be designed, as described above.
Conventional spray guns operating via pneumatic activation essentially
have a trigger mechanism similar in operation to pilot actuated valve 64
which mechanism is located within the gun. Such conventional design means
that when the pneumatic line activating the gun is shut off, then also the
flow of gas through the pneumatic trigger to the air cap assembly is shut
off. Thus, a different control circuit design is called for.
When the spray gun is not operated via a pneumatic line as described for
gun 50, then the system depicted at FIG. 2 cannot be used. Other means of
activating a spray gun include hand activation, electrical signal
activation, conventional pneumatic activation or the like. In those cases,
the purge gas flow system can be implemented as depicted at FIG. 3. Hand
operated spray gun 90 is fed paint via line 92 and is actuated via and
trigger 94. Vaporous amine gas is generated in vapor generator 96 which
conveys the catalytic amine vapors via line 98 through pressure regulation
means 100. The vaporous amine flow at pressure P.sub.a is passed via line
102 through unidirectional flow means 104 and thence into line 106 for
admission to spray gun 90. When trigger 94 is depressed, vaporous tertiary
amine catalyst borne by a carrier gas passes through gun 94 and is
delivered at pressure P.sub.a. When trigger 94 is released, the pressure
in lines 102 and 106 increases above its delivery pressure, P.sub.A, since
the flow is prohibited from passing out of gun 90. This pressure is
referred to as the surge pressure, P.sub. s.
The purge gas is heated and is at a pressure P.sub.p by means not shown and
then is passed via line 108 into line 110 and two-way pilot actuated valve
112. The purge gas flow is withdrawn from pilot actuated valve 112 via
line 114 and passed through unidirectional flow means 116 and thence into
line 106 via line 118.
A control gas line is estabilshed by the same purge gas flow from line 108
and is passed via line 120 through pressure regulation means 122 and
thence via line 124 into adjustable reference pressure control means 126.
It will be appreciated that for economy and efficiency, the control line
is established to be the same as the purge gas flow, though a separate
control flow can be provided additionally. Adjustable reference pressure
sensing control means 126 utilizes the pressure in amine gas flow line 102
as the sensed pressure input and utilizes the gas flow in control line 124
as the adjustable reference pressure. Biasing means within adjustable
reference pressure sensing control means 126 will permit or prevent the
control gas flow therethrough depending upon the pressure differential
established between the sensed pressure in line 102 and the adjustable
reference pressure in line 124. The control gas pressure, P.sub.c, in line
124 is adjusted via pressure regulation means 122 so that its pressure is
greater than the pressure of the vaporous amine gas flow, P.sub.a, in line
102 when gun 90 is being utilized and amine flow is being passed via line
106 into gun 90, but is set lower than the surge pressure, P.sub.s, which
develops in line 102 when trigger 94 is released and the gun is idle, i.e.
P.sub.s >P.sub.c >P.sub.a. Now, the pressure in the purge gas flow in line
110 is adjusted to be greater than the surge pressure in line 106, i.e.
P.sub.p >P.sub.s.
The foregoing pressure settings mean that when the sensed vaporous amine
pressure is less than the control reference pressure, there is an output
from adjustable reference pressure sensing control means 126 via line 128.
When the sensed amine pressure rises above the adjustable reference
control pressure (i.e., surge pressure), then the output in line 128
ceases. Now, the output in line 128 from adjustable reference pressure
sensing control means 126 is the control signal for two-way pilot operated
valve means 112. Valve 112 operates such that when a control signal or
pressure is sensed in line 128, there is no output flow in line 114.
Conversely, when the control signal in line 128 ceases, then the purge
flow passes via line 110 through valve 112 and into line 114. Such purge
flow will occur when there is no flow or pressure in line 128 which
corresponds to a surge pressure in line 102 being greater than the control
pressure in line 124. Since the purge pressure in line 110, 114, and 118
is set above the surge pressure in lines 102 and 106, when the gun is
idle, the purge flow via line 118 will flow via line 106 into gun 90. A
suitable adjustable reference pressure sensing control means for the
present invention can be a model 1043 pressure repeater supplied by
Clippard Instrument Laboratory, Inc. (Cincinnati, Ohio). While a suitable
normally open two-way pilot actuated valve can be a model 2012 valve
supplied by Clippard Instrument Laboratory, Inc.
Since the gun is idle when the purge flow via line 106 commences, gun 90
must be modified so that the purge flow can flow through the amine path in
gun 90 for maintaining the temperature of the flow path at a desired level
and for evaporating any amine condensate therein. One simple modification
is set forth at FIG. 4 wherein trigger 94 is seen to be connected via rod
140 to stopper 142 which is mounted against biasing means 144. The sealing
and remaining construction of the trigger mechanism housed within handle
146 of gun 90 is conventional and is not described in detail herein. Line
106 is in communication with cavity 148 which communicates with cavity 150
via the opening in which stopper 142 is disposed. Cavity 150 is in direct
communication with the air cap assembly of gun 90, such as described in
connection with FIG. 1, for example. It will be seen that grooves 152 and
154 have been placed in stopper 142 so that even when trigger 94 is not
depressed and biasing means 144 forces stopper 142 into the orifice
between cavities 148 and 150, gas can still flow therebetween through such
grooves. The two grooves depicted at FIG. 4 are merely representative of a
number which may be greater or lesser than that shown. Thus, when gun 90
is idle and the heated purge flow via line 108 passes into cavity 148 via
line 106, it will pass via grooves 152 and 154 into cavity 150 and thence
along the amine flow path within gun 90 for maintaining a desired
temperature therein and for purging any condensed amine therein.
A final spray gun design which needs to be specifically addressed for
implementing the novel heated gas purge technique of the present invention
is known as an air-assisted airless electrostatic spray gun which is
manufactured under one or more of the following U.S. Pat. Nos. 4,241,880,
4,335,851, 4,290,091, 4,219,865, 4,386,739, 3,8483,502, 4,462,061, and
4,497,447. An example of such spray gun is a Model PRO AA5000 as
manufactured by Graco, Inc. (Minneapolis, Minn.). A partial crosssection
elevational view of such an air-assisted airless electrostatic spray gun
is set forth at FIGS. 5-7. In simple terms, such an electrostatic spray
gun operates by atomizing the paint by a combination of two techniques.
One technique is hydraulically pumping the paint through an orifice while
the second technique is atomizing air. A second flow of fan air is used to
shape the spray pattern, thus the name air-assisted airless electrostatic
spraying. Referring specifically to FIG. 5, barrel 160 of the spray gun
(not fully shown) is shown at its two respective ends with the middle not
illustrated. The delivery end of barrel 160 is composed of retaining nut
162 which retains air cap 164 (see FIGS. 6 and 7), and spray tip assembly
166. Paint is delivered via a port (not shown) into chamber 170 which has
spring 172 disposed therein wherein an external pump in the paint line
hydraulically pressurizes the paint through orifice 174 of spray tip
assembly 166. Electrostatic needle 176 provides a charge to the spray
pattern of atomized paint exiting orifice 174 and is in electrical
connection through barrel 160 by means not illustrated. Fan air supplied
via channel 178 and flows into compartment 180 and out passageways 182 and
183 adjacent orifice 174.
Atomizing air enters the gun via port 184 and flows via channel 186 to
passages 188 and 190 located in horns 192 and 194, respectively, of air
cap 164, as illustrated specifically in FIG. 7. From FIG. 5 it will be
seen that a pair of passageways actually is provided in each horn 192 and
194, only one passageway being illustrated specifically in FIG. 7. This
atomizing flow of gas provides a second and distinct means for atomizing
the paint in addition to the hydraulic mechanism described above.
In conventional operation, the fan air and atomizing air intermix at the
intersection of channels 178 and 186 since air is fed via both the fan air
and the atomizing air delivery systems. In the spray catalyst adaptation
for the electrostatic spray gun illustrated at FIG. 5, vaporous tertiary
amine catalyst borne by a carrier gas can be passed through both the fan
air and the atomizing air delivery systems. Should any amine condense in
chamber 180, contact with the electrical delivery line to needle 176 may
occur with consequent possible shorting out of the electrostatic gun.
Accordingly, the present invention preferably utilizes plug 196 which is
screwed into channel 178 and separates the fan air from the atomizing air
throughout their respective flow paths in barrel 160. Now, the vaporous
amine catalyst can be provided separately through the fan air flow path or
through the atomizing air path as is necessary, desirable, or convenient
since the spray catalyst method operates effectively with the vaporous
amine catalyst supplied by either flow path. For minimizing possible
contact of amine condensate with the electrical flow path to needle 176,
it is preferred that the amine be supplied strictly through the atomizing
air flow path which then becomes the amine flow path in barrel 160 of the
electrostatic gun. The fan air flow path then can have heated air supplied
thereto continuously whether or not the gun is spraying or is idle for
insuring that the gun is maintained at a desirable high temperature
adequate for preventing amine condensate from remaining in the gun when
the gun is idle, and for suppressing the propensity for amine to condense
in a cold gun which has been idle for a period of time. Of course, use of
the purge air control schemes of FIGS. 2 or 3 could be applied to the
amine flow path in barrel 160 additionally or in lieu of continuously
supplying heated purge air through the fan air flow path.
In the preferred operation of this embodiment of the invention, however,
the vaporous amine and the fan air flows both are controlled by pressure
regulation means (not shown in the drawings) which ensure that the
pressure in the fan air flow is always maintained at a higher pressure
than the atomizing gas, i.e. vaporous amine flow. This means that the
amine in the amine flow path will not be able to penetrate into chamber
180 for contact with any electrical lines therein. Desirably also in order
to minimize the interference and diluting effect of the fan air and
maximize the catalyzing effect of the vaporous amine in the process, it
may be desirable also to reduce the fan air flow and increase the
atomizing amine gas flow. This can be accomplished by reducing the size of
orifice 182 and 183 (see FIG. 5) in communication with the fan air flow
path and increasing the orifice size of passages 188 and 190.
While any liquid can be delivered through the spray guns described herein,
typical utilization of the spray gun is for application of coating
compositions or paints onto substrates. In this regard, it will be
appreciated that any vaporous phase catalyst which can condense in the
catalyst flow path in the spray gun when the gun is idle can benefit by
application of the purge technique of the present invention. Preferably,
though, the coating composition will be one that can be catalyzed or
activated in the presence of an amine. Suitable such coating compositions
include the polymercapto/polyisocyanate coatings disclosed in
commonly-assigned application Ser. No. 06/905,700, filed Sept. 9, 1986;
and the polyol/polyisocyanate coating composition bearing a tin or bismuth
catalyst complex as disclosed in commonly-assigned application Ser. No.
06/931,610, filed 11/17/86 (a continuation-in-part of application Ser. No.
06/844,810, filed Mar. 27, 1986). Additional coating compositions suitable
for use in connection with the present invention include U.S. Pat. No.
3,409,579 which discloses a binder composition of a phenol-aldehyde resin
(including resole, novolac, and resitole), which preferably is a benzylic
ether or a polyether phenol resin. U.S. Pat. No. 3,676,392 discloses a
resin composition in an organic solvent composed of a polyether phenol or
a methylol-terminated phenolic (resole) resin. U.S. Pat. No. 3,429,848
discloses a composition like that in U.S. Pat. No. 3,409,579 with the
addition of a silane thereto.
U.S. Pat. No. 3,789,044 discloses a polyepoxide resin capped with
hydroxybenzoic acid. U.S. Pat. No. 3,822,226 discloses a curable
composition of a phenol reacted with an unsaturated material selected from
unsaturated fatty acids, oils, fatty acids esters, butadiene homopolymers,
butadiene copolymers, alcohols, and acids. U.S. Pat. No. 3,836,491
discloses a similar hydroxy-functional polymer (e.g. polyester, acrylic,
polyether, etc.) capped with hydroxybenzoic acid. British Pat. 1,369,351
discloses a hydroxy or epoxy compound which has been capped with
diphenolic acid. British Pat. No. 1,351,881 modifies a polyhydroxy,
polyepoxy, or polycarboxyl resin with the reaction product of a phenol and
an aldehyde.
U.S. Pat. No. 2,967,117 discloses a polyhydroxy polyester while U.S. Pat.
No. 4,267,239 reacts an alkyd resin with para-hydroxybenzoic acid. U.S.
Pat. No. 4,298,658 proposes an alkyd resin modified with
2,6-dimethylol-p-cresol.
U.S. Pats. Nos. 4,343,839, 4,365,039 and 4,374,167 disclose polyester resin
coatings especially adapted for flexible substrates. U.S. Pat. No.
4,374,181 discloses resins especially adapted for application to reaction
injection molded (RIM) urethane parts. U.S. Pat. No. 4,331,782 discloses a
hydroxybenzoic acid-epoxy adduct. U.S. Pat. No. 4,343,924 proposes a
stabilized phenol-functional condensation product of a phenol-aldehyde
reaction product. U.S. Pat. No. 4,366,193 proposes the use of
1,2-dihydroxybenzene or derivatives thereof in vapor permeation curable
coatings. U.S. Pat. No. 4,368,222 discloses the uniqueness of utilizing
vapor permeation curable coatings on surface-porous substrates of
fibrous-reinforced molding compounds (e.g. SMC). Finally, U.S. Pat. No.
4,396,647 discloses the use of 2,3'4-trihydroxy diphenyl.
It will be appreciated that the precepts of the present invention can be
implemented by a variety of techniques in addition to those explicitly
described herein. In this application, all citations are expressly
incorporated herein by reference.
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