Polymer-based material for carbon deposition during brazing operations

A polymeric-based material for use in brazing operations to prevent agglomeration and run-off of brazing material on exterior exposed surfaces of the workpiece containing an extrudable melt-processible thermoplastic material selected from the group consisting of polyethylene, polypropylene, ethylene vinyl alcohol, nylon, and mixtures thereof; and up to about 20% by weight, based on the thermoplastic material, of an inert inorganic particulate material such as carbon or carbon black capable of at deposition on and adherence to an underlying metallic surface. In the process of the present invention, the polymeric-based material is imparted by any suitable means to the exterior or exposed surface of the workpiece to be brazed prior to exposure of the workpiece to a suitable fluxing atmosphere and temperature. In tube forming processes, the polymeric-based material is applied to the outer circumferential area of the formed unsealed tubing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to brazing operations, particularly those relating 
to manufacture of double-wall tubes. More particularly this invention 
relates to a material suitable for use as an aid in brazing operations to 
prevent unwanted agglomeration and pooling of brazing material on exposed 
surfaces of the brazed workpiece. Even more particularly, this invention 
related to a material which can be employed as a substitute for 
solvent-based black paint containing elemental carbon or carbon black. 
2. Discussion of Relevant Art 
Various manufacturing operations employ brazing as the desired method of 
metal surface joining for reasons of economy, durability and the like. 
Brazing methods are not without drawbacks however. For example, safety 
standards in the automotive industry dictate that critical elements such 
as automotive brake lines be resistant to leakage, puncture and corrosion. 
In order to achieve these objectives, double-wall tubes for brake lines 
have been adopted as the industry standard. Such double-wall tubes consist 
of at least two thicknesses of a breakage-resistant metal material having 
sufficient properties to withstand fatigue due to prolonged vibration. The 
double walls of the tube employed in automotive vehicles are joined in 
such a manner to eliminate the possibility of leakage at any joined seam. 
Brazing has been found to be the most effective metal joining operation. 
In typical tubing manufacture operations, a suitable brazing material such 
as copper or various copper alloys is plated over the surface of a 
material such as carbon steel in either sheet or strip form prior to 
tubing formation to permit the ultimate formation of a leak-resistant 
joint. 
In conventional operations, the plated brazing material tends to 
agglomerate or pool on the exterior surface of the double-wall tube when 
the tube is subjected to the high temperatures necessary to achieve 
metallurgical brazing. This agglomeration phenomenon can interfere with 
the close tolerance conditions required for the finished product. 
Additionally the agglomeration and pooling of the excess material is not 
aesthetically pleasing. 
In order to prevent agglomeration, it has been necessary to apply one of a 
variety of solvent-based black paint products over the formed tube before 
the brazing operation. The solvent-based black paint typically contains 
elemental carbon or a suitably inert material such as carbon black which 
remains as a residue on the surface of the workpiece after the organic 
components of the solvent-based paint have been volatilized during the 
brazing process. 
Use of carbon-containing black paint in this manner is costly and 
hazardous. Exposure to the elevated temperatures at which brazing occurs 
causes the solvent and volatile portions of black carbon-containing paint 
to volatilize generating a host of volatile organic compounds (VOC) which 
must be handled in an appropriate manner. Various volatilized materials 
deposit and accumulate in and on various components of the brazing furnace 
which can impair the operational efficiency of the brazing furnace. For 
instance muffle furnace tubes must be cleaned of organic deposits from 
black paint components routinely to maintain optimum performance. The 
rigorous cleaning schedule reduces the life of furnace muffle tubes 
necessitating their frequent replacement. 
Conventional procedures also result in high paint and thinner usage. 
Additional difficulties exist in hazardous waste handling and disposal and 
as well as the associated issues pertaining to worker exposure and safety. 
Thus, it would be desirable to provide a material which could be used to 
coat workpieces prior to brazing operations in place of solvent-based 
black paint containing carbon which is capable of preventing or reducing 
agglomeration and run off of the brazing materials such as copper or 
copper-containing alloys. It is also desirable to provide a material which 
could be used as a carrier to impart carbon over a brazing material which 
would break down at brazing temperatures and result in various 
volatilization by-product materials, at least some of which may be 
consumed in the atmosphere present in the brazing furnace environment. It 
is desirable to provide a process for producing brazed workpieces which 
employs the polymeric material of the present invention to reduce or 
eliminate the occurrence of agglomeration or run-off of brazing material 
present on exposed surfaces. It is also desirable to provide a brazing 
process in which at least some of the undesirable volatile organic 
components generated during conventional brazing operations using black 
paint are reduced or eliminated. 
SUMMARY OF THE INVENTION 
The present invention is a polymeric-based material for use in brazing 
operations and a brazing method employing the same. The polymeric-based 
material consists essentially of: 
an extrudable melt-processible thermoplastic material selected from the 
group consisting of polyethylene, polypropylene, ethylene vinyl alcohol, 
nylon, and mixtures thereof; and 
up to about 20% by weight, based on the thermoplastic material, of an inert 
inorganic particulate material capable of deposition on and adherence to 
an underlying metallic surface. The inert inorganic material is, 
preferably, selected from the group consisting of carbon, carbon black, 
and mixtures thereof. 
In the process of the present invention, the polymeric-based material is 
imparted by any suitable means to the exterior or exposed surface of the 
workpiece to be brazed prior to exposure of the workpiece to the fluxing 
atmosphere and temperature. In tube forming processes, the polymeric-based 
material is applied to the outer circumferential area of the formed 
unsealed tubing in any suitable manner, i.e. by cross-head extrusion 
operations. The applied polymeric material is allowed to cool and solidify 
after which, the coated workpiece is introduced into a suitable gaseous 
brazing atmosphere and exposed to an elevated temperature sufficient to 
support metallurgical brazing. 
In the process of the present invention, the polymeric-based material in 
the coating is volatilized leaving residual carbon on the workpiece for 
the remainder of the brazing process. The elevated temperature necessary 
to support brazing triggers the chemical pyrolysis of the polymeric-based 
material in the coating yielding various volatile short-chain hydrocarbon 
molecules, as well as gaseous hydrogen and the like. The volatilized 
pyrolysis products may be further consumed in the brazing atmosphere or 
can be dealt with in other suitable manners. 
The present invention also includes an unbrazed workpiece comprising a 
first metallic substrate material; a second metallic brazing material 
overlaying and adhering to the first metallic substrate material, the 
second metallic brazing material capable of forming a fusion bond with the 
first metallic substrate material; and a polymeric-based material 
consisting essentially of an extrudable melt-processible thermoplastic 
material selected from the group consisting of polyethylene, 
polypropylene, ethylene vinyl alcohol, nylon, and mixtures thereof; and up 
to about 20% by weight, based on the thermoplastic material, of an inert 
inorganic particulate material capable of deposition on and adherence to 
the second metallic brazing material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is predicated on the unexpected discovery that 
polymeric material which optionally contains an inert particulate material 
can provide an effective substitute for conventional solvent-based black 
paint. In the present invention polymeric material is place in an 
overlying relationship to a metallic surface with a suitable brazing 
material thereon and, as a result of pyrolysis at brazing temperatures, 
can impart a particulate film or coating which can effectively prevent or 
minimize the agglomeration or run-off of brazing material on the exposed 
surface of the workpiece being brazed. 
The present invention encompasses polymeric-based coating which can be 
imparted on the surface of the workpiece prior to brazing to prevent 
agglomeration and run off of brazing material during the brazing process. 
The polymeric-based coating consists essentially of: 
an extrudable melt-processible thermoplastic material selected from the 
group consisting of polyolefin resins such as polyethylene and 
polypropylene, polyesters, ethylene vinyl alcohols, polyamides, acetal 
resins, and mixtures thereof; and 
up to 20% by weight, based on the weight of the extrudable thermoplastic 
material, of an inert particulate material capable of deposition on and 
adherence to the underlying metal surface. The particulate material is, 
preferably, selected from the group consisting of carbon, carbon black, 
and mixtures thereof. 
The thermoplastic material employed in the polymeric-based carrier system 
of the present invention is, preferably an extrudable melt processible 
material having a melt processing temperature in typical extrusion 
processing ranges. Suitable polymeric materials are capable of adhering to 
an underlying metal surface either by mechanical means, chemical means or 
a combination of the two. The metal-to-polymer adhesion is sufficient to 
maintain contact between the polymeric material and the underlying 
metallic surface for an interval sufficient to transfer the coated tube 
from suitable extrusion devices to a suitable brazing furnace. 
The polymeric material of choice has a molecular structure made up of 
carbon atoms, hydrogen atoms, and, optionally, oxygen atoms and nitrogen 
atoms. The polymeric material is one which can be successfully applied to 
the tubing surface in a manner which will permit sufficient adhesion until 
and during brazing operations. Various application methods including, but 
not limited to, dipping, spraying, and extrusion are contemplated to be 
within the purview of the present invention. In the embodiment present 
herein, cross-head extrusion application methods are preferred. 
Suitable polymeric materials which can be employed in the polymeric-based 
system of the present invention include extrusion grade polyolefin 
thermoplastic resins capable of maintaining structural adhesion 
characteristics and integrity when extruded as a thin film of less than 
250 microns (0.010 inches). Suitable material is widely available from a 
variety of commercial sources. Polyolefin thermoplastic resins suitable 
for use in the coating material of the present invention generally have a 
density between about 0.090 g/cm.sup.3 and about 1.18 g/cm.sup.3 ; an 
elongation at break between about 480% and 1000%. The polyolefin resin of 
choice can be employed alone or in combination with other polymeric 
materials outlined herein. 
The polymeric material can also be a melt-processible thermoplastic 
polyethylene material. Materials commonly referred to as either high 
density polyethylene or low density polyethylene can be successfully 
employed in the present invention. Suitable polyethylene materials are 
commercially available from a wide variety of sources. Suitable materials 
generally classified as either low density polyethylene or high density 
polyethylene can have the exemplary characteristics set forth in Table I. 
TABLE I 
______________________________________ 
Typical Characteristics of Suitable Low Density and 
High Density Polyethylene Materials 
______________________________________ 
Low density 
Density (g/cm) 0.910 to 0.925 
Crystal Melt Temperature (.degree.C.) 
95 to 130 
Tensile Strength M Pa 4.1 to 15.9 
Tensile Modulus M Pa 96.5 to 260 
Elongation at break % 90 to 800 
Hardness Shore D 41 to 50 
High density 
Density (g/cm) 0.941 to 0.965 
Crystal Melt Temperature (.degree.C.) 
120 to 140 
Tensile Strength M Pa 21.4 to 37.9 
Tensile Modulus M Pa 414 to 1250 
Elongation at break % 20 to 1300 
Hardness Shore D 50 to 70 
______________________________________ 
Polypropylene materials suitable for use in the carrier system of the 
present invention are thermoplastic melt processible materials which are 
conventionally suitable for use in conjunction with extrusion processes. 
Suitable materials are commercially available from a wide variety of 
sources. Exemplary characteristics include, but are not limited to, a 
processing temperature between about 205.degree. C. and about 221.degree. 
C.; density between about 0.895 and about 1.10 g/cm.sup.3 ; an elongation 
at break between about 200% and about 500%. 
Suitable acetal resin materials which can be employed in the composition of 
the present invention can include both the acetal homopolymeric compound 
as well as the copolymeric compound. Suitable homopolymeric materials are 
represented by materials marketed under the tradename DELRIN by DuPont 
which is characterized by a specific gravity of 1.42; a rockwell hardness 
of 94; and an elongation at break of 23 to 75%. Suitable copolymeric 
materials are represented by materials marketed under the tradename CELCON 
by Celanese which is characterized by a specific gravity of 1.41; a 
rockwell hardness of 80; and an elongation at break of 40 to 75%. Acetal 
resin materials are also commercially available from various other 
sources. 
Polyester materials which can be employed in composition of the present 
invention are melt-processible thermoplastic materials characterized by 
ester linkages distributed among the main molecular chain. Preferably, the 
thermoplastic material employed is characterized as a partly aromatic 
polyester obtained from the polymerization reaction of aliphatic glycols 
and aromatic dicarboxylic acids or esters with partly aromatic polyesters 
selected from the group consisting of polybutylene terepthalate, 
polyethylene terepthalate and mixtures thereof being preferred. In the 
composition and process of the present invention, polyethylene 
terepthalate is the preferred polyester. Suitable polyester materials are 
commercially available from a wide variety of sources. 
Ethylene vinyl alcohol materials suitable for use in the process an 
composition of the present invention are melt-processible thermoplastic 
copolymers of ethylene and vinyl alcohol having an ethylene content which 
is, preferably, between about 27% and about 32%. Examples of suitable 
commercially available material includes material produced by EVA/LA as 
well as from a wide variety of other commercial sources. 
Suitable melt-processible polyamide thermoplastics are, preferably selected 
from the group consisting of Nylon 11, Nylon 12, Nylon 6 and mixtures 
thereof. Nylon 6 materials are polyamide thermoplastic materials derived 
from the condensation polymerization of caprolactam. Nylon 12 and Nylon 11 
materials are respectively derived from the condensation reaction of 
either laurolactam or 11-aminoundecanoic acid. 
In the process of the present invention, Nylon 6 is the preferred polyamide 
material. However it is within the purview of this invention to employ a 
polyamide material which is a multi-component system compromising a Nylon 
6 copolymer blended with other nylons and/or olefinic materials. The 
preferred polyamide material, Nylon 6, generally has a melt-temperature 
between about 225.degree. C. and about 290.degree. C. with a temperature 
between about 220.degree. C. and about 240.degree. C. being preferred. 
Suitable Nylon 6 polyamide material is commercially available from various 
sources. 
In the present invention the polymeric material can be either virgin 
polymer, regrind or a mixture of virgin and regrind materials having 
suitable characteristics of melt-processibility, extrudability and 
strength to be applied to the unbrazed metallic material with its 
overlayment of suitable brazing material prior to the brazing operation. 
The polymeric material is optionally compounded with a suitable inert 
particulate material prior to application onto the surface to be brazed. 
The optional particulate material is one which is essentially inert and 
one which is capable of essentially uniform deposition on the exposed 
surface of the workpiece to be brazed. The inert material employed in the 
present invention preferably is selected from the group consisting of 
carbon, carbon black, and mixtures thereof. In the composition of the 
present invention, the suitable inert material can be incorporated into 
the polymeric substrate material by any conventional compounding method. 
When employed, the particulate material is employed at amounts sufficient 
to prevent agglomeration and run-off of the brazing material from the 
exposed surfaces of the workpiece. It is anticipated that particulate 
material in amounts up to and including about 20% by weight of the total 
polymeric material can be employed. Preferably an amount of particulate 
material between about 5% and about 20% is incorporated when the 
particulate material is employed. However amounts less than 5% can be 
effectively employed. 
When it is desirable to provide the surface of the workpiece with enhanced 
heat absorptive qualities, preferably, the composition of the present 
invention is provided with black pigments to enhance the heat adsorptive 
characteristics by a "black body" effect. This can be provided by the 
incorporation of a suitable pigmentation agent, or, more preferably, the 
incorporation of a black particulate material such as carbon black. Carbon 
black material employed is, preferably, present as a very fine powder. 
It is also within the purview of this invention, to provide a coating 
material for application on exposed surfaces of a suitable workpiece to be 
brazed which does not incorporate inert particulate material. It has been 
found that the application of a thin film of polymeric material overlying 
the exposed brazing alloy deposited on the metallic substrate of the 
workpiece to be brazed prior to brazing operations will prevent the 
agglomeration and run-off of the deposited brazing material as a result of 
the brazing operation. Without being bound to any theory, it is believed 
that the brazing operation results in pyrolysis of the polymeric coating 
leaving a thin residual carbon layer which alters the surface tension of 
the brazing material in its liquidus state sufficient to prevent the 
agglomeration and run-off phenomena. 
In the process of the present invention, the polymeric material may be 
applied to the workpiece to be brazed by any suitable method. In the 
manufacture of materials such as double-wall tube, the polymeric coating 
is applied by any suitable method to formed, unbrazed tubing. Application 
may be by any suitable means which will provide an essentially uniform 
homogeneous coating. Examples of such methods include spraying, dipping, 
extrusion or the like. In processes such as those involved in the 
manufacture of double-wall tube, the polymeric coating is preferably 
applied by conventional cross-head extrusion operations. 
The thickness of the polymeric coating layer overlaying the unbrazed 
workpiece is dependant on various factors such as the amount of carbon 
deposition required, concentration of the optional particulate material 
optionally present in the polymeric coating and the ease and efficiency 
with which the polymeric coating undergoes pyrolysis. The coating 
thickness is necessarily variable given the concentration of particulate 
material present or derivable from the polymeric coating. Further factor 
which influence coating thickness include the film forming characteristics 
of the polymeric material itself and the ability of the imparted coating 
to sustain and withstand normal handling during processing. Generally, 
thicknesses of the polymeric coating can be between about 25 and about 250 
microns (about 0.001 inches and about 0.010 inches); with coating 
thicknesses between about 25 and about 150 microns (about 0.001 and about 
0.006 inches) being preferred. In double-wall tubing manufacturing 
processes it is preferred that the coating have an average thickness 
between about 50 and about 75 microns (about 0.002 and about 0.003 
inches). 
The present invention also encompasses tubing stock having an outer 
polymeric coating consisting essentially of the polymeric material of the 
present invention. Such tubing 10, preferably, has a metal layer 12 
disposed radially innermost. The metal employed can be any material which 
is formable and amenable to brazing processes. Examples of such material 
include, but are not limited to, low carbon steel, conventional carbon 
steel, various grades of stainless steel and the like as would be known to 
the skilled artisan. 
In the preferred embodiment as shown in FIG. 1, the metal layer is actually 
composed of two metal sublayers 12a and 12b which are positioned in 
overlapping radial relationship to one another. Overlaying the exteriorly 
oriented surface of layers 12a and 12b is a layer 14 composed of a 
suitable brazing material. The brazing material is, preferably, imparted 
onto the surface of the metal layers 12a and 12b prior to final tubing 
formation. 
The brazing material layer 14 is generally composed of a metal alloy 
capable of being uniformly deposited on the surface of metal sublayers 12a 
and 12b. Deposition may be by any suitable mechanical, chemical and/or 
electrochemical process which will permit permanent or, at the minimum, 
semi-permanent mechanical adhesion of the selected metal alloy to the 
underlying metal surface. The brazing material employed may be any 
suitable metal or alloy which can form an appropriate fusion bond with the 
underlying metal. Preferred brazing materials employed in the brazing 
material layer 14 include, but are not limited to, brazing materials such 
as alloys of copper, alloys of silver as well as non-alloyed metals such 
as copper, silver. Other suitable brazing materials capable of forming a 
fusion bond with the metallic substrate would be known to those skilled in 
the art. 
As shown in FIG. 1, the brazing alloy material layer 14 is in overlying 
contacting relationship with the outwardly oriented surface of the 
respective metal layers 12a, 12b. The brazing material layer 14 can be 
attached to the metal layers 12a, 12b by any suitable mechanism such as 
electroplating, chemical deposition or the like. 
Where the objective is the formation of sealed double-wall tubing, the 
brazing metal alloy is, preferably, deposited on the metal surface prior 
to the formation of the double wall tubing. The brazing metal layer 14 can 
be attached to the metal layers 12a, 12b by any suitable mechanism such as 
electroplating, chemical deposition or the like. 
Where the objective is the formation of sealed double-wall tubing, the 
brazing metal alloy is, preferably, deposited on the metal surface prior 
to the formation of the double wall tubing. The metal with the brazing 
alloy thus deposited can be rolled and formed by any conventional method. 
An outer polymeric layer 16 overlies brazing metal layer 14 to completely 
enclose this surface. The thickness of the polymeric overlayer 16 is 
sufficient to provide a durable coating which can be handled and 
transferred with minimal risk to the structural integrity of the outer 
polymeric layer. In practice, the outer polymeric layer 16 has a thickness 
between about 25 and about 250 microns (about 0.001 inches and about 0.01 
inches). Preferably, the thickness is between about 25 and about 150 
microns (about 0.001 and about 0.006 inches) with an average thickness 
between about 50 and about 75 microns (about 0.002 and about 0.003 inches) 
being most preferred. Such material can be transferred into a suitable 
brazing environment in which the various multiple layers of metal can be 
fused to form a sealed double-wall tube. 
In the process of the present invention, unbrazed material such as unsealed 
double-wall metal tube with a suitable brazing alloy imparted thereon can 
be prepared by any suitable method. In the preferred process, unsealed, 
uncoated double-wall tubing is prepared from metal in strip or sheet form. 
The metal, preferably, has a thickness between about 0.25 mm and 0.35 mm 
(about 0.010 and about 0.012 inch) and has a width suitable for producing 
a double-wall tube of an appropriate diameter. The length of the metal 
strip or sheet is determined by handling constraints and requirement 
needs. In the process as depicted in FIG. 2, the metal sheet or strip to 
be formed is advanced to suitable roll forming machines 20 which 
subsequently produce unbrazed, unsealed double-wall tubing. If necessary, 
the tubing can be immediately advanced to a volatilization station 16 
which includes means for removing volatile portions of lubricating 
material employed during the roll forming steps (not shown). 
The metal to be formed may be prepared with the selected brazing material 
prior to placement of such prepared feedstock in a suitable feed-storage 
device such as payoff reel such as reel 18. The metal may be formed into 
tubing by any suitable formation operations as would be known to those 
skilled in the art such as roller forming operations depicted in FIG. 2. 
After the tube is formed into suitable unsealed double-wall tubing, it is 
conveyed to the polymeric coating station 24 where a suitable polymeric 
coating is applied to the exterior of the unsealed double-wall tube. It is 
to be understood that the unsealed double-wall tube may be immediately 
conveyed to the polymeric coating station or can be stored and accumulated 
for later coating depending upon the processing conditions in the given 
paint or location. 
As indicated previously, the polymeric coating may be applied by any of a 
number of methods which would be known to those skilled in the art of 
coating technology. Thus the polymeric coating station discussed in herein 
is taken to encompass any suitable means for applying polymeric coating to 
the exterior surface of the unsealed double-wall tubing. As depicted in 
FIG. 2, the polymeric coating station 24 is, preferably, a suitable 
conventional cross-head, extrusion mechanism with appropriate fixturing 
and conveying means as would be known to one reasonably skilled in the 
art. 
The material imparted onto the outermost surface of the unsealed, unbrazed 
metal tube is, preferably, a polymeric material having a thickness 
sufficient to permit handling of the coated tube during subsequent 
processing steps without compromising the structural integrity of the 
overlaying polymeric film. The thickness of the polymeric material is 
between about 25 and about 250 microns (0.001 inches and about 0.010 
inches), with an average thickness between about 50 and about 150 microns 
(about 0.002 to about 0.006 inches) being preferred. It is to be 
understood that the polymeric material applied to the exterior surface of 
the unsealed tubing is, preferably applied in its molten or liquid form, 
i.e. at or near its particular melt processing temperature. 
Upon exiting the polymeric coating station 24, the applied polymeric 
material is permitted to cool in a suitable cooling station 26 to permit 
the complete solidification of the polymeric film and its suitable 
adherence on the outer surface of the unsealed metal tubing. 
The coated unsealed tube can be conveyed as required to an appropriate 
brazing furnace 28. As employed herein the term "brazing furnace" is 
defined as any suitable device by which unsealed tubing of the type 
described herein can be brought to an appropriate temperature in an 
appropriate environment for an interval sufficient to achieve appropriate 
brazing. Such brazing furnaces include, but are not limited to 
conventional or modified muffle furnaces, induction-type furnaces and the 
like. In the embodiment described herein, a muffle furnace is effectively 
employed. 
Brazing furnace 28 includes means for raising the surface temperature of 
the tube to an elevated temperature sufficient to vaporize the polymeric 
material in the initial stages of the brazing operation. The elevated 
temperature employed in the muffle furnace 28 is one sufficient to trigger 
fusion between the metal surfaces and the selected brazing metals 
overlaying thereon. The muffle furnace also includes means for providing a 
suitable dry gaseous atmosphere within the furnace itself. The gas 
preferably employed is composed of a nitrogen atmosphere with sufficient 
hydrogen to achieve and maintain fluxing. In the preferred embodiment the 
gas is dry. The gas can be supplied by any conventional manner such as 
from gas bank 32 shown in FIG. 2. 
Upon exiting the brazing furnace 28, the metal tube has been fused into a 
leak-proof, double-wall material. At this point, it can be conveyed to 
appropriate post brazing steps such as suitable cool down and any suitable 
post-forming processes. 
In the process of the present invention, a metallic material having an 
outwardly oriented surface overlayed with a suitable metallic brazing 
material or alloy and coated with a melt-processible extrudable polymeric 
material is subjected to an elevated temperature sufficient to trigger 
fusion between the metallic brazing material or alloy and the underlying 
metallic material to be brazed. The term fusion as used herein is defined 
as the existence or establishment of a metallurgical bond between two 
dissimilar metals; i.e. the metallic material and the metallic brazing 
material or alloy. The elevated temperature to which the surface of the 
underlying metallic material is elevated is a temperature which is 
sufficiently higher than the liquidus temperature of the brazing material 
or alloy imparted on the metallic material to trigger and maintain the 
fusion process. "Liquidus temperature" is defined herein as the 
temperature at which a metal or metal alloy begins to enter the molten 
state. In the preferred embodiment, where copper is employed, the liquidus 
temperature of copper is 1083.degree. C. (1,981.degree. F.). Preferably, 
the elevated temperature is at or above a temperature between 1093.degree. 
C. and 1121.degree. C. (2,000.degree. and 2,050.degree. F.). The upper 
maximum for the elevated temperature is determined by both the properties 
of the underlying metallic material substrate and the selected brazing 
material employed. Ideally, the upper temperature is limited to a point 
below thermal degradation or melting point of the metallic material 
substrate and/or the degradation point of the selected brazing material. 
In the process of the present invention, metallurgical fusion occurs in an 
atmosphere which will support fluxing. Suitable atmospheres include 
reducing atmospheres as well as exothermic gaseous atmospheres. In the 
preferred embodiment, the atmosphere for supporting fluxing is an 
exothermic atmosphere formed by partial combustion of a mixture of air and 
natural gas (methane) in a ratio of between about 4:1 to about 8:1 air to 
natural gas respectively; with an air to natural gas ratio of about 4:1 
being preferred. The generated gas preferably has a controlled dew point. 
The exothermic fluxing gas employed, preferably provides an atmosphere in 
the fluxing environment which consists essentially of an anhydrous gas 
selected from the group consisting of nitrogen, hydrogen, carbon dioxide, 
carbon monoxide, and mixtures thereof. 
The atmosphere employed and the temperatures attained in the brazing 
process of the present invention permit and promote the volatilization of 
the polymeric material employed in outer layer 16. As used herein, the 
term "volatilization" is defined as the forced vaporization and 
evaporation of the polymeric material employed in the outer layer 16 
accompanied by the generation of volatilization products which are 
typically simple organic and inorganic molecule together with elemental 
carbon. 
The material to be brazed is conveyed through the brazing furnace at a rate 
sufficient to achieve effective brazing. Typically in brazing operation 
involving double-wall tubing, the unsealed coated tubing will be conveyed 
through the furnace at a rate sufficient to achieve appropriate brazing. 
Conveyance into the brazing furnace environment causes a rise in 
temperature of the coated metal to be fused. It is during this initial 
rise in temperature that the polymeric material employed in the outer 
layer 16 is volatilized. Preferably volatilization of the polymeric 
material employed in the outer layer 16 is essentially instantaneous such 
that the volatilization of the polymeric material occurs in the initial 
phase of the brazing process before the brazing material or alloy has 
achieved its liquidus temperature. 
Upon volatilization, a thin powder residue remains and adheres to the outer 
surface of the tubing to be brazed. Without being bond to any theory, it 
is believed that the residue consists essentially of elemental carbon; the 
volatilized hydrocarbon components of the polymeric material having either 
been consumed in the fluxing atmosphere or conveyed away from the tubing 
surface due to the relative movements of the fluxing gas and the tubing 
undergoing the brazing operation. 
It has been found that tubing prepared with the polymeric coating of the 
present invention does not experience the agglomeration and run off 
phenomenon found in untreated tubing overlayer with a brazing material. 
Without being bound to any theory, it is believed that this is due to 
changes in surface tension of the molten brazing alloy caused by the 
presence of the carbon powder. This beneficial aspect is attained even 
when the polymeric coating material is not compounded with additional 
carbon material. Without being bound to any theory, it is believed that 
the volatilization of the polymeric material results in the complete 
volatilization of sufficient amounts of the polymeric material to 
elemental compound residue to provide sufficient carbon film to prevent 
agglomeration and run-off. 
Having disclosed and discussed the forgoing invention, the following 
examples are included to further illustrate and demonstrate the present 
invention. The Examples are included for illustrative purposes and are not 
to be construed as limitative of the invention. 
EXAMPLE I 
The effects of pyrolysis of various polymeric compounds in a standard air 
atmosphere was investigated to determine the efficiency of the 
volatilization process and the breakdown products produced thereby. In 
all, six samples of various polymeric compounds were each compounded with 
20% by weight carbon black. Samples of each resulting material were 
subjected to pyrolysis in either an inert helium atmosphere or a standard 
air atmosphere to ascertain the pyrolysis products derived therefrom. 
Direct pyrolysis products were determined by thermogravimetry with on-line 
mass-spectrometry (TG/MS) using inert helium gas and a nominal heating 
rate of 30.degree. C./minute to 880.degree. C. These products and 
additional potential products of incomplete combustion (PIC) were further 
determined using laser pyrolysis/gas 
chromatography/mass-spectrometry(LPy-GC/MS) which involved very rapid 
surface heating to temperatures of 1,000.degree. C. or higher with a 50 or 
100 msec pulse from a carbon dioxide laser in ambient air. The laser 
technique examined the secondary reaction products formed at high heating 
rates or in the highly reactive atmosphere of the flame. The TG/MS 
technique further indicates the temperatures at which decomposition takes 
place and the amount of residue left from simple (inert atmosphere) 
pyrolysis. 
The results summarized in Table II include the temperature of the maximum 
weight of sample weight loss (max DTG), the approximate percent residue, 
the main TG pyrolysis products and the main laser pyrolysis products. 
The materials analyzed are as follows: 
Sample 1.--Nylon 6 with 20% carbon black (CAPRON I4094; Cabot monarch 120) 
Sample 2.--Low density polyethylene with 20% carbon black (recycled 
polyethylene derived from industrial processes; Cabot monarch 120); 
Sample 3.--Acetal (POM) with 20% carbon black (DELRIN 500; Cabot monarch 
120); 
Sample 4.--Low density polyethylene with 20% carbon black; polyethylene 
terepthalate with 20% carbon black (TIGER MB, lot J 3466; Cabot monarch 
120); 
Sample 5.--Nylon 6 with 20% carbon black (AP20BK, lot AV1403-0895; Cabot 
monarch 120). 
As illustrated in the data collected in Table II, the temperature of the 
maximum rate of sample weight loss (max DTG) is well below the liquidus 
temperature for conventional brazing metals or alloys such as copper. 
Thus, it can be assumed that the volatilization of the organic components 
due to pyrolysis occurs sequentially before the brazing material achieves 
its liquidus state. 
The approximate percent residue indicates that materials such as 
polyethylene and polyethylene terepthalate actually produce reside in 
excess of the amount contributed by the carbon black as would be expected. 
It is hypothesized that this additional residue is actually a result of 
the pyrolysis of the polymeric material. Such residue may assist in 
preventing agglomeration and run-off of brazing material during subsequent 
brazing processes. 
The thermogravometric pyrolysis products resulting from pyrolysis in an 
inert helium atmosphere are set forth in Table II. 
The laser pyrolysis products which were produced in air indicate more 
complex molecular structures as would be expected due to reaction with 
atmospheric oxygen. 
EXAMPLE II 
Unsealed 3/16 inch diameter double wall steel tube having an overlaying 
copper brazing material was coated with a thin film virgin extrusion-grade 
polyethylene compounded with 20% carbon black at an average thickness 
between about 0.002 and 0.003 inches applied by a conventional cross-head 
extrusion device. The coating was allowed to cool and was then fed to a 
conventional brazing furnace where the material was brazed at 2050.degree. 
F. at a furnace speed of 9 feet per minute in a brazing atmosphere. 
The resulting sealed double-wall tube was inspected after cooling. A visual 
inspection indicated a thin film of carbon dust-like material covering the 
exterior surface of the tube. The tubing was subjected to conventional 
industrial tests to determine its grade designation. The quality of the 
resulting tubing was found to be equal to or better than that prepared by 
conventional black paint processes. Visual inspection of the exterior 
surface of the tubing indicated no appreciable evidence of agglomeration 
or run-off of the copper brazing material. This indicates that employing 
the coating material of the present invention on unbrazed, double-wall 
tubing as a substitute for black paint provides an acceptable level of 
conforming parts. 
EXAMPLE III 
Unsealed tubing was prepared according to the process outlined in Example 
II. The polymeric coating material employed was extrusion grade virgin 
polyethylene containing 10% by weight carbon black. The resulting coated 
unsealed tubes were brazed in the manner described previously. The 
resulting sealed tubes were inspected and found to be of a quality equal 
to or better than that achieved by conventional black paint processes. 
Visual inspection of the exterior of the tubing showed no appreciable 
evidence of agglomeration or run-off of the copper brazing material. The 
visual inspection further indicated a thin film of an inert particulate 
material which was determined to be carbon dust. 
EXAMPLE IV 
Unsealed double-wall tubing having an intermediate bonding layer composed 
of copper was prepared by conventional processes. The tubing was coated 
with a film of virgin extrusion grade polyethylene by a cross-head 
extrusion method to an average thickness between 0.002 and 0.003 inches. 
The coated tubing was allowed to cool and then was fed into a suitable 
brazing furnace having a controlled exothermic atmosphere at a temperature 
of 2050.degree. F. and a furnace speed of 9 to 11 feet per minute. 
The resulting brazed tube was allowed to cool and was visually inspected. A 
thin film of dust was found to cover the exterior of the tube. No 
appreciable evidence of copper agglomeration or run-off was detected. 
Further performance tests were performed to determine that effectiveness 
of the brazing process. The resulting tubing was found to be of a quality 
equal to or superior to tubing produced by conventional black paint 
processes. 
This indicates that application of polymeric materials in the film coating 
does not interfere with the brazing process. The thin coating film appears 
to undergo an essentially complete thermal paralytic volatilization and/or 
decomposition with no organic carbonaceous residue on the tubing surface. 
Residue would appear to be essentially particulate carbon. Application of 
polymeric coating, even material which is not compounded with additional 
particulate carbon, can effectively reduce or eliminate agglomeration and 
run-off surface brazing material. 
TABLE II 
__________________________________________________________________________ 
Summary of Results 
Temp. 
Samp Max. DTG 
Residue 
TG Pyrolysis Laser Pyrolysis 
No. 
Polymer 
(approx) 
(approx %) 
Products (in He) 
Products (in air) 
__________________________________________________________________________ 
1 Nylon 6 
480 20 caprolactam (cyclic monomer) 
caprolactam 
(formaldehyde)* 
alkybenzenes (Co--C2) 
(pyridine)* 
other polymer fragments 
2 LDPE 
510 25 alkenes (C2 to --C30) 
alkenes (.apprxeq.C5 to --C12) 
alkanes (.apprxeq.C4--) 
alkanes (.apprxeq.C7--C12) 
alkadienes (.apprxeq.C6--) 
alkadienes (.apprxeq.C6--C12) 
alkylbenzenes (trace C1--C2) 
alkylbenzenes (CO--C2) 
phenylacetylene 
styrene 
3 Acetal 
350 20 formaldehyde hydroxyacetal (+/or 
hydroxyacetal (+/or 
hydroxyethyleneoxide) 
hydroxyethyleneoxide) 
4 LDPE 
510 40 alkenes (C2 to .apprxeq.C30) 
same as No. 2 
alkanes (=C4--) 
alkadienes (.apprxeq.C6--) 
alkylbenzenes (trace CO--C3) 
(alkyl-)styrenes + indenes 
5 PET 470 30 terephthalic acid 
alkylbenzenese (CO--C2) 
benzoic acid phenylacetylene 
ethylene styrene 
CO2 +/or ehtyleneoxide 
benzoic acid 
dimer + trimer 
phenyl-aldehydes + 
ketones 
6 Nylon 6 
470 20 same as No. 1 
same as No. 1 
__________________________________________________________________________ 
*Tentatively identified compounds.