Mar resistant, corrosion inhibiting, weldable coating containing iron powder for metal substrates

A weldable heat curable liquid coating composition for steel is provided that exhibits improved mar resistance without impairing the weldability characteristics of the coating. To this end, the composition contains a conductive welding aid of iron dust. The weldable coating when applied to steel and cured thereon to a dry film allows for spot welding of the coated steel without requiring special welding equipment and techniques.

FIELD OF THE INVENTION 
This invention relates to corrosion inhibiting organic coatings for metal 
substrates, and, more particularly to weldable corrosion inhibiting 
organic coatings having improved mar resistance without sacrificing 
weldability, and to a method of using the same to weld two pieces of metal 
together. 
BACKGROUND OF THE INVENTION 
For many years, corrosion inhibiting organic coatings have been applied to 
metal coils or sheets prior to forming into finished articles. Designing 
with prepainted metal provides the metal finisher with many benefits, such 
as elimination of in-house painting operations, reduction in associated 
environmental liabilities, and improvement in the quality of the paint 
finish. One of the problems encountered in using prepainted metal is that 
if such articles are to be assembled, they must be joined together by 
adhesives or weldless fasteners, since organic coatings are insulative in 
nature and are either not weldable or weldable with difficulty and only by 
employing special techniques and equipment. 
These techniques include spot welding with higher currents or longer weld 
times. However, such unorthodox methods are time-consuming and costly. 
Also, excessive temperatures are normally generated in the weld areas, 
which, in turn, causes vaporization and expulsion of the metal out from 
between the welding electrodes. This results in inferior welds as well as 
rapid deterioration of the electrode tips. Other techniques include 
decreasing the thickness of the protective film which sacrifices corrosion 
protection for weldability. 
Recently, there has been a growing demand for "weldable" organic coatings. 
Organic coatings which are electroconductive and allow for electric 
resistance welding through their cured coating films without resorting to 
special equipment and techniques are said to be "weldable" or have 
weld-through capability. Various types of weldable anticorrosive liquid 
coatings have been proposed which typically contain conductive powdered 
metals or alloys to reduce the electrical resistivity of the coating film 
U.S. Pat. No. 5,001,173 (Anderson et al.) discloses some commercially 
popular weldable primers which contain high concentrations of conductive 
powdered ferroalloys, such as ferrophosphorous (a mixture containing 
di-iron phosphide and iron phosphide), and powdered zinc. 
Zinc powder alone is not considered a good welding aid. Moreover, one of 
the problems encountered with ferrophos-rich weldable coatings in their 
appearance. Ferrophos is a very dark gray pigment, and when provided in 
the coatings in the high pigment to binder ratio necessary to impart 
desired weldability, it tends to produce very dark gray colored films, 
which are undesirable in certain applications. For instance, mar 
resistance is almost nil and even fingernail scratches are highly visible. 
In addition, the dark gray coating film tends to detract from the 
appearance of any topcoat finish applied thereover. Usually, it is 
necessary to topcoat at high film builds for adequate hiding or encryption 
of the primer, which, in turn, is very costly. Attempts have been made to 
lighten weldable primers to improve mar resistance and cryptability by 
adding standard light colored pigments, such as titanium dioxide, without 
much success. The standard pigments are inhibitively insulative, and the 
high pigment concentration needed to offset the darkness tends to impair 
weldability. 
One solution to this problem has been to return to the use of standard 
non-weldable mar resistant coatings. Yet without welding aids in the 
formulations, the very thin films (i.e., no greater than about 0.1 mil 
thick) required for weldability is usually below the minimum thickness 
needed to provide adequate film opacity and corrosion resistance. Another 
approach taken has been to use a two-coat weldable primer as disclosed in 
U.S. Pat. No. 5,260,120 (Moyle et al.), wherein a ferrophos-rich primer is 
top coated with an extremely thin layer of a non-weldable, titanium 
dioxide-rich, protective coating. The thin protective film provided does 
not significantly interfere with the weldability of the conductive primer, 
yet provides a light colored surface film which has greatly improved mar 
resistance. The protective film also smooths out the abrasiveness of the 
underlying ferrophos primer. However, it is time-consuming and costly to 
employ such a two-step coating procedure. 
Another problem encountered with weldable ferrophos-rich primers is their 
abrasiveness, which raises excessive stamping and forming die wear 
concerns during metal finishing operations. The abrasive, sandpaper, 
texture of the film finish is due to the hardness of the ferrophos. As 
mentioned above, the Moyle et al. patent provides a solution to this 
problem but again requires an undesirable two-step coating procedure. 
A further problem with ferrophos-rich primers is that during welding they 
produce toxic fumes, such as phosphine gas, along with objectionable odors 
when subject to the required welding temperatures. While the toxicity does 
not reach the environmentally harmful and physiologically unsafe levels, 
workers have been known to complain about unpleasant odors produced during 
welding. It is difficult, or course, to reduce toxic effluents and 
eliminate unpleasant odors produced by ferrophos primers without 
sacrificing weldability. 
Still another problem encountered with ferrophos-rich primers is that the 
film finish has a very high coefficient of friction. During metal 
finishing, the stamping and forming dies tend to scape off the coating 
film. Corrosion protection in these areas is thus lost. Also, the paint 
scrapings tend to build-up and eventually cause the finishing line to shut 
down. Internal lubricants, such as polytetrafluoroethylene, have been 
incorporated in conductive coatings to lower surface friction, allowing 
the finishing operations to proceed without destroying the coating, as 
disclosed in U.S. Pat. No. 5,624,978 (Soltwedel et al.). 
Weldable primers also invariably shorten the life of the welding 
electrodes. Copper tipped electrodes on resistance spot welders are easily 
degraded by coating pick-up during welding. The number of spot welds that 
can be made on precoated metal before corrective action is required is 
dramatically reduced in comparison to that for bare metal. This results in 
reduced productivity arising from the need to change or dress the 
electrodes more frequently as well as inconsistent weld quality. Weldable 
coatings which extend the electrode life are continually being sought. 
Other types of weldable liquid coatings have been disclosed which contain 
metallic welding aids other than ferrophos or zinc powders, but all of 
them suffer from drawbacks. For example, U.S. Pat. No. 5,047,451 (Barrett 
et al.) discloses a weldable liquid anticorrosive primer containing a 
welding agent of powdered nickel, a non-weldable corrosion inhibitor of 
powdered aluminum or stainless steel, a polyethylene suspension agent for 
preventing the finely divided metal from settling out, a silane-treated 
silicon dioxide thixotropic agent, a drawing agent of 
polytetrafluoroethylene, and a hygroscopic agent. Nickel powder, however, 
is dark gray and thus undesirable for improving mar resistance and topcoat 
crypt. Nickel powder is also an expensive material and uneconomical for 
use in weldable coatings. 
Earlier U.S. Pat. No. 2,666,835 (Elleman) discloses a weldable liquid 
anticorrosive zinc chromate primer containing up to 30 vol. % of primer 
solids of a non-oxidized, magnetic, metal powder, such as non-oxidized 
forms of nickel powder, soft iron powder, stainless steel powder, steel 
powder, and nickel alloy powder. Nickel powder, however, is clearly 
preferred due to its inherent possession of magnetic remanence, which 
causes the metal particles to naturally link together and form conductive 
chains in the paint film. While coatings containing soft iron powder are 
mentioned, Elleman suggests the need for chemically reducing the thin 
oxide layer normally present on iron powder before incorporating it in the 
coating. This special procedure, for inclusion of only substantially 
non-oxidized soft iron powder, is time-consuming and costly. 
Elleman also resorts to other special techniques for generating the 
weldable coating. For instance, Elleman suggests the need to expose the 
liquid coating to a magnetic field prior to drying on metal, in order to 
uniformly align the metal particles and thus impart the necessary 
conductivity to the film. This adds a time-consuming step to the welding 
process which, in turn, leads to reduced productivity and increased costs. 
These primers also require zinc chromate. While chromate pigments, 
including zinc chromate, strontium chromate, calcium chromate, and lead 
chromate, are excellent corrosion inhibitors, they are bright yellow 
insulative pigments and tend to produce darker coatings having reduced mar 
resistance and higher topcoat crypt. 
What is needed is a weldable liquid corrosion inhibiting coating which 
forms a dry, electroconductive film on metal substrates which has improved 
mar resistance, improved topcoat crypt, reduced abrasiveness, reduced 
friction, reduced toxic and unpleasant odor emissions, extended electrode 
life, without sacrificing weldability and corrosion resistance, and that 
can weld together, in its cured state, two pieces of metal, such as steel, 
coated with the same, without the need for special equipment and 
techniques. 
SUMMARY OF THE INVENTION 
It is an object of this invention, therefore, to provide a weldable liquid 
coating for metal substrates, such as steel, which does not suffer from 
the foregoing drawbacks. 
It is another object of this invention to provide a weldable coating that 
has improved mar resistance and topcoat crypt without sacrificing 
weldability and corrosion protection. 
Still another object of this invention is to provide a weldable coating 
that has a relatively non-abrasive texture to prevent die wear. 
Yet still another object of this invention is to provide a weldable coating 
that has a low coefficient of friction to prevent destruction of the 
coating film during metal finishing. 
And another object of this invention is to provide a weldable coating that 
emits low levels of toxic effluents and unpleasant odors during welding. 
A further object of this invention is to provide a weldable coating that is 
weldable without rapidly deteriorating the life of welding electrodes. 
It is a still another object of the present invention to provide a weldable 
coating that has excellent corrosion resistance. 
It is a related object to provide a method of welding together metal 
articles having coated and cured thereon a weldable coating of the 
aforesaid character without the need for special equipment or techniques. 
The aforesaid and other objects are achieved by providing a weldable liquid 
coating composition for metal in which a welding aid of conductive iron 
powder is incorporated in the liquid coating to impart weldability without 
substantially darkening the coating, such that when the coating is applied 
and cured on a metal substrate, the coating film not only has improved mar 
resistance and crypt, but also allows the coated metal to be electric 
resistance welded without requiring special welding equipment and 
techniques. The iron powder particles found most useful are shiny and 
smooth irregular spheroids produced by water jet atomization. No chemical 
reduction of the iron powders is required immediately prior to 
incorporation into the liquid coating. Furthermore, the iron powders 
require no magnetization and remain randomly oriented in the liquid 
coating. 
The preferred weldable liquid coating composition of this invention 
comprises a solvent-borne, thermosetting, epoxy-pendant, urethane coating 
which is characterized by a solvent blend of: a) a film-forming 
hydroxy-functional resin, preferably a mixture of hydroxy-functional 
polyester resins and bisphenol A epoxy resins; b) a crosslinker for the 
resin which effects a urethane cure, preferably a mixture of blocked 
isocyanate resins and aminoplast resins; c) a catalyst; d) a weldably 
effective amount of conductive iron powder particles of the aforesaid 
character randomly dispersed in the liquid to impart desired weldability 
to the coating film; e) optional yet preferred suspension aid to prevent 
the iron powder particles from settling out; f) optional yet preferred 
internal lubricant comprising polytetrafluoroethylene to lower the 
coefficient of friction of the film; and, g) minor amounts of insulative 
light colored pigments, wherein the composition is further characterized 
in that it is free of ferrophos and other ferroalloy and nickel welding 
aids, and it may also be free of non-weldable anticorrosive chromate 
pigments. 
This weldable coating not only has improved mar resistance and cryptability 
without sacrificing weldability and corrosion protection, but also 
exhibits reduced abrasiveness for preventing excessive die wear during 
finishing, reduced coefficient of fraction to prevent destruction of the 
film during finishing, emits few toxic effluents and unpleasant odors 
during welding operations, sustains the life of the copper-tipped welding 
electrodes, and has the ability to weld together two pieces of coated 
metal using a weld cycle similar to that for bare steel, 
The aforesaid and other objects are also achieved by using the liquid 
coating of the aforesaid character to weld together two pieces of metal. 
The liquid coating is applied to metal sheets or coil and heat cured 
thereon to form a hardened dry film. Two pieces of coated metal are then 
welded together, for example, using a standard spot welder, without 
requiring special equipment and techniques. 
The various objects, features and advantages of the invention will become 
more apparent from the following description and appended claims. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Throughout this specification, all parts and percentages specified herein 
are by weight unless otherwise stated. 
In this invention, a weldable corrosion inhibiting coating composition in 
liquid form is applied to a metal substrate. The liquid coating is 
converted to a solid dry film which is bonded to the metal substrate by 
heating at elevated temperatures. The heating evaporates the solvents in 
the liquid layer and initiates curing of a film-forming resin to provide a 
weldable protective coating film permanently adhered to the substrate. 
FILM-FORMING RESIN 
The weldable coating composition of this invention includes a film-forming 
resin component. A wide variety of traditional film-forming resins may be 
employed in this invention, such as polyester, epoxy, urethane, acrylic 
and methacrylic resins. These resins generally include a plurality of 
crosslinkable functional groups to initiate curing into a dry film. 
The preferred resin component is a hydroxy-functional resin. 
Hydroxy-functional resins provide the building blocks for producing 
flexible urethane coatings, which are desired in this invention. 
One suitable class of hydroxy-functional resins useful herein include 
hydroxy-functional polyester resins. These polyester resins can be 
prepared by any of the methods well known to those of ordinary skill in 
the art. For example, condensation reactions can be carried out between 
one or more aliphatic or cycloaliphatic di- or polyhydric alcohols and one 
or more aliphatic, cylcoaliphatic, or aromatic di- or polycarboxylic 
acids, or corresponding anhydrides. 
Among the polyester resins which are useful herein are linear polyesters 
derived from aromatic dicarboxylic acids and alkylene glycols. Examples of 
suitable aromatic dicarboxylic acids include terephthalic acid, bibenzoic 
acid, ethylene bis-p-oxy benzoic acid, tetramethylene bis-p-oxy benzoic 
acid, 2,6-naphthalic acid, orthophthalic acid, and isophthalic acid. 
Mixtures of terephthalic acid and isophthalic acid are particularly 
useful. Examples of suitable alkylene glycols include ethylene glycol, 
trimethylene glycol, pentamethylene glycol, 1,2-propanediol, 
1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 
1,4-cyclohexanedimethanol, and polyethylene glycol. 
The linear polyesters can also be derived from mixtures or aromatic 
dicarboxylic acids and aliphatic dicarboxylic acids and alkylene glycols. 
Examples of suitable aliphatic dicarboxylic acids include maleic acid, 
fumaric acid, adipic acid, pimelic acid, suberic acid, sebacic acid, 
azelaic acid, oxy-dibutyric acid, 5-oxa-1,10-decanedioic acid, 4-n-propyl 
suberic acid, dodecane dioic acid, and tridecane dioic acid. 
The relative amounts of aromatic dicarboxylic acid and aliphatic 
dicarboxylic acid may be varied in order to obtain polyesters having 
different characteristics. In general, the ratio of aromatic acid to 
aliphatic acid will be from about 2:1 to 1:2 and more often about 1:1 on 
an equivalent weight basis. The ratio of dicarboxylic acid to dihydric 
alcohol also may be varied, however, with the diol generally being present 
in excess. The ratio of dicarboxylic acid to diol is generally from about 
1:1 to 1:2 on a weight equivalent basis. 
The reaction between the dicarboxylic acid mixture and dihydric alcohol 
mixture is effected in the conventional manner, typically by heating the 
mixture to an elevated temperature in the presence of catalysts. Tin 
catalysts are especially useful, including dibutyltin oxide and dibutyltin 
dilaurate. Antimony oxide may also be used as a catalyst. 
The hydroxy-functional polyesters prepared in this manner will generally 
have molecular weights ranging between about 5,000 and 50,000, and will 
further have hydroxyl numbers of between about 5 and 35. 
In a preferred embodiment, the polyester resin comprises between about 20 
and 60 wt. % of total solids, and, more preferably, between about 35 and 
45 wt. %. 
The film-forming resin component of the weldable coating composition may 
also contain other resins that are capable of modifying the properties of 
the polyester-rich blend, such as epoxy resins, which improve the adhesion 
and flexibility of the coating film, through incorporation of pendant 
epoxy groups in the urethane compound. Epoxy resins generally refer to the 
condensation reaction products of an epihalohydrin and a 
hydroxy-containing compound or a carboxylic acid. The epoxy resins may be 
of the ether- or ester-types, although the ether-type epoxy resins are 
preferred. 
Ether-type epoxy resins are formed by reacting an epihalohydrin, such as 
epichlorohydrin, and a compound containing at least two free alcoholic 
hydroxyl and/or phenolic hydroxyl groups per molecule. The condensation 
reaction is typically carried out under alkaline conditions, or, in the 
alternative, in the presence of an acid catalyst. The products of such 
reactions are generally complex mixtures of glycidyl polyethers. 
The ether-type epoxies can be derived from aliphatic alcohols, such as 
ethylene glycol, diethylene glycol, and higher poly(oxyethylene) glycols, 
propane-1,2-diol and poly(oxypropylene)glycols, propane-1,3-diol, 
poly(oxytetramethylene) glycols, pentane-1,5-diol, hexane-2,4,6-triol, 
glycerol, 1,1,1-trimethylolpropane, pentaerythritol, sorbitol; from 
cycloaliphatic alcohols, such as resorcitol, quinitol, 
bis(4-hydroxycyclohexyl)methane, and 2,2-bis(4-hydroxycyclohexyl)propane; 
and, from alcohols having aromatic nuclei, such as 
n,n'-bis(2-hydroxyethyl)aniline and 
p,p'-bis(2-hydroxyethylamino)diphenylmethane. The esters may also be made 
from mononuclear phenols, such as resorcinol and hydroquinone; from 
polynuclear phenols, such as bis (4-hydroxyphenyl) methane (bisphenol F), 
4,4'-dihydroxydiphenyl, bis(4-hydroxyphenyl)sulfone, 
1,1,2,2-tetrakis(4-hydroxyphenyl)ethane, 2-2-bis(4-hydroxyphenyl)propane 
(bisphenol A), 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane; and, from 
novolaks formed from the condensates of aldehydes, such as formaldehyde, 
acetaldehyde, chloral, and furfuraldehyde, with phenol, chlorinated 
phenols, such as 4-chlorophenol, 2-methylphenol, and 4-tert-butylphenol. 
Particularly preferred epoxy resins useful herein are diglycidyl ethers of 
bisphenol A, which are formed from the condensation reaction of 
epichlorohydrin with bisphenol A in the presence of alkaline catalyst. 
Bisphenol A type epoxy resins are commercially available from a wide 
variety of sources. Exemplary of bisphenol A type epoxy resins include 
those sold under the trade name "Epon" by Shell Oil Company. Other 
desirable epoxy resins include the diglycidyl ethers of other bisphenol 
compounds, such as bisphenol B, F, G and H. 
Another suitable class of epoxy resins useful in the present invention are 
the epoxidized novolaks, such as the epoxy cresol- and epoxy 
phenol-novolak resins. Aliphatic or cycloaliphatic epoxy resins can also 
be utilized in the present invention. 
The epoxy resins prepared in this manner will generally have molecular 
weights ranging between about 300 and 100,000 and epoxide equivalent 
weights of between about 150 and 10,000. 
In a preferred embodiment, the epoxy resin comprises between about 0.5 and 
10 wt. % of total solids, and, more preferably, between about 1 and 2 wt. 
%. 
The total amount of film-forming resin in the weldable coating of this 
invention is usually between about 30 and 60 wt. % of total solids, and, 
preferably, between about 40 and 50 wt. %. 
It will be apparent to those skilled in the art that other suitable 
film-forming resins may be employed in the coating composition of this 
invention, although the aforesaid resins are most preferred. 
CROSSLINKER 
The curing agent or crosslinker for the film-forming resin component can be 
selected from a variety of curing agents traditionally known to be useful 
for curing such resins. As previously mentioned, a urethane curative 
system is preferred. Curing agents suitable for effecting a urethane cure 
include isocyanates and blocked isocyanates, although blocked isocyanates 
are most preferred. 
Free isocyanates are generally not used in this invention, since the 
weldable coating composition is usually coil coated onto the metal 
substrate from a reservoir. The coating, therefore, should have a suitably 
long pot life, such that is does not cure and harden prematurely in the 
reservoir. 
Blocked isocyanate resins are based on the reaction products of aliphatic, 
cycloaliphatic or aromatic di- and polyisocyanates and isocyanate blocking 
agents which prolong the pot life of the coating. Standard methods can be 
used to prepare the blocked isocyanates, for example, by biuretization, 
dimerization, trimerization, urethanization, and uretidionization of the 
starting monomeric isocyanates. 
Examples of suitable aliphatic diisocyanates, include 1,4-tetramethylene 
diisocyanate and 1,6-hexamethylene diisocyanate. Examples of suitable 
cycloaliphatic diisocyanates, include 1,4-cyclohexyl diisocyanate, 
isophorone diisocyanate, and 4,4'-methylene-bis-cyclohexyl isocyanate. 
Examples of suitable aromatic diisocyanates, include 4,4'-diphenylmethane 
diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, and 
2,4-toluene diisocyanate. Examples of suitable polyisocyanates, include 
1,2,4-benzene triisocyanate, polymethylene polyphenyl isocyanate, and the 
like. 
The blocking agent is typically selected from those materials that react 
with the functional groups of the isocyanate so as to form stable adducts 
at room temperature, but that can be dissociated at elevated temperature. 
Examples of suitable blocking agents, include lactams, such as caprolactam 
and butyrolactam, lower alcohols, such as methanol, ethanol, and isobutyl 
alcohol, oximes, such as methyl ethyl ketoxime and cyclohexanone oxime, 
phenols, such as phenol, p-t-butyl phenol and cresol, and pyrazoles, such 
as 3,5-dimethylpyrazole. 
In a preferred embodiment, the blocked isocyanate crosslinker comprises 
between about 0.5 and 10 wt. % of total solids, and, preferably, between 
about 2 and 5 wt. % 
In addition to the aforesaid crosslinkers, it is generally preferred to 
include other crosslinkers to provide the desired final urethane film 
properties, such as hardness, adhesion, flexibility and solvent 
resistance. One suitable class of crosslinkers are the aminoplast resins. 
Aminoplast resins are based on the reaction products of formaldehydes with 
amino- or amido-group carrying compounds. A wide variety of aminoplast 
resins are useful in the practice of this invention. Examples of suitable 
aminoplast resins, include condensation products of aldehydes, 
particularly formaldehyde, with melamine, urea, dicyanodiamide, 
benzoguanamine, and glycouril. Aminoplasts which are modified with lower 
alkanols having from about 1 to 4 carbon atoms are preferred. The 
melamine-formaldehyde condensates of hexamethoxymethyl melamine and 
butylated melamime-formaldehyde are especially preferred. The aminoplast 
resins facilitate hardening of the crosslinked urethane resin backbone. 
Phenoplasts and carbamates can also be used. 
In a preferred embodiment, the aminoplast crosslinker comprises between 
about 0.5 and 10 wt. % of total solids, and, preferably, between about 2 
and 5 wt. %. 
In order to achieve the outstanding properties which make these weldable 
coatings particularly useful, it is desirable that the amount of 
crosslinker be sufficient to substantially completely react with the 
functionalities present in the film-forming resin component. 
The total amount of crosslinker in the weldable coating of this invention 
is usually between about 0.5 and 10 wt. % of total solids, and, 
preferably, between about 2 and 5 wt. %. 
Other suitable crosslinkers will be apparent to those skilled in the art. 
CATALYST 
The coating composition of this invention may also include a cure catalyst 
or accelerator to increase the rate of the crosslinking reaction between 
the film-forming resin and the crosslinker. A wide variety of catalysts 
traditionally employed for urethane cure systems can be used. Examples of 
suitable catalysts include tertiary amines, such as triethylene diamine, 
organometallic salts, particularly organotin compounds, such as dibutyltin 
dilaurate, dibutyltin dilauryl mercaptide, dibutyltin maleate, dimethyltin 
dichloride, dibutyltin di-2-ethylhexoate, dibutyltin diacetate, stannic 
chloride, ferric chloride, potassium oleate, and acid catalysts, such as 
phosphoric acid, alkyl or aryl acid phosphates, such as butyl acid 
phosphate or phenyl acid phosphate, and sulfonic acids, such as methane 
sulfonic acid, benzene sulfonic acid, p-toluene sulfonic acid, naphthalene 
sulfonic acid, dodecylbenzene sulfonic acid, and dinonylnaphthalene 
sulfonic acid. Acid catalysts blocked with amines and pyridines are also 
useful for improving shelf stability. 
The catalyst is generally employed in an effective amount to initiate the 
crosslinking reaction at commercially acceptable rates. 
In a preferred embodiment, the catalyst comprises between about 0.1 and 1 
wt. % of total solids, and, more preferably, between about 0.3 and 0.5 wt. 
%. 
For a further description of particularly useful liquid, urethane 
film-forming, coating systems, reference can be made to U.S. Pat. Nos. 
5,001,173; 5,260,120; and 5,624,978, which disclosures are incorporated by 
reference herein in their entireties. 
WELDABILITY AGENT 
Conductive ferrous metal powder, particularly iron metal powder, is 
employed in this invention as the weldability agent or welding aid. 
Powdered iron is a very inexpensive material. Even more significantly, 
powdered iron offers very little color to the coating, which dramatically 
improves the mar resistance and cryptability of the weldable coating film 
without sacrificing weldability, film opacity, and corrosion protection. 
Powdered iron also reduces the abrasiveness of the finished coating, does 
not cause the coating to emit high levels of toxic effluents and 
unpleasant odors during welding, sustains the life of the welding 
electrodes and the forming dies, and converts the coating into a 
composition that has welding characteristics similar to that for bare 
steel. The desired adhesion, flexibility, formability of the coating are, 
furthermore, not impaired using powdered iron. 
The preferred iron powder employed in this invention comprises finely 
divided iron particles which have shiny, silvery, uncorroded, smooth 
surfaces and are in the form of irregular spheroids, resembling ball 
bearings. Such irregular spheroids are traditionally produced by water jet 
atomization methods. It should be understood that the geometry of the iron 
powder varies significantly with their production method. 
Water jet atomization, in particular, involves the introduction of a stream 
molten iron which is poured from a ladle into an atomizing chamber wherein 
the stream is directed past one or more nozzles which direct pressurized 
jets of water to impinge against the down pouring molten metal stream. The 
stream is caused to split up into multiple droplets which rapidly cool and 
coagulate, forming solid particles of iron powder that fall to the bottom 
of the atomizing chamber while solidifying. The iron particles thus formed 
are then collected, preferably in water, and subsequently separated from 
most of the water by, for example, heated drying followed by magnetic 
separation. The particles are usually screened at this point to eliminate 
undesirably large particles that can be reworked. The dried particles are 
then collected and passed through an annealing furnace at about 
1,400.degree. F. in a reducing atmosphere of hydrogen, and the iron dust 
particles are finally collected in the form of smooth, shiny, irregular 
(i.e., spattered) spheroids. For a further description of water jet 
atomization techniques, reference can be made to U.S. Pat. Nos. 3,764,295; 
3,909,239; and, 4,274,864, which disclosures are incorporated by reference 
herein in their entireties. 
The preferred iron particle size is less than about 100 mesh (150 microns), 
and, more preferably, less than about 325 mesh (45 microns). Commercial 
powders which contain about 85 to 95% of the iron particles smaller than 
325 mesh and the remainder between 100 mesh and 325 mesh are most 
desirable. The apparent density of the iron powder is preferably between 
about 2.85 and 3.30 g/cc. Exemplary of such atomized iron powder is that 
sold under the trade name "Anchor ATW-230" by Hoeganaes of Riverton, N.J. 
Iron powders can also be produced by other traditional methods which 
produce spheroids, such as air atomization which gives irregular spheroids 
or dissociation of iron carbonyls which gives more uniform ultra fine 
spheroids. Methods which produce spongy iron particles, such as the direct 
reduction of iron ore or scale are generally discouraged in this 
invention, since it has been found that iron with a shiny surface are far 
superior to spongy iron particles. Weldable coatings containing spongy 
iron powder are not easily spot welded under standard conditions. 
Moreover, a spongy, pumice-like surface tends to darken the iron powder 
and consequently reduces the mar resistance of the coating films. 
In a preferred embodiment, the powdered iron comprises up to about 50 wt. % 
of total solids, and, more preferably, between about 30 and 40 wt. %. 
The weldable coating composition of this invention is further characterized 
in that no chemical reduction of the natural oxide films on the surface of 
the iron powder is necessary prior to incorporation in the liquid coating. 
Furthermore, the liquid coating is not subject to a magnetic field after 
incorporation of the powdered iron and, therefore, the non-magnetized iron 
particles remain randomly oriented in the liquid. 
The weldable coating composition of this invention is even further 
characterized as being essentially free of dark gray welding aids, such as 
ferrophos and nickel powders. 
SUSPENSION AGENT 
Desirably, a suspension agent is used to ensure that the powdered iron 
remains stably suspended in the liquid coating and does not settle out and 
form a hard cake. A suitable suspension agent is polyamide wax. Exemplary 
of such suspension agents are those sold under the trade name "Disparlon 
6900-20X" by King Industries of Norwalk, Conn., which are dispersions of 
swollen particles of polyamide wax in low boiling alcoholic solvents such 
as xylene. Other suitable suspension agents include silicon dioxide, for 
example, fumed silica, silane treated silica, phosphoric acid, alkylated 
or arylated phosphoric acid, and quaternary amine treated magnesium 
aluminum silicate. The suspension agents also serve as thixotropic agents 
to prevent gelation of the coating before application. Silicone dioxide 
additionally functions as a hygroscopic agent or water scavenger in the 
coating composition. 
In a preferred embodiment, the amount of suspension agent present in the 
coating composition is between about 0.3 and 2 wt. % of total solids, and 
preferably between about 0.4 and 0.6 wt. %. 
INTERNAL LUBRICANT 
An internal lubricant may be incorporated in the coating composition to 
lower the coefficient of friction of the coating film. 
Polytertrafluoroethylene (PTFE) is the preferred internal lubricant due to 
its ability to dramatically lower the coefficient of friction of the film 
finish, thus allowing metal forming and finishing without destroying the 
coating film. Other halogen-containing thermoplastic polymers can also be 
used, although PTFE has superior lubricant properties. Blends of PTFE and 
polyethylene (PE) are also useful. Other suitable internal lubricants, 
such as glycerol esters, fatty acids, fatty acid esters, fatty acid 
amides, fatty acid salts, fatty alcohols, and molybdenum disulfide, may be 
used as well. 
Desirably, the PTFE has particles in the size ranging from about 0.01 to 30 
microns, and, more preferably, from about 1 to 15 microns. Suitable PTFE 
is sold under the trade name "Polyfluo 190" by Micro Powders of Scarsdale, 
N.Y. 
The weldable coating composition preferably contains internal lubricants, 
desirably PTFE, in an amount ranging between about 0.2 and 1.5 wt. % of 
total solids, and, more preferably, between about 0.5 and 1 wt. %. 
PIGMENT 
The weldable coating composition of this invention may also contain light 
colored insulative pigment powders to further improve the mar resistance, 
crypt, and opacity of the coating film, as well as to provide the desired 
final appearance, yet without sacrificing the weldability of the coating. 
The choice of pigment will depend on the particular color or colors 
desired in the coating. The pigments may be organic or inorganic pigments, 
although inorganic pigments are generally utilized. Suitable inorganic 
pigments include metal oxides, especially titanium dioxide. Other metal 
oxides include, zinc oxide, aluminum oxide, magnesium oxide, iron oxide, 
chromium oxide, lead oxide, nickel oxide, silver oxide, tin oxide, and 
zirconium oxide. Other inorganic pigments which may be utilized include 
inorganic sulfides, sulfoselenides, ferocyanides, aluminates, phosphates, 
sulfates, borates, carbonates and especially titanates. 
The pigment can be present in the coating in reduced concentrations and 
still achieve the desired mar resistance and crypt. The ability to lower 
the concentration of non-weldable pigments dramatically improves the 
weldability of the coating, especially at the high dry film builds desired 
for adequate coverage, opacity, and corrosion protection. In this 
invention, mar resistant coating film builds as high as about 1.0 mil 
thick coated on each side of the metal surface remain weldable without 
resorting to special equipment and techniques. 
In a preferred embodiment, the insulative pigment comprises no greater than 
about 25 wt. % of total solids, and, more preferably, between about 10 and 
20 wt. %. 
The pigment to binder ratio is usually no greater than about 2, and, 
preferably, between about 1 and 1.5 
CORROSION INHIBITING AGENT 
The coating composition of this invention may also contain a corrosion 
inhibiting agent to enhance corrosion protection of the underlying metal 
substrate. In this invention, a corrosion inhibiting agent is optional, 
since the conductive powdered iron welding aid also serves as a 
sacrificial anode and thus provides cathodic protection against galvanic 
corrosion of the metal substrate. 
Suitable corrosion inhibiting agents include finely divided metals, such as 
powdered zinc spheroids or flakes. Typically the zinc powder is prepared 
through distillation of zinc dust or by air atomization of molten zinc. 
Zinc powder typically has a particle size ranging from about 1 to 15 
microns, preferably from about 2 to 6 microns. Zinc powder, in particular, 
improves the corrosion resistance of the coating, yet without 
significantly darkening the coating film. 
Other corrosion inhibitors can be employed which include anticorrosive 
insulative chromate pigments, such as strontium chromate. It is generally 
preferred, however, that the weldable coating composition of this 
invention is further characterized as being essentially free of 
anticorrosive pigments, including strontium chromate, calcium chromate, 
zinc chromate and lead chromate, since these pigments impair the mar 
resistance and crytability as well as weldability of the coating film. 
However, in certain circumstances they may be desirable. 
The corrosion inhibiting agent, desirably zinc powder, if employed, may 
comprise up to about 10 wt. % of total solids, and, more preferably, 
between about 3 and 10 wt. %, although it is most preferred not to employ 
the same. 
OTHER ADDITIVES 
In addition to the above-described components, the weldable coating 
composition of this invention can contain the usual functional additives 
that are well known in the art, such as, the flow control agents, for 
example, polyacrylic resins. Flow control agents are usually present in 
amounts up to about 1 wt. % of total solids, and, preferably, between 
about 0.06 and 0.5 wt. %. The polyacrylic resins generally include 
methyl(meth)acrylate resins, ethylene vinyl acetate resins, and the like. 
Other thixotropic agents, light stabilizers, surfactants, wetting agents, 
dispersing aids, flattening agents, antioxidants, flocculating agents, 
foam control agents, etc., can also be employed. Inorganic fillers, such 
as calcium carbonate, may also be included in the coating. Adhesion 
promoters are usually present as well in amounts up to about 0.5 wt. % of 
total solids, and, preferably, between about 0.01 and 0.1 wt. %. One 
suitable class of adhesion promoters are the epoxy phosphate esters, which 
are generally prepared by reacting an epoxy resin with phosphoric acid. 
Phosphoric acid may also be considered an adhesion promoter. 
SOLVENT 
The aforesaid components of the coating composition are blended together in 
a suitable vehicle or carrier for the solids, such as an aqueous or 
organic solvent, to facilitate formulation and liquid application. 
Suitable organic solvents include aromatic and aliphatic petroleum 
distillates, such as Aromatic 100, Aromatic 150, Aromatic 200, dibasic 
esters (DBE), V M & P naptha, hexane, and the like, ketones, such as 
isophorone, methyl ethyl ketone, methyl isobutyl ketone, methyl isoamyl 
ketone, diisobutyl ketone, acetone, and the like, alcohols, such as ethyl 
alcohol, propyl alcohol, diacetone alcohol, 2-ethyl hexanol, n-butanol, 
and the like, dimethyl, phthalate, and mono- and dialkyl ethers of 
ethylene and diethylene glycol, such as ethylene glycol monoethyl ether, 
ethylene glycol monobutyl ether, ethylene glycol monoethyl ether acetate, 
propylene glycol monoethyl ether acetate, diethylene glycol monobutyl 
ether and diethylene glycol diethyl ether, xylene, and the like. Dibasic 
esters are especially useful solvents, which are typically available as 
mixtures of refined dimethyl esters of adipic, glutaric and succinic 
acids. 
The weldable coating composition typically contains sufficient solvent to 
produce the desired viscosity for the particular liquid coating method. 
In a preferred embodiment, the viscosity of the uncured liquid coating 
ranges preferably between about 20 and 50 #4 Zahn at 25.degree. C., and, 
more preferably, between about 28 and 32. 
COATING PREATION 
The constituents of the weldable coating composition are blended together 
in any convenient manner known to persons skilled in the art. Moreover, no 
chemical reduction of the powdered iron is necessary before incorporation 
into the liquid coating. Also, the liquid coating is not subject to a 
magnetic field after incorporation of the powdered iron. 
METAL SUBSTRATES 
The weldable coating composition is usually applied as a primer over a 
variety of metal substrates. In some cases, the primer may also serve as 
the final finish. Metal substrates of current interest include zinc-, 
zinc/nickel alloy, and zinc-iron alloy steels, which include various 
zinc-containing forms of galvanized steel, steel having a chrome 
conversion coating (with or without zinc therein) on its galvanized or 
ungalvanized surface, steel having a zinc-rich primer on either of such 
surfaces, and steel having FIRST COAT.RTM. aqueous epoxy resin/chromium 
trioxide coating pretreatment (as described in previously mentioned U.S. 
Pat. No. 5,001,173), that is commercially available from Morton 
International of Chicago, Ill., baked on either of such surfaces. Other 
suitable metal substrates include cold rolled and hot rolled steel, 
aluminized steel, and galvanized steel such as hot-dipped and 
electrogalvanized steel, galvalume, galvanneal, etc. 
COATING ON METAL 
The liquid weldable coating can be applied to the metal substrate by any 
conventional technique, including, for example, dipping, spraying, roll 
coating, and bar coating. It is preferred to use coil coating (reverse 
roll coating) techniques to apply the coating. 
After application, the weldable coating is heat cured and dried in an oven 
to a hardened cured film finish on the substrate surface. The wet coating 
is usually cured at elevated temperatures of between about 390 and 
500.degree. F. peak metal temperature, and, preferably, between about 435 
and 485.degree. F., for a suitable period of time to fully cure the 
coating, usually between about 20 and 60 seconds, and, preferably, about 
40 seconds. 
The weldable coating is generally applied on the substrate in sufficient 
amounts to provide a dry film coating having a thickness of up to about 
1.0 mil on each surface, and usually between about 0.4 and 0.6 mils. It 
has been advantageously found that two pieces of metal can be coated on 
both sides with about 1.0 mil of coating film and still be welded together 
using a weld cycle similar to that for bare steel. 
WELDING PRECOATED METAL 
After the coating is cured, the coated metal substrates can be welded 
together by any standard welding technique such as electric resistance 
(spot) welding, mig welding, tig welding, and arc welding. Spot welding, 
in particular, involves placing together two pieces of the precoated metal 
articles to form an assembly and then inserting the assembly between two 
copper-tipped electrodes of a spot welder. When the welder is turned on, 
an initial squeeze cycle is performed, wherein the two coated steel plates 
are further forced together between the welding electrodes. A subsequent 
weld cycle is performed where sufficient current flows through the 
assembly including the coating, and finally a hold and off cycle is 
completed before the welding electrodes are released and the welded 
assembly is removed from the machine. The formation of a nugget between 
the welded parts represents an excellent weld. 
Any of the standard non-weldable topcoats may be applied to the precoated 
metal surfaces after welding for a decorative appearance or enhanced 
corrosion protection.

The invention will be further clarified by a consideration of the following 
non-limiting examples. 
EXAMPLE 1 
Mar Resistant Weldable Urethane Primer 
The following ingredients were blended together in the order and manner 
given to provide a solvent borne weldable liquid primer of this invention. 
______________________________________ 
Ingredients Weight % 
______________________________________ 
CHARGE TO DISPERSING MILL 
30% Dynapol L-205 Polyester Solution.sup.1 
9.62 
TiPure R-900 (TiO.sub.2) Pigment.sup.2 8.42 
AeroSil 200 Fumed Silica.sup.3 0.10 
Mixed Dibasic Esters (DBE).sup.4 2.53 
Panasol AN-3N Solvent.sup.5 2.05 
Coroc A-620-A2 Acrylic Resin Solution.sup.6 0.17 
SANDMILL TO 7 HEGMAN GRIND 
RINSE SANDMILL 
30% Dynapol L-205 Polyester Solution 
0.82 
Mixed DBE 0.82 
RECHARGE TO DISPERSING MILL 
THEN ADD UNDER MEDIUM AGITATION 
30% Dynapol L-205 Polyester Solution 
23.33 
40% Mor-Ester 49001 Polyester Solution.sup.7 4.15 
55% Mor-Ester 4120 Polyester Solution.sup.8 18.26 
Mixed DBE 3.48 
Epon 828 Epoxy Resin.sup.9 0.84 
Desmodur BL 3175 Blocked Isocyanate.sup.10 1.08 
Resimene 747 Aminoplast Resin.sup.11 1.29 
Nacure 1051 (DNNSA) Catalyst.sup.12 0.22 
10% Phosphoric Acid Solution.sup.13 0.50 
Metacure T-12 (Dibutyl Tin Dilaurate).sup.14 0.10 
Anchor ATW-230 Iron Powder.sup.15 20.00 
Disparlon 6900-20X Suspension Agent.sup.16 0.34 
ADJUST VISCOSITY 
Mixed DBE 1.88 
Total Weight 100.00 
______________________________________ 
.sup.1 30% Dynapol L205 Polyester Solution is a solvent solution of 30% 
Dynapol L205 polyester resin of about 15,000 molecular weight and about 
5-10 hydroxyl number believed to be derived from isophthalic acid, 
terphthalic acid, ethylene glycol, and neopentyl glycol, and that is 
commercially available from Huls of Somerset, NJ, in DBE. 
.sup.2 TiPure R900 is a TiO.sub.2 pigment that is commercially available 
from DuPont of Wilmington, DE. 
.sup.3 AeroSil 200 is fumed silica that is commercially available Degussa 
of Ridgefield Park, NJ. 
.sup.4 Mixed Dibasic Esters (DBE) is a commercial mixture of dibasic 
esters that is commercially available from DuPont of Wilmington, DE. 
.sup.5 Panasol AN3N is a S100 Aromatic solvent that is commercially 
available from Ashland Chemical of Columbus, OH. 
.sup.6 Coroc A620-A2 acrylic resin solution is an acrylic flow modifier 
that is commercially available from Cook Paint & Varnish of Kansas City, 
MO. 
.sup.7 40% MorEster 49001 Polyester Solution is a solvent solution of 40% 
MorEster 49001 polyester resin of about 35,000 molecular weight and about 
9 hydroxyl number derived from terephthalic acid, isophthalic acid, 
azelaic acid and ethylene glycol, and that is available from Morton 
International of Chicago, IL, in MEK. 
.sup.8 55% MorEster 4120 Polyester Solution is a solvent solution of 55% 
MorEster 4120 polyester resin of about 13,000 molecular weight and about 
20-28 hydroxyl number derived from isophthalic acid, terephthalic acid, 
hexane diol and neopentyl glycol, and that is available from Morton 
International of Chicago, IL, in xylene. 
.sup.9 Epon 828 Epoxy Resin is a bisphenol A/epichlorohydrin based epoxy 
of about 350-450 molecular weight and about 175-210 epoxide equivalent 
weight, and that is commercially available from Shell Chemical Company of 
Houston, TX 
.sup.10 Desmodur BL 3175 is a blocked isocyanate crosslinker resin of 
methyl ethyl ketoxime blocked 1,6hexamethylene diisocyanate that is 
commercially available from Bayer of Pittsburgh, PA. 
.sup.11 Resimene 747 is an aminoplast crosslinker resin of 
hexamethoxymethyl melamine that is commercially available from Monsanto o 
St. Louis, MO. 
.sup.12 Nacure 1051 is a sulfonic acid catalyst of dinonylnaphthalene 
sulfonic acid (DNNSA) that is commercially available from King Industries 
of Norwalk, CT. 
.sup.13 10% Phosphoric Acid Solution is a solution of 10% phosphoric acid 
catalyst in isophorone. 
.sup.14 Metacure T12 is a dibutyltin dilauarte catalyst that is 
commercially available from Air Products of Allentown, PA. 
.sup.15 Anchor ATW320 is atomized iron powder that contains about 95% of 
the iron particles finer than 325 mesh and the remainder between about 10 
and 325 mesh, that is commercially available from Hoeganaes of Riverton, 
NJ. 
.sup.16 Disparlon 690020X is a suspension agent of a dispersion of swolle 
particles of polyamide wax in xylene that is commercially available from 
King Industries of Norwalk, CT. 
Two cold rolled steel panels were individually coated on each side with the 
foregoing liquid weldable primer and then baked at about 450.degree. F. 
peak metal temperature for about 45 seconds to yield a cured dry white 
coating film of about 1.0 mils thick on each side of the two panels. 
The weldabilty of the coating film deposited on the cold rolled steel 
panels was determined by attempting to spot weld the two coated panels 
together. The coated panels were successfully spot welded together between 
copper tipped 1/4" electrodes using a weld cycle similar to that for bare 
steel. 
The corrosion resistance characteristics of the coating film deposited on 
the cold rolled steel panels was determined by subjecting the coated 
panels to a salt water spray test under test method ASTM B-117. Despite 
the absence of anticorrosive chromate pigments in the primer composition, 
the corrosion resistance of the coating film was similar to that for 
chromated primer systems at 240 hours salt spray. The performance of the 
weldable primer at 580 hours salt spray was significantly worse as would 
be expected without the protection of chromates. Yet, the improvement in 
mar resistance and weldability are issues that cannot be obtained with 
standard chromated primer systems. 
EXAMPLE 2 
Mar Resistant, Internally Lubricated, Weldable Urethane Primer 
The following ingredients were blended together in the order and manner 
given to provide another solvent borne weldable liquid primer of this 
invention. 
______________________________________ 
Ingredients Weight % 
______________________________________ 
CHARGE TO DISPERSING MILL 
30% Dynapol L-205 Polyester Solution 
10.36 
Mixed Dibasic Esters (DBE) 2.47 
Panasol AN-3N Solvent 2.00 
TiPure R-900 (TiO.sub.2) Pigment 8.21 
11-3071 Fast Yellow HGR Pigment.sup.1 0.49 
Cab-O-Sil M-5 Fluffy Fumed Silica.sup.2 0.20 
Coroc A-620-A2 Acrylic Resin Solution 0.17 
SANDMILL TO 7 HEGMAN GRIND 
RINSE SANDMILL 
30% Dynapol L-205 Polyester Solution 
0.98 
Mixed DBE 0.98 
RECHARGE TO DISPERSING MELL 
THEN ADD UNDER MEDIUM AGITATION 
30% Dynapol L-205 Polyester Solution 
21.61 
Disparion 6900-20X Suspension Agent 0.49 
MIX WELL THEN ADD UNDER MEDIUM AGITATION 
40% Mor-Ester 49001 Polyester Solution 
4.05 
55% Mor-Ester 4120 Polyester Solution 17.81 
Mixed DBE 4.88 
Epon 828 Epoxy Resin 0.82 
Desmodur BL 3175 Blocked Isocyanate 1.05 
Resimene 747 Aminoplast Resin 1.25 
10% Phosphoric Acid Solution 0.49 
Nacure 1051 (DNNSA) Catalyst 0.22 
Metacure T-12 (Dibutyl Tin Dilaurate) 0.10 
Polyfluo 190.sup.3 0.50 
Anchor ATW-230 Iron Powder 19.51 
ADJUST VISCOSITY 
Mixed DBE 1.36 
Total Weight 100.00 
______________________________________ 
.sup.1 113071 Fast Yellow HGR is C.I. Pigment Yellow 191 (inorganic 
titanate) that is commercially available from Hoechst Celanese of 
Charlotte, NC. 
.sup.2 CabO-Sil M5 is fumed silica that is commercially available from 
Cabot Corporation of Bellerica, MA. 
.sup.3 Polyfluo 190 is an internal lubricant of PTFE that is commercially 
available from Micro Powders of Scarsdale, NY. 
Two cold rolled steel panels were individually coated on each side with the 
foregoing liquid weldable primer and then baked at about 450.degree. F. 
peak metal temperature for about 45 seconds to yield a cured dry putty 
yellow coating film of about 1.0 mils thick on each side of the two 
panels. 
The weldability of the coating film deposited on the cold rolled steel 
panels was determined by attempting to spot weld the two coated panels 
together. The coated panels were successfully spot welded together between 
copper tipped 1/4" electrodes using a weld cycle similar to that for bare 
steel. 
The invention having been disclosed in the foregoing embodiments and 
examples, other embodiments of the invention will be apparent to persons 
skilled in the art. The invention is not intended to be limited to the 
embodiments and examples, which are considered to be exemplary only. 
Accordingly, reference should be made to the appended claims to assess the 
true spirit and scope of the invention, in which exclusive rights are 
claimed.