FIELD OF THE INVENTION
This invention relates to a self-priming, air drying protective coating based on a vinyl
terpolymer of polyvinyl formal, polyvinyl alcohol and polyvinyl acetate, an alkyd resin,
and alkoxylated amino resins. The coating requires no chemical pre-treatment or primer
when applied on new as well as old pre-rusted substrates. It has good barrier properties,
chemical and thermal resistance, flexibility and hardness and is especially useful to
protect new as well as old pre-erected hot rolled steel structures exposed to
environmental degradation, without elaborate surface preparation. Specifically, on new
hot rolled steel structures no preparation other than oil/grease and moisture removal is
required while existing structures require the removal of loose scales and any other
surface contaminant No sand/shot blasting is required as the coating adheres to and
protects the oxidized metal surface even under severe, environmentally degrading
conditions. The coating may also be applied on existing structures that have tightly
adhering portions of alkyd or epoxy paint persisting after wire brushing. The coating
withstands sustained exposure to a wide temperature range (-50°C to 150°C) and is flame
retardant.

BACKGROUND OF THE INVENTION
World wide attention is focused on corrosion protection of metal structures. Hot rolled
steel structures remain the most important subject of study in this regard. While new
structures may be protected in a variety of ways, pre-erected structures pose problems
due to the extent of surface preparation often required by protective coatings/paints.
Adherence of coatings to pre-corroded steel surface and to remnants of the existing
coating remaining after surface preparation by mechanical cleaning or application of
paint remover, is far from satisfactory and is the cause of interfacial corrosion and
coating detachment from the metal surface. Hence sand/grit blasting is prescribed. This is
seldom practical in many situations and is polluting and expensive. New hot roiled steel
structures are routinely blasted and a primer is applied immediately after, to prevent
rusting before the application of protective coatings. Self priming coatings have been
developed for metal structures (U.S.Pat.Nos.5,539,032, 5,427,821, 5,403,880, 5,236,983,
5,491,185,5,202,367, 5,130,361, 5,274,021) but are not applicable to hot rolled steel with
mill scale, or pre-rusted steel with residual paint that is to be protected without blasting or
extensive surface preparation. Further, high performance protective coatings, especially
polyurethanes and epoxies normally exhibit poor recoatability.
Self priming chromate free rapid curing coating for ferrous and non-ferrous substrates
incorporating a specially structured vinyl terpolymer with randomly distributed hydroxyl,
acetyl and formal groups and a variety of co-resins have been developed (US Patent Nos.

7,205,353 and 7,208,537 of which the former corresponds to Indian Patent No.212698)
that exhibit good corrosion resistance in thin films (5-30 micrometers DFI) on cold
rolled, hot dip galvanized, and other types of coated steel. However, these coatings are
cured thermally preferably in extremely short cure duration of coil coating lines (60
seconds or less). Moreover, coated substrates are not easily recoatable during service.
Polyvinyl formal has previously been used as a thermoplastic resin in a variety of
insulating compositions where it imparts flexibility and toughness to the coatings. It has
also been used to coat cellulosic substrates such as fabric and wood. However in these
applications the polymer is used as a thermoplastic non-reactive ingredient with other
thermosetting resins in applications not related to long term corrosion protection of metal
structures.
In-plant preservative coating of new hot rolled steel structural members as manufactured
has been proposed for transit and storage protection, in a copending Indian patent
application no. 25/KOL/2007 by the same applicant in which the pre-coated hot rolled
steel need not be blasted during fabrication or erection. However, protection of existing,
pre-erected structures still remains a problem due to reasons cited earlier. Additionally,
chromates are still being used in some form, either as etch/wash primers or metal
conditioners or as part of the primer- intermediate coat- top coat system. Thus there was
long felt need for a protective coating that can perform on ill prepared surfaces, can be
recoated without resorting to complete paint removal during maintenance, and be free
from hexavalent chromium and provide for a cost effective, environment friendly and
practical corrosion protection practice.

The inventor of the present invention has found that a specially structured linear vinyl
terpolymer with randomly distributed acetyl, hydroxyl and formal groups and with
reactive pendant hydroxyls forms a lightly cross linked matrix with suitable co-resins in
presence of o-phosphoric acid catalyst and adheres tenaciously to a variety of metal
oxides that form part of a metal surface. This characteristic of the polymer has been used
to suggest pre-coating of freshly prepared black hot rolled steel containing a layer of mill
scale on its surface to protect the substrate from rust during transit and storage as
mentioned earlier which is the subject matter of copending Indian patent application
no.25/KOL/2007. It has now been found, rather unexpectedly, that the polymer in
question can be incorporated with a short to medium oil alkyd resin, alkoxylated amino
resins, extenders such as mica, zinc phosphate and talc and o-phosphoric acid catalyst to
form an air drying coating-for long lasting steel protection in hostile weather conditions,
that interacts strongly with an oxidized metal substrate including freshly rolled metal as
well as largely pre-rusted steel and imparts excellent barrier characteristics to the coating-
metal interface. The DFT (Dry Film Thickness) of this coating should be a minimum of
100 micrometers for protection of structural steel. The cured matrix is lightly cross linked
upon cure and provides a good balance of thermo-mechanical and dielectric properties
apart from flame retardance. Most importantly, the oxide layer present on most metals,
whether the oxides of freshly rolled metal or the rust of existing pre-erected structures,
becomes a part of this interface and therefore need not be removed. Thus mill scale on
hot rolled steel need not be removed by pickling or blasting and more significantly pre-
erected rusted structures also need not be sand/shot blasted prior to coating application.

Further this coating eliminates the requirement of etch/wash primers and tie coats used on
hot rolled and metalized steel. The composition includes a suitable metallic dryer for the
alkyd resin such as cobalt, manganese, zinc, naphthenate or octoate used singly or in
combination and p-toluene sulphonic acid to accelerate the drying/curing process. The
composition uses a mix of o-xylene and n-butanol in the weight ratio of 60:40 to dissolve
the resin polymer binder, and may be pigmented with a variety of inorganic pigments and
organic dyes. Excellent adhesion to mill scale covered hot rolled steel, pre-rusted steel
freed from loose scale, and surface painted with either itself or alkyd and epoxy paints is
obtained. Whenever extra weatherability and/or chemical resistance is desired in the
coating, especially in industrial and/or marine applications, a top coat of a thermosetting
acrylic or polyurethane paint may be recommended. The coating therefore addresses a
critical need in the protection of new as well as existing pre-erected ferrous structures
exposed to environmental attack.
GB 782393 discloses a similar composition but the difference arises from the specific
vinyl terpolymer chosen in the present composition which allows significant cross linking
with co-resins through the reactive hydroxyls. Moreover the reactive hydroxyls interact
with the oxides of steel substrate in the presence of phosphoric acid. In other words the
vinyl polymer used in '393 is a non reactive thermoplastic ingredient whereas here it is a
reactive thermosetting species that shows an unusual and unique interaction with metal
oxides in the presence of phosphoric acid.

OBJECTS OF THE INVENTION
The first object of the invention is to provide an air drying chromate free self-priming
coating with good barrier characteristics.
The second object of the invention is to provide a corrosion resistant coating on hot rolled
steel with minimum surface preparation for both new as well as old pre-rusted structures.
The third object of the invention is to provide a protective coating with good
recoatability.
The fourth object of the invention of the coating is to provide corrosion protection to steel
over a wide range of service temperature (-50°C to 150°C) and flame retardance.
The fifth object of the invention of the coating is to provide good mechanical properties
such as flexibility and hardness.
SUMMARY OF THE INVENTION
The invention in one embodiment provides a self-priming air drying chromate free
corrosion resistant coating composition comprising:
a) a linear vinyl terpolyrner made up pre-dominantly of polyvinyl formal with
polyvinyl alcohol and polyvinyl acetate as the two other co-polymers having
randomly distributed hydroxyl, acetyl and formal groups along the vinyl
backbone offering cross linking sites through the hydroxyl groups during ambient
curing;

b) an short to medium oil alkyd resin with hydroxyl number between 80 and 200
measured as mg. potassium hydroxide per gram of resin;
c) an alkoxylated urea formaldehyde resin
d) an alkoxylated melamine formaldehyde resin
e) ortho-phosphoric acid with other suitable acid(s) catalyst for accelerating ambient
drying/ cure
f) metal dryers for the alkyd resin;
g) ortho-xylene and normal butanol in a weight ratio of 60:40;
h) chromate free extenders such as talc, mica and zinc phosphate;
The preferred dryer used in this composition is cobalt naphthenate.
The preferred acid catalyst used with o-phosphoric acid, is p-toluene sulphonic acid.
The invention in another embodiment provides a two-pack composition comprising : the
ingredients (a), (b), (e), (f), (g) and (h) in the first part and the ingredients (c), (d) and (g)
in the second part which are mixed in a predefined weight ratio before use.
The invention in yet another embodiment provides a method of coating new mill scale
covered hot rolled steel as well as old pre-rusted steel structures.
DETAILED DESCRIPTION OF THE INVENTION
A linear vinyl terpolymer with randomly distributed pendant groups of hydroxyl, acetyl
and formal offering reactive condensation sites through the hydroxyl groups was chosen.
The terpolymer and its method of synthesis, have been described in US patents

7,205,353 (Indian patent no.2]2698) and 7,208,537 which are incorporated herein as a
reference. This terpolymer is used in combination with a short to medium oil alkyd resin
with a hydroxyl number between 80 and 200, alkoxy amino resins and o-phosphoric acid
as catalyst. The alkyd appropriate for this invention has also been described in US Patent
7,208,537 which is incorporated herein as a reference. The alkyd resin is produced from
linseed oil, glycerine, pentaerythritol, isophthalic acid, terephthalic acid. Cobalt
naphthenate dryer was used for the alkyd resin. Ortho xylene and normal butanol in a
60:40 weight ratio were used as the solvent. Talc, mica and zinc phosphate were used as
extenders. P-toluene sulphonic acid catalyst was used to accelerate cure.
Weight percentage of polyvinyl alcohol, polyvinyl acetate and polyvinyl formal in the
terpolymer are 6%-15%, 9%-15%, and 70-84%, respectively and more preferably 6%-
7 5%, 10%- 13%, and 80- 83% respectively. The weight average molecular weight ranges
between 20,000 and 120,000 and preferably between 25,000 and 70,000.
The alkyd resin has a hydroxyl number between 80 and 200 and preferably between 120
and 140 with a solid content of 60-75%.
The alkoxylated amino resins used are a combination of a partially isobutylated urea
formaldehyde resin supplied as CYMEL U-662 resin by Cytec Corporation, USA
containing 60% solids in a mix of xylene and isobutanol and a methylated melamine
formaldehyde resin supplied as CYMEL 303 resin by Cytec Corporation, USA
containing approximately 100% solids.
The terpolymer and alkyd resin is dissolved in the solvent mix to obtain a concentrated
solution which is used to make a mill base containing the pigment and extenders, in a
high speed attritor or a sand mill. O-Phosphoric acid and p-toluene sulphonic acid are

added to this mill base which is then diluted by adding more of the solvent mix to obtain
a sprayable/brushable consistency. The other part of the two component system
comprises the blend of the amino resins in the solvent mix. The two components are
required to be mixed together prior to application and the final wet product is stable for at
least 8 hours at 30°C. The product is normally formulated with a solid content of 40 to
45% by weight. The coating may be applied on a variety of steel substrates such as hot
rolled steel, cold rolled steel, galvanized and other coated steels without any chemical
pre-treatment except degreasing with xylene. For hot rolled steel protection a minimum
wet film thickness of 300 micrometers so as to obtain a dry film thickness of 100 microns
or more is recommended.
The coating provides excellent corrosion resistance to steel, both hot and cold rolled,
shows good scratch hardness, thermal and chemical resistance and flexibility. The dry
coating may also be top coated if required with high performance top coats such as
acrylic or polyurethane paints for extra chemical resistance or improved weatherability.
The following example of composition is illustrative of the invention and is not intended
to limit the scope of the invention as defined by the appended claims.
EXAMPLE 1
A pigmented coating composition obtained on mixing parts 1 and 2 as described earlier
contains the following ingredients in the assigned range of weight proportions to obtain
solids between 40 to 45% by weight

After preparing the two components, PART 1 and 2 of the coating individually as
described earlier, 100 parts of PART 1 was mixed with 8 parts of PART 2 to obtain the
final coating composition which contains approximately 45% solids by weight. The
individual components are stable for at least one year at 30°C. The viscosity of the final
mixed composition is 100+20 seconds measured by Ford Cup B4 at 30°C, and it is stable
and usable for at least 8 hours at 30°C.
Cold rolled steel panels were cleaned with xylene to remove oil and moisture. Hot rolled
steel panels were used a) as such retaining the mill scale and b) after abrading the surface
to remove the mill scale. Both sets of hot rolled steel panels were cleaned with xylene
prior to coating application. No chemical pre-treatment was done on any of these
substrates. Coating was applied by spray on all the panels. The DFT of coating on cold
rolled steel panels used for all tests except salt fog corrosion test was approximately 100
micrometers while that for salt fog corrosion test (ASTM Bl 17) was approximately 150
micrometers. DFT of coating on hot rolled steel panels subjected to all tests was
approximately 150 micrometers and the panels were further top coated with an air drying
thermosetting acrylic paint to obtain a total DFT of 200 micrometers. All panels were
force dried at 70°C for one hour. This curing procedure has been found to be equivalent
to 14 days of ambient cure at 30°C for this composition. The tests conducted on the three
kinds of substrates ie cold rolled steel, abraded hot rolled steel, and mill scale covered hot
rolled steel, and results obtained are summarized below.

TESTS AND RESULTS
a) Drying Time, maximum
i) tack free: 4 hours at 30°C
ii) hard dry: 8 hours at 30°C
iii) full cure: 14 days at 30°C
b) Adhesion: All three types of coated panels showed excellent adhesion passing ASTM
D3359 with classification 5B.
c) Hardness: Metal scratch hardness wherein a sharp steel needle loaded with a specified
weight is scratched on the coated surface and the minimum weight required to expose the
metal underneath the coating is recorded as scratch hardness. All types of coated panels
exhibited scratch hardness above 4Kg.
d) Flexibility: Coated cold rolled steel panels were bent upon a mandrel of size 6mm and
the coating examined around the bend. No visual damage or detachment of the coating
was observed.
e) Impact Resistance: Coated cold rolled steel panels passed a direct impact resistance
load of 75Kgcm, without any cracking or disbondment, when tested as per ASTM D-
2794.

f) Thermal Resistance: ASTM D 2485-91 Test Method A was followed and the panels
exposed to 150°C. No dulling, blistering, cracking, loss of adhesion or flexibility was
observed. Similar tests carried out at -50°C yielded identical results.
g) Recoatability: Some coated panels were exposed to normally prevailing weather
conditions for 6 months and coated thereafter with the same coating after light dry
sanding and cleaning the surface with a dry cloth. All such panels showed good adhesion
passing ASTM D3359 with classification 5B.
h) Chemical Resistance: Coated cold rolled steel panels were tested by a chemical soak
test in accordance with ASTM D1308 (modified version incorporated in ASTM D3730)
for 4 hours, with 5% aqueous solution of i) acetic acid, ii) sulfuric acid, iii) hydrochloric
acid, iv) sodium hypochlorite, v) sodium carbonate and vi) sodium hydroxide. All the
tests were satisfactory except sodium hydroxide where softening of the coating was
observed.
i) Corrosion Resistance (salt fog): Scribed panels of all three types were exposed to
neutral salt fog as per ASTM B117. Cold rolled steel panels showed no corrosion on the
coated face as well as the stripped metal face and less than 2mm creep at the scribe after
1000 hours. Hot rolled steel panels showed no corrosion on coated face as well as the
stripped metal face and no creep at the scribe after 1000 hours. No swelling or blistering
of coating was observed except in the case of abraded hot rolled steel panels where slight
swelling at some spots was observed.

j) Corrosion Resistance (prohesion): Prohesion tests as per ASTM G 85-94 A5 using
aqueous solution of 0.05% sodium chloride and 0.35% sodium sulfate (1 hour fog
exposure and 1 hour dry) on coated cold rolled steel and hot rolled steel panels for 1000
hours revealed no rusting on coated face nor on the stripped metal. No blistering or
swelling was observed on the cold rolled steel and the mill scale covered hot rolled steel
panels, however, some small swollen spots were observed on the abraded steel panels.
k) Corrosion Resistance (cyclic corrosion): Coated cold rolled steel panels were subjected
to cyclic humidity tests for 1000 hours as per ASTM G60-86 wherein dip aqueous
solution of 1% sodium chloride, 1% calcium chloride and 0.1% sulfuric acid was used.
The dip time was 5 minutes and the solution and chamber temperature was 52±2°C. No
blistering or rusting was observed though some discoloration of the coating was seen.
1) Electrochemical impedance spectroscopy: This test was done only on the coated hot
rolled steel panels to investigate performance of coating over mill scale and the tested
panels were top coated with a thermosetting acrylic paint of 50 micrometer DFT. Tests
were designed to simulate marine exposure. No detectable change in polarization
resistance was observed after 4350 hours of exposure to salt solution containing 3.5%
sodium chloride, and the value was maintained in the range of 1 to 2 x2010 ohmcm2 for
mill scale covered panels during the period of exposure. Consistently higher values were
obtained for this parameter on mill scale covered panels compared to the abraded panels
(1 to 4 xl09ohmcm2). At the conclusion of the test, loss of adherence of the coating on

abraded panels was observed while coating adherence was maintained in the case of the
mill scale covered panels. Cyclic D.C. Polarization tests in 3.5% aqueous solution of
sodium chloride for 4350 hours also revealed some hysteresis in loop indicating pitting
tendency on the abraded steel panels while no loop was seen on the mill scale covered
steel panels showing absence of pitting.
m) A largely rusted cold rolled steel showing more than 50% surface rust was coated to
achieve a DFT of 250-270 micrometers and exposed to neutral salt fog for 1500 hours.
On conclusion of the test it was noted that the coated face showed no rusting and the
coating maintained its adherence throughout the period of exposure. The stripped metal
face also revealed no growth of rusting whatsoever.
n) Flame Retardancy: Here general guidelines of ASTM D1360-98 for fire retardancy of
paints were followed using coated wood substrate. A cylindrical cup as specified was
filled with 5 ml of pure absolute ethanol. The same was ignited and the coated face
exposed to the flame as described in the test. The flame did not burn or char the coating
during duration of the test. However, weight loss examination was not conducted. This
constitutes a qualitative test for fire retardancy.

We Claim:
1. A self priming, air drying coating composition comprising:
a) a linear vinyl terpolymer having three randomly distributed pendant functional
groups comprising acetyl, formal and hydroxyl along the vinyl backbone, offering
cross linking sites through the hydroxyl groups during cure;
b) a short to medium oil alkyd resin with hydroxyl number between 80 and 200
measured as mg. potassium hydroxide per gram of resin;
c) a suitable metal dryer for the alkyd resin;
d) alkoxy amino resins used singly or in combination;
e) ortho-phosphoric acid;
f) mix of ortho-xylene and normal-butanol; and,
g) chromate free extenders and pigments.
2. The composition as claimed in claim 1, wherein the percentages by weight of
polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl
terpolymer are 6-15%, 9-15% and 70-84%, respectively.
3. The composition as claimed in claim 2, wherein the percentages by weight of
polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl terpolymer
are 6-7.5%, 10-13% and 80-83%, respectively.
4. The composition as claimed in claim 1, wherein the weight average molecular
weight of the vinyl terpolymer is between 20,000 and 120,000.

5. The composition as claimed in claim 4, wherein the weight average molecular
weight of the vinyl terpolymer is between 25000 and 70,000.
6. The composition as claimed in claim 1, wherein the hydroxy] number of the alkyd
resin is between 120 and 140.
7. The composition as claimed in claim 1, wherein the alkyd resin is produced from
linseed oil, glycerine, pentaerythritol, isophthalic acid, terephthalic acid and
cobalt naphthenate drier in xylene solvent.
8. The composition as claimed in claim 1, wherein the solid percentage of the alkyd
resin is 60-75%.
9. The compostion as claimed in claim 1, wherein the alkoxy amino resins comprise
a combination of partially butylated urea formaldehyde resin and methylated
melamine formaldehyde resin.
10. The composition as claimed in claim 1, wherein the weight ratio of o-xylene and
n-butanol is 60:40.
11. The composition as claimed in claim 1, wherein mica, talc and zinc phosphate are
used as extenders and red oxide is used as pigment.
12. The composition as claimed in claim 1, wherein the solid content: by weight is 40-
45%.
13. The composition as claimed in claim 1, wherein the ingredients are present in the
following weight proportions in the final coating:

15. The composition as claimed in claims 13 and 14, wherein it is formulated as a two
component liquid coating comprising a) resin polymer solution in o-xylene and n-
butanol solvent mix in 60:40 weight ratio, of the vinyl terpolymer, alkyd resin
with drier, o-phosphoric acid, extenders and pigments, and b) the combination of

butylated urea and methylated melamine resins dissolved in the same organic
solvent mix.
16. A method of coating a metal substrate wherein the coating composition as
claimed in claiml5 is applied to the metal substrates after mixing parts a) and b)
and using the liquid mix within 8 hours.
17. A method as claimed in claim 16, wherein the liquid coating obtained by mixing
parts (a) and (b) is applied, by brush or spray, without any chemical pre-treatment
or primer, on metal substrates cleaned with a wire brush and a volatile organic
solvent to remove surface contaminants including loose scale if any, oil or grease,
and moisture, and curing the coating by cured air drying or forced drying for one
hour at 70°C, to obtain a coating DFT of 100-250 micrometers.
18. A method of coating a hot rolled steel, either fresh with mill scale or pre-rusted
comprising the liquid coating obtained by mixing parts (a) and (b) as claimed in
claim 16, by brush or spray, without blasting the substrate, and curing the coating
by air drying or forced drying for one hour at 70°C, to obtain a coating DFT of
100-250 micrometers.
19. The method as claimed in claims 17 and 18, wherein a top coat of a highly
chemically resistant or a special weatherable coating/paint, preferably a
thermosetting acrylic, is applied on this coating after it is dry and cured.
20. An article comprising:

a) a ferrous metallic substrate; and
b) a coating applied to at least one surface of the substrate by a method as claimed in
claim 17.

c) a coated substrate with at least one surface coated with the coating as claimed in
claim 17 and a top coat of a thermosetting acrylic paint as claimed in claim 19
21. The article as claimed in claim 20, wherein the substrate is preferably hot rolled
steel and the method of application, is as claimed in claims 18 and 19.

The invention discloses a corrosion resistant self-priming chromate free air drying
coating composition and method of its application. The coating uses a binder and film
former thermosetting resin solution comprising a polyvinyl terpolymer containing acetyl,
hydroxyl, and formal groups randomly distributed along a vinyl chain, a medium to short
oil alkyd resin, a dryer for the alkyd resin, alkoxylated amino resins, ortho-phosphoric
and p-toluene sulphonic acid catalyst, extenders and pigments such as mica, talc, zinc
phosphate and red oxide, and organic solvents ortho-xylene and normal butanol. The
vinyl terpolymer is predominantly polyvinyl formal with polyvinyl acetate and polyvinyl
alcohol as its other constituents and is capable of cross linking through the pendant
hydroxyl groups. The alkyd resin has a hydroxyl number between 80 and 200. The amino
resins are a combination of butylated urea formaldehyde and methylated melamine
formaldehyde resins. Ortho-xylene and normal butanol are used as solvents in a 60:40
weight ratio. The coating may be applied on all forms of steel, fresh as well as pre-erected
and rusted, comprising hot rolled, cold rolled, coated or metalized steel without
chemical pre-treatment or primer, by spray or brush, with minimal surface preparation,
and cures at room temperature.