METAL PRESERVATIVE COATING COMPOSITION AND METHOD
FIELD OF THE INVENTION
The invention is in the field of chromate free ambient or on-line curing metal preservative coating
composition, particularly suitable for coating mill scale covered hot rolled steel, galvanized steel
and metalized steel as these products roll out from the steel mills and to coating method using
such composition. The coating thus applied protects the metal products from corrosion and
rusting during storage and transit.
This invention relates to a thin film ambient curing chromate free coating composition based on a
polymeric material, particularly a vinyl terpolymer of polyvinyl formal, polyvinyl alcohol and
polyvinyl acetate, and an alkoxy-amino resin cross-linker, particularly a butylated urea
formaldehyde resin. This coating composition can be applied on ferrous or non-ferrous metallic
substrates.
BACKGROUND OF THE INVENTION
Hot rolled steel structures are protected from environmental corrosion by means of variety of
conventional and high performance paints that are applied after fabrication of such structures.
Mill scale formed on the surface of hot rolled steel during its manufacture is inherently protective
in nature being cathodic to steel. However, it rapidly converts to rust during exposure to humidity
in the environment. Rust removal by mechanical or chemical methods becomes imperative prior
to paint application. Further, even non-rusted hot rolled steel with mill scale, cannot be
effectively protected by paint systems available. Hence, hot rolled steel is routinely blasted as an
essential prelude to paint application. This process is often followed by application of wash/shop
primers to enhance corrosion protection and adhesion of topcoats. While new structures may be
sand/grit blasted, old existing structures may not always be amenable to such techniques. Blasting
necessarily leads to loss of metal, is environmentally polluting, and blasted steel has to be

protected with primers/paints almost immediately after, else corrosion sets in quickly. Moreover,
blasting is rarely able to provide a completely uniform and clean surface free of contaminants.
In-plant coating of hot rolled steel as manufactured, with an inhibitive system, would therefore
provide an ideal way to prevent surface contamination and rust formation in high humidity field
environments. This practice would also ensure a reproducible surface, however such a coating
must necessarily have the abrasion resistance and adhesion to maintain its integrity during the
transportation and handling of pre-coated articles.
Chromate free wash primers are well known, for example as disclosed in US patents 5077332,
5891960, 6084036 and 6169150. These are principally used on galvanized steel and mostly use
polyvinyl butyral resin. Similarly shop primers, for example as disclosed in US patents 4888056
and 6468336, are extensively used on blasted hot rolled steel. Wash and shop primers provide
temporary protection to coated substrates and are useful tie coats for further painting. However no
composition is available for use on non blasted mill scale covered steel. Further, although metal
products coated with wash/shop primers are weldable, these may not have the required degree of
abrasion resistance to permit handling and transportation of pre-coated steel products, if such
coating is applied in steel mills.
The inventor of the instant invention has found that a tough thin film transparent coating based on
a vinyl terpolymer cross linked with an alkoxy amino resin forms a tightly adherent abrasion
resistant coat on mill scale covered hot rolled steel as well as galvanized steel and provides
excellent barrier characteristics to retard the ingress of moisture and air to the steel surface. The
thin film integrates itself with the mill scale of hot rolled steel, occupying the voids and
discontinuities in it, and cannot be removed from the coated surface except by strong mechanical
attrition. It also provides a surface of good lubricity and has excellent flexibility and aesthetic
appeal. Such a coated article will withstand normal environmental attack on extended storage and
limited exposure to hostile environs such as transit in high seas when steel is shipped overseas.
Hence the coating may be useful in providing a rust free substrate to users if it is applied over hot
rolled steel products both flat as well as tubular, as manufactured in steel mills, prior to dispatch,
preferably with online heating, where coated substrates may reach a temperature of about 80°C to
90°C, to facilitate drying. Such pre-coated hot rolled steel may also be welded upon and can be
effectively protected from long term corrosion, by applying conventional or high performance
paints, directly on the pre-coated substrate. In other words, the preservative vinyl coating need
not be stripped prior to paint application and acts as an excellent tie coat. This thin film coating

provides a rust and contaminant free substrate for painting and therefore may eliminate the
practice of blasting steel altogether. Further, by top coating with chromate free protective paints
the entire coating system becomes chromate free. The coating prevents formation of white rust
during transportation and storage of galvanized steel and also protects metallized steel.
The wet coating is formulated using a linear vinyl terpolymer containing randomly appended
formal, acetyl and hydroxyl groups, an alkoxy amino resin cross-linker such as a partially
isobutylated urea formaldehyde resin for curing at ambient temperature, and a mineral acid such
as phosphoric acid. It has excellent adhesion to variety of un-oiled/de-oiled metallic substrates
such as hot rolled and cold rolled steel, galvanized and aluminized steel, apart from exceptional
impact strength and scratch hardness, good corrosion, chemical, and thermal resistance in thin
films with dry film thickness (DFT) ranging from 5 to 25 micrometers. On galvanized substrates
it restricts the formation of white rust. It is also compatible with a variety of topcoats both
ambient curing as well as stoving. It will be obvious to someone skilled in the art that such
properties offer a viable method of rust/corrosion protection of hot rolled steel and white rust
prevention of galvanized steel, from the time of manufacture.
In an embodiment of the invention such coating composition satisfying the need of quick
curing in an online in-plant condition with oven heating is ensured by proper choice of alkoxy
amino resin cross-linker.
Outstanding adhesion to mill scale covered hot rolled steel, galvanized and metallized
steel, coupled with high flexibility and hardness, resistance to impact, wear, abrasion, chemicals
and corrosion, makes the coating composition of the present invention ideally suitable for
preservation of such products when exposed to vagaries of climatic conditions during transit and
storage.
OBJECTS OF THE INVENTION
The first object of the invention is to provide a quick ambient or in-plant online curing thin film
polymer coating composition that is suitable for rust protection of mill scale covered hot rolled
steel and cold rolled steel, and white rust protection of galvanized steel.

The second object of the invention is to provide a polymer coating, which has a high degree of
impact strength, flexibility and surface hardness so that coated products may easily be handled
during transportation.
The third object of the invention is to provide an environment friendly chromate free corrosion
resistant coating composition that may be applied online on steel products, both flat and tubular,
without blasting or chemical pre-treatment.
Another object of the invention is to provide a method for coating a ferrous substrate with a
corrosion resistant composition to eliminate blasting of steel altogether by offering pre-coated
rust and contaminant free steel substrate.
Yet another object of the invention is to provide coated articles with its surface coated by
applying the coating composition according to the invention.
SUMMARY OF THE INVENTION
Accordingly the present invention provides a thin film, ambient or on line heat curing, chromate
free, corrosion resistant coating composition comprising:
a) a linear vinyl terpolymer having three randomly distributed functional groups
comprising acetyl, formal and hydroxyl groups along the vinyl backbone that offers
cross-linking sites through the hydroxyl groups during cure;
b) one or more alkoxy amino resin(s);
c) a mineral acid catalyst;
d) a blend of organic solvents and optionally,
e) one or more chromate free inorganic and/or organic pigment.
The said alkoxy amino resin is preferably a medium molecular weight partially isobutylated urea
formaldehyde resin.

In the alternative embodiment, a suitable alkoxy amino resin may also be used to further increase
the cross linking if desired, when on line heating is possible.
The said mineral acid is preferably ortho-phosphoric acid.
The said vinyl terpolymer has 6-15% polyvinyl alcohol content, 9-15% polyvinyl acetate content,
and 70-84% polyvinyl formal content, by weight.
The organic solvent blend preferably is a mix of ortho-xylene and n-butanol in the weight ratio of
60:40.
The coating composition may be suitably pigmented with inorganic and/or organic pigment/dye
to obtain pigmented coatings in a choice of attractive colors with medium to high gloss.
The invention also provides a method of coating ferrous or non-ferrous metal substrates by
applying the composition according to invention on the surface of the said substrate in desired
thickness and curing the same.
The invention further provides a coated article comprising a ferrous substrate coated with the
composition according to the invention.
DETAILED DISCRETION OF THE INVENTION
In the composition of the present invention a linear vinyl terpolymer, with three functional groups
namely, acetyl, formal and hydroxyl groups randomly distributed along the vinyl backbone,
offers cross-linking sites through the hydroxyl groups. The density of cross-links in the cured
matrix may be controlled by the number and placement of hydroxyl groups on the vinyl backbone
of polymer. This polymer possesses a rare combination of mechanical, thermal, chemical and
dielectric properties. The functional groups along with its vinyl backbone confer to this polymer
the properties of adhesion, toughness, chemical inertness and heat stability while the long linear
chains contribute to the outstanding flexibility. The spatial structure of this thermoplastic
material helps to form a closely packed molecular structure, which provides excellent barrier
characteristics when coated on a wide array of substrates. The hydroxyl groups are fully

accessible for cross-linking and this makes the thermoplastic polymer heat curable in the presence
of a mineral acid. The chain length distribution of a poly disperse polymer is made such as to
permit film formation, migration by diffusion to the substrate and development of the required
cohesive strength in the coating. Specifically, the terpolymer used in the invention has weight
average molecular weight ranging between 15,000 and 80,000 preferably between 20,000 and
50,000. The content of polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl
terpolymer used for the invention is 6-15%, 9-15% and 70-84% respectively by weight and
preferably 6.0-7.5%, 10-13% and 80-83% respectively by weight.
The said terpolymer useable in the composition may be produced by simultaneous hydrolysis and
formalization of polyvinyl acetate in acetic acid media. For this purpose polyvinyl acetate of the
required weight average molecular weight (28,000-140,000) and of low to very low branching
frequency is dissolved in acetic acid and formalin (37% formaldehyde aqueous solution) at room
temperature. Dilute sulfuric acid (N/10 normality) is added to this solution with agitation. The
contents are well stirred, heated to 75°C and maintained at this condition for 20-24 hours. The
whole process is conducted in a homogenous solution state. By regulating quantities of acetic
acid, water and formaldehyde, the required composition of the functional groups of the vinyl
backbone viz. acetyl, hydroxyl and formal may be obtained. Typically for one part of polyvinyl
acetate, 1.65 parts of acetic acid, 0.55 part of water and 0.45 part of formalin (37% formaldehyde
solution in water) is used to obtain a composition comprising 6.0 - 6.5 % polyvinyl alcohol, 11.0-
12.0 % polyvinyl acetate and 81.5-83.0 % polyvinyl formal. The extent of reaction is determined
by the percentage of hydroxyl and acetyl groups in the extracted polymer and the reaction is
terminated at the desired point by neutralizing the acid catalyst with a dilute alkali. The polymer
is next precipitated from solution by adding water as non-solvent, washed and dried to 98-99%)
solids. The simultaneous hydrolysis and formalization reactions in a homogenous media prevent
the development of blocky sequences on vinyl chain and thus helps to get a random terpolymer.
The Tg of the terpolymer so produced ranges from 100°C -115°C.
The alkoxy amino resin used as cross-linker for ambient curing is a medium molecular weight
urea formaldehyde resin, which is partially alkylated. Specifically CYMEL U-662 of Cytec
Industries has been used in the examples. It is partially isobutylated and has a medium degree of
alkylation, a medium methylol content and a low imino functionaliy. The product has 60% solids,
contains xylene and isobutanol as solvents, and has viscosity of 1200-2400 mPa.s at 23°C.

For, an online coating composition curable at about 80°C to 90°C, through oven heating, other
alkoxoxy amine resin cross linkers like methylated melamine formaldehyde, CYMEL 303, may
additionally be used.
The coating is a two-component system comprising the vinyl terpolymer, in solution with the
catalyst, as component A, and the amino resin solution as the component B. Pigment when used
is also a part of component A. The two components may be mixed while stirring and used within
2-4 hours. The preferred proportion of the alkoxy urea formaldehyde amino resin is 10-20% of
the vinyl terpolymer by weight, more preferably 14-17% by weight, and that of the acid is 4-14%
of the polymer by weight, more preferably 10-12% by weight when ortho phosphoric acid is
used. If pigment is to be added a mill base is first prepared in the polymer solution. For this
purpose, a concentrated solution of the polymer in the solvent blend is used and the dry pigment
is added and milled in an attritor/ball mill for approximately 6-8 hours. Component B also
contains the same solvent blend. The final weight solids is adjusted by adding the requisite
amount of solvent blend to provide preferably 6-18% overall weight solids depending on the
mode of application.
Components of the solvent blend may be selected from aliphatic alcohols such as methanol,
ethanol, isobutanol, isopropanol, n-butanol and arornatics such as toluene, xylene. Other solvents
such as butyl cellosolve and its acetate, and solvent naphtha may also be used as co-
solvents/diluents. Preferably the solvent blend comprises ortho xylene and n-butanol in a 60/40
ratio by weight.
Phosphoric acid, para toluene sulfonic acid may be used as the acid catalyst for this formulation.
However best properties are obtained with ortho phosphoric acid. This ambient curable clear coat
system may be pigmented to impart color with medium to high gloss.
The coating can be formulated both as transparent clear as well as pigmented opaque coating in
volatile organic solvents and may be spray or flow coated on metal substrates.
The coating.composition of the instant invention may be applied to metal substrates including hot
rolled steel, cold rolled steel, hot dip galvanized steel, aluminized steel in thin films of DFT
ranging from 5-25 micrometers, preferably 5-15 micrometers by means of spray or flow coating
methods. The two-component system can be stored at room temperature for minimum one year.

Should storage temperature fall below 25°C component A may have to be warmed to 30 °C and
stirred well prior to use.
The following examples of compositions are illustrative of the invention and are not intended to
limit the scope of the invention as defined by the appended claims.
The polyvinyl terpolymer as used in these examples is prepared by the process as described
earlier with the weight content of polyvinyl alcohol, polyvinyl acetate and polyvinyl formal at
6.0-7.5%, 10-13% and 80-83% respectively and a weight average molecular weight between
25,000 and 30,000.
Example 1 - Clear Coat
A clear coat composition may be prepared from the listed ingredients in the following range of
weight ratios:


Example 2- Pigmented/Dyed Coat
Colors can be incorporated by adding inorganic or organic pigments/dyes in a proportion upto
40% by weight of the total polymer and resin content in solvent bearing compositions.


The non-volatile components of the coating composition of this invention including the polymer,
butylated urea formaldehyde resin and pigments can range from 6-18% and is typically between
8-15% by weight of the total composition.
Black hot rolled steel(HRS), fresh cold rolled steel(CRS), and hot dip galvanized steel(HDG)
panels were used immediately after rinsing with xylene to remove oil and grease. The
composition of TABLE 1 was applied by brush on HRS, CRS and HDG panels. When air dried at
25°C, tack free condition is achieved in one hour and hard dry condition in 4 hours for DFT of 25
micrometers. For testing purposes the coated panels were force cured at 70°C for two hours. All
panels passed water immersion test for 24 hours and showed excellent compatibility with stoving
paints such as alkyds and air drying paints such as polyurethane. All such top coated panels
passed adhesion tests as per ASTM D3359. Results of other tests conducted on coated panels are
summarized in the following table:


TESTING METHOD
Ten tests were carried out on each of the substrate and the average value has been quoted in
above table. The testing methods that were followed are given below:
i) Adhesion - Permacell brand tape was used as per ASTM D 3359, METHOD B.
ii) Flexibility - CRS and HDG coated substrate was bent on a mandrel of 1/8 inch diameter
and observed for any cracking visually. Next, Permacell tape was applied at the bend and
coating pick off on the tape was observed on its removal,
iii) Pencil Hardness - Mitsubishi Uni pencils were used, and the test was conducted as per
ASTM D 3363.
iv) Impact Resistance : Direct impact test was done as per ASTM D 2794.
v) Solvent resistance - A cloth soaked with methyl ethyl ketone was used to rub over coated
surface for the prescribed number of times as per ASTM D 740 and reported as double
rubs,
vi) Chemical resistance - Resistance to chemicals was tested as per ASTM D 1308 as
modified in ASTM D3730.
vii) Resistance to neutral salt fog (corrosion resistance) : This test was done as per ASTM B
117.

We Claim :
1. An ambient or online heat curing chromate free, corrosion resistant coating
composition comprising:
a) a linear vinyl terpolymer having three randomly distributed functional groups
comprising acetyl, formal and hydroxyl groups along the vinyl backbone that
offers cross-linking sites through the hydroxyl groups during cure;
b) one or more alkoxy amino resin(s);
c) a mineral acid catalyst;
d) a blend of organic solvents and optionally
e) one or more chromate free inorganic pigment and/or organic dyes

2. The coating composition as claimed in claim-1 wherein the percentages by weight of
polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl terpolymer are
6.0-15%, 9-15% and 70-84% respectively.
3. The coating composition as claimed in claim-2 wherein the percentages by weight of
polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl terpolymer are
6.0-7.5%, 10-13% and 80-83% respectively.
4. The coating composition as claimed in claim 1, 2 or 3 wherein the weight average
molecular weight of said vinyl terpolymer is 15,000 to 80,000.
5. The coating composition as claimed in claim 4 wherein weight average molecular
weight of said vinyl terpolymer is 20,000 to 50,000.
6. The coating composition as claimed in claim 1 wherein the alkoxy amino resin used is a
partially isobutylated urea formaldehyde resin for ambient curing.
7. The coating composition as claimed in claim 1, wherein the alokxy amino resin used are
isobutylated urea formaldeyde and methylated melamine formaldeyde for online in-plant
heat curing.
8. The coating composition as claimed in claim 7, wherein the partially isobutylated urea
formaldehyde resin used is CYMEL U-662 and has medium degree of alkylation,
medium methylol content, low imino functionality and a medium molecular weight,
with a viscosity of 1200-2400 mPa.s at 23°C, and the methylated melamine formaldehye
resin is CYMEL 303.
9. The coating composition as claimed in claim 1 wherein the components of the solvent
blend are selected from xylene, toluene, naphtha, isopropanol, n-butanol, isobutanol,
ethanol, methanol, butyl cellosolve, and butyl cellosolve acetate.
10. The coating composition as claimed in claim 9, wherein 60 parts of o-xylene and 40 parts
of n- butanol by weight are used in the solvent blend.
11. The coating composition as claimed in claim 1 wherein 4-14%) by weight of phosphoric
acid based on the total weight of the vinyl terpolymer is used as the catalyst.

12. The coating composition as claimed in claim 1 which includes 0- 40% of the polymer
and resin content of one or more chromate free inorganic pigment and/or organic dye.
13. The coating composition as claimed in claim 1 wherein the ingredients are present in
the following range of weight percentage in separate packages for components A and
B:
COMPONENT A
INGREDIENTS % BY WEIGHT
Polyvinyl Terpolymer 6-8
o-Phosphoric Acid(88-93%) 0.24-1.12
o-Xylene 41-56
n-Butanol 28- 37
COMPONENT B
CYMEL U-662 0.6-1.6
o-Xylene 3.6-6
n-Butanol 2.4-4
14. The coating composition as claimed in claim 13 wherein upon mixing components A
and B before application, the ingredients are present in the following percentage by
weight:
INGREDIENTS % BY WEIGHT
Polyvinyl Terpolymer 6.9
CYMEL U-662 1.83
o-Xylene 54.26
n-Butanol 36.18
Ortho Phosphoric Acid (88-93%) 0.83
15. The pigmented coating composition as claimed in claim 1, wherein the ingredients are
present in the following percentage by weight:
Polyvinyl Terpolymer 6.32
CYMEL U-662 1.69
Fire Red Organic Dye 0.21
o-Xylene 54.57
n-Butanol 36.38
o-Phosphoric Acid(88-93%) 0.83

16. A method of forming a coating on a ferrous or non-ferrous metallic substrate which
comprises:
degreasing the surface of said substrate on which coating is to be applied, preferably by
a volatile organic solvent such as xylene;
preparing the coating composition as claimed in claim 1 by dissolving the vinyl
terpoiymer in the solvent blend and aaamg the catalyst to prepare component A,
dissolving the alkoxy amino resin(s) in solvent blend to make component B, and
finally, mixing the two components together;
applying said coating composition by spray coating or flow coating method on un-
oiled/de- oiled HRS, CRS, HDG to attain the desired thickness;
curing coating either at ambient conditions or with mild heat at 80° to 90°C to
accelerate the drying process in on line, in plant conditions.
17. The method as claimed in claim 16, wherein the alkoxy amino resin used is partially
isobutylated urea formaldehyde resin fox ambient curing.
18. The method as claimed in claim 16, wherein the alkoxy amino resins used are
isobutylated urea formaldehyde and methylated melamine formaldehyde for online in-
• plant heat curing.
19. The method as claimed in claim 16, 17 or 18, wherein the DFT of the coating as
applied is 5-25 micrometers.
20. The method as claimed in claim 19 where the.DFT of the applied coating is 5-15
micrometers.
21. The method as claimed in claim 16,17 or 18, wherein the said coating is applied to the
bare un-oiled/de-oiled substrate as a rust preventive coating during transit and extended
storage.
22. The method as claimed in any of the claims 15 to 20, wherein the said coating is used as
tie coat for further application of an air drying paint or a heat curable stoving paint
23. An article comprising:'
a) a ferrous substrate
b) a coating applied to surface of the said substrate by a method as claimed in
claim 16.
24. The article as claimed in claim 23, wherein the said substrate is selected from the group
comprising of CRS, HDG, HRS.

25. The coating composition as claimed in claim 7, wherein the ingredients are present
in the following percentage by weight:
Ingredients % by weight
Polyvinyl Terpolymer 6.85
CYMEL U-662 1.82
CYMEL 303 0.69
o-Xylene 53.89
n-Butanol 35.93
Ortho Phosphoric Acid (88-93%) 0.82

An invention discloses an ambient or online heat curing chromate free, corrosion resistant
coating composition comprising (a) a linear vinyl terpolymer having three randomly distributed
functional groups comprising acetyl, formal and hydroxyl groups along the vinyl backbone that
offers cross-linking sites through the hydroxyl groups during cure; (b) one or more alkoxy amino
resin(s); (c) a mineral acid catalyst; (d) a blend of organic solvents and optionally (e) one or more
chromate free inorganic pigment and/or organic dyes.