Nonabrasive, corrosion resistant, hydrophilic coatings for aluminum surfaces, methods of application, and articles coated therewith

A nonabrasive, corrosion-resistant, hydrophilic coating on aluminum sheet such as fin stock, produced by applying to the sheet surface a coating material containing, in an aqueous vehicle, effective amounts of nitrilotrismethylenetriphosphonic acid, phosphoric acid, and borate material of the group consisting of zinc borate and sodium borate, and essentially free of silica, alumina and precursors thereof, and heating the surface to establish the coating thereon. The coating formulation may also contains up to about 1 wt. % of polyacrylic acid and a surfactant to aid in application.

BACKGROUND OF THE INVENTION 
This invention relates to the provision of corrosion resistant, hydrophilic 
coatings for surfaces of aluminum articles. In particular aspects it is 
directed to coating compositions, methods of applying them, and aluminum 
articles having surfaces so coated. Illustrative examples of articles that 
may be beneficially coated in accordance with the invention include, 
without limitation, aluminum foil, and aluminum sheet from which various 
types of components and products are formed. The term "aluminum" is used 
herein to refer to aluminum metal and aluminum-based alloys. 
For certain purposes, aluminum articles, e.g. sheet articles, are desirably 
provided with hydrophilic surfaces. One commercially important example is 
the aluminum fin stock (sheet aluminum, in final gauge) from which fins 
are made for heat exchangers in air conditioners. Water condensing on the 
surfaces of the closely spaced fins in an air conditioner tends to 
accumulate in the form of drops that impede airflow between the fins, 
thereby reducing heat exchange efficiency. This problem can be overcome by 
producing the fins from fin stock having a hydrophilic coating on its 
surfaces; the coating allows water to drain from the fin surfaces and 
largely prevents the development and retention of airflow-obstructing 
drops. Since the environment of use of the fins is relatively severe, it 
is desirable that the coating also afford protection against corrosion. 
A satisfactory hydrophilic and corrosion-resistant coating for fin stock or 
the like must be smooth and nonporous with relatively uniform thickness. 
To these ends, as well as to ensure that it remains durably on the fins 
which are formed from the stock, a strong bond must be formed between the 
material of the coating and the coated aluminum surface; otherwise, as the 
coating is dried or cured with heat after application, it may tend to move 
relative to the surface, developing regions of differing thickness and/or 
shrinkage cracks. In addition, the coating must maintain good corrosion 
resistant and hydrophilic properties over extended periods of exposure to 
water; it should be nontoxic and environmentally acceptable in 
application, use and recycling, as well as being inexpensive, easy to 
apply, and free from tackiness or stickiness. 
Heretofore, a variety of hydrophilic coating systems have been proposed for 
imparting hydrophilicity to aluminum surfaces. A serious difficulty 
presented by many of the known coating formulations is that oxide material 
(such as silica or alumina or their precursors), included therein to 
impart hydrophilicity, renders the produced coatings abrasive. The 
abrasive character of the coatings causes increased wear of the tooling 
used in air conditioner fabrication, i.e., incident to forming or other 
operations performed on fin stock thus coated. 
It is also known that polymers of a polar nature, such as polyvinyl alcohol 
and polyacrylic acid, can provide satisfactorily hydrophilic films. Such 
films, however, tend to absorb water and swell, and then afford little or 
no corrosion resistance. Attempts have been made to stabilize the polymers 
by cross-linking but these attempts have not yet achieved successful 
results. 
SUMMARY OF THE INVENTION 
The present invention, in a first aspect, broadly contemplates the 
provision of an aluminum article having a surface bearing a nonabrasive, 
corrosion-resistant, hydrophilic coating produced by applying to the 
surface a coating formulation comprising, in an aqueous vehicle, effective 
minor amounts of nitrilotrismethylenetriphosphonic acid, phosphoric acid, 
and borate material of the group consisting of zinc borate and sodium 
borate, and essentially free of silica, alumina and precursors thereof, 
and heating the surface to establish the coating thereon. 
Further in accordance with the invention, an effective minor amount of 
polyacrylic acid is advantageously incorporated in the coating material. 
Zinc borate, viz. 2ZnO.multidot.3B.sub.2 O.sub.3 .multidot.3.5H.sub.2 O, 
preferably together with additional ZnO, and optionally Na.sub.2 B.sub.4 
O.sub.7 .multidot.10H.sub.2 O, is currently preferred as the borate 
material. An effective minor amount of a surfactant (e.g. aluminum 
polymethacrylate, ethoxylated octyl phenol) to facilitate application can 
also be included in the formulation. 
The term "minor amount" as used herein refers to an amount of less than 
50%. All percentage values of coating formulation ingredients set forth 
herein are expressed as percent by weight of total coating material 
(including the aqueous vehicle) unless otherwise specifically stated. 
The amounts of the various ingredients used are those that are effective, 
in the formulations employed (i.e. in conjunction with the other 
ingredients present) to provide strongly bonded, smooth, nonporous 
hydrophilic and corrosion resistant coatings on aluminum surfaces, at 
least substantially free of tackiness or stickiness. Advantageously or 
preferably, the amounts of the ingredients used, in combination, are 
effective to provide a coating on said surface producing a stable contact 
angle with water of not more than about 15.degree. (preferably not more 
than about 10.degree.) and/or to produce corrosion resistance such that 
when the coated surface is exposed to a 10 weight percent copper 
sulfate--1 weight percent hydrochloric acid solution, a period of at least 
about one minute elapses before gas bubbles appear. 
The contact angle is a measure of hydrophilicity; i.e., the smaller the 
contact angle, the more hydrophilic the coating is. Stability of contact 
angle refers to the maintenance of the contact angle below the stated 
value (15.degree. or, preferably, 10.degree.) throughout a period of 
essentially continuous immersion in water up to about two weeks; when once 
the immersion period exceeds two weeks, the contact angle invariably 
decreases. 
Currently preferred broad limits or ranges for the various ingredients in 
the coating formulation or feed for application to the aluminum surfaces 
are as follows: about 2.5 to about 7.8 parts by weight of 
nitrilotrismethylenetriphosphonic acid measured as a solution at 50% 
concentration, about 1.7 to about 6.1 parts by weight of phosphoric acid 
measured as 85% concentration H.sub.3 PO.sub.4, about 0 to about 4.3 parts 
by weight of 2ZnO.multidot.3B.sub.2 O.sub.3 .multidot.3.5H.sub.2 O, about 
0 to about 2.6 parts by weight of ZnO, about 0 to about 4.3 parts by 
weight of sodium borate measured as Na.sub.2 B.sub.4 O.sub.7 
.multidot.10H.sub.2 O, about 0 to about 0.9 parts by weight of polyacrylic 
acid, about 0.008 to about 0.17 parts by weight of surfactant, balance 
essentially water, subject to the provisos that the total of 
nitrilotrismethylenetriphosphonic acid and phosphoric acid present is 
between about 7.7 and about 12.1 parts by weight, that the total of 
2ZnO.multidot.3B.sub.2 O.sub.3 .multidot.3.5H.sub.2 O, ZnO and sodium 
borate present is between about 1.3 and about 5.2 parts by weight, and 
that the amount of water present (exclusive of combined water, and water 
in the acid solutions) is between about 100-P and about 200-P parts by 
weight where P is the total parts by weight of ingredients other than 
water present in the formulation. 
The invention affords water-stable coatings that are desirably hydrophilic 
(typically characterized by a stable contact angle with water of 
10.degree. or less), satisfactorily corrosion resistant for use on fin 
stock (for example) or the like, nontoxic, and environmentally acceptable, 
as well as being adequately uniform and adherent to the aluminum surfaces 
to which they are applied, and free from tackiness or stickiness. At the 
same time, owing to the absence of silica, alumina, and precursors thereof 
from the coating formulation, they are advantageously nonabrasive, leading 
to reduced wear of tooling used to perform post-coating operations on the 
coated metal, as in the fabrication of air conditioners. 
A further advantage of the invention is that coatings having these 
attributes can be achieved with short curing times at relatively low 
temperatures. For instance, curing can be performed by heating the metal 
to reach a peak metal temperature of around 160.degree.-210.degree. C. 
This can be achieved by heating the sheet at an oven temperature of 
250.degree.-300.degree. C. for a few seconds of residence time. The peak 
metal temperature is in any event kept below about 225.degree. C., as 
curing at higher peak metal temperatures results in degradation of the 
organic components of the coating material and causes an increase in 
contact angle. 
The "peak metal temperature," as referred to herein, is the highest 
temperature reached by the metal sheet during the heating step, while the 
"oven temperature" is the temperature set on the control of the oven or 
furnace employed to provide the heating. It will be appreciated that 
although two ovens or furnaces can be set at the same temperature setting, 
the metal surface does not necessarily reach the same maximum temperature 
in each. For example, in a convective furnace, the metal surface will 
reach a higher temperature than in a nonconvective furnace. The data given 
in the detailed description below were obtained using a nonconvective 
laboratory furnace, but in industrial practice a moving web or sheet of 
aluminum will pass through a convective furnace. 
The articles coated in accordance with the invention, in each of the 
abovedescribed embodiments, may be aluminum sheet articles. In particular, 
the invention has been found highly advantageous for the coating of 
aluminum fin stock as used to produce heat exchanger fins for air 
conditioners. The coated surfaces of the fin stock or other aluminum sheet 
are satisfactorily hydrophilic and corrosion resistant, and these 
properties are maintained over extended periods of use in exposure to 
water. 
In additional aspects, the invention contemplates the provision of 
compositions and methods for producing a hydrophilic and corrosion 
resistant coating as described above on surfaces of aluminum articles, 
including aluminum sheet, and in particular aluminum fin stock. 
Further features and advantages of the invention will be apparent from the 
detailed description hereinbelow set forth. 
DETAILED DESCRIPTION 
For purposes of specific illustration, the invention will be particularly 
described with reference to the provision of hydrophilically coated 
aluminum fin stock for air conditioner heat exchangers. Such fin stock is 
aluminum sheet which has been rolled to final gauge and is ready for 
cutting to form heat-exchanger fins; suitable alloy compositions, gauges, 
and tempers of such stock are well-known in the art and accordingly need 
not be further specified. Thus, exemplary products of the invention are 
fin stock sheets bearing hydrophilic, corrosion resistant coatings in 
accordance with the invention; when the fin stock is cut and formed into 
fins, these coatings are retained on the fin surfaces to impart the 
desired hydrophilic and corrosion resistant properties thereto. However, 
while the coating of aluminum fin stock represents a currently important 
commercial application of the invention, it is to be understood that in a 
broader sense the invention may be employed in coating a wide variety of 
aluminum articles, notably including sheet articles, for which a 
hydrophilic coating that is also corrosion resistant is desired. 
The invention contemplates the provision of a coating feed (i.e. liquid 
coating material or composition, ready for application to aluminum fin 
stock or other aluminum surfaces) comprising, in an aqueous vehicle, 
effective minor amounts of nitrilotrismethylenetriphosphonic acid, 
phosphoric acid, and borate material of the group consisting of zinc 
borate and sodium borate, preferably also including an effective minor 
amount of polyacrylic acid, and essentially free of silica, alumina and 
precursors thereof. An effective minor amount of a surfactant is usually 
or preferably also incorporated in the formulation, to promote wetting of 
surfaces incident to application. 
The several ingredients of the coating composition will now be further 
described. 
Nitrilotrismethylenetriphosphonic acid--it is currently preferred to use a 
50 weight % aqueous solution of nitrilotrismethylenetriphosphonic acid 
(hereinafter sometimes abbreviated "NTPA") in the coating feeds of the 
invention, and amounts of NTPA are expressed herein as amounts of such 
solution. The NTPA contributes to the corrosion resistance of the produced 
coatings. For obtaining a stable coating, the amount of NTPA (i.e. 50% 
solution) present in the applied coating material should exceed 2.5%, and 
more preferably (in at least many instances) should be in a range of 2.9% 
to 7.8%. Amounts of NTPA above 7.8% tend to increase the tackiness of the 
produced coating on absorption of moisture, and also add unnecessarily to 
the cost of the coating. 
Phosphoric acid--It is currently preferred to use orthophosphoric acid 
(H.sub.3 PO.sub.4) in an 85 weight % aqueous solution, and amounts of 
phosphoric acid are expressed herein as amounts of such solution. The 
phosphoric acid content of the coating feed is essential to maintain 
contact angle stability over time. It is therefore generally preferred 
that the phosphoric acid content be at least about 1.7% and more 
preferably between 2.9% and 5.2%. 
Zinc borate--Zinc borate is conveniently employed in the form 
2ZnO.multidot.3B.sub.2 O.sub.3 .multidot.3.5H.sub.2 O (sometimes 
hereinafter abbreviated "ZB"). The zinc oxide:boric oxide mole ratio of 
the zinc borate material may be increased, above that of ZB, by adding 
zinc oxide powder (ZnO). As used herein, the term "zinc borate" embraces 
ZB with or without additional ZnO. It is necessary to include zinc borate 
and/or sodium borate in order to achieve the desired hydrophilic property 
of the coating, zinc borate being preferred because it gives better 
corrosion resistance than sodium borate. The amount used should not exceed 
the limit of solubility in the coating formulation, which is dependent on 
the concentration of acids (NTPA and phosphoric acid) present. 
Sodium borate--In addition to or in substitution for zinc borate, sodium 
borate (sometimes hereinafter abbreviated "NAB") may be used in the 
formulation, conveniently in the decahydrate form, Na.sub.2 B.sub.4 
O.sub.7 .multidot.10H.sub.2 O. Zinc borate and sodium borate may be used 
together, with or without added zinc oxide. 
Polyacrylic acid--The polyacrylic acid used may, for example, be the 
product commercially available under the trade name "Acusol" from Rohm & 
Haas. Polyacrylic acid (sometimes hereinafter abbreviated "PAA") 
contributes to the hydrophilicity (reduction in contact angle) of the 
coating. However, when its concentration in the coating feed exceeds about 
1%, the coated surface becomes tacky with time owing to absorption of 
moisture. This tackiness is undesirable as it can cause the coated sheet 
to stick to the rubber rolls used to advance the sheet during fabrication 
of fins or other elements. It is therefore preferred to maintain the 
polyacrylic acid concentration below about 1%. 
Surfactant--a surfactant is added only to facilitate wetting of surfaces 
during coating application. It does not impart hydrophilicity or otherwise 
affect the performance of the coating. Aluminum fin stock sheet in "O" 
temper (fully annealed) can be wetted by coating feeds of the invention 
containing polyacrylic acid without surfactant, but it is difficult to wet 
the chrome-plated rolls used in roll-coating application of the feed to 
the aluminum surfaces. Suitable surfactants are aluminum polymethacrylate 
(sometimes hereinafter abbreviated "APMA"), commercially available under 
the trade name "Darvan C" from R. T. Vanderbilt & Co., and ethoxylated 
octyl phenol (sometimes hereinafter abbreviated "EOP"), commercially 
available under the trade name "Nonidet P-40" from Sigma Chemicals. Only a 
very small amount of surfactant (usually less than 0.1%) is used. 
In the practice of the method of the invention, the coating composition or 
feed is first prepared by dissolving the described ingredients in water. 
The resulting aqueous feed is then applied to the fin stock or other 
aluminum surface to be coated, using any convenient application procedure, 
e.g., immersion, roller-coating, spin-coating, spraying, or painting, in 
accordance with techniques well-known in the art. 
After application of the feed, the fin stock or other coated aluminum 
article is heated (to remove water and other volatiles, and thereby to 
establish a dried coating on the aluminum surfaces) so as to reach a peak 
metal temperature of about 160.degree.-210.degree. C., and in any event 
below 225.degree. C. This typically involves placing the sheet, with the 
applied feed, in an oven maintained at 250.degree.-300.degree. C., for a 
few seconds of residence time. The drying of the applied coating by the 
described heating step completes the coating procedure. It is important 
that the peak metal temperature be kept below 225.degree. C. to prevent 
impairment of the hydrophilic properties of the coating. 
The coatings thus produced by the method of the invention are 
advantageously hydrophilic, characterized by a contact angle with water 
below 15.degree., and with preferred formulations, not more than about 
10.degree.. The contact angle does not increase significantly, i.e. above 
the maxima just mentioned, with extended exposure to water. The exposure 
time of concern is the period represented by up to about two weeks of 
continuous immersion in water, since the contact angle invariably 
decreases thereafter. The contact angle also remains adequately stable 
when exposed to cooling oils normally employed in the industry during 
fabrication of fins. 
Owing to the absence of silica, alumina and their precursors, the coatings 
are nonabrasive, and therefore do not cause tool wear during fabrication 
of fins or the like. In addition, they are inexpensive, do not contain any 
toxic substances, and do not present problems in application or use; in 
particular, they do not become inconveniently tacky or sticky. They also 
provide a satisfactory degree of corrosion resistance to the surfaces to 
which they are applied. 
Preferably the amounts or proportions of the several ingredients of the 
coating feed are such as to be effective, in combination, to provide a 
coating producing a contact angle with water of not more than about 
10.degree.. Preferably, also, these amounts or proportions are such as to 
be effective to provide a coating having corrosion resistance such that 
when the coated surface is exposed to a 10 weight percent copper 
sulfate--1 weight percent hydrochloric acid solution, a period of at least 
about one minute elapses before gas bubbles appear. 
The relative proportions of the various ingredients of the coating feed 
(other than water) are important for the attainment of the desired coating 
properties. Broad and currently preferred ranges of such relative 
proportions (expressed as parts by weight) are set forth in TABLE 1 below, 
which defines these relative proportions in terms of specifically 
identified, convenient or preferred forms of these ingredients. In 
addition to the ingredients listed, other components may be included in 
the coating feed formulation. Small amounts of substances such as 
inorganic salts, other acids or organic derivatives can also be added to 
or be present in the feed without adverse effects but do not appear to 
improve the properties of the coating. 
The balance of the coating feed (i.e., apart from the ingredients listed in 
TABLE 1) is essentially water. A currently preferred concentration for the 
aqueous coating feed is that at which the parts by weight listed in TABLE 
1 are in fact percentages by weight of the listed ingredients, the balance 
of the composition being water. However, in at least some instances this 
concentration may be diluted up to half strength by addition of water, 
such that the percentage by weight of each ingredient is numerically equal 
to half the value of parts by weight given in TABLE 1. That is to say, at 
least over this indicated wide range, the amount of water in the coating 
feed is not critical to the performance of the coating, although higher 
dilution results in a thinner coating and may consequently reduce the 
corrosion resistance and/or otherwise decrease the time the coating will 
last in service, which could nevertheless be within acceptable limits for 
some applications. 
Examples of five specific currently preferred coating formulations, within 
the ranges set forth in TABLE 1, are given in TABLE 2 below. Each of these 
preferred formulations is represented by one of the coating feeds 
described in the specific examples that follow. All of the formulations of 
TABLE 2 are given in % by weight (of the total coating feed, including 
water) at full-strength concentration. 
In these tables, and in the formulations given in the specific examples 
that follow, amounts and proportions of water set forth do not include 
water incorporated in the starting materials, e.g. in the acids. 
TABLE 1 
______________________________________ 
Parts by Weight 
Ingredient Broad Range 
Preferred Range 
______________________________________ 
(1) NTPA 2.5-7.8 2.9-7.8 
(2) H.sub.3 PO.sub.4 
1.7-6.1 2.9-5.2 
SUBTOTAL OF (1) + (2) 
7.7-12.1 7.7-11.2 
(3) ZB 0-4.3 0.8-2.2 
(4) ZnO 0-2.6 0.8-2.6 
SUBTOTAL OF 1.3-5.2 1.3-5.2 
(3) + (4) + NAB 
(5) PAA 0-0.9 0.07-0.43 
(6) Surfactant 0.008-0.17 0.008-0.10 
______________________________________ 
NTPA = nitrilotrismethylenetriphosphonic acid (50%, in water) 
H.sub.3 PO.sub.4 = orthophosphoric acid (85%, in water) 
ZB = 2ZnO.multidot.3B.sub.2 O.sub.3 .multidot.3.5H.sub.2 O 
ZnO = zinc oxide powder 
NAB = sodium borate decahydrate, Na.sub.2 B.sub.4 O.sub.7 
.multidot.10H.sub.2 O 
PAA = polyacrylic acid (trade name "Acusol") 
TABLE 2 
______________________________________ 
balance water, in all compositions 
% by weight 
Ingredient 
I II III IV V 
______________________________________ 
NTPA 5.19 6.94 3.12 5.18 5.19 
H.sub.3 PO.sub.4 
4.14 3.47 5.20 4.15 4.14 
ZB 1.73 1.73 1.73 1.16 1.73 
ZnO 2.02 1.02 2.03 1.35 2.02 
NAB 0 0 0 1.35 0.00 
PAA 0.43 0.35 0.28 0.43 0.43 
APMA 0.09 0.09 0.07 0.00 0.00 
EOP 0 0 0 0.017 
0.02 
______________________________________ 
APMA = aluminum polymethacrylate (trade name "Darvan C") 
EOP = ethoxylated octyl phenol (trade name "Nonidet P40") 
By way of further illustration of the invention, reference may be made to 
the following specific examples, wherein all ingredients used are those 
specifically identified in TABLES 1 and 2. Data for EXAMPLES 1-6 are set 
forth in TABLES 3 and 4 below, while data for EXAMPLES 7-9 are set forth 
in TABLES 5 and 6 below.

EXAMPLE 1 
Coating formulations 1--1 and 1-2 set forth in TABLE 3 were prepared and 
applied to surfaces of small aluminum fin stock sheets in "O" temper 
(fully annealed) by roll coating, using chrome-plated rolls. The coatings 
were dried by heating the sheets in an oven for a few seconds, to achieve 
a peak metal temperature of about 160.degree.-200.degree. C. 
Immediately thereafter, contact angles with water were measured for the 
coatings thus applied. Samples of the test sheets were then continuously 
immersed in water (which was changed daily) for periods of 4, 8, 12 and 16 
days. At the end of each of these periods, the contact angle with water 
was measured for each coating. The results are given, for coatings 1--1 
and 1-2, in TABLE 4, wherein "Initial" refers to the initial contact angle 
measurement (i.e., before any immersion in water) and the number of days 
of immersion before each subsequent test are indicated. 
This example illustrates the effect of the addition of polyacrylic acid on 
hydrophilicity. Although both coatings 1--1 and 1-2 met the requirement of 
providing stable contact angles (throughout a 2-week period) below 
15.degree., coating 1-2 (which contained 0.43% polyacrylic acid) exhibited 
a significant reduction in contact angle as compared to coating 1--1, 
which contained no polyacrylic acid. 
Coating 1-2 is the currently especially preferred composition I set forth 
in TABLE 2 above. 
EXAMPLE 2 
The procedure of EXAMPLE 1 above was repeated, using the coating 
formulations identified as 2-1, 2--2, and 2-3, to show the effect of 
phosphoric acid on maintenance of a stable low contact angle. As TABLE 4 
shows, in the case of coating 2-1, which contained no phosphoric acid, the 
contact angle was substantially higher than 15.degree. for much of the 
immersion test period, and progressively better results were achieved 
(coatings 2--2 and 2-3) as the proportion of phosphoric acid was 
increased. 
EXAMPLE 3 
Further samples of the "O" temper aluminum fin stock sheet were coated with 
coating formulations 3-1 and 3-2 set forth in TABLE 3, again using the 
applying and drying procedure described in EXAMPLE 1. 
To demonstrate the effect of NTPA in the composition on the corrosion 
resistance of the produced coatings, these samples, and also a sheet 
coated with formulation 1-2 as described in EXAMPLE 1, were tested for 
corrosion resistance by placing a drop of a solution containing 10 weight 
% copper sulfate and 1 weight % hydrochloric acid on the coated aluminum 
sheet, and observing the time elapsed before hydrogen bubbles became 
visible. 
The sample coated with formulation 3-1, containing no NTPA, exhibited the 
least corrosion resistance; hydrogen bubbles evolved after a lapse of 
about 15 seconds. In the case of the sample coated with formulation 3-2, 
containing 2.6% NTPA, hydrogen bubbles were seen after a lapse of 40 
seconds. The sample coated with formulation 1-2, containing 5.19% NTPA 
displayed superior resistance to corrosion, in that about 150 seconds 
elapsed before gas bubbles evolved. 
EXAMPLE 4 
The procedure described in EXAMPLE 1 above, including the contact angle 
stability tests, was again repeated, using coatings 4-1, 4-2, and 4-3 set 
forth in TABLE 3, and results were compared with those obtained for 
samples coated with formulations 1-2 (EXAMPLE 1) and 2-3 (EXAMPLE 2), to 
ascertain the effect of varying amounts of zinc borate and zinc oxide. In 
these compositions, the mole ratio of ZnO to B.sub.2 O.sub.3 was as 
follows: 
______________________________________ 
Coating No. ZnO/B.sub.2 O.sub.3 Mole Ratio 
______________________________________ 
4-1 0/0 
4-2 0.67 
2-3 1.5 
4-3 1.75 
1-2 2.75 
______________________________________ 
Coating 4-1, containing no ZB or ZnO, exhibited no corrosion resistance, 
and was not tested for contact angle. As shown in TABLE 4, of those that 
were tested, the lowest stable contact angle was achieved by coating 1-2, 
which had the highest concentration of zinc borate (ZB+ZnO=3.75%). It was 
also observed that when the overall concentration of zinc borate was below 
2%, the coating became tacky after exposure to air and moisture. Least 
tackiness was observed when the concentration of zinc borate exceeded 2%. 
The amount of zinc borate that could be dissolved in the coating 
formulation depended on the concentration of the two acids NTPA and 
H.sub.3 PO.sub.4. At the levels of acid concentration in the formulations 
tested, the maximum zinc borate concentration was limited to about 3.2%. 
It was also observed that when the coating feed (i.e., the initial 
formulation in water) was exposed to air for periods of 8 hours or more, a 
precipitate was formed. This can be avoided by replacing part of the zinc 
borate and zinc oxide with sodium borate. It is believed that formation of 
a precipitate also occurs on the coated sheet; the coating becomes 
increasingly insoluble in water with time when exposed to air. 
EXAMPLE 5 
The procedure of EXAMPLE 1 was repeated using coating formulation 5-1 of 
TABLE 3, with the results (contact angle stability) shown in TABLE 4. 
Coating 5-1 is the same as the preferred coating composition II of TABLE 
2. 
Aluminum sheet samples coated with each of formulations 1-2 (EXAMPLE 1) and 
5-1 were immersed in the cooling oil identified by trade name "Arrow 688" 
for 24 hours and then air dried. In the case of formulation 5-1, the 
contact angle with water increased from 8.2.degree. before oil immersion 
to 19.degree. after oil immersion. For the sample coated with formulation 
1-2, the contact angle with water increased from 5.4.degree. before oil 
immersion to 7.4.degree. after oil immersion. These results show that with 
the optimum formulation (1-2), the coating retains its hydrophilic nature 
even after exposure to cooling oil. 
EXAMPLE 6 
The procedure of EXAMPLE 1 was again repeated using coating 6-1 of TABLE 3. 
Contact angle stability results were as shown in TABLE 4. Coating 6-1 is 
the preferred composition III of TABLE 2. 
EXAMPLE 7 
To determine the effect of substituting sodium borate (NAB, as identified 
in TABLE 1) for zinc borate in the coating feed, two further coatings (A 
and B, TABLE 5) were prepared and applied to aluminum fin stock sheet 
samples in "O" temper by roll coating as in EXAMPLE 1. The coatings were 
then dried by heating the coated metal samples in an oven at an oven 
temperature of 300.degree. C. for various time periods ranging from 12 to 
15 seconds. The peak metal temperature varied between 200.degree. and 
220.degree. C. For samples of each coating, dried for each of four periods 
(12, 13, 14 and 15 seconds), contact angle stability was measured by the 
same immersion technique as in EXAMPLE 1, except that tests were made 
initially and after immersion periods of 1, 4, 8, 10 and 16 days. Results 
are shown in TABLE 6. 
These results indicate that coating B, which contained polyacrylic acid, 
was significantly better from the standpoint of hydrophilicity (lower 
stable contact angle) than coating A, which had no polyacrylic acid. When 
tested by the procedure of EXAMPLE 3, however, these samples exhibited 
inferior corrosion resistance, as hydrogen bubbles began to be generated 
within less than 60 seconds. 
EXAMPLE 8 
The procedure of EXAMPLE 1 was repeated once more with coating C of TABLE 
5, containing zinc borate and oxide and also sodium borate. This 
composition (preferred composition IV of TABLE 2) also gave satisfactory 
results, as TABLE 6 shows. 
TABLE 3 
______________________________________ 
balance water, in all compositions 
Coating 
% by weight 
No. NTPA H.sub.3 PO.sub.4 
ZB ZnO PAA APMA 
______________________________________ 
1-1 5.20 4.33 1.73 2.03 0.00 0.09 
1-2 5.19 4.14 1.73 2.02 0.43 0.09 
2-1 6.32 0.00 1.81 0.85 0.58 0.09 
2-2 6.17 1.94 1.76 0.83 1.06 0.09 
2-3 5.24 4.20 1.75 0.82 0.44 0.09 
3-1 0.00 4.89 1.81 2.12 0.44 0.09 
3-2 2.66 4.20 1.75 2.05 0.44 0.09 
4-1 5.38 4.31 0.00 0.00 0.45 0.09 
4-2 5.29 4.23 1.76 0.00 0.44 0.09 
4-3 5.24 4.36 1.75 1.03 0.35 0.09 
5-1 6.94 3.47 1.73 1.02 0.35 0.09 
6-1 3.12 5.20 1.73 2.03 0.28 0.07 
______________________________________ 
TABLE 4 
______________________________________ 
Coating 
contact angle, degrees 
No. Initial 4 days 8 days 12 days 
16 days 
______________________________________ 
1-1 11.2 10.6 12.6 10.0 10.0 
1-2 10.4 4.6 4.8 8.6 3.2 
2-1 7.2 18.0 24.8 21.6 3.4 
2-2 4.6 22.0 22.4 14.4 2.2 
2-3 6.8 12.8 12.8 12.4 12.4 
4-2 7.4 7.4 14.2 11.4 9.8 
4-3 3.8 4.0 17.0 12.8 2.6 
5-1 6.0 5.6 8.4 10.2 12.2 
6-1 11.8 8.3 9.4 not measured 
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TABLE 5 
______________________________________ 
balance water, in all compositions 
% by weight 
Ingredient 
Coating A Coating B Coating C 
______________________________________ 
NTPA 6.38 6.25 5.18 
H.sub.3 PO.sub.4 
6.38 6.25 4.15 
ZB 0.00 0.00 1.16 
ZnO 0.00 0.00 1.35 
NAB 2.13 2.08 1.35 
PAA 0.00 2.08 0.43 
APMA 6 drops 6 drops 0.00 
EOP 0.00 0.00 0.017 
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TABLE 6 
______________________________________ 
contact angle, degrees 
Coating Init. 1 day 4 days 
8 days 
10 days 
16 days 
______________________________________ 
A-12 11.4 14 13 5 13 5 
A-13 8.0 15 8 7 16 12 
A-14 19.0 21 4 4 12 5 
A-15 16.2 9 8 10 15 11 
B-12 6.0 4 8 2 4 8 
B-13 5.2 3 4 4 6 7 
B-14 4.8 3 4 10 6 4 
B-15 6.8 5 4 2 3 5 
C 4.6 not 8.8 11.4 not measured 
measured 
______________________________________ 
NOTE: The numerals 12, 13, 14 and 15 after "A" and "B" represent the 
number of seconds of drying time (at 300.degree. C. oven temperature) of 
sample aluminum sheets coated with coatings A and B 
It is to be understood that the invention is not limited to the features 
and embodiments hereinabove specifically set forth but may be carried out 
in other ways without departure from its spirit.