Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2001-06-13
2002-09-10
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S014050, C106S014120, C106S014210, C252S387000, C252S389200, C252S389230, C252S389240, C423S593100, C423S606000, C423S607000
Reexamination Certificate
active
06447594
ABSTRACT:
The present invention relates to novel modified pigment grade strontium chromate corrosion inhibitors characterized by reduced solubility in water and undiminished CrO
4
−−
content.
BACKGROUND OF THE INVENTION
Several pigment grade inorganic and organic chromates are known to the art and employed in organic based protective primer applications, most notably: SrCrO
4
and Ca
++
, Ba
++
, Zn
++
chromates.
Many chromate corrosion inhibitor pigments used to be ubiquitously applied in the paint and coating industry, worldwide as well as in the United States. Due to environmental concerns, however, most of them have been practically eliminated from general industrial use during the past two decades and replaced with less effective phosphate-, molybdate-, borate- or silicate-based products, which, notably, also represent all of the available possibilities offered by inorganic chemistry for this purpose. The elimination of chromate inhibitor pigments from the paint and coating industry appears to be a compromise allowed primarily by improvements in coating science and resin chemistry. Notable exceptions are high-performance organic coatings, especially aircraft and coil primers, which possess no significant barrier capacity and consequently, require the inhibitor efficiency of chromates without any possibility of compromise. Not surprisingly, the corrosion inhibitor pigment of choice for aircraft and coil primers is SrCrO
4
, a preference entirely justified, considering this product's remarkably advantageous pigment qualities, as subsequently discussed.
Corrosion inhibitor pigments function as reservoirs of inhibitor species built into organic primers. In general, anions are the active inhibitor species of corrosion inhibitor pigments, which, in the case of chromates, is CrO
4
−−
. It will be apparent, that high CrO
4
−−
concentration per unit volume of pigment, i.e., high “specific inhibitor capacity”, is the desirable property of pigment grade chromates. This is particularly true in view of the fact the average thickness of high performance aircraft and coil primers is approximately 10 to 20 microns.
Understandably, high inhibitor capacity is expected to be concentrated in the limited volume available for corrosion inhibitor pigments in high performance organic primer applications. This concept is quantified by
e
isp
=c
i
·g
sp
/w
m
.
e
isp
represents the specific inhibitor capacity of corrosion inhibitor pigments, where Ci and gsp are, respectively, the weight ratio of CrO
4
−−
in the pigment and the specific gravity of the pigment, and Wm is the molar weight of the inhibitor species, in this case of CrO
4
−−
=116. It will be apparent that a combination of high CrO
4
−−
content and high specific gravity results in high specific inhibitor capacity, as it can be seen by comparing the cases of CaCrO
4
and SrCrO
4
presented in Table 1. Due to higher specific gravity, the specific inhibitor capacity of SrCrO
4
(18.4 mmol/cm
3
) is higher at only 57% CrO
4
−−
content than at 74% CrO
4
−−
content of CaCrO
4
(e
isp
=13.1 mmol/cm
3
). Specific inhibitor capacity is not informative, however, with respect to inhibitive activity of corrosion inhibitor pigments. Corrosion and its inhibition are aqueous, dynamic processes and consequently, in coating applications, the kinetic availability of inhibitor species is essentially a function of the solubility of pigment grade corrosion inhibitors. It will be apparent that corrosion inhibitor pigments must possess effective, however limited, solubility in water, as well as to dissociate and hydrolyze as ordinary electrolytes:
SrCrO
4
⇄Sr
++
+CrO
4
−−
+H
2
O⇄HCrO
4
−
+OH
−
1.
The corrosion inhibitor activity of pigments can be conveniently qualified by I
i
=n·c
isat
/c
icrt
, the inhibitive activity parameter, where c
isat
and c
icrt
are the is pigments' solubility and, respectively, the critical concentration of the related inhibitor species, in this case CrO
4
−−
. A stoichometric parameter, n=1 in the case of alkaline earth chromates. As for c
icrt
, it characterizes distinct inhibitor anionic species and it is defined as the minumum concentration of anionic species necessary to maintain metal substrates in a passive state when in contact with aqueous phases. It will be apparent that I
i
<1 indicates low inhibitor activity of pigments and consequently, are unacceptable materials. Similarily, high values of I
i
are not acceptable, because highly soluble pigments promote degradation of organic coatings by osmotic blistering as well as by leaching, the latter resulting in increased porosity and increased water absorption in organic coatings. Consequently, it appears that organic coatings impose a limited window or range of solubilities for corrosion inhibitor pigments. This range is estimated to correspond to 1<I
i
<100 and preferably 1<I
i
<10. As can be seen from Table 1, SrCrO
4
is characterized by I
i
~5 and corresponding solubility of about c
isat
=5 mmoles/L.
TABLE A
ei
sp
General
c
isat
mmol/
Formula
c
i
g
sp
mmmol/L
pH
cm
3
I
i
CaCro
4
0.74
2.05
141
11.0 ±
13.1
141
3
SrCro
4
0.57
3.79
5
8.3 ±
18.6
5
0.5
K
2
CrO
4
*3ZnCro
4
0.53
3.35
11
6.8 ±
15.3
11
*Zn(OH)
2
*2H
2
O
0.2
BaCrO
4
0.46
4.4
0.2
7.8 ±
17.3
0.2
0.5
Zn
5
CrO
12
H
8
a
0.2
3.45
0.2
7.2 ±
5.95
0.2
0.2
PbCrO
4
0.36
6.12
1.3 ×
18.9
1.3 ×
10−3
10−3
Na
2
Cro
4
b
0.71
2.7
5.4 ×
16.1
5 × 102
104
a
basic zinc chromate
b
does not qualify as pigment
As can be seen, due to its high specific inhibitor capacity (e
isp
=18.6 mmol/cm
3
, the highest of all corrosion inhibitor pigment grade chromates) and with moderate yet effective solubility (I
i
~5), SrCrO
4
appears to be the most effective and valuable of all chromate corrosion inhibitor pigments. However, due to its solubility, SrCrO
4
is also moderately hygroscopic, an undesirable property of some characteristic finely divided pigment grade products. This generally results in unwanted water vapor absorption from the surrounding air, and consequently, a moderate tendency for agglomeration or clump formation, especially during transport or storage in high humidity conditions. Although clumping does not necessarily alter the initial particle size distribution or fineness of pigment grade SrCrO
4
, it is undesirable, nevertheless, for it adversely affects dispersion procedures and ultimately results in increased energy consumption in paint manufacture.
As intuitively expected, the kinetics and extent of the water absorption process by pigment grade SrCrO
4
is a function of the relative humidity (R.H.) of the surrounding air, among other factors. At 60% R.H., the extent of water vapor absorption by SrCrO
4
is negligible and independent of exposure time.
In contrast, however, the rate of water vapor absorption is considerable at 100% R.H. and a function of exposure conditions and exposure time. Such conditions can result in a significant increase (4-7% by weight) in the moisture content of pigment grade SrCrO
4
. As also expected, the process is found to be reversible. It is observed, that water absorbed by SrCrO
4
at 100% R.H., desorbs spontaneously at 60% R.H. and ambient temperature, until a corresponding equilibrium value of <0.1% moisture content is established. It is apparent that clumping of pigment grade SrCrO
4
is a consequence of the water absorption process' reversible character and, presumably, it specifically occurs during desorption.
DETAILED DESCRIPTION OF THE INVENTION
It is a principal object of the present invention to provide a process and a strontium chromate, calcium chromate or zinc (II) chromate product produced by the process having reduced solubility in water with undiminished CrO
4
−−
content. It is a further aspect of the invention tha
Denesha Michael
Sinko John
Green Anthony
Ryan Kromholz & Manion S.C.
Wayne Pigment Corporation
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