Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-05-15
2003-11-04
Mayekar, Kishor (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S506000, C523S415000
Reexamination Certificate
active
06641707
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to cationic electrodepositable compositions and, more particularly, to cationic electrodepositable compositions that are resistant to effects of biodegradation.
BACKGROUND OF THE INVENTION
Electrodeposition as a coating application method involves deposition of a film-forming composition under the influence of an applied electrical potential. Advantages of electrodeposition over non-electrophoretic coating processes include increased paint utilization, outstanding corrosion protection and low environmental contamination. Since its introduction in 1972, cationic electrodeposition has steadily gained in popularity over anionic electrodeposition and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
In preparing the electrodepositable composition used in the cationic electrodeposition process, a resinous binder which contains basic groups, such as basic nitrogen groups, is neutralized with an acid. The resultant cationic resin is dispersed in water and combined with pigment and other additives normally used in the cationic electrodeposition process to form a paint. The neutralizing acids may include organic acids such as acetic acid and lactic acid as well as inorganic acids such as sulfamic.
The electrodeposition process involves immersing an electroconductive substrate into a bath of an aqueous electrocoating composition, the substrate serving as a charged electrode in an electrical circuit comprising the electrode and an oppositely charged counter-electrode. In the case of a cationic electrocoat composition, the workpiece serves as a cathode. Sufficient electrical current is applied between the electrodes to deposit a substantially continuous, adherent film of the electrocoating composition onto the surface of the electroconductive substrate. The electrocoated substrate is then conveyed to a rinsing operation where it is rinsed with an aqueous rinsing composition. Typical rinsing operations have multiple stages which can include closed loop spray and/or dip applications such as are described below. For example, in a spray rinse application the electrocoated substrate exits the electrocoating tank and is conveyed over a rinse tank while an aqueous rinsing composition is spray applied to the electrocoated surfaces of the substrate. Excess rinsing composition is permitted to drain from the substrate into the rinse tank below. The rinsing composition is then recirculated to the spraying apparatus for subsequent spray applications.
Recirculating the coating or rinsing compositions is both economically and environmentally desirable. However, the combination of organic nutrients, warmth, aeration and recirculation in an aqueous coating system creates an environment conducive to bacterial and fungal growth. These microorganisms, if left unchecked, can adversely affect the quality and appearance of the electrodeposited coating. Microorganisms present in the coating or rinsing compositions can cause pH shifts, particulate “dirt” deposition and biofouling, which detrimentally affect the appearance of the coating and reduce system performance. Organic acids, such as lactic and acetic acids, commonly used to neutralize the basic groups of the cationic electrodeposition composition, for such microorganisms are a major nutrient source.
Also, ethylene glycol ether alcohols can suppress microorganism growth in electrocoating compositions, but are undesirable ecologically.
A microbiocide composition containing a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one (commercially available as KAYTHON® LX from Rohm and Haas Co.) has been used commercially in electrodeposition coatings and rinse compositions as the sole microorganism control composition. Although effective for inhibiting and/or controlling the growth of microorganisms in such systems, this microbiocide is relatively expensive and can cause a rougher appearance than a coating composition without this microbiocide. Moreover, such microbiocide compositions can contain, as inert ingredients, metal salts such as magnesium nitrate and magnesium chloride. The presence of metal ions of these salts in electrodeposition systems is undesirable because the metals can cause coating defects due to gas generation at the cathode. Furthermore, such a microbiocide typically is not included as a component in the coating composition, but, rather, is added to the electrodeposition system in an assembly plant setting. Microbiocides can lose their effectiveness over time as they are depleted from the bath and constant replenishment is necessary. Moreover, some of the microbiocides discussed above can require special handling and disposal means.
U.S. Pat. No. 6,017,431 discloses sulfamic acid (an inorganic acid) as a neutralizing agent for cationic electrocoating compositions and for the adjustment of pH of the electrodeposition bath compositions containing these compositions. Such electrodeposition baths are more resistant to the adverse effects of microorganism growth when the amount of sulfamic acid in the electrodepositable composition is greater than 90 up to 100 equivalent weight percent. However, due to certain processing issues which can arise during the preparation of electrodeposition composition components containing sulfamic acid as the neutralizing agent, the inclusion of an organic acid in such electrodepositable compositions often is desirable. As mentioned above, however, organic acids, which are present to rectify these difficulties, can be consumed by bacteria. Moreover, in such cases, the indigestible sulfamic acid can be post-added to the electrodepositable composition to replace the organic acids consumed by bacteria.
In view of the foregoing, a need exists for an inherently biodegradation resistant electrodepositable coating composition that requires minimal maintenance. The elimination of the necessity to handle toxic microbiocides that often are used in electrodeposition baths neutralized with organic acids is also desirable. It was surprising to find that cationic electrodeposition compositions containing certain &agr;-alkoxycarboxylic acids exhibit improved resistance to biodegradation with use of little or no microbiocide.
SUMMARY OF THE INVENTION
In accordance with the present invention, a biodegradation resistant cationic electrodeposition composition is provided. The biodegradation resistant cationic electrodeposition composition comprises a resinous phase dispersed in an aqueous medium. The resinous phase comprises an electrodepositable resin having cationic onium salt groups which have been at least partially solubilized with an &agr;-alkoxycarboxylic acid of formula (I),
where R is C
1
to C
6
alkyl or aryl and R′ is H or C
1
to C
2
alkyl.
Also provided is a method of electrocoating a conductive substrate serving as a cathode in an electrical circuit comprising the cathode and an anode which are immersed in an aqueous electrodeposition bath comprising the biodegradation resistant cationic electrodepositable composition described above, and substrates coated by the method.
DETAILED DESCRIPTION OF THE INVENTION
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding technique
Kaylo Alan J.
McCollum Gregory J.
Zawacky Steven R.
Altman Deborah M.
Mayekar Kishor
PPG Industries Ohio Inc.
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