Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1999-12-15
2002-01-29
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S486000, C204S484000
Reexamination Certificate
active
06342144
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to multilayer films or cured multilayer coatings and a process for the production of such. More particularly, the invention relates to multilayer films having a first layer applied by electrophoretic deposition processes and a second layer applied thereto wet-on-wet, followed by a joint and simultaneous curing of the first and second layers.
BACKGROUND OF THE INVENTION
Automotive manufacturers have traditionally relied upon electrophoretically deposited coatings, both cathodic and anodic, for significant protection against corrosion of the underlying metal automotive body. “Electrocoat” as used herein may refer to both curable coating compositions used in electrophoretic deposition processes and to cured coating films obtained from the curing of coating compositions applied by electrophoretic deposition processes.
During electrodeposition, an ionically-charged polymer having a relatively low molecular weight is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin, and applying an electrical potential between the substrate and a pole of opposite charge, usually a stainless steel electrode. This produces a relatively soft coating of low molecular weight on the substrate. Traditionally, this coating is converted to a hard high molecular weight coating by curing or crosslinking of the resin, usually upon exposure to elevated temperatures prior to any further application of subsequent coating layers.
However, automotive manufacturers have long desired to either lower the temperature required to cure the electrodeposited coating or to eliminate such a separate curing step all together. In particular, automotive manufacturers would like to use electrodeposited coatings as the first part of a wet-on-wet process. It will be appreciated that “wet-on-wet” typically refers to a coating application process wherein subsequent coatings are applied directly to a substantially uncured previously applied coating. The two or more uncured coatings are then jointly baked or cured. “Wet-on-wet” may encompass processes wherein the first coating is subjected to conditions which eliminate solvent and/or reduce the volume of the first film, but stop short of complete cure or crosslinking.
The elimination of the separate bake step following application of a traditional electrocoat composition would result in significant energy and space savings, due to the elimination of an entire baking oven.
Thus, it is desirable to provide a multilayer film composition having optimum performance properties which is at least partially obtained through the application of a curable basecoat, primer or sealer coating directly onto a substantially uncured or “wet” previously electrodeposited coating, followed by the joint or simultaneous curing of both the wet electrodeposited coating and the wet basecoat, primer or sealer coating.
The prior art has long attempted to provide methods to make such cured multilayer coating or film compositions and/or coating compositions for use in said methods.
For example, U.S. Pat. No. 3,998,716, Masar et al., discloses thermosetting coatings wherein multiple coats of thermosetting organic coating material, including a topcoat of powder paint are cured by employing a single baking step.
U.S. Pat. No. 5,507,928, Böhmert et al., discloses a process for the production of multi-layer lacquer coatings by electrophoretic deposition of a first coating layer of a first aqueous coating composition onto an electrically conductive substrate, application of a second coating layer based on a second, powder coating composition and joint baking of the coating layers so obtained, which process is characterized in that a powder coating composition is used for the second layer which is based on binders which contain no diene-based polymer units, wherein the coating composition is selected such that the minimum baking temperature range of the second coating layer is above that of the first coating layer or overlaps with this range in such a manner that the lower limit of the range of the second coating layer is above the lower limit of the range of the first coating layer.
However, the use of a powder coating composition as the second applied coating is not always desirable.
U.S. Pat. No. 5,376,457, Smith, discloses a process of applying a finish to an electrically conductive vehicle body wherein an electrocoated coating is dehydrated to a state of sufficient dryness to permit spray application of a water-based primer while maintaining the coat cool enough to avoid fusing the electrocoat coating. The electrocoat and subsequently spray applied water-based primer are then concurrently baked so as to “fuse both of them”.
In U.S. Pat. No. 5,869,198, Erne et al., a process is disclosed for the multi-layer coating of electrically conductive substrates by the electrophoretic deposition of a first coating layer comprising an electrophoretic deposition of a first coating layer comprising an electrophoretically depositable aqueous coating medium, and the subsquent application of further coating layers, which is characterized in that a second coating layer comprising a first color- and/or effect imparting base lacquer coating medium is applied wet-into-wet to the first coating obtained by electrophoretic deposition, and the first and second coating layers thus obtained are jointly stoved, whereupon a third coating layer comprising a second color- and/or effect-imparting base lacquer coating medium is applied and a fourth coating layer comprising a clear lacquer coating medium is applied wet-into-wet thereto and the third and fourth coating layers are stoved jointly, wherein the total dry coat thickness (the sum of the coat thickenesses) of the second and third coating layers produced from the base lacquer coating media is between 15 and 40 &mgr;m, and the proportion of the second coating layer is between 20 and 50% of the total dry coat thickness of the second and third coating layers.
U.S. Pat. No. 4,619,746, Delaney et al., discloses a process for applying a non-electrophoretic top-coating over an electrophoretically applied base coating and curing the composite coating by employing a single curing step. Isocyanates, and most preferably blocked isocyanates, are the preferred curing agent for the electrophoretically applied basecoat and the non-electrophoretic topcoating composition.
However, the use of isocyanate curing agents, particularly blocked isocyanate curing agents, is no longer favored, especially in electrocoat compositions. Blocked polyisocyanates require high temperatures (e.g., 176° C. or more) to unblock and begin the curing reaction. The resulting coating can also be susceptible to yellowing. Moreover, the volatile blocking agents released during cure can cause other deleterious effects on various coating properties, as well as increasing VOC. In addition, use of some of the volatile blocking agents may give rise to environmental concerns. Finally, the volatile blocking agents account for significant and disadvantageous weight loss upon crosslinking.
U.S. Pat. No. 5,389,406, Doebler et al., describes a process for producing multilayer coatings in which a first coating layer of a first aqueous coating medium is applied to an electrically conducting substrate by electrophoretic deposition, is provided wet-in-wet with a second coating layer of a second aqueous coating medium, followed by joint stoving, wherein a coating medium based on one or more vehicles stabilized by ionic groups and which crosslink on stoving with the formation of urethane groups is used for the second coating layer, and the coating media are selected so that the maximum pigment/vehicle weight ratio of the first coating medium is 1:1, that the ratio of the pigment/vehicle weight ratio of the first coating medium to that of the second coating medium is up to 1.8, and the minimum stoving temperature interval of the second coating layer is above that of the first coating layer or overlaps the latter so that the lo
BASF - Aktiengesellschaft
Golota Mary E.
Mayekar Kishor
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