Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-11-02
2002-08-13
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C525S185000
Reexamination Certificate
active
06432483
ABSTRACT:
The present invention generally relates to low VOC (volatile organic component) high solids coating compositions and more particularly to fast-drying two-component compositions suited for automotive coatings.
BACKGROUND OF THE INVENTION
The time needed before a primer-surfacer on an auto body surface can be sanded after application, without fouling the sandpaper, is becoming increasingly important since it determines how many cars can be repaired per day. It is typical for traditional two-pack (2K) non-isocyanate-based thermoset resin systems to employ polyacrylics containing pendant acetoacetate (AcAc) groups as zone component of the binder resin and an amine and/or ketimine oligomer and/or polymer as the other. However, such an approach does not provide sufficiently rapid formation of robust coatings that can be sanded easily since viscosity restrictions imposed by the Environmental Protection Agency of the United States (EPA) must be adhered to (≦4.8 lbs. volatile organic content (VOC)/gallon).
Several attempts have been made to produce rapid formation of coatings. U.S. Pat. No. 4,772,680 (hereafter '680 patent) discloses polymers of average molecular weight in the range 1000 to 100,000. However, to comply with the EPA regulation above, the average molecular weight must actually be less than about 45,000, otherwise the coating composition becomes too viscous to be of practical use or has a very short pot life of less than 30 minutes. One approach to overcome this problem is to utilize coatings composition containing polymers having high. glass transition temperatures (Tgs). However, with the ever increasing restrictions on the permissible amount of the VOC that can be released in the atmosphere and attendant molecular weight limitations, it is becoming more difficult to polymerize polymers having Tg's above 40° C. and having sufficient pendant AcAc groups (e.g., 10 to 40% by weight). Such polymers having the desired cross-link density are required to achieve good repair durability. Therefore, one cannot apply the teaching of the '680 patent for polymers with Tg's above about 40° C. and still meet the EPA restrictions. The instant invention provides for coating compositions containing high Tg polymers and it represents a significant advance over the teachings of the '680 patent.
SUMMARY OF THE INVENTION
The present invention is directed to a coating composition comprising:
a crosslinking component comprising a polyamine, a polyketimine, or a combination thereof, wherein said polyamine has an average of at least two amine functionalities per polyamine molecule and wherein said polyketimine has an average of at least two ketimine functionalities per polyketimine molecule; and
a binder component comprising:
a polyacetoacetate having at least two acetoacetate functionalities, said polyacetoacetate being polymerized from a monomer mixture comprising a methacrylate monomer, a styrene monomer, or a combination thereof, said methacrylate and styrene monomers each having a bulky pendant moiety, wherein a coating from said coating composition at a two-hour cure has a Persoz hardness of about greater than or equal to 60 for a dry film thickness of greater than or equal to 40 microns.
The present invention is also directed to a method of producing a coating on a substrate, said method comprising:
mixing a crosslinking component with a binder component to form a pot mix, said crosslinking component comprising a polyamine, a polyketimine, or a combination thereof, said polyamine having an average of at least two amine functionalities per polyamine molecule and said polyketimine having an average of at least two ketimine functionalities per polyketimine molecule; said binder component comprising a polyacetoacetate having at least two acetoacetate functionalities, said polyacetoacetate being polymerized from a monomer mixture comprising a methacrylate monomer, a styrene monomer, or a combination thereof, said methacrylate and styrene monomers each having a bulky pendant moiety;
applying a layer of said pot mix on said surface; and
curing said layer under ambient conditions to form said coating on said surface of said substrate, said coating at a two-hour cure has a Persoz hardness of about greater than or equal to 60 for a dry film thickness of greater than or equal to 40 microns.
One of the advantages of the present invention is its low VOC, which is significantly below the current guidelines of the EPA.
Another advantage of the composition of the present invention is that it is free from isocyanate groups. As a result, it has less toxicity than conventional polyurethane coatings prepared from isocyanate group-containing compounds. This reduced toxicity is particularly helpful for auto paint repair shops that may not have physical facilities required to handle more toxic compositions containing isocyanate functionalities.
Yet another advantage of the composition of the present invention is that it reduces the time-to-sand, before the coating can be sanded without fouling the sandpaper, thereby increasing the number of repairs that could be performed in a day.
Still another advantage of the composition of the present invention is that it does not release pungent odors often associated with low molecular weight compounds containing acrylate functional moieties. Compounds such as these are employed in some of the conventional low VOC non-isocyanate containing coating compositions.
DETAILS OF THE INVENTION
As defined herein:
“Two-pack coating composition” means a thermosetting composition comprising two components that are stored in separate containers, which are typically sealed for increasing the shelf life of the components of the coating composition. The components are mixed just prior to use to form a pot mix, which has a limited pot life, typically a few minutes, such as 15 minutes to 45 minutes to a few hours, such as 2 hours to 6 hours. The pot mix is applied as a layer of desired thickness on a substrate surface, such as an autobody. After application. the layer dries and cures to form a coating on the substrate surface having desired coating properties, such as solvent resistance.
“Low VOC coating composition” means a coating composition that is less than about 0.6 kilogram of organic solvent per liter (4.8 pounds per gallon) of the composition, as determined under the procedure provided in ASTM D3960.
“High solids composition” means a coating composition having a solids component of above 20 percent, preferably in the range of from 25 to 95 percent and more preferably in the range of from 30 to 80 percent, all in weight percentages based on the total weight of the composition.
“GPC weight average molecular weight” means a weight average molecular weight measured by utilizing gel permeation chromatography. A high performance liquid chromatograph (HPLC) supplied by Hewlett-Packard, Palo Alto, Calif. was used. Unless stated otherwise, the liquid phase used was tetrahydrofurane and the standard was polymethyl methacrylate.
“Polydispersity” means GPC weight average molecular weight divided by GPC number average molecular weight.
“Polymer solids” or “Binder solids” means a polymer or binder in its dry state.
“Bulky pendent moiety” means a group on a polymer that restricts segmented motion of the polymer backbone.
The present invention is directed to a coating composition suited for various coating processes, particularly in automotive refinishing processes used for coating autobodies. The composition is a two-pack composition, which includes a crosslinking component and a binder component.
Typically, about a 3-hour cure time is required for conventional polyacetoacetate-ketimine binder systems, before coatings therefrom can be sanded without fouling the sandpaper. However, a need exists for a coating composition that provides for a coating that can be sanded in 1 hour or less without adversely affecting holdout [i.e., distinctness of image (DOI) and feather-edge holdout (FIE)] once it is topcoated with a base coat and a clear coat, or with a single-s
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