Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-06-29
2001-07-03
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S222000, C525S227000, C525S230000, C525S418000, C525S424000, C525S426000, C525S427000
Reexamination Certificate
active
06255398
ABSTRACT:
The present invention is directed to thermosetting compositions in which acrylic copolymers are used as carriers for fluorocarbon resins. More particularly, it is directed to such compositions having a high solids content much desired but not achieved until now. The coatings are particularly useful in coil coating processes.
BACKGROUND OF THE INVENTION
Fluorocarbon resins, such as polyvinylidene fluoride (PVDF), are useful in formulating coatings with excellent weathering resistance. Because fluorocarbons have poor rheology and pigment wetting characteristics, it is common to add a modifying polymer to fluorocarbon coating compositions. A typical binder polymer is an acrylic polymer, and both thermosetting and thermoplastic acrylics resins have been utilized for this purpose.
Coatings containing fluorocarbons and thermosetting resins, wherein the acrylic polymers have functionality that renders them cross-linkable, and in which a cross-linker is included in the resin composition, are sold by several coating producers. U.S. Pat. No. 3,779,976 describes combinations of polyvinyl fluoride and a thermoplastic copolymer of methyl methacrylate and 3-(2-methacryloxyethyl)-2,2-spirocyclohexyl oxazolidine. Thermosetting binder polymers are preferred for many applications because of the improved mechanical and chemical resistance they provide.
As noted in U.S. Pat. No. 5,185,403, a high fluorocarbon resin content is required to achieve optimal weathering and chemical resistance. Many coating applications call for coating compositions in which the fluorocarbon resin comprises 70 wt. % or more of the resin and the binder polymer the remainder. Coating compositions containing fluorocarbon resins, particularly PVDF, and thermosetting acrylic resins tend, however, to have relatively high viscosities at the solids levels used. For some coating applications, particularly coil coating, it is desirable to have high fluorocarbon resin content, a high total solids content and a lower viscosity than is currently available.
In a coil coating operation, a coil of sheet metal is uncoiled as it is pulled through a series of rollers, one or more of which is a paint applicator roller, at up to 1000 feet per minute. The paint is picked up by a roller rotating in a paint pan and transferred to an applicator roller, thence to the moving sheet metal. It is then passed through a curing oven and coiled again for the market. The cost of coating could be lessened greatly if the solids content of the paint were high enough that a sufficiently thick layer could be applied in one pass through the coating process. But a high solids content usually means that the paint's viscosity would be too great for efficient transfer from roller to roller.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a fluorocarbon/acrylic resin coating composition having a combination of high fluorocarbon content, high solids content, and a low viscosity suitable for coil coating.
This and other objects which will be apparent from the following description of the invention are achieved by a coating composition having a volume solids content of from about 45 to about 55 percent and comprising a fluorocarbon resin, a cross-linkable acrylic binder copolymer having a weight average molecular weight (Mw) of from about 8,000 to about 18000, a cross-linking agent for the acrylic binder copolymer, and a solvent system consisting essentially of mixtures of 4 or more of the following solvents: aliphatic ketones having from 4 to 10 carbon atoms, alkyl esters having a total of from 5 to 7 carbon atoms, aromatic acid esters having from 8 to 12 carbon atoms, lactones, and aromatic solvents.
The binder copolymer is formed from acrylic monomers that include:
(a) acrylic esters having no additional functionality,
(b) acrylic esters such as 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxyethyl acrylate, and acetoacetoxyethyl methacrylate having functionality for reaction with cross-linking agents, such as aminoplasts, phenoplasts, isocyanates; and
(c) acryloxyalkyl-oxazolidines which function to lower and stabilize the viscosity of the fluorocarbon resin-based coating composition.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
Herein, unless stated otherwise, components of the coating composition will be stated in parts per hundred resin (phr) based upon the total weight of polymer (i.e., the fluorocarbon resin and the acrylic polymer); monomer components of polymers are expressed as mole percentages; and other percentages expressed are by weight.
Suitable fluorocarbon resins for use in the invention include polyvinylidene fluoride, such as those sold under the tradename Kynar; polyvinyl fluoride, polytetrafluoroethylene; copolymers of vinylidene fluoride and tetrafluoroethylene, such as that sold under the trademark Kynar SL; a fluoroethylene/vinyl ester/vinyl ether sold under the trademark Fluonate; proprietary vinylidene fluoride-based polymers sold under the trademarks Kynar 500 and Kynar SL; and mixtures of fluorocarbon resins. The fluorocarbons are high molecular wt., typically having molecular weights (weight average) above 250,000. The fluorocarbon resins are utilized in powder form. The powders are insoluble in the solvents used in the coating compositions of the present invention, but are swelled by the solvents. The fluorocarbon resin comprises between about 40 and 90 phr of the polymer, typically between about 70 and about 80 phr; 70 phr being considered to be near optimal for chemical and mechanical resistance. Above 70 phr, minor improvements in chemical and mechanical resistance may be achieved, but with increasing costs due to the high cost of the fluorocarbon resins.
In accordance with the present invention, a unique acrylic binder polymer is provided which is formed from three types of acrylate monomers. The first type of acrylate monomer (a) is an acrylic ester having no additional functionality. This monomer has the general formula:
R
1
2
C═CR
2
—COOR
3
wherein R
1
and R
2
are the same or different and are each selected from —H and C
1
-C
3
alkyl and R
3
is C
1
-C
6
alkyl. Examples of this type (a) of acrylate monomer include methyl methacrylate, ethyl methacrylate, ethyl acrylate, methyl acrylate, butyl acrylate and butyl methacrylate.
The second type of acrylate monomer (b) is an acrylic acid ester having the general formula:
R
1
2
C═CR
2
—COOR
4
wherein R
1
and R
2
are the same or different and are selected from H, and C
1
-C
3
alkyl, and R
4
is the residue of an alcohol having additional OH or beta-diketone functionality. More specifically, R
4
is preferably the residue of a C
2
or C
3
glycol or has the formula:
Examples of this type (b) of acrylate monomer include 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate,2-hydroxypropyl methacrylate and acetoacetoxyethyl methacrylate.
The third type (c) of acrylic monomer is an acryloxy-alkyl oxazolidine, as described in U.S. Pat. No. 3,037,006, the teachings of which are incorporated herein by reference. Specifically, acryloxyalkyl-oxazolidines useful in the invention have the formula:
wherein n is an integer having a value of 1 to 2, m is an integer having a value of 2 to 3; R
5
and R
6
are optionally directly joined together to form a 5- to 6-carbon ring with the attached carbon atom of the ring in the formula; R
5
, when not directly joined to R
6
, is selected from the group consisting of hydrogen, phenyl, benzyl, and (C
1
-C
1 2
) alkyl groups; R
6
, when not directly joined to R
5
, is selected from the group consisting of hydrogen and (C
1
-C
4
) alkyl groups. When joined together, R
5
and R
6
are selected from the group consisting of pentamethylene and tetramethylene. Representative of this type (c) of monomer is 3-(2-methacryloxyethyl)-2,2-spirocyclohexyl oxazolidine (MESO).
The first type (a) monomer provides flexibility to the polymer and is used at between about 85 and about 97.5 mole percent. This first type (a) monomer is necessary to provide the cur
Allman Jack C.
Lang James V.
Zupancic Joseph J.
BASF Corporation
Nutter Nathan M.
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