Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2001-05-15
2002-05-21
Sellers, Robert E. L. (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
Reexamination Certificate
active
06391391
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to thermosetting compositions of one or more carboxylic acid functional polymers and an epoxide functional crosslinking agent, such as tris(2,3-epoxypropyl) isocyanurate. The carboxylic acid functional polymer is prepared by atom transfer radical polymerization, and has well defined polymer chain structure, molecular weight and molecular weight distribution. The present invention also relates to methods of coating a substrate, and substrates coated by such methods.
BACKGROUND OF THE INVENTION
Reducing the environmental impact of coatings compositions, in particular that associated with emissions into the air of volatile organics during their use, has been an area of ongoing investigation and development in recent years. Accordingly, interest in powder coatings has been increasing due, in part, to their inherently low volatile organic content (VOC), which significantly reduces air emissions during the application process. While both thermoplastic and thermoset powder coatings compositions are commercially available, thermoset powder coatings are typically more desirable because of their superior physical properties, e.g., hardness and solvent resistance.
Low VOC coatings are particularly desirable in a number of applications, e.g., the automotive original equipment manufacture (OEM), industrial and appliance markets, due to the relatively large volume of coatings that are used. However, in addition to the requirement of low VOC levels, many manufactures have strict performance requirements of the coatings that are used. Examples of such requirements include, good exterior durability, solvent resistance, and excellent gloss and appearance. While liquid topcoats can provide such properties, they have the undesirable draw back of higher VOC levels relative to powder coatings, which have essentially zero VOC levels.
Powder coatings based on carboxylic acid functional polymers cured with epoxide functional crosslinkers, such as tris(2,3-epoxypropyl) isocyanurate, (“epoxy cured powder coatings”) are known and have been developed for use in a number of applications, such as industrial and automotive OEM topcoats. The epoxide functional crosslinker tris(2,3-epoxypropyl) isocyanurate is also commonly referred to as triglycidyl isocyanurate (TGIC). Such epoxy cured powder coatings, in which the crosslinking agent is TGIC, are described in, for example, U.S. Pat. Nos. 3,935,138, 4,242,253, 4,605,710, 4,910,287, 5,264,529 and 5,684,067. However, their use has been limited due to deficiencies in, for example, flow, appearance and storage stability. The binder of epoxy cured powder coatings compositions typically comprises polyester and/or acrylic polymers having carboxylic acid functionality. The carboxylic acid functional polymers used in such epoxy cured powder coatings compositions are typically prepared by standard, i.e., non-living, radical polymerization methods, which provide little control over molecular weight, molecular weight distribution and polymer chain structure.
The physical properties, e.g., glass transition temperature (Tg) and melt viscosity, of a given polymer can be directly related to its molecular weight. Higher molecular weights are typically associated with, for example, higher Tg values and melt viscosities. The physical properties of a polymer having a broad molecular weight distribution, e.g., having a polydispersity index (PDI) in excess of 2.0 or 2.5, can be characterized as an average of the individual physical properties of and indeterminate interactions between the various polymeric species that comprise it. As such, the physical properties of polymers having broad molecular weight distributions can be variable and hard to control.
The polymer chain structure, or architecture, of a copolymer can be described as the sequence of monomer residues along the polymer back bone or chain. For example, a carboxylic acid functional copolymer prepared by standard radical polymerization techniques will contain a mixture of polymer molecules having varying individual carboxylic acid equivalent weights. Some of these polymer molecules can actually be free of carboxylic acid functionality. In a thermosetting composition, the formation of a three dimensional crosslinked network is dependent upon the functional equivalent weight as well as the architecture of the individual polymer molecules that comprise it. Polymer molecules having little or no reactive functionality (or having functional groups that are unlikely to participate in crosslinking reactions due to their location along the polymer chain) will contribute little or nothing to the formation of the three dimensional crosslink network, resulting in less than desirable physical properties of the finally formed polymerizate, e.g., a cured or thermoset coating.
The continued development of new and improved epoxy cured powder coatings compositions having essentially zero VOC levels and a combination of favorable performance properties is desirable. In particular, it would be desirable to develop epoxy cured powder coatings compositions that comprise carboxylic acid functional polymers having well defined molecular weights and polymer chain structure, and narrow molecular weight distributions, e.g., PDI values less than 2.5. Controlling the architecture and polydispersity of the carboxylic acid functional polymer is desirable in that it enables one to achieve higher Tg's and lower melt viscosities than comparable carboxylic acid functional polymers prepared by conventional processes, resulting in thermosetting particulate compositions which are resistant to caking and have improved physical properties.
International patent publication WO 97/18247 and U.S. Pat. Nos. 5,763,548 and 5,789,487 describe a radical polymerization process referred to as atom transfer radical polymerization (ATRP). The ATRP process is described as being a living radical polymerization that results in the formation of (co)polymers having predictable molecular weight and molecular weight distribution. The ATRP process is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). The '548 and '487 patents and WO 97/18247 patent publication also describe (co)polymers prepared by ATRP, which are useful in a wide variety of applications, for example, with paints and coatings.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided, a thermosetting composition comprising a co-reactable solid, particulate mixture of:
(a) polycarboxylic acid functional polymer prepared by atom transfer radical polymerization initiated in the presence of an initiator having at least one radically transferable group, and in which said polymer contains at least one of the following polymer chain structures I and II:
—[(M)
p
—(G)
q
]
x
— I
and
—[(G)
q
—(M)
p
]
x
— II
wherein M is a residue, that is free of carboxylic acid functionality, of at least one ethylenically unsaturated radically polymerizable monomer; G is a residue, that has carboxylic acid functionality, of at least one ethylenically unsaturated radically polymerizable monomer; p and q represent average numbers of residues occurring in a block of residues in each polymer chain structure; and p, q and x are each individually selected for each structure such that said polycarboxylic acid functional polymer has a number average molecular weight of at least 250; and
(b) epoxide functional crosslinking agent having at least two epoxide groups.
In accordance with the present invention, there is also provided a method of coating a substrate with the above described thermosetting composition.
There is further provided, in accordance with the present invention, a multi-component composite coating composition comprising a base coat deposited from a pigmented film-forming composition, and a transparent top coat applied over the base coat. The transparent top coat comprises the above described thermosetting composition
Barkac Karen A.
Coca Simion
Franks James R.
Humbert Kurt A.
Lamers Paul H.
PPG Industries Ohio Inc.
Sellers Robert E. L.
Uhl William J.
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