Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2001-09-27
2003-02-11
Wilson, D. R. (Department: 1713)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S388200, C428S411100, C428S500000, C525S09200D, C525S09200D, C525S123000, C525S124000, C525S162000, C528S045000, C528S085000
Reexamination Certificate
active
06517905
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to thermosetting (curable) compositions of one or more crosslinking agents and one or more hydroxyl functional polymers. The hydroxyl 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, substrates coated by such methods, and color-plus-clear composite coating compositions.
BACKGROUND OF THE INVENTION
Reducing the environmental impact of curable compositions, such as that associated with emissions into the air of volatile organic compounds during the application of curable coating compositions, has been an area of ongoing investigation and development in recent years. Accordingly, interest in high solids liquid coating compositions has been increasing, due in part to their comparatively lower volatile organic content (VOC), which significantly reduces air emissions during the application process.
Lower VOC coating compositions are particularly desirable in the automotive original equipment manufacture (OEM) market, due to the relatively large volume of coatings that are used. However, in addition to the requirement of lower VOC levels, automotive manufacturers have very strict performance requirements for the coatings that are used. For example, automotive OEM top coats are typically required to have a combination of good exterior durability and excellent gloss and appearance.
Thermosetting coatings containing hydroxyl functional polymers have been used extensively as automotive OEM primers and top coats. Such coating compositions typically comprise a crosslinking agent having at least two functional groups that are reactive with hydroxyl groups, and a hydroxyl functional polymer. The hydroxyl functional polymers used in such coating 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., 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 viscosities. The physical properties of a polymer having a broad molecular weight distribution, e.g., having a polydispersity index (PDI) in excess of 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 polymer can be described as the sequence of monomer residues along the polymer back bone or chain. A hydroxyl functional copolymer prepared by standard radical polymerization techniques will contain a mixture of polymer molecules having varying individual hydroxyl equivalent weights and polymer chain structures. In such a copolymer, the hydroxyl functional groups are located randomly along the polymer chain. Moreover, the number of functional groups is not divided equally among the polymer molecules, such that some polymer molecules may actually be free of hydroxyl functionality. In a thermosetting composition, the formation of a three-dimensional crosslinked network is dependent on 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 locations along the polymer chain) will contribute little or nothing to the formation of the three-dimensional crosslinked network, resulting in decreased crosslink density and less than optimum physical properties of the finally formed polymerizate, e.g., a cured or thermoset coating.
The continued development of new and improved thermosetting compositions having lower VOC levels and a combination of favorable performance properties is desirable. In particular, it would be desirable to develop thermosetting compositions that comprise hydroxyl functional copolymers having well-defined molecular weights and polymer chain structure, and narrow molecular weight distributions, e.g., PDI values less than 2.5.
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 a living radical polymerization that results in the formation of polymers having predictable molecular weight and molecular weight distribution. The ATRP process of these publications is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). These patent publications also describe polymers prepared by ATRP, which are useful in a wide variety of applications, for example, with paints and coatings.
It would be desirable to develop thermosetting compositions that comprise hydroxyl functional copolymers prepared using atom transfer radical polymerization, thus having well-defined molecular weights and polymer chain structure, and narrow molecular weight distributions. Such compositions would have lower VOC levels due to lower viscosities, and a combination of favorable performance properties, particularly in coatings applications.
SUMMARY OF THE INVENTION
In accordance with the present invention, a thermosetting composition is provided comprising:
(a) a crosslinking agent having at least two functional groups that are reactive with hydroxyl groups; and
(b) a non-gelled, hydroxyl functional polymer prepared by atom transfer radical polymerization, in the presence of an initiator having at least one radically transferable group, and wherein the polymer contains at least one of the following polymer chain structures:
—{(M)
p
—(G)
q
}
x
—
or
—{(G)
q
—(M)
p
}
x
—
wherein M is a residue, that is free of hydroxyl functionality, of at least one ethylenically unsaturated radically polymerizable monomer; G is a residue, that has hydroxyl 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 independently selected for each structure such that the hydroxyl functional polymer has a number average molecular weight of at least 250.
Also provided by the present invention are methods of coating a substrate using compositions of the present invention, substrates coated by such methods, and color-plus-clear composite coating compositions.
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 modified in all instances by the term “about”.
DETAILED DESCRIPTION
As used herein, the term “polymer” is meant to refer to both homopolymers, i.e., polymers made from a single monomer species, and copolymers, i.e., polymers made from two or more monomer species, as well as oligomers.
The hydroxyl functional polymer used in the composition of the present invention is a non-gelled polymer prepared by atom transfer radical polymerization, in the presence of an initiator having at least one radically transferable group. The polymer contains at least one of the following polymer chain structures:
—{(M)
p
—(G)
q
}
x
— (I)
or
—{(G)
q
—(M)
p
}
x
— (II)
where M is a residue, that is free of hydroxyl functionality, of at least one ethylenically unsaturated radically polymerizable monomer; G is a residue, that has hydroxyl functionality, of at least one ethylenically unsaturated radically polymerizable monomer; p and q represe
O'Dwyer James B.
Simpson Dennis A.
White Daniela
PPG Industries Ohio Inc.
Uhl William J.
Wilson D. R.
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