Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-04-06
2001-10-02
Mulcahy, Peter D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S508000, C524S556000
Reexamination Certificate
active
06297311
ABSTRACT:
This invention relates to an aqueous coating composition containing an acrylic addition polymer which has carboxyl groups in which the carboxyl groups are derived substantially from a carboxyl functional chain terminating agent. It also relates to a processes for producing the coating composition to a process of coating using, the composition and to a coated substrate obtainable by the coating process.
One major known class of coating compositions comprises acrylic addition polymer as a film former. These can be carried in an organic solvent or an aqueous medium. The polymer can have various functional groups such as hydroxyl groups. The compositions are applied as a layer to the surface of a substrate and either left to dry and cure at room temperature or else heated to initiate or speed the drying and curing process. These compositions can also contain a crosslinker which reacts with functional groups on the polymer so as to crosslink the final coating film. The crosslinker can be for example a polyisocyanate or a melamine formaldehyde resin. During drying and curing the solvent or water evaporates and the polymer and cross-linker react together so as to produce a crosslinked coating film.
Due to environmental considerations there is a general trend in the coatings industry towards coating compositions with reduced organic solvent content. Coatings with a lower organic solvent content emit lower levels of solvent when they are used and so are less polluting of the atmosphere.
One way to achieve a lower solvent content is to use waterborne compositions. One method of incorporating acrylic addition polymers into water is to make them carboxyl (—COOH) functional by the incorporation of some carboxyl functional ethylenically unsaturated monomer such as acrylic acid during their manufacture, and neutralising at least some of the carboxyl groups in the aqueous composition by adding a base such as alkali metal hydroxide, ammonia or an amine. The resulting neutralised carboxyl groups stabilise the polymer in dispersion in water.
A problem which we have discovered with the use of carboxyl functional monomers is that a relatively high amount needs to be used if the polymers are to remain in stable solution or dispersion in water. This high level of carboxyl leads to poor water resistance when these polymers are made into coating compositions. Alternatively, when a lower level of carboxyl monomer is incorporated, the polymers exhibit poor storage stability.
Coatings used for the painting of motor vehicles are required to have very good physical properties such as hardness and resistance to water and solvents. The coating compositions are also required to have good application and appearance properties so that they are easy to apply to a substrate using spray application and result in final films having good gloss. It is very important that these compositions also have good storage stability so that these properties do not deteriorate in the period between manufacture and use, which is often many months and can be several years. Poor water resistance and/or poor storage stability has been a major problem preventing more widespread commercial acceptance of waterborne coatings comprising carboxyl functional acrylic addition polymers, particularly in the motor vehicle refinish field.
One further component of these types of addition polymers is a chain transfer agent. Chain transfer agents are compounds such as thiols which are added to the polymerisation mixture to control molecular weight during the process of making the polymer. They act by transferring a hydrogen radical to the end of the growing polymer chains which prevents further polymerisation and so limits molecular weight. The residue of the chain transfer agent then initiates a new polymer chain and becomes reacted onto the end of the new polymer chain and thus part of the polymer. Chain transfer agents are particularly important where polymers are required having a low molecular weight and hence a low solution viscosity.
We have found that we can produce water dispersible addition polymers which have carboxyl functionality, a proportion of which is derived from the use of a carboxyl functional chain transfer agent, and that these polymers will form stable dispersions in water at a surprisingly low level of carboxyl groups. Another way of looking at this is that polymers with comparable carboxyl group content are more stable if a proportion of the carboxyl groups are derived from a carboxyl functional chain transfer agent. Without being limited by this theory, it seems that the partially neutralised carboxyl groups stabilise the polymer more efficiently when they are placed on the ends of the polymer by being part of the chain transfer agent than when placed randomly along the polymer by being part of a carboxyl functional monomer. The use of a low level of carboxyl groups allows the formulation of stable aqueous coating compositions which have improved water resistance compared to those compositions of comparable storage stability produced from polymers having carboxyl functionality substantially derived from carboxyl functional monomer. The use of a carboxyl functional chain transfer agent also surprisingly gives rise to better properties than the use of a carboxyl functional initiator which might have been expected to be similar in effect.
According to the present invention there is provided an aqueous coating composition comprising a dispersion in an aqueous medium of an acrylic addition polymer having carboxyl groups in which a proportion of the carboxyl groups are derived from a carboxyl functional chain transfer agent, the composition also comprising a base in an amount sufficient to at least partially neutralise the carboxyl groups on the polymer. Preferably, the polymer also has hydroxyl groups.
The acrylic addition polymer is derived from polymerisable ethylenically unsaturated monomers such as vinyl or acrylic monomers. The polymer comprises structural units, carboxyl functional units derived from a carboxyl functional chain transfer agent and optionally comprises other carboxyl functional units, hydroxyl functional units and other functional units.
When used herein, the term acrylic monomer refers to acrylic or methacrylic acid or their esters. The term (meth) acrylate refers to both the acrylate and methacrylate equally and the term (meth)acrylic acid refers to acrylic or methacrylic acid equally.
The structural units are derived from monomers which are non-functional, that is they do not have reactive functional groups. Examples of non-functional monomers are alkyl esters of (meth)acrylic acid and non-functional vinyl monomers.
Examples of suitable alkyl esters of (meth)acrylic acid are C
1-12
alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, and n-propyl (meth)acrylate. Examples of non-functional vinyl monomers are styrene and alpha-methyl styrene.
Examples of other carboxyl functional units are those derived from unsaturated carboxyl functional monomers such as acrylic or methacrylic acid.
The optional hydroxyl functional units are derived from hydroxyl functional vinyl or acrylic monomers. An example of a hydroxyl functional vinyl monomer is vinyl alcohol. Examples of hydroxyl functional acrylic monomers are hydroxyethyl (meth)acrylate, and hydroxypropyl (meth)acrylate and hydroxybutyl (meth)acrylate.
Other examples of suitable hydroxyl functional acrylic monomers are the reaction products of glycidyl (meth)acrylate with mono-carboxylic acids, such as versatic acid and the reaction product of (meth)acrylic acid with monoepoxy compounds such as Cardura E (the glycidyl ester of versatic acid; trade mark of Shell).
Preferably, the polymer comprises 10 to 50% by weight of hydroxyl functional units, more preferably 10 to 40% by weight. Preferably the polymer has a hydroxyl value of 5 to 500 mgKOH/g of polymer, more preferably 50 to 250.
Other functional units are derived from monomers which carry reactive groups other than hydroxyl groups or carboxyl g
Casper James William
Fenn David Robert
Redman Richard Paul
Millman Dennis G.
Mulcahy Peter D.
PPG Industries Ohio Inc.
Uhl William J.
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