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
2001-02-16
2002-09-03
Cain, Edward J. (Department: 1714)
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
active
06444749
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to aqueous dispersions of particles of polymers with a glass transition temperature gradient, and to a method for the preparation of the same.
BACKGROUND OF THE INVENTION
Recent changes in the legislation concerning the emission of organic solvents have led to a growing interest in water borne coating systems for industrial applications at ambient temperature. Water borne coating systems have been used for a long time already in applications where the decorative aspects of the coating were more important than the protective properties. The aqueous polymer dispersions that are used as binders in these coatings are often prepared by means of the emulsion polymerization process. Often these binders are thermoplastic acrylic copolymers. The mechanical properties of these systems are related to the high molecular weights that can be obtained by the emulsion polymerization process. A serious drawback to the conventional emulsion polymerization process is the fact that substantial amounts of protective colloids and surfactants must be used. Conventional surfactants or emulsifiers and protective colloids are highly water sensitive and impart poor water resistance to coatings with binders thus prepared. Furthermore, conventional surfactants or emulsifiers and protective colloids often act as plasticizer for the polymers, resulting in reduced blocking resistance. Blocking refers to the tendency of painted surfaces to stick together when placed in contact with each other under pressure. It is important for a coating to have good resistance to blocking, especially when painted surfaces come into contact. The blocking resistance of a coating depends on the polymer's hardness and on external factors such as pressure, temperature, humidity, layer thickness, and drying conditions.
Aqueous polymer dispersions that are free of conventional surfactants using the salt of a monoester of an unsaturated dicarboxylic acid which is copolymerized with the other comonomers are known, for instance from U.S. Pat. No. 4,089,828. These emulsions may be used in textile finishing, paper sizing, industrial finishes, clear films, and air-dry paints. However, although the blocking resistance of these dispersions is satisfactory, paint formulations based on these dispersions possess a volatile organic content (VOC) that is too high according to the latest environmental legislation. Furthermore, paints based on dispersions disclosed in the patent above will lack the balance between hardness (blocking resistance) and flexibility that is required for surface coatings intended for substrates with low dimensional stability such as wood or for substrates that are deformed after the coating has been applied (e.g. metal).
Several techniques are known to those skilled in the art to improve the balance between the required hardness and the flexibility. Step-wise or staged addition procedures such as described in U.S. Pat. No. 3,448,173 or U.S. Pat. No. 3,562,235, where different reactants are introduced into the polymerization reactor at different stages of the reaction, result in the formation of polymer particles possessing a morphology referred to as “core-shell”. Other procedures that result in the formation of polymer particles with a non-homogeneous morphology are known, for instance from U.S. Pat. No. 3,804,881, where polymers are produced by continuously introducing a primary polymerizable feed composition from a primary feed source to a polymerization zone, which primary polymerizable feed composition is continually varying in compositional content during the continuous introduction. This variation is obtained by continuously adding to the primary feed source a different secondary polymerizable feed composition from a secondary feed source, so as to continually change the compositional content of the reactants in the primary feed source. Coating compositions formulated with polymer dispersions prepared according to the techniques described above may possess low VOC values and have a favorable balance between flexibility and blocking resistance, yet still contain substantial amounts of water-sensitive surfactants and/or protective colloids. The presence of these water-sensitive compounds in the final coating is undesirable, since it adversely influences properties such as water resistance and gloss retention upon weathering.
SUMMARY OF THE INVENTION
It has now been found that the aforementioned disadvantages can be circumvented by an aqueous dispersion of particles of a polymer having a functional group for cross-linking, which is obtainable by a free radical emulsion polymerization of at least two different ethylenically unsaturated monomers, a surfactant of the formula M
+
.
−
OOC—CH═CHCOOR, wherein R is C(6-22) alkyl and M
+
is Na
+
, K
+
, Li
+
, NH
4
+
, or a protonated or quaternary amine, and a co-reactive compound having at least two groups capable of reacting with the functional group for cross-linking, characterized in that the glass transition temperature (Tg) of the polymer at the center of the particle is different from the Tg of the polymer at the surface of the particle, the difference being at least 55° C., and the Tg of the polymer having the highest Tg is at least 40° C.
DETAILED DESCRIPTION OF THE INVENTION
Coatings having low VOC using these dispersions as binder show very good flexibility while maintaining superior blocking resistance and gloss retention properties.
A key feature of the present invention is the difference between the Tg of the polymer at the center and the Tg of the polymer at the surface of the particle, although it is not important whether the polymer at the surface has the higher or lower Tg. In principle, two types of particles are envisaged, i.e. core-shell particles with two different polymers in the core and the shell, respectively, having different glass transition temperatures (the gradient thus being a discontinuous gradient), and particles where the composition continuously varies in compositional content from the center to the surface of the particle. The concentration and the Tg gradient may have a linear character, but they may also be convex or concave or have any other character. Particles of this latter type are preferred and can be made by the method of U.S. Pat. No. 3,804,881.
The temperature gradient from the center to the surface leads to glass transition temperatures differing by at least 55° C., with the highest Tg being at least 40° C. More preferably, the difference in Tg is 75° C. and the highest Tg is at least 50° C.
The ethylenically unsaturated emulsion-polymerizable monomers are selected from a monovinylidene aromatic monomer, an &agr;,&bgr;-ethylenically unsaturated carboxylic acid ester monomer, a vinyl ester monomer, and combinations thereof.
Accordingly, preferred monovinylidene aromatic monomers include styrene, &agr;-methyl styrene, vinyl toluene, o-, m-, and p-methylstyrene, o-, m-, and p-ethylstyrene, and combinations thereof.
Preferred &agr;,&bgr;-ethylenically unsaturated carboxylic acid ester monomers include methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, butyl methacrylate, tert-butyl acrylate, 2-ethyl hexyl acrylate, and combinations thereof.
Preferred vinyl ester monomers include vinyl acetate, vinyl esters of versatic acid such as the monomers sold by Shell Chemicals as VEOVA™ 9, 10, and 11, acrylonitrile, and combinations thereof.
The monomeric mixture may, if desired, include a chain transfer agent (or more than one chain transfer agent), as well as minor amounts of monomers having more than one ethylenically unsaturated bond.
The polymers have pendant groups, preferably comprising a moiety selected from —COR′, —CO—CH
2
—COCH
3
and —CH
2
OH, wherein R′ is H or C(1-4) alkyl. These pending groups make the polymer cross-linkable with co-reactive compounds that are added to the polymer dispersion after the emulsion polymerization process.
The pendant groups can be obtained from the copolymerization of the et
Akzo Nobel N.V.
Cain Edward J.
McGillycuddy Joan M.
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