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
1999-12-27
2003-01-07
Wu, David W. (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...
C526S204000, C526S220000
Reexamination Certificate
active
06503983
ABSTRACT:
The present invention relates to a process of free-radically initiated aqueous emulsion polymerization for preparing an aqueous polymer dispersion, where compounds having at least one ethylenically unsaturated group (monomers) are emulsified in an aqueous medium by means of dispersants and are polymerized by means of a free-radical polymerization initiator in the presence of a N-oxyl radical (a compound having at least one
group) which is derived from a secondary amine which carries no hydrogens on the &agr; carbons (in other words, the N-oxyl groups are derived from corresponding secondary amino groups), to form an aqueous polymer dispersion. In this document, the abovementioned N-oxyl radicals will be referred to as stable N-oxyl radicals.
Aqueous polymer dispersions are fluid systems comprising, as the disperse phase, polymer particles in stable (storage stability in general ≧24 h, normally ≧2-3 days, usually ≧1 week) disperse distribution in an aqueous dispersion medium. The number-average diameter of the polymer particles is generally from 0.01 to 1 &mgr;m.
Like polymer solutions when the solvent is evaporated, aqueous polymer dispersions have the property, when the aqueous dispersion medium evaporates, of forming polymer films, which is why aqueous polymer dispersions are widely employed in direct form as binders, for example for paints or compositions for coating leather.
In many cases, however, the dispersed polymer is also separated off by coagulation and used as a constituent in polymer blends to modify the mechanical properties. For this purpose, the polymer separated off from the aqueous polymer dispersion is extruded together, for example, with other thermoplastics, with or without the customary additives such as dyes, pigments, lubricants, stabilizers or fillers.
Aqueous polymer dispersions are mostly prepared by free-radically initiated aqueous emulsion polymerization of compounds having at least one ethylenically unsaturated group at below 100° C. In this case, the monomers to be polymerized, which are mainly of only little solubility in water, are emulsified in the aqueous medium without any great effort, for example by customary stirring, with the addition of dispersant and are polymerized by the action of free-radical polymerization initiators.
The free-radical polymerization initiators are usually water-soluble peroxides, hydroperoxides and/or azo compounds which, above a certain temperature, generally ≦100° C., dissociate into reactive radicals which trigger the polymerization.
The term emulsion expresses the fact that the monomers and the water are present as a system of two liquids in more or less fine distribution and with little mutual solubility. The phrase aqueous emulsion expresses the fact that the aqueous phase forms the continuous phase. To prepare an aqueous monomer emulsion normally requires the addition of dispersants (for example, Ullmanns Encyklopädie der technischen Chemie, Vol. 10, 4th Edition, Verlag Chemie, Weinheim (1975), p. 449), which prevent the direct combination of two monomer droplets which happen to collide in the aqueous emulsion, and which ensure the stability of the resulting aqueous polymer dispersion.
As a result of the low dispersion effort, the aqueous monomer emulsion employed in connection with the free-radical aqueous emulsion polymerization usually consists predominantly of monomer droplets with a diameter >1 &mgr;m.
Like all free-radically initiated polymerizations of compounds having at least one ethylenically unsaturated group, the process of free-radically initiated aqueous emulsion polymerization also has the disadvantage that the molecular weight of the polymer chains does not normally increase with the degree of conversion and that the resulting polymer chains are not generally of uniform molecular weight. In other words, and in terms of its molecular weight, the polymer obtainable is generally not monodisperse but usually has a polydispersity index PDI in this regard of ≧2 (PDI={overscore (M)}
w
/{overscore (M)}
n
, where {overscore (M)}
w
=weight-average molecular weight and {overscore (M)}
n
=number-average molecular weight), which is attributed in particular to termination reactions as a consequence of the irreversible combination of growing free-radical polymer chain ends.
Another disadvantage of free-radically initiated aqueous emulsion polymerization is that a change made during polymerization to the monomers that are to be polymerized leads generally not to segmented copolymers (block polymers) but normally, at best, to dispersed core-shell polymer particles with a core composed of one type of monomer and a shell composed of the other type of monomer, the bond between core and shell being primarily not chemical but merely physical.
TRIP Vol. 4, No. 6, June 1996, p. 183 ff., U.S. Pat. No. 5,322,912, WO 96/24620, U.S. Pat. No. 4,581,429, U.S. Pat. No. 5,412,047, EP-A 135 280 and Prior Application DE-A 19602539 disclose that conducting free-radically initiated polymerizations at above 100° C. in the presence of a stable N-oxyl radical (that is, one essentially devoid of an initiating action) allows a certain degree of control of the free-radically initiated polymerization.
The mechanism on which the action is based is presumed to be that the stable N-oxyl radicals do not irreversibly terminate but merely temporarily block reactive radical ends of a growing polymer chain at elevated temperatures. The result of this is a reduction in the steady-state concentration of growing free-radical polymer chain ends, thereby reducing the possibility for the irreversible termination of the chain growth through the combination of two growing polymer chain ends. This leads on average to polymer chains which grow in (ideally linear) proportion with the polymerization conversion. The result of the latter is an average molecular weight which grows in (ideally linear) proportion with the polymerization conversion, with the resulting polymer having a polydispersity index of 1.
According to U.S. Pat. No. 5,322,912, column 10, line 65 et seq. suitable reaction media for a controlled free-radically initiated polymerization of this kind include an emulsion. Further details regarding the implementation of such a free-radically initiated emulsion polymerization are not given by U.S. Pat. No. 5,322,912. The same applies to DE-A 19602539. The only recommendation made in U.S. Pat. No. 5,412,047, column 18, lines 54 et seq. for the case where free-radically initiated polymerization takes place in a multiphase system, as is the case with the free-radically initiated aqueous emulsion polymerization, is to use stable N-oxyl radicals which are of particularly low solubility in water.
The availability of an easy-to-implement, controlled, free-radically initiated aqueous emulsion polymerization for preparing an aqueous polymer dispersion would be advantageous insofar as it would enable the molecular weight of the resulting polymer, present in disperse distribution, to be adjusted in a controlled manner. This controlled establishment determines, for example, the cohesion and adhesion of the resulting film of the aqueous polymer dispersion. In general there is a rise in the degree of cohesion as the molecular weight increases, whereas a decreasing molecular weight generally promotes the surface tack of the film. Moreover, the possibility of controlled establishment opens up direct access to aqueous dispersions of tailor-made block copolymers, since the free-radical polymer chain ends are not destroyed by combination but only blocked reversibly. In other words, following the consumption of a first type of monomer, the polymerization can be continued with the addition of further types of monomer.
To prepare an aqueous polymer dispersion by controlled initiated aqueous emulsion polymerization, Macromolecules 1997, 30, pp. 324-326 recommends implementing said polymerization such that a preformed aqueous polymer dispersion (seed latex) is charged to a polymerization vessel and that to this initial charge the
Durant Yvon
Fischer Michael
Mathauer Klemens
Morrison Bradley Ronald
Egwim Kelechi C.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wu David W.
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