Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1998-07-22
2001-04-17
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S089000, C525S241000, C525S242000, C525S243000, C525S262000, C525S445000, C525S447000, C525S451000
Reexamination Certificate
active
06218468
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the preparation of an aqueous polymer dispersion by polymerizing monomers having at least one vinyl group by the free radical aqueous emulsion polymerization method, in which an amphiphilic substance is added to the polymerization vessel before and/or during the polymerization.
2. Description of the Background
Aqueous polymer dispersions (latices) are generally known. They are fluid systems which contain, as the disperse phase in an aqueous dispersing medium, polymer coils (i.e. polymer particles) consisting of a plurality of intertwined polymer chains.
The diameter of the polymer particles is frequently from 10 to 2000 nm.
As in the case of polymer solutions on evaporation of the solvent, aqueous polymer dispersions have the potential to form polymer films on evaporation of the aqueous dispersing medium, and they are therefore used in particular as binders, for example for paints or for materials for coating leather, paper or plastics films. They are becoming increasingly important owing to their environmentally friendly properties.
An important feature of aqueous polymer dispersions is the diameter of the polymer particles present as the disperse phase, since the size of the polymer particles or their size distribution plays a role in determining a number of performance characteristics of aqueous polymer dispersions. For example, films of finely divided aqueous polymer dispersions have high gloss (cf. for example, Progress in Organic Coatings 6 (1978), 22). Furthermore, the power of finely divided aqueous polymer dispersions to penetrate into porous but relatively dense substrates, such as paper, leather or a render surface is greater than that of coarse-particled aqueous polymer dispersions (for example, Dispersionen synth. Hochpolymerer, Part II, Anwendung, H. Reinhard, Springer-Verlag, Berlin (1969), page 4).
On the other hand, coarse-particled aqueous polymer dispersions have, for example, lower flow resistance than finely divided aqueous polymer dispersions, the composition and solids concentration otherwise being identical (for example, Dispersionen synth. Hochpolymerer, Teil II, Anwendung, H. Reinhard, Springer-Verlag, Berlin (1969), page 5). Aqueous polymer dispersions whose polymer particle diameters are distributed over a relatively large diameter range also have advantageous flow behavior (cf. for example, DE-A 42 13 965).
Establishing the diameters of the dispersed polymer particles in a controlled, reproducible manner tailored to the particular intended use is therefore of key importance in the preparation of an aqueous polymer dispersion.
The most important method for the preparation of aqueous polymer dispersions is the free radical emulsion polymerization method, in particular the free radical aqueous emulsion polymerization method.
In the latter method, monomers having at least one vinyl group are usually subjected to free radical polymerization under the action of free radical polymerization initiators dissolved in the aqueous medium, to give polymer particles present directly as the disperse phase in the aqueous dispersing medium. The aqueous polymer dispersions prepared by the free radical aqueous emulsion polymerization method are usually referred to as aqueous primary dispersions, in order to distinguish them from the aqueous secondary dispersions. In the case of the latter, the polymerization is carried out in a nonaqueous medium. Dispersing in the aqueous medium is not effected until after the polymerization reaction is complete.
The monomers to be polymerized are distributed in the form of droplets (the droplet diameter is frequently from 2 to 10 &mgr;m) in the aqueous medium with formation of an aqueous monomer emulsion. However, these monomer droplets are not the sites of the polymerization but act merely as a monomer reservoir. Rather, the polymerization sites are formed in the aqueous phase, which always contains a limited amount of the monomers to be polymerized and the free radical polymerization initiator in dissolved form. Chemical reaction of these reactants present in solution results in the formation of oligomer radicals, which are precipitated as primary particles above a critical chain length (homogeneous nucleation). The formation of primary particles presumably takes place up to the point at which the rate of formation of the free radicals in the aqueous phase is equal to the rate of their disappearance due to free radical capture by polymer particles already formed. This polymer particle formation phase is then followed by the polymer particle growth phase, i.e. the monomers to be polymerized diffuse from the monomer droplets acting as a reservoir, via the aqueous phase, to the primary particles formed (whose number and surface area are very much greater than those of the monomer droplets), in order to be incorporated into said primary particles by polymerization (cf. for example, Faserforschung und Textiltechnik 28 (1977), Part 7, Zeitschrift für Polymerforschung, page 309). By controlled addition of suitable dispersants, both the disperse phase of the monomer droplets and the disperse phase of the polymer particles formed are, if required, stabilized.
While the process of polymer particle growth usually takes place systematically, the polymer particle formation is essentially a stochastic process, i.e. the number of primary polymer particles formed and hence the diameters of the final polymer particles resulting after the end of the polymerization fluctuate from polymerization batch to polymerization batch. The product quality fluctuates in a corresponding manner (identical reproduction is usually not possible). This applies very particularly in the case of a high solids volume content (≧50 Vol.-%) of the aqueous polymer dispersion, since, for example, the viscosity of highly concentrated aqueous polymer dispersions is particularly sensitive to the number and size of the polymer particles contained in dispersed form.
It is known that a controlled free radical aqueous emulsion polymerization procedure is possible by initiating it in the presence of a surfactant dissolved in the aqueous medium, the surfactant content of the aqueous medium being such that it is above the critical micelle formation concentration of said medium (cf. for example, High Polymers, Vol. IX, Emulsion Polymerization, Interscience Publishers, Inc., New York, Third Printing, 1965, page 1 et seq.).
The term surfactant means amphiphilic substances which, on dissolution in water, are capable of reducing the surface tension a of pure water significantly (as a rule by at least 25%, based on the a value of pure water) before reaching the critical micelle formation concentration.
The term “amphiphilic” indicates that surfactants have both hydrophilic and hydrophobic groups. Hydrophilic groups are those which are drawn into the aqueous phase, whereas hydrophobic groups are forced out of the aqueous phase.
In highly dilute aqueous solutions, surfactants are therefore present essentially independent molecules in solution, their amphiphilic structure resulting in accumulation at the water surface with oriented adsorption, which reduces the surface tension.
In concentrated aqueous solutions, on the other hand, surfactants are present predominantly as micelles in solution, i.e. the surfactant molecules are arranged in the aqueous solution predominantly in a state of relatively high aggregation, i.e. as micelles, in which they are oriented in such a way that the hydrophilic groups face the aqueous phase and the hydrophobic groups point toward the interior of the micelle. As the surfactant concentration increases further, essentially only the number of micelles per unit volume increases, but not the number of surfactant molecules dissolved in molecular form per unit volume.
The transition from the aqueous molecular solution to the aqueous micellar solution usually takes place relatively abruptly, as a function of the surfactant concentration, which is evident from correspond
Machtle Walter
Mathauer Klemens
Meyer Wolfgang
Rager Timo
Schrof Wolfgang
Asinovsky Olga
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Seidleck James J.
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