Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-07-14
2002-07-30
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S748000, C528S486000
Reexamination Certificate
active
06426377
ABSTRACT:
The present invention relates to a process for improving the stability of aqueous polymer dispersions to thermal and/or mechanical stress (=exposure).
Owing to the large surface area of the dispersed polymer particles, aqueous polymer dispersions constitute metastable systems. They have a propensity to reduce their surface area while enlarging the polymer particles present within them and in this way of attaining a more energetically advantageous state. In general, aqueous polymer dispersions are kinetically stable to these changes. If, however, they are subjected to thermal or mechanical stress, there is a risk of particle enlargement. Thermal stressing of polymer dispersions occurs, for example, in the course of physical deodorization: for example, when the aqueous dispersions are stripped with steam or when solvents are removed by distillation. Aqueous polymer dispersions undergo mechanical stress in the course, in particular, of processes which entail shear forces: for example, in the course of stirring, pumping or filtering the dispersions.
An uncontrolled change in the particle sizes of aqueous polymer dispersions is undesirable for a number of reasons. Firstly, there is the danger that microcoagulum (gel specks) will be formed or that the dispersion will coagulate and become unusable. Secondly, a number of applications-related properties, such as viscosity and film formation, depend on the specific size and/or size distribution of the polymer particles within the aqueous polymer dispersion. An irreproducible change in these particle sizes must therefore be avoided in view of the stringent applications-related requirements placed on aqueous polymer dispersions.
It is known fundamentally that surface-active substances such as emulsifiers and protective colloids (the latter can be interpreted as polymeric emulsifiers) improve the stability of aqueous polymer dispersions. The surface-active substances commonly used for this purpose, however, are unable to provide aqueous polymer dispersions with effective stabilization against changes in the particle size distribution of the dispersed polymer particles in the course of thermal and/or mechanical stress.
The use of sodium salts of sulfosuccinic acid dialkyl esters for improving the wetting power of aqueous polymer dispersions is known (Technical Information from BASF, TI/ED 1342d, 1992). J. Schwartz (J. Coating Tech. 64, No. 812, 1992, p. 62) describes the use of the sodium salt of di-n-octyl sulfosuccinate for improving the quality of coatings based on aqueous polymer dispersions.
It is an object of the present invention to provide a process which permits effective stabilization of aqueous polymer dispersions against changes in the particle size distribution.
We have found that this object is achieved by adding salts of the bis-C
4
-C
18
-alkyl esters of sulfonated dicarboxylic acids having 4 to 8 carbon atoms to aqueous polymer dispersions.
The present invention accordingly provides a process for improving the stability of aqueous polymer dispersions to thermal and/or mechanical effects which comprises adding at least one salt of a bis-C
4
-C
18
-alkyl ester of a sulfonated dicarboxylic acid having 4 to 8 carbon atoms (=salts S) to the aqueous polymer dispersion.
By C
4
-C
18
-alkyl is meant linear or branched alkyl having 4 to 18 carbon atoms, e.g. n-butyl, 2-butyl, isobutyl, 2-methylbutyl, 2-ethylbutyl, 1,3-dimethylbutyl, n-pentyl, 2-pentyl, 2-methylpentyl, n-hexyl, 2-methylhexyl, 2-ethylhexyl, n-heptyl, 2-heptyl, 2-methylheptyl, 2-propylheptyl, n-octyl, 2-methyloctyl, n-nonyl, 2-methylnonyl, n-decyl, 2-methyldecyl, n-undecyl, 2-methylundecyl, n-dodecyl, 2-methyldodecyl, n-tridecyl, 2-methyltridecyl, n-tetradecyl, n-hexadecyl and n-octadecyl. Examples of sulfonated dicarboxylic acids having 4 to 8 carbon atoms are sulfonated succinic, glutaric, adipic, phthalic and isophthalic acids.
Preferred salts S are the salts of bis-C
4
-C
18
-alkyl esters of sulfonated succinic acids, especially the salts of bis-C
6
-C
12
-alkyl esters of sulfonated succinic acid.
It is further preferred for the salts S to comprise the potassium, calcium, ammonium and, in particular, the sodium salts of the abovementioned sulfonated dicarboxylic esters. Especially preferred salts S are the sodium salts of the bis-n-octyl ester and of the bis-2-ethylhexyl ester of sulfosuccinic acid.
Depending on the stability of the aqueous polymer dispersion at least one salt S will be added to the aqueous polymer dispersion, generally in an amount of at least 0.1% by weight, preferably at least 0.2% by weight and, in particular, at least 0.3% by weight, based on the weight of the disperse polymer in the aqueous polymer dispersion. The amount of salt S employed will generally be not more than 5% by weight, preferably not more than 3% by weight and, in particular, not more than 2% by weight, based on the weight of the disperse polymer in the aqueous polymer dispersion. The amount of salt S required for stabilization will fall as the size of the particles in the aqueous polymer dispersion increases. Polymers having a large number of negative charge centers likewise require a smaller amount of salt S for stabilization than polymers having a small number of negative charge centers. Negative charge centers result from copolymerized monomers having acidic groups. The number of negative charge centers in the polymer is, of course, dependent on the pH of the aqueous polymer dispersion.
The stabilizing effect of the salts S, in accordance with the invention, is particularly marked with aqueous polymer dispersions in which the polymer particles have a ponderal median particle diameter (d
50
) in the range from 50 to 1000 nm and, preferably, in the range from 100 to 600 nm. The statistical distribution of the polymer particle diameters can be either monomerical or polymerical. Here and below, the ponderal median particle diameter which is indicated is that diameter which is exceeded and fallen short of by 50% by weight of the polymer particles in the aqueous dispersion (d
50
). The particle size distribution in the aqueous dispersion can be determined by customary methods; for example, using an analytical ultracentrifuge in accordance with the methods of W. Scholtan and H. Lange, Kolloid-Z. u. Z. Polymere 250 (1972), 782-796. The ultracentrifuge measurement gives the integral mass distribution of the particle diameter of a sample, from which it is easy to derive the d
50
. Reference may be made at this point to other methods of determining the polymer particle diameter (see Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol.
A21, p. 186).
In addition, the light transmissitivity of dilute samples of aqueous polymer dispersions constitutes an indirect measure of the size of the polymer particles (see Ullmann's Encyclopedia of Industrial Chemistry, loc. cit.). A change in the light transmissitivity by more than two percentage points, resulting from thermal or mechanical stress, indicates a change in the average particle size or in the particle size distribution in the aqueous polymer dispersion.
In general, the polymer in the aqueous polymer dispersion will have a glass transition temperature T
g
in the range from −80 to +100° C. The stabilizing effect of the process of the invention is manifested preferentially when the polymer in the aqueous polymer dispersion has a glass transition temperature of below 80° C. and, in particular, below 50° C. The process of the invention is employed with particular preference if the polymer in the aqueous polymer dispersion has a glass transition temperature of below 30° C., in particular below 20° C. and, with very particular preference, below 10° C. The polymer in the aqueous polymer dispersion will preferably have a glass transition temperature of above −60° C. and, in particular, above −50° C. One specific embodiment of the present invention relates to the stabilization of aqueous polymer dispersions whose copolymerized polymers have a glass transition temperature T
g
in the rang
Dames Burkhardt
Gerst Matthias
Schuler Bernhard
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Szekely Peter
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