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
1999-12-14
2002-07-23
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...
C525S327800, C525S330200, C526S078000, C526S087000, C526S173000, C526S203000, C526S317100
Reexamination Certificate
active
06423769
ABSTRACT:
The present invention relates to aqueous polymer dispersions wherein the polymer particles comprise at least two different polymers, P and P′. The present invention also relates to the use of said polymer dispersions in adhesive formulations, especially in pressure-sensitive adhesives.
Aqueous polymer dispersions find diverse application, for example, as coating compositions or as impregnants for paper or leather, as binders in, for example, emulsion paints, filling compounds or synthetic-resin-bound plasters, as modifiers for mineral construction binders, and as adhesives or adhesive base materials.
Polymer dispersions are variously proposed wherein the polymer particles comprise at least two different polymers. Such polymer dispersions are generally prepared by first of all preparing an aqueous polymer dispersion and then in a further step conducting an addition polymerization of essentially hydrophobic monomers in said dispersion. This procedure is also referred to as staged polymerization. Staged polymerization produces aqueous polymer dispersions in which the polymer particles contain predominantly both the polymer of the first polymerization stage and the polymer of the second polymerization stage. Through such a combination of two polymers it is possible to improve the performance properties of. coating compositions. In the sector of pressure-sensitive adhesives, this concept has not to date led to the desired success.
A fundamental problem with adhesives is that for firm and dimensionally stable bonding of the substrates it is necessary to ensure good adhesion of the adhesive to the adherend substrate and at the same time internal strength (cohesion) of the adhesive film. Adhesion and cohesion cannot generally be optimized independently of one another. The desire is for measures which either raise the level of both properties or at least maintain one property unchanged while improving the other. This problem plays a particular part in the case of pressure-sensitive adhesives.
Pressure-sensitive adhesives (PSAs) form a permanently tacky film which adheres to a very wide variety of surfaces even—as their name suggests—under slight pressure at room temperature. PSAs are used to produce self-adhesive products, such as labels, tapes and sheets. Such products are very easy to use and make it possible to work rapidly when bonding. In contrast to contact adhesive compositions, no ventilation periods are required. Moreover, there is no “open time” within which the adhesive bond must be made. The quality of a self-adhesive article depends critically on whether the inner strength (cohesion) of the film of adhesive and its-adhesion to the surface on which the bond is to be produced are in tune with one another in accordance with the application.
In the case of PSAs for labels, in particular, the level of cohesion must be sufficient for no stringing or glue emergence at edges to occur in the course of stamping and cutting, since otherwise the cutting tools become soiled and the cut faces sticky. At the same time, the adhesion should be situated at a high level in order to provide good sticking on the substrate on which a bond is to be produced.
DE 196 33 967 discloses a process for preparing highly concentrated aqueous PSA dispersions, in accordance with which the polymerization is conducted using a special feed technique in the presence of less than 50 ppm of polymerization inhibitor.
DE 196 32 203 discloses pressure-sensitive adhesives whose adhesion is enhanced through the presence of small amounts of copolymerized styrene.
From the earlier Patent Application DE 198 18 394.1 it is known that the relationship of adhesion to cohesion in aqueous PSA formulations based on aqueous polymer dispersions can be improved by using aromatic emulsifiers.
WO 98/16560 and U.S. Pat. No. 5,476,897 disclose processes for preparing highly concentrated polymer dispersions where the pH is altered during the polymerization.
It is an object of the present invention to provide aqueous polymer dispersions which are particularly suitable as adhesives or adhesive base materials, i.e., as the adhesive component in adhesive formulations. The adhesives or adhesive formulations should have a balanced profile of properties, i.e., good adhesion of the adhesive to the adherend substrates in combination with high strength of the adhesive film.
We have found that this object is achieved and that polymer dispersions wherein the polymer particles comprise at least two different polymers P and P′, have particularly advantageous properties when at least two different monomer mixtures comprising monomers having acid groups are polymerized in succession by the method of free-radical aqueous emulsion polymerization and a base is added during the changeover of polymerization stages.
The present invention accordingly provides aqueous polymer dispersions wherein the polymer particles comprise at least two mutually different water-insoluble polymers P and P′, said dispersions being obtainable by free-radical addition polymerization of ethylenically unsaturated monomers, comprising the following measures:
1. polymerizing a first monomer mixture M by the method of free-radical aqueous emulsion polymerization to give a polymer P,
2. adding a base to the dispersion of the polymer P, and
3. polymerizing a further monomer mixture M′, which is different from the monomer mixture M, in the dispersion of the polymer P, to give a polymer P′,
said monomer mixtures M and M′ independently of one another comprising from 0.1 to 10% by weight, based on the total amount of the monomers M and M′, respectively, of at least one monomer M1 which has at least one acid group and from 90 to 99.9% by weight of one or more essentially hydrophobic monomers M2. In the polymer dispersions of the invention, the polymer particles may also include more than two different water-insoluble polymers.
The weight ratio of monomer mixture M to monomer mixture M′, and thus the weight ratio of polymer P to polymer P′, is preferably in the range from 10:1 to 1:10, in particular in the range from 1:5 to 5:1 and, especially, in the range from 1:2 to 2:1.
Both the polymer P and the polymer P′ normally have a glass transition temperature less than 60° C. The average glass transition temperature {overscore (T)}
g
of the polymer particles will generally not exceed 50° C. and preferably will not exceed 20° C. Where the polymers of the invention are used as adhesives, or as the adhesive component in adhesive formulations, both the polymer P and the polymer P′ preferably have a glass transition temperature of less than 50° C., in particular less than 10° C., with particular preference less than 0° C., and with very particular preference in the range from −60° C. to −10° C. The average glass transition temperature {overscore (T)}
g
of the polymer particles in adhesive formulations will generally not exceed 50° C., preferably 10° C., in particular 0° C., and especially −10° C. {overscore (T)}
g
is especially in the range from −60° C. to −10° C.
It is of advantage in accordance with the invention if the glass transition temperature T
g
of the polymer P and the glass transition temperature T
g
′ of the polymer P′ differ from one another by at least 5 K. The difference in glass transition temperature can be up to 150 K. In the case of adhesives and adhesive formulations, especially pressure-sensitive adhesives, it has proven particularly advantageous for this difference to be in the range from 5 to 20 K and, in particular, in the range from 10 to 15 K. However, it is also possible for the glass transition temperatures to be approximately equal.
By the glass transition temperature T
g
here is meant the midpoint temperature determined by differential thermal analysis (DSC) in accordance with ASTM D 3418-82 (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A 21, VCH Weinheim 1992, p. 169, and Zosel, Farbe und Lack 82 (1976), pp. 125-134; see al
Centner Alexander
Gerst Matthias
Egwim Kelechi C.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wu David W.
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