Method for producing aqueous polymer dispersions

Details Method for producing aqueous polymer dispersions Method for producing aqueous polymer dispersions

2002-02-19

2004-07-06

Egwim, Kelechi C. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S087000, C526S088000, C526S318400, C526S318440, C526S318450, C526S318600, C526S329200

Reexamination Certificate

active

06759490

ABSTRACT:

The present invention relates to a process for preparing aqueous polymer dispersions comprising at least one copolymer CP in the form of dispersed polymer particles which is composed of ethylenically unsaturated monomers M. The invention also relates to the aqueous polymer dispersions obtainable by this process and to polymer powders prepared from them. The present invention further relates to the use of these 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 emulsion paints, filling compounds or synthetic-resin-bound plasters, for example, as modifiers for binding mineral building materials, and as adhesives or adhesive base materials.
In the case of adhesives a fundamental problem posed is that for firm and dimensionally stable bonding of the substrates it is necessary to ensure both good adhesion of the adhesive on the substrate to be bonded and at the same time to ensure internal strength of the adhesive film (cohesion). Adhesion and cohesion cannot generally be optimized independently of one another. There is a desire for measures which either raise the level of both properties or at least retain one property unchanged while improving the other. This problem has a role to play in the case, in particular, 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. Pressure sensitive adhesives are used to produce self-adhesive products, such as labels, tapes and films. Products of this kind are very easy to use and make it possible to work rapidly when bonding. In contrast to contact adhesive compositions, no ventilation times are necessary. Moreover, there is no “open time” within which the adhesive bond must be made. The quality of a self-adhesive article depends essentially on whether the cohesion (internal strength of the film of adhesive) and its adhesion (to the surface that is to be bonded) are in tune with one another in accordance with the application.
In the case of pressure sensitive adhesives for labels, in particular, the level of cohesion must be sufficient for no stringing and no emergence of glue at the 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 at a high level in order to provide good sticking on the substrate that is to be bonded.
DE 196 33 967 discloses a process for preparing highly concentrated aqueous dispersions of pressure sensitive adhesive in which the polymerization is conducted in accordance with 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 improved through the presence of small amounts of copolymerized styrene.
From the earlier patent application DE 198 18 394.1 it is known that in aqueous pressure sensitive adhesive formulations based on aqueous polymer dispersions the ratio of adhesion to cohesion can be improved by using aromatic emulsifiers.
WO 97/12921 describes a process for preparing aqueous polymer dispersions in which a monomer mixture which is altered during the polymerization is polymerized onto a seed latex. The polymers prepared by this process are used as coating materials. U.S. Pat. No. 3,804,881 describes a similar process operated in the absence of a seed latex.
It is an object of the present invention to provide aqueous polymer dispersions particularly suited to use as adhesives or adhesive base materials, i.e., as the adhesive component in adhesive formulations. The adhesives and adhesive formulations are to have a balanced profile of properties, i.e., good adhesion by the adhesive to the substrates to be bonded in combination with high strength of the adhesive film.
We have found that this object is achieved in that a process for preparing aqueous polymer dispersions by the method of free-radical aqueous emulsion polymerization of ethylenically unsaturated monomers M, in which the rate at which the polymerization initiator is supplied to the polymerization reaction is altered a number of times, or continuously, leads to aqueous polymer dispersions having improved technical performance properties, especially with respect to their use as pressure sensitive adhesives.
The present invention accordingly provides a process for preparing an aqueous polymer dispersion of a copolymer CP of at least two different monomers M1 and M2 by free-radical aqueous emulsion polymerization of the monomers M in the presence of at least one initiator, at least 80% of the monomers M and at least 75% of the initiator being supplied continuously to the polymerization reaction during its course, which comprises changing the rate at which the initiator is supplied to the polymerization reaction a number of times, or continuously, during the polymerization reaction.
Suitable monomers M are all mono- or polyethylenically unsaturated monomers which, alone or in combination with other monomers, may be polymerized by the method of a free-radical aqueous emulsion polymerization. Such monomers are known to the skilled worker and embrace vinyl, allyl and methallyl esters of aliphatic carboxylic acids having a linear or branched hydrocarbon structure, esters of monoethylenically unsaturated carboxylic acids with alkanols or cycloalkanols, olefins, conjugated dienes, vinylaromatic compounds, ethylenically unsaturated carboxylic acids, sulfonic acids, phosphonic acids, and functional monomers, e.g., crosslinkers and crosslinking monomers.
In general, the monomers M comprise at least one and preferably at least two hydrophobic monomers. Hydrophobic monomers are monomers whose solubility in water is generally less than 80 g/l, preferably less than 60 g/l, at 25° C. and 1 bar. The hydrophobic monomers generally make up at least 60% by weight, and preferably at least 70% by weight, of the monomers for polymerization.
With a view to the use of the polymer dispersions of the invention as adhesives or adhesive base materials, it has proven advantageous for the monomers for polymerization to comprise at least 60% by weight and up to 99% by weight, preferably from 70 to 98% by weight, and in particular from 80 to 95% by weight, of at least one hydrophobic monomer M1 whose homopolymer has a glass transition temperature T
g
≦0° C. Accordingly, the monomers M comprise at least 1% by weight and up to 40% by weight, preferably from 2 to 30% by weight, and in particular from at least 5 to 20% by weight, of monomer M2 which is copolymerizable with the monomers M1.
The glass transition temperature T
g
here is the midpoint temperature determined by differential thermoanalysis (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 also DIN 53765). The glass transition temperature may also be determined from the measurement of the modulus of elasticity in the creep test as a function of temperature. Glass transition temperatures for the homopolymers of numerous monomers are known and are compiled, for example, in Ullmann's Encyclopedia of Industrial Chemistry, VCH, Weinheim, Vol. A 21 (1992) p. 169 or in J. Brandrup, E. H. Immergut, Polymer Handbook 3
rd
ed, J. Wiley, New York 1989.
Hydrophobic monomers M1 of the invention are those having a limited solubility in water, as are commonly used for free-radical aqueous emulsion polymerization. Customary monomers M1 are selected from the vinyl esters of C
3
-C
18
monocarboxylic acids, such as vinyl propionate, vinyl butyrate, vinyl valerate, vinyl hexanoate, vinyl 2-ethylhexanoate, vinyl decanoate, vinyl laurate, vinyl stearate, and vinyl esters of Versatic acids (Versatic acids are branched, aliphatic car

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