Process for preparing an aqueous polymer dispersion for...

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

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Reexamination Certificate

active

06414087

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for preparing an aqueous polymer dispersion for removable pressure-sensitive adhesive films by means of free-radically initiated aqueous emulsion polymerization.
BACKGROUND OF THE INVENTION
Pressure-sensitive adhesives based on aqueous polymer dispersions have been known for a long time. Examples of pressure-sensitively adhering articles which can be prepared using them are labels and adhesive tapes. For example, U.S. Pat. No. 2,884,126 discloses acrylate-based pressure-sensitive adhesives containing 3 to 12% of polar comonomers. U.S. Pat. No. 4,181,752 describes UV-crosslinked, acrylate-based pressure-sensitive adhesives containing from 2 to 30% of polar comonomers. In the fields of use referred to it is desirable for the adhesive film to be detachable from the substrate, as far as possible without residue, even after a prolonged period. A problem in this respect is that the peel strength of conventional pressure-sensitive adhesive compositions, as are often used to produce labels or adhesive tapes, normally increases so greatly over time that they cannot be separated easily from the substrate. When separation takes place, the backings are torn off and/or large portions of the adhesive composition (adhesive film) remain adhering to the substrate. This problem is particularly prevalent in the case of systems of large surface area; for example, when carpets are bonded using carpet laying tapes. The adhering adhesive composition on the floor or parquet must then be removed by laborious cleaning with organic solvents.
U.S. Pat. No. 4,599,265 describes removable pressure-sensitive adhesives based on acrylate with up to 3% of polar comonomers whose redetachability is ensured by means of UV crosslinking. EP-A 224795 reports on earlier attempts in the prior art to improve the redetachability by adding plasticizer to the pressure-sensitive adhesive or by copolymerizing crosslinkable comonomers. For improving the redetachability it is recommended to copolymerize silane-functional comonomers. The document explicitly advises against copolymerizing polar monomers in removable pressure-sensitive adhesive compositions. For the preparation of removable pressure-sensitive adhesives, EP-B 287306 uses phosphate emulsifiers and copolymerizable emulsifiers. EP-A 736585 likewise advises against copolymerizing polar comonomers in pressure-sensitive adhesives for removable pressure-sensitive adhesive films. Copolymer compositions are recommended which are obtained by polymerizing a precursor mixture comprising acrylate monomers, without polar comonomers, and also hydrophobic silica and crosslinkers.
The object of this invention was to provide an aqueous dispersion system for use with a very wide variety of pressure-sensitively adhering articles in the label and adhesive tape sectors which possesses high cohesion and whose peel strength does not increase greatly after aging or prolonged bonding periods, so that the articles can be detached again virtually without residue.
BRIEF DESCRIPTION OF THE INVENTION
The invention provides a process for preparing an aqueous polymer dispersion for removable pressure-sensitive adhesive films by means of free-radically initiated aqueous emulsion polymerization of one or more ethylenically unsaturated monomers from the group consisting of esters of acrylic acid and methacrylic acid with branched or unbranched alcohols having 1 to 12 carbon atoms, vinyl esters of branched or unbranched carboxylic acids having 1 to 12 carbon atoms, vinyl-aromatics, vinyl halides, olefins and dienes, which comprises copolymerizing from 0.01 to 2.0% by weight of polar, ionic or nonionic comonomers or mixtures thereof, and if polar ionic comonomers, alone or in a mixture, are copolymerized, the proportion thereof is not more than 1.0% by weight, and the amounts in % by weight are based in each case on the overall weight of the comonomers.
DETAILED DESCRIPTION OF THE INVENTION
Preferred methacrylic esters or acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, and 2-ethylhexyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate.
Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of &agr;-branched monocarboxylic acids having 9 to 11 carbon atoms, an example being VeoVa9
R
or VeoVa10
R
(tradenames of Shell). Vinyl acetate is particularly preferred. Preferred vinylaromatics are styrene, methylstyrene and vinyltoluene. A preferred vinyl halide is vinyl chloride. The preferred olefins are ethylene and propylene and the preferred dienes are 1,3-butadiene and isoprene.
Particular preference is given to comonomer compositions in which the proportion of the esters of acrylic acid and methacrylic acid with branched or unbranched alcohols having 1 to 12 carbon atoms is ≧50% by weight. Most-preferred comonomer mixtures containing the specified proportions of polar ionic and nonionic comonomers are those including ethyl acrylate or n-butyl acrylate or
2
-ethylhexyl acrylate, or mixtures of methyl methacrylate with n-butyl acrylate and/or 2-ethylhexyl acrylate, or mixtures of methyl methacrylate with n-butyl acrylate and/or ethyl acrylate, or mixtures of methyl methacrylate with 1,3-butadiene, or mixtures of styrene with ethyl acrylate and/or n-butyl acrylate, or mixtures of acrylic esters such as ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate with vinyl esters such as vinyl acetate and/or vinyl esters of &agr;-branched monocarboxylic acids having 9 to 11 carbon atoms and, if desired, ethylene.
In general, the monomers and, if appropriate, the weight fractions of the comonomers are selected so as to give a glass transition temperature Tg of the polymers of from −60° C. to 0° C. The glass transition temperature Tg of the polymers can be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately in advance by means of the Fox equation. According to Fox T. G., Bull. Am. Physics Soc. 1, 3, page 123 (1956) the following is true: 1/Tg=x
1
/Tg
1
+x
2
/Tg
2
+ . . . +x
n
/Tg
n
, where x
n
represents the mass fraction (% by weight/100) of the monomer n and Tg
n
is the glass transition temperature, in Kelvin Degrees, of the homopolymer of the monomer n. Tg values for homo-polymers are listed in Polymer Handboook, 2
nd
Edition, J. Wiley & Sons, New York (1975).
Suitable polar ionic comonomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, itaconic acid, fumaric acid and maleic acid, and also ethylenically unsaturated sulfonic acids, preferably vinylsulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid. Ionic comonomers are those which, like the carboxylic acid group or sulfonic acid group, can be present in ionic form under certain pH conditions. Suitable polar nonionic comonomers are ethylenically unsaturated carboxamides and carbonitriles, preferably acrylamide and acrylonitrile, and also methacrylic and acrylic acid hydroxyalkyl esters with a C
1
to C
8
alkyl radical, preferably hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate. Acrylic acid and methacrylic acid are the most preferred polar ionic comonomers. The most preferred polar nonionic comonomers are acrylamide, methacrylamide and hydroxyethyl acrylate, hydroxypropyl acrylate and hydroxybutyl acrylate. The proportion of said polar ionic comonomers is preferably from 0.2 to 0.99% by weight. The proportion of said polar nonionic comonomers is preferably from 0.5 to 2.0% by weight, the proportions in % by weight being based in each case on the overall weight of the comonomers.
In the case of preparation in accordance with the emulsion polymerization process the polymerization temperature

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