Pressure-sensitive adhesives

Details Pressure-sensitive adhesives Pressure-sensitive adhesives

1999-04-12

2001-07-03

Cole, Elizabeth M. (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Adhesive outermost layer

C428S3550AC

Reexamination Certificate

active

06254985

ABSTRACT:

Pressure-sensitive adhesives comprising an aqueous polymer dispersion wherein the polymer is composed of
from 60 to 95% by weight
of a mixture comprising at least one
C
4
-alkyl acrylate and at least one
C
6
-C
12
-alkyl acrylate (monomers a)
from 5 to 30% by weight
of ethylenically unsaturated com-
pounds having a glass transition
temperature of above 0° C. and contain-
ing no functional groups other than
the ethylenically unsaturated group
(monomers b),
from 0 to 10% by weight
of ethylenically unsaturated com-
pounds having at least one acid
group or acid anhydride group (mono-
mers c),
from 0 to 20% by weight
of further ethylenically unsaturated
compounds (monomers d),
the weight percentages being based on the polymer and the polymer dispersion comprising an emulsifier consisting to the extent of at least 5% by weight of aromatic carbon atoms (and called aromatic emulsifier for short) or an emulsifier mixture consisting to the extent of at least 10% by weight of an aromatic emulsifier.
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 required. 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 films, adhesive tapes or 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.
In general, it is impossible to optimize adhesion and cohesion independently of one another. There is a desire for measures which either lift the level of both properties, or at least maintain one property unchanged while improving the other.
Adhesives, including pressure-sensitive adhesives, based on polyacrylate dispersions are known from EP-A-625 557 or PCT/EP 97/04102. Copolymers of this kind still do not have the desired level of adhesion and cohesion or the desired ratio of the one to the other. The unpublished PCT/EP 97/06424 discloses laminating adhesives based on a copolymer including C
4
and C
8
alkyl acrylates.
It is an object of the present invention to provide pressure-sensitive adhesives having improved adhesion and/or cohesion.
We have found that this object is achieved by the pressure-sensitive adhesives defined above.
The polymer of the aqueous polymer dispersion is composed of the monomers a) to d) defined at the outset.
The monomers a) comprise a mixture of at least one C
4
-alkyl acrylate and at least one C
6
-C
12
-alkyl acrylate.
The preferred C
4
-alkyl acrylate is n-butyl acrylate.
Suitable examples of C
6
-C
12
-alkyl acrylate are n-hexyl, 2-ethylhexyl, octyl, decyl or dodecyl acrylate.
Preference is given to C
8
-alkyl acrylates, especially 2-ethylhexyl acrylate.
The proportion of the C
4
-alkyl acrylate is preferably from 15 to 85% by weight, more preferably from 20 to 80% by weight and, with very particular preference, from 30 to 70% by weight, and that of the C
6
-C
12
-alkyl acrylates is preferably from 85 to 15% by weight, more preferably from 20 to 80% by weight and, with very particular preference, from 30 to 70% by weight, the weight percentages being based in each case on the overall amount of the monomers a).
Monomers a) have a glass transition temperature of below 0° C.
Monomers b) comprise monomers which contain no functional group other than the ethylenically unsaturated group, i.e. no hydroxyl, acid, acid anhydride, nitro or primary, secondary or tertiary amino groups, nor epoxy groups. Monomers b), or to be more precise the homopolymers of the monomers b), have a glass transition temperature of above 0° C., preferably above 15° C.
The glass transition temperature of the polymer can be determined by customary methods, such as differential thermal analysis or differential scanning calorimetry (see e.g. ASTM 3418/82, at midpoint temperature).
Examples of monomers b) are C
1
-C
20
-alkyl (meth)acrylates, vinyl. esters of carboxylic acids containing up to 20 carbon atoms, vinylaromatic compounds with up to 20 carbon atoms, ethylenically unsaturated nitriles, and vinyl halides, provided they have the glass transition temperature specified above.
Particularly suitable alkyl (meth)acrylates are methyl methacrylate, methyl acrylate, n-butyl methacrylate and tert-butyl acrylate.
Methyl acrylate, methyl methacrylate and butyl methacrylate are preferred.
Examples of vinyl esters of C
1
-C
20
carboxylic acids are vinyl laurate, stearate, propionate, the vinyl ester of Versatic acid and vinyl acetate.
Suitable vinylaromatic compounds are vinyltoluene, &agr;- and p-methylstyrene, &agr;-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene and, preferably, styrene. Examples of nitriles are acrylonitrile and methacrylonitrile.
Vinyl halides are ethylenically unsaturated compounds substituted by chlorine, fluorine or bromine, and are preferably vinyl chloride and vinylidene chloride.
Preferred monomers (b) are methyl (meth)acrylates and styrene.
Monomers c) are, for example, ethylenically unsaturated monomers having carboxyl groups, such as (meth)acrylic acid, maleic acid, ethylenically unsaturated acid anhydrides or monoesters, such as maleic anhydride or maleic or fumaric monoesters. Acrylic and methacrylic acid are preferred. The content of monomers c) is preferably not more than 5% by weight, based on the polymer. Further monomers d) can be of any kind. Preferred examples which may be mentioned are C
1
-C
10
-hydroxyalkyl (meth)acrylates or monomers listed under b) which have a glass transition temperature of less than 0° C.
The polymer of the invention consists preferably of
from 65 to 90
% by weight of monomers a)
from 5 to 25
% by weight of monomers b)
from 0 to 5
% by weight of monomers c)
from 0 to 10
% by weight of monomers d)
and with particular preference of
from 70 to 89.9
% by weight of monomers a)
from 10 to 20
% by weight of monomers b)
from 0.1 to 3
% by weight of monomers c)
from 0 to 10
% by weight of monomers d)
The glass transition temperature of the polymer is preferably from −60° C. to +10° C., more preferably from −50° C. to −10° C. and, with very particular preference, from −40° C. to −20° C.
The polymer is preferably prepared by emulsion polymerization and is therefore an emulsion polymer.
In emulsion polymerization it is common to use ionic and/or nonionic emulsifiers and/or protective colloids, or stabilizers, as surface-active compounds.
In the present case use is made in accordance with the invention of an emulsifier consisting to the extent of at least 5% by weight of aromatic carbon atoms (and called aromatic emulsifier for short) or an emulsifier mixture which comprises at least 10% by weight of such an aromatic emulsifier, based on the overall amount of emulsifier.
The aromatic emulsifier preferably consists to the extent of at least 10% by weight and more preferably at least 20% by weight of aromatic carbon atoms (by which are meant carbon atoms that are part of an aromatic ring system).
The proportion of aromatic carbon atoms is generally below 80% by weight.
The aromatic emulsifier is preferably an ionic emulsifier, especially one having one or two, preferably two, sulfate groups.
The molar weight of the emulsifier is preferably less than 2000 and especially less than 1000 g/mol.
With par

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