Contact bonding adhesives

Details Contact bonding adhesives Contact bonding adhesives

2001-11-06

2003-09-16

Egwim, Kelechi (Department: 1713)

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

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S558000, C524S577000, C524S833000, C526S911000, C526S932000

Reexamination Certificate

active

06620870

ABSTRACT:

The present invention relates to pressure-sensitive adhesives comprising an aqueous polymer dispersion, wherein the polymer is composed of
from 50 to 99.9% by weight of at least one C
1
-C
12
alkyl (meth)acrylate (monomers a)
from 0.05 to 20% by weight of at least one vinylaromatic compound (monomers b)
from 0.05 to 10% by weight of at least one ethylenically unsaturated hydroxy compound (monomers c)
from 0 to 10% by weight of an ethylenically unsaturated acid or an acid anhydride (monomers d) and
from 0 to 30% by weight of other ethylenically unsaturated compounds (monomers e)
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 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 sheets, 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, for example, from WO 98/06763, WO 98/23656 or EP-A-625 557. 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. DE 19818394 (OZ48974) discloses pressure-sensitive adhesives with an aromatic emulsifier.
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 e) defined at the outset.
The monomers a) comprise a C
1
-C
12
alkyl (meth)acrylate, including in particular mixtures of the alkyl (meth)acrylates.
Suitable preferably are C
1
-C
8
-alkyl (meth)acrylates.
Suitable examples are methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate and ethyl acrylate.
Monomers b) comprise preferably &agr;-methylstyrene or styrene.
Particular preference is given to styrene. The proportion of monomers b) in the polymer is preferably not more than 10%, in particular not more than 5% and, with particular preference, not more than 3% by weight, and is preferably more than 0.1% by weight, in particular more than 0.5.
Monomers c) are preferably C
1
-C
18
hydroxyalkyl (meth)acrylates.
Particular preference is given to C
2
to C
8
hydroxyalkyl (meth)acrylates. Suitable examples are hydroxypropyl acrylate and methacrylate, and hydroxy-n-butyl acrylate and methacrylate.
Hydroxypropyl acrylate or hydroxypropyl methacrylate is particularly preferred.
The proportion of the monomers c) in the polymer is preferably up to 10%, in particular 5% and, with particular preference, 3% by weight, preferably at least 0.1% by weight and, in particular, at least 0.5% by weight.
Monomers d) are, for example, ethylenically unsaturated monomers having in particular 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 proportion of monomers d) is preferably not more than 5% by weight, based on the polymer.
Further monomers, e), can be of any kind. Preferred examples which may be mentioned are vinyl esters, olefins, diolefins, ethylenically unsaturated amides or nitriles etc.
The polymer consists preferably of
from 60 to 99.5
% by weight of monomers a)
from 0.05 to 5
% by weight of monomers b)
from 0.05 to 5
% by weight of monomers c)
from 0 to 5
% by weight of monomers d)
from 0 to 25
% by weight of monomers e)
and with particular preference of
from 75 to 99.8
% by weight of monomers a)
from 0.1 to 3
% by weight of monomers b)
from 0.1 to 3
% by weight of monomers c)
from 0 to 5
% by weight of monomers d)
from 0 to 15
% by weight of monomers e)
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, midpoint temperature).
The glass transition temperature (Tg) 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 −50° 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 (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, with particular preference 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 particular preference, the aromatic emulsifier comprises an ionic compound having a molecular weight of less than 2000 g/mol and comprising at least one sulfate group or sulfonate groups, preferably two sulfate groups or sulfonate groups with two unsubstituted or substituted phenyl groups.
Preference is given to aromatic emulsifiers having one or two, preferably two, sulfonate groups.
The emulsifier particularly preferably comprises a compound of the formula
where X is O, S, CH
2
, NH or NR
7
, one or two of R
1
to R
6
are SO
3
⊖
K
⊕
and the others of R
1
to R
6
are H or C
1
-C
18
alkyl, R
7
is C
1
-C
18
alkyl and K is a countercation.
X is preferably O. Preferably, one or two of R
1
to R
6
are C
1
-C
18
alkyl, especially C
6
-C
18
alkyl, and the others of R
1
to R
6
are hydrogen atoms and the sulfonate groups.
K is preferably a countercation selected, for example, from the alkali metals, ammonium and hydrogen. Sodium is particularly preferred. Compounds of the formula I normally also comprise a mixture of compounds having different degrees of substitution (mono- or dialkylated) and different positions of the substituents (of the sulfonate groups and of the one or two alkyl

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