Pressure sensitive materials based on a multistep...

Details Pressure sensitive materials based on a multistep... Pressure sensitive materials based on a multistep...

1999-02-09

2001-10-16

Wu, David W. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S317100, C526S346000, C156S327000, C156S332000, C428S349000

Reexamination Certificate

active

06303726

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a polymer, which contains
from 50 to 99.98% by weight of a C
1
-C
12
alkyl (meth)acrylate a),
from 0.02 to 10% by weight of a vinylaromatic compound b),
from 0 to 10% by weight of an ethylenically unsaturated acid or of an ethylenically unsaturated acid anhydride c), and
from 0 to 40% by weight of further monomers d), the percentages by weight being based on the polymer and the polymer being obtainable by polymerizing the monomers a) to d) in at least two stages which differ in their content of vinylaromatic compounds in such a way that the content of vinylaromatic compounds in the vinylaromatics-rich stage is at least 5 times as great as in the low-vinylaromatics stage.
Furthermore, the invention relates to the use of the polymers or of their aqueous dispersions as pressure-sensitive adhesives.
2. Description of the Background
Pressure-sensitive adhesives (PSAs) form a permanent tacky film which at room temperature, even under slight pressure, sticks to a very wide variety of surfaces. Pressure-sensitive adhesives are used to produced self-adhesive products such as self-adhesive labels, tapes and films. Products of this kind are very simple 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 implemented. The quality of a self-adhesive article depends essentially on whether the cohesion and the adhesion of the film of adhesive 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.
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 polymers which can contain styrene are already known from DE-A-3 013 812. U.S. Pat. No. 5,416,134 and EP-A-625 557. The polymers prepared or used in the examples of the disclosures, however, contain at least 4% by weight of styrene. Copolymers of this kind still do not have the desired level of adhesion and cohesion or the desired ratio of adhesion to cohesion.
U.S. Pat. No. 4,939,190 and EP 215241 disclose pressure-sensitive adhesives comprising polymers of multistage construction.
SUMMARY OF THE INVENTION
It is an object of the present invention, therefore, to provide polymers of improved adhesion and/or cohesion.
We have found that this object is achieved by the polymers defined above and by their use as pressure-sensitive adhesives.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present polymer preferably contains at least 0.05% by weight of a vinylaromatic compound, or vinylaromatic for short, and preferably not more than 5% by weight, in particular not more than 3% by weight, particularly preferably not more than 2% by weight and, especially, not more than 1.5% by weight of the vinylaromatic compound.
Preferred polymers are composed of
from 50 to 99.98% of a C
1
-C
12
alkyl (meth)acrylate a) by weight
from 0.02 to 10% of a vinylaromatic compound b) by weight
from 0 to 10% by of an ethylenically unsaturated acid c) weight
from 0 to 40% by of further monomers d), which are different weight from a) to c).
Particularly preferred polymers are composed of
from 60 to 99.95
% by weight a)
from 0.05 to 5
% by weight b)
from 0 to 5
% by weight c)
from 0 to 35
% by weight d)
Very particularly preferred polymers are composed of
from 80 to 99.75
% by weight a)
from 0.25 to 1.5
% by weight b)
from 0 to 2
% by weight c)
from 0 to 18
% by weight d)
In one particular embodiment the content of monomer c) is at least 0.1% by weight, the maximum content of components a) being reduced accordingly.
The novel polymer is obtainable by polymerizing the monomers a) to d) in at least two stages which differ in their content of vinylaromatic compounds in such a way that the content of vinylaromatics in the vinylaromatics-rich stage is at least 5 times, preferably at least 10 times and, with particular preference, at least 20 times as great as in the low-vinylaromatics stage.
The content of vinylaromatics in the low-vinylaromatics stage is preferably below 0.2, in particular below 0.05 and, with particular preference, below 0.01% by weight, based on the polymer.
With particular preference, the low-vinylaromatics stage is essentially free from vinylaromatics.
The proportion of the vinylaromatics-rich stage is preferably 10-90% by weight, particularly preferably from 30 to 70% by weight and, with very particular preference, from 40 to 60% by weight; that of the low-vinylaromatics stage is, accordingly, preferably from 90 to 10% by weight, particularly preferably from 70 to 30 and, with very particular preference, from 60 to 40% by weight, the weight data being based in each case on the complete polymer.
In one preferred embodiment the proportion of the two stages is approximately equal, ie. about 50% by weight.
The sequence of polymerization of the individual stages is arbitrary. Thus it is possible, for example, first to polymerize the monomers of the vinylaromatics-rich stage and then the monomers of the low-vinylaromatics stage, or vice versa.
The monomer composition of the individual stages can be identical or different. In one possible embodiment, for example, the content of monomers c) in the 1st stage is higher than in the 2nd stage; for example, the amount of monomers c) in the 1st stage can be at least 1.2 times the amount of monomers c) of the 2nd stage.
The monomers of each individual stage are polymerized in the presence of the polymerization product of the previous stages.
The polymerization is preferably an emulsion polymerization, in which case aqueous polymer dispersions are obtained.
The monomers of one stage are preferably added to the reaction mixture of the emulsion polymerization only when the monomers of the preceding stage of the [sic] are already entirely in the reaction mixture and, with particular preference, when the monomers of the preceding stage have already polymerized to an extent of at least 60% by weight, very preferably at least 80% by weight, based on the monomers of the preceding stage (in other words, less than 40% by weight or, respectively, less than 20% by weight are still in monomer form).
Particularly suitable C
1
-C
12
alkyl (meth)acrylates are methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl acrylate and isooctyl acrylate.
In one particularly preferred embodiment the monomers a) necessarily include C
1
-C
2
-alkyl (meth)acrylates (monomers a
1
)) in addition to C
3
-C
12
-alkyl (meth)acrylates (monomers a
2
)).
In particular, from 5 to 50% by weight, preferably from 10 to 40% by weight and, with particular preference, from 10 to 20% by weight of the overall amount of monomers a) are monomers a
1
), ie. methyl (meth)acrylate or ethyl (meth)acrylate, preferably methyl acrylate.
Preferred vinylaromatic compounds are those having up to 20 carbons. Suitable examples are vinyltoluene, &agr;- and &bgr;-methylstyrene. Styrene is particularly preferred.
Examples of ethylenically unsaturated acids or anhydrides c) are maleic acid, fumaric acid, itaconic acid and, preferably, acrylic acid or methacrylic acid, (meth)acrylic anhydride or maleic anhydride.
Further monomers which may be mentioned are, in particular, alkyl (meth)acrylates having more than 12 carbons in the alkyl radical,

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