Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2002-06-12
2004-12-14
Harlan, Robert D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S129000, C522S130000, C522S133000, C522S904000
Reexamination Certificate
active
06831114
ABSTRACT:
The invention relates to a process for preparing UV-crosslinkable acrylic pressure sensitive adhesives (PSAs) and also to the use of acrylic PSAs of this kind.
Hotmelt PSAs are compounds which combine the properties of hotmelt adhesives with those of pressure sensitive adhesives. Hotmelt PSAs melt at elevated temperatures and cool to form a permanently tacky film which flows adhesively on contact with a substrate. In combination with various substrates, such as paper, fabric, metal, and polymer films, for example, it is possible to produce a large number of different products, especially pressure sensitive adhesive tapes and also labels. These pressure sensitive adhesive products have a broad field of application in the automobile industry, e.g. for fastening or for sealing, or in the pharmaceutical industry, e.g., for active substance patches.
The typical coating temperature for hotmelt PSAs is between 80 and 180° C. In order to minimize coating and processing temperatures, the molecular weight of the hotmelt PSA to be applied should be as low as possible. On the other hand, the PSA must also possess a certain level of cohesion, so that the pressure sensitive adhesive tape does not slip from the substrate in use. In order to increase the cohesion, therefore, a high molecular weight is essential.
In principle, the cohesion of acrylic PSAs can be raised by means of efficient crosslinking. In U.S. Pat. No. 5,536,759, polyacrylates containing hydroxyl or carboxylic acid groups were reacted with 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene (m-TMI). By functionalization with the double bonds, the polyacrylates described can be crosslinked very efficiently with UV light and thus produce PSAs possessing high cohesion. Disadvantages of this process include the high toxicity of the isocyanates used and also the complex two-stage process, since in a first step the polymerization is carried out and then in a second step the polymer-analogous reaction has to be carried out in a reactor. Moreover, double-bond-functionalized polyacrylates do not possess great thermal stability and are sensitive to shearing. Ultimately, these polymers are unsuitable for the hotmelt process, since the UV photoinitiators are not attached to the polymer and are therefore liable to undergo volatilization at the high temperatures normal for the hotmelt process.
In U.S. Pat. No. 5,741,543 double-bond-functionalized polyacrylates prepared by way of a polymer-analogous reaction are UV crosslinked. The polymers have been prepared by way of the UV prepolymerization technique and therefore have the known disadvantages such as slow process speeds and the free circulation of monomers.
The disadvantage of the volatile photoinitiators can be avoided by copolymerizing, for example, acrylated benzophenone derivatives as UV photoinitiators (U.S. Pat. No. 5,264,533). In U.S. Pat. No. 5,294,688 these copolymerizable UV photoinitiators were also described for acrylic PSAs. In U.S. Pat. No. 4,144,157, copolymerizable benzoin UV photoinitiators were described for acrylic PSAs. All of the methods have the disadvantage, however, that the UV photoinitiators used must first be synthesized in a separate, complex step, and are not available commercially. Moreover, the copolymerization of UV photoinitiators is hampered by problems, since acrylic PSAs are normally prepared by free-radical polymerizations and intermediate radicals may interact with the photoinitiator. The result is that instances of gelling occur which can only be compensated by means of high regulator fractions, which in turn reduces the average molecular weight. Moreover, following UV irradiation of the systems described, inter alia, in DE 27 43 979 A1, benzaldehyde is formed, and is perceptible as a disruptive odor.
U.S. Pat. Nos. 4,565,769 and 4,454,218 describe photosensitizers for photopolymerizations in photoresist technology. These photosensitizers are reacted with the polymer in a polymer-analogous reaction. The polymers described are not, however, used for acrylic PSAs. Instead, the photosensitizers are used for the polymerization and not for the UV crosslinking. Further disadvantages are that unbound UV photoinitiators are added as well and that no continuous process is possible.
Derived from the prior art described above, therefore, there is a need for a process for preparing UV-crosslinkable acrylic hotmelt PSAs which are easy to concentrate and can be processed by the hotmelt process in which for UV crosslinking the polymer is functionalized with UV photoinitiators and, following the coating operation, is crosslinked on the backing as an acrylic hotmelt PSA.
Surprisingly for the skilled worker, this object is achieved by the process of the invention depicted in the main claim. Further claims relate to advantageous developments of the process.
The invention accordingly provides a process for preparing UV-crosslinkable acrylic pressure sensitive adhesives, polyacrylates being reacted in a hotmelt process in the melt with UV-photoactive substances which are able to absorb radiation energy and transfer it to a reaction system.
In one particularly preferred development of the inventive process the polyarcrylates used are those having the following composition or those which can be prepared by polymerizing a monomer mixture comprising the following components:
(a) from 45 to 99.5% by weight of acrylic and/or methacrylic acid derivatives of the formula
CH
2
═CH(R
1
)(COOR
2
)
where R
1
is H or CH
3
and R
2
is an alkyl chain having from 1 to 20 carbon atoms,
(b) from 0.5 to 25% by weight of vinyl compounds containing functional groups Y selected such that they are able to enter into chemical coupling reactions with the UV-photoactive substances or parts thereof, in particular with the group X of the UV-photoactive substances which is defined in more detail below.
One further advantageous variant of the process starts from a monomer mixture to which as a further component there are added
(c) up to 30% by weight of olefinically unsaturated monomers, especially monomers containing functional groups.
In one very advantageous embodiment of the pressure sensitive adhesive, component (a) comprises one or more components which can be described by the following general formula
In this formula R
1
is H or CH
3
and the radical R
2
is selected from the group consisting of branched and unbranched saturated alkyl groups having from 4 to 14 carbon atoms. Acrylic monomers which are used with preference as component (a) for the inventive PSA comprise acrylic and methacrylic esters with alkyl groups composed of from 4 to 14 carbon atoms, preferably from 4 to 9 carbon atoms. Specific examples, listed without wishing to impose any unnecessary restriction, include n-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, and their branched isomers, such as 2-ethylhexyl acrylate, for example.
Examples of suitable vinyl compounds containing functional groups as component (b), capable in particular of a reaction with UV photoinitiators of type (I), include one or more carboxylic anhydrides, such as maleic anhydride or itaconic anhydride, for example, or acrylated or methacrylated carboxylic anhydrides, such as 4-(methacryloyloxyethyl)trimellitic anhydride, for example, or hydroxy-functionalized monomers, such as hydroxyethyl acrylate or hydroxyethyl methacrylate, for example, or carboxyl-functionalized monomers, such as acrylic and methacrylic acid, for example, or amine-functionalized monomers, such as dimethylformamide, for example, isocyanate-functionalized monomers, such as MOI, for example, or epoxidized monomers, such as glycidyl methacrylate, for example, or, generally, all monomers containing a functional group able to enter into a chemical coupling with UV photoinitiators and/or photosensitizers.
As monomers (b) it is possible to use compounds of the following general formula
where R
1
is H or CH
3
and the radical —OR
2
comprises or represents a functional group. Particularly preferred examples of com
Husemann Marc
Zöllner Stephan
Harlan Robert D.
Norris McLaughlin & Marcus PA
tesa AG
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