Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-07-17
2004-04-13
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...
C526S905000, C526S222000, C526S224000, C525S309000, C522S035000, C428S3550RA, C428S356000
Reexamination Certificate
active
06720399
ABSTRACT:
The invention relates to a polyacrylate pressure sensitive adhesive (PSA), to a process for preparing such a PSA, and to the use of such PSAs.
BACKGROUND OF THE INVENTION
For industrial PSA tape applications it is very common to use polyacrylate PSAs. Polyacrylates possess a variety of advantages over other elastomers. They are highly stable toward UV light, oxygen, and ozone. Synthetic and natural rubber adhesives normally contain double bonds, which make these adhesives unstable to the aforementioned environmental effects. Further advantages of polyacrylates include their transparency and their serviceability within a relatively wide temperature range.
Polyacrylate PSAs are generally prepared in solution by free radical polymerization. The polyacrylates are generally applied to the corresponding backing material from solution using a coating bar, and then dried. In order to increase the cohesion, the polymer is crosslinked. Curing takes place thermally or by UV crosslinking or by EB curing (EB: electron beams). The process described is fairly costly and ecologically objectionable, since as a general rule the solvent is not recycled and the high consumption of organic solvents represents a high environmental burden.
Moreover, it is very difficult to produce PSA tapes with a high adhesive application rate, without bubbles.
One remedy to these disadvantages is the hotmelt process. In this process, the PSA is applied to the backing material from the melt.
However, this new technology has its limitations. Prior to coating, the solvent is removed from the PSA in a drying extruder. The drying process is associated with a relatively high temperature and shearing effect, so that high molecular mass polyacrylate PSAs in particular are severely damaged. The acrylic PSA gels, or the low molecular mass fraction is greatly enriched as a result of molecular weight breakdown. Both effects are undesirable, since they are disadvantageous for the application. Either the adhesive can no longer be applied or there are changes in its technical adhesive properties, since, for example, when a shearing force acts on the adhesive the low molecular mass fractions act as lubricants and so lead to premature failure of the adhesive.
One solution to mitigating these disadvantages is offered by polyacrylate adhesives with a low average molecular weight and narrow molecular weight distribution. In this case the fraction of low molecular mass and high molecular mass molecules in the polymer is greatly reduced by the polymerization process. The absence of the high molecular mass fractions reduces the flow viscosity, and the adhesive shows less of a tendency to gel. As a result of the reduction in the low molecular mass fraction, the number of oligomers which reduce the shear strength of the PSA is lessened.
A further disadvantage of relatively low molecular mass acrylic PSAs is the relatively low crosslinking propensity. Short polymer chains are generally crosslinked less efficiently, since there is a lesser probability that a polymer radical will meet a second polymer chain. In order to increase the crosslinking efficiency, therefore, promoters are needed.
A variety of polymerization methods are suitable for preparing low molecular mass PSAs. The state of the art is to use regulators, such as alcohols or thiols, for example (Makromoleküle, Hans-Georg Elias, 5
th
Edition, 1990, Hüthig & Wepf Verlag Basel). These regulators reduce the molecular weight but broaden the molecular weight distribution.
Another controlled polymerization method used is that of atom transfer radical polymerization (ATRP), in which initiators used preferably include monofunctional or difunctional secondary or tertiary halides and, for abstracting the halide(s), complexes of Cu, Ni, Fe, Pd, Pt, Ru, Os, Rh, Co, Ir, Cu, Ag or Au [EP 0 824 111; EP 0 826 698; EP 0 824 110; EP 0 841 346; EP 0 850 957]. The various possibilities of ATRP are further described in U.S. Pat. Nos. 5,945,491, 5,854,364 and 5,789,487. Generally, metal catalysts are used, which have the side effect of adversely influencing the aging of the PSAs (gelling, transesterification). Moreover, the majority of metal catalysts are toxic, discolor the adhesive, and can be removed from the polymer only by means of complicated precipitations.
U.S. Pat. No. 4,581,429 discloses a controlled radical polymerization process. As its initiator the process employs a compound of the formula R′R″N—O—X, in which X denotes a free radical species which is able to polymerize unsaturated monomers. In general, however, the reactions have low conversion rates. A particular problem is the polymerization of acrylates, which takes place only with very low yields and molecular weights.
WO 98/13392 describes open-chain alkoxyamine compounds which have a symmetrical substitution pattern. EP 0 735 052 A1 discloses a process for preparing thermoplastic polymers having narrow polydispersities.
WO 96/24620 describes a polymerization process in which very specific radical compounds, such as phosphorus-containing nitroxides, for example, are described.
WO 98/30601 discloses specific nitroxyls, based on imidazolidine.
WO 98/4408 discloses specific nitroxyls based on morpholines, piperazinones, and piperazinediones.
DE 199 49 352 A1 discloses heterocyclic alkoxyamines as regulators in controlled radical polymerizations.
Corresponding further developments of the alkoxyamines or of the corresponding free nitroxides improved the efficiency for the preparation of polyacrylates. [Hawker, C. J., Paper, National Meeting of the American Chemical Society in San Francisco, Spring 1997; Husemann, M., IUPAC World Polymer Meeting 1998, Gold Coast, Australia, Paper on “Novel Approaches to Polymeric Brushes using ‘Living’ Free Radical Polymerizations” (July 1998).]
In the abovementioned patents and papers attempts were made to improve the control of radical polymerization reactions. There nevertheless exists a need for a nitroxide-controlled polymerization process which is highly reactive and can be used to realize high conversions in combination with high molecular weight and low polydispersity. These requirements have been met in DE 100 36 801.8.
Nevertheless, this type of compound does not promote efficient crosslinking. Instead, nitroxides generally act as carbon radical scavengers and therefore possess an inhibitory effect as far as crosslinking is concerned.
BASF AG offers UV AC-Resins™ containing copolymerizable photoinitiators based on benzophenone. By this route, the photoinitiators are attached to the polymer, are not volatile, and can be effectively UV-crosslinked as a result of the binding to the polymer. A similar path was taken by Guse (U.S. Pat. No. 4,144,157). These acrylic PSAs are readily UV-crosslinkable and can be processed as hotmelts, but as a result of the broad molecular weight distribution they do not possess good technical adhesive properties.
A further variant is the RAFT process (Reversible Addition-Fragmentation Chain Transfer). The process is described at length in WO 98/01478 and WO 99/31144, but in the manner set out therein is unsuited to the preparation of PSAs, since the conversions achieved are very low and the average molecular weight of the polymers prepared is too low for acrylic PSAs. Accordingly, the polymers described cannot be used as acrylic PSAs. An improvement was achieved with the process described by BDF in DE 100 30 217.3.
The above-described process cannot, however, be used for UV crosslinking, since the compounds described likewise possess a radical scavenger effect, so that the crosslinking efficiency following addition of the free UV photoinitiator is too low. Moreover, there is a risk that the abovementioned types of compound will be unstable over a prolonged period—such as is necessarily the case in a hotmelt process, for example—and would decompose.
A central problem which therefore remains is the efficient UV crosslinking of narrow-distribution acrylic PSAs for the purpose of preparing improved acrylic PSAs.
It is an object of the inve
Husemann Marc
Zöllner Stephan
Norris & McLaughlin & Marcus
Sastri Satiya
tesa AG
Wu David W.
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