Pressure sensitive adhesive, particularly for apolar surfaces

Details Pressure sensitive adhesive, particularly for apolar surfaces Pressure sensitive adhesive, particularly for apolar surfaces

2002-05-21

2003-11-25

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Adhesive outermost layer

C525S089000, C526S319000, C526S931000, C528S272000, C156S332000

Reexamination Certificate

active

06652963

ABSTRACT:

The invention relates to pressure sensitive adhesives based on block copolymers, said block copolymers comprising at least the unit P(A)-P(B)-P(A), composed of a middle polymer block P(B) and of two polymer blocks P(A) surrounding the middle polymer block P(B), or the unit P(B)-P(A)-P(B), composed of a middle polymer block P(A) and of two polymer blocks P(B) surrounding the middle polymer block P(A), to the use of such adhesives, and to a process for preparing them.
In industry, hotmelt processes operating with solventless coating technology are of growing importance in the preparation of pressure sensitive adhesives. In general, environmental regulations and increasing costs are forcing forward the development process of such adhesives. Besides SIS (styrene-isoprene-styrene copolymers) systems, acrylic polymers are increasingly being applied from the melt as a polymer film to backing materials. Moreover, for specialty applications, pressure sensitive adhesive tapes which feature very low outgassing are needed. This is something which can be ensured only by means of hotmelt processes, since conventional coatings applied from solution always still contain small fractions of residual solvent.
Additionally, there is a growing need for acrylic pressure sensitive adhesives on apolar surfaces. In industry, the use of plastics is on the increase, owing to the weight reduction they offer as compared with conventional metals, and, accordingly, adhesive bonds are being formed more and more on these substrates. Since the large number of their ester groups makes polyacrylates relatively polar, bond strength can be increased only by using resins of similar polarity. Although these resins exhibit good bond strength on polar surfaces such as steel, their bond strengths to apolar surfaces are mediocre and inadequate for the majority of applications.
U.S. Pat. No. 4,418,120 A describes crosslinked pressure sensitive adhesives comprising rosin ester resins, which possess good bond strength to polypropylene. Owing to the unsaturated nature of the resin, however, the UV stability of these adhesives is poor. The bond strengths to the apolar surfaces as well are relatively low.
U.S. Pat. No. 4,726,982 A describes crosslinked pressure sensitive adhesives featuring high bond strength to inks and paints. The copolymers of acrylic esters and N-vinyl-2-pyrrolidone are blended with tackifiers such as poly(isobornyl methacrylate), pentaerythritol esters of rosins, and mixed aliphatic/aromatic resins.
Here again, relatively polar resins are used to increase the bond strength to apolar surfaces. The tackifiers mostly described, however, possess unsaturated compounds which, in the hotmelt process, can lead to, instances of gelling; additionally, after bonding, the unsaturated compounds undergo aging or weathering under UV light and, accordingly, the adhesive properties suffer over a prolonged period of time.
In EP 0 707 604 A1, polyethylene/butylene macromonomers are used for copolymerization with acrylates. As a result, phases are formed which have a low glass transition temperature, which in turn allow the adhesives to flow on apolar surfaces and thus ensure high bond strengths to PE and PP. A disadvantage is the poor conversion of the polymerization process described. Furthermore, the macromonomers are copolymerized randomly, and so no domains are able to form. Such domains would offer regions in which even very apolar resins would be soluble. With these pressure sensitive adhesive tapes, therefore, only a relatively low bond strength to apolar surfaces is attained.
Furthermore, the polyacrylates described are very difficult to process as hotmelts because the high residual monomer fractions impact negatively on the concentration process, and migration within the pressure sensitive adhesive tape can have an adverse effect on the long-term adhesive properties.
In contrast, styrene-isoprene-styrene (SIS) block copolymers are widespread as elastomers for hotmelt-processable pressure sensitive adhesives [preparation processes:
U.S. Pat. No. 3,468,972 A; U.S. Pat. Nos. 3,595,941 A; use in pressure sensitive adhesives: 3,239,478 A; U.S. Pat. No. 3,935,338 A] with high bond strength to apolar surfaces. Their effective processability is achieved by means of a relatively low molecular weight and by means of a specific morphology [EP 0 451 920 B1]. These pressure sensitive adhesives can be crosslinked very effectively with UV light in the presence of photoinitiators or with electron beams, since the middle blocks contain a large number of double bonds.
Nevertheless, these elastomers are not without their disadvantages, such as, for example, severe aging under UV light (i.e., in daylight too) and in an oxygen/ozone atmosphere. Moreover, owing to the formation of hard block domains comprising the hard polystyrene end blocks, effective flow on apolar surfaces is prevented. The same applies to other block copolymers which possess a middle block containing at least one double bond [U.S. Pat. No. 5,851,664 A].
Another very unfavorable property of styrene-isoprene-styrene block copolymers for the application is the relatively low thermal shear strength. These pressure sensitive adhesives, accordingly, are unsuited to applications within relatively high temperature ranges.
U.S. Pat. No. 5,314,962 A describes A-B-A block copolymers as elastomers for adhesives, which possess A domain formation as their cohesion-forming feature. As a result of the selection of the comonomers used, however, only low bond strengths can be realized on apolar surfaces. Additionally, these polymers are not compatible with highly apolar resins.
EP 0 921 170 A1 describes A-B-A block copolymers which have been modified with additions of resin. Here again, owing to the selection of the comonomers and added resins, only low bond strengths can be realized on apolar surfaces.
EP 0 408 429 A1 and EP 0 408 420 A1 described A-B-A block copolymers which, however, were synthesized by living anionic polymerization. Because of the absence of an acrylic acid fraction, however, these polymers are unsuited to use as pressure sensitive adhesives, since the internal cohesion of the middle block is too low and at least a low fraction of polar monomers is required for bonds to steel. Because of the anionic polymerization, it is not possible to use acrylic acid comonomers or other hydroxyl-functionalized acrylate comonomers, such as hydroxyethyl acrylate, for example. And, again, highly efficient flow on apolar surfaces is prevented by the use of the conventional domain-forming polymers, such as PMMA or polystyrene.
In U.S. Pat. No. 5,166,274 A, this acrylic acid deficiency is compensated by hydrolyzing block copolymers of tert-butyl methacrylate in order to liberate the carboxylic acid function. On the industrial scale, however, this method cannot be applied to pressure sensitive adhesives, since the hydrolysis step is very costly and time consuming.
In U.S. Pat. No. 6,069,205 A1, diblock and triblock copolymers are described which are prepared by an atom transfer polymerization and utilized for adhesives. This method too is unsuitable for preparing pressure sensitive adhesives, since it uses relatively high catalytic amounts of heavy metal compounds which would have to be removed, in a cumbersome operation, by extraction processes.
EP 1 008 640 A1 describes styrene block copolymers comprising an acrylate middle block composed, however, of the common C
2
to C
14
alkyl acrylates. Because of the restriction of the comonomers and, associated therewith, the restriction of the resins which can be used, only low bond strengths to apolar substrates are achievable with these polymers. Moreover, metal salts are used to prepare these polymers too (in analogy to U.S. Pat. No. 6,069,205 A), which would have to be removed, again a cumbersome operation, for pressure sensitive adhesive tape applications.
It is an object of the invention to provide improved pressure sensitive adhesives based on polyacrylate which do not have the disadvantages of the pr

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