Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-08-11
2004-11-23
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S123000, C525S452000, C428S3550RA, C428S3550BL, C428S3550EN
Reexamination Certificate
active
06822048
ABSTRACT:
The present invention relates to the use of isocyanates for the chemical/thermal crosslinking of hot-melt pressure-sensitive adhesives (PSAS) based on non-thermoplastic elastomers, such as natural rubber, for example, using tackifying resins, optionally fillers and plasticizers, and to the application of these hot-melt PSAs to produce self-adhesive articles, especially for producing high-performance self-adhesive articles such as tapes or labels.
The hot-melt PSAs developed in recent years on the polymer basis of non-thermoplastic elastomers, such as, for example, natural rubber or other high molecular mass rubbers, in the absence of a crosslinking step after application lack sufficient cohesion for the majority of applications. This is manifested in inadequate shear strength of the self-adhesive tapes manufactured in this way and may even lead to the formation of disruptive residues of adhesive, which make it impossible to achieve a desired residueless redetachability after use.
For many years, this deficiency prevented the use of hot-melt PSAs based on natural rubber in the self-adhesive tape applications traditionally dominated heavily by natural rubber, such as masking tapes or adhesive tapes for packaging.
The crosslinking processes used to date for hot-melt PSAs based on non-thermoplastic elastomers, by means of ionizing radiation (electron beams=EBC or ultraviolet light=UV), require the presence of appropriate, cost-intensive installations such as radiation sources and complex protective equipment, especially at relatively high film thicknesses.
Furthermore, in the case of many customary ingredients such as fillers, non-transparent resins and pigments, and in the case of thick films of adhesive, UV crosslinking is possible only to an extremely limited extent.
The use of exclusively non-thermoplastic rubbers as an elastomer component in the formulation of PSAs with the existing cost advantage possessed, for example, by natural rubbers over the standard commercial block copolymers, and the outstanding properties, especially the shear strength of natural rubber and of corresponding synthetic rubbers, and also processes for preparing, applying and crosslinking hot-melt PSAs based on non-thermoplastic elastomers, are also set out at length in the patents WO 9411175 A1, WO 9525774 A1, WO 9707963 A1 and, correspondingly, U.S. Pat. No. 5,539,033, U.S. Pat. No. 5,550,175, and also EP 0 751 980 B1 and EP 0 668 819 B1.
In these cases, the additives customary in PSA technology, such as tackifier resins, plasticizers and fillers, are described.
The preparation process disclosed in each case is based on a twin-screw extruder which permits compounding to a homogeneous PSA blend with the chosen process regime, involving mastication of the rubber and subsequent gradual addition of the individual additives with an appropriate temperature regime.
The mastication step of the rubber, which precedes the actual production process, is described at length. It is necessary and characteristic of the process chosen, since with the technology selected therein it is indispensable to the subsequent integration of the other components and to the extrudability of the blended composition. Also described is the feeding-in of atmospheric oxygen, as recommended by R. Brzoskowski, J. L. and B. Kaivani in Kunststoffe 80 (8), (1990), p. 922 ff., in order to accelerate mastication of the rubber.
This procedure makes it absolutely necessary to practise the subsequent step of electron beam crosslinking (EBC), and to use reactive substances as EBC promoters in order to achieve an effective crosslinking yield.
Both process steps are described in the abovementioned patents, but the EBC promoters chosen also tend towards unwanted chemical crosslinking reactions at elevated temperatures, which limits the use of certain tackifying resins.
Owing to the unavoidable high product temperatures, compounding in a twin-screw extruder prevents the use of heat-activatable substances suitable for crosslinking the adhesive compositions, such as, for example, reactive (optionally halogenated) phenolic resins, sulphur or sulphur-donor crosslinker systems, since the chemical crosslinking reactions which ensue in the process result in such a great increase in viscosity that the coatability of the resulting pressure-sensitive adhesive composition is impaired.
The patent application JP 95 278 509 discloses a self-adhesive tape in whose production natural rubber is masticated to an average molecular weight M
w
=100,000 to 500,000 in order to obtain a coatable homogeneous mixture comprising hydrocarbon resins, rosin/rosin-derivative resins or terpene resins, which can be processed regularly at between 140° C. and 200° C. with a coating viscosity of from 10 to 50×10
3
cps, but subsequently requires an extremely high EBC dose (40 Mrad) in order to ensure the shear strength necessary for its use.
For backing materials such as impregnated and/or sized papers, and for woven backings based on viscose staple and the like, the system is not very suitable, since at the necessarily high beam doses there is significant deterioration of the backing.
A disadvantage of the crosslinking technologies (essentially EBC irradiation) described in the documents cited, in addition to the capital investment required, is the damage of certain sensitive backings by electron beams. This is manifested to a particular extent in the case of paper backings, viscose staple wovens, and siliconized release papers, but especially in the case of widespread film materials such as polypropylene, by a deterioration in the elongation-at-break properties.
Moreover, many standard commercial PVC films tend to discolour under EBC irradiation, such discoloration having a deleterious effect in the case of light-coloured or transparent film grades.
Furthermore, many of the release coatings which are customary in adhesive tape manufacture are damaged by electron beam irradiation and so are impaired in their effect. In an extreme case, this may result in the non-unrollability of adhesive tape rolls or in the non-reusability of transfer release papers, which are required in the adhesive tape production process.
Certain synthetic rubbers such as polyisobutylene (PIB), butyl rubber (IIR) and halogenated butyl rubber (XIIR), finally, are not amenable to electron crosslinking and are degraded under irradiation.
One way of minimizing these disadvantages consists in the use of certain substances which lessen the required beam dose and thus the concomitant damage. A range of such substances are known for use as EBC promoters. However, EBC promoters may also tend towards unwanted chemical crosslinking reactions at elevated temperatures, which limits the selection of the EBC promoters that can be used for hot-melt PSA production and, moreover, restricts the use of certain tackifying resins. These restrictions, and certain advantageous combinations of EBC promoters and non-crosslinking phenolic tackifier resins, are the subject-matter, in particular, of the document WO 97/07963.
The use of non-thermoplastic elastomers is also described in JP 95 331 197, where use is made of natural rubber having an average molecular weight (weight average) M
w
<1 million g/mol with aliphatic, non-reactive hydrocarbon resins, which is blended with blocked isocyanates, precrosslinked at 150° C. for five minutes, and, following its subsequent coating onto PET film, is cured at 180° C. for several minutes (for example 15 minutes).
A disadvantage of this process, firstly, is the blocking agent released during the crosslinking reaction, which on the one hand, if it remains in the adhesive composition, may impair the adhesion properties of the tape in a variety of respects, and on the other hand, on its escape in vapour form, leads to coating defects such as porosities and necessitates complex technology in order to draw off these blocking agents under suction and remove them.
Of particular disadvantage, however, is the high crosslinking temperature, which rules out temperature-sensitive
Burmeister Axel
Hansen Sven
Kreft Christian
Leydecker Heiko
Stähr Jochen
Lee Rip
Norris McLaughlin & Marcus PA
tesa Aktiengesellschaft
Wu David W.
LandOfFree
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