Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reissue Patent
1995-06-01
2002-03-26
Zirker, Daniel (Department: 1771)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S198000, C428S339000, C428S343000, C428S345000, C428S3550RA
Reissue Patent
active
RE037612
ABSTRACT:
The present invention relates to contact-adhesive sheetlike structures which, after attachment, can be residuelessly redetached and, if desired, attached again. Sheet-like structures of this type have long been used in practice, examples thereof being residuelessly detachable reversibly adherent contact-adhesive labels, protective films, masking papers, note sheets and advertising materials and residuelessly detachable and reattachable cohesive plasters and dressings.
A number of different contact-adhesive materials and manufacturing methods have already been described for these purposes.
German Offenlegungsschrift No. 2,407,494 describes a weakly contact-adhesive emulsion polymer which is prepared by copolymerizing 2-ethylhexyl acrylate, methyl methacrylate, (meth)acrylic acid and 1,4-butanediol diacrylate in the presence of plasticizers such as paraffin oil, squalene or low molecular weight polyisobutylene and, owing to its relatively low adhesivity, permits redetachability of the contact-adhesive articles equipped therewith.
Japanese Pat. No. 82/87,481 describes an aqueous contact-adhesive dispersion which is prepared by copolymerizing esters of acrylic acid with alkyl radicals of 4 or more carbon atoms, methacrylic acid and N-methylolacrylamide in the presence of special plasticizers. After addition of thickening agents these dispersions are suitable for equipping reversibly adherent paper labels. Japanese Pat. No. 82/70,162 describes a similar process.
Japanese Pat. No. 82/31,972 describes an emulsion copolymer which is prepared from 2-ethylhexyl acrylate, methyl methacrylate, acrylic acid, itaconic acid and ethylene dimethacrylate and, after buffering with sodium acetate and thickening with hydroxyethyl cellulose, is suitable as a contact adhesive for redetachable contact adhesive articles.
Japanese Pat. No. 82/42,778 describes a weakly contact-adhesive bead copolymer which is prepared in aqueous dispersion from (meth)acrylic esters in the presence of a dispersant. Said water-soluble dispersant consists of a copolymer which is obtained by free-radical copolymerization from a low proportion of butyl acrylate and a high proportion of acrylic acid.
U.S. Pat. No. 3,691,140 and German Offenlegungsschrift No. 2,417,312 describe the production of redetachable, repeatedly usable contact-adhesive articles whose contact-adhesive layer consists of contact-adhesive microspheres. Said microspheres are synthesized in aqueous dispersion in the presence of anionic emulsifiers by copolymerizing alkyl acrylates, special water-soluble ionic monomers and maleic anhydride. To improve the anchoring, the microspheres are affixed to the carrier materials by means of special anchoring layers.
In U.S. Pat. No. 2,510,120, the redetachability of contact-adhesive sheetlike structures is improved by coating part of the surface, for example in the form of strips or other two-dimensional patterns.
German Offenlegungsschrift No. 2,535,897 describes a part-surface application of contact-adhesive compositions to label paper by using an engraved roll to coat only the label area in full before the punching out.
Contact-adhesive articles which are equipped with suitable contact-adhesive compositions by the cited methods have not only advantageous properties but also serious shortcomings.
1. Plasticizers, which are frequently contained in the contact-adhesive compositions described, for example paraffin oils, tend to bleed out of the composition and can stain the carrier materials, for example paper. This also affects the adhesive properties. Finally, on prolonged attachment they can also penetrate into the attached substrates.
2. Thickeners and emulsifiers settle out on the contact-adhesive particles as the emulsion dries and forms a film; they reduce the adhesivity and promote, in particular, the absorption of water by the contact-adhesive films in dependence upon the ambient humidity and the moisture content of the attached substrates. The plasticizing action of even small amounts of water affects the adhesion properties in an uncontrollable manner; this impairs the usability of contact-adhesive articles of this type.
3. The adhesivity of a whole-surface coat of contact adhesive increases with the duration of attachment, since, to form a bond, films of contact adhesive must possess some flow, albeit limited, because without that property the substrate on which attachment is to take place cannot be wetted. In the case of short-lived attachment, only the protuberances of the substrate surface are wetted, and the strength of attachment is correspondingly low. As the duration of attachment increases, the contact adhesive flows also into the indentations of the substrate surface, which is normally not microscopically smooth. As a result, the force required to separate the attached surfaces rises to two to three times the starting values. As a consequence, the carrier material or the contact adhesive film can split (cohesive failure) on breaking the attachment. The substrate is soiled, and the contact-adhesive article is unsuitable for reuse. In some instances, moreover, the surfaces of less strong substrates, for example papers, can be damaged. As contact-adhesive articles are used on substrates of all kinds of roughness, strength and chemical composition, a satisfactory match between adequate initial adhesivity and reversibly rebreakable end adhesivity is possible for whole-surface smooth coatings only in specific cases.
4. Part-surface coatings of contact adhesive which are applied to the carrier materials in the form of strips or other patterns can be advantageous, in particular for carriers having lower tear strength, such as, for example, paper. Owing to the smaller area of attachment, the tensile forces exerted on the carrier material in the course of separating the attached surfaces are kept in check, and splitting of the carrier material is avoided. However, in the areas of whole-surface attachment the force of adhesion to the substrate increases with time by the mechanism described in the preceding section, so that on suitably long attachment there is a danger that the contact-adhesive film will split on separation of the attached surfaces, in particular when the attached surfaces are separated in the direction of the strips.
5. Coatings of contact-adhesive microspheres of a suitable size (50-150 &mgr;m) show the time-dependent increase in the strength of attachment to a small extent. The elastomeric recovery forces of the spheres flattened off in the course of the pressure-sensitive attachment to the substrate tend toward the re-formation of the originally curved surface and thereby counteract any stronger flow of the contact adhesive into the microinch indentations of the substrate. The force employed in separating the attached surfaces acts concentratedly on the relatively small, circular, discrete attached flattened parts of the spheres and is distributed from there into the much bigger sphere volumes. As a result, the desired adhesive failure is brought about preferentially, and the undesirable cohesive failure is suppressed. Besides these advantages, this process also has some shortcomings and/or limitations. Anchoring the microspheres to the carrier requires a binder layer in which the spheres are partly submerged and which, owing to the larger wetting area, effects anchoring to the carrier surface. However, this principle requires a flat surface of limited absorbency; in order to prevent the binder from penetrating into the substrate and thus becoming ineffective in the sense described above. The choice of the carriers which are coatable by this principle is limited as a result and/or necessitates an additional, smoothing coat, for example for paper.
Moreover, application of the microspheres to the carrier material must be done in such a way as to produce a monoparticulate coating. In areas where microspheres are positioned on top of one another, reliable anchorage is no longer assured, since the anchoring mechanism described above then becomes ineffective. This condition is difficult to meet
Behrend Ekkehard
Gleichenhagen Peter
Jauchen Peter
Beiersdorf AG
Norris & McLaughlin & Marcus
Zirker Daniel
LandOfFree
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