Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1998-09-24
2002-06-11
Morris, Terrel (Department: 1771)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S229000, C156S249000, C156S345420
Reexamination Certificate
active
06402875
ABSTRACT:
The invention relates to the repeated use of an adhesive-film laminate for redetachable bonds, said redetachment taking place by pulling on the laminate in the direction of the bond plane, and to adhesive-film laminates suitable for this purpose.
Adhesive films for redetachable bonds which can be redetached by pulling in the direction of the bond plane are known and are commercially available under the designation “tesa Power-Strips”. Bonds produced therewith offer a powerful hold and yet can be redetached without a trace and without damaging the substrate or the bonded parts, as is described in DE 33 31 016 C2. Moreover, DE 42 33 872 C2 describes adhesive films of this kind together with a hook, the hook itself being completely reusable. Despite the many undisputed advantages of these products, they have the disadvantage that the adhesive films cannot be used again and thus repeatedly. Given the considerable price of these adhesive films, this is a significant disadvantage for the user.
Nor from other prior art is it possible to find any such products which can be used again, and thus repeatedly.
WO 92/11333 and WO 93/01979 describe redetachable adhesive tapes with an intermediate support comprising an extensible but not resilient film. Such films, although they can be removed again by pulling in the direction of the bond plane, are in no case reusable, since these products, after “stripping”, are unable to relax back into their original state. Admittedly, WO 92/11332 describes redetachable adhesive tapes which also use extensible, resilient films as intermediate supports, but exclusively photopolymerized pressure-sensitive acrylate adhesives are used, which give such products disadvantages whose consequences in practice are decisively disruptive. Reproducible crosslinking of the pressure-sensitive adhesive composition is difficult to attain, resulting in corresponding fluctuations in the product properties. Moreover, a residual content of photo-initiator is unavoidable, which, especially in the case of subsequent bonding under the action of sunlight, for example on window panes, leads to distinct changes in the layer of adhesive; to after crosslinking, yellowing and film formation, so that stripping without residue is no longer possible. In addition, the unavoidable content of residual monomer (at least 1%) is objectionable on health grounds, especially for interior applications. Similarly, follow-on products of the photoinitiator, especially methyl benzoate, may lead to migration and alteration of the product properties. The heat of reaction which is liberated during two-dimensional UV polymerization of acrylates may damage or corrugate the support. Compounding with, for example, resins is possible only to a very limited extent, since these resins disrupt UV polymerization. Also, the inevitably obtained crosslinking profile of the adhesive composition is a further disadvantage: normally, UV polymerization is carried out by UV irradiation on the composition side, leading to layers of adhesive with greater crosslinking on the surface of the adhesive than on the support. The result is reduced tack and poor anchorage of the composition. If UV irradiation is carried out through the support, which brings about better tack and improved adhesion and anchorage of the composition, the support must be UV-permeable. However, many supports and many SBS/SIS block copolymers do not possess particularly good UV permeability, or are damaged by UV light.
Overall, the product properties mean that prolonged bonding in constant quality is not ensured, and stripping without residue is in particular not possible, as demonstrated in WO 92/11332 on p. 19, Table 2, column 6, where residues of the composition remain at the edges of the bond (footnote a), if indeed the products do not tear (footnote b). Products which can be used more than once are therefore neither described nor suggested.
Furthermore, U.S. Pat. No. 4,024,312 describes highly extensible adhesive films which have a support consisting of highly elastic, thermoplastically processable styrene block copolymers of type A-B-A (where A=poly(-styrene), B=poly(isoprene), poly(butadiene) or hydrogenation products thereof). Resins which can be mixed with the block polystyrene domains can be added, optionally, to the support, preferably in a quantity of from 85 to 200 parts per 100 parts of elastomer. The support is coated on at least one side with a pressure-sensitive adhesive composition. The elastic deformation of the adhesive tapes is at least 200%, and the modulus at 50% extension is <13.8 MPa (2000 lbs/inch
2
). Elastic reformation after stretching by 50% is at least 75%. The pressure-sensitive adhesive compositions employ either polyisoprene (e.g. natural rubber) or the synthetic rubbers which are also employed for the support material and are based on styrene block polymers, blended with adhesive resins and, if desired, with further blend components. Adhesive tapes can be removed readily from the substrate by stretching parallel to the bond surface. As a result of the migration capacity of the low molecular mass constituents (resins, plasticizers) between pressure-sensitive adhesive composition and support, such self-adhesive tapes are not constant in their product properties: the mechanical strengths of the support and properties of the pressure-sensitive adhesive compositions used are irreversibly altered by the diffusion of the resins. Controlled setting and regulation of the product properties, as is essential for industrial bonds, is therefore not possible. Repeated use is impossible since the support materials coated with rubber adhesive, when stretched parallel to the bond surface, and immediately after detachment from the substrate, shoot back and stick together on the adhesive side with such force that these rubber coatings cannot be detached from one another. Products which can be used more than once are therefore neither described nor suggested.
The object of the invention was to remedy this situation and, in particular, to enable an adhesive-film laminate to be used more than once.
The invention relates accordingly to the repeated use of an adhesive-film laminate, as characterized in more detail in the claims.
Elastic supports are, in particular, elastomers with a resilience of >50%, preferably >80%; during the process of detaching the bonded products, extensions of from 100 to 400% occur on average. Values of up to 1500% may also arise. In this context, the extension which occurs is essentially a function of the tensile expansion behaviour of the type of elastomer employed, of the thickness of the support and of the bond strength which is attained by the adhesive films.
In every case, a prerequisite for residue-free redetachment from the substrate or from the bonded joint is that the tear strength of the adhesive film must be higher than its stripping force. The ratio of tearing force to stripping force is preferably >1.5, particularly preferably >2.5.
The tear strength of the adhesive films described here is determined in particular by the nature and thickness of the support films used. For supports based on styrene block copolymers with a linear three-block structure and block polystyrene contents of from about 15 to 40% by weight, the minimum support thickness for double-sided pressure-sensitive adhesive films, with adhesive forces of > about 5 N/cm, is about 50 &mgr;m (see examples). Below this limit, the tear strength is normally too low for redetachment without residue. For substrates with a high degree of adhesion relative to the adhesive films employed (high adhesion forces), the minimum support thickness required is increased correspondingly.
Preferred elastomers are:
1. Styrene Block Copolymers.
Suitable such copolymers are styrene-isoprene and styrene-butadiene block copolymers and their hydro-genation products, styrene-ethylene/butylene and styrene-ethylene/propylene block copolymers. Block copolymers according to the invention can be linear SES (S denotes the polystyrene block, E
Harder Christian
Lühmann Bernd
Storbeck Reinhard
Zöllner Stephan
Norris & McLaughlin & Marcus
Pratt Christopher C.
testa AG
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