Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1998-04-21
2003-03-04
Aftergut, Jeff H. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S244240, C156S244260
Reexamination Certificate
active
06527899
ABSTRACT:
The invention relates to a process for producing a rubber-based double-sided pressure-sensitive adhesive tape.
American Patent U.S. Pat. No. 4,440,829 describes a sticky-both-sides bonding tape, and its process of production, whose backing is a milled mixture essentially comprising crosslinked and uncrosslinked butyl rubber, pigments and plasticizers. These starting materials are processed in a mixer to form a composition. The composition is subsequently extruded to form a backing which is in tape form, is millable and extrudable, and which also has sufficiently high tensile strengths.
Extrusion takes place directly into an organic solution of the pressure-sensitive adhesive, preferably based on acrylic copolymers. The adhesives selected are those having not only a high bond strength and sufficient shear strength but also enough adhesion to the backing to cause the bonding tape to fail cohesively when peeled from a substrate.
Excess adhesive is washed off, and the resulting adhesive tape is placed on a release layer (liner) and subsequently, if desired, wound up into a roll.
The procedure described has the disadvantage that large amounts of plasticizer must be used in order to ensure the-possibility of processing by milling and extrusion. However, these large amounts of plasticizer reduce the desired high strength of the rubber backing. In addition, it is known from experience that plasticizers migrate to the surfaces of the backing and so hamper the anchoring of the adhesive. Moreover, they may also migrate directly into the adhesive, thereby reducing its bonding strengths and shear strength.
In addition, the application of the adhesive takes place from an organic solution after extrusion, which involves a high energy input to remove the organic solvent from the applied layers. Finally, complex drier designs are necessary in order to allow economic belt speeds.
Then, also, account must be taken of the explosion and fire prevention provisions in force.
Since there is no possibility of subsequent crosslinking, moreover, the tensile strengths which the product is able to achieve are predetermined by the maximum processing viscosities of the mills and of the extruder.
It was an object of the invention to provide a process which allows a double-sided pressure-sensitive adhesive tape with a rubber-based backing to be produced in the most efficient manner possible.
This object has been achieved by a process as set out in more detail below.
Accordingly, the process for producing a double-sided pressure-sensitive adhesive tape features the process steps set out below.
First of all, a ready-made, rubber-based backing material is placed in a screw-type extruder, preferably a single-screw extruder. This material can be fed in in the form of feed strips.
The manufacture of the feed strips is familiar to the skilled worker, beginning as it does with the mixing of the individual components of the backing material, preferably batchwise, in an internal mixer of the type typical for elastomer compounding (for example, a Banbury internal mixer); alternatively, in-line preparation by means of screw machines is possible. The backing material mixture can then be milled to the desired thickness on customary equipment, provided with an internal release agent, and cut into feed strips of a geometry appropriate for the extruder intake. Finally, these strips can be laid as wigwag rough sheet on pallets.
In the extruder, the backing material is picked up by the single or two or more rotating screw(s) and transported downstream. In addition to the shear energy deployed, heating elements disposed on the screw extruder barrel, above all, ensure that the backing material is heated.
In a commercially customary degassing zone, the backing material is freed from volatile components by means of the application of reduced pressure.
Downstream of the degassing station the backing material is transported further, homogenized and brought to an intended temperature of 120° C. to 160° C.
In the second step of the process, the backing material is supplied to a co-extrusion adapter customary in the plastics processing industry, in which a barrier layer is placed over both sides of the backing layer by means of an appropriate manifold insert. the barrier layer here is fed in by means of a further extruder or another suitable apparatus for the melting and conveying of thermoplastic media. This apparatus can comprise, for example, the drum melters, premelters, melt pumps or other melting and conveying units that are customary in the adhesives industry.
The term extruder is to be understood below as including other suitable above-mentioned melting and conveying units. It also refers to the combination of extruder and melt pump that can be employed in this case in order to improve the constancy of conveying. Suppliers of such melt pumps include, for example, the companies Maag (Zürich, Switzerland) and Witte (Itzehoe, Germany).
The resulting intermediate laminate is supplied to a three-channel slot die and is uniformly extruded in web form.
In the three-channel slot die, a pressure-sensitive hotmelt adhesive, supplied in each case with the aid of an appropriate melting and conveying unit, is applied in an appropriate coat thickness, top and bottom, over the whole area of the intermediate laminate.
The adhesive tape thus produced is placed onto an antiadhesive substrate, preferably siliconized paper.
If desired, the adhesive tape is cooled and crosslinked.
In this case crosslinking of the backing material takes place preferably with by means of electron beams in the dose range from 30 to 150 kGy, preferably from 50 to 100 kGy, with acceleration voltages of between 200 kV and 1000 kV, depending on the weight per unit area of the layer through which irradiation is to take place.
An alternative option is thermal crosslinking. If thermal crosslinking is provided in place of electron beam crosslinking, then the desired processes can be realized by means of suitable thermal activation sections, such as contact dryers, convection dryers, IR sources or microwave excitation.
By crosslinking it is possible to tailor the mechanical properties of the backing material to the properties required in the product.
The resulting adhesive tape can be wound up into bales of the desired length and processed further to the end product with the aid of widespread customary end-processing operations.
In an advantageous embodiment of the process, between process steps a) and b), an adhesion promoter is additionally applied to both sides of the backing material by means of an appropriate manifold.
Advantageously, the hotmelt pressure-sensitive adhesive is supplied to a single melting and conveying unit, such as an extruder, is melted, transported and applied uniformly to both sides of the intermediate laminate by means of appropriately placed pipelines; in other words, in particular, with symmetrically guided pipelines and die.
Also necessary for the process of the invention is adaptation to the processing viscosities and processing temperatures of the backing material and the hotmelt pressure-sensitive adhesive. As usual in co-extrusion, the processing viscosities should be matched as well as possible to one another within the relevant shear gradient range. The viscosity ratio of backing material to hotmelt pressure-sensitive adhesive should not exceed 10, advantageously 8. The processing temperatures of the materials employed should differ by not more than 50° C., preferably by less than 30° C., from one another.
The rubbers used as backing material are, in particular, a natural rubber, a butyl rubber, an acrylonitrile-butadiene rubber, a random styrene-butadiene rubber or a blend of these rubbers, preferably a blend of a natural rubber and a random styrene-butadiene rubber, and with very particular preference a natural rubber.
The rubber advantageously has a Mooney value ML
(1+4)
(100° C.) of from 40 to 80.
The rubbers used as backing material can have been blended with one or more additives, such as anti-ageing agents, crosslinkers, light s
Dietz Bernd
Henke Frank
Karmann Werner
Kluge-Paletta Werner
Szonn Bodo
Aftergut Jeff H.
Norris & McLaughlin & Marcus
tesa AG
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