Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
2000-12-14
2003-07-29
Zirker, Daniel (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S423700, C428S354000, C428S423100, C428S480000
Reexamination Certificate
active
06599624
ABSTRACT:
DESCRIPTION
Adhesive Tape with Thermally Curable Backing for Masking a Cathodic Electrocoat Primer
The invention relates to an adhesive tape, especially for masking off window flanges, preferably in automotive body shells coated with cathodic electrocoat, and to processes for producing the adhesive tape. The purpose of the adhesive tape is to protect the window flanges against overpainting during the subsequent coating and baking processes such that following the removal of the adhesive tape an automotive glass window can be installed onto the surfacer- and topcoat-free window flange using a reactive PU window adhesive.
BACKGROUND OF THE INVENTION
Automobile glass windows are conventionally mounted in the painted vehicle body using rubber seals. In recent years, this method has been increasingly replaced by the installation of the windows using reactive adhesives (based, for example, on polyurethane). The window is coated with the adhesive and placed on the body such that the adhesive bead is pressed onto the window flange.
The installed windows, especially the windshields, nowadays act as reinforcing elements of the body. In the extreme case, that of the vehicle turning over, they prevent the roof columns from buckling. Consequently, a sufficient bond strength is critical to the safety of a modern motor vehicle in an accident situation.
Modern automotive finishes comprise a variety of coats, which are applied to the primed bodywork metal in the following order (schematically):
electrophoretic coat, usually cathodic electrocoat
surfacer or functional coat
color coat
topcoat
According to Römpp Lexikon Chemie (Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998), electrophoretic coating (electrodeposition coating; electrocoating) is a technique in which coating takes place by the action of an electrical field (from 50 to 400 V). The article to be coated, which conducts electric current, is introduced into the paint bath as an anode or cathode, with the tank wall in practice acting as the second electrode.
The amount of paint deposited is directly proportional to the amount of current supplied. Electrophoretic coating is used especially for priming, in the automotive industry, for example. There are no spray losses, and the coatings obtained are highly uniform, even in difficult-to-reach areas. Where the substrates are not conductive, such as plastics, glass, ceramic, etc., coating is carried out by way of the electrostatic charging of the paint particles (known as electrostatic coating).
If the automobile window is bonded onto the painted window flange after the painting process has been completed, the following disadvantages arise.
Since the window adhesive has to be matched to the topcoat as its adhesion substrate, an unnecessarily high degree of complexity may result given the large number of topcoats used by a manufacturer, since it is necessary to hold a large number of appropriate adhesives in stock. More significant, however, is the fact that the total bond strength of the automobile window depends on the weakest point in the multicoat paint system, and may therefore be much lower than the bond strength of the adhesive to the topcoat.
It is therefore advantageous to apply the window to the bottommost paint coat, the cathodic electrocoat. The number of cathodic electrocoat preparations used by a manufacturer is usually lower than the number of topcoats. Firstly, there are few defined adhesion substrates for the window adhesive, and secondly the system comprising primed metal/cathodic electrocoat/window adhesive, with two boundary layers, harbors a lower risk of fracture than a complex overall coating system.
To mask the window flange following the application of the cathodic electrocoat it is possible to use a PVC plastisol as described in EP 0 655 989 B1. This plastisol is applied in liquid form to the window flange, painted over and gelled during the baking phase at temperatures of at least 163° C. to give a solid film. A disadvantage of this process is that for the purpose of demasking after baking has taken place it is necessary for a “grip tab” to be mechanically exposed, in which case the cathodic electrocoat may also easily be damaged, something which harbors the danger of subsequent corrosion.
On the window flanges, the plastisol strip crosses, in some cases more than once, PVC seam sealants which fill weld seams. On gelling, a frequent observation is of instances of severe sticking between seam sealants and PVC plastisol window flange masking, which prevent easy demasking. Another observation is of plastisol-related contamination of the adhesion substrate, giving rise to an adhesion failure at the boundary between window adhesive and formerly plastisol-masked cathodic electrocoat.
As a result, the required bonding security of the window is not ensured.
Although this drawback can be countered by using a primer, such a step is labor-intensive, leads to unwanted solvent emissions, and may necessitate repair to the paint, as a result of accidental splashing or dripping on the topcoat.
A more advantageous possibility for the masking of window flanges is the use of adhesive tapes. For a number of years, masking has been carried out using a PVC-polyester laminate, laminated with natural rubber adhesive and rendered self-adhesive with a natural rubber adhesive composition. Similarly to the plastisol bead, this adhesive tape is bonded to the cathodically electrocoated window flange before surfacing and coating, and following baking is removed together with the paint coats applied to it. Application may be made by hand or else, in an automated process, by robot.
The principal disadvantage of this last-mentioned product is the frequency of errors during demasking, since the laminate tends toward complete and partial splicing and also toward tearing. Partial splicing, in particular, represents a safety risk, since the polyester film which remains on the window flange is little different in color from the cathodic electrocoat and is therefore easily missed. These residues of polyester are a wholly inappropriate adhesion medium for window adhesives and, accordingly, may result in inadequate window adhesion.
A further disadvantage both of the last-mentioned product and of the PVC plastisol are the disposal difficulties. Land filling is injudicious for reasons of the biological persistence of the PVC. Recycling is not possible, owing to contamination with the various paint coats. Incineration harbors the hazard of the emission of hydrochloric acid and dioxins. To avoid this requires complex measures in the incinerators, such as flue-gas scrubbing and very high incineration temperatures, for example.
It is an object of the invention to develop an adhesive tape, especially for window flange masking, with a backing material which does not exhibit the disadvantages of the prior art, or not to the same extent. In particular, on demasking, the adhesive tape should neither tear nor undergo complete or partial splicing. Moreover, the adhesive tape should not contain PVC.
Suitable backing materials, in principle, for an adhesive tape which for this application is required to withstand typical high temperatures of up to 180° C. for approximately 30 minutes are films made of polyester (polyethylene terephthalate PET, polyethylene naphthalate PEN), and also fibrous materials such as cotton, aromatic polyamide and polyester wovens or nonwovens. A disadvantage of fiber materials is their inherent low extensibility, which makes it more difficult to stick the tape on in curves.
If the tape is to be processed by robot, it is subjected to strong tensile forces and must therefore possess a high tensile modulus in order to prevent early extension, which under the hot conditions leads to a high recovery tension.
The requirement for high tensile strength in combination with temperature resistance for a number of minutes is met by biaxially oriented PET or PEN films having a draw ratio lengthwise (MD: machine direction) to crosswise (CD: cross direction) of from about 3-4:3-4. A problem of orien
Böhm Nicolai
Krupke Siegfried
Schümann Uwe
Wappler Ulrike
Norris & McLaughlin & Marcus
tesa AG
Zirker Daniel
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