Adhesive tape

Details Adhesive tape Adhesive tape

2000-11-17

2003-01-28

Zirker, Daniel (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Adhesive outermost layer

C428S354000, C428S910000, C156S244110, C156S244250, C264S171110, C264S177100

Reexamination Certificate

active

06511742

ABSTRACT:

The invention relates to a film oriented in the machine direction, having plane-parallel outer faces and at least two coextruded layers of different composition whose interface is not planar but instead has a nonlinear course in cross section, which continues laminarly in the machine direction, and to a process, by coextrusion and orientation, for its preparation, and to an adhesive tape produced therefrom.
The particular internal structure of the film derives from the fact that the thickness of one layer varies periodically or irregularly in the cross-direction and the second layer compensates the fluctuations in thickness such that the overall thickness is essentially constant.
The essential technical characteristic of the invention is the combination of lateral tear resistance (tear propagation force and residual strength) and longitudinal strength (tensile strength and stress at 10% elongation) in the machine direction.
PRIOR ART
Films possessing high longitudinal strength are normally obtained by orienting melt-extruded partially crystalline thermoplastics such as polypropylene or polyester. The orientation in question is predominantly biaxial orientation; in exceptional cases, the films are oriented in the machine direction only for the purpose of further increasing the longitudinal strength. The reduced extensibility on tensile stress and improved strength brought about by the orientation have the simultaneous effect, however, of a drastically reduced tear propagation force and residual strength. This leads to tearing and tensile load in the case, for example, of rough edges of film and, respectively, adhesive tape (caused by blunt blades during cutting or subsequent unintended damage to the cut edge). For simple applications, it is sufficient to use rigid PVC films (EPVC and impact-modified SPVC), which offer a compromise between lateral tear resistance and longitudinal strength. The use of woven fabrics offers a comprise at a higher level, albeit with the disadvantages of a very much higher price and further operations such as coating in order to seal the surface.
DE-A 36 40 861 describes a tear-open strip whose tendency to tear off is reduced through the use of a film, oriented in the machine direction, which is produced by coextruding raw materials differing in toughness. The tough and soft coextrusion layer reduces the formation of microcracks when the product is cut and so enhances the lateral tear resistance. However, it does not prevent tearing at edges damaged subsequently; this necessitates a drastic improvement in the tear propagation resistance, possessed only by the coextruded film of the invention with an internal structure. The raw materials specified for the abovementioned invention, for increasing the toughness, however, lead to markedly reduced longitudinal strength, especially the raw materials of the tough coextrusion layer. The films described have a reduced force per 4 mm width even at 10% elongation, and on fracture; if the cross sectionally related values of tensile strength, tear strength and stress at 10% elongation are considered, then the drop in performance becomes even more marked.
Under high load, films and, respectively, adhesive tapes must be reinforced with filaments or with networks of filaments of glass or plastic. In terms of equipment, the production of such filament adhesive tapes is very complex and thus expensive, and susceptible to faults. In addition to the base film, there is an additional requirement for the filaments and laminating adhesive (or additional pressure-sensitive adhesive coating), which makes the products more expensive and ties up a greater quantity of raw material resources. Further disadvantages of such filament adhesive tapes are the low crease fracture stability, the unclean cut edges, and the absence of weldability and recyclability. The preparation is described, for example, in U.S. Pat. No. 4,454,192.
U.S. Pat. No. 4,536,362 describes a method of producing stretch films for pallet wrapping. The ribs which extend in the machine direction are intended to accommodate impurities in the polymer, in order to prevent the formation of holes and other defects in the thin film. The films are unoriented and offer no reinforcing effect.
U.S. Pat. No. 4,592,938 describes a method for producing films for plastic bags, handles, and stretch packaging. A reinforcing effect is obtained in the interior of the film by virtue of coextruded filament like strips extending in the machine direction. The method is not applicable to polypropylene, since the customary gel fraction automatically blocks the filament channels of the die. The films are unoriented and therefore of only low longitudinal strength.
EP 0 411 830, EP 0 343 896, U.S. Pat. Nos. 5,145,544 and 5,173,141 describe an adhesive tape comprising monoaxially oriented film having a rib structure for reinforcement, some of the ribs projecting from the surface and some being embedded in the film surface, with notch-like joints being formed between film and ribs. The invention achieves high lateral tear strength, although the tensile strength and extensibility are still needy of improvement. The essential defect of this invention, however, is that it cannot be produced on an industrial scale. The reason for this is the poor orientability in the standard width. When producing a primary film having a structured (rib) surface in a width of 25 cm, the orientation produces on both sides an indentation which, taking into account the industrially required edge trimming, gives a maximum useful width of approximately 15 cm, which is no longer reasonable commercially for coating with adhesive composition. Despite the high edge trim, which is already unacceptable for economic reasons, the film still has an extremely poor flatness, so that its ability to be coated with pressure-sensitive adhesive is no longer ensured. At relatively large widths, moreover, the flatness is further impaired as a result of nonuniform and inadequate adhesion (owing to the non-plane-parallel outer faces of the film) to the stretching rolls in the subsequent orienting process. When the primary film is oriented longitudinally, the film contracts sharply in the edge region, with the rib structure being essentially retained. In the case of production in standard industrial width, the film is held in the middle region on the stretching rolls, in the cross-direction, as a result of which the rib structure alters because of orientation and the entire quality of the product becomes nonhomogeneous. A further disadvantage is the need for at least 50% embedding of the ribs by means of a calender, which is a very expensive capital item and makes the process much more complex. Comparative Examples I and II of EP 0 411 820 show that excessive impression impairs the cross-direction properties of the film. The rib structure on the surface also readily gives rise to coating errors during the application of release agents or primers in the course of subsequent processing to give adhesive tapes, since the application techniques for films require a smooth surface.
A further disadvantage of films having reinforcing strips or rib structures in or on the surface is in relation to printing, which requires planar surfaces. Especially when the film of the invention is used for an adhesive packaging tape, printability is an important criterion for the customer. The disadvantages of the prior art technical solutions have been overcome by the invention described below.
The invention relates to a film oriented in the machine direction, having plane-parallel outer faces and at least two coextruded layers of different composition whose interface is not planar but instead has a nonlinear course in cross section, which continues laminarly in the machine direction, and to a process, by coextrusion and orientation, for its preparation, and to an adhesive tape produced therefrom.
The essential technical feature of the invention is a combination of lateral tear resistance (tear propagation force and residual strength) and longitudinal strength

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