Coating processes – With post-treatment of coating or coating material – Solid treating member or material contacts coating
Reexamination Certificate
2002-08-08
2004-11-30
Bareford, Katherine A. (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Solid treating member or material contacts coating
C427S358000, C427S407100, C427S411000, C427S412100, C427S412200, C427S412500, C427S402000, C427S420000
Reexamination Certificate
active
06824828
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to release surfaces of the type used with pressure-sensitive adhesive (PSA) constructions. More particularly, the present invention relates to multilayer release liners and their methods of manufacture.
BACKGROUND OF THE INVENTION
A pressure-sensitive adhesive construction, such as a label, generally comprises a facestock or label surface, an adhesive composition adhered to the facestock, and a release liner. The adhesive composition is typically coated on a silicone-containing release surface of the liner. Alternately, the PSA can directly be coated onto the facestock and then be laminated to the release liner. In these combinations, the release liner protects the PSA prior to the label being used and is removed immediately prior to application of the label to another surface. Additionally, the release liner serves to facilitate cost effective manufacture of rolls or sheets of labels. The release liner also functions as a carrier of labels for dispensing in automatic labeling operations and for computer printing in EDP applications. The performance attributes of a release liner are critical to both the manufacture and end-use application of adhesive labels.
In conventional practice, the release liner is provided as a silicone layer on a paper or film surface having high holdout, i.e., the surface of the paper on which the silicone layer is deposited is resistant to silicone penetration. This is preferred because silicone tends to be an expensive component of a release liner, and it is therefore desirable to minimize the amount of silicone coated. High quality papers conventionally used in the manufacture of release liners, such as a super-calendered or densified glossy paper, achieve this goal by providing a surface which absorbs much less silicone than regular open paper. However, use of these high quality papers increases the cost of the end product adhesive construction, because such papers are typically much more expensive than regular open papers.
One currently accepted way of applying a silicone release composition to a high holdout paper is by solvent coating. Growing concern over the environment has imposed stringent restrictions regarding recovery of the solvent used in applying the solvent based silicone to the high-holdout backing paper or other materials. An alternative to this is to use 100% solids silicone release compositions. These are supplied with a viscosity (usually <2000 cps) suitable for roll-coating techniques. When applied to porous low cost papers, such as machine finished (MF) or machine glazed (MG) papers, these materials soak into the paper (penetrate the paper surface) to give ineffective coverage of the paper fibers unless excessively high quantities of expensive silicone are used. Ineffective coverage of the paper fibers provides unsuitable release liners for PSA applications, especially where high speed convertibility is an essential performance feature.
One proposed prior art solution to these problems is to use low cost open papers which have been pre-coated with a support layer comprising an inexpensive filler material, and then to coat the silicone layer onto the support layer. The filler material of the support layer flows into the pores and interstices of the open paper surface which would otherwise absorb silicone if the silicone were directly coated onto the paper. Consequently, less silicone is needed to form an adequate release surface. An example of this approach may be found in U.S. Pat. No. 4,859,511 to Patterson. However, there are several drawbacks to this prior art process. First, additional costs are incurred because the prior art methods require two coating steps. The prior art teaches that the support layer must first be coated and then dried, cured or hardened before the silicone layer may be coated. Otherwise, there is a potential for undesirable intermixing or destruction of the respective layers. Second, because two separate coating steps are required, more time is needed for the overall formation of the release liner. These additional processing costs somewhat offset the savings realized in materials by using support layers in combination with lower cost open papers.
Thus, there is a need for improved methods of forming multilayer release surfaces in which a support layer is used in conjunction with a silicone layer to form a release liner.
SUMMARY OF THE INVENTION
The present invention advantageously provides an efficient method of creating multilayer release liners, thereby overcoming the problems resulting from the prior art processes. The present invention achieves these benefits by providing a method of coating both a support layer and a release layer on a substrate substantially simultaneously. Consequently, separate coating steps are eliminated, and a corresponding savings in both time and costs are achieved.
Generally, these advantageous results may be achieved by at least two different coating methods. The first method achieves these results by modifying the die used to coat the support layer and a release layer (e.g., silicone layer) so that the die can dispense the fluids of both layers substantially simultaneously at a single coating station. The die dispenses the support layer onto the substrate, and substantially simultaneously, the release layer on the support layer. There is no need for a separate drying, hardening or curing step to prevent the layers from intermixing. By controlling the coating gap between the die and substrate, the processing conditions of the modified die may be optimized to achieve the most stable and efficient deposition of these layers. In some embodiments, application of controlled vacuum to the dual die coating process may be used to improve coating efficiency, increase coating tolerances and provide for less penetration of coated fluids onto the substrate to be coated. The multilayer release surfaces resulting from the simultaneous dual die coating of support layer and silicone-containing layer are also believed to have a unique morphology and advantageous properties.
Simultaneous coating of the support and release layers to form a multilayer release surface may also be achieved by curtain coating. For example, a slide coat die may be modified to have two slots, with the upper slot metering the release layer and the lower slot the support layer. The release layer and support layer combine on the die face surface, and fall to the moving substrate as a multilayer liquid sheet. The distance between the die and the substrate may range from 5 cm to 50 cm, and more preferably, from 10 cm to 30 cm. Advantageously, curtain coating techniques do not require as precise an optimization of the coating gap between the die and the substrate to generate high speed coatings, and high coating speeds are easily obtained.
With respect to simultaneous coatings using a dual die, the present invention provides a method that is capable, at steady state coating conditions, of precisely controlling the interface or “separating streamline” between the support layer and silicone-containing layer as these layers are being coated onto the substrate. Unlike single-layer coating, the stability of the flow (i.e., its tendency to exhibit only a steady, two-dimensional flow) particularly at the separating streamline between the two layers, is extremely important. Advantageously, this method can be used to substantially simultaneously coat a support layer on a backing in conjunction with a silicone-containing release layer on the support layer. As used herein, substantially simultaneously refers to two or more liquid layers being deposited at a single coating station without an intermediate drying, curing, or hardening step for the support layer. For die coating, preferably, the single coating station comprises the dual die described herein, although this is not essential to the present invention. For example, the single coating station may comprise two separate dies located close enough spatially to achieve the benefits of a dual die.
The present
Dordick Robert
Guo Hongjie
Huff Stephen
Hulme Adrian
Jansen Alexander
Avery Dennison Corporation
Bareford Katherine A.
Casella Anthony J.
Hespos Gerald E.
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