Stock material or miscellaneous articles – Layer or component removable to expose adhesive – Release layer
Reexamination Certificate
2001-03-08
2002-02-19
Ahmad, Nasser (Department: 1772)
Stock material or miscellaneous articles
Layer or component removable to expose adhesive
Release layer
C428S040100, C428S041300, C428S041500, C428S041700, C428S218000, C428S219000, C428S220000, C428S343000, C428S352000, C428S354000, C428S35500R, C428S516000, C428S517000, C428S518000, C428S906000
Reexamination Certificate
active
06348249
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a release liner for pressure sensitive adhesive tapes.
BACKGROUND OF THE INVENTION
Release coatings are well known in the pressure sensitive adhesive tape industry to provide a release surface so that a tape can be unwound from a roll without the adhesive sticking to the backside of the tape. The release coating can also be coated onto a liner which serves as a carrier for a pressure sensitive adhesive transfer tape or a double coated tape, both of which are tacky on both sides of the tape.
Materials commonly used for release surfaces and coatings are silicone compositions because they can be formulated to provide varying levels of release from a “premium” release (i.e., the force to unwind the tape is very low) to a “low adhesion backsize” release which typically requires a greater force to unwind.
The primary deficiency of silicone release coatings is the potential contamination that may be associated with them. When workers handle a silicone release coated tape or liner, silicone can transfer to their hands, work gloves, or clothing. The silicone can be further transferred from their hands to surfaces to be painted, and cause problems such as poor adhesion of the paint to the contaminated surface, and nonwetting of the paint in areas where the silicone is present on the surface. Silicone release materials also tend to be costly, and silicone coated materials may be difficult to recycle.
Polyethylene has also been used as a release material and can be coated onto Kraft papers for a release liner or it can be used as a single layer film, or a multi-layer film with polyethylene co-extruded with or laminated to a base layer such as high density polyethylene, and the like. Higher density polyethylenes are limited in use to certain adhesives and are found to be unsuitable for very aggressive pressure sensitive adhesives because the adhesion between the polyethylene and the adhesive is so high that it can be difficult to start removing the liner. Since the release force is high, the adhesive and/or the liner can also be damaged as one tries to separate the adhesive from the liner.
There remains an ongoing need for controlled release liners for pressure sensitive adhesives that do not have a silicone release layer.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a novel sheet material comprising i) a pressure-sensitive adhesive film comprising an acrylate-based pressure-sensitive adhesive polymer; and ii) a release film comprising a first layer comprising polyolefin polymer having a density of no greater than about 0.90 g/cc and a CDBI of greater than about 70 percent, the pressure-sensitive adhesive film being in contact with the first layer of the release film.
In a preferred embodiment there is provided a sheet material which is substantially free of any silicone release coating.
In another preferred embodiment the release film comprises a second layer which is preferably a polybutylene.
DETAILED DESCRIPTION
In the practice of the invention, the materials useful for the first layer of the release film are copolymers of ethylene and alpha-olefins having from 3 to about 10 carbon atoms. Suitable alpha-olefins include butene-1, hexene-1, octene-1, and combinations thereof. Copolymers of ethylene and butene-1 are preferred for use with acrylate-based pressure sensitive adhesives.
The copolymers are generically described as very low density polyethylene (VLDPE) and have been described as “plastomers”, a polymer having thermoplastic and elastomeric characteristics. Preferred copolymers have a density no greater than 0.90 grams per cubic centimeter (g/cc). More preferred are copolymers having a density of no greater than 0.89 g/cc and most preferred are those having a density no greater than 0.88 g/cc. Lower density copolymers are found to provide a lower release value, and values can be changed by blending copolymers of varying types.
The useful copolymers preferably have a narrow molecular weight distribution as defined by having a polydispersity of between about 1 and 4, and more preferably between about 1.5 and 3.5. Polydispersity is defined as the ratio of the weight average molecular weight to the number average molecular weight.
The useful copolymers can be defined by a parameter which has been characterized as the “composition distribution breadth index” (referred to hereinafter including the claims as “CDBI”). The CDBI is defined as the weight percent of the copolymer molecule, having a comonomer content within 50 percent (i.e., ±50%) of the median total molar comonomer content. The CDBI and the method for its determination is described in U.S. Pat. No. 5,206,075, incorporated herein by reference. The CDBI of suitable copolymers for use in this invention is preferably greater than about 70 percent, more preferably is greater than about 80 percent, and most preferably is greater than about 90 percent.
Suitable copolymers are commercially available from Exxon under the EXACT tradename. These copolymers form films that are not fusible at ambient temperature, and will not block, i.e., stick to underlying layers, when the film is wound into a roll.
In a preferred embodiment of the first layer of the release liner consists essentially of the copolymer having a density of no greater than 0.90 g/cc, and is substantially free of any polyethylene having a density of 0.91 g/cc or greater.
As used herein, the terms release liner, liner, and release films will be used interchangeably. The release film can be prepared by extruding the copolymers as a single self supporting film where there is no second layer present. The film thickness may range from about 0.1 millimeter to about 0.4 millimeters, although the thickness will generally depend upon considerations such as the strength needed in the release liner, ease of handling the film, flexibility needed for the liner, and the like.
The release film may also be prepared by extruding the copolymer onto a base material, or by co-extruding the copolymer with one or more other polymers to form multi-layer films to provide additional strength and/or stiffness. Examples of suitable base materials include cellulosic materials such as paper, woven nonwoven fabrics, films such as nylon, polyester, polyolefins, acrylonitrile butadiene styrene, and sheet materials made of materials such as metal, ceramic, or plastic. The copolymer film can also be attached to the base material using an appropriate laminating adhesive. The base material can also be a sheet material suitable for thermoforming or vacuum forming so that trays can be formed having a release surface for holding pressure sensitive adhesive coated parts.
The thickness of the copolymer film on a base layer should be sufficient to provide the desired release force, and may be less than for a self-supporting sheet.
The films are typically extruded onto chill rolls having various surfaces such as a matte finish or a smooth finish to provide matte or glossy surfaces on the release liner. Chill rolls can be treated or coated with materials to prevent sticking of the extrudate to the roll surface, such as Teflon™ or a plasma coating. Silicone rubber rolls may also be used for this purpose. The extruded films can also be extruded between a chill roll and a smooth polyester film to provide a glossy surface on one side and a matte surface on the other side, or between two polyester liners for two glossy surfaces.
Multi-layer films may be prepared by known co-extrusion processes. Other polymers that can be co-extruded with the copolymer include polyolefins such as polypropylene, polyethylene, polybutylene, and mixtures thereof, and polyesters. Co-extrusion with polyolefins is especially useful for making recyclable or re-usable liners.
In a multi-layer film construction, the polymers and co-polymers can be selected to exhibit differential release, i.e., the pressure sensitive adhesive will release from one side with a noticeably lower force than from the other side.
Preferred polymers for co-extrusion with the copolymer of the first
3M Innovative Properties Company
Ahmad Nasser
Knecht III Harold C.
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