Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having a component wherein a constituent is liquid...
Reexamination Certificate
1997-08-29
2002-09-03
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having a component wherein a constituent is liquid...
C428S3550AC, C525S193000, C525S223000, C525S226000, C525S903000
Reexamination Certificate
active
06444305
ABSTRACT:
TECHNICAL FIELD
This invention relates to adhesives and adhesive compositions, particularly to adhesive compositions containing microspheres and more particularly to printable adhesive compositions containing microspheres and methods thereof.
BACKGROUND OF THE INVENTION
A variety of methods to deliver printable or patternable adhesives exist and offer several advantages over conventional flood coating. These advantages include selective placement of adhesive on the substrate that results in reduced waste and improved handling. Each printing method requires that the printable material that is used possess particular rheological characteristics.
The following table describes a range of viscosities and characteristics of printable material classified by printing method.
Characteristics of Printing Material
Printing Method
Viscosity @ 25° C. (Pas)**
Description*
Lithography
15-35
Heavy or thick
Gravure
~0.015
Quite fluid
Flexography
~0.015
Relatively fluid
Screen
2
Short and buttery
*Graphics Art Manual, Arno Press, Ed. Janet N. Field, pages 416-418, 492-496, 1980
**Printing Inks, Pira International, Ronald Todd, pages 296-297, 1994
Printing methods can be classified into two general fields, contact (impression) and non-contact (impactless) printing. Contact printing describes the processes in which an image is transferred through direct contact between the printing plate or image carrier and the substrate. Contact printing methods include lithography, gravure, flexography, and screen printing. In contrast, non-contact printing methods describe processes in which little or no contact is made with the substrate. Non-contact printing methods include electrophotography, thermal imaging, jet printing, and electrographic methods.
Screen-printing is a popular contact printing method due to the relative simplicity of both the printing process and equipment and the variety of surfaces that can be imprinted using this method. The screen printing process first involves making a stencil on a screen mesh that defines the pattern, text, image, etc. that is to be printed on a particular surface. Once the screen is prepared, a fluid (usually ink) is squeezed through the open areas of the stenciled mesh to transfer the pattern or image to this surface. As a final step, the fluid must be dried or otherwise fixed to prevent the distortion and preserve the integrity of the printed pattern or image.
Screen printing is unique among other printing methods as it can be used to deliver relatively thick fluid films, anywhere from 8 micrometers to over 30 micrometers in thickness. This thick coating capability renders screen printing an ideal method for coating discrete patterns of adhesives. Adhesive materials, particularly pressure sensitive adhesives, generally exhibit their desirable properties of high internal strength and clean removability from a adherend only if coated or printed at a sufficient thickness.
Screen printable fluids, including adhesives, must possess a particular balance of rheological properties that permits both sufficient fluid flow when squeezed through the screen mesh during the printing process and adequate resistance to flow to prevent smearing or soak through of the printed pattern. Such rheological characteristics can be expressed in terms of the viscosity and yield point of the printing fluid. Viscosity is generally defined as the printing fluid's relative resistance to continual shear or flow. When the viscosity of a screen printing fluid is too low, excessive flow following printing causes poor image resolution. On the other hand, a printing fluid having too high a viscosity flows unevenly through the screen, resulting in poor transfer of the image or pattern to the target surface.
The yield point represents the printing fluid's relative resistance to initial shear. Fluids having acceptable yield point for screen-printing possess high apparent viscosity when stationary, but readily flow when exposed to shear forces.
Part of the poor transfer of the image associated with high viscosity materials is due to “stringing” of the high viscosity materials. “Stringing” is the visible strands of material as the screen pulls away from the substrate. Some adhesive compositions, particularly pressure sensitive adhesive compositions, are prone to stringing, which makes them undesirable for screen-printing. Stringing can be reduced or eliminated by controlling the molecular weights of the polymers and prepolymers in the composition. However, elimination of high molecular weight polymers from the composition may limit adhesive performance.
Attempts to modify adhesive and other coatings to meet these rheological requirements for screen printability have taken several routes. Addition of solvents or other carrier liquids to coating formulations has traditionally been one means for adjusting rheology of coating formulations. Although effective in providing screen printable compositions, use of solvents has several undesirable drawbacks, including potential for screen clogging due to solvent volatility and the need to recover and dispose of the solvent after printing. When water-based printable compositions are used, similar water removal and disposal process steps and equipment are required. Such drying steps can also limit the use of types of surfaces or substrates: thermally sensitive surfaces or substrates must often be avoided.
Screen printable formulations that can be cured after printing by exposure to actinic or other radiation sources eliminate some of the processing difficulties and concerns associated with solvent- or water-based compositions. Using this method, a polymerizable mixture of monomers, initiators, crosslinkers and other reactive components is combined with other coating additives and this mixture is screen printed onto a surface. Once printed, the image or pattern is irradiated and cured using a radiation source such as electron beam, ultraviolet light or gamma radiation.
Several approaches have been described to obtain radiation curable formulations having the requisite rheological properties for screen-printing. Co-reactive oligomers or polymers, such as oligomers or polymers having one or more functional groups that can copolymerize with monomeric components, can be added to tailor the viscosity of the reactive mixture. Similarly, nonfunctional oligomers or polymers can also be added to the mixture. With both of these approaches, the compatibility of these co-reactive and nonfunctional additives with both the unreacted mixture and the cured product can be a critical consideration. Lack of compatibility of these oligomeric or polymeric rheological additives can result in cured products having compromised appearance and physical properties.
Attempts have also been made to provide suitable viscosities for screen printable adhesive formulations by partially polymerizing the reactive mixture. Once the partially polymerized mixture is printed, the image or pattern is then exposed to radiation to complete the polymerization and cure.
However, incorporation of polymeric materials of too high molecular weight, either by direct addition or partial polymerization of the reactive mixture, may cause “stringing”. The weight average molecular weight of the polymers is typically below 1,000,000 and preferably below 600,000 to limit the elongation viscosity and result in less stringing of the adhesive during screen-printing. Limitation of molecular weight, however is not a complete solution, because such changes can diminish adhesive properties.
Addition of inorganic particulate fillers, such as silica, is another approach to controlling viscosity and yield point of radiation curable screen printable compositions. While providing suitable rheological characteristics, such inorganic particulate additives are difficult to evenly disperse in the coating and can lead to diminished weatherability and appearance of printed pattern or image. In addition, for adhesive applications, addition of silica or other inorganic particles can drastically influence the adhesive pro
Banovetz John P.
Delgado Joaquin
Silver Spencer F.
Garrett Dawn
Kelly Cynthia H.
Peters Carolyn V.
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