Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2000-12-05
2002-09-24
Mulvaney, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S065200
Reexamination Certificate
active
06455121
ABSTRACT:
1. FIELD OF INVENTION
This invention relates to cationic radiation-curable adhesive formulations and to hybrid cationic and free radical radiation-curable adhesive formulations useful for bonding together surfaces of digital versatile discs.
2. DESCRIPTION OF RELATED ART
The compact disc, or CD as it is commonly known, revolutionized the recording and computer industries, making the storage of enormous amounts of data, such as music, possible in an inexpensive, readily available medium. The technology behind the compact disc has been improved and expanded to meet the increasing storage needs of the computer and entertainment industries, culminating in the creation of digital versatile discs, or DVDs. While compact discs and digital versatile discs store information in the same general manner, the DVD design exploits CD technology to create a superior product.
Structurally, digital versatile and compact discs are very similar to one another. The information bearing surfaces of both discs are marked with indentations, or pits, arranged in a continuous spiral pattern. As the drive laser moves across the pits, the laser beam is reflected back to the driver, which receives the light signal and converts it into an appropriate format, for example, audio, video, graphic or textual format. DVDs store more data than equivalent CDs because, inter alia, the information-carrying pits are smaller and are intimately spaced in tight tracks, as opposed to the wide tracks of CDs. DVD players utilize lasers which emit red light at 650 nm and 635 nm, which are shorter wavelengths than the infrared light used in conventional CD players. These shorter wavelengths enable DVD players to accurately read the smaller, more densely packed pits of the digital versatile discs.
The compact and digital versatile discs, composed of a core member around which the information bearing surface is symmetrically arranged, are the same diameter (120 mm), and the same thickness (1.2 mm). However, instead of a single layer characteristic of traditional compact discs, digital versatile discs are made of two 0.6 mm layers of polycarbonate. This reduces the amount of distance between the surface of the discs and the pits, such that the laser penetrates less plastic in the DVDs than in CDs when accessing information. Consequently, the thinner DVD substrate enhances the read accuracy of the laser. The two bonded sides of the DVDs serve to strengthen the discs, preventing warping. Thus, digital versatile discs have greater capacity and reduced responses to environmental factors than compact discs.
Digital versatile discs may be created by variations on a few basic processes, as disclosed, for example, by U.S. Pat. No. 4,310,919 and U.S. Pat. No. 4,423,137. For example, during production of digital versatile discs, a master glass disc with the desired information is created, using a laser beam to record data from the center of the master glass disc to the outer edge of the master glass disc in a spiral pattern. After recording, the master glass disc is developed by spinning a sodium hydroxide solution over the glass surface, revealing the pits created by the laser. The developed master glass disc is then metallized with a coating of silver, followed by a coating of nickel. The nickel layer is then separated from the silver-coated master glass disc, forming a nickel reverse image of the data, known as the father copy. One or more nickel copies of this father may be generated, which can be used as a stamper in an injection molding machine to mass produce discs. Molten polycarbonate is then shot into molds containing the stamper, creating polycarbonate discs carrying the desired information. The discs are then removed from the molds with the lacquer layer adhered thereto, and a reflective metal, usually aluminum, is evaporated or sputtered on top of the polycarbonate first layer containing the information. A protective coating of lacquer is then applied over the reflective layer and dried or cured, forming a single sided disc. The stamped side of the single-sided DVD is backed by a dummy layer, onto which graphics may be applied.
The basic DVD configuration is usually modified to further enhance the capacity of the discs. The capacity of a single sided disc may be almost doubled by applying a semi-reflective data layer zero, comprising, for example, gold, over the reflective aluminum layer one. The gold layer may be read by the driver laser on a low power setting, while the aluminum layer may be accessed by increasing the power of the laser. This results in a double layer of information on a single side of a disc, imparting the DVD with currently about 8.5 GB of capacity.
Two of these single sided, dual layer discs may be bonded together back to back with a thick layer of adhesive, creating double sided, double layer digital versatile discs with currently about 17 GB of storage space. The first and second disc layers are bonded such that they are parallel to and equidistant from the core member of the disc. The adhesive employed must provide high shear strength, while keeping the information layers uniformly equidistant from each other.
Three technologies are currently employed for DVD bonding, namely contact adhesives, cationic or PSA UV bonding, and free radical UV bonding. The formulations must provide adhesion between the aluminum and polycarbonate layers, the gold and polycarbonate layers, and the lacquer and the polycarbonate layers, and various combinations thereof. Furthermore, the adhesive coatings must have a high cure speed and must wet the substrate. Following cure, these materials must have high dimensional stability and durability.
However, strong, long-lasting adhesion between DVD component layers, without compromising the other desirable properties, such as dimensional stability of the disc, is not achievable with the existing systems.
Contact adhesives are applied to discs in a hot melt process, during which temperatures are kept between 120° C. and 160° C. The adhesive is spread on the discs as a thin layer by roll coating both inner bonding surfaces. The halves are then pressed together and the adhesive is allowed to set. Flat discs may be produced at high yield rates via this method, but these discs tend to warp when stored above 70° C. or in humid environments.
During free radical UV bonding, acrylate lacquer is placed on the leading edge of a disc, after which a second disc is placed on top, and the pair is spun. The weight of the second disc promotes the movement of the lacquer toward the inner edge of the metal layer, while the spinning causes the lacquer to move to the outer edge. The adhesive is cured via UV irradiation after the spin coating process is completed. The curing process is most effective when one of the substrate layers is semi-transparent or transparent. When both substrates are opaque, high UV energy is required for curing and excess heat is generated in the process. Heat adversely affects the mechanical properties of bonded discs, such as radial tilt, tangential tilt and birefringence. Excessive heating may also negatively impact the dye and phase change layers of recording media such as DVD-R or DVD-RAM. Furthermore, acrylates shrink upon cure, often to substantial degrees, thereby preventing the formation of flat discs. This shrinkage may also reduce the environmental stability of bonded discs.
During cationic UV bonding, the adhesive is screened onto both the discs, UV irradiated, and then pressed together. The bond strengthens with time due to aging, such that after approximately 24 hrs, the disc halves are permanently attached to each other. Screening methods are milder bonding processes than are spin coating techniques. The discs produced by screening methods are flatter than with other processes, but cationic UV bonding is often plagued by air entrapment during lamination and undesirable screen textures. The cationic adhesive material produces Lewis and Bronsted acids upon exposure to actinic sources, and thus pure cationic adhesives may be corrosive to the bonded recording subs
Ha Chau Thi Minh
Sullivan Michael G.
DSM N.V.
Mulvaney Elizabeth
Pillsbury & Winthrop LLP
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