Data storage medium containing polyestercarbonate

Details Data storage medium containing polyestercarbonate Data storage medium containing polyestercarbonate

1999-11-15

2002-08-20

Evans, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400

Reexamination Certificate

active

06436503

ABSTRACT:

FEDERALLY SPONSORED RESEARCH
Not applicable
BACKGROUND OF THE INVENTION
This invention relates to data storage media, and more particularly to data storage media having both a data storage layer and an adjacent transparent overlayer wherein the data storage layer is capable of reflecting an energy field incident upon said transparent overlayer prior to being incident upon said data layer. Specifically, this invention relates to data storage media having thin transparent polyestercarbonate overlayers.
Use of optical storage media has become common since the advent of the compact disc (CD) widely used for the storage of music, the digital versatile disk (DVD) for video or other data, and the CD-ROM for computer files. Optical storage media of this type require a transparent overlayer with excellent optical properties, which covers a data layer. The data layer is encoded with information, typically in the form of a series of pits or depressions, in the case of pre-recorded media, or continuous grooves in the case of recordable and rewritable media. In the above conventional formats, the media is disk-shaped. The media is conventionally formed by patterning the overlayer, and later forming a data layer over the overlayer. A laser and an optical pickup system can recover the information stored on the disc by detecting pits or grooves on a suitably coated substrate. With storage devices of this type, there is a growing need to store more and more information in a smaller space.
There are several requirements for the transparent overlayer to ensure that the optical storage media can be manufactured efficiently, will be durable and will function properly. For example, the transparent overlayer must have low birefringence, which is defined as the difference between refractive indices along two perpendicular axes. The measured birefringence of an optical disk has an intrinsic component, which is a characteristic of the material used for the transparent overlayer, and an extrinsic component, which is a function of how much stress is introduced when molding the disk. Materials which are more viscous (i.e., have lower values of melt flow index) tend to produce disks having higher birefringence at similar molding conditions. Large values for birefringence are undesirable because they interfere with reading the data, and thus limit the density of readable information that can be encoded on a disk. Also, it is desirable to move towards relatively thinner transparent overlayers to enable even higher data storage densities. In general, the stresses introduced when injection molding thinner items are greater, and therefore it becomes even more important to find methods for reducing the extrinsic birefringence when creating high density data storage media.
In general, one can reduce the extrinsic birefringence created during a molding operation by using a higher melt flow index (MFI) material because less viscous materials are better able to flow during the molding operation. However, plastics having a higher MFI often are too brittle to provide acceptable durability in optical media applications.
Bisphenol-A Polycarbonate (BPA-PC) has thus far been widely used for optical storage media applications such as the CD and DVD applications. However BPA-PC has some limitations which may make it inadequate or less desirable for some present, and next generation applications. Specifically, BPA-PC that meets acceptable brittleness specifications, is relatively viscous, and therefore is rather difficult to process by injection molding. This difficulty limits the speed with which discs can be manufactured. Also, these processing challenges increase the difficulty of obtaining good pit or groove replication, thus limiting the quality and density of information that can be stored on a disk. Therefore, optical data storage media made from BPA-PC are limited, in terms of their maximum data density, by their birefringence and capability to reproduce pits or grooves on the surface of the plastic substrate. Poor replication of pits or grooves which is influenced by Theological and thermal properties of the polymer melt, as well as process conditions during injection molding, can also lead to increased noise during readout of an optical disk.
Typical CDs utilize a substrate which is 120 mm in diameter and 1.2 mm thick. More recently, the DVD has been introduced. The DVD has two substrates, each 120 mm in radius and only 0.6 mm thick. These substrates are bonded together to make a double-sided optical medium. The decreased thickness requirement for DVD has increased the difficulty of injection molding an overlayer having the necessary birefringence and pit or groove replication because thinner molded parts are subject to greater stress, which increases the extrinsic birefringence. In the future, higher capacity optical storage devices will use even thinner overlayers. Thinner overlayers having acceptable birefringence will be increasingly difficult to fabricate via injection molding of thermoplastic resin.
Better replication and lower stress can be obtained in a hotter molding process using longer cycle times, but BPA-PC is prone to degradation and yellowing at higher temperatures and greater residence times. Alternatively, the molecular weight of BPA-PC can be lowered, but this tends to increase brittleness of the finished disk.
Numerous structural variations of BPA-PC have been tested in an effort to overcome the limitations of BPA-PC in optical media, but many of these variations do not meet all requirements for a successful optical data storage device material. Most variations are either too brittle, have poor optical properties (low transmittance and/or high haze) or are difficult to process due to their high glass transition temperature (Tg). High processing temperatures can also lead to degradation of the polymer chain, which causes loss of mechanical properties, color formation (especially yellowing) and generation of gaseous byproducts impairing optical properties.
Other potential low birefringence optical materials are unacceptable because they are too floppy (have a flex modulus below about 150,000 psi), or have a low thermal capability: (Tg below about 80° C.).
Therefore there is a need to find a transparent overlayer material for optical media that will be transparent, have good melt processability, have low birefringence, have high thermal capability and maintain flatness when used to produce data storage medium having an overlayer thickness of equal to or less than the thickness of today's DVD's (about 0.6 mm).
BRIEF SUMMARY OF THE INVENTION
It has been discovered that polyestercarbonates having an MFI of greater than 14 are capable of providing a satisfactory solution to the above mentioned challenges when used to form overlayer thicknesses of about 0.6 mm or less (such as found in the DVD format). This is somewhat surprising since, as shown below, polyestercarbonates are an inferior solution to these problems when used to make overlayer thicknesses of about 1.2 mm (such as found in the CD format).
In a first aspect, the invention refers to a data storage media which has a data layer and a transparent overlayer adjacent to the data layer. The transparent overlayer has a thickness of about 0.6 mm or less. The storage media is configured such that the data layer will reflect an energy field incident upon the transparent overlayer prior to being incident upon the data layer. The transparent overlayer comprises a polyestercarbonate having a MFI greater than 14.
In a second aspect, the invention refers to a storage media for data which is readable using relative motion between the media and a reading light beam for retrieving the data carried by the media. Said media has a first, disc shaped transparent overlayer which comprises a polyestercarbonate. The polyestercarbonate has monomer units derived from a dihydric phenol and an aliphatic dicarboxylic acid. This polyestercarbonate has an MFI greater than 14. The first transparent overlayer has an entrance surface and an exit surface. The entranc

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