Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Reexamination Certificate
2001-02-16
2004-08-10
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C264S211120, C264S284000, C425S363000
Reexamination Certificate
active
06773649
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for producing low-birefringent and/or low stress plastic film or sheet having a high surface-polish and is suitable for optical media applications as well as low stress film for non optical applications either one using a continuous extrusion process. Optical media applications include such items as compact discs (CD), digital video discs (DVD), liquid crystal displays (LCD) or any other optical media applications which require a transparent substrate with low birefringence low stress and a high surface-polish. Non optical applications using low stress film or sheet for use in such applications as automobile dash board overlays or other uses for opaque film or sheet which require tight graphics registration. Birefringence is not measurable in opaque film or sheet.
More particularly, this invention relates to a particular calendering or process finishing roll stack wherein the structure of at least one of the finishing rolls is comprised of an inner steel shell, a resilient covering thereover and a multi layer metal sleeve outer covering. Film or sheet produced using the roll structure of this invention has low-birefringence, low stress and is highly polished on at least one surface i.e. a surface having a low roughness of 4 microinches or less which film or sheet is suitable for optical media applications or opaque film or sheet for such other non optical applications. Such film or sheet is produced in a one step continuous extrusion process.
Currently, polycarbonate is used as the polymeric material for optical media applications such as CD's and are made by injecting molding. The process is relatively slow and expensive. In addition, it is difficult to produce CD's of very low-birefringence which will be required to reach higher data densities in the future. CD's currently produced today have a retardation value of 25-30 nm. (nanometers), which is birefringence times thickness. Stress and birefringence are inherent in injection molding CD's because the melt starts to solidify on the inside mold wall as the mold is filling, and then additional melt is forced into the mold cavity to compensate for shrinkage of the disc as it solidifies. In opaque film or sheet, birefringence is not measurable but low stress is wanted for applications in vehicles, computer housings, etc. that require tight (0.4 mm/MAX) graphics registration.
Birefringence is defined as the difference between the refractive indices along two perpendicular directions as measured with polarized light along these directions. It results from molecular orientation, and the measurement of birefringence is the most common method of characterising polymer orientation. It is determined by measurement of the retardation distance by either a compensation or a transmission method. Positive birefringence results when the principal optic axis lies along the chain; negative birefringence when transverse to the chain. In Cartesian coordinates there are three birefringences, two being independent. Thus &Dgr;xy=n
x
−n
y
, the differences in refractive indices along the x and y axes. Uniaxial orientation only requires one of these to describe the orientation. Therefore, in order to obtain a uniform homogeneous polycarbonate, the lower the birefringence (the differences between the refractive indices) the more homogeneous the polymer composition of the product and thus the more uniform properties of the product. This is critical, particular on CD's, DVD's or LCD wherein the Laser read out must have minimal or zero distortion. The lower birefringence, the less is the variation in polymer homogeninity and Laser distortion.
Another parameter for optical materials is Cg which is the stress-optical coefficient of material in the glassy state. It can be measured with a molded part such as a small bar or disc. Birefringence can be measured by the method described above. When a stress is applied to the bar, the birefringence will change by an amount B. The stress-optical coefficient, which has units of Brewsters, is given by:
B=Cg &dgr;
The stress-optical coefficient (Cg) should be less than or equal to about 70 Brewsters.
Improvements in optical data storage media, including increased data storage density, are highly desirable, and achievement of such improvements is expected to improve well established and new computer technology such as read only (ROM), write once, rewritable, digital versatile and magneto-optical (MO) disks.
In the case of CD ROM technology, the information to be read is imprinted directly into a moldable, transparent plastic material, such as bisphenol A (BPA) polycarbonate. The information is stored in the form of shallow pits embossed in a polymer surface. The surface is coated with a reflective metallic film, and the digital information, represented by the position and length of the pits, is read optically with a focused low power (5 mW) laser beam. The user can only extract information (digital data) from the disk without changing or adding any data. Thus, it is possible to “read” but not to “write” or “erase” information.
The operating principle is a write once read many (WORM) drive is to use a focused laser beam (20-40 mW) to make a permanent mark on a thin film on a disk. The information is then read out as a change in the optical properties of the disk, e.g., reflectivity or absorbance. These changes can take various forms: “hole burning” is the removal of material, typically a thin film of tellurium, by evaporation, melting or spalling (sometimes referred to as laser ablation); bubble or pit formation involves deformation of the surface, usually of a polymer overcoat of a metal reflector.
Although the CD-ROM and WORM formats have been successfully developed and are well suited for particular applications, the computer industry is focusing on erasable media for optical storage (EODs). There are two types of EODs: phase change (PC) and magneto-optic (MO).
Generally, amorphous materials are used for MO storage and have a distinct advantage in MO storage as they do not suffer from “grain noise”, spurious variations in the plane of polarization of reflected light caused by randomness in the orientation of grains in a polycrystalline film. Bits are written by heating above the Curie point, T
C
, and cooling in the presence of a magnetic field, a process known as thermomagnetic writing. In the phase-change material, information is stored in regions that are different phases, typically amorphous and crystalline. The film is initially crystallized by heating it above the crystallization temperature. In most of these materials, the crystallization temperature is close to the glass transition temperature. When the film is heated with a short, high power focused laser pulse, the film can be melted and quenched to the amorphous state. The amorphized spot can represent a digital “1” or a bit of information. The information is read by scanning it with the same laser, set at a lower power, and monitoring the reflectivity.
In the case of WORM and EOD technology, the recording layer is separated from the environment by a transparent, non-interfering shielding layer. Materials selected for such “read through” optical data storage applications must have outstanding physical properties, such as moldability, ductility, a level of robustness compatible with particular use, resistance to deformation when exposed to high heat or high humidity, either alone or in combination. The materials should also interfere minimally with the passage of laser light through the medium when information is being retrieved from or added to the storage device.
As data storage densities are increased in optical data storage media to accommodate newer technologies, such as DVD and higher density data disks for short or long term data archives, the design requirements for the transparent plastic component of the optical data storage devices have become increasingly stringent. Materials displaying lower birefringence at current, and in the
Bourne Robert Allan
Coyle Dennis Joseph
Lilly Kenneth L.
Morrow Robert
Schenk Brian Scott
Eashoo Mark
General Electric Company
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