Preparation of a recording carrier

Details Preparation of a recording carrier Preparation of a recording carrier

2000-04-21

2002-08-06

Evans, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S457000

Reexamination Certificate

active

06428872

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical information recording carriers. In particular, this invention relates to two-sided optical information carriers, also known as digital video discs, of certain acrylic polymers.
2. Description of the Prior Art
Polymers have become the medium for embedding information as optical information recording carriers. Polycarbonate has dominated the compact disc market, while acrylic polymers, based on polymers of methyl methacrylate, have dominated the laser-readable video disc market.
Two-sided optical information carriers (also called digital video discs, digital versatile discs, “DVD”, “DVD-ROM”, “DVD-RAM” or “DVD-R”) hold much more information than compact discs. Compact discs are based on a single molded disc having a metal facing on one surface, while the final DVD assemblage is two-sided (two discs). As DVDs hold much more information than compact discs, DVDs require precise copying of the pit depths and dimensions of the master facing mold during molding of the individual discs.
In order to produce DVDs on which information can be encoded and read with a low error rate, a number of variables must be controlled. The purity of the starting molding resin must be carefully controlled as impurities can affect reading of the encoded data. The molding resin must be able to flow sufficiently in order to properly fill the mold. The molding resin must be able to precisely replicate the pit depths and dimensions with minimal deformation around the pit. A molding resin that does not precisely replicate the pit depths and dimensions will have increased error in reading the encoded data.
Birefringence of the molded disc must be minimized for high-quality discs and can be controlled by avoiding molded-in stresses in the disc. Birefringence is a measure of optical retardation due to small non-uniformities in disc composition resulting in small, but important differences in the index of refraction. These small non-uniformities are often due to polymer orientation which occurs as a result of the injection molding operation. Birefringence values differing from zero by more than 100 nm are unacceptable. Birefringence leads to a loss of signal intensity, and therefore increased error in reading the encoded data.
Birefringence is a problem with polycarbonate discs because polycarbonate's high stress optical coefficient and stiff flow make it difficult to mold discs with low birefringence values. The low stress optical coefficient and easy flow of acrylic polymers generally result in significantly less birefringence, and offer better replication of the master facing mold.
Discs made from acrylic polymers are less rugged than those made from polycarbonate and are prone to breakage and warpage. A solution to the breakage and warpage problem is to increase the molecular weight of the acrylic polymer used. However, as the molecular weight of the acrylic polymer increases, its flowability decreases. A decrease in the polymer's flowability decreases the polymer's ability to properly fill the mold and, therefore, to precisely copy the pit depths and dimensions of the master facing mold. This results in a disc that has a higher error rate in reading the encoded data.
Japanese unexamined Patent Application 08-132,496 A discloses a method for molding optical discs with specific control of melt and mold temperatures and control of mold pressure in a two-step operation. Their application does not disclose the specific compositions and molecular weights of the acrylic polymers used, except to list a broad list of monomer combinations, and to list a preferred molecular weight range for a copolymer with no more than 80% of polymerized units of methyl methacrylate, and an exemplification of a methyl methacrylate copolymer with 10 weight percent methyl acrylate. The glass transition temperatures of the polymers disclosed in this application will be less than 100° C. Such glass transition temperatures are not high enough to provide an acrylic polymer that sufficiently resists warpage. This application implies that the composition and molecular weight are less important than the specific molding conditions.
Japanese Patent 85-045,649 B2 (Arakawa et al.) discloses an optical information recording carrier, containing a copolymer of methyl methacrylate and ethyl acrylate having a specific composition and intrinsic viscosity. The polymers disclosed in Arakawa et al. have an intrinsic viscosity within the range of greater than or equal to 0.4 dl/g and less than or equal to 0.62 dl/g, as defined by the formula 0.4≦y≦({fraction (1/45)})x+0.334, where “y” is the intrinsic viscosity and “x” is the weight percent of ethyl acrylate in the copolymer. As long as the copolymer meets the above criteria for intrinsic viscosity the weight percent of ethyl acrylate in the copolymer can vary from 3 to 13. Arakawa et al. suggest that an acrylic polymer having an intrinsic viscosity more than value defined by the formula ({fraction (1/45)})x+0.334 cannot be used as an optical information recording carrier; thus, Arakawa teaches that when x is 3%, then y (intrinsic viscosity) cannot not be more than 0.40 dl/g which would result in a molecular weight maximum of 80,000.
Japanese patent application 09-48827 A (Ohtani et al.) discloses methyl methacrylate copolymers having 3-10%, preferably 4-8%, by weight of a C1 to C8 alkyl acrylate. The only copolymer of MMA/EA exemplified has 4% EA. Ohtani et al. does not disclose that any copolymer any having less than 4% by weight ethyl acrylate can be used as an optical information recording carrier.
There is, therefore, a continuing need to develop acrylic polymers useful in double-sided optical information carriers that resist breakage and warpage, still have sufficient flowability such that they precisely replicate pit depths and dimensions in the master facing mold and can be molded in a very short mold cycle time.
STATEMENT OF THE INVENTION
The present invention is directed to an optical information recording carrier as a component of a digital video disc, the optical information recording carrier comprising a copolymer of methyl methacrylate with no more than 3% wt of a copolymerized ethyl acrylate, the molecular weight of the copolymer being in the range of from 90,000 to 115,000, the copolymer having a Tg in the range of from 105° C. to 115° C.
DETAILED DESCRIPTION OF THE INVENTION
The following terms shall have the following meanings, unless the context clearly indicates otherwise.
The term “molecular weight” means “weight average molecular weight,” abbreviated “MW.” Molecular weight is estimated by conventional gel permeation chromatographic methods, using poly(methyl methacrylate) standards for calibration.
“Lower alkyl” means linear or branched C1-C4 alkyl, such as methyl, ethyl, n-propyl, iso-propyl, n-butyl and iso-butyl.
“Tilt” is a measurement of flatness made on the assembled two-sided optical information carrier by determining the angular deviation of an incident light beam. Considered acceptable for the optical information recording carrier of the present invention are tilt values of less than 0.8° in the radial direction and of less than 0.30° in the tangential direction after exposing the carrier to 60° C. and 50% relative humidity for 96 hours.
The term “polymer” and “copolymer” are used interchangeably, and the former term encompasses the latter term.
As used herein, the term “acrylic” polymer means a polymer derived from acrylate or methacrylate monomers, or both, such as methyl acrylate, methyl methacrylate, ethyl acrylate and butyl acrylate.
All amounts are percent by weight (“% wt”), unless otherwise noted, and all % wt ranges are inclusive.
As used herein, the following abbreviations are applied: Tg=glass transition temperature; “nm”=nanometer; “mm”=millimeter; “dl”=deciliter; “g”=gram; “MMA”=methyl methacrylate; “EA”=ethyl acrylate; and “AFM”=atomic force microscopy.
Birefringence is measured in terms o

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