Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Light polarizing article or holographic article
Reexamination Certificate
2000-08-17
2001-10-09
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Light polarizing article or holographic article
C264S328100
Reexamination Certificate
active
06299802
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing optical media discs.
Optical media discs have been prepared previously using molding techniques such as those described in The Compact Disc Handbook, 2nd edition, by Pohlmann. Methods of molding optical media discs are well known in the art and include injection and injection-compression molding. Compact discs (CD's) have a thickness of approximately 1.2 mm and are typically made using standard injection molding techniques. However, Digital Versatile Discs (DVD's) have a layer thickness of 0.6 mm and require injection compression-molding as described in
Injection Molding An Introduction
, pgs. 171-172 Hanser/Gardner/ Publication, Inc., Cincinnati, 1995 by Potsch and Michaeli. Injection-compression is an enhanced injection molding process where the mold is left slightly ajar at the beginning of the cavity fill stage. The cavity is clamped completely closed to the desired final part thickness of 0.6 mm during the filling phase. Injection-compression is required in order to achieve adequate data transfer to a 0.6 mm substrate surface while achieving low birefringence in polycarbonate discs. Unfortunately compression-injection molding requires increased processing complexity, thus requiring purchase of new processing equipment or costly modification of existing standard injection molding equipment.
Therefore, there remains a need for a process for producing DVD's using standard injection molding equipment while maintaining acceptable data transfer, cycle time and low birefringence.
SUMMARY OF THE INVENTION
It has been surprisingly discovered that DVD's having a thickness of approximately 0.6 mm can be manufactured using an injection molding process, without the use of compression, when the discs are made from a polymer having
1) a glass transition temperature (Tg) of greater than 110° C. as measured by Differential Scanning Calorimetry (DSC) at 10° C./min.,
2) an elastic modulus (G′) of less than or equal to 1000 dynes/cm
2
at a temperature of 240° C., as measured by Dynamic Mechanical Spectroscopy measurements at a temperature ramp rate of 3° C./min., and
3) a shear rate of 1 radian/sec using a parallel plate geometry (under nitrogen), and a complex viscosity (Eta*) of less than 2000 poise, as measured according to Dynamic Mechanical Spectroscopy measurements, using shear rate sweeps at 280° C., and a shear rate of 1 radian/sec using a parallel plate geometry (under nitrogen).
It is surprising that a injection molding process, without compression, can be used to prepare DVD's having a thickness of approximately 0.6 mm, while maintaining good data transfer and low birefringence.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a process for producing DVD's using injection molding techniques, without the use of compression, from a polymer having a glass transition temperature (Tg) of greater than 110° C., an elastic modulus (G′) of less than or equal to 1000 dynes/cm
2
at a temperature of 240° C., and a complex viscosity (Eta*) of less than 2000 poise.
In the process of the present invention, an injection molding process is used to produce a DVD having a single disc layer thickness of approximately 0.6 mm. Injection molding techniques are well known in the art and described in
The CompactDisc Handbook
, 2
nd
edition by Pohlman, pages 295-296 and in
Injection Molding An Introduction
, pgs. 1-12, Hanser/Gardner/ Publication, Inc., Cincinnati, 1995 by Potsch and Michaeli. In one embodiment, molten polymer is injected into a mold cavity, with the stamper on one face producing a clear plastic disc with the pits impressed on one side.
The injection molding process of the present invention does not use compression during molding. Instead, the mold is closed prior to injection of the molten polymer.
Surprisingly, it as been found that DVD's can be produced using this process from a polymer having:
1) a glass transition temperature (Tg) of greater than 110° C. as measured by Differential Scanning Calorimetry (DSC) at 10° C./min.,
2) an elastic modulus (G′) of less than or equal to 1000 dynes/cm
2
at a temperature of 240° C., preferably at 225° C., more preferably at 215° C. and most preferably at 210° C., as measured by Dynamic Mechanical Spectroscopy measurements at a temperature ramp rate of 3° C./min., and a shear rate of 1 radian/sec using a parallel plate geometry (under nitrogen), and
3) a complex viscosity (Eta*) of less than 2000 poise, preferably less than 1300 poise, and more preferably less than 600 poise, as measured according to Dynamic Mechanical Spectroscopy measurements, using shear rate sweeps at 280° C., and a shear rate of 1 radian/sec using a parallel plate geometry (under nitrogen).
Polymers which can be used in the process of the present invention include saturated hydrocarbon thermoplastics. The term saturated refers to the amount of olefinic bonds within the chemical structure. As used herein, saturated refers to a polymer wherein less than 10 percent of the carbon-carbon bonds are olefinic or unsaturated in nature, preferably less than 7.5 percent, more preferably less than 5 percent, even more preferably less than 2 percent, and most preferably less than 1.5 percent. These types of polymers include hydrogenated aromatic/conjugated diene block copolymers, cyclic-olefin-copolymers and hydrogenated ring opening metathesis polymers.
Aromatic/conjugated diene block copolymers include block copolymers of a vinyl aromatic monomer and a conjugated diene monomer. The vinyl aromatic monomer is typically a monomer of the formula:
wherein R′ is hydrogen or alkyl, Ar is phenyl, halophenyl, alkylphenyl, alkylhalophenyl, naphthyl, pyridinyl, or anthracenyl, wherein any alkyl group contains 1 to 6 carbon atoms which may be mono or multisubstituted with functional groups such as halo, nitro, amino, hydroxy, cyano, carbonyl and carboxyl. More preferably Ar is phenyl or alkyl phenyl with phenyl being most preferred. Typical vinyl aromatic monomers include styrene, alpha-methylstyrene, all isomers of vinyl toluene, especially paravinyltoluene, all isomers of ethyl styrene, propyl styrene, butyl styrene, vinyl biphenyl, vinyl naphthalene, vinyl anthracene and the like, and mixtures thereof. Block copolymers containing 5 or more blocks can contain more than one specific polymerized vinyl aromatic monomer. In other words, for example, a pentablock copolymer can contain a polystyrene block and a poly-alpha-methylstyrene block. The hydrogenated vinyl aromatic polymer block may also be a copolymer of a vinyl aromatic wherein the vinyl aromatic portion is at least 50 weight percent of the copolymer. Preferably, the vinyl aromatic polymer block is a styrene polymer block.
The conjugated diene monomer can be any monomer having two conjugated double bonds. Such monomers include for example 1,3-butadiene, 2-methyl-1,3-butadiene, 2-methyl-1,3 pentadiene, isoprene and similar compounds, and mixtures thereof. Preferably, the conjugated diene is a butadiene.
The conjugated diene polymer block can be prepared from materials which remain amorphous after the hydrogenation process, or materials which are capable of crystallization after hydrogenation. Hydrogenated polyisoprene blocks remain amorphous, while hydrogenated polybutadiene blocks can be either amorphous or crystallizable depending upon their structure. Polybutadiene can contain either a 1,2 configuration, which hydrogenates to give the equivalent of a 1-butene repeat unit, or a 1,4-configuration, which hydrogenates to give the equivalent of an ethylene repeat unit. Polybutadiene blocks having at least approximately 40 weight percent 1,2-butadiene content, based on the weight of the polybutadiene block, provides substantially amorphous blocks with low glass transition temperatures upon hydrogenation. Polybutadiene blocks having less than approximately 40 weight percent 1,2-butadiene content, based on the weight of the polybutadiene block, provide crystalline blocks upon
Maher James P.
Parsons Gary D.
The Dow Chemical Company
Vargot Mathieu D.
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