CA2252947C

CA2252947C - Process and apparatus for forming plastic sheet

Info

Publication number: CA2252947C
Application number: CA002252947A
Authority: CA
Inventors: Steven David Fields; James Ralph Galione
Original assignee: Rohm and Haas Co
Current assignee: Rohm and Haas Co
Priority date: 1997-11-07
Filing date: 1998-11-05
Publication date: 2001-10-30
Anticipated expiration: 2018-11-05
Also published as: CN1225307A; CN1147390C; JPH11227026A; JP2010162899A; JP4519211B2; KR19990045095A; CA2252947A1; US6183829B1; DE69807816D1; CN100480027C; CN1196574C; US6451403B1; CN1361001A; US6472031B1; DE69807816T2; CN1500623A; EP0914926B1; TW442384B; EP0914926A3; KR100562051B1

Abstract

Disclosed is a process and apparatus for formation of optical quality plastic sheet in a continuous fashion, wherein the plastic sheet produced is capable of use in a variety of optical and electronic display applications.

Description

PROCESS AND APPARATUS FOR FORMING PLASTIC SHEET

BACKGROUND OF THE INVENTTON
The present invention relates to a process and apparatus for forming plastic sheet. In partic~llar, the present invention relates to a process and apparat~ls for 5 forming plastic sheet having low residual stress and high surface quality. Plastic sheet formed according to the process of the present invention is partic~llarly useful in optical and electronic display applications, such as, for example, optical windo~vs, optical filters, recording media, and liquid crystal displays (LCD).
Sheets of optical quality glass or quartz are used in electronic display 10 applications as "substrates." In such applications, a "substrate" is a sheet of material used to build an electronic display. Such substrates can be transparent, translucent or opaque, but are typically transparent. In general, such sheets have conductive coatings applied thereto prior to use as substrates. Such substrates often have stringent specifications for optical clarity, flatness and minimal birefringence, and 15 typically must have high resistance to gas and solvent permeation. Mechanicalproperties such as flexibility, impact resistance, hardness and scratch resistance are also important considerations. Glass or quartz sheets have been used in display applications because these materials are able to meet the optical and flatness requirements and have good thermal and chemical resistance and barrier properties;
20 however, these materials do not have some of the desired mechanical properties, most notably low density, flexibility and impact resistance.
Because of the mechanical limitations of glass or quartz sheet in optical or display applications, it is desirable to use plastic sheet in S~lCh applications.
Although plastic sheets hclve greater flexibility, are more resistant to breakage, and 25 are of lighter weight than glass or quartz sheets of eq~lal thic~ness, it has been ~ ery difficult to produce plastic sheet having the requisite optical specifications needed ror use in optical and display applications at reasonable costs. i~loreover, many types of plastic sheet undergo unacceptable dimensional distortion ~vhen subjected to substrate processing conditions during manufact~lre of the display devices, 30 particularly wlth réspect to temperature.

There are several commercially utilized methods for producing plastic sheet, including casting, extrusion, molding, and stretching operations. Of these methods, several are not suitable for producing high quality plastic sheet. As used throughout this specification, the term "high quality" is used to describe plastic sheet having the 5 following characteristics: low surface roughness, low w aviness, low thicknessvariation, and minimal amount of polymer chain orientation (for example, as measured by asymmetric physical properties, birefringence or thermal shrinkage).
For example, injection molding is likely to produce high amounts of polymer chain orientation, especially for thin sheets (i.e ,1 mm thickness or less), due to the 10 flow of molten plastic into the mold, which unacceptably increases birefringence for polymers with non-negligible photoelasticity coefficients. Injection compressionmolding is an improved molding process which allows squeezing of the polymer after injection for the purpose of improving surface quality and reducing polymer chain orientation. However, even with these improvements, injection compression 15 molding has limited ability to produce high quality sheet.
Compression molding and press polishing may be used to produce sheets with good surface quality; however, the squeezing flow inherent in such processes results in polymer chain orientation which results in unacceptable shrinkage during thermal cycling. Moreover, these processes are not continuously operable and 20 therefore increase labor and production costs.
Stretching operations (for example, for the production of uniaxially- or biaxially-oriented films) and blown film extrusion inherently introduce large amounts of polymer chain orientation and are unsuited for the production of highq~:ality plastic sheet.
''5 Solvent casting can be ~Ised to produce high quality film; however, there are practical limitations to the maximum film thickness which can be E?roduced by this method. In addition, the solvent used in the casting must be removed after formation of the sheet.

. . .

Sheet extrusion is run as a continuous operation, but this process introduces unacceptable polymer chain orientation due to the nature of the polymer flow in the die and between the polished rollers in the roll stack.
There is therefore a continuing need for a method for producing relatively 5 inexpensive, high quality plastic sheet in a continuous fashion, wherein the resultant plastic sheet is capable of use as a substrate in optical and electronic displayapplications.

STATEMENT OF THE INVENTION
The present invention is directed to a method for producing high quality 10 plastic sheet, including the steps of: a) providing molten plastic resin; b) directing the molten plastic resin to an overflow die having an inlet and an outlet; c) shaping the molten plastic resin into a molten web using said overflow die; d) guiding said molten web away from said overflow die; and e) cooling said molten web to form asolid sheet.
The present invention is also directed to an apparatus for producing high quality plastic sheet, including: a) a source for providing molten plastic resin; b) an overflow die having a length and a width, comprising a substantially egg-shaped cross-section culminating in an apex, a conduit opening, and a metering arrangement connected with said conduit opening, wherein the molten plastic resin 20 flows into the die through the conduit opening, out of the die through the metering arrangement, and around the sides of the die to form a molten web at said apex; c) means for delivering said molten plastic resin from said source to said overflow die;
and d) guidance means for guiding said molten web away from said o~ erflow die.
The present invention is also directed to an overflolvv die useful in forming 25 high quality plastic sheet, said o~ erflow die having an interior and an exterior and including: a) an overflow surface formed by the exterior of the die and comprising a pair of die lips; b) two exterior sides connected to said over~low surface; c) an apex formed by the con~luence of said two exterior sides and located in substantial opposition to said overflow surface, wherein the die has a substantially egg-shaped 30 cross-section; d) a conduit opening from the exterior to the interior, and e) a , metering arrangement located in the interior, wherein such metering arrangement is connected with said conduit opening and said overflow surface.

BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a frontal view of a typical apparahls of the present invention.
Figure 2 is a side view of the apparatus of Figure 1.
Figures 3A-3C are close-ups of overflo~v die 20. Figure 3A is a perspective view of the die ~vith heating manifold attached. Figure 3B is a top view of the die;
and Figure 3C is a side view of the die.
Figure 4 is a cross-sectional view of overflow die 20.
Figures 5-7 are alternate embodiments of the overflow die of the present invention. Figure 5 illustrates an overflow die having a series of holes in place of the slot 22 of die 20; Figure 6 illustrates an overflow die having a non-tapering slot; and Figure 7 illustrates an overflow die having a "coathanger" arrangement.

DETAILED DESCRIPTION OF THE INVENTION
As used in this specification, the following terms have the following definitions, unless the context clearly indicates otherwise. "Glass transition temperature" or "Tg" is the midpoint of the narrow temperature range over which polymers change from being relatively hard and brittle to relatively soft and viscous (rubbery). "Plastic" refers to polymer, such as thermoplastic polymers, which can form sheets. The terms "polymer" and "resin" are used interchangeably throughoutthe specification. "Sheet" refers to a sheet having a thickness of about 25 mm or less, and is intended to include "films" (sheets having thickness of ~ 0.5 mm).
"Shrinkage" refers to an irreversible dimensional change that occurs in a sheet subjected to a heat-cool cycle. The follo~ving abbreviations are used in the specification: cm = centimeter(s); mm = millimeter(s); nm = nanometer(s); u =
rricron(s) (micrometers); g = gram(s); mL = milliliters; Pa = Pascclls; kPa =
kiloPascals; Pa-s = Pascal-seconds; sec = second(s); min = minute(s); hrs = hour(s);
UV= ultraviolet, and IR = infrared. All temperature references are ~C unless otherwise specified. Ranges specified are to be read as inclusive, unless specifically identified otherwise.
The high quality plastic sheet formed by the process of the present invention can be used in a number of applications, including but not limited to: substrates for electronic display devices such as LCD and electroluminescent displays; optical windovvs and filters; substrates for optical, magnetic, chemical or other types of recording media; substrates for imaging, such as for photographic or x-ray applications. Depending on the particular use for sheet produced by the method of the present invention, sheet characteristics such as low shrinkage, low birefringence, 10 and surface quality may vary in relative importance. Desired sheet thickness ~vill also vary depending on the particular use, but will generally be about 25 mm or less, preferably 10 - 5000 ~, and most preferably 50 - 1000 ,u. Sheet thickness can beadjusted by varying the speed of delivery of the molten polymer to the die or byvarying the speed of the take-off means. Thickness variation over a sample length of 15 400 mm should be generally 10% or less, preferably 5~O or less, and most preferably 1% or less.
A typical apparatus of the present invention is shown in Figures 1 - 4. As will become clear to those skilled in the art, variations from the apparatus illustrated in these Figures may be made within the scope of the present invention.
Molten polymer from a source 10 is delivered to an overflow die 20 ~ ia channel 12 (preferably controlled by delivery means 1~ here it is introd~lced tothe die 20 through conduit opening 21 to conduit 22. The temperature of the molten polymer as it is delivered to die 20 is maintained by use of heaters 15 located in close proximity to die 20. As the molten polymer fills the opening 21, it is forced O~lt 25 through the meterinD arrangement, slot 23, onto the die lips 40 and 41, and flows out around the sides 2~ and 25 of the die 20. At the ape~ 26 of the die 20, the molten polymer flowing from sides 2~ and 25 con~ erge to form the beginning of molten web 27.
The molten web 27 is picked up at its edges by two pairs of g~lidance means, 30 (e.~., tank treads 31, 32, 33 and 34) whicll guide the molten web away from die 20.

As molten web 27 is guided away from die 20, the temperature of the web gradually falls below the glass transition temperature of the polymer, and results in cooled sheet 40. In an optional embodiment, cooling means 36 located in close proximity to the guidance means 31, 32, 33, 34 aid in lowering the temperahlre of the web.
Molten resin can be supplied in any of a number of ways. For example, the molten resin may be supplied from a polyrnerization reactor, a mixer, a devolatilization device (e.g., a flash column, falling strand devolatilizer or wiped film evaporator), or an extruder. An extruder is preferred, as it can also act as a polymer delivery means (see discussion below). It is most preferred to use a single screw extruder, although a double (twin) screw extruder or a multiple screw extruder may also be used. If a twin or multiple screw extruder is used, it can be of any type, for example, counter-rotating, co-rotating, intermeshing or non-intermeshing.
The molten resin may contain one or more plastic additives such as antioxidants, ultra-violet ('W') absorbers, W stabilizers, fluorescent or absorbing dyes, anti-static additives, release agents, fillers and particulates. The type and amount of additive used ~ith particular resins for particular purposes is known to those skilled in the plastic arts and will not be further detailed herein.
The temperature at which the resin is processed will depend upon the composition of the resin and may vary during processing. The temperat~lre must be sufficiently high that the resin will flow but not so high as to degrade the resin.
Operating conditions ~vill vary depending on the type of polymer to be processed, and are within ranges known to those skilled in the art. However, as a general guideline, the operating temperature will be between 100 and ~00 ~C. For example, I'MMA may be processed in an extruder with the extruder karrel temperature of 150 to 260 ~C and a melt temperature of 150 to 260 ~C. Other polymers such as polycarbonate or poly methylmethacrylimide can also be used at appropriately higher melt temperatures (200 - 330 ~C). It is preferred that volatile materials and undesired particulate matter be removed from the molten plastic resin prior to sheet , formation. This may be accomplished in accordance with methods known to those skilled in the art.
Delivery means 14 for delivering constant flow of the molten polymer are required for the purpose of regulating the flow rate and providing the pressure required to deliver the molten polymer through the channel 12, conduit opening 21 and conduit 22, to the die 20. The delivery means may include any type of mechanical melt pump, including, but not limited to any appropriate extruder (asdescribed above), gear pump, or combinations thereof. In simple form, the delivery means may be a gravity feed, or hydrostatic pressure. The delivery means may be 10 selected in accordance with methods known to those skilled in the art. The use of a gear-type melt pump is preferred because it provides control of flow rate and minimizes flow rate fluctuations, resulting in more uniform sheet thickness. In addition, the use of a melt pump may reduce degradation of the molten resin by reducing the shear heating of the polymer. Temperatures for the melt pump are 15 determined by the plastic resin used, and are similar to those used in standard extrusion processes, typically between 50 and 200 ~C above the Tg of the resin. More than one delivery means may be used, for example, for the production of wide sheets. In the present invention, the delivery means should provide molten polymer to the inlet of the overflow die in the range of 50 to 70,000 kPa, preferably 300 to 7000 20 kPa, and most preferably 1000 to 3500 kPa.
The overflow die is used to form a sheet from the molten plastic resin. The die includes a metering arrangement and an overflow surface with converging sides which in cross section culminate in an apex. The die in length~ ise fashion can be substantially linear, curved, oval or circular. The die height to ~vidth ratio should ~5 generally be in the range of 1:1 to 10:l, preferably 2:1 to 5:1, and most preferably 2.5:1 to ~:1. The length (or circumfertnct) to height ratio sho~ 1 generally be at least 1:2, preferably at least 2:1, and most preftrably at least 3:1.
The meterinc7 arrangement portion of the o~ erflo~vv die consists of flow distribution elements such as, for example, holes, slot, "coathanger" arrangement or 30 combinations thereof, which control the flo~vv distribution of the molten resin across the die, thereby controlling the sheet thickness profile. Examples of such metering arrangements are illustrated in Figures 5-7. Other metering arrangements may be used as known to those skilled in the art. A slot arrangement is preferred. The length of the die will depend upon the width of the sheet to be made, but the ratio of the mean slot gap (mean width of the slot 23) to mean conduit diameter (mean diameter of the conduit 22) should generally be at least 1:5, preferably at least 1:10, and most preferably at least 1:20. For sheets having a finished thickness of 1 mm or less, a substantially constant slot width across the die is preferred. For greater thicknesses, a tapered slot is preferred wherein the slot is thinner at the feed end, 10 and thicker at the opposing end. If a wide sheet is desired, cond~lit openings 21 and 21' (see Figure 6) can be located at both ends of the die, and it is possible to have the slot 23 tapered at both ends.
The overflow surface is formed by the exterior of the die 20 and consists of a pair of die lips, 40 and 41, which connect with the metering arrangement and direct 15 the molten polymer to the converging sides, 24 and 25. The converging sides direct the melt flow to the apex 26, where the melt web exits from the die. Although the overflow surface can be textured or smooth, it is preferably smooth. Moreover, the overflow surface is preferably highly polished to minimize variations and defects in the sheet. The overflow surface may be treated with a coating (for example, 20 electroplating or other depositing techniques) to improve die surface smoothness, provide corrosion resistance, or improve the flow properties over the die.
The material of construction of the die is important. Metals are preferred due to their high thermal conductivity, good corrosion resistance, high modulus, andabi]ity to be polished. However, other materials such as glass and ceramics can, in 25 principle, be used. It is preferred to uje stainless or tool ,grade steel.
If a non-planar sheet is desired, the die geometry may be modified accordingly, using methods known to those skilled in the art. For example, if a curved sheet is desired, the die can be curved along its longitudincll axis.
In general, it is desired to maintain the v iscosity of the molten plastic (for a 30 shear rate of 10 sec-') bet~veen 10 and 100,000 poise, preferably bet~veen ~0 and -CA 022~2947 1998-11-0~

10,000 poise, and most preferably between 100 and 5000 poise. In addition, the melt flow rate per unit die length (flow rate divided by the length) is typically in the range of 1.0 x 10-3 to 10 g/s/cm, preferably 1.0 x 10-~ to 1.0 g/s/cm, and most preferably 2.0 x 10-2 to 2.0 x 10-l g/s/cm. The viscosity can be controlled by ~arying 5 the temperature. Depending on the die design, the temperature control may be more or less important. The more even the temperature across the die, the more even the thickness of the sheet. Thickness variation resulting from uneven temperature distribution down the length of the die can be minimized by changingthe design of the slot or other metering arrangement. Temperature control may be10 accomplished, for example, by one or more of the following: electric cartridge heaters, infrared lamp heaters, heated oil (or other heat transfer fluid), heat pipes, or microwave heaters. Heated oil or other heat transfer fluids are preferred because the temperature may be controlled by a thermostat and uniformity of temperature may be readily accomplished. The die is preferably housed within a partially enclosed 15 area in order to minimize temperature fluctuations.
It is preferred, but not essential, that the molten plastic flows in a downward direction after passing over the die, since the downward flow is affected by gravity.
The rate of flow is determined by a combination of the effect of gra~ity, and the tension applied by the takeoff means. By conducting the plastic flow in a downward 20 direction over the die, gravity acts in the same direction as the sheet flow, thereby reducing the tension needed in the takeoff means and impro~ ing sheet quality. The molten plastic after passing through the die is in a form known as a "web."
The takeoff means transports the molten plastic web from the die at a controlled speed and allows the web to cool. The takeoff means may be, for 25 example, rollers or a "tank tread" arrangement, whereby only the outer ed,,es of the sheet come into contact with the takeoff means. A "tank tread" arrangement is preferred, as this maximizes the smoothness of the sheet surface. A tank tread arrangement is illustrated as part of the apparatus of Figures 1 and ~ as 31, 3~, 33 and 34.

.

The takeoff means controls the speed at which the plastic sheet is produced, which at a given polymer flow rate determines the thickness of the sheet; therefore, control of the speed of the takeoff means is quite important. The takeoff means also supports the weight of the sheet, thereby maintaining consistent sheet width and5 thickness. It is desirable to position the takeoff means as close as possible to the die so that the amount of molten resin that is unsupported is minimized. The distance from the apex of the die to the takeoff system (e.g., the nip area at the top of the tank tread arrangement) is typically <25 cm, preferably <10 cm, and most preferably <5 cm.
The sheet takeoff speed will vary depending on the type of sheet desired, and the thickness. For example, for a sheet having 0.4 mm thickness, the sheet takeoff speed will generally be in the range of 10 to 1000 cm/min, preferably 20 to 200 cm/min, and most preferably 50 to 100 cm/min; whereas for a sheet having 1 mm thickness, the takeoff speed will generally be in the range of 5 to 500 cm/min, preferably 10 to 100 cm/min, and most preferably 25 to 50 cm/min. In like fashion, the residence time during cooling in the takeoff system before bending will vary.
For example, for a sheet having 0.4 mm thickness, the residence time before bending will generally be >10 sec, preferably >1 min, and most preferably >2 min; whereas for a sheet having 0.2 mm thickness, the residence time before bending will generally be 25 sec, preferably 230 sec, and most preferably 21 min.
The plastic sheet may be allowed to cool by natural convection during transport by the takeoff system, or by forced convection. Nahlral convection consists of passive cooling of the sheet during passage through air or a fluid bath.
Forced convection is accomplished by pumping or blowing a heat transfer ~1~lid 25 along or against the sheet to enhance heat transfer. Forced gas con~ection utilizing a blower and plen~lm arrangement is preferred for minimizing sheet ripples and s~lrface marks. It is preferred to use a clean fl~lid (free from particulates) for cooling the sheet to prevent surface contamination or defects. For example, HEPA filtersmay be used with air or gas cooling for this purpose. Any ~ id or combinations of 30 fluids can be ~l~td for sheet cooling, provided that the fluid ~Ised is not detrimental to the plastic material being processed. E~amples of usef-ll cooling fluids are: air, nitrogen, water, oils, and glycols. It is possible to combine the cooling process with a coating process by using a suitable coolant which acts as a coating and is deposited as a film on the plastic sheet as it leaves the cooling bath.
It will be recognized by those skilled in the art that a variety of optional 5 equipment may be used following the takeoff means. Examples of optional equipment include conventional film handling equipment such as film winders, edge cutters, sheet cutters, and packaging equipment. In addition, other downstream devices can be utilized, for example, forming equipment, coating equipment, decorating equipment, and laminating equipment.
The process of the present invention may be used with any suitable plastic resin, and is preferably used with thermoplastic resins. A thermoplastic resin is a polymeric resin which reversibly softens when exposed to heat and hardens upon cooling. Thermoplastic resins may be linear or branched polymers that are not substantially cross-linked. It is preferred that the thermoplastic resins useful in the 15 process of the present invention have virtually no crosslinking and have thermal stability (for residence time of up to 10 min or more) at melt processing temperahlres (i.e., having a viscosity on the order of 10~ poise). Examples of thermoplastic resins for which the process of the present invention is useful include but are not limited to:
homopolymers or copolymers of acrylic acid, methacrylic acid and their esters, 20 including but not limited to copolymers formed with styrene and its derivatives, N-alkyl maleimides, acrylonitrile, and vinyl acetate; phenoxy ethers; polyphenylene o~cide resins, epoxy resins; cellulosic resins; vinyl polymers such as polyvinylchloride ("PVC"); fluoropolymers such as fluorinated ethylene-propylene and poly(vinylidene fluoride); polycarbonates; polystyrenes; polyolefins such as 25 polyetllylene, polypropylene, poly-~-methylpentene-l, and including cyclic polyolefins; polysulfones; polyether sulfones; polyether ketones; polyether imides, polyphenyleIle sulfides; polyarylene ester resins; polyesters; homopolymers or copolymers of N-H and/or N-alkyl glutarimide; acrylonitrile-butadiene-styrene resins ("ABS"); ~tyrene-acrylonitrile resins ("SAN"); styrene-maleic anhydride resins 30 ("SMA"); imidized SMA; and polyamides ("Nylons"). ~lixtures of thermoplastic resins may also be used. Particularly useful thermoplastic resin mixtures include, for example: SAN-polyglutarimide, polycarbonate-polyester, PMMA-poly(vinylidene fluoride) and polystyrene-poly(phenylene oxide). Preferred resins for use in theprocess and apparatus of the present invention are: polycarbonates; linear acrylic homopolymers and copolymers; cyclic polyolefins; and linear imidized acrylic homopolymers and copolymers such as those described in US 4,727,117 (Hallden-Abberton et al.) and US 4,246,374 (Kopchik).
The plastic resins useful in the present invention typically result from addition polymerization or condensation polymerization processes. Addition polymerization processes include bulk polymerization and solution or dispersion 10 polymerization in water or organic solvent media; such processes are ~vell known in the art and may incorporate cationic, anionic, or free radical initiation and propagation reactions. Condensation polymerization processes include bulk, solution and dispersion polymerization processes. Plastic resins formed by polymerization processes other than bulk polymerization may require subsequent 15 treatment in order to isolate the resin.
The following examples are presented to illustrate further various aspects of the present invention, but are not intended to limit the scope of the invention in any respect.

Example 1: Preparation of Acrylic Film This example illustrates the method of the present invention used to produce optical quality acrylic sheet.
PMMA resin having an average molecular weight of 110,000 was starve-fed into a 2 inch (5 cm) diameter single screw vented two-stage extruder having a 30:1 L:D ratio at a rate of 3.1 g/s u~ing a ~olumetric feeder. The extruder barrel had a 25 temperature profile from 20~ GC at the feed end to 27~ ~C at the discharge end. The resin ~vas devolatilized using a devolatilization vent operating at 720 - 750 mm Hg.
The screw was rotated at 30 rpm. A gear-type melt pump was used to pump the molten resin through a screen pack filter to a 12" (30 cm) long o~erflow die ha~ing a 1.27 cm diamei~. internal conduit and a series of 22 metering holes with a spacing of 30 1.27 cm. The diameter c f the metering holes increased from the feed end of the die . .

CA 022~2947 l998-ll-0 to the downstream end from 3.18 mm to 3.73 mm. The melt pump temperature was 274 ~C. The melt pump suction pressure was 2100 kPa. and the melt pump discharge pressure was approximately 4100 kPa. The overflow die was heated internally using three electric cartridge heaters and externally using three IR heating 5 units to a temperature of 274 ~C. The molten web formed at the apex of the die was conveyed using two pairs of tank treads, and cooled using cooled forced air which was applied using two air plenums.
The resultant sheet had average thickness of 0.325 mm, surface roughness Rq of 14.6 nm and an optical retardance of <5 nm.

Example 2: Preparation of Imidized Acrylic Sheet This example illustrates the method of the present invention used to produce optical quality imidized acrylic sheet.
A capped imidized acrylic resin having an weight average molecular weight of 97,500 and a glass transition temperature of about 180 ~C was starve-fed into a 2 inch (5 cm) diameter single screw vented two-stage extruder having a 30:1 L:D ratio at a rate of 2.5g/s using a gravimetric feeder. The extruder barrel had a temperature profile from 246 ~C at the feed end to 329 ~C at the discharge end. The resin was devolatilized using a devolatilization vent operating at 720 - 750 mm Hg. The screw was rotated at 30 rpm. A gear-type melt pump was used to pump the molten resin through a screen pack filter to a 25.5 inch (65 cm) long overflow die with a 1.588 cm diameter internal conduit and a 16 inch (40 cm) long slot tapering from 0.038 to 0.042 inch (0.965 to 1.067 mm). The melt pump temperature was 329 ~C. The melt pump suction pressure was approximately 4100 kPa. The melt pump dischclrge pressure was approximately 1650 kPa. The die was heated using a hot oil system (oil temperature = 3~3 ~C) via internal holes in the die, and the air aro~lnd the die was heated with a forced-air oven (temperature = 280 ~C). The molten web formed at the apex of the die was conveyed using two pairs of tank treads operating at a speed of 1.2 cm/s, and cooled by natural convection of room air.
A 200 mm x 200 mm piece was cut from the cooled sheet and tested. The resultant sheet had a thickness of 0.390 mm, with a variation of + 0.015 mm. The surface waviness Wy and Wq were <0.5 ~ and 0.18 ,u respectively, surface roughness Rq was 7.6 nm, and the optical retardance was <6 nm. The thermal shrinkage, measured at a temperature of 160 ~C, was 0.03~/O or less.

Example 3: Preparation of Polycarbonate Sheet This example illustrates the method of the present invention used to produce optical quality polycarbonate sheet.
Extrusion-grade polycarbonate resin (GE Lexan 101) was starve-fed into a 2 inch (5 cm) diameter single screw vented two-stage extruder having a 30:1 L:D ratio at a rate of 4.4 g/s using a gravimetric feeder. The extruder barrel had a 10 temperature profile from 232 ~C at the feed end to 315 ~C at the discharge end. The resin was devolatilized using a devolatilization vent operating at 720 - 750 mm Hg.
The screw was rotated at 30 rpm. A gear-type melt pump was used to pump the molten resin through a screen pack filter to a 37.5 inch (95 cm) long overflow die with a 1.905 cm diameter internal conduit and a 28 inch (71 cm) long slot tapering 15 from 0.038 to 0.045 inch (0.965 to 1.143 mm). The melt pump temperature was 315 ~C. The melt pump suction pressure was approximately 3400 kPa. The melt pump discharge pressure was approximately 1300 kPa. The die was heated using a hot oil system (oil temperature = 315 ~C) via internal holes in the die, and the air around the die was heated with a forced-air oven (temperature = 260 ~C). The molten web 20 formed at the apex of the die ~as conveyed using two pairs of tank treads operating at a speed of 1.2 cm/s, and cooled by natural convection of room air.
~ 400 mm x 400 mm piece was cut from the cooled sheet and tested. The resultant sheet had an average thickness of O.d~3 mm, with a variation of _ 0.02 mm in both the transverse and machine directions. Wy was <1 ~I, Wq ~vas 0.15 ~I, the 25 surface roughness Rq was <10 nm, and the average optical retardance was 20 nmwith a variatioll of 10 nm. Thermal shrinkage, measured at 130 ~C, was 0.02"~O.

Example 4: Preparation of Polycarbonate Film This example illustrates the method of the present invention used to produce optical quality polycarbonate film.

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CA 022S2947 l998-ll-OS

Extrusion-grade polycarbonate resin (GE Lexan 101) was starve-fed into a 2 inch (5 cm) diameter single screw vented two-stage extruder having a 30:1 L:D ratio at a rate of 2.5 g/s using a gravimetric feeder. The extruder barrel had a temperature profile from 232 ~C at the feed end to 315 ~C at the discharge end. The resin was devolatilized using a devolatilization vent operating at 720 - 750 mm Hg.
The screw was rotated at 30 rpm. A gear-type melt pump was used to p~lmp the molten resin through a screen pack filter to a 37.5 inch (95 cm) long overflow die with a 1.905 cm diameter internal conduit and a 28 inch (71 cm) l~ ng slot tapering from 0.038 to 0.045 inch (0.965 to 1.143 mm). The melt pump temperature was 315 10 ~C. The melt pump suction pressure was approximately 3400 kPa. The melt pump discharge pressure was approximately 1300 kPa. The die was heated using a hot oil system (oil temperature = 315 ~C) via internal holes in the die, and the air around the die was heated with a forced-air oven (temperature = 250 ~C). The molten web formed at the apex of the die was conveyed using two pairs of tank treads operating 15 at a speed of 3.1 cm/s, and cooled by natural convection of room air.
A 400 mm x 400 mm piece was cut from the cooled sheet and tested. The resultant film had an average thickness of 54 ~1, with variation + 4 ,u in both the transverse and machine directions, and an optical retardance of < 10 nm.

Test Methods The following test methods ~vere used to test the sheets made in the Examples above. It is understood in the art that these test methods are exemplary in nature, and that the results are not method-dependent.

Optical retardance:
The retardance of light at 632.8 nm wavelength ~vas determined in the 25 ~ollowing manner. A polarized laser beam (polarized at -45~ ~vith respect to the laboratory frame) w as passed through the plastic sheet, and then thro~lgh a photoelastic mod~llator (PEM) (Model PEM-90, Hinds InstrLlments, Inc.; ~Iillsboro, Oregon) oriented witll optical aYis set to 0~ in the lab frame. The PEM ~voltage ~vas set at 1/4 wa~ retardan~e (158.2 nm). The light then was passed through a second30 linear polarizer (polarization axis +45~) and intensity detected by a silicon diode CA 022S2947 l998-ll-OS

detector (Model PDA-50, ThorLabs Inc.; Newton, New Jersey). The PEM and detector were modulated, and the signal from the detector processed by a lock-inamplifier (Model 5210, E G & G Princeton Applied Research; Princeton, New Jersey).
The plastic sheet was rotated perpendicular to the laser beam to find the maximum 5 signal. The retardance was determined by comparing the maximum signal to that measured for a standard 1/4 ~vave plate.
Birefringence of a material can be obtained by dividing the optical retardance of a material by its thickness. For example, if the optical retardance for a 0.4 mm thick sheet of plastic is 4 nm, the birefringence of the materials is 0.00001. For optical 10 quality plastic sheet made by the method of the present invention, birefringence of a material is considered to be low if it is <0.0002, preferably <0.00005, and mostpreferably <0.00001.

Sheet waviness:
Sheet waviness (Wy and Wq) ~vas measured using a stylus profiler 15 (Surfanalyzer System 5000, Federal Products; Providence, Rhode Island) with aprocedure similar to that of SEMI Standard D15-1296. The measured profile was digitally filtered with a Gaussian long wavelength cutoff (8 mm). Wy is the difference bet~veen maximum and minimum values in an 20 mm sampling length, and Wq is the root mean square average deviation of the filtered profile from the 20 mean line calculated over 8 mm, and averaged over a 80 mm eval~lation length. For optical q~lality sheet produced by the method of the present invention, Wy should be <1.0 ,u, preferably <0.2 ,u, and most preferably <0.05 ,u.

Sheet roughness:
Sheet roughness (Rq) was measured using a styl~ls profiler (Dektak 3-30, 25 Veeco/Sloan; Santa Barbara CA) w ith a procedure similar to that of SE~vfl Standard D7-94. The measured profile ~- as digitally filtered ~,vith a Gaussian long ~ avelengtl c~1toff (0.0~ mm) and a short ~vavelengtll cutoff (0.0025 mm). The evaluation length was 0.4 mm. The roughness parameter (Rq) is the root mean square average deviation of the filtered profile from a mean line. The average value frorm three 30 different measurements ~vas reported. For optical quality sheet produced by the method of the present invention, Rq should be '50 nm, preferably <10 nm, and most preferably <5 nm.

Shrinkage:
Shrinkage was determined by directly measuring the sample length before 5 and after heat treatment. Multiple measurements were made to determine the length of a dry piece of plastic. The accuracy of the measurement was 0.005O/O. The sample was heated to a set temperature below its T~ for 4 hours. Upon cooling toroom temperature, the length was again determined by multiple measurements.
The percentage change in length before and after the heating cycle w as reported as 10 the shrinkage. For optical quality sheet produced by the method of the present invention, the shrinkage should be <0.05%, preferably '0.02%, and most preferably <0.005%.

,

Claims

WHAT IS CLAIMED IS:

1. A method for producing high quality plastic sheet, comprising the steps of:
a) providing molten plastic resin;
b) directing the molten plastic resin to an overflow die having an inlet and an outlet;
c) shaping the molten plastic resin into a molten web using said overflow die;
d) guiding said molten web away from said overflow die; and e) cooling said molten web to form a solid sheet.

2. The method of claim 1 wherein the source of molten plastic resin is provided by extrusion.

3. The method of claim 1 wherein the resin is a thermoplastic resin selected from the group consisting of: homopolymers or copolymers of acrylic acid, methacrylicacid and their esters; phenoxy ethers; polyphenylene oxide resins, epoxy resins;cellulosic resins; vinyl polymers; fluoropolymers; polycarbonates; polystyrenes;polyolefins; polysulfones; polyether sulfones; polyether ketones; polyether imides, polyphenylene sulfides; polyarylene ester resins; polyesters; homopolymers or copolymers of N-H and/or N-alkyl glutarimide; acrylonitrile-butadiene-styrene resins; styrene-acrylonitrile resins; styrene-maleic anhydride resins; imidized styrene-maleic anhydride; polyamides; and mixtures thereof.

4. The method of claim 3, wherein the thermoplastic resin is selected from the group consisting of: polycarbonates; linear acrylic homopolymers and copolymers;cyclic polyolefins; and linear imidized acrylic homopolymers and copolymers.

5. The method of claim 3 wherein the copolymers of acrylic acid, methacrylic acid and their esters comprise acrylic acid, methacrylic acid or their esters copolymerized with styrene and its derivatives, N-alkyl maleimides, acrylonitrile, or vinyl acetate.

6. The method of claim 3, wherein the thermoplastic resin is a mixture of resinsselected from the group consisting of: styrene-acrylonitrile-polyglutarimide, polycarbonate-polyester, polymethylmethacrylate-poly(vinylidene fluoride) and polystyrene-poly(phenylene oxide).

7. An apparatus for producing high quality plastic sheet, comprising:
a) a source for providing molten plastic resin;
b) an overflow die having a length and a width, comprising:
a substantially egg-shaped cross-section culminating in an apex, a conduit opening, and a metering arrangement connected with said conduit opening, wherein the molten plastic resin flows into the die through the conduit opening, out of the die through the metering arrangement, and around the sides of the die to form a molten web at said apex;
c) means for delivering said molten plastic resin from said source to said overflow die; and d) guidance means for guiding said molten web away from said overflow die.

8. The apparatus of claim 7, further comprising delivery means to deliver said molten plastic resin from said source to said overflow die.

9. The apparatus of claim 8, wherein the delivery means comprises a gear-type melt pump.

10. The apparatus of claim 7, further comprising a filter to remove foreign particles, said filter is located between said source and said overflow die.

11. The apparatus of claim 7, further comprising cooling means in close proximity to said guidance means.

12. An optical quality plastic sheet produced by the method of claim 1.

13. An overflow die useful in forming high quality plastic sheet, said overflow die having an interior and an exterior and comprising:
a) an overflow surface formed by the exterior of the die and comprising a pair of die lips;
b) two exterior sides connected to said overflow surface;
c) an apex formed by the confluence of said two exterior sides and located in substantial opposition to said overflow surface, wherein the die has a substantially egg-shaped cross-section;
d) a conduit opening from the exterior to the interior, and e) a metering arrangement located in the interior of the die, wherein such metering arrangement is connected with said conduit opening and said overflow surface.

14. An optical quality plastic sheet having a thickness of 25 mm or less with a thickness variation over a 400 mm length of 10% or less, shrinkage of ~0.05%, a waviness value Wy ~1.0µ and a roughness value Rq ~ 50 nm.

15. The plastic sheet of claim 14, wherein the thickness is 10 - 5000 µ.

16. The plastic sheet of claim 15, wherein the thickness is 50 -1000 µ.

17. The plastic sheet of claim 14, wherein the birefringence is 0.0002 or less.

18. The plastic sheet of claim 17, wherein the birefringence is 0.00005 or less.