U.S. patent application number 10/189677 was filed with the patent office on 2003-11-20 for polycarbonate films prepared by coating methods.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Bermel, Marcus S..
Application Number | 20030215581 10/189677 |
Document ID | / |
Family ID | 29423126 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030215581 |
Kind Code |
A1 |
Bermel, Marcus S. |
November 20, 2003 |
Polycarbonate films prepared by coating methods
Abstract
A method of film fabrication is taught that uses a coating and
drying apparatus to fabricate resin films suitable for optical
applications. In particular, polycarbonate films are prepared by
simultaneous application of multiple liquid layers to a moving
carrier substrate. After solvent removal, the polycarbonate films
are peeled from the sacrificial carrier substrate. Polycarbonate
films prepared by the current invention exhibit good dimensional
stability and low birefringence.
Inventors: |
Bermel, Marcus S.;
(Pittsford, NY) |
Correspondence
Address: |
Thomas H. Close
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
29423126 |
Appl. No.: |
10/189677 |
Filed: |
July 3, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60381931 |
May 20, 2002 |
|
|
|
Current U.S.
Class: |
428/1.31 |
Current CPC
Class: |
B29D 7/01 20130101; B29C
48/21 20190201; B29K 2705/02 20130101; B29C 48/154 20190201; B29K
2001/12 20130101; B29K 2033/08 20130101; G02B 5/3083 20130101; B29C
41/32 20130101; B29C 48/08 20190201; B29K 2081/06 20130101; B29L
2009/00 20130101; B32B 2307/40 20130101; Y10T 428/24802 20150115;
B29C 41/12 20130101; B29C 48/0014 20190201; B29C 48/35 20190201;
B29K 2995/0026 20130101; B29K 2033/12 20130101; B29K 2069/00
20130101; C08J 5/18 20130101; B29C 48/304 20190201; B32B 27/30
20130101; B32B 2307/734 20130101; B29K 2001/00 20130101; B29K
2067/00 20130101; B29K 2027/00 20130101; B29L 2011/00 20130101;
C09K 2323/031 20200801; C08J 2369/00 20130101; B29C 41/26 20130101;
B29K 2029/00 20130101 |
Class at
Publication: |
428/1.31 |
International
Class: |
C09K 019/00 |
Claims
What is claimed is:
1. A coating method for forming a polycarbonate film comprising the
steps of: (a) applying a liquid polycarbonate/solvent mixture onto
a moving, discontinuous carrier substrate; and (b) drying the
liquid polycarbonate/solvent mixture to substantially remove the
solvent yielding a composite of a polycarbonate film adhered to the
discontinuous carrier substrate, the polycarbonate film being
releasably adhered to the discontinuous carrier substrate thereby
allowing the polycarbonate film to be peeled from the discontinuous
carrier substrate.
2. A coating method as recited in claim 1 wherein: the liquid
polycarbonate/solvent mixture is applied using slide bead coating
die with a multi-layer composite being formed on a slide surface
thereof.
3. A coating method as recited in claim 2 wherein: the viscosity of
each liquid layer of the multi-layer composite is less than 5000
cp.
4. A coating method as recited in claim 1 wherein: the carrier
substrate is polyethylene terephthalate.
5. A coating method as recited in claim 1 wherein: the carrier
substrate has a subbing layer applied to the coated surface.
6. A coating method as recited in claim 2 wherein: an uppermost
layer of the multi-layer composite contains a surfactant.
7. A coating method as recited in claim 1 wherein: the first drying
section is operated at a temperature between 25 and 95.degree.
C.
8. A coating method as recited in claim 1 further comprising the
step of: winding the composite into at least one roll before the
polycarbonate sheet is peeled from the discontinuous carrier
substrate.
9. A coating method as recited in claim 1 further comprising the
steps of: (a) separating the polycarbonate film from the carrier
substrate immediately after the drying step; and (b) winding the
polycarbonate film into at least one roll.
10. A coating method as recited in claim 8 further comprising the
step of: (a) unwinding at least a portion of at least one roll of
the composite; and (b) separating the polycarbonate film from the
carrier substrate.
11. A coating method as recited in claim 1 wherein: the
polycarbonate film is adhered to the carrier substrate with an
adhesive strength of less than about 250 N/m.
12. A coating method as recited in claim 9 further comprising the
step of: reducing residual solvent in the polycarbonate film to
less than 10% by weight prior to the separating step.
13. A coating method as recited in claim 10 further comprising the
step of: reducing residual solvent in the polycarbonate film to
less than 10% by weight prior to the separating step.
14. A coating method as recited in claim 8 further comprising the
step of: delivering the composite to a user of the polycarbonate
film, the carrier substrate acting as a protective support for the
polycarbonate film prior to the polycarbonate film being separated
from the substrate carrier.
15. A coating method as recited in claim 1 further comprising the
step of: including a plasticizer in the liquid
polycarbonate/solvent mixture.
16. A coating method as recited in claim 1 wherein: the
polycarbonate film has an in-plane retardation of less than 20
nm.
17. A coating method as recited in claim 1 wherein: the
polycarbonate film has an in-plane retardation of less than 10
nm.
18. A coating method as recited in claim 1 wherein: the
polycarbonate film has an in-plane retardation of less than 5.0
nm.
19. A coating method as recited in claim 1 further comprising the
step of: applying at least one additional polycarbonate layer to
the composite after the drying step.
20. A coating method as recited in claim 1 wherein: the
polycarbonate film has a thickness in the range of 1 to 500
.mu.m.
21. A composite film comprising: a polycarbonate film coated on a
discontinuous carrier substrate, the polycarbonate film having a
thickness in the range of from about 1 to about 500 .mu.m, the
polycarbonate film having an in-plane retardation that is less than
20 nm, the polycarbonate film being adhered to the carrier
substrate with an adhesive strength of less than about 250 N/m.
22. A composite film as recited in claim 21 wherein: the
polycarbonate film has an in-plane retardation that is less than 10
nm.
23. A composite film as recited in claim 21 wherein: the
polycarbonate film has an in-plane retardation that is less than
5.0 nm.
24. A composite film as recited in claim 21 wherein: the
polycarbonate film is adhered to the carrier substrate with an
adhesive strength of at least about 0.3 N/m.
25. A composite film as recited in claim 21 wherein: the
polycarbonate film is peelable from the carrier substrate.
26. A composite film as recited in claim 21 wherein: the
polycarbonate film is a multi-layer composite.
27. A composite film as recited in claim 26 wherein: at least a top
layer of the multi-layer composite includes a surfactant
therein.
28. A composite film as recited in claim 21 wherein: a plasticizer
is incorporated in the polycarbonate film.
29. A polycarbonate film made by the method of claim 1 wherein: the
in-plane retardation is less than 20 nm.
30. A polycarbonate film comprising: a layer of polycarbonate
formed by a coating operation, the polycarbonate film having a
thickness in the range of from about 1 to about 500 .mu.m, the
polycarbonate film having an in-plane retardation that is less than
20 nm.
31. A polycarbonate film as recited in claim 30 further comprising:
a plasticizer incorporated in the polycarbonate film.
32. A polycarbonate film as recited in claim 30 wherein: the
polycarbonate film having an in-plane retardation that is less than
10 nm.
33. A polycarbonate film as recited in claim 30 wherein: the
polycarbonate film having an in-plane retardation that is less than
5.0 nm.
34. A coating method as recited in claim 1 further comprising the
step of: using the polycarbonate film to form a light
polarizer.
35. A display device including a polycarbonate film therein made by
the method of claim 1.
36. A polycarbonate film comprising: a layer of polycarbonate
formed by a coating operation, the polycarbonate film having an
in-plane retardation that is less than about 20 nm.
37. A polycarbonate film as recited in claim 36 wherein: the
polycarbonate film having an in-plane retardation that is less than
10 nm.
38. A polycarbonate film as recited in claim 36 wherein: the
polycarbonate film having an in-plane retardation that is less than
5.0 nm.
39. A composite film as recited in claim 26 wherein: only a top
layer of the multi-layer composite includes a surfactant
therein.
40. A composite film as recited in claim 26 wherein: at least a top
layer of the multi-layer composite includes a fluorinated
surfactant therein.
41. A coating method as recited in claim 2 wherein: an uppermost
layer of the multi-layer composite contains a fluorinated
surfactant.
42. A coating method as recited in claim 2 wherein: an uppermost
layer of the multi-layer composite contains a polysiloxane
surfactant.
43. An electronic display device having view screen comprising: a
polycarbonate film having an in-plane retardation that is less than
about 10 nm.
44. An electronic display device as recited in claim 43 wherein:
the polycarbonate film having an in-plane retardation that is less
than about 5 nm.
45. An polycarbonate film comprising: a coated layer of
polycarbonate having an in-plane retardation that is less than
about 20 nm.
46. A coating method as recited in claim 7 wherein: the drying step
is initially performed at a temperature in the range of from about
25.degree. C. to less than 95.degree. C.
47. A coating method as recited in claim 7 wherein: the drying step
is initially performed at a temperature in the range of from about
30.degree. C. to about 60.degree. C.
48. A coating method as recited in claim 7 wherein: the drying step
is initially performed at a temperature in the range of from about
30.degree. C. to about 50.degree. C.
49. An electronic display device as recited in claim 43 wherein:
the polycarbonate film has a light transmittance of at least about
85 percent and a haze value of less than about 1.0 percent.
50. A polycarbonate film as recited in claim 45 wherein: the coated
layer of polycarbonate has a light transmittance of at least about
85 percent and a haze value of less than about 1.0 percent.
51. A coating method as recited in claim 1 wherein: the optical
resin film has a light transmittance of at least about 85 percent
and a haze value of less than about 1.0 percent.
52. A coating method as recited in claim 1 wherein: the optical
resin film has an average surface roughness of less than about 100
nm.
53. A coating method as recited in claim 1 wherein: the optical
resin film has an average surface roughness of less than about 50
nm.
54. A coating method as recited in claim 1 wherein: the optical
resin film has an average surface roughness of not more than about
1 nm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a 111A Application of Provisional Application,
Serial No. 60/381,931, filed on May 20, 2002.
FIELD OF THE INVENTION
[0002] This invention relates generally to methods for
manufacturing resin films and, more particularly, to an improved
method for the manufacture of optical films, and most particularly,
to the manufacture of polycarbonate films used as substrates,
polarizer plates, compensation plates, and protective covers in
optical devices such as light filters, liquid crystal displays and
other electronic displays.
BACKGROUND OF THE INVENTION
[0003] Polycarbonates (PC) are used to produce films that are noted
for their transparency, mechanical strength, and thermal stability.
As a result, polycarbonate films have a number of optical
applications. In particular, transparent polycarbonate films have
recently been suggested for use as protective covers for light
polarizers, as polarizer sheets, as compensation plates, and as
electrode substrates in optical displays. In this regard,
polycarbonate films are intended to replace glass and less stable
polymeric films to produce lightweight, flexible optical display
screens. These display screens may be utilized in liquid crystal
displays, OLED (organic light emitting diode displays, and in other
electronic displays found in, for example, personal computers,
televisions, cell phones, and instrument panels.
[0004] Polymers of the polycarbonate type are available in a
variety of molecular weights as well as in numerous permutations
around the basic molecular structure. Common to all polycarbonates
are the carbonate linkages and usually the presence of stabilizing
phenyl groups (Ph) in the polymer backbone. In terms of
commercially significant polycarbonates, the condensation product
of the dihydridic phenol, 2,2-bis-(4-hydroxyphenyl)-propane
(Bisphenol-A), with a carobonate precursor such as phosogene forms
a polymer having recurring units of
-O-Ph-C(CH.sub.3).sub.2-Ph-O-CO-. Polycarbonates of the Bisphenol-A
type are both readily available and relatively inexpensive.
[0005] In general, resin films are prepared either by melt
extrusion methods or by casting methods. Melt extrusion methods
involve heating the resin until molten (approximate viscosity on
the order of 100,000 cp), and then applying the hot molten polymer
to a highly polished metal band or drum with an extrusion die,
cooling the film, and finally peeling the film from the metal
support. For many reasons, however, films prepared by melt
extrusion are generally not suitable for optical applications.
Principal among these is the fact that melt extruded films exhibit
a high degree of optical birefringence. In the case of
polycarbonate polymer, there is the additional problem of melting
the polymer. Polycarbonate films have exceptionally high melting
temperatures of approximately 230.degree. C. and may require very
high processing temperature in excess of 300.degree. C. At these
high temperatures, polycarbonates are vulnerable to hydrolysis and
discoloration. For these reasons, melt extrusion methods are
generally not suitable for fabricating many resin films, including
polycarbonate films intended for optical applications. Rather,
casting methods are generally used to produce these films.
[0006] Resin films for optical applications are manufactured almost
exclusively by casting methods. Casting methods involve first
dissolving the polymer in an appropriate solvent to form a dope
having a high viscosity on the order of 50,000 cp, and then
applying the viscous dope to a continuous, highly polished metal
band or drum through an extrusion die, partially drying the wet
film, peeling the partially dried film from the metal support, and
conveying the partially dried film through an oven to more
completely remove solvent from the film. Cast films typically have
a final dry thickness in the range of 40-200 .mu.m. In general,
thin films of less than 40 .mu.m are very difficult to produce by
casting methods due to the fragility of wet film during the peeling
and drying processes. Films having a thickness of greater than 200
.mu.m are also problematic to manufacture due to difficulties
associated with the removal of solvent in the final drying step.
Although the dissolution and drying steps of the casting method add
complexity and expense, cast films generally have better optical
properties when compared to films prepared by melt extrusion
methods and problems associated with high temperature processing
are avoided.
[0007] Examples of optical films prepared by casting methods
include: 1.) Polyvinyl alcohol sheets used to prepare light
polarizers as disclosed in U.S. Pat. No. 4,895,769 to Land and U.S.
Pat. No. 5,925,289 to Cael as well as more recent disclosures in
U.S. patent application Ser. No. 2001/0039319 A1 to Harita and U.S.
patent application Ser. No. 2002/001700 A1 to Sanefuji, 2.)
Cellulose triacetate sheets used for protective covers for light
polarizers as disclosed in U.S. Pat. No. 5,695,694 to Iwata, 3.)
Polycarbonate sheets used for protective covers for light
polarizers or for retardation plates as disclosed in U.S. Pat. No.
5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180,
both to Taketani, and 4.) Polysulfone sheets used for protective
covers for light polarizers or for retardation plates as disclosed
in U.S. Pat. Nos. 5,611,985 to Kobayashi and U.S. Pat. Nos.
5,759,449 and 5,958,305 both to Shiro.
[0008] The manufacture of polycarbonate films by the casting method
is confounded by abrasion, scratch and wrinkle artifacts that may
be created during conveyance of the film as described in U.S. Pat.
No. 6,222,003 to Hosoi. These artifacts are created while the film
passes over numerous conveyance rollers in the final drying and
winding operations of the casting method. To overcome these
problems, cast films may contain additives that act as lubricants,
may be laminated with a protective sheet, or may have the edges
knurled to minimize damage to the polycarbonate film.
Alternatively, U.S. Pat. No. 6,222,003B1 to Hosoi discloses a
method of creating small irregularities on the surface of the cast
polycarbonate film to minimize contact with the conveyance rollers
and hence minimize scratching and wrinkling. These small
irregularities are said to be formed by the use of non-solvents in
the casting dope along with special drying conditions. However,
lubricants are known to compromise film clarity. Moreover,
lamination and edge knurling devices are expensive and add
complexity to the casting process. Finally, the deliberate
formation of surface irregularities on a film to be used for
optical applications is complicated and undesirable. In general,
optical films are preferred to be very smooth with low haze.
[0009] Another disadvantage to the casting method is that cast
films have significant optical birefringence. Although films
prepared by casting methods have lower birefringence when compared
to films prepared by melt extrusion methods, birefringence remains
objectionably high. For example, cellulose triacetate films
prepared by casting methods exhibit in-plane retardation of 7
nanometers (nm) for light in the visible spectrum as disclosed in
U.S. Pat. No. 5,695,694 to Iwata. A polycarbonate film prepared by
the casting method is disclosed as having an in-plane retardation
of 17 nm in U.S. Pat. Nos. 5,478,518 and 5,561,180 both to
Taketani. U.S. patent application Ser. No. 2001/0039319 A1 to
Harita claims that color irregularities in stretched polyvinyl
alcohol sheets are reduced when the difference in retardation
between widthwise positions within the film is less than 5 nm in
the original unstretched film. For many applications of optical
films, low in-plane retardation values are desirable. In
particular, values of in-plane retardation of less than 10 nm are
preferred.
[0010] Birefringence in cast films arises from orientation of
polymers during the manufacturing operations. This molecular
orientation causes indices of refraction within the plane of the
film to be measurably different. In-plane birefringence is the
difference between these indices of refraction in perpendicular
directions within the plane of the film. The absolute value of
birefringence multiplied by the film thickness is defined as
in-plane retardation. Therefore, in-plane retardation is a measure
of molecular anisotropy within the plane of the film.
[0011] During the casting process, molecular orientation may arise
from a number of sources including shear of the dope in the die,
shear of the dope by the metal support during application, shear of
the partially dried film during the peeling step, and shear of the
free-standing film during conveyance through the final drying step.
These shear forces orient the polymer molecules and ultimately give
rise to undesirably high birefringence or retardation values. To
minimize shear and obtain the lowest birefringence films, casting
processes are typically operated at very low line speeds of 1-15
m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line
speeds generally produce the highest quality films.
[0012] Low birefringence polycarbonate films are exceptionally
difficult to manufacture. This is due to the fact that
polycarbonates are rigid polymers and readily align or orient when
exposed to shear forces in the casting process. While polycarbonate
films have been prepared with low in-plane retardation using a
batch casting method, continuously cast polycarbonate films have
objectionably high retardation. For example, although batch-cast
polycarbonate films have been described with in-plane retardation
values of 4-8 nm, continuous-cast films are considerably higher at
17 nm as disclosed in U.S. Pat. Nos. 5,478,518 and 5,561,180 both
to Taketani. Batch casting is primarily a laboratory method for
preparing short experimental samples for physical analysis and is
not suitable for large-scale manufacture of polycarbonate
films.
[0013] Another drawback to the casting method is the inability to
accurately apply multiple layers. As noted in U.S. Pat. No.
5,256,357 to Hayward, conventional multi-slot casting dies create
unacceptably non-uniform films. In particular, line and streak
non-uniformity is greater than 5% with prior art devices.
Acceptable two layer films may be prepared by employing special die
lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but
the die designs are complex and may be impractical for applying
more than two layers simultaneously.
[0014] Another drawback to the casting method is the restrictions
on the viscosity of the dope. In casting practice, the viscosity of
dope is on the order of 50,000 cp. For example, U.S. Pat. No.
5,256,357 to Hayward describes practical casting examples using
dopes with a viscosity of 100,000 cp. In general, cast films
prepared with lower viscosity dopes are known to produce
non-uniform films as noted for example in U.S. Pat. No. 5,695,694
to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity
dopes used to prepare casting samples are approximately 10,000 cp.
At these high viscosity values, however, casting dopes are
difficult to filter and degas. While fibers and larger debris may
be removed, softer materials such as polymer slugs are more
difficult to filter at the high pressures found in dope delivery
systems. Particulate and bubble artifacts create conspicuous
inclusion defects as well as streaks and may create substantial
waste.
[0015] In addition, the casting method can be relatively inflexible
with respect to product changes. Because casting requires high
viscosity dopes, changing product formulations requires extensive
down time for cleaning delivery systems to eliminate the
possibility of contamination. Particularly problematic are
formulation changes involving incompatible polymers and solvents.
In fact, formulation changes are so time consuming and expensive
with the casting method that most production machines are dedicated
exclusively to producing only one film type.
[0016] Finally, cast films may exhibit undesirable cockle or
wrinkles. Thinner films are especially vulnerable to dimensional
artifacts either during the peeling and drying steps of the casting
process or during subsequent handling of the film. In particular,
the preparation of composite optical plates from resin films
requires a lamination process involving application of adhesives,
pressure, and high temperatures. Very thin films are difficult to
handle during this lamination process without wrinkling. In
addition, many cast films may naturally become distorted over time
due to the effects of moisture. For optical films, good dimensional
stability is necessary during storage as well as during subsequent
fabrication of composite optical plates.
SUMMARY OF THE INVENTION
[0017] It is therefore an object of the present invention to
overcome the limitations of prior art casting methods and provide a
new coating method for preparing amorphous polycarbonate films
having very low in-plane birefringence.
[0018] It is a further object of the present invention to provide a
new method of producing highly uniform polycarbonate films over a
broad range of dry thicknesses.
[0019] Yet another object of the present invention is to provide a
method of preparing polycarbonate films by simultaneously applying
multiple layers to a moving substrate.
[0020] Still another object of the present invention is to provide
a new method of preparing polycarbonate films with improved
dimensional stability and handling ability by temporarily adhering
the polycarbonate film to a supporting carrier substrate at least
until it is substantially dry and then subsequently separating the
carrier substrate from the polycarbonate film.
[0021] A further object of the present invention is to overcome the
limitations of the prior art casting method and define a new
coating method for preparing resin films without the need for
co-solvents, lubricants, or protective laminates as converting aids
to minimize scratch and abrasion artifacts.
[0022] Briefly stated, the foregoing and numerous other features,
objects and advantages of the present invention will become readily
apparent upon review of the detailed description, claims and
drawings set forth herein. These features, objects and advantages
are accomplished by applying a low viscosity fluid containing
polycarbonate resin onto a moving carrier substrate by a coating
method. The polycarbonate film is not separated from the carrier
substrate until the coated film is substantially dry (.ltoreq.10%
residual solvent by weight). In fact, the composite structure of
polycarbonate film and carrier substrate may be wound into rolls
and stored until needed. Thus, the carrier substrate cradles the
polycarbonate film and protects against shearing forces during
conveyance through the drying process. Moreover, because the
polycarbonate film is dry and solid when it is finally peeled from
the carrier substrate, there is no shear or orientation of polymer
within the film due to the peeling process. As a result,
polycarbonate resin films prepared by the current invention are
remarkably amorphous and exhibit very low in-plane
birefringence.
[0023] Polycarbonate films can be made with the method of the
present invention having a thickness of about 1 to 500 .mu.m. Very
thin polycarbonate films of less than 40 microns can be easily
manufactured at line speeds not possible with prior art methods.
The fabrication of very thin films is facilitated by a carrier
substrate that supports the wet film through the drying process and
eliminates the need to peel the film from a metal band or drum
prior to a final drying step as required in the casting methods
described in prior art. Rather, the polycarbonate film is
substantially, if not completely, dried before separation from the
carrier substrate. In all cases, dried polycarbonate films have a
residual solvent content of less than 10% by weight. In a preferred
embodiment of the present invention, the residual solvent content
is less than 5%, and most preferably less than 1%. Thus, the
present invention readily allows for preparation of very delicate
thin films not possible with the prior art casting method. In
addition, thick films of greater than 40 .mu.m may also be prepared
by the method of the present invention. To fabricate thicker films,
additional coatings may be applied over a film-substrate composite
either in a tandem operation or in an offline process without
comprising optical quality. In this way, the method of the present
invention overcomes the limitation of solvent removal during the
preparation of thicker films since the first applied film is dry
before application of a subsequent wet film. Thus, the present
invention allows for a broader range of final film thickness than
is possible with casting methods.
[0024] In the method of the present invention, polycarbonate films
are created by forming a single or, preferably, a multi-layer
composite on a slide surface of a coating hopper, the multi-layer
composite including a bottom layer of low viscosity, one or more
intermediate layers, and an optional top layer containing a
surfactant, flowing the multi-layer composite down the slide
surface and over a coating lip of the coating hopper, and applying
the multi-layer composite to a moving substrate. In particular, the
use of the method of the present invention is shown to allow for
application of several liquid layers having unique composition.
Coating aids and additives may be placed in specific layers to
improve film performance or improve manufacturing robustness. For
example, multi-layer application allows a surfactant to be placed
in the top spreading layer where needed rather than through out the
entire wet film. In another example, the concentration of
polycarbonate in the lowermost layer may be adjusted to achieve low
viscosity and facilitate high-speed application of the multi-layer
composite onto the carrier substrate. Therefore, the present
invention provides an advantageous method for the fabrication of
multiple layer composite films such as required for certain optical
elements or other similar elements.
[0025] Wrinkling and cockle artifacts are minimized with the method
of the present invention through the use of the carrier substrate.
By providing a stiff backing for the polycarbonate film, the
carrier substrate minimizes dimensional distortion of the
polycarbonate resin film. This is particularly advantageous for
handling and processing very thin films of less than about 40
microns. Moreover, scratches and abrasion artifacts that are known
to be created by the casting method are avoided with the method of
the present invention since the carrier substrate lies between the
polycarbonate film and potentially abrasive conveyance rollers
during all drying operations. Thus, the method of the present
invention does not require the use of co-solvents, lubricants or
protective laminates as converting aids as are needed in casting
operations to minimize abrasion artifacts. In addition, the
restraining nature of the carrier substrate also eliminates the
tendency of polycarbonate films to distort or cockle over time as a
result of changes in moisture levels. Thus, the method of the
current invention insures that polycarbonate films are
dimensionally stable during preparation and storage as well as
during final handling steps necessary for fabrication of optical
elements.
[0026] In the practice of the method of the present invention it is
preferred that the substrate be a discontinuous sheet such as
polyethylene terephthalate (PET). The PET carrier substrate may be
pretreated with a subbing layer or an electrical discharge device
to modify adhesion between the polycarbonate film and the PET
substrate. In particular, a subbing layer or electrical discharge
treatment may enhance the adhesion between the film and the
substrate, but still allow the film to be subsequently peeled away
from the substrate.
[0027] Although the present invention is discussed herein with
particular reference to a slide bead coating operation, those
skilled in the art will understand that the present invention can
be advantageously practiced with other coating operations. For
example, freestanding films having low in-plane retardation should
be achievable with single or multiple layer slot die coating
operations and single or multiple layer curtain coating operations.
Moreover, those skilled in the art will recognize that the present
invention can be advantageously practiced with alternative carrier
substrates. For example, peeling films having low in-plane
birefringence should be achievable with other resin supports [e.g.
polyethylene naphthalate (PEN), cellulose acetate, PET], paper
supports, resin laminated paper supports, and metal supports (e.g.
aluminum).
[0028] Practical applications of the present invention include the
preparation of polycarbonate sheets used for optical films,
laminate films, release films, photographic films, and packaging
films among others. In particular, polycarbonate sheets prepared by
the method of the present invention may be utilized as optical
films in the manufacture of electronic displays such as liquid
crystal displays. For example, liquid crystal displays are
comprised of a number of film elements including polarizer plates,
compensation plates and electrode substrates. Polarizer plates are
typically a multi-layer composite structure having dichroic film
(normally stretched polyvinyl alcohol treated with iodine) with
each surface adhered to a protective cover. The polycarbonate films
prepared by the method of the present invention are suitable as
protective covers for polarizer plates. The polycarbonate films
prepared by the method of the present invention are also suitable
for the manufacture of compensation plates and electrode
substrates.
[0029] The polycarbonate film produced with the method of the
present invention is an optical film. As produced, the
polycarbonate films made with the method of the present invention
will have a light transmittance of at least about 85 percent,
preferably at least about 90 percent, and most preferably, at least
about 95 percent. Further, as produced, the polycarbonate film will
have a haze value of less than 1.0 percent. In addition, the
polycarbonate films are smooth with a surface roughness average of
less than 100 nm and most preferably with a surface roughness of
less than 50 nm
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic of an exemplary coating and drying
apparatus that can be used in the practice of the method of the
present invention.
[0031] FIG. 2 is a schematic of an exemplary coating and drying
apparatus of FIG. 1 including a station where the polycarbonate web
separated from the substrate is separately wound.
[0032] FIG. 3 is a schematic of an exemplary multi-slot coating
apparatus that can be used in the practice of the method of the
present invention.
[0033] FIG. 4 shows a cross-sectional representation of a
single-layer polycarbonate film partially peeled from a carrier
substrate and formed by the method of the present invention.
[0034] FIG. 5 shows a cross-sectional representation of a
single-layer polycarbonate film partially peeled from a carrier
substrate and formed by the method of the present invention wherein
the carrier substrate has a subbing layer formed thereon.
[0035] FIG. 6 shows a cross-sectional representation of a
multi-layer polycarbonate film partially peeled from a carrier
substrate and formed by the method of the present invention.
[0036] FIG. 7 shows a cross-sectional representation of a
multi-layer polycarbonate film partially peeled from a carrier
substrate and formed by the method of the present invention wherein
the carrier substrate has a subbing layer formed thereon.
[0037] FIG. 8 is a schematic of a casting apparatus as used in
prior art to cast polycarbonate films.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Turning first to FIG. 1 there is shown a schematic of an
exemplary and well known coating and drying system 10 suitable for
practicing the method of the present invention. The coating and
drying system 10 is typically used to apply very thin films to a
moving substrate 12 and to subsequently remove solvent in a dryer
14. A single coating apparatus 16 is shown such that system 10 has
only one coating application point and only one dryer 14, but two
or three (even as many as six) additional coating application
points with corresponding drying sections are known in the
fabrication of composite thin films. The process of sequential
application and drying is known in the art as a tandem coating
operation.
[0039] Coating and drying apparatus 10 includes an unwinding
station 18 to feed the moving substrate 12 around a back-up roller
20 where the coating is applied by coating apparatus 16. The coated
web 22 then proceeds through the dryer 14. In the practice of the
method of the present invention the final dry film 24 comprising a
polycarbonate resin film on substrate 12 is wound into rolls at a
wind-up station 26.
[0040] As depicted, an exemplary four-layer coating is applied to
moving web 12. Coating liquid for each layer is held in respective
coating supply vessel 28, 30, 32, 34. The coating liquid is
delivered by pumps 36, 38, 40, 42 from the coating supply vessels
to the coating apparatus 16 conduits 44, 46, 48, 50, respectively.
In addition, coating and drying system 10 may also include
electrical discharge devices, such as corona or glow discharge
device 52, or polar charge assist device 54, to modify the
substrate 12 prior to application of the coating.
[0041] Turning next to FIG. 2 there is shown a schematic of the
same exemplary coating and drying system 10 depicted in FIG. 1 with
an alternative winding operation. Accordingly, the drawings are
numbered identically up to the winding operation. In the practice
of the method of the present invention, the dry film 24 comprising
a substrate (which may be a resin film, paper, resin coated paper
or metal) with a polycarbonate coating applied thereto is taken
between opposing rollers 56, 58. The polycarbonate film 60 is
peeled from substrate 12 with the polycarbonate film going to
winding station 62 and the substrate 12 going to winding station
64. In a preferred embodiment of the present invention,
polyethylene terephthalate (PET) is used as the substrate 12. The
substrate 12 may be pretreated with a subbing layer to enhance
adhesion of the coated film 60 to the substrate 12.
[0042] The coating apparatus 16 used to deliver coating fluids to
the moving substrate 12 may be a multi-layer applicator such as a
slide bead hopper, as taught for example in U.S. Pat. No. 2,761,791
to Russell, or a slide curtain hopper, as taught by U.S. Pat. No.
3,508,947 to Hughes. Alternatively, the coating apparatus 16 may be
a single layer applicator, such as a slot die hopper or a jet
hopper. In a preferred embodiment of the present invention, the
application device 16 is a multi-layer slide bead hopper.
[0043] As mentioned above, coating and drying system 10 includes a
dryer 14 that will typically be a drying oven to remove solvent
from the coated film. An exemplary dryer 14 used in the practice of
the method of the present invention includes a first drying section
66 followed by eight additional drying sections 68-82 capable of
independent control of temperature and air flow. Although dryer 14
is shown as having nine independent drying sections, drying ovens
with fewer compartments are well known and may be used to practice
the method of the present invention. In a preferred embodiment of
the present invention the dryer 14 has at least two independent
drying zones or sections.
[0044] Preferably, each of drying sections 68-82 have independent
temperature and airflow controls. In each section, temperature may
be adjusted between 5.degree. C. and 150.degree. C. To minimize
drying defects from case hardening or skinning-over of the wet
polycarbonate film, optimum drying rates are needed in the early
sections of dryer 14. There are a number of artifacts created when
temperatures in the early drying zones are inappropriate. For
example, fogging or blush of polycarbonate films is observed when
the temperature in zones 66, 68 and 70 are set at 25.degree. C.
This blush defect is particularly problematic when high vapor
pressure solvents (methylene chloride and acetone) are used in the
coating fluids. Aggressively high temperatures are also associated
with other artifacts such as case hardening, reticulation patterns
and microvoids in the polycarbonate film. In a preferred embodiment
of the present invention, the first drying section 66 is operated
at a temperature of at least about 25.degree. C. but less than
95.degree. C. with no direct air impingement on the wet coating of
the coated web 22. In another preferred embodiment of the method of
the present invention, drying sections 68 and 70 are also operated
at a temperature of at least about 25.degree. C. but less than 95
.degree. C. It is preferred that initial drying sections 66, 68 be
operated at temperatures between about 30.degree. C. and about
60.degree. C. It is most preferred that initial drying sections 66,
68 be operated at temperatures between about 30.degree. C. and
about 50.degree. C. The actual drying temperature in drying
sections 66, 68 may be optimized empirically within these ranges by
those skilled in the art.
[0045] Referring now to FIG. 3, a schematic of an exemplary coating
apparatus 16 is shown in detail. Coating apparatus 16,
schematically shown in side elevational cross-section, includes a
front section 92, a second section 94, a third section 96, a fourth
section 98, and a back plate 100. There is an inlet 102 into second
section 94 for supplying coating liquid to first metering slot 104
via pump 106 to thereby form a lowermost layer 108. There is an
inlet 110 into third section 96 for supplying coating liquid to
second metering slot 112 via pump 114 to form layer 116. There is
an inlet 118 into fourth section 98 for supplying coating liquid to
metering slot 120 via pump 122 to form layer 124. There is an inlet
126 into back plate 100 for supplying coating liquid to metering
slot 128 via pump 130 to form layer 132. Each slot 104, 112, 120,
128 includes a transverse distribution cavity. Front section 92
includes an inclined slide surface 134, and a coating lip 136.
There is a second inclined slide surface 138 at the top of second
section 94. There is a third inclined slide surface 140 at the top
of third section 96. There is a fourth inclined slide surface 142
at the top of fourth section 98. Back plate 100 extends above
inclined slide surface 142 to form a back land surface 144.
Residing adjacent the coating apparatus or hopper 16 is a coating
back up roller 20 about which a web 12 is conveyed. Coating layers
108, 116, 124, 132 form a multi-layer composite which forms a
coating bead 146 between lip 136 and substrate 12. Typically, the
coating hopper 16 is movable from a non-coating position toward the
coating backing roller 20 and into a coating position. Although
coating apparatus 16 is shown as having four metering slots,
coating dies having a larger number of metering slots (as many as
nine or more) are well known and may be used to practice the method
of the present invention.
[0046] In the method of the present invention, the coating fluids
are comprised principally of a polycarbonate resin dissolved in an
organic solvent. Polymers of the polycarbonate type are available
in a variety of molecular weights as well as in numerous
permutations around the basic molecular structure. Common to all
polycarbonates are the carbonate linkages and usually the presence
of stabilizing phenyl groups (Ph) in the polymer backbone. In terms
of commercially significant polycarbonates, the condensation
product of the dihydridic phenol, 2,2-bis-(4-hydroxyphenyl)-propane
(Bisphenol-A), with a carbonate precursor, such as phosogene or
diphenyl carbonate, forms a polymer having recurring units of
-O-Ph-C(CH.sub.3).sub.2-Ph-O-CO-. Polycarbonates of the Bisphenol-A
type are both readily available and relatively inexpensive. Less
readily available and more expensive are the numerous polycarbonate
copolymers that may be formed by the addition of various dihydric
phenol derivatives during polymer synthesis. Examples of such
derivatives are 1,1-bis-(4-hydroxyphenyl)cyclohexane (Bisphenol Z),
1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethyl cyclohexane,
2,2-bis-(3-methyl-4-hydroxyphenyl)propane (Bisphenol C),
1,1-bis-(4-hydroxyphenyl)-1-phenyl ethane (Bisphenol P),
bis-(4-hydroxyphenyl)-diphenyl methane, among others. These
co-polymeric polycarbonates may be formulated to alter material
properties such as thermal stability, impact resistance and the
like, while maintaining good optical properties. In the method of
the present invention, there are no particular restrictions as to
the type of polycarbonate or blend of polycarbonate co-polymers
used to form a film. Polycarbonate resins are commercially
available from General Electric and Bayer.
[0047] In terms of organic solvents for polycarbonates, suitable
sovlents include, for example, chlorinated solvents (methylene
chloride and 1,2 dichloroethane), alcohols (methanol, ethanol,
n-propanol, isopropanol, n-butanol, isobutanol, diacetone alcohol,
phenol, and cyclohexanol), ketones (acetone, methylethyl ketone,
methylisobutyl ketone, and cyclohexanone), esters (methyl acetate,
ethyl acetate, n-propyl acetate, isopropyl acetate, isobutyl
acetate, and n-butyl acetate), aromatics (toluene and xylenes) and
ethers (tetrahydrofuran, 1,3-dioxolane, 1,2-dioxolane, 1,3-dioxane,
1,4-dioxane, and 1,5-dioxane). Polycarbonate solutions may be
prepared with a blend of the aforementioned solvents. Preferred
primary solvents include methylene chloride and 1,3-dioxolane.
Preferred co-solvents include toluene, tetrahydrofuran,
cyclohexanone, methanol, ethanol, and isopropanol.
[0048] Coating fluids may also contain small amounts of
plasticizers. Appropriate plasticizers for polycarbonate films
include phthalate esters (diethylphthalate, dibutylphthalate,
dicyclohexylphthalate, dioctylphthalate, didecylphthalate and butyl
octylphthalate), adipate esters (dioctyl adipate), carbonates
(dicetyl carbonate and distearyl carbonate) and phosphate esters
(tricresyl phosphate and triphenyl phosphate). Plasticizers are
normally used to improve the flow characteristics of polycarbonates
processed by the melt extrusion method. However, plasticizers may
be used here as coating aids in the converting operation to
minimize premature film solidification at the coating hopper and to
improve drying characteristics of the wet film. In the method of
the present invention, plasticizers may be used to minimize
blistering, curl and delamination of polycarbonate films during the
drying operation. In a preferred embodiment of the present
invention, plasticizers may be added to the coating fluid at a
total concentration of up to 5% by weight relative to the
concentration of polymer in order to mitigate defects in the final
polycarbonate film.
[0049] Coating fluids may also contain surfactants as coating aids
to control artifacts related to flow after coating. Artifacts
created by flow after coating phenomena include mottle,
repellencies, orange-peel (Bernard cells), and edge-withdraw.
Surfactants used control flow after coating artifacts include
siloxane and fluorochemical compounds. Examples of commercially
available surfactants of the siloxane type include: 1.)
Polydimethylsiloxanes such as DC200 Fluid from Dow Corning, 2.)
Poly(dimethyl, methylphenyl)siloxanes such as DC510 Fluid from Dow
Corning, and 3.) Polyalkyl substituted polydimethysiloxanes such as
DC190 and DC 1248 from Dow Corning as well as the L7000 Silwet
series (L7000, L7001, L7004 and L7230) from Union Carbide, and 4.)
Polyalkyl substituted poly(dimethyl, methylphenyl)siloxanes such as
SF1023 from General Electric. Examples of commercially available
fluorochemical surfactants include: 1.) Fluorinated alkyl esters
such as the Fluorad series (FC430 and FC431) from the 3M
Corporation, 2.) Fluorinated polyoxyethylene ethers such as the
Zonyl series (FSN, FSN100, FSO, FSO100) from Du Pont, 3.)
Acrylate:polyperfluoroalkyl ethylacrylates such as the F series
(F270 and F600) from NOF Corporation, and 4.) Perfluoroalkyl
derivatives such as the Surflon series (S383, S393, and S8405) from
the Asahi Glass Company. In the method of the present invention,
surfactants are generally of the non-ionic type. In a preferred
embodiment of the present invention, non-ionic compounds of either
the siloxane or fluorinated type are added to the uppermost
layers.
[0050] In terms of surfactant distribution, surfactants are most
effective when present in the uppermost layers of the multi-layer
coating. In the uppermost layer, the concentration of surfactant is
preferably 0.001-1.000% by weight and most preferably 0.010-0.500%.
In addition, lesser amounts of surfactant may be used in the second
uppermost layer to minimize diffusion of surfactant away from the
uppermost layer. The concentration of surfactant in the second
uppermost layer is preferably 0.000-0.200% by weight and most
preferably between 0.000 -0.100% by weight. Because surfactants are
only necessary in the uppermost layers, the overall amount of
surfactant remaining in the final dried film is small.
[0051] Although surfactants are not required to practice the method
of the current invention, surfactants do improve the uniformity of
the coated film. In particular, mottle nonuniformities are reduced
by the use of surfactants. In transparent polycarbonate films,
mottle nonuniformities are not readily visualized during casual
inspection. To visualize mottle artifacts, organic dyes may be
added to the uppermost layer to add color to the coated film. For
these dyed films, nonuniformities are easy to see and quantify. In
this way, effective surfactant types and levels may be selected for
optimum film uniformity.
[0052] Turning next to FIGS. 4 through 7, there are presented
cross-sectional illustrations showing various film configurations
prepared by the method of the present invention. In FIG. 4, a
single-layer polycarbonate film 150 is shown partially peeled from
a carrier substrate 152. Polycarbonate film 150 may be formed
either by applying a single liquid layer to the carrier substrate
152 or by applying a multiple layer composite having a composition
that is substantially the same among the layers. Alternatively in
FIG. 5, the carrier substrate 154 may have been pretreated with a
subbing layer 156 that modifies the adhesive force between the
single layer polycarbonate film 158 and the substrate 154. FIG. 6
illustrates a multiple layer film 160 that is comprised of four
compositionally discrete layers including a lowermost layer 162
nearest to the carrier support 170, two intermediate layers 164,
166, and an uppermost layer 168. FIG. 6 also shows that the entire
multiple layer composite 160 may be peeled from the carrier
substrate 170. FIG. 7 shows a multiple layer composite film 172
comprising a lowermost layer 174 nearest to the carrier substrate
182, two intermediate layers 176, 178, and an uppermost layer 180
being peeled from the carrier substrate 182. The carrier substrate
182 has been treated with a subbing layer 184 to modify the
adhesion between the composite film 172 and substrate 182. Subbing
layers 156 and 184 may be comprised of a number of polymeric
materials such as polyvinylbutyrals, cellulosics, acrylics, gelatin
and poly(acrylonitrile-co-vinylidene chloride-co-acrylic acid). The
choice of materials used in the subbing layer may be optimized
empirically by those skilled in the art.
[0053] The method of the present invention may also include the
step of coating over a previously prepared composite of
polycarbonate film and carrier substrate. For example, the coating
and drying system 10 shown in FIGS. 1 and 2 may be used to apply a
second multi-layer film to an existing polycarbonate film/substrate
composite. If the film/substrate composite is wound into rolls
before applying the subsequent coating, the process is called a
multi-pass coating operation. If coating and drying operations are
carried out sequentially on a machine with multiple coating
stations and drying ovens, then the process is called a tandem
coating operation. In this way, thick films may be prepared at high
line speeds without the problems associated with the removal of
large amounts of solvent from a very thick wet film. Moreover, the
practice of multi-pass or tandem coating also has the advantage of
minimizing other artifacts such as streak severity, mottle
severity, and overall film nonuniformity.
[0054] The practice of tandem coating or multi-pass coating
requires some minimal level of adhesion between the first-pass film
and the carrier substrate. In some cases, film/substrate composites
having poor adhesion are observed to blister after application of a
second or third wet coating in a multi-pass operation. To avoid
blister defects, adhesion must be greater than 0.3 N/m between the
first-pass polycarbonate film and the carrier substrate. This level
of adhesion may be attained by a variety of web treatments
including various subbing layers and various electronic discharge
treatments. However, excessive adhesion between the applied film
and substrate is undesirable since the film may be damaged during
subsequent peeling operations. In particular, film/substrate
composites having an adhesive force of greater than 250 N/m have
been found to peel poorly. Films peeled from such excessively,
well-adhered composites exhibit defects due to tearing of the film
and/or due to cohesive failure within the film. In a preferred
embodiment of the present invention, the adhesion between the
polycarbonate film and the carrier substrate is less than 250 N/m.
Most preferably, the adhesion between polycarbonate film and the
carrier substrate is between 0.5 and 25 N/m.
[0055] The method of the present invention is suitable for
application of polycarbonate resin coatings to a variety of
substrates such as polyethylene terephthalate (PET), polyethylene
naphthalate (PEN), polystyrene, and other polymeric films.
Polymeric substrates may be unstretched, uniaxially stretched or
biaxially stretched prior to application of the polycarbonate
coatings. Additional substrates may include paper, laminates of
paper and polymeric films, glass, cloth, aluminum and other metal
supports. In some cases, substrates may be pretreated with subbing
layers or electrical discharge devices. Substrates may also be
pretreated with functional layers containing various binders and
addenda.
[0056] The prior art method of casting resin films is illustrated
in FIG. 8. As shown in FIG. 8, a viscous polymeric dope is
delivered through a feed line 200 to an extrusion hopper 202 from a
pressurized tank 204 by a pump 206. The dope is cast onto a highly
polished metal drum 208 located within a first drying section 210
of the drying oven 212. The cast film 214 is allowed to partially
dry on the moving drum 208 and is then peeled from the drum 208.
The cast film 214 is then conveyed to a final drying section 216 to
remove the remaining solvent. The final dried film 218 is then
wound into rolls at a wind-up station 220. The prior art cast film
typically has a thickness in the range of from 40 to 200 .mu.m.
[0057] Coating methods are distinguished from casting methods by
the process steps necessary for each technology. These process
steps in turn affect a number of tangibles such as fluid viscosity,
converting aids, substrates, and hardware that are unique to each
method. In general, coating methods involve application of dilute
low viscosity liquids to thin flexible substrates, evaporating the
solvent in a drying oven, and winding the dried film/substrate
composite into rolls. In contrast, casting methods involve applying
a concentrated viscous dope to a highly polished metal drum or
band, partially drying the wet film on the metal substrate,
stripping the partially dried film from the substrate, removing
additional solvent from the partially dried film in a drying oven,
and winding the dried film into rolls. In terms of viscosity,
coating methods require very low viscosity liquids of less than
5,000 cp. In the practice of the method of the present invention
the viscosity of the coated liquids will generally be less than
2000 cp and most often less than 1500 cp. Moreover, in the method
of the present invention the viscosity of the lowermost layer is
preferred to be less than 200 cp. and most preferably less than 100
cp. for high speed coating application. In contrast, casting
methods require highly concentrated dopes with viscosity on the
order of 10,000-100,000 cp for practical operating speeds. In terms
of converting aids, coating methods generally involve the use of
surfactants as converting aids to control flow after coating
artifacts such as mottle, repellencies, orange peel, and edge
withdraw. In contrast, casting methods do not require surfactants.
Instead, converting aids are only used to assist in the stripping
and conveyance operations in casting methods. For example, lower
alcohols are sometimes used as converting aids in cast
polycarbonate films to minimize abrasion artifacts during
conveyance through drying ovens. In terms of substrates, coating
methods generally utilize thin (10-250 micron) flexible supports.
In contrast, casting methods employ thick (1-100 mm), continuous,
highly polished metal drums or rigid bands. As a result of these
differences in process steps, the hardware used in coating is
conspicuously different from those used in casting as can be seen
by a comparison of the schematics shown in FIGS. 1 and 8,
respectively.
[0058] The advantages of the present invention are demonstrated by
the following practical examples given below. In these examples,
the polycarbonate (PC) was the Bisphenol-A homopolymer with a
weight average molecular weight of 54,000 daltons as determined
with polystyrene equivalent weight distributions using size
exclusion chromatography.
EXAMPLE 1
[0059] This example describes the single pass formation of a very
thin polycarbonate film. The coating apparatus 16 illustrated in
FIG. 1 was used to apply four liquid layers to a moving substrate
12, 170 of untreated polyethylene terephthalate (PET) to form a
single layer film as illustrated earlier in FIG. 6. The substrate
speed was 25 cm/s. All coating fluids were comprised of PC
dissolved in methylene chloride. The lowermost layer 162 had a
viscosity of 17 cp. and a wet thickness of 14 .mu.m on the moving
substrate 170. The second 164 and third 166 layers each had a
viscosity of 660 cp. and had a combined final wet thickness of 27
.mu.m on the moving substrate 170. In addition, the third layer 166
also contained a fluorinated surfactant (Surflon S8405) at
concentration of 0.02 %. The uppermost layer 168 had a viscosity of
107 cp. and a wet thickness of 22 .mu.m on the moving substrate
170. The uppermost layer 168 also contained a fluorinated
surfactant (Surflon S8405) at a weight percent of 0.10%. Coatings
were applied at a temperature of 16.degree. C. The gap between the
coating lip 136 and the moving substrate 12 (see FIG. 3) was 200
.mu.m. The pressure differential across the coating bead 146 was
adjusted between 0-10 cm of water to establish a uniform coating.
The temperature in the drying sections 66 and 68 was 40.degree. C.
The temperature in the drying section 70 was 50.degree. C. The
temperature in the drying sections 72, 74, 76, 78, 80 was
120.degree. C. The temperature in the drying section 82 was
25.degree. C. The composite of PC film and PET substrate was wound
into rolls. When peeled from the untreated PET substrate, the final
dry film had a thickness of 10 .mu.m. The peeled PC film was free
from scratch and wrinkle artifacts and had an in-plane retardation
of less than 5.0 nm. Properties of this polycarbonate film are
summarized in Table I.
EXAMPLE 2
[0060] This example describes the single pass formation of a thin
PC film. The conditions were identical to those described in
Example 1 except that the combined wet thickness of the second and
third layers 164 and 166 was increased to 73 .mu.m. The composite
of PC film and PET substrate was wound into rolls. When peeled from
the subbed PET substrate, the final dry film had a thickness of 20
.mu.m. The peeled PC film had a good appearance, was smooth, was
free from scratch and wrinkles artifacts, and had an in-plane
retardation of less than 5.0 nm. Properties of this PC film are
summarized in Table I.
EXAMPLE 3
[0061] This example describes the single pass formation of a thin
PC film. The conditions were identical to those described in
Example 1 except that the combined wet thickness of the second and
third layers 164 and 166 was increased to 120 .mu.m. The composite
of PC film and PET substrate was wound into rolls. When peeled from
the subbed PET substrate, the final dry film had a thickness of 30
.mu.m. The PC film had a good appearance, was smooth, was free from
scratch and wrinkle artifacts, and had an in-plane retardation of
less than 5.0 nm. Properties of this PC film are summarized in
Table I.
EXAMPLE 4
[0062] This example describes the single pass formation a PC film.
The conditions were identical to those described in Example 1
except that the combined wet thickness of the second and third
layers 164 and 166 was increased to 166 .mu.m. The composite of PC
film and PET substrate was wound into rolls. When peeled from the
subbed PET substrate, the final dry film had a thickness of 40
.mu.m. The peeled PC film had a good appearance, was smooth, was
free from scratch and wrinkle artifacts, and had an in-plane
retardation of less than 5.0 nm. Properties of this PC film are
summarized in Table I.
EXAMPLE 5
[0063] This example describes the formation of a thin PC film using
two pass coating operation. The conditions were identical to those
described in Example 1 except that the wound composite of PC film
and PET substrate of Example 1 was subsequently over-coated with an
additional pass. The second pass was conducted with the combined
wet thickness of the second and third layers at 27 .mu.m as
described in Example 1. The composite of PC film and PET substrate
was wound into rolls. When peeled from the untreated PET substrate,
the final dry film had a thickness of 20 .mu.m. The peeled PC film
had a good appearance, was smooth, was free from scratch and
wrinkle artifacts, and had an in-plane retardation of less than 5.0
nm. Properties of this polycarbonate film are summarized in Table
I.
EXAMPLE 6
[0064] This example describes the formation of a PC film using a
three-pass coating operation. The conditions were identical to
those described in Example 2 except that the wound composite of PC
film and PET substrate of Example 2 was subsequently over-coated
with two additional passes. Each additional pass was conducted with
the combined wet thickness of the second and third layers at 73
.mu.m as described in Example 2. The final composite of PC film and
PET substrate was wound into rolls. The final dry film had a
thickness of 60 .mu.m. The peeled PC film was smooth, was free from
scratch and wrinkle artifacts, and had an in-plane retardation of
less than 5.0 nm. Properties of this PC film are summarized in
Table I.
EXAMPLE 7
[0065] This example describes the formation of a PET/PC composite
having optimal peeling properties. In this example, the PET support
has a subbing layer applied to the coated side. The subbing layer
is polyvinylbutyral (.about.12% vinyl alcohol content) having a dry
thickness of 10 .mu.m and a surfactant content of 500 mg/sq-m of
Surflon S-8405. This polyvinylbutyral layer is adhered to the
subbed PET substrate. Otherwise, the conditions were identical to
those described in Example 2. The final composite of PC film and
subbed PET substrate was wound into rolls. The final dry film had a
thickness of 20 .mu.m. When peeled from the subbed PET substrate,
the PC film was found to separate very smoothly from the carrier
support. The average adhesive strength of the PC film to the subbed
PET substrate was found to be 1.8 N/m with a standard deviation of
0.4 N/m. This smooth peeling process contrasted noticeably with the
more hesitant peeling properties of the untreated PET substrate
described earlier in Example 2. For untreated PET substrate of
Example 2, the average adhesive strength of the PC film to the
subbed PET substrate was found to be 3.0 N/m with a higher standard
deviation of 1.4 N/m. This feature of smooth peeling is reflected
in the smaller standard deviation values found with the adhesion
measurements. Similar results where observed with thicker PC films
of 40 microns. For 40 micron PC films prepared under the conditions
of Example 4, samples prepared using untreated PET and the
polyvinylbutyral subbed PET described here in Example 7 had
standard deviations of adhesive strength of 2.5 and 0.1 N/m,
respectively. The PET substrate treated with the polyvinylbutyral
subbing layer displayed very smooth peel characteristics having
very low standard deviation values of adhesive strength.
COMPARATIVE EXAMPLE 1
[0066] This example describes the formation of a PET/PC composite
having poor peeling properties. In this example, the PET support
has a subbing layer of poly(acrylonitrile-co-vinylidene
chloride-co-acrylic acid) with a dry coverage of 100 mg/sq-m.
Otherwise, the conditions for Comparative Example 1 were identical
to those described in Example 1. The final dry film had a thickness
of 20 .mu.m. When dried, the PC film could not be peeled from the
subbed PET substrate. For this composite film, the adhesive
strength of the PC film to the subbed PET substrate was greater
than 250 N/m.
COMPARATIVE EXAMPLE 2
[0067] This example describes defects formed as a result of poor
drying conditions during a single pass operation. The conditions
for Comparative Example 2 were identical to those described in
Example 2 except that the drying conditions were adjusted such that
the temperature in the first three drying zones 66, 68, 70 was
decreased to 25.degree. C. When peeled from the subbed PET
substrate, the final dry film had a thickness of 20 .mu.m. The
peeled PC film was of unacceptable quality due to fogging of the
film.
COMPARATIVE EXAMPLE 3
[0068] This example describes defects formed as a result of poor
drying conditions during a single pass operation. The conditions
for Comparative Example 3 were identical to those described in
Example 2 except that the drying conditions were adjusted such that
the temperature in the first three drying zones 66, 68, 70 was
increased to 95.degree. C. When peeled from the subbed PET
substrate, the final dry film had a thickness of 20 .mu.m. The
peeled PC film was of unacceptable quality due to a reticulation
pattern in the film as well as to blister artifacts.
1TABLE I Example Thickness Retardation Transmittance Haze Roughness
1 10 .mu.m 2.0 nm 92.1% 1.0% 1.3 nm 2 20 3.8 92.0 1.0 1.0 3 30 2.5
92.3 0.8 0.9 4 40 2.8 92.3 0.7 0.7 5 20 3.8 92.0 0.6 1.1 6 60 4.5
92.1 0.8 0.7
[0069] The following tests were used to determine the film
properties given in Table I.
[0070] Thickness. Thickness of the final peeled film was measured
in microns using a Model EG-225 gauge from the Ono Sokki
Company.
[0071] Retardation. In-plane retardation (R.sub.e) of peeled films
were determined in nanometers (nm) using a Woollam M-2000V
Spectroscopic Ellipsometer at wavelengths from 370 to 1000 nm.
In-plane retardation values in Table I are computed for
measurements taken at 590 nm. In-plane retardation is defined by
the formula:
R.sub.e=.vertline.n.sub.x-n.sub.y.vertline..times.d
[0072] where R.sub.e is the in-plane retardation at 590 nm, n.sub.x
is the index of refraction of the peeled film in the slow axis
direction, n.sub.y is the index of refraction of the peeled film in
the fast axis direction, and d is the thickness of the peeled film
in nanometers (nm). Thus, R.sub.e is the absolute value of the
difference in birefringence between the slow axis direction and the
fast axis direction in the plane of the peeled film multiplied by
the thickness of the film.
[0073] Transmittance and Haze. Total transmittance and haze are
measured using the Haze-Gard Plus (Model HB-4725) from BYK-Gardner.
Total transmittance is all the light energy transmitted through the
film as absorbed on an integrating sphere. Transmitted haze is all
light energy scattered beyond 2.5.degree. as absorbed on an
integrating sphere.
[0074] Surface Roughness. Surface roughness was determined in
nanometers (nm) by scanning probe microscopy using TappingMode.TM.
Atomic Force Microscopy (Model D300 from Digital Instruments).
[0075] Adhesion. The adhesion strength of the coated samples was
measured in Newtons per meter (N/m) using a modified 180.degree.
peel test with an Instron 1122 Tensile Tester with a 500 gram load
cell. First, 0.0254 m (one inch) wide strips of the coated sample
were prepared. Delamination of the coating at one end was initiated
using a piece of 3M Magic Tape. An additional piece of tape was
then attached to the delaminated part of the coating and served as
the gripping point for testing. The extending tape was long enough
to extend beyond the support such that the Instron grips did not
interfere with the testing. The sample was then mounted into the
Instron 1122 Tensile Tester with the substrate clamped in the upper
grip and the coating/tape assembly clamped in the bottom grip. The
average force (in units of Newtons) required to peel the coating
off the substrate at a 180.degree. angle at speed of 2 inches/min
(50.8 mm/min) was recorded. Using this force value the adhesive
strength in units of N/m was calculated using the equation:
S.sub.A=F.sub.p(1-cos .theta.)/w
[0076] wherein S.sub.A is the adhesive strength, F.sub.p is the
peel force, .theta. is the angle of peel (180.degree.), and w is
the width of the sample (0.0254 m).
[0077] Residual Solvent. A qualitative assessment of residual
solvents remaining in a dried film is done by first peeling the
film from the carrier substrate, weighing the peeled film,
incubating the film in an oven at 150.degree. C. for 16 hours, and
finally weighing the incubated film. Residual solvent is expressed
as percentage of the weight difference divided by the
post-incubation weight.
[0078] From the foregoing, it will be seen that this invention is
one well adapted to obtain all of the ends and objects hereinabove
set forth together with other advantages which are apparent and
which are inherent to the apparatus.
[0079] It will be understood that certain features and
subcombinations are of utility and may be employed with reference
to other features and subcombinations. This is contemplated by and
is within the scope of the claims.
[0080] As many possible embodiments may be made of the invention
without departing from the scope thereof, it is to be understood
that all matter herein set forth and shown in the accompanying
drawings is to be interpreted as illustrative and not in a limiting
sense.
PARTS LIST:
[0081]
2 10 coating and drying system 12 moving substrate/web 14 dryer 16
coating apparatus 18 unwinding station 20 back-up roller 22 coated
web 24 dry film 26 wind up station 28 coating supply vessel 30
coating supply vessel 32 coating supply vessel 34 coating supply
vessel 36 pumps 38 pumps 40 pumps 42 pumps 44 conduits 46 conduits
48 conduits 50 conduits 52 discharge device 54 polar charge assist
device 56 opposing rollers 58 opposing rollers 60 polycarbonate
film 62 winding station 64 winding station 66 diying section 68
drying section 70 drying section 72 diying section 74 drying
section 76 drying section 78 dtying section 80 drying section 82
drying section 92 front section 94 second section 96 third section
98 fourth section 100 back plate 102 inlet 104 metering slot 106
pump 108 lower most layer 110 inlet 112 2nd metering slot 114 pump
116 layer 118 inlet 120 metering slot 122 pump 124 form layer 126
inlet 128 metering slot 130 pump 132 layer 134 incline slide
surface 136 coating lip 138 2nd incline slide surface 140 3rd
incline slide surface 142 4th incline slide surface 144 back land
surface 146 coating bead 150 polycarbonate film 152 carrier
substrate 154 carrier substrate 156 subbing layer 158 polycarbonate
film 160 multiple layer film 162 lower most layer 164 intermediate
layers 166 intermediate layers 168 upper most layer 170 carrier
support 172 composite film 174 lower most layer 176 intermediate
layers 178 intermediate layers 180 upper most layers 182 carrier
substrate 184 subbing layer 200 feed line 202 extrusion hopper 204
pressurized tank 206 pump 208 metal drum 210 drying section 212
drying oven 214 cast film 216 final drying section 218 final dried
film 220 wind up station
* * * * *