U.S. patent application number 10/279891 was filed with the patent office on 2004-04-29 for process for making polyester photographic film base and photographic element comprising said base.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Brickey, Michael R., Greener, Jehuda, Rao, Yuanqiao.
Application Number | 20040081928 10/279891 |
Document ID | / |
Family ID | 32106819 |
Filed Date | 2004-04-29 |
United States Patent
Application |
20040081928 |
Kind Code |
A1 |
Rao, Yuanqiao ; et
al. |
April 29, 2004 |
PROCESS FOR MAKING POLYESTER PHOTOGRAPHIC FILM BASE AND
PHOTOGRAPHIC ELEMENT COMPRISING SAID BASE
Abstract
A process for making polyester film base material comprising
more than 65 mol % of 1,4-cyclohexanedimethanol (CHDM) of the total
glycol component in the polyester material. The film base is
prepared by sequentially carrying out the following steps: (a)
casting a molten polyester resin in a machine direction onto a
casting surface to form a continuous sheet, (b) drafting the sheet
by stretching in the machine direction at a stretch ratio of from 2
to 4, and at a temperature ranging from 70.degree. C. to
130.degree. C., (c) tentering the sheet in the transverse direction
by stretching at a stretch ratio of from 2 to 4, and at a
temperature ranging from 70.degree. C. to 130.degree. C. to obtain
a biaxially oriented film, (d) heat-setting the oriented film at an
actual temperature of from 160.degree. C. to 200.degree. C., and
(e) cooling the heat-set film without substantial detentering to
obtain a biaxially oriented, heat-set polyester-based film having a
cutting index of 1 to 1.5 and a tensile toughness ranging from 15
to 55 MPa. The invention is also directed towards a silver halide
light sensitive photographic element comprising the PET-based film
base made by the present process. The photographic film base of the
invention exhibits excellent dimensional stability, optical clarity
and mechanical strength while also possessing a crystalline
morphology that enables finishing by cutting, chopping or
perforating techniques at reduced cutting index without requiring a
detentering step after heat-setting.
Inventors: |
Rao, Yuanqiao; (Rochester,
NY) ; Greener, Jehuda; (Rochester, NY) ;
Brickey, Michael R.; (Rochester, NY) |
Correspondence
Address: |
Paul A. Leipold
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
32106819 |
Appl. No.: |
10/279891 |
Filed: |
October 24, 2002 |
Current U.S.
Class: |
430/533 ;
264/210.5; 264/210.7; 264/212; 264/216; 264/290.2; 264/342RE;
264/40.1; 264/DIG.73; 428/480 |
Current CPC
Class: |
C08J 2367/02 20130101;
G03C 1/498 20130101; G03C 1/7954 20130101; G03C 2005/168 20130101;
C08J 5/18 20130101; G03C 5/14 20130101; B29C 55/143 20130101; G03C
5/16 20130101; G03C 5/17 20130101; Y10T 428/31786 20150401; B29K
2067/00 20130101 |
Class at
Publication: |
430/533 ;
428/480; 264/040.1; 264/210.5; 264/210.7; 264/212; 264/216;
264/290.2; 264/342.0RE; 264/DIG.073 |
International
Class: |
G03C 001/795; G03C
011/22; B32B 027/06; B32B 027/36; B29C 055/00; B29C 071/00; B29C
049/08; B29D 007/00; B29B 017/00 |
Claims
What is claimed is:
1. A process for making a biaxially stretched PET-based polyester
photographic film base comprising the steps of: (a) casting a
molten form of the polyester material comprising more than 65 mol %
of 1,4-cyclohexanedimethanol onto a casting surface in a machine
direction to form a continuous solid sheet, (b) drafting the sheet
by stretching in the machine direction at a stretch ratio of from 2
to 4, and at a temperature ranging from 80.degree. C. to
130.degree. C., (c) tentering the sheet in a transverse direction
by stretching at a stretch ratio of from 2 to 4, and at a
temperature ranging from 80.degree. C. to 130.degree. C. (d)
heat-setting the tentered sheet at an actual heat-set temperature
in the range of from 160.degree. C. to 200.degree. C., wherein the
actual heat-set temperature is determined from a differential
scanning calorimetry thermogram of the biaxially-stretched sheet
and is represented by the secondary low-temperature melting peak
recorded on the thermogram, and. (e) cooling the heat-set sheet to
obtain a stretched, heat-set PET-based polyester film having a
cutting index of 1 to 1.5 and tensile toughness of 15 to 55
MPa.
2. The process of claim 1 wherein the sheet is stretched in the
machine direction at a temperature ranging from 85.degree. C. to
110.degree. C. and in the transverse direction at a temperature
ranging from 90.degree. C. to 130.degree. C.
3. The process of claim 1 wherein the actual heat-set temperature
is from 160.degree. C. to 180.degree. C.
4. The process of claim 1 wherein the drafting and tentering steps
are performed simultaneously.
5. The process of claim 1 wherein no detentering is applied after
the heat-setting step.
6. The process of claim 1 wherein the stretch ratio in the machine
direction is 2.8 to 3.8 and the stretch ratio in the transverse
direction is 2.8 to 3.8.
7. The cutting index of the film prepared in accordance of the
process of claim 1 is from 1 to 1.3.
8. The process of claim 1, further comprising heat treating the
film base at temperatures ranging from Tg-50.degree. C. up to Tg
for times ranging from 1 hr to 1000 hrs, where Tg is the glass
transition temperature of said material.
9. A film base prepared in accordance with the process of claim 1,
wherein the film base comprises a PET-based polyester material in
which material: a. the level of repeat units derived from
1,4-cyclohexane dimethanol is in the range of from 65 to 100 mol %
based on the total glycol component in the material, b. the level
of repeat units derived from terephthalic acid is in the range of
from 70 to 95 mol % based on the total acid component in the
material, c. the level of repeat units derived from isophthalic
acid is in the range of from 5 to 30 mol % based on the total acid
component in the material.
10. A film base prepared in accordance with the process of claim 9,
wherein the film base comprises a PET-based polyester material in
which material: a. the level of repeat units derived from
1,4-cyclohexane dimethanol is in the range of from 85 to 100 mol %,
based on the total glycol component in the material, b. the level
of repeat units derived from terephthalic acid is in the range of
from 80 to 90 mol %, based on the total acid component in the
material, c. the level of repeat units derived from isophthalic
acid is in the range of from 10 to 20 mol %, based on the total
acid component in the material.
11. An imaging element comprising at least one imaging layer and
the film base prepared in accordance with the process of claim
1.
12. The imaging element of claim 11 wherein the imaging layer
comprises a silver-halide emulsion.
13. The imaging element of claim 11 wherein the light-sensitive
imaging layer is sensitive to X-ray exposure or to exposure of
emissions from phosphor intensifying screens.
14. The imaging element of claim 11 wherein the element is a
photographic film or a photothermographic film.
15. The imaging element of claim 11 wherein the element is a 35 mm
photographic film.
16. The imaging element of claim 11 further comprising a
photographically acceptable subbing layer and backing layers on the
film base.
17. The imaging element of claim 11 wherein the film base bears a
magnetic or optical recording layer.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a process for making a polyester
photographic film base having improved properties. More
particularly, the invention relates to a process involving
heat-setting a biaxially stretched CHDM-containing PET-based
photographic film base having improved properties with regard to
cutting, chopping, and perforating.
BACKGROUND OF THE INVENTION
[0002] Poly(ethylene terephthalate) (PET) films exhibit excellent
properties for use as photographic film base with regard to
transparency, dimensional stability, mechanical strength, and
resistance to thermal deformation. However, PET films are extremely
tough and not well suited for finishing operations, i.e., slitting,
chopping, and perforating processes, which are required in the
preparation of photographic films. Moreover, such films are not
well suited for cutting processes in various photofinishing steps
such as notching, splicing, and sleeving.
[0003] Several polyester-based materials with improved cutting
characteristics were proposed for use as photographic film base
materials in commonly assigned copending U.S. Ser. No. 10/036,668
and U.S. Ser. No. 10/027,023 hereby incorporated by reference in
their entirety. This improvement was accomplished through
incorporation of 1,4-cyclohexane dimethanol (CHDM) either by
copolymerization as a glycol comonomer or by blending with
CHDM-containing copolyesters. Although significant improvement in
cutting performance has been shown with these PET-modified
polyesters, further improvement may be needed to meet the stringent
requirements of certain cutting steps in photofinishing
operations.
[0004] The process for making a polyester-based photographic film
base typically comprises the steps of casting a molten polyester
resin in a machine direction onto a casting surface to form a
continuous sheet, drafting the sheet by stretching in the machine
direction, tentering the sheet by stretching in the transverse
direction, heat-setting the drafted and tentered sheet, and cooling
the heat-set sheet to form a stretched, heat-set film, such as
described in, e.g., U.S. Pat. No. 4,141,735, the disclosure of
which is incorporated by reference herein. U.S. Pat. Nos. 5,385,704
and 5,607,826 disclose a method for improving the finishing
characteristics of photographic materials employing a PET film base
involving lowering the planar birefringence of the film base to
below 0.150 by performing a detentering step which allows the
tentered film to shrink in width by 2 to 20% (pref. 10-18%) after
the heat-setting step during film manufacturing. Improvement in
finishing characteristics of PET-based photographic film, as
manifested by decrease in dirt and debris generated during
finishing operations, is also disclosed in U.S. Pat. No. 6,228,569
and U.S. Ser. No. 09/223,876 hereby incorporated by reference in
their entirety. These latter inventions disclose a method utilizing
relatively high heat-set temperatures (>220.degree. C.) applied
during the film manufacturing process, which substantially improves
the finishing and cutting characteristics of PET-based photographic
supports. However, even with the demonstrated improvements in
finishability, the PET-based film is still difficult to cut in
various steps of the photofinishing process. Furthermore, applying
this method to the manufacture of high-CHDM PET-based photographic
film base does not provide the desired improvement in cutting and
finishing characteristics. Therefore, a clear need exists to
further improve the cutting characteristics of high-CHDM PET-based
photographic supports.
[0005] U.S. Pat. No. 5,034,263 to Maier et al. disclosed a
laminated film comprising a PET core and, on at least one surface
thereof, an overcoat of a poly(1,4-cyclohexylene dimethylene
terephthalate) polyester, in order to allow the laminated film to
be readily slit and perforated using techniques commonly employed
with consumer photographic film. Maier et al. states that the CHDM
component should comprise at least 70 mol % of the glycol component
of the polyester. However, such laminates have been found prone to
delamination.
[0006] U.S. Pat. No. 5,925,507 to Massa et al. discloses a PET
film-base material having less tendency to core set, comprising
polyester containing at least 30 weight % CHDM, which polyester is
blended with a polycarbonate that contains bisphenol. U.S. Pat. No.
4,141,735 to Schrader et al. discloses a polyester film base having
improved core-set curl, involving the use of heat tempering, in one
example using poly(1,4-cyclohexylene dimethylene terephthalate).
However, this polymer crystallizes rapidly, therefore, the making
of its oriented film is difficult. Also, the polymer becomes opaque
or hazy and useless for photographic applications where
transparency is required.
[0007] The blending or copolymerizing of conventional polyester
with other polyester constituents, in order to improve the cutting
performance of a film, has also been proposed for poly(ethylene
naphthalate) (PEN)-based polyester films, as disclosed in U.S. Pat.
No. 6,232,054 B1 to Okutu et al. However, PEN is generally
considerably more costly and more difficult to manufacture than
PET, so a clear need exists for improving the cuttability of
PET-based polyester supports.
[0008] Outside the photographic field, PET and PEN are valuable
commercial semi-crystalline polyesters, which are widely used for
packaging materials due to the combination of desirable properties
that they possess. The high oxygen barrier properties of these
polyesters render them particularly valuable for packaging
oxygen-sensitive food and other goods and materials. PEN has
advantages over PET due to its higher Tg and higher oxygen barrier
properties, although PEN, as mentioned above, is considerably more
costly and is somewhat harder to process than PET.
[0009] The toughness and cutting difficulty of PET and similar
polyesters is generally attributed to the crystal structure and
molecular orientation of the film. It is known that changes in
these factors, driven either by formulary changes or by modified
process conditions, can be used to lower the toughness and improve
the cutting performance of PET. Generally, the crystallinity of PET
can be lowered or altogether eliminated by adding suitable
crystallization modifiers. Crystallization modifiers like
isophthalic acid (IPA) and CHDM are often copolymerized into PET
and PEN polyesters to form copolyesters that have better processing
properties. Modest levels of IPA slow down crystallization and
raise the oxygen barrier properties. Higher levels of IPA break up
crystallinity and lead to amorphous copolyesters with good barrier
properties, but these copolyesters, are known to those skilled in
the art, to possess poor impact and other mechanical properties.
Modest levels of CHDM also slow down crystallization, but decrease
oxygen barrier properties. Higher levels of CHDM are well known to
form families of amorphous copolyesters, which are widely used in
commerce in a multitude of applications including heavy gauge
sheet, signage, medical packages, etc. These copolyesters have
excellent impact resistance and other mechanical properties, but
have lower oxygen barrier properties than IPA-modified copolyesters
and lower oxygen barrier properties than PET.
[0010] Photographic film requires a strict control of the thickness
uniformity and surface flatness. One method of control is through
stretching of a polymer sheet into a semi-crystalline state. For
CHDM-modified polyester, only when the concentration of
CHDM-comonomer units relative to total glycol content is less than
about 30 mol % or greater than about 65 mol % is the resulting
polyester crystalline. For materials in which the content is less
than about 30 mol %, however, the material does not become
sufficiently crystalline for dimensional stability and thickness
uniformity until the concentration of CHDM comonomer relative to
total glycol content is less than 25 mol %. Amorphous polyester
film or insufficiently crystalline film presents dimensional
stability and thickness uniformity problems, and it possesses
relatively low stiffness.
[0011] Amorphous copolyesters are generally defined as copolyesters
that do not show a substantial melting point by differential
scanning calorimetry (DSC). These copolyesters are typically based
on terephthalic acid, IPA, ethylene glycol, neopentyl glycol and
CHDM. It is known that amorphous copolyesters possess a combination
of desirable properties, such as excellent clarity and color,
toughness, chemical resistance and ease of processing. Accordingly,
such copolyesters are known to be useful for the manufacture of
extruded sheets, packaging materials, and parts for medical
devices. For example, U.S. Pat. Nos. 5,385,773 and 5,340,907 to Yau
et al. disclose polyesters of CHDM in an amount of 10-95 mol % of
the glycol component, and a process for producing such copolymers
by esterification. U.S. Pat. No. 6,183,848 B1 to Turner et al.
discloses an amorphous copolyester comprising various amounts of
comonomers derived from CHDM which, because of improved gas barrier
properties, are useful for packaging perishable goods. In one
embodiment, the copolyester is disclosed as a biaxially oriented
sheet.
Problem to be Solved by the Invention
[0012] Accordingly, it would be desirable to provide a PET-based
film base with improved physical properties. In particular, it
would be desirable to obtain a PET-based film base that is less
tough and better suited for finishing operations, i.e., slitting,
chopping, and perforating processes, which are required in the
preparation of photographic films. Moreover, it would be desirable
to obtain a PET-based film base that is easier to cut in various
steps of the photofinishing process, such as splicing, notching,
and sleeving. Additionally, it would be desirable to be able to use
PET-based polyester film as a film base in certain consumer
photographic film applications and in films processed in a minilab
setting. It would also be desirable for such a PET-based film base
to have other advantageous properties such as dimensional stability
and a reduced tendency to take up high levels of curl during
storage in cartridges at high temperatures and/or is better able to
lower this curl during photoprocessing.
SUMMARY OF THE INVENTION
[0013] The invention provides an improved method of making a
polyester photographic film base having excellent dimensional
stability, optical clarity and mechanical strength while also
possessing a crystalline morphology that enables finishing by
cutting, chopping, or perforating techniques at reduced cutting
index. This is achieved by controlling the base-making process to
produce a biaxially oriented high-CHDM PET-based film having actual
heat-set temperature of under 200.degree. C. In accordance with one
embodiment of the invention, the film base is prepared by
sequentially carrying out the following steps: (a) casting a molten
polyester resin onto a casting surface to form a continuous sheet
in a machine direction, (b) drafting the sheet by stretching in the
machine direction at a stretch ratio of from 2 to 4, and at a
temperature ranging from 80 to 130.degree. C., (c) tentering the
sheet in a transverse direction by stretching at a stretch ratio of
from 2 to 4, and at a temperature ranging from 80 to 130.degree.
C., (d) heat-setting the tentered sheet at an actual temperature
sensed by the sheet of from 160 to 200.degree. C., and (e) cooling
the heat-set sheet without substantial detentering to obtain a
stretched, heat-set polyester-based film having a cutting index of
1 to 1.5 and tensile toughness of less than 55 MPa.
[0014] The actual heat-set temperature of the film may be
determined from a secondary melting endothermic peak of a DSC
thermogram for the film.
[0015] A further embodiment of the invention is directed towards a
method of making a photographic element comprising at least one
light sensitive silver halide-containing emulsion layer and a
high-CHDM PET-based film base produced in accordance with the above
embodiments.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Definitions of terms, as used herein, include the
following:
[0017] In the present invention, monomeric units derived from
1,4-cyclohexane dimethanol (CHDM) are also referred to as "CHDM
repeat units" or "CHDM-comonomer units" or "CHDM."
[0018] By "terephthalic acid," suitable synthetic equivalents, such
as dimethyl terephthalate, are included. It should be understood
that "dicarboxylic acids" includes the corresponding acid
anhydrides, esters and acid chlorides for these acids. Regarding
the glycol component or acid component in a polymer or material,
the mol percentages referred to herein equal to a total of 100 mol
%.
[0019] "PET, " "PET polymer," "PET resin," "poly(ethylene
terephthalate) resin," and the like refers to a polyester
comprising at least 98 mol % terephthalic acid comonomer units,
based on the total acid component, and comprising at least 98 mol %
of ethylene glycol comonomer units, based on the total glycol
component. This includes PET resins consisting essentially of about
100 mol % terephthalic acid comonomer units, based on the total
acid component, and consisting essentially of about 100 mol % of
ethylene glycol comonomer units, based on the total glycol
component.
[0020] The term "modified PET polymer," "modified PET resin," or
the like is a polyester comprising at least 70 mol % terephthalic
acid comonomer units, based on the total acid component, that has
been modified so that either the acid component is less than 98 mol
% of terephthalic acid ("TA") comonomer units or the glycol
component is less than 98 mol % of ethylene glycol ("EG") comonomer
units, or both the TA and EG comonomer units are in an amount less
than 98 mol %. The modified PET polymer is modified with, or
copolymerized with, one or more comonomers other than terephthalic
acid comonomers and/or ethylene glycol comonomers in an amount of
greater than 2 mol % (including greater than 5 mol %), of either
the acid component and/or the glycol component, for example, to
improve the cuttability of a film base or otherwise change the
properties of the film base in which it is used. The "modified PET
resin" does not necessarily need to contain any ethylene glycol
comonomer units, and it does not necessarily need to contain any
acid component other than terephthalic acid comonomer units.
[0021] In one embodiment, the modified PET polymer is a polyester
comprising at least 80 mol % terephthalic acid comonomer units,
based on the total acid component, and at least 35 mol % ethylene
glycol comonomer units, further modified with or copolymerized with
one or more additional types of comonomers, preferably in the
amount of greater than 5 mol % of the acid component and/or glycol
component.
[0022] The term "CHDM-modified PET" or "CHDM-modified-PET
polyester" refers to a modified PET polymer modified by the
inclusion of at least 3.5 mol % CHDM comonomer units, based on the
total glycol component.
[0023] Similarly, the term "CHDM-modified polyester" refers to a
polyester comprising at least 3.5 mol % CHDM comonomer units, based
on total glycol component, but not necessarily comprising any
specific amount of terephthalic acid component.
[0024] The term "high-CHDM-modified PET" refers to a CHDM-modified
PET polyester in which the level of CHDM comonomer units is equal
to or greater than 65 mol %, based on the total glycol component.
This includes both "PCT" (polycyclohexylene dimethylene
terephthalate) and "PCTA," which is a copolymer of three monomers:
terephthalic acid, isophthalic acid and CHDM, with 100 mol % of the
CHDM, based on its total glycol component.
[0025] "PET-based polyester material" is a semi-crystalline
material comprising one or more polymers wherein at least 70% by
weight of the material is one or more polymers that are either a
PET polymer or modified PET polymer. Optionally, the material may
also include addenda such as silica beads, plasticizers, and the
like. The addenda may include inorganic particles, which can be
larger than, but may have at least one dimension describing
particle size ranging from 0.1 to 100 nm.
[0026] A film base is made using a PET-based polyester material in
the present invention. In one embodiment, preferably greater than
80% by weight, more preferably greater than 90% by weight, of the
PET-based polyester material used in this invention are one or more
polymers that are a high-CHDM PET-based polyester or a
CHDM-modified PET polyester.
[0027] The film base comprising said materials may be manufactured
by a process of casting, biaxial stretching and heat-setting. The
process for making PET-based film base typically comprises the
steps of casting a molten PET-based polyester resin onto a casting
surface to form a continuous sheet along a machine direction,
drafting the sheet by stretching in the machine direction,
tentering the sheet by stretching in a transverse direction,
heat-setting the drafted and tentered sheet, and cooling the
heat-set sheet to form a stretched, heat-set polyester film, such
as described in, e.g., U.S. Pat. No. 4,141,735 to Schrader et al.,
the disclosure of which is incorporated in its entirety by
reference herein. Alternately, the stretching of the film in the
machine and transverse directions can be performed simultaneously
using appropriate machinery.
[0028] Preferably, in order to improve its dimensional stability,
the multilayer film base is heat treated at temperatures from Tg
-50.degree. C. up to Tg for times ranging from 1 hr to 1000 hrs,
where Tg is the glass transition temperature of the polyester
material.
[0029] In one particular embodiment, the process for preparing
films from the resin compositions of this invention comprises the
following steps:
[0030] (1) The resin is cast under molten conditions upon a cooling
surface to form a continuous cast sheet. Preferably, the molten
polyester resin has an inherent viscosity of from 0.5 to 0.8 dl/g,
and is cast at a temperature of from 250 to 310.degree. C. while
the casting surface has a temperature of from 40 to 70.degree. C.
The inherent viscosity (IV) is measured at 25.degree. C. in a
solvent mixture of phenol/chlorobenzene (60/40 by weight) at a
concentration of 0.25 g/dl with an Ubbelhode glass viscometer.
[0031] (2) The continuous sheet is removed from the casting surface
and passed into a drafting zone where it is first preheated and
then stretched in the machine direction at a stretch ratio of 2.0
to 4.0, at a temperature of from about 80.degree. C. to 110.degree.
C. The drafting zone typically includes two sets of nipped rollers,
the first being the entrance to the drafting zone and the second is
the exit from the drafting zone. To achieve the stretch ratios
necessary for the practice of this invention, the exit nip rollers
are rotated at a speed greater than the entrance nip rollers. The
film may be cooled in the last stage of the drafting zone to
25.degree. C. to 40.degree. C.
[0032] (3) The film moves from the drafting zone into a tentering
zone where it is preheated and stretched in the transverse
direction at a stretch ratio of 2.0 to 4.0, at a temperature of
from about 80.degree. C. to 115.degree. C. The tentering zone
typically includes a means for engaging the film at its edges and
stretching such that the final width is from 2.0 to 4.0 times that
of the original width.
[0033] (4) The film is next heat-set by maintaining it at a
temperature of at least 160.degree. C., preferably at least
160.degree. C. to 180.degree. C., while being constrained as in the
tentering zone for a time sufficient to affect heat-setting. Times
longer than necessary to bring about this result are not
detrimental to the film; however, longer times are undesirable as
the lengthening of the zone requires higher capital expenditure
without achieving additional advantage. The heat-setting step is
typically accomplished within a time period of 0.1 to 15 seconds
and preferably 0.1 to 10 seconds. Finally, the film is cooled
without substantial detentering (the means for holding the edges of
the film do not permit greater than 2% shrinkage thereof).
[0034] The actual temperature sensed by the film during the
heat-setting step ("actual heat-set temperature") may be determined
by the DSC technique. The DSC heat-set temperature represents the
actual heat-set temperature. The actual temperature sensed by the
film is often different from the set heat-set temperature applied
in the process due to heat transfer inefficiencies, and it
sometimes depends on the position of the material sample across the
web. The DSC heat-set temperature may be determined by scanning a
test sample (as-received) by a conventional DSC apparatus (e.g.,
DuPont 990 Thermal Analyzer) at a rate of 10.degree. C./min from
ambient to approximately 300.degree. C. The thermogram produced by
the scan will contain two distinct endothermic peaks: (1) a high
temperature peak ranging from 240 to 280.degree. C., which
represents the primary melting range of the PET-based polyester;
and (2) a much smaller peak detected at a lower temperature for
films heat-set under temperatures between the glass transition and
the primary melting transition for the material. The position of
this secondary melting peak is closely dependent on the heat-set
temperature applied in the process and it represents the actual
temperature sensed by the material in the heat-set section of the
film-making machine.
[0035] Photographic film requires a strict control of the thickness
uniformity and surface flatness. One method of control is through
stretching of a polymer sheet into a semi-crystalline state.
Typically, biaxially stretching the material causes amorphous
material to become semi-crystalline. In a typical embodiment, the
crystallinity is at least 10%. For CHDM-modified polyesters, only
when the concentration of CHDM comonomer units relative to the
total glycol content is less than about 30 mol % or greater than
about 65 mol % is the resulting polyester crystalline. For
materials in which the content is less than about 30 mol %,
however, the material does not become sufficiently crystalline for
dimensional stability and thickness uniformity until the
concentration of CHDM comonomer relative to the total glycol
content is less than 25 mol %. Amorphous polyester film or
insufficiently crystalline film presents dimensional stability and
thickness uniformity problems, and it possesses relatively low
stiffness.
[0036] In one embodiment of the invention, the high-CHDM-modified
PET polyesters used in the film base comprise copolyesters having a
dicarboxylic acid component and a glycol component, the
dicarboxylic acid component comprising repeat units from at least
80 mol % terephthalic acid (or its ester) and the glycol component
comprising at least 65 mol %, preferably more than 75 mol %, of
repeat units from CHDM and from about 0 to 35 mol % from another
glycol, preferably from 0 to 25 mol % from ethylene glycol.
[0037] The high-CHDM-modified PET polyesters used in making the
articles of this invention preferably have about 100 mol % of a
dicarboxylic acid portion and about 100 mol % of a glycol portion.
Less than about 30 mol %, preferably not more than about 20 mol %
of the dicarboxylic acid repeat units may be from conventional
acids other than terephthalic acid such as those selected from
succinic, glutaric, adipic, azelaic, sebacic, fumaric, maleic,
itaconic, 1,4-cyclohexane-dicarboxylic, phthalic, isophthalic, and
naphthalene dicarboxylic acid.
[0038] Preferably, the glycol component of the
high-CHDM-modified-PET polyesters contains repeat units comprising
from 65 to 100 mol % of CHDM and from about 0 to 35 mol % of
ethylene glycol, based on the total glycol component. The glycol
component may optionally include less than 35 mol %, preferably not
more than about 10 mol % of other conventional glycols such as
propylene glycol, 1,3-propanediol;
2,4-dimethyl-2-ethylhexane-1,3-diol, 2,2-dimethyl-1,3-propanediol,
2-ethyl-2-butyl-1,3-propanediol,
2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,4-butanediol,
neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol,
2,2,4-trimethyl-1,6-hexanediol, thiodiethanol,
1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol,
2,2,4,4-tetramethyl-1,3-cyclobutanediol and the like.
[0039] In one embodiment, a preferred high-CHDM-modified PET for
use in the present invention is represented by the following
structure: 1
[0040] In Structure (I) above, the subscripts `a` and `b` represent
the mol %, based on the total acid component of the copolyester.
Preferably, as indicated above, `a` is 70 to 95 mol % and b is
between 5 to 30 mol %; the subscripts x and y represent the mol %,
based on the total glycol component of the copolyester. Preferably,
as indicated above, x is 0 to 35 mol % and y is between 65 and 100
mol %.
[0041] In the case of embodiments involving blends, a blend
comprising at least one high-CHDM-modified PET polymer blended with
a suitable CHDM-modified polyester, such that the total content of
the CHDM-comonomer units in the blend is 65 to 100 mol %,
preferably at least 75 mol %, more preferably at least 85 mol %,
based on the total glycol component. In the CHDM-modified
polyester, any of the above-mentioned acid components may be used
and any of the above glycol components may be used in addition to
the CHDM component.
[0042] In accordance with one embodiment of the invention, a high
CHDM-modified PET resin is used to prepare a biaxially stretched
and heat-set film or sheet material under conditions as described
above. In another embodiment of this invention, a CHDM-modified PET
resin is blended using a suitable compounding method with a high
CHDM-modified PET resin at a sufficient level, and this blend is
then used to prepare a biaxially stretched and heat-set film or
sheet material under conditions as described above.
[0043] More particularly, the film base used in the present
invention comprises a PET-based polyester material comprising one
or more polyester resins, in which material the level of repeat
units derived from CHDM, based on the total glycol component, is 65
to 100 mol %, and the level of repeat units derived from an acid
component other than terephthalic acid or its ester is in the
amount of 5 to 30 mol %, preferably 10 to 20, based on the total
acid component, and wherein the cutting index of the film base as
defined below is preferably in the range 1.0 to 1.5, optimally in
the range 1 to 1.3, and the tensile toughness is in the range 15 to
55 MPa. Preferably, the film base comprises a material in which the
level of repeat units derived from CHDM is 85 mol % or greater,
based on the total glycol component in the material. Also,
preferably, less than 25 mol % of the total glycol units is
aromatic.
[0044] The acid component other than terephthalic acid can be, for
example, isophthalic acid (IPA), dimethyl isophthalate,
1,4-cyclohexanedicarboxylic acid (1,4-CHDA), 1,4
cyclohexanediacetic acid, diphenyl-4,4-dicarboxylic acid,
dimethyl-2,6-naphthalene-dicarboxyl- ate, succinic acid, glutaric
acid, adipic acid, azelaic acid, sebacic acid,
paraphenylenedicarboxylic acid (PPDA), naphthalenedicarboxylic acid
(NDA), and mixtures thereof. Preferably, the other acid component
is IPA, 1,4-CHDA, PPDA, NDA, and the like, and mixtures
thereof.
[0045] In one embodiment, the film base used in the present
invention comprises a high-CHDM modified PET-based polyester
material with the following chemical structure: 2
[0046] In another embodiment, the film base used in the present
invention comprises a blend comprising a poly(cyclohexane
dimethylene terephthalate) (PCT) polymer and a CHDM-modified
polymer in the ratio of 95:5 to 30:70, more preferably 95:5 to
80:20. Preferably, the level of the CHDM comonomer units in the
CHDM-modified polymer is 50 to 100 mol %, based on the total glycol
component. All polyester materials in the blend must be miscible,
that is, the film produced from said blend must be optically clear
to meet the stringent optical requirements of high transparency and
low haze placed on photographic film bases.
[0047] Both of the polyester and the copolymerized polyester to be
used in the support for a photographic material of the present
invention may contain phosphoric acid, phosphorous acid and esters
thereof, and inorganic particles (e.g. silica, kaolin, calcium
carbonate, calcium phosphate and titanium dioxide) during
polymerization, and inorganic particles may be blended with the
polymer after polymerization. The inorganic particles can be larger
than, but may have at least one dimension ranging from 0. 1 to 100
nm. A dye, a UV absorber or an antioxidant may also be suitably
added at any stage during polymerization and after polymerization.
As indicated above, the film base is useful in a photographic
element comprising at least one silver-halide imaging layer over a
support comprising said film base of the present invention. Such a
photographic element can be a photographic film or a
photothermographic film.
[0048] In addition to the PET-based layer made according to the
present invention, the support can further comprise one or more
photographically acceptable subbing layers, backing layers, tie
layers, magnetic layers, and the like.
[0049] Subbing layers are used for the purpose of providing an
adhesive force between the polyester support and an overlying
photographic emulsion comprising a binder such as gelatin, because
a polyester film is of a very strongly hydrophobic nature and the
emulsion is a hydrophilic colloid. If the adhesion between the
photographic layers and the support is insufficient, several
practical problems arise such as delamination of the photographic
layers from the support at the cut edges of the photographic
material, which can generate many small fragments of chipped-off
emulsion layers, which then cause spot defects in the imaging areas
of the photographic material.
[0050] Various subbing processes and materials have, therefore,
been used or proposed in order to produce improved adhesion between
the support film and the hydrophilic colloid layer. For example, a
photographic support may be initially treated with an adhesion
promoting agent such as, for example, one containing at least one
of resorcinol, catechol, pyrogallol, 1-naphthol,
2,4-dinitro-phenol, 2,4,6-trinitrophenol, 4-chlororesorcinol,
2,4-dihydroxy toluene, 1,3-naphthalenediol, 1,6-naphthalenediol,
acrylic acid, sodium salt of 1-naphthol-4-sulfonic acid, benzyl
alcohol, trichloroacetic acid, dichloroacetic acid,
o-hydroxybenzotrifluoride, m-hydroxybenzotrifluoride,
o-fluorophenol, m-fluorophenol, p-fluorophenol, chloralhydrate, and
p-chloro-m-cresol. Polymers are also known and used in the subbing
layer for promoting adhesion between a support and an emulsion
layer. Examples of suitable polymers for this purpose are disclosed
in U.S. Pat. Nos. 2,627,088; 2,968,241; 2,764,520; 2,864,755;
2,864,756; 2,972,534; 3,057,792; 3,071,466; 3,072,483; 3,143,421;
3,145,105; 3,145,242; 3,360,448; 3,376,208; 3,462,335; 3,475,193;
3,501,301; 3,944,699; 4,087,574; 4,098,952; 4,363,872; 4,394,442;
4,689,359; 4,857,396; British Patent Nos. 788,365; 804,005;
891,469; and European Patent No. 035,614. Often these include
polymers of monomers having polar groups in the molecule such as
carboxyl, carbonyl, hydroxy, sulfo, amino, amido, epoxy, or acid
anhydride groups, for example, acrylic acid, sodium acrylate,
methacrylic acid, itaconic acid, crotonic acid, sorbic acid,
itaconic anhydride, maleic anhydride, cinnamic acid, methyl vinyl
ketone, hydroxyethyl acrylate, hydroxyethyl methacrylate,
hydroxychloropropyl methacrylate, hydroxybutyl acrylate,
vinylsulfonic acid, potassium vinylbenezensulfonate, acrylamide,
N-methylamide, N-methylacrylamide, acryloylmorpholine,
dimethylmethacrylamide, N-t-butylacrylamide, diacetonacrylamide,
vinylpyrrolidone, glycidyl acrylate, or glycidylmethacrylate, or
copolymers of the above monomers with other copolymerizable
monomers. Additional examples are polymers of, for example, acrylic
acid esters such as ethyl acrylate or butyl acrylate, methacrylic
acid esters such as methyl methacrylate or ethyl methacrylate or
copolymers of these monomers with other vinylic monomers; or
copolymers of polycarboxylic acids such as itaconic acid, itaconic
anhydride, maleic acid or maleic anhydride with vinylic monomers
such as styrene, vinyl chloride, vinylidene chloride or butadiene,
or trimers of these monomers with other ethylenically unsaturated
monomers. Materials used in adhesion-promoting layers often
comprise a copolymer containing a chloride group such as vinylidene
chloride. The PET-based polyester material comprising the film base
of the present invention can be made by conventional synthetic
processes. In general, as is well known by the skilled artisan,
polyesters comprise the reaction product of at least one
dicarboxylic acid and at least one glycol component. The
dicarboxylic acid component can typically comprise residues of
terephthalic acid, isophthalic acid, 1,4-cyclohexanedicarboxylic
acid, 2,6-naphthalenedicarboxylic acid, and/or mixtures thereof.
Also suitable are the anhydrides thereof, acid chlorides thereof,
and lower, e.g., C1-C8 alkyl esters thereof. Any isomers of the
dicarboxylic acid component or mixtures thereof may be used. For
example, cis, trans, or cis/trans mixtures of
1,4-cyclohexanedicarboxylic acid may be employed. Examples of
suitable naphthalene dicarboxylic acid isomers include
1,4-naphthalenedicarboxylic acid, 2-6-naphthalenedicarboxylic acid,
2,7-naphthalenedicarboxylic acid, or mixtures thereof.
[0051] The polyester polymers used in the present invention can be
prepared by a process comprising reacting the dicarboxylic acid
component and the glycol component at temperatures sufficient to
effect esterification or ester exchange and polycondensing the
reaction product under an absolute pressure of less than 10 mm Hg
for a time of less than about 2 hours in the presence of a catalyst
and inhibitor system. An example of a preferred catalyst and
inhibitor system is about 0-75 ppm Mn, about 50-150 ppm Zn, about
5-200 ppm Ge, about 5-20 ppm Ti and about 10-80 ppm P, all parts by
weight based on the weight of the copolyester.
[0052] Either dimethyl terephthalate (or other lower dialkyl
terephthalate ester) or terephthalic acid can be used in producing
the copolyester. Thus, the term "terephthalic acid component,
monomer, repeat unit, or portion" herein is meant to include either
the acid or ester form. These materials are commercially available.
The glycols CHDM and ethylene glycol are also commercially
available. Either the cis or trans isomer of CHDM, or mixture
thereof, may be used in accordance with the present invention.
[0053] Generally, the copolyesters may be produced using
conventional polyesterification procedures described, for example,
in U.S. Pat. Nos. 3,305,604 and 2,901,460, the disclosures of which
are incorporated herein by reference. The amorphous or
semi-crystalline copolyesters according to the invention are
prepared by conventional polymerization processes known in the art,
such as disclosed by U.S. Pat. Nos. 4,093,603 and 5,681,918, the
disclosures of which are herein incorporated by reference. Examples
of polycondensation processes useful in making the modified-PET
material of the present invention include melt phase processes
conducted with the introduction of an inert gas stream, such as
nitrogen, to shift the equilibrium and advance to high molecular
weight or the more conventional vacuum melt phase polycondensation,
at temperatures ranging from about 240.degree. C. to about
300.degree. C. or higher which are practiced commercially. Although
not required, conventional additives may be added to the
copolyester materials of the invention in typical amounts. Such
additives include pigments, colorants, stabilizers, antioxidants,
extrusion aids, slip agents, carbon black, flame-retardants and
mixtures thereof.
[0054] Various modified-PET polyesters comprising repeat units from
CHDM, which can be used in the present invention, are commercially
available from Eastman Chemical Company (Kingsport, Tenn.) under
the trademark EASTAPAK and EASTAR copolyester, as described at
http://www.eastman.com.
[0055] Photographic elements of this invention can have the
structures and components shown in Research Disclosure, Vol. 370,
No. 37038 (Feb. 1996) and can be imagewise exposed and processed
using known techniques and compositions, including those described
in the Research Disclosure No. 37038 cited above.
[0056] With regard to cuttability, it is generally known in the art
of sheet material cutting that the cutting process combines crack
formation and propagation. To form a crack, one needs to apply
cutters to cause tension on the surfaces of the sheet material
until the material is deformed and its break point is reached. Once
the material's break point is reached, a crack would be formed,
which starts the second stage of cutting--crack propagation. One
can maintain and eventually complete the cutting process by
straining the sheet material further using the cutters. Eventually,
the cutting would be completed as cracks propagate through the
sheet thickness.
[0057] To evaluate the cuttability of a given material, one needs
to evaluate how the material behaves during the crack formation and
propagation stages. If the material absorbs and dissipates more
mechanical energy during the crack formation and propagation
processes, it is said to be more difficult to cut and will have a
poorer cuttability. Two standard tests can be used to evaluate how
much mechanical energy a material absorbs and dissipates during the
said crack formation and propagation steps. One is the tensile test
(ASTM D882) and the other is the tear test (ASTM D1938). The former
can be used to evaluate the crack formation part of the cutting
process, and the latter can be used to assess the crack propagation
part of the cutting process.
[0058] The mechanical and cutting properties of the polyester films
of the present invention were evaluated in accordance with the
following procedures:
[0059] Tensile Properties: Modulus and tensile toughness can be
determined using a tensile test such as that described in ASTM
D882-80a. A tensile test consists of pulling a sample of material
with a tensile load at a specified rate until it breaks. From the
load and elongation history, a stress-strain curve is obtained with
the strain being plotted on the x-axis and stress on the y-axis.
The modulus is defined as the slope of the initial linear portion
of the stress-strain curve. The modulus is a measure of the
stiffness of the material. The tensile toughness is defined as the
area under the entire stress-strain curve up to the fracture point.
The tensile toughness is a measure of the ability of a material to
absorb energy in a tensile deformation. Both modulus and tensile
toughness are fundamental mechanical properties of the
material.
[0060] Tear Strength: The resistance to tear can be determined
using a tear test such as that described in ASTM D1938. The test
measures the force to propagate tearing in a fracture mode III. The
test sample used has a rectangular shape and a sharp long cut in
the middle. The separated two arms are then fixed in a conventional
tensile testing machine. The fixtures move at constant crosshead
speed to prolong the preexisting cut and the steady state force of
tearing is recorded.
[0061] Cutting Index: It is generally known that tensile toughness
represents the energy required to initiate a crack, while fracture
toughness determines the energy needed to further propagate the
crack. As typical cutting processes involve both crack initiation
and crack propagation, a quantity of cuttability can be defined
based on these two fundamental material quantities. Tensile
toughness can be evaluated through tensile testing. Fracture
toughness G.sub.c can be calculated from the tear strength:
G.sub.c=2P.sub.c.vertline.b (1)
[0062] where P.sub.c is the load at tear crack growth and b is the
specimen thickness. (See Rivlin, R. S. & Thomas, A. G., (1953),
J. Polym. Sci., 10, 291).
[0063] For practical simplicity, a dimensionless quantity of
cutting index is defined as follows,
C=0.5*W.sub.t/W.sub.tr+0.5*G.sub.c/G.sub.cr (2)
[0064] where C is the cutting index, W.sub.t is tensile toughness
and G.sub.c is fracture toughness, and W.sub.tr and G.sub.cr are
the corresponding properties of a reference material, where
conventional cellulose triacetate (CTA) film is selected as the
reference material of this invention. The cutting indices of
commonly used film base materials such as PET, PEN and CTA
correspond well to their practical cutting performance. Generally,
it is desirable for C to be close to 1 (CTA value).
[0065] The polyester films having the properties set forth above
and prepared by the process described above are less likely to fail
and more likely to produce cleaner cut surfaces in various cutting
operations. In fact, the films prepared in accordance with this
invention compare favorably with CTA, which has been the
traditional film base of choice in the photographic industry
because of its special physical characteristics.
[0066] The present invention is described in detail below by
referring to the Examples. However, the present invention should
not be construed as being limited thereto.
EXAMPLES
Materials
[0067] The polymer films described in the following examples are
prepared using a commercial polyester resin with the trade name of
EASTAR A150 (Trademark of Eastman Chemical Company, U.S.A). The
material is a copolymer of poly(cyclohexane dimethylene
isophthalate) and poly(cyclohexane dimethylene terephthalate) and
it comprises approximately 17 mol % of isophthalic diacid in its
acid component and 100 mol % of CHDM comonomer in its glycol
component.
Physical Property Evaluation
[0068] The physical properties of the films prepared in the
Examples below were evaluated as follows:
[0069] Tensile Toughness: All tests were performed in accordance
with ASTM D 882-80a in a standard environment of 50% RH and
23.degree. C. The tensile test is conducted using a Sintech.RTM.2
mechanical testing system with Testworks.RTM. version 4.5 software.
The specimen size is 1.5 cm wide by 10.2 cm long (gauge length).
The crosshead speed is 5.1 cm/min. Five specimens are tested per
film sample. The reported tensile toughness is the area under the
stress-strain curve.
[0070] Tear Strength: All tear tests were performed in accordance
with ASTM D1938 in a standard environment of 50% RH and 23.degree.
C. The tear test is conducted using a Sintech.RTM.2 mechanical
testing system with Testworks.RTM. version 4.5 software. The
specimen size is 2.5 cm wide by 7.6 cm long. A 2.5 cm long cut is
first made in the specimen at the center of the width using a pair
of sharp scissors, creating two distinct arms. The arms are placed
between two flat-faced grips of the mechanical test frame and
stretched apart. The crosshead speed is 25 cm/min. The tear
strength is reported by normalizing the average peak load by the
thickness of the film.
Example 1
[0071] Eastar.RTM. A150 resin was converted into film using the
process of melt extrusion, drafting, tentering, and heat-setting
steps. The resin was extruded through a sheet-forming die at a
temperature of 277.degree. C. and cast onto a cooling surface at
60.degree. C. to form a continuous cast sheet. The sheet was
drafted and tentered at temperatures of 100.degree. C. and
110.degree. C., respectively. The cast sheet was stretched to 3.4
times its original dimensions in both the machine and transverse
directions and immediately following stretching, the film was
heat-set at an actual temperature of 170.degree. C. for
approximately 10 sec. The tensile toughness and the tear strength
of the film were measured in accordance with procedures as
described above. The cutting index was determined according to
Equation 2. The results are shown in Table 1.
Examples 2-4
[0072] Films of Eastar.RTM. A150 resin were produced in the same
manner as described in Example 1 but the actual heat-set
temperatures were set at the following values: 175, 176, and
191.degree. C. for Examples 2-4, respectively. The tensile
toughness and the tear strength of the corresponding films were
measured as described above. The cutting index was determined
according to Equation 2. The results are shown in Table 1.
Comparative Examples 1-4
[0073] In comparative Examples 1-4, Eastar.RTM. A150 resin is
converted into film in the same manner as described in Example 1
except that the films were heat-set at the following temperatures:
150.degree. C., 204.degree. C., 220.degree. C., and 240.degree. C.
for Comparative Examples 1-4, respectively. The tensile toughness
and the tear strength of the film were measured as described above
and the cutting index was determined according to Equation 2. The
results are shown in Table 1.
Comparative Example 5
[0074] Comparative Example 5 is a conventional cellulose triacetate
(CTA) film used as a support for 35 mm photographic elements. This
film has been prepared by a conventional solvent-casting process
well known to the skilled artisan.
1TABLE 1 Heat-Set Tensile Tear Film Temperature Toughness Strength
Cutting Example .degree. C. MPa g/100 .mu.m Index 1 170 44.5 15.4
1.3 2 175 42.6 12.0 1.2 3 176 43.4 11.2 1.2 4 191 52.2 14.2 1.4
Comparative 1 150 24.9 8.2 0.7 Comparative 2 204 59.4 16.2 1.6
Comparative 3 220 71.2 21.3 2.0 Comparative 4 240 82.9 32.4 2.5
Comparative 5 (CTA) NA 23 25 1.0
[0075] Examples 1 to 4 in Table 1 show that the desired film
cutting index (1-1.5) and tensile toughness for EASTAR A1500
polyester film base (15-55 MPa) can be achieved by selecting the
proper heat-set temperature. Unexpectedly, the desired properties
are obtained when the material is heat-set at relatively low
temperatures. However, a heat-set temperature that is too low, such
as 150.degree. C. used in Comparative Example 1, results in film
with very low tear strength and low cutting index. Such a film,
while easy to cut and finish, is potentially susceptible to tearing
in cameras and photofinishing equipment. Conventionally used
heat-set temperatures such as 204.degree. C., 220.degree. C. and
240.degree. C. in Comparative Examples 2, 3 and 4, respectively,
result in a film base that is potentially difficult to cut. The
preferred cutting index for use in cameras and photofinishing
equipment ranges from 1 to 1.5 is obtained only when unusually low
heat set temperature is employed. This range is based on the
well-established cutting performance of CTA in the photographic
industry. In general, films with a cutting index of less than 1 are
easily torn during transport in cameras and may be undesirable.
Films with a cutting index of greater than 1.5 may be difficult to
cut in some photofinishing equipment.
[0076] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *
References