Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BIAXIALLY ORIENTED ORDERED POLYMER PILMS
FIE~D OF THE INVENTION
This invention relate~ in general to the formation
of thlck ti.e., greater than about 0.10, preferably
0.20 mm) films having a controlled biaxial molecular
orientation. Such films are preferably prepared in
accordance with the present invention from rod-like
extended-chain aromatic-heterocycl ic ordered polymers.
Such f~lms have a controllable coefficient of thermal
expansion (CTE~, low dielectric constant, low moisture
pick~p charac~eri~tics, low outgas ing, high tensile
strength, high modulus, and ~uperlor environmental
reslstance charac~er:lstics in comparison to uniaxial
films of similar compo~ition. The films of the presen~
invention exhibit thermal ~tability, ~hemical
res~stance and toughness, even at low temperatures.
BAC~GROUND OF THE INV~NTION
Ordered polymers are polymers having an nordered, n
~-~2~
--2--
orientation in space i.e., linear, circular, star
shaped, or the like, imposed thereon by the nature of
the monomer units making up the polymer. Most ordered
polymers possess a linear "order" due to the linear
nature of the monomeric repeating units comprising the
polymeric chain. Linear ordered polymers are also
known as "rod-like" polymers.
For example, U.S. Patent No. 4,423,202 to Choe,
discloses a process for the production of
para-ordered, aromatic heterocyclic polymers having an
average molecular weight in the range of from about
10,0~0 to 30,000.
U.S. Patent No. 4,377,546 to ~elminiak, discloses a
process ~or the preparation of composite films prepared
from para-ordered, rod-like, aromatic, heterocyclic
polymers embedded in an amorphous heterocyclic system.
U.S. Patent Nos. 4,323,493 and 4,321,357 to Keske
et al., disclose melt prepared, ordered, linear,
crystalline injection moldable poly~ers containing
aliphatic, cycloaliphatic and araliphatic moieties.
U.S. Patent No. 4,229,566 to Evers et al.,
describes para-ordered aromatic heterocyclic polymers
characterized by the presence of diphenoxybenzene
"swivel" sections in the polymer chain.
U.S. Patent No. 4,207,407 to Helminiak et al.,
discloses composite films prepared from a para-ordered,
rod-liXe aromatic heterocyclic polymer admixad with a
flexible, coil-like amorphous heterocyclic polymer.
U.S. Patent No. 4,108,835 to Arnold et al.,
describes para-ordered aromatic heterocyclic polymers
containing pendant phenyl groups along the polymer
chain backbone.
U.S. Patent No. 4,051,108 to Helminiak et al.,
discloses a process for the preparation of films and
.. . .
_3- ~32~
coatings from para-ordered aromat~c heterocyclic
polymers.
Ordered polymer solution~ in polyphosphoric acids
(including PB~ compositions) use~ul as a dope in the
production of polymeric fibers and films are described
in U,S. Patent Nos. 4,533,692, 4,533,693 and 4,533,724
(to Wol~e et al.).
Film processi~g methods and apparatus have been
available ~or a ~umb~r o~ years. For example, U.S. .
Patsnt No, 4,370,293 to Petersen-Ho; describes a method
and apparatus for the manu~acture of biaxially oriented
plastic Pilms, parkicularly polyester films. The
process described for polyester comprises extruding
polyester through an annular die to form a seamless
tube and inflating the tube by means of a pressurized
gas. The ~xpanded tube thus ~ormed is drawn out in a
longitudinal direction, cooled and flattened. The
flattened tube i~ heated to the orientation tem~er~ture
Or the film, expanded again, and stretched in its
longitudinal direction. These stretching techniques
are said to impart a biaxial orientation to the
polym~ric backbone of the film~
Similarly, U.S. Patent No. 4,011,128 to Suzu~i
describes a m~thod and apparatus for forming a
cro~s-ori~nted film, wherein a non-oriented film to be
tr~a~ed is ~irst ~ormed by conventional methods, then
cross-oriented by stretching and twisting. In addition
the cross-oriented ~il~ i8 flattened so as to
continuously form a laminated cross-orienked film.
U.S. Patent ~o. 4,358,330 to Aronovici describes a
m~thod and apparatu6 for manufacturing films having
pairs o~ adjacent layers who~R molecular oxien~ation is
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~l32~6~L
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in different directions. ~he method employed is a
modification of the conventional ~blown film~ technique
such that the molecular chains forminy the layers of
film are oriented substantially immediately prior to
their solidifying.
U.S. Patent No. 4,~96,413 to Sharps, Jr., describes
a process and apparatus for the preparation of a
blocked cross-plied polymer film which involves the
extrusion of a polymer melt through a tubular rotary
die. The rotation of a single member o~ the die is
said to impart a molecular orientation to the polymer
in a transverse direction during the extrusion. The
film is blocXed by expanding the film and then pressing
opposing walls together to produce a composite film
having at least two layers, each having a transverse
molecular orientation opposing the other. The
composite film is said to have a balanced cross-ply.
The disclosures of each of the above described
patents are incorporated herein by reference.
The degree of molecular orientation achieved duxing
tbe rotating die extrusion of thermoplastic polymers is
very low, since random coil thermoplastic melts are not
oriented to any great extent by shear,unless the melts
are anisotropic (such as Xydar). Minimal biaxial
orientation of thermoplastics is obtained by blowing
tubular films of the melt. Even then, the preferential
molecular orientation in blown thermoplastic films is
in the machine direction~
on the other hand, anisotropic dopes of ordered,
rigid-rod polymexs contain isolated bundles of oriented
molecules suspended in solvent. It has been discovered
that counter-rotating tubular Pxtrusion of these
polymers orients these crystallites in the direction of
shear. Stretchin~ of biaxially-oriented ~ubular films
'
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~ ~ 3 ~
5~
., .
: of anisoptropic dope by blowing further increases the
degree of orientat~on in ~uch materlal~.
. . .
SUMMARY oP THE INvENTION
. 5
iThe present invention is dlrected to the produckion
.of films having hereto~ore unava$1able strength
;,'characteristics in more than one direction. The
startlng materials u~eEul hereln include those
lyotropic or ~hermotropic polymeric materials in which
strain produces a materlal orlentation in the
micro~cale structure and which are relatively weak if
.thi~ orientation is ~n only one dlrection, l.e.,
uniax~al. The present invention i8 particularly
15 appllcable to dopes and like material~ made from
ordered polymers, or o~cher rlgid rod-lilte molecules.
The method of the pre~ent invention comprises
~irst producing a certain mlcro~cale structural
or~entation within a polymer dope by a sequence of
,.20 ~training methods, followed by solidlfying th~s ordered
~tructure by a sequence of thermal and/or chemical
conditioning operatlons.
~ he present invention is e~pecially directed to
biaxially oriented fllms, Goatin98 and 1 ike materials
~ormed from ordered polymers. A preferred ordered
polymer ~or use in th~ present lnventlon is poly 5~
;- phenyle~ebenzo blsthiazole), (P~T), a compound havlng
:.the structure:
- -
~n
;
-6- ~32~
Biaxially oriented polymeric films of PBT are
especially preferred embodiments of the present
invention . These films possess unique properties
including:
(a) high tensile strength (most preferably,
greater than 100,000 psi ultimate tensile
stress in one direction and ~ot less than
40,000 psi ultimate tensile stress in any
direction);
(b) high modulus (most preferably, greater than 5
x lo6psi tensile modulus in one direction
and not less than 8 x 105psi tensile modulus
in any direction);
(c) controllable coefficient of thermal expansion
(CTE) either negative, positive or zero in any
particular direction in the plane of the film;
(d) low dielectric constant (most preferably, less
than 3.0);
(e) low outgassing ~most preferably, less than
0.1% weight loss in a vacuum at 125C for 24
hours;
(f) low moisture pickup (most preferably, less
than 0.5% weigh~ gain in water at 100C for
24 hours.
The present invention is also directed to methods
and apparatus suitable for produciny biaxially oriented
films, coatings, and like materials from ordered
polymers, preferably PBT.
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_7_ ~3~8~ ~
The preferred films, methods and apparatus of the
present invention are described in greater detail i~
the accompanying drawings and in the detailed
description o the invention which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram depicting the process
of the present invention for the formation of biaxially
oriented films from ordered polymers.
Figure 2 is a schematic representation of a single
screw extruder apparatus for the degassing and
preconditioning of PBT dope;
Figure 3 is a schematic representation of a counter
rotating tube die apparatus for producing a biaxially
oriented film from an ordered polymer~
Figure 4 is a schematic representation of one
preferred drying/heat treating apparatus used for
producing a biaxially oriented film from an ordered
polymer.
Figure 5 is a schematic representation of an
apparatus incorporating the die of Figure 4
constructed in accord with the present invention.
Figura 6 is a schematic representation of a counter
rotating plate apparatus for producing a biaxially
oriented film from an ordered polymer;
Figure 7 is a schematic representation of a roller
die apparatus for producing a biaxially oriented film
from an ordered polymer.
Figure 8 is a schematic representation of the
proce~sing apparatus preferably employed in the present
invention.
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-8- ~328~
Figure 9 illustrates various orientations of
polymer films. Figure 9A represents uniaxial
orientation, i.e., that imposed on polymers by typical
slit-die extrusion or fiber spinning. Figure 9B
represents the random disorder of ordered polymer films
that are coagulated without pre-orientation. Figure sc
illustrates the biaxial order imposed on the polymer of
Fig. ga by treatment in accord with the present
invention.
Figure 10 illustrates an end-attachment for the
tube-die of Figure 8 to reduce the die gap thereof.
DETAILED DESCRIPTION OF T~E PREFERRED EMBODIMENTS
The present invention is directed to the production
of film having controlled anisotropic properties from
ordered polymers.
When ordered polymers are subjected to a shear
field they become highly aligned in the direction of
the applied field. By imparting to such polymers a
preferred orientation one obtains material with a high
tensile value which is the basis or producing fibers
of high strength.
2S Similar orientation imparted in the machine
direction during the production of ordered polymer
films results in films having a very high tensile
strength in the machine oriented direction but very
poor physical properties in the transverse direction.
In some cases highly oriented polymer films will lose
their film integrity by simply coming apart along the
machine direction orientation.
`- 13~6~
The present invention is thus directed to the
productior. of ordered polymer films that have highly
controlled orientation resulting in films that have
property balances that are much more useful from a
practical standpoint. Films can be produced having
high tensile values in the machine direction and
substantial strength in the transverse direction.
These films maintain their film integrity and as a
result are useful in many applications requiring good
film properties.
The process of the present invention affords films
that have strength characteristics making them suitable
for the production of laminate film composites and like
structures.
The essential strength chara~teristics of these
films are the result of a two stage orientation process
followed by post treatment to optimize the film
property balance. In preferred embodiments, the
biaxial molecular orientation is achieved by utilizing
a homogenized dope consisting of PBT in polyphosphoric
acid~ The term "polyphosphoric acid" as used herein,
means any of the members of the continuous series of
amorphous condensed phosphoric acid/water mixtures,
generally given by the ~ormula:
Hn+2Pn~3n+1
wherein thP value of n depends upon the molar ratio of
water to phosphorous pentoxide present. Such
compositions are described in U.S. Patent Nos.
4,533,6~2, 4,533,724, and 4,533,693 (to Wolfe et al.).
Referring to Figure 1, there is illustrated a block
diagram of the principal steps of the method of thP
present invention for the formation of biaxially
oriented films from the preferred ordered polymer, PBT.
```` ~328~
-- 10 --
As illustrated at 10 the first step comprises a
conditioning of the polymer which preferably is about a
10 to 30 weight percent solution in poly(phosphoric
acid), or PPA. PPA is the preferred solvent, although
methanesul~onic acid (MSA) or chlorosulfonic acid (CSA)
may also be used. The degassing step is employed to
prevent interference of entrapped gas within the polymer
solution with the molecular orientation of the film.
The second step (12) comprises the orientation
step. This may be accomplished by the use of any of the
extrusion means which induce shear flow, stretching, and
the like. Preferred extrusion means of the present
invention include counter rotating tube dies, plates, or
roller dies. It has been discovered that such extrusion
means, preferably combined with subsequent stretching of
the extrudate, may be employed to impart varying degrees
of biaxial orientation to ordered polymers.
A third step (14) comprises coagulation of the
polymeric solution.
The fourth s~ep (16) is a densification step
wherein the PPA is removed.
The penultimate step (18) is generally a drying
and heat treatment step.
In the final step, the product film is packaged.
Each of these general steps will be elucidated
further in the description of the preferred apparatus for
co~ducting the above described processing conditions.
In Figure 2 there is illustrated one preferred
embodiment of an extruder apparatus for the degassing of
PBT dope. After homogenization (as described in the
Wolfe et al. patents) the dope is fed by means of a
; ~ ~
3 2 8 ~
heated pressure pot t22) to the inlet of a slow heated
extruder (24) which in turn feeds a positive
displacement pump (26~.
The positive displacement pump t26) of Fig. 2 feeds
a film die (28) as illustrated in Figure 3. The film
die (28) has two counter rotating barrels, 30 and 32
respectively, whose purpose is to create a shear field
through the cross section of the extruded dope
composition. This shear field is at right angles to
the axial shear field produced by forcing the dopa
axially through the annulus of the die. Counter
rotating die members are necessary to prevent a
screw-like rotation of ~he orientation and twist-off of
the extrudate which occurs if only one member of the
die is rotatedO This combination of shear fields is
necessary prior to the blowing operation in order to
permit blowing o~ the tube without fracturing the
extrudate, and hence, to produce material with integral
biaxial film properties.
Upon exit from counter rotating die (28) the film
is treated to a blowing operation. Here, the film is
expanded under internal pressure, further orienting the
molecules throughout the film cross section. Control
of the die RPM, extrusion rate, film windup rate, and
degree of expansion rasults in a precisely aligned,
blown PBT dope composition film. The top and bottom
surfaces of the film are aligned at approximately equal
but opposite angles to the machine direction.
As described above, the processing variables of die
speed (RPM), extrusion rate, and degree of film
extension and expansion during the blowing operation,
all can be varied to achieve any desired degree of
biaxial molecular orientation.
~ .
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Control of the degree of molecular orientation
results in attractive film properties. Blown dope
compositions that have not been subjected to controlled
shear fields prior to expansion do not have physical
property balances anywhere approaching those of the
films of the present invention. Furthermore, films
extruded by the counter rotated die but not wi~h the
blowing process do not have good property balances. It
is the combination of shear field extrusion followed by
internal expansion and extension that yields films with
a useful property balance.
The extruded, sheared and blown film is quenched,
both on the internal and external surfaces, by an
aqueous coagulation bath or other controlled aqueous
coagulant composition. This que~ching operation serves
to "gel" the polymer dope composition, producing a
strong, tough, solution-filled film. By controlling
the ccmposition of the coagulation bath many other
materials can be incorporated into the film
microstructure~
In addition to causing the film microstructure to
gel and become strong, the aqueous solution serves to
hydrolize the polyphosphoric acid to phosphoric acid,
facilitating its removal from the film. The
solution-filled film i~ then wa~hed free of phosphoric
acid before it is subjected to controlled drying
conditions.
As illustrated in Figure 4, the film is preferably
dried under controlled internal pressure, also known as
a res~rained drying process. This is accomplished by
drying the film under a regulated air or nitrogen
pressure of from about 5 to 10 psi as illus~rated. The
pressurized film tube in the example may have about 1.5
to 3 inches diameter and a length of from about 5 to 12
.
~ 3 2 ~
-13-
inches. Drying under such conditions results in a
highly oriented film of high strength characteristics.
Figure 5 illustrates schematically the above
described processing steps. As illustrated, the
conditioning and degassing step is conducted by the
apparatus (34), which sends the homogenized dope to the
extruder means (36) whereupon shear is imparted to the
dope. The dope is then blown using conventional film
blowing equipment (3B) and the blown tube enters
coagulation zone (40). The coagulation zone ~40)
comprises a water tank ~41) and may include additives
useful in imparting specialized characteristics to the
~ilm. The coagulation zone acts to stabilize the
molecular orientation imparted to the film by the
extrusion and blowing processes. The water and/or
additives in the bath spread into the microstructure of
the film. ~ollowing the coagulation zone, there is
shown an exchange bath t42). Here the acid solvent
used to prepare the polymer dope (PPA, MSA, CSA, etc.)
is removed by repeated water washings. Following
removal of the acid solvent from the film, the film can
be exposed to other solutions that may include
additives useful in imparting special characteristics
to the film. Afterward, the film is dried under
appropriate stress conditions in a drying oven (44).
After drying, the film is packaged using conventional
means (46).
When tube-blowing is amployed, if the tube is not
slit after coagulation but is merely collapsed flat for
water-solution and drying trPatments, it can then be
re-blown and stretched biaxially in a tower- or
tunnel-oven. The tube is slit into tape and
roll-packaged just downstream of a central plug mandrel
and guide rolls. Tube-blowing gas is advantageously
introduced through the mandrel.
.
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-14- ~ 3 ~
Figure 6 illustrates another means of imparting
shear stress to the polymer dope ~hat is useful in the
method of the present invention. As illustrated, the
stress means (48) comprises counterrotating pressure
plates (47 and 49). Polymer dope, such as PBT is
inserted between the plates, pressure is applied and
the plates are rotated in opposite directions.
Another means for imparting shear stress to a
polymer dope in accord with the method of the prese~t
invention is the apparatus illus~rated in Figure 7. As
illustrated, a laterally spreading die (50) having open
top and bottom is contoured to fit in the convergence
of two pinch rolls (52 and 54). The extrudate enters
the die as a high and narrow flow, then undergoes
progressive lateral and axial direct strains to emerge
as a thin and wide strip. This strip then undergoes
some further axial extension to become a film on one of
the rolls, depending on the balance between roll
surface velocity and supply pressure induced flow.
Process variables inc~ude the proportions and internal
shape of the die, the supply pressure, and the film
tension.
The apparatus illustrated in Figure 8 represents a
counter-rotating tube die which comprises an rotatable
cylindrical inner shaft (56) having a smooth surface
encased in an independently rotatable cylinder ~58)
having a plurality o~ passagaways therein (59~. A
space (60) is provided between the shaft and the
cylinder to allow for the introduction of polymer and
to allow independent movement of shaft (56) and
cylinder (58). Cylinder (58) and shaft (56) are
rotated in opposite directions. Ordered polymer is
fed through passageways ~5~) to the space (60j between
shaft (56) and cylinder (58). The polymeric mass
i
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-15- ~ 3 ~
strikes shaft (56) and is subjected to orientation
forces by the opposing movement of cylinder ~58) and
shaft (56). Drive gears (~2) and (64) are shown,
attached to outer cylinder (58) and inner shaft (56)
raspectively. Also illustrated is housing (66) which
surrounds the tube die and serves to control the
temperature of the extrusion system. water inlet (68)
and outlet (70) are provided to maintain the
composition of the coagulation znne and to exert
pressura on the interior o~ the blown film tube.
Nitrogen inlet (72~ serves to maintain an inert
atmosphere within the die and also provides the means
for blowing (i.e., expanding) the film into a tube upon
its exiting the die. Drive gears (62) and (64) are
operated by independent variable speed drive means such
as electric motors (not shown).
Ordered polymer is pumped through passageway (74)
in housing (66) whereupon it impinges upon the surface
of rotating cylinder (58). The polymer flows throug~
the plurality of passages (59) in cylinder (58) into
the space (60) between cylinder (58) and rotating shaft
(56). Sinc~ the top of the die is sealed, the polymer
flows to the outlet at the bottom (~). As the polymer
flows toward the outlet (76) counteracting shear forces
imparted by revolving cylinder (58) and revolving shaft
(56) impart a degree o~ biaxial orientation to the
~ilm's molecular structure.
Previous attempts to rotate only one cylinder of
the tube die wh le maintaining the other in a
stationary condition caused uncontrolled twisting and
tearing of the dope extruding from the die.
Transverse shear, longitudinal flow sh~ar, axial
stretch, and radial expansion forces all interact in
the dies illustrated in Figures 3, 6, 7 and 8 to impart
.
... ...
-16- ~ 3 2 ~
a partial biaxial orientation to the ordered polymer
fed therethrough. Variation of the speed of the
movement of the shaft and cylinder of the illustrated
die, as well as flow rate, temperature, etc. effect the
degree of orientation imparted to the ordered polymer
feedstock. Additional orientation is imparted to the
extruded film by virtue of the blowing processes, both
following the extrusion and as a part of the h~at
treatment.
Figure 9 illustrates the various orientations
imparted to polymers by stress conditions. Typically,
polymers subjected to shear stress assume a uniaxial
orientation as illustrated in Fig. 9A. Ordered
polymers in solution hav~ the scattered or random
nematic orientation illustrated in Fig. 9B. Figure 9C
illustrates the twisted nematic (or cholesteric)
orientation imparted to ordered polymers by processing
under the method of the present invention.
In the preparation of twisted nematic orientation
with P3T by solution processing, molecules in adjacent
planes with twisted orientation are not be abla to pack
closely on solvent removal. Thus, each "layer" will
have to densify by diffusion transverse to the rod
axis, an unlikely process on the microscopic scale of
the sheet. Consequently, if twisted nematic
orientation is smooth and gradual through the film
thic~ness, the densification can occur with the least
amount of strain or disruption between adjacent layers.
Biaxial shearing as well as biaxial direct stresses
30 and strains can be imposed and ~ontrolled in ~his
system. A useful combination of strain patterns is
achieved by the apparatus of Figure 8 where f irst a
twisted nematic tcholesteric) orientation is promoted
in the dies and then a uniform biaxial strain is
.
- 17 -
promoted in the blow/stretch. The former provides enough
bi-directional strength for the latter, as well as near-
order of layers, conducive to densification in the normal
(thickness) direction. The biaxial strain can be
symmetric or asymmetric. If this system is operated with
low strain in the dies, then biaxial blow/stretch will
promote biaxial nematic orientation rather than twisted
nematic.
of course, the system of the present invention
could be used to produce uniaxial nematic tube or film as
well.
A common characteristic of laminates of the
prsferred biaxial film materials is that they can be weak
in the transverse direction (i.e., perpendicular to the
plane of the laminated film). It is therefore desirable
to increase the so called trans-laminar strength of
biaxial films by using additional processing steps in the
manufacture of the film. These additional steps scan be
during the preparation of the dope or in the washing or
solution processing of the coagulated film. Trans-laminar
strength of the film can be increased either by increasing
the cohesivity between the ordered, rigid-rod polymer
structure, and/or by enclosing the ordered structure in a
binding, surro-7nding network of the added material. This
added material typically does not interfere with the rest
of the processing steps, because the added material is not
rendered strong and cohesive except by a subsequent
processing step, e.g., heat treating or chemical
conversion.
An important aspect of the methods envisioned
for increasing trans-laminar film strength is that the
added material is not necessarily intended to be a major
fraction of the final structural material or
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3 ~ 8 11 ~ 7
film; the added material can be a very minor
constituent of the final structure and still provide
sub~tantial trans-laminar cohesivity or strength. In
fact, since the rigid-rod ordered polymeric structure
is relatively very competent, the added material most
preferably is a very minor component, such that the
final overall material has the highest specific
strength and stiffness properties, i~e., highest
strength a~d stiffness per weight and volume.
One method of increasing the trans-laminar strength
of biaxial PBT film is to blend a finely divided powder
of compatible material with the PBT dope during the
dope-preparation step of the total process. A
preferred material is polyphenylene sulfide (PPS), at
about 10 percent by volume tor more) of the final
dope. PPS is a strong, highly resistant, thermotropic
polymer. This powder remains in the dope and the
prepared film through all of the processing steps up to
the f inal drying stage. During drying and heat
treating, the film is heated to a temperature that
melts the PPS, causing it to flow around and between
the PBT rod-like microscale structure. Subsequent
pressing or rolling and cooling produces a structure
that îs strong in all directions of stress.
Another method of increasing translaminar strength
is to diffuse a precursor of a strong binder material
into the PBT ~ilm during the washing stage of the
process. This precursor can be an organometallic
precursor of an inorganic glass, such as
tetramethoxysilane; or an organically-modified glass
precursor that has reactive organic groups incorporated
therein, such as expoxides; or a precursor of a
thermotropic plastic, such as caprolactam as a
precursor for nylon, or polyamic acid as a precursor
-19- ~3~8~ ~
for polyimide. After the precursor has diffused in~o
the washed but still swollen PBT film, e.g., via
various sequential solvent exchanges, the ~ilm is dried
and heat treated, causing a transformation of the added
material to its final form as a strong trans-laminar
binder material. As a final ~inder material, glasses
and polyimides are preferred over nylons, because the
former materials more nearly complement the high
temperature and strength properties of the PBT film
structure.
The processing equipment of the present invention
is straightforward in design and fabrication, with the
exception of the counter-rotating die assembly. The
storage tank must be heated, is preferably made of
stainless steel (e.g~, type 316L suitable for PPA
process~s), and is pressurized with dry/inert gas
(e.g., N2) in order to prevent both coagulation of
PBT and/or starvation of the pump. The pump is
typically a precision-gear type (e.g., Zenith). Other
types of pump, such as piston-ram, extruder, or
traveling-cavity (Moyno~, are possible.
While other counter-rotating tube-dies exist, the
design of the die of the present invention is
specialized in that a wide range of parameters can be
explored by using di~ferent speeds and die-inserts.
Sealing between the hot blo~k and die cylinders is
affected by spring loaded face-bushing (Te~lonR or
graphite~, and alignment is maintained by remote collar
bearlngs. Because the extrudate undergoes so much
densification to final thickness, the die annulus is
usually large, moderating die pressure required. The
central gas for film blowing (N2) is provided through
a remote, cooler, standard rotating coupli~g.
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-20-
Function and operation of the extrusion-blowing
system are thus straightforward:
Counter-rotation of the dies generates
transverse shear without any net twist or torque on the
extruded tube.
The pump generates the axial flow and, in
combinatian with the annular gap, determines the axial
shear (flow profile).
Draw~down of the tube at a linear rate greater
than die-discharge causes an axial strain in the hot,
uncoagulated extrudate.
~ lowing of the film tube causes
circumferential stress and strain in the extrudate.
Immersion in a water bath after blow/stretch
causes coagulation and, below the central water level,
a balance o~ pressure and nulling of pressure
di~ferantial, unless the tube is pinched closed at the
bottom.
Xey processing parameters for ~uccessful extrusion
of biaxial film from PBT/PPA dope with a tube die
substantially as depicted in Figure 8 are listed in
Table I. This tube die has adaptors at the exit of the
die to allow for two different annular diameters and
gap distan es. Referring to Figure ~, the shear zone
length is the distance between the inlet passageways
~59) and the exit o~ the tube die (76)~ Shear rates
are calculated as the linear velocity difference
between the revolving cylinders divided by the gap
distance. Blow ratio is defined as the final diameter
of the coagulated PBT~PPA tube divided by the initial
diameter of the PBT/PPA tube at the exit of the die.
The draw ratio i~ de~ined as the linear bulk velocity
of the PBT/PPA extrudate, at the exit of the die,
divided by the wind-up roller linear velocity,
.. . . .
:
.
: - ;
' ' : : ' :
-21- ~ 32~
referring to (Part ~6) of Figure 5. The linear bulk
velocity is defined as the volumetric output of the
extruder dived by the cross-sectional area of the
annular gap of the die. For typical PBT/PPA dope, the
extruder zone temperature was 120C and the die zone
temperature was 80C; i.e., the PBT/PPA dope was
cooler in the die than the extruder.
TABLE I
Tube Die Specifications:
Annular Gap 0.040", 0.080"
Annulus Diameter 0.80", 1.5"
Shear Zone Axial Length 4 inches
Processing Conditions:
Shear rate ls-1 to
greater than
3s--1
Blow ratio 1:1 to 3:1
Draw ratio 8:1 to 20:1
2~ Extruder Temperature 120C
Die Temperature 80C
Applications of the high-strength, high-modulus,
thermally-stable, chemically resistant, microporous PBT
polymer films of the present invention include the
following: (1) multi-layered, structural composites
molded to complex shapes, (2~ rigid, glass-containing
composites, ~3) filters of controlled porosity for use
.. . . . . .
o
-22~
in harsh environments; ~4) gas separation membranes;
(5) water-purification membranes; (6) electronic
circuit board structures; (7) lightweight space
structures; (8) multi-layered, electrically conducting
structural composites; (9) ionizing radiation-resistant
composites; (10) low radar profile structures; (11)
zero coefficient of expansion structural composites;
(12) porous substrates for controlled release of
volatile materials in harsh environments; (13) leaf
springs, helical springs and (14) capacitors.
The method of the present invention will be further
illustrated with reference to the following examples
which are intended to aid in the understanding of the
present invention, but which are no~ to be construed as
a limitation thereof. All percentages reported herein,
unless otherwise specified, are percent by weight. All
temperatures are expressed in degrees Celsius and are
uncorrected.
EXAMPLE 1
.
The coagulation and take-up system substantially as
described in Figure 5 was used, and ~lown tube films
were extruded under the following conditions:
extrusion die: 3.81 cm diam. x 1.02 mm
gap
extrusion rate: 3 cc/min
air gap: 11.7 cm
coagulation zone: 18.8 cm
take-up speed*: 24.6. cm/min.
counter-rotating
shear rate: 4 sec~
blow-up ratio: 1.5:1
,
::
-23- ~ ~ 2 g~
draw ratio: 10:1
*speed with empty packaye roll
The PBT/PPA dope of this example had an intrinsic
viscosity (IV) of 19 as measured by the method
described in the Wolfe et al. patents tsupra).
An attachment was made for the tube die to reduce
the extruded tube diamet~r to 2 cm (see Figure 10).
10 This allows greater blow-up ratios using the same
take-up system, which is limited to a 7.6 cm maximum
bubble diameter. The die gap was 1.02 mm and the
counter rotating shear rate was about 4.5 sec~l.
The die was also operated at the full 2.04 mm gap,
without any end ~ixtures, to determine whether the
variation in extruded wall thickness was due to the
inner and outer mandrels of the die, or to the
attachments. These films, which are twice as thick
(approximately 0.003 in, 0.076 mm), are fairly uniform
in thickness once the system reaches steady-state, and
they do not exhibit any spiral pattern.
Uniform operation can be restored by a combination
oP reducing internal pressure, increasing longitudinal
draw, decreasing internal water height, and spraying
thin sections with water to "freeze" that section of
the bubble. Other blown tube processes (high molecular
weiqht polyethylene, for example) encounter similar
bubble stability and film thickness problems Internal
mandrels ~within the bubble) can be used to direct cold
air at the blown film to chill it (analogous to
coagulation). Driven pinch rolls could also be used to
provide more controllable draw.
. : ,
-24- ~32~
Converging plates were used on one run to collapse
the bubble and reduce folding and creasing o~ the
tube. The plates were made from clear acrylic sheets
and were attached about 2 c~ above the pinch so that
the tops o~ the plates were above the water level and
the PBT film would ~e coagulated before touching the
inclined plate. In operation, the coagulated PBT tube
tended to stick then slip on the plates, causing some
vibration in the take-up system. This was the result
of unexpected friction between the tube and the
plates, and could be remedied by using Teflon plates,
or going to a roller or belt converging system.
Otherwise, the convarging plates worked well to
maintain bubble diameter and alignment, and resulted
in smooth surface films.
Washing and Dryina Films
All film samples were collected on a wide spool
under water and were kept under water without air
contact and interleaving corsely woven material was
used to allow water circulation. Samples were washed
for at least 48 hours before drying.
The samples measured 0.8% phosphorous after 24 wash,
and 4% on samples with only 5 minutes wash.
Several drying methods were attempted including:
- Clamping wet films in 7.6 cm square frames
- applying internal pressure of 5 to 9 psi to
the wet tube
- using rods inside the tube with variable
- spring-load between the rods.
The clamped frame method works well, is simple and is
convenient to hold samples for subsequent heat
treatement trials. The internal gas pressure methods
requires a pressure regulator, crimping seal at two
ends of the tube and a pressure relief valve to allow
.: :
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-25- ~3~8~1
passage of gas and water from the inside of the tube.
Internal pressure causes thin film sections to be more
highly stressed than thick ones, but the stress is
more predictable than in the case of the clamping
frame. Stress was calculated during drying trials at
3000 to 5000 psi in the hoop direction, and half that
in the longitudinal direction.
EXA~PLE 2
The PBT/PPA dope of this example was obtained from
DuPont, and identified as follows:
- SRI code 5103-28
- 50 KG (110 lb~ of PBT/PPA
- 13.7% P~T
- Intrinsic viscosity (IV) = 40, as measured
by SRI
- DuPont has measured 35 to 40, indicatinq
variability in the dope. The viscosity is
stable with temperature, they report).
- 82.7% P20s
This materials is much more viscous than the 19 IV
2~ dope used in Example 1.
The dope preparation system was assembled as in
Example 1, with vacuum degassing, a 50-hole ~0.36 mm
diameter) spinneret, and a sintered metal filter (80
micron) spin pack. The major difference between this
and Example 1 (using l9 IV polymer) was than the feed
pot piston pressure and temperature were increased.
The 40 IV dope required 100 psi instead of 20 psi, and
240F instead of 200~. The flow from the feed
pot to the extruder was slower because there was
almost no shear on the dope and the viscosity remained
-26- ~ 3 2g ~ 6 1
quite high. Once in the barrel of the extruder, the
screw provided shear, and the 40 IV material extruded
easily at about 230F and 1000 psi barrel pressure
-- very similar to conditions with the 19 IV polymer.
The extruder was operated at 3 cm/min; faster rates
may require more temperature and pressure at the feed
pot to avoid starving the extruder.
Degassing proceed in much the same way as the 19
IV dope. The filaments falling into the vacuum showed
"graininess~' indicating entrained gases were being
removed. About 2.5L ~5 kg or 11 lb) of PBT/PPA dope
was prepared by one pass through the system.
As described in Example 1, the tube die was
modifiPd to reduce bearing clearances; improve
alignment, and reduce corrosion problems.
About 50 ft of high quality film was produced from
this example. Relatively high draw ratios were used,
in the range of 13:1 to 21:1 because bubble stability
improved markedly when take-up was increased. Blow-up
ratios were maintained at about 2 :1. All successful
film trials were made with the 0. 040 in. gap. The~e
conditions resulted in relatively thin ~ilm, 0.1 to
0.6 mil (2.5 to 15.2 micron) in thickness. These thin
films could be produced consistently at steady-state
because o~ the "toughness" of the 40 IV polymer dope
and the improved precision of the die.
The die rotation rate was varied from 0.5 to 2 rpm
with a die temperature of 190F.
The most successful extrusion conditio~s are0 summarized below:
throughputo 3 cc/min
barrell pressure: 1000 psi
die pressure: 50 to 75 psi (estimated)
barr~l temperature: 230F
." ~ . . ..
.
-27- ~32~61
die temperature: 190~
draw ratio: 13:1 to 21:1
blow-up ration: 2:1
die rotation: 0.5 rpm
An ice-water coagulation bath was used to achieve
better ~ilm properties. The low temperature bath
provided a slower coagulation which was less
disruptive to the oriented PBT polymer network.
Film ~ ty_Measurements
Tensile tests showed impxoved strength and modulus
for biaxially oriented films. Heat treatment was
conducted at 400C for 2 hours. Higher temperature
heat treatment will be evaluated at temperatures up to
650C for brief periods, typically 30 to 60 seconds.
EXAMPLE 3
Rolle~ rusions
In this example about 3.0 liters of 13.7~ solids
PBT/PPA dope (40 I.V.) was extruded using a roller die
substantially as illustrated in Figure 7. The
processing conditions were those listed in Table II.
Approximately 1.0 liter of dope was extruded in each
O~ the three trial extrusionsO
.. ~
- , -., . ~ .:
-28~ 6 ~
TABLE II
Processing conditions for roller-die extrusions
of 40 I.V. PBT/PPA dope
Run Throuqhput cc/min ~E~ Die Tem~erature F
1,2 4.8 - 8.0 - 190 230
312.8 4.6:1 " 230
15.8 3.8:1 It 230
The first two runs gave useful operating
information on the roller die system, but did not
produce high quality film. The roller die required
higher throughputs than the tube die used in the
previous examples, and at throughputs around 10 cc/min,
the feed vessel could not feed the extruder screw
sufficiently. Therefore the feedpot system was
modified from the air drived piston of the previous
Examples to a hydraulic-ram-driven one.
The uncoagulted PB$/PPA extrudate could be released
from th~ rollers of the die~ with applica~ion of a mold
release agent "ReleassaGen H-1501", formerly produced
by General Mills.
In run #3, about 15-20 feet of 2 in. wide x 0.075
in. thick (washed state) sheet-like PBT was extruded.
3 0 This thick film had a regular V~shaped, ridged
pattern. By visual inspection, the film had a
microfibrillar structure in the machine direction~
However it was more difficult to split parallel to the
machine direction in comparison to a uniaxial film.
~- . . . . .
.
-29- ~ ~2
The film thus has the desired improvedment in
transverse strength~
The present invention has been described in detail,
including the preferred embodiments thereof. However,
it will be appreciated that those skilled in the art,
upon consideration of the present disclosure, may make
modifications and/or improvements on this invention and
still be within the scope and spirit of this invention
as set forth in the following claims.
29
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