Note: Descriptions are shown in the official language in which they were submitted.
CA 02778513 2012-05-28
INDUSTRIAL TEXTILES COMPRISED OF BI-AXIALLY ORIENTED,
HYDROLYTICALLY STABILIZED POLYMER FILM
FIELD OF THE INVENTION
The invention generally concerns hydrolytically stabilized, bi-axially
oriented polymeric
films. It is particularly concerned with the use of such films in the
manufacture and
production of components for industrial textiles and other applications where
durability
and stability in adverse environmental conditions are important.
BACKGROUND OF THE INVENTION
Industrial fabrics used in filtration, conveyance and similar processes have
been and are
typically manufactured by weaving, whereby synthetic yarns are interwoven to
provide
either the entire fabric, or only a base portion which may subsequently be
either
encapsulated (e.g. with polyurethane or other similar rugged material) or
needled to
attach a nonwoven ban. material. Such fabrics have been satisfactory for these
uses, but
the cost of their production is high, particularly when the fabrics must be
finely and
precisely woven using relatively small yarns. Further, these fabrics must be
rendered
endless in some manner, either by installing a seaming element at their
opposed
longitudinal ends, or by re-weaving the longitudinal yarns back into the
fabric structure to
form seaming loops or similar joining means, for secure connection by a
pintle, coil or
similar securing means. It is also known to weave such fabrics in an endless
manner, so
that there is no seam, or to interweave the yarns from one longitudinal end
into the yarns
of the opposed end to forni a woven seam. These fabrics are expensive to
produce and
require a high capital investment in wide industrial looms and similar related
equipment
for subsequent processing, as well as a skilled workforce to operate the
equipment and
produce an acceptable finished product.
It has recently been proposed by Manninen (WO 2011/069259) to construct
nonwoven
fabrics suitable for the same or similar processes as those which currently
use woven
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fabrics by using selectively slit and embossed polymeric films. It is also
proposed to form
components from suitably shaped and processed film for use in seaming both
these
nonwoven and woven textiles (see WO 2010/121360, WO 2011/069258, or CA
2749477).
The '259 document proposes that thermoplastic polymeric films used in such
fabrics
described by Manninen in WO 2011/069259, WO 2010/121360, WO 2011/069258, or
CA 2749477 and which are comprised of bi-axially oriented hydrolytically
stabilized
polymer films which are resistant to heat, humidity and abrasive wear.
Bi-axially oriented hydrolytically stabilized polyester films are known. For
example, US
6,855,758 to Murschall et al. discloses a hydrolysis resistant bi-axially
oriented film
made from a crystallizable thermoplastic polyester. This patent teaches that
phenolic
stabilizers, in particular the 3,5-di-tert-butyl-4-hydroxyphenyl propionates
of
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monomeric carbodiimides such as dicyclohexylcarbodiimide, or aromatic
polymeric
carbodiimides having a molecular weight of from 2,000 to 5,000 obtainable as
Stabaxol0
P from Rhein Chemie GmbH of Mannheim, Geimany may also be used. In a preferred
embodiment, the film includes from 0.1% to 5% pbw of aromatic polymeric
carbodiimides and from 0.1% to 5% pbw of a blend made from 30% to 90% pbw
organic
phosphite (in particular a triaryl phosphite) and from 70% to 10% pbw of a
hydroxyphenyl propionate. The film as claimed includes a crystallizable
polyester or
copolyester and a hydrolysis stabilizer consisting essentially of (1) either a
monomeric
carbodiimide, an aromatic polymeric carbodiimide or oxazolines, and (2)
optionally at
least one of either a phenolic compound or an organic phosphite. The examples
noted in
the reference do not include any film of a thickness exceeding 100 m, in
particular in a
range suitable for use in industrial textiles and components thereof
US 6,020,056 to Walker et al. discloses a bi-axially oriented hydrolytically
stabilized
PET film which does not employ end-capping agents such as carbodiimides. The
PET has
an initial IV of from 0.95 to 1.1 and, when cast, has an IV of from about 0.8
to 1.0; the
resulting film is stretched and oriented at least 2 times in the MD and CD,
and its
intended end use is as a motor insulation.
US 7,229,697 to Kliesch et al. discloses a hydrolysis stabilized PET film
whose thickness
is from 0.4 to 500 pan, using alternatives to carbodiimides as hydrolysis
stabilizers for
PET to address health issues relating to the off-gassing of isocyanates during
extrusion
and increases in the molecular weight of the PET and extrusion problems
associated with
it.
WO 2011/030098 to Brennan et al. discloses a bi-axially oriented PET film
further
including a hydrolysis stabilizer which is a glycidyl ester of a branched
monocarboxylic
acid having from 5 to 50 carbon atoms and which is present in the film as its
reaction
product along with some of the polyester end-groups. Use of the film as a
layer in a
photovoltaic cell is disclosed.
It is known from US 5,885,709 to Wick et al. to provide hydrolysis resistant
polyester
fibers and filaments which have capped carboxyl end groups following reaction
with
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carbodiimides. The polyesters can include any aliphatic or aromatic filament
forming
polyester (e.g. PET) with preference given to those having a molecular weight
corresponding to an intrinsic viscosity (IV) of at least 0.64 and preferably
at least 0.7
dL/g as measured in dichloroacetic acid at 25 C.
None of the aforementioned prior art proposes a bi-axially oriented,
hydrolytically
stabilized polymeric film with physical characteristics making it suitable for
use as
components of nonwoven industrial textiles. In order to be useful and have
physical
properties sufficient to allow such textiles to survive the various rigors of
the
environment for which they are intended, a suitable film should be formed from
a
medium to high IV polyester; the IV should be between about 0.55 and 1Ø The
PET
must also be hydrolytically stabilized to prevent premature depolymerization
in hot and
moist environments due to hydrolytic degradation; carbodiimides are preferred
for this
application. The film must be stretched and oriented as it is produced so as
to increase
and maximize its elastic modulus and other physical properties. The film
should have a
thickness of from about 100 up to 500 um, but ideally in the range of about
250 to
350 um and this caliper should be uniform throughout. None of the prior art
describes
such a film.
SUMMARY OF THE INVENTION
In a first broad embodiment, the present invention seeks to provide a
biaxially oriented
multilayer thermoplastic film, wherein
(i) each layer comprises a polyester having an intrinsic viscosity (IV) of at
least
0.5;
(ii) at least one layer comprises a hydrolytic stabilizer comprising a
carbodiimide;
and
(iii) the film has a thickness of at least 100um.
Preferably, the polyester for each layer is selected from one of polyethylene
terephthalate
(PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN) and
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poly(cyclohexylene dimethylene terephthalate) acid (PCTA). Preferably, the
polyester
for each layer is PET.
Preferably, the PET has an intrinsic viscosity (IV) of from about 0.55 to at
least 1.0; more
preferably the PET has an IV in the range of about 0.5 to 0.8 when measured at
30 C in a
solution of trifluoroacetic acid and dichloromethane.
Preferably, the film consists of at least two layers of miscible polymeric
extrudate. When
the film is comprised of two layers, one layer will have a thickness of from
5% to 15% of
the overall film thickness, and the second layer will have a thickness of from
85% to 95%.
More preferably, the thickness of the first film layer is 10% of the overall
film thickness
and the thickness of the second layer is 90% of the film thickness.
Preferably, the film is comprised of at least three coextruded layers. Where
the film is
comprised of three coextruded layers, each of the outer layers preferably
comprises from
5% to 20% of the overall film thickness and the inner layer comprises from 60%
to 90%
of the film thickness. More preferably, each of the outer layers comprises
from 10% to
15% of the final film thickness.
Preferably, at least one outer film layer includes an antiblock agent.
Preferably, at least one film layer includes a colorant or other additive,
such as titanium
dioxide, carbon black, or a dye. Alternatively, at least one outer layer
includes a radiant
energy absorbing material.
Preferably, the overall thickness of the finished film is at least 100 pm;
more preferably,
the film thickness is between 100 ¨ 500 p.m. Most preferably, the film
thickness is
between about 175 and about 350 m.
Preferably, the hydrolysis stabilizer is a carbodiimide and is added to and
blended with
the polyester in masterbatch form sufficient to comprise from about 0.1% to 5%
pbw
(parts by weight), preferably about 0.5% to 3% pbw and more preferably about
1.5% to 3%
pbw based on the weight of the material in each film layer. Preference is
presently given
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to aromatic polymeric carbodiimides; alternatively the carbodiimide is
monomeric.
Preferably, the carbodiimide is incorporated as a masterbatch in the polymer
melt.
Preferably, the film is oriented and stretched in each of the machine
direction (MD) and
transverse direction (TD) by a factor of from about 2 to about 4 times its
original
dimension; preferably it is oriented by a factor of at least 3. The resulting
film is
subsequently annealed, cooled and formed into rolls for later use.
When intended for use as components in nonwoven industrial textiles, the film
can be
processed in one of several ways. It may be first cut to a desired size and
then a desired
topography imparted by means of a thermoforming process whereby heat and
pressure
are used to deform the film out of plane in a desired shape. The film is then
slit by either
mechanical means or by means of radiant energy such as from tuned laser.
Alternatively,
the film is first slit or perforated by chosen means, and then embossed with a
suitable
pattern. In either case, the slit and profiled film sections are then
assembled into
nonwoven industrial textiles and associated components using known means. As a
further
alternative, the film may first be embossed according to a desired pattern,
and then
assembled in two or more layers, and finally perforated as desired.
DETAILED DESCRIPTION OF THE INVENTION
The fabrics and seaming components of the invention are formed from an
extruded and
bi-axially oriented film which is comprised of at least one, and preferably
three
coextruded polymeric layers that are oriented and heatset together, and which
optionally
include a hydrolysis stabilizer in the form of a monomeric or polymeric
carbodiimide.
As examples only of polyesters suitable for use in the films of the invention,
it has been
found that Invista Type 4027 PET (available from Invista S.a.r.l. of Wichita,
Kansas)
having an IV of about 0.60, Invista Type 8326 PET at an IV = 0.55, and DAK
Americas
LLC of Charlotte, NC Type 80 PET which has an IV of 0.80 have proven suitable.
These
polyesters are commercially available in dry pelletized form from the supplier
with the
specified IV. If necessary or desired, the IV of these or any other PET resins
can be
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increased by means of known solid state polymerization processes whereby the
polyester
is exposed to high temperatures and vacuum (or inert gas, to prevent oxidative
degradation); the result is a relatively higher molecular weight polyester in
comparison to
that of the starting material. In general, higher IV PET resins will allow for
the
production of a film with improved physical properties, in particular
resistance to
abrasion and hydrolysis, when compared to films produced from resins of lower
IV. High
intrinsic viscosity polymer resins will allow the resulting films to better
withstand the
rigorous demands of certain of the industrial environments to which it may be
exposed,
such as in the hot and moist dryer section of a papermaking machine, or
continuous
exposure to sunlight on a solar panel.
Depending on the intended end use of the film, it may be either desirable or
necessary to
increase its resistance to hydrolysis. Hydrolysis is a chemical process by
which a water
molecule is added to a substance resulting in that substance splitting into
two parts. It is
the type of reaction that will break down certain polymers, especially those
such as PET
which are made by condensation polymerization. Hydrolysis stabilizers are
often added
to PET resins when the intended end product will be used in hot and moist
environments.
Hydrolysis stabilization additives are well known and function by reacting
with free
polymeric carboxyl end groups in the polymer melt prior to extrusion. One such
additive,
which has proven successful when incorporated into polyester monofilaments, is
Stabaxol KE7646. This additive is commercially available from Rhein Chemie
Corp. of
Chardon, OH and is comprised of from about 10% to 30% pbw of a polymeric
carbodiimide in 70 to 90% pbw of a high IV PET (IV approx. 0.80). A monomeric
form
of the Stabaxol additive is also available and is anticipated to be equally
as successful
in imparting hydrolysis resistance as the polymeric form; either form is
suitable for use in
the polymeric films of the invention.
The Stabaxol KE7646 is the masterbatch form of a polycarbodiimide; according
to the
manufacturer, it contains 15% Stabaxol P100 (the active ingredient) uniformly
blended
in the PET. Addition of 10-20 pbw Stabaxol KE7646 per 100 pbw PET should
provide
an active ingredient content of Stabaxol P100 of 1.5% to 3%, which is the
preferred
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range of carbodiimide hydrolysis stabilizer in the films and components of the
present
invention. It has been found that compositions including relatively high IV
PET (e.g. 0.8,
such as is found in the DAK Type 80 resin) and about 5% pbw carbodiimide, or
more,
may become difficult to reliably extrude; thickness variation, shrinkage and
film
orientation are problematic because the stabilizer appears to increase
crosslinking and
extensional viscosity of melt.
The films of the invention are made as follows. A desired PET or other
polyester resin is
first obtained and an appropriate amount of hydrolysis stabilizer is added
according to
normal blending processes as are known in the art. As previously mentioned, if
the end
use of the film is as a belt component in a hot and/or humid environment, the
polymer
should be hydrolysis stabilized and the IV selected as appropriate. The
polymer is
preferably obtained as resin pellets which are then loaded into the hoppers of
the film
extruder(s). Once heated to the melt point, the polymer melt is then extruded
through a
slot die according to techniques and equipment common in the industry. The
amorphous
prefilm is subsequently quenched on a chill roll and then reheated and
oriented in both
the MD (machine direction) and TD (transverse direction) so as to impart
stretch-induced
structure through biaxial orientation. This step is important in order to
provide a
mechanically stable film as the stretching process will straighten out the
polymer chains
in the film and provide crystals with the desired morphology. Stretching
temperatures are
normally above the glass transition temperature T, by at least 10 C. The
stretching ratio
in each of the MD and TD will be about 3, but may range from 2 to about 4 as
required.
There are essentially two film stretching processes in use at present:
simultaneous
stretching, in which the film is exposed to both MD and TD force
simultaneously; and
sequential stretching, in which the film is exposed first to MD and then TD
stretch forces.
The films of the invention can be made using the simultaneous stretch process,
or
sequential stretching. Depending on the end use of the film, it may be desired
to
preferentially stretch the film in one of these directions over the other. A
second
subsequent stretch in either or both the MD and TD may be employed as needed.
The
MD and TD shrinkage can be adjusted as appropriate by temperature settings and
frame
geometry. It should be noted that, as film thickness and carbodiimide content
increases, it
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becomes increasingly difficult to reliably and uniformly control properties,
particularly
thickness. Heat setting or annealing of the film at oven temperatures of about
180 C to
260 C follows stretch and orientation; the film is then cooled and wound. The
oriented
film has a final thickness of from about 175 um to 350um; depending on the
intended
end use, the film thickness may be increased or decreased around these limits
as
necessary.
The polyester films of the invention may be monolayer films, but are
preferably
multilayer and more preferably are comprised of three layers. In experimental
trials, the
film was extruded using a three layer die with a feedblock designed to feed
both outside
skins from one extruder and the core layer from another. The resultant film
thus included
three polymer layers arranged according to an A-B-A configuration in which
each of A
and B are essentially the same polymer but are of two differing thicknesses.
It was found
that multilayer films of the invention in which the layers A each accounted
for 15% of the
overall film thickness and the layer B provided the remaining 70% were
suitable for use
as components in industrial textiles, however, other film thickness ratios
such as 10-80-
10 may also prove suitable. Preference is given to A-B-A or A-B-C three layer
structures.
In such structures, it is possible for at least one and preferably both of the
outer layers,
and an intermediate layer to include the hydrolysis stabilizer. The
concentration of the
stabilizer may vary from one layer to the next. However, in such structures,
it is
important that the adjacent layers be compatible, or miscible; alternatively a
so-called "tie
layer" may be located in between the adjacent layers to prevent layer
separation and
provide a unified film structure.
The hydrolytically stabilized film of the present invention bears some
similarities to the
coextruded laser weld enabled film described in CA 2758622 (Manninen). As
described
in that document, one layer of the film is different from the others in that
it includes a
laser weld enabling material. In the present invention, the film is comprised
entirely of
essentially the same polymer (although a dye or other common additive may be
included
in one or more of the layers). For example, it may be necessary to provide an
antiblock
agent, such as Invista V388 (available from Invista S.a.r.l. of Wichita,
Kansas) at a 5%
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pbw concentration, to the outer "A" layers of the film to prevent them from
sticking to a
roll or other component of the extruder and/or stretching arrangement.
The polymer films of the present invention are of particular importance to the
industrial
textile industry for several reasons. First, they are formed from a higher IV
PET resin
than others that have been used previously and which are commercially
available PET
films. It has been found that the high IV PET retards brittle crystal
formation during
heatsetting/thermoforming steps. Commercially available PET films are farmed
from
polyester resins whose IV is less than 0.5; if exposed to heat in the range of
about 200 C
or more, or prolonged exposure to sunlight, such films will become very
brittle and fail in
various ways (their tensile strength will diminish, they will become prone to
stress
cracking, etc.) whereas the films of the present invention will not degrade in
this manner.
Also, the grade of the PET resin is important when the end use application
involves
hydrolysis; generally the resin should have a relatively low carboxyl end
group
concentration and contain low residual diethylene glycol. Further, the
hydrolysis
stabilizer is reactive and affects the extensional viscosity of melt of the
blend ¨ therefore
the stabilizer loading affects the processability of the film. It has been
found that the
quality of the film is significantly affected by the line process parameters,
i.e.
temperatures, stretching ratio, etc.
The final film is now available for use in the manufacture of industrial
textiles which are
uniquely suitable for conveyance, filtration and separation processes. For
example,
hydrolytically stabilized PET film having a thickness of from about 250 m to
about
350 pm is suitable for use in the production of selectively slit and embossed
films
intended for subsequent assembly as nonwoven papermaking fabrics. Similar
films of
greater or lesser thickness will be appropriate for the manufacture of seaming
components which will be used to join the opposing ends of these fabrics on
the
machines for which they are intended. The films will also be suitable for use
in other
applications, such as in solar panels, where resistance to degradation and
embrittlement
are important.
