Note: Descriptions are shown in the official language in which they were submitted.
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1007-319
Polymer Blends o~f Trimethylene Terephthalate and a Polyamide
Field of the Invention
The present invention relates to a polymer blend consisting
essentially of from about 50~ to about 95~ of polytrimethylene
terephthalate and of from about 5~ to about 50~ a polyamide,
based on the total weight of the polymer composition. The
polymer blend can be used in the fabrication of industrial
fabrics that are particularly suitable for use in rigorous
environments, where the dimensional stability and mechanical
properties of the fabrics are important.
~ackQround of the Invention
Modern industrial fabrics are commonly assembled by weaving,
braiding, knitting, knotting and other known methods from
polymeric monofilament or multifilament yarns. It is also known
from EP 802280 to assemble such fabrics from a plurality of
extruded polymeric strips or panels. The chosen polymers are
most frequently polyesters, copolyesters, polyamides,
polyphenylene sul:Eides, polyphenylene oxides, fluoropolymers or
polyketones. Selection of any particular polymer for a specific
application will generally be dictated by the physical and
mechanical properties desired, the cost of the polymer, and the
prevailing environmental conditions of the end use.
The present i.nventi.on is primarily concerned with a polymer
blend for use in fabricating industrial fabrics intended for
environments where the dimensional stability and resistance to
repetitive compressive stress of the fabrics are important. The
invention is thus particularly relevant to papermaking fabrics
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which are used to form, drain, dewater and convey a paper web as
it is created within a paper making machine.
For the purposes of this application, the term "fabric" is
taken to mean an assembly of components. The term "component" is
taken to mean any of t:he components from which a fabric can be
assembled, such as yarns (including monofilament, multifilament
and spun forms) oz: extrusions. The components forming the fabric
can be arranged by interlacing, entangling or engagement so as to
form an integrated cohesive structure, such as nets, cloth,
felts, textiles and the like, which. are created by weaving,
knitting, knotting, joining, felting, needling, spiral winding,
bonding, or similar methods. Typical components include
individual monofilaments, multifilaments, spiral coils, and
profiled plastics extrusions such as strips, tiles or panels.
These components are generally fabricated by an. appropriate
method such as melt extrusion, melt spinning, casting or slitting
from an extruded :film. The fabricated components are then joined
to form an integrated cohesive structure.
In a papermaking machine, a paper web is created in three
stages. In the forming section, a water based stock of
papermaking components is discharged onto a moving continuous
forming fabric. As the fabric conveys the stock through the
forming section, :it is .drained and agitated to provide a somewhat
self supporting wet paper web. Drainage of the stock is augmented
by various stationary elements with which the forming fabric. is
in moving contact:. The web is then transferred to the press
section where a major proportion of the remaining water is
removed by mechanical pressing in a series of high pressure nips
between opposed press rolls. Press fabrics are used both to
convey the web, and to receive expelled water. The web then
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passes to the dryer section, in which it is conveyed on at least
one dryer fabric over a series of heated cylinders where the
remaining water is removed by evaporation. The resulting paper
is then calendered, slit and wound onto reels.
Papermaking fabrics must ideally possess multiple
characteristics simultaneously:
(a) they must: be resistant to abrasive wear caused by their
passage over the various stationary elements in the paper
making machine, and by contact with solids in the stock
which they a.re to convey:
(b) they must be structurally stable, so as to function as
designed within the range of stresses imposed during their
use;
(c) they must: resist dimensional r_hanges in the plane of the
fabric due to moisture absorption over a wide range of
moisture contents;
(d) they must, resist stretching under the tension imposed by
the powered rolls which drive the fabrics on the machine:
and
(e) they muat be resistant to degradation caused by the
various materials present in both the fiber-water slurry and
in the materials. used to clean the fabrics, at the
prevailing temperatures of use.
In addition, some industrial fabrics, such as press felts used in
the press section of a papermaking machine, must be resistant_to
compaction and repetitive cyclic compressive stress.
Of the various polymers available for industrial fabrics
applications, those most commonly used in paper making are:
- polyesters, in particular polyethylene terephthalate (PET) and
various copolymers thereof, and
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- polyamides, particularly polyamide-6 (also known as
polycaprolactam), and polyamide-6/6 (also known as
polyhexamethylene adipamide).
Although yarns and extrusions formed from both polymer types
offer certain advantageous characteristics, there are essentially
two difficulties associated with their use:
(i) while PET generally provides adequate chemical resistance
and dimensional stability, and is also amenable to weaving,
having good crimpability and heatsetting behaviour, its
abrasion and compaction resistance are not always adequate,
especially in higher speed paper machines; and
(ii) although both polyamide-6 and polyamide-6/6 have adequate
abrasion and. comp~action resistance, they do not possess
adequate dimE~nsional stability in the moisture range found
in the paper making environment, and the. mechanical
properties of yarn~~ aid fabrics made from them are known to
change.
US 5,137,601 to Hsu discloses papermaking fabrics, in
particular press felts, whose component fibers and filaments are
fabricated from polypropylene terephthalate, herein after
referred to as PfT. In view of the use of "propylene" in the
polymer name, this appears to be a polymer of terephthalic acid
and 1,2-propanediol. The fabrics are alleged to have chemical
resistance properties similar to polyester, and physical
properties comparable to polyamide-6. There is no disclosure of
suitable intrinsic: viscosities for the PPT, no identification of
suitable grades, no discussion of the possibility of blending PPT
with a second poJ_ymer, nor are there any teachings to suggest
that this polymer may be' suitable to withstand repetitive cyclic
compressive stress.
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Best, in EP 844320, discloses monafilaments for use in paper
making machine clothing whose principle :.component is
polytrimethylene terephthalate (described by Best as PTMT, and
stated to be a polymer of terephthalic acid and 1,3-propanediol).
In a preferred embodiment, the PTT may be blended with up to 45$
by weight of polyurethane so as to improve the abrasion
resistance of the monofilaments. There is no disclosure of the
appropriate grade or intrinsic viscosity for a suitable PTT, nor
does the disclosure teach that blending of PTT with any polymer
other than polyurethane will improve the ability of the
components to withstand repetitive cyclic compressive stresses
and therefore increase the service life of the components.
Specifically, there is no disclosure or suggestion that other
polymers may prov_Lde satisfactory results.
Despite these innovations, and for various other reasons
including the cost of the raw materials, neat polyamide yarns are
still preferred for many industrial fabric applications. The
term "neat" as used herein refers to a polymer system containing
only one polymer, e.g. polyamide-6, and nothing else. The
compaction and abrasion resistance of these polyamide based yarns
is useful in physically demanding applications, such as paper
makers press felts, filtration fabrics, and the like.. However,
the fact that thE~se polyamide yarns absorb moisture, and thus
undergo physical changes in their mechanical properties,
dimensions and weight when the environment of use exposes them to
different moisture conditions, limits their use in moist
environments where such variations often cannot be tolerated.
This difficulty is discussed inter olio in US 4,529,013, US
4,289,173 and DE 2,502,,466 which teach that fabrics including
more than 50~ pol.yamide yarns tend to grow or stretch as they
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absorb moisture in a wet environment, and will shrink as they dry
out, thus rendering the fabric unstable on the machine.
To circumvent these dimensional stability problems, fabric
manufacturers have turned to substantially more costly
polyamides, such ass polyamide-6/10 and polyamide-6/12, especially
in instances where wet-to-dry dimensional stability is crucial.
However, the cost of these polymer resins is approximately three
times greater than either polyamide-6 or polyamide-6/6. E5lrther,
polyamide-6/10 and polyamide-6/12 yarns have comparatively poorer
thermal properties, making them attractive for only a limited
number of very specific: applications.
While the moisture stability performance of polyamide-6 and
polyamide-6/6 yarns has been improved, yarn manufacturers have as
yet been unable to address effectively the special combination of
requirements necessary for paper machine clothing applications,
in particular: cost, wet-to-dry dimensional stability, and
resistance to both abrasive wear and repetitive compressive
stress cycling.
Polytrimethylene terephthalate, herein referred to as PTT,
which is also known as poly(1,3-propanediol terephthalate), is a
polymer that has recently become commercially available. PTT
appears to combine a number of the mechanical properties of both
polyesters and polyamides. PTT is commercially available from
Shell Chemical Co. of Houston, Texas under the trade name
CORTERRA'n' and is stated to be the reaction product of purified
terephthalic acid and 1,3-propanediol. Two grades of PTT are
currently available in bulk: carpet grade, which has an
intrinsic viscosity as supplied of about 0.92 dL/g, and a second
grade which has an intrinsic viscosity as supplied of about 1.3
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dL/g. In both cases, the intrinsic viscosity quoted is measured
according to ASTM D4603-96 on the bulk resin prior to extrusion
into monofilament.s: the intrinsic viscosity when measured on
monofilaments of extruded PTT is somewhat lower. Unless
indicated otherwise, all intrinsic viscosity values quoted herein
are determined by this ;procedure.
The present inventors have discovered that industrial
fabrics whose components are comprised of at least 50~ to about
95~ by weight of :PTT, and from about .i0$ to about 5$ by weight,
based on the total. weight of the composition, of a polyamide, are
as resistant to compaction, and are able to withstand cyclic
repetitive stresses as well as components fabricated from
polyamide-6/10 or polyamide-6/12 while retaining their
dimensional stability. The components are found to be as
chemically stable as those formed from PET and are.amenable to
weaving when formed into monofilament or multifilament yarns.
The finished PTT Y>lend components are thus particularly suitable
for use in the assembly of industrial fabrics such as press felts
for papermaking machines. Components fabricated from neat PTT
having an intrinsic viscosity of about 0.92 dL/g or less cannot
survive exposure to about 9,000 cycles of repetitive cyclic
compressive stress (under the test conditions set out below)
without catastrophic failure. We have now discovered that if at
least about 5~ by weight of a polyamide is added to the PTT, then
components formed from. the resulting blend exhibit sufficient
resiliency to survive exposure to at least 54,000 cycles of
compressive strE:ss. The resistance to repetitive cyclic
compressive stresa exhibited by components formed from a blend of
PTT having an ini~rinsic viscosity of about 0.87 dL/g, and from
about 5~ up to at least about 30~ of a polyamide, is about equal
to that of similar components formed from neat PTT resin having
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an intrinsic viscosity of about 1.3 dL/g. Components fabricated
from a blend of from about 95$ to about 50~ by weight PTT (having
an intrinsic viscosity before processing of from about 0.87 dL/g
to at least 1.3 dL/g), end from about 50~ to about 5~ by weight
based on the total composition of a polyamide appear to exhibit
resistance to repetitive cyclic compressive stress that is
roughly equivaleni~ to that obtained from components formed from
neat polyamide-6, polyamide 6/6 or polyamide-6/12. Thus
components formed from blends of PTT having a bulk intrinsic
viscosity in the range of from about 0.87 dL/g to at least 1.3
dL/g appear to be suitable for use in industrial fabrics intended
for environments. where resistance to repetitive cyclic
compressive stre:>s, abrasive wear, chemical degradation and
dimensional. stability under changing moisture conditions are
important.
Rricf Descr~nt~on sf th Drawing
Figure 1 shows diagrammatically the wet-to-dry dimensional
stability of monofilaments prepared from neat polyamides, neat
PTT, and from a PTT/pol.yamide blend.
y,~ry of the Invention
In its broadest embodiment, this invention seeks to provide
a thermoplastic polymer blend consisting essentially of from
about 95~ to aboui~ 50~ by weight, based on the total composition,
of polytrimethylene terephthalate(PTT), having an intrinsic
viscosity from about 0.87 dL/g to at least about 1.3 dL/g, and
from about 5~ to about 50~ of a polyamide.
In a first more narrow embodiment, the present invention
seeks to providE: components which can be assembled into an
industrial fabric which are fabricated from a thermoplastic
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polymer blend consisting essentially of from about 95~ to about
50~ by weight, based on the total composition, of
polytrimethylene terephthalate(PTT), having : an intrinsic
viscosity from about 0.87 dL/g to at least about 1.3 dL/g, and
from about 5~ to .about 50$ of a polyamide.
In a second narrow embodiment the present invention seeks to
provide industrial fabrics including components fabricated from
a polymer blend consisting essentially of from 95~ to about 50~
by weight, based on the total composition, of polytrimethylene
terephthalate(PTT), having an intrinsic viscosity from about 0.87
dL/g to at least about 1.3 dL/g, and from about 5~ to about 50~
of a polyamide.
Components such as yarns(monofilament, multifilament and
spun) and extrusions formed from this polymer blend can be
assembled either alone, or in admixture with components
fabricated from other materials, to provide fabrics which are
resistant to cyclic repetitive compressive stress and
fibrillation, as well as to permanent deformation caused by
compressive stress.
Preferably, the intrinsic viscosity of the PTT resin from
which the yarns or extrusions is produced is at least 0..87 dL/g:
more preferably, the intrinsic viscosity is at least about 0.92
dL/g. _
Preferably, the polyamide is an aliphatic polyamide.
The polymer blend may also contain from about 0.05 to about
5$ by weight based on t:he total weight of the composition of one
or more conventional additives, such as stabilizers, plastic
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processing aids, colourants, and inhibitors of oxidative,
hydrolytic or thermal degradation. The amount and type of
additive used blend will be dependent upon the intended end use
of the polymer blend.
Preferably, t:he components of the industrial fabrics of this
invention are monofilaments or multifilaments which are assembled
by weaving. Alternatively, the components are extruded strips or
panels which are assembled by snap/press fitting, spiral winding
or by rapier insertion according to the methods described by
Baker in EP 802280.
Preferably, all of the components of the industrial fabrics
of this invention are fabricated from the PTT blend.
Alternatively, at least some of the components that are oriented
in the direction of a tensional load placed on the fabric under
its conditions oiE use are fabricated from the PTT blend. As a
further alternative, air least some of those components that are
oriented in a direct=ion within the plane of the fabric
substantially per~oendicular to the direction of a tensional load
placed on the fabric under its conditions of use are fabricated
from the PTT blend.
D_Ptey ~ ed DeSCr'l~f'' ~'n of tha TnvantlOn '
Components of the industrial fabrics of this invention are
produced using known methods of melt extrusion from a blend_of
PTT polymeric resin having an intrinsic viscosity of at least
about 0.87 dL/g together with a thermoplastic polyamide, using a
conventional single or twin screw extruder under conditions
suitable for the polymer blend involved. From about 95$ to about
50~ by weight based on the total composition of the PTT resin,
together from about 5$ to about 50~ by weight of the polyamide,
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and from about 0.5~ to about 5$ by weight of any additives (so as
to produce a blend whose components total 100 by weight), are
metered to the e~arudei: either as separate streams or as a
mixture. The polymer resins and any additives are mixed and
melted in the extz:uder at a temperature of from about 230°C to
about 310°C . As the extruder screw ( s ) conveys the components
forward, the molten and thoroughly blended resin is fed into a
metering pump that drives the blended resin mixture through a die
to form a molten extrudate. The extrusion temperature is
preferably from aY>out 2:30°C to about 310°C, and more
preferably
is from about 240°C to about 280°C.
The molten e:~trudate can be quenched in air or watery water
quenching is preferred. For the production of monofilaments, the
solid extrudate is drawn into yarn at room or an elevated
temperature in a multi-stage draw process. For the compositions
disclosed herein, the preferred range for the drawing temperature
is from about 65°C to about 260°C, with energy being supplied
between independently speed controlled godets that provide a
final draw ratio of from about 3:1 to about 6:1. The final yarns
are allowed to relax about 0-25$ by passing them through a
relaxing stage at. a temperature of from about 65°C to about
260°C: the yarns aoe then wound onto spaols. The preceding normal
processing conditions have been found to provide acceptable
results others may prove suitable.
PTT yarns produced from neat PTT resin whose intrinsic
viscosity is from about 0.86 dL/g to about 0.92 dL/g or less,
known as carpet grade, do not offer the necessary mechanical
integrity required for repetitive compressive stress
applications. Although the wet-to-dry dimensional stability of
yarns formed from this range of intrinsic viscosities is
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excellent, as well as their mechanical properties as measured by
traditional test methods (e.g. percent elongation) compared to a
neat polyamide-6 yarn, the performance of these PTT yarns under
repetitive cyclic compressive stress is poor.
This is demonstraated in the following tests, where the
resistance of yarns formed from neat polyamide-6 to repetitive
compressive stress is compared to the following:
(a) yarn extruded from neat polyamide-6/6;
(b) yarn extruded from neat polyamide-12~
(c) yarn extrudE:d from neat PTT palymeric resin having an
intrinsic viscosity of 0.87 dL/g,:
(d) yarn extrudE~d from neat PTT polymeric resin having an
intrinsic viscosity of 1.3 dL/g,;
(e) yarn extrudE:d from a blend of PTT having an intrinsic
viscosity of about 0.87 dL/g with from 5~ to 30~ by weight
of polyamide-12~ and
( f ) yarn extrudESd from a blend of PTT having an intrinsic
viscosity of about 0.87 dL/g and about 5$ polyamide-6.
In these and later tests, the control yarns of neat
polyamide-6 resin. were extruded from Ultramid~ X-301 available
from BASF, the control yarns of neat polyamide-6/6 were extruded
from DuPont FE3077L polyamide-6/6 resin available from E.L. DuPont
de Nemours Co., Vilmington, Delaware, and the control yarns of
polyamide-12 were extruded from Rislan Aesno polyamide-12,
available from Elf Atochem North America of Philadelphia,
Pennsylvania. Twro samples of PTT were used. One sample had an
intrinsic viscosity of 0.87 dL./g, and was obtained from the
Degussa Co.: as supplied this polymer was not identified by a
grade or other designation. The other sample had an intrinsic
viscosity as supplied of 1.3 dL/g, and was Corterra CP51300,
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r
which is available from Shell Chemical Company, of Houston,
Texas.
In these tests, yarns were produced from PTT blended with
polyamide in amounts of from about 5~ to about 30$ by weight,
based on the total composition. The polyamides used in the
blends were polyamide-6 and polyamide-12.
The monofilaments were produced as follows. Each of the
polyamide-6, polyamide-6/6, polyamide-12, neat PTT samples and
PTT resin/polyamide-12 blends, were fed through a single screw
extruder at a melt temperature of about 280°C. Oriented strands
were produced from the extrudate using conventional godet and hot
air oven technology to achieve the necessary drawing. The oven
temperatures were about 200°C and overall draw ratios of about 4
were used. The final monofilaments had a diameter of about 0.20
mm.
The monofila:ment samples were first tested to determine
their wet-to-dry dimensional stability. The wet-to-dry
dimensional stability of a monofilament was determined using the
following method. Fiber samples greater than 1 meter in length
were obtained from each of the control and test monofilaments.
These samples were first conditioned by exposing them to air at
23°C and 50~ relative humidity for 24 hours. Each sample was
then cut to a convenient length, typically l meter. The samples
were then immersed in distilled water at room temperature for 24
hours, removed from the water, blotted surface dry, and their wet
length accurately measured. The samples were then again exposed
to air at 23°C and 50$ relative humidity for 24 hours, and their
dry length then measured. The wet and dry cycle measurements
were then repeated. The changes in length are expressed as a
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percentage, and the: average percent change in length over the two
wet to dry cycles is taken as being indicative of the stability
of the monofilament under these conditions. -
Figure 1 shows that the percent change in the length of the
control monofilament made of neat polyamide-6 was about 3.5$ and
that of the monofilamen,ts made from the PTT blends was about
0.3$. This test demonstrates that the monofilaments produced
from the PTT blends have superior wet-to-dry dimensional
stability when compared to monofilaments produced from a neat
polyamide-6 resin.
The resistance to repetitive compressive stress cycling of
monofilaments formed from neat PTT whose intrinsic viscosity is
from about 0.87 to about 0.92 dL/g (the intrinsic viscosity of
the PTT in the finished yarn is lower) is poor and their physical
response, as determined by their loss in diameter and gross
failure by fibrillation, is unacceptable for paper machine
clothing applications. 'This was surprising, as this grade of PTT
is supplied as "carpet grade", which was expected to withstand
repetitive compressive stress . There was no way to predict
priori that this material would not be suitable in applications
where resistance to repeated compressive stress is important,
such as clothing i:or a papermaking machine.
Table 1 provides test data to illustrate this case. In this
test, monofilaments formed from the polyamide-6 control resin,
neat polyamide-6/Ei, newt polyamide-12, two grades of neat PTT "
and various blends of PTT with polyamide-12, were each subjected
to a repetitive cyclic compressive stress test to simulate the
compressive stress cycling experienced. by a paper machine press
felt passing through a nip under load. In this test, a pressure
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of approximately 7.3 MPa at 40°C was applied at a frequency of
300 compression per minute for 180 minutes using a laboratory
scale dynamic compaction tester to simulate 54,000 compression
cycles. The samples are each wrapped around a notched crossover
plate so as to create a grid which simulates a press felt base
weave with a mes'.h of :L6 x 16 yarns per cm. The samples and
crossover plate are then placed in the dynamic compaction tester
where they are subjected to repeated compression under constant
load and recovered under zero load. The samples are removed at
specific time intervals and examined for damage. Table l shows
the calculated physical response of all of the monofilaments,
based on a summary of test samples, as determined by their
percent loss in original diameter following 54,000 cycles of
compression and relaxation.
In Table 1, PA-6 refers to polyamide-6; PA-6/_6 refers to
polyamide-6/6; PA--12 refers to polyamide-12; PTT(A) refers to PTT
yarns formed from resin having an intrinsic viscosity of 0.87
dL/g as supplied: PTT (l3) refers to PTT yarns formed from resin
having an intrin~;ic viscosity of 1.3 dL/g as supplied; and all
blends total 100$.
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Percent Diameter Loss of Monofilaments under
Repetitive Compressive Stress.
Number
of Compression
Cycles
Yarn Composition 9,000 18,000 27,000 36,000 54,000
PA-6 33.7 38.1 40.9 43.1 46.2
PA-6/6 28.8 33.3 36.3 38.6 42.0
PTT(A)~ Samples
failed
before
9,000
cycles
95$ PTT(A),5$ P.A-12 33.6 38.3 42.1 44.7 48.6
90$ PTT (A) , 10$ F?A-1233. 5 39..3 43. 0 46. 0 50.4
80$ PTT(A),20$ ~?A-12 38.6 52.5 46.2 49.0 53.3
70$PTT(A), 30$ ~'A-12 41.5 46.5 49.7 52.2 55.8
95$ PTT(A), 5$ PA-6 31.2 37.0 40.8 43.7 48.3
PA-12 21.1 26.0 29.4 32.2 36.4
PTT(B) 18.9 23.6 26.8 29.3 33.3
Table 1 showa that yarns fabricated from PTT(A), which has
an intrinsic viscosity of about 0.87 dL/g as supplied, do not
survive the repetitive cyclic compressive stress test. However,
following addition. of from 5$ to about 30$ polyamide-12,.or of 5$
polyamide-6, to PTT(A), the yarns are now able to survive the
test and suffer a diameter loss that is comparable to that found
for the neat polyamide-6. and polyamide-6/6 control yarns, and-is
similar to that for PTT(B). However, as shown in Figure 1, the
yarns based on PTT do not suffer rom the dimensional stability
problems of neat ~polyamide-6.
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The results displayed in Table 1 also show that a yarn
product formed from the PTT having an intrinsic viscosity of
about 0.87 to about 0.92 dL/g as taught by the prior art is not
viable in applications where resistance to repeated compressive
stress is important, such as in papermaking machine press felt
fabrics.
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