Note: Descriptions are shown in the official language in which they were submitted.
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POLYPROPYLENE FIBER AND PREPARATION THEREOF
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates, in general, to a
polypropylene fiber and, more particularly, to a
polypropylene fiber which is useful as a material for
non-woven fabrics, thereby allowing the non-woven fabrics
to be smooth and excellent in strength and providing
workability and physical properties for the non-woven
fabrics during after-processes. Also, the present
invention is concerned with a method for preparing such
fibers.
2. Description of the Prior Art
In order that staple fibers are prepared from
polyolefin polymers, they have to undergo a series. of
processes: the polyolefin polymers are generally
compounded with some amount of additives and the
resulting mixtures are melt-extruded in ordinary
commercial processes to give fibers, which are crimped
and cut into predetermined lengths.
When being applied for the making of non-woven
fabrics, polyolefin staples are typically processed in a
carding machine to give non-woven webs which are then
thermally bonded.
For thermal bonding, a pair of calender rollers,
ultrasonification, or hot air is usually used.
Particularly as for polypropylene filaments or
staples, they are arranged after opening and carding
processes, and bridged to afford webs. These webs are
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thermally bonded with the aid of a calender roller with
diamond or delta type patterns to produce non-woven
fabrics which are industrially useful in various fields.
Alternatively, hot air may be utilized instead of
calender rollers. In this case, after being allowed to
undergo a carding process, webs are bonded to give non-
woven fabrics by means of hot air which is circulated in
a porous drum.
Polypropylene non-woven fabrics find numerous
applications in the disposable diaper, diaper for
patients suffering from urinary incontinence, hygienic
band, mask, and medical fabric industries. Although not
demanding strength as high as that of woven fabrics, the
non-woven fabrics used in these purposes have to be soft
and satisfy the requirement of safety to skin because
they are in direct contact with the skin.
The strength of non-woven fabrics varies depending
on their preparation processes as well as on physical
properties of material fibers.
With the aim of improving productivity, non-woven
fabric producing manufacturers generally try to make high
production speed. The high production speed, however,
demands more excellent physical properties for the fibers
for non-woven fabrics.
SUMMARY OF THE INVENTION
Leading to the present invention, the intensive and
thorough research on polypropylene yarns or staples
suitable for non-woven fabrics, repeated by the present
inventors aiming to overcome the above problems
encountered in prior arts, resulted in the finding that
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isotactic polypropylene homopolymers, which are found to
have two endothermic peaks as measured by a differential
scanning calorimeter (DSC), allow the production of novel
fibers, which have been not yet reported in the arts, and
guarantee the excellent strength and softness of the non-
woven fabrics prepared from the fibers. In addition, the
fibers of such a structure were found to be obtained by
controlling melting indexes and polydispersity indexes in
each process step through the total procedure.
Therefore, it is an obj ect of the present invention
to provide polypropylene fibers for non-woven fabrics,
which can be applied to high-speed carding machines and
guarantee the excellent strength and softness of the non-
woven fabrics after thermal bonding.
It is another object of the present invention to
provide a method for preparing such polypropylene fibers.
It is a further object of the present invention to
provide a non-woven fabric prepared from such
polypropylene fibers.
In accordance with an embodiment of the present
invention, there is provided a polypropylene fiber, which
is obtained from an isotactic polypropylene homopolymer
with an isotactic index of 90 to 99 o through melt-
spinning or through drawing after melt-spinning, and
shows two differential scanning calorimeter (DSC)
endothermic peaks between 155 and 170 °C.
In accordance with another embodiment of the present
invention, there is provided a method for preparing
polypropylene fibers, comprising the steps of: (a)
melting an isotactic polypropylene homopolymer with an
isotactic index of 90-99 0, a melting index (MIa) of 10.0-
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40.0, preferably 10.0-25.0 and a polydispersity index
(PIa) of 2.5-6.0, preferably 2.8-5.0 and more preferably
3.5-4.3 to give a molten polymer which has a melt index
(MIb) with the ratio of MIb/MIa ranging from 1.01 to 1.50
and a polydispersity index (PIb) narrower by 10 o or less
than the PIa; (b) spinning the molten polymer to produce
fibers with a melting index (MIA) of 16.5-80.0 and a
polydispersity index (PIE) narrower by 20 0 or less than
the PIa, the ratio of MI~/MIa ranging from 1.65 to 7.50;
and (c) optionally drawing the fibers.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other
advantages of the present invention will be more clearly
understood from the following detailed description taken
in conjunction with the accompanying drawings, in which:
Fig. 1 is a DSC endothermic curve in which two
endothermic peaks apparently appear in a polypropylene
homopolymer fiber of the present invention as measured by
a DSC;
Fig. 2 is a DSC endothermic curve in which two
endothermic peaks apparently appear with the secondary
peak being in a shoulder form of the primary peak; and
Fig. 3 is a DSC endothermic curve in which only one
DSC endothermic peak appears in a conventional
polypropylene homopolymer fiber.
DETAILED DESCRIPTION OF THE INVENTION
The present invention pertains to polypropylene
fibers which are prepared from polypropylene homopolymers
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with an isotactic index of 90 to 99 o by melt-spinning or
by melt-spinning and drawing and have two differential
scanning calorimeter (DSC) endothermic peaks in the range
from 155 to 170 °C. Preferably, the polypropylene fibers
of the present invention show a primary endothermic peak
at 160~3 °C and a secondary endothermic peak at 165~3 °C.
Where non-woven fabrics are prepared from the
polypropylene fibers of the present invention by thermal
bonding, the above physical properties allow the non-
woven fabrics to be smooth with excellent strength. This
advantage is believed to result from the fact that, while
the fibers thermally fused due to the heat or the heat
and the pressure between rolls are solidified again,
rapid recrystallization occurs in the regions which are
high in melting points.
Useful as materials to prepare the fibers of the
present invention are polypropylene homopolymers with an
isotactic index of 90 to 99 0.
The polypropylene fibers of the present invention
have a melt index (MIA) of 16.5-80.0, which is preferably
1.65-7.50 times as large as that (MIa) of the material
isotactic polypropylene.
Preferably, the polypropylene fibers of the present
invention range, in polydispersity index (PIE), from 2.1
to 5.7 and, more preferably from 3.5 to 4.3 with a value
being narrower by 20 o than the PIa of the material
isotactic polypropylene.
A preferable fineness that the polypropylene fibers
of the present invention have falls in the range of 1.0
to 80.0 deniers.
As for the isotactic polypropylene used in the
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present invention, it preferably ranges, in melt index
(MIa), from 10 to 40 and, in polydispersity index (PIa),
from 2.5 to 6Ø
When the polypropylene is melted in an extruder, a
stabilizer or an antioxidant is preferably formulated at
an amount of 0 . 03 to 2 . 0 wt o, preferably 0 . 03 to 0 . 7 wt o
and more preferably 0.03 to 0.4 wto.
In addition to stabilizers or antioxidants, ordinary
additives in the art, such as a deoxidant agent, a
colorant, metal carboxylates, etc., may be used for the
preparation of the fibers of the present invention. The
metal carboxylate available in the present invention is
selected from the group consisting of nickel salts of 2-
ethyl hexanoic acid, caprylic acid, decanoic acid, and
dodecanoic acid, Fe, Co, Ca and Ba salts of 2-ethyl
hexanoic acid, and combinations thereof. As the deoxidant
agent or the colorant, calcium stearate, which is usually
used to prepare polypropylene homopolymers in
petrochemical plants, may be selected. A variety of
additives available in the present invention may be
referred to European patent No. 279,511.
The isotactic polypropylene useful in the present
invention, as previously mentioned, preferably has a melt
index (MIa) of 10 to 40. For example, when the MIa is
below 10, an increase occurs in the spinneret pressure
upon spinning, giving rise to a decrease in productivity.
A high heat is required for the melt-spinning of such
polypropylene, resulting in an increase in energy
consumption. In addition, the fibers obtained under such
a high heat show increased tenacity, so that they are not
suitable for the non-woven fabric uses which require
smoothness. On the other hand, when the isotactic
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polypropylene has an MIa of greater than 40, the resulting
fibers are not suitable for non-woven fabrics in terms of
strength. Further, incompletion frequently occurs in the
fulfillment of quenching after the spinning, leading to
fusion between neighboring fibers.
It should be noted that the fibers of the present
invention include those which are obtained through
melting, spinning, solidifying and taking-up processes as
well as those which are obtained through a drawing
process after melting and spinning processes and
necessarily have undergone the processes of crimping,
thermal fixing and cutting into staples. The fibers which
experience melt-spinning are almost identical to those
which further experience drawing in MI, PI and DSC
endothermic peak.
In an embodiment of the preparation of polypropylene
filaments or staples according to the present invention,
the material polymer is melted in an extruder to give a
molten polymer which has a melt index (MIb) with the ratio
of MIb/MIa ranging from 1.01 to 1.50 and a polydispersity
index (PIb) being narrower by 10 o than PIa and more
preferably by 5 0. A preferable PIb falls in the range of
2.4 to 5Ø
For example, if MIb exceeds 1.5 times of MIa, the
molecular chain of the polypropylene is so cleaved that
its inherent strength cannot be maintained. In addition,
such cleavage leads to an insufficient viscosity for the
molecular chains to orient at the nozzle, as well as
cannot maintain a pressure suitable for spinning. Further,
the fibers obtained are poor in strength, so that the
non-woven fabrics prepared from the fibers feel harsh to
the touch. In result, the productivity becomes poor. An
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MI change as large as or larger than 1 o naturally occurs
in the polypropylene upon extrusion. When MIb is changed
to less than 1.01 times of MIa, serious difficulty is
found in the preparation process of fibers. Particularly,
a high viscosity at the nozzle appears to increase the
pressure of the nozzle, making the spinning process very
unstable. Accordingly, the production yield is lowered
with a serious deviation in fiber quality.
By controlling the quenching condition after the
spinning, the polymer which has undergone an MI change in
the extruding process, is allowed to be secondarily
changed in MI. The MI change at a quenching step is
determined depending on the temperature of the delayed
quenched region, the atmosphere, the temperature, speed
and quantity of quenching air. U.S. Pat. No. 4,193,961
describes the use of delay quenching and quenching air,
which may also be referred to other documents, for
example, M. Ahmed "Polypropylene Fibers-Science and
Technology" sponsored by Society of Plastics Engineers,
I nc .
In accordance with the preparation of the present
invention, the fibers which experience the quenching step
are preferably controlled to have a melt index (MIA) 1.65-
7.50 times as large as the melt index (MIa) of the
material polymer and a polydispersity index (PIE) narrower
by 20 0 or less than that (PIa) of the material polymer
(that is, amounts to 0.80xPIa or wider). The fibers
preferably range, in PIE, from 2.1 to 5.7, more preferably
from 2.3 to 4.5 and most preferably from 3.0 to 4Ø
When the MIA is over the above range, the strength
of the grey yarns is deteriorated. The making of non-
woven fabrics from the grey yarns suffers from poor
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workability because the non-woven fabrics are apt to be
contaminated with card clothing and partially melted on
the calender roll. In detail, if the MI~ deviates from
the upper limit, the yarn has a too greatly decreased
molecular weight and the quenching effect after the
spinning from a nozzle is decreased to generate fusion
between yarns. Where the yarns are used to make non-woven
fabrics after being forcibly prepared in spite of the
above conditions, much powder is generated from the poor
yarns in an opening and a carding process, having a
negative influence on the making process. In addition,
heat-vulnerable portions of the poor yarns are melt out
upon calendering, making dirty the surface of the
calender roll which plays a role in the final thermal
bonding of the non-woven fabrics.
On the other hand, if the MIA is beyond the lower
limit, the strength of the grey yarn is improved, but it
is difficult for such grey yarns to improve the thermal
bonding index (hereinafter referred to as "TBI") to a
desired extent. That is, the non-woven fabrics obtained
show low TBI and feel harsh to the touch. Although the
strength or TBI of the non-woven fabrics can be improved
by increasing the temperature of the calender roll or the
thermal bonding area, the non-woven fabrics still remain
harsh .
Upon the making of non-woven fabrics, their machine
direction orientation and cross direction strengths vary
depending on the kinds and arrangements of carding
machines. Differences may be found in the machine
direction and cross direction strengths of the non-woven
fabrics which have passed through carding machines if
these machines are manufactured by different
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manufacturers. Even in the carding machines manufactured
by the same manufacturers, the non-woven fabrics show
different physical properties in dependence on the shape
and material of carding clothing and the presence of
random rolls. In addition, the non-woven fabrics are
different in plan weight, depending on the requirements
for the after-process. The measured strength values of
the non-woven fabrics represent simple tenacity and their
units are characteristically different from one company
to another. Therefore, since there may occur a case in
that superiority cannot be discriminated therebetween,
the simple tenacity is unsuitable to determine whether
the physical properties of the non-woven fabrics are
improved. However, the structure and inherent physical
properties of the yarn or staple can be compared as to
the influence on the non-woven fabrics by reference to
the bonding indexes of the non-woven fabrics prepared
although a difference may exist in kinds or arrangements
of the carding machines.
In accurately determining the influence of the
physical properties of yarns or staples on the non-woven
fabrics thereof, therefore, the concept of TBI is
recognized as very proper. A detail of TBI is described
in an article concerning Polypropylene Fibres and
Textiles, reported in the Fourth International Conference
held by The Plastics and Rubber Institute. Indeed, TBI is
introduced in the present invention as the most valuable
parameter to comparatively determine the influence of the
physical properties of yarns or staples on the non-woven
fabrics.
Of the fibers of the present invention, the non-
woven fabrics can be made which are 2.0 or higher in TBI
CA 02372453 2003-10-23
with good softness.
A better understanding of the present invention may
be obtained in light of the following examples which are
set forth to illustrate, but are not to be construed to
limit the present invention.
In the following Examples, fibers and non-woven
fabrics suggested in the present invention were analyzed
for physical properties.
DSC endothermic Peaks: fiber samples were
sufficiently washed to remove oiling agents. After being
dried for 30 min in the air, the samples were vacuum
dried for 1 hour in a decicater and cut into a length or
2-9 mm. The cut samples of 5mg were put on a measuring
pan which was then subjected to thermal analysis using
the. Perkin Elmer 7(*) series Thermal Analysis System in
which the temperature was elevated at a rate of 5 °C/min
from 30 °C to 190 °C, so as to obtain endothermic curves .
Other conditions of this measurement were accorded with
ASTM 3418-82 method. Conventional ~ polypropylene
homopolymer fibers showed single endothermic peaks while
the fibers of the present invention have double
endothermic peaks, as exhibited in the accompanying
drawings. Fig. 1 shows two apparent DSC endothermic peaks
of the fiber according to the present invention and Fig.
2 shows that a secondary DSC endothermic peak appears in
a shoulder form of a primary DSC endothermic peak. Fig. 3
is an endothermic curve showing that only one DSC
endothermic peak appears in a conventional fiber.
Denier of Yarn and Staple: measured using Vibroskop(*),
manufactured by henzing.
Strength and Elongation.of Yarn and Staple: measured
(*: trade-mark)
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,~ , ~.
....... v
using Vibrodyn(*), manufactured by Lenzing, according to
ASTM D 638.
Melt Index (MI): measured using Model MP 993(*) of
Tinius Glsen, according to ASTM D 1238. For the
measurement. of MI, the fiber samples were washed with
copious water, centrifuged, dried at 105 °C for 15 min in
an oven and cut into 1 cm.
Polydispersity Index (PI): using Model RMS-800(Disk;
parallel plate) (*) of Rheometrics, U. S.A. , G~ was measured at
200 °C at a shear rate , of 0. 1-100 under a 10°s strain
condition and substituted into the following equation.
PI =
~° .
wherein G~ is a modulus of a point at which a
storage modulus (G') and a loss modulus (G") are crossed
with each other at two. to six frequencies in a frequency
range of S-250 Hz. When there occurred no cross points,
. G~ was determined by extrapolation.
Isotactic Index (I.I.): a polypropylene homopolymer
sample was cut into a length of 5 mm, washed with water,
and dried at 105 °C for 1 hour in an oven. After taking
about 5g and then being accurately weighed, a portion of
the dried sample was boiled for about 5 hours in heptane
for extraction. After completion of the extraction, the
sample was sufficiently washed with water, dried at 105 °C
for 1 hour in an oven and then weighed. The weights
measured before and after the extraction were substituted
in the following equation to yield the isotactic index.
( * : trade-mark)
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Isotactic Index (%) = Weight After Extraction x 100
Weight Before Extraction
Thermal Bonding Index (TBI) of Non-Woven Fabric:
calculated according to the following mathematical
equation:
TBI = (MD x CD)"' x 20
Plan Weight
wherein, MD is a machine direction strength (kg/50
mm), CD is a cross direction strength (kg/50 mm), and
plan weight is a weight per area of a non-woven fabric.
Strength of Non-Woven Fabric: samples with a
dimension of 50 mm width and 140 mm length were measured
at a tensile speed of 100 mm/min using an instron.
Softness . the feeling to the touch was graded: 1
very harsh; 2 harsh; 3 ordinary; 4 soft; 5 very soft.
EXAMPLES 1 TO 7 AND COMPARATIVE EXAMPLES 1 TO 7
Isotactic polypropylene homopolymers with an
isotatic index of 97o and MI as indicated in Table l,
below, containing an antioxidant and stabilizer at an
amount of 0.09 wto, were melt-spun at an extruder
temperature from 250 to 290 °C while the heating in the
range from the extruder to the nozzle was controlled in
the range of 285-310 °C by means of a heating medium to
allow the melt to have MIb as indicated in Table l, below.
For the comparison of MI between the raw material and the
melt before the nozzle, a bypass was set to take the
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samples while a minimization was provided to the pressure
just before a gear pump which served to constantly feed
the melt into the nozzle.
Next, the melt was extruded at a spinning rate of
1,500 m/min through a spinneret, passed through a heat
reserving zone for delayed quencr;ng and then quenched to
afford primary yarns of 2.4 deniers, which had Mic, PIc
and DSC endothermic peaks as shown in Table I.
The primary yarns thus obtained were collected in a
bundle and drawn at a drawing ratio of 1.5 times while
being crimped in a crimper, following cutting them to
staples 40 mm long.
Given in Table 2 are MI, PI, fiber strength, number
of crimps and DSC endothermic peaks of the staples
obtained.
In order to prepare non-woven fabrics, the staples
were applied to the carding machines according to
manufacturers. The upper roll used for the preparation of
non-woven fabrics was of a diamond type with a sealing
area amounting to 22 o while the calender roll performed
its function at 147 °C with a pressure of 95 kg/cm.
The non-woven fabrics obtained are described
concerning plan weight, strength of machine direction and
cross direction, TBI and softness in Table 3, below.
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TABLE 1
Nos.Of Mate rials Me lts Fib ers
MI,,/MI~MI~/MI
Example ML. PL. ML. PL, MI. PI.
1 17.2 4.1 18.2 3.9 52.0 3.8 1.1 3.0
2 22 3.2 23.0 3.1 56.0 2.9 1.0 2.5
3 12.2 5.5 18.0 5.1 57.0 4.5 1.5 4.7
4 12.2 6.0 17.0 5.5 36.5 4.9 1.4 3.0
13.2 4.3 15.0 3.9 45.0 3.6 1.1 3.4
6 11.0 3.8 14.6 3.6 46.0 3.5 1.3 4.2
7 110 58 136 56 31 4 1 2
2 9 2 8
C.l 12.2 2.4 17.0 2.3 . . . .
24.3 2.2 1.4 2.0
C.2 17.1 10.8 29.2 9.5 106.5 6.5 1. i 6.2
C.3 3.9 6.5 5.6 6.4 32.5 5.1 1.4 8.3
C.4 12.2 5.5 15.4 4.9 80.2 4.7 1.3 6.6
C.5 8.1 5.6 11.6 5.1 28.0 4.2 1.4 3.5
C.6 10.8 12.0 19.1 11.0 39.5 10.2 1.8 3. r
C 7 42 5 ~ 62 5 i 698.0 5.1 1.5 4.7
5 4
TABLE 1 (continued)
5
DSC
Nos. Of ~l-(PI,,/PI~)~x100 ~l-(Pl~/PI~)}x100 Endothermic Peaks
Examples (%) (%) of primary yarn
1 4.9 7.3 159.6 164.2
2 3.1 9.4 160.2 164.5
3 7.3 18.2 159.8 165.2
4 8.3 18.3 158.9 164.8
5 9.3 16.3 160.1 163.9
6 5.3 7.9 160.0 166.2
r 3 5 159 164
4 15 5 2
C.l . . . .
4.2 8.3 159.8 -
C.2 12.0 39.8 160.2 -
C.3 1.5 21.5 160.2 -
C.4 10.9 14.5 159.2 -
C.5 8.9 25.0 160.0 -
C.6 8.3 15.0 159.9 -
C7 84 136 159.8 -
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TABLE 2
DSC
Endothermic
Nos. MI~ PI~ Strength Peaks staple
of
Crimp
of of Fiber (C )
Nos.
Exmpl.FiberStaple drawnFiberStaple (g/d) ,
drawn '
SpunAfter shin spun After spin 1
1 52.051.0 3.8 3.7 1.9 7.7 159.4 164.2
2 56.055.0 2.9 2.8 2.2 6.2 160.1 164.5
3 57.057.6 4.5 4.5 2.3 6.4 159.7 165.8
4 36.536.9 4.9 5.0 1.9 6.4 158.9 165.1
45.044.0 3.6 3.8 2.4 7.2 160.2 164.1
6 46.047.3 3.5 3.4 2.2 6.2 159.8 165.8
7 2 0 9 4 2 5 159 163
i 33 4 8 6 9 9
2
. . . . . . .
C.1 24.324.3 2.2 2.3 2.2 7.5 160.1 -
C.2 106.5104.2 6.5 6.5 1.7 7.6 160.2 -
C.3 32.333.6 5.1 5.2 2.2 6.5 160.3 -
C.4 80.281.2 4.7 4.9 1.9 8.1 160.4 -
C.5 28.029.3 4.2 4.3 2.1 6.9 160.1 -
C.6 39.641.2 10.2 10.3 1.6 7.2 160.2 -
C 7 201 1 c37 0 5 5.2 1.7 6.5 160.1 -
0 1
T ~ R T F.' ~
Strength
Nos. Carding Soft-
of TBI Notes
Exmpl.Machines Speed plan Wt.(g/mz)MD CD ness
i 4 -
2 T ~ -~
w
-
vv
N ' ~ 1r ~ 4 1 r
4 NN T 4 ~ r
vv
vv
c c
C.7 SPINNBAU 90 19.5 3.8 0.91.9 3 Difficult,
much
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As apparent from the above examples, the non-woven
fabrics, which are prepared by thermally bonding the
isotactic polypropylene homopolymer fibers having two DSC
endothermic peaks in accordance with the present
invention, show excellent strength in addition to being
soft. Also, the non-woven fabrics can be produced in high
speed carding machines. Consequently, the present
invention allows the production of high quality non-woven
fabrics with a high yield.
The present invention has been described in an
illustrative manner, and it is to be understood that the
terminology used is intended to be in the nature of
description rather than of limitation. Many modifications
and variations of the present invention are possible in
light of the above teachings. Therefore, it is to be
understood that within the scope of the appended claims,
the invention may be practiced otherwise than as
specifically described.
17
