Note: Descriptions are shown in the official language in which they were submitted.
2 ~ 3 9
BICONSTITUENT POLYPROPYLENE/POLYETHYLENE BONDED FIBERS
Blends consisting of po]ypropylene and
polyethylene are spun into fibers having improved
bonding properties and lower shrinkage.
Polypropylene (PP) fibers and filaments are
items of commerce and have been used in making products
such aq ropes, non-woven fabrics, and woven fabrics.
In conformity with commonly accepted vernacular
or jargon of the fiber and filament industry, the
following definitions apply to the terms u3ed in this
disclo~ure:
A "monofilament" (a.k.a. monofil) refers to an
individual strand of denier greater than 15, usually
greater than 30.
A "fine denier fiber or filament" refers to a
strand of denier less than about 15.
A "multi-filament" (a.k.a. multifil) refers to
simultaneously formed fine denier filaments spun as a
bundle of fibers, generally containing at leaqt 3,
preferably at least about 15 to 100 fibers and can be
several hundred or several thousandO
37 , 7 35 -F - 1 -
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"Staple fibers" refer to fine denier strandq
which have been formed at, or cut to, staple lengths o~
ganerally about 1 to about 8 inches.
An "extruded strand" refers to an extrudate
formed by passing polymer through a forming-orifice,
such as a die.
A "fibril" refers to a superfine discrete
filament embedded in a more or less continuous matrix.
Whereas it is known that virtually any
thermoplastic polymer can be extruded as a coarse strand
or monofilament, many of these, such as polyethylene and
some ethylene copolymers, have not generally been found
to be suitable for the making of fine denier fibers or
multi-filaments at feasibly high production speeds.
Practitioners are aware that it is easier to make a
coarqe monofilament yarn of 15 denier than to make a
multi-filament yarn of 15 denier, especially where high-
speed spinning is needed to obtain economical productionrates. It is also recognized that the mechanical and
thermal conditions experienced by a bundle of filaments,
whether in spinning staple fi-bers or in multi-filaments
yarns, are very different to those in spinning
monofilaments. The fact that a given man-made polymer
can be extruded as a monofilament, does not necessarily
herald its use in fine denier or multi-filament
spinning. Whereas an extruded monofilament which has
been cooled can usually be cold-drawn (stretched) to a
finer denier size, even if it does not have sufficient
melt-strength to be melt-drawn without breaking9 it is
apparent that a polymer needs to have an appreciable
melt-strength to be melt-drawn to fine denier sizes. _
37,735-F -2-
2 ~ 9
Low density polyethylene (LDPE) is prepared by
polymerizing ethylene using a free-radical initiator,
e.g. peroxide, at el~vated pressures and temperatures,
having densities in the range, generally, o~ about 0.910
to 0.935 gmstcc. The LDPE, sometimes called "I.C.I~-
type" polyethylene is a branched (i.e. non-linear~
polymer 7 due to the presence of short-chains of
polymerized ethylene units pendent from the main polymer
backbone. Some of the older art refers to these as high
pressure polyethylene (HPYE).
High density polyethylene (HDPE) is prepared
USil'lg a coordination catalyst, such as a "Ziegler-type"
or "Natta-type" or a "Phillips-type" chromium oxide
compound. These have densities generally in the range
of about 0.94 to about 0.98 gms/cc and are called
"linear" polymers due to the substantial absence of
short polymer chains pendent from the main polymer
backbone.
Linear low density polyethylene (LLDPE) is
prepared by copolymerizing ethylene with at least one a-
olefin alkylene of C3 to C12, especially at lea~t one of
C4 to C8 7 using a coordination catalyst such as is used
in making HDPE. LLDPE is "linear", but has alkyl groups
of the ~-olefin pendent from the polymer chain. These
pendent alkyl groups usually cause the density to be in
about the same density range (o.88 to 0.94 gms/cc) as
the LDPE; thus the name "linear low density
3 polyethylene" or LLDPE is used in the industry in
referring to these linear low density copolymers of
ethylene.
Polypropylene (PP) is known to exist as atactic
(largely amorphous), syndiotactic (largely crystal-
37,735-F -3-
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line), and isotactic (also largely crystalline), some of
which can be processed into fine denier fibers. It is
preferable, in the present invention, ~o use the largely
crystalline types of PP grades, sometimes re~erred to as
constant rheology ("CR"), which are suitable for
spinning fibers, especially fine denier fibers.
~ It was ~ound that improvements are made in
polypropylene fibers if the polypropylene is first
blended with 20 percent to 90 percent by weight of a
polyethylene, especially a linear low density ethylene
copolymer (LLDPE) containing, generally, 3 percent to 20
percent of at least one a-olefin alkylene of 3 to 12
carbon atoms. It was also ~ound that certain
polyethylenes (more specifically LLDPE's) can be blended
in a molten state with polypropylene in all proportions
and then melt spun into fine denier fibers, some of
which offer improved propertie~ over polyethylene and
polyprop~lene alone.
In accordance with this invention heat-bonded
articles having excellent bond strength when bonded over
a wide range of temperatures are prepared from ~ibers
comprising a dynamically-mixed melt-spun blend o~
polypropylene (PP) and polyethylene (PE~, said blend
comprising a PP/PE ratio in the range of 0.6 to 1.5,
said fiber having a substantially co-continuous domains
morphology.
The heat-bonded articles, including those
wherein the above fibers are used alone or are blended
with other fibers or other materials, can take a number
of shapes and sizes including, e.g., various non-woven
fabrics, composites and other items in which bonding
37,735-F _~_
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into a unit is accomplished using the above described
fibers.
It will be understood that the present
invention is not limited to only neat PP and PE7 but
also includes polymers containing additives that are
often uqed in such polymers, suoh as, stabili~ers, dyes,
- colorants, pigments, wetting agents, water-proofing
agents, soil-proofing agents, and the like, so long as
the additives have no substantial detrimental effect of
the fiber-making ability of the polymers. Considering
that most fibers produced on a commercial scale for
ordinary application in fabrics and the like are drawn
as fibers in the presence of air while they are hot, and
considering that the surface area to volume ratio of
fine fibers is quite high, then it will be understood
that an antioxidant is often used to avoid, or at least
reduce, oxidation of the polymer during the fiber-making
process.
Useful and novel fibers, especially fine denier
fibers, are prepared from blends of polypropylene (PP)
and polyethylene (PE) ? especially linear low density
ethylene copolymer (LLDPE) which have been melt blended
in an intensive mixer just ahead of the melt spinning of
the fibers when using ratios of PP and PE which result
in co-continuous zones in the resulting fiber, said co-
continuous zones being microscopically detectable in the
sectioned fibers when cooled. Generally, these co-
3 continuous zones are produced when the ratio of PP/PE isin the range of 0.6 to 1.5, especially in the range of
0.8 to 1.2, most especially in the range of 0.9 to 1.1.
Such fiber3 have unexpectedly been found to exhibit
appreciably stronger fiber-to-fiber bonds over a wide
temperature range employed when heat bonding, as
37,735-F -5-
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compared wi~h PP alone. The tenacity and softness of
the fibers is improved over that of the polypropylene or
the polyethylene alone.
The polyethylene for use in this invention may
be LDPE or HDPE, but is preferably LLDPE. The molecular
weight of the polyethylene should be in the moderately
- high range, as indicated by a melt index, M.I., (a.k.a.
melt fiow rate, M.F.R.) value in the range of 12 to 120,
preferably 20 to 100, most preferably 50 ~ 20 gmsf10
min. as measured by ASTM D-1238(E) (190~C~2~16 Kg).
Regarding the use of preferred LLDPEI it is
preferred that the comonomer a-olefin alkylenes in the
upper end of the C3 to C1z range be used, especially 1-
octene. Butene (C4) is preferred over propylene (C3)but is not as preferred as 1-octene. Mixtures of the
alkylene comonomers may be used, such as butene/octene
or hexene/octene in preparing the ethylene/alkylene
copolymers. The density of the LLDPE is dependent on
the amount oP, and the molecular size (i.e. the number
of carbons in the alkylene molecule) of, the alkylene
incorporated into the copolymer. The more alkylene
comonomer used, the lower the density; also, the larger
the alkylene comonomer, the lower the density.
Preferably an amount of alkylene comonomer is used which
results in a density in the range of 0.88 to 0.94, most
preferably 0.92 to 0.93 gms/cc. An ethylene/octene
copolymer having a dansity of about 0.925 gms/cc, an
octene content in the range of 10 to 15 percent and a
M.F.R. at or near 50 gms/10 min~ is very effective for
the purposes of this invention.
The method of melt-mixing is important due to
generally acknowledged immiscibility of the PP and PE.
37,735-F -6-
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An intensive mixer-extruder is required which causes, in
the blender, on the one hand, molten PE to be dispersed
in the molten PP and the dispersion maintained until the
mixture, as an extrudate, is expelled from the extruder.
On the other hand, molten PP is dispersed in molten PE
when the amount of PE exceeds the amount of PP.
- The following chart IS provided as a means for
describing the results believed to be obtained for the
various ratio ranges of PP/PE, when using PE (e~p.
0 LLDPE) having an M.F.R. in the range of about 12 to
about 120 gms./10 min., and a crystalline PP, where the
melt viscosity and melt strength are such that
reasonably good melt-compatibility and miscibility are
achieved by use of the high-intensity mixer-extruder:
Approx. Range of
Ratio of PP/PE General Results One May Obtain*
4.0 - 1.5 Mostly PE fibrils dispersed in PP
continuous matrix.
1.5 - 1.2 Mostly co-continuous domains of
PP and PE with some PE fibrils.
1.2 - 0.8 Nearly all co-continuous domalns
of lamellar structure.
0.8 - 0.6 Mostly co-continuous domains of
PP and PE with some PP fibrils.
0.6 - 0.1 Mostly PP fibrils dispersed in PE
continuous matrix.
*Obviously the results in or around the central
ratio ranges are overlapping and are ambiguous in
that some of the results obtained are from both
sides of the overlap.
Polymer blends of PP and PE prepared in such a
mixer are founcl to be useful, strong, and can be
extruded into products where the immiscibility is not a
37,735-F ~7-
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problem. As the so-formed extrudate of a mixture which
contains more PP than PE is spun and drawn into fibers,
the molten PE globules become extended into fibrils
within the polypropylene matrix. An important, novel
feature of the fibers is that the fibrils o~ PE are
diverse in thelr orientation in the PP matrix. A larger
fraction of PE particles is found close to the periphery
of the cross section of the fibers, and the remaining PE
particles are spread in the inner portions of the fiber.
The size of the PE particles is smallest at the
periphery of the fiber's cross-section and a gradual
increase in size is evidenced toward the center of the
fiber. The frequency of small particles at the
periphery is highest, and it decrease~ toward the center
where the PE particles are largest, but spread apart
more. The PE fibrils near the periphery of the fiber's
cross-section are diverse in the direction in which they
are oriented or splayed, whereas close to the center of
the fiber the orientation is mostly coaxial with the
fiber. For the purpose of being concise, these fibers
will be referred to herein as blends conqisting of PP as
a continuous phase, and containing omni-directionally
splayed PE fibrils as a dispersed phaseO Microscopic
examination reveals that the PE fibrils, when viewed in
a cross-section of the biconstituent ~iber, are more
heavily populated near the outer surface than in the
middle. The shape of each PE fibril in the cross
section is dependent on whether one is viewing a PE
fibril sliced at right angles to the axis of the PE
fibril at that point or at a slant to the axis of the PE
fibril at that point. An oval or elongate shaped
section indicates a PE fibril cut at an angle. An
37,735-F -8-
2 ~ 9
elongate shaped section indicates a PE fibril which has
skewed from axial alignment to a transverse position.
The mixer for preparing the molten blend of
PP/PE is a dynamic mixer, especially one which provide~
3-dimensional mixing. Insufficient mixing will cause
non-homogeneous dispersion of PE in PP resulting in
~ fibers of inconsistent propertiets7 and tenacities lower
than that of the corresponding PE' fibers alone. A 3-
dimensional mixer suitable for use in the present
invention is disclosed in a publication titled
"Polypropylene--Fibers and Filament Yarn With Higher
Tenacity", presented at International Man-Made Fibres
Con~ress, September 25-27, 1985, Dornbirn~Austria, by
Dr. Ing. Klaus Schafer of Barmag, Barmer Maschinen-
Fabrik, West Germany.
The distribution of PE fibrils in a PP matrix
are studied by using the following method: The fibers
are prepared for transverse sectioning by being attached
to strips of adhesive tape and embedded in epoxy resin.
The epoxy blooks are trimmed and faced with a glas~
kni~e on a Sorvall MT-6000 microtome. The block3 are
soaked in a mixture of 0.2 gm ruthenium chloride
dissolved in 10 ml of 5.25 percent by weight aqueous
sodium hypochlorite for 3 hours. This stains the ends
of the fibers with ruthenium to a depth of about 30
microns. The blocks are rinsed well and remounted on
the microtome Transverse sections of fibers in epoxy
3 are microtomed using a diamond knife, floated onto a
water trough, and collected onto copper TEM grid The
grids are examined at 100 KV accelerating voltage on a
JEOL 100C transmission electron microscope (TEM).
Sections taken from the first few microns, as well as
approximately 20 microns from the end are examined in
37,735-F _g
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the TEM at magnifications of 250X to 66,000X. The
polyethylene component in the samples are preferentially
stained by the ruthenium. Eiber sections microtomed
near the end of the epoxy block may be overstained,
whereas sections taken about 20 microns away from the
end of the fibers are more likely to be properly
stained. Scratches made by the microtome knife across
the face of the section may also contain artifacts of
the stain, but a skilled operator can distinguish th~
artifacts from the stained PE. The diameter of PE
fibrils near the center of the F'P fiber have been found
to be, typically, on the order of 350 to 500 angltrom~
whereas the diameter of the more populace fibrils near
the periphery edge of the PP fiber have been found to
be, typically, on the order of 100 to 200 angstrom.
This is in reference to those which appear under high
magnification to be of circular cross-section rather
than oval or elongateO
At le~s than 20 percent polyethylen~ in the
polypropylene one obtains better "hand" than with
polypropylene alone, but without obtaining a significant
increase in tenacity and without obtaining a
dimensionally stable fiberO By the term "dimensionally
stable" it is meant that upon storing a measured fiber
~or several months and then remeasuring the tenacity,
one does not encounter a significant change in the
tenacity. A change in tenacity indicates that stress
relaxation has occurred and that fiber shrinkage has
taken place. In many applications, such as in non-woven
fabrics, such shrinkage is considered undesirable.
By using 20 percent to 45 percent polyethylene
in the polypropylene one obtains increased tenacity as
well as obtaining better "hand" than with polypropylene
37,735-F -10-
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alone. By using between 25 percent to 35 percent,
especially 28 percent to 32 percent, of polyethylene in
the polypropylene one also obtains a substantiallr
dimensionally stable fiber. A substantially
dimensionally stable fiber is one which undergoes very
little, if any, change in tenacity during storage. A
ratio of polypropylene/polyethylene of about 70/30 is
especially beneficial in obtaining a dimensionally
stable fiber. By using 50 percent to 90 percent
polyethylene in the blend, a reduction in tenacit~ may
be observed, but the "hand" is noticeably softer than
polypropylene alone.
A greater draw ratio gives a higher tenacity
than a lower draw ratio. Thus, for a given PP/PE ratio,
a draw ratio of, say 3.0 may yield a tenacity greater
than PP alone, but a draw ratio of, say 2.0 may not give
a greater tenacity than PP alone.
In order to establish a nominal base point for
making comparisons, several commercially available PP's
are spun into fine denier ~ibers and the results are
averaged. The average denier size is ~ound to be 2.1,
the average elongation is found to be 208 percent and
the average tenacity at the break point is 2.26
gm/denier.
Similarly, to establish a nominal base point,
several LLDPE samples are spun into fine denier fibers
and the result~ are averaged. The average denier size
is found to be 2.84, the average elongation is found to
be 141 percent, and the average tenacity at the break
point is 2.23 gmJdenier.
37,735-F
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~ he following examples illustrate particular
embodiments, of the invention.
Biconstituent PP/PE fiber~ prepared as
described above and heat-bonded at temperatures
sufficient to melt the polymers, or at least soften them
enough for bonding, exhibit heat-bonding ranges over a
- surprisingly wide range of temperatures, and the bond
strengt~ obtained when heat-bonded over a wide range is
unexpectedly high.
EXAMPLE 1 (Heat-bonded fibers)
This example illustrates the broad temperature
range over which strong bonds are obtained by using the
biconstituent PP/PE fibers as compared with PP alone.
The fabric samples are of 1 ounce/yard2 (about
33.9gm/m2) weight and are made using a heated flat top
calendar roll and a heated, embossed bottom calendar
roll. The top calendar roll temperatures are maintained
about 4F (about 2C) lower than the bottom calendar roll
temperatures. Cutting the 4" X 1" (10 x 2.54 cm) strips
in the machine direction is done in such a way that the
most uniform portions of the-fabrics are u~ed before
pulling them apart on an Instron ten~ile te~ter. The
force to cause failure is measured as gram-force. Each
datapoint is the average of 8 sample, and a standard
deviation if observed in the range of 5 percent to 15
percent.
Commercially available LLDPE (26.5 MFR and
0.940 g/cc density, 1-octene comonomer) is blended with
equal parts of commercially available PP (CR fiber
grade) and extruded at 2X stretch ratio as continuous
biconstituent fine fibers using an intensive mixer
37,735-F -12-
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extruder. The 50/50 PP/PE biconstituent fibers are made
into staple fibers used in making non-woven fabrics at a
variety of embossed ro l temperatures. An example of a
neat PP ~without PE) is included as a "Control" for
comparison. Table IV below demonstrates the MD strip
tensile strength (grarn-force) needed to tear the non-
woven fabric. The temperatures in the table are the
embossed roll temperatures, adjusted to the nearest
whole number.
TABLE I
PE/PP ~atio in
Temp.Biconstituent Fibers Control
( C )
15Approx. 40/60 50/50 60/40 0/100
122 -- - ---- 285~
124 ---- 3450 3523 _~__
127 ____ 4061 3521 ----
20129 ---- 4230 3373 ----
131 ---- 4310 3847 __ _
133 ____ 4402 4113 ----
136 -~-- 4475 4031 ____
25138 ---- 4593 ~~~- 1865
140 3626 4422 ---- 2696
142 3943 4629 ____ 368~
144 414~ 4272 ---- 3903
30147 4029 4219 ---~ 3528
149 3809 ~180 ~ 3498
Table I clearly shows that the mid-ranges of
the ratios of the PE/PP biconstituent fibers not only
produce stronger fiber bonds, but also provide lower
37,735-F -13-
-14
effective bonding temperatures, a wider effective
bonding range of temperatures, and softer fabrics.
Advantages are found in range including and between the
~0/60 to 60/40 ratios. Having found this phenomenon,
one may extrapolate the range of ratios a little beyond
each end of the mid-range. Thus a PP/PE ratio range of
about 1.5 (i.e. about 60/40) to about 0.6 (i.e. about
40/60) is operable, with a ratio somewhere around 50/50
being most preferable.
EXAMPLE 2 (Heat-bonded fibers)
Similarly to Example 1 above, additional data
is collected for LLDPE (12 MFR, 1-octene9 0.935 density)
in Table II, LLDPE (98 MFR, 1-octene, 0.936 density) in
Table III, and LLDPE (25 MFR, propene, 0.955 density) in
Table IV. These tables show the improved results
obtained when operating within the range of 40/60 to
60/40 PE/PP ratio.
3o
37,735-F -14-
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~.E~l:
PE/PP Ratio o~ Bicon~tituent Fibers
Temp.
Approx. 30/70 40/6050/50 60/40 70/30
118 ---- --~- ~~~~ ~~~~ ~~~~
- 120 ---~ ~ ~~~~ ~~~~ 2743
12c ~~~~ 3382 3046
124 ---~ --__ _._--_ 3332 2913
127 ____ ____3538 3593 28~3
129 ---- ____3671 3589 3196
131 ---- ---- 4047 3688 ___
133 ____ ____3724 --__ __~_
136 1609 ---- 4090 ---- ---
138 1825 ---- 2568 4156 ----
140 1991 31404169
142 2645 33104155 ---- ____
144 3173 34154738 ---- ____
3o
37,735-F -15-
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TABLE I I I
PE/PP Ratio in
Biconstituent Fibers
T emp,
(C)
Approx. 30/70 50/50 70/30
-
118 ____ ____ 2387
120 - - - - - - - - 2480
122 - - - - - - - - 2714
124 - - - - - - - - 2905
127 - ~ 2842
129 - ___ ____ ____
131 - - - - ____ ____
133 ---- 3491 ----
136 1134 3299 ____
138 1283 3642 ____
140 1585 3578 ____
142 2078 - 3413 ----
3o
37,735-F ~16-
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TABLE IV
PE~PP ~atio in Bicon t_~,uent Fibers
Temp.
(C)
Approx. 30/70 40/60 50/50 60/40 70/~0
124 ---- ____ _.__ 2896 2753
- 127 -- - ____ _.__ 3659 2892
129 ---- ____ 3796 3854 3156
131 -- - - -- 1~008 3774 3201
133 ____ ____ 4066 3924 3149
6 -~-- 4049
138 ---- ---- 4032 --~
140 2961 4049 ----
142 3363 3947 4331 --_~ ____
144 3557 4147 4579 ___~ ____
147 4060 4223 4333 --__ ____
EXAMPLE 3 (Heat shrinkage)
Fibers of PP/LLDPE of various ratios between
the range 60/40 to 40/60, are tested in comparison with
PP fibers alone and L.LDPE alone9 by being subjected to
boiling water for 5 minutes and the shrinkage measured.
It is Yound that in this range there is little or no
increase in shrinkage when compared with PP. Thus the
benefits of adding LLDPE to PP are not substantially
3 compromised by the greater tendency of the neat LLDPE
fibers to undergo shrinkage in boiling water,
The heat-bonding capabilities of thesa
biconstituent PP/PE fibers are useful in blends with
other fibers, both natural and synthetic~ especially
37,735-F -17-
when staple fibers are blended and then heat-bonded at
temperatures favorable for the particular blend being
employed. Also, the heat~bondable PP/PE fibers can be
employed as the bonding agent when in admixture with, or
place between, other materials which are not
thermoplastic in or near melt or softening point of the
PP/PE biconstituent~ Other materials, such as
cellulosic fibers, metal fibers, mineral fibers, wood
fibers, high melting synthetic Yibers, and other
particulate material can be mixed with, and thermally
bonded into a unit by, the PP/PE biconstituent fibers.
3o
37,735-F -18-
