Note: Descriptions are shown in the official language in which they were submitted.
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CONSOLIDATED FIBROUS STRUCTURE
TECHNICAL FIELD
[0001] The present application is directed to consolidated fibrous
structures, fibrous layers, and fibers making up the fibrous structures. Also
disclosed are methods of making the same.
BACKGROUND
[0002] Consolidation of thermoplastic fibers into a fibrous structure such
as the consolidation of polypropylene fibers for molded parts and other
applications is typically accomplished by hot pressing under pressure at
typically
high temperatures (about 300 F or more) and typically high pressures of (about
300 psi or more) to obtain consolidated fibrous structures having the desired
performance attributes. The high temperature requirement hinders the ability
to
co-process the polypropylene fibers with other materials such as polymer
fibers
that are compromised at or above 250 F. The high pressure requirements
prevent these materials from being processed in cost-effective autoclavable
processes where the maximum application pressure is often about 45-100 psi.
Due to the existing method of high temperature-high pressure consolidation,
manufacturability of large parts requires large metal moulds which
significantly
add to the cost of the machinery and finished parts.
[0003] Thus, there is a need for high performance thermoplastic fibrous
layers and structures that are able to be processed at lower temperatures,
pressures, and/or dwell times while having the same or better performance
characteristics than the prior art materials processed at much higher
temperatures and pressures.
BRIEF SUMMARY
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[0004] The present disclosure provides a consolidated fibrous structure of
fibrous layers in cohesive adjoined relation.
[0005] According to one aspect, a consolidated fibrous structure
comprising a multiplicity of fibrous layers is provided. Each fibrous layer
contains fibers. The fibers of each fibrous layer contain a core and a skin
layer.
The core has an exterior surface portion containing polypropylene. The skin
layer is disposed on at least a portion of the core and contains a first
polymer
and a second polymer. The first polymer contains a polymer having at least 70%
a-olefin units and is characterized by a melting temperature lower than the
melting temperature of the exterior surface portion of the core. The second
polymer contains a co-polymer having at least 50% a-olefin units and is
characterized by a number-average molecular weight of about 7,000 g/mol to
50,000 g/mol, a viscosity of between about 2,500 and 150,000 cP measured at
170 C, and a melting temperature lower than the melting temperature of the
exterior surface portion of the core. The viscosity of the second polymer is
not
greater than 10% of the viscosity of the first polymer measured at 170 C. At
least a portion of the skin layers of the fibers within each fibrous layer are
fused
to at least a portion of other skin layers of fibers within the same fibrous
layer, at
least a portion of the skin layers of the fibers of each fibrous layer are
fused with
at least a portion of the skin layers of the fibers in an adjacent fibrous
layer, and
the stiffness of the consolidated fibrous structure is at least 1 N-m measured
by
ASTM D 790.
[0006] According to a second aspect, The present disclosure provides a
multi-layered fiber. According to one aspect, a multi-layered fiber comprising
a
core and a skin layer is provided. The core has an exterior surface portion
containing polypropylene. The skin layer is disposed on at least a portion of
the
core and contains a first polymer and a second polymer. The first polymer
contains a polymer having at least 70% a-olefin units and is characterized by
a
melting temperature lower than the melting temperature of the exterior surface
portion of the core. The second polymer contains a co-polymer having at least
50% a-olefin units and is characterized by a number-average molecular weight
of about 7,000 g/mol to 50,000 g/mol, a viscosity of between about 2,500 and
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150,000 cP measured at 170 C, and a melting temperature lower than the
melting temperature of the exterior surface portion of the core. The viscosity
of
the second polymer is not greater than 10% of the viscosity of the first
polymer
measured at 170 C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Figure 1 illustrates schematically a cross-section of an exemplary
consolidated fibrous structure.
[0008] Figure 2 illustrates schematically a cross-section of an exemplary
fiber of tape construction having a skin layer on one side of a core.
[0009] Figure 3 illustrates schematically a cross-section of an exemplary
fiber of tape construction having a skin layer on both sides of a core.
[0010] Figure 4 illustrates schematically a cross-section of an exemplary
fiber of tape construction having a skin layer on both sides of a core where
the
core has an inner portion and an exterior surface portion.
[0011] Figure 5 illustrates schematically a cross-section of an exemplary
fiber of circular cross-section having a skin layer surrounding a core.
[0012] Figure 6 illustrates schematically a cross-section of an exemplary
fiber of oval cross-section having a skin layer surrounding a core.
[0013] Figure 7 illustrates schematically a cross-section of an exemplary
fiber of oval cross-section having a skin layer surrounding a core where the
core
has an inner portion and an exterior surface portion of the core.
[0014] Figure 8 illustrates schematically a cross-section of an exemplary
fibrous layer having a woven construction.
[0015] Figure 9 illustrates schematically a cross-section of an exemplary
fibrous layer having a unidirectional construction.
[0016] Figure 10 illustrates schematically a cross-section of an exemplary
fibrous layer having a knit construction.
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[0017] Figure 11 illustrates schematically a cross-section of an exemplary
fibrous layer having a non-woven construction.
[0018] Figure 12 illustrates schematically a consolidated fibrous structure
in a three-dimensional shape.
[0019] Figure 13 illustrates schematically a cross-section of two fibrous
layers consolidated together showing polymer at least partially filling the
interstices of the fibrous layers.
[0020] Figure 14 illustrates schematically a cross-section of an exemplary
fibrous layer having a unidirectional construction where the fibers have one
continuous skin layer and one discontinuous skin layer.
[0021] Figure 15 illustrates schematically a cross-section of a fibrous layer
showing 2 fibers fused together.
[0022] Figure 16 is a 100x image of a fibrous layer as illustrated in Figure
illustrating the peeling failure mode.
15 [0023] Figure 17 is an illustrative version of the image of Figure 16.
[0024] Figure 18 is a 200x image of fibers peeled apart having the
interface between the skin layer and the skin layer fail.
[0025] Figure 19 is an illustrative version of the image of Figure 18.
DETAILED DESCRIPTION
[0026] Referring now to Figure 1, there is shown one embodiment of the
consolidated fibrous structure 200 which is formed from three (3) fibrous
layers
100. Each fibrous layer 100 contains fibers 10 having a core 12 with an
exterior
surface portion comprising polypropylene and skin layers 14 and 14' comprising
a first polymer and a second polymer. The stiffness of the consolidated
fibrous
structure is at least about 1 N-m, more preferably at least 5 N-rn measured by
ASTM D 790.
[0027] Figures 2-7 illustrate different configurations of the fibers 10. For
purposes of the present application, fiber is defined as an elongated body,
the
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length at least 100x the square root of the cross-sectional area. Accordingly,
the
term "fiber" as used herein includes a monofilament elongated body, a
multifilament elongated body, ribbon, strip, tape, and the like. The term
fibers
include a plurality of any one or combination of the above. Figure 2 shows a
5 fiber 10 having a tape construction with a core 12 and a skin layer 14 on
one
side of the core 12. Figure 3 shows a fiber 10 being tape elements having a
core 12 and two skin layers 14, 14' sandwiching the core 12. Figure 4
illustrates
an embodiment where the core has an inner portion 12" and an exterior surface
portion 12' including polypropylene. The inner portion 12" may be, but is not
limited to, polypropylene, polyethylene, polyester, polyamides, polyethers, or
copolymers thereof; glass fiber, aramid, carbon fiber, ceramic fiber, nylon,
polyetherimide, polyamide-imide, polyphenylene sulfide, polysulfones,
polyimide,
conjugated polymers, mineral fiber, natural fibers, metallic fiber or mixtures
thereof. In one embodiment, the inner portion 12" of the core 12 has a tensile
modulus of greater than 15 grams per denier as measured by ASTM method
3811-07. The inner portion 12" and exterior surface portion 12' of the core
may
be of the same or similar chemical composition or may be of different chemical
composition with or without the use of surfactants, copolymers, or other means
of reducing the surface energy difference between the inner portion 12" and
exterior surface portion 12' of the core 12. In one embodiment, the fiber 10
is a
high modulus fiber, being defined as fibers having a tensile modulus of
greater
than 10 grams per denier as measured by ASTM method 3811-07 and
preferably have a tensile strength of at least 100 MPa.
[0028] Figure 5 shows a fiber 10 being a fiber having a circular cross-
section with a core 12 and a skin layer 14 surrounding the core. Figure 6
shows
a fiber 10 being a fiber having an oval or oblong cross-section having a core
12
and a skin layer 14 surrounding a section of the core, the skin layer 14 being
discontinuous at least in the direction around the circumference of the core
12.
Figure 7 illustrates the embodiment where the core has an inner portion 12"
and
an exterior surface portion 12' being polypropylene. The exterior surface
portion
12' of the core 12 may be continuous or discontinuous. The inner portion 12"
may be, but is not limited to, polypropylene, polyethylene, polyester,
polyamides,
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polyethers, or copolymers thereof; glass fiber, aramid, carbon fiber, ceramic
fiber, nylon, polyetherimide, polyamide-imide, polyphenylene sulfide,
polysulfones, polyimide, conjugated polymers, mineral fiber, natural fibers,
metallic fiber or mixtures thereof. In one embodiment, the inner portion 12"
of
the core 12 has a tensile modulus of greater than 15 grams per denier as
measured by ASTM method 3811-07. The inner portion 12" and exterior surface
portion 12' of the core may be of the same or similar chemical composition or
may be of different chemical composition with or without the use of
surfactants,
copolymers, or other means of reducing the surface energy difference between
the inner portion 12" and exterior surface portion 12' of the core. The
interface
between the inner portion 12" and the exterior surface portion 12' should have
sufficient physical interlocking as to prevent delamination between the inner
portion 12" and the exterior surface portion 12' of the fiber 12 in the final
product.
[0029] The core 12 of the fiber 10 has a high modulus (greater than 10
grams per denier) to provide stiffness for the fiber 10. It is contemplated
that the
core 12 of the fibers 10 is preferably made up of a molecularly-oriented
thermoplastic polymer. The core 12 may account for about 50-99 wt. % of the
fiber 10. Preferably, the core 12 of the fiber 10 has a tensile modulus of at
least
greater than 10 grams per denier as measured by ASTM method 3811-07, and
more preferably greater than 40 grams per denier. According to one practice,
the core 12 of the fiber 10 is polypropylene and is highly drawn with a draw
ratio
greater than 10:1. The core 12 (inner portion 12" and exterior surface portion
12') of fiber 10 has a peak melting temperature equal to or higher than the
all the
skin layers 14, 14'. Preferably, the core 12 (inner portion 12" and exterior
surface portion 12') of the fiber 10 has a peak melting temperature of at
least 5 F
greater than the skin layers 14, 14', more preferably at least 10 F greater
than
the skin layers 14, 14'.
[0030] In one embodiment, the fibers 10 are tape elements having a core
12 and at least one skin layer 14, 14'. The tape fibers 10 may be formed by
slitting a film. The film may be formed by any conventional means of extruding
such multilayer polymeric films. By way of example, and not limitation, the
film
may be formed by blown film or cast film extrusion. The film is then cut into
a
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multiplicity of longitudinal strips of a desired width by slitting the film to
yield tape
fibers 10 having cross-sections in the thickness dimension as shown in Figures
2-4. The tape fibers 10 are then preferably drawn in order to increase the
orientation of the core 12 so as to provide increased strength and stiffness
of the
material. After the drawing process is complete, in one embodiment the
resulting strips are in the range of about 1 to about 5 millimeters wide.
[0031] Fiber elements being tape elements, core-shell elements, and their
textile layer constructions are believed to be more fully described in U.S.
Patent
Publication No. 2007/0071960 (Eleazer et al.), US Patent Application No.
11/519,134 (Eleazer et al.), and US Patents 7,300,691 (Eleazer et al.),
7,294,383 (Eleazer et al.), and 7,294,384 (Eleazer et al.), each of which is
incorporated by reference.
[0032] The core 12 (or exterior surface portion 12' of the core 12) is
compatibly bonded to each of skin layers 14, 14' along their respective
surfaces.
The skin layers 14, 14' contain a first polymer and a second polymer. The skin
layers 14, 14' may be continuous or discontinuous on the core 12. Preferably,
the skin layers 14, 14' account for about 1-50 vol. % of the fiber 10. The
core 12
and skin layer(s) 14, 14' may be co-extruded together, or the skin layer(s)
layer
14, 14' may be applied to the core 12 after the core 12 has been formed.
Additionally, a portion of the skin layer(s) may be applied during or after
core
formation with the balance of the skin layer(s) being introduced at a later
point.
The fiber 10 may be drawn or oriented before or after the skin layer(s) 14,
14'
are formed in order to increase the orientation of the core 12 so as to
provide
increased strength and stiffness or achieve a targeted core dimension.
[0033] The first polymer comprises a polymer having at least about 70%
a-olefin units and is compatible with the polypropylene of the exterior
surface
portion 12' of the core 12. Preferably, the first polymer is a co-polymer
having at
least about 70% a-olefin units. "Compatible", in this application, is defined
as
two or more polymers that are inherently or enhanced to remain mixed without
objectionable separation over the range of processing conditions that will
form
the final product. "a-olefin", in this application, is defined as 1 -alkene
olefin
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monomer units other than ethylene such as propylene, butylene, 4-methyl-1-
pentene, pentene, 1-octene, and the like.
[0034] It may be preferred for the first polymer to have an ethylene
content of about 1-25 mol. %, and a propylene content of about 75-99 mol. %.
It
may be further preferred for the first polymer to have a ratio of about 95
mol. %
propylene to about 5 mol. % ethylene. In one embodiment, the first polymer is
a
terpolymer, one example being a terpolymer of ethylene, propylene, and
butylene. The first polymer has a viscosity of greater than 150,000 cP
measured
at 170 C and a melting temperature lower than the melting temperature of the
exterior surface portion of the core. In one embodiment, the first polymer has
a
viscosity of greater than 1,000,000 cP measured at 170 C. The first polymer
has
at least ten times the viscosity of the second polymer, measured at 170 C. In
one embodiment, the first polymer has a melting temperature of between about
120 and 140 C, a weight average molecular weight of between about 300,000
and 350,000, a viscosity of about 4,000,000 to 7,000,000 cP at 170 C, a melt
flow index of between about 4 and 8 grams/10 minutes measured at 230 C, and
a polydispersity of between about 3 and 6. "Melting temperature", in this
application, is defined to be the lower of the peak melting temperature or the
temperature at which 50% of the polymer has melted from the solid state as
measured by Differential Scanning Calorimetry (DSC). Preferably, the first
polymer as a melting temperature of at least about 10 C lower than that of the
exterior surface portion 12' of the core 12, and preferably between about 15-
40 C lower. In one embodiment, the tensile modulus of the first polymer is
greater than about 100 MPa, preferably greater than about 500 MPa and most
preferably greater than about 1 GPa.
[0035] The skin layer(s) 14, 14' also contain a second polymer. The
second polymer comprises a co-polymer having at least 50% a-olefin units.
Preferably, the second polymer comprises a co-polymer having at least 50%
propylene units, more preferably 80% propylene units, most preferably more
than 82% propylene units. The second polymer comprises a number-average
molecular weight of between about 7,000 g/mol and 50,000 g/mol, a viscosity of
between about 2,500 and 150,000 cP measured at 170 C, and a melting
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temperature lower than the melting temperature of the exterior surface portion
of
the core. The second polymer has a viscosity not greater than about 10% of the
viscosity of the first polymer measured at 170 C. If the polymer has a number-
average molecular weight lower than 7,000 g/mol or a viscosity lower than 2500
cP, the molecular chains of the second polymer are too short to entangle with
one another and are too weak to effectively fuse with the first polymers. A
polymer with a viscosity of greater than 150,000 cP has a reduced ability to
flow
into the interstices of the fibrous layers under lower temperature and
pressure
consolidation conditions. The melting temperature of the second polymer is
preferably lower than the core in order to fuse the layers together without
compromising the integrity of the core. In one embodiment, the second polymer
has a weight average molecular weight of between about 20,000 and 40,000
g/mol, a number average molecular weight of between about 7,000 and 22,000.
[0036] In one embodiment, the viscosity of the second polymer is between
about 4,000 and 120,000 cP measured at 170 C, more preferably between
about 4,000 and 16,000 cP. In one embodiment, elongation at break is greater
than about 200%, more preferably greater than about 400%. In one potentially
preferred embodiment, the second polymer has a lower melting temperature
than the first polymer. This aids in allowing the second polymer to flow into
the
interstices. In one embodiment, the second polymer has a cohesion strength of
at least about 1 MPa, more preferably greater than about 4 MPa. Having a
cohesion strength in this range provides a polymer that resists tearing. In
one
embodiment, the viscosity of the second polymer is between about 0.005 to 10%
the viscosity of the first polymer measured at 170 C, more preferably about
0.1
and 10%. The large difference in viscosity is believed to help facilitate the
structural adhesion between the first polymer and the core and the filling of
the
interstices of the fibrous layers by the second polymer. Generally it has been
found that an increase in the molecular weight leads to an increase in the
cohesion strength and the viscosity of the polymer. The first polymer provides
the strength of the reinforcement and is expected to have a high molecular
weight and hence a higher cohesion strength and viscosity. The second polymer
aids in processing of the skin layer and also fills the interstices of the
fibrous
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layers to provide additional reinforcement. In one embodiment, the tensile
modulus of the second polymer is greater than about 0.1 MPa, preferably
greater
than about 0.5 MPa and most preferably greater than 1 MPa. The percentage
by weight of the second polymer to the total weight of the fiber is between
about
5 0.25%wt and 50%wt, more preferably between about 1%wt and 25%wt. In one
preferred embodiment, the second polymer comprises a metallocene catalyzed
propylene-ethylene co-polymer.
[0037] The skin layers 14, 14' have an interior margin adjacent the core
12 and an exterior margin in the skin layer 14, 14' remote from the core 12.
The
10 interior margin of the skin layer comprises the first polymer and the
exterior
margin of the skin layer comprises the second polymer. In some embodiments,
the first polymer contributes significantly to the adhesion between the core
and
the skin layer and the second polymer contributes significantly to the
adhesion of
the skin layer to skin layer of adjacent fibers 10.
[0038] In one embodiment, the skin layers 14, 14' contain a generally
homogenous blend of the first and second polymers. However, in one
embodiment, the first and second polymers form concentration gradients through
the thickness of the skin layer 14, 14'. The first polymer has a higher
concentration at the inner margin, decreasing concentration through the skin
layer to a lower concentration at the exterior margin of the skin layer. The
second polymer forms a concentration gradient from a higher concentration at
the outer margin decreasing through the thickness of the skin layer to the
inner
margin. Preferably, the first polymer is a majority component by weight of the
skin layer 14, 14' at the inner margin and the second polymer is a majority
component by weight of the skin at the exterior margin. This gradient may be
formed, for example, by co-extruding the skin layer and core together having
the
second polymer bloom towards the surface of the skin layer or applying the
first
and second polymers as discrete layers then heating them to form one layer
where a portion of the first polymer migrates into the second and a portion of
the
second polymer migrates into the first.
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[0039] The fibers 10 are in fibrous layers 100. These fibrous layers 100
may contain fibers in woven, non-woven, knit, or unidirectional constructions
(or
mixtures of these constructions). Referring now to Figure 8, there is shown
fibers 10 in a woven construction. While the fibers 10 shown in Figures 8 are
tape fibers having a core 12 and two skin layers 14 and 14', other fiber types
or
shapes may be utilized. As illustrated, the fibrous layer 100 preferably
includes
a multiplicity of fibers 10 running in the warp direction interwoven with
fibers 10
running in the fill direction in transverse relation to the warp fibers. As
shown,
the fill fibers are interwoven with the warp fibers such that a given fill
extends in a
predefined crossing pattern above and below the warp. In one embodiment, the
fill fibers and the warp fibers are formed into a so called plain weave
wherein
each fill fiber passes over a warp fiber and thereafter passes under the
adjacent
warp fiber in a repeating manner across the full width of the fibrous layer
100.
However, it is also contemplated that any number of other weave constructions
as will be well known to those of skill in the art may likewise be utilized.
By way
of example only, and not limitation, it is contemplated that the fill fibers
may pass
over two or more adjacent warp fibers before transferring to a position below
one
or more adjacent warp fibers thereby forming a twill weave. The term
"interwoven" is meant to include any construction incorporating interengaging
formation fibers.
[0040] Figure 9 illustrates a fibrous layer 100 having a unidirectional
construction formed from a multiplicity of fibers 10 being tape elements. The
fibers are aligned parallel along a common fiber direction of the fibrous
layer
100. In one embodiment, the fibers 10 in the fibrous layer 100 do not overlap
one another, and may have gaps between the fibers 10. In another
embodiment, the fibers overlap one another up to 90% in the fibrous layer 100.
One approach for aligning the fibers (especially tape elements) is to align
the
fibers into a sheet by pulling the fibers from a creel. Using a roll-off creel
is
helpful to reduce twist in the fibers. The common fiber direction of the
fibers 10
in one fibrous layer 100 is the same (parallel to) as the common fiber
direction of
the fibers 10 of the adjacent fibrous layer. In the consolidated fibrous
structure
200, the common fiber direction of the fiber 10 in one layer 100 may be
parallel,
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perpendicular, or have any other rotational degree relative to the fiber
direction in
the adjacent fibrous layers 100.
[0041] Figure 10 illustrates a fibrous layer 100 having a knit construction.
The warp and weft fibers are circular cross-section fibers 10. There is also a
stitching fiber 15 which may be the same as fiber 10 or may be another
thermoplastic or non-thermoplastic fiber. The knit fibrous layer 100 may be
any
known knit, including, but not limited to circular knit, warp knit, weft-
inserted warp
knit, double needle bar, and jacquard knit fabrics.
[0042] Figure 11 illustrates a fibrous layer 100 having a non-woven
construction. The fibers 10 are tape fibers. The core 12 and skin layers 14
and
14' are not shown for simplicity. The term "non-woven" refers to structures
incorporating a mass of fiber elements that are entangled and/or heat fused so
as to provide a coordinated structure with a degree of internal coherency. Non-
woven fibrous layers or webs may be formed from many processes such as for
example, meltspun processes, hydroentangling processes, mechanically
entangled processes, needle punched processes, air-laying processes and wet
laying processes, laid scrims and the like.
[0043] In the fibrous layers, the fibers 10 may be unattached from one
another or fused to one another. In the fused configuration, at least a
portion of
the skin layers 14, 14' of the fibers 10 within the fibrous layer 100 are
fused to
one another. The fibrous layer 100 is heated, preferably under pressure, to a
softening temperature below that of the core 12. In so doing, the skin layers
14,
14' will melt while the core 12 will remain substantially solid and highly
oriented.
As the fibrous layer 100 then cools, the skin layers 14, 14' will fuse
together,
thereby forming a solid matrix. Fused or un-fused individual fibrous layers
100
may be stacked and reheated to form the consolidated fibrous structure 200.
[0044] At least 3 of the fibrous layers 100 are stacked together and
consolidated using heat and/or pressure to form a consolidated fibrous
structure
200 such as shown in Figure 1. While the consolidated fibrous structure 200
has
been depicted in Figure 1 as including three (3) fibrous layers 100, those of
ordinary skill in the art will readily appreciate that the consolidated
fibrous
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structure 200 can comprise any suitable number of fibrous layers 100. In one
embodiment, at least two (2) fibrous layers are consolidated together. In
other
embodiments, the consolidated fibrous structure 200 can comprise greater than
three fibrous layers. By way of example only and not limitation, such
structures
may have ten fibrous layers, twelve fibrous layers, eighteen fibrous layers,
twenty fibrous layers, thirty fibrous layers, or forty fibrous layers. While
Figure 1
shows each of the fibrous layers 100 containing the same type of fibers 10 and
layer construction (woven in the case of Figure 1), the consolidated fibrous
structure 200 may contain many different fibrous layer 100 constructions and
fibers 10 in the structure 200.
[0045] Several layers of fibrous layers 100 may be stacked in layered
relation prior to the application of heat and pressure in order to form the
consolidated fibrous structure 100. The layers of the fibrous layer 100 may be
formed from a single sheet of a fibrous layer that is repeatedly folded over
itself,
or from several discrete overlaid fibrous layers. Alternatively, the structure
200
may be formed by reheating several previously fused fibrous layers 100. Any of
these methods may be employed to form a structure 200 with any desired
thickness or number of layers.
[0046] Consolidation of multiple fibrous layers 100 is preferably carried out
at suitable temperature and pressure conditions to facilitate both interface
bonding fusion and partial migration of the melted skin layer material between
the layers. It has been found that having a both the first polymer and the
second
polymer in the skin layer aids in the ability to use lower temperature, lower
pressure, and/or shorter dwell time consolidation conditions with the same or
better properties as compared to a fiber having only a first polymer in the
skin
layer. Heated batch or platen presses may be used for multi-layer
consolidation.
In one exemplary practice, autoclaves or vacuum bags may be used to provide
the pressure during consolidation. Continuous consolidation methods such as
calendaring or use of a single or double belt laminator may likewise be
employed. It is contemplated that any other suitable press may likewise be
used
to provide appropriate combinations of temperature, pressure, and residence
time. According to a potentially preferred practice, heating is carried out at
a
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temperature of about 195-325 F and a pressure of about 15-400 psi. When
exposed to such an elevated temperature and pressure, the skin layers 14, 14'
will melt while the core 12 will remain substantially solid. Upon cooling, the
skin
layers 14, 14' will fuse thereby forming a matrix through which the stiff core
12
are distributed. "Fuse" is defined as being joined as if by melting together.
At
least a portion of the skin layers of the fibers 10 in each fibrous layer 100
are
fused within the same fibrous layer 100 and at least a portion of the skin
layers
of the fibers 10 of each fibrous layer 100 are fused with at least a portion
of the
skin layers 14, 14' of the fibers 10 in an adjacent fibrous layer 100.
[0047] According to a potentially preferred practice, cooling is carried out
under pressure to a temperature lower than about 100 F. It is contemplated
that
maintaining pressure during the cooling step tends to inhibit shrinkage of the
core and ensures no loss of consolidation. Without being limited to a specific
theory, it is believed that higher pressures may facilitate polymer flow at
lower
temperatures. Thus, at the higher end of the pressure range, (greater than
about 200 psi) the processing temperature may be about 80-140 C.
[0048] The consolidated fibrous structure 200 may thereafter be subjected
to three-dimensional molding under heat and pressure at temperatures above
the softening point of the skin layers 14, 14' so as to yield complex shapes.
The
fibrous structures may also be consolidated and molded in a single step
forming
structure 200 simultaneous with the formation of a subtle or complex shape. An
example of a plurality of fibrous layers 100 formed into a consolidated
fibrous
structure 200 having a three-dimensional shape is shown in Figure 12.
[0049] In one embodiment, the fibrous layers 100 contain interstices 22, or
voids, between the fibers as shown, for example, in Figure 13. It has been
found
that the second polymer is more mobile during the fusing process and more
easily migrates into the interstices 22 of the fibrous layer 100. In some
embodiments, there are also interstices formed between fibers of adjacent
fibrous layers. Preferably, when the interstices are at least partially filled
with the
first and second polymer, the filled interstices contain at least about 80% by
weight the second polymer. At least partially filling the interstices 22 of
the
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fibrous layers 100 creates better adhesion between the fibrous layers 100 and
a
better performing end product.
[0050] In some embodiments, during consolidation of the fibrous layers
100 or structures 200, a portion of the second polymer and first polymer
intermix
5 forming a blend of the first and second polymers.
[0051] Semicrystalline polymers like the polyolefins involved in this patent
comprise oriented crystallites separated by amorphous chains. During the melt-
welding of polyolefins, especially polypropylene, the amorphous chains migrate
towards the interface to maximize the entropy. Due to the accumulations of
short
10 amorphous chains, the strength of the interface can be very weak. In one
embodiment, the first polymer and second polymer can be found to co-
crystallize
with one another resulting in a polymer blend with a higher crystallinity than
either polymer alone. The complementary crystallization is believed to enhance
the physical properties of the skin yielding an improved skin layer. This is
15 achieved through the second polymer increasing the mobility of the first
polymer
chains and allowing a more extensive crystallization. The second polymer,
being
compatible with the first, is also able to incorporate into the crystal
structure of
the first without the introduction of significant additional defects.
[0052] While not being bound to any theory, it is believed that typically, if
a
mixture of two polymers is melted and cooled one would expect the
crystallinity
to be a simple average rule of mixtures. However, if a lower molecular weight
species (preferably metallocene polyolefins) is present at the interface, the
shorter molecular chains co-crystallize with the amorphous chains at the
interface. In this case the final crystallinity of the polymer mixture is
higher than
either of the two polymers. In the preferred embodiment the crystallinity of
the
heated-cooled mixture of the skin layer is higher than any of the two polymers
separately. This co-crystallization phenomenon imparts great strength to the
interface between the skin layers 14, 14'. The degree of co-crystallization is
dependent on the molecular chain length of the amorphous segments of the first
polymer and on the crystallizable chain length of the second polymer. Another
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phenomenon that could control the degree of co-crystallization is the rate of
cooling. The rate of cooling is dependent on the consolidation conditions.
[0053] The skin layers 14, 14 may be applied to the core 12 of the fibers
in any known method. The skin layers may be co-extruded or otherwise
5 formed at the same time as the core of the fibers or may be applied to a
core
after the core are formed. The skin layers 14, 14' may be applied to
individual
cores or onto cores that have already been formed into a fibrous layer. The
skin
layers may be continuous or discontinuous. Figure 14 illustrates a fibrous
layer
100 being a unidirectional layer having a continuous skin layer 14 on one side
of
10 core and a discontinuous skin layer 14' on the opposite side of the core.
These
discontinuous regions may be along a single fiber 10 or across multiple fibers
10.
This application may be conducted by any known means including, but not
limited to, solvent coating, curtain coating, extrusion coating, inkjet
printing,
gravure printing, solvent coating, powder coating, covering the core with a
spunbond skin layer, or covering the core with a separate film layer
comprising
the skin polymers.
[0054] One contemplated practice to form multi-layered fiber comprises
providing an elongated core having an exterior surface portion comprising
polypropylene and applying a skin layer to at least a portion of the core,
where
the skin layer comprises a first polymer and a second polymer. In one
embodiment, the first polymer contains a co-polymer having at least 70% a-
olefin
units and is characterized by a viscosity of greater than 1,000,000 cP
measured
at 170 C and a melting temperature lower than the melting temperature of the
exterior surface portion of the core. The second polymer contains a propylene
co-polymer having at least 80% propylene units and is characterized by a
number-average molecular weight of about 7,000 g/mol to 50,000 g/mol, a
viscosity of between about 4,000 and 10,000 cP measured at 170 C, and a
melting temperature lower than the melting temperature of the first polymer.
In
this embodiment, the ratio by weight of the second polymer to the first
polymer is
between about 1:20 to 20:1.
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[0055] How well the consolidated fibrous layer performs can be evaluated
by testing stiffness and peel strength (both of which are described in detail
in the
examples section). During the peel test, the fiber-fiber unit fails either at
the
core-skin layer interface or the skin layer-skin layer interface.
[0056] A section of the consolidated fibrous structure has two interfaces
as shown in Figure 15. The core-skin layer adhesion is fixed by the method at
which the skin layer was applied to the core (for example, co-extruded or
coated
on). When the skin layer contains the first and second polymers, the skin
layer-
skin layer interface is strong resulting in a failure between the core and the
skin
layer or within the core itself. This indicates a very strong bond between the
skin
layers. This can be seen in the 100x micrograph of Figure 15. An illustration
of
the image is shown in Figure 17. One is able to determine where in the
structure
the failure mode happened by the appearance, or lack thereof, of oriented
fibers
at the split. These oriented fibers are parts of the oriented core stripping
away
from the core.
[0057] If the skin layer-skin layer cohesion is not stronger than the core-
skin layer adhesion, the failure will occur between the skin layers and
resulting in
a smooth split as can been seen in the 200x micrograph of Figure 18 and as an
illustration of the micrograph in Figure 19. This indicates a poor bond
between
the fibers within fibrous layers or between fibrous layers. The skin layer-
skin
layer adhesion may be influenced by various material and processing
parameters.
[0058] The fibrous layers 100 and consolidated fibrous structure 200 may
contain additional fibers or layers. Examples of additional fibers that may be
incorporated include, but are not limited to fibers made from highly oriented
polymers, such as gel-spun ultrahigh molecular weight polyethylene fibers,
melt-
spun polyethylene fibers, melt-spun nylon fibers, melt-spun polyester fibers,
sintered polyethylene fibers, rigid-rod polymers, carbon fibers, aramid
fibers,
glass fibers, polylactic acid fibers, and natural fibers such as cotton.
[0059] Additional films such as polycarbonate films, polyester films,
polyethylene films, and polypropylene films may be included into the structure
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200. In some embodiments, additional panels may be included with the
structure 200 such as ballistics panels or the like.
[0060] The fibers, fibrous layers, and consolidated fibrous structures may
be used for many purposes in many different applications that would be
unavailable to fibers having a core and a skin having only the first polymer.
Having the skin layer contain both the first polymer and the second polymer
allows for the same or better performance of the element or structure as
fibers
with only the first polymer, but processed at much lower temperatures,
pressures, and/or dwell times.
EXAMPLES
[0061] Various embodiments are shown by way of the Examples below,
but the scope of the invention is not limited by the specific Examples
provided
herein.
Test Methods
[0062] Stiffness was measured by ASTM D 790. For the peel strength
test, the areal density of the specimen is adjusted to 0.21 psf (Typically 10
layer
consolidated stacks are used). The samples are cut to 2" width strips. The
peel
specimens are prepared by leaving a 1" wedge at one end in the center of the
stack (between layers 5 and 6) to grip the ends using fixtures mounted on the
cross-head of an Instron. A 180 degree peel test is conducted with a crosshead
speed fixed at 12"/min. The initial gauge length is 1". The peel start point
is 1"
and the end point is 9". The peel strength is calculated by measuring the
average peel force divided by the sample width. Melting temperatures were
measured by DSC using a scanning rate of 20 C/min.
Control Example 1
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[0063] Woven fibrous layers were formed from monolayer polypropylene
tape fibers in a plain weave with a fabric weight of 0.3 lb/yd2. Ten (10)
fiber
layers were cut to a foot square (12" x 12") and stacked. The layers were
placed
between two aluminum platens consolidated at various temperatures (T = 320 F,
300 F) and pressures (P = 2500 psi, 300 psi) for 10 minutes of heating time.
The
sample was then subsequently cooled to 100 F. The cooling rate was
approximately 20 F/minute.
Working Example I
[0064] Woven fibrous layers were formed as described in Control
Example 1. A propylene copolymer having Mw of 280,000 g/mol, vicat softening
point of 107 C, a melting temperature of bout 117 C, and viscosity of
5,700,000
cP at 170 C together with Licocene 2602, a metallocene type of propylene-
ethylene co-polymer obtained from Clariant were in dissolved at 80 C in
toluene
under continuous stirring to give a uniform solution comprising 2% of each
component (by weight; total weight of solids = 4%). Licocene 2602 had a
viscosity of 6000 cP measured at 170 C and a melting temperature of 75 C. The
tensile modulus of Licocene 2602 is measured to be approximately 0.7 MPa;
has an elongation at break of 760% and cohesion strength of 9 MPa. The
crystallinity of the polymer is - 17%.
[0065] The coating solution was padded on uniformly onto ten 12" x 12"
woven tape layers. The percent add-on on the fibrous layers was approximately
2% dry add on. The layers were then dried overnight at room temperature. After
drying the woven tape fibrous layers they were stacked and consolidated by
being placed between heated aluminum platens at a temperature of 300 F and a
pressure of 300 psi for 10 minutes of heating time. The sample was then
subsequently cooled to 100 F. The cooling rate was approximately 20 F/minute.
Working Example 2
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[0066] Woven fibrous layers were formed as described in Control
Example 1. Dow 6D83K (a propylene-ethylene random copolymer) obtained
from Dow was melted at 225 C in the melt bath of a film blowing line. Dow
6D83K had a weight average molecular weight of 360,000 and a melting
5 temperature of 140 C. 25 micron thick films were blown and cut into 12" x
12"
sheets. Next, they were padded with a solution of a 2%wt Licocene 2602, a
metallocene type of propylene-ethylene co-polymer obtained from Clariant in
toluene solution. Licocene 2602 had a viscosity of 6000 cP measured at
170 C and a melting temperature of 75 C. The tensile modulus of Licocene
10 2602 is measured to be approximately 0.7 MPa; has an elongation at break of
760% and cohesion strength of 9 MPa. The crystallinity of the polymer is -
17%.
These treated films were layered in between each of 10 woven fibrous layers of
Control Example 1. It is believed that the coated film formed one layer. The
stack was then placed between two platens at a temperature of 300 F and a
15 pressure of 300 psi for 10 minutes of heating time. After this the cooling
cycle
was started and the composite was removed when the temperature was 100 F.
Working Example 3
[0067] Woven fibrous layers were formed as described in Control
20 Example 1. A propylene copolymer having Mw of 280,000 g/mol, vicat
softening
point of 107 C, a melting temperature of about 117 C, and viscosity of
5,700,000
cP at 170 C together with Vestoplast 708, a co-polymer obtained from Evonik
Degussa Corporation were in dissolved at 80 C in toluene under continuous
stirring to give a uniform solution comprising 2% of each component (by
weight;
total weight of solids = 4%). The Vestoplast 708 had a number average
molecular weight of 11,500 g/mol, a viscosity of 8,000 cP measured at 170 C,
and a melting temperature of 56 C.
[0068] The coating solution was padded on uniformly onto ten 12" x 12"
woven tape layers. The percent add-on on the fibrous layers was approximately
2% dry. The layers were then dried overnight at room temperature. After drying
the woven fibrous layers they were stacked and consolidated by being placed
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between heated aluminum platens at a temperature of 300 F and a pressure of
300 psi for 10 minutes of heating time. The sample was then subsequently
cooled to 100 F. The cooling rate was approximately 20 F/minute.
Results
Peel Strength when Peel Strength when
Example consolidated at a temp of 320 F consolidated at a temp of 300 F
and a pressure of 2,500 psi and a pressure of 300 psi
Control Ex. 1 2.0 lbf/in No consolidation
Working Ex.1 -- 5.5lbf/in
Working Ex. 2 -- 3.2 lbf/in
Working Ex. 3 -- 5.7lbf/in
Table I - Peel Strength of Examples
[0069] A peel strength of "No consolidation" indicates that fibrous layers
did not hold together well enough to perform the peel strength test.
[0070] Working Examples 1 and 3 had at least 85% greater peel strength
processed at lower conditions (300 F, 300 psi) than Control Example 1
processed at much higher conditions (320 F, 2,500 psi). Control Example 1
would not even consolidate at the lower temperatures and pressures.
Additionally, stiffness was measured for all samples and the Working Examples
were at least as stiff as or stiffer than the Control Examples processed at
the
same conditions.
[0071] Having a skin layer containing a first polymer and a second
polymer enables the working examples to be consolidated at much lower
temperatures, pressures and/or dwell time saving time and energy over prior
art
materials.
[0072] All references, including publications, patent applications, and
patents, cited herein are hereby incorporated by reference to the same extent
as
if each reference were individually and specifically indicated to be
incorporated
by reference and were set forth in its entirety herein.
[0073] The use of the terms "a" and "an" and "the" and similar referents
(especially in the context of the following claims) are to be construed to
cover
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both the singular and the plural, unless otherwise indicated herein or clearly
contradicted by context. The terms "comprising," "having," "including," and
"containing" are to be construed as open-ended terms (i.e., meaning
"including,
but not limited to,") unless otherwise noted. Recitation of ranges of values
herein are merely intended to serve as a shorthand method of referring
individually to each separate value falling within the range, unless otherwise
indicated herein, and each separate value is incorporated into the
specification
as if it were individually recited herein. All methods described herein can be
performed in any suitable order unless otherwise indicated herein or otherwise
clearly contradicted by context. The use of any and all examples, or exemplary
language (e.g., "such as") provided herein, is intended merely to better
illuminate
the invention and does not pose a limitation on the scope of the invention
unless
otherwise claimed. No language in the specification should be construed as
indicating any non-claimed element as essential to the practice of the
invention.
[0074] Preferred embodiments of this invention are described herein,
including the best mode known to the inventors for carrying out the invention.
Variations of those preferred embodiments may become apparent to those of
ordinary skill in the art upon reading the foregoing description. The
inventors
expect skilled artisans to employ such variations as appropriate, and the
inventors intend for the invention to be practiced otherwise than as
specifically
described herein. Accordingly, this invention includes all modifications and
equivalents of the subject matter recited in the claims appended hereto as
permitted by applicable law. Moreover, any combination of the above-described
elements in all possible variations thereof is encompassed by the invention
unless otherwise indicated herein or otherwise clearly contradicted by
context.
