U.S. patent application number 11/491657 was filed with the patent office on 2008-01-24 for hybrid fiber tows containning both nano-fillers and continuous fibers, hybrid composites, and their production processes.
Invention is credited to Jiusheng Guo, Bor Z. Jang, Lulu Song, Aruna Zhamu.
Application Number | 20080020193 11/491657 |
Document ID | / |
Family ID | 38971789 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080020193 |
Kind Code |
A1 |
Jang; Bor Z. ; et
al. |
January 24, 2008 |
Hybrid fiber tows containning both nano-fillers and continuous
fibers, hybrid composites, and their production processes
Abstract
Disclosed is a hybrid fiber tow that comprises multiple
continuous filaments and nanoscale fillers embedded in the
interstitial spaces between continuous filaments. Nanoscale fillers
may be selected from a nanoscale graphene plate, non-graphite
platelet, carbon nano-tube, nano-rod, carbon nano-fiber, non-carbon
nano-fiber, or a combination thereof. Also disclosed are a hybrid
fiber tow impregnated with a matrix material and a composite
structure fabricated from a hybrid fiber tow. The composite
exhibits improved physical properties (e.g., thermal conductivity)
in a direction transverse to the continuous fiber axis. A
roll-to-roll process for producing a continuous fiber tow or
matrix-impregnated fiber tow and an automated process for producing
composite structures containing both continuous filaments and
nanoscale fillers are also provided.
Inventors: |
Jang; Bor Z.; (Centerville,
OH) ; Zhamu; Aruna; (Centerville, OH) ; Guo;
Jiusheng; (Centerville, OH) ; Song; Lulu;
(Centerville, OH) |
Correspondence
Address: |
Bor Z. Jang
9436 Parkside Drive
Centerville
OH
45458
US
|
Family ID: |
38971789 |
Appl. No.: |
11/491657 |
Filed: |
July 24, 2006 |
Current U.S.
Class: |
428/292.1 |
Current CPC
Class: |
B29B 15/12 20130101;
B29C 70/025 20130101; D02J 1/18 20130101; D04H 3/04 20130101; B29C
70/20 20130101; D06B 1/04 20130101; D04H 3/12 20130101; B29B 15/125
20130101; B29K 2105/162 20130101; Y10T 428/249924 20150401; D04C
1/12 20130101 |
Class at
Publication: |
428/292.1 |
International
Class: |
D04H 13/00 20060101
D04H013/00 |
Goverment Interests
[0001] This invention is based on the research results of a project
supported by the U.S. Department of Energy (DOE) SBIR-STTR Program.
The US Government has certain rights on this invention.
Claims
1. A hybrid fiber tow comprising multiple continuous filaments and
nanoscale fillers embedded in interstitial spaces between said
continuous filaments, wherein said nanoscale fillers comprise a
nanoscale graphene plate, non-graphite platelet, carbon nano-tube,
nano-rod, carbon nano-fiber, non-carbon nano-fiber, or a
combination thereof.
2. The hybrid fiber tow as defined in claim 1, wherein said
nano-fillers comprise a nanoscale graphene plate or a non-graphite
platelet with a width or length smaller than 10 .mu.m.
3. The hybrid fiber tow as defined in claim 1, wherein said
nano-fillers comprise a nanoscale graphene plate or non-graphite
platelet that has a length or width smaller than 500 nm.
4. The hybrid fiber tow as defined in claim 1, further comprising a
matrix-forming material embedded in interstitial spaces or coated
on a surface of said continuous filaments.
5. The hybrid fiber tow as defined in claim 4, wherein said
matrix-forming material comprises a thermoplastic, a thermoset, or
a combination thereof.
6. The hybrid fiber tow as defined in claim 1, wherein said
continuous filaments comprise a polymer fiber, ceramic fiber,
carbon fiber, graphite fiber, glass fiber, or a combination
thereof
7. The hybrid fiber tow as defined in claim 1, wherein said
nano-fillers are preferentially oriented in a direction
substantially non-parallel to a continuous filament axial
direction.
8. A hybrid composite structure comprising a hybrid fiber tow as
defined in claim 1 and a matrix material.
9. The hybrid composite as defined in claim 8, wherein said matrix
material comprises a polymer, glass, carbon, ceramic, metal, or a
combination thereof; said continuous filaments comprise a polymer
fiber, ceramic fiber, carbon fiber, graphite fiber, glass fiber, or
a combination thereof; and said nano-fillers comprise a nanoscale
graphene plate, carbon nano-tube, carbon nano-fiber, or a
combination thereof.
10. A process for producing the hybrid fiber tow of claim 1, said
process comprising a) spreading a continuous fiber tow into
multiple, separated filaments that define interstitial spaces
between said filaments; b) exposing said separated filaments to a
fluid medium or fluidized medium containing said nanoscale fillers
suspended therein under a flow condition for a duration of time
sufficient to cause said nanoscale fillers to be trapped and stay
in said interstitial spaces; and c) moving said separated filaments
with said trapped interstitial nanoscale fillers away from said
medium to produce said hybrid fiber tow.
11. The process of claim 10, wherein said step of exposing
comprises moving said separated filaments through a fluidized bed
comprising a fluidized medium that contains said nanoscale
particles suspended in said medium.
12. The process of claim 11, wherein said fluidized bed is
provisioned with electrostatic charging means to facilitate
attraction of said nanoscale fillers to said filaments.
13. The process of claim 10, wherein said step of exposing
comprises moving said separated filaments through a fluid medium
that contains said nanoscale particles suspended in a liquid or
solution.
14. The process of claim 10, wherein said step of exposing
comprises moving said separated filaments at a desired speed in a
desired direction while directing a stream of a liquid medium
containing said nanoscale fillers to impinge upon said filaments in
such a manner that said fillers are trapped in said interstitial
spaces to form said hybrid fiber tow.
15. The process of claim 10, wherein said step of exposing
comprises moving said separated filaments at a desired speed in a
desired direction while directing a stream of a gaseous medium
carrying said nanoscale fillers to impinge upon said filaments in
such a manner that said fillers are trapped in said interstitial
spaces to form said hybrid fiber tow.
16. The process of claim 10 wherein said fluid medium or fluidized
medium further contains a matrix-forming material and said step of
exposing comprises causing both said nanoscale fillers and said
matrix-forming material to stay in said interstitial spaces to form
a matrix-forming material-impregnated hybrid tow, herein referred
to as a hybrid fiber towpreg.
17. The process of claim 10, further comprising a step of reeling
said continuous fiber tow from a roller or spool prior to the fiber
tow spreading step and a step of winding said hybrid fiber tow on a
roller or drum.
18. The process of claim 16, further comprising a step of reeling
said continuous fiber tow from a roller or spool prior to the fiber
tow spreading step and a step of winding said hybrid fiber towpreg
on a roller or drum.
19. The process of claim 10, further comprising d) reeling said
continuous fiber tow from a roller or spool prior to the fiber tow
spreading step; e) impregnating said hybrid fiber tow obtained in
step (c) with a matrix material to form a matrix-impregnated hybrid
fiber tow; f) subjecting said matrix-impregnated hybrid tow to a
shape-forming operation to form a composite shape; and g)
consolidating said composite shape through heating, curing, and/or
cooling said matrix material to form a hybrid composite
structure.
20. The process of claim 19 wherein said shape-forming operation
comprises a filament winding, fiber placement, prepreg-forming,
pultrusion, freeform fabrication step, or a combination
thereof.
21. The process of claim 10, further comprising d) reeling said
continuous fiber tow from a roller or spool prior to the fiber tow
spreading step; e) subjecting said hybrid fiber tow obtained in
step (c) to a shape-forming operation to form a composite preform;
f) impregnating said preform with a matrix material; and g)
consolidating the matrix-impregnated preform through heating,
curing, and/or cooling said matrix material to form a hybrid
composite structure.
22. The process of claim 21 wherein said shape-forming operation
comprises a step of filament winding, fiber placement, freeform
fabrication, weaving, braiding, stitching, knitting, or a
combination thereof.
23. The process of claim 16, further comprising d) reeling said
continuous fiber tow from a roller or spool prior to the fiber tow
spreading step; e) subjecting said hybrid fiber towpreg to a
shape-forming operation to form a composite shape; and g)
consolidating said composite shape through heating, curing, and/or
cooling said matrix-forming material to form a hybrid composite
structure.
24. The process of claim 23 wherein said shape-forming operation
comprises a step of filament winding, fiber placement,
prepreg-forming, freeform fabrication, weaving, braiding,
stitching, knitting, or a combination thereof.
25. The process of claim 19 wherein said step of consolidating
comprises melting a matrix material, cooling or solidifying a
matrix material, curing a resin, polymerizing or cross-linking a
resin precursor, converting an organic or polymeric material to a
carbonaceous material, or a combination thereof.
26. The process of claim 21 wherein said step of consolidating
comprises melting a matrix material, cooling or solidifying a
matrix material, curing a resin, polymerizing or cross-linking a
resin precursor, converting an organic or polymeric material to a
carbonaceous material, or a combination thereof.
27. The process of claim 23 wherein said step of consolidating
comprises melting a matrix material, cooling or solidifying a
matrix material, curing a resin, polymerizing or cross-linking a
resin precursor, converting an organic or polymeric material to a
carbonaceous material, or a combination thereof.
28. The hybrid composite of claim 8 wherein said nano-fillers are
present at a loading of greater than 5% by weight based on the
total weight of nano-fillers plus the matrix material.
30. The hybrid composite of claim 8 wherein said nano-fillers are
present at a loading of at least 15% by weight based on the total
weight of nano-fillers plus the matrix material.
30. The hybrid composite of claim 8 wherein said nano-fillers have
an elongate axis that is inclined at an angle of at least 45
degrees with respect to a longitudinal axis of said continuous
fibers.
Description
FIELD OF INVENTION
[0002] The present invention is related to a hybrid composite
containing both a nano-filler and a continuous fiber dispersed in a
matrix material. The nano-filler comprises nano-scaled graphene
plates (NGPs), carbon nano-tubes (CNTs), carbon nano-fibers (CNFs),
nano-clay platelets, nano-rods, or any other nanoscale
reinforcement with at least an elongate axis. The matrix material
comprises a polymer, organic, metal, ceramic, glass, carbon
material, or a combination thereof. The nano-filler can be made to
be substantially oriented in a preferred direction (e.g., with an
elongate axis perpendicular to the continuous fiber).
BACKGROUND
[0003] Advanced composites, containing continuous fibers dispersed
in a matrix material, are widely used in aerospace, sports
equipment, infrastructure, automotive, and other transportation
industries, as both primary and secondary load-bearing structures.
These composite materials derive their excellent mechanical
strength, stiffness, electrical conductivity, and thermal
conductivity from the reinforcement fibers. However, using polymer
matrices and carbon or graphite fibers are examples, continuous
carbon fiber reinforced polymer composites exhibit these good
properties only in the directions parallel to the fiber axial
directions. In other words, these composites have excellent
in-plane properties and relatively poor thickness-direction and
shear properties.
[0004] Specifically, the thickness-direction and shear strengths
and moduli of continuous carbon fiber reinforced polymer composite
laminates are relatively poor. Poor interlaminar shear strengths in
turn lead to poor delamination resistance. Incorporation of
reinforcements that are oriented in a direction perpendicular to
the continuous fiber axis can significantly improve these
mechanical properties.
[0005] In addition, the longitudinal conductivities (both thermal
and electrical) of a carbon fiber are orders of magnitude greater
than its corresponding transverse conductivities. Hence, the
transverse conductivities of a composite laminate are also much
lower than the longitudinal properties. The transverse
conductivities of a continuous fiber composite can be significantly
improved by incorporating a reinforcement phase perpendicular to
the continuous fiber axis. However, the addition of transverse
reinforcements such as short (chopped) fibers is known to create
processing difficulties. Even with nanoscale fillers like carbon
nano-tubes (CNTs) and carbon nano-fibers (CNFs), a small amount of
these nano-fillers could dramatically alter the flow properties
(e.g., increased viscosity) of a matrix resin. The resulting matrix
material, a nano-filler/resin mixture, is typically so viscous that
it becomes extremely difficult to disperse continuous fibers in
this matrix.
[0006] Further specifically, on the one hand, the loading of
conductive nano-fillers (e.g., >5 wt. % of CNTs or CNFs)
increases the viscosity of the matrix resin to a level that is not
conducive to filament winding and other automated composite
manufacturing techniques. On the other hand, a low percentage
(<5% by wt.) of nano-fillers normally does not provide adequate
through-thickness thermal or electrical conductivity to a composite
structure for certain engineering applications. A need exists to
develop an approach that can resolve these conflicting issues; i.e.
a process capable of combining continuous fibers with a matrix that
features an adequate proportion of nano-fillers dispersed in a
resin or other material with these fillers preferentially oriented
along a desired direction for improved transverse or shear
properties.
[0007] Generally, the advantages of nanoscale reinforcements in
polymer matrices are fourfold: (1) when nanoscale fillers are
finely dispersed in the matrix, the tremendous surface area
developed could contribute to both polymer chain confinement and
load transfer effects, leading to higher glass transition
temperature, stiffness, and strength; (2) nanoscale fillers provide
an extraordinarily zigzagging, tortuous path that leads to enhanced
resistance to micro-cracking; (3) nanoscale fillers can also
enhance the electrical and thermal conductivities; and (4)
carbon-based nanoscale fillers have excellent thermal protection
properties and, when incorporated in a matrix material, could
eliminate the need for a thermal protective layer--for instance, in
missile and rocket applications.
[0008] Fabrication of carbon nanotubes (CNTs) is expensive,
particularly for the purifying process required to make them useful
in applications. Instead of trying to discover lower cost processes
for CNTs, we have been seeking to develop an alternative nanoscale
carbon material with comparable properties that can be produced
cost-effectively and in larger quantities. This development work
has led to the discovery of processes for producing a new class of
nano material herein referred to as nanoscale graphene plates (NGP)
[Jang, et al., "Nano-scaled Graphene Plates," U.S. Pat. No.
7,071,258 (Jul. 4, 2006) and "Process for Producing Nano-scaled
Graphene Plates," U.S. patent pending, Ser. No. 10/858,814 (Jun. 3,
2004)]. An NGP is a nanoscale platelet composed of one or more
layers of graphene plane. In a graphene plane, carbon atoms occupy
a 2-D hexagonal lattice. These carbon atoms are bonded together
through strong covalent bonds lying on this plane. In the c-axis
direction, several graphene planes may be weakly bonded together
primarily through van der Waals forces. An NGP may be viewed as a
flattened sheet of a CNT. Although NGP and CNT are geometrically
different in architecture, preliminary calculations have indicated
very similar mechanical properties (in-plane stiffness and
strength) and thermal and electrical conductivities (Table 1).
TABLE-US-00001 TABLE 1 Estimated physical constants of CNTs and
NGPs. Property Single-Walled CNTs NGP Specific Gravity 0.8
g/cm.sup.3 1.8 2.2 g/cm.sup.3 Elastic modulus ~1 TPa ~1 TPa
(in-plane) Strength 50 500 GPa ~100 400 GPa Resistivity 5 50
.mu..OMEGA. cm 50 .mu..OMEGA. cm (in plane) Thermal Up to 1,500 W
m.sup.-1 K.sup.-1 3,000 W m.sup.-1 K.sup.-1 (in-plane) Conductivity
(estimated) 6 30 W m.sup.-1 K.sup.-1 (c-axis) Magnetic 22 .times.
10.sup.6 EMU/g (.perp. to 22 .times. 10.sup.6 EMU/g (.perp. to the
Susceptibility the plane) plane) 0.5 .times. 10.sup.6 EMU/g (|| to
0.5 .times. 10.sup.6 EMU/g (|| to the the plane) plane) Thermal
Negligible (theoretical) -1 .times. 10.sup.-6 K.sup.-1 (in-plane)
expansion 29 .times. 10.sup.-6 K.sup.-1 (c-axis) Thermal stability
>700.degree. C. (in air); 450 650.degree. C. (in air)
2800.degree. C. (in vacuum) Specific surface Typically 100 500
m.sup.2/g Up to 1,500 m.sup.2/g area
[0009] NGP-reinforced composites are also expected to exhibit
similar properties compared to CNT-reinforced composites. When the
NGP-resin mixture, a nanocomposite, is incorporated as a matrix for
forming a continuous fiber reinforced composite, the resulting
hybrid composite (containing both the NGP and the continuous fiber
as reinforcement phases) is expected to have improved mechanical
and physical properties compared to the conventional fiber
composite (containing only continuous fiber, no NGP). A need exists
for incorporating both NGPs and continuous fibers in a matrix
material to make a hybrid composite. Further, as indicated earlier,
the loading of nano-fillers (e.g., CNTs and CNFs) increases the
viscosity of the matrix resin to a level that is not conducive to
subsequent filament winding and other automated composite
manufacturing techniques. A need exists to develop a process that
is capable of combining both continuous fibers and CNTs, CNFs,
other nano-rods, or nano-platelets with a matrix material to make a
hybrid composite. Preferably, such a process can be automated.
SUMMARY OF THE INVENTION
[0010] The present invention provides a process that is capable of
producing a hybrid composite that contains both nano-fillers (e.g.,
NGPs, CNTs, CNFs, or a combination thereof) and continuous fibers
as reinforcement phases dispersed in a matrix material. The
nano-fillers can be oriented in a direction that is non-parallel to
the longitudinal axis direction of the continuous fiber (e.g.,
preferably perpendicular to the continuous fiber axis). The process
begins with spreading a continuous fiber tow separate continuous
filaments from each other and then incorporating nano-fillers in a
continuous fiber tow with individual nano-fillers embedded in the
interstitial spaces between continuous filaments. The resulting
hybrid fiber tow is then impregnated with a resin to produce a
resin-pre-impregnated hybrid tow or hybrid towpreg. This wet hybrid
towpreg can then go through a filament winding, fiber placement,
prepregging, or pultrusion process for making a composite
structure.
[0011] Alternatively, the hybrid fiber tow can be woven, braided,
knitted, or stitched into a textile-structured preform, which is
then impregnated with a resin.
[0012] The process may involve incorporating continuous fibers,
nano-fillers, and a matrix-making material in a powder form
concurrently to form a matrix-forming powder-impregnated hybrid
fiber tow, a dry hybrid towpreg. The dry hybrid towpreg can then go
through a filament winding, fiber placement, weaving, braiding,
knitting, or stitching to form a structured preform, which is then
converted to become a hybrid composite structure by heating and
consolidating the matrix-forming material to become the solid
matrix material.
[0013] The present invention also provides hybrid fiber tows,
hybrid fiber towpregs, and resulting hybrid fiber composites that
can be composed of a wide range of fibers, fillers, and matrix
materials.
[0014] The versatility of the invented process opens up a window of
many application opportunities for hybrid composites containing
continuous fibers and nano-fillers such as NGPs, CNTs, and
CNFs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 (a) Schematic of a hybrid fiber tow containing
nano-fillers 16,18 embedded in the interstitial spaces 14 between
continuous filaments 12 and (b) schematic of a hybrid fiber towpreg
containing both nano-fillers and a matrix-forming material (e.g.,
thermoplastic powder particles 20) embedded in the interstitial
spaces between continuous filaments.
[0016] FIG. 2 Schematic of a roll-to-roll process for continuous
production of hybrid fiber tows or towpregs.
[0017] FIG. 3 Schematic of a process for continuously producing a
towpreg and fabricating a composite structure.
[0018] FIG. 4 Schematic of another version of the roll-to-roll
process for the continuous production of hybrid fiber tows or
towpregs.
[0019] FIG. 5 Transverse thermal conductivity of hybrid composites
containing continuous carbon fibers and NGPs (7 compositions) and
CNTs (2 compositions).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] The conventional approach to fabricating composite materials
containing both continuous fibers and fillers (such as nanoscale
fillers, short fibers, etc.) typically involves mixing the fillers
with a resin first, followed by impregnating the continuous fiber
tows with the resin/filler mixture. It is now well-recognized that
a small amount of nano-fillers like carbon nano-tubes (CNTs) and
carbon nano-fibers (CNFs) could dramatically increase the viscosity
of a matrix resin. The resulting nano-filler/resin mixture is
typically so viscous that it becomes extremely difficult to
disperse continuous fibers in this matrix. Hence, it is also
commonly believed that only a small amount of nano-fillers can be
incorporated in a hybrid composite.
[0021] Furthermore, the prior-art sequence of mixing nano-fillers
with a resin and then impregnating continuous fibers with the
nano-filler/resin mixture tends to produce a hybrid composite with
fillers oriented along the continuous fiber axis. Such an
orientation does not improve thickness-direction properties and
shear properties of a composite laminate with continuous fibers
lying on a laminar plane.
[0022] Contrary to what composite materials experts would or might
expect, we have developed an approach that enables the fabrication
of hybrid composites containing a high proportion of nano-fillers
with a preferential orientation that is substantially perpendicular
to the continuous fiber axis (FIG. 1(a)). Instead of mixing the
nano-fillers with a matrix resin first and then impregnating
continuous fibers with the nano-filler/resin mixture, we took a
novel and innovative approach that involved mixing the nano-fillers
with continuous fibers first to produce a hybrid fiber tow (with
nano-fillers such as NGPs 16 and/or CNTs 18 embedded in the
interstitial spaces 14 between continuous fibers 12) and then
impregnating the hybrid fiber tow with a resin. An important step
was to spread a continuous fiber tow into separated, individual
continuous filaments prior to nano-filler mixing or addition. It
turns out that, with this sequence, the resin can readily penetrate
into the interstitial spaces and wet out both the continuous fibers
and nano-fillers. Any resin that is commonly used in a filament
winding, prepregging, fiber placement, or pultrusion operation can
be used in the invented process. This process surprisingly but
elegantly accomplishes the task of fabricating advanced composites
containing both continuous filaments (fibers) and nano-fillers as
reinforcement phases.
[0023] This process also enables impregnation of continuous
fiber/nano-filler preform shape with a ceramic, glass, or carbon
matrix via a specialized technique like chemical vapor infiltration
to produce corresponding hybrid composites, which otherwise would
be difficult to obtain.
[0024] As an example to illustrate this process, a fluidized-bed
powder impregnation or coating process (FIG. 2) has been
successfully adapted for incorporating a controlled percentage of
nano-fillers (e.g., NGPs, CNFs, and/or CNTs) in a continuous fiber
tow (consisting of multiple filaments) to produce a hybrid fiber
tow containing nano-fillers residing in the interstices between
individual filaments. The process begins with continuously feeding
a fiber tow into a fiber tow spreader to separate individual carbon
filaments from one another, opening up interstitial spaces between
filaments to accommodate nano-fillers. The separated filaments are
fed into a fluidized bed chamber in which nano-fillers are driven
by an air flow to move around like a fluid. When the air-suspended
fillers impinge upon the filaments with desired interstitial space
sizes the nano-fillers are trapped in the interstitial spaces with
an elongate axis of the filler typically oriented in the fluid flow
direction. This direction can be controlled to ensure that a
majority of the fillers are oriented perpendicular to the
longitudinal axis of the continuous fibers. The resulting structure
is a continuous hybrid fiber tow, as schematically shown in FIG.
1(a).
[0025] Electrostatic charges may be imparted to nano-fillers to
facilitate attraction of nano-fillers to the carbon fiber tow. This
is analogous to the conventional towpreg production operation by
which micron-scaled thermoplastic powder particles, serving as a
precursor to the composite matrix, are incorporated into a
continuous fiber tow [e.g., J. D. Muzzy, et al., U.S. Pat. No.
5,094,883, Mar. 10, 1992]. No nanoscale filler was involved in this
earlier process.
[0026] In one embodiment of the present invention, a matrix-forming
material (e.g., thermoplastic powder particles 20 in FIG. 1(b)), in
addition to nano-fillers, is also suspended in the fluidized
medium. The resulting structure, composed of continuous fibers,
nano-fillers, and a matrix-forming material, is hereinafter
referred to as a hybrid towpreg. The hybrid towpreg may then be
subjected to a weaving, winding, or any other textile
structure-forming procedure to produce a composite preform, which
is heated (to melt out the thermoplastic powder) and consolidated
to obtain a composite structure.
[0027] In the presently invented hybridfiber tow approach, the
continuous hybrid fiber tow may be directed to enter a resin bath
for impregnation with a matrix resin in a filament-winding,
prepreg-forming, pultrusion, or fiber placing operation (FIG. 3).
This resin can be a thermosetting, cyclic, or thermoplastic resin.
This is an automated process that is suitable for mass production
of hybrid composites.
[0028] Alternatively, the hybrid fiber tow may be subjected to
weaving, winding, braiding, stitching, knitting, freeform
fabrication, and/or other textile-forming procedures to produce a
dry composite preform, which is then impregnated with a matrix
material to obtain a composite structure. With a polymer matrix,
the preform can be impregnated through resin transfer molding,
reaction injection molding, vacuum-assisted transfer molding,
pressure-assisted liquid resin impregnation, etc. For a metal
matrix, the preform can be impregnated through microwave-assisted
infiltration, liquid metal impregnation, etc. For a glass or
ceramic matrix, the preform can be impregnated through chemical
vapor deposition or chemical vapor infiltration. A
resin-impregnated preform can be subjected to a heat treatment
(pyrolization) that converts a polymer into a carbonaceous
matrix.
[0029] The nanoscale filler that can be used in the presently
invented hybrid fiber tow, towpreg, or composite can be a nanoscale
graphene plate, non-graphite platelet, carbon nano-tube, nano-rod,
carbon nano-fiber, non-carbon nano-fiber, or a combination thereof.
These entities all have one thing in common--they have at least on
elongate axis. For instance, CNTs have one elongate axis (in the
tube axial direction) and platelets have two elongate axes (in the
length and width direction). The resulting hybrid composite can
easily have nano-fillers that are present at a loading of greater
than 5% by weight based on the total weight of nano-fillers plus
the matrix material. The nano-fillers in many cases exceed 15% by
weight. A majority of these nano-fillers have an elongate axis
oriented at an angle of at least 45 degrees with respect to the
continuous fiber axis. If improved transverse thermal or electrical
conductivities are desired, carbon-based nano-fillers are
preferred.
[0030] The NGPs obtained in our facilities typically have a
platelet thickness of 1-100 nm and length and width of 0.1-10
.mu.m. These rigid two-dimensional platelets appear to be conducive
to fitting into inter-filament interstices. The nanoscale graphene
plate or non-graphite platelet that has a length or width smaller
than 500 nm is particularly well-suited to the present application.
The flexibility of both the CNT and the CNF afforded to by their
large length-to-diameter ratios makes these one-dimensional
structures tend to assume curved or coiled shapes and should make
it more difficult to be incorporated in a hybrid composite.
However, surprisingly, the presently invented process is capable of
incorporating CNTs and CNFs into the inter-filament spaces.
[0031] A fluidized-bed powder impregnation apparatus, schematically
shown in FIG. 2, was designed and constructed for incorporating a
controlled percentage of nano-fillers in a continuous fiber tow to
produce a hybrid fiber tow. The apparatus is mainly composed of a
feeder roller, a fiber tow spreader, a fluidized bed chamber, and
an optional fiber tow consolidator. The fiber tow (or strands of
fibers or filaments) is reeled from a fiber spool or feeder roller
and directed to go through a tow spreader in which individual
filaments are separated from each other. The separated fibers are
fed into the fluidized bed chamber in which the nano-fillers,
"fluidized" by an air flow, are introduced to impinge upon the
fibers. The nano-fillers may be electrostatically charged and the
fiber tow grounded to promote nano-filler impregnation of the fiber
tow, with nano-fillers residing in interstices between fibers while
being re-merged or compacted.
[0032] The fluidized bed powder coating apparatus are well-known in
the art. For instance, these apparatus were successfully used to
prepare a towpreg that is composed of reinforcing filaments coated
with matrix-forming resin powder as a precursor to a plastic matrix
composite [J. Lamanche, et al., U.S. Pat. No. 3,703,396 (Nov. 21,
1972)]. A key component in the system is a tow spreader. Spreading
of the filaments can be achieved by vibrating the graphite fiber
tow in air pulsating at a frequency and intensity sufficient to
couple the energy of the pulsating medium to the graphite tow
[e.g., S. Iyer, et al., U.S. Pat. No. 5,042,122 (Aug. 27, 1991)].
Spreading may also be facilitated or promoted by using air currents
or electrostatic charges of the same polarity.
[0033] An optional filament re-merger or compactor may be used to
facilitate the merging of separated filaments, along with the
embedded nano-fillers, into a more compact fiber tow. This filament
re-merging step can occur before, during, and after the resin
impregnation step. Resin impregnation can be part of a
filament-winding, prepreg-forming, fiber-placing, or pultrusion
process.
[0034] The continuous filament can be a polymer fiber, ceramic
fiber, carbon fiber, graphite fiber, glass fiber, or a combination
thereof. In the hybrid fiber tow, the nano-fillers are preferably
oriented in a direction substantially non-parallel to the
continuous filament axial direction and further preferably
perpendicular to the filament axis.
[0035] In summary, the process for producing a hybrid fiber tow
comprises (a) spreading a continuous fiber tow into multiple,
separated filaments that define interstitial spaces between the
filaments; (b) exposing the separated filaments to a fluid medium
or fluidized medium containing nanoscale fillers under a flow
condition for a duration of time sufficient to cause the nanoscale
fillers to stay in the interstitial spaces; and (c) moving the
separated filaments with the interstitial nanoscale fillers away
from the medium to produce the hybrid fiber tow. The step of
exposing can comprise moving the separated filaments through a
fluidized bed comprising a fluidized medium that contains the
nanoscale particles suspended in the medium, as illustrated in FIG.
2. The fluidized bed may be provisioned with electrostatic charging
means to facilitate attraction of the nanoscale fillers to the
filaments.
[0036] Alternatively, the step of exposing comprises moving the
separated filaments through a fluid medium that contains the
nanoscale particles suspended in a liquid or solution. In other
words, the fluidized-bed powder coater device shown in FIG. 2 is
now replaced by a tank of liquid with nano-fillers suspended in the
liquid which is driven (e.g., pumped) to flow around in the tank
with a desired flow pattern that enables impingement of
nano-fillers with the continuous filaments at a desired
direction.
[0037] Further alternatively, as schematically shown in FIG. 4, the
step of exposing can comprise moving the separated filaments at a
desired speed in a desired direction while directing a stream of a
liquid medium (or gaseous medium such as air) containing the
nanoscale fillers to impinge upon the filaments in such a manner
that the fillers are trapped in the interstitial spaces to form a
hybrid fiber tow. Again, the process begins with reeling a
continuous fiber tow 54 from a fiber spool 32, feeding the tow into
a tow spreader 56 to obtain separated filaments 58, which are
impinged upon by a fluid (liquid or air) suspending nano-fillers 60
therein. The nano-fillers are trapped between filaments, allowing
the liquid or air to filter through the gaps between filaments. The
hybrid tow 62 can be optionally compacted by a compactor or
consolidator 64 to become consolidated hybrid fiber tow which is
then collected on a take-up roller 46. Again, such a roll-to-roll
process is suitable for mass production of composites.
[0038] The fluid medium (e.g., in FIG. 4) or fluidized medium
(e.g., in FIG. 2 or 3) can further contain a matrix-forming
material. The step of exposing then comprises causing both the
nanoscale fillers and the matrix-forming material to stay in the
interstitial spaces to form a matrix-forming material-impregnated
hybrid tow, referred to as a hybrid fiber towpreg. The
matrix-forming material may also be coated onto the surface of
continuous filaments.
[0039] In another embodiment of the present invention, in addition
to the aforementioned steps (a), (b), and (c), the process further
comprises (d) reeling the continuous fiber tow from a roller or
spool prior to the fiber tow spreading step; (e) impregnating the
hybrid fiber tow with a matrix material to form a
matrix-impregnated hybrid fiber tow; (f) subjecting the
matrix-impregnated hybrid tow to a shape-forming operation to form
a composite shape; and (g) consolidating the composite shape
through heating, curing, and/or cooling the matrix material to form
a hybrid composite structure. The shape-forming operation can
comprise a filament winding, fiber placement, prepreg-forming,
pultrusion, freeform fabrication step, or a combination thereof.
Freeform fabrication involved computerized deposition of a material
point-by-point and layer-by-layer. The process is also commonly
referred to as rapid prototyping.
[0040] In yet another embodiment of the present invention, the
process comprises, in addition to aforementioned steps (a), (b),
and (c), the following steps: (d) reeling the continuous fiber tow
from a roller or spool prior to the fiber tow spreading step; (e)
subjecting the hybrid fiber tow to a shape-forming operation to
form a composite preform; (f) impregnating the preform with a
matrix material; and (g) consolidating the matrix-impregnated
preform through heating, curing, and/or cooling the matrix material
to form a hybrid composite structure. The shape-forming operation
can comprise a step of filament winding, fiber placement, freeform
fabrication, weaving, braiding, stitching, knitting, or a
combination thereof.
[0041] In still another embodiment of the present invention, the
process comprises, in addition to the aforementioned steps (a),
(b), and (c), the following steps: (d) reeling the continuous fiber
tow from a roller or spool prior to the fiber tow spreading step;
(e) subjecting the hybrid fiber towpreg to a shape-forming
operation to form a composite shape; and (f) consolidating the
composite shape through heating, curing, and/or cooling the
matrix-forming material to form a hybrid composite structure. The
shape-forming operation can include a step of filament winding,
fiber placement, prepreg-forming, freeform fabrication, weaving,
braiding, stitching, knitting, or a combination thereof.
[0042] In all of the aforementioned versions of the invented
process, the step of consolidating can comprise melting a matrix
material, cooling or solidifying a matrix material, curing a resin,
polymerizing or cross-linking a resin precursor, converting an
organic or polymeric material to a carbonaceous material, chemical
vapor infiltration, or a combination thereof.
[0043] As examples to illustrate the utility value of the developed
hybrid composites, we obtained the thermal conductivity values of a
series of hybrid composites containing continuous graphite fibers
(approximately 60% by volume of the total composite) and NGP/epoxy
matrix materials (with several NGP weight fractions based on the
total NGP/epoxy weight) or CNT/epoxy matrices. As shown in FIG. 5,
the transverse thermal conductivity values of hybrid composites
(with NGPs substantially perpendicular to the continuous graphite
fiber direction) can be improved from approximately 9.1
Wm.sup.-1K.sup.-1 for a continuous graphite fiber/epoxy composite
(0% NGP) to 193 Wm.sup.-1K.sup.-1 for a hybrid composite (15% NGP).
The longitudinal thermal conductivity values, parallel to the
continuous graphite fiber direction, for all these composites are
in the range of 125-180 Wm.sup.-1K.sup.-1, relatively independent
of the NGP content in the matrix. Addition of CNTs has also
significantly improved the transverse thermal conductivity of the
hybrid composite, albeit to a smaller extent. The high transverse
as well as longitudinal thermal conductivity for these composites
is a highly significant result since thermally conductive
composites can be used as thermal management materials for
microelectronic devices and rocket motor cases, just to cite two of
many examples.
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