U.S. patent application number 10/149216 was filed with the patent office on 2006-03-02 for oriented nanofibers embedded in polymer matrix.
Invention is credited to EnriqueV Barrera, Luis Paulo Felipe Chibante, Karen Lozano, FernandoJ Rodriguez-Macias, David Harris Stewart.
Application Number | 20060047052 10/149216 |
Document ID | / |
Family ID | 22614952 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060047052 |
Kind Code |
A1 |
Barrera; EnriqueV ; et
al. |
March 2, 2006 |
Oriented nanofibers embedded in polymer matrix
Abstract
A method of forming a composite of embedded nanofibers in a
polymer matrix is disclosed. The method includes incorporating
nanofibers in a plastic matrix forming agglomerates, and uniformly
distributing the nanofibers by exposing the agglomerates to
hydrodynamic stresses. The hydrodynamic said stresses force the
agglomerates to break apart. In combination or additionally
elongational flow is used to achieve small diameters and alignment.
A nanofiber reinforced polymer composite system is disclosed. The
system includes a plurality of nanofibers that are embedded in
polymer matrices in micron size fibers. A method for producing
nanotube continuous fibers is disclosed. Nanofibers are fibrils
with diameters 100 nm, multiwall nanotubes, single wall nanotubes
and their various functionalized and derivatized forms. The method
includes mixing a nanofiber in a polymer; and inducing an
orientation of the nanofibers that enables the nanofibers to be
used to enhance mechanical, thermal and electrical properties.
Orientation is induced by high shear mixing and elongational flow,
singly or in combination. The polymer may be removed from said
nanofibers, leaving micron size fibers of aligned nanofibers.
Inventors: |
Barrera; EnriqueV; (Houston,
TX) ; Rodriguez-Macias; FernandoJ; (Houston, TX)
; Lozano; Karen; (McAllen, TX) ; Chibante; Luis
Paulo Felipe; (Houston, TX) ; Stewart; David
Harris; (Houston, TX) |
Correspondence
Address: |
Robert C Shaddox;Winstead Sechrest & Minick
2400 Bank One Center
910 Travis Street
Houston
TX
77002
US
|
Family ID: |
22614952 |
Appl. No.: |
10/149216 |
Filed: |
December 7, 2000 |
PCT Filed: |
December 7, 2000 |
PCT NO: |
PCT/US00/33291 |
371 Date: |
November 15, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60169273 |
Dec 7, 1999 |
|
|
|
Current U.S.
Class: |
524/495 |
Current CPC
Class: |
Y10T 428/31855 20150401;
Y10T 428/13 20150115; Y10T 428/2929 20150115; Y10T 442/30 20150401;
C08K 7/06 20130101; B29C 48/37 20190201; Y10T 428/249934 20150401;
C08K 3/04 20130101; B33Y 70/00 20141201; Y10T 428/249948 20150401;
D01F 6/04 20130101; C08K 7/24 20130101; D01F 6/06 20130101; B82Y
30/00 20130101; C08K 2201/011 20130101; C08K 7/04 20130101; B29C
48/92 20190201; Y10T 428/249986 20150401; D01F 1/10 20130101 |
Class at
Publication: |
524/495 |
International
Class: |
C08K 3/04 20060101
C08K003/04 |
Goverment Interests
[0001] This invention was made with Government support under NSF
Grant No. DMR-9357505 awarded by the National Science Foundation
and NASA Grants No. NCC9-77 and STTR Grant NAS 9 99129, awarded by
the National Aeronautics and Space Administration. The Government
may have certain rights in the invention.
Claims
1. A method for forming a composite of embedded (0-100%) nanofibers
in a polymer matrix, comprising: incorporating a plurality of
nanofibers in a plastic matrix, said incorporation forming a
plurality of agglomerates; and uniformly distributing said
nanofibers by exposing the agglomerates to hydrodynamic stresses,
said stresses forcing the agglomerates to break apart.
2. The method of claim 1, further comprising: processing the
composite material in a high shear condition using a capillary
rheometer or extruder or other fiber spinning processes.
3. A nanofiber reinforced polymer composite system comprising: a
plurality of nanofibers, said nanofibers embedded in polymer
matrices in micron size fibers.
4. A method for producing nanotube continuous fibers, comprising:
mixing at least one nanofiber selected from the group consisting of
carbon fibrils, multi-walled nanotubes, and single wall nanotubes
in a polymer; where these nanofibers may be functionalized or
derivatized. inducing an orientation of the nanofibers that enables
said nanofibers to be used to enhance mechanical, thermal and
electrical properties.
5. The method of claim 4, further comprising: removing said polymer
or binder from said nanofibers, said removal leaving micron size
fibers of nanofibers.
14. The method of claim 4 wherein said polymer is selected from the
group consisting of PP, ABS, PE and UHMW PE.
34. A composite comprising a network of isotropic dispersions of
aligned nanofibers in a polymer matrix for ESD applications.
35. A composite comprising a network of isotropic dispersions of
aligned nanofibers in a polymer matrix for EMURFI applications.
36. A highly conducting composite comprising isotropic dispersions
of aligned nanofibers in a polymer matrix for conducting electrical
wire applications.
37. A composite comprising a isotropic dispersions of aligned
nanofibers in a polymer matrix filament for structural
applications.
38. A composite comprising isotropic dispersions of aligned
nanofibers in a polymer matrix for thermal applications.
39. A composite comprising aligned nanofiber reinforced polymer
formed by a FDM processing.
40. FDM components formed from aligned nanofibers in a polymer
matrix.
41. A composite comprising aligned nanofiber reinforced polymer
formed by a FDM processing for ESD applications.
42. A composite comprising aligned nanofiber reinforced polymer
formed by a FDM processing for EMLRFI applications.
43. A composite comprising aligned nanofiber reinforced polymer
formed by a FDM processing for thermal applications.
44. A composite comprising aligned nanofiber reinforced polymer
formed by a FDM processing for mechanical applications.
45. A composite comprising aligned nanofiber reinforced polymer
wherein said nanotubes are integrated into a polymer matrix.
46. The composite of claim 45 wherein said nanotube integration is
by tip attachment and/or side wall functionalization, coincident
polymerization, or high shear alignment.
47. The method of claim 2 wherein said processing the composite
material in a high shear condition using a capillary rheometer or
extruder or other fiber spinning processes comprises wet spinning,
dry spinning, melt spinning or gel spinning.
48. A tailored multifunctional reinforced polymer composite system
comprising: a plurality of nanofibers, said nanofibers embedded in
polymer matrices in micron size fibers.
49. A multifunctional reinforced polymer composite system
comprising a plurality of nanofibers, said nanofibers embedded in
polymer matrices in micron size fibers, suitable for further
processing to provide composite forms including weaves, mats,
plies, filament wound tubing and vessels.
50. The method of claim 4 further comprising the steps of: mixing
one or more nanofibers selected from said group in a polymer to
disperse said nanofibers to the desired range of dispersion and to
provide a mix; processing said mix in a high shear condition;
inducing an orientation of the nanofibers while extruding at least
one continuous filament selected from the group consisting of
fibers, films, and tapes.
51. The method of claim 50 further comprising subjecting said at
least one continuous filament to elongational flow.
52. The method of claim 50 wherein said mixing and nanofiber
dispersion are provided by a Banbury-type mixing.
53. The method of claim 50 wherein said mixing and the extruding
are accomplished in a multiple zone compounding extruder where the
mixing residence is held for sufficient time followed by extrusion
of a dispersed nanofiber system.
54. The method of claim 51 wherein said mixing and the extruding
are accomplished in a multiple zone compounding extruder where the
mixing residence is held for sufficient time followed by extrusion
of a dispersed nanofiber system.
55. The method of claim 4 further comprising the step of purifying
as-received single wall nanotubes.
56. The method of claim 50 further comprising the step of purifying
as-received single wall nanotubes.
57. The method of claim 4 further comprising the steps of selecting
a polymer in powder form; drying the polymer powder; mixing said
powder with said nanofibers in a solvent to form a slurry; drying
said slurry to remove all said solvent to form chunks of
agglomerated powder with highly dispersed nanofibers.
58. The method of claim 50 further comprising the steps of
selecting a polymer in powder form; drying the polymer powder;
mixing said powder with said nanofibers in a solvent to form a
slurry; drying said slurry to remove all said solvent to form
chunks of agglomerated powder with highly dispersed nanofibers.
59. The method of claim 57 wherein said solvent is toluene.
60. The method of claim 58 wherein said solvent is toluene.
61. The method of claim 57 wherein said solvent is dimethyl
formamide (DMF).
62. The method of claim 58 wherein said solvent is dimethyl
formamide (DMF).
63. Aligned nanofibers packaged in a polymer matrix for subsequent
handling and processing formed by: mixing at least one nanofiber
selected from said group in a polymer to disperse said nanofibers
to the desired range of dispersion; processing said mix in a high
shear condition; inducing an orientation of the nanofibers while
extruding at least one continuous filament selected from the group
consisting of fibers, films, and tapes.
64. The packaged aligned nanofibers of claim 65 wherein said
formation process further comprises subjecting said extruded
filament or filaments to elongational flow.
65. A woven composite comprising the packaged nanofibers of claim
63 or 64.
66. A laid up composite comprising the packaged nanofibers of claim
63 or 64.
67. A bundled composite comprising the packaged nanofibers of claim
63 or 64.
68. A composite comprising rows of the packaged nanofibers of claim
63 or 64.
69. A composite comprising bundles of the packaged nanofibers of
claim 63 or 64.
70. A yarn comprising the packaged nanofibers of claims 63 or
64.
71. A thread comprising the packaged nanofibers of claim 63 or
64.
72. The composite of claims 63 through 71 wherein said polymer
binder has been removed leaving micron size fibers of only
nanofibers.
73. the composite of claims 63 through 72 wherein said polymer is
selected from the group of Acetal, PP, ABS, ASA, PE, PEK, PEEK, PET
and UHMW PE.
74. The composite of claims 63 through 72 wherein said matrix is
selected from the group of epoxies and resins.
75. The method of claim 4 or 5 further comprising the step of fully
integrating said composites by one or more of the processes of
integration, dispersion and alignment, derivatization,
functionalization, and polymerization so that said nanotubes are
part of said matrix.
77. A fully integrated nanofiber composite comprising the composite
of claims 63 through 74 further subjected to one or more of the
processes of integration, dispersion and alignment, derivatization,
functionalization, and polymerization to integrate said nanofibers
into part of said matrix.
78. The composite of claim 77 comprising gas permeable polymer for
gas sensor applications.
79. The composite of claim 77 for electronic, wiring or
interconnecting applications.
80. The composite of claim 79 comprising 10% byweight of SWNT.
81. The fully integrated nanofiber composite of claim 77 further
subjected to a toughening process to form a fully integrated
toughened nanotube composite surpassing the limits of the rule of
mixtures.
82. The composite of claims 81 wherein said matrix material is PP
or nylon.
83. A shielding material extending to hypervelocity impact
applications formed from the composite of claim 82.
84. The method of claims 4, 5 or 14 wherein said orientation is
induced by fused deposition modeling processing.
85. The method of claims 50-62 wherein said orientation is induced
by fused deposition modeling processing.
86. Aligned nanofibers packaged in a polymer matrix for subsequent
handling and processing formed by: mixing one or more nanofibers
selected from the group consisting of carbon fibrils, multi-walled
nanotubes, and single wall nanotubes in a polymer to disperse said
nanofibers to the desired range of dispersion and to provide a mix;
processing said mix in a high shear condition; and inducing an
orientation of the nanofibers while extruding at least one
continuous filament selected from the group consisting of fibers,
films, and tapes.
87. The packaged nanofibers of claim 86 wherein said process
further comprises subjecting said at least one continuous filament
to elongational flow.
88. A woven composite comprising the packaged nanofibers of claim
86.
89. A laid up composite comprising the packaged nanofibers of
claims 86.
90. A bundled composite comprising the packaged nanofibers of claim
86.
91. A composite comprising rows of the packaged nanofibers of claim
86.
92. A composite comprising bundles of the packaged nanofibers of
claim 86.
93. A yarn comprising the packaged nanofibers of claim 86.
94. A thread comprising the packaged nanofibers of claim 86.
95. The composite of claim 86 wherein said polymer has been removed
leaving micron size fibers of nanofibers.
96. The composite of claim 87 wherein the polymer has been removed
leaving micron size fibers of nanofibers.
97. The composite of claim 88 wherein said polymer has been removed
leaving micron size fibers of nanofibers.
98. The composite of claims 89 wherein said polymer has been
removed leaving micron size fibers of nanofibers.
99. The composite of claim 90 wherein said polymer has been removed
leaving micron size fibers of nanofibers.
100. The composite of claim 91 wherein said polymer has been
removed leaving micron size fibers of nanofibers.
101. The composite of claim 92 wherein said polymer has been
removed leaving micron size fibers of nanofibers.
102. The composite of claim 93 wherein said polymer has been
removed leaving micron size fibers of nanofibers.
103. The composite of claim 94 wherein said polymer has been
removed leaving micron size fibers of nanofibers.
104. The composite of claim 86 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
105. The composite of claim 87 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
106. The composite of claim 88 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
107. The composite of claim 89 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
108. The composite of claim 90 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
109. The composite of claim 91 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
110. The composite of claim 92 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
111. The composite of claim 93 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
112. The composite of claim 94 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
113. The composite of claim 95 wherein said polymer is selected
from the group consisting of Acetal, PP, ABS, ASA, PE, PEK, PEEK,
PET and UHMW PE.
114. The composite of claim 86 wherein said matrix is selected from
the group consisting of epoxies and resins.
115. The composite of claim 87 wherein said matrix is selected from
the group consisting of epoxies and resins.
116. The composite of claim 88 wherein said matrix is selected from
the group consisting of epoxies and resins.
117. The composite of claim 89 wherein said matrix is selected from
the group consisting of epoxies and resins.
118. The composite of claim 90 wherein said matrix is selected from
the group consisting of epoxies and resins.
119. The composite of claim 91 wherein said matrix is selected from
the group consisting of epoxies and resins.
120. The composite of claim 92 wherein said matrix is selected from
the group consisting of epoxies and resins.
121. The composite of claim 93 wherein said matrix is selected from
the group consisting of epoxies and resins.
122. The composite of claim 94 wherein said matrix is selected from
the group consisting of epoxies and resins.
123. The composite of claim 95 wherein said matrix is selected from
the group consisting of epoxies and resins.
124. The method of claim 4 further comprising the step of
integrating said nanofibers by a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
125. An integrated nanofiber composite comprising the composite of
claim 86 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
126. An integrated nanofiber composite comprising the composite of
claim 87 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
127. An integrated nanofiber composite comprising the composite of
claim 88 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
128. An integrated nanofiber composite comprising the composite of
claim 89 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
129. An integrated nanofiber composite comprising the composite of
claim 90 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
130. An integrated nanofiber composite comprising the composite of
claim 91 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
131. An integrated nanofiber composite comprising the composite of
claim 92 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
132. An integrated nanofiber composite comprising the composite of
claim 93 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
133. An integrated nanofiber composite comprising the composite of
claim 94 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
134. An integrated nanofiber composite comprising the composite of
claim 95 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
135. An integrated nanofiber composite comprising the composite of
claim 104 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
136. An integrated nanofiber composite comprising the composite of
claim 114 further subjected to a process selected from the group
consisting of integration, dispersion and alignment,
derivatization, functionalization, polymerization, and combinations
thereof.
137. The method of claim 4 wherein said orientation is induced by
fused deposition modeling processing.
138. The method of claim 5 wherein said orientation is induced by
fused deposition modeling processing.
139. The method of claim 14 wherein said orientation is induced by
fused deposition modeling processing.
140. The method of claim 50 wherein said orientation is induced by
fused deposition modeling processing.
141. The method of claim 51 wherein said orientation is induced by
fused deposition modeling processing.
142. The method of claim 52 wherein said orientation is induced by
fused deposition modeling processing.
143. The method of claim 53 wherein said orientation is induced by
fused deposition modeling processing.
144. The method of claim 54 wherein said orientation is induced by
fused deposition modeling processing.
145. The method of claim 55 wherein said orientation is induced by
fused deposition modeling processing.
146. The method of claim 56 wherein said orientation is induced by
fused deposition modeling processing.
147. The method of claim 57 wherein said orientation is induced by
fused deposition modeling processing.
148. The method of claim 58 wherein said orientation is induced by
fused deposition modeling processing.
149. The method of claim 59 wherein said orientation is induced by
fused deposition modeling processing.
150. The method of claim 60 wherein said orientation is induced by
fused deposition modeling processing.
151. The method of claim 61 wherein said orientation is induced by
fused deposition modeling processing.
152. The method of claim 62 wherein said orientation is induced by
fused deposition modeling processing.
153. The composite of claim 125 comprising a gas permeable polymer
for gas sensor applications.
154. The composite of claim 125 for electronic, wiring or
interconnecting applications.
155. The composite of claim 125 comprising 10 percent by weight of
SWNT.
156. The composite of claim 125 further subjected to a toughening
process to form a toughened nanotube composite surpassing the
limits of the rule of mixtures.
157. The composite of claim 156 wherein said matrix materials
comprise PP or nylon.
158. A shielding material extending to hypervelocity impact
applications formed from the composite of claim 157.
159. The method of claim 1 wherein the concentration of said
nanofibers is in a range of from about 0 to about 100 weight
percent.
160. The method of claim 1 wherein said composite provides a
delivery system for handling said nanofibers.
161. The method of claim 1 wherein said composite provides a
package for handling said nanofibers.
162. The method of claim 1 wherein said polymer comprises a gas
permeable polymer that provides for said composite capable of being
utilized as a gas sensor.
163. The method of claim 1 wherein said nanofibers provide for
enhancing the thermophysical characteristics of said polymer.
164. The method of claim 1 wherein said nanofibers provide for an
increase in the degradation temperature of said polymer.
165. The method of claim 1 wherein said nanofibers are linked to a
polymer.
166. The method of claim 1 wherein said nanofibers are linked
together.
167. The method of claim 1 wherein said nanofibers are optimized
for non-wetted or unbound conditions.
168. The method of claim 1 wherein said composite is further
chemically treated.
169. The method of claim 1 wherein said composite is capable of
being used as a wire or electrical interconnect.
170. The method of claim 1 wherein said composite achieves
conduction via said nanofibers.
171. The method of claim 1 wherein said uniformly distributing
comprises gel-spinning.
172. The method of claim 1 wherein said uniformly distributing
comprises the use of a fused deposition modeling system.
173. The method of claim 1 wherein said process further comprises
quenching.
174. The method of claim 1 wherein said composite is used for the
production of electrostatic discharge materials by integration.
175. The method of claim 1 wherein said nanofibers act as
nucleation sites.
176. The method of claim 1 wherein said nanofibers affect
crystallization of said polymer.
177. The method of claim 1 wherein said nanofibers affect the
molecule morphology of said polymer.
178. The method of claim 1 wherein said process further comprises
thinning to help provide for a translucent composite providing for
increased visibility.
179. The method of claim 1 wherein the toughness of said composite
is lower and the strength and rigidity of said composite are higher
compared to pure ABS.
180. The method of claim 1 wherein a ceramic is used in place of
said polymer.
181. The method of claim 1 wherein connections are present between
said nanofibers and said polymer and further wherein said
connections provide for enhanced mechanical properties.
182. The method of claim 1 wherein said composite provides for a
system having properties similar to Kevlar or ultra-high-density
polyethylene.
183. The method of claim 1 wherein said polymer comprises a gas
permeable polymer and further wherein said gas permeable polymer
provides for altering the electrical conduction of said polymer
when in contact with a gas.
184. The method of claim 1 wherein carbon sheets that mimic
nanotubes are used in place of said nanofibers.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the development of a
nanofiber reinforced polymer composite system with control over the
orientation of the nanofibers dispersed in the system.
[0004] Most synthetic and cellulosic manufactured fibers are
created by "extrusion"--forcing a thick, viscous liquid through a
spinneret to form continuous filaments of semi-solid polymer. In
their initial state, the fiber-forming polymers are solids and
therefore must be first converted into a fluid state for extrusion.
This is usually achieved by melting, if the polymers are
thermoplastic synthetics (i.e., they soften and melt when heated),
or by dissolving them in a suitable solvent if they are
non-thermoplastic cellulosics. If they cannot be dissolved or
melted directly, they must be chemically treated to form soluble or
thermoplastic derivatives. Recent technologies have been developed
for some specialty fibers made of polymers that do not melt,
dissolve, or form appropriate derivatives. For these materials, the
small fluid molecules are mixed and reacted to form the otherwise
intractable polymers during the extrusion process.
[0005] The spinnerets used in the production of most manufactured
fibers have from one to several hundred holes. As the filaments
emerge from the holes in the spinneret, the liquid polymer is
converted first to a rubbery state and then solidified. This
process of extrusion and solidification of endless filaments is
called spinning, not to be confused with the textile operation of
the same name, where short pieces of staple fiber are twisted into
yarn. There are four methods of spinning filaments of manufactured
fibers: wet, dry, melt, and gel spinning.
[0006] Wet spinning is the oldest process. It is used for
fiber-forming substances that have been dissolved in a solvent. The
spinnerets are submerged in a chemical bath and as the filaments
emerge they precipitate from solution and solidify. Because the
solution is extruded directly into the precipitating liquid, this
process for making fibers is called wet spinning. Acrylic, rayon,
aramid, modacrylic and spandex are produced by this process.
[0007] Dry spinning is also used for fiber-forming substances in
solution. However, instead of precipitating the polymer by dilution
or chemical reaction, solidification is achieved by evaporating the
solvent in a stream of air or inert gas. The filaments do not come
in contact with a precipitating liquid, eliminating the need for
drying and easing solvent recovery. This process is used for the
production of acetate, triacetate, acrylic, modacrylic, PBI,
spandex, and vinyon.
[0008] In melt spinning, the fiber-forming substance is melted for
extrusion through the spinneret and then directly solidified by
cooling. Nylon, olefin, polyester, saran and sulfur are produced in
this manner. Melt spun fibers can be extruded from the spinneret in
different cross-sectional shapes (round, trilobal, pentagonal,
octagonal, and others).
[0009] Gel spinning is a special process used to obtain high
strength or other special fiber properties. The polymer is not in a
true liquid state during extrusion. Not completely separated, as
they would be in a true solution, the polymer chains are bound
together at various points in liquid crystal form. This produces
strong inter-chain forces in the resulting filaments that can
significantly increase the tensile strength of the fibers. In
addition, the liquid crystals are aligned along the fiber axis by
the shear forces during extrusion. The filaments emerge with an
unusually high degree of orientation relative to each other,
further enhancing strength. The process can also be described as
dry-wet spinning, since the filaments first pass through air and
then are cooled further in a liquid bath. Some high-strength
polyethylene and aramid fibers are produced by gel spinning.
[0010] While extruded fibers are solidifying, or after they have
hardened, the filaments may be drawn or elongated to impart
strength. Drawing pulls the molecular chains together and orients
them along the fiber axis, creating a considerably stronger
yarn.
SUMMARY OF THE INVENTION
[0011] A method of embedding nanofibers in a polymer matrix so that
high degrees of alignment may be achieved is accomplished by a
nanofiber continuous fiber ("NCF") system. These micron-size fibers
provide for easy handling of nanofibers, provide for control of
their distribution, including high degrees of alignment, for
subsequent manipulation, and for ease of processing and
manufacturing into a range of parts for mechanical, electrical and
thermal applications. NCFs are continuous fibers with dispersed
nanofibers which can be produced in continuous fiber lengths (1000s
of km for example) to be filament wound, woven, laid up, processed
in rows or bundles, used for thread or yarn to produce a range of
products requiring the enhancement from the nanofiber additions.
The polymer matrix is a system that can easily be processed into
various shapes or with other polymer systems or non-polymeric
additions. The NCFs are a system that can deliver aligned nanotubes
for strengthening (including improved impact strength), or
electrical or thermal anisotropic features (properties varying in
different directions). By extension, the NCF system can also be
applied to nanofibers embedded in matrix and then formed into a
tape or film to provide control of distribution and alignment and
to enable a variety of subsequent processing steps.
[0012] The present invention relates to the development of a
nanofiber reinforced polymer composite system with control over the
orientation of the nanofibers dispersed in the system. The system
is a nanofiber continuous fiber system where nanofibers are
embedded in polymer matrices in micron size fibers. Nanofibers are
carbon fibrils with diameters in the 100 nm and less range,
multi-walled nanotubes (MWNTs) and single wall nanotubes (SWNTs),
including ropes and their various derivatives with a range of
functionalizations, which can be, but are not always, exclusively
carbon. Through a polymer/nanofiber mixing, induced orientation of
the nanofibers can be processed into a range of micron size
diameter fibers that enable nanofibers to be used to enhance
mechanical, thermal and electrical properties. It can include a
nanotube system, where NCFs are made up of nanotubes without a
polymer binder. The nanotube continuous fibers would originate from
a nanotube/polymer precursor system where the polymer has been
removed leaving micron size fibers of only nanotubes. The NCFs can
be prepared with isotropic dispersions of nanofibers or with highly
aligned nanofibers that can easily be handled and processed with
conventional composite manufacturing technologies to deliver high
performance structures. This method of producing nanofibers in a
continuous polymer system could have highly dispersed nanofibers or
a range of dispersion conditions to suit special property needs.
Polymer systems are expected to be those, which can undergo high
shearing and elongational flow conditions including, but not
limited to block copolymers, various thermoplastics, liquid crystal
polymers, thermosets, gel processed polymers and elastomers.
Rheological studies identified the key steps to dispersing both
VGCFs and SWNTs in polymers. A number of suitable polymer/nanofiber
systems exist, including but not limited to: Acetal, ABS, ASA, PE,
PEEK, PET, PP, and Epon epoxy. For some applications epoxies and
resins may be desired. Using the methods of the present invention
which control the orientation of the nanofibers, materials can be
prepared with specifically enhanced structural, electrical, and
thermal properties. The NCFs provide a delivery system or package
for handling nanofibers, and specifically SWNTs. The NCFs are a
structure in and of themselves, a material useful for further
processing into other forms, and a method of aligning nanofibers.
The NCFs can be effectively used to process a range of composite
forms including weaves, mats, plies, filament wound tubing and
vessels and for a range of applications including wires and
electrostatic discharge materials.
[0013] The invention extends to further processing of the nanofiber
composites. To achieve the full potential of nanotubes for micro to
macro scale applications such as wiring and interconnects, single
wall nanotubes (SWNTs) can be developed into fully integrated
nanotube composites (FINCs). Full integration of nanotubes requires
their development beyond conventional composites so that the level
of the non nanotube material is designed to integrate well with the
amount of nanotubes so that the nanotubes are part of the matrix
rather than a differing component. This development of
multifunctional materials from nanotubes, produces fully integrated
nanotube composites (FINCs), a nanotube hybrid material system
designed to surpass the limits of rule of mixtures engineering and
composite design, implementing designs to fully mimic nanotubes on
larger scales. This new approach involves integration, dispersion
and alignment, functionalization, and polymerization to achieve
total integration. Some cases achieve conduction through well
designed networks but the goal is conduction through chemistry. The
material systems described in this invention have application as
highly conducting plastic wires and interconnects for
multifunctional device and electronic applications. The basis for
these materials will be nylon, PMMA, and conducting epoxy.
Producing FINCs with gas permeable polymers provides new gas sensor
capabilities. Key applications will be the lightweight
multifunctional interconnects for electronics and wiring.
[0014] The invention further outlines applications for the full
integration of nanotubes. The lengths of SWNTs are such that
toughening can easily be achieved and our calculations indicate
that processing beyond rule of mixtures approaches is necessary to
gain the full potential of the SWNTs. The invention and methods
produce toughened-fully integrated nanotube composites (T-FINCs), a
nanotube hybrid material system designed to surpass the limits of
rule of mixtures engineering and composite design, to implement
designs to fully mimic nanotubes on larger scales for enhanced
mechanical properties. The basis for these materials will be two
standard polymers: polypropylene (PP) and nylon. Shielding systems
in the forms of panels and woven shields are typical applications
for the new materials. Nanotube shielding has a wide range of
applications, as intermediate bumpers and rear wall panels,
extending to hypervelocity impact applications and shielding for
space applications.
[0015] Material systems (in sheet form) are produced that see ten
orders of magnitude drop in electrical resistivity when using VGCFs
and fourteen orders of magnitude drop in electrical resistivity
when using SWNTs. The 10 wt. % SWNT material has been used to
produce the first of our wire systems and has an electrical
resistivity in the 600 ohm-square range. These systems have highly
dispersed nanofibers otherwise the percolation would be even lower
(when a segregated network is used).
[0016] Enhancements have also been seen in improved stiffness (as
high as 350% for VGCF systems), increased strength (50% increase
for the 10 wt. % SWNT material) and with 100% elongation to failure
for the SWNT system with the highest loading of nanotubes of 10 wt.
%.
[0017] Specific fibers are made from ABS with 10 wt. % VGCFs, PE
with 5 wt. % SWNTs, Pe and PP with 1-3 wt. % HiPco, or other SWNTs.
Alignment of VGCFs and SWNTs is observed. The ability to make
composite fibers extends to making T-FINCs in fiber form.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows (a) volume resistivity versus carbon nanofiber
content VGCF (in wt. %) in the PP matrix at room temperature, (b)
surface resistivity versus carbon nanofiber content (in wt. %) in
the PP matrix at room temperature, and(c) I wt. % SWNTs volume
resistivity.
[0019] FIG. 2 is a TEM micrograph of a 20% carbon nanofiber
reinforced PP composite from which isotropic dispersion of the
individual fibers can be observed.
[0020] FIG. 3 are SEM micrographs of a 10% carbon nanofiber
reinforced PP composite in which wetting of the fibers by the PP
matrix can be observed.
[0021] FIG. 4 is a comparison of two micron size fiber composite
diameters.
[0022] FIG. 5 shows viscosity versus shear rate for various
concentrations of VGCFs in a polypropylene matrix demonstrating the
reduction in viscosity with increasing shear rate.
[0023] FIG. 6 shows the change in viscosity with increasing shear
rate for 3, 5 and 10 wt. % VGCF-polyethylene systems.
[0024] FIG. 7 shows electrical resistivity as a function of SWNT
composition.
[0025] FIG. 8 shows surface resistivity of ABS with VGCF and
SWNTs.
[0026] FIG. 9 shows a micrograph of a 10 wt. % SWNT/ABS polymer
fractured at room temperature.
[0027] FIG. 10 shows torque data for the PP12 and PE10 systems.
[0028] FIG. 11 shows the significant, 100.degree. C. higher,
increase in the degradation temperature for PP with just 2 wt. %
SWNTs.
[0029] FIG. 12 shows the increase observed in the modulus of
polymers when SWNTs are added.
[0030] FIG. 13 shows property enhancement due to high extruder
speed and take up speed.
[0031] FIG. 14 shows strength of fibers processed at different
speeds.
[0032] FIG. 15 shows a plot indicating the change in diameter of a
fiber system at slow extruder speeds and subsequent slow take up
speeds.
[0033] FIG. 16 shows extrusion data collected while producing
fibers from a Haake extruder.
[0034] FIGS. 17, 18, and 19 are rheological results for filled and
unfilled ABS, polypropylene, and polyethylene, at temperatures and
flow rates found to be favorable for fiber spinning with the
RH-7.
[0035] FIG. 20 shows an industrial polymer fiber spinning
process.
[0036] FIG. 21 shows a micrograph of PP1000 showing well aligned
SWNTs.
[0037] FIG. 22 shows tensile tests on polypropylene samples.
[0038] FIG. 23 shows single fiber strength.
[0039] FIG. 24 shows products and processes that can be achieved
using NCFs.
[0040] FIG. 25 shows ABS with dispersed and aligned VGCFs (10 wt.
%).
[0041] FIG. 26 shows PE with dispersed and aligned VGCFs (a) 5 wt.
% and (b) 2 wt. %.
[0042] FIG. 27 shows desired nanofiber alignment by inducing
directionality by shearing and or elongational flow into
fibers.
[0043] FIG. 28 shows nanofiber dispersion issues where the initial
preparation of the nanofibers influences the degree of alignment
and dispersion obtained fibers.
[0044] FIG. 29 shows tensile test results following correction for
the cross-plying
[0045] FIG. 30 shows a micrograph of the polymer with as-received
not dispersed SWNT.
[0046] FIG. 31 shows longitudinal cross section of the wire
feedstock.
[0047] FIG. 32 shows inter trace fusion.
[0048] FIG. 33 shows inter trace fusion in cross ply.
[0049] FIG. 34 shows the two conditions of FIG. 33.
[0050] FIG. 35 on the right shows attachment of polymer chains to
the oxidized edge of a cut SWNT via amide linkages.
[0051] FIG. 35 on the left shows attached organic molecules.
[0052] FIG. 37 shows nanotubes crosslinking polymer chains.
[0053] FIG. 38 shows nanotube block copolymers.
[0054] FIG. 39 shows possible configurations for alternating
polymer blocks.
[0055] FIG. 40 shows functionalized SWNTs in graft
copolymerization.
[0056] FIG. 41 shows graft copolymerization on only one side.
[0057] FIG. 42 shows a randomly crosslinked chain.
[0058] FIG. 43 shows substrate attached nanotubes.
[0059] FIG. 44 shows nanotubes shear oriented and chemically bonded
to the polymer matrix.
[0060] FIG. 45 shows a typical composite design system.
[0061] FIG. 46 shows a comparison of a stress-strain curve for (a)
a typical epoxycomposite compared to that for (b) a nanotube
composite where significantstrength of the nanotube is discarded.
SWNTs are expected to have a high degree of elongation to
failure.
[0062] FIG. 47 shows parameters used to calculate the properties of
SWNT composites by Rule of Mixture Calculations based on an ABS
polymer as the matrix.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0063] NCFs are a continuous fiber system, typically in the 1 to
150 micron diameter size range, made up of polymers and nanofibers,
where the composition of the nanofibers range from above zero to
100 wt. %. Processing of nanofibers into polymer matrices in a
continuous fiber form will provide control over the nanofiber
orientation and offer advanced properties compared to conventional
materials. The structural, electrical and thermal properties can be
significantly enhanced. NCFs made up of 100% nanofibers are to be
processed by mixing with a polymer matrix or binder that is
subsequently removed to leave a nanofiber system. This process of
making 100% micron size NCFs is especially directed toward
development with SWNTs and their various functionalized
derivatives.
[0064] Currently, nanofibers are not easily handled or manipulated
into various engineered forms for a range of scales. Nanotubes can
be obtained in low and high purity dried paper forms, called "Bucky
Paper," or may be purchased in various solutions that do not easily
lend themselves to processing large scale products. Nanotubes are
also available in the as-processed, unpurified condition which
carry with them numerous unwanted impurities that affect composite
properties. In this disclosure, a form of nanotubes is presented
which can easily be handled in much the same way as thread, yarn,
fabrics, etc. are handled. The processing to achieve this end
involves dispersing nanofibers in polymeric matrices that are
formed into continuous fibers. The method of forming the fibers
involves high shear processing, where polymers are a liquid, molten
or melted and extruded through small orifices to produce high shear
and small fiber systems. Elongational flow is subsequently used to
further reduce the size of the fibers and to ensure aligned
nanofibers (aligned parallel to the length of the continuous fiber
system). NCFs using vapor-grown carbon fibers have already been
processed using ABS with 10 wt. % nanofibers and PE matrices with
2, 5 and 10 wt. % nanofibers. NCFs have also been processed using
ABS with 5 and 10% wt. % SWNTs and using UHMW (ultra high molecular
weight) PE with 2% wt. % SWNTs. Additional systems produced
included PP 1000 and 12 wt. % SWNTs with nanotube concentrations up
to 6 wt. %.
[0065] Spools of NCFs can be manufactured to use in the as-received
form, or may be designed to further activate, functionalize,
surface treat, pyrolize, modify, cross link, etc. the fiber system.
Through chemistry, the nanofibers may be further linked to the
polymer, to each other, or optimized for non-wetted or unbound
conditions according to the needs of the specific application. The
ability to provide for aligned nanofibers in a range of polymeric
materials and to further chemically treat the fiber systems opens a
range of opportunities for new product development and materials
enhancement. This includes the processing of FINC and T-FINC
fibers.
[0066] For an application relating to electrical properties
conducting polymers, composites can be processed with isotropic
dispersions of nanofibers when a network is formed. The material
may be designed for Electrostatic Discharge (ESD) applications,
primarily used to prevent the buildup of static electrical charges.
Static electricity is not current or flow electricity; rather it is
electricity that is at rest. The static dissipative range is known
to be between an insulator (surface resistivities higher than
10.sup.13 ohms/square) and a conductor (surface resistivities lower
than 10.sup.5 ohms/square). That is, values of surface resistivity
between 10.sup.5 and 10.sup.12 ohms/square will provide static
electrical dissipation. The values are low enough to provide
dissipation, but not too low where sparking can occur. The volume
and surface resistivities of the isotropic nanofiber reinforced
composites were measured. The results of the isotropic composites
are plotted in FIG. 1 for (a) volume and (b) surface resistivity,
respectively. A percolation threshold occurs as it is expected for
plastic reinforced with conductive materials. The nanofiber
reinforced polypropylene composite starts to percolate at around
9%, and by 18%, a network of fibers has been formed with no
fuirther decrease in resistivity. This system with this level of
percolation has the opportunity of providing strengthening over
other filled polymers for ESD, where the filler networks only lead
to ESD properties.
[0067] To extend the range of that isotropic material, the
conducting polymers can be processed into NCFs, where the
nanofibers are well-aligned and can easily be aligned in bulk form
by easy handling of micron size fibers. The material, which has
highly anisotropic electrical properties, has the further potential
of being used as an Electromagnetic Interference/Radio Frequency
Interference (EMI/RFI) material, since the electrical conductivity
can be processed to a higher electrical conductivity. The nanofiber
network that is formed is highly aligned parallel to the length of
the fibers. An EMI/RFI material has electrical resistivities in the
conductor range (<10.sup.5 ohms/square) and can even be used for
wires and electrical interconnects particularly when SWNTs are
used. The interconnected network for conduction is maintained but
the high degree of alignment further enhances the electrical
properties and strength.
[0068] The processing method used in the present invention to
incorporate the nanofibers in the plastic matrix was a Banbury-type
mixing where high shear stresses and high power were incorporated
into the system. Given the size and tendency of agglomeration of
the nanofibers, the Banbury-type mixing was selected to provide a
uniform distribution of the nanofibers by exposing the agglomerates
to hydrodynamic stresses forcing the agglomerates to break apart
without, in most cases, damaging the nanofibers. In some cases,
high shear can be used to shorten the fibers, particularly those
with a high number of defects. Composites with carbon nanofiber
concentration ranging from 0-60 wt. % were prepared demonstrating
the high degree of fiber concentrations that can be achieved.
Nanofiber/tube dispersion is a key aspect in the sample
preparation, since the physical properties of the finished
composite are strongly governed by the dispersion of the fibers in
the matrix. FIG. 2 shows a transmission electron microscopy
micrograph of vapor grown carbon fibers in a polypropylene matrix.
These nanofibers with an average diameter of 100 nm are highly
dispersed and there are no indications of porosity in the composite
system. FIG. 3 shows the scanning electron micrograph of a fracture
surface of a composite sample tested in tension showing regions of
wetting as indicated by the deformation of the polymer around the
nanofibers.
[0069] The NCFs were obtained by subsequently processing the
composite material in a high shear condition using a capillary
rheometer. In the rheometer, the viscosity of the polymer/nanofiber
system is significantly reduced to promote easy processing and high
degrees of alignment. FIG. 5 shows the reduction in viscosity that
can be accomplished as shear rate is increased for a
VGCF-polypropylene system. The degree of alignment is processed
into the micron size fiber system according to the level of shear
used (see FIG. 4 for micron size fiber composite). FIG. 6 shows a
similar reduction in viscosity obtained with increasing shear taken
from the capillary rheometer where high degrees of alignment of the
nanofibers are achieved. The degree of wetting and bonding can be
processed into the initial NCF or may subsequently be altered
following NCF assembly into various parts.
[0070] To achieve NCFs polymer mixing procedures were developed to
achieve high SWNT dispersion with reduced tangles. The rheology of
mixing and melt spinning was studied Continuous polypropylene (PP)
and Polyethylene (PE) fibers with and without single wall nanotube
(SWNT) additions were produced at lengths of thousands of
kilometers (quantities which could easily be spooled on bobbins or
spools). Aligned SWNTs were observed in nanotube continuous fibers.
Improved fiber properties were demonstrated via a rheology study,
thermophysical analyses, microscopy, and mechanical testing.
Improved properties included strength higher than the unfilled
fibers, improved modulus, significant changes in glass transition
temperature and degradation temperature, and indications of
sustained strength at high temperatures. The addition of SWNT
increased the tensile strength by 743% and the elongation to
failure by 2964% for the fiber forms. Production equipment was
built including an elongational flow apparatus and a take up wheel
system.
[0071] The identified mixing approaches are successful for
producing highly dispersed SWNTs in polymers where porosity and
mixing viscosity is not an issue (typically high shear conditions).
The process outlined in this invention identifies below key factors
for achieving alignment, to producing continuous fibers, and for
producing small diameter fibers.
[0072] As-received SWNTs do not produce the same results as
purified SWNTs in polymer systems. Initial polyethylene (PE) with
as-received SWNTs were evaluated and showed to be poorly mixed. The
amorphous carbon and impurities limited the mixing. ABS was
processed with 10 wt. % SWNTs as a test system. The mixing
procedure was augmented to use ABS powder rather than pellets.
Powder provides for the best initial dispersion level of nanotubes
once they were dispersed over the powders uniformly. Powder was
dried and mixed with purified SWNTs in toluene. The slurry was
dried where all the solvent was removed. The dried material formed
chunks of agglomerated powder with highly dispersed nanotubes that
were very easy to handle. A composition of 5 wt. % SWNTs in ABS was
processed using the torque rheometer. The composite was hot pressed
into a sheet for subsequent processing. The sheets showed good
flexibility and could be cut into pieces without cracking. The
VGCF/polymer composite systems would many times crack when
sectioned from the sheet form. PP and PE were processed with
dispersed as-received and purified SWNTs.
[0073] Polymer powder mixed with SWNTs in solution route proved to
be an optimal approach for dispersing nanotubes in a polymer. PE
powder was used but polymer pellets were used for the PP. The
solvent was dimethyl formamide (DMF) since it has a low solubility
in PE and PP, adequately disperses SWNTs and it boils off at a
temperature below the softening temperature of the polymers. The
coverages were uniform and this method provided a step where
unwanted impurities in the as-received SWNT material (whether
as-received or purified) could be removed. Unwanted contaminants
had to be removed even after purification of the nanotubes. These
powders formed agglomerates upon drying and the chunks were easy to
handle and removed concern of having airborne particles. The
agglomerates loaded very easily into a torque rheometer for mixing
and compounding. Mixed composite was easy to process without any
significant increase in torque (this means at low viscosity).
Although the viscosity increases with the added SWNTs, the torque
requirements for mixing the material are over an order of magnitude
lower than the instrument limitations. When mixing the PP (melt
flow index of 12) and PE (melt flow index of 10) materials, the
torque conditions were the lowest observed.
[0074] SEM analyses showed the SWNTs in the PP and PE to be highly
dispersed, to have a lesser degree of entanglement, and to have
some indication of wetting. The composite was porosity free.
Compositions of 0, 5 wt. % as-received SWNTs (Tubes@rice) in PE
(melt flow index of 10), 0, 2 wt. % SWNTs from HiPco in PP1000 and
PE ultra-high molecular weight, and 1.7 wt. % SWNTs from NTT in
PP12 and 3 wt. % SWNTs in PE10 were used. The 10 wt. % SWNTs from
Tube@grice in ABS were processed to learn more about the mixing
process and the electrical properties of these systems. Typically a
concentration of 1 wt. % SWNTs well dispersed in a matrix will lead
to electrostatic discharge (ESD) conditions. It is likely that any
concentration less than that that produces conduction is likely to
be a segregated network. FIG. 7 shows electrical resistivity as a
function of SWNT composition. It is expected that the matrix does
not play a significant role in the current processing methods.
[0075] The processing route using powders and SWNTs in solution
works very well and all compositions processed to date show good
matrix/nanotube properties. These enhanced materials (the
as-prepared aggregate and the mixed composite sheets) and nanotube
coated powders, composite prepeg, and final sheet material can be
provided for end use or further processing. Fibers have been
produced from various systems. Long continuous fiber forms, and
over a 1000 meters of fiber were produced. FIG. 8 illustrates the
surface resistivity of ABS with VGCFs and SWNTs. Note that only 1%
SWNTs are needed to achieve conduction from a well dispersed
composite system.
[0076] There are a number of ways to achieve conduction via SWNTs.
First, a network must be established and maintained in the polymer.
Second, when alignment is achieved, either end to end contact or
tunneling across small gaps must occur. Alignment that leads to
high dispersion and no contacting nanotubes may result in poor
conduction. The nanotube ropes are useful for achieving conduction
because they lead to a high aspect ratio filler. The metal in the
HiPco nanotubes will also contribute to a reduced percolation
threshold for conduction.
[0077] From various mixing runs and from measurements of the peak
and steady-state torque for each run, the average and peak torque
levels for composite processing were determined. Data showed that
the mixing conditions were far lower than the limits of the torque
rheometer and that the mixing viscosity was significantly lower
than other investigators had encountered. A viable commercial
approach to producing nanotube composites at low costs is achieved
by the present invention since the Banbury mixing is already a low
cost commercial manufacturing method. Only costs added besides that
of the SWNTs is the cost of the solvent. Water or other low costs
solvents or surfactants are part of the future work. FIG. 9, a
micrograph of a 10 wt. % SWNT/ABS polymer which was fractured at
room temperature shows good dispersion, absence of porosity, and
the reduced entanglement of the nanotubes. The degree of
entanglement is reduced since the high shear processing during
mixing aids in dispersing the nanotubes from each other. SWNTs are
wetted by the polymer, as indicated by places where nanotubes
appear to be pulled out of the polymer. Some nanotubes are not
coated by the polymer, indicating that the fiber/matrix shear
strength and normal stress are still relatively low.
[0078] PP with the following formula was used in this project:
[--CH.sub.2CH(CH.sub.3)--]n (1) that was (1) isotactic, Melt flow
index 1000, melting point of 160.degree. C., and density of 0.900
and (2) isotactic, Melt flow index 12, melting point of 165.degree.
C. and density of 0.900. PE with the following formula was used in
this project: (--CH.sub.2CH.sub.2--)n (2) that was (1) ultra-high
molecular weight, melting temperature of 130.degree. C. and density
of 0.940 and (2) Melt flow index 10, Marlex from the Phillips Co.,
density of 0.94. PP1000 was not an optimum polymer to use for melt
spinning but it was used to explore the limits of process rheology.
The ultra high molecular weight PE was also not optimal for melt
spinning but could be mixed with the SWNTs by a gel-spinning
approach and thus show the extension of the process of this
invention to gel-spinning applications. Polymers were typically
prepared by drying in a hot box at 90-100.degree. C. The slurries
of SWNTs, and polymer were dried in a furnace at temperatures below
the melting temperature of the polymers to remove the solvent. The
resulting materials were polymers overcoated with an even
distribution of nanotubes. This step provided the initial
dispersion condition and one that would lead to easier dispersion
of the nanotubes during the mixing of the melt. Numerous mixing
runs of the polymer systems were prepared to provide for extrusion,
use in the capillary rheometer, rheology, tensile testing, and
thermal analysis. The materials were cut into pellets for
subsequent extrusion or use in the capillary rheometer.
[0079] The mixing conditions for the polymers with SWNTs were at
torque conditions far below the limits of equipment shutdown (Haake
Polylab rheometer). FIG. 10 gives torque data for the PP12 and PE10
systems which show very low values of torque indicating that mixing
has ho problems with excessive viscosity as reported by others.
Note that the torque is relatively low and very steady state once
the initial mixing was conducted. In some cases, mixing speeds were
ramped up from 60-65 rpm to 90 rpm to enhance mixing. The mixing
parameters including temperature, time, and mixing rate were
selected based on a process optimization study and on a database
for Banbury mixing of thermoplastic polymers. The mixing and the
extrusion could be accomplished in a multiple zone compounding
extruder where the mixing residence could be held for sufficient
time followed by extrusion of a well dispersed SWNT system. This
provides a cost effective approach to making continuous fibers.
[0080] Thermal physical measurements and rheology were conducted on
the polymer blends. Thermal degradation, creep behavior, storage
modulus as a function of temperature and frequency were all
measured. The change in the glass transition temperature, where
applicable, was also measured. PP and PE saw increases in the
degradation temperature with SWNT additions. These improvements are
significant to the polymer industry since only small amounts of
SWNTs led to these increases. FIG. 11 shows the significant,
100.degree. C. higher, increase in the degradation temperature for
PP with just 2 wt. % SWNTs.
[0081] Early studies worked with compositions of 1-10 wt. % in ABS.
Small compositions usually around 2 wt. % ensured the processing of
multiple runs for use in several polymers. Some of the small
concentration samples were in part, amorphous carbon and catalysts
since they were used to ensure numerous runs. Compositions shown on
plots identify the composition of added nanotube material whether
they were purified or not. FIG. 12 shows the type of increase
observed in the modulus of polymers when SWNTs are added. Although
this change for PE appears small (25%), the potential for creating
a tough fiber exists over that seen for dispersed VGCFs due to
remaining elongational properties of this polymer system.
[0082] It is known in the art of fiber spinning that selection of
extrusion speed and subsequent draw speed (allowed elongational
flow) effect general fiber properties especially final fiber size.
FIG. 13 shows the enhancement in properties that would be obtained
of high extruder speed and take up speed are used. FIG. 13 details
the strength of various fibers when processed at different extruder
and take up speeds. High extruder speed coupled to high take up
speed leads to stronger fibers. It is also known that use of a fast
extruder speed with a take up speed that is high can lead to very
small diameter fibers. FIG. 14 shows the fiber sizes for these
processing conditions, more particularly a plot indicating the
change in diameter of a fiber system when processed at high
extruder speeds and subsequent high take up speeds. Small diameter
fibers are sought since defects in these systems tend to be
minimized. Further, use of a slow extruder speed with a slow take
up speed leads to larger diameter fibers as seen in FIG. 15. FIG.
15 provides a plot indicating the change in diameter of a fiber
system when processed at slow extruder speeds and subsequent slow
take up speeds. Larger diameter fibers result. Thus a range of
fiber sizes can be processed according to customer needs.
[0083] Fibers were analyzed using optical microscopy and SEM and
were observed in some cases to have aligned nanotubes. The samples
with compositions greater than 1 wt. % purified SWNTs were
considered either ESD or EMI conducting. The samples with
compositions greater than 2 wt. % purified SWNTs were EMI. The 10
wt. % sample was highly conducting and could be used as a plastic
wire. A thick plastic wire was placed in an electrical circuit to
light up LED's when powered by a DC power supply. This demonstrated
a significant application for these new fibers that are being
produced according to the invention.
[0084] Both an extruder and a capillary rheometer were used to make
fibers in this period of the program. A Stratasys Fused Deposition
Modeling system could also be used to make continuous fibers. Shown
below in FIG. 16 is the extrusion data collected while producing
fibers from a single bore Haake extruder. Fibers were processed
with the extruder. Using the extruder, one must consider die size,
screw rate, and extruder temperature as well as the take up speed
when the fibers are drawn out of the extruder. Processing must be
at a condition where additional drawing out of the fiber can occur.
A Rosand RH7 capillary extrusion rheometer, was used to investigate
the rheological behavior of nanotube-filled and unfilled polymers.
It was also used, in conjunction with a spinning wheel take up
system (not pictured), to extrude and extend filled and unfilled
polymer fibers.
[0085] Below in FIG. 17, 18 and 19 are rheological results for
filled and unfilled ABS, polypropylene, and polyethylene, at
temperatures and flow rates found to be favorable for fiber
spinning with the RH-7.
[0086] Industrial melt spinning of polymer fibers is a highly
controlled process, with many operating parameters that must be
precisely maintained in order to achieve continuous, consistent
fibers. The process most often begins with dry polymer chips. These
are melted and transported with a screw extruder, which also mixes
and homogenizes the melt. The molten polymer is forced at very high
pressure through a fine filter, removing impurities, degraded
polymer pieces, and gas bubbles. The filter further homogenizes the
melt. An accurate metering gear pump is used to force the material
through the filter at a precise flow rate. See FIG. 20.
[0087] The now uniform, temperature controlled melt is forced at a
consistent flow rate through a capillary in a spinneret die. The
hole is 100-500 .mu.m diameter, and the polymer is squeezed out as
a stream of fluid. The shear in the spinneret partially aligns the
polymer molecules, but much of that alignment is lost when as the
extrudate swells on exit from the die. The extrudate is quenched
and drawn off from the bottom, stretching it thinner and longer.
There are many types of quenching methods, but the essential
characteristic of all of them is the controlled cooling of the
filament as it is drawn. The stretching provides a degree of
alignment that depends on the haul-off speed:
[0088] Low Orientation--below 1800 m/min
[0089] Medium Orientation--1800-2800 m/min
[0090] Partial Orientation--2800-3500 m/min
[0091] High Orientation--4000-6000 m/min
[0092] Full Orientation--not presently obtainable m/min
[0093] After spinning, the fiber is cold-drawn--a vital step in
orienting the polymer molecules. The higher the orientation from
spinning, the lower the draw ratio in post drawing will be. An
undrawn fiber will be extended by a factor of up to four in length
as the molecules that make it up are further aligned.
[0094] In any fiber drawing process, process control is of utmost
importance. The melt must be consistent, the flow rate through the
spinneret must be uniform, and the temperature must be well
controlled. In addition, stresses in the melt which cause fracture
or rupture must be avoided, as must a drawing tension which exceeds
the strength of the filament. The best orientation of molecules in
spinning comes when the extensional flow rates are high enough to
provide alignment, and the relaxation time of the polymer is long
enough so that any aligned molecules do not have a chance to
recoil. Thus, alignment is associated with high spin speeds, high
molecular weights, and good quenching efficiency. However, low
speed spinning and subsequent cold drawing often produces more
alignment than just high speed spinning. Low speed spinning and
high speed spinning, with additional annealing, hot drawing does
even better.
[0095] The RH7 was used as the extruder for a low-speed
fiber-spinning process. The essential needs of melt spinning were
met, albeit with less process control over some areas of the
process. The RH7 step motor provides excellent control over flow
rates even while delivering well-controlled force to the melt. The
dies found most suitable for fiber drawing were of 0.5 mm diameter;
the best fibers were drawn through a 8-mm.times.0.5-mm die, held in
a "flat exit" die holder to allow the extrudate easy passage out of
the barrel without danger of it getting stuck to the walls of a
conventional die holder. However, the flat entry of the die is
unlike that of a melt-spinning spinneret. The RH7 temperature
control is accurate to 0.1.degree. C.
[0096] To ensure that flow rates were kept at a level where the
required pressure would not cause melt fracture, rheological
testing was done not just for viscosity but also for the melt
strength. The following figures show the melt fracture and the melt
condition for the polymer systems studied.
[0097] No quench zone was applied when drawing the fiber other than
that of ambient temperature, and the spinning rates themselves were
much lower than those seen in industrial processes, about 100-200
m/min.
[0098] Fibers with diameters of 35-50 .mu.m fibers were produced
and were reduced to .about.25 .mu.m by stretching. Uniform
thickness fibers with well dispersed SWNTs were produced. A number
of the fibers had less than homogeneous dispersion due to the use
of unpurified SWNTs in many of the cases. Use of the capillary
rheometer led to uniform fibers but use of the extruder tended to
produce fibers less uniform and in some cases with small enriched
thickness sections. It is known in the art that a process assembly
using an extruder calls for extruded melt to go into a fluid
metering pump so that a constant pressure is maintained on the die
or spinneret. The extruder was useful in showing the ability to use
an extruder particularly since extruders are more continuous in
their processing (manufacturing) than the capillary extruder. FIG.
21 shows a micrograph of PP1000 showing well aligned SWNTs.
Alignment was achieved in several fiber systems. The fibers
produced using NTT nanotubes also showed alignment but on a limited
basis because the SWNTs were only 25% of the starting material.
[0099] Tensile tests were performed on eight samples of
polypropylene samples as shown in FIG. 22. Four of these samples
were pure polypropylene and four of these samples were
polypropylene containing SWNT. The polypropylene was purchased from
Aldrich and the SWNT were provided by Nanotechnologies of Texas.
The four samples containing SWNT were die cut from compression
molded sheet and possess Type V geometry. The four samples of pure
polypropylene were die cut from a thinner compression molded sheet
and do not meet Type V geometry specifications with regard to
thickness. Of the eight samples, six provide valid data. One of the
two samples not analyzed broke as a result of set up error and the
other sample fractured outside the gage length. Both of the
disregarded samples were pure polypropylene. The test rate used was
0.5 inches per minute, and the strain was calculated over the gage
length (0.3 inches) because extensive deformation was not observed.
The tensile strength was not improved. The average tensile
strengths of both materials were 28 MPa. However, the strain to
failure of the PP sample with SWNTs was improved by 115%. The pure
polypropylene sample failed in steps as shown by the stress/strain
curve. The filled polypropylene samples failed in one step.
[0100] The fibers were tested in bundles of 20 fibers. The diameter
of the pure polypropylene fibers was 0.0040 inches and the diameter
of the filled polypropylene fibers was 0.0015 inches. The test rate
used was 30 inches per minute. The strain is calculated over the
entire length of the fiber (4 inches). FIG. 23 shows the strength
of a single fiber. The graph goes up to the highest load the bundle
could withstand. Following this load, individual fibers began to
fail. The addition of SWNT increased the tensile strength by 743%
and the elongation to failure by 2964%. One important note to
consider is that as these fibers get further developed they will
surpass the bulk properties of PP even by aligning the polymer
system itself. Aligned SWNTs along with aligned polymer may well
produce enhanced strength rivaling that expected from single
nanotubes themselves however, the elongation features of this
system may be reduced to that of a well aligned polymer system.
[0101] Tensile tests in accordance with ASTM D638 were performed on
eight samples with Type V geometry. Four samples were made from
pure polyethylene, and the remaining four samples were SWNTs in
polyethylene. The polymer matrix material was Phillips Marlex
polyethylene in pellet form. Nanotechnologies of Texas supplied the
SWNT used. The test specimens were cut using a die from compression
molded sheet. The test rate used was 0.5 inches per minute. Strain
was calculated over the entire length of the sample, 2.5 inches,
because extensive plastic deformation was observed in the majority
of the samples. The addition of SWNT decreased the tensile strength
by 17% and elongation by 37%.
Utility
[0102] The utility of Nanofiber Continuous Fibers is far-reaching.
FIG. 24 shows various products and processing that can be achieved
using NCFs. These commercial avenues for NCFs impact the highest
percentage of the composite manufacturing industry. Beyond this,
they impact the textile and fabric industry as well.
[0103] Example Systems
[0104] (Electrostatic Veil and Surfacing Mats).
[0105] ESD materials are needed for packaging purposes and also for
garments, dissipative chairs, work benches, carpets and floor mats
for the personnel working in the electronic manufacture site. ESD
is a part of everyday life, but in the electronics industry, the
cost of damage and rework due to static electricity is estimated to
be of billions of dollars annually. Of all the failures in the
electronic industry, approximately 40% are ESD related.
[0106] As micron-size fibers for structural applications, the NCFs
can be woven into crossplies for preforms, vacuum bagging and hand
lay-ups. Aligned nanofibers are expected to deliver the most
optimum mechanical properties for high performance composites.
Assembled cross-ply systems still offer superior performance to
isotropic composites since a higher degree of alignment can be more
easily controlled for these systems.
[0107] Based on accepted models, there is little question that
these materials will have significant strategic potential, produce
performance enhancement and present immediate industrial benefits
(i.e., greater strength/weight ratio). The composite fiber
manufacturing technologies currently in place do not have to be
significantly altered, although some alterations may occur for
specialty fiber systems, so they will be readily accepted from the
government and civilian composite communities. These fibers can be
used in a wide variety of applications with processes shown in FIG.
24.
[0108] The present invention provides a revolutionary approach of
processing nanofibers in an isotropic or anisotropic form as a
continuous fiber composite providing control over the dispersion
and alignment of the nanosize fibers in various polymer matrices.
It illuminates a clear path to making aligned nanofibers available
to the composite manufacturing community and other industries. It
further provides for the opportunity to easily manipulate the
nanofibers through additional processing like annealing, reactions,
pyrolisys, further modification and functionalization. Polymers can
be chosen to enhance or inhibit nanofiber alignment, or selected to
be crosslinked or strongly bond to the nanofibers. The polymers
themselves may be enhanced and altered by the nanofibers acting as
nucleation sites and effecting crystallization and final polymer
molecular morphologies. It solves the inherent problem of
manipulating nanosize structures while taking advantage of
commercial filament and fiber technology.
[0109] The present invention provides an enhanced approach to
achieve surface resistivities in the desired ESD range for the
prevention of static electricity buildup. It provides a novel
composition that can be prepared by conventional plastic processing
technologies eliminating or ameliorating many of the problems
associated with prior art fibers incorporation in plastic matrices
and with processing of conductive plastics filled with metal
fibers, flakes and powders or chemically modified polymers.
[0110] Rheological analyses have shown that the practical
possibility of scaling up the composite manufacturing according to
the nanofiber weight percentage is at hand.
[0111] The present invention shows purified and unpurified SWNTs,
and vapor grown carbon nanofiber reinforced thermoplastic
composites made into Nanofiber Continuous Fibers. These composites
were prepared by mixing the purified and unpurified SWNTs, or vapor
grown carbon nanofibers with the thermoplastic matrix in a Haake
miniaturized internal mixer (MIM). The mixing process consisted of
distributive mixing where the SWNTs or nanofibers were spread over
different positions within the chamber and dispersive mixing where
the application of high shear conditions and energy were required
to overcome the nanofiber agglomerates. Different compositions by
weight percent were prepared ranging from 0 to 60% of SWNTs or
nanofibers (up to 10 wt. % SWNTs has been used). After mixing, the
obtained composite material was then hot pressed to a temperature
of 150-200.degree. C. to form thin sheets. These sheets were then
pelletized for subsequent extrusion and fiber forming. The
composites were processed as continuous micron size fibers in which
alignment of the nanofibers was obtained, thus promoting then the
feasibility to be further processed with conventional composite
manufacturing technologies to deliver high performance structures
for multiple uses (space, defense and commercial applications).
FIGS. 25-26 show NCFs with aligned nanofibers or SWNTs. Surface
conditions of the NCFs can be varied as can degree of alignment and
deagglomeration. FIGS. 27 and 28 depict these various
conditions.
[0112] Since the nanofibers are black materials, the possibility of
having transparent composite materials for specific applications
becomes an issue. The composite can be thinned to be translucent so
some visibility can be obtained (not that this applies to sheet
forms).
[0113] Having prepared the composites as micron size fibers it
provides the feasibility to be used for structural applications,
thermal and electrical applications. Highly anisotropic thermal
systems are of significant interest.
EXAMPLE
Application of Nanotube Reinforced Polymers for Fused Deposition
Modeling
[0114] The alignment that occurs with nanotube continuous fiber
processing can be achieved by Fused Deposition Modeling (FDM)
processing. A spool of nanofiber reinforced polymer is formed into
wire feedstock for the FDM process, a rapid prototyping (RP)
process, also sometimes identified as a free form fabrication
technique, is processed into final parts using computer generation
of slices of a three dimensional (3D) image for layer by layer
manufacturing. The alignment is due to shear processing by
extrusion and some extensional flow while spooling (to achieve the
desired diameter) and enhanced in the hot extrusion aspect of the
FDM process. In this process, the nanofiber composite wire
(.about.2 mm in diameter) is extruded through millimeter-size dies
to generate rows that build sheets and 3D parts. FDM feedstock can
be made by producing wire by extrusion or by grouping NCFs with
further processing to make wire.
[0115] Another application related to FDM is to make the spools of
materials (having reinforcing nanotubes which add to, in some
cases, electrical and thermal properties) to manufacture small
batches of plastic parts for commercial purposes and a range of
applications, with FDM, parts with intricate internal shapes can be
manufactured which in the case of other traditional technologies is
either not possible, or it will be too costly since, in order to
compensate the price, a few thousand parts have to be made.
Examples of these parts include medical tools, electronic,
replacement parts, etc. An example of space applications of FDM
parts made with the material is the use of FDM on the Space Station
where the superior feedstock (nanotube filled polymers) is used to
make replacement parts, rather than storing supplies of parts in
the limited space on the Space Station.
[0116] Vapor grown carbon fibers (VGCFs), mixed with various
polymers are proving to be a good approach to producing polymeric
composites using nanotubes. VGCFs can be mixed through shear
processing without nanofiber breakage and high degrees of
dispersion can be achieved originating from a tangled mass.
Similarly, functionalized nanotubes can be more fully integrated
into polymeric systems. In the present invention, the processing
"Bandury" mixing was used to produce feedstock continuous filament
for Fused Deposition Modeling (FDM), enabling composite part
manufacturing for a range of applications and well past material
property evaluation. Nanofiber composites significantly enhance
rapid prototyping techniques like FDM, SLS, etc. because of their
potential for enhancing polymeric properties as a multifunctional
material (structural/electrical, structural/thermal,
structural/impact).
[0117] FDM is a manufacturing process that takes a feedstock
(continuous filament or billet) and hot extrudes it to make
continuous traces of polymer material. The traces map out a layer
and subsequent layers can be deposited on top to build up
3-dimensional shapes. Any number of sample forms can be produced as
long as a 3-D computer image can be generated. That file is then
cut into slices to match the process parameters for the FDM. The
FDM unit then builds the part ( bottom up technology") layer by
layer starting from a removable support. FDM parts are generally
used for models, molds and for some part applications where
material usage is optimized (waste is minimized) and tooling and
part finishing is reduced. A number of feedstock materials are
available and include wax-filled plastic adhesives, nylon
copolymers, investment casting waxes and
acrylonitrile-butadiene-styrene (ABS). ABS is the more optimal for
part applications because of its superior strength to the other
materials available. Composite feedstocks filled with various
reinforcements, including chopped carbon fibers, are now being
developed with the purpose of enhancing mechanical strength of FDM
parts. Typically, filled polymers are of interest because they have
the potential of improving interlayer strength, many times the weak
link in FDM parts, and to enhance stiffness which is often seen in
filled polymers. The availability of high strength polymers for FDM
extends its use in part manufacturing and extends its application
range.
[0118] In the present invention, three feedstocks were produced
using (1) purified VGCFs, (2) pelletized VGCFs, and (3) as-received
SWNTs. Pelletized, in this case, is a processing method to improve
handling where a latex sizing is placed on the nanofibers from the
commercial producer. The as-received SWNTs provided illustrative
processing of SWNTs with ABS using high shear and FDM processing. A
number of different parts were produced including three different
tensile tests part sizes. Parts were also made connecting plain ABS
to the VGCF composite to show that the new materials can work in
conjunction with the currently available ABS. The processability of
FDM parts using nanofiber/polymer composites was evaluated, tensile
tests were conducted and the fractured regions of the samples were
analyzed using electron microscopy. XRD was used to verify the
presence of aligned nanofibers in the wire feedstock and FDM parts.
The enhancements observed for the nanofiber filled polymers
demonstrate that these composite materials can enhance the use of
rapid prototyping, such as FDM.
[0119] In the FDM process the wire is fed between two friction
bearing rollers that account for velocity control. This process is
conducted in a temperature control chamber that terminates in a
circular die. The temperature is maintained just above the
solidification point. When the material exits the die it solidifies
as it is directed into place with the X--Y controlled extruding
nozzle. FDM consists of the deposition of continuous layers next to
and on top of each other to build the specified model under CAD
control. The successive layers are bonded by thermal fusion. The
heat capacity of the material is important to the amount of
shrinkage and the degree that the material fuses to itself.
[0120] FDM work was conducted involving filled feedstocks for
producing ceramic filled polymers for prepreg applications. The
filled polymers are used to trace out a mold that is subsequently
sintered to form a ceramic part. The polymer is burned out, leaving
behind a porous ceramic pre-form which can be infiltrated with
metals of other matrix materials.
[0121] VGCFs known as Pyrograph III with an average diameter of 100
nm were obtained from Applied Science, Inc. in as-received and
pelletized forms. The as-received VGCFs were purified and
functionalized before the compounding stage according to previously
developed procedures. The ABS GMID #31875 was obtained from Magnum.
ABS was chosen because of its high strength and use in FDM. Typical
properties for ABS are listed in Table I along with those for the
VGCFs.
[0122] Composite preparation was conducted using a HAAKE torque
rheometer using a 30 g mixing bowl. ABS was compounded with the
various nanofibers at high shear rates to achieve a homogeneous
dispersion. Dispersion, in this case, means homogeneous spread of
the fibers and individually isolated fiber forms (either individual
VGCFs or individual ropes). Starting sample compositions of 10 wt.
% were prepared. Samples were hot compression molded and pelletized
(chopped up into small pellets) to use as a feed material for the
wire extrusion. Composite batches were then extruded at a rate of 5
rpm and spooled on to a reel-type container while maintaining a
constant cross-section for the length of the wire extruded.
[0123] The extruded samples of plain ABS, with VGCFs and with SWNTs
had a diameter of 1.7.+-.0.1 mm which was optimum for the FDM
processes. Extrusion to form the wire for all composites was
conducted by starting the extrusion with plain ABS pellets,
followed by the composite material, and finished off with plain ABS
again to sufficiently fill the extruder barrel. This approach
caused the composite of the wire feedstock to vary over the length
of the wire so that the maximum composition achieved was 10 wt. %
at the middle of the extrusion run.
[0124] A Stratasys Inc. FDM 1600 that operates using spooled
feedstock material and at relatively low shear rates was used to
manufacture several parts. Parts were made in dome shapes,
spacecraft models, logos, and tensile tests samples of different
sizes. Straight bar and dog-bones tensile samples were fabricated
for the various strength measurements. The samples consisted of 12,
9, or 10 deposited layers in (I) flat bar, (II) cross section #1
dog bone or (III) cross section #2 dog bone. The insert shows the
schematic of the FDM process showing the extruder tip drawing out
polymeric traces being built into layers. The arrows indicate the
ability of the extruder to traverse the surface. FDM traces were,
in all cases, cross-plied with either 90.degree./180.degree. or
45.degree./45.degree. orientations. Mechanical tests were conducted
using a MTS hydraulic tester. The tests were carried out at a
strain rate of 2.54 cm/min at room temperature. Care was taken in
the placement and orientation of the samples to better understand
alignment of the sample effects and those that might be associated
with the orientation of the FDM growth process (top and bottom
orientations).
[0125] Cross sections of composite filaments and tensile fracture
surfaces were analyzed with a JEOL scanning electron microscope.
The samples were gold coated even though 10 wt. % VGCFs leads to
electrostatic dissipative conduction in these materials. The wire
fracture surfaces were taken from samples submerged in liquid
nitrogen to promote a highly brittle fracture and to reduce induced
alignment during fracture. Nanofiber alignment was studied by
observation of fracture surfaces. Generally samples are fractured
at liquid nitrogen temperatures to prevent alignment from occurring
during the fracture process and distorting the results. Significant
care must be taken to ensure alignment from deformation does not
occur.
[0126] The nanofiber/ABS composites have showed improvements in
tensile properties over the unfilled ABS processed at the same
conditions. Composites tested were from the pelletized VGCFs. These
samples were processed with 90.degree./180.degree. cross-ply
conditions. The data for the ABS is relatively consistent, as would
be expected from a FDM process. The tensile test results for the
VGCF/ABS data showed significant scatter related to concentration
changes and the lower amount of swelling that occurs in the filled
ABS. Scatter in the data is also thought to be related to the
variation in the feedstock diameter which would also occur in the
traces as the samples are processed. Feedstock wire (filament) was
hand-spooled on the spools to be used in FDM and this would lead to
variations in the tensile test properties. Measured tensile
strengths tended to be 50% less than published ABS data which is
listed in Table 1. FIG. 29 shows tensile test results following
correction for the cross-plying where only half of the sample is
effectively tested due to 90.degree./180.degree. cross-plies. Note
again the consistency in the ABS data and the variation in the
VGCF/ABS results. This difference is a 65% increase over the
unfilled ABS, a significant increase even though there was limited
wetting. Table 2 shows the results for the SWNT/ABS material. FIG.
30 shows a micrograph of the polymer composite microstructure where
the as-received SWNTs are not well dispersed. Non-nanotube material
(amorphous carbon and metal catalyst) hinder the shear mixing of
the nanotubes. The improvement in strength is important yet not
critical since the mixing was not homogeneous.
[0127] Various samples tested showed moduli up to a 150% increase
in stiffness. The SWNT/ABS samples saw a 100% increase in
stiffness. A decrease in ductility for the reinforced ABS is
observed where nanofiber reinforced samples had a brittle fracture
with limited signs of ductility, where SEM examinations revealed
that crack propagation had localized yielding restricted basically
to the filament layers oriented parallel to the applied stress.
This localized yielding has the form of craze formation.
[0128] The VGCF/ABS feedstock was analyzed by SEM after the
extrusion process. FIG. 31 shows the longitudinal cross-section of
the wire feedstock. The arrow indicates the axial direction of the
wire. The extrusion high shear condition and the extensional flow
during wire spooling caused alignment of the nanofibers. Shown are
a high degree of aligned VGCFs which are well dispersed and not
bunched up with each other. Since the fibers appear very clean and
the polymer is not highly deformed around the fibers, wetting is
described as poor with a low level of resistant occurring with
fiber pull-out. The fibers have similar lengths to the starting
conditions and are undamaged from the high shear that occurs in the
mixing and extrusion processes. XRD was used to further evaluate
the VGCF alignment and Table 3 shows the results for several
samples of the feedstock evaluated. The existence of the
preferential fiber orientation definitively contributed to strength
enhancement since isotropic samples showed no strength variations
in a previous study.
[0129] The ABS samples consistently showed good inter-trace fusion
as seen in FIG. 32 although it was not ideal. The VGCF/ABS samples
showed a variation in fusion conditions which tended to match the
tensile results and the scatter observed. Shown in FIG. 32 are
samples with six layers,(which were cross-plied
90.degree./180.degree.. Note that interlayer strength is still low
for the VGCF/ABS, in part because the FDM is not optimized in its
process parameters for the lower swelling of the feedstock material
(associated with different thermal properties of the filled
material). Limited porosity is also observed in some cases in the
VGCF/ABS traces which is attributed to the extrusion process and
not to the initial Banbury mixing.
[0130] Optimization of the extrusion process where more composite
material is used, eliminating the use of plain ABS, is expected to
eliminate this defect condition. Looking at the poor fusion sample
in FIG. 32 and FIG. 33 which shows the directions of traces in a
90.degree./180.degree. cross-ply samples is can be shown that the
traces during FDM processing also resulted in conditions of aligned
VGCFs. FIG. 34 shows the two conditions depicted in FIG. 33.
Micrograph (a) shows a trace produced with the axial direction of
the trace shown as the area indicates. Figure (b) shows a condition
where the trace is effectively pointed out of the page so that the
axial direction of the trace is coming out of the page. Note again
the alignment conditions, the dispersion and the absence of
clustering of the VGCFs. Note also the poor wetting observed as
either open space (troughs) around the VGCFs or left on the polymer
surface where VGCFs once were (micrograph a) or as VGCFs easily
pulled away from the matrix showing gaps between the VGCF and the
polymer matrix (micrograph b). Note also that some nanofiber
breakage likely occurred for those aligned with the tensile test
condition (the tensile axis was aligned with the micrograph (b)
condition) since the VGCFs in micrograph (a) are much longer then
many of those seen in micrograph (b). In the case of the
transversely oriented layers it can be observed that failure occurs
mostly by matrix failure. The micron size circular particles are
expected to be the segregated butadiene phase of the ABS. These
features were also observed in plain ABS. The figures further show
the degree of alignment with the direction of the traces. The holes
seen on the sample surfaces are produced from VGCF pull-out and not
from process porosity. Process porosity tended to show surface
morphological differences in the polymer and was only observed on a
few of the early processed samples.
[0131] The conferred effect of the nanofibers on the fracture
behavior of ABS is similar to the fracture behavior of tightly
cross-linked resins where the molecular network is unable to deform
sufficiently. In this case the nanofibers decreased the resistance
to yield acting as constraints for chain mobility. The decrease in
chain mobility increased the stiffness of the material which was
first observed by the differences in swelling on the extrusion
process. The toughness of the composite is therefore lower than
that of the pure ABS but the strength and rigidity are
improved.
[0132] Along with this application in the use of FDM, the material
of the present invention has the potential to provide
multifunctional properties. This is to say that, while the material
is structural, it may also be a thermal management system and or an
electrostatic discharge material (or electromagnetic interference
material). This new material, initially described as NCFs, would be
a new multifunctional material system that could be manufactured
into a range of parts for mechanical, electrical, thermal, or
combined applications. Examples of NCFs are mechanical/electrical,
mechanical/thermal, electrical/mechanical, impact/strength, and
impact/electrical or thermal. NCF can be made with a ceramic
matrix, and there are FDM-like systems (a robocaster) for making
parts with a ceramic matrix.
[0133] The FDM can further be used to make nanofiber continuous
fibers. By starting with the nanofiber composite feedstock (wire or
filament) the FDM can be used to extrude out fiber where high shear
and elongational flow are implemented. In the die the shearing
action takes place and subsequent elongational flow can be achieved
as the material leaves the die. Conitinuous fibers with 10 wt. %
VGCFs and 10 wt. % as-received SWNTs have been processed from a
FDM.
EXAMPLE
Application of Fully Integrated Nanotube Composites for
Multifunctional Applications
[0134] Fully integrated nanotube composites (FINCs) are implemented
as plastic lightweight wires and interconnects. The goal of this
research is to mimic on a larger scale the properties of nanotubes
so that their properties can be used more aggressively on micro and
macro scales. The idea of mimicking nanotubes involves two
concepts. The first is the single wall nanotube (SWNT) integration
by tip attachment (and/or side wall functionalization), coincident
polymerization, and high shear alignment. This path will provide
for hybrid materials that will be designed to expand out the
properties of the single nanotubes by translating their properties
to each other. This is to say, material that might be considered
the matrix must be nanotube like as well. In this system, nanotube
to polymer connections are designed to foster enhanced mechanical
properties. The first step in this path is through mimicking a
polymer (polymer processing) followed by designing an architecture
for mimicking nanotubes. The second concept leading to the goal of
mimicking nanotubes involves identifying the properties of
nanotubes in other systems where those properties can be fostered
for materials enhancement. The starting system is a nanosize
graphitic material similar to SWNTs in that it must be separated
from itself before subsequent processing. Consider that small
flakes of graphite can be separated on the individual flake basis
where no defects occur in the flake (much like for nanotubes). The
flakes would be bonded through available bonds on the outside of
the flakes in a similar way that we would functionalize the tips or
the outside wall of the nanotubes. In short, nanometer size carbon
sheets rather than tubes are identified which mimic nanotubes in a
number of ways. A number of bonding conditions for the
nanotube-like materials translate back to nanotube advanced
materials developments.
[0135] The present disclosure describes the methods to produce a
highly conductive polymeric wire and interconnect system with good
stability of the conduction properties and permeable plastics to
alter conduction by gas interaction.
[0136] Materials produced demonstrated significant reductions in
resistivity with only 10 wt. % SWNTs added to a thermoplastic
polymer system. The key was to gain homogeneous dispersion so that
conduction could occur with an opportunity for structural
enhancement as well. Segregated dispersion of nanotubes would lead
to a low percolation threshold but with little to no structural
enhancement.
[0137] Methods of the present invention provide highly conducting
polymers resembling metallic-like conduction properties such as
that for polyacetylene (PA) but more stable or with higher
environmental stability. Processing from a polymer basis of nylon,
poly(methyl methacrylate) (PMMA), and conducting epoxy develops a
polymer architecture for mimicking nanotubes. Conducting epoxy is a
polymer that can be processed with insulating, semiconducting or
conducting properties. Processing further will enhance its
stability, its conducting properties and provide a high temperature
polymer system. These materials can be used for wire and other
interconnects. Extending their properties for multifunctional use
also enhance the thermal and structural properties of these
material. Gas permeable polymers are considered to alter the
electrical conduction by gas exposure.
[0138] The derivatization or functionalization of SWNTs opens the
way to new approaches in making advanced hybrid materials that are
a cross between polymers and composites. The traditional composite
mixing with carbon fibers is limited in the amount of interaction
and bonding of the nanotubes with the polymer matrix. Sidewall and
tip functionalization provide attachment of different organic
groups to modify and control the bonding and interactions with the
polymer. The purified nanotube materials as currently available,
consist of tangled bundles of SWNTs ropes. Mixing such material
with polymers gives physical linking with the random polymer coils
and the random nanotube tangles becoming intermingled during
mixing. This limits the dispersability of the nanotubes, and the
homogeneity of the resulting composites. Cutting the SWNTs in
oxidizing environments not only gives shorter tube segments, which
are more easily dispersible, but opens the nanotube tips, allowing
for chemical modifications to be made. Sidewall fluorination of the
SWNTs also breaks the ropes so that individual tubes are soluble in
alcohol, and opens the door to the attachment of organic molecules
to the tube wall.
[0139] SWNTs can be derivatized on the open edges, which become
decorated with oxygen groups (hydroxyl, ketone, carboxylic acids)
by sonication treatment in acid. The carboxylic acid group can
react with amines to form amide bonds, which allows the attachment
of compatibilizing groups to increase the interactions with the
polymer matrix. Aliphatic and aromatic chains of different lengths
can be attached and these organic chains can also have polar groups
such as ester, ether, amides, or terminal amino groups, which can
be used to create hydrogen bonding interactions with suitable
polymer matrices. Organic chemistry can be used to modify the
functional groups in the attached molecules, allowing for complete
control of the interactions with the polymer. The left side of FIG.
35 shows the attachment of polymer chains to the oxidized edge of a
cut SWNT via amide linkages. The left part shows the attachment of
an organic molecule that can act as compatibilizer for a polymer
matrix. The center of the figure shows how the attachment of a
reactive molecule (in this case a double bond) can be used as the
starting point of a polymer chain. The polymer chain will be
directly attached to the SWNT tip, as shown in the right part of
the figure.
[0140] The attached organic molecules can also serve as a starting
point for polymerization, and an example is shown in FIG. 35 where
attaching a molecule with a double bond can be used for the radical
polymerization of polyethylene. The polymer chains formed will be
directly attached to the open nanotube tip. During polymerization,
two growing polymer chains may join, creating random length polymer
links between SWNTs. It is also likely that more than one polymer
chain will start from an open nanotube tip. This is expected to
result in the nanotube acting as a crosslinking agent between
polymer chains, as exemplified in FIG. 37, a representation of
crosslinking that may result from random polymerization starting at
SWNT tips.
[0141] Producing different linkages to nanotubes would be like
producing different blocks for block copolymers. In FIG. 38, a
representation of nanotube block co-polymers, envisions activating
the nanotube tips by adding a functional group (x), that will react
with functional groups (y) at the ends of polymer block of a
monomer A. The resulting polymer would be a "nanotube block
copolymer", where the nanotube is an integral part of the
composite, and chemically bonded to the polymer matrix. These
reactions can proceed in a manner analogous to the attachment of
simple organic molecules using amide linkages.
[0142] The polymer blocks can be selected to have different
properties, chemical backbones, reactivity, lengths and length
distributions. Varying the nature and length of the polymer chains
between the nanotubes will allow us to selectively vary properties
of the composite. It is also possible to have a different polymer
block (B) to have further modifications and control of the
properties of the nanotube block copolymer. Different alternations
of the blocks (polymer A, polymer B, nanotube) are possible, as
shown in FIG. 39, two different possible configurations for
nanotube block co-polymers, with alternating polymer blocks.
[0143] Crosslinking can also occur in these cases, and the amount
of crosslinking can be controlled by limiting the amount of reagent
that will create the functional group x on the open nanotube tips.
In this way the average amount of "activated" sites is controlled.
Closed SWNT tips can also be modified by applying the knowledge of
fullerene chemistry which would allow more options for the
attachment of functional groups that will link the polymer to the
nanotube. Another possibility is to use functionalized SWNTs in a
graft copolymerization as shown in FIG. 40, nanotube graft
copolymer with the nanotubes acting as crosslinking agents. The
polymer with a chain containing reactive groups (the amount of
which can be controlled) can be mixed with "activated" nanotubes,
which can be made to bond to the sides of the chain. This approach
can also result in crosslinks, with the nanotubes bridging between
polymer chains. It is also possible to create polymer chains with
nanotubes only on the sides, by controlling the reaction, or the
reactive groups in the nanotube tips. For example, the tips can be
attached to a solid support, the exposed tip "deactivated" and the
other tip derivatized to react with the polymer chain as shown in
FIG. 41, nanotube graft copolymer where only one side of the
nanotube can attach to the polymer chain.
[0144] A second approach is to use sidewall derivatization of
single-walled nanotubes. The sidewalls of the nanotubes can be
fluorinated, and control of the amount of fluorination is possible
(up to a stoichiometry of C2F). The fluorine groups can be
substituted by organic groups, as previously shown for alkyl
chains. Several different terminal organic groups could be attached
to the sidewall, and further modified by organic chemistry
techniques. In the simplest approach, modification of the sidewall
can be used to control the interactions of the polymers and the
nanotubes, making the SWNTs compatible with several polymer
matrices with or without polar groups. The fluorinated and
alkylated SWNTs would also be dispersed as individual tubes to form
stable suspensions in polar solvents (e.g. alcohols), allowing for
easier manipulation, dispersability, as well as doing chemistry on
the sidewalls.
[0145] Having reactive groups on the walls allows for
polymerization to start from the nanotubes, in a way similar to the
one sketched in the right side of FIG. 35. In the case of sidewall
derivatization, however, the nanotube acts as a multifunctional
starter for polymerization, and the resulting polymer nanotube
composite will most likely be heavily crosslinked, possibly
resulting in a thermoset nature of the final product. The expected
structure is sketched in FIG. 42, side-wall attached
polymer-nanotube composite, with random polymerization creating
crosslinking. One of the proposed ways to minimize or control this
random crosslinking could be to deposit (and attach) the nanotubes
on a substrate, such that part of the nanotube wall will not be
covered with polymer chains, as shown in the left panel of FIG. 43.
FIG. 43 illustrates "Hairy-tube composites". Left side: SWNTs over
a substrate, only the exposed surface is covered with polymer.
Right side: Tips of SWNTs attached to a solid support, with the
possibility of controlling the length and crosslinking of the
polymer chains. Another possibility is to use the chemistry of the
tips to attach the nanotubes to solid supports, creating
"hairy-tube composites". The amounts of polymerization initiation
groups in the tube walls can be varied, the probability of
crosslinking can be controlled by spacing the attachment points of
the nanotubes, and by controlling the degree of polymerization. A
concept sketch of this approach is shown in the right panel of FIG.
43. Control of the length of polymer chains attached to the
nanotubes allows control over the properties of the final
composite. Several different polymer chains can be attached this
way, without concern over the adequacy of the solvent for
suspending or dissolving the nanotubes.
[0146] The attachment of the SWNT tips to a solid support can also
be an approach to create nanotube block co-polymers, like those
shown in FIGS. 39 and 40. Polymerization can be started from the
exposed tip, or a polymer block can be attached to it. This
approach can also be used to polymerize alternating monomers, in a
way analogous to the Merrifield solid phase syntheses of peptides
in biochemistry. After detaching the nanotube from the solid
support it is possible to attach another polymer to the unreacted
tip, or to use it to graft the nanotube-block copolymer to side
groups in a different polymer.
[0147] The aligned SWNTs advanced polymers that can be made in this
way are shown in one example in FIG. 45, nanotubes shear oriented
and then chemically bonded to the polymer matrix. Combining
polymerization with high shear alignment methods we can produce a
system analogous to Kevlar or ultra high density polyethylene where
the nanoscale features are highly aligned and therefore lead to
enhanced properties. At this point the idea of mimicking SWNTs
starts to take over. Providing bonding to continue the enhancements
that can be achieved by the nanotubes is the important selection
point for this research.
[0148] It is important to note that sidewall derivatization of the
nanotubes will disrupt the continuity of the graphene sheet, which
will have an effect on the electrical, thermal and mechanical
properties of the SWNTs. Using the tips as anchor points may be a
better choice in several cases. Another important point will be the
feasibility of sorting tubes by length, diameter of type, to ensure
that the tubes used will be of those for promoting electrical
conduction. Mimicking of the nanotubes on larger scales means
designing the chemical attachment to provide for good stability of
properties of the nanotubes. Integration means reducing the
possibility of free matrix (material without integrated SWNTs).
Continuation of this concept leads to the use of gas permeable
plastics. Since the conducting polymers will have a significant
conducting range as to how they are processed, coupling to gas
permeable plastics so that gas detection can occur is possible.
Interactions with gases will alter the conduction providing a
sensing capability to the wire of sheet configurations. Through
SWNT purification, separation, functionalization, and combining
SWNTs with gas permeable polymers, and tailoring conduction and
percolation, these systems may well be used to alter electrical
conduction of the conducting polymers when in contact with various
gases.
EXAMPLE
Shielding for Micrometeroid Protection Developed from Toughened
Fully Integrated Nanotube Composites
[0149] Fully integrated nanotube composites are described above to
mimic on a larger scale the properties of nanotubes so that their
properties can be used more aggressively on micro and macro scales.
The general discussion of the previous section on nanotube tip
attachment, crosslinkink, block co-polymers, graft copolymers, side
wall attachment, and substrates is equally applicable here, and is
incorporated by reference. An example application is shielding for
space applications where extremes of lightweight and strength and
toughness are desired.
[0150] Although composite materials are seen as a methodology for
producing advanced materials with nanotubes, conventional composite
processing may not be the way to go when it comes to ultra high
strength systems. If one considers that epoxies are the matrix of
choice for many composite applications since higher strength
features are usually realized, one must also acknowledge that the
elongation features of the epoxies are significantly lower than
those expected for SWNTs. FIG. 46 shows how one would typically
design a composite system compared to that expected for SWNTs in an
epoxy matrix. Note that for the nanocomposite, of is not the
ultimate strength of the nanotube but .sigma..sub.f(.epsilon.)
since the matrix fails at a much lower strain than the
reinforcement. Where typical reinforcements see similar elongation
capability between the matrix and the fibers, the SWNT system is
unusual because SWNTs are expected to have elongations up to 5
percent. At the onset, a significant portion of the strength
capability of nanotubes appears to be discarded when coupled with
typical epoxy resins. Even so, strength enhancements when using
this approach are projected to be three orders of magnitude greater
than the unfilled polymer when considering rule of mixtures
calculations which are only used here to serve as order of
magnitude, "back of the envelope" calculations. Table 1 shows the
parameters for the nanotubes and matrices used in the various
calculations of mechanical strength. Although the resulting
property enhancements seem high, one should acknowledge that high
strength epoxies that provide ample strength enhancements over the
unfilled epoxy do already exist. Therefore, conventional composites
development are not the way to go since so much of the potential of
the nanotubes is discarded.
[0151] Calculations indicate that the critical length of a nanotube
to get full use for strengthening (based on
.sigma..sub.f(.epsilon.)) should be about six microns, assuming a
fully aligned discontinuous system occurs. Current understanding of
the length of SWNTs is that they are 0.3-0.6 microns in length and
far less than that needed for preferred strength enhancements. Note
however that short fibers do lead to toughness enhancements. Note
also that these calculations assume complete bonding between the
nanotubes and the matrix even though this may not be the case.
Bonding through functionalization is likely to be intermittent
along the nanotube and for unfunctionalized SWNTs is expected to be
solely by van der Waals forces which only becomes significant for
longer nanotubes. One additional consideration to make includes
exploring the fact that a majority of the SWNT composites processed
to date are not aligned systems. FIG. 47 shows by means of the
Kelly-Tyson equation the expected composite strength of a SWNT
composite with off axis alignment. Note the further reduction in
composite strength due to low shear strength and normal stresses.
For these various reasons, extensions in processing with SWNTs to
produce advanced materials must be accomplished by looking beyond
what has been the typical approach of the past. Although the use of
rule of mixture calculations and the Kelly-Tyson equations may not
be ideal or used appropriately for nanotube composites, one gains a
sense that there are significant shortcomings by more conventional
composite manufacturing approaches. Composite strength vs. the
orientation of the SWNTs when considered to be fully aligned. Note
that initially when all fibers are aligned with the applied load
the strength of the composite is rather high. As the fibers become
misoriented with the load (as in the case of an isotropic composite
where SWNTs are randomly dispersed), the composite strength goes
way down due to low shear and normal strength contributions. On one
hand you might use conventional processing to improve the shear and
normal stresses where as the approach of the current invention is
to fully integrate and therefore remove defects that might also
result in the matrix. As to the manner of operation and use of the
present invention, the same is made apparent from the foregoing
discussion. With respect to the above description, it is to be
realized that although an enabling embodiment is disclosed, the
enabling embodiment is illustrative, and the optimum relationships
for the steps of the invention and calculations are to include
variations in size, material, shape, form, function and manner of
operation, assembly and use, which are deemed readily apparent to
one skilled in the art in view of this disclosure, and all
equivalent relationships to those illustrated in the drawings and
encompassed in the specifications are intended to be encompassed by
the present invention.
[0152] Therefore, the foregoing is considered as illustrative of
the principles of the invention and since numerous modifications
will readily occur to those skilled in the art, it is not desired
to limit the invention to the exact construction and operation
shown or described, and all suitable modifications and equivalents
may be resorted to, falling within the scope of the invention.
[0153] What is claimed as being new and desired to be protected by
Letters Patent is as follows:
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