U.S. patent application number 09/840317 was filed with the patent office on 2001-08-16 for hollow polymeric fibers.
This patent application is currently assigned to ZMS, LLC. Invention is credited to Houston, Michael R., Soane, David S..
Application Number | 20010014394 09/840317 |
Document ID | / |
Family ID | 23819864 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010014394 |
Kind Code |
A1 |
Soane, David S. ; et
al. |
August 16, 2001 |
Hollow polymeric fibers
Abstract
The present invention is directed to thermo-expandable fibers
and to the expanded hollow fibers or microtubes, microcellular foam
or foamed composite material that results upon heating the
expandable fibers. The thermo-expandable fiber of the present
invention is characterized by having a polymeric wall surrounding
one or more pockets or particles of blowing agent or propellant
within the fiber. The polymeric wall may have reactive functional
groups on its surface to give a fusible fiber. When the expandable
fibers are heated, they expand to form hollow fibers or microtubes
comprising polymeric shells surrounding one or more internal
gaseous voids, and when the fibers are expanded while in contact
with each other, a microcellular foam may be formed. The foam
consists of a plurality of hollow fibers fused together, optionally
aided by functional groups present on the surface of the heated
fibers that act to crosslink the material. When expandable
microspheres are mixed with a matrix, which can optionally react
with functional groups on the fiber surface, and the resulting
combination is heated, the fibers expand to give a foamed composite
material in which the hollow fibers or microtubes may optionally be
fused or chemically crosslinked to the matrix.
Inventors: |
Soane, David S.; (Piedmont,
CA) ; Houston, Michael R.; (Berkeley, CA) |
Correspondence
Address: |
JACQUELINE S LARSON
P O BOX 2426
SANTA CLARA
CA
95055-2426
US
|
Assignee: |
ZMS, LLC
|
Family ID: |
23819864 |
Appl. No.: |
09/840317 |
Filed: |
April 23, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09840317 |
Apr 23, 2001 |
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09458220 |
Dec 9, 1999 |
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6221486 |
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Current U.S.
Class: |
428/364 |
Current CPC
Class: |
D01F 1/08 20130101; Y10T
428/249921 20150401; Y10T 428/2927 20150115; Y10T 428/2935
20150115; Y10T 428/2975 20150115; D01D 5/247 20130101; C08J 9/04
20130101; Y10T 428/2913 20150115; Y10T 428/249971 20150401; Y10T
428/249972 20150401; C08J 2201/03 20130101; Y10T 428/249962
20150401 |
Class at
Publication: |
428/364 |
International
Class: |
D02G 003/00 |
Claims
What is claimed is:
1. A thermo-expandable plastic fiber comprising a polymeric wall
surrounding one or more pockets of blowing agent within the
fiber.
2. A thermo-expandable fiber according to claim 1 wherein the
polymeric wall comprises reactive functionalities.
3. A thermo-expandable fiber according to claim 1 wherein the
blowing agent is a liquid.
4. A thermo-expandable fiber according to claim 1 wherein the
blowing agent is a solid at room temperature.
5. A thermo-expandable fiber according to claim 4 wherein the
blowing agent is insoluble and is in the shape of a strand or a
fiber.
6. A thermo-expandable fiber according to claim 1 wherein the
polymeric wall comprises an engineering thermoplastic polymer.
7. A thermo-expandable fiber according to claim 1 wherein the
polymeric wall comprises a copolymer, multiblock polymer, or
polymer blend.
8. A thermo-expandable fiber according to claim 1 wherein the
polymeric wall comprises a naturally occurring polymer.
9. A thermo-expandable fiber according to claim 8 wherein the
naturally occurring polymer is selected from the group consisting
of polysaccharides, lipids, and proteins.
10. A thermo-expandable fiber according to claim 8 wherein the
naturally occurring polymer is Zein.
11. A thermo-expandable fiber according to claim 1 wherein the
polymeric wall comprises a polymer and one or more reactive
oligomers or crosslinkable moieties capable of forming a
crosslinked, interpenetrating, or semi-interpenetrating polymeric
network within the polymeric wall.
12. A hollow plastic fiber comprising a polymeric shell surrounding
one or more internal gaseous voids, the polymeric shell comprising
polymer chains that are at least partially radially oriented.
13. A hollow fiber according to claim 12 wherein the hollow fiber
is derived from a thermo-expandable fiber, the thermo-expandable
fiber characterized by a polymeric wall surrounding one or more
pockets of blowing agent, the polymeric wall comprising reactive
functionalities.
14. A hollow fiber according to claim 12 wherein the hollow fiber
is derived from a thermo-expandable fiber, the thermo-expandable
fiber characterized by a polymeric wall comprising a polymer and
one or more reactive oligomers or crosslinkable moieties capable of
forming a crosslinked, interpenetrating, or semi-interpenetrating
polymeric network within the polymeric wall.
15. A hollow fiber according to claim 12 wherein the polymeric
shell comprises an engineering thermoplastic polymer.
16. A hollow fiber according to claim 12 wherein the polymeric
shell comprises a copolymer, multiblock polymer, or polymer
blend.
17. A hollow fiber according to claim 12 wherein the polymeric
shell comprises a naturally occurring polymer.
18. A hollow fiber according to claim 17 wherein the naturally
occurring polymer is selected from the group consisting of
polysaccharides, lipids, and proteins.
19. A hollow fiber according to claim 17 wherein the naturally
occurring polymer is Zein.
20. A microcellular foam comprising expanded hollow fibers fused to
each other, each hollow fiber comprising a polymeric shell
surrounding one or more internal gaseous voids, the polymeric shell
comprising polymer chains that are at least partially radially
oriented.
21. A microcellular foam according to claim 20 wherein the hollow
fibers are derived from thermo-expandable fibers, the
thermo-expandable fiber characterized by a polymeric wall
comprising reactive functionalities.
22. A foamed composite material comprising expanded hollow fibers
fused to a surrounding matrix, each hollow fiber comprising a
polymeric shell surrounding one or more internal gaseous voids, the
polymeric shell comprising polymer chains that are at least
partially radially oriented.
23. A foamed composite material according to claim 22 wherein the
hollow fibers are derived from thermo-expandable fibers, the
thermo-expandable fiber characterized by a polymeric wall
comprising reactive functionalities.
24. An insulating material comprising a plurality of expanded
hollow fibers, each hollow fiber comprising a polymeric shell
surrounding one or more internal gaseous voids, the polymeric shell
comprising polymer chains that are at least partially radially
oriented.
25. An insulating material according to claim 24 wherein the hollow
fibers are fused to each other.
26. A non-woven fabric comprising expanded hollow fibers fused to
each other, each hollow fiber comprising a polymeric shell
surrounding one or more internal gaseous voids, the polymeric shell
comprising polymer chains that are at least partially radially
oriented.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the field of plastic fibers, more
specifically to the fields of expandable, fusible, and hollow
plastic fibers or microtubes, and composite materials produced
therefrom.
BACKGROUND OF THE INVENTION
[0002] In the current state of the art, the production of hollow
fibers is greatly hindered by the required extrusion process using
an annular die, which makes the production of fibers having a small
cross-sectional area very difficult. Such processes are also
sensitive to the polymer composition being extruded, limiting the
variety of compositions available for practice. Hollow fiber
production is further limited because hollow fibers are especially
prone to breakage, rupture, or other defects during any draw-down
(spinning) process, which further limits the size, geometries, and
other physical properties typically achieved during a conventional
spinning process.
[0003] Another limitation of the current art is the control (or
lack thereof of the polymer molecule orientation in the final
hollow fiber. In conventional hollow fiber production, the polymer
molecules become oriented, at least partially, in the longitudinal
direction by the extrusion and/or spinning process. While this may
benefit the strength of the fiber in the longitudinal direction, it
actually can degrade other properties such as its resistance to
collapse, crushing, fraying , crimping or other failure modes of
the fiber. Thus, a technique capable of providing or imparting some
degree of radial orientation to the polymer molecules in the shell
wall of the fiber is desirable. Such radial orientation of the
polymer chains is not currently achievable in the current art, yet
would provide the hollow fibers with heretofore unattainable
properties, even if only partial radial orientation could be
achieved.
SUMMARY OF THE INVENTION
[0004] The present invention is directed to thermo-expandable,
thermoplastic or thermosettable fibers, their method of production,
and the microtubes or hollow fibers that result upon heating said
thermo-expandable fibers. The thermo-expandable fibers of the
present invention are characterized by having a polymeric wall
surrounding a core of liquid or solid blowing agent or propellant
within the fiber. The fibers may be cylindrical in shape, having a
circular cross-section. Or they may have other various geometries,
such as oval, star-shaped, or triangular cross-sections. The
"fibers" may even possess a cross-section that has a large aspect
ratio so that they resemble a sheet. The fibers may additionally be
designed to possess multiple, parallel cores instead of a single
core. The fibers themselves can be singular and independent from
one another, or they may be agglomerated together and pressed flat
to form non-woven sheets or membranes consisting of many fibers. In
one embodiment of the invention, the expandable fibers of the
present invention are also fusible or crosslinkable with each other
and/or with a surrounding matrix material in which the fibers are
mixed.
[0005] The terms "thermo-expandable fiber" and "expandable fiber"
as used herein and in the appended claims, mean a strand that is
many times longer than it is wide and is capable of increasing in
size upon heating due to the formation of one or more continuous or
segmented voids or bubbles in the interior of the fiber. The terms
"hollow fiber" and "microtube" as used herein and in the appended
claims, mean a hollow fiber defined by having a polymeric shell
wall surrounding one or more continuous or segmented internal,
gaseous voids. The term "fusible", as used herein and in the
appended claims, means able to fuse together into an agglomerated
or connected mass.
[0006] The present invention discloses a unique approach that
overcomes the drawbacks of commercially established processes for
the production of microtubes or hollow fibers. It is unique in that
it uses only physical processes and solution thermodynamics to
create novel expandable fibers. The expandable fibers in turn form
the hollow fibers or microtubes upon heating and/or
depressurization. Polymerization is an optional but not requisite
step in the fiber formation process of this invention. The process
is also unique in that any pre-polymerized material having a
suitable solvent may be used to form the fiber shell walls,
irrespective of the polymerization technique used to synthesize the
polymer. By selecting functional polymers that have reactive sites
within the polymer chain, and/or by incorporating crosslinking
agents into the polymer walls, expandable fibers may be readily
produced which are fusible or crosslinkable with adjacent fibers or
with a matrix material in which the fibers have been incorporated.
For these reasons and others that will become clear, the present
invention is an extremely economical process suitable for mass
production.
[0007] In one embodiment of the process of the invention, a
polymer, co-polymer, or polymer blend is solvated by an appropriate
solvent, and combined with an inert liquid, which serves as a
blowing agent or propellant. Optionally, crosslinkers, catalysts,
plasticizers, stabilizers, pigments, and other desirable additives
may be added to the mixture. Fiber spinning proceeds by extruding
or ejecting the mixture through an orifice into air or a second
immiscible liquid to produce strands. Either by evaporation or
liquid-liquid extraction, the solvent is then removed from the
strands, precipitating the polymer from solution and effectively
solidifying the strands into hard-walled expandable polymeric
fibers containing liquid blowing agent cores and other optional
additives.
[0008] In another embodiment of the process of the invention, a
polymer, co-polymer, or polymer blend is solvated by an appropriate
solvent, and is combined with a solid that produces a gas upon
heating. Optionally, crosslinkers, catalysts, plasticizers,
stabilizers, pigments, and other desirable additives may be added
to the mixture. Fiber spinning proceeds by extruding or ejecting
the mixture through an orifice into air or a second immiscible
liquid to produce strands. Either by evaporation or liquid-liquid
extraction, the solvent is then removed from the strands,
precipitating the polymer from solution and effectively solidifying
the strands into rigid-walled expandable polymeric fibers
containing blowing agent cores that are solid at room
temperature.
[0009] Utilizing either method, the final product is a fiber that
is dispersible, residue-free, thermoplastic or thermosetting, and
expandable. The polymer, co-polymer, or polymer blend may be chosen
from any existing polymers, provided there exists a suitable
solvent capable of dissolving said polymer. The fibers may also
beneficially possess reactive functionalities, either built into
the polymer chains or added to the formulation in the form of
crosslinking or other reactive groups, that allow the fibers to
fuse to each other or to a surrounding matrix upon expansion into
hollow fibers or microtubes.
[0010] Expansion of the fibers of the invention into hollow
microtubes occurs with the reduction in pressure, application of
heat, or some other triggering energy such that the pressure
differential between the inside and outside walls of the fiber(s)
is great enough to expand the fiber walls to form a hollow fiber.
For example, in one preferred embodiment of this invention, a
liquid propellant trapped inside a fiber may be heated sufficiently
to generate a vapor pressure capable of expanding the fiber walls
outward, thereby producing a hollow fiber.
[0011] This invention makes possible a broad selection of fiber
compositions. It may be used to obtain hollow fibers made from
conventional thermoplastics, thermosets, elastomers, naturally
occurring polymers, engineering thermoplastics, or mixtures of
these or other polymers. The polymer and propellant may further be
chosen to give a wide range of blowing temperatures at which the
polymer softens and the fibers expand to produce hollow fibers.
[0012] These and other benefits of the invention will be made
apparent in the detailed description of the invention that
follows.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The present invention is directed to the production of
expandable fibers capable of expanding into hollow fibers and,
optionally, fusing with adjacent hollow fibers or the surrounding
matrix upon the application of heat. These expandable, optionally
fusible fibers are synthesized by using purely physical processes.
The present invention is unique in that it uses commercially
available, pre-polymerized materials in a physical process governed
solely by solution thermodynamics for the economical manufacture of
expandable and optionally fusible fibers.
[0014] A. Formulation
[0015] I. With Liquid Blowing Agents
[0016] Polymeric fibers containing liquid blowing agents are
produced by first forming a solution consisting of the polymer that
is to form the hollow fiber shell walls and a suitable solvent. The
wall-forming polymer may be a thermoplastic, a thermoset, an
elastomer, or a mixture of polymers, optionally mixed with
cross-linkers (i.e., thermosettable precursors), plasticizers, or
other desirable additives. A suitable solvent is one capable of
dissolving the polymer to form a polymer solution that is
homogeneous on a macroscopic scale and is free-flowing. The polymer
may be solvated to a concentration between about 0.5 and 95 wt %.
More preferably, the polymer is solvated at as high a concentration
as possible where the mixture may still be extruded at the desired
fiber size and geometry, and at a suitable rate. Typically, less
concentrated solutions (i.e., less polymer) will allow for faster
extrusion and smaller diameter fibers due to their lower viscosity.
More concentrated mixtures have the advantage of providing a higher
solution viscosity and requiring less solvent removal. For
practical application, these trade-offs usually result in a
solution having between about 20% and 60% polymer, although
exceptions will exist for certain formulations. When the fibers are
drawn at elevated temperatures, the solvent usage can be greatly
minimized.
[0017] In accordance with the present invention, an inert liquid
blowing agent (or propellant) is also added to the solvated polymer
solution. By "inert" it is meant that the blowing agent does not
chemically react with either the polymer solvent or the polymer
during the normal course of fiber formation and processing. The
blowing agent is selected such that it is miscible with the polymer
solvent, but incompatible with the pure polymer, i.e., it does not
act as a solvent for the polymer. The blowing agent is also
typically selected such that it disperses in the polymer-solvent
solution with no phase separation up to its desired concentration.
The blowing agent is further chosen such that it produces a vapor
pressure sufficient to expand the polymer walls at or above the
temperature at which the polymer softens.
[0018] The concentration of blowing agent in the polymer-solvent
solution is chosen according to the desired amount of fiber
expansion, i.e. the desired aspect ratio of the void inside the
fiber versus the fiber diameter. The blowing agent concentration
must be high enough that the desired amount of expansion is
achieved upon vaporizing the blowing agent. However, the blowing
agent concentration must be low enough that the polymer is able to
form a coherent wall around the blowing agent within the fiber. The
polymer wall must also be sufficiently thick so as to withstand,
without rupture, the wall thinning which accompanies expansion. It
is recognized that the concentration of the blowing agent necessary
to provide a given amount of expansion will depend on the volume
change upon vaporization of the liquid propellant, as well as the
resistance to stretching exhibited by the polymer. It is also
recognized that some of the polymer solvent may remain entrapped
within the dried fiber along with the propellant itself, which will
act to provide additional expansion in conjunction with the liquid
blowing agent. Therefore, exact blowing agent concentrations
providing a given amount of expansion must to some extent be
empirically determined and optimized for each system in order to
achieve the best possible performance. Methods for doing so are
known in the art and do not require undue experimentation. For the
purposes of this invention and with these constraints in mind, the
blowing agent concentration, as given by the ratio of its weight to
the weight of polymer added to the solution, is preferably within
the range of 1 to 200%. More preferably, the blowing agent to
polymer weight ratio shall be between 2 and 100%, and most
preferably between 5 and 100%.
[0019] Thus, the solution used to produce expandable, optionally
fusible fibers containing liquid blowing agents in accordance with
this invention comprises the following components: a polymer or
polymer mixture that is to form the fiber walls, a liquid blowing
agent or propellant that creates a vapor pressure sufficient to
expand the fiber walls upon heating, optional additives to promote
crosslinking or other desirable properties, and a solvent that
solvates both the polymer and the liquid propellant to form a
homogeneous, free-flowing solution.
[0020] There are also systems where the solvent may serve the role
of an expanding agent. Alternatively, where small-molecule
crosslinkers or precursors are added, the system may not need a
solvent because the reactive diluents may lower the mixture
viscosity sufficiently to allow fiber spinning without the need for
further viscosity reduction or compatibilization.
[0021] II. With Solid Blowing Agents
[0022] Polymeric fibers having blowing agent cores that are solid
at room temperature are produced by first forming a solution
consisting of the polymer that is to form the hollow fiber
microtube shell walls and a suitable solvent. The wall-forming
polymer may be a thermoplastic, a thermoset, an elastomer, or a
mixture of polymers, optionally mixed with cross-linkers (i.e.,
thermosettable precursors), plasticizers, or other desirable
additives. A suitable solvent is one capable of dissolving the
polymer to form a polymer solution that is homogeneous on a
macroscopic scale and is free-flowing. The polymer may be solvated
to a concentration between about 0.5 and 95 wt %. More preferably,
the polymer is solvated at as high a concentration as possible
where the mixture may still be extruded at the desired fiber size
and geometry, and at a suitable rate. Typically, less concentrated
solutions (i.e., less polymer) will allow for faster extrusion and
smaller diameter fibers due to their lower viscosity. More
concentrated mixtures have the advantage of providing a higher
solution viscosity and requiring less solvent removal. For
practical application, these trade-offs usually result in a
solution having between about 20% and 60% polymer, although
exceptions will exist for certain formulations. When the fibers are
drawn at elevated temperatures, the solvent usage can be greatly
minimized.
[0023] In accordance with the present invention, an inert solid
blowing agent (or propellant) is also added to the solvated polymer
solution. By "inert" it is meant that the blowing agent does not
chemically react with either the polymer solvent or the polymer
during the normal course of fiber formation and processing. The
blowing agent may be insoluble in the polymer solvent, in which
case it should be in the form of a finely divided powder, strands,
or fibers having a sufficiently small particle size. In this case,
efficient dispersion of the solid propellant may be aided by first
treating the propellant with a surface-active agent before mixing
with the polymer solution, or by adding surfactants into the total
mixture. Ultrasonic agitation can also be used to speed up
dispersion. Alternatively, the blowing agent may be soluble in the
polymer solvent such that it dissolves in the polymer-solvent
solution with no phase separation up to its desired
concentration.
[0024] The blowing agent is further chosen such that it produces a
vapor pressure sufficient to expand the polymer shell walls at or
above the temperature at which the polymer softens. The vapor
pressure generated upon heating may be caused by the evaporation or
sublimation of the propellant (physical blowing agent), or may be
generated by a thermally-induced chemical decomposition of the
propellant (chemical blowing agent), in which case a sufficient
quantity of gas is produced by the decomposition reaction to expand
the fiber walls.
[0025] The concentration of blowing agent in the polymer-solvent
solution is chosen according to the desired amount of fiber
expansion, i.e. the desired aspect ratio of the void inside the
fiber versus the fiber diameter. The blowing agent concentration
must be high enough that the desired amount of expansion is
achieved upon vaporizing the blowing agent. However, the blowing
agent concentration must be low enough that the polymer is able to
form a coherent wall around the blowing agent within the fiber. The
polymer shell wall must also be sufficiently thick so as to
withstand, without rupture, the shell wall thinning which
accompanies expansion. It is recognized that the concentration of
the blowing agent necessary to provide a given amount of expansion
will depend on the volume of gas produced upon vaporization or
decomposition of the propellant, as well as the resistance to
stretching exhibited by the polymer. It is also recognized that
some of the polymer solvent may remain entrapped within the dried
fiber along with the propellant itself, which will act to provide
additional expansion in conjunction with the solid blowing agent.
Therefore, the exact blowing agent loading level which provides a
given amount of expansion must to some extent be empirically
determined and optimized for each system in order to achieve the
best possible performance. Methods for doing so are known in the
art and do not require undue experimentation. For the purposes of
this invention and with these constraints in mind, the blowing
agent concentration, as given by the ratio of its weight to the
weight of polymer added to the solution, is preferably within the
range of 1 to 200%.
[0026] Thus, the solution or mixture used to produce expandable,
optionally fusible fibers in accordance with this invention
comprises, in another embodiment, the following components: a
polymer or polymer mixture that is to form the hollow fiber shell
walls, a blowing agent or propellant that generates a vapor
pressure sufficient to expand the fiber walls upon heating (said
blowing agent or propellant being normally a solid at or near
ambient pressure and temperature), optional additives to promote
crosslinking or other desirable properties, and a solvent that
solvates the polymer (and optionally the solid propellant) to form
a free-flowing mixture.
[0027] B. Fiber Formation
[0028] In accordance with this invention, expandable, optionally
fusible fibers are manufactured by extruding or otherwise ejecting
the solution or mixture from Section A above through an orifice
having the desired cross-sectional geometry. The purpose of this
step is to form strands of the solution having the desired
cross-sectional shape. The strands may be nominally continuous or
may be chopped or pinched at regular intervals to produce fibers of
a desired length. The fibers may be spun through a single orifice
or may be ejected through multiple orifices to produce a plurality
of fibers concurrently.
[0029] By controlling the orifice size, the viscosity of the
solvent-polymer-propellant solution, and the extrusion rate of the
solution, the fiber diameter may be closely regulated. In
particular, by extruding solutions with moderate to low viscosities
through a small orifice, very small fibers may be produced, which
fibers are not easily produced by conventional means. The fibers
may further achieve small dimensions by virtue of their resistance
to rupture or breakage. Because the fibers are solid (not hollow)
upon initially exiting the orifice, the fibers may be drawn
(stretched) to an even smaller diameter than that dictated by the
orifice opening. Once the ejection and draw-down process have been
substantially completed, the fiber may then be expanded to create a
hollow, tubular geometry. This is a significant advantage over
conventional technology because the limitations of the annular
extrusion process and difficulties associated with the draw-down of
hollow fibers severely limit the economical production of
small-diameter hollow fibers.
[0030] Extruding the polymer mixture into strands provides an
efficient avenue for the removal of the solvent from the mixture by
evaporation or extraction. As the solvent is removed from the
strands, phase separation occurs whereby the polymer no longer
stays in solution within each strand. Because solvent removal
occurs at the strand surface during drying or extraction processes,
the polymer will typically phase separate at the strand surface
first, creating a thin polymer wall surrounding the core.
Alternatively, the polymer may precipitate within the strand and
migrate to the strand surface. Further drying occurs as the
remaining solvent in the strand interior diffuses through the
polymer wall to the surface and evaporates or is extracted.
Diffusion of the solvent through the polymer occurs readily in this
case because the solvent is compatible with the polymer, and
because the radial diffusion distance in the strand is short.
[0031] Where the blowing agent is a liquid, the agent is
essentially trapped within the strand during drying due to its
incompatibility with the polymer, which greatly hinders its
permeability through the polymer shell wall.
[0032] Where the blowing agent is a solid, the agent does not leave
the strand to any appreciable degree during drying due to the
non-volatility of the solid blowing agent. In the case where a
polymer solvent-soluble solid propellant is used, the solid
propellant will precipitate out of solution as the solvent leaves
the strand. In the case where the solid propellant is insoluble in
the polymer solvent, the solid particles will be initially coated
with the polymer solution and a polymer coating will be left behind
as the solvent is dried or extracted. The use of
cylindrically-shaped solid propellants in this case will facilitate
their orientation parallel to the direction of fiber extrusion.
Thus, drying or extraction of the polymer solvent leads to the
formation of a polymeric wall. When most of the solvent has been
removed, the final product consists of a polymer wall surrounding a
continuous series of pockets or particles of the solid blowing
agent.
[0033] In one embodiment of this invention, the propellant may
exist as a strand itself. For example, a cloth thread may be
impregnated with conventional liquid and/or solid propellants to
produce a continuous thread containing the blowing agent(s). This
thread may then be coated with the polymer solution by extrusion
techniques similar to those used for wire coating applications.
Upon evaporating the solvent, the resultant product is a fiber
consisting of a polymer wall surrounding a blowing agent core,
which may subsequently be triggered to expand and form a hollow
fiber.
[0034] The extent to which additional components remain in the
strands during solvent removal will depend on their volatility,
compatibility with the polymer and blowing agent, and diffusivity
in the polymer, among other things. Additives such as crosslinkers,
catalysts, plasticizers, pigments, etc., will generally remain
within the strand upon solvent removal since these agents typically
have low volatilities.
[0035] In a presently preferred embodiment of this invention, the
formation of strands is achieved by ejecting the polymer solution
through a circular orifice into a drying gas atmosphere where
solvent removal proceeds by simple evaporative drying. A particular
advantage of this embodiment is that the strand is automatically
cooled as the solvent evaporates due to the latent heat of
vaporization associated with the solvent phase change. Thus, heated
gases may be used to facilitate fiber drying without unduly raising
the fiber temperature and thereby expanding the fibers prematurely.
Alternatively, fiber expansion may be desirably triggered in
conjunction with the solvent removal process. For instance, if a
sufficiently hot gas stream is flowed over the strands after
extrusion (and after or during an optional draw-down process), or
an initially hot mixture is extruded out of an orifice, the solvent
will be evaporated from the strands, and the strands will be
sufficiently heated to trigger the blowing agent. In yet another
embodiment, the mixture may be extruded from a state of high
pressure through an orifice to a state of low pressure, where the
pressure change will aid in the expansion of the fiber after
exiting the orifice. By judicial choice of the solvent, polymer,
and blowing agent combination, the system may be designed to expand
while a significant fraction of solvent remains within the strand,
which can be advantageously used as a plasticizer for the polymer
wall to facilitate the wall expansion. Further heating will remove
the rest of the solvent, yielding a hollow fiber produced in a
single, continuous, economical process.
[0036] Strand formation may be alternatively accomplished by
introducing the polymer-propellant-solvent solution into a second,
immiscible liquid in which the polymer and propellant are
immiscible and the polymer solvent is only slightly soluble. The
extrusion of the polymer solution occurs just as described above,
with optional draw-down of the strands to produce small diameters
as desired. The second liquid shall be chosen such that it is not a
solvent for the polymer, and is somewhat incompatible with the
solvent such that the overall polymer solution remains as discrete
strands with the second liquid. The second liquid must, however,
provide a reasonable solubility for the polymer solvent such that
the polymer solvent is extracted from the strands in a manner
analogous to evaporative drying. That is, as the strands make
contact with the second, immiscible liquid, the polymer solvent is
extracted from the strands at their surfaces.
[0037] Once sufficient solvent has been removed, the polymer will
phase separate and form a polymer wall at the strand surface, as in
the case of evaporative drying. Further extraction of the solvent
through the polymer wall results in fibers composed of a polymer
wall surrounding, in one embodiment, the liquid blowing agent. The
liquid propellant will not be substantially extracted into the
second liquid due to its incompatibility and low permeability in
the polymeric walls. In another embodiment, removal of the solvent
results in fibers composed of a polymer wall surrounding a
continuous series of pockets or particles of the solid blowing
agent. The solid propellant will not be substantially extracted
into the second liquid due to its extremely low permeability in the
polymeric walls. The extent to which additional components remain
in the strands during solvent extraction will depend on the their
diffusivity in the polymer and compatibility with the polymer, the
blowing agent, and the second liquid medium, among other things.
Additives such as crosslinkers, catalysts, plasticizers, pigments,
etc., will generally remain within the strand upon solvent removal
since these agents are typically chosen to be compatible with the
polymer or polymer solvent, and diffusion constraints will limit
their extraction into the second liquid even if compatibility
exists.
[0038] This second liquid may be advantageously selected to be
water, as many of the known polymer solvents are immiscible with
and only slightly soluble in water. Other examples include
hydrophobic liquids such as fluorocarbons and silicone fluids. One
knowledgeable in the art will be able to select other liquids which
meet these criteria, and which will advantageously perform in the
manner described above in accordance with this invention.
[0039] In order to achieve sufficient pressure within the fiber to
expand the polymer walls, it may be advantageous to pinch off the
fiber at its ends so that the propellant is effectively sealed
inside the fiber. For this purpose the strand may be pinched at
each end after extrusion from the orifice, either during or after
solvent removal. Alternatively, the strand may, at regular
intervals, be crimped only enough to seal off the internal
propellant cavities from each other, but not so much as to actually
break the fiber. This would have the effect of producing a fiber
that has a series of propellant pockets sealed off and independent
from one another, while the fiber remains a single, continuous
strand.
[0040] C. Hollow Fiber or Composite Formation
[0041] The hollow fibers or microtubes of the invention are
prepared by heating the thermo-expandable fibers, either by
themselves or admixed with a matrix resin or other binder
composition. Upon heating, the polymeric wall material of the fiber
softens and stretches under the expansion force created by the
blowing agent, forming a hollow fiber or microtube characterized by
one or a series of gaseous interior voids surrounded by a polymer
shell. Having gone through this expansion, the hollow fibers are
larger in cross-sectional area than the unexpanded fibers and have
a lower true density.
[0042] In one embodiment of the present invention, before, during,
or after microtube formation (i.e., fiber expansion), the shell
walls of the microtubes join with one another (melt or fuse
together) to form a semi-continuous sheet or foam. Upon cooling,
the microtube walls harden to give a microcellular foam, said foam
being characterized by a plurality of fused hollow fibers. Such a
foam can be formed that is at least two to three times the original
volume of the unexpanded fibers. Alternatively, a given geometry
can be produced where, when produced using the hollow fibers, the
overall weight is less than if the equivalent geometry were
produced with conventional solid (non-hollow) fibers.
[0043] Such a foam, consisting of fused microtubes, will exhibit
voids associated with the microtube interiors. Additional voids may
exist in the extra-cellular regions where the microtube shells do
not merge completely. While the voids associated with the microtube
interiors will be singular and discrete (i.e., generally not
connected to each other), the voids attributable to the
extracellular regions may be interconnected to the extent that a
semi-continuous void structure is formed. Fusing of the microtubes'
walls may occur by simple physical means (in the case of
semi-molten surfaces coming into contact), or may occur with the
help of crosslinking reactions between the adjacent hollow
fibers.
[0044] The unexpanded fibers may be blown, sprayed, dusted, or
otherwise spread onto curved or flat surfaces, stamped into
cavities or molds, into tubes or pipes, or otherwise into
difficult-to-reach places. Once in place, the fibers may be heated
to create a microcellular foam which fills in the voids within a
given constrained space to give the foam in a desired geometry. The
expandable fibers may also be expanded independent of one another
and subsequently bunched together to give an ultra-low-weight
insulating material. A layered sheet or bunch of such fibers will
provide a material with desirable properties like thermal
insulation, acoustic damping (sound insulation), and vibration
damping (mechanical insulation), among others, while contributing
relatively little to the overall weight of the final article. Such
materials may find beneficial use in apparel, construction,
automotive, aerospace or other industries.
[0045] A particularly significant feature of the hollow fibers
produced by the present invention is that such hollow fibers have
beneficial properties compared to the hollow fibers produced by
conventional means. The expansion process is significant because
the resulting polymeric shell walls of the hollow fibers will
consist of biaxially oriented polymer chains. That is, the polymer
chains in the shell walls of the hollow fibers produced by this
invention will not only be oriented longitudinally by the
extrusion/draw-down process (as with conventionally produced hollow
fibers), but will also be oriented radially due to the radial
stretching accompanying the expansion process. Conventional hollow
fibers that are produced in the current art by extrusion through an
annular die and subsequent draw-down will necessarily have polymer
chains that are uniaxially oriented in the longitudinal direction.
Such hollow fibers will not have any radial orientation among the
polymer chains making up the shell wall. This is an important
feature of the articles produced by the presently disclosed
invention because hollow fibers or microtubes made from polymer
chains which are at least partially oriented in the radial
direction can be much stronger and much less prone to crushing,
kinking, collapse, fraying or other failures or defects than their
uniaxially-oriented counterparts.
[0046] Alternatively, in another embodiment of the present
invention, when the fibers are expanded within a surrounding
matrix, the fiber polymeric walls may react with the matrix
formulation to create a shell wall that is fused with the
surrounding matrix. The result is a microcellular, foamed composite
material where the voids within the resin are individual microtubes
having walls that are integrally bonded with the surrounding
matrix. Thus, conventional composites that use both solid
(non-hollow) fibers for strength and expandable microspheres for
density reduction as two separate additives may now be produced
with only a single additive - expandable fibers. Furthermore, by
utilizing fusible, expandable fibers that react with the matrix
material, an overall stronger composite material may be produced
because bonding with the matrix can eliminate delamination between
the resulting hollow fibers or microtubes and the matrix resin.
Such bonding may also help prevent crack initiation within the
resin, or may help mitigate crack propagation once a crack tip
develops.
[0047] Alternatively, the matrix resin may be thought of as a
binder that bonds to or holds together the expanded microtubes to
give a foamed composite material. The resulting composite may or
may not contain voids outside the microtubes, depending on the
nature of the matrix resin and the mixing conditions used. Such
composites may be thought of as non-woven fabrics or sheets that
provide the strength of traditional fabrics, and have the added
benefit of a low-density material with good insulation properties
that is strong, lightweight, yet is so produced without the
time-consuming weaving process.
[0048] D. Material Selection
[0049] The polymers that may be used to form expandable fibers in
accordance with this invention are numerous. In short, any polymer
or polymer mixture for which there is a suitable solvent or solvent
mixture, and which softens and is at least slightly stretchable
upon the application of heat, may be formed into an expanding,
optionally fusible fiber using the techniques provided by this
disclosure. Examples of the polymers which may be used include
homopolymers such as, but not limited to, polystyrene
(.alpha.-methyl, brominated), polybutadiene, poly(meth)acrylates,
poly(meth)acrylic acids, poly(meth)acrylamides,
poly(meth)acrylonitrile, polyethylene (propylene or butylene),
polyesters, polyolefins, polyvinylidene fluoride or chloride,
polyvinyl alcohol, polyvinyl acetate, polyvinyl ether, polyethylene
terephthalate (PET), polybutylene terephthalate (PBT), or blends or
copolymers of these or other homopolymers thereof. Such polymers
enabled by the technology disclosed in the present invention may be
blocky (diblock, triblock, or multiblock), alternating or random
copolymers, terpolymers, starpolymers, etc., such as
styrene-butadiene (SBR), styrene-acrylonitrile,
butadiene-acrylonitrile, styrene-maleic anhydride (SMA),
ethylene-(meth)acrylic acid, acrylonitrile-butadiene-styrene (ABS),
and other copolymers or blends of polymers and/or copolymers
thereof.
[0050] Naturally occurring polymers, such as polysaccharides (e.g.,
celluloses, modified celluloses, starches, chitin, chitosan, etc.),
lipids, or proteins or other polypeptides for example, may also be
used in accordance with the present invention. This class of
polymers is significant in that they are not man-made, but rather
are obtained from renewable resources, and are generally not
reproducible by synthetic means. Another benefit of naturally
occurring polymers is their inherent biocompatibility and
biodegradability, properties not often achieved with conventionally
polymerized thermoplastics.
[0051] Another very important class of polymers which becomes
available for use in the production of expandable fibers by the
present invention is the class of polymers known as engineering
thermoplastics. These polymers have a special significance because
of their high dimensional stability, good chemical resistance, good
impact strength, high strength at elevated temperatures, and other
superlative engineering properties, which may be advantageously
incorporated into hollow fibers or microcellular foams for further
property enhancement. Examples of such thermoplastics enabled for
use by the present invention include, but are not limited to,
polymers such as nylon, polycarbonate, polyamide, polysulfone
(polyethersulfone, polyphenylsulfone, polyphenylene ether-sulfone,
etc.), polyetherimide, polyketone, polyetherketone, and other
engineering thermoplastics thereof.
[0052] Polymers which contain one or more reactive functionalities
built into the polymer chains will be particularly beneficial in
the practice of this invention because such groups can react with a
surrounding matrix or can react with suitable crosslinkers to more
effectively fuse together the microtubes walls with any adjacent
constituents. Such reactive groups could be, for example, alcohols,
anhydrides, vinyls, amines, caboxylates, sulfhydryls, aldehydes,
epoxies, etc. Specific polymer examples include styrene-maleic
anhydride co-polymers (maleic anhydride functionalities),
hydrolyzed poly vinyl acetates/poly vinyl alcohol (hydroxyl
functionalities), polyethylene imines (primary, secondary, and
tertiary amine functionalities), and naturally occurring polymers
such as the celluloses and proteins (hydroxyl, carboxyl, amine,
sulfhydryl, and other functionalities). The reactive
functionalities enable the bonding or reaction of the microtube
shell walls to an appropriately chosen surrounding matrix or to
each other. Alternatively, two types of fibers can be manufactured,
one possessing one type of functionality (primary or secondary
amines, for example) and another possessing a different type of
functionality (anhydrides, for example). The two types of fibers
may then be mixed with each other by simple mechanical means. When
this mixture is heated, the microtubes so produced will fuse and
react with adjacent microtubes having the opposite functionality,
forming a crosslinked microcellular foam.
[0053] A particular fiber composition which may be advantageously
exploited by the present invention is one in which the fiber walls
are comprised of one or more polymers mixed with one or more
reactive components. The reactive components may be of the monomer,
crosslinker, reactive oligomer, or oligomeric crosslinker types.
Such reactive components may be mono- or multi-functional, having
one or more reactive groups per entity. They may be incorporated
into the fiber wall material in order to provide crosslinking
between the polymer chains, or to form an interpenetrating polymer
network (IPN) or semi-interpenetrating polymer network
(semi-IPN--produced by the polymerization of the reactive
components primarily with themselves) within and/or across the
microtube walls to adjacent microtubes. The reactive components may
also be used to provide reaction with and bonding to the
surrounding matrix.
[0054] By adding small quantities of thermal or photo-initiators,
catalysts or other synergists, the crosslinking reaction or IPN or
semi-IPN formation may be designed to proceed during or after the
heating and expansion of the fibers. For example, a thermal
initiator may be used which activates crosslinking at a temperature
about equal to or higher than the fiber expansion temperature, such
that the fiber shell walls crosslink only during or after
expansion. Photo-initiators may also be incorporated, which make
the crosslinking reactions mostly independent of the temperature
and allow them to proceed only when the fibers are exposed to a
source of polymerizing energy either before or after expansion has
taken place.
[0055] The advantages of incorporating reactive components into the
polymeric wall material are primarily attributable to the formation
of a crosslinked polymer shell wall (i.e., thermoset formation).
For some applications thermosetting polymers have more desirable
properties than thermoplastic-type polymers, including increased
dimensional stability, high-temperature performance, chemical
resistance, and durability. A lightly crosslinked shell wall, IPN,
or semi-IPN can also provide improved solvent resistance prior to
fiber expansion when the crosslinking reactions or IPN formation
are triggered independently from, just prior to, or concurrently
with the fiber expansion step.
[0056] Examples of reactive components that may be mixed with one
or more polymers to make up a fiber shell wall material are
numerous, and only a few will be listed here. However, this
invention is not limited to only those listed. For certain
crosslinking reactions, di-functional crosslinkers may be employed
such as diols, diepoxies, diisocyanates, di-anhydrides, aldehydes,
acrylates, methacrylates, melamines, etc., such as: ethylene
glycol, propylene glycol, triethylene glycol, tetraethylene glycol,
butanediol diglycidyl ether, bisphenol A diglycidyl ether,
partially or completely methylated or butylated melamines,
epichlorohydrin, glutaraldehyde, and many others. For IPN or
semi-IPN formation, the reactive functional groups may be chosen
from acrylate, methacrylate, vinyl ether, vinyl, diene, allyl,
epoxy, alcohol, amine, caboxyl, isocyanate, melamine, or others.
The reactive components may be used singly or in mixtures. Below
are listed acrylate-functional components, but similar structures
with other reactive groups could alternatively be used in their
place. These include, but are not limited to: ethyl acrylate,
propyl acrylate, butyl acrylate, isodecyl acrylate, hexadecyl
acrylate, isobornyl acrylate, tetrahydrofurfural acrylate, methyl
methacrylate, hydroxy ethyl methacrylate, hydroxy propyl acrylate,
polyethylene glycol diacrylate, methylene bisacrylamide, hexanediol
diacrylate, polybutadiene diacrylate, bisphenol A diacrylate,
trimethylolpropane triacrylate, pentaerythritol tetraacrylate, etc.
Other examples include silicates such as trimethoxy-silane,
dimethoxy-silane, triethoxy-silane, trichlorosilane, etc. These and
other suitable reactive compounds are commercially available from
companies such as Sartomer, Henkel, Radcure, Gelest, Allied Signal,
Aldrich, and others.
[0057] Photo-initiators such as the Irgacure and Darocure series
are well-known and commercially available from Ciba Geigy, as is
the Escacure series from Sartomer. Thermal initiators such as
azobisisobutyronitrile (AIBN) benzoyl peroxide, dicumyl peroxide,
t-butyl hydroperoxide, and potassium persulfate are also well-known
and are available from common chemical suppliers, as are catalysts
which may be desirably incorporated to facilitate the crosslinking,
IPN, or semi-IPN reactions.
[0058] These and other desirable formulations of polymers, polymer
mixtures, or polymer compositions with reactive components may not
be readily used in the economical manufacture of expandable fibers
using conventional technology because, to the authors' knowledge,
no such technology currently exists. Furthermore, as to the
production of hollow fibers, the current state of the art is
greatly hindered by the required extrusion process using an annular
die, which makes the production of fibers having a small
cross-sectional area very difficult. Such processes are also
sensitive to the polymer composition being extruded, limiting the
variety of compositions available for practice. Thus, the present
invention provides a much-needed and much-desired method for
incorporating such polymers or polymer mixtures into the shell wall
materials of hollow fibers or microtubes. After fiber expansion,
the resulting microtube will greatly benefit from the favorable
properties exhibited by such polymer compositions, yielding hollow
fiber shell walls and/or microcellular foams with properties
heretofore unattainable through conventional technologies
prominently known in the art.
[0059] The single requirement with respect to polymer selection in
accordance with the current invention is that a suitable solvent or
solvent combination must exist for the said polymer or polymer
mixture. The solvent must be capable of solvating the polymer to
form a nominally homogenous, free-flowing liquid, which in turn
must be capable of being extruded into strands of the desired size.
Solvent selection will depend on the polymer chosen to form the
fiber shell walls, and may also be influenced by factors such as
volatility, flammability, viscosity, toxicity, chemical reactivity,
recoverability, cost, and interactions with the blowing agent or
other components. Typical solvents which may be used in the
practice of this invention include, but are not limited to:
acetone, methyl ethyl ketone, ethyl ether, tetrahydrofuran,
dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, methyl
acetate, ethyl acetate, propyl acetate, butyl acetate, methanol,
ethanol, iso-propanol, toluene, methylene chloride, chloroform,
dichloroethane, trichloroethane, tetrachloroethylene, water, and
others. The present invention places no requirements on the solvent
selected, although it is recognized that certain solvents will be
more preferable than others based on the above-stated criteria. It
is also recognized that in some situations it may be advantageous
or necessary to use a mixture of two or more solvents instead of a
single solvent in order to obtain the desired solvation,
evaporation, and/or extraction properties.
[0060] The final component, which must be added to the
polymer-solvent mixture, aside from any of the optional additives
as mentioned above, is the blowing agent or propellant. When a
liquid blowing agent is used, it is typically chosen to be
incompatible with the polymer. That is, it does not solvate the
polymer to any appreciable degree. This is not an absolute
requirement in accordance with the present invention; however, the
use of blowing agent-polymer combinations which are incompatible
will generally lead to a longer shelf-life for the polymer fibers
since, in this case, the blowing agent will exhibit greatly reduced
diffusion through the polymer shell walls. Thus, the choice of
expanding agent will vary depending on the nature of the polymer
chosen to form the fiber walls, as well as the solvent chosen to
solvate said polymer. When solid propellants are to be used, the
interactions with the polymer are much less important since the
solid propellants will not typically act to solvate the polymer
wall.
[0061] One important consideration when selecting a liquid blowing
agent or propellant is that it must produce a vapor pressure
sufficient to expand the polymer walls once the polymer has
softened due to heating. Propellant selection in this regard will
thus depend on the softening temperature of the polymer chosen to
form the fiber walls as well as the vapor pressure of the liquid
propellant at this softening temperature. Typically, a solvent that
has a boiling point at atmospheric pressure of no more than 10
degrees above the softening temperature (or glass transition
temperature, T.sub.g) of the polymer will provide a sufficient
vapor pressure to expand said polymer walls upon heating the fibers
to the boiling point of the blowing agent. More preferably, the
boiling point of the propellant liquid at atmospheric pressure will
be equal to or less than the polymer T.sub.g, and even more
preferably, the propellant boiling point will be at least 10
degrees lower than the polymer T.sub.g. Note that the glass
transition temperature of the polymer in question will be altered
if a significant amount of solvent remains within the polymer
walls. In most cases this will be beneficial to the process by
lowering the glass transition temperature at which the polymer
softens, thus requiring less heat consumption to achieve expansion
of the fibers. In cases where lower glass transition temperatures
are not desirable, the T.sub.g depression can be avoided by
removing substantially all of the solvent from the fiber
composition prior to expansion. For this purpose, a particularly
volatile solvent may be beneficially employed as the polymer
solvent.
[0062] Particularly preferred liquid blowing agents are the small
chain hydrocarbons since they are inert towards most polymers,
miscible with most solvents, and have boiling points near ambient
temperatures. For liquid blowing agents that have boiling points
below ambient temperatures, the process may advantageously be
carried out at low temperatures and/or under a pressurized
atmosphere. Examples of liquid propellants that may be used in
conjunction with the polymers and solvents listed above include,
but are not limited to, hydrocarbons (n-butane, iso-butane,
n-pentane, iso-pentane, trimethyl-2-pentene, hexane, heptane,
n-octane, iso-octane, nonane, decane, benzene, toluene, etc.),
ethers and ketones (ethyl ether, isopropyl ether, acetone, methyl
ethyl ketone, etc.), alcohols (methanol, ethanol, iso-propanol,
etc.), halogentated hydrocarbons (methylene chloride, chloroform,
carbon tetrachloride, dichloroethane, trichloroethane,
tetrachloroethane, tetrachloroethylene, trichlorofluoromethane,
dichlorodifluorodimethane, etc.), ammonia or ammonia-based liquids,
silane or siloxane-based liquids (hexamethyl disilane, hexamethyl
disiloxane), and water or other aqueous mixtures. These examples
are not meant to be exhaustive, for one skilled in the art will
know of many liquids which will exhibit miscibility with a given
polymer-solvent mixture while also exhibiting incompatibility with
the pure polymer, and at the same time exerting a vapor pressure
sufficient to expand the polymer walls at or above the softening
temperature of the polymer.
[0063] The two main types of solid propellants are physical blowing
agents and chemical blowing agents. Physical blowing agents are
those which produce a vapor by changing phase upon heating. There
are a vast number of chemicals that exist as a solid at room
temperature, yet vaporize upon reaching temperatures typically used
to soften most polymers. Some solid blowing agents of this type
pass through an intermediate liquid state upon heating, while
others sublime directly to a gas upon heating. Examples of suitable
physical blowing agents include, but are not limited to: neopentyl
alcohol, hexamethyl ethane, tertiary-butyl carbazate,
tertiary-butyl dimethylsilyl chloride, tertiary-butyl
N-allylcarbamate, and tetramethyl-1,3-cyclobutanedione, etc. This
list is not meant to be exhaustive as one knowledgeable in the
field of chemistry will find many substances that meet the criteria
so-described. In selecting a suitable physical blowing agent,
consideration may be given to toxicity, polymer compatibility,
solvent compatibility, melting point, boiling point, vapor
pressure, or other issues, depending on the particular
polymer-solvent system under consideration.
[0064] Chemical blowing agents, typically solid at ambient pressure
and temperature, undergo decomposition or other chemical reactions
that produce gaseous vapors as at least one of the reaction
by-products. These reactions are most often triggered by heat, but
can alternatively be triggered by the presence of a co-reactant.
For instance, a chemical blowing agent could be triggered by the
presence of water, whereby water is included in the formulation but
only becomes available for reaction upon the addition of heat.
(Such would be the case for certain hydrated salt compounds mixed
with the chemical blowing agent sodium borohydride, for example.)
Chemical propellants can be categorized as either organic or
inorganic chemical blowing agents. Inorganic chemical blowing
agents typically decompose to give off carbon dioxide gas in an
endothermic reaction. Organic chemical blowing agents typically
decompose to give off nitrogen gas (which has a lower diffusion
rate in most polymers) in an exothermic reaction.
[0065] Examples of chemical blowing agents include, but are not
limited to: sodium bicarbonate, potassium hydrogencarbonate, sodium
borohydride (decomposes upon the addition of a proton donor such as
water), polycarbonic acid, ammonium carbonate, ammonium carbamate,
ammonium acetate, ammonium diethyldithiocarbamate,
dinitrosopentamethylene-tetraam- ine, p-toluenesulfonyl hydrazide,
4,4'-oxybis(benzenesulfonyl hydrazide), azodicarbonamide,
p-toluenesulfonyl semicarbazide, 5-phenyltetrazole,
diazoaminobenzene, etc. One advantage of chemical blowing agents is
that the carbon dioxide or nitrogen gas typically evolved is inert,
nonflammable, and nontoxic. Another advantage is that the inorganic
blowing agents can themselves be very inert and nontoxic, which
makes them easy and safe to work with during production and in the
end-use products.
[0066] Solid blowing agents, both physical and chemical (organic
and inorganic) avoid the inherent hazards associated with volatile,
flammable liquids. Another advantage to be realized by the solid
propellants is that the temperature at which fiber expansion occurs
may be altered independent of the polymer used to make the fiber
walls. In fibers containing liquid propellants, the temperature at
which expansion occurs is typically determined by the softening
temperature of the polymer. That is, expansion occurs when the
polymer walls soften, allowing the vapor pressure of the volatile
liquid to stretch the walls outward.
[0067] Using the solid propellants described above and in
accordance with this invention, however, the polymer-propellant
combination may be chosen so that the expansion temperature is
dictated by the decomposition temperature of the solid propellant
rather than the softening temperature of the polymer. This will
occur when the softening temperature of the polymer is below the
decomposition temperature of the propellant. As the fibers are
heated the polymer may soften, but as long as no gas is generated,
no expansion will occur. Only upon heating further, to the
decomposition temperature of the propellant, will a vapor pressure
sufficient to expand the polymer walls be generated. Thus, by using
solid-phase blowing agents which exert virtually no vapor pressure
prior to the onset of decomposition, the temperature at which fiber
expansion occurs may be controlled by the selection of the
propellant rather than by the softening temperature of the polymer.
This feature can provide added flexibility in designing the
temperature ramp-up cycle during the molding processes used to
produce final products.
[0068] The greatly decreased volatility of the solid propellants
used in accordance with this invention further helps to preserve
the shelf-life of the expandable fibers. Since the solid
propellants have virtually no vapor pressure and little propensity
to permeate through the polymer walls, the long-term shelf-life of
unexpanded fibers is expected to be substantially increased.
Chemical blowing agents offer an additional advantage over physical
blowing agents (liquid or solid) in that they are capable of
generating a higher expansion pressure than their physical blowing
agent counterparts. This is because physical blowing agents will
always be in a state of reversible equilibrium between the liquid
and vapor phases. In contrast, the chemical blowing agents
decompose to form inert gases in an essentially irreversible
process. Because the decomposition is virtually irreversible and
the gases produced are very volatile, chemical blowing agents are
capable of producing much greater pressures than those generated by
even the most volatile physical blowing agents.
[0069] In order to incorporate the solid propellants into
expandable fibers, they must be mixed with the polymer-solvent
mixture. Whether physical or chemical blowing agents are used, the
solid propellant may optionally be dissolved in the polymer-solvent
mixture. In the case of soluble systems, the blowing agent is
simply dissolved up to its desired composition; the mixture is
extruded into strands; and the solvent is dried or extracted from
the strands. When the solid propellant is not soluble in the
polymer-solvent mixture, the solid propellant is typically added to
the mixture in the form of a finely divided powder, strands, or
fibers. In this case, the size of the propellant particles will be
as small as or smaller than the desired fiber size, at least in the
cross-sectional dimensions when fiber or strand-shaped propellants
are used. If the solid propellant particles are significantly
smaller than the strands formed by extrusion through an orifice,
then the final fibers will typically contain many solid propellant
particles contained within the fibers in a serial fashion. In this
case, a hollow fiber may be produced after expansion that comprises
many non-connected voids within the interior of the hollow
fiber.
[0070] Solubility of the propellant in the polymer-solvent mixture,
the amount of gas generated, the vapor pressure generated, and the
temperature at which vapor generation occurs are all parameters
that will influence the selection of an appropriate solid
propellant for use in accordance with this invention.
[0071] The matrix materials with which these fibers may be
incorporated are numerous. Example matrices suitable for fiber
incorporation include paints, inks, epoxies, sealants, insulation,
potting compounds, spackling compounds, underbody coatings, pulp
fibers, dielectric laminates, prosthetic devices, synthetic foams,
cultured marble, polymer concretes, and synthetic cements. The
matrices may be simple one or two-component mixtures, or may
contain any number of additives such as crosslinkers, catalysts,
initiators, stabilizers, pigments, fibers, inert fillers, etc.
[0072] In one preferred embodiment of this invention, the matrix is
an epoxy-based resin mixed with a suitable initiator package. When
expandable fibers made from the copolymer styrene-maleic anhydride
(also containing crosslinkers, catalysts, etc.) are incorporated
into such a matrix and expanded by heating, crosslinking occurs in
the bulk of the epoxy matrix, within the hollow fiber shell walls,
and across or between the hollow fiber walls and the epoxy matrix.
The final product is a fully crosslinked, light-weight composite
system wherein the hollow fibers are integrally bonded to the
surrounding matrix.
[0073] In another preferred embodiment of the present invention,
the matrix is a mixture of pulp fibers and, optionally, a sucrose
solution. Expandable fibers made from a prolamine, a protein
derived from corn also known as Zein (containing suitable
crosslinkers, catalysts, etc.) are mixed into the fiber-sucrose
solution, and the mixture is transferred to a mold, filling about
half of the total mold cavity volume. When the mold is closed and
heated, the mixture expands, fills in the entire mold cavity, and
sets with crosslinking within the hollow fiber shell walls and
between the hollow fiber walls, the pulp fibers, and the sucrose
molecules. The resultant material is a crosslinked, biodegradable,
light-weight, non-woven fabric that is suitable for use as
insulation, drink cups, food containers, packaging material, paper
or cardboard products, and other products.
EXAMPLES
[0074] The examples below are meant to show representative polymer,
blowing agent, and solvent combinations that are capable of forming
expandable and optionally fusible fibers. These examples further
illustrate the formation of expandable, optionally fusible fibers,
as well as the resulting hollow fibers and microcellular foam or
foamed composite using commercially available polymer resins and
only physical processes governed by solution thermodynamics. Other
multi-component mixtures can be formulated and processed in
accordance with this invention by those skilled in the art of
polymer solution thermodynamics.
Example 1
Expandable Fibers Using a Copolymer
[0075] Polystyrene-co-maleic anhydride (SMA) polymer is obtained
from Aldrich Chemical Company, Inc., Milwaukee, Wis., Catalog
#44,238-0. This product has about a 1.3-to-1 styrene-to-maleic
anhydride ratio, and a softening temperature of 154.degree. C.
Approximately 2.0 grams of SMA is dissolved in 3 mL of ethyl
acetate, and an additional 0.2 mL of iso-octane is dissolved to
serve as the blowing agent. After mixing, this solution becomes
clear and free-flowing. The solution is ejected through an orifice
having a circular opening with a diameter of 0.165 inches, forming
a strand of polymer solution. The polymer-blowing agent-solvent
solution is extruded into a tall box which has a fan at the bottom
blowing ambient air up through the box, out the top of the box
where the strand enters, and into a chemical fume hood. The dried
fiber is collected at the bottom of the box. The fiber consists of
a small, nominally cylindrical fiber ranging in size from about
0.08 to about 0.14 inches in diameter depending on the ejection
speed through the orifice. Upon placing into a convection oven and
heating to 160.degree. C., the fiber expands to approximately
double its original size to give a hollow fiber or microtube. When
multiple fibers are bunched together and heated, the fibers expand
and coalesce to form a fused, foamed material that occupies several
times the original volume of the unexpanded fibers. The foamed
material consists of individual fused microtubes having thin shell
walls surrounding one or more internal voids.
Example 2
Expandable Fibers Using Polycarbonate
[0076] Polycarbonate resin is obtained from Bayer Corporation,
Pittsburgh, Pa., under the tradename Makrolon. Approximate 2 grams
of polycarbonate is solvated in 5 mL of chloroform, and 0.7 mL of
2,4,4 trimethyl-1-pentene is dissolved in the mixture to serve as
the liquid blowing agent. After mixing, this solution becomes clear
and free-flowing. The solution is ejected through an orifice having
a circular opening with a diameter of 0.165 inches, forming a
strand of polymer solution. The polymer-blowing agent-solvent
solution is extruded into a tall box which has a fan at the bottom
blowing ambient air up through the box, out the top of the box and
into condenser where the chloroform gas is condensed back into a
liquid and recovered. The dried fiber is collected at the bottom of
the box. The fiber consists of a small, nominally cylindrical fiber
ranging in size from about 0.06 to about 0.12 inches in diameter
depending on the ejection speed through the orifice. Upon placing
into a convection oven and heating to 160.degree. C., the fiber
expands to approximately double its original size to give a hollow
fiber or microtube. When multiple fibers are bunched together and
heated, the fibers expand and coalesce to form a fused, foamed
material that occupies several times the original volume of the
unexpanded fibers. The foamed material consists of individual fused
microtubes having thin shell walls surrounding one or more internal
voids.
Example 3
Expandable Fibers Using a Copolymer Mixed with a Reactive
Component
[0077] Polystyrene-co-maleic anhydride (SMA) polymer is obtained
from Elf-Atochem North America, Philadelphia, Pa., Product #
SMA3000. This product has about a 3-to-1 styrene-to-maleic
anhydride ratio, and a softening temperature of about 110.degree.
C. Approximately 4.0 grams of SMA is dissolved in 6 mL of ethyl
acetate, and an additional 0.2 mL of iso-octane is dissolved to
serve as the blowing agent. Also added are 0.4 grams of hexanediol
diacrylate (difunctional polymerizable group, HDODA; Radcure) and
0.02 grams of AIBN (Aldrich). After mixing, this solution becomes
clear and free-flowing. The solution is ejected through an orifice
having a circular opening with a diameter of 0.165 inches, forming
a strand of polymer solution. The polymer-blowing agent-solvent
solution is extruded into a tall box which has a fan at the bottom
blowing ambient air up through the box, out the top of the box
where the strand enters, and into a chemical fume hood. The dried
fiber is collected at the bottom of the box. The fiber consists of
a small, nominally cylindrical fiber ranging in size from about
0.08 to about 0.14 inches in diameter depending on the ejection
speed through the orifice.
[0078] The fibers are collected and placed into a convection oven
at 90.degree. C. (below the softening temperature of the SMA) for
four minutes to cure and crosslink the HDODA. Upon placing into a
convection oven and heating to 120.degree. C., the fiber turns
clear and expands to approximately double its original size to give
a hollow fiber or microtube. When multiple fibers are bunched
together and heated, the fibers expand and coalesce to form a
fused, foamed material that occupies several times the original
volume of the unexpanded fibers. The foamed material consists of
individual fused microtubes having thin shell walls surrounding one
or more internal voids.
Example 4
Expandable Fibers Using a Copolymer Mixed with Crosslinking
Components
[0079] Polystyrene-co-maleic anhydride (SMA) polymer is obtained
from Elf-Atochem North America, Philadelphia, Pa., Product #
SMA3000. This product has about a 3-to-1 styrene-to-maleic
anhydride ratio, and a softening temperature of about 110.degree.
C. Approximately 2.0 grams of SMA is dissolved in 4 mL of methyl
acetate, and an additional 0.5 mL of iso-octane is dissolved to
serve as the blowing agent. Also added are 0.5 grams of a
plasticizer (Santicizer S160, Solutia, Inc. St. Louis, Mich.), 0.2
grams of maleic anhydride-grafted polybutadiene resin (Ricon
131MA5; Ricon Resins Inc., Grand Junction, Colo.) as a crosslinker,
0.07 grams of AIBN (initiator, Aldrich), 0.1 gram of tetraethylene
glycol (Aldrich) as a crosslinker, and 0.05 grams of a catalyst,
2,4,6-tris(dimethylaminometh- yl)phenol (DMP-30, Aldrich). After
mixing, this solution becomes clear and free-flowing. The solution
is processed as stated in Example 3 with a similar fiber
resulting.
[0080] Upon placing into a convection oven and heating to
120.degree. C., the fiber turns clear and expands to approximately
double its original size to give a hollow fiber or microtube. When
multiple fibers are bunched together and heated, the fibers expand
and coalesce to form a fused, foamed material that occupies several
times the original volume of the unexpanded fibers. The foamed
material consists of individual fused microtubes having thin shell
walls surrounding one or more internal voids.
[0081] When unexpanded fibers are mixed into an epoxy-based resin
and heated to 120.degree. C. in a test tube, the resin rises in the
test tube (increased in volume), then cures into a hard, foamed
composite. The microtubes within the composite are bonded to the
epoxy matrix by means of the maleic anhydride groups in the polymer
chains, and the polymer chains are crosslinked to each other by the
reaction of the tetraethylene glycol with the maleic anhydride
units.
Example 5
Expandable Fibers Using Liquid-Liquid Extraction and a
Homopolymer
[0082] Poly-.alpha.-methylstyrene (PMS) polymer is obtained from
Aldrich Chemical Company, Inc., Milwaukee, Wis., Catalog #19,184-1.
Approximately 2 grams of PMS is solvated in 3 mL of tetrahydrofuran
along with 0.9 mL of 2-methyl butane as the blowing agent. The
mixture forms a clear, homogeneous solution. Separately, 100 mL of
an aqueous solution with 2 wt % PVA (Mowiol surfactant, 88%
hydrolyzed PVA, Aldrich Chemical Company, Catalog # 32,459-0) is
made and is placed into a trough approximately three feet long.
[0083] The polymer solution is slowly ejected using a 10 mL syringe
with a 29 gauge, 1.5" needle, forming a thin strand of the polymer
solution. As the polymer solution strand is contacted with and
pulled through the aqueous phase, the THF solvent is extracted from
the strand into the water. The strand turns translucent and then
opaque as it passes through the water, indicative of the THF
extraction. At the other end of the trough, the strand emerges as a
fiber, which can be optionally dried with an air flow.
[0084] Upon placing the unexpanded fiber into a convection oven and
heating to 120.degree. C., the fiber expands to approximately
double its original size to give a hollow fiber or microtube. When
multiple fibers are bunched together and heated, the fibers expand
and coalesce to form a fused, foamed material that occupies several
times the original volume of the unexpanded fibers. The foamed
material consists of individual fused microtubes having thin shell
walls surrounding one or more internal voids.
Example 6
Expandable Fibers Using a Naturally-Occurring Polymer Mixed with a
Crosslinking Component
[0085] A natural protein extracted from corn gluten, known as
prolamine or zein (Zein), is obtained from Freeman Industries, LLC,
Tuckahoe, N.Y., Product # F4000. This product has a softening
temperature of about 105.degree. C. Approximately 1 gram of Zein is
dissolved in 0.7 grams of isopropyl alcohol, 0.7 grams of ethyl
alcohol, and 0.6 grams of water. About 0.2 grams of glutaric
dialdehyde (50% in water, Aldrich product # 34,085-5) and 0.15
grams of ethylene glycol are added to crosslink and plasticize the
polymer, respectively. An additional 0.2 grams of
1,1,1-trichloroethane are added to serve as the blowing agent. The
solution is processed as stated in Example 3 with a similar fiber
resulting.
[0086] Upon placing into a convection oven and heating to
120.degree. C., the fiber expands to approximately double its
original size to give a hollow fiber or microtube. When multiple
fibers are bunched together and heated, the fibers expand and
coalesce to form a fused, foamed material that occupies several
times the original volume of the unexpanded fibers. The foamed
material consists of individual fused microtubes having thin shell
walls surrounding one or more internal voids.
[0087] When unexpanded fibers are mixed into an epoxy-based resin
and heated to 120.degree. C. in a test tube, the resin rises in the
test tube (increased in volume), then cures into a hard, foamed
composite. Upon filling a rectangular-shaped mold about half full
with the fibers so produced, sealing the mold shut, and heating to
approximately 120.degree. C., the fibers expand and coalesce,
filling the mold cavity. After removal from the mold, the resultant
article is a biodegradable foamed sheet having the shape of the
internal mold cavity, comprised of fused microtubes which could be
seen to have thin shell walls surrounding one or more internal
voids.
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