U.S. patent number 3,632,092 [Application Number 04/865,334] was granted by the patent office on 1972-01-04 for stabilization procedure and apparatus for polymeric fibrous materials.
This patent grant is currently assigned to Celanese Corporation. Invention is credited to Dagobert E. Stuetz.
United States Patent |
3,632,092 |
Stuetz |
January 4, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
STABILIZATION PROCEDURE AND APPARATUS FOR POLYMERIC FIBROUS
MATERIALS
Abstract
An improved continuous process and apparatus are provided for
the uniform stabilization of a strand of polymeric fibrous material
which is capable of undergoing thermal stabilization. The strand is
continuously wound in a plurality of turns and continuously unwound
from at least one rotating roll having a porous surface while a gas
at an elevated temperature is expelled outwardly through the
surface of the porous roll and penetrates the fibrous configuration
of the strand wound upon the roll. In a preferred apparatus in
accordance with the present invention the porous roll situated
within a heat treatment chamber is internally provided with a
plurality of individually adjustable heating elements along its
length. The resulting stabilized material retains its original
fibrous configuration essentially intact, exhibits enhanced thermal
stability, and is capable of undergoing carbonization. In a
particularly preferred embodiment of the invention the precursor is
an acrylonitrile homopolymer and air having a temperature of at
least about 260.degree. C. is expelled through the surface of the
rotating porous roll.
Inventors: |
Stuetz; Dagobert E. (Westfield,
NJ) |
Assignee: |
Celanese Corporation (New York,
NY)
|
Family
ID: |
25345272 |
Appl.
No.: |
04/865,334 |
Filed: |
October 10, 1969 |
Current U.S.
Class: |
432/60; 28/219;
28/281; 28/244; 34/629 |
Current CPC
Class: |
D01F
9/14 (20130101); D01D 10/0436 (20130101); B29C
71/02 (20130101); B29B 13/023 (20130101); D01D
10/02 (20130101); D01F 9/225 (20130101) |
Current International
Class: |
B29B
13/02 (20060101); D01F 9/22 (20060101); B29C
71/02 (20060101); D01F 9/14 (20060101); D01D
10/00 (20060101); D01D 10/04 (20060101); B29B
13/00 (20060101); F27b 009/28 (); F26b
013/08 () |
Field of
Search: |
;263/3,6C
;34/155,160 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
2701765 |
February 1955 |
Codichini et al. |
3161484 |
December 1964 |
Bagnoli et al. |
3387833 |
June 1968 |
Whittaker et al. |
|
Primary Examiner: Camby; John J.
Claims
I claim:
1. An apparatus for the continuous stabilization of a strand of
polymeric fibrous material comprising a substantially enclosed heat
treatment chamber, means for continuously introducing a strand of
fibrous material into said heat treatment chamber, means for
continuously withdrawing a strand of fibrous material from said
heat treatment chamber, at least one rotatable roll situated within
said chamber having a porous surface, drive means capable or
rotating said roll, conduit means essentially coextensive with the
length of said roll in a communicative relationship to said porous
surface of said roll, heating means situated within said rotatable
roll, and means for supplying a gas to said conduit means.
2. An apparatus according to claim 1 wherein said rotatable roll
having a porous surface has a tapered configuration.
3. Apparatus according to claim 1 wherein said rotatable roll
having a porous surface has an essentially uniform cylindrical
configuration, and said apparatus is additionally provided with a
rotatable skewed roll in a paired relationship to said rotatable
roll.
4. An apparatus according to claim 1 wherein means are provided for
supplying said gas at successively elevated temperatures along said
rotatable roll.
5. An apparatus for the continuous stabilization of a strand of
polymeric fibrous material comprising a substantially enclosed heat
treatment chamber, means for continuously introducing a strand of
fibrous material into said heat treatment chamber, means for
continuously withdrawing a strand of fibrous material from said
heat treatment chamber, at least one rotatable roll situated within
said chamber having a porous surface, drive means capable of
rotating said roll, central conduit means essentially coextensive
with the length of said roll, dividing means positioned about the
circumference of said central conduit means capable of dividing
means capable of dividing the interior of said roll into a
plurality of modules which communicate with the porous surface of
said roll and said central conduit means, resistance heating means
situated within said modules, means for supplying current to said
resistance heating means, and means for supplying current to said
resistance heating means, and means for supplying a gas to said
central conduit means.
6. An apparatus according to claim 5 wherein said rotatable roll
having a porous surface has a tapered configuration.
7. An apparatus according to claim 5 wherein said rotatable roll
having a porous surface has an essentially uniform cylindrical
configuration, and said apparatus is additionally provided with a
rotatable skewed roll in a paired relationship to said rotatable
roll having a porous surface,
8. An apparatus according to claim 5 wherein said means for
continuously introducing a strand of fibrous material into said
heat treatment chamber is a bobbin.
9. An apparatus according to claim 5 wherein said means for
continuously withdrawing a strand of fibrous material from said
heat treatment zone is a bobbin.
10. An apparatus according to claim 5 wherein exhaust means are
provided to withdraw off gasses generated within said heat
treatment chamber.
11. An apparatus according to claim 5 wherein said means for
supplying a gas to said central conduit means is a fan.
Description
BACKGROUND OF THE INVENTION
In the past procedures have been proposed for the conversion of
organic polymeric fibers to a modified form possessing enhanced
thermal stability. Such modification has generally been
accomplished by heating the fibrous material in an appropriate
atmosphere at moderate temperatures for extended periods of time.
The product may be suitable for use as an intermediate in the
formation of carbonized fibrous materials, or for direct
utilization in applications where fibers of enhanced thermal
stability are required.
U.S. Pat. No. 2,913,802 to Barnett and U.S. Pat. No. 3,285,696 to
Tsunoda disclose representative processes for the conversion of
fibers of acrylonitrile homopolymers of copolymers to a heat
resistant form wherein the fibers are heated in an
oxygen-containing atmosphere. Such prior art stabilization
techniques have commonly been directed to batch operations which
require relatively long heating periods.
The more recent Belgian Pat. No. 700,655 discloses a procedure
whereby a continuous length of an acrylonitrile copolymer may be
continuously subjected to a stabilization treatment to produce
essentially complete oxygen saturation while maintained in air at a
temperature not exceeding 250.degree. C., e.g., three hours or more
at 220.degree. C. Commonly assigned U.S. Ser. No. 749,957, filed
Aug. 5, 1968 of Dagobert E. Stuetz additionally discloses an
improved generically defined process wherein certain acrylic
fibrous precursors surprisingly may be continuously stabilized at
more highly elevated temperatures than previously deemed
possible.
Heretofore a common technique for the continuous stabilization of a
continuous length of fibrous material has involved the passage of a
length of the material through the interior of a tubular muffle
furnace provided with an appropriate atmosphere. Rollers have
commonly been situated at each end of the furnace to direct
movement of the strand so that it is aligned along the axial center
of the furnace. The residence time achieved within such a heat
treatment zone is by necessity determined by the length of the
furnace and by the strand speed. In order to achieve a commercially
acceptable production level the oven (1) must be extremely long in
order to make possible a relatively high strand speed, or (2) the
strand must be returned for a multiplicity of passes. When using
such an arrangement, the stringing of the apparatus prior to
carrying out the process is often tedious and time consuming.
In continuous preoxidation systems of the prior art in which the
strand being treated is wound upon a pair of spaced converging
rolls and heated air is supplied to the same from an external
source, it has been essential to provide a plurality of ovens in
series if the strand is to be subjected to a plurality of
successively elevated temperatures. Also, an appreciable time lapse
may be required for the oven to attain the operating temperature
desired.
When stabilization of a strand of polymeric fibrous material is
conducted in accordance with the teaching of U.S. Ser. No. 865,333,
filed Oct. 10, 1969, in the name of William M. Cooper, by contact
with the surface of a rotating heated roll having an internal heat
source and a gas impervious surface, the stabilization reaction has
a tendency to be at least initially concentrated upon the side of
the fibrous strand where direct physical contact is made with the
heated roll.
It is an object of the invention to provide an efficient continuous
process and apparatus for the stabilization of a strand of
polymeric fibrous material in which the strand undergoes
stabilization on a highly uniform basis.
It is an object of the invention to provide an improved
stabilization apparatus capable of accomplishing the continuous
stabilization of a strand of a polymeric fibrous material while
accommodating a relatively large precursor inventory per unit of
area.
It is an object of the invention to provide an efficient process
for the preoxidation of an acrylic yarn to render the same capable
of undergoing carbonization.
It is another object of the invention to provide a compact
stabilization apparatus capable of penetrating a fibrous strand
with a hot gas at successively elevated temperatures as the strand
travels along the surface of a porous roll.
These and other objects, as well as the scope, nature, and
utilization of the invention will be apparent from the following
detailed description and appended claims.
SUMMARY OF THE INVENTION
It has been found that a process for the stabilization of a strand
of polymeric fibrous material capable of undergoing thermal
stabilization comprises passing the strand through a heat treatment
zone wherein the strand is continuously wound in a plurality of
turns and continuously unwound from at least one rotating roll
having a porous surface while a gas at an elevated temperature is
expelled outwardly through the surface of the porous roll and
penetrates the fibrous configuration of the strand wound upon the
roll, and continuously withdrawing the resulting stabilized strand
of fibrous material from the heat treatment zone which retains its
original fibrous configuration essentially intact, exhibits
enhanced thermal stability, and is capable of undergoing
carbonization.
An apparatus for the continuous stabilization of a strand of
polymeric fibrous material comprises a substantially enclosed heat
treatment chamber, means for continuously introducing a strand of
fibrous material into the heat treatment chamber, means for
continuously withdrawing a strand of fibrous material from the heat
treatment chamber, at least one rotatable roll situated within the
chamber having a porous surface, drive means capable of rotating
the roll, conduit means essentially coextensive with the length of
the roll in a communicative relationship to the porous surface of
the roll, heating means situated within the rotatable roll, and
means for supplying a gas to the conduit means.
FIG. 1 is a perspective view partially cut away of a preferred
apparatus of the present invention showing the positioning of a
strand of polymeric fibrous material upon a rotatable tapered
driven roll having a porous surface while a gas at an elevated
temperature is expelled outwardly through the surface of the porous
roll and penetrates the fibrous configuration to the strand of
produce stabilization.
FIG. 2 is a cross section of the tapered porous roll shown in the
apparatus of FIG. 1.
FIG. 3 is a perspective view of a portion of an alternative
apparatus arrangement in which the strand undergoing treatment is
continuously wound in a plurality of turns and continuously unwound
from a rotatable porous roll having an essentially cylindrical
configuration as well as upon a rotatable skewed roll in a paired
spaced relationship to the porous roll.
DETAILED DESCRIPTION OF THE INVENTION
The stabilized materials formed in accordance with the present
invention retain the original fibrous configuration of the organic
polymeric precursor, possess an enhanced thermal stability, and are
capable of undergoing carbonization by heating in a nonoxidizing
atmosphere according to procedures known in the art, e.g., heating
in a nitrogen atmosphere at 1,000.degree. C. The stabilized strands
formed in the present process may alternatively be used as
heat-resistant fibrous insulative materials, or in other
applications where a thermally stable fibrous material is required.
Additionally, the fibrous materials when derived from an acrylic
precursor are nonburning when subjected to an ordinary match flame
and find utility in the formation of flame-resistant fabrics.
The organic polymeric materials which are treated in accordance
with the present invention may be of varied composition. Those
precursors which are capable of undergoing thermal stabilization
may be selected. For instance, the organic polymeric precursor may
be an acrylic polymer, a cellulosic polymer, a polyamide, a
polybenzimidazole, polyvinyl alcohol, etc. As discussed hereafter,
acrylic polymeric materials are particularly suited for use in the
present process. Illustrative examples of suitable cellulosic
materials include the natural and regenerated forms of cellulose,
e.g., rayon. Illustrative examples of polyamide precursors include
the aromatic polyamides such as nylon 6T which is formed by the
condensation of hexamethylenediamine and terephthalic acid. An
illustrative example of a suitable polybenzimidazole is poly-2,2'
-m-phenylene-5,5' -bibenzimidazole.
The acrylic polymer selected for use as the precursor may be formed
primarily of recurring acrylonitrile units. For instance, the
acrylic polymer should generally contain not less than about 85 mol
percent of recurring acrylonitrile units with not more than about
15 mol percent of a monovinyl compound which is copolymerizable
with acrylonitrile such as styrene, methyl acrylate, methyl
methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride,
vinyl pyridine and the like, or a plurality of such monomers.
In a preferred embodiment of the invention the starting material is
selected in accordance with the teachings of U.S. Ser. No. 749,957,
filed Aug. 5, 1968, of Dagobert E. Stuetz, which is assigned to the
same assignee as the instant invention, and is herein incorporated
by reference. More specifically, the copolymer should contain no
more than about 5 mol percent of one or more monovinyl comonomers
copolymerized with acrylonitrile. In a particularly preferred
embodiment of the invention the acrylic polymer is an acrylonitrile
homopolymer.
The strand of polymeric fibrous material which is treated in
accordance with the present invention is preferably a continuous
multifilament yarn which may be formed by conventional techniques.
Various threads, ropes, and cables, or continuous lengths of
similar fibrous configurations may be selected. The strand which
serves as the starting material may optionally be provided with a
twist which tends to improve its handling characteristics. For
instance, a twist of about 0.1 to 5 t.p.i., and preferably about
0.3 to 1.0 t.p.i. may be utilized.
The fibrous material which serves as the starting material
optionally may be highly oriented. Such orientation is generally
capable of enhancing the tensile properties of the resulting
stabilized fibrous material, as well as of any carbon materials
derived therefrom. An acrylic starting material may be oriented by
hot drawing to a relatively high single-filament tensile strength
of at least about 4 grams per denier prior to stabilization. For
instance, fibrous acrylic starting materials which possess a
single-filament tensile strength of about 4 to 9 grams per denier
may be selected for use in the process.
As will be apparent to those skilled in the art, the atmosphere
provided in the heat treatment zone may be varied. For instance, a
cellulosic precursor is commonly stabilized in (1) an
oxygen-containing atmosphere or in (2) an inert or nonoxidizing
atmosphere, such as nitrogen, helium, argon, etc. Additionally,
precursors such as an acrylic polymer, a polyamide, a
polybenzimidazole, or polyvinyl alcohol are commonly stabilized in
an oxygen-containing atmosphere. Air may be conveniently selected
as the oxygen-containing atmosphere for use in the process. If
desired, the precursor may be preliminarily treated with catalytic
agents which are capable of promoting the stabilization reaction.
When the stabilization treatment is conducted in an
oxygen-containing atmosphere, it is commonly termed a
"preoxidation" treatment particularly when followed by
carbonization which is conducted in an inert atmosphere.
The stabilization zone is substantially enclosed in order to
facilitate the confinement and withdrawal of off gases and/or the
maintenance of an appropriate atmosphere. When a nonoxidizing
atmosphere is desired within the heat treatment chamber the strand
may pass through a seal as it continuously enters and leaves the
heat treatment chamber in order to exclude oxygen.
The stabilization of fibers of acrylonitrile homopolymers and
copolymers in an oxygen-containing atmosphere involves (1)an
oxidative cross-linking reaction of adjoining molecules as well as
(2) a cyclization reaction of pendant nitrile groups to a condensed
dihydropyridine structure. While the reaction mechanism is complex
and not readily explainable it is believed that these two reactions
occur concurrently, or are to some extent competing reactions.
The cyclization reaction involving pendant nitrile groups which
occurs upon exposure of an acrylic fibrous material to heat is
generally highly exothermic and, if uncontrolled, results in the
destruction of the fibrous configuration of the starting material.
In some instances this exothermic reaction will occur with
explosive violence and result in the fibrous material being
consumed by flame. More commonly, however, the fibrous material
will simply rupture, disintegrate and/or coalesce when the critical
temperature is reached. As the quantity of comonomer present in an
acrylonitrile copolymer is increased, a fibrous material consisting
of the same tends to soften at a progressively lower temperature
and the possible destruction of the original fibrous configuration
through coalescence of adjoining fibers becomes a factor of
increasing importance. The "critical temperature" referred to
herein is defined as the temperature at which the fibrous
configuration of a given sample of acrylic fibrous starting
material will be destroyed in the absence of prior
stabilization.
In a preferred embodiment of the invention the acrylic starting
material exhibits a critical temperature of at least about
300.degree. C., e.g., about 300.degree. C. to 330.degree. C. In
addition to visual observation, the detection of the critical
temperature of a given acrylic fibrous material may be aided by the
use of thermoanalytical methods, such as differential scanning
calorimeter techniques, whereby the location and magnitude of the
exothermic reaction can be measured quantitatively.
The strand of polymer fibrous material is continuously wound a
plurality of turns in a single strand thickness and continuously
unwound from at least one cantilevered rotating roll having a
porous surface while a gas at an elevated temperature is expelled
outwardly through the surface of the porous roll and penetrates the
fibrous configuration of the strand wound upon the roll. The roll
or rolls are preferably positioned within an essentially enclosed
heat treatment chamber or zone whereby stabilization off gases may
be confined and withdrawn and/or the appropriate atmosphere
maintained. As the fibrous strand travels a circuitous path around
the periphery of one or more porous roll it is stabilized. The
porous roll may be in the configuration of (1) a truncated cone
[see FIGS. 1 and 2] or (2) it may by cylindrical having an
essentially uniform diameter [see FIG. 3].
As illustrated in FIG. 1, when the porous roll has a conical or
tapered configuration, the strand is passed onto the rotating roll
surface at one end and is wound around the roll in a plurality of
turns prior to leaving the porous roll surface at the other end. A
strand of substantial length may be continuously stabilized within
a relatively small area. The strand is wound about the periphery of
the roll in such a manner that it is in intimate contact with the
rotating porous surface of the roll and moves along the rotating
roll in a helical or spiral path. A heated gas is continuously
expelled through the surface of the porous roll which effectively
penetrates the fibrous configuration of the strand. If desired, a
plurality of porous conical or tapered rolls may be provided within
the heat treatment zone. The rolls may be intergeared for actuation
in unison according to known techniques. The strand may accordingly
pass from the narrow end of one rotating roll to the wide end of an
adjacent rotating roll where the stabilization is continued. If
desired, the gas may be expelled from the surfaces of a series of
rolls at successfully elevated temperatures.
In another embodiment of the invention at least one cylindrical
porous rotatable driven roll 1 is provided in a paired relationship
to a rotatable skewed roll 2, i.e. an inclined roll, as illustrated
in FIG. 3. Alternatively, tapered roll pairs may be utilized with
the taper of each roll being in the same ratio within a given pair.
The resulting pair of rolls may be positioned in the same or a
different plane. The pair of rolls is aligned so that when rotated
a fibrous strand wound or looped about the same will track from one
end of the porous roll to the other while retaining a thickness of
a single strand. Preferably the porous roll is of a substantially
larger diameter than the skewed roll. The skewed roll optionally
may be driven and provided with a porous surface through which a
gas at an elevated temperature is also expelled. If not driven the
skewed roll may idle, i.e. be rotated by the movement of the
fibrous strand wound thereon. The rolls are preferably provided in
a single plane and aligned so that their axes converge slightly. As
will be apparent to those skilled in the fiber art, the strand will
track the length of the heated roll and move towards the end where
the axes converge. The lesser the degree of convergence the lesser
the rate of the lateral movement of the strand along the length of
the heated roll.
The strand is passed to each roll at an angle essentially
perpendicular thereto. When a skewed roll is utilized, the strand
tends to leave each roll for passage to the roll paired therewith
at an angle other than 90.degree.. If desired, the strand may be
introduced into the heat treatment zone for a plurality of passes
in the event the size of the heat treatment chamber is inadequate
to complete stabilization during a single pass.
The strand is treated while in contact with one or more rotating
roll having a porous surface while a gas at an elevated temperature
is expelled outwardly through the surface of the porous roll and
penetrates the fibrous configuration of the strand wound upon the
roll as described at least until the strand attains a stabilized
form which retains it original fibrous configuration essentially
intact. The resulting stabilized strand is commonly black in
appearance. The treatment times during which the strand contacts a
porous roll and temperatures will vary depending upon the
composition and the exact configuration of the strand of polymeric
fibrous material. Temperatures are selected which may be withstood
by the strand without the destruction of its original fibrous
configuration, e.g., through softening, melting, an uncontrolled
exothermic reaction, or decomposition. The higher the temperature
of the gas expelled through the porous roll generally the greater
the rate at which the stabilization reaction occurs. When the
fibrous strand is cellulosic in nature, e.g., rayon or cellulose
acetate, the stabilization treatment may commonly be conducted at
about 200.degree. to 320.degree. C. for a porous roll contact time
of about 5 to 180 minutes. When the fibrous strand is a polyamide,
e.g., nylon 6T, the stabilization treatment may commonly be
conducted at about 200.degree. to 350.degree. C. for a porous roll
contact time of about 5 to 120 minutes. When the fibrous strand is
a polybenzimidazole, e.g., poly-2,2' -m-phenylene-5,5'
-bibenzimidazole, the stabilization treatment may commonly be
conducted at about 400.degree. to 550.degree. C. for a porous roll
contact time of about 2 to 30 minutes. When the fibrous strand is
polyvinyl alcohol, the stabilization treatment may commonly be
conducted at about 180.degree. to 200.degree. C. for a porous roll
contact time of several hours. More highly elevated temperatures
may optionally be utilized during the final portion of the
stabilization treatment.
When the strand of polymeric fibrous material is an acrylonitrile
homopolymer or an acrylonitrile copolymer containing at least about
95 mol percent of one or more monovinyl units copolymerized
therewith, the stabilization temperature is preferably selected in
accordance with the teachings of commonly assigned U.S. Ser. No.
749,957 of Dagobert E. Stuetz which is herein incorporated by
reference. With such starting materials the gas expelled through
the surface of the porous roll may be provided at a temperature of
about 260.degree. C. up to about 10.degree. C. below the critical
temperature of the starting material. The entire stabilization
treatment may be conducted by heating the fibrous material at a
temperature substantially within the above range. In a preferred
embodiment of the invention a strand of an acrylonitrile
homopolymer fibrous material is heated at about 260.degree. C. to
about 300.degree. C. and at a temperature of about 270.degree. C.
to 290.degree. C. in a particularly preferred embodiment of the
invention.
The stabilization treatment of the present process may be expedited
if the gas is expelled through the surface of one or more porous
roll over which the strand passes at successively elevated
temperatures as the strand travels through the heat treatment
chamber. For instance, the gas which penetrates the surface of a
given porous rotating roll may be supplied to the roll surface at
successively elevated temperatures along the length of the roll.
Alternatively, a plurality of porous rolls may be provided in
series with each successive roll expelling a gas through its
surface at a more highly elevated temperature than the previous
roll over which the strand passed. If desired, when stabilizing a
strand of the preferred acrylic precursor the gas may be initially
expelled at a temperature slightly below 260.degree. C. and
subsequently increased to at least about 260.degree. C. where a
substantial portion of the stabilization reaction occurs. During
the final portion of the stabilization reaction the critical
temperature exhibited by the unmodified starting material may
commonly be exceeded. When it is desired to produce a strand of
stabilized acrylic product which is capable of being carbonized or
carbonized and graphitized to a product of high tensile strength,
the present continuous process optionally may be conducted on a
multiple stage basis in accordance with the teaching of U.S. Ser.
No. 749,959 of Michael J. Ram, filed Aug. 5,1968, now U.S. Pat. No.
3,539,295, which is assigned to the same assignee as the instant
invention, and is herein incorporated by reference.
A strand of acrylic fibrous material is subjected to the
stabilization treatment for a residence time sufficient to impart a
bound oxygen content of at least about 7 percent by weight. Higher
bound oxygen contents, e.g., up to 15 percent or more, may be
achieved upon the extended treatment. The weight percentage of
bound oxygen present in the material may be determined by routine
analytical techniques, such as the Unterzaucher analysis. When
operating with a strand of acrylonitrile homopolymer or an
acrylonitrile copolymer containing at least about 95 mol percent of
recurring acrylonitrile units and up to about 5 mol percent of one
or more monovinyl units copolymerized therewith at a stabilization
temperature of at least about 260.degree. C., stabilization
residence times may commonly range from about 10 minutes to about
150 minutes. More extended residence times (e.g. up to 24 hours)
are commonly required when utilizing acrylonitrile copolymers
containing a higher percentage of comonomer and stabilization
temperatures substantially below 260.degree. C., e.g., 200.degree.
C.
When utilizing a plurality of rotating porous rolls in the heat
treatment zone, it is possible to vary slightly their relative
speeds of rotation or the relative roll circumferences in order to
accommodate shrinkage or stretching of the strand as it passes
between the rolls should this be desired. When the strand is wound
along the circumference of a single-tapered porous roll, shrinkage
or stretching of the strand optionally may be carried out depending
upon whether the strand initially contacts the porous-tapered roll
at its wide end or its narrow end respectively, and whether the
particular strand has a propensity to shrink or to stretch at the
temperature of the gas being expelled.
The gas expelled through the surface of the porous roll when
conducting the present process may be raised to the desired
elevated temperature by any convenient means. In a preferred
embodiment of the invention the gas passes over one or more
electric-resistance heating units positioned within the interior of
each porous roll.
The strand of stabilized fibrous material produced in the present
process may optionally next be carbonized or carbonized and
graphitized at more highly elevated temperatures in a nonoxidizing
or inert atmosphere, according to techniques known in the art.
During the carbonization reaction elements present in the strand of
stabilized material other than carbon, e.g., nitrogen, hydrogen and
oxygen, are substantially expelled. The term "carbonized" as used
herein is defined to describe a product consisting of at least
about 90 percent carbon by weight. Suitable atmospheres include
nitrogen, argon, helium, etc. A particularly preferred
carbonization or carbonization and graphitization process is
disclosed in commonly assigned U.S. Ser. No. 777,275, filed Nov.
28, 1968 of Charles M. Clarke which is herein incorporated by
reference.
The following examples are given as specific illustrations of the
inventions. It should be understood, however, that the invention is
not limited to the specific details set forth in the examples.
EXAMPLE I
An acrylonitrile homopolymer is dry spun to produce a 40 fil
continuous yarn, and is hot drawn at a draw ratio of about 7.5:1 to
obtain a highly oriented fibrous material having a single filament
tenacity of about 7 grams per denier. Twenty ends of this yarn are
then plied to produce the 800 fil strand having a total denier of
about 1,150 which is utilized as the starting material.
As shown in FIG. 1, the strand of acrylic fibrous material may be
provided on a bobbin 4 located outside the substantially enclosed
treatment chamber 6 having walls 8. Situated within the heat
treatment chamber 6 is provided a cantilevered rotatable roll 10
having the configuration of a truncated cone and a porous surface.
The wide end 12 of the roll has a diameter 5 inches, and the narrow
end 14 of the roll has a diameter of 4.5 inches. The length of the
roll 10 is 24 inches. The rotatable roll 10 is mounted upon a
hollow conductive shaft 16 which extends through a seal in the wall
of the heat treatment chamber 6, and is driven by the aid of pinion
18 and worm gear 20. The hollow conductive shaft is bolted to
supporting member 21 with the aid of fixed end plate 22 and is
rendered rotatable by bearing 24.
Air is continuously supplied at an adjustable overpressure to the
end 28 of the hollow conductive shaft 16 which extends through
supporting member 21 by a fan (not shown). A multilead cable 30
extends into hollow shaft 16 and supplies electrical current to
resistance heaters (not shown) situated within rotatable roll 10.
Air introduced into hollow shaft 16 at end 28 passes over
resistance heaters and is continuously expelled outwardly through
the porous surface 32 of rotatable roll 10.
As shown in FIG. 2, the hollow conductive shaft of conduit 16 is
coextensive with the length of rotatable roll 10 and further
extends through seal 34 in the opposite wall of the heat treatment
chamber 6 where it is capped. Such extension of hollow conductive
shaft 16 through the wall of the chamber lends additional support
to the rotatable roll 10. Nonconductive end plates 36 and 38 are
situated at the wide and narrow ends respectively of rotatable roll
10. Additional dividers 40, 42, 44 and 46 of a nonconductive
material are positioned around the circumference of central conduit
16 and define a series of heating modules 48, 50, 52, and 56 within
the interior of rotatable roll 10. The porous surface 32 of roll 10
is formed of a sintered ceramic material through which air may be
readily expelled outwardly. Situated within heating modules 48, 50,
52, 54, and 56 are conductive screens 58, 60, 62, 64, and 66 which
are welded to pairs of conductive rings 68, 70, 72, 74, and 76. One
member of each pair of conductive rings is welded to hollow
conductive shaft 16.
Multilead cable 30 includes a series of five individual leads 78,
80, 82, 84, and 86 which pass through oversized openings in hollow
conductive shaft 16 lined with insulative bushings 88, 90, 92, 94,
and 96. The individual power leads 78, 80, 82, 84, and 86 are
welded to one member of each pair of conductive rings 68, 70, 72,
74, and 76 respectively. Electric current is supplied to each
individual power lead 78, 80, 82, 84, and 86 from a variable power
source 98 with a sliding connector or wiper arrangement 100 of the
concentric ring type.
As the strand of acrylic fibrous material is unwound from bobbin 4
at a rate of 2 meters/minute, it passes through an opening in the
wall 8 of the heat treatment chamber 6 and meets the porous surface
32 of rotating roll 10 at its wide end 12. The strand winds 11
turns per inch along the axial length of rotating roll 10 in the
absence of strand overlap. Electric current is supplied to
individual leads 78, 80, 82, 84, and 86 so that conductive screens
56, 60, 62, 64, and 65 impart a uniform temperature of 270.degree.
C. to the air which is expelled outwardly through the porous
surface 32 of roll 10. Such air penetrates the fibrous
configuration of the strand wound upon roll 10 and stabilizes the
same. The contact time of the strand upon roll 10 is 50 minutes. As
shown in FIG. 1 the resulting stabilized strand is continuously
unwound from the narrow end 14 of rotating roll 10 and withdrawn
from heat treatment chamber 6 through opening 102 and wound upon
bobbin 104.
Off gases from the stabilization treatment are removed from heat
treatment chamber 10 through exhaust duct 106, with the aid of fan
108. If desired, gases removed from duct 106 may be scrubbed and at
least partially recycled to the end 28 of hollow conductive shaft
16.
The resulting stabilized strand present upon bobbin 104 is black in
appearance, exhibits a bound oxygen content of 8 percent by weight
as determined by the Unterzaucher analysis, retains its original
fibrous configuration essentially intact, and is nonburning when
subjected to an ordinary match flame.
The stabilized strand may be woven to form a fire-resistant fabric,
or carbonized or carbonized and graphitized in an inert atmosphere
in accordance with the teachings of U.S. Ser. No. 777,275, filed
Nov. 20, 1968 which is herein incorporated by reference. The
carbonized or carbonized and graphitized fibrous products find
particular utility as a reinforcing medium when suspended in a
suitable matrix material to form an article useful as a strong
lightweight structural component.
EXAMPLE II
Example I is repeated with the exception that the air expelled
outwardly through the porous surface 32 of roll 10 at heating
modules 48, 50, 52, 54, and 56 is at 260.degree. C., 275.degree.
C., 310.degree. C., and 350.degree. C. respectively. The fibrous
strand penetrated by the heated air is passed continuously over
each module for a residence time of approximately 5 minutes.
Substantially similar results are achieved.
EXAMPLE III
The stabilization procedure of example I is repeated with the
exception that the strand of starting material is a 740 fil rayon
continuous filament yarn having a total denier of about 2,200, the
air expelled outwardly through the surface 32 of roll 10 is
maintained at a uniform temperature of 220.degree. C., and the
strand is penetrated by the heated air while passing along the roll
for a residence time of 60 minutes.
The resulting strand retains its original fibrous configuration
essentially intact, exhibits enhanced thermal stability, and is
capable of undergoing carbonization.
EXAMPLE IV
The stabilization procedure of example I is repeated with the
exception that the strand of starting material is a 300 fil
continuous filament yarn of nylon 6T (condensation product of
hexamethylenediamine and terephthalic acid) having total denier of
about 900, the air expelled outwardly through the surface 32 of
roll 10 is maintained at a uniform temperature of 315.degree. C.,
and the strand is penetrated by the heated air while passing along
the roll for a residence time of 90 minutes.
The resulting stabilized strand retains its original fibrous
configuration essentially intact, exhibits enhanced thermal
stability, and is capable of undergoing carbonization.
EXAMPLE V
The stabilization procedure of example I is repeated with the
exception that the strand of starting material is a 400 fil
continuous filament yarn of poly-2,2' -m-5,5' -bibenzimidazole (a
preparation of which is described in example II of U.S. Pat. No.
3,174,497) having a total denier of about 1,600 the air expelled
outwardly through the surface 32 of roll 10 is maintained at a
uniform temperature of 470.degree. C., and the strand is penetrated
by he heated air while passing along the roll for a resident time
of 5 minutes.
The resulting stabilized strand is nonburning when subjected to an
ordinary match flame and retains its original fibrous configuration
essentially intact.
Although the invention has been described with preferred
embodiments, it is to be understood that variations and
modifications may be resorted to as will be apparent to those
skilled in the art. Such variations and modifications are to be
considered within the purview and scope of the claims appended
hereto.
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