Process for the thermal stabilization of acrylic fibers and films

An improved process for the thermal stabilization of an acrylic fibrous material or film is provided. A tetrasubstituted phosphonium salt which is capable of promoting the thermal stabilization is incorporated in a solution of the acrylic polymer prior to extruding the same to form a fibrous material or film, and the acrylic fibrous material or film having the tetrasubstituted phosphonium salt incorporated therein is heated in an oxygen-containing atmosphere until a thermally stabilized fibrous material or film is formed which is black in appearance, non-burning when subjected to an ordinary match flame and which is capable of undergoing carbonization. The presence of the tetrasubstituted phosphonium salt has been found to cause the thermal stabilization reaction to proceed at an accelerated rate.

BACKGROUND OF THE INVENTION 
In the past procedures have been proposed for the conversion of fibers and 
films formed from acrylic polymers to a modified form possessing enhanced 
thermal stability. Such modification has generally been accomplished by 
heating a fibrous material or film in an oxygen-containing atmosphere at a 
moderate temperature for an extended period of time. 
U.S. Pat. Nos. 2,913,802 to Barnett, 3,285,696 to Tsunoda, and 3,539,295 to 
Ram disclose processes for the conversion of fibers of acrylonitrile 
homopolymers or copolymers to a heat resistant form. The stabilization of 
fibers of acrylonitrile homopolymers and copolymers in an 
oxygen-containing atmosphere involves (1) a chain scission and 
crosslinking reaction of adjoining molecules as well as (2) a cyclization 
reaction of pendant nitrile groups. It is generally recognized that the 
rate at which the stabilization reaction takes place increases with the 
temperature of the oxygen-containing atmosphere. However, the 
stabilization reaction must by necessity be conducted at least initially 
at relatively low temperatures (i.e., below about 300.degree. C.), since 
the cyclization reaction is exothermic in nature and must be controlled if 
the original fibrous configuration of the material undergoing 
stabilization is to be preserved. Accordingly, the stabilization reaction 
tends to be time consuming, and economically demanding because of low 
productivity necessitated by the excessive time requirements. Prior 
processes which may shorten the period required by the stabilization 
reaction include those disclosed in U.S. Pat. Nos. 3,416,874, 3,592,595, 
3,647,770, 3,650,668, 3,656,882, 3,656,883, 3,708,326, 3,729,549, 
3,767,773, 3,813,219, 3,814,577, 3,820,951, 3,850,876, 3,917,776, 
3,923,950, 3,961,888, 4,001,382, 4,002,426, and 4,004,053; British Pat. 
Nos. 1,280,850; 1,471,066; 1,478,775; and 1,578,094; and Soviet Author's 
Certificate No. 389,012. 
The above-identified U.S. Pat. No. 4,001,382 and British Pat. Nos. 
1,478,775; 1,471,066; and 1,578,094 contemplate the presence of ammonium 
salts in acrylic fibers during the heat treatment thereof. 
While stabilized acrylic fibrous materials may be used directly in 
applications where a non-burning fiber is required, demands for the same 
have been increasingly presented by manufacturers of carbonized fibrous 
materials. Carbonized fibrous materials are commonly formed by heating a 
stabilized acrylic fibrous material in a non-oxidizing atmosphere such as 
nitrogen or argon, at a more highly elevated temperature. During the 
carbonization reaction elements such as nitrogen, oxygen, and hydrogen are 
substantially expelled. Accordingly, the term "carbonized" as used in the 
art commonly designates a material consisting of at least about 90 percent 
carbon by weight, and generally at least about 95 percent carbon by 
weight. Depending upon the conditions under which a carbonized fibrous 
material is processed, it may or may not contain graphitic carbon as 
determined by the characteristic x-ray diffraction pattern of graphite. 
See, for instance, commonly assigned U.S. Pat. Nos. 3,656,904, 3,723,605, 
3,775,520, 3,900,556, and 3,954,950. 
It is an object of the present invention to provide an improved process for 
forming thermally stabilized acrylic fibers and films. 
It is an object of the present invention to provide an improved process for 
forming a thermally stabilized acrylic fibrous material or film which 
satisfactorily can be carried out on an accelerated basis and optionally 
at a lower stabilization temperature. 
It is an object of the present invention to provide an improved process for 
forming thermally stabilized acrylic fibers and films in which the 
undesirable exothermic nature of the stabilization reaction is modified. 
It is another object of the invention to provide an improved process for 
forming stabilized fibrous materials or films derived from acrylic 
polymers which results in a product which is suitable for carbonization, 
or carbonization and graphitization. 
It is a further object of the invention to provide an improved process for 
forming stabilized fibrous materials or films derived from acrylic 
polymers which provides a product which may be carbonized to form a 
carbonized product in an improved yield. 
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 
A process for the production of stabilized acrylic fibers and films which 
are capable of undergoing carbonization is provided comprising: 
a. providing a solution consisting essentially of (1) an acrylic polymer 
selected from the group consisting of an acrylonitrile homopolymer and 
acrylonitrile copolymers containing at least about 85 mole percent of 
acrylonitrile units and up to about 15 mole percent of one or more 
monovinyl units copolymerized therewith, (2) about 0.5 to 10 percent by 
weight based upon the weight of the acrylic polymer of a tetrasubstituted 
phosphonium salt which is capable of promoting the stabilization of the 
acrylic polymer, and (3) a solvent for the acrylic polymer and the 
tetrasubstituted phosphonium salt selected from the group consisting of 
N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, 
butyrolactone, and N-methyl-2-pyrrolidinone, with the acrylic polymer 
being present in the solution in a concentration of about 10 to 25 percent 
by weight based upon the weight of the solvent, 
b. extruding the solution through a shaped orifice via solution spinning to 
form an acrylic fibrous material or film having incorporated therein about 
0.5 to 10 percent by weight based upon the weight of the acrylic polymer 
of the tetrasubstituted phosphonium salt which was initially present in 
the solution of step (a), and 
c. heating the acrylic fibrous material or film having about 0.5 to 10 
percent by weight based upon the weight of the acrylic polymer of the 
tetrasubstituted phosphonium salt which was initially present in the 
solution of step (a) incorporated therein in an oxygen-containing 
atmosphere at a temperature of about 200.degree. to 350.degree. C., until 
a thermally stabilized fibrous material or film is formed which is black 
in appearance, non-burning when subjected to an ordinary match flame and 
which is capable of undergoing carbonization, with the thermal 
stabilization being conducted at an accelerated rate because of the 
presence of the tetrasubstituted phosphonium salt. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The acrylic polymer utilized as the starting material is formed primarily 
of recurring acrylonitrile units. For instance, the acrylic polymer should 
either be an acrylonitrile homopolymer of an acrylonitrile copolymer which 
contains not less than about 85 mole percent of acrylonitrile units and 
not more than about 15 mole percent of units derived from 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. 
The stabilization promoting agent employed in the process of the present 
invention is a tetrasubstituted phosphonium salt which is capable of 
accelerating the thermal stabilization and which is capable of dissolution 
in the solution of the acrylic polymer. Such tetrasubstituted phosphonium 
salt should not vaporize too readily during the course of the thermal 
stabilization reaction so as to become unavailable and thereby be 
incapable of modifying such reaction. Representative tetrasubstituted 
phosphonium salts suitable for use in the process of the present invention 
include tetraphenylphosphonium bromide, tetraphenylphosphonium chloride, 
(carboethyoxymethyl)triphenylphosphoniuium bromide, 
carboethoxymethyl)triphenylphosphonium chloride, 
phenacyltriphenylphosphonium bromide, phenacyltriphenylphosphonium 
chloride, acetonyltriphenylphosphonium bromide, 
acetonyltriphenylphosphonium chloride, benzyltriphenylphosphonium bromide, 
benzyltriphenylphosphonium chloride, bisxylenetriphenylphosphonium 
bromide, bisxylenetriphenylphosphonium chloride, tetrakis(2-cyanoethyl) 
phosphonium bromide, tetrakis(2-cyanoethyl)phosphonium chloride, and 
mixtures of the foregoing. The salt preferably is a halide salt such as a 
chloride or a bromide. In a particularly preferred embodiment the salt is 
a bromide, such as tetraphenylphosphonium bromide, which offers 
significant resistance to vaporization during the thermal stabilization 
reaction. 
Suitable solvents which may be utilized in the present process are capable 
of dissolving both the acrylic polymer and the tetrasubstituted 
phosphonium salt. Representative organic solvents include 
N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 
butyrolactone, and N-methyl-2-pyrrolidinone. Such solvents should be 
substantially dry but not necessarily anhydrous. The solvent should be 
capable of forming a stable dope with the polymer and tetrasubstituted 
phosphonium salt. The preferred solvents are those which are commonly 
utilized during the spinning of fibers from acrylonitrile homopolymers and 
copolymers. The particularly preferred solvent is N,N-dimethylacetamide. 
The concentration of the acrylic polymer in the solvent may be varied 
widely, e.g. about 10 to 25 percent by weight based upon the weight of the 
solvent. Preferred concentrations range from 16 to 22 percent acrylic 
polymer based upon the weight of the solvent. Preferred inherent 
viscosities for the solution range from approximately 1.1 to 1.7 dl./g. 
The tetrasubstituted phosphonium salt is present in the solution in a minor 
concentration, i.e., about 0.5 to 10 percent by weight based upon the 
weight of the acrylic polymer. The larger concentrations of 
tetrasubstituted phosphonium salt may be used with advantage in those 
embodiments wherein the solution is extruded into a coagulation bath 
wherein a portion of the tetrasubstituted phosphonium salt is removed and 
is not present within the resulting acrylic fibrous material or film. 
In a preferred embodiment of the process the solution of acrylic polymer 
and tetrasubstituted phosphonium salt additionally contains 0.1 to 5.0 
percent by weight based upon the total weight of the solution (0.5 to 2.0 
percent in a particularly preferred embodiment) of lithium chloride 
dissolved therein. The incorporation of lithium chloride serves the 
function of lowering and preserving upon standing the viscosity of the 
solution. The desired solution fluidity and mobility for extrusion are 
accordingly efficiently maintained even upon the passage of time. 
The solution of the acrylic polymer and tetrasubstituted phosphonium salt 
suitable for extrusion may be formed by any convenient technique. For 
instance, the acrylic polymer while in particulate form together with the 
tetrasubstituted phosphonium salt may be added to the solvent with 
stirring while maintained at about 50.degree. to 90.degree. C. It is 
recommended that any heating of the solution in excess of about 
100.degree. C. be of limited duration, i.e., no more than a few minutes in 
order to avoid undesirable reactions that could contribute to gelling or 
instability in the spinning dope. 
The solution is preferably filtered such as by passage through a plate and 
frame press provided with an appropriate filtration medium, prior to 
extrusion in order to assure the removal of any extraneous solid matter 
which could possibly obstruct the extrusion orifice. 
The solution containing the acrylic polymer and the tetrasubstituted 
phosphonium salt is extruded through a shaped orifice to form fibers or 
films by conventional solution spinning techniques (i.e., may be dry spun 
or wet spun). As is known in the art, dry spinning is commonly conducted 
by passing the solution through an opening of predetermined shape into a 
suitable evaporative atmosphere, and wet spinning is commonly conducted by 
passing the solution through an opening of predetermined shape into a 
suitable coagulation bath. 
When wet spinning is utilized in the fiber or film forming step of the 
process, a coagulation bath is selected which is capable of preserving the 
requisite quantity of the tetrasubstituted phosphonium salt within the 
resulting fibrous material or film. More specifically, the bath preferably 
exhibits no propensity to leach out and dissolve the tetrasubstituted 
phosphonium salt below the minimum level required during subsequent heat 
treatment step (described hereafter). Such coagulation bath may inherently 
possess no substantial tendency to dissolve the stabilization promoting 
agent. Alternatively, the coagulation bath which is selected may have its 
inherent tendency to dissolve the stabilization promoting agent diminished 
by preliminary dissolving a substantial quantity of the tetrasubstituted 
phosphonium salt or other compound therein. A representative coagulation 
bath is a 50/50 mixture of N,N-dimethylacetamide and methanol. A preferred 
wet spinning technique is disclosed in commonly assigned U.S. Pat. No. 
3,567,409. 
The shaped orifice or spinneret utilized during the extrusion may contain a 
single hole through which a single filament is extruded, and preferably 
contains a plurality of holes whereby a plurality of filaments may be 
simultaneously extruded in yarn form. The spinneret preferably contains 
holes having a diameter of about 50 to 150 microns when producing 
relatively low denier fibers having an as-spun dienier of about 8 to 24 
denier per filament. Alternatively, acrylic films of relatively thin 
thickness, e.g., about 1 to 10 mils, may be formed, when the extrusion 
orifice is a rectangular slit. 
The resulting as-spun fibrous material or film is preferably maintained in 
a continuous length configuration throughout the process. At an 
intermediate point prior to heat treatment (described hereafter) the 
fibrous material may alternatively be transformed into another fibrous 
assemblage, e.g., a tow, fabric, or yarn of greater total denier. 
When the fibrous material is a continuous multifilament yarn, a twist may 
be imparted to the same to improve the handling characteristics. For 
instance, a twist of about 0.1 to 5 tpi (turns per inch), and preferably 
about 0.3 to 1.0 tpi may be utilized. Also a false twist may be used 
instead of or in addition to a real twist. Alternatively, one may select 
bundles of fibrous material which possess essentially no twist. 
The fibrous material may be drawn in accordance with conventional 
techniques in order to improve its orientation. For instance, the fibrous 
material may be drawn by stretching while in contact with a hot shoe at a 
temperature of about 140.degree. to 160.degree. C. Additional 
representative drawing techniques are disclosed in U.S. Pat. Nos. 
2,455,173; 2,948,581; and 3,122,412. It is recommended that fibrous 
materials prior to the heat treatment (described hereafter) be drawn to a 
single filament tenacity of at least about 3 grams per denier. If desired, 
however, the fibrous material may be more highly oriented, e.g., drawn up 
to a single filament tenacity of about 7.5 to 8 grams per denier, or more. 
In a preferred embodiment the as-spun fibers are drawn to a relatively 
fine denier of approximately 0.6 to 2.0, e.g., 0.7 to 1.7. Additionally, 
the acrylic films optionally may be either uniaxially or biaxially 
oriented prior to the heat treatment (described hereafter). 
Immediately prior to the heat treatment step the acrylic fibrous material 
or film preferably contains the tetrasubstituted phosphonium salt 
incorporated therein in a concentration of about 0.1 to 10 percent by 
weight based upon the weight of the acrylic polymer and most preferably in 
concentration of about 2 to 6 percent by weight (e.g., 5 percent by 
weight) based upon the weight of the acrylic polymer. 
The resulting acrylic fibrous material or film containing the 
tetrasubstituted phosphonium salt incorporated therein is heated in an 
oxygen-containing atmosphere at a temperature of about 200.degree. to 
about 350.degree. C. until a stabilized fibrous product or film is formed 
which retains its original configuration essentially intact and which is 
non-burning when subjected to an ordinary match flame. In a preferred 
embodiment of the process, the oxygen-containing atmosphere is air. 
Preferred temperatures for the oxygen-containing atmosphere range from 
about 250.degree. to 350.degree. C., and most preferably about 260.degree. 
to 290.degree. C. If desired, the fibrous material or film may be exposed 
to a temperature gradient wherein the temperature is progressively 
increased. 
For best results, uniform contact during the stabilization reaction with 
molecular oxygen throughout all portions of the acrylic material is 
encouraged. Such uniform reaction conditions can best be accomplished by 
limiting the mass of fibrous material or film at any one location so that 
heat dissipation from within the interior of the same is not unduly 
impaired, and free access to molecular oxygen is provided. For instance, 
the acrylic fibrous material or film may be placed in the 
oxygen-containing atmosphere while wound upon a support to a limited 
thickness. In a preferred embodiment of the invention, the acrylic fibrous 
material or film is continuously passed in the direction of its length 
through the heated oxygen-containing atmosphere. For instance, a 
continuous length of the acrylic fibrous material or film may be passed 
through a circulating oven or the tube of a muffle furnace. The speed of 
passage through the heated oxygen-containing atmosphere will be determined 
by the size of the heating zone and the desired residence time. A 
particularly preferred continuous heat treatment is disclosed in commonly 
assigned U.S. Ser. No. 749,957, filed Aug. 5, 1968, now abandoned, which 
is herein incorporated by reference. 
The period of time required to complete the stabilization reaction within 
the oxygen-containing atmosphere is generally inversely related to the 
temperature of the atmosphere, and is also influenced by the denier of the 
acrylic fibrous material or the thickness of the film undergoing 
treatment, and the concentration of molecular oxygen in the atmosphere. 
Treatment times in the oxygen-containing atmosphere accordingly commonly 
range from about 15 minutes to several hours. Regardless of the 
stabilization temperature selected within the range of about 200.degree. 
to 350.degree. C., the presence of the tetrasubstituted phosphonium salt 
within the acrylic fibrous material or film results in an accelerated 
stabilization reaction for a given temperature. 
For instance, polyacrylonitrile homopolymer fibers of 1.7 denier per 
filament which incorporated tetraphenylphosphonium bromide in a 
concentration of only one percent by weight based upon the weight of the 
polymer have been examined in nitrogen by differential scanning 
calorimetry employing a DuPont Model 1090 thermal analysis system in the 
temperature scanning mode and the results compared with similar control 
fibers which lack the tetraphenylphosphonium bromide additive. In this 
mode of operation the instrument was programmed to scan the temperature 
range from 25.degree. to 450.degree. C. The output of the instrument in 
this mode was the magnitude of the characteristic exotherm of 
polyacrylonitrile materials which diminishes during the course of the 
stabilization reaction. The stabilization temperature of 265.degree. C. 
was maintained for two and one-half hours, and the residual exotherm 
results are reported below during this period: 
______________________________________ 
Fiber Which 
Incorporated 
Tetraphenylphosphonium 
Fiber 
Bromide Salt Control 
______________________________________ 
1.0 hour 250 J./g. 275 J./g. 
1.5 hours 130 J./g. 240 J./g. 
2.0 hours 100 J./g. 145 J./g. 
2.5 hours 65 J./g. 75 J./g. 
______________________________________ 
The fiber was sufficiently stabilized to undergo carbonization after about 
1.5 hours in spite of the fact that it contained only one percent by 
weight of tetraphenylphosphonium bromide. The control fiber required in 
excess of two hours of heating to achieve the same level of stabilization. 
Had a greater concentration of the tetraphenylphosphonium bromide been 
present in the acrylonitrile hompolymer then the desired thermal 
stabilization would have been accelerated even more. When the 
stabilization is conducted in an oxygen-containing atmosphere in 
accordance with the present invention, such stabilization is similarly 
accelerated. Also, the use of an oxygen-containing atmosphere instead of a 
nitrogen atmosphere contributes to the enhancement of the physical 
properties of the resulting stabilized acrylic fibers and films. 
The stabilized acrylic fibrous materials or films formed in accordance with 
the present process are black in appearance, retain essentially the same 
configuration as exhibited prior to heat treatment, are non-burning when 
subjected to an ordinary match flame, commonly have a bound oxygen content 
of at least 7 (e.g., 7 to 12) percent by weight as determined by the 
Unterzaucher or other suitable analysis, and commonly contain from about 
50 to 65 percent carbon by weight. 
The theory whereby the tetrasubstituted phosphonium salt serves to 
accelerate the stabilization reaction is considered complex and incapable 
of simple explanation. It is believed, however, that the oxygen 
cross-linking and incorporation as well as the cyclization reaction are 
catalyzed and proceed at an accelerated rate. 
Since the stabilization reaction is accelerated in the present process, one 
optionally may elect to carry out the stabilization reaction at a less 
severe temperature than heretofore commonly utilized. Under milder 
temperature conditions a more uniform stabilized product may be achieved 
in the absence of undue chain degradation. 
The stabilized fibrous material resulting from the stabilization treatment 
of the present invention is suitable for use in applications where a fire 
resistant fibrous material is required. For instance, non-burning fabrics 
may be formed from the same. The stabilized film resulting from the 
stabilization treatment is suitable for use in applications where a fire 
resistant sheet material is required. As previously indicated, the 
stabilized acrylic fibrous materials and films are particularly suited for 
use as intermediates in the production of carbonized fibrous materials and 
films (e.g., by heating in a non-oxidizing atmosphere such as nitrogen, 
argon, or helium at a temperature of at least 1000.degree. C. until a 
carbonized fibrous material or film is formed which contains at least 90 
percent carbon by weight). Such amorphous carbon or graphitic carbon 
fibrous products may be incorporated in a binder or matrix to serve as a 
reinforcing medium. The carbon fibers may accordingly serve as a 
lightweight load bearing component in high performance composite 
structures which find particular utility in the aerospace industry. The 
carbonized films may be utilized in the formation of lightweight high 
temperature resistant laminates when incorporated in a matrix material 
(e.g., an epoxy resin).

The following examples are given as specific illustrations of the 
invention. It should be understood, however, that the invention is not 
limited to the specific details set forth in the examples. 
EXAMPLE I 
A solution of acrylic polymer is formed while employing 
N,N-dimethylacetamide as a solvent which is maintained at 50.degree. C. 
Particulate acrylonitrile copolymer containing 98 mole percent 
acrylonitrile units and 2 mole percent methyl acrylate units is added to 
the solvent with stirring in a concentration of 22 percent by weight based 
upon the weight of N,N-dimethylacetamide, and tetraphenylphosphonium 
bromide is dissolved in the acrylic polymer solution with stirring in a 
concentration of 5 percent by weight based upon the weight of the acrylic 
polymer. 
Following filtration, the solution is promptly elevated to 140.degree. C. 
and is fed to a standard cup type spinneret having a circle of 10 holes 
each having a diameter of 10 microns. The jet temperature and the 
temperature of the dry spinning column into which the solution is extruded 
are maintained at 180.degree. C. The spinning column contains circulating 
nitrogen which substantially evaporates the N,N-dimethylacetamide solvent. 
The resulting fibers possess a denier per filament of about 8, are 
subsequently washed to remove residual solvent, and are drawn at a draw 
ratio of about 5:1 by passage over a hot shoe at a temperature of about 
140.degree. C. to yield fibers having a denier of about 0.9, and a single 
filament tenacity of at least 3 grams per denier. 
The resulting fibers contain tetraphenylphosphonium bromide incorporated 
therein in a concentration of about 5 percent by weight, and the pendant 
nitrile groups of the acrylonitrile units present therein are 
substantially uncyclized. 
The fibrous material is next stabilized on a continuous basis by heating 
while continuously passing for 30 minutes in the direction of its length 
through a circulating air atmosphere provided in a circulating air furnace 
provided at 265.degree. C. 
The resulting stabilized fibrous material is black in appearance, flexible, 
has a textile-like hand, retains its original fibrous configuration 
essentially intact, is non-burning when subjected to an ordinary match 
flame, retains strength after glowing in a match flame, and has an oxygen 
content in excess of 10 percent by weight. 
The stabilized acrylic fibers are capable of undergoing carbonization by 
passage for 2 minutes through a resistance heated furnace provided with a 
circulating nitrogen atmosphere at 1350.degree. C. 
In a control run, an identical sample of the acrylonitrile homopolymer 
fibrous material is passed through the circulating air furnace in an 
identical manner with the exception that it contains no 
tetraphenylphosphonium bromide. The resulting fibrous material still 
exhibits a substantial residual exotherm indicating that the stabilization 
reaction failed to progress to the level achieved in Example I wherein the 
tetraphenylphosphonium bromide serves to promote the same. 
EXAMPLE II 
Example I is substantially repeated with the exception that a stabilized 
film is formed instead of a stabilzied acrylic fibrous material. More 
specifically, the solution containing the acrylic copolymer and 
tetraphenylphosphonium bromide is extruded via a standard film extrusion 
procedure and is subsequently hot drawn to form a film having a thickness 
of 0.75 mil. During the thermal stabilization reaction sufficient tension 
is exerted upon the film to maintain a constant longitudinal dimension. 
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.