Filament of polyimide from pyromellitic acid dianhydride and 3,4'-oxydianiline

Polypyromellitimide filaments having high strength and high modulus are obtained by dry-spinning a solution of polyamide-acid of polypyromellitic dianhydride, 3,4'-oxydianiline and paraphenylenediamine, substituted derivatives thereof or 4,4'-diaminobiphenyl or 4,4'-diaminobenzanilide, then converting the polyamide-acid to the polyimide and drawing.

BACKGROUND OF THE INVENTION 
This invention relates to certain high strength and high modulus 
polypyromellitimide filaments and to a method of obtaining them. Aromatic 
polyimides, and particularly polypyromellitimides, are well known for 
their high temperature physical and chemical stability. Certain polyimides 
or modified polyimides have been commercialized as high temperature fibers 
but their tenacity or modulus are inadequate for reinforcement purposes. 
U.S. Pat. No. 3,415,782 discloses a process whereby a polyamide-acid is 
formed into a shaped article and the article then treated to convert the 
polyamide-acid polymer to the polyimide thereof. The polyimide polymers 
described therein were prepared from pyromellitic dianhydride and 
bis(4-aminophenyl)ether or bis(4-aminophenyl)sulfide. 
In the case of filaments comprising polyimide polymers of the 
aforementioned U.S. Pat. No. 3,415,782, it has been found that in some 
applications these filaments have insufficiently high modulus for 
applications in plastics reinforcement where carbon, glass and Kevlar.RTM. 
aramid fibers are currently used. 
SUMMARY OF THE INVENTION 
The filaments of this invention consist essentially of a polyimide of 
recurring units of the formula 
##STR1## 
with from 0 to 60 mol %, preferably from 20 to 30 mol %, of units of the 
formula 
##STR2## 
where n is 0 or 1; R and R.sub.1 which may be the same or different are 
selected from halogen, lower alkoxy, hydrogen or lower alkyl. Filaments of 
the invention typically exhibit a tenacity of at least about 10 g./d., and 
a modulus of at least about 250 g./d. Filaments of the copolymers, 
particularly where 20 to 30 mol % of units of formula B are present, 
typically exhibit tenacities in excess of 12 g./d. and moduli in excess of 
350 g./d. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a preferred embodiment of the process of this invention pyromellitic 
dianhydride is first reacted with 3,4'-oxydianiline or a mixture thereof 
with paraphenylene diamine in an organic solvent under anhydrous 
conditions while maintaining the temperature throughout the reaction below 
50.degree. C., advantageously below 30.degree. C. In place of the 
paraphenylene diamine which is preferred, there may be employed the 
halogen (preferably chloro-), lower alkyl (i.e., 1-4 carbon alkyl), or 
lower alkoxy (i.e., 1-4 carbon alkoxy), substituted derivatives thereof or 
4,4'-diaminobiphenyl. There is thus provided a spinning solution of an 
amide-acid polymer of fiber forming molecular weight, having the formula 
##STR3## 
where R, R.sub.1 and n have the previously ascribed meanings. 
The paraphenylenediamine may be replaced by 4,4'-diaminobenzanilide if 
desired yielding a polyamide-acid of the formula 
##STR4## 
A highly suitable process for the preparation of spinning solutions of the 
above amide-acid polymers involves addition of about 0.97 equivalent of 
solid pyromellitic dianhydride to a solution of about 1.0 equivalent of 
one or both of the appropriate diamines dissolved in an organic solvent. 
The contents of the reaction vessel are desirably kept below 50.degree. 
C., preferably below 30.degree. C. at all times during the addition by 
externally cooling the contents so as to remove the evolved heat of 
polymerization. The resulting polymer solution is then adjusted to a 
suitable solution viscosity by incremental addition of a solution or 
slurry of pyromellitic dianhydride in the organic solvent. Reagents and 
solutions should be essentially anhydrous because of the detrimental 
effect of water upon the polymerization process, and at all times should 
be kept under nitrogen to exclude moisture. For the same reason the 
reaction vessel should be also flushed with nitrogen prior to dissolution 
of the diamine in the solvent. 
While many organic solvents are useful for preparing amide-acid polymers in 
solution form, those found especially suitable for use in the process of 
the invention for the preparation of polymer solutions and for the direct 
dry spinning of strong filaments therefrom are dimethylformamide, 
N,N-dimethylacetamide, pyridine, and mixtures of pyridine with either of 
the other two solvents. A mixture of N,N-dimethylacetamide-pyridine is the 
preferred solvent in most cases. 
Polyamide-acid spinning solutions containing from 10 to 26% or greater by 
weight solids can be employed in the process of the invention although 
spinning solutions of about 20% solids content are particularly well 
suited. Solution viscosity is an important factor in preparing tough 
polyimide fibers by the process of this invention. For example, in the 
spinning of dimethylacetamide solution of the polyamide-acid it has been 
found that suitable filaments can be dry spun from solutions wherein the 
solution viscosity varies between 800 and 1000 poises and preferably 
1500-5000 poises at 30.degree. C. The solution viscosity, as measured in 
poise units with a Brookfield viscometer, increases sharply as equivalence 
is approached and in accordance with the preferred process of preparing 
these solutions, final additions of the pyromellitic dianhydride solution 
or slurry must be made with care. Solution viscosity drops as the 
equivalence point is exceeded. It is, therefore, important for the 
stability of the polymer to avoid using an excess of pyromellitic 
dianhydride in preparing the spinning solutions. 
When the desired solution viscosity has been attained, the preferred 
polyamide-acid spinning solution in the N,N-dimethylacetamide-pyridine 
mixture (which preferably is not handled for prolonged periods of time at 
temperatures above 60.degree. C. prior to spinning) is extruded through a 
spinneret into a heated spinning column which is continuously being swept 
by a steady co-current flow of dry gas, e.g. nitrogen. Normally the gas 
temperature will be well below 200.degree. C., usually 
100.degree.-150.degree. C. for a low number of filaments. Spinning 
conditions should be adjusted so that the freshly spun filaments contain 
about 20% to about 35%, preferably about 20% to 30% of the spinning 
solvent based on the total weight of the filament. The filaments may be 
withdrawn from the spinning column at a rate of about 100-200 yds./min. 
and may then be treated, after they emerge from the spinning chamber, with 
water or a composition relatively free of metal salts and preferably 
volatile under imidization conditions. 
The subsequent steps of the process for obtaining the strong, thermally 
stable polyimide fibers from the as-spun filaments need not be carried out 
in a fixed sequence, since several variations are possible wihin the scope 
of the invention. As previously noted, these post spinning steps involve 
complete conversion of the as-spun filaments to the polyimide species and 
filament drawing at elevated temperatures. 
Conversion of the as-spun filaments to strong, thermally stable polyimide 
filaments by imidization of the polyamide-acid linkages in the polymer can 
be effected by thermal means. Filament drawing can be accomplished after 
the conversion process, i.e., after the as-spun filaments have been heat 
treated to form the imide. The filaments are ordinarily drawn at least 
3.times. at temperatures in the range of 450.degree. C. to 700.degree. C. 
Polymer composition, draw ratio and temperature are variables which must 
be taken into account when preparing the filaments of this invention. For 
example drawing the filaments of Example 3 below, 6.times. at 550.degree. 
C. results in fusion of the filaments. 
The filaments of the invention may be of denier up to 10 denier per 
filament or more, and of various shapes. Typical uses for the filaments 
because of their excellent tensile properties include protective clothing 
and reinforcement for polymer matrix composites. 
The filaments of the invention may be modified by typical additives such as 
pigments, finishes and the like. In some cases these may be advantageously 
provided in the filaments by inclusion directly in the spinning solution. 
Fiber properties of tenacity, elongation, and initial modulus, are coded as 
T/E/Mi and are in conventional units of grams per denier (g/d), percent, 
and grams per denier (g/d), respectively. 
The following examples will further illustrate the invention. It will be 
understood that these examples are deemed to be representative of this 
invention but do not constitute all of the runs performed and results 
obtained. Variations in results are believed to be due primarily to use of 
different draw temperatures and draw ratios and variability in fiber 
denier occasioned by the small-scale, experimental nature of the equipment 
.

EXAMPLE 1 
This example illustrates the preparation of polyamide-acid filaments from a 
copolymer of pyromellitic dianhydride, 3,4'-oxydianiline and 
paraphenylenediamine, conversion to the imide and drawing at elevated 
temperatures. The mole ratio of the 3,4'-oxydianiline to the 
paraphenylenediamine used was 3:1, hence 75 mol % of unit A (as previously 
described) is present in the copolymer and 25 mol % of unit B (as 
previously described) is present. 
Under anhydrous conditions 48 g. 3,4'-oxydianiline (0.24 mole) and 8.64 g 
paraphenylenediamine (0.08 mole) was dissolved in a mixture of 396 ml dry 
pyridine and 44 ml dry N,N'-dimethylacetamide. (The latter is necessary to 
prevent gelation of the spin dope.) To this solution, at room temperature, 
was added, all at once, 67.68 g. predried pyromellitic dianhydride (0.32 
mole) with good stirring. When all of this had reacted (dissolved), a 
solution/slurry of 5 g. pyromellitic dianhydride/50 ml dimethylacetamide 
was added portionwise until the solution viscosity had built up to a level 
(estimated 3000-4000 poises) suitable for dry spinning. 
This solution containing 22% polymer, was dry spun under the following 
conditions: 
______________________________________ 
Spinneret 10 holes of 0.005 inch diameter 
Pack Screens 
Solution temp. 30.degree. C. 
Column temp. 135.degree. C. 
Spinneret temp. 60.degree. C. 
Gas temperature 110.degree. C. 
______________________________________ 
Operability was excellent. The as-spun fiber, without finish, was piddled 
into a perforated drum container and heat-treated: 20 min at 150.degree. 
C.+20 min at 200.degree. C.+30 min at 300.degree. C. There was no fiber 
sticking. The fiber was backwound onto cones and drawn through an 18 inch 
hot tube in nitrogen at various temperatures and draw ratios as shown in 
Table I. The feed rate was about 5 ft/min. As-spun dpf was about 8 dpf. 
TABLE I 
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AVERAGE BEST BREAK 
Temp. Draw Ratio T/E/Mi (g/d) T/E/Mi (g/d) 
______________________________________ 
-- As-spun 1.8/125/2 2.0/134/28 
550.degree. C. 
4X 12.6/7.1/354 12.8/7.4/365 
575 4.75X 14.7/4.7/427 18.1/4.9/574 
600 4.7X 14.5/3.7/492 15.1/3.8/-- 
650 6.0X 12.8/2.8/519 14.1/3.3/555 
675 6.1X 15.6/3.3/570 18.7/4.0/717 
700 6.8X 13.1/3.0/592 13.6/3.6/635 
700 10X 15.5/3.4/534 16.2/3.6/561 
750 5X 3.6/3.7/168 4.0/4.1/172 
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EXAMPLE 2 
For comparison with the foregoing copolyimide, the homopolymer was prepared 
by similar procedure as a 22% solution of the polyamide-acid in 
dimethylacetamide/pyridine (10/90). This was dry spun under similar 
conditions to the copolymer above and cycloimidized as a piddled package 
in the same way. Table II shows that the best tensile properties obtained 
from the homopolymer were below those of the copolymer. Whereas, the 
maximum drawability of the latter was almost 10.times., the homopolymer 
was not drawable by more than 5.times.. 
TABLE II 
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DRAWABILITY AND PROPERTIES OF HOMOPOLYMER 
AVERAGE BEST BREAK 
Temp. Draw Ratio T/E/Mi T/E/Mi 
______________________________________ 
500.degree. C. 
.sup. 3.6X 9.5/8.6/248 11.7/11.5/286 
550 4.5 8.9/2.7/404 10.1/3.3/450 
550 4.0 10.7/5.5/302 12.3/6.5/396 
575 4.0 8.3/4.2/321 9.1/5.9/368 
______________________________________ 
This example illustrates the preparation of a copolymer of pyromellitic 
dianhydride, 3,4'-oxydianiline and paraphenylenediamine. Filaments were 
spun therefrom, converted to the imide and then hot drawn. 
Polymerization 
A 21.7% solution of the polyamide-acid in pyridine-dimethylacetamide 
(72/28) was prepared by dissolving 19.2 g. 3,4'-oxydianiline (0.096 mole) 
and 6.92 g. paraphenylenediamine (0.064 mole) in a mixture of dry pyridine 
(154 ml) and dry dimethylacetamide (66 ml), i.e., a 72:28 weight ratio. At 
10.degree.-20.degree. C., with external cooling as necessary and under a 
slow current of dry nitrogen to exclude atmospheric moisture, 32.0 g. 
pyromellitic dianhydride (0.147 mole; 92% of theoretical) was mixed in 
rapidly and quantitatively. The polymer solution was brought to a high 
bulk viscosity by portion-wise addition of a slurry/solution of 5.0 g. 
pyromellitic dianhydride (0.023 mole; 6.5% excess of stoichiometric 
amount) until the desired viscosity was reached. 
Spinning and Heat-Treatment 
This polymer solution as dry-spun by extuding it from a reservoir, in which 
it was held at 55.degree. C., through a 10-hole spinneret with each hole 
0.001 in. diameter, into a concurrent flow of nitrogen at 125.degree. C. 
at a rate of 6.0 ft.sup.3 /min, inside a column 6 in. diameter.times.16 
ft. long with walls heated uniformly at about 125.degree. C. Fiber was 
wound up at 375 ft./min. 
The yarn was piddled into a perforated metal basket which was heated, under 
nitrogen, in an oven at 150.degree. C./20 min.+200.degree. C./20 
min.+300.degree. C./30 min. At this point the filaments had 
T/E/Mi/dpf=1.60/136/36.8//7.18. The fibers were manually stretched 
-6.times. over a short hot-plate under nitrogen at 500.degree. C. to give 
average T/E/Mi=12/3.2/472 and highest individual filament break of 
15/3.6/557. At 550.degree. C. and above, the fibers fused. 
EXAMPLES 4-7 
Copolyamide-acid solutions similar to those of Example 3 and containing the 
molar percentages of 3,4'-oxydianiline comonomer as shown in Table III, 
were prepared, spun, and heat-treated similarly to Example 3. The 
as-imidized fibers were single-stage drawn in a similar manner across a 
hot-plate, with approximately one inch contact distance at various 
temperatures between 450.degree. C. and 600.degree. C. Table III showed 
maximum draw ratios within this temperature range, the maximum average (of 
five or more breaks) tensile properties, and the best single break 
properties for each composition. 
TABLE III 
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Ex- 
am- Mole % 3,4.sup.1 - 
Max. Average Best Break 
ple oxydianiline 
Draw Ratio T/E/Mi T/E/Mi 
______________________________________ 
4 100 5X 11/3/5.1/297 
12.7/5.4/326 
5 60 6X 11.8/3.2/472 
14.8/3.6/557 
6 50 6X 10.1/4.4/401 
10.6/4.9/414 
7 40 4X 9.3/2.8/428 
11.2/2.9/575 
-- 0 1.4X 2.6/3.3/118 
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The filaments had appreciable non-uniformity.