Field of Invention
[0001] This invention relates to the thermal stabilization of fibers formed from a copolymer
of acrylonitrile and vinyl bromide.
Background of Invention
[0002] The first carbon fibers ever appear to have been made by Edison, who used them as
electrical resistance in Light bulbs. Prepared by pyrolysis of cellulose threads,
these carbon fibers had poor mechanical properties.
[0003] In modern times, the interest in carbon fibers is based mainly on their use as reinforcement
in epoxy or polyester resins (composites). Early in the development of composites,
in the forties, glass fibers were used to provide tensile strength to the formable
matrix, which acts as_an agglomerant, and transfers stress to the fiber. Glass fibers
have a high tensile strength, but a retatively poor elastic modulus, so their use
is confined to applications where high modulus is not required, for example, for silos,
tanks and boat bodies. As increasingly more demanding end uses for composites were
aspired, for example by the automotive and aeronautic industry, other fibrous materials
had to be developed, which would offer not only high tensile strength, but also high
elastic modulus. Such fibers consist mainly of Light elements such as boron, carbon
or beryllium, but also of carbides, nitrides, silicides and oxides. Among these, carbon
fibers are potentially the most interesting, in particular for the automotive industry,
because of their outstanding strength-to-weight and stiffness- to-weight ratios. Carbon
fibers consist essentially (>99.5% by weight) of carbon. They can, in principle, be
made from many organic, fiber forming materials, however only three such materials
have gained industrial importance: rayon, polyacrylonitrile (PAN) and pitch. Rayon
is injured by a relatively high carbon loss during carbonization as the oxygen contained
in rayon fibers tends to be released as CO or C0
2. Pitch based carbon fibers have relatively poor tensile properties, unless they are
prepared from extremely purified (expensive) mesophase pitch. At present, polyacrylonitrile
appears to be the most widely used starting material for carbon fibers.
[0004] Fibers formed from an acrylic homopolymer or copolymer can be modified to enhance
their thermal stability by heating in an oxygen-containing atmosphere at a moderate
temperature for prolonged periods of time. Mechanistically the modification involves
the oligomerization of the nitrile groups to form the so-called ladder structure comprising
dihydropyridine moieties. Intermolecular reaction of nitrile groups also occurs resulting
in crosslinking. As all free radical polymerizations of double and triple bonds, this
reaction is strongly exothermic.
[0005] The resulting structure contains conjugated -C
=N- groups which cause color formation. A light yellow color can be observed at about
150°C, and after heating in a vacuum at temperatures of 180 - 200°C, polyacrylonitrile
has a copper color. The cyclization reaction becomes particularly critical between
200 and 300°C in an oxygen containing atmosphere. If uncontrolled the exothermic oligomerization
of nitrile groups can become explosive and the fibers fuse. However, if a suitable
temperature regimen and sufficient time are provided, an acrylic precursor fiber can
become black, infusible and resistant to flame. Such a fiber is said to be stabilized
and can be further heat treated to form a carbon fiber or a graphite fiber. Heating
such stabilized fibers up to about 1400°C results in high strength carbon fibers while
heating up to about 3000
0C results in high modulus carbon fibers or graphite fibers.
[0006] Control of the thermal stabilization step has been achieved heretofore by heating
the precursor fiber at moderate temperatures over a long period of time extending
up to several hours which becomes a very expensive procedure. The present invention
markedly reduces the time required for stabilization of the fiber and the stabilized
product results, upon additional heat treatment, in carbon fibers with excellent properties.
Summary of Invention
[0007] The present invention comprises a process for the thermal stabilization of a fiber
derived from a copolymer containing from up to about 15 percent by weight of vinyl
bromide and not less than about 85 percent by weight of acrylonitrile whereby said
fiber is heated at a temperature of about 220° to 330°C in an oxygen atmosphere for
about 10 to 30 minutes.
Detailed Description
[0008] The acrylic polymer utilized as the starting material is formed primarily of recurring
acrylonitrile units. In general the starting material acrylic polymer contains not
less than 85 percent by weight of acrylonitrile units and not more than about 15 percent
by weight of vinyl bromide units. Preferably the polymer precursor consists of from
about 85 to 98 percent by weight of acrylonitrile and from about 2 to 15 percent by
weight of vinyl bromide. More preferably the vinyl bromide constitutes 4 to 6 percent
by weight of the starting material polymer. A vinyl bromide content of 4.2 percent
by weight is the most desirable.
[0009] The acrylic precursor typically is provided as a continuous length of a fibrous material
and may be in a variety of physical configurations, such as, for example multifilament
tows, yarns, strands or similar fibrous forms. The fibrous polymer material generally
is comprised of 0.7 to 2.1 denier filaments,preferably 1.5 denier filaments.
[0010] The polymeric fibrous precursor is heated in a continuous furnace, featuring a temperature
profile ranging from about 220° to 330
0C, in an oxygen containing atmosphere, until there is formed a stabilized fibrous
material which retains its original configuration substantially intact and which is
non-burning when subjected to an ordinary match flame. Typically the fibrous material
requires a residence time in the furnace of about 10 to 30 minutes.
[0011] Preferably the oxidizing atmosphere is air however other such atmospheres may be
employed. For example,an oxidizing atmosphere comprised of from 2 to 50 percent oxygen
and an inert gas, such as nitrogen, argon or helium may be utilized.
[0012] The precursor polymeric fibrous material is highly oriented and this characteristic
is maintained or enhanced by stretching the precursor during stabilization which ultimately
enhances the tensile strength of the carbon fibers produced therefrom. Orientation
of the fibrous material is achieved primarily by stretching (6-13 fold) during the
spinning of the filamentary material. The optimum amount of stretching which may be
applied during stabilization depends upon not only the amount of stretch applied during
spinning of the polymeric material but also upon the particular vinyl bromide content.
A lower spinning stretch and a higher vinyl bromide content permit higher stabilization
stretch. Up to 15X stretch may be imparted during stabilization to the fibrous material
employed in the present invention.
[0013] Generally the desirable draw-ratio during stabilization is determined by increasing
the stretch level until filament breakage appears, and backing off so that undamaged
fiber is produced.
[0014] Typically in practicing the present invention the polymeric fibrous precursor is
passed through a heated furnace provided with an oxygen-containing atmosphere by conventional
means. For example a bobbin of spun fibrous precursor may be mounted on a free wheeling
mandrel and redirected through a nip roll. The nip roll is closed and the motor speed
is adjusted to obtain the desired feed rate into the furnace. A similar drive system
is mounted at the furnace exit, and each drive system is independently controlled.
By varying the linear speed at the upper and lower nip rolls the amount of stretch
imparted to the fiber may be controlled. The furnace employed may be,for example,
a vertical, tubular furnace of about 20 feet in length. The furnace temperature profile
can be established by wiring and controlling the heaters in five independent, approximately
equally long zones with Zone 1 being the entrance zone, and Zone 5 being the exit
zone.
[0015] After the fibers have been stabilized they may be completely carbonized in times
as short as 2 minutes without detrimental effects on the resultant carbon fibers by
heating in an inert atmosphere at a temperature which may vary from about 1200° to
about 1450°C. In order to maintain the high orientation, shrinkage should be avoided
during this step. The properties of the final carbon fibers, all other conditions
being essentially the same, depend considerably on the temperature profile in the
furnace and on the residence time of the fiber therein as illustrated in the following
Table I.
[0016] It is apparent that utilization of profiles having a higher entrance and end temperature
renders possible shorter stabilization times and also yields carbon fibers having
excellent tensile properties. Carbon fibers having the best tensile properties result
from precursor fibers which were stabilized in 15 to 20 minutes.
[0017] The acrylonitrile/vinyl bromide polymeric precursor offers certain advantages not
found in known precursor materials. Vinyl bromide has two important properties which
are not found combined in any of the other potential comonomers,for example, vinyl
chloride, methyl methacrylate, methyl acrylate or vinyl acetate. The first such property
is a small molar volume. Due to the small volume of the bromine side chain, vinyl
bromide as a comonomer does not essentially reduce the high molecular order, which
can be obtained in stretched polyacrylonitrile fibers. Since good carbon fiber tensile
properties are a direct consequence of molecular orientation, the advantage is evident.
[0018] Intimately related to the greater molecular order is the relatively high melting
point, which helps to prevent fiber fusion, thus permitting stabilization at higher
temperatures than with other precursors. Since higher reaction temperature means higher
reaction rate, it is possible to achieve shorter stabilization times.
[0019] The second such property is the very weak C-Br bond. The weak C-Br bond (
==65 kcal/mol) is the first bond to break during heat treatment, at a temperature significantly
below that required for main chain scission (main chain C-C bond in polyacrylonitrile:
71 kcal/mol). The breaking of the C-Br bond gives radicals, without fragmentation
of a macromolecule. The radicals may initiate the oligomerization of CN groups at
a lower temperature, compared to the cases where main chain scission is the lowest
energy radical source, for example, acrylonitrile/methyl methacrylate, acrylonitrile/
methyl acrylate, or acrylonitrile/vinyl acetate. This not only spreads the heat evolution
over a greater temperature range, thus preventing fiber fusion, but also takes care
of early crosslinking of macromolecules. As a consequence, the formation of small
fragments is reduced in the temperature range of main chain scission.
[0020] The very reactive and mobile bromine radical most probably does not take part directly
in the oligomerization reaction. Presumably it abstracts the nearest hydrogen it finds,
creating a new carbon radical able to initiate the CN oligomerization:
Hydrobromic acid is very likely to initiate, additionally, the CN oligomerization
by a different, ionic mechanism, as known for succinotrile:
Since part of the bromine appears again as covalently bonded, there might be even
a truly catalytic effect of the bromine.
[0021] The advantage of the small molar volume of vinyl bromide is shared by monomers Like
ethylene, vinyl chloride or vinyl iodide. But, the first two do not have a labile
bond which could give radicals before main chain scission takes place. (C-C in polyethylene:
82 kcaL/moL; C-C1 in polyvinylchloride: 78 kcal/mol). Vinyl iodide, on the other hand,
has too labile a bond (C-1: 53 kcal/mol) which probably would not even survive dope
preparation temperatures.
[0022] VinyL chloride, in a polymer or copolymer, is known to decompose thermally by dehydrochlorination
rather than by a radical break. However, HCl is considerably Less prone than HBr to
attack CN groups. Hence, for several reasons vinyl bromide may be expected to be superior
to vinyl chloride.
[0023] The acrylonitrile/vinyl bromide precursor fiber permits rapid stabilization without
fiber damage, and consequently with resulting carbon fibers (1400°C) of high quality:
sonic modulus about 276 GN/m
2; tensile strength about 2.76 GN/m
2 density 1.7 g/mL. Prior to our invention we have been unable to obtain such high
quality carbon fibers with the reduced stabilization times as herein disclosed.
[0024] The polymeric precursor material described herein may be prepared by any conventional
polymerization procedure, such as mass polymerization methods, solution polymerization
methods, or procedures wherein the monomers are dispersed in the reaction medium,
either by suspension or emuLsion. The polymerization is normally catalyzed by known
catalysts and is carried out in equipment generally used in the art. However, the
preferred practice utilizes suspension polymerization wherein the polymer is prepared
in finely divided form for immediate use in the filament forming operations. Suspension
polymerization according to batch or semi-continuous methods can be used. The preferred
method however is continuous polymerization involving the gradual addition of monomers
and the continuous withdrawal of polymer.
[0025] The polymerization is catalyzed by means of conventional free radical catalysts well
known in the art. Included among these are organic and inorganic peroxides containing
the peroxy group:
A wide variation in the quantity of peroxy compound is possible. For example, from
0.1 to 3.0 percent based on the weight of the polymerizable monomer may be used.
[0026] The well known redox catalyst system also may be used. Redox agents are generally
compounds in a lower valence state which are readily oxidized to the higher valence
state under the conditions of reaction. Through the use of this reduction-oxidation
system it is possible to obtain polymerization to a substantial extent at lower temperatures
than otherwise would be required. Suitable redox agents are sulfur dioxide, the alkali
metal and ammonium bisulfites, and sodium formaldehyde sulfoxylate. The catalyst may
be charged at the outset of the reaction, it may be added continuously or in increments
throughout the reaction for the purpose of maintaining a more useful concentration
of catalyst in the reaction mass. The latter method is preferred because it tends
to make the resultant polymer more uniform in regard to its chemical and physical
properties.
[0027] Although the uniform distribution of the reactants throughout the reaction mass for
the suspension polymerization technique can be achieved by vigorous agitation, it
is generally desirable to promote the uniform distribution of reagents by using inert
wetting agents, emulsion stabilizers, or dispersing agents. Suitable reagents for
this purpose are the water soluble salts or fatty acids, such as sodium oleate and
potassium stearate, mixtures of water soluble fatty acid salts, such as common soaps
prepared by the saponification of animal and vegetable oils, the amino soaps such
as salts of triethanolamine and dodecylmethylamine, salts of resin acids and mixtures
thereof. The water soluble salts of half esters of sulfonic acids and long chain aliphatic
alcohols, sulfonated hydrocarbons, such as alkyl aryl sulfonates, and others of a
wide variety of wetting agents, which are in general organic compounds, containing
both hydrophobic and hydrophilic radicals. The quantity of emulsifying or dispersing
agent will depend upon the particular agents selected, the ratio of monomer to be
used and the conditions of polymerization. In general, however, from 0.1 to 1.0 weight
percent based on the weight of the monomers can be employed.
[0028] The dispersion polymerizations are preferably conducted in stainless steel or glass-lined
vessels provided with means for agitating the contents therein. Generally, rotary
stirring devices are the most effective means of incurring the intimate contact of
the reagents, but other methods may be successfully employed, for example, by rocking
or rotating the reactors. The polymerization equipment generally used is conventional
in the art.
[0029] The polymers from which the filaments are produced in accordance with the present
invention have specific viscosities within the range of 0.1 to 0.3. The specific viscosity
value as employed herein is represented by the formula:
[0030] Viscosity determinations of the polymer solutions and solvents are made by allowing
said solutions flow by gravity at 25°C. through a capillary viscosity tube. In the
determinations herein a polymer solution containing 0.1 gram of the polymer dissolved
in 100 ml of N,N'dimethylformamide is employed. The most effective polymers for the
preparation of filaments are those of uniform physical and chemical properties and
of relatively high molecular weight.
[0031] Filaments prepared from the copolymers of the present invention possess excellent
properties of strength and dimensional stability as well as properties of heat and
light stability which carry over from the polymer. The polymer dopes comprising 10-25
percent solids of copolymers may be spun according to conventional wet, dry or dry
jet-wet methods. In general useful filaments have been manufactured by dissolving
the vinyl bromide copolymers in a polar organic solvent such as dimethylacetamide,
dimethylformamide or dimethylsulfoxide and adjusting the polymer solids to about 25
percent. The polymer dope can then be extruded into a coagulation bath, washed, stretched
and passed through a finish bath before drying. Variations in the process for fiber
preparation are well known in the art.
[0032] In general wet-spinning techniques can be used for the acrylonitrile/vinyl bromide
copolymers, if the somewhat Lower solubility (as compared to copolymers such as acryLonitriLe/vinyL
acetate or acrylonitrile/ methyl acrylate) is taken care of. Solution, for example,
in dimethylacetamide, storage and pumping are made at elevated temperature. Typically,
a 25 percent solution of acrylonitrile/vinyl bromide copolymer is prepared at 112°C.
The dope is deaerated in a vacuum (22 mm Hg) at 85°C for one hour and spun into a
sLow-coaguLant medium (e.g., 60X dimethylacetamide, 40% water, at 40°C) with a jet
stretch of 0.7 to 1.25 X. Orientation is accomplished using a boiling water cascade,
steam tube, and/or a surface heated godet. Stretching factors from 6 to 13 X are suitable.
EXAMPLE 1
[0033] A copolymer consisting of 95.8% acrylonitrile and 4.2% vinyl bromide, with a specific
viscosity η
sp=0.173, is wet-spun from dimethylacetamide into a fiber, with a spin stretch of 13
X. The fiber is subsequently stabilized by passing it through a tubular, 20 ft. Long
furnace, featuring 5 approximately equally Long temperature zones. The applied temperature
profile is as follows:
Zone 1: heating up to 260°C, zone 2: plateau at 260°C, zone 3: heating to 300°C, zone
4: plateau at 300°C, zone 5: heating to 330°C. The residence time of the fiber in
the furnace is 15 minutes. A stabilization stretch of 15% is applied. The atmosphere
is air. The stabilized fiber has a tenacity of 1.8 g/denier and an elongation of 4.8%.
After carbonization at 1400°C, the carbon fiber has a sonic modulus, Es, of 292 GN/m2, a tensile strength σ of 2.95 GN/m and a density ρ of 1.73 g/ml.
EXAMPLE 2
[0034] The precursor fiber of ExampLe 1 is stabilized with the following temperature profile:
Zone 1: heating up to 260°C, zone 2: plateau at 260°C, zone 3: heating to 330°C, zones
4 and 5: plateau at 330°C. The residence time is 11 minutes; 14X stabilization stretch.
Carbon fiber properties:
Es = 262 GN/m2, δ = 2.29 GN/m2 ρ = 1.72 g/ml.
EXAMPLE 3
[0035] The precursor fiber of Example 1 is stabilized with the following temperature profile:
Zone 1: heating up to 260°C, zone 2: plateau at 260°C, zone 3: heating to 310°C, zone
4: plateau at 310°C, zone 5: heating to 330°C. Residence time: 20 minutes, stretch
15%. Carbon fiber properties: Es = 292 GN/m2, δ= 3.07 GN/m2, ρ = 1.72 g/ml.
EXAMPLE 4
[0036] A copolymer consisting of 93.6% of acrylonitrile and 6.4% of vinyl bromide is spun
into a fiber, with spinning stretch 10%. The fiber is stabilized with the following
temperature profile:
Zone 1: heating up to 260°C, zone 2: plateau at 260°C, zone 3: heating to 300°C, zones
4 and 5: plateau at 300°C. The residence time is 30 minutes, stabilization stretch
is 7.5%. Carbon fiber properties: Es = 234 GN/m2, δ= 2.90 GN/m2, ρ = 1.70 g/mL.