CARBON FIBERS OBTAINED FROM SILICON TREATED POLYOLEFIN

PRECURSOR FIBERS

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a process for producing stabilized fibers and carbon fibers.

[0002] Carbon fibers serve an ever increasing need. The world production of carbon fiber in 2010 was 40 kilometric tons (KMT) and is expected to grow to 150 KMT in 2020.

Industrial-grade carbon fiber is forecasted to contribute greatly to this growth, wherein low cost is critical to applications. The traditional method for producing carbon fibers relies on polyacrylonitrile (PAN), which is solution-spun into fiber form, oxidized and carbonized. Approximately 50 percent of the cost of the carbon fiber is associated with the cost of the polymer itself and solution-spinning.

[0003] In an effort to produce low cost industrial grade carbon fibers, various groups studied alternative precursor polymers and methods of making the carbon fibers. Precursor alternatives to PAN fibers have included cellulosic yarns, nitrogen-containing polycyclic polymers and pitch. Preparing carbon fibers from each different precursor entails challenges unique to the precursor and the carbonization process for each precursor has to be designed for the chemistry of the particular precursor

[0004] More recent efforts have included work with stabilized polyolefin (S-PO) fibers such as sulfonated polyethylene fibers. US 4,070,446 and WO 92/03601, for example, both teach methods for sulfonation of polyethylene fibers and subsequent conversion to carbon fibers and even further conversion to graphitized carbon fibers. The use of S-PO fibers to produce carbon fibers is a relatively new technology and historically has produced carbon fibers with lower tensile strength and Young's modulus compared to carbon fibers from other known precursors. High temperature graphitization (typically in excess of 2000 degrees Celsius (°C)) of S-PO fibers can help increase the resulting carbon fiber Young's modulus, but also increases the processing cost and complexity.

[0005] Lubricants, such as silicon-based lubricants, are used in the production of carbon fibers. Such lubricants are known to be beneficial when added to a carbon fiber process, such as before the air oxidation process or before the carbonization process. It is generally accepted that such coatings do not substantially permeate the interior structure of the carbon fiber.

[0006] It is desirable to provide a process for creating carbon fibers from S-PO fibers, such as sulfonated polyolefin fibers, that increases Young's modulus and preferably also tensile strength of the resulting carbon fiber without requiring heating to temperatures in excess of 2000°C, or even 1800°C.

BRIEF SUMMARY OF THE INVENTION

[0007] A carbonaceous article is prepared from a polyolefin fabricated article. The polyolefin fabricated article is stabilized and is treated with a silicon source. In one instance the silicon source is a siloxane. In one instance the fabricated article is stabilized by sulfonation. In one instance the carbonaceous article comprises a fiber having 90-100 weight percent Carbon, 0.1-1 weight percent Silicon, 0.1-1 weight percent Nitrogen.

DETAILED DESCRIPTION OF THE INVENTION

[0008] Test methods refer to the most recent test method as of the priority date of this document when a date is not indicated with the test method number. References to test methods contain both a reference to the testing society and the test method number.

[0009] "And/or" means "and, or as an alternative". All ranges include endpoints unless otherwise indicated.

[0010] "Elastic modulus" and "Young's modulus" are interchangeable.

[0011] The process of the present invention is useful for preparing carbon fibers, preferably graphitized fibers from stabilized polyolefin fiber.

[0012] "Carbon fiber" is a fiber comprising an excess of 70 wt% carbon, preferably 80 wt% carbon or more, still more preferably 90 wt% carbon or more by weight of the fiber and wherein the carbon weight exceed hydrogen weight by a factor of twenty or more, preferably fifty or more.

[0013] "Graphite fiber" is a form of carbon fiber this is characterized by ordered alignment of hexagonal carbon rings - a crystal-like structure and order. A carbon fiber becomes more graphite in nature as the amount and organized arrangement of hexagonal rings increases in the carbon fiber. [0014] "Graphitized carbon fibers" are carbon fibers demonstrating some degree of crystallike structure and order.

[0015] Stabilized polyolefin (S-PO) fibers are polyolefin fibers that have been chemically modified so as to experience less than 10 weight-percent (wt%), more preferably less than 5 wt%, or even more preferably less than 1 wt% and preferably no detectable hydrocarbon loss at temperatures up to 600°C by thermogravimetric analysis based on fiber weight. Polyolefin (PO) fibers can be converted into S-PO fibers by crosslinking, oxidizing (for example air oxidation) or sulfonating the polyolefin fibers.

[0016] The polyolefin fiber that is chemically modified so as to become an S-PO can be a polyolefin homopolymer or multipolymer, including multipolymers comprising both olefins and non-olefins. Herein, "multipolymer" refers to polymers of more than one type of monomer such as copolymers, terpolymers and higher order polymers. Desirably, the polyolefin fiber is a homopolymer or copolymer comprising one or any combination or more than one of ethylene, propylene, butadiene and/or styrene units.

[0017] Polyethylene homopolymer and multipolymers, particularly copolymers, are especially desirable polyolefin fibers. Preferable polyethylene copolymers include ethylene/octene copolymers, ethylene/hexene copolymers, ethylene/butene copolymers, ethylene/propylene copolymers, ethylene/styrene copolymers, ethylene/butadiene copolymers, propylene/octene copolymers, propylene/hexene copolymers, propylene/butene copolymers, propylene/styrene copolymers, propylene/butadiene copolymers,

styrene/octene copolymers, styrene/hexene copolymers, styrene/butene copolymers, styrene/propylene copolymers, styrene/butadiene copolymers, butadiene/octene copolymers, butadiene/hexene copolymers, butadiene/butene copolymers, butadiene/propylene copolymers, butadiene/styrene copolymers, ethylene/ethylidene norbornene copolymers, ethylene/propylene/ethylidene norbornene copolymers or a combination of two or more thereof. As used herein, octene may refer to any octene isomer. In one instance octene refers to 1 -octene.

[0018] The polyolefin is desirably a multipolymer, preferably a copolymer of ethylene and octene.

[0019] Polyolefin multipolymers can have any arrangement of monomer units. For example, the polyolefin multipolymer can be linear or branched, alternating in monomer units or blocks of monomer units (such as diblock or triblock polymers), graft multipolymer, branch copolymers, comb copolymers, star copolymers or any combination of two or more thereof.

[0020] The polyolefin fiber and S-PO fiber can be of any cross-sectional shape such as circular, oval, star-shaped, that of a hollow fiber, triangular, rectangular and square.

[0021] The S-PO fibers are desirably sulfonated polyolefin fibers. Sulfonated polyolefin fibers are polyolefin fibers that are stabilized by being sulfonated and comprising sulfate functionalities. Any means of sulfonating a polyolefin fiber is suitable for preparing the sulfonated polyolefin fiber for use in the process of the present invention. For example, a suitable means of sulfonating a polyolefin fiber is by exposing the polyolefin fiber to a sulfonating agent such as concentrated and/or fuming sulfuric acid, chlorosulfonic acid, and/or sulfur trioxide in a solvent and/or as a gas. Preferably, prepare the sulfonated polyolefin fiber by treating the polyolefin fiber with a sulfonating agent selected from fuming sulfuric acid, sulfuric acid, sulfur trioxide, chlorosulfonic acid or any combination thereof. Sulfonation can be a step-wise process during which a polyolefin fiber is exposed to a first sulfonating agent and then a second sulfonating agent and the, optionally, a third and optionally more sulfonating agents. The sulfonating agent in each step can be the same or different from any other step. Typically, sulfonating occurs by running a polyolefin fiber through one or more than one bath containing a sulfonating agent.

[0022] One desirable method for sulfonating a polyolefin fiber is to treat the polyolefin fiber with fuming sulfuric acid (first step), then with a concentrated sulfuric acid (second step) and then by a second concentrated sulfuric acid treatment (third step). The temperature during each of the three steps can be the same or different from one another. Preferably, the temperature in the first step is lower than the temperature during the second step. Preferably the temperature during the second step is lower than the temperature during the third step. Examples of suitable temperatures include: for the first step: zero degrees Celsius (°C) or higher, preferably 30 °C or higher and more preferably 40 °C or higher and at the same time desirably 130 °C or lower, preferably 100 °C or lower; desirably 105-130 °C for the second step and desirably 130-150 °C for the third step. Residence times in each step can range from 5 minutes or more to 24 hours or less.

[0023] It is desireable to treat the S-PO fiber with a siloxane source. Suitable siloxane sources include siloxane, cyclic siloxane, and polydiorganosiloxane oligomer (cyclic or linear hydroxyl end-terminated). In one instance, an aqueous siloxane solution is selected as the siloxane source and to treat a S-PO fiber by exposing the S-PO fiber to the aqueous siloxane solution. The concentration of siloxane in the aqueous siloxane solution is typically 0.1 weight percent to 1.0 weight percent. Most preferably the siloxane solution is a saturated siloxane solution at the temperature of exposure to the S-PO fiber. In another instance, a neat siloxane solution is used to treat the S-PO fiber. Commercially available siloxanes are suitable, for example, Advalon CF 3295 (octamethyl cyclotetrasiloxane).

[0024] It is desirable to expose the S-PO fiber to a siloxane source for a sufficient period of time so as to incorporate sufficient siloxane with the S-PO fiber to obtain a silicon concentration in the S-PO fiber of at least 1 mole-percent. Further, it is desired to incorporate sufficient silicon in the S-PO fiber to yield a final carbon fiber as described below with the description of the carbon fiber. Treatment time of S-PO fibers with a siloxane source can range from 0-180 min.

[0025] Heat the S-PO fiber that has been treated with a siloxane source in an inert atmosphere in order to convert the S-PO fiber into a carbon fiber. Heating in an inert atmosphere prevents oxidative degradation of the S-PO fiber during carbonization. An inert atmosphere contains less than 100 parts per million by weight oxygen based on total atmosphere weight. The inert atmosphere can contain inert gasses (gasses that will not oxidize the PO fiber during the heating process). Examples of suitable inert gasses include nitrogen, argon, and helium. The inert atmosphere can be a vacuum, that is a pressure lower than 101 kiloPascals. The oxygen level can be reduced purely by purging with one or more than one inert gas, by purging with inert gas and drawing a vacuum, or by drawing a low enough vacuum to reduce the oxygen level to a low enough concentration to preclude an undesirable amount of oxidation of the S-PO fiber during heating.

[0026] Heat the S-PO fiber in an inert atmosphere to a temperature of 1000°C or higher in order to carbonize the S-PO fiber. Preferably, heat the S-PO fiber in an inert atmosphere to a temperature of 1150°C or higher, more preferably 1600°C or higher, still more preferably 1800°C or higher. Heating can be to a temperature of 2000°C or higher, 2200°C or higher, 2400°C or higher and even 3000°C or higher. However, generally heating is to a temperature of 3000°C or lower. In some instances, higher heating temperatures are desirable for carbonizing S-PO fibers because higher temperature can convert the fiber to a graphite fiber having higher strength, higher Young's modulus or both than non-graphite carbon fiber.

[0027] Heat the fiber as long as necessary to achieve desired properties. Generally, the longer a fiber is heated the more complete the carbonization and more aligned the carbon becomes. Generally, the duration of heating is a balance of processing the fibers fast enough to be commercially viable while still heating long enough to achieve desired fiber properties.

[0028] It is observed that treating a S-PO fiber with siloxane prior to carbonization allows production of carbon fibers having higher strength, higher Young's modulus or both higher strength and higher Young' s modulus than carbon fibers produced from S-PO fibers that do not contain siloxane and that are carbonized at the same carbonization temperature. For the sake of this invention strength refers to tensile strength. It is observed that the overall carbon fiber mass yield is improved when treating with siloxane as compared to a fiber which has been similarly prepared but not treated with siloxane.

[0029] In one instance, a carbonaceous article is provided having a fiber having 90-100 weight percent carbon. In one instance, a carbonaceous article is provided having a fiber having 94.4- 100 weight percent carbon. In one instance, a carbonaceous article is provided having a fiber having 0-1 weight percent silicon. In one instance, a carbonaceous article is provided having a fiber having 0.004-0.410 weight percent silicon. In one instance, a carbonaceous article is provided having a fiber having 0- 1 weight percent nitrogen. In one instance, a carbonaceous article is provided having a fiber having 0-0.9 weight percent nitrogen.

Examples

[0030] In the Examples, carbon fiber mass yield, YCF is calculated according to the following equation:

[0031] In this equation, p _CF is the calculated linear density of the carbonaceous article, p _FA is the calculated linear density of the fabricated article prior to treatment, S _s (stabilization shrinkage) is as calculated according to the equation herein, and S _c (carbonization shrinkage) is as calculated according to the equation herein.

[0032] In the Examples, shrinkage of the stabilized article is calculated according to the following equation:

Length of S— PO Article

s Length of Fabricated Article

[0033] In this equation, the length of S-PO Aricle refers to the length of the fabricated article following the stabilization step, for example, following sulfonation. In this equation, the Length of Fabricated Article refers to the length of the fabricated article prior to any treatment steps.

[0034] In the Examples, shrinkage of the carbonaceous article is calculated according to the following equation:

Length of Carbonaceous Article

Length of S— PO Article

[0035] In this equation, the length of the Carbonaceous Article refers to the length of the fabricated article following carbonization. In addition to carbon fiber mass yield, there is also interest in quantifying the relative change in mass yield, %YCF, and linear density, %PCF- For this calculation, we determine the linear density, p _C F , and carbonization shrinkage, Sc, for an untreated (superscript C) and treated sulfonated polyethylene

(superscript T) sample. Therefore, relative improvement in mass yield carbon fiber linear density are defined as

[0036] As the same starting SPE material is used for comparison tests, terms for linear density of polyethylene and sulfonation shrinkage cancel.

[0037] Percentages used in these examples are by weight unless specifically stated otherwise.

[0038] Example 1

[0039] A polyethylene/l-octene copolymer (density = 0.955 g/cm ³ ; MI = 30 g/10 min, 190°C/2.16 kg) is melt- spun to form a continuous fiber tow containing 1758 filaments (diameter = 8.8 microns; tenacity = 4.54 g/denier; elongation-to-break = 7.61%). The fiber tow is then sulfonated in a 4-bath continuous process under constant tension. The first bath contains 6% fuming sulfuric acid (maintained at 120 °C). The second bath contains 1% fuming sulfuric acid (maintained at 120 °C). The third bath contains 95-98% sulfuric acid (Sulfuric acid 95-98%, obtained from EMD, UN1830, SX1244-6, Lot 51320; MW 98.08; CAS: 766493-9, maintained at 140 °C). Each bath is maintained at a constant temperature. The fourth bath contains deionized water. An average fiber tension of 151.5 gf (gram- force) is maintained in the first bath. An average fiber tension of 125 gf is maintained in the second, third, and fourth baths. The fiber tow is advanced sequentially through the first, second, third and fourth baths such that the fiber tow resides in each bath for 15 minutes, thereby providing a stabilized fabricated article. Seven segments of the stabilized fabricated article are then treated at room temperature with Advalon CF 3295 (manufactured by Wacker) for varying lengths of time without tension, thereby providing seven siloxane- treated stabilized fabricated articles. Each of the segments is then carbonized in a continuous oven up to 1150°C with 0.5 N tension to yield a carbonaceous article, with properties as identified in Table I.

Table I

Segment Siloxane Young's Tensile Strain

Treatment Modulus Strength (%)

Time (min) (GPa) (GPa)

1 0 56.20 0.897 1.605

2 1 64.785 1.039 1.601

3 10 68.774 1.122 1.640

4 60 74.568 1.539 2.058

5 85 70.935 1.454 2.042

6 100 81.394 1.364 1.674

7 180 81.126 1.350 1.665

[0040] Example 2

[0041] A polyethylene/l-octene copolymer (density = 0.955 g/cm ³ ; MI = 30 g/10 min, 190°C/2.16 kg) is melt- spun to form a continuous fiber tow containing 1758 filaments (diameter = 9.1 microns; tenacity = 3.989 g/denier; elongation-to-break = 8.51%). The fiber tow is then sulfonated in a 4-bath continuous process under constant tension. The first bath contains 20 to 30% free oleum (maintained at 50 °C). The second and third baths both contain 95-98% sulfuric acid (Sulfuric acid 95-98%, manufactured by EMD, UNI 830, SX1244-6, Lot 51320; MW 98.08; CAS: 766493-9, bath 2 maintained at 120 °C, bath 3 maintained at 140 °C). Each bath is maintained at a constant temperature. A constant fiber tension of 250 gf (gram-force) is maintained in the first bath. A constant fiber tension of 150 gf is maintained in both the second and third baths. The fiber tow is advanced sequentially through the first, second, third and fourth baths such that the fiber tow resides in each bath for 60 minutes, thereby providing a stabilized fabricated article. Segments of the stabilized fabricated article are then treated according to three treatment regimes.

[0042] In regime 1, four segments (A, B, C, D) of the stabilized fabricated article are not treated with a siloxane-containing solution. Here the fourth bath is a deionized water bath with a residence time of 60 minutes.

[0043] In regime 2, four segments (E, F, G, H) of the stabilized fabricated are treated at room temperature with Advalon CF 3295 (manufactured by Wacker) in the fourth bath with a residence time of 60 minutes. Tension is maintained at 150 gf, following treatment the segments are dried in-line (-0.25-1 min) with nitrogen drying jets, thereby providing four siloxane-treated stabilized fabricated articles.

[0044] In regime 3, four segments (I, J, K, L) of the stabilized are treated at room temperature with Advalon CF 3295 (manufactured by Wacker) in the fourth bath with a residence time of 60 minutes. Tension was maintained at 150 gf, thereby providing four siloxane-treated stabilized fabricated articles.

[0045] Each of the segments prepared according to regimes 1, 2 and 3 are then carbonized in a continuous oven at a temperature listed in Table II with 0.5 N tension to yield a carbonaceous article having the properties described in Table III. It is noted that the segments of regime 1 do not have strength or modulus improvement data since these segments are the control against which such data is measured for the other regimes.

Table II

Segment Carbon Fiber Carbonization

Diameter (microns) Temperature (°C)

A 8.42 1,150

B 7.40 1,800

C 7.38 2,200

D 6.91 2,400

E 8.50 1,150

F 8.55 1,800

G 8.12 2,200

H 7.43 2,400

I 8.14 1,150

J 7.95 1,800

K 7.83 2,200

L 7.56 2,400

Table III

Segment Strain (%) Tensile Young's % Strength % Modulus

Strength Modulus Improvement Improvement

(GPa) (GPa)

A 1.389 1.086 78.048

B 1.316 1.365 104.162

C 1.065 1.431 134.246

D 0.920 1.560 167.350

E 1.531 1.417 93.231 30.5 19.5

F 1.165 1.192 101.201 -12.7 -2.8

G 1.206 1.759 145.731 22.9 8.6

H 0.876 1.611 185.458 3.3 10.8

I 1.648 1.477 89.962 36.0 15.3

J 1.112 1.204 108.447 -11.8 4.1

K 1.027 1.472 144.583 2.9 7.7

L 0.661 1.142 174.360 -26.8 4.2

[0046] Example 3

[0047] A polyethylene/l-octene copolymer (density = 0.955 g/cm ³ ; MI = 30 g/10 min, 190°C/2.16 kg) is melt- spun to form a continuous fiber tow containing 1758 filaments (diameter = 8.5 microns; tenacity = 4.434 g/denier; elongation-to-break = 8.483%). The fiber tow is then sulfonated in a 4-bath continuous process under constant tension. The first bath contains 20 to 30% free oleum (manufactured by Acros) maintained at 50 °C). The second and third baths both contain 95-98% sulfuric acid (Sulfuric acid 95-98%, manufactured by EMD, UN1830, SX1244-6, Lot 51320; MW 98.08; CAS: 766493-9, bath 2 maintained at 120 °C, bath 3 maintained at 140 °C). Each bath is maintained at a constant temperature. The fourth bath contains deionized water. A constant fiber tension of 292.5 gf (gram-force) is maintained in the first bath. A constant fiber tension of 194 gf is maintained in both the second, third, and fourth baths. The fiber tow is advanced sequentially through the first, second, third and fourth baths such that the fiber tow resides in each bath for 60 minutes, thereby providing a stabilized fabricated article. Eight segments (M, N, O, P, Q, R, S, T) of the stabilized fabricated article are then treated at room temperature with Advalon CF 3295 (manufactured by Wacker) for varying lengths of time without tension listed in Table IV, thereby providing eight siloxane-treated stabilized fabricated articles. Each of the segments is then carbonized in a continuous oven at a temperature listed in Table IV with 0.5 N tension to yield a carbonaceous article having the properties described in Table V and Table VI. It is observed that tensile strength and Young's modulus is improved with the introduction of silicon.

Table IV

Segment Siloxane Carbonization

Treatment Temperature (°C)

Time (min)

M 0 1150

N 0 1800

0 0 2400

P 100 1150

Q 100 1800

R 100 2400

S 50 1150

T 50 1800

Table V

Segment S _c p _FA p _CF %p _CF %Y _CF

(mg/mm) (mg/mm)

M 0.117 0.0849 0.11923 - -

N 0.085 0.0849 0.10459 - -

O 0.051 0.0849 0.09960 - -

P 0.081 0.0849 0.14097 18.2 17.3

Q 0.063 0.0849 0.13256 26.7 30.0

R 0.018 0.0849 0.12354 24.0 28.3

S 0.124 0.0849 0.14725 23.5 28.5

T 0.064 0.0849 0.13175 26.0 28.8 Table VI

Segment Carbon Young's Tensile Strain (%)

Fiber Modulus Strength

Diameter (GPa) (GPa)

(microns)

M 8.27 76.34 1.24 1.64

N 7.77 96.70 1.26 1.30

0 7.27 161.33 1.38 0.86

P 8.55 84.85 1.43 1.68

Q 8.70 109.44 1.33 1.22

R 7.82 174.49 1.45 0.83

S 8.87 90.61 1.46 1.61

T 8.26 101.59 1.43 1.40

[0048] The carbon, hydrogen, nitrogen, and sulfur content of carbonaceous articles is determined by elemental analysis, as summarized in Table VII (wherein ND refers to a non- detectable amount of the given element, or less than 0.5 weight percent). Of particular interest here is the silicon concentration in the fiber. The silicon composition of carbonaceous articles was determined by X-ray fluorescence (XRF) - direct fiber analysis, XRF - fused bead analysis, and inductively coupled plasma - atomic emission spectrometry (ICP-AES), summarized in Table VIII. As used herein for XRF - fused bead analysis, ND refers to a non-detectable amount of the given element, or less than 0.014 weight percent. As used herein for ICP - fused bead analysis, ND refers to a non-detectable amount of the given element, or less than 0.05 weight percent. Silicon content measured with each technique is consistent; further silicon analysis is reported by XRF - direct fiber analysis. The silicon composition, determined by XRF - direct fiber analysis, of carbonaceous articles prepared according to given examples is summarized in Table IX. Untreated fibers - fibers not treated with a silicon source such as Advalon, are measured by XRF as having non-detectable amounts of silicon (approximately 0.01 wt% silicon), and this value is used as a baseline for comparison. It is observed that Si concentration decreases (constant treatment time) as carbonization temperature increases. Table VII

Segment C (wt%) H (wt%) N (wt%) S (wt%)

M 94.4 ND 0.9 1.7

N 99.0 ND ND ND

0 100 ND ND ND

P 97.2 ND 0.5 ND

Q 99.0 ND ND ND

R 100 ND ND ND

S 96.1 ND 0.7 ND

T 99.1 ND ND ND

Table VIII

Segment XRF XRF ICP

(Fiber) Si (Fused) Si (wt%)

(wt%) Si (wt%)

M 0.013 ND ND

P 0.18 0.21 0.21

S 0.41 0.43 0.48

Table IX

Segment Si (wt%)

M 0.013

N 0.004

0 0.012

P 0.180

Q 0.130

R 0.010

S 0.410

T 0.072 [0049] The surface chemical composition of carbonaceous articles, determined by X-ray photoelectron spectroscopy (XPS), prepared according to give examples is summarized in Table X. As used herein for XPS analysis of surface chemical composition, ND refers to a non-detectable amount of the given element, or less than 0.1 atom percent.

Table X

Segment C (atom%) N (atom%) O (atom%) S (atom%) Si (atom%) M 93 ^~ 9 ND 5 05 05

N 97.9 0.3 1.5 ND 0.2

P 71.1 6.1 14.4 ND 6.8

Q 96.6 0.7 2.4 ND 0.2

[0050] Scanning electron microscopy and energy dispersive X-ray spectrometry (SEM- XEDS) is used to scan a cross-section of the prepared carbonaceous articles to characterize the radial distribution of silicon. The distribution of silicon is surprising and unexpected. At 1150°C, there is a silicon coating on the carbon fiber. At 1800°C, SEM-XEDS reports that silicon is uniformly distributed throughout the fiber, thereby showing that silicon is mobile within the fiber at higher carbonization temperatures.

[0051] EDS is used to determine the silicon content at various depths within a fiber at different carbonization temperatures, as reported in Table XI. As used in Table XI, the position of measurement references the distance between the outer edge of the fiber and the center of the fiber, as measured along a radius of the fiber, for example, 25% refers to 25% of the distance between the outer edge of the fiber and the center. Table XI illustrates that silicon migration in the fiber is dependent on temperature, where silicon is primarily at the outer edge of the fiber for lower carbonization temperatures and silicon is roughly uniformly distributed at higher carbonization temperatures. Table XI

Silicon content relative to fiber edge

Position of measurement 1150 °C carbonization 1800 °C carbonization

temperature temperature

Outer Edge of fiber 1.00 1.00

25% 0.22 1.11

50% 0.24 0.89

75% 0.23 1.06

Center 0.21 1.06

[0052] Example 4

[0053] A polyethylene/l-octene copolymer (density = 0.955 g/cm ³ ; MI = 30 g/10 min, 190°C/2.16 kg) is melt- spun to form a continuous fiber tow containing 1758 filaments (diameter = 8.0 microns; tenacity = 4.957 g/denier; elongation-to-break = 7.273%). The fiber tow is then sulfonated in a 4-bath continuous process under constant tension. The first bath contains 20 to 30% free oleum (manufactured by Acros) maintained at 50 °C). The second and third baths both contain 95-98% sulfuric acid (Sulfuric acid 95-98%, manufactured by EMD, UN1830, SX1244-6, Lot 51320; MW 98.08; CAS: 766493-9, bath 2 maintained at 120 °C, bath 3 maintained at 140 °C). Each bath is maintained at a constant temperature. The fourth bath contains deionized water. A constant fiber tension of 292.5 gf (gram-force) is maintained in the first bath. A constant fiber tension of 194 gf is maintained in both the second and third baths. The fiber tow is advanced sequentially through the first, second, third and fourth baths such that the fiber tow resides in each bath for 60 minutes, thereby providing a stabilized fabricated article.

[0054] Segments of the stabilized fabricated article are then treated according to two treatment regimes.

[0055] In regime 1, four segments (A, B, C, D) of the stabilized fabricated article are not treated with a siloxane-containing solution.

[0056] In regime 2, four segments (E, F, G, H) of the stabilized fabricated are treated at room temperature with Advalon CF 3295 for 100 min.

[0057] All segments are then carbonized to 1150, 1400, 1600, or 1800°C according to conditions listed in Table XII. Segment properties are reported in Table XIII Table XII

Segment Siloxane Treatment Final Carbonization

(100 min) Temperature (°C)

A N 1150

B N 1400

C N 1600

D N 1800

E Ϋ ΪΪ50

F Y 1400

G Y 1600

H Y 1800

Table XIII

Segment Carbon Fiber Young's Tensile Strain (%)

Diameter Modulus (GPa) Strength (GPa)

(micron)

A 7.79 88.1 1.70 1.92

B 7.53 90.7 1.65 1.83

C 7.84 96.6 1.59 1.64

D 7.34 113 1.73 1.53

E 7770 9Ϊ 1/77 L9T

F 7.71 93.7 1.66 1.77

G 7.55 102 1.72 1.68

H 7.59 112 1.75 1.56

[0058] Example 5

[0059] Samples A-H from Example 4 are used in this example. Thermo-oxidative stability experiments are conducted on the resulting carbon fibers using TGA. Control and silicon- containing carbon fibers are heated in an air environment from 25-800°C at 10°C/min. The measured properties of the segments are reported in Table XIV.

Table XIV

Example Siloxane Char yield at Temp at Temp at Final

Treatment 800°C (%) 10% mass 50% mass Carbonization

(100 min) (°C) (°C) Temperature

(°C)

A N 0.5433 635.26 604.21 1150

B N 0.1714 667.02 637.51 1400

C N 0.3081 724.59 684.61 1600

D N 7.738 796.96 752.82 1800

E Y 0.3785 647.12 612.78 1150

F Y 0.3266 676.40 650.52 1400

G Y 0.9722 732.63 687.32 1600

H Y 20.80 »800 766.95 1800